Junpei Akamatsu

A Method to Estimate Phase Velocities of Rayleigh Waves Using Microseisms Simultaneously Observed at Two Sites

The frequency-wave number (F-K) method and the spatial autocorrelation (SPAC) method can both be applied to estimate the subsurface structure by using array observations of long-period (1–10 sec) microtremors. Since these methods require more than four to seven sets of simultaneous recordings, the allocation of many resources, not only of instruments, but also of personnel, must be provided for the observations. To avoid this difficulty, we propose an alternative method based on the conventional SPAC method. It is called the two-site SPAC (2sSPAC) method and requires only two sets of seismographs under the assumption that the wave fields are spatially and temporally stationary. After formulating the analytical background of the 2sSPAC method, we use data from actual microtremors to discuss the limitations of the assumption and the applicability of this method through a comparison with conventional methods. This discussion leads to the conclusion that the 2sSPAC method provides reasonable values for phase velocities in the frequency range lower than about 1.0 Hz at the sites studied.

The CLIO project

The CLIO project including a 100 m baseline cryogenic gravitational wave laser interferometer and a 100 m baseline geophysical strain meter was conducted in the Kamioka mine in Japan to investigate the technical feasibility of the large-scale cryogenic gravitational wave telescope (LCGT), which is planned to be constructed in the same Kamioka mine with 30 times longer baseline than CLIO, and to demonstrate the collaborative operation between these instruments about long-term continuous operation and gravitational wave signal veto analysis. About the cryogenic gravitational wave interferometer, the whole vacuum system and four cryostats, which house and cool sapphire mirrors, were constructed, and the required vacuum level of 10 −6 mbar and the temperature of 8 K at the inner radiation shield in the cryostat were achieved. About the geophysical strain meter, the obtained geophysical strain in the Kamioka mine was successfully simulated with a finite element model with a good agreement with less than 5% error. The strain meter also verified a permanent ground step change of micrometre order due to some earthquakes. We present the recent progress about the CLIO project.

Status of the CLIO project

The CLIO project involves the Cryogenic Laser Interferometer Observatory (CLIO) detector complex for gravitational wave detection and the Kamioka Laser Interferometric Strainmeter for the acquisition of geophysical data. CLIO has been constructed to demonstrate the feasibility of a future project, the Large-scale Cryogenic Gravitational wave Telescope (LCGT). It will utilize the low seismic and stable environment of the Kamioka mine as well as sapphire mirrors and suspension fibres at low temperature to reduce thermal noise. We designed CLIO to have a noise level limited by the thermal noise of sapphire mirrors and sapphire suspension fibres, which vary from 3 × 10−19 m Hz−1/2 at 300 K to 2 × 10−20 m Hz−1/2 at 20 K around 100 Hz. The strainmeter has already succeeded in monitoring the Earths tidal motion with a strain sensitivity of 2 × 10−12. The seismic noise veto between these same-scale interferometers is expected to provide an effective means of data selection for the gravitational wave signal analysis, and the ground motion data obtained by the strainmeter will help to maintain the stable operation of CLIO.

Design and construction status of CLIO

Construction of CLIO (cryogenic laser interferometer observatory) with 100 m baseline length has begun in the Kamioka mine. The tunnel for CLIO has been dug and infrastructure work is now in progress. CLIO is the final step to LCGT (large scale cryogenic gravitational wave telescope) and the first practical construction of a cryogenic interferometer in the world. The objective of CLIO is to demonstrate two of three features of LCGT, which are to utilize the quietness and stable environment of the underground site and to adopt cryogenic sapphire mirrors for thermal noise reduction. Also, it is a joint project by gravitational wave and geophysics researchers. CLIO has a locked Fabry–Perot configuration equipped with ring mode cleaners and cryocoolers to cool the sapphire mirrors to 20 K. The noise level of CLIO is designed to trace the thermoelastic noise of sapphire mirrors which varies from 10−18 m Hz−1/2 at 300 K to 10−19 m Hz−1/2 at 20 K around 100 Hz. A 7 m single-arm cryogenic test facility has been built at ICRR (Institute for Cosmic Ray Research), while the 20 m room temperature interferometer is in operation at Kamioka. Technical knowledge developed by these prototypes will be leveraged to realize CLIO.

Current status of the CLIO project

CLIO (Cryogenic Laser Interferometer Observatory) is a Japanese gravitational wave detector project. One of the main purposes of CLIO is to demonstrate thermal-noise suppression by cooling mirrors for a future Japanese project, LCGT (Large-scale Cryogenic Gravitational Telescope). The CLIO site is in Kamioka mine, as is LCGT. The progress of CLIO between 2005 and 2007 (room- and cryogenic-temperature experiments) is introduced in this article. In a room-temperature experiment, we made efforts to improve the sensitivity. The current best sensitivity at 300 K is about 6 × 10-21/√Hz around 400 Hz. Below 20 Hz, the strain (not displacement) sensitivity is comparable to that of LIGO, although the baselines of CLIO are 40-times shorter (CLIO: 100m, LIGO: 4km). This is because seismic noise is extremely small in Kamioka mine. We operated the interferometer at room temperature for gravitational wave observations. We obtained 86 hours of data. In the cryogenic experiment, it was confirmed that the mirrors were sufficiently cooled (14 K). However, we found that the radiation shield ducts transferred 300K radiation into the cryostat more effectively than we had expected. We observed that noise caused by pure aluminum wires to suspend a mirror was suppressed by cooling the mirror.

