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International audience; Editors of JGR-Solid Earth express their appreciation to those who served as peer reviewers for the journal in 2021.

International audience; As faults grow over time and become more “mature,” some of their geometrical and mechanical properties evolve, and these changes modify earthquake behavior. It is thus of prime importance to know the degree of structural maturity of a fault that is likely to produce large earthquakes. Although this concept is extensively used, there is no common definition or metric to measure the structural maturity of a fault. We analyzed the heterogeneity of the surface traces of 13 large seismogenic faults whose maturity is known qualitatively. We measured the corrugations and step-over segmentation of the traces from ∼100 m to the fault length scale. Corrugations and some properties of the segmentation are found to vary with fault structural maturity. We provide scaling relationships that quantify the structural maturity of a fault based on its surface trace. These results should help in parameterizing source faults in earthquake models.

International audience; Our planet is in crisis! The latest report of the United Nations Intergovernmental Panel on Climate Change (IPCC AR6) confirms that human influence is causing widespread, rapid, and intensifying changes in our weather and climate that are affecting every region on Earth in multiple ways. With every additional ton of carbon we emit, the frequency and intensity of storms, floods, droughts, and fires become greater and the effects on the environment and on human health and civilization become more severe.As geoscientists and journal editors, most of us have been accustomed to being on the leading edge of human knowledge and understanding of climate change, where we deal in objectivity, uncertainty, and debate, but now we find ourselves at the core of this climate crisis. It is no longer scientific discoveries at stake, but also humanity itself. This is an uncomfortable place for many of us. We are trained to be dispassionate observers and cautious thinkers, yet the alarming rate of recent climate change impels us to turn our attention directly toward mitigating this impending crisis.We are making a plea for collective action: we must make the switch to a green economy, put a just and effective price on carbon now, and consider a portfolio of other equitable public investments in climate solutions. These actions will ensure that the true costs and risks of burning fossil fuels are accounted for and global carbon emissions are rapidly reduced. Rich countries must lead the way in making drastic cuts to carbon emissions and in helping low- and middle-income countries to develop sustainably. We are running out of time.For decades, American Geophysical Union (AGU) journals have been at the forefront of documenting human-caused climate change and warning of a worsening climate crisis. Over 2,000 publications from AGU journals are cited in the new IPCC AR6 report. But we too can do more than just document and scientifically explain the ongoing crisis—our profession must help lead the way to solutions.Finding solutions and adapting to change have become not only necessary, but essential in ensuring safe, sustainable, and healthy human communities in the future. The geosciences have an essential role to play in these efforts by pivoting toward more cross-sector, solution-based science. To help lead this vision, the AGU is adding a new publication forum for community science in partnership with associations outside the geosciences. This forum will enhance interactions among AGU's existing, more disciplinary journals and give local communities a voice in leading solutions to global challenges.We are scientists, but we also have families and loved ones alongside our fellow citizens on this planet. The time to bridge the divide between scientist and citizen, head and heart, is now. The lead-up to the 2021 UN Climate Change Conference, COP26, being held in Glasgow in November, is our “last best chance” to urge world leaders to come together and commit to keeping climate change and its devastating impacts in check.

International audience; The editors of JGR: Solid Earth announce that Plain Language Summaries will be required for all manuscripts.

International audience; Faults form dense, complex multi‐scale networks generally featuring a master fault and myriads of smaller‐scale faults and fractures off its trace, often referred to as damage. Quantification of the architecture of these complex networks is critical to understanding fault and earthquake mechanics. Commonly, faults are mapped manually in the field or from optical images and topographic data through the recognition of the specific curvilinear traces they form at the ground surface. However, manual mapping is time‐consuming, which limits our capacity to produce complete representations and measurements of the fault networks. To overcome this problem, we have adopted a machine learning approach, namely a U‐Net Convolutional Neural Network, to automate the identification and mapping of fractures and faults in optical images and topographic data. Intentionally, we trained the CNN with a moderate amount of manually created fracture and fault maps of low resolution and basic quality, extracted from one type of optical images (standard camera photographs of the ground surface). Based on a number of performance tests, we select the best performing model, MRef, and demonstrate its capacity to predict fractures and faults accurately in image data of various types and resolutions (ground photographs, drone and satellite images and topographic data). MRef exhibits good generalization capacities, making it a viable tool for fast and accurate mapping of fracture and fault networks in image and topographic data. The MRef model can thus be used to analyze fault organization, geometry, and statistics at various scales, key information to understand fault and earthquake mechanics.

International audience

Measuring fault offsets preserved at the ground surface is of primary importance to recover earthquake and long‐term slip distributions and understand fault mechanics. The recent explosion of high‐resolution topographic data, such as Lidar and photogrammetric digital elevation models, offers an unprecedented opportunity to measure dense collections of fault offsets. We have developed a new Matlab code, 3D_Fault_Offsets, to automate these measurements. In topographic data, 3D_Fault_Offsets mathematically identifies and represents nine of the most prominent geometric characteristics of common sublinear markers along faults (especially strike slip) in 3‐D, such as the streambed (minimum elevation), top, free face and base of channel banks or scarps (minimum Laplacian, maximum gradient, and maximum Laplacian), and ridges (maximum elevation). By calculating best fit lines through the nine point clouds on either side of the fault, the code computes the lateral and vertical offsets between the piercing points of these lines onto the fault plane, providing nine lateral and nine vertical offset measures per marker. Through a Monte Carlo approach, the code calculates the total uncertainty on each offset. It then provides tools to statistically analyze the dense collection of measures and to reconstruct the prefaulted marker geometry in the horizontal and vertical planes. We applied 3D_Fault_Offsets to remeasure previously published offsets across 88 markers on the San Andreas, Owens Valley, and Hope faults. We obtained 5,454 lateral and vertical offset measures. These automatic measures compare well to prior ones, field and remote, while their rich record provides new insights on the preservation of fault displacements in the morphology.

Members of the editorial board of JGR-Solid Earth express their appreciation to those who served as peer reviewers for the journal in 2020.

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.

