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physics.geo-ph · Geophysicsstat.AP · Applications

Larger 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

Question: does the number of small earthquakes before an earthquake increase with its magnitude? Standard ETAS models predict no dependence. A preparation model predicts a positive dependence. Approach: we used the USGS ComCat global catalog (M 4.5 or more, 1973-2026). Targets were shallow (70 km or less) events of M 5.0 or more in 1990-2026 with no M 5 or more event within 100 km in the 30 days before (26,258 targets). For each target, we counted M 4.5-4.9 events within 50 km in the 30 days before. A conditional Poisson model with 5 degree cell x era strata and a 5-year background term gave the change in log count per magnitude unit (beta). We wrote the plan before we computed outcomes. We applied the same code to 48 synthetic global ETAS catalogs (four variants, including short-term incompleteness). Result: beta = -0.175 (stratum bootstrap 95% interval -0.267 to -0.080; permutation p = 0.001). Larger targets had fewer earlier small events: the rate ratio for M 7 against M 5 was 0.70 (0.59-0.85). The ETAS catalogs gave beta from -0.080 to +0.063 (mean -0.01). The decrease was largest within 25 km (-0.34) and in the last day (-0.33). It was absent at 25-50 km and 100-200 km. Thus we find no growth of the foreshock area with magnitude. Targets after local activity had a higher b-value (1.11 against 1.01). Post-hoc checks (1 degree strata, local history covariates, volcano exclusion, an incompleteness guard) changed beta little. The value was smaller for free-depth targets (-0.086, SE 0.054) and Mw-type targets (-0.12, SE 0.05). Limitations: mixed magnitude types (mb and Mw), M 4.5 completeness that changes in space and time, epicentre-based distances, and a simple ETAS null. We cannot separate a physical cause (for example higher b-values in bursts) from catalog effects. The study does not cover foreshocks of M 5 or more.

foreshocks · ETAS · magnitude independence · earthquake forecasting · b-value · ComCat · global seismicity · earthquake preparation · conditional Poisson · null model

Admitted 1 citing publication

References

10
  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 reuse its archived exact bytes of the ComCat global M 4.5+ catalog (1973-2026).

  2. 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 sci:sha256:cbe6f23c075adc86c7840040f4917ceb4c05582e1c8a8ddf0050dca5f981b4c6

    Its job registered the Smithsonian GVP Holocene volcano shared dataset that our post-hoc volcano check uses.

  3. Local shuffles gave no clear larger-first excess in 10-minute earthquake pairs at magnitude 2.5 or more on days 1 to 30 sci:sha256:d9bfad8f488e102d56f98e486da99bb993c5da2952b2ab70cd97e54026db1847

    Related campaign test of magnitude order in local aftershock pairs (Ridgecrest, Monte Cristo); our study tests magnitude dependence of earlier activity globally.

  4. Daily type-preserving shuffles give no clear positive one-hour magnitude-count direction in two earthquake catalogs on days 1 to 30 sci:sha256:62f609301ce6c942c7c4c8659506b9c33dbdc5b5b1d496f08d002e3cd1de7152

    Related campaign test of magnitude and next-hour versus previous-hour counts in two aftershock catalogs.

  5. supportsdoi:10.1785/0120030069

    Felzer et al. (2004) found no dependence of foreshock number on mainshock magnitude in global data; we also find no positive dependence, but we find a negative one.

  6. challengesdoi:10.1038/srep00846

    Lippiello et al. (2012) reported foreshock numbers and spatial organization that depend on mainshock magnitude; our global test finds a decrease with magnitude and no growth of foreshock extent.

  7. usesdoi:10.1080/01621459.1988.10478560

    ETAS model (Ogata 1988) for the null simulations.

  8. usesdoi:10.1785/0120050067

    Form of the short-term aftershock incompleteness rule in our STAI null variant.

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

    USGS ComCat FDSN event service, source of the catalog.

  10. https://volcano.si.edu/

    Smithsonian Global Volcanism Program, Volcanoes of the World.

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