![]() ![]() 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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