The 2019 Earthquake Sequence in the San Rafael Swell Region of the Colorado Plateau, Utah

Between 13 March 2019 and 27 May 2019, a sequence of over 180 earthquakes occurred on the northwest edge of the San Rafael Swell near the town of Clawson in east-central Utah. The San Rafael Swell is an asymmetrical Paleocene anticline in the northwestern Colorado Plateau, with no mapped Quaternary faults, that was the site of an M_w 5.2 earthquake in 1988. The largest event in the 2019 sequence was an M_L 3.2 on 10 April 2019. Owing to the small magnitudes of these earthquakes, and their remote location, the sequence was only lightly felt. Absolute locations were determined by University of Utah Seismograph Stations analysts using the program HYPOINVERSE with a local 1D velocity model. These initial locations indicated that the sequence consists of two separate clusters, one to the northeast with a median depth of around 3 km and one to the southwest with a median depth of around 15 km. The event depths, however, are relatively poorly constrained because of azimuthal gaps up to 176° and closest station distances of 22 to 27 km. We are currently refining event locations with GrowClust, a cluster-based relative relocation program, using differential P- and S-wave arrival times determined by cross correlations of waveforms filtered from 1 to 10 Hz. For the cross-correlations we used time windows of -0.6 to 1.4 seconds and -1.2 to 3.8 seconds relative to the predicted P- and S-wave arrivals, respectively. We calculated over 342,000 differential P travel times and 99,500 differential S travel times; approximately 44% had correlation coefficients above 0.7 and were used in relocations. Preliminary results show that the sequence collapses into a single cluster, indicating that the bimodal depth distribution in the initial, single-event locations could be an artifact from the relatively poor depth control. Following relocations, we will use template matching techniques to try to detect more events associated with the sequence and then perform the relocation procedure on these new detections. We plan to use our results to make inferences about local tectonic structures to better understand the threat posed by earthquakes in the Colorado Plateau. Additionally, we will seek to determine if the recent Clawson sequence is related to the 1988 M_w 5.2 earthquake located about 10 km to the northeast.