Source parameters of March 10 and September 13, 2007, United Arab Emirates earthquakes

On March 10 and September 13, 2007 two earthquakes with moment magnitudes 3.66 and 3.94, respectively, occurred in the eastern part of the United Arab Emirates (UAE). The two events were widely felt in the northern Emirates and Oman and were accompanied by a few aftershocks. Ground motions from these events were well recorded by the broadband stations of Dubai (UAE) and Oman seismological networks and provide an excellent opportunity to study the tectonic process and present day stress field acting in this area. In this study, we report the focal mechanisms of the two main shocks by two methods: first motion polarities and regional waveform moment tensor inversion. Our results indicate nearly pure normal faulting mechanisms with a slight strike slip component. We associated the fault plane trending NNE–SSW with a suggested fault along the extension of the faults bounded Bani Hamid area. The seismicity distribution between two earthquake sequences reveals a noticeable gap that may be a site of a future event. The source parameters (seismic moment, moment magnitude, fault radius, stress drop and displacement across the fault) were also estimated from displacement spectra. The moment magnitudes were very consistent with waveform inversion. The recent deployment of seismic networks in Dubai and Oman reveals tectonic activity in the northern Oman Mountains that was previously unknown. Continued observation and analysis will allow for characterization of seismicity and assessment of seismic hazard in the region.     

Improved characterization of local seismicity using the Dubai Seismic Network,United Arab Emirates

In April 2006, Dubai Municipality established a broadband seismological network in Dubai Emirate, United Arab Emirates (UAE). This network was the first seismic network in UAE and consists of four remote seismic stations to observe local and regional seismic activity that may have an effect on Dubai Emirate and the surrounding areas. The network exchanges real-time data with the National Center of Meteorology and Seismology in Abu Dhabi, the Earthquake Monitoring Center in Oman and imports in real-time data from few Global Seismic Network stations, which increases the aperture of the network. In April 2012, Dubai Municipality installed an additional five free-field strong motion stations inside the urban area to estimate and publish real-time ShakeMaps for public and decision makers. Although the local seismic activity from April 2006 to June 2013 reflects low seismic activity with the Emirate, it indicates active tectonics in the relatively aseismic northern Oman Mountains region. A few inland clusters of micro-to-small earthquakes have been identified with the new network. A clear cluster of small-to-moderate earthquakes took place in the eastern part of UAE to the east of Masafi, while two clusters of micro-to-small earthquakes took place at Wadi Nazwa and northern Huwaylat. Focal mechanisms of few well recorded earthquakes in this region indicate normal faulting, generally trending NE in parallel to the transition shear zone between the collision at Zagros and the subduction at the Makran zone.     

A source study of the 6 July 2003 (Mw 5.7) earthquake sequence in the Gulf of Saros (Northern Aegean Sea): Seismological evidence for the western continuation of the Ganos fault

The July 2003 sequence in the Gulf of Saros (Northeastern Aegean Sea) is investigated, in terms of accurate event locations and source properties of the largest events. The distribution of epicenters shows the activation of a 25-km long zone, which extends in depth between 9 and 20 km. The major slip patch of the 6 July 2003 M_w 5.7 mainshock is confined in a small area (45 km²), which coincides with the deeper (12–20 km) part of the activated zone. The epicenters of the sequence follow the northern margin of the Saros depression. This observation supports recent studies, according to which the continuation of the Ganos fault in the Gulf of Saros does not coincide with the fault along the northern coast of the Gelibolu peninsula, but it is located at the northern boundary of the Saros depression. This is further supported by the fact that the focal mechanisms of the mainshock and of the largest aftershocks of the 2003 sequence imply almost pure dextral strike-slip faulting, whereas the fault bounding the Gulf of Saros to the south appears as a normal fault on seismic sections. Thus, we infer that the principle deformation zone consists of a major strike-slip fault, which lies close to the northern margin of the Saros depression and this fault could be regarded as the continuation of the northern branch of the North Anatolian Fault into the Saros Gulf and North Aegean Trough as suggested by regional tectonic models. The northeastern extent of the 2003 sequence marks the western termination (at 26.3° E) of a long-term seismic quiescence observed in the period following the 1912 Ganos earthquake, which may be associated with the extend of the rupture of the particular earthquake.     

