Detailed Fault Structure of the Eastern Tennessee Seismic Zone using Matched Filter Detection on 15 Years of Seismicity

Abstract:

Small to moderate-size earthquakes have occurred along the Eastern Tennessee Seismic Zone (ETSZ) for decades, resulting in the second highest seismicity rate in Central and Eastern US (CEUS) following the New Madrid Seismic Zone. The current seismicity forms a nearly 250-km long diffuse zone from northeastern Alabama to southwestern Virginia, without clear lineation of fault structures. The largest present-day earthquakes are about M~4.6, however causes of seismicity in the ETSZ remain unclear. A few faults have been identified in the region, but many remain unknown or poorly constrained at depth. Our objective is to gain new insight into the structures of seismogenic faults in the ETSZ, as faults in this region typically do not extend to the surface. We use a matched filter technique to detect earthquakes not previously catalogued. We apply a bandpass filter of 2-16 Hz to over 900 catalogued templates spanning over 15 years (January 2005 to May 2020), within 150 km of the center of seismicity. These templates are then used to detect missing events within the same 15 years. We also focus on the time frame that spans the December 2018 Mw 4.4 earthquake near Decatur, TN, during which there is an transient increase in seismicity following the Mw4.4 mainshock. Cross-correlation based arrival time refinements are calculated to relocate the events. We use two relocation algorithms, XCORLOC and HYPODD, to comparatively relocate detected events in the ETSZ. Relocated hypocenters are examined in an effort to resolve fault structures and orientations. Our findings on the orientation of seismogenic faults are compared to previous seismic studies. We also calculate the magnitudes of new events using principle-component fitting between templates and newly detected events. Our ultimate goal is to use spatio-temporal changes of seismicity during the last 15 years reveal subsurface fault structures and the driving forces of seismicity along the ETSZ.