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: