Homework 6
Earthquake Location
EPSc 1021, Spring 2007
Due: Monday, March 26
Name: ________________________
Because P waves travel faster than S waves through rock, the difference in their arrival times can be
used to estimate the distance to an earthquake’s epicenter. However, with only one seismograph,
the location of the earthquake cannot be determined (in other words, a seismograph can’t determine
which direction the seismic waves came from). Multiple seismographs are therefore used to
triangulate earthquake locations.
In this exercise, P-S travel time differences are used to locate an earthquake in the New Madrid
Seismic Zone. This 5.0 magnitude earthquake occurred in June 2002 and could be felt in St. Louis.
Use the following figures to find the earthquake’s epicenter. We assume a P wave velocity of 6.5
km per second and an S wave velocity of 3.5 km per second through the crust.
Figure 1. Seismograms at St. Louis, Nashville, and Memphis. The P and S wave arrivals are
denoted by vertical red lines. The PS interval is the time between P and S arrivals.
60
55
ave
s
50
Sw
45
time (s)
av
e
s
40
w
al
rv
P
35
PS
30
e
int
25
20
15
10
5
0
0
50
100
150
200
250
distance (km)
300
350
400
Figure 2. Travel-time plot for P and S waves. The line marked “PS interval” is the time difference
between P and S wave arrivals. This interval gives the distance from the epicenter.
Figure 3. Map of the New Madrid Seismic Region. Each segment of the scale bar represents 100
km. Use the seismogram and travel-time plots on the previous page to triangulate the location of
the June 2002 earthquake. Indicate the epicenter on the map.
Draw a line indicating the position of a plate boundary, and arrows indicating plate motions, had the
mid-continent rift fully developed.
If a powerful earthquake occurred in the seismic zone above, how/why would the Mercalli intensity
rating for that earthquake be different in St. Louis than, say, Memphis?