On 28 September 2018, a shallow, large earthquake struck in the neck of the Minahasa Peninsula, Indonesia, with its epicentre located in the mountainous Donggala Regency, Central Sulawesi. The magnitude 7.5–7.6 quake was located 70 km (43 mi) away from the provincial capital Palu and was felt as far away as Samarinda on East Kalimantan and also in Tawau, Malaysia. This event was preceded by a sequence of foreshocks, the largest of which was a magnitude 6.1 tremor that occurred earlier that day.
Following the mainshock, a tsunami alert was issued for the nearby Makassar Strait. A localised tsunami struck Palu, sweeping shore-lying houses and buildings on its way. The combined effects of the earthquake and tsunami led to the deaths of an estimated 4,340 people. This makes it the deadliest earthquake to strike the country since the 2006 Yogyakarta earthquake, as well as the deadliest earthquake worldwide in 2018, surpassing the previous earthquake that struck Lombok nearly two months earlier, killing more than 500. The Indonesian Agency for Meteorology, Climatology and Geophysics (BMKG) confirmed that a tsunami had been triggered, with its height reaching an estimated maximum of 4 to 7 metres (13 to 23 ft), striking the settlements of Palu, Donggala and Mamuju along its path.
The earthquake caused major soil liquefaction in areas in and around Palu. In two locations this led to mudflows in which many buildings became submerged causing hundreds of deaths with many more missing. The liquefaction was considered to be the largest in the world and was deemed as rare.
Tectonic setting
Sulawesi lies within the complex zone of interaction between the Australian, Pacific, Philippine and Sunda plates in which many small microplates are developed. The main active structure onshore in the western part of Central Sulawesi is the left-lateral NNW-SSE trending Palu-Koro strike-slip fault that forms the boundary between the North Sula and Makassar blocks. The rate of slip along this fault is estimated to be in the range 30–40 mm per year. At Palu, there are several parallel fault strands defining the margins of a pull-apart basin, each of which take up some of the overall slip. Several major earthquakes have been attributed to movement on this fault since 1900 and three larger events in the last 2,000 years have been deduced from studies of the fault zone. In 2017, a study recognised this fault as representing the greatest seismic risk in eastern Indonesia.
Earthquake
Foreshocks
The mainshock was preceded by a 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 Mw. 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 struck 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 resumed.
The mainshock magnitude was initially reported as Mw 7.7 by both the United States Geological Survey (USGS) and BMKG. The USGS later revised their estimate to Mww 7.5. The International Seismological Centre (ISC) reported a magnitude of Mw 7.6. 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. The possibility of supershear rupture on the Palu-Koro Fault was raised in 2017.
Indonesian authorities confirmed that no one had been able to contact a single person from Donggala, the closest town to the epicentre. As of 1 October, contacts were still hampered by downed telecommunication lines.
Aftershocks
The mainshock was followed by a series of aftershocks, with 14 of M≥5.0 in the first 24 hours. Since then, a total of 150 aftershocks have struck the region.
Intensity
The earthquake was felt over a wide area. The strongest shaking (MMI X) was felt in some parts of Palu City. In other areas of the city, the Mercalli intensity was evaluated to be VII, based on damage in single and double-story buildings. Four-story structures like a hotel and shopping mall moderate to heavy damage and the intensity was assigned VIII. Major damage to the Palu IV Bridge was eventually given MMI IX.
Tsunami
A tsunami warning was issued in Palu and Donggala at 18:07 WITA. Warnings were sent via SMS by the Indonesian Ministry of Communication and Information. Residents of Donggala were told to expect tsunamis with heights of 0.5 to 3 metres, while residents in Palu were told to expect tsunamis with heights of less than 0.5 metres. The tsunami, however, struck higher than expected. Residents of Palu reported waves with heights of more than 2 metres while several others stated that the waves managed to reach the second floor.
Indonesian officials calculated the estimated time of arrival of the tsunami. Calculations suggested that the tsunami would arrive in Palu approximately 20 minutes after the earthquake. Around 18:27 WITA, the BMKG in Mamuju detected tsunamis. A tsunami warning was issued, and later lifted. The BMKG claimed the warning was lifted after the tsunami had struck, the timing is clear, but since there was a one-hour time difference between the sunset in Palu and Java where many Indonesian in social media reside, many thought the warning was lifted before the tsunami hit, because the video that circulating around was after the sunset, while the warning lifted around 18:27 WITA it was 17:27 WIB for most of these people reside where the sun is still high up, resulting in the agency being criticized on social media.
Even though officials did send warnings via telephone and television, tsunami alarms were not activated. The Palu Nomoni Festival in Talise Beach continued and most of the festival attendees did not realize that there was a possibility of a tsunami. Eyewitnesses even stated that some people were still strolling on the beach when the tsunami struck. It was estimated that hundreds were caught off guard and swept away by the waves.
