PROBABILISTIC SEISMIC HAZARD ANALYSES AND DEAGGREGATION FOR MAPPING LIQUEFACTION AND EARTHQUAKE-INDUCED LANDSLIDE HAZARD ZONES

Transcription

1 Eleventh U.S. National Conference on Earthquake Engineering Integrating Science, Engineering & Policy June 25-29, 2018 Los Angeles, California PROBABILISTIC SEISMIC HAZARD ANALYSES AND DEAGGREGATION FOR MAPPING LIQUEFACTION AND EARTHQUAKE-INDUCED LANDSLIDE HAZARD ZONES R. Chen 1, T.P. McCrink 2, M.A. Silva 3, and C.R. Real 4 ABSTRACT Probabilistic seismic hazard analyses (PSHA) and hazard deaggreagtions are performed to derive ground motion parameters required for developing liquefaction and earthquake-induced landslide hazard zone maps in accordance with the California Seismic Hazards Mapping Act of The analyses are based on the U.S. Geological Survey PSHA model for the 2014 Update of the United States National Seismic Hazard Maps and incorporate potential effects of near surface geologic conditions. For liquefaction hazard mapping, an earthquake magnitude weighting factor (MWF) is calculated based on binned magnitude-distance deaggregation to incorporate a magnitude-correlated duration effect. At each location, the MWF is the sum of probabilistic hazard-weighted MWFs from all magnitude-distance bins so that all magnitudes contributing to ground motion hazard are considered, effectively causing the cyclic stress ratio liquefaction threshold curves to be scaled probabilistically when computing factor of safety. For landslide hazard mapping, a probabilistic peak ground acceleration and a predominant earthquake magnitude (modal magnitude) are calculated at each location in order to estimate cumulative Newmark displacement for a given rock strength and slope gradient condition. We present ground motion parameters for four selected quadrangles and discuss pros and cons in the use of each parameter for hazard mapping. We further discuss potential improvements to meet ground motion needs in creating regulatory hazard zone maps in California. 1 Senior Seismologist, California Geological Survey (CGS), Sacramento, CA ( Rui.Chen@conservation.ca.gov) 2 Supervising Engineering Geologist, CGS, Sacramento, CA ( Tim.McCrink@conservation.ca.gov) 3 Senior Engineering Geologist, CGS, Sacramento, CA ( Michael.Silva@conservation.ca.gov) 4 Retired, CGS, Sacramento, CA ( Chuck.Real@conservation.ca.gov) Chen R., McCrink T.P., Silva M.A., and Real C.R. Probabilistic Seismic Hazard Analyses and Deaggregation for Mapping Liquefaction and Earthquake-Induced Landslide Hazard Zones. Proceedings of the 11 th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Los Angeles, CA

2 Probabilistic Seismic Hazard Analyses and Deaggregation for Mapping Liquefaction and Earthquake-Induced Landslide Hazard Zones R. Chen 1, T.P. McCrink 2, M.A. Silva 3, and C.R. Real 42 ABSTRACT Probabilistic seismic hazard analyses (PSHA) and hazard deaggreagtions are performed to derive ground motion parameters required for developing liquefaction and earthquake-induced landslide hazard zone maps in accordance with the California Seismic Hazards Mapping Act of The analyses are based on the U.S. Geological Survey PSHA model for the 2014 Update of the United States National Seismic Hazard Maps and incorporate potential effects of near surface geologic conditions. For liquefaction hazard mapping, an earthquake magnitude weighting factor (MWF) is calculated based on binned magnitude-distance deaggregation to incorporate a magnitude-correlated duration effect. At each location, the MWF is the sum of probabilistic hazard-weighted MWFs from all magnitude-distance bins so that all magnitudes contributing to ground motion hazard are considered, effectively causing the cyclic stress ratio liquefaction threshold curves to be scaled probabilistically when computing factor of safety. For landslide hazard mapping, a probabilistic peak ground acceleration and a predominant earthquake magnitude (modal magnitude) are calculated at each location in order to estimate cumulative Newmark displacement for a given rock strength and slope gradient condition. We present ground motion parameters for selected quadrangles and discuss pros and cons in the use of each parameter for hazard mapping. We further discuss potential improvements to meet ground motion needs in creating regulatory hazard zone maps in California. Introduction The Seismic Hazards Mapping Act of 1990 (Public Resources Code, Chapter 7.8, Division 2) directs the California Geological Survey (CGS) to compile maps that identify Seismic Hazard Zones consistent with requirements and priorities established by the California State Mining and Geology Board [1], including zones of potential liquefaction and earthquake-induced landslide hazards. The Act requires site-specific geotechnical investigations for most urban development projects in seismic hazard zones before lead agencies can issue building permits. The Act also requires sellers of real property within these zones to disclose hazard potential at the time such property is sold. 1 Senior Seismologist, California Geological Survey (CGS), Sacramento, CA ( Rui.Chen@conservation.ca.gov) 2 Supervising Engineering Geologist, CGS, Sacramento, CA ( Tim.McCrink@conservation.ca.gov) 3 Senior Engineering Geologist, CGS, Sacramento, CA ( Michael.Silva@conservation.ca.gov) 4 CGS (retired), Sacramento, CA ( Chuck.Real@conservation.ca.gov) Chen R., McCrink T.P., Silva M.A., and Real C.R. Probabilistic Seismic Hazard Analyses and Deaggregation for Mapping Liquefaction and Earthquake-Induced Landslide Hazard Zones. Proceedings of the 11 th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Los Angeles, CA

