Moment-Tensor Determination by Nonlinear Inversion of Amplitudes

Abstract

We propose an inversion method of the direct wave amplitudes P, SV, and SH to determine the general moment tensor (MT) and estimate the associated uncertainties. This method is a generalization of the one we developed to determine the double-couple (DC) or fault-plane solution from sparse observations in reservoir contexts (Godano et al., 2009). Like the previous one, it is based on a simulated annealing inversion algorithm. First, we test the reliability of this new method on synthetic data. The inversion allows retrieval of the expected moment tensor, but the analysis of the uncertainty associated with the solution shows that the resolution of the inversion is sensitive not only to the direct wave amplitudes but also to uncertainties in the velocity model and the event location. Next, we test the method on four induced microearthquakes recorded by the Soultz-sous-Forets hot-dry-rock (HDR) reservoir network. The inversion correctly converges for three events, and the obtained MTs display a dominant DC part (70 to 90%). The uncertainty associated with the non-DC part is relatively high. This prevents us from determining if the small non-DC part is significant or an artifact related to noise in the data and/or uncertainties in the velocity model and at the event location.