Knowing the slip amplitudes that large earthquakes produced in prehistorical times is one key to anticipate the magnitude of large forthcoming events. It is long known that the morphology is preserving remnants of paleoearthquake slips in the form of fault-offset landforms. However, the measured offsets that can be attributed to the most recent paleoearthquakes are generally few along a fault, so that they rarely allow recovering the slip distributions and largest slips of these earthquakes. We acquired ~1 m resolution airborne lidar data on a 30 km stretch of a fast-slipping strike-slip fault (eastern Hope Fault, New Zealand) located in a region of high alluvial dynamics where landforms are rapidly evolving. Data analysis reveals >200 offset landforms; only 30% allow a good to moderate quality offset measurement. From these good to moderate quality measures, we recover the slip-length distributions and largest slips of the four most recent large paleoearthquakes and find evidence of 4–6 prior events. The record suggests that large earthquake slip recurred in multiples of about 4 m along the 30 km stretch. Although they have larger uncertainties, the more numerous lower-quality offsets that we also measured reveal a similar earthquake slip record. This shows that, although offset landforms are partly degraded in dynamically active landscapes, they store valuable information on paleoearthquake slips. This information might be recovered provided that the morphology is analyzed at high resolution and " continuously " over a significant fault length. Remote lidar data are powerful to perform such analyses.

International audience; Natural earthquake slip profiles have a generic triangular shape which the available rupture dynamics models fail to reproduce. Long-term faults are embedded in long-damaged crustal material, and the properties of the long-term damage vary both across and along the faults. We examine the effects of the predamaged state of the medium on the earthquake slip distributions. We simulate long-term damage by the decrease in the elastic modulus of the medium around the fault. We model the dynamic crack-like rupture of a slip-weakening planar, right-lateral strike-slip fault, and search which geometries and elastic properties of the long-term damage produce a triangular slip profile on the rupture. We find that such a profile is produced only when a laterally heterogeneous preexisting damage zone surrounds the ruptured fault. The highest on-fault slip develops in the most compliant region of the damage zone, and not necessarily above the earthquake hypocenter. The coseismic slip decreases in zones of stiffer damage. The amount of coseismic slip dissipated in the damage zone is large, at least 25-40% of maximum on-fault slip, and can occur over large distances from the fault. Our study thus emphasizes that off-fault preexisting damage should be considered for an accurate description of earthquake ruptures. It also motivates a reformulation of the available earthquake source inversion models since most of them do not include the inelastic deformations that occur in the near field of the earthquake ruptures.

We analyze new GPS data spanning 11 years at 92 stations in eastern Iran. We use these data to analyze the present-day kinematics and the slip rates on most seismogenic faults in eastern Iran. The east Lut, west Lut, Kuhbanan, Anar, Dehshir, and Doruneh faults are confirmed as the major faults and are found to currently slip laterally at 5.6 ± 0.6, 4.4 ± 0.4, 3.6 ± 1.3, 2.0 ± 0.7, 1.4 ± 0.9, and 1.3 ± 0.8 mm/yr, respectively. Slip is right-lateral on the ~NS striking east Lut, west Lut, Kuhbanan, Anar, and Dehshir faults and left-lateral on the ~EW Doruneh fault. The ~NS faults slice the eastern Iranian crust into five blocks that are moving northward at 6–13 mm/yr with respect to the stable Afghan crust at the eastern edge of the collision zone. The collective behavior of the ~NS faults might thus allow the Arabian promontory to impinge northward into the Eurasian crust. The ~NS faults achieve additional NS shortening by rotating counterclockwise in the horizontal plane, at current rates up to 0.8°/Ma. Modeling the GPS and available geological data with a block rotation model suggests that the rotations have been going on at a similar rate (1 ± 0.4°/Ma) over the last 12 Ma. We identify large strains at the tips of the rotating east Lut, west Lut, and Kuhbanan faults, which we suspect to be responsible for the important historical and instrumental seismicity in those zones.

We recover the Holocene earthquake history of seven seismogenic normal faults in the Fucino system, central Italy. We collected 800 samples from the well-preserved limestone scarps of the faults and modeled their 36Cl concentrations to derive their seismic exhumation history. We found that > 30 large earthquakes broke the faults in synchrony over the last 12 ka. The seven faults released strain at the same periods of time, 12-9 ka, 5-3 ka, and 1.5-1 ka. On all faults, the strain accumulation and release occurred in 3-6 ka supercycles, each included a 3-5 ka phase of slow (≤ 0.5-2 mm/yr) strain accumulation in relative quiescence, followed by a cluster of three to four large earthquakes or earthquake sequences that released most of the strain in

[1] Past earthquake slips on faults are commonly determined by measuring morphological offsets at current ground surface. Because those offsets might not always be well preserved, we examine whether the first 10 m below ground surface contains relevant information to complement them. We focus on the Te Marua site, New Zealand, where 11 alluvial terraces have been dextrally offset by the Wellington fault. We investigated the site using pseudo-3D Ground Penetrating Radar and also produced a high-resolution digital elevation model (DEM) of the zone to constrain the surface slip record. The GPR data reveal additional information: (1) they image the 3D stratigraphic architecture of the seven youngest terraces and show that they are strath terraces carved into graywacke bedrock. Each strath surface is overlain by 3–5 m of horizontally bedded gravel sheets, including two pronounced and traceable reflectors; (2) thanks to the multilayer architecture, terrace risers and channels are imaged at three depths and their lateral offsets can be measured three to four times, constraining respective offsets and their uncertainties more reliably; and (3) the offsets are better preserved in the subsurface than at the ground surface, likely due to subsequent erosion-deposition on the latter. From surface and subsurface data, we infer that Te Marua has recorded six cumulative offsets of 2.9, 7.6, 18, 23.2, 26, and 31 m (± 1–2 m). Large earthquakes on southern Wellington fault might produce 3–5 m of slip, slightly less than previously proposed. Pseudo-3D GPR thus provides a novel paleoseismological tool to complement and refine surface investigations.

