Sumatran segment of the Indonesian subduction zone: morphology of the Wadati-Benioff zone and seismotectonic pattern of the continental wedge

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

“…2a-c), parameters of the WadatiBenioff zone beneath volcanoes, depth extent of the intermediate-depth aseismic gap in the Wadati-Benioff zone, and information on the occurrence of the seismically active column in the lithospheric wedge beneath volcanic domains. Vaněk, 1976, 1978) and later confirmed in several active subduction zones (Hanuš and Vaněk, 1985;Vaněk et al, 1987Vaněk et al, , 1994Delouis et al, 1996;Hanuš et al, 1996). It is difficult to interpret this aseismic gap in the Wadati-Benioff zone beneath volcanic chain from seismic observations only.…”

“…It should be stressed that the Wadati-Benioff zone is understood as that part of the subducting slab where earthquakes are observed; the varying thickness of the Wadati-Benioff zone, schematically shown by parallel lines At depths between 100 and 200 km, the Wadati-Benioff zone is characterized by a region without any strong teleseismically recorded events. Such a gap in seismic activity is characteristic for many Wadati-Benioff zones and correlates spatially with active calc-alkaline volcanism Vaněk, 1985, 1988;Delouis et al, 1996;Hanuš et al, 1996;Š pičák et al, 2002;Vaněk et al, 1987Vaněk et al, , 1994. The depth and thickness of the gap varies laterally along individual arcs.…”

“…The distribution of earthquake foci at convergent plate margins reveals that the contemporary tectonic processes are also manifested by relatively high seismicity in the lithospheric wedge above the Wadati-Benioff zone (Hanuš and Vaněk, 1977-78, 1979, 1984Hanuš et al, 1996). It appears that the foci of these earthquakes are not distributed randomly, but instead show a tendency to occur in well-separated linear zones.…”

“…During our previous investigations into the seismicity pattern of different convergent plate margins by means of narrow vertical cross sections perpendicular to pertinent oceanic trenches, we identified a regularly occurring intermediate-depth aseismic gap in the Wadati-Benioff zone directly situated beneath active calc-alkaline volcanoes. This led us to the conclusion that the aseismic part of the subducted slab was likely to represent a partially melted medium and the site of the main source of primary magma for active volcanoes at convergent plate margins (Hanuš and Vaněk, 1976, 1985Vaněk et al, 1987;Hanuš et al, 1996).…”

“…The results of our previous studies on the deep structure and seismotectonics of Sumatra (Hanuš et al, 1996) and on the seismicity pattern beneath and around Krakatau Volcano (Š pičák et al, 2002) inspired us to apply the set of relocated ISC data (Engdahl et al, 1998) in the regions of Java and Nusa Tenggara to study the morphology and internal structure of the Wadati-Benioff zone, the active tectonic structures of the lithospheric wedge and the possible relationship of seismic and volcanic activity. Our study on Krakatau (Š pičák et al, 2002) pointed to an important phenomenon-the presence of an uninterrupted column of earthquakes down to the upper boundary of the subducting slab at a depth of 100 km, which indicated the brittle character of the mantle wedge beneath the volcano.…”