A formulation of directivity for earthquake sources using isochrone theory by Paul Spudich and others (2004, 10.3133/ofr20041268)

United States Geological Survey Open-File Report 2004-1268. Paul Spudich and coauthors derive a finite-source directivity measure from the distribution of arrival times across a rupturing fault. The explanatory discussion links apparent pulse compression to the rate at which radiation from successive patches reaches a receiver. pubs.usgs.gov/of/2004/1268/of2004-1268tagged.pdf