sequence of foreshocks starting about three hours earlier with a M6.1 event, followed by several others, all located in the area immediately south of the mainshock epicentre. The first foreshock occurred at 15:00 WITA (Indonesia Central Standard Time). The earthquake struck at a shallow depth of 10 km with a magnitude of 5.9 (BMKG). The USGS registered the size of the quake as 6.1 . Strong shaking was felt in Donggala. At least 1 person was killed by fallen debris while 10 others were injured. Authorities confirmed that dozens of structures were either damaged or destroyed in the quake. Mainshock The mainshock occurred at 18:02 WITA, during rush hour traffic. The shaking was felt as far away as Samarinda, the capital of East Kalimantan, to the west and Makassar in the south. It was also felt in several parts in Gorontalo, located on the north and was also felt in Tawau, Malaysia. Violent shaking was reported in Donggala and Palu. Blackouts occurred throughout the area and telecommunications went down. The state-owned Telkomsel reported that more than 500 wireless communication towers had been damaged by the earthquake. The earthquake happened during the 40th anniversary of the city of Palu. To commemorate the anniversary, the Government of Palu on each year holds a festival, the Palu Nomoni Festival, with its starting point from Wina Hotel to Talise Beach, stretching almost 3.8 kilometres on the coast of Palu. Hundreds of people had gathered on the beach. The festival was expected to attract international paragliders as an international paragliding event was also being held as a part of the festival. Hotel owners in Palu reported that the number of guests had risen significantly due to the festival. In the immediate aftermath of the earthquake, the festival was resumed. The mainshock magnitude was initially reported as M7.7 by both the USGS and BMKG. The USGS later revised their estimate to M7.5. The focal mechanism of the earthquake showed that it was caused by strike-slip faulting on faults trending either roughly north–south or west–east. Further analysis of seismic waveforms suggests that a fault trending north–south gives a better fit. The earthquake was caused by movement on the Palu-Koro fault. Geodetic evidence suggests that about 150 km of the fault ruptured. During the mainshock, the rupture propagated at speeds in excess of the S-wave velocity as determined by back-projection of teleseismic data, and supported by satellite geodesy.