Large earthquakes are thought to occur more or less regularly in space along major fault systems and, in time, as a result of gradual stress buildup and sudden release by failure. This repetitive cycle of strain accumulation and release, termed the seismic or earthquake cycle, is driven by plate tectonics along the worlds major plate boundaries and fault systems.
(National Research Council, 55)
California has had at least eighty magnitude 6.0 or larger earthquakes in the past two hundred years or so, since the first European settlements on the land. A number of these earthquakes, including the 8.3 magnitude 1906 San Francisco disaster, have occurred along the San Andreas fault, a tectonic plate boundary that is an earthquake threat for both Los Angeles and San Francisco. The San Andreas Fault runs through central and southern California, spanning hundreds of kilometers (Fradkin 4). We can better understand what the San Andreas fault is and why areas flanking this fault line are forever prone to earthquakes by exploring the fundamental geological dynamics of earthquakes, i.e., plate tectonics. The study of plate tectonics helps us have a better notion of how the San Andreas Fault could cause such a huge disaster in the past and can very likely cause an even bigger one in the near future,
Plate tectonics is the study of subterranean movements of the planet. The interior of the Earth is divided into four distinct layers. In the center of Earth is the spherical core, which is made of solid nickel and iron. This is surrounded by outer core, a layer of molten nickel and iron. Thickly wrapped around the Earths liquid and solid cores is the mantle, which is made of molten rock. The top layers of the mantle are called the asthenosphere and lithosphere, which are chiefly responsible for creating earthquakes.
(Source Klous, 24)
Above these layers is the Earths periphery called crust, which is made of solid rock, and rests on top of the uppermost mantle. The part of the crust that makes the continents is 35 to 70 km thick. The part under the oceans, the ocean crust, is much thinner and is only about 5 to 10 km thick. The huge pressure form the crust stops the incredibly hot rock in the mantle from turning into liquid. The solid-rock crust is only the thin outer layer of the Earth, with the mantle taking up nearly of half the earths radius and the outer and inner cores taking up about another half. (Geology.com).
You can think of Earth as something like at giant piece of fruit. Its inner core is like the pit of a cherry or peach. The mantle is like the layer of juicy pulp in the middle of the fruit. The crust is lie the outer skin of the fruit. Compared to the size of the whole earth, the crust is a rather thin skin. (Silverstein et al., 9)
4.5 billion years ago, the Earth was formed out of hot gases. As the Earth cooled, the lithosphere cracked and split into seven large and twelve small floating pieces of a jigsaw puzzle, with uneven and ragged edges. These huge pieces of crust and the upper part of the lithosphere are called tectonic plates, and continuously move over the viscous mantle rubbing and pushing against each other. These plates move at somewhat the same speed as human fingernails grow, ranging from 10 to 130 millimeters per year. The tectonic plates have to move because of the slow swirling movements of semi-molten rock deep in the mantle.
Plate tectonics studies the crunching, grinding and jostling and other dynamics of friction between the giant rocky plates under the Earths surface, on which are situated continents and oceans. The boundaries between two such massive, constantly moving tectonic plates are called faults. When the Earths plates move against each other, the lithosphere absorbs much stress. However, when this stress accumulates up to a breaking point, the lithosphere breaks or shifts (Glasscoe).
An earthquake is caused by a sudden, violent shifting of the tectonic plates which releases stress that accumulates along geologic faults. The areas lying in vicinity of the fault line are especially susceptible to earthquakes (although significant tremors can occur in relatively stable interior of continents). The San Andreas Fault is a fracture appearing where the North American and the Pacific plates slide past each other. Passing through south California, it is perhaps the most well-known of faults, infamous for causing severe earthquakes.
The theory of plate tectonics came into its own by 1970s, developing from the continental drift theory which was originally proposed by Alfred Wegner in 1912 and being further bolstered by the Sea Floor Spreading Theory that emerged in the 1960s. Plate tectonics is the science of earthquakes, it also explains the sweeping geological transformation of the planetary landscape over hundreds of millions of years. The theory of plate tectonics has revolutionized the thought paradigm in earth sciences during the past couple of decades. Most of the geological action such as the formation of mountains, volcanoes, rift valleys, is caused by the complex interactions happening at plate boundaries.
Although a knowledge of the planets past and future geological evolution is crucial for understanding our biological evolution as well as for a better understanding of our world in general, the study of plate tectonics is much more relevant and urgent because it explains why earthquakes occur, and why they do not occur randomly, with a majority of them being distributed across relatively narrow and clearly demarcated areas of earths surface called seismic zones.
