Modelo tectónico reciente de la Cuenca Alta del Tajo (parte central española)

Active tectonics within the Upper Tagus Basin is related to the lithospheric flexure affecting the Palaeozoic basement of the basin. This flexure displays NE-SW trending. Besides, this structure is in agreement with the regional active stress field defined by the maximum horizontal stress with NW-SE trending. In this tectonic framework, irregular clusters of instrumental seismicity (Mw< 5.0) fade in the zone bounded by the Tagus River and the Jarama River valleys. These clusters are related to major NW-SE trending faults of suspected strike-slip kinematics. Moreover, reverse faults with NE-SW trending are affected by the strike-slip system as well. Despite the reverse faults are in agreement with the present SHMAX orientation, though, they apparently are blocked as seismogenic sources (scarce instrumental seismicity recorded today). In addition, we have determined the regional and local stress/ strain fields and two different fracture patterns were observed. Hence, we have divided the area in two zones: (1) the lateral bands of the basin, defined by reverse faulting (NE-SW trending) and strike-slip faulting (NW-SE trending) and (2) the central zone of the basin characterized by shallow normal faulting and NE-SW trending strike-slip faults. Furthermore, surface faulting and liquefaction structures are described affecting Middle to Late Pleistocene fluvial deposits, suggesting intrabasinal palaeoseismic activity (5.5 < M < 6.5) during the Late Quaternary. The obtained structural and tectonic information has been used to classify and characterize the Upper Tagus Basin as a semi-stable intraplate seismogenic zone, featured by Pleistocene slip-rates < 0.02 mm/yr. This value is low but it affords the occurrence of Pleistocene paleoearthquakes.La tectónica activa de la Cuenca Alta del río Tajo está caracterizada por la presencia de una flexión litosférica localizada en el basamento paleozoico, y cuyo eje se orienta según NE-SW. Esta flexión es coherente con el estado de esfuerzos actual en la zona, definido mediante la orientación del esfuerzo máximo horizontal, SHMAX, según NW-SE. En este contexto, la sismicidad en el borde sur del Sistema Central aparece concentrada en fallas direccionales según NW-SE, registrándose terremotos de magnitud < 5.0 (S. XX). Estos desgarres sectorizan el sistema de fallas inversas (NE-SW) que estructuran la cadena montañosa, las cuales a pesar de estar orientados coherentemente con el estado actual de esfuerzos, aparecen bloqueados o desactivados como fuentes sismogénicas. La geometría de la flexura condiciona la ocurrencia de una zona de concentración anómala de esfuerzos que resulta en la agrupación de eventos sísmicos localizada entre los valles de los ríos Tajo y Jarama. El estudio sismotectónico indica que el campo actual de deformación se encuentra distribuido en dos zonas relacionadas con la flexión: (1) una zona central caracterizada por la presencia de fallas normales superficiales y desgarres orientados según NE-SW; y (2) zona lateral caracterizada por fallas de tipo inverso (NE-SW) y desgarres (NW-SE) y que bordea a la zona anterior. Estudios paleosísmicos en la zona muestran la ocurrencia de diferentes niveles de liquefacción afectando a los depósitos fluviales del Pleistoceno Medio y Superior en los valles de los ríos Manzanares, Jarama y Tajo, así como una importante variedad de estructuras de deformación. En general la granulometría licuefactada muestra diques de arena, aunque en algún caso aparecen niveles decimétricos de gravas. A partir de los datos sismotectónicos, geológicos y paleosísmicos descritos y recopilados para esta zona, la Cuenca alta del Tajo puede clasificarse como una zona sismogénica intraplaca con velocidades de deformación pleistocenas inferiores a 0,02 mm/año y sismicidad instrumental difusa de tamaño moderado-bajo

Costa Recta beach, Deception Island, West Antarctica: A retreated scarp of a submarine fault?

Deception Island (South Shetlands, Antarctica) is one of the most active volcanoes in Antarctica, having erupted recently in 1967, 1969 and 1970, damaging scientific stations on the island. It is also seismically very active. The island has attracted the attention of many researchers as it constitutes an undisturbed natural laboratory to study seismo-volcanic events and how they affect landscape modelling and evolution. One of the most remarkable geological and geomorphological features on Deception Island is the linearity of its easternmost coastal landform, the origin of which remains unknown. Some answers, based on presence of strike-slip fault or on the ice cap and beach geomorphological dynamics, have been reported in the literature. Our new work provides several indications of the existence of a dip-slip submarine fault, parallel to the coast (NNW-SSE), which suggests a tectonic origin for this morphological feature. Uplifted marine terraces, incision of a fluvial network over the ice cap, normal faulting parallel to the coast in the north and south rock heads bounding the beach and sharp shelf-break with rather constant slope, constitute some of this evidence. Terrace uplift and fluvial channel incision decreasing southward from Macaroni Point, indicates possible tilt movement across this inferred fault plane. © Antarctic Science Ltd.Fil:Caselli, A.T. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Evidencias paleosísmicas y geomorfológicas de actividad tectónica reciente de la Falla de Pozohondo (Cordillera Bética, SE de España)

