As we settle into 2015, the world keeps turning and the earthquakes keep coming.
This week (2-8 January) the United States Geological Survey’s real time earthquake map (which includes all earthquakes in the US and its territories and those greater than magnitude 4 elsewhere) included 22 earthquakes of at least M5.0.
As usual, most of these were in the western Pacific (15 out to the 22); but the largest of the week was on the other side of the ocean, off Central America.
The eastern Pacific also produced earthquakes of interest off California and Canada; and there were smaller tremors along the Mediterranean basin, where Africa is moving against Eurasia.
The Week’s Largest Earthquake: M6.6, Panama
Central and South America are classic earthquake territory, with subduction of both the Cocos and Nazca plates beneath central and south America creating the conditions for major, and sometimes mega, earthquakes.
Despite being relatively close to both of these subduction zones, this week’s largest earthquake, an M6.6 some 250 miles or so off the Panama coast, was not subduction-related but occurred at the margin between the two subducting plates (Nazca and Cocos).
To understand why the tremor occurred, we need to look at the tectonic setting.
The Cocos plate is converging with central America in a (roughly) NNE direction at a rate of around 91mm per year. The Nazca plate, which adjoins it, is moving more slowly (around 32mm per year) roughly WNW. The differences of speed and direction must, inevitably, create tensions in the crust and in this case generate a fracture zone as the two plates move apart.
The shallow earthquake, and its M5.0 aftershock, both occurred on the margin between these plates and are associated with lateral, rather than convergent, movement.
M5.4, off California
The largest earthquake in the US this week, and the joint third largest overall, took place off California and has a similar tectonic origin to the Panama ‘quake.
The earthquake occurred off the Californian coast around 130km from Ferndale, also along a fracture zone — the Mendocino Fracture Zone.
In this case, rather than separate two subduction zones, the MFZ lies between the Cascadia subduction zone and the conservative boundary of the San Andreas fault zone.
The relationship between the subducting Gorda microplate to the north and the Pacific plate to the south creates significant stresses, with the result that the area frequently experiences shallow, moderate-to-large earthquakes which don’t cause major damage or generate tsunamis.
The map of seismicity over the past century shows that in relative terms, this week’s earthquakes is small — the USGS earthquake database shows 23 earthquakes of at least M6.0 since 2015, seven of which were M7.0 or more.
US Earthquakes: Shakin’ All Over
It isn’t just California, which quivers most of the time, albeit at a low level. Nor is it Alaska, which is the same. The earthquake swarm in Nevada keeps going, as does that in Oklahoma.
To these we can add an M4.9 and associated tremors in Idaho and a (possibly anthropogenic) cluster of minor tremors north west of Dallas, Texas.
Perhaps nothing outstanding — but a good illustration of just how much otherwise-solid areas can shake.
Triple Junctions and Earthquakes
Where two plates meet, you get a plate margin. Where three meet, you get a triple junction.
This week’s featured earthquakes both occurred close to triple junctions, both involving subduction and fracture zones — illustrating that it isn’t just subduction which causes significant seismic activity.
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