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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
Copiapó
Copiapó
São Paulo
São Paulo
Buenos Aires
Buenos Aires
Valdivia
Valdivia
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
Duration69 seconds[1]
Magnitude8.0 Mw[2][3]
Depth35.0 km (21.7 mi)[2]
Epicenter33°15′25″S 71°51′29″W / 33.257°S 71.858°W / -33.257; -71.858[2]
FaultAtacama Trench
TypeMegathrust
Areas affectedCentral Chile
Total damage1.5–1.8 billion US dollars[4][5]
Max. intensityMMI IX (Violent) (Llolleo)
Peak acceleration.86 g (Llolleo)[6]
TsunamiYes
LandslidesYes
Casualties177–200 dead
2,483–2,575 injured
372,532–500,000 homeless[7][4][a]

A megathrust earthquake measuring 8.0 Mw  struck just offshore the Greater Valparaíso area of Central Chile on 3 March 1985. The event followed a ten-day period of moderate and non-destructive foreshocks and left at least 177 people dead and about 2,500 injured. According to several reports, a small sector of damage in Llolleo corresponded to a maximum Mercalli intensity of IX (Violent), though the vast majority of damage was considered to align with intensity VIII (Severe) effects or less. The damage was significant and widespread, and was similar to numerous previous events that had severely impacted the Valparaíso and Santiago metropolitan areas. Great earthquakes related to the subduction zone have occurred directly under populated areas or very close offshore since records began beginning with the arrival of Europeans in the sixteenth-century.

Because of its heavy impact (financial losses of 1.5–1.8 billion US dollars) in the Valparaíso, Santiago, O'Higgins, and Maule Regions, the event has been thoroughly studied across various disciplines. Numerous scientific and academic papers published beginning in the late 20th-century and continuing into the 21st-century have explored the its features, including the geological, seismological, and epidemiological aspects of the event, and have improved the understanding of the complex plate boundary and how this specific event relates to the many that came before it. Most have been primarily of the megathrust type, but Central Chile is also subject to other types of earthquakes that are related to the relatively deep subduction zone as well as shallow and onshore crustal faults.

A significant aftershock sequence followed that included many large (M6+) and very large (M7+) shocks. Only a small portion of these had destructive characteristics, including the twin 6.7 Mw  shocks on 17 and 19 March and the 7.2 Mw  Rapel Lake earthquake on 9 April. Multiple international scientific groups convened in the area to assist local universities with seismological, engineering, and geological surveys, including the study of a moderately-destructive and basin-wide tsunami that caused several million dollars worth of damage along the Central Chilean coast.

History

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See also: Timeline of Santiago de Chile, List of megathrust earthquakes, Regions of Chile, and Types of earthquake

Central Chile (roughly between 32° and 35° south latitude) has been repeatedly affected by great earthquakes. Since the arrival of Europeans in the sixteenth-century, onshore and offshore events have been documented in this region from the coastal cities of Valparaíso in the north and Concepción in the south. The majority of these events have been of the megathrust/interplate type; a limited number of them have been identified as normal/intraplate. It was one of these normal (dip-slip) events that was Chile's deadliest; the 1939 Chillán event resulted in 28,000–30,000 deaths (without the influence of a tsunami). The inland city of Santiago was heavily damaged in the 1730, 1822, and 1906 megathrust events. With a recurrence interval of a little more than 80 years, the 1985 event was not unexpected.[8][9][10]

Tectonic setting

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See also: Chile triple junction, Convergent boundary, List of tectonic plate interactions, and Geology of Chile

The Peru–Chile Trench, also known as the Atacama Trench, is the primary tectonic feature off the west coast of South America and has contributed to Chile being one of the most seismically active countries in the world. The Nazca plate is subducting to the east under the South American plate, with high speed convergence and continuous stick-slip action resulting in two types of earthquakes along the trench. There are the typical interplate events that occur at the subduction interface and the less common intraplate events that occur within the downgoing Nazca slab. Magnitudes around eight are typical for the subduction interface events, while the intraplate type can result in a magnitude of a half unit less.[9][11]

Foreshocks

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See also: Moment magnitude scale and Seismic magnitude scales

