## Geography 260: NATURAL HAZARDS

### Dr. Rodrigue

### Graded Lab 1: Simplified Epicenter Triangulation

### Corrected Version

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.

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

- 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.

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

- 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.

- About how far was the focus, in kilometers? ______________

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

Epicentral area and time of the event:

- Mark the probable epicentral area on the map.

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

**Map of Seismographic Stations**

##### Document maintained by Dr.
Rodrigue

Last revised: 10/01/99