Rotational Motions Excited by Earthquakes

11.5 Conclusions

We extend the dislocation model of earthquakes to cover phenomena generated by rotational motions in the source area. The extended model includes defects, dislocations and disclinations, which are shown to be completely characterized by geometrical quantities, a torsion tensor and a curvature tensor. We derive a set of continuity equations among densities of dislocation and disclination and their currents.

Employing the continuity equations, we derive a simple expression for the rotational velocity of seismic waves. Combining the rotational motions of seismic waves with the translational motions, we can estimate the tensors γ*_0mn and ∂_mν_n, i.e., the rotational strain tensor and the spatial variation of slip velocity. These quantities will be large at the edges of a fault plane due to spatially rapid changes in slip on the fault and/or a formation of tensile fractures. We estimate from a simulation that the angular sensor now available will detect the rotational motions from earthquakes with magnitude 6 or larger if the hypocentral distance is shorter than 25 km.