Long shallow earthquake swarms in Japan (JMA, 1998-2024) were followed by M6+ events no more often than their own M2+ event counts predict; Noto 2021-2023 was one of two such cases

Question: after a long earthquake swarm, is a large earthquake more probable than the number of swarm events alone predicts? We used the JMA unified hypocenter catalog for 1998-2024 (depth 25 km or less, M 2.0 or more, far-offshore regions removed). An objective rule found sequences with high monthly rates in 0.2-degree windows that lasted 6 months or more. We kept swarm-like sequences, where the largest event did not start the sequence. The outcome was the first M 6.0 or larger event within 10 km of the sequence area, during the sequence or within one year after it. Of 26 swarm-like sequences, 2 had this outcome (7.7%, 95% interval 0.9-25%): Niijima 1998-1999 (followed by the M 6.5 Izu Islands event of July 2000) and Noto 2021-2023 (followed by the M 6.5 event of May 2023 and the M 7.6 event of January 2024). This count was 4.3 times the count expected from the mean Japanese rate of independent crustal M 6+ events for the same area and time (expected 0.47, p = 0.08). But it was 0.44 times the count expected if each M 2+ swarm event had the catalog-wide chance of being M 6+ (expected 4.5, 95% interval of the ratio 0.05-1.6). It was also 0.8 times the count expected from the long-term activity of the same areas. In 18 design variants, the ratio to the event-count expectation was 0.26-0.89 and never significantly above 1. The ratio to the area expectation was 2.4-8.1 (p < 0.05 in 8 variants). Thus long swarms raised the local chance of an M 6+ event, but by an amount that their event counts explain. We found no extra swarm-specific precursor effect. Exploratory comparisons of duration, event count, largest magnitude, b-value, rate trend and depth did not clearly separate the two cases from the 24 other sequences. Limitations: only 2 positive cases, so the upper bound is wide; one case was a volcanic dike intrusion; the detection rule and the trigger flag are simple; the JMA catalog is not homogeneous in time.

earthquake swarm · Noto Peninsula · JMA catalog · large earthquake probability · Gutenberg-Richter · seismic hazard · precursor test · Japan