The GEOsensor Project: Rotations — a New Observable for Seismology

Abstract

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.

Broad-band observations of earthquake-induced rotational ground motions

“…Nigbor 1994) close to seismic sources to 10 −11 rad s −1 observed for large earthquakes at teleseismic distances (e.g. Igel et al 2005; Schreiber et al 2005, 2006), thus spanning more than 10 orders of magnitude. It is unlikely that one instrument (or one instrumental technology) will ever be capable of providing accurate measurements over such a large range of amplitudes.…”

“…tilt) will contribute to the vertical component of rotation rate. This contribution was investigated by McLeod et al (1998) and Schreiber et al (2005, 2006) who concluded that it is one to two orders of magnitude lower than the rotation due to the seismic signal. At present it is not clear whether standard tiltmeters are providing accurate enough information in the relevant frequency band as their signals are polluted by transverse acceleration.…”

“…Levevre 1993) and ring laser based sensors (e.g. Stedman et al 1995; Stedman 1997; Schreiber et al 2005, 2006). …”

“…Following the above mentioned first observations of distant earthquake‐induced rotations, the ring laser technology and data acquisition system was continuously improved and modified to fulfil the needs of seismology. This implies particularly precise timing and observations at sufficiently high sampling rate (Schreiber et al 2005, 2006). Those modifications were carried out on the 4 × 4 m ring laser system that was installed at the geodetic observatory Wettzell, SE‐Germany in 2002 (Fig.…”

A 1.82 m2 ring laser gyroscope for nano-rotational motion sensing

“…In order to improve this contact as much as possible, a liquid, fast-setting, concrete has used to fill cracks and gaps between the floor and the monument basis. The optical cavity design is based on the GEOsensor [13] project. As stated before, it is a square optical cavity, 5.40 m in perimeter and 1.82 m 2 in area, enclosed in a vacuum chamber entirely filled with the active medium gas.…”