Oriented Fault Zone Research Articles

Travertine and calcareous tufa occurrences as an indicator of the ongoing tectonic activity of the Central Slovak Fault System inferred from airborne laser scanning data, geomorphometric, and structural analysis (Northern Slovakia)

The travertine formation process, active over the Quaternary, has enabled the origin of one of the largest travertine accumulations in the Western Carpathians. Travertines and calcareous tufas are associated with the tectonic activity of the Central Slovak Fault System, a 45 km wide and 140 km long N-S oriented fault zone. As the geological mapping, structural analysis, geomorphometric and isobase surface analysis revealed, the development of faults was polyphase and the travertine/tufa precipitation was not a continuous process. In addition, numerous travertine/tufa morphofacial varieties have been recognized within the study area, such as travertine/tufa terraces and tufa cascades, hillslope paludal tufa crust, phytoherm framework tufa barrages, lacustrine microdetrital tufas, bryophyte-dominated self-built channel, and fluvial tufa crust, reflecting its local geomorphology and precipitation factors. In the Revúcka dolina Valley, the travertine/tufa geobodies thus represent a reliable indicator of the tectonic activity over the Neogene up to Quaternary. The structural data provided relevant evidence confirming the character of orogen-parallel extension for the current tectonic/neotectonic regime in the Inner Western Carpathians. High-resolution topography derived from airborne laser scanning and the subsequent application of various geomorphometry techniques allowed quick identification of the edges of the terrace cascade systems and morpholineaments. Based on our observations, in this zone, the significant vertical movements of normal faults took place with a maximum estimated offset of 800–1000 m. The achieved results also support the use of the presented multifaceted methodology to understand the interplay between geomorphogenesis, hydrology, and tectonics.

Oblique Collision of the Bahamas Platform at the Northern Boundary of the Caribbean Plate Recorded by the Late Cenozoic Coastal Terraces of SE Cuba

AbstractThe southeastern tip of Cuba Island is limited to the south by the N‐Caribbean boundary. By revisiting the impressive sequences of coastal terraces of this region, we decipher the Quaternary deformation pattern of this plate boundary. We present a detailed mapping of coastal terraces uplifted over a hundred kilometers of coastline, and U/Th dating. At Punta de Maisí, the deformation pattern shows (a) a faster uplift close to the transform boundary and (b) a northward propagation of folding produced by the convergence of the Bahamas platform toward the Caribbean plate. Along the southern coast of Punta de Maisí, the sequence displays 29 coastal terraces up to 520 m in elevation and a upper Pleistocene uplift rate of 0.23 ± 0.07 mm yr−1. We interpret this deformation as resulting from an offshore north‐dipping reverse fault near the coast. This uplift rate corresponds to 3% to 1.6% of the short‐term horizontal slip rate of Septentrional Oriente Fault Zone (10 ± 0.1 mm yr−1). Along the northern coast of Punta de Maisí, the sequence displays height coastal terraces up to 220 m in elevation and the uplift rates amount to 0.1 ± 0.05 mm yr−1 and likely result from the reverse faulting and folding associated with the offshore North Hispaniola Fault Zone. Uplift rates quickly decrease to the West, in agreement with the westward decrease in the activity of the North Hispaniola Fault Zone due to the docking of the Bahamas Platform against Cuba, while the platform more gently underthrusts Cuba to the East.

Geochemistry and U–Pb (SHRIMP) geochronology of Grupelli Granite: New constraints on the cessation of felsic magmatism in the Pelotas Batholith, Dom Feliciano Belt

