INTRODUCTIONVolcanic processes are known to facilitate the

outgassing of some regions of the lithosphere ormantle of the Earth. It is not known, however, howmuch outgassing occurs in non-volcanic regions,either as a general diffusion through the ground, ora s s o c i ated with major fault lines and with earthquakes.Limits can of course be placed on the rates at which certaingases could be added to the atmosphere at the present time, orin geologic history, but such limits would admit the possibilityof a range of interesting effects. In particular, a set ofphenomena that are now well recognized to be associatedwith earthquakes have no adequate explanations without theassumption that large quantities of high-pressure gas escapefrom the ground into the atmosphere at such times. Thesephenomena include sudden and surprisingly largechanges in ground conductivity preceding and duringearthquakes, even hundreds of kilometers from theepicenter; changes in ground water level; changes inthe ratio of compressional-to-shear wave velocities;sharp increases in the radon concentration in groundwater and in the atmosphere; and in the case of largeearthquakes, frequently the appearance of startlingluminous effects in the sky fanning out from thesurface.

The radon phenomenon clearly implies thepresence of a carrier gas, and in adequate amountssuch a gas can account for all the other phenomena.Gases escaping from deep below and at very highpressures can pervade the porosity of the shallower

ground, rapidly displace ground water, change thepressure in microcracks, and of course transport radonto the surface in a time short compared with its 3.8 day radioactive half-life.

No general studies of the possibility ofearthquake-related gas emission seem to have beenundertaken, and there is li t t le direct evidenceconcerning the chemical nature of such gases. Gasesthat are known to be associated with deeper layers ofthe Earth’s crust are CO2, CO, CH 4, H2, N2, H2O andH2S, as major components, as well as small quantitiesof noble gases. It is the phenomenon of luminosity inthe air which gives the only direct clue, namely thatthe escaping gases generally are combustible.

Although attempts have been made to discuss theluminosity effects in terms of electrical atmosphericphenomena, the high conductivity of the ground reallyrules out any such explanation. We have pursuedmany detailed eyewitness accounts of the luminosityphenomena, and these leave little doubt, as we shallsee, that we are concerned with the combustion ofgases coming out of the ground. Methane, carbonmonoxide, hydrogen and hydrogen sulfide arecandidates, and some of those must be involved incombustible concentrations. We believe that ignitionof any combust ib le gases coming out of theground from a high-pressure source will alwaystend to occur through the frictional electricity ofdus t pa r t i c l e s ca r r i ed in the s t r eam. Of thecombustible gases, we consider methane

Journal of Petroleum Geology, 1, 3, pp. 3-19, 1979 3

TERRESTRIAL SOURCES OF CARBON ANDEARTHQUAKE OUTGASSING

Thomas Gold*

The Earth has replenished its surface with carbon throughout geologic time. The supply may havecome from hydrocarbons originally included in the body and outgassing largely through faults in thecrust. Combustible gases are frequently released in earthquakes and seem to be an essential part ofthat phenomenon. New sources of fuel and improved earthquake prediction may come from a betterunderstanding of the processes.

*Director, Center for Radiophysics and Space Research, Space Sciences Building, Cornell University, Ithaca,New York. 14853, U.S.A.

the one that is likely to be the most abundant, and inthe following discussion we concentrate attention on this;however, we recognize that any of the other combustiblegases may also be involved and that non-combustible gasesmay dilute the medium, but generally not to the extent ofmaking combustion impossible.

Continuous outgassing may have importantconsequences for tectonic processes. Thus, it has beenstressed by several investigators of seismic phenomena thatthe deeper earthquakes cannot be understood except with thehypothesis that a pore fluid is present and at a pressure of atleast the lithostatic value (Anderson and Whitcomb, 1975;Griggs and Handin, 1960; and Evison, 1963). Only then can ashear stress result in a sudden slippage along a surface;without such a pore fluid, the high rock pressure would causeshear stresses to be discharged by a gradual and distributeddeformation of the rock only. It does not appear to have beendiscussed what the rate of loss of such a pressurizedfluid would be, and how it is replenished.

Since any such fluid—H2O, CO2, CO, CH4, N2— w o u l dalways be lighter than rock, one has to suppose that infractured rock it will generally move upwards. Even withoutthe fractures caused by an earthquake, the tendency forupward migration must always be there, as soon as the porespaces interconnect over some distances, for then thepressure head in the fluid cannot be balanced everywhere bythe rock pressures. In particular, the upper zone of aninterconnected domain will generally have an excessive, andthe lower part an insufficient pressure to balance that of therock; the lower pores will thus tend to close and the upperones open, making for a general upward migration of thefluid. A downward transport of fluids, to make up any losses,can only occur with a subduction of a mass of rock. Th u s ,subducted sedimentary rocks may supply water, orC O2 from the dissociation of limestone, or othercontained volatiles. But deep earthquakes occur inareas where there has been no large-scale subduction,and where such a source of fluids cannot be expected.If fluids are present there, a supply of volatiles in theprimeval material of the Earth must be suspected.

A continuous (though by no means steady)o u t gassing process may account for the ability of rocks tofracture suddenly, and discharge shear strain by slipping alonga surface, even at great depth (earthquake foci occur down to700 km). But several other features of earthquakes are nowknown that may well depend upon the supply of gases. Thus,the usual occurrence of aftershocks following majorearthquakes requires a time constant for a redistribution ofstresses; but this cannot be found in plastic flow, since this

would generally allow the gradual discharge of the strainin the first place. If pore fluid pressure is involved thepossibility suggests itself that after a major earthquake and thesubsequent escape of fluids through the cracked rocks, theremaining fluids will redistribute themselves with speedsdependent on the porosity. Subsequent shocks may representthe consequences of this gradual redistribution.

The range of other earthquake-related phenomenathat have been identified (changes in the water table,escape of radon from the ground, changes in theseismic velocities, flaming and other pseudo-volcanicphenomena, and also the well-documented strangebehaviour of some animals preceding an earthquake) maywell all be caused by the gases from the deep ground. Firstly,there may be a slight leakage at the top as a gas mass makesits way upwards at some depth; and later, if an earthquakehas indeed been facilitated by this gas, there will bethe more violent effects of gas escaping through thenew cracks generated. The precursory effects willf r e q u e ntly involve no more than the slow displacement ofthe gases normally present in the porosity of the shallowground, but this may be a process that is readily perceived bythe sense organs of some animals (i.e. sense of smell of dogsand pigs, low frequency acoustical effects, asphyxiation forground-dwelling animals, etc.) .

Much of the hydrocarbon supply of the Earth thatis at present being exploited shows strong evidence ofbiogenic origin. Petroleum deposits, and gases associated withsuch deposits must have derived, at least in part, frombiological materials. The evidence for this is strongest for theyoungest petroleum, but gets weaker for the oldest (Robinson,1966). Hydrocarbon gases unassociated with petroleum andcoming from depth carry no definitive information to identifytheir origin. Such non-associated natural gas is known inmany regions, and in exploitable quantities, where noready explanation for a biogenic source is at hand.Even in cases where hydrocarbon gases are associatedwith petroleum, it is not certain that they derived fromit or from a common biogenic source. It is also possible thatpetroleum reservoirs become augmented through absorbingand polymerizing hydrocarbon gases that enter the reservoirfrom below. The disposition of oilfields along major faultlines can perhaps be understood in that way, and also the factthat the older oils are generally the more hydrogen-rich. If, indeed, some outgassing of primeval materialsis taking place, one may suspect hydrocarbon gases tobe a component of these, since carbon in the earlys o l a r s y s t e m w a s p r o b a b l y i n h y d r o c a r b o nc o m p o u n ds to a major extent, and is still found

Thomas Gold4

in that form in the most primitive meteorite material. Thus,terrestrial hydrocarbons may indeed have two different typesof origins, as suggested by Robinson.

