Research

Overview of main results on Himalayan Tectonics and Seismicity

As geodetic data is becoming more and more common there is a need for some simple models relating crustal deformation and seismicity that would provide some physical basis to help assess the frequency and size of major earthquakes. TheHimalaya, which is the most active intracontinental mountain range on earth, is a most appropriate case study to address this question.In collaboration with Nepali colleagues from the Department of Mines and Geology and many French colleagues, we have carried on various investigations in the Himalaya of Nepal.Some idea of the seismic cycle could be derived that relate mountain building in theHimalaya and the large M>8 Himalayan earthquakes (See Fig. 1) [Avouac et al., 2001].

Figure 1 - Major earthquakes along the Himalayan arc

The seismic permanent network, initially settled around Katmandu was extended in 1994 and now consists of 23 short period stations. This network has revealed a belt of intense microseismic activity which follows the front of the high range [Pandey et al., 1994; 1999] (See Fig. 2).

Figure 2 - Seismicity of Nepal

In the sub-Himalaya, abandoned Holocene terraces have recorded active folding with uplift rates up to 1.5 cm/yr (See Fig. 3). It shows that as much 21.5+/-1.5 mm/yr of horizontal shortening is accommodated by localized slip along the MFT [Lavé and Avouac, 2000]. Structural geology suggests that this fault emerges from a decollement at the top of the Indian basement, at 5-6 km depth, extends northwards beneath the Lesser Himalaya and roots into a mid-crustal sub-horizontal shear zone beneath the Higher Himalaya and southern Tibet that could be imaged from INDEPTH seismic experiments in southern Tibet. Incision rates along the major rivers across the Himalaya of Nepal are keeping with this geometry : little incision in the Lesser Himalaya and Southern Tibet, is observed and up to 5-10mm/yr in the Higher Himalaya, a pattern consistent consistent with 20-23mm/yr of slip along the ramp-and-flat geometry of the MHT (See Fig. 4) [Lavé and Avouac, 2001].

Figure 3 - Folded Terraces	Figure 4 - Structural section across central Nepal Himalaya and incision rates

By contrast, geodetic data shows that, during the interseismic period, horizontal shortening is mainly absorbed within a 100 km wide zone that spans over the Higher Himalaya, while deformation in the sub-Himalaya and southern part of the Lesser Himalaya seems negligible. It indicates that the fault is fully locked with horizontal shortening being mainly absorbed within a 100 km wide zone that spans over the Higher Himalaya. This information is reconciled from a mechanical FEM model in which we consider a lithospheric section with a realistic rheology, submitted to horizontal shortening [Cattin and Avouac, 2000]. The model accounts for surface processes and the thermal structure of the lithosphere. In this model, on the long term, provided that friction on the MHT is low (less than 0.2-0.3) shortening across the Himalaya is essentially accommodated by localized frictional slip along the MHT in the brittle upper crust and by ductile flow in the lower crust beneath the high range and southern Tibet (See Fig. 5). During the interseismic period, the MHT is fully locked from the sub-Himalaya to beneath the Higher Himalaya. Microseismic activity is enhanced in the zone of increasing Coulomb stress (See Fig. 6).

Interseismic stress build-up by elastic straining of the upper crust is probably the main process responsible for the observed belt of microseismicity that can be traced along the front of the high range all along the Himalayas of Nepal. This is consistent with the geodetic data that also suggest that the MHT is locked everywhere including the seismic gap between the rupture areas of 1905 and 1934. It seems highly probable that this portion of the Himalayan arc also produces large recurrent earthquakes similar to the 1934 and 1905 events. Motion along the MHT is probably stick-slip as a result of recurring large earthquakes similar to the 1934 Bihar-Nepal or 1905 Kangra events. By comparing the distribution of background seismicity with Coulomb stress variations due to interseismic strain, we show that deviatoric stresses at seismogenic depth (~10 km) are modulated by topographic stresses and do not exceed about 35MPa. Such low stressed also constrain the friction on the Main Himalayan Thrust to be lower than about 0.3.

Additional information comes from the result of a MT sounding experiment also carried on across the Himalaya of central Nepal. This experiment has revealed a deep conductor, with a resistivity of the order of 30 W.m, (resistivity across the Himalaya) that roughly coincides with the zone of intense microseismic activity but apparently extends to greater depth [Lemmonnier et al., 1999] (See Fig. 7). This pattern suggests that the high conductivity results from a well interconnected fluid phase that would be fed from metamorphic reactions as the Indian crust is thrusted under the mid-crustal ramp, channeled upward in the zone of interseismic straining.

Figure 7 - Resistivity model of the crust across the Himalaya of central Nepal

We have investigated thermal metamorphism in the Lesser Himalaya taking advantage of a recently calibrated technique based on Raman Spectrometry on Carbonaceous Matter [Beyssac et al, 2004]. The combination with structural investigations shows an inverted metamorphic gradient throughout the Lesser Himalaya which can be explained by a model in which crustal thickening in the Himalaya would have resulted primarily from underplating rather than from fluvial accretion [Bollinger et al, 2004a).

