The week of 9-16 November 2017 comes with a dubious claim to fame — it’s the most deadly week, in earthquake terms, of the year so far; and we can only hope that it isn’t surpassed. At the time of writing the death toll from the M7.2 quake on the border of Iran, which struck on Sunday, was over 400 and likely to rise.
This tremor, unsurprisingly, dominated the United States Geological Survey’s interactive earthquake map for the week, but there was plenty of seismic activity elsewhere. The map, which includes earthquakes of all magnitudes (which is not the same as all earthquakes) in the US and its territories and those of at last magnitude 4 elsewhere, included four earthquakes of at least M6.0 in its total of just over 1900 tremors.
In addition to the largest quake, on the Iran-Iraq border, there were significant tremors in Costa Rica, the mid-Atlantic and Japan, with 30 earthquakes of ≥M5.0 and 142 of ≥M4.0.
The Week’s Biggest Earthquake: M7.2, Iran
The south-western border of Iran, both where it marches with Iraq to the north and where it borders the Persian Gulf, is marked by a significant mountain zone know to the lay observer as the Zagros Mountains and to the geologist as the Zagros thrust belt. Caused by the coming together of Arabia and Eurasia, it’s an area of active tectonic uplift on a huge scale and is dominated by parallel faults as the Earth fractures and slabs of crust build up on top of one another.
The available data on this week’s earthquake from the USGS confirm that the driving force behind this week’s earthquake is compressional, with movement occurring as “the result of oblique-thrust faulting at mid-crustal depth (~25 km)”. In such an active seismic environment, where compression causes uplift and slopes are unstable, the risk to life is heightened by the probability of landslides being triggered.
Early indications are that in addition to lives lost in buildings, there may be more casualties arising from these landslides, many of which may have occurred in remote areas, with reports slow to come in.
Seismologist Robert Yeats quotes the rate of convergence as around 20mm per year and the USGS figure is higher, at 26mm per year. With this mind, it’s no surprise that Iran has a daunting history of earthquakes, although the immediate vicinity of this week’s tremor has been relatively quiescent, with the last earthquake of at least M6.0 occurring half a century ago, in 1967.
M6.5 Earthquake, Costa Rica
On the other side of the world, an earthquake of M6.5 shook Costa Rica. Like the Iran tremor, this was the result of convergent movement, but in this case the collision between the oceanic Cocos plate and the continental crust of Central America results in subduction rather than uplift. (Oceanic crust, being dense, descends beneath the more buoyant continental rocks.)
USGS data on depth (around 20km) and location (around 100km inside the overriding plate) combined with the convergent direction, imply that this earthquake was the result of deformation within the overriding plate rather than movement at or near the plate interface.
This week’s earthquake was much smaller than that in Iran, but the USGS suggests that just shy of two million people will have experienced moderate or higher shaking — compared with a staggering 55 million who may have experienced the same level of shaking in Iran. The difference in the numbers affected is probably key to explaining why the Costa Rica ‘quake hasn’t been widely reported internationally, and so there’s no indication of any casualties.
US Earthquakes: California
I haven’t called in to the Sunshine State in a while, but this week it’s time for a visit. When I talk about Californian earthquakes I’m usually at pains to point out that the San Andreas is a fault zone, not just a single fault, and most of the earthquakes that occur here are actually on one of the many parallel faults that mark the laterally-moving boundary between North America and the Pacific plate.
This section of the fault, between Los Angeles and San Francisco, is constantly moving, or ‘creeping’, so that small earthquakes here are by no means unknown. This week saw series of 83 of them, showing in the USGS map as between M0.8 and M4.6.
Last Thoughts: How Bad Can It Get?
I’m not a natural pessimist, but the thought of an earthquake in certain parts of the world worries me intensely, and this week reminded me why. In his book, The Million Death Quake, seismologist Roger Musson speculated on the impact of a large earthquake on a major city and cited Tehran as a possible location for this (there are others).
Tehran is some way from this week’s earthquake — roughly 500km — and although not in the Zagros Thrust Belt, it’s also in a tectonic setting that reflects the collisional impacts of Arabia and Eurasia. And while this week’s earthquake occurred in a (relatively) sparsely populated area and didn’t strike any major settlements, that won’t necessarily be the case in future.
Add in the absence of earthquake resistant buildings, which will increase levels of damage, and the human impact of an equivalent tremor in Tehran is almost unthinkable.