Earthquake Spectra

On 25 April 2015, a M_w7.8 earthquake struck near Gorka, Nepal. The earth-quake and its aftershocks caused over 8,790 deaths and 22,300 injuries; a half a million homes were destroyed; and hundreds of historical and cultural monuments were destroyed or extensively damaged (NPC 2015). Triggered landslides blocked access to road networks, and other lifelines were significantly impacted. Damage occurred in the capital of Kathmandu and the surrounding valley basin, but the most heavily affected areas were in more rural regions of central Nepal where losses to some towns were severe. Recovery has been slow, but progress is being made in rebuilding and repairing lost and damaged buildings and infrastructure. This Earthquake Spectra special issue provides a compendium of research papers on the Gorkha earthquake. They are organized into five topics: (1) seismology, ground motion, and geotechnical issues; (2) lifelines; (3) buildings; (4) cultural heritage structures; and (5) social science and public policy related topics. This overview summarizes key aspects of the earthquake and highlights findings from the special issue papers.

We develop a unified near-field shaking intensity map for the 25 April 2015 M_w 7.8 Gorkha, Nepal, earthquake by synthesizing intensities derived from macroseismic effects that were determined by independent groups using a variety of approaches. Independent assessments by different groups are generally consistent, with minor differences that are likely due in large part to differences in spatial sampling. Throughout most of the near-field region, European Macroseismic Scale (EMS-98) intensities were generally close to 7 EMS. In the Kathmandu Valley, intensities were somewhat higher (6.5–7.5) along the periphery of the valley and in the adjacent foothills than in the central valley, where they were ≈6. The results are consistent with instrumental intensity values estimated from available data using a published relationship between peak ground acceleration (PGA) and intensity. Using this relationship to convert intensities to PGA, we estimate strong-motion PGA de-amplification factors of ≈0.7 in the central Kathmandu Valley, with amplification of ≈1.6 in adjacent foothills. The results support the conclusion that the Kathmandu Valley experienced a pervasively nonlinear response during the Gorkha main shock.

The M7.8 Gorkha, Nepal main shock ruptured a segment of the Main Himalayan Thrust (MHT) directly below Kathmandu Valley, causing strong shaking levels across the valley. Strong-motion data reveal an initial 6 s source pulse that was amplified and reverberated within the basin. One of the striking features of the observed ground motions in the valley was the exceptionally low energy of periods less than 2 s, which likely limited the extent and severity of structural damage in Kathmandu compared with alternative rupture scenarios of the same magnitude in the region. Isolated cases of liquefaction and lateral spreading of unconsolidated sediments were also observed, but have not yet revealed a systematic damage pattern. Initial analysis of available data suggests that several different factors, including source and path as well as site effects, were responsible for the unusual ground motions characteristics. In this paper, we provide a short description of the Kathmandu Valley geology and analyze available strong-motion records from the main shock and three strong aftershocks, with the intent to shed light on earthquake reconnaissance observations from this earthquake.

This paper presents application of microtremor (ambient vibration) and surface wave field techniques for post-earthquake geotechnical reconnaissance purposes in Kathmandu, Nepal. Horizontal-to-vertical spectral ratios (HVSR) are computed from microtremor recordings at 16 individual measurement locations to obtain an estimate of fundamental frequency (site period) of the subsurface soils. A combination of active- and passive-source surface wave array testing was accomplished at five key sites including Kathmandu's Durbar Square and Airport. Joint inversion of each site's higher frequency dispersion and lower frequency HVSR data sets provides an estimate of subsurface material stiffness [i.e., shear wave velocity (V _S ) depth profiles]. Direct comparison of our V _S profiling at Kathmandu Durbar Square and that accomplished by downhole V _S and/or standard penetration testing (SPT) profiling yield similar results. Classification of the five sites based on average V _S , site period, and/or basin depth is presented. There is little differentiation in these site classification designations amongst the five sites, which does not capture significant differences in observed earthquake damage.

