Despite the large magnitude of the earthquake and the shallow depth of its focus, not a consistent surface faulting was observed, except one site at the city of Kathmandu. The city is about 76 km to the SE-E of the epicenter of the main shock. This distance, in general, might be considered as rather long, but the observed faulting could be justified due to the very small inclination (dipping angle about 100) of the faulting plane towards north, in combination with the depth and the location of the focus.
The incurred structural damage was extended over a rather large area along the main axis of the country from about 120 km (E-W) to 50 km (N-S). The intensity of the damage was strongly varying all over this vast region, even among adjacent plots, and closely lying structures. On the other hand, it was not observed the well known diminuation or the differentiation of damage as a basic function of epicentral distance up to around 80 km. Even more, the field observations of the response of structures led to a general conclusion that either we had very small no damage, or collapse/heavy damage. Statistically speaking, not intermediate level of damage was observed.
For most of the buildings with fired clay brick masonry and wooden roofs and floors, the observed damage started from the roof and the top floors. This type of structural response implies that the structures have been excited dominantly along the vertical direction, in addition to the fact that they have been collapsed symmetrically around a vertical axis and/or inside their plan. In some cases intermediate floors have been squeezed. In almost all partially collapsed buildings no window glass panels were broken in the uncollapsed part of them. These phenomena are tried to be explained by considering not only the structural characteristics and deficiencies, but also the convolution of different types of emerging seismic waves. The basic characteristics of the ground motion were estimated by an appropriate consideration of each building structure as a kind of a 3-D seismoscope. Its frozen final response is recorded and the causing ground excitation is inferred by applying basic principles of structural mechanics. This methodology may be applied where strong motion records are not available, or are not representative, as the present case is.
Although forward directivity phenomena occurred towards Kathmandu (the rupture was directed towards Kathmandu producing velocity pulses with period larger than 4 sec), they area as proved from the observations of the structural response, a fact that it is tried to be explained. The arguments are supported by the least possible number of characteristic pictures and drawings.