Large Overpressures Research Articles

Reconciling patterns of interseismic strain accumulation with thermal observations across the Carrizo segment of the San Andreas Fault

The Carrizo segment of the San Andreas Fault separates rocks of the Salinian Block southwest of the fault characterized by high heat flow (~ 75–95 mW/m 2) and shallow seismicity (<~ 10 km depth), from rocks of the Franciscan Complex and Great Valley Group northeast of the fault associated with low heat flow (50–60 mW/m 2) and deeper seismicity (<~ 20 km depth). GPS data from this region suggest that the northeast side of the fault accommodates more interseismic strain than the southwest side. We show that by incorporating variations in depth to the brittle ductile transition inferred based on heat flow and seismicity data, we fit the geodetic data well and achieve agreement with geologically accepted long-term average slip rates with a ~ 20 km wide zone with low Young's modulus NE of the fault. This wide compliant zone is consistent with observations suggesting that the presence of large overpressures may be decreasing the elastic moduli in this region.

High-resolution simulations of planetesimal formation in turbulent protoplanetary discs

AbstractWe present high resolution computer simulations of dust dynamics and planetesimal formation in turbulence triggered by the magnetorotational instability. Particles representing approximately meter-sized boulders clump in large scale overpressure regions in the simulation box. These overdensities readily contract due to the combined gravity of the particles to form gravitationally bound clusters with masses ranging from a few to several ten times the mass of the dwarf planet Ceres. Gravitationally bound clumps are observed to collide and merge at both moderate and high resolution. The collisional products form the top end of a distribution of planetesimal masses ranging from less than one Ceres mass to 35 Ceres masses. It remains uncertain whether collisions are driven by dynamical friction or underresolution of clumps.

Potential role of mantle‐derived fluids in weakening the San Andreas Fault

On the basis of both geomechanical and thermal data, the San Andreas Fault (SAF) has been interpreted to act as a weak plane within much stronger crust, allowing it to slip at very low shear stresses. One explanation for this weakness is that large fluid overpressures exist locally within the fault zone. However, mechanisms for generating, maintaining, and localizing pressures within the fault are poorly quantified. Here we evaluate whether realistic sources of mantle‐derived fluids, proposed on the basis of high mantle helium signatures near the SAF, can generate localized fluid pressures within the fault zone in a manner consistent with a wide range of observations along the fault and in the San Andreas Fault Observatory at Depth borehole. We first calculate a reasonable estimate of the magnitude and location of a mantle‐derived flux of water into the crust. This fluid flux results from dehydration of a serpentinized mantle wedge following the northward migration of the Mendocino Triple Junction and the transition from subduction to strike‐slip tectonics. We then evaluate the potential effect of this water on fluid pressures within the crust using 2‐D cross‐sectional models of coupled fluid flow and heat transport. We show that in models with realistic permeability anisotropy, controlled by NE dipping faults and fractures within the country rock, large localized fluid pressures can be focused within a SAF acting as a hydrologic barrier. Our results illustrate a simple and potentially plausible means of weakening the SAF in a manner generally consistent with available hydrologic, thermal, and mechanical constraints.

Kinetics of the post-garnet transformation: Implications for density and rheology of subducting slabs

Mechanisms and kinetics of the post-garnet transformation in natural pyrope were examined at 28.1–31.3 GPa and 1320–2150 K by time-resolved X-ray diffraction measurements using synchrotron radiation combined with microstructural observations of the recovered samples. The water content of samples recovered was estimated to be approximately 400 wt. ppm H 2O from infrared spectroscopic measurements. The transformation occurs by the grain-boundary reaction and reaction rims are formed along the grain boundaries of parental garnet grains. Decomposed post-garnet assemblages consisting of Mg- and Ca-perovskites, aluminous phase, and stishovite show very fine-grained symplectic texture. Analyses of kinetic data obtained have revealed that growth rates in the post-garnet transformation are strongly time-dependent. Extrapolations of the present kinetic data suggest that the post-garnet transformation cannot complete at less than ca. 1730 K, even under geological timescales, because of the slow growth kinetics. Due to the presence of the metastable garnet, subducting slabs become buoyant in the lower mantle at less than ca. 1730 K for the basaltic crust, and at ca. 1400–1600 K for the underlying peridotite layer. Rheological weakening of the basaltic crust is expected in the lower mantle through grain-size reduction by the metastable post-garnet transformation occurring under large overpressure conditions. The kinetics of the post-garnet transformation significantly affect both the density and rheology of subducting slabs in the lower mantle, and possibly has important roles in the mixing and survival of chemically differentiated slab materials.

