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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] Regional and local variations in mantle helium provide insight into the coupling of mantle-crust tectonics, and heat and/or mass transfer from the Earth's interior. In order to further elucidate the geographic distribution of 3 He/ 4 He ratios in southwest Japan, the data from a total of 924 sites were compiled and synthesized. These include data from 48 additional hot spring and drinking water well sites on the northern Kyushu Island and in the northern Chugoku region. There appears to be good correlation between variations in helium isotope ratios and the geophysical evidence used to determine the configuration of the subducting Philippine Sea plate (PHS). Seismological studies reveal that the leading edge of the aseismic slab does not extend to the northern Chugoku region nor to the Osaka Bay area, where gas samples with significantly elevated 3 He/ 4 He ratios occur. This is consistent with a mantle-derived helium in these areas, from melts and/or mantle fluids ascribed to upwelling asthenosphere without being hindered by the descending PHS slab. In contrast, gas samples in the regions where the overriding crust comes into direct contact with the subducting PHS are dominated by radiogenic helium derived from the crust because of the absence of a mantle wedge, the most plausible source of mantle helium. Owing to the abrupt changes in the seismicity and focal mechanisms of intraplate earthquakes, the PHS is considered to have slab tears beneath the Kii Channel and/or the eastern Kii Peninsula oriented in a NW-SE direction. However, the lenear alignment of anomalously high 3 He/ 4 He ratios does not appear to be NW-SE trending along the assumed slab tears but rather forms an broad, ENE-WSW trending zone between the tears where low-frequency events occur. The emanation of gas with elevated 3 He/ 4 He ratios in the central peninsula can be explained by the upward mobilization of mantle volatiles derived from the mantle wedge above the PHS and/or transferred from the hydrated slab mantle through the N-S trending fractured medium within the PHS. Accordingly, the helium isotopes observed on the Earth's surface may be efficient geochemical indicators of the configuration of a relatively younger, warm aseismic slab, and be especially useful in seismically inactive areas.

No abstract

The Northeast Japan arc, a mature volcanic arc with a back-arc marginal basin (Japan Sea), is located on a convergent plate boundary along the subducting Pacific plate and the overriding North American plate. From a compilation and analysis of stratigraphy, radiometric age and data on erupted magma volumes, 176 eruptive episodes identified from 69 volcanoes so far, indicate that notable changes in eruption style, magma discharge rates and distribution of eruptive centres occurred around 1.0 Ma. Before ca. 1.0 Ma, large-volume felsic eruptions were dominant, forming large calderas in the frontal arc, a region of low crustal strain rate. After ca. 1.0 Ma to the present, the calc-alkaline andesite magma eruptions in the frontal and rear arcs, synchronous with crustal shortening characterized by reverse faulting, resulted in stratovolcano development along narrow uplifted zones. Although, it is widely assumed that magma cannot rise easily in a compressional setting, some of the magma stored within basal sills could be extruded where N-S-trending uplifted mountains bounded by reverse faults formed since about ca. 1.0 Ma.