Seismology Introduction PDF

54 INTRODUCTION TO SEISMOLOGY 24. It is the study of elastic or sound wave in the solid earth. Seismic waves are generated at source. These are of two types natural of artificial. The resulting wave propagates through the medium (some portion of the ear...

54 INTRODUCTION TO SEISMOLOGY 24. It is the study of elastic or sound wave in the solid earth. Seismic waves are generated at source. These are of two types natural of artificial. The resulting wave propagates through the medium (some portion of the earth) and recorded at receiver (called seismometer). Seismogram (the record of the ground motion at the receiver), thus contains information about both the source and the medium. 25. The wave provides the information about the location and nature of the source. The travel time (arrival time original time) gives the information about the speed at which they travel in the medium. Thus the physical properties of the medium become important. Signal observed (amplitude and phase) on seismograms provides additional information about the medium. 26. accessible to direct observation. Seismology is the most powerful indirect method to existence of shallower crust, deeper mantle, liquid outer core and solid inner core are inferred from variations in seismic velocity with depth. Ideas about their chemical composition, presumed location of changes in mineral structure due to increase of pressure with depth, and also based on seismological data. 27. Near surface, the seismology provides detailed crustal images that reveal information about the location of economic resources like oil and minerals. Deeper in earth, seismology provides the basic dat history and evolution including the process of mantle convection. 28. Seismology is the primary method for studies of earthquakes. Most of the information about the nature of faulting during an earthquake is determined from the resulting seismograms. These observations are useful for several purposes:- (a) Because earthquakes generally result from the motions of the plates making up the earth, lithosphere which is surface expressions of convention Mantle.  55 (b) Knowledge of the direction and amount of motion is valuable for describing plate motions and the forces giving rise to them. (c) Analysis of seismogram also makes it possible to investigate the physical processes that occurs prior the, during and after faulting. (d) Such studies are helpful in assessing the societal hazard posed by earthquakes. 29. The basic data, the seismograms are the record of the motion of ground resulting from the arrival of refracted, reflected, and diffracted seismic waves. Seismograms incorporate precise time so that the travel time can be determined. If actual ground motion. As the ground motion is a vector, three different components (north-south, east-west and up-down) are recorded. Seismogram, which appears a simply wiggly line, contains interesting and useful information. 30. Several seismic wave arrivals called phases are identified using simple nomenclature that describes path each followed from the source to the receiver. If the distribution of seismic velocity near the source is known, the depth of the between the direct P and pP phases. 31. Energy released in large earthquake. The largest earthquake of the 1960 Chile earthquake: (a) About 21 m of slip occurred on a fault of 800 Km long and 200 km. across.  56 (b) Released about 1019Joule of elastic energy. (more than 2000 Mt bomb) (released energy more than all the nuclear bombs ever exploded) (c) Fortunately, the largest earthquake is infrequent. (d) Earthquake of magnitude 7 occurs approximately, monthly while the earthquake of magnitude 6 or greater occurs on average every three days. (e) Earthquake of given magnitude occurs above ten times less frequently than those one magnitude smaller. Sumatra (2004) Sumatra 32. The bound between seismology and earthquake is so strong that seismology is viewed as the science of earthquakes rather than elastic waves in the earth. Earthquake invariably occur on fault, surfaces on the earth on which one side moves with respect to other. Typically earthquakes occur on fault previously identified by geological mapping. Earthquakes which occur on land and close to the surface often leave visible ground breakage along the fault. 33. Seismic Zoning Map of India. In India, the main seismic zone runs along:- (a) Himalayan mountain range, (b) Northeast India, (c) Andaman-Nicobar islands and (d) Rann of Kutch region  57 34. Earthquakes Waves - Determination of epicenter of an earthquake. 35. The time separation between the P- and S-wave arrivals multiplied by the ratio VP·VS/ (VP-VS) of the P- and S-wave velocities gives us the epicentral distance (distance from the station to the projection of the earthquake's focus at the surface). 36. The epicenter is found within the black area where the circles cross. 37. Basic earthquake parameters. Although in fact the earthquake involves as part of the fault-plane measuring many square kilometer in area, from the point of view of an observer at a distance of hundreds or even thousands of kilometers the earthquake appears to happen at point. 38. This point is called the focus or hypocenter of the earthquake. It generally occurs at focal depth many kilometers below the Earth surface. The point on the 39. Elastic Rebound Theory. H.Reid proposed the elastic rebound theory of earthquakes on a fault. In this model, materials at distances on opposite sides of prevents the sides form slipping. Eventually the strain accumulated in the rock is more than the rock on the fault can withstand, and the fault slips resulting an earthquake.  58 40. Types of body waves. There are two types of body waves (a) P waves arrive first. Primary, pressure waves. Analogous to sound waves. (b) Particle motion is along the direction of travel (propagation) of the wave, i.e., longitudinal waves. (c) P waves can travel through solids, liquids or gases. 41. S Waves Arrive Second. These are secondary, shear waves and are slower than P waves. (a) S waves vibrate perpendicular to the direction of propagation. (b) A shear wave can be split into orthogonal, i.e., horizontal and vertical, components. (c) have any shear strength. 