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ROMY Report Summary

Project ID: 339991
Funded under: FP7-IDEAS-ERC
Country: Germany

Mid-Term Report Summary - ROMY (ROtational Motions: a new observable for seismologY)

The ROMY project aims at understanding any aspect of rotational ground motions at the Earth’s surface. Earthquakes, the atmosphere, and the oceans permanently excite the Earth to vibrate. Classic seismometers measure translational motions routinely. However, every measurement point also undergoes rotational motions and these have never been observed with the exception of single-component measurement using a highly accurate ring laser system. One of the most important ROMY project goals is the installation of a first-of-its kind multicomponent ring laser system with which the Earth’s rotation as a whole and local ground rotations can be observed with unprecedented detail. With additional support by the Ludwig-Maximilians-University Munich a unique construction was designed that now hosts the first large tetrahedral ring laser with full operation being expected in 2017. This instrument will be sensitive enough to observe the Earth’s free oscillations, ocean generated noise, and teleseismic events. In addition we expect to observe temporal variations of Earth’s rotation rate.
In several theoretical studies the potential of additional rotational ground motion measurements for seismology could be proven. For example, earthquake source processes can be better constrained when observing both translational and rotational measurements. In addition, local sub-receiver tomography is possible even with a single station. This is particularly interesting in situations, when seismic arrays are not possible (e.g., in planetary seismology, ocean floor, boreholes). However, to demonstrate these concepts in the field we need a portable, high-sensitive rotation sensor that – at the beginning of the ROMY project – was not in sight. In a unique science-industry collaboration the French company iXBlue decided to open a new branch in seismology developing a rotation sensor based on fibre-optic technology for the specific requirements in seismology. The sensor is called BlueSeis ( with a first prototype shipped directly to a field installation at Stromboli volcano end of September 2016. As previously shown we expect to improve the imaging the magmatic processes inside volcanoes with this new sensor type. Further field experiments (seismic exploration, seismically active regions, volcanic activity, building vibrations) are scheduled for the 2nd project part.
It is well known that ocean-bottom seismometer (OBS) recordings are extremely noisy for the horizontal motion components. For decades it had been postulated that rotational motions of the ground (and/or induced by currents) cause these noisy signals. In a simple experiment – putting a low-resolution rotation sensor otherwise used for navigation on an OBS system – we could measure for the first time directly rotations on the seafloor showing that indeed these rotational (tilt) motions couple into the classic seismometer records. Observing them directly implies that they can be corrected for improving the quality of OBS recordings, and substantially improving science output of very expensive experiments.
One of the most enigmatic observations in seismology in the past decade is the observation of unexpected high amplitudes of transversal (Love) waves generated through ocean waves. The ring laser observations provide useful information here, as they filter out these specific motion type and help quantifying this observation. In connection with seismic monitoring of storms in the oceans, their connection with global change, and the ubiquitous use of noise signals for seismic tomography, the observations of rotational ground motions with our new ring laser system should help solving this puzzle.

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