Earthquakes

Earthquakes
Earthquakes occur when built-up energy releases inside the earth, and "snaps." They usually occur at plate boundaries, where plates converge, diverge, or slip past each other. The starting point of an earthquake in the ground is called the **Focus**. Right above that, on the surface, is the **Epicenter.**
 * P and S Waves** originate from this place and you can calculate the distance from and the origin of the start of the quake with these P and S waves.

Here is an example:

The arrival of the P wave is at 0 minutes, then the arrival of the S wave is at 5 minutes. The difference between the two is five minutes.

Now you would go to your reference table (Page 11):

On the Y axis you have to mark a line at the five(on a separate piece of paper) and then move it along the line until the span on the paper(from 0-5 in this case) lines up with in between the P and S waves: In this case the span is right above the 3600 kilometer mark on the X axis, so the epicenter distance is 3,600 kilometers.

If you wanted to find the arrival time of a P or S wave, you first have to find the travel time:

Let's say that //the epicenter distance was 5,000 kilometers and we want to find when the P-wave arrives if the quake starts at 6:00 AM.//

It sounds like a lot, but it really isn't, just break it down: On your ESRT find the 5 on the X axis and go up to the P wave: It intersected at the :08:20 travel time, so you would just add that time the the start time (6:00) to get 6:08:20 as the arrival tim eof the P wave. You do the same thing for the S wave, but you keep going up until the S wave line on the graph.


 * Other Problems:**

If you want to find to the arrival time of the S wave after the P wave and the question gives you the P wave time, you find the difference between the two times and add that to the time of the P wave. This one's tricky cause you can't find the time from when the quake started; you have to find the time //after// the P wave comes.

Just remember: The farther away from the epicenter, the bigger the circle and time of arrival of the waves. Then for the lesser amount of kilometers from the epicenter, the circles are smaller, and so are the arrival times!