_09_Earthquakes_Tsunami_09 - GEL 1 Lecture 9 Earthquakes...

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1 GEL 1: Lecture 9: (Ch. 10) “Earthquakes posing a potentially greater hazard than the Loma Prieta (M=7.1) shock in 1989 are likely to occur within California during the lifetime of most residents.” (USGS, 2003) - see these estimates for the Bay Area at <quake.usgs.gov> - the probabilities for Southern California are comparable to those for the Bay Area Earthquakes occur along faults . . . fault = plane of weakness along which movement has occurred (i.e., rock on one side of the fault plane moves in some direction - up, down, lateral, oblique - relative to rock on the other side of the fault plane) Elastic Rebound Theory : fundamental model for earthquakes Most earthquakes occur along plate boundaries because of proximity to tectonic stress (but many other earthquakes may occur far away from plate boundaries) Tectonic motion causes stress on rocks (stress is the push, pull or shear that a material feels when subjected to a force) - with continued plate motion, rocks accumulate strain energy elastically (strain is the change in shape of the material in response to the application of stress) - strain energy builds until the frictional strength of the rocks is overcome – rocks rupture at a small region along the fault plane (the focus) with the rupture extending outward along the plane at about 3 to 4 km/sec - strain energy is abruptly released as spherical vibrations (seismic waves) that emanate outward away from the fault plane (this is the earthquake) - the rocks on either side of the fault elastically ‘rebound’ to an undeformed, unstressed state after the earthquake Earthquakes follow a pattern called “stick-slip motion” - the blocks on either side of the fault 'stick' for extended periods of time, as they accumulate strain energy. This may take decades, centuries or even millennia. - eventually, as soon as the frictional strength is exceeded, they 'slip' in a rapid, violent release of energy called an 'earthquake'. - repetitive earthquake activity along faults follows the earthquake cycle ” (as yet unpredictable - we’ll talk more about this later) - the problem is that stick-slip motion and the resulting earthquake cycle is not periodic in occurrence – in fact, it may be chaotic and thus unpredictable - there is no average time between earthquakes, only a range of times (tens to hundreds to thousands of years) within which a fault may rupture
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2 Earthquake terminology earthquake = sudden motion in the Earth caused by abrupt release of slowly accumulated strain (typically along faults) seismicity = the occurrence of earthquakes in space and time seismology = study of earthquakes, and of the structure of the Earth, by analysis of seismic waves focus = location within the Earth where the original rupture along a fault begins. (also called the hypocenter ) epicenter = point on Earth’s surface that lies vertically above the focus - epicenter typically doesn’t occur along the surface trace of the fault . . . displacement
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_09_Earthquakes_Tsunami_09 - GEL 1 Lecture 9 Earthquakes...

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