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1969 Sulawesi earthquake
UTC time	1969-02-23 00:36:58
ISC event	812497
USGS-ANSS	ComCat
Local date	February 23, 1969
Local time	08:56
Magnitude	7.0 Mw
Depth	15.0 km (9 mi)
Epicenter	3°12′04″S 118°54′14″E / 3.201°S 118.904°E
Fault	Majene Thrust
Type	Thrust
Max. intensity	MMI VIII (Severe)
Tsunami	yes
Casualties	664

The western coast of West Sulawesi was struck by a major earthquake on 23 February 1969 at 00:36 UTC. It had a magnitude of 7.0 M_w and a maximum felt intensity of VIII on the Modified Mercalli intensity scale.^[2]^[1] It triggered a major tsunami that caused significant damage along the coast of the Makassar Strait. At least 64 people were killed, with possibly a further 600 deaths caused by the tsunami.^[1]

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 Palu-Koro strike-slip fault that forms the boundary between the North Sula and Makassar blocks and was responsible for the destructive Palu earthquake in 2018.^[3]^[4] According to the interpretation of GPS data, the Makassar block is currently rotating anticlockwise, with its northwestern margin showing convergence with the Sunda block across the Makassar Strait. The main structure in that part of Sulawesi is the offshore, north–south trending, moderately east-dipping Makassar Thrust, also known as the Majene Thrust.^[5] The GPS data also support the presence of a seismically "locked" fault in the Makassar Strait.^[3] Seismic reflection data from the Makassar Strait support the presence of active thrusting west of the Makassar block. The Majene/Kalosi fold and thrust belt is exposed onshore between Majene and Mamuju. The northern part of the Makassar Strait is interpreted as a foreland basin, with its subsidence caused by the loading of this active thrust belt.^[6]

Earthquake

The earthquake had an estimated magnitude of 7.4 M_s,^[1] later recalculated to be 7.0 M_w .^[2] It was caused by movement on the Majene Thrust, which was also responsible for the 2021 West Sulawesi earthquake 52 years later.^[7] Modelling of the earthquake by Pranantyo and others suggested rupture occurred across a rectangular fault plane measuring 40 km by 20 km. A maximum coseismic slip of 1.5 meters was estimated. The maximum Modified Mercalli intensity was VIII (Severe), causing serious damage at the coast.^[8]

Tsunami

The earthquake triggered a significant tsunami, with a maximum run-up of 4 meters at Peletoang and 1.5 meters at both Parosanga and Palipi.^[9] The same modelling by Pranantyo and others could not replicate the 4-meter tsunami run-up along Peletoang. Simulations of various scenarios for the earthquake and tsunami only generated a maximum tsunami height of 2.25 meters. It is possible that a submarine landslide triggered the large tsunami.^[8]

Damage

The city of Majene was particularly badly affected, with the foundations of four out of five of tile brick buildings suffering serious effects. Wooden buildings escaped with only minor effects, but unreinforced masonry walls were badly damaged. A number of bridges were damaged beyond repair.^[1]

The tsunami caused local flooding, damaging banana plantations along the coast. Many wooden buildings were washed away by the waves. At Majene's harbour, the pier was seriously affected due to the effects of subsidence. Witnesses mention a "roaring sound" just before the tsunami.^[9]

At least 64 deaths were reported, with one news story also mentioning another 600 deaths due to the tsunami. A further 97 injuries were also reported.^[1]

References

^ ^a ^b ^c ^d ^e ^f ^g National Geophysical Data Center / World Data Service (NGDC/WDS): NCEI/WDS Global Significant Earthquake Database. NOAA National Centers for Environmental Information. (1972). "Significant Earthquake Information". NOAA National Centers for Environmental Information. doi:10.7289/V5TD9V7K. Retrieved 16 January 2020.
^ ^a ^b ANSS. "Sulawesi 1969 : M 7.0 – Sulawesi, Indonesia". Comprehensive Catalog. U.S. Geological Survey. Retrieved 16 January 2020.
^ ^a ^b Socquet A.; Simons W.; Vigny C.; McCaffrey R.; Subarya C.; Sarsito D.; Ambrosius B.; Spakman W. (2006). "Microblock rotations and fault coupling in SE Asia triple junction (Sulawesi, Indonesia) from GPS and earthquake slip vector data". Journal of Geophysical Research: Solid Earth. 111 (B8): B08409. Bibcode:2006JGRB..111.8409S. doi:10.1029/2005JB003963.
^ ANSS. "Sulawesi 2018: M 7.5 – 70 km N of Palu, Indonesia". Comprehensive Catalog. U.S. Geological Survey. Retrieved 28 September 2018.
^ Bellier, O.; Sébrier, M.; Seward, D.; Beaudoin, T.; Villeneuve, M.; Putranto, E. (2006). "Fission track and fault kinematics analyses for new insight into the Late Cenozoic tectonic regime changes in West-Central Sulawesi (Indonesia)". Tectonophysics. 413 (3–4): 201–220. Bibcode:2006Tectp.413..201B. doi:10.1016/j.tecto.2005.10.036.
^ Bergman, S.C.; Coffield, D.Q.; Talbot, J.P.; Garrard, R.A. (1996). "Tertiary Tectonic and magmatic evolution of western Sulawesi and the Makassar Strait, Indonesia: evidence for a Miocene continent-continent collision". In Blundell, D.J.; Hall, R. (eds.). Tectonic Evolution of Southeast Asia. Geological Society, Special Publications. Vol. 106. pp. 391–429. doi:10.1144/GSL.SP.1996.106.01.25. ISBN 9781897799529. S2CID 140682113.
^ "BMKG : Gempa Majene Merusak" (in Indonesian). Media Indonesia. Retrieved 15 January 2021.
^ ^a ^b I.R. Pranantyo; A. Cipta; H.A. Shiddiqi; M. Heidarzadeh (2021). "Source reconstruction of the 1969 Majene, Sulawesi earthquake and tsunami: A preliminary study". IOP Conference Series: Earth and Environmental Science. 873 (1): 012054. Bibcode:2021E&ES..873a2054P. doi:10.1088/1755-1315/873/1/012054. S2CID 240430042.
^ ^a ^b National Geophysical Data Center / World Data Service (NGDC/WDS): NCEI/WDS Global Significant Earthquake Database. NOAA National Centers for Environmental Information. (1972). "Tsunami Event Information". NOAA National Centers for Environmental Information. doi:10.7289/V5TD9V7K. Retrieved 16 January 2020.