From: stead@skadi.CSS.GOV (Richard Stead)
Newsgroups: ca.earthquakes
Subject: Re: Newly Mapped "San Jose Fault?"
Date: 15 Dec 92 00:23:12 GMT
Distribution: ca

In article <1gihhuINN78h@morrow.stanford.edu>, jeff@pangea.Stanford.EDU (Jeff Freymueller) writes:
> In article <-288709981@hpcupt1.cup.hp.com> stachnik@hpcupt1.cup.hp.com (George Stachnik) writes:
> >newly discovered "San Jose Fault" - reported in Saturday's
> >1)      The article said this fault was first identified
> >	using data from an instrument "towed behind an
> >	aircraft."  Any details?
> The instrument was a magnetometer. The fault was identified by noting a
> discontinuity in the magnetic properties on either side of a more-ore-less
> linear boundary.

> >2)      The article indicated that this fault has not 
> >	moved in a long time (1000+ years?  My memory fails
> >	me...).  Would that suggest a fault that is
> >	dangerously locked?  Or one that's dormant?  (The
> >	article characterized it, if my memory serves me
> >	well, as "active".  How can it be active if it
> >	hasn't moved in 1000 years?)  
> This I can't answer, since I haven't heard anything directly from the
> group that found this fault. "A long time" may just mean that it hasn't
> produced an earthquake in historical times.

I don't know how they can tell the age of faulting from aeromag, I assume
they did other work (at least checking historical seismicity).  There
are a couple things that can be done to see if the fault is active.
the best would be to trench it (see below), but it would also be productive
to investigate and pre-development maps of the region.  This technique
was used to help identify the two new faults in LA.  The old maps often
indicate scarps, offset streams, sag ponds, etc., that existed along
a fault trace before these indicators were oblitorated.  This is something
that an amatuer with a basic understanding of fault geomorphology and access
to old maps can do - it doesn't necessarily take a PhD.

> >	indicated that the fault can't be mapped accurately
> >	because it hasn't moved, and because the area it is
> >	in is so highly developed (unlike San Andreas which
> The discontinuity in magnetic properties can be tracked, although the
> lack of ground control makes it difficult to see the details. The resolution
> of the magnetic survey is determined by a number of factors - I would
> imagine that the location of the fault would be known to at best a few
> hundred meters, perhaps no better than a kilometer or two.

They probably did not fly this survey at more than a few hundred meters
elevation, and should know the fault location to within a few 10s or hundreds
of meters.  I don't know for sure, having not read the report, but that's
how these things normally run.  It looks likes it's time for one of the
Geophysics profs in the area to take a geophysical field methods class
out there and do a simple survey with a portable magnetometer or perhaps
magnetotelluric survey.  That would pinpoint the fault.  Then it could be
trenched and its activity evaluated that way.  Any of the .edu folks out
there know a prof that needs a simple and interesting project?

> >"real"?   How big of an event could it cause?  Incidentally,

I could hazard a guess on this - I would peg the maximum at about 7.  That's
an educated guess based on the maximum length of fault I could put in the
region you described, and assumes the entire fault breaks in the quake.
If the fault is really an extension of other faults north and south, then
maybe 7.5.  No more than that, because I'm somewhat familiar with the known
faults in the surrounding area and can't construct a through-going fault
much larger than that.

> >San Jose - the Santa Theresa fault for example.  Where can I
> >get a comprehensive map that shows all the known faults on
> >the peninsula?  (Could one be posted?)  

Andy can answer you on computer maps, but a good paper map of all the known
faults (apart from the new one) can be obtained from the CDMG.


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