Introducing The Robinson Crusoe Island Tsunami

[BrickFielder]

Introduction
My intention is to produce a series of articles (depending on interest) which introduces ideas about volcanic and earthquake activity. I want to go slightly off the beaten track to explore oddities, volcano hazards, analysis methods, Wonders and Mankind’s impacts. Keep in mind I am not expert (corrections gratefully received), but hopefully these will at least give a flavour of some different places in the world and provide a few minutes escape from people’s troubles.

Introducing The Robinson Crusoe Island Tsunami

Reason for Interest
I had never heard of the Juan Fernández Islands or Robinson Crusoe Island but the accounts of the tsunami were so vivid that I thought it was important to remind us that lives can be devastated by disaster. Throw in a little mythical history and I find it fascinating.

The Island
Robinson Crusoe Island is the second largest of the Juan Fernández Islands, situated 670 km west of San Antonio Chile. It is the most populated of the islands with most living in the town of San Juan Bautista at Cumberland Bay on the north coast. From 1704 to 1709, the island was home to the marooned sailor Alexander Selkirk, who at least partially inspired novelist Daniel Defoe's fictional Robinson Crusoe in his 1719 novel.

 

Robinson Crusoe Island lies to the west of the boundary between the Nazca Plate and the South American Plate. A volcanic eruption on the island was reported in 1743 from El Yunque, but this event is unconfirmed. The Fuerte Santa Barbara fort once used by 18th century Spanish to protect themselves from raucous pirates.

 

The islands have a subtropical Mediterranean climate moderated by the cold Humboldt Current, which flows northward to the east of the islands, and the southeast trade winds. Temperatures range from 10 °C to 22 °C. In 1977 the archipelago was named a biosphere reserve and now forms part of the enormous Juan Fernandez Marine Park. A particular success has been the South American fur seal, a species that was almost extinct 40 years ago, which now has a population of 30,000 on the islands. 

Just offshore in the harbour can be found the wreck of the German light cruiser SMS Dresden. Having escaped the battle of the Falklands she eluded her British pursuers until she put into Robinson Crusoe Island in March 1915. Her engines were worn out and she had almost no coal left for her boilers. After The British violated Chilean neutrality and opened fire on the ship the Germans scuttled the Dresden and the majority of the crew escaped. 

The Tectonic environment
The west coast of South America is a subduction zone where the Nazca Plate is plowing under the South America Plate at an average rate of 3 inches per year. The boundary where the two plates converge is marked by a trench located about 62 miles offshore where the Nazca Plate begins its descent beneath the South America Plate. When plates become locked together for a time and earthquake may occur as the plates lurch past each other violently. If this occurs at a subduction zone then crust one side of the earthquake will drop and on the other rise and it is this movement that causes a tsunami.

A First hand Account
British couple Rhian Salmon and Andy Whittaker embarked on the trip of a lifetime in their yacht. Their first port of call was the volcanic paradise of Robinson Crusoe Island where they moored near to the village of San Juan Bautista. The island is part of the Juan Fernández Islands 400 miles off the South American coast. In the early hours of 27 February, the day they were due to set sail again, a devastating earthquake struck Chile, and unleashed a massive tsunami westwards across the Pacific directly towards the island. 

At 4am they work up to the sound of water rushing under the boat. Then they heard an almighty roar resonating around the bay. Water is flying past the yacht in big whirls, carrying trees and what looks like roofs. The water sweeps back towards the open ocean carrying with it all objects in its path. They hear cries and calls from people on the roofs and trapped in the houses which fly past them.

Andy reaches over the side and pulls a boy on to the deck shivering, covered in oil and cuts, looking for his family. They are surrounded by strong fuel fumes and there is a strong noise of gas hissing from gas bottles that have been ripped from the houses they used to supply.

Another older boy climbs on board. There are other boats in the water now and a Navy boat. The Navy boat is unoccupied and dragging hard on its mooring. Andy and the two boys fend of the boat off while on the other side a rooftop is pushing up against us, and next to it a whole house.

Further away, a family are stranded on the top floor of a floating house. They see the yacht and start to swim. Andy throws a rope to them and they grab on. Andy pulls up a young girl but the other three are swept away.

As dawn approaches they start to see the damage. The whole town front has been wiped out, the navy boat is wrecked on the rocks, the shops where they bought supplies is gone, as are the school and town square. 

 

The Maule Chile Earthquake

The 2010 Chile Maule earthquake occurred off the coast of central Chile on a Saturday in the early hours of the morning. The magnitude of the earthquake was recorded as 8.8 on the moment magnitude scale, with intense shaking lasting for nearly four minutes. Since the earthquake was in the middle of the night on a weekend injury due to falling damaged infrastructure like bridges and roads was minimized. The tsunami waves that followed this event affected the coastal regions between the cities of Valparaiso and Valdivia, with minor effects as far as Coquimbo. In Chile 525 people lost their lives, 25 people went missing and about 9% of the population in the affected regions lost their homes. 

The focus of the earthquake occurred at a depth of about 22 miles below the surface of the Pacific Ocean. The earthquake—resulting from the rupture of a 300- to 375-mile stretch of the fault that separates the South American Plate from the subducting Nazca Plate. Stress brought on by the convergence of the two tectonic plates caused rocks to shatter along the boundary between them. This forced a portion of the seabed upward, displacing the water above and triggering a tsunami. The Chilean town of Constitución was inundated by waves as high as 50 feet and the port of Talcahuano was damaged by a wave measuring nearly 8 feet high. Traveling across the Pacific Ocean at nearly 450 miles per hour the tsunami encountered the Robinson Crusoe Island approximately 420 miles off the coast of Chile.

The tremor shook the capital Santiago for a minute bringing down telephone and power lines. Santiago's international airport was forced to close, a highway bridge collapsed and rubble from damaged buildings fell on to the streets. The largest earthquake ever recorded struck the same area of Chile on 22 May 1960. The magnitude 9.5 quake killed 1,655 people and left 2 million homeless, and caused a tsunami that killed people in Hawaii, Japan and the Philippines.

Several factors contributed overall to the low casualty rate and rapid recovery. A major factor 
was the strong building code in Chile and its comprehensive enforcement. A second factor was the  limited number of fires after the earthquake due to the shut down of the electricity grid early in the earthquake. Third factor was the practiced close coordination between emergency management teams.

Discussion
 While an earthquake of the same magnitude in the same area is unlikely there will be other earthquakes along the fault line in the future. History suggests the next one could be even bigger and reach right across the Pacific ocean.