Incremental growth of normal faults: Insights from a laser-equipped analog experiment

International audienceWe conducted a laser-equipped analog experiment aimed at quasi-continuously monitoring the growth of a dense population of normal faults in homogeneous conditions. To further understand the way geological faults progressively gain in slip and length as they accumulate more strain, we measured with great precision the incremental slip and length changes that the analog faults sustain as they grow. These measurements show that the analog faults share common features with the natural ones. In particular, during their growth, the faults develop and maintain cumulative slip profiles that are generally triangular and asymmetric. The growth takes place through two distinct phases: an initial, short period of rapid lateral lengthening, followed by a longer phase of slip accumulation with little or no lengthening. The incremental slip is found to be highly variable in both space (along the faults) and time, resulting in variable slip rates. In particular, ‘short- and long-term' slip rates are markedly different. We also find that slip measurements at local points on fault traces do not contain clear information on the slip increment repeat mode. Finally, while the fault growth process is highly heterogeneous when considered at the scale of a few slip events, it appears homogeneous and self-similar at longer time scales which integrate many slip increments. This is likely to be the result of a feedback between stress heterogeneities and slip development. The long-term scale homogeneity also implies that the long-term faulting process is primarily insensitive to the short-term heterogeneities that are rapidly smoothed or redistributed. We propose a new conceptual scenario of fault growth that integrates the above observations and we suggest that faults grow in a bimodal way as a result of a self-driven and self-sustaining process

Location of largest earthquake slip and fast rupture controlled by along-strike change in fault structural maturity due to fault growth

Earthquake slip distributions are asymmetric along strike, but the reasons for the asymmetry are unknown. We address this question by establishing empirical relations between earthquake slip profiles and fault properties. We analyze the slip distributions of 27 large continental earthquakes in the context of available information on their causative faults, in particular on the directions of their long-term lengthening. We find that the largest slips during each earthquake systematically occurred on that half of the ruptured fault sections most distant from the long-term fault propagating tips, i.e., on the most mature half of the broken fault sections. Meanwhile, slip decreased linearly over most of the rupture length in the direction of long-term fault propagation, i.e., of decreasing structural maturity along strike. We suggest that this earthquake slip asymmetry is governed by along-strike changes in fault properties, including fault zone compliance and fault strength, induced by the evolution of off-fault damage, fault segmentation, and fault planarity with increasing structural maturity. We also find higher rupture speeds in more mature rupture sections, consistent with predicted effects of low-velocity damage zones on rupture dynamics. Since the direction(s) of long-term fault propagation can be determined from geological evidence, it might be possible to anticipate in which direction earthquake slip, once nucleated, may increase, accelerate, and possibly lead to a large earthquake. Our results could thus contribute to earthquake hazard assessment and Earthquake Early Warning

Développement d'une approche de paléosismologie géophysique par imagerie Géoradar. Applications aux failles décrochantes actives de Nouvelle Zélande

