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MONTEREY PENINSULA COLLEGE GEOLOGY 2 HOMEWORK: DETERMINE THE EPICENTER OF THE 1994 NORTHRIDGE EARTHQUAKE Seismic waves, felt as the shaking motion when an earthquake occurs, are generated when energy is released during an earthquake. The difference in time is takes for the two primary types of seismic body waves—P waves and S waves—to travel to and be recorded by a seismometer, is used to determine where the epicenter of the earthquake is located. Primary waves (P waves) travel faster than secondary waves (S waves), therefore the P waves will arrive at a recording station first. First, visit http://www.sciencecourseware.com/VirtualEarthquake/VQuakeExecute.html, read each web page carefully, and determine the epicenter for at least one earthquake in one of the four regions. Attach a print-out of your epicenter triangulation page and Richter magnitude confirmation page to this homework. (You may also apply for a Certificate of Completion as a Virtual Seismologist.) Next: View Figures 1 through 3 (attached), and carefully read instructions below. To use a seismogram (the picture generated by a seismograph) to calculate the distance between the recording station and the epicenter of an earthquake: a) Identify the first arrival of the P waves (the first appearance of a wave image; Figure 1); b) Identify the first arrival of the S waves (when the wave image is amplified; Figure 1); c) Measure the lag time (∆T) between the P and S wave arrivals in seconds (ts - tp); d) Find the distance (D) that corresponds to that lag time using a travel-time curve (Figure 2); or use the following equation: D = ∆T/(1/vs – 1/vp), Where D = distance, ∆T = the lag time, vs = the speed of the S wave in the crust, and vp = the speed of the P wave in the crust; e) Using a compass, draw a circle with a radius equal to D and its center at the seismic station that recorded the P and S waves; and, f) Repeat steps (a) through (e) for at least 2 more stations, and you will have “triangulated” the location of the earthquake epicenter. 1. Complete the following table using steps a, b, and c (above). Station Code tp (seconds) ts (seconds) ∆T (ts – tp) Distance (km) Barrett Dam BAR 35 64 29 245 Goldstone GSC Needles NEE Pasadena PAS Pinion Flat PFO Seven Oaks Dam SVD Victorville VTV 2. Using the data from the table above and following steps d, e, and f, determine the epicenter location of the 1994 Northridge earthquake in latitude and longitude (plot it on Figure 3). 3. Is the earthquake’s epicenter located on one of the major faults shown in Figure 3?