Low-angle normal faults (i.e. with a dip < 30°) were assumed to have a very low seismic potential (Sibson et al., 1985). However, several observations have shown that earthquakes and aseismic slip can occur along such faults. For instance, the Alto Tiberina Fault (ATF), a 60-km long normal fault with a 15° low angle dip located in the active sector of the Northern Apennines (Italy), is seismically active as well as is actively accommodating part of the Apennines extensional strain. However, the relative contribution of seismic and aseismic slip on it is still unclear. The central and northern Apennines experienced several seismic sequences in the recent decades and a Mw ∼ 4.6 aseismic event accompanied by a seismic swarm of similar or smaller size was also recorded in 2013-2014 along two synthetic and antithetic fault in the hanging-wall of the ATF (Gualandi et al., 2017). The interactions between such minor conjugate faults and the ATF compose a system undergoing complex behavior making the area an ideal candidate to improve our understanding of interactions between different slipping modes. We benefit from data of the Alto Tiberina Near Fault Observatory (TABOO-NFO; Chiaraluce et al., 2014) looking for aseismic events on the ATF and its surrounding faults. The dense network of GNSS, seismometers and borehole strainmeters provides a rarely attained high spatial (inter-distance < 10km) and temporal (from 2009 to nowadays) resolution framework enabling the study of the ATF fault system slip history. We search for transients with a semi-automatic detection tool of slow slip events based on kinematic inversions of strainmeters time series. We also test if these events interact with larger seismic events of the region. We present the strain time series processed with the EarthScope Strain Tools (EarthScope Consortium) and the preliminary signals detected with our tool. The fine analysis of the ATF would help better constraining the behavior of faults and more generally large events. 
Abstract. What controls the location and segmentation of mega-earthquakes in subduction zones is a long-standing problem in earth sciences. Prediction of earthquake ruptures mostly relies on interplate coupling models based on Global Navigation Satellite Systems providing patterns of slip deficit between tectonic plates. We here investigate if and how the seismic and aseismic patches revealed by these models relate to the distribution of deformation along the plate interface, i.e. basal erosion and/or underplating. From a mechanical analysis of the topography applied along the Chilean subduction zone, we show that extensive plate interface deformation takes place along most of the margin. We show that basal erosion occurs preferentially at 15 km depth while underplating does at 35 ± 10 and 60 ± 5 km depth, in agreement with P-T conditions of recovered underplated material, expected pore pressures, and spatial distribution of marine terraces and uplift rates. Along southern Chile, large sediment input favors shallow accretion and underplating of subducted sediments, while along northern Chile, extensive basal erosion provides material for the underplating. We then show that all major earthquakes of southern Chile are limited along their down-dip end by underplating while, along northern Chile, they are surrounded by both basal erosion and underplating. Segments with heterogeneously distributed deformation largely coincide with lateral earthquake terminations. We therefore propose that long-lived plate interface deformation promotes stress build-up and leads to earthquake nucleation. Earthquakes then propagate along fault planes shielded from this long-lived permanent deformation, and are finally stopped by segments of heterogeneously distributed deformation. Slip deficit patterns and earthquake segmentation therefore reflect the along-dip and along-strike distribution of the plate interface deformation. Topography acts as a mirror of distributed plate interface deformation and should be studied systematically to improve the prediction of earthquake ruptures.
Abstract. What controls the location and segmentation of mega-earthquakes in subduction zones is a long-standing problem in Earth sciences. Prediction of earthquake rupture extent mostly relies on interplate coupling models based on Global Navigation Satellite Systems providing patterns of slip deficit between tectonic plates. We here investigate if and how the strongly and weakly coupled patches revealed by these models relate to the distribution of deformation along the plate interface, i.e., basal erosion and/or underplating. From a mechanical analysis of the topography applied along the Chilean subduction zone, we show that extensive plate interface deformation takes place along most of the margin. We show that basal erosion occurs preferentially at 15âkm depth while underplating does at 35â±â10 and 60â±â5âkm depth, in agreement with P-T conditions of recovered underplated material, expected pore pressures and the spatial distribution of marine terraces and uplift rates. South of the Juan Fernández Ridge, large sediment input favors shallow accretion and underplating of subducted sediments, while along northern Chile, extensive basal erosion provides material for the underplating. We then show that, along the accretionary margin, the two last major earthquakes were limited along their down-dip end by underplating while, along the erosive margin, they were surrounded by both basal erosion and underplating. Segments with heterogeneously distributed deformation largely coincide with lateral earthquake terminations. We therefore propose that long-lived plate interface deformation promotes stress build-up and leads to earthquake nucleation. Earthquakes then propagate along fault planes shielded from this long-lived permanent deformation, and are finally stopped by segments of heterogeneously distributed deformation. Slip deficit patterns and earthquake segmentation therefore reflect the along-dip and along-strike distribution of the plate interface deformation. Topography acts as a mirror of distributed plate interface deformation and should be more systematically studied to improve the prediction of earthquake ruptures.
