The 25 April 2015 Nepal MS8.1 earthquake slip distribution from joint inversion of teleseismic, static and high-rate GPS data

On 25 April 2015, a devastating (M(S)8.1) earthquake struck the central Nepal, causing severe damages in Kathmandu. The earthquake is believed to occur on a basal detachment fault along which the Indian plate plunged under Tibet, providing a rare opportunity to understand seismicity of the continental plate boundary. Strong ground motions and permanent surface displacements induced by this event were observed unprecedentedly by continuous GPS networks in Nepal and Tibet, and these geodetic observations close to the rupture zone are important as such when a finite fault model of rupture is constructed to characterize rupture processes and source properties. In this work, we focus on retrieving the slip distribution and temporal history of this earthquake through a joint inversion of teleseismic waveforms and near-field GPS data. We derived 12 static coseismic offsets of GPS sites in Nepal and Tibet and retrieved 5 seismograms of strong motions recorded by high-rate (1 Hz) GPS sites in Tibet. In addition, we chose a total of 43 P-wave waveforms from global seismic networks to enhance the spatiotemporal resolution of source model. The fault geometry is prescribed on a subsurface plane that is buried at 5 similar to 30 km depths with a dip of 11 degrees to the north and a strike of 295, consistent with the USGS CMT solution and structural geology. This rectangular model plane in dimensions of 210 km X 160 km was further divided into 21X8 matrix of sub-faults. The finite source modeling assumes that the rupture processes can be approximated by abrupt rise of slip on these subfaults in the wake of rupture front that passages successively through them from the hypocenter. The rupture velocity across adjacent subfauts is assumed to be a constant at 2. 5 km s(-1). For each subfault, the slip growth is represented by a source time function that is parameterized by 5 overlapping triangles with a 2 sec half-time duration, each shifted by 2 sec. Seismic moments of all triangles, each corresponding to a subevent, are unknown parameters to be solved with the non-negative least squares algorithm. The slip magnitude, rake and rise time for each subfault are derived from the estimates of the associated subevents, all together to minimize postfit residuals of the waveforms and static offsets while maintaining smoothness of seismic moment over the model plane for which a Laplace operator is used to achieve spatial regularization. Green' s functions were generated assuming a one-dimensional structure model. The frequency-wavenumber integration algorithm was used for GPS dynamic waveforms and static offsets, and a reflectivity method developed by Kikuchi for teleseismic data. The joint inversion shows that the detachment fault fails unilaterally from the hypocenter with slip extending eastward over an area of 100 km in along-strike length by 130 km in downdip width. The best-fitting model indicates that the seismic moments were released largely by thrusting motions within duration of 80 sec. In the first 40 sec, slip propagated essentially all the way to the Kathmandu. The slip model shows one major asperity between the hypocenter and Kathmandu, on which a peak slip of 4. 3 m is found at 11 km depth, 35 km away from the hypocenter. During 40 to 75 sec, the rupture extends downward to the bottom of the model plane and slip attains the local maximum at 18 km depth and 60 km away from the hypocenter. Slip of 0. 5 similar to 1. 0 m is found at 25 similar to 30 km depths beneath the Gyrong town. Slip continues also upward but stops approximately at 5 km shallow depth. The slip model does not indicate that the earthquake has broken the surface, suggesting that a significant fraction of the basal detachment fault remain locked at shallower depths. The unlocked part of the detachment fault has yielded an averaged slip of 2.4 m with a total seismic moment of 9. 4 X 10(20) N . m, which gives M-w =7. 9. If the asperity of this event corresponds to the ruptur e. zone of the 1833 M(w)7. 7 similar to 7. 8, its recurrence in the same rupture area would be every 150 similar to 200 year.