Seismic Wave Strain, Rotation, and Gradiometry for the 4 March 2008 TAIGER Explosions

Acceleration spatial gradients, horizontal strains and horizontal rotation were computed using strong motion array data from the 4 March 2008 Taiger explosions in northeastern Taiwan, and used in conjunction with the original 3 component acceleration data to perform a gradiometric analysis of the strong ground motion wave train. The analysis yields a complex, frequency-dependent view of the nature of seismic wave propagation over short propagation distances that imply significant lateral velocity changes in structure. Areal strain and rotation about the vertical have equal amplitudes and suggest significant wave scattering within the confines of the river valley where the experiment was performed and/or significant departure from an axisymmetri explosion source. Gradiometry shows that the P wave arrives at the array 35 degrees off-azimuth clockwise from the straight-line path and appears to have been refracted from the northern side of the valley. Large, slowly propagating secondary surface waves initially arrive 45 degrees counter-clockwise from the straight- line path but later arrivals are seen to propagate in all directions, including back towards the explosion source. Frequency-dependent radiation pattern for the three-borehole explosion in comparison to the single-borehole explosion explains the differences in maximum amplitudes between sources seen in the acceleration data. The use of seismic strain and rotation with standard particle motion wave fields at a single location allows for a direct view of seismic wave propagation that illuminates the true nature of the seismogram.