Western Quebec seismic zone (Canada): Clustered, midcrustal seismicity along a Mesozoic hot spot track

The western Quebec seismic zone (WQSZ) is a 160-km-wide band of intraplate seismicity extending 500 km from the Adirondack Highlands (United States) to the Laurentian uplands (Canada). Previous authors have proposed that the WQSZ is localized over the Mesozoic track of the Great Meteor hot spot. Here we explore this hypothesis further by investigating regional seismicity characteristics. Focal mechanisms for WQSZ earthquakes, including a new mechanism for a moderate (mN 4.5) earthquake, reveal a pattern of reverse-sense faulting with SW trending P axes changing to E-W in the southern part of the zone. We introduce a simple box-counting method to delineate spatial clusters, based on exceedance of random seismicity density. Combining this approach with focal depths from regional depth phase analysis, we find that seismicity with shallow focus (0-7 km) is characterized by a random spatial distribution, whereas earthquakes with an intermediate focal depth (8-18 km) are strongly clustered along a diffuse linear band trending N50°W. Earthquakes deeper than 18 km are confined to a few distinct clusters. These clusters are characterized by differing b values and, for at least one cluster, repeating events. Projection of hypocenters onto a deep seismic profile and comparison with preexisting crustal structures suggest that local reactivation of Precambrian structural features may have occurred; however, the Great Meteor hot spot track remains the only compelling explanation for the overall distribution of earthquakes. Proximity of seismicity clusters to historic and prehistoric earthquakes lends support to the hypothesis that modern seismicity may represent exceptionally long-lived aftershocks of large past events.