1992 Flores earthquake and tsunami

Tectonic setting

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 =6.5 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 Flores thrust ruptured in the 1992 Flores earthquake, while the western segment has remained intact. However, the only instrumentally recorded quake caused by the Flores thrust was the 1992 quake.

Earthquake

The earthquake was the result of shallow thrust faulting along a fault located in the back-arc region between the Sunda and Banda island arcs. The Flores Thrust represents a back-arc thrust fault that accommodated tectonic deformation between the Australian and Sunda plates. This structure runs for over 1,500 km. Its eastern segment was responsible for producing the earthquake. The earthquake occurred at 05:29 UTC and was followed by several serious aftershocks. Extensive soil liquefaction was observed along the Flores Sea, including in the Maumere Port, where it caused "extensive damage". A peak ground acceleration of 1.0860 g and a peak ground velocity of 116.03 cm/s were observed. – 148 aftershocks with magnitudes above 3.0 were recorded. The largest recorded aftershock was rated 6.1. It has been estimated that a 140 x area with two major slip regions was ruptured in the quake.

Tsunami

The tsunami was reported to have arrived within five minutes of the earthquake. Wave run-up heights of 3–4 m occurred along the eastern coast of Maumere. The maximum run-up height was measured at Riangkroko village at 26 m; the tsunami killed 137 villagers. Located along the Nipah River, the tsunami traveled 600 m inland. It washed away the whole village and stripped away coconut trees, leaving only roots. In another village 6 km north, Bou-Tanabeten, the tsunami run-up was 12 m. Along other locations hit by tsunamis on Flores Island, most trees remained standing. At Wuhring, a village 5 km from Maumere, 3 m waves swept through the village and killed 100 people. On Babi Island, the tsunami left human remains hanging on trees; 263 deaths occurred on the island, representing nearly a quarter of the island's population, and two villages were badly damaged. The maximum run-up on the island was recorded at 7.3 m. Wave reflection off Flores Island may have partly contributed to the tsunami's destructive effects on Babi Island. Babi Island was likely inundated by two separate tsunami episodes. The earthquake produced the deadliest and largest tsunami in the Flores Sea area in modern history.

The tsunami has been described as having "enormous size", suggesting a potential submarine landslide. Major subaqueous slumping was observed near Leworaharag. In addition, there exists evidence of a submarine landslide or rockslide caused by the earthquake. Seismic stratigraphy was used to identify two areas of submarine rockfall likely caused by the earthquake. Submarine landslide-induced tsunamis have occurred in the past in Indonesia, with one notable example being the 1674 Ambon earthquake and megatsunami.

Aftermath and relief efforts

At least 2,500 people were killed or went missing near Flores, including 1,490 at Maumere and 700 on Babi. More than 500 people were injured and 90,000 were left homeless. 19 people were killed and 130 houses were destroyed on Kalaotoa. Damage was assessed at approximately US$100 million. Approximately 90% of the buildings were destroyed at Maumere, the hardest-hit town, by the earthquake and ensuing tsunami while 50% to 80% of the structures on Flores were damaged or destroyed. Electricity in the general area and the port of Maumere was shut down. Maumere's hospital was completely destroyed, and patients were treated in tents. Damage also occurred on Sumba and Alor.

The Indonesian government sent rescue and relief missions to earthquake-stricken areas and declared the earthquake a national disaster through Presidential Decree (Keppres) Number 66 of 1992, due to its widespread impact and the need for cross-ministerial response and international assistance. The Indonesian Air Force provided assistance, mostly transporting medicine and clothing.

References

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