Segment Of Plate Boundary Research Articles

Widespread fracture movements during a volcano-tectonic unrest: the Reykjanes Peninsula, Iceland, from 2019–2021 TerraSAR-X interferometry

Tectonic controls on dyke emplacements, eruption dynamics and locations have been observed in multiple volcanic areas worldwide. Mapping of active structures is therefore key for assessing potential tectonic and volcanic hazards in active regions. We used wrapped interferograms from the TerraSAR-X satellite to map active fracture movements over a 2-year period of a volcano-tectonic unrest at the onshore Reykjanes Peninsula plate boundary in SW Iceland. As of 1 December 2023, the unrest has included at least six inflation events and five dyke injections resulting in three eruptions of the Fagradalsfjall volcanic segment. In addition to the deformation associated with the 2019–2021 inflation events and intrusions, the interferograms reveal fracture movements over a wide area surrounding the active plate boundary segment. This first-order mapping of active fractures complements previously mapped structures, as InSAR allows for the detection of subtle ground movements, even in areas where young lava flows cover older structures. Our fracture data therefore fill in some of the apparent voids in previous fracture and fault maps of SW Iceland. Furthermore, our investigation reveals aseismic movement on previously unknown fractures directly beneath the town of Grindavík, as well as a N45∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^\\circ $$\\end{document} E striking fracture co-located with the longest lasting volcanic vent of the subsequent 2021 eruption. The mapping method we present in this study is relevant for active volcano-tectonic regions where InSAR can be applied to detect small-scale fracture movements to advance understanding of ongoing unrest and volcano-tectonic hazards.

Seismicity Induced at the Northern Dead Sea Transform Fault, Kinneret (Sea of Galilee) Basin, by Shallow Creep Involving a Salt Body

AbstractIn October 2013 and July–August 2018, two extensive earthquake swarms shook the northern reaches of Lake Kinneret (Sea of Galilee). Former studies explored the swarms, resulting in discrepant depths and mechanisms. Here, we attempt to settle the discrepant interpretations using alternative seismological methods and some unpublished data from borehole seismometers. We propose a hypothesis for the faulting phenomenon focusing on the interaction of the two plate boundary segments that step‐over the northern Kinneret depocenter: a creeping segment in the south and a locked segment in the north. The energy accumulated from the interaction induces earthquake swarms from time to time. A shallow fault patch (from the surface down to 1.5 ± 1.0 km) south of the swarms is thought to creep in association with a salt formation underlying some of the basin fill. We use regional seismograms, including two near‐source borehole stations, to refine the characteristics of the swarms. We test hypocentral and centroid depths using several methods and different velocity models and corroborate shallow ruptures: the majority are shallower than 6 km (all shallower than ≤10 km). The hypocentral locations and focal mechanisms suggest shallow NW‐SE normal faults splaying from the tip of the creeping segment in a horsetail pattern. We test the hypothesis by Coulomb stress calculations that show stress concentration at the step‐over interaction zone, consistent with focal locations and mechanisms of the earthquake swarms.

Reverse Faulting Within a Continental Plate Boundary Transform System

AbstractContractional deformation is common along transform plate margins where plate motion is oblique to the plate boundary. While faults that accommodate this deformation are often inferred to be subsidiary to the main plate boundary fault, we typically lack direct geometric or kinematic information. Here we investigate the timing of faulting relative to changes in the orientation of the North American‐Pacific plate boundary. Coeval with development of an oblique plate boundary segment (i.e., the “Big Bend” of the San Andreas fault), active shortening is inferred to have initiated at ∼5 Ma in the Western Transverse Ranges (WTR). However, new low‐temperature thermochronometric data yield Miocene to Pleistocene apatite (U‐Th‐Sm)/He cooling ages and partially reset zircon (U‐Th)/He ages. Inverse thermal modeling indicate that reverse faulting initiated as early as 10 Ma, several million years prior to our current understanding of the timing of the Big Bend. Reverse faults in the WTR also propagate from west to east, toward the San Andreas Fault, rather than outwards from it. New and existing thermochronometry data delineate the WTR as the locus of rapid post‐Miocene exhumation, and demonstrate that similar exhumation is not present in the broader region surrounding the Big Bend. We posit that reverse faulting is localized in the WTR because of a weak underlying lithosphere and predates the more recent geometric anomaly of the restraining bend in the transform margin.

