AN IMPROVED SOURCE MECHANISM FOR THE 1935 TIMISKAMING, QUEBEC EARTHQUAKE FROM REGIONAL WAVE-FORMS

The Timiskaming earthquake, which occurred near the Quebec-Ontario bor der at the northwest end of the Western Quebec seismic zone in 1935, i s one of the five largest instrumentally recorded southeastern Canadia n earthquakes. Previous studies of this earthquake concentrated on mod eling teleseismic waveforms from which a unique source mechanism could not be determined. By modeling seismograms recorded at regional dista nces, a better constrained focal mechanism is obtained. The waveforms indicate thrust faulting on a moderately dipping northwest striking pl ane at a depth of IO km. The M(W) of 6.1 determined in this study is i n good agreement with previous magnitude estimates (m(b), 6.1, M(S) 6. 0, and m(bLg) 6.2-6.3). The focal mechanism is similar to those of man y recent small to moderate earthquakes in the region, and the inferred (from the P axis) acting stress of northeast compression is consisten t with the overall eastern North American stress field. The ''Lake Tim iskaming Rift Valley'' in which the earthquake occurred, comprises sev eral northwest striking faults consistent with the strike of the 1935 event. Thus, the 1935 earthquake appears to be a result of faulting on the reactivated Timiskaming graben.