2 Delaware Earthquake of 30 November 2017 by

The 30 November 2017 Delaware earthquake with magnitude Mw 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 midAtlantic. 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° from that of the Mw 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. Electronic Supplement: Peak amplitude and instrumental intensity maps of the 30 November 2017 Delaware earthquake. INTRODUCTION On 30 November 2017, a moderate earthquake of magnitude Mw 4.2 (this study) occurred about 10 km northeast of Dover, Delaware, beneath the west coast of Delaware Bay (Fig. 1). The earthquake was felt throughout Delaware and in neighboring New Jersey, Maryland, and Pennsylvania, and the ground motion near the epicenter attained a maximum intensity of V (modified Mercalli intensity [MMI] scale), moderate shaking (Community Internet Intensity Map [CIIM], see Data and Resources). Light ground shaking was reported in Wilmington, Baltimore, and Philadelphia and as far away as New York City and Washington, D.C. (“Did You Feel It?,” see Data and Resources). This event is the largest magnitude earthquake in Delaware at least in the past 150 yrs of record. The location and magnitude of the event were determined by the Lamont Cooperative Seismographic Network (LCSN) operated by more than 40 partner educational organizations in the northeastern United States and led by the Lamont–Doherty Earth Observatory (LDEO). The seismic waves generated by the 2017 Delaware earthquake were well recorded by broadband seismographic stations in the central and eastern United States (CEUS). Although the Atlantic Ocean occupies the vast majority of area to the east of the hypocenter, more than 380 broadband seismographic stations from distances of 70 to 2800 km provided regional Lg-wave peak amplitude measurements with signal-tonoise ratio greater than 2. That the event was so well recorded is largely due to funding from the 2009 American Recovery and Reinvestment Act that allowed upgrading nearly all existing seismic stations in the eastern United States in 2010 and the continued deployment of new broadband stations by regional networks. The retention of 159 temporary (18-month deployment) USArray Transportable Array (TA) stations of the EarthScope project supported with National Science Foundation and U.S. Geological Survey (USGS) funds beginning in 2013 helped fill the gap in seismic station coverage in the eastern United States. doi: 10.1785/0220180124 Seismological Research Letters Volume XX, Number XX – 2018 1 Downloaded from https://pubs.geoscienceworld.org/ssa/srl/article-pdf/doi/10.1785/0220180124/4336457/srl-2018124.1.pdf by Columbia University, 10831 on 19 September 2018 ▴ Figure 1. Historical earthquakes that occurred in and around Delaware since 1785 from earthquake catalogs are plotted with hexagons; earthquakes since 1972 from Lamont Cooperative Seismographic Network catalog are plotted with circles. Permanent seismographic stations used to locate small earthquakes around Delaware are plotted with solid triangles. 1871 is the epicenter of the largest known earthquake (M 4.1) in Delaware, and 1879 is an M 3.3 earthquake that occurred close to the 2017 Delaware event. 1984 M 4.1 Lancaster, Pennsylvania, and 1994M 4.6 Reading, Pennsylvania, earthquake sequences are indicated. Focal mechanism of the mainshock and trend of the subhorizontal P axis is indicated by thick arrows. Shaded area is Atlantic Coastal Plain strata covering bedrock. 2 Seismological Research Letters Volume XX, Number XX – 2018 Downloaded from https://pubs.geoscienceworld.org/ssa/srl/article-pdf/doi/10.1785/0220180124/4336457/srl-2018124.1.pdf by Columbia University, 10831 on 19 September 2018 The 2017 Delaware earthquake presents an opportunity for detailed study of a mainshock–aftershock sequence in a passive margin setting. The earthquake was followed by a rapid deployment of portable seismic stations by research and educational institutions in the region using limited resources but a collaborative effort. Within 24 hrs of the mainshock, 18 stations were deployed in the epicentral area on both shores of Delaware Bay. These local network stations operated for six weeks during the coldest winter in recent years. This article focuses on a description of the mainshock of the 2017 Delaware earthquake and analyses of seismic data recorded on the permanent seismographic stations in the mid-Atlantic States. More thorough analyses of the aftershock data are underway. The data from the temporary deployment are archived at the Incorporated Research Institutions for Seismology Data Management Center (see Data and Resources).