In the seven days from 7-13 February 2014 there were three earthquakes recorded on the United States’ Geological Survey real time earthquake map, in western China, Vanuatu and the South Atlantic that had a magnitude of at least 6 (≥M6.0).
This map shows all seismic events in the US and its territories and those of ≥M4.0 elsewhere.
There were 28 tremors of ≥M5.0, the majority of them around the Pacific, in an overall total of 1,744.
The Week’s Largest Earthquake: M6.9, Western China
It’s common for the largest earthquake in a given week to occur around the margins of the Pacific Ocean; but this week it occurred as a result of continental collision. The northward movement of the Indian subcontinent against Eurasia has, over millions of years, uplifted both the Himalayas and the Tibetan Plateau to the north, creating a diffuse margin with complex lateral and thrust faults, across which large earthquakes are frequent.
This week’s M6.9 in China occurred at the northern edge of this margin. Latest information from the USGS bears out the initial suggestion that the cause of the fault was movement along a strike-slip fault and, although the USGS doesn’t specify the exact source, maps of the region suggest that it probably occurred along the major Altyn Tagh fault or a smaller, similar fault associated with it. Despite intense localised shaking, there were no reports of death or injuries at the time of writing.
M6.5, Vanuatu in the Western Pacific
The complex tectonics of the western Pacific provided the setting for the second largest earthquake of the week, an M6.5 tremor in the island state of Vanuatu on 7 February. Compared to the difficult and geologically-diffuse boundaries to both east and west, the tectonic setting is relatively straightforward. Convergence of the Australian and Pacific plates results in subduction of the former beneath the latter.
The resulting subduction zone is the source of many earthquakes. Both the depth (118km) and epicentre (in the overriding plate) suggest that the tremor resulted from movement at or near the plate interface.
Although earthquakes in this region often occur offshore and frequently generate tsunamis, the magnitude of the earthquake was insufficient and no tsunami followed.
Earthquakes in the US
One of the largest earthquakes in the US this week, at just M3.6, occurred close to Yellowstone crater, on 11 February. The area, which overlies a hotspot (where hot magma from the mantle rises to the surface) is characterised by frequent earth tremors, which often occur in clusters (or ‘swarms’) such as those of 2004, 2009 and 2010.
Tectonically speaking, Yellowstone lies to the east of most of the major earthquake zones which characterise western North America and the earthquake swarms around Yellowstone are related to movements of the magma which lies beneath.
This Week’s Quakes
This week’s featured earthquakes cover two forms of convergent boundaries – subduction zones and zones of continental uplift. But the occurrence of smaller earthquakes, such as those around Yellowstone. provide a reminder that earthquakes can have a volcanic, as well as a seismic, origin.