Several major earthquakes (Mw>7) have occurred in this gap since 1850 (Fig. 1); the largest until now was the Mw 7.7 Tocopilla earthquake in 2007, which broke the southern rim of this segment beneath and north of Mejillones Peninsula along a total length of 150 km. Only the downdip end of the locked zone slipped in this event, and the total slip in the rupture area was less than 2.6 m 6,7 leaving most of the past slip deficit of c. 8-9 m accumulated since 1877 3 approaches. First, we performed waveform modelling of local strong motion seismograms and teleseismic body waves to constrain the kinematic development of the rupture towards the final displacement in a joint inversion with continuous GPS data of static displacements (Fig. 1, 2a). Second, we use the backprojection technique applied to stations in North America to map the radiation of high frequency seismic waves (HFSR; 1-4 Hz) 9,10 . The latter technique is not sensitive to absolute slip amplitudes, but rather to changes in slip and rupture velocity.During the first 35-40s the rupture propagated downdip with increasing velocity, nearly reaching the coastline (Fig. 2a,b). Surprisingly, towards the end of the rupture, the area near the epicenter was reactivated. In spite of the relatively complicated kinematic history of the rupture the cumulative slip shows a simple 'bull's eye' pattern with a peak coseismic slip of (Fig. 3a). The Iquique main shock nucleated at the 4 northwestern border of a locked patch and ruptured towards its center (Fig. 2a, 3a). The downdip end of the main shock as well as for the large Mw 7.6 aftershock rupture mapped both by the HFSR and co-seismic slip agrees quite accurately with the downdip end interseismic coupling (Fig. 2a,c 3a). The accelerated downdip rupture propagation for both earthquakes closely followed the gradient towards higher locking. Therefore, the Iquique event and its largest aftershock appear to have broken the central, only partly locked segment of the Northern Chile Southern Peru seismic gap releasing part of the slip deficit accumulated here since 1877 (cf. Fig. 1).The seismicity before the Iquique earthquake also concentrates in this zone of intermediate locking at the fringe of the highly locked -high slip patch (Fig. 3a). Starting in July 2013, three foreshock clusters with increasingly larger peak magnitudes and cumulative seismic moment occurred here (Fig. 2c, 3a,c). The mainshock rupture started at the northern end of the foreshock zone, inside the region of intermediate locking (Fig. 2c, 3a). Interestingly, the second foreshock cluster (January 2014) is associated with a weak transient deformation, whereas the third cluster (March 2014) shows a very distinct transient signal. GPS displacement vectors calculated over the times spanning these foreshock clusters point towards the cluster epicentres (Extended Data Figure 4). Deformation for both transients is entirely explained by the cumulative coseismic displacement of the respective foreshock clusters (Fig. 3d inset, Extended Data Figure 4). The ar...

. (2006): PSGRN/PSCMP -a new code for calculating co-and post-seismic deformation, geoid and gravity changes based on the viscoelasticgravitational dislocation theory.

S U M M A R YRecently, the S receiver function method has been successfully developed to identify upper mantle interfaces. S receiver functions have the advantage of being free of S-wave multiple reflections and can be more suitable than P receiver functions for studying mantle lithosphere. However, because of specific ray geometry and interference of diverse phases, the S receiver function method has some technical difficulties and limitations. We use synthetic seismograms to demonstrate the feasibility and limitations of S receiver functions for studying mantle structures. Full-wavefield seismograms were calculated using the reflectivity method and processed to generate synthetic S receiver functions for S, SKS and ScS waves. Results show that S receiver functions can be obtained from waveforms of S, SKS and ScS waves. The synthetic S receiver functions for these incident waves show S-to-P converted phases at all discontinuities in the crust and upper mantle. Useful ranges of epicentral distances for calculation of S receiver functions are: 55 • -85 • for S, >85 • for SKS and 50 • -75 • for ScS waves. We apply both the S and P receiver function methods to data recorded at broadband station YKW3 in Northwest Canada. The study shows that there is significant agreement among different receiver function methods, and demonstrates the usefulness of S receiver functions for imaging the mantle lithosphere.

We present a fast, powerful numerical scheme to compute poroelastic solutions for excess pore pressure and displacements in a multilayered half‐space. The solutions are based on the mirror‐image technique and use an extension of Haskell's propagator method. They can be applied to assess in‐situ formation parameters from the surface deformation field when fluids are injected into or extracted from a subsurface reservoir, or they can be used to simulate changes in pore‐fluid pressure resulting from matrix displacements induced by an earthquake. The performance of the numerical scheme is tested through comparison with observations of the surface deformation as recorded by tiltmeters in the vicinity of an iteratively pumped well. Modeling of near‐surface tilt data around a productive well is useful in constraining hydraulic diffusivity in the layered subsurface.

Large tectonic earthquakes lead to significant deformations in the months and years thereafter. These so-called post-seismic deformations include contributions mainly from afterslip and viscoelastic relaxation, quantification of their relative influence is of importance for understanding the evolution of post-seismic crustal stress, strain and aftershocks. Here, we investigate the post-seismic deformation processes following the 2011 M w 9.0 Tohoku earthquake using surface displacement data as observed by the onshore global positioning system network in the first ∼1.5 yr following the main shock. We explore two different inversion modelling strategies: (i) we simulate pure afterslip and (ii) we simulate the combined effect of afterslip and viscoelastic relaxation. By assuming that the afterslip is solely responsible for the observed post-seismic deformation, we find most afterslip activities to be located close to the downdip area of the coseismic rupture at 20-80 km depth with a maximum cumulative slip of ∼3.8 m and a seismic moment of 2.3 × 10 22 Nm, equivalent in moment to an M w 8.84 earthquake. By assuming a combination of afterslip and viscoelastic components, the best data fit is found for an afterslip portion that is spatially consistent with the pure afterslip model, but reveals a decreased seismic moment of 2.1 × 10 22 Nm, or M w 8.82. In addition, the combined model suggests an effective thickness of the elastic crust of ∼50 km overlying an asthenosphere with a Maxwell viscosity of 2 × 10 19 Pa s. Temporal analysis of our model inversions suggests that the rate of afterslip rapidly decreases with time, consistent with the state-and rate-strengthening frictional law. The spatial pattern of afterslip coincides with the locations of aftershocks, and also with the area of coseismically increased Coulomb failure stress (CFS). Only a small part of the coseismically increased CFS was released by the afterslip in 564 d after the event. The effect of the viscoelastic relaxation within this initial stage only plays a secondary role, but it shows an increasing tendency, that is, the contribution of viscoelastic relaxation increases with time. Further geodetic observations are needed for a robust quantification of the role of the viscoelastic relaxation in the post-seismic deformation.

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