SCEC Award Number 21175 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Seismic Indictors of Fault Maturity
Investigator(s)
Name Organization
Thorne Lay University of California, Santa Cruz
Other Participants Huiyun Guo
SCEC Priorities 1d, 2d, 2e SCEC Groups Seismology, Geology, Geodesy
Report Due Date 03/15/2022 Date Report Submitted 03/14/2022
Project Abstract
 Earthquake generating fault systems in California span a wide spectrum of fault maturity, from develop-ing segmented fracture systems to strongly localized primary faults with large cumulative offsets. Nu-merous globally distributed large continental strike-slip earthquakes (M > 6.0) on faults with varying de-gree of maturity have been well studied, by both field observations and seismic and geodetic analyses that characterize their rupture patterns and ground deformations. A moderate number of these events have occurred in California. Guided by field observations that indicate relative maturity of the fault zone (cumulative displacement, number of segments ruptured in a given event, offsets between segments, angles between segments, damage zone extent, afterslip, and ratio of longest segment to total length) and rupture attributes inferred from seismic and geodetic analysis (radiated seismic energy, aftershock productivity, average rupture velocity), we are exploring whether the degree of fault zone maturity is em-pirically manifested in remotely observable source characteristics. Progress on data analysis is being made, after delays in contract initiation associated with COVID response, and preliminary results are cur-rently available for some measures. Extensive review of the literature for 15 large strike slip events has compiled observed attributes from which an overall assessment of fault maturity can be made, although there are many differences in the details of field measurements available for different earthquakes, as well as differences in seismic parameter estimates. Preliminary analysis supports the hypothesis of fault maturity influence on seismic parameters, although there is substantial scatter in the data.
Intellectual Merit Damaging earthquakes occur on faults involving greatly varying maturity, in the sense of degree of localization and cumulative slip history. Geological constraints on the maturity state of various strike-slip fault systems ruptured by large events is correlated with the seismic radiation characteristics for the earthquakes, with some correlation being found for attributes such as aftershock productivity and radiated energy. By calibrating these connections, relative seismic radiation attributes from future failures of faults with different maturity can be anticipated, providing guidance on relative seismic hazards in diverse source regions.
Broader Impacts The project is supporting part of Huiyun Guo’s Ph.D. thesis work at the University of California Santa Cruz. Thus, it is directly supporting a promising young Asian female seismologist working on earthquake science. The fundamental science addressed in this project may guide regionalized seismic hazard assessments, which likely differ in regions with mature or immature faults. The main long-term benefit could be improved confidence in scenario earthquakes and ground motion predictions for earthquakes likely to strike in California, with improved input to earthquake engineering, strong-motion modeling, and earthquake warning procedures.
Exemplary Figure Figure 1. Comparisons of scaled aftershock productivity (observed number of aftershocks divided by the magnitude-predicted value from global compiled productivity measures) for 15 large strike-slip events versus observed faulting measures indicative of fault maturity: (left) ruptured segment number for each mainshock, and (right) cumulative fault offset. A priori assessment of fault maturity is classified by a bimodal pattern (color) based on multiple faulting parameters, with blue + signs indicating intermediate to ambiguous maturity. Less mature faulting, with higher segment number and lower cumulative offset tends to have higher aftershock productivity.