Coda attenuation in the Lützow-Holm Bay region, East Antarctica

The frequency-dependent coda Qc−1 was estimated for the Lutzow-Holm Bay region, East Antarctica, on the basis of the single-scattering model, and discussed together with Qc−1 data of Kyoto, Japan, as an example of an active region. Coda waves from six shallow earthquakes, observed with a local telemetry seismic network installed along the Soya Coast, were analyzed with narrow band-pass filters. Qc−1 for a lapse time of 20–40 s was estimated to be 0.00495 ƒ−0.776 for a frequency of 4–16 Hz, and Qc−1 for 150–210 s was 0.00335 ƒ−0.926 for 1–24 Hz. In comparison with Qc−1 data for Kyoto and other regions with various tectonic conditions, Qc−1 in the Lutzow-Holm Bay region was characterized by large values at low frequency (around 1 Hz) and smaller values at higher frequency. From the single-scattering model, Qc−1 suggests a strong frequency dependence of intrinsic absorption for S waves. Assuming that the observed Qc−1 reflects scattering loss of energy, and taking account of the geological conditions and the extremely low seismic activity in the East Antarctic continental shield, it is suggested that the frequency-dependent Qc−1 is attributable to large scattering loss in the lower-frequency range caused by large-scale heterogeneities as a result of velocity and/or density perturbations.

Chapter 13 Bedrock structure around faults and its relation to earthquake disaster

Abstract The frequency-wavenumber and horizontal-to-vertical ratio spectral analyses of microseisms (long-period microtremors), and the bandpass filtering of Bouguer gravity anomalies, were conducted to elucidate the subsurface structure around the eastern end of the severely damaged belt-like zone in the Kobe-Hanshin area which appeared during the 1995 Hyogokennanbu (Kobe) earthquake. A close relationship was found between the fault-related bedrock configuration thus obtained and the distribution of earthquake damage: a narrow zone of severe damage was located 1.5–2 km apart from the basin edge where the depth to bedrock changes abruptly by several hundreds to a thousand meters. The relationship was atributed to the amplification of ground motions due to the specific configuration of bedrock, that is, focusing of seismic waves and/or interference between incident S-waves and surface-waves secondarily generated at the basin edge. It is pointed out, as a lesson learned from the Hyogoken-nanbu earthquake disaster, that the investigation of fault-related 3-D bedrock configuration is an urgent issue for hazard mapping of an urban area on a sedimentary basin.

Application of Microseisms to Microzoning of an Urban Area in a Sedimentary Basin

Taking account of the necessity for extending the frequency range to lower frequency in the seismic microzoning in an urban area, a comparative analysis of microseisms (long-period microtremors) and gravity data was carried out in the Kyoto basin, southwest Japan. The stability of spectral ratios of microseisms (soil site/rock site) was first examined through repeated simultaneous observation at soil sites and rock sites. Then spatial variation of the spectral ratios in the whole basin was mapped and compared with the characteristic feature of Bouguer gravity anomaly which was bandpass filtered using the upward-continuation technique. Main results obtained are as follows: (1) spectral features of microseisms observed both at soil sites and rock sites are remarkably affected by weather condition, but when averaged for different weather conditions, the spectral ratios (soil site/rock site) exhibit similar features, which enable us to take them as time-invariant and site-specific amplification factors; (2) the amplification factors for both horizontal and vertical components become larger stepwise from north to south in the basin, and correspondingly the frequency range having large amplification extends to lower frequency; (3) the amplification factors for horizontal components are larger than those for vertical component in the low frequency range; (4) frequency characteristics of the amplification factors for vertical component exhibit a similar shape to those for horizontal components with one-octave shift in frequency; (5) horizontal component of vibration oriented parallel to the long axis of the basin is amplified more than that oriented normal to the axis; (6) the spatial distribution of amplification factors has a close correlation with the general trend of the bandpass-filtered Bouguer gravity anomaly. These features of amplifications are interpreted from the viewpoint of Rayleigh surface waves incident to the basin with 3-D structure of bedrock, though it is suggested that some of them may be explained approximately by 1-D resonance of S and P waves in the soil sediments. It is pointed that comparative analysis of microseisms and gravity data are very useful for microzoning of an urban area with irregular configuration of bedrock, especially in the case where the information of subsurface structure is not available.