The February 11, 2004 Dead Sea earthquake ML = 5.2 in Jordan and its tectonic implication

A moderate-sized (M_w  5.3) earthquake occurred in the Dead Sea basin on February 11, 2004. A rigorous seismological analysis of the main shock and numerous aftershocks suggests that seismogenic structure was a secondary, antithetic fault within the Dead Sea fault system. The main shock is well located using all available regional seismic stations, and 43 aftershocks were precisely located relative to the main shock using a double difference algorithm. The first motion, focal mechanism for this earthquake demonstrates NNW–SSE and ENE–WSW striking nodal planes, and the aftershocks distribution is consistent with the latter — indicating a right-lateral sense of displacement. This orientation and sense of shear are consistent with similarly oriented geological faults around the Dead Sea basin — these structures are likely antithetic faults within the transform system. Although moderate in size, earthquakes that occur very close to the large Dead Sea fault system warrant consideration in the earthquake hazard assessment of the region: For example, owing to the proximity to the main fault, moderate earthquakes such as this may produce static changes in Coulomb stress along the main fault.     

Constraints on the location and mechanism of the 1511 Western-Slovenia earthquake from active tectonics and modeling of macroseismic data

The 1511 Western Slovenia earthquake (M = 6.9) is the largest event occurred so far in the region of the Alps–Dinarides junction. Though it strongly influences the regional seismic hazard assessment, the epicenter and mechanism are still under debate. The complexity of the active tectonics of the Alps–Dinarides junction is reflected by the presence of both compressional and transpressional deformations. This complexity is witnessed by the recent occurrence of three main earthquake sequences, the 1976 Friuli thrust faulting events, the 1998 Bovec–Krn Mountain and the 2004 Kobarid strike-slip events. The epicenters of the 1998 and 2004 strike-slip earthquakes (M_s = 5.7 and M_s = 4.9, respectively) lie only 50 km far from the 1976 thrust earthquake (M_s = 6.5).We use the available macroseismic data and recent active tectonics studies, to assess a possible epicenter and mechanism for the 1511 earthquake and causative fault. According with previous works reported in the literature, we analyze both a two-and a single-event case, defining several input fault models. We compute synthetic seismograms up to 1 Hz in an extended-source approximation, testing different rupture propagations and applying a uniform seismic moment distribution on the fault segments. We extract the maximum horizontal velocities from the synthetics and we convert them into intensities by means of an empirical relation. A rounded-to-integer misfit between observed and computed intensities is performed, considering both a minimized and a maximized databases, built to avoid the use of half-degree macroseismic intensity data points. Our results are consistent with a 6.9 magnitude single event rupturing 50 km of the Idrija right-lateral strike-slip fault with bilateral rupture propagation.     

Cenozoic noncoaxial transtension along the western shoulder of the Ross Sea, Antarctica, and the emplacement of McMurdo dyke arrays

Field data on Cenozoic faults and the McMurdo dyke arrays in the Reeves Glacier–Mawson Glacier area, Victoria Land, Antarctica, allow us to support noncoaxial transtensional tectonics along the N–S-trending western shoulder of the Ross Sea. Dyke injection within a crustal-scale right-lateral strike-slip shear zone is testified by magma filled, tension gash-like arrangements within some master fault zones, and by the left-stepping arrangements of dykes in the intrafault zones. The noncoaxiality of deformation is shown by the re-activation of many dyke walls as right-lateral strike-slip faults. This suggests an increase in the strike-slip component over time along the western shoulder of the Ross Sea. Our data support the relevance of transtensional to strike-slip tectonics for triggering melting and controlling the geometry and modes of magma emplacement.     

