Improved Understanding of Intraplate Earthquakes in the Southeastern USA with Matched Filter Detection

Clara Daniels

Most earthquakes occur along plate boundaries and are caused by the repeated accumulation and release of strain in the rocks of tectonic plates moving past one another. However, the same forces driving these interplate earthquakes does not account for intraplate earthquakes, which are located within the interiors of a tectonic plate. The relatively long recurrence intervals between large earthquakes, causal fault locations, and driving mechanisms of intraplate earthquakes present a challenge to understanding their physical mechanisms and the seismic hazard in intraplate regions.

To better understand earthquake and fault properties in intraplate settings, a more complete detection of earthquakes, precise locations, and magnitude estimations are critical. Traditional earthquake catalogs tend to miss smaller earthquakes due to high background noise or during intensive sequences, which results in an incomplete catalog. To overcome this, we use a matched filter method to detect microseismicity and build a more complete catalog. This technique applies cross correlation to detect previously uncatalogued events in continuous data by using the waveforms of known earthquakes as templates.

This thesis focuses on earthquake detection in the Southeastern United States, an intraplate region on the North American Plate, which hosts several seismic zones not well understood yet. In particular, I focus on the Piedmont Province in Georgia and South Carolina, Eastern Tennessee Seismic Zone (ETSZ), and Middleton Place-Summerville Seismic Zone (MPSSZ) near Charleston, South Carolina.

            In the Piedmont Province, I found that the 2014 Mw 4.1 Edgefield, South Carolina in the Piedmont Province had a deficient aftershock sequence, suggesting that most of the strain was released during the mainshock. The mainshock and its largest M3.0 aftershock also had relative shallow hypocentral depth of 3-4 km, which may account for their low stress drops and the low number of aftershocks. I also examined the recent 2018 Mw 4.4 earthquake in the ETSZ, and detected very few aftershocks. In addition, the mainshock had a depth range of 5-6 km, shallower than the typical background seismicity in the ETSZ. By performing rupture directivity analysis, I found that the mainshock appears to rupture bi-laterally along an E/NE trending strike-slip fault, which is consistent with the relocated aftershock locations and one of the nodal planes of the mainshock. The occurrence of the 2018 Mw 4.4 mainshock at a relatively shallow depth and its close proximity to the nearby Watts Bar Nuclear Power Plant, highlight the need to reevaluate seismic hazard associated those moderate-size earthquakes along the ETSZ.

In the next chapter, I detected microseismicity in the ETSZ using over 15 years of continuous data, yielding the most high-resolution catalog yet for this seismic zone. I found the greatest concentration along or to the east of the NY-AL Lineament, as defined by the magnetic anomaly, supporting the evidence that this feature’s origin is linked to seismicity in the ETSZ. I examined seismicity around the Watts Bar Reservoir, near which the Mw 4.4 mainshock occurred, and did not find strong support for Reservoir Induced Seismicty around this reservoir. I also found limited evidence for hydrologically-driven seismicity in the shallow portion of the ETSZ, which contradicts some previous studies which hypothesize that some intraplate earthquakes are associated with the dynamics of hydrologic cycles.

            In the MPSSZ, near Summerville, South Carolina, I detected new microseismic events during a temporary seismic deployment in 2011-2012 and relocated them. I found deep clusters and linear features which were not in line with the the previous interpretation of a major westward dipping fault in this region at depth. This has implications for the fault structures which are responsible for hosting the 1886 M~7 Summerville, SC earthquake, the largest historical earthquake east of the Mississippi river. I expect that the fault structure will be further refined by an ongoing temporary seismic deployment in this region.

committee members are: Dr. Zhigang Peng (advisor), Dr. Andrew Newman, Dr. James McClellan, Dr. Samer Naif, and Dr. Yao Xie.