WHAT CAN BE LEARNED FROM ROTATIONAL MOTIONS EXCITED BY EARTHQUAKES

One answer to the question posed in the title is that we will have mor e accurate data for arrival times of SH waves, because the rotational component around the vertical axis is sensitive to SH waves although n ot to P-SV waves. Importantly, there is another answer related to seis mic sources, which will be discussed in this paper. Generally, not onl y dislocations commonly used in earthquake models but also other kind of defects could contribute to producing seismic waves. In particular, rotational strains at earthquake sources directly generate rotational components in seismic waves. Employing the geometrical theory of defe cts, we obtain a general expression for the rotational motion of seism ic waves as a function of the parameters of source defects. Using this expression, together with one for translational motion, we can estima te the rotational strain tensor and the spatial variation of slip velo city in the source area of earthquakes. These quantities will be large at the edges of a fault plane due to spatially rapid changes of slip on the fault and/or a formation of tensile fractures.