Hisashi Utada

Magma ascent beneath Unzen Volcano, SW Japan, deduced from the electrical resistivity structure

The resistivity structure of Unzen Volcano has been revealed by extensive magnetotelluric surveys since the first eruption on November 17, 1990. This structure comprises a highly resistive surface layer, a low-resistive second layer at several hundred meters depth, interpreted as a water-saturated layer, a resistive third layer, and a low-resistive fourth layer at 10 km depth, possibly related to the deep magmatic activity. The structure has influenced the volcanic activity of Unzen. This activity was characterized by a series of dramatic changes in eruption type: a minor phreatic eruption on November 17, 1990; phreatic eruptions after February 12, 1991, preceded by several weeks of volcanic tremor; phreatomagmatic eruptions after April 9, and dome effusion beginning May 19, 1991. This paper presents a hypothesis in which the top of the magma column rose about 20 m/day, reached the base of the water-saturated layer at the end of January, 1991, and approached the upper boundary of this layer on April 9. Thus, the temporal change of eruption type and associated phenomena are systematically explained by an interaction between magma and groundwater contained in the saturated layer.

Upper mantle electrical resistivity structure beneath the central Mariana subduction system

This paper reports on a magnetotelluric (MT) survey across the central Mariana subduction system, providing a comprehensive electrical resistivity image of the upper mantle to address issues of mantle dynamics in the mantle wedge and beneath the slow back-arc spreading ridge. After calculation of MT response functions and their correction for topographic distortion, two-dimensional electrical resistivity structures were generated using an inversion algorithm with a smoothness constraint and with additional restrictions imposed by the subducting slab. The resultant isotropic electrical resistivity structure contains several key features. There is an uppermost resistive layer with a thickness of up to 150 km beneath the Pacific Ocean Basin, 80–100 km beneath the Mariana Trough, and 60 km beneath the Parece Vela Basin along with a conductive mantle beneath the resistive layer. A resistive region down to 60 km depth and a conductive region at greater depth are inferred beneath the volcanic arc in the mantle wedge. There is no evidence for a conductive feature beneath the back-arc spreading center. Sensitivity tests were applied to these features through inversion of synthetic data. The uppermost resistive layer is the cool, dry residual from the plate accretion process. Its thickness beneath the Pacific Ocean Basin is controlled mainly by temperature, whereas the roughly constant thickness beneath the Mariana Trough and beneath the Parece Vela Basin regardless of seafloor age is controlled by composition. The conductive mantle beneath the uppermost resistive layer requires hydration of olivine and/or melting of the mantle. The resistive region beneath the volcanic arc down to 60 km suggests that fluids such as melt or free water are not well connected or are highly three-dimensional and of limited size. In contrast, the conductive region beneath the volcanic arc below 60 km depth reflects melting and hydration driven by water release from the subducting slab. The resistive region beneath the back-arc spreading center can be explained by dry mantle with typical temperatures, suggesting that any melt present is either poorly connected or distributed discontinuously along the strike of the ridge. Evidence for electrical anisotropy in the central Mariana upper mantle is weak.

Magnetic and electric field observations during the 2000 activity of Miyake-jima volcano, Central Japan

Magnetic and electric field variations associated with the 2000 eruption of Miyake-jima volcano are summarized. For about 1 week prior to the July 8 phreatic explosion, significant changes in the total intensity were observed at a few stations, which indicated uprising of a demagnetized area from a depth of 2 km towards the summit: this nonmagnetic source can be regarded as a vacant space itself. Electric and magnetic field variations were observed simultaneously associated with the tilt-step event, which was the abrupt (V50 s) inflation at a few km depth within the volcano followed by gradual recovery (Vseveral hours). The electric field is ascribed to the electrokinetic effect most probably due to forced injection of fluids from the source, while the magnetic field to the piezomagnetic effect due to increased pressure. Large magnetic variations amounting to a few tens of nT were observed at several stations since July 8, and they turned almost flat after the August 18 largest eruption. Magnetic changes are explained mostly by the vanishing of magnetic mass in the summit and additionally by the thermal demagnetization at a rather shallow depth. A large increase in the self-potential by 130 mV was also observed near the summit caldera associated with the August 18 eruption, which suggests that the hydrothermal circulation system sustained within the volcano for the past more than 10 years was destroyed by this eruption. 9 2002 Published by Elsevier Science B.V.