New focal mechanisms reveal fragmentation and active subduction of the Antalya slab in the Eastern Mediterranean

In the Eastern Mediterranean, the convergence of Nubia and Eurasia is accommodated along the Hellenic and Cyprus Trenches. The plate geometry and mode of convergence to the east of the Hellenic Trench remains poorly understood. Specifically, along the west of Cyprus, tomography models reveal NW striking high velocity anomaly, which is interpreted as a slab named as the Antalya slab. However, near surface observations pertaining to the top 15 km of the convergence zone show no evidence of an active subduction in the region. In this study, we investigate the seismic activity along the Antalya slab in order to constrain the mode of convergence and the geometry along this plate boundary. Towards this end, we obtain a new focal mechanism catalogue with reliable centroid depths for earthquakes Mw > 4 from regional waveforms by using path dependent seismic velocity models. Our results indicate that the subduction is still active along the plate boundary segment between Cyprus and Anaximander seamount. The earthquakes along the slab interface reveal a NE-dipping slab with a dip angle of ~35°. Slip vectors show a pure convergent motion which is consistent with relative motion of Anatolia with respect to Nubia. We also identify a number of intraslab earthquakes down to a depth of 120 km along the Antalya slab. The T-axis of intraslab earthquakes show down-dip extension consistent with negatively buoyant subducting slab. The Paphos fault bounds the Antalya slab to the east where previously large right-lateral earthquakes occurred, which marks the relative motion of the Antalya slab with respect to the eastern Cyprus slab. Based on slab isodepth contours of the slab interface, we calculate ~100 km offset between the 80 km depth contours of the Antalya and Cyprus slabs. The western boundary of the Antalya slab possibly lies between the Anaximander and Anaximenes seamounts, where several left-lateral earthquakes are obtained. The western boundary of the Antalya slab can also be identified from orientation change of normal fault earthquakes along overriding plate which shows the limit of N-S extension due to roll-back of the neighboring Hellenic slab.

Seismicity, Stress State, and Style of Faulting of the Ridgecrest-Coso Region from the 1930s to 2019: Seismotectonics of an Evolving Plate Boundary Segment

ABSTRACTDecadal scale variations in the seismicity rate in the Ridgecrest-Coso region, part of the Eastern California Shear Zone, included seismic quiescence from the 1930s to the early 1980s, followed by increased seismicity until the 2019 Mw 6.4 and 7.1 Ridgecrest sequence. This sequence exhibited complex rupture on almost orthogonal faults and triggered aftershocks over an area of ∼90 km long by ∼5–10 km wide, which is a fraction of the area of the previously seismically active Indian Wells Valley and Coso range region. During the last 40 yr, the seismicity has been predominantly the result of strike-slip motion, extending north from the Garlock fault, along the Little Lake and Airport Lake fault zones, and approaching the southernmost Owens Valley fault to the north. The Coso range forms an extensional stepover between these two strike-slip fault systems. This evolution of a plate boundary zone is driven by the northwestward motion of the Sierra Nevada, and crustal extension along the southwestern edge of the Basin and Range Province. Stress inversion of focal mechanisms shows that the postseismic stress state consists of almost horizontal σ1 and vertical σ2. The σ1 is spatially rotated across the Coso range stepover with σ1-trending ∼N17° E to the north, whereas, along the Mw 7.1 mainshock rupture, the trend is ∼N6° E. The friction angles as measured between fault strikes and the σ1 trends correspond to a frictional coefficient of 0.75, suggesting average fault strength. In comparison, the mature Garlock fault has a smaller frictional coefficient of 0.28, similar to weak faults like the San Andreas fault. Thus, it appears that the heterogeneously oriented and spatially distributed but strong Ridgecrest-Coso faults accommodate seismicity at seemingly random places and times within the region and are in the process of self-organizing to form a major throughgoing plate-boundary segment.

