Constraining the geotherm beneath the Japanese islands from Curie point depth analysis and comparison with seismicity and drillhole data in the Kakkonda geothermal field

Abstract

This paper shows results of Curie point depth (CPD) analysis of the Japanese Islands using magnetic data. The analysis method is Bouligand et al. (2009) based on fractal theory. This analysis result was combined with the cutoff depth of ordinary earthquakes, low frequency earthquakes and drillhole data. CPD by magnetic data (Dcmag) performed in the case of 50 km window size ranges 6–9 km deep in volcanic areas and is deeper than 9 km in non-volcanic areas. The results are concordant with the previous study by Okubo et al. (1989). Cutoff depths calculated by JMA unified catalog are 10 km or shallower in the volcanic area, deeper than 20 km in the forearc side and concordant with Dcmags. Dcmags across the Kakkonda GP gradually become shallower from 6.8 km west Akita-Komagatake toward to 4.6 km near Iwate volcano. The depth-to-temperature curves from drillhole database around the Kakkonda GP show that thermal gradients between Iwate volcano and the Kakkonda GP range 0.157 - 0.222 K/m. Assuming that these thermal gradients continue from the bottom of hole to the deeper part, the depths reaching 580 °C (Dchole) are determined. Dchole is concordant with Dcmag in the west side of the Kakkonda GP and much shallower than Dcmag in the east side. This suggests that Dcmags may miss to delineate a local high temperature area smaller than 50 km square in the east because of the average effect of CPD analysis. Dchole is shallower than 3 km west Iwate volcano and deepens at about 5 km west the Kakkonda GP. Combining the low frequency earthquakes data from the JMA unified earthquake catalog and the drillhole data of the Kakkonda geothermal field, we find the Curie point depths are consistent with those data.