Validation of continuous horizontal background oscillations of the Earth with the Kamioka laser strainmeters

Recently the existence of the continuous toroidal modes of the Earth free oscillation was found from horizontal seismometers at several quiet stations [Kurrle and Widmer-Schnidrig, 2008]. The sources of Earth background oscillation have not sufficiently been understood although there has been considerable research on the continuous spheroidal oscillation. At present the signal-to-noise ratio for toroidal events is not so much high as for spheroidal ones because the horizontal component of crustal movement has much higher noise level than the vertical component. More investigation of continuous toroidal mode leads to further insights into the Earth background phenomenon. We examined to search the continuous toroidal modes with the Kamioka laser strainmeters although few time period are available to analysis since there are a lot of earthquake events in recent years. Kamioka station in the deep tunnel underground about 1,000m is very low noise environment. The Kamioka 100m laser strainmeters have high sensitivity in the horizontal-component of crustal strain by high level stability of laser frequency. The noise level at quiet time is 5-6× 10-11 strain/rHz at 3-4mHz and about below 3× 10-11 strain/rHz at over 4.5mHz. One type in our strainmeters is an equal arm interferometer detecting difference of strains in the NS and EW directions. This strainmeter is highly sensitive to shear strain, that is toroidal modes, and is less effect for atmospheric pressure. It is possible to detect the continuous horizontal modes since the power spectrum density is estimated about 5 × 10-11 strain/rHz. For pre-analysis with FFT using the period of first half year in 2007, we found the spectrum peaks at the frequencies coincident with fundamental spheroidal and toroidal modes. Some observed spectral lines laid on the frequencies of toroidal modes, 0T29 and 0T33 and so on in the frequency bands where spheroidal and toroidal modes are clearly separated. As another extraction method we also discuss the availability of the phase sensitive detector method applying lock-in detection.