After the strong earthquake of magnitude Mw=6.3 (source: NEIC and Harvard) on 26th of May 2006 on the island of Java, an expert group of 4 members of the German Task Force for Earthquakes left for Indonesia. This group includes 3 scientists of GFZ Potsdam (Germany's national research centre for geosciences) and 1 scientist of the University of Munich (LMU). All are familiar with seismic activity in the surrounding environment of Merapi volcano. The major task of this group in the first days after their arrival in the epicentral area of Bantul will be the installation of a seismological network of 12 short period stations to record the aftershock activity following the main shock. These recordings will be used to determine precise aftershock hypocenters thus helping to reveal the structure of the fault system on which the earthquakes' main shock occurred.

Figure 1. Digital elevation model of Java island with the marked region under study (see also Figure 2a and b). The left-lateral Central Java fault (CJF) is probably related to a transition zone between the continental and oceanic crust (Hoffmann-Rothe et al. 2001). The faults are compiled from Armion et al. (1999) and Hoffman-Rothe et al. (2001). The white rectangle marks the region shown in Figure 2a and b.

Responsible for the characteristic tectonic and geomorphologic features in the region is the subduction of the oceanic Australian plate beneath the continental Eurasian plate with a velocity of approximately 6 cm/year. The subducting plate reaches a depth between 100 and 200 km beneath the island of Java. In this depth range the down-going plate gets partially melted, producing magma which ascends and forms volcanoes. Many earthquakes occur in the seismogenic zone of the interface between subducting and overriding plate. However, the recent earthquake has a much shallower depth and occurred in a fault system well above this subduction zone (see explanations in figures below).

Figure 2a. Digital elevation model of the region under study. The yellow stars mark the epicentres of the earthquake located by USGS (Preliminary determination of epicentre - PDE, Fast moment tensor solution - FMT) and by the Harvard University (Centroit moment tensor solution - CMT).

Figure 2b. Remote sensing data (Landsat5) of the considered region. The black lines mark the dominant faults in the region (Armin et al., 1999).

The city of Yogjakarta in the region of Bantul is framed in the north by the high risk volcano Merapi and in the south by the epicentral area of Saturday's earthquake. The entire region suffered of intensive damages resulting in a high death toll and thousands of homeless people. The small distance between Merapi and the epicentral area suggests the existence of possible relations between the volcano and the recent strong earthquake. So one important part of the Task Force investigations will include the study of changes in stress and strain produced by the earthquake which might influence volcanic activity. Ash layers and fluvial sediments constitute the soil conditions in the area of Yogjakarta. Therefore, a first estimation of site effects which probably contributed to the severe damage will be another important direction of investigation.

The Task Force mission is planned in cooperation with the BMG Jakarta (Meteorological and Geophysical Agency of Indonesia), who is the project partner of GFZ in the German Indonesian Tsunami Early Warning System (GITEWS) project. Indonesian experts of BMG Jakarta and other Indonesian institutions will enlarge the Task Force group and join the German scientists in the next days. Most of the financial needs for this mission are covered by GFZ; major support in the private sector is coming from the German insurance company Hannover Re (see also press release from 31st of May 2006).

Task Force members on duty in Indonesia:

Birger Lühr (Head)	Seismologist, Merapi expert	GFZ
Dr. Thomas Walter	Volcano seismologist	GFZ
Erwin Guenther	Task Force engineer	GFZ
Dr. Joachim Wassermann	Seismologist, Merapi expert	LMU

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