Although there is no evidence of volcanism in the Noto Peninsula since the late Miocene, the peninsula has long been known to be unusual and atypical of non‐volcanic regions, as indicated by high‐temperature hot springs and a geothermal gradient greater than 50 K/km. In order to provide geochemical constraints on the heat source for amagmatic hydrothermal activity, the chemical and isotopic compositions of 14 gas and water samples from hot springs were measured. The observed 3He/4He ratios of most hot spring gases range from 0.03 to 1.2 Ra (Ra denotes the atmospheric 3He/4He ratio of 1.4 × 10−6). In these samples, mantle helium composes less than 10% of the total helium, indicating an insignificant contribution of mantle‐derived volatiles from, for example, newly ascending magma and/or aqueous fluid generated by dehydration of the subducting slab. The Noto Peninsula mainly consists of Neogene volcaniclastic and sedimentary rocks overlying Paleozoic to Mesozoic basement rocks associated with uranium‐bearing granite pegmatite containing elevated concentrations of heat generating elements such as uranium, thorium, and potassium. A plausible heat source for the amagmatic hydrothermal activity can be attributed to this distinctive geological environment where high heat producing granitic rocks are buried under Neogene sedimentary rocks with low thermal conductivities that act as thermal blankets. Most Noto Peninsula hot springs are found in areas of active faulting, where stress concentrations since the late Miocene could open an existing fault pathway because the fluid pressure is close to the lithostatic pressure. Meteoric waters circulating through hot basement rock come to the surface along these permeable conduits with minimal mixing with shallow groundwater, resulting in emanation of high‐temperature hot springs with low 3He/4He ratio gases along active fault zones.

A magnitude (Mj) 7.3 intraplate earthquake occurred in the western Tottori area, southwest Japan, on 6 October 2000, where there was no apparent prefaulting subsurface indication of the source fault of the 2000 earthquake. Magnetotelluric soundings were taken in and around where the aftershocks occurred in order to image three‐dimensional electrical resistivity structure at depths of up to 40 km, which could indicate an anomalously conductive body in the middle crust to the upper mantle on the southwestern side of the source fault. Free gas and dissolved gases collected from groundwater wells around the seismic source region are characterized by 3He/4He ratios several times higher than the atmospheric value; the highest value of 5.1RA is similar to those of typical arc‐related volcanic gases. Although alkali basalts of early Pleistocene age are sparsely distributed in the western Tottori area, the observed 3He/4He ratios are higher than the calculated 3He/4He ratio derived from the ancient magmatism, considering postextrusive radiogenic ingrowth of 4He by decay of U and Th included in the magma. Therefore, it is concluded that the geophysical anomaly imaged to the southwest of the source fault is attributed to latent magmatism in the present‐day subduction system. Aqueous fluids separated from the cooling crustal magma could cause deep low‐frequency earthquakes around the Moho discontinuity and migrate into the brittle upper crust. In addition, the presence of aqueous fluid is expected to weaken the crustal materials. Locally anelastic deformation, implying notable compressive deformation in the E‐W direction, was observed in the region where aftershocks were distributed. Under overpressure conditions, the existing fault could serve as a pathway for aqueous fluids expelled from magma with high 3He/4He ratios, so the upwelling of overpressurized fluids toward the Earth's surface results in the emanation of groundwaters with high 3He/4He ratios along the trace of the source fault segments.

[1] 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 Wm) 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 3 He/ 4 He 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.Components: 4221 words, 5 figures.

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