Seismotectonic pattern and the source region of volcanism in the central part of Sunda Arc

The seismotectonic pattern in the central part of the Sunda Arc (Java, Nusa Tenggara) was studied in relation to the distribution of active calc-alkaline volcanoes, using global seismological data. Hypocentral determinations of the International Seismological Centre from the period 1964-1999, as relocated by Engdahl, and Harvard Centroid Moment Tensor Solutions from the period 1976-2003 were used. The following phenomena, which could assist the location of the source region of primary magma for island arc calc-alkaline volcanism, were observed: (1) An aseismic gap without any strong teleseismically recorded earthquakes was found in the Wadati-Benioff zone of the subducting slab along the whole investigated region of the Sunda Arc, forming a continuous strip of laterally variable depth and shape, at depths between 100 and 200 km. The absence of strong earthquakes (with m(b) > 4.0) indicates a significant change in the mechanical properties of the subducting slab at intermediate depths. All active calc-alkaline volcanoes in the Sunda Arc are located above this gap. (2) The majority of earthquakes occurring in the lithospheric wedge of the Eurasian Plate above the subducted slab could be attributed to several deep-rooted seismically active fracture zones of regional extent. All delineated active fracture zones display a thrust tectonic regime as shown by the available fault plane solutions. (3) Clusters of earthquakes were found beneath active volcanoes of western Java, Bali and Nusa Tenggara in the lithospheric wedge above the slab and identified as seismically active columns. These clusters occur only beneath the volcanoes that are located at the outcrops of seismically active fracture zones. We interpret the earthquakes in these clusters beneath volcanoes as events induced by magma transport through the medium of the lithospheric wedge that has been subcritically pre-stressed by the process of plate convergence. (4) Beneath the volcanoes of central Java no seismically active columns were observed. The latter volcanoes are not located at the outcrop of any seismically active fracture zone. The presence of the intermediate-depth aseismic gap in the Wadati-Benioff zone and the presence of seismically active columns above the gap beneath some active volcanoes support the concept that the subducted oceanic lithosphere is the source region for the primary magma for calc-alkaline volcanoes at convergent plate margins. However, for volcanoes without any seismically active columns and fracture zones beneath them, a source region for the primary magma in the lithospheric wedge above the subduction zone cannot be excluded. (c) 2004 Elsevier Ltd. All rights reserved.