Subsidence history of the Bahama Escarpment and the nature of the crust underlying the Bahamas

Subsidence of the Bahama Escarpment, determined from deep-diving submersible and dredge samples, is used to constrain the nature of crust underlying the Bahamas. Horizontal disposition of the Hauterivian/Barremian (125 Ma) age boundary along the Bahama Escarpment is inconsistent with an underlying oceanic crust (either normal or thickened) here, and suggests that thinned continental crust underlies the Bahamas. Subsidence curves are then fit based on a stretched lithosphere model to a stratigraphic section (2000–4000 m) off Cat Island. This analysis indicates crustal thinning by a factor (β) of 2.0–2.5, resulting in present crustal thicknesses of 10–12 km. We propose that rifting beneath the Bahamas occurred from middle (175–180 Ma) to late (160 Ma) Jurassic time. The pre-extension Bahamas fit between North and South America and Africa in Early Jurassic time, eliminating overlap of the present Bahamas onto Africa in reconstructions of the North Atlantic.     

On the Structure of Small-Scale Motion in the Core of the Earth

In 1996, St Pierre (1996) reported numerical simulations of a buoyant blob migrating across the earth's outer core and subject to the combined effects of rotation and an azimuthal magnetic field. He noted that the blob rapidly fragments into a series of plate-like structures. Quite independently, Davidson (1995, 1997) discovered a similar behaviour in the context of low- R m turbulence (without a Coriolis force) and showed that this phenomenon has its roots in the destruction of angular momentum by the Lorentz force. The purpose of this paper is to pull together these earlier studies and, in particular, to determine whether or not St. Pierre's platelets are also a consequence of the destruction of angular momentum. We confirm that this is indeed the case.     

On the variation in the geomagnetic dipole over the geological history of the Earth

The global database on the paleointensity, containing determinations of the virtual dipole moment (VDM) for a stable (normal) regime of the geomagnetic field in a time interval of up to 3.5 Ga, is supplemented by new VDM determinations and analyzed. The field generation process started no later than 3–3.5 Ga (earlier data are absent) at the stage of the Earth’s core formation. Since that time, the dipole value has differed from its present value by no more than an order of magnitude, and the deviations that have already been detected tend toward smaller values. The distribution of VDM values in the time interval 0–400 Ma is bimodal, which apparently reflects the presence of two different generation levels of the geomagnetic field distinguished by a relatively large value (close to the present field value) and a relatively small value (approximately half as large as the present value). The total duration of decreased VDM values appreciably exceeds that of increased VDM values (179.1 and 28.6 Myr, respectively). On the whole, data on the paleointensity do not contradict the hypothesis about the dipole nature of the field over the last 400 Myr; however, the number of determinations at high paleolatitudes is too small to draw decisive conclusions on the validity (or invalidity) of the dipole field approximation based solely on paleointensity data.     

On the variability of the flow along the Meso-American Barrier Reef system: a numerical model study of the influence of the Caribbean current and eddies

A high resolution (3–8 km grid), 3D numerical ocean model of the West Caribbean Sea (WCS) is used to investigate the variability and the forcing of flows near the Meso-American Barrier Reef System (MBRS) which runs along the coasts of Mexico, Belize, Guatemala and Honduras. Mesoscale variations in velocity and temperature along the reef were found in seasonal model simulations and in observations; these variations are associated with meandering of the Caribbean current (CC) and the propagation of Caribbean eddies. Diagnostic calculations and a simple assimilation technique are combined to infer the dynamically adjusted flow associated with particular eddies. The results demonstrate that when a cyclonic eddy (negative sea surface height anomaly (SSHA)) is found near the MBRS the CC shifts offshore, the cyclonic circulation in the Gulf of Honduras (GOH) intensifies, and a strong southward flow results along the reef. However, when an anticyclonic eddy (positive SSHA) is found near the reef, the CC moves onshore and the flow is predominantly westward across the reef. The model results help to explain how drifters are able to propagate in a direction opposite to the mean circulation when eddies cause a reversal of the coastal circulation. The effect of including the Meso-American Lagoon west of the Belize Reef in the model topography was also investigated, to show the importance of having accurate coastal topography in determining the variations of transports across the MBRS. The variations found in transports across the MBRS (on seasonal and mesoscale time scales) may have important consequences for biological activities along the reef such as spawning aggregations; better understanding the nature of these variations will help ongoing efforts in coral reef conservation and maintaining the health of the ecosystem in the region.     

