INTRAPLATE SEISMICITY IN THE MID-ATLANTIC US

Seismicity in the US Mid-Atlantic in the 2014 US NSHM appears to spatially correlate with a region of strong lateral gradient in crustal thickness, imaged by receiver functions, in the transition from the Coastal Plain-Piedmont into the Valley & Ridge. The steep gradient in crustal thickness follows the orientation of the Appalachian orogen and a gradient in the gravity field. This suggests that lateral variations in crustal structure, inherited from past orogenic processes, play a role in the concentration of stresses, and the genesis and spatial distribution of seismicity on this region.

To further explore the relationship between seismicity and crustal structure we characterize the spatio-temporal distribution of events in this region. Due to the low strain rates it is critical to develop a robust catalog of tectonic events with a low magnitude threshold. We integrate locations from the NSHM, ISC, ANSS, and USArray ANF into a uniform moment magnitude catalog (1568-2016). The ANF catalog (2013-2015) includes 508 events and shows a Mc of 2.2 in the region. The ANSS catalog, built from local/regional networks, includes 45 events (M 1.5-3.4) during that same period. Only 14 of the 45 events in the ANSS catalog appear in the ANF catalog primarily due to the lower magnitude threshold between the two catalogs. The difference in number of events between the two catalogs within the two year time window is likely attributed to mining activity; the time of day distribution shows clustering of events in the ANF catalog between 9am-5pm local time while events in the ANSS catalog show random distribution. To decluster our catalog, we combine waveform analysis and temporal and spatial distribution of events relative to mine sites. Efforts to lower the magnitude threshold and assess location uncertainty in a uniform catalog will improve analysis of the spatio-temporal characteristics and magnitude-frequency distribution of seismicity.

Preliminary analysis shows that seismicity occurs as a band following the transition from thinner to thicker crust while the thicker crust of the orogen is relatively aseismic. This suggests that lateral thickness/strength variations resulting in stress concentration is a viable mechanism for the observed distribution of seismicity.