Optimization of Nozzle Design for Pulse Cleaning of Ceramic Filter

The experimental study was carried out to optimize the nozzle shape and dimension for the pulse cleaning of a ceramic filter candle. A bench scale unit of ceramic filter consisting of four commercial filter elements was used to measure the traces of the transient pressure around the nozzle and the overpressure in the filter cavity during the pulse-jet injection of pulse gas. Overpressure in the filter cavity is related to the pulse cleaning force. Nozzle design is concerned to increase the overpressure at the open end of filter element of pulse cleaning inlet, as well as to minimize the consumption of pulse gas. Convergent nozzle induces more secondary flow and generates higher pulse cleaning effect than straight nozzle. Nozzles of different convergent ratio (ratio of outlet to inlet diameter of nozzle) by changing the convergent angle and height were tested. The outlet diameter of convergent nozzle seriously influences the cleaning effect. The optimum convergent ratio increases with the increase of pulse gas pressure. The nozzle position (distance of nozzle tip from the open end of filter inlet) is also important to decide the nozzle dimension. Nozzle of large outlet diameter accepts high pressure of pulse gas to provide large overpressure in the filter cavity of top position by applying long distance.

Nonlinear oscillations and collapse of elongated bubbles subject to weak viscous effects: Effect of internal overpressure

The details of nonlinear oscillations and collapse of elongated bubbles, subject to large internal overpressure, are studied by a boundary integral method. Weak viscous effects on the liquid side are accounted for by integrating the equations of motion across the boundary layer that is formed adjacent to the interface. For relatively large bubbles with initial radius R0 on the order of millimeters, PSt=PSt′∕(2σ∕R0)∼300 and Oh=μ∕(σR0ρ)1∕2∼200, and an almost spherical initial shape, S∼1, Rayleigh-Taylor instability prevails and the bubble breaks up as a result of growth of higher modes and the development of regions of very small radius of curvature; σ, ρ, μ, and PSt′ denote the surface tension, density, viscosity, and dimensional static pressure in the host liquid while S is the ratio between the length of the minor semiaxis of the bubble, taken as an axisymmetric ellipsoid, and its equivalent radius R0. For finite initial elongations, 0.5⩽S&amp;lt;1, the bubble collapses either via two jets that counterpropagate along the axis of symmetry and eventually coalesce at the equatorial plane, or in the form of a sink flow approaching the center of the bubble along the equatorial plane. This pattern persists for the above range of initial elongations examined and large internal overpressure amplitudes, εB⩾1, irrespective of Oh. It is largely due to the phase in the growth of the second Legendre mode during the after-bounce of the oscillating bubble, during which it acquires large enough positive accelerations for collapse to take place. For smaller bubbles with initial radius on the order of micrometers, PSt∼4 and Oh∼20, and small initial elongations, 0.75&amp;lt;S⩽1, viscosity counteracts P2 growth and subsequent jet motion, thus giving rise to a critical value of Oh−1 below which the bubble eventually returns to its equilibrium spherical shape, whereas above it collapse via jet impact or sink flow is obtained. For moderate elongations, 0.5⩽S⩽0.75, and large overpressures, εB⩾0.2, jet propagation and impact along the axis of symmetry prevails irrespective of Oh. For very large elongations, S&amp;lt;0.5, and above a certain threshold value of Oh the counterpropagating jets pinch the contracting bubble sidewalls in an off-centered fashion.

Some aspects of the seismicity associated with the 1982 eruption of El Chichon Volcano, Chiapas, Mexico

We review the different phases of the seismicity related to the 1982 eruption of El Chichon Volcano, Chiapas, Mexico. The pre-eruption seismicity was already anomalous by late 1980, became significant by late 1981 and increased towards 28 March, 1982, when the first eruptive event occurred. A noticeable feature within the 7-day period of unrest is the occurrence of three earthquake swarms before the devastating explosions of 3 and 4 April 1982 (local time). The periodicity and appearance of the swarms, close to the time of maximum tidal strain, suggests a large overpressure in the magmatic system, and the triggering of the events by the earth tides. The post-eruption seismicity occurred mostly in a radius 5 km from the crater and a depth range 11 to 15 km suggesting that this region was a deeper reservoir of the erupted magma.

