Tectónica activa de la Cuenca de Málaga: evidencias en marcadores morfotectónicos (Cordillera Bética Occidental, España)

The Malaga Basin is located in the westernmost part of the Betic Cordillera. This alpine cordillera in the south of Spain is the most active region of the Iberian Peninsula. Some of the most destructive earthquakes occurred historically in Spain took place within the Malaga Basin. In this work we focus on geomorphic and morphotectonic observations in the aim of finding active tectonic structures that could be seismogenic sources. First, we study the spatial arrangement and age of the Quaternary alluvial fan system as well as the drainage pattern of the basin, followed by the analysis of the distribution of regional markers like marine erosive surfaces and the extend of Pliocene marine deposits in the Malaga Basin. The tectonic structures inferred as active by the morphotectonic analysis are grouped into four main families: N60º-85ºE folds associated with blind thrust faults, N20º-30ºE and N40º-50ºE high angle dip-slip faults, and N165º-170ºE tear faults. Finally, their seismic potential in terms of maximum moment magnitude (Mw) is assessed by means of empirical relationships, varying between 6.0 and 7.0 depending on the hypothesis considered.La cuenca de Málaga se sitúa en la parte más occidental de la cordillera Bética. En esta cordillera alpina del sur de España se han localizado algunos de los terremotos más destructivos ocurridos históricamente en la península Ibérica. En este trabajo se estudian algunos de los aspectos geomorfológicos de la cuenca del Málaga con el fin de identificar las estructuras tectónicas activas que pudiesen ser fuentes sismogénicas. Se estudia la distribución y edad del sistema de abanicos aluviales de la cuenca, así como la red de drenaje. También se analiza la distribución regional de marcadores tectónicos pliocenos, como superficies de erosión marina y los depósitos sedimentarios de este periodo transgresivo. Se han identificado varias estructuras tectónicas activas que han sido agrupadas en cuatro familias: pliegues N60º-85ºE asociados con falla inversas ciegas, fallas de alto buzamiento con salto en la vertical N20º-30ºE y N40º-50ºE, y fallas de transferencia N165º-170ºE. En base a relaciones empíricas se ha estimado un potencial sísmico para las fallas identificadas, obteniéndose valores de magnitud momento (Mw) entre 6,0 y 7,0

Modelling earthquake rupture rates in fault systems for seismic hazard assessment: the Eastern Betics Shear Zone.

Earthquake surface fault ruptures can show very complex geometries and involve different faults simultaneously. Consequently, modern fault-based probabilistic seismic hazard assessments (PSHA) need to account for such complexities in order to achieve more realistic modellings that treat fault systems as a whole and consider the occurrence of earthquake ruptures as aleatory uncertainties. We use SHERIFS, a recent approach of modelling annual rates of complex multi-fault ruptures, to obtain system-level magnitude-frequency distributions (MFDs) for the Eastern Betics Shear Zone (EBSZ, Spain) considering four fault rupture hypotheses. We then analyze the consistency of each scenario based on data from the earthquake catalogue and paleoseismic studies. The definition of the different rupture hypotheses was discussed within the frame of Fault2SHA ESC working group and critical fault input data is extracted from previous published studies. The four rupture hypotheses are defined as incremental scenarios based on fault geometry and kinematics, with lengths varying from minimal fault sections to a rupture of nearly the whole system. The results suggest that multi-fault ruptures involving lengths up to single to several whole faults are consistent with the annual rates from both the instrumental catalogue and paleoseismic record. The method does not allow to completely discard any hypothesis, but it allows to weight the different models in a logic tree for seismic hazard assessment. The approach is revealed as a practical tool for obtaining fault-system MFDs and as a useful tool for highlighting limitations and uncertainties in geological and paleoseismic data to be assessed. This study aims to constitute a step forward in the consideration of complex multi-fault ruptures for future seismic hazard assessments in the region

New insights on the seismogenic potential of the Eastern Betic Shear Zone (SE Iberia): Quaternary activity and paleoseismicity of the SW segment of the Carrascoy Fault Zone

