3TU Munich www.geophysik.uni-muenchen.de www.rotational-seismology.orgDepartment of Earth Sciences

1IntroductionThe collocated observations of rotations around a vertical axis and translations obtainedby a 4x4m ring laser and traditional broad band seismometer recently opened opportunities tofind out additional information that may not be contained in classical three-componentrecordings. By investigating the cross correlation between transverse acceleration and rotationrate around a vertical axis we recognized that there is significant rotational motion component in the P coda of seismic signals. Theoretically, in spherically symmetric isotropic media we should not observe a vertical component of rotation before the onset of SH waves. Two possible explanations for this phenomenon are: (1) Tilt of the Earth’s surface caused by P waves polluted the ring laser measurements; (2) P-SH converted energy (because of 3D effects) close to the receiver. In this study, tilt in the P coda of the seismic signal as well as its effects on the Ring Laser records have been calculated from translation data of a 3-component-seismometer. These estimates indicate that the P coda rotations are due to both P-SH scattering and ring laser – tilt coupling, however, P-SH scattering being orders of magnitude stronger. The result may eventually provide a means to estimate near receiver P-SH scattering in a quantitative way, thereby separating out the SH contribution in the scattering field