Low Aftershock Productivity and Fault Geometry of the 2017 Delaware Earthquake

Central and Eastern United States (CEUS) earthquakes are less common than those in the tectonically active West Coast, but their significance is elevated due to higher population densities, less-attenuating bedrock geology, variable site-amplification effects, and a higher proportion of structures prone to damage from shaking. Associating CEUS earthquake focal mechanisms with causative crustal faults is challenging due to a lack of mapped faults. Aftershock productivity of CEUS earthquakes is difficult to predict because it is highly variable, displaying globally typical behavior in some regions (Wu et al., 2015; Wu and Chapman, 2017) and low decay rates (Stein and Liu, 2009; Calais et al., 2016; Toda and Stein, 2018) in others. Here, we study the aftershock sequence of an unusual Mw 4.24 CEUS earthquake that occurred below the Atlantic Coastal Plain east of Dover, Delaware, in late 2017. We analyze data from a temporary 14-station network and use template matching to search for aftershocks, which we locate using a custom 1D velocity model. We find aftershock locations favoring slip on a northwest–southeast-striking fault oblique to the presumed fault where the mainshock was located. We document an unusually low a value and large magnitude difference between the mainshock and the largest aftershock, as well as an average aftershock decay p value. Factors proposed to explain variations in aftershock productivity include fault alignment relative to the prevailing stress field (Hardebeck, 2010) and low productivity after a high stress drop (Wetzler et al., 2018). We test these hypotheses in relation to the 2017 Delaware earthquake aftershocks, showing the Delaware earthquake had a stress drop of 35 MPa, normal for an intraplate region (Boyd et al., 2017), and had favorable alignment for aftershock thrust faulting. We therefore propose a small fault of possible pre-Mesozoic origin, limiting the productivity observed.