The Carrascoy Fault (CAF) is one of the main active faults that form part of the Eastern Betic Shear Zone, a 450 km fault system that accommodates most of the convergence between the Eurasian (Iberia) and Nubian plates in the Betic Cordillera, south Spain. Although the CAF represents a major earthquake threat to the nearby City of Murcia, studies on its Quaternary tectonics and seismogenic potential are scarce to date. We present evidence that supports the division of the CAF into two overlapping segments with contrasting tectonic structure, Quaternary activity, and landform control: a SW segment, characterized by a broad fold-and-thrust zone similar to the forebergs defined in the Gobi-Altai region, and a NE segment, characterized by a sharp mountain front controlled by strike-slip tectonics. We attribute the differentiation into these two segments to the stresses associated with topography, which in turn is a consequence of the shortening component, at the middle Pleistocene, after circa 217.4 ka. For the SW segment we infer the occurrence of 9 to 11, Mw 6.7 paleoearthquakes in the last 30.2 kyr, and a slip rate of 0.37 ± 0.08 m/kyr. We date the occurrence of the last surface rupture event after 2750 B.P., and we estimate an average recurrence period of major events of 3.3 ± 0.7 kyrThis work was supported by SISMOGEN (IGME, 2279) and FASEGEO (CGL2009-09726) research projects and a technical assistance of the Civil Protection Service of Murci

Paleoseismological evidence of holocene activity of the Los Tollos fault (Murcia, se Spain): a lately formed quaternary tectonic feature of the eastern betic shear zone

The Los Tollos Fault is a recent and important feature of the Eastern Betic Shear Zone, one of the major tectonic structures in South Iberia accommodating the convergence between Nubian and Eurasian plates in the western Mediterranean. The Los Tollos Fault became active by the end of Middle Pleistocene introducing some paleogeographical modifications. Previously mapped as a secondary normal fault related to the Carrascoy Fault, recent research evidences that the Los Tollos Fault is an independent Holocene active left-lateral reverse fault extending for at least 15 km. Data analysis from 4 trenches dug across the fault has revealed the occurrence of at least two paleo-earthquake events during the Holocene. The most recent event is dated between 2,740 and 2,140 yr BP (8th to 2nd centuries BC). The size of the paleoevents is calculated to be Mw 6.3 – 6.6 following empirical regressions on surface rupture length. The recurrence interval is estimated to be between 2,200-6,860 years, fitting a slip rate for the fault between 0.12 and 0.17 mm/yr. Such parameters highlight the Los Tollos Fault as a tectonic structure with a considerable activity located relatively close to densely populated areas. These seismogenic parameters should be considered in future reassessments of the seismic hazard of the regionThis work forms part of the research activities carried out in the FASEGEO Project (CGL2009-09726) funded by the Spanish Ministry of Science and Innovatio

Characterization of the shallow subsurface structure across the Carrascoy Fault System (SE Iberian Peninsula) using P-wave tomography and Multichannel Analysis of Surface Waves

The seismicity in the SE Iberian Peninsula is distributed parallel to the coast in a well-developed strike-slip fracture system known as the Eastern Betic Shear Zone (EBSZ). This work focuses on the characterization of the shallow subsurface structure of the Algezares-Casas Nuevas Fault, within the Carrascoy Fault System of the EBSZ. The Carrascoy Fault borders the Guadalentín Depression to the south, which is a densely populated area with extensive agricultural activity. Therefore, this faults system represents a seismic hazard with significant social and economic implications. We have constructed two velocity-depth models based on P-wave tomography and Multichannel Analysis of Surface Waves (MASW) acquired from seismic reflection data. The resulting velocity models have allowed us to interpret the first ~250m depth and have revealed: i) the thickness of the critical zone; ii) the geometry of the Algezares-Casas Nuevas Fault; iii) the depth of the Messinian/Tortonian contact and iv) the presence of blind thrusts and damage zones under the Guadalentín Depression. Our results have also helped us to estimate an apparent vertical slip rate of 0.66±0.06m/ky for the Algezares-Casas Nuevas Fault since 209.1±6.2ka. Our results provide a methodological and backflow protocol to study the shallow subsurface of active faults, complementing previous geological models based on paleoseismological trenches, and can be used to improve the seismic hazard assessment of tectonically active regions around the world

