タイトル：

Electrical Resistivity Structure of Fluid and Magma Supply Systems in Subduction Zones: A Comparison between Kyushu–KPR and the North Island, New Zealand

要旨：

In subduction zones, fluids transported into the Earth’s interior by subducting oceanic plates play a fundamental role in driving both magmatic and seismic activity. These fluids are released through dehydration reactions within the slab under specific pressure–temperature conditions and promote partial melting in the mantle, thereby generating magma supply systems for arc volcanism. Consequently, volcanic chains in island arcs generally develop along depth contours of the subducting slab. However, some arcs, including Kyushu in Japan and the North Island of New Zealand, contain non-volcanic segments of approximately 100 km length along the volcanic front, and the mechanisms responsible for such spatial discontinuities remain poorly understood.

To investigate the relationship between subsurface heterogeneity and arc segmentation, we compare three-dimensional resistivity structures in Kyushu and the North Island of New Zealand. In Kyushu, analyses using various electromagnetic datasets reveal crustal low-resistivity anomalies that are focused beneath individual active volcanic centers. At depths near the crust–mantle boundary, pronounced low-resistivity zones are observed beneath volcanic segments in both northern and southern Kyushu, whereas no clear anomalies are detected beneath non-volcanic segments. At depths exceeding approximately 100 km, a distinct conductive region is observed only beneath the southern volcanic segment, indicating strong spatial heterogeneity in both magmatic activity and the contribution of slab-derived fluids.

In the North Island of New Zealand, long-period magnetotelluric observations were conducted across a ~300 km × 150 km transitional region spanning the southern Taupo Volcanic Zone and adjacent non-volcanic segments along the Hikurangi subduction system. Three-dimensional resistivity modeling is underway based on preliminary one-dimensional results to better resolve structural differences between volcanic and non-volcanic segments. In the crust, a prominent low-resistivity zone is observed around the Taupo Volcanic Zone, suggesting an active thermal and fluid system. In the mantle beneath volcanic segments, a structure potentially corresponding to the oceanic slab–mantle boundary is inferred, although it is less clearly resolved compared to Kyushu. In addition, resistivity contrasts near the crust–mantle transition zone appear to correlate with the distribution of slow earthquakes and slip behavior.

These results suggest that spatial variations in slab-derived fluid distribution control both the presence or absence of arc volcanism and the segmentation of volcanic arcs, and further imply that such fluid distributions impose important constraints on seismic processes in subduction zones.