Acoustic measurements of the 1999 basaltic eruption of Shishaldin volcano, Alaska2. Precursor to the Subplinian phase

The 1999 eruption of Shishaldin volcano (Alaska, USA) displayed both Strombolian and Subplinian basaltic activity. The Subplinian phase was preceded by a signal of low amplitude and constant frequency (≈2 Hz) lasting 13 h. This “humming signal” is interpreted as the coalescence of the very shallow part of a foam building up in the conduit, which produces large gas bubbles before bursting. The acoustic waveform of the hum event is modelled by a Helmholtz resonator: gas is trapped into a rigid cavity and can only escape through a tiny upper hole producing sound waves. At Shishaldin, the radius of the hole (≈5 m) is close to that of the conduit (≈6 m), the cavity has a length of ≈60 m, and gas presents only a small overpressure between (≈1.2×10 −3 and 4.5×10 −3 MPa). Such an overpressure is obtained by the partial coalescence of a foam formed by bubbles with a diameter from ≈2.3 mm at the beginning of the episode towards ≈0.64 mm very close to the end of the phase. The intermittency between hum events is explained by the ripening of the foam induced by the H 2 O diffusion through the liquid films. The two extreme values, from 600 to 10 s, correspond to a bubble diameter from 2.2 to 0.3 mm at the beginning and end of the pre-Subplinian phase, respectively. The extremely good agreement between two independent estimates of bubble diameters in the shallow foam reinforces the validity of such an interpretation. The total gas volume lost at the surface during the humming events is at most 5.9×10 6 m 3 . At the very end of the pre-Subplinian phase, there is a single large bubble with an overpressure of ≈0.42 MPa. The large overpressure suggests that it comes from significant depth, unlike other bubbles in the pre-Subplinian phase. This deep bubble may be responsible for the entire foam collapse, resulting in the Subplinian phase.

Acoustic measurements of the 1999 basaltic eruption of Shishaldin volcano, Alaska: 2. Precursor to the Subplinian phase

The 1999 eruption of Shishaldin volcano (Alaska, USA) displayed both Strombolian and Subplinian basaltic activity. The Subplinian phase was preceded by a signal of low amplitude and constant frequency (≈2 Hz) lasting 13 h. This “humming signal” is interpreted as the coalescence of the very shallow part of a foam building up in the conduit, which produces large gas bubbles before bursting. The acoustic waveform of the hum event is modelled by a Helmholtz resonator: gas is trapped into a rigid cavity and can only escape through a tiny upper hole producing sound waves. At Shishaldin, the radius of the hole (≈5 m) is close to that of the conduit (≈6 m), the cavity has a length of ≈60 m, and gas presents only a small overpressure between (≈1.2×10 −3 and 4.5×10 −3 MPa). Such an overpressure is obtained by the partial coalescence of a foam formed by bubbles with a diameter from ≈2.3 mm at the beginning of the episode towards ≈0.64 mm very close to the end of the phase. The intermittency between hum events is explained by the ripening of the foam induced by the H 2O diffusion through the liquid films. The two extreme values, from 600 to 10 s, correspond to a bubble diameter from 2.2 to 0.3 mm at the beginning and end of the pre-Subplinian phase, respectively. The extremely good agreement between two independent estimates of bubble diameters in the shallow foam reinforces the validity of such an interpretation. The total gas volume lost at the surface during the humming events is at most 5.9×10 6 m 3. At the very end of the pre-Subplinian phase, there is a single large bubble with an overpressure of ≈0.42 MPa. The large overpressure suggests that it comes from significant depth, unlike other bubbles in the pre-Subplinian phase. This deep bubble may be responsible for the entire foam collapse, resulting in the Subplinian phase.

