By essentially taking the logarithm of the moment, we can create a magnitude rating. Every earthquake has a unique total energy, and therefore a unique magnitude.
Question: what principle says that plate tectonics should operate now as it has in the past? Could there be exceptions?
Question: what does the term elastic imply about the mathematical relation between stress and strain?
Question: does the length of the rebound curvature tell you anything about the fault above?
By essentially taking the logarithm of the moment, we can create a
magnitude rating. Every earthquake has a unique total energy,
and therefore a unique magnitude.
Question: what is the magnitude of the largest earthquake ever? The largest possible? The smallest?
(J. Louie)
The part of the elastic energy radiated from an earthquake as seismic
waves takes two forms, compressional and shear.
(J. Louie)
Sensitive seismographs record the passing ground motions of the seismic
waves with respect to time, noting the motions of an inertial mass against
the moving ground.
On seismograms it is easy to separate P-waves from S-waves because
they travel at different speeds, Vp and Vs:
where k is the incompressibility property of the rock,
is the rigidity (or resistance to shear), and 
is the
density.
Question: why will P-waves arrive first at a seismograph, and S-waves later?
Question: as rocks age and become more deeply buried, Vp and Vs generally increase. Why?
), this fact means that
you can multiply the S-P time by a factor a 8 km/s to get a generally
reliable estimate of the distance of the earthquake from the seismograph.
and distance, you can use the above empirical equation with a calibrated
seismograph to estimate an earthquake's magnitude.
(from Kearey & Vine, copyright Blackwell Sci. Publ.)
Of course the distance we get from the S-P time is the distance along the
travel path of the seismic waves, so we have to adjust for the earth's
spherical shape.
(J. Louie)
Given the S-P times and distances of an earthquake from three stations,
we can triangulate to find its location.
Question: why do we need at least three stations to get a location?
Question: how would location errors appear? What earthquakes are likely to have the greatest location errors?
(from USGS)
Simply locating earthquakes around the world over the past 100 years
(black dots above) produces a striking pattern.
Earthquakes are not evenly spread around the earth, but occur in continuous
but thin belts or zones surrounding areas of far lower seismicity.
Thus on the earth tectonic deformation is largely confined to zones of
interaction between apparently rigid regions.
Note that the pattern is more diffuse on the continents than it is in the oceans.
Question: are there any cultural or technological factors affecting the earthquake distributions on this map?
(J. Louie)
Knowing where the seismically-active zones are tells us what regions
are currently deforming tectonically, but it does not tell us anything
about the nature of the deformation. There are these three fundamental
types of deformation; we can create any type of faulting by mixing them
in different proportions. All of these faults are shear or sliding
dislocations that produce no net volume change or net rotation.
Only two opposing force couples can produce such motion.
Very few earthquakes have been shown
to result from volume changes, or from single forces.
Question: what are the directions of the maximum and minimum principal strains for each mechanism?
(from Kearey & Vine, copyright Blackwell Sci. Publ.)
The body waves do not radiate in all directions with the same strength,
however. Above are radial plots of relative wave amplitude in all directions
in a plane through a shear dislocation, or double couple.
The P-wave radiation pattern at left shows that the strongest compressions
(C) and dilatations (D) radiate at 45 degree angles from the fault plane.
The S-wave radiation pattern at right shows that the strongest shear
waves radiate at directions parallel and perpendicular to the fault plane.
(from Kearey & Vine, copyright Blackwell Sci. Publ.)
The double-couple origin of earthquake motions divides the area around the
focus into quadrants revealing different directions of motion.
For the P-wave recordings above, initial motions will be up if the wave
originated in a compressive quadrant, and down if from a dilatational
quadrant. Note that two planes separate the quadrants: the real fault plane;
and an indistinguishable auxiliary plane. The object of finding an
earthquake focal mechanism is to describe the orientations of these planes.
(from Kearey & Vine, copyright Blackwell Sci. Publ.)
What seismographs record radiation from which quadrant depends on the
location and depth of the focus, the orientation of the fault plane, and the
paths the waves take to the seismographs.
(from Kearey & Vine, copyright Blackwell Sci. Publ.)
To estimate a focal mechanism we pick the direction of P-wave motion
(up and compressional, or down and dilatational) at each seismograph,
find the azimuth of each ray from the locations of the event and the
seismograph, and estimate the takeoff angle of each ray from the
locations and ray projections through earth structure models.
Each C or D pick is plotted in lower-hemisphere projection on an equal-area
stereonet. With that data we try to find a unique strike and dip for the
mutually-perpendicular fault and auxiliary planes on the stereonet.
Question: why is it most convenient to use the lower-hemisphere projection?
(from Kearey & Vine, copyright Blackwell Sci. Publ.)
At left is the result of the fitting process for some strike-slip earthquake.
All the compressional picks should land in the dark compressional quadrants,
and all the dilatational picks should land in the light quadrants.
Note the double-couple, and the fact that we cannot tell whether this
was a north-striking left-lateral fault, or an east-striking right-lateral
fault.
(from Kearey & Vine, copyright Blackwell Sci. Publ.)
At left is the focal mechanism of a thrust earthquake, and below are
cross sections showing the two possibilities for fault motion allowed
by this mechanism. From the mechanism alone, the strike and dip
of the actual fault break is ambiguous.
Note that the centers of the compressional wave quadrants are at a
direction perpendicular to the axis of the maximum compressional strain.
So the compressional axes appear in the dilatational wave quadrants.
(from Kearey & Vine, copyright Blackwell Sci. Publ.)
At left is the mechanism of an extensional normal fault earthquake,
with the two faulting possibilities below.
Question: what other data could we use to resolve the ambiguity between the fault and auxiliary planes?
Question: identify a strike-slip, a normal-fault, and a thrust-fault mechanism on the map above, and describe at least three different mixed-mode mechanisms.