Seismic rotation waves related to volcanic, mining and seismic events: Near-field and micromorphic motions
Rotation and twist waves recorded in Hokkaido (Japan), Ojców and Książ (Poland) and L'Aquila (Italy) bring a new insinght into the features of these motions for volcanic, mining and tectonic events. The rotation seismograph system used at the observatories in Hokkaido differs from that observatories in Poland and Italy. Nevertheless, the results obtained, for all observatories, are compatible. The levels of rotation and twist motions depend distinctly on the back-ground properties of the recording sited. Moreover, at a given recording station we have noticed two different levels of these motions; for some near events the records indicate very small rotation motions, while for other, also near events, these motions are distinctly greater and remain on a similar level as those for records from the distant sources. For the near events the evident differences in magnitudes and character of these motions suggest a distinct relation between the rotation and twist motions and the source mechanism. Similarly, an analysis of records for the volcanic events indicates existence of two kinds of events: for the first group, the rotation components reach the order of magnitude much greater than those for the group, for which the rotation and twist motions are quite negligible. The records related to this group, representing probably the near-surface volcanic explosions, confirm at the same time the reliability of results obtained for the rotation motions belonging to the first group, as the technique of recording and distances remain for both groups the same. The records from several stations give grounds for searching for as influence local structure on rotation and twist motions. The data treatment applied for local events made it possible to separate the recorded motions into the micromorphic and source related parts.
Bibliogr. 17 poz.