John Baldwin and Brian Gray Lettis Consultants International, Inc. Alexis Lavine AMEC Josh Goodman and Janet Sowers Fugro Consultants, Inc. December 1, 2011 Pacific Gas & Electric SSHAC Workshop 1 San Luis Obispo, CA 1
Los Osos Fault Data Compilation Purposes: Develop Fault Parameter Information for the Revised DCPP Seismic Source Model (1) Location and activity of fault strands (L.Quat-Holo) (2) Rate of slip (Late Quaternary-Holocene) (3) Fault zone geometry (style of deformation, fault dip) (4) Segmentation (5) Recommendation(s) for future studies to refine 1 thru 4 2
Los Osos Fault Data Compilation Geologic and geomorphic maps of area (published and unpublished) (3) Borehole, oil well, and water well data (2) Unpublished trenching studies (Alquist-Priolo) (1a) Geophysical seismic reflection and refraction surveys (1) Develop 3-D model consistent with seismotectonic model Plesch et al., 2005 Fault Dip and Style of Faulting 3
Los Osos Fault Interpreted as a northeast verging thrust/reverse fault that forms the NE boundary of the uplifted Irish Hills sub-block. Composed of four segments that may or may not represent rupture segmentation boundaries Segmentation of Lettis and Hall (1994) based on:(1) an association with regional sub-blocks (i.e., Irish Hills/Edna sub-blocks);(2) geomorphology (range-front character);(3) slip rate and recency of slip Multiple Quaternary active fault traces Poorly constrained SW dip 4
Seismic Source Model Inputs Los Osos Fault Geometry (Dip) Sense of Slip Slip Rate (mm/yr) Segmentation Length PG&E (2010/11) 45 to 75 SW Reverse to Oblique 0.4v; 0.2h Yes 49-57 km CGS/USGS (96;02;07) 45 SW Reverse 0.5 +-0.4 mm/yr* NA 44-48 km *Assumed dip of 30 to 60 degrees with predominant vertical displacement (Lettis and Hall, 1994) Source Model Uses a Mw6.8 Slip rate from trench, marine terrace and fluvial terrace mapping (Lettis and Hall, 1994) 5
Los Osos Fault Data Compilation Compiled Published and Unpublished Existing Geologic and Geotechnical Data Geologic and geomorphic maps of area (published and unpublished) Borehole, oil well, and water well data Unpublished trenching studies (Alquist-Priolo; LTSP) Geophysical seismic reflection and refraction surveys 6
1.0 Geologic Map Compilation of the Irish Hills Region-Draft Output Estero Bay Irish Hills Lopez Reservoir Newsom Ridge 7
New Mapping Data -Light Detection and Ranging (LiDAR) Data 8
Los Osos Fault Data Compilation Compiled Published and Unpublished Existing Geologic and Geotechnical Data Geologic and geomorphic maps of area (published and unpublished) Borehole, oil well, and water well data Unpublished trenching studies (Alquist-Priolo; LTSP) Geophysical seismic reflection and refraction surveys Compiled 395 boreholes between Morro Bay and Arroyo Grande and tens of oils wells drilled for Santa Maria Basin area (Sweetkind et al., 2010) (focused on Irish Hills and Edna Subblocks) Table of borehole data with stratigraphic interpretation (ID, elevation [NAD88], stratigraphy, groundwater, presence of fossils and woody debris, rank of data [A-D], data documentation, and cross sections) linked to ArcGIS shapefiles 9
2.0 Subsurface Data Compilation Borehole and Oil Well Data Cl Data Sources: Cleath-Harris Geologists, Inc., Sweetkind et al. 2010; California Oil and Gas, Department of Water Resources, and Caltrans 10
2.0 Initial Findings of Subsurface Compilation Los Osos Basin 11
2.0 Los Osos Basin Preliminary AT2 (Base) Structure Contour Ma 12
2.0 Fault Geometry: Spooner Well Depth (ft) ft Depth (m) Formation Lithology Dips 0 to? 0 to? Qe Sand dunes ND 200 to 780 61 Pismo or Paso Robles? Coarse gravel described at 200 ft. ND 785 to 795 239 to 242 Pismo? Sand, well sorted, permeable, described in core ND 800 to 1,300 418 to 423 Monterey? No core description. ND 1,300 to 1,745 423 to 532 Monterey? Franciscan? Sand and shale logged in core at 1370-1,386 ft. ND 1,745 to 1,749 BOH 532 to 533 BOH Basement Serpentinite ND E Log Spooner well is challenging to interpret as no geologist s log is available and no descriptions to 200 ft in depth Possible Franciscan -1300 ft Serpentine -1745 ft. Use Paso Robles as marker? Multiple interpretations (Hall, Sowers, AMEC, Morro Group) Incorrect location in Sweetkind (2010) and relocated upslope. 13
