Seismic velocity recovery in the subsurface: transient damage and groundwater drainage following the 2015 Gorkha earthquake, Nepal

Shallow earthquakes frequently disturb the hydrological and mechanical state of the subsurface, with consequences for hazard and water management. Transient post-seismic hydrological behaviour has been widely reported, suggesting that the recovery of material properties (relaxation) following ground shaking may impact groundwater fluctuations. However, the monitoring of seismic velocity variations associated with earthquake damage and hydrological variations are often done assuming that both effects are independent. In a field site prone to highly variable hydrological conditions, we disentangle the different forcing of the relative seismic velocity variations $\delta v$ retrieved from a small dense seismic array in Nepal in the aftermath of the 2015 Mw 7.8 Gorkha earthquake. We successfully model transient damage effects by introducing a universal relaxation function that contains a unique maximum relaxation timescale for the main shock and the aftershocks, independent of the ground shaking levels. Next, we remove the modeled velocity from the raw data and test whether the corresponding residuals agree with a background hydrological behaviour we inferred from a previously calibrated groundwater model. The fitting of the $\delta v$ data with this model is improved when we introduce transient hydrological properties in the phase immediately following the main shock. This transient behaviour, interpreted as an enhanced permeability in the shallow subsurface, lasts for $\sim$ 6 months and is shorter than the damage relaxation ($\sim$ 1 year). Thus, we demonstrate the capability of seismic interferometry to deconvolve transient hydrological properties after earthquakes from non-linear mechanical recovery.

Liquefaction hazard assessment and ground failure probability analysis in the Kathmandu Valley of Nepal

During the 2015 Gorkha Earthquake (Mw7.8), extensive soil liquefaction was observed across the Kathmandu Valley. As a densely populated urban settlement, the assessment of liquefaction potential of the valley is crucial especially for ensuring the safety of engineering structures. In this study, we use borehole data including SPT-N values of 410 locations in the valley to assess the susceptibility, hazard, and risk of liquefaction of the valley soil considering three likely-to-recur scenario earthquakes. Some of the existing and frequently used analysis and computation methods are employed for the assessments, and the obtained results are presented in the form of liquefaction hazard maps indicating factor of safety, liquefaction potential index, and probability of ground failure (PG). The assessment results reveal that most of the areas have medium to very high liquefaction susceptibility, and that the central and southern parts of the valley are more susceptible to liquefaction and are at greater risk of liquefaction damage than the northern parts. The assessment outcomes are validated with the field manifestations during the 2015 Gorkha Earthquake. The target SPT-N values (Nimproved) at potentially liquefiable areas are determined using back analysis to ascertain no liquefaction during the aforesaid three scenario earthquakes.

Study on overstrength and ductility of reinforced concrete building with different infill through nonlinear analysis

The building structure with infill wall shows higher global stiffness along with the uncertain behaviour during 2015 Gorkha earthquake. It significantly increased the collapse rate of structures during earthquakes. The response of buildings with different infills during seismic excitations is not completely accounted by current seismic codes in the region. On the other hand, due to the different geological region, availability of infill materials for reinforced concrete building also differs on region to region. In most of the situations the burnt clay brick, concrete blocks and stone block are used as infill materials during building construction. In this scenario, this study explores the importance of selection of right infill material for better seismic performance during earthquakes. For this, building constructed at Pokhara Metropolitan City is considered for case study. The structural model is prepared with and without considering infills. The solid, hollow concrete block and clay brick masonry are taken as infill material during analysis. The structural behaviour during earthquakes is studied with non-linear static pushover. The result shows that the hollow concrete block masonry infill (INHB) shows better structural performance compared to other infill types.

