We investigate the crustal seismic structure of the Garhwal Himalayan region using regional and teleseismic earthquake waveforms, recorded over 19 closely spaced broad-band seismic stations along a linear profile that traverses from the Sub Himalayas to Higher Himalayas. The regional earthquake traveltime analysis provides uppermost mantle P- and S-wave velocities as 8.2 and 4.5 km s⁻¹, respectively. The calculated receiver functions from the teleseismic P waveforms show apparent P-to-S conversions from the Moho as well as from intracrustal depths, at most of the seismic stations. These conversions also show significant azimuthal variations across the Himalayas, indicating complex crustal structure across the Garhwal Himalaya. We constrain the receiver function modelling using the calculated uppermost mantle (Pn and Sn) velocities. Common conversion point stacking image of P-to-S conversions as well as receiver function modelling results show a prominent intracrustal low shear velocity layer with a flat–ramp–flat geometry beneath the Main Central Thrust zone. This low velocity indicates the possible presence of partial melts/fluids in the intracrustal depths beneath the Garhwal Himalaya. We correlate the inferred intracrustal partial melts/fluids with the local seismicity and suggest that the intracrustal fluids are one of the possible reasons for the occurrence of upper-to-mid-crustal earthquakes in this area. The results further show that the Moho depth varies from ∼45 km beneath the Sub Himalayas to ∼58 km to the south of the Tethys Himalayas. The calculated lower crustal shear wave velocities of ∼3.9 and 4.3 km s⁻¹ beneath the Lesser and Higher Himalayas suggest the presence of granulite and partially eclogite rocks in the lower crust below the Lesser and Higher Himalayas, respectively. We also suggest that the inferred lower crustal rocks are the possible reasons for the presence and absence of the lower crustal seismicity beneath the Lesser and Higher Himalayas, respectively.

中文翻译：

---

使用区域和远震波形模拟的Garhwal Himalaya下方的地壳地震结构

我们使用区域和远震地震波形调查了Garhwal喜马拉雅地区的地壳地震结构，沿从南喜马拉雅山脉到高喜马拉雅山脉的线性剖面记录了19个近距离的宽带地震台站。区域地震传播时间分析提供的最高地幔P波和S波速度分别为8.2和4.5 km s ^-1。根据远震P波形计算出的接收器函数显示出明显的P对S大多数地震台站的莫霍面以及地壳内深度的转换。这些转换还显示出喜马拉雅山脉的明显方位角变化，表明整个Garhwal喜马拉雅山脉的地壳结构复杂。我们使用计算出的最高地幔速度（Pn和Sn）约束接收器函数模型。P-to-S的通用转换点叠加图像转换以及接收器函数建模结果表明，主中心冲断带下方有一个显着的壳内低剪切速度层，具有平坦-斜坡-平坦的几何形状。这个低速度表明在Garhwal喜马拉雅山下面的地壳内深度可能存在部分熔体/流体。我们将推断的壳内部分熔体/流体与当地地震活动性联系起来，并认为壳内流体是该地区发生中上地壳地震的可能原因之一。结果进一步表明，莫霍面深度从喜马拉雅山以下的约45 km到特提斯喜马拉雅山的南部的约58 km。计算得出的下地壳剪切波速度约为3.9和4.3 km s ^-1小喜马拉雅山脉和高喜马拉雅山脉下方的岩石表明，小喜马拉雅山脉和高喜马拉雅山脉下方的下部地壳中分别存在花岗石和部分榴辉岩。我们还建议，推断的下地壳岩石可能分别是小喜马拉雅山和上喜马拉雅山下存在和不存在下地壳地震活动的可能原因。