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.
Highlights
Recent progress in rotational sensor technology has made it possible to directly measure rotational ground-motion induced by seismic waves
Research on potential applications of rotational motion measurements was primarily driven by the earthquake seismology and earthquake engineering communities. This is mainly due to the rapid development of the ring laser technology, which enabled the direct observation of rotational motions in the frequency-band relevant for earthquake seismology
The reason for this is that the 6DOF ground-motion pattern is unique for each wave type [25], meaning that, if another wave type is dominant in the analysis time-frequency window, no parametrization of the Rayleigh wave 6DOF polarization vector v exists that causes it to lie parallel to the dominant polarization direction in the data e1
Summary
“Instruments ought to be devised [...] to ends [...], such as the measurement of horizontal concussions, of vertical elevation, and of heaving or angular motion of the surface. Until recently, an instrument capable of recording angular ground motions, as envisaged by Forbes and Milne-Home, was still missing It took 150 years before Robert Nigbor could report on the first successful observations of such ‘rotational’ ground-motions using a solid-state rotational velocity sensor [4]. It was Nigbor who coined the term ’six-degree-of-freedom (6DOF) measurement’ for combined recordings of the three components of translational and the three components of rotational ground-motion. For further information on the history of rotational seismology, we would like to refer the reader to the following reviews found in References [15,16,17,18,19,20,21]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
