Investigation of Damage in and Around Kathmandu Valley Related to the 2015 Gorkha, Nepal Earthquake and Beyond
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An earthquake with a magnitude of 7.8 (Mw) occurred at 11:56 NST (local time) on 25 April 2015, in the central part of Nepal (Gorkha).We organized a damage survey team and dispatched it to the affected area for several periods following the earthquake (May 26 to June 3: first trip, June 17 to 24: second trip, August 16 to 21: third trip and October 27 to November 2: forth trip) to investigate the damage and collect data. We found traditional construction methods are stronger than imagined. Many traditional earthquake-resistance technologies exist in Nepal. The first and second surveys were to collect timely statistical information on the damage to brick and stone masonry buildings and to confirm the availability of data and their sources for subsequent surveys. We also carried out a first-hand building damage survey in selected areas. The investigation of the strong-motion data set from the USGS Center for Engineering Strong Motion Data includes information from stations in Nepal that continued to function throughout the main shock and the several subsequent strong aftershocks of the 2015 earthquake. The third and fourth surveys were to collect the every building damage survey in selected areas. The motivation behind the survey was to obtain ground truth data for the calibration and improvement of a wide-area damage estimation system that uses satellite data; the system is currently under development by National Research Institute for Earth Science and Disaster Prevention (NEID) and the Japan Aerospace Exploration Agency (JAXA). A survey of the degree of damage was conducted for every house in Sankhu and Khokana by the European Macroseismic Scale (EMS) -98. This report outlines the findings of this investigation team into various aspects of the earthquake disaster in the Kathmandu Valley. The motivation behind the survey was to obtain ground truth data for the calibration and improvement of a wide-area damage estimation system that uses satellite data. Field surveys confirmed that the severely damaged urban area was well detected by the decrease derived from the ALOS-2 satellite SAR data. The higher classification accuracy for non-damaged area helps to detect the damaged urban area using this technique, immediately after a disaster.
KeywordsGorkha Nepal earthquake Kathmandu Masonry Ground truth
An earthquake with a magnitude of 7.8 (Mw) occurred at 11:56 Nepal Standard Time (NST), (local time) on 25 April 2015, in the central part of Nepal (Gorkha). The epicenter was east-southeast of Lamjung, 77 km south-west of Kathmandu, 28.15 at the north latitude and 84.71 at the east longitude, and the depth was 15 km (USGS). According to the statistics by The Nepal Police on 22 June the number of deaths 8660 and injured 21,952 for the main shock and deaths 172 and injured 3470 for the aftershock. It was also reported that >5,000,000 buildings and houses were damaged and about half those which of had collapsed. This earthquake was officially named as The 2015 Gorkha, Nepal earthquake, since the hypocenter was located in the Gorkha region.
A major aftershock with a moment magnitude of 7.3 (Mw) occurred at 12:51 NST on 12 May 2015. The epicenter was 75 km north-east of Kathmandu and near the Chinese border, 27.82 at the north latitude and 86.08 at the east longitude, and the depth was 19 km (USGS).
We surveyed to the affected area during 26 May to 3 June and 17 to 24 to investigate the damage and collect data. The findings of this investigation undertaken by this team on the various aspects of the earthquake disaster in the Kathmandu valley. Third damage survey and fourth damage were conducted for every house in Sankhu on 17–18 Aug/29 Oct 2015 and in Khokana on 19–20 Aug/30–31 Oct 2015.
We found traditional construction methods are stronger than imagined. Many traditional earthquake-resistance technologies exist in Nepal. This is an important factor in maintaining traditional construction methods to preserve such technologies.
The maintenance of traditional buildings contributes not only to the maintenance of world heritage but also improvements in the earthquake resistance of cities. The problem of the masonry structure and in maintenance is shown in this study.
Surveys of building types and damage extent were conducted, for every house in Sankhu and Khokana, Kathmandu, after a second survey term. For building types, we used building classification surveys by the Japan International Cooperation Agency, commonly called JICA (2002). Building damage magnitudes were classified using European Macroseismic Scale (EMS)-98 (Grünthal 1998). A high-resolution image from March 12 (before the disaster) was obtained from Google Earth™ prior to the survey. This image was used to identify the position of each building before the earthquake, based on which the damage to each building was estimated in accordance with EMS-98.
