Re: What is the probability of Mount Huascaran having an avalanche occur again?

Hello, Nicole!

Estimating the time scales and probabilities of massive landslides on a 
particular mountain is not an exact science. Keep that in mind if the 
answer below starts to sound "authoritative."

Let's start with some general facts about the region. The Andes, in which 
Nvdo. Huascaran lies, are one of the most active geological areas on 
Earth, being formed by the subduction of the ocean floor under the 
advancing western edge of the South American continent. The Andes are 
rising extremely fast in geological terms, and thus are steep and very 
prone to landslides.  Also, subduction zones are some of the most highly 
active regions on the Earth for earthquakes.  The particular one that 
caused the 1970 avalanche was a 7.8. There has been another equally large 
(7.9) further south in Peru just this year that killed several thousand 
people.  

What little I know about subduction earthquakes tells me that they tend to 
be extremely violent, and occur on time scales of every 1 to a few 
centuries in any given part of a subduction zone.  We can thus expect a 
high probability of another earthquake in the Huascaran area, of similar 
size to the 1970 event, in perhaps 100 to 300 years. Will this cause an
avalanche? Almost certainly yes, but the size(s) of it(them) is quite 
arguable. There will certainly be many small ones, but predicting 
a "whopper" is much more difficult. It may depend on such items as the
recent amount of snowfall on the peak, which over a century or more is a
highly random variable. Bear in mind also that what is a "whopper" to 
tiny, frail humans, is just a sloughing of a tiny bit of "skin" to the 
massive bulk of all of Nvdo. Huascaran...essentially insignificant by 
itself in geological time/size scales.

Since you are asking specific questions on a specific mountain, the best 
thing to do is go to the experts on the ground.  They will have studied 
the region, finding evidence of many past avalanches and dating them. 
Deciphering the past history can build up a picture of how many and what 
size of avalanches have occured over perhaps the past few thousand years. 
This kind of data leads to a far better estimate than mine of the actual 
frequency and sizes of future events. The nation of Peru has both an 
Institute of Geology, Mines, and Metallurgy and an Institute of
Geophysics.  You should contact these people, because they are without 
doubt the world's experts on the region in question. Here are their
web addresses (These pages are of course in Spanish. If you do not read 
Spanish yourself, find another student in your school who does to help on 
the project, or seek the help of a Spanish instructor.):

     http://www.igp.gob.pe        (geophysics institute)
     http://www.ingemmet.gob.pe   (geology, mines & metallurgy)

You may wish to write them letters asking for reports on the Huascaran 
area. I'd be extremely surprised if there have not been several master's 
or doctoral theses done that study the area.

The USGS does have some stuff on of Huascaran in English. The moderator 
who sent me your question suggests the following link to start from:

     http://landslides.usgs.gov/html_files/landslides/slides/slide5.htm

In addition, the USGS has some good documents that can help you get a
basic understanding of landslides (not necessarily on Huascaran but in
general):  

     USGS Professional Papers #367, #1183, and #1264 
     USGS Open File Report 91-239

There is also an interesting article on the danger of landslides/lahars on 
Mt. Ranier in one of the very recent issues of "Sciene News."  While these 
stem from a somewhat different mix of mechanisms (volcanism along with 
earthquakes) it will be very instructive nonetheless.