At a particular seismic station, a seismologist determined the difference in arrival times between the P and S waves from a recent earthquake to be 00:04:40. How far away was the seismic station from the epicenter?

Aim: How do seismologists determine the distance to the epicenter of an earthquake? (Continued)

I. Lets Review the Steps for determining the distance to an epicenter:

1. Read the seismogram and determine the PA and the SA

2. Subtract the PA from the SA to get your DA (difference in arrival times).

3. Have your Earthquake P and S wave travel time graph available for use

4. Using a piece of scrap paper, mark off your arrival time difference.

5. Slide your scrap paper (“Electric Slide”) until the top mark is on the S-wave line and the bottom mark is on the P-wave line simultaneously.

6. Follow that line down and note the distance. This is your distance to the epicenter.

II. How do we subtract time?

1. Always start from the right side (seconds column) and work your way to the left. How is subtracting time different than subtracting normal numbers?

Sample Problem:

SA = 05:53:30

PA = 05:50:00

A. Subtract your PA from your SA – WHY?

Try this one on your own.

SA = 10:23:45

PA = 10:18:15

Volunteer to come up and complete the subtraction?

S.A. = 06:03:00

P.A.= 05:58:201. Always start from the right (seconds)

2. If you cannot subtract the seconds, you need to borrow a minute, which is 60 seconds

3. If you cannot subtract the minutes, you need to borrow an hour, which is 60 minutes.

Complete Part II on your Travel Time Curve Worksheet – BUT…

I want you to complete the time difference column first. If you have time, go on to finding the epicenters.

Lets do the first one together. Who wants to volunteer to lead the way?

SA = 14:08:00

PA = 14:05:25

Check your answers:

2. 00:09:05

3. 00:04:50

4. 00:03:35

5. 00:06:40

Lets Review:

1. How do we subtract time?

2. How do we borrow time?

III. How do we determine the origin time (the time the earthquake occurred at)?

A. Determine the P travel time.

How is the P Travel time different than the P Arrival time?

B. Subtract the P Travel time from the P Arrival time, and this gives you the origin time of the earthquake. Just like that!!!

You want to know what time a student left his house before arriving to school. What information would you need?

Lets try one together, ok?

PA = 07:45:30

The distance to the epicenter is 5000 km.

P travel time = ? John Mellusi?

Correct, the P Travel time = 00:08:20. Nice job John!

Origin time? Tom Snyder, please come up and show us how you arrived at your answer.

PA = 07:45:30

PT = 00:08:20

Origin Time = 07:37:10

Excellent as usual Tom!!

Complete Part III of your Travel Time Curve Worksheet.

Lets check your answers:

Complete ticket to leave problem.