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1985 Algarrobo earthquake

Coordinates: 33°15′25″S 71°51′29″W / 33.257°S 71.858°W / -33.257; -71.858
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1985 Algarrobo earthquake
1985 Algarrobo earthquake is located in South America
1985 Algarrobo earthquake
1985 Algarrobo earthquake is located in Chile
1985 Algarrobo earthquake
UTC time1985-03-03 22:47
ISC event529084
USGS-ANSSComCat
Local date3 March 1985 (1985-03-03)
Local time19:47 UTC−03:00
Duration65–70 seconds[1][2]
Magnitude8.0 Mw[3]
Depth35.0 km (21.7 mi)[3]
Epicenter33°15′25″S 71°51′29″W / 33.257°S 71.858°W / -33.257; -71.858[3]
FaultAtacama Trench
TypeMegathrust
Areas affectedChile
Total damage1.5 billion US dollars[4]
Max. intensityMMI VIII (Severe)
Peak acceleration.67 g[5]
TsunamiYes[4]
LandslidesYes
Casualties177–200 dead[4]
2,483–2,575 injured[4]

A megathrust earthquake measuring 8.0 Mw  struck just offshore the Greater Valparaíso area on 3 March 1985. The event followed a ten-day period of moderate foreshocks and left at least 177 people and injured about 2,500 others. It was felt with a maximum intensity of VIII (Severe) on the Mercalli intensity scale.

The complexity of the earthquake's mechanism and occurrence evolved over time, with initial reports describing an earthquake doublet, while later reports proposed an alternative approach, describing the initial shock as a precursory event.

A significant aftershock sequence followed that included many large and very large shocks. Only a few had destructive characteristics, including the 9 April Rapel Lake earthquake. A destructive tsunami caused several million dollars worth of damage along the coast, and multiple international scientific groups convened in the area to assist local universities with seismological, engineering, and geological surveys.

Tectonic setting

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The Peru–Chile Trench, also known as the Atacama Trench, is the primary tectonic feature off the coasts of Peru in the north and Chile to the south. The Nazca plate is subducting to the east under the South American plate at 10 centimeters (3.9 in) per year. This continuous stick-slip movement results in two types of earthquakes around the trench. There are interplate events along the subduction interface and intraplate events within the downgoing Nazca plate. Magnitudes around eight are typical for the subduction interface events, while the intraplate type can result in a magnitude of half unit less.[6]

Foreshocks

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Foreshock activity occurred for eleven days prior to the mainshock, beginning on 21 February with a 4.7 mb  event at 18:53 UTC. Over the course of the eleven days, 360 events over 3.0 Mc  (earthquake duration magnitude). The frequency of the activity caused alarm in Valparaiso.[7]

Earthquake

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The primary event in the sequence was initially described as comprising two shocks that were very close together in time and space.[8][9]

Casualties

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The United States Geological Survey' Preliminary Determination of Epicenters and Utsu 2002 list 177 fatalities and 2,575 injuries, while the Belgian Centre for Research on the Epidemiology of Disasters' EM-DAT database lists slightly higher losses, with 180 dead and a slightly lower number of injured at 2,483. The National Geophysical Data Center again lists a slightly higher death toll of 200 and repeats the Utsu figure of 2,575 injured. President Augusto Pinochet said in a speech regarding the state of the nation that "Esta tragedia provocó en la zona central del país la pérdida de numerosas vidas humanas, más de dos mil quinientos heridos" (This tragedy caused the loss of numerous human lives in the central part of the country, more than two thousand five hundred injured).[4][10]

Aftershocks

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A number of large and very large aftershocks occurred in the following month, although the majority of them did not cause any additional damage and any deaths that occurred were not directly related. On the day of the mainshock, two events of at least 6.4 Ms  occurred and the following day at least thirteen shocks took place, including a 7.4 Mw  event. Six aftershocks took place on 5 March, then activity continued at a decreasing frequency, including the 6.7 Mw  shocks on 17 and 19 March, and the 7.2 Mw  Rapel Lake event on 9 April. These last three events had intensities of VII (Very strong), VI (Strong), and VI in Valparaiso. One heart attack and damage in Valparaiso was attributed to the 17 March event, and two deaths, several injuries, and additional damage occurred as result of the Rapel Lake shock on 9 April.[11][12][13][a]

Ground effects

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Reports by local residents in the coastal area from Matanzas to several kilometers north of Algarrobo indicated unusually low tides for a period of three to five days. After this, the tides reportedly returned to normal. This suggests that there was earthquake-related uplift along parts of the coast which was recovered in three to five days. Near Algarrobo, an estimate of the change is about 20 cm uplift.

