On 15 August 2026, at 05:58 WITA (UTC+8), a earthquake struck off the coast of the island of Flores in East Nusa Tenggara, Indonesia. The earthquake was caused by movement on the Flores back-arc thrust fault located north of the island. Seismic observatories recorded a maximum Modified Mercalli intensity of VIII (Severe) on Flores. It triggered a tsunami with a maximum recorded height of . At least 136 people were killed in the earthquake while another 1,776 people were injured. Approximately 98,900 homes were damaged, 27,400 of which collapsed or were badly damaged.
Tectonic setting
Indonesia lies on the Pacific Ring of Fire, an area of significant volcanic and tectonic activity. The archipelago is located between the Eurasian, Pacific and Australian tectonic plates. The Flores area in particular contains both strike-slip faults and thrust faults. The Flores backthrust (FBAT), located north of the island arc (Bali, Lombok, Sumbawa, Flores Sumba, Rote and Timor), is entirely within the backarc region. It formed from subduction of the Indo-Australian plate underneath the Eurasian plate while west of Lombok, involves continental collision of the Australian plate and Banda Arc, south of Rote Island. It is characterized by intermediate-depth down-dip subduction. It is a part of the larger backthrust of the back-arc region of Indonesia. Earlier marine geophysical studies have suggested that the back-arc backthrust consists of two regions: the Wetar thrust and the FBAT. However, newer research suggests that the whole thrust is continuous, spanning a distance of about . The Sunda and Australian plates converge at a rate of per year, in which the Sunda megathrust takes per year of the movement. The remaining per year rate is accommodated by the FBAT. It runs off the north coast of the Lesser Sunda Islands, in the Banda Sea.
The area is seismically active, with 79 earthquakes larger than 6.0 recorded in the area between 1900 and 2022. Eleven of those quakes generated tsunamis. In addition, the area has generated at least six tsunamigenic quakes of ≥ since 1800. There exists substantial tsunami risk associated with the back-arc thrust of Indonesia. The lesser-studied eastern part of Indonesia has been hit by more tsunamis than the more-studied western part. Despite back-arc thrust tsunamis not being as common as megathrust tsunamis, they can still inflict severe damage. The eastern segment of the FBAT ruptured in the 1992 Flores earthquake, while the western segment has remained unruptured. Corresponding segments of the FBAT in Bali and Lombok were responsible for very destructive earthquakes in 1815, 1857, 1917, 1979 and 2018. The segment east of Flores was responsible for the 2004 Alor earthquake.
Earthquake
The earthquake occurred at 05:58 WITA. The USGS measured it at at a depth of with an epicenter off the northern coast of East Nusa Tenggara, north-northwest of Ende. It had a focal mechanism corresponding to shallow thrust faulting on an east-striking fault dipping at a shallow angle to the south. The Global Centroid Moment Tensor (GCMT) assigned the earthquake a moment magnitude of . Germany's GFZ Helmholtz Centre for Geosciences reported the event at and its depth at . The United States Geological Survey (USGS) estimated that the earthquake had a rupture size of . Wijayanto, the director of the Meteorology, Climatology, and Geophysical Agency (BMKG), said the earthquake was caused by movement of the FBAT. The segment that ruptured had been previously evaluated by geologists to generate a maximum possible magnitude of 7.8 to 7.9.
Aftershocks As 5 September, the BMKG recorded 13,000 aftershocks with magnitudes ranging from 1.0 to 6.2 with depths no more than , of which 177 were felt. At least 134 aftershocks exceeding were recorded by the USGS, the largest measuring . The BMKG said a high rate of aftershocks is expected to continue for the next two or three weeks after the mainshock before it gradually decays. Faisal Fathani, the head of the BMKG, said it is typical for damaging earthquakes around the North Maluku region to be followed by many aftershocks.
The BMKG inferred four clusters of concentrated activity. Cluster 1 is located off the north coast of Nagekeo. Because the focal mechanisms of the mainshock and 17 August aftershock featured thrust faulting on an east–west trending fault, this cluster is likely on the FBAT. The other clusters are located west and north of Manggarai. Cluster 2 lies further offshore, north of the surface expression of the FBAT. One of the larger aftershocks in this cluster, measuring , was caused by normal faulting on a northwest–southeast trending fault. Cluster 3 is near Reo and displayed high seismicity from 19 to 21 August. These aftershocks are associated with north–south trending normal faults. Cluster 4 is located at the western edge of the rupture.
Fault model
The USGS published a preliminary finite-fault rupture model indicating that the primary slip patch measured approximately 150 by 40 km (93 by 25 mi), with the total rupture lasting about 51 seconds. The fault was oriented east-northeast and dipped southward at an angle of 32°. Although the rupture propagated bilaterally, most of the displacement occurred near the hypocenter, particularly down-dip and to its west. The shallower (>) parts of the fault up-dip and northwest of the hypocenter experienced the greatest slip of . The primary slip zone propagated to the seafloor while extending to depths of within the crust. There is also an isolated slip patch at depth east-southeast of the hypocenter where the slip peaked at .
