Seismic Traveltime Tomography of Southern California Using Poisson‐Voronoi Cells and 20 Years of Data. Issue 5 (17th May 2022)

• Record Type: Journal Article
• Authors: Fang, Hongjian; White, Malcolm C. A.; Lu, Yang; Ben‐Zion, Yehuda
• Is Part Of: Journal of geophysical research. Volume 127:Issue 5(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-05-17
• Subjects: Southern California -- seismic tomography -- Poisson's ratio; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2021JB023307
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: We derive new, 3D, isotropic models of seismic compressional and shear wavespeeds, Vp and Vs, respectively, their ratio, Vp/Vs, and a catalog of relocated earthquakes for Southern California from more than 10 million P‐ and S‐wave arrivals associated with over 0.3 million earthquakes that occurred between 2000 and 2020. We augment high‐quality analyst‐reviewed phase arrival picks from the Southern California Earthquake Data Center with S‐wave arrival picks obtained with an automated algorithm, and we derive new wavespeed models via traveltime tomography formulated using Poisson‐Voronoi cells (Fang et al., 2020, https://doi.org/10.1785/0220190141 ). The results contribute to improved regional wavespeed models, particularly the Vp/Vs model, and absolute event locations. The obtained models correlate well with regional geological features and yield more accurate synthetic waveforms than other regional models do for waves with periods shorter than 5 s in much of the modeled region. The derived event catalog exhibits tighter spatial clustering than the standard regional catalog, thereby helping to characterize subsurface features of major faults. The regional 1D averaged Vp/Vs ratio shows high values at shallow depths, decreases to a minimum at about 10 km, then increases again at greater depths below 15 km. Deep seismicity correlates well with regions of Vp/Vs ratio lower than 1.75, which may indicate an increased brittle‐to‐ductile transition depth with an important influence on crustal mechanics. The new wavespeed models and seismic catalog can be useful for various studies including analyses of seismicity patterns and simulations of crustal deformation and ground motion. Plain Language Summary: Motion along the tectonic plate boundary between the Pacific and North American plates is deforming the Earth's crust in Southern California and causing ongoing seismicity including numerous micro earthquakes not felt by humans. These earthquakes alter the physical properties of the crustal rocks that host them. In this study, we compile observations from over 0.3 million earthquakes in Southern California to construct new 3D models of the elastic properties of the crust, and to improve earthquake locations. The results show significant lateral and depth variations of seismic wavespeed consistent overall with previous studies. Earthquakes deeper than 15 km tend to concentrate in regions with low values of the elastic parameter Poisson's ratio. The models show that earthquakes in the Eastern California Shear Zone—a broad zone of distributed crustal deformation east of the main plate boundary—occur at unusually shallow depths, possibly as a result of weakened crust between 10 and 15 km depth. This zone of weak crust may also be partially responsible for the networks of closely spaced, perpendicular faults found in the area. A section in the central part of the San Jacinto fault with very few earthquakes may be a favorable site for a future large earthquake. Key Points: New Vp, Vs, and Vp/Vs models for Southern California are derived from 20 years of seismic data with a recently developed tomography method Sharper images of fault‐zone structures are revealed by the relocated earthquakes The depth‐averaged Vp/Vs is higher than 1.77 in top 3 km, reaches a minimum of about 1.73 around 10 km, and increases again at greater depth