The Pelotas Batholith forms the eastern portion of the Neoproterozoic Dom Feliciano Belt and records the final stages of the Neoproterozoic Brazilian–Pan-African orogeny. This granite-dominated belt experienced a complex tectonomagmatic evolution, yet constraints on the nature, and especially, the timing of the different magmatic stages are still scarce. This study provides new geochemical and geochronological constraints on the Grupelli Granite, a post-deformational pluton located in the rural district of Quilombo, Pelotas, RS. This granite crops out as a series of sub-kilometric solid bodies along a NE–oriented fault zone, intruding the Pinheiro Machado Complex that dominates the central and southeastern portion of the Pelotas Batholith. The studied granite is widely used as an ornamental rock due to its extremely leucocratic and isotropic appearance. The Grupelli Granite is homogeneous, relatively undeformed, inequigranular and typically medium-to coarse-grained (1–5 mm), with porphyritic and hypidiomorphic textures. Based on modal mineralogy the rock is a monzogranite with 40–50% quartz, 19–25% K-feldspar, and 15% oligoclase, 3–5% biotite, 2–3% magnetite and ilmenite, and accessory chlorite, zircon, apatite, chalcopyrite, and pyrite. Geochemically, the Grupelli Granite is characterized by high SiO2 (74–77 wt%) and K2O (5.08–5.65 wt%) contents combined with low FeOt (1.71–1.93 wt%), MgO and TiO2 (<0.2 wt%) concentrations. This highly evolved granite has a high-K calc-alkaline affinity, is weakly peraluminous, with I-type fingerprints. The combined data are consistent with formation in a post-tectonic environment by anatexis of mainly calk-alkaline sources, with a potential minor contribution of a metapelitic source. The low Sr-contents and concomitant high Ba/Sr ratios likely reflect plagioclase fractionation, while ilmenite fractionation may account for the pronounced negative Nb, Ta, and Ti anomalies. Although the rare earth element (REE) and other trace element patterns tend to parallel the patterns in the ca. 600 Ma Dom Feliciano Suite and the Cerro Grande Suite magmatism, the Grupelli Granite presents significantly lower heavy REE (HREE) concentrations and a smaller negative Eu anomaly. The HREE depletion can be attributed to retention of garnet in the source. These geochemical characteristics distinguish it from the earlier magmatic pulses in the Pelotas Batholith. The Grupelli Granite yielded a crystallization age of 578 ± 3.9 Ma (U–Pb zircon SHRIMP), and is thus younger than the ca. 600 Ma Dom Feliciano and Cerro Grande suites. This crystallization age places new constraints on the felsic post-collisional magmatism during the final stages of the evolution of Pelotas Batholith.

Late Quaternary uplift along the North America-Caribbean plate boundary: Evidence from the sea level record of Guantanamo Bay, Cuba

The tectonic setting of the North America-Caribbean plate boundary has been studied intensively, but some aspects are still poorly understood, particularly along the Oriente fault zone. Guantanamo Bay, southern Cuba, is considered to be on a coastline that is under a transpressive tectonic regime along this zone, and is hypothesized to have a low uplift rate. We tested this by studying emergent reef terrace deposits around the bay. Reef elevations in the protected, inner part of the bay are ∼11–12 m and outer-coast, wave-cut benches are as high as ∼14 m. Uranium-series analyses of corals yield ages ranging from ∼133 ka to ∼119 ka, correlating this reef to the peak of the last interglacial period, marine isotope stage (MIS) 5.5. Assuming a span of possible paleo-sea levels at the time of the last interglacial period yields long-term tectonic uplift rates of 0.02–0.11 m/ka, supporting the hypothesis that the tectonic uplift rate is low. Nevertheless, on the eastern and southern coasts of Cuba, east and west of Guantanamo Bay, there are flights of multiple marine terraces, at higher elevations, that could record a higher rate of uplift, implying that Guantanamo Bay may be anomalous. Southern Cuba is considered to have experienced a measurable but modest effect from glacial isostatic adjustment (GIA) processes. Thus, with a low uplift rate, Guantanamo Bay should show no evidence of emergent marine terraces dating to the ∼100 ka (MIS 5.3) or ∼80 ka (MIS 5.1) sea stands and results of the present study support this.

GEOLOGICAL STRUCTURE AND MORPHOGENESIS OF THE ELBLĄG ELEVATION IN THE LIGHT OF DIVERSIFIED DYNAMICS OF the VISTULian ICE SHEET

