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Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania, USA
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- PublicationOpen AccessA pressure vessel for true-triaxial deformation and fluid flow during frictional shear(2007-09-25)
; ; ; ; ; ; ; ; ; ;Marone, C.; Penn. State Univ., USA ;Carperter, B.; Penn. State Univ., USA ;Elsworth, D.; Penn. State Univ., USA ;Faoro, I.; Penn. State Univ., USA ;Ikari, M.; Penn. State Univ., USA ;Knuth, M.; Penn. State Univ., USA ;Niemeijer, A.; Penn. State Univ., USA ;Saffer, D.; Penn. State Univ., USA ;Samuelson, J.; Penn. State Univ., USA; ; ; ; ; ; ; ; ; ; ;Vinciguerra, S.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italiasee Abstract Volume168 493 - PublicationRestrictedMultiphase flow dynamics of pyroclastic density currents during the May 18, 1980 lateral blast of Mount St. Helens(2012-06-26)
; ; ; ; ; ;Esposti Ongaro, T.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italia ;Clarke, A. B.; School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, USA ;Voight, B.; Department of Geosciences, Penn State University, University Park, Pennsylvania, USA ;Neri, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italia ;Widiwijayanti, C.; Earth Observatory of Singapore, Nanyang Technological University, Singapore; ;; ; The dynamics of the May 18, 1980 lateral blast at Mount St. Helens, Washington (USA), were studied by means of a three-dimensional multiphase flow model. Numerical simulations describe the blast flow as a high-velocity pyroclastic density current generated by a rapid expansion (burst phase, lasting less than 20 s) of a pressurized polydisperse mixture of gas and particles and its subsequent gravitational collapse and propagation over a rugged topography. Model results show good agreement with the observed large-scale behavior of the blast and, in particular, reproduce reasonably well the front advancement velocity and the extent of the inundated area. Detailed analysis of modeled transient and local flow properties supports the view of a blast flow led by a high-speed front (with velocities between 100 and 170 m/s), with a turbulent head relatively depleted in fine particles, and a trailing, sedimenting body. In valleys and topographic lows, pyroclasts accumulate progressively at the base of the current body after the passage of the head, forming a dense basal flow depleted in fines (less than 5 wt.%) with total particle volume fraction exceeding 10−1 in most of the sampled locations. Blocking and diversion of this basal flow by topographic ridges provides the mechanism for progressive current unloading. On ridges, sedimentation occurs in the flow body just behind the current head, but the sedimenting, basal flow is progressively more dilute and enriched in fine particles (up to 40 wt.% in most of the sampled locations). In the regions of intense sedimentation, topographic blocking triggers the elutriation of fine particles through the rise of convective instabilities. Although the model formulation and the numerical vertical accuracy do not allow the direct simulation of the actual deposit compaction, present results provide a consistent, quantitative model able to interpret the observed stratigraphic sequence.213 32 - PublicationRestrictedThe role of fault zone fabric and lithification state on frictional strength, constitutive behavior, and deformation microstructure(2011)
; ; ; ;Ikari, M. J.; Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania, USA ;Niemeijer, A. R.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Marone, C.; Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania, USA; ; We examine the frictional behavior of a range of lithified rocks used as analogs for fault rocks, cataclasites and ultracataclasites at seismogenic depths and compare them with gouge powders commonly used in experimental studies of faults. At normal stresses of ∼50 MPa, the frictional strength of lithified, isotropic hard rocks is generally higher than their powdered equivalents, whereas foliated phyllosilicate-rich fault rocks are generally weaker than powdered fault gouge, depending on foliation intensity. Most samples exhibit velocity-strengthening frictional behavior, in which sliding friction increases with slip velocity, with velocity weakening limited to phyllosilicate-poor samples. This suggests that lithification of phyllosilicate-rich fault gouge alone is insufficient to allow earthquake nucleation. Microstructural observations show prominent, throughgoing shear planes and grain comminution in the R1 Riedel orientation and some evidence of boundary shear in phyllosilicate-poor samples, while more complicated, anastomosing features at lower angles are common for phyllosilicate-rich samples. Comparison between powdered gouges of differing thicknesses shows that higher Riedel shear angles correlate with lower apparent coefficients of friction in thick fault zones. This suggests that the difference between the measured apparent friction and the true internal friction depends on the orientation of internal deformation structures, consistent with theoretical considerations of stress rotation.114 17 - PublicationOpen AccessFrictional Strengthening Explored During Non‐Steady State Shearing: Implications for Fault Stability and Slip Event Recurrence Time(2020)
