The GEOsensor Project: Rotations — a New Observable for Seismology

Over the last 40 years ring laser gyroscopes became one of the most important instruments in the field of inertial navigation and precise rotation measurements. They have a high resolution for angular velocities, a very good scale factor stability and a wide dynamic range. These properties made them suitable for aircraft and autonomous submarine navigation. Over the last decade we have developed several very large perimeter ring laser gyroscopes for the application in geodesy and geophysics (Schreiber et al., 2001). Because of a substantial upscaling of these ring lasers, their sensitivity to rotations has been increased by at least 5 orders of magnitudes. At the same time the instrumental drift was reduced by about the same amount. This progress in rotational sensor technology led to the successful detection of rotational signals caused by earthquakes (Pancha et al., 2000) several thousands kilometers away. These observations stimulated the development of a highly sensitive ring laser gyro for specific seismological applications. The GEOsensor provides rotational motions along with the usual translational motions at a high data acquisition rate of at least 20 Hz. Observations of seismic induced rotations show that they are consistent in phase and amplitude with the collocated recordings of transverse accelerations obtained from a standard seismometer over a wide range of distances and frequencies.