Variations of b-values at the western edge of the Ryukyu Subduction Zone, north-east Taiwan

Using earthquakes relocated in north-east Taiwan, we estimated b-value distribution along a cross-section located near the Ryukyu slab edge, and four b-value anomalous areas are evidenced: (1) a high b-value body lying on top of a low Vp, low Vs and high Vp/Vs sausage-like body was considered as a region of enhanced partial melt or water supply above which seismicity occurs; (2) beneath the Ilan Plain, an anomalous area characterized by b-values slightly higher than 1.1 might give evidence to the magma conduits to the Kueishantao Island; (3) above the Ryukyu Wadati-Benioff zone, at depths ranging from 90 to 110km, a high b-value anomaly might correspond to the depth where dehydration occurs in the subducting oceanic plate; and (4) a low b-value area located within the Ryukyu slab, at depths ranging from 70 to 90km, might be linked to the compressive mechanisms shown by focal mechanisms and the bending of the subducting plate.