2000 Fall MeetingDecember 15-19,2000

San Francisco, California

EOS

v.81:no.18:suppl. (2000:Nov. 28)JRNLSXReceived: 12-07-2000

. •,&•*.•: ' -..-?-/?.i(*«-:~-.̂ »>A>5'>vKfci

Published as a supplement toEos, Transacf/ons, American Geop/iy/sca/ L/n/on

Vol. 81, No. 48, , November 28, 2000

2000 Fall Meeting F1197

I rJ , ,f the. Pn phase between 200 and 500km offset.I I | , > w ve loc i ty zone migh t be a t rans ien t feature such

f , . ,-^nt t h e r m a l perturbation, or it might be a zone

i U , t l < r metmomatiBed during Farallon plate subduc-

but ion, and composit ional va r ia t ion . Previoui studieshave shown tha t the t e m p e r a t u r e derivative* of seis-mic wave velocities can have large errors if the anelaa-tic effects are not considered. Therefore , accurate esti-

M A , t h i n the. crust and at the Moho to modulate the

' ' . , , the c rua t toward the south . In th i s case there is

of both elocity and a t tenua t ion . The RMF

ray of t h i r t y broadband, three-co

.,[)["•' m a n t l e benea th the Colorado p la teau . BothJf., ,>f model allow the en t i r e prof i le to be in iso-.,. , - c ) u i l i b r i u m w i t h a reasonable conversion from

stations were chosen to tha t the S phase was clear andhad a s i g n i f i c a n t l y larger a m p l i t u d e t h & n the sur round-ing noise. Di f fe ren t ia l t" measurements are made us ing

locity and he :e densi ty; low densities w i t h i i

oci ty-densi ty mappings .

ved. Absolute at-

TllG-05 0940h

Velocity Structure of the SouthernRocky Mountains Based onRefraction/Wide Angle ReflectionSeismic Data (CD-ROM'99)

ji3,,:t a- Maria Rumpel 1

, H.in na-Maria. Rum pel Q g p i . u n i - k a r l s r u h e . d e )

- A t K e r i n e M. Snelson2 (snelson'Ogeo,utep.edu)

.•!»u* Prodehl1

, C l aus .P rodeh lT igp i . un i -ka r l s ruhe .de )

,j Randy Keller2 (kellerageo.utep.edu)

K a t * C. Mi l le r 2 (mi l l e rOgeo .u tep .edu)

IG-- >phy3 ' c a l I n s t i t u t e , U n i v e r s i t y of K a r l s r u h e ,H 'Ttzs t r . ie , K a r l s r u h e 76187, Germany

Texas at El Paso. 500 West Univers i ty , El Paao, TX7'>n68, United States

D u r i n g the CD-ROM Exper iment in 1999, a 950km-.nt, refract ion /wide angle reflection seismic profile

*aj acquired in the southern Rocky Mountains. The.^ ismic l ine trended south to north from northern NewMexico to central Wyoming. An i n s t r u m e n t spacing of...Him along the line and 10 chemical sources consist-n$ of charges between 1000kg-4500kg (100km spacing)^rovuled a relat ively high resolution data set. A pre-iminary P-wave velocity model was derived based onsnm.iry and secondary arrivals. The model domain isoOkm long and 65km deep i n c l u d i n g the extreme topo-g r a p h i c variat ions between New Mexico (1000m) up to•he Cont inenta l Divide in Colorado (3500m). There are•wo s -d imentary layers at the top of the model. Their•hicktiess varies between some 100m near the ArkansasFiner . Colorado to 5km in the Nor th Park Basin. Col-orado The surface velocities are low in New Mexico3 f - k m / a ) , decrease in southern Colorado and increase

Ham i n no r the rn Colorado and Wyoming to 5 .7km/s.The -ed iments reach a max imum veloci t iy of 5.8km/s in•he ^outh Park Bas,n, Colorado. Due to the different• h i c k n e s s of the sediment , the Precambr ian basementn \-w Mexico is about 2km deep. In Colorado and

W y o m i n g portions of the line are located on basement-xposurea . However, basins like the North Park Basin,Colorado or the Hanna /Kind t Basin. Wyoming are asieep as 5km. The velocities at the top of the base-mem vary between 5.8km/s and 6 2km/s and increase' l o w l y w i t h dep th . The overall crus ta l thickness is lessThan -10km in Wyoming and increases to 45-50km inColorado and New Mexico. A h igh <ca . 7 . 2 k m / a ) ve-

jf ; he model.

'.'RL: h t tp : / /www.geo .u tep .edu /CDROM/

T11G-06 1015h

Attenuation Tomography Under theRocky Mountains: Implications forthe Physical State of the UpperMantle

O h v e r S Doyd1 (303-492-7296;

factor to the upper m a n t l e below western Kansas. Ourresul ts show a region of elevated t* below the RockyMounta ins tha t t rends to the northeast. This geo-graphic pattern of h igh t" correlates well w i th previ-ously mapped low seismic velocities in the region. Theeastern extent of the array, in the Great Plains of west-ern Kansas, exhib i t s the smallest relative t" values. Weat t r ibute th i s to stable cont inenta l l i thosphere .