The narrow belts of seismic zones happen to be along the boundaries of plates or fault lines, with the interior of these plates being relatively safe and free from earthquakes. Anomalous earthquakes that occur in the interior region of a tectonic plate, despite constituting less than one percent of worlds total number of earthquakes, can still pose significant threat in terms of damage they cause.
The planet Earth has seven major crustal plates, which are subdivided into many smaller plates. The pattern of their movement is highly complex and shows very little symmetry. The complexity involved is of such a degree that the more we learn about the structure and behavior of the major plates, the more complicated and intricate the motions and maneuvers of these plates have become.
The forbidding complexity of the movement of earths tectonic plates is sadly disappointing because it severely limits our abilities to predict earthquakes in the near future. Although it is easy to point out the general regions on the Earths surface where large-scale earthquakes can be expected, it is still nearly impossible to predict when and where exactly a large earthquake would occur, with any significant degree of accuracy. This is because the general time frame for notable changes happening in plate tectonics is very large, spanning millions of years. Over the span of merely a year or a decade, the minute motions of the plates amount to only several centimeters. Therefore it would be difficult to estimate where exactly an earthquake-prone area such as San Francisco or Los Angeles is positioned in the gradual worldwide processes of strain buildup and strain release.
Close monitoring of movements of stress and strain in local areas can considerably increase our chances of predicting the onset of renewed activity at a fault line, but accuracy still remains an elusive goal. Plate tectonics can lead us to develop more sophisticated techniques for earthquake prediction, and there is still vast scope of improvement in the earthquake predicting technologies we currently have. By the late twentieth century, earthquake predicting technologies were considered to be still in their infancy. Although there have been significant developments in the past decade in this direction, they have not yet led to a dramatic breakthrough, or do not even promise one yet.
Geological technology developed from our knowledge of plate tectonics may reach a point within the foreseeable future at which scientifically credible earthquake predictions can be made. Some scientists such as Robert J.Geller, a US geologist at the University of Tokyo, however, suggest that earthquake prediction may be inherently impossible. While it remains to be seen how far we can improve our capacity of earthquake prediction in the next several years, the potential havoc that earthquakes can wreak is rapidly increasing, as the tension keeps building up under major metropolitan areas such as Los Angeles and San Francisco. If a major earthquake were to occur in heavily populated areas lying in the vicinity of the San Andreas in the near future and major earthquakes are already overdue in many parts of California overlying the fault zone it could lead to a tragedy that may dwarf the great 1906 San Francisco earthquake in comparison. As scientists keep trying to render the predictive methods more effective, the stakes in terms of lives and property that could be saved are increasing at an alarming pace.
(National Research Council, 55)
California has had at least eighty magnitude 6.0 or larger earthquakes in the past two hundred years or so, since the first European settlements on the land. A number of these earthquakes, including the 8.3 magnitude 1906 San Francisco disaster, have occurred along the San Andreas fault, a tectonic plate boundary that is an earthquake threat for both Los Angeles and San Francisco. The San Andreas Fault runs through central and southern California, spanning hundreds of kilometers (Fradkin 4). We can better understand what the San Andreas fault is and why areas flanking this fault line are forever prone to earthquakes by exploring the fundamental geological dynamics of earthquakes, i.e., plate tectonics. The study of plate tectonics helps us have a better notion of how the San Andreas Fault could cause such a huge disaster in the past and can very likely cause an even bigger one in the near future,
Plate tectonics is the study of subterranean movements of the planet. The interior of the Earth is divided into four distinct layers. In the center of Earth is the spherical core, which is made of solid nickel and iron. This is surrounded by outer core, a layer of molten nickel and iron. Thickly wrapped around the Earths liquid and solid cores is the mantle, which is made of molten rock. The top layers of the mantle are called the asthenosphere and lithosphere, which are chiefly responsible for creating earthquakes.
(Source Klous, 24)
Above these layers is the Earths periphery called crust, which is made of solid rock, and rests on top of the uppermost mantle. The part of the crust that makes the continents is 35 to 70 km thick. The part under the oceans, the ocean crust, is much thinner and is only about 5 to 10 km thick. The huge pressure form the crust stops the incredibly hot rock in the mantle from turning into liquid. The solid-rock crust is only the thin outer layer of the Earth, with the mantle taking up nearly of half the earths radius and the outer and inner cores taking up about another half. (Geology.com).
You can think of Earth as something like at giant piece of fruit. Its inner core is like the pit of a cherry or peach. The mantle is like the layer of juicy pulp in the middle of the fruit. The crust is lie the outer skin of the fruit. Compared to the size of the whole earth, the crust is a rather thin skin. (Silverstein et al., 9)
4.5 billion years ago, the Earth was formed out of hot gases. As the Earth cooled, the lithosphere cracked and split into seven large and twelve small floating pieces of a jigsaw puzzle, with uneven and ragged edges. These huge pieces of crust and the upper part of the lithosphere are called tectonic plates, and continuously move over the viscous mantle rubbing and pushing against each other. These plates move at somewhat the same speed as human fingernails grow, ranging from 10 to 130 millimeters per year. The tectonic plates have to move because of the slow swirling movements of semi-molten rock deep in the mantle.
