May 20, 2013 – KAMCHATKA – A massive earthquake swarm has occurred in the region of Petropavlovsk-Kamchatskiy, Russia. More than 20 earthquakes have struck the region in the last 22 hours- the strongest of which was a 5.9 magnitude earthquake, which rattled the seafloor at a depth of 16.5 km deep. The latest seismic flare-up along this region reflects growing agitation on one of the planet’s largest tectonic plates- the Pacific plate. Increasing seismic volatility along this region of Kamchatka could have serious ramifications for every tectonic plate that comes into contact with the Pacific plate- from the South Pacific to South America. A similar, more intense earthquake swarm occurred along the periphery of the Pacific Plate near the Santa Cruz Islands in February of this year. Is the Pacific plate on the verge of a large-scale change?
The geology of the region: The Kuril-Kamchatka arc extends approximately 2,100 km from Hokkaido, Japan, along the Kuril Islands and the Pacific coast of the Kamchatka Peninsula to its intersection with the Aleutian arc near the Commander Islands, Russia. It marks the region where the Pacific plate subducts into the mantle beneath the Okhotsk micro-plate, part of the larger North American plate. This subduction is responsible for the generation of the Kuril Islands chain, active volcanoes located along the entire arc, and the deep offshore Kuril-Kamchatka trench. Relative to a fixed North America plate, the Pacific plate is moving towards the northwest at a rate that increases from 75 mm/year near the northern end of the arc to 83 mm/year in the south. Plate motion is predominantly convergent along the Kuril-Kamchatka arc with obliquity increasing towards the southern section of the arc. The subducting Pacific plate is relatively old, particularly adjacent to Kamchatka where its age is greater than 100 Ma. Consequently, the Wadati-Benioff zone is well defined to depths of approximately 650 km. The central section of the arc is comprised of an oceanic island arc system, which differs from the continental arc systems of the northern and southern sections. Oblique convergence in the southern Kuril arc results in the partitioning of stresses into both trench-normal thrust earthquakes and trench-parallel strike-slip earthquakes, and the westward translation of the Kuril forearc. This westward migration of the Kuril forearc currently results in collision between the Kuril arc in the north and the Japan arc in the south, resulting in the deformation and uplift of the Hidaka Mountains in central Hokkaido. The Kuril-Kamchatka arc is considered one of the most seismically active regions in the world. Deformation of the overriding North America plate generates shallow crustal earthquakes, whereas slip at the subduction zone interface between the Pacific and North America plates generates interplate earthquakes that extend from near the base of the trench to depths of 40 to 60 km. At greater depths, Kuril-Kamchatka arc earthquakes occur within the subducting Pacific plate and can reach depths of approximately 650 km. -USGS