Earthquake genesis in Nepal Himalaya: A perspective from imaging of the 25th April 2015 Mw 7.8 earthquake source zone
Graphical abstract
Introduction
The 25 April 2015 earthquake of magnitude (Mw) 7.8 that occurred on a gently (7–10°) north dipping, near E–W trending thrust fault, at about 15 km depth (Fig. 1; USGS, 2015, Galetzka et al., 2015), is the largest event recorded in the Nepal Himalaya during the modern instrumental era. The rupture propagated eastward within ∼40 s (Avouac et al., 2015, Yagi and Okuwaki, 2015) where most of the aftershocks were clustered and the largest aftershock/triggered earthquake of Mw 7.3 occurred on 12th May 2015 at a depth of 17 km. Over 500 local earthquakes/aftershocks with magnitude >4.0 occurred within 45 days of the main event (Fig. 1; USGS, 2015, Adhikari et al., 2015). It is construed that the hypocentre lies within the mid crustal ramp on the Main Himalayan Thrust (MHT) (Avouac et al., 2015), which is well established in this region (Ni and Barazangi, 1984, Schelling, 1992, Pearson and DeCelles, 2005, Avouac, 2015). The spatial distribution of the aftershocks defines a nearly 140 × 40 km2 rectangular fault zone that ruptured during the 2015 Nepal earthquakes (Fig. 1) abutting the presumed extension of the rupture zone of the 1934-M 8.4-Great Bihar Nepal earthquake (Adhikari et al., 2015). Results from InSAR line-of-sight displacement data gleaned from the ALOS-2 data show that the slip during the main shock and the largest aftershock/triggered event extends for over >140 km with a peak slip of 5.5–6.5 m on the MHT between 5 and 15 km depth and a >1 m co-seismic uplift just north of Kathmandu and a corresponding depression on either side of the uplift zone (Lindsey et al., 2015, Avouac et al., 2015, Elliott et al., 2016, Sreejith et al., 2016). The main shock and the aftershocks of the 2015-Nepal earthquake did not produce any surface rupture and the observed ground deformation (Lindsey et al., 2015, Sreejith et al., 2016, Elliott et al., 2016) largely remains confined to the Lesser Himalayan region, which constitutes the locked zone of the MHT having an interseismic strain deficit rate of around 17.8 ± 1.5 mm/yr (Ader et al., 2012, Bilham et al., 1997) for over a decade in the region. The seismogenesis in the Himalayan region is controlled by the mid–crustal ramp on the MHT, which is the source zone of the earthquakes and microseismicity (Pandey et al., 1999, Schulte-Pelkum et al., 2005, Avouac, 2015), though its configuration is not well constrained. The large and great earthquakes rupture the southern flat of the MHT and emerge towards the Himalayan front depending on the co–seismic slip distribution and the geometry of the structures along the rupture zone (Mugnier et al., 2013, Bollinger et al., 2014, Avouac, 2015). The 25th April 2015 earthquake affected zone also experienced the Mw 7.6 – 1833 Nepal earthquake (Bilham, 1995, Mugnier et al., 2013) and the 1934-Bihar Nepal Great earthquake, which produced surface rupture and ground deformation along the MFT in the adjoining region (Sapkota et al., 2013, Bollinger et al., 2014).
In this paper, we performed common conversion point (CCP) receiver function analysis of the HiCLIMB (Nábělek et al., 2009) and HIMNT (Schulte-Pelkum et al., 2005) data along three sections AB-traversing the epicentre of the Mw 7.8 main event, CD- across Kathmandu, where maximum ground deformation is observed, and EF – across the epicentre of the largest aftershock/triggered earthquake (Fig. 1). We correlated seismic receiver function analysis results with the crustal structures along the respective sections (Pearson and DeCelles, 2005, Schelling, 1992) and integrated them with the coseismic geodetic observations (Lindsey et al., 2015) to understand the seismogenesis and rupture during the 2015 Nepal earthquake.
Section snippets
Receiver function imaging of the crust
The data accrued from the HiCLIMB and HIMNT networks offered an excellent opportunity to interrogate and validate the structure of the seismogenic zones in the Himalayan mountain belt using the receiver function (RF) analysis. Waveforms used in this study are extracted from the data base of HiCLIMB and HIMNT seismological experiments in the Nepal Himalaya and southern Tibet, archived in the IRIS Data Management Centre. However, only a subset of data from stations of the Hi-CLIMB experiment,
Seismic structure and seismogenesis
The CCP images are overlaid with the aftershock distribution and fault plane solutions of M > 5 events derived from the USGS catalogue (Fig. 3). To understand the seismogenesis in Nepal, we interpreted the seismic images in conjunction with focal mechanisms of the Mw > 5 earthquakes that occurred during April–May 2015 and the published crustal structures along sections CD (after Pearson and DeCelles, 2005) and EF (Schelling, 1992; Fig. 4). Since the seismic station coverage along section A-B
Coseismic slip and uplift pattern vis-a-vis lateral variability in the rupture
The Mw 7.8 earthquake ruptured the mid-crustal MHT for more than 140 km, with the largest aftershock/triggered event of Mw 7.3 defining the eastern limit (USGS; Fig. 1). Results of InSAR interferometry from the earthquake affected region show coseismic ground deformation for over 100 km along a strike parallel narrow zone to the north of Kathmandu with a maximum uplift that is >1 m and a corresponding depression towards north during the Mw 7.8 event (Fig. 5a and b) and ∼50 cm uplift associated with
Discussion
The Himalaya of Central Nepal has experienced several large earthquakes during the last millennium. Notable among them are the earthquakes in 1934, 1833, ∼1255, and ∼1100 CE, that severely affected the Kathmandu valley despite being at varied distance from the epicentres (Pandey and Molnar, 1988, Bilham, 1995, Mugnier et al., 2013, Sapkota et al., 2013, Bollinger et al., 2014). The rupture zone of the 2015 earthquake (Fig. 1; Avouac et al., 2015, Yagi and Okuwaki, 2015) overlies the relocated
Conclusions
We synthesize the salient observations and interpretations in a schematic diagram (Fig. 7). The decreasing thickness, increasing depth of the low velocity layer, increasing density and depth of aftershocks towards east and the interpreted structure on the seismic profiles (Fig. 3, Fig. 6) in the earthquake rupture zone suggest lateral variation in the upper crustal structure and the MHT configuration. The rupture propagated eastward from the locus of the Mw 7.8 earthquake and decelerated after
Acknowledgements
Authors acknowledge funding from the GENIAS and HEART programs of CSIR-NGRI. IRIS-DMC is gratefully acknowledged for making the data available. We highly appreciate the critical and constructive comments from guest editor J.P. Mugnier, Roland Burgmann and an anonymous reviewer, that significantly improved the manuscript.