Acquérir des informations sur les forts séismes passés est crucial pour anticiper les caractéristiques des forts séismes futurs. Une partie des traces laissées par les forts séismes passés sont enfouies dans les premiers mètres du sol et sont en général révélées par des tranchées de quelques mètres de profondeur ouvertes à travers les failles sismogènes. Bien que pertinente, cette méthode est destructive. L'objectif a été de développer une nouvelle forme de paléosismologie, non destructive, basée sur l'imagerie géoradar pseudo 3D, capable de retrouver ces traces enfouies des séismes passés. Dans ce travail, cinq sites d étude sont présentés, situés le long de failles actives décrochantes de Nouvelle Zélande. Notre nouvelle approche débute, dans un premier temps, par l analyse classique de la morphologie de surface à partir de données LiDAR et de MNT GPS haute résolution. Ceci nous permet d identifier l'ensemble des marqueurs morphologiques préservés à la surface et les déplacements horizontaux qu ils ont enregistrés. Dans un second temps, l analyse des profils GPR pseudo-3D acquis en chacun des sites révèlent des réflecteurs principaux dans les premiers 5-10 m du sol recoupés par un grand nombre de marqueurs morphologiques, partiellement ou totalement invisibles en surface. La plupart de ces marqueurs enfouis sont coupés et décalés par la faille considérée. Les mesures de ces décalages fournissent des collections denses de déplacements cumulés sur chacune des failles investiguées avec généralement un nombre de mesures effectués en sub-surface 10 à 20 fois plus important qu en surface et couvrant une plus large gamme de valeurs. L application sur la faille de Hope de cette approche a notamment permis de mettre en évidence un déplacement latéral caractéristique de 3.2 +- 1 m lors des 30-35 derniers forts séismes. Ce travail démontre le potentiel de l'imagerie géoradar pseudo-3D à détecter une partie de l'histoire sismique des failles et, ce faisant, à fournir des informations sur les caractéristiques des forts séismes passés.Collecting information on past strong earthquakes is crucial to anticipate the characteristics of the future strong earthquakes that threaten us. A part of the traces left by the past earthquakes remains hidden in the first few meters of the ground. Until now, paleoseismological trenches across faults have been used to search for these traces. Though relevant, this method is destructive and allows, at best, detecting the few most recent events. The objective of my PhD work, done in the framework of the ANR project CENTURISK, was to develop a novel form of paleoseismology, of geophysical type, based on multi-frequency, pseudo-3D GPR surveys. The idea is to image at high-resolution the architecture of the first 10 m of the ground over wide areas along active faults, in order to detect the possibly buried traces, especially the offsets, produced by the last 10-20 strong earthquakes on the fault. We have first developed the approach by adapting the acquisition and processing of GPR data to the selected targets. We have then applied the approach on some of the largest active strike-slip faults in New Zealand, where sedimentation conditions are ideal. Twelve sites were investigated, 5 of them are presented in this work. At each site, we first analyzed the surface morphology in the greatest detail on LiDAR data and high resolution GPS DEMs. This analysis allowed us to identify all the morphological markers preserved at the ground surface, and being offset by the fault. We measured these surface offsets, doing so collecting a dense population of cumulative displacement values. We then surveyed each site with 40-60, 100 and 250 MHz, hundreds of meters long GPR profiles, parallel to the fault and regularly spaced by 5-10 m on either side of the fault trace. At each site, the processing of the GPR data revealed a large number of buried markers palaeosurfaces and incision features, hidden in the first 5-10 m of the ground. Most of the buried markers were observed cut and laterally displaced by the fault, and these offsets could be measured. The measures provide a dense collection of cumulative offsets on each investigated fault, generally 10-20 times more than ever reported. To analyze these dense surface and sub-surface data collections, we used statistical methods made to define and retain only the best constrained offset values. These best values are separated by slip increments that are directly related to the successive coseismic slips that we search. The entire analysis revealed that the offsets measured in the sub-surface fill the gaps in the surface record, and that the surface offsets are systematically lower than those measured in the sub-surface on the same markers. Additionally, the buried record is longer than the surface record. Applied to the Hope Fault, our novel approach allowed identifying the last 30-35 strong earthquakes that broke the fault, each had produced a lateral offset at surface of 3.2 +- 1 m and got a magnitude Mw 7.0-7.4. Applied to the Wellington Fault (at Te Marua site), the approach allowed identifying a minimum of 15 past strong earthquakes, each had produced a lateral offset at surface of 3.7 +- 1.7 m and got a magnitude Mw 6.9-7.6. My PhD work thus confirms the great potential of pseudo-3D Ground Penetrating Radar survey to detect a significant part of the fault seismic history, and thus to provide critical information to determine the displacements and magnitudes of the past strong earthquakes on faults. Applied to seismogenic faults worldwide, in complement to surface approaches, the geophysical GPR paleoseismology should help better assessing seismic hazard.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

Digital surface model generation from multiple optical high-resolution satellite images

International audienceThe modern optical satellite sensors capture images in stereo and tri-stereo acquisition modes. This allows reconstruction of high-resolution (30-70 cm) topography from the satellite data. However, numerous areas on the Earth exhibit complex topography with a lot of "discontinuities". One case is tectonic fault sites, which form steep topographic escarpments enclosing narrow, deep corridors that mask parts of the ground. Built with common approaches (stereo or tri-stereo), a digital surface model (DSM) would not recover the topography in these masked zones. In this work, we have settled on a new methodology, based on the combination of multiple satellite Pleiades images taken with different geometries of acquisition (pitch and roll angles), with the purpose to generate fully-resolved DSMs at very high-resolution (50 cm). We have explored which configurations of satellites (i.e., number of images and ranges of pitch and roll angles) allow to best measure the topography inside deep and narrow canyons. We have collected seventeen Pleiades Images with different configurations over the Valley of Fire fault zone, USA, where the fault topography is complex. We have also measured sixteen ground control points (GCPs) in the zone. From all possible combinations of 2 to 17 Pleiades images, we have selected 150 combinations and have generated the corresponding DSMs. The calculations are done by solving an energy minimization problem that searches for a disparity map minimizing the energy, which depends on the likelihood for pixels to belong to a unique point in 3D as well as regularization terms. We have statistically studied which combinations of images deliver DSMs with the best surface coverage, as well as the lowest uncertainties on geolocalisation and elevation measures, by using the GCPs. Our first results suggest that an exceeding time between our acquisitions leads to DSM with a low covered area. We conclude that Stereo and Tri-Stereo acquisition in one-single pass of the satellite will systematically generate a better DSM than multi-date acquisition. We also conclude that in some cases, multi-date acquisitions with 7-8 images can improve the DSM robustness compared to multi-date acquisitions with fewer images

