A long foreshock series unlocked the South American plate boundary until eventually initiating the M 8.1 Iquique, Chile, earthquake. Two groups publishing in this issue of Nature analyse the seismic context of the Iquique earthquake that occurred off the coast of northern Chile on 1 April 2014 in a seismic zone that had been quiescent since a significant event in 1877. Gavin Hayes et al. identify areas of remaining or elevated earthquake hazard along the megathrust fault in the region, and conclude that the 2014 Iquique event was not the earthquake that had been anticipated. Given that significant sections of the northern Chile subduction zone have not ruptured in almost 150 years, they suggest that it is likely that future megathrust earthquakes will occur south and potentially north of the 2014 Iquique sequence. Bernd Schurr et al. show that the April 2014 earthquake broke a central fraction of the 'northern Chile seismic gap', the last major segment of the South American plate boundary that had yet to rupture in the past century. From July 2013 up to the April earthquake they identify three seismic clusters along this part of the plate boundary, each lasting a few weeks, with earthquakes of increasing peak magnitudes. They conclude that these seismic clusters and their slip transients reflect a gradual weakening of the central part of the seismic gap that was instrumental in initiating the final failure. On 1 April 2014, Northern Chile was struck by a magnitude 8.1 earthquake following a protracted series of foreshocks. The Integrated Plate Boundary Observatory Chile monitored the entire sequence of events, providing unprecedented resolution of the build-up to the main event and its rupture evolution. Here we show that the Iquique earthquake broke a central fraction of the so-called northern Chile seismic gap, the last major segment of the South American plate boundary that had not ruptured in the past century1,2. Since July 2013 three seismic clusters, each lasting a few weeks, hit this part of the plate boundary with earthquakes of increasing peak magnitudes. Starting with the second cluster, geodetic observations show surface displacements that can be associated with slip on the plate interface. These seismic clusters and their slip transients occupied a part of the plate interface that was transitional between a fully locked and a creeping portion. Leading up to this earthquake, the b value of the foreshocks gradually decreased during the years before the earthquake, reversing its trend a few days before the Iquique earthquake. The mainshock finally nucleated at the northern end of the foreshock area, which skirted a locked patch, and ruptured mainly downdip towards higher locking. Peak slip was attained immediately downdip of the foreshock region and at the margin of the locked patch. We conclude that gradual weakening of the central part of the seismic gap accentuated by the foreshock activity in a zone of intermediate seismic coupling was instrumental in causing final failure, distinguishing the Iquique earthquake from most great earthquakes. Finally, only one-third of the gap was broken and the remaining locked segments now pose a significant, increased seismic hazard with the potential to host an earthquake with a magnitude of >8.5.

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Gradual unlocking of plate boundary controlled initiation of the 2014 Iquique earthquake

The bearing capacity and failure mechanism of a vertically loaded strip footing placed on the top of slopes

Site amplification effects of a radially multi-layered semi-cylindrical canyon on seismic response of an earth and rockfill dam

Face stability analysis of a shallow horseshoe-shaped shield tunnel in clay with a linearly increasing shear strength with depth

New formulation of Cholesky decomposition and applications in stochastic simulation

The series solution of wave functions for 2D scattering and diffraction of plane SH (shear horizontal) waves induced by a U‐shaped canyon is proposed herein to account for the topographic effect of such a canyon. The wave function expansion method has been frequently employed to study the topographic effect because it can reveal the physics of the wave scattering and can test the accuracy of other methods. Through a new domain decomposition strategy, the half‐space having a U‐shaped canyon is divided into three subregions. Hence, we defined three cylindrical coordinate systems. In each coordinate system, the wave field satisfying the Helmholtz equation was represented by means of the separation of variables method, in terms of the series of both Bessel functions and Hankel functions with unknown complex coefficients. Then three wave fields are all represented in the same coordinate system using the Graf addition theorem. The unknown coefficients are solved by satisfying the continuity conditions of the auxiliary boundary and the traction‐free boundary conditions on the bottom of the canyon. To show the effects of symmetrical and nonsymmetrical U‐shaped canyons on the surface ground motion, a parametric analysis is carried out in the frequency domain. Surface and subsurface transient responses in the time domain demonstrate the phenomenon of wave propagating and scattering. It is found that a zone of amplification can obviously take place at the bottom of a U‐shaped canyon with nearly vertical walls.

Effects of Topographic Amplification Induced by a U‐Shaped Canyon on Seismic Waves

SUMMARY

Topographies have significant effects on seismic waves. The wavefunction expansion method has been frequently employed to study the topographic effect because this method can reveal the physics of the wave scattering and can testify the accuracy of numerical methods. The 2-D scattering and diffraction of plane SH waves induced by a non-symmetrical V-shaped canyon is examined here by using the wavefunction expansion method. Through a new domain decomposition strategy, the half-space containing a V-shaped canyon is divided into three subregions. Then the wavefield for every subregion is constructed in terms of an infinite series of wavefunctions with unknown coefficients in three coordinate systems, respectively. After that, three wavefields are all represented in the same coordinate system using the Graf addition theorem. The unknown coefficients are obtained by satisfying the continuity conditions of the auxiliary boundary. The proposed series solution of wavefunctions for a non-symmetrical V-shaped canyon can be reduced to a symmetrical case. Results in this study for the symmetrical case agree very well with those in the published literature. To show the effects of the non-symmetrical-geometry topography on the surface ground motion, a parametric study is carried out in the frequency domain. Surface and subsurface transient responses in the time domain demonstrate the phenomenon of wave propagating and scattering. Finally, the proposed model is successfully applied to two idealized cross sections of Pacoima canyon and Feitsui canyon.

Scattering of SH waves induced by a non-symmetrical V-shaped canyon

An exact solution for SH-wave scattering by a radially multilayered inhomogeneous semicylindrical canyon

A three-dimensional (3D) kinematically admissible rotational failure mechanism is extended from toe failure to include face failure and base failure for homogeneous slopes in both purely cohesive and frictional/cohesive soils. In the strict framework of limit analysis, an analytical approach is derived to obtain the upper bounds on slope stability and the corresponding type of the critical failure mechanism. Compared with the available results from the finite-element limit-analysis method, the 3D rotational failure mechanisms give the best estimate on the upper bound. A set of stability charts is presented in a wide range of parameters for 3D homogeneous slopes under both static and pseudostatic seismic loading conditions. This set is useful in assessing the safety of the 3D homogeneous slopes in practical applications.

Ning Zhang

Papers

Effects of Topographic Amplification Induced by a U‐Shaped Canyon on Seismic Waves

Scattering of SH waves induced by a non-symmetrical V-shaped canyon

Site amplification effects of a radially multi-layered semi-cylindrical canyon on seismic response of an earth and rockfill dam

An exact solution for SH-wave scattering by a radially multilayered inhomogeneous semicylindrical canyon

Stability Charts for 3D Failures of Homogeneous Slopes