Earthquake genesis in Nepal Himalaya: A perspective from imaging of the 25th April 2015 Mw 7.8 earthquake source zone

https://doi.org/10.1016/j.jseaes.2016.12.039Get rights and content

Highlights

  • Correlation between the seismic structure and seismogenesis in the Nepal Himalaya.

  • Delineation of a low velocity layer that corresponds to the Lesser Himalaya sedimentary duplex above the MHT flat.

  • Lateral variations in upper crustal structure control rupture propagation.

  • Tectonic loading on the duplex caused internal deformation localizing bulk of the aftershocks.

Abstract

The Mw 7.8 earthquake in Central Nepal nucleated in the mid-crustal ramp zone of the Main Himalayan décollement Thrust (MHT) and propagated eastward for >140 km where the largest triggered event of Mw 7.3 occurred without any surface rupture. Although it is advocated that the slip and rupture dynamics are controlled by the structural configuration of the MHT and the upper crust, precise correlation between the seismic structure and seismogenesis is hitherto scarce in the Himalaya. To address the issue, we imaged the crustal structure along three profiles covering the earthquake source region using receiver function analysis of the seismic data from the HiCLIMB and HIMNT seismic networks to understand the lateral variability. A ∼5 km thick, low velocity layer is observed at the mid–crustal level, that steepens in the MHT ramp zone. The bulk of the seismicity including large shocks after the 2015 Nepal earthquake lies in the vicinity of this low velocity layer. Correlation of the seismic structure and aftershock distribution with the published crustal structure clearly suggests that the rupture involves a thicker zone extending for >40 km to the south of the source zone in the MHT ramp. We refined the structure of the MHT zone incorporating published coseismic slip and ground deformation to suggest that the rupture terminated at the footwall imbricate (horse) on the floor thrust below the zone of maximum coseismic uplift and there was a two stage rupture towards the eastern margin of the rupture zone.

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.

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