SCEC Award Number 20124 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Estimation of physical scattering parameters related to shallow crustal heterogeneity in Southern California
Name Organization
Nori Nakata Massachusetts Institute of Technology
Other Participants Tong Bai (postdoc associate)
SCEC Priorities 4a, 4b, 4d SCEC Groups Seismology, GM
Report Due Date 03/15/2021 Date Report Submitted 03/18/2022
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 using a rock samples to develop a method, and then apply it to the data in southern California especially in the area around the San Jacinto fault zone, which is still ongoing and for the subject of extension. 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 tomography at different frequencies to reveal the small-scale heterogeneities using a rock sample to develop the method before we apply it to the San Jacinto fault zone. This laboratory experiment is essential to understand the complex phenomena of scattering. The tomograms show the high heteroge- neities are concentrated around the faults, which correspond to the CT scan of the rock sample. Also we numerically calculate the wavefields according to different scattering properties. We will use this tomog- raphy method to reveal the scattering properties around the southern California.
Intellectual Merit Small-scale heterogeneities are necessary parameters for high-frequency ground motion prediction, but the estima- tion of them is not trivial because of the physical limitation of our tomography methods. Therefore we need to rely on stochastic 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 earth- quakes can be in the very small scale. The estimated heterogeneities can be related to stress state and/or friction in the subsurface structure, and hence our results can be used for earthquake source physics and prediction as well.
Exemplary Figure Figure 4: Simplified CT image of internal fracture network (left) and spatial distribution of 𝛥𝑙𝑜𝑔(𝑡!) values for Darley Dale Sandstone. Logarithmic peak delay variations are shown in the lower color bar while dia- monds show the PZT positions. Only regions crossed by a minimum of 5 rays are displayed. Azimuthal cov- erage of model blocks (and so confidence) is reduced towards the edges and outside the region delineated by the receivers. Diamonds indicate receivers used in mapping. The bounding boxes indicate the dimensions of the sample.