ALBERT

Description: In 2007 a M7.7 earthquake occurred near the town of Tocopilla within the northern Chile seismic gap. Main shock slip, derived from coseismic surface deformation, was confined to the depth range between 30 and 55 km. We relocated ∼1100 events during six months before and one week after the main shock. Aftershock seismicity is first congruent to the main shock slip and then it spreads offshore west and northwest of Mejillones Peninsula (MP). Waveform modeling for 38 aftershocks reveals source mechanisms that are in the majority similar to the main shock. However, a few events appear to occur in the upper plate, some with extensional mechanisms. Juxtaposing the Tocopilla aftershocks with those following the neighboring 1995 Antofagasta earthquake produces a striking symmetry across an EW axis in the center of MP. Events seem to skirt around MP, probably due to a shallower Moho there. We suggest that the seismogenic coupling zone in northern Chile changes its frictional behavior in the downdip direction from unstable to mostly conditionally stable. For both earthquake sequences, aftershocks agglomerate in the conditionally stable region, whereas maximum inter-seismic slip deficit and co-seismic slip occurs in the unstable region. The boundary between the unstable and conditionally stable zones parallels the coastline. We identify a similar segmentation for other earthquakes in Chile and Peru, where the offshore segments break in great M 〉 8 earthquakes, and the onshore segments in smaller M 〈 8 earthquakes. Using critical taper analysis, we demonstrate a causal relationship between varying slip behavior on the interface and forearc wedge anatomy that can be attributed to spatial variations in the rate-dependency of friction.

Description: Abstract Several great earthquakes occur on thrust faults along both subduction and continental collision zones. These events often feature a large shallow slip patch, an asymmetric pattern for the ground motion, and the static deformation between the hanging wall and footwall of the fault. From a mechanical point of view, this asymmetry can be partially explained taking into account the interaction between the fault and the seismic radiation emitted during rupture propagation and stored in the hanging wall in the vicinity of the free surface. We numerically investigate the rupture dynamics along a thrust dipping fault impacting onto the free surface at a dip angle of δ = 20°, in a 2‐D elastic model. We show how the wave interaction of the rupture with the free surface leads to a breaking of the reflection symmetry. Compared to a rupture propagating in an infinite medium, this interaction enhances the slip rate in the updip direction as an effect of the coupling between slip and normal traction around the crack front. The breaking of symmetry leads to sizeable acceleration of the rupture toward asymptotic speed with inertia acquisition, and dependence of the rupture dynamics on the level of friction along the interface might produce an interface opening over a finite length in the vicinity of the surface. We finally explore how the wave interaction drives amplification and asymmetry of the shallow slip and the vertical displacement at the surface. The described effects should be considered in various numerical approaches and in interpretation of geophysical observations.

Description: Large earthquakes produce crustal deformation that can be quantified by geodetic measurements, allowing for the determination of the slip distribution on the fault. We used data from Global Positioning System (GPS) networks in Central Chile to infer the static deformation and the kinematics of the 2010 moment magnitude (M(w)) 8.8 Maule megathrust earthquake. From elastic modeling, we found a total rupture length of ~500 kilometers where slip (up to 15 meters) concentrated on two main asperities situated on both sides of the epicenter. We found that rupture reached shallow depths, probably extending up to the trench. Resolvable afterslip occurred in regions of low coseismic slip. The low-frequency hypocenter is relocated 40 kilometers southwest of initial estimates. Rupture propagated bilaterally at about 3.1 kilometers per second, with possible but not fully resolved velocity variations.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Vigny, C -- Socquet, A -- Peyrat, S -- Ruegg, J-C -- Metois, M -- Madariaga, R -- Morvan, S -- Lancieri, M -- Lacassin, R -- Campos, J -- Carrizo, D -- Bejar-Pizarro, M -- Barrientos, S -- Armijo, R -- Aranda, C -- Valderas-Bermejo, M-C -- Ortega, I -- Bondoux, F -- Baize, S -- Lyon-Caen, H -- Pavez, A -- Vilotte, J P -- Bevis, M -- Brooks, B -- Smalley, R -- Parra, H -- Baez, J-C -- Blanco, M -- Cimbaro, S -- Kendrick, E -- New York, N.Y. -- Science. 2011 Jun 17;332(6036):1417-21. doi: 10.1126/science.1204132. Epub 2011 Apr 28.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Laboratoire de Geologie de l'Ecole Normale Superieure, UMR CNRS 8538, Paris, France. vigny@biotite.ens.fr〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/21527673" target="_blank"〉PubMed〈/a〉

Description: 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. 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.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Schurr, Bernd -- Asch, Gunter -- Hainzl, Sebastian -- Bedford, Jonathan -- Hoechner, Andreas -- Palo, Mauro -- Wang, Rongjiang -- Moreno, Marcos -- Bartsch, Mitja -- Zhang, Yong -- Oncken, Onno -- Tilmann, Frederik -- Dahm, Torsten -- Victor, Pia -- Barrientos, Sergio -- Vilotte, Jean-Pierre -- England -- Nature. 2014 Aug 21;512(7514):299-302. doi: 10.1038/nature13681. Epub 2014 Aug 13.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉GFZ Helmholtz Centre Potsdam, German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany. ; School of Earth and Space Sciences, Peking University, Beijing 100871, China. ; Centro Sismologico National, Universidad de Chile, Facultad de Ciencias Fisicas y Matematicas, Blanco Encalada 2002, Santiago, Chile. ; Institut de Physique du Globe de Paris, 1, rue Jussieu, 75238 Paris cedex 05, France.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/25119049" target="_blank"〉PubMed〈/a〉