Information that combustible gases are a widespreadconstituent in the crust of the Earth cannot fail to be of interestfrom the point of view of the availability of natural fuels. Thequantities that may have escaped in this way and contributedto the carbon on the surface of the Earth in all of geologic timemay be very large when compared with fuel requirements.Thus, if primeval methane had been the chief source ofcarbon, the amount necessary to produce all the carbon in thesediments would be the equivalent of 20 million years ofpresent-day fuel consumption. There is no clear indicationthat the outgassing of carbon has come to an end, and theamounts remaining in the Earth may still be very largecompared with any foreseeable human requirements. Nodoubt most of it is too deep and probably too diffuselydistributed to be accessible to exploitation; but even a verysmall fraction that may have become concentrated in thevicinity of faults and temporarily stored at a shallow depthmay still be a major item when compared with the quantitiesof the known fossil fuels. The detailed investigations ofearthquakes and of many fault lines will therefore be ofi n t e r e s t .

THE SOURCES OF TERRESTRIALC A R B O N

The derivation of all the carbon on the surface of theEarth and in the biosphere is not known with any certainty.The bulk of it is in the form of carbonates, chiefly CaCO3,whose derivation as an ocean precipitate is clear, withatmospheric CO2 being the source material for the carbon.The two basic schemes that can be discussed are: (a) that allthe biospheric carbon came into the atmosphere early in thehistory of the Earth, that it was then quickly precipitatedchiefly as carbonates, and that these carbonates subsequentlywere reworked by erosion and by volcanic heating to producea continuous (but by no means constant) supply of CO2 to theatmosphere ever since; or (b) that the source material for theatmospheric CO2 has remained locked up in the body of theEarth, being released gradually at a slight but significant rate.The evidence of the deposits seems to favor the secondalternative. However, the method by which carbon from theinterior of the Earth slowly became concentrated on thesurface is still quite uncertain.

If the meteorites are in any way representative of thematerials that contributed to the construction of the Earth, thenthere are two major possibilities: either the carbon was

supplied in comparatively high concentration as hydrocarboncompounds, as in the carbonaceous chondrites, and thenoutgassing from a comparatively small amount of suchmaterial and from shallow depths would suffice to producethe observed amounts; or the carbon was supplied in the formof carbonates, carbides and elemental carbon, as present inmany meteorites but in much smaller concentrations, and thenoutgassing from a volume approximating the entire mantle ofthe Earth would have been needed.

Such extensive outgassing would have required an epochduring which the mantle of the Earth was largely molten. Onewould have expected that most outgassing would haveoccurred then, and very little after it solidified. Yet theevidence of the deposits of carbon seems to favor a muchmore continuous supply (Rubey, 1951).

The case for an early complete outgassing, and as u b s e quent reworking of the sediments resulting froman early massive CO2 atmosphere has been argued(Fanale, 1971). But there is no evidence for such largequantities of very early carbonate deposits, and theywould need to have been rather completely subductedto escape detection. Also, a massive CO2 atmospherewould need to be coupled with a substantially weakerSun in order to avoid so high a temperature on theEarth that would preclude the deposition ofcarbonates. Still, these are possibilities that cannot beruled out—but there is no case here that is so strong thatthe alternative does not need to be consider e d .

The atmospheric content of the noble gases has beenused to estimate what fraction of the Earth has beeno u t g a ssed. This is dependent upon the crit icalassumption that the building material of the Earth wassimilar in its rare gas content to that of the smallsamples of apparently primitive material that present-day meteorites provide. If the Earth was constructedfrom material that had a more complex history, anda c c r e ted inhomogeneously, these considerations would notbe applicable. Even using such assumptions, the range ofmeteorite rare gas concentrations (Wasson, 1969) wouldallow estimates ranging from 4% to 100% of the Earth havingbeen outgassed.

Urey (1952) gave reasons for considering that theoriginal condition of carbon in the accretion of the Earth wasin the form of hydrocarbons, and many authors have sincefollowed this line of reasoning. Whether this implies that thesupply to the surface over geologic time was largely methane,or whether earlier processes had oxidized this, is not clear. Ineach case, CO2 would be made available in the atmospherevery quickly, and provide the source material

Terrestrial Sources of Carbon and Earthquake Outgassing 5

for laying down the carbonate deposits that now form morethan two-thirds of all surface carbon. Methane is estimated tobe destroyed in the atmosphere in a period of between fourand seven years (Ehhalt, 1974), with essentially all the carbonending up as CO2. If methane from the interior formed themain source of a continuous carbon supply to the surface,because of its short atmospheric residence time, it would stillimply only a small methane concentration in the atmosphere,and, as we shall see, the present atmospheric compositiondoes not rule this out.

The view that carbon had emerged as CO2 and not ashydrocarbons appears to have been based chiefly on theobservation that CO2 is abundant and hydrocarbons rare involcanic gases. This observation has nothing to say as to theoriginal form in which the carbon was held in the Earth beforeemerging through a volcanic zone; one understands that theequilibrium between hydrocarbons and CO2 in the presenceof potential oxygen donors, such as water and metal oxides inthe magmas, would greatly favor almost all carbon emergingas CO2, irrespective of the original form. Outgassing by otherpathways, where the temperature is low when the pressure islow, would retain hydrocarbons if they were present in theoriginal source, and among those methane would be favoredas being the most stable against dissociation.

Discussions of the cosmochemistry of the early solarsystem make it seem probable that carbon was initially muchmore in the form of hydrocarbons than in oxidized form. Ifthe meteorites are in any way representative of materials inthe early solar system, then it is relevant that the ones that arerich in carbon, namely the carbonaceous chondrites, have itmostly in hydrocarbon form. At temperatures and pressuresoccurring down to several hundred kilometers, somehydrocarbons, including methane, would still be stable, andone has every reason therefore to inquire whetherhydrocarbon outgassing is taking place.

The total quantity of carbon that has been supplied to thes u r face has been estimated by various authors. Weshall consider the range given by Rubey’s estimate(1951) of 2.5 X 10 1 6 tons and that of Galimov,Migdisov and Ronov (1975) of 7.4 X 101 6 tons oftotal carbon in sediments (the atmospheric, oceanicand biomass carbon is a small quantity bycomparison ) .

For this to be supplied steadily over 4.5 billion years,between 1 and 3 X 101 0 cubic meters of CO2 or CH4 w o u l dneed to be provided on an average per year.

