شبيه‌سازي توليد، انتشار و بالاروي سونامي در منطقه مكران غربي، قسمت اول: شبيه‌سازي توليد

Simulation of tsunami generation, propagation and run-up in the western Makran, Part 1: Simulation of the generation

درحالي‌كه توجه جهاني به سمت پديده سونامي به‌خصوص در مناطق شرقي اقيانوس هند در نزديكي اندونزي افزايش يافته، مكران غربي كه در شمال غرب اقيانوس هند و در درياي عمان واقع است به‌عنوان يك ناحيه داراي پتانسيل خطر سونامي كمتر مورد توجه واقع شده است. عدم رخداد زمين‌لرزه طي ساليان طولاني در اين ناحيه احتمال قفل‌شدگي مكران غربي را افزايش داده كه در اين صورت مي‌توان انتظار داشت كه خطر سونامي بالقوه‌اي در آينده سواحل مكران و درياي عمان را تهديد خواهد كرد. هدف اين مطالعه شبيه‌سازي عددي فرآيند توليد سونامي محتمل در منطقه مكران غربي با هدف محاسبه شرايط اوليه براي انتشار سونامي مي‌باشد. براي اين منظور با در نظر گرفتن يك سناريو با پارامترهاي مشخص در درياي عمان ميدان دگرشكلي حاصل از اين چشمه سونامي با استفاده از الگوريتم اكادا محاسبه شد. نتايج حاصل از اين مطالعه الگوي مقطع اوليه سونامي را كه شامل بالاآمدگي و فروافتادگي است در سرتاسر ناحيه مورد مطالعه نشان مي‌دهد. خروجي حاصل از اين مدل‌سازي به‌عنوان ورودي مدل‌سازي انتشار سونامي استفاده مي‌شود.

Tsunami is an oceanic gravity wave generated by the displacement of huge volumes of water. There are three main types of disturbances: underwater earthquakes, submarine landslides and sudden earth surface movements adjacent to the ocean (volcanoes, meteorites, rock falls, sub-aerial landslides and ship sinking). Most tsunamis are caused by large shallow earthquakes in subduction zones (Satake and Tanioka, 1999). Sumatra–Andaman (2004) and Honshu, Japan (2011) tsunami events and following widespread damages and tragic consequences demonstrated the need of worldwide attention, awareness and preparedness for tsunami hazard mitigation. While the world draws its attention to tsunamis in the Indian Ocean, further attention is increased in the eastern areas of the Indian Ocean near Indonesia. Western Makran is located in the northwestern Indian Ocean basin. It has received less attention as a potential tsunamigenic zone. The Makran region is a 1000-km section of the Eurasian-Arabian plate boundary and located offshore Pakistan in the northwestern Indian Ocean where the oceanic crust of Arabian plate is being subducted beneath Eurasian plate since the Early Cretaceous along a north dipping subduction zone (Byrne et al., 1992; Smith et al., 2013). Following the great earthquake in Pasni-Ormara on 1945.11.27, Mw=8.1 (Byrne et aL, 1992), the coastline uplifted by about 2 m (Page et al., 1979). This event was accompanied by a significant regional tsunami, with run-up in the 5–10 m range which caused about 4000 deaths along the very sparsely populated Makran coast (Heck, 1947; Ambraseys and Melville, 1982; Okal and Synolakis, 2008). The Makran may be seismically segmented along its length into a western and an eastern segment, distinguished by different levels of seismicity (lower in the west). Moderate to large magnitude earthquakes are either related to the down going slab at intermediate depths or superficial in the eastern Makran (e.g. 1765, 1851 and 1945 earthquakes), while western Makran is marked with almost no seismicity in the coastal area at present but might have experienced a strong earthquake in 1483 (Byrne et al., 1992; Zarifi, 2006). The lack of earthquakes for many years has increased the possibility of locking the western Makran segment. This means that, it could generate a potential tsunami event in the future that can threat the Gulf of Oman and the Makran coastlines. Because of the tsunamigenic potential of Makran subduction zone, also importance of strategic geographic location, financial role of Makran coast in Iran, accessibility to international waters, ability to communicate with other countries and its cultural, natural and historical tourism potential along with the establishment of ports and coastal and offshore installations in the region, tsunami can be a real threat. Consequently, it is indispensable to have accurate studies and estimates for tsunami risk mitigation. The aim of this study is to simulate tsunami generation in western Makran numerically for estimating the initial condition for tsunami propagation. Tsunami generation mechanism should be modeled as the first step in the process of tsunami modeling. The generation modeling problem should be studied geophysically and geologically, therefore it is a very important and vital stage in tsunami simulation. To estimate the static uplift of seafloor, we can use the fault models e.g., Okada (1985) and Mansinha and Smylie (1971) which are the analytical solution of deformation field caused by instantaneous rupture on an elastic finite fault plane. The theory was proposed originally by Mansinha and Smylie (1971) and then improved by Okada (1985). We need the fault parameters (Hypocenter (Latitude, Longitude and Depth), Length and Width of Fault Plane, Dislocation (Slip), Strike direction, Dip angle and Rake (slip) angle) to compute the deformation. A tsunami scenario with defined source parameters was constructed in the Gulf of Oman to compute the deformation field based on the Okada algorithm. The source model was based on Okal and Synolakis (2008) and Smith et al. (2013) with a length of 450 km, a width of 100 km and a dislocation of 10 m which has a moment magnitude (Mw) of 8.7. The result of this study represents the initial profile of the tsunami while including the uplift and subsidence in the study area. The earthquake scenario predicted maximum seafloor uplift of 3.5 m and maximum subsidence of 2.4 m. The deformation field covered an area from 23.5° N to 27.5° N and from 56° E to 63° E. The southern coastal areas of Iran and Pakistan experienced subsidence and the northern coastlines of Oman experienced uplift. The outcome can be used as the input in the simulation of tsunami propagation