Earthquake Epicenters

An earthquake is a shaking motion of a portion of the earth's crust. This motion results from the passage of waves traveling through the earth away from a place of rupture or faulting. The location of the rupture or faulting, which may be within or below the earth's crust, is called the focus. The point on the earth's surface directly above the focus is called the epicenter. In this exercise you will use the records of an earthquake from several different locations to find the epicenter of the earthquake.

A seismograph is used to record several types of waves radiating through the earth away from the focus. Two of these waves can be used to locate earthquake epicenters. These are:

1) "P"- waves or longitudinal wave (also called primary waves because they are the first to appear on a seismogram).

2) "S"- waves or transverse waves (also known as secondary waves because they are the second major wave to appear on the seismogram).

The P waves travel through the crust at a velocity of about 5 miles per second and the S waves at a velocity of about 3 miles per second. Because of this velocity difference, the seismogram will record the arrival of the P- wave first. At a later period of time the seismogram will record the S-wave arrival. The difference in time between the S-wave arrival time and the P- wave arrival time will be a measure of the distance of the seismograph from the epicenter.

Travel time curves are graphs that show how long it takes for a particular wave to travel a certain distance. If the difference in the arrival times for the P and S at some particular station can be determined, then by using the travel time curves, the distance from that station to the epicenter can be obtained.

For example, in Figure 6-2 ( a sample seismogram recorded in Dallas) notice that the P wave arrives ~ 2.1 minutes and the S wave arrives ~ 5.4 minutes. The difference in arrival times (5.4-2.1 = 3.3) is 3.3 minutes.

To get the distance, you must use the travel time curves given in Figure 6-1:

1) Lay a sheet of blank tracing paper along the S-P (time) vertical axis in Figure 6-1, and draw a vertical line on the tracing paper

2) Using the vertical scale on Figure 6-1, mark two dots on the vertical line on the tracing paper so there is a gap of 3.3 minutes (a 3.3 minute time interval) between the dots.

3) Keeping the vertical line of the tracing paper parallel to the vertical lines on Figure 6-1, slide the tracing paper across Figure 6-1 until the two dots on the tracing paper lie exactly on the S and P curves.

4) Draw a vertical line through the S and P curves at these points until it intersects the horizontal axis and gives you a distance in miles.

This gives the distance between the station and the epicenter. For the sample given in Figure 6-2, the epicenter is 1200 miles from Dallas.

Exercises

Using the sections of seismograms from three stations that are given in Figure 6-3:

1 Estimate to the nearest tenth of a minute the arrival times of the P and S waves at each station.

Site Name Time of P arrival Time of S arrival Difference (S-P)

Sitka ______

Columbia ______

Honolulu ______

2. Using the S minus P times, determine the distances corresponding these values

Site Name Distance to epicenter

Sitka ______miles.

Columbia ______miles.

Honolulu ______miles. 3) Next you will find the location of the earthquake epicenter distances just obtained.

a) Locate the three sites on the world map (Figure 6-4). The latitude and longitude of each site is as follows:

Site Name Lat. Long.

Sitka 57 N 135 W

Columbia 35 N 81 W

Honolulu 21 N 158 W

b) Draw a circle about each site with a radius equivalent to the distance to the epicenter determined for that site. Use a drawing compass and the scale given on the map for this purpose. The three circles should intersect at one point. The location of the epicenter is at the following:

Lat.______Long.______.

Source: http://mimp.mems.cmu.edu/~ordofmag/earthqua/earqua.htm