Local site effect on seismic hazard of the relocated new Balakot town

Abstract

This paper evaluates the effect of local site conditions on the seismic hazard of the relocated new Balakot town as a result of the complete destruction of old Balakot city due to the 2005 Muzaffarabad Earthquake. New Balakot town has been placed in a high seismic zone according to the Building Code of Pakistan because of its vicinity to active seismogenic faults. Representative sites from the new Balakot town have been selected to evaluate the local site effects. The acceleration time series was used as suite input motion in the ground response analysis selected from the worldwide earthquake catalog and matched to the target spectrum of Balakot. The site-specific equivalent linear (EL) and nonlinear (NL) analyses were conducted for which the ground model was developed based on the field and laboratory geotechnical test results. The mean surface ground response spectrum and also the mean Amplification Factor showed that the suite input motions were de-amplified and amplified respectively below and above the fundamental period of the site. The mean maximum amplification factor at 0.4 s shows that a four-story building may be subjected to resonant conditions. The transfer function that relates the amplitude of incident and transmitted motion at interface resulted in different mean peak ground acceleration. The higher surface peak ground acceleration value was obtained from that of suit base motions. The NL analysis was observed to yield more conservative maximum AF and PGA values in comparison to the EL analysis. This study is helpful to provide improved and site-specific values of ground acceleration based on local site effects of the new Balakot town, which is an important locality in the 2005 Muzaffarabad earthquake-affected region.