Kinichiro Kusunose

Detailed analysis of acoustic emission activity during catastrophic fracture of faults in rock

Abstract The detailed time-space distribution of acoustic emission (AE) events during the catastrophic fracture of rock samples containing a pre-existing joint or potential fracture plane is obtained under triaxial compression using a high-speed 32-channel waveform recording system, and the results are discussed with respect to the prediction and characterization of catastrophic fault failure. AE activity is modeled quantitatively in terms of the seismic b -value of the magnitude–frequency relation, the self-excitation strength of the AE time series, and the fractal dimension of AE hypocenters. Consistent with previous studies on rock samples containing a fracture plane with several asperities, the present analyses reveal three long-term phases of AE activity associated with damage creation on heterogeneous faults, each clearly identifiable based on the above parameters. A long-term decreasing trend and short-term fluctuation of the b -value in the phase immediately preceding dynamic fracture are identified as characteristic features of the failure of heterogeneous faults. Based on the experimental results it is suggested that precursory anomalies related to earthquakes and other events associated with rock failure are strongly dependent on the heterogeneity of the fault or rock mass. A homogeneous fault or rock mass appears to fracture unpredictably without a consistent trend in precursory statistics, while inhomogeneous faults fracture with clear precursors related to the nature of the heterogeneity.

The hierarchical rupture process of a fault: an experimental study

Abstract We describe the detailed faulting process of a naturally healed fault containing geometric and mechanical asperities in a granitic porphyry sample, based on data collected with a high-speed acoustic emission (AE) waveform recording system. Asperity failure is examined using the detailed spatio-temporal distribution of AE hypocenters. The initial phase of AE activity is also examined using high dynamic range waveforms. Our experimental results indicate that quasi-static nucleation of the heterogeneous fault is associated with the failure of asperities on the fault plane. The fracturing of an asperity is characterized by a dense spatial clustering of AE events and a changing b -value ( b , hereinafter), which is manifest in three typical stages of failure as follows: (1) foreshocks exhibiting a decrease in b , (2) a period of mainshocks corresponding to a minimum in b , and (3) aftershocks of increasing b . The progressive fracture of several coupled asperities results in short-term precursory fluctuations in both b and AE rate. Furthermore, some AE events possess similar dynamic rupture features to those of earthquakes, having an initial phase associated with the transition from quasi-dynamic to dynamic rupture. We conclude based on these experimental observations that fault rupture has hierarchical characteristics. Quasi-static nucleation of fault rupture represents dynamic fracture of the asperities on the fault plane; likewise, a quasi-static nucleation process characterized by dynamic microfracturing precedes the fracture of an asperity. Since dynamic motions are easier to detect remotely than static deformations, understanding the hierarchical processes underlying fault rupture may thus be helpful for elucidating quasi-static nucleation at larger scales in terms of the dynamic rupture of the asperities at smaller scales. Careful studies of asperity failure in the lab may guide future seismic studies of large asperities on natural faults, potentially making it possible to recognize the final preparation stage before a large earthquake.

On the spatio‐temporal distribution of acoustic emissions in two granitic rocks under triaxial compression: The role of pre‐existing cracks

We used a new rapid data acquisition system to examine the role of pre-existing crack density in determining the mechanical properties of crystalline rock. Two similar coarse-grained samples, Tsukuba granite (TG) and granitic porphyry (GP), were used in triaxial compression tests. The main difference between the two rocks is that TG contains main large pre-existing cracks, whereas GP is almost crack-free. In our tests, the two rocks showed significantly different behaviors. In GP, before the fault nucleation, AE activity was low and showed increasing b-values with increasing stress. But in TG, a large number of AEs were observed and showed short-term b-value anomalies - mutual fluctuations on a decreasing background. The short-term fluctuations of b-values are closely correlated with those of the spatio-temporal clustering of AE locations. Our results demonstrate: 1) pre-existing cracks are the most dominant factor of all heterogeneities that govern the fault nucleation process in laboratory rock samples; 2) failure predictability is dependent on the pre-existing crack density.

Effect of grain size on fractal structure of acoustic emission hypocenter distribution in granitic rock

Abstract Triaxial compression tests were carried out on Inada and Oshima granodiorites under constant loading rate conditions at a confining pressure of 50 MPa. Acoustic emissions (AE) were monitored and recorded using a multichannel system, and the spatial distributions of AE in the two samples were examined and compared as the experimental conditions were identical. Approximately 3000 and 1000 events were located in Oshima (fine-grained) and Inada (coarse-grained) granodiorites respectively, and the fractal dimensions of their spatial distributions were estimated by Grassbergers method. The AE hypocenter distribution of the fine-grained granodiorite showed a single fractal structure with a fractal dimension of 2.7, whereas that of the coarse-grained granodiorite showed a band-limited fractal structure. The fractal dimension of the spatial distribution of Inada granodiorite changed from 2.0 to 2.4 when the length scale r became nearly equal to the average grain size (5 mm) of the rock. These results imply that the spatial distribution of newly developed microcracks, which are the sources of AE and the scatterers of elastic wave, can be influenced by the grain of the rock.

