The 2007 Magnitude 7.5 Earthquake in the Volcano Islands Region

On 28 September 2007 at 13:38 UTC, a magnitude 7.5 earthquake struck the Volcano Islands region of Japan. The event originated at a depth of 260 km and was the sole strong earthquake recorded in the area since 1 January 2000. The Volcano Islands form part of the Izu-Bonin-Mariana arc system in the western Pacific Ocean. This intra-oceanic arc results from the subduction of the Pacific Plate beneath the Philippine Sea Plate at rates of approximately 5–6 cm per year. The subducting slab descends steeply, generating a well-defined Wadati-Benioff zone that extends to depths exceeding 500 km. Earthquakes at intermediate depths, such as the 260 km focus of the 2007 event, occur within the cold interior of the slab where brittle failure can still take place despite surrounding high temperatures and pressures. Geologically, the region is characterized by active submarine and subaerial volcanism. Islands such as Iwo Jima and Nishinoshima sit atop stratovolcanoes that have produced andesitic to basaltic eruptions throughout the Holocene. The arc’s volcanic front lies roughly 100–150 km above the subducting slab, consistent with the depth at which hydrous fluids released from the slab trigger mantle melting. Historical records document multiple eruptions and associated seismic swarms, underscoring the coupled volcanic-seismic behavior of the arc. The 2007 earthquake released strain accumulated along the slab interface and within the descending plate. Its great focal depth attenuated surface shaking, limiting damage on the remote islands. No significant tsunami was generated, as expected for an intraslab event lacking substantial vertical seafloor displacement at shallow depths. Continued subduction ensures that the Volcano Islands region will experience recurrent intermediate-depth seismicity. Monitoring by regional seismic networks provides essential data for understanding slab dynamics and improving hazard assessments for this tectonically active margin.

References

USGS Earthquake Catalog Japan Meteorological Agency Seismic Data Global Volcanism Program, Smithsonian Institution