Many landslides were registered too, pavement breaks with the destruction of the Pan-American Highway in several points, broken-down bridges and considerable damage in affected town's infrastructure, with a long interruption on basic services.

Tsunami

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A destructive local tsunami was observed.[14]

Response

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Multiple agencies from around the world were sent to assist professor Nicolas Velasco Saragoni from the University of Chile and professor Patricio Bonelli from Federico Santa María Technical University, as well as staff from Universidad Católica de Chile. The foreign teams assisted with damage, engineering, geological, and intensity surveys. Seismologists Mehmet Çelebi and George Plafker arrived on 21 March from the United States Geological Survey. Additional equipment was brought (seismometers and accelerographs) to capture as much data as possible, and they were operated by the National Autonomous University of Mexico and University of Chile. Other teams included the Earthquake Engineering Research Institute and the New Zealand National Society for Earthquake Engineering.[15][16]

See also

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Notes

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  1. ^ Magnitudes for these aftershocks are taken from Nealy 2017; intensity details are from Wood, Wight & Moehle 1987; damage and effects are from Stover & Brewer 1991

References

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  1. ^ Bravo et al. 2019, p. 1792
  2. ^ Mendoza, Hartzell & Monfret 1994, p. 269
  3. ^ a b c International Seismological Centre (2024), ISC-GEM Earthquake Catalogue (Data set), Version 11.0, doi:10.31905/D808B825
  4. ^ a b c d e PAGER-CAT Earthquake Catalog (Data set), Version 2008_06.1, United States Geological Survey, 4 September 2009
  5. ^ Algermissen, S. T.; Kausel, E.; Sembera, E.; Thenhaus, P. C. (1985), "Site Selection and Field Experiments", in Algermissen, S. T. (ed.), Investigations of the Central Chile earthquake of March 3, 1985 (PDF), United States Geological Survey, p. 27
  6. ^ Wood, Wight & Moehle 1987, pp. 6, 7
  7. ^ Comte, D.; Eisenberg, A.; Lorca, E.; Pardo, M.; Ponce, L.; Saragoni, R.; Singh, S. K.; Suárez, G. (1986), "The 1985 Central Chile Earthquake: A Repeat of Previous Great Earthquakes in the Region?", Science, 233 (4762): 449–453, doi:10.1126/science.233.4762.449, ISSN 0036-8075
  8. ^ Algermissen, S. T.; Kausel, E. (1985), "The earthquakes of March 3, 1985, and the seismicity of Chile", Investigations of the Central Chile earthquake of March 3, 1985 (PDF), United States Geological Survey, p. 3
  9. ^ Mendoza, Hartzell & Monfret 1994, p. 271
  10. ^ Mensaje Presidencial 11 Septiembre 1984 al 11 de Septiembre 1985, Ministerio de Hacienda, 11 September 1985, p. XX
  11. ^ Wood, Wight & Moehle 1987, p. 61
  12. ^ Nealy, J. L. (2017), 2017 Valparaiso, Chile earthquake data (Data set), 1985_Valparaiso_catalog.csv, doi:10.5066/F71Z439C
  13. ^ Stover, C. W.; Brewer, L. R. (1991), United States earthquakes, 1985, Bulletin 1954, Table 7, doi:10.3133/b1954
  14. ^ Lander, J. & Whiteside, L. & Lockridge, P. (2003). Two decades of global tsunamis 1982–2002. Science of Tsunami Hazards. 21. pp. 30, 31
  15. ^ Algermissen, S. T. (1985), "Introduction", Investigations of the Central Chile earthquake of March 3, 1985 (PDF), United States Geological Survey, p. 1
  16. ^ Booth & Taylor 1988, p. 12

Sources

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Further reading

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