Tenth U.S. National Conference on Earthquake EngineeringFrontiers of Earthquake Engineering July 21-25, 2014 Anchorage, Alaska 10NCEE

IMPLICATION OF UPDATED SUBDUCTION ZONE EARTHQUAKE

GROUND MOTION PREDICTION EQUATIONS AND SEDIMENTARY BASIN

EFFECTS TO THE BUILDING CODE DESIGN SPECTRA IN THE PACIFIC

NORTHWEST

K.H. Chin1 and J. Hooper2

ABSTRACT This paper presents the results of the site-specific ground motion study completed for a project and discusses the implication of the results to the building code design spectra in the Pacific Northwest. The project site is located in Bellevue, Washington, and within the Seattle sedimentary basin in the Puget Sound. The seismic hazard at the project site is dominated by the Cascadia Subduction Zone and the Seattle Fault Earthquake sources. The project site is classified as Site Class C per ASCE 7-10. The site-specific ground motion study completed for the project included the updated subduction zone earthquake ground motion prediction equations (GMPEs) developed by Zhao et al. (2006) and Abrahamson et al. (2012). The sedimentary basin effects (Frankel et al., 2007) were also incorporated in the development of the site specific design spectra developed for the project. The study evaluated effects of using the GMPEs currently used by United States Geological Survey (USGS) and the updated GMPEs developed for subduction zone earthquakes. This paper also presents a comparison of the results of the ground motion study to the results computed using the 2008 USGS probabilistic seismic hazard model and the ASCE 7-10 generalized design spectra.

1Principal, GeoEngineers, Inc., 8410 154th Avenue NE, Redmond, WA 98052 2Senior Principal and Director of Earthquake Engineering, Magnusson Klemencic Associates, 1301 Fifth Avenue, Suite 3200, Seattle, WA 98101 Chin, K.H. and Hooper, J. Implication of Updated Subduction Zone Earthquake Ground Motion Prediction Equations and Sedimentary Basin Effects to the Building Code Design Spectra in the Pacific Northwest. Proceedings of the 10th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.

Implication of Updated Subduction Zone Earthquake Ground Motion Prediction Equations and Sedimentary Basin Effects to the Building

Code Design Spectra in the Pacific Northwest

K. H. Chin1 and J. Hooper2

ABSTRACT This paper presents the results of the site-specific ground motion study completed for a project and

discusses the implication of the results to the building code design spectra in the Pacific Northwest. The project site is located in Bellevue, Washington, and within the Seattle sedimentary basin in the Puget Sound. The seismic hazard at the project site is dominated by the Cascadia Subduction Zone and the Seattle Fault Earthquake sources. The project site is classified as Site Class C per ASCE 7-10. The site-specific ground motion study completed for the project included the updated subduction zone earthquake ground motion prediction equations (GMPEs) developed by Zhao et al. (2006) and Abrahamson et al. (2012). The sedimentary basin effects (Frankel et al., 2007) were also incorporated in the development of the site specific design spectra developed for the project. The study evaluated the effects of using the GMPEs currently used by United States Geological Survey (USGS) and the updated GMPEs developed for subduction zone earthquakes. This paper also presents a comparison of the results of the ground motion study to the results computed using the 2008 USGS probabilistic seismic hazard model and the ASCE 7-10 generalized design spectra.

Introduction

The Puget Sound and the Pacific Northwest are located near the convergent continental boundary known as the Cascadia Subduction Zone (CSZ). The CSZ is an area where the westward advancing North American Plate is overriding the subducting Juan de Fuca Plate. The CSZ extends from mid-Vancouver Island to Northern California. The interaction of these two plates results in two potential seismic source zones: (1) the Benioff source zone, and (2) the CSZ interplate source zone. A third seismic source zone, referred to as the shallow crustal source zone, is associated with the north-south compression resulting from northerly movement of the Sierra Nevada block of the North American Plate. Figure 1 presents the results of the seismic deaggregation completed as part of the site-specific probabilistic seismic hazard analyses for the subject site in Bellevue, Washington. As presented

1Principal, GeoEngineers, Inc., 8410 154th Avenue NE, Redmond, WA 98052 2Senior Principal and Director of Earthquake Engineering, Magnusson Klemencic Associates, 1301 Fifth Avenue, Suite 3200, Seattle, WA 98101 Chin, K.H. and Hooper, J. Implication of Updated Subduction Zone Earthquake Ground Motion Prediction Equations and Sedimentary Basin Effects to the Building Code Design Spectra in the Pacific Northwest. Proceedings of the 10th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.

in Figure 1, the seismic hazards of the Pacific Northwest are controlled by three earthquake sources for structures or buildings of all periods. The contribution of the CSZ sources to the seismic hazard is especially significant at periods longer than 1 second. Therefore, in order to develop a realistic design spectrum for tall buildings and long period structures, and to avoid bias due to ground motion data from different regions, it is critical to use a suite of ground motion prediction equations (GMPEs) for the earthquake sources that include the model uncertainties. This is especially important in the selection of the CSZ GMPEs because the available models were developed mainly based on the ground motion data from other regions and compute response spectra values that can be different by a factor of up to 4. The subject site is located in Bellevue and within the Seattle sedimentary basin (Li et al., 2006) as shown in Figure 2. Recent studies (Frankel et al., 2007 and Day, et al., 2008) has shown that it is critical to incorporate the amplification effects of the sedimentary basin in developing the seismic design spectra, especially for tall buildings and other long period structures.

