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Fault Segmentation: User Perspective

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Fault Segmentation:User Perspective

Norm Abrahamson

PG&E

March 16, 2006

- Pre ~1985
- Used 1/3 to 1/2 fault length for the characteristic (max) earthquake
- Implies segmentation but unknown segmentation points

- 1985-1995
- Geologist identified segmentation points
- Used full length of segments for characteristic earthquake
- Typically considered segmented or unsegmented cases through logic tree (e.g. epistemic uncertainty), but not aleatory variability in segmentation

- 1995 - Current
- Begin to include aleatory variability in segmentation (sometimes individual segment rupture, sometimes multi-segment ruptures

- Truncated exponential model for mag pdf is sometimes used as proxy for segmentation

- Fault Models
- Need rates of ruptures on faults
- Develop credible models of the fault behavior in terms of segmentation that are consistent with observations

- Avoid Bias
- Should not include intentional conservatism
- Not clear what is conservative

- Should not include intentional conservatism
- Uncertainty
- Uncertainty in segmentation is handled using alternative models of fault behavior
- Should not just be single segments ruptures and all segment ruptures

- Uncertainty in segmentation is handled using alternative models of fault behavior

- Hecker evaluated variability of surface slip at a point
- Found that the truncated exponential model will produce much larger variability than observed
- Concluded that truncated exponential model is not applicable to individual faults

- Earthquake probabilities for ruptures are useful, but they need to be converted to PSHA inputs
- More useable information is the equivalent slip-rate that for each rupture (single segment or multi-segment)
- Allows direct input into standard PSHA codes

- Forward Modeling of expected observations of slip at a point
- Prob (M) (from mag recurrence model)
- Prob (rupture to surface given M)
- Prob (rupture past site given Rup Length(M))
- Prob (amount of surface slip given M)
- Prob (detection) including effect of adding slip from non-detected events to the detected events

- Magnitude recurrence models
- Truncated exponential
- Youngs & Coppersmith Characteristic
- Max Mag = 7.5, MinMag = 6.0

- Average Displacement
- Use Wells and Coppersmith for all fault types
- log(AD) = -4.8 + 0.69M ± 0.36 (±0.82 ln units)

- Variation in Displacement along Strike
- Use results from (ref?)
- Sigma along strike approx 0.7 natural log units

- Total standard deviation of slip-at-a-point
- Sqrt(0.822+0.702)=1.07

- Variability of slip at a point must be much smaller than expected using global models
- The Y&C mag recurrence model can give C.V. values similar to observed values if small variability in slip for given mag is used.
- The truncated exponential mag recurrence model gives much larger C.V. values than observed even with reduced variability in slip given mag.
- The truncated exponential model is not consistent with the observed C.V. values.

- Use Monte Carlo
- Population C.V.=0.4
- Same sampling as in data set

- Result:
- Average C.V.=0.41 (small bias toward larger CV)

- Smallest slip is more often the most recent event
- Accommodate effect by overprinting
- Calibrate levels for overprinting (e.g. 40% of amplitude) using observed frequencies