Introduction

The Lg wave is a super critically reflected S-wave, travels within the continental crust, between the free surface and the Moho1,2 and is the most prominent seismic phase recorded at regional to teleseismic distances provides an important insight on nature of lithology and its tectonic environments3. The attenuation of Lg wave vary significantly across major continents due to the presence of partial melting, fluid contents and different compositions of rocks from one region to another4. In several continental regions where there are considerable fluctuations in crustal thickness, such as the mountain belts in central Asia, the Tibetan, the Bolivian Altiplano and close to the Alpine ranges, strong attenuation of Lg has been reported. In tectonically active areas such as Tibet, scattering of Lg energy from fractures within the shallow crust, partial melting and low S-wave velocity layer within the crust may be a significant factor for strong Lg attenuation5,6,7,8,9. The values of Lg Q vary up to an order of magnitude from tectonically stable (> 700) to unstable regions (< 200)3,10. There have been significant studies worldwide to characterize the attenuation property of a medium through Quality factor (Q) estimation utilizing different types of seismic waves for e.g. coda-wave, Lg-wave, P-wave and S-wave, etc. recorded at local and regional distances from an earthquake location4,11,12,13,14,15,16,17,18,19. The effect of fluid or melt fraction is more dominant in attenuation property (1/Q) compared to the S-wave velocity. While the S-wave velocity in crust varies by less than 20%, Q varies by a factor of 3 on major continents20.

The present profile, which lies in the Kumaon sector of the northwest Himalaya stretches to about 200 km in length along the northeast-southwest direction. The transect cuts perpendicular across the major fault structures in the region that have a strike directing northwest-southeast.

The present seismic profile consists of stations that are fit by the great-circle path under a least square criterion3 and thus provide us with the opportunity to invert for the lateral variation of Lg Q along this best-fit profile in the Kumaon section of the northwest Himalaya (Fig. 1). We have utilized the interstation Q0 computation approach3 to image the attenuation across the major structural features of the Kumaon Himalaya that extends from the Indo-Gangetic plain in the south across the Main Frontal Thrust (MFT) to the north of the South Tibetan Detachment (STD) fault in the Tethys Himalaya. The first step involves calculation of the interstation Q0 values from the selected pair of stations. The interstation Q0 values are then used to map the lateral variations of Lg Q0 using a back projection method. We find low Lg Q0 along the entire length of the profile, which we interpret in terms of crustal ramps and /or trapped partial melts, which supports the outward channel flow of the Tibetan crust. Attenuation measurement is important in investigating the presence of aqueous fluid/partial melt in the crust, and can provide constraints on the quantification of ground motions from future earthquakes in the Himalaya and neighbouring regions.

Figure 1
figure 1

Regional tectonic map of the Kumaon Himalaya showing major thrust systems. MFT Main frontal thrust, MBT Main Boundary Thrust, SAT South Almora Thrust, NAT North Almora Thrust, BT Berinag Thrust, SCT South Chippalkot Thrust, CCT Central Chippalkot Thrust, NCT North Chippalkot Thrust, MT Munsiari Thrust, VT Vaikrita Thrust, STD Southern Tibetan detachment. Broadband seismographs operated during 2018–2022 is marked as filled triangle (blue). Inset 1 shows the Indian map with the study area marked as a red square. Inset II shows the geographical location of the five regional earthquakes (solid star) used in analysis and the recording stations (filled triangle). Three earthquakes in the Tibet are represented in the form of solid stars with different sizes and colours. Rest two earthquakes from Indian shield are represented as solid blue star. The figure has been generated using the Generic Mapping Tool (GMT), version 6.0 (https://docs.generic-mapping-tools.org/6.0/).

