Seismic hazard in central southern Africa

Seismic hazard maps of central-southern Africa where hazard has been expressed in terms of peak ground acceleration for an annual probability in excess of 10^-1 show relatively high values that distinguish the seismic hazard potential of the Deka fault zone, the mid-Zambezi basin-Luangwa rift and western central Mozambique. In areas such as central-southern Africa where little is known about the geology of the region and the fault systems have not been fully mapped, seismic hazard potential may be estimated from seismicity and broad-scale fault features. For this region, such potential is based on earthquake magnitude M_s ≥ 6. Events of such magnitude have recently occurred in the mid-Zambezi basin, southern Zimbabwe and western-central Mozambique. This paper follows the conventional probabilistic hazard analysis procedure, defining seismic source zones from seismicity based on instrumental records from a cataloque that spans a period of 83 years. Geological and geomorphological features in the region are described on the mesoscale and are correlated with the seismicity as broad fault zones. The scarcity of strong-motion accelerogram data necessitated the formulation of attenuation values based on random vibration theory (RVT).     

Late Quaternary kinematics of the Pallatanga strike-slip fault (Central Ecuador) from topographic measurements of displaced morphological features

The northeast-trending Pallatanga right-lateral strike-slip fault runs across the Western Cordillera connecting N50E-N70E trending normal faults in the Gulf of Guayaquil with N-S reverse faults in the Interandean Depression. Over most of its length, the fault trace has been partly obscured by erosional processes and can be inferred in the topography only at the large scale. Only the northern fault segment, which follows the upper Rio Pangor valley at elevations above 3600 m, is prominent in the morphology. Valleys and ridges cut and offset by the fault provide an outstanding record of right-lateral cumulative fault displacement. The fault geometry and kinematics of this particular fault segment can be determined from detailed topographic levellings. The fault strikes N30E and dips 75 to the NW. Depending on their size and nature, transverse morphological features such as tributaries of the Rio Pangor and intervening ridges, reveal right-lateral offsets which cluster around 27 ± 11m, 41.5 ± 4 m, 590 ± 65 m and 960 ± 70 m. The slip vector deduced from the short-term offsets shows a slight reverse component with a pitch of about 11.5 SW. The 41.5 ± 4 m displacements are assumed to be coeval with the last glacial termination, yielding a mean Holocene slip-rate of 2.9- 4.6 mm yr⁻¹. Assuming a uniform slip rate on the fault in the long term, the 27 m offset appears to correlate with an identified middle Holocene morphoclimatic event, and the long term offsets of 590 m and 960 m coincide with the glacial terminations at the beginning of the last two interglacial periods.     

Characterization of active fault scarps from LiDAR data: a case study from Central Apennines (Italy)

A high-resolution digital elevation model (DEM, 1 ms spacing) derived from an airborne light detection and ranging (LiDAR) campaign was used in an attempt to characterize the structural and erosive elements of the geometry of the Pettino fault, a seismogenic normal fault in Central Apennines (Italy). Four 90- to 280 m-long fault scarp segments were selected and the surface between the base and the top of the scarps was analyzed through the statistical analysis of the following DEM-derived parameters: altitude, height of the fault scarp, and distance along strike, slope, and aspect. The results identify slopes of up to 40° in faults lower reaches interpreted as fresh faces, 34° up the faces. The Pettino fault maximum long-term slip rate (0.6–1.1 mm/yr) was estimated from the scarp heights, which are up to 12–19 m in the selected four segments, and the age (ca. 18 ka) of the last glacial erosional phase in the area. The combined analysis of the DEM-derived parameters allows us to (a) define aspects of three-dimensional scarp geometry, (b) decipher its geomorphological significance, and (c) estimate the long-term slip rate.     

