Probabilistic seismic hazard assessment for Nepal

Abstract

Written history of great earthquakes in excess of magnitude M8 and recently identified 92 small faults around underlying big three fault systems parallel to the Himalayas show a high seismicity in Nepal. However, since faults are so closed that it is difficult to judge which earthquake belongs to which fault and even some of the faults do not hold earthquakes, the usual method of assigning the earthquakes to the nearest fault developing magnitude-frequency relationship is not applicable. Thus, an attempt has been made here to address the problem considering area sources with different densities at each location based upon historical earthquakes and faults which is real evidence of the seismicity of the region. Separate earthquake densities are calculated based upon historical earthquakes and maximum magnitudes of faults using the kernel estimation method which accounts the significance of both the number of earthquakes and size. Since there is no specific attenuation laws developed for the Himalayan region, five attenuation laws developed for seduction zone are selected and used, giving equal weight to all to minimize the uncertainties. Then, the probabilistic spectra for various return periods are calculated, compared with previous estimates and various aspects discussed.

Design Earthquake Ground Motions From Probabilistic Response Spectra: Case Study of Nepal

“…Magnitude frequency relationship (eq. 2) has been developed (Parajuli et al 2007) by calculating slope of the relationship from existing earthquakes and considering 50% intra-plate slip which clearly satisfies (Fig. 3) the rate of smaller and great earthquakes which is explained in the following paragraphs.…”

Design earthquake ground motions for Kathmandu

“…Considering the lower threshold magnitude M5, maximum magnitude M8.8 (Lave et al 2005), and 50% intra plate slip, the magnitude-frequency relationship was developed (Figure 2) following Stepp (1972). Refer to the work done by Parajuli et al (2007Parajuli et al ( , 2008 for details.…”

“…The recurrence equation is applicable for the whole region, but the earthquake data are not uniformly distributed over the entire area ( Figure 1). Considering each area cells as sources, probabilistic seismic hazard assessment was done by taking both the weightages of historical earthquakes and faults (Parajuli et al 2007(Parajuli et al , 2008 in order to obtain the probabilistic spectra for the city (Figure 3). …”