Background Seismic-Wave Noise and Elimination of Noise Transmitted through Water in a Bore Hole at the Iwatsuki Observatory

The Iwatsuki Crustal Activity Observatory was established by the National Research Center for Disaster Prevention for the purpose of earthquake prediction research in and around Tokyo.Because of high-level artificial noise generated by industrial activity and concentrated transportation, it is very difficult to observe microearthquakes and crustal movement with highly sensitive instruments at the surface around Tokyo.A two-component set of tiltmeters, three-component sets of velocity seismometers and acceleration seismometers were settled at the bottom of the bored hole of 3500m depth at the Iwatsuki observatory, so that these high sensitivity observations might not be affected by the disturbance at the surface.Contrary to our expectation, remarkable noise, however, appeared on the vertical component seismogram. It has been shown that this noise was caused by trafic cars passing by the observatory and transmitted from the surface to the bottom through the water filled in the hole.Particularly deviced shelters were made and set in the middle of the hole to decrease the noise transmitted from the surface. Finally, satisfactory effectiveness was obtained after improvement of the shelters.Background seismic-wave noise was simultaneously measured at the surface and at the bottom of the hole. The spectral amplitude of background noise at the bottom of the hole was about 1/301/1000 compared with that at the surface.