The September 2004 Earthquakes Near Shingu, Japan

On September 5, 2004, two major earthquakes struck the region southeast of Shingu, Japan, highlighting the persistent seismic hazards along the Nankai Trough subduction zone. The first event, a magnitude 7.2 quake, occurred at 10:07 local time with a focal depth of 14 km, centered 94 km southeast of Shingu. Later that day, a magnitude 7.4 event took place 118 km east-southeast of Shingu, approximately 44 km from the earlier rupture.

These earthquakes originated in the Philippine Sea Plate subduction interface beneath the Eurasian Plate, where convergence rates average 4–6 cm per year. The shallow depths amplified ground shaking across the Kii Peninsula and adjacent coastal areas. Historical records document recurrent megathrust activity in this segment of the Nankai Trough, including the 1944 Tonankai (M8.0) and 1946 Nankai (M8.1) events, which produced significant tsunamis and coastal deformation.

Geologically, the region features accretionary prism sediments and forearc basins shaped by long-term subduction. The 2004 pair illustrated segmented rupture behavior, with the two events occurring on adjacent patches of the plate boundary without triggering a larger cascade. Post-event studies confirmed minor seafloor deformation consistent with thrust faulting, though no major tsunami was generated.

Japan’s seismic monitoring network, upgraded after the 1995 Kobe earthquake, provided high-quality recordings that refined understanding of stress transfer in the trough. Updated assessments from national agencies continue to identify this zone as capable of future M8+ events, underscoring the importance of preparedness along the Pacific coast of central Japan.

References

USGS Earthquake Catalog
Japan Meteorological Agency historical records