Strike-slip related folding within the Malvinas/Falkland Trough (south-western Atlantic ocean)

The Malvinas/Falkland Trough (M/FT) is an E-W trending bathymetric depression along which runs the present-day South American-Scotia plate boundary. To the north of the M/FT lies the southern sector of the South Malvinas/Falkland Basin, and to the south lies the Burdwood Bank, an elongated morphological high constituting part of the South Scotia Ridge. Analysis of bathymetric and slope maps, integrated with seismic reflection profiles, have allowed describing in detail the M/FT in the sector comprised between 60° and 57° W. Data show the presence of an array of folds forming a thin-skinned fold-and-thrust Belt (FTB), previously interpreted as an active compressional field. The thin-skinned FTB is developed in a triangular-shaped area, which extends ca. 100 km in the E-W direction and 16 km in the N-S direction. Four parallel, asymmetric folds that develop to progressively greater water depths (between 1600 and 2600 m depth) towards the north, with orientations ESE-WNW and lengths that vary between 22 and 98 km, have been recognized. These folds are constituted by Late Cenozoic sediments that are undeformed in the Malvinas/Falkland Trough. The development of folds, which began in the late Miocene (ca. 7 Ma), was associated with the left-lateral strike-slip tectonic regime of the Magallanes-Fagnano-Malvinas lineament, the western segment of the South American-Scotia plate boundary.

Evidence of Segmentation in the Iberia–Africa Plate Boundary: A Jurassic Heritage?

The present structure of the Iberia–Africa plate boundary between the Gorringe Bank and the Algerian Basin is characterized by a highly segmented geometry and diffused seismicity. Filtered Bouguer gravity data show conspicuous highs coinciding with the Gorringe Bank, the Guadalquivir–Portimao Bank, and the Ronda/Beni–Bousera massifs, reflecting the current geometry of the plate boundary segments. The Africa–Eurasia Alpine convergence produced crustal-scale thrusting in the Atlantic segments and roll-back subduction in the Ligurian–Tethys segments. Despite the growing consensus that the Gorringe and the Guadalquivir–Portimao Banks resulted from tectonic inversion of hyperextended margin structures inherited from the Early Jurassic, this heritage is more debatable for the Ronda/Beni–Bousera massifs lacking models linking the Atlantic and Mediterranean realms. On the basis of gravity analysis combined with plate reconstruction models, geological cross-sections, and recent local tomography, we infer a strong Jurassic heritage of the present-day segmentation and substantiate a comprehensive tectonic evolution model of the Iberia–Africa plate boundary since the Early Jurassic to Recent that includes the Atlantic and the Mediterranean domains.

Rupture of the Indian slab in the 2011 Mw 6.9 Sikkim Himalaya earthquake and its tectonic implications

Unlike the other Himalayan plate boundary segments, the eastern Nepal to Bhutan Himalayan region is not known to have generated prominent shallow thrust faulting earthquakes, typical of the ongoing convergence. This region features strike-slip earthquakes over the depth ranges of 40�120 km, indicating intraslab deformation. Here we present for the first time a slip distribution model for the largest recorded intraslab strike-slip earthquake in this region, the Mw 6.9 Sikkim event that occurred on 18 September 2011. Relying on kinematic source process modeling, our results indicate a NE-SW trending, steeply dipping sinistral source zone within the underthrusting Indian slab. The rupture propagated radially, with a low rupture velocity of 1.7 km/s, breaking a large asperity of 20 A� 20 km 2 with a maximum slip of 1.6 m. The rupture nucleated at a depth of 45 km and reached upper mantle depths. The computed coseismic stress drop value is 13.6 MPa. We suggest that most of the aftershocks occurred on the conjugate plane, possibly due to stress triggering. Stress inversion of focal mechanisms indicates a transpressive stress regime throughout the crust and pure strike-slip regime in the upper mantle. We observed a unimodal distribution of earthquakes beneath the Higher Himalaya. This indicates a strong, brittle Indian slab and unravels a scenario of an eventual breakup of the lithosphere; the key trigger might be variation in the convergence rates along the Himalayan arc. ©2019. American Geophysical Union. All Rights Reserved.

