Scholarlink

<p>The collision between the Indian and Eurasian plates has significantly shaped the crustal structure of the Uttarakhand Himalaya, crucially affecting seismic hazard potential. We conduct a comprehensive local earthquake seismic velocity tomography to evaluate the seismic hazard potential of the region. Analysing our seismic tomography data, we observe distinct high-velocity anomalies in the upper and lower crust, as well as along the MHT, indicating robust, heterogeneous lithospheric blocks. Between 0 and 20 km depth, the observed 5–20% increase in dVp and 4–16% increase in dVs, coupled with a low Vp/Vs ratio of 1.5–1.8, is characteristic of a high-velocity upper crust. This typically reflects the presence of compact, less porous, and often felsic or metamorphic rocks, which are common in the upper continental crust and result in elevated seismic velocities. In contrast, between 20 and 40 km depth, the reduction in dVp (–10%) and dVs (–4 to 12%) along with a higher Vp/Vs ratio (1.65–2.0) indicates a low-velocity lower crust. Such features are usually associated with mafic or partially molten rocks, increased fluid content, or high porosity, all of which lower seismic velocities and raise the Vp/Vs ratio. The Main Himalayan Thrust (MHT) is commonly modelled as a low-angle, north-dipping layer situated between 8 and 20 km depth, characterised by a 10–20% reduction in both Vp and Vs, and a 10–15% increase in Vp/Vs. Pockets with low velocities may result from partial melts in the lower crust, whilst those in the upper and intermediate crust may arise from meteoric water and aqueous or metamorphic fluids, respectively. Our simulations show that the 1803 M<sub>w</sub>7.8 Garhwal, 1991 M<sub>w</sub>6.8 Uttarkashi, and 1999 M<sub>w</sub>6.5 Chamoli earthquakes occurred in low-velocity zones along various segments of the MHT, implying that fluids or elevated pore fluid pressure were responsible for these seismic events. Our common conversion point (CCP) stacking of radial P-Receiver Functions (PRFs) indicates a north-dipping crust–mantle boundary characterised by larger positive Ps/P amplitudes between depths of 30 and 55&#xa0;km. In addition, these CCP images reveal a north-dipping zone with larger negative Ps/P amplitudes between 8 and 20&#xa0;km depth, potentially signifying the low-velocity MHT. Our findings, based on a joint inversion of radial PRFs and Rayleigh wave group velocity dispersion data, provide robust evidence for a doubling of the Moho interface and a north-dipping Main Himalayan Thrust (MHT) with a dip angle of 1–3°.</p>

错误：搜索内容不能为空，请输入英文关键词
错误：关键词超出字数限制，请精简
高级检索

Seismic evidence for the Main Himalayan Thrust and the crust–mantle boundary in the Kumaon–Garhwal Himalaya

  • 期刊论文

  • 发表日期：

    2025年12月19日

查看全文
  • Prantik Mandal,
  • R Raghavan,
  • Satish Saha,
  • Raju Prathigadapa

摘要

The collision between the Indian and Eurasian plates has significantly shaped the crustal structure of the Uttarakhand Himalaya, crucially affecting seismic hazard potential. We conduct a comprehensive local earthquake seismic velocity tomography to evaluate the seismic hazard potential of the region. Analysing our seismic tomography data, we observe distinct high-velocity anomalies in the upper and lower crust, as well as along the MHT, indicating robust, heterogeneous lithospheric blocks. Between 0 and 20 km depth, the observed 5–20% increase in dVp and 4–16% increase in dVs, coupled with a low Vp/Vs ratio of 1.5–1.8, is characteristic of a high-velocity upper crust. This typically reflects the presence of compact, less porous, and often felsic or metamorphic rocks, which are common in the upper continental crust and result in elevated seismic velocities. In contrast, between 20 and 40 km depth, the reduction in dVp (–10%) and dVs (–4 to 12%) along with a higher Vp/Vs ratio (1.65–2.0) indicates a low-velocity lower crust. Such features are usually associated with mafic or partially molten rocks, increased fluid content, or high porosity, all of which lower seismic velocities and raise the Vp/Vs ratio. The Main Himalayan Thrust (MHT) is commonly modelled as a low-angle, north-dipping layer situated between 8 and 20 km depth, characterised by a 10–20% reduction in both Vp and Vs, and a 10–15% increase in Vp/Vs. Pockets with low velocities may result from partial melts in the lower crust, whilst those in the upper and intermediate crust may arise from meteoric water and aqueous or metamorphic fluids, respectively. Our simulations show that the 1803 Mw7.8 Garhwal, 1991 Mw6.8 Uttarkashi, and 1999 Mw6.5 Chamoli earthquakes occurred in low-velocity zones along various segments of the MHT, implying that fluids or elevated pore fluid pressure were responsible for these seismic events. Our common conversion point (CCP) stacking of radial P-Receiver Functions (PRFs) indicates a north-dipping crust–mantle boundary characterised by larger positive Ps/P amplitudes between depths of 30 and 55 km. In addition, these CCP images reveal a north-dipping zone with larger negative Ps/P amplitudes between 8 and 20 km depth, potentially signifying the low-velocity MHT. Our findings, based on a joint inversion of radial PRFs and Rayleigh wave group velocity dispersion data, provide robust evidence for a doubling of the Moho interface and a north-dipping Main Himalayan Thrust (MHT) with a dip angle of 1–3°.