10th Asian Regional Conference of IAEG (2015)

Engineering Geological Issues after Gorkha Earthquake 2015 in Nepal - a preliminary
understanding
Ranjan Kumar Dahal
Himalayan Geodisaster Research Center, Central Department of Geology, Tribhuvan University, Kirtipur,
Kathmandu, Nepal
E-mail: ranjan@ranjan.net.np, rkdahal@gmail.com
Abstract
Nepal is one of the earthquake-prone countries in the Himalayan region andearthquakes in Nepal have been reported since 1255. Recent Gorkha Earthquake measuring
Mw7.8 occurred at 11:56 AM Nepal Standard Time on 25 April 2015 with an epicenter 77
km northwest of Kathmandu at Barpak village of Gorkha district and killed more than 8900
people. This earthquake was the one of the most powerful earthquakes to strike central
Nepal since the 1934 Nepal-Bihar earthquake. Earthquake-induced landslides, land
subsidence, and liquefaction are major engineering geological issues after the 2015 Gorkha
Earthquake in central Nepal. In the case of the Gorkha Earthquake, topographic effect isquite well observed in many towns and villages on ridge in mountainous regions. Many
houses on the ridge were damaged and many tension cracks were observed on ridges. Rockfall, shallow landslides and dry debris fall, deep seated landslides, debris flow andavalanche, valley fill collapse and cut-and-fill failure are major earthquake-induced
landslides in the affected area of the Gorkha earthquake. More than 40% of house damage
in Kathmandu valley was found to be caused by differential settlement of the land in
Kathmandu Valley lacustrine sediments. Along with land subsidence, a few sand boils wereobserved in Kathmandu valley due to liquefaction of the lower strata.
Keywords: Gorkha Earthquake, Nepal, Himalaya, Earthquake-induced landslide, Liquefaction,
Land subsidence
1.

Introduction

Earthquake-induced landslides, land subsidence,and liquefaction are major engineering geologicalissues after the 2015 Gorkha Earthquake.
Earthquake-induced slope failures are one of the mostdamaging natural disasters. Commonly, damage fromearthquake-induced slope failures is worse thandamage related to the shaking and rupture of theearthquake
itself.
Whenearthquake-induced
landslides in Nepal is concerned, there were many
incidents. But, the record on the earthquake-induced
slope failures is not well documented. Few
documentations has been done during 2011
Sikkim-Nepal border earthquake in Taplejung district(Dahal et al., 2013). In the Gorkha Earthquake also,
many landslides are occurred in the major hit area andbasically the distribution of landslides is concentratedin Barpak to Dolakha, mainly northern part ofKathmandu valley. This paper will share preliminary
understanding of landslides in earthquake affecteddistricts. It also deal with a few land subsidence
problems and damages in infrastructures in andaround Kathmandu valley.

Being located in central part of the Himalayan
ranges, Nepal is regarded as one of theearthquake-prone countries in the region. Earthquake
is a major concern of Nepal because of rapid
population growth, poor land use planning, precarious
settlement patterns, and poorly implemented building
code. Earthquakes in Nepal have been reported since1255 while major earthquakes were recorded in 1408,
1681, 1810, 1833, and 1866, 1934, 1980, 1988, and2011. Recent Gorkha Earthquake measuring Mw7.8,
killed more than 8900 people and injured thousandsof people. The Gorkha Earthquake occurred at 11:56
AM Nepal Standard Time on 25 April 2015 with an
epicenter 77 km northwest of Kathmandu at Barpak
village. This earthquake was the one of the mostpowerful earthquakes to strike Nepal since the 1934
Nepal-Bihar earthquake. It is estimated that morethan eighty five million people have been affected by
the Gorkha Earthquake in Nepal. This is equal to thequarter of the Nepal’s population.

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10th Asian Regional Conference of IAEG (2015)

2.

Geological and tectonic settings of the Nepal
Himalaya2) as South Tibetan Detachment System (STDS),
Main Central Thrust (MCT), Main Boundary Thrust(MBT) and Main Frontal Thrust (MFT). Likewise,
geomorphologically, Nepal is divided into eight units
running east-west, namely, Terai, Churia Range, Dun
Valley, Mahabharat Range, Midland, Fore Himalaya,
Higher Himalaya, Inner and Trans Himalaya (Dahal
and Hasegawa, 2008).

