People tremble when they hear the word; destruction, mayhem, and tragedy: all words that come to mind when “earthquake” is heard. They occur without warning and cause millions of dollars in destruction and numerous deaths. For these reasons and more, earthquakes are one of the most unpredictable and devastating occurrences Mother Nature dishes out. Earthquakes usually occur without warning. There is a sudden slip in the earth’s crust, which makes the earth shake causing mass destruction to buildings and people in the surrounding areas. These areas in the earth’s crust are called faults. When the earth “faults” the ground bends to a certain limit until the point of breaking. When it finally snaps, it sends vibrations up to the earth’s surface where the earthquake occurs. In a nutshell, an earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time. Earthquakes have been a source of terror and wonder for as long as people have inhabited the earth. Earthquake is one of the major natural hazards in Bangladesh like many other countries threatening life, property, and economic well being. Bangladesh is an earthquake-prone country. Historical seismic catalogues (ISET, 1993) reveal that Bangladesh has been affected by earthquake disasters since ancient times. Although in recent past no major earthquake has affected this country, a major event may affect the country any moment. As Bangladesh is the world’s most densely populated area, any future earthquake shall affect more people per unit area than any other seismically active regions of the world.
2. Causes of Earthquake
2.1. Plate Tectonics
An earthquake is the violent shaking of the Earth caused by a sudden movement of rock beneath its surface. Rocks respond to stress (squeezed or pulled apart) near the Earth's surface by breaking, and when rocks move along either side of a fracture, it is called a fault. The land around a fault may shift horizontally, vertically, or a combination of these motions. Hence, there are three basic types of faults: normal, reverse, and strike-slip (lateral). (A) A normal fault is one in which the rocks above the fault plane, the hanging wall, move down relative to the rocks below the fault plane in the footwall. (B) A reverse fault is one in which the hanging wall moves up relative to the footwall. (C) When rocks on either side of a nearly vertical fault plane move horizontally, the movement is called strike-slip.
Fig. 2.1. The three basic types of faults
For hundreds of millions of years, the forces of continental drift have reshaped the Earth. Continental drift is based on the idea that the continents bumped into, and slid over and under each other and at some later time broke apart. Today, most people accept the theory that the Earth's crust is on the move, and we call this theory plate tectonics. The crust (lithosphere) is broken into about 12 enormous plates that float on hotter, softer rocks in the underlying mantle (asthenosphere). The Earth's heat drives convection currents in the asthenosphere, moving the plates past one another very slowly. Plates move mere inches annually, carrying the continents and ocean basins with them as they drift about. The majority of earthquakes worldwide occur at plate boundaries when plates stick and then jump past each other. These quakes often are the ones that are the most destructive and well understood in terms of plate tectonics.
Fig. 2.2. The plates of the Earth's crust
Earthquakes are three dimensional events, the waves move outwards from the focus, but can travel in both the horizontal and vertical plains. This produces three different types of waves which have their own distinct characteristics and can only move through certain layers within the Earth. These are as follows: A)...