Source parameters from locally recorded aftershocks of the 9 January 1982 Miramichi, New Brunswick, earthquake

Abstract Source parameters are derived from 111 locally recorded three-component seismograms of 40 aftershocks (magnitude = 1.0 to 3.5) of the 9 January 1982 Miramichi, New Brunswick, earthquake. This data set greatly expands the set of three-component data recorded at small source-to-receiver distances in eastern North America. Preliminary analysis showed that some aftershock spectra were modified by strong site effects. The need to understand and account for site effects, when analyzing the aftershock data in terms of simple source models, led to a two-part data analysis. First, site effects were modeled as a seismic-wave resonance in a thin layer (0 to 10 m) of low-seismic-impedance glacial till over a high-seismic-impedance bedrock half-space. Second, we calculated source parameters from SH-wave seismograms and amplitude spectra, incorporating what had been learned about site resonances. Correcting for obvious site effects partially succeeded in reducing scatter in the source-parameter determinations but some site-dependent discrepancies remain. Seismic moments range from 1 × 1018 to 5 × 1020 dyne-cm and corner frequencies from 5 to 45 Hz. Brune-model stress drops primarily range between 5 and 300 bars; stress drop apparently increases with increasing seismic moment. Trends in apparent stress calculated from radiated energy and seismic moment parallel trends in stress drop. Corner frequencies from these locally recorded data are greater than those derived from regionally recorded Lg waves for earthquakes of comparable moment. This result contradicts a recently proposed M0ω4 = constant source-scaling law based primarily on Lg data.