Southern Sumatra, Indonesia, September 30, 2009, 10:16 (UTC) (Mw 7.5)






1. Introduction

On September 30 2009 at 5:16 (local time), an earthquake with magnitude (Mw) 7.5 occurred in southern Sumatra, Indonesia. Our waveform analysis shows that this earthquake ruptured a reverse fault at a depth of 70-80 km. This earthquake caused severe damages in Sumatra.


2. Location of this earthquake

The Indo-Australia plate is subducting northward beneath the Sumatra island, which is located on the Eurasia plate (Figs. 2-1, 2-2). Since the focal mechanism of this earthquake shows that the compression axis is oriented in NW-SE direction, this event occurred in the subducting plate, not on the plate boundary.

Fig. 2-1. Epicenter distribution obtained from USGS PDE (circles, between 1973 and 2009). The source centroid location obtained by this study is shown by the focal mechanism symbol.

Fig. 2-2. Vertical cross-section of the hypocenter distribution (USGS PDE) along the profile A-B shown in Fig. 2-1. The star indicates the source centroid location obtained by this study.


3. Rupture duration

Our inversion analysis shows that the fault rupture during this earthquake took about 12 s (Fig. 3). This value is smaller than the typical duration for earthquakes of this magnitude (about 28 s for Mw=7.5 events, after Ekstrom and Engdahl, 1989, JGR).

Fig. 3. The estimated moment function (blue line).


Last update: October 1, 2009

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