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 C. Furthermore, new helium isotope data from hot springs around the volcano were obtained. The spatial distribution of the observed / 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. 1. Introduction Naruko volcano is an active volcano situated in the volcanic front of the northeastern Japan Arc (Figure 1), where the Pacific Plate is subducting along the Japan Trench beneath the North American Plate. A number of seismological studies have investigated the structure of the crust and mantle beneath northeastern Japan, including Naruko volcano. These include studies investigating seismic velocity structure (e.g., [1, 2]), attenuation structure (e.g., [3]), distribution of S-wave reflectors (e.g., [4]), and low-frequency micro-earthquakes (e.g., [5, 6]). These studies generally confirm the existence of partial melts and related aqueous fluids in the crust and upper mantle that are imaged as seismic low-velocity and high-attenuation zones with distinct S-wave reflectors and are associated with low-frequency micro-earthquakes. Figure 1: (a) Distribution of plate boundaries in northeastern Japan. The rectangle denotes the study area shown as Figure 1(b). Solid triangles show the Quaternary volcanoes [ 7]. (b) Distribution of volcanoes (solid lines) and active faults (broken lines) in the study area. The MT sites (open squares) are also shown. A: Akakura Caldera Volcano (Pliocene to 1？Ma), H: Hanayama Caldera volcano (Miocene), K: Kurikoma Volcano (0.5？Ma to present), M: Mukaimachi Caldera Volcano (Pliocene to 0.6？Ma), N: Naruko Caldera Volcano (0.05？Ma to present), O: Onikobe Caldera Volcano (0.5 to 0.2？Ma), S: Sanzugawa Caldera Volcano (Late Miocene to Pliocene) [ 8– 10]. Magnetotelluric (MT) soundings can image
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