Rotational components of the surface ground motion during an earthquake

Abstract Under the assumption of an immovable point source for the earthquake energy release, a mathematical model is developed to compute the components of the free field surface rotation consistent with assigned translational components. Experimental evidence on these quantities is offered by the strong‐motion instrument arrays. However, lack of adequate identification of the different wave contributions prevents in general a direct measurement of rotational and torsional components, allowing only a qualitative assessment of just how large these components might be. The only exceptions are the records of SMART‐1, which offer the entire information to derive rotations. The available data of strong‐motion instrument arrays for this problem are summarized in Table I. They are in qualitative agreement with the suggestions of the mathematical model. Earthquake response spectra for the rotational components are developed for a few selected cases, in particular a rotational spectrum consistent with the prescription of USNRC Regulatory Guide 1.60. The correlations between the translational and rotational components of the ground motion are also shown.