Magnetotelluric transect of Unzen graben, Japan: conductors associated with normal faults

Abstract

We conducted a broadband magnetotelluric (MT) survey along a north-south transect across Unzen graben, Japan. The MT survey line is located ~ 2 km west of the most recent lava dome and consisted of 27 stations along a 9-km profile. We estimated the 3-D resistivity structure and correlated it with the seismic reflection structure obtained by the same survey line as in the present study. The best-fit resistivity structure shows an upper resistive layer underlain by a moderately conductive layer. The resistive layer, which is interpreted as a cold groundwater zone, is cut by four faults marked by their relatively high conductivity. The underlying layer, which is interpreted as a hydrothermal-waterrich layer, also shows relatively conductive values near the faults. By assuming that the faults are imaged as relatively conductive zones, we infer the dip and depth extent of fracture zones around the faults. Beneath the Chijiwa Fault, which is the longest and most active fault of Unzen graben, the dominant conductor (C1) has a width of 2 km and extends down to below 4 km depth. C1 corresponds to a zone of strong seismic reflection and is located close to one of the pressure sources causing surface deformation. In this study, we interpret C1 as a network of fractures generated by the Chijiwa Fault to which magmatic volatiles are supplied from a deeper pressure source. Given that C1 extends to a greater depth and its resistivity is lower than other conductive zones, it is possible that earthquakes have occurred repeatedly on the Chijiwa Fault. In the center of the study area, we identify a vertically oriented body of high resistivity (R1) that corresponds to a zone of low seismic reflectivity. We interpret R1 as a cooled dyke complex that may have acted as a volcanic conduit.

The economic potential of metalliferous sub-volcanic brines

“…Such high melt fraction is commonly inconsistent with other geological evidence, such as shear wave speeds (e.g. [125,128]). Finally, the conductor may be composed, in part, of electrically conductive minerals such as sulfides or magnetite, which can have conductivities as high or higher than brines (e.g.…”

“…From the perspective of sub-volcanic brine lenses, electrical conductivity has the greatest potential as an imaging and mapping tool. Electromagnetic surveys, including magnetotelluric (MT) and time-domain (TDEM), have been conducted at numerous active and dormant volcanoes, including Mount Fuji, Japan [122]; Taal, The Philippines [123]; Kusatsu-Shirane, Japan [124]; Uturuncu, Bolivia [125]; Unzen, Japan [126–128]; Taupo Volcanic Zone, New Zealand [129–131]; and Mount St Helens, USA [132,133]. These surveys consistently reveal prominent, electrically conductive (greater than or equal to 1 S m −1 ) regions at depths greater than or equal to 2 km, interpreted by Afanasyev et al .…”

Caracterización magnetotelúrica de las fallas y sistema de fallas en el valle del Cusco, sur del Perú

“…Por ejemplo, los estudios realizados por Triahadini (2019), Balasco et al (2015), Pavez (2015), Unsworth & Bedrosian (2004), Hoffmann-Rothe (2002, Unsworth (2002), entre otros, han identificado una estrecha correlación entre las zonas conductivas con las fallas activas.…”

Hydrothermal and Magmatic System of a Volcanic Island Inferred From Magnetotellurics, Seismicity, Self‐potential, and Thermal Image: An Example of Miyakejima (Japan)

“…In Miyakejima, a sharp resistivity contrast exists between the superficial high-resistive materials ('u' unit) and the underlying conductive ones ('c' unit). Such vertical variation is often observed on active volcanoes that include significant topography and meteoric recharge (e.g., Aizawa et al, 2005;Gailler et al, 2018;Piña-Varas et al, 2014;Revil et al, 2011;Rosas-Carbajal et al, 2016;Triahadini et al, 2019;Usui et al, 2016). This change in electrical resistivity is generally interpreted as a shift from unsaturated to water-saturated regime, delimiting the water table boundary of the volcano (e.g., Aizawa et al, 2009;Hurwitz et al, 2003).…”