3 Current CGS methods for analyses of liquefaction and earthquake-induced landslide hazards for regulatory seismic hazard zone mapping require ground motion parameters obtained from probabilistic seismic hazard analyses (PSHA) and hazard deaggreagtions. These data include peak ground acceleration (PGA), PGA scaled by an earthquake magnitude weighting factor (MWF), and modal magnitude. For seismic hazards mapping, CGS uses source, ground motion, and PSHA models that are consistent with those used by the U.S. Geological Survey (USGS) for its national Seismic Hazard Maps (NSHM). For the latest releases, the USGS PSHA model for the 2014 update of the NSHM [2, 3] are used. Ground motion parameters are derived for the preparation of new and revised Seismic Hazard Zone Maps for ten 7.5-minute quadrangles in California. Ground motions are also calculated at borehole locations for future refinement of liquefaction hazard analysis. Site effects are incorporated via scaling by ground motion attenuation equations using time-averaged shearwave velocity in the top 30 meters (VS30). A VS30 value at each borehole location or grid point is obtained from the 2015 version of the VS30 map for California developed by Wills and others [4]. CGS methods for liquefaction and landslide hazard analyses are summarized in this paper. Maps of selected key results that are important to liquefaction and landslide hazard zone mapping are presented and their application in hazard analyses is discussed. Our intention is to increase awareness of ground motion data needs in creating regulatory hazard zone maps in California and to solicit suggestions for potential improvements in seismic hazard analyses for zoning purposes. Liquefaction and Landslide Hazard Assessment for Hazard Zone Mapping The California Code of Regulations (CCR Section 3722) and the recommendations of the Seismic Hazards Mapping Act Advisory Committee [1] specify that CGS prepare statewide PSHA ground shaking maps, and that maps having a 10% probability of being exceeded in 50 years are to be used to delineate seismic hazard zones for liquefaction and earthquake-induced landslides. Liquefaction Hazard Analyses For liquefaction hazard analyses, PGA is scaled by earthquake magnitude to incorporate a magnitude-correlated duration weighting factor [1, 5]. There are two essential parameters in liquefaction hazard analyses: cyclic stress ratio (CSR), and cyclic resistance ratio (CRR). CSR is the seismic demand on a soil layer and CRR is the capacity of the soil to resist liquefaction. When the demand exceeds the resistance, liquefaction is expected to occur. CSR is calculated using the following equations by Seed and Idriss [6]: CSR = 0.65 ( a max ) g (σ v σ v ) r d (1) where amax is PGA at the ground surface; g is acceleration of gravity; σv and σv' are total and effective vertical overburden stresses, respectively; and rd is stress reduction coefficient. CRR is calculated empirically based on field test data, including the standard penetration test, cone penetration test, shear-wave velocity measurements, and Becker penetration test [7]. Existing empirical relationships, however, are based on clean-sand curves that only apply to M7.5

