Summary of Shoreline Fault Lloyd Cluff and Norman Abrahamson

PG&E Geosciences Department June 10, 2009

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Summary of Shoreline Fault

Alignment of epicentersoffshore DCPP

Alignment

DCPP

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2008 USGS Marine Survey Area

Blue – 800 m Seismic/ Magnetic

Orange – 400 m Magnetic

Summary of Shoreline Fault

Comparison of Earthquake Locations with Marine Magnetic Data

DCPP

Courtesy, Janet Watt (USGS)

Hosgri fault zone

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Seismicity cross sectionsOctober 1987

toMarch 2007

Earthquake locationscourtesy of J. Hardebeck (USGS)

Earthquake locationscourtesy of J. Hardebeck (USGS)

Black: good quality single- event mechanisms

Gray: lower quality single- event mechanisms

Red: composite mechanisms

Focal mechanismsshow predominantly strike-slip motionin the offshore region

DCPP

Hosgri fault zone

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Initial Evaluation of Seismic Demands from Assumed Shoreline Fault Zone

• Magnitude– Based on the size of the fault

• 24 km length (extended to the intersection with Hosgri)• 12 km depth

– M = 6.5

• Distance– 1 km from DCPP (assumed)

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Initial Evaluation of Ground Motion (84th Percentile Deterministic)

Assumed Shoreline Fault Zone

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Summary of Shoreline Fault

Assumed Shoreline Fault

– Methods to estimate and constrain:

Location Variations in seismicity alignment Style of faulting Length and rupture area Magnitude of potential earthquakes Constraints on activity and fault slip-rate

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Summary of Shoreline Fault Action Plan

• Assumed Faults Length and Width– Does Shoreline fault zone extend to Hosgri?– Does Shoreline fault zone extend to the SE?– Is the fault zone continuous over its length

• Fault zone may be broken into small discontinuous segments

• Closest distance offshore DCPP– Assuming 1 km, could be less– Epicenters uncertain to 1 km

• Activity Rate– Slip-rates not available yet– Appear to be low

Topics being reviewed in PG&E’s Action Plan

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Summary of Shoreline Fault

• Synchronous rupture of Hosgri and Shoreline FZ (splay)?– Geometry is unfavorable for Splay faulting

• Rupture stepping from Hosgri onto Shoreline FZ ?– Rupture Length would be reduced

• North end of Hosgri (San Simeon Flt) to SE end of SFZ is about 60 km• M=6.85 (W=12), M=7.0 (W=16)

– Distance to DCPP• Offshore geophysical data needed to constrain the location offshore

and the distance to DCPP

Topics being reviewed in PG&E’s Action Plan

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Summary of Shoreline Fault

Ground Motion Sensitivity (emp)

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Summary of Shoreline Fault

There appears to be various alignments

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Pt. Buchon Multibeam Bathymetry

collected as part ofCA State Waters Mapping Program by CSU Monterey BaySeafloor Mapping Lab 2006-2007

Summary of Shoreline Fault

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Summary of Shoreline Fault

Paleo-wave-cut platforms

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Summary of Shoreline Fault

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Summary of Shoreline Fault

LTSP - Small Faults in Southwest Boundary Zone

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Summary of Shoreline Fault

Wave-cut Shorelines near DCPP usedas Strain Gauges to Constrain Fault Deformation

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Summary of Shoreline Fault

Predicted Paleoshorelines in Point Buchon Area

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Summary of Shoreline Fault

Geologic Map of Point Buchon Area

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Summary of Shoreline Fault

50,00050,000--yearyear--old old wavecut platform wavecut platform

across the N40W Faultsacross the N40W Faults

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Summary of Shoreline Fault

Within Data resolution, Paleo-Platform is not Deformed

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Summary of Shoreline Fault

Seismic Line PBS-32 Across the N40W Faults

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Summary of Shoreline Fault

Hosgri fault

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Summary of Shoreline Fault

2009 Multibeam Survey Area

3 priority areas

1. Shoreline ‘surf zone’ 0 – 10 m2. Pecho Fault Zone 10 – 50 m 3. Hosgri Fault Zone 50 – 150 m

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Schedule for Shoreline Fault Studies

Schedule for 2009 Activities• July 2009

– Complete offshore geophysical data collection• August 2009

– Complete peer review of USGS seismicity studies• November 2009

– Complete initial integration of new offshore geophysical data with the magnetic, seismic, geologic, and gravity data

– Updated interpretation of geometry, segmentation, and slip-rate of the SFZ

– Estimate ground motion at DCPP for possible rupture stepping from Hosgri to SFZ

• December 2009– Issue interim report to NRC summarizing updated information and

interpretation (source characterization and ground motion)

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Schedule for Shoreline Fault Studies

Schedule for 2010 Activities• June 2010

– Complete geologic mapping of onshore faults that may interact with the SFZ

– If needed, collect additional offshore geophysical data• November 2010

– Complete final integration and interpretation of offshore geophysical data with the magnetic, seismic, geologic, and gravity data to characterize the SFZ

– Complete final estimate of deterministic ground motions at DCPP for the updated characterization of the SFZ

– Complete updated probabilistic seismic hazard analysis• December 2010

– Issue final report to NRC summarizing updated information and interpretation of the SFZ

• Location, geometry, segmentation, and activity rate• Ground motions at DCPP (deterministic and probabilistic)

• Note: Results will be incorporated into the updated tectonic model for the region to be completed 2012