The United States Geological Survey’s real time earthquake map for the week 2-8 May 2014 shows once again how the complex tectonic margins of the western Pacific Ocean are the focus of much of the planet’s seismic activity.
Over half (17 out of 30) of tremors larger than M5.0 on the map (which shows earthquakes of all magnitudes in the US and its territories and those of at least M4.0 elsewhere) are in this region, including five out of seven with magnitude of at least 6 (≥M6.0).
Elsewhere, the map shows the usual sprinkling of earthquakes along the Pacific margin of South America and the southern margin of the Euarasian continent, where continental collision is marked by tremors in Thailand, central Asia and Turkey.
In total the map recorded 1,634 tremors, 124 of them with magnitudes ≥M4.0.
The Week’s Largest Earthquake: M6.6, Fiji
The largest tremor recorded on the USGS map was an M6.6 to the south of the Fiji Islands. Although its epicentre is (relatively!) closer to Fiji than to anywhere else, the earthquake, which occurred in the South Fiji ocean basin, is actually associated with subduction of the westward moving Pacific plate beneath the Australian plate along the Tonga Trench.
The Tonga Trench offers a classic example of a subduction feature known as a Wadati-Benioff zone. The depth of earthquakes increases as the subducting plate descends – a situation which is elegantly illustrated on the accompanying map of depth and location of earthquakes. Put simply, the further from the subduction zone an earthquake epicentre (location on the surface) is, the deeper its actual location will be. This week’s M6.6 was around 600km from the trench and over 525km deep.
M6.0 Japan: Complex Tectonic Setting
The Japanese archipelago is one of the most earthquake-prone zones in the world – something which is hardly surprising when we look at the tectonic setting. The country lies across the junction between four separate plates: the Pacific plate to the west; the Philippine Sea plate to the south; the Eurasian plate to the west; and the Okhotsk microplate (often shown as a southward extension of the North America plate) to the north.
With so many forces at work, it’s unsurprising that Japan experiences regular large earthquakes.
This week’s M6.0, which occurred along the Sagami Trough, a narrow subduction zone at the apex of the Philippine Sea plate where it subducts beneath the Japanese archipelago south of Tokyo, is a relative minnow. But the country’s vulnerability to major tremors cannot be overstated.
US Earthquakes: Oklahoma (Still) Shaking
Minor earthquakes continue to plague central and northern parts of Oklahoma. Though theoretically tectonically stable, this region has been trembling for some time and so has been subject to close monitoring by The USGS and Oklahoma Geological Survey.
This week the agencies produced a conclusion – and it isn’t good news. “The rate of earthquakes in Oklahoma has increased by about 50 percent since October 2013,” they said “significantly increasing the chance for a damaging quake in central Oklahoma”.
More controversially, the concluded that that “a likely contributing factor to the increase in earthquakes is wastewater disposal by injection into deep geologic formations”.
In other words, the increase in minor earthquakes in the area appears not to be natural but the result of human activity – a sobering thought.
Interplate and Intraplate Activity
Most seismic activity, including pretty much all larger earthquakes, is associated with major plate boundaries. But the experience of Oklahoma shows that even small old faults can become active again under certain conditions.