URL: http://ucsu.colorado.edu/-oliverb/r m f a t t e n . h t m l

T11G-07 1030H

Azimuthally Anisotropic Shear WaveStrcuture Beneath Colorado RockyMountains

Aibing Li1 (1-508-289-3423; aibingOwhoi.edu)

Donald W Forsyth2 (1-401-863-1699:Donald.Foraythabrown.edu)

Karen M Fischer2 (1-401-863-1360:karen ttemma.geo.brown.edu)

on the

phys ics ( M G C ) recorded theie event* du r ing a 10-yearperiod w h i l e monitoring the central Front Range forthe Denver Water Department. Over 1,100 event* werelocated d u r i n g the moni to r ing period, many occurr ing

al loca-tions reveal some s t eep ly -d ipp ing clusters of events that

traces and subsurface projections of Tertiary and Qua-

K i r k h a m and Rogers (1981) . About 50 stations fromthe Front Range network were used in thia i tudy, whichspans a region west of Denver w h i c h stretches 140 km

sen data set. MGC used a layer a ha l f space and

we use an in i t i a l ve loci ty model derived from seismicrefraction profiles of Prodehl and Lipman (1989). AID joint hypocenter-velocity inversion is then used t

andthe data set. This ii then used a* a s t a r t ing model fora 3D invers ion. Final relocations are compared to thoseof MGC and t h e i r potential association wi th the surfacetraces of any Tertiary and Quaternary faults. Implica-tions to seismic hazard of the Denver Metropoli tan areaare discussed, a region which has only experienced onelarge na tu r a l l y occurr ing historic ea r thquake , the 1882MW 6.6 earthquake, which is thought to have occurredsomewhere along the nor thern Front Range (Spenc« etal., 1996).

lWoof Geology and Geophysics, Woods Hole. MA 02543,United States

2 Brown Universi ty, Department of Geological Sci-ences. Providence, RI 02912. United States

Phase velocities and shear-wave velocity s t ruc tu reobtained from Ray lei gh waves recorded at the RockyMounta in Front (RMF) Broadband Seismic Experimentprovide clear images of velocity variation across threetectonic provinces: the Colorado Plateau, the RockyMounta ins , and the Great Plains. The RMF marks thewestern edge of the North American craton and sepa-rates the region of low seismic velocities in the tectonicmantle to the west from the h i g h velocities of the t h i c kcratonic l i thosphere to the east. Beneath the most el-evated part of the Rockies around 39° N. we observeanomalously low crus ta l velocities and a s l i g h t l y th in-ner c rus t . The var ia t ion in c rus ta l th ickness from theColorado Plateau to Rocky M o u n t a i n s is i n s u f f i c i e n t tosupport the topography change. The large contrast incrustal velocity between the two regions suggests thatdensity variations within the crust may play an impor-tant role in compensat ing the topography. Velocitiesin the upper 100 km of the mant le are s imi l a r beneaththe Colorado Plateau and the Rockies, wh ich are bothmuch slower than that beneath the Great Plains. Weobserved a s imple pa t t e rn of az imu tha l anisotropy inthe Rocky Mountain region with fast directions close tothe absolute plate motion of the Nor th America plateand s t r eng th i n c r e a s i n g w i t h period. suggest ing tha tdeep asthenoaphere is h i g h l y an i so t rop ic w i t h flow/ dom-inated by the c u r r e n t plate motion.

T11G-08 1045h

T11G-09 HOOh

A Seismic Reflection, Gravity andRemote Sensing Study of theSoutheastern Sangre de CristoMountains, New Mexico

Oscar A. Quezada1 ((915)747-5501;

oscar Qgeo.utep.edu)

G. Randy Kel ler 1 ((915)747-5501:keHer 3geo.utep.edu)

Chr is topher L. Andronicos 1 ((915)747-5501;chriiOgeo.utep.edu)

Kate C. Mil ler 1 ((915)747-5501;mi l le r Qgeo.utep.edu I

1 Univers i ty of Texas a t -El Paso. 500 W. Univers i tyDept. of Geological Sciences. El Paao. TX 79968.United StatesAs part of the CD-ROM project, an integrated

geophysical and remote sensing investigation of theLaramide and Precambrian influenced tectonic featuresin the southeastern Sangre de Criato Mountains ofnor the rn New Mexico was under taken. This research isan at tempt to correlate gravity signatures, seismic re-flection data and surface expressions from satellite im-agery in order to unders tand the s t ruc tu r e under th iscomplexly deformed section of the Rocky Mountains .Despite abundan t metamorphic and igneous basementoutcrops, the Taos-Mora area is the site of one of themoat p rominen t low gravi ty anomalies in the RockyM o u n t a i n region. Based on previous research and ear lyresults, the anomalous gravity values could be producedby one of three possible scenarios:

-A c rus ta l welt of Precambrian or Laramide age.-An allochtonous Precambrian sheet t h r u s t e d over

low dens i ty sed imentary rocks.-The presence of an intra-basement ba thol i th of

lower dens i ty than the s u r r o u n d i n g metamorphic and

-The presence of a low dens i t y p l u t o n i c body at thesurface in the Rincon Range.

