Passive Seismic Imaging of Near Vertical Structures Around the SAFOD Site, California, Jointly Using Scattered P and SH Waves

Abstract

AbstractTo better illuminate structural discontinuities around the San Andreas Fault Observatory at Depth (SAFOD) site, following the previous study of Zhang et al. (2009), we have extended the Generalized Radon Transform (GRT) method to jointly use scattered P and SH waves from abundant local earthquakes recorded by a local dense seismic network. A hybrid imaging condition is applied to extract consistent structures from scattered P and SH images. In this way, more robust structure information can be determined from separate images with potential artifacts. For the transverse component waveforms, coherent S‐S scattered waves after the direct SH waves can be clearly identified, and they are less interfered with by other scattered and converted waves compared to scattered P‐P waves on the vertical component. Similar to Zhang, Wang, et al. (2009), near‐vertical reflectors are imaged on both sides of the San Andreas Fault (SAF) with scattered SH waves, which is generally consistent with the imaging results using scattered P‐P waves from local earthquakes and a wide‐angle active seismic reflection profile. Compared to the P‐P scattering imaging result, the imaging result using scattered SH waves has higher resolution due to shorter S wavelength and cleaner S‐S scattered waveforms, and the SAF, as well as other reflectors around it, is better resolved. Although the resolution of the joint image obtained by combining separate imaging results from different scattered waves may be degraded, it is able to more robustly characterize strong structure discontinuities using passive seismic sources.