Warmup 11/4/15 If you were a scientist, what would you do or look for to try and find out where the center of the earthquake was? Objective Tonight’s Homework To learn how earthquakes move and how we locate them p 133: 1, 2, 3, 4, 5, 6

Seismic Waves and Epicenters When an earthquake happens, the very center of it is usually less than 45 miles underground. We call this spot the focus. Waves radiate out from this point in all directions. Directly above the focus is the strongest point on the surface. We call this point the epicenter. There are several kinds of waves that can radiate out from the center of our earthquake.

Seismic Waves and Epicenters The first 2 kinds of waves are called body waves. These waves travel straight through the earth. The first type of body waves are the P waves, which stands for “primary waves”. P waves are called compressional waves. This means they move in a squishing and stretching motion. The second type of body wave is called an S wave, for “secondary wave”. These waves move the ground up and down, like someone shaking a blanket to get dust off.

Seismic Waves and Epicenters We have another type of wave besides body waves. These are sometimes called “L waves” or surface waves. (These are not S waves) These usually do the most damage because they travel on the surface. The back-and-forth kinds are called “Love waves” and the up-and-down ones are called “Rayleigh waves”. Rayleigh waves move the ground in loops, the same way ocean waves move water.

Seismic Waves and Epicenters How do we measure how strong these waves are? Scientists do this with something called a seismometer. A seismograph uses a hanging weight with a pen attached to something solidly on the ground. A piece of paper slides by all the time, making the pen draw a line. If an earthquake comes, it shakes the ground, but not the weight, making the straight line become squiggly.

Seismic Waves and Epicenters So how do scientists know exactly where an earthquake starts? All seismometers do is record how strong a quake is, not where it comes from. Seismometers can actually tell us a bit more. If scientists look at the wiggles closely, they can tell the difference between P and S waves. With a bit of math, they can use this information to find out how far away the earthquake was.

Seismic Waves and Epicenters But this isn’t good enough. If the black dot is our seismometer, and if scientists were to graph this, the earthquake could have come from anywhere on the circle shown. But if we have 3 seismometers all do the same kinds of measurements, we can find the one spot they meet. That one spot is our epicenter.

Seismic Waves and Epicenters So now we know how earthquakes work and where they come from. There’s just one piece missing. What are aftershocks? We said that when an earthquake happens, it happens because rocks under pressure finally broke or moved. Our earthquake happens and energy ripples through the ground. What if this energy hits another area where rocks are under pressure and about to break? The earthquake could push them over the edge and cause these other areas to snap and shift too. This is what an aftershock is.

Earthquake Lab Tomorrow you’re going to be doing a lab on locating earthquake epicenters given data. Mr. C. is going to hand out the form for the lab and go through the first set of earthquake data to show you what you’ll be doing tomorrow.

Exit Question What kind of earthquake waves do we label as “S waves”? Surface waves Secondary waves Sonic waves Slipping waves Mystery waves None of the above