Clustered activity of Intraplate Faults: the silent and slow active faults of southern Germany

Silent and slow faults pose a particularly fascinating challenge in the field of active tectonic studies, especially in regions characterized as Stable Continental Regions (SCR) or Active Intraplate Regions (AIR), such as Central Europe. The term "silent faults" encompasses a broad spectrum of meanings, with "silent" denoting either the absence of seismic activity, the lack of faulting that generates earthquakes (though possibly involving creep), or the limited visibility of these faults in terms of their geomorphological and geological features. Slow active faults, generally defined by slip rates of ≤ 0.1 mm per year, typically do not produce noticeable topographic features in regions with a humid or moderate climate. The slip rate of a fault is a critical parameter governing the occurrence of earthquakes and seismic hazard in a given area. Lower slip rates result in longer intervals between earthquakes of a specific magnitude. Owing to these prolonged recurrence intervals, earthquakes occurring on low slip-rate faults are often absent from historical records and standard processes for assessing seismic hazard. In our study, we present new fault data obtained from the AIR region of the Rhine Graben rift within the German SCR region. Generally, Holocene surface rupturing events are exceedingly rare in these areas, with recurrence intervals spanning approximately 1,000-10,000 years. The associated slip rates are notably low, often below 0.5 mm per year, or even ≤ 0.1 mm per year. We also observe secondary earthquake effects in a broader context, extending beyond our study regions. However, some faults exhibit distinct linear scarps and topographic variations, as revealed through high-resolution Digital Elevation Models (DEMs) and geophysical field surveys. It is indeed puzzling how these small-scale scarps, with heights of around 50 cm and formed during single events, have managed to persist for 1,000-10,000 years, particularly in agricultural areas. Nevertheless, recurring paleo-earthquakes are recorded in surface deposits, as demonstrated by our trench excavations in the Upper Rhine Graben. Geodetic techniques, such as GPS and Differential Interferometric Synthetic Aperture Radar (DInSAR), offer valuable tools for detecting silent faults. However, their signals can be influenced by factors like groundwater extraction, seasonality, vegetation, and other obstacles. Additionally, in the Upper Rhine Graben, some normal faults display evidence of "Clustering and Quiescence" in terms of earthquake occurrences, potentially explaining the persistence of these scarps over time. This contrasts with the "One Shot" hypothesis sometimes applied to SCR faults. To further our understanding of the history of silent and slow faults, we propose the integration of high-resolution geodetic techniques, like GPS and DInSAR, with traditional paleoseismological investigations. Despite potential signal distortions caused by groundwater and vegetation, this combined approach holds promise in unveiling the hidden history of these slow and silent faults

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

Present-day deformation of the Pyrenees revealed by GPS surveying and earthquake focal mechanisms until 2011

The Pyrenean mountain range is a slowly deforming belt with continuous and moderate seismic activity. To quantify its deformation field, we present the velocity field estimated from a GPS survey of the Pyrenees spanning 18 yr. The PotSis and ResPyr networks, including a total of 85 GPS sites, were installed and first measured in 1992 and 1995 1997, respectively, and remeasured in 2008 and 2010. We obtain a deformation field with velocities less than 1 mm yr−1 across the range. The estimated velocities for individual stations do not differ significantly from zero with 95 per cent confidence. Even so, we estimate a maximum extensional horizontal strain rate of 2.0 ± 1.7 nanostrain per year in a N S direction in the western part of the range. We do not interpret the vertical displacements due to their large uncertainties. In order to compare the horizontal strain rates with the seismic activity, we analyse a set of 194 focal mechanisms using three methods: (i) the 'r' factor relating their P and T axes, (ii) the stress tensors obtained by fault slip inversion and (iii) the strain-rate tensors. Stress and strain-rate tensors are estimated for: (i) the whole data set, (ii) the eastern and western parts of the range separately, and (iii) eight zones, which are defined based on the seismicity and the tectonic patterns of the Pyrenees. Each of these analyses reveals a lateral variation of the deformation style from compression and extension in the east to extension and strike-slip in the west of the range. Although the horizontal components of the strain-rate tensors estimated from the seismic data are slightly smaller in magnitude than those computed from the GPS velocity field, they are consistent within the 2σ uncertainties. Furthermore, the orientations of their principal axes agree with the mapped active faults

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

Emergence of Zaire Ebola Virus Disease in Guinea - Preliminary Report

In March 2014, the World Health Organization was notified of an outbreak of a communicable disease characterized by fever, severe diarrhea, vomiting, and a high fatality rate in Guinea. Virologic investigation identified Zaire ebolavirus (EBOV) as the causative agent. Full-length genome sequencing and phylogenetic analysis showed that EBOV from Guinea forms a separate clade in relationship to the known EBOV strains from the Democratic Republic of Congo and Gabon. Epidemiologic investigation linked the laboratory-confirmed cases with the presumed first fatality of the outbreak in December 2013. This study demonstrates the emergence of a new EBOV strain in Guinea