Reconstruction of heritage in Bhaktapur, Nepal: examining tensions and negotiations between the “local” and the “global”

PurposeCultural heritage, specifically built heritage – including monuments, urban ensembles, religious and palatial complexes – has emerged as a central focus of tensions and negotiations within the post-disaster recovery landscape in Nepal following the 2015 Gorkha Earthquake. This not only reflects a growing recognition of heritage within international disaster risk management frameworks, but also responds to the critical role played by heritage at national, regional and local levels. The paper aims to examine the entangled realities of “local” and “global” operating in ongoing reconstruction of built heritage in Bhaktapur, Nepal.Design/methodology/approachThe paper is an account of reconstruction practices observed in Bhaktapur between 2018 and 2020. It is based on data collected by layering ethnographic methods with textual and historical analysis. In seeking to analyse manifestations of global and local, the author presents reflections from fieldwork carried over seven months in Bhaktapur and describes the micro-politics enacted out between researcher, heritage custodians, translators, intermediaries and participants.FindingsReconstruction of built heritage in Bhaktapur negotiates between developmentalist-paradigms of post-disaster recovery, heritage conservation discourses as well as religious and quotidian practices of care. It is simultaneously informed by global institutions and policy and local politics and aspirations that operate in constant tension and negotiation.Originality/valueThe current study responds to the call for reframing research agendas and practices set out in the Disaster Studies Manifesto by critically engaging with ideas of local and global. The study builds on the growing body of research linking heritage with disaster risk management.

Assessment of earthquake-induced landslide inventories and susceptibility maps using slope unit-based logistic regression and geospatial statistics

Inventories of seismically induced landslides provide essential information about the extent and severity of ground effects after an earthquake. Rigorous assessment of the completeness of a landslide inventory and the quality of a landslide susceptibility map derived from the inventory is of paramount importance for disaster management applications. Methods and materials applied while preparing inventories influence their quality, but the criteria for generating an inventory are not standardized. This study considered five landslide inventories prepared by different authors after the 2015 Gorkha earthquake, to assess their differences, understand the implications of their use in producing landslide susceptibility maps in conjunction with standard landslide predisposing factors and logistic regression. We adopted three assessment criteria: (1) an error index to identify the mutual mismatches between the inventories; (2) statistical analysis, to study the inconsistency in predisposing factors and performance of susceptibility maps; and (3) geospatial analysis, to assess differences between the inventories and the corresponding susceptibility maps. Results show that substantial discrepancies exist among the mapped landslides. Although there is no distinct variation in the significance of landslide causative factors and the performance of susceptibility maps, a hot spot analysis and cluster/outlier analysis of the maps revealed notable differences in spatial patterns. The percentages of landslide-prone hot spots and clustered areas are directly proportional to the size of the landslide inventory. The proposed geospatial approaches provide a new perspective to the investigators for the quantitative analysis of earthquake-triggered landslide inventories and susceptibility maps.

Revisiting the Fault Locking of the Central Himalayan Thrust with a Viscoelastic Earthquake-Cycle Deformation Model

Based on a viscoelastic earthquake-cycle deformation model, we revisit the fault locking of the central Himalayan thrust using geodetic data acquired in the past three decades. By incorporating the viscoelastic relaxation effect induced by stress buildup and release, our viscoelastic model is capable of explaining the far-field observation with similar fault locking width obtained in previous studies. Elastic models underestimate the far-field deformation and consequently underestimate the fault slip rate by attributing the far-field deformation to stable intraplate deformation. A steady-state viscosity of ∼1019 Pa·s is required for the lower crust beneath south Tibet to best fit the crustal velocity. The optimal slip rate and locking width of the central Main Himalayan Thrust are estimated to 18.8 ± 1.6 mm/a and 85 ± 2.1 km, respectively. The inferred fault locking width, along with the down-dip rupture extension of the 2015 Gorkha earthquake, agrees well with the identified mid-crustal ramp, which leads to an interpretation that the fault geometry of the central Himalayan thrust plays an important role on fault kinematics. Our results highlight that viscoelastic relaxation during the earthquake cycle should be incorporated for robust estimation of fault locking parameters and reasonable data fitting.

