Peter Sammonds

Evidence for seismogenic fracture of silicic magma

It 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 lavas 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 spines, leading to models of seismogenic stick-slip along shallow shear zones in the magma. 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.

Experimental and theoretical fracture mechanics applied to Antarctic ice fracture and surface crevassing

Recent disintegration of ice shelves on the Antarctic Peninsula has highlighted the need for a better understanding of ice shelf fracture processes generally. In this paper we present a fracture criterion, incorporating new experimental fracture data, coupled with an ice shelf flow model to predict the spatial distribution of surface crevassing on the Filchner-Ronne Ice Shelf. We have developed experiments that have enabled us to quantify, for the first time, quasi-stable crack growth in Antarctic ice core specimens using a fracture initiation toughness, Kinit, for which crack growth commences. The tests cover a full range of near-surface densities, ρ = 560–871 kg m−3 (10.9–75.7 m depth). Results indicate an apparently linear dependence of fracture toughness on porosity such that Kinit = 0.257 ρ-80.7, predicting a zero-porosity toughness of Ko = 155 kPa m1/2. We have used this data to test the applicability to crevassing of a two-dimensional fracture mechanics criterion for the propagation of a small sharp crack in a biaxial stress field. The growth of an initial flaw into a larger crevasse, which involves a purely tensile crack opening, depends on the size of the flaw, the magnitude of Kinit and the nature of the applied stress field. By incorporating the criterion into a stress map of the Filchner-Ronne Ice Shelf derived from a depth-integrated finite element model of the strain-rate field, we have been able to predict regions of potential crevassing. These agree well with satellite imagery provided an initial flaw size is assumed in the range 5–50 cm.

Comparison of earthquake strains over 102 and 104 year timescales: Insights into variability in the seismic cycle in the central Apennines, Italy

In order to study the existence of possible deficits or surpluses of geodetic and earthquake strain in the Lazio-Abruzzo region of the central Apennines compared to 15 +/- 3 kyrs multi seismic cycle strain-rates, horizontal strain-rates are calculated in 5 km x 5 km and 20 km x 20 km grid squares using slip-vectors from striated faults and offsets of Late Pleistocene-Holocene landforms and sediments. Strain-rates calculated over 15 +/- 3 kyrs within 5 km x 5 km grid squares vary from zero up to 2.34 +/- 0.54 x 10(-7) yr(-1) and resolve variations in strain orientations and magnitudes along the strike of individual faults. Surface strain-rates over a time period of 15 +/- 3 kyrs from 5 km x 5 km grid squares integrated over an area of 80 km x 160 km shows the horizontal strain-rate of the central Apennines is 1.18(-0.04)(+0.12)x10(-8) yr(-1) and -1.83(-4.43)(+3.80) x 10(-10) yr(-1) parallel and perpendicular to the regional principal strain direction (043 degrees-223 degrees+/-1 degrees), associated with extension rates of Ms 6.0) magnitude historical earthquakes have been reported to be as low as zero. This demonstrates the importance of comparing the exact same areas and that strain-rates vary spatially on the length scale of individual faults and on a timescale between 10(2) yr and 10(4) yr in the central Apennines. We use these results to produce a fault specific earthquake recurrence interval map and discuss the regional deformation related to plate boundary and sub-crustal forces, temporal earthquake clustering and the natural variability of the seismic cycle.

Fracture toughness measurements on igneous rocks using a high-pressure, high-temperature rock fracture mechanics cell

Abstract A sound knowledge of mechanical properties of rocks at high temperatures and pressures is essential for modelling volcanological problems such as fracture of lava flows and dike emplacement. In particular, fracture toughness is a scale-invariant material property of a rock that describes its resistance to tensile failure. A new fracture mechanics apparatus has been constructed enabling fracture toughness measurements on large (60 mm diameter) rock core samples at temperatures up to 750°C and pressures up to 50 MPa. We present a full description of this apparatus and, by plotting fracture resistance as a function of crack length, show that the size of the samples is sufficient for reliable fracture toughness measurements. A series of tests on Icelandic, Vesuvian and Etnean basalts at temperatures from 30 to 600°C and confining pressures up to 30 MPa gave fracture toughness values between 1.4 and 3.8 MPa m1/2. The Icelandic basalt is the strongest material and the Etnean material sampled from the surface crust of a lava flow the weakest. Increasing temperature does not greatly affect the fracture toughness of the Etnean or Vesuvian material but the Icelandic samples showed a marked increase in toughness at around 150°C, followed by a return to ambient toughness levels. This material also became tougher under moderate confining pressure but the other two materials showed little change in toughness. We describe in terms of fracture mechanics probable causes for the changes in fracture toughness and compare our experimental results with values obtained from dike propagation modelling found in the literature.

