Pre- and post-seismic deformation related to the 2015, Mw7.8 Gorkha earthquake, Nepal

We analyze time series from continuously recording GPS stations in Nepal spanning the pre- and post-seismic period associated to the M_w7.8 Gorkha earthquake which ruptured the Main Himalayan Thrust (MHT) fault on April 25th, 2015. The records show strong seasonal variations due to surface hydrology. After corrections for these variations, the time series covering the pre- and post-seismic periods do not show any detectable transient pre-seismic displacement. By contrast, a transient post-seismic signal is clear. The observed signal shows southward displacements consistent with afterslip on the MHT. Using additional data from stations deployed after the mainshock, we invert the time series for the spatio-temporal evolution of slip on the MHT. This modelling indicates afterslip dominantly downdip of the mainshock rupture. Two other regions show significant afterslip: a more minor zone updip of the rupture, and a region between the mainshock and the largest aftershock ruptures. Afterslip in the first ~ 7 months after the mainshock released a moment of [12.8 ± 0.5] × 10^(19) Nm which represents 17.8 ± 0.8% of the co-seismic moment. The moment released by aftershocks over that period of time is estimated to 2.98 × 10^(19) Nm. Geodetically observed post-seismic deformation after co-seismic offset correction was thus 76.7 ± 1.0% aseismic. The logarithmic time evolution of afterslip is consistent with rate-strengthening frictional sliding. According to this theory, and assuming a long-term loading velocity modulated on the basis of the coupling map of the region and the long term slip rate of 20.2 ± 1.1 mm/yr, afterslip should release about 34.0 ± 1.4% of the co-seismic moment after full relaxation of post-seismic deformation. Afterslip contributed to loading the shallower portion of the MHT which did not rupture in 2015 and stayed locked afterwards. The risk for further large earthquakes in Nepal remains high both updip of the rupture area of the Gorkha earthquake and West of Kathmandu where the MHT has remained locked and where no earthquake larger than M_w7.5 has occurred since 1505.

[1]  J. Avouac,et al.  Seasonal variations of seismicity and geodetic strain in the Himalaya induced by surface hydrology as revealed from GPS monitoring, seismic monitoring and GRACE measurements , 2007 .

[2]  L. Rivera,et al.  The 2015 Gorkha earthquake: A large event illuminating the Main Himalayan Thrust fault , 2016 .

[3]  Michael Bevis,et al.  Trajectory models and reference frames for crustal motion geodesy , 2014, Journal of Geodesy.

[4]  Walter H. F. Smith,et al.  New, improved version of generic mapping tools released , 1998 .

[5]  L. Bollinger,et al.  Slip deficit in central Nepal: omen for a repeat of the 1344 AD earthquake? , 2016, Earth, Planets and Space.

[6]  B. N. Upreti,et al.  Structural interpretation of the great earthquakes of the last millennium in the central Himalaya , 2013 .

[7]  Masanobu Shimada,et al.  Line‐of‐sight displacement from ALOS‐2 interferometry: Mw 7.8 Gorkha Earthquake and Mw 7.3 aftershock , 2015 .

[8]  Enrico Serpelloni,et al.  Blind source separation problem in GPS time series , 2016, Journal of Geodesy.

[9]  P. Shearer,et al.  Dynamics of the 2015 M7.8 Nepal earthquake , 2015 .

[10]  Yann Klinger,et al.  Estimating the return times of great Himalayan earthquakes in eastern Nepal: Evidence from the Patu and Bardibas strands of the Main Frontal Thrust , 2014 .

[11]  Richard Styron,et al.  Database of Active Structures From the Indo‐Asian Collision , 2010 .

[12]  Kelin Wang,et al.  Viscoelastic relaxation following subduction earthquakes and its effects on afterslip determination , 2015 .

[13]  M. Cheng,et al.  GGM02 – An improved Earth gravity field model from GRACE , 2005 .

[14]  U.,et al.  Slip pulse and resonance of Kathmandu , 2015 .

[15]  W. Szeliga,et al.  Intensity, magnitude, location and attenuation in India for felt earthquakes since 1762 , 2010 .

[16]  F. Cotton,et al.  Spatial and temporal evolution of a long term slow slip event: the 2006 Guerrero Slow Slip Event , 2011 .

[17]  Nadia Lapusta,et al.  Towards inferring earthquake patterns from geodetic observations of interseismic coupling , 2010 .

[18]  John Douglas,et al.  Magnitude calibration of north Indian earthquakes , 2004 .

[19]  J. Genrich,et al.  Modeling deformation induced by seasonal variations of continental water in the Himalaya region: Sensitivity to Earth elastic structure , 2011 .

[20]  J. Avouac,et al.  Inverting geodetic time series with a principal component analysis-based inversion method , 2010 .

[21]  Jérôme Lavé,et al.  Interseismic strain accumulation on the Himalayan crustal ramp (Nepal) , 1995 .

[22]  Lin Ding,et al.  Convergence rate across the Nepal Himalaya and interseismic coupling on the Main Himalayan Thrust: Implications for seismic hazard , 2011 .

