GEOL 240Lg 1st Edition Lecture 16 Outline Current Lecture I Locating Earthquakes this is the main focus on current lecture II Diagrams Current 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 Pwave 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 wave
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