1857 Rupture

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1857 Rupture
300 km
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1812 Rupture
300 km
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1690
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By pushing events into the tails of the age
distributions and by careful choices one can make
a highly periodic, characteristic-looking
earthquake model (1812 does not seem to work, but
this model is supported by slip rate and
displacement data).
However, one can make many other models,
including some that are essentially random in
space, time and rupture length. In fact, the
more variable the model, the easier it is to fit
the age data because the ages tend to vary
considerably from site to site, in a way that
makes regular models difficult to fit.
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Conditional Probability Depends on Scenario For
30 years Poisson model with fewest quakes
20 Maximum reasonable Poisson
model 40 Periodic (lognormal) model North
Bend event 10 South Bend
event 60 Fault Models Depend on
Scenario Poisson behavior supports models in
which fault processes are most important (e.g.
self healing pulses). Periodic behavior supports
models with stress thresholds (e.g. time or slip
predictable models).
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Prehistoric earthquakes are represented at a site
by probability density functions on age. To
correlate sites we must compare these pdfs from
site to site. This is the Wrightwood record.
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Overlapping pdfs from 4 sites can be correlated
by progressively linking sites. These 4
paleoquakes can represent 4 different
earthquakes, a single earthquake and many
different 2 to 3 earthquake combinations in
between
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Example of the pdfs that contribute to a single
rupture. In our analysis we allow single sites
to be skipped if there is no evidence at that
site, but the next site has overlapping evidence.
We assume that it is possible that the evidence
was missed or not recorded. This parameter can
be adjusted in the analysis (ie 0 skips, 2 skips,
etc.)
Because pdfs have long tails, the choice of a
minimum overlap criteria greatly affects how many
ruptures you can make. This parameter is
adjustable in the analysis.
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Once we have all ruptures, we string them
together to make scenarios ie possible
earthquake histories that describe the complete
paleoseismic dataset. This is done by drawing
randomly from our pool of ruptures, removing all
other ruptures that share site events already
chosen, choose again, etc. until all site data is
used to make a possible history.
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The upper part of the record cannot be run until
we combine historical and C-14 data
Before about 600 AD there is not enough data to
compare sites.
1857
1812
1690
This is extremely preliminary to show that we
have a program up and running!!!
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We score each scenario 3 ways 1) Degree of
total age overlap, 2) consistency with site
displacement (each rupture has a simple slip
distribution), 3) slip rate/moment rate by
summing vertically at all points). So far very
few possibilities match the data well there is
drop off after about 25/50,000 scenarios. This
may mean that we will end up with a relatively
small number of possible histories.
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How will we use the possible event
histories? 1) Simply revise the existing fault
segmentation models to reflect the best available
data. 2) Take a Working Group 02 approach with a
lot more branches weight and moment balance the
branches. 3) Use a relatively full suite of
likely histories to run a large number of ground
motion models and do statistics there.
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Middle ground approach is likely to be most
fruitful. I.e. extend Bay Area approach to have
some 25-50 scenarios, fix details to back
calculate each segment RI, an interaction
coefficient (likelihood that neighbor goes, and
make sure the total is moment balanced (weights
can be based on likelihood ranking using our 3
criteria, or an expert opinion system).
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• Data needs
• complete set of site pdfs for all paleoseismic
  events (done for the Southern San Andreas but
  will need to be constructed for the Northern
  after the basic data is compiled).
• b) complete set of ruptures for the San Andreas
  (done for the Southern
• requires (a) for the Northern).
• c) all available displacement per event data for
  the San Andreas (largely compiled for the
  Southern San Andreas in progress for Northern)
• d) slip rates for all paleoseismic sites. While
  slip rates are available for the entire fault,
  there are a number of studies in progress
  (including several in the Bay Area, the San
  Bernardino portion of the San Andreas by McGill
  Weldon and Littlerock on the Mojave portion of
  the San Andreas by Weldon Fumal) that we would
  like to include in the analysis.
• e) complete set of weighted rupture scenarios for
  the San Andreas (almost
• done for the Southern, and the methodology will
  make doing the northern
• easy once we've constructed pdfs for the site
  event data).

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