basic seismology

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Chris Goldfinger Burt 282 7-5214 gold_at_coas.or
egonstate.edu http//activetectonics.coas.oregonst
ate.edu Reading for Thursday Miller et al., 2001
McCaffrey et al., 2000 Hawkes et al., 2005,
Natawidjaja et al., 2003 Suggested
Supplements Moores and Twiss Chapter 7
Convergent Margins
OCE 661 Plate Tectonics
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Seismology for Dummies (or why Agassiz lost it in
1906)
Some of todays material comes from Seth Steins
web site http//www.earth.northwestern.edu/people
/seth/
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Earthquakes of a given magnitude are 10 times
less frequent than those one magnitude smaller.
An M7 earthquake occurs approximately monthly,
and an earthquake of Mgt 6 about every three days.
Hence although earthquake predictor I. Browning
claimed to have predicted the 1989 Loma Prieta
earthquake, he said that near a date there would
be an M6 earthquake somewhere, a prediction
virtually guaranteed to be true. Magnitude is
proportional to the logarithm of the energy
released, so most energy released seismically is
in the largest earthquakes. An M 8.5 event
releases more energy than all other earthquakes
in a year combined. Hence the hazard from
earthquakes is due primarily to large (typically
magnitude gt 6.5) earthquakes.
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EARTHQUAKE MAGNITUDE
Earliest measure of earthquake size Dimensionless
number measured various ways, including ML
local magnitude mb body wave magnitude Ms surface
wave magnitude Mw moment magnitude Easy to
measure No direct tie to physics of faulting
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Moment magnitude Mw Magnitudes saturate No
matter how big the earthquake mb never exceeds
6.4 Ms never exceeds 8.4 Mw defined from moment
so never saturates
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The moment magnitude scale (a successor to the
Richter scale), was introduced in 1979 by Tom
Hanks and Hiroo Kanamori and is used by
seismologists to compare the energy released by
earthquakes. The moment magnitude Mw is a
dimensionless figure defined by
where M0 is the seismic moment. An increase of 1
step on this scale corresponds to a 101.5 31.6
times increase in the amount of energy released,
and an increase of 2 steps corresponds to a 103
1000 times increase in energy. The constants in
the equation are chosen so that estimates of
moment magnitude roughly agree with estimates
using other scales such as the Richter magnitude
scale. One advantage of the moment magnitude
scale is that, unlike other magnitude scales, it
does not saturate at the upper end. That is,
there is no particular value beyond which all
large earthquakes have about the same magnitude
(as does the surface wave magnitude for example).
For this reason, moment magnitude is now the most
often used estimate of large earthquake
magnitudes. The USGS does not use this scale for
earthquakes with a magnitude of less than
3.5. Hanks TC, Kanamori H (1979). A moment
magnitude scale. Journal of Geophysical Research
84 (B5) 2348-50.
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Another measure of earthquake energy, the
Mercalli Intensity Scale

• People do not feel any Earth movement.
• A few people might notice movement if they are at
  rest and/or on the upper floors of tall
  buildings.
• Many people indoors feel movement. Hanging
  objects swing back and forth. People outdoors
  might not realize that an earthquake is
  occurring.
• Most people indoors feel movement. Hanging
  objects swing. Dishes, windows, and doors rattle.
  The earthquake feels like a heavy truck hitting
  the walls. A few people outdoors may feel
  movement. Parked cars rock.
• Almost everyone feels movement. Sleeping people
  are awakened. Doors swing open or close. Dishes
  are broken. Pictures on the wall move. Small
  objects move or are turned over. Trees might
  shake. Liquids might spill out of open
  containers.
• Everyone feels movement. People have trouble
  walking. Objects fall from shelves. Pictures fall
  off walls. Furniture moves. Plaster in walls
  might crack. Trees and bushes shake. Damage is
  slight in poorly built buildings. No structural
  damage.
• People have difficulty standing. Drivers feel
  their cars shaking. Some furniture breaks. Loose
  bricks fall from buildings. Damage is slight to
  moderate in well-built buildings considerable in
  poorly built buildings.
• Drivers have trouble steering. Houses that are
  not bolted down might shift on their foundations.
  Tall structures such as towers and chimneys might
  twist and fall. Well-built buildings suffer
  slight damage. Poorly built structures suffer
  severe damage. Tree branches break. Hillsides
  might crack if the ground is wet. Water levels in
  wells might change.
• Well-built buildings suffer considerable damage.
  Houses that are not bolted down move off their
  foundations. Some underground pipes are broken.
  The ground cracks. Reservoirs suffer serious
  damage.
• Most buildings and their foundations are
  destroyed. Some bridges are destroyed. Dams are
  seriously damaged. Large landslides occur. Water
  is thrown on the banks of canals, rivers, lakes.
  The ground cracks in large areas. Railroad tracks
  are bent slightly.
• Most buildings collapse. Some bridges are
  destroyed. Large cracks appear in the ground.
  Underground pipelines are destroyed. Railroad
  tracks are badly bent.
• Almost everything is destroyed. Objects are
  thrown into the air. The ground moves in waves or
  ripples. Large amounts of rock may move.

