Describe the global distribution of earthquakes Essay

According to plate tectonics, the global distribution of epicentres is related to boundaries between lithospheric plates. Earthquakes at plate boundaries are called interplate earthquakes. Less commonly, earthquakes also take place in plate interiors and these are called intraplate earthquakes. The most active region in the world corresponds to the margins of the Pacific Ocean. Earthquakes with large magnitudes take place along this zone in the Americas from the Aleutian Islands to southern Chile and from the Kamchatka peninsula in Asia to New Zealand. Besides shallow earthquakes, throughout most of this long region, intermediate and deep shocks take place along the margin of Central and South America and on the other side of the Pacific along the systems of island arcs (Aleutians, the Kuriles, Japan the Philippines) Another large seismically active region is known as the Mediterranean-Alpine-Himalayas region and extends from West to East from the Azores to the eastern coast of Asia. This region is related to the boundary between the plates of Eurasia to the North and Africa, Arabia, and India–Australia to the South. Its seismicity involves shallow, intermediate, and deep earthquakes. A third seismic region is formed by earthquakes located on ocean ridges that form the boundaries of oceanic plates, such as the Mid-Atlantic Ridge, East Pacific Rise, etc. In these regions earthquakes of shallow depths are concentrated in relatively narrow bands following the trend of the oceanic ridges. In general, boundaries between oceanic plates and between oceanic and continental plates have simpler distributions of seismicity than do boundaries between continental plates. Name two pieces of evidence that can be used to show the scale of the hazard at any one place. Comment on the reliability of such evidence. The most well known method of measuring the intensity of an earthquake is the Richter scale. The Richter scale is named after an American seismologist named Charles Francis Richter, and measures the amount of energy released at the focus of a quake. It uses a logarithmic scale that runs from â€Å"1† to â€Å"9. † Because this scale is logarithmic, each number is actually an increase of ten times than the number which precedes it. Thus, a 7. 0 earthquake is ten times more powerful than a 6. 0 and 100 times more powerful than a 5. 0. To allow a greater degree of precision, a decimal equivalent was provided. At one time it was believed that an earthquake with a magnitude of 8. 5 was the most powerful possible but new seismic measuring techniques have revealed that it is possible to reach 9. 5. This is reliable source as to how destructive an earthquake can be, although it does not specifically relate to how much damage will be caused, for example a less economically developed area which has a high population density will suffer greater loss than a more economically developed area which has better education, more stable buildings and emergency plans as well as sufficient communication. The intensity of an earthquake is a more reliable source of evidence as to how destructive an earthquake has been. Intensity of an earthquake depends on the distance from epicentre, and also on the local soil conditions, geology and topography. In a typical case, however, the largest intensity is observed in the vicinity of epicentre and it diminishes with the distance. It measures the total number of deaths and building failures. I believe this is more reliable as it measures the direct effect of the earthquake, for example, the total destruction of the land etc if directly proportional to the intensity and does not take into account the land use. Describe the effects of the hazard in the areas where it occurs. How earthquakes affect humans, buildings, and bridges depends on many factors. The most important factors are earthquake magnitude, the distance from the earthquake centre (called the epicentre), and the geologic conditions at a site Primary effects of earthquakes are caused directly by the earthquake and can include violent ground shaking motion accompanied by surface rupture and permanent displacement. The most significant societal impact of the Kobe earthquake was the tremendous loss of human life. In addition, for more than 300,000 survivors in the heavily impacted cities of Kobe, Ashiya, and Nishinomiya who were displaced from their homes, there were the hardships of finding shelter; securing food and water; locating friends and family members; and acquiring warm clothing for the cold, damp winter weather. Although relatives and friends took some of the displaced people in, and others possessed the means to relocate to hotels, those requiring emergency shelter reached a peak of 235,443 on the evening of January 17. Many camped in public parks or assembled makeshift shelters from materials salvaged from the wreckage of their homes. The 1,100 shelters included community centres, schools, and other available and undamaged public buildings. Facilities were too few to avoid severe crowding in some shelters, however, causing sanitation problems and increased risk of