Location of epicentre of the earthquake and its aftershock, Main Central Thrust, Main Boundary Thrust, Main Frontal Thrust and the towns visited in Nepal and India. 

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... The disaster killed more than 8000 people, destroyed about half a million buildings completely and disrupted the road network in the mountainous terrain by surface ruptures and landslides. This communication aims at providing a brief overview of the earthquake and its effects on built environment, especially residential buildings, as observed in the affected areas of Nepal and adjoining Indian states of Uttar Pradesh and Bihar, during the field trip undertaken by the present authors during 3–9 May 2015 traversing over 2200 km. The M 7.8 earthquake of 25 April 2015 struck at 11 : 41 am IST (11 : 56 am local time), with its epicentre located in Gorkha district (28.15  N 84.7  E) in the central Nepal, about 80 km NW of the capital Kathmandu (Figure 1). This event occurred as the result of thrust faulting on or near the Main Himalayan/Frontal Thrust (MFT) interface between the Indian plate and the Eurasian plate 1 . The strong aftershock of M 7.3 occurred on 12 May 2015, 17 days after the main shock, which was located at about 80 km NE of Kathmandu (Figure 1). In Nepal, the earthquake caused unprecedented loss of life and devastation. A large part of northern India, especially eastern Uttar Pradesh, Bihar and north Bengal, also experienced moderate shaking during these earthquakes. The Himalayan region is one of the most seismically active regions in the world producing significant number of earthquakes of M 8.0+ magnitude in the past. The largest M 8.1 event, known as the 1934 Nepal–Bihar earthquake caused widespread damage in Nepal and Bihar, and around 10,000 fatalities were reported. The M 7.8 earthquake was not completely unexpected in the Central Nepal region, as several studies had indicated the likelihood of earthquakes of magnitude greater than 8.0 based on the slip deficit estimation and accumulation of strain energy in the region. This has been anticipated in early 1990s and further confirmed by recent studies 2–7 . As shown in Figure 2, the major part of Nepal, including Kathmandu, lies in zone A on the seismic zoning map of Nepal 8,9 , whereas the districts of north Bihar adjoining the Nepal border lie in zones IV and V on the Indian seismic zone map 10 . The seismic zone A of Nepal is equivalent to the most severe Indian seismic zone V liable to shaking intensity of IX on the MSK scale. The ground motion of the main event was recorded at USGS station KATNP (27.71N, 85.32E) in Kathmandu 11 . The reported values of peak ground acceleration (PGA) and velocity were 0.164 g and 107.30 cm/s respectively (see Figure 3 a for acceleration and velocity time histories). It should be noted that the peak ground velocity is much higher than typically expected for the observed PGA of 0.164 g (ref. 12). This is noteworthy as peak ground velocity is better correlated with the damage statistics of mid- to high-rise buildings and it could have played a significant role in the unexpected degree of damage to many structures 13 . In Figure 3 b , acceleration response spectra of the recorded ground motions are compared with the code- prescribed elastic design response spectrum correspond- ing to zone A of the Nepal seismic code and zone V of the Indian seismic code, for the design basis earthquake (DBE) in soft-soil site. The USGS global V S30 server in- dicates that the central part of Kathmandu valley has soft- soil deposits which are typically NEHRP site class D ( V S30 between 180 and 360 m/s) 14 . It is clear that in the acceleration-controlled regime (i.e. short-period range which is typical for low-rise unreinforced masonry and infilled RC-frame construction), the ground motion has higher acceleration demand than the code-expected demand in the most severe seismic zone. Geologic studies show that the Kathmandu valley is covered by thick semi-consolidated quaternary sediments with a maximum depth of 550 m in the central part of the valley 15 . An ear- lier study on local site amplifications due to unconsoli- dated quaternary sediments of Kathmandu valley has indicated that the resonant frequencies were in the range 0.5 to 8.9 Hz, with the maximum amplification occurring at 0.5 Hz (2 sec) in the central lacustrine area 16 . However, in addition to the amplification at 0.5 sec (2 Hz), unusual higher spectral amplification was observed in the range 3–6 sec (0.17 to 0.33 Hz), which could also be due to the complex influence of underlying unconsolidated quaternary sediments in the basin. Similar basin effect has been observed in some of the past few earthquakes, including the notable 1985 Mexico City earthquake, where the ground acceleration was amplified by about 10 times at 2 sec period due to the presence of lake deposits, which resulted in large devastation even at a distance of 300 km from the epicentre 17 . During 3–9 May 2015, the present authors undertook a reconnaissance survey of the earthquake