Dynamics of the Zones of Strong Earthquake Epicenters in the Arctic–Asian Seismic Belt

Our comprehensive study of the Russian Arctic region aims to clarify the features and types of seismotectonic deformation of the crust in the Arctic–Asian Seismic Belt, specifically in the zones of strong earthquakes in the Laptev Sea Segment, the Kharaulakh Segment, and the Chersky Seismotectonic Zone. We have analyzed modern tectonic structures and active fault systems, as well as tectonic stress fields reconstructed by tectonophysical analysis of the Late Cenozoic faults and folds. The investigated neotectonic structures are ranked with respect to the regional classification principles. Changes in the crustal stress–strain state in the lithospheric plate boundaries between the Eurasian, North American, and Okhotsk Sea Plates are analyzed, and regularities of such changes are discovered. A set of models has been constructed for the studied segments of plate boundaries with account of the dynamics of the regional geological structures. The models can give a framework for the assessment of potential seismic risks of seismogenerating structures in the Russian Arctic region.

REGAT: A permanent GPS network in Algeria, configuration and first results

The REGAT (“REseau Géodésique de l'ATlas”) geodetic network is composed of 53 continuously–recording GPS stations distributed in the Algerian Atlas. It spans the whole width of the Algerian coast and reaches 300 km inland, with inter-sites distance of about 100 km. One additional site is located in Tamanrasset in the southernmost part of the country. The network, whose oldest stations started operating in 2007, encompasses the main active tectonic features of the most seismically active segment of the Nubia-Eurasia plate boundary in the Western Mediterranean. Here we describe the network configuration, the data collection and analysis strategy, as well as some preliminary results on horizontal GPS velocities. A detailed analysis of the velocity field in terms of plate boundary kinematics is the topic of a separate publication. The REGAT network fills an important gap in our knowledge of present-day plate boundary deformation in the Western Mediterranean. It will soon be enhanced by an additional 100 sites in order to improve deformation monitoring with a higher spatial resolution for a better assessment of the regional seismic hazard.

Rupture of the Indian Slab in the 2011 Mw 6.9 Sikkim Himalaya Earthquake and Its Tectonic Implications

AbstractUnlike the other Himalayan plate boundary segments, the eastern Nepal to Bhutan Himalayan region is not known to have generated prominent shallow thrust faulting earthquakes, typical of the ongoing convergence. This region features strike‐slip earthquakes over the depth ranges of 40–120 km, indicating intraslab deformation. Here we present for the first time a slip distribution model for the largest recorded intraslab strike‐slip earthquake in this region, the Mw 6.9 Sikkim event that occurred on 18 September 2011. Relying on kinematic source process modeling, our results indicate a NE‐SW trending, steeply dipping sinistral source zone within the underthrusting Indian slab. The rupture propagated radially, with a low rupture velocity of 1.7 km/s, breaking a large asperity of 20 × 20 km2 with a maximum slip of 1.6 m. The rupture nucleated at a depth of 45 km and reached upper mantle depths. The computed coseismic stress drop value is 13.6 MPa. We suggest that most of the aftershocks occurred on the conjugate plane, possibly due to stress triggering. Stress inversion of focal mechanisms indicates a transpressive stress regime throughout the crust and pure strike‐slip regime in the upper mantle. We observed a unimodal distribution of earthquakes beneath the Higher Himalaya. This indicates a strong, brittle Indian slab and unravels a scenario of an eventual breakup of the lithosphere; the key trigger might be variation in the convergence rates along the Himalayan arc.

Formation and migration of hydrocarbons in deeply buried sediments of the Gulf of Cadiz convergent plate boundary - Insights from the hydrocarbon and helium isotope geochemistry of mud volcano fluids

