Hypocentral relocation using clustering-along-planes constraints: implications for fault geometry

Abstract

SUMMARY Hypocentre location is an ill-posed inverse problem even assuming that the velocity model is known, because different sets of hypocentre locations may satisfy the fitting criterion. We present a regularized hypocentre inversion in which the constraints of spatial proximity of the hypocentres to target planes are used. This constraint introduces the geological bias that earthquakes might occur along fault planes. Here, the target planes may be either (1) planes specified by the interpreter or (2) planes fitting groups of events. We assume also that initial estimates of hypocentres and origin times are available. Then, the initial hypocentre estimates, origin times and target planes are used as input to an inversion problem to relocate the hypocentres so that the maximum-possible clustering of events along the given planes is attained, matching the observed traveltimes. We use L1 norm for data fitting, L2 norm for the plane proximity criterion and a polytope algorithm to minimize the functional. Results from synthetic and real data indicate that the plane proximity constraint allows for hypocentre relocation presenting a high degree of clustering along planes. The real-data example is an intraplate earthquake sequence in NE Brazil. Our methodology defined the geometry and strike of fault segments close to known geology and focal mechanism data. In addition, the new method indicates that the fault is characterized by a splay geometry in its southern end and that more than three fault segments are necessary to explain the hypocentre distribution.