In order to evaluate the activity of the Hawaiian hot spot, we calculate both the magma production rate, associated with volcanism, and the rate of swell formation, characteristic of the plume behavior. Both computations are made along the Emperor‐Hawaii track. Depth anomaly is calculated by correcting the 2′ bathymetry grid of Smith and Sandwell [1997] from thermal subsidence. A new filtering method is then used to separate the topography associated with volcanism and the swell surrounding the hot spot chain. The volume of magma includes the compensating root underlying the volcanoes, computed assuming either an Airy compensation (local) or a flexural root (regional) associated with the volcanic load. Next, the volume corresponding to the swell is calculated between the swell amplitude map and the zero value of the depth anomaly. Temporal variations of both volumes are then computed by the means of 1° × 10° windows translated along the hot spot track. Both volume fluxes are correlated through time and present (1) a general increase in amplitude for the last 30 Ma, indicating an increase in hot spot activity, and (2) short‐wavelength oscillations with a 5 m.y. period, which may reflect the presence of solitary waves in the plume conduit. Contrary to the swell volume flux, the magma production rate estimation is not dependent on the subsidence model and is still valid for the older part of the chain. It is thus the most relevant parameter to describe the temporal variation of the Hawaiian hot spot behavior.

Triggering large-scale motion by imposing vibrations to a system can be encountered in many situations, from daily-life shaking of saltcellar to silo unclogging or dynamic earthquakes triggering. In the well-known situation of solid or granular friction, the acceleration of imposed vibrations has often been proposed as the governing parameter for the transition between stick-slip motion and continuous sliding. The threshold acceleration for the onset of continuous slip motion or system unjamming is usually found of the order of the gravitational acceleration. These conclusions are mostly drawn from numerical studies. Here, we investigate, in the laboratory, granular friction by shearing a layer of grains subjected to horizontal vibrations. We show that, in contrast with previous results, the quantity that controls the frictional properties is the characteristic velocity and not the acceleration, of the imposed mechanical vibrations. Thus, when the system is statically loaded, the typical acceleration of the vibrations which trigger large slip events is much smaller than the gravitational acceleration. These results may be relevant to understand dynamic earthquake triggering by small ground perturbations.

We report on the fluidization dynamics of an attractive gel composed of non-Brownian particles made of fused silica colloids. Extensive rheology coupled to ultrasonic velocimetry allows us to characterize the global stress response together with the local dynamics of the gel during shear startup experiments. In practice, after being rejuvenated by a preshear, the gel is left to age for a time t_w before being subjected to a constant shear rate . We investigate in detail the effects of both t_w and on the fluidization dynamics and build a detailed state diagram of the gel response to shear startup flows. The gel may display either transient shear banding towards complete fluidization or steady-state shear banding. In the former case, we unravel that the progressive fluidization occurs by successive steps that appear as peaks on the global stress relaxation signal. Flow imaging reveals that the shear band grows until complete fluidization of the material by sudden avalanche-like events which are distributed heterogeneously along the vorticity direction and correlated to large peaks in the slip velocity at the moving wall. These features are robust over a wide range of t_w and values, although the very details of the fluidization scenario vary with . Finally, the critical shear rate * that separates steady-state shear-banding from steady-state homogeneous flow depends on the width of the shear cell and exhibits a nonlinear dependence with t_w. Our work brings about valuable experimental data on transient flows of attractive dispersions, highlighting the subtle interplay between shear, wall slip and aging whose modeling constitutes a major challenge that has not been met yet.

Air is injected locally at the base of an immersed granular bed. The gas, which is forced to flow gently through the material, creates several paths between the grains. We observe that the latter gas venting results in the emission of bubbles in a localized region at the free surface. Additional experiments, performed in two dimensions, permit a direct visualization of the paths, and a theoretical approach shows that the typical size of the region at the free surface can be accounted for by a diffusionlike process. The diffusion coefficient is expressed as a function of the system parameters.

Summary. Different types of 4-component ocean bottom seismometers (OBS) were deployed for variable durations ranging from 1 week to about 4 months in 2007, ove

We report the formation of a crater at the free surface of an immersed granular bed, locally crossed by an ascending gas flow. In two dimensions, the crater consists of two piles which develop around the location of the gas emission. We observe that the typical size of the crater increases logarithmically with time, independently of the gas emission dynamics. We describe the related granular flows and give an account of the influence of the experimental parameters, especially of the grain size and of the gas flow.