2.0 Borehole Data Compilation San Luis Basin 14
2.0 San Luis Basin Preliminary Findings Top of Bedrock 15
Los Osos Fault Data Compilation (cont.) Compiled Published and Unpublished Existing Geologic and Geotechnical Data Geologic and geomorphic maps of area (published and unpublished) Borehole, oil well, and water well data Unpublished trenching studies (Alquist-Priolo; LTSP) Geophysical seismic reflection and refraction surveys Trench Data Compilation: Tabulated trench data into an ArcGIS database (location, fault strike and dip, style of deformation, sense of apparent displacement, kinematic data, age of displaced units, data ranking [A-D]). 1. Database consists of 57 trenches with 31 providing direct evidence of faulting 2. Honoring LTSP model of reverse faulting, recent data consist primarily of faults located within the hanging wall of the LOF with little to no evidence of a shallow SW dipping fault (few trenches cross or deposits too young). 16
2.0 Fault Trench Database 17
2.0 Subsurface Data Compilation- Trenching D Data Sources: PG&E (1988), Earth Systems Pacific, and Geosolutions 18
2.0 Irish Hills Estates AP Fault Studies 19
Source: Lettis, W.R., and Hall, N.T., 1994, Los Osos Fault Zone, San Luis Obispo County, California in Alterman, I.B., McMullen, R.B., Cluff, L.S., and Slemmons, D.B., eds., Seismotectonics of the Central California Coast Ranges: Geological Society of America Special Paper 292.
Los Osos Fault Data Compilation (cont.) Compiled Published and Unpublished Existing Geologic and Geotechnical Data Geologic and geomorphic maps of area (published and unpublished) Borehole, oil well, and water well data Unpublished trenching studies (Alquist-Priolo; LTSP) Geophysical seismic reflection and refraction surveys 21
4 Onland Geophysical Data Acquisition 22
Los Osos Fault Summary Broad zone of deformation (flower structure vs. rev flt) Geophysics (Estero Bay-1-2 km zone of faulting) Cross Sections (Irish Hills, Lopez Reservoir) Multiple Quaternary active fault traces Hall steeply dipping Secondary hanging wall deformation Valleyward low-angle reverse faults (Holocene) Localized basins along Irish Hills and Lopez Reservoir segments Seismicity appears to support primarily reverse faulting Down-dip geometry difficult to characterize with existing shallow information (need to combine deep geophysics, oil well and surficial mapping/trenching and consistent tectonic model) 23
Recommendations for Future Studies GIS Database Perform a QA/QC review of the GIS database, including stratigraphic framework Fill in data gaps (Los Osos Valley; San Luis Basin south of LOF) Identify areas where age dating information can be obtained Location and Activity of fault strands Refine GIS fault line work from trench, borehole and geophysics Analyze and Interpret 2011 LiDAR data Fault Zone Geometry Refine and prepare new geologic cross sections using all existing data Collect additional existing water well and geotechnical borehole information Deep and shallow geophysical profiles in key areas (basins with drainages) Borehole transects - Drill deep and shallow borings to intersect fault Excavate shallow paleoseismic trenches across multiple fault traces Slip Rate Identify paleoseismic trench and borehole transect sites LiDAR mapping trench sites, fluvial and marine terrace mapping Correlate buried aquitards with marine high stands Segmentation Develop a structure contour map for Edna Valley hole(?) Construct segmentation model using slip rate and earthquake recurrence data 24
THANK YOU! 25