Glaciotectonic and neotectonic processes are among the key factors influencing the geological structure of the Elbląg Elevation. The former is undoubtedly interrelated with the latter. Intensity and scale of glaciotectonic processes were for the first time examined from the point of view of transgression dynamics of the Vistulian continental glaciers, advancing from the Baltic Sea area. Geological structure, morphology and bathymetry of the eastern part of the Central Baltic Sea bottom show that main elements of the dynamics were shaped in this area. The diversified dynamics resulted in the formation of ice streams, carrying the major part of ice mass out of the ice sheet. It also caused that the advance of the ice sheet over the Polish Lowlands was not synchronous and occurred with various intensity. The Elbląg Elevation was subjected to these rules, because it was the first to be situated within the diverse force of interaction field with ice stream B3 (streams according to Punkari, 1997), advancing towards the Polish Lowlands from the area of a submeridional megadepression of the Gotland Deep and the Gdańsk Deep (south-eastern part of Central Baltic Sea). As regards the Elbląg Elevation area, an evaluation of the scale and intensity of glaciotectonic processes in time has been performed. Especially strong glaciotectonic disturbances, appearing on the western slope of this elevation, have been related to the presence of the distinct, submeridionally oriented fault zone Iława–Elbląg (its northern part), which coincides with the western boundary of this elevation. The likely reason for the development of glaciotectonic deformation during the middle and upper parts of the Vistula Glaciation might habve been the change of the regime of neotectonic stress field, operating within the broad foreland of the Kuyavian-Pomeranian Swell, which probably took place at the end of the Eemian Interglacial.

Morphotectonic control of the Białka drainage basin (Central Carpathians): Insights from DEM and morphometric analysis.

Abstract The Białka river valley is directly related to a deep NNW-SSE oriented fault zone. According to the results of previous morphometric analyses, the Białka drainage basin is one of the most tectonically active zones in the Central Carpathians. It is also located within an area of high seismic activity. In this study Digital Elevation Model (DEM) based, morphometric analyses were used to investigate the morphotectonic conditions of the watershed. The results reveal the relationships between the main tectonic feature and the landforms within the research area. The lineaments, as obtained from the classified aspect map, seem to coincide with the orientation of the main structures as well as the trends revealed by the theoretical Riedel-Skempton shear model. Base-level and isolong maps support the conclusion that the Białka and Biały Dunajec fault zones exert a strong influence on the morphology of the adjacent area.

Segmentation and kinematics of the North America‐Caribbean plate boundary offshore Hispaniola

AbstractWe explored the submarine portions of the Enriquillo–Plantain Garden Fault zone (EPGFZ) and the Septentrional–Oriente Fault zone (SOFZ) along the Northern Caribbean plate boundary using high‐resolution multibeam echo‐sounding and shallow seismic reflection. The bathymetric data shed light on poorly documented or previously unknown submarine fault zones running over 200 km between Haiti and Jamaica (EPGFZ) and 300 km between the Dominican Republic and Cuba (SOFZ). The primary plate‐boundary structures are a series of strike‐slip fault segments associated with pressure ridges, restraining bends, step overs and dogleg offsets indicating very active tectonics. Several distinct segments 50–100 km long cut across pre‐existing structures inherited from former tectonic regimes or bypass recent morphologies formed under the current strike‐slip regime. Along the most recent trace of the SOFZ, we measured a strike‐slip offset of 16.5 km, which indicates steady activity for the past ~1.8 Ma if its current GPS‐derived motion of 9.8 ± 2 mm a−1 has remained stable during the entire Quaternary.

Possible natural fluid pathways from gravity pseudo-tomography in the geothermal fields of Northern Alsace (Upper Rhine Graben)

BackgroundThis study aims on investigating the regional flow field of the Soultz and adjacent geothermal fields located on the western side of the central Upper Rhine Graben and thus to provide insight into the origin of the 70% of the geothermal fluid coming from the regional inflow in the deep reservoir of the Soultz site. In an integrative approach, we consolidate conceptual models on fluid flow in the central Upper Rhine Graben.MethodsBased on a 3D geological model and a new 3D temperature interpolation, we tackle the relation between tectonic structures and the occurrence of advection/convection along favourably oriented fault zones. Using sequential Butterworth filters, we study the distribution of negative residual anomalies in a pseudo-tomography down to a depth of about 6 to 8 km.ResultsWe derived N-S-striking V-shaped negative anomalies that are consistent with the orientation of fault zones revealing major temperature anomalies to their east.ConclusionsFollowing the concept of negative anomalies revealing zones of increased fracture porosity, and in agreement with fluid-chemistry, our findings suggest infiltration of meteoric water through the graben boundary fault and along preferential flow pathways that merge at intermediate depth. Up-flow of thermal water mixed most likely with brine from the deeper eastern part of the graben occurs along W-dipping typically rather steep structures.