The BMKG later admitted that the tsunami detectors in Palu were damaged in the earthquake. The shaking was violent enough to damage the tsunami detectors in Palu, as further examination revealed that none of the detectors were able to send signals to alert residents.
As Palu was located at the end of a narrow bay, the tsunami's force was intensified when the waters entered. Officials confirmed that the waves that struck Palu had actually reached a height of 5 metres, with some as nearly as high as 6 metres.
The size of the tsunami surprised geologists. It was expected to be at a low height, no more than about two metres, since the earthquake was a strike-slip earthquake, which usually do not have enough vertical movement to create large tsunamis. One explanation is that the earthquake triggered underwater landslides, causing the tsunami. A study of coastal liquefaction has suggested that 84% of the tsunami amplitude was a result of the combined effect of coastal and submarine landslides, based on an analysis of
tide gauge records at Pantoloan. A 2019 research suggests the earthquake created about 1.5 meters of vertical seafloor movement, which, combined with a very fast rupture speed, could have produced the tsunami, regardless of any landslides.
Damage
More than 70,000 houses are reported to be damaged, forcing tens of thousands of people to live in shelters and tents. In Palu, the capital of Central Sulawesi, a mosque and a major section of the Antapura Hospital collapsed. Palu's Tatura Mall, one of the oldest shopping centres in Palu, collapsed, trapping dozens of people. An eight-story hotel, identified as the Roa-Roa Hotel, had also collapsed, trapping numerous hotel guests, several of whom were participants of the Palu's annual paragliding festival. At the time of the earthquake, 76 of the 80 bedrooms of the hotel were occupied.
Palu's Mutiara SIS Al-Jufrie Airport was forced to close as large cracks, one of which was 500 meters long, had been formed on the runway. Airport officials confirmed that the navigation system was damaged and the control tower of the airport collapsed. An air traffic controller, who was watching the take-off roll of Batik Air Flight 6231 during the earthquake, was killed due to internal injuries. Major structural damage was also reported in the airport, with the roof the airport's terminal reportedly caved in. The airport was reopened for limited operation on 29 September 2018. Due to the damage in the airport, hundreds of passengers were stranded and were told to wait in the airport's apron. Normal operation returned on service on 30 September.
Authorities confirmed that numerous settlements and residential areas, including more than a thousand homes, had been destroyed by the earthquake and tsunami. The tsunami also caused major damage to ports in Palu. In Pantoloan Harbour, a Quay crane collapsed and was swept by the tsunami. In Wani Harbour, structural damage was reported, while Ogoamas Harbour moved 3 cm to its right due to the earthquake. The IAIN Datokarama Palu, a university in Palu, was severely damaged by the tsunami.
Officials confirmed that Palu's iconic bridge, the Kuning Ponulele Bridge, also called Palu IV Bridge, which was also the first arch bridge in Indonesia, was destroyed by the earthquake and the ensuing tsunami. Roads to and from the city, connecting it to Makassar and Poso, were also severely damaged. There were also reports of landslides.
Of the 24 mosques in Palu, 20 were severely damaged. Only a few survived, including the iconic Floating Mosque of Palu which was partially submerged underwater. The worst-hit mosque was the Baiturrahman Mosque where 300 worshippers were killed during the evening prayers.
Communication to the affected area was severely hampered, with President Joko Widodo being initially unable to contact governor Longki Djanggola.
Ministry of Communication and Information Technology announced that 1,678 base transceiver station in Central Sulawesi were damaged by the earthquake and disaster. Local hospitals were damaged, with the Director of the Palu Undata Hospital opting to treat victims outside of the hospital and made a public plea for tents, medicine, canvas, and nurses. A warden for a prison in Palu reported that more than half of its 560 inmates fled in panic as the prison walls collapsed.
An assessment made by the Ministry of Education and Culture showed that nearly 3,000 schools had been damaged by the earthquake and tsunami and more than 100,000 students and 20,000 teachers were affected. Minister of Education and Culture Muhadjir Effendi stated that a period of minimum 1 year would be needed to rebuild and repair the damaged schools in the affected areas.
In the immediate aftermath of the earthquake, prisoners in the Donggala Penitentiary in Central Sulawesi rioted and set the prison on fire as they demanded to be met by their families. At least 100 prisoners managed to escape. The prison reportedly suffered severe damage.
Minister of Agriculture Amran Sulaiman stated that the earthquake, tsunami and subsequent soil liquefaction damaged 9,718 hectares of crops throughout Palu, Sigi and Donggala, costing 36 billion rupiah in damage.
Overall damage from the earthquake, tsunami and liquefaction was Rp24.6 trilion (US$1.71 billion), while damage in Palu alone reached Rp23.9 trillion (US$1.66 billion).
Mudflows
Parts of Central Sulawesi had been hit by major mudflows following the earthquake as a result of liquefaction. The most affected areas were the Petobo sub-district in southern Palu and the village of Balaroa, just outside the city, both locations some distance from the coast.