Abstract

Thirteen strong-motion accelerographs sited in a permanent array at the Idaho National Engineering Laboratory (INEL) were the closest instruments to the MS = 7.3 1983 Borah Peak, Idaho, earthquake. They recorded the event at epicentral distances of 90 to 110 km. The peak horizontal accelerations or peak ground accelerations (PGAs) at the basement and free-field sites of the array ranged from 0.022 to 0.078 g. For 2 weeks after the main shock, the U.S. Geological Survey maintained an array of digital seismographs in the epicentral area; these instruments recorded six large aftershocks at epicentral distances of 4 to 45 km. The largest of these aftershocks also triggered four accelerographs in the INEL array. PGAs for the main shock are estimated by two separate analyses. First, the attenuation of the PGAs from the largest aftershock is used to extrapolate the far-field PGA into the near-field, obtaining estimates of 0.54, 0.58, 0.21, and 0.24 g at epicentral distances of 11, 12, 16, and 18 km, respectively. Second, paired recordings for a set of small aftershocks, obtained from a near-field station (U.S. Geological Survey portable instrument) and a far-field station (INEL telemetered station), were used together with an INEL main shock acceleration time history to generate four synthetic accelerograms for a hypothetical recording site 18 km southeast of the main shock. The PGAs measured from these synthetic accelerograms are 0.08, 0.14, 0.15, and 0.23 g.

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