Distribution of Volcanoes and Hot Low-Velocity Zones, and Mantle Dynamics
Some interesting features observed in the 3D structure beneath NE Japan are summarized as follows. At 110 km depth, fairly low-velocity regions (velocity perturbations below -4%) appear in the mantle wedge and exist to shallow depths right beneath the Moho discontinuity. Extremely low-velocity regions (velocity perturbations below -5%) are observed around 70 km depth beneath lat. 40.2 N, long. 140 E, and around 40 km depth right beneath the volcanoes. These low-velocity regions appear to be localized. Low velocity indicates high temperature (and so low density) and the possible presence of magma in the region. Diapir-like melting regions of up to a few tens of kilometers are expected along the middle of the mantle wedge. The most extensive melting may occur in the uppermost mantle (~40 km depth) beneath the volcanoes. The continuous supply of magma from melting zones may cause the surface volcanic activity of the island arc. However, most of the magma will cool at depth and form the island arc crust. The local existence of hot low-velocity zones observed in this study may account for the lifetime of the island arc volcanoes. Once a melting zone cools down, volcanic activity will cease. New volcanic activity may start again after another diapir in the mantle wedge rises up to the Moho discontinuity.
The contours of -3% velocity perturbation are also shown in Figure 1 and Movies 1 and 2. Within the contours, velocity perturbations are below -3%, and thus higher temperatures and lower densities than average are expected. These high-temperature regions indicate the upwelling of hot mantle materials from beneath. Two major upwelling regions exist below 60 km depth at 38N and 40N, branch off at the uppermost mantle, and eventually underplate the Moho right beneath the volcanoes. The results indicate that melting zones localized in the upwelling hot mantle materials cause volcanic activity in the NE Japan arc.
In between the volcanoes and the low-velocity zones, mid-crustal S wave reflectors exist, and low-frequency microearthquakes occur in the crust, as also shown in Figure 1. The close relationship of magma source regions with S wave reflectors and microearthquakes indicates that magma ascent from the uppermost mantle to the ductile lower crust may cause low-frequency microearthquakes, and magma penetration into the brittle upper crust may produce the mid-crustal reflectors, as has been discussed previously . Magma movement in the brittle upper crust may increase stress (or at least change the state of stress) in the crust, and may contribute to the occurrence of crustal earthquakes including destructive earthquakes. Interestingly, large crustal earthquakes () also occur around the volcanoes and the low-velocity zones (Figure 1, Movies 1 and 2).
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