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Strong seismic coupling underneath Garhwal–Kumaun region, NW Himalaya, India
Earth and Planetary Science Letters 506 (2019) 8–14
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Earth and Planetary Science Letters www.elsevier.com/locate/epsl
Strong seismic coupling underneath Garhwal–Kumaun region, NW Himalaya, India Rajeev Kumar Yadav a , Vineet K. Gahalaut b,∗ , Amit Kumar Bansal c , S.P. Sati d , Joshi Catherine c , Param Gautam e , Kireet Kumar f , Naresh Rana b a
Institute of Seismological Research, Raisan, Gandhinagar 382009, India National Centre for Seismology, New Delhi, India c CSIR-National Geophysical Research Institute, Hyderabad, India d Department of Basic and Social Science, V.C.S.G. Uttarakhand University of Horticulture and Forestry, Bharsar, Pauri Garhwal, India e Wadia Institute of Himalayan Geology, Dehradun, India f G.B. Pant National Institute of Himalayan Environment & Sustainable Development, Almora, India b
a r t i c l e
i n f o
Article history: Received 28 March 2018 Received in revised form 14 October 2018 Accepted 15 October 2018 Available online xxxx Editor: J.P. Avouac Keywords: Himalaya GPS measurements crustal deformation strain accumulation locking
a b s t r a c t No great earthquake has occurred in the Garhwal–Kumaun region of NW Himalaya in the past 500 years or more. We report results of continuous GPS measurements from 28 sites from the region to suggest that the convergence rate in this part of the Himalaya is about 18 mm/yr which is leading to strain accumulation in the region. The Main Himalayan Thrust (MHT) in the frontal part of the Himalaya under the Outer and southern Lesser Himalaya is strongly coupled for a width of about 85 km. The midcrustal ramp where earthquakes of Himalayan seismic belt occur, exhibits low coupling. Strong coupling on the MHT beneath the Outer and Lesser Himalaya is homogeneous except in the very shallow updip part of the MHT. Subduction of sediments of the Indo-Gangetic plains or the Delhi Hardwar ridge does not seem to influence coupling. A high rate of strain accumulation, which has continued for more than 500 years on a strongly coupled MHT makes this one of the most earthquake-vulnerable segments of the Himalayan arc. © 2018 Elsevier B.V. All rights reserved.
1. Introduction More than two thirds of the Himalayan arc has not experienced a great earthquake in the past 200 years (Bilham and Wallace, 2005) and most of the unruptured Himalayan segments may have the potential to generate great earthquakes (Bilham et al., 2001). The Garhwal–Kumaun segment of the NW Himalaya in India and part of western Nepal has not experienced a great earthquake since at least 1505. In fact, it has been debated whether the rupture of the 1505 earthquake in western Nepal actually extended westward into the Garhwal–Kumaun Himalaya (Bilham and Ambraseys, 2005; Kumar et al., 2006; Rajendran et al., 2015; Jayangondaperumal et al., 2017) and if it didn’t, then the Garhwal– Kumaun segment of the Himalayan arc has not ruptured since at least ∼1344 (Rajendran et al., 2015) and could be the most vulnerable Himalayan segment along its 2400-km long length. Geodetic measurements in this area and elsewhere in the Himalayan region have provided constraints on the rate of convergence/strain accu-
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Corresponding author. E-mail address: [email protected] (V.K. Gahalaut).
https://doi.org/10.1016/j.epsl.2018.10.023 0012-821X/© 2018 Elsevier B.V. All rights reserved.
mulation and the width of locked zone (Jackson and Bilham, 1994; Bilham et al., 1997; Gahalaut and Chander, 1997, 1999; Jouanne et al., 1999; Banerjee et al., 2008; Gautam et al., 2017). With increasing precision and resolution, and establishment of a large number of observation sites, GPS measurements have now been used to map the spatial variation of locking on the Main Himalayan Thrust, MHT (Stevens and Avouac, 2015; Jouanne et al., 2017). However previous estimates of coupling model for the Garhwal–Kumaun Himalaya were based on sparse and survey mode GPS measurements and it is possible that the coupling might be more heterogeneous than that derived from these observations. A coupling model derived from a dense network of continuous measurements may reveal heterogeneity of coupling arising due to the influence of basement features (Gahalaut and Kundu, 2012), sediments and may also constrain the ramp-and-flat structure (Srivastava and Mitra, 1994; Pandey et al., 1995; Célérier et al., 2009) of the Himalayan thrust faults system. A robust estimate of convergence and slip deficit rate from these measurements may confirm whether the seismic moment release during large Himalayan earthquakes balances the slip deficit rate (Stevens and Avouac, 2016). Here, we report continuous GPS measurements from 30 sites in the Garhwal– Kumaun Himalaya and the adjoining Indo-Gangetic plains spanning
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Fig. 1. Horizontal displacement rates in the Indian reference frame at continuous GPS sites are shown with black arrows. Survey mode GPS sites (Banerjee et al., 2008) are shown in grey. Open circles are the earthquakes from ISC catalogue during period 1964–2015. Black dots are the micro-earthquakes during 2005–2008, reported by Mahesh et al. (2013) from a dense seismological network in the region. Contour of 3.5 km topography is shown by a light blue line. PT2 is also shown with brick red colour. MFT – Main Frontal Thrust, MBT – Main Boundary Thrust, MCT – Main Central Thrust. (For interpretation of the colours in the figure(s), the reader is referred to the web version of this article.)