One method of estimating at least limits on the amount ofprimeval methane that may be entering the atmosphere is toobserve the proportion of 1 4C in atmospheric methane, and

compare that with the proportion in atmospheric CO2. Since1 4C has a half-life of 5730 years, this will define whatcontribution is derived from sources that have notinterchanged carbon with the atmosphere for many thousandsof years and therefore lack 1 4C. Methane from petroleumoperations and industrial processes is in that category, as isalso the product of decay of some buried biogenic deposits, inaddition to any primeval methane. The estimates given byEhhalt (1974) are that 20% of the atmospheric methane lackssufficient 1 4C to be of recent biological origin, but that onlybetween 7.4 and 38% of that can be accounted for byindustrial and known natural sources. His estimates wouldentitle one to consider that unknown sources of 1 4C-free CH4

amount to between 2.7 and 1.1 X 101 1 cubic meters per yearat the present time. This may be compared with estimates ofthe time average of the total carbon supply. According tothese, if methane were the chief source, an average ofbetween 1 and 3 X 101 0 cubic meters per year would need tobe supplied. This is still only a small fraction of the“unaccounted for” atmospheric 1 4C-free methane.

We cannot judge at the moment whether indeed there issuch a primeval methane supply that is at present a little aboveits long-term mean, or whether the discrepancy is due touncertainties in the estimates. But the figures serve to showthat contributions of primeval methane, very significant forthe total surface carbon content, would still not have producedany very prominent effect in the atmospheric methane budget.

The other carbon isotope determination, the ratio of 1 2Cto 1 3C, is also not conclusive in deciding whether anymethane that is found constitutes a primeval supply oris the result of a biogenic deposit. Carbon that isbiogentically deposited is generally isotopically light,i.e. contains less 13C, by between ten and thirty partsper thousand compared with marine limestone (this isusually expressed on the PDB carbon isotope scale (Craig,1953) as δ1 3C, which is the deviation, expressed inparts per thousand from the adopted standard 13C/12Cratio of 1123.72 X 10-5). In almost all circumstances,however, the oxidized carbon is found to be heavy andthe reduced carbon light, and this can be understood,at least in part, as an equilibration process, dependenton the temperature at which reactions take p l a c e(Bottinga, 1969). The fact that most methane coming fromdepth is isotopically light has been interpreted as implying abiogenic origin, since biological processes are known to selecti n favor of the light isotope for reduced carbon.However, a range of light carbon also exists in thehydrocarbons of carbonaceous chondrites, moreover,the process of cracking that would be r e s p o n s i b l e

Thomas Gold6

7

for generating methane from complex hydrocarbons inthe crust is known to favor the light isotope, and theprocess of slow diffusion of methane throughmicropores in the rocks may further contribute to suchselection. (For the heavy CO2 molecule, this effectwould be small.) It is not at all clear that biology isunique in its ability to shift the δ13C value by ten ortwenty parts per mil towards the light side (Hoefs,1972) which is all that is involved in the case ofsources of methane that may be candidates for aprimeval supply. (Fuex, 1977, gives a range of —25to —30o/ o o for the δ1 3C value of “geothermalmethane”, while the mean of all terrestrial carbonappears to have a value around —10o/oo).

The meteorites demonstrate a large amount ofisotopic fractionation, with the carbonates heavy andthe reduced carbon light, but with a range betweenthem still larger than generally occurs in terrestrial

carbon. Carbonaceous chondrites (Krouse andModzeleski, 1970) have a range of δ13C values from+60o/oo for the carbonates to —30o/oo, for the reducedcarbon. with the mean for most samples being around—10o/oo), similar to the terrestrial average. In thiscase, equilibration and diffusion are generallydiscussed as responsible, and no t b i o l o gi c a lp r o c e s se s .

Terrestrial igneous rock is reported to have arather evenly distributed concentration of 200 ppm ofreduced carbon with a δ1 3C value around —25 to — 2 8o/ o o. Hoefs (1972) considers that this uniformdistribution would be difficult to understand on a basisof biogenic sources and thinks it more likely that it isof primary origin. He concludes that such isotopicselection may arise abiogenically. This range of δ13Cvalues is very small, compared with most othermaterials, and it is just the same as the small

Terrestrial Sources of Carbon and Earthquake Outgassing

range quoted by Fuex (1977) for geothermal methane. Onemay wonder whether a generic connection exists betweenthese two forms of reduced carbon.

There are many other problems in the interpretation ofthe terrestrial carbon isotope variations that we have nottouched upon here; they leave an uncertainty as to whetherthe supply isotope ratio has been constant over geologic time(Galimov and others, 1975; Eichmann and Schidlowski,1975; Veizer and Hoefs, 1976), and they suggest that thepresent supply is lighter than re-cycled archaen carbonateswould have made it.

It has sometimes been stated that hydrocarbons cannot bepresent at great depth in the crust, since the temperatures therewould be high and cause decomposition. However, thestabilizing effect of pressure is very important. We mayassume that a temperature of 1300°K is generally reached at adepth of approximately 100 km and therefore at a pressure of33 kilobars (Jeffreys, 1959, p. 305). It appears that someh y d r o c arbons are quite stable under these conditions.Detailed thermodynamic calculations have beencarried out by French (1966) and Karzhavin andVendillo (1970) of the equilibrium between CO2, CO,C H4, H2, H2O for temperatures up to 1500°K andpressures up to 5 kilobars, and these show theimportant stabilizing effect of pressure, in particularon methane, which would be the major component in the1500°K and 5 kilobar condition, and probably even at muchhigher temperatures. If methane were the supply at depth, awide range of temperature-pressure conditions on the way tothe surface would maintain the gas in that form.

Oxidation of methane to CO and CO2 would take placeen route in circumstances where firstly an oxygen donor wasavailable, and secondly where the temperature was high whenthe pressure was low. These conditions are most likelyto be met along a volcanic pathway. In addition toproviding the high temperature near the top, theprocess of bubbling gases through liquid rock willallow oxygen to be extracted from dissolved water orfrom oxides. In pathways through pores in solid rockthe available oxygen from surfaces is much morelimited. On this basis, one could understand that thegas emission from volcanic vents contains most of thecarbon in oxidized form, even if it originated fro mhydrocarbons. (Of course, there is always the possibility thatsome volcanic regions provide CO2 and CO that was derivedfrom the decomposition of subducted limestone, ratherthan from a primeval source of c a r b o n . )

Karzhavin and Vendillo (1970) also provide calculationsfor the comparative stability of methane, ethane, propane and

butane. These show that methane would be greatly favored asthe gas to escape on top, at low temperatures and pressures,even if the other hydrocarbons were present at hightemperatures and pressures. This can account for the findingthat the deepest sources of hydrocarbon gases appear tosupply a very high proportion of methane (Tiratsoo, 1976).

Out of all this, no compelling reason emerges foridentifying the gas that has been responsible for a continuoussupply of carbon to the surface. Many other considerationscan be introduced, the most obvious being the question of theoxygen supply.

The two sources of supply of oxygen to the atmosphere(other than the mere recycling of oxygen in the biosphere) arethe photosynthetic reduction of the carbon that emerged asC O2 but was laid down as organic carbon, and thephotodissociation of water in the upper atmosphere, coupledwith the escape of hydrogen. Brinkmann (1969) discusses theEarth’s oxygen balance, and in contrast to the earlier work ofBerkner and Marshall (1966), he concludes that a verysubstantial sink for oxygen over geologic time has to be foundfor the atmospheric oxygen to be restricted to the low valuesin pre-Cambrian times that most investigations consider to beindicated by the biological evidence. He notes that theoxidation of methane might constitute such a sink, butconsiders that the quantities that would have been required aree x c e s s i v e .