3-D seismic velocities calculated from lattice-preferred orientation and reflectivity of a lower crustal section: examples of the Val Sesia section (Ivrea zone, northern Italy)

To quantify the seismic properties of lower crustal rocks and to better constrain the origin of the lower crustal seismic reflectivity, we determined the complete 3-D seismic properties of a lower crustal section. Eight representative samples of the main lithologic and structural units outcropping in the Val Sesia (Ivrea zone) were studied in detail. The seismic velocities were calculated using the single crystal stiffness coefficients and the lattice preferred orientation (LPO) of each mineral in all samples. The 21 stiffness coefficients characterizing the elastic behaviour of each rock are determined. Mafic and ultramafic rocks such as pyroxenite and pyroxene-bearing gabbros display complex shear wave properties. These rocks are weakly birefringent (maximum 0.1 kms⁻¹) and it is difficult to find consistent relationships between the seismic properties and the rock structure. On the other hand, seismic properties of deformed felsic rocks are essentially controlled by mica. They display strong S -wave birefringence (0.3 km s⁻¹) and relatively high V _p anisotropy (7.6 per cent). Amphibole also strongly influences the rock birefringence patterns. For both kind of rocks, the foliation is highly birefringent and the fast polarized shear wave is systematically oriented parallel to the foliation. We show that the number of mineral phases in the rock strongly controls the anisotropy. The seismic anisotropy has a complex role in the P -wave reflectivity. Compared to the isotropic case, anisotropy enhances the reflection coefficient for about 60 per cent of the possible lithological interfaces. For 40 per cent of the interfaces, the reflection coefficient is much lower when one considers the medium as anisotropic.     

Late Quaternary palaeohydrology of Lake Pergusa (Sicily,southern Italy) as inferred by stable isotopes of lacustrine carbonates

Ten meters of lacustrine deposits retrieved from Lake Pergusa (Sicily, southern Italy) were investigated through stable isotope composition (carbon and oxygen) of authigenic carbonate (calcareous muds) and freshwater shells. The core chronology was established through three AMS dates, and by correlation with a previously dated nearby core. Stable isotope data show that the lake water evolution was mainly dominated by evaporation. Between ca. 20 and 28 ka the recovered sediments have very high δ¹⁸O values, likely corresponding to very dry climatic conditions. The observed rapid oscillations in the δ¹⁸O of the recovered sediments during this period also suggest important climatic fluctuations. More humid conditions dominated during the Holocene period, with the wettest interval occurring between ca. 9000 and 3000 years BP. Late Holocene sediments represent a substantial return to drier conditions. The available pollen data from a nearby core substantially confirm this general climatic trend during the Holocene. The positive correlation between δ¹³C of the calcareous muds and carbonate content suggests that biological activity played a key role in the carbon isotope evolution of dissolved inorganic carbon. However, a clear climatic signal is not evident from the δ¹³C record.     

Microearthquake seismicity and fault-plane solutions in the southern Aegean and its geodynanic implications

Seismicity ( Ml <3.5) in the southern Aegean, located using data collected during seven weeks of recording by a temporary network of seismological stations, largely follows the Hellenic arc; the Sea of Crete is nearly aseismic, and only little activity is located south of the Hellenic trench, within the African plate. Focal mechanisms exhibit reverse faulting in the external part of the arc and normal faulting inside it. This normal faulting indicates N-S extension in the northern Aegean, the Gulf of Corinth, the Cyclades and Dodecanese Islands, but NW-SE extension in southern Peloponnese and western Crete and E-W extension in eastern Crete. This non-uniform strain pattern suggests that the Aegean region not only extends in a N-S sense, with the Hellenic arc moving south-westward relative to the Eurasian plate, but also by E-W extension of its southern margin, so that there is a net divergence of material.     

