Shinichi Takakura

Resistivity structure across Itoigawa-Shizuoka tectonic line and its implications for concentrated deformation

We investigated the deep crustal resistivity structure across Itoigawa-Shizuoka Tectonic Line (ISTL), one of the most dangerous active intraplate faults in Japan, by use of wide-band magnetotelluric (MT) method. The 28 MT stations were aligned perpendicular to the ISTL. A two-dimensional model was created in transverse magnetic (TM) mode where electric currents flow in N60°W-N120°E directions. The model showed good correlations with the surface geology. In particular, we found a thick (∼6 km) surface conductor to the east of ISTL which corresponds to the heavily folded sedimentary layer. The Japan Alps to the west of the ISTL is characterized by the resistive upper crust, where the pre-Tertiary rocks crop out. The Japan Alps is underlain by a conductor below 15–20 km depth, which is consistent with the low seismic velocity anomaly. We also found a localized shallow conductor corresponding to the Mt. Tateyama volcano. The most important feature is the conductor in the mid-crust directly under the area of active folding to the east of the ISTL. This may imply a localized zone of fluids because of the enhanced porosity in a shear zone. The recent seismicity clusters in the resistive crust underlain by the conductor, and this suggests the fluid involvement in earthquake generation processes.

Wide‐band magnetotelluric measurements across the Taupo Volcanic Zone, New Zealand‐Preliminary results

The Taupo Volcanic Zone (TVZ) of New Zealand is characterised by intensive geothermal activity and frequent rhyolitic volcanism. Sixteen wide-band (0.01-1,800 s) magnetotelluric soundings were measured along a 110 km-long profile approximately perpendicular to the strike of the TVZ. A model obtained from 2D inversion of the soundings shows two near-surface regions of high conductance which correspond to low density volcaniclastic sediments, up to 3 km thick, which infill a sequence of collapse calderas. At deeper levels (approximately 5-10 km) a resistive layer underlies the entire TVZ. Modelling shows other conductive zones occur beneath the TVZ, with the shallowest lying below the central part at a depth of 10 -15 km. Given the high heat flux and volcanic history of the TVZ, the high conductivity at depth may indicate the presence of connected melt. At greater depth (20-30 km) the upper mantle beneath the TVZ appears to be anomalously conductive, consistent with observed high seismic attenuation.

A resistivity cross-section of Usu volcano, Hokkaido, Japan, by audiomagnetotelluric soundings

We collected audio-magnetotelluric (AMT) data across Usu volcano, Hokkaido, Japan, which erupted in 1977 and is still active. We had a profile of 17 sites perpendicular to the regional tectonic strike, which crossed the 1977 cryptodome, Usu-Shinzan. Tensor-decomposed data were interpreted by a two-dimensional inversion. Outside the crater rim, the resistivity structure is simple. The resistive somma lava is underlain by a conductive substratum, implying altered Tertiary or Quaternary rocks. In the crater, there are two resistive bodies bisected by a vertical conductor, which corresponds to Usu-Shinzan fault, located at the foot of the uplift. The vertical conductor was not detected in the AMT sounding in 1985. One of the possible causes of the development of the vertical conductor is a cold water supply from the surface into the vapor dominant fracture zone. One of the resistive bodies is located beneath Usu-Shinzan and implies an intrusive magma body which caused the 1977 uplift.

ζ potential measurement of volcanic rocks from Aso caldera

[1] We deduced the z potential of various rocks in Aso caldera from streaming potential measurements in laboratory. In contrast to the conventional observation that the z potential of crustal rocks is mostly negative under typical geologic conditions, almost half of the samples equilibrated with dilute KCl solution show positive values of z potential at pH 4.5–5.5. The samples showing positive z are characterized by relatively low SiO2 content and abundance of elements having high isoelectric points, and found to be localized around the summit of Mt. Takadake where the positive correlation between self-potential (SP) and altitude is observed. We emphasize that measurements of z potential of volcanic rocks are very important to give more appropriate interpretations of SP data from volcanic fields. INDEX TERMS: 0634 Electromagnetics: Measurement and standards; 0925 Exploration Geophysics: Magnetic and electrical methods; 5199 Physical Properties of Rocks: General or miscellaneous; 8499 Volcanology: General or miscellaneous; 1832 Hydrology: Groundwater transport. Citation: Hase, H., T. Ishido, S. Takakura, T. Hashimoto, K. Sato, and Y. Tanaka, z potential measurement of volcanic rocks from Aso caldera, Geophys. Res. Lett., 30(23), 2210, doi:10.1029/2003GL018694, 2003.

Crustal magma pathway beneath Aso caldera inferred from three‐dimensional electrical resistivity structure

At Naka-dake cone, Aso caldera, Japan, volcanic activity is raised cyclically, an example of which was a phreatomagmatic eruption in September 2015. Using a three-dimensional model of electrical resistivity, we identify a magma pathway from a series of northward dipping conductive anomalies in the upper crust beneath the caldera. Our resistivity model was created from magnetotelluric measurements conducted in November–December 2015; thus, it provides the latest information about magma reservoir geometry beneath the caldera. The center of the conductive anomalies shifts from the north of Naka-dake at depths >10 km toward Naka-dake, along with a decrease in anomaly depths. The melt fraction is estimated at 13–15% at ~2 km depth. Moreover, these anomalies are spatially correlated with the locations of earthquake clusters, which are distributed within resistive blocks on the conductive anomalies in the northwest of Naka-dake but distributed at the resistive sides of resistivity boundaries in the northeast.