La Base de Datos de Fallas Activas en el Cuaternario de Iberia (QAFI v.2.0)

ABSTRACT. The Quaternary Active Faults Database of Iberia (QAFI) is an initiative lead by the Institute of Geology and Mines of Spain (IGME) for building a public repository of scientific data regarding faults having documented activity during the last 2.59 Ma (Quaternary). QAFI also addresses a need to transfer geologic knowledge to practitioners of seismic hazard and risk in Iberia by identifying and characterizing seismogenic fault-sources. QAFI is populated by the information freely provided by more than 40 Earth science researchers, storing to date a total of 262 records. In this article we describe the development and evolution of the database, as well as its internal architecture. Additionally, a first global analysis of the data is provided with a special focus on length and slip-rate fault parameters. Finally, the database completeness and the internal consistency of the data are discussed. Even though QAFI v.2.0 is the most current resource for calculating fault-related seismic hazard in Iberia, the database is still incomplete and requires further review.RESUMEN. La Base de Datos de Fallas Activas de Iberia (QAFI) es una iniciativa promovida por el Instituto Geológico y Minero de España (IGME) para construir un repositorio público de información científica sobre fallas con actividad en los últimos 2,59 Ma (Cuaternario). Además, la QAFI persigue establecer una base sobre la que facilitar la transferencia de conocimiento geológico al ámbito tecnológico de la gestión del riesgo sísmico en Iberia, en particular en la identificación y caracterización de fuentes sismogénicas tipo falla. La QAFI se ha construido a partir de la información proporcionada de modo altruista por más de 40 investigadores en ciencias de la Tierra conteniendo actualmente un total de de 262 registros. En este artículo se describe la concepción y evolución de la base de datos, y su arquitectura interna. Además, se ofrece un primer análisis global de los datos que contiene, con especial interés en parámetros tan importantes como la longitud y tasa de deslizamiento de las fallas. Finalmente se discuten dos temas cruciales en cualquier base de datos: su completitud y la homogeneidad de los datos. Se concluye que QAFI v.2.0, pese a ser la fuente más actualizada de información disponible en Iberia sobre peligrosidad sísmica de fallas concretas, dista aun de ser completa, por lo que nuevas revisiones y versiones deberán seguir llevándose a cabo en el futuro

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

Geological evidences of surface rupture related to a seventeenth century destructive earthquake in Betic Cordillera (SE Spain): constraining the seismic hazard of the Alhama de Murcia fault

Constraining the date of the last major event occurred in a fault is of paramount importance in probabilistic seismic hazard assessment when time-dependent models are considered. Eight of the twelve destructive earthquakes occurred in the eastern Betic Cordillera since sixteenth century, are located less than 10 km away from the Alhama de Murcia fault (AMF). Up to now, it has not been identified any geological evidence on the ground surface to associate these events with the activity of specific fault sections of the AMF. In this work we present the first geological evidence of the catastrophic 1674 event occurred at Lorca (SE Spain). The excavations carried out at La Torrecilla Creek exposed archaeological remains from the Islamic period (VIII-XIII centuries in this region) affected by 55 ± 20 cm offset by the AMF fault. This event reached intensity VIII and produced 30 fatalities at Lorca for an estimated population of 7300 inhabitants. This supports the occurrence of earthquakes with surface rupture in the historical epoch on the Alhama de Murcia fault and reinforces the results obtained in previous paleoseismological work. The theoretical scenarios of maximum magnitudes and recurrence time obtained by combining this historical event with the fault slip rate allow us to conclude that the seismic hazard associated with maximum magnitude events in this section could be high. In addition, the static Coulomb stress transferred to the Góñar-Lorca section by the 2011 (Mw 5.2) Lorca earthquake may have significantly increased the hazard