A geochemical study of the composition of hydrocarbon gases and helium isotopes (3He/4He) in fluids from Mud Volcanoes (MVs) located on and out of the active accretionary wedge of the Gulf of Cadiz (GoC) provides information on fluid sources and migrations in deeply buried sediments. The GoC is a tectonically active segment of the Africa-Iberia plate boundary occluded beneath the thick sediments of an accretionary wedge dissected by crustal-scale strike-slip faults. Initially built during the Miocene Gibraltar Arc subduction, the wedge has since developed toward the W-NW in an oblique convergent setting. Interstitial water expelled from clays undergoing diagenesis in buried sediments drives mud volcanism on the wedge, with MVs located along strike-slip faults mediating fluid ascent. The large excess of radiogenic helium (4He) in all GoC fluids agrees with a clay mineral dehydration source of water. Hydrocarbon gases from all deepwater MVs bear methane having similar stable carbon isotope compositions of ~−50‰VPDB whether fluids are highly enriched in methane relative to heavier homologues (C2+) or not (Methane / (Ethane + Propane) ~10 to 10,000). We suggest that methane with −50‰VPDB was largely diffused out of early generating source rocks, and became dissolved in the water expelled by the buried sediments. Consistently, low 3He/4He ratios suggest an open hydrocarbon system: Petroleum accumulations and 3He dissolved in the original sedimentary pore water have mostly escaped into the water column during the major Late Neogene compressional events.At present, some MVs vent CH4-rich fluids from dewatering sediments, while other structures located on active thrusts additionally vent C2+-rich gases generated by active Cretaceous source intervals. By contrast, evaporitic seals preserved petroleum accumulations on the shallow Moroccan Margin, while the westernmost MVs located out of the accretionary wedge vent microbial gas.

Seismicity of the Reykjanes Peninsula 1971–1976

The Reykjanes Peninsula Oblique Rift, a segment of the mid-Atlantic plate boundary, connects the Reykjanes Ridge to the rift zones of Iceland. The axis of the deformation zone of the plate boundary can be traced as a rather narrow epicentral zone of earthquakes that is highly oblique to the spreading direction of the North America Plate with respect to the Eurasia Plate. The seismicity of the area is episodic. Active periods with a duration of about a decade are separated by more quiet periods of one to two decades. Our observation period spans a good part of an episode that began in 1967 and ended in 1975. Nine earthquake swarms illuminate a 50 km long segment of the plate boundary. The epicentral zone cuts obliquely across the fissure swarms of the volcanic systems of the Peninsula. Geothermal areas are preferentially located at the intersection of the fissure swarms with the seismic zone. Outside the intersections with the seismic zone the fissure swarms show very low seismicity. The tectonic characteristics gradually change along the plate boundary segment, from being similar to those of a spreading segment in the western part to being more like a transform zone in the eastern part. This is seen in the type of earthquake sequences, maximum magnitude, and type of faulting.

Cenozoic tectonic and thermal history of the Nenana basin, central interior Alaska: new constraints from seismic reflection data, fracture history, and apatite fission-track analyses

The Nenana basin of interior Alaska forms a segment of the diffuse plate boundary between the Bering and North American plates and is located within a complex zone of crustal-scale strike-slip deformation that accommodates compressional stresses in response to oblique plate convergence to the south. The basin is currently the focus of new oil and gas exploration. Integration of seismic reflection and well data, fracture data, and apatite fission-track analyses with regional data improves our understanding of the tectonic development of this continental strike-slip basin. The Nenana basin formed during the Late Paleocene as a 13 km wide half-graben, affected by regional intraplate magmatism and localized crustal thinning across the Minto Fault in south-central Alaska. The basin was uplifted and exhumed along this faulted margin in the Early Eocene through to Late Oligocene in response to oblique subduction along the southern Alaska margin. This event resulted in the removal of up to 1.5 km of Late Paleocene strata from the basin. Renewed rifting and subsidence during the Early Miocene widened the basin to the west resulting in deposition of Miocene non-marine clastic rocks in reactivated and newly formed extensional half-grabens. In the Middle to Late Miocene, left lateral strike-slip faulting was superimposed on this half-graben system, with rapid subsidence beginning in the Pliocene and continuing to the present day. At present, the Nenana basin is in a zone of transtensional deformation that accommodates compressional stresses in response to oblique plate convergence and allows tectonic subsidence by oblique extension along major basin-bounding strike-slip faults.