an area of ∼300 × 200 km2 (Fig. 1). We provide the most robust estimates of the convergence rate, locking width and spatial variation in coupling on the MHT under the Garhwal–Kumaun Himalaya. 2. GPS measurements in Garhwal–Kumaun Himalaya In 2013 CSIR-NGRI Hyderabad installed 20 continuous GPS sites in the Garhwal–Kumaun Himalaya. These sites were in addition to five already existing sites operated by WIHG, Dehradun and 3 sites operated by GBPIHED, Almora. We processed daily observation files from these sites using GAMIT/GLOBK GLORG software (King and Bock, 2006; Herring et al., 2010a, 2010b) in ITRF2008 (Altamami et al., 2011) and estimated the site velocity by stabilizing core IGS reference sites (Mahesh et al., 2012). The ocean-loading and poletide correction model FES2004 (Lyard et al., 2006) is applied and Global Mapping Function (GMF, Boehm et al., 2006) is used for the hydrostatic and nonhydrostatic components of the tropospheric delay model. The ITRF2008 velocity estimates at these sites vary from ∼50 mm/yr towards the NE at sites located in the Indo-Gangetic plains and Outer Himalaya to ∼36 mm/yr towards the NE at sites located in the Higher Himalaya (Table S1). All the sites exhibit seasonal variations which were removed by fitting an analytical function consisting of sine and cosine functions with annual and biannual periodicities (Herring, 2003; Yoshioka et al., 2004). Similar to the observation in Nepal Himalaya (Ader et al., 2012), we too did not find any evidence of slow slip events (SSE) or episodic tremor and slip (ETS) from Garhwal–Kumaun Himalaya. However, as our observations are of relatively short period (≤5 yr), it may not imply that SSE or ETS do not ever occur in the Himalayan region. Panda et al. (2018) have proposed that seasonal hydrologi-
cal load modulates slip on the mid-crustal ramp which may cause higher transients at some GPS sites in the ramp region. In the Indian plate reference frame (Ader et al., 2012), estimates of horizontal velocity vary from 0.5 ± 0.4 in the Indo-Gangetic plains and Outer Himalaya to 14 ± 0.3 mm/yr in Higher and Tethys Himalaya, predominantly towards south in the arc normal direction (Fig. 1, Table S1). Sites located in the Indo-Gangetic plains and Outer Himalaya do not exhibit any significant vertical motion, however those in the Lesser and Higher Himalaya exhibit uplift with rate of up to 5–7 ± 1 mm/yr. Subsidence at a few sites in the Indo-Gangetic plains is mainly driven by the irrigation activities (e.g., Fu et al., 2013). 3. Strain accumulation in the Garhwal–Kumaun Himalaya The dense network of continuous GPS sites in the Garhwal Himalaya, the densest anywhere along the Himalayan arc, provides the most reliable estimate of spatial variations of site velocities in the Himalayan arc. Gradual increase in the southward site velocity implies strain accumulation in the region. To further densify the spatial coverage, we included some survey-mode estimates of site velocity which are reliable, consistent and have low uncertainties (Banerjee et al., 2008) after converting them into ITRF2008 and then the Indian reference frame (Ader et al., 2012). We used these observations of horizontal velocity to estimate the spatial variation in the locking on the Main Himalayan Thrust (MHT). As it involved estimation of several parameters, we first estimate the slip deficit rate and the width of the locked zone along an arc normal profile in the Garhwal–Kumaun region and later use these values to estimate variation of locking on the MHT. We use Okada’s formulation (Okada, 1992) of elastic dislocation, which
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Fig. 2. Result of grid search method. Arc normal site velocity along a composite profile in the arc normal direction (Fig. 1). The continuous curve simulates the effect of full locking of a 100 km wide Main Himalayan Thrust (MHT) under the Outer and Lesser Himalaya with a slip deficit rate of 18 mm/yr. Grey diamonds denote the survey mode GPS sites of Banerjee et al. (2008) while black diamonds are from this study. The inset shows result of grid search exercise in terms of reduced χ 2 value.
provides surface displacements due to slip across a planar rectangular fault in an elastic half space, to estimate the total convergence rate across the Garhwal–Kumaun Himalaya along an arc normal profile. We use the deep slip model (Savage, 1983) in which it is assumed that while the shallow MHT is perfectly locked, the deeper part of the MHT slips aseismically and its response can be simulated by assuming a deep dislocation. We perform a grid search to estimate the dip, the location of the downdip edge of the locked shallow MHT or the updip edge of the aseismically slipping deeper MHT and the slip rate on it (Fig. 2). By assuming that the MHT extends right up to the Main Frontal Thrust (MFT) and that the strain accumulation rate, or locking, is uniform on the shallow part of the MHT, we estimate the width of the locked zone as 100 ± 15 km and the slip deficit rate as 18 ± 1 mm/yr. These estimates are consistent with that derived from the survey-mode GPS observations earlier (Banerjee et al., 2008) and also with that estimated by Lindsey et al. (2018) from neighbouring western Nepal. However, the estimate of slip deficit rate is more than the estimated lower bound quaternary slip rate of ∼14 mm/yr along the MFT in the region (Wesnousky et al., 1999). Next, we follow Ader et al. (2012) and Stevens and Avouac (2015) and use weighted least square inversion scheme (Kositsky and Avouac, 2010) with Laplacian regularization which inverts estimates of site velocity for slip distribution on a given geometry of the subsurface fault. We allow for a strike-slip component with rake varying from 110◦ to 70◦ . We estimated slip distribution on the MHT by dividing it into rectangular subfaults of size 20 × 10 km2 and used the above derived estimate of 18 mm/yr for the total convergence rate across the Garhwal–Kumaun. 3.1. Geometry of MHT We considered several models of the MHT, with and without a mid-crustal ramp (Srivastava and Mitra, 1994; Pandey et al., 1995; Célérier et al., 2009). Overall there is insignificant reduction (in comparison to the error of observations) in the misfit if we assume a planar MHT. However, if we consider misfit only at sites which are located in the possible ramp region of northern Lesser Himalaya, there is a slight decrease in the misfit when the ramp is considered, however, even this improvement is insignificant compared to the error in GPS measurements. In our subse-
