Abstract
Intermediate-depth earthquakes (>40 km) have been observed beneath the central Himalayas over decades, with little known about their nature and characteristics. Here, we apply a state-of-the-art systematic processing routine, starting from continuous waveform data, to obtain the most comprehensive high-quality earthquake catalog with a focus on the intermediate-depth seismicity beneath the central Himalayas. We construct a catalog containing 414 robust earthquake locations with depths ranging from 40 to 110 km spanning from late 2001 till mid-2003. We calculate earthquake magnitudes in a consistent way and obtain values ranging between ML0.8 and 4.5 with a magnitude of completeness of Mc2.4. This information allows us to study the spatiotemporal characteristics of the seismicity in great detail. Earthquakes mainly take place in a cluster, consisting of two linear segments at ca. 35° azimuth difference, situated beneath the high Himalayas in NE Nepal and adjacent S. Tibet. Seismicity there does not feature any mainshock-aftershock patterns but presents a few sequences with potential seismicity migration rates compatible with linear or diffusive migration. This result, along with previous studies in the lower Indian crust, allows interpreting these events as related to metamorphic reactions involving dehydration processes. However, given the geodynamic context, a tectonic interpretation with a dextral basement fault zone propagating beneath the Himalaya and continuing as a westward propagating tear fault would also be possible. This represents a continuous fault zone from the deep crust in S. Tibet, across the Himalaya along the Dhubri-Chungthang fault zone (DCFZ) to the Shillong plateau, which could be an inherited tectonic feature.
Highlights
The Himalayan orogen is a unique and complex geological structure that comprises the highest mountain chain on Earth
We present 414 high-quality intermediate-depth earthquake locations beneath the central Himalayas orogen spanning from late 2001 through mid-2003
The nearly continuous seismic activity and the evidence of potential seismicity migration within sequences of succeeding events at reasonable rates we observe here allow for the interpretation that fluids and metamorphic dehydration reactions to play a role in seismogenesis within a hydrous Indian lower crust (ILC) beneath the high Himalayas
Summary
In addition to shallow events, which are responsible for the seismic hazard, intermediate-depth earthquakes (40–100 km) take place beneath the orogen near the plate interface and intra-slab regions (Figure 1; Chen and Molnar, 1983; Monsalve et al, 2006; Priestley et al, 2008; Paul et al, 2015; Schulte-Pelkum et al, 2019; Wang et al, 2020). These events have been identified over decades (e.g., Chen et al, 1981; Chen and Molnar, 1983), and their significance and location relative to the Moho contributed to advancing the understanding of the rheology of the continental lithosphere (e.g., Chen and Molnar, 1983; Maggi et al, 2000; Jackson, 2002; Jackson et al, 2004)
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