Seismic wave generation by a nonisotropic explosion source

We develop a general solution for seismic wave generation by an explosion source in a spherical cavity, and then a specific solution for a point explosion source offset from the cavity center. We find that the offset causes a dipole component to the source which generates S waves and causes angular variation in P wave amplitude and shape. The S waves vanish at zero frequency but may be a sizable fraction of the P waves at frequencies of interest. We also perform a nonlinear axisymmetric calculation of an offset explosion in a spherical cavity, which causes a complex source due to cavity reverberations and generates S waves comparable to those found from the analytic solution. The solution has application to shear wave generation by decoupled explosions, and explains at least part of the shear waves observed from such explosions. Data from a pair of explosions centered and offset from the center of a spherical cavity show the expected enhancement of shear waves from the offset source.