Cataclastic rocks, as clay-rich fault gouges, are commonly present in brittle rock masses when fault zones appear during geological engineering projects. Highly deformed rocks that are of poor ...mechanical quality can lead to technical, safety, and economic problems in rock engineering. The aim of this study is to characterise the resistant behaviour of a highly deformed clay-rich gouge >40 m wide with a marked tectonic fabric that indicates strength anisotropy. We present the results of consolidated-undrained (CU) triaxial tests that were performed at low confining pressures (50, 150, and 300 kPa) on several sets of foliated gouge specimens with four different orientations in the tectonic fabric. Specimens were collected from the encapsulated rock cores of two research boreholes drilled through the Alhama de Murcia Fault (AMF), a main regional fault located in SE Spain. The strain–stress relationships and failure modes were established, indicating that the gouge behaves as hard soil or very soft rock. The test results were adjusted at each orientation using the non-linear Hoek and Brown criteria by considering the fault gouge as an intact material or as a tectonised rockmass. Here, we use the Geological Strength Index (GSI) as an indicator of the rockmass strength that depends on the direction of the tectonic fabric. However, the results from specimens with tectonic fabric that is oriented most favourably for failure were not the weakest in terms of rock strength. Such an anomalous result could be the result of asymmetry in the roughness of the weakness planes that is related to the original gouge microstructure characterised by the strong reorientation of clays in an S-C′ like tectonic fabric.
Our results will be useful for practical applications that are related to the stability of slopes and/or shallow underground excavations in brittle fault zones, and provide an inexpensive and easy way to preliminarily evaluate the anisotropic behaviour of this type of brittle fault zones for future engineering projects.
•Strength of a foliated clay-rich fault gouge is controlled by roughness within the weak planes.•Gouge microstructure defined by strong reorientation of clays in a S-C’ fabric controls the roughness of the weakness planes•Strength of clay gouges shows a medium to low anisotropy.•GSI is a sensitive index with which the strength anisotropy of tectonised rocks can be investigated.
Fault slip rate is one of the most crucial parameters to characterize earthquake occurrence in fault‐based seismic hazard assessments (SHA). Accordingly, paleoseismic studies have increasingly ...focused on constraining this parameter in active faults worldwide. We present a comprehensive paleoseismic study in the Alhama de Murcia Fault (AMF), one of the most active faults in SE Spain and source of destructing earthquakes such as the 2011 Mw 5.2 Lorca event. Contrasting with previous studies, we integrate paleoseismic data from four fault strands in the AMF and, based on trench slip analysis and numerical dates, we derive slip rate estimates of each strand over the whole transect and assess their time variability. The AMF has a minimum net slip rate between 1.35+0.16/−0.10 and 1.64+0.16/−0.11 mm/yr for the past 18 ± 1 to 15.2 ± 1.1 ka. These results prove the importance of accounting for the complete sections of a geological structure as they are almost twice the previous estimates for a single fault branch. Slip rate variability is identified in the AMF, with cyclic acceleration‐quiescence patterns that could be related to stress field changes driven by fault interaction or synchronicity with neighboring faults (e.g., Carrascoy). We hope that the data presented here motivates their inclusion into forthcoming fault‐based SHAs. In this regard, limitations related to the lack of paleoseismic data for one fault strand, along with poor characterization of the strike component of slip and insufficient age control of the units for another strand are highlighted and need to be accounted for by modelers.
Key Points
Multi‐site paleoseismic surveys allow to improve the estimations of the geological slip rates in the Alhama de Murcia Fault (AMF)
The AMF shows a geological slip rate between 1.35 and 1.64 mm/yr for the past 18–15 ka
Late Quaternary slip rate fluctuations suggest strain rate variations and highlight possible synchronicities with neighboring faults
This paper on the Baza Fault provides the first palaeoseismic data from trenches in the central sector of the Betic Cordillera (S Spain), one of the most tectonically active areas of the Iberian ...Peninsula.
With the palaeoseismological data we constructed time-stratigraphic OxCal models that yield probability density functions (PDFs) of individual palaeoseismic event timing. We analysed PDF overlap to quantitatively correlate the walls and site events into a single earthquake chronology. We assembled a surface-rupturing history of the Baza Fault for the last ca. 45,000 years. We postulated six alternative surface rupturing histories including 8–9 fault-wide earthquakes. We calculated fault-wide earthquake recurrence intervals using Monte Carlo. This analysis yielded a 4750–5150 yr recurrence interval. Finally, compared our results with the results from empirical relationships.
Our results will provide a basis for future analyses of more of other active normal faults in this region. Moreover, our results will be essential for improving earthquake-probability assessments in Spain, where palaeoseismic data are scarce.
•The first palaeoseismic analysis of the Baza Fault (SE Spain) is presented.•We postulate six alternative fault-wide surface rupturing histories for the Baza fault.•Chronologies suggests 8–9 surface-rupturing earthquakes in the last ca. 45,000 yr.•Recurrence interval of the Baza Fault is 4750–5150 yr.•Results agree with palaeoseismic parameters obtained from empirical regressions.