The Himalaya belongs to very high seismic
region and has a history of devastating earthquakes.
The major source of earthquakes in Nepal and theHimalayan Region is the subduction of the Indian
plate underneath the Eurasian plate, which causes
contraction and stress concentration. Seismicity of theHimalayan region has been studied in terms of its
relationship with known tectonic activities (Rai et al.,2004). During the past 100 years, three greatearthquakes occurred along the Himalayan front(Ambraseys and Bilham, 2000). From west to east,
they includes the 1905 Kangra Earthquake (Ms ~7.8),
the 1934 Bihar-Nepal Earthquake (Mw = 8.1), andthe 1950 Assam Earthquake (Mw ~8.6). Althoughnone of these earthquakes is reported to haveproduced primary surface rupture (Seeber andArmbruster, 1981), on the basis of isoseismal map
and location of damage, it is estimated that theseearthquakes are the result of slip on the Main Frontal
Thrust (MFT). Same case appeared in the cases of the
Gorkha Earthquake, where seismologists still could
not trace out primary surface rupture. Therefore, thisearthquake also regarded as the case of the strain
release on blind thrust and it can be expressed as
anticline growth rather than primary surface rupture
or co-seismic surface rupture in and around
Kathmandu valley (Fig. 3). However, Avouac et al.
(2001) have mentioned that previous earthquake
events have produced a rupture of 250-300 km in the
Himalayan arc together with a co-seismic slip ofestimated average length of 5 km.
The southern part of the Himalaya, i.e. Ganges
Delta of northern India has extremely high population
density is in an area very close to historic greatearthquakes, demonstrated high strain accumulation(Fig. 1).

Fig. 2 Geological map of Nepal with major road
networks (modified after Dahal and Hasegawa(2008)
Generalized cross section is shown in Fig. 3 and it
illustrates the approximate locations of slip during the
25 April and 12 May 2015 ruptures on the Main
Himalayan Thrust, and approximate aftershock
locations of both events.
3.

Earthquake
Characteristics
Displacement

and

Fault

As mentioned in earlier section, 7.8 Mw was main
shock on 04-25 at 77 km NW of Kathmandu. Morethan ~100 subsequent aftershocks were occurred
eastward of main shock. M 7.3 Mw aftershock hit on
05-12 at 80 km NE of Kathmandu. Most earthquakes
are shallow angle thrust faulting on decollement ofMain Frontal Thrust (see Fig. 3). Main shock slip was
directed towards east from hypocenter with peak slip
about 4 m (USGS, 2015). The block dimensions isapproximately 120 x 80 km. Shaking estimates in
epicenter region are poorly constrained due to fewer
intensity observations and absence of strong groundmotion data. Likewise, predominantly strong to verystrong shaking were focused around aftershock
hypocenter and severe shaking was elongatedeastward from Barpak (epicenter of April 25, 2015)
to Dolakha (May 12, 2015) as shown in Fig. 3.
The aftershock distribution outlines the rupturezone of the main shock. The rupture during the main
shock initiated beneath the epicenter and propagatedtoward the southeast. Fig. 4 shows fault displacement
during the Gorkha Earthquake. The star is theepicenter while the thick arrow shows the direction of
rupture propagation towards the south east direction.

Fig. 1 Major earthquakes in the Himalayan region,
modified after Zurick et al.(2005) and USGS
(2015)
Geologically and tectonically, Nepal is divided
into five major tectonic zones, namely, Terai Zone,
Siwalik Zone, Lesser Himalayan Zone, Higher
Himalayan Zone and Tibetan-Tethys Himalayan Zone
(Ganser, 1964). These tectonic zones are separated bymajor thrusts and faults of the Himalaya and thosefaults and thrusts are named from north to south (Fig.

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Fig. 3 Block diagram of area affected by 2015 Gorkha Earthquake, Generalized cross section showing theapproximate locations of slip during the 25 April and 12 May 2015 ruptures on the Main Frontal Thrust(MFT). Locations of both Barpak and Dolakha epicenters as well as aftershocks are schematic only and all
are projected on vertical plane of sight. The elongated area faced strong-severe damages/shaking and it isextended towards east up to Dolakha. The block model is modified after (Hasegawa et al. 2009).

4.

Ground response in Kathmandu valley

Kathmandu valley is an intermontane basin and it
has soft soil layers of lacustrine origin. Borehole log
analysis in different region of the valley shows thatthe lacustrine soil is very complex in nature, andproperties and soil thickness is vary from place toplace. From borehole data, the maximum depth of thedeposit goes as high as 500 m in central part of thevalley. These large variations in sediment thickness
and in soil properties are responsible for change incharacteristics of the seismic waves and amplification
process of the strong ground motion. Not only in the
Gorkha earthquake but also in the Bihar-Nepal Great
Earthquake (Mw = 8.1) of 1934 ground shaking of
the Kathmandu valley from earthquakes suggest thatspectral amplification is common in Kathmandu andlacustrine sediments in the valley is playing the majorrole in the intensification of ground motion. From therapid damage assessment in the valley after the
Gorkha Earthquake, it is found that the most ofdamages were concentrated in northern part of thevalley. Center part of valley faced damaged on highrise structures. Southern part of the valley also faced
damages but basically damages were concentrated on
adobe old buildings and temples only.
This earthquake was destructive because it isshallow earthquake (15 km), and Kathmandu valley itself lies in a basin filled with about 300 m of soft

Fig. 4 Fault displacement during Gorkha
Earthquake (IRIS, 2015)
Contours show the rupture front in 5 second
increments after rupture initiation. Small red arrows
show the direction and amount of motion of the rocks
above the fault with respect to the rocks below thefault. The amount of slip is shown by color of
shading. Maximum fault displacement of about 3
meters occurred in the rupture zone in 20 km north ofKathmandu (IRIS, 2015).

