Taiwan, Chile and (Not) California: Earthquakes 4-10 February 2016


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The map shows earthquakes of at least M4.5 in the week of 4-10 February 2016

The map shows earthquakes of at least M4.5 in the week of 4-10 February 2016. Image by USGS

The week of 4-10 February 2016 was a sobering one. It may not have been anything particularly special in numerical terms — the United States Geological Survey’s real time earthquake map recorded just three earthquakes larger than magnitude 6 and the largest of these was just M6.3. But reports indicate that this tremor killed dozens of people and the death toll may yet rise.

In the light of this, the week’s earthquake statistics scarcely matter, but here they are anyway:

The USGS map, which includes earthquakes of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, included a total of over 1500 tremors worldwide. Of these, 37 were ≥M5.0 and 120 ≥M4.0.

The distribution showed the usual concentration of more significant activity around the plate margins, but there were a few outliers — including an M4.5 in the usually stable Baffin Bay and a cluster of small but noteworthy temblors (nothing larger than M3.3) in sedate New Brunswick.

The Week’s Largest Earthquake: M6.4, Taiwan

The week's largest - and most damaging - earthquake occurred in Taiwan

The week’s largest – and most damaging – earthquake occurred in Taiwan. Image by USGS.

Strictly speaking, the M6.4 which struck Taiwan on 5 February was only the joint largest of the week, but it certainly caused more damage and destruction than the other M6.4 (in Papua New Guinea). Taiwan is caught in a tectonic trap, between two subduction zones which, as Robert Yeats remarks, “are in great contrast in their behaviour”.

The details aren’t important in an article of this length, but the key thing to this quake is that the contrasting direction of the two (the northern zone subducting beneath the Pacific plate and the western one subducting eastwards) creates a complicated tectonic setting where both collisional and extensional tectonics are at play.

The USGS earthquake summary identifies the cause of this week’s tremor as being: “oblique thrust faulting at shallow-mid crustal depths”. At this location of the earthquake the Pacific and Eurasian plates are converging at a rate of 80-82mm per year. The collision has caused uplift and crustal shortening — generating the faults on which the earthquake occurred.

The Taiwan earthquake was deadly. The USGS summary notes that: “At least 40 people were killed, 525 people injured and 108 people missing in the Tainan area”. It’s a reminder, if we needed it, that our planet is a powerful place.

M6.3 Earthquake, Chile

This week's M6.3 in Chile was one of an ongoing series

This week’s M6.3 in Chile was one of an ongoing series. Image by USGS

The tectonic setting of our second earthquake is much simpler — a single trench along the western coast of South America, along which the Nazca plate subducts beneath the South American continent. Earthquakes are common here, and it is the tectonic setting of the largest recorded earthquake — an M9.5 in 1960, which sent a tsunami across the Pacific.

In truth, at M6.3 this week’s earthquake may have been one of the largest of the week, but for its tectonic setting, it was small. The map shows the number of earthquakes of at least M6.0 on the USGS map for the sector of the subduction zone since mid-September 2015: this week’s was the thirteenth largest.

Why September? That was when the region was hit by not one but two tremors of M8.3. That’s 100 times larger than that experienced in the area this week. This part of the world is used to major earthquakes — and just as well, because they keep on coming.

US Earthquakes: California

Not much activity in California's Ventura Basin this week - but the hazard may have been underestimated

Not much activity in California’s Ventura Basin this week – but the hazard may have been underestimated. Image by USGS

This week’s US feature is a bit of a cop-out, in the sense that there were no significant earthquakes there (the map is blank but for one tiny shiver of M2.9). It isn’t always so: the Ventura Basin, off Los Angeles, where there are active reverse faults both onshore and offshore, is seismically active.

This week, a new study suggests that the potential for a significant earthquake in the area is larger than previously thought — with the implication that the associated hazards, such as tsunamis, are likewise increased.

Last Thoughts: Back to Taiwan

For major earthquakes the USGS produces a ‘Pager’ summary — an assessment of the potential damage from the event. For this week’s Taiwan earthquake, the Pager summary indicated “a low likelihood of casualties”, further noting that: “Overall, the population in this region resides in structures that are resistant to earthquake shaking, though some vulnerable structures exist”.

At the time of writing, at least 40 deaths have been confirmed. All but one were in an apartment building which collapsed and whose developers have now been arrested for what the news media describe as “allegations of shoddy building practices”.

I’ve said it many times before; I imagine I’ll say it many times more. Earthquakes don’t kill people. Buildings do. It’s a lesson we can’t overlook.

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