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Unformatted text preview: Earthquakes 07:20 Basic considerations-earthquakes Earthquakes can be understood in terms of the basic mechanisms of deformation Most earthquakes occur at plate boundaries (convergent, divergent, and strike-slip) Earthquakes cannot yet be reliably predicted or mitigated. 1. What is an Earthquake? Earthquake - Violent oscillatory motion of ground caused by passage of seismic waves radiating from a fault associated with sudden movement. Elastic rebound theory - faults remain locked while strain energy accumulates in rock, and then suddenly slips and releases energy. Focus - the point at which fault slips. Epicenter - geographic point directly above focus of earthquake. Seismic waves - elastic energy released as seismic vibrations that propagate in all directions. o Travel at ~2500-6000 m/sec and up to 1000 km. Foreshock - relatively smaller earthquakes occurring in vicinity of main shock- currently not useful for earthquake prediction. Aftershock - smaller shocks that follow main shock and distributed throughout fault plane. Damage - generally greatest damage near epicenter, but other factors may influence this e.g. ground type or bedrock. Seismographs - measures magnitude of vertical and horizontal motion related to earthquakes. o Three types of seismic waves P-waves (primary) - compressional waves- travel fastest. S-waves (secondary) - shear waves-travel about one half as fast as P waves. Surface waves - surface motion- may move in a rolling motion or sideways motions. 2. Studying Earthquakes Locating earthquake epicenters (how to do it) o 1. Time interval between arrival of P and S waves and compare to time- travel chart This relates to distance o 2. Multiple seismographs at different locations allows epicenter to be located at intersection of circles. Determining the size of an earthquake o 1. A geologist measures the amplitude of the largest seismic wave... o 2. And the time interval between the P-wave and S-wave arrivals to determine the distance from the epicenter to the station(210 km). o 3. By plotting the two measurements on these graphs and connecting the points, the geologist deter the Richter magnitude of the earthquake (5.0). o 4. Developed Richter Scale- logarithmic (i.e. unit represents 10x more ground motion than previous one) or a fator of 33X more energy. o 5. Another measure of damage-shaking intensity determined by the modified Mercalli Scale (I-XII). 3. Earthquakes and patterns of faulting...
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- Spring '07