All is quiet, as if the Earth is in post-Easter slumber. There isn’t a lot of seismic activity showing up on the United States Geological Survey’s real time earthquake map for the week of 4-10 April.
Although the map, which shows earthquakes of all magnitudes in the US and its territories and those of at least M4 (≥M4.0) elsewhere, mustered a (broadly standard) 1,671 tremors in all, just 17 of those were ≥M5.0 and the largest of those was just M5.8.
The number of tremors of ≥M4.0 was also generally as we might expect in a typical week, if such a thing exists in seismology, and the distribution of these also offered no surprises with the concentration of larger events around the planet’s plate boundaries.
It appears, then, that the Earth isn’t sleeping, but it also wasn’t in any great hurry to stir this week – merely opening one eye and deciding it just wasn’t worth the effort.
The Week’s Largest Earthquake: M5.8, Tonga
If it’s unusual for an earthquake of M5.8 to be the largest of the week, it certainly isn’t unusual for it to occur in the western Pacific, an area which (to extend the rather whimsical analogy) rarely sleeps soundly.
This week’s tremor, which occurred at the northern end of the Tonga Trench, isn’t even the largest in recent weeks in its area, but is one of a series of shocks over the past 30 days (the largest, at M6.5, featured in last week’s digest).
The Tonga Trench is part of the western Pacific margin, an area where the dense oceanic crust of the Pacific plate sinks beneath the Australian plate. Further west the boundary is complicated, but here it’s relatively straightforward. I say ‘relatively’ because a little to the north the boundary curves round to the west and loses definition so that earthquakes in this area may be the result either of subduction or of deformation within one of the plates.
The shallow depth of this earthquake (just 10km) suggests that it, along with most of the others recorded in the area recently, was the result of the latter process, influenced by the change in direction of the plate boundary.
M4.5 Quake, Greece
Plate boundaries come and go (admittedly over timescales of millions of years). The Mediterranean is a collisional zone as Africa careens towards Eurasia at speeds of up to 10mm per year (that’s a simplification, because there’s a bit a traffic jam with smaller pieces of crust caught between the two, trying to find a way out).
The tremor which occurred this week in the Aegean between Greece and Turkey is, however, relatively straightforward (in so far as we can describe any of the tectonics of the Mediterranean Basin as such).
The African plate subducts beneath the Eurasian plate south of Crete. This week’s M4.5 in the region took place towards the eastern end of the zone, the Pliny Trench. As with Tonga’s tiny tremor, the depth (17km) suggests that this tremor was probably the result of deformation rather than movement at or near the plate interface.
US Earthquakes: Hawaii
Not one to miss an opportunity, Hawaii popped up this week with the largest US earthquake.
In fairness, the quake was pretty respectable, at M4.5 — the largest on the islands since one of the same size in August 2014. Hawaii’s a long way from any plate boundaries but it overlies (and is the result of) a volcanic hotspot. Movement of large amounts of magma beneath the islands fractures rock and causes earthquakes. Mostly they’re pretty small, but they can be very significant indeed — Hawaii’s largest, in 1868, is thought to have been around M7.9. So even this week’s isn’t as surprising as it might look.
Information on Deformation
This week’s featured earthquakes all appear to be the result of deformation — no surprise in Hawaii but perhaps more so for the other two, given that they occurred at subduction zones. They demonstrate clearly that earthquakes are the response to stresses and strains and that these can come from many directions. Even relatively straightforward boundaries have their twists and turns.