Laura P. Perucca

Late Pleistocene-Holocene earthquake-induced slumps and soft-sediment deformation structures in the Acequion River valley, Central Precordillera, Argentina

Abstract Evidence of earthquake-induced liquefaction features in the Acequión river valley, central western Argentina, is analysed. Well-preserved soft-sediment deformation structures are present in Late Pleistocene deposits; they include two large slumps and several sand dikes, convolutions, pseudonodules, faults, dish structures and diapirs in the basal part of a shallow-lacustrine succession in the El Acequión River area. The water-saturated state of these sediments favoured deformation. All structures were studied in a natural trench created as a result of erosion by a tributary of the Acequión River, called El Mono Creek. They form part of a large-scale slump system. Two slumps occur in the western portion of the trench and must have moved towards the ENE (70°), where the depocentre of the Boca del Acequión area is situated. Considering the spatial relationship with Quaternary faults, the slumps are interpreted as being due to a seismic event. The thickest dikes in the El Mono Creek trench occur in the eastern portion of the trench, indicating that the responsible earthquake was located to the east of the study area, probably at the Cerro Salinas fault system zone. The slumps, sand dikes and other soft-sediment deformation features are interpreted as having been triggered by earthquakes, thus providing a preliminary palaeoseismic record of the Cerro Salinas fault system and extending the record of moderate-to high-magnitude earthquakes in central western Argentina to the Late Pleistocene.

Determination of seismogenic structures and earthquake magnitud from seismites in the Acequion river, Precordillera Range, central-western Argentina

Evidences for paleoearthquake induced liquefaction features in the Central Western Argentina were determined in the Acequion river valley. Well-preserved liquefaction structures were found in a Holocene lake dammed by rock avalanches. At least five paleoearthquakes affected this region during the Pleistocene-Holocene according to the temporal sequence established for these seismic structures and the rock avalanches recognized in the Acequion river. The magnitude of these triggering paleoevents and probable seismic source are analysed and discussed, concluding that liquefaction features and rock avalanches should be generated by M>5 earthquakes related to the nearby Cerro Salinas Fault belonging to Precordillera Oriental fault system. These findings allow us to extend the record of moderate to high magnitude-earthquakes to the Pleistocene-Holocene.

Indicative Structures of Paleoseismicity in the Acequion River Valley, San Juan Province, Central-Western Argentina

Evidence for paleo-seismicity has been discovered in the Acequión river valley, in West central Argentina. Two Holocene rock avalanches have been observed; the most recent of these dammed a lake, whose sediments contain liquefaction structures. At least five paleo-earthquakes affected this region during the late Quaternary, as deduced from the succession of their secondary effects. The magnitude and the probable tectonic source of these paleo-events are discussed. The observed liquefaction features associated with slumps, joints, fractures, and faults, should be generated by M>5 earthquakes related to the nearby quaternary Cerro Salinas fault, which belongs to the Eastern Precordillera fault system. These data extend the regional seismicity record to the Holocene and highlight the high seismic hazard in this part of Argentina.

Neotectonics, Seismology and Paleoseismology

Publisher Summary This chapter focuses on the neotectonics of Argentine–Chilean Patagonia and overviews the active tectonics of the southernmost region of South America, which constitutes a field of relatively recent development on a national standard. The presence of remarkable morphotectonic features associated with active faulting fronts in several of the defined seismotectonic regions indicates modern tectonic activity for this region of Patagonia. From the point of view of seismic risk studies, these regions have been subjected to various seismic effects and can be considered as units of potential risk. Once the different aspects of tectonics, large morphological features, seismic activity, and active volcanism have been analyzed individually, it is important to consider in combination because their occurrence indicates those regions where the earths crust is being exposed to greater stresses and the resulting tension release. To properly estimate seismic risk in the Patagonian region, it is necessary to delimit these regions in such a way that they can be identified as independent units called “seismotectonic regions.” The four identified Patagonian seismotectonic regions are (1) Liquine–Ofqui–Fagnano, (2) Somuncura (3) Agnia, and (4) Deseado.

