Earthquakes
The northern two-thirds of Chile lie on top of the telluric Nazca Plate, which, moving eastward about ten centimeters a year, is forcing its way under the continental plate of South America. This movement has resulted in the formation of the Peru-Chile Trench, which lies beyond a narrow band of coastal waters off the northern two-thirds of the country. The trench is about 150 kilometers wide and averages about 5,000 meters in depth. At its deepest point, just north of the port of Antofagasta, it plunges to 8,066 meters. Although the ocean's surface obscures this fact, most of Chile lies at the edge of a profound precipice.
The clash between the earth's surface plates has also generated the Andes, a geologically young mountain range that, in Chilean territory alone, includes about 620 volcanoes, many of them active. Almost sixty of these had erupted in the twentieth century by the early 1990s. More than half of Chile's land surface is volcanic in origin.
Subduction zones such as the South America arc are geologically complex and generate numerous earthquakes from a variety of tectonic processes that cause deformation of the western edge of South America. Crustal deformation and subsequent mountain building in the overriding South America plate generate shallow earthquakes. Slip along the dipping interface between the two plates generates frequent and often large interplate earthquakes between depths of approximately 10 and 50-60 km.
The telluric displacements that created the Peru-Chile Trench make the country highly prone to earthquakes. Since the middle of the 16th century, there is a comprehensive written record of other large damaging earthquakes throughout the region. Notable likely mega-thrust earthquakes prior to 1900 include earthquakes in 1868 and 1877 offshore southern Peru and northern Chile. In the vicinity of the 2010 earthquake, damaging earthquakes were reported in 1751, near Concepcion, and one further to the north in 1730. Tsunamis from the 1730, 1751, 1868, and 1877 earthquakes produced Pacific-wide tsunamis as evidenced from detailed records of flooding and damage in Japan. An 1835 Concepcion earthquake is notable because the great Explorers Charles Darwin and Robert FitzRoy provided observations and comments.
During the twentieth century, Chile was struck by more than two dozen major earthquakes, all with a force greater than 6.9 on the Richter scale. The strongest of these occurred in 1906 (registering an estimated 8.4 on the Richter scale) and in 1960 (reaching 8.75, more recenly estimated at 9.5). This latter earthquake occurred on May 22, the day after another major quake measuring 7.25 on the Richter scale, and covered an extensive section of south-central Chile. The giant 1960 earthquake was the largest 20th-century earthquake worldwide, a product of a fault rupture nearly 1,000 km long south of Concepcion. It caused a tidal wave that decimated several fishing villages in the south and raised or lowered sections of the coast as much as two meters. An estimated 1600 lives were lost to the 1960 earthquake and tsunami in Chile, and the 1960 tsunami took another 200 lives among Japan, Hawaii, and the Philippines.
At Valparaiso, a plate-boundary earthquake of magnitude 8.0 occurred in 1985. North of Valparaiso, a magnitude 8.5 earthquake in 1922 generated a Pacific Ocean tsunami that reached heights of 9 m on the Chilean coast near Coquimbo and washed away boats in Hilo harbor, Hawaii. Still farther north along the coast of Chile and Peru, great earthquakes with trans-Pacific tsunamis occurred are known from 1586, 1687, 1868, and 1877. Overall along the Chilean coast there have been 13 earthquakes of magnitude 7.0 or greater since 1973.
On 27 February 2010 a Magnitude 8.8 off-shore earthquake resulted in over 500 people killed, dozens missing, about 12,000 injured, 800,000 displaced and at least 370,000 houses, 4,000 schools, 79 hospitals and 4,200 boats damaged or destroyed by the earthquake and tsunami in the Valparaiso-Concepcion-Temuco area. At least 1.8 million people affected in Araucania, Bio-Bio, Maule, O'Higgins, Region Metropolitana and Valparaiso. The total economic loss in Chile was estimated at 30 billion US dollars. Electricity, telecommunications and water supplies were disrupted and the airports at Concepcion and antiago had minor damage. The tsunami damaged or destroyed many buildings and roads at Concepcion, Constitucion, Dichato and Pichilemu and also damaged boats and a dock in the San Diego area, USA. Maximum acceleration of 0.65g was recorded at Concepcion and more than 2 m of uplift along the coast was observed near Arauco.
The earthquake was generated at the gently sloping fault that conveys the Nazca plate eastward and downward beneath the South American plate. The two plates are converging at 7 meters per century. The fault rupture, largely offshore, exceeded 100 km in width and extended nearly 500 km parallel to the coast. The rupture began deep beneath the coast and spread westward, northward, and southward. As it spread, the fault slip generated earthquake shaking. The fault slip also warped the ocean floor, setting off the tsunami along the fault-rupture area.
The written history of very large plate-boundary earthquakes between Valparaiso and Concepcion probably begins with a shock near Concepción in 1562. Another earthquake eight years later set off a tsunami that led Spaniards to rebuild Concepcion on higher ground. A 1730 earthquake probably centered near Valparaiso generated a tsunami that caused flooding and damage in Japan, as did the great Concepción earthquake of 1751. Charles Darwin and Robert FitzRoy observed the next major Concepcion earthquake in 1835. The northern part of the 2010 rupture overlaps with the probable source area of the 1906 Valparaiso earthquake of magnitude 8.2. The tsunami associated with the 1906 earthquake produced damage in Hawaii, with reported run-up heights as great as 3.5 m.
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