; ; ; ; ; ; ; ; ; On natural faults that host repeating slip events, the inter‐event loading time is quite large compared to the slip event duration. Since most friction studies focus on steady‐state frictional behavior, the fault loading phase is not typically examined. Here, we employ a method specifically designed to evaluate fault strength evolution during active loading, under shear driving rates as low as 10−10 m/s, on natural fault gouge samples from the Waikukupa Thrust in southern New Zealand. These tests reveal that in the early stages of loading following a slip event, there is a period of increased stability, which fades with accumulated slip. In the framework of rate‐ and state‐dependent friction laws, this temporary stable phase exists as long as slip is less than the critical slip distance and the elapsed time is less than the value of the state variable at steady state. These observations indicate a minimum earthquake recurrence time, which depends on the field value of the critical slip distance and the background slip rate. We compare estimates of minimum earthquake recurrence times with the recurrence times of repeating large earthquakes on the Alpine Fault in southern New Zealand and repeating small‐magnitude earthquakes on the San Andreas Fault system in California. We find that the observed recurrence times are mostly longer than the predicted minimum values, and exceptions in the San Andreas system may be explained by elevated slip rates due to larger earthquakes in this region.62 14 - PublicationOpen AccessStability of Clay-rich Fault Gouge at Intermediate to High Shear Strain(2007-09-25)
; ; ; ;Ikari, M.; Penn State, USA ;Marone, C.; Penn State, USA ;Saffer, D.; Penn State, USA; ; ; ; ; ;Vinciguerra, S.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Bernabé, Y.; MIT, USA; see Abstract Volume122 138 - PublicationRestrictedFluid dynamics of the 1997 Boxing Day volcanic blast on Montserrat, West Indies(2008-03-21)
; ; ; ; ; ;Esposti Ongaro, T.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italia ;Clarke, A. B.; Department of Geological Sciences, Arizona State University, Tempe, Arizona, USA ;Neri, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italia ;Voight, B.; Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania, USA ;Widiwijayanti, C.; Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania, USA; ;; ; Directed volcanic blasts are powerful explosions with a significant laterally directed component, which can generate devastating, high-energy pyroclastic density currents (PDCs). Such blasts are an important class of eruptive phenomena, but quantified understanding of their dynamics and effects is still incomplete. Here we use 2-D and 3-D multiparticle thermofluid dynamic flow codes to examine a powerful volcanic blast that occurred on Montserrat in December 1997. On the basis of the simulations, we divide the blast into three phases: an initial burst phase that lasts roughly 5 s and involves rapid expansion of the gas-pyroclast mixture, a gravitational collapse phase that occurs when the erupted material fails to mix with sufficient air to form a buoyant column and thus collapses asymmetrically, and a PDC phase that is dominated by motion parallel to the ground surface and is influenced by topography. We vary key input parameters such as total gas energy and total solid mass to understand their influence on simulations, and we compare the simulations with independent field observations of damage and deposits, demonstrating that the models generally capture important large-scale features of the natural phenomenon. We also examine the 2-D and 3-D model results to estimate the flow Mach number and conclude that the range of damage sustained at villages on Montserrat can be reasonably explained by the spatial and temporal distribution of the dynamic pressure associated with subsonic PDCs.141 16 - PublicationRestrictedMultiphase-flow numerical modeling of the 18 May 1980 lateral blast at Mount St. Helens, USA(2011-06)
; ; ; ; ; ;Esposti Ongaro, T.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italia ;Widiwijayanti, C.; Earth Observatory of Singapore, Nanyang Technological University, Singapore ;Clarke, A. B.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italia ;Voight, B.; Department of Geosciences, Pennsylvania State University, Pennsylvania, USA; U.S. Geological Survey, Cascades Volcano Observatory, Vancouver, Washington, USA ;Neri, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italia; ; ;; Volcanic lateral blasts are among the most spectacular and devastating of natural phenomena, but their dynamics are still poorly understood. Here we investigate the best documented and most controversial blast at Mount St. Helens (Washington State, United States), on 18 May 1980. By means of three-dimensional multiphase numerical simulations we demonstrate that the blast front propagation, final runout, and damage can be explained by the emplacement of an unsteady, stratified pyroclastic density current, controlled by gravity and terrain morphology. Such an interpretation is quantitatively supported by large-scale observations at Mount St. Helens and will influence the definition and predictive mapping of hazards on blast-dangerous volcanoes worldwide.193 24