Crustal deformation
Crustal deformation refers to the changes in shape structure and position that occur on the Earths surface and the rocks underneath the ground due to tension and stress resulting from movement of Earths crust. This movement occurs as a result of movement of continental plates adjacent to each other along plate boundaries. Grounds adjacent to active volcanoes deforms as a result of frequent shakings during the eruptions and changes in land masses below and above the earth crust. Crustal deformation involves a comparative movement of locations on the surface of the earth or its crust, tilting of the ground, strain and slipping due to internal forces emanating from the earths crust (George, 21). In most cases, earths crust deforms due to movement of the continental plates. The crust adjacent to fault lines deforms in periods preceding, during and immediately after the incidence of an earthquake.
There are two kind s of deformations, elastic and plastic deformations which are dependent on the type of rocks the forces are being subjected to the earth crust. In elastic deformation, the rocks changes in their shape but the change is reversible as the rocks regain their original shape and structure once the stress is removed. In the plastic deformation, the damage is an irreversible deformation as the rocks changes in their shape permanently. However, brittle rocks behave differently from ductile rocks in the even t of a deformation. Brittle rocks break once they are subjected to a stress while ductile rocks bend producing visible folds. These breaks occur in form of faults. At some moments, large amounts of stresses are subjected on a vast rock to an extent that it exceeds their yield limit. In this case, these rocks results in faulting along the surface next or at a distance far away where there a weak point develops. Movement occurs along these fault lines (Grant, 189). In another scenario, prolonged uniform stresses on ductile rocks results to folding of rocks. This type of crustal deformation occurs due to compression and is common along the plate boundaries that are under collision. This results in folds that turn upwards and are called anticlines while the downward folds are known as synclines. Along the axial plane of such a fold, one can divide the fold into two symmetrical folds.
In the event of an earthquake, volcanic eruption and other plate tectonics, abnormal amounts of stress are subjected on the crust of the earth along the boundaries of the plates of the lithosphere and regions exposed to convectional currents. This results in a pull and tearing of the crustal layer above these surfaces. Three types of stress are involved during crustal deformations. First the compression stress occurs in an event when a series of rocks are pushed against each other (Williams, 18). This is what happens when two plate tectonics collide with each other. As a result, the rock adjacent to each other tends to reduce in size laterally and thicken vertically due to these compressional stresses. Secondly, tensional stresses which results in pulling or stretching the crust apart. This is what happens when crust is moving away from each other resulting to thinning and lengthening of the crust. In this case, fault lines occurs and as the two plates rifts apart a forming a depression like the Great Rift Valley of Africa. Thirdly, shearing stress results from two plates sliding past each other in opposing directions along their fault lines or shearing boundaries (Koster, 28). This results in horizontal displacement of land marks and positions.
Application of Global Positioning Systems
Global Positioning System (GPS) involves use of a set of 24 satellites which are orbiting around the earth and can be used to give data about what is happening on the earths surface. They transmit messages to receivers on the earth using two radio frequencies which can later be analysed to get accurate and comparative information about what is happening on the earth. Traditionally, GPS was designed for use for military operations but has eventually found a lot of use in other scientific and geo-science physics applications. Global Positioning System data in geo-science has become a more specific way of detecting where there is presence and absence of tectonic movement. Nowadays, the earth scientists have the ability to arrive at more accurate information as a result of improved processing technologies applied by the GPS signals. The application of carrier tracking technique allows earth scientists to establish the length of a given baseline to an accuracy of few millimetres (Smith, 45). Using this methodology to determine changes in locations of coordinates and their baseline lengths in tri-orthogonal directions which is computed using data from GPS enables one to assess the extent of crustal deformations.
The analysis of changes in rates of such deformation has essential value in understanding the geo-physics of earthquakes. Studies in crustal deformation have gained momentum across different regions on the world due to the efficient coverage of GPS system. In addition, there has been an improved access to receivers of a low cost, increased cooperation from international Global Positioning system services for Geodynamics (IGS). In some regions, GPS networks has been designed and applied in monitoring the strain on earth crust for earthquake research work, forecasting and sensing crustal deformations and displacements (Redform, 32). In most of these countries, these receivers are applied permanently in form of a network to collect and process data endlessly so as to have an upgraded baseline vectors.
