25 research outputs found
A Nonextensive Statistical Physics Analysis of the 1995 Kobe, Japan Earthquake
No full textThis paper presents an analysis of the distribution of earthquake magnitudes for the period 1990–1998 in a broad area surrounding the epicenter of the 1995 Kobe earthquake. The frequency–magnitude distribution analysis is performed in a nonextensive statistical physics context. The nonextensive parameter q M , which is related to the frequency-magnitude distribution, reflects the existence of long-range correlations and is used as an index of the physical state of the studied area. Examination of the possible variations of q M values is performed during the period 1990–1998. A significant increase of q M occurs some months before the strong earthquake on April 9, 1994 indicating the start of a preparation phase prior to the Kobe earthquake. It should be noted that this increase coincides with the occurrence of six seismic events. Each of these events had a magnitude M = 4.1. The evolution of seismicity along with the increase of q M indicate the system’s transition away from equilibrium and its preparation for energy release. It seems that the variations of q M values reflect rather well the physical evolution towards the 1995 Kobe earthquake
Experimental investigation of the mechanical properties of synthetic magnesium sulfate hydrates: Implications for the strength of hydrated deposits on Mars
No full textWe have carried out uniaxial compression experiments to determine the mechanical properties of three crystalline magnesium sulfate hydrates that may be present in the near-surface environment of Mars. Our synthetic samples of kieserite (MgSO4 center dot H2O), epsomite (MgSO4 center dot 7H(2)O), and meridianiite (MgSO4 center dot 11H(2)O) have mean values of unconfined compressive strength of 6.3 +/- 0.7, 12.9 +/- 1.8, and 30.1 +/- 4.5 MPa, respectively, Young's modulus of 0.8 +/- 0.1, 2.9 +/- 0.4, and 5.9 +/- 0.8 GPa, respectively, and mean porosity values of 47.8% +/- 0.5%, 11.1% +/- 0.6%, and 2.9% +/- 0.2%, respectively. Although our tests cannot quantify a systematic relationship between hydration state and mechanical properties, the different porosities produced by consistent sample preparation methods suggest that the addition of non-cation-coordinated water molecules likely reduces the strength of individual sulfate hydrate phases. However, the bulk mechanical properties of our synthetic specimens are instead controlled predominantly by the sample porosity; generally, the strength increases as the porosity decreases. We expect the mechanical properties of sulfate hydrate deposits on Mars to be governed by the bulk porosity rather than the strength of the pure solid phase. We have performed cyclic stressing tests, replicating possible periodic depositional and erosional periods on Mars resulting from obliquity changes. A gradual compaction and reduction in sample porosity, rather than an increase in crack damage, is observed with each loading cycle, suggesting that the evolution of mechanical properties will depend on local factors such as bulk density, in addition to the overall stress history
Electric potential changes prior to shear fracture in dry and saturated rocks
No full textElectric potential changes before shear rupture were measured using Darley Dale sandstone (quartz-rich) and Icelandic basalt (quartz-free) on both dry specimens and in the presence of pore fluid. We find that electric potential changed markedly just prior to dynamic rupture in dry and saturated sandstones and saturated basalt but we did not detect precursory signals in dry basalt. The absence of signals in dry basalt provides strong evidence that the piezoelectric effect and electrokinetic effect are dominant sources for precursory signals. Moreover we find that the amplitude of the precursory signals due to electrokinetic effect in saturated sandstone were as large as the coseismic signals. We propose that this signal is caused by accelerating evolution of dilatancy as cracks grow in the rock before rupture, resulting in water flow into the dilatant region with an electric current produced concurrently
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A rate and state friction law for saline ice
