Tsunami risk assessment through numerical modeling of branch fault for Makran subduction zone

Dissertation - Master

Geophysics - Seismology

Winter  2012

Abstract

In this research, the effect of a branch fault in a subduction zone on tsunami waves has been studied using numerical modeling of tsunami waves. For modeling, we considered the Makran subduction zone in the northwest of the Indian Ocean. The Makran subduction zone is formed by the Arabian plate to the north under the Eurasian plate. This area extends from the east, from the Strait of Hormuz in Iran to near Karachi in Pakistan, with a length of about 900 kilometers. The selection of Makran area for study has two reasons, firstly, evidence is provided that a branch fault may be the cause of the large uplift observed in the near field during the earthquake and tsunami of November 27, 1945. And the second reason is the studies that show the presence of a branch fault in Makran subduction zone using two-dimensional reflection profiles. In addition, the Makran area is one of the largest accretionary prisms on earth with a thickness of sediments of about 7 km, and there is a high probability for the branching of the branch fault during large earthquakes in this subduction zone. Therefore, the branching of branch faults is a worrying issue in the Makran subduction zone, and it is possible that this phenomenon will occur in large earthquakes in the subduction zones in this area. We quantitatively investigated the effect of branch fault on tsunami waves. We considered the numerical model of a large earthquake and subsequent tsunami in Makran subduction zone with and without branch fault. We used the Geowave model for tsunami modeling and showed that the branching of a branch fault from the boundary of the subduction plane during large earthquakes can locally increase the wave height by 1.5 times.

Key words: branch fault, tsunami, Makran subduction zone, numerical modeling, earthquake

Introduction

Tsunami is always one of the most terrible disasters Natural has been discussed throughout history. which threatens most of the coastal areas on the edge of the oceans. According to Bernard's report, since 1850, tsunamis have killed more than 450,000 people and caused billions of dollars in damage to coastal areas around the world. The great tsunami of December 26, 2004 in the Indian Ocean region, which killed about 230,000 people, is considered the most destructive and worst recorded tsunami event in history. Considering that in most parts of the world, reliable earthquake data is only about a hundred years old, and historical and paleontological studies of tsunamis sometimes provide information about the history of earthquakes and tsunamis, but such information usually lacks the necessary details, as a result, the risk of large earthquakes and tsunamis in some subduction zones is not recognized correctly. One of the subduction zones of the world is located on the southern coasts of Iran and Pakistan called the Makran subduction zone. On November 28, 1945, a large earthquake with an estimated magnitude of 1.8 occurred in this area, causing tsunami waves and killing 4,000 people in this area. According to the existing records, the Makran region has the ability to create large tsunami-causing earthquakes in terms of tectonics, so the danger of tsunami caused by this region is considered a serious danger for the southern coast of Iran. In the first meeting of the intergovernmental coordination group for the development of the tsunami warning system in the Indian Ocean region, which was held by the Intergovernmental Oceanographic Commission affiliated to UNESCO on March 3-8, 2005 in Paris, the Makran region was officially recognized as one of the main tsunami-prone regions of the Indian Ocean. The occurrence of tsunami cannot be prevented like other natural disasters, but by using a set of measures, possible financial and human damages can be reduced, among the most important measures that can be taken in this direction are tsunami hazard and risk analysis. Tsunami risk is usually measured according to the parameter of the maximum vertical and horizontal rise of tsunami waves on the coastline. In this regard, it is more common to use the vertical height parameter to assess the tsunami risk.Obviously, with the increase of the high value on the waves, the risk of tsunami also increases. In general, the high value of the vertical waves of more than one meter can be dangerous. The amount of vertical rise of the waves on the coastline strongly depends on the changes in the depth of the ocean water in the areas near the coast, the topography of the area and the shape of the coastline. Since the topography and water depth of the ocean are different in different regions, the high zinc value expected from an earthquake with the same characteristics can be significantly different from another region. In addition, another issue that is effective on the top of the waves is secondary tsunami generating sources including submarine landslides and branch faults. The problem of branch faulting usually occurs in subduction zones of the world due to the occurrence of large earthquakes. In fact, branch faulting is one of the ways to discharge earthquake energy locally during large earthquakes. which has locally caused a sharp increase in the rise of the ocean floor, which causes a significant increase in the height of the tsunami waves on the coasts near these points. During the tsunami of December 26, 2004, in some coastal areas, the height of the waves caused by the tsunami and the speed of the water flow were much higher than the values ??expected from tectonic tsunamis. The studies conducted by the researchers showed that secondary sources, especially branch faults, have played an important role in the significant increase in the waves in the near field. Therefore, according to the studies conducted regarding the presence of branch faults in the Makran region and the identification of branch faults in this region, and international experiences, branch fault modeling using the real parameters of branch faults in the Makran region seems necessary. Chapter 1: Generalities-1- Introduction The theory of sheet tectonics [1] is based on the earth model, with a number of sheets. The lithosphere is characterized by a thickness of 70 to 250 km, it floats on the lower viscous layer called the asthenosphere [2] These sheets cover the entire surface of the earth, including continents and seabeds. They move relative to each other at a speed of more than 10 cm per year. The area where two sheets come into contact with each other is called a sheet boundary. The way one plate moves relative to the other determines the type of boundary. 2-Tectonic plate boundaries 1-2-1-Divergent boundaries Where the plates are separated from each other and cause material to move from the Earth's mantle upwards, thus forming the new ocean floor. The destruction of the spheres slide from one another. By right-slip faults, the crust is transported to the place of their destruction, i.e. deep ocean trenches.

1-2-3- Convergent boundaries

Where the plates move towards each other, as a result, one of the pieces of spheroid goes into the mantle while subducting one plate under the opposite plate and is digested. Although all converging zones are generally similar to each other, the way the plates collide often depends on the materials that make up the crust. The encounter may be in one of the following ways:

1-2-3-1– Oceanic – Oceanic

If both plates are oceanic, the older plate is denser and heavier, slides under the younger plate, which is relatively lighter, and creates the subduction zone.

1-2-3-2- Oceanic – Continental

In this case, the oceanic plate is heavier than the continental plate, so the oceanic plate slides under the continental plate, and as a result, the heavier plate is curved and moves towards the center of the earth with a steep slope. As a result of this movement, an ocean pit hundreds of kilometers long, tens of kilometers wide and several kilometers deep usually appears in the subduction zones. Meanwhile, many subduction zones are associated with volcanic activities. Convergent boundaries where one of the converging oceanic plates is called subduction.

  1-2-3-3- continental Continental

When the oceanic plate is subducted under the continental plate, the Andes mountain range is formed on the edge of the continent. If the subducting plate also includes continental crust, the continuation of subduction will cause the continents to converge and eventually bring them together.