4 earthquakes. To adjust the clean-sand CRR curves to magnitudes smaller or larger than M7.5, Seed and Idriss [8] introduced correction factors known as magnitude scaling factors (MSF). Alternatively, magnitude weighting factor (MWF), which is the inverse of MSF may be applied to correct CSR for magnitude [7]. The factor of safety (FS) against liquefaction is defined as: FS = CRR 7.5 (MSF) CSR = CRR 7.5 (CSR)(MWF) (2) Because CSR is proportional to amax as shown in Eq. 1, MWF can be applied to amax in hazard calculations, resulting in magnitude-corrected amax for liquefaction hazard analyses. The magnitude-corrected amax is referred to as magnitude-weighted PGA, pseudo PGA, or liquefaction opportunity [7]. The need for magnitude correction is due to the influence of earthquake duration on liquefaction potential at a site [8]. For a given PGA, sites experiencing a longer duration of strong shaking are more likely to liquefy. Because larger earthquakes shake longer, magnitude weighting is a proxy for shaking duration. Therefore, MWF is also known as a magnitude-correlated duration weighting factor. MWFs have been developed using different approaches for mutually exclusive liquefaction potential assessment schemes [7, 9, 10]. CGS currently uses the revised scaling factor presented by Youd and others [7]. Landslide Hazard Analyses The susceptibility of a slope to earthquake-induced landsliding is a function of the composition and strength of the underlying earth materials and the slope gradient. Susceptibility is defined by yield acceleration (ay), the inertial force required to initiate slope movement, calculated from Newmark s equation [11]: a y = (FS 1)g sin α (3) where g is acceleration due to gravity, α is the slope of movement of the slide mass (in degrees) measured from the horizontal when displacement is initiated, and FS is factor of safety. For the purpose of delineating seismic hazard zones, it is assumed that an infinite-slope failure model under unsaturated slope conditions applies, therefore α is taken as the slope gradient angle (β). Also, slope material is assumed to be cohesionless to simplify analysis procedure and to be conservative, so its strength is defined by angle of internal friction (ϕ). Under these simplifying assumptions, FS can be calculated as: FS = tan φ tan β (4) The current CGS method for earthquake-induced landslide hazards mapping is based on the Newmark displacement concept [12] with Newmark displacement (DN in cm) estimated using a regression equation developed by Jibson [12]: logd N = log [(1 a c a max ) ( a c a max ) ] M (5)

5 where amax is PGA, ac is critical (or yield) acceleration (Eq. 3), and M is earthquake moment magnitude. The regression has a standard deviation of in log units. CGS earthquake-induced landslide hazard zones are designed to encompass all areas that have calculated Newmark displacement of 5 cm or greater. This threshold value is selected based on studies with extensive calibrations by McCrink and Real [13] and McCrink [14]. In CGS s mapping applications, probabilistic PGA with 10% probability of being exceeded in 50 years is used, and M is the modal magnitude from PGA hazard deaggregation. Ground Motion Parameters for Hazard Zone Mapping Three ground motion parameters are required in the current CGS methodology for liquefaction and earthquake-induced landslide hazard zone mapping. These are PGA, magnitude-weighted PGA (also known as pseudo PGA), and modal magnitude. These parameters are calculated using nshmp-haz, a Java library developed by the USGS National Seismic Hazard Mapping Project, publically available on GitHub at: The 2014 version of the USGS source model for the conterminous U.S. is adopted to be consistent with the source models, ground motion models, and logic trees used for the 2014 update of the National Seismic Hazard Maps [2, 3]. This 2014 version of the USGS source model is also available on GitHub at: /. Unlike the National Seismic Hazard Map that is for a uniform site condition, our ground motion calculations incorporate local site effects via scaling by ground motion prediction equations using VS30 as a proxy. VS30 values at each grid point and borehole location are obtained from the 2015 version of the VS30 map for California developed by Wills and others [4] by grouping geologic units based on their shear-wave velocity characteristics. The 2015 version of the VS30 map is a significantly improved version of the previous VS30 maps for California in that it makes use of the most detailed available large-scale geologic maps and improved classification within young alluvium units based on surface slope. Hazard deaggregation separates contributions to the total hazard by a selected number of bins of magnitude, distance, and ground motion uncertainty (ε or epsilon, usually measured in number of ground motion standard deviation or σ) [15, 16, 17]. Deaggregation is performed on PGA with 10% exceedance probability in 50 years using the deaggregation code (DeaggCalc), which is part of the USGS Java library, to derive magnitude-weighted PGA and modal magnitude. Modal magnitude is chosen to be the mean magnitude of the distance-magnitude bin that contributes the most to the total hazard. Magnitude-weighted PGA is obtained by multiplying the probabilistic PGA by MWF at each grid point or borehole location without altering the PSHA calculations. Because earthquakes of different magnitudes and distances contribute differently to the total hazard, calculation of MWF uses a hazard-weighted approach that relies on detailed deaggregation results. For each magnitudedistance bin, a MWF is calculated using the mean magnitude for that bin weighted by the contribution of that bin to the total hazard at the chosen probabilistic PGA level. The total MWF is the sum of probabilistic hazard-weighted MWFs from all magnitude-distance bins:

