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Author(s): Bernd Schurr [sup.1] , Günter Asch [sup.1] , Sebastian Hainzl [sup.1] , Jonathan Bedford [sup.1] , Andreas Hoechner [sup.1] , Mauro Palo [sup.1] , Rongjiang Wang [sup.1] , Marcos [...], A long foreshock series unlocked the South American plate boundary until eventually initiating the M 8.1 Iquique, Chile, earthquake. 2014 Iquique earthquake examined 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 century.sup.1,2. 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.

[1] We present a quantitative reconstruction of uplift of the western flank of the Altiplano plateau (central Andes), one of the largest monoclines on the Earth, on the basis of an analysis of tectonic structures, syntectonic deposits, and geophysical data. Uplift occurred on a west vergent, slowly propagating system of high-angle reverse faults merging into a joint detachment that ramps down to midcrustal levels below the plateau edge. The upper ramps determine local fold geometries while the lower ramp controls large-scale surface tilting and uplift. At 20[degrees]S, this fault system was active between ~30 Ma and 5-10 Ma, with maximum shortening rates of 0.22 mm/yr between 17 and 10 Ma. It generated some 2600 m of surface uplift with only minor shortening of ~3000 m. Its activity was largely synchronous to eruption of large-volume ignimbrites from a midcrustal source. Geophysical data indicate that the fault system localized deformation at the boundary between a cool, strong forearc crust and a presumably fluid-rich and/or partially molten zone underneath the plateau area. The systematic relation between crustal melting and shortening with uplift at the western plateau margin can be traced along most of the plateau flank, with a stepwise decrease in age of deformation and magmatism toward the south indicating discontinuous addition of plateau segments. Crustal thickening to as much as 70 km from westward underthrusting in the back arc parts of the plateau isostatically compensated the tectonic surface uplift and monocline formation with respect to a stable forearc, which only reacted with minor tilting. INDEX TERMS: 1035 Geochemistry: Geochronology: 8107 Tectonophysics: Continental neotectonics; 8015 Structural Geology: Local crustal structure; 9360 Information Related to Geographic Region: South America; 9604 Information Related to Geologic Time: Cenozoic; KEYWORDS: Andes, plateau formation. Citation: Victor, P., O. Oncken, and J. Glodny (2004). Uplift of the western Altiplano plateau: Evidence from the Precordillera between 20[degrees] and 21[degrees]S (northern Chile), Tectonics, 23, TC4004, doi:10.1029/2003TC001519.

Surface rupturing earthquakes occur in a wealth of plate tectonic settings and pose important hazard to nearby cities and infrastructure. Nevertheless mechanical models to describe the slip behavior and displacement accumulation pattern of the shallow fault zone are contradictory. Recent observations suggest that, despite low confining pressure and phyllosilicate rich fault gouges at shallow crustal levels, faults can exhibit velocity weakening behavior all the way to the surface. We use continuous time series from the IPOC Creepmeter Array (www.ipoc-network.org/observatory/creepmeter/creepmeter-at-ipoc/) in N-Chile, monitoring 4 segments of the Atacama Fault System, to investigate the slip mode of the different fault segments.This instrumental dataset covers a time span of 8 years at 11 monitoring sites and comprises both triggered shallow slip events and creep signals. The recorded shallow slip events range from μm to mm scale in fault parallel displacement. Slip velocities for these events range between 1 -2000 mm/year therefore representing slow slip events. In fact, the current sampling rate of 2 samples/min does not allow for the detection of seismic velocities for the size of the so far recorded slip events.Decomposing the time series at the different monitored fault segments we are able to differentiate between slow creeping fault segments not accumulating displacement by triggered shallow slip events and others which are accumulating displacement solely by triggered shallow slip events. Comparing these observations with the fault gouge thickness at the monitored sites, we find a strong correlation of segments exhibiting 10 cm thick fault gouge at the monitoring site and creeping fault behavior. Whereas these fault segments, accumulating displacement only via triggered shallow slip events, do not have a fault gouge developed. Instead those fault contacts are developed in a strongly fragmented basement/basement contact zone or show a submillimeter thin sharp contact in alluvial fan sediment. These first results demonstrate the ability to extract fault frictional behavior from time series collected with the IPOC Creep Observatory, that match fault gouge properties studied in the field directly at the monitoring sites. [ABSTRACT FROM AUTHOR]