Full size image

Seismotectonics and previous geophysical studies

The spectacular Himalaya, mountain chain resulted from the northwards underthrusting of the Indian plate with the Eurasia since 50 Ma21,22. The subduction of the Indian plate beneath the Himalaya caused lateral deformations giving rise to fault structures from north to south. These crustal scale faults have a NW–SE strike and are designated as the Indus Tsangpo Suture Zone (ITSZ), the Main Central thrust (MCT), the Main boundary thrust (MBT) and the Main frontal thrust (MFT)23,24. The MFT bounds the Himalaya from the adjoining Indo-Gangetic Plain (IGP) to its south. Four different tectonic units referred to as the Siwalik Himalaya; the Lesser Himalaya; the Higher Himalaya and the Tethys Himalaya are separated from each other by thrust faults at the respective southern and northern boundaries by the MFT, MBT, MCT, STD and ITSZ respectively (Fig. 1). The other major thrust faults between the MBT and the MCT are the Almora thrust which is further divided into a north Almora thrust (NAT) and south Amotha thrust (SAT) and the Barinag Thrust (BT). These surface faults penetrate in depth to about 5 km25.

The western Kumaon region lies in the central seismic gap (CSG) and several geological and geophysical studies suggest a ramp structure involving the Main Himlayan Thrust (MHT) at mid-crustal depth that is responsible for major and great earthquakes in the Himalaya26. Recently, crustal investigations using receiver function analysis along the Kali River of the Kumaon Himalaya have shown variations in the crustal thickness of ~ 38 km to 41 km from the Indo-Gangetic Plain (IGP) to the Higher Himalaya and have also detected partial melts at mid-crustal depths27. A study to the west of the Kali river region28 found a flat-ramp-flat structure of the MHT and a low shear wave velocity zone (LVZ) beneath the Lesser Himalaya between the Main Frontal Thrust (MFT) and the Main Central Thrust (MCT). This LVZ is well corroborated by low resistivity found in magnetotelluric studies29,30. Receiver function analysis on the eastern side of the Kali river in Nepal reported that the Moho beneath western Nepal is gently dipping northward, from a depth of ~ 40 km beneath the foothills to ~ 58 km beneath the Higher Himalaya and then even deeper beneath southern Tibet 31. A mid-crustal low-velocity zone at ~ 12–18 km depth, beneath the Lesser Himalaya in western Nepal was also reported31. This LVZ lies in between the MBT and the MCT and is likely caused by fluids expelled from underthrust sedimentary rocks trapped at the MHT31. The spatial distribution of simulated crustal Vp/Vs ratios reveals a large lateral variation in the crustal composition of the Uttarakhand Himalayan region32. Large variations in the crust's thickness (28.3 to 52.9 km) and Poisson’s ratio (0.17 to 0.36) indicate the presence of serpentinization and high pressure metamorphic fluids in the crust32.

In a Lg Qo attenuation study33 conducted along a profile in the Garhwal Himalaya area of the northwest Himalaya found Indian shield-like Qo values of 742 ± 235 in the Lower Himalaya, whereas the High Himalaya is distinguished by an exceptionally low Qo value of 30–60. Their results suggest the presence of a low viscosity channel in the Higher Himalaya. A similar study34 for the Ladakh–Karakoram and southern Tibet suggests high crustal seismic attenuation (Qo∼70) beneath the region with possible presence of aqueous fluid/partial melt in the Ladakh crust.

All of the above mentioned geophysical studies in the Garhwal and Kumaon region of Uttarakhand, India highlights the fact that this region has been seismically very active in the recent decades and involves a structurally complex and heterogenous crust. According to the 2011 census this region has a population density of 69.77% residing in rural villages with poor living conditions and with bare minimum services. There is also very poor enforcement of earthquake resistant building practices, which makes the region highly vulnerable to future seismic hazards of ground motion. An ability of accurately estimate ground motions from future earthquakes is needed to address the seismic hazard of a region requires estimation of ground motion from future earthquakes. This ability requires knowledge of the crustal velocity structure, characterization of earthquake source zones, and quantification of seismic wave attenuation from the source to any point on the surface of the Earth. The first two requirements have been extensively studied for the Garhwal-Kumaon Himalaya, over the past decade26,27,35,36,37,38,39,40 but gaps exist in our knowledge of the attenuation structure. The present study focuses on estimation of seismic wave attenuation using the Lg wave spectra from earthquakes recorded at regional distances to ascertain the quality factor (Q) for the crust beneath the Kumaon Himalaya, India. This should provide more constraints on assessment of future seismic hazard scenarios in the Kumaon sector of the northwest Himalayas.

