Characteristics of rotational earthquake ground motions and their effects on structural response are not yet well-defined. Recording rotational ground motions directly in free field is in its infancy, and simultaneous sixcomponent earthquake measurements are being accumulated slowly. A six-degree-of-freedom (6DOF) ground motion observation system was installed in the Garner Valley Downhole Array (GVDA), a very well-characterized and wellinstrumented geotechnical array in Southern California. Since 2008, six-component free-field earthquake ground motions have been recorded from hundreds of earthquakes with a relatively wide range of hypocentral distances and magnitudes. In this paper, analysis was conducted to develop the characteristics of these measured rotational ground motions. Linear relationships between peak rotation velocity and peak ground acceleration were found, similar to previous 6DOF measurements in Taiwan and Japan. Ratios of rotation to translation as a function of hypocentral distance show larger ground rotations at closer distance, and that rotational ground motions tend to attenuate more rapidly than corresponding translational ground motions. Measured rotational motions show differences from estimations using elastic plane wave theory when using simple local apparent wave velocities. Finally, preliminary empirical relationships for rotational response spectra are estimated for earthquake engineering applications.

Earthquake insurance has become a topic of major importance in mainland China ever since the Mw 7.9 Wenchuan earthquake in 2008. Site‐condition amplification is one of the essential factors that influences the prediction of strong ground motion and the assessment of damages of buildings and structures. The site effects should hence be included in calculating earthquake damage and determining the insurance rate for insurance companies. In particular, large‐scale site classification is required for the implementation of earthquake insurance. This study first used a slope method for site classification in China through the VS30 values from the topographic slope data and the correlation between VS30 and the topographic slope. A novel site classification method, called the slope–geological method, was then proposed in this study to improve the slope method by combining geological characteristics, including geological age, the depositional environment, and soil properties. Four hundred and fifty three pieces of borehole data in Northern Plain and Sichuan of China were used to verify the effectiveness and difference of the slope method and the slope–geological method proposed in this study. Site‐condition classification maps of China were drawn using the slope method and slope–geological method, respectively.