1992 Flores earthquake and tsunami

{{Infobox earthquake

| title = 1992 Flores earthquake and tsunami

| timestamp = 1992-12-12 05:29:26

| anss-url = usp0005j5a

| isc-event = 257091

| local-date = {{start date and age|1992|12|12|df=yes}}

| local-time = 13:29 WITA (Indonesia Central Standard Time)

| map2 = {{Location map | Indonesia Flores#Indonesia

| relief = 1

| label =

| lat = -8.48

| long = 121.896

| mark = Bullseye1.png

| marksize = 40

| position = top

| width = 250

| float = right

| caption = }}

| magnitude = 7.8 {{M|w|link=y}}

| depth = {{convert|27.7|km|abbr=on}}

| location = {{coord|8.480|S|121.896|E|display=inline,title}}

| tsunami = Yes (26m)

| intensity = {{MMI|VIII}}

| countries affected = Flores
Indonesia

| casualties = 2,080 - 2,500 dead
500 - 2,144 injured

}}

The 1992 Flores earthquake and tsunami occurred on 12 December on the island of Flores in Indonesia. With a magnitude of 7.8 and a maximum Mercalli intensity of VIII (Severe), it was the largest and also the deadliest earthquake in 1992 and in the Lesser Sunda Islands region, with at least 2,500 people were killed. The earthquake generated an unusually large tsunami for its size; a submarine landslide is suspected.

The earthquake was caused by slip on the Flores Thrust fault. This fault dips to the south underneath Flores Island, and is part of the back-arc thrust of Indonesia. The epicenters of most earthquakes on the Flores Thrust are on Flores Island. The area is no stranger to earthquakes, and multiple have impacted the area since 1992, such as the 2018 Lombok earthquakes, which were produced by the Flores thrust, and the 2021 Flores earthquake. The Flores thrust itself has produced at least six tsunamigenic earthquakes since 1800.

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 {{cvt|1,500|km}}. Its eastern segment was responsible for producing the earthquake.{{cite journal|last1=Felix|first1=Raquel P.|last2=Hubbard|first2=Judith A.|last3=Bradley|first3=Kyle E.|last4=Lythgoe|first4=Karen H.|last5=Li|first5=Linlin|last6=Switzer|first6=Adam D.|title=Tsunami hazard in Lombok and Bali, Indonesia, due to the Flores back-arc thrust|journal=Nat. Hazards Earth Syst. Sci.|date=2022|volume=22|issue=5|pages=1665–1682|doi=10.5194/nhess-22-1665-2022|doi-access=free|bibcode=2022NHESS..22.1665F}} The earthquake occurred at 13:29:26 WITA and was followed by several serious aftershocks.[http://www.reliefweb.int/rw/RWB.NSF/db900SID/OCHA-64BHB5?OpenDocument Reports 1–9] United Nations Department of Humanitarian Affairs In total, 102{{Cite journal |last1=Jufriansah |first1=Adi |last2=Khusnani |first2=Azmi |last3=Pramudya |first3=Yudhiakto |last4=Afriyanto |first4=Mulya |date=March 27, 2023 |title=Comparison of aftershock behavior of the flores sea 12 december 1992 and 14 december 2021 |url=https://www.researchgate.net/publication/377456099 |journal=Journal of Physics: Theories and Applications |volume=7 |issue=1 |pages=65–74|doi=10.20961/jphystheor-appl.v7i1.71609 }} - 148{{Cite web |date=March 24, 1993 |title=Indonesia Flores Earthquake Reconstruction Project |url=https://documents1.worldbank.org/curated/en/263861468039852272/pdf/multi-page.pdf |access-date=April 1, 2025 |website=World Bank |page=7}} aftershocks with magnitudes above {{Earthquake magnitude|w}}3.0 were recorded. It has been estimated that a 140 km by 40 km area with two major slip regions was ruptured in the quake.{{Cite web |last1=Pranantyo |first1=Ignatius |last2=Cummins |first2=Phil |date=July 2019 |title=Multi-Data-Type Source Estimation for the 1992 Flores Earthquake and Tsunami |url=https://www.researchgate.net/publication/330117953 |access-date=April 1, 2025 |website=ResearchGate|page=11}}