Mesozoic structural-magmatic pattern and metallogeny of the western part of the Pacific belt

The distribution of magmatism and related endogenous metallogeny within short intervals of geological time displays strong lateral zonal pattern governed by the positions of contemporaneous eugeosynclines, i.e. previous oceanic basins. This pattern includes: (1) the eugeosyncline with ultramafics and mafics, and with Cu, Au, Cr, Pt; (2) the amagmatic back troughs filled by clastic sediments; (3) a zone of granite-granodiorite batholiths with Au, Mo; (4) a zone of diorite-monzonite with PbZn; (5) a zone of standard and LiF granites with Sn, W, Mo; and (6) a zone of alkaline plutons.The zones in (3)–(5) correspond to calc-alkaline volcanism, and the zone in (6) to alkaline volcanism. The zonal pattern is related to the activity along fossil Benioff zones. Great transversal faults displaced structural-magmatic and metallogenic zonality far inside continents. They are interpreted as transform faults. The existance of a zonal pattern is discussed in terms of plate tectonics.     

Effectiveness of tune mass damper in the reduction of the seismic response of the structure

A shaking table testing of a 16.6 t five storey steel frame structure with and tuned mass damper (TMD) named as Tuned Mass Control System (TMCS) installed at the top has been carried out in the Dynamic Testing Laboratory at the Institute of Earthquake Engineering and Engineering Seismology (IZIIS) in Skopje, Republic of Macedonia. For estimation of the effectiveness of Tuned Mass Control System (TMCS) large number of shaking table experiments have been performed. Simulating different earthquake time histories on the model structure with and without TMCS it has been demonstrated that this system is capable to reduce the responses in order from 10% to more than 50% depending on the frequency content of the seismic input and the corresponding sensitivity of the structure. Given a high quality analytical model of a structure and a dynamic absorber, a series of variant analyses have been performed within the study to investigate the effect of the individual parameters and evaluate the efficiency of the dynamic absorber. The analyses have been performed to define the effect of the location of the absorber, also, upon the dynamic behaviour of the structure in the case the absorber is installed at the different level (storey) of the structure. Comparative analysis of the structure with TMCS having optimally tuned its mechanical properties versus structure that has TMCS having the same mechanical properties as tested specimen showed that the TMCS additionally improves the structural behaviour, depending on frequency content of earthquake excitation.     

Configuration of the upper boundary of the ionosphere

Variations of the upper boundary of the ionosphere (UBI) are investigated based on three sources of information: (i) ionosonde-derived parameters: critical frequency foF2, propagation factor M3000F2, and sub-peak thickness of the bottomside electron density profile; (ii) total electron content (TEC) observations from signals of the Global Positioning System (GPS) satellites; (iii) model electron densities of the International Reference Ionosphere (IRI^*) extended towards the plasmasphere. The ionospheric slab thickness is calculated as ratio of TEC to the F2 layer peak electron density, NmF2, representing a measure of thickness of electron density profile in the bottomside and topside ionosphere eliminating the plasmaspheric slab thickness of GPS-TEC with the IRI^* code. The ratio of slab thickness to the real thickness in the topside ionosphere is deduced making use of a similar ratio in the bottomside ionosphere with a weight Rw. Model weight Rw is represented as a superposition of the base-functions of local time, geomagnetic latitude, solar and magnetic activity. The time-space variations of domain of convergence of the ionosphere and plasmasphere differ from an average value of UBI at ∼1000 km over the earth. Analysis for quiet monthly average conditions and during the storms (September 2002, October–November 2003, November 2004) has shown shrinking UBI altitude at daytime to 400 km. The upper ionosphere height is increased by night with an ‘ionospheric tail’ which expands from 1000 km to more than 2000 km over the earth under quiet and disturbed space weather. These effects are interposed on a trend of increasing UBI height with solar activity when both the critical frequency foF2 and the peak height hmF2 are growing during the solar cycle.     