[23]  Michael Bevis,et al.  Coseismic and postseismic slip associated with the 2010 Maule Earthquake, Chile: Characterizing the Arauco Peninsula barrier effect , 2013 .

[24]  Jean-Paul Ampuero,et al.  Lower edge of locked Main Himalayan Thrust unzipped by the 2015 Gorkha earthquake , 2015 .

[25]  Yehuda Bock,et al.  Frictional Afterslip Following the 2005 Nias-Simeulue Earthquake, Sumatra , 2006, Science.

[26]  Hugo Perfettini,et al.  Postseismic relaxation driven by brittle creep: A possible mechanism to reconcile geodetic measurements and the decay rate of aftershocks, application to the Chi-Chi earthquake, Taiwan , 2004 .

[27]  Jeffrey T. Freymueller,et al.  Seasonal and long-term vertical deformation in the Nepal Himalaya constrained by GPS and GRACE measurements , 2012 .

[28]  C. Scholz,et al.  Correction to “On the mechanics of earthquake afterslip” by Chris J. Marone, C.H. Scholz, and Roger Bilham , 1991 .

[29]  Albert Tarantola,et al.  Inverse problem theory - and methods for model parameter estimation , 2004 .

[30]  J. Avouac,et al.  Interseismic coupling on the main Himalayan thrust , 2015 .

[31]  J. Galetzka,et al.  Slip pulse and resonance of the Kathmandu basin during the 2015 Gorkha earthquake, Nepal , 2015, Science.

[32]  J. Avouac,et al.  Millenary Mw > 9.0 earthquakes required by geodetic strain in the Himalaya , 2016 .

[33]  S. Rajaure,et al.  Seismic structure of the crust and the upper mantle beneath the Himalayas: Evidence for eclogitization of lower crustal rocks in the Indian Plate , 2008 .

[34]  Tomokazu Kobayashi,et al.  Detailed crustal deformation and fault rupture of the 2015 Gorkha earthquake, Nepal, revealed from ScanSAR-based interferograms of ALOS-2 , 2015, Earth, Planets and Space.

[35]  Stephen J. Roberts,et al.  Variational Mixture of Bayesian Independent Component Analyzers , 2003, Neural Computation.

[36]  Enrico Serpelloni,et al.  Vertical GPS ground motion rates in the Euro‐Mediterranean region: New evidence of velocity gradients at different spatial scales along the Nubia‐Eurasia plate boundary , 2013 .

[37]  Göran Ekström,et al.  The global CMT project 2004–2010: Centroid-moment tensors for 13,017 earthquakes , 2012 .

[38]  Yoann Cano,et al.  The aftershock sequence of the 2015 April 25 Gorkha–Nepal earthquake , 2015 .

[39]  L. Bollinger,et al.  Surface ruptures of large Himalayan earthquakes in Western Nepal: Evidence along a reactivated strand of the Main Boundary Thrust , 2016 .

[40]  J. Avouac,et al.  Modeling afterslip and aftershocks following the 1992 Landers earthquake , 2007 .

[41]  R. Bürgmann,et al.  Dynamics of Izmit Earthquake Postseismic Deformation and Loading of the Duzce Earthquake Hypocenter , 2002 .

[42]  V. E. Levin,et al.  Interseismic coupling and asperity distribution along the Kamchatka subduction zone , 2005 .

[43]  H. Perfettini,et al.  Dynamics of a velocity strengthening fault region: Implications for slow earthquakes and postseismic slip , 2008 .

[44]  Vinod K. Gaur,et al.  One‐Dimensional Reference Velocity Model and Precise Locations of Earthquake Hypocenters in the Kumaon–Garhwal Himalaya , 2013 .

[45]  Anthony Sladen,et al.  Seismic and aseismic slip on the Central Peru megathrust , 2010, Nature.

[46]  J. Avouac,et al.  Himalayan megathrust geometry and relation to topography revealed by the Gorkha earthquake , 2016 .

[47]  M. Simons,et al.  Post-seismic and interseismic fault creep; II, Transient creep and interseismic stress shadows on megathrusts , 2010 .

[48]  Paul Segall,et al.  Rapid afterslip following the 1999 Chi‐Chi, Taiwan Earthquake , 2002 .

[49]  L. Bollinger,et al.  Rupture process of the Mw = 7.9 2015 Gorkha earthquake (Nepal): Insights into Himalayan megathrust segmentation , 2015 .

[50]  Yehuda Bock,et al.  Parkfield earthquake: Stress-driven creep on a fault with spatially variable rate-and-state friction parameters , 2009 .

[51]  Enrico Serpelloni,et al.  Space–time evolution of crustal deformation related to the Mw 6.3, 2009 L'Aquila earthquake (central Italy) from principal component analysis inversion of GPS position time-series , 2014 .

[52]  H. Kanamori,et al.  A moment magnitude scale , 1979 .

[53]  Jean-Louis Mugnier,et al.  Postseismic deformation in Pakistan after the 8 October 2005 earthquake: Evidence of afterslip along a flat north of the Balakot‐Bagh thrust , 2011 .

[54]  John H. Woodhouse,et al.  Determination of earthquake source parameters from waveform data for studies of global and regional seismicity , 1981 .