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10th Asian Regional Conference of IAEG (2015)

sediment. Sedimentary basins can have a large effecton ground motion above them. Earthquake waves
travel at high velocity through the stiff, metamorphic
rock of the valley but slow dramatically whenentering the basin possessing long period-low
frequency nature. This increases the amplitude of theearthquake waves within the basin fill. In addition,
the sharp density contrast of the soft basin rocks withsurrounding material can cause waves to reflect,
trapping energy in the basin for a period of time. Thisextends the duration of shaking (IRIS, 2015).
Being a bowl shaped valley with thick (more than
300 m in many places) lacustrine sediment,
Kathmandu valley is in great risk of earthquake. The
soil type, ground response, building types and soil
depth are not well known to the concerned
governmental agencies and even to the engineers.
Still researches are very few and evidences andgeotechnical clues of the hidden materials beneath thevalley are still not well known. The variation of
predominant frequency (Fig. 5) determined by
Poudyal et al. (2012) for Kathmandu valley fits
approximately with the damage distribution in thevalley after the Gorkha Earthquake. After the Gorkha
Earthquake, the Government of Nepal is realizing aurgent need of detail investigation and research forearthquake risk in the Kathmandu valley.

earthquake, the 1999 Chi-Chi earthquake of Taiwan,
the 2004 Chuetsu earthquake of Niigata Prefecture,
Japan, the 2005 Kashmir earthquake of Pakistan andthe 2011 Sikkim-Nepal Earthquake of Nepal also
indicate the occurrence of intense shaking in elevatedridges of rugged topography.
Geli et al. (1988) have reported that buildings on
crests suffer more damage than those located at thebase and they conclude that there is always
significant
amplificationoffrequencies
corresponding to wavelengths about equal tomountain width at hilltops with respect to the base. In
the case of the Gorkha Earthquake, it is quite wellobserved in Chautara town in Sidhupalchowk District.
Many houses on the ridges were damaged and many
cracks were observed on ridge (Photo 1). Similarly,
an amplification-deamplification pattern on slopes
leads to a strong energy differential on the upper partof the slope. For the preliminary site visit of the
Gorkha Earthquake, it is understood that landslide
frequency is much higher on or near the crests of hills
and mountains.

Photo. 1 Old Chautara town on ridge and damaged
houses6.

Three major engineering geological issues
observed after the Gorkha Earthquakes areearthquake-induced landslides, land subsidence andliquefaction. Brief information of each issue aredescribed below.

Fig. 5 Dominant natural period of ground of
Kathmandu from microtremor analysis (after
Poudyal et al. 2012)
5.

Engineering Geological issues

Topographic effect and earthquake damages

6.1 Earthquake-Induced landslide
As in the other part of the world, earthquake
induced landslides after Gorkha Earthquake is also
can be classified in terms of six categories as follow:
1. Rock fall
2. Shallow landslides and dry debris fall
3. Deep seated landslides
4. Debris flow and mud flow
5. Valley fill collapse
6. Cut-and-fill failure
Rock fall were very common along the road sideslope after the Gorkha Earthquake. Especially, Liping

The various investigations showed that
topographic features are basically responsible fordissipation of seismic energy (e.g. Gilbert andKnopoff, 1960) and extremely high accelerations areusually observed at sites located on topographic
ridges (Ambraseys and Srbulov, 1995; Miles and
Keefer, 2000; Lin et al., 2003; Uchida et al., 2004).
Observations of the damage patterns of earthquakes,
such as the 1987 Whittier Narrows, California
earthquake, the 1989 Loma Prieta (California)
earthquake, the 1994 Northridge, California

410th Asian Regional Conference of IAEG (2015)

Bazaar area, near to the Nepal-China border faced
tremendous damages due to rock fall and boulder
falls (Photo 2). Shallow landslides were common
problems in the mountains at northern part ofKathmandu valley. The damages were mainly
concentrated on Dhading, Nuwakot, Rasuwa andSindhupalchowk, Dolakha districts.
Deep-seated landslide is was not much after the
Gorkha Earthquake, but few failures are seen in oldlandslide mass (Fig. 5) in the Rasuwa district, northof Kathmandu.