We can draw up an oxygen balance sheet for theatmosphere and the sediments, for the period before thewidespread release of oxygen by photosynthesis, and forvarious assumptions concerning the quantities of thesediments and the nature of the supply of carbon.Brinkmann’s calculations then give for each case theapproximate value of the oxygen pressure that would havehad to have existed over most of geologic time (4.6 billionyears) in order that photodissociation of water would haveprovided the required oxygen. (The photodissociation rate ofwater in the atmosphere is high when the oxygen pressure isl o w . )

Case I: Adopting Rubey’s (1951) estimate of thequantities of the carbon deposits, and assuming, arbitrarily,that half the carbon emerged as CH4 and half as CO2.

Case II: Like Case I, but adopting estimates for thecarbon deposits three times as high as those of Rubey,approximately in accord with some more recent estimates(Galimov and others, 1975).

Case III: Adopting again the higher estimate forthe deposits, but including the possible oxygendemand of the lithosphere, estimated

Thomas Gold8

by Brinkmann as due to the oxidation of all lithosphericFeO to Fe2O3.

Case IV: Rubey’s values for the deposits, no lithosphericoxygen demand, but all carbon supplied unoxidized as CH4.

Case V: Like case IV, but with the high values for thed e p o s i t s .

Case VI: The high values for the deposits and t h elithospheric oxygen demand, with again CH4 as the supply.This represents the highest oxygen requirement to be suppliedby photodissociation of water with any of the possibilitiesd i s c u s s e d .

From Table 1 it is evident that methane may well havebeen a major source of all surface carbon, and that theoxidized state of pre-Cambrian deposits can be accounted forby the photodissociation of water. Indeed, if the pre-Cambrianatmosphere had little oxygen, as has frequently beensuggested based on the biological evidence, then a verysubstantial methane supply is indicated.

In the course of geologic time, there may well have beenlarge changes in both the rate of carbon outgassing and in theproportions supplied as CH4 and CO2.

The composition of the present atmosphere is difficult toaccount for if all carbon had come up as CO2. The oxygenresulting from the photosynthetic reduction of CO2 (in theperiod since widespread photosynthesis came into existence)and the permanent deposition of reduced carbon in sediments,amounts to much more than appears to be present in theatmosphere or in additional oxidation of sediments. While thequantitative estimate of the various deposits is perhaps notvery certain, it is at least clear that the data favor rather thandeny the possibility that some of the carbon has emerged inunoxidized form.

The next question we shall turn to concerns the processesof outgassing through faults in the crust and upper mantle. It hassometimes been stated that at a depth of more than 10 km or so allporosity in the rocks must be crushed out, suggesting an absence of anypathways for gases. This is not so if any gases or liquids are present atpressure equalling or exceeding that of the local rock. Onepresumes that shear faults would greatly facilitate the movementof such high-pressure gases, and particularly favorablecircumstances for their escape would occur at the instant ofmajor earthquakes. We shall now discuss the evidence thatindeed large quantities of gas, and usually of a combustiblegas, escape during major earthquakes.

THE EARTHQUAKE EVIDENCE FOR THEESCAPE OF GASES

Many features of major earthquakes are known that seempuzzling and for which no completely satisfactory

explanations have yet been offered. A list of these phenomenamight be tabulated as follows:

1. lights in the air associated with earthquakes;2. large changes in the electrical conductivity of the

ground preceding and following earthquakes;3. changes in ground water levels;4. radon2 2 2 excesses in the atmosphere preceding or

during earthquakes;5. changes in the ratio of the seismic velocities;6. aftershocks;7. the “visible waves” phenomenon;8. Large volumetric changes associated with earthquakes.All these phenomena are very well documented, and we

shall discuss their interpretations in terms of the escape ofg a s e s .

Earthquake lightsMost major earthquakes that occurred at night appear to

have been accompanied by a display of luminescence of theair near the epicenter (and sometimes even as much as ahundred kilometers away) described by most observers aslight fanning out from the ground and getting weaker withheight. The patterns of light are sometimes described as a setof beams radiating from a point on the horizon, likesearchlights turned to the sky, and the color is usuallydescribed as blue or white. The phenomenon is seen at seaalso, and has there been described to look like a burning shipon the horizon. Such displays in general seem to have lastedsome minutes, usually following but sometimes evenpreceding the major earthquake. Richter (1958) states that“rarely have they (earthquake lights) been missing fromreports of any large earthquake in a populated area”. Derr(1973) gives a review of such observations that leave littledoubt that the phenomenon is real, and indeed frequent.Photographs of such events are available, chiefly from Japanin the mid-1960’s, showing a bright hemisphericalluminescence based at ground level.

The number and the consistency of such reports fromm o s t major earthquakes is quite remarkable. There isno question that the explanation of earthquakephenomena must encompass this effect. Someattempts at an explanation are in the recent literature,mostly considering the phenomenon as amanifestation of atmospheric electricity. These havebeen criticized, and it is indeed clear that no electricalphenomena causing a breakdown in the atmosphere can arisefrom the ground in any circumstances in view of the values ofthe ground conductivity. Before any atmospher i cbreakdown could be generated, Earth currents

Thomas Gold10

would reach values at which very large magnetic effectswould be a major consequence: yet no significant magneticdisturbances are reported in association with earthquakes,even from nearby magnetic observatories. The well-documented occurrence of similar phenomena at sea makesthe same point even more clearly. Electrical breakdown in theatmosphere has also been attributed to some unspecifiedconsequence of atmospheric pressure waves, generated by theearthquake, but no such mechanism is known; furthermore,the purely mechanically generated disturbances in theatmosphere are quite small compared with the normalmeteorological disturbance level.

Combustion of gases is the obvious alternative, and thereis a large amount of evidence, in many cases quite decisive,that this was involved. Very detailed descriptions exist offlames seen issuing from the ground during great earthquakesin different parts of the world. For example, during the OwensValley (California) earthquake of 1872, the San FranciscoChronicle reported (April 2,1872) that “people living nearIndependence, at points where they could see plainly the sidesof the mountains on either hand [said] that at everysucceeding shock they could plainly see in a hundred placesat once, bursting from the rifted rocks great sheets of flameapparently thirty or fifty feet in length, and which would coiland lap about a moment and then disappear.”

Or, from the Inyo Independent (April 20, 1872):“Immediately following the great shock, men, whosejudgment and veracity is beyond question, while sitting on theground near the Eclipse mine, saw sheets of flame on therocky sides of the Inyo mountains but a half a mile distant.These flames, observed in several places, waved to and froapparently clear of the ground, like vast torches; theycontinued for only a few minutes.” Similar observations werereported (Stoqueler, 1756; Hamilton, 1783, and Humboldt,1881, p. 163-164) during the great earthquakes at Lisbon in1755, at Calabria in 1783, and at Cumana in 1797.

