Why Chile Has So Many Earthquakes & Tsunamis

The Magnitude 8.2 quake that struck off the coast of Chile on April 1, 2014, was the latest in a series of major earthquakes and tsunamis to hit that area in recent years. The undersea quake and resulting 7 ft. (2.1m) tsunami killed 7, toppled buildings, and severely damaged the Chilean fishing fleet.  Earthquake/tsunami events in 2010 (M8.8), 2007 (M7.7), 2005 (M7.8), and 2001 (M8.4) killed more than 1,000 and inflicted billions of dollars in property damage .

The most powerful earthquake ever recorded, a Magnitude 9.5, hit the coast of Chile on May 22, 1960. The monster quake triggered an 82 ft (25m) tsunami that not only battered the west coast of South America, but rolled across the Pacific Basin, devastating Hilo, Hawaii, and damaging coastal villages as far away as Japan and the Philippines. Some sources estimate 6,000 dead and $800 million in property loss (6 billion in 2014 dollars).

Why does this area of planet earth spawn so many high-magnitude earthquakes and punishing tsunamis?

One explanation is that the collision of the two tectonic plates that meet off the South American west coast occurs, in geologic terms, at a very high rate of speed. The oceanic Nazca Plate and the continental South American Plate converge in the Peru-Chile trench that lies about 100 mi (160km) off the coast. The overriding South American Plate moves eastward at 10cm a year, while the subducting Nazca Plate pushes west at 16cm/y, a closing velocity of 26cm/y (about 10 in.), one of the fastest absolute motions of any tectonic plate. The Africa Plate, for example, moves approximately 7 times slower.

This high closing velocity builds up fault line strain much faster than it does when slower-moving plates converge. Every few years, tension on the Peru-Chile fault line builds up to a breaking point. In this latest earthquake on April 1, a 100 mi. (160km) section of the fault line ruptured, allowing the Nazca Plate to ram under the South American Plate. This sudden violent action 12.5 mi (20.1km) below the ocean floor triggered the tsunami and the 8.2 earthquake, and at the same time wedged the South American Plate higher. Uplifting from frequent fault line failures continues to build the Andes Mountain Range into one of the highest in the world. During the 1960 M9.5 quake, some coastal areas uplifted as much as 10 ft. (3m).

As long as the two tectonic plates that meet off the South American coast move geologically at such high speed, major earthquakes and tsunamis will keep happening. We hope the zoning laws and building codes put in place by the governments of Chile and Peru will keep the damage and loss of life to a minimum.  

Why Did the Hill Come Down?

As of this writing, 21 people have been confirmed dead and 30 are missing in the disastrous March 22, 2014, Oso, Washington mudslide. We send our condolences to all those affected by this terrible tragedy.

At the same time, we have to ask ourselves why a forest-covered mountainside would suddenly shear off and bury an entire community of 30 homes under a 1 square mile (2.6km²) mud and debris slide 40 ft (12m) deep.

Two main reasons have been given. One is that the hill had become saturated after weeks of heavy rainfall. The rainfall in that area during the month of March was 200% of normal. Although the soil there is compacted clay that tends to be impermeable, it is believed there were cracks at the top that allowed the rain to penetrate. The other reason for the failure is that the swollen Stillaguamish River at the base was undercutting the toe of the hill. With the base of the hill weakened and the slope heavy with soaked-in rain, the hill collapsed.

After a number of landslides had been reported in that area during the prior 40 years, the US Army Corps of Engineers did a survey there in 1999 and issued a report warning of “the potential for catastrophic failure.” In 2006, a section of that same hill collapsed and blocked the course of the river. Other state and local agencies had examined the hill at various times and all concluded it was unstable. Whether the permit-issuing authorities were aware of those findings is not known. What is known is that building permits for that location continued to be issued, even after the 2006 slide.

The last compilation of world landslide statistics was posted by the American Geographical Union for the year 2010. In that year, 6,211 people died in 494 landslide events worldwide. 83,275 landslide deaths were reported for the period September, 2002 to December 2010, an average of a little more than 10,000 a year. People living in the mountains of China, India, Central America, the Philippines, Taiwan, and Brazil were the most vulnerable during that period. Landslides and mudslides often occur when intense rainfall from tropical storms and monsoons saturate hillsides that have been compromised by logging, farming, and construction. Although not as highly dramatic as earthquakes and tsunamis, landslides may be the most costly of all natural disasters in loss of life and property.

In the United States, landslide fatalities average between 25 and 50 a year, according to the Centers for Disease Control and Prevention. Using airborne Lidar, a laser-based mapping system, it is now possible to set up a national data bank on areas throughout the US that are susceptible to hillside failure, but it would be a long and very costly project. Until such a survey is done, local jurisdictions will have to rely on other methods to determine landslide-prone areas. Even knowing the possible dangers, people will still build homes below unstable hillsides, in fire areas, and flood plains. It is up to local zoning authorities to prohibit building in these hazardous places.

 

 

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