Probabilistic Seismic Hazard Assessment for Jamaica

A probabilistic seismic hazard analysis was performed to generate seismic hazard maps for Jamaica. The analysis was then conducted using a standard logic-tree approach that allowed systematically taking into account the model-based (i.e., epistemic) uncertainty and its influence on the computed ground motion parameters. Hazard computations have been performed using a grid of sites with a space of 0.05 degrees. Two different computation methodologies have been adopted: the standard approach based on the definition of appropriate seismogenic sources and the zone-free approach, which overcomes the ambiguities related with the definition of the seismic sources solely reflecting the characteristics of the earthquake catalogue. A comprehensive and updated earthquake catalogue for Jamaica has been compiled for the years 1551-2010 and new empirical relationships amongst magnitudes MW-MS and MW-mb have been developed for the region. Uniform hazard spectra and their uncertainty have been calculated for the horizontal component of ground motion for rock site conditions and five return periods (95, 475, 975, 2,475 and 4,975 years) and spectral accelerations for 34 structural periods ranging from 0 to 3 s, and 5% of critical damping. The spectral accelerations have been calculated to allow the definition of seismic hazard in Jamaica according to the International Building Code 2012. The disaggregation analysis for Kingston Metropolitan Area suggests that the magnitude-distance pair that contributes most to the hazard corresponds to events with M 7.8 and M 7.0 in the Enriquillo Plantain Garden Fault and the Jamaican Faults at a distance of 28 km and 18 km for short and long period structures respectively corresponding to 2,475 years return period. However, for long period structures, a substantial contribution is found for a M 8.2 at a distance of 198 km in the Oriente Fault Zone.

Geological development of the Central and South Vietnamese margin: Implications for the establishment of the South China Sea, Indochinese escape tectonics and Cenozoic volcanism

The Vietnamese margin forms a key region to the understanding of escape tectonics and the development of the South China Sea (SCS). The existing geological reconstructions of the region are restricted to studies of single basins, are based on limited amounts of geophysical data or analysis of onshore geological features. These models are critically assessed on the basis of interpretation of the most comprehensive 2-D digital seismic database published to date within the area combined with a thorough analysis of existing literature, and a new model is presented. The Vietnamese margin is underlain by a series of Paleogene rift basins established through southeastward extrusion of Indochina. The East Vietnam Boundary Fault (EVBF) forms the almost 1000 km long seaward continuation of the left-lateral Ailao Shan-Red River Shear Zone (ASRRSZ). Toward the southern half of the Phu Khanh Basin the EVBF breaks up into discrete segments and splays into the SE-directed Tua Hoa Fault Zone. Paleogene faults splayed from the EVBF and the Mae Ping Shear Zone and accommodated the coeval motion of these two major left-lateral structural lineaments. During the late Oligocene, basin inversions offshore occurred contemporaneously with initial right-lateral inversion along the Mae Ping Shear Zone and the onset of major uplift of the metamorphic core complexes along the ASRRSZ. It is suggested that a dramatic change of the regional stress pattern occurred in response to the northward movement of India and the effective coupling of the West Burma Block and India, the later resulting in broadening of the indenting continental mass. After the mid-Oligocene, left-lateral movements across the offshore EVBF decreased and eventually ceased. Later, onshore sinistral movements were accommodated by internal shortening and local clockwise block rotations within the Shan-Thai Terrain. Renewed rifting offshore south Vietnam resulted from the jump of the SCS spreading axis and subsequent Neogene southwestward propagation of continental break-up. The Neogene opening of the SCS is viewed as a consequence of a slab-pull associated with subduction of the proto-SCS underneath Borneo as offshore Neogene escape tectonism had virtually ceased. During this phase of rifting, right-lateral transtension is inferred along N to NW oriented fault zones in the Phu Khanh and the Nam Con Son basins. Rifting and by inference seafloor spreading continued until the end of middle Miocene time although at significantly reduced rates during the final 5–10 Ma. Termination of seafloor spreading is marked by a distinct latest middle Miocene unconformity in the Nam Con Son and the southern Phu Khanh basins. Neogene volcanism resulted in the build-up of several magmatic structures across the central and South Vietnamese margin and induced reactivation of older faults in the Phu Khanh Basin. The offshore magmatism heralded widespread volcanism onshore southern Indochina associated with a latest Neogene uplift and denudation event. The uplift led to significantly increased siliciclastic accumulation rates offshore, which repressed widespread carbonate deposition.

