Reconnaissance microearthquake survey of Sulawesi, Indonesia

Abstract

Several hundred earthquakes were recorded during the operation of a two‐week five‐station portable seismograph network on Sulawesi, Indonesia. Forty one of these events were locatable and half of these occurred beneath the eastern Gorontalo Basin in a north dipping zone which extends from the Batui Thrust on the East Arm of Sulawesi to about 100 km depth beneath the Gorontalo Basin. The Batui Thrust is the site of thrusting of the Banggai Islands continental fragment beneath the East Arm ophiolite and oceanic crust of the Gorontalo Basin. The observation of a zone of earthquakes dipping to the north from this thrust zone suggests that the leading edge of the Banggai Island block was subducted to at least 100 km depth. The eastern Gorontalo Basin earthquake zone may connect with a deep seismic zone beneath the Celebes Basin. Beneath the western Gorontalo Basin, a very narrow zone of earthquakes dips to the south, probably within lithosphere of the Celebes Basin subducted at the North Sulawesi Trench. Three shallow earthquakes occurred near Lake Matano in central Sulawesi, possibly on the Matano Fault, and their composite focal plane solution suggests east‐west extension. The occurrence of only one earthquake on the Palu Fault and the lack of short S‐P times at a station operated on the fault indicate that this feature was very quiet during the survey period.