Lateral Ground Spread at Kausaltar, Kathmandu that Appeared in the 2015 Gorkha Earthquake

The April 25, 2015 Gorkha earthquake jolted the central region of Nepal causing extensive damage to buildings and earthen structures in both mountainous and urban areas of Nepal. Kathmandu-Bhaktapur road, one section of the Araniko Highway, crosses a small valley in the center of the Kathmandu Basin with an embankment. This embankment and adjacent area were deformed in the earthquake. To examine the cause of this ground deformation, several in-situ tests such as micro-tremor measurements, standard penetration tests (SPT), multi-channel analyses of surface waves (MASW), and C 14 dating were conducted. These tests show that a silty sand layer with low plasticity has most likely been liquefied 5 to 8 meters underground. It is also shown that groundwater lowering using existing wells can decrease the liquefaction-prone area by 81%.

Management of Orthopaedic Injuries following 2015 Gorkha Earthquake: Our Experience in Public Health Concern Trust Nepal

Background: On April 25 2015, a magnitude of 7.8 earthquake struck in central Nepal, causing a huge physical and social disturbances. Physical impacts comprised casualties with deaths and injuries and damage to infrastructure, cultural heritage and natural environment while social impacts are demographic, psychological and economic consequences. We report our experience in treating victims who were brought at our centre via different means.Objectives: to provide an overview the caseload and provide analysis of earthquake victims for improving the future strategies in similar scenarios.Methods: A retrospective study of the clinical characteristics and patterns of hospitalised patient after the 2015 earthquake was conducted. Demographic evaluation, surgical procedures and morbidities were reviewed. The patients were followed up for an average of 24 months, detail records were kept on their recovery and function.Results: A total of 85 patients were treated with subsequent follow-up. The proportion of males admitted was similar to that of females (49.4% and 50.5% respectively). The highest number of admitted age group ranges (17- 45) was about 37.64%. Most injured site was lower limbs (68.23%) where fracture tibia and fibula had the highest incidence (56.89%). Out of all, 14.11% of cases were open fractures. Trauma severity was assessed with injury severity score and most of them categorized as mild one (95.29%). The most common procedure performed was closed reduction and pinning (n=28), followed by open reduction and fixation (n=24). Overall, mortality rate was 2.35% (2 of 85). Total 25 implants were removed within three years of period and 28.6% of patients were not returned to date.Conclusions: The injury epidemiology reported in this study showed quite congruence with most other earthquake related studies. Analysis profiles of injuries and clinical features of earthquake victims will definitely impact rescue efforts and treatment of fracture injuries in possible future natural calamities.

On the impact of earthquake-induced landslides on Red Panda Ailurus fulgens (Mammalia: Carnivora: Ailuridae) habitat in Langtang National Park, Nepal

In addition to the threats of human encroachment, infrastructure development, tourism activities, habitat fragmentation, and human-wildlife interactions, natural disasters also pose a threat to the habitat of endangered species such as the Red Panda. This study aims to assess the impact of the 2015 Gorkha earthquake-induced landslides on the Red Panda’s habitat in Langtang National Park (LNP), central Nepal Himalaya. Remote sensing and geographical information system were applied to estimate the potential and core habitats of the Red Panda, and collect information on earthquake-induced landslides. Field sampling and verification of remotely collected data were done within a year of the earthquake. Considering preferred vegetation types, elevation range, aspects, distance from water sources, and Red Panda presence points, an area of 214.34 km2 was estimated as the potential habitat of Red Panda in the Park. Thirty-nine landslides were identified in LNP triggered by the Gorkha earthquake, 14 of which occurred in the core Red Panda habitat. As a result of the earthquake-induced landslides, a significant decrease in tree density was observed in the areas affected by the landslides. Similarly, the bamboo cover was observed to be significantly lower in the areas affected by landslides compared to the unaffected adjacent areas. The average size of the landslide, causing damage to the Red Panda habitat was 0.8 ha. The potential habitat damaged by the earthquake-induced landslide was estimated to be 11.20 ha which is equivalent to the habitat required by one Red Panda. The findings could be useful in initiating restoration of the damaged Red Panda habitat in LNP.