Ground rotational motions excited by earthquakes

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What can be learned form rotational motions excited by earthquakes? One answer to the question posed in the title is that we will have more accurate data for arrival times of SH waves, because the rotational component around the vertical axis is sensitive to SH waves though not to P-SV waves. Importantly there is another answer related to seismic sources.Generally, not only 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 defects, we obtain a general exptession for rotational motion of seismic waves as a function of the parameters of source defects. Using this expression, together with one for translational motion, we can ectimate the rotational strain tensor and the spatial variation of slip velocity in the source area of earthquakes.Large rotational motions excited by earthquakes are recorded during an earthquake swarm of March, 1997, offshore Ito in Izu peninsula, Japan. The largest rotational velocity recorded at KAW is 2.6 x 10^2 rad/s around the north-south axis during an earthquake of magnitude 5.2m at 14 : 09 (GMT) on March 3. The rotational motions around the vertical axis excited by the largest event is 3.3 x 10^<-3> rad/s. Considering a spatial variation of slip velocity which directly ralates to excitation of a rotational motion, we apply a simple point source model to the largest event and suceed in explaining the observed rotational motion around the vertical axis.