From: stead@skadi.CSS.GOV (Richard Stead)
Newsgroups: ca.earthquakes
Subject: Re: types of earthquake predictions
Date: 28 Dec 92 16:28:17 GMT

In article <1hb56dINN7h@jethro.Corp.Sun.COM>, barroux@licancabur.Corp.Sun.COM (Juan Carlos Barroux - Technical Consultant - Latin America) writes:
> In article 51678@seismo.CSS.GOV, stead@skadi.CSS.GOV (Richard Stead) writes:
> >
> >The 1960 Chile quake had Mw=9.6 (Mw is an open-ended magnitude scale,
> >Richter magnitude saturates at about 8.5 and cannot get larger).  It
> >is the biggest quake during instrumental seismology.  But there is evidence
> >it may well be the largest quake in several centuries.  It broke a fault
> >area 800 km long and 200 km wide, and the sides slipped about 20 meters.
> >Hard to beat that for size.  It was so big it made the earth wobble measurably,
> >and the rang like a bell for weeks.  
> 
>   I just went back from a trip to Chile and some friends told me that
> they are expecting another earthquake on the same fault as the 1960
> one. They told me that only the lower portion of the fault did fracture
> and not the upper part. They also are worried about a quake in the
> north part of the country, near the frontier with Peru. 

The fault is the subduction zone that extends along the entire west coast
of South and Central America.  Chile gets the worst of it, however.  Chile
has the distinction of experiencing more great earthquakes (M>8.5) than any
other nation in the world.

The same area that broke in 1960 also broke in 1575, 1737 and 1837.  Thus,
it is only a little more than 100 years between quakes, as with the rest
of the subduction zone along Chile.  The segment immediately to the North
broke in 1939.  The 1939 segment also broke in 1570, 1657, 1751 and 1835.
Just north of that, the next segment broke in 1647, 1730, 1822 and 1906.
Continuing north, the next segment broke in 1604, 1868, 1922 and 1943.

latitude  Geography                     last quake
18        Peru-Chile border
24        Antofagasta
 |                                      1943
30
 |                                      1906 (1928?)
33        Valparaiso, Santiago
 |                                      1939
37        Concepcion
 |                                      1960
45        Archipelago de los Chonos
54        Cape Horn

So you can see, if 100 years is typical between quakes on any segment of
the zone along Chile, then the segment most at risk currently is the segment
near Valparaiso and Santiago, which last broke in 1906 (There was an M 8.0
quake in 1928, but it was probably too small).  The other segments
are much less at risk, particularly the 1960 segment.  However, Chile's
densest population is in the vicinity of Valparaiso and Santiago, so this
is a significant hazard.  The population in the vicinity of the 1960 quake
is much smaller.  I did not mention quakes south of 45 degrees south latitude,
because that is where the Chile Rise, a major tectonic feature, intersects
the subduction zone.  Seismic activity is very low south of 45 degrees (and
population is very small).  North of 24 degrees, the seismic history is
less well-known, and seismic activity is apparently somewhat less.

Some more information on the 1960 quake:
It killed 2000 people (loss of life was small due to small population).
It caused $550 million in damage in Chile (1960 dollars).  The tsunami
caused $125 million in damage at coasts throughout the Pacific (Primarily
Japan, Hawaii and Phillipines), and killed 230 people.  Virtually every
great Chilean quake is associated with a very destructive tsunami.
The quake was preceded by large foreshocks starting 33 hours before the
main event (at Concepcion).  The most lethal quake in Chile history was
the 1939 quake which killed 28000.


--
Richard Stead
Center for Seismic Studies
Arlington, VA
stead@seismo.css.gov