Shear-wave velocity structure of the south-eastern part of the Iberian Peninsula from Rayleigh wave analysis

In this study, we present the lithospheric structure of the south-eastern part of the Iberian Peninsula by means of a set of 2D images of shear velocity, for depths ranging from 0 to 50 km. This goal will be attained by means of the inversion of the Rayleigh wave dispersion. For it, the traces of 25 earthquakes occurred on the neighbouring of the study area, from 2001 to 2003, will be considered. These earthquakes have been registered by 11 broadband stations located on Iberia. All seismic events have been grouped in source zones to get an average dispersion curve for each source-station path. The dispersion curves have been measured for periods between 2 and 45 s, by combination of two digital filtering techniques: Multiple Filter Technique and Time Variable Filtering. The resulting set of source-station averaged dispersion curves has been inverted according to the generalized inversion theory, to get S-wave velocity models for each source-station path. Later, these models have been interpolated using the method of kriging, to obtain a 2D mapping of the S-wave velocity structure for the south-eastern part of Iberia. The results presented in this paper show that the techniques used here are a powerful tool to investigate the crust and upper mantle structure, through the dispersion analysis and its inversion to obtain shear velocity distributions with depth. By means of this analysis, principal structural features of the south-eastern part of Iberia, such as the existence of lateral and vertical heterogeneity in the whole study area, or the location of the Moho discontinuity at 30 km of depth (with an average S-velocity of uppermost mantle of 4.7 km/s), have been revealed. Other important structural features revealed by this analysis have been that the uppermost of Iberian massif shows higher velocity values than the uppermost of the Alpine domain, indicating that the massif is old and tectonically stable. The average velocity of the crust in Betic cordillera is of 3.5 km/s, while in the Iberian massif is 3.7 km/s. All these features are in agreement with the geology and other previous geophysical studies.     

Meteorological factors associated with floods in the north-eastern part of the Iberian Peninsula

The main meteorological features of catastrophic rainfall events in Catalonia are described. Data come from several sources listed in the text. Surface and upper air synoptic and some subsynoptic conditions under which these events occur are described. Two kinds of events are identified, depending on the amount of forced lift required to release potential instability: Type A events, which take place on the coastal area when the forcing due to littoral and prelittoral hills is enough, and type B events require a large forced lift and occur near the Pyrenees. Local topographical and mesoscale meteorological conditions turn out to have a relevant role in connection with such events.     

Thermal and mechanical structure of the central Iberian Peninsula lithosphere

The central Iberian Peninsula (Spain) is made up of three main tectonic units: a mountain range, the Spanish Central System and two Tertiary basins (those of the rivers Duero and Tajo). These units are the result of widespread foreland deformation of the Iberian plate interior in response to Alpine convergence of European and African plates. The present study was designed to investigate thermal structure and rheological stratification in this region of central Spain. Surface heat flow has been described to range from 80 to 60 mW m⁻². Highest surface heat flow values correspond to the Central System and northern part of the Tajo Basin. The relationship between elevation and thermal state was used to construct a one-dimensional thermal model. Mantle heat flow drops from 34 mW m⁻² (Duero Basin) to 27 mW m⁻² (Tajo Basin), and increases with diminishing surface heat flow. Strength predictions made by extrapolating experimental data indicate varying rheological stratification throughout the area. In general, in compression, ductile fields predominate in the middle and lower crusts and lithospheric mantle. Brittle behaviour is restricted to the first 8 km of the upper crust and to a thin layer at the top of the middle crust. In tension, brittle layers are slightly more extended, while the lower crust and lithospheric mantle remain ductile in the case of a wet peridotite composition. Discontinuities in brittle and ductile layer thickness determine lateral rheological anisotropy. Tectonic units roughly correspond to rheological domains. Brittle layers reach their maximum thickness beneath the Duero Basin and are of least thickness under the Tajo Basin, especially its northern area. Estimated total lithospheric strength shows a range from 2.5×10¹² to 8×10¹² N m⁻¹ in compression, and from 1.3×10¹² to 1.6×10¹² N m⁻¹ in tension. Highest values were estimated for the Duero Basin.Depth versus frequency of earthquakes correlates well with strength predictions. Earthquake foci concentrate mainly in the upper crust, showing a peak close to maximum strength depth. Most earthquakes occur in the southern margin of the Central System and southeast Tajo Basin. Seismicity is related to major faults, some bounding rheological domains. The Duero Basin is a relative quiescence zone characterised by higher total lithospheric strength than the remaining units.     

