This week’s review of earthquakes recorded on the United States Geological Survey’s real time earthquake map (which shows all earthquakes in the US and its territories and those of at least magnitude 4 elsewhere in the world) shows a number of medium-sized seismic events.
The 34 tremors of ≥M5.0 earthquakes followed the usual pattern, concentrated in the western Pacific with a scattering at ocean ridges and continental collisions zones.
There were seven tremors of ≥M5.0 in the week’s total of 1,663 recorded earthquakes, the largest of them registering M6.6.
Interestingly, only two pairs of these – an M6.4 and its M6.0 aftershock off Mexico and and M6.6 and M6.1 in Micronesia – appear to be associated. The others occurred around the margins of the Pacific – in Panama, on the South Eastern Pacific Rise, and in the Philippines.
Week’s Largest Earthquake: M6.6 Micronesia
The largest earthquake this week is a little unusual in that it occurred pretty much in the middle of nowhere, and is unrelated to any plate boundary. Most of the planet’s major earthquakes are associated with plate margins of one sort or another and although large intraplate earthquakes do occur, relatively little is known about them.
(In their book Global Tectonics, Kearey et al devote just a short paragraph to such events, though they acknowledge that they’re are significant in indicating areas of stress within plates.)
This week’s M6.6 and its aftershock occurred within the Pacific plate, around 750km from the nearest plate boundary.
Despite the distance from known seismic zones, it’s hardly surprising that stress builds up in the crust as the complex movements of the many microplates in the western Pacific create contrasting movements. Such earthquakes are typically shallow and indeed this was the case this week, with both mainshock and aftershock occurring at a depth of around 10km.
In contrast to the Micronesia event, the M6.5 which occurred off Panama was associated with a plate boundary. This tremor, however, is possibly not as simple as it might appear, with the fracturing occurring close to the junction between the Cocos, Nazca and Caribbean plates.
Although this region is broadly described as a subduction zone, there are significant components of lateral movement involved.
A closer look at fault maps of the region suggests that the tremor was not a subduction earthquake but is more likely to be the result of faulting along one of three north-south trending fault zones which mark the boundary between the Cocos and Nazca plates. The depth of the earthquake – also shallow, at around 10km – gives further weight to this theory.
US Earthquakes: Alaska
Alaska wasn’t still this week – the USGS map shows a mere 550 tremors or so in the state, both on the mainland and along its associated island arcs. None of these was particularly large, with the biggest measuring in at M5.5.
A map of Alaska’s tremors, however, gives a good idea of the stresses generated by the convergence of the Pacific and North American plates and the different components of movement which result from the change in direction of the margin.
Quake Map: Not As Simple As It Looks
In explaining an earthquake it’s often necessary to simplify – sometimes, if time is short, oversimplify. Each of this week’s featured earthquake zones indicates that simplicity is is by no means always the case.
The earth’s movements are complicated; the stresses and strains they generate are complicated as a result. Even apparently simple earthquakes may have a convoluted tale to tell.