Instrumental and historical seismicity in the Albacete province (External Prebetic Zone) has been scarcely recorded. However, major strike-slip faults showing NW-SE trending provide geomorphologic and paleoseismic evidence of recent tectonic activity (Late Pleistocene to Present). Moreover, these faults are consistently well oriented under the present stress tensor and therefore, they can trigger earthquakes of magnitude greater than M6, according to the lengths of surface ruptures and active segments recognized in fieldwork. Present landscape nearby the village of Hellin (SE of Albacete) is determined by the recent activity of the Pozohondo Fault (FPH), a NW-SE right-lateral fault with 90 km in length. In this study, we have calculated the Late Quaternary tectonic sliprate of the FPH from geomorphological, sedimentological, archaeoseimological, and paleoseismological approaches. All of these data suggest that the FPH runs with a minimum slip-rate of 0.1 mm/yr during the last 100 kyrs (Upper Pleistocene-Holocene). In addition, we have recognized the last two major paleoearthquakes associated to this fault. Magnitudes of these paleoearthquakes were gretarer than M6 and their recurrence intervals ranged from 6600 to 8600 yrs for the seismic cycle of FPH. The last earthquake was dated between the 1st and 6th centuries, though two earthquakes could be interpreted in this wide time interval, one at the FPH and other from a far field source. Results obtained here, suggest an increasing of the tectonic activity of the Pozohondo Fault during the last 10,000 yrs.A pesar de la escasa sismicidad instrumental e histórica en el Prebético Externo, que comprende la provincia de Albacete, existen tres grandes fallas con evidencias de actividad reciente y cuya orientación es compatible con el campo de esfuerzos activo. Estas fallas consisten en desgarres dextrorsos de orientación NO-SE cuya longitud de traza sobrepasa los 50 km. Este trabajo se centra en el análisis del segmento Tobarra-Cordovilla de la Falla de Pozohondo (FPH) (90 km de longitud), mediante estudios paleosísmicos, geomorfológicos, sedimentológicos e incluso arqueosísmicos. De todos estos análisis es posible afirmar que dicha falla (FPH) presenta una tasa de actividad cuaternaria superior a 0,1 mm/año durante los últimos 100 ka (Pleistoceno Superior-Holoceno). Además, se han descrito los últimos dos paleoterremotos asociados a este segmento con una tasa de recurrencia para magnitudes superior a M6 de 6600-8600 años, habiendo tenido lugar el último evento entre los siglos I y VI. Sin embargo, los datos no permiten fijar de forma más precisa dicho evento, e incluso es posible que hayan tenido lugar dos eventos diferenciados durante este periodo, uno relacionado con la FPH y el otro de origen de campo lejano. En cualquier caso, se evidencia un suave incremento en la actividad tectónica en la zona durante los últimos 10.000 años

Estimation of the tectonic slip-rate from Quaternary lacustrine facies within the intraplate Albacete province (SE of Spain)

The Quaternary lacustrine basin of Cordovilla (CB) represents one of the most active tectonic areas of the Prebetic Zone (Albacete, SE of Spain). The Quaternary sedimentary deposits of this basin are mainly endoreic lacustrine carbonate and alluvial deposits, developed in a semi-arid climate (Pleistocene-present). The basin is a NW–SE-elongated graben bounded by a major right-lateral oblique-fault, the Pozohondo Fault. This fault trendsNW–SE, with an approximate trace of 55 km, and is composed of various segments which are identified by fault scarps. In order to establish the slip-rate of the most active segment of the Pozohondo Fault, called the Cordovilla segment, we carried out a detailed study of the affected Quaternary lacustrine deposits. We found that the lacustrine facies could be related to episodic moderate paleoearthquakes. The slip-rate is calculated to be 0.05 and 0.09 mm/yr, using radiometric dating for the vertical offsets of the lacustrine facies. A trenching study at the northern part of the Cordovilla segment revealed two events caused by paleoearthquakes, with the most recent expressed as an oblique-fault off-setting a poorly-developed soil. The magnitude of the last event was greater than 6, using various empirical relationships for the fault displacement and the surface-length rupture. We estimate episodic activity across the Cordovilla segment, to be characterized by moderate-sized paleoearthquakes (M6), which is in agreement with the tectonic context of an intraplate zone of the Iberian plate

The 1970’s volcanic eruption at Deception Island (West-Antarctica) and fault-related activity: the Maria’s Fault

