Gloria Fault Research Articles

On the focal mechanism of the 26.05.1975 North Atlantic event contribution from tsunami modeling

The 1975 May 26 earthquake, of magnitude Ms = 7.9, occurred in the North Atlantic close to the Azores Archipelago. Its epicentre, as given by US Geological Survey, was 17.5° W, 35.9° N, 200 km south of the Gloria Fault. Several authors determined the focal mechanism as a dextral strike-slip event with no significant dip-slip component, compatible with the relative motion between Eurasia and Nubia plates but away from the presumed plate boundary. The 1975 earthquake generated a tsunami of small amplitude, recorded at the Portuguese tide-gauge network, in Spain and Northern Africa. The peculiar location of the earthquake and tsunami source and the generation of a noticeable tsunami were already discussed by several authors, but up to now, no direct modelling of the tsunami generation and propagation was made to judge the set of source solutions obtained by seismological analysis. In this paper, we present tsunami simulations, backward ray tracing and forward non-linear shallow water simulations using data from Iberia and Azores and Northern Africa. We show that a good fit between observed data and synthetic waveforms can be obtained with a focal mechanism with no significant dip-slip component, favouring its interpretation as almost pure dextral strike-slip event located in an old fracture zone south of Gloria Fault.

Inversion of moment tensor focal mechanisms for active stresses around the microcontinent Iberia: Tectonic implications

The Iberian microcontinent and its connected oceanic crust are affected by deformations related to the Eurasian‐African plate boundary. Active stress inversions from populations of moment tensor focal mechanisms have been performed around and inside the Iberian peninsula, using a total of 213 moment tensor estimates. Main results are as follows: (1) The tensorial solutions show better consistency and lower misfits compared to those obtained previously from first P arrival focal mechanisms. (2) Along the Eurasia‐Africa western boundary, the type of active stresses progressively changes easternward from triaxial extension to uniaxial compression along the Terceira Ridge, the Gloria Fault zone, and the Gulf of Cadiz. (3) In the Betics‐Alboran‐Rif zone, uniaxial extension predominates with Shmax N155°E trending. (4) In northern Algeria, uniaxial compression reappears. (5) The Iberian foreland is currently under strike‐slip to uniaxial extension tensorial conditions.

Evidence for a post-3.16-Ma change in Nubia–Eurasia–North America plate motions?

We combine updated GPS velocities from the Nubian (NU), Eurasian (EU), and North American (NA) plates with 500 new 3.16-Myr-average seafloor spreading rates and nine transform fault azimuths from the northern Atlantic and Arctic basin seafloor spreading centers to estimate and test for changes in the relative motion between these plates. The numerous new seafloor spreading rates and GPS velocities improve our ability to detect recent changes in the relative motions of these plates. The angular velocity vector that best fits the EU–NA GPS velocities lies significantly north of the 3-Ma-average pole, in accord with previously published geologic evidence that the EU–NA pole has migrated northward since ∼3 Ma. Although we also find evidence for a significant post-3-Ma change in NU–NA motion, it is less compelling because the Nubian plate GPS velocity field is sparse and NU–NA seafloor spreading rates appear to have remained steady within the 1 mm yr −1 uncertainties if we systematically decrease the seafloor spreading rates to correct for outward displacement of seafloor spreading magnetic lineations. The NU–EU pole derived from GPS site velocities lies more than 30 angular degrees south of the tightly constrained 3-Ma-average estimate and predicts significantly slower and more oblique present-day NU–EU convergence in the Mediterranean. Both models for NU–EU motion pass a key test for their accuracy, namely, they correctly predict strike-slip motion along the well-mapped Gloria fault east of the Azores. The change to more oblique NU–EU motion may reflect increasing difficulty in maintaining margin-normal convergence within this continent–continent collision zone.

