1960 Valdivia Earthquake

The 1960 Valdivia earthquake and tsunami ( es, link=no, Terremoto de Valdivia) or the Great Chilean earthquake (''Gran terremoto de Chile'') on 22 May 1960 was the most powerful earthquake ever recorded. Various studies have placed it at 9.4–9.6 on the moment magnitude scale. It occurred in the afternoon (19:11 GMT, 15:11 local time), and lasted for approximately 10 minutes. The resulting tsunamis affected southern Chile, Hawaii, Japan, the Philippines, eastern New Zealand, southeast Australia, and the Aleutian Islands.

The epicenter of this megathrust earthquake was near Lumaco, approximately south of Santiago, with Valdivia being the most affected city. The tremor caused localised tsunamis that severely battered the Chilean coast, with waves up to . The main tsunami traveled across the Pacific Ocean and devastated Hilo, Hawaii, where waves as high as were recorded over from the epicenter.

The death toll and monetary losses arising from this widespread disaster are not certain.
Various estimates of the total number of fatalities from the earthquake and tsunamis have been published, ranging between 1,000 and 6,000 killed. Different sources have estimated the monetary cost ranged from 400 million to $800 million (or $4.01 billion to $8.021 billion in 2022, adjusted for inflation).

Earthquake sequence

Concepción earthquakes

The 1960 Chilean earthquakes were a sequence of strong earthquakes that affected Chile between 21 May and 6 June 1960, centered in the Araucanía, Aysén, and Bío Bío regions of the country. The first three quakes, all registering in the planet's top 10 by magnitude for 1960, are grouped together as the 1960 Concepción earthquakes. The first of these was the 8.1 M_w Concepción earthquake at on 21 May 1960. Its epicenter was near Curanilahue. Telecommunications to southern Chile were cut off and President Jorge Alessandri cancelled the traditional ceremony of the Battle of Iquique memorial holiday to oversee the emergency assistance efforts. The second and third Concepción earthquakes occurred the next day at (7.1 M_w) and (7.8 M_w) on 22 May. These earthquakes formed a southward migrating foreshock sequence to the main Valdivia shock, which occurred just 15 minutes after the third event.

The earthquake interrupted and effectively ended Lota's coal miners' march on Concepción as they demanded higher salaries.

Valdivia earthquake

The Valdivia earthquake occurred at 15:11 UTC-4 on 22 May, and affected all of Chile between Talca and Chiloé Island, more than . Coastal villages, such as Toltén, were struck. At Corral, the main port of Valdivia, the water level rose before it began to recede. At 16:20 UTC-4, a wave of struck the Chilean coast, mainly between Concepción and Chiloé. Another wave measuring was reported ten minutes later.

Hundreds of people were already reported dead by the time the tsunami struck. One ship, ''Canelos'', starting at the mouth of the Valdivia River, sank after being moved backward and forward in the river; as of 2005, its mast was still visible from the road to Niebla.

Soil subsidence also destroyed buildings, deepened local rivers and created wetlands in such places as the Río Cruces and Chorocomayo, a new aquatic park north of the city. Extensive areas of the city were flooded. The electricity and water systems of Valdivia were totally destroyed. Witnesses reported underground water flowing up through the soil. Despite the heavy rains of 21 May, the city was without a water supply. The river turned brown with sediment from landslides and was full of floating debris, including entire houses. The lack of potable water became a serious problem in one of Chile's rainiest regions.

The earthquake did not strike all the territory with the same strength; measured with the Mercalli scale, tectonically depressed areas suffered heavier damage. The two most affected areas were Valdivia and Puerto Octay, near the northwest corner of Llanquihue Lake. Puerto Octay was the center of a north–south elliptical area in the Central Valley, where the intensity was at the highest outside the Valdivia Basin. East of Puerto Octay, in a hotel in Todos los Santos Lake, stacked plates were reported to have remained in place. Excepting poor building sites, the zone of Mercalli scales intensities of VII or more all lay west of the Andes in a strip running from Lota (37° S) southwards. The area of intensities of VII or more did not penetrate into the Central Valley north of Lleulleu Lake (38° S) and south of Castro (42.5° S).

Two days after the earthquake Cordón Caulle, a volcanic vent close to Puyehue volcano, erupted. Other volcanoes may also have erupted, but none were recorded because of the lack of communication in Chile at the time. The relatively low death toll in Chile (5,700) is explained in part by the low population density in the region, and by building practices that took into account the area's high geological activity.

Aftershocks

One of the main aftershocks occurred on 6 June in Aysén Region. This earthquake probably occurred along the Liquiñe-Ofqui Fault, meaning in this case that the fault would have moved as a consequence of the 22 May Valdivia earthquake.

