From: stead@skadi.CSS.GOV (Richard Stead)
Newsgroups: ca.earthquakes
Subject: Re: Sea Earthquakes in SoCal
Date: 4 Sep 92 14:38:06 GMT

In article <1992Sep4.002034.28539@gordian.com>, johnk@gordian.com (John Kalucki) writes:
> The devastation in Nicaragua this past week has a number of folks
> here in SoCal thinking about a similar event occuring here. I, for
> one, live only 300 yards from the Pacific. Observationally my house
> is just barely 4 feet above higher high water (+7.1 high tides) and
> there isn't a whole lot physically between it and the sea.
> 
> I understand that, in general, a long fault is need to cause a
> large earthquake. My fault chart for CA (1975!) shows several large,
> active, undersea faults in the area. The San Clemente fault (adjacent
> to the island of the same name) appears pretty long. The CalTech
> tables also show an occasional quake out there. This week had
> one 11 miles south of Avalon, Catalina.
> 
> Could this fault, or others like it generate a 6.5+ quake? Would a 
> large sea swell be the result? What type of fault is the one
> in Nicaragua (slip-strike, etc.)?

That is a hard pronouncement to make.  Most of these faults are dominantly
strike-slip, and as such will not directly cause a tsunami.  The ocean floor
has to be moved vertically to cause a tsunami.  The one in Nicaragua is simply
a standard subduction event - the ocean floor moves up on the Nicaragua side
of the fault, and down on the ocean side.  The movement of the fault is quick,
and that means all the water above moves as well.  Then the water rushes
back to its mormal level, causing a "shallow water wave", that is, a wave
so deep that the ocean seems shallow to it.  The speed of shallow water waves
is determined by the depth of the water - out in the deep ocean, tsunamis
travel at a few hundred mph, but they move the water only a little, so ships
in the deep ocean are completely unaffected.  However, as the waves approach
land, they have to slow down (the water gets shallower) - the wave begins
to pile up, because it is moving slower in front and faster behind.  It builds
to the point that it "breaks" (the path of the water is no longer closed -
the water on top moves faster than the water at the sea bed).  The wave speed
at this point is no longer limited by depth and the water will rush onto the
land at speed exceeding 100 mph.  Note that this is not your usual sea wave
where you would expect the wave to go by and the water level to return to
normal.  It is more like a step - there is a wall of water rushing on land,
and the water for miles behind the wall is as high as the top of the wall.

So you are right to be concerned about tsunamis.  They are nothing to mess
with.  They usually do give a bit of warning however - generally you will
see a relatively rapid draw down of the water level - well below low tide
and at an unusual time.  It is shortly after this draw down that the tsunami
will hit.  (the draw down will take several minutes to an hour).

Also, the faults offshore can definitely produce M7 events.

Even though they a mostly strike-slip, they can still cause a tsunami by
inducing large undersea landslides.  That is really the main concern
in the s. CA borderland.



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