Yasukuni Okubo

Estimation of Curie point temperature and geothermal structure of island arcs of Japan

Abstract Curie depth analysis from aeromagnetic data was performed in a nationwide project to assess the geothermal resources of Japan. Creation of a Curie depth map of Japan was completed in 1984 and it agrees well with thermal structures such as the high-temperature gradient regions of volcanic chains, low-temperature regions of the forearc basins and the intermediate-temperature regions of the backarc basins. Measured temperature-gradient ( ▽T ) data were gathered in order to estimate the Curie temperature from the ▽T and the Curie depths. When ▽T is assumed to be constant with respect to depth, a remarkable difference is found in the estimated Curie temperature ( T e c ) of the backarc and forearc side of the basins. In the backarc side an average T e c of 450 °C is reasonable for both the Curie depth estimated from aeromagnetic data (average depth of about 10 km in the backarc side) and the temperature gradients (average gradient of 4.5° C/100 m). The average T e c on the forearc side was found to be low, at about 300 °C. The difference in the Curie temperatures between the two areas may be due to a regional difference in rock type (e.g., granite as opposed to basalt). In the case of the Northeast Japan arc, as suggested by some authors, the isotherms may suddenly fall between the volcanic front and the trench. Based on model studies, we recognized that a Curie depth map obtained by the algorithm we developed should include an averaging effect. This suggests that the inferred Curie depth curve across the line marking the sudden fall in the Curie isotherm should gradually increase towards the trench. Therefore, the low Curie temperature that we estimated may indicate an apparent shallow Curie depth caused by the averaging effect against an area bearing deep Curie depths. Hence, the reasons for the inconsistency of the estimated Curie temperature between the forearc side and backarc side may be as follows: (1) the averaging effect of the Curie depth estimate, which makes the Curie depth apparently shallower in the forearc side and (2) the lower Curie temperature of basaltic rocks which are assumed to be distributed along the east side of the aseismic front.

Magnetic model of the subduction zone in the northeast Japan Arc

Abstract Magnetic modeling of the forearc side of the Tohoku (northeastern Japan) Arc was carried out using spectral analysis of shipborne magnetic data. The original data show anomalies caused by volcanic rocks on the western, shallow side and linear marine magnetic anomalies on the eastern, deeper side throughout the forearc region. As calculated from temperature gradient data of heat flow measurements and deep drilling, the Curie depth is deeper than the Moho on the eastern, deeper side. This suggests that the Moho may correspond to the base of the magnetic layer, because analysis of upper mantle xenolith suites indicates that the upper mantle should be non-magnetic. In addition, the Conrad discontinuity may correspond to the apparent top of magnetic layer because the lower crustal layer would be more mafic than the upper layer. It was estimated by 2D spectral analysis that the magnetic base and surface lie at about 20 km and 10 km respectively. This suggests that the Moho and the Conrad may roughly correspond to the magnetic base and surface. The magnetic anomalies in the Pacific basin are traced towards and over the inner side of the trench, and show a gradual decrease in amplitude, disappearing eventually about 100 km west of the trench. This reflects the descent of the oceanic slab. The 1D spectral analysis indicates that the marine magnetic layer is descending at an angle of about 4° in the Japan Trench, this angle increasing to 20 ° degrees below the coast, where the marine magnetic layer reaches a depth of more than 30 km. This agrees with the seismic reflection data from the trench, and because the magnetic layer can be traced down to a depth of 30 km it may be suggested that the Curie isotherm deepens to more than 30 km just east of the coast.

Thermal and crustal structure of the Aso volcano and surrounding regions constrained by gravity and magnetic data, Japan