Tectonic interpretation

The earthquake was a megathrust earthquake resulting from the release of mechanical stress between the subducting Nazca Plate and the South American Plate, on the Peru–Chile Trench. The focus was relatively shallow at , considering that earthquakes in northern Chile and Argentina may reach depths of . A recent 2019 research paper postulates that the Liquine-Ofqui fault had a 9.07 strike-slip subevent along with the main 9.37 main thrust sub-event which could help account for how the plate boundary event seemingly "over spent" its tectonic budget. That is to say, the previous and to this day more widely accepted explanation for the earthquake involves the Peru-Chile Trench slipping further than its accumulated slip deficit (the amount of slip available for an earthquake to tap into) should allow. The alternative explanation, with two faults slipping near simultaneously could help explain the true mechanism of the earthquake.

Subduction zones are known to produce the strongest earthquakes on earth, as their particular structure allows more stress to build up before energy is released. Geophysicists consider it a matter of time before this earthquake will be surpassed in magnitude by another. The earthquake's rupture zone was long, stretching from Arauco (37° S) to Chiloé Archipelago (43° S). Rupture velocity, the speed at which a rupture front expands across the surface of the fault, has been estimated as per second. The average slip across all 27 Nazca subfaults was estimated to be , with of slip south of the epicenter on offshore subfaults.

While the Valdivia earthquake was extraordinarily large, the 2016 Chiloé earthquake hints that it did not release all the potential slip in that segment of the plate interface.

Natural disasters

Tsunamis

Earthquake-induced tsunamis affected southern Chile, Hawaii, Japan, the Philippines, China, eastern New Zealand, southeast Australia, and the Aleutian Islands. Some localized tsunamis severely battered the Chilean coast, with waves up to . The main tsunami crossed the Pacific Ocean at a speed of several hundred km/h and devastated Hilo, Hawaii, killing 61 people. Most of the tsunami-related deaths in Japan occurred in the northeast Sanriku region of Honshu.

The Chilean coast was devastated by a tsunami from Mocha Island (38° S) to Aysén Region (45° S). Across southern Chile, the tsunami caused huge loss of life, damage to port infrastructure, and the loss of many small boats. Further north, the port of Talcahuano did not suffer any major damage, only some flooding. Some tugboats and small sailboats were stranded on Rocuant Island near Talcahuano.

After the 21 May Concepción earthquake, people in Ancud sought refuge in boats. A carabinero (police) boat, ''Gloria'', was towing a few of these boats when the second earthquake struck on 22 May. As the sea regressed ''Gloria'' became stranded between Cerro Guaiguén and Cochinos Island. The stranded boat was wrecked when a tsunami wave engulfed it.

All the new infrastructure of the small port of Bahía Mansa was destroyed by the tsunami, which reached heights of up to 10 metres above sea level there. The boat ''Isabella'' in Bahía Mansa quickly left the port but lost its anchors.

In the Valdivia River and Corral Bay, several vessels were wrecked by the earthquake, among them ''Argentina'', ''Canelos'', ''Carlos Haverbeck'', ''Melita'', and the salvaged remnants of ''Penco''. ''Canelos'' was anchored at Corral when the quake struck, filling a cargo of wood and other products destined for northern Chile. The engine of ''Canelos'' was warmed up after the earthquake. After hours of drifting around in Corral Bay and Valdivia River, the ship was wrecked and subsequently abandoned by its crew at 1800 local time. Two men on board ''Canelos'' died in the incident. As of 2000, the remnants of ''Canelos'' were still visible.

''Santiago'', another ship anchored at Corral at the time of the quake, managed to leave Corral in a bad state but was wrecked off the coast of Mocha Island on 24 May. The schooner ''La Milagrosa'' departed from Queule on 22 May to load a cargo of ''Fitzroya'' wood shingles in a small port south of Corral. ''La Milagrosa'' was battered by the currents and waves of the tsunami for four days while moving south. Outside Corral the crew rescued six nearly unconscious and dehydrated children on board two boats. The boats found were used to navigate in Valdivia River and Corral Bay but had drifted into the high sea.

At the coastal town of Queule, a carabinero reported hundreds of people dead or missing some days after the tsunami. Historians Yoselin Jaramillo and Ismael Basso report that people in Queule decades later know about 50 people to have died because of the earthquake and tsunami.

Landslides

The earthquake triggered numerous landslides, mainly in the steep glacial valleys of the southern Andes. Within the Andes, most landslides occurred on forested mountain slopes around the Liquiñe-Ofqui Fault. Some of these areas remain sparsely vegetated while others have naturally developed more or less pure stands of ''Nothofagus dombeyi''. These landslides did not cause many fatalities nor significant economic losses because most of the areas were uninhabited, with only minor roads.

One landslide caused destruction and alarm following its blockage of the outflow of Riñihue Lake (see below). About south of Riñihue Lake, landslides in the mountains around Golgol River

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