An active zone characterized by slow normal faults, the northwestern margin of the València trough (NE Iberia): a review

The northwestern margin of the Valencia trough is an area of low strain characterized by slow normal faults and low to moderate seismicity. Since the mid 1990s this area has been the subject of a number of studies on active tectonic which have proposed different approaches to the location of active faults and to the calculation of the parameters that describe their seismic cycle. Fifty-six active faults have been found and a classification has been made in accordance with their characteristics: a) faults with clear evidence of large paleo-, historic or instrumental earthquakes (2/56); b) faults with evidence of accumulated activity during the Plio-Quaternary and with associated instrumental seismicity (7/56); c) faults with evidence of accumulated activity during the Plio-Quaternary and without associated instrumental seismicity (17/56); d) faults with associated instrumental seismicity and without evidence of accumulated activity during the Plio-Quaternary (30/56), and e) faults without evidence of activity or inactive faults. The parameters that describe the seismic cycle of these faults have been evaluated by different methods that use the geological data obtained for each fault except when paleoseismological studies were available. This classification can be applied to other areas with low slip faults because of the simplicity of the approaches adopted. This study reviews the different approaches proposed and describes the active faults located, highlighting the need a) to better understand active faults in slow strain zones through paleoseismological studies, and b) to include them in seismic hazard studies

Sedimentary evidence of historical and prehistorical earthquakes along the Venta de Bravo Fault System, Acambay Graben (Central Mexico)

The Venta de Bravo normal fault is one of the longest structures in the intra-arc fault system of the Trans-Mexican Volcanic Belt. It defines, together with the Pastores Fault, the 80 km long southern margin of the Acambay Graben. We focus on the westernmost segment of the Venta de Bravo Fault and provide new paleoseismological information, evaluate its earthquake history, and assess the related seismic hazard. We analyzed five trenches, distributed at three different sites, in which Holocene surface faulting offsets interbedded volcanoclastic, fluvio-lacustrine and colluvial deposits. Despite the lack of known historical destructive earthquakes along this fault, we found evidence of at least eight earthquakes during the late Quaternary. Our results indicate that this is one of the major seismic sources of the Acambay Graben, capable of producing by itself earthquakes with magnitudes (MW) up to 6.9, with a slip rate of 0.22-0.24 mm yr− 1 and a recurrence interval between 1940 and 2390 years. In addition, a possible multi-fault rupture of the Venta de Bravo Fault together with other faults of the Acambay Graben could result in a MW > 7 earthquake. These new slip rates, earthquake recurrence rates, and estimation of slips per event help advance our understanding of the seismic hazard posed by the Venta de Bravo Fault and provide new parameters for further hazard assessment

Quaternary active tectonic structures in the offshore Bajo Segura basin (SE Iberian Peninsula Mediterranean Sea).

The Bajo Segura fault zone (BSFZ) is the northern terminal splay of the Eastern Betic shear zone (EBSZ), a large left-lateral strike-slip fault system of sigmoid geometry stretching more than 450 km from Alicante to Almería. The BSFZ extends from the onshore Bajo Segura basin further into the Mediterranean Sea and shows a moderate instrumental seismic activity characterized by small earthquakes. Nevertheless, the zone was affected by large historical earthquakes of which the largest was the 1829 Torrevieja earthquake (IEMS98 X). The onshore area of the BSFZ is marked by active transpressive structures (faults and folds), whereas the offshore area has been scarcely explored from the tectonic point of view. During the EVENT-SHELF cruise, a total of 10 high-resolution single-channel seismic sparker profiles were obtained along and across the offshore Bajo Segura basin. Analysis of these profiles resulted in (a) the identification of 6 Quaternary seismo-stratigraphic units bounded by five horizons corresponding to regional erosional surfaces related to global sea level lowstands; and (b) the mapping of the active sub-seafloor structures and their correlation with those described onshore. Moreover, the results suggest that the Bajo Segura blind thrust fault or the Torrevieja left-lateral strike-slip fault, with prolongation offshore, could be considered as the source of the 1829 Torrevieja earthquake. These data improve our understanding of present deformation along the BSFZ and provide new insights into the seismic hazard in the area

Evidence of recent ruptures in the central faults of the Acambay Graben (central Mexico)

