Philip M. Benson

Laboratory simulation of volcano seismicity

The physical processes generating seismicity within volcanic edifices are highly complex and not fully understood. We report results from a laboratory experiment in which basalt from Mount Etna volcano (Italy) was deformed and fractured. The experiment was monitored with an array of transducers around the sample to permit full-waveform capture, location, and analysis of microseismic events. Rapid post-failure decompression of the water-filled pore volume and damage zone triggered many low-frequency events, analogous to volcanic long-period seismicity. The low frequencies were associated with pore fluid decompression and were located in the damage zone in the fractured sample; these events exhibited a weak component of shear (double-couple) slip, consistent with fluid-driven events occurring beneath active volcanoes.

Reconstructing magma failure and the degassing network of dome-building eruptions

Volcanic eruptions are regulated by the rheology of magmas and their ability to degas. Both detail the evolution of stresses within ascending subvolcanic magma. But as magma is forced through the ductile-brittle transition, new pathways emerge as cracks nucleate, propagate, and coalesce, constructing a permeable network. Current analyses of magma dynamics center on models of the glass transition, neglecting important aspects such as incremental strain accommodation and (the key monitoring tool of) seismicity. Here, in a combined-methods study, we report the first high-resolution (20 μm) neutron-computed tomography and microseismic monitoring of magma failure under controlled experimental conditions. The data reconstruction reveals that a competition between extensional and shear fracturing modes controls the total magnitude of strain-to-failure and importantly, the geometry and efficiency of the permeable fracture network that regulates degassing events. Extrapolation of our findings yields magma ascent via strain localization along conduit margins, thereby providing an explanation for gas-and-ash explosions along arcuate fractures at active lava domes. We conclude that a coupled deformation-seismicity analysis holds a derivation of fracture mechanisms and network, and thus holds potential application in forecasting technologies.

Imaging slow failure in triaxially deformed Etna basalt using 3D acoustic-emission location and X-ray computed tomography

We have deformed basalt from Mount Etna (Italy) in triaxial compression tests under an effective confining pressure representative of conditions under a volcanic edifice (40 MPa), and at a constant strain rate of 5 similar to 10(-6) s(-1). Despite containing a high level of pre-existing microcrack damage, Etna basalt retains a high strength of 475 MPa. We have monitored the complete deformation cycle through contemporaneous measurements of axial strain, pore volume change, compressional wave velocity change and acoustic emission (AE) output. We have been able to follow the complete evolution of the throughgoing shear fault without recourse to any artificial means of slowing the deformation. Locations of AE events over time yields an estimate of the fault propagation velocity of between 2 and 4 mm. s(-1). We also find excellent agreement between AE locations and post-test images from X-ray microtomography scanning that delineates deformation zone architecture.

Laboratory simulations of fluid/gas induced micro-earthquakes: application to volcano seismology

Understanding different seismic signals recorded in active volcanic regions allows geoscientists to derive insight into the processes that generate them. A key type is known as Low Frequency or Long Period (LP) event, generally understood to be generated by different fluid types resonating in cracks and faults. The physical mechanisms of these signals have been linked to either resonance/turbulence within fluids, or as a result of fluids ‘sloshing’ due to a mixture of gas and fluid being present in the system. Less well understood, however, is the effect of the fluid type (phase) on the measured signal. To explore this, we designed an experiment in which we generated a precisely controlled liquid to gas transition in a closed system by inducing rapid decompression of fluid-filled fault zones in a sample of basalt from Mt. Etna Volcano, Italy. We find that fluid phase transition is accompanied by a marked frequency shift in the accompanying microseismic dataset that can be compared to volcano seismic data. Moreover, our induced seismic activity occurs at pressure conditions equivalent to hydrostatic depths of 200 to 750 meters. This is consistent with recently measured dominant frequencies of LP events and with numerous models.

Imaging compaction band propagation in Diemelstadt sandstone using acoustic emission locations

We report results from a conventional triaxial test performed on a specimen of Diemelstadt sandstone under an effective confining pressure of 110 MPa; a value sufficient to induce compaction bands. The maximum principal stress was applied normal to the visible bedding so that compaction bands propagated parallel to bedding. The spatio-temporal distribution of acoustic emission events greater than 40 dB in amplitude, and associated with the propagation of the first compaction band, were located in 3D, to within +/- 2 mm, using a Hyperion Giga-RAM recorder. Event magnitudes were used to calculate the seismic b- value at intervals during band growth. Results show that compaction bands nucleate at the specimen edge and propagate across the sample at approximately 0.08 mm s(-1). The seismic b-value does not vary significantly during deformation, suggesting that compaction band growth is characterized by small scale cracking that does not change significantly in scale.