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SEISMIC WAVES COMPRESSIONAL (P) AND SHEAR (S)
WAVES P waves longitudinal waves S waves
transverse waves P waves travel faster S waves
from earthquake generally larger
Stein Wysession, 2003
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SYNTHETIC SEISMOGRAM AS CONVOLUTION
Regard ground motion recorded on seismogram as a
combination of factors - earthquake source -
earth structure through which the waves
propagated - seismometer Create synthetic
seismogram as Fourier domain convolution of these
effects
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SEISMIC RAY PATHS BEND AS VELOCITY INCREASES WITH
DEPTH
i
Stein Wysession, 2003
Snells law for spherical earth with velocity v
at radius r Ray turns to incidence angle i so ray
parameter p is constant along the ray
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SEISMIC RAY PATHS BEND AS VELOCITY INCREASES WITH
DEPTH
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SEISMIC WAVE PATHS Body waves through Earths
interior Surface waves along surface Ray path
travel time reflect velocity structure on path
Stein Wysession, 2003
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EARTHQUAKE LOCATION Least squares fit to travel
times
Accuracy (truth) depends primarily on velocity
model Precision (formal uncertainty) depends
primarily on network geometry (close stations
eq within network help) Locations can be accurate
but imprecise or precise but inaccurate (line up
nicely but displaced from fault) Epicenters
(surface positions) better determined than depths
or hypocenters (3D positions) because
seismometers only on surface
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IMPROVE EARTHQUAKE LOCATION
Precision can be improved by relative location
methods like Joint Epicenter Determination (JED)
or master event
Or via better velocity model, including methods
that simultaneously improve velocity model
(double-difference tomography)
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EARTHQUAKES TECTONICS
Locations map plate boundary zones regions of
intraplate deformation even in underwater or
remote areas Focal mechanisms show strain
field Slip seismic history show deformation
rate Depths constrain thermo-mechanical structure
of lithosphere
NORTH AMERICA
36 mm/yr
PACIFIC
San Andreas Fault, Carrizo Plain
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Earthquake locations map narrow plate boundaries,
broad plate boundary zones regions of
intraplate deformation even in underwater or
remote areas
DIFFUSE BOUNDARY ZONES
INTRAPLATE
NARROW BOUNDARIES
Stein Wysession, 2003
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LARGER EARTHQUAKES GENERALLY HAVE LONGER FAULTS
AND LARGER SLIP
Wells and Coppersmith, 1994
M7, 100 km long, 1 m slip M6, 10 km long,
20 cm slip Important for
tectonics, earthquake source physics, hazard
estimation
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SOURCE TIME FUNCTION DURATION PROPORTIONAL TO
FAULT LENGTH L AND THUS CONSTRAINS IT
Also depends on seismic velocity V and rupture
velocity Vr
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• EARTHQUAKES MAP FAULTS WELL AND DELINEATE STABLE
  BLOCKS
• Sometimes, some places
• How well depends on
• - magnitude threshold
• - rate of motion
• duration of earthquake history
• Some parts of SAF dont show up due presumably to
  short history

