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Source parameters and effects of bandwidth and local geology on high-frequency ground motions observed for aftershocks of the northeastern Ohio earthquake of 31 January 1986

UNITED STATES GEOLOGICAL SURVEY, 345 MIDDLEFIELD ROAD—MS 977, MENLO PARK, CALIFORNIA 94025

Abstract

A 10-station array (GEOS) yielded recordings of exceptional bandwidth (400 sps) and resolution (up to 96 dB) for the aftershocks of the moderate (m_b 4.9) earthquake that occurred on 31 January 1986 near Painesville, Ohio. Nine aftershocks were recorded with seismic moments ranging between 9 x 10¹⁶ and 3 x 10¹⁹ dyne-cm (M_w: 0.6 to 2.3). The two largest aftershocks (depth 5.3, 5.6 km; oblique right slip, rake 30°, strike N25°E) yielded seismic signals above background noise at frequencies as high as 130 Hz at epicentral distances up to 17 km. The aftershock recordings at a site underlain by 8 m of lakeshore sediments show significant levels of high-frequency soil amplification of vertical motion at frequencies near 8, 20, and 70 Hz. Viscoelastic models for P and SV waves incident at the base of the sediments yield estimates of vertical P-wave response consistent with the observed high-frequency site resonances, but suggest additional detailed shear-wave logs are needed to account for observed S-wave response. Peak acceleration values obtained from the broadband recordings are about two and four times as large as those that would be recorded on strong-motion recorders or short-period networks with upper bandwidth limits of 30 and 15 Hz, respectively. Attenuation-corrected acceleration spectra are used to reduce the influence of high-frequency (up to 100 Hz) local site effects on corner frequency estimates. The moment versus source radius trend inferred for events with moments as small as 9 x 10¹⁶ dyne-cm, based on the Brune source model, extends previous relations inferred for the central United States, shows little evidence for a minimum source radius, and suggests that stress drops for the smaller events (M₀ < 10¹⁹ dyne-cm) decrease with decreasing moment.

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