The effects of seismic rotations on inertial sensors
Résumé
With the important increase in the number of instruments, especially in the near field of quite significant earthquakes, unsaturated traces have led seismologists to question what they are really measuring. We have performed a review of previous studies related to the effects of rotations on both horizontal and vertical components of various sensors. Illustrations of near-field records show that the recovering static displacement requires an accurate rotation estimation that present-day seismometers cannot achieve. Estimations of coseismic tilts (rotation around an horizontal axis) using seismometers cannot be achieved independently of translational motion. Therefore, for specific configurations such as near-field or long-period far-field, reconstruction of the ground motion requires specific rotation measurements. Effects of the Chi-Chi earthquake (1999) on accelerograms make the static displacement estimation unreliable. For long-period background noise, far-field horizontal seismic signals present unexpected N45° polarization, which is explained by the similar influence of the rotation around the vertical axis on the two horizontal components. For the GEOSCOPE network, this feature has been seen for stations with a Streckeisen STS-1 sensor, while those with a Streckeisen STS-2 sensor do not show this polarization feature. This study suggests that sensor installation should follow a protocol that will better guarantee the verticality of the sensors. Moreover, rotational recordings with ad hoc sensors are necessary, and by adequately correcting the traces, they will enable us to reconstruct the translational motion from recorded seismic signals.
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