A New Approach To Paleoseismic Event Correlation

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A New Approach To Paleoseismic Event Correlation

• Glenn Biasi and Ray Weldon
• University of Nevada Reno
• Acknowledgments
• Tom Fumal, Kate Scharer, SCEC and the USGS.

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Big Question

• How do we estimate seismic hazard when we cant
  prove events correlate between paleoseismic
  sites?
• Why it matters (1)
• If they correlate longer, less frequent
  ruptures
• If not shorter, more frequent ruptures.
• Why it matters (2)
• Support for future paleoseismic investigations.

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0
2000
Calendar Year
The Data Paleoseismic event date pdfs.
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Correlating Events Between Paleoseismic Sites

• Time correlation is not likely to ever be
  entirely convincing.
• Example two exactly overlapping uniform date
  pdfs six years wide give 1/6 chance that the
  events are in the same year.
• Probabilities of correlation based on
  displacement fall off with site separation (ask
  me later what can be done).

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Ways to build a rupture history (1)
N. Bend/ S. Bend
No pattern
Big events
Example from Weldon et al. (2004) Science
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Ways (2) Pearls to Scenarios

• Find all ruptures consistent at some level with
  the data
• Build a large suite of rupture scenarios
• Select likely histories using other constraints
  (slip rate, dating consistency, etc.)
• Study the properties of likely histories for
  recurrence, segmentation, etc.

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Linking involves some rules for overlap. Rules
dont seem to dominate results.
Pair-wise joint probability range 3e-2 to 3e-3
-gt absolute likelihoods are all small
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Example ruptures.
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Building Scenarios from Ruptures

• Draw from all possible ruptures until each
  paleoquake is included exactly once.
• Scenario is one possible history of rupture on
  the fault. Construct 10,000 scenarios.
• Core rupture lengths set by sites in rupture.
• Tails added by drawing around the average
  per-event displacement d, then tapering by
  9900d. Use measured d where available.
• Tail truncated if rupture would cross a
  neighboring site.
• Tails can extend into creeping zone and Bombay
  Beach.

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Scoring Scenarios

• Degree of time agreement in ruptures
• Total displacement compared to rupture prediction
  in some time.
• Recurrence rate in light of hiatus since 1857
  (number of ruptures)

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The lack of a southern SAF earthquake since 1857
constrains the probable number of ruptures in
viable scenarios. E.g., 15 rupture scenarios are
twice as likely as 22 rupture scenarios.
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Segment bounds (WGCEP) - frequency of single-
and multiple segment ruptures fall out directly.
Fewest ruptures case. Two wall-to-wall (W2W)
ruptures since AD 900. Displacement scales with
length.
W2W cause serious over- prediction of total
displace- ment. (14.7 m mean).
How much misfit is too much?
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WW several short events - un- correlated.
No wall-to-wall events 2 pre-1857
north-bend events, one bigger.
Best case 1.43 m. avg. misfit.
Max age of complete record.
predicted from slip rate
Coachella misfit
maximum displacement among all ruptures.
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Time overlap score
Displ. score vs. rupture number
Product time score vs. displace- ment fit.
Best time scores
Fewest ruptures, okay displ. scores
Time Score
Best displ. scores
Product score vs. of ruptures
Displ. vs. of ruptures trend below 21
ruptures is too few.
Displ. Score
No. Ruptures
No. Ruptures
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Applications of Rupture Scenarios (2)
segs 1, 2
no segs
L1
L2
seg 2, but L1gtL2
Ensembles of scenarios gt probabilities of single
and multiple-segment ruptures. 20 scenarios -
450 ruptures. Rules for counting segments are
required.
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Conclusions for Data Collection

• Complete count and rough event dates are most
  valuable
• Dont need great dating precision (but dont stop
  trying to get it)
• Slip-per event measurements are valuable
• New sites are most valuable in large spatial
  gaps.
• Do need geologic or geodetic slip rates

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Conclusions for Hazard Estimation

• Scenarios include earthquake location, magnitude,
  and frequency essentials for seismic hazard
  estimation
• Ensembles of likely scenarios support hazard
  calculations without having to resolve the
  per-event correlation issues
• Scenarios are data-based - the paleoseismic
  record.
• Can quantify single- and multi-segment rupture
  frequency for the whole fault (or use it to
  question segmentation!)

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Surface Slip versus Rupture Length
Relationship for reverse and normal may be linear
but may NOT for strike-slip Rate of increase of
strike-slip decreases with length for strike-slip
without apparently reaching a plateau.
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Having rupture displacements helps. Even with
displacement measurements, P(correl) is often
small.
This case all magnitudes equally likely
Pallett Cr. - Wrightwood
Pallett Cr. - Carrizo