Nepal Earthquake Aftermath- What Is Happening Underneath Kathmandu?

We continue to learn more about the buildup and release of strain along the master detachment fault underneath the Himalaya. The master detachment fault known as the Main Himalayan Thrust is the surface along which the Indian plate is sliding underneath Asia.

This data using GPS records of surface motion in the area affected by the 2015 Gorkha (Nepal) earthquake:

Himalayan strain reservoir inferred from limited afterslip following the Gorkha earthquake -David Mencin et al 2016 (behind paywall)

The magnitude 7.8 Gorkha earthquake in April 2015 ruptured a 150-km-long section of the Himalayan décollement terminating close to Kathmandu. The earthquake failed to rupture the surface Himalayan frontal thrusts and raised concern that a future Mw ≤ 7.3 earthquake could break the unruptured region to the south and west of Kathmandu. Here we use GPS records of surface motions to show that no aseismic slip occurred on the ruptured fault plane in the six months immediately following the earthquake. We find that although 70 mm of afterslip occurred locally north of the rupture, fewer than 25 mm of afterslip occurred in a narrow zone to the south. Rapid initial afterslip north of the rupture was largely complete in six months, releasing aseismic-moment equivalent to a Mw 7.1 earthquake. Historical earthquakes in 1803, 1833, 1905 and 1947 also failed to rupture the Himalayan frontal faults, and were not followed by large earthquakes to their south. This implies that significant relict heterogeneous strain prevails throughout the Main Himalayan Thrust. The considerable slip during great Himalayan earthquakes may be due in part to great earthquakes tapping reservoirs of residual strain inherited from former partial ruptures of the Main Himalayan Thrust.

The Himalaya cross section below shows seismicity along the interface of the Indian plate boundary faults.  MFT is the Main Frontal Thrust, MBT is the Main Boundary Thrust and MCT is the Main Central Thrust. These south younging sequence of thrust faults are thought to absorb the crustal shortening in the Himalayas. The youngest, the Main Frontal Thrust is thought to be active today, i.e. great earthquakes that nucleate underneath the Himalaya, rupture the surface along this fault system. All these thrusts faults are inferred to be splays of the Main Himalayan Thrust. Red and Orange dots are instrumentally recorded earthquakes.

Source: Bollinger et al 2014: Estimating the return times of great Himalayan earthquakes in eastern Nepal: Evidence from the Patu and Bardibas strands of the Main Frontal Thrust

Return Times Of Great Himalayan Earthquakes

Bollinger, L., S. N. Sapkota, P. Tapponnier,Y. Klinger, M. Rizza, J. Van der Woerd,D. R. Tiwari, R. Pandey, A. Bitri, and S. Besde Berc (2014), Estimating the return times of great Himalayan earthquakes in eastern Nepal: Evidence from the Patu and Bardibas strands of the Main Frontal Thrust, J. Geophys. Res. Solid Earth, 119, doi:10.1002/2014JB010970.

This is a very detailed study taken up along  two strands of the Main Frontal Thrust in Nepal south east of Kathmandu. Along this fault, the Neogene foreland basin Siwaliks are thrust over the Indo-Gangetic alluvium.

Crustal shortening taking place in the Himalayas as a result of convergence between India and Asia is accommodated along a sequence of south younging thrust faults; The Main Central Thrust, The Main Boundary Thrust and the southernmost Main Frontal  Thrust, which is active today. All these thrusts are interpreted to merge into a single decollment called The Main Himalayan Thrust (a "master fault" along which India subducts underneath Asia). The cross section below shows these major thrust faults flattening at depth and merging into the Main Himalayan Thrust. Earthquake clusters in red dots shows a region of the Indian slab which as it slides under Asia, often (over decadal to millenial times scales) gets locked. Rupture follows, thus releasing that accumulated slip. These ruptures propagate southwards and occasionally break the surface along the Main Frontal Thrust.