Liquid Helium up to 160 bar

We have used an acoustic technique to pressurize liquid helium 4 up to 163 ± 20 bar. This is far above the liquid–solid equilibrium pressure Pm, which is 25.3 bar in the low, temperature domain, where the experiment was performed (0.05 K < T < 1 K). This is also far above 65 bar, the prediction of the standard theory for homogeneous nucleation of solid helium. However, no solidification was observed. We discuss our experimental method and the metastability of liquid helium at such very large overpressures. We also propose improvements of our experiment, in order to reach a possible instability limit of liquid helium 4 around 200 bar.

Tympanometry accurately measures middle ear underpressures in monkeys.

Tympanometry is useful for evaluating middle ear (ME) status, but its accuracy in estimating true ME pressure has been questioned. We evaluated the accuracy of tympanometry in 6 monkeys. Direct application and measurement of ME pressure were achieved with a probe introduced into the mastoid antrum, and tympanometry was done over a large range of applied ME pressures. For all ears, tympanometric pressure was a linear function of applied pressure. At large overpressures, the tympanometric pressure was approximately 40 mm H2O greater than the applied pressure, but there was little error in the measurement for applied underpressures. The measurement error was proportional to the ME pressure multiplied by the ratio of the extant volume displacement of the tympanic membrane to ME volume. These results show that in monkeys, tympanometry provides an accurate, relatively unbiased estimate of ME underpressure and suggest that the measurement error for tympanometry can be predicted for MEs of other species.

Near-surface sediment mobilization and methane venting in relation to hydrate destabilization in Southern Lake Baikal, Siberia

Abstract Four seeps and mud extrusion features at the lake floor were discovered in August 1999 in the gas hydrate area in Lake Baikal’s South Basin. This paper describes these features in detail using side-scan sonar, detailed bathymetry, measurements of near-bottom water properties, selected seismic profiles and heat flow data calculated from the depth of the hydrate layer as well as obtained from in situ thermoprobe measurements. The interpretation of these data is integrated with published geochemical data from shallow cores. The seeps are identified as methane seeps and appear as mud cones (maximum 24 m high, 800 m in diameter) or low-relief craters (maximum 8 m deep, 500 m in diameter) at the lake floor. Mud cones (estimated to be approximately 50–100 ka old) appear to be older than the craters and have a different structural setting. Mud cones occur at the crest of rollover structures, in the footwall of a secondary normal fault, while the craters occur at fault splays. The seeps are found in an area of high heat flow where the base of the gas hydrate layer shallows rapidly towards the vent sites from about 400 m to about 160 m below the lake floor. At the site of the seep, a vertical seismic chimney disrupts the sedimentary stratification from the base of the hydrate layer to the lake floor. Integration of these results leads to the interpretation that focused destabilization of gas hydrate caused massive methane release and forced mud extrusion at the lake floor and that the gas seeps and mud diapirs in Lake Baikal do not have a deep origin. This is the first time that methane seeps and/or mud volcanoes associated with gas hydrate decomposition have been observed in a sub-lacustrine setting. The finding suggests that gas hydrate destabilization can create large pore fluid overpressures in the shallow subsurface (&lt;500 m subsurface) and cause mud extrusion at the sediment surface.

Earthquake scaling and the strength of seismogenic faults

Two important and unresolved issues in tectonics and earthquake mechanics are the strength of seismogenic faults, and scaling relationships between the seismic moment of an earthquake and the area or length of the rupture. These two issues, usually treated separately, are shown here to be fundamentally related. It is shown that the reported scatter in moment‐area and moment‐length data of strike‐slip and dip‐slip earthquakes is not scatter, but instead reflects the strength of the fault that failed. Relationships that exhibit continuous scaling between small and large earthquakes are derived, and demonstrate that fault zone pore pressure is the scaling parameter that collapses the combined catalogs of strike‐slip and dip‐slip earthquakes to a single function. It is shown that for large earthquakes overpressures vary continuously between hydrostatic and near‐lithostatic above about 15 km, with evidence for a clear transition to near‐lithostatic pore pressures below this depth. These results have significant implications for plate tectonics, earthquake source physics, and mechanistic seismic hazard assessment.