Data

CSIR-NGRI initiated a project to study the seismic hazard aspect of the Kumaon Himalaya referred to as “Synthesis of Earthquake Hazard scenario in NW Himalaya by Investigating the multi-scale Variations in structural and seismotectonic Assemblages” (SHIVA). The seismic attenuation study in the Kumaon Himalaya focused on the central seismic gap zone along the SW-NE transect covering MFT, MBT, MCT and STD and used 32 broadband seismic stations that operated September-2018 to March-202241,42. All the stations were equipped with CMG-3 T (120 s period) sensor and REFTEK (RT 130–01) data loggers with 4 GB swappable hard disk and GPS. Details of individual stations are presented in Table 1.

Table 1 List of broadband seismic stations and locations.
Full size table

The Lg wave recorded on the seismograms of regional earthquakes (Table 2) are extracted with corner at 0.5 Hz using in the time window appropriate for a wave travelling with a group velocity between 3.0 and 3.6 km s−13 (Fig. 2).

Table 2 Details of events used in the study.
Full size table
Figure 2
figure 2

Record section of earthquakes from (a) Southern Tibet and (b) Indian Shield. Seismograms that have been high passed with corner frequency at 0.5 Hz. The Pn velocity for these two earthquakes is 8.7 km s−1. A Strong Lg wave is recorded in seismograms for Southern Tibet (a), whereas it is feeble for the Indian shield earthquake (b). Sn velocities for these two earthquakes are 4.5 km s−1 and 4.7 km s−1.

Full size image

The Lg wave was then cut out and a cosine taper of width 0.05 was applied to avoid spectral leakage. The instrument response was removed after transforming to the frequency domain using the fast Fourier transform (FFT). To stabilize the measurement of Qo, we applied a 15-point smoothing to the ground displacement spectra3 and computed the spectral ratio in the frequency band 0.4–2 Hz. Here we have utilized a total five regional events of which three events were in the Tibet region and two were from the Indian shield region. Figure 3a,b shows the plot of example of Lg amplitude spectra of selected stations.

Figure 3
figure 3

Amplitude spectra for an earthquake from (a) Southern Tibet on 2018 November 20 and (b) Indian shield on 2019 June 5 at selected stations.

Full size image

Methodology

Lg is a regional seismic wave comprised of multiple shear wave reverberations trapped in the crustal waveguide, and is important for studying the crustal structure. The Lg Q at frequencies greater than 1 Hz is observed to obey a frequency dependence power law of the form

$$ {\text{Q}}_{{{\text{Lg}}}} \left( {\text{f}} \right) = {\text{Q}}_{{\text{o}}} {\text{f}}^{\eta } $$
(1)

where Qo and ƞ are Lg Q at 1 Hz and its power law frequency dependence, respectively. The value for Lg Q for tectonically active regions has been observed to be approximately ≤ 200 and that for stable regions the value is ≥ 6503,20. The quality factor (Qo) generally increases with frequency. We analysed the Lg Q at 1.0 Hz to maintain the uniformity within the analysis and to accurately determine the relative variation within different lithotectonic units.

Estimation of Lg Qo

We have utilized the two-station technique3 to compute the Lg Q attenuation structure beneath the Kumaon Himalaya. If two-stations denoted as station i and station j then the method collects the Lg amplitude spectra, which is denoted as Ai(f) and Aj (f) at a particular frequency, f respectively. Then we calculate the scaled amplitude ratio following the methodology3, which states that

$$ {\text{R}}\left( {\text{f}} \right) = \left( {\Delta_{{\text{i}}}^{{{1}/{2}}} /\Delta_{{\text{j}}}^{{{1}/{2}}} } \right) \, \left[ {{\text{A}}_{{\text{i}}} \left( {\text{f}} \right)/{\text{A}}_{{\text{j}}} \left( {\text{f}} \right)} \right], $$
(2)

where Δi and Δi represents the epicentral distances and the effect of geometrical spreading on the readings are cancelled by taking the square root of their ratio in 1-D computations.

