SCEC Award Number 21083 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title A Technical Activity Group for the Community Stress Drop Validation Study
Investigator(s)
Name Organization
Annemarie Baltay United States Geological Survey Rachel Abercrombie Boston University Taka'aki Taira University of California, Berkeley Clara Yoon United States Geological Survey William Ellsworth Stanford University Christine Ruhl University of Tulsa
Other Participants Peter Shearer, Doug Dreger, Wenyuan Fan, Daniel Trugman, Ralph Archuleta, Chen Ji, student at Stanford, student at University of Tulsa
SCEC Priorities 2d, 1d, 4a SCEC Groups Seismology, FARM, GM
Report Due Date 03/15/2022 Date Report Submitted 03/16/2022
Project Abstract
 We report on the progress in Year 1 in 2021 of the Community Stress Drop Validation Technical Activity Group (TAG). The TAG seeks to understand the physical controls and methodological reasons for similarity or differences in estimates of earthquake stress drops, so that they can be used reliably by the earthquake science community. Under the TAG, many researchers from the community independently calculate and submit stress drop estimates using a consistent dataset of the 2019 Ridgecrest sequence of earthquakes. In Year 1, we put together and distributed this common data set of ~13,000 earthquakes recorded on ~100 stations from the 2019 Ridgecrest sequence, and associated metadata. Following Workshop #1, we have identified a subset of 55 earthquakes for particular focus in the second year of the TAG. In only one year, the TAG has gained huge momentum, with 117 members on the mailing list, 101 participants at Workshop #1, and 11 research groups submitting preliminary results. The TAG activities are three-fold: (1) Coordination of the study and generation and upkeep of a common dataset; (2) Analysis of the common dataset to estimate stress drops; and (3) Meta-analysis of the resulting stress drops to understand their correlation or why differences arise. As the TAG developed, it became clear it is also serving a real need in informing potential users of stress drop-type measurements what they should be careful about and how to understand published stress drop values, when incorporating them into their own work.
Intellectual Merit SCEC seeks to understand both the characteristics and uncertainty of earthquake stress drop as it relates to basic source physics, rupture modeling and ground motion prediction, and has a focus on collaborative stress drop studies under Seismology Research Priority 4.1.3: Collaborative Earthquake Stress Drop and Source Study. This TAG specifically meets this research priority.
The motivation for the SCEC Community Stress Drop Validation TAG is focused on understanding the nature and causes of discrepancies in earthquake stress drop, as well as where random and physical variability arises. In this context, the main goals for the TAG are to use a common data set of records from the 2019 Ridgecrest earthquake sequence (consisting of over 12,000 events of M1 to M7.1) to address the questions:
● How do differing methods and model assumptions affect stress drop estimates?
● How do different researchers approach similar methods?
● How do data quantity, quality, selection and processing affect stress drop estimates?
● How do physical source (mechanism, depth, radiation pattern, directivity), path (geometrical spreading, attenuation), and site (soil conditions, site attenuation) features affect the estimates?
● What measurements, and uncertainties, would be most useful for the broader community?
Broader Impacts Workshop 1 was held as a Zoom meeting on November 4, 2021. Anyone currently working on, or interested in, the science questions noted above and/or learning about Ridgecrest earthquake studies was encouraged to apply to participate, especially researchers and young investigators. We received 132 registrations, and 101 participants from 14 countries on 5 continents. We solicited preliminary results estimating stress drop using the common dataset, and used those results in the workshop to start discussion. We received preliminary results from 11 groups, of which, one was directly led by an undergraduate and four by graduate students. We expect to have even more geographically diverse research groups joining in the project this spring.
As one overarching goal for the workshop was simply to facilitate communication between different research groups and disciplines, and to build (and maintain) project momentum, we experimented with formats in different sessions, built in lots of time for discussion and breakout rooms, and had fewer long talks. We also worked to ensure a diverse group of speakers and moderators, prioritizing early-career researchers and gender equity; 1 speaker was an undergraduate, two were graduate students and 2 were post-docs. The decision to include both observational seismologists making measurements, and users of those measurements was clearly successful with both groups contributing and learning from one another. The workshop also proved beneficial to the many students who participated, looking to understand the problems and meet the community as they begin work in this field.
Exemplary Figure Figure 1. Preliminary submitted stress drop results from 11 research groups (legend), vs catalog magnitude. Generalized Inversion Technique (GIT)/ Spectral Decomposition (SD) methods are shown with diamond symbols; empirical Green’s function (eGf) spectral ratio methods shown in dots; eGf finite fault in stars; and other methods in triangles.

Preliminary results provided by research groups led by: The 11 groups who are led by: Ian Vandevert, Dino Bindi, Xiaowei Chen, Emma Devin, Bill Ellsworth, Killian Kemna, Meichen Liu, Christine Ruhl, Oliver Boyd, Haoran Meng and Doug Dreger.
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