High velocity ring under caldera flanks

Isosurface plot of velocity anomalies under the volcano summit. Depths are measured below sea surface (Axial summit is 1.46 km deep). The red body is the limit of the central low velocity zone drawn on the -1.0 km/s surface. Blue marks the region where velocities are anomalously high by 0.3 km/s or more.

The low velocity zone is clearly the result of melt in the crust. The high velocity ring feature encircles the low velocity zone and extends up toward the surface. This feature is robust in different versions of the tomography and is visible in the raw data as well (see figure below).

Previous experiments have shown that the periphery of the caldera is responsible for providing much of the eruptive material to the summit (the 1998 eruption, vent locations and sidescan images). The coincidence of high velocity material with known magmatic activity forces one to rethink how melt transport manifests itself in seismic properties.

This fan shot through the caldera from a range of 20-25 km. The rays cross the caldera region a few kilometers below seafloor. The effect of the LVZ is clearly visible causing traveltime delays of up to 0.5 seconds. On each side of the LVZ there is a small bump of early arrivals. These data show the effect of the high velocity "curtain" around the magma body.

http://www.ldeo.columbia.edu/~mwest/AX/F/ring