October 26, 2015
Indian slab lurches downward beneath Afghanistan
Posted by Austin Elliott
As I walked into the department this bright brisk morning, coffee cheerily in hand, the live global seismogram display in the atrium caught my eye with an alarming event that had just happened during my bike ride into work.
BIG earthquake, somewhere in the vicinity of Central/Southern Asia. Indeed, an earthquake deep (>200 km) beneath the Hindu Kush mountains of Afghanistan had shaken a huge swath of Central and South Asia. The great depth of the earthquake meant less extreme shaking at the epicenter (nobody lives closer than 212 km from the source), but wider zones of strong and perceptible shaking. Indeed this M7.5 earthquake was felt in most of the Central Asian capitals–Dushanbe, Tashkent, Bishkek, Ashgabat–and throughout South Asia, from strong shaking in the nearby Kabul and Islamabad, to mild but perceptible ground motion in Kathmandu, Kolkata, and capitals as far west as Doha and Kuwait City. This impressive range is consistent with previous earthquakes in the region, and may in fact reveal the configuration of tectonic elements at depth (pdf).
A whole host of largely unsurprising earthquake environmental effects and secondary hazard phenomena have already been documented, from ground water changes to landslides. The twitter feed of @pamirtimes is a great source for timely info and media on these local effects.
18. Dust rising from the mountains in Skardu due to landslide triggered by earthquake #Skardu #Baltistan #Earthquake pic.twitter.com/pH5iMxx7O6
— pamirtimes (@pamirtimes) October 26, 2015
19. More pics of landslide near Ganish bridge. Road to upper parts of Nagar district is closed #Hunza #Nagar pic.twitter.com/u64TD64MIm — pamirtimes (@pamirtimes) October 26, 2015
Water of a natural spring in Gircha village, Hunza has turned muddy after the earthquake. #earthquake Pic: Amir Ali pic.twitter.com/BahX0NjR8k
— pamirtimes (@pamirtimes) October 26, 2015
An observer in Pakistan filmed the Rawalpindi Metro Bus bridge shaking during the earthquake in a very threatening scene, but perhaps it importantly shows the strength and resilience of this fragile structure to strong ground motion: [youtube=https://www.youtube.com/watch?v=DhYrqq3MMBs]
Although the shaking appears dramatic, the death toll so far appears mercifully small for the size of the earthquake and the standard of construction in the region it impacted. This is largely due to its depth. The depth of the earthquake is apparent in seismograms recorded by the Global Seismic Network stations closest to the epicenter, which show a fairly long delay between P and S wave arrivals despite their relative proximity to the epicenter. For example the GSN station in Kabul, 212 km from the epicenter (thus roughly 301 km distant from the hypocenter at depth) has a ~32 second delay between the arrival of vibrating P waves and the more severe shaking of the S waves:
Aside from its remarkable felt extent, this earthquake serves as a significant illustration of the regional tectonics beneath the Hindu Kush and Pamir mountains. The USGS has analyzed Global Seismic Network waveforms and determined that the causative fault rupture occurred on a steeply south-dipping plane beneath the Hindu Kush (illustrated by the second nodal plane presented here), a region with complicated and controversial tectonics within the collision zone between India and Asia. Despite controversy over what is being subducted beneath Asia (continental or oceanic crust), this earthquake rupture is consistent with prior seismicity, highlighting the uncontroversial–if complex–structure of the Indian lithospheric slab at depth. Here the subducted crustal slab of the Indian plate is oriented nearly vertically, producing a narrow zone of earthquakes at rather substantial depth beneath the surface. An illuminating paper by Sippl et al., (2013) describes the slab geometry inferred from these earthquakes. Other papers have described this same configuration of the subducted Indian crust but interpreted quite differently what it represents, from a “curtain” of remnant oceanic crust, stalled because the Indian continent can’t get pulled down behind it (Pavlis and Das, 2000), to a huge, bent, rapidly-subducting slab of the Indian continent itself (Negredo et al., 2007).
Deep continental subduction #earthquake Mw7.5-7.6 in Hindu-Kush. See https://t.co/5prjTbJA0I pic.twitter.com/r1hAMLsVIB — Robin Lacassin (@RLacassin) October 26, 2015
Whichever the case, this earthquake fits neatly into the picture from prior earthquakes beneath the region, and will only help supply more data to understand just how far the Indian subcontinent has gone as it has crumpled its way beneath the highest mountain ranges on Earth.
Khalturin, V. I., Rautian, T. G., and Molnar, P. (1977), The spectral content of Pamir-Hindu Kush intermediate depth earthquakes: evidence for a high-Q zone in the upper mantle, J. Geophys. Res. Solid Earth, 82(20), 2931-2943.
Negredo, A. M., et al. (2007), Modeling the evolution of continental subduction processes in the Pamir–Hindu Kush region, Earth and Planetary Science Letters, 259(1–2), 212-225, doi:10.1016/j.epsl.2007.04.043.
2000), The Pamir-Hindu Kush seismic zone as a strain marker for flow in the upper mantle, Tectonics, 19(1), 103–115, doi:10.1029/1999TC900062., and (
2013), Geometry of the Pamir-Hindu Kush intermediate-depth earthquake zone from local seismic data, J. Geophys. Res. Solid Earth, 118, 1438–1457, doi:10.1002/jgrb.50128., et al. (