[S-CG10_2AM1] Microcracks preceding ruptures in the crust related to earthquakes, volcanic eruptions and landslide

★ [SCG10-01] Microcracks preceding ruptures: insights gained from laboratory acoustic emission study

*Xinglin LEI¹ (1.Geological Survey of Japan, AIST, Japan)

Keywords:Acoustic emission (AE), Microfracture, Pre-failure damage, Fault nucleation

Earthquakes in the crust are caused by the rapid shear fracture of a fault. Thus, understanding the source processes of earthquakes relies on the understanding of shear fracturing in rocks. Abundant experimental evidence shows that macroscopic shear fracturing within rocks and other brittle materials does not occur by the growth of a single shear crack in its own plane. Rather, it is preceded by a very complex pervasive evolution of some pre-failure damage. Therefore, studies focusing on both fracture dynamics and pre-failure damage are a subject of interest in seismology. Fracturing dynamics and the pre-failure damage can be inferred from AE statistics as the number of AE events is proportional to the number of growing cracks, and the AE amplitudes are proportional to the length of crack growth increments in the rock. Therefore, the AE technique, which monitors the spatiotemporal distribution of AE events, is applied to the analysis of the micro-cracking activity inside the sample space, and it can be performed under an artificially controlled pressure, which is very important for the simulation of underground conditions.The fracture of intact rocks as well as rocks containing natural structures (joints, faults, foliations) under constant stress rate loading or creep conditions is generally characterized by typical stages with different underlying physics. Through an integrated analysis of several AE statistics obtained from AE data collected with the high-speed AE waveform recording system, a three-phase pre-failure-damage model has been proposed and further enforced with new data. The primary phase reflects the initial rupture of pre-existing microcrack population in the sample or in the fault zone. Sub-critical growth dominates the secondary phase. The third phases termed nucleation phase corresponds to the initiation and accelerated growth of the ultimate fracture. In earthquake seismology, researchers have a special interest with the nucleation phase since faulting nucleation governs the predictability of earthquakes. Lithology, density and size distribution of pre-existing cracks, meso-scale and macro-scale heterogeneities all have an overall role in AEs. There are some cases in which some phases are not clear. In general, homogeneous (both fine-grained and coarse-grained) rocks with pre-existing cracks likely show all phases. Heterogeneous or weak rocks such S-C cataclasite normally show a lack of the primary phase. Samples with few pre-existing cracks and samples containing optimally oriented weak structures, likely show an unpredictable fracturing behaviour as well as a lack of primary and secondary phases, in addition the nucleation phase has a small number of AEs.Rules obtained at the laboratory scale are helpful for understanding natural earthquakes on a significantly larger scale. However, we cannot simply bridge laboratory scale to a scale several orders larger. At every step up from a smaller scale to a larger scale, we encountered something different. The difference could be small for each step but, after many steps, we could see something quite different. Studies on all scales are important. Quantitative investigation of rock fracture using AE techniques is still an interesting field for the future. On one hand, it may shed some light on earthquake seismology. On the other hand, it may provide a fundamental technical background promoting applications including: enhanced geothermal systems (EGS), extraction of shale gas and core bed gas, and CO2 geological storage. The latter of which involves fluids being intensively pumped into the deep Earth under high pressure; injection-induced earthquakes would be a problem that must be well-addressed.