Low-velocity zone parameters as functions of focal depth

Abstract

Summary. Explicit inversion for the velocity-depth structure within a lowvelocity zone (LVZ) can be made from surface observations of the variations of the travel-time intercept (a) and the epicentral distance extent (y) of the shadow zone as functions of focal depth. Prior knowledge of the focal depths is not required. The observed a-y curve can be converted into the corresponding travel-time curve that crosses the shadow zone in the surface focus travel-time curve. The inversion of this travel-time curve requires the construction of the LVZ ray parameter-epicentral distance @-A) curve by the relation p-pL = da/dy where p~ is the p at which the LVZ occurs. Inversion is then performed by treating each layer in the LVZ as a separate LVZ.