quent models we considered the mid-crustal ramp in our model and instead of constraining its location, we assumed that it follows the front of the Higher Himalaya and use the 3.5 km elevation contour to define its extent as it also marks the northward extent of belt of seismicity (e.g., Cattin and Avouac, 2000; Morrell et al., 2017). The PT2 (physiographic transition, Hodges et al., 2001; Morell et al., 2017) approximately coincides with the southern extent of the seismicity belt and with the 2.5 km topography contour (Fig. 1). Thus, all the small and moderate magnitude earthquakes in this part of the Himalaya appear to occur on or near the ramp, similar to that in the Nepal Himalaya (Pandey et al., 1995). We also include a frontal ramp simulating the Main Frontal Thrust (MFT). 3.2. Coupling ratio map For the inversion of the GPS site velocity for slip distribution on the MHT, we use a relatively weak smoothing constraint, 1/β , (Fig. S1) in order to detect small scale variations in slip that may reflect the existence of asperities/barriers on the MHT and estimated slip distribution in the arc-normal (thrust slip) and arcparallel (strike slip) direction (Fig. S2). However, the arc parallel slip in the Garhwal–Kumaun Himalaya is insignificant (varying within ±2 mm/yr) and henceforth we discuss only the arc normal component. We converted the arc normal slip distribution into a locking ratio by dividing the slip deficit rate on each of the subfault with the long term slip rate of 18 mm/yr. The locking ratio varies between 0 to 1, 0 being the uncoupled and 1 being the fully coupled state. Our inverted map of variable locking on the MHT brings out several features (Fig. 3). The MHT under the Outer and Lesser Himalaya is strongly coupled (ratio >0.6), i.e., the region between MFT and the earthquake belt, is strongly coupled. The choice of high coupling threshold of 0.6 was based upon the observation that 2015 Gorkha earthquake rupture extended in the region of coupling of up to ∼0.6 (Elliot et al., 2016). The ramp appears to be weakly coupled, suggestive of it being the transition between the strongly coupled MHT to the south and uncoupled MHT to the north. Coupling appears to be rather uniform and strong (more than 0.8) in the frontal part of the MHT under the Outer and southern Lesser Himalaya. Although, we did not use the vertical velocity from GPS measurements in the inversion as they have rel-
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Fig. 3. The superimposed colour image and contours over Fig. 1 represent the seismic coupling derived from the slip deficit rate (Fig. S2) obtained after inverting the GPS displacement rates. The convergence of 18 mm/yr across the Garhwal–Kumaun Himalaya is estimated using grid search method (Fig. 2). Black and green vectors are the observed and simulated horizontal velocity estimates. Top right inset shows the configuration of Main Himalayan Thrust (MHT) used in the analysis with the dip of each segment indicated.
atively larger uncertainty in comparison with that in the horizontal estimates, the model derived from the inversion of horizontal velocity using the same geometry of MHT fits the vertical rates reasonably well (Fig. 4). 4. Discussion 4.1. Seismic hazard The derived coupling map from GPS measurements has implications for seismic hazard in the Garhwal–Kumaun region. The width of the coupled (ratio >0.5) region is 90–100 km with very strongly coupled region lying under the Outer and Lesser Himalaya. Some shallow parts of MHT under the Outer Himalaya exhibit slightly lower coupling. Further north of the strongly coupled MHT, the mid-crustal ramp is partially coupled (ratio <0.4). The regions of high coupling would be the regions of high seismic energy release during future thrust earthquakes on the MHT. Depending upon the local site conditions, such regions may be the locales of strong ground motions and extensive damage. Taking the analogy from the 2015 Gorkha earthquake and 1985 Mexico earthquake, the sediment filled valleys, e.g., the Dehradun valley in Garhwal region, are expected to be the regions of potentially more damage in future events. Indeed ambient noise analysis from Dehradun valley shows high amplification in the region at resonant frequency of 1.1 Hz (Sathyaseelan et al., 2017). The strongly coupled region (coupling of ∼0.8) must have accumulated slip of more than 7 m since 1505 and more than 9 m since 1344, which is enough to produce a great earthquake in the region. Ruptures of such earthquakes generally nucleate close to the mid-crustal ramp and extend up to the MFT. However, in case of major earthquake, rupture may not extend right up to the MFT, e.g., the case of 2015 Gorkha earthquake (Avouac et al.,
2015). A shallow ramp, without any surface expression (Michel et al., 2017) below the Lesser Himalaya and just north of the Main Boundary Thrust (MBT) may have arrested the rupture (Hubbard et al., 2016). However, it may not be necessary that such a structural control would explain similar partial ruptures in the context of subduction megathrusts (Michel et al., 2017). We are not aware whether the shallow and small ramp, inferred in the Nepal region, extends below the Garhwal–Kumaun region, as it does not have any surface expression. Moreover, even if it exists, it is unlikely that its presence can actually be resolved by the GPS measurements of interseismic deformation. Thus we did not incorporate it in the MHT geometry. 4.2. Influence on coupling due to subduction of Indo-Gangetic plains sediments The Himalayan arc abuts the Indo-Gangetic plains with sediment thickness of up to 5 km (Srinivas et al., 2013). Part of these sediments on the Indian plate could be subducting beneath the Himalaya making the shallower part largely aseismic, causing low coupling, a typical feature of subduction zones (e.g., Byrne et al., 1988; Oleskevich et al., 1999; Polet and Kanamori, 2000). However, in the case of the Garhwal–Kumaun Himalaya, we do not see any evidence of low coupling at shallow depth, probably implying that subduction of Indo-Gangetic sediments beneath the Garhwal–Kumaun Himalaya region does not seem to decrease the shallow coupling on the MHT. A contrasting view is that their subduction could in fact be leading to high coupling (Ruff, 1989; Wang et al., 2012; Wang and Bilek, 2014). In recent years an alternate view that has emerged is that the zone of apparent high coupling at shallow updip part of the MHT doesn’t necessarily mean that it is accumulating strain and it will participate in future earthquakes. It could be a zone which appears
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Fig. 4. Fitting of vertical rates (black arrows – observed, green arrows – simulated) due to the derived slip distribution with the same fault geometry as used for inverting horizontal rates. Fitting along an arc normal profile is also shown in the inset. The black and green curves represent the along-strike average in the observed (black diamonds) and simulated (green diamonds) uplift rate, respectively.