Collection of GPS data and analysis
This system applies the exposed crust or bed rocks or other permanent pillars on whose view from the sky is not obstructed in absence of any reflective environment as their sites. Most of the monuments made on expansive bedrocks are always taken to have minimal instabilities. The data from these sites is received in two frequencies by the Global Positioning System satellite receivers at a dual frequency. The resulting distance from the site on the earth surface to the satellite is derived by taking the time each radio signal from the satellite takes to reach to the collection site. The computation of this distance is done through a cross correlation of simulated random code produced by the satellite to the receiver. The signals from these sites are best transmitted when the sky is clear due when there ids no interference from the clouds. Such data is organized into a segment of 24 hour sections. The data collected is thereafter analysed using GAMIT software which was developed specially for analysing GPS data (Hsu, et al, 9). The data gives estimates with an associated variance one site to the other with a non-fixed restriction on the set measurement. Such distances measured using this kind of ranges are called code pseudo ranges. This acts as the basis of analysis through their changes. Year after year, these data is compared and the resulting variation is taken to predict the reasons behind such tendencies.
The occurrence of earthquakes on the Pacific Northwest of United states has gained a lot of focus which has brought about a lot of concern about the possibility of this region being struck by an earthquake of strong magnitude. Earlier occurrences of earthquakes of high magnitudes in the Pacific Northwest had no recognition in the periods preceding 1980s. In recent years, scientists have unearthed evidence of occurrence of multiple great earthquakes with a magnitude ranging from 8.0 to 9.0 in this region during the past thousand years and in the recent three centuries (Abidin, et al, 4). This has had a lot of effects in the change of building codes as a precautionary measure for any eventuality of an earthquake of similar magnitude. In the recent days, the resident s of this region has taken precautionary steps which will reduce their vulnerability in terms of losses in case such an earthquake occurs.
Information gathered from GPS data from a quite number of sites over the western edge of Washington reveals that, there has been a slippage that has been occuring along the boundary line between the tectonic plates of Juan de Fuca and the Northern America. This slippage is occurring at a depth of 25 kilometres beneath the earths surface. The bad thing is that, this fault is likely to trigger an earthquake with a magnitude above 9.0 with an epicentre located at a distance more than 60 kilometres in the inland. The likely damage would be enormous. The GPS data has also revealed an existence of seafloor spread or stretching due to tensional stresses which is pushing the eastern boundary of Juan de Fuca plate eastward underneath the North American tectonic plate (Gandalfi, Vittuari, 1). The Juan de Fuca plate spreads from Northern part of California extending to Southern part of British Columbia.
A long term examination shows that, these two plates are coming together at a mean rate between three to four centimetres per year. By use of GPS system, this unique occurrence can be observed on weekly basis. These two plates merge within a shallow depth a point where the crust of the earth is moderately cold and the tectonic boundary becomes locked. As a result, there occurs a build up of seismic stress till it gets released through a quake. At the boundaries of the western part of Washington, at a depth of 25 to 45 kilometres, these two plates have been sliding against each other producing silent quakes of very low magnitudes between few days of every fortnight or so (Hsu, et al, 13). However, if these plates were to release this energy at a go, it would result on an earthquake of a magnitude of 6.0 to 7.0.
In the long-term, the GPS data gives information regarding the positions where this slippage is happening and the resultant deformation from this slippage. Combining the present trends with the previous data from seismic records, the study has shown that there is a stress build up occurring along the boundaries of these two tectonic plates at a depth lower than 25 kilometres especially at points where these plates have locked on each other. From this data, if a major quake raptures the over three hundred kilometre length of Cascadia subduction area running along the coastal zone of Washington it would result to an earth quake of over 8.5 in magnitude. In addition, if the whole length under seduction slips at one moment, this would result in an earthquake of a magnitude of over 9.0 in magnitude (Abidin, et al, 7). As mentioned earlier, scientists have discovered that earthquakes of this kind of magnitude occur in this area after a period of four hundred to six hundred years. This is evidenced by the discovery of remains of submerged forests along the costal of Washington, Northern California and Oregon in 1980s. In their findings, they also found layers of sand which were deposited by tsunamis. The presence of sand-filled cracks could also have been as a result of high magnitude earthquakes. Most tsunamis results from large quakes (Williams, 49). These quakes could have had a magnitude of over 8.0. From these findings, it is very true that, the Northwest Pacific coast had been struck by a series of high magnitude earthquakes over the past number of thousand years. The most recent similar quake could have occurred at around 1700.
In a more recent past such as in 1989, a quake measuring about 7.1 in magnitude struck thee northern part of California. The damage from the quake heightened worries of most of the Northern Pacific residents over eminent danger posed by a similar earthquake. This resulted in concerted efforts from all stakeholders involved in building and construction industry. This saw some revisions in Uniform Building Codes (UBC) for Washington and Oregon to accommodate the safety factor due to threat of such earthquakes (Koster, 81). In itself, the UBC contains a set of standards that are applied in the whole nation as a guide during designing of structures. The variation in these standards across the states specially the states adjacent to the Pacific Northwest is based on a consideration of expected vibrations on structures resulting from shakes from quakes.
