The earthquake which occurred in the north Pacific Ocean on 14 August is the third largest to be recorded in 2012 to date, with a magnitude of 7.7 (M7.7). This significant seismic event was exceeded only by two earthquakes of M8.6 and M8.2, both of which occurred on 11 April off the west coast of Sumatra.
The Sea of Okhotsk Earthquake of 14 August
The earthquake occurred in the Sea of Okhotsk off the island of Sakhalin, north of Japan and in a relatively remote area of the Pacific. Initial reports from the United States Geological Survey indicate that its epicentre was around 150 km from the nearest settlement, Poronoyask. The quake took place at a depth of over 600km.
Despite the magnitude of the earthquake, and the fact that it occurred offshore, no tsunami warning was issued by the Pacific Tsunami Warning Center, whose bulletin stated that “a destructive tsunami was not generated based on earthquake and historical tsunami data.” Although the earthquake was significant, magnitude is not the sole factor involved in generating tsunamis, which also require a significant vertical displacement along a fault or fault zone.
August 14th Earthquake: The Tectonic Setting
The Sea of Okhotsk lies in the northern Pacific Ocean. Although the region is, at first glance, associated with the so-called Pacific Ring of Fire, the details of the tectonic setting are more complicated. The large-scale dynamics of the western Pacific Ocean is are complex, and characterised by significant and continuing seismic and volcanic activity.
In a paper on Tectonic Evolution of the Japanese Island Arc System, Asahiko Taira writes that “the current tectonics of the Japanese arc system can be explained by the interaction of five plates: the Eurasia, Amur, Okhotsk, Pacific, and Philippine Sea plates.” Lying just to the north, the Sea of Okhotsk is also likely to be affected by the movement of these plates.
Although at the time of writing no detailed information on the mechanics of the earthquake is available, the location and pattern of past seismic activity suggests that the August 14 event is likely to be related to the interaction of these plates, rather than activity on the dominant subduction zone in the area, the Kuril Trench, which lies some way to the south.
Sea of Okhotsk: Tectonic History
The August 2014 earthquake in the Sea of Okhotsk is worthy of note not only in its magnitude, given that it lies at such a distance from a major subduction zone, but also in its depth. A glance at the USGS map of seismic activity in the region since 1990 suggests that tremors deeper than 500km are typically much smaller – usually of M4-5.
Though unusual, however, the August 14 event is not without precedent. Longer-term data mapped by the USGS shows that it is the third earthquake of greater than M7.0 and a depth greater than 500km to have occurred in the region since 1900 and the sixth to occur at a depth of over 300km during this time period.
NOAA Pacific Tsunami Warning Center. Tsunami Information Statement. (2012). Accessed August 14, 2012.
USGS. Magnitude 7.7 – Sea of Okhotsk. (2012). Accessed August 14, 2012.
Asahiko Taira. Tectonic Evolution of the Japanese Island Arc System in Annual Review of Earth Planetary Science. (2001.) Accessed August 14, 2012.