Comparisons of Ground Motions from Colocated and Closely Spaced One-Sample-per-Second Global Positioning System and Accelerograph Recordings of the 2003 M 6.5 San Simeon, California, Earthquake in the Parkfield Region

The 2003 San Simeon, California, earthquake (M 6.5) generated a set of colocated and closely spaced high-rate (1-sample-per-second) Global Positioning System (GPS) positions and ground motions from digital accelerographs in the Park- field region (at epicentral distances of 50 to 70 km). The waveforms of displacements derived from the 13 GPS receivers in the region have dominant periods between about 7 and 18 sec. The waveforms are similar in shape, with a systematic change in waveform as a function of distance from the source. The GPS motions are smaller than the accelerograph motions for periods less than about 2 sec. From this we conclude that the 1-sample-per-sec GPS receivers provide a good representation of ground motion at periods longer than about 2 sec. Perhaps more important for earth- quake engineering is that the accelerograph data are similar to the GPS data for periods as long as 30 sec, if not longer. This means that data from digital accelero- graphs can provide reliable relative-displacement response spectra at the periods needed in the design of large structures, at least for earthquakes with magnitudes of 6.5 or above at distances within 70 km. We combine the colocated or very closely spaced GPS and accelerograph data sets in the frequency domain to obtain a single broadband time series of the ground motion at each accelerograph station. These broadband ground motions may be useful to seismologists in unraveling the dynamic process of fault rupture and to engineers for designing large structures with very- long-period response.

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