The subduction zone in northern Chile is a well-identified seismic gap that last ruptured in 1877. The moment magnitude (Mw) 8.1 Iquique earthquake of 1 April 2014 broke a highly coupled portion of this gap. To understand the seismicity preceding this event, we studied the location and mechanisms of the foreshocks and computed Global Positioning System (GPS) time series at stations located on shore. Seismicity off the coast of Iquique started to increase in January 2014. After 16 March, several Mw > 6 events occurred near the low-coupled zone. These events migrated northward for ~50 kilometers until the 1 April earthquake occurred. On 16 March, on-shore continuous GPS stations detected a westward motion that we model as a slow slip event situated in the same area where the mainshock occurred.

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S U M M A R YThe kinematic fractal source model presented in this study is able to simulate broad-band accelerograms with spectral amplitudes proportional to a fraction of the directivity coefficient C d in the far-field approximation. This approach is based on a composite source description, where subevents are generated using a fractal distribution of sizes, and, by summation, produces k-square space distribution of the slip. Each elementary source is described as a crack-type slip model growing circularly from a nucleation point when the rupture front reaches it. In order to better control the directivity effect, the location of the nucleation point for an elementary source is assumed to be scale-dependent. For the larger sources, the nucleation point is located near the intercept of the crack with the rupture front, whereas for smaller sources, it is randomly chosen within the crack. For simplicity, a constant rupture velocity is assumed. Each subevent is set up with scale-dependent rise-time, assuming a boxcar source-time function, hence filtering out its own high frequency radiation. The resulting mean slip-velocity functions are very similar to the ones derived from dynamic rupture modelling. Ground motion synthetics are computed by convolving the slip-velocity functions with the Green's functions. It is demonstrated that, in the far-field approximation, accelerogram spectra follow the ω 2 model with amplitudes controlled by the frequency-dependent directivity effects. In particular, spectral amplitudes at high-frequencies are proportional to a fraction of C d . These results were verified for few earthquake magnitudes. In addition, a validation exercise was made in the near-fault region by modelling the complete wavefield of strong ground motion at a few receivers and for several rupture scenarios. The synthetic strong-motion parameters are compared to the ones predicted by empirical attenuation relationships. It is shown that calculated standard deviations are in good agreement with the empirical ones, as well as the ground-motion parameter amplitudes predicted as a function of distance for whole interval of source distance considered in modelling. Minor differences were found in peak ground-accelerations computed at large distance from the fault, a problem related to the simplified response of the medium.

A large seismic gap lies along northern Chile and could potentially trigger a M w * 8.8-9.0 megathrust earthquake as pointed out in several studies. The April 1, 2014, Pisagua earthquake broke the middle segment of the megathrust. Some slip models suggest that it ruptured mainly from a depth of 30 to 55 km along dip and over 180 km in length, reaching a magnitude M w 8.1-8.2. The northern and southern segments are still unbroken; thus, there is still a large area that could generate a M w [ 8.5 earthquake with a strong tsunami. To better understand the effects of source parameters on the impact of a tsunami in the near field, as a case study, we characterize earthquake size for a hypothetical and great seismic event, M w 9.0, in northern Chile. On the basis of physical earthquake source models, we generate stochastic k -2 finite fault slips taking into account the non-planar geometry of the megathrust in northern Chile. We analyze a series of random slip models and compute vertical co-seismic static displacements by adding up the displacement field from all point sources distributed over a regular grid mesh on the fault. Under the assumption of passive generation, the tsunami numerical model computes the runup along the shore. The numerical results show a maximum peak-runup of *35-40 m in the case of some heterogeneous slip models. Instead, the minimum runup along the coast, from the heterogeneous slip models tested, almost coincides with the runup computed from the uniform slip model. This latter assumption underestimates the runup by a factor of *6 at some places along the coast, showing agreement with near-field runups calculated by other authors using similar methodologies, but applied in a different seismotectonic context. The statistical estimate of empirical cumulative distribution functions conducted on two subsets 123Nat Hazards (2015) 79:1177-1198 DOI 10.1007/s11069-015-1901 of slips, and their respective runups, shows that slip models with large amount of slip near the trench are more probable to produce higher runups than the other subset. The simple separation criterion was to choose slip models that concentrate at least 60 % of the total seismic moment in the upper middle part of the non-planar rupture fault.

The Mw 8.8 megathrust earthquake that occurred on 27 February 2010 offshore the Maule region of central Chile triggered a destructive tsunami. Whether the earthquake rupture extended to the shallow part of the plate boundary near the trench remains controversial. The up-dip limit of rupture during large subduction zone earthquakes has important implications for tsunami generation and for the rheological behavior of the sedimentary prism in accretionary margins. However, in general, the slip models derived from tsunami wave modeling and seismological data are poorly constrained by direct seafloor geodetic observations. We difference swath bathymetric data acquired across the trench in 2008, 2011 and 2012 and find ~3–5 m of uplift of the seafloor landward of the deformation front, at the eastern edge of the trench. Modeling suggests this is compatible with slip extending seaward, at least, to within ~6 km of the deformation front. After the Mw 9.0 Tohoku-oki earthquake, this result for the Maule earthquake represents only the second time that repeated bathymetric data has been used to detect the deformation following megathrust earthquakes, providing methodological guidelines for this relatively inexpensive way of obtaining seafloor geodetic data across subduction zone.