Seismological Processing of Six Degree-of-Freedom Ground-Motion Data

Abstract

Recent progress in rotational sensor technology has made it possible to directly measure rotational ground-motion induced by seismic waves. When combined with conventional inertial seismometer recordings, the new sensors allow one to locally observe six degrees of freedom (6DOF) of ground-motion, composed of three orthogonal components of translational motion and three orthogonal components of rotational motion. The applications of such 6DOF measurements are manifold—ranging from wavefield characterization, separation, and reconstruction to the reduction of non-uniqueness in seismic inverse problems—and have the potential to revolutionize the way seismic data are acquired and processed. However, the seismological community has yet to embrace rotational ground-motion as a new observable. The aim of this paper is to give a high-level introduction into the field of 6DOF seismology using illustrative examples and to summarize recent progress made in this relatively young field. It is intended for readers with a general background in seismology. In order to illustrate the seismological value of rotational ground-motion data, we provide the first-ever 6DOF processing example of a teleseismic earthquake recorded on a multicomponent ring laser observatory and demonstrate how wave parameters (phase velocity, propagation direction, and ellipticity angle) and wave types of multiple phases can be automatically estimated using single-station 6DOF processing tools. Python codes to reproduce this processing example are provided in an accompanying Jupyter notebook.

Rotation, Strain, and Translation Sensors Performance Tests with Active Seismic Sources

“…It is clear that rotation and strain measurements attract more and more interest for geophysical research [ 17 ]. Rotation and strain sensing instruments have to be tested and compared extensively in order to establish reliable measurement techniques.…”

“…The first observations of earthquake-induced rotational ground motions by large ring laser gyroscopes [ 2 , 3 ], as well as the observation of crustal coseismic deformation during the Landers earthquake sequence in 1992 by long baseline strainmeters [ 4 ], certainly count to the major milestones in seismic wavefield gradient observation. However, only very recent developments of portable seismic rotation and strain sensors made the direct observations of seismic wavefield gradients possible for a broad range of applications, such as volcanology [ 5 , 6 , 7 ], ocean bottom seismology [ 8 , 9 , 10 ], structural health monitoring [ 11 , 12 , 13 ], seismic exploration [ 14 , 15 , 16 , 17 ], microzonation in urban environments [ 18 ], and glaciology [ 19 ]. The most commonly used technologies for seismic ground rotation sensing are fiber-optic Sagnac interferometry [ 20 ], micro-electro mechanical systems [ 21 ], small-scale finite differencing within a rigid configuration of translation sensors [ 22 ], and liquid-based electrochemical transducers [ 23 ].…”

Validation of Rayleigh Wave Theoretical Formulation with Single-Station Rotational Records of Mine Tremors in Lower Silesian Copper Basin

“…In order to separate surface waves from the overall ground motion, the qualitative methodology of the time-frequency decomposition was used. A sophisticated algorithm related to this topic was presented in the paper by Sollberger et al [47,48], where a timefrequency decomposed representation of the 2018 Gulf of Alaska earthquake by computing the S-transform was performed. This approach seems to be very promising in terms of the automatic detection of surface waves, but at this stage, it has not been verified yet in the LSCB geological mining conditions, where the epicentral distance of the source of the tremors is often lower than 7 km.…”

Automated Quality Assessment of Interferometric Ring Laser Data

“…In seismology, rotational motion observations supplement the conventional observation of translational motions and extend the scope of insight granted by seismic observations (e.g., [ 1 , 2 , 3 , 4 ]). Array derived rotations, based on classical translational seismometer array measurements (e.g., [ 5 , 6 , 7 ]) and recent efforts to derive rotational motions using high-rate GNSS measurements [ 8 ], were complemented by rotational sensors in the course of the last decades (e.g., [ 9 , 10 ]).…”