Plate tectonics studies the crunching, grinding and jostling and other dynamics of friction between the giant rocky plates under the Earths surface, on which are situated continents and oceans. The boundaries between two such massive, constantly moving tectonic plates are called faults. When the Earths plates move against each other, the lithosphere absorbs much stress. However, when this stress accumulates up to a breaking point, the lithosphere breaks or shifts (Glasscoe).
An earthquake is caused by a sudden, violent shifting of the tectonic plates which releases stress that accumulates along geologic faults. The areas lying in vicinity of the fault line are especially susceptible to earthquakes (although significant tremors can occur in relatively stable interior of continents). The San Andreas Fault is a fracture appearing where the North American and the Pacific plates slide past each other. Passing through south California, it is perhaps the most well-known of faults, infamous for causing severe earthquakes.
The theory of plate tectonics came into its own by 1970s, developing from the continental drift theory which was originally proposed by Alfred Wegner in 1912 and being further bolstered by the Sea Floor Spreading Theory that emerged in the 1960s. Plate tectonics is the science of earthquakes, it also explains the sweeping geological transformation of the planetary landscape over hundreds of millions of years. The theory of plate tectonics has revolutionized the thought paradigm in earth sciences during the past couple of decades. Most of the geological action such as the formation of mountains, volcanoes, rift valleys, is caused by the complex interactions happening at plate boundaries.
Although a knowledge of the planets past and future geological evolution is crucial for understanding our biological evolution as well as for a better understanding of our world in general, the study of plate tectonics is much more relevant and urgent because it explains why earthquakes occur, and why they do not occur randomly, with a majority of them being distributed across relatively narrow and clearly demarcated areas of earths surface called seismic zones.
The narrow belts of seismic zones happen to be along the boundaries of plates or fault lines, with the interior of these plates being relatively safe and free from earthquakes. Anomalous earthquakes that occur in the interior region of a tectonic plate, despite constituting less than one percent of worlds total number of earthquakes, can still pose significant threat in terms of damage they cause.
The planet Earth has seven major crustal plates, which are subdivided into many smaller plates. The pattern of their movement is highly complex and shows very little symmetry. The complexity involved is of such a degree that the more we learn about the structure and behavior of the major plates, the more complicated and intricate the motions and maneuvers of these plates have become.
The forbidding complexity of the movement of earths tectonic plates is sadly disappointing because it severely limits our abilities to predict earthquakes in the near future. Although it is easy to point out the general regions on the Earths surface where large-scale earthquakes can be expected, it is still nearly impossible to predict when and where exactly a large earthquake would occur, with any significant degree of accuracy. This is because the general time frame for notable changes happening in plate tectonics is very large, spanning millions of years. Over the span of merely a year or a decade, the minute motions of the plates amount to only several centimeters. Therefore it would be difficult to estimate where exactly an earthquake-prone area such as San Francisco or Los Angeles is positioned in the gradual worldwide processes of strain buildup and strain release.
Close monitoring of movements of stress and strain in local areas can considerably increase our chances of predicting the onset of renewed activity at a fault line, but accuracy still remains an elusive goal. Plate tectonics can lead us to develop more sophisticated techniques for earthquake prediction, and there is still vast scope of improvement in the earthquake predicting technologies we currently have. By the late twentieth century, earthquake predicting technologies were considered to be still in their infancy. Although there have been significant developments in the past decade in this direction, they have not yet led to a dramatic breakthrough, or do not even promise one yet.
Geological technology developed from our knowledge of plate tectonics may reach a point within the foreseeable future at which scientifically credible earthquake predictions can be made. Some scientists such as Robert J.Geller, a US geologist at the University of Tokyo, however, suggest that earthquake prediction may be inherently impossible. While it remains to be seen how far we can improve our capacity of earthquake prediction in the next several years, the potential havoc that earthquakes can wreak is rapidly increasing, as the tension keeps building up under major metropolitan areas such as Los Angeles and San Francisco. If a major earthquake were to occur in heavily populated areas lying in the vicinity of the San Andreas in the near future and major earthquakes are already overdue in many parts of California overlying the fault zone it could lead to a tragedy that may dwarf the great 1906 San Francisco earthquake in comparison. As scientists keep trying to render the predictive methods more effective, the stakes in terms of lives and property that could be saved are increasing at an alarming pace.
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