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Re: Mistreatment of Seismology Issues

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Dear All,

I just couldn't resist throwing my two cents in.  I may have come into
this string a little late, but here goes.  When determing the risk at
any site not only one fault is considered but all the faults with a
reasonable distance.  This called a probabolistic approach.

The acceleration (pga) at any site due to any fault is derived from
attenuation equations.  Ricter scale the measurment of energy
realeased after an earthquake is not even used to consider the energy
in an earthquake. Today they use a term which expresses energy
potential or Moment magnitude.  The moment magnitude scale is similar
to the richter scale magnitude but not exactly. The energy realease is
a funcrion of te length of fualt and the measured slip rate of the
fault.

The code level earthquake UBC is the earthquake that has 10% chance of
exceedance in 50 years.  This corresponds to an earthquake that occurs
about every 475 years from any adjacent fault.  What this means that
if you consider that all fualts have quakes everyday, and there are
many low magnitude quakes, but rarely high magnitude quakes. The PGA
at this site corresponds to the highest possible PGA in a 475 year
period with 90 % certainty.  There is a 10% chance that a higher PGA
might occur one for example that might correspond to a 1200 year event. 

Areas in the San Fernando Valley may have PGA's of .55g or greater. 
When compared to Bakersfield you will see PGA's at .12g or San Deigo
at .25g.  All of the sites have been grouped into one seismic zone 4.
However the mathematical risk is much less in San diego than
Northridge or Chatsworth.

Structural Engineers still design with forces.  We apply a spectrum of
accelerations to our models to derive different forces but we are
still using forces.

