On the estimation of focal depth from macroseismic data*

It has long been known that deep earthquakes are felt more widely, for a given epicentral intensity, than shallow. Several attempts have been made to express this tendency in precise form for the estimation of focal depth. The purpose of this paper is to discuss the shortcomings of the formulas that have been tried and certain ways in which they may be improved. It is noteworthy that while the inverse square of the distance of the observer from the earthquake focus is insufficient to account for the falling off of the intensity, the introduction of an absorption coefficient is not a satisfactory method of dealing with the lack.