Foreshock activity occurred for eleven days prior to the mainshock, beginning on 21 February with a 5.7 Mw  event at 18:53 UTC. This magnitude was presented in a 2017 United States Geological Survey (USGS) study and data release when the mainshock, foreshocks, and aftershocks were relocated. The National Earthquake Hazards Reduction Program/National Earthquake Information Center (Preliminary Determination of Epicenters) placed the intensity of this initial foreshock at VI (Strong). Over the course of the eleven days, 360 events over 3.0 Mc  (earthquake duration magnitude) occurred, but none with a higher intensity than the initial event. This included more than 50 events on 21 February and more than 100 the following day. The frequency of the activity caused great alarm in Valparaíso, to the extent that funds were immediately sought by Mexican and Chilean scientists for travel expenses and to bring additional equipment to set up portable seismograph stations. Prior to their arrival on 5 March, the final foreshock in the sequence arrived as a 5.2 Mw  event just 10 seconds prior to the mainshock.[10][3][12][13][b]

Earthquake

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Characteristics

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See also: Strike and dip, Rake (geology), and Wadati–Benioff zone

The Centro Sismológico Nacional, the USGS, and the International Seismological Centre all place the magnitude at 8.0 Mw . Other figures exist, like the initial 1985 observation from the Preliminary Determination of Epicenters, which placed it at 7.8 Ms  and the Harvard–Adam Dziewonski Observatory, which places it at 7.9 Mw . Harvard's slip parameters for the strike, dip, and rake are 11°, 26°, and 110°. A 1986 study, with only a one-degree change in the dip angle, placed the figures at 11°, 25°, and 110°. A 1994 study compared their own figures regarding rupture length, depth, and duration, with those of nine previous studies. Of the six studies that contributed duration information that was gathered using different seismic networks, a range was given from 40 to 80 seconds, with a mode of 69 seconds. The focal depth was equally diverse, with five kilometers on the low end to 60 km on the high end. The fault's rupture length had a range of 75 to 250 kilometers, with the highest figure derived from geodetic information alone.[4][1][14]

Intensity

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See also: Modified Mercalli intensity scale and Seismic intensity scales

Compared with on-the-ground observations of damage and ground effects, isoseismal maps provide a rudimentary overview of the intensity that was experienced. Professor Rodolfo Saragoni from the University of Chile provided one of these maps that shows zones of mixed intensities within roughly 150 kilometers (93 mi) of the epicenter,[c] where mixed groups include V–VI, VI–VII, or VII–VIII. These areas are relatively small and isolated areas of higher intensities embedded in much larger areas of lower intensity. For example, Rengo is in a very small intensity VIII (Severe) zone, but is itself surround by a slightly larger zone of VII (Very strong) intensity that includes Curicó, San Fernando, and Rancagua. Both of these areas are embedded within a VI–VII zone that stretches roughly 320 km (200 mi) from Talca to La Ligua. Another example is that the city of Algarrobo is in an area of VI–VII shaking, while embedded in a zone of VII–VIII shaking that includes nearby communities of Valparaíso, San Antonio, Llolleo, and Viña del Mar. Santiago was marked as intensity VII. Cities designated as having experienced intensity VI shaking included Linares and Illapel. The map shows a more linear pattern of attenuation farther from the epicentral area.[15][16]

On the ground, the shock's area of perceptibility was from Copiapó in the north to Valdivia in the south, a distance of more than 1,400 kilometers (870 mi).[d] Some people in high rises felt their buildings sway on the east coast of South America, both at Buenos Aires, Argentina (1,200 km (750 mi) distant) and São Paulo in Brazil (2,700 km (1,700 mi) distant). Several other surveys provided a bit more specific details when describing the observed intensity than the isoseismal maps. The Earthquake Engineering Research Institute's report agreed with the University of Illinois study that the intensity was generally VII in Santiago. An intensity of VII is stated as the most appropriate intensity of shaking for Valparaíso and Viña del Mar, but an accommodation for intensity VIII effects was made, based on several instances of considerable damage or partial collapse of buildings. With areas of liquefaction, considerable damage and rock slides, San Antonio was placed at intensity VIII. The same could be said for Llolleo, but an exception was made for small areas of intensity IX (Violent) effects. A separate strong ground motion study also published a figure of IX for a heavily damaged school at Llolleo.[17][18][19]

Casualties

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See also: Earthquake casualty estimation

Deaths

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The USGS's 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][e][21]