Alhama de Murcia slip rate based on paleoseismology and a morphotectonic analysis

Se calcula la velocidad de desplazamiento de la falla de Alhama de Murcia mediante la aplicación de dos técnicas: a) la paleosismología y b) el análisis morfotectónico. Estas metodologías basan el cálculo en la velocidad en la dislocación tectónica de un elemento lineal de edad conocida. Se han excavado trincheras 3D en el segmento Lorca-Totana con el fin de identificar y medir la dislocación de un paleocanal enterrado. La edad del canal ha sido delimitada gracias a dataciones de OSL, radiocarbono y series del uranio aplicadas a pequeñas cantidades de carbonato pedogénico. La velocidad total obtenida es 0.9 mm/a. En la terminación sur de la falla (segmento Goñar-Lorca) se han analizado ocho canales dislocados en superficie. Las edades máximas de estos canales (edad de la superficie en la que se encajan) se han aproximado gracias a dataciones publicadas basadas en la termoluminiscencia, junto con nuevas dataciones de carbonato pedogénico. La velocidad lateral mínima obtenida con esta metodología es de 1.7-1.6 mm/a. Estos valores, coincidentes con los de las trincheras 3D y mayores a los obtenidos hasta la fecha, reafirman la falla de Alhama de Murcia como una de las más activas de la Zona de Cizalla de las Béticas Orientales.We calculated the slip rate of the Alhama de Murcia fault by means of two techniques: a) paleoseismology, and b) a morphotectonic analysis. These methodologies measure the offset of a linear feature whose age is known. In the Lorca-Totana segment of the fault, we carried out a three-dimensional trenching campaign to identify and measure the offset of a buried paleochannel. The age of the paleochannel was constrained by new approaches of OSL, radiocarbon and U-series applied to small amounts of pedogenic carbonate datings. The mean net slip rate obtained is 0.9 mm/a. In the southernmost termination of the fault (Goñar-Lorca segment), we analysed eight offset channels in surface. The maximum ages of the channels (age of the surface in which they are entrenched) were estimated in base of previous geomorphological maps and of published ages (thermoluminescence) together with new datings of pedogenic carbonate. The minimum lateral slip rate for the fault in this area is 1.7-1.6 mm/a. These slip rate values position the Alhama de Murcia fault as one of the most active faults in the Eastern Betics Shear Zone.Depto. de Geodinámica, Estratigrafía y PaleontologíaFac. de Ciencias GeológicasTRUEMinisterio de Economía y Competitividad (MINECO)Generalitat de Catalunyapu

Combining dating techniques in paleo-earthquake chronologies: the example of El Saltador (Alhama de Murcia fault)

Presentamos la cronología de paleoterremotos establecida para la rama sur de la Falla de Alhama de Murcia (Béticas Orientales) a partir del estudio paelosismológico de la localidad de el Saltador (Lorca). Mostramos la aplicación combinada de cuatro métodos de datación; luminiscencia en cuarzo y feldespato (OSL en cuarzo y pIRIRSL), datación de series de Uranio en carbonatos pedogénicos, y radiocarbono. Para acotar la edad de los paleoterremotos con la mejor definición posible, se han integrado las edades de las capas que definen temporalmente los paleosismos en un modelo deposicional. Para ello se usó el programa OxCal (versión 4.2), que realiza un tratamiento estadístico bayesiano de las funciones de probabilidad de las edades de materiales afectados según un orden estratigráfico conocido y calcula la recurrencia media y la recurrencia particular entre eventos sucesivos. La secuencia de materiales aluviales muestra el registro de ocho eventos paleosísmicos ocurridos desde hace 54,3 ± 9,1 ka. El tiempo de recurrencia medio varía entre 3,5- y 5,3 ka , localizándose el último evento entre el s. XIII y el s. XVIII.We present a chronology of paleo-earthquakes established at El Saltador (Lorca), a paleoseismical site in the southern branch of the Alhama de Murcia Fault (Eastern Betics). Because of the difficulties in dating the affected sediments using a single dating approach, we combined four types of methods; Two of them based on the luminescence of quartz and feldspar (Quartz-OSL and pIR-IRSL), U-series on pedogenic carbonates and radiocarbon dating. To obtain the best possible accuracy for the age of each paleoearthquake, we integrated the age of the layers that constrain the seismic events into a depositional model. To this end, we relied on the OxCal program (version 4.2), which performs a Bayesian statistical treatment of the probability functions of the ages assigned to a sequence of layers of known stratigraphical order, at the time that it provides the average recurrence and the single recurrence between successive events. The sequence of alluvial deposits analyzed shows the record of eigth paleoseismic events occurred since 54,3 ± 9.1 ka. The average recurrence intervals ranges between 3,5- and 5,3 ka with the last event taking place between the XIII and XVII centuries.Depto. de Geodinámica, Estratigrafía y PaleontologíaFac. de Ciencias GeológicasTRUEMinisterio de Ciencia e Innovación (MICINN)Generalitat de Catalunyapu