Dedicated to the people who lost their lives and injured by the 2015 Gorkha Earthquake

A QUICK REPORT

ON

THE 2015 GORKHA (NEPAL) EARTHQUAKE

AND

ITS GEO-ENGINEERING ASPECTS

*mer AYDAN

University of the Ryukyus, Dept. of Civil Engng. & Architecture, Okinawa, JAPAN

*Resat ULUSAY

Hacettepe University, Dept. of Geological Engng., 06800 Beytepe, Ankara, TURKEY

(* Member of IAEG)

INTRODUCTION

The Gorkha (Nepal) earthquake (Mw7.8) occurred at 11:56 NST on 25 April 2015 with an

epicentre 77 km (48 miles) northwest of Kathmandu, the capital city of Nepal, that is home to

nearly 1.5 million inhabitants, and at a focal depth of approximately 10-15 km (Figure 1). This

earthquake was the one of the most powerful earthquakes to strike Nepal since the 1934

Nepal-Bihar earthquake (Mw8.1). Based on the information by the United Nations, eight

million people have been affected by the massive 2015 earthquake in Nepal, more than a

quarter of the Nepals population.

Figure 1. Map showing the epicentre of the 2015 Nepal earthquake

The April 25, 2015 earthquake occurred about 200 km west of the 1934 earthquake. Nepal,

which constitutes a part of Himalaya region, was also hit by other earthquakes occurred in

1964, 1988 during the instrumental period. The region belongs to Himalaya Arc, which was

suffered very large earthquakes with a moment magnitude of 7.5 or more in 1100, 1505, 1555,

1724, 1803, 1833, 1897, 2005, 1947, 1950, 2005, 1833 during the instrumental period and

historical period, respectively (Bilham, 2004, 2009; Bilham et al., 2001).

The earthquake mainly resulted in about loss of more than 7400 people as of May 4, 2015 and

it may rise again in the coming days as collapsed and heavy damaged structures cleared away

(Figure 2). Particularly in Nepal, historic buildings and temples were destroyed, leaving massive

piles of debris in streets.

Figure 2. Damage to buildings and historical monuments

This earthquake induced many mass movements in mountainous areas and resulted in

landslide lakes, which could be another cause of secondary disasters. The mass movements

and deformation of weathered soft soil cover are the main causes of the collapse or heavy

damage to buildings and heavy casualties in mountainous areas (Figure 3a). In addition, the

earthquake also triggered a major avalanche on the south slopes of Mt. Everest, located

approximately 160 km east-northeast of the epicentre. The avalanche destroyed the base

camp of climbers. According to reports, the avalanche killed at least 17 people and injured 61

others. The earthquake also triggered avalanches in Himalayas, killing some people. Some

other adjoining countries such as India, China and Bangladesh were also affected by the

earthquake with causalities (Figure 3b).

Figure 3. Collapsed buildings due to surficial plastic deformation of slopes and the avalanche at

Mt. Everest

GLOBAL AND REGIONAL TECTONICS OF NEPAL AND ITS CLOSE VICINITY

Apparently, about 225 million years ago, the Indian continent was a large island situated off

the Australian coast. A vast ocean called the Tethys Sea separated the Indian continent from

the rest of the Asian continent. Later when Pangea began to break apart, India began to move

northward. About 80 million years ago, India was located just south of the Asian continent,

moving northward at a rate of about 9 m a century. Eventually India collided with Eurasia

about 40 to 50 million years ago, and its northward advance slowed by about half. The

Himalayas are also in continuous motion. Himalaya mountain range constitutes the northern

plate boundary of the Indian plate. Chaman fault in the west and Sagaing fault in the east is

the transform plate boundaries. While Chaman fault is a sinistral fault, the Sagaing fault is a

dextral fault. The indentation of the Indian plate into Euroasia resulted in the formation of

Altyn Tagh and Karakorum faults in the central Asia (Figure 4).

Himalayan Frontal Thrust (HFT), the Main Boundary Thrust (MBT) and Main Central Thrust

(MCT) are the main faults in the region and they are part of the Great Himalayan range (Figure

5). Presently the main tectonic displacement zone is the Himalayan Frontal Thrust Fault (HFTF)

System, which comprises Himalayan Frontal Fault at the edge of the Indo-Gangetic plains, and

several active anticlines and synclines to the north. The Himalayan front in the western Nepal

is characterized by several discontinuous segments of the HFT and its subsidiary faults (Figure

6).

