Change font in deepfocus4/20/2023 ![]() ![]() These results showed that the AE events were linked to the final failure. Scanning and transmission electron microscopy (SEM and TEM) confirmed the XMT results and allowed more details to micrometer scales. X-ray microtomography (XMT) on the recovered samples revealed conjugated faults. All the AE events were located within the sample. The onset of the olivine-spinel transformation was observed to correlate closely with an abrupt jump in AE occurring rate and a marked stress drop. Recently, we conducted transformational faulting experiments on Mg 2GeO 4 olivine with controlled deformation and phase transformation by combining in situ synchrotron x-ray diffraction (XRD) and imaging along with acoustic emission (AE) monitoring (see Materials and Methods and fig. Without these details, it is difficult to develop realistic physical models for the failure process and scale laboratory observations to geological activities. In all of these proposed mechanisms, there is a marked lack of knowledge as to how fracture self-organizes and propagates, leading to ultimate failure. Once instability is triggered, shear failure may propagate as ductile faulting or plastic instabilities ( 23– 25) rather than as brittle failure, and rupture propagation may be facilitated by melt production at the tip of the fracture zone ( 26) or failure of the surrounding mantle materials ( 27). Transformational faulting, in which a metastable mineral, such as olivine, transforms into a denser phase, such as wadsleyite and/or ringwoodite, triggering instability by strain localization and stress concentration in the mantle transition zone (MTZ), has been suggested to explain deep-focus earthquakes (DFEQs depths, >350 km) ( 15– 22). A number of dehydration reactions have been shown to trigger rock failure in the laboratory ( 12– 14). High fluid pressure resulting from dehydration reactions may reduce the effective normal stress and allow movement on preexisting faults ( 9– 11). Several mechanisms have been proposed to explain deep faulting. Furthermore, higher temperatures at depths render rocks more ductile ( 6– 8). Brittle-frictional processes, which produce shallow seismogenic faults through open cracks and frictional sliding via mode-I and mode-II/III fractures, are suppressed by conditions at depths greater than ~70 km, because high pressure inhibits opening cracks and frictional sliding on existing fractures ( 4, 5). The physical processes that permit the occurrence of deep earthquakes remain poorly understood. Deep earthquakes pose significant seismic hazards and have played a key role in illuminating the structure of Earth’s mantle and core ( 3). They occur in association with convergent margins, defining planar regions known as the Wadati-Benioff zones ( 2), which delineate the cold, down-going cores of subducting slabs. Future numerical analyses may help resolve scaling issues between laboratory AE events and deep-focus earthquakes.ĭeep earthquakes, that is, those with hypocenter depths greater than ~70 km, constitute about a quarter of all recorded events, with moment magnitudes greater than 5 in the International Seismological Centre catalog ( 1). A rupture propagation model based on strain localization theory is proposed. The seismic relation between magnitude and rupture area correctly predicts AE magnitude at millimeter scales. AEs follow the Gutenberg-Richter statistics with a well-defined b value of 1.5 over three orders of moment magnitudes, suggesting that laboratory failure processes are self-affine. Several source parameters of AE events were extracted from the recorded waveforms, allowing close tracking of event initiation, clustering, and propagation throughout the deformation/transformation process. ![]() This precursory seismic process leads to ultimate macroscopic failure of the samples. These nanoshear bands have a near constant thickness (~100 nm) but varying lengths and self-organize during deformation. Microstructure analysis shows that AEs are produced by the dynamic propagation of shear bands consisting of nanograined spinel. ![]() AEs’ focal mechanisms, as well as their distribution in both space and time during deformation, are carefully analyzed. We report nanoseismological analysis on high-resolution acoustic emission (AE) records obtained during ruptures triggered by partial transformation from olivine to spinel in Mg 2GeO 4, an analog to the dominant mineral (Mg,Fe) 2SiO 4 olivine in the upper mantle, using state-of-the-art seismological techniques, in the laboratory. ![]() How fractures initiate, nucleate, and propagate at these depths remains one of the greatest puzzles in earth science, as increasing pressure inhibits fracture propagation. Global earthquake occurring rate displays an exponential decay down to ~300 km and then peaks around 550 to 600 km before terminating abruptly near 700 km. ![]()
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