RUPTURE HISTORIES OF EASTERN NORTH-AMERICAN EARTHQUAKES

Digital, worldwide records of P and SH waves are used to invert for ru pture histories of the recent, larger, eastern North American earthqua kes. 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 e mploy a finite-fault, waveform inversion scheme that discretizes the s lip history as a function of position on the fault. This formulation a llows estimation of rise time and slip velocity, in addition to the sp atial distribution of slip. Static stress drops averaged over the enti re rupture surface range from a few tens of bars to just over 100 bars and are similar to values estimated for western United States earthqu akes. Estimates of stress drop for spatially limited asperities are in the range of a few hundred bars, with the value for the Saguenay eart hquake 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 velo cities 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 eart hquakes lying at the upper end of this range. Of the events studied, t he Saguenay earthquake is unique in terms of its greater depth, spatia lly concentrated source, and large asperity stress drop.