Determination of stresses from heterogeneous focal mechanism data: An adaptation of the multiple inverse method

A new method is proposed to separate stresses from earthquake focal mechanism data (double couple component of focal mechanisms) from spatially or temporarily varying state of stress. The underlying multiple inverse method was originally devised for geological fault-slip data, but we have adapted its resampling scheme to focal mechanism data to cope with the nodal planes ambiguity problem. The stresses compatible with a dataset are recognized as the clusters of reduced stress tensors. The clusters are identified with the k-means clustering. The uncertainty of the stresses are evaluated from the spread of the clusters. The technique is shown to successfully determine valid stresses from simulated data. We have also applied the technique to natural data from southwestern Japan.