A METHOD FOR DETERMINING SIGNIFICANT STRUCTURES IN A CLOUD OF EARTHQUAKES

The interpretation of a cloud of earthquake hypocenters in terms of ca usative structures is not a simple task. Locations are subject to unce rtainties, which will not be the same for every earthquake. The data s hould therefore not be interpreted simply by inspection, which is diff icult in the case of three-dimensional data anyway. Instead, we propos e using the location uncertainties as a guide in processing the data. Earthquake locations are moved inside their uncertainty or confidence ellipsoids until a simplified picture of the earthquake cloud is obtai ned, which can then be interpreted in terms of some simplified structu re such as faults. The aim of the approach is to give the simplest pos sible structure that is consistent with all the location and confidenc e ellipsoid data. The method is applied to three synthetic sets of dat a. These illustrate the potential and limitations of the method. Appli cation to a real earthquake data set from Rabaul Caldera in Papua New Guinea gives an image of the caldera ring fault that suggests departur es from the simple ring-fault structure previously assumed. Sensitivit y analysis on the Rabaul data shows that the method is not unduly sens itive to the assumptions that have to be made in applying it.