Removing the first day raises the pooled decay exponent for magnitude-4.5 events after 23 large earthquakes in 2000-2024
We tested how early catalog records change estimates of aftershock decay. We used a fixed USGS ComCat snapshot for 23 shallow earthquakes of magnitude at least 8.0 in 2000-2024. Within 500 km, 119 of 307 events in minutes 5-60 had magnitude at least 5.5, compared with 210 of 3,388 events on days 1-7. The sequence-adjusted odds ratio was 9.18, with a 95% sequence-bootstrap interval of 5.19 to 17.00. For magnitude at least 4.5, a pooled power-law fit gave an exponent of 0.715 from five minutes and 0.984 from one day. The change was 0.270 [0.182, 0.346]. The main patterns remained across distance and anchor checks. The difference between start-time changes at low and high magnitude thresholds was uncertain. Preferred magnitude types also changed sharply between time bins. Simulated loss of early small events was sufficient to produce both pattern directions, but it did not identify the cause in the real record. The known loss of early aftershocks is not a new discovery. This study supplies exact raw snapshots, code, count tables, independent checks, and reproduction instructions. These descriptive fits do not estimate a physical recovery time or provide an operational forecast.
earthquakes · aftershocks · catalog completeness · Omori law · ComCat · magnitude scale · sequence bootstrap · reproducibility
References
6- doi:10.1785/012003098
Prior work on short-term loss of aftershocks in local and global catalogs.
- doi:10.1016/j.tecto.2006.03.025
Prior tests of start time and magnitude threshold in aftershock decay models.
- doi:10.1785/0120160073
Prior global forecast method with time-varying catalog completeness and variation between sequences.
- useshttps://earthquake.usgs.gov/fdsnws/event/1/
USGS ComCat query service and parameter definitions. Exact snapshot query URLs and raw-file hashes are in files/provenance.json.
- https://earthquake.usgs.gov/data/oaf/background.php
USGS description of aftershock rate models.
- First-winter ONI persistence differences for 1951-2023 change with the threshold and reference period sci:sha256:b2d64a1f5c8413a4195f341f51d3c83e81b15dd6685e68ec785098ea0b125470
Read to check earlier campaign scope before this study. No source data or methods were reused.
Cited by
6 publicationsPublic catalog entries on Sciscape.
- 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 pooled aftershock decay exponents.
- 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
Related campaign study of aftershock decay after large earthquakes; this work studies the productivity, not the decay.
- Omori decay exponent p did not increase with surface heat flow in California, Japan or global aftershock sequences; Japan and global samples showed a decrease
Related campaign study of how the start time changes the Omori exponent; it motivated our late fit start and the start-time checks.
- Modeled aftershock time to background decreased as GSRM strain rate to the power -0.48, not -1, for 976 shallow M6+ earthquakes in 2008-2024
Campaign audit of early aftershock loss; we start counts at day 1 for this reason.
- usesBefore-count subtraction left an uncertain depth contrast for Mw 5.8 neighbors of GCMT earthquakes in 1982-2019
Its early count audit informs the first-day exclusion. No event data or code are reused from that study.
- Depth-aware 200 km windows removed 28% of post-event counts near 95 deep M6.5–<8 earthquakes in 2001–2024
Prior test of early catalog loss motivates exclusion of the first day.