EASTERN CALIFORNIA SHEAR ZONE
MENDOCINO FZ
SAN ANDREAS
STABLE SIERRA NEVADA BLOCK
UCB website
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RECENT SEISMICITY MAY NOT REFLECT
LONG-TERM PATTERN WELL
Random seismicity simulation for fault along
which probability of earthquake is
uniform Apparent seismic gaps develop May take
long time to fill compared to length of
earthquake record
Stein Wysession, 2003
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SOME PLACES SEISMICITY INADEQUATE TO RESOLVE
Hard to tell if Okhotsk distinct from North
America
Apel et al.
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SUBDUCTION ZONES Can see details Shallow thrust
interplate Shallow intraslab overriding plate
eqs Dipping intraslab Wadati-Benioff Zone
Hasegawa et al., 1978
TOHOKU, JAPAN
Best images use local network good velocity
model
Stein Wysession, 2003
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SEISMICITY DIFFERENCE SLOW VERSUS FAST SPREADING
CENTERS
Fast East Pacific Rise has only strike-slip
earthquakes on the transforms.
AXIAL GRABEN
Slow Mid-Atlantic Ridge has earthquakes on both
active transform and ridge segments. Strike-slip
faulting occurs on a plane parallel to the
transform. On ridge segments, normal faulting
occurs with nodal planes parallel to the ridge
trend.
AXIAL HIGH
Stein Wysession, 2003
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STUDYING EARTHQUAKE FAULTING FROM THE SEISMIC
WAVES IT GENERATES IS AN INVERSE PROBLEM

• Arrival time of seismic waves at seismometers at
  different sites
• is first used to find the location and depth of
  earthquake
• Amplitudes and shapes of radiated seismic waves
  used to study
• - size of the earthquake
• - geometry of the fault on which it occurred
• direction and amount of slip
• Seismic waves give an excellent picture of the
  kinematics of faulting, needed to understand
  regional tectonics
• They contain much less information about the
  actual physics, or dynamics of faulting

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Stein Wysession, 2003
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INFER STRESS ORIENTATIONS FROM FOCAL
MECHANISMS Simple model predicts faulting on
planes 45 from maximum and minimum compressive
stresses These stress directions are halfway
between nodal planes Most compressive (P) and
least compressive stress (T) axes can be
found by bisecting the dilatational and
compressional quadrants
Stein Wysession, 2003
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• SOMETIMES FIRST MOTIONS DONT CONSTRAIN FOCAL
  MECHANISM
• Especially likely when
• Few nearby stations, as in the oceans, so
  arrivals are near center of focal sphere
• Mechanism has significant dip-slip components,
  so planes dont cross near center of focal sphere
• Additional information is obtained by comparing
  the observed body and surface waves to
  theoretical, or synthetic waveforms computed for
  various source parameters, and finding a model
  that best fits the data, either by forward
  modeling or inversion.
• Waveform analysis also gives information about
  earthquake depths and rupture processes that
  cant be extracted from first motions.

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ACTUAL EARTHQUAKE FAULT GEOMETRIES CAN BE MUCH
MORE COMPLICATED THAN A RECTANGLE
Fault may curve, and require 3D-description.
Rupture can consist of sub-events on different
parts of the fault with different orientations.
Can be treated as superposition of simple
events.
Sumatra rupture model, plus very slow northern
slip (seismic part from Chen ji, Caltech)
Heterogeneous nature of structure in the upper
plate in Cascadia
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• SEISMIC MOMENT TENSOR
• Represents other types of seismic sources as
  well as slip on a fault
• Gives additional insight into the rupture
  process
• Simplifies inverting (rather than forward
  modeling ) seismograms to estimate source
  parameters
• Used to produce global data set of great value
  for tectonics