affected regions and visited (by road) major towns in Bihar (India) and Nepal (visited towns are marked in Figure 1). During the 25 April 2015 earthquake, the Kathmandu valley experienced intensity IX shaking on MSK scale, which left many buildings and temples in ruins. The regions around Kathmandu reported an intensity of VII in Nepal. Kathmandu, with a zone factor 1.0 according to the Nepal seismic code NBC 105, is expected to experience PGA higher than the recorded value 9 . Though it is about 80 km away from the epicentre, it experienced a shaking intensity higher than the regions around the epicentre. From the structural damage evaluation, it has been found that the damage was concentrated in few pockets of the Kathmandu valley such as Khadka Gaon, and banks of Bish- numati River in Machha Pokhari. Similar difference in site responses was observed in Los Angeles, USA during the 1994 Northridge earthquake and in Mexico City during the 1985 earthquake, due to the basin/site effect 18 . The extensive damage in few regions of Kathmandu valley can also be because of the amplification due to the soft-soil deposits, as observed in the response spectra of recorded ground motion (Figure 3 b ). The valley sur- rounded by four mountains is also susceptible for focusing of seismic waves. There could be other factors which have resulted in concentrated damage, but due to the lack of sufficient number of ground motion records, the soil amplification and focusing effect cannot be proven cur- rently. The cultural heritage structures suffered extensive damage during this earthquake. Especially the historical temples and palaces in the urban centres of Kathmandu, Bhaktapur and Patan suffered severe damage (Figure 4). In India, the maximum shaking intensity of VI was observed in some parts of Northern Bihar, and since the intensity of shaking was small (less than VI), even poorly built structures escaped serious damage during this event; however, damages were reported in kaccha houses (non- engineered masonry buildings constructed from stone/ bricks and mud mortar) in Sitamarhi district, north Bihar. In addition, damage to vulnerable, free-standing masonry walls was also reported in parts of Bihar and Uttar Pra- desh. A great majority of buildings affected in the northern region of Bihar are not constructed according to the Indian code of practice, and the presence of serious structural deficiencies makes them highly vulnerable to severe damage under expected shaking intensity of IX (corre- sponding to zone V). This region had already witnessed the maximum shaking intensity of X on Mercalli scale during M 8.1 1934 Nepal–Bihar earthquake which caused widespread damage in the north Bihar districts and lique- faction of soils extending from Motihari to Sitamarhi to Madhubani (a slump belt 300 km long and 60 km wide) 19 . Unreinforced masonry buildings were the most prevalent building type before masonry infilled RC structures became popular in Nepal. The traditional Newari type buildings were made of multi-leaf unreinforced masonry with the outer leaf made of fired clay bricks, neatly finished inner leaf of sun-dried bricks, and rammed earth or random bricks filled in the cavity with no interconnec- tion between inner and outer leaf 20 . The walls were generally thick (450–750 mm), made of clay brick units with thin mud mortar and were unsupported over a large height. Many such 50–60-year-old unreinforced masonry buildings in Bhaktapur were severely damaged not only due to their deteriorated strength but also due to their inherent structural seismic defects. The box-like action achieved by integrating peripheral walls in unreinforced masonry buildings is an important earthquake-resistant feature. The provision of continuous horizontal bands at different levels of the building helps in maintaining structural integrity with all walls and floor diaphragms acting together as a single unit under lateral loads. However, in most of the collapsed buildings, it was observed that there were no horizontal bands connecting the wall units (Figure 5 a ). The cross walls in this type of construction were simply butt-jointed and had no interlocking features, which resulted in their separation by the formation of vertical cracks at the corners (Figure 5 b ). However, according to the present Nepal National Building Code 21 , at the junction of two or more walls, reinforcement in the form of timber or steel should be provided to integrate the box action for the peripheral walls (Figure 6). Due to the absence of positive connection between the walls at the corners and at T-junctions, these behave as free-standing slender walls subjected to large out-of-plane seismic forces due to their heavy mass which often exceeded their capacity. Thus these separated walls were vulnerable to out-of-plane collapse and many failed during the shaking (Figure 7). The floor and roof diaphragms were made of timber joists with timber planks and were embedded in the masonry. Wooden pegs were generally used to prevent the dislodgement of the roof from the wall in ...

