Abstract:
Gilgit and the surrounding areas of the Gigilt-Baltistan region are located in the seismic zone, which presents a major risk to the population and the infrastructure of the area. Significant work on seismic risk evaluation has not been undertaken previously based on the results of the true seismic monitoring approach and ground motion modeling techniques. Therefore, the current study is an attempt based on newly available seismological data and waveform modeling techniques in order to develop a new seismo-tectonic model near Gilgit city and the surrounding areas. The area possesses a complex geologic and tectonic history where rocks of the two continental plates and a sandwiched island arc along with two major tectonic boundaries, namely the Main Karakoram Thrust, and the Main Mantle Thrust zones exposed. Frequent occurrence of strong magnitude earthquakes (Mw ≥ 6.0) in the northern Pakistan region indicates a higher level of seismic risk to Gilgit city and the surrounding areas. Seismological data from the Pakistan Meteorological Department local seismic network have been used for knowing the crustal thickness and structure of the S-wave velocity underneath the Gilgit area. Primary wave receiver function, analysis, H-K stacking, and inversion techniques are applied for this purpose. A new crustal model for S-wave velocity structure has been generated through waveform inversion. Results of waveform modeling show ~ 63 ± 2 km Moho crust thickness and a sedimentary rock layer at a depth ~15 km beneath the Gilgit seismic station, respectively. The Time Domain Moment Tensor inversion analysis has been applied to determine the source mechanism solution provided convincing evidence of prevailing transpressive and transtensive stresses along several faults. Major seismic activities in the area are linked to the Nanga Parbat syntaxis region, which passes along the Main Mantle Thrust-Raikot active fault zone near Gilgit and Astore cities. For the risk assessment findings, firstly, ground motion shake maps using the SIGMA tool and GMT mapping software were generated. Later, the exposure of buildings, selected earthquake scenarios, and vulnerability information were provided as input parameters to the SELENA software to obtain potential damage and human and economic loss due to three scenario earthquakes. The first earthquake, Mw 7.0, induced maximum damage to the infrastructure with human causalities and injuries in Gilgit city. The second earthquake scenario, Mw 6.8, also resulted in high damage and casualties. However, the third scenario caused maximum shaking with low damage to the infrastructure. Based on these scenarios, six building typologies are considered, among which RC-L, BrMsn, and adobe structures in Zone 1, Zone 5, and Zone 6 experienced maximum damage both in the first and second scenarios, indicating the vulnerability for these zones. The main reason for higher damage in these zones is the proximity of the earthquake source, the presence of buildings along the slope, poor BrMsn and RC-L structures, and, of course, higher ground motion. There was also observed influence of the unfavorable soil conditions, and trapping of seismic waves resulted in amplification in Zone 5 and Zone 6. The risk assessment study will help the people of the area to mitigate earthquake hazards more efficiently by adopting proper measures per recommendation related to pre- and postearthquake scenarios. This study, besides the local population, will also help Pakistan and the China Economic Corridor (CPEC) counter earthquake hazards by adopting proper measures according to the earthquake zonation classification in the area of investigation.
