Earthquakes
The study ofplate tectonicsmakes it clear that the crust of theEarthis in
long-term motion being driven by giant convective currents in the materials of the
mantle. The average rate of motion is small but able to account for massive crustal
change on the order of tens of millions of years. Rather than moving slowly and
smoothly in all places, the crust is subject to differential motions because of irregu-
larities in the convection of the flow of plastic materials in the mantle and variations
in resistance of rock formations to pressures. In many instances rock is able to
accommodate the pressures and will warp andfoldover long amounts of time.
Sometimes, rocks will suddenly snap and crack propagating underground motion
in the form of seismic waves. Such events can be very large. In 2004 the western
edge of the Burma Plate jerked to the west and upward tearing some 400 km of
crust and causing massive destruction via tsunamis. In other circumstances small
pieces of the crust are rearranged a fewmillimeters along previously existing
faults. Sometimes, ground motion can be felt hundreds of kilometers away.
Earthquakes are measured using devices called seismometers, which record
time series of data called seismographs. Seismometers measure motion in the
crust. In the simplest incarnation, the frame of the device is anchored to a hanging
pendulum. As earthquake waves move the crust underneath, the pendulum
maintains its position by inertia and the relative motion of the crust is traced onto
a seismograph.
The center of earthquake motion is thefocus while the epicenter is the surface
position directly about the focus. Most earthquake energy is used in the deforma-
tion of rock material, but up to 10 percent of the energy is released as shock
waves. These seismic waves emanate from the focus with their speed governed
by the rock materials in which they are working. The fastest waves are known as
P (primary) waves and propagate at speeds between roughly 1.8 and 8.0 km per
second. S (secondary) waves include side-to-side and up-and-down motions
and travel at 60 percent to 70 percent of the speed of P waves. P and S waves both
register on seismographs with P waves arriving before S waves. Their speeds of
travel can range widely, but their ratio of speeds is always within a few percent
and this allows the establishment of the distance of the focus from a seismom-
eter. So, distance to the epicenter is readily found with greater distances being
associated with the greater difference in arrival times between S and P waves.
Exact epicenter location is unknown until it is “triangulated” from the data of
at least three seismographs. The GlobalSeismographic Network encompasses
over 150 stations capable of rapid exchange of standardized data so that events
are very well monitored. In addition there are country-based and local networks
in operation.108 Earthquakes