What the exercise of the SPICE source inversion validation BlindTest 1 did not tell you

Abstract

Uncertainties of earthquake finite-fault inversions based upon strong motion data are investigated using the source inversion validation BlindTest 1 exercise of the SPICE (Seismic Wave Propagation and Imaging in Complex Media: A European Network) project, motivated by previous counterintuitive results. The distribution of slip and the shapes of asymmetric slip rate functions are simultaneously inverted by matching 10 or 33 broad-band three-component velocity waveforms within the period ranging from 0.02 to 2 Hz, using a finite-fault method that carries out the waveform inversion in the wavelet domain. The effects of subfault size, data noise and the number of stations have been explored. Our results suggest: (1) Although there are inevitable discrepancies between the inverted model and the target model because of ignoring the spatial slip variations within individual subfaults, the fault slip and rise-time distributions can be well constrained even with the data including large Gaussian noise. (2) It is crucial for source studies to develop new inversion schemes that can properly honour the frequency- and time-dependent energy distribution of seismic radiation and data noise. For instance, inversions using the variance reduction function of velocity waveforms as the objective function have low sensitivities to the total seismic moment and peak slip. (3) Although the relative value of the objective function is guided in the inversion, the absolute value of the objective function cannot be used to evaluate the quality of an inverted model. (4) Because the source inversion is based on surface observations, the spatiotemporal resolution of source inversion is affected not only by the data quality but also by the earthquake itself. For vertical strike-slip faults, the along-strike resolution is better than that along the downdip direction.