Figure 4. Tectonic features of Indian plate and its close vicinity (Aydan, 2006)

Figure 5. Main tectonic elements in the earthquake affected area (from Sapkota et al., 2013)

Nepal has been divided into three major tectonic zones (Figure 6), namely, Main Central Thrust

(MCT), Main Boundary Thrust (MBT) and Himalayan Frontal Fault (HFF) (Piya, 2004). According

to Nakata and Kumahara (2002), many active faults are distributed along the major tectonic

boundaries. These faults were produced by the collision of the Indian Plate with the Eurasian

Plate. A cross-section of the Nepal Himalayas running from SSW-NNE is shown in Figure 10.

Saijo et al. (1995) and Yagi et al. (2000) reported the existence of active faults in the SW part

the Kathmandu basin and they are cutting the Late Pleistocene sediments and have a vertical

displacement rate of 1 mm/yr.

Figure 6. Distribution of the active faults in and around Nepal Himalaya (from Nakata and

Kumahara 2002)

Crustal deformation measurements have been carried out to observe the motions of crustal

plates by International GPS service. Although some local GPS networks are used in both

Pakistan and India, the measurements at GPS stations are not always continuous. A rough

estimation of the crustal straining in the vicinity in the Indian Plate and close vicinity was

carried out using the measured annual deformation rates of GPS stations, namely, BAHR

(Bahrain), IISC (India), KIT3 (Uzbekistan) and LHAS (Tibet) by Aydan (2006). The strain rates of

the elements are given in Table 1. The annual deformation rates and strains are shown in

Figure 7. As noted from the computational results, the principal direction of crustal strain is

NE-SW and it ranges between 12 and 28 degrees from north.

Table 1. Computed annual strain rates

Element 1

( years / ) 3

( years / )

(radian)

1 7.45044 -20.1116 -50.0149E-02

2 8.73246 13.3154 -22.4190E-02

Figure 7. Crustal deformation rates and strain rates in the Indian plate and its close vicinity

(from Aydan, 2006)

GENERAL CHARACTERISTICS OF THE EARTHQUAKE

The earthquake occurred in Gorkha district near the village of Barpak, which was completely

destroyed by the earthquake. The distribution of aftershocks, which extend up to 130 km to

the east of the epicenter, suggests that the rupture have propagated from west to east,

potentially leading to more severe destruction in Kathmandu. Most of the aftershocks were at

the relatively shallow depth of less than 15 kms below the Earth's surface (Figure 8).

Figure 8. The location of the main shock and the distribution of aftershocks (data from USGS

and base map from Google-Earth)

The estimated magnitude of the earthquake varies from 7.7 to 7.9 depending upon the

institutes and USGS assigned the moment magnitude of the 2015 Gorkha (Nepal) earthquake

as 7.8. The epicentre of the earthquake estimated from the different institutes is located in

Gorkha district while the epicentre by Harvard is very close to Kathmandu (Figure 9). The 2015

Nepal earthquake occurred as the result of thrust faulting on or near the main frontal thrust

between the subducting India plate and the overriding Eurasia plate to the north. The rupture

plane strikes parallel to the Himalayan Belt WNW to ESE, and dips with 11 to the North. The

rupture duration and relative slip range between 45-60 seconds and 4-5 m. The estimated

length, slip and rupture duration of the earthquake fault for a moment magnitude of 7.8 are

132 km, 6 m and 67 seconds from the empirical relations developed by Aydan (2007, 2012),

respectively. The preliminary location, size and focal mechanism of the April 25 earthquake are

consistent with its occurrence on the main subduction thrust interface between the India and

Eurasia plates (i.e. Bilham et al., 2001; Bilham, 2004, 2009).

Figure 9. Focal plane solutions, depth and magnitude of the 2015 Gorkha earthquake (from

EMSC, 2015) (Red line corresponds to the causative Himalaya Frontal Thrust Fault (HFTF))

(www.emsc-csem.org, EMSC, 2015)

There are several preliminary intensity maps based on the accessible districts. According to

these preliminary intensity maps on the MMI scale, intensity VIII was reported for Kathmandu

(Figure 10). However, it is very likely that it may be up to IX-X in Gorkha district when the

information becomes available from presently inaccessible regions. For example, the aerial

photos imply that the Barpak village was completely destroyed and this may imply that the

intensity may be almost X on MMI scale.