Neotectonic modelling of the western part of the Africa–Eurasia plate boundary: from the Mid-Atlantic ridge to Algeria

In this work we use the thin-shell approximation to model the neotectonics of the western part of the Africa–Eurasia plate boundary, extending from the Mid-Atlantic ridge to Tell Atlas (northern Algeria). Models assume a nonlinear rheology and include laterally variable heat flow, elevation, and crust and lithospheric mantle thickness. Including the Mid-Atlantic ridge permits us to evaluate the effects of ridge push and to analyse the influence of the North America motion on the area of the Africa–Eurasia plate boundary. Ridge push forces were included in a self-consistent manner and have been shown to exert negligible effects in the neotectonics of the Iberian Peninsula and northwestern Africa. Different models were computed with systematic variation of the fault friction coefficient. Model quality was scored by comparing predictions of anelastic strain rates, vertically integrated stresses and velocity fields to data on seismic strain rate computed from earthquake magnitude, most compressive horizontal principal stress direction, and seafloor spreading rates on the Mid-Atlantic ridge. The best model scores were obtained with fault friction coefficients as low as 0.06–0.1. The velocity boundary condition representing spreading on the Mid-Atlantic ridge is shown to produce concentrated deformation along the ridge and to have negligible effect in the interior of the plates. However, this condition is shown to be necessary to properly reproduce the observed directions of maximum horizontal compression on the Mid-Atlantic ridge. The maximum fault slip rates predicted by the model are obtained along the Mid-Atlantic ridge, Terceira ridge and Tell Atlas front. Relatively high slip rates are also obtained in the area between the Gloria fault and the Gulf of Cadiz. We infer from our modelling a significant long-term seismic hazard for the Gloria fault, and interpret the absence of seismicity on this fault as possibly due to transient elastic strain accumulation. The present study has also permitted better understanding of the geometry of the Africa–Eurasia plate boundary from the Azores triple junction to the Algerian Basin. The different deformational styles seem to be related to the different types of lithosphere, oceanic or continental, in contact at the plate boundary.

Thin‐shell modeling of neotectonics in the Azores‐Gibraltar Region

We applied the thin‐shell neotectonic modeling method to study the neotectonics of the Africa/Eurasia plate boundary in the Azores‐Gibraltar region. The plate boundary consists of a simple fault system running from Azores to the Gorringe Bank where it branches along the Betics and Rift‐Tell thrust fronts. Major faults in west Iberia and NW Africa have also been incorporated. Results are compared with seismic strain rates, fault slip rates and stress orientations. The best estimate for the fault friction coefficient is 0.1–0.15 meaning that the plate‐boundary is only about 1/4 as strong as the adjacent lithosphere. The largest fault slip rates (>1.5 mm/yr) are obtained along the Gloria fault (strike‐slip), and the Betic (transpressive) and Rif‐Tell (compressive) thrust systems. Whereas tectonic activity in the Atlas region is comparable to that obtained along the plate boundary, the fault slip rates in the west Iberia fault systems are one order of magnitude less.

Morpho-tectonic analysis of the Azores Volcanic Plateau from a new bathymetric compilation of the area

The existing studies of the Azores triple junction, although based on specific geological or geophysical data, largely rely upon morphological considerations. However, there is no systematic bathymetric coverage of this area, and the quality of the available bathymetric charts does not allow consistent morpho-structural analysis. In this work we present a new bathymetric grid elaborated with all the available data sources in an area comprised between 24° W to 32° W and 36° N to 41° N. The basic data set corresponds to the merge of NGDC data with new swath profiles. This new map, included as an Appendix, combined with other results from seismology and neotectonics, is the basis for the study of the morpho-structural pattern of the Azores area, the present day stress field and its implications on the current view of the Azores geodynamics. As a major result, we conclude that the Azores region is controlled by two sets of conjugated faults with 120° and 150° strikes that establish the framework for the onset of volcanism, expressing as linear volcanic ridges or as point source volcanism. This interaction develops what can be considered as the morphological signature of the Azores Spreading axis segmentation. We argue that the Azores domain, presently in a broad transtensional regime, is acting simultaneously as a ultra slow spreading centre and as a transfer zone between the MAR and the dextral Gloria Fault, as it accommodates the differential shear movement between the Eurasian and African plates.

