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physics.geo-ph · Geophysics

Aftershock counts and one b-value explained the faulting-style difference in Bath's law for 1,271 global shallow M 6.2+ mainshocks in 1990-2024

Question: thrust, normal and strike-slip earthquakes have different gaps between the mainshock and the largest aftershock (Bath's law). Does this come only from different aftershock counts, or does the aftershock magnitude distribution change with faulting style? Data: ComCat global M4.5+ events and Global CMT mechanisms. We used 1,271 isolated mainshocks of ComCat magnitude 6.2 or more, centroid depth 40 km or less, 1990-2024. We counted M5.0+ events within 365 days and max(40 km, 2 rupture lengths). Results: a largest aftershock within 1.2 units of the mainshock occurred after 68% of thrust, 49% of normal and 37% of strike-slip mainshocks (class p = 4e-25). Strike-slip mainshocks had 0.32 times (0.28-0.37) and normal mainshocks 0.74 times (0.60-0.90) the aftershock count of thrust mainshocks of equal magnitude. Gutenberg-Richter order statistics with the observed count and one b-value (1.14) predicted each class: observed/expected 1.04 (0.99-1.08) thrust, 0.94 (0.84-1.03) normal, 1.05 (0.99-1.12) strike-slip. Class terms added no clear effect after this prediction (p = 0.14). This repeated in 1990-2007 and 2008-2024, with a 30-day window, depth to 70 km, a smaller radius, Mc 5.3 and GCMT Mw only (p = 0.08-0.81). Out of sample, a count model with class and one b-value forecast large aftershocks within 0.01 nats of a direct class model; both beat a constant Bath probability. Limitations: the test cannot exclude class departures smaller than about 5-10%. ComCat gave normal-fault aftershocks a higher b-value (1.34) that GCMT magnitudes did not confirm. We did not explain why productivity changes with faulting style. Code, sequence tables and figures are included.

Bath's law · largest aftershock · aftershock productivity · faulting style · Gutenberg-Richter · focal mechanism · ComCat · Global CMT · aftershock forecasting

Admitted 4 citing publications

References

11
  1. usesEarlier aftershock counts within 200-1000 km improved out-of-sample 10-day aftershock count forecasts for 1,992 global shallow M6+ mainshocks in 2000-2026 beyond depth, mechanism and background rate sci:sha256:2182948ccf9d3297ee3ca28cdd0204ea9bf8162558495eb3b423983ebea9368c

    We read its archived ComCat global M4.5+ CSV files (sha256 19e97634...bfb0) and use them as our earthquake catalog. Its productivity results are related; data overlap.

  2. usesGRACE seasonal water load gave a weak thrust-timing signal that did not replicate, and no fault-type sign contrast, in global shallow GCMT Mw 5+ mainshocks, 2002-2026 sci:sha256:7184e3ca8e705c7f1821efa1d5a15e4c4783218097677a59a269fcc8859179c1

    We read its archived GCMT monthly NDK files for 2021-2026.

  3. doi:10.1016/0040-1951(65)90003-X

    Origin of Bath's law.

  4. doi:10.1029/2011GL050604

    Earlier global finding that the largest-aftershock gap differs between reverse and strike-slip faulting.

  5. doi:10.1029/2019JB018111

    Earlier study of controls on aftershock productivity.

  6. doi:10.1029/2004JB003110

    Low aftershock productivity on oceanic transform faults.

  7. doi:10.1785/BSSA0840040974

    Rupture length scaling used for window radii.

  8. doi:10.1016/0031-9201(92)90130-N

    Plunge-based mechanism classes.

  9. https://www.globalcmt.org/

    Global CMT catalog.

  10. https://earthquake.usgs.gov/fdsnws/event/1/

    USGS ComCat event service.

  11. https://www.naturalearthdata.com/downloads/10m-physical-vectors/10m-land/

    Land polygons for the exploratory oceanic strike-slip split.

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