CMG40T地震計の傾斜応答に関する簡易試験

抄録

Deformation under the ground excites not only translational motion but also tilt changes, although seismologists usually analyze recorded waveforms under an assumption that seismogram only expresses translational motion. Since broadband seismometers have high enough sensitivity to tilt changes, the seismograms are sometimes contaminated by tilting motion at a seismic station. To check the effect of tilt change on seismogram recorded by a broadband velocity seismometer, we performed a simple experiment with a CMG40T seismometer and a tilt sensor of surface mount type. Tilt changes yield one-sided oscillation of the pendulums in a horizontal transducer. Such particular oscillation is clearly identified in the displacement trace calculated from the original velocity seismogram. Theoretical consideration predicts that slow temporal changes in tilt angle correlate with a time function of the apparent displacement of seismogram. Since results of our experiment well agree with the prediction, we estimate the conversion coefficient from the apparent displacement to tilt angle and confirm that the coefficient obtained experimentally well corresponds with the value from theoretical prediction. As a possible example in an active volcano, we consider slow isotropic volume change near a seismic station and calculate synthetic seismogram due to the volume change.