Not only are there many eyewitness reports describingthe phenomena in terms of flames of a burning gas, butevidence of burning could subsequently be found along faultlines. For example, Goodfellow (1888) describes the greatSonora earthquake of 1887: “The Sierra Madre fires were,beyond question, synchronous . . . The evidence of gaseousirruption were few but striking. Primarily were thestatements of many who claim to have seen streaks offlame at different points, in the course of the firstnight in particular, and several times thereafter duringsucceeding days and nights while the heavy shockscontinued . . . the evidence [for ignited gas] was found inseveral places, both in the river beds and in the hills along the

line of faulting. This consisted of cinders about the marginsand on the walls of the river fissures, and the discovery ofburnt branches overhanging the edges of such places, as wellas the same testimony on some of the hills and mountainsnear the main fault.” In the Owens Valley earthquake,there are also reports of fires having been started inthe mountains by such sources.

Since unstable burning of gases frequently results inexplosions, one might expect loud airborne explosive noisesto have been associated with the phenomenon. Indeed, thereare many reports of this kind also. For example, Kingdon-Ward (1951) reported of the great 1950 Assam earthquake:“From high up in the sky to the northwest (as it seemed) camea quick succession of short, sharp explosions—five or six—clear and loud, each quite distinct, like ‘ack-ack’ shellsbursting”. Kingdon-Ward was a few miles from the epicenterbut the sounds were heard as far as 750 miles away(Mukherjee, 1952). Other accounts of earthquakes havereferred to airborne noises “like the whistling and rush ofwind” (Thompson, 1929), or “like the escape of steamfrom a boiler” (Fuller, 1912).

If the cause of these phenomena is indeed the escape ofcombustible gases at high pressure out of fissures in theground, we can describe the events that would be expected.The very high velocity gas in the cracks would pick upparticulate matter, and, as in practically every industrial dustpumping process, electric sparks would be generated throughthe frictional charging of such particles and the transport ofthis charge against the electric field set up (Van deGraaf action). These will serve to set fire if acombustible mixture exists. If the velocity of flow ishigh, the flames would usually be disconnected fromthe orifice and continue burning only at that levelabove the ground where mixing with atmosphericoxygen is sufficient to produce a flame that can burnback at the local stream speed of the gases, so as toremain stationary (just as in the case of a bunsenburner, whose air intake is closed). It is to this modeof burning that we attribute the evidence that althoughflames are seen coming from the ground, in many cases thereis an absence of evidence of scorching of material on theground; although in other cases, as we have said, there clearlyis such evidence.

In the descriptions of great earthquakes in cities, therehave been many reports of fires shooting out of fissures in thestreets. Usually, this has been interpreted as due to gas mainsbursting, and this interpretation may indeed be correct. It isalso possible, however, that some of these fires have to beattributed to the same causes as we have just discussed.

Terrestrial Sources of Carbon and Earthquake Outgassing 11

Many of the reports of the flames or the light make apoint of stressing the preponderance of the phenomenon onthe hillsides and in regions of bare rock. One can understandthat in terms of the outflow of gases, since fissures in thebrittle rock would provide much better escape routes wherethey are not overlaid by a thick alluvial deposit that does notcrack nearly so readily.

Fires of combustible gases associated with earthquakeswere regarded as well-documented occurrences in much ofthe earlier literature on the subject; von Humboldt (1849, p. 209), for example, cited several examples from differentareas. It is strange that this interpretation of the “earthquakelights” has been forgotten to the extent that it is not evenmentioned among the possibilities in modern articles on thesubject. In fact, the general association of earthquakes withgas emission seems to have been generally recognized as farback as classical times (Adams, 1938, p. 399). Since thebeginning of the present century, however, little attentionseems to have been given to this type of evidence and to theearlier discussions.

Earthquake Precursory EffectsA number of different physical measurements of the

ground have been identified in recent times as showingprecursory effects for earthquakes, and usually large changesat the time of the ‘quake (Scholz, Sykes and Aggarwal, 1973).These measurements concern the electrical conductivity ofthe ground, ground water levels, the transport of radon gasthrough the ground, and variations in the ratio of thecompressional to the shear wave seismic velocity. All theseeffects are in a sense remarkably large. For example,precursory changes in electrical conductivity exceeding 20%have been measured (Mazzella and Morrison, 1974). In aregion in which changes in strain of the ground amount to nomore than 10- 9, changes in electrical conductivity of 10- 5 a r eobserved (Yamazaki, 1975); such effects are seen as clearprecursors to the earthquakes that may be as much as severalhundred km away, occurring usually a few hours after theonset of the conductivity change. Similarly, the precursoryeffects of a decrease of the ratio of compressional to shearvelocity occupies a very large region, and not just the one thatwill subsequently be involved in the fracturing process of theearthquake. Equally, the ground water level changes are seenover a large region, most of which will not take any activepart in the generation of the earthquake. If, as has beenproposed (Scholz and others, 1973), all these effects were dueto a dilation of the rock due to the widening of microcrackswhen the shear strain approaches the critical value forfracture, then the phenomena should be confined to the region

that initiates the eventual fracture. It would seem most

improbable that a very large region would all reach the critical

stress for fracture at so nearly the same time. in the case of a

very slow build-up of stress, and in view of the scale of

unevenness of the material, the temperature, the topography

of the over-burden and many other factors. In any fracture

phenomenon, it is more usual for a small region to reach the

critical stress, but for the resulting crack to propagate into

regions in which the stress was originally much below critical,

but which became locally stressed through the propagation of

that crack. In any such picture of fracture we would not

understand how regions of hundreds of kilometers would be

simultaneously affected shortly before the earthquake.

The rock dilatency model clearly gives a good

explanation for the relationship between all the observable

quantities mentioned in terms of a change of the amount of

porosity of the ground, or of the content of the pores. Thus,

sudden changes in ground water level can readily be

correlated with sudden changes in ground conductivity, or

with a change in the ratio of the sonic velocities. It is only the

origin of these porosity changes over wide areas, and the

sudden onset that are not satisfactorily accounted for.

The large changes of the radon content of ground water,

or of the air above (Smith, 1976; Mogro-Camparo and

Fleischer, 1977), cannot be understood without another gas

serving as the transport agent. Such changes have been

observed a few days before an earthquake, and at distances of

the order of one hundred kilometers from the epicenter.

R a d o n2 2 2 has a half-life of only 3.8 days, and the amount of

diffusion that it could suffer in periods of that order in the

ground would be restricted to a few meters. This is true for

any realistic value of the porosity of the ground, and therefore

a mere change in the dimensions of the pores is not the

explanation. Even the transport of ground water, which may

change its level in the same time by distances usually not

exceeding one meter, cannot be held responsible for the

enrichment of radon in the water, let alone for its escape into

the air above. Mogro-Camparo and Fleischer (1977) discuss

this problem but confine themselves to the gaseous transport

provided by a thermally-driven convective motion. The

distances over which such convection could drive radon in a

few days are very small (the authors quote 100 m in 20 days

in loose sand). In compacted soil at some depth, this can

certainly not account for large changes as are observed in a

few days as precursors to earthquakes. It is clear that a gas

driven by a large pressure gradient has to be the vehicle.