Trans-Pacific temperature field in the mantle transition region derived from seismic and electromagnetic tomography

Abstract The trans-Pacific temperature field for the depth range 350–850 km was inferred from global seismic tomography and semi-global electromagnetic tomography. The seismic tomography incorporated millions of reported first arrival times and 7000 PP–P differential travel times measured on broadband seismograms. The electromagnetic tomography used voltage data from trans-Pacific submarine cables and magnetic field data from circum-Pacific geomagnetic observatories. The resultant P-wave velocity anomalies and electrical conductivity anomalies were converted to temperature anomalies using a proposed conversion formula and experimental results for mantle minerals, respectively. These conversions show consistently high-temperature anomalies of 200–300 K in the mantle transition region beneath the Hawaiian hotspot. At subduction zones, where slab-related cold anomalies and wedge mantle-related hot anomalies are likely to coexist in close proximity, the seismic and electromagnetic tomography did not always give consistent features, in part because of the preferred sensitivity of electromagnetic tomography to hot anomalies. Low-temperature anomalies of 200–300 K associated with subducted slabs are clearly resolved in the seismic tomography, but are less apparent in the electromagnetic tomography. The high-temperature anomaly in the intervening zone between the Mariana and Philippine slabs is very pronounced in the electromagnetic tomography but is marginal in the seismic tomography.

Upper mantle conductivity structure of the back‐arc region beneath northeastern China

The upper mantle electrical conductivity structure of the pacific back-arc beneath northeastern China was investigated through long-period magnetotelluric (MT) and geomagnetic depth sounding (GDS) experiments. MT and GDS responses were obtained up to periods of 105 ∼ 106 seconds, and were inverted to one-dimensional (1-D) models with minimum and smooth structure constraints, respectively. The resulting conductivity model was compared with past models and the mantle transition zone beneath northeastern China is more conductive than those of other tectonic settings by almost one order of magnitude in the depth range of 400∼600 km. This feature may correspond to the presence of a stagnant slab beneath this region. In the mantle at a depth of less than 400 km, the conductivity profile has a similar feature to that in the thermal and extensional tectonic region in the southwestern United States.

Water content in the mantle transition zone beneath the north pacific derived from the electrical conductivity anomaly

Fukao et al. (2004) inverted semi-global electromagnetic network data for threedimensional electrical conductivity structure in the mantle transition zone beneath the north Pacific. In this paper we interpret the electrical conductivity structure in terms of the water distribution in the mantle transition zone, using partial derivatives determined by laboratory experiments on mantle materials. Fukao et al. (2004) explained both electrical conductivity and seismic P-wave velocity anomalies with thermal anomalies because of the overall coincidence of high electrical conductivity with low seismic velocity. However, a significant discrepancy is found beneath the Mariana islands where the seismic tomography would indicate little temperature anomaly, while electromagnetic tomography implies high temperatures. Despite limitations and differences in spatial resolution, this result indicates that this particular feature may not be explained by only a thermal effect. Taking into consideration that this region is well populated by subducted slabs, we further assume that this discrepancy is caused by water dehydrated from those slabs. Under this assumption, by combining the Nernst-Einstein relationship (e.g. Karato, 1990) and the recent result of laboratory measurements of hydrogen diffusivity in wadsleyite (Hae et al., 2006), the water content anomaly was estimated from the electrical conductivity anomalies. We find that the mantle transition zone beneath Mariana islands could contain about 0.3 weight % water.

