Overview of the Seismic Source Characterization for the Palo Verde Nuclear Generating Station Scott Lindvall SSC TI Team Lead Palo Verde SSC SSHAC Level 3 Project Tuesday, March 19, 2013 1
Questions from SWUS GMC TI Team Summarize tectonic setting. What is the range of dip angles (min and max) and faulting styles? What is the seismogenic thickness? Are you including deep ruptures in the upper mantle (~30 km)? What is the largest magnitude in your sources (due to linked faults)? Do you have a complex multi-segment rupture with different rake/dip along strike? Do you have splay faults or overlapping segments? 2
Regional Tectonics and Geologic Setting The Palo Verde Nuclear Generating Station (PVNGS) site region encompasses a number of physiographic and tectonic provinces in Arizona, Nevada, California, and Mexico: Basin and Range Colorado Plateau Arizona Transition Zone So. California/Baja Gulf of California 3
Regional Tectonics and Geologic Setting Structures in the northern portion of the PVNGS site region reflect Mesozoic-Cenozoic subduction of the Farallon plate: Sevier orogeny characterized by a relatively narrow band of (generally) thin-skinned thrust faulting. Laramide orogeny produced basins and basement-cored uplifts for 500 km east of the Sevier thrust belt. Post-Laramide Basin and Range extension. Major tectonic provinces of the western U.S. from Decelles, 2004 4
Regional Tectonics and Geologic Setting The southern portion of the PVNGS site region records post-laramide epeirogenic extension and the late Miocene to Recent evolution of the Pacific-North American plate boundary. Extension of the southern Basin and Range initiated within the site region ~25 Ma and migrated north. Complete subduction of the Farallon plate resulted in a transition from compression to oblique divergence along the plate boundary, leading to rifting in the Gulf of California. Tectonic features of the Gulf of CA from Dorsey and Umhoefer, 2012 5
Regional Tectonics and Geologic Setting Geophysical data document the transition from thickened crust in the NE to thinned crust in SE. Crustal thickness, interpolated from data provided by Levander et al. (2008). 6
Regional Tectonics and Geologic Setting Geophysical data document the transition from thickened crust in the NE to thinned crust in SE. Surface heat flow (after Blackwell et al., 2011). 7
Regional Tectonics and Geologic Setting Geophysical data document the transition from thickened crust in the NE to thinned crust in SE. Arizona Bouguer gravity anomaly map (Sweeney and Hill, 2001). 8
Geologic Setting and Site Geology B&R contains pc, K, Mz, T, and QT bedrock, with Q alluvial fill in basins. PVNGS site consists of alternating sandy, clayey, and gravelly alluvium overlying Miocene volcanic bedrock with interbedded sedimentary layers. Depth to volcanic bedrock ranges from 95m to 110m beneath the 3 units. Pre-Cambrian basement rock (granite and schist) underlies the volcanics. Arizona geology from Hirschberg and Pitts, 2000 9
Regional Seismicity SSHAC2 Catalog SSHAC Level 2 project earthquake catalog based on information from seven national and regional catalogs. New earthquake catalog will be developed for SSHAC Level 3 project. Moderate to large earthquakes concentrated along plate margin and N. Arizona Seismic Belt. 10
Regional Seismicity - Instrumental Seismicity is concentrated at the western periphery of the site region. 11
Regional Seismicity Hypocentral depths Earthquake hypocentral depths from April 2006 to March 2009 (Lockridge et al., 2012). 12
Historical Surface Ruptures Historical surface ruptures and earthquakes >M6 are concentrated at the periphery of the site region. Notable ruptures include: 1887 Sonora M7.6 2010 El Mayor-Cucapah M7.2 13
Regional Faulting Highest slip-rate faults are the at western periphery of the site region along plate boundary. Green < 0.2 mm/yr Red > 5 mm/yr Quaternary faults from USGS (2012) 14
