The GEOsensor: A measurement device for rotations caused by seismic sources

U. Schreiber (1), H. Igel (2), F. Vernon(3), A. Velikoseltsev(2),  G. Sasagawa(3), T. Hansen(3)

1. Department für Geo- und Umweltwissenschaften LMU
2. Forschungseinrichtung Satellitengeodäsie der TU-München
3. Scripps Institution of Oceanography

Recent advances in ring laser technology have led to the successful construction of highly sensitive gyroscopes suitable for the study of seismic induced rotation. By vastly up-scaling the size of navigational ring lasers, one obtains improved instrument sensitivity several orders of magnitude above commercially available sensors. We measured rotations from teleseismic earthquakes, which are consistent in waveform and amplitude with collocated recordings of transverse accelerations of a standard seismometer. This achievement marks an important step towards a complete six degrees of freedom measurement of seismic events. The GEOsensor is a seismic measurement platform, comprising a large ring laser, a standard seismometer, and auxiliary devices like a GPS referenced time base, a temperature, and a pressure sensor, etc. Following completion of the system at the Geodetic Observatory Wettzell (Germany), we installed the GEOsensor at a UCSD seismic observatory located in Piñon Flats, CA (USA). The ring laser, installed in Winter 2005, measures the vertical component of the rotation vector (i.e. rotation, twist, or spin around a vertical axis). We will present technical details of the installation and instrument properties and show observations from the instrument before and after it was installed in California. The database up to now consists of a few dozen seismic events with epicentral distances ranging from a few hundred to a few thousand kilometers. Collocated observations of rotations, strain, and translations will allow for the theoretical estimation of horizontal phase velocities of transversely polarized waves as well as the direction of propagation. Additionally, we will compare the accuracy of these estimates with respect to those derived from standard seismic array measurements.