Long Period Seismic Source Inversions Using Global Tomography Models

We have implemented a Centroid Moment Tensor (CMT) inversion scheme to determine source parameters of large global earthquakes. The method uses full ray theory to simulate surface waves in a laterally varying Earth model and to calculate linearised differential kernels with respect to the source location and seismic moment tensor. Calculations are made using a variety of global tomography Earth models. Moreover, we also employ the great circle approximation to study the impact of using different theoretical approaches in seismic source inversions. Synthetic tests suggest that even source parameters which fit the data very well may have very large errors due to incomplete knowledge of lateral heterogeneity. The method is applied to 31 real shallow, large earthquakes. For a given earthquake, the focal mechanisms calculated using different Earth models and different forward modelling techniques can significantly vary. We provide a range of selected solutions based on the fit to the data, rather than one single value. Difficulties in constraining the dip-slip components of the seismic moment tensor often produce overestimated seismic moments, leading to near vertical dip-slip mechanisms. This happens more commonly for the Earth models not fitting the data well, confirming that more accurate modelling of lateral heterogeneity can help constrain the dip-slip components of the seismic moment tensor.