The 2016Mw 6.7 Imphal Earthquake in the Indo‐Burman Range: A Case of Continuing Intraplate Deformation within the Subducted Slab

The 2016 M-w 6.7 Imphal earthquake is one of the largest and instrumentally well-recorded seismic events to have occurred on the segment of the Indo-Burman plate boundary, where Indian and Burman plates converge in a roughly northeast direction (similar to 3-4 cm/yr). This segment of the plate boundary has not generated any great earthquakes in the documented history but is noted for its complex intraslab deformation, arc-parallel compression, and slip partitioning with the continental strike-slip Sagaing fault. Previous studies have noted a mix of faulting styles and hypocentral distribution, indicating the inherent complexities within this segment. Based on the teleseismic moment inversion of 88 P and SH waveforms from 70 global broadband seismic stations, we use the source model of the 2016 earthquake to corroborate the continuing intraslab deformation. The earthquake is sourced at a depth of 55 km, where the dip of the slab changes to similar to 50 degrees with the P axis oriented in the north-northeast-south-southwest direction. Our analysis of azimuths and plunges of P axes of 214 earthquakes (M-w >= 4: 5, Global Centroid Moment Tensor) is consistent with this trend. Previous studies have noted two distinct types of intraplate earthquakes along this segment: shallower events (25-75 km) due to arc-parallel bending and the deeper ones (90-150 km) in response to slab pull. Interplate seismicity is almost absent, and the potential for great earthquakes seems much lower compared with the rest of the eastern India plate boundary.

Lithospheric deformation in the Africa‐Iberia plate boundary: Improved neotectonic modeling testing a basal‐driven Alboran plate

AbstractWe present an improved neotectonic numerical model of the complex NW Africa‐SW Eurasia plate boundary segment that runs from west to east along the Gloria Fault up to the northern Algerian margin. We model the surface velocity field and the ongoing lithospheric deformation using the most recent version of the thin‐shell code SHELLS and updated lithospheric model and fault map of the region. To check the presence versus the absence of an independently driven Alboran domain, we develop two alternative plate models: one does not include an Alboran plate; another includes it and determines the basal shear tractions necessary to drive it with known velocities. We also compare two alternative sets of Africa‐Eurasia velocity boundary conditions, corresponding to geodetic and geological‐scale averages of plate motion. Finally, we perform an extensive parametric study of fault friction coefficient, trench resistance, and velocities imposed in Alboran nodes. The final run comprises 5240 experiments, each scored to geodetic velocities (estimated for 250 stations and here provided), stress direction data, and seismic strain rates. The model with the least discrepancy to the data includes the Alboran plate driven by a basal WSW directed shear traction, slightly oblique to the westward direction of Alboran motion. We provide estimates of long‐term strain rates and slip rates for the modeled faults, which can be useful for further hazard studies. Our results support that a mechanism additional to the Africa‐Eurasia convergence is required to drive the Alboran domain, which can be related to subduction processes occurring within the mantle.

Estimated Casualties in a Possible Great Earthquake along the Pacific Coast of Mexico

Faults may rupture across several segments of plate boundaries, generating earthquakes of M 9 class. Along the Pacific coast of Mexico, a rupture of 450 km on 28 March 1787 generated an M w 8.6 earthquake. Intensity values ( I ) of VII were reported at the Atlantic coast, and the maximum was XI in Oaxaca. Here, we estimate the number of casualties if an M 9 rupture were to occur at present. We verified that our software tool, QLARM, with its database, estimates intensities and fatalities approximately correctly for historic earthquakes along the Pacific coast. Our test set consists of 10 earthquakes ( M w 7.3–8.6) that caused between 0 and ∼10,000 fatalities between 1787 and 2003. Requirements for a satisfactory match are that the maximum I agrees within 0.5 units, the extent of the I =VII area agrees approximately, and the fatality count differs by not more than a factor of 2 or by 200 fatalities, whichever is larger. Results for Mexico, without the special case of Mexico City, suggest that an M 9 earthquake would cause approximately 27,000 fatalities and 120,000 injuries. Applying amplification factors to the ground motion in 11 of 15 districts, we estimate that, in an M 9 along the Pacific coast, 100,000 people may perish and 480,000 may be injured in Mexico City. The number of people in settlements with fewer than 20,000 inhabitants exposed to large I exceed the exposed population in larger cities by factors of 3–10. The estimated mortality rate in rural areas is several times higher than in major population centers.