Relating pore fabric geometry to acoustic and permeability anisotropy in Crab Orchard Sandstone: A laboratory study using magnetic ferrofluid

Pore fabric anisotropy is a common feature of many sedimentary rocks. In this paper we report results from a comparative study on the anisotropy of a porous sandstone (Crab Orchard) using anisotropy of magnetic susceptibility (AMS), acoustic wave velocity and fluid permeability techniques. Initially, we characterise the anisotropic pore fabric geometry by impregnating the sandstone with magnetic ferro-fluid and measuring its AMS. The results are used to guide subsequent measurements of the anisotropy of acoustic wave velocity and fluid permeability. These three independent measures of anisotropy are then directly compared. Results show strong positive correlation between the principal directions given from the AMS, velocity anisotropy and permeability anisotropy. Permeability parallel to the macroscopic crossbedding observed in the sandstone is 240% higher than that normal to it. P and S-wave velocity anisotropy and AMS show mean values of 19.1%, 4.8% and 3.8% respectively, reflecting the disparate physical properties measured.

Role of void space geometry in permeability evolution in crustal rocks at elevated pressure

A key consequence of the presence of void space within rock is their significant influence upon fluid transport properties. In this study, we measure changes in elastic wave velocities (P and S) contemporaneously with changes in permeability and porosity at elevated pressure for three rock types with widely different void space geometries: a high-porosity sandstone (Bentheim), a tight sandstone (Crab Orchard), and a microcracked granodiorite (Takidani). Laboratory data are then used with the permeability models of Gueguen and Dienes and Kozeny-Carman to investigate the characteristics that different void space geometries impart to measured permeabilities. Using the Kachanov effective medium theory, elastic wave velocities are inverted, permitting the recovery of crack density evolution with increasing effective pressure. The crack densities are then used as input to the microcrack permeability model of Gueguen and Dienes. The classic Kozeny-Carman approach of Walsh and Brace is also applied to the measured permeability data via a least squares fit in order to extract tortuosity data. We successfully predict the evolution of permeability with increasing effective pressure, as directly measured in experiments, and report the contrast between permeability changes observed in rock where microcracks or equant pores dominate the microstructure. Additionally, we show how these properties are affected by anisotropy of the rock types via the measured anisotropic fabrics in each rock. The combined experimental and modeling results illustrate the importance of understanding the details of how rock microstructure changes in response to an external stimulus in predicting the simultaneous evolution of different rock physical properties.

Volcanic conduit failure as a trigger to magma fragmentation

In the assessment of volcanic risk, it is often assumed that magma ascending at a slow rate will erupt effusively, whereas magma ascending at fast rate will lead to an explosive eruption. Mechanistically viewed, this assessment is supported by the notion that the viscoelastic nature of magma (i.e., the ability of magma to relax at an applied strain rate), linked via the gradient of flow pressure (related to discharge rate), controls the eruption style. In such an analysis, the physical interactions between the magma and the conduit wall are commonly, to a first order, neglected. Yet, during ascent, magma must force its way through the volcanic edifice/structure, whose presence and form may greatly affect the stress field through which the magma is trying to ascend. Here, we demonstrate that fracturing of the conduit wall via flow pressure releases an elastic shock resulting in fracturing of the viscous magma itself. We find that magma fragmentation occurred at strain rates seven orders of magnitude slower than theoretically anticipated from the applied axial strain rate. Our conclusion, that the discharge rate cannot provide a reliable indication of ascending magma rheology without knowledge of conduit wall stability, has important ramifications for volcanic hazard assessment. New numerical simulations are now needed in order to integrate magma/conduit interaction into eruption models.

The propagation and seismicity of dyke injection, new experimental evidence

To reach the surface, dykes must overcome the inherent tensile strength of the country rock. As they do they generate swarms of seismic signals, frequently used for forecasting. In this study we pressurize and inject molten acrylic into an encapsulating host rocks of 1) Etna basalt and 2) Comiso limestone; at 30 MPa of confining pressure. Fracture was achieved at 12 MPa for Etna basalt, 7.2 MPa for Comiso limestone. The generation of radial fractures was accompanied by acoustic emissions (AE) at a dominant frequency of 600 kHz. During “magma” movement in the dykes, AE events of approximately 150 kHz dominant frequency were recorded. We interpret our data using AE location and dominant frequency analysis, concluding that the seismicity associated with magma transport in dykes peaks during initial dyke creation but remains significant as long as magma movement continues. These results have important implications for seismic monitoring of active volcanoes.

On the generation mechanisms of fluid‐driven seismic signals related to volcano tectonics

The generation mechanics of fluid-driven volcano seismic signals, and their evolution with time, remains poorly understood. We present a laboratory study aiming to better constrain the time evolution of such signals across temperature conditions 25 to 175 °C in order to simulate a “bubbly liquid”. Simulations used pressures equivalent to volcanic edifices up to 1.6 km in depth using a triaxial deformation apparatus equipped with an array of Acoustic Emission (AE) sensors. We investigate the origin of fluid driven seismic signals by rapidly venting the pore pressure through a characterized damage zone. During the release of water at 25 °C broadband signals were generated, with frequencies ranging from 50 to 160 kHz. However the decompression of a water/steam phase at 175 °C generated a bi-modal spectrum of different signals, in the range 100 kHz and 160 kHz. These new results are consistent with natural signals from active volcanoes, such as Mt. Etna, and highlight the role of fluid and gas phases (such as “bubbly liquids”) in generating different types of volcano-tectonic seismicity.