[1] We document geodetic strain across the Nepal Himalaya using GPS times series from 30 stations in Nepal and southern Tibet, in addition to previously published campaign GPS points and leveling data and determine the pattern of interseismic coupling on the Main Himalayan Thrust fault (MHT). The noise on the daily GPS positions is modeled as a combination of white and colored noise, in order to infer secular velocities at the stations with consistent uncertainties. We then locate the pole of rotation of the Indian plate in the ITRF 2005 reference frame at longitude = À 1.34 AE 3.31 , latitude = 51.4 AE 0.3 with an angular velocity of W = 0.5029 AE 0.0072 /Myr. The pattern of coupling on the MHT is computed on a fault dipping 10 to the north and whose strike roughly follows the arcuate shape of the Himalaya. The model indicates that the MHT is locked from the surface to a distance of approximately 100 km down dip, corresponding to a depth of 15 to 20 km. In map view, the transition zone between the locked portion of the MHT and the portion which is creeping at the long term slip rate seems to be at the most a few tens of kilometers wide and coincides with the belt of midcrustal microseismicity underneath the Himalaya. According to a previous study based on thermokinematic modeling of thermochronological and thermobarometric data, this transition seems to happen in a zone where the temperature reaches 350 C. The convergence between India and South Tibet proceeds at a rate of 17.8 AE 0.5 mm/yr in central and eastern Nepal and 20.5 AE 1 mm/yr in western Nepal. The moment deficit due to locking of the MHT in the interseismic period accrues at a rate of 6.6 AE 0.4 Â 10 19 Nm/yr on the MHT underneath Nepal. For comparison, the moment released by the seismicity over the past 500 years, including 14 M W ≥ 7 earthquakes with moment magnitudes up to 8.5, amounts to only 0.9 Â 10 19 Nm/yr, indicating a large deficit of seismic slip over that period or very infrequent large slow slip events. No large slow slip event has been observed however over the 20 years covered by geodetic measurements in the Nepal Himalaya. We discuss the magnitude and return period of M > 8 earthquakes required to balance the long term slip budget on the MHT.
Movie S1Correction: In table S1, the displacement at station SNDL was reported erroneously. The correct displacement is: east, 0.047 ±0.002 m; north, -0.223 ±0.003 m; vertical, 0.003 ±0.003 m. The PDF has been corrected.
SUMMARY
Underthrusting of the Indian lithosphere beneath the Himalayas occurs during the Quaternary period along a gently north‐dipping main basal detachment (main Himalayan thrust: MHT), from which the southernmost emergent ramp (main frontal thrust: MFT) branches. Historical seismicity shows that slip on the MHT is frequently accommodated through M > 8 shallow earthquakes, but shows a seismic gap in western Nepal. This absence of major historical earthquakes in western Nepal can be explained either by an aseismic slip on the MHT or a long‐lived elastic strain accumulation. To test these hypotheses, the present‐day displacement field has been measured for a GPS network formed of 35 sites. The updated solution presented in this paper combines data from 1995, 1997, 1998 and 2000 measurements. The lack of deformation (less than 3 · 10−8 yr−1) through the outer belt does not fit with a regional aseismic slip along the southern part of MHT. A less than 3 mm yr−1 aseismic slip could nonetheless affect restricted areas of the outer belt. In contrast, a strain accumulation of more than 30 · 10−8 yr−1 is measured south of the Higher Himalayas, in a zone where an intense microseismicity reflects a stress build‐up. It is presumably generated by locking of the aseismic creep that occurs along the MHT beneath the Higher Himalayas and Tibet. The displacement field is simulated by a dual‐dislocation model that takes into account the pattern of microseismicity, and particularly a segmentation between central and western Nepal. The best fit between the measured and simulated displacement fields is obtained with 19 mm yr−1 thrust and 0–1 mm yr−1 dextral strike‐slip components along a 117°NE dislocation locked to a depth of 20–21 km beneath western Nepal, and 19–20 mm yr−1 thrust and 0–2 mm yr−1 dextral strike‐slip components along a 108°NE dislocation locked to a depth of 17–21 km beneath central Nepal. The width of the locked zone between the main frontal thrust and the creeping zone is of the same order, but rather greater, in western Nepal than in central Nepal. Therefore it is expected that M > 8 earthquakes could occur in western Nepal.
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