Fig. 5. Along strike coupling variation in the rows of first five shallow subfaults on the updip part of the MHT (corresponding to the centroid depth of subfaults in each row as 2.5, 5.6, 6.8, 8.0, 9.3 km, respectively) to analyse the influence of Delhi Hardwar ridge (DHR) and Moradabad fault (MF) on the coupling.
to be locked because of stress shadow effect of the downdip locked portion of the MHT and it could be only partially locked and may slip aseismically after an earthquake on the deeper locked portion of the MHT (Gualandi et al., 2017; Almeida et al., 2018). Nevertheless, the inference of apparent high coupling is consistent with the paleoseismological observations which suggest that the ruptures of a number of great Himalayan earthquakes have extended right up to the MFT (Rajendran, 2016). This has serious implications in terms of seismic hazard as the population density is higher in the frontal Himalaya and its contiguous Indo-Gangetic plains, thus exposing more people to high, nearfield ground shaking. Besides this, the unconsolidated sediments in the Indo-Gangetic plains can cause more structural damage during earthquakes. The presence of these unconsolidated sediments could also cause liquefaction during the next great earthquake, as happened in the 1934 Nepal Bihar earthquake (Rana, 1935; Pandey and Molnar, 1988).
4.3. Subduction of Delhi Hardwar ridge The influence of the subduction of paleo-topographic aseismic ridges and faults of the Indian basement beneath the Himalayan arc has been discussed extensively (Gahalaut and Kundu, 2012; Godin and Harris, 2014). Such features are proposed to potentially limit the extent of Himalayan earthquake ruptures and act as barriers that may be characterised by low friction or coupling (Bilek, 2007; Das and Watts, 2009; Wang and Bilek, 2014). Godin and Harris (2014) were able to track some of these ridges and faults beneath the Himalayan arc using gravity measurements. In the Garhwal–Kumaun Himalaya region two major features are suggested to be subducting beneath the Himalayan arc, the Delhi– Hardwar ridge and the Moradabad fault (Fig. 1). Unfortunately, our GPS measurements do not span the Delhi Hardwar ridge under the Himalayan arc quite adequately. But in both the regions, coupling on the MHT appears to be unaffected by these features (Fig. 5), though the strike slip component of slip distribution (Fig. S2b)
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shows some variations corresponding to the Delhi Hardwar ridge. But even this is not apparent at deeper depths, i.e., further north. So either, these features are not that prominent at deeper depth or they are not sufficiently illuminated by our GPS measurements. In order to verify this further we even varied the smoothing constraint, 1/β (Fig. S3), while inverting for slip distribution on the MHT. But the high and almost homogeneous coupling on the MHT under the Outer and Lesser Himalaya persisted, confirming that these features do not influence the coupling on the MHT. 4.4. Interseismic uplift rate The interseismic uplift rate in the Garhwal–Kumaun Himalaya increases northward from the MFT and reach a maximum value of ∼5–7 ± 1 mm/yr in the mid-crustal ramp region (Fig. 4). These rates and their spatial variations are consistent with that derived from levelling (Jackson and Bilham, 1994), GPS measurements (Ader et al., 2012), InSAR analysis (Grandin et al., 2012), mechanical modelling (Cattin and Avouac, 2000) from Nepal Himalaya and slip/coupling model derived from the GPS measurements of horizontal deformation rates. Because of point positioning measurements, although the spatial resolution in GPS measurements cannot be as good as in InSAR or levelling measurements, so as to precisely infer the location of maximum uplift rate and investigate the mode of deformation (i.e., out-of-sequence thrusting vs ramp migration, Grandin et al., 2012), the region of maximum uplift rate in our case approximately coincides with the earthquakes of Himalayan seismic belt and also with the PT2. The coincidence in the region of maximum interseismic uplift with the region of maximum uplift derived from long term deformation (i.e., PT2) and erosion, implies that some of the interseismic deformation contributes to the long term uplift. In other words, some of the interseismic uplift is inelastic, confirming an earlier proposal from the Nepal region (Grandin et al., 2012). 5. Conclusion Analysis of continuous GPS measurements of crustal deformation in the Garhwal–Kumaun Himalaya suggest strong and homogeneous coupling on the MHT beneath the Outer and Lesser Himalaya which does not seem to be affected by the subduction of sediments of the Indo-Gangetic plains or the ridges/faults on the Indian plate. The slip deficit rate of 18 mm/yr is consistent with that in the neighbouring western Nepal Himalaya. High rate of strain accumulation for the past 500 or more years with homogeneous and high coupling on this segment of the MHT may make this the seismically most vulnerable section of the Himalayan region. Acknowledgements This work was financially supported by the Ministry of Earth Sciences, Government of India through grant no. MoES/PO(Seismo)/ 1(116)/2010. Roland Bürgmann provided comments on an initial draft of the article. J.-P. Avouac provided the slip inversion code. Bhaskar Kundu, Deepak Singh Negi, several students and local residents helped in the GPS installation and data acquisition. We greatly appreciate constructive comments from two anonymous reviewers. Editor, Jean-Philippe Avouac’s suggestions and encouragement significantly raised the impact of the article. This work is a part of the Ph.D. thesis of RKY submitted at the Council of Scientific and Industrial Research-National Geophysical Research Institute (CSIR-NGRI), Hyderabad. All the GPS time series are available in this thesis which is available at http://www.ngri.org.in/cms/ phd-thesis-of-acsir-students.php. The CSIR-NGRI reference number of the article is NGRI/Lib/2018/Pub-105.
