Systematic high-resolution imaging of fault zone structures

The subsurface structures of earthquake fault zones are investigated with systematic analysis of seismic fault zone (FZ) trapped waves and shear wave splitting in large waveform data sets. Contrary to previous claims on the existence of ∼100 m wide FZ seismic waveguide extending to the bottom of the seismogenic zone (e.g., > 10 km), an objective and quantitative analysis of FZ waveform data collected in the rupture zone of the 1992 Landers, California earthquake suggests that the ∼100 m wide waveguide extends only to the depth of ∼3 km. Results from a systematic shear wave splitting study along the Karadere-Duzce branch of the north Anatolian fault, which ruptured during the 1999 Mw7.4 Izmit, and Mw7.1 Duzce, Turkey, earthquake sequences, indicate the existence of ∼1 km wide belt of strongly anisotropic rock around the FZ. The anisotropic layer is confined primarily to the same depth extent (∼3 km) of the narrower (∼100 m) seismic trapping structure.; These results indicate that major strike-slip faults tend to have multiple near-vertical FZ layers. The shallow structures responsible for generating both FZ trapped waves and shear wave splitting effects may be related to the top part of a flower-type structure of the FZ that contains highly damaged materials with intense microcracks and resides above the active portion of seismogenic crust where earthquakes nucleate.; The spatio-temporal variations of crustal anisotropy are investigated from similar earthquake clusters that are identified using a waveform cross-correlation technique. Splitting parameters averaged within each cluster show significant variations for different propagation paths, indicating strong spatial variations and multiple mechanisms in the study area. Apparent temporal changes of up to 30% of splitting delay times is observed at stations near the epicentral region of the Duzce mainshock. However, the changes can be mostly explained by the spatial variations of propagation paths due to the changing seismicity, instead of changes in properties of the anisotropic medium. Delay times measured within similar earthquake clusters indicate at most 2% co-seismic changes associated with the occurrence of the Duzce earthquake. The results do not show systematic precursory changes before the Duzce mainshock.