The BMKG also published its own finite fault model featuring an east–west oriented plane dipping to the south at 30°. In their model, the hypocenter depth was placed at and the rupture propagated over westward. The earthquake ruptured three asperities; the first was in the hypocenter area while the two adjacent ones were located up-dip and to the west. The deeper portions of the fault to the south did not exhibit significant slip. A maximum slip of was projected at shallow depths of the fault. The rupture sequence took place with a velocity of per second and was complete in about 75 seconds with three or four episodes of large energy release.
The 1992 and 2026 earthquakes although occurring on adjacent segments, initiated from a central location before propagating east or west. BMKG also made similar observations during their analysis of the 2018 Lombok earthquake sequence where all these events initiated from a common rupture point before propagating away. They inferred that major earthquakes can first rupture a common highly-stressed region.
Ground motion Seismic observations at BMKG reported a maximum Modified Mercalli intensity of VIII (Severe) in Manggarai Regency and VII (Very strong) in West Manggarai Regency and Bima Regency. In the epicenter areas of Ngada, Nagekeo and Sikka, and further west in Sumbawa, the intensity was VI (Strong). Intensity V (Moderate) was recorded in East Sumba, East Flores and III–IV (Weak–Light) in some regencies of southern Sulawesi. A maximum peak ground acceleration of 0.635 g was recorded at Kuwus district in West Manggarai, while a peak ground velocity of per second was recorded by a seismic station at Ruteng.
Tsunami The BMKG issued a tsunami warning for South Sulawesi, Southeast Sulawesi, East Nusa Tenggara, and West Nusa Tenggara two minutes and sixteen seconds after the earthquake. Evacuation orders were issued because of the warning. The BMKG later declared these warnings over at 07:30 WIB. A tsunami with a maximum height of was reported in Reok, Manggarai Regency. A tsunami was recorded at Maurole District in Ende Regency while at Labuan Bajo in East Nusa Tenggara, a tsunami was recorded. A tsunami occurred in Dompu.
Authorities said the tsunami struck ten locations across East Nusa Tenggara and South Sulawesi provinces. On Buton, South Sulawesi, the tsunami flooded a beach and village in Siontapina Sub-District. In the Selayar Islands, some coastal villages were flooded by the tsunami. At Anakoli village in Nagekeo, the earthquake and tsunami struck during a school scouting activity at a beach attended by more than 350 people. Fears of a tsunami caused a stampede as attendants rushed for higher ground and several were trampled or injured. The tsunami also swept several people away but there were no deaths.
In explaining the drastic tsunami effects between the 2026 and nearby 1992 earthquakes, New Zealand-based Indonesian geophysics Aditya Gusman said standard tsunami modelling could not replicate the unusually large tsunami run-up during the earlier event. In an earlier research by Pranantyo and Cummins, the 1992 tsunami which reached up to run-up was caused by concentrated slip in a small area of the fault, the wrap-around effect on Babi Island, and a submarine landslide at Hading Bay. Gusman said because most of the 2026 rupture occurred beneath Flores rather than at sea, the tsunami was smaller. Gusman further emphasized the need for tsunami preparedness because Flores and the surrounding region have experienced historic tsunamis before. In 1928, an eruption on Paluweh triggered a tsunami that killed about 260 people. Another earthquake between Flores and Sumbawa in 1820 also triggered a tsunami reaching high at South Sulawesi.
Impact
According to the National Agency for Disaster Countermeasure (BNPB), at least 136 people were killed and 1,776 others were injured, 336 of them seriously. It is the deadliest earthquake in Indonesia since the 2022 West Java earthquake. East Nusa Tenggara governor Emanuel Melkiades Laka Lena said most of the victims died while they were asleep as rubble fell onto them. Manggarai and East Manggarai regencies recorded 41 and 36 deaths, respectively; 13 deaths were reported in Nagekeo, and the remaining 15 deaths were from Sikka, Ende, Ngada and West Manggarai.
A total of 95,567 homes were damaged, of which 27,414 were heavily damaged while the remaining had moderate or light damage. At least 1,918 educational facilities, 495 water infrastructure units, 371 sanitation facilities, and 257 offices were also affected. Twenty of 52 petrol stations in Flores were closed due to earthquake damage. Seventeen road sections and 14 bridges were affected. Around 566,797 people lost electricity. Around 812 cellular sites were also disabled. One death was that of a drowned fisherman whose body was found in the sea off Lamba Leda in East Manggarai. On 21 August, during relief work, a volunteer from Jakarta died from suspected exhaustion. Two children also died while at an evacuation center, which an Indonesian Child Protection Commission official attributed to an illness which worsened due to the conditions at the camp.
Initial power outages made sharing information about the earthquake damage difficult. At least eight people died in Reok District of Manggarai. Two people were killed and five others were injured in Maumere. A maritime passenger terminal in the city collapsed, causing hundreds of passengers inside to be struck by falling debris. Also at the terminal, some people fell into the sea after a gangway to an inter-island ferry collapsed as they were boarding. Among the injured were two Chinese tourists at a hotel in Labuan Bajo who were fleeing in panic. On Padar Island, 986 tourists were stranded after maritime port services were suspended. A landslide occurred at the crater wall of Kelimutu National Park.