“Great earthquakes” of Mw 8.0 or more occur only about once a year and stronger earthquakes are scarcer still. The most powerful quake recorded in the last century or so, when accurate estimates of magnitude have been possible, was a truly massive event estimated by the United States Geological Survey to have had a magnitude of Mw 9.5.
This enormous earthquake struck 60 years ago, off the western coast of South America—part of the “Ring of Fire” and one of the most seismically active regions in the world. It was a typical subduction zone earthquake, occurring the near the shore at a focal depth of approximately 33 km (20 miles) and about 900 km (435 miles) south of the Chilean capital, Santiago.
Shaking lasted 10 minutes or so as more than 1,609 km (1,000 miles) of fault ruptured along the subduction zone at the interface of the Nazca and South American plates—one of the longest ruptures ever reported. The main quake was accompanied by nine fore- and aftershocks greater than Mw 7.0; weaker aftershocks continued for a month.
The Associated Press reported that recording needles in Buenos Aires, Argentina, “jumped off the recording paper at the start of the quake so that scientists could not register the violence.” More than 200 km (124 miles) from the epicenter the shaking caused a seiche on an Argentinian lake, and 47 hours later it likely triggered an eruption of Puyehue, a volcano in the Andes of southern Chile. Seismic waves from the mainshock were recorded traveling around the globe and the planet was evidently shifted slightly off its axis, shortening the length of the day by 1.26 microseconds.
After a series of foreshocks, many people were fortunately outside their buildings when the mainshock struck midafternoon. Nevertheless, almost 2,000 people lost their lives and 3,000 people were injured. There was devastation throughout Chile, particularly along the coast from Concepción to Isla Chiloe.
The town of Puerto Montt was devastated, for example, and the village of Tolten was almost completely reduced to rubble. About 145,000 homes were destroyed or damaged and half of the buildings in Valdivia were rendered uninhabitable. Landslides destroyed colonial Spanish forts in the city of Valdivia and dammed waterways throughout central Chile. Major flooding and disruptions in telecommunications services hampered rescue and recovery efforts.
Just 10 to 15 minutes after the mainshock a tsunami came ashore in the vicinity of Lebu, where runup was measured as high as 25 meters (82 feet). About 800 km (500 miles) of the coast from Concepcion to the south end of Isla Chiloe was impacted by the tsunami, and debris was carried as far as 3 km (2 miles) inland in places.
The tsunami also impacted much of the Pacific Basin, with buildings damaged and lives lost as far afield as Hawaii and Japan; even Easter Island and Samoa saw runups exceeding 4 meters (13 feet). AIR’s recently updated Earthquake Model for New Zealand incorporates the first probabilistic trans-ocean basin tsunami model because large far-field tsunamigenic earthquakes like Valdivia could impact that country, even from as far away as South America.
AIR estimates that if the Mw 9.5 Valdivia earthquake were to occur today, insured losses arising directly from shake damage and excluding demand surge would be more than USD 18 billion.