SCEC Award Number 19174 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Shallow Crustal Heterogeneity in Southern California Estimated from Earthquake Coda Waves
Investigator(s)
Name Organization
Nori Nakata Massachusetts Institute of Technology Arben Pitarka Lawrence Livermore National Laboratory
Other Participants N/A
SCEC Priorities 4a, 4b, 4d SCEC Groups Seismology, GM
Report Due Date 04/30/2020 Date Report Submitted 04/30/2020
Project Abstract
 High-frequency scattered waves contain important information to reveal small-scale structure such as heterogeneities of velocities and attenuation, which should be considered and included for high-frequency ground motion. In this study, we image the 3D small-scale scatterer structure in southern California especially in the area around the San Jacinto fault zone. The small-scale heterogeneities are difficult to image using wavefield migration or waveform inversion, but coda waves contain such information. For example, the peak amplitude time of waveform envelope is delayed when structures along the ray path is highly scattered. We use this phenomenon and apply peak-delay-time tomog-raphy at different frequencies to reveal the small-scale heterogeneities around the San Jacinto fault zone. The tomograms show the high heterogeneities are concentrated around the faults. Interest-ingly, low-frequency tomography results (i.e., long wavelength) show larger velocity variations, which is counter intuitive, but possibly related to high attenuation of high-frequency waves. The 3D small-scale heterogeneity images are the first step of incorporating such heterogeneities into velocity models for numerical approaches of high-frequency ground motion prediction.
Intellectual Merit Small-scale heterogeneities are necessary parameters for high-frequency ground motion prediction, but the estimation of them is not trivial because of the physical limitation of our tomography methods. Therefore we need to rely on stochas-tic approaches, and here we use envelope peak-delay time. The results of peak-delay-time tomography are encouraging to show the high heterogeneities around the fault zones.
Broader Impacts Small-scale subsurface information is usually not available for earthquake physics, but the nucleation of earthquakes can be in the very small scale. The estimated heterogeneities can be related to stress state and/or friction in the subsur-face structure, and hence our results can be used for earthquake source physics and prediction as well.
Exemplary Figure Figure 7: Vp variation estimated from envelope tomography at 16-32 Hz. The variation is calculated from the velocity model shown in Figure 1 in the unit of km/s.