From: stead@skadi.CSS.GOV (Richard Stead)
Newsgroups: ca.earthquakes
Subject: Re: Questions raised by SoCal Activity
Keywords: usgs refs, landers
Date: 17 Aug 92 00:40:52 GMT

In article <jester.713904311@crash>, jester@crash.cts.com (Ken Bibb) writes:
> 1)  Refer to book _Earthquake_Hazards_in_the_Los_Angeles_Region,
>     USGS, 1985, map pages 34 and 35 showing "...1970 to 1981 record of
>     well-located epicenters of M>=3 earthquakes...".  Note the number
>     of epicenters along the Desert Hot Springs-Yucca Valley-Landers
>     alignment, so well known now since June 28.  Also note the cluster
>     of epicenters around Lat N 34 35' Long W 116 18' compared to
>     recent activity.  What would a similar map for 1981 to 1991 show?
>     If the 1970-1981 level of activity was maintained through the
>     80's, should such areas of high seismic activity be placed on some
>     alert basis?  It appears that the June 28 earthquake was
>     telegraphed for several decades.  Are there other areas in

Not really.  That zome of faulting through the Mojave is one of the most
seismically active areas of CA.  The zone shows up as early as sytematic
maps of seismicity were available.  I'm looking at a map of S. CA quakes
of 1932 right now - the zome is clearly visible, despite the limited
location accuracy possible with the few instruments available at the time.
That's the earliest year for which such observations are available.
So the association of the large quake with the many smaller ones is just that
that region has very high seismicity at all magnitudes.  Not far from the
Landers quake, there was a big quake in 1947 - the Manix quake.

>     California where similar long-term clusters of epicenters might
>     indicate future large-scale earthquakes (other than Long Valley
>     Caldera)?  Are such observations a part of the development of a
>     method of seismic prediction?  Why is a relatively quiet portion

No.  only if seismicity turns on suddenly or dramatically changes in the
number or magnitude of events should there be any concern.  This area
has continued high seismicity - since it is always there, how can it be
used to predict?

>     of a fault considered to be a risk when the Landers experience
>     shows a great deal of activity may instead be the precursor of a
>     major quake?

Andy answered this already.

> 2)  At the end of the California portion of "The Making of A
>     Continent" there is a computer generated map with a 'clock'
>     showing millions of years.  This map proposes that in about 3
>     million years all of California west of the Sierra Nevada fault
>     and our recent Desert Hot Springs-Yucca
>     Valley-Landers-Barstow-Ridecrest line of epicenters will swing out
>     on the Pacific Plate and drift off to the north.  Why, the, all of
>     the media attention to northward movement just westerly of the
>     currently-designated San Andreas Fault?

What it shows is a theory - one that is in "vogue" and receiving a lot
of interest from tectonicists.  It is certainly a good theory, but it is
only that.  One must resist the temptation to view such graphics as
depicting the actual future.  The interest in the movement of the San
Andreas is due to the known and well-measured movement along that fault.
And the long history of movement along that fault as recorded in the
surrounding geology shows about 400 km of movement in the last 5 million
years.  Thus the San Andreas is the known and continuing threat and
is where the majority of the plate motion is taken up.  This will
continue to be true as long as you and I live and for many centuries
thereafter.  So why should the media care what _might_ happen in a few
million years?

> 3)  Can you give me references for the role of plate tectonics in the
>     creation of the Sierra Nevada, Owens Valley, White and Panamint ranges
>     and Death Valley?  Note:  The Front Range of the Rocky Mountains also
>     lines up between the Mendocino and Murray fracture zones projected
>     eastward.  Has the eastward movement of the Sierra Nevada stopped?  Is
>     that why the beginnings of movement with the Pacific Plate?

A good layman's book on CA geology is "Geology of California" by Norris and
Webb (publisher is Wiley, original copyright is 1976).  Plate tectonics
is responsible for virtually all crustal movements on earth.  In the case
of the Basin and Range (the province that all the features you mention belong
to, and it extends through Utah and north into Idaho), that region is
undergoing thinning and extension.  The Sierra Nevada has not been moving
east in this context - it has been and continues to move west.  The ranges
and basins are horsts and grabens in an extensional regime (there also
half-grabens).  The difference between spreading of the basin and range
and the lack of such spreading in the Mojave is taken up along the Garlock
fault.  As to the lineup of the Rockies and the Fracture zones:  1)
the lineup isn't very good as far as I can tell  2) The rockies have had
several periods of orogeny, but these all pre-date the meeting of the
Pacific and North American plates  3) Any lineup would be purely coincidental
since the locations of those fracture zones have moved dramatically relative
to North America during the last 15 million years.

A good reference for the tectonics of the meeting of the Pacific and North
American plates is "Implications of Plate Tectonics for the Cenozoic
Tectonic Evolution of Western North America", Tanya Atwater, Bulletin of
the Geological Society of America, vol 81, p. 3513-3536, 1970.  It's a
technical paper, but well-written and I believe accessible to the layman.

> 4)  A glance at the map of North America shows:
> 	a) a row of volcanos inland from a subduction trench from the
> 	   Straits of Juan de Fuca to Cape Mendocino

Atwater's paper mentions this.

> 	b) The North American Plate overrides the Pacific Plate and
> 	   then is carried back eastward on the Pacific Plate from
> 	   Cape Mendocino to Point Concepcion

North America is not "carried back eastward on the Pacific Plate".

> 	c) From Point Concepcion to the Salton Basin is
> 	   faulting--sliding along a subducted fracture zone

No - strike-slip faulting has nothing to do with subduction.  It is a
fundamentally different type of plate boundary.  Also the relative
motion between North America and the Pacific makes the SanAndreas
a transform fault, but the orientation of NA-PAC transforms is
very different from PAC-JDF transforms (JDF fro Juan De Fuca plate).
I could go into spherical geometry and velocity vectors at this point,
but I'll spare you.  Just suffice it to say that each of the three
plates is moving independantly, but between any two plates of any
relative motion there is a unique direction (imagine parallel lines
between the plates) along which transforms can form.

> 	d)  the Salton Basin/Gulf of California spreading, with a
> 	    portion of the Cocos (?) Plate carrying northern Mexico. 

Yes, the Salton Trough/Gulf of California is spreading.  No, the Cocos
Plate is an independent oceanic plate, much like the Juan De Fuca plate.
it is subducting under Mexico, which is part of the North American plate.
It is spreading away from the Pacific Plate.  Only the North American plate
is "carrying" Mexico (or Washington and Oregon, for that matter) anywhere.

>     A reference, please, for the location of the East Pacific Rise
>     under California.

No such reference - mainly because the East Pacific Rise is not under
California.  the East Pacific Rise is a manifestation of the relative
motion of the Cocos and Pacific plates.  It stops where the Cocos plate
stops.  At that point, the relative motion of the Pacific Plate and
North America is manifested along the contact between those two plates.
The meeting of the three plates is called a "triple junction".  triple
junctions are special features of plate tectonics and invaluable for
plate reconstruction and plate motion determination - they have well-defined
geometric constraints.


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