Tsunami hazard and enhanced vulnerability assessment along the Makran coast, Pakistan

The active Makran Subduction Zone and transform tectonics in the Arabian Sea makes the region seismically active and prone to tsunamis. The region has witnessed several catastrophic earthquakes and tsunamis in the past. The thesis deals with the tsunami potential and enhanced vulnerability assessment along the Makran Coast of Pakistan. The first part of the thesis encompasses the tsunami potential assessment through a cross-analysis of multidisciplinary proxies. The second part deals with vulnerability and preliminary risk assessment through a detailed analysis of tsunami modelling and inundation for various hazard scenarios. In the third part an attempt is being made to comprehend and understand the four puzzling dynamics of the 1945 Tsunami event at Pasni. For tsunami hazard and potential assessment an integrated assessment of various approaches and proxies is carried out. It involves a comprehensive compilation, review and cross-analysis of geophysical, probabilistic-deterministic modelling, sedimentological-geomorphical extreme wave evidence, and historical records around the Arabian coastline. Also, the tsunamites (tsunami deposits) exhibiting worst-case morphological modifications and impacts around the Arabia Sea are modelled to estimate the tsunami wave potential. These morphodynamic changes are modelled backwards in terms of wave height, wavelength, flow depth, flow velocity and inundation extents. The analysis also resulted in a recurrence interval between 500 and 1000 years for a mega-tsunami event (≥12 m). The second part deals with vulnerability and preliminary risk assessment though a detailed tsunami generation and inundation analysis for various hazard scenarios. It utilizes both static and dynamic inundation models for comparative analysis and for better understanding. The vulnerability assessment of the Makran Coast particularly focuses on the towns of Pasni and Gwadar in detail. Based on the evaluation of hazard potential, three different tsunami wave scenarios (7m, 10m, 15m) are modelled for each town. In addition to simulations, spot observations of key areas in the study region are presented, detailing flow velocity and depth. The tsunami modelling also evaluates the effectiveness of the early warning system by comparing tsunami generation, arrival, and evacuation timings for near-field and far-field tsunamis. In the third part an attempt is being made to comprehend four puzzling facts of the 1945 Tsunami event at the Pasni Coast. These are i) arrival time delay of 1-2 hours, ii) wave height ≥10 m which is exceptionally high as compared to earthquake size, iii) occurrence of three waves, and iv) temporary beach line regression or the initial draw-down. Moreover, the mystery deepened as deterministic numerical models from various studies failed to align with reports especially arrival timings and wave height. The models failed to achieve a wave height exceeding 5 meters, accompanied by a delay of over an hour. This discrepancy prompted the consideration of a submarine landslide-generated triggered subsequent tsunami. We discuss that the earthquake was the sole tsunami source, capable of elucidating the reported tsunami characteristics. The factors contributing to the delay and exceptionally high wave height are explained. In brief, we argue that the reflection-amplification phenomenon between the coastal cliffs and continental shelf edge is responsible for all these anomalous values. Chapter 4 presents the paleotsunami deposits (tsunamites) in the Pasni area, marking the first study conducted within this domain in Pakistan. We detected two tsunamites older than that of 1945 Tsunami horizon. The event dated 1539 CE is very close to an already reported 1524 CE Tsunami in the Arabian Sea. The second event could date back to 17th BCE maximum. These deposits need further investigations. The synthesis of the thesis concludes the hazard vulnerability assessment including effects of tsunamis along the coastline of Pakistan. The study underscores the importance of comprehensive mitigation strategies to enhance coastal resilience and minimize the impacts of future tsunamis along the coastline. As one outlook, we encourage continued research efforts to deepen our understanding of these complex phenomena and improve disaster preparedness and response capabilities in the region.