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.

*gasp* that looks bad

*gasp* that looks bad

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).

Testimonies from people who felt the earthquake, received by the European-Mediterranean Seismic Center

Felt reports submitted to the USGS Did You Feel It system, displayed in Google Earth

Felt reports submitted to the USGS Did You Feel It system, displayed in Google Earth. Note testimonies of perceptible shaking from eastern India and west in the UAE.

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.

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:

Close-up of P wave onset in Kabul, with S waves just arriving at the end of the plotted record here.

Close-up of clear P-wave onset in Kabul, with stronger S-waves just arriving at the end of the record plotted here. Note 30 second “warning” time during P-wave rattling.

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).

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.

Google Earth perspective view of the shaking intensity from this deep earthquake beneath the Hindu Kush, at the northwestern syntaxis of India-Asia collision

Google Earth perspective view of the shaking intensity from this deep earthquake beneath the Hindu Kush, at the northwestern syntaxis of India-Asia collision

 

References:

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.

Pavlis, G. L., and S. Das (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.

Sippl, C., et al. (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.


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