Active crustal deformation from the Azores triple junction to the Middle East

The co-seismic crustal deformation along the Africa-Eurasia, Eurasia-Arabia plate boundary, from the Azores triple junction (30°W) up to the Karliova triple junction (41°E) is examined, based on the moment tensor summation of mainly post-1964 earthquakes with M S ⩾ 5.0. Strain rates range from 10 −8/yr to 10 −6/yr, while values exceeding 10 −7/yr were calculated for the back-arc Aegean area and for the North Anatolian Fault Zone. A remarkable stability in the azimuth of the component of compression and of extension is observed in the area that extends west of Italy (17°E) up to the Azores triple junction. Along this region the deformation is taken up by compression along a mean direction of N140°, with rates from 1.1 to 2.8 mm/yr, and by extension along a mean direction of N50°, with rates from 2.4 to 12.0 mm/yr. At the Gloria fault and in the surrounding area dextral strike-slip motion occurs at a rate of 11 mm/yr. From Sicily up to the eastern end of the Hellenic Arc the crust is compressed at azimuths in the range N20–N31°, with an average value of 26 ± 5°. This compression is occurring at a rate of 1–21 mm/yr with an average value of 9 ± 8 mm/yr. In the back-arc Aegean area extension is occurring along a mean azimuth of N156° with a rate of 5.2 ± 3.8 mm/yr. The average shortening rate along the convex side of the Hellenic arc is 12 mm/yr. From the eastern end of the North Anatolian Fault (41°E), up to the Northern Aegean trough, extension along N68–N179° and compression along N158-N89° occurs, which results in dextral strike-slip motion. The rate of extension reduces from east to west from 30 mm/yr to 5 mm/yr, while the rate of compression reduces from 17 mm/yr to 5 mm/yr. The northwards motion of Arabia results in a N31° trending compression at a rate of 7 mm/yr along the East Anatolian Fault Zone.

Geodynamic models for the Azores triple junction: A contribution from tectonics

The American, Eurasian and African lithospheric plates meet at the Azores triple junction. The nature of the northern and southern branches of the junction is well known and uncontroversial, the American plate is separated from Eurasia and Africa by the Mid-Atlantic Ridge (MAR). The western group of islands of the Azores archipelago (Flores and Corvo) lies on the American plate, just west of the rift, and their tectonic features agree with that location. The nature of the third branch of the junction, to the east of the MAR, is however still controversial. Some authors believe that a ridge-ridge-ridge junction occurs in the Azores area and adopt the structure known as the “Terceira Ridge” (working as a simple rift boundary) as the third branch. Another model proposes a triangular microplate in the Azores region bounded by the Mid-Atlantic Rift to the west, the East Azores Fault Zone to the south and a fault crossing S. Miguel and the Terceira islands to the northeast; in this model the fault zones are pure dextral strike-slip boundaries. Both models would create space problems at Gloria Fault, a well-known pure dextral strike-slip structure. In the first model transtension would be necessary at Gloria Fault to accommodate plate motion, while the second would require a transpressive regime on that structure. Neotectonic and seismotectonic data in the Azores indicate a transtensile regime in the central and eastern island groups in present and recent times. This is compatible with a “leaky transform” structure acting as the third branch of the junction in the Azores area, a model which has already been presented by several authors in the past. Stress trajectories deduced from neotectonic studies in some of the islands suggest that the main plate boundary passes between the islands of S. Jorge and Pico south of S. Miguel and joins Gloria Fault east of Santa Maria. This model and boundary location is also compatible with seismotectonic and magnetic data.

