S31C-01 08:00h
Japanese Strategy and Technology of Active Monitoring of the Earth's Interiors
The difficulty of the research towards the highly nonlinear phenomena such as earthquakes motivated us to explore an essential tool demanded inevitably beyond the level of technology and budget available at present. Intuitively it must be the extensive and intensive monitoring of the target with higher resolution. Naturally it is the remote sensing by utilizing actively and continuously all possible types of `wave', i. e., seismic wave and electromagnetic wave. The developmental study of the active tools has been made since 1994 in Japan for the precise structure exploration and continuous state monitoring targeted to the tectonically active parts in the Earth's lithospheres. It was promoted after the disastrous Kobe Earthquake of 1993. Basic necessity for the technology is the robustness against the natural and artificial noise in remote observation to obtain data with larger S/N under non-destructive condition for environments. The data to be obtained is the transfer function between many transmitter sites and many receiver sites. We believe that our science would change drastically, once we come to obtain the highly reliable data with reliable estimate of observational error smaller than before by one order of magnitude. The technical demand is the precision of transmitting signal and receiving device, which are stable and accurate enough both in synchronization and reproducibility so as to allow the effective data stacking of long period of time in obtaining the transfer function data between the source and receiver. A special emphasis is placed on the polarization and dispersion (color) of the transfer function, in order to extract the data of anisotropy, scattering and attenuation that are expected to carry the information on the physical states within the polycrystalline rocks bearing the structural sensitivity under stress. A review is made on the current state of the technology called ACROSS (Accurately Controlled Routinely Operated Signal System) and several applications in Japan and also a proposal is made for establishing the underground monitoring system of Japanese islands. The relevant test site is set to Tokai area, where a disastrous Earthquake is expected to take place at the plate boundary between Philippine Plate and Asian Plate.
S31C-02 08:15h
Vibration Geotechnologies In III Millennium
In seventies of last century scientists of the Academy of sciences USSR together with other interested industrial institutes started to create new technologies of lithosphere monitoring. It was supposed to use high-power vibration sources of seismic waves - vibrators - in the basis of the technologies. Appearance of new class instruments allowed starting deep investigations in real earnest. Just in this period the theoretical methods of active seismology began to develop actively, they became the base of new direction in geophysics. The paper presents new vibration geotechnologies based on the using of powerful seismic vibrators: detail deep seismic investigations of the Earth's crust and the upper mantle, diagnostics of physical state of buildings and structures, active vibroseismic monitoring of the seismic-prone zones and studying of the geodynamic processes, vibroseismic calibration of international network stations, seismic ecology.
S31C-03 08:30h
Vibroseis Monitoring of San Andreas Fault in California: What Can Be Done Next.
In the Vibroseis monitoring experiment, seismic waves repeatedly illuminated the epicentral region of the expected M6 event San Andreas Fault (SAF) at Parkfield from June 1987 until November 1996. For this effort, a large shear-wave vibrator was interfaced with the 3-component (3-C) borehole High-Resolution Seismic Network (HRSN), providing precisely timed collection of data for detailed studies of changes in wave propagation associated with stress and strain accumulation in the fault zone (FZ). The investigations reported significant traveltime changes in the S coda for paths crossing the fault zone southeast of the epicenter and above the rupture zone of the 1966 M6 earthquake. Analysis and modeling of these data and comparison with observed changes in creep, water level, microseismicity, slip-at-depth and propagation from characteristic repeating microearthquakes showed temporal variations in a variety of wave propagation attributes that were synchronous with changes in deformation and local seismicity patterns. Numerical modeling suggests 200 meters as an effective thickness of SAF. The observed variations can be explained by velocity 6% velocity variation within SAF core. Guided wave amplitude tomographic inversion for SAF using microearthquakes, shows clearly that FZGW are significantly less attenuated in a well-defined region of the FZ. This region plunges to the northwest along the northwest boundary of the region of highest moment release and separates locked and slipping sections of the SAF at depth, as determined independently from geodesy, seismicity and the recurrence rates of characteristically repeating microearthquakes. Temporal changes of FZGW correlates with changes in overall seismicity. Active monitoring of changes in FZGW has a potential for imaging and detecting of changes in strees within FZ cores.
S31C-04 08:45h
Active Monitoring With The Use Of Seismic Vibrators: Experimental Systems And The Results Of Works
Active methods of geophysical monitoring with the use of powerful seismic vibrators play an important role in the investigation of changes in the medium's stressed-deformed state in seismic prone zones for problems of seismic hazard prediction. In the last three decades, this scientific direction has been actively developed at institutes of Siberian Branch of Russian Academy of Sciences. In this period, experimental systems for the active monitoring of the medium, which include powerful vibrational sources with computer control systems, mobile specialized complexes for the precision recording of vibrational seismic signals, and data processing systems have been created. A review of various constructions of resonant vibrational seismic sources with a vibrational force of 100 tons in the frequency range from 5 to 15 Hz and the principles of creation of precision computer control systems and low-frequency three-component recording systems VIRS-M, VIRS-K, and ROSA is presented. A method for the active monitoring of the medium with the use of wideband sweep signals and narrow-band harmonic signals radiated by seismic vibrators has been developed. To determine the sensitivity of the active monitoring system, some experiments to detect the influence of the Earth's crust tidal deformations (of the order of 10-7) on seismic wave velocities have been performed. A 100-ton seismic vibrator and recording systems were located at a distance of 356 km. The radiation sessions of harmonic and sweep signals were repeated every 3 hours during 8 days. This made it possible to construct the time series of variations in the amplitudes and phases of the signals and wave arrival times. Both 12-hour and 24-hour periodicities correlated with the earth's tides were distinguished in the spectrum of variations of the recorded signals. The experiment has shown that the active monitoring system makes it possible to detect relative variations of the seismic wave velocities of the order of 10-5 - 10-6 in an area of 300-400 km around the source. This makes possible the direct monitoring of the state of stresses in an area of 100 thousand km2 to detect the regions and phases of the critical stress as an earthquake precursor. In recent years, works on the use of the method of vibroseismic interferometry for the active monitoring of the lake Baikal region have been started. The method is based on the seismic sounding of the region by powerful seismic vibrators with a long radiation of narrow-band harmonic signals. The changes in the stressed-deformed state are determined through the variations of the amplitude-phase characteristics of stationary wave fields, which are excited in the medium due to a long-time radiation of harmonic signals of constant frequency from the vibrator. The method of vibroseismic interferometry has a high sensitivity to the time changes of the parameters of the medium in the case of long-distance observations. A peculiarity of the experiments is the simultaneous use of the data of regional seismic stations and mobile recording complexes.