The intraplate Porto dos Gaúchos seismic zone in the Amazon craton — Brazil

The largest earthquake observed in the stable continental interior of the South American plate occurred in Serra do Tombador, Mato Grosso state — Brazil, on January 31, 1955 with a magnitude of 6.2 m b. Since then no other earthquake has been located near the 1955 epicentre. However, in Porto dos Gaúchos, 100 km northeast of Serra do Tombador, a recurrent seismicity has been observed since 1959. Both Serra do Tombador and Porto dos Gaúchos are located in the Phanerozoic Parecis basin. Two magnitude 5 earthquakes occurred in Porto dos Gaúchos, in 1998 and 2005, with intensities up to VI and V, respectively. These two main shocks were followed by aftershock sequences lasting more than three years each. Local seismic stations have been deployed by the Seismological Observatory of the University of Brasilia since 1998 to study the “Porto dos Gaúchos” seismic zone (PGSZ). A local seismic refraction survey was carried out with two explosions to help define the seismic velocity model. Both the 1998 and 2005 earthquake sequences occurred in the same WSW–ENE oriented fault zone with right-lateral strike-slip mechanisms. The epicentral zone is in the Parecis basin, near its northern border where there are buried grabens, generally trending WNW–ESE, such as the deep Mesoproterozoic Caiabis graben which lies partly beneath the Parecis basin. However, the epicentral distribution indicates that the 1998 and 2005 sequences are related to a N60°E fault which probably crosses the entire Caiabis graben. The 1955 earthquake, despite the uncertainty in its epicentre, does not seem to be directly related to any buried graben either. The seismicity in the Porto dos Gaúchos seismic zone, therefore, is not directly related to rifted crust. The probable direction of the maximum horizontal stress near Porto dos Gaúchos is roughly E–W, consistent with other focal mechanisms further south in the Pantanal basin and Paraguay, but seems to be different from the NW–SE direction observed further north in the Amazon basin. The recurrent seismicity observed in Porto dos Gaúchos, and the large 1955 earthquake nearby, make this area of the Parecis basin one of the most important seismic zones of Brazil.

Emplacement of the northern ophiolites of Cuba and the Campanian-Eocene geological history of the northwestern Caribbean-SE Gulf of Mexico region

Abstract The Mesozoic Proto-Caribbean Plate was consumed in the subduction zone of the Greater Antilles volcanic arc until the Campanian. At this time, volcanic arc magmatism ceased along Cuba. From Late Campanian to Danian, Cuba and its surroundings were a collision zone where the GAC accreted to the North American palaeomargin. In the Danian the almost east–west trending SE Cuba–Cayman Ridge–Hispaniola? volcanic arc was born. The related north dipping subduction zone acted as the SE North American plate boundary. From the Paleocene to Middle Eocene dense Caribbean lithosphere travelled northwards. The location, strike and subduction polarity of the assumed subduction zone are very different from those described by other models. Almost simultaneously the Cuban Orogeny developed in western and central Cuba. During the orogeny the northern ophiolite belt of Cuba and the Cretaceous volcanic rocks were thrust northwards tens of kilometres, onto the Mesozoic North American palaeomargin. In the Middle Eocene subduction stopped. Simultanously(?) a change in the regional stress field originated the near east–west trending sinistral Oriente fault zone, whose position and origin are probably tied to the weakened hot crust to the south of the Palaeogene volcanic arc axis.

Synsedimentary faults and amalgamated unconformities: Insights from 3D-seismic and core analysis of the Lower Triassic Middle Buntsandstein, Ems Trough, north-western Germany

The Late Permian/Early Triassic succession of the Central European Basin (CEB) was repeatedly affected by the tectonic pulses associated with the earliest phases of Tethyan and Arctic–North Atlantic rifting. Effects of the differential tectonic subsidence are particularly well recorded by unconformities, which form widespread sequence boundaries. Such unconformities are most obvious in areas occupied by fault-controlled intra-basinal highs (swells). In that areas, stratigraphic loss may comprise entire Lower and Middle Buntsandstein formations and in places remnant Middle Buntsandstein successions directly rest on Permian strata. Analysis of 3D-seismic data and well logs combined with high-resolution sedimentological logging of drillcores at the western margin of the Ems Trough (NW Germany) reveals details of synsedimentary tectonic control on sequence development. Early Triassic extensional faulting of basement blocks provided stepwise addition of accommodation space for continental sequences by growth faulting along north–south oriented fault zones blocks on the flanks of the East Netherlands High. This process is most evident during the development of the Hardegsen Unconformity, which is characterised by an amalgamation of succeeding unconformity surfaces in areas of structurally controlled intrabasinal highs.

