SCEC Award Number 19078 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Improved lithospheric structure and Moho across Southern California from joint inversion of broadband Rayleigh wave ellipticity/phase dispersion and harmonic receiver functions
Investigator(s)
Name Organization
Fan-Chi Lin University of Utah Vera Schulte-Pelkum University of Colorado, Boulder
Other Participants PhD student Elizabeth Berg
SCEC Priorities 4a, 3a, 3b SCEC Groups Seismology, CXM, GM
Report Due Date 03/15/2020 Date Report Submitted 03/23/2020
Project Abstract
 This study is a continuation of the study “Tomography of Southern California via Bayesian Joint Inversion of Rayleigh Wave Ellipticity and Phase Velocity from Ambient Noise Cross-Correlations” published in Berg et al. (2018). Using over 300 stations throughout southern California, we have measured Rayleigh wave phase maps between 20 and 80 second period with signals from earthquakes that are most sensitive to the lower crust/uppermost mantle structure. We have also updated the receiver function database and constructed a set of receiver functions unbiased by out-of-plane conversions (which significantly distort Moho estimates) to use in the inversion with surface waves. These new measurements complement the existing short period surface measurements made through ambient noise analysis and will be used to refine the SCEC CVMS model.

We have also studied detailed fault zone damage structure of the San Jacinto Fault near Anza. We applied double beamforming tomography to a month long linear dense seismic array to construct a 2D shear velocity profile. Within 300 meters in depth, we observed a ~200m wide low velocity zone that narrows with depth, which we interpret as the main damage zone, in addition to two other ~100m wide subsidiary zones corresponding to secondary damaged structures, agreeing with the distribution of fault zone trapped waves produced by local earthquakes. The primary damage zone asymmetry indicates that materials on the northeast side of the fault are stiffer at seismogenic depth and suggests that large San Jacinto earthquakes tend to nucleate to the southeast and propagate to the northwest.
Intellectual Merit We demonstrate 24-60 sec Rayleigh wave phase velocity maps can be constructed across Southern California by applying eikonal/Helmholtz tomography on earthquake data. The result shows lower phase velocities at 24s and 30s between the San Jacinto and San Andreas faults, near the San Bernardino basin, down to near the Salton Trough, as well as north of the Garlock fault. At 40s, a fast anomaly is observed beneath the Transverse Ranges. In addition, we show that it is common for receiver functions in Southern California to have a strong backazimuthal dependence and azimuthally averaged receiver functions should be used when jointly invert with surface wave measurements. On a local scale, we demonstrate that detailed fault zone structure can be imaged by applying double beamforming tomography on ambient noise data from a temporary dense array.

Broader Impacts The project supported the training of a PhD student, Elizabeth Berg. The student was also supported to present her work at the SCEC meeting and interact with other members of the SCEC community. The work on imaging the San Jacinto fault damage zone with a dense geophone array has been published by GRL. The 3D model constructed through this study will be share with the SCEC community through the IRIS Earth Model Collaboration (EMC) web portal.
Exemplary Figure Figure 1. Phase velocity maps from Eikonal tomography at periods of (a) 24s, (b) 30s, (c) 40s, (d) 50s, and (e) 60s. Each sub-figure is labeled with the number of contributing earthquake events. (Figure by Elizabeth Berg)