Toward Understanding Anomalously Low Aftershock Productivity

Understanding aftershock seismicity is important for seismic hazard assessment and for understanding earthquake stress changes and triggering. Analyses of global and regional catalogs of seismicity have confirmed that descriptive statistics such as Omori's Law, the Gutenberg-Richter distribution, and Båth's Law seem to hold across diverse tectonic settings. By investigating the productivity of aftershock sequences in intraplate regions with few tectonic events and long interevent times, we assess the applicability of common earthquake statistics to these low seismicity regions, compared to areas of induced and plate boundary seismicity. We document multiple mainshocks with anomalously low productivity and lower than anticipated maximum magnitude aftershocks that substantially deviate from Båth's Law. These include the M4.2 earthquake that occurred in 2017 outside Dover, DE, and the M4.2 earthquake that occurred in 2015 in south-central Michigan. We discuss the viability of hypotheses to explain the low aftershock productivity, including mainshock alignment with the regional stress tensor, fault complexity and orientation, as well as pore fluid diffusion.