• journal_title：Geophysics
• Contributor：C. Hewitt Dix
• Publisher：Society of Exploration Geophysicists
• Date：1955-
• Format：text/html
• Language：en
• Identifier：10.1190/1.1438128
• journal_abbrev：Geophysics
• issn：0016-8033
• volume：20
• issue：1
• firstpage：87
• section：Articles

Cagniard's method is applied to the numerical calculation of the vertical displacement due to a point source in a semi-infinite elastic solid medium at three points on a vertical line through the source. The source is a step in the scalar displacement potential. From these calculated responses the response for any physically possible spherically symmetric source can be computed by application of the Duhamel integral. Clear evidence of backward transmission of transverse wave energy is found along the vertical axis through the source. This, together with the energy of the longitudinal waves, also transmitted backwards, accounts for the mechanism by which energy is held near the source and near the free surface long enough to account for the generation of long period surface waves. This mechanism of generation of long period surface waves is not restricted to the free surface case. Any good reflector, which also generates secondary transverse waves from longitudinal primary waves, will serve the purpose. It is suggested that this gives a clue to the mechanism of the formation of "ground roll" in many practical cases.