UPDATED: Chile earthquake and tsunami on April 1; 900,000+ evacuated

Map of South American
subduction zone and significan
earthquakes. From
Susan Bilek reference listed
below at **.

A powerful earthquake off shore of Chile has generated a tsunami that has already produced six-to-seven-foot waves that have struck the beaches. The earthquake was centered about 60 miles northwest of Iquique and at a depth of about 12-13 miles. A tsunami warning has been issued for Colombia, Panama and Costa Rica, and geoscientists are working to determine the magnitude of tsunami waves at Hawaii and as far north as North America. UPDATE: Tuesday afternoon--The government of Chile has reported over 900,000 evacuated; CNN has reported a million. The major problem appears to be serious structural damage to poorly built homes.
      Chile has been subjected to powerful earthquakes several times over the past century: November 11, 1922, a magnitude 8.5; May 22, 1960 a magnitude 9.5; February 27, 2010 a magnitude 8.8, and today, a magnitude 8.0 or 8.2 (magnitudes are being revised as I write this). Charles Darwin experienced the 1835 earthquake in southern Chile in 1835, one that had a magnitude of 8.1 or 8.2. It triggered a tsunami that destroyed Talcahuano and devastated Concepcion. In this tsunami, a schooner was swept 200 meters inland. The earthquake took place in the middle of the day and inhabitants had time to run into the hills so the death toll was fortuitously low. Historical records going back to the 1500's suggest other great earthquakes.

Geometry of the plates around South America
From Wiki here

     Why is Chile so prone to these quakes? Off the west coast of Chile, the Nazca plate is diving down (being subducted) below the South American plate.  The Nazca plate is relatively young, having formed when the now-defunct Farallon plate split about 22.8 Mya split into the Nazca and Cocos plates. The plate is being subducted at a rate of 3.7 cm/year, one of the fastest motions of any tectonic plate. It dives so deeply under the South American plate that it even influences the geology and geography of Bolivia far inland to the east. The 1994 Bolivia earthquake of magnitude 8.2 occurred on this place and is renowned as the strongest earthquake occurring deeper than 300 km.

Susan Bilek's model of the role of heterogeneities
in subduction zone dynamics.

    In a nice review paper**, Susan Bilek discussed how the heterogeneity of the subducted plate causes there to be a wide variety of rupture modes along this zone, ranging from "magnitude >8 events during one century followed by smaller ones in other time periods, as well as unusual tsunami events." Her idea of the effect of heterogeneity is shown on the figure attached. The idea is that as various geographic features on the subducting plate--such as seamounts and ridges--enter the subduction zone, they change the friction in the zone. This acts, along with variations in the thickness of sediments in the overriding plate and in pore pressure in the sediments, to produce variability in the slip mechanisms along the fault.

**Bilek, Susan L., Invited Review Paper: Seismicity along the South American subduction zone: Review of large earthquakes, tsunamis, and subduction zone complexity, Tectonophysics, 495 (2010), 2-14.