The Acambay Graben, within the central part of the Trans-Mexican Volcanic Belt, is one of the major sources of continental earthquakes in Mexico. To date, the activity and paleoseismological history of the axial faults of the graben are not well constrained. We provide morphological, structural and sedimentological evidence of the seismogenic nature of two of the axial structures, the Temascalcingo and the Tepuxtepec fault systems. Faults consist of multiple parallel scarps with en echelon and horse-splay patterns. Fault systems extend for 60 km and displace Quaternary to Upper Miocene volcanic edifices and volcano-sedimentary materials. Surface lengths of individual fault traces range between 3 and 25 km, and observed throws reach a minimum of 150-200 m. The long-term and short-term slip rate of the Temascalcingo fault system in the studied section presents similar values, ranging from 0.06 ± 0.02 (minimum long term) to 0.12 ± 0.02 mm y−1 (maximum value of average short-term). Only the long-term slip rate of the Tepuxtepec system could be constrained in 0.01-0.02 mm/y, being a minimum estimate. The Holocene fault rupture history at two sites provided evidence of six ruptures since 12,500-11,195 BCE, among which three ruptures should have occurred between 11,847 ± 652 BCE and 11,425 ± 465 BCE Variable single event displacements (SEDs, between 6 and 77) are interpreted as the result of fault interdependences and/or the interaction with the latest volcanic activity. Also, small displacements triggered by activity on other faults probably contributed to slip variability, i.e., faults display primary and secondary behavior

The North Maladeta fault (Spanish Central Pyrenees) as the Vielha 1923 earthquake seismic source: recent activity revealed by geomorphological and geophysical research.

The Spanish Central Pyrenees have been the scenario of at least two damaging earthquakes in the last 800 years. Analysis of macroseismic data of the most recent one, the Vielha earthquake (19 November 1923), has led to the identification of the North Maladeta Fault (NMF) as the seismic source of the event. This E-W trending fault defines the northern boundary of the Maladeta Batholith and corresponds to a segment of the Alpine Gavarnie thrust fault. Our study shows that the NMF offsets a reference Neogene peneplain. The maximum observed vertical displacement is ~ 730 m, with the northern downthrown sector slightly tilting towards the South. This offset provides evidence of normal faulting and together with the presence of tectonic faceted spurs allowed us to geomorphically identify a fault trace of 17.5 km. This length suggests that a maximum earthquake of Mw=6.5 ± 0.66 could occur in the area. The geomorphological study was improved with a resistivity model obtained at Prüedo, where a unique detritic Late Miocene sequence crops out adjacent to the NMF. The section is made up of 13 audiomagnetotelluric soundings along a 1.5 km transect perpendicular to the fault trace at Prüedo and reveals the structure in depth, allowing us to interpret the Late Miocene deposits as tectonically trapped basin deposits associated with normal faulting of the NMF. The indirect age of these deposits has been constrained between 11.1-8.7 Ma, which represents a minimum age for the elevated Pyrenean peneplain in this part of the Pyrenees. Therefore, we propose the maximum vertical dip slip rate for the NMF to be between 0.06-0.08 mm/a. Normal faulting in this area is attributed to the vertical lithospheric stress associated with the thickened Pyrenean crust

First evidence of paleoearthquakes along the Carboneras Fault Zone (SE Iberian Peninsula): Los Trances site

Seismogenic faults that have not produced historical large earthquakes remain unnoticed and, thus, are dangerously left out from seismic hazard analyses. The seismogenic nature of the Carboneras Fault Zone, a left-lateral strikeslip fault in the Eastern Betic Shear Zone (southeastern Spain), has not been fully explored to date in spite of having a morphological expression equivalent to the Alhama de Murcia Fault, a seismogenic fault in the same tectonic system. This study provides the first paleoseismic evidence of the seismogenic nature of the Carboneras Fault Zone, based on the analysis of 3 trenches at Los Trances site, on the northwestern edge of the La Serrata Range. Cross cutting relationships and numerical dating, based on radiocarbon, thermoluminescence and U-series, reveal a minimum of 4 paleoearthquakes: Paleoearthquake1 (the oldest) and Paleoearthquake2 took place after 133ka, Paleoearthquake3 occurred between 83-73ka and Paleoearthquake4 happened after 42.5ka (probably after 30.8ka), resulting in a maximum possible average recurrence of 33ka. This value, based on a minimum amount of paleoearthquakes, is probably overestimated, as it does not scale well with published slip-rates derived from offset channels or GPS geodetical data. The characterization of this fault as seismogenic, implies that it should be considered in the seismic hazard analyses of the SE Iberian Peninsula

La Zona de Falla del Bajo Segura: cabalgamiento activo “ciego” en la Cordillera Bética oriental