My main contributions on the subject :

Avouac, J.P., and E. G. Burov, Erosion as a driving mechanism of intracontinental growth?, J. Geopys. Res., 101, 17,747-17,769, 1996. [PDF]

Avouac, J.P., L. Bollinger, J. Lavé, R. Cattin and M. Flouzat, Le cycle sismique en Himalaya, C. R. Acad. Sc., 333, 513-529, 2001. [PDF]

Avouac, J.P., Mountain Building, Erosion, and the Seismic Cycle in the Nepal Himalaya, Advances in Geophysics, Vol. 46, 10.1016/S0065-2687(03)46001-9, December 2003. [PDF]

Bettinelli, P., M. Flouzat, L. Bollinger, and G. R. Chitrakar, Plate motion of India and interseismic strain in the Nepal Himalaya from GPS and DORIS measurements, J. Geod., 2006. [PDF]

Beyssac, O., L. Bollinger, J.P. Avouac, and B. Goffe, Thermal metamorphism in the lesser Himalaya of Nepal determined from Raman spectroscopy of carbonaceous material, EPSL, 225, 233-241, 2004. [PDF

Bollinger, L, F. Perrier, J.P. Avouac, S. Sapkota, U. Gautam, and D.R. Tiwari, Seasonal modulation of seismicity in the Himalaya of Nepal, Geophysical Research Letters, 34, L08304, doi: 10.1029/2006GL029192, 2007. [PDF]

Bollinger, L., P. Henry, and J.P. Avouac, Mountain building in the Nepal Himalaya: Thermal and kinematic model, EPSL, 244, 58-71, 2006. [PDF]

Bollinger, L., J.P. Avouac, O. Beyssac, E.J. Catlos, T.M. Harrison, M. Grove, B. Goffe, and S. Sapkota, Thermal Structure and exhumation history of the Lesser Himalaya in central Nepal, Tectonics, 23, 2004. [PDF]

Bollinger, L., J.P. Avouac, R. Cattin, M.R. Pandey, Stress buildup in the Himalaya, J. Geophys. Res, 109, 2004. [PDF]

Cattin, R. & Avouac, J.P. Modeling mountain building and the seismic cycle in the Himalaya of Nepal. J. Geophys. Res, 105, 13,389-13,407, 2000.[PDF]

Cattin R., Martelet, G., Henry, P., Avouac, J.P., Diament, M. and Shakya, Gravity anomalies, crustal structure and thermo-mechanical support of the Himalaya of Central Nepal, Geophys. J. Int., 147, 381-392, 2001. [PDF]

Jouanne, F., J.L. Mugnier, J.F. Gamond, P. Le Fort, M.R. Pandey, L. Bollinger, M. Flouzat, and J.P. Avouac, Current Shortening across the Himalayas of Nepal, Geophys. J. Int., 157, 1-14, 10.111/j.1365-246X.2004.02180, 2004. [PDF]

Jouanne, F., J.L. Mugnier, M.R. Pandey, J.F. Gamond, P. Le Fort, L. Serrurier, C. Vigny, J.P. Avouac, Oblique convergence in the Himlayas of western Nepal deduced from preliminary results of GPS measurements, Geophys. Res. Lett., 13, 1933-1936, 1999. [PDF]

Lavé, J., J.P. Avouac, R. Lacassin, P. Tapponnier and J.P. Montagner, Seismic anisotropy beneath Tibet : evidence for eastward extrusion of the tibetan lithosphere? Earth, Planet. Sc. Let., 140, 83-96, 1996.[PDF]

Lavé J., and J.P. Avouac, Active folding of fluvial terraces across the Siwalik Hills Himalaya of central Nepal, J. Geophys. Res., 105, 5735-5770, 2000. [PDF]

>LavéJ., and J.P. Avouac, Fluvial incision and tectonic uplift across the Himalayas of Central Nepal, J. Geophys. Res., 106, 26,561-26,592, 2001. [PDF]

Lemmonier, C., G. Marquis, F. Perrier, J.P. Avouac, G. Chitrakar, B. Kafle, S. Sapkota, U. Gautam and D. Tiwari, Electrical structure of the Himlaya of central Nepal: high conductivity around the mid-crustal ramp along the MHT, Geophys. Res. Lett., 21, 3261-3264, 1999. [PDF]

Pandey, M.R., R.P. Tandukar, J.P. Avouac, J. Lavé and J.P. Massot, Interseismic strainAccumulation on the HimalayaCrustal Ramp,Nepal, Geophys. Res. Lett., 22, 751-754, 1995. [PDF]

Pandey, M.R., R.P.Tandukar, J.P. Avouac, J. Vergne and Th. Héritier, Seismotectonics of Nepal Himalayas from a local seismic network J. Asian Earth Sciences, 17, 703-712, 1999. [PDF]

Toussaint, G., E. Burov, and J.P. Avouac, Tectonic evolution of a continental collision zone: A thermomechanical numerical model, Tectonics, 23, TC6003, doi: 10.1029/2003TC001604, 2004. [PDF]

Vergne J., R. Cattin and J.P. Avouac, On the  use of dislocations to model interseismic strain and stress build-up at intracontinental thrust faults, Geophys. J. Int.,147, 155-162, 2001. [PDF]

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