SCEC Award Number 21169 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Rupture Complexity of Small and Moderate Earthquakes in the 2019 Ridgecrest Earthquake Sequence: Implications for Variabilities in Stress Drop Estimates and Ground Motions
Investigator(s)
Name Organization
Qimin Wu Lettis Consultants International, Inc. Xiaowei Chen University of Oklahoma
Other Participants One graduate student of PI Chen at OU
Dr. Rachel Abercrombie at Boston University
SCEC Priorities 1d, 2d, 4a SCEC Groups Seismology, GM, FARM
Report Due Date 03/15/2022 Date Report Submitted 03/14/2024
Project Abstract
 It is well known that many larger earthquakes have highly complex rupture processes with significant variations in slip and stress drop over the rupture plane. As the quantity and quality of recordings of small-to-moderate earthquakes increase, it is becoming clear that the simple source models in common use are inadequate in explaining the observations, and thus limiting progress in understanding earthquake source physics and ground-motion predictions. In this study, we systematically investigate the source processes for small to moderate earthquakes (M ~2.5-5) in the 2019 Ridgecrest earthquake sequence. Work to date has mainly focused on spectral analysis to classify earthquakes with different spectral characteristics as compared with simple omega-square source model. We perform the multiple spectral ratio analysis based on the empirical Green’s function (EGF) technique on selected potential target events with M ≥ 2.5 in the Ridgecrest sequence to measure corner frequencies, and quantify the spectral complexity by examining the deviation of the observed spectral ratios from the omega-square source model. “Complex” earthquakes with spectra that deviate significantly from the omega-square model have been identified across the entire magnitude range, and they show larger variability of stress drop estimates when compared to their simple counterparts. Our results contribute to a better understanding of fundamental earthquake rupture processes and the limitations of the Brune-type simple source model in estimating source parameters and the influence on ground motion predictions.
Intellectual Merit Our results contribute to a better understanding of fundamental earthquake rupture processes and the limitations of the Brune-type simple source model in estimating source parameters such as stress drop and source dimension and the influence on ground motion predictions. 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 (fc) in the frequency domain (Aki, 1967; Brune, 1970). fc is interpreted as corresponding to the source dimension (radius or length, here L) of the area ruptured during the earthquake. This type of model is typically referred to as a Brune-type, or “omega-square” model, and has been commonly used to measure source parameters such as stress drop (e.g., Abercrombie, 1995; Baltay et al., 2011) and to simulate high-frequency ground motion using the stochastic method (e.g., Hanks and McGuire, 1981; Boore, 1983, 2003). As the quantity and quality of recordings of small-to-moderate earthquakes increase, it is becoming clear that the simple source models in common use are inadequate in explaining the observations, and thus limiting progress in understanding earthquake source physics and ground-motion predictions. There have been an increasing number of studies observing complex rupture processes across a wide range of magnitude (M 3-8) that deviate significantly from omega-square models (e.g., Archuleta and Ji, 2016; Denolle and Shearer, 2016; Uchide and Imanishi, 2016; Wu et al., 2019). A recent study by Shearer et al. (2019) demonstrated that deviation of source spectra from the simple source models is a major source of uncertainty and bias in stress drop estimates based on the corner frequency measurements.
Broader Impacts This project facilitates the collaboration between university and the consulting industry, and funding for this project partially supported a graduate student at OU.
Exemplary Figure Figure 3. Summary of the results: (a). Spatial distribution of the classified simple and complex earthquakes; (b). Stress drop versus Mw scaling for the classified simple and complex earthquakes.