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Re: Re[2]: 7.0 Landers Aftershock

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>From John McRaney at SCEC he reports:

The 40-km-long surface rupture was
arbitrarily divided into 3-5 km-long sections and a group of 2-3 scientists
was sent to each section to perform reconaissance-level mapping and slip
measurements.  Mapping group leaders were:  Katherine Kendrick (USGS),
James Dolan (USC), Michael Rymer (USGS), Sally McGill (San Bernardino
State), Bill Bryant (CDMG), Tom Fumal (USGS), Scott Lindvall (William
Lettis and Assoc.), Jerry Trieman (CDMG), Bill Bryant (CDMG), and Chris
Hitchcock (William Lettis and Assoc.).  Preliminary results of this first
phase of the ground mapping effort include a refinement of Tom Rockwell,
Katherine Kendrick, and Ken Hudnut's (USGS) aerial estimate of the slip
distribution.  Maximum recorded slip of approximately 3.5 m right lateral
strike-slip was recorded a few kilometers south of Lavic Lake.  Also
recognized in the field was a complex faulting pattern, with many en
echelon steps, local normal and reverse components of slip, and localized,
broad (up to 200-m-wide) zones of distributed deformation.

Fred Turner, Staff Structural Engineer, California Seismic Safety
Commission, 1755 Creekside Oaks Drive Suite 100, Sacramento, CA 95833
916-263-0582 Work Phone, 916-263-0594 Fax fturner(--nospam--at)quiknet.com
-----Original Message-----
From: TBenson(--nospam--at)lowney.com <TBenson(--nospam--at)lowney.com>
To: seaint(--nospam--at)seaint.org <seaint(--nospam--at)seaint.org>
Date: Monday, October 18, 1999 6:10 PM
Subject: Re[2]: 7.0 Landers Aftershock


>I saw Lucy Jones (USGS Cal Tech) on the news this weekend, and she reported
that
>this 10/16/99 M 7.0 was not a Landers aftershock.  This is clear now since
we
>know that the earthquake was the result of strike slip rupture on another
fault
>to the east (Pisgah), not the Emerson or similar faults.  Basically, the
>earthquake was at 29 Palms (MCAGCC), rather than Yucca Valley.  I
understand
>that they had to stop live fire exercises by the Marines, so geologists
could
>look for surface rupture.
>
>I'm sorry to say I'm not sure if your comment about "aftershocks" was
levity or
>not.  But to answer your question, there have been many Landers aftershocks
even
>this year, month and week, as defined (or reported) by USGS and others.  My
>understanding is that an aftershock is simply energy released on the same
fault
>zone as an earlier earthquake.  Since this was on a different fault to the
east,
>the earthquake this weekend was not an aftershock.  However, I don't think
you
>need to be a geophysicist to see that strike slip stress release on the
Emerson
>and associated faults, could be transferred to the Pisgah and associated
faults
>that are roughly parallel.  It is interesting to note that there are small
>segments of the Pisgah Fault that moved in 1992, as mapped on Jennings 1994
>"Fault Activity Map of California."  Other than this map, all my data is
from
>local (LA) TV, so consider the early source.
>
>Anybody know more about where there was surface rupture, and how much?  I
don't
>think we can just drive on to 29 Palms MCAGCC to look for rupture.  We
would
>more likely find unexploded ordnance.
>
>Tom Benson at Lowney Associates, (626) 396-1490
>
>____________________Reply Separator____________________
>Subject:    Re: 7.0 Landers Aftershock
>Author: <seaint(--nospam--at)seaint.org>
>Date:       10/18/99 4:04 PM
>
>     Can there REALLY be an aftershock from an EQ that happened in '92?
>
>