Seismic Swarm PS20240101.1: Noto Peninsula Region Analysis

A seismic swarm designated PS20240101.1 was recorded 42 km northeast of Anamizu on Japan’s Noto Peninsula. The sequence began at 07:06 on 1 January 2024 and concluded at 01:17 on 2 January 2024, spanning 18 hours and 11 minutes. During this interval, 13 earthquakes were registered, with the largest event reaching magnitude 7.5 at a focal depth of 10 km.

The temporal distribution shows an initial energetic phase followed by sustained moderate activity. The sequence opened with a magnitude 5.8 foreshock at 07:06, immediately succeeded by the mainshock of magnitude 7.5 at 07:10. Subsequent events included magnitudes 6.2, 5.2, 5.6, 5.0, 5.1, 5.5, 5.6, 5.2, 5.0 and 5.1, all at 10 km depth. The final event, magnitude 5.4, occurred at 01:17 on 2 January at a slightly shallower depth of 6 km. This pattern is characteristic of swarm sequences in which no single event dominates aftershock decay but rather multiple comparable shocks occur within a compressed time window.

The Noto Peninsula lies within a tectonically active zone on the eastern margin of the Sea of Japan. The region experiences deformation driven by reverse faulting along northwest-southeast trending structures. Crustal shortening in this area arises from the broader convergence between the Amurian and Okhotsk plates, producing frequent moderate-to-large earthquakes. Historical records document damaging events in 1729, 1833 and 1892, confirming long-term seismic productivity. Depths consistently near 10 km align with the brittle upper crust where these reverse faults are optimally oriented for slip.

Since 1 January 2000, only two prior swarms have been identified in the immediate vicinity: one in 2004 comprising a single significant event and another in 2023. The 2024 swarm therefore represents the third such episode in more than two decades, yet it produced the strongest recorded shock in the instrumental catalog for the peninsula—the magnitude 7.5 Noto Peninsula earthquake of 1 January 2024, located 23 km from the swarm centroid.

The combination of shallow focal depths, repeated moderate-to-large magnitudes and spatial clustering underscores elevated seismic hazard in the Noto region. Continued monitoring of microseismicity remains essential for refining fault models and assessing the potential for future sequences.

References

SeismoSight internal swarm classification PS20240101.1
Japan Meteorological Agency earthquake catalog
Geological Survey of Japan, active fault database (updated 2024)