Perhaps the world has other things to think about (preoccupied by the coming holidays?) but it’s more likely that it’s just normal variation. Either way, the planet was relatively quiet in seismic terms this week.
The United States Geological Survey’s real time earthquake map, which records tremors of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, showed an unusual scarcity of larger tremors this week.
What we mean by large is, of course, relative. Generally speaking this digest recognises a large earthquake as being M6.0 or more but the truly large earthquakes can be 100 times bigger.
This week’s biggest tremor was a puny M5.8 off Tonga, one of 30 of ≥M5.0. All of them occurred in association with the major tectonic boundaries.
The Week’s Largest Earthquake: M5.8, Tonga
The largest earthquake of the week was an M5.8 which occurred in the western Pacific around 150 miles west of the Tongan archipelago.
The location and depth point to a subduction origin for the earthquake, which was around 200 miles west of the Tonga Trench, which forms the surface expression of the subduction zone between the Pacific and Australian plates.
As the subducting plate descends over a distance of hundreds of miles, the depths of earthquakes which occur at or near the plate interface will increase. At almost 200 miles, the depth of the tremor is consistent with this.
Other tremors in the area also lend weight to this theory. This week saw several earthquakes of this nature to the west of the Tonga Trench — and, as expected, the further from the trench, the deeper the earthquake. Maps of historic earthquakes elegantly demonstrate this relationship.
M5.7 South Atlantic
Another example of subduction-related seismicity was to be found in the south Atlantic this week. The remote South Sandwich Islands, a volcanic arc resulting from the subduction of the South American plate beneath the Scotia microplate are one of just two subduction zones surrounding the Atlantic Ocean (the other is in the Caribbean).
The M5.7 tremor, which was the second largest of the week, had its epicentre close to the trench and occurred at a depth of around 6 miles.
This time, however, there’s a little more ambiguity about its cause, and while it may be that it’s a subduction zone earthquake we can’t say for certain without knowing the angle of subduction and the actual location of the plate interface.
Although it’s possible that the earthquake was caused by movement at the plate interface, we must also consider the possibility that the wider stresses of subduction have generated fractures in the over-riding plate — and that this mechanism is the trigger for this week’s earthquake.
US Earthquakes: Shaking Everywhere
This week’s largest tremor in the contiguous US was M4.3 in Oklahoma — something which, while a few years ago would have raised eyebrows, has become more or less a normal part of the ongoing, probably human-induced, earthquake swarm in the area.
This week there was a sprinkling of minor, but nevertheless unusual, earthquakes across much of the central and eastern states, with Alabama (M3.4), North Carolina (M3.0), Texas (M2.6) Missouri (M2.5), Tennessee (M1.9) and even New Jersey (M1.9) all appearing on the USGS map. There’s no detailed information available for any of these, but the chances are that they’re associated with normal movement on existing faults and, as such, nothing to be concerned about.
Quakes: Depth Matters
This week’s Tongan earthquakes provide a clear illustration of increasing depth along a descending plate interface; but the shallow earthquakes of the Scotia arc show that, close to a plate interface, the relationship between depth and epicentre isn’t always clear.
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