With the above approach, we determined damage in Sankhu and Khokana related to the 2015 Gorkha Earthquake in core areas, using the damage function indicated by JICA (2002) .
2 Earthquake Recorded in Kanti Path (Katnp), Central Kathmandu
In Fig. 1, the vertical velocity motion waveform has two pulse-like ground motions. The main parts of the velocity waveform can be seen centered at two points: 45.08 and 53.07 s. The difference between the rupture start time and arrival time for the S-wave is 8 s. The dominant period of the body wave is about 4 s.
These sizes can be estimated for each strong motion generation area (SMGA) by direct interpretation of body waves.
Thus, two SMGA might existed near the city of Kathmandu.
3 Damage in Kathmandu Valley
Sankhu is an old town and locating on a small hill in the north east part of the Kathmandu Valley. Houses damaged by the earthquake have been demolished. In general, RC buildings were partially damaged, whereas masonry buildings were severely damaged.
4 Traditional Construction Methods in Kathmandu Vally
4.1 Chowks, a Type of Courtyard
The traditional method of brick making is stronger than imagined. It is considered that buildings surrounding a courtyard have high rigidity. This is important to save the lost courtyard Fig. 8 shows typical courtyard in Lalitpur and Bhaktapur.
4.2 Typical Masonry Building Four-Story Structures
4.3 The Timber Repair
4.4 The Latticed Window and the Outer Frame Effect
5 Manintenance Technology
5.1 Roof Repair
5.2 Renovation of the Royal Palace
6 Building Types in the Kathmandu Valley
Brick with mud mortar, poorly built
Brick with mud mortar, well built
Brick with cement or lime mortar
Reinforced concrete (RC) frame with masonry of four stories or more
RC frame with masonry of three stories or less
BC and BM
BC buildings, also constructed in the past 30 or 40 years, comprise about half the buildings in the valley. This type of building is still weak regarding horizontal rigidity, owing to poor workmanship and lack of structural consideration of the joints from wall to wall, wall to wooden floor and roof, and non-integration of the masonry wall itself. Although buildings of this type that are less than four stories are generally constructed with suitable workmanship and adequate wall balance, those more than four stories tall have great fragility during a powerful earthquake.
AD and ST
AD and ST buildings have great fragility during a moderate earthquake.
7 Classification of Damage to Masonry Buildings
Classification of damage to masonry buildings by EMS-98 (Grade1–5) (Grünthal 1998)
Classification of damage to masonry buildings
Grade 1: Negligible to slight damage (no structural damage, slight non-structural damage)
Hair-line cracks in very few walls. Fall of small pieces of plaster only. Fall of loose stones from upper parts of buildings in very few eases
Grade 2: Moderate damage (slight structural damage, moderate nonstructural damage)
Cracks in many walls
Fall of fairly large pieces of plaster
Partial collapse of chimneys
Grade 3: Substantial to heavy damage (moderate structural damage, heavy non-structural damage
Large and extensive cracks in most walls. Roof tiles detach. Chimneys fracture at the roof line; failure of individual non-structural elements (partitions, gable walls)
Grade 4: Very heavy damage (heavy structural damage, very heavy non-structural damage)
Serious failure of walls; partial structural failure of roofs and floors
Grade 5: Destruction (very heavy structural damage)
Total or near total collapse
- Grade 5
Very heavy structural damage
- Grade 4
Very heavy structural and non-structural damage
- Grade 3
Moderate structural damage and heavy non-structural damage
- Grade 2
Slight structural damage and non-structural damage
- Grade 1
No structural damage and slight non-structural damage
Collapsed or un-repairable (unsuitable for living)
Collapsed or un-repairable (unsuitable for living) + 1/2 repairable (available for temporary living)
This standard is difficult to correspond with Nepalese building conditions. Figure shows the building state of the site after the earthquake. In this study, Grade 5 or upper Grade 4 was determined by complete or non-complete collapse.
Grade 3 or upper Grade 4 was determined by moderate structural damage or not enough. A grade >3 was not determined for ground truth distinction by a satellite image, and was assigned a corresponding category.
7.1 Survey of Building Damage in Sankhu Core Area
This town was the ancient trade route with Tibet. Many Tibetan is living even at present. However these, buildings lose a source of income by present and decline, and are superannuated, and a renovation is not performed.
