Seismic coupling and megaquakes along the Himalayan arc – BHUTANEPAL

To address these questions our methodology encompasses a large panel of earth science techniques based on the acquisition of original datasets, the development of innovative methods in data processing and the establishment of new dedicated numerical models. Compared to subduction zones the great advantage of studying the Himalayas is to perform measurements near the seismic structures, which is a key point to improve seismic hazard assessment related to major thrust fault system.
The two main originalities of the project are (1) to quantify the lateral variation of seismic coupling along a continental thrust fault system and (2) the large set of complementary and innovative approaches that we propose to conduct (combination of GOCE data with ground gravity, seismology, geodesy, morpho-tectonics, paleo-seismology and numerical modelling) in both Nepal and Bhutan to obtain the first real 3D image of the Himalayas. Furthermore one of the strengths of the present project relies on the proper characterization and integration of deformation of the Himalayan arc over several spatial and temporal scales.

. Gravimetric measurements
Absolute gravimetry campaign (FG5) was conducted in Bhutan in March 2015 to define the first reference frame for this country. A first approach for working with gradients GOCE was also implemented.

. Seismic imaging
The temporary seismological network of associated project worked with 38 stations between January 2013 and April 2014 and with 13 stations up to December 2014. The first images are based on an analysis for each station and show a deepening of the crust-mantle interface.

. GPS measurements
A re-measurement and an extension of the GPS network in Bhutan was made in April 2014. A new campaign is scheduled for spring 2016 in order to obtain a GPS velocity field across Bhutan.
Following the earthquake of Gorkha in Nepal April 25, 2015, 14 new GPS points were installed and measured during a post-seismic survey in June 2015.

. InSAR Measurements
Several interferograms were obtained following the earthquake Gorkha

. Structure essain mid-crustal seismicity
A temporary seismological network of 15/17 stations was deployed in the West of Nepal in November-December 2014.

. Holocene deformation and paleoseismology
Since the beginning of the project, four field missions were carried out in Bhutan: two in the front for paleoseismology, one at the center of the range for a morphostructural study and one to quantify denudation rate along a noth-south profile in western Bhutan.

. Modeling
A master internship on modeling stress transfer was made in 2015. He suggested that the sequence of the 2015 earthquakes in Nepal can be interpreted in terms of Coulomb stress changes. The results also show that Bhutan is characterized by a very high slip deficit.

In the next months, our efforts will particularly focus on the imaging of crustal structures (receiver function, location of seismicity, integration of satellite gravity data), and measurements of deformation using both a new GPS campaign and a long-term study of the accommodation of convergence in Bhutan.

Publications
Vernant, P., Bilham, R., Szeliga, W., Drukpa, D., Kalita, S., Bhattacharyya, A. K., Gaur, V. K., Pelgay, P., Cattin, R., Berthet, T.. Clockwise rotation of the Brahmaputra Valley relative to India : tectonic convergence in the eastern Himalaya, Naga Hills and Shillong Plateau, J. Geophys. Res., doi:10.1002/2014JB011196, 2014
Berthet, T.. Ritz, J.F., Ferry, M., Pelgay, P., Cattin, R., Drukpa, D., Braucher, R. and Hetényi, G., Active tectonics in eastern Himalaya : new constraints from the first morphotectonic study in southern Bhutan, Geology, 42 (5), 427-430, 2014
Le Roux-Mallouf, R., Godard, V. Cattin, R., Ferry, M., Gyeltshen, J., Ritz, J.F., Drupka, D., Guillou, V., Arnold, M., Aumaître, G. Bourlès, D. L., Keddadouche, K., Geophys. Res. Lett., Evidence for a wide and gently dipping Main Himalayan Thrust in western Bhutan, doi : 10.1002/2015GL063767, 2015

Congrès
Hoste-Colomer, R. et al., (2015) , seventh Nepal Geological congress, April 7-9, 2015, Kathmandu, Nepal
Le Roux-Mallouf, et al., EGU2015-15056, 2015.
Le Roux Mallouf et al., HKT, 2014.
Diehl T et al. (2015) . Abstract IUGG-4704 at the IUGG 26th General Assembly, 26 June - 2 July 2015, Prague, Czech Republic.
Diehl et al. (2014) . AGU Fall Meeting T11E-03.
Singer et al. (2014) . AGU Fall Meeting T21B-4573.
Hetényi et al. (2014) 29th Himalaya-Karakoram-Tibet Workshop, 2-4 September 2014, Lucca, Italy.
Singer et al. (2014) . 29th Himalaya-Karakoram-Tibet Workshop, 2-4 September 2014, Lucca, Italy
Ferry, et al., EGU, EGU2014-16401, 2014.

As tragically demonstrated by the 2011 Tohoku Japan earthquake, one of the most challenging issues in improving seismic hazard assessment consists in better forecasting the size/magnitude of future great (M>8) earthquakes. This requires exploring many fundamental but unresolved questions in earth sciences: What controls the lateral variation of large earthquakes occurrence along major seismic faults? What governs the transition from stick-slip behaviour to steady sliding? How do earthquake rupture zones recover and reload? How do large and small earthquakes fundamentally differ, if they do?
To answer these questions active oceanic subduction zones are obvious targets because most of the great earthquakes occur there. However the presence of water prevents from measuring surface deformation within the frontal most ~100 km of major seismic structures. Much effort has been done to deploy ocean bottom instrumentation (including seismic or geodetic measurements), but the sparse coverage of these datasets still prevents detailed studies of seismic zones and of their state of stress. Here, we will rather favour the study of an emerged area: the Himalayan belt, which – as a former oceanic subduction zone – exhibits a first-order along-strike continuity of major faults and tectono-stratigraphic units that can be mapped over an E-W distance of ~2500 km. Some recent major historical earthquakes have been documented. However both the maximum earthquake size that struck the Himalayan front in the past and the probability of occurrence of a magnitude 9 megaquake in the next decades are still debated. Over the last decades most studies along the Himalayas have focused only on Central Nepal. Therefore lateral variations of the state of stress on frontal faults and the size of great Himalayan earthquakes are still poorly constrained. Based on previous studies and our own work in Nepal and Bhutan we have decided to address the question of lateral variations in seismic coupling along the Himalayan arc by extensive and detailed description of local loading (present-day convergence and seismicity rate, late Quaternary shortening rate, past seismic events), and crustal structural geometry (major faults, Moho depth, Indian plate flexure) from western Nepal to Bhutan.
The proposed approach is clearly multi-disciplinary and aims at integrating deformation of the Himalayan arc over various spatial and temporal scales. Our methodology encompasses a large panel of up-to-date and innovative complementary techniques in gravity, seismology, geodesy, morpho-tectonics, paleo-seismology and thermo-mechanical numerical modelling.
Gathering a high level of expertise from national (Montpellier, Paris, Nancy, Chambéry-Grenoble) and international (Switzerland, USA, India, Nepal and Bhutan) co-operation, this project will also contribute to the development of innovative methods in the InSAR processing chain, analysis of GOCE gravity data, determining accurate hypocentral location and numerical modelling.
Ultimately, this project will contribute to provide the first real 3D image of the state of stress along a continental thrust-fault system, which is a crucial step in improving seismic hazard assessment in the areas producing most of the largest earthquakes.