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[SGD01-15] Detection of local displacement of GEONET stations using spatial high-pass filtering
Keywords:GEONET, crustal deformation, spatial high-pass filtering, landslide, ground subsidence
Introduction
The Geospatial Information Authority of Japan (GSI) has been operating continuous observation reference stations (CORS) network (GEONET), and GEONET is a powerful infrastructure that can obtain positions and their displacements throughout Japan, and is used in a wide range of applications from surveying, information-based construction to crustal deformation analysis. Although the F5 solution obtained daily at the GEONET stations has an accuracy of several millimeters, the Japanese archipelago, which exists in the plate boundary region, has large crustal deformations over a wide area. Therefore, it is difficult to evaluate the accuracy of each station. In this presentation, by applying high-pass filtering to the spatial distribution of GEONET displacements, we will remove wide-area crustal deformations and extract local displacements.
Results
After removing artificial fluctuations due to antenna replacement, etc., because Figure 1 includes crustal deformations over a wide area, it is difficult to detect local displacements, except for large displacements caused by large earthquakes. A spatial high-pass filter is applied to remove wide-area displacements. Figure 2 shows the results after spatial high-pass filtering from the southern part of Tohoku to Shikoku, Japan, and the local displacements are clearly visible from Figure 2.
By checking the time series and the environment at the installation location, we estimated the cause of local displacement.
-Earthquake (EQ) Displacement caused by the 2014 Nagano-ken-hokubu Earthquake (M6.7), the 2016 Tottori-ken-chubu Earthquake (M6.6) and the December 28, 2016 northern Ibaraki-ken earthquake (M6.3) can be seen.
-Volcanic activity (VC) Displacement due to volcanic activity can be seen in Izu-oshima, Miyake-jima and around Hakone region.
-Landslide (LS) There are stations that are continuously displaced in a direction different from surrounding stations. Most of them are located in areas judged to be landslide terrain, which are topographical traces formed by landslide movement.
-Ground Subsidence (GS) Ground subsidence associated with the pumping of water-soluble natural gas in Chiba Prefecture, and widespread subsidence in the volcanic areas of the Tohoku region, which has continued after the 2011 Tohoku Earthquake, can be seen. Ground subsidence associated with the pumping of groundwater for snow removal during the winter often returns to its original level in the summer, with almost no change seen over in Figure 2. The ground subsidence in Chiba Prefecture is uniform over time, and the horizontal component is also continuously displaced toward the subsidence zone (Figure 3).
-Snowfall (SN) Displacement due to the weight of snowfall in winter cannot be fully recovered in summer and remains as a displacement.
-Trees (TR) Overgrowth of trees causes blocking, delay, and diffuse reflection of GNSS radio waves, making them unsuitable for handling as deformation data.
Discussions
The cause of displacement due to earthquakes and volcanic activity is easy to identify based on the time and location of the displacement, and even if the displacement is local, it is distributed continuously over a certain range.
The problem is that there are a large number of stations that have been displaced by landslides. A landslide causes block-like displacement of an area of several hundred meters to several kilometers, including the station, and the displacement is found only inside the landslide area. Landslides are complex, including those that continue to slide constantly, those that slide intermittently due to the effects of earthquakes or rainfall (Figure 4), and those that cause displacement that is a combination of both.
Furthermore, although vertical displacement has been the focus of ground subsidence, it is clear that horizontal displacement is also occurring toward the center of the subsidence zone.
The Geospatial Information Authority of Japan (GSI) has been operating continuous observation reference stations (CORS) network (GEONET), and GEONET is a powerful infrastructure that can obtain positions and their displacements throughout Japan, and is used in a wide range of applications from surveying, information-based construction to crustal deformation analysis. Although the F5 solution obtained daily at the GEONET stations has an accuracy of several millimeters, the Japanese archipelago, which exists in the plate boundary region, has large crustal deformations over a wide area. Therefore, it is difficult to evaluate the accuracy of each station. In this presentation, by applying high-pass filtering to the spatial distribution of GEONET displacements, we will remove wide-area crustal deformations and extract local displacements.
Results
After removing artificial fluctuations due to antenna replacement, etc., because Figure 1 includes crustal deformations over a wide area, it is difficult to detect local displacements, except for large displacements caused by large earthquakes. A spatial high-pass filter is applied to remove wide-area displacements. Figure 2 shows the results after spatial high-pass filtering from the southern part of Tohoku to Shikoku, Japan, and the local displacements are clearly visible from Figure 2.
By checking the time series and the environment at the installation location, we estimated the cause of local displacement.
-Earthquake (EQ) Displacement caused by the 2014 Nagano-ken-hokubu Earthquake (M6.7), the 2016 Tottori-ken-chubu Earthquake (M6.6) and the December 28, 2016 northern Ibaraki-ken earthquake (M6.3) can be seen.
-Volcanic activity (VC) Displacement due to volcanic activity can be seen in Izu-oshima, Miyake-jima and around Hakone region.
-Landslide (LS) There are stations that are continuously displaced in a direction different from surrounding stations. Most of them are located in areas judged to be landslide terrain, which are topographical traces formed by landslide movement.
-Ground Subsidence (GS) Ground subsidence associated with the pumping of water-soluble natural gas in Chiba Prefecture, and widespread subsidence in the volcanic areas of the Tohoku region, which has continued after the 2011 Tohoku Earthquake, can be seen. Ground subsidence associated with the pumping of groundwater for snow removal during the winter often returns to its original level in the summer, with almost no change seen over in Figure 2. The ground subsidence in Chiba Prefecture is uniform over time, and the horizontal component is also continuously displaced toward the subsidence zone (Figure 3).
-Snowfall (SN) Displacement due to the weight of snowfall in winter cannot be fully recovered in summer and remains as a displacement.
-Trees (TR) Overgrowth of trees causes blocking, delay, and diffuse reflection of GNSS radio waves, making them unsuitable for handling as deformation data.
Discussions
The cause of displacement due to earthquakes and volcanic activity is easy to identify based on the time and location of the displacement, and even if the displacement is local, it is distributed continuously over a certain range.
The problem is that there are a large number of stations that have been displaced by landslides. A landslide causes block-like displacement of an area of several hundred meters to several kilometers, including the station, and the displacement is found only inside the landslide area. Landslides are complex, including those that continue to slide constantly, those that slide intermittently due to the effects of earthquakes or rainfall (Figure 4), and those that cause displacement that is a combination of both.
Furthermore, although vertical displacement has been the focus of ground subsidence, it is clear that horizontal displacement is also occurring toward the center of the subsidence zone.