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Figure 5. Seismograms (a) through (e) were recorded by stations in Washington and Oregon and illustrate the range of ground motion frequencies commonly recorded. The seismometers that recorded these motions are similar, and all had natural periods of 1.0 second. The seismograms for (a), (b), and (c) are expanded as (A), (B), and (C) in the lower part of the figure. P and S waves are marked on all seismograms. (a) Seismogram of a small (magnitude 1.2) volcanic earthquake at Mount St. Helens on November 23, 1987. The focus was less than I km below the surface, and the epicenter was less than I km from the station. (b) and (c) Seismograms of a magnitude 0.9 earthquake in the Cascade Range on November 18, 1987. The focus was at a depth of 17 km, and the epicenter was 13 km from the station that recorded (b) and 47 km from the station that recorded (c). (d) and (e) Seismograms from a magnitude 6.3 earthquake in the Imperial Valley of California on November 24, 1987 (d) shows the P wave as recorded at a station in northern Oregon, 1427 km from the epicenter. (e) shows the surface waves, which have lower frequencies, recorded at the same station. The surface waves arrived about 5-1/2 minutes after the P waves.

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