Parsimonious Velocity Inversion Applied to the Los Angeles Basin, CA. Issue 2 (28th January 2022)

• Record Type: Journal Article
• Authors: Muir, Jack B.; Clayton, Robert W.; Tsai, Victor C.; Brissaud, Quentin
• Is Part Of: Journal of geophysical research. Volume 127:Issue 2(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-01-28
• Subjects: Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2021JB023103
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: The proliferation of dense arrays promises to improve our ability to image geological structures at the scales necessary for accurate assessment of seismic hazard. However, combining the resulting local high‐resolution tomography with existing regional models presents an ongoing challenge. We developed a framework based on the level‐set method that infers where local data provide meaningful constraints beyond those found in regional models ‐ for example the Community Velocity Models (CVMs) of southern California. This technique defines a volume within which updates are made to a reference CVM, with the boundary of the volume being part of the inversion rather than explicitly defined. By penalizing the complexity of the boundary, a minimal update that sufficiently explains the data is achieved. To test this framework, we use data from the Community Seismic Network, a dense permanent urban deployment. We inverted Love wave dispersion and amplification data, from the Mw 6.4 and 7.1 2019 Ridgecrest earthquakes. We invert for an update to CVM‐S4.26 using the Tikhonov Ensemble Sampling scheme, a highly efficient derivative‐free approximate Bayesian method. We find the data are best explained by a deepening of the Los Angeles Basin with its deepest part south of downtown Los Angeles, along with a steeper northeastern basin wall. This result offers new progress toward the parsimonious incorporation of detailed local basin models within regional reference models utilizing an objective framework and highlights the importance of accurate basin models when accounting for the amplification of surface waves in the high‐rise building response band. Plain Language Summary: Los Angeles is a major city of the United States that is at high risk of damage due to earthquakes, due to the large number of nearby active faults and its location on a deep bowl of weak rock, which tends to amplify earthquake damage. We use a large number of instruments located in Los Angeles district schools to make measurements of earthquakes that occurred near Ridgecrest, California in July 2019. These earthquakes generated a type of energy that is particularly useful for studying the structures responsible for amplification of earthquakes. Using this data, we applied a new imaging technique to create a local model of the northeast Los Angeles basin at higher resolution than had been previously available. Our imaging technique appropriately balances information from previous, lower resolution inversions with the new data obtained in this study. Key Points: We generate a new velocity model of the northeastern Los Angeles Basin using data from the Community Seismic Network Using a level‐set framework, we parsimoniously balance the existing Community Velocity Models with new data constraints The new model indicates a steeper and deeper basin underneath downtown Los Angeles, significantly amplifying 4–6 s Love waves