On the influence of earthquake energy range on the estimated seismicity parameters before the magnitude 7.9 Kronotskii earthquake of 1997

Results are reported from a detailed study of central Kamchatka seismicity for the period 1962–1997 based on a modification of the traditional approach. The approach involves (a) a detailed structure of the seismic region that recognizes the Kronotskii and Shipunskii geoblocks and two further blocks, the continental slope, and the offshore portion, (b) a study of variations in the rate of M = 3.0–7.2 earthquakes and the amount of seismic energy released at depths of 0–50 and 51–100 km, (c) a study of seismicity variability, and (d) separate estimates of the recurrence of crust-mantle earthquakes (depths 0–50 km) and mantle events (51–100 km). As a result, apart from corroborating the fact of a quiescence preceding the December 5, 1997 Kronotskii earthquake (M 7.9), we also found that a relationship exists between its beginning and the position of the earthquake-generating region relative to the mainshock epicenter. The quiescence dominates the seismic process during the pre-mainshock period and is characterized by a decreased rate of earthquakes (the first feature) and a decreased amount of seismic energy release (the second feature). Based on the first feature, we found that the quiescence started in 1987 throughout the entire depth range (0–100 km) in both parts of the Kronotskii geoblock close to the rupture zone of the eponymous earthquake. As to the Shipunskii geoblock, which is farther from the rupture zone, the quiescence began in the mantle of the inner area first (1988) and somewhat later at depths of 0–50 km within the continental slope (1989). By the second feature, the quiescence began at shallower depths in the inner area of the Kronotskii geoblock at the same time and later on (a year later) in the mantle (1988). Under the continental slope of the trench in the Shipunskii geoblock the shallower quiescence also began in 1987, while it was 3 years late in the inner zone (1990) and involved the earthquake-generating earth volume at depths of 0–100 km. These data are identical with or sufficiently close to the estimate for the beginning of this quiescence using a circular area of radius 150 km that combines the Kronotskii and Shipunskii geoblocks by the RTL method (1990).     

Petrology of the Easter microplate region in the South Pacific

Submersible investigations along the East Rift segments, the Pito Deep and the Terevaka transform fault of the Easter microplate eastern boundary, and on a thrust-fault area of the Nazca Plate collected a variety of basalts and dolerites. The volcanics consist essentially of depleted (N-MORB), transitional (T-MORB) and enriched (E-MORB) basalts with low (0.01−0.1, < 0.7), intermediate (0.12–0.25, 0.7–1.2) and high (> 0.25, > 1.2–2) K/Ti and(La/Sm)_N ratios, respectively. The Fe-Ti-rich ferrobasalt encountered among the N-MORBs are found on the Pito Deep Central volcano, on the Terevaka intra-transform ridge, on the ancient (< 2.5 Ma) Easter microplate (called EMP, comprising the East Rift Inner pseudofaults and Pito Deep west walls) and on thrust-fault crusts. The most enriched (T- and E-MORB) volcanics occur along the East Rift at 25 °50′–27 °S (called 26 °S East Rift) and on the Pito seamount located near the tip of the East Rift at 23 °00′–23 °40′S (called 23 °S East Rift). The diversity in incompatible element ratios of the basalts in relation to their structural setting suggests that the volcanics are derived from a similar heterogenous mantle which underwent variable degrees of partial melting and magma mixing. In addition the Pito seamount volcanics have undergone less crystal fractionation (< 20%) than the lavas from the other Easter microplate structures (up to 35–45%). The tectonic segmentation of the East Rift observed between 23 and 27 °S corresponds to petrological discontinuities related to Mg# variations and mantle melting conditions. The highest Mg# (> 61) are found on topographic highs (2000–2300 m) and lower values (Mg# < 56) at the extremities of the East Rift segments (2500–5600 m depths). The deepest area (5600 m) along the East Rift is located at 23 °S and coincides with a Central volcano constructed on the floor of the Pito Deep. Three major compositional variabilities of the volcanics are observed along the East Rift segments studied: (1) the 26 °S East Rift segment where the volcanics have intermediate Na₈ (2.5–2.8%) and Fe₈ (8.5–11%) contents; (2) the 23 °S East Rift segment (comprising Pito seamount and Pito Deep Central volcano) which shows the highest (2.9–3.4%) values of Na₈ and a low (8–9%) Fe₈ content; and (3) the 25 °S (at 24 °50′–26 °10′S) and the 24 °S (at 24 °10′–25 °S) East Rift segments where most of the volcanics have low to intermediate Na₈ (2.6–2.0%) and a high range of Fe₈ (9–13%) contents. When modeling mantle melting conditions, we observed a relative increase in the extent of partial melting and decreasing melting pressure. These localized trends are in agreement with a 3-D type diapiric upwelling in the sense postulated by Niu and Batiza (1993). Diapiric mantle upwelling and melting localized underneath the 26, 25 and 23 °S (Pito seamount and Central volcano) East Rift segments are responsable for the differences observed in the volcanics. The extent of partial melting varies from 14 to 19% in the lithosphere between 18 and 40 km deep as inferred from the calculated initial (P_o=16kbar) and final melting (P_f=7kbar) pressures along the various East Rift segments. The lowest range of partial melting (14–16%) is confined to the volcanics from 23 °S East Rift segment including the Pito seamount and the Central volcano. The Thrust-fault area, and the Terevaka intra-transform show comparable mantle melting regimes to the 25 and 26 °S East Rift segments. The older lithosphere of the EMP interior is believed to have been the site of high partial melting (17–20%) confined to the deeper melting area (29–50 km). This increase in melting with increasing pressure is similar to the conditions encountered underneath the South East Pacific Rise (13–20 °S).     

