The 2008 Sea of Okhotsk Deep-Focus Earthquake

On 5 July 2008 at 02:12 UTC, a magnitude 7.7 earthquake struck beneath the Sea of Okhotsk at a focal depth of 632.8 km. The event originated directly within the subducting Pacific slab and was recorded as the sole magnitude-7.0 or greater earthquake in the region since 1 January 2000.

The Sea of Okhotsk lies above the Kuril-Kamchatka subduction zone, where the Pacific Plate descends beneath the Okhotsk Plate at rates of approximately 8–9 cm per year. Deep-focus seismicity occurs within the Wadati-Benioff zone as the slab encounters increasing pressure and temperature at depths exceeding 300 km. At 632.8 km the 2008 rupture took place near the base of the mantle transition zone, where olivine undergoes phase changes to wadsleyite and ringwoodite. These mineral transformations can generate metastable faulting, allowing brittle failure even under extreme confining pressure.

Historical records document recurrent deep earthquakes along the same slab segment. Notable prior events include the 1994 M 8.3 event farther south and several M 7+ shocks in the 1960s and 1970s, all occurring between 500 and 650 km depth. The 2008 earthquake fits this pattern, confirming persistent seismic activity at the base of the transition zone.

Because of its great depth, the 2008 mainshock produced only moderate shaking at the surface. Peak ground accelerations remained below 0.1 g across the Kuril Islands and Kamchatka Peninsula, and no significant tsunami was generated. Teleseismic body-wave analyses indicate a compact rupture area and a source duration of roughly 15 seconds, consistent with high-stress-drop failure in cold slab material.

Continued monitoring by global seismic networks shows that the Sea of Okhotsk slab remains seismically active at transition-zone depths. The 2008 event therefore serves as a key data point for understanding the mechanisms that permit brittle failure at depths where ductile flow would otherwise dominate.

References
USGS Earthquake Catalog, event page for 2008-07-05 M 7.7 Sea of Okhotsk.
International Seismological Centre, On-line Bulletin, reviewed hypocenters 2000–2023.
Hayes et al., Slab seismicity and phase changes in the mantle transition zone, Journal of Geophysical Research, 2012.