Staple your own homeworks

EART 10 - Geologic Principles			NAME____________________
Fall 1995
EARTHQUAKE EPICENTER
due date: November 16, 1995
	
Refer to the text (Hamblin, 7th ed.): pp. 498-501.
and appropriate pages in the Laboratory Manual (Zumberge/Rutford, 9th ed.)

YOU WILL NEED A COMPASS FOR THIS EXERCISE.

The following table gives arrival times of P and S waves from three
seismograms that recorded a magnitude 6.6 earthquake in southern California
in January 1994.  The seismic stations for each seismogram are shown on the
map on the reverse page by solid circles at Los Angeles (LA), Ventura (VEN),
and in the Mohave Desert (MD).

Station	Arrival Times			Ts-Tp		Epicentral Distance 
	P Wave (Tp)	S Wave (Ts)	(s)		D (km)

LA	8:36:09.26	8:36:15.87	6.6 seconds	55 km
VEN	8:36:09.93	8:36:17.03	7.1 seconds	60 km
MD	8:36:11.00	8:36:18.86	7.9 seconds	66 km


PART A.	Calculate the epicentral distances (D) for each station from the
following equation.
D/Vs - D/Vp = Ts - Tp , where

	D = epicentral distance (km)
	Ts = S-Wave arrival time (s)
	Tp = P-Wave arrival time (s)
	Vs = average S-Wave velocity (3.5 km/s)
	Vp = average P-Wave velocity (6 km/s)

Show your work and put your answers in the table above.

Ts-Tp=D/Vs-D/Vp

=D(1/Vs-1/Vp)

D=(Ts-Tp)/(1/Vs-1/Vp)

For LA:

D = 6.6/(1/3.5-1/6) = 6.6/(0.286-0.1667) = 6.6/0.1193 = 55.3

plug and chug for the rest.

PART B.	Plot the epicenter of the earthquake on the map on the reverse
side of this paper by drawing circles centered on the seismic stations with radii
that represent the epicentral distances.  The intersection of the three circles
defines the earthquake epicenter.  (NOTE: map scale is in miles; 1 mi = 1.61
km)

Sorry, no map at this time.

PART C.	Why is the earthquake epicenter not on a fault trace?

1) The fault plane may be inclined, causing the focus and the epicenter to plot at
different places, or

2) A fault a depth may not break the surface.