Luminescence and radiocarbon chronologies for the Xindian Culture site of Lamafeng in the Guanting Basin on the NE edge of the Tibetan Plateau

The archaeological Lajia and Lamafeng sites were situated in the Guanting Basin in the northeastern edge of the Tibetan Plateau, and were in the second terrace of the Yellow River. The Lamafeng site is only 1500 m away from Lajia site. The Lajia site belongs to typical Qijia culture (4100–3600 cal a BP). Many human skeletons were found in the Lajia site. These remains presented a state of unnatural deaths. They were all buried in-situ. The Lajia site recorded a prehistoric natural disaster, and the cause of which was probably a combination of earthquake and flooding due to the outburst of a lake that had been dammed by a landslide on the Yellow River. The Lajia site was abandoned by the prehistoric people after the disaster, but follow-up cultural relics were not found. Lamafeng site is a representative of Xindian Culture (3400–2600 cal a BP). The investigation of the Xindian Culture is intensive. However, the chronological data is still very limited, and no luminescence data has been reported so far. In the current study, both optically stimulated luminescence (OSL) (three samples) and accelerator mass spectrometry (AMS) (five charcoal samples) dating are applied to samples from Lamafeng archaeological site. The results showed that: (1) For the sample from the cultural layer, within error margins OSL and ¹⁴C ages are in good agreement. The ¹⁴C ages show that the cultural layer has an age range of ∼2400–2300 cal a BP. (2) The Lamafeng site was inhabited by the people of Xindian Culture, which indicated that the natural disaster occurred in the Qijia culture period did not completely destroy the prehistoric culture in the basin. (3) There was an enhanced extreme flood activity during Qijia and Xindian Culture periods, and the extreme flood period was ended at 2.6 ± 0.2 ka.     

海南琼海加积井水位对远大震的同震响应特征研究

In this study,we analyze the co-seismic response of water levels in the Jiaji well to strong earthquakes(MS≥7.8) from 2001 to 2010 at an epicentral distance less than 8000km.We investigated the co-seismic variation form of water levels,and analyzed the relationship between the amplitude of water level variation and the magnitude and the epicentral distance.We then checked the seismic wave phases when the changes of water level occurred.It was shown that:(1) the water level’s co-seismic response is mainly characterized by escalation with no oscillation;(2) the amplitude of water level change has a certain connection with epicentral distance and magnitude;(3) co-seismic response of water levels in the Jiaji well shows a certain directivity;(4) most of the co-seismic responses were caused by surface waves,and some by long-period S waves.     

