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Earthquakes Chapter 4 Lesson 2 Objectives • Describe how earthquakes occur. • Name and describe three types of faults. • Describe three types of seismic waves and explain how a seismograph works. Main Idea • Earthquakes are caused by sudden_______________ along breaks in the crust called _______________. Vocabulary • _________________________ – (noun), energy released by the movement of breaking of rock along a fault during an earthquake. • _________________________ – (noun), an instrument used to detect and measure the ground motion. Building Background Earthquakes are the result of movement of rock at_______________. This movement produces several types of seismic waves that caused the ground motion that result in____________________ damage. At the Faults • ____________________ in Earth’s crust are usually so small and slow you can’t even feel them. But at times, these movements can be quick and_______________, causing sudden and unexpected changes to Earth’s surface. Such movements typically happen at_______________. Types of Faults • _______________ _______________ occur at or near divergent boundaries, places where the Earth’s crust is being stretched apart. Eventually, the rock breaks. A fault forms when one block moves down the edge of a sloping_______________. Most normal faults are_______________, with blocks of rock moving down by just a meter or so. • _______________ _______________ usually occur at convergent plate boundaries, where rock is being squeezed. As the rock breaks, one block is pushed_______________ along a 1 sloping crack. This type of fault often occurs in regions where one plate_______________ below the other. • ____________________ _______________ occur where tectonic plates slide past each other and transform-fault boundaries. In contrast to the other two types of faults, the movement is____________________, is one block grinds past the other. California’s_______________ _______________ _______________ is a strike-slip fault. Seismic Waves • Remember that tectonic plates move very_______________. Sometimes rocks move along easily with the plates, but they can also jam up against a plate or between two_______________. Over time, stress builds up within the rock at the plates_______________ against each other. • Eventually, the stress becomes so great that the rocks_______________. The plates shoulder and_______________ into a new position. This causes an_______________. • The released energy during an earthquake travels and waves in all directions, including up to the_______________. These waves are called_______________ _______________. They cause the_______________ that is felt as an earthquake. • Seismic waves that travel through Earth’s interior are called_______________ _______________. There are two kinds of body waves: _______________ and _______________. P-waves or _______________ _______________, are an example of longitudinal waves. Rocks move back and forth in the same direction the wave _______________. P-waves are the fastest kind of seismic wave traveling at about ____________________ through the rock. • During an earthquake, the first jolt or bad you feel is the arrival of the_______________. The P-waves _______________ and _______________ on rock along their path. As P-waves reach the surface, the ground cracks and crumples. P-waves can travel through solids and liquids, including Earth’s liquid_______________ _______________. • The second type of body waves is the S-wave or _______________ _______________. Swaves are_______________ waves. Rocks move up and down or____________________ at right angles to the direction the wave travels. S-waves are_______________than P-waves. When S-waves it, you might feel a strong____________________ motion. S-waves can only travel through solid_______________. 2 Surface Waves • Surface waves travel only across Earth’s surface. The L-wave or_______________ wave is the slowest. L-waves arrive just after both P-waves and S-waves. _______________ _______________ do not travel as far from an earthquake’s origin as body waves. They cause a rolling motion on the surface that can cause great damage to_______________. L-waves are often the most_______________. • Although each type of wave differs in_______________, all seismic waves travel very fast. • Compare the surface movements caused by P-waves and S-waves. ________________________________________________________________________________ ________________________________________________________________________________ ________________________________________________________________________________ ________________________________________________________________________________ Recording Seismic Waves • When an earthquake strikes, sciences can quickly locate its_______________ and measure its_______________. ____________________ are the main tools for measuring the strength and duration of an earthquake. A_______________ detects and measures the amount of ground motion during a quake. Scientists interpret these data to measure the strength of an earthquake, also called its_______________. • A seismograph produces a written trace of seismic waves called a_______________. Data are recorded as lines on a drum or by a computer. One type of seismograph measures_______________ motion. It consists of a rotating drum attached firmly to the bedrock_______________. A weighted pen is suspended over the drum like a_______________. The pen marks the paper wrapped around the_______________ _______________. • When an earthquake strikes, the drum attached to the bedrock moves with Earth’s_______________. The_______________ pen does not move. So it traces the movement of the drum beneath it. Seismographs also record aftershocks, which are smaller_______________ that follow the first one. Aftershocks can occur for several days after an earthquake. 3 • In what order do waves appear on a seismogram? ________________________________________________________________________________ ________________________________________________________________________________ ________________________________________________________________________________ ________________________________________________________________________________ 4