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

Abstract

[1] 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.

Scenario shakemaps for Montreal

“…9, locations of areas that are more active than the average background rate (seismic activation) are shown in red (Web version only), those that are less active than the average background rate (seismic quiescence) are shown in blue (Web version only). In this figure, the linear pattern running northwest toward Ottawa coincides with a seismicity trend along an ancient hot-spot track, as discussed by Ma and Eaton (2007).…”

“…Scenario 2 was chosen because it has the largest potential magnitude of all the events shown in Fig. 9 and corresponds with the seismicity trend identified by Ma and Eaton (2007). Scenarios 3 and 4 were chosen because they represent moderate size events to the northeast and southeast of the city, locations with which we have limited prior experience.…”

“…Based on the exposed population and on the probability of earthquake occurrence, Montreal ranks second in Canada after Vancouver for seismic risk (Adams et al 2002). The seismic activity in the region is attributed to the reactivation of ancient normal rift faults along the St. Lawrence and Ottawa Rivers as well as the passage of an ancient hot spot beneath the region (Adams and Basham 1991;Ma and Eaton 2007). The city is particularly vulnerable to seismic events since the city is largely built on recent unconsolidated marine and river deposits and much of its infrastructure is old and may have limited seismic resistance.…”

Focal Depth Determination for Moderate and Small Earthquakes by Modeling Regional Depth Phases sPg, sPmP, and sPn

“…The relative errors due to crustal model are estimated as within 15% when the error in the crustal velocity model is 10%, or when the thicknesses of layers in the crustal model have some errors, or the crustal medium differs from the assumed Poisson medium. We have analyzed the regional depth phases for many earthquakes (Ma and Atkinson, 2006;Kim et al, 2006;Dineva et al, 2007;Ma and Eaton, 2007;Atkinson et al, 2008). For this chapter we selected some special cases to show that our modeling method is simple, reliable, and suitable for all regions where regional depth phases are developed.…”

“…2; model 1 in Table 1). Because the focal mechanisms in western Quebec are predominantly thrust type (e.g., Adams et al, 1989;Bent and Perry, 1999;Ma and Eaton, 2007), we used a thrust type focal mechanism as the default (Fig. 2, bottom left).…”