Source parameters from shallow events in the Rocky Mountain House earthquake swarm

Abstract

We report here source parameters of the Rocky Mountain earthquake swarm derived from three-component digital data. During 6 days in October 1980, 21 events were recorded. Focal depths for these events are in the range of 1 ± 0.8 to 2 ± 2 km. Eleven events with local magnitudes from 2.1 to 2.8 yielded source parameters. Corner frequencies of the S-wave spectra were found in the range 6.2 ± 0.5 Hz, giving source dimensions of 160 ± 10 m. The corresponding P-wave corner frequencies are in the range 8.6 ± 3 Hz. The ratio of P to S corner frequencies varies from 0.9 to 2.1. There is a path effect between 13 and 16 Hz that could have affected these ratios. The average falloff over three components at high frequencies varies from −1.8 to −2.3. High stress drops, ranging from 47 to 263 bar (4.7–26.3 MPa), and apparent stresses, from 2.5 to 23 bar (0.25–2.3 MPa), were calculated. Five events have remarkably similar characteristics in the frequency and time domains. For these events the ratio of minimum strain energy W0, according to Kanamori, to the energy calculated using the integration scheme of Hanks and Thatcher was 3.7 ± 0.5. A theoretical value gives 3.1. The seismic efficiency ranges from 0.2 ± 0.04 to 0.17 ± 0.8. Large seismic moments for relatively small magnitudes were found. Some of these spectral characteristics are best explained as the result of displacement along a smooth fault.