Thomas Gold12

Faults in the crust will assist in the outgassing processfrom great depths by letting high-pressure gases migrateupwards along them. Any crack held apart by gas andspanning some interval of height then possesses thehydrostatic pressure gradient in the rock over this height, but avery much smaller pressure gradient in the gas, correspondingto its lower density. If the rock can deform, the tendencywould be therefore to widen and extend the crack at the topand to close it at the bottom. In the course of this upwardmigration, in a deformable rock, the gas will be maintained atapproximately the ambient rock pressure until it reaches thelevel at which there is naturally a porosity in the ground. Atthat level, generally between two and 10 km below thesurface, the gas can spread out and move rapidly through thepre-existing porosity over large regions. If, for example, thisgeneral porosity becomes momentarily connected to afissure that extends to a depth of a km lower, the gasin it may supply a pressure of 300 atm above that ofthe ambient rock. On this basis, one could understandthat it would move over large distances rapidly andthat it would dilate the pores. Where there is water it wouldtend to displace it and therefore temporarily raise the watertable, until the hydrostatic instability causes the lighter gas toget above the heavier water. In the fine pores, surface tensionand viscosity will delay this instability from acting, andwe estimate that it could easily be much slower thanthe period of days that is involved in some of theprecursor effects. Only such a gas can be heldresponsible for transporting the radon over largedis tances in a short t ime, and s ince i t wouldeffectively collect the radon from a large surface area ofporosity below, it must in general cause an increase in thesurface radon concentration. The radon thus merely acts as aconvenient tracer, showing that an event of high-speed gasflow through the rock pores has taken place.

The mechanical consequences of a gas invading a bodyof rock under high pressure are likely to be very important. Ata certain depth, in the absence of gas, a rock will haveinsufficient strength to maintain any vacant volumes. Whilesuch a rock would acquire a porosity and dilate if subjected toa certain shear stress under low hydrostatic pressure, at asufficiently high hydrostatic pressure no value of shear stresscan create porosity. Under those circumstances, thephenomenon of the propagation of cracks must be greatlyimpeded; rocks are known to become ductile (Griggs andHandin, 1960). Evison (1963) discussed the serious problemsthis raises for the widely-held view that brittle fracture andelastic rebound provide the mechanism for earthquakes; heconcluded that another mechanism is required. The

occurrence of earthquakes down to a depth of 700 km is cited,where the frictional forces in a crack would be too large by afactor of 1,000 to allow crack propagation. He also considersthat the striking similarity of the seismic signals ofearthquakes with those of subterranean atomic bomb blastsprovides a further argument against that theory.

In the presence of a gas at the hydrostatic pressure of therock, the situation is completely changed. Any pores that thegas can reach can open up, with the displacement energybeing provided almost entirely by the gas, because of its“soft” equation of state (i.e. large volume change correspondsto a small pressure change). If the rock is under a large shearstress that would have caused the development of porosityunder zero hydrostatic rock pressure, the gas will effectivelybear all the pressure in any pores it can invade and againallow the growth of porosity. One would assume that aconnected pattern of pores would then grow, pervading such abody of rock, and that the effects will be a general volumetricexpansion, accompanied by a rapid change in the mechanicalproperties from a ductile to a brittle material. The invasion bya gas is then a precursor of an earthquake, because it is thisthat suddenly converts the rock into a material capable ofshear fracture and elastic rebound. If then the shocks socreated are strong enough to fracture the ground up to thesurface, this same gas will find a rapid escape route, andproduce the large surface effects of gas emission that arereported for almost all major earthquakes. (In somewhatdifferent discussions of the problem, Orowan (1960), Mogi(1967), Griggs and Handin (1960) and Evison (1963) havealso stressed the importance of pore fluid pressure tofacilitate sudden slip.)

A f t e r s h o c k sLarge earthquakes generally have a series of aftershocks

decaying over a period of months or years. This is usuallyattributed to a gradual “settling down” of the stressdistribution in the ground that takes new areas to breakingpoint in the general vicinity of the original epicenter. Thenature of the time constants, of the order of months, for thissettling effect is not clear. If these are time constants derivedfrom plastic deformation, they are hard to reconcile, at least ina sensibly homogeneous rock, with the occurrence of brittlefracture. The amount of redistribution of the stress has to bevery large so that rock will later initiate a fracture, whenearlier at the time of the main ‘quake it did not break andrelieve its stress, despite the severity of the shock. If thephenomenon is to be interpreted in terms of somecombination of rheologic properties of rock, this explanationmust suffice for many different types of rock

Terrestrial Sources of Carbon and Earthquake Outgassing 13

under different pressure and temperature conditions. We thinkit unlikely that sufficiently universal properties of rock leadingto aftershocks can be found.

On the basis that the escape of gases is a majorcomponent of the earthquake phenomenon, one can invoketime constants of the gas flow through pores. The mainearthquake presumably corresponds to the escape of gas fromunderground chambers, released by the sudden opening offracturing rock. After this release of a gas mass, the internalgas pressure distribution in the remaining porosity is changed,and probably a fairly good exit path continues to exist forsome time. We may imagine that this will lead to the drainingof gas from pockets over a gradually widening zone, and inturn that the withdrawal of pressure in such pockets will causelocal fractures and collapse. We would attribute the aftershockphenomenon to this type of gas migration, and thereforeconsider that it should be possible to detect a continuedoutflow of gases in the general region for the entire durationof the aftershock sequence.

‘The “Visible Waves” PhenomenonIn detailed reports of many major earthquakes, a very

remarkable and unexpected phenomenon is described,whereby large waves are seen to progress on the surface ofthe ground with a height sometimes declared to be as much as1-2 ft, a wavelength ill-defined but of the general order of 10-100 m, at a speed of propagation also perhaps ill-defined butat any rate low enough for the progress of individual wavecrests to be clearly seen. Eyewitness accounts generallydescribe this phenomenon as quite distinct from the “hammerblows” of the sharp shocks, and some descriptions comparethe land surface with that of waves in the sea (Sloane, 1694;MacDonald, 1918; Thomas and Witts, 1971, p. 69).

Fuller (1912) quotes one eyewitness to the New Madridearthquake who says that “the Earth rolled in waves severalfeet high with a visible depression between the swells, finallybursting and leaving parallel fissures extending in a north-south direction for distances as great as five miles in somecases.” Richter (1958) writes that “visible waves are mostcommonly reported from the meizoseismal areas of greatearthquakes, particularly on soft or alluvial ground.Consequently, many of the clearer and more accessibleaccounts come from India”. He quotes one (Oldham, 1899):“The ground rocked violently and we were both throwndown . . . We saw a series of earthwaves approaching over thesurface of the ground, exactly like rollers on the sea. As thesepassed us, we had some difficulty in standing, but none of thewaves reached the intensity of the first which had overthrownus . . . As the waves above subsided, the ground began to

crack at our feet . . . This was immediately followed by theemergence on the spot of earthquake fountains . . . Thisoccurred in an alluviated region . . .”. Another observer of thesame earthquake “saw the ground in every direction shakinglike soft jelly.”

Similar observations were made during the earthquakes

of 1886 at Charleston, South Carolina (Dutton, 1889, p. 265-268) and of 1906 in San Francisco (Lawson, 1908).