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Contents lists available at ScienceDirect
Earth and Planetary Science Letters www.elsevier.com/locate/epsl
Strong seismic coupling underneath Garhwal–Kumaun region, NW Himalaya, India Rajeev Kumar Yadav a , Vineet K. Gahalaut b,∗ , Amit Kumar Bansal c , S.P. Sati d , Joshi Catherine c , Param Gautam e , Kireet Kumar f , Naresh Rana b a
Institute of Seismological Research, Raisan, Gandhinagar 382009, India National Centre for Seismology, New Delhi, India c CSIR-National Geophysical Research Institute, Hyderabad, India d Department of Basic and Social Science, V.C.S.G. Uttarakhand University of Horticulture and Forestry, Bharsar, Pauri Garhwal, India e Wadia Institute of Himalayan Geology, Dehradun, India f G.B. Pant National Institute of Himalayan Environment & Sustainable Development, Almora, India b
a r t i c l e
i n f o
Article history: Received 28 March 2018 Received in revised form 14 October 2018 Accepted 15 October 2018 Available online xxxx Editor: J.P. Avouac Keywords: Himalaya GPS measurements crustal deformation strain accumulation locking
a b s t r a c t No great earthquake has occurred in the Garhwal–Kumaun region of NW Himalaya in the past 500 years or more. We report results of continuous GPS measurements from 28 sites from the region to suggest that the convergence rate in this part of the Himalaya is about 18 mm/yr which is leading to strain accumulation in the region. The Main Himalayan Thrust (MHT) in the frontal part of the Himalaya under the Outer and southern Lesser Himalaya is strongly coupled for a width of about 85 km. The midcrustal ramp where earthquakes of Himalayan seismic belt occur, exhibits low coupling. Strong coupling on the MHT beneath the Outer and Lesser Himalaya is homogeneous except in the very shallow updip part of the MHT. Subduction of sediments of the Indo-Gangetic plains or the Delhi Hardwar ridge does not seem to influence coupling. A high rate of strain accumulation, which has continued for more than 500 years on a strongly coupled MHT makes this one of the most earthquake-vulnerable segments of the Himalayan arc. © 2018 Elsevier B.V. All rights reserved.
1. Introduction More than two thirds of the Himalayan arc has not experienced a great earthquake in the past 200 years (Bilham and Wallace, 2005) and most of the unruptured Himalayan segments may have the potential to generate great earthquakes (Bilham et al., 2001). The Garhwal–Kumaun segment of the NW Himalaya in India and part of western Nepal has not experienced a great earthquake since at least 1505. In fact, it has been debated whether the rupture of the 1505 earthquake in western Nepal actually extended westward into the Garhwal–Kumaun Himalaya (Bilham and Ambraseys, 2005; Kumar et al., 2006; Rajendran et al., 2015; Jayangondaperumal et al., 2017) and if it didn’t, then the Garhwal– Kumaun segment of the Himalayan arc has not ruptured since at least ∼1344 (Rajendran et al., 2015) and could be the most vulnerable Himalayan segment along its 2400-km long length. Geodetic measurements in this area and elsewhere in the Himalayan region have provided constraints on the rate of convergence/strain accu-
*
Corresponding author. E-mail address: [email protected] (V.K. Gahalaut).
https://doi.org/10.1016/j.epsl.2018.10.023 0012-821X/© 2018 Elsevier B.V. All rights reserved.
mulation and the width of locked zone (Jackson and Bilham, 1994; Bilham et al., 1997; Gahalaut and Chander, 1997, 1999; Jouanne et al., 1999; Banerjee et al., 2008; Gautam et al., 2017). With increasing precision and resolution, and establishment of a large number of observation sites, GPS measurements have now been used to map the spatial variation of locking on the Main Himalayan Thrust, MHT (Stevens and Avouac, 2015; Jouanne et al., 2017). However previous estimates of coupling model for the Garhwal–Kumaun Himalaya were based on sparse and survey mode GPS measurements and it is possible that the coupling might be more heterogeneous than that derived from these observations. A coupling model derived from a dense network of continuous measurements may reveal heterogeneity of coupling arising due to the influence of basement features (Gahalaut and Kundu, 2012), sediments and may also constrain the ramp-and-flat structure (Srivastava and Mitra, 1994; Pandey et al., 1995; Célérier et al., 2009) of the Himalayan thrust faults system. A robust estimate of convergence and slip deficit rate from these measurements may confirm whether the seismic moment release during large Himalayan earthquakes balances the slip deficit rate (Stevens and Avouac, 2016). Here, we report continuous GPS measurements from 30 sites in the Garhwal– Kumaun Himalaya and the adjoining Indo-Gangetic plains spanning
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Fig. 1. Horizontal displacement rates in the Indian reference frame at continuous GPS sites are shown with black arrows. Survey mode GPS sites (Banerjee et al., 2008) are shown in grey. Open circles are the earthquakes from ISC catalogue during period 1964–2015. Black dots are the micro-earthquakes during 2005–2008, reported by Mahesh et al. (2013) from a dense seismological network in the region. Contour of 3.5 km topography is shown by a light blue line. PT2 is also shown with brick red colour. MFT – Main Frontal Thrust, MBT – Main Boundary Thrust, MCT – Main Central Thrust. (For interpretation of the colours in the figure(s), the reader is referred to the web version of this article.)