Injuries

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A study on the epidemiology of the injured, authored by staff from Chile's Ministry of Health and Department of Planning, acknowledged the profound difficulties in establishing a complete understanding of the situation. Contributing to the problems were sometimes as simple as medical personnel failing to provide complete records or as complex as the complete loss of hospital or clinical facilities. Rengo Hospital, for example, collapsed entirely and 12 other hospitals required replacing due to structural damage, though no fatalities occurred at these facilities. The reduction in the number of beds available, misplaced files, and incomplete records all contributed to inconsistent numerical data. Despite these challenges, the authors presented an overview of the injured, and acknowledged a total figure of 2,575. Their work comprised a subset of those, which represented 811 women and 784 men for a total of 1,623.[22]

Damage

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Further information: Basílica del Salvador; Hospital del Salvador; St. Joseph's Cathedral, Melipilla; and Parroquia Nuestra Señora de Andacollo, Santiago

Aftershocks

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Further information: 1985 Rapel Lake earthquake

A number of large (M6+) and very large (M7+) aftershocks occurred in the following month, although the majority of them were moderate in intensity and did not cause any additional damage. Also, any casualties that occurred were not directly related and instead were attributed to sudden death from cardiac causes. 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, but 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 Valparaíso. One heart attack and damage in Valparaíso 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.[23][3][24][f]

Tsunami

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See also: List of tsunamis, Subsidence, and Tectonic uplift

A destructive local tsunami was observed.[25][26]

Aftermath

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Scientific response

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Further information: Strong ground motion

Multiple seismological 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. Brought with them were additional seismometers and accelerographs, which were delivered to (and operated by) the National Autonomous University of Mexico and the University of Chile. These instruments were deployed at temporary locations to augment the existing Chilean seismograph network to improve its ability to locate aftershocks and gather strong motion data. Other foreign teams included the Earthquake Engineering Research Institute, the British Institution of Structural Engineers, the New Zealand National Society for Earthquake Engineering, and a trio from University of California, Berkeley, University of Michigan, and the University of Illinois.[10][27][28]

