Post on 04-Jan-2016
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transcript
A New Approach To Paleoseismic Event
Correlation
Glenn Biasi and Ray Weldon
University of Nevada RenoAcknowledgments:
Tom Fumal, Kate Scharer, SCEC and the USGS.
Big Question
• How do we estimate seismic hazard when we can’t 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.
0 2000Calendar Year
The Data: Paleoseismic event date pdf’s.
Correlating Events Between Paleoseismic Sites
• Time correlation is not likely to ever be entirely convincing.
• Example: two exactly overlapping uniform date pdf’s 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).
Ways to build a rupture history (1)
N. Bend/S. Bend
No pattern
Bigevents
Example from Weldon et al. (2004) Science
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.
Pair-wise joint probability range: ~3e-2 to 3e-3; -> absolute likelihoods are all small
Linking involves some rules for overlap.
Rules don’t seem to dominate results.
Example ruptures.
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 9900*d. Use measured d where available.
• Tail truncated if rupture would cross a neighboring site.• Tails can extend into creeping zone and Bombay
Beach.
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)
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.
Fewest rupturescase.
Two wall-to-wall(W2W) ruptures since AD 900.Displacementscales with length.
W2W causeserious over-prediction oftotal displace-ment. (14.7 mmean).
How muchmisfit is toomuch?
Segmentbounds(WGCEP) -frequencyof single- and multiplesegment rupturesfall outdirectly.
predicted fromslip rate Coachella
misfit
No wall-to-wallevents; 2 pre-1857 north-bendevents, onebigger.
WW:several shortevents - un-correlated.
maximum displacement among all ruptures.
Best case: 1.43 m. avg. misfit.
Max age of complete record.
Time overlapscore
Product timescore vs.displace-ment fit.
Displ. vs. # of ruptures;trend below 21 rupturesis too few.
Productscore vs. # of ruptures
Displ. scorevs. rupture number
No. Ruptures
Dis
pl. S
core
Tim
e S
core
No. Ruptures
Best time scores
Fewest ruptures, okaydispl. scores
Best displ. scores
Ensembles of scenarios => probabilities of single and multiple-segment ruptures.
20 scenarios - ~450 ruptures.
Rules for counting segments are required.
segs 1, 2
no segs
seg 2, but L1>L2
Applications of Rupture Scenarios (2)
L1 L2
Conclusions for Data Collection
• Complete count and rough event dates are most valuable
• Don’t need great dating precision (but don’t 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
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!)
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.
Pallett Cr. - Wrightwood
Pallett Cr. - Carrizo
Having rupture displacements helps.
Even with displacement measurements, P(correl) is often small.
This case: all magnitudes
equally likely: