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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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P ‐Wave Reflectivity of the Crust and Upper Mantle Beneath the Southern Appalachians and Atlantic Coastal Plain Using Global Phases
Abstract Reflection profiles generated using PKPdf as a virtual source show laterally continuous reflections from structures at depths less than 1 km to roughly 200 km beneath the southern Appalachian orogen and Atlantic coastal plain. Arrivals interpreted as reflections from the Moho increase in time from ~10 s beneath the coastal plain to 17.4 s (~57 km) beneath the Blue Ridge Mountains, providing additional evidence that the southern Appalachians are in rough isostatic equilibrium. Reflections at 32–36 s (120–135 km) are consistent with the depth to the base of the lithosphere found in recent inversions of Ps arrivals and surface waves. Alternatively, these and later reflections at times up to 58 s (~224 km) may be due to layering associated with drag‐induced flow in the asthenosphere, suggesting largely horizontal rather than vertical flow for depths less than 225 km beneath the Georgia coastal plain.
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- Award ID(s):
- 1830182
- PAR ID:
- 10449770
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 47
- Issue:
- 18
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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