Unusual Findings After the 2015 Nepal Gorkha M 7.8 Earthquake From Engineering and Geological Points of View

Abstract eng:
Despite the large magnitude and the shallow depth of the 2015 Nepal earthquake, no consistent surface rupture was observed, except from a highly disrupted zone in Araniko Highway probably attributed to thrust faulting directly located under the Kathmandu basin. Kathmandu city is situated at about 76 km to the ESE of the epicenter of the main shock. This distance, in general, might be considered as rather long, but the observed disrupted zone may be due to the very small northward dip of the fault combined with the hypocenter depth and location. The incurred structural damage was extended over a rather large area along the main axis of the country. The damage intensity varied strongly over a vast region, even among adjacent structures. On the other hand, attenuation of the damage intensity as a function of the epicentral distance up to around 80 km was not observed. Field observations included cases of negligible to slight damage as well as cases of heavy damage to destruction. Intermediate level of damage was not observed. Most buildings were constructed with fired clay brick masonry and wooden roofs and floors. In these cases, the observed damage started from the roof and the top floors. In case of collapse, debris was distributed symmetrically around the vertical axis of the building. This type of structural response implied that the structures have been excited dominantly along the vertical direction. In some cases, intermediate floors have been crushed. Window glass panels were not broken in the intact parts of the almost all partially collapsed buildings. These phenomena can be explained by considering not only the structural characteristics and deficiencies, but also the convolution of different types of emerging seismic waves. Despite the inadequate aseismic design, the great height and the flexibility of the majority of the reinforced concrete buildings, they performed well during the earthquake. The ground motion was characterized by low ground accelerations due to filtering of the deep soil deposits of the site as well as large velocity pulses with periods over 4 sec caused by directivity effects. These directivity effecs occurred because the rupture was directed towards Kathmandu, but they had no effect on structures since they presented very low frequency content. The recorded ground motion was dominated by predictable values of frequency and amplitude due to the source mechanism of the event.

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