Equation (2) can be written as.

$$ \begin{aligned} & \left( {{\text{V}}_{{{\text{Lg}}}} /\pi \Delta _{{{\text{i}},{\text{j}}}} } \right){\text{ ln}}\left[ {{\text{R}}\left( {\text{f}} \right)} \right]\, = \,{\text{f}}^{{{\text{1}} - \eta }} /{\text{Q}}_{{\text{o}}} \quad {\text{or}} \\ & {\text{R}}\left( {\text{f}} \right) = \left( {\Delta _{{\text{i}}} ^{{{\text{1}}/{\text{2}}}} /\Delta _{{\text{j}}} ^{{{\text{1}}/{\text{2}}}} } \right){\text{ }}\left[ {{\text{A}}_{{\text{i}}} \left( {\text{f}} \right)/{\text{A}}_{{\text{j}}} \left( {\text{f}} \right)} \right], \\ \end{aligned} $$
(3)

where P(f) = (VLg/πΔi,j) ln[R(f)]

VLg is the typical Lg group velocity. The Qo and η values can be calculated by fitting a line using the least-squares criterion. While the computation of Qo is stable, the measurement of stable interstation η is difficult. Further, due to a limited availability of earthquake-station pair measurements, we restricted this study to Qo variation only. Accordingly, we used η = 0 in Eq. (3) and computed the Qo along with the associated error in individual two-station data.

Initially, we selected all possible two-station pairs for the analysis to estimate the interstation spectral ratios but later on we applied the suggested criterion3 to examine the final two-station pairs for the Lg Q analysis.

$$ \delta {\text{Q}}_{{\text{o}}} /{\text{Q}}_{{\text{o}}} \approx {1}.{1}\left( {{\text{Q}}_{{\text{o}}} /\Delta_{{{\text{i}},{\text{j}}}} } \right) \, \delta {\text{x}} $$
(4)

There is another important limitation to this method, which assumes that the recording stations are aligned exactly with the source, but in a real scenario, a perfect alignment of stations and source is usually not possible3. To define the alignment, an angle δθ, which is the difference between the azimuths from the source and the two stations, is used. A previous study43 allowed δθ up to 10°, whereas another study3 used 15°. The amplitude spectra of two events one from the north-eastern and other from the south-western side of the profile are shown in Fig. 3a and b, respectively.

Inversion for regional variation of Qo

The inversions are composed of two steps. In the primary step, interstation Qo is measured from selected pairs of two stations (Table 3). In the second step, the interstation Qo values are mapped using inversion. We have applied the singular value decomposition method44 to compute the variations of Lg Qo attenuation along the profile in the Kumaon Himalaya. We have divided our NE-SW profile in to four different cells with the M denoting the discrete Lg Qo values in different grids, and where N is the number of observations (Q value along a two-station path, so N = number of paths). If there are N interstation Qo measurements and the region is divided into M cells, the quality factor value for individual cells (Qm) is related to the two-station measurements of quality factor (Qn) as:

$$ \Delta_{{\text{n}}} /{\text{Q}}_{{\text{n}}} = \, \Sigma \, \Delta_{{{\text{mn}}}} /{\text{Q}}_{{\text{m}}} + \varepsilon {\text{n}}, \,{\text{n}}\, = \,{1},{2},{3}, \ldots {\text{N,}} $$
(5)

where Δmn is the length of the nth ray in the mth cell. εn is the error for each Qn measurement.

Table 3 Two-station Q0 measurements.
Full size table

The resulting system of N equations is solved through a singular value decomposition algorithm in MATLAB to obtain the quality factor values for each individual cells. The economy-size decomposition algorithm of MATLAB removes extra rows or columns of zeros from the diagonal matrix of singular values along with the columns in either of data (Qn) or kernel (G) matrices that multiply those zeros in the expression

$$ {\text{Q}}_{{\text{m}}} = {\text{ G}}*{\text{m}}*{\text{Q}}_{{\text{n}}} $$
(6)

Removing these zeros and columns can improve execution time and reduce storage requirements without compromising the accuracy of the decomposition. The four blocks are shown in Fig. 4 with different colours.

Figure 4
figure 4

Spatial parameter used to compute the Lg Q value in different blocks N = 1,2,3,4. This figure has been generated using the GMT, version 6.0 software (https://docs.generic-mapping-tools.org/6.0/).

