Abstract

Arrays of highly portable, exploration style seismographs were deployed following the 16 October 2012 Mw 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 mb 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.

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