2008 NSHMP Source Model Few fault sources modeled near the PVNGS site. NSHMP fault sources from Petersen et al. (2008) 15
CA Fault Model UCERF3 PVNGS SSHAC3 SSC model will consider UCERF3 UCERF3 fault sources, FM 3.1 16
Existing SSHAC Level 2 SSC for Palo Verde Developed in 2011-2012. Seismic source model extends to >400 km from the site to include most of AZ, southern NV, southeastern CA, western New Mexico, and northern Mexico. Model components and parameters will change as part of ongoing SSHAC Level 3 project. Palo Verde SSC Workshop #1 scheduled for April 2013. 17
Master Logic Tree SSC includes both areal and fault sources to model the location, rate, and size of future earthquakes. Areal sources include coarse (5 sources) and fine (11 sources) zonation alternatives. Roughly 140 fault sources included (but most were insignificant hazard contributors). 18
Areal Sources Logic Tree Defined based on differences in earthquake recurrence rate, Mmax, and expected future rupture characteristics. Coarse zonation uses spatially smoothed seismicity approach (spatially varying a- and b- values in each zone). Fine zonation uses uniform rate (constant a- and b- values in each zone). 19
Areal Sources Coarse Zonation Coarse areal seismic sources represent the primary tectonic provinces of the PVNGS site region: Colorado Plateau Transition Zone Basin and Range (host zone) Southern California and Baja Gulf of California Spatial smoothing of seismicity within zones. Note lack of seismicity near site. 20
Areal Sources Fine Zonation Fine areal seismic sources represent differences in seismicity, geology, fault activity, topography, and structural style: Colorado Plateau Northern Arizona Seismic Belt Transition Zone A Transition Zone B Transition Zone C Nevada Basin and Range Sonoran Basin and Range (host zone) Mexican Highlands Basin and Range Southern California Baja Gulf of California Uniform seismicity rates within each zone. Slightly preferred (weight 0.6). 21
Fault Sources Branch of Master Logic Tree CA Type A faults (San Andreas, San Jacinto, Elsinore): characterization simplified after 2008 NSHMP and UCERF2. NV faults and non-type A CA faults: adopt characterization from 2008 NSHMP. AZ and Mexico faults: detailed characterization. 22
AZ and Mexico Fault Sources Logic Tree 23
Fault Sources (1 of 3) Fault sources in California, Baja, and southwest Arizona. Cerro Prieto fault source (#105): 220 km from site 267-km-long s/s fault Slip rate ~40 mm/yr Sand Tank fault source (#82): 60 km from site (closest) Length? Slip rate ~0.2 mm/yr 24
Fault Sources (2 of 3) Fault sources in southwest Arizona and Mexico. 1887 EQ ruptured 3 separate faults; however, given significant distance to site fault trace was simplified into a single fault source 25
Fault Sources (3 of 3) Fault sources in northern Arizona and Nevada. 26
Summary Summarize tectonic setting. What is the range of dip angles (min and max) and faulting styles? Strike-slip: 90 Normal: 50 ± 15 What is the seismogenic thickness? Variable. Areal sources: Maximum: 20 ± 3 km (CO Plateau, AZ Transition Zone) Host zone: 15 ± 3 km (Basin and Range) Minimum: 12 ± 3 km (Gulf of California) Fault sources: Maximum: 20 ± 3 km (fault sources in AZ Transition Zone) Minimum: 12 ± 3 km (fault sources in Gulf of California) 27
Summary (cont d) Are you including deep ruptures in the upper mantle (~30 km)? No not in current model. What is the largest magnitude in your sources (due to linked faults)? Over a dozen fault sources have upper tails of magnitude distributions in the M7.8 to M8.2 range. However, all are more than 240 km from site. Host zone has upper limit of M7.5. Do you have a complex multi-segment rupture with different rake/dip along strike? No not in current model. Do you have splay faults or overlapping segments? No not in current model. 28
Questions? 29