• i l i v e r b Oterra .colorado.edu)

A n n e F Sheehan1 (303-492-4597;a fa f l rnant te .colorado.edu)

! D-pt . of Geological Sciences and CIRES. Un ive r s i tyif Colorado. Boulder , CO 80309. Uni ted States

U t i l i z i n g the Rocky Mountain Front ( R M F ) broad-bind seismic dataset acquired in 1992, we determinese i smic a t t enua t ion s t ruc ture u n d e r l y i n g the southernRorky Mountains and v ic in i ty . Previous P, S and sur-?ac- wave tomography studies in the region indicatelow upper mantle velocities beneath the Rockies. Thelar;e magnitude of the low velocity anomaly suggests

Upper Crustal Velocities andEarthquake Relocations in theCentral Front Range, Colorado, Froma Joint HypocenterrVelocity Inversion.,

Jacquel ine DJ Bott1 ' O i l 44 131 650 0224:

jbott abgs.ac.uk }

Ivan G Wong- ( i g w o n g f i u r a c o r p . c o m )

Jon Ake3 ( j a k e a s e i s m o . u s b r . g o v )

burgh EH9 SLA. Uni ted Kingdom

2URS Corporat ion. 500 12th Street, Oakland, CA94607. Uni ted States

3USBR, Denver Federal Center. Denver, CO. UnitedStates

4 Denver Water Department . 1600 W 12th Avenue.Denver, CO 80254, Uni ted States

P- and S-wave arr ival t imes from a well-recorded set

This page may be freely copied.

t i b l edeep reflectors f rom e i t h e r a g ran i ta l lochtonous sheet of Precambrian a t e r i a l . Port ions

ment unde* the Sangre de Cristo Uplift interpreted as

t h r u s t f au l t s on the west cent ra l s ide of the southernSangre de Cristo Mountains . The t h i c k n e s s of the Pa-leozoic sect ion on the cen t r a l part of the gravi ty pro-file was also constrained by the seismic reflection data.Model ing of a deep c rus ta l fea ture as a cause of the iowgravi ty values qu ick ly ruled out t h i s poss ib i l i ty sincesuch a s t r u c t u r e produces a broad tnonialy tha t doesnot match the observed data. The cons idera t ion ofthese cons t ra in t s and the i r i n t eg ra t i on w i t h the surfacefeatures visible in remote sensing were the bases for the

shows a low densi ty metamorphosed i n t r u s i v e body atthe sur face in the Rincon Range tha t corresponda withthe location of a g r an i t i c gneiss exposure . Well datafrom the adjacent Rainsvi l le t rough , where the grav-ity values are lowest, has reported high amounts of im-mature sediments eroded from these exposures. Lowdensi ty sediments from the Rincon Range are thereforeaffecting the overall density of the trough and could bea c o n t r i b u t i n g cause of the low gravi ty anomaly.

URL: http:/ /www.geo.utep.edu/CDROM/

UPPER CRUSTAL VELOCITIES ANDEARTHQUAKE RELOCATIONS INTHE CENTRAL FRONT RANGE,

COLORADO, FROM A JOINTHYPOCENTER-VELOCITY

INVERSION

Jacqueline Bott1, Ivan Wong2,

Jon Ake3, Susan Steele-Weir4

1British Geological Survey, 2URS Corporation,3Bureau of Reclamation,4Denver Water Department

This work was funded by the NEHRP grant No. 99HQGR0028

SEISMICITY OF THE FRONT RANGECOLORADO, 1983-1993

0 LONGMONT

BOULDER

-105.5 -105.0 -104.5

MGC monitoring project forDWD1100+ located events in 10y<ear periodLargest event in last 20 yearsis ML 4.0 on 25 December1994 near Castle RockLargest historical event inColorado was Mw 6.6 in 1882Potential reactivation ofTertiary and Quaternary faults- will affect seismic hazardVelocity inversion to improvelocations

Analysis

• There is significant blast contamination in the data set:so removed events from data set around known blastsites which occurred during the day and with depths lessthan 2 km.

• Then selected data set for inversion by location quality:rms < 0.25, gap < 180, No. of P arrivals > 7 and total No.of P and S arrivals > 10. This left 198 events with a totalof 3756 ray paths (both P and S).