Siberian paleomagnetic data and the problem of rigidity of the Northern Eurasian continent in the post-Paleozoic

The Meso-Cenozoic paleomagnetic poles from the Siberian platform and its folded margins, which comply with the modern technical and methodological standards, are analyzed. The analysis suggests the following conclusions. (1) The geometrical relationship between the Permo-Triassic poles of the Stable Europe and Siberian Platform prohibits the possibility of relative displacements of these platforms in the post-Paleozoic time. (2) The Mesozoic paleomagnetic poles of the Siberian Platform support the hypothesis of rigid Northern Eurasia. (3) The paleolatitudes of the Mesozoic sections located on the folded margins of the Siberian Platform closely agree with the Apparent Polar Wandering Path (APWP) for Europe. (4) The available data indicate that the vertical-axis rotation of separate local blocks within the folded margins of the Siberian Platform was a widespread phenomenon. Therefore, (1) the modern paleomagnetic data quite certainly show that consolidation of the Northern Eurasian continent was completed by the end of Permian, and, since the very beginning of the Mesozoic, the Siberian and East-European platforms have been parts of a single rigid megablock. (2) The Meso-Cenozoic segment of the APWP for Europe can be used as reference for the Siberian platform.     

An assessment of conditions before and after the 1998 Tam Pokhari outburst in the Nepal Himalaya and an evaluation of the future outburst hazard

On 3 September 1998, a glacial lake outburst flood (GLOF) that originated from Tam Pokhari occurred in the Hinku valley of the eastern Nepal Himalaya. This study analyses the lake's geomorphic and hydrologic conditions prior to the outburst, and evaluates the conditions that could contribute to a future flood through photogrammetric techniques. We processed high‐resolution Corona KH‐4A (2.7 m) and ALOS PRISM (2.5 m) stereo‐images taken before and after the GLOF event, and produced detailed topographic maps (2‐m contour interval) and DEMs (5 m × 5 m). We (re‐) constructed lake water surfaces before (4410 ± 5 m) and after (4356 ± 5 m) the outburst, and reliably estimated the lake water surface lowering (54 ± 5 m) and the water volume released (19.5 ± 2.2 × 10⁶ m³) from the lake, showing good agreement with the results obtained from ground‐based measurements. The most relevant conditions that may have influenced the catastrophic drainage of Tam Pokhari in 1998 include the presence of: (i) a narrow (75 ± 6 m), steep (up to 50°) and high (120 ± 5 m) moraine dam; (ii) high lake level (8 ± 5 m of freeboard) and (iii) a steep overhanging glacier (>40°). The lake outburst substantially altered the immediate area, creating a low and wide (>500 m) outwash plain below the lake, a wide lake outlet channel (~50 m) and a gentle channel slope (~3–5°). Our new data suggest that the likelihood of a future lake outburst is low. Our results demonstrate that the datasets produced by photogrammetric techniques provide an excellent representation of micro‐landform features on moraine dams, lake water surfaces and the changes in both over time, thereby allowing highly accurate pre‐ and post‐GLOF (volumetric) change analysis of glacial lakes. Furthermore, it enables precise measurement of several predictive variables of GLOFs that can be useful for identifying potentially dangerous glacial lakes or prioritizing them for detailed field investigations. Copyright © 2015 John Wiley & Sons, Ltd.     

Ecological status in the Ligurian Sea: the effect of coastline urbanisation and the importance of proper reference sites

The assessment of the ecological status, as required by the Water Framework Directive (WFD), plays an important role in coastal zone management, but only a small number of ecological indices are applicable on rocky bottoms. In this study, we apply a previously defined ecological quality index based on the cartography of littoral and upper-sublittoral rocky-shore communities (CARLIT), based on the sensitivity of algae dominated communities to anthropogenic impacts along a moderate urban gradient. We also apply this index in four Marine Protected Areas (MPAs), proposed as reference sites at a regional scale. After comparing the outputs with water variables and other quality indices, we can affirm that (1) the CARLIT index is suitable to detect different kinds of anthropogenic pressures, that (2) the choice of proper reference sites is a focal point in the fulfilment of the WFD (Water Framework Directive) and that (3) historical data are important to define reference conditions and the degradation of ecological status.     