Figure 10. Intensity map for the 2015 Nepal earthquake (www.mapsoftworld.com, 2015)

The strong motion network of Nepal is quite limited. Nevertheless, there are strong motion

stations at Gorkha, Kathmandu and Everest in the earthquake-affected area. However, the

records at these stations are not available yet. Nevertheless, the Kanti-Path (Kathmandu)

recorded the maximum ground acceleration of 0.164 g. It was noted that the record was

dominated by the long-period components of acceleration, which may be affected by the soft

sedimentary basin effects on the duration and amplification of shaking in Kathmandu Valley.

The USGS preliminary estimation of the maximum ground acceleration (PGA) in the epicentral

area was about 0.35g and 0.1-0.15g for Kathmandu as shown in Figure 11. The authors tried to

estimate the ground motions using the empirical relations developed by Aydan (Aydan and

Ohta, 2011; Aydan, 2007, 2012) as shown in Figure 12. The estimations shown in Figure 12 are

for base accelerations and the estimated ground acceleration are high at the epicentral area.

Figure 11. Maximum ground acceleration (www.usgs.gov; USGS, 2015)

Figure 12. Maximum ground acceleration based on the relations of Aydan (2007, 2012)

Although the available data are quite limited, an attempt was made on the attenuation of

maximum ground accelerations. Figure 13 shows the attenuation of strong motion together

with observed and inferred data from the collapsed or toppled structures. The data is roughly

consistent with available empirical relations proposed by various researchers.

Figure 13. Comparison of various empirical relations for attenuation of maximum ground

acceleration with observed and inferred data

India and China also have their own strong motion networks. It was reported that none of the

strong motion sensors of the Indian strong motion network was triggered due to poor

maintenance of the network sensors. There is no data from the network of China as it is not

open to international access, which happened to be the case in Wenchuan earthquake also.

BRIEF GEOLOGY OF NEPAL AND KATHMANDU AND SURROUNDINGS

Nepal is located in the centre of the Himalayan concave chain, and is almost rectangular in

shape with about 870 km length in the NWW-SEE and 130-260 km in N-S direction. The Main

Frontal Thrust (MFT) system consists of two or three thrust sheets composed entirely of

Siwalik rocks, from bottom to top mudstone, multi-storied sandstone and conglomerate

(Chamlagain and Gautam, 2015). These sedimentary foreland basin deposits form an archive of

the final stage of the Himalayan upheaval and record the most recent tectonic events in the

entire history of Himalayan evolution since ~14 Ma. The northernmost thrust sheet of the MFT

is truncated by the Lesser Himalayan sequence and overlain by unmetamorphosed to weakly

metamorphosed rocks of the Lesser Himalaya, where the Lesser Himalayan rock package is

thrust over the Siwalik Group along Main Boundary Thrust (MBT). In western Himalaya

crystalline thrust sheets are frequently observed within the Lesser Himalaya (LH). The Lesser

Himalayan zone generally forms a duplex above the mid crustal ramp (Schelling and Arita,

1991; Srivastava and Mitra, 1994; Decelles et al., 2001). The Main Central Thrust (MCT) system

overlies the Lesser Himalayan MBT system and was formed in ca. 24 Ma. This MCT system

consists of high-grade rocks, e.g. kyanite-sillimanite gneiss, schist and quartzite and is mostly

characterized by ductile deformation.

Figure 14. Geological map of Nepal (after Upreti and Le Fort, 1999) (LH: lesser Himalaya; HH:

higher Himalaya; TTS: Tibetan-Tethys sequence; STDS: South Tibetan detachment

system)

The Kathmandu valley, where the 2015 Nepal earthquake caused heavy damage, comprises of

thick semi-consolidated fluvio-lacustrine Quaternary sediments on the top of basement rocks

(Figure 15). Piya (2004) reports that the maximum thickness of the valley sediments reaches

up to 550 m at the central part of the valley and the basement rocks composed of Precambrian

to Devonian rocks, such as limestone, dolomite, slate, marble, schist, meta-sandstone, phylitte,

quartzite. The shear wave velocity of the soft sedimentary deposits ranges between 167 m/s

and 297 m/s and ground amplification may be ranging between 1.9 and 7.9 according to

Chamlagain and Gautam (2015).

