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EARTHQUAKES AT DEPTH

Learn about earthquakes at depth and the Richter Scale, measure the magnitude of earthquakes, earthquake depth zones, and the different types of seismic waves. Understand how scientists study earthquake frequency and predict future earthquakes. Also, discover what tsunamis are and how they are generated.

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EARTHQUAKES AT DEPTH

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  1. EARTHQUAKES AT DEPTH Veronica Ann Zabala Department of Geological Sciences Arizona State University PO Box 871404 Tempe, Arizona 85287-1404 480.965.7029 Veronica.Zabala@asu.edu

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  3. EARTHQUAKES AT DEPTH The Richter Scale, named after Dr. Charles F. Richter of the California Institute of Technology, is the best known scale for measuring the magnitude of earthquakes. The scale is logarithmic so that a recording of 7, for example, indicates a disturbance with ground motion 10 times as large as a recording of 6. A quake of magnitude 2 is the smallest quake normally felt by people. Earthquakes with a Richter value of 6 or more are commonly considered major; great earthquakes have magnitude of 8 or more on the Richter scale.

  4. The earthquake depth range of 0 - 700 km is divided into three zones: shallow, intermediate, and deep. Shallow earthquakes are between 0 and 70 km deep; intermediate earthquakes, 70 - 300 km deep; and deep earthquakes, 300 - 700 km deep. The evidence for deep-focus earthquakes was discovered in 1922 by H.H. Turner of Oxford, England. Previously, all earthquakes were considered to have shallow focal depths. The existence of deep-focus earthquakes was confirmed in 1931 from studies of the seismograms of several earthquakes. Surface-wave patterns generally indicate that an earthquake is either shallow or may have some depth. The most accurate method of determining the focal depth of an earthquake is to read a depth phase recorded on the seismogram. The depth phase is the characteristic phase pP-a P wave reflected from the surface of the Earth at a point relatively near the hypocenter. EARTHQUAKES AT DEPTH

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  6. EARTHQUAKES AT DEPTH Seismic P-waves, or primary waves, have a strong back-and-forth pattern of motion which produces areas of compression and dilation in the earth, spreading outward from the earthquake's epicenter. The P-waves are longitudinal waves which mean that their motion are parallel to the direction that the waves are traveling. The back-and-forth, up-and-down motions produce the S-waves, the secondary or shear waves. The S-waves are transverse waves; the “earth motion” it produces is perpendicular to the direction that the waves are traveling. S-waves can be horizontal or vertical.

  7. EARTHQUAKES AT DEPTH Scientists study the past frequency of large earthquakes in order to determine the future likelihood of similar large shocks. For example, if a region has experienced four magnitude 7 or larger earthquakes during 200 years of recorded history, and if these shocks occurred randomly in time, then scientists would assign a 50 percent probability (that is, just as likely to happen as not to happen) to the occurrence of another magnitude 7 or larger quake in the region during the next 50 years. Another way to estimate the likelihood of future earthquakes is to study how fast strain accumulates. When plate movements build the strain in rocks to a critical level, like pulling a rubber band too tight, the rocks will suddenly break and slip to a new position. Scientists measure how much strain accumulates along a fault segment each year, how much time has passed since the last earthquake along the segment, and how much strain was released in the last earthquake.

  8. WHAT IS A TSUNAMI? EARTHQUAKES AT DEPTH Tsunami is a Japanese word with the English translation, "harbor wave." Represented by two characters, the top character, "tsu," means harbor, while the bottom character, "nami,“ means "wave." In the past, tsunamis were sometimes referred to as "tidal waves" by the general public, and as "seismic sea waves" by the scientific community. Tsunamis can be generated when the sea floor abruptly deforms and vertically displaces the overlying water. Waves are formed as the displaced water mass, which acts under the influence of gravity, attempts to regain its equilibrium. When large areas of the sea floor elevate or subside, a tsunami can be created.

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  10. EARTHQUAKES AT DEPTH http://www.usgs.gov/ http://www.jpl.nasa.gov/earth/natural_hazards/natural_hazards_index.html http://www.geophys.washington.edu/tsunami/intro.html

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