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Encyclopedia of geology, five volume set, volume 1 5 (encyclopedia of geology series) ( PDFDrive ) 2963

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426 TECTONICS/Neotectonics sea-level rise, etc.), because it focuses on crustal movements that can be expected to recur within a future interval of concern to society Contemporary crustal movements may be discerned in Earth surface processes and landforms, such as in the sensitivity of alluvial rivers to crustal tilting In addition, geomorphological and geological studies are important in recording the surface expression of Earth movements such as earthquake ground ruptures, which, due to their subtle, ephemeral, or reversible nature, are unlikely to have been preserved in the geological record However, active tectonics also employs an array of high-technology investigative practices; prominent among these are the monitoring of ongoing Earth surface deformation using space-based or terrestrial geodetic methods (tectonic geodesy), radar imaging (interferometry) of ground deformation patterns produced by individual earthquakes and volcanic unrest, and the seismological detection and measurement of earthquakes (seismotectonics) These techniques are applied globally via the World-Wide Standardized Seismograph Network and regionally via local seismographic coverage The modern snapshots of tectonism can be pushed back beyond the twentieth century through the analysis of historical accounts and maps to infer past land surface changes or to deduce the parameters of past seismic events (historical seismology) In addition, earthquakes can leave their mark in the mythical practices and literary accounts of ancient peoples, recorded in the stratigraphy of their site histories and in the damage to their buildings (archaeoseismology) The time covered by such human records varies markedly, ranging from many thousands of years in the Mediterranean, Near East, and Asia to a few centuries across much of North America Generally records confirm that regions that are active today have been consistently active for millennia, thereby demonstrating the long-term nature of crustal deformation, but occasionally records reveal that some regions that appear remarkably quiet from the viewpoint of modern seismicity (such as the Jordan rift valley) are capable of generating large earthquakes In reality, the distinction between neotectonics and active tectonics is artificial; the terms simply describe different time slices of a continuum of crustal movement This continuum is maintained by the persistence of the contemporary stress field, which means that inferences of past rates and directions of crustal movement from geological observations can be compared directly with those measured by modern geodetic and geophysical methods Although the terms ‘neotectonic’ and ‘active’ are somewhat blurred and are often used interchangeably, societal demands (for instance, regulatory authorities for seismic hazard, nuclear safety, etc.) often require the incidence of tectonic movements to be defined strictly For instance, in the United States, under California law, an ‘active fault’ is presently defined as one that has generated surface-rupturing earthquakes in the past 11 000 years (i.e., the time period was established to relate to the time when the Holocene was considered to have begun) Other regulatory bodies recognize a sliding scale of fault activity: Holocene (activity in the past 10 000 years), Late Quaternary (activity in the past 130 000 years), and Quaternary (activity in the past 1.6 million years) Neotectonic faults, by comparison, are simply those that formed during the imposition of the current tectonic regime ‘Real’ structures, of course, are unconstrained by such legislative concerns Many modern earthquakes rupture along older (i.e., palaeotectonic) basement faults Indeed, it is important to recognize that any fault that is favourably oriented with respect to the stress currently being imposed on it has the potential to be activated in the future, regardless of whether it has moved in the geologically recent past Global Tectonics A useful way to differentiate styles and degrees of neotectonic activity is in terms of tectonic strain rate, which is a measure of the velocity of regional crustal motions and, in turn, of the consequent tectonic strain build-up Crustal movements are most vigorous, and therefore most readily discernible, where plate boundaries are narrow and discrete In these domains of high tectonic strain, frequent earthquakes on fast-moving (>10 mm year 1) faults ensure that a century or two of historical earthquakes and a few years of precise geodetic measurements are sufficient to capture a consistent picture of the active tectonic behaviour Intermediate tectonic strain rates characterize those regions where plate–boundary motion is distributed across a network of slower moving (0.1–10 mm year 1) Examples of such broad deforming belts are the Basin and Range Province of the western United States or the Himalayan collision zone, where earthquake faults rupture every few hundred or thousand years, ensuring that the Holocene period is a reasonable time window over which to witness the typical crustal deformation cycle In contrast, areas with low strain rates ensure that intraplate regions, often referred to as ‘stable continental interiors’, are low-seismicity areas with slow-moving (

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