Figure 15. (a) Geological map of the Kathmandu valley, (b) schematic geological cross-section

along N-S (after Sakai, 2001)

GEO-ENGINEERING ASPECTS OF THE EARTHQUAKE

(a) Mass Movements (Landslides, Slope Failures)

The epicentral area is very mountainous and valleys are steep. Furthermore, sedimentary rocks

are heavily folded and faulted resulting from the tectonic movements and subjected to

weathering due to intense freezing-thawing cycles as well as water-content variations. The

mass movements are quite similar to those observed recently in the 2005 Kashmir earthquake

and 2008 Wenchuan earthquake (Aydan et al., 2009a, 2009b, 2009c). According to satellite

imagery and aerial photographs (ReliefWeb, NASA, Mass media reports), huge mass

movements were caused by the earthquake (Figure 16). The common forms of mass

movements can be categorized as surficial plastic deformations of top soil or weathered zone,

planar and wedge sliding and flexural or block toppling. When these mass movements are of

large scale, there is a strong possibility of large valley-blocking mass movements triggered by

the Nepal earthquake in the high mountainous areas. Such debris dams were already spotted

in satellite images (ReliefWeb, NASA and Indian Space Research Organization). The mass

movements were observed in Gorkha district as well as Tibetan side of Himalayas (Figure 16-

19).

Figure 16. A satellite image of mass movements (28.36N;85.35E) (from Indian Space Research

Organization (ISRO): nrsc.gov.in, 2015)

Figure 17. Surficial failure or surficial plastic deformation

Figure 18. Various scale rock slope failures and rock falls

Figure 19. Large-scale mass movements

(b) Liquefaction and Liquefaction Induced Ground Failures

After the devastating 1934 Nepal earthquake, Rana (1935) reported about the occurrence of

liquefaction at some of the places in the Kathmandu valley in his book entitled Great

Earthquake of Nepal. Reports from previous major earthquakes of Nepal, such as the one

from 1934, give evidence that substantial damage to buildings and infrastructures occurred in

Kathmandu valley as a result of widespread liquefaction (Piya, 2004). The comparison between

the two liquefaction susceptibility maps prepared for the Kathmandu valley (Figure 20)

indicates that in this region, a large area is susceptible to liquefaction and its effects.

Although detailed information on the occurrence of liquefaction during this earthquake has

not been reported yet, CEDIM (2015) reports that besides the impact caused directly by

ground shaking, secondary effects like liquefaction posed an additional threat. Some pictures

from Kathmandu clearly confirmed that liquefaction did occur in Kathmandu (Figures 21 and

22). In addition, ground liquefaction did occur in even Bihar region of India.

Figure 20. Comparison between the liquefaction susceptibility map prepared by JICA (2002) for

the 1934 Nepal earthquake and that prepared by Piya (2004) for the Kathmandu

valley

Figure 21. Views of liquefaction induced by the 2015 Gorkha earthquake

Figure 22. Location of liquefaction observed in Kathmandu on the liquefaction susceptibility

map by OCHA, 1993)

(c) Roadway Damages

The roadway embankments in Kathmandu City suffered some damage in the form of

subsidence and lateral spreading as seen in Figure 23. As Kathmandu City is on the hanging-

wall side of the earthquake fault, permanent deformations resulting from the faulting may

cause some damage. Nevertheless, the sedimentary deposits beneath Kathmandu City are

more than 500 m, surface ruptures likely to be diluted within the sedimentary deposits. It is

very likely that improper compaction, lateral spreading of sidewalls and subsidence of

saturated soil beneath the embankments would be the potential causes as expected from

various geotechnical investigations in Kathmandu.

(d) Damage to Bridges

There is no report yet about the damage to bridges. Figure 24 shows two bridges. The bridge in

Kathmandu City is a two span-simply supported reinforced concrete structure. The bridge

appears to be functional despite a nearby reinforced concrete building collapsed in a pan-cake

mode. Bridge in Gorhka district is a single-span truss bridge. This bridge is also non-damaged

despite it was located in the epicentral area.

Figure 23. Damage and settlement of roadway embankments

(a) Kathmandu (b) Gorhka district

Figure 24. Views of bridges in Kathmandu and Gorhka district

(d) Utility Poles

Utility poles are either made of reinforced concrete or steel. The damage to these utility poles

was generally caused by the collapse of nearby buildings. However, some reinforced concrete

utility poles having rectangular cross section were toppled down during earthquake shaking.

