GEOL 240Lg 1st Edition Lecture 16 Outline Current Lecture I. Locating Earthquakes, this is the main focus on current lectureII. DiagramsCurrent Lecture- A seismogram is a recording of ground motion at a seismic station during some time period. - The ground motion can be measured in terms of displacement, velocity (displacement per unit time), or acceleration (velocity per unit time). - You should really try and memorize the formula for ground motion. - timing of P- and S-wave arrivals tells us how far the hypocenter is from the station. - The P-wave arrival time, tP, is generally clear and easy to identify. - The S-wave arrival time, tS, is often less obvious, because there is usually still some P-wave energy at the time of the S-wave arrival. - Most of today's earthquakes are located using COMPUTERS.- This is done by fitting the P-wave arrival time data to a "least-squares best fit" hypocenter and origin time. - the hypocenter is only an approximate location of the initial break of a large earthquake.- For smaller earthquakes (M<5), where the rupture plane is only a few square kilometers,the hypocenter is a more adequate description of the “source” of an earthquake.-Interpreting Earthquakes-Focus (hypocenter)- Point at which earthquake originates.- Epicenter - Point on the earth's surface directly above the focus.Locating Earthquakes- Difference between arrival times of P and S waves is determined. This gives distance to the epicenter from the seismograph.- Three seismographs are triangulated to give actual location of the epicenter (Figure).Once distance to epicenter is known a correction factor is applied to amplitude of largest
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