The difference in damage between the various building types is remarkable. Generally, the damage in Sankhu was extensive. Brick and cement mortar houses without RC columns had considerable damage, whereas damage to RC structures—particularly those erected in recent years—was generally minor. These latter structures were mainly five- to six-story buildings, whereas many of the non-engineered masonry structures that experienced complete collapse or partial damage had two to four stories.
7.2 Survey of Building Damage in Khokana Core Area
Khokana is a traditional old town on a small hill in the southwestern Kathmandu Valley. It is a municipality in the Lalitpur District of Bagmati Prefecture, central Nepal, ~10 km south of the city of Kathmandu.
According to an interview survey of the inhabitants, all buildings collapsed during the 1934 Bihar Earthquake in Khokana, except for one, which is being renovated. Thus, the historical buildings are <80 years old. Well-built historical buildings had expensive improvements for earthquake resistance, which included mixing plaster with mud joints.
7.3 Survey of Damage for Every House in Sankhu and Khokana
NIED and JAXA surveyed of damage for every house in Sankhu and Khokana (Ohsumi et al. 2015; Watanabe et al. submitted). A survey of the degree of damage was conducted for every house in Sankhu and Khokana by the European Macroseismic Scale (EMS)-98.
Khokana was once Newari village and has retained its tradition and culture as a World Heritage Site in Nepal. A damage survey was conducted for every house in Khokana on August 19–20, 2015 (Fig. 21b).
We performed a helicopter and unmanned aerial vehicle (UAV) photogrammetry survey in Sankhu and Khokana, to map buildings and damage related to the 2015 Nepal Earthquake. NIED and National Society for Earthquake Technology (NSET) Nepal will use the data for research on building damage distribution, calibration of satellite imagery by ground truth data, and more detailed risk assessment of cities and rural communities as a survey in addition to the ground truth survey.
3D digital surface models of buildings will be created to measure building heights and shapes. Raw photos with oblique views can be used to investigate damage in more detail and structures. This technique was applied to the helicopter and UAV investigation. The helicopter flight was used for a photogrammetry survey of Sankhu on 20 Aug 2015. We used a helicopter (Bell Jet Ranger 206B).
The UAV flight was used for a photogrammetry survey of Khokana on 23 Nov 2015. We used a compact UAV, a battery-powered aircraft with onboard digital camera in auto-pilot mode to take aerial photos from 50 to 150 m above ground. The craft cruises at a speed ~60 km/h for about 20 min, covering an area of 2 km2 in one flight. Pictures are processed by photogrammetry software to create Orthomosaic photos for mapping. Safety of the flights and compliance with regulations were of primary importance. Our fixed-wing foam aircraft is much safer than popular multi-rotor drones. It will not injure people if it crashes because: (1) the fuselage is made of soft Styrofoam; (2) the propeller is rear-facing; and (3) it can glide when falling. Autopilot flight can be programmed to avoid prohibited zones and is above the height limit directed by the Civil Aviation Authority.
EMS level for each ratio and building type in Sankhu
EMS level for each ratio and building type in Khokana
The total number of buildings for each area are shown in the upper stage, and the percentages of each item are shown in the lower stage. There was no damage to 94 % of the surveyed RC buildings. From the comparison of BM and BC, the collapse ratio was improved by 27 % in Sankhu and 16 % in Khokana for BC houses. There was 0 % BM Well with very heavy structural damage in both areas.
7.4 Completion with Fragility Curves
In the aforementioned JICA study, fragility curves for buildings in the valley were adjusted by calibrating existing fragility curves for Indian buildings from Arya (2000) and those for West Nepal from a UNDP/UNCHS (1994) study. For this calibration, damage from the 1988 Udayapur earthquake cited in Murakami et al. (1990) and Dikshit (1991) were analyzed. These papers present very large seismic intensity distributions compared to the expected peak ground accelerations. We reanalyzed the intensity considering building weakness in our damage area, most of which are AD or poor BM. Fragility curves of the relationship between damage rate and peak ground acceleration for each building type are shown in Fig. 23.
