Observations and Modeling of Rotational Signals in the P Coda: Constraints on Crustal Scattering

Abstract In addition to three classical components (vertical, north–south, and east–west) of ground translations recorded by a broadband seismometer, a component of earthquake induced rotational ground motions around the vertical axis is consistently measured by a ring laser sensor located in Wettzell, southeast Germany. Significant rotations around the vertical axis in the P coda of teleseismic signals are either directly visible or can be inferred through the investigation of cross correlation between the transverse component (the component that is perpendicular to the great circle connecting the earthquake and the seismometer) of translation acceleration and ring laser rotation rate. Theoretically, in spherically symmetric isotropic media, we should not observe rotational signals around the vertical axis before the onset of SH waves. Possible causes for the observed rotations in the P coda are: (1) tilt–ring laser coupling, (2) anisotropy, (3) topographic scattering, and (4) P - SH scattering in the crust. Here we show that P - SH scattering in the 3D random crust can explain the observations and allow us to constrain crustal scattering properties.