EARTHQUAKES.pdf

EARTHQUAKES - is the sudden vibration of the earth’s surface by rapid release of energy. This energy is released when two parts of rock masses move suddenly in relation to each other along a fault. ORIGIN OF EARTHQUAKES Earthquakes can originate from sudden motion along a fault, from a volcanic eru...

EARTHQUAKES - is the sudden vibration of the earth’s surface by rapid release of energy. This energy is released when two parts of rock masses move suddenly in relation to each other along a fault. ORIGIN OF EARTHQUAKES Earthquakes can originate from sudden motion along a fault, from a volcanic eruption, bomb blasts, landslides, or anything else that suddenly releases energy on or in the Earth. Not every fault is associated with active earthquakes. Most faults are in fact no longer active but were active at some time in the geologic past of the active faults, only some are particularly prone to earthquakes. Some faults are slippery, and the two blocks on either side just slide by each other passively without producing major earthquakes. In other cases, however, the blocks stick together and deform until they reach a certain point at which they suddenly snap, releasing energy in an earthquake event. Rocks and materials are said to behave in a brittle way when they respond to built-up tectonic pressures by cracking, breaking, or fracturing. Earthquakes represent a sudden brittle response to built-up stress and are almost universally activated in the upper few kilometers of the earth. Deeper than this, the pressure and temperature are so high that the rocks simply deform like silly putty and do not snap, but are said to behave in a ductile manner. EFFECTS OF EARTHQUAKE 1. Buildings are damaged and people get frightened or hurt. 2. Roads are fissured, railway lines are twisted and bridged are destroyed. 3. In cities, ground waves disrupt underground services and start fires. 4. Rivers change their courses, Fissures are opened up in the ground which may cause spring. 5. Permanent tilting of the land mass may occur in certain areas. Landslides may occur in hilly regions. CLASSIFICATION OF EARTHQUAKES Classification - I. Depending upon their mode of origin, the earthquakes are classified into the follow groups. 1. Earthquakes Due to Surface Causes – Earthquakes may be generated by landslips and collapse of the roof of underground caverns. Most of these are very minor. 2. Earthquakes Due to Volcanic Causes – Volcanic eruptions may also produce earthquakes, but such earthquakes are generally feeble. 3. Earthquakes Due to Tectonic Causes – Tectonic earthquakes are the most numerous and usually the most disastrous. They are caused by shocks which originate in the earth’s crust due to sudden movement along faults. Classification – II. On the basis of the depth of focus, the earthquakes are divided into the following groups. 1. Shallow Focus Earthquakes - Earthquakes having depth of focus up to 55 km. 2. Intermediate Focus Earthquakes - Earthquakes having depth of focus between 55 to 300 km. 3. Deep Focus Earthquakes - Earthquakes having depth of focus between 300 to 650 km. TERMINOLOGIES 1. Focus (hypocenter) – the point of origin of an earthquake within the earth’s crust. From the focus, the earthquake waves radiate in all directions. 2. Epicenter – The point lying vertically above on the earth’s surface and directly above the focus. 3. Isoseismal lines – The lines connecting points of equal intensity on the ground surface. If the focus is a point, the isoseismal lines will be circles. From the distribution of the lines, the epicenter of an earthquake can be determined. 4. Earthquake Intensity - it is a measure of the degree of destruction caused by an earthquake to humans and their structures. - is expressed by a number (written in Roman numeral). Several scales exist, but the ones most commonly used in the United States are the Modified Mercalli scale and the Rossi-Forel scale. 5. Magnitude - is a number that characterizes the relative size of an earthquake. - is based on measurement of the maximum motion recorded by a seismograph. Several scales have been defined, but the most commonly used are (1) local magnitude (ML), commonly referred to as "Richter magnitude", (2) surface-wave magnitude (Ms), (3) body-wave magnitude (Mb), and (4) moment magnitude (Mw). 6. Tsunamis - are giant sea waves generated by earthquakes on the floor. - They cause heavy destruction in area lying near the sea shore. - Sea tsunamis have wave lengths as great as 200 km and can travel at a speed of 800 kph. Seismographs - Are instruments which detect and record earthquake waves. - Most seismographs contain a heavy weight suspended from a support which is attached to bed rock. When waves from a distant earthquake reach the instrument, the inertia of the weight keeps it stationary, while the earth and the support vibrate. The movement of the earth in relation to the stationary weight is recorded on a rotating drum. Some seismographs detect horizontal motion while others detect vertical motion. Seismogram - It is the recording of the ground shaking at the specific location of the seismograph. On a seismogram, the HORIZONTAL axis = time (measured in seconds) and the VERTICAL axis= ground displacement (usually measured in millimeters). Earthquake waves During an earthquake, seismic waves are generated which spread out wards in all directions from the focus. THREE TYPES OF SEISMIC WAVES 1. P-waves 2. S-waves 3. L-waves or surface waves Surface waves are responsible for causing earthquakes while the main significance of P and S waves is in the study of the earth’s interior. P- waves or Primary waves - These are compressional waves which cause the particles of rocks to vibrate in the longitudinal direction. - The P waves travel fastest, therefore it reaches the seismic station first. - Their velocity is 1.7 times that of the S-waves. - They pass through solids and liquids S- waves or Secondary waves - These are shear waves are transverse in nature. - Their velocity is less than the P waves. - The S waves travel through solids only and do not pass through liquids. L- waves of Surface waves - The L waves travel along the surface and cause earthquakes. - They are transverse in nature and their velocity is much less than the P and S waves.