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... Earthquake effects, reinforced concrete frame, seismic vulnerability, unreinforced masonry. N EPAL and the neighbouring regions suffered a major earthquake on 25 April 2015, which was followed by strong aftershocks even after a fortnight of the main event. The disaster killed more than 8000 people, destroyed about half a million buildings completely and disrupted the road network in the mountainous terrain by surface ruptures and landslides. This communication aims at providing a brief overview of the earthquake and its effects on built environment, especially residential buildings, as observed in the affected areas of Nepal and adjoining Indian states of Uttar Pradesh and Bihar, during the field trip undertaken by the present authors during 3–9 May 2015 traversing over 2200 km. The M 7.8 earthquake of 25 April 2015 struck at 11 : 41 am IST (11 : 56 am local time), with its epicentre located in Gorkha district (28.15  N 84.7  E) in the central Nepal, about 80 km NW of the capital Kathmandu (Figure 1). This event occurred as the result of thrust faulting on or near the Main Himalayan/Frontal Thrust (MFT) interface between the Indian plate and the Eurasian plate 1 . The strong aftershock of M 7.3 occurred on 12 May 2015, 17 days after the main shock, which was located at about 80 km NE of Kathmandu (Figure 1). In Nepal, the earthquake caused unprecedented loss of life and devastation. A large part of northern India, especially eastern Uttar Pradesh, Bihar and north Bengal, also experienced moderate shaking during these earthquakes. The Himalayan region is one of the most seismically active regions in the world producing significant number of earthquakes of M 8.0+ magnitude in the past. The largest M 8.1 event, known as the 1934 Nepal–Bihar earthquake caused widespread damage in Nepal and Bihar, and around 10,000 fatalities were reported. The M 7.8 earthquake was not completely unexpected in the Central Nepal region, as several studies had indicated the likelihood of earthquakes of magnitude greater than 8.0 based on the slip deficit estimation and accumulation of strain energy in the region. This has been anticipated in early 1990s and further confirmed by recent studies 2–7 . As shown in Figure 2, the major part of Nepal, including Kathmandu, lies in zone A on the seismic zoning map of Nepal 8,9 , whereas the districts of north Bihar adjoining the Nepal border lie in zones IV and V on the Indian seismic zone map 10 . The seismic zone A of Nepal is equivalent to the most severe Indian seismic zone V liable to shaking intensity of IX on the MSK scale. The ground motion of the main event was recorded at USGS station KATNP (27.71N, 85.32E) in Kathmandu 11 . The reported values of peak ground acceleration (PGA) and velocity were 0.164 g and 107.30 cm/s respectively (see Figure 3 a for acceleration and velocity time histories). It should be noted that the peak ground velocity is much higher than typically expected for the observed PGA of 0.164 g (ref. 12). This is noteworthy as peak ground velocity is better correlated with the damage statistics of mid- to high-rise buildings and it could have played a significant role in the unexpected degree of damage to many structures 13 . In Figure 3 b , acceleration response spectra of the recorded ground motions are compared with the code- prescribed elastic design response spectrum correspond- ing to zone A of the Nepal seismic code and zone V of the Indian seismic code, for the design basis earthquake (DBE) in soft-soil site. The USGS global V S30 server in- dicates that the central part of Kathmandu valley has soft- soil deposits which are typically NEHRP site class D ( V S30 between 180 and 360 m/s) 14 . It is clear that in the acceleration-controlled regime (i.e. short-period range which is typical for low-rise unreinforced masonry and infilled RC-frame construction), the ground motion has higher acceleration demand than the code-expected demand in the most severe seismic zone. Geologic studies show that the Kathmandu valley is covered by thick semi-consolidated quaternary