Velocity gradient

About: Velocity gradient is a research topic. Over the lifetime, 3013 publications have been published within this topic receiving 77120 citations.

A low velocity zone underlying a fast-spreading rise crest

Abstract: WE present the results of an unreversed seismic refraction profile on the East Pacific Rise near the Siqueiros Fracture Zone recorded using a digital ocean bottom seismograph (OBS)1. An analysis of P wave arrival times and amplitudes indicates a velocity gradient in the top 2 km, with the velocity reaching 6.7 km s−1. This is underlain by a low velocity channel some 1.4 km thick in which the velocity decreases to around 4.8 km s−1. Below this low velocity region there is a velocity gradient from 6.2 to 6.8 km s−1 and mantle velocities of 7.7 km s−1 are reached at a depth of 6 km below the sea bed.

Capillary flow instability of ethylene polymer melts

Abstract: The capillary flow instability resulting in extrudate distortion has been studied for ethylene polymer melts using a molecular structure approach. It is found that the instability initiates at a critical value of elastic strain energy independent of (average) molecular weight for linear polyethylene. Once the flow breaks down, a slip interface within the melt is formed near the capillary wall, causing an abrupt increase in volumetric throughput. The velocity gradient within the melt remains continuous through the instability, however. Low molecular weight species present in the molecular weight distribution of linear polyethylene tend to suppress slip. Blends of linear and branched polyethylene exhibit instability behavior characteristic of both components throughout the entire range of composition. Results are discussed in terms of specific molecular mechanisms.

Ubiquitous low-velocity layer atop the 410-km discontinuity in the northern Rocky Mountains

Abstract: Receiver functions from three 30-station IRIS-PASSCAL small-aperture arrays (2–15 km station spacing) operated for 10 months each in the northern Rocky Mountains show a ubiquitous negative polarity P to S conversion just preceding the 410-km discontinuity arrival. Data from the three arrays were sorted into NW, SE, and SW back-azimuth quadrants and stacked to form nine quadrant stacks. Remarkably, the negative polarity arrival (NPA) is apparent in 8 of the 9 quadrant stacks, with 7 of the 8 having well-correlated waveforms. Each quadrant stack also contains clear P to S conversions from the 410- and 660-km discontinuities. Moveout analysis shows that all the major phases display the correct moveout for forward scattered P-S phases. The waveshapes for the seven similar NPA-410 km discontinuity arrivals are modeled with a five-parameter “double gradient slab” model that is parameterized as follows: a top gradient thickness and shear velocity decrease; a constant velocity layer; bottom gradient thickness; and shear velocity increase. Model misfit is assessed via a grid search over the model space using a reflectivity code to calculate synthetic seismograms. Model likelihood is determined by calculating 1- and 2-D marginal probability density functions (PDF) for the five parameters. The 1-D marginals display a range of peak values, although significant overlap is observed for the top gradient thickness and its associated velocity decrement. From the peak value of the summary PDF, we find the top velocity gradient to be sharp (<6.4 km) and the shear velocity decrement to be large (8.9% Vs). Defining an effective thickness of the low-velocity layer as the mean layer thickness plus half the mean gradient thicknesses, the 410 low-velocity layer thickness is found to be 22 km. A review of changes in the physical state required to match our new 410-LVL constraints suggests that the water-filter model remains an operative hypothesis to test.