Detection of magma bodies beneath Krakatau volcano (Indonesia) from anomalous shear waves

Abstract The seismograms of 14 local earthquakes recorded by a temporary network in the Sunda Strait were used to construct an image of shear-wave attenuating bodies in the vicinity of the Krakatau volcanic complex. Seismograms for all ray paths going beneath the Krakatau complex show a diminution of the amplitude of S waves. These diminutions cannot be attributed to source mechanisms or station effects. Combination of all ray paths, although in limited number, demonstrates that the attenuating body is located in two zones, which are probably disconnected, although the limited set of data available does not allow us to draw a definite conclusion. The upper zone is about 9 km deep and the lower one is at least 22 km deep. The upper zone contains probably numerous irregular pockets of magma. The deep zone appears to be a very wide body, related to the extensional nature of the Sunda Strait. The existence of a large, deep reservoir should be taken into account for the mitigation of future hazards.