In the field of vibration control, accelerometers are widely used as feedback sensors. In addition, according to state feedback control theory, detections of velocity and displacement signals are required to improve anti-vibration performance. Then, an absolute displacement sensor is proposed by modifying a control loop of a seismo-type velocity sensor. Basically, the proposed displacement sensor has the same mechanical structures having a pendulum as a commercial seismo-type velocity sensor. In our previous works, the proposed sensor has been successfully applied as feedback and feedforward sensors. However, it is revealed that bandwidth expansion is necessary as a substitute of the accelerometer. The bandwidth is limited by high frequency resonances observed in high frequency dynamics. In this paper, causes of the resonances are investigated in terms of mechanical resonances of the velocity sensor. At first, operating principles of the velocity and the displacement sensors are explained to show the mechanical structures, and the high frequency dynamics are determined by measuring frequency response functions. Next, experiments on the modal analysis are conducted to specify vibration modes of the pendulum. Finally, the high frequency resonances are suppressed by loading viscoelastic materials.