Wrench (transcurrent) fault System of the southwestern Iberian Peninsula,paleogeographic and morphostructural implications

Mapping of the Southwestern Hesperian Massif (South Western Spain) has permitted the definition of two principal systems of wrench faults in conjugate directions, SW-NE and NNW-SSE. By their dimensions and their association with deep basic rocks, they may be considered as accidents which interest the entirety of the crust. The directions of these systems are enclosed in a scheme of late-Hercynian wrench faults produced in the dismantling of the Iberian Peninsula at the end of the orogenic Hercynian phases. According to these large lines of fracture it is possible to relate a) the areas of Mesozoic sedimentation, pre-Alpine distension, b) the reliefs of the Mesozoic covering of the interior of the Hesperic Massif, c) the reliefs and the tectonic basins of the interior of the Meseta and d) the possible evolution of the Iberian Plate with relation to the Bay of Biscay and the areas of Alpine compression of the meridional edge, Betic Chains.     

The Weichselian Late-glacial in southwestern Europe (Iberian Peninsula,Pyrenees, Massif Central,northern Apennines)

This paper presents a summary of Late-glacial environmental changes in southwestern Europe (lberian Peninsula, Pyrenees, Massif Central and the northern Apennines). The emphasis is on palaeoclimatic interpretations inferred from key sites in the region from which the most detailed records are available and which have been radiocarbon dated. The earliest evidence for climatic improvement following the end of the last glacial stage is dated to ca. 15 ka BP and is found at a few sites only. By 13 ka BP, a more widespread and marked climatic improvement is evident, although it is difficult to be precise in the timing and magnitude of the event. There are significant variations in detail between the Late-glacial records, but evidence for a significant cooling correlated with the Younger Dryas event is widespread throughout the region. Just two sites in the region provide evidence for an earlier, less emphatic phase of climatic cooling, which is tentatively equated with the ‘Older Dryas’ of continental northern Europe. Dry conditions appear to have predominated throughout the region in the later part of the Younger Dryas and the early Holocene.     

Short-period surface waves across the Iberian Peninsula area

A large number of records of near earthquakes, for the period 1950–1975, have been revised at Toledo Observatory (I.G.C.), looking for clear trains of surface waves.All records used correspond to the Wiechert, WWSS or HGLP seismometers of this station.Multiple filtering techniques have been applied to the selected seismograms to obtain Love and Rayleigh fundamental mode dispersion curves.As most of the paths are of mixed continental and oceanic structure, the first interpretation has been made considering an average model for each set of similar seismic paths. However, for some regions, the separation of the two structures (pure continental and pure oceanic) has been attempted.     

Application of FLATModel, a 2D finite volume code, to debris flows in the northeastern part of the Iberian Peninsula

FLATModel is a two-dimensional shallow-water approximation code with corrections and modifications that create a simulation tool adapted to debris-flows behaviour. FLATModel uses the finite volume method with the numerical implementation of the Godunov scheme and includes correction terms regarding the effect of flow over high slopes and curvature. Additionally, the stop-and-go phenomenon, the basal entrainment and a correction regarding the front inclination of the final deposit are incorporated into FLATModel. In addition, different flow resistance laws were integrated in the numerical code including Bingham, Herschel–Bulkley and Voellmy fluid model. Firstly, our numerical model was validated using analytical solutions of a dam-break scenario and published data on a laboratory experiment. Secondly, three real events, which occurred in the northeastern part of the Iberian Peninsula, were back-calculated. Although field observations of the three events are not very detailed, the back-analyses revealed interesting patterns on the flow dynamics, and the numerical results generally showed good agreement with field data. Comparing the different flow resistance laws, the Voellmy fluid model presents the best behaviour regarding both the flow behaviour and the deposit characteristics. Preliminary simulation runs incorporating the effect of basal entrainment offered satisfactory results, although the final volume is rather sensitive on the selected friction angle of channel-bed material. The outcomes regarding the correction of the calculated front inclination of the final deposit showed that this implementation strongly improves the simulation results and better represents steep fronts of final deposits.     