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REPRESENTING EARTHQUAKE WITH MOMENT
TENSOR Simple representation yields seismic
waves produced by a complex rupture involving
displacements varying in space and time on
irregular fault First, approximate rupture with
a constant average displacement D over a
rectangular fault Approximate further as a set
of force couples. Approximations are
surprisingly successful at matching observed
seismograms.
Stein Wysession, 2003
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MOMENT TENSOR ADVANTAGES FOR SOURCE
STUDIES Analyze seismograms without assuming
that they result from slip on a fault. In some
applications, such as deep earthquakes or
volcanic earthquakes, we would like to identify
possible isotropic or CLVD components. Makes it
easier to invert seismograms to find source
parameters, because seismograms are linear
functions of components of the moment tensor,
but are complicated products of trigonometric
functions of the fault strike, dip, and slip
angles. This is not a problem in forward
modeling, but makes it hard to invert the
seismograms to find the fault angles.
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MOMENT TENSOR DATA FOR TECTONIC
STUDIES Globally-distributed broadband digital
seismometers permit reliable focal mechanisms to
be generated within minutes after most
earthquakes with Ms gt 5.5 and made available
through the Internet. Several organizations
carry out this service, including the Harvard CMT
(centroid moment tensor) project. CMT
inversion yields both a moment tensor and a
centroid time and location. This location often
differs from that in earthquake bulletins, such
as that of the International Seismological Centre
(ISC), because the two locations tell different
things. Bulletins based upon arrival times of
body wave phases like P and S give the
hypocenter the point in space and time where
rupture began. CMT solutions, using full
waveforms, give the centroid or average location
in space and time of the seismic energy
release. The availability of large numbers of
high-quality mechanisms (Harvard project has
produced over 17,000 solutions since 1976) is of
great value in many applications, especially
tectonic studies.
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SEISMICITY DIFFERENCE SLOW VERSUS FAST SPREADING
CENTERS
Fast East Pacific Rise has only strike-slip
earthquakes on the transforms. No normal faulting
since there is no axial graben.
AXIAL GRABEN
Slow Mid-Atlantic Ridge has earthquakes on both
active transform and ridge segments. Strike-slip
faulting occurs on a plane parallel to the
transform. On ridge segments, normal faulting
occurs with nodal planes parallel to the ridge
trend, showing extension in the spreading
direction.
AXIAL HIGH
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TRENCH-NORMAL CONVERGENCE - ALEUTIAN TRENCH 54
mm/yr
MECHANISMS SHOW BOTH NOMINAL PLATE
BOUNDARY Aleutian Trench thrust San
Andreas strike slip Gulf of
California normal strike slip AND OTHER
BOUNDARY ZONE DEFORMATION Basin Range normal
Los Angeles Basin thrust
PACIFIC wrt NORTH AMERICA pole
BASIN RANGE EXTENSION
STRIKE SLIP - SAN ANDREAS
LA BASIN SHORTENING
EXTENSION - GULF OF CALIFORNIA
Bawden et al., 2001
Stein Wysession, 2003
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Los Angeles Basin Thrust earthquakes indicate
shortening 1994 Northridge Ms 6.7
Caused some of the highest ground accelerations
ever recorded. It illustrates that even a
moderate magnitude earthquake can do considerable
damage in a populated area. Although the loss of
life (58 deaths) was small due to
earthquake-resistant construction the 20B damage
makes it the most costly earthquake to date in
the U.S.
AFTTERSHOCKS
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NORTH AMERICA
EURASIA
STRIKE-SLIP GLORIA TRANSFORM
EXTENSION TERCEIRA RIFT
OBLIQUE CONVERGENCE NORTH AFRICA
NUBIA
Argus et al., 1989
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MECHANISMS FAULTS SHOW CONVERGENCE DIRECTION
Meghraoui et al. (2004)
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NUBIA-SOMALIA SPREADING Normal fault mechanisms
show extension across East African Rift
Rift Valley, Lake Bogoria, Kenya. Area is
30kmx30km and contains numerous faults on the
downthrown side of a large bounding fault with
maximum displacement 1km.
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DEFORMATION IN NZ-SA PLATE BOUNDARY ZONE
Thrust faulting at trench due to interplate
motion Thrust faulting shows compression causing
shortening in foreland thrust belt Aseismic
Altiplano acts as rigid block between forearc
thrust belt
Norabuena et al., 1998
GPS site vectors relative to stable South America
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WORLD STRESS MAP Combines earthquake
mechanisms other stress indicatorsi.e borehole
breakouts, strain meters, creep meters, volcanic
alignments etc.
ALPINE CONVERGENCE
DINARIDES COMPRESSION
HELLENIC ARC EXTENSION
APPENNINE EXTENSION
NUBIA-EURASIA CONVERGENCE COMPRESSION
DEAD SEA TRANSFORM SINAI-ARABIA STRIKE SLIP
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SEISMOLOGY GIVES FOCAL MECHANISMS, SEISMIC
MOMENTS, SOME INFORMATION ABOUT FAULT
DIMENSIONSOur goal is to use these to
understand tectonics
Loma Prieta 1989 Ms 7.1