Quaternary evolution of the Cordillera Frontal piedmont between c. 33° and 34°S Mendoza, Argentina

Abstract The piedmont of Cordillera Frontal between c. 33° and 34°S (Mendoza, Argentina) is a highly populated area deeply modified by human activities, known as Valle de Uco. It is situated within the borderland region of the geological provinces of Cordillera Frontal and Cuyo basin. The landscape is dominantly composed of both erosional and depositional landforms made of fluvio-aeolian deposits fractured and folded by tectonic processes together with some landforms of volcanic origin. Alluvial fans, related to several aggradational cycles of Quaternary age, are the most remarkable geomorphological units. Several tectonic features are present giving rise to conspicuous morphological features. Some of the streams are structurally controlled by faults while several drainage anomalies that indicate active tectonic processes have been identified. The Late Quaternary alluvial sequences, dominantly comprising sandy and silty deposits of volcaniclastic composition and secondarily metamorphic rocks, represent the fine-grained sedimentary facies of the fluvial systems accumulated in a distal fan environment. The alluvial deposits have been incised by several episodes of erosion since Pleistocene time.

Catastrophic flash flood triggered by an extreme rainfall event in El Rodeo village, January 2014. Northwestern Pampean Ranges of Argentina

ABSTRACT Flash floods occur periodically in the hilly areas of Argentina, but since they generally occur at inhabited areas, these are rarely registered. Several factors contribute to the occurrence of flash floods: heavy seasonal rains, rugged topography and geological structures. On Thursday night, 23 January 2014 an extreme rainfall event, took place in El Rodeo, Catamarca Province, triggering an unexpected flash flood. This flood caused 14 fatalities and great economic losses in the main localities of the Ambato Department. Steep slope gradients facilitated the quick drainage of the precipitated volumes. Water, boulders and debris channeled downstream caused severe stream bank erosion and major damage to houses roads and bridges. This paper presents a morphometric analysis in order to determine some geomorphic characteristics of the Ambato River basin, together with a preliminary analysis of the flood hazard potentially damaging to inhabitants and infrastructure to downstream villages. The study was carried out using high-resolution satellite data acquired before and after the event. The drainage networks were derived from Aster GDEM V2 and satellite images.

Propuesta Metodológica para el Análisis de la Licuefacción de Suelos Asociada a Sismos Destrucutivos. Pre-Andes Centrales, Argentina (31º 30’ S y 68º 25’ O)

In this paper we evaluate the susceptibility to the occurrence of soil liquefaction processes associated with destructive earthquakes in the north of the San Juan River area. The earthquakes of 1894, 1944, 1952 and 1977 triggered liquefaction processes in central west Argentina and particularly in many places of the San Juan Province, being the northern area of the Tulum valley, one of the most affected. Historical records of soil liquefaction processes (PLS) occurred during the above mentioned earthquake include cracks, sand volcanoes, craters and differential settlements, which caused major damage to housing and agro industrial activities. In this study, we made an analysis of the key factors that influence their occurrence, as well as the effects of liquefaction. The methodology used to estimate liquefaction of soils consisted in evaluating the conditioning factors such as origin and grain size of the deposits, age of the deposits, phreatic level depth, and historical records, among others. From the analysis of these factors resulted a map associated with an evaluative table. The influence of each factor in the evaluation of the susceptibility was performed using an iterative process of overlapping maps. Once achieved the optimum combination, a final map of susceptibility to liquefaction was obtained. Zonation obtained was related to a susceptibility index (IS), qualitatively classified a Very High, High, Moderate and Low. The obtained results allow to conclude that in the middle portion of the alluvial fan of the San Juan River, whereurban centers and most agricultural activities are concentrate, is where the condition of Very High susceptibility are exposed, while the High susceptibility occupy some distal to medium alluvial fan portion and Moderate to Low circumscribe to the proximal area of the alluvial fan.