The eruption of Soufrière Hills Volcano, Montserrat (1995-1999): overview of scientific results

Abstract The eruption of Soufrière Hills Volcano, Montserrat (1995-1999) has displayed a wide range of volcanic phenomena: growth of an andesitic lava dome, generation of pyroclastic flows by lava dome collapse and by fountain collapse in explosive eruptions, Vulcanian and sub-Plinian explosivity with accompanying tephra fall, entrance of pyroclastic flows into the sea, sector collapse with formation of a debris avalanche and a high-velocity pyroclastic density current, and generation of lahars. New phenomena include: cyclic patterns of ground deformation linked with shallow seismicity and eruptive activity; pyroclastic flows formed by rapid sedimentation from pyroclastic surges; and an unprecedented slow escalation of eruption intensity. Magma pulsations with timescales of hours to years have been recognized. Transitions from extrusive to explosive activity were triggered by major dome collapses. Relationships between magma ascent dynamics and geophysical signals have been elucidated. Ascending water-rich andesitic magma becomes Theologically stiffened by degassing and groundmass crystallization. Large magma overpressures are consequently developed in the upper conduit, causing shallow seismicity, radial patterns of ground deformation, and cyclic pulsations of eruptive activity. Lava dome growth involved heterogeneous deformation with formation of spines and lobes along shear zones. Collapse of the pressurized dome resulted in substantial pyroclastic surges forming above pyroclastic flows. Influx of hydrous mafic magma remobilized hot crystalline igneous rocks at depths of 5 to 6 km to form crystal-rich andesitic magma and triggered the eruption.

Mechanisms and kinetics of the post-spinel transformation in Mg 2SiO 4

Mechanisms and kinetics of the post-spinel transformation in Mg 2SiO 4 were examined at 22.7–28.2 GPa and 860–1200 °C by in situ X-ray diffraction experiments using synchrotron radiation combined with microstructural observations of the recovered samples. The post-spinel phases nucleated on spinel grain boundaries and grew with a lamellar texture. Under large overpressure conditions, reaction rims were formed along spinel grain boundaries at the initial stage of the transformation, whereas under small overpressure conditions, the transformation proceeded without formation of reaction rims. Mg 2SiO 4 spinel metastably dissociated into MgSiO 3 ilmenite and periclase, and stishovite and periclase as intermediate steps in the transformation into the stable assemblage of MgSiO 3 perovskite and periclase. Topotactic relationships were found in the transformation from spinel into ilmenite and periclase. Kinetic parameters in the Avrami rate equation, time taken to 10% completion, and the growth rate were estimated by analysis of the kinetic data obtained by in situ X-ray observations. The empirical activation energy for 10% transformation decreases with increasing pressure because the activation energy for nucleation becomes smaller at larger overpressure conditions. Extrapolations of the 10% transformation to ∼700 °C, which is the lowest temperature expected for the cold slabs at ∼700 km depth, suggest that overpressure of more than ∼1 GPa is needed for the transformation. Because the growth rate is estimated to be large even at low-temperatures of ∼700 °C and overpressures of 1 GPa, the depth of the post-spinel transformation in the cold slabs is possibly controlled by nucleation kinetics.

Stress-dependent permeability of a de-mineralised fracture in shale

The paper presents the results of laboratory direct shear tests on the hydro-mechanical behaviour of an extensional fracture in shale. The tests were conducted on fracture specimens obtained by mechanically splitting blocks of a naturally fractured shale along the cemented fracture and dissolving the calcite fracture filling with a strong acid. The experiments consisted of radial fluid flow tests from a single fluid source along the fracture surface. Fracture samples were tested under different levels of effective normal stress and shear displacement. The experimental results show significant reduction of fracture permeability during increasing contact normal stress across the fracture and after shearing of the fracture under constant high normal stress. However, the tested fractures never completely closed even under normal stresses close to or higher than the unconfined compressive strength of the intact shale. The fracture permeability remained much higher than the intact shale (or matrix) permeability even when the fracture surface has undergone some gouge formation from local asperity failure during shearing. The results indicate that fractures once created are difficult to close by mechanical loading. In tight formations like shales, fractures will always be conduits for fluid flow unless closed by cementation. Currently, it is widely assumed that fractures in sedimentary basins stay open only when there is an overpressure which could keep the fractures hydraulically parted. However, the results presented here indicate that given a sufficient hydraulic gradient, fluid can still flow along a fracture even in the absence of large overpressures.