Resistivity and self-potential changes associated with volcanic activity: The July 8, 2000 Miyake-jima eruption (Japan)

Abstract The Miyake-jima volcano abruptly erupted on July 8, 2000 after 17 years of quiet and gave birth to a crater, 1 km in diameter and 250 m deep. This expected unrest was monitored during the years 1995–2000 by electromagnetic methods including DC resistivity measurements and self-potential (SP) surveys. Beneath the 2500 yr old Hatcho-Taira summit caldera audio-magnetotelluric soundings made in 1997–98 identified a conductive medium, 200–500 m thick (within the 50 Ω m isoline) located at a few hundred metres depth. It was associated with the active steady-state hydrothermal system centred close to the 1940 cone and extending southward. A DC resistivity meter set in a Schlumberger array with 600, 1000 and 1400 m long injection lines evidenced strong resistivity changes between September 1999 and July 3, 2000 in the vicinity of the newly formed crater. The apparent resistivity has reached about three times its initial values on the 1400 m long line and has lowered to about 20% on the 600 m line. Just prior to the July 8, 2000 eruption SP mapping made inside the summit Hatcho-Taira caldera revealed negative anomalies where positive ones had occurred during the previous tens of years. The largest negative anomaly, −225 mV in amplitude, mainly took place above the 1940 cone which collapsed in the crater formation. A permanent 1 km long SP line across the caldera suggests accelerating changes during the 3 months preceding the eruption. On a larger scale, the comparison between 1995 and 2000 surveys has shown a global increase of the hydrothermal activity beneath the volcano. Its source could have been 250 m to the south of the crater. These observations suggest that the hydrothermal system was slowly disturbed in the months preceding the eruption while drastic changes have occurred during the 2 weeks before the summit collapse when tectonic and volcanic swarms have appeared.

Ocean Bottom Array Probes Stagnant Slab Beneath the Philippine Sea

Global seismic tomography has revealed the existence of a now-stagnant subducted slab (Figure 1c, inset) in the mantle transition zone (MTZ) of the western Pacific Ocean [e.g., Fukao et al., 2001], where the old Pacific plate subducts at the Kuril/Japan/Mariana Trench system. Fukao et al. [2001] showed that a stagnant slab is a common feature of many subduction systems around the world, although it is still not well understood why and how a subducted slab may or may not become stagnant. In addition to a subducted slab itself, water entrained with the slab may also accumulate in the MTZ in association with stagnation. The MTZ has been thought to play a key role in the Earths water cycle, with water transported to the MTZ possibly being released and supplied to the upper mantle to drive back-arc volcanism [e.g., Ichiki et al., 2006].

Electrical conductivity anomalies beneath the Western Sea of Kyushu, Japan

Geomagnetic depth soundings have been made in the area covering Kyushu island and its neigh- boring islands in the western sea of Kyushu to investi- gate the back-arc conductivity structure. The in-phase induction arrows derived from geomagnetic variations are found to point roughly southwestwards at most sites on Kyushu island for the period range 300-7200 sec. This is quite different from some other areas of Japan. The non-uniform thin sheet model calculation indicates that the induction arrows in this area are strongly influ- enced by the surrounding seas. However, the observed arrows have much more intense westward components than indicated by the model calculations. This strongly suggests existence of highly conducting layers (HCLs) beneath the East China Sea. And the existence of HCLs in the upper mantle are confirmed by a two-dimensional finite element modeling. These HCLs may be consid- ered to be a part of mantle upwelling extended to East

Possible effects of lateral heterogeneity in the D″ layer on electromagnetic variations of core origin

This paper examines the consistency of a model for typical spatial features of rapid geomagnetic secular variations in which electromagnetic (EM) scattering due to the lateral heterogeneity of mantle conductivity plays an important role. When geomagnetic field variations of a core origin were separated into poloidal and toroidal modes, previous studies suggested that the scattering effect on the poloidal mode is probably weak. However, the toroidal field originating in the core may extend to the (very) deep mantle and can be converted into a poloidal field of observable intensity by scattering. In this paper, scattering of the EM field due to the D �� layer, where the lateral heterogeneity is supposed to be most significant in the mantle, is studied using three-dimensional numerical calculations. The results show that the spatial features of 60-year variations at the surface can be explained by scattering, although it is possible to interpret that they are directly reflecting those of poloidal field variations originating in the core. The same explanation may be applicable to geomagnetic jerks, assuming that they are represented by a shorter time-scale (1 year) variation. Although a magnetic field observation itself does not enable us to distinguish whether or not observed variations originated from poloidal field variations in the core or by a scattering of the toroidal field in the D �� layer, our calculation results indicate that detection of electric field and LOD variations of corresponding periods provide strong constraints on the mechanisms of these phenomena, especially the spatial pattern of decadal variations.