One of the largest earthquakes of the year struck in north-eastern Afghanistan on 26 October 2015. At the time of writing, news media such as BBC News were already reporting over 100 deaths with the number likely to rise.
The earthquake occurred, according to the United States Geological Survey’s initial earthquake summary: “as the result of reverse faulting at intermediate depths, approximately 210 km below the Hindu Kush Range in northeastern Afghanistan”.
Seismologist Robert Yeats notes that the area is unusual in experiencing damaging earthquakes at such a depths (up to 300km).
The shaking from the tremor was felt across large areas of north-eastern Afghanistan and into Pakistan and Tajikistan, as well as some parts of northern India.
The USGS Pager report (which summarises and predicts likely earthquake impacts) lists 11 towns and cities with a population of at least 1000 people as having experienced moderate or stronger shaking.
These areas include Kabul, with over three million residents, and the likelihood is that well over four million will have experienced significant shaking with many others also experiencing tremors.
Afghanistan Earthquake: Tectonic Setting
The Himalayas and the mountains which run to the east and west (including the Hindu Kush) include the highest peaks in the world. This huge mountain chain is the result of the relatively rapid (in geological terms) collision between the Indian subcontinent and Eurasia which began 50-70 million years ago and continues today at a rate of up to 50mm per year.
When oceanic crust collides with a continent, the relatively dense oceanic crust descends below the more buoyant continental material. When continents collide subduction does not occur but instead the two land masses come together, with rock sliding over rock in a process of uplift. As the pressure increases the rocks fracture and fault, piling up in layers.
Although the process is more complicated, with different factors at play, the evidence suggests that the tremor of 26 October, 2015 was the result of just such faulting, with compression resulting in “rupture … on either a near-vertical reverse fault or a shallowly dipping thrust fault” according to the USGS. (Both reverse and thrust faults are the outcome of compression tectonics and they differ in the angle of dip.)
Why Was the Afghanistan Quake So Deadly?
The seismologists’ saying can never be repeated often enough: earthquakes don’t kill people — buildings do. This is certainly largely true for this week’s earthquake, which took place in a area where: “Overall, the population … resides in structures that are highly vulnerable to earthquake shaking, though some resistant structures exist”.
In areas of steep slopes and active erosion, the problem is further complicated by damage caused by landslides, which not only cause direct fatalities but can also hamper rescue efforts by blocking access to remote areas.
Yeats refers, in his work on the earth’s tectonics, to three major earthquakes in the Hindu Kush since 2000. The most deadly, an M6.1, killed 1200 people while larger tremors ((M7.4, also in 2002 and M6.7 in 2005) were deeper and so killed fewer people. And of course, in April 2014 an earthquake of M7.8 in Nepal killed several thousand people.
The USGS impact forecasts suggest a significant likelihood of over a hundred deaths and this seems likely to be the case on the basis of the reported death toll alone, which will almost certainly rise. The relatively deep location of the actual earthquake itself, at 210km, may in fact prove a blessing as greater depth allows the earthquake’s energy to dissipate and reduces its impact on the surface.