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Location:
9 km ENE of Uto, Japan
Period:
14 Apr 2016 12:26:35 - 14 Apr 2016 15:34:17 (3 hours 7 minutes)
Volcanoes in 100km radius:
Asosan(37km), Unzendake(41km), Kujusan(62km), Kirishimayama(87km), Yufu-Tsurumi(87km)
Earthquakes:
5
M 7.0+:
M 7.0; 6 km ESE of Kumamoto, Japan; (15 Apr 2016) (8km from the swarm center)
No swarms nearby.
AI-generated article — for informational and entertainment purposes only. May contain inaccuracies. Full disclaimerFound an error?

Seismic Swarm PS20160414.1: Foreshock Activity Near Uto, Japan

A seismic swarm designated PS20160414.1 was recorded on 14 April 2016 near Uto in central Kyushu, Japan. The sequence began at 12:26 and concluded at 15:34 local time, encompassing five earthquakes within a three-hour, seven-minute window. The events were centered approximately 9 km east-northeast of Uto, with hypocentral depths ranging from 8 to 10 km.

The recorded events displayed the following parameters:

  • 14 April 2016 12:26:35, magnitude 6.2, depth 9 km
  • 14 April 2016 13:07:35, magnitude 5.4, depth 10 km
  • 14 April 2016 15:03:47, magnitude 6.0, depth 8 km
  • 14 April 2016 15:06:22, magnitude 5.3, depth 10 km
  • 14 April 2016 15:34:17, magnitude 4.7, depth 10 km

This swarm formed part of the early foreshock activity preceding a magnitude 7.0 earthquake that struck 6 km east-southeast of Kumamoto on 15 April 2016, only 8 km from the swarm centroid. The close spatiotemporal clustering and consistent shallow depths point to rapid stress transfer along a segmented fault system.

Central Kyushu occupies a tectonically active zone within the Beppu-Shimabara graben, where oblique subduction of the Philippine Sea plate drives right-lateral strike-slip motion along the Futagawa-Hinagu fault zone. These faults accommodate east-west extension and have produced recurrent moderate-to-large earthquakes throughout the Holocene. Paleoseismic studies indicate recurrence intervals of several hundred to a few thousand years for surface-rupturing events on the Futagawa segment.

The April 2016 sequence highlighted the segmented nature of the fault system, with initial ruptures on the Hinagu fault propagating northeastward toward the Futagawa fault. Depths of 8–10 km align with the brittle-ductile transition in the regional crust, where most seismicity concentrates. Historical records document damaging events in 1889 and 1929 near Kumamoto, underscoring the long-term seismic hazard.

Swarm activity of this intensity and brevity is characteristic of foreshock sequences in subduction-related volcanic arcs. The five events released significant moment in rapid succession, likely promoting further failure on adjacent fault patches. Depths remained stable near 10 km after the initial event, suggesting a consistent seismogenic layer.

Post-sequence monitoring revealed afterslip and triggered seismicity extending into adjacent segments, consistent with viscoelastic relaxation models for the region. The proximity of Uto to the active faults emphasizes the need for continued geodetic and seismic surveillance to refine rupture forecasts.

References

Japan Meteorological Agency Earthquake Catalog
Geological Survey of Japan Active Fault Database
USGS Earthquake Hazards Program records for Kyushu tectonics