Homework tonight: Read p. 370-372, p. 372#1-5. On #5, change the wording from "mantle" to "mantle's thickness". (thanks to Maura for pointing that out).
Our test will likely be next Friday, the 20th. Topics will be: Earth's interior, plate tectonics, earthquakes, and volcanoes. You need to know vocabulary, understand concepts related to our understandings, be able to cite and explain evidence supporting our knowledge, and use math to solve related problems.
Today in class we worked a problem to start out: An Earthquake is 150 km away. Find:
1. time for P wave to arrive
2. time for S wave to arrive
3. P-S delay
4. Equation relating delay to P&S wave speeds & distance
For answers to 1&2, we used the basic "speed = distance / time" equation and some algebra to solve for time:
1. 18.75 s
2. 37.5 s
For 3, subtract the P time from the S time and get a familiar number: 18.75s. The reason this comes out the same as the P time is due to the numbers we're assuming for P wave and S wave speed, which aren't exactly the exact values we would have in the Crust, but close enough that the math is easier and you hopefully get the concepts.
For #4, we worked with different ideas in different classes, but eventually came up with the simplest equation that works for our numbers: distance to epicenter = P-S delay * 8 km/s
Finally, we looked at a sample seismograph and:
a. labeled P,S, surface waves
b. used P-S delay to find distance to epicenter from seismograph station
c. found time of earthquake origination.
Afterward we continued on with our plate tectonics notes, found here.
Tomorrow we'll work on more notes, answer the homework questions, and work more epicenter location practice before embarking on a large project to use real data to find real earthquakes.
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