RELATIVE LOCATIONS OF EARTHQUAKES AND POTENTIAL DRIVING MECHANISMS OF SEISMICITY ALONG THE NORTHEAST U.S. ATLANTIC PASSIVE MARGIN

Understanding the possibility of a large magnitude, tsunamigenic earthquake along the continental shelf break of a passive margin is important as events similar to the 1929 M7.2 Grand Banks earthquake can unexpectedly devastate coastline towns. The tsunamigenic Grand Banks event raises the question, what is the potential for such an event to occur along other parts of the northeast Atlantic margin? Moderate seismicity has been observed near the continental shelf break east of the Gulf of Maine (~1000km southwest of the Grand Banks event). From 2008-2013 there were 16 earthquakes ranging in magnitude from 2.0-3.9 along the Gulf of Maine continental shelf break, with 10 of the earthquakes occurring on 12 April 2012. This research focuses on identifying the spatial and temporal migration patterns of these earthquakes calculated using a double-difference relative location analysis. In this procedure two waveforms are cross-correlated to determine the difference of arrival times of the P-waves and of the S-waves. Then the location of one event relative to another is calculated based on these differences in arrival times. The data show that generally as the earthquakes occurred further northwest they also happened deeper in the crust. Plotting the relative longitude versus relative latitude versus relative depth shows that the earthquakes map out a planar surface. If the events occurred on a single fault, then the preliminary interpretation of that fault is the strike is in the range from N60°E to N66°E, the dip is in the range from 23°-27.5° and the faults total extent is ~2.4km by ~2.7km. If this entire area ruptured in one earthquake, the event would be about a magnitude 5.0. The seismic hazard in the Gulf of Maine region will be better constrained with continued research on focal mechanisms of the 2012 events and by comparing the results of the locations and magnitudes of the 2012 events to those of the 1929 tsunamigenic Grand Banks event.