ALBERT

Description: Nature Geoscience 8, 388 (2015). doi:10.1038/ngeo2401 Authors: A. Koptev, E. Calais, E. Burov, S. Leroy & T. Gerya Although many continental rifts and passive margins are magmatic, some are not. This observation prompted endmember views of the mechanisms driving continental rifting, where magma-rich or active rifts would be caused by deep mantle plumes, whereas magma-poor or passive rifts would result from the stretching of the lithosphere under far-field plate forces. The Central East African Rift provides a unique setting to investigate the mechanisms of continental rifting because it juxtaposes a magma-rich (eastern) branch and magma-poor (western) branch on either side of the 250-km-thick Tanzanian craton. Here we investigate this contrasted behavior using a high-resolution rheologically consistent three-dimensional thermo-mechanical numerical model. The model reproduces the rise of a mantle plume beneath a craton experiencing tensional far-field stress. In our numerical experiments the plume is deflected by the cratonic keel and preferentially channelled along one of its sides. This leads to the coeval development of magma-rich and magma-poor rifts along opposite craton sides, fed by melt from a single mantle source. Our numerical experiments show strong similarities to the observed evolution of the Central East African Rift, reconcile the passive and active rift models, and demonstrate the possibility of developing both magmatic and amagmatic rifts in identical geotectonic environments.

Description: Earthquake activity in parts of the central United States has increased dramatically in recent years. The space-time distribution of the increased seismicity, as well as numerous published case studies, indicates that the increase is of anthropogenic origin, principally driven by injection of wastewater coproduced with oil and gas from tight formations. Enhanced oil recovery and long-term production also contribute to seismicity at a few locations. Preliminary hazard models indicate that areas experiencing the highest rate of earthquakes in 2014 have a short-term (one-year) hazard comparable to or higher than the hazard in the source region of tectonic earthquakes in the New Madrid and Charleston seismic zones.

Description: The Red Sea arm of the triple junction in northeastern Ethiopia provides an opportunity to investigate rift-forming processes at divergent boundaries. In an attempt to study the subsurface, especially the distribution and role of melt in the rifting process, we carried out a high-precision gravity survey with a mean-square error of 0.011 mgal, assisted by differential global positioning system measurements. The profile is 162 km long and strikes ENE–WSW across the southern part of the Red Sea rift at a latitude of approximately 11.75° N. Modelling of the Bouguer anomaly, constrained by a priori information, showed detailed in-rift variations in the crustal structure and the distribution of melt beneath the rift axis. Our interpretation suggested that the process of continental break-up is governed by crustal stretching and rifting accompanied by the emplacement of melt into the lower crust above a lower density upper mantle. In addition, we interpreted the thickness of the crust beneath this part of the rift axis to be 25 km. The subsurface distribution of density beneath the profile shows that the south-central part of the Red Sea rift has modified thinned crust, intruded by high-density material, which resembles the crust formed during seafloor spreading.

Description: 〈span〉〈div〉SUMMARY〈/div〉The present-day kinematics of plate boundary deformation in the western Mediterranean is now well described by space geodetic measurements, except for the Algeria–Tunisia part of north Africa where information is still lacking. Yet, that portion of the Nubia–Eurasia Plate boundary likely concentrates most of the oblique plate convergence, with an on-going debate on the role of offshore versus on-land active—and seismogenic—structures. Here we use 10 yr of continuous GPS data from the Algerian REGAT network to compute the first geodetic velocity field in Algeria. In the west, velocities are uniform and collinear to the plate convergence trend, with a sharp gradient at the coast, while in the east they indicate that deformation involves a broader region. Using a simple block kinematic approach, we show that the velocities are consistent with the presence of an active, reverse, offshore fault system that runs along the toe of the Algerian margin, with a slip rate decreasing from west to east. In the western half of Algeria, the GPS data do not require additional faults system on-land. In the eastern half, GPS velocities require an E–W-trending strike-slip fault separating two blocks that accounts for the strike-slip component of the overall plate motion. We also observe significant shortening between the Saharan platform and the Aures range in southeastern Algeria. This first-order description of the Eurasia–Nubia oblique convergence in Algeria provides new guidelines for seismic hazard assessment but still requires denser geodetic measurements over this vast territory.〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉The present-day kinematics of plate boundary deformation in the western Mediterranean is now well described by space geodetic measurements, except for the Algeria-Tunisia part of north Africa where information is still lacking. Yet, that portion of the Nubia-Eurasia plate boundary likely concentrates most of the oblique plate convergence, with an on-going debate on the role of offshore versus on-land active – and seismogenic – structures. Here we use 10 years of continuous GPS data from the Algerian REGAT network to compute the first geodetic velocity field in Algeria. In the west, velocities are uniform and collinear to the plate convergence trend, with a sharp gradient at the coast, while in the east they indicate that deformation involves a broader region. Using a simple block kinematic approach, we show that the velocities are consistent with the presence of an active, reverse, offshore fault system that runs along the toe of the Algerian margin, with a slip rate decreasing from west to east. In the western half of Algeria, the GPS data do not require additional faults system on-land. In the eastern half, GPS velocities require an E-W-trending strike-slip fault separating two blocks that accounts for the strike-slip component of the overall plate motion. We also observe significant shortening between the Saharan platform and the Aures range in southeastern Algeria. This first-order description of the Eurasia-Nubia oblique convergence in Algeria provides new guidelines for seismic hazard assessment but still requires denser geodetic measurements over this vast territory.〈/span〉