A morphotectonic study of the Central System,Iberian Peninsula

The Central System (CS) or Spanish Central System forms part of two of the five morphotectonic mesoblocks that make up the Intermediate macroblock (number 6) of the Iberian plate. The combination of geological, geophysical and geomorphological data, used in accordance with the Rantsman methodology (1979), served as the basis for obtaining a regional analysis. The cartography obtained shows territorial units (4 macroblocks, 4 mesoblocks, 35 blocks, 85 microblocks and 162 nanoblocks), morphotectonic alignments (quantity/order: 3/2, 4/3, 5/23, 6/48, 7/93 and 8/164) and morphotectonic knots (quantity/order: 1/2, 5/3, 35/4, 85/5, 162/6, 324/7, 816/8). The number of delimited morphostructures increases from the central part towards the east. At the block level, one may distinguish a transverse differentiation of the territorial units and alignments, which is interpreted as an expression of the region’s lithospheric heterogeneity. There is a close relationship between the morphostructures and seismicity, indicating that greater activity occurs in the blocks of the eastern and northeastern sectors.     

Fluvial networks of the Iberian Peninsula: a chronological framework

Knowledge of the evolution of Spanish fluvial networks has improved during recent years as more river systems have been studied and more geochronological data has become available. However, the chronological framework is a major issue as the range of applications is limited by methodological constraints and spatial coverage is sparse. Integration of ‘absolute’ dating methods with biostratigraphy and palaeomagnetism permits the recent evolution of these river systems to be reviewed. The timing of incision from the Late Neogene to the present varies between the major Iberian fluvial systems, depending on the substrata and tectonic settings. Early Pleistocene and older fluvial sequences in the core areas of the Iberian Peninsula provide a more extensive record of fluvial evolution and are better preserved than the terrace flights in the coastal lowlands. Middle Pleistocene sequences are well developed in most of the major river systems in Iberia, particularly those of the Tajo, Guadalquivir and Aguas River, and frequently represent the principal climatic cycles of that period, although tectonic and sea-level effects can also be seen. For Late Pleistocene to Holocene times, the scheme becomes more complex. Our review suggests that each river system has responded differently to local and regional climate control, glacial and periglacial processes in headwaters in high mountain areas, glacio-eustatic sea-level changes and local and regional tectonic patterns.     

Crustal structure in Alaska

Knowledge of the crustal structure is still fragmentary, despite the stimulus to geophysical work provided by the earthquake of March 28, 1964 (GMT), the underground nuclear explosion LONGSHOT, and the June 1967 series of earthquakes in the Fairbanks area. The most reliable information about struc ture has come from a combination of seismic explosion-refraction profiles, gravity surveys, and magnetic surveys. This report is a summary of recent investigations, but the results are not adequate to permit unambiguous generalizations about crustal structure.     

Crustal structure of the Balearic sea

Within the frame of the German-French project ANNA-1970, two long refraction profiles were investigated north and south of the island of Majorca.

For the southern Balearic Basin an oceanic crust can be derived from the travel-time curves consisting of a 4.0 km thick Cenozoic sedimentary layer with: V_p = 2.35 (km/sec) + 0.35 (sec⁻¹) × Z (km) and a 5 km thick layer with: V_p = 4.0 (km/sec) + 0.28 (sec⁻¹) × Z (km)

The transition to the upper mantle takes place at a depth of 12 km. Directly south of Majorca a crustal thickening was measured which may be caused by the process of crustal shortening. P]In the northern Balearic Basin a faulted transitional type of crust has been observed indicating probably an embryonic and juvenile ocean expansion.     

Crustal structure of the Rhinegraben area

Numerous ge ological and geophysical investigations within the past decades have shown that the Rhinegraben is the most pronounced segment of an extended continental rift system in Europe. The structure of the upper and lower crust is significantly different from the structure of the adjacent “normal” continental crust.

Two crustal cross-sections across the central and southern part of the Rhinegraben have been constructed based on a new evaluation of seismic refraction and reflection measurements. The most striking features of the structure derived are the existence of a well-developed velocity reversal in the upper crust and of a characteristic cushion-like layer with a compressional velocity of 7.6–7.7 km/sec in the lower crust above a normal mantle with 8.2 km/sec. Immediately below the sialic low-velocity zone in the middle part of the crust, an intermediate layer with lamellar structure and of presumably basic composition could be mapped.

It is interesting to note that the asymmetry of the sedimentary fill in the central Rhinegraben seems to extend down deeper into the upper crust as indicated by the focal depths of earthquakes. The top of the rift “cushion” shows a marked relief which has no obvious relation to the crustal structure above it or the visible rift at the surface.     