Full size image

Results and discussion

Assessment of seismic hazard in an earthquake prone area involves quantification of its crustal velocity structure, earthquake source properties and the seismic wave attenuation in the area. The latter requires estimates of the seismic quality Q. Estimation of Q at 1 Hz (Q0) of the Lg waves recorded from regional earthquakes is useful because of Q0Lg is sensitive to crustal properties3,6,19. Lg attenuation is also an important factor in determining seismic hazard because Lg of the large amplitude at regional distances45. The seismic risk in the Himalaya is high as it is expected to host a Mw 8.0 or greater anytime in future46. The present study area, which forms the eastern part of the CSG in the Himalaya has hosted many moderate to strong earthquakes in the past with the two most significant earthquakes being the 1991 Mw 6.8 Uttarkashi earthquake and the 1999 Mw 6.6 Chamoli earthquakes. Quantifying the obvious hazard is an essential to prerequisite to reducing the seismic risk to the population in this region. We computed the Lg attenuation characteristics using five regional events with Mw 4.0 to 5.5 (Table 2) along the NE-SW profile of ∼200 km along the Tanakpur-Dharchula-Dharma transect in eastern Kumaon Himalaya. We utilized the interstation Q0 computation approach3 to quantify the attenuation at different places along the 200 km seismic profile. We find significant differences of Lg Qo along the profile.

The individual Lg Qo values resulting from the various station pairs are represented in Table 3 and some examples for linear regression fit for two station pairs are shown in Fig. 5.

Figure 5
figure 5

Figures showing some of the linear regression fit to compute the interstation Qo values for selected source and station pair. The obtained Qc and its standard deviation are also shown.

Full size image

The value of Lg Qo from different events and different pairs of stations are plotted in Fig. 6. The Lg Qo values vary 20 to 203 for 64 combinations of two stations along the profile.

Figure 6
figure 6

Plot of all the computed interstation Qo values with different stations along the profile used for modelling the attenuation in individual parameterized cell.

Full size image

To better understand the lateral variability of the Qo along the profile, we divide the profile into four blocks. From north to south, first block covers the IGP and Siwalik Himalaya and the second and third blocks cover the lesser Himalaya and the fourth block consists of the higher Himalaya and the Tethys Himalaya. The linear inversion of Qo values computed using Eqs. (5) and (6), utilizing all the individual interstation Lg Qo values (Fig. 6) resulted in a minimum value of 63 ± 2 and a maximum of 203 ± 25 for four individual blocks along the profile (Fig. 7). The Lg Qo values along the profile are nearly equal to 200 for all the blocks except for a part of the second block lying between the South Almora Thrust (SAT) and the Berinag Thrust (BT), which shows characteristically low Qo value of 63.

Figure 7
figure 7

Lg Qo along with its standard deviation values for the individual blocks obtained from inversion of several two-station pair data. The relative position of stations and the corresponding geological block are shown. Faults, SAT South-Almora Thrust, NAT North-Almora Thrust, BT Berinag Thrust, MT Munsiari Thrust, VT Vaikrita Thrust.

Full size image

Our maximum Lg Qo values resulting from the inversion of interstation Lg Qo values is 203 ± 25, observed for the first block of the profile that includes some stations from Indo-Gangetic Plain, Siwalik Himalaya and some part of the Lesser Himalaya to the south of the SAT (South Almora Thrust). The reason for such low Lg Qo values in the region can be attributed to the presence of younger tertiary sediments, which has characteristically low Vp/Vs ratios and cause decay of Lg wave amplitude below the paired stations. Another possibility is the lateral variation of crustal structure and sedimentary thickness (2–5 km)47.

This second block in the profile shows the low value of Lg Qo 63 which reflects strong attenuations which consists of major secondary fault structures of the lesser Himalayan tectonic units and is mainly bounded between the South Almora Thrust (SAT) in the south and the Berinag Thrust (BT) in north. The low value of Qo correlates with low resistivity and a low seismic velocity zone28,29,48 detected to the west.

