Empirically determined finite frequency sensitivity kernels for surface waves

S U M M A R Y We demonstrate a method for the empirical construction of 2-D surface wave phase traveltime finite frequency sensitivity kernels by using phase traveltime measurements obtained across a large dense seismic array. The method exploits the virtual source and reciprocity properties of the ambient noise cross-correlation method. The adjoint method is used to construct the sensitivity kernels, where phase traveltime measurements for an event (an earthquake or a virtual ambient noise source at one receiver) determine the forward wave propagation and a virtual ambient noise source at a second receiver gives the adjoint wave propagation. The interference of the forward and adjoint waves is then used to derive the empirical kernel. Examples of station–station and earthquake–station empirical finite frequency kernels within the western United States based on ambient noise and earthquake phase traveltime measurements across USArray stations are shown to illustrate the structural effects on the observed empirical sensitivity kernels. We show that a hybrid kernel constructed from the empirical kernel and the kernel for a reference model can be used to compute traveltimes accurate to second order in model perturbations for an earth-like model. A synthetic test demonstrates the application of such hybrid kernels to predict surface wave phase traveltimes.