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The 1996 Earthquakes in Sulawesi, Indonesia

Unidad de Investigación en Ciencias de la Tierra
Campus Juriquilla-UNAM
Querétaro 76230, Qro. México
Apdo. Postal 1-742, C.P. 76001
gomez{at}unicit.unam.mx

École Normale Supérieure, Lab. de Géologie
U.R.A. 1316, 24, Rue Lhomond,
75231 Paris Cedex 05, France

École des Mines de Paris, Centre de Recherche en Geophysique
35, rue Saint Honoré
F-77305 Fontainebleau Cedex, France

We study the rupture process of the 1 January (M_w = 7.9), 16 July (M_w = 6.6), and 22 (M_w = 7) July 1996 earthquakes in Sulawesi, Indonesia. A teleseismic body waveform inversion of very broadband records shows that these events occurred beneath the accretionary prism. The main shock was due to a well determined fault with a strike of 53°N and a very shallow dip of 7°. From its source-time function duration (30 sec) and the aftershock distribution we estimate a rupture area of 90 x 60 km² and an average slip of 1.80 m. The surface displacement computed with our best model fits well the displacement vector at the only available GPS station at Tomini. The tsunami generated by the mainshock had an approximate source radius of 45 km which roughly agrees with the rupture size estimated above. The earthquakes took place in a relay zone between the trench and the Palu-Koro transcurrent fault. They ruptured a shallow dipping thrust fault which corresponds to the subduction interface under the North Sulawesi arm. The slip vectors of all the events have a NNW orientation parallel to the direction of convergence between the North Sulawesi arm and the Celebes Sea. Among the peculiarities of the rupture process, we found that the July 22 aftershock was the only event to have a well defined precursor 1.8 sec before the main P-wave onset.