With the United States Geological Survey’s real time earthquake map recording no tremor of magnitude 6 or greater (≥M6.0) the week of 5-11 December 2013 was relatively quiet, seismically speaking.
At least one tremor of ≥M6.0 occurs somewhere in the world in a given week, but a look a the earthquake map shows that there was nevertheless a significant scattering of earthquakes worldwide. As usual, most occurred along the margins of the planet’s tectonic plates, but a scattering in the continental interiors is also worthy of note.
The Week’s Largest Earthquake: M5.9, Kuril Island Chain
Subduction zones, where one of the Earth’s tectonic plates subsides below another, are the source of most major earth tremors and the week’s largest, an M5.9 which occurred off the Russian coast in the north west Pacific, is typical. The largest of the six tremors to occur this week along the Kuril-Kamchatka trench, the earthquake occurred at a depth of 25 km just off the Kuril islands and some 300 km from Japan. Despite its offshore location, this quake was too small to generate a significant tsunami and no warning was issued.
The Kuril-Kamchatka island arc is the product of the subduction of the Pacific plate beneath the Okhotsk microplate and is subject to frequent earthquakes. The depth of the earthquake, and the location of its epicentre on the over-riding plate, suggest that it was the result of movement at or near to the plate boundary itself, rather than of deformation within the Okhotsk microplate.
Earthquakes in Central Asia
Most major earthquakes occur at subduction zones where oceanic crust is subducted and are characterised by long, linear earthquake belts. Where continents collide the processes involved are different. Overall the ground is lifted rather than forced downwards and the patterns of seismicity are complex and diffuse.
This week the earthquake map shows a range of earthquakes across central Asia, all of which are likely to be related to the collision between Arabia and India in the south and Eurasia to the north. The location of these tremors (all of which have their epicentres hundreds of kilometres from the zone of convergence), alongside a topographical map, illustrates the impacts of continental collision on geography with a vast area of central Asia uplifted as a consequence.
US Earthquakes: Oklahoma Again
The two largest tremors in the US this week both occurred in Alaska – but the most noteworthy is the M4.5 which struck in Oklahoma. Although it lies in stable continental setting, where significant seismic activity might not be expected, Oklahoma has, over recent years, experienced significant low-level earthquake activity. There is much current scientific debate about the causes of these tremors.
Although earthquakes do occur naturally in such areas as a result of slippage on old faults, there is increasing concern about the level of induced seismicity – i.e., that generated by humans.
The USGS notes that “activities that have induced felt earthquakes in some geologic environments have included impoundment of water behind dams, injection of fluid into the earth’s crust, extraction of fluid or gas, and removal of rock in mining or quarrying operations.” Furthermore, the agency notes that statistical analysis of Oklahoma earthquakes since 2009 suggests that the pattern is not random but may relate to human activity and is continuing to monitor and investigate the levels and causes of seismic activity in the state.
Last Thoughts: Many Causes of Earthquakes
Earthquakes come with many causes and this week’s tremors clearly demonstrate that. Continental collision produces a range of patterns and tremors also occur within stable continental interiors with both natural and human causes.