Ionic surface electrical conductivity in sandstone

Recent analyses of complex conductivity measurements have indicated that high-frequency dispersions encountered in rocks saturated with low-salinity fluids are due to ionic surface conduction and that the form of these dispersions may be dependent upon the nature of the pore and crack surfaces within the rock (Ruffet et al., 1991). Unfortunately, the mechanisms of surface conduction are not well understood, and no model based on rigorous physical principles exists. This paper is split into two parts: an experimental section followed by the development of a theoretical description of adsorption of ions onto mineral surfaces. We have made complex conductivity measurements upon samples of sandstone saturated with a range of different types and concentrations of aqueous solution with a frequency range of 20 Hz to 1 MHz. The frequency dependence of complex conductivity was analyzed using the empirical model of Cole and Cole (1941). The “fractal” surface models of Le Mehaute and Crepy (1983), Po Zen Wong (1987), and Ruffet et al. (1991) were used to calculate apparent fractal pore surface dimensions for samples saturated with different solution types and concentrations. These showed a pronounced decrease of apparent fractal surface dimension with decreasing electrolyte concentration and a decrease of apparent fractal dimension with increasing relative ionic radius of the dominant cation in solution. A model for ionic surface concentration (ISCOM I) has been developed as the first step in producing a rigorous physicochemical model of surface conduction in quartz-dominated rocks. The results from ISCOM I show that quartz surfaces are overwhelmingly dominated by adsorbed Na+ when saturated with NaCl solutions of salinities and pH found in actual geological situations. ISCOM I also shows that the concentration threshold for dominance of surface conduction over bulk conduction is aided by depletion of ions from the bulk fluid as a result of their adsorption onto the mineral surfaces as well as by changes in the ionic mobility in the surface conduction double-layer as the wetting solution becomes more dilute.

Temporal variations in seismic event rate and b-values from stress corrosion constitutive laws

Abstract Main, I.G., Meredith, P.G. and Sammonds, P.R., 1992. Temporal variations in seismic event rate and b-values from stress corrosion constitutive laws. In: T. Mikumo, K. Aki, M. Ohnaka, L.J. Ruff and P.K.P. Spudich (Editors), Earthquake Source Physics and Earthquake Precursors. Tectonophysics, 211: 233–246. A model is developed to characterise the state of damage for a fractal population of cracks in terms of a mean energy release rate 〈G〉which depends on the stress σ, the event rate N and the seismic b-value. N is assumed to be proportional to the total number of potentially active cracks, and b to be proportional to the exponent D of the crack length distribution. 〈G〉is then positively correlated with N and negatively correlated with b, consistent with experimental observation based on stress intensity as a constitutive variable. The model predicts that higher b-values are associated either with lower stress intensity or greater material heterogeneity. Both experiment and theory predict intermediate-term seismic quiescence of similar magnitude to that observed in the field (45–90%), for only a small simultaneous reduction in stress and stress intensity (2–7%) or the equivalent mean energy release rate. The theoretical model predicts three different types of quiescence associated with increasing, constant or decreasing b-value. The first two are associated with stable intermediate-term quiescence due to a reduction in 〈G〉, and the third is associated with unstable short-term quiescence associated with constant or increasing 〈G〉. The theory can be used to infer relative changes in 〈G〉 from acoustic emissions in the laboratory. Experiments to date show that quiescence of the order 10% can be seen in the laboratory at strain rates of 10−5 s−1, but that this has the characteristics of short-term rather than intermediate-term quiescence.

Electric potential changes prior to shear fracture in dry and saturated rocks

Electric 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.

