Equivalent forces for colliding ocean waves

Colliding ocean waves is considered to be the major source of seismic noise. In this paper, we examine the question, what would be the equivalent forces for such colliding ocean waves? In addition to the vertical pressure force term pointed out by Longuet-Higgins, we find that a lateral pressure gradient term is included in this system. This leads to a lateral dipole force and the excitation of noise that is different from vertical forcing. Numerical evaluation with PREM-like models shows that this term may become more important than vertical pressure term for frequencies below 5 millihertz (mHz). Importance of this horizontal term at low frequencies is related to the fact that horizontal motions of ocean waves become dominant in the low frequency band and its square is proportional to the excitation.