Dr. Nori Nakata of Stanford University will give a talk at Speakers Club entitled Body-Wave Imaging Using the Ambient Seismic Field on Thursday at 2:00 PM in Webb 1100.

Abstract:

Correlation-based analyses of ambient seismic wavefields are powerful tools for retrieving subsurface information such as stiffness, anisotropy, and heterogeneity at a variety of scales. Studies of ambient-field tomography have focused on the surface waves, especially fundamental-mode Rayleigh waves.  Although the surface-wave tomography is useful to model 3D velocity variations, spatial resolution is limited due to the diffuse depth sensitivity of the surface wave measurements. Moreover, to represent elastic media, we need at least two stiffness parameters (e.g., shear and bulk moduli).  I develop a technique to retrieve diving P waves from the ambient field and apply it to the data observed by the dense geophone networks at Long Beach, California (~2500 receivers with 100-m spacing), and at Piton de la Fournaise, ReUnion island, France (~300 stations with 80-m spacing). With two-step filtering, I improve the signal-to-noise ratio of body waves sufficiently to extract P waves that I use for 3D tomography of P-wave velocity structure.  The small scale-length heterogeneity of the velocity model follows a power law with ellipsoidal anisotropy of heterogeneity.  These small-scale heterogeneities are important for high-frequency motion prediction for earthquakes.