15:30 〜 16:30
[G04-P-07] Identifying the land subsidence attributed to the natural gas mining in GNSS time series in Japanese actively crustal deformed area
Large-scale land subsidence has been observed in the regions of Kujukuri and Shimousa, Kanto District, Japan, caused by continuous mining of groundwater which is refined for natural gas and iodine uses since 1950's. In terms of advanced sustainable development of the mining, urgent research from the view point of geodesy is highly required to evaluate the amount of subsidence caused by the mining. We have successfully identified the land subsidence attributed to the groundwater mining for refining natural gas and iodine from the pumping of groundwater for the agricultural usages and crustal deformations driven by tectonic factors; secular tectonic dynamics, co-seismic offsets, slow earthquake wandering, and the post-seismic deformations of huge earthquakes, applying the time series of GNSS observations analyzed applying GAMIT/GLOBK software. We detach the land subsidence due to groundwater pumping for agriculture uses by focusing their annual periodicity. Then we represent the comparatively short period of time slow earthquakes as Markov process, and segregate it from other causes. We evaluate the amount of crustal subsidence originated from plate collisions under the Kanto District, which widely seen in the Kanto plane before the 2011 Mw9.0 Tohoku-Oki earthquake, and we disregarded it since this kind of land subsidence is an order of magnitude smaller than them caused by the human activities. Finally, we express the time series of the post-seismic activities after the 2011 Tohoku-Oki earthquake as logarithmic, exponential, or combined logarithmic and exponential function models. Thus we enable complicated crustal deformations arising around Kujukuri and Shimousa area to be segregated into the artificial and the tectonic factors, and evaluate the amount of the subsidence originated by the groundwater mining for refining natural gas and iodine.