The volcano of Capelinhos (Azores), the solar activity and the earth-tide

Bulletin of Volcanology - Correlations on the eruptive behaviour of the volcano of Capelinhos (Fayal island) in 1957–58, with (a) solar activity, and (b) earth-tide amplitudes, by way of a...     

Numerical simulation of the variations in the total electron content of the ionosphere observed before the Haiti earthquake of January 12, 2010

We present the results of a study of the abnormal variations in the total electron content (TEC) of the ionosphere observed before the earthquake of January 12, 2010, in Haiti. Global and regional maps of TEC relative (%) deviations from the quite background state are built for January 9–12, 2010. Using the UAM (Upper Atmosphere Model) global numerical model of the upper atmosphere of the Earth, the variations in the electric potential in the ionosphere and TEC are calculated using external seismic current above faults between the Earth and the ionosphere as a lower boundary condition. The numerical simulation results are compared with observations. It is shown that the simulated variations in the TEC at a specified current density of about 1 × 10^?8 A/m² on an area of 200 km (latitude) × 4000 km (longitude) above the focus represent all main features of the observations: prevalence of increased TEC values (positive disturbances), neighboring negative disturbances of lower magnitudes, localization, magnetic conjugacy of high-intensity effects in the Southern Hemisphere, and disappearance of disturbances around midday. Methodological recommendations are given to reveal variations in the TEC related to the preparation of seismic events.     

A Fractal Interpretation of the Topography of the Geomagnetic Scalar Potential at the Core-mantle Boundary

—The spatial power spectrum of the scalar potential (V) of the main geomagnetic field shows a power-law behaviour at the core-mantle boundary (CMB) and an almost uniform distribution of the corresponding phases. This is strong evidence for a fractal topography of V having a non-integer dimension of 2.2 (with an uncertainty of ± 0.1) which is, indeed, found from an analysis of the power spectra of 32 spherical harmonic models of V spanning the interval 1647 to 1990.     

Origin and distribution of the terrigenous component of the unconsolidated surface sediment of the Aegean floor: A synthesis

The Aegean Sea covers an area of some 160×10³ km² and receives the water/sediment fluxes from a mountainous drainage basin of >200×10³ km². On the basis of its morphodynamic characteristics, the Aegean Basin could be divided into: (1) the North Aegean Sea, an elongated region (trending between N50° and N70°) including the extensive northern shelves and the Deep Aegean Trough; (2) the Central Aegean, which includes: the Cyclades Plateau, a relatively shallow (average depth <350 m) submerged platform, surrounded by small basins (up to 1000 m depth), including also the relatively extended eastern shelf of Asia Minor, and (3) the Southern Aegean Sea, located southwards of the Hellenic volcanic arc, which presents the characteristics of a true back-arc basin (the Cretan Sea).The surficial unconsolidated sediments of the north Aegean floor are dominated by the terrigenous component (from 50% up to >90%) due to the large terrigenous riverine fluxes. The South Aegean presents high percentages (>50%) of biogenic material, due to the small terrigenous inputs and despite the fact that it is more oligotrophic than the North Aegean. The Central Aegean presents a transitional character with the terrigenous influxes being imported along its eastern part and quantitatively being in between those of the North and South Aegean Sea sub-regions.The coarse-grained materials in shallow (shelf) areas are attributed to ‘relict’ deposits, while those in large water depths are almost exclusively biogenic products. The offshore distribution of the fine-grained terrigenous material is dominated by the overall circulation pattern, while meso-scale eddies may, locally, either enhance (anticyclones) or reduce (cyclones) settling rates. Moreover, the spatial distribution of the predominant clay minerals (illite and smectite) and of kaolinite and chlorite is governed by the lithology and proximity to land source areas, the water circulation and the processes of differential settling and flocculation.Overall, the North Aegean is characterised by sedimentation processes similar to those of a ‘continental margin’, primarily neritic and secondarily hemipelagic, the Central Aegean region mostly by hemipelagic and the South Aegean, behaving more like an ‘oceanic margin’, mostly by pelagic processes.