New portable sensor system for rotational seismic motion measurements.

A new mechanical sensor system for recording the rotation of ground velocity has been constructed. It is based on measurements of differential motions between paired sensors mounted along the perimeter of a rigid (undeformable) disk. The elementary sensors creating the pairs are sensitive low-frequency geophones currently used in seismic exploration to record translational motions. The main features of the new rotational seismic sensor system are flat characteristics in the wide frequency range from 1 to 200 Hz and sensitivity limit of the order of 10(-8) rad/s. Notable advantages are small dimensions, portability, easy installation and operation in the field, and the possibility of calibrating the geophones in situ simultaneously with the measurement. An important feature of the instrument is that it provides records of translational seismic motions together with rotations, which allows many important seismological applications. We have used the new sensor system to record the vertical rotation velocity due to a small earthquake of M(L)=2.2, which occurred within the earthquake swarm in Western Bohemia in autumn 2008. We found good agreement of the rotation record with the transverse acceleration as predicted by theory. This measurement demonstrates that this device has a much wider application than just to prospecting measurements, for which it was originally designed.