Earthquakes and Seismic Waves An earthquake is the shaking and trembling that results from the movement of rock beneath Earth's surface. The point beneath Earth s surface where rock under stress breaks to cause an earthquake is called the focus. The point on the surface directly above the focus is called the epicenter. During an earthquake, vibrations called seismic waves move out from the focus in all directions. Seismic waves carry the energy of an earthquake away from the focus, through E a r t h s interior, and across the surface. There are three categories of seismic waves: P waves, S waves, and surface waves. P waves compress and expand the ground like an accordion. S waves vibrate from side to side and up and down. When P waves and S waves reach the surface, some become surface waves. Surface waves move more slowly than P waves and S waves. Three commonly used methods of measuring earthquakes are the Mercalli scale, the Richter scale, and the moment magnitude scale. The Mercalli scale was developed to rate earthquakes according to the level of damage at a given place. An earthquake s magnitude is a number that geologists assign to an earthquake based on the earthquake's strength. The Richter scale is a rating of an earthquake s magnitude based on the size of the earthquake's Seismic waves. The seismic waves are measured by a seismograph. A seismograph is an instrument that records and measures seismic waves. Geologists today often use the moment magnitude scale, a rating system that estimates the total energy released by an earthquake. An earthquake s magnitude tells geologists how much energy was released by the earthquake. The effects of an earthquake increase with magnitude. Geologists use seismic waves to locate an earthquake's epicenter. When an earthquake strikes, P waves arrive at a seismograph first and S waves next. The farther away the epicenter is, the greater the difference between the two arrival times. This time difference tells scientists how far from the seismograph the epicenter is. The scientists then use the information from three different seismograph stations to plot circles on a map. Each circle shows the distance from one seismograph station to all the points where the epicenter could be located. The single point where the three circles intersect is the location of the earthquake s epicenter. Questions: 1. What are seismic waves? 2. In what order do the three types of seismic waves arrive at the seismograph? 3. Which type of seismic wave produces the most severe ground movements? 4. What are the three scales used for measuring earthquakes? 5. How do geologist locate the epicenter of an earthquake? 6. Describe the moment magnitude scale and explain why it is useful in measuring earthquakes? Matching a. Focus b. epicenter c. Seismograph d. magnitude e. surface waves 1. records ground movements caused by surface waves as they move through the Earth 2. slowest seismic waves that produce the most severe ground movements 3. the point beneath Earth s surface at which rock under stress breaks and triggers an earthquake 4. a measurement of earthquake strength 5. the point on the surface directly above the point at which an earthquake occurs
Earth s Crust in motion An earthquake is the shaking and trembling that results from the movement of rock beneath Earth's surface. Powerful forces caused by the movement of Earth s plates squeeze rock and pull it in different directions. A force that acts on rock to change its shape or volume is called stress. Stress adds energy to the rock. The energy is stored in the rock until it changes shape or breaks. The change in the shape or volume of the crust is called deformation. Three kinds of stress cause deformation: shearing, tension, and compression. Shearing pushes a mass of rock in two opposite directions. Tension pulls on the crust, stretching rock. Compression squeezes rock until it folds or breaks. Shearing, tension, and compression work over millions of years to change the shape and volume of rock. When stress builds up in rock, it fractures along a fault. A fault is a break in Earth s crust where slabs of crust slip past each other. Faults usually occur along plate boundaries, where the forces of plate motion compress, pull, or shear the crust so much that the crust breaks. There are three main types of faults. In a strike-slip fault, the rocks on either side of the fault slide past each other sideways with little up-or-down motion. Strike-slip faults are caused by shearing. In a normal fault, one block of rock lies above the fault while the other block lies below it. The block that lies above the fault is the hanging wall, and the block below the fault is the footwall. Normal faults are caused by tension. In a reverse fault, the hanging wall slides upward past the footwall. Reverse faults are caused by compression. How blocks of rock move along a fault depends on how much friction there is between them. If friction is low, the blocks slide constantly with- out sticking. If friction is high, the blocks lock together. When the amount of stress is greater than the amount of friction, the blocks unlock suddenly, setting off an earthquake. The forces of plate movement can change Earth's surface. Over mil- lions of years, fault movement can change a Bat plain into a towering mountain range. When normal faults uplift a block of rock, a fault- block mountain forms. Sometimes plate movement bends Earth's crust instead of breaking it along a fault. A fold is a bend in rock that forms when compression shortens and thickens part of the crust. A fold that bends upward in an arch is an anticline. A fold that bends downward in the middle to form a bowl is a syncline. When a fault pushes up a large, flat block of rock, a plateau can form. A plateau is a large area of flat land elevated high above sea level. Use the diagrams above to answer 1-3 1. Diagram A - Type of fault: Stress force: 2. Diagram B - Type of fault: Stress force: 3. Diagram C - Type of fault: Stress force: 4. The shaking and trembling that results in the movement of rock beneath the earths surface is called an. 5. Define the three types of stress that occur in the earth s crust. a. b. c. 6. is the term that means any change in the volume or shape of the Earth s crust.
7. A break in the earth s crust is called a. 8. What can form when a fault pushes up a large block of rock?