Earthquakes and Seismic Waves Chapter 5 Section 2

Key Terms Earthquake Focus Epicenter P Wave S Wave Surface Wave Mercalli Scale Magnitude Richter Scale Seismograph Moment Magnitude Scale

A Bit About Earthquakes Earthquake: the shaking that results from the sudden movement of rock along a fault Most are too small to notice. Large ones can change the earth’s surface. The forces of plate movement cause earthquakes. Focus: the area beneath the earth’s surface where rock that is under stress breaks, triggering an earthquake. Epicenter: the point on the surface directly above the focus.

Types of Seismic Waves Earthquakes produce vibrations called waves. These waves race out in all directions. Seismic waves carry energy from an earthquake away from the focus, through the earth’s interior, and across the surface. There are three categories of waves: P waves, S waves, and surface waves.

P Waves P waves are known as primary waves, the ones that arrive first. They compress and expand like an accordion. P waves can damage buildings. They can move through both solids and liquids. https://www.youtube.com/watch?v=2rYjlVPU9 U4

S Waves S waves are known as secondary waves. They come after the P waves. They vibrate from side to side as well as up and down. When S waves hit the surface, they shake structures violent. They can’t move through liquids. https://www.youtube.com/watch?v=en4HptC0 mQ4

Surface Waves When P waves and S waves hit the surface, some of them become surface waves. Surface waves move slower than P and S waves, but they can produce severe ground movements. Some roll and some shake from side to side. https://www.youtube.com/watch?v=x1tb6Hwy aHg

Measuring Earthquakes “How big was that earthquake??” There are three commonly used methods of measuring earthquakes: the Mercalli scale, the Richter scale, and the moment magnitude scale.

The Mercalli Scale The Mercalli scale was developed to rate earthquakes according to their intensity, or strength, at a given place. The same earthquake can have different Mercalli ratings because its intensity varies at different locations. There are 12 steps to this scale.

The Richter Scale Magnitude: the number that geologists assign to an earthquake based on the earthquake’s size. The Richter scale assigns a magnitude number to an earthquake based on the size of the seismic waves. Seismograph: measures the seismic waves. This scale does not work well for large or distant earthquakes.

The Moment Magnitude Scale Moment magnitude scale: estimates the total energy released by an earthquake. This can measure earthquakes of all sizes, near or far. Geologists study the seismic waves and how much movement occurred along the fault to create the rating.

Comparing Earthquakes An earthquake's magnitude tells geologists how much stored energy was released by the earthquake. Magnitude values can range from near zero to 9.0 and 10.0. The effects of an earthquake increase with magnitude. Below 5=small with little damage 5 to 6= moderate with moderate damage Above 6=large with great damage 8 and above=most powerful but are rare https://www.youtube.com/watch?v=TBIu5JYjxHg

Locating the Epicenter Geologists use seismic waves to locate an earthquake’s epicenter. To tell how far the epicenter is from the seismograph, scientists measure the difference between time of the P waves and S waves. Geologists then draw at least 3 circles using the data from different seismographs set up at stations all over the world. https://www.youtube.com/watch?v=oBS7BKqH Rhs