In places where sedimentation and erosion compete at fast rates, part of the record of past earthquakes on faults may be buried, hence hidden, in the first few metres below the surface. We developed a novel form of palaeoseismology, of geophysical type, based on the use of a dense pseudo-3-D Ground Penetrating Radar (GPR) survey to investigate such possible buried earthquake traces, on a long, fast-slipping strike-slip fault (Hope fault, New Zealand), at a site (Terako) where marked alluvial conditions prevail. We first used LiDAR data to analyse the ground surface morphology of the 2 km2 site at the greatest resolution. Nineteen morphological markers were observed, mainly alluvial terrace risers and small stream channels that are all dextrally offset by the fault by amounts ranging between 3 and 200 m. The measurements document about 10 past earthquake slip events with a mean coseismic slip of 3.3 +/- 1 m, with the most recent earthquake event having a slip of 3 +/- 0.5 m. We then investigated a detailed area of the site (400 x 600 m2) with pseudo-3-D GPR. We measured 56, similar to 400 m long, 510 m spaced GPR profiles (250 MHz), parallel to the fault and evenly distributed on either side. The analysis revealed the existence of a palaeosurface buried at about 3 m depth, corresponding to the top of alluvial terraces of different ages. That buried surface is incised by a dense network of stream channels that are all dextrally offset by the fault. We measured 48 lateral offsets in the buried channel network, more than twice than at the surface. These offsets range between 6 and 108 m, as observed at the surface, yet provide a more continuous record of the fault slip. The similarity of the successive slip increments suggests a slip per event averaging 4.4 +/- 1 m, fairly similar to that estimated from surface data. From the total surface and buried 67 offset collection, we infer that a minimum of 30 large earthquakes have broken the Hope fault at the Terako site in the last about 67 kyr, with an average coseismic slip of 3.2 +/- 1 m, a minimum average recurrence time of about 200 yr, and a magnitude of at least Mw 7.07.4. Our study therefore confirms that part of the record of past earthquakes may indeed reside in the first few metres below the surface, where it may be explored with geophysical, GPR-based palaeoseismology. Developing such a new palaeoseismological tool should provide rich information that may complement surface observations and help to document the past earthquakes on faults.

SUMMARY A new detailed palaeomagnetic study of Tertiary volcanics, including extensive K-Ar and 40 Ar/ 39 Ar dating, helps constrain the deformation mechanisms related to the opening processes of the Afar depression (Ethiopia and Djibouti). Much of the Afar depression is bounded by 30 Myr old flood basalts and floored by the ca 2 Myr old Stratoid basalts, and evidence for pre-2 Ma deformation processes is accessible only on its borders. K-Ar and 40 Ar/ 39 Ar dating of several mineral phases from rhyolitic samples from the Ali Sabieh block shows indistinguishable ages around 20 Myr. These ages can be linked to separation of this block in relation to continental breakup. Different amounts of rotation are found to the north and south of the Holhol fault zone, which cuts across the northern part of the Ali Sabieh block. The southern domain did not record any rotation for the last 8 Myr, whereas the northern domain experienced approximately 12 ± 9 ◦ of clockwise rotation. We propose to link this rotation to the counter-clockwise rotation observed in the Danakil block since 7 Ma. This provides new constraints on the early phases of rifting and opening of the southern Afar depression in connection with the propagation of the Aden ridge. A kinematic model of propagation and transfer of extension within southern Afar is proposed, with particular emphasis on the previously poorly-known period from 10 to 4 Ma.

Recent measurements of slip profiles on normal faults have found that they are usually triangular in shape. This has been explained to be a consequence of on-fault processes such as slip-dependent friction. However, the recent observation that cumulative slip profiles on normal faults and fault systems in Afar are both triangular and self-similar excludes this explanation and requires some form of off-fault deformation. Here, we use elastic modelling to show that large triangular zones of off-fault damage can explain the observed triangular slip profiles provided damage is anisotropic in the form of cracks sub-parallel to the fault. Our modelling suggests that these triangular damage zones result from the enlargement of the crack tip damage area as the fault (or system) lengthens. Our modelling also demonstrates that different types of ‘barriers’ can cause the slip profiles to terminate abruptly at one or both fault ends, as observed in Afar and elsewhere.

International audience; Understanding how natural faults are segmented along their length can provide useful insights into fault growth processes, stress distribution on fault planes, and earthquake dynamics. We use cumulative displacement profiles to analyze the two largest scales of segmentation of 900 normal faults in Afar, East Africa. We build upon a prior study by Manighetti et al. (2009) and develop a new signal processing method aimed at recovering the number, position, displacement, and length of both the major (i.e., longest) and the subordinate, secondary segments within the faults. Regardless of their length, age, geographic location , total displacement, and slip rate, 90% of the faults contain two to five major segments, whereas more than 70% of these major segments are divided into two to four secondary segments. In each hierarchical rank of fault segmentation, most segments have a similar proportional length, whereas the number of segments slightly decreases with fault structural maturity. The along-strike segmentation of the Afar faults is thus generic at its two largest scales. We summarize published fault segment data on 42 normal, reverse, and strike-slip faults worldwide, and find a similar number (two to five) of major and secondary segments across the population. We suggest a fault growth scenario that might account for the generic large-scale segmentation of faults. The observation of a generic segmentation suggests that seismogenic fault planes are punctuated with a deterministic number of large stress concentrations, which are likely to control the initiation, arrest and hence extent and magnitude of earthquake ruptures.

We investigate fault growth in Afar, where normal fault systems are known to be currently growing fast and most are propagating to the northwest. Using digital elevation models, we have examined the cumulative slip distribution along 255 faults with lengths ranging from 0.3 to 60 km. Faults exhibiting the elliptical or “bell-shaped” slip profiles predicted by simple linear elastic fracture mechanics or elastic-plastic theories are rare. Most slip profiles are roughly linear for more than half of their length, with overall slopes always

International audience; A detailed geophysical survey of the Ghoubbet Al Kharab (Djibouti) clarifies the small‐scale morphology of the last submerged rift segment of the propagating Aden ridge before it enters the Afar depression. The bathymetry reveals a system of antithetic normal faults striking N130°E, roughly aligned with those active along the Asal rift. The 3.5 kHz sub‐bottom profiler shows how the faults cut distinct layers within the recent, up to 60 m thick, sediment cover on the floor of the basin. A large volcanic structure, in the centre of the basin, the ‘Ghoubbet’ volcano, separates two sedimentary flats. The organization of volcanism and the planform of faulting, with en echelon subrifts along the entire Asal–Ghoubbet rift, appear to confirm the westward propagation of this segment of the plate boundary. Faults throughout the rift have been active continuously for the last 8400 yr, but certain sediment layers show different offsets. The varying offsets of these layers, dated from cores previously retrieved in the southern basin, imply Holocene vertical slip rates of 0.3–1.4 mm yr−1 and indicate a major decrease in sedimentation rate after about 6000 yr BP, and a redistribution of sediments in the deepest troughs during the period that preceded that change.

