Investigating the crustal and upper mantle structure of the central Java subduction zone with marine wide-angle seismic and gravity data

Offshore wide-angle seismic data recorded on ocean bottom instruments of a combined onshore- offshore investigation on the tectonic framework of central Java are presented in this study. The joint inter- disciplinary project MERAMEX (Merapi Amphibious Experiment) was carried out to characterize the subduction of the Indo-Australian plate beneath Eurasia. Three marine wide-angle profiles are analyzed by combined forward- and inverse modeling of first and later arrival traveltimes and are integrated to- gether with gravity data. The results of this study are compared with former investigations off southern Sumatra, western Java and eastern Java to obtain a detailed image of the Java margin. The subduction of the oceanic Roo Rise plateau, located south of central Java, with its thickened and buoyant crust, strongly influences subduction dynamics. The trench is retreated about 60 km in a land- ward direction. Large scale forearc uplift is manifested in isolated forearc highs, reaching water depths of only 1000 m compared to 2000 m water depth off western Java, and results from oceanic basement relief subduction. The dip angle of the underthrusting oceanic lithosphere is 10◦ underneath the marine forearc and its crustal thickness increases eastward from 9 - 10 km over a distance of 100 km between both dip profiles off central Java, which is thicker than the global average of 7.4 km. The incipient subduction of a broad band of seamounts off central Java causes frontal erosion of the margin here and leads to mass wasting due to oversteepening of the upper trench wall. The well-developed accretionary wedge off southern Sumatra and western Java diminishes into a small frontal prism with steep slope angles of the upper plate off central Java. This causes a persistent threat for generating tsunamis, which may also be triggered by smaller (Mw8) earthquakes. The rough surface of the Indo-Australian plate with its volcanic edifices strongly influences the interplate coupling. A subducted and dismembered seamount is revealed on the eastern profile at the toe of the backstop in 15 km depth. This seamount and similar features present on the megathrust may potentially act as asperities or as barriers to seismic rupture, limiting lateral rupture propagation in the co-seismic phase. Subduction earthquakes with a magnitude ≥ 8 are not observed, while smaller earthquakes fre- quently occur. A remarkable clustering of earthquakes in the forearc mantle wedge below the shallow forearc Moho may be the seismic expression of seamount detachement.


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