Comparison of the microfracture localization in granite between fracturation and slip of a preexisting macroscopic healed joint by acoustic emission measurements

Experiments of fracturation and slip of a preexisting macroscopic healed joint have been performed under triaxial deformation on granite from Mayet de Montagne (France). This granite shows high grain-scale inhomogeneity. Acoustic emissions have been recorded and hypocenters have been determined during the entire experiments. For both rupture experiment and slip experiment, precursory localization of microfractures in the final rupture plane has been observed in the early stage of deformation, well before the dilatancy. It is likely that not only initial closure of favorably oriented cracks but also breaking of partially cemented grains or slipping between grains may occur in the pseudoelastic phase and are already localized on the final rupture plane where the shear stress seems to be concentrate. This behavior is observed in both cases where stress heterogeneity and rupture nucleation are controlled by (1) medium-scale heterogeneity at the grain scale (HS sample) or (2) macroscopic heterogeneity in the form of a preexisting healed joint (JS sample). The sample with the healed joint exhibited ~ 1.6 times more acoustic emission events than the intact sample. The presence of the healed joint significantly weakened the sample.

Precursory localization and development of microfractures along the ultimate fracture plane in amphibolite under triaxial creep

In a triaxial creep experiment in amphibolite, we clearly found a precursory localization and development of microfractures along the final fracture plane using an AE (acoustic emission) source location technique. The precursory localization of AE hypocenters first nucleated near a pre-existing macroscopic defect and then extended gradually along the final fracture plane prior to failure On the other hand, no significant precursory localization of AE hypocenters on the final fracture plane before failure has been reported in rock samples free of pre-existing macroscopic defects. This difference in AE occurrence patterns before failure could be explained by the difference in the degree of damage in the portion of the rock surrounding the localization zone when it nucleates.

Geological Disposal of Carbon Dioxide and Radioactive Waste in the Geotectonically Active Country of Japan

In Japan, site selection for geological disposal of radioactive waste (RW) and carbon dioxide (CO2) is very important because of the large regional differences in tectonic activity. Assessment of the long-term stability of geological environments is key to geological RW disposal in Japan. A comprehensive system of long-term prediction of crustal movement and the groundwater regime around the virtual RW disposal sites has been developed in Japan. CO2 is naturally abundant, but geological disposal of the gigantic volumes of CO2 may have big impacts on the environment. One of the adverse effects of underground fluid injection is that it may induce earthquakes. Underground carbon microbubble injection accelerates advanced geological disposal mechanisms. The autogenously sealed ‘CO2 capsules’ can be formed in large basaltic sheets, ophiolite complex and oceanic crust. Sub-seabed aquifers under the deep sea floor can provide very safe and virtually limitless CO2 disposal. Different disposal strategies for CO2 and RW are needed because of the extreme difference in their toxicity and volume. The dispersion and dilution principle is possible for the CO2 disposal, while RW is strictly contained by the multiple barrier system. The stability of the geological environment is important for both CO2 and RW disposal.

Frequency-dependence of velocity and attenuation of elastic waves in granite under uniaxial compression

Velocity as well as attenuation factorQ−1 ofP-wave in a dry granitic rock sample under uniaxial compressions were measured in the range of frequency between 100 kHz and 710 kHz by using the pulse transmission technique. Above the stress of 0.5 σf, where σf is the fracture stress, theP-wave velocity decreases with increasing axial stress, whereasQ−1 increases. Particularly, the change ofQ−1 is greater for high frequency than for low frequency. At a given stress level, the higher the frequency, the higher theP-wave velocity and the largerQ−1. This result means that the velocity decrease with increasing stress is smaller for higher frequency. Because of this frequency-dependence of velocity decrease, theP-wave in the rock under dilatant state shows dispersion. The body wave dispersion is more remarkable at higher stress, and is not found in a homogeneous material with no cracks. Thus the disperison is attributed to the generation of cracks. When the frequency-dependence ofQ−1 is approximated asfn in the present frequency range, the exponentn takes a value from 0.63 to 0.77.

Temporal and spatial distribution of microfractures in granites of different structures under triaxial compression and its significance in seismology

The temporal and spatial distribution of microfracturing activity in two kinds of granite under triaxial compression has been studied by using a new acoustic emission system. For Inada granite, there is no clear clustering of acoustic emission events in time and space, thus it is difficult to exactly deduce the time and position of the major fracturing. While for Mayet granite, acoustic emission events are clustered in time and space, so the time and position of the major fracturing can be exactly predicted according to microfracturing process. Such a difference may result from the difference in deformation mode caused by different rock structures.