Many ground failures resulted from the 2015 Nepal earthquake sequence, including landslides, rockfalls, liquefactions, and cyclic failures. And whereas the amount and extent of landsliding were relatively consistent with predictions for a M_w 7.8 main shock, the amount and extent of liquefaction were not. We present a summary of liquefaction field observations that we made as part of the Geotechnical Extreme Events Reconnaissance (GEER) investigations. The liquefaction that did occur in the Kathmandu Valley was limited in its spatial extent, and the postliquefaction deformations were small. Prior earthquakes in this region have been reported to have caused greater liquefaction-related failures, and liquefaction hazard–mapping studies predicted widespread liquefaction hazard from an event of this size. We explore two possible reasons at the regional scale for the limited liquefaction from this earthquake sequence: drawdown of the groundwater table and high near-surface shear wave velocity. Our study finds that pumping has depressed the groundwater table across the Kathmandu Valley by 13–40 m since 1980, thereby decreasing the amount of near-surface liquefiable material and increasing the nonliquefiable “crust” layer. The regional slope-based V _S30 for the valley is on average higher than that for liquefaction sites in a global database of observed liquefaction. A global geospatial model for liquefaction occurrence shows low liquefaction potential in the Kathmandu Valley consistent with the observed patterns.

Array microtremor observations were conducted in Kathmandu Valley close to six seismic stations. Sedimentary layers from surface to deep basement rock were modeled according to the derived velocity structures for site response analysis. The records in horizontal component from the 2015 Gorkha earthquake main shock at deep sedimentary sites were compared with the predictions from analysis using the records from a shallow sedimentary site as input motions. Generally, the comparisons are in good agreement where spectral amplification at long periods and suppression at short periods could be justified by the velocity models.

Thousands of landslides occurred during the April 2015 Gorkha earthquake in Nepal. Previous work using satellite imagery mapped nearly 25,000 coseismic landslides. In this study, the satellite-based mapping was analyzed in three areas where field deployment was also conducted—the Budhi Gandaki, Trishuli, and Indrawati river valleys—to better characterize the landslides. Unmanned aerial vehicles (UAVs) were deployed to map the three-dimensional (3-D) geometry of failed slopes using photogrammetry, as well as to characterize rock structure and strength. The majority of landslides were rock slides along the ridges and the steeper portions of the basins primarily involving the weathered rock zone. Additional landslides included rock falls and soil failures. Satellite imagery analysis indicated that landsliding was concentrated north of the physiographic transition, in steep areas, and in close proximity to the major rivers. The Trishuli area experienced the lowest landslide density in terms of number of landslides compared to the Budhi Gandaki and Indrawati areas, although all three areas had similar density in terms of total landslide area and other landslide statistics.

Hydropower infrastructure, the primary source of electricity in Nepal, experienced severe damage following the 2015 Gorkha earthquake sequence, resulting in a 15% loss in the country's energy production. The performance of hydropower infrastructure during and after the sequence was one of the unique focuses of the Geotechnical Extreme Events Reconnaissance (GEER) study. The GEER team visited damaged hydropower projects along the Trishuli and Sunkoshi rivers by road and on foot, along with the ongoing 465-MW Upper Tamakoshi hydropower project by helicopter. The primary cause of damage to the hydropower infrastructure was landslide and rockfall debris falling on powerhouses, penstocks, and dam structures. Moreover, landslides blocked road access to many sites, delaying necessary repairs to damaged structures and resumption of power generation. Power production in Nepal before and after the 2015 Gorkha earthquake sequence, seismic performance of visited hydropower projects, and short- and long-term effects, together with residual risks for Nepal's hydro-power infrastructure, are discussed in this paper.

The 2015 Nepal earthquake destroyed over half a million buildings including the drinking water and sanitation infrastructures, causing the displacement of around 2.8 million people. However, knowledge of how individuals coped with water, sanitation, and hygiene (WASH) inadequacies following the earthquake remains incomplete. We conducted focus group discussions and detailed interviews with 30 participants in the affected areas of Kavrepalanchowk and a temporary settlement in Bhaktapur to assess their response and access to WASH after the earthquake. The data were analyzed based on the cultural empowerment domain of the PEN-3 cultural model. Results show that responses to WASH include the provision of water from public and private resources (positive response), the provision of chlorine tablets for treating drinking water (unique response), and limited water supply for household chores and limited sanitation and hygiene resources (negative response). These findings underscore the need to understand how individuals and households cope with WASH following an earthquake. It also highlights the need for targeted interventions focused on building community resilience in addition to providing critical relief efforts.

