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Aron F.,Cornell University | Cembrano J.,University of Santiago de Chile | Cembrano J.,National Research Center for Integrated Natural Disasters Management | Cembrano J.,University of Los Andes, Chile | And 4 more authors.
Bulletin of the Geological Society of America | Year: 2015

We present new field structural data from the Chilean Coastal Cordillera located above the northern and central parts of the interplate contact ruptured by the A.D. 2010 Mw 8.8 Maule earthquake. The northern study area contains the northwest-striking Pichilemu normal fault, an intraplate structure reactivated after the megathrust event by crustal earthquakes up to Mw 7.0. The structural style of this region is dominated by kilometer-scale normal faults that have been active at least throughout the Quaternary. The orientations of these main faults define three structural systems: (1) northeast- and (2) northwest-striking margin-oblique faults, and (3) north- to north-northeast-striking margin-parallel faults. From north to south, these three systems vary in their predominant occurrence, starting with bimodal orientations of groups 1 and 2, followed by predominantly single north to north-northeast orientations of group 3. Reverse faults coexist in time and space with the normal structures, but are scarce and display variable, apparently random orientations. The shallow crustal normal faults, including the Pichilemu fault, show a persistent kinematic history probably spanning thousands of subduction seismic cycles. Though historically smaller in magnitude than those of the triggered normal faults, interseismic forearc thrust events were recorded above the rupture area prior to the Maule earthquake. The Quaternary reverse faults identified in our study regions may be preserving interseismic, slow-strain-rate, permanent deforma tion signature in the structural grain. Analogous observations along the A.D. 2011 Tohoku earthquake rupture in Japan imply that such a link between the short- and longterm deformation patterns of the forearc is not exclusive of the Maule earthquake region. © 2014 Geological Society of America.

Aron F.,Cornell University | Allmendinger R.W.,Cornell University | Cembrano J.,University of Santiago de Chile | Cembrano J.,National Research Center for Integrated Natural Disasters Management | And 3 more authors.
Journal of Geophysical Research: Solid Earth | Year: 2013

Geologists have long known that young normal faults are an important structural element of the Andean Coastal Cordillera, but their relationship to the subduction seismic cycle is still unclear. Some of the largest aftershocks of the 2010 Mw8.8 Maule earthquake in central Chile were nucleated on upper plate normal faults, including the Mw6.9 and 7.0 events of the Pichilemu earthquake sequence. We use the available coseismic GPS displacements, moment tensor sums, and slip distribution models for the Maule earthquake to compute the static strain and stress fields imposed on the upper plate by slip on the subduction interface. The extensional strains calculated from coseismic GPS and from a moment tensor sum of the Pichilemu events have similar orientations and orders of magnitude. The normal Coulomb stress increment (CSI) on the Pichilemu fault has maximum positive stresses as high as 4.9 MPa. Regionally, the Maule event produced a semi-elliptical, radial pattern of static extension and deviatoric tension (CSI > 1.5 MPa) along the Coastal Cordillera enclosing the rupture area. This elliptical pattern mimics the trends of the major upper-crustal structures. The static deformation field produced by a great subduction earthquake is an effective mechanism for generating permanent extension above the seismogenic zone, reactivating suitably oriented, long-lived normal faults. We suggest that the semi-elliptical outline of the first-order structures along the Coastal Cordillera may define the location of a characteristic, long-lived megathrust segment. This observation implies a persistence at least over the Quaternary of great subduction ruptures along the Maule segment. ©2012. American Geophysical Union. All Rights Reserved.

Schmelzbach C.,Free University of Berlin | Schmelzbach C.,ETH Zurich | Kummerow J.,Free University of Berlin | Wigger P.,Free University of Berlin | And 5 more authors.
Geophysical Journal International | Year: 2016

The coda of passive seismic recordings is often rich in arrivals that are coherent across several stations. If reflections can be extracted, then they may be used for seismic reflection subsurface imaging. With the objective to image the upper crust of the North Chilean Precordillera (Central Andes; approximate location 21°S 69° W), we developed a workflow to process passive seismic data into subsurface reflection images.We analysed thewaveformrecordings of several hundred microseismic events using signal processing and imaging techniques adapted from active (controlled source) seismic imaging as used in the oil industry. Key processing steps involved precise arrival time picking and hypocentre determination, removing signal amplitude variations due to varying source radiation patterns, identification and separation of reflections from coherent noise, and transformation of the processed waveforms into images of the subsurface reflectivity.When designing our microseismic reflection imaging workflow, we took advantage of the fact that the passive seismic recording geometry with the hypocentres located at depth and the receivers positioned at the surface resembles a reverse vertical-seismic profiling experiment. The resultant P- and S-wave reflection images reveal several reflective features, such as an approximate 15° westward dipping reflector over the 5-25 km depth range that largely coincides with a distinct seismicity boundary. We interpret the imaged interface as the brittle-ductile transition zone boundary, possibly enhanced by a tectonic shear zone. For the area of the North Chilean Precordillera, the deduced microseismic reflection sections with horizontal extensions of about 50 km represent the first high-resolution images of the shallow crust, which could not be obtained from previous active-source seismic-reflection data. © The Authors 2015. Published by Oxford University Press on behalf of The Royal Astronomical Society.

Bloch W.,Free University of Berlin | Kummerow J.,Free University of Berlin | Salazar P.,Católica del Norte University | Salazar P.,National Research Center for Integrated Natural Disasters Management | And 2 more authors.
Geophysical Journal International | Year: 2014

We obtained high-precision locations for 5250 earthquakes in the Iquique segment of the northern Chilean subduction zone from two temporary local seismic networks around 21°S. A double seismic zone in the downgoing Nazca slab can be clearly identified. One band of seismicity is located at the plate interface and a second one 20-25 km deeper in the oceanic mantle. It can be traced updip to uncommonly shallow levels of 50 km. A combined interpretation of seismicity and reflectivity along the seismic ANCORP'96 experiment suggests the prevalence of fluid processes in the subducted oceanic crust as well as in the uppermost 20 km of the mantle. Crustal seismicity is pervasive below the Coastal Cordillera. Beneath the Precordillera, the lower bound of crustal seismicity delineates a sharp west-dipping boundary down to 20 km depth, consistent with earlier findings indicating a rheological boundary. © The Authors 2014. Published by Oxford University Press on behalf of The Royal Astronomical Society.

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