an area of ∼300 × 200 km2 (Fig. 1). We provide the most robust estimates of the convergence rate, locking width and spatial variation in coupling on the MHT under the Garhwal–Kumaun Himalaya. 2. GPS measurements in Garhwal–Kumaun Himalaya In 2013 CSIR-NGRI Hyderabad installed 20 continuous GPS sites in the Garhwal–Kumaun Himalaya. These sites were in addition to five already existing sites operated by WIHG, Dehradun and 3 sites operated by GBPIHED, Almora. We processed daily observation files from these sites using GAMIT/GLOBK GLORG software (King and Bock, 2006; Herring et al., 2010a, 2010b) in ITRF2008 (Altamami et al., 2011) and estimated the site velocity by stabilizing core IGS reference sites (Mahesh et al., 2012). The ocean-loading and poletide correction model FES2004 (Lyard et al., 2006) is applied and Global Mapping Function (GMF, Boehm et al., 2006) is used for the hydrostatic and nonhydrostatic components of the tropospheric delay model. The ITRF2008 velocity estimates at these sites vary from ∼50 mm/yr towards the NE at sites located in the Indo-Gangetic plains and Outer Himalaya to ∼36 mm/yr towards the NE at sites located in the Higher Himalaya (Table S1). All the sites exhibit seasonal variations which were removed by fitting an analytical function consisting of sine and cosine functions with annual and biannual periodicities (Herring, 2003; Yoshioka et al., 2004). Similar to the observation in Nepal Himalaya (Ader et al., 2012), we too did not find any evidence of slow slip events (SSE) or episodic tremor and slip (ETS) from Garhwal–Kumaun Himalaya. However, as our observations are of relatively short period (≤5 yr), it may not imply that SSE or ETS do not ever occur in the Himalayan region. Panda et al. (2018) have proposed that seasonal hydrologi-
cal load modulates slip on the mid-crustal ramp which may cause higher transients at some GPS sites in the ramp region. In the Indian plate reference frame (Ader et al., 2012), estimates of horizontal velocity vary from 0.5 ± 0.4 in the Indo-Gangetic plains and Outer Himalaya to 14 ± 0.3 mm/yr in Higher and Tethys Himalaya, predominantly towards south in the arc normal direction (Fig. 1, Table S1). Sites located in the Indo-Gangetic plains and Outer Himalaya do not exhibit any significant vertical motion, however those in the Lesser and Higher Himalaya exhibit uplift with rate of up to 5–7 ± 1 mm/yr. Subsidence at a few sites in the Indo-Gangetic plains is mainly driven by the irrigation activities (e.g., Fu et al., 2013). 3. Strain accumulation in the Garhwal–Kumaun Himalaya The dense network of continuous GPS sites in the Garhwal Himalaya, the densest anywhere along the Himalayan arc, provides the most reliable estimate of spatial variations of site velocities in the Himalayan arc. Gradual increase in the southward site velocity implies strain accumulation in the region. To further densify the spatial coverage, we included some survey-mode estimates of site velocity which are reliable, consistent and have low uncertainties (Banerjee et al., 2008) after converting them into ITRF2008 and then the Indian reference frame (Ader et al., 2012). We used these observations of horizontal velocity to estimate the spatial variation in the locking on the Main Himalayan Thrust (MHT). As it involved estimation of several parameters, we first estimate the slip deficit rate and the width of the locked zone along an arc normal profile in the Garhwal–Kumaun region and later use these values to estimate variation of locking on the MHT. We use Okada’s formulation (Okada, 1992) of elastic dislocation, which
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Fig. 2. Result of grid search method. Arc normal site velocity along a composite profile in the arc normal direction (Fig. 1). The continuous curve simulates the effect of full locking of a 100 km wide Main Himalayan Thrust (MHT) under the Outer and Lesser Himalaya with a slip deficit rate of 18 mm/yr. Grey diamonds denote the survey mode GPS sites of Banerjee et al. (2008) while black diamonds are from this study. The inset shows result of grid search exercise in terms of reduced χ 2 value.
provides surface displacements due to slip across a planar rectangular fault in an elastic half space, to estimate the total convergence rate across the Garhwal–Kumaun Himalaya along an arc normal profile. We use the deep slip model (Savage, 1983) in which it is assumed that while the shallow MHT is perfectly locked, the deeper part of the MHT slips aseismically and its response can be simulated by assuming a deep dislocation. We perform a grid search to estimate the dip, the location of the downdip edge of the locked shallow MHT or the updip edge of the aseismically slipping deeper MHT and the slip rate on it (Fig. 2). By assuming that the MHT extends right up to the Main Frontal Thrust (MFT) and that the strain accumulation rate, or locking, is uniform on the shallow part of the MHT, we estimate the width of the locked zone as 100 ± 15 km and the slip deficit rate as 18 ± 1 mm/yr. These estimates are consistent with that derived from the survey-mode GPS observations earlier (Banerjee et al., 2008) and also with that estimated by Lindsey et al. (2018) from neighbouring western Nepal. However, the estimate of slip deficit rate is more than the estimated lower bound quaternary slip rate of ∼14 mm/yr along the MFT in the region (Wesnousky et al., 1999). Next, we follow Ader et al. (2012) and Stevens and Avouac (2015) and use weighted least square inversion scheme (Kositsky and Avouac, 2010) with Laplacian regularization which inverts estimates of site velocity for slip distribution on a given geometry of the subsurface fault. We allow for a strike-slip component with rake varying from 110◦ to 70◦ . We estimated slip distribution on the MHT by dividing it into rectangular subfaults of size 20 × 10 km2 and used the above derived estimate of 18 mm/yr for the total convergence rate across the Garhwal–Kumaun. 3.1. Geometry of MHT We considered several models of the MHT, with and without a mid-crustal ramp (Srivastava and Mitra, 1994; Pandey et al., 1995; Célérier et al., 2009). Overall there is insignificant reduction (in comparison to the error of observations) in the misfit if we assume a planar MHT. However, if we consider misfit only at sites which are located in the possible ramp region of northern Lesser Himalaya, there is a slight decrease in the misfit when the ramp is considered, however, even this improvement is insignificant compared to the error in GPS measurements. In our subse-
