The week of 2-8 December 2015 proved to be seismically … interesting.
Two large earthquakes — of at least magnitude 7 (≥M7.0) — dominated the United States Geological Survey’s real time earthquake map this week, but there was nothing else greater than or equal to M6.0.
The map, which shows tremors of all magnitudes in the US and its territories and those of ≥M4.0 elsewhere, showed a total of over 1400 tremors in the week, with 30 of them ≥M5.0.
Most of the tremors were associated with the planet’s major tectonic plate boundaries, but both of the week’s largest earthquakes (described below) had their epicentres at some distance away.
The Week’s Largest Earthquake: M7.2, Tajikistan
The collision between the Indian subcontinent and Eurasia is the result of the northwards movement of the former plate, which has travelled (approximately) 45 degrees of latitude over (approximately) 70 million years. That’s fast: and the current rate of collision is an estimated 36-40mm per year.
Continental collision leads to mountain uplift and the uplift of the Himalayas, the highest mountain range in the world (and theTibetan Plateau to the north) inevitably creates huge stresses over a very large area.
Earthquakes associated with the Indian-Eurasian collision occur at significant distances from the contact between them and this week’s M7.2 in Tajikistan is an example, located around 500km to the north.
The USGS gives the case as strike-slip (lateral) faulting, although fault maps indicate that thrust faults dominate in this area (Yeats). The complicated relationship between speed and direction of movement, however, means that both types of fault are active in the region.
The past 50 years have seen eight tremors of at least M7 in this broad area, though most of them were further south — and these included a tremor of M7.5 in Afghanistan, as recently as October 2015.
M7.1 Earthquake, Southern Indian Ocean
Something very unusual happened this week in the depths of the remote Indian Ocean, something so remote, isolated and unusual that I have to depend largely upon the USGS event summary for information on it.
The USGS description (for simplicity’s sake, one presumes) places it on the Southeast Indian Ridge but the actual epicentre was some around 700km away.
So what does the USGS say? They remark that “Earthquakes away from the Southeast Indian Ridge plate boundary in this region are quite rare; no other events larger than M5 are known to have occurred within 400 km of the December 4, 2015 event over the previous century”.
What the USGS don’t do is provide a possible reason for it, which is hardly surprising given that the area so remote.
I can only speculate that the huge stresses generated by the spreading ridges which run around the southern oceans have generated stresses and caused major faulting within the plate. Intra-plate earthquakes, after all, aren’t unusual in themselves, even if tremors of this magnitude are.
US Earthquakes: Nevada
And so to the States, where nothing of great significance happened this week. The larger tremors again appear on the map around the two ongoing swarms, in Oklahoma and Nevada. The first is almost certainly human-induced: the second is natural but its exact cause is not clear. I expect we’ll come back to both of them before very long.
Last Thoughts: Not Where You Might Think
Large earthquakes are usually found at subduction margins but this week clearly demonstrates that this is not always the case. The Tajikistan earthquake is less anomalous than that in the Indian Ocean, because of its association with collision. But both illustrate the principle that the forces of tectonic movement are so significant that major stresses can build up many hundreds of kilometres from a plate boundary, even of the majority of larger earthquakes occur much closer to the margins.