Felipe Leyton

Intense foreshocks and a slow slip event preceded the 2014 Iquique Mw 8.1 earthquake

The earthquake that rocked northern Chile Subduction zones often produce the largest earthquakes on Earth. A magnitude 8.2 earthquake (Iquique) occurred in one such zone off the coast of northern Chile on 1 April 2014, in a seismic gap that had not experienced a large earthquake since the 9.0 one in 1877. Ruiz et al. analyzed continuous GPS data to monitor the movement of plates over time in this region, including before and after major earthquakes. The most recent large quake was preceded by an extended series of smaller earthquakes and creeping westward movement of the coastline. Science, this issue p. 1165 The intense and anomalous seismicity preceding the mainshock was the final step of a slow slip event. 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.

Inner‐core fine‐scale structure from scattered waves recorded by LASA

[1] Recent observations of inner-core scattering (ICS) waves provide evidence that the outermost 300 km of the inner-core has strong heterogeneities with a length scale of a few kilometers. These waves follow a path similar to that of the inner-core–reflected waves PKiKP and were originally observed in data from 16 events in the distance range 58 to 73 recorded by the Large Aperture Seismic Array (LASA). Here we present additional observations of the ICS waves from a total of 78 events recorded by LASA at distances from 18 to 98. We use a modified version of the Generic Array Processing software package to identify ICS waves on the basis of travel time, back azimuth, ray parameter, amplitude, and coherence. There are 44 events that produce clear ICS waves. We then perform forward modeling of the observed ICS waves using a Monte Carlo seismic phonon method that allows for multiple scattering along the raypath. Most of the ICS waves appear without a visible PKiKP phase, initially grow in time, and have a spindleshaped envelope. The duration, risetime, and decay rates of the observed ICS waves can be best explained by small-scale volumetric heterogeneities in the outermost few hundred kilometers of the inner core. The average Qc value for the 44 events is � 600. Most clear ICS waves are found for raypaths sampling the Pacific Ocean and Asia, and relatively few observations are from the Atlantic Ocean, roughly consistent with the recently observed hemispheric difference in the inner-core structure.

Reawakening of large earthquakes in south central Chile: The 2016 Mw 7.6 Chiloé event

On 25 December 2016, the Mw 7.6 Chiloe earthquake broke a plate-boundary asperity in South- Central Chile near the center of the rupture zone of the Mw 9.5 Valdivia earthquake of 1960. To gain insight on decadal-scale deformation trends and their relation with the Chiloe earthquake, we combine geodetic, teleseismic and regional seismological data. GPS velocities increased at continental scale after the 2010 Maule earthquake, probably due to a readjustment in the mantle flow and an apparently abrupt end of the viscoelastic mantle relaxation following the 1960 Valdivia earthquake. It also produced an increase in the degree of plate locking. The Chiloe earthquake occurred within the region of increased locking, breaking a circular patch of ~15 km radius at ~30 km depth, located near the bottom of the seismogenic zone. We propose that the Chiloe earthquake is a first sign of the seismic reawakening of the Valdivia segment, in response to the interaction between postseismic viscoelastic relaxation and changes of interseismic locking between Nazca and South-America.

Empirical Site Classification of CSN Network Using Strong‐Motion Records

The National Seismological Center of the University of Chile (CSN, Centro Sismologico Nacional) has been operating more than 400 seismic stations throughout the country. The data collected from this network, from March 2012 to August 2017, includes more than 4000 Chilean strong‐motion records, from more than 1000 events (magnitudes ranging from 4.0 up to 8.3). In this study, we use this data set and classify 118 stations from this network, using the horizontal‐to‐vertical response spectral ratio (HVRSR) of strong‐motion records. This classification considers not only the predominant period obtained from the average HVRSR but also the peak amplitude from this curve. The results indicate no correlation between the two parameters, despite the common practice of combining all curves with similar predominant periods. Even more, we believe that relevant information of the site’s impedance contrast between the soil and bedrock is lost in the process of averaging HVRSR curves from different stations.

Geophysical Characterization of the Chilean Seismological Stations: First Results

The Chilean Seismological Network has been rapidly growing in recent years, going from a few dozens stations working before 2010 to nearly a 100 installed all over the territory. Even more, nearly 300 strong-motion stations from the Accelerographic National Network have recently complemented this network, mainly deployed in large cities, at a variety of site conditions. All of these stations are currently providing useful information of Chilean earthquakes and are expected to record moderate-to-large events. However, the lack of appropriate site characterization sets an important limit to its usefulness. In this work, we present the geophysical characterization of 163 stations, the first results of larger effort to complete the characterization at all sites, based on array measurements of microtremors and horizontal-to-vertical spectral ratios. This information will help improve our understanding of the dynamic behavior of soils during earthquakes, providing relevant information for seismic design and seismic codes. Electronic Supplement: Figures showing distribution of the geophones, and table with nearly 400 seismic stations managed by the Centro Sismológico Nacional of Universidad de Chile (CSN), along with their location and, the average S-wave velocity in the upper 30 m and the horizontal-to-vertical spectral ratio.