Temporal coincidence between continental flood basalts and breakup has been noted for almost three decades. Eight major continental flood basalts have been produced over the last 300 Ma. The most recent, the Ethiopian traps, erupted in about 1 Myr at 30 Ma. Rifting in the Red Sea and Gulf of Aden, and possibly East African rift started at about the same time. A second trap-like episode occurred around 2 Ma and formation of true oceanic crust is due in the next few Myr. We find similar relationships for the 60 Ma Greenland traps and opening of the North Atlantic, 65 Ma Deccan traps and opening of the NW Indian Ocean, 132 Ma Parana traps and South Atlantic, 184 Ma Karoo traps and SW Indian Ocean, and 200 Ma Central Atlantic Margin flood basalts and opening of the Central Atlantic Ocean. The 250 Ma Siberian and 258 Ma Emeishan traps seem to correlate with major, if aborted, phases of rifting. Rifting asymmetry, apparent triple junctions and rift propagation (towards the flood basalt area) are common features that may, together with the relative timings of flood basalt, seaward dipping reflector and oceanic crust production, depend on a number of plume- and lithosphere- related factors. We propose a mixed scenario of `active/passive' rifting to account for these observations. In all cases, an active component (a plume and resulting flood basalt) is a pre-requisite for the breakup of a major oceanic basin. But rifting must be allowed by plate-boundary forces and is influenced by pre-existing heterogeneities in lithospheric structure. The best example is the Atlantic Ocean, whose large-scale geometry with three large basins was imposed by the impact points of three mantle plumes.

Following a sequence of earthquakes in the Asal Rift in November 1978, the deformation resulting from dyke emplacement and associated faulting was measured geodetically and using teleseismic data. Using this information we have computed the resulting Coulomb stress changes, which can explain the features of the distribution of seismicity for the following six weeks. A further major shock then occurred. When this event is included in the modelling, the distribution of all the seismicity within a radius of 40 km in the following 4 years is predicted. The study confirms that increases of Coulomb stress as small as 0.3 bars are sufficient to trigger earthquakes and shows that dykes as well as fault slip can result in stress changes that influence the seismicity distribution over a broad region.

International audience; The temporal evolution of the mantle melting processes in the Asal Rift is evaluated from the chemical composition of 56 new lava flows sampled along 10 km of the rift axis and 9 km off-axis (i.e., erupted within the last 620 kyr). Petrological and primary geochemical results show that most of the samples of the inner floor of the Asal Rift are affected by plagioclase accumulation. Trace element ratios and major element compositions corrected for mineral accumulation and crystallization show a symmetric pattern relative to the rift axis and preserved a clear signal of mantle melting depth variations. While FeO, Fe8.0, Zr/Y, and (Dy/Yb)N decrease from the rift shoulders to the rift axis, SiO2, Na/Ti, Lu/Hf increase and Na2O and Na8.0 are constant across the rift. These variations are qualitatively consistent with shallow melting beneath the rift axis and deeper melting for off-axis lava flows. Na8.0 and Fe8.0 contents show that beneath the rift axis, melting paths are shallow, from 81 ± 4 to 43 ± 5 km. These melting paths are consistent with adiabatic melting in normal-temperature fertile asthenosphere, beneath an extensively thinned mantle lithosphere. On the contrary, melting on the rift shoulders (from 107 ± 7 to 67 ± 8 km) occurred beneath thicker lithosphere, requiring a mantle solidus temperature 100 ± 40°C hotter. In this geodynamic environment, the calculated rate of lithospheric thinning appears to be 4.0 ± 2.0 cm yr-1, a value close to the mean spreading rate (2.9 ± 0.2 cm yr-1) over the last 620 kyr.

International audience; We use 36Cl surface exposure dating to determine the slip release pattern over the last ~ 15 kyr for the Velino-Magnola fault, a major active normal fault in Central Italy. We sampled the fault at five well-separated sites along its length, and modeled the 36Cl concentrations measured in the 376 samples. We find that the fault broke in at least 9 large earthquakes that occurred in two 5-6 ka-long supercycles. Each cycle included a 4-5 ka-long phase of relative quiescence, followed by a cluster of at least 3 large earthquakes or earthquake sequences that released most of the accumulated strain in ~ 1 ka. All 9 identified events broke the entire fault and produced maximum surface slips of 2-3 m. Though the Velino-Magnola fault seems presently in a stage of relative quiescence, it may re-enter a phase of paroxysmal seismic activity in a few hundred of years

In August 1989, an earthquake sequence including ten events with 6:3 ! M ! 5:5 in the first two days produced widespread ground deformation in the Dobi graben of central Afar. Numerous surface breaks with complex geometry, including fresh scarplets with vertical throws up to 30 cm high and open fissures up to 30 cm wide, were observed. Coseismic slip incremented the deformation (normal faulting, block tilting, and counterclockwise rotation of basaltic slices) accumulated in the last 2 m.y. in the transfer zone between the Dobi and Hanle grabens. By combining maps of surface ruptures, relative event relocations with the local Djibouti network, published focal mechanisms, and source sizes, we tentatively relate most of the main- shocks of the sequence to slip on individual faults. The largest shocks at 11h16 on 20 August 1989 (MS 6:2) and at 1h09 on 21 August 1989 (MS 6:3) ruptured southern segments of the southwestern bounding fault of the graben. A dozen other faults also slipped along the edges of, and inside, the graben. On average, triggered seismic fault- ing propagated about 35 km northwestward along the graben in about 20 hr. Slip on the main faults was coupled with slip on secondary antithetic faults branching from them at depth. Although the Dobi earthquakes ruptured part of the fault array between the Asal rift (1978 sequence) and the Serdo region (1969 sequence), an approximately 50-km-long gap subsists along the Der'ela half-graben. We infer the patterns of sur- face faulting in the Dobi sequence, which coinvolved bookshelf-faulting about both horizontal and vertical axes, to typify the complexity of coseismic stress release in central Afar and in other active zones of distributed extension (e.g., Iceland, Abruzzi, Basin and Range). Online Material: Additional photos and descriptions of surface effects of the Dobi earthquake sequence.

Cosmic-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.