6 n i=1 (6) MWF = [(MWF) i ( ν i )] v where (MWF)i is magnitude weighting factor for the i th magnitude-distance bin, νi is hazard contribution of the i th magnitude-distance bin, and ν is the total hazard. For seismic hazard zoning purposes, ground motions are calculated at each grid point on a degree grid (approximately 500-m spacing). Ground motions are also calculated at locations of geotechnical boreholes. Calculated ground motion parameters are presented and discussed for four selected adjacent quadrangles where seismic hazard zones have recently been prepared: Bachelor Mountain, Murrieta, Pechanga, and Temecula. Official hazard zone maps and reports are available on the CGS website at: ( The Elsinore fault zone traverses the selected area diagonally (Figure 1), cutting through geologic units and Figure 1. Map of V S30 groups and corresponding geologic units in a four-quadrangle area of western Riverside County, California, extracted from the statewide V S30 map developed by Wills et al. (2015). Seven of the fifteen V S30 groups in California occur in the map area. Mean and standard deviation (SD) for each group are indicated in the legends. Quadrangle names are shown. Abbreviation definitions: Qal2 - Quaternary (Holocene) alluvium in areas of moderate slopes ( %); Qal3 - Quaternary (Holocene) alluvium in areas of steep slopes (greater than 2%); Qoa - Quaternary (Pleistocene) alluvium; QT - Quaternary to Tertiary (Pleistocene - Pliocene) alluvial deposits; Tss - Tertiary sandstone units; Tv - Tertiary volcanic units, serpentine - Serpentine; Crystalline - crystalline bedrock.

7 controlling topographic characteristics. The northwest-trending Elsinore fault separates bedrock of the Peninsular Ranges Province into two distinct structural blocks, the Santa Ana Mountains block to the southwest and the Perris block to the northeast. Faults of the Elsinore fault zone also control the topography of the Murrieta-Temecula Valley, a broad linear valley along the Elsinore fault zone referred to as the Elsinore Trough. Probabilistic PGA is depicted in Figure 2. Ground motion distribution is controlled by the Elsinore fault zone and by local site conditions. PGA in the area ranges from a quarter to about half of gravitational acceleration (g). It is highest along the fault zone in the Murrieta-Temecula Valley and in softer Quaternary deposits in a broad area to the northeast of the fault zone. PGA decreases with increasing fault distance and is lowest in harder Tertiary and crystalline bedrock southwest of the fault zone. Higher PGA in the northeast corner comes from greater contributions to the ground motion hazards from the southern San Andreas fault to the northeast of the area. Modal magnitude in much of the area is 7.7 (Figure 3), reflecting the characteristic earthquake magnitude for the Elsinore fault zone. An exception is the northeast corner, where modal magnitude is 8.1, reflecting the effect of the southern San Andreas fault zone on ground motion hazards in this area. MWF ranges from 64% to 76% (Figure 4), indicating general reduction of liquefaction potential compared to a M7.5 earthquake for which clean-sand CRR curves are developed. If MWF were Figure 2. Probabilistic peak ground acceleration (PGA) of the four-quadrangle area of western Riverside County used for earthquake-induced landslide hazard zone mapping analysis.

8 Figure 3. Modal magnitude for earthquake-induced landslide hazard zone mapping analysis. estimated using modal magnitude for simplicity, MWF and, therefore, liquefaction potential would be overestimated compared to hazard-weighted MWF in this area because modal magnitudes are greater than 7.5, as indicated in Figure 3. Incorporating site conditions in probabilistic ground motion calculations for CGS liquefaction and landslide hazard zone mapping is a new development. In previously published hazard zone maps, a uniform soft rock site condition was assumed with VS30 of 760 m/s, an equivalent of a condition between the National Earthquake Hazard Reduction Program Site Classes B and C, even though the likely site effects of site-specific response characteristics, including amplification due to soft soils, were recognized (CGS, 2008)[5]. Explicitly incorporating the impact of the location site conditions on earthquake ground motions is also recommended by the National Academies of Sciences, Engineering, and Medicine [18] in developing methods to evaluate liquefaction triggering and its consequences. Future Improvements Improvements are envisioned for liquefaction and earthquake-induced landslide hazard analyses for zoning purposes including using ground motion parameters calculated at borehole locations for liquefaction hazard analyses. In addition, CGS continues to monitor and evaluate new

