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Rupturing of the primary fault may trigger secondary fault activity in a complex multi-fault system. The mainshock focal mechanism and well-constrained aftershock locations reveal vital information about aftershock migration patterns, the regional tectonic stress field, and the seismogenic process. Aftershock relocations of the events in the Utah Mw5.7 earthquake sequence that occurred on March 18, 2020, indicate that the mainshock ruptured a listric normal fault; this fault is characterized by a shallow concave upward fault surface and a dip that decreases with depth. The aftershocks were largely organized into two clusters. The first small cluster, which is located ~ 8 km east of the mainshock, resides on part of a fault that is steeply dipping to the east; the second larger cluster is spread across multiple conjugate faults. Using local and regional waveforms to estimate the seismic moment tensor solutions, we determined that the Utah MW5.7 mainshock was a shallow crustal normal faulting event. Furthermore, 11 selected aftershocks (MW ≥ 3.3) are also characterized by normal faulting. The nearly EW-oriented extensional stress pattern in the focal area has an orientation that is similar to that of the regional maximum horizontal extensional stress field. Our analysis of the Coulomb stress variations showed that the Utah Mw5.7 earthquake triggered the aftershocks that formed along conjugate normal faults near the West Valley fault zone. These results provide evidence for the triggering hypothesis, wherein subsequent seismic events are caused by static stress changes triggered by large mainshock earthquakes. [ABSTRACT FROM AUTHOR]

A major left-lateral strike-slip Mw7.7 earthquake occurred in the vicinity of the Caribbean Sea on January 28, 2020. As a result, a small-scale tsunami was generated. The properties of the seismogenic source were described using observational data gathered for the earthquake and tsunami, as well as information on the regional tectonic setting. The tsunami was simulated with the COMCOT model and Okada's dislocation model from finite fault solutions for MW7.7 Caribbean Sea earthquakes published by the United States Geological Survey. The simulation results were compared to tide gauge records to validate whether the seafloor's vertical displacement generated by the strike-slip fault caused a small-scale tsunami. We conducted a spectral analysis of the tsunami to better understand the characteristics of tsunami records. The tsunami simulation results showed that the co-seismic vertical displacement caused by a strike-slip MW7.7 earthquake could have contributed to the small-scale tsunami, but the anomalously large high-frequency tsunami waves recorded by the George Town tide gauge 11 min after the earthquake were unrelated to the earthquake-generated tsunami. According to the spectrum analysis, the predominant period of noticeable high-frequency tsunami waves recorded by the George Town tide gauge occurred only two minutes after the earthquake. This indicates that the source of the small-scale tsunami was close to the George Town station and the possible tsunami source was 150 km away from George Town station. These facts suggest that a submarine landslide was caused by the strike-slip earthquake. The comprehensive analysis showed that the small-scale tsunami was not caused solely by co-seismic seafloor deformation from the strike-slip event but that an earthquake-triggered submarine landslide was the primary cause. Therefore, the combined impact of two sources led to the small-scale tsunami. [ABSTRACT FROM AUTHOR]