Structural performance and associated lessons to be learned from world earthquakes in Nepal after 25 April 2015 (MW 7.8) Gorkha earthquake
Introduction
On 25th April 2015, a strong earthquake of MW 7.8 hit central Nepal and neighboring areas including the capital city at 11:56 local time. The epicenter was located near the Barpak village of Gorkha districts, around 77 km NW from the capital city, Kathmandu and the focal depth was about 15 km depth. The devastating main shock event was followed by more than 400 aftershocks as of November 2015. Three major aftershocks after Gorkha earthquake 2015 also accelerated the damage in already jolted structures. The aftershock of May 12 (MW 7.3) which occurred in the border between Dolakha and Sindhupalchowk districts was largely responsible for the aggravated damage scenario in Sindhupalchowk and Dolakha districts. Out of 75 administrative districts, 31 were affected by Gorkha earthquake and 14 out of 31 districts in central Nepal were the worst affected in terms of casualties, infrastructural losses and lifeline damages. Distribution of earthquake effects as per categories endorsed by Government of Nepal for the affected 31 districts is reported in Fig. 1. In particular, the impacts of earthquake are congregated only towards east and south east of the epicenter and interestingly, no damage is cases were recorded even in neighboring districts towards west of epicenter. Details regarding the seismological reconnaissance and directivity effects are reported in several contributions (e.g. [3], [9], [26]). The official death toll was 8790, with 22,300 injured, almost eight million people were affected and millions were homeless [49]. Due to directivity effects and site conditions of settlements, the damage was primarily concentrated in central Nepal. In terms of structural damages, four major events MW 7.8 (25th April), (MW 6.7 (25th April), MW 6.8 (26th April) and MW 7.3 (12th May) governed almost all fraction of major structural collapse and minor to severe damage. The preliminary intensity distribution map prepared by Martin et al. [75] designates the near-field regions at around 8 in 1988 European Macroseismic Scale (EMS-98) and for the case of capital city districts; the intensity is designated to be 6–7. The severe losses in near-field regions of the April 25 main shock and 12 May aftershock is largely attributed by the flattened rubble stone masonry buildings ranging from recent constructions to centuries old unrepaired houses continuously used for several generations. For all of these events till May 12, the Post Disaster Need Assessment (PDNA) conducted by government of Nepal depicts the overall loss of around US$ 7 billion and there is no any case of strong shaking after the May 12 event till date, however aftershocks of magnitude 4–5.5 are occurring until April 2016 exactly one year after the main shock event.
Nepal, being situated in one of the most active seismic zone in the world, and continuously hit by strong earthquakes, still lacks proper framework in structural preparedness to strengthening measures. In addition to this, the lessons and preventive measures after every earthquake events are not accounted in policies and planning, thus the losses in case of earthquakes are always devastating. Moreover, lessons of the each earthquake events are not well documented till date, so it is imperative to document the causes behind the damage, lessons to be learned and improvement scenario for possible replication in case of future planning. Some contributions are made regarding various aspects of Gorkha earthquake, however exhaustive coverage in affected areas, comparative analysis of structural performance with past major earthquakes in from last century and comparative structural performance in terms of structural performance and associated lessons to be learned were not disseminated. This contribution covers the scenario of 21 affected districts including the entire crisis hit and severely hit districts in central Nepal. In particular, this paper outlines the brief summaries of significant world earthquakes and 1934 and 1988 earthquakes in Nepal with special focus on structural performance and associated casualties and draws comparisons with significant world earthquakes to infer the lessons to be learned by Nepal for improving the performance of structures in case of future earthquakes in Nepal. Reviews regarding the state-of-the art of building construction and possible improvement measures are discussed in light of prevalent guidelines along with the sufficiency of guidelines.
Section snippets
Seismicity and occurrence of earthquakes in Nepal
Nepal Himalaya is characterized by the frequent occurrence of earthquakes of varying magnitudes from east to west. In general, strong earthquakes in Nepal occur within every 60–80 years (Table 1) causing enormous loss of life and properties in terms of infrastructure and human casualties. In addition to this, losses are also government by attributed by moderate size earthquakes having relatively less return period and occurred in areas where large population concentration is prevalent in the
Current scenario of Nepalese buildings
Majority of Nepalese people reside in rural neighborhoods wherein reinforced concrete (RC) concentration is quite rare. In addition to this, RC construction system is relatively new for Nepal and is being practiced only after 1980s. The rural setups in Nepal constitute dominantly the building forms practiced for many centuries and it is very common to use rubble stone houses by several generations without any routine maintenance and strengthening. People in rural neighborhoods hire local masons
1934 Bihar-Nepal earthquake
Among the historical earthquakes in Nepal, the great Bihar-Nepal earthquake of 1934 (MW 8.1) is considered as the most devastating in terms of loss of life and property. The epicenter of this event was located in eastern Nepal near Everest with its damaging impacts in eastern and central Nepal as well as neighboring Indian state of Bihar [2], [34], [52]. The maximum intensity of MMI-X was assigned in some parts of Kathmandu valley that experienced enormous damages in building stocks along with
Some major world earthquakes in last two decades: brief overview and comparison of structural performance
Many earthquakes hit on various part of the world in last two decades, however fault mechanism, seismicity, preparedness, source to site distance among others parameters are not similar for all thus damage statistics are also largely scattered. Many of the developing states like Nepal in highly active seismic zones have also varied statistics and some key lessons that could be insightful for other countries in terms of structural preparedness and earthquake engineering aspects as well as for
Comparative structural performance analysis during major world earthquakes
After brief overview of Koceali, Bhuj, Kashmir, L'Aquila, Haiti, Chile and Gorkha earthquake in preceding section, behavior of structures during these earthquakes have been discussed on comparative basis in Table 5. The major types of structures, typical failure mechanisms and their causes are considered as major bass of comparison along with the seismic design and detailing criteria. The lessons from comparison are discussed in following section outlining possible remedial issues in case of
Building regulations in Nepal
Following the mandate of Building Code Act-1990, Nepal Building Code was promulgated in 1994 and legal enforcement was done in 2003 for some urban areas of Nepal. As of 2014, only 20 out of 58 municipalities have been following mandatory rules of thumb (MRT) suggested by NBC-201 [45]); though design optimization and performance based analysis are not practiced yet in Nepal. Majority of the structural design and analysis, which is limited within Kathmandu valley are done with the help of Indian
Comparative forensic analysis of observed damage
Forensic analysis in terms of structural performance is particularly pivotal for countries like Nepal where buildings codes are not adequate, seismicity is not properly understood, very few cases of damage statistics and uneven damage are occurred in almost all earthquakes. Past earthquakes are not documented properly in Nepal in terms of structural performance, strong ground motion as well lessons to be learned after the events; in this context earthquakes across the world could disseminate
Concluding remarks
Gorkha seismic sequence is the most devastating event in Nepal after the 1934 Bihar-Nepal earthquake. As many as 755,549 building in central, eastern and western Nepal were either completely collapsed or partially damaged; structural damage in affected areas is attributed to structural and material deficiencies as well as effect of local amplification, topographical and ridge effects. Marginal construction practices and inferior quality of construction materials, age of buildings and
Acknowledgement
Authors express sincere gratitude to Dr. Giovanni Forte (DICEA Department, University of Naples Federico II, Italy) for his kind support and the unknown reviewer is greatly acknowledged for providing constructive ideas and comments to improve the quality of paper. We are thankful to National Seismological Center, DMG for strong ground motion data records.
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