Ask A Scientist General Science Archive

Question:
I'm interested in hearing from any of the geologists in California
about the 1/17 earthquake...it was a shallow thrust, correct? Can you
remind me about the p and s waves..and were there l waves..or have I
forgotten all truthes? I would like to have my 7th gr math students
try to locate the epicenter by computing the arival times of the waves..
any ideas? This may not be the place to ask this general a question..
Thanks for any info I can share with students and MYSELF!
 catherine adele

Answer:
The magnitude 6.6 Northridge, California earthquake was located
at 34.22 degrees north latitude, 118.54 degrees west longitude
at a depth of 17 kilometers (10.5 miles).  The earthquake
occurred on a thrust fault which means that the earth on one
side of the fault was thrust over the earth on the other side
of the fault.  This type of faulting occurs where the earth is
being compressed together.  The depth of the earthquake was not
unusual for thrust earthquakes occurring in California but is
shallow compared to thrust earthquakes that occur in many other
parts of the world (for example, Alaska, Mexico, Chile, and Japan).
                               ______
                        __    /       \     __
                       /  \  | O  O  O |   /  \
      __              |    | |         |  |    |
     /  \            _|_[]_|_|__[]_[]__|__|_[]_|__
    |    |           \
____|_[]_|____________\
                       \           NORTHRIDGE
                        \          EARTHQUAKE
                         \   ___
        thrust fault ---> \  \
                           \  \
                            \  \
                             \  \


The thrust movement of this earthquake is in contrast to
movement on the San Andreas Fault (the major fault
in California) which is a strike-slip fault.
Movement on a strike-slip fault is horizontal
to the surface of the ground.
 
                               _______
      __                __    /       \     __
     /  \              /  \  | O  O  O |   /  \
    |    |            |    | |         |  |    |
____|_[]_|____________|_[]_|_|__[]_[]__|__|_[]_|__
                    |
                    |
                    |     San Andreas Fault
                    |      (Strike-slip)
                    |
Movement away from  |      Movement towards
 computer screen    |      computer screen
                    |
                    |
 

The final type of fault is called a normal fault.  This
type of fault occurs where the ground is being pulled
apart.  This type of faulting occurs in the Great Basin
region of the United States between the Sierra Nevada
Mountains in California and the Rocky Mountains.
 
 
                               _______
                        __    /       \     __
                       /  \  | O  O  O |   /  \
                      |    | |         |  |    |
       __             |_[]_|_|__[]_[]__|__|_[]_|__
      /  \           /
     |    |         /
_____|_[]_|________/
                  /      Normal fault
               / /
              / /
             / /
         \  / /
          \/ /
            /
           /
 

There are three kinds of seismic waves generated by an
earthquake: P-waves, S-waves, and surface waves.  The P-waves
travel fastest and are the first to be felt (P stands
for Primary).  The S-waves (Secondary waves) are the next
to be felt and usually have the greatest amplitudes
so they cause the most damage.  The last waves to be
felt are the surface waves--these waves feel something
like being on a boat in heavy seas. The farther you are
from the earthquake, the greater is the time separation
between the arrival of the P- and and S-waves.  Here in
Santa Barbara, about 108 kilometers (65 miles) from the Northridge
earthquake, we were awakened by the P-waves and roughly
15 seconds later we felt the S-waves.  The velocity of
P-waves in the earth is about 6.1 km/second (3.7 miles/second)
and the velocity of S-waves is roughly 3.5 km/sec (2.1 miles/sec).
You can estimate the distance to the earthquake by taking the
difference in time between the P- and S-wave arrivals and
multiplying by 8 to get kilometers or 4.8 to get miles.  In the
example above, to estimate how far the earthquake was from Santa
Barbara, multiply 4.8 times 15 seconds to get an estimate of 72
miles.  The value of 72 miles is reasonably close to the
actual distance of 65 miles.  By doing this type of calculation
for several different locations, you can find the location
of the epicenter.  This is basically the way scientists locate
earthquakes.
 
-Grant Lindley