Abstract The Aso volcano forms a large caldera that is one of the most active volcanoes in Japan. Much evidence suggests the existence of a high-temperature zone beneath the active central cones of the Aso volcano and a recent study using seismic observations revealed an attenuation in amplitude of seismic waves that pass through the central region of the Aso caldera. The SWW-NEE-trending Futagawa active fault, a part of the Oita-Kumamoto Tectonic Line, which dips north, lies to the west of the caldera and joins the caldera rim but there are no scars within the caldera. An elongate magnetic positive anomaly from the central cones to the west exists close to the Futagawa fault, this anomaly connects the central cones and the two lateral volcanoes west to the caldera. Magnetic analyses through spectral method and forward modeling delineate a highly magnetized two-dimensional buried prism source dipping north beneath the fault. The lower bounds are about 10 km depth outside the caldera and quite shallow depths of about 3 km inside the caldera. The area to the north of the Oita-Kumamoto Tectonic Line forms a gravity depression whereas the area to the south is a regional gravity high. A two-layered two-dimensional gravity model yields a normal fault dipping north with a throw of 1.3 km, whereas a multi-layered two-dimensional gravity model gives a depression of high-density basement within the caldera. The greatest depth is about 2 km below the surface. Available temperature-depth data suggest the possibility of a sudden change between the inside (high-temperature zone) and the outside (low-temperature zone) of the caldera. Therefore, the difference in the lower boundary of magnetic sources between the area inside and outside the caldera is possibly attributed to the difference of depth of the Curie point. Magnetic sources along the Futagawa fault are likely to be related to materials produced by the volcanic activity. If so, the passage for magma for Aso volcanism could be along the Futagawa fault.

Magnetic spectral analysis in Portugal and its adjacent seas

Abstract Magnetic spectral analysis, which has often been applied to estimate Curie point depths, was used to delineate thermal and crustal structures of Portugal and its adjacent seas. The magnetic data set was the grid file of 5 km interval. Assuming random samplings from magnetic prisms, we obtained power spectra of two-dimensional square areas by the double Fourier transform and estimated statistical average depths. The cell-size for the area we used is 320 km. The average centroid depth, z 0 , was obtained automatically from the gradient of natural log of [(power spectrum)/(radial frequency)] in the range between 0.005 and 0.02 km −1 in frequency (50–200 km in wavelength). The average depth to the top of the magnetic layer, z t , was estimated from the gradient of the power spectrum between 0.02 and 0.04 km −1 in frequency (25–50 km in wavelength). The average depth to the bottom of the magnetic layer is calculated from 2 z 0 − z t . The results show that the bottom depths of land areas range between 20 and 55 km below sea level. The statistical errors of centroid depth are within 8 km. Marine linear magnetic anomalies widely cover the offshore areas, where random samples of prisms is inconsistent. We take one-dimensional analysis for the areas. The south of Portugal marks shallow depths of 20–30 km, while deep depth areas greater than 30 km spread widely in the north. One-dimensional analysis of the offshore area indicates the depth of marine magnetic layer of oceanic crust. The wide variation of depth is interpreted to be a reflection of the crustal and the thermal structures. Since the Moho must correspond to the base of the magnetic layer, the results could be the depths to the Moho instead of the Curie point isotherm in the northern Portugal. South Portuguese Zone is in the area ranging from 20 to 30 km in depth. These depths are shallower than the Moho. Two-dimensional forward modeling confirms that there is a throw of the bottom of the magnetic layer across the Ficalho over thrust from 35 to 40 km in the Ossa Morena Zone to 20–30 km in the South Portuguese Zone. There is a geothermal anomaly of high heat flow density values more than three times the normal average value. The Curie point isotherm estimated by the anomaly is shallower than the depth to the bottom of the magnetic layer. One interpretation is that the bottom of the magnetic layer corresponds to a Moho depth that could be 28–30 km in the Ossa Morena Zone and to a Curie point isotherm that could be 20–30 km in the South Portuguese Zone. The high heat flow density values of this area could be produced by hydrothermal convection along tectonic lines such as the Ferreira–Ficalho over thrust.

Seismic reflector imaging by prestack time migration in the Kakkonda geothermal field, Japan

We show that a prestack migration method improves the S/N ratio of seismic reflection profiling in the Kakkonda geothermal field where seismic reflection data are of poor quality. We use non-iterative prestack time migration (PSTM), which does not require multiple iterations to determine the velocity structure for prestack time migration. The optimum constant migration velocity can be determined at each image point from a migration velocity analysis based on primary diffraction patterns. Our results delineate a strong reflector beneath a zone of high seismicity. According to the correspondence between the fracture distribution, the distribution of microearthquakes, and geothermal structure, this reflector is interpreted to be a zone of low-angle fractures saturated with hydrothermal fluids, and to be strongly controlled by the geothermal structure.

