SCEC Award Number 24130 View PDF
Proposal Category Individual Research Project (Single Investigator / Institution)
Proposal Title Constraining Crustal Elasticity by Sweeping Lamb Waves over California : Testing the Community Earth Models
Investigator(s)
Name Organization
Toshiro Tanimoto University of California, Santa Barbara
SCEC Milestones A1-1, A2-1, A2-2 SCEC Groups Seismology, CEM, PBS
Report Due Date 03/15/2025 Date Report Submitted 02/26/2025
Project Abstract
 We have developed a new method for testing the shallow elastic structures in the uppermost 5–10 km of the Community Velocity Models (CVMs). This method uses the compliance parameter derived from atmospheric Lamb waves that propagate over seismic networks. By comparing the measured compliance data to the theoretical compliance for each CVM, this method can evaluate the suitability of each model’s shallow elastic structure.

In 2024, we applied this test to three southern California models—CVM-S4, CVM-S4.26.M01 (Lee et al., 2014), and CVM-H (Shaw et al., 2015)—and one model for northern California, CenCalVM (Brocher et al., 2006). These models were obtained from the SCEC Unified Community Velocity Model (UCVM) website (https://southern.scec.org/software/ucvm).

Our findings indicate that CVM-H (v15.11) provides the best overall fit to the compliance data among three southern California models, followed by CVM-S4.26.M01 and CVM-S4. While CVM-S4.26.M01 includes valuable improvements in the shallow crust compared to CVM-S4, its basin structures—particularly in the Ventura and Los Angeles basins—seem to have the average shear modulus that is too low, potentially leading to overestimated ground motions in these areas. The model for northern California, CenCalVM, shows a larger scatter compared to southern California CVMs, highlighting a greater need for refinement to improve its accuracy in predicting realistic ground-motion amplitudes.
SCEC Community Models Used Community Velocity Model (CVM)
Usage Description We developed a new method and applied it to test three community velocity models in southern California (CVM-H, CVM-S4.26.M01, CVM-S4) and one model for northern California (CenCalVM). These models were obtained from the SCEC Unified Community Velocity Model (UCVM) website (https://southern.scec.org/software/ucvm).
Intellectual Merit The method we developed is based on signals from atmospheric (Lamb) waves, which have been rarely used. The compliance ratio of these waves is sensitive to shallow elastic structures, providing a new tool for testing community velocity models. Our analysis demonstrates that these waves can serve as an important test for shallow seismic structures, particularly in basins, which may be critical for predicting ground motions in future earthquakes.
Broader Impacts I collaborated with Jakub Bystricky, a graduate student from the UCSB physics department. The grant funded him during the fall quarter, allowing him to acquire fundamental knowledge of wave propagation in both the solid Earth and the atmosphere. Jakub also assisted me in analyzing the data, including downloading SCEC velocity models and computing theoretical compliance values for the models. Additionally, the project involved Miguel Alvarez, an undergraduate student at UCSB, who was supported by a different funding source. This project greatly motivated him to pursue research in Earth science.
Project Participants In addition to PI, one graduate student (Jakub Bystricky) and one undergraduate student (Miguel Alvarez) were involved. Jakub was supported by this grant but Miguel had support from a different source.
Exemplary Figure Figure 2: (a) Histograms of 〖R=η〗_obs (f)/η_theo (f). Vertical dashed line is shown at R=1. Results for three models in Southern California are shown. (b) Geographical distribution of R values. CVM-S4.26.M01 shows systematically low values in the Ventura basin and the LA basin. The results indicate that shear modulus in CVM-S4.26.M01 is too small (red and magenta) for these basins.
Linked Publications

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  • Tanimoto, T. (2024). Deformation of solid earth by surface pressure: equivalence between Ben-Menahem and Singh’s formula and Sorrells’ formula. Geophysical Journal International, 238(2), 820-826. SCEC Contribution Number 14156