The Bajo Segura Fault Zone, located at the NE end of the Eastern Betic Shear Zone, has been the site of some of the most intense seismic activity on the Iberian Peninsula in the historical and instrumental time periods. This structure is an active blind fault that does not show any surface rupture. It is characterised by a set of ENE-WSW trending blind thrust faults that offset the Triassic basement and cause active folding of the Upper Miocene-Quaternary sedimentary cover. The main active structures of this fault zone are two ENE-WSW striking reverse blind faults, the Torremendo and the Bajo Segura Faults, and several secondary NW-SE striking dextral faults (San Miguel de Salinas, Torrevieja and Guardamar Faults). These structures continue offshore to the east. From geological, geomorphological and geodetic data, we obtain fault slip rates between 0.2 and 0.4 mm/yr, whereas other authors have proposed higher values ranging between 0.75 and 1 mm/yr. The fault zone can generate earthquakes with maximum estimated magnitudes (Mw) from 6.6 to 7.1 and has approximate recurrence intervals between 4.500 and 21.500 years.La Zona de Falla del Bajo Segura, localizada en el extremo NE de la Zona de Cizalla de la Bética Oriental, es una de las estructuras tectónicas de la Península Ibérica con mayor actividad sísmica asociada durante el periodo histórico e instrumental. Es una zona de falla ciega, sin ruptura en superficie, caracterizada por varios pliegues que deforman las rocas sedimentarias de edad Mioceno Superior a Cuaternario. Su traza principal tiene varios segmentos aproximadamente paralelos entre sí de dirección ENE-WSW entre los que destacan la Falla del Bajo Segura y la Falla de Torremendo. Además, tiene asociadas varias fallas de transferencia dextrorsas de dirección NW-SE (San Miguel de Salinas, Torrevieja y Guardamar). Estas fallas ENE-WSW y NW-SE tienen su continuidad hacia el este, en el Mar Mediterráneo. Algunos marcadores geológicos y geomorfológicos junto con un perfil de nivelación de alta precisión, han permitido obtener tasas de desplazamiento que varían entre 0.2 y 0.4 mm/año, aunque otros autores han propuesto valores más altos de entre 0.75 y 1 mm/año. A partir de estas tasas de desplazamiento y de la cartografía de la zona de falla, se estima que la máxima magnitud Mw varía entre 6.6 y 7.1 con periodos de recurrencia aproximados entre 4.500 y 21.500 años.Financial support for this work was provided by the research projects CGL2011-30153-C02-02, FASEGEO (CGL2009-09726), EVENT (CGL 2006-12861-C02-02) and SHAKE (CGL 2011-30005-C02-02) and Acciones Complementarias EVENT-SHELF (CTM 2008-03346-E/MAR) and SPARKER (CTM 2008-03208-E/MAR)

Refining seismic parameters in low seismicity areas by 3D trenching: The Alhama de Murcia fault, SE Iberia.

Three-dimensional paleoseismology in strike-slip faults with slip rates less than 1 mm per year involves a great methodological challenge. We adapted 3D trenching to track buried channels offset by the Alhama de Murcia seismogenic left-lateral strike-slip fault (SE Iberia). A fault net slip of 0.9 +/- 0.1 mm/yr was determined using statistical analysis of piercing lines for one buried channel, whose age is constrained between 15.2 +/- 1.1 ka and 21.9-22.3 cal BP. This value is larger and more accurate than the previously published slip rates for this fault: The minimum number of five paleo-earthquakes identified since the deposition of dated layers suggests a maximum average recurrence interval of approximately 5 ka. The combination of both seismic parameters yields a maximum slip per event between 53 and 63 m. We show that accurately planned trenching strategies and data processing may be key to obtaining robust paleoseismic parameters in low seismicity areas. (C) 2016 Elsevier B.V. All rights reserved

First evidence of paleoearthquakes along the Carboneras Fault Zone (SE Iberian Peninsula) : Los Trances site

Seismogenic faults that have not produced historical large earthquakes remain unnoticed and, thus, are dangerously left out from seismic hazard analyses. The seismogenic nature of the Carboneras Fault Zone, a left-lateral strikeslip fault in the Eastern Betic Shear Zone (southeastern Spain), has not been fully explored to date in spite of having a morphological expression equivalent to the Alhama de Murcia Fault, a seismogenic fault in the same tectonic system. This study provides the first paleoseismic evidence of the seismogenic nature of the CarbonerasFault Zone, based on the analysis of 3 trenches at Los Trances site, on the northwestern edge of the La Serrata Range. Cross cutting relationships and numerical dating, based on radiocarbon, thermoluminescence and U-series, reveal a minimum of 4 paleoearthquakes: Paleoearthquake1 (the oldest) and Paleoearthquake2 took place after 133ka, Paleoearthquake3 occurred between 83-73ka and Paleoearthquake4 happened after 42.5ka (probably after 30.8ka), resulting in a maximum possible average recurrence of 33ka. This value, based on a minimum amount of paleoearthquakes, is probably overestimated, as it does not scale well with published slip-rates derived from offset channels or GPS geodetical data. The characterization of this fault as seismogenic, implies that it should be considered in the seismic hazard analyses of the SE Iberian Peninsula

The Quaternary Active Faults Database of Iberia (QAFI v.2.0)

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. Aditionally, 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