Source: Bollinger et al. 2014

So, along this fault in front of the Siwalik  hills, there is evidence in the form of fault scarps and fault traces, offset and deformed strata, uplifted river terraces and stacks of colluvial deposits (sediments eroded from a fault scarp) of past earthquakes. This study examines this record in detail going back several thousand years. Its a long paper and there were sections where the reading is a hard slog and when my eyes glazed over, but it is rewarding to understand the techniques (geomorphic measurements, geochronology and shallow seismic profiling)  that have been applied to reconstruct earthquake history.

Abstract:

The return times of large Himalayan earthquakes are poorly constrained. Despite historical devastation of cities along the mountain range, definitive links between events and specific segments of the Main Frontal Thrust (MFT) are not established, and paleoseismological records have not documented the occurrence of several similar events at the same location. In east central Nepal, however, recently discovered primary surface ruptures of that megathrust in the A.D. 1255 and 1934 earthquakes are associated with flights of tectonically uplifted terraces. We present here a refined, longer slip history of the MFT’stwo overlapping strands (Patu and Bardibas Thrusts) in that region, based on updated geomorphic/neotectonic mapping of active faulting, two 1.3 km long shallow seismic profiles, and logging of two river-cut cliffs, three paleoseismological trenches, and several pits, with constraints from 74 detrital charcoals and 14 cosmogenic nuclide ages. The amount of hanging wall uplift on the Patu thrust since 3650 ± 450 years requires three more events than the two aforementioned. The uplift rate (8.5 ± 1.5mm/yr), thrust dip (25° ± 5°N), and apparent characteristic behavior imply 12–17.5m of slip per event. On the Bardibas thrust, discrete pulses of colluvial deposition resulting from the coseismic growth of a flexural fold scarp suggest the occurrence of six or seven paleo-earthquakes in the last 4500 ± 50 years. The coeval rupture of both strands during great Himalayan earthquakes implies that in eastern Nepal, the late Holocene return times of such earthquakes probably ranged between 750 ± 140 and 870 ± 350 years.

And in conclusion:

Certainly, the best path toward fully understanding whether and where great or giant earthquakes are likely to occur along the foothills of the highest mountain range on Earth will be to combine many exhaustive geomorphological and paleoseismological field investigations such as that presented here with extensive, long-term geodetic measurements, capable of narrowing uncertainties in estimates of the full seismic moment deficit.

Eyewitness Account Of A Great Nepal Earthquake

From 1934 - as reported by military officer Bhrama Shumsher Rana in his book  Mahabhukamp (The Great Earthquake) -

“The trees were moving as if they were agitated by a storm and it seemed that the tree-tops would touch the ground … Pillars and walls of houses were cracking,doors and windows slamming. With movements up and down, houses collapsed. Statues and decorations placed on top of temples and houses fell to the ground. The noise made by the houses collapsing was reminiscent of canon fire, as…during festivities. Because of the dust, it was darker and no one was able to see at more than a distance of 8 to 10 hands apart. This cloud of dust came from the city itself, invading open areas such as the Thundikhel—a large open and unconstructed space in the centre of Kathmandu—which was such as lost in a fog. People rushed to all these open spaces. Those who could not move themselves were seizing pillars, while others were searching to hide in shelters or ran to the fields. People would run on all fours like animals…”

“Cracks opened in the fields and roads. Water spurted from these cracks. There was flooding in all streams.Rivers like the Bagmati and Bishnumati were invaded by black muddy water. At some places, the water rose 8to 10 hands above the cracks. Many fields were flooded with water. Warm water and sand spurted from some of the cracks. The roads toward Balaju and Shankhamul were affected by a subsidence as large as one or both hands in height. There were few roads that were not cracked.”

via Bollinger et al 2014 

Kathmandu has been leveled and rebuilt several times over the past 1000 years or so. There are a series of posts on Dot Earth about damage done by the recent 7.8 mag Nepal earthquake, on building codes and earthquake preparedness.