Figure 1. Seismic hazard deaggregation results for the subject site in Bellevue, Washington

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Figure 2. Subject site location and Seattle sedimentary basin The planned development at the subject site consists of a tower ranging from 300 to 400 feet above grade with a seven-level below-grade parking garage. The predominant vibration period of the tower is 4.6 seconds. A site-specific ground motion hazard analysis was completed to determine the seismic design response spectra and to develop earthquake time histories for use in the structural design of the tower. As noted above, the selection of the CSZ GMPEs and incorporation of the basin effects are two of the critical considerations in the site-specific ground motion hazard analysis. This paper focuses on the evaluations completed on the various CSZ GMPEs considered and the approach used to incorporate the basin effects. Our opinions on the implications of the new CSZ GMPEs and the basin effects to the seismic design of buildings in the Puget Sound and Pacific Northwest are also provided.

Site Specific Ground Motion Hazard Analysis The site-specific ground motion hazard analysis was completed using the commercial software EZ-FRISK developed by Risk Engineering, Inc. The seismic source geometries and recurrence models developed by the United States Geological Survey (USGS) in the 2008 Update of the United States National Seismic Hazard Maps (Petersen et al., 2008) were used in the analysis. A

Subject Site

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Shear wave velocity contours (Li et al., 2006)

Outline of Seattle Basin (Li et al., 2006)

peer review of the site-specific ground motion hazard analysis was performed by a third-party technical reviewer retained by the City of Bellevue. Site Conditions The subsurface soil conditions at the site were evaluated using the results of the down-hole shear wave velocity (Vs) test completed and boring information obtained at the project site. Figure 3 presents the best estimate and the lower- and upper-bound Vs profiles developed at the project site. The average shear wave velocity based on the upper 100 feet (30 meters) of the soil (Vs-30) beneath the foundation level per Section 20.4.1 of American Society of Civil Engineering (ASCE) 7-10 for the lower-bound, best estimate and upper-bound Vs profiles were computed to be 463 m/s, 515 m/s and 565 m/s, respectively. The project site is classified as Site Class C. The depth to the bedrock is estimated to be at depth of about 1,570 feet (Jones, 1996). Based on the published shear wave velocity models by Frankel et al. (2007) and Pratt et al. (2003), the shear wave velocity of the bedrock ranges from 2,600 ft/s to 5,300 ft/s (0.8 km/s to 1.6 km/s).

Figure 3. Shear wave velocity profiles Ground Motion Prediction Equations The Maximum Considered Earthquake (MCE) hazard with 2 percent probability of exceedance in 50 years at the project site was computed using a suite of selected GMPEs for the seismic sources identified and the site conditions encountered. For the crustal earthquake sources, the Next Generation Attenuation (NGA) GMPEs developed by Abrahamson and Silva (A&S2008),

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Boore and Atkinson (B&A2008), Campbell and Bozorgnia (C&B2008) and Chiou and Youngs (C&Y2008) were used with equal weight. For the CSZ sources, the GMPEs developed by Youngs et al. (Yea1997), Atkinson and Boore (A&B2003), Zhao et al. (Zea2006) and Abrahamson et al. (Aea2012) were considered in the analysis.

Evaluation of CSZ GMPEs Prior to selecting the suite of CSZ GMPEs for use in the site-specific ground motion analysis, we completed an evaluation to compare the response spectra computed by the four CSZ GMPEs considered. The Cascadia model of the Abrahamson et al. (2012) GMPE was considered and was used in nine combinations with different weights in order to capture the epistemic uncertainty both on magnitude scaling and median ground motions (Gregor, 2013). Figure 4 shows the logic tree of the nine combinations completed using the Abrahamson et al. (2012) GMPE along with the weights assigned to each of the nine combinations.

Figure 5 presents the response spectra computed for two scenario CSZ earthquake events that produce the highest ground motion amplitudes: (1) Mw = 7.2, R = 50 km CSZ intraplate event and (2) Mw = 9.0, R = 96.8 km CSZ interplate event. As presented in Figure 5, the GMPEs considered compute the response spectra that vary significantly (up to a factor of about 3) at periods of 0.2 and 1.0 seconds. In general, the updated CSZ GMPEs such as Zhao et al. (2006) and Abrahamson et al. (2012) compute higher short period spectral accelerations for the CSZ earthquakes.

