Determination of hypocentral parameters of local earthquakes using weighting factor based on take-off angle

Abstract

This paper introduces an improved method of determining hypocentral parameters of local earthquakes by applying a weighting factor based on take-off angle at a source. The hypocentral parameters determined by the existing methods contain errors if the true velocity structure is not used, and they are not identical depending on the velocity models used. Variation in the traveltime caused by a focal depth may indicate the degree of focal depth information contained in the data. The degree of information will decrease rapidly as the epicentral distance increases. The take-off angle at the source is related to the epicentral distance. Therefore, a weighting factor representing the degree of focal depth information could be described in terms of the take-off angle at the source. The accuracy of hypocentral parameters, especially the focal depth and the origin time, is improved by applying the take-off angle as the weighting factor and two-point ray tracing to the existing methods. Synthetic traveltime data both with noise and without noise are generated using the two-point ray tracing technique with a model in which velocity structure and hypocentral parameters are known. The effect of the weighting factor is examined by comparing the estimated hypocentral parameters with the true, known values. The computational results show that the focal depth and the origin time estimated by the improved method (MHYPO) in this work are more accurate than those estimated by other existing methods.