The earthquakes recorded on the United States Geological Survey’s real time earthquake map for the week of the 22-28 October 2015 share an unusual pattern with those of the previous week. Each week included a tremor of at least magnitude 7 (≥M7.0) – but nothing else of M6.0 or more.
This week’s map (which shows all tremors in the US and its territories and those of ≥M7.0 elsewhere) reveals the domination of the deadly earthquake which struck in northeastern Afghanistan on 26 October, of which more is below.
Taking this tremor out, the planet was relatively quiet, with 21 events exceeding M5.0, 107 ≥M4.0 and just over 1500 in total.
The earthquake distribution, as in most weeks, reflected the locations of the Earth’s plate margins.
The Week’s Largest Earthquake: M7.5, Afghanistan
Subduction zones, where oceanic plates collide with continents (or with each other) typically produce the planet’s largest (megathrust) earthquakes. But when continents collide with continents over periods of millions of years, they raise mountain ranges, and large earthquakes are, if not as common as at subduction zones, by no means unusual.
This week’s earthquake was a killer: latest reports indicate about 260 deaths and the number is almost certain to rise. Although large, the earthquake’s impact may have been mitigated by its depth (210km) — deeper earthquakes lose more energy on the way to the surface so that there’s a reduction in shaking.
But the other aspect of a continent-continent collision is that the high mountains resulting from the event have steep slopes which are vulnerable to collapse in a large earthquake. This causes landslides which are not only the direct cause of loss of life but also restrict access to remote areas, meaning that rescue can take a long time and further loss of life may ensue.
That geography plays a significant part in the death toll of an earthquake is evident from the comparison of two other major earthquakes of 2015. The megathrust subduction earthquake of September 16, off Chile, had a magnitude of M8.3 and occurred at a depth of just 22.4 km. (It killed fewer than 20 people).
In April this year, an earthquake of M7.8 in the high Himalayan kingdom of Nepal, at a depth of just 8km, killed over 8,000 people.
Earthquakes in the Eastern Mediterranean
Further west, the African continent is closing in on Eurasia. It isn’t there yet, but the Mediterranean basin, especially in the east, is fracturing and breaking under the strain.
This week, the creaking of the Earth’s bones manifested itself in a series of small tremors (M4.3-M4.7) along the Hellenic Trench. This minor subduction zone runs from the west of Greece to the southern coast of Turkey and is the regular source of intermediate earthquakes.
Although there are no reports of injuries as a result of this low-key trembling, seismologists are well aware of the potential for large earthquakes — and significant damage — in this region.
As recently as August 2015, geologist and natural hazards lecturer Matthew Blackett blogged on the subject of tidal waves in the Mediterranean — identifying the Hellenic Trench as a possible source.
After the drama of Afghanistan, it’s a relief to look at the earthquake map of the US and find that all is (relatively) quiet. The earthquake swarms in California (natural) and Oklahoma-Kansas (induced) continued as they have done for weeks; the odd small earthquake shook the Rockies; and an M2.5 quarry blast in Idaho reminds us that manmade earthquakes have been with us since before fracking.
A quiet week: long may it continue.
(Sombre) Last Thoughts: Earthquakes Cost Lives
In an otherwise quiet week, it’s difficult not to reflect on the loss of life in Afghanistan, in a region both politically and tectonically unstable. The Chile experience shows us that, with forethought and planning (and, of course, resources) a large earthquake doesn’t have to be a killer. But some areas of the world are more vulnerable to disaster — not just to earthquakes — than others.