Shear-wave anisotropy: Spatial and temporal variations in time delays at Parkfield, Central California

Yun Liu*, Stuart Crampin, Ian Main

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract / Description of output

Shear-wave splitting is analysed on data recorded by the High Resolution Seismic Network (HRSN) at Parkfield on the San Andreas fault, Central California, during the three-year period 1988-1990. Shear-wave polarizations either side of the fault are generally aligned in directions consistent with the regional horizontal maximum compressive stress, at some 70° to the fault strike, whereas at station MM in the immediate fault zone, shear-wave polarizations are aligned approximately parallel to the fault. Normalized time delays at this station are found to be about twice as large as those in the rock mass either side. This suggests that fluid-filled cracks and fractures within the fault zone are elastically or seismically different from those in the surrounding rocks, and that the alignment of fault-parallel shear-wave polarizations are associated with some fault-specific phenomenon. Temporal variations in time delays between the two split shear-waves before and after a ML = 4 earthquake can be identified at two stations with sufficient data: MM within the fault zone and VC outside the immediate fault zone. Time delays between faster and slower split shear waves increase before the ML = 4 earthquake and decrease near the time of the event. The temporal variations are statistically significant at 68 per cent confidence levels. Earthquake doublets and multiplets also show similar temporal variations, consistent with those predicted by anisotropic poroelasticity theory for stress modifications to the microcrack geometry pervading the rock mass. This study is broadly consistent with the behaviour observed before three other earthquakes, suggesting that the build-up of stress before earthquakes may be monitored and interpreted by the analysis of shear-wave splitting.

Original languageEnglish
Pages (from-to)771-785
Number of pages15
JournalGeophysical Journal International
Issue number3
Publication statusPublished - 1 Jan 1997

Keywords / Materials (for Non-textual outputs)

  • Anisotropy
  • Fault gouge
  • Pore-fluid pressure
  • Shear-wave splitting
  • Temporal variations


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