Theoretical study of GaN molecular beam epitaxy growth using ammonia: A rate equation approach

III–V nitrides are intensely researched for optoelectronic applications spanning the entire visible spectrum. In spite of realization of commercial devices and advances in processing of materials and devices, the understanding of the processing and epitaxial growth of these materials is incomplete. In this study, a rate equation approach is proposed based on physically sound surface processes to investigate the molecular beam epitaxy growth of GaN using ammonia. A surface riding layer of Ga and ammonia and its associated dynamics such as incorporation of Ga and N in to the crystal and desorption are included in the model. Rates of all surface processes are assumed Arrhenius type. The simulated Ga incorporation rate as a function of ammonia pressure and substrate temperature are in excellent agreement with the experimental data. Ga incorporation increases with increasing NH3 overpressure and saturates at a maximum value at large NH3 overpressure. The Ga incorporation rate exhibits a peak at 820 °C due to competition between thermally activated pyrolysis of NH3 and reevaporation of Ga from the surface. The simulated Ga desorption parameter versus time data is also in good agreement with the experimental data. These observations will be explained based on the interplay of competing surface processes such as reevaporation and incorporation.

Control on the emplacement of the andesite lava dome of the Soufriere Hills volcano, Montserrat by degassing‐induced crystallization

Lava solidification is controlled by two mechanisms: external cooling and gas exsolution, the latter inducing crystallization due to increasing liquidus temperature. The andesite lava dome of the Soufriere Hills Volcano, Montserrat, is an extrusion dominated by crystallization caused by gas exsolution where cooling is unimportant in controlling emplacement. In the magma chamber the magma has an estimated viscosity of 7 × 106 Pa s. During ascent, gas exsolution caused the magma to extrude in a highly crystalline state, with only 5–15% residual melt, viscosities in the range 1013–1014 Pa s and mechanical strength &gt; 1 MPa. Deformation can be heterogeneous with extrusion along shear zones. Rheological stiffening in the upper conduit also causes large overpressures, shallow seismicity, and cyclic patterns of dome extrusion. Gas‐rich porphyritic andesites tend to be the least mobile kind of lava, because transition from magma into hot crystalline material was reached during ascent.

Nonlinear dynamics of lava dome extrusion

During the eruption of the Soufriere Hills volcano, Montserrat (1995–99), and several other dome eruptions, shallow seismicity, short-lived explosive eruptions and ground deformation patterns indicating large overpressures (of several megapascals) in the uppermost few hundred metres of the volcanic conduit have been observed. These phenomena can be explained by the nonlinear effects of crystallization and gas loss by permeable flow, which are here incorporated into a numerical model of conduit flow and lava dome extrusion. Crystallization can introduce strong feedback mechanisms which greatly amplify the effect on extrusion rates of small changes of chamber pressure, conduit dimensions or magma viscosity. When timescales for magma ascent are comparable to timescales for crystallization, there can be multiple steady solutions for fixed conditions. Such nonlinear dynamics can cause large changes in dome extrusion rate and pulsatory patterns of dome growth.

Migration and accumulation of hydrocarbons in the Swiss Molasse Basin: implications of a 2D basin modeling study

This 2D modelling study attempted to quantify the processes of hydrocarbon (HC) generation, expulsion and migration along a regional section (NW–SE) in the North Alpine Foreland Basin of Western Switzerland. Modelled excess pressures increase towards the Alpine front, and are mainly related to the lithology distribution; excess pressure compartments are centred around shaly or evaporitic intervals of low permeability. By late Jurassic times, a first major phase of HC generation is initiated in the deepest part of the Permo-Carboniferous grabens in the external part (i.e. NW) of the Molasse basin. A second more important generation phase starts in Oligocene–Miocene times in the internal parts close to the Alpine front. Migration of HC seems to be controlled predominantly by the layer-cake geometry of the Mesozoic passive margin sequence. In contrast to many studies where vertical buoyancy driven migration dominates, the main driving mechanism for the migration and accumulation of HC is the excess pressure evolution. Large overpressure zones in the frontal part of the orogen (i.e. Subalpine Molasse) can drive deep fluids far updip into the foreland. The build-up of overpressured zones depends strongly on the subsidence rates, lithology and the occurrence of heterogeneities such as faults. Modelling results suggest that the presence of vertical fault zones have a dramatic influence on the pore pressure evolution (pressure drain-off), and in consequence on the HC accumulation pattern.