# 1985 Algarrobo earthquake

A megathrust earthquake measuring 8.0 M$_w$  struck just offshore the Greater Valparaíso area of 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. It was felt with a maximum intensity of VIII (Severe) on the Mercalli intensity scale. Damage was significant and widespread, and was similar to numerous previous events. The Valparaiso and Santiago metropolitan areas have been repeatedly and severely impacted by great earthquakes that have occurred very close offshore or directly under populated areas.
Because of its heavy impact on the Central Chile region (with financial losses of 1.5–1.8 billion US dollars) and complex seismological factors, the event has been thoroughly studied, with numerous scientific and academic papers published beginning in the mid-1980s and into the mid-2010s. This has proved to aid specifically in understanding not only the complex plate boundary where it originated, but especially the earthquake's initiation phase and precursors.
A significant aftershock sequence followed that included many large (M6+) and very large (M7+) shocks. Only a few had destructive characteristics, including the 9 April Rapel Lake earthquake. 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

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/intraplate types; 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 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 at 10 centimeters (3.9 in) per year. This continuous stick-slip movement results 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.

## Foreshocks

Foreshock activity occurred for eleven days prior to the mainshock, beginning on 21 February with a 5.7 M$_w$  event at 18:53 UTC, as determined by several products of the United States Geological Survey (USGS). During a 2017 study and data release, the mainshock, foreshocks, and aftershocks were relocated, and an agency seismologist in the Earthquake Hazards Program placed the intensity of this initial foreshock at VI (Strong). Over the course of the eleven days, 360 events over 3.0 M$_c$  (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 Valparaiso, 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 M$_w$  event just 10 seconds prior to the mainshock.

## Earthquake
### Characteristics

Centro Sismológico Nacional, the USGS, the International Seismological Centre, and numerous other independent studies all place the magnitude at 8.0 M$_w$ . Other figures vary, like the initial 1985 observation from the Preliminary Determination of Epicenters, which placed it at 7.8 M$_s$  and the Harvard CMT project, which places it at 7.9 M$_w$ . Harvard's slip parameters for the strike, dip, and rake are 11°, 26°, and 110°, which varies only slightly from a 1986 study which placed the figures at 11°, 25°, and 110°. A 1994 study summarized compared their own work with that of nine previous studies regarding rupture length, depth, and duration. 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 the mode being 69 seconds. The focal depth was equally diverse, with five kilometers on the low end to 60 km on the high end. Rupture length had a range of 75 to 250 kilimeters, with the highest figure derived from geodetic information alone.

### Intensity

An isoseismal map (a visual representation of shaking intensity that is divided by lines of equal intensity—and in this case—a high level overview) was presented. It includes two types of zones; mixed and unmixed. A mixed zone represents intensities that could be V–VI, VI–VII, or VII–VIII. There are also relatively small and isolated zones 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 Valparaiso, 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.

### Casualties

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).

### Ground effects

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.

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

## Tsunami

A destructive local tsunami was observed.

## 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. 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 the University of Chile. Other teams included the Earthquake Engineering Research Institute and the New Zealand National Society for Earthquake Engineering.