We explore the extent to which two properties of Earth's present‐day rotational state may be applied to constrain the viscosity of the lower regions of the lower mantle. The analysis builds upon recent advances in understanding the radial resolving power of the relative sea level data of postglacial rebound which have demonstrated anew that no significant increase of viscosity across the 660‐km seismic discontinuity appears to be allowed by these data. Such observations do not constrain the viscosity of the mantle below about 1400 km depth, however, and here we estimate the average viscosity in this deepest region by invoking observations of the present‐day rate and direction of polar wander and the present‐day magnitude of the nontidal acceleration of the rate of planetary rotation. Since the rotational response to the glaciation cycle depends only upon the degree 2 spherical harmonic components of the induced deformation, this response is expected to provide the most specific sampling possible of deep mantle properties. Our analysis is based upon the use of glacial excitation functions calculated from complete gravitationally self‐consistent solutions of the sea level equation for the most recent refinement of the history of ice sheet thickness variations across the last glacial‐interglacial transition, namely, the ICE‐4G model. This analysis shows that when Earth rotation observations are combined with relative sea level constraints, the viscosity of the deepest mantle is required to be ∼1 order of magnitude higher than the viscosity of the upper part of the lower mantle. Such significant elevation of viscosity in the lowermost region of the lower mantle is also in accord with recent inferences based upon analysis of the nonhydrostatic geoid. The analyses presented herein therefore suffice to effect a reconciliation between the requirements of these distinct geophysical observations.
The street-crossing behavior of pedestrians was time dependent. Pedestrians behave differently under the effects of various factors. Pedestrian safety interventions that aim at reducing pedestrian injuries may need to consider these effects. The pedestrians' behavioral modifications, such as enhancing the safety awareness, might be the most efficient means to reducing the likelihood of pedestrian violation, though environmental modifications also worked well in improving pedestrian safety.
We present a device for stretching cells adhering to elastic membranes in equiaxial or uniaxial mode, meanwhile allowing real-time imaging of molecular dynamics of live cells at high resolution on an inverted microscope during the entire process of the stretch. We obtained high-resolution images of stress fibers at each stage of the stretch, and found that stress fibers were shortened after one stretching cycle. We, for the first time, captured real-time images of the process of stress fiber disassembly during stretching. Several adjacent stress fibers appeared to reassemble into a single one after stretching. All these indicated that mechanical stretching played important roles in the rearrangement of actin filaments. This device will be especially useful in studies of the molecular dynamics in the process of mechanotransduction. The device is fabricated on a glass slide through a simple procedure and is adaptable to most ordinary laboratories.
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