Method and apparatus are provided for generating in situ in a zone of earth surrounding a bore hole both longitudinal and shear waves and of predetermined relative amplitude, entirely mechanically, avoiding use of explosives, and with relatively low power requirements for the stress waves generated. Subterranean impacts generating high amplitude stress waves equivalent to those generated by fifteen pounds of TNT or multiples of such quantities of TNT are realized. These high amplitude waves are produced in an annular zone ten feet or more from the earth bore, where the earth specimen has not been previously disturbed and its measured properties are therefore truly representative of the medium. In addition to being a non-explosive earth stress wave generator, the present device, and method herein, enables controllable variation in the amplitude of the shear wave relative to the longitudinal wave and vice versa by control of operating parameters using the same equipment, including rate of rotation, travel distance and mass/area relationship.