The M (sub w) 4.2 Delaware earthquake of 30 November 2017

The 30 November 2017 Delaware earthquake with magnitude M (sub w) 4.2 occurred beneath the northeastern tip of the Delmarva Peninsula near Dover, Delaware. The earthquake and its aftershocks provide an opportunity to evaluate seismicity in a passive margin setting using much improved coverage by high-quality permanent broadband seismometers at regional distance ranges in the central and eastern United States. This is the largest instrumentally recorded earthquake in Delaware, and it triggered a collaborative rapid-response effort by seismologists at five institutions along the mid-Atlantic. As a result of this effort, 18 portable seismographs were deployed in the epicentral region within 24 hrs of the mainshock. High-quality seismic recordings at more than 380 permanent regional broadband seismographic stations in the eastern United States show a remarkably small decrease in amplitude with distance between 800 and 2000 km. The mainshock focal mechanism shows predominantly strike slip with a significant thrust component. The orientation of the subhorizontal P axis is consistent with that of earthquakes in the nearby Reading-Lancaster seismic zone in Pennsylvania, but the trend is rotated counterclockwise about 45 degrees from that of the M (sub w) 5.8 Mineral, Virginia, earthquake. We detected small aftershocks below the normal event detection threshold using a waveform cross-correlation detection method. This demonstrated the effectiveness of this approach for earthquake studies and hazard evaluation in the eastern United States. Based on their waveform similarities, repeating earthquakes with magnitudes greater than 1.5 are detected in 2010, 2015, and 2017. Although there is a large time interval between events, 5 and 2.2 yrs, respectively, the events occur within a spatially tight cluster located near the 2017 Dover, Delaware, earthquake mainshock.