High Resolution Relocations of the 2019 Bluffdale, Utah, Earthquake Sequence

On February 15, 2019, an M w 3.7 earthquake occurred beneath the city of Bluffdale, Utah, and was reported felt by nearly 9,000 people in the greater Salt Lake City and Provo regions. A moment tensor solution for this event shows normal faulting on a plane dipping 57º west or 35º east. This mechanism, together with the location of the epicenter 10 km WSW of the surface trace of the west-dipping Wasatch fault, raises the question of whether or not the Bluffdale earthquake might have been on the Wasatch fault. The University of Utah Seismograph Stations used conventional single-event methods to detect and locate a total of 191 earthquakes that occurred in the Bluffdale region from Feb. 13 through April 15. There were 13 foreshocks, the mainshock, and 177 aftershocks. We relocated the 2019 Bluffdale events with a cluster-based relative relocation algorithm, GrowClust, using differential travel times calculated by cross correlating waveforms bandpass filtered from 1-10 Hz. For the P-wave and S-wave cross-correlations, we used time windows of 2 s and 4 s, respectively, both centered on the theoretical arrival times. Of the 558,076 differential P times and 376,267 differential S times that we measured, approximately 12% had correlation coefficients above 0.8 and were used in the relocation. Preliminary results show that the hypocenters form an east-dipping zone that is approximately 1 km wide downdip and 2 km long. These results suggest that the Bluffdale mainshock occurred on a minor, unnamed, east-dipping fault which lies within the area of complex structural geology at the transition between the Salt Lake City and Provo segments of the Wasatch Fault. For future work, we intend to use template matching techniques to detect earthquakes associated with this sequence that were missed by the conventional location methods. Our main goals are to investigate further the possible relationship of this sequence to the Wasatch fault, analyze the complex nature of the tectonic setting surrounding the Bluffdale region, and improve our understanding of earthquake hazards in this densely populated area.