sediments with a maximum depth of 550 m in the central part of the valley 15 . An ear- lier study on local site amplifications due to unconsoli- dated quaternary sediments of Kathmandu valley has indicated that the resonant frequencies were in the range 0.5 to 8.9 Hz, with the maximum amplification occurring at 0.5 Hz (2 sec) in the central lacustrine area 16 . However, in addition to the amplification at 0.5 sec (2 Hz), unusual higher spectral amplification was observed in the range 3–6 sec (0.17 to 0.33 Hz), which could also be due to the complex influence of underlying unconsolidated quaternary sediments in the basin. Similar basin effect has been observed in some of the past few earthquakes, including the notable 1985 Mexico City earthquake, where the ground acceleration was amplified by about 10 times at 2 sec period due to the presence of lake deposits, which resulted in large devastation even at a distance of 300 km from the epicentre 17 . During 3–9 May 2015, the present authors undertook a reconnaissance survey of the earthquake affected regions and visited (by road) major towns in Bihar (India) and Nepal (visited towns are marked in Figure 1). During the 25 April 2015 earthquake, the Kathmandu valley experienced intensity IX shaking on MSK scale, which left many buildings and temples in ruins. The regions around Kathmandu reported an intensity of VII in Nepal. Kathmandu, with a zone factor 1.0 according to the Nepal seismic code NBC 105, is expected to experience PGA higher than the recorded value 9 . Though it is about 80 km away from the epicentre, it experienced a shaking intensity higher than the regions around the epicentre. From the structural damage evaluation, it has been found that the damage was concentrated in few pockets of the Kathmandu valley such as Khadka Gaon, and banks of Bish- numati River in Machha Pokhari. Similar difference in site responses was ...

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... Earthquake effects, reinforced concrete frame, seismic vulnerability, unreinforced masonry. N EPAL and the neighbouring regions suffered a major earthquake on 25 April 2015, which was followed by strong aftershocks even after a fortnight of the main event. The disaster killed more than 8000 people, destroyed about half a million buildings completely and disrupted the road network in the mountainous terrain by surface ruptures and landslides. This communication aims at providing a brief overview of the earthquake and its effects on built environment, especially residential buildings, as observed in the affected areas of Nepal and adjoining Indian states of Uttar Pradesh and Bihar, during the field trip undertaken by the present authors during 3–9 May 2015 traversing over 2200 km. The M 7.8 earthquake of 25 April 2015 struck at 11 : 41 am IST (11 : 56 am local time), with its epicentre located in Gorkha district (28.15  N 84.7  E) in the central Nepal, about 80 km NW of the capital Kathmandu (Figure 1). This event occurred as the result of thrust faulting on or near the Main Himalayan/Frontal Thrust (MFT) interface between the Indian plate and the Eurasian plate 1 . The strong aftershock of M 7.3 occurred on 12 May 2015, 17 days after the main shock, which was located at about 80 km NE of Kathmandu (Figure 1). In Nepal, the earthquake caused unprecedented loss of life and devastation. A large part of northern India, especially eastern Uttar Pradesh, Bihar and north Bengal, also experienced moderate shaking during these earthquakes. The Himalayan region is one of the most seismically active regions in the world producing significant number of earthquakes of M 8.0+ magnitude in the past. The largest M 8.1 event, known as the 1934 Nepal–Bihar earthquake caused widespread damage in Nepal and Bihar, and around 10,000 fatalities were reported. The M 7.8 earthquake was not completely unexpected in the Central Nepal region, as several studies had indicated the likelihood of earthquakes of magnitude greater than 8.0 based on the slip deficit estimation and accumulation of strain energy in the region. This has been anticipated in early 1990s and