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Main Himalayan Thrust – Rate of Convergence Across the Nepal Himalaya and Interseismic Coupling on the MHT, implications for Seismic Hazard

In this paper Ader et al. document that geodetic strain across the Nepal Himalaya by the use of GPS time series from 30 stations located in Nepal and southern Tibet, as well as previously published campaign GPS points and levelling data  to determine the pattern of interseismic coupling on the Main Himalayan Thrust fault (MHT). To infer secular velocities at the stations with consistent uncertainties the noise on the daily GPS positions was modelled by Ader et al. as a combination of white and coloured noise. The pole of rotation of the Indian Plate was then located in the ITRF 2005 reference frame at longitude = - 1.34^o ± 3.31^o and an angular velocity of Ω = 0.5029 ± 0.0072^o/Myr. The coupling pattern of the MHT was computed on a fault that dipped 10^o to the north which had a strike that roughly follows the arcuate shape of the MHT. The MHT is indicated by the model to be locked from the surface to about 100 km down dip, which corresponds to a depth of 15-20 km. The transition zone of the MHT, in map view, between the portion that is locked and the portion that is creeping at the long term slip rate appears to be a few 10s of km wide at the most, and coincides with the midcrustal microseismicity belt that is located beneath the Himalaya. This transition appears to occur in a zone in which the temperature reaches 350^o C, according to an earlier study that was based on thermokinematic modelling of thermochronological and thermobarometric data. India is continuing to converge with South Tibet at a rate of 17.8 ± 0.5 mm/yr in central and eastern Nepal and 20.5 ± 1 mm/yr in western Nepal. As a result of the MHT locking in the interseismic period the moment of deficit accrues at a rate of 6.6 ± 0.4 x10⁹ Nm/yr on the MHT beneath Nepal. Compared with this, the moment that has been predicted by the seismicity over the past 500 years, which includes 14 M_w ≥ 7 earthquakes with moment magnitudes up to 8.5, which amounts to only 0.9 x 10¹⁹ Nm/yr, indicates a large deficit of seismic slip over the period during which large slow slip events were very infrequent. During the 20 years that were covered by geodetic measurements in the Nepal section of the Himalaya there have been no large slow slip events that have been observed. Ader et al. discuss the magnitude and the return period of earthquakes of M > 8 that are required to balance the long term slip budget on the MHT.

Conclusion

In Nepal the denser network of geodetic data brings better kinematic constraints on convergence of India beneath the Tibetan Plateau. According to Ader et al. the MHT appears to be fully locked over a width of about 100 km, from the surface to beneath the front of the high Himalaya. Within a transition zone that is probably less than 30 km wide interseismic coupling decreases rapidly. This transition occurs at about 15-20 km depth, at which point the temperature on the MHT has been estimated to reach 350^o C. Ader et al. suggest that this might be a reflection of aseismic sliding being promoted where the temperature exceeds 350^o C, as has been inferred from lab experiments and observations in other continental contexts (Blanpied et al., 1995; Marone, 1998; Hsu et al., 2009). According to Ader et al. this favours a scenario in which temperature is the primary controller of the locked-creeping transition, which is similar to the conclusion reached by Hsu et al. (2009) on the Chelungpu Fault in Taiwan and on the Mandeyapecua Thrust Fault (Brooks et al., 2011) in the central Andean backarc. Ader et al. suggest that microseismicity on the MHT appears to cluster where the accumulation of shear stress is at its greatest, and beneath topography of greater than 3,500 m elevation, i.e. where the principle stresses become vertical. As a result of the competition between the relative positions of the 3,500 m contour line and the transition between locked and creeping, which is where the rate of stress reaches its greatest. Ader et al. describe as an interesting result the lack of any apparent variation of coupling, as it is different from observations at subduction zones, locations where coupling patterns exhibit noticeable segmentation (Chlieh et al., 2008; Moreno et al., 2010; Suwa et al., 2006; Ozawa et al, 2000; Wallace et al., 2004). This is suggested to possibly point to a fundamental difference between intracontinental and subduction megathrusts. In any case, the rate of accumulation on the MHT of the moment deficit within Nepal is large (6.6 ± 0.4 x 10¹⁹ Nm/yr) and it suggested by comparison with the historical seismicity that has been suggested that infrequent events (with a return period above 1,000 yr) with an above M_w ~8 value assigned to the largest known earthquakes of 1505 and 1934 needs to be taken into consideration, as inferences that are based on palaeoseismological investigations have also suggested (Lavé et al., 2005). Ader et al. suggest that it should be kept in mind, however, that those seismic hazard assessments rely on a few hypotheses (no significant moment release by afterslip events or slow slip events) that could alter the conclusions if proven correct.

Sources & Further reading

1. Ader, T., J.-P. Avouac, J. Liu-Zeng, H. Lyon-Caen, L. Bollinger, J. Galetzka, J. Genrich, M. Thomas, K. Chanard, S. N. Sapkota, S. Rajaure, P. Shrestha, L. Ding and M. Flouzat (2012). "Convergence rate across the Nepal Himalaya and interseismic coupling on the Main Himalayan Thrust: Implications for seismic hazard." Journal of Geophysical Research: Solid Earth 117(B4): B04403.