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 400∘C. Furthermore, new helium isotope data from hot springs around the volcano were obtained. The spatial distribution of the observed 3He/4He 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

Tomographic imaging of arc magma and subducting slab beneath the Kyushu subduction zone

The Kyushu subduction zone in southwest Japan is characterized by active subduction of the young Philippine Sea slab beneath the Eurasian plate. We have determined the detailed three-dimensional P-wave velocity structure of the Kyushu subduction zone to a depth of 120 km. We used 9,041 P and S wave arrival times from 486 shallow and intermediate-depth earthquakes, which occurred in and around Kyushu. The arrival times were recorded by the Japan University Seismic Network, which covers the entire Japan Islands densely and uniformly. Our results show that the subducting Philippine Sea slab is imaged clearly as a high-velocity zone. It has a thickness of about 35 km and a P wave velocity 3-5% faster than the initial 1-D velocity model used in this study. Very slow velocity anomalies (3-5%) exist in the mantle wedge and extend to the forearc region down to the subducting Philippine Sea slab beneath northern Kyushu. These results indicate that regimes of melting and magmatism in a subduction zone with a young slab are different from those with old slabs. Dehydration and melting occur beneath the arc and forearc regions above a young slab, such as in northern Kyushu (age<26 Ma), while they occur beneath the volcanic front and back arc above old slabs, such as Northeast Honshu and South Kyushu. The formation of magmatism and volcanism in Kyushu is considered to be related to the convective circulation process in the mantle wedge and the dehydration of the subducting Philippine Sea slab. Large crustal earthquakes during the period 1886-1996 are found to occur in the vicinity of the low-velocity zones, indicating that volcanoes and magma chambers weaken the brittle seismogenic crust, and cause those weak areas to be subject to tectonic stress and prone to large earthquakes