Holocene activity of the Scilla Fault, Southern Calabria: Insights from coastal morphological and structural investigations

Integration of on-land and offshore geomorphological and structural investigations coupled to extensive radiometric dating of co-seismically uplifted Holocene beaches allows characterization of the geometry, kinematics and seismotectonics of the Scilla Fault, which borders the eastern side of the Messina Strait in Calabria, Southern Italy. This region has been struck by destructive historical earthquakes, but knowledge of geologically-based source parameters for active faults is relatively poor, particularly for those running mostly offshore, as the Scilla Fault does. The  30 km-long normal fault may be divided into three segments of  10 km individual length, with the central and southern segments split in at least two strands. The central and northern segments are submerged, and in this area marine geophysical data indicate a youthful morphology and locally evidence for active faulting. The on-land strand of the western segment displaces marine terraces of the last interglacial (124 to 83 ka), but seismic reflection profiles suggest a full Quaternary activity. Structural data collected on bedrock faults exposed along the on-land segment provide evidence for normal slip and  NW-SE extension, which is consistent with focal mechanisms of large earthquakes and GPS velocity fields in the region. Detailed mapping of raised Holocene marine deposits exposed at the coastline straddling of the northern and central segments supplies evidence for two co-seismic displacements at  1.9 and  3.5 ka, and a possible previous event at  5 ka. Co-seismic displacements show a consistent site value and pattern of along-strike variation, suggestive of characteristic-type behaviour for the fault. The  1.5–2.0 m average co-seismic slips during these events document M_e  6.9–7.0 earthquakes with  1.6–1.7 ka recurrence time. Because hanging-wall subsidence cannot be included into slip magnitude computation, these slips reflect footwall uplift, and represent minimum average estimates. The palaeoseismological record based on the palaeo-shorelines suggests that the last rupture on the Scilla Fault during the February 6, 1783 M_w = 5.9–6.3 earthquake was at the expected time but it may have not entirely released the loaded stress since the last great event at  1.9 ka. Comparison of the estimated co-seismic extension rate based on the Holocene shoreline record with available GPS velocities indicates that the Scilla Fault accounts for at least  15–20% of the contemporary geodetic extension across the Messina Strait.     

The April 2007 earthquake swarm near Lake Trichonis and implications for active tectonics in western Greece

We investigate the properties of the April 2007 earthquake swarm (Mw 5.2) which occurred at the vicinity of Lake Trichonis (western Greece). First we relocated the earthquakes, using P- and S-wave arrivals to the stations of the Hellenic Unified Seismic Network (HUSN), and then we applied moment tensor inversion to regional broad-band waveforms to obtain the focal mechanisms of the strongest events of the 2007 swarm. The relocated epicentres, cluster along the eastern banks of the lake, and follow a distinct NNW–ESE trend. The previous strong sequence close to Lake Trichonis occurred in June–December 1975. We applied teleseismic body waveform inversion, to obtain the focal mechanism solution of the strongest earthquake of this sequence, i.e. the 31 December 1975 (Mw 6.0) event. Our results indicate that: a) the 31 December 1975 Mw 6.0 event was produced by a NW–SE normal fault, dipping to the NE, with considerable sinistral strike-slip component; we relocated its epicentre: i) using phase data reported to ISC and its coordinates are 38.486°N, 21.661°E; ii) using the available macroseismic data, and the coordinates of the macroseismic epicentre are 38.49°N, 21.63°E, close to the strongly affected village of Kato Makrinou; b) the earthquakes of the 2007 swarm indicate a NNW–SSE strike for the activated main structure, parallel to the eastern banks of Lake Trichonis, dipping to the NE and characterized by mainly normal faulting, occasionally combined with sinistral strike-slip component. The 2007 earthquake swarm did not rupture the well documented E–W striking Trichonis normal fault that bounds the southern flank of the lake, but on the contrary it is due to rupture of a NW–SE normal fault that strikes at a  45° angle to the Trichonis fault. The left-lateral component of faulting is mapped for the first time to the north of the Gulf of Patras which was previously regarded as the boundary for strike-slip motions in western Greece. This result signifies the importance of further investigations to unravel in detail the tectonics of this region.     

