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1. Lower-crustal earthquakes in southern Tibet are linked to eclogitization of dry metastable granulite

   https://doi.org/10.1038/s41467-018-05964-1  

   Shi, Feng; Wang, Yanbin; Yu, Tony; Zhu, Lupei; Zhang, Junfeng; Wen, Jianguo; Gasc, Julien; Incel, Sarah; Schubnel, Alexandre; Li, Ziyu; et al (August 2018, Nature Communications)

   Abstract Southern Tibet is the most active orogenic region on Earth where the Indian Plate thrusts under Eurasia, pushing the seismic discontinuity between the crust and the mantle to an unusual depth of ~80 km. Numerous earthquakes occur in the lower portion of this thickened continental crust, but the triggering mechanisms remain enigmatic. Here we show that dry granulite rocks, the dominant constituent of the subducted Indian crust, become brittle when deformed under conditions corresponding to the eclogite stability field. Microfractures propagate dynamically, producing acoustic emission, a laboratory analog of earthquakes, leading to macroscopic faults. Failed specimens are characterized by weak reaction bands consisting of nanometric products of the metamorphic reaction. Assisted by brittle intra-granular ruptures, the reaction bands develop into shear bands which self-organize to form macroscopic Riedel-like fault zones. These results provide a viable mechanism for deep seismicity with additional constraints on orogenic processes in Tibet. 

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2. Metamorphism-facilitated faulting in deforming orthopyroxene: Implications for global intermediate-depth seismicity

   https://doi.org/10.1073/pnas.2112386119  

   Shi, Feng; Wang, Yanbin; Wen, Jianguo; Yu, Tony; Zhu, Lupei; Huang, Taizi; Wang, Kelin (March 2022, Proceedings of the National Academy of Sciences)

   Significance The exothermic metamorphic reaction in orthopyroxene (Opx), a major component of oceanic lithospheric mantle, is shown to trigger brittle failure in laboratory deformation experiments under conditions where garnet exsolution takes place. The reaction product is an extremely fine-grained material, forming narrow reaction zones that are mechanically weak, thereby facilitating macroscopic faulting. Oceanic subduction zones are characterized by two separate bands of seismicity, known as the double seismic zone. The upper band of seismicity, located in the oceanic crust, is well explained by dehydration-induced mechanical instability. Our newly discovered metamorphism-induced mechanical instability provides an alternative physical mechanism for earthquakes in the lower band of seismicity (located in the oceanic lithospheric mantle), with no requirement of hydration/dehydration processes. 

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3. Upper Mantle Earthquakes Along the Edge of the Wyoming Craton

   https://doi.org/10.1029/2024GL114073  

   Hutchings, Sean_J; Koper, Keith_D; Burlacu, Relu; Zeng, Qicheng; Lin, Fan‐Chi; Zandt, George (May 2025, Geophysical Research Letters)

   Abstract Earthquakes in continental regions overwhelmingly occur in the crust where low pressure and temperature promote brittle failure in response to tectonic stress. In rare cases, primarily in the thickened lithosphere near the Himalayas and Tibet, continental earthquakes occur in the uppermost mantle, perhaps implying an abnormally deep brittle‐ductile transition zone created by relatively low temperatures (≲600°C) and the increased strength of olivine‐rich mantle rocks. Here we present evidence for nine mantle earthquakes—only four of which were previously recognized—along the edge of the Wyoming Craton in the western U.S. Eight of the nine earthquakes occurred >15 km beneath the Moho where temperatures are likely above 700°C. We infer a mixture of brittle and ductile (thermal runaway) source processes facilitated by elevated strain rates from regional or edge‐driven mantle convection, which is thought to be a primary force behind crustal seismicity in the Intermountain West. 

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4. Mantle earthquakes in the Himalayan collision zone

   https://doi.org/10.1130/G46378.1  

   Schulte-Pelkum, Vera; Monsalve, Gaspar; Sheehan, Anne F.; Shearer, Peter; Wu, Francis; Rajaure, Sudhir (June 2019, Geology)

   Abstract Earthquakes are known to occur beneath southern Tibet at depths up to ∼95 km. Whether these earthquakes occur within the lower crust thickened in the Himalayan collision or in the mantle is a matter of current debate. Here we compare vertical travel paths expressed as delay times between S and P arrivals for local events to delay times of P-to-S conversions from the Moho in receiver functions. The method removes most of the uncertainty introduced in standard analysis from using velocity models for depth location and migration. We show that deep seismicity in southern Tibet is unequivocally located beneath the Moho in the mantle. Deep seismicity in continental lithosphere occurs under normally ductile conditions and has therefore garnered interest in whether its occurrence is due to particularly cold temperatures or whether other factors are causing embrittlement of ductile material. Eclogitization in the subducting Indian crust has been proposed as a cause for the deep seismicity in this area. Our observation of seismicity in the mantle, falling below rather than within the crustal layer with proposed eclogitization, requires revisiting this concept and favors other embrittlement mechanisms that operate within mantle material. 

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5. Crustal Anisotropy Across Eastern Tibet and Surroundings Modeled as a Depth-Dependent Tilted Hexagonally Symmetric Medium

   https://doi.org/10.1093/gji/ggx004  

   Xie, Jiayi; Ritzwoller, Michael H.; Shen, W.; Wang, Weitao (January 2017, Geophysical Journal International)

   null (Ed.)

   Two types of surface wave anisotropy are observed regularly by seismologists but are only rarely interpreted jointly: apparent radial anisotropy, which is the difference in propagation speed between horizontally and vertically polarized waves inferred from Love and Rayleigh waves, and apparent azimuthal anisotropy, which is the directional dependence of surface wave speeds (usually Rayleigh waves). We show that a new data set of Love and Rayleigh wave isotropic phase speeds and Rayleigh wave azimuthal anisotropy observed within and surrounding eastern Tibet can be explained simultaneously by modeling the crust as a depth-dependent tilted hexagonally symmetric (THS) medium. We specify the THS medium with depth-dependent hexagonally symmetric elastic tensors tilted and rotated through dip and strike angles and estimate these quantities using a Bayesian Monte Carlo inversion to produce a 3-D model of the crust and uppermost mantle on a 0.5° × 0.5° spatial grid. In the interior of eastern Tibet and in the Yunnan-Guizhou plateau, we infer a steeply dipping THS upper crustal medium overlying a shallowly dipping THS medium in the middle-to-lower crust. Such vertical stratification of anisotropy may reflect a brittle to ductile transition in which shallow fractures and faults control upper crustal anisotropy and the crystal-preferred orientation of anisotropic (perhaps micaceous) minerals governs the anisotropy of the deeper crust. In contrast, near the periphery of the Tibetan Plateau the anisotropic medium is steeply dipping throughout the entire crust, which may be caused by the reorientation of the symmetry axes of deeper crustal anisotropic minerals as crustal flows are rotated near the borders of Tibet. 

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