This code recognizes six levels of hazardous earthquake shakings. Prior to 1994, the standard had placed the Puget Sound region of Washington and Seattle at the second threat level with other areas of Washington an d Seattle at the third level. However, the revised code has placed has raised their levels due to their proximity to the earthquake prone zones. This has made the new designs to be done in a way that ca help them withstand more than fifty percent more shakes from earthquakes than before the revision of the code was implemented. This is a vital step towards achieving the preparedness on the Pacific Northwest region which is now a high earthquake threat area (Grant, 194). These discoveries have played a big role in convincing the public officials and other executives of corporate in this region to reinforce the structures of their buildings, schools, bridges, dams, and other basic residential structures. Over 130 million US dollars has been sent on such upgrades all over the Pacific Northwest region.
The recent Chilean earthquake has also awakened the attention of the ongoing undersea fault along the Pacific Northwest which has al the potential of producing a similar earthquake. A similar quake would transmit enormous waves of ocean waters over the inland next to the Pacific Ocean in a matter of minutes. This brings a more concern as it brings about a possibility of the locked portion of the Northern part of California and the Southern Oregon breaking away in the next fifty years to eighty percent. This means that, the Pacific Northwest should be prepared for a great quake before the end of the next half century (George, 56). However, this region is very familiar to violent earth quakes as in 2001, a quake with a magnitude of 6.8 occurred in this region of Pacific Northwest with its epicentre centred close to Olympia.
In summary, crustal deformation is a resultant of plate tectonics which is an on-going process within different plates that makes earths crust. Resultant deformations can vary depending on the magnitude and frequency of the stress tat they are exposed to. Volcanic eruptions have been found to have a significant effect on the stability of the earths crust and can result to varying deformations. Earthquakes, is a representation of high level of plate tectonics with severe effects in changing landforms. The study of occurrence and predictability of earthquake occurrence has been made possible by the emergence and application of Global positioning systems. This system has the advantage of carrying out studies of earth surface from a periphery such as space through the help of a satellite.
Crustal deformation refers to the changes in shape structure and position that occur on the Earths surface and the rocks underneath the ground due to tension and stress resulting from movement of Earths crust. This movement occurs as a result of movement of continental plates adjacent to each other along plate boundaries. Grounds adjacent to active volcanoes deforms as a result of frequent shakings during the eruptions and changes in land masses below and above the earth crust. Crustal deformation involves a comparative movement of locations on the surface of the earth or its crust, tilting of the ground, strain and slipping due to internal forces emanating from the earths crust (George, 21). In most cases, earths crust deforms due to movement of the continental plates. The crust adjacent to fault lines deforms in periods preceding, during and immediately after the incidence of an earthquake.
There are two kind s of deformations, elastic and plastic deformations which are dependent on the type of rocks the forces are being subjected to the earth crust. In elastic deformation, the rocks changes in their shape but the change is reversible as the rocks regain their original shape and structure once the stress is removed. In the plastic deformation, the damage is an irreversible deformation as the rocks changes in their shape permanently. However, brittle rocks behave differently from ductile rocks in the even t of a deformation. Brittle rocks break once they are subjected to a stress while ductile rocks bend producing visible folds. These breaks occur in form of faults. At some moments, large amounts of stresses are subjected on a vast rock to an extent that it exceeds their yield limit. In this case, these rocks results in faulting along the surface next or at a distance far away where there a weak point develops. Movement occurs along these fault lines (Grant, 189). In another scenario, prolonged uniform stresses on ductile rocks results to folding of rocks. This type of crustal deformation occurs due to compression and is common along the plate boundaries that are under collision. This results in folds that turn upwards and are called anticlines while the downward folds are known as synclines. Along the axial plane of such a fold, one can divide the fold into two symmetrical folds.
In the event of an earthquake, volcanic eruption and other plate tectonics, abnormal amounts of stress are subjected on the crust of the earth along the boundaries of the plates of the lithosphere and regions exposed to convectional currents. This results in a pull and tearing of the crustal layer above these surfaces. Three types of stress are involved during crustal deformations. First the compression stress occurs in an event when a series of rocks are pushed against each other (Williams, 18). This is what happens when two plate tectonics collide with each other. As a result, the rock adjacent to each other tends to reduce in size laterally and thicken vertically due to these compressional stresses. Secondly, tensional stresses which results in pulling or stretching the crust apart. This is what happens when crust is moving away from each other resulting to thinning and lengthening of the crust. In this case, fault lines occurs and as the two plates rifts apart a forming a depression like the Great Rift Valley of Africa. Thirdly, shearing stress results from two plates sliding past each other in opposing directions along their fault lines or shearing boundaries (Koster, 28). This results in horizontal displacement of land marks and positions.