Anatolian surface wave evaluated at GEOFON Station ISP Isparta, Turkey

We have studied seismic surface waves of 255 shallow regional earthquakes recently recorded at GEOFON station ISP (Isparta, Turkey) and have selected these 52 recordings with high signal-to-noise ratio for further analysis. An attempt was made by the simultaneous use of the Rayleigh and Love surface wave data to interpret the planar crust and uppermost mantle velocity structure beneath the Anatolian plate using a differential least-square inversion technique. The shear-wave velocities near the surface show a gradational change from approximately 2.2 to 3.6 km s^− 1 in the depth range 0–10 km. The mid-crustal depth range indicating a weakly developed low velocity zone has shear-wave velocities around 3.55 km s^− 1. The Moho discontinuity characterizing the crust–mantle velocity transition appears somewhat gradual between the depth range  25–45 km. The surface waves approaching from the northern Anatolia are estimated to travel a crustal thickness of  33 km whilst those from the southwestern Anatolia and part of east Mediterranean Sea indicate a thicker crust at  37 km. The eastern Anatolia events traveled even thicker crust at  41 km. A low sub-Moho velocity is estimated at  4.27 km s^− 1, although consistent with other similar studies in the region. The current velocities are considerably slower than indicated by the Preliminary Reference Earth Model (PREM) in almost all depth ranges.     

Scenarios of Seismic Risk in the United Arab Emirates,an Approximate Estimate

We estimate the losses due to 10 scenario earthquakes in 150 settlements of the United Arab Emirates (UAE). For southern Iran, we use four source zones and the maximum magnitudes in them as determined by GSHAP (7.2 M 8.1). For six local scenario earthquakes, we use the range 5.5 M 6.5, place the sources mainly on mapped faults and vary the distance to major cities from 10 to 60 km. In the test case of the Masafi earthquake (M5, 11 March 2002), the method and data bank we use yield the correct results, suggesting that our approach to the problem is valid for the UAE. The sources in Iran are expected to cause only minor damage, except for an M8.1 earthquake in the Makran region. For such an event we expect some deaths, several hundred injured and a loss of 3–6% of the value to the building stock in the northeastern UAE, including Oman. The losses for local scenarios with epicenters in the unpopulated areas of the UAE and for scenarios with M < 5.8 are estimated to be minor. Because the two major mapped faults run through several of the large cities, scenarios with short epicentral distances from cities have to be considered. Scenarios with M6 near cities lead to estimates of about 1000 ± 500 deaths, and several thousand injured. Most buildings are expected to be damaged to a moderate degree and the loss to buildings is estimated around 1/4of their value. If the magnitude should reach 6.5, the losses to humans and to building value could be staggering. These estimates are approximate because: (1) there exists no local seismograph network that could map active faults by locating microseismicity; (2) there exist no historically old buildings that could serve as tests for effects due to strong ground motion in the past; (3) there exist no microzonation of the subsurface properties in this region of unconsolidated building ground; (4) there exist no detailed inventory of building fragility. Nevertheless, our conclusion that there exists a substantial seismic risk in the UAE is reliable, because our method yields accurate results in the cases of earthquakes with known losses during the last several decades in the Middle East.     

Analysis of crustal deformation in Luzon, Philippines using geodetic observations and earthquake focal mechanisms

We utilize regional GPS velocities from Luzon, Philippines, with focal mechanism data from the Harvard Centroid Moment Tensor (CMT) Catalog, to constrain tectonic deformation in the complex plate boundary zone between the Philippine Sea Plate and Eurasia (the Sundaland block). Processed satellite imagery and digital elevation models are used with existing gravity anomaly, seismicity, and geologic maps to define a suite of six elastic blocks. Geodetic and focal mechanism data are inverted simultaneously to estimate plate rotations and fault-locking parameters for each of the tectonic blocks and faults comprising Luzon. Major tectonic structures that were found to absorb the plate convergence include the Manila Trench (20–100 mm yr^− 1) and East Luzon Trough ( 9–15 mm yr^− 1)/Philippine Trench ( 29–34 mm yr^− 1), which accommodate eastward and westward subduction beneath Luzon, respectively; the left-lateral strike-slip Philippine Fault ( 20–40 mm yr^− 1), and its northward extensions, the Northern Cordillera Fault ( 17–37 mm yr^− 1 transtension), and the Digdig Fault ( 17–27 mm yr^− 1 transpression). The Macolod Corridor, a zone of active volcanism, crustal thinning, extension, and extensive normal and strike-slip faulting in southwestern Luzon, is associated with left-lateral, transtensional slip of  5–10 mm yr^− 1. The Marikina Fault, which separates the Central Luzon block from the Southwestern Luzon block, reveals  10–12 mm yr^− 1 of left-lateral transpression. Our analysis suggests that much of the Philippine Fault and associated splays are locked to partly coupled, while the Manila and Philippine trenches appear to be poorly coupled. Luzon is best characterized as a tectonically active plate boundary zone, comprising six mobile elastic tectonic blocks between two active subduction zones. The Philippine Fault and associated intra-arc faults accommodate much of the trench-parallel component of relative plate motion.     