Magnetic structure and evolution of the island of Santa Maria, Azores

A palaeomagnetic study of the major volcanic units of Santa Maria reveals dominantly reverse characteristic magnetizations with a mean direction of D = 179, I = −49 and α 95 = 2.1° (85 specimens from 20 sites). Sporadically occurring normal polarity remanences are interpreted as an overprinted component imposed by the island's terminal volcanic phase (the Feteiras Formation) now primarily exposed as strongly oxydized pyroclasts. The palaeomagnetic results are combined with available radiometric age estimates and local magnetic field information, and an integrated interpretation of all the data yields the principal conclusion that the Facho and Pico Alto volcanic sequences, covering the major part of the island, were laid down at around the Miocene/Pliocene boundary (∼ 5 Ma ago), i.e. immediately succeeding the local Messinian sedimentary horizon. The Feteiras Formation is assigned to the Gauss normal polarity chron (3.4-2.5 Ma) in the Upper Pliocene. It is tentatively inferred that the Feteiras event represented the final tectonomagmatic link between the Gloria Fault and the East Azores Fracture Zone. The question of a realistic lower age assessment for the older (pre-Messinian) volcanic complex remains undecided.

Closure of the Africa‐Eurasia‐North America Plate motion circuit and tectonics of the Gloria Fault

We examine the closure of the current plate motion circuit between the African, North American, and Eurasian plates to test whether these plates are rigid and whether the Gloria fault is an active transform fault. We also investigate the possible existence of microplates that have been previously proposed to lie along these plate boundaries, and compare the predicted direction of motion along the African‐Eurasian plate boundary in the Mediterranean with the direction of slip observed in earthquakes. From marine geophysical data we obtain 13 transform fault azimuths and 40 3‐m.y.‐average spreading rates, 34 of which are determined from comparison of synthetic magnetic anomaly profiles to ∼140 observed profiles. Slip vectors from 32 earthquake focal mechanisms further describe plate motion. Detailed magnetic surveys north of Iceland provide 11 rates in a region where prior plate motion models had few data. Magnetic profiles north of the Azores triple junction record a rate of 24 mm/yr, 4 mm/yr slower than used by prior models. Gloria and Sea Beam surveys accurately measure the azimuths of seven transform faults; our plate motion model fits six of the seven within 2°. Two transform faults surveyed by Gloria side scan sonar lie near FAMOUS area transform faults A and B and give azimuths 13° clockwise of them. Because recent studies show that short‐offset transforms, such as transforms A and B, are in many places oblique to the direction of plate motion, we exclude azimuths from transforms with less than 35‐km offset. The best fitting and closure‐enforced vectors fit the data well, except for a small systematic misfit to the slip vectors: On right‐lateral slipping transforms, slip vectors tend to be a few degrees clockwise of plate motion and mapped fault azimuths, whereas on left‐lateral slipping transforms, slip vectors tend to be a few degrees counterclockwise of plate motion and mapped fault azimuths. We search the long Eurasia‐North America boundary for evidence of an additional plate, but find no systematic misfits to the data. In particular, if a Spitsbergen plate exists and moves relative to Eurasia, its motion is less than 3 mm/yr. An Africa‐Eurasia Euler vector determined by adding the Eurasia‐North America and Africa‐North America Euler vectors is consistent with the Gloria fault trend and with slip vectors from eastern Azores‐Gibraltar Ridge focal mechanisms. A small circle, centered at the Africa‐Eurasia closure‐enforced pole, fits the trace of the Gloria fault. The model in which closure was enforced predicts ∼4 mm/yr slip across the Azores‐Gibraltar Ridge, and west‐northwest convergence near Gibraltar, ∼45° more oblique than suggested by a recent model based on compressive axes of focal mechanisms. Moreover, our model predicts directions of plate motion that agree well with northwest trending slip vectors from thrust earthquakes between Gibraltar and Sicily. Because closure‐enforced vectors fit the data nearly as well as the best fitting vectors, we conclude that the data are consistent with a rigid plate model and with the Gloria fault being a transform fault.