quent models we considered the mid-crustal ramp in our model and instead of constraining its location, we assumed that it follows the front of the Higher Himalaya and use the 3.5 km elevation contour to define its extent as it also marks the northward extent of belt of seismicity (e.g., Cattin and Avouac, 2000; Morrell et al., 2017). The PT2 (physiographic transition, Hodges et al., 2001; Morell et al., 2017) approximately coincides with the southern extent of the seismicity belt and with the 2.5 km topography contour (Fig. 1). Thus, all the small and moderate magnitude earthquakes in this part of the Himalaya appear to occur on or near the ramp, similar to that in the Nepal Himalaya (Pandey et al., 1995). We also include a frontal ramp simulating the Main Frontal Thrust (MFT). 3.2. Coupling ratio map For the inversion of the GPS site velocity for slip distribution on the MHT, we use a relatively weak smoothing constraint, 1/β , (Fig. S1) in order to detect small scale variations in slip that may reflect the existence of asperities/barriers on the MHT and estimated slip distribution in the arc-normal (thrust slip) and arcparallel (strike slip) direction (Fig. S2). However, the arc parallel slip in the Garhwal–Kumaun Himalaya is insignificant (varying within ±2 mm/yr) and henceforth we discuss only the arc normal component. We converted the arc normal slip distribution into a locking ratio by dividing the slip deficit rate on each of the subfault with the long term slip rate of 18 mm/yr. The locking ratio varies between 0 to 1, 0 being the uncoupled and 1 being the fully coupled state. Our inverted map of variable locking on the MHT brings out several features (Fig. 3). The MHT under the Outer and Lesser Himalaya is strongly coupled (ratio >0.6), i.e., the region between MFT and the earthquake belt, is strongly coupled. The choice of high coupling threshold of 0.6 was based upon the observation that 2015 Gorkha earthquake rupture extended in the region of coupling of up to ∼0.6 (Elliot et al., 2016). The ramp appears to be weakly coupled, suggestive of it being the transition between the strongly coupled MHT to the south and uncoupled MHT to the north. Coupling appears to be rather uniform and strong (more than 0.8) in the frontal part of the MHT under the Outer and southern Lesser Himalaya. Although, we did not use the vertical velocity from GPS measurements in the inversion as they have rel-
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Fig. 3. The superimposed colour image and contours over Fig. 1 represent the seismic coupling derived from the slip deficit rate (Fig. S2) obtained after inverting the GPS displacement rates. The convergence of 18 mm/yr across the Garhwal–Kumaun Himalaya is estimated using grid search method (Fig. 2). Black and green vectors are the observed and simulated horizontal velocity estimates. Top right inset shows the configuration of Main Himalayan Thrust (MHT) used in the analysis with the dip of each segment indicated.
atively larger uncertainty in comparison with that in the horizontal estimates, the model derived from the inversion of horizontal velocity using the same geometry of MHT fits the vertical rates reasonably well (Fig. 4). 4. Discussion 4.1. Seismic hazard The derived coupling map from GPS measurements has implications for seismic hazard in the Garhwal–Kumaun region. The width of the coupled (ratio >0.5) region is 90–100 km with very strongly coupled region lying under the Outer and Lesser Himalaya. Some shallow parts of MHT under the Outer Himalaya exhibit slightly lower coupling. Further north of the strongly coupled MHT, the mid-crustal ramp is partially coupled (ratio <0.4). The regions of high coupling would be the regions of high seismic energy release during future thrust earthquakes on the MHT. Depending upon the local site conditions, such regions may be the locales of strong ground motions and extensive damage. Taking the analogy from the 2015 Gorkha earthquake and 1985 Mexico earthquake, the sediment filled valleys, e.g., the Dehradun valley in Garhwal region, are expected to be the regions of potentially more damage in future events. Indeed ambient noise analysis from Dehradun valley shows high amplification in the region at resonant frequency of 1.1 Hz (Sathyaseelan et al., 2017). The strongly coupled region (coupling of ∼0.8) must have accumulated slip of more than 7 m since 1505 and more than 9 m since 1344, which is enough to produce a great earthquake in the region. Ruptures of such earthquakes generally nucleate close to the mid-crustal ramp and extend up to the MFT. However, in case of major earthquake, rupture may not extend right up to the MFT, e.g., the case of 2015 Gorkha earthquake (Avouac et al.,
2015). A shallow ramp, without any surface expression (Michel et al., 2017) below the Lesser Himalaya and just north of the Main Boundary Thrust (MBT) may have arrested the rupture (Hubbard et al., 2016). However, it may not be necessary that such a structural control would explain similar partial ruptures in the context of subduction megathrusts (Michel et al., 2017). We are not aware whether the shallow and small ramp, inferred in the Nepal region, extends below the Garhwal–Kumaun region, as it does not have any surface expression. Moreover, even if it exists, it is unlikely that its presence can actually be resolved by the GPS measurements of interseismic deformation. Thus we did not incorporate it in the MHT geometry. 4.2. Influence on coupling due to subduction of Indo-Gangetic plains sediments The Himalayan arc abuts the Indo-Gangetic plains with sediment thickness of up to 5 km (Srinivas et al., 2013). Part of these sediments on the Indian plate could be subducting beneath the Himalaya making the shallower part largely aseismic, causing low coupling, a typical feature of subduction zones (e.g., Byrne et al., 1988; Oleskevich et al., 1999; Polet and Kanamori, 2000). However, in the case of the Garhwal–Kumaun Himalaya, we do not see any evidence of low coupling at shallow depth, probably implying that subduction of Indo-Gangetic sediments beneath the Garhwal–Kumaun Himalaya region does not seem to decrease the shallow coupling on the MHT. A contrasting view is that their subduction could in fact be leading to high coupling (Ruff, 1989; Wang et al., 2012; Wang and Bilek, 2014). In recent years an alternate view that has emerged is that the zone of apparent high coupling at shallow updip part of the MHT doesn’t necessarily mean that it is accumulating strain and it will participate in future earthquakes. It could be a zone which appears
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Fig. 4. Fitting of vertical rates (black arrows – observed, green arrows – simulated) due to the derived slip distribution with the same fault geometry as used for inverting horizontal rates. Fitting along an arc normal profile is also shown in the inset. The black and green curves represent the along-strike average in the observed (black diamonds) and simulated (green diamonds) uplift rate, respectively.