Late Mesozoic tectonic structure and evolution along the present-day northeastern South China Sea continental margin

The northeastern South China Sea continental margin holds the key to understanding Late Mesozoic tectonics and evaluating hydrocarbon potentials in Mesozoic tectonic and stratigraphic structures offshore southeast China. With newly obtained and processed seismic data, and new drilling and logging data, we correlate regional Mesozoic stratigraphy and analyze major Mesozoic tectonic events and structures. In particular, we focus our study on the three major tectonic units in the area, the Chaoshan Depression, the Tainan Basin, and the Dongsha–Penghu Uplift, which are separated by basement high, thrust fold, and (or) faults. Stratigraphic correlations suggest a major phase of southeastward regression, spanning in time from the late Early Jurassic (180 Ma) to the Early Cretaceous (120 Ma). Seismic data reveal two major tectonic events, with the first one in the Late Jurassic to the Early Cretaceous, contemporary with the regression, and the second one in the Late Cretaceous. Regional magnetic anomaly map after the reduction to the pole clearly reveals the boundary between the Dongsha–Penghu Uplift and the Chaoshan–Tainan depositional system. The seismic and magnetic data also suggest that, while the Dongsha–Penghu Uplift has highly magnetized sources buried mostly in the upper crust at depths from about 2 km to about 20 km, the Chaoshan–Tainan depositional system has thick Mesozoic sediments of low magnetization.     

Archean greenstone-tonalite duality: Thermochemical mantle convection models or plate tectonics in the early Earth global dynamics?

Mantle convection and plate tectonics are one system, because oceanic plates are cold upper thermal boundary layers of the convection cells. As a corollary, Phanerozoic-style of plate tectonics or more likely a different version of it (i.e. a larger number of slowly moving plates, or similar number of faster plates) is expected to have operated in the hotter, vigorously convecting early Earth. Despite the recent advances in understanding the origin of Archean greenstone–granitoid terranes, the question regarding the operation of plate tectonics in the early Earth remains still controversial. Numerical model outputs for the Archean Earth range from predominantly shallow to flat subduction between 4.0 and 2.5 Ga and well-established steep subduction since 2.5 Ga [Abbott, D., Drury, R., Smith, W.H.F., 1994. Flat to steep transition in subduction style. Geology 22, 937–940], to no plate tectonics but rather foundering of 1000 km sectors of basaltic crust, then “resurfaced” by upper asthenospheric mantle basaltic melts that generate the observed duality of basalts and tonalities [van Thienen, P., van den Berg, A.P., Vlaar, N.J., 2004a. Production and recycling of oceanic crust in the early earth. Tectonophysics 386, 41–65; van Thienen, P., Van den Berg, A.P., Vlaar, N.J., 2004b. On the formation of continental silicic melts in thermochemical mantle convection models: implications for early Earth. Tectonophysics 394, 111–124]. These model outputs can be tested against the geological record. Greenstone belt volcanics are composites of komatiite–basalt plateau sequences erupted from deep mantle plumes and bimodal basalt–dacite sequences having the geochemical signatures of convergent margins; i.e. horizontally imbricated plateau and island arc crust. Greenstone belts from 3.8 to 2.5 Ga include volcanic types reported from Cenozoic convergent margins including: boninites; arc picrites; and the association of adakites–Mg andesites- and Nb-enriched basalts.Archean cratons were intruded by voluminous norites from the Neoarchean through Proterozoic; norites are accounted for by melting of subduction metasomatized Archean continental lithospheric mantle (CLM). Deep CLM defines Archean cratons; it extends to  350 km, includes the diamond facies, and xenoliths signify a composition of the buoyant, refractory, residue of plume melting, a natural consequence of imbricated plateau-arc crust. Voluminous tonalites of Archean greenstone–granitoid terranes show a secular trend of increasing Mg#, Cr, Ni consistent with slab melts hybridizing with thicker mantle wedge as subduction angle steepens. Strike-slip faults of 1000 km scale; diachronous accretion of distinct tectonostratigraphic terranes; and broad Cordilleran-type orogens featuring multiple sutures, and oceanward migration of arcs, in the Archean Superior and Yilgarn cratons, are in common with the Altaid and Phanerozoic Cordilleran orogens. There is increasing geological evidence of the supercontinent cycle operating back to  2.7 Ga: Kenorland or Ur  2.7–2.4 Ga; Columbia  1.6–1.4 Ga; Rodinia  1100–750 Ma; and Pangea  230 Ma. High-resolution seismic reflection profiling of Archean terranes reveals a prevalence of low angle structures, and evidence for paleo-subduction zones. Collectively, the geological–geochemical–seismic records endorse the operation of plate tectonics since the early Archean.     

