THE EFFECTIVE DURATION OF EARTHQUAKE STRONG MOTION

The duration of strong ground shaking during earthquakes can play an important role in the response of foundation materials and structures, particularly when strength or stiffness degradation is encountered. A thorough seismic hazard assessment should therefore include an estimation of the expected duration of strong motion, which first requires criteria to define the part of an accelerogram considered to represent the duration of strong ground motion. Some 30 different definitions of strong motion duration are reviewed and classified into generic groups. Problems that arise with the use of these definitions for duration are highlighted. A new definition of duration is presented using a previously unexplored option which identifies the part of the record where the main energy is contained and constrains this strong shaking phase by absolute criteria. This new definition is shown to give consistently meaningful durations for strong earthquake accelerograms from an engineering viewpoint. The correlations between the new definition of duration and magnitude, soil conditions and distance are explored as a first step towards the development of predictive equations.     

Thermal waters of ‘tectonic origin’: the alkaline,Na‐HCO3 waters of the Rio Valdez geothermal area (Isla Grande de Tierra del Fuego,Argentina)

The geothermal area of Rio Valdez is located in the central portion of the Isla Grande de Tierra del Fuego (South Argentina), ten kilometers south of the southeastern sector of the Fagnano Lake. It consists of a series of thermal springs with low discharge rates (≤1 L/s) and temperatures in the range of 20–33°C distributed in an area of <1 km². The thermal springs are characterized by alkaline, Na‐HCO₃ waters with low salinity (0.53÷0.58 g/L), but relatively high fluoride contents (up to 19.4 mg/L). Their composition is the result of a slow circulation at depth, possibly through deep tectonic discontinuities connected with the Magallanes‐Fagnano Fault (MFF) system. According to geothermometric calculations, thermal waters reach temperatures in the range of 100–150°C and an almost complete chemical equilibrium with the alkali‐feldspars in the metavolcanic country rocks. The relatively high fluorine contents can be explained by the slow ascent and cooling of deep groundwaters followed by a progressive re‐equilibration with F‐bearing, hydrated Mg‐silicates, such as chlorite, which has been recognized as an abundant mineral in the metavolcanics of the Lemaire Formation and metapelites and metagraywackes of the Yahgán Formation. Finally, the isotopic composition of the investigated samples is consistent with the infiltration from local snow melting at altitudes in the range of 610–770 m asl. The comparison of our data with those collected in 1991 seems to suggest a possible progressive decline of the bulk thermal output in the near future. This possibility should be seriously considered before planning a potentially onerous exploitation of the resource. Presently, the only ways to exploit this geothermal resource by the population scattered in the area are the direct use of thermal waters and/or spa structures.     

Taking sides: cosmopolitanism,internationalism and ‘complex solidarity’ in the work of Fred Halliday

Fred Halliday's life and work were intimately associated with the theory and practice of internationalism. In his later writings, the notion of ‘complex solidarity’ emerges as a key component of Halliday's worldview. This article explores the conceptual interconnections between different historical expressions of internationalism, cosmopolitanism and solidarity. It considers the intricate relationship between these categories and their place in our understanding of international affairs, emphasizing the divergence between liberal and revolutionary conceptions of internationalism and cosmopolitanism. The article discusses diverse understandings of ‘solidarity’ in International Relations, arguing that beyond the cosmopolitan and communitarian approaches, there exist other ‘Grotian’ and ‘republican’ ideas of solidarity. Halliday drew on these to present his own defence of universal human rights and solidarity, arguably developing a distinctive brand of republican internationalism. The latter part of the article gives content to ‘complex solidarity’ by suggesting it is built on three inter‐related components: a methodological internationalism, an egalitarian reciprocity and a critique of global capitalism. Overall, these guiding features of complex solidarity deliver a unique rendition of internationalism which reflect Halliday's eclectic combination of radical liberalism with a residual historical materialism.