Davidson et al., 2002
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Location magnitude and mechanism
summary Earthquake locations and focal
mechanisms can give crucial insight into regional
tectonics Integration with plate motion,
geologic, and geodetic data is very powerful
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Supplementary material from here on
down Directivity SOURCE TIME FUNCTION DURATION
ALSO VARIES WITH STATION AZIMUTH FROM FAULT, AND
THUS CAN CONSTRAIN WHICH NODAL PLANE IS THE FAULT
PLANE
Directivity similar to Doppler Shift, but differs
in requiring finite source dimension
Stein Wysession, 2003
Analogous effect thunder generated by sudden
heating of air along a lightning channel in the
atmosphere. Observers in positions perpendicular
to the channel hear a brief, loud, thunder clap,
whereas observers in the channel direction hear a
prolonged rumble.
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BODY WAVE MODELING FOR SHALLOW EARTHQUAKE Initial
portion of seismogram includes direct P wave and
surface reflections pP and sP Hence result
depends crucially on earthquake depth and thus
delay times Powerful for depth determination
Stein Wysession, 2003
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IMPULSES
SYNTHETIC BODY WAVE SEISMOGRAMS Focal depth
determines the time separation between
arrivals Mechanism determines relative
amplitudes of the arrivals Source time function
determines pulse shape duration
WITH SEISMOMETER AND ATTENUATION
Okal, 1992
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BODY WAVE MODELING FOR DEPTH DETERMINATION Earthq
uake mechanism reasonably well constrained by
first motions. To check mechanism and estimate
depth, synthetic seismograms computed for various
depths. Data fit well by depth 30 km. Depths
from body modeling often better than from
location programs using arrival
times International Seismological Center gave
depth of 0 17 km Modeling shows this is too
shallow Depth constrains thermomechanical
structure of lithosphere
Stein and Wiens, 1986
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EARTH SEISMOMETER FILTER OUT HIGH FREQUENCY
DETAILS
Stein and Kroeger, 1980
High frequencies determining pulse shape
preferentially removed by attenuation. Seismogram
smoothed by both attenuation and
seismometer. Pulses at teleseismic distances can
look similar for different source time functions
of similar duration. Best resolution for details
of source time functions from strong motion
records close to earthquake.
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MODEL COMPLEX EVENT BY SUMMING SUBEVENTS
1976 Guatemala Earthquake Ms 7.5 on Motagua
fault, transform segment of Caribbean- North
American plate boundary Caused enormous damage
and 22,000 deaths
Kikuchi and Kanamori, 1991
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San Fernando earthquake on buried thrust fault in
the Los Angeles area, similar to Northridge
earthquake. Short faults are part of an oblique
trend in the boundary zone, so fault areas are
roughly rectangular. The down-dip width seems
controlled by the fact that rocks deeper than 20
km are weak and undergo stable sliding rather
than accumulate strain for future
earthquakes. San Francisco earthquake ruptured a
long segment of the San Andreas with
significantly larger slip, but because the fault
is vertical, still had a narrow width. This
earthquake illustrates approximately the maximum
size of continental transform earthquakes.
Alaska earthquake had much larger rupture area
because it occurred on shallow-dipping
subduction thrust interface. The larger fault
dimensions give rise to greater slip, so the
combined effects of larger fault area and more
slip cause largest earthquakes to occur at
subduction zones rather than transforms.
THREE EARTHQUAKES IN NORTH AMERICA - PACIFIC
PLATE BOUNDARY ZONE Tectonic setting
affects earthquake size
Stein Wysession, 2003
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EARTHQUAKE SOURCE PARAMETER ESTIMATES HAVE
CONSIDERABLE UNCERTAINTIES FOR SEVERAL
REASONS - Uncertainties due to earth's
variability and deviations from the mathematical
simplifications used. Even with high-quality
modern data, seismic moment estimates for the
Loma Prieta earthquake vary by about 25, and Ms
values vary by about 0.2 units. - Uncertainties
for historic earthquakes are large. Fault length
estimates for the San Francisco earthquake vary
from 300-500 km, Ms was estimated at 8.3 but now
thought to be 7.8, and fault width is
essentially unknown and inferred from the depths
of more recent earthquakes and geodetic data. -
Different techniques (body waves, surface waves,
geodesy, geology) can yield different
estimates. - Fault dimensions and dislocations
shown are average values for quantities that can
vary significantly along the fault Hence
different studies yield varying and sometimes
inconsistent values. Even so, data are sufficient
to show effects of interest.
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• Ok so there you have a quick review of basic
  seismology, much of which youve seen in some
  form
• Now lets look at some recent developments as
  they relate to plate tectonics
• Earthquake prediction and forecasting
• Slow earthquakes
• Shear wave splitting and attenuation