Crustal velocity inhomogeneities along the Hirapur–Mandla profile, central India and its tectonic implications

Wide-angle seismic and gravity data across the Narmada-Son lineament (NSL) in central India are analyzed to determine crustal structure, velocity inhomogeneities and hence constrain the tectonics of the lineament. We present the 2-D crustal velocity structure from deep wide-angle reflection data by using a ray-trace inverse approach. The main result of the study is the delineation of fault-bounded horst raised to a subsurface depth (1.5 km) and the Moho upwarp beneath the NSL. The crust below the basement consists of three layers with velocities of 6.45–6.7, 6.2–6.5 and 6.7–6.95 km/s and interface depths of about 5.5–8.7, 14–17 and 18–23 km along the profile. The low-velocity (6.2–6.5 km/s) layer goes up to a depth of 5 km and becomes the thickest part (13 km), while the overlying high-velocity (6.45–6.7 km/s) layer becomes the thinnest (3 km) and upper boundary lies at a depth of 1.5 km beneath the NSL. The overall uncertainties of various velocity and boundary nodes are of the order of ±0.12 km/s and ±1.40 km, respectively. The up-lifted crustal block and the up-warping Moho beneath the NSL indicate that the north and south faults bounding the NSL are deeply penetrated through which mafic materials from upper mantle have been intruded into the upper crust. Gravity modeling was also undertaken to assess the seismically derived crustal features and to fill the seismic data gap. The lateral and vertical heterogeneous nature of the structure and velocity inhomogeneities in the crust cause instability to the crustal blocks and played an important role in reactivation of the Narmada south fault during the 1997 Jabalpur earthquake.     

Kinematics and Ar/Ar geochronology of the Gaoligong and Chongshan shear systems, western Yunnan, China: Implications for early Oligocene tectonic extrusion of SE Asia

The Gaoligong and Chongshan shear systems (GLSS and CSSS) in western Yunnan, China, have similar tectonic significance to the Ailaoshan–Red River shear system (ASRRSS) during the Cenozoic tectonic development of the southeastern Tibetan syntaxis. To better understand their kinematics and the Cenozoic tectonic evolution of SE Asia, this paper presents new kinematic and ⁴⁰Ar/³⁹Ar geochronological data for these shear systems. All the structural and microstructural evidence indicate that the GLSS is a dextral strike-slip shear system while the CSSS is a sinistral strike-slip shear system, and both were developed under amphibolite- to greenschist-grade conditions. The ⁴⁰Ar/³⁹Ar dating of synkinematic minerals revealed that the strike-slip shearing on the GLSS and CSSS at least began at  32 Ma, possibly coeval with the onset of other major shear systems in SE Asia. The late-stage shearing on the GLSS and CSSS is dated at  27–29 Ma by the biotite ⁴⁰Ar/³⁹Ar ages, consistent with that of the Wang Chao shear zone (WCSZ), but  10 Ma earlier than that of the ASRRSS. The dextral Gaoligong shear zone within the GLSS may have separated the India plate from the Indochina Block during early Oligocene. Combined with other data in western Yunnan, we propose that the Baoshan/Southern Indochina Block escaped faster southeastward along the CSSS to the east and the GLSS to the west than the Northern Indochina Block along the ASRRSS, accompanying with the obliquely northward motion of the India plate during early Oligocene (28–36 Ma). During 28–17 Ma, the Northern Indochina Block was rotationally extruded along the ASRRSS relative to the South China Block as a result of continuously impinging of the India plate.     

