Evidence for the splitting of shear waves from waveform and focal mechanism analyses

A temporary seismograph station was sited above the aftershock area of an intermediate crustal earthquake (M = 4.9) in the Kinki district, Japan. The results from analysing digital three-component seismograms and numerically determining focal mechanisms of aftershocks have provided clear evidence for the splitting of shear waves attributed to crustal anisotropy. Faster shear waves from most of the aftershocks are polarized WNW-ESE, regardless of their azimuths and incident angles. Such shear wave motions are often significantly inconsistent with the focal mechanisms of the aftershocks determined using P-wave first motions. For many of the events, slower shear waves, polarized orthogonally to the faster shear wave direction (NNE-SSW), are observed to arrive about 0.1 s later. Two vectors showing particle motions of faster and slower shear waves in the horizontal plane are summed, to reconstruct polarizations of shear waves before splitting. For events with sufficiently high signal-to-noise ratios of shear waves and well determined focal mechanisms, the reconstructed polarizations are consistent with the focal mechanisms within the range of uncertainty. Such consistency is not clearly detected for the other events, possibly because of large noise effects and poorly constrained fault-plane solutions. The direction of faster shear wave polarization (WNW-ESE) generally coincides with that observed at three other stations located within 15 km of the temporary station. In the study area, the fabric of crustal anisotropy is almost uniform over the scale of 15 km, but appears to exhibit small local fluctuations.