= Tectonic setting =

File:Southeastern Indonesia Tectonic Map.png 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.{{cite news |date=18 March 2011 |title=RI sits on Pacific 'ring of fire' : official |url=https://en.antaranews.com/news/69302/ri-sits-on-pacific-ring-of-fire--official |url-status=live |archive-url=https://web.archive.org/web/20180806145803/https://en.antaranews.com/news/69302/ri-sits-on-pacific-ring-of-fire--official |archive-date=6 August 2018 |access-date=6 August 2018 |work=ANTARA News}} The Flores area in particular contains both strike-slip faults and thrust faults.{{Cite book |last=Handayani |first=Lina |title=Proceedings of the Proceedings of the 7th Mathematics, Science, and Computer Science Education International Seminar, MSCEIS 2019, 12 October 2019, Bandung, West Java, Indonesia |chapter=Seismic Hazard Analysis of Maumere, Flores: a Review of the Earthquake Sources |date=2020 |chapter-url=https://eudl.eu/pdf/10.4108/eai.12-10-2019.2296247 |access-date=April 1, 2025 |page=1|doi=10.4108/eai.12-10-2019.2296247 |isbn=978-1-63190-256-7 }} The Flores backthrust is north of the island arc of Indonesia, and is formed by the subduction of the Indo-Australian plate underneath the Eurasian plate west of Lombok while being formed by the Australian plate and Banda Arc south of Rote Island.{{Cite journal |last1=Xiaodong |first1=Yang |last2=Singh |first2=Satish |last3=Tripathi |first3=Anand |date=January 6, 2020 |title=Did the Flores backarc thrust rupture offshore during the 2018 Lombok earthquake sequence in Indonesia? |url=https://academic.oup.com/gji/article/221/2/758/5757908 |journal=Geophysical Journal International |pages=758–759 |access-date=2024-02-12}} It is characterized by intermediate-depth down-dip subduction.{{Cite web |last=Eric S. Jones, Gavin P. Hayes, Melissa Bernardino, Fransiska K. Dannemann, Kevin P. Furlong, Harley M. Benz, and Antonio Villaseñor |title=USGS Open-File Report 2010–1083-N: Seismicity of the Earth 1900–2012 Java and Vicinity |url=https://pubs.usgs.gov/of/2010/1083/n/ |access-date=2025-03-30 |website=pubs.usgs.gov}} 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 Flores thrust. However, newer research suggests that the whole thrust is continuous, spanning a distance of about {{Convert|2000|km|mi}}. The Sunda and Australian plates converge at a rate of 80 mm/yr, in which the Sunda megathrust takes 70 mm/yr of the movement. The remaining 10 mm/yr rate is accommodated by the Flores back-arc thrust. It runs off the north coast of the Lesser Sunda Islands, in the Banda Sea.{{cite journal |last1=Sarjan |first1=Achmad |last2=Nugraha |first2=Andri |display-authors=1 |date=22 April 2021 |title=Delineation of Upper Crustal Structure Beneath the Island of Lombok, Indonesia, Using Ambient Seismic Noise Tomography |url=https://dr.ntu.edu.sg/bitstream/10356/151064/2/feart-09-560428.pdf |journal=Frontiers in Earth Science |volume=9 |page=2 |bibcode=2021FrEaS...9..269S |doi=10.3389/feart.2021.560428 |access-date=16 October 2021 |doi-access=free}}

The area is seismically active, with 79 earthquakes larger than {{Earthquake magnitude|w}}6.0 recorded in the area between 1900 to 2022. Eleven of those quakes generated tsunamis.{{Cite web |last1=Citrosiswoyo |first1=Wahyudi |last2=Mulyadi |first2=Yeyes |display-authors=1 |date=July 2024 |title=Numerical Modelling of the Potential Hazard Due to Future Flores Back Arc Thrust Earthquake Generated Tsunami |url=https://www.researchgate.net/publication/382708658 |access-date=April 6, 2025 |website=ResearchGate |page=2}} In addition, the area has generated at least six tsunamigenic quakes of {{Earthquake magnitude|s}}>=6.5 since 1800.{{Cite journal |last1=Felix |first1=Raquel P. |last2=Hubbard |first2=Judith A. |last3=Bradley |first3=Kyle E. |last4=Lythgoe |first4=Karen H. |last5=Li |first5=Linlin |last6=Switzer |first6=Adam D. |date=2022-05-18 |title=Tsunami hazard in Lombok and Bali, Indonesia, due to the Flores back-arc thrust |url=https://nhess.copernicus.org/articles/22/1665/2022/ |journal=Natural Hazards and Earth System Sciences |language=English |volume=22 |issue=5 |pages=1665–1682 |bibcode=2022NHESS..22.1665F |doi=10.5194/nhess-22-1665-2022 |issn=1561-8633 |access-date=March 31, 2025 |doi-access=free}}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-studies western part.{{Cite web |last1=Pranantyo |first1=Ignatius |last2=Heidarzadeh |first2=Mohammad |last3=Cummins |first3=Phil |date=May 2021 |title=Complex tsunami hazards in eastern Indonesia from seismic and non-seismic sources: Deterministic modelling based on historical and modern data |url=https://www.researchgate.net/publication/351374631 |access-date=March 31, 2025}} 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.{{Cite web |last1=Pranantyo |first1=Ignatius |last2=Cummins |first2=Phil |date=July 2019 |title=Multi-Data-Type Source Estimation for the 1992 Flores Earthquake and Tsunami |url=https://www.researchgate.net/publication/330117953 |access-date=April 1, 2025 |website=ResearchGate|page=3}}