communicable disease. Indeed, two weeks after the earthquake, reports of influenza and pneumonia were common. Food, water for drinking and sanitation, blankets, and warm clothing were in short supply for at least the first few days after the earthquake, and many people from the hardest-hit wards made the long walk to the Nishinomiya Railway Station, journeyed to Osaka for necessities, then returned via rail with whatever they were able to transport by hand. Short-term secondary effects of earthquakes include liquefaction, landslides, fires, seismic sea waves (tsunami), and floods (following collapse of dams). Long-term secondary effects include regional subsidence or emergence of landmasses and regional changes in groundwater levels. Liquefaction is defined as the transformation of water saturated granular material from solid to a liquid state. During earthquakes, this may result from an increase in pore – water pressure caused by compaction during intense shaking. Liquefaction of near – surface water – saturated silts and sand causes the materials to lose their shear strength and flow. As a result, buildings may tilt or sink into the liquefied sediments; tanks or pipelines buried in the ground may float to the surface. Also the pressure generate by the shaking, forces the sand to loose its cohesive strength and to work more like a dense liquid. This leads to buildings collapsing and for sand to explode onto the surface to create ‘sand volcanoes’ and ‘boils’. Earthquake shaking commonly triggers many landslides (a comprehensive term for several types of hill slope failure) in hilly and mountainous areas. Landslides can be extremely destructive and cause great loss of life. Fire is a major secondary hazard associated with earthquakes. Shaking of the ground and surface displacements can break electrical power and gas lines and ignite fires. The threat from fire is doubled because fire-fighting equipment may be damage and water mains may be broken. The major cause of death form earthquakes is due to the collapse of buildings. The number of buildings destroyed by the Kobe earthquake exceeds 100,000, or approximately one in five buildings in the strongly shaken area. An additional 80,000 buildings were badly damaged. The large numbers of damaged traditional-style Japanese residences and small, traditional commercial buildings of three stories or less account for a great deal of the damage. In sections where these buildings were concentrated in the outlying areas of Kobe, entire blocks of collapsed buildings were common. The fires following the earthquake also destroyed several thousand buildings. Discuss the degree to which the hazard can be predicted and managed. Effective management of geological hazards is still an exclusive object for countries throughout the world. Experience has shown that, even in the most technologically developed countries, much remains to be achieved. Although considerable advances have been made in the field of geological hazard prediction, many geophysicists feel that accurate prediction of earthquakes may no longer be regarded as an achievable goal. Increasingly scientists and hazard managers are turning their attention to improving and adapting buildings and infrastructures that will withstand earthquakes. Hazard mapping, and land use zoning have important parts to play in the reduction of losses from earthquakes. The proper co-ordination of community awareness, evacuation procedures and effective response by public services is acquiring a much higher profile as a result of shortcomings revealed in recent events such as the Kobe and Armenian earthquakes. Administration of aid and relief programmes during the vital days after the occurrence of a disaster has often been criticised, particularly in the less economically developed countries, and much more competent use of resources is clearly required in many cases. Predictions of earthquakes are based largely on past patturns and generally tend to be imprecise. They are usually long term, and as we have seen, in the case of earthquakes it is unlikely that the location and magnitude of an event can be predicted with any accuracy. Forecasts are based on the evolution of an event through a series of stages that are increasingly well understood. In contrast to predictions, forecasts are often short-term and thus offer little time for effective warning to be given. Again little progress has been possible with seismic hazard forecasting. There has been considerable investment into the scientific prediction of earthquakes in areas such as the Kanto and Tokai regions of Japan and in California. In such densely urbanised and technologically complex areas the search for accurate prediction methods clearly justifies research costs. Seismic variations in the San Andreas Fault are well known. The section around the town of Parkfield is currently the site for an ongoing seismic prediction experiment. It appears that slips occur along this section of the fault at fairly regular intervals, averaging out at 22 years. The window of occurrence for the latest slip and earthquake was between1987 and 1993, but no major seismic event has yet occurred.