Abstract

Strong motion data for aftershocks of the 1982 Miramichi earthquake are analyzed to determine site effects on recorded waveforms. Although aftershocks vary in source mechanism, the complicated vector particle motion of P and S arrivals are often coherent between events recorded at the same station. This suggests that near-surface receiver structure has a significant effect on the recorded waveforms. Resonance features dominate the wavetrains with P reverberations existing on the vertical components and S reverberations prominent on the radial components indicating near vertical incidence. These reverberations take the form of peaks in the amplitude spectra which may complicate corner frequency interpretation. As an example, a corner frequency shift, fc(S) < fc(P), is evident and is attributed to structure induced effects. Synthetic seismograms computed using the propagator matrix method indicate that a thin low velocity layer of unconsolidated glacial till located directly beneath the receivers is responsible for most waveform effects. Attenuation effects in the thin layer are included in the computation of the synthetics in order to help match amplitude ratios. The geometry of the till-bedrock interface is interpreted as being three dimensional due to high tangential P amplitudes and anomalous particle motions. The observed waveforms are believed to consist mainly of direct waves, reverberations within the thin layer, and scattered arrivals resulting from the nonplanar interface. A careful analysis of three component waveform data in earthquake aftershock data sets such as this one considered here may be useful in determining local site conditions independent of assumed source models.

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