Earthquake magnitude

Most related to maximum amplitudes in seismograms.

Local Magnitude (ML): Richter, 1930ies

Noticed similar decay rate of log10A (displacement) versus distance

Defined distance-independent magnitude estimate by subtracting a log10A for reference event recorded on a Wood-Anderson seismograph at the same distance

ML=log10A(in 10-6m)-log10 A0(in 10-6m)

=log10A(in 10-6m)+2.56log10 dist (in km) -1.67

for 10<dist<600km only

Geol 600 Notable Historical Earthquakes

Source mechanisms and body wave

radiation patterns

http://www-rohan.sdsu.edu/~kbolsen/geol600_nhe_magnitude.ppt

Earthquake magnitude

Body wave magnitude (mb): (used for global seismology)

mb=log10(A/T)+Q(h,)

T is dominant period of the measured waves (usually P, 1s)

Q is an empirical function of distance and depth h

(details versus amplitude versus range)

Surface wave magnitude (Ms): (used for global seismology, typically using Rayleigh waves on vertical components)

Ms=log10(A/T)+1.66 log10 + 3.3 = log10A20+1.66 log10 + 2.0

(shallow events only) QuickTime™ and aTIFF (Uncompressed) decompressor

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• Insert table showing saturation

Earthquake magnitude

Saturation problem motivated the moment magnitude Mw

Mw=2/3 log10M0-10.7 (M0 moment in dyne-cm, 107dyne cm=1Nm)

= Mw=2/3 log10M0-6.1 (M0 moment in Nm)

Scaling derived so Mw agree with Ms for small events

More physical property, does not saturate for large events

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Earthquake magnitude

Intensity scale

Measures damage to structures

Often used is Mercalli I-XII

Can be used to examine historic earthquakes without seismic records

For example, eastern US.

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Intensity Observed Effects

I Not felt at all

II Felt only by a few individuals, indoors and ﾊ at rest, usually on upper floors of tall buildings.

III Felt indoors by many persons, but not necessarily recognized as an earthquake. Chandeliers and hanging plants swing. IV Felt both indoors and out. Feels like the vibration caused by a heavy truck or train passing. Windows rattle.

V Strong enough to awaken sleeping persons. Small objects knocked off shelves. Beverages may splash out of cups or glasses on tables.

VI Perceptible to everyone. May cause public fright. Pictures fall off walls. Weak masonry cracks. Some plaster may fall from ceilings.

VII Difficult to stand upright. Ornamental masonry falls from buildings. Waves may be seen in ponds and swimming pools.

VIII Mass panic may occur. Chimneys, smoke stacks and water towers may lean and fall. Unsecured frame houses slide off foundations.

IX Panic is general. Heavy damage to masonry structures and to ﾊ underground pipes. Large cracks open in ground.X Many buildings collapse. Water splashes over riverbanks.

XI-XII Virtually total destruction.

Radiated Energy

Log10Es (ergs) ~ 5.8+2.4mb~11.8+1.5Ms

Es(Ms=7)/Es(Ms=6)~32

Es(Ms=7)/Es(Ms=5)~1,000

Es~0.5DaveA=M0/2

Es/M0