The week of 1-7 December proved deadly in earthquake terms. Very often, medium-to-large earthquakes cause relatively little damage and high death tolls (in the thousands) are mercifully rare.
This week there were four earthquakes of at least magnitude 6 and the largest of those caused significant death and destruction.
The United States Geological Survey’s real time earthquake map records (broadly speaking) tremors of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere.
Of the four ≥M6.0, two were in Indonesia, one in Peru and one in Alaska — all subduction zones where significant seismic activity is common.
The USGS map recorded a total of just over 1450 tremors, of which 30 were ≥M5.0. As we would normally expect, all of these larger tremors were associated with the margins of the planet’s major tectonic plates.
The Week’s Biggest Earthquake: M6.5, Sumatra
When an earthquake off the Indonesian island of Sumatra makes the news, it’s difficult not to fear the worst. In this area, where the Indo-Australian plate subducts beneath the Sunda microplate along the Sunda Trench, earthquakes can be very large and very damaging indeed. This week, an earthquake of M6.5 in the north of the island caused the deaths (at the time of writing) of at least 52 people in Indonesia’s Aceh province.
Though devastating, the earthquake could have been a lot worse. All the evidence suggests that it was not caused by movement directly at the subduction zone, which has been the source region for several so-called megathrust earthquakes over the years, but by movement along the Sumatran Fault. This runs onshore rather than off it, and is a strike-slip fault, along which movement is predominantly lateral rather than vertical.
The earthquake, being relatively small, onshore and with no vertical movement, didn’t generate a tsunami as large subduction events often do but (as noted above) nevertheless caused considerable damage locally. This can be attributed to local building styles: as the USGS notes: “the population in this region resides in structures that are vulnerable to earthquake shaking.”
M5.9 Earthquake, Trinidad and Tobago
Most larger earthquakes occur in the subduction zones around the Pacific and (like that above) Indonesia. In the Atlantic, which has just two short lengths of subducting crust, they are much rarer. This week’s M5.9, between the islands of Trinidad and Tobago, occurred at one of those two, the Lesser Antilles subduction zone.
In this area the South American plate converges (very slowly) with the almost static Caribbean plate, subducting beneath it and raising the chain of volcanic islands that characterises the eastern margin of the Caribbean.
At the southern end of this zone, the boundary curves round and the direction of movement becomes lateral, with the two plates separated by a broad zone of strike-slip (and some reverse) faults.
As in the Sumatran earthquake above, the evidence available points to an origin for this tremor along a strike-slip fault rather than subduction, again indicating that because an earthquake occurs at or close to a subduction zone, that isn’t necessarily the cause of the tremor.
US Earthquakes: Alaska
And so to America (though only just: this week’s M6.0 was very far west along the Aleutian Island chain). Again the tectonic setting is one where subduction is dominant, this time with the Pacific plate moving northwards and descending beneath the North American plate.
Unlike the other two earthquakes, this tremor looks like a subduction earthquake from every angle, with an epicentre in the over-riding plate — though perhaps a little too far from the plate margin to be caused by movement directly at the margin — and evidence that it was caused by compressional movement.
No surprises, there, though, as the Alaskan subduction zone is a relatively uncomplicated one.
Last Thoughts: How Big is Big?
In a typical week, I regard a large earthquake as something in excess of M6.0. But of course everything is relative: an M6.0 in (say) Washington would make news headlines worldwide while I doubt if anyone else has given news time to an M6.0 in Alaska.
I commented on the number of significant earthquakes along the Sunda Trench. Remembering that the earthquake scale is logarithmic (i.e. an M7.5 is ten times larger than an M6.5) this week’s deadly earthquake in Sumatra was small for its setting.
Here’s an eye-opening statistic: since 1 January, 2000, in the Sumatran section of the subduction zone, there have been a dozen earthquakes at least ten times larger than this week’s. Some have proved harmless; others, including the Boxing Day earthquake of 2004, have caused major loss of life.
The Earth is a very powerful thing.