Despite many studies on infrastructure resilience in the existing literature, there is a limited empirical understanding of disaster resilience in the context of intermittent infrastructure systems. To fill this knowledge gap, our study provides an example assessment of the resilience of Kathmandu Valley's electricity and water supply infrastructure systems in the 2015 Gorkha earthquake. The study is based on qualitative data collected over a period of one year following the earthquake, obtained through in-depth interviews (n = 52), a focus group, and a review of secondary sources. A resilience assessment framework that includes eight factors adapted from existing studies—vulnerability, anticipation, redundancy, adaptive capacity, rapidity, resourcefulness, cross-scale interactions, and learning culture—was used for the data analysis. The characteristics of intermittent infrastructure systems pertaining to resilience identified in this study could have important implications for engineers and decision makers in developing communities to better design and maintain infrastructure in the face of disasters.

The April 2015 Gorkha earthquake in Nepal revealed the relative effectiveness of the Nepal Standard or the national building code (NBC), and irregular compliance with it in different parts of Nepal. Much of the damage to more than half a million residential structures in Nepal may be attributed to the prevalence of owner-built or owner-supervised construction and the lack of owner and builder responsiveness to seismic risk and training in the appropriate means of complying with the NBC. To explain these circumstances, we review the protracted implementation of the NBC and the role played by one organization, the National Society for Earthquake Technology—Nepal (NSET), in the implementation of the NBC. We also share observations on building code compliance made by individuals in Nepal participating in workshops led by the Earthquake Engineering Research Institute's 2014 class of Housner Fellows.

This paper takes the 2015 Nepal earthquake as a case study to explore the use of post-event dual polarimetric synthetic aperture radar images for earthquake damage assessment. The radar scattering characteristics of damaged and undamaged urban areas were compared by using polarimetric features derived from PALSAR-2 and Sentinel-1 images, and the results showed that distinguishing between damaged and undamaged urban areas with a single polarimetric feature is challenging. A split-based image analysis, feature selection, and supervised classification were employed on a PALSAR-2 image. The texture features derived from the intensity of cross-polarization show higher correlations with the damage class. Additionally, feature selection revealed a positive influence on the overall performance. Employing 70% of the data for training and 30% data for testing, the support vector machine classifier achieved an accuracy of 80.5% compared with the reference data generated from the damage map that was provided by the United Nations Operational Satellite Applications Programme.

Following the 25 April 2015 M_w 7.8 Gorkha, Nepal, earthquake and subsequent aftershocks, field surveys were conducted on medium-to-high rise reinforced concrete (RC) frame buildings with masonry infill located in the Kathmandu Valley. Rapid visual assessment, ambient vibration testing, and ground-based lidar (GBL) showed that these buildings suffered damage ranging from light to severe, where damage occurred in both structural and nonstructural elements, but was most prevalent in nonstructural masonry infills. Finite-element structural analyses of selected buildings corroborate field observations of only modest structural damage. The lack of severe structural damage in this relatively limited class of engineered medium-to-high rise RC infill frame buildings illustrates the impact of modern seismic design standards and stands in stark contrast to the severe damage and collapse observed in low-rise nonengineered RC infill frame buildings. Nonetheless, the nonstructural damage hindered many of these buildings from being occupied for many months following the earthquake and subsequent aftershocks.

A rapid visual damage assessment of buildings around four strong-motion seismic stations in Kathmandu Valley was carried out after the damaging Gorkha, Nepal earthquake (M_w7.8) of 25 April 2015. The waveforms of the main shock recorded at these stations were compared with the damage to buildings around the stations. The damage was found to be related to strong-motion characteristics of the earthquake. A dominance of long-period oscillation could be observed in the records. The damage to low-rise buildings in the valley was less than anticipated from an earthquake of this magnitude given that the majority of buildings were built without proper engineering consideration. The acceleration response spectra of one of the sedimentary sites show high response in the 1–2 s period range, and nearly 10% of the buildings, which were all low-rise, suffered damage around this site.