Some corrosion of reinforcement bars was noticed in the poles collapsed due to shaking.

Figure 25 shows some examples of damage to utility poles in Kathmandu City.

Figure 25. Damage to utility poles in Kathmandu City

(e) Damage to Dams

Nepal has been trying to improve the energy shortage by building dams along major rivers.

However, these dams along major rivers are all located within the earthquake-prone areas.

Mass media reported that two Chinese workers were killed by the falling rocks at the

construction site of the Rasuwagadhi hydropower dam, on the upper reaches of the Trishuli

River, a tributary of the Narayani. There is no report of damage by the earthquake at other

hydropower dam sites, yet.

STRUCTURAL DAMAGES

(a) Building Damages

The shallow depth of the quake and the nature of Kathmandu valley have contributed to the

high losses in the capital of Nepal. However, it should be noted that the quality of construction

and materials of buildings is very poor. Many recently built reinforced concrete structures

failed in a pan-cake mode due to improper column-beam connections (Figure 26). Furthermore,

many brick structures collapsed or heavily damaged due to the use of poor mortar (mainly

earth) material and tie-beams and slabs within the walls (Figure 27). The walls of houses were

built as dry-masonry and their resistances are mainly due to frictional forces. In addition,

plastic deformation of their foundation on sloping ground due to ground shaking was another

cause of collapse and heavy damage as seen in Figures 2 and 17. Although Nepal has been

trying to improve its safety and infrastructure by updating building codes for more than two

decades, the efforts were not sufficient.

Figure 26. Views from RC building damages caused by the 2015 Nepal earthquake

(b) Damage to Monuments

Historical monuments as well as religious structures associated with budism suffered

tremendous damage induced by the 2015 Gorhka earthquake. Most of these structures are of

masonry type using bricks and earth-mortar as a bonding-agent. Figure 28 shows the damage

to major historical momental structures in Kathmandu. Among them Dharahara Tower, which

was 9 story high and built in 1832, was completely destroyed by the earthquake as seen In

Figure 28a. Similarly Maju Deval Temple completely collapsed as seen in Figure 28b.

Figure 27. Views from brick masonry building damages caused by the 2015 Nepal earthquake

(a)

(b)

Figure 28. Before and after pictures of Nepal showing the extent of the devastation: (a)

Dharahara Tower, (b) Maju Deval Temple-Durbar Square

ATTENTION

This quick note is prepared with a sole purpose of summarizing the information available from

various sources, which may be useful for damage investigation teams to be dispatched to the

earthquake stricken area. The pictures and figures are obtained from various sources with due

references available in various web-sites. If anything is not referred, it is not done intentionally

and the authors apologize for that.

ACKNOWLEDGEMENTS

The information provided by J. Itoh of Orient Consultants, Tokyo (Japan) is gratefully

acknowledged. The major sources of pictures relevant to the aspects of this document are

obtained from the following web-sites and they are gratefully acknowledged for providing and

sharing the information through images and reports of the earthquake.

Institute or Establishment URL Address (http://www.)

Google-Earth earth.google.com

Google-map maps.google.co.jp/

Kashmir kashmir3d.com

NOAA noaa.gov/

NASA earthobservatory.nasa.gov

UN-ReliefWeb reliefweb.int

EERI eeri.org

PEER peer.berkeley.edu

JSCE jsce.org.jp

AIJ aij.or.jp

JAEE jaee.or.jp

EFFIT istructe.org/knowledge/EEFIT/

MCEER mceer.buffalo.edu

FLICKR (Yahoo) flickr.com

PICASAWEB picasaweb.google.com

MSNBC msnbc.msn.com

USGS earthquake.usgs.gov

EMSC emsc-csem.org

HARVARD globalcmt.org

TU-ERI eri.u-tokyo.ac.jp

IPGP geoscope.ipgp.fr

New York Times nytimes.com

BBC bbc.com

CNN cnn.com

Washington Post washingtonpost.com

National Geographic nationalgeographic.com

Huffington Post huffingonpost.com

Telegraph telegraph.co.uk

UNAVCO unavco.org

Indian Space Research Organization nrsc.gov.in

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