Review Session 3 Earthquakes

Continental Drift The Theory – In 1915, Alfred Wegner believed that the continents were once a supercontinent that he called Pangea – He thought that the continents are still moving! – something that most people did not believe! – Wegner thought that the crust was broken up into pieces called plates that move relative to each other

The Evidence…

Seafloor Spreading Hess proposed that hot, less dense material below Earth’s crust rises toward the surface at the mid- ocean ridges. Then, it flows sideways, carrying the seafloor away from the ridge in both directions.

Plate Tectonic Theory Both Hess’s discovery and Wegner’s continental drift theory combined into what scientists now call the Plate Tectonic Theory. Theory of plate tectonics: The Earth’s crust and part of the upper mantle are broken into sections, called plates which move on a plastic-like layer of the mantle

Paleomagnetism

Primary Waves also called compressional waves -travel faster= arrive first -can travel through solids and liquids -particles move parallel to the direction of wave motion

Secondary waves Secondary waves  shear waves -travel slower= arrive second -can not travel through liquids -particles move perpendicular to the direction of wave motion. ch?v=gl4FvHKzAlU

GO TO PAGE 11 OF ESRT Shows the time it takes for S and P waves to travel away from the epicenter (location) of an earthquake Distances are in terms of THOUSANDS OF KILOMETERS “Travel Time” is the time it takes for the wave to go a certain distance

QUESTION TYPES 1 &2 Finding Time and Distance

We know the distance (3000km) We’re trying to find the time it will take P waves to travel this distance. Find 3 (really 3000km) on the bottom axis and, using a straight edge (a ruler) draw a line up to meet the P wave line.

We know the P wave travel time: 5 minutes and 40 seconds. We are trying to find the distance between the epicenter and the seismic station. draw a line from the time (5:40) to the P wave line. Now draw a line straight down to the bottom axis and read the distance kilometers! All of these problems are the same. Given the time, find the distance. Given the distance, find the time. And it doesn’t matter whether you are given P or S wave travel time as long as you are careful to use the correct line and, most important: TAKE THE TIME TO BE CAREFUL AND ACCURATE!

QUESTION TYPE 3 Twin Questions

QUESTION TYPE 4 GAP Questions

The Get-A-Paper G – GET A – A P – PAPER You need to get a piece of scrap paper for these kinds of problems!!

Question Type #4 – GAP Questions Another type of question involves the “difference in travel time between P waves and S waves”. Whenever you are given the difference in arrival times OR you are given both times so that you can subtract and find the difference, you should immediately realize that you will be dealing with the shaded area (yellow) between the two lines. Here’s a sample problem: P waves arrive at a seismic station 5:20 (5 minutes, 20 seconds) that’s totally What this said before… before the arrival of S waves from the same earthquake. How far from the epicenter is the seismic station?

Here’s what you know: P & S waves arrive at a seismic station 5 minutes and 20 seconds (5:20) apart. You are asked to find the distance to the epicenter. Here’s how: 1) Take a sheet of paper and line up the left edge with the vertical axis (time). Be sure that most of the paper is hanging down below the graph. This is important. 2) Make a small, thin, and accurate mark on the paper at 0 time. Make another small, thin, and accurate, mark at 5:20 (5 minutes, 20 seconds). Your 2 marks are now 5:20 apart!

Now slide your paper to the right until one of your marks is exactly on the S wave line and the other is exactly on the P wave line. It is very important to be sure your paper is straight (vertical). Now look to see where the bottom of your paper crosses the lower (epicenter distance) axis. In this case it crosses at exactly 3000 km which is the answer. When P & S waves arrive 5 minutes and 20 seconds apart it means that the seismic station is exactly 3700 km from the epicenter of the quake. Of course we can do the same problem in reverse!