Crustal structure of Antarctica

Seismic refraction profiles completed in the past twenty years reveal that the top of the basement complex generally lies near sea level in East Antarctica but typically 2 or 3 km below sea level in West Antarctica. Throughout much of East Antarctica the thickness of the layer overlying the basement complex is less than half a kilometer, although a Phanerozoic sequence more than 1 km thick probably underlies the ice at the South Pole. Throughout central West Antarctica, on the other hand, a section one to several kilometers thick generally overlies the basement complex. The observed sedimentary section is no more than one half kilometer thick on either side of the Transantarctic Mountains. Rocks with high seismic velocities typical of the lower continental crust occur within a few kilometers of the surface on both sides of the Transantarctic Mountains. This occurrence lends support to the hypothesis of an abrupt increase in crustal thickness between West and East Antarctica.

In 1969, deep seismic soundings were carried out by the 14th Soviet Antarctic Expedition near the coast of Queen Maud Land. The crustal thickness was found to be about 40 km near the mountains, decreasing to about 30 km near the coast. In the top 15 km of the crust there is a gradual downward increase in P-wave velocity from 6.0 to 6.3 km/sec. The average velocity through the crust is 6.4 km/sec and the measured velocity below the M-discontinuity is 7.9 km/sec.

At the southwestern margin of the Ronne Ice Shelf, near-vertical reflections from the M-discontinuity have been recorded. A mean P-wave velocity of 6 km/sec in the crust was measured, leading to an estimated depth to M of 24 km below sea level.

Seismic surface wave dispersion studies indicate a mean crustal thickness of about 30 km in West Antarctica and about 40 km in East Antarctica. The dispersion data also show that group velocities across East Antarctica are much closer to those along average continental paths than to those across the Canadian shield. The results thus support other indications that central East Antarctica is not a simple crystalline shield.

P′P′-reflections beneath the continent support the existence of a low-velocity channel for P-waves, but show no significant difference in deep structure between Antarctica and other continents.     

Crustal structure of the Indian subcontinent

Studies carried out by various investigators up to 1971 to delineate the Indian crustal structure using body wave travel times, surface wave dispersion and gravity methods are summarised and reviewed. The average crustal thickness is found to be 35–40 km in the Indian peninsular shield, 30–35 km in the Indo Gangetic plains and 60–80 km in the Himalayas and the Tibetan plateau region. The limitations of the various methods used and the errors in the estimation of crustal thickness by them are discussed. As no deep refraction work for crustal studies has been carried out so far in India, this topic is not covered in this study.     

A site amplification factor map of the Iberian Peninsula and the Balearic Islands

Considering the lack of site effect cartography in the Iberian Peninsula region to be related to the strong motion data, we present a site amplification factor map covering the Iberian Peninsula and the Balearic Islands. Steps leading to produce the map have included the classification of geological units from the 1:1,000,000 scale Geological Map of Spain into six site classes characterized on a seismic response basis, and the calculation, for every site class, of both short-period and mid-period amplification factors. In order to test the validity and applicability of the map, we have calculated the synthetic seismic intensity in the particular localities where several past earthquakes were felt, supposing a point source approximation. The synthetic intensities have been obtained with and without site amplification factors, resulting in two types of synthetic isoseismals maps that have been compared with the corresponding observed isoseismals. As the amplification factors obtained are only applicable to the linear domain, the synthetic intensities greater than VII are only illustrative. A main conclusion has been drawn about the fact that synthetic isoseismals with site amplification factor approach to the related observed isoseismals in a higher degree than synthetic isoseismals without site amplification factor. In addition, the resemblances between the synthetic isoseismals produced with site amplification factors and the corresponding observed isoseismals have been found to be more evident at shorter epicentral distances, provided that intensities are lower than VIII, and therefore, we remain in the linear domain.     

Surging of the southwestern part of the Laurentide Ice Sheet

The southwestern part of the Laurentidc Ice Sheet, in central North America, repeatedly surged during the last part of the Wisconsin Glaciation. Evidence includes the extreme lobation of the ice margin, the gentle slopes of lateral moraines and other marginal features, a radiocarbon chronology indicating extremely rapid marginal advance and retreat, and the abundance of supraglacial flow till. Rapid ice movement was caused by subglacial water and was probably limited to areas of slowly permeable substrate, which slowed the escape of the water.