3:00 PM - 3:15 PM
[SCG50-06] Thermal Structure in North East Japan
Keywords:Heat flow, Thermal structure, Hi-net
Measurements of crustal heat flow and estimates of subsurface temperature structure based on these measurements are essential for determining the depth of the brittle/ductile transition in the crust, which is considered to be the lower limit of the seismogenic layer of the crust (Sibson,1984), understanding trench and slow earthquakes (e.g., Yoshioka et al., 2013), and many other essential applications.
On the other hand, crustal heat flow data around the Japanese archipelago were first presented by Uyeda and Horai (1964), and many measurements were made in the terrestrial and oceanic regions (e.g., Yamano 1995). However, the terrestrial data were mainly measured from wells drilled in geothermal areas. In some non-geothermal areas, heat flow data is scarce. To compensate for this heterogeneity in the spatial distribution, we investigated the temperature profiles of 132 borehole wells (100-200 m deep) in Hi-net, a uniformly distributed seismic network over land in the Japanese archipelago. From the results of processing the data, taking into account the effects of climate change and other factors, a new heat flow map was created to estimate the subsurface temperature structure.
For the correction of climate change, differences between non-urban and urban areas were taken into account. Regarding climate change in Japan over the past 100 years, Fujibe (2012) quantitatively assessed the warming trend in the background (non-urban) and urban areas and found that the national average warming trend in the background (non-urban) daily mean temperature was 0.88°C/100yr, with the trend being stronger at stations with higher population density in the surrounding area. Based on this model, we estimated the temperature increase over the past 100 years according to the population density around the boreholes, using the Regional Economic Analysis System (https://resas.go.jp/) operated by the Cabinet Office for data on population density as of 2005.
We obtained heat flow data over a large area of the Tohoku region. It seems that there is a trend of less heat flow in the fore-arc and more heat flow in the volcanic regions, and some regions have slightly less heat flow in the back-arc. The high heat flow region extends along the central axis of the Ou Mountains, and local high heat flow anomalies exist around volcanoes distributed along this axis. There are many places where the depth to 650°C is less than 5 km in these regions. On the other hand, in the coastal area on the forearc side where low heat flow regions of about 50 mW/m2 exist, a low-temperature structure of about 300°C to 400°C is estimated at a depth of 30 km.
There seems to be a good correlation between the temperature structure and the spatial distribution of D90, and the isotherm at a depth of 400°C is more correlated than the isotherm at 300°C.
The area around the Niigata Plain, where the sedimentary layer is as thick as 5 km, is a low heat flow region on the back-arc side, with a temperature structure of about 400°C at a depth of 10-13 km. The effect of introducing detailed crustal structure can be seen from the excellent correlation in the plains of the southern part of the Japan Sea, the epicenter of the 2004 Niigata-Chuetsu earthquake.
On the other hand, crustal heat flow data around the Japanese archipelago were first presented by Uyeda and Horai (1964), and many measurements were made in the terrestrial and oceanic regions (e.g., Yamano 1995). However, the terrestrial data were mainly measured from wells drilled in geothermal areas. In some non-geothermal areas, heat flow data is scarce. To compensate for this heterogeneity in the spatial distribution, we investigated the temperature profiles of 132 borehole wells (100-200 m deep) in Hi-net, a uniformly distributed seismic network over land in the Japanese archipelago. From the results of processing the data, taking into account the effects of climate change and other factors, a new heat flow map was created to estimate the subsurface temperature structure.
For the correction of climate change, differences between non-urban and urban areas were taken into account. Regarding climate change in Japan over the past 100 years, Fujibe (2012) quantitatively assessed the warming trend in the background (non-urban) and urban areas and found that the national average warming trend in the background (non-urban) daily mean temperature was 0.88°C/100yr, with the trend being stronger at stations with higher population density in the surrounding area. Based on this model, we estimated the temperature increase over the past 100 years according to the population density around the boreholes, using the Regional Economic Analysis System (https://resas.go.jp/) operated by the Cabinet Office for data on population density as of 2005.
We obtained heat flow data over a large area of the Tohoku region. It seems that there is a trend of less heat flow in the fore-arc and more heat flow in the volcanic regions, and some regions have slightly less heat flow in the back-arc. The high heat flow region extends along the central axis of the Ou Mountains, and local high heat flow anomalies exist around volcanoes distributed along this axis. There are many places where the depth to 650°C is less than 5 km in these regions. On the other hand, in the coastal area on the forearc side where low heat flow regions of about 50 mW/m2 exist, a low-temperature structure of about 300°C to 400°C is estimated at a depth of 30 km.
There seems to be a good correlation between the temperature structure and the spatial distribution of D90, and the isotherm at a depth of 400°C is more correlated than the isotherm at 300°C.
The area around the Niigata Plain, where the sedimentary layer is as thick as 5 km, is a low heat flow region on the back-arc side, with a temperature structure of about 400°C at a depth of 10-13 km. The effect of introducing detailed crustal structure can be seen from the excellent correlation in the plains of the southern part of the Japan Sea, the epicenter of the 2004 Niigata-Chuetsu earthquake.