Earth and Space Science Open Archive This preprint has been submitted to and is under consideration at Journal of Geophysical Research - Solid Earth. ESSOAr is a venue for early communication or feedback before peer review. Data may be preliminary.Learn more about preprints preprintOpen AccessYou are viewing the latest version by default [v1]Afterslip of the Mw8.3 2015 Illapel earthquake imaged through a time-dependent inversion of continuous and survey GNSS dataAuthorsRoxaneTissandieriDJean-MathieuNocquetEmilieKleinChristopheVignyJavierOjedaSergioRuizSee all authors Roxane TissandieriDCorresponding Author• Submitting AuthorInstitut de physique du globe de Paris, Université de ParisiDhttps://orcid.org/0000-0003-3438-8288view email addressThe email was not providedcopy email addressJean-Mathieu NocquetInstitut de physique du globe de Paris, Université de Paris & Université Côte d'Azurview email addressThe email was not providedcopy email addressEmilie KleinScripps Institution of Oceanography, UCSDview email addressThe email was not providedcopy email addressChristophe VignyEcole Normale Superieureview email addressThe email was not providedcopy email addressJavier OjedaInstitut de physique du globe de Paris, Université de Paris & Departamento de Geofısica, Universidad de Chileview email addressThe email was not providedcopy email addressSergio RuizUniversidad de Chileview email addressThe email was not providedcopy email address
<p><span lang="EN-US">The Western Kunlun Range (WKR) is a slowly converging orogen located along the northwestern edge of the Tibetan Plateau, facing the Tarim Basin. The recent Mw 6.4 2015 Pishan earthquake along the mountain front recalls that this region remains seismically active, despite little or moderate historical seismicity. Its low deformation rates can be hardly retrieved from current geodetic data, placing limited constraints on the potential interseismic loading of the region. This is particularly critical as recent structural investigations report the existence of an extremely wide (~150-180 km) frontal thrust sheet, whose dimensions would imply the possibility of major M &#8805; 8 earthquakes in the case that it is locked and slips during one single seismic event.</span></p> <p><span lang="EN-US">To place further constraints on the seismic hazards of this region, we have conducted morphological and structural analyses of active faults to unravel the geomorphic record of active deformation cumulated other multiple seismic events at specific sites. To do so, field observations, seismic profiles and high-resolution Pl&#233;iades images and DEMs were combined together with the dating of fluvial terraces. We find that shortening rates have been of 0.5-2.5 mm/yr, with most probable values of ~2 mm/yr over the last ~300-500 kyr. Our detailed morphological investigations further indicate that this shortening is variably partitioned on one or several blind ramps along the mountain front, and from there is transmitted forward all the way to the deformation front, ~150-180 km further north. As such, this extremely wide single frontal thrust sheet stands most probably as the largest active thrust sheet in the world!</span></p> <p><span lang="EN-US">Finally, previously published GPS velocity fields highlight a 2-3 mm/yr gradient in horizontal velocities across the WKR and southern Tarim basin when combined and expressed in a stable Tarim reference. Such gradient, unseen from previous analyses, is consistent with our morphological results on shortening rates. Most importantly, this spatial gradient in velocities may suggest that the frontal thrust sheet is presently partly locked, questioning the possibility of mega-earthquakes in the region.</span></p>
<div> <div> <div> <p>The Mw 8.3 2015 Illapel earthquake ruptured a 190 km long segment of the Chilean subduction zone. In the past, this area ruptured several times through large and great earthquakes, the most recent event before 2015 being a Mw 7.9 earthquake in 1943. Here, we combine continuous and survey GPS ground displacements to perform a kinematic inversion of the two-months afterslip following the mainshock. We show that the postseismic slip developed South and North of the coseismic rupture, but also overlaps the deeper part of it. We estimate that two months after the large mainshock, the postseismic moment released represents 13% of the coseismic moment (the mainshock released 3.16x10<sup>21</sup> N.m whereas the afterslip released 3.98x10<sup>20</sup> N.m). At a first order, seismicity and areas experiencing afterslip match together and are concentrated at the edges of the coseismic rupture between 25 and 45 km depth. One interesting feature is the occurrence of two moderate size aftershocks on November, 11<sup>th</sup> at shallow depth North of the rupture. We investigate the relationship between the evolution of afterslip and these aftershocks. Finally, we interpret the result in the light of past earthquakes history and calculate the moment balance through the last centuries.</p> </div> </div> </div>
