Comparative Measurements of Local Seismic Rotations by Three Independent Methods.

Johana Brokešová,Jiří Málek

doi:10.3390/s20195679

Abstract

A comparative active experiment that is aimed at collocated measurement of seismic rotation rates along three orthogonal axes by means of three different methods is described. The rotation rates in a short-period range of 6–20 Hz were obtained using three different methods: the 6C Rotaphone sensor system developed by the authors, the commercial R-1 rotational sensor by Eentec, and a small-aperture array of twelve standard velocigraphs in a rectangular arrangement. Those three methods are compared and discussed in detail. A medium-size quarry blast was used as a seismic source. At a distance of approximately 240 m, the rotation rates reached an amplitude of the order of magnitude of 10–10 rad/s. The array derived rotation rates displayed serious limitations, as clearly documented. The R-1 instruments have shown certain technical problems that partly limit their applicability. The measured rotation rates were compared to the relevant acceleration components according to rotation-to-translation relations. Out of all the three methods, the records best matching the acceleration components were made by Rotaphone. The experiment also revealed that rotation rates in the given short-period range noticeably changed over a distance as short as 2 m.

Highlights

Observational seismology mainly relies on measuring translational ground motions by traditional seismometers
There is a third type of seismic motion, seismic rotations [2], which should be measured together with translations and strains in order to obtain a complete description of the seismic wavefield in a close vicinity of an observation point
A growing interest in detecting and interpreting seismic rotational motions gave rise to new seismological disciplines, rotational seismology and rotational seismometry, and it has been demonstrated by the scientific journals publishing several special issues exclusively dedicated to these new disciplines

Summary

Introduction

Observational seismology mainly relies on measuring translational ground motions by traditional seismometers. By translational motions, we mean the three Cartesian components of ground velocity v (Figure 1), as measured by conventional inertial seismographs that consist of a mass attached to a fixed frame. Another kind of seismic instruments, known for decades, are strainmeters [1], which are used to measure the deformation of the Earth by detecting changes in the distance between two points. A growing interest in detecting and interpreting seismic rotational motions gave rise to new seismological disciplines, rotational seismology and rotational seismometry, and it has been demonstrated by the scientific journals publishing several special issues exclusively dedicated to these new disciplines

Methods

Results

Conclusion