42. General Conclusions. µ, selected Earth materials. The following general conclusions can be drawn from it: (a) For the same material, shear waves travel always slower than compressional waves; (b) The higher the rigidity of the material, the higher the P- and S-wave velocities; (c) (d) This explains why denser rocks have normally faster wave propagation (e) Fluids (liquids or gasses) have no shear strength (µ= 0) and thus do not propagate shear waves; (f) For the same material, compressional waves travel slower through its core, through the liquid outer and solid inner iron core, respectively).  59 43. Intensity. One of the most commonly used intensity scale is Modified Mercalli intensity (MMI) scale. Uses roman numeral ranging from I (generally unfelt) to XII (total destructive). Intensity is not uniquely related to acceleration (which is the numerical parameter the seismologist compute for the earthquakes and engineers use to describe the building effects. It is often the best information available about the historical earthquakes. The variation of intensities with distance from an earthquake can be seen by plotting line of constant intensity known as Isoseismals 44. Modified Mercalli Intensity (abridged) Scale (Ref: Earthquakes by Bruce A. Bolt) Average Average peak peak Intensity Description accelerati velocity Value (cm/sec) on 2 (g=9.8m/sec ) I Not felt except by a very few under especially favorable circumstances. II Felt only by a few persons at rest, especially on upper floors of buildings. Delicately suspended objects may swing III Felt quite noticeably indoors, especially on upper floors of buildings, but many people do not recognize it as an earthquake. Standing automobiles may rock slightly. Vibrations like passing of truck. Duration estimated. IV During the day felt indoors by many, 0.015g- 1-2 outdoors by few. At night some 0.02g awakened. Dishes, windows, doors disturbed; walls make creaking sound. Sensation like heavy truck striking building. Standing automobiles rocked noticeably. V Felt by nearly everyone, many 0.03g- 2-5 awakened. Some dishes, windows and 0.04g so on broken; cracked plaster in a few places; unstable objects overturned. Disturbances of trees, poles, and other tall objects sometimes noticed. Pendulum clocks may stop. VI Felt by all, many frightened and run 0.06g- 5-8 outdoors. Some heavy furniture moved; 0.07g a few instances of fallen plaster and damage chimneys. Damage slight.  60 VII Everybody runs outdoors. Damage 0.10g- 8-12 negligible in buildings of good design 0.15g and construction; slight to moderate in well built ordinary structures; considerable in poorly build or badly designed structures; some chimneys broken. Noticed by persons driving cars. VIII Damage slight in specially designed 0.25g- 20-30 structures; considerable in ordinary 0.30g substantial buildings with partial collapse; great in poorly built structure. Panel walls thrown out of frame structures. Fall of chimneys, factory stacks, columns, monuments, and wall. Heavy furniture overturned. Sand and mud ejected in small amounts. Change in well water. Persons driving cars disturbed. IX Damage considerable in specially 0.50g- 45-55 designed structures; well-designed 0.55g frame structures thrown out of plumb; great in substantial buildings, with partial collapse. Buildings shifted off foundations. Ground cracked conspicuously. Under Ground pipes broken. X Some will-built wooden structures More than More than 60 destroyed; most masonry and frame 0.60g structures destroyed with foundations; ground badly cracked. Rail bent. Land slides considerable from riverbanks and steep slopes. Shifted sand and mud. Water splashed, slopped over banks. XI Few, if any (masonry structures remain standing. Bridges destroyed. Broad fissures in ground. Underground pipelines completely out of service. Earth slumps and land slips in soft ground. Rail bent greatly. XII Damage total. Waves seen on ground surface. Lines of sight and level distorted. Objects thrown into the air. 45. Tsunami. It is a system of gravity waves formed in the sea due to large scale disturbance of sea level over a short duration of time. It is caused by:-  61 (a) Earthquakes under the sea bottom (b) Submarine volcanic eruptions (c) Displacement of submarine sediments (d) Coastal landslides (e) Meteor impact 46. However, not all coastal earthquakes produce Tsunamis. 47. Tsunami velocity is dependent on the depth of water through which it travels (Velocity equals the square root of water depth h times the gravitational acceleration Tsunamis travel approximately 700 kmph in 4000 m depth of sea water. In 10 m of water depth the velocity drops to about 36 kmph.Period of Tsunami waves is 10-45 minutes. 48. Early Warning System for Tsunami. The Early Warning System for Tsunami and Storm Surges being setup by the Ministry of Earth Science would enable generation and issue of timely and reliable warning and watch advisories on: (a) Tsunami (e.g. potential for Tsunami, confirmation of Tsunami, estimated arrival times of initial wave, forecast of Tsunami strength, surge height at landfall points, extent of inundation, warning cancellation),  62 (b) Storm Surges (e.g. forecast of surge height at landfall points, extent of inundation, warning cancellation) and Storm Surge / Tsunami Vulnerability Maps 49. The building blocks of Early Warning System for Tsunami and Storm Surges. 50. Steps In Tsunami Warning. When a tsunami event occurs, the first information available about the source of the tsunami is based only on the available seismic information for the earthquake event. 51. As the tsunami wave propagates across the ocean and successively reaches the Deep Ocean Assessment and Reporting Systems (DOARS), Tide Gauge Subsystem, Coastal Radar (CODAR) Subsystem, these systems report sea level information measurements back to theTsunami Warning Centers, where the information is processed to produce a new and more refined estimate of the tsunami source. The result is an increasingly accurate forecast of the tsunami that can be used to issue watches, warnings or evacuations.