The 2004 Kii Peninsula Earthquake Sequence and Regional Seismicity

On 5 September 2004 at 14:57 local time, a magnitude 7.4 earthquake struck 118 km east-southeast of Shingu, Japan, at a focal depth of 10 km. The event formed part of a closely spaced sequence that included a magnitude 7.2 earthquake 94 km southeast of Shingu on the same day and a magnitude 7.1 event 161 km south of Oyama on 9 August 2009. These earthquakes occurred within the complex tectonic setting offshore the Kii Peninsula, where the Philippine Sea Plate subducts beneath the Eurasian Plate along the Nankai Trough. The Kii Peninsula lies above one of Japan’s most active subduction zones. Convergence rates of approximately 4–6 cm per year generate megathrust earthquakes and frequent moderate-to-large intraslab events. The 2004 sequence originated within the subducting Philippine Sea Plate, consistent with the region’s history of both shallow crustal and deeper intraslab seismicity. Historical records document major Nankai Trough events in 684, 1096, 1498, 1707, 1854, and 1946, each producing widespread strong shaking and, in several cases, destructive tsunamis along the Pacific coast of southwestern Japan. Geological studies show that the overriding plate in this area comprises the Cretaceous–Paleogene accretionary complex of the Shimanto Belt, which records long-term subduction-related deformation. GPS measurements indicate ongoing strain accumulation along the plate interface, with episodic slow-slip events detected both before and after the 2004 sequence. The relatively shallow 10 km depth of the mainshock contributed to locally felt intensities reaching 5 on the Japan Meteorological Agency scale in parts of Wakayama and Mie prefectures. Since 2000, the region has experienced several additional earthquakes exceeding magnitude 7.0, underscoring persistent seismic hazard. Post-2004 aftershock studies refined fault-plane solutions and confirmed that the September events ruptured separate but adjacent segments of the intraslab stress field. Updated seismic hazard maps issued by the Japanese government incorporate these data, assigning high probabilities for future magnitude 8+ events along the Nankai interface within the next several decades. Continued monitoring by dense seismic and geodetic networks provides real-time constraints on strain release and helps refine probabilistic forecasts for the Kii Peninsula and surrounding areas. References: USGS Earthquake Catalog Japan Meteorological Agency Headquarters for Earthquake Research Promotion (Japan)