INFLUENCE OF FRACTAL FLAW DISTRIBUTIONS ON ROCK DEFORMATION IN THE BRITTLE FIELD

Abstract The geometrical distribution of flaws plays a crucial role in the physical behaviour of geological materials under stress. Flaws are present in the earth on all scales, from microcracks to plate-rupturing faults. They may be distributed on one characteristic length scale (e.g. joints, ‘characteristic’ earthquakes), or more commonly exhibit scale-invariance over a specified range of sizes. Scale-invariance implies that the discrete length distribution in a finite range is a power law of negative exponent D, where 1 ≤ D < 3. Fault systems where motion is concentrated on a dominant fault (e.g. San Andreas) have D ≈ 1, but more diffuse fault systems have D near 2. D is one of the fractal dimensions of the fracture system. The length distribution of faults or microcracks may be inferred from the slope b of the log-linear frequency—magnitude distribution of earthquakes, or laboratory-scale acoustic emissions, since it can be shown that D = 3b/c. The scaling factor c depends on the relative time constants of the seismic event and the recording instrument, and is usually equal to 3/2. b is found experimentally to be negatively correlated with the stress intensity on the dominant flaw, which depends in turn on the applied stress and the flaw length. Thus a fracture mechanics model of rock failure which includes a range of flaw sizes can be tested by seismic monitoring. We describe a fracture mechanics model of rock failure for a variety of styles of deformation, ranging from elastic failure to quasi-static cataclastic flow, and predict the time-dependence of D and the seismic b-value at different times up to and including failure. Critical coalescence of microcracks during dynamic failure (e.g. earthquake foreshocks) occurs when D = 1 (b = 0.5); random processes (e.g. cataclastic flow, background seismicity) are associated with D = 2 (b = 1); positive feedback in the concentration of stress on the dominant flaw (e.g. during strain softening and shear localisation) occurs when D < 2 (b < 1); negative feedback in stress concentration (e.g. during the early stages of dilatancy), and where a highly diffuse fracture system is produced, occurs at low stress intensities and is associated with D > 2 (b > 1). It has long been a goal of structural geologists to measure stress on rocks, since most geometrical signatures of deformation are strain-related. We show that stress is not usually as significant in rock fracture as stress intensity, and furthermore that the geometric signature of the length distribution of microcracks is well-correlated with the stress intensity.

Pathways for degassing during the lava dome eruption of Mount St. Helens 2004–2008

The ability of volatiles to escape rising magma regulates the explosivity of a volcanic system. During silicic lava dome eruptions, strain localization at the conduit margin occurs during magma ascent, creating a damage halo with implications for gas escape. Here we report the first systematic study of permeability network anisotropy across the marginal shear zone of the A.D. 2004–2008 lava dome at Mount St. Helens (Washington State, USA). The results show increasingly large permeability anisotropy of as much as four orders of magnitude (over ∼4 m) moving from the interior of the spine through the damage halo. We find the permeability to be essentially isotropic in the spine interior but highly anisotropic in the damage zone and fault core. Our examination of the dome rocks reveals that the permeability anisotropy depends strongly on the presence of vertically oriented shear layers. Here we show that the rate of escape of volatiles will be several orders of magnitude higher vertically through a conduit margin shear zone than horizontally into the conduit wall.

Fracturing of Etnean and Vesuvian rocks at high temperatures and low pressures

Abstract The mechanical properties of volcanic rocks at high temperatures and low pressures are key properties in the understanding of a range of volcanological problems, in particular lava flow dynamics. The measurement of these properties on extrusive volcanic samples under the appropriate pressure and temperature conditions has a direct application in the assessment of volcanic hazards. A new triaxial deformation cell has been designed to obtain mechanical strength data on rock samples at temperatures up to 1000°C and pressures up to 30 MPa. Significantly, the cell uses large cylindrical rock specimens, 25 mm diameter by 75 mm long, never previously employed in such a high-temperature apparatus. The large specimen size is necessary to test volcanic rocks with their large crystals and vesicles. The design of this novel apparatus is presented. Its operating temperature and pressure range encompasses the conditions of an advancing flow from the vent to the front, as well as the conditions of the volcanic rocks hosting magma at equivalent depths of up to 2 km. Experimental results are presented for tests on Vesuvian and Etnean rocks. Results show that the Vesuvius and the Etnean rocks remain fully brittle up to 600°C with typical strengths of 90 MPa and 100 MPa and Young’s moduli of 60 GPa and 40 GPa, respectively. Above these temperatures the elastic modulus and compressive strength decreases steadily in both the Vesuvian and Etnean rocks, reaching 10% of the original values at 900°C and 800°C, respectively, when partial melting occurred. Full melting occurs at 1100°C in the Vesuvian rock and at 1040°C in the Etnean rock. Results also show that confining pressure has only a small effect on the strength of the rock at these low pressures, and that strain rates are important at high temperatures. Fracture energy release rates have been calculated and show an inversely proportional relationship with temperature. Results reveal why fracturing is important on the crust of the lava flow as much as at the flow front where the flow has almost completely solidified but still maintains high temperatures. These results put quantitative limits on models where fracturing processes play an important role.