Ground rotational motions excited by earthquakes

从地震激发的旋转运动中可以学到什么？对标题中提出的问题的一个答案是，我们将有更准确的SH波到达时间的数据，因为围绕垂直轴的旋转分量对SH波敏感，但对P-SV波不敏感。重要的是，还有一个与震源有关的答案。一般来说，除了地震模型中常用的位错外，其他类型的缺陷也会导致地震波的产生。特别是震源处的旋转应变直接在地震波中产生旋转分量。利用缺陷的几何理论，我们得到了地震波旋转运动作为震源缺陷参数函数的一般表达式。利用该表达式，结合平移运动表达式，我们可以估算出地震震源区的旋转应变张量和滑动速度的空间变化。1997年3月，日本伊豆半岛伊藤近海的地震群记录了地震引起的大旋转运动。在3月3日14点09分（格林尼治标准时间）发生的5.2米级地震中，在KAW记录的最大旋转速度是围绕南北轴的2.6 x 10^2 rad/s。由最大事件激发的绕垂直轴的旋转运动为3.3 x 10^<-3> rad/s。考虑到滑移速度的空间变化直接与旋转运动的激发有关，我们将一个简单的点源模型应用于最大事件，并成功地解释了观测到的绕垂直轴的旋转运动。

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.