Stress is released when an earthquake occurs. This ‘stress drop’ is a key parameter for earthquake source physics, and determines ground motion amplitude and thus potential extent of damage. Unfortunately, stress drop is notoriously difficult to measure and involves large uncertainties. Different studies of the same earthquakes often yield very different estimates of stress drop, as well as different dependence on depth and magnitude of the earthquakes. With little agreement on how much stress is released for individual events, it is hard to know what to believe. A new SCEC/USGS Technical Activity Group (TAG) has been formed to focus community efforts on resolving discrepancies in earthquake stress drop. By determining whether the stress drop varies with tectonic setting, orientation, earthquake magnitude or any other observable parameter, we can improve ground motion prediction and provide constraints on the factors controlling earthquake rupture.

In recent years, interest has grown for a community study to investigate the source of differences in stress drop measurements and associated uncertainties. The large and well-recorded 2019 Ridgecrest earthquake aftershock sequence provides the ideal opportunity for this community study.

The new SCEC/USGS Stress Drop TAG has launched a validation study of the 2019 Ridgecrest earthquake sequence. Using a common set of waveforms from the Ridgecrest sequence, participants will estimate stress drop or other earthquake source parameters by any method or means available to them. They will be asked to submit their results to a shared platform for comparison and analysis. This Ridgecrest Stress Drop Community Validation Study will be an iterative community process. That is, once the first round of estimates are submitted, initial meta-analysis may indicate that researchers need to return to their methods and try something new. We will work together as a community to get the best understanding of earthquake stress drop.

We invite anyone interested to join this community validation study and the new Stress Drop TAG research efforts. See the SCEC Stress Drop TAG website for more information. Interested participants should sign up for the USGS group mailing list to be informed of upcoming activities, events and results. The first organizational workshop to compare initial results is planned for fall of 2021, and we will convene a special AGU session in December.

The long-term goal of this community research effort is to provide guidance on best practices for (1) estimating stress drop and associated uncertainties and (2) assessing the reliability of published stress drop measurements for future studies of earthquake source physics and ground motion prediction. Through this community activity, we intend to improve predictive models for seismic hazard analysis and to reduce their uncertainties. Ideally, individual stress drops and/or location and fault specific estimates can be incorporated into hazard analysis.

About the Authors

Rachel Abercrombie is a research professor in the Department of Earth and Environment at Boston University. Her research interests are focused on detailed measurements of earthquake source properties to improve our understanding of earthquake nucleation and rupture. She has worked on earthquakes from small to large, in diverse tectonic settings to probe the controlling factors. She is on the SCEC Board of Directors, and is a co-leader of the new Stress Drop Validation TAG.

Annemarie Baltay is an observational seismologist at the U.S. Geological Survey (USGS). She is interested in how earthquake source physics and earth materials and structure control ground motions recorded from an earthquake, and how we can use observations of ground motion to infer information about the physics of the earth. She is also involved in the development of the USGS led ShakeAlert earthquake early warning project. She serves on the SCEC Science Planning Committee as the Ground Motion Focus Group co-lead, and is co-leader of this new Stress Drop Validation TAG.

Acknowledgements

This research is supported by the Southern California Earthquake Center and the U.S. Geological Survey. SCEC is funded by NSF Cooperative Agreement EAR-1600087 and USGS Cooperative Agreement G17AC00047. We thank all colleagues collaborating in this research and for everyone’s upcoming participation and support.

For More Information

• Visit the SCEC Stress Drop Technical Activity Group website.
• Join the group mailing list to be informed of upcoming activities, events and results.

References

• Parker, G., A. Baltay, J. Rekoske, E. M. Thompson (2020), Repeatable Source-, Path-, and Site-Effects from the 2019 Ridgecrest M7.1 Earthquake Sequence, Bull. Seis. Soc. Am. doi: 10.1785/0120200008
• Pennington, C., X. Chen, R. E. Abercrombie, and Q. Wu (2020). Cross Validation of Stress Drop Estimates and Interpretations for the 2011 Prague, OK, Earthquake Sequence Using Multiple Methods, J. Geophys. Res., https://doi.org/10.1029/2020JB020888
• Rosas, V. G. and A. S. Baltay (2020), Analyzing Stress drops and other earthquake parameters from the 2019 Ridgecrest Earthquake Sequence. Poster Presentation at 2020 SCEC Annual Meeting. SCEC Contribution 10738
• Trugman, D. T. (2020), Stress‐Drop and Source Scaling of the 2019 Ridgecrest, California, Earthquake Sequence. Bulletin of the Seismological Society of America 2020; 110 (4): 1859–1871. doi: 10.1785/0120200009