Editors’ Highlights are summaries of recent papers by AGU’s journal editors.
Source: Journal of Geophysical Research: Solid Earth
Foreshocks are smaller earthquakes that sometimes occur before bigger ones and studying them could help give early warnings of large earthquakes and understand how large earthquakes occur. But, because scientists use different ways to define and find foreshocks, estimates for how often they happen before big earthquakes in Southern California vary a lot—from 19% to 72%.
Khan et al. [2025] looked at both regular earthquake catalog and special “enhanced” catalogs with more small events to figure out why these estimates are so different. They found that using a simple method, just by checking small quakes near big ones in space and time, could lead to high foreshock rate, but the rate is comparable between standard and enhanced catalogs. Using statistics of past seismicity to define foreshock is better, but the choice of statistical representation matters. Assuming a constant average rate of past earthquakes (using a Poisson distribution) produces the highest foreshock rates and makes the results most sensitive to magnitude cut-offs and catalog choice. Their preferred method uses statistical distributions that account for variations in past earthquake rates, resulting in more reliable foreshock rates that are less sensitive to the magnitude cut-off or the type of earthquake catalog used.
This study clears up confusion about the wide range of foreshocks rates from previous studies in the same region and is the most thorough review of foreshock studies in Southern California so far. The authors also provide clear definitions, guidelines, and computer codes for other researchers to use. The authors emphasize the need to carefully consider biases in data and statistical methods in searching for precursory signals before large earthquakes and offer useful tips for improving short-term earthquake forecasts in the future.
Citation: Khan, R. A., Werner, M. J., Biggs, J., & Fagereng, Å. (2025). Effect of mainshock selection, earthquake catalog and definition on foreshock rate estimates in Southern California. Journal of Geophysical Research: Solid Earth, 130, e2024JB030733. https://doi.org/10.1029/2024JB030733
—Xiaowei Chen, Associate Editor, JGR: Solid Earth
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