7 research outputs found
Characteristics of direct human impacts on the rivers Karun and Dez in lowland south-west Iran and their interactions with earth surface movements
Two of the primary external factors influencing the variability of major river systems, over river reach scales, are human activities and tectonics. Based on the rivers Karun and Dez in south-west Iran, this paper presents an analysis of the geomorphological responses of these major rivers to ancient human modifications and tectonics. Direct human modifications can be distinguished by both modern constructions and ancient remnants of former constructions that can leave a subtle legacy in a suite of river characteristics. For example, the ruins of major dams are characterised by a legacy of channel widening to 100's up to c. 1000 m within upstream zones that can stretch to channel distances of many kilometres upstream of former dam sites, whilst the legacy of major, ancient, anthropogenic river channel straightening can also be distinguished by very low channel sinuosities over long lengths of the river course. Tectonic movements in the region are mainly associated with young and emerging folds with NWâSE and NâS trends and with a long structural lineament oriented EâW. These earth surface movements can be shown to interact with both modern and ancient human impacts over similar timescales, with the types of modification and earth surface motion being distinguishable. This paper examines the geomorphological evidence and outlines the processes involved in the evolution of these interactions through time. The analysis shows how interactions between earth surface movements and major dams are slight, especially after ancient dam collapse. By contrast, interactions between earth surface movements and major anthropogenic river channel straightening are shown to be a key factor in the persistence of long, near-straight river courses. Additionally, it is suggested that artificial river development, with very limited river channel lateral migration, may promote incision across an active fold at unusually long distances from the fold âcoreâ and may promote markedly increased sinuosity across a structural lineament
A 1500âyear record of North Atlantic storm flooding from lacustrine sediments, Shetland Islands (UK)
Severe storm flooding poses a major hazard to the coasts of northâwestern Europe. However, the longâterm recurrence patterns of extreme coastal flooding and their governing factors are poorly understood. Therefore, highâresolution sedimentary records of past North Atlantic storm flooding are required. This multiâproxy study reconstructs stormâinduced overwash processes from coastal lake sediments on the Shetland Islands using grainâsize and geochemical data, and the reâanalysis of historical data. The chronostratigraphy is based on Bayesian ageâdepth modelling using accelerator mass spectrometry 14 C and 137 Cs data. A high XRFâbased Si/Ti ratio and the unimodal grainâsize distribution link the sand layers to the beach and thus stormâinduced overwash events. Periods with more frequent storm flooding occurred 980â1050, 1150â1300, 1450â1550, 1820â1900 and 1950â2000 ce, which is largely consistent with a positive North Atlantic Oscillation mode. The Little Ice Age (1400â1850 ce ) shows a gap of major sand layers suggesting a southward shift of storm tracks and a seasonal variance with more storm floods in spring and autumn. Warmer phases shifted winter storm tracks towards the northâeast Atlantic, indicating a possible trend for future stormâtrack changes and increased storm flooding in the northern North Sea region
New Geological Evidence of Past Earthquakes and Tsunami Along the Nankai Trough, Japan
The east coast of Japan is prone to tsunamigenic megathrust earthquakes, as tragically demonstrated in 2011 by the TĆhoku earthquake (Mw 9.0) and tsunami. The Nankai Trough subduction zone, to the southwest of the area affected by the TĆhoku disaster and facing the densely populated and heavily industrialized southern coastline of central and west Japan, is expected to generate another megathrust earthquake and tsunami in the near future. This subduction zone is, however, segmented and appears to be characterized by a variable rupture mode, involving single- as well as multi-segment ruptures, which has immediate implications for their tsunamigenic potential, and also renders the collection of sufficiently long time records of past earthquakes and tsunami in this region fundamental for an adequate hazard and risk assessment. Over the past three decades, Japanese researchers have acquired a large amount of geological evidence of past earthquakes and tsunami, in many cases extending back in time for several thousands of years. This evidence includes uplifted marine terraces, turbidites, liquefaction features, subsided marshes and tsunami deposits in coastal lakes and lowlands. Despite these efforts, current understanding of the behaviour of the subduction zone still remains limited, due to site-specific evidence creation and preservation thresholds and issues over alternative hypotheses for proposed palaeoseismic evidence and insufficiently precise chronological control. Within the QuakeRecNankai project we are generating a long and coherent time series of megathrust earthquake and tsunami recurrences along the Nankai Trough subduction zone by integrating all existing evidence with new geological records of paleo-tsunami in the Lake Hamana region and of paleo-earthquakes from selected lakes in the Mount Fuji area. We combine extensive fieldwork in coastal plain areas and lakes, with advanced sedimentological and geochemical analyses and innovative dating techniques.QuakeRecNanka