Along-Arc Variation in the Magma Distribution Beneath NE Japan Inferred From Travel Time Tomography

Volcanoes in active plate margins are generally considered as being regularly spaced along the strike of the arc [e.g., Marsh, 1979, JGR], and this has been used to derive constraints on the characteristics of the deep source of magma and on the mechanisms of magma ascent. However, Bremond d_fArs et al. [1995, JGR] investigated the distribution of volcanoes in 16 active plate margins and concluded that volcanoes are randomly distributed. Recently, Tamura et al. [2002, EPSL] investigated spatial distribution of Quaternary volcanoes in the NE Japan arc and pointed out that Quaternary volcanoes in the NE Japan arc are grouped into 10 volcanic clusters striking transverse to the arc. They have an average width of 50 km, and are separated by 30-75 km wide gaps. Tamura et al [2002] proposed that these observations are surface manifestations of locally developed hot regions having the form of inclined, about 50 km wide fingers extending from >150 km depth below the back-arc region to approximately 40 km depth below the volcanic front within the mantle wedge. To confirm this interpretation by Tamura et al. [2002], we conducted a specialized tomographic inversion of S-wave velocity structure of the mantle wedge beneath NE Japan. Only S-wave velocities of the mantle wedge portion were estimated by using the same data set of our previous study [Nakajima et al., 2001, JGR] but with those of the other portions being fixed. Obtained images have higher spatial resolutions of ~10 km in both the horizontal and vertical directions. As predicted by Tamura et al. [2002], velocity decrease in the inclined low-velocity zone has an along-arc variation and is locally large below the areas where Quaternary volcanoes are clustered. In other words, the hot mantle fingers have extremely low S-wave velocities. Moreover, deep, low-frequency microearthquakes are also clustered above such locally low velocity areas. Prominent S-wave low-velocity zones in the mantle wedge, distribution of Quaternary volcanoes at the surface, and deep, low-frequency microearthquakes are closely correlated in space, suggesting that the spacing of Quaternary volcanoes at the surface is determined by along-arc variation of melt contents in the low-velocity zones in the mantle wedge.