The effect of velocity structure errors on double-difference earthquake location

We show that relative earthquake location using
double-difference methods requires an accurate knowledge of the velocity structure throughout the study region to prevent artifacts in the relative position of hypocenters. The velocity structure determines the ray paths between hypocenters and receivers. These ray paths, and the corresponding ray take-off angles at the hypocenters, determine the partial derivatives of travel time with
respect to the hypocentral coordinates which form the inversion kernel that maps double-differences into hypocentral perturbations. Thus the large-scale velocity structure enters into the core of the double-difference
technique. By employing a 1D layered model with sharp
interfaces to perform double-difference inversion of
synthetic data generated using a simple, 1D gradient
model; we show that inappropriate choice of the velocity
model, combined with unbalanced source-receiver
distributions, can lead to significant distortion and bias in
the relative hypocenter positions of closely spaced
events.

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