Archean Granitoids at the Contact of Eastern Ghat Granulite Belt and Singhbhum-Orissa Craton, in Bhuban-Rengali Sector, Orissa, India

The small granite plutons occurring at the contact of the Singhbhum-Orissa Iron Ore craton (IOC) to the north and the Eastern Ghat Granulite Belt (EGGB) to the south in eastern Indian shield are characterised by the presence of enclaves of the granulites of EGGB and the greenschist facies rocks of IOC. These granites also bear the imprints of later cataclastic deformation which is present at the contact of the IOC and the EGGB. In situ Pb-Pb zircon dating of these granites gives minimum age of their formation 2.80 Ga. A whole-rock three point Rb-Sr isochron age of this rock is found to be 2.90 Ga. Therefore, the true age of formation of these granites will be around 2.90–2.80 Ga. These granitic rocks also contain xenocrystic zircon components of 3.50 Ga and show a later metasomatic or metamorphic effect 2.48 Ga obtained from the analyses on overgrowths developed on 2.80 Ga old zircon cores. The presence of granulitic enclaves within these contact zone granite indicates that the granulite facies metamorphism of the EGGB is 2.80 Ga or still older in age. The cataclastic deformations observed at the contact zone of the two adjacent cratons is definitely younger than 2.80 Ga and possibly related to 2.48 Ga event observed from the overgrowths. As 2.80 Ga granite plutons of small dimensions are also observed at the western margin of the IOC; it can be concluded that a geologic event occurred 2.80 Ga over the IOC when small granite bodies evolved at the marginal part of this craton after its stabilisation at 3.09 Ga.     

Kinematics of the Iberia–Maghreb plate contact from seismic moment tensors and GPS observations

The Iberian Peninsula and the Maghreb experience moderate earthquake activity and oblique,  NW–SE convergence between Africa and Eurasia at a rate of  5 mm/yr. Coeval extension in the Alboran Basin and a N35°E trending band of active, left-lateral shear deformation in the Alboran–Betic region are not straightforward to understand in the context of regional shortening, and evidence complexity of deformation at the plate contact. We estimate 86 seismic moment tensors (M_W 3.3 to 6.9) from time domain inversion of near-regional waveforms in an intermediate period band. Those and previous moment tensors are used to describe regional faulting style and calculate average stress tensors. The solutions associated to the Trans-Alboran shear zone show predominantly strike-slip faulting, and indicate a clockwise rotation of the largest principal stress orientation compared to the regional convergence direction (σ₁ at N350°E). At the N-Algerian and SW-Iberian margins, reverse faulting solutions dominate, corresponding to N350°E and N310°E compression, respectively. Over most of the Betic range and intraplate Iberia, we observe predominately normal faulting, and WSW–ENE extension (σ₃ at N240°E). From GPS observations we estimate that more than 3 mm/yr of African (Nubian)–Eurasian plate convergence are currently accommodated at the N-Algerian margin,  2 mm/yr in the Moroccan Atlas, and  2 mm/yr at the SW-Iberian margin. 2 mm/yr is a reasonable estimate for convergence within the Alboran region, while Alboran extension can be quantified as  2.5 mm/yr along the stretching direction (N240°E). Superposition of both motions explains the observed left-lateral transtensional regime in the Trans-Alboran shear zone. Two potential driving mechanisms of differential motion of the Alboran–Betic–Gibraltar domain may coexist in the region: a secondary stress source other than plate convergence, related to regional-scale dynamic processes in the upper mantle of the Alboran region, as well as drag from the continental-scale motion of the Nubian plate along the southern limit of the region. In the Atlantic Ocean, the  3.5 mm/yr, westward motion of the Gibraltar Arc relative to intraplate Iberia can be accommodated at the transpressive SW-Iberian margin, while available GPS observations do not support an active subduction process in this area.     

