Date:

Graded Assignment

Lab Report

Answer the questions below. When you are finished, submit this assignment to your teacher by the due date for full credit. You may type in the answers, but you will need to print this lab report to draw the epicenter.

In this laboratory experiment, you will be working as an amateur seismologist to locate the epicenter of a fictional earthquake. Your task will involve interpreting seismograms from three seismograph stations; determining the difference in time between the arrival of the P waves and arrival of the S waves from the earthquake; using a travel-time graph to determine how far each seismograph station is from the epicenter; and using the technique of triangulation to pinpoint the location of the quake.

Step 1: Interpreting the Seismograms

1. Estimate the times of the first arrival of the P waves and the S waves at each seismograph station. Enter these times into your data table. Reminder: You are trying to find the difference between the arrival time of the P wave and the S wave.

2. Determine the difference between the arrival of the P wave and the arrival of the S wave and enter this difference into the data table.

Example:

The P wave arrived at 8:08:00 and the S wave arrived at 8:10:30.

The difference is 2 minutes and 30 seconds, or 2½ minutes.

Step 2: Determining the Distance to the Epicenter

You now know the difference between the arrival of the first P wave and the first S wave for each seismic station. Because the waves travel at a known speed, this interval can be converted to distance using the graph below.

Example:

The P wave arrived at 8:08:00 and the S wave arrived at 8:10:30.

The difference is 2 minutes and 30 seconds, or 2½ minutes.

S-P Wave Interval Chart

Find 2 minutes and 30 seconds on the chart, then drop down to the distance when you intersect the S-P wave interval line = 1,600 km.

Reading the Graph: Tips and Practice

The major lines you read on the y-axis (vertical axis) are minutes.