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Precursory signals Mostly seen after the fact
though
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Precursory signals Mostly seen after the fact,
but not always.
The Philippine Sea coast of central-southwest
Japan tilts oceanward during interseismic periods
and peninsulas uplift suddenly at the time of
great earthquakes. Tide gauge data indicate that
precursory uplifts of the peninsulas occurred
during about the decade prior to the occurrence
of recent earthquakes. Uplifts due to slow
failures of smaller asperities contained in the
rupture zone may be the cause. Applications to
tide gauge data before the 1923 and 1946
earthquakes give tf 1923.2 (1.6), and 1943.7
(2.7), respectively. For the expected Tokai
earthquake, tf 2007.6 (-5.4, 2.8) using
precise leveling data. The intermediate-term
precursors of a decade may be useful to limit
the expected time of occurrence of coming great
earthquakes, filling the gap between long-term
and short-term earthquake predictions. (after
Seno, 2004).
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Precursory signals Mostly seen after the fact
though
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Precursory signals Mostly seen after the fact
though
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Precursory signals Mostly seen after the fact but
maybe not all
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SLOW EARTHQUAKES
Compared to ridge earthquakes, transform
earthquakes often have large Ms relative to mb
and large Mw relative to Ms suggesting that
seismic wave energy is relatively greater at
longer periods. Earthquakes that preferentially
radiate at longer periods are called "slow"
earthquakes. Underlying physics unclear
Stein and Pelayo, 1991
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D? Anisotropy

• Lattice-Preferred Orientation (LPO)?
• Shape-Preferred Orientation (SPO)?
• Result of a Chemical Boundary Layer?
• Related to Slabs?

McNamara, van Keken, and Karato 2002
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D? Anisotropy

• Often most intense at the top of D?.
• Most always with SH faster than SV (?
  transverse isotropy)
• Shows large lateral variation.
• Often associated with the D? discontinuity.

Fouch et al. 2001
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FORCES REPRESENTING SEISMIC SOURCES
SINGLE FORCE - Landslide (Grand Banks slump) or
Explosion (Mt. St. Helens) SINGLE COUPLE - add 3
for isotropic explosion DOUBLE COUPLE - slip on
fault
Stein Wysession, 2003
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SEISMIC MOMENT TENSOR General representation of
seismic source using 9 force couples
Stein Wysession, 2003