SCEC Award Number 18087 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Probing characteristics of earthquake source complexity in an area of structural complexity
Investigator(s)
Name Organization
Xiaowei Chen University of Oklahoma Rachel Abercrombie Boston University
Other Participants
SCEC Priorities 1d, 2d, 4a SCEC Groups Seismology, FARM, GM
Report Due Date 03/15/2019 Date Report Submitted 04/08/2019
Project Abstract
 The increased quantity and quality of recordings of small and moderate earthquakes (i.e., M~2 to 5) now available suggest that the simple source models in common use are inadequate, and are limiting progress in understanding earthquake source physics and ground motion patterns. High-resolution imaging of earthquake rupture processes and earthquake sequences also reveal correlations between geological structure and earthquake rupture dynamics. In this study, we focus on the “trifurcation” area of the San Jacinto Fault Zone (SJFZ), which is a region of both structural and geometric complexity. It also has a high rate of seismicity and is one of the best-recorded areas in southern California. We combine both frequency and time domain analysis to understand the characteristics of source complexity for earthquakes, and potential relationship with structural complexity. Our analysis has identified different types of complex earthquakes, differing from previous studies that report similar spectral complexity in the same tectonic region. F
Intellectual Merit This research contributes to better understanding of earthquake source processes, and factors that influence rupture. For decades, the default model of earthquake sources has been that of self-similarity, with the source process controlled by one characteristic time scale that corresponds to the corner-frequency in the frequency domain. This corner-frequency (fc) is interpreted as corresponding to the source dimension (radius or length, here L) of the area ruptured during the earthquake, and is the frequency of maximum ground velocity, and radiated energy at the source. Following results from dynamic rupture simulations (Boatwright, 1984; Brune, 1970; Madariaga, 1976), the single corner frequency has been used to estimate the stress drop in earthquakes from all over the world, in an attempt to find relations between stress drop and magnitude, tectonic setting, and temporal changes in stress. However, recent spectral analyses have demonstrated that deviations from the simple source model are common, for earthquakes of a wide magnitude range (Archuleta & Ji, 2016; Denolle & Shearer, 2016; Uchide & Imanishi, 2016). Applying simple source model to complex earthquake sources will not only bias source parameter estimations, but also limit the progress of understanding earthquake source processes. A combination of both frequency domain and time domain analyses are needed to understand causes of deviations from simple spectral models, and any possible links with geological structural complexity.
This work is directly relevant to SCEC priorities P1.d, P2.d and P4.a in understanding earthquake source complexity and its contribution to spatial/temporal stress heterogeneity and high-frequency ground motion.
Broader Impacts The project results are beneficial for learning earthquake hazards, risks and earthquake physics, and have been presented at international conferences (e.g. Chen & Abercrombie, 2018; Wu et al., 2018; Zhang et al., 2018). The project contributed to the training and education of a postdoc scholar and students at OU. Postdoc Qimin Wu contributed to the study of developing forward modeling codes to model sub-events of moderate earthquakes using a Mw 4 earthquake in central Oklahoma (manuscript under revision with GRL, Wu et al., 2019)), and a Mw 5 earthquake in San Jacinto earthquake (Wu and Chen, 2018). Graduate student Colin Pennington at OU received training for spectral ratio analysis. The project also supported collaboration between SCEC PIs, and is closely related to the ongoing SCEC Project of Peter Shearer and Rachel Abercrombie to improve routine methods of estimating reliable source parameters. Source complexity is shown to be a major of disagreement between different approaches in their recently accepted paper (Shearer et al., 2019, SCEC Contribution #8916).
Exemplary Figure Figure 3.