ALBERT

Description: Arrays of highly portable, exploration style seismographs were deployed following the 16 October 2012 M w  4.0 Waterboro, Maine, earthquake with the goal of improving hypocenter locations and source mechanisms of aftershocks, lowering the detection threshold for such aftershocks, and using the aftershocks as sources to image subsurface structure in the hypocentral volume. Based on the Aftershock Imaging with Dense Arrays (AIDA) experience following the Mineral, Virginia, earthquake of August 2011, the Maine deployment consisted of three complementary arrays totaling 110 stations: one array was for event detection and location, another for structural imaging, and a third for measuring regional attenuation. Although the arrays operated from day 3 through day 10 following the mainshock, only one aftershock was detected by the 43 stations that had been deployed at the time of the event. This aftershock was located at a focal depth of 6.9±0.15 km, with a horizontal uncertainty of ~0.15 km. Although the strike-slip focal mechanism for this aftershock differs from the thrust mechanism of the mainshock, both share an east–west maximum stress direction. Recordings of the aftershock from the high-density portion of the deployment show possible upper- and lower-crustal reflections that we attempt to image using vertical seismic profiling methods. However, the lack of redundant sources and the limited areal extent of the imaging arrays hinders any useful interpretation of the nature of the crustal reflections. Based on comparison with the Virginia experiment, in which numerous aftershocks were recorded, and from examining the signal-to-noise ratio of the single aftershock recorded, we believe the AIDA Maine deployment should have detected any aftershocks greater than magnitude m b of approximately –2.5. That only one aftershock was detected supports the observation that earthquakes in some intraplate areas are notable for their lack of aftershock activity down to this level.

Description: The Charlevoix, Quebec, earthquake of 20 October 1870 caused damage to several towns in Quebec and was felt throughout much of southeastern Canada and along the U.S. Atlantic seaboard from Maine to Maryland. Site-specific damage and felt reports from Canadian and U.S. cities and towns were used in analyses of the location and magnitude of the earthquake. The macroseismic center of the earthquake was very close to Baie-St-Paul, where the greatest damage was reported, and the intensity magnitude M I was found to be 5.8, with a 95% probability range of 5.5–6.0. After corrections for epicentral-distance differences are applied, the modified Mercalli intensity (MMI) data for the 1870 earthquake and for the moment magnitude M  6.2 Charlevoix earthquake of 1925 at common sites show that on average, the MMI readings are about 0.8 intensity units smaller for the 1870 earthquake than for the 1925 earthquake, suggesting that the 1870 earthquake was M I  5.7. A similar comparison of the MMI data for the 1870 earthquake with the corresponding data for the M  5.9 1988 Saguenay event suggests that the 1870 earthquake was M I  6.0. These analyses all suggest that the magnitude of the 1870 Charlevoix earthquake is between M I  5.5 and M I  6.0, with a best estimate of M I  5.8. Online Material: Earthquake catalogs with distance to assignment epicenter and modified Mercalli intensity estimates.

Description: A swarm of 21 small earthquakes, with the largest being M Lg  1.7, was recorded by regional seismic network monitoring from near Searsport, Maine, in April and May 2011. An additional five events were detected by two portable seismic instruments that were installed in the Searsport area for the later part of the swarm. Relative locations of the larger events of the swarm, computed in relation to a selected master event, showed that the swarm events extended for a distance of about 2.5 km and migrated from northeast to southwest. The events also became shallower toward the southwest. If the area of the swarm had ruptured in a single earthquake, the magnitude of the event would have been about M  5.1–5.5. The S - P time of only about 0.34 s at one of the portable seismic stations for the detected events from the swarm indicates that the station was located about 2.7 km from the hypocenters, thus constraining the location of the southwest end of the swarm. The events took place within the Devonian Mount Waldo pluton, a granitic body that locally cuts northeast–couthwest-oriented thrust faults that parallel the Norumbega fault zone. The trend of the swarm events is parallel to and on-strike with the trend of a thrust fault mapped to the southwest of the Mount Waldo pluton. The seismic data suggest that the fault might be seismically active, although the modern seismotectonic relationship of the fault and the pluton is far from clear.