Relative Location Analysis and Moment Tensor Inversion of the 2012 Gulf of Maine Earthquake Swarm

摘要

Large magnitude offshore passive margin earthquakes are rare, making small magnitude events (M_w <4) the predominant data available to study the mechanisms of seismicity along passive margins. This study is focused on a swarm of events (M_(Lg) 2.1-3.9) that occurred from 2012 to 2013 in the Gulf of Maine (GM) along the Atlantic Passive Margin (APM) shelf break, a region with few previously recorded earthquakes. Relative locations were calculated for the earthquakes of the GM swarm and a moment tensor inversion method was used to calculate focal mechanisms for the two largest events in the swarm. The results of the relative location method constrained a fault orientation to a strike of 243° ± 3° and a dip of 25° ± 3°. From the regional moment tensor analysis, the focal depths of the largest events were between 12 and 18 km. For the largest event (M_(Lg) 3.9), the strike is 235° ± Io, with a dip of 77.7° ± 0.8° and a rake of —116.5° ± 3°, and for the second largest event (M_(Lg) 3.7) the strike is 259° ± 3°, with a dip of 78° ± 2° and a rake of —58.8° ± 7°. By mapping the spatial extent of the relative hypocenters, a potential fault size of 2.7 km by 2.4 km is inferred. If this entire area were to rupture at once in a single earthquake it would have M_w 4.9-5.0. Based on Gutenberg-Richter relations (Gutenberg and Richter, 1944) from the eastern Canadian APM, if an M_w 7 can occur in the GM, its estimated mean repeat time is 2120-22,800 yrs. Because the geologic setting of the GM swarm is similar to that of the 1929 M_w 7.2 Grand Banks earthquake, an earthquake of ≥ 7 in the GM swarm area could be tsunamigenic.