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.

S U M M A R YThe aftershock sequence of the Spitak earthquake, as recorded by a dense portable network deployed around the source region, is analysed in order to obtain a precise description of the mechanics of the rupture. A collection of 708 well-recorded events corresponding to a period of about two weeks is studied, their hypocentres are accurately located, and their focal mechanisms are calculated individually as well as by a joint procedure that permits us to estimate the stress regime.The epicentral distribution of these aftershocks indicates that the fault at depth extends well beyond the surface rupture towards the west and northwest. Neotectonic and seismic observations permit us to identify five segments broken during the earthquake. The southeastern one, oriented N140", corresponds to the surface ruptures along the Alavar right lateral shear fault. The main surface ruptures, between Spitak and Gekhasar, correlate well with aftershocks showing a N120" trending fault surface, dipping 50" to the NE and acting as a thrust with a right lateral component. Two similar segments towards the west, are offset and hidden under active folds. A fifth segment, towards the NW, corresponds to a right lateral blind shear fault buried in depth, in the vicinity of the large Pambak-Sevan fault. 14 vertical sections across the fault, including focal mechanisms, illustrate these features.A relocation of the main shock and aftershocks for the period before the installation of the portable network, confirms the spatial extent of the seismicity. The foreshock, the main shock and the strongest aftershock (4min 20s after the main shock) have approximately the same epicentre. Thus the rupture started at the crossing of the Pambak-Sevan and Alavar faults and propagated bilateraly from there, although the seismic moment associated to the Alavar branch is only about one sixth of the total moment.The stress regime is one of triaxial compression with a o1 axis oriented N344". The same orientation is given by the microtectonic observations made on the central segment of the surface ruptures, a result that agrees with previous estimates for the Georgian Caucasus.

In many cases, it takes several minutes after an earthquake to publish online a seismic location with confidence. Via monitoring for specific types of increased website, app, or Twitter usage, crowdsourced detection of seismic activity can be used to “seed” the search in the seismic data for an earthquake and reduce the risk of false detections, thereby accelerating the publication of locations for felt earthquakes. We demonstrate that this low-cost approach can work at the global scale to produce reliable and rapid results. The system was retroactively tested on a set of real crowdsourced detections of earthquakes made during 2016 and 2017, with 50% of successful locations found within 103 s, 76 s faster than GEOFON and 271 s faster than the European-Mediterranean Seismological Centre’s publication times, and 90% of successful locations found within 54 km of the final accepted epicenter.

Artículo de publicación ISIWe studied the correlation between the final event magnitude and four parameters obtained from the early portion of P and S phases for a set of high quality subduction events. These relationships are used in the framework of earthquake earlywarning systems for real-time magnitude estimation. The investigated parameters are the low-pass-filtered peak displacement (PD), the integral of the velocity squared (IV2), and the predominant and characteristic periods (τp and τc). We created a dataset from the continuous records of the first two weeks following the 14 November 2007Mw 7.8 Tocopilla (Chile) earthquake. The dataset includes 69 events with magnitudes greater than 4, among them the main event (Mw 7.8), the main aftershocks ofMw 6.7 occurred on November 15, and 4 events with magnitude greater than 6. The low-pass-filtered PD read on short P-phase and S-phase windows is well correlated with the finalmagnitude, confirming previous results. Indeed when examining 2-s time windows of P waves, we did not observe any saturation effect for magnitudes greater than 6.5; however, there is a slope change in the regression curve. A similar result is obtained from the integral of squared velocity computed over short windows around P and S waves. The characteristic and predominant periods are correlated with magnitudes up to Mw 6; but they clearly do not scale with the magnitude for the stronger events. Our observations offer insight into the feasibility of an early-warning system in Chile