Abstract
Returning to the old problem of observed rotation effects, we present the recording system and basic elements of the theory related to the rotation fi eld and its association with seismic waves. There can be many different causes leading to observed/recorded rotation effects; we can group them as follows: generation of micro-displacement motion due to asymmetry of source processes and/or due to interaction between seismic body/surface waves and medium structure; interaction between incident seismic waves and objects situated on the ground surface. New recording techniques and advanced theory of deformation in media with defects and internal (e.g., granular) structure make it possible to focus our attention on the fi rst group, related to microdisplacement motion recording, which includes both rotation and twist motions. Surface rotations and twists caused directly by the action of emerging seismic waves on some objects situated on the ground surface are considered here only in the historical aspects of the problem. We present some examples of experimental results related to recording of rotation and twist components at the Ojcow Observatory, Poland, and L'Aquila Observatory, Italy, and we discuss some prospects for further research.
Highlights
Droste and Teisseyre (1976) derived the rotation seismogram from the azimuth recording system at a site very close (1-3 km) to mine
The ring coil laser interferometer, based on the Sagnac effect, seems to be the most advanced (Takeo and Ito, 1997; Jaroszewicz et al, 2001; Cochard et al, 2003), but some problems arise with application of such gyroscopes to the extremely high sensitivity required in seismology
We shall note in advance that the observed rotation and twist effects as recognized by special recording system bring very small motions in the range of 10 -8 rad/s; it remains an open question whether such motions can cause the macro-rotation effects as experienced by some surface structures after strong earthquakes
Summary
Rotation effects at the ground surface associated with earthquakes have been described in several classical monographs, for instance Hobbs (1907) and Davison (1927); the cited examples concern, among other things, twisting of some obelisks, stone graves and segments of columns. Gutenberg (1927) remarked that such effects are observed, they cannot be explained as being caused by rotation waves, because such. In our present study we refer to the rotation-seismograph system consisting of two oppositely oriented seismographs, having pendulums suspended on a common axis Such a seismograph system can record rotation, twist and stretching effects. We will not enter here into the theories of continua with internal structure and of continua containing a dense distribution of internal nuclei (dislocations, disclinations, vacancies, thermal nuclei, or electric nuclei) which may be the objects/sources creating internal/self stresses Some of these internal sources, like dislocations, disclinations and rotation nuclei (rotating grains) cause deviation from stress and strain symmetry; for reference see e.g., Anthony et al (1968), Kleman (1980), Kossecka and DeWitt (1977) and Teisseyre (2002). We shall note in advance that the observed rotation and twist effects as recognized by special recording system bring very small motions in the range of 10 -8 rad/s; it remains an open question whether such motions can cause the macro-rotation effects as experienced by some surface structures after strong earthquakes
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