ANALYZING TIME-VARYING SEISMICITY AND AFTERSHOCK BEHAVIOR IN CENTRAL AND EASTERN UNITED STATES

Abstract

Central and Eastern United States (CEUS) earthquakes are far less common than those in the tectonically active west coast, but the significance is elevated for a few reasons. Due to older, harder, and often denser rocks making up the bedrock geology east of the Rockies, seismic waves can travel much further without losing energy. Poor construction, efficient transmission of seismic waves, and site amplification effects can make even light to moderate earthquakes pose high risk within the CEUS. The CEUS has significant aging infrastructure and some of the highest population densities in the country, which would lead to great economic losses and even the potential for human injury if hazards are not properly identified and communicated. Aftershock sequences are governed by several descriptive statistical “laws,” each with one or more characteristic parameters. These parameters are used to illustrate factors such as the overall productivity, the rate of decay, and the relative frequency of larger and smaller magnitude aftershocks. Variations in these parameters can relate to the geologic region being studied, the tectonic environment, the driving force of seismicity (i.e. induced earthquakes, volcanic, or geothermal-related), and more. This work discusses the aftershock sequences of two unusual CEUS earthquakes from the past five years. The first earthquake I study is a M4.2 earthquake that occurred east of Dover, DE, in late 2017. I continue by studying the aftershocks in the six weeks following a M5.8 earthquake that occurred near Pawnee, OK, in autumn 2016. Both of these earthquakes experienced below-normal aftershock productivities. I explore the challenges of analysis when station coverage is heterogeneous for the period of aftershock analysis. From there, I discuss the limitations of some statistical tests for special cases such as aftershock decay. The work concludes by highlighting additional CEUS earthquakes exceeding M4 that have occurred in the past 20 years and discussing some preliminary analytical findings.