Using in situ Chlorine-36 cosmonuclide to recover past earthquake histories on limestone normal fault scarps: a reappraisal of methodology and interpretations

International audienceCosmic-ray exposure dating of preserved, seismically exhumed limestone normal fault scarps has been used to identify the last few major earthquakes on seismogenic faults and recover their ages and displacements through the modelling of the content of in situ [36Cl] cosmonuclide of the scarp rocks. However, previous studies neglected some parameters that contribute to 36Cl accumulation and the uncertainties on the inferred earthquake parameters were not discussed. To better constrain earthquake parameters and to explore the limits of this palaeoseismological method, we developed a Matlab® modelling code (provided in Supplementary information) that includes all the factors that may affect [36Cl] observed in seismically exhumed limestone fault scarp rocks. Through a series of synthetic profiles, we examine the effects of each factor on the resulting [36Cl], and quantify the uncertainties related to the variability of those factors. Those most affecting the concentrations are rock composition, site location, shielding resulting from the geometry of the fault scarp and associated colluvium, and scarp denudation. In addition, 36Cl production mechanisms and rates are still being refined, but the importance of these epistemic uncertainties is difficult to assess. We then examine how pre-exposure and exposure histories of fault-zone materials are expressed in [36Cl] profiles. We show that the 36Cl approach allows unambiguous discrimination of sporadic slip versus continuous creep on these faults. It allows identification of the large slip events that have contributed to the scarp exhumation, and provides their displacement with an uncertainty of +/- ~25 cm and their age with an uncertainty of +/-0.5-1.0 kyr. By contrast, the modelling cannot discriminate whether a slip event is a single event or is composed of multiple events made of temporally clustered smaller size events. As a result, the number of earthquakes identified is always a minimum, while the estimated displacements are maximum bounds and the ages the approximate times when a large earthquake or a cluster of smaller earthquakes have occurred. We applied our approach to a data set available on the Magnola normal fault, Central Italy, including new samples from the buried part of the scarp. Reprocessing of the data helps to refine the seismic history of the fault and quantify the uncertainties in the number of earthquakes, their ages and displacements. We find that the Magnola fault has ruptured during at least five large earthquakes or earthquake clusters in the last 7 ka, and may presently be in a phase of intense activity

Semiautomatic algorithm to map tectonic faults and measure scarp height from topography applied to the Volcanic Tablelands and the Hurricane fault, western US

Observations of fault geometry and cumulative slip distribution serve as critical constraints on fault behavior over temporal scales ranging from a single earthquake to a fault's complete history. The increasing availability of high-resolution topography (at least one observation per square meter) from air and spaceborne platforms facilitates measuring geometric properties along faults over a range of spatial scales. However, manually mapping faults and measuring slip or scarp height is time-intensive, limiting the use of rich topography datasets. To substantially decrease the time required to analyze fault systems, we developed a novel approach for systematically mapping dip-slip faults and measuring scarp height. Our MATLAB algorithm detects fault scarps from topography by identifying regions of steep relief given length and slope parameters calibrated from a manually drawn fault map. We applied our algorithm to well-preserved normal faults in the Volcanic Tablelands of eastern California using four datasets: (1) structure-from-motion topography from a small uncrewed aerial system (sUAS; 20 cm resolution), (2) airborne laser scanning (25 cm), (3) Pleiades stereosatellite imagery (50 cm), and SRTM (30 m) topography. The algorithm and manually mapped fault trace architectures are consistent for primary faults, although can differ for secondary faults. On average, the scarp height profiles are asymmetric, suggesting fault lateral propagation and along-strike variations in the fault's mechanical properties. We applied our algorithm to Arizona and Utah with a specific focus on the normal Hurricane fault where the algorithm mapped faults and other prominent topographic features well. This analysis demonstrates that the algorithm can be applied in a variety of geomorphic and tectonic settings.Peer reviewedGeolog

Thank You to Our 2022 Peer Reviewers

Editors of JGR-Solid Earth express their appreciation to those who served as peer reviewers for the journal in 2022