Broad-band observations of earthquake-induced rotational ground motions

It has been noted by theoretical seismologists for decades that-in addition to translations and strains-the rotational part of ground motions should also be recorded. It is expected that collocated measurements of translations and rotations may (1) allow transformation of translational seismograms to the complete ground motion of an observation point; (2) help to further constrain rupture processes and (3) provide additional hazard-relevant information to earthquake engineers. The lack of instrumental sensitivity used to be the main obstacle to observing rotational motions. Recently, ring laser technology has provided the means to develop instruments that allow in principle the observation of rotational motions in a wide frequency band and epicentral distance range. Here we investigate whether this technology-originally designed for geodesy-is capable of providing accurate and useful observations for seismology. We report observations of rotations around a vertical axis of several earthquakes obtained by a 4 x 4 m ring laser installed in SE-Germany and compare them to collocated broad-band translations. Assuming plane transverse wave propagation, acceleration and rotation rate should be in phase and their amplitude ratio proportional to horizontal phase velocity. Here we show that most of the observations can be explained under these assumptions and that the collocated observations allow the estimation of wavefield properties (e.g. phase velocities, propagation directions), otherwise only accessible through seismic array measurements, polarization analysis, or additional strain measurements.