1857 Rupture

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Kate Scharer Glenn Biasi Tom Fumal Ray Weldon
1857 Rupture
Missing or extra paleoseismic events do they
matter?
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Issues to address

• There are missing events in paleoseismic
  records how many depends upon the quality of the
  site, and the number and diversity of the
  exposures. A good, well studied site probably
  misses about 1 in 10 even a cursory
  investigation of a decent site will probably find
  more than half the events.
• A more serious problem is the over-interpretation
  of paleoearthquakes individual pieces of
  evidence that can be interpreted as a
  paleoearthquake can be found at almost every
  stratigraphic horizon, so the number and quality
  of the evidence at each stratigraphic horizon
  must be assessed to distinguish the real
  earthquakes.
• Careful study of a good site produces results
  robust enough to answer critical questions about
  earthquake recurrence.

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Wrightwood Earthquake Chronology
105 years (10-300)
120 years (10-210)
Average recurrence (ranges)
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Old Section Intervals
EQ 1

EQ 2

Int 1-2
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0.06 cm/yr 15 yr/cm
Peat accumulation rates
Example of a missing event Between 1300 and
1400 AD, there was virtually no clastic
sedimentation at Wrightwood, the swamp dried up,
peat stopped accumulating and the ground surface
was bioturbated. Clearly if there was an
earthquake at the site we would have trouble
seeing it.
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0.06 cm/yr 15 yr/cm
Peat accumulation rates
At Pallett Creek (20 kms to the NW) and at
Pitman Canyon (25 kms to the SE) there is a
paleo-earthquake at about 1360 AD, with the age
range shown as the dashed pdf to the left.
Because this event seen to the N and S cannot
correlate with other events at Wrightwood, the
stratigraphic hiatus, and this time interval
corresponds to the longest interval in the
chronology, we are quite certain the event
occurred at Wrightwood but cannot be seen.
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Individual pieces of paleoseismic data are often
equivocal this leads to interpretation of too
many events.
Possible event horizon if layers above 570 are
folded
590
Inferred event horizon offset occurred in peat
570, debris flow c571 incompletely buried scarp,
silts and peats above 580 draped the scarp that
was finally buried by debris flow above 590.
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Did the event occur at peat 590? Or at 592? Or
are there two events? Could the left crater
wall, formed in 590, have grown up to 592 by
later shaking, compaction in the crater, wetting
and drying?
Again, not every call is simple.
Craterlet Charleston, 1886
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1. Photographing
3. Map on prints at 17
2. Interpretation
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Catalogue of deformation (excerpt)
Scharer et al., in review
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Catalogue of deformation (excerpt)
Scharer et al., in review
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(Scharer et al., in press BSSA)
sum of rank
rank sum - obs
observations
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(Scharer et al., in press BSSA)
sum of rank
rank sum - obs
observations
11 preferred, 3 maybe, 15 very unlikely
paleoearthquakes
Only about 10 of the event indicators are
associated with nonevent horizons BUT about 50
of all horizons with gt 1 piece of evidence are
not earthquakes, SO you need a thorough study to
get a useable record, and there will always be
some events that are not certain.
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Lets see how much the uncertainty matters, by
asking a simple question is the Wrightwood
series of ground ruptures consistent with a
Poisson process?for Dave Jackson, this is the
recurrence of ground rupture at one point on the
fault individual earthquakes could have been
large or small, centered to the north or south,
whatever. We are working on turning this data
into earthquakes, but I will not talk about our
correlation approach today
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Percentage of tests that are more regular than
Poisson
Splice
2006
6000 BP
young section
old section
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Percentage of tests that are more regular than
Poisson
Splice
2006
6000 BP
young section
old section
98
80
Not poisson
poisson
29 events 98 of possible series are
inconsistent with poisson at the 80 confidence
level
Method of Biasi et al., 2002, BSSA
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Percentage of series that are more regular than
Poisson (at 80 confidence level)
6000 BP
2006
young section
T
63
WW Record
81
WW PC Event T
Missing events (can) make a difference,
especially in a short series.
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2006
6000 BP
young section
old section
88
Percentage of tests that are more regular than
Poisson
380
73
Remove earthquake 380 (/- other unlikely events)
95
Retain 380, remove others
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Percentage of tests that are more regular than
Poisson
2006
6000 BP
young section
old section
98
98
88
Once we get a long series, the presence or
absence of a single event doesnt matter even
the presence or absence of all questionable
events matters little.

• Very, very unlikely to be random.

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Back to the recurrence of what. We know that
there are different kinds of earthquakes
represented however, it is hard to understand
how combining a set of Poisson series conspires
to make a pretty nonPoisson behavior. So, what
kind of time dependent models are consistent with
the data?
combined
old
young
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Empirical CDF
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Lognormal Distribution (Savage,1991)
Avg. lognormal recurrence interval 86 years (68
110)
Method of Biasi et al., 2002, BSSA
Lognormal COV 0.62 (0.85 - 0.49)
30-yr conditional probability 35
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86 year RI (68 110)
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0 20 40 60 80
youngest
Sum of Rank
Number of indicators
All layers
oldest
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Earthquake Quality vs. Debris Flow Thickness
(Scharer et al., in review)
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Summary for Policymakers

• the following levels of confidence have been
  used to express expert judgements on the
  correctness of the underlying science
• Very high confidence at least a 9 out of 10
  chance
  of being correct
• High confidence 8 out of 10

the following terms have been used to indicate
the assessed likelihood, using expert judgement,
of an outcome or a result Virtually certain gt
99 probability of occurrence, Extremely likely
gt 95, Very likely gt 90, Likely gt 66,
More likely than not gt 50, Unlikely lt 33,
Very unlikely lt 10, Extremely unlikely lt 5.
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3. Calibration and trimming
1. Physical pretreatment
2. Chemical Pretreatment
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From peat dates to
paleoearthquake event ages
Peat a

Peat b
Stratigraphic order

EQ ab
Calendar Years (AD)
Methodology in Biasi et al., 2002
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1857 Earthquake
http//wwwflag.wr.usgs.gov/USGSFlag/Data/maps/Cali
fornia.gif
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1812 Rupture
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Need many 14C dates
Examples peat site (Wrightwood)
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Liquefaction
Craterlet Charleston, 1886