Passive seismic imaging of structure discontinuities around the active fault using scattered earthquake waveforms

For seismically active fault with relative dense seismic network, seismic tomography using first arrival times from earthquakes is routinely applied to image fault zone structures. To characterize structure discontinuities around the active fault, seismic scattering imaging using scattered waves can be applied. This technique has been extensively used with active seismic sources, especially in oil/gas industry. However, scattering imaging using waveforms from earthquakes is rare. Here we present the imaging results using scattered SH waves from earthquakes around the SAFOD, California. Near vertical reflectors are clearly imaged around the San Andreas fault (SAF), similar to results obtained using scattered P-P waves. However, for strike-slip focal mechanisms for earthquakes along the SAF, first arrival polarities for P waves are different at different regions. In comparison, SH waves have the same polarities. Because wavelength for P waves is longer than S waves, making the resolution of the imaging result is higher when using scattered S waves. For this study, we found coherent phases (scattered SH waves) after direct SH waves. Overall, the imaging results using different types of waves are bery similar, thus supporting the reliability and usefulness of using passive seismic events to image structure discontinuities.