Abstract. What controls the location and segmentation of mega-earthquakes in subduction zones is a long-standing problem in Earth sciences. Prediction of earthquake rupture extent mostly relies on interplate coupling models based on Global Navigation Satellite Systems providing patterns of slip deficit between tectonic plates. We here investigate if and how the strongly and weakly coupled patches revealed by these models relate to the distribution of deformation along the plate interface, i.e., basal erosion and/or underplating. From a mechanical analysis of the topography applied along the Chilean subduction zone, we show that extensive plate interface deformation takes place along most of the margin. We show that basal erosion occurs preferentially at 15âkm depth while underplating does at 35â±â10 and 60â±â5âkm depth, in agreement with P-T conditions of recovered underplated material, expected pore pressures and the spatial distribution of marine terraces and uplift rates. South of the Juan Fernández Ridge, large sediment input favors shallow accretion and underplating of subducted sediments, while along northern Chile, extensive basal erosion provides material for the underplating. We then show that, along the accretionary margin, the two last major earthquakes were limited along their down-dip end by underplating while, along the erosive margin, they were surrounded by both basal erosion and underplating. Segments with heterogeneously distributed deformation largely coincide with lateral earthquake terminations. We therefore propose that long-lived plate interface deformation promotes stress build-up and leads to earthquake nucleation. Earthquakes then propagate along fault planes shielded from this long-lived permanent deformation, and are finally stopped by segments of heterogeneously distributed deformation. Slip deficit patterns and earthquake segmentation therefore reflect the along-dip and along-strike distribution of the plate interface deformation. Topography acts as a mirror of distributed plate interface deformation and should be more systematically studied to improve the prediction of earthquake ruptures.
An earthquake sequence occurred in the Atacama region of Chile throughout September 2020. The sequence initiated by a mainshock of magnitude Mw6.9, followed 17 hours later by a Mw6.4 aftershock. The sequence lasted several weeks, during which more than a thousand events larger than Ml 1 occurred, including several larger earthquakes of magnitudes between 5.5 and 6.4. Using a dense network that includes broad-band, strong motion and GPS sites, we study in details the seismic sources of the mainshock and its largest aftershock, the afterslip they generate and their aftershock, shedding light of the spatial temporal evolution of seismic and aseismic slip during the sequence. Dynamic inversions show that the two largest earthquakes are located on the subduction interface and have a stress drop and rupture times which are characteristics of subduction earthquakes. The mainshock and the aftershocks, localised in a 3D velocity model, occur in a narrow region of interseismic coupling (ranging 40%-80%), i.e. between two large highly coupled areas, North and South of the sequence, both ruptured by the great Mw~8.5 1922 megathrust earthquake. High rate GPS data (1 Hz) allow to determine instantaneous coseismic displacements and to infer coseismic slip models, not contaminated by early afterslip. We find that the total slip over 24 hours inferred from precise daily solutions is larger than the sum of the two instantaneous coseismic slip models. Differencing the two models indicates that rapid aseismic slip developed up-dip the mainshock rupture area and down-dip of the largest aftershock. During the 17 hours separating the two earthquakes, micro-seismicity migrated from the mainshock rupture area up-dip towards the epicenter of the Mw6.4 aftershocks and continued to propagate upwards at ~0.7 km/day. The bulk of the afterslip is located up-dip the mainshock and down-dip the largest aftershock, and is accompanied with the migration of seismicity, from the mainshock rupture to the aftershock area, suggesting that this aseismic slip triggered the Mw6.4 aftershock. Unusually large post-seismic slip, equivalent to Mw6.8 developed during three weeks to the North, in low coupling areas located both up-dip and downdip the narrow strip of higher coupling, and possibly connecting to the area of the deep Slow Sleep Event detected in the Copiapo area in 2014. The sequence highlights how seismic and aseismic slip interacted and witness short scale lateral variations of friction properties at the megathrust.
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