1. Earthquake Seismology
2017/4/17
Earthquake Seismology
Earthquake descriptors
Seismic waves
Earthquake location

2. 2017/4/17
Snell’s Law

3. Reflection and refraction of seismic waves
2017/4/17
Reflection and refraction of seismic waves

4. 2017/4/17
Seismic Phases

5. Near-earthquake phases
2017/4/17
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.

6. Phases of distant shallow earthquakes
2017/4/17
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.

7. Phases of deep-focus earthquakes
2017/4/17
Phases of deep-focus earthquakes
The 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

8. Surface waves L G LQ LR Lg
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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).

9. Locating the source of earthquakes
2017/4/17
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

10. Single Station Location
2017/4/17
Single Station Location
Estimate Distance From S – P travel time
Estimate azimuth from P-wave polarization in 3 dimensions

11. Hypocenter Location Techniques
2017/4/17
Hypocenter Location Techniques
Linearized Least Squares
Use of Depth Phases
Centroid Moment Tensor (waveform modelling)
see

12. Earthquake Seismology -II
2017/4/17
Earthquake Seismology -II
Intensity and magnitude
Earthquake focal mechanism
Velocity of the Earth
Seismic tomography

13. Measuring the size of earthquakes
2017/4/17
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

14. Measuring the size of earthquakes
2017/4/17
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

15. Measuring the size of earthquakes
2017/4/17
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

16. Measuring the size of earthquakes
2017/4/17
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

17. Earthquake Magnitude Ml - Local (Richter) magnitude
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Earthquake Magnitude
Ml - Local (Richter) magnitude
Mb - Body wave magnitude
Ms - Surface wave magnitude
Mw - Seismic moment magnitude

18. Seismic Moment Moment Magnitude Shear modulus (~3x1010 N/m2)
2017/4/17
Seismic Moment
Shear modulus (~3x1010 N/m2)
Average amount of slip on the fault plane
Area of the fault plane that ruptured
Moment Magnitude

19. Example: 1994 Northridge earthquake (Los Angles)
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Example: 1994 Northridge earthquake (Los Angles)
Estimated rupture area: A=430 km2
Average slip = 1.5 m
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

20. Magnitude and Energy Log E = 11.8 – 1.5 Ms
2017/4/17
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!

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