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

Large magnitude offshore passive margin earthquakes are rare, making small magnitude events (Mw <4) the predominant data available to study the mechanisms of seismicity along passive margins. This study is focused on a swarm of events (MLg 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 (MLg 3.9), the strike is 235° 1°, with a dip of 77:7° 0:8° and a rake of −116:5° 3°, and for the second largest event (MLg 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 Mw 4.9–5.0. Based on Gutenberg–Richter relations (Gutenberg and Richter, 1944) from the eastern Canadian APM, if an Mw 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 Mw 7.2 Grand Banks earthquake, an earthquake of Mw ≥7 in the GM swarm area could be tsunamigenic