Transcript Principles of Geology

Earthquake Seismology
I.
II.
III.
Earthquake descriptors
Seismic waves
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


PKP (or P')


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 or S waves reflected at the Earth's core boundary.
SKS


P or S waves reflected once or twice at the Earth's surface.
PcP, ScS


Core P phase through the inner core.
PP, PPP, SS, SSS


Direct longitudinal waves traversing the Earth's core without detailed identification.
PKIKP (or P')


Direct longitudinal or transverse waves.
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
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
Surface waves

L


G


Love waves.
LR


A group of long-period Love waves often in the form of a large pulse for
transoceanic paths.
LQ


Long waves, unidentified, the beginning of the surface wave group.
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
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•
•
•
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
– 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
 Intensity
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
M o   SA
Shear modulus (~3x1010 N/m2)
Average amount of slip on the fault plane
Area of the fault plane that ruptured
Moment Magnitude
2
M w  log M o  6.0
3
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!