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U.S. Geological Survey Branch of Earthquake and Landslide Hazards, Denver, Colorado 80225
U.S. Geological Survey National Earthquake Information Center, Denver, Colorado 80225
Abstract
Digital, worldwide records of P and SH waves are used to invert for rupture histories of the recent, larger, eastern North American earthquakes. The data include a broad bandwidth to facilitate recovery of both source details and total moment. The following events are studied- 9 January 1982 New Brunswick; 5 October and 23 December 1985 Nahanni; 25 November 1988 Saguenay; and 25 December 1989 Ungava earthquakes. We employ a finite-fault, waveform inversion scheme that discretizes the slip history as a function of position on the fault. This formulation allows estimation of rise time and slip velocity, in addition to the spatial distribution of slip. Static stress drops averaged over the entire rupture surface range from a few tens of bars to just over 100 bars and are similar to values estimated for western United States earthquakes. Estimates of stress drop for spatially limited asperities are in the range of a few hundred bars, with the value for the Saguenay earthquake asperity being the largest (approaching 1 kbar). The variation in stress drop is considerable, but no evidence is seen for a scaling relation in which stress drop increases with moment. Maximum slip velocities on the fault also have a wide range (from 50 cm/sec to perhaps greater then 200 cm/sec), with the Saguenay and 5 October Nahanni earthquakes lying at the upper end of this range. Of the events studied, the Saguenay earthquake is unique in terms of its greater depth, spatially concentrated source, and large asperity stress drop.
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