The present-day active volcanic front associated with the Ryukyu subduction zone extends from Japan to the Ilan plain (northern Taiwan) and is located within the Okinawa Trough, 80--100 km above the Ryukyu slab. An abnormal amount of arc volcanism, which consists of basalt, andesite and rhyolite occurs within the southwestern Okinawa Trough, above a slab tear of the Ryukyu subduction zone (CBVT). The power spectrum analysis of magnetic data shows the occurrence of a thin crust above the slab tear and a thick crust beneath this volcanic area. We suggest that an excess of H2O-rich fluid might occur at the slab tear and might increase the melt flux. Both are conveyed obliquely to the uppermost mantle and lower crust CBVT magmas. After interactions, basaltic magmas would rise up, accounting for the contrast of magnetization between this volcanic body and the adjacent OT crust.