Koichi Asamori

Seismic structure and magmatism of the Young Kyushu Subduction Zone

Seismic tomography shows that slow velocity anomalies exist in the mantle wedge and extend to the forearc region down to the subducting Philippine Sea slab under northern Kyushu. We also conducted numerical simulations with petrologic data to estimate the fluid distribution in the mantle wedge. The seismic and simulation results indicate that regimes of melting and magmatism in a subduction zone with a young slab are different from those with old slabs. Dehydration and melting occur beneath the arc and forearc above a young (and warm) slab such as in northern Kyushu (≤ 26 Ma), while fluids (aqueous solution and melt) occur mainly beneath the back arc above old slabs.

Three-dimensional magnetotelluric imaging of crustal fluids and seismicity around Naruko volcano, NE Japan

We analyzed the 3-D resistivity structure beneath Naruko volcano, northeastern Japan, with the aim of imaging 3-D distribution of fluids in the crust for its volcanic and seismogenic implications. The data were recorded at 77 sites in total: 30 sites are new and are arranged in an approximately 5 × 5 km grid whereas the remaining older sites constitute two separate east-west profiles. We ran a 3-D inversion using full components of impedance tensors in the period range between 0.13 and 400 s. The resulting model showed that a sub-vertical conductor exists a few kilometers below Naruko volcano. The conductor extends from the surface of the volcano and dips towards the south, away from the volcano towards the backbone range. High levels of seismicity are observed in the upper crust above and around the conductors. We suggest that the seismicity is fluid driven and that a fluid trap is created by the precipitation of quartz owing to a reduction in solubility at shallow depth. The Quaternary volcanic front is characterized by a sharp resistivity contrast and a high-resistivity zone and extends 10 to 15 km towards the east. A fore-arc conductor was observed at mid-crustal levels even farther towards the east. The sub-vertical conductors along the arc and the fore-arc conductor have resistivities of 1 to 10 Ωm. Assuming a Hashin-Shtrikman model with saline fluids of 0.1-Ωm resistivity, a porosity of 1.5% to 15% is required to explain the observed conductive anomalies.

Variations in the 3He/4He ratios of hot springs on Shikoku Island, southwest Japan

New helium isotopic data from hot springs on Shikoku Island were obtained from a larger number of sampling points than previous studies in order to elucidate the geographical distribution of 3He/4He ratios in the fore-arc regions of southwest Japan. Systematic, regional variations in 3He/4He ratios were observed on both Shikoku Island and the Kii Peninsula. The hot springs on Shikoku Island have 3He/4He ratios between 0.21 RA and 2.5 RA. On the other hand, the majority of hot springs in the Kii Peninsula have significantly higher 3He/4He ratios than those on Shikoku Island. It has been suggested that the anomalously high 3He/4He ratios in the fore-arc region of southwest Japan are related to the migration of MORB-type helium with aqueous fluids generated by dehydration of the subducting Philippine Sea plate and mixing with terrestrial waters (e.g., Matsumoto et al., 2003). According to recent seismological observations of crustal microearthquakes and their focal mechanisms, the geotectonic environment beneath Shikoku Island is subject to a stress field with no normal faulting and with extremely low crustal seismicity. This argues against migration of aqueous fluids from the subcrustal lithosphere, resulting in a relatively small influence of mantle-derived helium at the surface. As a consequence, the regional variation in 3He/4He ratios of hot springs on Shikoku Island is interpreted as a three-component mixture of mantle-derived helium associated with magmatism of middle Miocene of age (<3.4 RA), crustal, radiogenic helium and atmospheric helium.

Magnetotelluric imaging of crustal magma storage beneath the Mesozoic crystalline mountains in a nonvolcanic region, northeast Japan

Mesozoic crystalline mountains (Iide Mountains) in a nonvolcanic region of the Northeast Japan Arc were imaged by wide-band magnetotelluric soundings. A two-dimensional model shows that an anomalous conductive body (<10 Ωm) is clearly visible beneath the Iide Mountains. The conductor widens with increasing depth and extends from the near-surface down to the base of the crust and perhaps into the upper mantle. The location of the conductive body correlates with high-temperature hot springs with high 3He/4He ratio, thinning of the brittle seismogenic layer, and anomalies of low seismic velocity. We conclude that the conductor reflects the presence of partial melts in the crust, related to renewed magmatism in the present-day subduction system.

