ABSTRACT: Source complexity of the Cap-Rouge earthquake: moderate earthquakes yield their secrets.

CGRG Bibliography of Canadian Geomorphology
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Abstract

The mbLg 5.1 Cap-Rouge earthquake is the largest well-instrumented earthquake in southeastern Canada since the 1990 Mont-Laurier earthquake. It occurred on 19971106 at 0234, 22 km under the western suburbs of Quebec City (46.80N, 71.42W). The mainshock (Mw ca. 4.5) was well recorded by seismographs at >100 km, and by the Quebec City analog strong motion instrument (11 km). The network recorded 15 aftershocks of magnitude 2.9 - 1.3 and field instruments recorded 20 smaller aftershocks. A P-nodal mechanism (by A. Bent) indicates primarily reverse-faulting, from geological arguments likely on a plane dipping 70 degrees southeast and striking N50E. We have interpreted spectral ratios and modelled the source. Derived source spectra vary with azimuth and show spectral scalloping, implying a double event. Spectral modeling indicates a pair of sources with the second about 1.0 km southwest of the first and delayed by 0.6 sec. The strong motion time history supports a double source. Consequences of the interference for moment determination and Q determination will be presented. The spectral characteristics of this model are fully consistent with the Haddon 1996 source scaling model. Such complexity makes simplistic interpretations of "the" source in terms of Brune-type models extremely misleading. Taken with the results of the Mont-Laurier mainshock, Saguenay mainshock and foreshock, and other mbLg>4.5 earthquakes studied, we find this complexity pervasive for eastern Canada. If such complexity exists for many Mw 4.5 earthquakes in other areas, conclusions about average rupture velocities inferred from the size and duration of rupture will be inaccurate and may account for the unusual properties claimed for the "nucleation zones" of M>6 earthquakes.

Bibliography of Canadian Geomorphology