Figure 4. Logic tree for Abrahamson et al. (2012) CSZ GMPE

Site-Specific MCE Response Spectra Ground hazard analyses were completed using two combinations of the CSZ GMPEs (Approach 1 and 2) and using the 2008 USGS Probabilistic Seismic Hazard Model (https://geohazards.usgs.gov/deaggint/2008/). Table 1 below presents the GMPEs and their assigned weights used in the three approaches.

Figure 5. Comparison of response spectra computed using the various CSZ GMPEs considered

We computed the MCE response for the lower-bound, best estimate and upper-bound Vs-30 values determined based on the site-specific Vs measurement. The site-specific MCE response spectrum was computed by assigning a 0.50 weighting factor to the results using the best estimate shear wave velocity profile and a 0.25 weighting factor to both the lower- and upper-bound shear wave velocity profiles. Figure 6 presents the site-specific MCE response spectra computed using Approach 1, Approach 2 and the 2008 USGS Model. Also presented in Figure 6 is the Site Class C General MCE response spectrum computed using the Ss and S1 values obtained from the 2008 USGS Model and the Fa and Fv factors from the building code.

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Table 1. GMPEs used in the ground motion seismic hazard analysis

Analysis GMPEs

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Crustal: A&S2008, B&A2008, C&B2008, and C&Y2008

CSZ Intra- and Interplate: Yea1997, A&B2003 (Cascadia), and Zea2006

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CSZ Intra- and Interplate: Zea2006 and Aea2012 (Cascadia)

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CSZ Intraplate: Yea1997 (0.50 wt), A&B2003 (Global) (0.25 wt), and A&B-2003 (Cascadia) (0.25 wt)

CSZ Interplate: Yea1997 (0.25 wt), A&B2003 (Global) (0.25 wt) and Zea2006 (0.50 wt)

*GMPEs used in Approach 1 and 2 are equally weighted. The comparison of the MCE response spectra presented in Figure 6 shows that the updated CSZ GMPEs (i.e., Zea2006 and Aea2012) result in higher spectral accelerations for the short periods (less than about 0.4 second) when compared to the MCE response spectra computed with lesser weights assigned to these updated GMPEs. As presented in Figure 6, the ASCE 7 general response spectrum provides the short period spectral acceleration (SMS) value that is 15 to 34 percent lower than the site-specific MCE response spectra. This suggests that buildings with period less than 0.4 seconds (e.g., buildings with four stories or less) designed using the ASCE 7’s general design spectrum may need to be designed for higher seismic forces.

Figure 6. Comparison of MCE Response Spectra

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Seattle Basin Ground Motion Amplification Effects The amplification factors associated with the effect of Seattle basin were computed using the semi-empirical approach included in the NGA GMPEs (A&S2008, C&B2008 and C&Y2008) to compute the basin effects on ground motion amplifications. Two sets of calculations were completed, one with the Z1.0 for A&S2008 and C&Y2008 GMPEs and the other using the Z2.5 for C&B2008 GMPE. For the subject site, the Z1.0 and Z2.5 at the site are estimated to be 480 m and 5 km, respectively. The site-specific basin amplification factors were taken as the weighted average of the results obtained with the three GMPEs, as presented in Figure 7. Figure 8 presents the site-specific MCE response spectra with basin effects and the Site Class C General MCE response spectrum without the basin effects.

Figure 7. Site-specific basin amplification factors

Figure 8. Comparison of MCE response spectra with basin effects

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The comparison of the site-specific MCE response spectra presented in Figure 8 shows that the building code general spectrum underestimate the spectral accelerations at periods less than about 4.0 seconds. Note that the Site Class C general response spectrum shown in Figure 8 does not include the basin effects.

Conclusions Based on the results of the study completed for the subject site, we concluded that both the updated CSZ GMPEs and the Seattle basin amplification effects have a significant effect on the seismic design for buildings and structures in the Puget Sound area and the Pacific Northwest. The results show that the general design spectrum developed using ASCE 7 may potentially underestimate the seismic force used in building design. In order to accommodate the effects of the updated CSZ GMPEs and the basin effects, it is recommended that a site specific study be completed for each project in the Pacific Northwest region. At a minimum, the results of the 2008 USGS Probabilistic Seismic Hazard Model should be evaluated and considered in adjusting the ASCE design spectrum. In addition, analytical methods such as performing site specific response analysis should be considered to better accommodate the epistemic uncertainties in ground motion hazard estimate especially since all of the GMPEs were developed based on ground motion data obtained from other regions. Lastly, for sites located within a sedimentary basin or at a basin edge, incorporation of the basin amplification effects should also be considered in the site specific study.

