Error analysis in hypocentral locations at Phlegraean Fields

Abstract

The location reliability of the earthquakes occurred at Phlegraean Fields has been analyzed, and the theoretical errors, inferred from the diagonal elements of the covariance matrix, have been estimated. Using only first P-phase arrivals to the local network (22 stations) and assuming a reading error of 0.05 sec., the average error on the spatial coordinates is estimated to be of the order of 0.2 km.

Shallow events (depth<1 km) are very poorly constrained in depth at the borders of the network. The use of both P and S arrival times, recorded by a smaller three component network (10 stations), improves the depth determination.

Further analysis has been performed on a set of about 350 selected earthquakes, using two different velocity models.

Differences in depth considerably greater than the theoretical errors, and showing highly different patterns have been found.

Tests with artificial events, randomly distributed in space, indicate that the observed depth distribution is essentially due to the used velocity model.