5 points

This page, 1 point eachEpicenters

P wave travel time3000 km.______

S wave travel time3000 km.______

Station / P Arrival / Arrival / Difference / Distance
(Pa) / (Sa) / (Sa – Pa) / km
A / 14:05:25 / 14:08:00
B / 14:13:15 / 14:22:20
C / 14:08:15 / 14:13:05

Earthquake AData Table

Station / P Arrival / Arrival / Difference / Distance / P travel / Origin Time
(Pa) / (Sa) / (Sa – Pa) / km / Pt / (Pa – Pt)
San Jose
New York
San Fran

Earthquake BData Table

Station / P Arrival / Arrival / Difference / Distance / P travel / Origin Time
(Pa) / (Sa) / (Sa – Pa) / km / Pt / (Pa – Pt)
San Jose
New York
San Fran

2 points each

Earthquake / Latitude / Longitude / Country/State

A

B

QUESTIONS:3 points each Answer questions # 1 – 3 in complete sentences.

1.

2.

3.

4.ABCD

5.ABCD

6.ABCD

7.ABCD

8.ABCD

Earthquake A
Draw the 3 lines and place an “X” at the epicenter (2 points each).


Earthquake B Draw the 3 lines and place an “X” at the epicenter (2 points each).

Lab #21Epicenters

INTRODUCTION:

Earthquakes occur in many distant places, and many of them occur under oceans. Seismologists need to be able to determine where an earthquake has occurred even though it is far away.

Waves radiate away from where an earthquake has occurred (focus). The first wave to arrive is the P-wave or Primary wave. It arrives first because it is the fastest. The S-wave or Secondary wave, arrives shortly after the P-wave. Seismometers can detect these waves and seismographs record the times of arrival of these waves. With three of these records (seismograms), a seismologist can determine the location of the epicenter (the point on the earth directly above the focus) and the magnitude (strength) of the earthquake.

PROCEDURE:

Use the graph on page 11 of The Earth Science Reference Tables, to determine the travel time of a P-wave and the S-wave if the seismometer is 3000 km away. This involves reading the Time v. Distance graph.

The answer should have been 5 minutes 35 seconds for the P wave and 10 minutes 10 seconds for the S wave. S waves always travel slower than P waves.

Since both P wave and S wave speeds are predictable and P waves are always faster, it is possible to tell how far away an earthquake is by the difference in arrival times of the P wave and S wave. For example in the previous problem the Sa – Sp is as follows:

00:10:10

-00:05:35

______

00:04:35

For the next data table, subtract Sa – Sp to determine the difference. Place the edge of a piece of paper against the time scale on the travel time graph on page 11 of the Earth Science Reference Tables. Mark off the Sa – Sp distance calculated for this station. Move the edge of the paper on the graph until the lower mark is touching the P wave graph line and the top mark is touching the S wave graph line. When you find where the marks touch both lines, read the distance on the graph, that corresponds to that point.

The table below shows the answers:

Station / S-P / Distance
A / 00:02:35 / 1500
B / 00:09:05 / 7700
C / 00:04:50 / 3300


To locate a quake, three stations are needed. One station yields a circle of possible locations; a second station narrows the location to two points; and the third station locates the epicenter. The diagram below shows how to locate the epicenter.

Using the simplified seismograms on the next two pages, determine where the epicenters of earthquake A and earthquake B are located. You must first read the arrival times of the P and S waves from the seismogram. Once the arrival times are determined, calculate the difference, distance and use the maps to locate the epicenters. Draw the circles, using a compass, on the maps provided. The circles will actually be too large to fit on the page so just draw the part of the circle that does fit on the paper.

QUESTIONS:

  1. Are these earthquakes located in recognized seismic areas (areas of earthquake activity)? Explain your answer.
  1. Using the chart below, what are the possible dates of these earthquakes?

Richter Magnitudes of some Major Earthquakes

Location / Date / Magnitude
India / 1897 / 8.7 (est.)
San Francisco / 1906 / 8.3 (est.)
Japan / 1923 / 8.3
Chile / 1960 / 8.5
Alaska / 1964 / 8.4
China / 1976 / 8.2
Mexico City / 1985 / 8.3
  1. Both earthquakes were of similar magnitude. Which earthquake do you think caused more deaths? Explain your answer.
  1. How much time is required for a P wave to travel 6000 kilometers?

A) 6 min00 sec

B)9 min 20 sec

C)10 min30 sec

D)16 min50 sec

  1. If a seismograph station is located 3800 kilometers from the earthquake, approximately how long after the arrival of the P wave will the S wave arrive

A)5 min20 sec

B)6 min40 sec

C)8 min20 sec

D)14 min50 sec

For questions #6 – 8, use the diagram below


  1. At which location was the difference in arrival times of the P and S waves the greatest?

A)A

B)B

C)C

D)D


  1. At which location could the seismogram below have been recorded?

A)A

B)B

C)C

D)D

  1. The earthquake epicenter is located at point

A)A

B)B

C)C

D)D

Earthquake A

Partial seismogram records showing the same earthquake as recorded by three seismograph stations


Earthquake B

Partial seismogram records showing the same earthquake as recorded by three seismograph stations