Active And Passive Seismic Monitoring Of Temporal Variations Of Fault Zone Properties

Most great historical earthquakes occurred on active faults. The stress within seis-mically active fault zones typically changes associated with preseismic, coseismic and post-relaxation processes, causing detectable physical properties variations. Therefore, measuring such changes is crucial for understanding the physical mechanisms of earth-quakes. Benefiting from the depth penetration of seismic waves and technical improve-ment of monitoring, the seismic velocity and attenuation, among all the elastic properties, can be measured with the highest accuracy and are frequently utilized to monitoring the active fault zone down to seismogenic depth. However, robust detection of the subtle and secular variations of physical properties within the fault zone remains challenging, and requires further improvements in both observation technology and weak signal detection methodology.In this thesis, focusing on improving the detecting accuracy of physical properties, we have investigated the theory and methodology of probing the variations of seismic velocity and attenuation within the fault zone. By exploiting the man-made active source and ambient noise simultaneously, we have developed a high-performance monitoring system jointly using the active and passive signals. We have carried out several research experiments within the fault zone of Wenchuan earthquake, including the comparative observation experiment of transportable array, cross-fault monitoring experiment with ac-tive source, comparative monitoring experiment with active and passive source (ambient noise). We acquired the temporal variations of seismic velocity and attenuation within the fault zone of Wenchuan earthquake and the adjacent areas, and discussed possible mechanisms responsible for the observed physical properties variations. Generally, the major work of this dissertation can be summarized as follows:1. Ambient noise characteristics and denoising technology of transportable array. Transportable seismic array can be deployed flexiblely to densely monitoring specific regions of interest, and ambient seismic noise is one of the critical factors that influ-ence seismic recording quality, especially in sedimentary cover and noisy condition. We have conducted a comparative observation experiment to characterize the ambient noise of transportable stations located at different geologic settings and background noise lev-els by computing the power spectral densities and the corresponding probability density functions. The result shows that high-frequency culture noise and long-period natural environmental noise are the main source of transportable seismic noise, which can be significantly reduced with specific vault construction. We have proposed the reference station scheme for transportable array based on the noise analysis.2. Methodology of obtaining physical properties variation within the fault zone. We have established a complete physical properties monitoring system and the corre-sponding analyzing methods, based on the studies on seismic signal receiving, source signal extraction and time-lapse analysis of physical properties. We used transportable array with high-standard vault construction to monitor the target fault zone. Based on the different characteristics of continuous and impulse active source and ambient noise source, we employed cross-correlation and waveform stacking methods to extract high SNR (signal to noise ratio) empirical green’s function between the source and seismic stations (or station pairs). According to the features of different phases (e.g. body wave, surface wave and coda wave), we measured the accurate velocity variation with waveform cross-correlation method. And we acquired the relative attenuation variation with single station spectral ratio method.3. Monitoring temporal variation of subsurface attenuation using active source. After the occurrence of the devastating2008Mw7.9Wenchuan earthquake, we have con-ducted a1-month active source monitoring experiment at the northeast end of the major fault (Beichuan—Yingxiu fault) with an electric drop hammer. With the highly repeatable active source signals, we obtained the continuous temporal variation of seismic velocity and attenuation independently. The result shows that:(1) The shallow fault zone, which is generally composed of sedimentary cover and unconsolidated rocks, exhibits relatively low velocity and strong attenuation with a quality factor Q of approximately10;(2) The temporal variation of seismic velocity and attenuation are remarkably consistent, which both strongly correlated with the variation of in-situ barometric pressure, suggesting that plausible mechanisms might be the opening and closing of compliant microcracks as-sociated with pressure variation;(3) We detected obvious coseismic velocity drop and attenuation increase due to the shallow subsurface medium damage associated with the strong ground motion of a moderate aftershock (ML4.7), which gradually recovered to the pre-event level.4. Comparatively monitoring the velocity variation with active and passive sources. As a sub-project of the integrated Wenchuan earthquake Fault Scientific Drilling (WFSD) project, we have carried out cross-fault monitoring experiment at the Guanxian—Anxian fault. With the signals from the ACROSS active source and noise cross-correlation func-tions of ambient noise, we obtained the continuous velocity changes within the fault zone and the adjacent areas independently during the period from June2009to January2012. The result reveals that:(1) While different phases and frequency band are used, the seismic velocity variations from the active and passive methods are quite consistent, which both are characterized by±0.2%seasonal variation, with peak and trough at win-ter and summer respectively;(2) The velocity change is not homogeneously distributed and highly localized in the damaged fault zone and the adjacent areas;(3) The periodic velocity variation within fault zone exhibits remarkably positive correlation with in-situ barometric pressure with stress sensitivity in the order of10-6Pa-1;(4) Similar to result of the electronic hammer experiment, the most plausible mechanism might be the crack density variation of the shallow subsurface medium of the damaged fault zone in response to the cyclic barometric pressure loading.5. Technical approaches that enhance monitoring accuracy and time resolution. It has been well demonstrated that we can obtain the time-lapse changes of the physical properties with both active and passive sources. The core technology of dynamic mon-itoring lies in the enhancement of accuracy and time resolution, as well as the improve-ment of reliability. In our study, while the seismic velocity variation from the active and passive methods are quite consistent, the measurement precision of the active source is significantly higher. The active sources shows superior performance over the passive in-terferometry imaging method, and we can even further enhance the monitoring accuracy and time resolution through enlarging the exciting energy, widening the frequency band and improving the signal repeatability.