28 September 2018, Mw 7.5, Palu, Indonesia Tsunami

The 28 September 2018 magnitude 7.5 Palu, Indonesia earthquake (0.178°S, 119.840°E, depth 13 km) occurred at 1002 UTC. The major earthquake triggered catastrophic liquefaction, landslides, and a near-field tsunami that resulted in direct damage, impact, economic loss, and loss of life.

As of 22 October 2018 according to the One ASEAN One Response (AHA) Centre (Sit Update 14), the earthquake and subsequent tsunami that had hit Palu and Donggala in Central Sulawesi Indonesia, Friday 28 September 2018, has killed 2077 people. 680 people are missing, 4,438 with major injuries, 205,870 displaced, and 688,451 houses damage. The three most affected areas were Donggala, Palu City, and Sigi.

A recently retrieved marigram from the Pantoloan - Sulteng tide gauge shows a 3.8 m trough-to-peak tsunami that arrived six (6) minutes after the earthquake’s origin. Several analyses based on pictures, post disaster information, and video clips suggest the first tsunami wave hit the Palu beach area 7-12 minutes after the earthquake. Preliminary field surveys conducted by the BMKG and led by Tohoku Univ/Indonesia Ministry of Environment and Forestry/Chuo Univ report an eyewitness height of up to 11.3 m in Palu and 1.57-10 m on the west and east sides of the bay.

Preliminary US Geological Survey (USGS) analysis showed shallow left-lateral, strike-slip faulting, presumably along the Palu-Koro Fault that trends generally north-south from the North Sulawesi subduction zone in the Sulawesi Sea into the Makassar Strait just west of Sulawesi and into Palu Bay.  Extensive liquefaction occurred in Balarao, Petobo, and Sidera in Palu City.  An updated USGS Finite Fault solution, consistent with the geodetic data showing rupture far to the south on land in Palu, shows the greatest slip (7-8 m) concentrated south of the epicenter, and a rupture time of about 25-30 sec.  Normally, such this type of faulting does not generate a significant tsunami in the regional and far field.  IRISDES (Tohoku University) preliminary modeling using various source configurations, including uniform slip and finite fault solutions, predict local inundation heights up to 3.9 m.

Five minutes after the earthquake, the Indonesia Agency for Meteorology, Climatology and Geophysics (Badan Meteorologi Klimatologi dan Geofisika - BMKG) issued a tsunami warning for a local tsunami.  Recently retrieved marigram from the Pantoloan - Sulteng tide gauge shows a 3.8 m trough-to-peak tsunami that arrived six (6) minutes after the earthquake’s origin. There were no other nearby instrumental observations.  Several analyses based on pictures, post disaster information, and video clips suggest the first tsunami wave hit the Palu beach area 7-12 minutes after the earthquake. Preliminary field surveys conducted by the BMKG and IRIDeS/Indonesia Ministry of Environment and Forestry/Chuo Univ report an eyewitness height of up to 11.3 m in Palu and 3-10 m on the west and east sides of the bay. 

The last significant tsunami that occurred in this region was a M7.6 earthquake on 14 August 1968 in the region of Manimbaja Baynorth of the 28 September 2018 epicenter. According to the Indonesia Hydrographic Service, the tsunami measured 9-10 m and inundated 500 m inland in the Donggalaregion; 160 people died and 40 remain missing; 800 coastal homes were destroyed and coconut plantations were flooded. Tambu and Mapaga villages were most affected. This earthquake was determined to have a normal faulting mechanism (strike 119 deg, dip 71 deg., Fitch, JGR, 1972).

The largest historical earthquake in this region was a M7.9 on 1 January 1996, about 100 km north of the 2018 earthquake.  This shallow thrust fault event also generated a tsunami that killed 9 persons in Tonggolobibi village, Donggala regency.  The ICSU/WDS (NOAA NCEI) Historical Tsunami Database lists a total of seven  historical tsunamis in the Sulawesi (Makassar Straight) region.

The government of Indonesia (GoI), other governments, and the wider scientific community are asking many questions about what, why, how and what can be done to reduce future losses for similar events.  National and International Tsunami Survey Teams (NTST and ITST-Palu) are now undertaking science surveys to collect perishable data to address these and other tsunami recovery questions.

Over the next month, between October 18 and November 30, 2018, national and international post-tsunami surveys (ITST-Palu) are planned to collect data that can answer questions on how and why the deadly earthquake and tsunami occurred, what its impact was, and how we can better reduce future losses for similar events. For more information, visit the ITST-Palu survey pages.