Application of Global Positioning Systems
Global Positioning System (GPS) involves use of a set of 24 satellites which are orbiting around the earth and can be used to give data about what is happening on the earths surface. They transmit messages to receivers on the earth using two radio frequencies which can later be analysed to get accurate and comparative information about what is happening on the earth. Traditionally, GPS was designed for use for military operations but has eventually found a lot of use in other scientific and geo-science physics applications. Global Positioning System data in geo-science has become a more specific way of detecting where there is presence and absence of tectonic movement. Nowadays, the earth scientists have the ability to arrive at more accurate information as a result of improved processing technologies applied by the GPS signals. The application of carrier tracking technique allows earth scientists to establish the length of a given baseline to an accuracy of few millimetres (Smith, 45). Using this methodology to determine changes in locations of coordinates and their baseline lengths in tri-orthogonal directions which is computed using data from GPS enables one to assess the extent of crustal deformations.
The analysis of changes in rates of such deformation has essential value in understanding the geo-physics of earthquakes. Studies in crustal deformation have gained momentum across different regions on the world due to the efficient coverage of GPS system. In addition, there has been an improved access to receivers of a low cost, increased cooperation from international Global Positioning system services for Geodynamics (IGS). In some regions, GPS networks has been designed and applied in monitoring the strain on earth crust for earthquake research work, forecasting and sensing crustal deformations and displacements (Redform, 32). In most of these countries, these receivers are applied permanently in form of a network to collect and process data endlessly so as to have an upgraded baseline vectors.
Collection of GPS data and analysis
This system applies the exposed crust or bed rocks or other permanent pillars on whose view from the sky is not obstructed in absence of any reflective environment as their sites. Most of the monuments made on expansive bedrocks are always taken to have minimal instabilities. The data from these sites is received in two frequencies by the Global Positioning System satellite receivers at a dual frequency. The resulting distance from the site on the earth surface to the satellite is derived by taking the time each radio signal from the satellite takes to reach to the collection site. The computation of this distance is done through a cross correlation of simulated random code produced by the satellite to the receiver. The signals from these sites are best transmitted when the sky is clear due when there ids no interference from the clouds. Such data is organized into a segment of 24 hour sections. The data collected is thereafter analysed using GAMIT software which was developed specially for analysing GPS data (Hsu, et al, 9). The data gives estimates with an associated variance one site to the other with a non-fixed restriction on the set measurement. Such distances measured using this kind of ranges are called code pseudo ranges. This acts as the basis of analysis through their changes. Year after year, these data is compared and the resulting variation is taken to predict the reasons behind such tendencies.
The occurrence of earthquakes on the Pacific Northwest of United states has gained a lot of focus which has brought about a lot of concern about the possibility of this region being struck by an earthquake of strong magnitude. Earlier occurrences of earthquakes of high magnitudes in the Pacific Northwest had no recognition in the periods preceding 1980s. In recent years, scientists have unearthed evidence of occurrence of multiple great earthquakes with a magnitude ranging from 8.0 to 9.0 in this region during the past thousand years and in the recent three centuries (Abidin, et al, 4). This has had a lot of effects in the change of building codes as a precautionary measure for any eventuality of an earthquake of similar magnitude. In the recent days, the resident s of this region has taken precautionary steps which will reduce their vulnerability in terms of losses in case such an earthquake occurs.
Information gathered from GPS data from a quite number of sites over the western edge of Washington reveals that, there has been a slippage that has been occuring along the boundary line between the tectonic plates of Juan de Fuca and the Northern America. This slippage is occurring at a depth of 25 kilometres beneath the earths surface. The bad thing is that, this fault is likely to trigger an earthquake with a magnitude above 9.0 with an epicentre located at a distance more than 60 kilometres in the inland. The likely damage would be enormous. The GPS data has also revealed an existence of seafloor spread or stretching due to tensional stresses which is pushing the eastern boundary of Juan de Fuca plate eastward underneath the North American tectonic plate (Gandalfi, Vittuari, 1). The Juan de Fuca plate spreads from Northern part of California extending to Southern part of British Columbia.