No full textSea ice friction models are necessary to predict the nature of interactions between sea ice floes. These interactions are of interest on a range of scales, for example, to predict loads on engineering structures in icy waters or to understand the basin-scale motion of sea ice. Many models use Amonton's friction law due to its simplicity. More advanced models allow for hydrodynamic lubrication and refreezing of asperities; however, modeling these processes leads to greatly increased complexity. In this paper we propose, by analogy with rock physics, that a rate-and state-dependent friction law allows us to incorporate memory (and thus the effects of lubrication and bonding) into ice friction models without a great increase in complexity. We support this proposal with experimental data on both the laboratory (similar to 0.1 m) and ice tank (similar to 1 m) scale. These experiments show that the effects of static contact under normal load can be incorporated into a friction model. We find the parameters for a first-order rate and state model to be A = 0.310, B = 0.382, and mu(0) = 0.872. Such a model then allows us to make predictions about the nature of memory effects in moving ice-ice contact
Evidence for seismogenic fracture of silicic magma
No full textIt has long been assumed that seismogenic faulting is confined to cool, brittle rocks, with a temperature upper limit of 600 °C (ref. 1). This thinking underpins our understanding of volcanic earthquakes, which are assumed to occur in cold rocks surrounding moving magma. However, the recent discovery of abundant brittle–ductile fault textures in silicic lavas2, 3, 4 has led to the counter-intuitive hypothesis that seismic events may be triggered by fracture and faulting within the erupting magma itself. This hypothesis is supported by recent observations of growing lava domes, where microearthquake swarms have coincided with the emplacement of gouge-covered lava spines5, 6, leading to models of seismogenic stick-slip along shallow shear zones in the magma7. But can fracturing or faulting in high-temperature, eruptible magma really generate measurable seismic events? Here we deform high-temperature silica-rich magmas under simulated volcanic conditions in order to test the hypothesis that high-temperature magma fracture is seismogenic. The acoustic emissions recorded during experiments show that seismogenic rupture may occur in both crystal-rich and crystal-free silicic magmas at eruptive temperatures, extending the range of known conditions for seismogenic faulting
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Critical slip and time dependence in sea ice friction
No full textRecent research into sea ice friction has focussed on ways to provide a model which maintains much of the clarity and simplicity of Amonton's law, yet also accounts for memory effects. One promising avenue of research has been to adapt the rate- and state- dependent models which are prevalent in rock friction. In such models it is assumed that there is some fixed critical slip displacement, which is effectively a measure of the displacement over which memory effects might be considered important. Here we show experimentally that a fixed critical slip displacement is not a valid assumption in ice friction, whereas a constant critical slip time appears to hold across a range of parameters and scales. As a simple rule of thumb, memory effects persist to a significant level for 10 s. We then discuss the implications of this finding for modelling sea ice friction and for our understanding of friction in general
Evolution of the mechanics of the 2004–2008 Mt. St. Helens lava dome with time and temperature
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Experimental and theoretical fracture mechanics applied to fracture of the crust of Venus
No full textMapping of closely spaced, parallel extensional fractures in the Guinevere and Sedna Planitia regions of Venus reveals a concentric pattern of fractures around the edge of the large topographic rise of Western Eistla Regio. We have constructed 13 transects through these closely spaced parallel fractures (CSPF) and find a mean spacing of between 0.8 and 1.2 km. A two-dimensional, nonlayered, fracture mechanics computer model for the formation of CSPF is described. For cracks extended by a remote tensile stress the stress intensity factor controls the depth of penetration, which in turn, governs the spacing between adjacent cracks based upon the stress-shadow principle. The stress required to initiate cracks depends upon the strength of the material; thus spacing of CSPF depends
only on the physical properties of the preexisting rock mass. Using a new fracture mechanics apparatus designed to simulate Venusian conditions (90 bar CO2, 450C) the
fracture toughness of basalt was measured for confining pressures up to 20 MPa and for temperature up to 600C. Fracture toughness was found to increase from 2.4 MPam1/2 at ambient pressure to 3.0 MPam1/2 at 10 MPa confining pressure. Fracture toughness showed no clear trend with temperature. The experimental results for fracture toughness suggest that the preexisting rock mass that fits best with observations contains an inverse square distribution of flaws with maximum sizes of only 20–80 cm and that a remote tensile stress of 3–6 MPa is required to form the observed CSPF
