MS Thesis Defense Presentation of Lindsay Blankenship

Signal Analysis of the M5.1 Sparta Aftershock Sequence to Identify Clustered Waveforms and Changes in Seismic Velocity Associated with the Little River Fault

Following the M5.1 earthquake near Sparta, NC on August 9, 2020, UNC-CH installed four seismic stations that utilized Wildlife Acoustics recording devices outfitted with 4.5 Hz geophones that were online from August 9 through August 13, overlapping with the CERI deployment utilizing five stations by four days. Over six days following the main shock, 786 events were cataloged, where only 86 events that registered on six or more stations were chosen as the focus of this study. Prior to locating these events, a scheduled quarry blast located 7.6 km from the epicenter of the main shock was recorded and used to derive a 1-D velocity model of the Sparta region, where this velocity was used to locate the 86 aftershocks. The seismic signals from all the stations deployed were extensively compared to each other to detect any clustered waveforms, where 19 of the 86 events were identified as belonging to a doublet or cluster. The centroids of these doublets and clusters along with the distribution of all the aftershocks about the fault trace were compared to the variable vertical displacement of the fault scarp, where a couple of the larger spikes corresponded to a doublet pair and the main shock but did not correspond to

a large grouping of events. The travel time residuals were analyzed at each station that was deployed for any changes in the seismic velocity across the fault plane, where Stations EAI and S2NC consistently experienced observed P-wave arrival times arriving faster than expected for events that were located on the opposite side of the fault plane from these stations. However, more aftershocks will need to be studied for the remaining stations to discern whether this change in velocity is due to the fault plane.