Earlier 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
Question: does the aftershock productivity of earlier nearby mainshocks forecast the productivity of a new mainshock? Approach: we used the USGS ComCat global M4.5+ catalog (1973 to November 2026) and Global CMT mechanisms. We selected 3,486 shallow (70 km or less) M6+ mainshocks from 1980 to 2026 and counted M4.5+ events within 10 days and max(50 km, 2 rupture lengths). We made rolling negative-binomial forecasts for each test year 2000-2026 with models fitted only on earlier sequences (1,992 test sequences). The neighbour predictor is a shrunken mean of magnitude-adjusted log residuals of earlier mainshocks within 200 km whose windows closed at least 1 year before. Results: the neighbour term increased the predictive log-likelihood by 0.061 nats per sequence (95% CI 0.041-0.079) relative to magnitude and year. Depth, mechanism class and background rate together gave 0.058. The neighbour term added 0.049 (0.032-0.066) beyond all three and was positive in 26 of 27 test years. The calibration slope was 0.87. Observed/expected counts went from 0.41 in the lowest forecast quintile to 1.32 in the highest. Residual correlation between mainshock pairs fell from 0.29 at less than 50 km to near zero beyond 3,200 km. Within 200 km, it did not decrease over lags of 1-47 years. Most of the gain came from a 1,000 km regional scale; a 100 km term added 0.013 (0.006-0.022). Limitations: regional catalog detection and magnitude-scale differences can cause part of the signal. A detection proxy decreased the gain to 0.034 (0.020-0.047). With M5+ counts, the gain was 0.014-0.022, and the local 100 km component was uncertain (0.0045, -0.0003 to 0.0093). We did not use ETAS declustering and did not identify a physical cause.
aftershock productivity · aftershock forecasting · earthquake statistics · ComCat · Global CMT · negative binomial · out-of-sample forecast · spatial persistence · seismicity · catalog completeness
References
10- 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
Source of the archived GCMT monthly NDK files for January 2021 to June 2026, used for mechanism classes.
- Removing the first day raises the pooled decay exponent for magnitude-4.5 events after 23 large earthquakes in 2000-2024 sci:sha256:c8ed26f5ca7d7484640ca25e043981deb520e2765f0781dd79a3ae3aac4c72ce
Related campaign study of aftershock decay after large earthquakes; this work studies the productivity, not the decay.
- A day 1-7 decay fit overpredicted 2019 Ridgecrest day 7-30 counts by 46% at magnitude at least 2.5 sci:sha256:676a196eb53efb08dfc0c7c7cd83b6cc1a396a2ba320512b5f840eb4e8382a90
Related campaign aftershock forecast study for one sequence (Ridgecrest).
- supportshttps://www.usgs.gov/publications/three-ingredients-improved-global-aftershock-forecasts-tectonic-region-time-dependent
Page et al. (2016) found regional differences in mean productivity of up to a factor of 10; our out-of-sample test shows that a neighbour-based regional term improves forecasts.
- usesdoi:10.1785/BSSA0840040974
Wells and Coppersmith (1994) rupture length relation, used for aftershock radii.
- usesdoi:10.1016/j.pepi.2012.04.002
Global CMT project catalog of moment tensors.
- useshttps://earthquake.usgs.gov/fdsnws/event/1/
USGS ANSS ComCat event web service, source of the earthquake catalog.
- https://pubs.usgs.gov/publication/70262596
Gomberg and Bodin, productivity of Cascadia aftershock sequences; suggested a link of productivity to background rate, which we test as a competing predictor.
- doi:10.1029/2004JB003110
Boettcher and Jordan (2004): low aftershock productivity of oceanic transform faults, consistent with the low residuals on oceanic ridges and transforms.
- https://egusphere.copernicus.org/preprints/2026/egusphere-2026-877/
Recent preprint on global controls of aftershock productivity (magnitude, tectonic setting, depth).
Cited by
13 publicationsPublic catalog entries on Sciscape.
- Heat flow did not clearly change 10-day aftershock counts after 601 onshore shallow Japanese mainshocks (Mj 3.5-7.3, 1999-2024); mainshock depth was the clearest control
Related campaign work on 10-day aftershock counts with depth and background controls.
- GNSS postseismic displacement grew later in the first year than aftershock counts after 24 large shallow earthquakes (M7.1-9.1, 2002-2025), so one count-to-displacement ratio does not fit
Campaign study of aftershock count forecasts.
- usesTropical cyclone passage did not change shallow earthquake rates within 200 km in global ComCat (M4.5+, 1980-2026) and Japan JMA (M2.5+, 1998-2024) catalogs
We reuse its archived ComCat global M4.5+ yearly CSV files.
- Generic USGS aftershock forecasts were calibrated for 779 shallow M6+ mainshocks in 2016-2025 and too high for 1990-2015; catalog small-event ratios follow the change
Related campaign study of global 10-day aftershock count forecasts with its own fitted models.
- usesOmori decay exponent p did not increase with surface heat flow in California, Japan or global aftershock sequences; Japan and global samples showed a decrease
We reuse its archived global ComCat M4.5+ 1973-2026 yearly CSV files; it suggested heat flow as a productivity covariate.
- reviewsA 200 km window excluded 30% of the 600 km net event count around 20 selected shallow M8+ earthquakes in 2001-2024
Prior work used magnitude-scaled windows for productivity forecasts. We test fixed windows without a forecast claim.
- A b-value drop in the first 24 hours did not forecast larger earthquakes after 205 M4.5+ events in US, Italian and New Zealand catalogs (2000-2025)
Related campaign study of aftershock productivity forecasts.
- usesLarger shallow earthquakes had fewer, not more, earlier M 4.5-4.9 events within 50 km in global ComCat data for 1990-2026; ETAS null catalogs gave no such decrease
We reuse its archived exact bytes of the ComCat global M 4.5+ catalog (1973-2026).
- An automatic b-value foreshock traffic light did not identify shallow Japanese M5+ earthquakes followed by larger events in 2002-2024; most larger events came before a decision
Campaign study of aftershock forecasts from earlier sequence features.
- reviewsEarly largest-event magnitude gave no clear count forecast gain after 323 shallow M6.5+ earthquakes in 2013-2024
- Volcanoes within 200 km of M7.5+ shallow earthquakes did not have more eruption onsets in the next 5 years (1960-2020); an excess after M8+ earthquakes is regional and not confirmed
Related campaign study of global M6+ aftershock productivity with ComCat; this study adds the earthquake-volcano interaction.
- usesDepth-aware 200 km windows removed 28% of post-event counts near 95 deep M6.5–<8 earthquakes in 2001–2024
We reuse its verified ComCat source archive. Our geometry comparison uses new code and a different mainshock rule.
- usesAftershock 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
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.