A long term examination shows that, these two plates are coming together at a mean rate between three to four centimetres per year. By use of GPS system, this unique occurrence can be observed on weekly basis. These two plates merge within a shallow depth a point where the crust of the earth is moderately cold and the tectonic boundary becomes locked. As a result, there occurs a build up of seismic stress till it gets released through a quake. At the boundaries of the western part of Washington, at a depth of 25 to 45 kilometres, these two plates have been sliding against each other producing silent quakes of very low magnitudes between few days of every fortnight or so (Hsu, et al, 13). However, if these plates were to release this energy at a go, it would result on an earthquake of a magnitude of 6.0 to 7.0.
In the long-term, the GPS data gives information regarding the positions where this slippage is happening and the resultant deformation from this slippage. Combining the present trends with the previous data from seismic records, the study has shown that there is a stress build up occurring along the boundaries of these two tectonic plates at a depth lower than 25 kilometres especially at points where these plates have locked on each other. From this data, if a major quake raptures the over three hundred kilometre length of Cascadia subduction area running along the coastal zone of Washington it would result to an earth quake of over 8.5 in magnitude. In addition, if the whole length under seduction slips at one moment, this would result in an earthquake of a magnitude of over 9.0 in magnitude (Abidin, et al, 7). As mentioned earlier, scientists have discovered that earthquakes of this kind of magnitude occur in this area after a period of four hundred to six hundred years. This is evidenced by the discovery of remains of submerged forests along the costal of Washington, Northern California and Oregon in 1980s. In their findings, they also found layers of sand which were deposited by tsunamis. The presence of sand-filled cracks could also have been as a result of high magnitude earthquakes. Most tsunamis results from large quakes (Williams, 49). These quakes could have had a magnitude of over 8.0. From these findings, it is very true that, the Northwest Pacific coast had been struck by a series of high magnitude earthquakes over the past number of thousand years. The most recent similar quake could have occurred at around 1700.
In a more recent past such as in 1989, a quake measuring about 7.1 in magnitude struck thee northern part of California. The damage from the quake heightened worries of most of the Northern Pacific residents over eminent danger posed by a similar earthquake. This resulted in concerted efforts from all stakeholders involved in building and construction industry. This saw some revisions in Uniform Building Codes (UBC) for Washington and Oregon to accommodate the safety factor due to threat of such earthquakes (Koster, 81). In itself, the UBC contains a set of standards that are applied in the whole nation as a guide during designing of structures. The variation in these standards across the states specially the states adjacent to the Pacific Northwest is based on a consideration of expected vibrations on structures resulting from shakes from quakes.
This code recognizes six levels of hazardous earthquake shakings. Prior to 1994, the standard had placed the Puget Sound region of Washington and Seattle at the second threat level with other areas of Washington an d Seattle at the third level. However, the revised code has placed has raised their levels due to their proximity to the earthquake prone zones. This has made the new designs to be done in a way that ca help them withstand more than fifty percent more shakes from earthquakes than before the revision of the code was implemented. This is a vital step towards achieving the preparedness on the Pacific Northwest region which is now a high earthquake threat area (Grant, 194). These discoveries have played a big role in convincing the public officials and other executives of corporate in this region to reinforce the structures of their buildings, schools, bridges, dams, and other basic residential structures. Over 130 million US dollars has been sent on such upgrades all over the Pacific Northwest region.
The recent Chilean earthquake has also awakened the attention of the ongoing undersea fault along the Pacific Northwest which has al the potential of producing a similar earthquake. A similar quake would transmit enormous waves of ocean waters over the inland next to the Pacific Ocean in a matter of minutes. This brings a more concern as it brings about a possibility of the locked portion of the Northern part of California and the Southern Oregon breaking away in the next fifty years to eighty percent. This means that, the Pacific Northwest should be prepared for a great quake before the end of the next half century (George, 56). However, this region is very familiar to violent earth quakes as in 2001, a quake with a magnitude of 6.8 occurred in this region of Pacific Northwest with its epicentre centred close to Olympia.
In summary, crustal deformation is a resultant of plate tectonics which is an on-going process within different plates that makes earths crust. Resultant deformations can vary depending on the magnitude and frequency of the stress tat they are exposed to. Volcanic eruptions have been found to have a significant effect on the stability of the earths crust and can result to varying deformations. Earthquakes, is a representation of high level of plate tectonics with severe effects in changing landforms. The study of occurrence and predictability of earthquake occurrence has been made possible by the emergence and application of Global positioning systems. This system has the advantage of carrying out studies of earth surface from a periphery such as space through the help of a satellite.
No comments:
Post a Comment