Tsunami

File:1992_Flores_tsunami_damage.jpg

The tsunami was reported to have arrived within five minutes of the earthquake. Wave run-up heights of {{cvt|3–4|m}} occurred along the eastern coast of Maumere. The maximum run-up height was measured at Riangkroko village at {{cvt|26|m}}; the tsunami killed 137 villagers. Located along the Nipah River, the tsunami traveled {{cvt|600|m}} inland. It washed away the whole village and stripped away coconut trees, leaving only roots. In another village {{cvt|6|km}} north, Bou-Tanabeten, the tsunami run-up was {{cvt|12|m}}. Along other tsunami-hit locations, most trees remained standing. At Wuhring, a village {{cvt|5|km}} from Maumere, {{cvt|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, making nearly a quarter of the island's population, and two villages were badly damaged. The maximum run-up on the island was recorded at {{cvt| 7.3|m}}. Wave reflection effect off Flores Island may have partly contributed to the tsunami's destructive effects on Babi Island.{{cite journal |last1=Yeh |first1=Harry |last2=Imamura |first2=Fumihiko |last3=Synolakis |first3=Costas |last4=Tsuji |first4=Yoshinobu |last5=Liu |first5=Philip |last6=Shi |first6=Shaozhong |date=1993 |title=The Flores Island tsunamis |journal=Eos |volume=74 |issue=33 |pages=369–373 |bibcode=1993EOSTr..74..369Y |doi=10.1029/93EO00381}} The earthquake produced the deadliest and largest tsunami in the Flores Sea area in modern history.{{Cite web |last1=Pranantyo |first1=Ignatius |last2=Heidarzadeh |first2=Mohammad |last3=Cummins |first3=Phil |date=May 2021 |title=Complex tsunami hazards in eastern Indonesia from seismic and non-seismic sources: Deterministic modelling based on historical and modern data |url=https://www.researchgate.net/publication/351374631 |access-date=March 31, 2025 |pages=5}}

The tsunami has been described as having "enormous size", suggesting a potential submarine landslide.{{Cite web |title=12 December 1992, Mw 7.7, Flores, Indonesia - International Tsunami Information Center |url=https://legacy.itic.ioc-unesco.org/legacy.itic.ioc-unesco.org/indexecb6.html?option=com_content&view=article&id=1101&Itemid=3127 |access-date=2025-04-01 |website=legacy.itic.ioc-unesco.org}} 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.{{Cite web |last=Budioni |first=Kris |title=Submarine Lanslides On The Sea Bottom Of Maumere Bay, Flores, Based On The Interpretation Of Seismic Reflection Records |url=https://www.iagi.or.id/web/digital/17/SUBMARINE-LANDSLIDES-ON-THE-SEA-BOTTOM-OF-MAUMERE-BAY,-FLORES,-BASED-ON-THE-INTERPRETATION-OF-SEISMIC-REFLECTION-RECORDS.pdf |access-date=April 1, 2025 |website=IAGI |page=369}} Submarine landslide-induced tsunamis have occurred in the past in Indonesia, with one notable example being the 1674 Ambon earthquake and megatsunami.{{Cite journal |last1=Khusnani |first1=Azmi |last2=Jufriansah |first2=Adi |last3=Thalo |first3=Ota |date=September 30, 2023 |title=Tsunami event in Flores: literature review |url=https://jurnal.uns.ac.id/jphystheor-appl/article/download/74625/42251 |journal=Journal of Physics: Theories and Applications |volume=7 |issue=2 |pages=139–151|doi=10.20961/jphystheor-appl.v7i2.74625 }} A combined fault and landslide model explains the tsunami height well, although with minor underestimations.{{Cite web |last=Sinki |first=Kian |title=Numerical Simulations Of The 1992 Flores Tsunami Using Earthquake Fault And Landslide Models |url=https://www.grips.ac.jp/cms/wp-content/uploads/2021/10/Synopsis_MEE20716_Kian_rev.pdf |access-date=April 1, 2025 |website=GRIPS |pages=4–5}}

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.[https://earthquake.usgs.gov/eqcenter/eqarchives/significant/sig_1992.php Significant Earthquakes of the World in 1992] {{webarchive|url=https://web.archive.org/web/20090912001941/http://earthquake.usgs.gov/eqcenter/eqarchives/significant/sig_1992.php |date=2009-09-12 }} United States Geological Survey A total of over 30,000 homes were destroyed in the earthquake.{{Cite web |title=NCEI Hazard Earthquake Information |url=https://www.ngdc.noaa.gov/hazel/view/hazards/earthquake/event-more-info/5343 |access-date=April 1, 2025 |website=NCEI}} Nineteen 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. The Indonesian Air Force provided assistance, mostly transporting medicine and clothing. Malaria and influenza rose significantly after the disaster. Babi Island survivors were evacuated since all the houses on the island were demolished. International relief efforts were hindered by the onset of the rainy season. The Indonesian government also requested assistance from the international community. Agencies such as the Asian Development Bank, along with the World Bank, the Australian Agency for International Development, the Japanese Overseas Cooperation Fund, and others, made a coordinated effort to support reconstruction. Each agency concentrated on sectors and locations in which it had experience.Some details are provided in Asian Development Bank, [https://www.adb.org/sites/default/files/evaluation-document/35256/files/pe552.pdf Project Performance Audit Report on the Flores Emergency Reconstruction Project], October 2000.