We observed higher Qo values of 200 towards the north of the BT in the third block of the profile bounded by BT in the south and Chippalkot thrust (CT) of the Dharchula area in the north. To the north of the CT, we observed very little decrease in the Qo value which is equal to 166 for the fourth block. This block extends between the CT and the South Tibetan Detachment fault (STD) and consists of two seismic stations towards the north of the STD in the Tethys Himalaya. The Qo values observed along this Tanakpur-Dharchula-Dharma NE-SW profile are similar to values observed along the INDEPTH III profile in Tibet by many researchers in the past3,49,50,51. The Lg Q0 values observed along the present profile depicts values between 63 and 203 respectively for the four different blocks in the Kumaon sector of the northwest Himalayas. The INDEPTH III experiment, which was conducted in the central Tibet region, extends further to the northeast of the present profile across the Bangong-Nujiang Suture (BNS). This area has been studied previously for seismic attenuation using the Fourier spectra of the Lg waves3. The results show low Lg Q0 values with higher attenuation characteristics, with an average Q0 value of ~ 158 for the entire profile length of ~ 200 km. The resultant high attenuation along the profile can be compared to the Lg Q0 values obtained for the INDEPTH II profile for the Southern Tibet region traversing the Indus-Yalong Suture to the north and south and extending further south into the Himalayas. Though the study in Southern Tibet and further south in the High Himalayas shows higher Lg Q0 values of ~ 300, these values are associated with large errors due to sparse path coverage as reported in the study3. The low Lg Q0 values of ~ 191 obtained to the north of the Indus-Yalong Suture in Southern Tibet are comparable to what we have obtained for our Higher Himalaya region in the Dharma sector of the Kumaon Himalaya. The study reports a gradual increase in the Lg Q0 values to the south of the Indus-Yalong Suture, which can be further explained in terms of underthrusting of the Indian Lithosphere beneath the Tibet6. Very low Qo values are also reported for the Higher Himalaya to the north of the MCT zone33, which lies further to the northwest of the present profile in the Garhwal Himalaya region. The observations of such high attenuation in the Higher Himalaya can be attributed to the presence of a low velocity zone in the High Himalaya. The present study also supplements the study conducted earlier33 for the Higher Himalayas section of the Garhwal Himalaya and further confirms the presence of the low viscous partial melts in the form of Miocene leucogranite plutons, which resulted out of the Indo-Asian collision. Another significant study34 along a profile from the southern edge of the northwestern Himalaya to Ladakh and Karakoram indicates some interesting lower Lg Q0 values, which gradually decreases northwards from ∼700 in Himalaya and ∼400 beneath the Indus Tsangpo suture (ITS) to ∼70 in Ladakh–Karakoram. The study further suggested an efficient transmission of seismic waves beneath the Himalayas and the IZS as compared to the high attenuation under Ladakh–Karakoram. It is impossible to postulate high Lg Q0 beneath High Himalayas as the crust beneath that is extremely fractured due to ongoing convergence between the India and the Eurasia plates. A significant study to the east of the Ladakh Himalaya using coda waves reported an average Qc between 47 for a 10 s lapse time with a 10-s coda window and 204 for a 50 s lapse time and 50 s window length52. Those coda Q values are similar to the Lg Q0 values found in the present study, which range between 63 and 200. The study conducted for the NW Himalaya52 as well as the present study find results similar those conducted in the adjoining Western Himalaya53,54, which report a low Q0 ≈ 44 and Q0 ≈ 74 respectively.

The study38 investigated the seismotectonic perspective for the Kumaon Himalaya and reported a complex faulting pattern, which is correlated with our reported low Q0 values to the south of the VT in the MCT zone. The reason for observing such structural or crustal scale heterogeneities can be ascribed to the presence of fluid-rich zone as well as strain localization and large stress build-up due to the locking in the ramp structure on the MHT in the Dharchula or the Chipalkot crystalline belt. We believe that the ramp structure extends further south into the region between BT and SAT in the Inner lesser Himalayas and leads to lowering of the Q0 values in this part of the lesser Himalayas.