It is not the magnitude of the displacements that are soremarkable, but rather the apparently slow wave speed. If allphenomena occurred at the speeds of the appropriate seismicwaves, these speeds would be far too great for the approach ofindividual wave crests to be seen. Richter considers thephenomenon real, although frequently somewhat exaggeratedin the reports. Just as in the case of the earthquake lights, it isthe consistency of the descriptions from different parts of theEarth that are most persuasive of their reality.

In terms of the gas release interpretation of earthquakes,

this phenomenon can be accounted for in the followingmanner. As a result of the major fissure in the underlyingrock, a large amount of high-pressure gas comes up withpressures that may well be of the order of hundreds orthousands of atmospheres. The alluvial fill covering thefractured bedrock is generally less brittle and will not soreadily open large fissures. It therefore acts as an extraimpediment to the outflowing gases whose pressure is easilysufficient to lift it entirely. When so lifted from the bedrock,the rigidity of this material is low and it is of course quiteunstable. A phenomenon of the ground “shaking like a jelly”

or “rolling like waves at sea” then seems entirely possible.(We find that Michell (1761) already offered a similarexplanation.) We may well presume that the concomitantfissures were the exit paths that the high-pressure gas hadmade. In the earthquakes of 1783 in Calabria (Hamilton, 1783and Lyell, 1892) and of 1887 in Sonora (Goodfellow, 1888,and MacDonald, 1918), both the “visible waves” and theflaming phenomenon were reported, showing that acombustible gas was involved.

We do not know at present how much of the devastationof earthquakes is generally caused by this phenomenon andhow much by the sharper shocks transmitted at seismic

speeds. Perhaps cities that are built on an alluvial fill of novery great depth could take precautions against thephenomenon by digging a pattern of trenches down to thebedrock, thus allowing a rapid escape of gas from there.

A quantitative estimate can be made for the mass of gasnecessary to cause the phenomenon.

Thomas Gold14

Thus, for example, for an area of l0 x l0 km and an alluvialfill 30 m deep to the bedrock, we would require a minimumof 50,000 tons of gas to raise this so that waves 30 cm highwould form. Probably, several times more is really required,allowing for some escape. In any case, these quantities, inevidence in large earthquakes every few years, would amountto only a small fraction of the mean terrestrial outgassing rate,which we estimated as between 7 and 21 million tons per year(10-30 billion cubic meters), if in the form of CH4. We haveno estimate of the amounts of gas that may be escaping at thesame time without causing this type of phenomenon.

Volumetric ChangesLarge volumetric changes of the ground are related to

e a r t h q uakes. Very long period precursory phenomenahave been observed, involving the raising of hundredsof sq km of ground by several centimeters, takingplace over periods of many years (Wyss, 1975),sometimes followed by a sudden collapse at the timeof the earthquake. In other cases, sudden raises orsudden falls of the land occurred at the time of theearthquake (Lyell, 1892, and Rikitaki, 1976), in somecases by as much as several meters.

These volumetric changes have been attributed in recenttimes mainly to dilation phenomena due to opening ofmicrocracks under shear-stress, or the rapid closing of themafter the annihilation of such a stress. With the evidence thathigh-pressure gas is frequently involved, we may considerthat the porosity increases and the consequent surface lifts aredue to the migration of a gas through layers of rock,expanding as it ascends, and that a surface fall thencorresponds to the escape of that gas. Such an interpretationhas the advantage that it can account for volumetric changeseven in cases where the phenomena are taking place at adepth at which the maximum shear-strain energy would beinsufficient to create porosity in the presence of the pressureof the overburden; also a wider range of temperature andmaterial properties can be accommodated by such ane x p l a n a t i o n .

The large uplift in Southern California, referred to as the“Palmdale Bulge” (Castle, Church and Elliott, 1976, Smith,1976), appears to have taken place mostly over the lastfifteen years, and may be a similar phenomenon toother precursor uplif ts . If i t is , i t would be asubstantially larger effect than has been reported in theother cases (25 cm maximum rise, and lateraldimensions of 200 x 100 km). If it is interpreted as anexpansion of rock due to invasion by gas, thenquantities of the order of 109 tons would be implied. If

CO2 and CH4 are major components, then their escapewould correspond to several hundred years of themean supply rate. While the nature of the Palmdale Bulgeis not clear, an interpretation along these lines would make itan unusually large phenomenon.

The production of tsunamis (“tidal waves”) in the oceanby large earthquakes is common, and indicative of large andrapid volumetric changes. A vertical displacement of the seafloor is usually considered as the cause (Press, 1963, and Iida,1963). Good estimates of the tsunami energy can be made,and these have been compared with the estimated earthquakeenergy. For large earthquakes, the tsunamis seemed to containapproximately one-tenth of the total earthquake energy, and asomewhat smaller fraction for smaller earthquakes. However,the calculation of the volumetric displacement necessary toproduce the wave leads to values that are quite outside therange of known landbased displacements (Kravtsov andVoitov, 1976). In turn, such sea floor displacements as wouldbe required would imply very much larger values for theearthquake energy. This can be seen by the following simplec o n s i d e r a t i o n .

The energy of the tsunami is given by the volume ofwater in the wave, multiplied by its height, or Ew = Ah2 Wg, where A is the area and h the height of thewater displaced, w the density of water and g thegravitational acceleration. For sea floor displacement as thecause, the displaced volume Ah has to be the same. But theenergy in the ground implied by this is then Er = Adh rg,where d is the depth of the plug of rock that has beendisplaced, and r the rock density. d is presumably similar tothe depth of the epicenter, a dimension usually of some tensof km, while h, the wave height in the deep ocean, is of theorder of one metre. Thus the ratio of the energies is:—

Er = d rEw h w

a quantity of the order of 30,000 or more, and not 10 asotherwise estimated. Not only is the required sea floordisplacement much too large to compare with otherearthquake displacements, but the energy implied by suchdisplacements would be immense compared with otherestimates of earthquake energy.

The conclusion is that a sea floor displacement is unlikelyas the cause of tsunamis; instead, a large volumedisplacement has to be responsible that is connectedwith much less energy, and a release of gas willsatisfy this condition. The wave volume can then be equalto the gas volume when the gas has expanded

Terrestrial Sources of Carbon and Earthquake Outgassing 15

almost to atmospheric pressure. In this interpretation, thetsunami at sea is just the equivalent of the “visible wave”phenomenon on land.

An estimate of the wave volume and hence of the gasmasses involved can be made for tsunamis. Taking theobserved period, the wave propagation velocity and the totalwave energy quoted by Iida (1963) for the largest eventrecorded in 70 years (Sanriku, March 3, 1933), we obtain avolume of the order of 101 2m3 and hence a gas mass of theorder of 109 tons. (For all other events the figures would bemuch smaller). If it is thought that this is a large figure, it isstill the method of displacing the required amount of waterwith the least amount of energy possible; the energy suppliedby the gas may be only marginally larger than the waveenergy produced. An earthquake energy only ten times largerthan the tsunami energy is then a possibility.

Detailed studies of this phenomenon will allow muchbetter estimates to be made of the gas masses involved in anyone case; also the nature of the gases could be determinedfrom a subsequent analysis of the dissolved components inthe ocean water where dispersal would be much slower thanin the air.