AB: The accurate prediction of near-source strong-ground motions in future earthquakes is a major challenge for seismologists and earthquake engineers. Such a prediction is however difficult, partly because ground motions are highly variable while the reasons for such a variability are basically ignored. In particular, though source effects are likely an important factor, the major properties of the earthquake source that control the ground motions are not clearly defined. Manighetti et al. (2007) have suggested that some of the large earthquake source properties are intimately controlled by intrinsic properties of the long-term faults broken by the earthquakes, such as the fault structural maturity. Following that suggestion, we examine the dependency of the strong ground motion variability on the long-term fault maturity. We analyze the near-field ground motions recorded at rock sites for 30 large (M 5.6-7.8) crustal earthquakes of various mechanisms. On the other hand, the structural maturity of the broken faults is determined and defined into three classes (mature, intermediate, immature) based on the combined knowledge of the age, slip rate, cumulative slip and length of the faults. The ground motions are determined in the period range 0-2 seconds and compared to the empirical attenuation equation of Boore et al. (1997). Residuals between observed and predicted ground motions are calculated following the methods of Kagawa et al. (2004; residuals not weighted by the number of records per earthquake) and of Spudich et al. (1999; residuals weighted by the number of records per earthquakes). Whatever the method, the strong motions produced by earthquakes on immature and mature faults are found to be larger and lower, respectively, than those predicted from the empirical law. Strong motions on immature faults result to be systematically larger by a factor of about 1.15 than those on mature faults. When earthquakes are distinguished from their focal mechanism, a same order difference is observed, with ruptures on reverse faults producing larger strong motions than earthquakes on strike-slip faults. Yet, when only strike-slip earthquakes are considered, a large difference in strong motion amplitude (averaging a factor 1.1) is still observed between immature and mature faults. By contrast, whether they produce surface breaks or not, earthquakes on faults of similar structural maturity produce ground motions of similar amplitude. We conclude that the strong ground motions generated in the near-field by earthquakes occurring on immature faults are higher than the strong motions generated by earthquakes on mature faults. The effect of fault structural maturity is of the same magnitude than that classically attributed to focal mechanism. The later effect may actually be only apparent, simply resulting from the maturity control. The structural maturity of long-term faults is thus an important parameter that should be considered in seismic hazard assessment.

article i nfo Article history: Earthquakes are sensitive to the along-strike segmentation of the faults they break, especially in their initiation, propagation and arrest. We examine that segmentation and search whether it shows any specific properties. We focus on the largest-scale fault segmentation which controls the largest earthquakes. It is well established that major segments within faults markedly shape their surface cumulative slip-length profiles; segments appear as large slip bumps separated by narrow, pronounced slip troughs (inter-segments). We use that property to examine the distribution (location, number, length) of the major segments in 927 active normal faults in Afar (East Africa) of various lengths (0.3-65 km), cumulative slips (1-1300 m), slip rates (0.5-5 mm/yr), and ages (10 4 -10 6 yr). This is the largest fault population ever analyzed. To identify the major bumps in the slip profiles and determine their number, location and length, we analyze the profiles using both the classical Fourier transform and a space-frequency representation of the profiles, the S-transform, which is well adapted for characterizing local spectral properties. Our work reveals the fol- lowing results: irrespective of their length, 70% of the slip profiles have a triangular envelope shape, in conflict with the elastic crack concept. Irrespective of their length, the majority of the faults (at least 50-70%) have a limited number of major segments, between 2 and 5 and more commonly equal to 3-5. The largest- scale segmentation of the faults is thus self-similar and likely to be controlled by the fault mechanics. The slip deficits at the major inter-segment slip troughs tend to smooth as the faults accumulate more slip resulting in increased connection of the major segments. The faults having accumulated more slip therefore generally appear as un-segmented (10-30%). Our observations therefore show that, whatever the fault on which they initiate, large earthquakes face the same number of major segments to potentially break. The number of segments that they eventually break seems to depend on the slip history (structural maturity) of the fault.

International audience; A normal fault scarp exhumed by repeated strong earthquakes is made of a series of rupture zones that were exposed, thus weathered, over significantly different time spans. We show that such differential weathering can be detected in the chemical content of the fault scarp rocks, and its signature used as a base to decipher the past earthquake history of the fault. We focus on the Magnola normal fault, Central Italy, whose Holocene seismic slip history has already been determined by Palumbo et al. (ESPL, 225, 163–176, 2004) from in situ 36Cl cosmic ray exposure dating of the fault limestone scarp surface. Five major earthquakes were found to have occurred over the last 12 ka, with slips of 1.5–3 m and recurrence times of 0.7–3.1 ka. We analyze the major and trace element concentrations of 15 carbonate samples collected from base to top of the 10 m-high Magnola Holocene scarp, next to the previous sampling done by Palumbo et al. [Palumbo, L., Benedetti, L., Bourlès, D., Cinque, A., Finkel, R., 2004. Slip history of the Magnola fault (Apennines, Central Italy) from 36Cl surface exposure dating: evidence for strong earthquake over Holocene. Earth Planet. Sci. Lett. 225, 163–176.]. We find that most element concentrations decrease upscarp at a rate averaging 5%/m. This decrease is attributed to leaching and re-precipitation of purer calcite that increase with exposure time. Superimposed to the overall leaching, concentration peaks are found at the transition zones separating the earthquake ruptures. These concentration peaks likely result from enrichment of the scarp sections that remained stuck in the 30–50 cm-thick impurity-rich upper soil during the quiescence periods that separated the earthquakes. Because the rare earth elements (REE) are among those most significantly enriched at the earthquake transition zones, they are the best chemical markers of past large seismic events. We finally propose a first-order model that reproduces adequately the observations. Our preliminary work thus suggests that geochemical analyses of preserved, seismically exhumed limestone fault scarp rocks may help deciphering the seismic slip history of a fault.

International audience; We 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.

Lateral slip on initially rift-parallel normal faults may be a particularly efficient mechanism to accomodate strain between overlapping oceanic rifts. It occurs in southern Afar, where clockwise block rotations result from distributed dextral shear between the overlapping Ghoubbet Asal-Manda Inakir and Manda Hararo-Abhe Bad rifts. Faulting observed during the 1969, Serdo earthquakes and on SPOT images is consistent with the shear being taken up by left-lateral slip on steep NW-SE striking faults, which formed as normal faults before extensional strain became localized in the two rifts. This bookshelf faulting accounts quantitatively for the 14.5°± 7.5° rotation documented by paleomagnetism in the 1.8 ± 0.4 Ma old Afar stratoid basalts, given the 17.5 ± 5 mm/yr rate of separation between Arabia and Somalia.

