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Figure 6 A method for calculating the epicentral distance and magnitude of a local earthquake (ML) ftom the wave amplitude recorded on a seismogram. The seismograph that recorded this seismogram is a standard Wood-Anderson seismograph. In this ex- ample, the ruler below the seismogram in (a) indicates the time in seconds between the arrivals of the P and S waves; here, S - P = 24 seconds. This difference between arrival times can be used to calculate the distance between the epicenter and the recording sta- tion. The arrival-time difference is shown on the vertical scale (b) and corresponds to an epicentral distance of about 214 km. The amplitude of the seismic waves, 23 mm, is measured on the vertical scale in (a); this measurement is noted on the vertical scale (c, on the right). The magnitude is determined by drawing a line that connects the points on vertical scales (b) and (c). This line passes through 5 on vertical scale (d), cor- responding to a magnitude of 5.0 (Modified from "Earthquakes" by Bruce A. Bolt. Copyright @1978, 1988 W. H. Freeman and Company; reprinted with permission

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