Gloria fault in Talave tunnel, Spain — used methods and consequences : Garcia Yague, A Proc 5th International Congress International Association of Engineering Geology, Buenos Aires, 20–25 October, 1986V1, P283–290. Publ Rotterdam: A A Balkema, 1986

Gloria fault in Talave tunnel, Spain — used methods and consequences : Garcia Yague, A Proc 5th International Congress International Association of Engineering Geology, Buenos Aires, 20–25 October, 1986V1, P283–290. Publ Rotterdam: A A Balkema, 1986

Gloria fault in Talave tunnel, Spain - used methods and consequences : Garcia Yague, A Proc 5th International Congress International Association of Engineering Geology, Buenos Aires, 20–25 October, 1986V1, P283–290. Publ Rotterdam: A A Balkema, 1986

Gloria fault in Talave tunnel, Spain - used methods and consequences : Garcia Yague, A Proc 5th International Congress International Association of Engineering Geology, Buenos Aires, 20–25 October, 1986V1, P283–290. Publ Rotterdam: A A Balkema, 1986

Source process and tectonic implications of the great 1975 North Atlantic earthquake

Summary. The Atlantic segment of the Africa-Europe plate boundary has usually been interpreted as a transform boundary on the basis of the bathymetric expression of the Gloria fault and dextral strike-slip first-motion mechanisms aligned along the Azores-Gibraltar line of seismicity. The 1975 May 26 earthquake (Ms=7.9) was assumed to fit into this framework because it occurred in the general area of this line and has a similar first-motion focal mechanism (strike=288, dip=72, slip angle=184). However, several anomalies cast doubt on this picture: the event is abnormally large for an oceanic transform event; a sizeable tsunami was excited; the aftershock area is unusually small for such a large event; and most significantly, the epicentre is 200 km south of the presumed plate boundary. The Rayleigh wave radiation pattern indicates a change in focal mechanism to one with a significant dip-slip component. The short duration of the source time history (20 s, as deconvolved from long-period P-waves), the lack of directivity in the Rayleigh waves, and the small one-day aftershock area suggest a fault length less than 80 km. One nodal plane of the earthquake is approximately aligned with the trace of an ancient fracture zone. We have compared the Pasadena 1-90 record of the 1975 earthquake to that of the 1941 North Atlantic strike-slip earthquake (200km to the NNW) and confirmed the large size of the 1941 event (M=8.2). The non-colinear relationship of the 1975 and 1941 events suggests that there is no well-defined plate boundary between the Azores and Gibraltar. This interpretation is supported by the intraplate nature of both the 1975 event and the large 1969 thrust event 650 km to the east. This study also implies that the largest oceanic strike-slip earthquakes occur in old lithosphere in a transitional tectonic regime.

Side-scan sonar studies of North Atlantic fracture zones

The Institute of Oceanographic Sciences has studied zones in 3 locations along the N mid-Atlantic Ridge and one on the Azores-Gibraltar plate boundary. 2 areas have short (~20 km) offsets and the others large ones (> 100 km). An important new observation is that probably all of these fracture zones are associated with en echelon , 20 km long scarps striking about 45 ° to the fracture zone. These scarps are believed to develop from oblique faults formed in the early stages of shearing. In addition, true transform faults associated with long, linear valleys occur in the large-offset fractures. Their appearance is similar to that of large continental strike-slip faults. The observed morphology and tectonics of the fracture-zone valleys generally accords with the description of Francheteau et al . (1976). The small-offset fracture zones have asymmetric cross sections which are attributed to the interplay of median valley and fracture-zone tectonics. Only one case of a possible intrusive ridge has been tentatively identified, in the inactive portion of Charlie-Gibbs Fracture Zone. An improved instantaneous rotation pole for Africa/Eurasia has been determined from the trace of the Gloria Fault on the Azores-Gibraltar plate boundary.