Hery Harjono

Seismicity of the Sunda Strait: Evidence for crustal extension and volcanological implications

The Sunda Strait is located in the transitional zone between two different modes of subduction: the Java frontal subduction and the Sumatra oblique subduction. This setting implies that the Sunda Strait region is a key to the understanding of the geodynamic processes involved. In order to study the shallow seismicity, a microearthquake survey was carried out in that region. Twelve stations, accurately located by the aim of satellite positionning, recorded about 300 local events in the summer 1984. From this set, 174 shallow earthquakes have been precisely located. The results of this study reveal that the crustal earthquakes in the Sunda Strait area occurs in three main areas: (1) beneath the Krakatau complex, where earthquakes are generated by double-couples and are of tectonic origin; (2) inside a graben in the western part of the strait; and (3) in a more diffused zone to the south of Sumatra. The individual and composite focal mechanisms from the events inside the strait show an extensional regime. A stress tensor, which have been deduced from the individual focal mechanisms of earthquakes of the Krakatau group shows that the tensional axis is oriented N130oE. This study confirms that the Sunda Strait is in an extensional tectonic regime as a result of the northwestward movement of the Sumatra sliver plate along the Semangko fault zone.

Inner structure of the Krakatau volcanic complex (Indonesia) from gravity and bathymetry data

Abstract On 27 August 1883, the Krakatau volcanic complex (Indonesia) was the site of one of the most destructive historical eruptions. Most of the volcano was destroyed and a new caldera also formed during this catastrophic event. Since the date of the eruption, many geological studies of the superficial structures and eruption products have been carried out. A debate on the scenario of the eruption and the way the volcano collapsed has developed and still is unresolved. In order to assess the inner structure of the volcanic complex, we carried out a detailed land and marine geo-physical survey in the summer 1990. In this paper, bathymetry and gravity data collected during the survey as well as literature data are compiled and analysed. Bathymetric data show that the caldera is characterized by a flat seabottom at 240 m below sea level and by steep linear walls suggesting that the caldera collapse has been controlled by pre-existing features. Moreover, the build-up of the young active volcano, Anak Krakatau, on the very edge of the caldera could lead to mechanically unstable conditions which must be considered for hazard mitigation. The Bouguer anomaly of the volcanic complex is characteristic of volcanoes with an explosive behaviour. 3-D gravity modelling reveals the previously unknown geometry of the dense substratum of the proto-Krakatau and evidences the presence of a collapsed structure beneath the caldera filled up with low-density material. Finally, we point out a major weakness zone, oriented N150 ° on a line passing through the old and recent vents of the volcano. This zone could have guided both the development of the volcanic activity and the emplacement of the 1883 caldera. Furthermore, this weakness zone passing through the summit line of the pre-1883 Krakatau volcano has been introduced as a significant disruption surface of the volcanic edifice in the updated scenario of the 1883 eruption that we propose.

Aftershock sequence of the 1994, Mw 6.8, Liwa Earthquake (Indonesia): Seismic Rupture process in a volcanic arc

We present the aftershock activity following the February 15, 1994, Mw 6.8 earthquake which was strongly felt in southern Sumatra, Indonesia, near the Great Sumatran Fault (GSF). At this place, the slip rate is supposed to be low; neverthless, three M>6 events occurred along this segment during this century. No significant instrumental microseismicity has ever been recorded there. We use data from both the regional Indonesian network and a local seismic array operating 11 days after the mainshock during one month. Aftershocks mostly locate in a broad zone of 55 × 20 km² near two active NW-trending strike-slip segments of the GSF separated by a recent caldera, Suwoh. During the experiment, the NW segment (from Suwoh up to 15 km SE of the Ranau lake caldera, an old right-stepover pull-apart) was very active. As first suggested by the aftershock distribution and the lack of coseismic rupture at the surface, the 20 focal mechanisms determined provide evidence for various post-seismic stress adjustments on secondary faults located in the Ranau-Suwoh paleo-pull-apart graben. Less than 20% of the aftershocks are directly linked to the main rupture, a nearly pure right-lateral strike-slip faulting reaching 25 km depth. A narrow seismic gap underlines the active volcanic area of Suwoh. We conclude that the rupture process along the GSF is controlled both by volcanism and structures, and that the volcanic activity affects the mechanical properties of the crust only in a narrow zone.