Rheological implications of the strong seismic reflector in the Kakkonda geothermal field, Japan

Abstract We re-processed the seismic reflection survey data of the Kakkonda geothermal field. The pre-stack migration delineates a strong and continuous reflector between 1800- and 2800-m depth, below which formations are not reflective. Earthquake data exhibit seismicity in the upper crust. The lower boundary of seismogenic layer is interpreted as the brittle–ductile transition. The thermal structure is thought to be the major factor controlling its depth. We compared the strong reflector with the thermal and rheological structure from drillholes. The depth of the reflector corresponds to the top of the highly–very highly fractured zone observed from formation microscanner imagery (FMI) logging in the Miocene formations. The density of fracture in the Kakkonda granite is very low, suggesting that granite corresponds to the nonreflective zone. The temperature–depth profile of well WD-1a shows that the temperature at the highly–very highly fractured zone is about 350 °C. This corresponds to a hydrothermal convection zone filled with two-phase geothermal fluid. The cut-off depth of seismicity that indicates the brittle–ductile transition lies at the isotherm of 300–350 °C near the reflector. We conclude that the strong seismic reflector is a strong contrast in acoustic impedance at the top of the fractured layer. The fractured layer could be a decoupling plane caused by different tectonic behaviors between the upper brittle and the lower ductile layers or a dehydration front by thermal diffusion. The similarity between the strong reflector and K-horizon, the strong reflector, found in southern Tuscany, Italy suggests that the P-wave reflector at the top of highly fractured zone at the brittle–ductile transition be common in areas with magmatic activity.

Reconstruction of the ground surface temperature history from the borehole temperature data in the southeastern part of the Republic of Korea

The changes in the temperature on the Earths surface in the past have penetrated into the subsurface and have been recorded as transient temperature perturbations to the background thermal field. In this study, we reconstruct the ground surface temperature (GST) history of the last 300 years by analysing three borehole temperature profiles in Ulsan, the southeastern part of the Republic of Korea. The borehole temperature profiles show positive temperature anomalies caused by recent warming. The reconstructed GST history showed a cold period in the late 19th century and subsequent warming in the present time. After the cold event, the GST increased by 1.5 K up to 1980. The warming trend from 1900 to 1980 was 2.0 K/century. We compare the GST history with proxy temperature reconstructions obtained by other studies in Northeast Asia. The result suggests spatial variability of the climate in Northeast Asia.

Repeated seismic reflection measurements in the Kakkonda geothermal field

Temporal variations in seismic reflection responses apparently caused by changes within a geothermal reservoir have been detected in the Kakkonda field, where production wells were shut in prior to annual power plant maintenance. Three seismic surveys were carried out during a 12-day period spanning the shut-in period. Receivers were deployed without replanting throughout the surveys, providing good data acquisition repeatability. We applied prestack time migration (PSTM) to each of the three seismic data sets and calculated the cross-correlation coefficients between PSTM sections. Our results indicate that the reservoir changes associated with shut-in are large enough to be seismically detectable. The region over which the seismic response changed corresponds to the zone of geothermal fluid flow paths inferred from reservoir temperatures and geochemical data. We have also compared our cross-correlation maps with the epicenters of micro-earthquakes, which are inferred to indicate the existence of fractured zones. Dense regions of micro-earthquake activity lie within the seismically identified zone of changes in reservoir properties. We demonstrate the feasibility of repeated seismic surveys in providing substantial improvements in temporal resolution during geothermal reservoir monitoring.

Depth estimate of a two‐dimensional source using the spectrum of one‐dimensional linear trending magnetic anomaly

Linear trending anomalies can be found in aeromagnetic anomaly maps. For such a type of anomaly, previously developed algorithms using two-dimensional data sets are not applicable because the assumption on which the algorithms are based does not hold. We have developed a new algorithm to invert the one-dimensional data, such as linear trending anomalies, based upon two kinds of two-dimensional structural model. One is an inclined prism model and the other is a fault model. The algorithm estimates the depths of the top bound (2,) and the centroid (zO) of the magnetized body by analysing power spectra of magnetic anomalies. The depth to the bottom of the prism is given by z~=~z,J-z,, Testing of this method proved that the algorithm gives satisfactory results for an inclined model when the length of onedimensional data is at least about ten times as long as the depth to lower bound of magnetized body. The results of a case study in the Aso caldera suggests shallowing of the lower bounds within the caldera and deepening to the west of the caldera.