This paper documents and analyzes the seismic behavior of unreinforced masonry (URM) buildings that were damaged by the 2015 Gorkha earthquake in Nepal, and reports on the performance of palaces, giving an overview on the failures suffered by significant examples of these monumental buildings. Field reconnaissance was completed through both rapid, in-situ visual assessment and state-of-the-art procedures utilizing light detection and ranging (lidar) and virtual reality (VR) technologies. Both the visual and virtual assessments were compared for 20 structures and were generally consistent; however, the virtual assessment process enabled detection of damage that could not be captured or was difficult to distinguish in the field observations. Further, both in-plane and out-of-plane mechanisms were analyzed and attributed to specific structural deficiencies that usually characterize poorly detailed masonry buildings. Moreover, wall overturning was correlated with the peculiarities of the pseudo-accelerations and rocking response spectra of the earthquake.

Low-rise reinforced concrete (RC) frames with brick masonry infill walls up to five stories high have been used for housing construction in Nepal since the late 1980s. Many buildings of this type were damaged and/or collapsed in the 25 April 2015 Gorkha earthquake (M 7.8), even in areas characterized with moderate shaking intensity such as Kathmandu Valley. Due to inadequate design and/or construction of RC frame components, these buildings essentially behave like masonry shear wall structures with a shear-dominant failure mechanism. The paper presents the findings of a field survey of 98 RC buildings affected by the 2015 earthquake. The main objective of the study was to correlate the observed damage in the buildings using the modified European macroseismic scale (EMS)-98 and the wall index (defined as the wall area in the direction of shaking divided by the total building plan area above the level of interest). The results can be used to help establish recommendations regarding the required wall index for low-rise RC buildings in Nepal.

Since 1255, major earthquakes have struck Nepal. This article looks at the history of these earthquakes and how they impacted the region and its heritage. The recent April 2015 earthquake was characterized by the widespread destruction of historic buildings. It is worth noting that not all of the historic buildings succumbed to the earthquake. In the Kathmandu Valley, more than a handful of restored or reconstructed historic structures survived the force of the quake. Structures such as the Cyasilin Mandap, Patan Museum, 55 Windows Palace, and the south wing of the Sundari Chowk stood their ground. However, the Nepalese government would like to reconstruct the destroyed heritage using the traditional methods and materials. So what can we learn from the past? Can the past guide our future reconstruction? Is there a method that is traditional, and, at the same time, resistant to earthquakes?

This paper studies damage to a few specific monuments in the Kathmandu Valley that were either partially or completely destroyed during the 2015 Gorkha earthquake. Three of these structures—namely, the Basantapur Column, the Dharahara Tower, and the Narayan Temple—were modeled both analytically using rocking dynamics and computationally using discrete element modeling (DEM). The results emphasize the importance of large low frequency content within the ground motion, demonstrating that the Dharahara Tower could have collapsed due to the primary long-period ground motion pulse alone. In addition, comparison of analytical and computational modeling to the observed response enables evaluation of structural behavior, including discussion of the importance of elastic amplification and column embedment on performance during the earthquake.

The Rana dynasty ruled Nepal from 1846 to 1951 and was responsible for the construction of a number of private and government Neoclassical- or Baroque-style palaces in Kathmandu and other parts of the country. Following the 2015 Gorkha (Nepal) earthquake, detailed damage assessments of these buildings were undertaken by local and international teams. Two case study buildings that suffered moderate structural damage are presented herein, the Kaiser Mahal Palace and the Ananda Niketan Palace. Kaiser Mahal was assessed prior to the 2015 Gorkha earthquake in order to develop potential seismic retrofitting options, and the results are compared with the damage observations made following the Gorkha earthquake. Ananda Niketan was only assessed after the Gorkha earthquake with an extensive damage evaluation, in-situ material testing and sample extraction, and the undertaking of a comprehensive detailed seismic assessment. The two case studies are presented herein, followed by a comparison between the two buildings.