Mental health effects

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See also

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Notes

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  1. ^ For the number of people made homeless, seismologist Bruce Bolt provides the minimum figure of 372,532, and EM-DAT (in the PAGER-CAT data set) provides the maximum figure of 500,000
  2. ^ The two USGS-related sources are the data release (Nealy 2017) which provides refined magnitudes and locations for the 1985 sequence, and the ANSS/Earthquake Hazards Program, which supplies an identical magnitude figure, plus intensity details. Algermissen 1985 gives the origin time of the final foreshock and the mainshock
  3. ^ Professor Saragoni's map is not available as it was first published in printed form in Saragoni, González & Fresard 1985 but it appears on page 72 of Wood, Wight & Moehle 1987 and a slightly modified version appears in figure 4.1 of Booth & Taylor 1988
  4. ^ The distance from Copiapó to Valdivia (1,407 km (874 mi)), is comparable to that of Seattle to Las Vegas (1,406 km (874 mi)), Dublin to Milan (1,418 km (881 mi)), or Baghdad to Dubai (1,381 km (858 mi))
  5. ^ PAGER-CAT is a compilation of parametric earthquake catalogs. Included are the Preliminary Determination of Epicenters, PAGER, EM-DAT (from the Centre for Research on the Epidemiology of Disasters), and the National Geophysical Data Center Significant Earthquake Database, and others[20]
  6. ^ 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. ^ a b Mendoza, C.; Hartzell, S.; Monfret, T. (1994), "Wide-band analysis of the 3 March 1985 central Chile earthquake: Overall source process and rupture history", Bulletin of the Seismological Society of America, 84 (2): 270, Bibcode:1994BuSSA..84..269M, doi:10.1785/BSSA0840020269 (inactive 1 April 2025){{citation}}: CS1 maint: DOI inactive as of April 2025 (link)
  2. ^ a b c International Seismological Centre (2024), ISC-GEM Earthquake Catalogue (Data set), Version 11.0, doi:10.31905/D808B825
  3. ^ a b c Nealy, J. L. (2017), 2017 Valparaiso, Chile earthquake data (Data set), U.S. Geological Survey data release, United States Geological Survey, 1985_Valparaiso_catalog.csv, doi:10.5066/F71Z439C
  4. ^ a b c d PAGER-CAT Earthquake Catalog (Data set), Version 2008_06.1, United States Geological Survey, 4 September 2009
  5. ^ Comte et al. 1986, p. 452, Note 4
  6. ^ Booth & Taylor 1988, p. 8
  7. ^ Bolt, B. (2005), Earthquakes: 2006 Centennial Update – The 1906 Big One (Fifth ed.), W. H. Freeman and Company, p. 156, ISBN 978-0-7167-7548-5
  8. ^ Algermissen, S. T.; Kausel, E. (1985), "The earthquakes of March 3, 1985, and the seismicity of Chile", Preliminary Report of Investigations of the Central Chile earthquake of March 3, 1985 (PDF), United States Geological Survey, p. 4, Bibcode:1985usgs.rept...29A, doi:10.3133/ofr85542
  9. ^ a b Wood, Wight & Moehle 1987, pp. 6, 7, 55
  10. ^ a b c Comte et al. 1986, pp. 449–452
  11. ^ Hussain, E.; Elliott, J. R.; Silva, V.; Vilar-Vega, M.; Kane, D. (2020), "Contrasting seismic risk for Santiago, Chile, from near-field and distant earthquake sources", Natural Hazards and Earth System Sciences, 20 (5): 1533, 1534, Bibcode:2020NHESS..20.1533H, doi:10.5194/nhess-20-1533-2020, ISSN 1684-9981
  12. ^ ANSS. "Valparaíso 1985: M 5.7 – 22 km SW of Valparaíso, Chile". Comprehensive Catalog. U.S. Geological Survey.
  13. ^ Algermissen, S. T.; Kausel, E. (1985), "The earthquakes of March 3, 1985, and the seismicity of Chile", Preliminary Report of Investigations of the Central Chile earthquake of March 3, 1985 (PDF), United States Geological Survey, p. 3, Bibcode:1985usgs.rept...29A, doi:10.3133/ofr85542
  14. ^ Comte et al. 1986, p. 450, Figure 1
  15. ^ Wood, Wight & Moehle 1987, p. 72
  16. ^ Booth & Taylor 1988, Figure 4.1
  17. ^ Earthquake Engineering Research Institute (1986), "The Chile Earthquake of March 3, 1985—Seismological Features", Earthquake Spectra, 2 (2): 258, 259, Bibcode:1986EarSp...2..253., doi:10.1193/1.1585313
  18. ^ ANSS. "Valparaíso 1985: M 8.0 – 25 km WSW of Valparaíso, Chile". Comprehensive Catalog. U.S. Geological Survey.
  19. ^ Algermissen, S. T.; Kausel, E.; Sembera, E.; Thenhaus, P. C. (1985), "Site Selection and Field Experiments", in Algermissen, S. T. (ed.), Preliminary Report of Investigations of the Central Chile earthquake of March 3, 1985 (PDF), United States Geological Survey, p. 27, Bibcode:1985usgs.rept...29A, doi:10.3133/ofr85542
  20. ^ Allen et al. 2009.
  21. ^ Mensaje Presidencial 11 Septiembre 1984 al 11 de Septiembre 1985, Ministerio de Hacienda, 11 September 1985, p. XX
  22. ^ Ortíz, M. R.; Roman, M. R.; Latorre, A. V.; Soto, J. Z. (1986), "Brief description of the effects on health of the earthquake of 3rd March 1985 – Chile", Disasters, 10 (2): 125–140, Bibcode:1986Disas..10..125O, doi:10.1111/j.1467-7717.1986.tb00578.x, ISSN 0361-3666
  23. ^ Wood, Wight & Moehle 1987, p. 61
  24. ^ Stover, C. W.; Brewer, L. R. (1991), "United States earthquakes, 1985", USGS Report, Bulletin 1954, Table 7, Bibcode:1991usgs.rept....7S, doi:10.3133/b1954
  25. ^ Lander, J. & Whiteside, L. & Lockridge, P. (2003). Two decades of global tsunamis 1982–2002. Science of Tsunami Hazards. 21. pp. 30, 31
  26. ^ Plafker, G. (1985), "Geologic Reconnaissance of the March 3, 1985 Chile earthquake", in Algermissen, S. T. (ed.), Preliminary Report of Investigations of the Central Chile earthquake of March 3, 1985 (PDF), United States Geological Survey, pp. 13–17, Bibcode:1985usgs.rept...29A, doi:10.3133/ofr85542
  27. ^ Algermissen, S. T. (1985), "Introduction", Preliminary Report of Investigations of the Central Chile earthquake of March 3, 1985 (PDF), United States Geological Survey, p. 2, Bibcode:1985usgs.rept...29A, doi:10.3133/ofr85542
  28. ^ Booth & Taylor 1988, p. 1

Sources

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Further reading

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Wikimedia Commons has media related to 1985 Santiago earthquake.
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