Improved Understanding of Intraplate Earthquakes in the Southeastern USA with Matched Filter Detection
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Daniels, Clara
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Abstract
Most earthquakes occur along plate boundaries (also known as interplate earthquakes) and are caused by repeated accumulation and release of strain in tectonic plates moving past one another. However, the same driving forces causing 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 the seismic hazard in intraplate regions.
To better understand earthquake 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 buried in the background noise or coda waves of larger earthquakes, which results in an incomplete catalog. To overcome this, I 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 within the North American Plate hosting several seismic zones that are not well understood. In particular, I focus on the Piedmont Province, Eastern Tennessee Seismic Zone (ETSZ), and Middleton Place-Summerville Seismic Zone (MPSSZ).
In the Piedmont Province, I found that the 2014 Mw 4.1 Edgefield, South Carolina had an aftershock deficiency, suggesting that most of the strain was released during the mainshock. The mainshock also had a low stress drop which may account for the shallow calculated depth of this earthquake and the low number of aftershocks.
I examined the recent 2018 Mw 4.4 earthquake in the ETSZ, which had a similarly shallow depth, and very few aftershocks. I resolved the fault orientation on which the mainshock lies, which was previously unknown by finding focal mechanism of earthquakes near the mainshock and by performing rupture directivity analysis.
I detected and relocated microseismicity in the ETSZ using over 15 years of continuous data, yielding the most detailed complete catalog yet for this seismic zone, as well as magnitude estimations and more defined structure at depth. 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 also examined seismicity around the Watts Bar Reservoir, near which the Mw 4.4 mainshock occurred, and found some evidence for Reservoir Induced Seismicity around this region. I also found limited evidence for hydrologically-driven seismicity due to seasonal rainfall in the shallow portion of the ETSZ, which contradicts some previous studies which hypothesize that most intraplate earthquakes are associated with the dynamics of hydrologic cycles.
In the MPSSZ, near Summerville, South Carolina, I detected new events during a temporary seismic deployment in 2011-2012 and relocated them. I found deep clusters and linear features which are not in line with the currently hypothesized extrapolation of the major fault plane in this region at depth. This has implications for the fault structures which are responsible for the 1886 M~7 Summerville, SC earthquake to be compared with future studies.
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2022-07-30
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Dissertation