Abstract
The 25th April 2015 M7.6 Gorkha earthquake caused significant damage to buildings and infrastructure in both Kathmandu and surrounding areas as well as triggering numerous, large landslides. This resulted in the loss of approximately 8600 lives. In order to learn how the impact of such events can be reduced on communities both in Nepal and elsewhere, the Earthquake Engineering Field Investigation Team (EEFIT) reconnaissance mission was undertaken, aiming to look at damage patterns within the country. Passive, microtremor recordings in severely damaged areas of the Kathmandu Valley, as well as at the main seismic recording station in Kathmandu (USGS station KATNP) are used to determined preliminary shear wave velocity (Vs) profiles for each site. These profiles are converted into spectral acceleration using the input motion of the Gorkha earthquake. The results are limited, but show clear site amplification within the Siddhitol Region. The resulting ground motions exceed the design levels from the Nepalese Building Codes, indicating the need for site-specific hazard analysis and for revision of the building code to address the effect of site amplification.
Highlights
The epicentre of the 25th April 2015, M7.6 Gorkha, Nepal earthquake was approximately 80 km north-west of Kathmandu, Fig. 1 (NSET 2015)
The resulting ground motions exceed the design levels from the Nepalese Building Codes, indicating the need for site-specific hazard analysis and for revision of the building code to address the effect of site amplification
Passive microtremor measurements, obtained from several sites in the Kathmandu Valley following the M7.6 Gorkha earthquake, have been used to estimate Vs velocity profiles for significantly damaged areas as well as near the KATNP station. These results indicate areas of likely site amplification, such as the Siddhitol Region near the edge of the Kathmandu Valley
Summary
The epicentre of the 25th April 2015, M7.6 Gorkha, Nepal earthquake was approximately 80 km north-west of Kathmandu, Fig. 1 (NSET 2015). Only 15 km below the surface, this caused high intensity shaking, up to MMI IX (NSET 2015) in the epicentral zone, as well as triggering a considerable number of large landslides, within the northern half of the country. The earthquake occurred at the end of the dry season and, further impacts during the monsoon season could be expected as the heavy rain acts on slopes weakened by the original shaking. An Earthquake Engineering Field Investigation Team (EFFIT) was deployed to Nepal in June 2015, just in advance of the monsoon. EEFIT is a UK-based collaboration between industry and academia, to conduct field investigations in regions affected by major earthquakes. Despite arriving over a month after the initial earthquake, aftershocks over M5.0 were still being experienced (NSET 2015)
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