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 restitution of seismograms to the complete ground motion of an observation point; (2) help to further constrain rupture processes; (3) provide additional hazard-relevant information to earthquake engineers. The lack of instrumental resolution used to be the main obstacle to observing rotational motions. Recently, ring laser technology has provided the means to develop instruments that allow the observation of rotational motions in a wide frequency band and epicentral distance range. We report observations of rotations around a vertical axis of several large earthquakes obtained by a 4x4m ring laser installed in SE-Germany and compare them to broadband translations. Assuming plane transverse wave propagation (e.g. Love waves), rotation rate and acceleration should be in phase and amplitudes scale linearly with the horizontal phase velocity. This implies that - in principle - collocated measurements of translations and rotations would allow estimation of Love-wave dispersion and thus provide additional information not contained in classical three-component recordings. We show observations from a data base with now approxl 40 seismic events. The development of a prototype ring-laser based instrument specifically designed for seismology  has been completed and is installed since early 2005 at Pinon Flat observatory in Southern California for testing. We will report preliminary observations.

Preliminary version of figures.


Fig. 1. Magnitude distribution of processed events as a function of epicentral distance.



Fig. 2: Comparison of transverse acceleration (black, left axis) and rotation rater (red, right axis) for several events and frequency bands.

Fig. 3: Maximum (positive) cross-correlation as a function of time for two events between rotation rate (red) and transverse acceleration (black). Note the excellent correlation for shear wave arrivals and surface waves as well as the overall increased correlation after the first break.

Fig. 4a: Earthquake in Greece. Comparison of transverse acceleration (black, left axis) and rotation rater (red, right axis) as a function of frequency using a very narrow band filter with a dominante period given left.

Fig. 5a: Comparison of transverse acceleration (black, left axis) and rotation rater (red, right axis) and determination of phase velocities as a function of time.

Fig. 5b: Comparison of transverse acceleration (black, left axis) and rotation rater (red, right axis) and determination of phase velocities as a function of time.

Fig. 5c: Comparison of transverse acceleration (black, left axis) and rotation rater (red, right axis) and determination of phase velocities as a function of time.

Fig. 5d: Comparison of transverse acceleration (black, left axis) and rotation rater (red, right axis) and determination of phase velocities as a function of time.

Fig. 6b: Comparison of transverse acceleration (black, left axis) and rotation rater (red, right axis) and determination correlation as a function of backazimuth and time. The black line indicates the calculated backazimuth for Station Wettzell.

Fig. 6c: Comparison of transverse acceleration (black, left axis) and rotation rater (red, right axis) and determination correlation as a function of backazimuth and time. The black line indicates the calculated backazimuth for Station Wettzell.