This laboratory exercise will familiarize you with the general process by which earthquake epicenters are determined. The method takes advantage of the fact that there are relatively constant ratios between the velocities of different pairs of seismic wave types. The difference between the arrival times of these different wave types at a seismic recording station can then be converted into distance from the focus. By triangulating among at least three such stations, it is possible to delimit the probable epicentral area.

P waves generally travel between 5.95 and 6.75 kilometers per second in the crust, depending on compressibility, rigidity, uniformity, and density of the materials traversed. S waves tend to move at velocities between 2.9 and 4.0 km/sec in the crust. Rayleigh waves travel somewhere between 2.7 and 3.7 km/sec. Expressed as ratios, these are V_p:V_s = 1:1.73 and V_s:V_r = 1:0.92.

Time and distance can be graphed for each of the wave types. By comparing the time between pairs of wave types on the graph, a rough estimate of the distance to the earthquake focus can be made. In each of the following sets of problems, you need to convert the time difference given between the arrivals of each of the two wave types into kilometers. To do this, measure the difference in time on the Y axis, marking it on a ruler or piece of paper. Then, find the distance on the X axis where the two relevant curves diverge by just that time difference on your marked paper or ruler. Give your answer rounded to the nearest 250 km.

Simplified Graph of P, S, and R Waves through Time

PROBLEMS

Station A records the first arrival of P waves at 14:05 GMT. The S waves arrive at 14:09.

1. About how far away was the earthquake, in kilometers? ______________

2. Draw a circle on the map around Station A with that radius.

Station B records the S waves at 14:10 GMT and the R waves at 14:16:30.

1. About how far away was the focus, in kilometers? ______________

2. Draw a circle on the map around Station B with that radius.

Station C records the P waves at 14:01 and the R waves at 14:03:30.

1. About how far was the focus, in kilometers? ______________

2. Draw a circle on the map around Station B with that radius.

Epicentral area and time of the event:

1. Mark the probable epicentral area on the map.

2. Give the approximate Greenwich Mean Time of the event. ______________

Map of Seismographic Stations

Document maintained by Dr. Rodrigue
Last revised: 10/01/99