International audience; [1] We seek to characterize how magmatic and tectonic activities combine and interact during the continental rifting process. We address this question in two companion papers. In both, we analyze the seismicity that occurs in an active magmato-tectonic rift, Asal-Ghoubbet (East Africa), to identify the features and/or processes responsible for its activity. Here, we report results from a 5-month experiment that we conducted in the rift. Eleven seismometers were deployed to complement the eight-station permanent network. This allowed recording $400 earthquakes in the rift; 200 events could be well located (precision 5–6 km) magma reservoir. Most events concentrate at the roof of the pipe (at 3–4 km) and result from up and down slip ruptures on both the volcanic (ring) and tectonic faults that enclose the pipe at depth. The up and down motions are likely driven by pressure changes in the magma reservoir. Hence, although a few rift faults were associated with seismicity, most remained seismically silent during the experiment. In the companion paper, we analyze the seismic activity in the rift over the 23 years that followed its last rifting episode. This confirms the importance of the Fieale-Shark Bay plumbing system in the overall rift behavior. structure and magmato-tectonic processes in an active rift (Asal-Ghoubbet, Afar, East Africa): 1. Insights from a 5-month seismological experiment

International audience; We seek to understand how the stress interactions and the slip-weakening process combine within a non-coplanar, normal fault network to allow a slip instability to develop, and shape the final slip distribution on the system. In a first part, we perform a non-linear spectral analysis to investigate the conditions of stability and the process of slip initiation in an antiplane non-coplanar fault system subject to a slip-dependent friction law. That numerical model allows determining the zones that are able to slip within a fault network, as well as the location of the stress singularities. The resulting slip profiles on the faults show only a few different shapes, some of them with long, linear sections. This leads to formulate a general classification of slip profiles that can be used to infer the degree of fault interaction within any non-coplanar system. In a second part of work, we use our modelling to try reproducing the cumulative slip profiles measured on three real normal interacting faults forming a large-scale en echelon system. For that, we assume that cumulative slip profiles can be compared to the first static modal solution of our conceptual model. We succeed reproducing the profiles quite well using a variable weakening along the faults. Overall, the weakening rate decreases in the direction of propagation of the fault system. Yet, modelling the slip along the propagating, isolated termination segment of the system requires an unlikely distribution of weakening. This suggests that factors not considered in our analysis may contribute to slip profile shaping on isolated, propagating faults.

SUMMARY The evolution of the Gulf of Aden and the Anatolian Fault systems are modelled using the principles of elastic fracture mechanics usually applied to smaller scale cracks or faults. The lithosphere is treated as a plate, and simple boundary conditions are applied that correspond to the known plate boundary geometry and slip vectors. The models provide a simple explanation for many observed geological features. For the Gulf of Aden the model predicts why the ridge propagated from east to west from the Owen Fracture Zone towards the Afar and the overall form of its path. The smaller en echelon offsets can be explained by upward propagation from the initially created mantle dyke while the larger ones may be attributed to the propagating rupture interacting with pre-existing structures. For Anatolia the modelling suggests that the East Anatolian Fault was created before the North Anatolian Fault could form. Once both faults were formed however, activity could switch between them. The time scales over which this should take place are not known, but evidence for switching can be found in the historical seismicity. For Aden and Anatolia pre-existing structures or inhomogeneous stress fields left from earlier orogenic events have modified the processes of propagation and without an understanding of the existence of such features the propagation processes cannot be fully understood. Furthermore a propagating fault can extend into an active region where it would not have initiated. The North Anatolian Fault encountered slow but active extension when it entered the Aegean about 5 Ma and the stress field associated with the extending fault has progressively modified Aegean extension. In the central Aegean activity has been reduced while to the north-west on features such as the Gulfs of Evvia and Corinth activity has been increased. The field observation that major structures propagate and the success of simple elastic models suggest that the continental crust behaves in an elastic-brittle or elastic-plastic fashion even though laboratory tests may be interpreted to suggest viscous behaviour. There are major problems in scaling from the behaviour of small homogeneous samples to the large heterogeneous mantle and large-scale observations should be treated more seriously than extrapolations of the behaviour of laboratory experiments over many orders of magnitude in space and time. The retention of long-term elasticity and localised failure suggests a similar gross rheology for the oceanic and continental lithospheres. Even though it is incorrect to attribute differences in behaviour to the former being rigid (i.e. elastic) and the latter viscous, oceanic and continental lithosphere behave in different ways. Unlike oceanic crust, continental crust is buoyant and cannot be simply created or destroyed. The process of thickening or thinning works against gravity preventing large displacements on extensional or contractional features in the upper mantle. The equivalents of ridge or subduction systems are suppressed before they can accommodate large displacements and activity must shift elsewhere. On the other hand, strike-slip boundaries and extrusion processes are favoured.

International audience; We characterize average slip distributions on earthquakes beyond their individual heterogeneity. For that, we analyze a large number of seismic slip distributions both measured at the surface after earthquakes (44 profiles) and derived from slip inversion models (76 models). Investigating the overall shape of these slip profiles, we find that they are roughly triangular both along strike and dip, and most of them (70–80%) are asymmetric. Long linear slopes and high slip gradients therefore are the key ingredients to describe earthquake slip profiles. The scaling relations between maximum displacement and length (or width) suggest furthermore that the triangular slip profiles are self-similar. Such slip patterns make earthquakes dominated by one major zone of maximum slip hence one major “asperity.” Analyzing the position of hypocenters with respect to these “asperities,” we find that earthquakes nucleate at a distance from them that averages 20–30% of their total length. Compiling observations on 56 earthquakes, we show that this distance (i.e., the asperity size) is structurally defined. We then compare the earthquake slip profiles to cumulative slip profiles measured on long-term faults of various ages and sizes and find that all profiles have a similar shape, triangular and asymmetric. Hence combining data for a large number of earthquakes leads to point out average, generic characteristics of the coseismic slip that are similar to those that emerge from the accumulation of events with time on a single fault. This suggests that these characteristics result from robust physical properties.

[1] The Guadeloupe islands are cut by normal faults that accommodate oblique convergence between the North American and Caribbean plates. Such faults are responsible for part of the shallow seismicity and have produced M ≥ 5 damaging earthquakes. To better assess the seismic hazard in Guadeloupe, we quantify the slip rate on one of the largest fault (Morne-Piton). This roughly E-W fault crosses the island of Marie-Galante and uplifts a flight of reef terraces. From geomorphic analysis, we mapped three main terraces. New U/Th datings show that they formed during the latest interglacials, ∼120 and ∼240 kyr ago. Correlation with SPECMAP isotopic records implies that the Marie-Galante plateau emerged during the ∼330 ka highstand. Topographic profiles show that the terraces and the plateau are deformed by faulting. Elastic modeling of their shape constrains the geometry of the fault (70–80° dip, 5 km depth) and its slip rate (0.5 ± 0.2 mm/yr). Given its length (50 km), depth, and slip rate, this fault might produce maximum M ∼ 6.5 earthquakes with a recurrence time of 1400 to 3300 years, or more likely smaller events such as the M ∼ 5.5 16 May 1851 and 3 August 1992 shocks that might recur every 400 to 1000 years. We also show that all the islands and terraces are tilted westward perpendicularly to the trench. La Desirade closest to the trench is uplifted by 276 m, whereas subsidence (−70 m) is observed 10 km east of the volcanic arc. This tilt probably resulted from a transient deformation episode at the subduction interface that predated the late Pleistocene.

