Recent seismic activity beneath the Tibetan Plateau is prompting scientists to re-evaluate the boundaries between the Earth’s crust, and mantle. A growing number of earthquakes originating deeper than the Moho discontinuity – the boundary between the crust and the mantle – are challenging conventional understanding of how these regions interact and contribute to seismic events. These “sub-Moho” earthquakes, concentrated in a roughly 300-kilometer segment of southern Tibet, are reaching depths of up to 110 kilometers, raising questions about the processes driving them and the composition of the material at those depths.
The Himalayas and the Tibetan Plateau are formed by the ongoing collision of the Indian and Eurasian tectonic plates, a process that generates immense stress within the Earth’s crust. Although earthquakes are common in this region, the discovery of significant seismic activity below the Moho is forcing geologists to consider more complex models of the lithosphere. Researchers are investigating whether these events are caused by faults penetrating into the mantle, or by the movement of dense, solidified crustal material sinking into the upper mantle – a process known as “dripping.”
What are Sub-Moho Earthquakes?
Traditionally, the Moho discontinuity, defined by changes in seismic wave speed and density, was considered a clear demarcation between the crust and the mantle. Yet, the distinction between a ‘Moho’ (a wavespeed/density boundary) and the ‘crust-mantle boundary’ (CMB, a petrological distinction) complicates the interpretation of these deeper earthquakes, as noted in research published by ScienceDirect. Sub-Moho earthquakes are those that originate below this boundary, suggesting that seismic activity isn’t limited to the crustal layers. More than 100 such earthquakes have been detected along the 2,000-kilometer Himalayan arc, according to a study published in Nature.
One proposed explanation involves the formation of eclogite, a high-pressure metamorphic rock, in the lower crust. As the Indian crust subducts beneath the Eurasian plate, mafic granulites within the Indian lower crust can undergo eclogitization, becoming denser and potentially sinking into the mantle in a process similar to Rayleigh-Taylor instability. Numerical modeling suggests that the viscosity of this eclogite must be relatively low – less than or equal to 1–5 × 1021 Pa⋅s – for this process to occur within a geologically reasonable timeframe of 5–20 million years.
The Role of Eclogite and Faulting
While eclogite drips can create density anomalies and initiate instability, researchers have found that an isolated drip alone cannot explain the observed focal mechanisms – the type of fault movement that generates the earthquake. The dominant pattern observed is dextral-slip, indicating a sideways movement. The study suggests that eclogitization occurs along lower-crustal shear zones associated with active faults and fluid intrusion, creating the density anomaly that drives the sinking process. As the eclogite drip grows, the resulting strain can create brittle faulting at upper-mantle depths, even within what were originally crustal rock types.
However, the research also indicates that a deeply penetrating fault alone is unlikely to be the sole cause of the deep seismicity. Brittle failure in the ultramafic mantle – the rock type typically found in the upper mantle – is considered implausible at the depths and temperatures found beneath the Tibetan Plateau. The Moho lies at approximately 70 kilometers depth in this region, and temperatures increase with depth, making brittle failure less likely.
Implications for Understanding Earth’s Interior
The findings have implications for understanding the rheology – the flow and deformation properties – of the lithosphere. Mapping earthquake depths relative to the Moho across the entire India/Asia convergent orogen, and eventually worldwide, is crucial for refining models of the Earth’s interior. The 2025 Tibet earthquake, a magnitude 7.1 event that struck Tingri County, serves as a stark reminder of the seismic hazard in this region. The earthquake, which occurred at a depth of 10 kilometers, caused between 126 and 400 fatalities and injured hundreds more in Tibet, Nepal, and India. It was the largest earthquake in China since May 2021 and the deadliest since December 2023.
Further research is needed to fully understand the interplay between faulting, eclogitization, and mantle dynamics in driving sub-Moho earthquakes. The code used for the drip modeling is publicly available on Github (https://github.com/xhsongstanford/Ecogite_Drip), allowing other researchers to build upon these findings. Detailed focal mechanism data and earthquake information are also available in supplementary materials.
The continued monitoring of seismic activity in the Himalayas and Tibetan Plateau, coupled with advanced modeling techniques, will be essential for improving our understanding of these complex geological processes and mitigating the risks associated with future earthquakes.
Disclaimer: This article provides informational content about geological and seismic events and should not be considered medical or emergency advice. If you have been affected by the recent earthquake, please consult official sources for assistance and support.
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