PACIFIC EARTHQUAKE ENGINEERING RESEARCH CENTER Empirical Characterization of Site Conditions on Strong Ground Motion Jonathan P. Stewart Yoojoong Choi University of California, Los Angeles and Robert W. Graves URS Corporation Pasadena, California PEER 2005/01 JUNE 2005 Empirical Characterization of Site Conditions on Strong Ground Motion Jonathan P. Stewart Yoojoong Choi Department of Civil and Environmental Engineering University of California, Los Angeles and Robert W. Graves URS Corporation, Pasadena, California PEER Report 2005/01 Pacific Earthquake Engineering Research Center College of Engineering University of California, Berkeley June 2005 ABSTRACT Empirical relationships are developed to predict amplification factors for 5% damped response spectral acceleration as a function of site condition. Amplification factors are evaluated as residuals between ground motion recordings and predictions from modified rock attenuation relationships. Both shallow and deep characteristics of site condition are considered to identify those parameters that are most effective from the standpoint of bias and dispersion reduction. The parameterization of shallow site condition is based on (1) surface geology, (2) NEHRP classification, (3) geotechnical site categories, and (4) average shear wave velocity in upper 30 m of site (Vs-30). Also considered are parameters that reflect the relatively deep sedimentary structure at many of the strong motion sites, including depth to the 1.5 km/s shear wave isosurface (z1.5) as well as the location of the source inside or outside of the basin in which the site is located. Sites located in a basin overlaying the source are denoted as having coincident source and site basin locations (CBL) and are differentiated from distinct source and site basin locations (DBL). It is found that standard deviation is minimized with the use of detailed surface geology or Vs-30 as the site parameter for shallow site condition. The Vs-30-based amplification model has several innovative features, including Vs-30-dependent nonlinearity and standard deviation and quantification of the reference velocity for a number of rock attenuation relationships. For all site categories, standard deviation was found to increase with period, being as much as 0.3 larger at long periods than short periods. The work on basin parameters utilized residuals calculated with respect to ground motion predictions derived using rock attenuation relations coupled with amplification factors for shallow site condition. Models relating amplification to z1.5 were developed for the CBL and DBL data groups. The results indicate that the use of basin models is generally worthwhile for periods T ≥ 0.75 s. At those long periods, residuals are significantly sensitive to z1.5 for CBL but not for DBL when shallow site condition is parameterized based on Vs-30. Standard deviation is also reduced at long periods, such that the increase with period is significantly reduced. iii ACKNOWLEDGMENTS This work was supported by the Earthquake Engineering Research Centers Program of the National Science Foundation under award number EEC-9701568 through the Pacific Earthquake Engineering Research (PEER) Center. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect those of the National Science Foundation. Many individuals assisted the authors during this study. We would especially like to thank Caroline Wang, Annie Kwok, Andrew Liu, and Mehmet Baturay for their assistance. We are also grateful to Rie Von Eyben and Xiao Chen who provided valuable consultation regarding the statistical analyses. We would like to extend our thanks to Dr. Kenneth Campbell for his assistance in compiling distance parameters for strong motion stations. iv CONTENTS ABSTRACT ................................................................................................................................ iii ACKNOWLEDGMENTS ........................................................................................................... iv TABLE OF CONTENTS ..............................................................................................................v LIST OF FIGURES .................................................................................................................... vii LIST OF TABLES ....................................................................................................................... ix 1 INTRODUCTION .................................................................................................................1 1.1 Statement of the Problem................................................................................................1 1.2 Organization of the Report..............................................................................................2 2 DATA RESOURCES ............................................................................................................5 2.1 Strong Motion Database..................................................................................................6 2.2 Shallow Site Condition Database....................................................................................9 2.2.1 Surface Geology..................................................................................................9 2.2.2 Near-Surface Shear Wave Velocity ..................................................................15 2.2.3 Geotechnical Data .............................................................................................20 2.3 Deep Basin Structure Parameter Database....................................................................26 2.3.1 Introduction.......................................................................................................26 2.3.2 Available Basin Models ....................................................................................26 2.3.3 Parameterization of Basin Geometry ................................................................35 2.3.4 Uncertainty in Basin Depths .............................................................................36 3 GROUND MOTION AMPLIFICATION FACTORS FOR VARIOUS CLASSIFICATION SCHEMES ........................................................................................45 3.1 Introduction...................................................................................................................45 3.2 Databases ......................................................................................................................47 3.2.1 Strong Motion Data...........................................................................................47 3.2.2 Site Classifications............................................................................................47 3.3 Data Analysis ................................................................................................................49 3.3.1 Amplification Factors from Individual Recordings ..........................................49 3.3.2 Regression Procedure........................................................................................50 3.3.3 Example Results and Statistical Testing of Results ..........................................52 3.4 Results...........................................................................................................................56 v 3.4.1 Synthesis of Results for Each Classification Scheme.......................................56 3.4.2 Comparison to Previous Studies .......................................................................62 3.4.3 Magnitude- and Distance-Dependence of Results ............................................65 3.4.4 Intercategory Error Terms.................................................................................67 3.5 Conclusions and Recommendations .............................................................................70 4 NONLINEAR SITE AMPLIFICATION AS FUNCTION OF SHALLOW SHEAR WAVE VELOCITY (Vs-30)...................................................................................73 4.1 Statement of the Problem..............................................................................................73 4.2 Existing NEHRP Amplification Factors .......................................................................76 4.3 Data Resources..............................................................................................................78 4.4 Development of Amplification Model..........................................................................79 4.4.1 Amplification within Vs-30 Categories...............................................................80 4.4.2 Variation of Amplification with Vs-30 ...............................................................86 4.4.3 Mixed Effects Regression for Unified Model...................................................88 4.4.4 Comparisons of Model Predictions to Data ......................................................92 4.4.5 Analysis of Standard Deviation Terms .............................................................96 4.5 Comparison to Amplification Factors by Others ........................................................102 4.5.1 Velocity Dependence of Amplification ..........................................................102 4.5.2 Amplification Levels within NEHRP Categories ...........................................103 4.5.3 Standard Deviation Terms...............................................................................106