Este trabajo analiza la formación de un escarpe de falla asociado a la erupción freatomagmática que ocurrió en 1970 en la Isla Decepción (Antártida Occidental). El hecho de tener información de fotografías aéreas previas a su desarrollo (1968) y una imagen de satélite de alta resolución (2003) permite establecer la formación de dicho escarpe, la evolución de la red de drenaje y la presencia de terrazas fluviales levantadas. A partir de esta información, llevamos a cabo una campaña de campo durante el verano austral de 2007, con el fin de realizar una serie de trincheras a lo largo del escarpe. De toda la información expuesta, se constata un movimiento de componente inverso a lo largo de la falla con 1 km de ruptura superficial y 20 cm de salto, afectando a los depósitos piroclásticos de la erupción de 1970. Tres modelos podrían explicar dicha actividad: (1) Terremoto inverso de tamaño pequeño-moderado (5<M<6) posterior a la erupción de 1970, (2) Reactivación de una falla previa asociada a la inyección o migración de un dique somero durante el proceso eruptivo y (3) Actividad asísmica de tipo creep a lo largo del plano de falla post terremoto.This work addresses the presence of a superficial rupture fault scarp in relationship with the phreatomagmatic eruption occurred in 1970 at Deception Island, West Antarctica. We have analysed aerial photographs (1968) previous to the eruption and contemporaneous high resolution satellite image (Quickbird, 2003). This information was relevant to establish the scarp morphology, fluvial network variations and uplifted fluvial terraces. Accordingly and during the austral summer of 2007, we carried out a field survey with the purpose to perform several paleoseismic trenches across the fault scarp. This analysis showed a reverse fault movement through 1 km of superficial rupture with 20 cm of vertical throw. This activity affected to the pyroclastic volcano sedimentary sequence of the 1970’s eruption. We propose three models that could explain this activity: (1) Reverse small-moderate earthquake (5<M<6) triggered by the 1970`s eruption, (2) Reactivation of a previous fault as a consequence of dike injection and/or magma migration in relationship with the eruptive process and (3) Aseismic fault creep activity post-earthquake.Depto. de Geodinámica, Estratigrafía y PaleontologíaFac. de Ciencias GeológicasTRUEMinisterio de Educación y Ciencia (MEC)pu

Compilation of parameterized seismogenic sources in Iberia for the SHARE European-scale seismic source model.

Abstract: SHARE (Seismic Hazard Harmonization in Europe) is an EC-funded project (FP7) that aims to evaluate European seismic hazards using an integrated, standardized approach. In the context of SHARE, we are compiling a fully-parameterized active fault database for Iberia and the nearby offshore region. The principal goal of this initiative is for fault sources in the Iberian region to be represented in SHARE and incorporated into the source model that will be used to produce seismic hazard maps at the European scale. The SHARE project relies heavily on input from many regional experts throughout the Euro-Mediterranean region. At the SHARE regional meeting for Iberia, the 2010 Working Group on Iberian Seismogenic Sources (WGISS) was established; these researchers are contributing to this large effort by providing their data to the Iberian regional integrators in a standardized format. The development of the SHARE Iberian active fault database is occurring in parallel with IBERFAULT, another ongoing effort to compile a database of active faults in the Iberian region. The SHARE Iberian active fault database synthesizes a wide range of geological and geophysical observations on active seismogenic sources, and incorporates existing compilations (e.g., Cabral, 1995; Silva et al., 2008), original data contributed directly from researchers, data compiled from the literature, parameters estimated using empirical and analytical relationships, and, where necessary, parameters derived using expert judgment. The Iberian seismogenic source model derived for SHARE will be the first regional-scale source model for Iberia that includes fault data and follows an internationally standardized approach (Basili et al., 2008; 2009). This model can be used in both seismic hazard and risk analyses and will be appropriate for use in Iberian- and European-scale assessments

Neotectonics of the eastern border of the Madrid basin

An analysis of the neotectonic activity and the current stress state of the eastern border of Madrid Basin has been carried ont using structural-methods (fault population analysis), as well as seismological methods. Two stress tensors have been established: - Tensor 1: with a N1500E shortening trend, characterized mainly by strikeslip faults. This tensor corresponds to the «Guadarrama» stress field (Capote et al., 1990), wihch causes the Central System structure; dated middle Aragonian -lower Pleistocene in the eastern border ofMadrid Basin (Muñoz Martín, 1993). - Tensor 2: dated post-lower Pleistocene, with a N150° E extension trend, characterized basically by normal faults. Actual stress tensor deduced focal mechanisms by the seismological method is compatible with the paleostress field deduced for the middle-lower Pleistocene (tensor 2).España. Dirección General de Investigación Científica y TécnicaDepto. de Geodinámica, Estratigrafía y PaleontologíaFac. de Ciencias GeológicasFALSEpu