Overpressure

About: Overpressure is a research topic. Over the lifetime, 3236 publications have been published within this topic receiving 34648 citations.

Hydrodynamic aspects of caldera‐forming eruptions: Numerical models

Abstract: Comparison of results from a two-dimensional numerical eruption simulation (KACHINA) to calculations based upon a shock tube analog supports the conclusion that the hydrodynamics during the initial minutes of large caldera-forming ash flow eruptions may be dominated by blast wave phenomena. Field evidence for this phenomenology is pyroclastic surge deposits commonly occurring both directly below caldera-related ash flow sheets, on top of a preceding Plinian fall deposit (ground surge), and separating individual ash flow units. We model the eruption of the Tshirege member of the Bandelier Tuff (1.1 Ma B.P.) from the Valles caldera, New Mexico. In the model a magma chamber at 100 MPa (1 kbar) and 800°C is volatile rich, with an average H2O abundance above saturation greater than 8.7 wt % increasing to nearly 100 wt % near the very top of the chamber. Using a shock tube analogy, decompression of the chamber through a wide-open dikelike vent 0.1 km wide and 1 to 5 km long forms a shock wave of 3 MPa (≃3O atm) with a velocity greater than 1.0 km s−1. Steady flow of material erupted from the vent begins after 20 to 100 s based upon a 7-km depth from the ground surface to a reflective (density) boundary in the chamber and a rarefaction wave velocity of 100 to 600 m s−1. The velocity of the ash front behind the shock wave is 300 to 500 m s−1. The shock tube model serves as a basis to evaluate the consistency of the KACHINA code results which are similar to a one-dimensional problem along the symmetry axis. The results of the KACHINA simulation show in some detail the effect of multiple reservoir rarefaction reflections and possibly Prandtl-Meyer expansion in generating compressive wave fronts following the initial shock. The rarefaction resonance not only prolongs unsteady flow in the vent but tends to promote surging flow of ash behind the leading shock. Furthermore, these results are consistent with a blast wave characterized as a shock front followed by one or more pulses of entrained ash. The blast wave shocks ambient air to higher pressures and temperatures, the magnitudes of which depend strongly on the initial chamber overpressure, distance, and direction from the vent. In consideration of volcanic hazards our numerical model shows that a shock wave compressed the atmosphere to pressures of ≃0.2 to 0.7 MPa (2–7 atm) and temperatures of ≃200° to 300°C for distances to 10 km from the Bandelier vent(s).

Overpressure and earthquake initiated slope failure in the Ursa region, northern Gulf of Mexico

Abstract: [1] We use fluid flow and slope stability models to study the evolution of overpressure and slope stability in the Ursa region, northern Gulf of Mexico. Our predictions match measured overpressures (pressures in excess of hydrostatic) from Integrated Ocean Drilling Program Expedition 308 Site U1324 above 200 m below seafloor (mbsf) but overpredict deeper (200–610 mbsf) overpressures by 0.4–1.1 MPa. Modeled overpressure at Site U1322 matches measurements for the entire section (0–240 mbsf) with exception of the measurement at 240 mbsf. Slope stability models that integrate modeled overpressure, vertical stress, and effective stress during deposition predict slope failure at 61 ka on the eastern end of the region. This failure corresponds to the base of a mass transport deposit that has been interpreted as a retrogressive failure initiated by high overpressure. Overpressure alone could not drive failure of a second mass transport deposit (MTD2) that has its base along the 27 ka horizon. With an earthquake acceleration model coupled with our slope stability model, we predict that horizontal acceleration from a magnitude 5 earthquake within 140 km of the Ursa region at 27 ka would initiate the failure that created MTD2; the same earthquake at 20 ka would have to be within 40 km for failure. This magnitude and maximum rupture distance are consistent with seismicity near the Ursa region. We therefore propose that in some cases, overpressure drives failure on low-angle slopes; however, earthquakes, even on passive margins, may play a critical role in initiating slope failure in sediments weakened by overpressure.