References 1. Zhao J.X., Zhang, J., Asano, A., Ohno, Y., Oouchi, T., Takahashi, T., Ogawa, H., Irikura, K., Thio, H.,

Somerville, P., Fukushima, Y., and Fukushima, Y., 2006 Attenuation relations of strong ground motion in Japan using site classification based on predominant period: Bulletin of the Seismological Society of America, v. 96, p. 898–913.

2. Abrahamson, N.A., N. Gregor, and K. Addo (2012). “BCHydro Ground Motion Prediction Equations for Subduction Earthquakes.” Submitted to Earthquake Spectra.

3. Li, Q., W.S.D. Wilcock, T.L. Pratt, C.M. Snelson and T.M. Brocher (2006). “Seismic Attenuation Structures of the Seattle Basin, Washington State, from Explosive-Source Refraction Data.” Bulletin of the Seismological Society of America, Vol. 96, No. 2, pp. 553-571.

4. Frankel, A., W.J. Stephenson, D.L. Carver, R.A. Williams, J.K. Odum, and S. Rhea (2007). “Seismic hazard maps for Seattle, Washington, incorporating 3D sedimentary basin effects, nonlinear site response, and rupture directivity.” U.S. Geological Survey Open File Report 2007-1175.

5. Day, S.M., Graves, R., Bielak, J., Dreger, D., Larsen, S., Olsen, K.B., Pitarka, A. and Ramirez-Guzman, L. (2008). “Model for Basin Effects on Long-Period Response Spectra in Southern California.” Earthquake Spectra 24, 257-275.

6. Risk Engineering, Inc. “EZ-FRISK: Software for Earthquake Ground Motion Estimation.”

7. Petersen, Mark D., Frankel, Arthur D., Harmsen, Stephen C., Mueller, Charles S., Haller, Kathleen M., Wheeler, Russell L., Wesson, Robert L., Zeng, Yuehua, Boyd, Oliver S., Perkins, David M., Luco, Nicolas, Field, Edward H., Wills, Chris J., and Rukstales, Kenneth S. (2008). “Documentation for the 2008 Update of the United States National Seismic Hazard Maps.” U.S. Geological Survey Open-File Report 2008-1128.

8. ASCE, 2010. “SEI/ASCE 7-10, Minimum Design Loads for Buildings and Other Structures,” American

Society of Civil Engineers.

9. Jones,M.A. (1996). Thickness of Unconsolidated Deposits in the Puget Sound Lowland, Washington and British Columbia, 1996, U.S. Geological Survey - Water-resources Investigations Report 94-4133.Abrahamson, N.A., and W.J. Silva (2008). “Summary of the Abrahamson and Silva NGA Ground-Motion Relations.” Earthquake Spectra 24, 67-97.

10. Pratt, T.L., T.M. Brocher, C.S. Weaver, K.C. Creager, C.M. Snelson, R.S. Crosson, K.C. Miler, and A.M. Trehu (2003). “Amplification of seismic waves by the Seattle Basin, Washington State.” Bulletin of the Seismological Society of America, 93(2), pp. 533-545.

11. Abrahamson, N.A., and W.J. Silva (2008). “Summary of the Abrahamson and Silva NGA Ground-Motion Relations.” Earthquake Spectra 24, 67-97.

12. Boore, D.M. and G.M. Atkinson (2008). “Ground Motion Prediction Equations for the Average Horizontal Component of PGA, PGV, and 5% Damped PSA at Spectral Periods between 0.01s and 10.0 s.” Earthquake Spectra 24, 99-138.

13. Campbell, K.W. and Y. Bozorgnia (2008). “NGA Ground Motion Model for the Geometric Mean Horizontal Component of PGA, PGV, PGD and 5% Damped Linear Elastic Response Spectra for Periods Ranging from 0.01 to 10s.” Earthquake Spectra, Vol. 24, No. 1, 139-171.

14. Chiou, B.S.J. and R.R. Youngs (2008). “An NGA Model for the Average Horizontal Component of Peak Ground Motion and Response Spectra.” Earthquake Spectra, Vol. 24, No. 1, 173-215.

15. Youngs, R.; Chiou, S-J.; Silva, W.; Humphrey, J. 1997. “Strong ground motion attenuation relationships for subduction zone earthquakes.” Seismological Research Letters 68. 58-73.

16. Atkinson, G.M., Boore, D.M. (2003). “Empirical ground-motion relations for subduction-zone earthquakes and their application to Cascadia and other regions.” Bulletin of the Seismological Society of America, v. 93, n. 4, p. 1703-1729.

17. Gregor, N. (2013), Personal communications.

18. United States Geologic Survey, 2008 Interactive Deaggregation, accessed from USGS web site: https://geohazards.usgs.gov/deaggint/2008/.