JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. Bll, PAGES 22,363-22,371, NOVEMBER 10, 1994 Curie point depth in northeast Japan and its correlation with regional thermal structure and seismicity Yasukuni Okubo I and Tsuneo Matsunaga Geological Survey of Japan, Higashi, Tsukuba, Japan Abstract. Heat flow measurements are high on the back arc side and low on the forearc side of the Tohoku arc, but convective heat transfer in volcanic areas affects the heat flow measurements, complicating the determination of the thermal structure from heat flow measurements alone. Curie point depths deduced from previous magnetic analyses show a sudden change of the Curie isotherm between the volcanic front and the Japan Trench. A magnetic belt extends from west Hokkaido to the eastern limit of Tohoku. The location is west of and parallel to the Cretaceous subduction zone. To determine the geometry of the source of the magnetic belt, detailed magnetic analyseswere carried out using forward modeling and recently developed spectral analysis techniques. The magnetic analyses indicate that sources of the magnetic belt extend to significant depths. The result reveals that the Curie isotherm varies from 10 km in the back arc to 20 km or deeper at the eastern limit of Tohoku. The boundary between seismic and aseismic zones in the overriding plate correlates with the inferred Curie isotherm, indicating that seismicity in the overriding plate is related to temperature. Introduction The Japanese Islands are a typical island arc system consisting of five active island arcs. Tohoku, the northern part of Honshu Island, and west Hokkaido trend north-south and lie in one of the island arcs, the northeastern Japan arc (Tohoku arc). To the east of the arc, the Pacific plate has been subducting westward from the Japan Trench under Japan (Figure 1). Many Quaternary volcanoes occur along the Tohoku arc about 300 km west of the trench. The crustal structure of the Tohoku arc has been explored using seismic studies. Seismic and aseismic zones have been mapped using the distribution of earthquakes. The Wadati- Benioff zone along the subducting Pacific plate is character- ized by inclined double-planed layers dipping arcward at an angle of 30 ø [Hasegawa et al., 1978]. Another seismic zone lies in the upper 15 km of the overriding plate, overlying an aseismic zone. Detailed seismic studies by Yoshii [1975] have delineated the frontal margin of the aseismic zone located in the mantle wedge facing the seismic subduction zone (Figure 1). Beyond the aseismic front, the lower limit of earthquakesin the crust deepensseaward and connectswith the Wadati-Benioff zone. The thermal structure of the Tohoku arc has been charac- terized on the basis of available thermal data. The thermal data reveal high vertical temperature gradients in the back arc and low gradients in the forearc [e.g., Hasebe et al., 1970]. The thermal structure of volcanic arcs, however, is di•cult to obtain from heat flow data alone because heat flow in volcanic areas is often distorted by convective heat 1Now at Coordinating Committee forCoastal and Offshore Geo- science Programmes in East and Southeast Asia, Bangkok, Thai- land. Copyright 1994 by the American Geophysical Union. Paper number 94JB01336. 0148-0227/94/94JB-01336505.00 transfer. Recent work in the Cascade Range, north-central Oregon, has revealed that anomalously high heat flow is related to groundwater circulation, which indicates that any deeper anomaly may be relatively narrow and spatially variable [Ingebritsen et al., 1989]. This complicated nature requires detailed heat flow stud- ies and comparison with other geophysical evidence. Based on the tomographic images of velocity structure determined by Zhao et al. [1992], Hasegawa et al. [1991] argued that low-velocity zones beneath the Tohoku arc represent magma reservoirs at depth. Thus it may be possible to delineate deep thermal structure from seismicity. The thermal structure correlates well with seismicity, implying that rock properties vary with temperature, as predicted by laboratory observation [e.g., Stesky et al., 1974; Brace, 1977]. This correlation suggestsa causal con- nection between the thermal structure and seismicity. In the land area of Tohoku, shallow crustal seismicity is confined to the upper 15 km of the crust, and even shallower around the volcanoes [Hasegawa et al., 1991], suggesting that the brittle- ductile transition controls the lower limit of seismicity. The spectral properties of aeromagnetic data have often been applied to estimate Curie point depths [Bhattacharyya and Leu, 1975; Byerly and Stolt, 1977; Shuey et al., 1977; Smith et al., 1977; Boler, 1978; Connard et al., 1983; Okubo et al., 1985, 1989, 1991; Blakely, 1988]. This is the depth at which crustal temperatures reach the Curie point of the dominant magnetic minerals, probably titanomagnetite with a Curie temperature of 580øCor less [Nagata, 1961]. Curie point depths in Japan estimated in this way [Okubo et al., 1989, 1991] have inferred Curie point depths of about 10 km or less below sea level along the Tohoku volcanic arc, deepeningto 30 km or greater 200 km east of the volcanic front and 100 km west of the Japan Trench [Okubo et al., 1991]. These results suggest a sudden change in the Curie point depth between the Tohoku arc and the trench. 22,363