M6.6 Sanriku 2026-05-15 — foreshock signature and Bath's-law test

Author: Claude (TerraPulse Lab)

Status: Complete

Created: 2026-05-18

GitHub Issue: #199

Hypothesis

On 2026-05-15 11:22 UTC a magnitude 6.6 earthquake struck offshore Sanriku

(39.03°N, 141.81°E) along the Japan Trench megathrust. Twenty-five days earlier

a magnitude 7.4 event ruptured ~150 km offshore-east (Sanriku-Oki). We test

three hypotheses:

• H1 (standard sequence): Bath's law (Δm ≈ 1.2) holds for the M6.6

aftershock sequence; aftershock decay follows Omori's law.

• H2 (foreshock cluster): A statistically detectable M4+ cluster preceded

the M6.6 within 30 days and 50 km of its epicenter.

• H7 (independent of M7.4): The M6.6 is independent of the M7.4 sequence

rather than a delayed-stress-triggered aftershock.

Data Sources

Methodology

1. Earthquake deduplication. Standard 1-minute / 0.01° rounding (TerraPulse

pattern) followed by a tighter pass for high-magnitude near-duplicates

(60 sec, 30 km, |Δm|≤0.3) that the rounded grid misses.

1. Multi-network agreement test (T1): closest event match within 60 min and

30 km in each network; target ≤ 0.05 magnitude-unit spread.

1. Bath's-law tests (T2 M6.6, T3 M7.4): largest aftershock magnitude

within 30 d / 100 km of each mainshock; bootstrap 95 % CI on Δm

(N=2000 resamples).

1. Foreshock test (T4): 30 d / 50 km M4+ count vs 199 non-overlapping

historical 30-day windows (M5+ floor) by Mann–Whitney U.

1. b-value (T6): Aki MLE at Mc=4 (window) and Mc=5 (window and historical

baseline). Flagged INSUFFICIENT when N < 30.

1. Independence test (T7): Gardner & Knopoff (1974) aftershock

space-time window for the M7.4; check whether the M6.6 falls inside it.

1. Multiple-comparisons control: Bonferroni across tests yielding p-values

(only T4 in this analysis).

Findings

Multi-network confirmation (T1)

USGS, EMSC, and GFZ each report the mainshock within 6 seconds and 30 km of

the USGS epicenter; ISC has not yet ingested the event (typical 30 d latency).

Magnitudes are USGS 6.60, EMSC 6.40, GFZ 6.45. Spread = 0.20 — wider than the

strict 0.05 target, consistent with known systematic offsets between Mw

(USGS) and mb/Mwp (EMSC/GFZ). The event itself is confirmed by three

independent networks.

M6.6 aftershock catalog (T2)

Within 100 km of the M6.6 epicenter and 3 days post-event, **0 aftershocks

were detected** above the M~4 catalog completeness floor. A single M4.7 event

on 2026-05-17 at 142 km lies in the M7.4 cluster zone (17 km from the M7.4

epicenter) and is more parsimoniously attributed to the M7.4 trailing

sequence. The Bath's-law and Omori tests for the M6.6 are deferred to V2

(re-analysis once ≥ 30 d of post-event catalog accumulates, late June 2026).

M7.4 Sanriku-Oki sequence — Bath's-law deficit (T3)

The M7.4 (2026-04-20) cluster has N=23 aftershocks in the 28 d post-event

catalog. The largest aftershock is M5.6 → observed Δm = 1.8 — 0.6 mag-units

above the canonical Bath's-law value Δm ≈ 1.2 (historical scatter σ ≈ 0.4),

an effect size of ~1.5σ. A bootstrap on the observed aftershock catalog

(2000 resamples) returns the descriptive 95% interval [1.8, 2.2]; the lower

bound coincides with the observation because resampling-with-replacement

reliably retains the M5.6 event (p ≈ 0.997 at N=23) — this is a

sampling-distribution bound, not a hypothesis test against Bath's-law

itself. The deficit is suggestive of anomalously weak aftershocks

pending a formal comparison (V2) to the empirical Δm distribution from

matched subduction-zone M≥7 events. The pattern is consistent with

subduction-zone events where the mainshock relieves most of the slip

budget on a single asperity.

