What is the use of rotation sensing for passive seismology?

Heiner igel, Felix Bernauer, Joachim wassermann, Shihao Yuan

Department of Earth and Environmental Sciences, Ludwig-Maximilians-University, Theresienstr 41, Munich, Germany (heiner.igel@lmu.de).

There are several ways to answer this question: 1.) Make your broadband seismic displacement observations without tilt contamination. Any inertial displacement sensor currently is sensitive to tilt leading to apparent translational motions. The only way to fix this, is to observe all 6 degrees-of-freedom components (6 DoF), i.e., 3 additional rotation components, and correct for it using the seismometer equation. In many cases this might not be relevant, in some cases (e.g., ocean bottom recordings, strong ground motions, long-period observations, volcanoes) tilt corrections are highly relevant. 2.) Turn your single-point measurement into a mini-array.  It is well known from theory and recently documented with field observations that 6 DoF measurements allows access to wavefield properties otherwise only accessible with arrays (e.g., wave type separation, propagation directions, phase velocities, incidence angles). This is particularly useful in situations where dense arrays are not possible (e.g., ocean floors, remote or challenging environments, boreholes, volcanoes, planetary objects), and/or where velocity information on sub-receiver structure is sought. 3.) Decrease your station density while keeping (or improving) your resolution power for seismic inverse problems. It has been shown theoretically that additional gradient observations (in this case rotational ground motions) allows beating Nyquist for wavefield reconstruction of surface observations, and that earthquake source properties can be recovered with equivalent or better resolution when half the number of 6 DoF measurements are done rather than classic 3C measurements. However, this still needs to be demonstrated with field data. There are some other interesting aspects concerning scattering/ in strongly heterogeneous media, rupture tracking that will be discussed in the talk. We will also report on the status of portable broadband rotation sensors, a technology that is currently emerging.

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