Late Pleistocene–Holocene landscape evolution in Fossa Bradanica, Basilicata (southern Italy)

Studies in the middle Basento river basin supported by reliable chronological data (tephra layers and a number of absolute datings) have allowed the reconstruction of Late Pleistocene–Holocene geomorphological evolution of the middle to low Fossa Bradanica area (Basilicata, southern Italy). The original Upper Pleistocene hillslope has been dissected by deep gullies leaving relict slope pediments. Holocene filling of the Basento river valley and gullies occurred as a succession of downcut and fill episodes. A first phase of accumulation occurred in the Late Neolithic, which was followed by a downcutting between 4500 and 3700 cal. yr BP. A second deposition phase took place in the Greek–Roman period between 2800 and 1620 cal. yr BP, which was interrupted at around 2500 cal. yr BP. Another downcutting phase took place between 1620 and 1500 cal. yr BP, followed by a deposition phase between 1440 and 1000 cal. yr BP. After 1000 cal. yr BP a deep downcutting took place. Evidence collected with this study, coupled with climate data recorded in other Italian and European locations, suggests that filling and downcutting episodes in Fossa Bradanica were predominantly climate-driven. Anthropogenic impact only intensified or weakened these processes.     

Influence of structural framework on mountain slope deformation in the Maiella anticline (Central Apennines, Italy)

Relationships between tectonic framework and gravity-driven phenomena have been investigated in an area of the Central Apennines (Italy) characterised by high relief. The north–south, half-dome shaped Maiella anticline lies in the easternmost part of the Apennine fold-and-thrust belt. Its backlimb is bordered by the Caramanico Fault, a normal fault with a maximum downthrown of about 3.5 km that separates the western slope of the Maiella Massif from the Caramanico Valley. The southwestern Maiella area is affected by deep-seated gravitational slope deformation indicated by major double crest lines, down-hill and up-hill facing scarps, a pattern of crossing trenches, bulging at the base of slopes and the presence of different types of landslide and talus slope deposits.The onset and development of deep-seated gravitational slope deformations and the location of Quaternary, massive rockslope failures have been strongly influenced by the structural framework and tectonic pattern of the anticline. Deep-seated gravitational slope deformation at Mt. Macellaro–Mt. Amaro ridge has developed along the Maiella western, reverse slope in correspondence with the anticline axial culmination; it is bordered at the rear by a NNW–SSE oriented, dextral, strike-slip fault zone and has an E–W direction of rock mass deformation. Closer to the southern plunging area of the anticline, gravity-driven phenomena show instead a N–S and NW–SE direction, influenced by bedding attitude.3D topographic models illustrate the relationship between deep-seated gravitational slope deformation and massive rockslope failures. The Campo di Giove rock avalanche, a huge Quaternary failure event, was the result of an instantaneous collapse on a mountaine slope affected by a long-term gravity-driven deformation.     

The S reflector west of Galicia: the seismic signature of a detachment fault

The so-called S reflector is a group of bright, continuous reflections underlying the landward-tilted fault blocks of the west Galicia rifted margin, S has been interpreted as the brittle-ductile transition, the lop of an intrusion, a detachment fault, and the crust-mantle boundary. To constrain the internal structure of the reflector, we have carried out detailed analyses of these reflections. We compare the waveforms of the seafloor reflection and its first multiple, both to determine the amplitude of the seafloor reflection and to show (hat the seafloor is in effect a spike in the reflectivity series so that the seafloor reflection can be used as the far-field wavelet, including both source and receiver ghosts. We compare (he waveform of the seafloor and 5 and show that, within the resolution of our data, S is a reflection from a step increase in acoustic impedance. This result is confirmed through complex trace analysis, and in particular the determination of the apparent polarity of S, and the examination of the instantaneous frequency function: S is consistently positive polarity, and shows no significant frequency anomaly. Simple modelling shows that S is very unlikely to be a reflection from a thin layer. We thus conclude that S is probably a single steplike interface. From the varying frequency content of the data, we determine a value for the effective Q between S and the seafloor, and use this to assess the amplitude loss due to attenuation and scattering. We use a comparison between the seafloor and the S reflection to constrain the amplitude of S, and estimate a reflection coefficient for S of at least 0.2 in places, decreasing landwards. By analogy with structures developed in the highly extended regions of the western United States, we consider that the most likely interpretation of S is as a sharp west-dipping detachment fault separating a 'granitic' upper plate from a higher-velocity lower plate, locally probably serpentinized mantle.