S31C-05 09:00h
Evaluation of a dense seismic array for acquisition of high quality data in the ACROSS observation
ACROSS is an active monitoring methodology to detect any subtle temporal change of physical properties in the Earth's interior. We demonstrate the potentiality of the ACROSS observation with a dense sensor array. We have conducted a dense seismic array observation at the distance of 1 km from the ACROSS source since 2003. The array is composed of 36 three-component velocity seismometers buried at 1.8 m deep in an area 25 m square. All the data are recorded accurately referring to a GPS clock. We derived and analyzed a transfer function (TF) from the source to a receiver by the following steps: (1) evaluating a force vector as source characteristics, (2) converting the observed data to the displacement vectors by incorporating all the corrections of the instruments, (3) stacking the observed data for an enough time to suppress the temporal noise, (4) extracting ACROSS signal and evaluating noise level, (5) representing TF in a tensor form with the estimated errors, (6) slant-stacking with variable ray parameters, (7) estimating the travel times and amplitudes of the wave arrivals by Sompi Event Analysis (Hasada et al., 2001) and representing the result by a pulse sequence, and (8) deriving the polarization vector for each arrival to identify all the wave modes. We analyzed TF of SH-wave component from 16 to 20 Hz as an example. We obtained good quality TF with S/N ratio up to $10^4$ by stacking for 12 days at the step (3). The spatial noise originated from the local heterogeneity around the array was eliminated by the step (6). Several arrivals were recognized within the time windows from 0.6 to 1.8 s. The maximum amplitude of event traces was detected at the travel time of 1.064 s with a ray parameter of $7.9x10^{-4}$ s/m. We also found the scattered waves probably generated by the heterogeneities around the array. The ACROSS dense array observation would provide a lot of information on the underground heterogeneities. Consequently, we have the important and challenging subjects: (1) optimum designing of ACROSS array to acquire the better data and (2) development of new theoretical method to deal with the variable types of the wave.
S31C-06 09:15h
Deep Seismic Researches Of Seismic-Active Zones With Use Of High-Power Vibrators - Technique, Outcomes, Outlooks
There are presented the materials of deep vibroseism researches, carried out in seismic active regions of Siberia with use of stationary (100-tos power) and moveable vibration sources (40-60tons power) and mobile digital recording equipment. There are given some examples of unique, have no world analogues, correlograms from high-power vibrators on distances to 400km and more. Using new vibroseismic technology of deep seismic researches, there were got detail deep sections of the Earth's crust and upper mantle, including time-sections of CDP-DSS up to depth of 80km. Materials of vibroseismic investigations on 2500km of seismic profiles in hard-to-reach regions of the Altay-Sayan region, the Baikal rift zone and Okhotsko-Chukotski regions are evidence of high cost efficiency, ecological safety, possibility to be realized in hard-to-reach region and finally of availability of deep seismic investigations with use of high-power vibration sources.
S31C-07 09:30h
A plan of continuous monitoring of seismic signature at the top of the subducting Philippine Sea Plate using the ACROSS active monitoring method
Recently, a very strong reflected PP phase from the subducting plate boundary was found beneath the aseismic forearc slope of the Japan Trench (Fujie et al., 2003, Kasahara et al., 2003). This strong PP reflection is interpreted as a reflection from the non-asperity region along the plate boundary caused by the presence of low-Vp material. Similar strong PP reflection phase was observed near the plate boundary of the subducting Philippine Sea Plate and the Eurasian Plate, in the western part of Shizuoka Prefecture along the NS seismic survey line across the central part of Japan using explosive sources (Iidaka et al., 2003, Kodaira et al., 2004). This region is match to the aseismic slip zone since 2000 observed by GPS (Ozawa et al., 2002). This region is supposed to the lower edge of future Tokai or Tonankai earthquake along the Nankai Trough. The "ACROSS" (Accurately Controlled and Routinely Operated Signal System) (Kunitomo and Kumazawa, 2004) can continuously transmit accurately-controlled seismic waves in amplitude, frequency and phase. Using continuous transmission of seismic waves from the ACROSS source located in Toki City, Japan and /or the new ACROSS source under planning in the western part of Shizuoka Prefecture and the high sensitivity seismometers along the NS traverse line, we can delineate the temporal variation of seismic wave properties of the anomalously reflected PP phase along the boundary of subducting Philippine Sea Plate. This observation can give information on variation of the physical state of frictional slip and clues for the earthquake generation. It is also very useful for the monitoring of precursory slip and/or after-slip of the future Tokai and Tonankai earthquakes (or any large earthquakes) with sources along the Philippine Sea Plate.