further confirmed by recent studies 2–7 . As shown in Figure 2, the major part of Nepal, including Kathmandu, lies in zone A on the seismic zoning map of Nepal 8,9 , whereas the districts of north Bihar adjoining the Nepal border lie in zones IV and V on the Indian seismic zone map 10 . The seismic zone A of Nepal is equivalent to the most severe Indian seismic zone V liable to shaking intensity of IX on the MSK scale. The ground motion of the main event was recorded at USGS station KATNP (27.71N, 85.32E) in Kathmandu 11 . The reported values of peak ground acceleration (PGA) and velocity were 0.164 g and 107.30 cm/s respectively (see Figure 3 a for acceleration and velocity time histories). It should be noted that the peak ground velocity is much higher than typically expected for the observed PGA of 0.164 g (ref. 12). This is noteworthy as peak ground velocity is better correlated with the damage statistics of mid- to high-rise buildings and it could have played a significant role in the unexpected degree of damage to many structures 13 . In Figure 3 b , acceleration response spectra of the recorded ground motions are compared with the code- prescribed elastic design response spectrum correspond- ing to zone A of the Nepal seismic code and zone V of the Indian seismic code, for the design basis earthquake (DBE) in soft-soil site. The USGS global V S30 server in- dicates that the central part of Kathmandu valley has soft- soil deposits which are typically NEHRP site class D ( V S30 between 180 and 360 m/s) 14 . It is clear that in the acceleration-controlled regime (i.e. short-period range which is typical for low-rise unreinforced masonry and infilled RC-frame construction), the ground motion has higher acceleration demand than the code-expected demand in the most severe seismic zone. Geologic studies show that the Kathmandu valley is covered by thick semi-consolidated quaternary sediments with a maximum depth of 550 m in the central part of the valley 15 . An ear- lier study on local site amplifications due to unconsoli- dated quaternary sediments of Kathmandu valley has indicated that the resonant frequencies were in the range 0.5 to 8.9 Hz, with the maximum amplification occurring at 0.5 Hz (2 sec) in the central lacustrine area 16 . However, in addition to the amplification at 0.5 sec (2 Hz), unusual higher spectral amplification was observed in the range 3–6 sec (0.17 to 0.33 Hz), which could also be due to the complex influence of underlying unconsolidated quaternary sediments in the basin. Similar basin effect has been observed in some of the past few earthquakes, including the notable 1985 Mexico City earthquake, where the ground acceleration was amplified by about 10 times at 2 sec period due to the presence of lake deposits, which resulted in large devastation even at a distance of 300 km from the epicentre 17 . During 3–9 May 2015, the present authors undertook a reconnaissance survey of the earthquake affected regions and visited (by road) major towns in Bihar (India) and Nepal (visited towns are marked in Figure 1). During the 25 April 2015 earthquake, the Kathmandu valley experienced intensity IX shaking on MSK scale, which left many buildings and temples in ruins. The regions around Kathmandu reported an intensity of VII in Nepal. Kathmandu, with a zone factor 1.0 according to the Nepal seismic code NBC 105, is expected to experience PGA higher than the recorded value 9 . Though it is about 80 km away from the epicentre, it experienced a shaking intensity higher than the regions around the epicentre. From the structural damage evaluation, it has been found that the damage was concentrated in few pockets of the Kathmandu valley such as Khadka Gaon, and banks of Bish- numati River in Machha Pokhari. Similar difference in site responses was observed in Los Angeles, USA during the 1994 Northridge earthquake and in Mexico City during the 1985 earthquake, due to the basin/site effect 18 . The extensive damage in few regions of Kathmandu valley can also be because of the amplification due to the soft-soil deposits, as observed in the response spectra of ...