Nucleation Phase and Dynamic Inversion of the Mw 6.9 Valparaíso 2017 Earthquake in Central Chile

The Valparaiso 2017 sequence occurred in the Central Chile mega-thrust, an active zone where the last mega-earthquake occurred in 1730. Intense seismicity started 2 days before the Mw 6.9 main-shock, a slow trench-ward movement was observed in the coastal GPS antennas and was accompanied by foreshocks and repeater-type seismicity. To characterize the rupture process of the main-shock, we perform a dynamic inversion using the strong-motion records and an elliptical patch approach. We suggest that a slow slip event preceded and triggered the Mw 6.9 earthquake, which ruptured an elliptical asperity (semi-axis of 10 km and 5 km, with a sub-shear rupture, stress drop of 11.71 MPa, yield stress of 17.21 MPa, slip weakening of 0.65 m and kappa value of 1.98). This earthquake could be the beginning of a long-term nucleation phase to a major rupture, within the highly coupled Central Chile zone where a mega-thrust earthquake like 1730 is expected.

The 2014 Earthquake in Iquique, Chile: Comparison between Local Soil Conditions and Observed Damage in the Cities of Iquique and Alto Hospicio

In this study, we present comparisons between the local soil conditions and the observed damage for the 1 April 2014 Iquique earthquake. Four cases in which site effects may have played a predominant role are analyzed: (1) the ZOFRI area, with damage in numerous structures; (2) the port of Iquique, in which one pier suffered large displacements; (3) the Dunas I building complex, where soil-structure interaction may have caused important structural damage; and (4) the city of Alto Hospicio, disturbed by the effects of saline soils. Geophysical characterization of the soils was in agreement with the observed damage in the first three cases, while in Alto Hospicio the earthquake damage cannot be directly related to geophysical characterization.

Focal Mechanism, Magnitude, and Finite‐Fault Rapid Estimation Using the Elliptical Patch Method in Chile

Nowadays, most fast, automatic algorithms estimate the magnitude and location of earthquakes assuming a point-source approximation that might produce large errors in the predicted shaking. In this study, using a kinematic approach, we modeled the fault plane as an elliptical patch with a Gaussian slip distribution to perform a rapid estimation of the finite fault, focal mechanism, and moment magnitude. We implemented this finite-fault kinematic inversion using data from the Centro Sismológico Nacional of the Universidad de Chile. We used near-field data, up to 600 km from the epicenter, mostly from accelerograms and a few nonsaturated broadband records. By considering data up to 5 min from the origin time, we are able to retrieve relevant source parameters within 10–15 min from the occurrence of the event. This elliptical patch method (EPM) successfully resolves the source parameters for 45 moderate-to-large events, from 2013 to 2017 (magnitudes ranging from 6.0 to 8.3). These results encourage the use of the EPM in the rapid response to moderate-to-large earthquakes in subduction zones. Electronic Supplement: Figures showing results of an intermediate depth normal event (11 April 2016 16:20:45 UTC, Mw 6.4) processed with the elliptical patch method (EPM) and tables of models used to compute Green’s functions used in this study and model parameters for 45 events.

The Chilean GNSS Network: Current Status and Progress toward Early Warning Applications

Chile is one of the world’s most seismically active regions and is therefore extensively studied by the earthquake sciences. The great length of the country hosts a variety of measurement systems allowing for the characterization of earthquake processes over a wide range of timescales and in different phases of the seismic cycle. Starting in the early 1990s, several research groups began to deploy continuously operating geodetic networks in Chile, forming the core of the modern network of Global Navigation Satellite Systems (GNSS) receivers used to monitor geodynamics from the southern tip of the Americas to the central Andes. Today, the Centro Sismologico Nacional (CSN) of the Universidad de Chile maintains and improves this network, increasing its coverage and spatial density while greatly reducing solution latency. We present the status of the GNSS network, its data streams, and the real‐time analysis system used to support real‐time modeling of earthquakes. The system takes 2 s, on average, to collect raw data, estimate positions, and stream results. Such low latency is essential to enabling early warning of earthquakes and tsunamis in Chile.

A preliminary study of seismic microzonation of Concepción based on microtremors, geology and damages patterns

Fecha de entrega: 18 de enero 2012 Fecha de aceptacion: 5 de junio 2012 We perform microtremors measurements at Concepcion and compute the predominant frequency using horizontal-to-vertical spectral ratio (HVSR). We compare these results with the surface geology and several geotechnical surveys existing in the region, enabling a general characterization of the area. We present and complement these results with observations of damage produced by the M w 8.8 Maule 2010 earthquake. Preliminary results show the presence of fine-grained materials in the area characterized by very low predominant frequency (lower than 1.5 Hz), which might explain the extensive damage observed at Concepcion and surroundings.