Electrical Resistivity Structure and Helium Isotopes around Naruko Volcano, Northeastern Japan and Its Implication for the Distribution of Crustal Magma

The two-dimensional electrical resistivity structure beneath Naruko volcano was determined using magnetotelluric soundings. The resulting model shows that a prominent conductor exists through the middle crust to the uppermost mantle beneath the volcano. The location of the conductor agrees closely with a seismic low-velocity zone. Low-frequency microearthquakes occur near the conductor around the Moho depth. The cutoff depth of crustal earthquakes is coincident with the upper boundary of the conductor, implying that the conductor has a temperature appreciably higher than 400∘C. Furthermore, new helium isotope data from hot springs around the volcano were obtained. The spatial distribution of the observed 3He/4He ratios reveals the extent of mantle-derived materials beneath Naruko volcano. Consequently, it is apparent that the conductor determined beneath the volcano reflects the presence of high-temperature mantle-derived materials such as magmas and/or related fluids derived from active magmatism in the northeastern Japan subduction zone.

Three-dimensional magnetotelluric inversion using a heterogeneous smoothness-constrained least-squares method

This paper presents a fast algorithm for electromagnetic data inversion to three-dimensional (3D) resistivity models. The algorithm is distinctive for the level of accuracy it attains while bypassing the sensitivity matrix update. A common sensitivity matrix for homogeneous half-space is used in all iterations. Instead of updating the sensitivity matrix, the smoothness filter coefficients at each model element are updated, based on the spatial variations in resistivity in the model derived from the latest iteration. This substitution is expected not only to reduce the computation time required for large-scale inversions, such as those for 3D surveys, but also to allow the resolution of sharp boundaries in resistivity structures. Our algorithm was applied to 3D magnetotelluric inversion in order to confirm its effectiveness. Using synthetic examples under several conditions, we demonstrated that the method can reduce the number of forward calculations required to reduce data misfits to noise level, and that the method is robust for constructing target models even with sharp boundaries without generating fatally false resistivity structures or boundaries under noisy conditions.

Current R & D Activities in the Study on Geosphere Stability

The Japanese islands are located in a tectonically active zone. The scientific base is required for assessing the geosphere stability for long-term isolation of radioactive waste in Japan. JAEA is promoting the establishment of investigation method for geotectonic events affecting geosphere stability and prediction model for the future changes of geological environments, that is necessary for site selection and safety assessment of the HLW geological disposal. For seismicity and faulting, detection techniques for active faults without topographic surface expression, such as using helium isotope ratios in hot spring gases or detection of hydrogen gas, and studies on the assessment of fault evolution have been developed. For volcanism and geothermal activity, heat sources for anomalous geothermal activity in non-volcanic regions are considered. Detection techniques for high-temperature fluids and magma deep underground, using geophysical and geochemical approaches, were constructed. For uplift, denudation and climatic/sea-level changes, a methodology to predict the future topographic change was developed. Also, for dating techniques as an essential part to proceed on these studies, C-14 and Be-10 dating using AMS and (U-Th)/He dating using QMS and ICP-MS have been developed. We are planning the establishment of assessment methods for geosphere stability including assessment of the activity of faults encountered in underground excavations, development of long-term prediction model of volcanism and hydrothermal activities, and hydrogeological analyses considering topographic change.© 2010 ASME

An Attempt to Evaluate Horizontal Crustal Movement by Geodetic and Geological Approach in the Horonobe Area, Northern Hokkaido, Japan

In this study, we present the preliminary results for the estimation of a horizontal crustal movement by using geodetic and geological approach in the Horonobe area, northern Hokkaido, Japan. The estimations have been carried out by using a GPS data and a geological cross section obtained by applying balanced-section method. As results of this study, both of the shortening rates estimated by GPS data and balanced-section method indicate several millimeters per year. Namely, there is no contradiction between geodetic and geological data, and it is considered that Horonobe area is still situated similar tendency and magnitude of a crustal movement. It is seemingly considered that geodetic data is unhelpful for estimating the long-term crustal movement, because period of geodetic observations is a very short. However, the result of this study indicates that geodetic data provide valuable information for estimating the long-term crustal movement in the area, and it is considered that geodetic approach play an important role in improvement of the credibility of evaluation for prediction of long-term stability.Copyright

Relationship Between Hypocentral Distribution and Geological Structure in the Horonobe Area, Northern Hokkaido, Japan

In this paper, we discuss the relationship between the accurate hypocentral distribution and three-dimensional (3-D) geological structure in and around the Horonobe area in northern Hokkaido, Japan. The multiplet-clustering analysis was applied to the 421 micro-earthquakes which occurred from 1 September, 2003 to 30 September, 2007. The 3-D geological structure model was mainly constructed from previous seismic reflection profiles and borehole data. As a result of this analysis, although with slight differences in depth between them, the hypocenters were found to be distributed in the NNW-SSE direction and become deeper from the west toward the east. The distributed pattern of the hypocenters is similar to that of the geological structure. These results indicate that the hypocentral distribution may represent existence of active zone related to the geological structure, and provide effective information which can contribute to establishing methods for estimating the future evolution of the geological environment.