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ROtational Motions: a new observable for seismologY

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

Seismology is a science based on the observation of ground motions using seismometers. Seismometers measure three orthogonal components of linear motions (up-down and in NS and EW directions). However the ground also undergoes three independent rotational motions that have never been routinely observed. These motion types are difficult to measure. The starting point of the proposal was the fact that no sensor dedicated to seismology existed. Prior studies of this motion type was possible with a single-component high-end, very expensive instrument developed for the purpose of geodesy: a ring laser, the socalled G-ring.

The primary goals of the ROMY project was to build on this experience and construct a first-of-its kind multicomponent ring laser system at the Geophysical Observatory in Munich. With a substantial further contribution by the host institution (Ludwig-Maximilians-University Munich) a tetrahedral shaped 4-component ringlaser (one component for redundancy) could be designed and constructed that was installed in 2016. The ring laser technology requires nano-precision to allow the establishment of the optical path of laser light around each triangular “ring” with 12m side length. For each of the four rings this optical path could be established. This implies that for the first time, the complete vector of Earth’s rotation could be directly observed with this instrument. While currently the resolution of Earth’s polar motion is still in the area of O(100m), further stabilization measures may bring the resolution down by several orders of magnitude, making it comparable and complementary to Very Long Baseline Interferometry (VLBI).

When ring laser operate well for geodesy, they clearly fulfill the requirements for seismology. The ROMY ring has allowed observing earthquake and ocean-related ground motions with all 6 motion components with unprecedented sensitivity. The instrumental setup helps in particular solving the current enigma (or at least providing high-quality additional observations) to understand the amount of Love waves observed in the ocean generated noise. We are still waiting to observe the first free oscillation of the Earth that are excited by large earthquakes.

An unexpected development during the first half of the project led to an exciting new product: the first commercial broadband portable rotation sensor for seismology. The French company iXblue, a world leader in navigation technology for marine, air and space applications, accepted our promotion for a sensor in geophysics and started a new product line called blueSeis-3A (www.blueseis.com). The ROMY Team provided the detailed technical specifications for this sensor and carried out the first laboratory characterizations and field tests. The sensor solves some fundamental issues in seismology such as correcting the tilt-contamination of classic broadband seismometers that is highly relevant for strong motions (near field of earthquakes or volcanic signals), ocean-bottom observations and long-period observations. The sensor has now been extensively tested, calibrate, and validated and is ready for geophysical applications. It is important to now that – through this development – new flavors of fibre-optic based sensors for ground motion sensing are being developed in particular for geophysical observatories (larger, higher resolution) and earthquake engineering (smaller, lower resolution) in both cases with a wide range of applications.

The ROMY project has established a new instrumentation strategy both for geophysics and geodesy. Several new projects have emerged that would not have been possible without ROMY. Notably the H2020 funded PIONEERS project that aims at bringing the 6C concept to planetary exploration. New projects aim at studying active fault zones and the vibration of building with the 6C instrumentation concept. Further info at www.romy-erc.eu).