With not a lot happening, the most significant event this week (18-24 December) has probably been the extensive reflections in the media on the tenth anniversary of the Boxing Day earthquake and tsunami — certainly one of the largest and most deadly seismic events in history.
Against this sobering background, this week’s earthquakes are minor.
The largest shown on the United States Geological Survey’s real time earthquake map, which shows tremors of all magnitudes in the US and its territories and those of larger than magnitude 4 (≥M4.0) elsewhere, is M6.6.
(For the record, the M9.1 Boxing Day earthquake released over 5,600 times as much energy.)
As usual, most of the larger tremors recorded this week occurred around the margins of the planet’s tectonic plates. The total of 1,461 tremors included 37 earthquakes of ≥M5.0 and 123 which were ≥M4.0.
The Week’s Largest Earthquake: M6.6, Indonesia
Indonesia covers some of the most earthquake-prone parts of the world, in a jumble of plates and microplates along the northern margin of the Australian plate; and it’s here that the 2004 earthquake took place.
This week’s M6.6 earthquake took place on the other side of the Indonesian archipelago, in a different tectonic setting, and is the latest (and one of the largest) in a series of aftershocks which followed an M7.1 tremor in mid-November.
The tremor, which occurred in the Molucca Sea, is the product of complex faulting and microplate movement; the available evidence suggests that the series of shocks in the area over the past few weeks is most likely to be associated with movement along one of many thrust belts which result from subduction zones which lie both to the east and the west.
M5.0, New Zealand
This week, the North Island of New Zealand was struck by a small earthquake of just M5.0; but in this week of anniversaries, it’s worth noting that it occurred seven years to the day after an M6.6 earthquake very close by.
This week’s tremor, near Gisborne, was minor by all standards and there were no reports of damage or injuries; but it’s a reminder that New Zealand, too, is prone to earthquake activity.
Geologically speaking, the region is associated with the offshore Hikurangi subduction zone, along which the Pacific plate subducts beneath the Australian plate.
The depth of the tremor (46km) and location relative to the subduction zone, however, suggest that it may have been the result of deformation in the over-riding plate rather than movement at the interface between the two plates.
While Idaho isn’t the first US state which springs to mind when earthquakes are mentioned, its position in the Basin and Range tectonic province of the US, where extensional tectonics lead to (usually small) movement along normal faults, means that minor earthquakes aren’t uncommon – and major ones, though rare, are possible.
This week the state shuddered a little, with tremors of between M3-M3.7… These quakes are minnows compared with the state’s largest earthquake of historic times, an M6.9 at Borah Peak.
Deadly Boxing Day Quake
It’s purely chance, of course, but the Boxing Day earthquake of 2004 was just the largest of three major and deadly tremors to fall on 26 December. Over 31,000 people were killed by an M6.6 earthquake in Iran in 1983 and 32,700 by an M7.8 tremor in Turkey in 1939.
Christmas Day isn’t immune either, being the anniversary of fatal tremors in California (1899), China (1932) and eastern Indonesia, not too far from the epicentre of this week’s largest earthquake (1982).