Seismic microzoning from synthetic ground motion parameters: case study, Santiago de Cuba

Synthetic seismograms (P-SV and SH waves) have been calculated along six profiles in Santiago de Cuba basin, with a cutoff frequency of 5 Hz, by using a hybrid approach (modal summation for a regional 1D structure plus finite differences for a local 2D structure embedded in the first). They correspond to a scenario earthquake of M S=7 that may occur in Oriente fault zone, directly south of the city. As initial data for a seismic microzoning, the characterisation of earthquake effects has been made considering several relative (2D/1D) quantities (PGDR, PGVR, PGAR, DGAR, I A R—ratios of peak ground values of displacement, velocity and acceleration, and of design ground acceleration and Arias intensity-, etc.) and functions representative of the ground motion characteristics in soil (2D) with respect to bedrock (1D). The functions are the response spectra ratio RSR( f), already routinely used in this kind of work, and the elastic energy input ratio E I R( f), defined, for the first time, in this paper. These data, sampled at 115 sites within all the profiles have been classified in two steps, using logical combinatory algorithms: connected components and compact sets. In the first step, from the original ground motion parameters or functions extracted from the synthetic seismograms, nine sets have been classified and the partial results show the spatial distribution of the soil behaviour as a function of the component of motion. In the second step, the results of the classification of the nine sets have been used as input for a further classification that shows a spatial distribution of sites with a quasi-homogeneous integral ground motion behaviour. By adding the available geological surface data, a microzoning scheme of Santiago de Cuba basin has been obtained.

Ground Motion Zoning of Santiago de Cuba: An Approach by SH Waves Modelling

The expected ground motion in Santiago de Cuba basin from earthquakes which occurred in the Oriente fault zone is studied. Synthetic SH-waves seismograms have been calculated along four profiles in the basin by the hybrid approach (modal summation for the path source-profile and finite differences for the profile) for a maximum frequency of 1 Hz. The response spectra ratio (RSR) has been determined in 49 sites, distributed along all considered profiles with a spacing of 900 m. The corresponding RSR versus frequency curves have been classified using a logical-combinatorial algorithm. The results of the classification, in combination with the uppermost geological setting (geotechnical information and geological geometry of the subsoil) are used for the seismic zoning of the city. Three different main zones are identified, and a small sector characterized by major resonance effects, due to the particular structural conditions. Each zone is characterized in terms of its expected ground motion parameters for the most probable strong earthquake (M s = 7), and for the maximum possible (M s = 8).

The New Cuban Seismograph Network

Evidence of instrumental seismic data can be found in Cuba as early as the beginning of the twentieth century. The first Cuban seismological station was installed in 1907 by the Jesuits of Belen Observatory in Luyano, Ciudad Havana and worked until the 1920's. The station was equipped with two Bosch-Omori-type seismometers recording on smoked paper (Cotilla, 1998). Some moderate earthquakes were recorded during that period but unfortunately the seismograms from those instruments were lost. The second station, Soroa (SOR), the first of the current seismograph network (Figure 1), began operation in 1964. It was located in west Cuba to monitor the seismic activity of the Pinar Fault (Figure 2), responsible for the 1880 San Cristobal earthquake (Table 1). One year later, in 1965, station Rio Carpintero (RCC) began operation in eastern Cuba (Figure 1), the most seismically active part of the country. View this table: TABLE 1 Historic Cuban Earthquakes Macroseismic determination is represented by parentheses. Only earthquakes Ms > 6 (Santiago de Cuba) and Ms > 5 (rest of the country) are shown. Figure 1. New Cuban Seismograph Network. Activity in eastern Cuba is generated mostly by a major transform fault, the Oriente Fault Zone (OFZ) (Figure 2). To monitor the OFZ activity and improve the coverage of the area, three more stations were installed in eastern Cuba in 1979—Pinares (PIN), Las Mercedes (LMG), and Maisi (MAS)—and three years later Cascorro (CCC) was added (Figure 1). Most of the stations were equipped with SKM-3 short-period three-component electromagnetic seismometers (of Russian manufacture) recording on photographic paper. The seismograph network consisted of eighteen stations in 1989, including a local telemetered network of eight stations in the area surrounding Santiago de Cuba. The locations of these stations can be found in Cotilla (1998). The local telemetered network was equipped with one vertical-component short-period seismometer with visual recording. …