A new micromammal site at Alhaurín el Grande (Málaga, southern Spain) located above early Pliocene marine deposits allows an approach to the marine-continental correlation for this age. The early ...Pliocene marine filling throughout the Málaga Basin is developed in three transgressive-regressive sequences (Pl-1, Pl-2, and Pl-3 units) bounded by discontinuities. At the top of the intermediate sequence Pl-2, peaty sediments have yielded fossils of Rodentia, Lagomorpha, Insectivora, and Crocodylia. The presence of Cricetus barrieri Mein & Michaux, 1970 in combination with murids, both of primitive morphology, such as Apodemus gudrunae Van de Weerd, 1976, and more advanced forms (i.e. Occitanomys brailloni Michaux, 1969 and Stephanomys donnezani cordii Ruiz Bustos, 1986), points to an early Ruscinian age (MN 14 biozone). Based on the planktonic foraminifers, the biostratigraphic data indicate that marine sediments just below the micromammal beds belong to the MPl-2 biozone of the early Zanclean. Available paleomagnetic data from the marine sediments show that the micromammal bed must be located between the normal geomagnetic subchron C3n3n (4.89-4.80 Ma) and the subchron C3n2n (4.63-4.49 Ma), limiting the age of this site to the late part of the early Zanclean.
This work summarises the seismic hazard analysis performed for the complete characterisation of strong ground-motion at the site of the Itoiz dam (Western Pyrenees, Spain). The hazard analysis ...includes the compilation of a composite catalogue from French and Spanish agencies, the definition of an original hybrid seismogenic source model (including zones and major faults) and the selection of ground motion prediction equations (GMPEs). Hazard results are provided as hazard curves and acceleration response spectra on rock for the 1000- and 5000-year return periods, which correspond respectively to the operating basis earthquake (OBE) and safety evaluation earthquake (SEE). The impact of truncating GMPEs at a number of standard deviations (epsilon) has been found not critical here for the return periods targeted. Subsequently, an analysis of the contribution of each source to total hazard and a hazard disaggregation analysis are performed in order to establish the earthquake-source parameters for both the OBE and SEE scenarios consistently with the seismotectonics of the region. The European Strong Motion database is then searched and a selection of records is proposed for each of the scenarios. Our results suggest that seismic hazard in the region is underestimated by the official Spanish seismic hazard map included in the current version of the code (NCSE-02), which is the reference document for the definition of seismic actions for dam projects in the whole Pyrenees.
Different kinematic models have been proposed for the triple junction between the North American, Cocos and Caribbean plates. The two most commonly accepted hypotheses on its driving mechanism are ...(a) the North American drag of the forearc and (b) the Cocos Ridge subduction push. We present an updated GPS velocity field which is analyzed together with earthquake focal mechanisms and regional relief. The two hypotheses have been used to make kinematic predictions that are tested against the available data. An obliquity analysis is also presented to discuss the potential role of slip partitioning as driving mechanism. The North American drag model presents a better fit to the observations, although the Cocos Ridge push model explains the data in Costa Rica and Southern Nicaragua. Both mechanisms must be active, being the driving of the Central American forearc towards the NW analogous to a push-pull train. The forearc sliver moves towards the west-northwest at a rate of 12-14 mm/yr, being pinned to the North American plate in Chiapas and western Guatemala, where the strike-slip motion on the volcanic arc must be very small.
Un nuevo yacimiento de micromamíferos en Alhaurín el Grande (Málaga, Sur de España) situado sobre depósitos marinos del Plioceno inferior permite una aproximación a la correlación marino-continental ...para esa época. El relleno sedimentario marino del Plioceno inferior en el conjunto de la cuenca de Málaga comprende tres secuencias transgresivas-regresivas (unidades Pl-1, Pl-2 y Pl-3) limitadas por discordancias. A techo de la secuencia intermedia Pl-2, sedimentos turbosos han liberado restos de Rodentia, Lagomorpha, Insectivora y Crocodylia. La presencia de Cricetus barrieri Mein & Michaux, 1970 en asociación con dos múridos, uno con morfología primitiva (Apodemus gudrunae Van de Weerd, 1976) y otros más evolucionados próximo a Occitanomys brailloni Michaux, 1969 y Stephanomys donnezani cordii Ruiz Bustos, 1986, permiten inferir una edad Rusciniense inferior (biozona MN14). En función de los foraminíferos planctónicos, los datos bioestratigráficos indican que los sedimentos marinos justo por debajo del nivel de micromamíferos pertenecen a la biozona MPL-2 del Zancliense inferior. Los datos paleomagnéticos disponibles muestran que el nivel de micromamíferos se localiza entre el subcron geomagnético normal C3n3n (4.89-4.80 Ma) y el subcron C3n2n (4.63-4.49 Ma), limitando la edad del yacimiento a la parte superior del Zancliense inferior.
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