1985 Algarrobo 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. == Tectonic setting ==

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 sustained 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. An additional earthquake hazard comes in the form of shallow crustal faults. The San Ramón Fault, for example, is a thrust fault that is positioned along the easternmost districts of Santiago. Despite estimated slip rates of per year, paleoseismologists have uncovered evidence of several very large events ( of slip). They acknowledged that a more typical event on this fault may only produce a large shock in the range of 6.2–6.7 as the fault is segmented. ==Foreshocks==

Foreshock activity occurred for eleven days prior to the mainshock, beginning on 21 February with a 5.7 event. This magnitude was supplied by the National Earthquake Hazards Reduction Program (NHRP)/National Earthquake Information Center (NEIC) and a 2017 United States Geological Survey (USGS) study and data release. The NHRP/NEIC placed the intensity of this initial foreshock at VI (Strong). Over the course of the eleven days, 360 events over 3.0 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 just seconds prior to the mainshock. == Earthquake ==

Characteristics The Centro Sismológico Nacional de la Universidad de Chile, the USGS, and the International Seismological Centre all place the magnitude at 8.0 . Other figures exist, like the initial 1985 observation from the Preliminary Determination of Epicenters, which placed it at 7.8 and the Harvard–Adam Dziewonski Observatory, which places it at 7.9 . 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. The authors of the same 1994 study presented more details from earlier workers, as well as their own analysis of the mainshock characteristics, which included data from both the Global Digital Seismograph Network and the GEOSCOPE Observatory. Their results showed a bi-lateral rupture on the Nazca–South American plate boundary, though only three of the ten previous studies shared similar information regarding rupture directivity. They revealed that it primarily ruptured to the south of the epicenter, with just of the rupture to the north, over a total duration of 70 seconds. Intensity 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 of the epicenter, 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 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. 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 . Some people in high rises felt their buildings sway on the east coast of South America, both at Buenos Aires, Argentina ( distant) and São Paulo in Brazil ( distant). Several other reconnaissance and damage surveys provided 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. Injuries 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. Damage ==Aftershocks==

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 occurred and the following day, at least thirteen shocks took place, including a 7.4 event. Six aftershocks took place on 5 March, then activity continued, but at a decreasing frequency, including the 6.7 shocks on 17 and 19 March, and the 7.2 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. ==Tsunami==

A destructive local tsunami was observed. ==Aftermath==

Scientific response 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. Mental health effects ==See also==