So, normal service has resumed, in so far as the (variable) pattern of earthquakes in any given week can be regarded as normal. It’s probably fairer to say that this week provided no surprises and no real anomalies.
Of the 1,658 earthquakes shown on 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) all of those of at least M4.5 were associated with the margins of the planet’s tectonic plates, the majority of them (as usual) in the western Pacific).
The largest seismic event of the week — an M7.0 — was in this area, in eastern Indonesia; the west of the country saw the second largest, an M6.1. Though they occurred in the same country, these two quakes have different tectonic settings.
The Week’s Biggest Earthquake: M7.0, Indonesia
This week’s biggest earthquake presents something of a challenge, occurring as it does in a complicated region of the western Pacific where it’s tectonically unclear as to exactly what’s going on. To try and explain, we must (inevitably) simplify somewhat.
The two red lines shown on the map are both subduction zones, with differing directions.
- The earthquake’s epicentre (the point on the Earth’s surface immediately above where it occurred) lies to the north of one of these, where the Philippine Sea plate is being forced southwards beneath the Australian plate.
- The second, to the south and terminating a little to the west, is the Java Trench, where subduction goes the opposite way, the Australian plate descending northwards beneath the Philippine Sea plate.
Although the map suggests an association between the earthquake and the first subduction zone, there are other clues. The great depth of the earthquake — over 550km — suggests that it’s actually the result of the second, larger, subduction and may be the result of movement at depth along the interface between the two plates.
M6.1 Earthquake, Sumatra
If the largest earthquake is a bit of a teaser, the second is much more straightforward. This event, an M6.1, occurred off the island of Sumatra along the Sunda Trench, the northern extension of the Java Trench. This area is notorious for its large and potentially tsunamigenic earthquakes (it has produced five of M8.5 or more this century, including the M9.0 Boxing Day earthquake of 2004) making this week’s a relative minnow.
The dominant tectonic mechanism is subduction, and again the depth provides a clue as to the origin of the earthquake. Faulting occurred at a depth of 28km, which implies that the cause may have been faulting within the over-riding plate rather than along the plate interface.
US Earthquakes: The Reelfoot Rift
Possibly the largest earthquakes on the US mainland, a series of tremors estimated at up to M8.7 in 1811-12, occurred in a deeply-buried ancient rift system which underlies the boundary between Arkansas, Missouri, Kentucky and Tennessee.
This area still suffered from occasional earth tremors which may be aftershocks from these long-ago megaquakes.
This week saw a tremor of M3.1 at a depth of 13.4 km in this area, in the extreme south-west of Missouri — one which may well be another in this series.
Earthquakes: Deep Thoughts
We view earthquakes on a surface map; but our planet is three-dimensional and earthquakes can occur at great depths (the USGS reports that they can occur up to 700km) which, at an inclined subduction zone, can mean the epicentres are at significant distances from the surface expression of the fault. The Flores Sea earthquake appears to be a classic example of this. So the message to the earthquake detective is to look at the location on the map — and then dig deeper.
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