The other gas-related phenomena we have discussed arenot easily used for a quantitative estimate. Sampling of theupper atmosphere over the region and soon after anearthquake may yield some such data. At the present time, allthat can be said is that the quantities of gas escaping atearthquakes may be a significant item for the mean carbonsupply to the surface. Of course the rate of this supply at thepresent time may deviate substantially in either direction fromthe mean rate over geologic time that we have quoted; ashappens often in the geologic record, the short-term meanmay be quite misleading.

The fact that earthquake-related gas seems to befrequently or usually combustible, even where the site isunrelated to any known deposits of natural gas, suggests thathydrocarbons are widespread in lower levels of the crust.Methane, because of its pressure-induced stability, is thenlikely to be the major component to emerge. Non-combustible gases like nitrogen and CO2 must generally bepresent in insufficient concentration to prevent burning. Watervapor also cannot generally be so abundant that it would formclouds above exit points, due to decompression and cooling,since this is not a part of the usually observed set ofphenomena (although this may fit some of the accounts).Hydrogen sulfide odours seem to have been reported oftenfrom earthquake sites. The frequent reports that animals,especially dogs, seem to have been disturbed for some periodpreceding a major earthquake can be attributed to the escape

of gases from the ground, detected by the superior sense ofsmell of these animals. Global surveys of atmosphericmethane (Ehhalt, 1974) have been carried out, and show asignificantly higher concentration in northern high latitudesthan in southern latitudes. There is a suggestion of anunexplained source in the northern Pacific Ocean. We maywish to relate this to the generally high tectonic activity of thenorthern high latitudes compared with the stability of thesouthern latitudes; and the survey data could be substantiallyinfluenced by individual events of a release of gas.

Systematic and long-duration observations of lithosphericoutgassing in relation to faults and earthquakes have beencarried out in southern Dagestan, in the fault system flankingthe NE part of the Caucasus, by Kravtsov and Voitov (1976).The reports of the observation at many sites there indicate thatdeep faults are the escape routes of gases; that thehydrocarbon component is sometimes remarkably puremethane (98.5%); and that the effect of earthquakes is tochange the composition of escaping gases over wide regions.Of particular interest is the observation that the carboncomponent of gas may change from CO2 to CH4 with theflow of other gases remaining nearly constant. This would bethe situation expected if the supply started as CH4, and if theoxygen supply of the pathway had become depleted.

The authors of that study give reasons why they considerthe gases to come principally from high-temperature zones,particularly those gases that appear in fault zones and arecharacterized by a predominantly methane composition.Detailed studies of the isotopic composition and of the rare-gas content all confirm a high-temperature, deep source. Thedepths of the Dagestan earthquakes are mostly 30-35 km.

They also noted that at the epicenter of the earthquake ofMay 14, 1970, an intensive release into the atmosphere of H2,C O2 and He was observed for a period of 40 days.Thus the observations in southern Dagestan givestrong support to the view that there are deep andpresumably abiogenic sources of hydrocarbons in thecrust; that faults and earthquakes play a role in theescape of gases from great depths; and that gasemissions over regions of hundreds of km can all beaffected by a single event. The authors suggest indeed thatlithospheric outgassing of carbon through faults may be amajor factor in the supply of surface carbon.

Prior to the February 4, 1975, earthquake in Hai-cheng,China, elevated temperatures were observed in the entire faultregion for several weeks, and for a few hours before the‘quake a low-lying, foul-smelling fog was also observed(Liao-ling Province Meteorological Station, 1977).

Thomas Gold16

C O N C L U S I O N

The importance of understanding the sources of thecarbon on the surface of the Earth is great, both for a generalunderstanding of the geologic and biologic past and also formany purposes related to mineral prospecting. If some ormuch of this carbon originates from unoxidized sources atsome depth in the crust, as many of the present data suggest,then one will be interested in the possibility of exploiting thisreservoir for fuel supplies. The largest mean rates of escape ofhydrocarbons that are permitted by the considerations of thepresent atmospheric composition and processes are not large,and are certainly smaller than 1% of industrial fuelconsumption (Perry and Landsberg, 1977, pp. 35-50). On theother hand, the size of the original reservoir, if itsupplied the entire surface carbon, is of the order of 20 million years of present-day usage of natural fuels.We may wonder, therefore, whether some smallfraction of this large reservoir is still available and at suchdepths, or communicating by natural channels to such depths,that recovery is possible.

In any case, the information that deep and presumablyabiogenic hydrocarbons exist would affect many aspects ofthe search for natural fuels. Entirely new techniques both forsearch and recovery would need to be invented. Igneous rocksand fault systems would come under investigation, not onlysedimentary regions. A new significance would be seen forthe cases where a reducing process has appeared to beassociated with faults or with igneous processes,without the recognizable intervention of any organicreducing agents. An example of this is the generaldistribution of reduced carbon in igneous rocks thatwe have mentioned (Hoefs, 1972); others are reportsof reduced minerals without any apparent reducingagent, such as the native iron on Disko Island WesternGreenland (Bird and Weathers, 1977); there is also theinteresting relation between methane and a rift provided bythe anomalous large source of methane of Lake Kivu(Deuser, Degens, Harvey and Rubin, 1973), a lake of thegreat African rift valley system.

The supply of carbon by tectonic releases of gases mayhave been a very unsteady process in different geologic eras.Both the primeval supply and the recycled supply may haveundergone substantial changes, and consequently thecomposition of the ancient atmosphere and the climate mayhave depended greatly upon these.

Many of the earthquake phenomena can be accounted forby large quantities of gases in the crust and upper mantle.Their presence must greatly modify the rheological properties

of the rocks. The escape routes of such gases would befavorable locations for earthquakes, and earthquakes in turnwill facilitate their escape. Although large amounts of gasmay have escaped undetected in the past, or been noticed onlyin a qualitative manner, there can be no serious difficulty insubjecting this entire process to good quantitative analysis.While a range of gas detectors may not be available at the siteof the next major earthquake, they can certainly be deployedin the period of the aftershocks, and the quantities andchemical nature of the gases can be established. Permanentgas detection equipment can be emplaced at known activefaults. Seawater gas analyses and upper atmosphere gasanalyses can be carried out after major earthquakes. Theindications of the masses involved that are obtainedfrom volumetric changes on land or at sea tend to givefigures somewhat larger than the estimates based onthe carbon budget; either the present epoch isparticularly active, or the estimates are inexact, or alarge proportion of the gases appearing at the presenttime are not primeval, but recycled deposits such asC O2 derived from limestone that has been subducted andheated (Anderson, 1975). Water vapor, the fluid whosesupply can most easily go unnoticed, may also play a largerrole than we have discussed.

The process of earthquake prediction can be sharpenedup greatly if indeed the escapes of gas from deep levels arethe precursory effects. Much more direct methods can then beused to detect gas flow in the ground, and perhaps the entirechain of events can be better understood. “Defusing” animpending earthquake is not out of the question.

The author wishes to acknowledge many helpfuldiscussions with colleagues, including Sir Fred Hoyle, and DrG. F. MacDonald, that have contributed to theviewpoint presented. The extensive help and keeninterest of Dr S. Soter is gratefully acknowledged; heis responsible for finding much of the supportingmaterial cited and for many improvements in the text.A fuller joint account with him concerning the relation ofgas emission to earthquakes is under preparation.

The work was supported by a grant from the NationalScience Foundation (AST-17838 A02).

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