The Gorkha, Nepal, earthquake and the series of aftershocks that followed have damaged many heritage structures in and around Kathmandu Valley, including UNESCO World Heritage Sites (WHSs). This paper summarizes observed damage to the heritage structures of diverse typologies within the UNESCO WHSs of Kathmandu Valley. As a part of the investigation, inspection survey and damage assessment were carried out for Jagannath Temple, one of the partially damaged monuments in the Kathmandu Durbar Square WHS. Ambient vibration and in-situ tests using the pendulum hammer, the rebound hammer, and in-place push on masonry walls were performed. Finite-element models of the structure were developed, and the results were analyzed and compared with field observations. Based on the observed damages and the results obtained from numerical modeling, the primary causes of the damage are discussed.

This paper presents a novel methodology to combine ambient vibration-based operation modal analysis with three-dimensional ground-based lidar data to study damage on the Nyatapola Temple, which is a Bhaktapur UNESCO World Heritage Site that was damaged during the 2015 Gorkha, Nepal, earthquake. The post-earthquake ambient vibration data, collected via accelerometers placed on various levels of the temple, are used to estimate the vibrational properties via operational modal analysis. These properties are then compared to the pre-earthquake dynamic characteristics collected in 2002. The lidar data provide a geometric assessment of the current condition of the temple, capturing post-earthquake drift as a function of height as well as significant cracks present in the facade. The lidar data also inform the numerical models implemented for the post-earthquake condition assessment of the temple.

The transition from response to recovery in Nepal following the 25 April and 12 May 2015 earthquakes represents an unusual set of tensions among political, economic, geographic, social, technical, and physical constraints. We examine this set of tensions in interorganizational, interjurisdictional decision making to assess how interlocking constraints stalled the recovery process following the severe earthquakes. We use a mixed-methods research design, drawing on data from a review of documentary sources regarding Nepali laws, policies, and procedures in reference to disaster mitigation and response; content analysis of reports from local newspapers and professional organizations; and direct observations from two field trips to Nepal: the first from June to early July of 2015, and the second, one year later from April to May of 2016. Using these sources, we identified a network of influential organizations operating in disaster decision making and the constraints that shaped this process. We conclude that transition from response to recovery in Nepal represents a complex, dynamic process involving actors at different scales of operation—from local to global—that exceeded the capacity of any single actor to guide or control.

People in Nepal labeled as low-caste carry the double burden of simultaneous denial of access to resources and to public space in which to discuss caste issues. Following the 2015 earthquake, issues regarding caste were pushed onto the national agenda through various groups seizing the window of opportunity to bring awareness to address issues related to caste rights and discrimination. Positioned in the Focusing Events Theory, we seek to understand how groups may have used the Nepal 2015 earthquake to mobilize to increase issue attention to caste and disaster vulnerability. To identify themes around these areas, we reviewed key news articles that were published in Nepalese and international media in the two-year time frame after the earthquake. We identified issue framing with several major themes: opportunities for lessons learned, outsiders and higher caste as “rescuing” lower caste, and disaster vulnerability linked to caste.

A Rapid Visual Damage Assessment was initiated in the direct aftermath of the 2015 Gorkha earthquake to assess the safety and damage of residential buildings in the areas affected by the earthquake. Over 30,000 paper assessment forms have been subsequently digitized. The collected data set allows comparison of the observed damage to the residential building stock to the damage expected using existing fragility curves. Under certain conditions and respecting certain limitations, the post-earthquake building safety and damage data can be used to update the existing fragility functions for the Nepalese building stock. Recommendations are made for the improvement of post-earthquake building safety assessments in Nepal in order to: (1) make data collection more consistent, (2) increase the accuracy of the collected data, and (3) make more effective use of the collected data after future earthquakes.

In the wake of large earthquake disasters, governments, international agencies, and large nongovernmental organizations scramble to conduct impact and damage assessments that help them understand the nature and scale of the emergency in order to orchestrate a complex series of emergency, response, and recovery activities. Using the Gorkha earthquake as a case study, this research seeks to provide greater clarity into the types of post-disaster damage assessments, their purposes, and their potential as catalysts for critical recovery activities. We argue that damage assessment methodologies need to be tailored to the diverse information needs in post-disaster contexts, which vary by user group and change over time. This research builds upon the authors’ direct experience supporting the government of Nepal in the Post-Disaster Needs Assessment (PDNA) process, support with the rapid visual inspections conducted by the National Engineering Association, and interviews with humanitarian organizations who conducted damage assessment in Nepal.