Instructions for Locating an Earthquake Epicenter

Earthquake Waves• By determining the time of arrival for both the

P and S waves at a location we can determine how far away the Earthquakes epicenter is from the seismograph.

• This does NOTNOT tell us what direction the waves are coming from.

Locating the Epicenter of an EarthquakePart I: Finding the distance to the epicenter

• Step 1:– Determine the difference in arrival time for

your P-wave, and your S-wave.P-wave

03:21:15

S-wave

03:27:15

Calculation – difference in arrival time

03:27:15

-03:21:15

00:06:00 minutes

Locating the Epicenter of an EarthquakePart I: Finding the distance to the epicenter

• Step 2:– Take out your ESRT.

Open to the Travel Time Graph on page 11.

• Step 3:– Use the vertical scale

(time) to mark off the difference in arrival time on a scrap sheet of paper.00

:06:

00 m

inu

tes

Locating the Epicenter of an EarthquakePart I: Finding the distance to the epicenter

• Step 4:– Make sure to keep

your scrap paper vertical! Slide it along the curves until it lines up on each of the curves.

– Be very accurate!!!!!

Locating the Epicenter of an EarthquakePart I: Finding the distance to the epicenter

• Step 5:– Read off the distance

from the horizontal axis that corresponds to this spot. This is the distance between the epicenter and your seismograph location.

– This distance is the radius of a circle around the seismograph.

Locating the Epicenter of an EarthquakePart I: Finding the distance to the epicenter

• Step 6:– Repeat steps 1-5 for at least 2 other

seismograph locations.

– It is necessary to have at least 3 stations, if you do not you can not be sure of the exact location of the epicenter.

Common Regents Question

Locating the Epicenter of an EarthquakePart II Locating the Epicenter

• Step 1:– Find the location of your first seismographic

station on the map.• Step 2:

– Use a compass or string to create a circle with its center at your seismograph location, and a radius equal to the distance you found.

Locating the Epicenter of an EarthquakePart II Locating the Epicenter

Step 3:• Repeat this procedure around two other

seismographic stations.• Where the three circles all intersect

(cross) is where your epicenter is located.

• If the circles do not all intersect, but form a small triangle, the epicenter is the center of the triangle.

Locating the Epicenter

Epicenter Location

Figure 2.13

Locating the Epicenter of an EarthquakePart III: Origin timeOrigin time of the Earthquake

Step 1:

• From the distances determined in Part I, determine how long it would take a P-wave to travel that distance.

Origin Time of Earthquake• Step 2:

• Find the distance on the horizontal axis.

• Go up to the point where you hit the P-wave travel line.

• Go over to the vertical axis and read off the travel time for the P-wave.

Distance = 4,400 km

P-wave Travel Time = 00:07:40

Origin Time Story• We have the P-wave arrival time at the

seismograph, and the time that it took to get there, so we need to find the time the wave left the focus of the earthquake (origin time of the seismic waves).

• You went to a party, you arrived at 06:30:00, it took 00:12:15 to get there, what time did you leave your house?

Calculation:

06:30:00 – arrival time

-00:12:15 – travel time

06:17:45 – time of departure

Check your work:

06:17:45 – departure

+00:12:15 – travel time

06:30:00 – arrival time

Origin Time of Earthquake

• Step 3:– Taking the time found in step 1, subtract this

from the arrival of the P wave and that is the original time of the earthquake.

– Calculation 03:21:15 - P-wave Arrival Time

-00:07:40 - Travel Time for P-wave @ 4,400km

03:13:35 - Origin Time (when the earthquake occurred)