Convergence rate across the Nepal Himalaya and interseismic coupling on the Main Himalayan Thrust: Implications for seismic hazard

Abstract

[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.

Investigation of Coulomb stress changes in south Tibet (central Himalayas) due to the 25th April 2015 M W 7.8 Nepal earthquake using a Coulomb stress transfer model

“…The MCT dips 30°-45°, along which there are some geological indications of minor recent movement (e.g., Valdiya 1980). The MHT, dipping at about 10°to the north, exhibits a downdip end of the locked part of the fault about 100 km along dip from its surface trace (Ader et al 2012). The STDS is a major normal fault system that runs parallel to the Himalayan range for more than 1500 km and dips gently 5°-15°to the north ).…”

“…Earthquakes occur along the thrust faults when stress accumulation is highly enough. A previous study shows the inference that large earthquakes that are known to recur along the Himalayan front must be associated with ruptures of the MHT, which emerges at the surface along the front of the Himalayan foothills and is a major basal thrust fault (Ader et al 2012). Besides this, to the north of the Himalayan Orogen, there are normal faults and graben structures, generally at N-S strike, which show west-east extension and are relevant to shallow normal style earthquakes (Ni and Barazangi 1984).…”

“…The MHT fault emerges at the surface along the front of the Himalayan foothills (e.g., Avouac 2003;Ader et al 2012). The MFT dips about 30°(e.g., Rajendran et al 2015) and the MBT about 60°-90°(e.g., Ni and Barazangi 1984).…”

“…These earthquakes break the MHT up to the near surface at the front of the Himalayan foothills and results in incremental activation of the MFT (ibid.). 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 Himalayas (Ader et al 2012). The MHT reaches the surface at the MFT (Nakata 1989) (Fig.…”

“…Based on the focal mechanism information from the U.S. Geological Survey (USGS) and the finite fault inversion results by Gavin Hayes (USGS/NEIC, http://earthquake.usgs.gov/earth quakes/eventpage/us20002926#scientific_finitefault), it seems that the mainshock of the event in Nepal ruptured along a shallow-dipping reverse fault to the north of the Main Frontal Thrust (MFT) between the Indian and Eurasian plates. The MFT seems to merge into the Main Himalaya Thrust (MHT) at a depth of * 5 km Ma and Gao 2011;Lavé and Avouac 2000), and the MHT is inferred to be the rupture fault along which many large earthquakes have occurred (Ader et al 2012). Therefore, according to the inversion results from USGS finite faults, we set the source fault to be the MHT, where the M W 7.8 Nepal event occurred, and the rupture fault dips approximately 10°to the north near the surface to about 30 km depth under the Himalayan orogen.…”