[CH] OT: The Chile Earthquake

AndyB (CH2060@xemaps.com)
Sat, 27 Feb 2010 16:02:37 -0500 

I post this here because I know that some subscribers would have 
specific concerns about the devastating earthquake that occurred in 
Chile early this morning.  I offer some research that I just have done.

The epicenter was just offshore Maule, Chile.  It was a magnitude 8.8 
event, and occurred over a very wide area.  It produced not only 
wide-spread earth shaking in Chile, but also a general Pacific tsunami, 
which is still in progress.

The _earthquake magnitude_ is a measure of the total energy involved. 
For the Chile quake, it is very large, M 8.8, because of the large 
earthquake area involved: a Pacific plate that is subducting under the 
South American plate (producing the Andes Mountain range} suddenly 
"rebounded", lifting an enormous amount of Pacific water. 

What is important to people and buildings is the _shaking intensity_. 
The shaking intensity of this earthquake, while severe, was 
significantly less than that of the Haitian one.

For most Chilean cities, the shaking intensity varied from "very strong" 
to "severe",  VII and VIII on the MMI (Modified Mercalli Intensity) 
scale. For the zones affected, see the automatically generated maps at

http://earthquake.usgs.gov/earthquakes/pager/events/us/2010tfan/index.html

Note that the rating for a zone is the maximum that may occur in a zone, 
so not all cities in the zone would necessarily experience this maximum.

Many newer buildings in Chile were built as "resistant" to earthquakes, 
for these the "potential structural damage" is: minimal for well-build 
wooden homes, moderate for others, and perhaps heavy for some in the 
"severe" zone.  Damage to older "vulnerable" buildings is likely to be 
heavy, particularly in the "severe" zone. There was also coastal damage 
due to the tsunami, with some homes "disappearing".

A summary of the _perceived_ intensity, as reported by victims, may be 
found at

http://earthquake.usgs.gov/earthquakes/dyfi/events/us/2010tfan/us/index.html

Fortunately, from newscasts, the Chilean government seems well prepared 
for the disaster -- although likely they will need some specific forms 
of assistance.

=== === ===
The Haitian quake, because of the small earthquake area involved, had 
only a M 7.0 rating, However, the shaking intensity was far greater that 
that for Chile. In Haiti, there were over 2 million people living in the 
"violent" quake zone (IX on the MMI scale), for which "heavy" to "very 
heavy" damage could be expected. Further, there were over 300 thousand 
in the "extreme" zone (X on the MMI scale) for which very heavy damage 
could be expected throughout the zone.

Additionally, Haiti had many buildings that were highly vulnerable to 
earthquake which were destroyed even in the less severe intensity zones.

AndyB