Six-degree-of-freedom near-source seismic motions II: examples of real seismogram analysis and S-wave velocity retrieval

Near-source records obtained by the mechanical seismic sensor Rotaphone are presented. The Rotaphone can measure six components of seismic movements, three translational and three rotational. The apparent S-wave phase velocity is determined and the possibility to obtain the wavepath S-wave velocity directly under the receiver is discussed. Rotation-to-translation ratios (RTRs) characterize the strength of rotations compared to translations. The Rotaphone records of local microearthquakes were obtained in various European seismoactive regions over the last few years. Three case studies, analyzed in detail, include various geological structures and seismograms recorded at various epicentral distances from 0.7 to 14.9 km. Also, the source depth varies from 4.8 to 10.4 km. The first case is an event from the West Bohemia intraplate seismic swarm region. The seismogram was recorded only 0.7 km from the epicenter. This case shows the complexity of rotation-to-translational relations near the epicenter. The second case is from the Corinthian Gulf active-rift region. The study confirms the expectation of the theory concerning rotations connected with the direct S wave; however, difficulties follow from a very complex 3D geological structure in the vicinity of the station, complicated by a distinctive topography with steep slopes of the hills. The third example is from South Iceland, near the active Katla volcano. The data in this case satisfy the rotation-to-translation relations very well, which is probably caused by the relatively simple geological setting and appropriate source-to-receiver configuration. The RTRs are computed for all three cases, and their frequency dependence is discussed.