What's Shaking Trailblazers?

We've been talking a lot about earthquakes. Let's take some time to review what we've learned...

As plates move, stress builds up in the earth. There are 3 types of stress; compression, tension and shearing. Compression squeezes rock, tension pulls rock, and shearing pushes rock in opposite directions. When the stress is released, this caused earthquakes to occur.

When an earthquake occurs, a fault is created. A fault is a crack in the ground where blocks of rock have moved past each other. There are 3 types of faults. Normal faults occur as a result of tension. The hanging wall moves down relative to the footwall. Reverse faults are caused by compresion. In this case, the hanging wall moves up. Shearing causes rocks to slide past each other and creates a strike-slip fault. The San Andreas fault is a strike-slip fault.

During an earthquake, seismic waves move out from the focus in all directions. Primary waves (or p-waves) are the fastest. They move in a straight line and can travel through solids and liquids. Secondary waves (or S-waves) move more slowly. They travel in a side-to-side motion and can only travel through solids. When the p-waves and s-waves hit Earth's surface, they combine to form surface waves (or L-waves). These are the waves that cause the most amount of damage.

These seismic waves can tell us a lot about an earthquake. When detected at a seismograph, scientists can use the data to help determine the size of the earthquake and the epicenter. The determine the distance to the epicenter, first determine the time between the arrival of the p-wave and s-waves at a seismograph station. Then use the graph to determine the distace to the epicenter. Next, use a compass to draw a circle around the location of the seismograph station. Repaet this process for 2 other siesmograph stations. The epicenter is located where the 3 cirlces overlap. To practice this, visit the virtual earthquake website.

At this time, scientist cannot predict when an earthquake will occur. However, scientists can determine the seismic risk of an area. Maps are created to show where earthquakes are most likely to occur. Most of these areas are along plate boundaries; however, some earthquake do occur away from plate boundaries.

Hope this summary is helpful to you. Our test will be next week. To study, visit some of the important links posted to the right.