ALBERT

Description: Glass formation in colloidal suspensions has many of the hallmarks of glass formation in molecular materials. For hard-sphere colloids, which interact only as a result of excluded volume, phase behaviour is controlled by volume fraction, phi; an increase in phi drives the system towards its glassy state, analogously to a decrease in temperature, T, in molecular systems. When phi increases above phi* approximately 0.53, the viscosity starts to increase significantly, and the system eventually moves out of equilibrium at the glass transition, phi(g) approximately 0.58, where particle crowding greatly restricts structural relaxation. The large particle size makes it possible to study both structure and dynamics with light scattering and imaging; colloidal suspensions have therefore provided considerable insight into the glass transition. However, hard-sphere colloidal suspensions do not exhibit the same diversity of behaviour as molecular glasses. This is highlighted by the wide variation in behaviour observed for the viscosity or structural relaxation time, tau(alpha), when the glassy state is approached in supercooled molecular liquids. This variation is characterized by the unifying concept of fragility, which has spurred the search for a 'universal' description of dynamic arrest in glass-forming liquids. For 'fragile' liquids, tau(alpha) is highly sensitive to changes in T, whereas non-fragile, or 'strong', liquids show a much lower T sensitivity. In contrast, hard-sphere colloidal suspensions are restricted to fragile behaviour, as determined by their phi dependence, ultimately limiting their utility in the study of the glass transition. Here we show that deformable colloidal particles, when studied through their concentration dependence at fixed temperature, do exhibit the same variation in fragility as that observed in the T dependence of molecular liquids at fixed volume. Their fragility is dictated by elastic properties on the scale of individual colloidal particles. Furthermore, we find an equivalent effect in molecular systems, where elasticity directly reflects fragility. Colloidal suspensions may thus provide new insight into glass formation in molecular systems.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Mattsson, Johan -- Wyss, Hans M -- Fernandez-Nieves, Alberto -- Miyazaki, Kunimasa -- Hu, Zhibing -- Reichman, David R -- Weitz, David A -- England -- Nature. 2009 Nov 5;462(7269):83-6. doi: 10.1038/nature08457.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Physics and Harvard School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA. johanm@chalmers.se〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/19890327" target="_blank"〉PubMed〈/a〉

Description: The first main shock of the off-Sanriku earthquake sequence (02/11/89, M 7.1; 18/07/92, M 6.9; 28/12/94, M 7.5) was preceded by a precursory seismic quiescence lasting 2.5 ± 1 year and up to this main shock. The detailed properties of this quiescence were mapped as a function of time and space by a gridding technique using the ZMAP computer code, and the statistical significance was estimated by generating a synthetic catalog based on the microearthquake catalog of Tohoku University, which was the data set used. The statistically most significant expression of this precursory quiescence has a probability of 0.1% to have occurred at random and was located in the eastern part of the 1989 aftershock area, at a point to which the 1994 aftershocks extended also. If we define the dimensions of the quiescence anomaly by a vertical cylinder with the depth of the entire seismogenic layer, centered at the point of most significant quiescence and showing a rate decrease of 75%, then we find its radius is 25 ± 9 km. If we allow other shapes, such as the simplified aftershock volume of 1989, or other simple geometric figures, to define the rate decrease we find dimensions of 80 by 80 km. The characteristics of the quiescence anomaly do not depend strongly on the choice of free parameters within the following ranges: 100 ≤ number of events ≤ 400, 2.0 ≤ Mmin £ 3.0, 1 ≤ time window ≤ 3 years. With our method, a thorough analysis of the period before the 1994 main shock is not possible because of the interference of the extended aftershock sequence of 1989. Nevertheless, we identified a quiescence of nearly zero earthquakes located near the center of the 1994 aftershock area that lasted for one year up to that main shock. However, this quiescence period ranked only 46th in significance, behind other quiescences of equal duration and similar dimensions distributed in time and space through the data set. Because of the ubiquitous existence of periods of near zero activity during short periods like one year, we find that quiescences shorter than about 1.5 years cannot be defined with high statistical significance in most earthquake catalogs. In the last two years of the data (1995.3-1997.3) we see no extensive quiescence of high significance off the east coast of Honshu between 36.5° and 42°N in the currently available data.

Description: JCR Journal

Description: open

Description: The earthquake catalog in southern Iceland between 20.25 degrees W and 21.3 degrees W is complete down to approximately M (sub w) 0 and shows no evidence for magnitude shifts as a function of time since 1991. This means that 6902 earthquakes (M〉 or =-0.1, depth 〈 or =20 km) are available for studies of seismicity patterns preceding the two M (sub S) 6.6 mainshocks in 2000, in their source volumes and vicinity (20.25 degrees W and 20.9 degrees W). The power law of the frequency-magnitude relationship holds down to M (sub 0) . Detailed mapping (including 3D mapping) of the b-value of the frequency- magnitude distribution shows that variations from b = 0.6 to 1.4 exist over distances of about 2 km. This suggests that the tectonic fabric in southern Iceland is heterogeneous on a small scale. The hypocenters of the two M (sub S) 6.6 mainshocks and one additional M 4.5 mainshock are associated with low b-values (0.6-0.8), statistically significantly different from volumes in their vicinity. Therefore, one can expect future mainshocks in southern Iceland to also emanate from volumes characterized by low b-values. However, the small dimensions of these asperities may render it difficult to identify them before the mainshocks occur. Low values of estimated local recurrence times (500-2000 years) map the two fault zones that ruptured in 2000 against a background of local recurrence times longer than 5000 years. The first of the two M 6.6 mainshocks emanated from a volume of short estimated local recurrence time. This supports the hypothesis that asperities with short local recurrence times control locations of major ruptures. Mapping of b-values in cross sections shows anomalies of high b at the bottom of the seismogenic crust, correlating with the change of its thickness in the middle of the study area. These high b-value anomalies, and other pockets of such anomalies at shallower depths, are best interpreted as caused by high pore pressure because evidence of fluids under high pressure is ubiquitous in this area.