Abstract

Two earthquake doublets and two multiplets recorded by the Charlevoix Telemetered Network (CLTN) in the Charlevoix Seismic Zone (CSZ) of southern Quebec, Canada, have been analyzed using an empirical Green's function (EGF) method to derive the relative source time functions (RSTF's) of seven master events with MbLg = 1.2 to 4.4. We identified the doublets and multiplets using a waveform cross-correlation and relative event location technique to verify that each earthquake pair had similar focal mechanisms and hypocentral locations. Three-component S waveforms recorded by the high dynamic range (126 dB) instrumentation of the CLTN were used to extract the RSTF's.

The RSTF's reveal that six of the seven events are simple with single-source pulses having durations of 0.05 to 0.2 sec. Another earthquake (920310-0545, M 3.3) appears to be a double event with two episodes of rupturing. Azimuthal variations of the RSTF pulse amplitudes and widths provide strong evidence for the rupture directivities of five of the earthquakes (M = 1.2 to 4.4). The azimuthal variations in the RSTF pulse amplitudes were used to estimate the rupture directions and rupture velocities. Lower-bound estimates of the rupture velocity range from 0.5 to 0.7 Vs. Estimates of the rupture direction were combined with P-wave focal mechanisms for the four largest events (M 3.3 to 4.4) to identify the fault plane for these earthquakes. Source parameters were measured for the RSTF's of the master events, including seismic moments of 3.5 × 1018 to 5.3 × 1021 dyne-cm, fault radii of 100 to 330 m, and static stress drops of 2 to 90 bars. The fault radii and stress-drop estimates for M > 3 events agree well with estimates obtained by other researchers for M ∼ 3 to 4.5 earthquakes in the CSZ. We also observed apparent scaling between the stress drop and the earthquake size, which has been reported in other studies of stress drop in northeastern North America.

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