References (38)
Mountain building: from earthquakes to geologic deformation
- et al.
Characterizing the Main Himalayan Thrust in the Garhwal Himalaya, India with receiver function CCP stacking
Earth Planet. Sci. Lett.
(2013) - et al.
Structural interpretation of the great earthquakes of the last millennium in the central Himalaya
Earth Sci. Rev.
(2013) - et al.
Seismotectonics of the Nepal Himalaya from a local seismic network: Jour
Asian Earth Sci.
(1999) An overview of the stratigraphy and tectonics of the Nepal Himalaya
J. Asian Earth Sci.
(1999)- et al.
Sourch scaling of earthquakes in the Shumagin region, Alaska: time-domain inversions of regional waveforms
Geophys. J. Int.
(1995) - et al.
Convergence rate across the Nepal Himalaya and interseismic coupling on the Main Himalayan Thrust: implications
J. Geophys. Res.
(2012) - et al.
The aftershock sequence of the 2015 April 25 Gorkha-Nepal earthquake
Geophys. J. Int.
(2015) - et al.
Magnitude calibration of north Indian earthquakes
Geophys. J. Int.
(2004) - et al.
On the non-uniqueness of receiver function inversions
J. Geophys. Res.
(1990)
Lower edge of locked Main Himalayan Thrust unzipped by the 2015 Gorkha earthquake
Nat. Geosci.
Location and magnitude of the 1833 Nepal earthquake and its relation to the rupture zones of contiguous great Himalayan earthquakes
Curr. Sci.
GPS measurements of present day convergence across the Nepal Himalaya
Nature
Estimating the return times of great Himalayan earthquakes in eastern Nepal: evidence from the Patu and Bardibas strands of the Main Frontal Thrust
J. Geophys. Res.
The 2015 Gorkha earthquake: a large event illuminating the Main Himalayan Thrust fault
Geophys. Res. Lett.
Himalayan megathrust geometry and relation to topography revealed by the Gorkha earthquake
Nat. Geosci.
Slip pulse and resonance of Kathmandu basin during the 2015 Mw 7.8 Gorkha earthquake, Nepal imaged with geodesy
Science
Initiation of the Himalayan orogen as an early Palaeozoic thin skinned thrust belt
GSA Today
Rupture process of the Mw = 7.9 2015 Gorkha earthquake (Nepal): Insights into Himalayan megathrust segmentation
Geophys. Res. Lett.
Cited by (4)
Interplay of fragmentation, flow, mingling, and mixing in composite pseudotachylyte-ultracataclasite veins – An example from the Main Central Thrust (central Nepal)
2023, Journal of Asian Earth SciencesCitation Excerpt :Along the MCT, medium to high-grade metamorphic rocks of the Higher Himalaya are thrust over low-grade rocks of the Lesser Himalaya. At depth, the MCT, the MBT and the MFT merge to the Main Himalayan Thrust (MHT) (Zhao et al., 1993; Jouanne et al., 2004), along which pre-historic (Bollinger et al., 2016) and historic earthquakes took place, including the 2015 Gorkha Earthquake (Pandey et al., 1995, 2017; Paudel and Arita, 2000; Elliott et al., 2016; Mugnier et al., 2017). The southern part of the MCT is inactive while the northern part shows seismicity at depth (GEER, 2015, Fig. 2-1).
Reflection on earthquake damage of buildings in 2015 Nepal earthquake and seismic measures for post-earthquake reconstruction
2021, StructuresCitation Excerpt :The Himalayan seismic belt belongs to the strong seismicity zone. Nepal has been a seismically active region for centuries [3–5]. Within the epicentre area 100 km from the historical record, a M7.8 earthquake occurred in the area 37 km from the epicentre (28° N, 85° E) in 1833 and 81 km (27.7° N, 85.3° E) from the epicentre in 1966.
Geophysical Studies for the Crust and Upper Mantle Structure of the Himalaya: Contributions of CSIR-NGRI
2021, Journal of the Geological Society of India