Photo. 4 Failure in deep-seated old landslide

Photo. 2 Rock fall in Liping Bazar, near to
Nepal-China border

Photo. 3 Shallow failures in Sindhupalchowk
district

Photo. 5 Debris flow and avalanche in Langtang
village, source area also is shown in photo.

Deep-seated landslide is was not much after the
Gorkha Earthquake, but few failures are seen in oldlandslide mass (Photo 4) in the Rasuwa district, northof Kathmandu.
Debris flow and mud flow type ofearthquake-induced landslide was not observed in theaffected area due to dry season in the area, but rockfall triggered debris flow and avalanche was observedin the Langtang valley and killed more than 200
persons on the spot. The Langtang valley wasvanished after this incident (Photo. 5).
Valley fill collapse was observed in many areas,
especially on road. But in Kathmandu such failures
were observed only on Kathmandu airport road andthe road was damaged due to valley collapse (Photo.
6).

Photo. 6 Valley fill collapse in the Kathmandu
airport road.

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10th Asian Regional Conference of IAEG (2015)7.

Concluding remarks

This study evaluates features of the engineering
geological issues in central Nepal after April 25, 2015
Gorkha Earthquake. In fact, this Gorkha Earthquake
illustrates recent scenario of earthquake risk of whole
Himalayan region. Primarily, this preliminary study
concludes that the Nepal Himalaya has great risk ofearthquake-induced slope failures. Especially many
mountainous roads passing through the highly
elevated Himalayan ridges as well as river valley areextensively damaged in this Gorkha Earthquake.
Likewise, the lacustrine deposit of the capital city
Kathmandu has shown different site response
characteristic during the Gorkha Earthquake.
Therefore, the behavior of the surface layer oflacustrine deposit as well as the layer underneathshould be taken into consideration during seismic risk
studies in the Kathmandu valley. Problem ofliquefaction is well observed in the lacustrine
sediment and river channel deposit. For the futureplanning of the Kathmandu city, liquefaction hazard
needs to evaluate with recently developed
geoscientific tools. Topographical effect onmountainous settlement during earthquake is anothersignificant problem which need to response throughsuitable structural design of rural houses and otherrural infrastructures.

Photo. 7 Failure in Cut-and-fill part of rural roadCut-and-fill failures were very common in
mountain roads in the affected area. Cut-and-fill land
were also damaged in settlement area of Kathmandu(Photo 7).
6.2 Land subsidence in Kathmandu
The Gorkha Earthquake was destructive not only
due to structural weakness of the construction but
there were many problem of differential settlement of
the ground. Field visit during rapid damage
assessment in Kathmandu valley suggested thatnearly 40% of damages were due to land subsidence.
Similarly, land subsidence due to liquefaction oflower sandy strata was also another problems
observed in various location of the Kathmandu valley(Photo. 8).

% Passing

Photo. 9 Sand boiling site at southern Kathmandu
valley (Bagdol area)
100
Photo. 8 Land subsidence and tilting of house due
to liquefaction of lower strata
6.3 Liquefaction in Kathmandu
Along with land subsidence, a few sand boils
were appeared in Kathmandu due to liquefaction of
the lower strata (Photo 9). Although houses were notsubmerged inside the liquefied layer, but tilting androad collapse were appeared in many area ofKathmandu. Sand boils were appeared in southernpart of the valley, mainly on the old river channel.
The percentage of sand is very high in the sample
taken from sand boil area (Fig. 6) shown in Photo 9.

90
80
70
60
50
40
30
20
10
0
0.01

0.1

1

Diameter, mm

Fig 6 Grain size distribution curve of soil
sample from sand boil
Acknowledgements
I would like to acknowledge Dr. Manita Timilsina

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10th Asian Regional Conference of IAEG (2015)

and Mr. Saurav Neupane for their cooperation in field
investigation and data collection.

Valley using microtremor analysis, Earthq Eng &Eng Vib, Vol. 11, pp. 257-268.
Rai, S.M., Upreti, B.N., Guillot, S., Pêcher, A. andFort, P.L. (2004): Mineral chemistry (biotite,muscovite, garnet, and plagioclase) in the
Kathmandu and Gosainkund regions, central
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Seeber, L. and Armbruster, J. (1981): Great
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Uchida, T., Kataoka, S, Iwao, T., Matsuo, O., Terada,
H., Nakano, Y., Sugiura, N. and Osanai, N.
(2004): A study on methodology for assessing thepotential of slope failures during earthquakes.
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USGS (2015): The April-May 2015 Nepal
Earthquake Sequence, document available in
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Zurick, D., Pacheco, J., Shrestha, B. and Bajracharya,
B. (2005): Atlas of the Himalaya, International
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