In the 2015 Gorkha Earthquake, there were no acceleration observation points in Sankhu and Khokana. The USGS maximum acceleration record was 160 Gal at the US embassy in the city of Kathmandu. The distance between that embassy and Sankhu is ~15 km, and that between the embassy and Khokana is ~5 km. According to Takai et al. (2015), the main shock in the observation results was 151 Gal at Lalitpur and 146 Gal at Thimi. Estimation of main shock acceleration values was 150 Gal in Sankhu and Khokana.
Damage ratio of RC is 7–8 % and damage ratio of BM is the 20 %.
Fragility curves (JICA, 2002) and damage and collapse rate
BM is composite structure including the sun-dried brick (AD). The building classification is different in ages and referred earthquakes (i.e., the 1934 Bihal Earthquake, the 1988 Udayapur Earthquake).
The complicated composite structure is being classified in BM by the damage function. On the other hand, RC is actual damage rate and the damage function rate are good agreement (Table 4).
7.5 SAR Analysis
7.5.1 Reason for Slight Damage at Kirtipur
The epicentral distance of the Kirtipur core area was 79.74 km, compared with 80.69 km for the epicentral distance of the Khokana core area. Both core areas are old traditional towns on small hills in the southwestern Kathmandu Valley. According to Takai et al. (2015), acceleration was amplified in Patan (Khokana) located in the Kathmandu Valley. In contrast, acceleration was less amplified in Kirtipur, which is located on outcropping basement rock units outside the valley.
8 After the 2015 Gorkha, Nepal Earthquake and Beyound
The recovery and reconstruction processes following the 2015 Gorkha, Nepal Earthquake are ongoing. The damage to RC structures was generally minor. Many of the non-engineered masonry structures that experienced complete collapse or partial damage were two- to four-story buildings. Although many of the masonry structures will be reconstructed as RC structures, traditional building methods should be sustained by traditional communities not only to conserve world heritage but also to improve the earthquake resistance of cities. In the following subsections we present a method for the constructing earthquake-resistant buildings using traditional methods.
8.1 Stabilized Mud Mortar with Lime
8.2 Horizontal Timber Beam Reinforcement
A number of documents discussing improvements in seismic-resistant construction methods for masonry structure were prepared under the National Building Code Development Project (NEP/88/054/21.03) in 1993.
- NBC 202
Mandatory Basic Rules for Load Bearing Masonry
- NBC 203
Guidelines for Earthquake Resistant Building Construction: Low Strength Masonry
- NBC 204
Guidelines for Earthquake Resistant Building Construction: Earthen Buildings (EB)
Traditional construction methods should be sustained by traditional communities not only to conserve world heritage but also to improve the earthquake resistance of cities. Technologies for the construction of earthquake-resistant buildings using traditional methods such as stabilizing buildings using mud mortar with lime and horizontal timber beams are both effective and sustainable.
RC buildings were partially damaged, and many masonry buildings were severely damaged. The difference in damage extent between the various building types was remarkable.
Along the west side of the Kathmandu Ring Road, RC frame buildings were damaged in some cases by during the 2015 Gorkha, Nepal Earthquake. The building damage might be caused by the soft ground conditions near the river branch as well as inappropriate construction methods.
BM buildings survived in urban and rural areas. These buildings have very poor horizontal rigidity because of low bond strength and strong moisture absorption in mud joints, wooden floors and roofs.
Damage extent and building type classification were surveyed for every house in Sankhu and Khokana. There was no damage to 94 % of surveyed RC buildings. The collapse ratio was improved using BC relative to BW.
Some buildings survived the 1934 Bihar Earthquake and the 2015 Gorkha Earthquake in Sankhu. These building structure types were classified as well built, using a type of brick with mud mortar.
For RC, the actual damage rate and damage function rate were in good agreement. The damage curve of BM exceeded that of ST and AD.
Tradition chowks are observed to have excellent earthquake resistance. However, chowks that have been altered and/or changed were damaged.
Comparison of the coherence magnitude with a survey of the damage extent for every house in Sankhu and Khokana detected damage well. Greatest damage correlated with the decreases of coherence (γ) in SAR imagery observed before and after the disaster. The brightest reflections occurred for severely damaged core areas, such as those in Khokana and Bungamati. Regions of less-bright reflections, such as those observed in Kirtipur, corresponded to only slightly damaged core areas.
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