Earthquake Science Center Seminars
Teleseismic body wave retrieval from ambient seismic noise correlation
Cross-correlation of seismic noise is now an established method to observe the propagation of surface waves between pairs of sensors without involving transient sources. These observed surface waves are routinely used to depict high-resolution image of the crust and upper mantle, or mapping the velocity changes associated with tectonic events. Recent works highlight more challenging observations using similar techniques, such as attenuation/amplitude measurements and body waves retrieval at various scales of the Earth. In order to better understand the 3D Earth structure, we aim at producing new data for the imaging by improving our capability to reconstruct Green's function between any combinations of sensors. Here we focus on the detection of body waves at teleseismic distances using a continuous one-year global broadband dataset. We first show that body waves emerge from cross-correlation of continuous records in the 5s to 100s period band. Then we demonstrate that these reconstructed phases can be used as a complement to earthquake data for imaging purposes from the crust to the deepest structure of the Earth. In this context, we analyze the contribution of large earthquakes, and particularly their long lasting reverberated coda and compare it to the contribution to correlations of the continuous background sources associated with the ocean-crust interaction. For the long period band (25s to 100s), we show that very late coda of large earthquakes, not excluded by standard preprocessing of noise records, produces highly correlated signals which contribute to large amplitude spurious arrivals. This point is also illustrated by numerical modeling. When using a careful processing based on coherence analysis of the daily records, we show that global sections of body wave propagation can be retrieve from actual ambient noise, with realistic amplitude ratios between the phases and true polarizations. At shorter periods (5s to 10s), standard procedures developed for surface waves reconstruction are sufficient to remove the effects of earthquakes on the reconstruction of deep phases.
Pierre Boue, Stanford University
June 4, 2014