Earthquake Seismology

I. Earthquake descriptors

II. Seismic waves

III. Earthquake location

Snell’s Law

Reflection and refraction of seismic waves

Seismic Phases

Near-earthquake phases

Pg, Sg (or p, s) Waves in the upper crust.

Pn, Sn Longitudinal and transverse waves refracted below the

Mohorovicic discontinuity (head waves).

Pb, Sb (or P*, S*) Waves in the lower crust or along the Conrad

discontinuity.

PmP, SmS Waves reflected from the Mohorovicic discontinuity.

Phases of distant shallow earthquakes

P, S Direct longitudinal or transverse waves.

PKP (or P') Direct longitudinal waves traversing the Earth's core without detailed identification.

PKIKP (or P') Core P phase through the inner core.

PP, PPP, SS, SSS P or S waves reflected once or twice at the Earth's surface.

PcP, ScS P or S waves reflected at the Earth's core boundary.

SKS S waves passing through the core as P waves, transformed back into S waves on

emergence. PS, SP, PPS, SPP, PSPS, PPSS, SPSP, etc.

P and S waves reflected and transformed at the Earth's surface. SKP

S wave transformed into P on refraction into the core.

Phases of deep-focus earthquakesThe major branches of the travel-time curves carry the same

descriptions as for shallow-focus events. Waves leaving the focus in an upward direction, and reflected at the surface are described by the letters p, s, as follows:

pP, pPKP, sP, sPP, etc. P or S waves reflected from the surface as P waves.

pS, sS, pSS, etc. P or S waves reflected from the surface as S waves

Surface waves

L Long waves, unidentified, the beginning of the surface wave group.

G A group of long-period Love waves often in the form of a large pulse for

transoceanic paths. LQ

Love waves. LR

Rayleigh waves. Lg

Crustal channel wave with characteristics similar to surface waves, it travels only along continental paths; in research papers the subdivision is more detailed (Lg1, Lg2, Li, Rg) (Bath, Oliver).

Locating the source of earthquakes

Earthquake epicenters: plate tectonics Earthquake depths

Earthquake foci arbitrarily classified as shallow (surface to 70 kilometers), intermediate (between 70 and 300 kilometers), and deep (over 300 kilometers)

Earthquakes originate at depths ranging from 5 to nearly 700 kilometers

Single Station Location

Estimate Distance From S – P travel time

Estimate azimuth from P-wave polarization in 3 dimensions

Earthquake Seismology -II

• Intensity and magnitude• Earthquake focal mechanism• Velocity of the Earth• Seismic tomography

Measuring the size of earthquakes

Two measurements that describe the size of an earthquake are

Intensity – a measure of the degree of earthquake shaking at a given locale based on the amount of damage

Magnitude – estimates the amount of energy released at the source of the earthquake

Measuring the size of earthquakes

Intensity scales Modified Mercalli Intensity Scale was

developed using California buildings as its standard

The drawback of intensity scales is that destruction may not be a true measure of the earthquakes actual severity

Measuring the size of earthquakes

Magnitude scales Richter magnitude - concept introduced

by Charles Richter in 1935 Richter scale

Based on the amplitude of the largest seismic wave recorded

Accounts for the decrease in wave amplitude with increased distance

Measuring the size of earthquakes

Magnitudes scales Other magnitude scales

Several “Richter-like” magnitude scales have been developed

Moment magnitude was developed because none of the “Richter-like” magnitude scales adequately estimates the size of very large earthquakes

Derived from the amount of displacement that occurs along a fault

Earthquake Magnitude

Ml - Local (Richter) magnitude Mb - Body wave magnitude Ms - Surface wave magnitude Mw - Seismic moment magnitude

Seismic Moment

2log 6.03w oM M

Moment Magnitude

oM SAShear modulus (~3x1010 N/m2)

Average amount of slip on the fault planeArea of the fault plane that ruptured

Example: 1994 Northridge earthquake (Los Angles) Estimated rupture area: A=430 km2

Average slip = 1.5 m = 3x1010 N/m2

Mo = 430x106 (m2) x 1.5 (m) x 3x1010 N/m2=1.9x1019 (N.m)

Mw=(2/3)log (1.9x1019) -6.0 = 6.8

Magnitude and Energy

Log E = 11.8 – 1.5 Ms

Note: because of the logarithm relation, a M=7 event is 101.5, or ~ 32 times, larger than a M=6 event in terms of energy release, and ~1000 times larger than a M=5 event!