Chile Earthquake and Tsunami

The great magnitude 9.5 Chilean earthquake of May 22, 1960, generated a huge tsunami that killed more than 1,000 people near the earthquake epicenter and almost 1,000 more as the wave propagated across the Pacific Ocean. This tsunami was generated along the convergent boundary between the small Nazca oceanic plate in the Pacific Ocean and southern South America. This part of the "Ring of Fire" convergent boundary generates more tsunamogenic earthquakes than anywhere else on the planet, unleashing a large tsunami about every 30 years. Damage from the

1960 earthquake led directly to the establishment of the modern Pacific tsunami warning system.

Saturday, May 21, began as a normal day in Chile, a morning soon interrupted by a series of about 50 significant earthquakes that shook the continent beginning at 6:02 a.m. The first tremor destroyed much of the area around ConcepciĆ³n, with a sequence of aftershocks that continued until 3:11 p.m. the following day. Then, on May 22, there were two massive earthquakes with magnitudes of 8.9 and 9.5, with their hypocenters located a mere 20 miles (33 km) below the surface. A section of the seafloor and coast nearly 200 miles (300 km) long experienced sudden uplift of 3.3 feet (1 m), and subsidence of the land of 5 feet (1.6 m) occurred across an area of 5,000 square miles (13,000 km2), extending 18 miles (29 km) inland.

since the area was (and still is) prone to tsunamis generated from earthquakes, local coastal fishermen knew that such a large earthquake would likely be followed by a giant tsunami, so they rapidly took their families and ran their boats to the open ocean. This knowledge of how to respond to tsunamis undoubtedly saved many lives initially, since 10 to 15 minutes after the large quake, a 16-foot- (5-m-) high tsunami rolled into many shoreline areas, causing destruction of dock areas and coastal villages. However, after this wave washed back to sea, many fishermen returned, or stayed close to shore. This was a mistake, since 50 minutes after the first wave retreated, another, larger crest struck, this time as a 26-foot- (8-m-) tall wall of water that crashed into the shore at a remarkable speed of 125 miles per hour (200 km/hr). Many of the deaths in Chile were reportedly related to the fact that after the first tsunami crest passed, many Chileans assumed the danger was over and returned to the shoreline. The second crest had a run-up of 26 feet (7.8 m) and was followed by a third crest with a height of 36 feet (11 m) that moved inland at about half the speed of the second wave, but was so massive that it inflicted considerable damage. For the next several hours the coast was pounded by a series of waves that virtually destroyed most of the development along the coastline between ConcepciĆ³n and Isla Chiloe. Run-up heights were variable along the coast in the southern part of Chile, with most between 28 and 82 feet (8.5 and 25 m). The number of people that were killed in Chile by this tsunami is unknown because of poor documentation, but most estimates place it between 5,000 and 10,000 people.

The massive earthquakes of May 22 sent a series of tsunami crests racing across the Pacific Ocean at 415-460 miles per hour (670-740 km/hr) and around the world for the next 24 or more hours. The tsunami train had wavelengths of 300-500 miles

(500-800 km), periods of 40-80 minutes between passing crests, and was only a little more than a foot high (40 cm) in the open ocean. The waves swept up the coast of South America, then the western United States, where run-up heights were small, typically less than 4 feet (1.2 m), but locally up to 12 feet. The tsunami was accurately predicted to hit Hawaii 14.8 hours after the earthquake, and it arrived within a minute of the predicted time. This should have saved lives, but 61 people were killed by this tsunami in Hawaii, including many who heard the warnings but rushed to the coast to watch the waves strike.

The response of the population of Hilo to tsunami warnings in 1960 is a lesson in the need to understand the hazards of tsunamis to save lives. As the tsunami approached Hawaii, the wave crests refracted around to the north side of the islands and hit the city of Hilo particularly hard, because of the shape of the shoreline features in Hilo Bay. Even though residents were warned of the danger, less than one-third of the population evacuated the coastal area when the warnings were issued, and about half remained after the first crest hit Hilo. Like many tsunamis, the first crests to invade Hilo were not the largest, and sightseers were surprised, and many killed, when the third crest pounded the downtown harbor and business district with a 20-foot (6-m) wall of water that carried 20-ton projectiles of pieces of the city wharf and waterfront buildings. The city suffered an inundation of 5 city blocks as the run-up reached 35 feet (10.7 m). Amazingly, about 15 percent of the city population stayed even through the largest waves. More than 540 homes and businesses were destroyed with tens of millions of dollars of damage (24 million 1960 dollars), and 61 residents of Hilo were dead.

Islands in the Western Pacific were widely affected, with the height of the waves largely determined by factors such as the shape of the shoreline, slope of the seafloor, and orientation of the beach with respect to the source in Peru. Pitcairn Island was the strongest hit, with run-up heights of more than 40 feet (12.2 m) reported. The effects of wave refraction around some Pacific islands focused some of the tsunami energy on Japan, where the runup heights exceeded 20 feet (6.4 m). The combined effects of this refraction of wave energy from the distant earthquake, and natural resonance effects of some harbors that further amplified the waves, made the 1960 earthquake an unexpectedly large and devastating disaster in Japan. Approximately 22 hours after the earthquake the coast of Japan began to feel its destruction. The resonance effects caused the largest waves to grow hours after the initial wave crests hit Japan, whose east coast saw an average run-up of 9 feet (2.7 m). The resonance effects and seiching in the harbors caused the greatest damage, with 5,000

homes destroyed in Hokkaido and Honshu, 191 people killed and another 854 injured, and 50,000 left homeless. Property damage in Japan was estimated to exceed 400 million (1960) dollars.

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