Wave-equation-based travel-time seismic tomography – Part 2: Application to the 1992 Landers earthquake (<i>M</i><sub>w</sub> 7.3) area

Abstract

Abstract. High-resolution 3-D P and S wave crustal velocity and Poisson's ratio models of the 1992 Landers earthquake (Mw 7.3) area are determined iteratively by a wave-equation-based travel-time seismic tomography (WETST) technique. The details of data selection, synthetic arrival-time determination, and trade-off analysis of damping and smoothing parameters are presented to show the performance of this new tomographic inversion method. A total of 78 523 P wave and 46 999 S wave high-quality arrival-time data from 2041 local earthquakes recorded by 275 stations during the period of 1992–2013 are used to obtain the final tomographic models, which cost around 10 000 CPU hours. Checkerboard resolution tests are conducted to verify the reliability of inversion results for the chosen seismic data and the wave-equation-based travel-time seismic tomography method. Significant structural heterogeneities are revealed in the crust of the 1992 Landers earthquake area which may be closely related to the local seismic activities. Strong variations of velocity and Poisson's ratio exist in the source regions of the Landers and three other nearby strong earthquakes. Most seismicity occurs in areas with high-velocity and low Poisson's ratio, which may be associated with the seismogenic layer. Pronounced low-velocity anomalies revealed in the lower crust along the Elsinore, the San Jacinto, and the San Andreas faults may reflect the existence of fluids in the lower crust. The recovery of these strong heterogeneous structures is facilitated by the use of full wave equation solvers and WETST and verifies their ability in generating high-resolution tomographic models.

Wave equation‐based reflection tomography of the 1992 Landers earthquake area

“…Tens of thousands of earthquakes have been recorded by the Southern California Seismic Network (SCSN), which provides us abundant data to explore the seismic structures of this region. Following the pioneering seismic tomography work of Aki and Lee [], many studies have been conducted to explore the subsurface structure in Southern California [e.g., Dziewonski et al , ; Zhao and Kanamori , ; Zhao et al , ; Tape et al , , ], revealing structural heterogeneities in the crust and upper mantle of this region [e.g., Lin et al , ; Tape et al , ; Tong et al , ]. The results provided by Tong et al [] are consistent with the other studies, indicating the validity of the WETST method in real applications.…”

“…Table shows quantitatively the structural similarity (SSIM) indices defined by Tong et al [] between the recovered P or S velocity models and the input checkerboard models at seven layers. As discussed in Tong et al [], the inverted model is better recovered when the SSIM index is close to 1.0. It can be observed that the SSIM indices are above 0.75 at the depths less than 21.0 km regardless of whether Moho‐reflected phases are used or not.…”

“…The relatively low computational costs allow this 2‐D‐3‐D seismic tomography method to use as many seismic records as the conventional ray‐based tomographic methods. To show the validity of the WETST method, Tong et al [] chose the 1992 Landers earthquake area in Southern California as the test field.…”

“…The results provided by Tong et al [] are consistent with the other studies, indicating the validity of the WETST method in real applications. However, only the first P and/or S wave arrivals were used in most tomographic inversions concerning the Southern California region [e.g., Lin et al , ; Allam and Ben‐Zion , ], including the one of Tong et al []. Since the crustal earthquakes mainly occurred in the upper and middle crust (Figure b), the tomographic resolution of the lower crust is generally poor [ Zhao et al , ; Lin et al , ; Tong et al , ].…”

“…However, only the first P and/or S wave arrivals were used in most tomographic inversions concerning the Southern California region [e.g., Lin et al , ; Allam and Ben‐Zion , ], including the one of Tong et al []. Since the crustal earthquakes mainly occurred in the upper and middle crust (Figure b), the tomographic resolution of the lower crust is generally poor [ Zhao et al , ; Lin et al , ; Tong et al , ]. Adding Moho‐reflected phases (such as P m P , p P m P , S m S , and s S m S ) to tomographic inversion is an efficient way to improve the data coverage in the lower crust.…”