Implications from the study of ambient noise velocity drop on the footwall and hanging wall of the M5.7 Magna earthquake on the Wasatch fault

Shankho Niyogi, Abhijit Ghosh, Alexander Yates, & David D. Oglesby

Published September 11, 2022, SCEC Contribution #12325, 2022 SCEC Annual Meeting Poster #232

We report ambient noise analysis results from single and multiple stations around the M5.7 Magna earthquake epicenter in Utah, which was generated by the Wasatch fault system. This earthquake took place on 18th March, 2020 at a depth of approximately 11.9 km. The earthquake and its aftershocks took place on the Wasatch fault, which trends north-south for a distance of about ~350 km and accommodates about 2-3 mm/yr east-west extension. We conduct cross-correlations of waveforms in single and multiple stations to target ambient noise and measure seismic velocity changes before and after the Magna M5.7 event. Single station velocity change results from cross component analysis in the frequency range of 1-9 Hz show that stations on the hanging wall block of the Wasatch fault experienced a noticeably larger velocity decrease during the M5.7 earthquake compared to those on the footwall block. We relate this observation to USGS shakemaps, past literature regarding normal faulting earthquake simulation results (Oglesby et al., 1998) and field studies (Brune, 2000) which showcase less shaking in the footwall block compared to the hanging wall block. We select stations that are at comparable distance from epicenter and located on ridges so that any parameter which influences ground shaking such as proximity and presence of sediments can be avoided. Our results show that that the higher amount of ground shaking in the hanging wall compared to the footwall affects the drop in ambient noise velocity differently in the two fault blocks. We also analyze interstation ambient noise to show that the velocity decrease is almost 0.05% when analyzed in 0.01-0.6 Hz. It was inferred that the interstation velocity decrease can be only seen in the region around the epicenter on the Salt Lake city segment of the Wasatch fault. Our study relates ground shaking to ambient noise velocity drops, and the results may have implications for the seismic hazard in the area.

Key Words
ambient noise analysis, velocity drop, Magna earthquake

Citation
Niyogi, S., Ghosh, A., Yates, A., & Oglesby, D. D. (2022, 09). Implications from the study of ambient noise velocity drop on the footwall and hanging wall of the M5.7 Magna earthquake on the Wasatch fault. Poster Presentation at 2022 SCEC Annual Meeting.

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