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Earthquake Common Terminology.: Understanding the terminology related with earthquakes is very important to deal with earthquake issues, even for a generalist involved in planning and execution of projects. |
Geological Faults: The release of energy resulting into an earthquake begins somewhere on a Geological Fault, which is a surface (vertical or inclined from the vertical) embedded in the earth, along which differential movement has take place. |
Earthquake Ground Motion: Energy released from an earthquake is transmitted in the form of vibrations. In the earthquake affected area every point on the ground vibrates in a certain manner, the particle motion depending on the type of vibration it receives. The particle motion can be subjected to harmonic analysis, characterized by amplitude and frequency of the ground motion. It is possible to estimate the amplitude of the earthquake ground motion in terms of the displacement, velocity and acceleration of the particle on the ground surface. |
Plate Tectonics: The upper portion of the Earth, including the crust and the upper most upper mantle is in a mobile state with respect to the lower portion of the Earth. The surface of the Earth is divided into segments called plates. Plates are separated by boundaries, and are moving relative to each other along the boundaries. Major earthquakes occur along plate boundaries. Many seismological and geological phenomena are explained using what is now called the theory of plate tectonics. |
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Earthquake
Prediction:
Scientific
research is on to predict earthquake occurrences, and some
successes have also been achieved. There are several instances where earthquakes were
predicted with varying degree of success. Earthquake prediction is based on certain precursors, which were
identified after studies of data on past earthquakes. Some changes were observed
before the arrival of a strong earthquake, and the duration for which the change
existed, was found to be dependent on the magnitude of the earthquake. |
Associating Earthquakes With Faults: Earthquake epicenters, when plotted on a tectonic map, rarely lie along the lineaments of known faults. This results from several factors, e.g. inaccuracy in the earthquake source location, a less accurately defined fault lineament, branching along the fault and dipping of the fault surface away from vertical. Associating the observed earthquakes with known faults is a difficult, but important , step in fixing the earthquake design basis. |
| Seismic Waves: Earthquake Energy released at the earthquake source travels in different directions in the form of seismic waves. These waves are of different types, e.g. body waves traveling through the Earth's interior, surface waves traveling along the surface. These studies have enabled precise estimates of travel times of the earthquake signals to earthquake recoding stations on the one hand and velocities of the different types of seismic waves as a function of depth leading to detailed internal structure of the Earth. | Seismicity Studies: Earthquakes occurrences have shown a certain pattern, larger the fewer. It has been possible to determine earthquake magnitude frequency relationships on global as well as regional basis. It has also been possible to study the attenuation of earthquake ground motion as it travels from the earthquake source to a receiving station. Studies of earthquake ground motion and earthquake frequencies and relating these to different earthquake environments, e.g. source and transmission path characteristics have been clubbed under what are called seismcity studies. Seismicity studies play an important role in estimating the probability of a site experiencing a certain level of ground motion from future earthquakes.. |