SCEC Award Number 22101
Proposal Category Workshop Proposal
Proposal Title Two 2022 Workshops for the Community Stress Drop Validation Study TAG
Name Organization
Annemarie Baltay United States Geological Survey Rachel Abercrombie Boston University
Other Participants Other TAG participants
SCEC Priorities 1d, 2d, 4a SCEC Groups Seismology, FARM, GM
Report Due Date 02/27/2023 Date Report Submitted No report submitted
Project Abstract
The September 2022 Stress Drop Validation TAG workshop was held in person at the Hilton Palm Springs on Saturday September 10, prior to the SCEC Annual Meeting. The workshop was relatively focused, with the aim of engaging those who make stress drop measurements in detailed discussion and comparison of the submitted results from the various methods. 24 people attended in person, with an additional 6 active on Zoom. Prior to the workshop, we received results of estimated stress drop, for a total of 21 submissions by various research groups. Researchers were asked to include a selected 55 events for focus in their studies. At a pre-meeting Zoom one week period, we identified 6 events to discuss in detail at the workshop.
At the workshop, Abercrombie, Baltay and Chu presented meta-analysis of the individual results, focused on the selected events for comparison. Direct comparison of the reported stress drops reveals considerable scatter, which was not surprising, and also significant scatter in estimated seismic moments. Each research team presented a lightning overview of their methods, and all were also invited to present more about their comparisons in the afternoon. Early-career researcher Hao Guo gave an invited talk on 3D attenuation tomography, to encourage the community to think about the impacts of the tradeoff in attenuation and path effects, with stress drop.
By the conclusion of the workshop, the group agreed to continue working on their methods, share spectra and more detailed results from a handful of selected events, and embark on two benchmark studies.
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 This workshop was held in person in Palm Springs, and attracted 30 participants, 6 of whom were able to participate on Zoom. At least three of those participants flew in from outside of the US, and many participants had never been to a SCEC meeting (or hadn’t been in a very long time). Prior to the workshop, we solicited preliminary results estimating stress drop using the common dataset, and used those results in the workshop to start discussion. We have now received results for 21 methods, of which, one was directly led by an undergraduate and four by graduate students. We also received submission of four results from groups in Europe. Leading up to the workshop, we have held ~monthly Zoom meet ups. Each time, we host the Zoom calls at two different times of day (usually in the morning and evening Pacific time) to enable researchers from around the globe to join. We have been successful in getting participants from Australia, Hong Kong, Taiwan, Japan, Colombia, Mexico, Europe, Israel and Saudi Arabia. Thus, we have been able to create a large, diverse, global community interested in resolving the issue of stress drop estimation.
At the workshop, we had only one invited talk, from Hao Guo, a graduate student. We invited and encouraged all participants to present their ideas as short talks or slide sets. We left a lot of time for discussion, and encouraged folks to mix at lunch, to foster the community.
The aims and results of the workshop were presented at a plenary session of the main SCEC meeting, and also ~40 people attended the follow-up zoom sessions we hosted to update the wider community.
Exemplary Figure Figure 1. Submitted stress drop results from 21 research group-methods (legend; some groups submitted multiple methods), 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. (left) for the whole set of events of M2+; (right) for the subset of 55 events.