9 Figure 4. Magnitude weighting factor (MWF) for liquefaction hazard zone mapping analysis. developments in analysis methods that are based on quantitative ground deformation associated with the occurrence of liquefaction and landslides. CGS will also evaluate probabilistic liquefaction and earthquake-induced landslide hazard analyses for potential adaptation for developing seismic hazard zone maps. Summary Ground motion parameters required for delineating regulatory seismic hazard zone maps by CGS are derived using the PSHA model developed by the USGS for the 2014 update of the NSHM. These parameters include PGA with 10% exceedance probability in 50 years and associated modal magnitude and MWF. Calculated values for each parameter in four selected quadrangles are presented and the uses of each parameter in the current simplified approaches for liquefaction and landslide hazard zone development are briefly discussed. The calculation of probabilistic PGA and its deaggregation incorporate potential effects of near surface geologic conditions. The calculation of MWF is based on results from binned magnitude-distance deaggregation and is weighted by hazard contribution for each magnitude-distance bin so that all magnitudes contributing to ground motion hazard are considered. Cyclic stress ratio liquefaction threshold curves are thus scaled probabilistically when computing factors of safety. CGS currently relies on simplified analysis

10 methods and a limited number of ground motion parameters, but monitors new developments closely for potential adaptation for zoning purposes. Acknowledgments We thank John Parrish for his careful review of this manuscript to ensure its compliance with the high levels of technical and editorial standards practiced at California Geological Survey. We are grateful to Mark Petersen, Peter Powers, and others at USGS for their continuing support and for making the USGS PSHA models and codes available. We also thank William M. Murphy for effective suggestions and editorial corrections. References 1. California Geological Survey, Recommended Criteria for Delineating Seismic Hazard Zones in California. California Geological Survey Special Publication 118, Available on-line at: 2. Petersen MD, Moschetti MP, Powers PM, Mueller CS, Haller KM, Frankel AD, Zeng Y, Rezaeian S, Harmsen SC, Boyd OS, Field N, Chen R., Rukstales KS, Luco N, Wheeler RL, Williams RS, Olsen AH. Documentation for the 2014 Update of the United States National Seismic Hazard Maps. U.S. Geological Survey Open-File Report, , doi: /ofr Petersen MD, Moschetti MP, Powers PM, Mueller CS, Haller KM, Frankel AD, Zeng Y, Rezaeian S, Harmsen SC, Boyd OS, Field N, Chen R., Rukstales KS, Luco N, Wheeler RL, Williams RS, Olsen AH. The 2014 United States national seismic hazard model. Earthquake Spectra 2015; 31 (S1): S1 S30, doi: /120814EQS210M. 4. Wills CJ, Gutierrez CI, Perez FG, Branum DM. A next-generation V S30 map for California based on geology and topography. Bulletin of the Seismological Society of America 2015; 105 (6): , doi: / California Geological Survey, Guidelines for Evaluating and Mitigating Seismic Hazards in California. California Geological Survey Special Publication 117a, Available on-line at: 6. Seed HB, Idriss IM. Simplified procedure for evaluating soil liquefaction potential, Journal of the Soil Mechanics and Foundation Division 1971; 97(9): Youd TL, Idriss I.M, Andrus RD, Arango I, Castro G, Christian JT, Dobry R, Finn WDL, Harder LF, Hynes ME, Ishihara K, Koester JP, Liao SSC, Marcuson WF, Martin GR, Mitchell JK, Moriwaki Y, Power MS, Robertson PK, Seed RB, Stokoe KH. Liquefaction resistance of soils: summary report from the 1996 NCEER and 1998 NCEER/NSF workshops on evaluation of liquefaction resistance of soils. Journal of Geotechnical and Geoenvironmental Engineering 2001; October: Seed HB, Idriss IM. Ground Motion and Soil Liquefaction During Earthquakes, Monograph. Earthquake Engineering Research Institute, Oakland, California, Cetin KO, Seed RB, Der Kiureghian A, Tokimatsu K, Harder LF, Jr., Kayen RE, Moss RES. Standard penetration test-based probabilistic and deterministic assessment of seismic soil liquefaction potential. Journal of Geotechnical and Geoenvironmental Engineering 2004; 130(12): Moss RES, Seed RB, Kayen RE, Stewart JP, Der Kiureghian A, Cetin KO. CPT-based probabilistic and deterministic assessment of in situ seismic soil liquefaction potential. Journal of Geotechnical and Geoenvironmental Engineering, 2006; 132(8): Newmark NM. Effects of earthquakes on dams and embankments. Geotechnique 1965; 15(2): Jibson RW. Regression models for estimating coseismic landslide displacement. Engineering Geology 2007;

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