1. Laser-interferometer application to broadband observations Akito ARAYA (Earthquake Research Inst., Univ. of Tokyo, Japan) Broadband observation … requires low-frequency sensitivity Advantages of Laser Interferometers: ・ high resolution owing to short wavelength of light ・ low drift using frequency-stabilized laser ・ wide range by counting fringes ・ in-situ calibration with reference to wavelength of light ・ free from EM noise or heating based on optical sensing ・ operation at high-temperature (deep borehole)

2. Application I ・・・・・・ an inertia accelerometer Features of the laser-interferometric seismometer ・ Laser-diode as a light source (l=850nm, 5mW) ・ Long-period pendulum (f 0 =7s with a 10-cm mass) ・ Self calibration (with reference to the laser wavelength) ・ Wideband feedback (UGF=1kHz, phase delay < 0.2deg. below 100Hz) Prototype laser seismometerBlock diagram

3. Comparison with an STS-2 seismometer ・ Accuracy of the self calibration ----- ~1% as compared with an STS-2 seismometer ・ Self-noise level ----- well below the Low Noise Model (50mHz ~ 100Hz) estimated from two identical laser seismometers Self-noise estimation (measured at Black Forest Obs.)

4. Optical-fiber-linked version of a laser seismometer (under development) Accelerometer (vertical) Accelerometer and an air-tight case No electronic components are installed in the accelerometer

5. Application II ・・・・・・ a long-baseline strainmeter Laser strainmeter as a broadband seismometer ・ Iodine-stabilized Nd:YAG laser (l =532nm, dn/n=2×10 -13 ) ・ 100-m baseline, 1000-m underground (in Kamioka Mine)

6. Tokachi (Hokkaido, Japan) earthquake (M=8.0) Comparison with CMG seismometers No saturation by fringe counting

7. Summany Laser interferometer … promising method for ・ opening up a new window (deep borehole, ocean floor, etc.) optical-fiber-linked borehole seismometer, tiltmeter, and strainmeter ・ improving sensitivity long-baseline extensometer with a highly-stabilized laser The details are shown in POSTER.