An abandoned rift in the southwestern part of the South Scotia Ridge (Antarctica): Implications for the genesis of the Bransfield Strait

AbstractThe western segment of the South Scotia Ridge is a narrow submarine ridge composed of continental crustal blocks, which hosts the left‐lateral boundary between the Scotia and Antarctic plates. This plate boundary segment gradually merges westward with the Bransfield Strait, a young (<4 Ma) basin located between the South Shetland Islands and the northern segment of the Antarctic Peninsula. Here we present a series of multichannel seismic profiles that reveal the presence of a linear chain of submarine volcanic edifices running parallel to the present‐day Scotia‐Antarctic plate boundary. These edifices consist of alkali basalts (~4 Ma), as shown by published data from a dredge haul. The occurrence, distribution, and shape of these magmatic manifestations suggest that they were emplaced in a rift‐related environment, associated with the activity of the former western segment of the Scotia‐Antarctic plate boundary. When spreading centers of the Phoenix‐Antarctic ridge NW of the South Shetland Islands became inactive at about 3.3 Ma, the former western Scotia‐Antarctic plate boundary was abandoned, and it moved to the present‐day location of the Bransfield Strait, where the motion was accommodated, and where active extension is ongoing. The proposed mechanism for the development of the Bransfield Strait, consistent with the temporal sequence of tectonic events, reconciles discrepancies in tectonic evolutionary models presented till now and does not require the presence of slab‐pull and/or slab retreat forces. The Bransfield Strait is not a classic back‐arc basin, but rather an actively extending marginal basin at the transition from mature rifting to incipient, punctiform spreading.

Gradual unlocking of plate boundary controlled initiation of the 2014 Iquique earthquake.

On 1April 2014, Northern Chile was struck by a magnitude 8.1 earthquake following a protracted series of foreshocks. The Integrated Plate Boundary Observatory Chile monitored the entire sequence of events, providing unprecedented resolution of the build-up to the main event and its rupture evolution. Here we show that the Iquique earthquake broke a central fraction of the so-called northern Chile seismic gap, the last major segment of the South American plate boundary that had not ruptured in the past century. Since July 2013 three seismic clusters, each lasting a few weeks, hit this part of the plate boundary with earthquakes of increasing peak magnitudes. Starting with the second cluster, geodetic observations show surface displacements that can be associated with slip on the plate interface. These seismic clusters and their slip transients occupied a part of the plate interface that was transitional between a fully locked and a creeping portion. Leading up to this earthquake, the bvalue of the foreshocks gradually decreased during the years before the earthquake, reversing its trend a few days before the Iquique earthquake. The mainshock finally nucleated at the northern end of the foreshock area, which skirted a locked patch, and ruptured mainly downdip towards higher locking. Peak slip was attained immediately downdip of the foreshock region and at the margin of the locked patch. We conclude that gradual weakening of the central part of the seismic gap accentuated by the foreshock activity in a zone of intermediate seismic coupling was instrumental in causing final failure, distinguishing the Iquique earthquake from most great earthquakes. Finally, only one-third of the gap was broken and the remaining locked segments now pose a significant, increased seismic hazard with the potential to host an earthquake with a magnitude of >8.5.

Earth science: Warning signs of the Iquique earthquake.

An earthquake off Chile in 2014 occurred in a region where a great seismic event was expected. Two studies reveal that months of foreshocks and slow slip on the associated plate-boundary fault preceded the event. See Letters p.295 & p.299 Two groups publishing in this issue of Nature analyse the seismic context of the Iquique earthquake that occurred off the coast of northern Chile on 1 April 2014 in a seismic zone that had been quiescent since a significant event in 1877. Gavin Hayes et al. identify areas of remaining or elevated earthquake hazard along the megathrust fault in the region, and conclude that the 2014 Iquique event was not the earthquake that had been anticipated. Given that significant sections of the northern Chile subduction zone have not ruptured in almost 150 years, they suggest that it is likely that future megathrust earthquakes will occur south and potentially north of the 2014 Iquique sequence. Bernd Schurr et al. show that the April 2014 earthquake broke a central fraction of the 'northern Chile seismic gap', the last major segment of the South American plate boundary that had yet to rupture in the past century. From July 2013 up to the April earthquake they identify three seismic clusters along this part of the plate boundary, each lasting a few weeks, with earthquakes of increasing peak magnitudes. They conclude that these seismic clusters and their slip transients reflect a gradual weakening of the central part of the seismic gap that was instrumental in initiating the final failure.