Fig. 5. Along strike coupling variation in the rows of first five shallow subfaults on the updip part of the MHT (corresponding to the centroid depth of subfaults in each row as 2.5, 5.6, 6.8, 8.0, 9.3 km, respectively) to analyse the influence of Delhi Hardwar ridge (DHR) and Moradabad fault (MF) on the coupling.
to be locked because of stress shadow effect of the downdip locked portion of the MHT and it could be only partially locked and may slip aseismically after an earthquake on the deeper locked portion of the MHT (Gualandi et al., 2017; Almeida et al., 2018). Nevertheless, the inference of apparent high coupling is consistent with the paleoseismological observations which suggest that the ruptures of a number of great Himalayan earthquakes have extended right up to the MFT (Rajendran, 2016). This has serious implications in terms of seismic hazard as the population density is higher in the frontal Himalaya and its contiguous Indo-Gangetic plains, thus exposing more people to high, nearfield ground shaking. Besides this, the unconsolidated sediments in the Indo-Gangetic plains can cause more structural damage during earthquakes. The presence of these unconsolidated sediments could also cause liquefaction during the next great earthquake, as happened in the 1934 Nepal Bihar earthquake (Rana, 1935; Pandey and Molnar, 1988).
4.3. Subduction of Delhi Hardwar ridge The influence of the subduction of paleo-topographic aseismic ridges and faults of the Indian basement beneath the Himalayan arc has been discussed extensively (Gahalaut and Kundu, 2012; Godin and Harris, 2014). Such features are proposed to potentially limit the extent of Himalayan earthquake ruptures and act as barriers that may be characterised by low friction or coupling (Bilek, 2007; Das and Watts, 2009; Wang and Bilek, 2014). Godin and Harris (2014) were able to track some of these ridges and faults beneath the Himalayan arc using gravity measurements. In the Garhwal–Kumaun Himalaya region two major features are suggested to be subducting beneath the Himalayan arc, the Delhi– Hardwar ridge and the Moradabad fault (Fig. 1). Unfortunately, our GPS measurements do not span the Delhi Hardwar ridge under the Himalayan arc quite adequately. But in both the regions, coupling on the MHT appears to be unaffected by these features (Fig. 5), though the strike slip component of slip distribution (Fig. S2b)
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shows some variations corresponding to the Delhi Hardwar ridge. But even this is not apparent at deeper depths, i.e., further north. So either, these features are not that prominent at deeper depth or they are not sufficiently illuminated by our GPS measurements. In order to verify this further we even varied the smoothing constraint, 1/β (Fig. S3), while inverting for slip distribution on the MHT. But the high and almost homogeneous coupling on the MHT under the Outer and Lesser Himalaya persisted, confirming that these features do not influence the coupling on the MHT. 4.4. Interseismic uplift rate The interseismic uplift rate in the Garhwal–Kumaun Himalaya increases northward from the MFT and reach a maximum value of ∼5–7 ± 1 mm/yr in the mid-crustal ramp region (Fig. 4). These rates and their spatial variations are consistent with that derived from levelling (Jackson and Bilham, 1994), GPS measurements (Ader et al., 2012), InSAR analysis (Grandin et al., 2012), mechanical modelling (Cattin and Avouac, 2000) from Nepal Himalaya and slip/coupling model derived from the GPS measurements of horizontal deformation rates. Because of point positioning measurements, although the spatial resolution in GPS measurements cannot be as good as in InSAR or levelling measurements, so as to precisely infer the location of maximum uplift rate and investigate the mode of deformation (i.e., out-of-sequence thrusting vs ramp migration, Grandin et al., 2012), the region of maximum uplift rate in our case approximately coincides with the earthquakes of Himalayan seismic belt and also with the PT2. The coincidence in the region of maximum interseismic uplift with the region of maximum uplift derived from long term deformation (i.e., PT2) and erosion, implies that some of the interseismic deformation contributes to the long term uplift. In other words, some of the interseismic uplift is inelastic, confirming an earlier proposal from the Nepal region (Grandin et al., 2012). 5. Conclusion Analysis of continuous GPS measurements of crustal deformation in the Garhwal–Kumaun Himalaya suggest strong and homogeneous coupling on the MHT beneath the Outer and Lesser Himalaya which does not seem to be affected by the subduction of sediments of the Indo-Gangetic plains or the ridges/faults on the Indian plate. The slip deficit rate of 18 mm/yr is consistent with that in the neighbouring western Nepal Himalaya. High rate of strain accumulation for the past 500 or more years with homogeneous and high coupling on this segment of the MHT may make this the seismically most vulnerable section of the Himalayan region. Acknowledgements This work was financially supported by the Ministry of Earth Sciences, Government of India through grant no. MoES/PO(Seismo)/ 1(116)/2010. Roland Bürgmann provided comments on an initial draft of the article. J.-P. Avouac provided the slip inversion code. Bhaskar Kundu, Deepak Singh Negi, several students and local residents helped in the GPS installation and data acquisition. We greatly appreciate constructive comments from two anonymous reviewers. Editor, Jean-Philippe Avouac’s suggestions and encouragement significantly raised the impact of the article. This work is a part of the Ph.D. thesis of RKY submitted at the Council of Scientific and Industrial Research-National Geophysical Research Institute (CSIR-NGRI), Hyderabad. All the GPS time series are available in this thesis which is available at http://www.ngri.org.in/cms/ phd-thesis-of-acsir-students.php. The CSIR-NGRI reference number of the article is NGRI/Lib/2018/Pub-105.
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