Ken Tarlow


---Yank2002(--nospam--at)aol.com wrote:
>
> On 10-23-1998 Ted Smith of  California Department of Conservation
wrote:
> 
> > Richter magnitude doesn't tell you anything about distance to the
fault or
> > epicenter, subsurface conditions, or your structural design
parameters.
> > Thus, were someone to attempt a simple declaration that the
potential for a
> > 7.5 M earthquake is seismically unacceptable, you'd have to ask
"At what
> > distance, for what ground conditions, and for what type of
structure,
> > occupancy, and use?" This essentially is the reason that the state
codes
> > (Seismic Hazards Mapping Act, for example) use the term
"acceptable level of
> > risk" < 
> 
> To put it in a very simple way, Ted Smith says that there are four (4)
> parameters which must be considered to determine the seismicity
issues which
> should be taken into account for structures built in California. 
> 
> 1. distance from the fault or epicenter
> 2. subsurface conditions
> 3. structural design parameters (type of structure)
> 4. occupancy and use for the structure.
> 
> I am looking at the "Fault Activity Map of California and Adjacent
Areas"
> published in 1994 by the Department of Conservation, Division of
Mines and
> Geology, compiled by Charles W. Jennings. 
> 
> To begin with, from the de-facto seismology point of view,
California is
> divided into two parts. Part One: All areas of California west from
the San
> Andreas fault, and Part Two: The remaining areas of California east
from the
> San Andreas fault. The reason for this division is: about 95% of all
active or
> inactive seismic faults in California find themselves "west from the
San
> Andreas fault". The exceptions are: The Greater San Francisco Bay
Area East,
> White Wolf Fault, Garlock Fault, and numerous historically active
and inactive
> faults north-east from Yucaipa-Cabazon (near Riverside). 
> 
> The "Fault Activity Map of California" distinguishes the following
types
> (kinds) of faults:
> 
> 1. Late Quaternary Period / Historic; Years Before Present (YBP) = 200
>     Evidence of displacements during historic time exists
> 2. Late Quaternary / Holocene; YBP = 10,000
>     Evidence of displacements during this time exists
> 3. Late Quaternary / Pleistocene; YBP = 700,000
>     Evidence of displacements exists
> 4. Early Quatemary / Pleistocenel; YBP = 1,600,000
>     Evidence of faults within Sierra Nevada exists; but is not
restricted to
> Foothills fault system
> 5. Pre-Quaternary; YBP > 1,600,000
>     There is evidence of recognized displacements. Faults are not
necessarily
> inactive.
> 
> Looking at the "Fault Activity Map of California" with a magnifying
glass or
> without (the map is about 5'-6" ft tall and 4 ft wide), one can
clearly see
> that there is not one point on the surface of the "west of the San
Andreas
> part of California" that is located further than 10 miles away FROM
ANY MAP
> IDENTIFIED FAULT. It means that there is not one-100 square miles
(10 miles by
> 10 miles) land surface in "western" California which is "fault-free"
(immune
> to seismic faults, immune to a possibility of an earthquakes
occuring at any
> time, anywhere in "western" California). One does not have to have an
> overblown imagination to see "western" California as an irregularly
formed
> spider web consisting of hundreds of seismic faults stretching from
the
> Mexican border to Mendocino and beyond. All these faults can be
designated as
> "non-San-Andreas-faults". They can generate earthquakes of magnitude
about 6.6
> / 6.8 anywhere in "western" California. 
> 
> Just to make things more interesting, to the best of my recollection
(which
> can be varified very easily), the City of Northridge was not
positioned on an
> active or inactive fault, or on any KNOWN active or non-active
fault, as per
> the U.S. Geological Survey or Department of Conservation maps. It
means the
> Northridge fault was a newly "discovered" fault that should not have
been
> there to begin with, and which could not be detected by current
methods used
> by modern seismologists. The lesson of this event teaches us that
regardless
> of the yet undetermined type of structure which is to be built in a
certain
> yet undetermined locality, the magnitude of future earthquake on map
> identified or unidentified fault anywhere in California could follow
a similar
> traumatic experience. The only exception is the San Andreas "tunnel"
> (corridor) where the expected magnitude is much higher.
> 
> Now, let us look at earthquakes occuring ON the San Andreas fault. 
Since the
> San Andreas fault is a major fault historically capable of generating
> earthquaes up to 8.6 (or more), this fault is prominantly displayed
and
> identified on the Fault Activity Map of California. The magnitude of
shaking
> at any given locality, some distance away from the fault but caused
by seismic
> energy release of the San Andreas is inversely proportional to the
distance
> from its epicenter (either lineary, parabolically, elliptically, or
anywhere
> in between). Example: The investigated location (say, Pasadena) is
35 miles
> away from San Andreas M7.4 quake at its epicenter. The distance
between the
> epicenter perpendicularly to the "long axis of the fault" and the
point (on
> the undefined shape line/curve) where the magnitude of the quake
equals
> zero/near zero is (say) 280 miles. Mathematically expressed: (M7.4 x
> (280-35))/280 = M6.5 using linear magnitude dissipation (it is an
unreasonably
> rosy picture). When parabollic / elliptical magnitude dissipation is
assumed
> (much more realistic assumption), the resulting magnitude in
Pasadena would be
> M6.9 / M7.2. (see Properties of Parabolla / Ellipse in any "steel
book"). Thus
> we can establish the magnitude of shaking at any location in
California due to
> the San Andreas fault seismic energy release mathematically.
> 
> The Tunnel -  By comparing numbers of earth shakings generated by
the San
> Andreas fault anywhere within say, a 70 mile wide tunnel running on
top of the
> San Andreas (35 miles on each side of the San Andreas longitudinal
axis)
> against expected earthquakes magnitude in the "western" part of
California, it
> becomes obvious that California, from the structural engineering
point of view
> should be divided into three parts.
> 
> Part 1.  The land areas within say, a 70 mile wide corridor running
on top of
> the San Andreas longitudinal axis. Minimum design magnitude should
be greater
> than 7.3. (Historical earthquakes that occured within this corridor
area are:
> San Francisco, 1906 - M8.2; Kern County, 1952 - M7.7; Loma Prieta,
1989 -
> M7.1; Landers, 1992 - M7.3). 
> 
> Part 2.  All areas west from the San Andreas fault (reasonable design
> magnitude should be somewhere around M6.8) except for the corridor
(tunnel)
> area described under Part 1. 
>  
> Part 3.  Relatively small areas designated as "exceptions" in the
fifth
> paragraph of this message. The "exceptions" require individual
treatment as
> far as the magnitude of the earthquakes is concerned.
> 
> What is interesting about the myth of "acceptable level of risk"
concept is
> that after you took a walk, after the Northridge disaster, along
Ventura Blvd
> in Sherman Oaks or Encino which are located about 10 miles south from
> Northridge (all three communities lie within the San Fernando
Valley, near Los
> Angeles), and observed hundreds of badly damaged or partially
collapsed
> commercial structures, then walked along the side streets where many
single
> story wood construction residences and almost EVERY 2, 3 or 4 story
apartment
> structure (and there were hundreds of them) were red-tagged and
every tenant
> was forced to leave the area, you knew that something very basic is
wrong with
> the concept of "acceptable level of risk". In my judgement, the
codes under
> which all these structures were designed were wrong, and in many
cases, they
> were deadly wrong. For all practical purposes, many of these areas
became
> ghost towns until they were rebuilt which took on the average 9-12
months or
> longer. I do not want to talk here about collapsed bridges and other
"heavy
> duty" damage all over the Los Angeles county as everyone knows what
happened
> by having watched television after the event. One characteristic of
the damage
> was clearly visible: the uneven distribution of damage was very much
related
> to soil characteristics which differed from area to area (GEOLOGY
part of the
> problem). 
> 
> The above comments are related to Ted Smith's positions 1, 3 and 4.
With due
> respect to his position and experience, my opinion still stands that
the
> "acceptable level of risk" concept is misleading, harmful and
meaningless. 
> 
> What remains to be determined is the "subsurface conditions" (Ted
Smith's
> position # 2). Since I am not a geologist I cannot say anything
about the
> subject. However, it seems that under the worst but realistic and
proven
> seismic conditions as described above, the "subsurface conditions"
is the only
> variable that has to be individually established for every structure
built in
> California. 
> 
>  
> 
> 
> 

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