Shear wave splitting in the Eastern Alps observed at the TRANSALP network

Data from the temporary TRANSALP seismic network were analysed to investigate the seismic anisotropy in the upper mantle beneath the Eastern Alps. We operated mostly short period and some broadband stations in a dense linear array, which transects the Eastern Alps at 12° E longitude. Recorded SKS and SKKS phases with different backazimuths were used simultaneously to calculate the splitting parameters of delay time and fast axis direction for each seismic station. While we found variations in the delay times between 0.8 and 2.0 s, the determined fast axis directions prove to be rather consistent along the profile (60°–70° N). They coincide well with the trend of the Eastern Alps, thus suggesting orogen-parallel flow in the upper mantle. Our findings support the earlier proposed idea that the Adriatic indenter which was forced northwards into the European plate during the late stage of the Alpine orogeny, triggered an escape movement to the less constrained Pannonian basin to the east.     

3D seismic analysis of the Coast Shear Zone in SE Alaska and Western British Columbia: Broadside analysis of ACCRETE wide-angle data

The multidisciplinary ACCRETE project addresses the question of continental assemblage in southeast Alaska and western British Columbia by terrane accretion and magmatic addition. The previous studies of this project yielded important information for understanding the structures across the Coast Shear Zone (CSZ) and the formation of the CSZ and the Coast Mountains Batholith (CMB). The present study extends these interpretations into pseudo-3-D by using two additional wide-angle ACCRETE seismic lines. By analyzing the broadside wide-angle data using a series of laterally homogeneous 2-D models, we derive a lower-resolution 3-D velocity model of the outboard terranes and constrain variations in crustal thickness across and along the CSZ. Models of the broadside data confirms major structural and compositional trends extend along strike to the northwest. The key features are: a) a steep Moho ramp only  15-km wide is coincident with the CSZ and divides thin (25 ± 1 km) crust to the west below the west-vergent thrust belt (WTB) from thicker ( 31 ± 1 km) crust to the east below the CMB, (b) low-velocity mantle (7.7--7.9 km/s) extends beneath the entire study region indicating high crustal and upper-mantle temperatures below the WTB and CMB, and (c) the Alexander terrane is characterized by strong mid-crustal reflectivity and high lower crustal velocities that are consistent with gabbroic composition. This study extends the earlier interpretation and implies that the ramp is indeed likely associated with transpressional tectonics and magmatic crustal addition east of the CSZ.     

Holocene Relative Sea Level Changes along the Seattle Fault at Restoration Point, Washington

At a marsh on the hanging wall of the Seattle fault, fossil brackish water diatom and plant seed assemblages show that the marsh lay near sea level between 7500 and 1000 cal yr B.P. This marsh is uniquely situated for recording environmental changes associated with past earthquakes on the Seattle fault. Since 7500 cal yr B.P., changes in fossil diatoms and seeds record several rapid environmental changes. In the earliest of these, brackish conditions changed to freshwater 6900 cal yr B.P., possibly because of coseismic uplift or beach berm accretion. If coseismic uplift produced the freshening 6900 cal yr B.P., that uplift probably did not exceed 2 m. During another event about 1700 cal yr B.P., brackish plant and diatom assemblages changed rapidly to a tidal flat assemblage because of either tectonic subsidence or berm erosion. The site then remained a tideflat until the most recent event, when an abrupt shift from tideflat diatoms to freshwater taxa resulted from 7 m of uplift during an earthquake on the Seattle fault 1000 cal yr B.P. Regardless of the earlier events, no Seattle fault earthquake similar to the one 1000 cal yr B.P. occurred at any other time in the past 7500 years.