6 MARCH 2005, RIVIERE-DU-LOUP, QUEBEC, CANADA EARTHQUAKE

KIM, WY., Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA, wykim@ldeo.columbia.edu.

Focal mechanism determined by analyzing regional waveform data from the 6 March 2005, Riviere-du-Loup, Quebec, Canada earthquake suggests a thrust faulting mechanism along N-S trending nodal planes. The Mw 4.7 earthquake occurred in the northern edge of the northeast trending Charlevoix seismic zone along the St. Lawrence River. It is the largest earthquake that occurred in the Charlevoix seismic zone since modern seismic networks began recording earthquakes in early 1970s in Eastern North America. Comparison of observed regional waveform data with corresponding synthetics calculated for various focal depths indicates that the focal depth is about 15 km. The P axis trends ENE-WSW and plunges 8 degree which is consistent with a broad-scale stress field in Eastern North America. Aftershocks and other earthquakes that occurred since 1990 in the area suggest that a nodal plane dipping moderately to east (45 degree) is the likely fault plane. The magnitude reported for the event is Mn 5.3 and, hence the difference between the magnitude obtained from the seismic moment and amplitudes of the regional signals is 0.6 magnitude units, which is somewhat greater than average differences.