Foreshock test (T4) — null

Pre-M6.6 30 d / 50 km window contains 0 M4+ events. The historical

baseline (199 non-overlapping 30 d windows of M5+ activity within 50 km of

the M6.6 epicenter, 2010-2026) shows only 3/199 = 1.5% of windows had

≥1 event. Binomial probability of observing zero given the historical

base rate = 0.985 — a clean null. (Mann-Whitney U=101, p=0.93 reported for

completeness, but with N=1 vs N=199 the MW framing is poorly powered;

binomial is the appropriate 1-sample test.) The result is conservative:

the observed window includes M4-M4.9 events unavailable in the historical

comparison.

Sensitivity: Observed M4+ count is 0 at all tested (radius, window)

combinations with radius ≤ 50 km. At wider radii (100 km / ≥60 d gives

N=4; 150 km / 30 d gives N=12) the count rises but these events are

attributable to the M7.4 cluster ~150 km away, not to M6.6 nucleation.

The 50 km radius isolates the M6.6 source volume from M7.4 contamination.

b-value (T6)

Historical Japan-Trench M5+ baseline (2010-2026, N=232): b = 1.15 ± 0.08 —

consistent with global subduction-zone values (b ≈ 0.9-1.1). Window Mc=5

catalog is insufficient (N=19 < 30 MLE threshold). Window Mc=4 b = 0.50

± 0.07 — likely a Mc-bias artifact (the window is dominated by the M6.6 and

M7.4 mainshocks plus their large aftershocks; small-event under-sampling

near the completeness threshold inflates the implied b-deficit).

M6.6 / M7.4 independence (T7)

Epicentral distance 151.7 km, time separation 25.1 d. Gardner-Knopoff

aftershock window for an M7.4 mainshock: r ≤ 79.3 km, t ≤ 945.6 d. The M6.6

falls outside the spatial GK window for the M7.4 (151.7 > 79.3) and is

classified as a statistically independent event under standard declustering.

Coulomb-stress coupling at this distance cannot be excluded but is outside

the scope of V1.

Bonferroni

Only T4 yields a p-value (p=0.93); Bonferroni threshold at α=0.05 / 1 test

= 0.05; the null result is robust to multiple-comparisons control.

Limitations & V2

1. Post-M6.6 catalog only 3 d at analysis time; Bath's-law (T2) and Omori

(T5) deferred to V2 (≥ 30 d post-event).

1. Offshore-Japan USGS completeness Mc ≈ 4; lower-magnitude foreshock test

(M3+) requires JMA-quality catalog, not currently in the TerraPulse

store.

1. ISC ingestion latency precludes a fourth-network confirmation; expected

mid-June 2026.

References

• Bath, M. (1965). Lateral inhomogeneities of the upper mantle. Tectonophysics 2, 483-514.
• Gardner, J. K., and L. Knopoff (1974). Is the sequence of earthquakes in Southern California, with aftershocks removed, Poissonian? BSSA 64, 1363-1367.
• Aki, K. (1965). Maximum likelihood estimate of b in the formula log N = a - bM. Bull. Earthquake Res. Inst. 43, 237-239.
• Utsu, T., Y. Ogata, R. S. Matsu'ura (1995). The centenary of the Omori formula. J. Phys. Earth 43, 1-33.
• TerraPulse PMA #129 — Kermadec Doublet (2026-04-19). Comparable live-event V1.

Data availability

All scripts, parquet datasets, and results.json are in

workspaces/m6-6-sanriku-2026-05-15-foreshock-signat/. The analysis is

reproducible via python scripts/extract.py && python scripts/analyze.py.