Description: Kinematics of divergent boundaries and Rift-Rift-Rift junctions are classically studied using long-term geodetic observations. Since significant magma-related displacements are expected, short-term deformation provides important constraints on the crustal mechanisms involved both in active rifting and in transfer of extensional deformation between spreading axes. Using InSAR and GPS data, we analyse the surface deformation in the whole Central Afar region in detail, focusing on both the extensional deformation across the Quaternary magmato-tectonic rift segments, and on the zones of deformation transfer between active segments and spreading axes. The largest deformation occurs across the two recently activated Asal-Ghoubbet (AG) and Manda Hararo-Dabbahu (MH-D) magmato-tectonic segments with very high strain rates, whereas the other Quaternary active segments do not concentrate any large strain, suggesting that these rifts are either sealed during interdyking periods or not mature enough to remain a plate boundary. Outside of these segments, the GPS horizontal velocity field shows a regular gradient following a clockwise rotation of the displacements from the Southeast to the East of Afar, with respect to Nubia. Very few shallow creeping structures can be identified as well in the InSAR data. However, using these data together with the strain rate tensor and the rotations rates deduced from GPS baselines, the present-day strain field over Central Afar is consistent with the main tectonic structures, and therefore with the long-term deformation. We investigate the current kinematics of the triple junction included in our GPS data set by building simple block models. The deformation in Central Afar can be described by adding a central microblock evolving separately from the three surrounding plates. In this model, the northern block boundary corresponds to a deep EW-trending trans-tensional dislocation, locked from the surface to 10–13 km and joining at depth the active spreading axes of the Red Sea and the Aden Ridge, from AG to MH-D rift segments. Over the long-term, this plate configuration could explain the presence of the en-échelon magmatic basins and subrifts. However, the transient behaviour of the spreading axes implies that the deformation in Central Afar evolves depending on the availability of magma supply within the well-established segments.

Description: We use recently published, high-resolution reconstructions of the Southwest Indian Ridge to test whether a previously described systematic difference between Global Positioning System (GPS) and 3.16-Myr-average estimates of seafloor spreading rates between Antarctica and Africa is evidence for a recent slowdown in Southwest Indian Ridge seafloor spreading rates. Along the Nubia-Antarctic segment of the ridge, seafloor opening rates that are estimated with the new, high-resolution reconstructions and corrected for outward displacement agree well with geodetic rate estimates and reduce previously reported, highly significant non-closure of the Nubia-Antarctic-Sur plate circuit. The observations are inconsistent with a slowdown in spreading rates and instead indicate that Nubia-Antarctic plate motion has been steady since at least 5.2 Ma. Lwandle-Antarctic seafloor spreading rates that are estimated from the new high-resolution reconstructions differ insignificantly from a GPS estimate, thereby implying steady Lwandle-Antarctic plate motion since 5.2 Ma. Between the Somalia and Antarctic plates, the new Southwest Indian Ridge reconstructions eliminate roughly half of the systematic difference between the GPS and MORVEL spreading rate estimates.We interpret the available observations as evidence that Somalia-Antarctic spreading rates have been steady since at least 5.2 Ma and postulate that the remaining difference is attributable to random and/or systematic errors in the plate kinematic estimates and the combined effects of insufficient geodetic sampling of undeforming areas of the Somalia plate, glacial isostatic adjustment in Antarctica and transient deformation triggered by the 1998 M w = 8.2 Antarctic earthquake, the 2004 M w = 9.3 Sumatra earthquake, or possibly other large historic earthquakes.

Description: The northeastern Caribbean island arc, which materializes the boundary between the North American and Caribbean plates, is particularly exposed to large earthquakes and tsunamis. The low level of preparedness of a large part of its population and the lack of risk reduction provisions in public policies in many countries of the region put their population and economy at high risk in case of large telluric events. Here, we investigate the impact of three possible earthquake scenarios, consistent with the regional seismotectonic setting, on northern Haiti through inundation by tsunami waves. These scenarios simulate the effect of a M 8.0 earthquake on the Septentrional strike-slip fault (possibly similar to the 1842 earthquake), a M 8.1 earthquake on the offshore thrust fault system north of Haiti, and an earthquake rupturing a large portion of the offshore thrust fault system north of Haiti and the Dominican Republic. We calculate run-up heights along the northern coast of Haiti, in particular in the densely populated Cap Haitien. We find that the rupture of the offshore North Hispaniola thrust fault could result in wave heights up to 10 m with inundation up to 4 km inland, with only 10–15 min between ground shaking and the first wave arrivals. The city of Cap Haitien is particularly exposed, with potential flooding of most of the city and its suburbs, including the international airport. We also find that the historical reports available for the 1842 earthquake, when compared to our simulations, favor a rupture of the North Hispaniola thrust fault, although much uncertainty remains. If the 1842 earthquake did not rupture the Septentional fault offshore Haiti, then it is currently capable of at least a M w 7.7 earthquake, significantly larger than previously thought. The simulations presented here provide a basis for developing conservative maps of run-up heights that can be transferred, with added factors of safety, into practical implementation for tsunami preparedness and protection.

Description: Large earthquakes in stable continental regions remain puzzling as; unlike at plate boundaries, they do not result from the local buildup of strain driven by plate tectonics. The 2017 Mw6.5, Bostwana normal faulting earthquake occurred in a region devoid from recent tectonic activity and where present-day deformation is negligible. The depth of the event (29 ± 4 km), in a felsic lower crust where ductile deformation is expected, likely requires a transient pulse of fluids from a deep source to activate brittle faulting. The mainshock was preceded by two foreshock swarm-like sequences that may be further evidence for fluid movement in a critically loaded fault network. Contrary to plate boundary events, the Mw6.5 Botswana earthquake did not require prior localized stress or strain accumulation. We propose that the crust in stable continental regions, even long after the last tectonic episode, constitutes a reservoir of elastic stress that can be released episodically, for instance, as a result of deep fluid migration. ©2018. American Geophysical Union. All Rights Reserved.