[1] We present here geomorphic evidence for an emergent active thrust along the northwestern Apennine front about 50 km south of Milano. Fieldwork combined with SPOT image analysis attests to the presence of a ∼35-km-long, en echelon, cumulative fault scarp, that cuts E-W across Quaternary surface deposits from Casteggio to Sarmato. The scarp offsets vertically alluvial fans and terraces emplaced by tributaries of the Po River that flow northward from the Apennines. Incision by such tributaries occurs only south of the scarp. Valleys of smaller streams stand in hanging position. To the northeast, the buried, east dipping Pavia lateral ramp bounds the San Colombano anticline. We interpret the Montebello-Sarmato scarp to result from the emergence of an active thrust fault dipping south, the Stradella thrust, which splays eastward from that ramp. Total station profiles leveled perpendicular to the Stradella thrust show variable cumulative surface throws ranging from 2 to 25 m. Scarp degradation analysis indicates fairly recent offset of the terrace surfaces (10–100 kyr) suggesting a minimum uplift rate of 0.3 mm/yr. The underlying fault might thus be one of the most active thrust ramps along this segment of the Apennines. N-S to N15°W shortening is consistent with both the E-W strike of this thrust ramp and with coeval oblique sinistral motion on the NNE-SSW trending Pavia lateral ramp. The Vogherese-Bobbiese earthquake of 1828 (I ≈ IX) south of Casteggio and Broni may have activated one segment of this lateral ramp rather than the Stradella thrust. Other historical events (I > 7) that caused damage in Pavia, Piacenza, Lodi, and Milano in the last 2000 years (290 A.D., 1249 A.D., 1276 A.D., 1473 A.D.), may have occurred on the Broni-Stradella thrust or on the blind San Colombano and Lodi thrusts farther north. This makes detailed paleoseismological studies and trenching across its trace mandatory.

[1] Subduction of Atlantic seafloor under the Caribbean plate causes shallow earthquakes within the Lesser Antilles volcanic arc. Such earthquakes, above the subduction interface, show strike-slip or normal fault plane solutions, the latter with ∼E-W striking nodal planes. To better assess seismic hazard and the coupling between volcanism and tectonics, we investigated faulting related to overriding-plate deformation in the Guadeloupe archipelago. Using aerial photographs, satellite SPOT images, and topographic maps (1/25000 scale), we mapped active and middle to late Pleistocene fissures and normal fault systems that cut the uplifted coral platforms Grande-Terre and Marie-Galante and the volcanic rocks of Basse-Terre. The available marine geophysical data show that the faults extend offshore to bound submarine rifts. The E-W striking, 1500 m deep, V-shaped Marie-Galante rift separates the two islands of Marie-Galante and Grande-Terre. Normal faults in the north of Grande-Terre appear to mark the similarly V-shaped, western termination of the 5000 m deep, N°50E to N130°E striking Desirade graben. Three shallow, M ∼ 5.5 earthquakes (6 May 1851, 29 April 1897, 3 August 1992) appear to have ruptured segments of the Marie-Galante rift boundary faults. The young “La Grande Decouverte” volcanic complex of Basse-Terre, including the 1440 A.D. Soufriere dome, lies within the western termination of the Marie-Galante rift. The ancient volcanic shoulders of the rift buttress the active dome to the north and south, which may explain why major prehistoric sector collapses and pyroclastic avalanches have been directed southwestward into the Caribbean Sea, or southeastward into the Atlantic Ocean. The Marie-Galante rift is typical of other troughs transverse to the northeastern edge of the Caribbean plate. We interpret such troughs, which are roughly orthogonal to the arc, to result from slip-partitioning and extension perpendicular to plate convergence. That they disappear southward implies that they result from interaction between the Caribbean and North American plates.

The Montello is an elongated hill about 15 km long and 5 km wide located south of the Venetian Alps front and ∼100 km southwest of Gemona, site of the destructive Ms ∼6, 1976 earthquake sequence. Mio-Pliocene strata in the core of the hill are folded. Seven Quaternary terraces across the western termination of the anticline have also been folded and uplifted. The terraces flank the abandoned Biadene valley, a former course of the Piave river which now flows eastwards along the north side of the hill. Topographic profiles along and transverse to the valley and terraces are used to measure the progressive development of the anticline. Fossil remains and archaeological sites dated with 14C suggest that the Biadene paleovalley was abandoned between 14 and 8 ka (11±3 ka). The successive terraces appear to have been emplaced at the onset of interglacials and interstadials, since about 350 ka. The best fitting terrace ages suggest vertical uplift rates of about 0.5 mm/yr before 172 ka and of about 1 mm/yr after 121 ka. The Montello thus appears to be a growing ramp anticline on top of an active, north dipping thrust that has migrated south of the mountain into the foreland. Modeling the deformation of the terraces as a result of motion on such a thrust ramp requires that it propagated both south and upwards with time but with a constant slip rate (1.8–2 mm/yr). For at least 300 kyr the lateral growth of the anticline kept pushing the course of the Piave river southwestwards, at a rate at first of 10 mm/yr, and then 20 mm/yr. Though the growth rate doubled more than 120 kyr ago, the anticline kept a constant height/length growth ratio (≃20) implying self-similar depth/length growth of the thrust underneath. The clustering of historical earthquakes north of Treviso suggests that the thrust responsible for ongoing folding of the Montello slipped seismically three times (778, 1268, 1859 A.D.; intensity I ≥ VIII) in the last 2000 years, with events of maximum magnitude close to 6 and with average recurrence time between 500 and 1000 years. NW shortening on NE-SW trending thrusts along the Venetian Alps front is compatible with the direction of convergence between Africa and Europe but does not suffice to absorb this convergence.

