This was a busy week for seismologists, with the United States Geological Survey’s real time earthquake map showing a total of over 1,650 earth tremors (all magnitudes in the US and its territories and at least magnitude 4 elsewhere).
Of these an unusually high number – 43 – were larger than M5.0, while there were 159 of at least M4.0 and 301 of M2.5 or more.
The increased number of relatively larger tremors stems from two clusters of aftershocks – 13 off Chile, following last week’s M6.7 there, and a further five in the Andaman Sea.
The rest of the week’s larger earthquakes were spread, as expected, along the planet’s convergent plate boundaries (both subduction and continental collision zones) with just one larger event at a mid-ocean ridge (an M5.0 in the Indian Ocean).
The Week’s Largest Earthquake: M6.5 Andaman Sea
This week saw two earthquakes tied in magnitude at M6.5; but that which occurred in the Andaman Sea on March 21 gets top billing on account of its supporting cast- the 13 associated shocks of at least M4.5.
The tectonic setting of these earthquakes is a subduction zone capable of generating some of the largest and potentially most devastating tremors on the planet as a result of the subduction of the Indo-Australian plate beneath the Sunda microplate.
In this case, however, a look at the epicentres of the earthquakes and their depths indicates that they did not occur as a direct result of movement along the plate boundary.
All are shallow and this suggests that they are actually the result of faulting within the over-riding plate rather than at the plate boundary.
M6.5 Quake, Fiji
The M6.5 earthquake which occurred 550 miles south of Fiji offers a direct contrast. Though this also has its epicentre some distance from a subduction zone in the overriding plate, the map gives its depth as almost 500 km compared with around 10km for those in the Andaman Sea.
This provides a neat illustration of a key concept of plate tectonics – the Wadati-Benioff zone.
Subduction zones dip and epicentres of earthquakes which occur as a direct movement at the plate interface (rather than deformation in the over-riding plate) increase in distance from the surface expression of the boundary as they get deeper.
Analysing these earthquakes by depth produces clearly-defined bands, the width varying from subduction zone to subduction zone (as not all have the same angle of dip).
The surface expression of the Wadati-Benioff zone can be seen in the location of volcanoes. At a critical death (around 100-110km) the subducted rock melts and generates magma, which rises to reach the surface as volcanoes. The steeper the dip, the closer to the subduction zone this line of volcanoes (the volcanic front) will be.
Earthquakes in the US
The US was quiet this week. If you were to ask about earthquakes in the last seven days I might be tempted to reply with a line from an old joke: “Idaho, Alaska.” Alaska produced the largest but at M5.2 that was by no means out of the ordinary; and the second largest, one of a cluster in Idaho, was not especially large. The Idaho earthquake is interesting, however, because though distant from tectonic boundaries it’s a reminder that the many tectonic blocks which make up the western US are still shifting.
Deep vs. Shallow Earthquakes
Like everything else, seismology is based on essentially simple concepts with many variations. This week’s earthquakes illustrate that – the relatively straightforward tremors below the Tonga Trench are countered by shallow earthquakes in another subduction zone and other shallow tremors where no obvious plate boundary exists at all.