The world was seismically, if not meteorologically quiet this week.
The United States Geological Survey’s real time earthquake map (which records earthquakes of all magnitudes in the US and its territories and those of at least magnitude 4 elsewhere) showed a total just short of 1,600 tremors this week.
Of these, there was just one quake of magnitude 6 or greater (≥M6.0) along with 21 of ≥M5.0.
As usual, there was a concentration in the western Pacific, where 13 of the 25 tremors occurred, including nine of the top ten. These earthquakes stretched along the western margin of the Pacific plate from Japan to Tonga and included an M5.3 in Vanuatu — though, given the damaging cyclone experienced in the islands this week, an earthquake is likely to be among the least of the community’s concerns.
The Week’s Biggest Earthquake: M6.2, Indonesia
The earthquake of M6.2 which struck in the Molucca Sea is, in the longer-term context, nothing unusual. This area of the western Pacific is highly seismically active.
The convergence of the Pacific and Philippine Sea plates has led to the development of two thrust zones (where the crust is forced upwards), one on each of the two colliding plates.
Tectonic maps of the region show complex fault zones in the area but the epicentre of the tremor occurred between them and this, along with its depth of just under 50km, suggests that it may be the result of crustal deformation — unsurprising, considering the complex stresses at work.
It’s tempting, in a weekly digest, to look at earthquakes in isolation – but a review of tremors of ≥M4.0 in the area since the beginning of December 2014 shows that eastern Indonesia is regularly shaken by earthquakes, with almost 400 shown on the map. This week’s is not the largest in the immediate area, but so frequent are the tremors that it’s reasonable to suggest that it’s just part of the area’s normal activity rather than an aftershock to the largest tremor shown (which was an M6.3 in mid-December 2014).
Earthquakes in North Africa
Another area of complicated collisional tectonics, though rather less active, is the Mediterranean Basin. Dominated by the convergence of Africa and Eurasia, this area, too, has a mix of local tectonic motions.
This week there were four tremors of ≥M4.0 in northern Algeria. As in Indonesia, the collisional motion has generated a thrust belt as the northern margin of Africa crumples upwards to form the Atlas Mountains.
In this case the earthquakes, all of which occurred at relatively shallow depths (around 10km), seem most likely to be the result of thrust faulting along these (broadly parallel) belts.
US Earthquakes: Keeping an Eye on Oklahoma
While there’s not really anything new to report on the ongoing earthquake swarm in Oklahoma (research is ongoing and Decoded will keep you informed of any groundbreaking information) it’s worth a look at how the swarm has developed.
In the period from 1 January-17 March 2014 the USGS map shows 178 earthquakes of ≥M2.5. For the equivalent period this year (2015) there were 548. I don’t know about you, but even allowing for possible under-recording of the earlier period, I think that’s a pattern worth watching.
Last Words: Colliding Continents
It’s so simple in the school textbooks — tectonic plates collide and build mountains or island arcs (or both). But in fact the process is slow and tortuous, with deformation of the crust occurring not just along the plate boundaries but within the crust and at sometimes large distances from the actual margins. That’s what we saw this week in both Algeria and Indonesia.