The localization and propagation of rifting between Arabia and Somalia are investigated by assessing the deformation geometry and kinematics at different scales between the eastern Gulf of Aden and the Gulf of Tadjoura, using bathymetric, magnetic, seismological, and structural evidence. Large-scale, southwestward propagation of the Aden ridge, markedly oblique to the Arabia-Somalia relative motion vector, began about 30 Myr ago between the Error and Sharbithat ridges. It was an episodic process, with stages of rapid propagation, mostly at rates >10 cm/yr, interrupted by million year pauses on transverse discontinuities coinciding with rheological boundaries between different crustal provinces of the Arabia-Somalia plate. The longest pause was at the Shukra-El Sheik discontinuity (≈45°E), where the ridge tip stalled for ≈13 Myr, between ≈17 and ≈4 Ma. West of that discontinuity, rifting and spreading took place at an azimuth (≈N25°±10°E) and rate (1.2±0.3 cm/yr) different from those of the global Arabia-Somalia motion vector (≈N39°, ≈1.73 cm/yr), implying an additional component of movement (N65°±10°E, 0.7±0.2 cm/yr) due to rotation of the Danakil microplate. At Shukra-El Sheik, the typical oceanic ridge gives way to a narrow, WSW trending axial trough, resembling a large fissure across a shallow shelf. This trough is composed of about eight rift segments, which result from normal faulting and fissuring along N110°–N130°E trends. All the segments step to the left southwestward, mostly through oblique transfer zones with en echelon normal faults. Only two segments show clear, significant overlap. There is one clear transform, the Maskali fault, between the Obock and Tadjoura segments. The latter segment, which encroaches onland, is composed of two parallel subrifts (Iboli, Ambabbo) that propagated northwestward and formed in succession. The most recent, southwestern subrift (Ambabbo) represents the current tip of the Aden ridge. We propose a mechanical model in which the large-scale propagation of the ridge followed a WSW trending zone of maximum tensile stress, while the small-scale propagation of its NW trending segments was dictated by the orientation of that stress. Oblique propagation was a consequence of passive lithospheric necking of the Arabia-Somalia plate along its narrow section, in map view, between Socotra and the kink of the Red Sea-Ethiopian rift, above the Afar plume. Individual ridge segments oriented roughly perpendicular to plate motion, like lithospheric cracks, were forced to jump southward because of confinement within the necking zone. Self-sustaining, plate-scale necking may explain why the Aden ridge did not connect with the Red Sea through Bab El Mandeb but continued straight into Afar.

We showed before that both the Aden and Red Sea plate boundaries are currently rifting and propagating along two distinct paths into Afar through the opening of a series of disconnected, propagating rifts. Here we use new geochronological, tectonic, and paleomagnetic data that we acquired mostly in the southeastern part of Afar to examine the geometry, kinematics, and time-space evolution of faulting related to strain transfer processes. It appears that transfer of strain is accommodated by a bookshelf faulting mechanism wherever rifts or plate boundaries happen to overlap without being connected. This mechanism implies the rotation about a vertical axis of small rigid blocks along rift-parallel faults that are shown to slip with a left-lateral component, which is as important as their normal component of slip (rates of ∼2–3 mm/yr). By contrast, where rifts do not overlap, either a classic transform fault (Maskali) or an oblique transfer zone (Mak'arrasou) kinematically connects them. The length of the Aden-Red Sea overlap has increased in the last ∼0.9 Myr, as the Aden plate boundary propagated northward into Afar. As a consequence, the first-order blocks that we identify within the overlap did not all rotate during the same time-span nor by the same amounts. Similarly, the major faults that bound them did not necessarily initiate and grow as their neighboring faults did. Despite these variations in strain distribution and kinematics, the overlap kept accommodating a constant amount of strain (7 to 15% of the extension amount imposed by plate driving forces), which remained distributed on a limited number (seven or eight) of major faults, each one having slipped at constant rates (∼3 and 2 mm/yr for vertical and lateral rates, respectively). The fault propagation rates and the block rotation rates that we either measure or deduce are so fast (30–130 mm/yr and 15–38°/Myr, respectively) that they imply that strain transfer processes are transient, as has been shown to be the case for the processes of tearing, rift propagation, and strain jumps in Afar.

It is generally accepted that the Aden ridge has propagated westward from ∼58°E to the western tip of the Gulf of Aden/Tadjoura, at the edge of Afar. Here, we use new tectonic and geochronological data to examine the geometry and kinematics of deformation related to the penetration of that ridge on dry land in the Republic of Djibouti. We show that it veers northward, forming a narrow zone of dense faulting along the northeastern edge of the Afar depression. The zone includes two volcanic rifts (Asal-Ghoubbet and Manda Inakir), connected to one another and to the submarine part of the ridge by transfer zones. Both rifts are composite, divided into two or three disconnected, parallel, NW-SE striking subrifts, all of which appear to have propagated northwestward. In Asal-Ghoubbet as in Manda Inakir, the subrifts appear to have formed in succession, through north directed jumps from subrifts more farther south. At present, the northernmost subrifts (Manda and Dirko Koma) of the Manda Inakir rift, form the current tip of the northward propagating Arabia-Somalia plate boundary in Afar. We account for most observations by a mechanical model similar to that previously inferred for the Gulf of Aden, in which propagation is governed by the intensity and direction of the minimum horizontal principal stress, σ3. We interpret the northward propagation on land, almost orthogonal to that in the gulf, to be related to necking of the Central Afar lithosphere where it is thinnest. Such necking may be a consequence of differential magmatic thickening, greater in the center of the Afar depression where the Ethiopian hot spot enhanced profuse basaltic effusion and underplating than along the edges of the depression. The model explains why the Aden ridge foregoes its WSW propagation direction, constant from ∼58°E to Asal-Ghoubbet. At a smaller scale, individual rifts and subrifts keep opening perpendicular to the Arabia-Somalia (or Danakil-Somalia) motion vector and propagate northwestward. Concurrently, such lithospheric cracks are forced to jump northward, such that the plate boundary remains inside the regional N-S necking zone. Changes of obliquity between the directions of overall and local propagation may account for different segmentation patterns, a small angle promoting long, en echelon subrifts, and a high-angle, smaller, nested, “subrifts within subrifts.” The propagation mechanism is thus similar, whether in oceanic or continental lithosphere, the principal change being the overall propagation path, here governed by thickness changes rather than by the geometry in map view as previously inferred for the rest of the Aden ridge. Finally, because the same mechanism has led rifting along the Red Sea to propagate southward and jump to the western edge of Afar, the Arabia-Somalia and Arabia-Nubia plate boundaries tips have missed each other and keep overlapping further, leading to strain transfer by large-scale bookshelf faulting.