Modelling of Seismic Ground Motion in Santiago de Cuba City from Earthquakes in Oriente Fault Seismic Zone

— We present the results of complete P-SV and SH waves modelling, up to a maximum frequency of 1 Hz, along two profiles in Santiago de Cuba city. The seismic sources are located in the depth range from 10 to 40 km on the Oriente fault zone at distances of several tens of kilometres from the city. The calculation has been made by a hybrid method: Modal summation in the regional anelastic model (one-dimensional) where the source is buried, and finite differences in the local sedimentary anelastic models (two-dimensional). The analysis of the influence of the depth and of the distance of the source on the site effects shows that standard traditional methods, based on the deconvolution analysis of the rock outcrop motion, can lead to erroneous results.

Shallow seismic reflection profiles and geological structure in the Benton Hills, southeast Missouri

During late May and early June of 1993, we conducted two shallow, high-resolution seismic reflection surveys (Mini-Sosie method) across the southern escarpment of the Benton Hills segment of Crowleys Ridge. The reflection profiles imaged numerous post-late Cretaceous faults and folds. We believe these faults may represent a significant earthquake source zone. The stratigraphy of the Benton Hills consists of a thin, less than about 130 m, sequence of mostly unconsolidated Cretaceous, Tertiary and Quaternary sediments which uncomfortably overlie a much thicker section of Paleozoic carbonate rocks. The survey did not resolve reflectors within the upper 75–100 ms of two-way travel time (about 60–100 m), which would include all of the Tertiary and Quaternary and most of the Cretaceous. However, the Paleozoic-Cretaceous unconformity (Pz) produced an excellent reflection, and locally a shallower reflector within the Cretaceous (K) was resolved. No coherent reflections below about 200 ms of two-way travel time were identified. Numerous faults and folds, which clearly offset the Paleozoic-Cretaceous unconformity reflector, were imaged on both seismic reflection profiles. Many structures imaged by the reflection data are coincident with the surface mapped locations of faults within the Cretaceous and Tertiary succession. Two locations show important structures that are clearly complex fault zones. The English Hill fault zone, striking N30°–35°E, is present along Line 1 and is important because earlier workers indicated it has Pleistocene Loess faulted against Eocene sands. The Commerce fault zone striking N50°E, overlies a major regional basement geophysical lineament, and is present on both seismic lines at the southern margin of the escarpment. The fault zones imaged by these surveys are 30 km from the area of intense microseismicity in the New Madrid seismic zone (NMSZ). If these are northeast and north-northeast oriented fault zones like those at Thebes Gap they are favorably oriented in the modern stress field to be reactivated as right-lateral strike slip faults. Currently, earthquake hazards assessments are most dependent upon historical seismicity, and there are little geological data available to evaluate the earthquake potential of fault zones outside of the NMSZ. We anticipate that future studies will provide evidence that seismicity has migrated between fault zones well beyond the middle Mississippi Valley. The potential earthquake hazards represented by faults outside the NMSZ may be significant.

The Prina Complex in eastern Crete and its relationship to possible Miocene strike-slip tectonics

The basal Neogene formations in the Ierapetra region, eastern Crete, are strongly influenced by a Late Serravallian tectonic phase which resulted in the breakup of pre-existing palaeogeographic patterns. Important vertical movements caused the southward emplacement of Neogene sediments, together with parts of the underlying pre-Neogene nappe pile. The resulting chaotic association of exotic blocks and sediments, known as the Prina Complex, has the properties of a sedimentary mélange. It can be traced for more than 15 km from north to south. In the north a relatively coherent accumulation of large slide masses overlies deformed Neogene coarse clastics and pre-Neogene rocks. Distally it comprises a poorly stratified sequence of breccias and intermixed finer grained sediments, which locally contains olistostromes and debris-flows and interfingers to the south with submarine fan deposits. The intricate relation of faulting and gravity sliding in a rapidly subsiding basin can be explained by generation in a strike-slip setting. It is suggested that the Ierapetra basin and its offshore extension, the South Cretan trough, were initiated by sinistral movements along a NE-SW oriented fault zone. Implications of this model for the geodynamic evolution of the south Aegean area are discussed.