Another significant study39 utilizing the P-wave receiver functions reported the presence of a ramp in the crust, which dips at about ∼20°. This ramp or duplex structure is responsible for introducing crustal heterogeneities within the crust. The study also reports the presence of a low velocity layer in the lesser Himalaya with aqueous fluids that can result in high attenuation of the Lg spectra below the lesser Himalayan crust. The presence of aqueous fluid is well correlated with the mechanism of decompression melting55, which can be associated with the melting of the lower crust in the region.

High Qo were reported for the southern part of the Lesser Himalaya zone between MBT and MCT in the Garhwal section, to the northwest of the present profile33.

For our profile, we observed a characteristically low Qo values and high attenuation in the range from 63 to 200 for the Lesser Himalayan tectonic unit, which lies between the two crustal scale faults MBT and the MCT. Our observations in the Higher Himalaya tectonic unit are similar to those from the previous study33, which has also reported low Lg Q0 values between 30 and 60 in the actively deforming Higher Himalaya. To the south of the MCT zone in the Lesser Himalaya they have reported high Lg Q0 values of 742 ± 235, which also shows a large error associated with the value and this is not an acceptable value for the actively deforming Himalayas. This large error reported for the Lesser Himalaya section in Garhwal Himalaya33 does not support the underthrusting mechanism of the Indian plate beneath Himalayas, which makes the region seismically very active and contradicts the presence of a stable lithosphere like the Indian Shield beneath the Himalayas.

Our findings show low Qo values and high attenuation for the Lesser Himalayan tectonic unit in accordance with LVL and low resistivity in between MBT and MCT29. A previous study of this region also reported high Poisson’s ratio due to the presence of fluids/partial melts at mid-crustal depths, which in turn controls the rheological strength of the crust leading to high crustal attenuation in the region32. The presence of fluids beneath the Higher Himalaya and the Tethys Himalaya tectonic unit has led to the long-term structural and compositional evolution of the active fault zones through decrease in fault friction28. This decrease in frictional strength of the fault causes a more heterogeneous fault behaviour and leads to high seismic attenuation of the Lg waves56,57,58. The Tethys Himalaya north of the STD is reported to be actively deformed and consists of abundant normal faults with east–west extension59, which can lead to the presence of strong small-scale structural heterogeneities within the crust that might have also contributed observed low Lg Qo values in the region.

There have been other studies available from the investigation of seismic attenuation utilizing different types of waves for the Himalaya-Tibet region (Fig. 8). Similar results of lower Qp (≈ 44) and Qs (≈ 87) at 1 Hz utilizing body waves were reported for the northeast Himalaya60. The results from most of the seismic attenuation studies reports low Qo values for the Himalaya-Tibet orogenic belt and the possible reasons include presence of crustal heterogeneities and trapped partial melts in the mid-crust61. Another significant reason controlling the seismic Q factor beneath Himalaya can be due to the crustal thickening and presence of thick sedimentary layers of 2–5 km47 below the Himalaya and the adjoining Indo-Gangetic Plain. The results obtained here for the Himalaya are similar to those found for other active seismic zones of the world.

Figure 8
figure 8

Map showing the comparison plot for the attenuation studies carried out in different part of the Himalaya-Tibet (References there-in). Figure generated using the GMT, version 6.0.

Full size image

Conclusions

Lg spectra recorded along the Tanakpur-Dharchula-Dhrama transect is utilized to study the seismic attenuation or the Q structure at 1 Hz beneath the western Kumaon sector of the NW Himalaya. A two-station method and a back projection algorithm are used to obtain models of laterally varying Lg Q0 (Lg Q at 1 Hz). This resulted in Q0 values to be low (∼ 63 -203) for the entire profile. We interpret these extremely low Lg Q0 values in the Kumaon Himalaya region being caused by the presence of crustal heterogeneities due to ramp structures and/or trapped fluids/partial melts caused due to high temperature in the crust.

Our results are consistent with the previous findings of strong attenuation beneath Higher Himalaya and Tethys Himalaya in Tibet as well as other sections of the northwest and central Himalayas but somehow differs for the Lesser Himalaya, which earlier reported low attenuation. Though this study has been conducted with a good quality of seismic datasets but there is a scope of obtaining Lg Q0 tomographic images along the profile, which would be more useful in having better constraints on the crustal velocity structure variations in the actively deforming Himalaya collision zone.