•Program Velest (Kissling, 1988) was used to first solvefor 1-D VP and Vs model; velocities of 2 upper layerswere fixed based analysis of blast data and using aVP/Vsof 1.71

Removing Suspected BlaststBTO tsar

4TCD

39T30

3700 3ETQO

KCTD' XFOO

Before50 -1

45

40

2 35

•5 25

a 20

10

5

0

i

1̂*V ") V

J-i-i-

v, fc *, * *»

O1 nJT-"iflr

0 - >V 1

R» •>

-« A,

Hour of Day

-

-

-

-

--

-

-

-

-:

.

///////

After

-i

Hour of Day

Data set and ray pathsKBDD UJ30 raca

3730

37CD

MagH 0-0.5

S 0.4 - 0.9

S 1 - 1.4

S 1.5-1.9

S 2 - 2 . 4

g 2.5 - 2.9

§ 3-3.5

N = 198MD 0.8-3.0

Boulder

Denver

Aurora

ColoradoSprings

10 0 10 20 30 40 Kilometers

4OT

3700

1C6TD

IGOttJ ICKU XBCD

4TCD

3733

370)

S-wave-ray pathsP-wave ray paths Boulder

Denver

Aurora

ColoradoSprings

43CD

373J

370D

KBOD KBOJ

Travel time Curves and VD/VC ratio

Travel times for Front Range

25

20

-2. 15VE

5 10

•» •

20 40 60

Mypocentral Distance (km)

80 100

P-time vs S-time for the Front Range Dataset

30

25

20 ]

o>in

E

V)

15

10

Vp/Vs = 1.71

R' = 0.998

10

P-time (sec)

15 20

P-wave Velocity Models for Front Range

MGC(1983)PRODEHLANDLIPMAN (1989)

-2.5 km

•0.55 km5.1 km/sec

5.95 km/sec

-2.5 km

0.5 km

29.5 km

4.3-5.6 km/sec

6.0 km/sec

P-wave Velocity (km/sec)

5 5.5 6

IDfromVelest

MGC

FVodehl & Lipman

4JCD

Earthquake Relocations

.Colorado[Springs

0 10y 20 30 40 Kilometers

Arvada , Denver

« 'Lakewood Aurora

Station Corrections and GeologyP-wave

KF3JtGID

icrool

3ET33I

3WD

KF3J

S-waveXBXD

I era

StationCorrections

(sec)3T -0.55- -03

3TTTTTTTT

-0.3- -0.2

-0.2- -0.12

•0.12- -0.05

-Q 05 -0.05

0.05-0.12

0.12-02

0.2-0.3

0.3-0.55

3BPOO

4TOO

3HD

10737KEW

3D tomographic inversion

Used same data set of 198 events andprogram Fdtomo (Benz et al., 1996) for 3Dsimultaneous tomographic inversion for thevelocity field and hypocenters.Used 1-D velocity model as starting model.Used 10 km by 10 km by 2 km grid spacing.P-wave rms was reduced from 0.18 sec to0.10 sec over 10 iterations.A larger data set of events was then relocatedusing the final velocity model.

40' 00'

39' 00' -

3D inversion results - velocitiesand relocations

254*30' B C255'00'

C'

50

Distance (km)

Distance (km)

4.0 4.5 4.6 4.7 4.8 49 5.0 5.1 5.2 53 5.4 5.5 5,6 5.7 518 5.9 60 6.1 62 6.3 64 6 5 70

Velocity (km/s) Plots made using GMT

Relocation of larger data set

Distance (km

50 100

Distance (km)

4.0 4.5 4 6 4.7 4.8 4.9 5.0 5.1 S3 5.3 5.4 5.5 5.6 5.7 5.8 5.9 6.0 6.1 62 8.3 64 6.5 7.0

Velocity (km/s)

150

Plots made using GMT

SUMMARY

• Computed 1D and 3D velocity models using ajoint hypocenter-velocity inversion.

• 1D model P- and S-wave station correctionscorrelate with surface geology.

• Variations in velocity in 3D model correlate withthe station corrections noted in the 1D model andextend to depth.

• Relocations show tightening of clusters withinwell-located data set, and for the larger data setshow a variation in depth distribution and stylefrom north to south along the Front Range.

SUMMARY CONTD.

This variation in seismicity may be related to thechange in style of Laramide deformation andunderlying geology along the Front Range, with achange from backthrusting to forethrusting from Nto S (Erslev and Selvig, 1997) and a change fromPC metamorphic sediments and volcanics in theN to PC granites in S. To the N, the seismicityoccurs at depths < 12 km and is distributed moreevenly throughout the upper crust. This area isdissected by many NW-trending faults. Incomparison, in the S, seismicity is occurring atdepths > 5 km and often in clusters.