Greywacke Fault Zone Elastic Properties: Field and Laboratory Constraints

<p><strong>Straddling the Australian-Pacific plate boundary, New Zealand has high rates of tectonic deformation. This deformation produces earthquakes on faults in the brittle upper crust. Within New Zealand, faults are most commonly located in greywacke bedrock, low-grade metamorphosed sandstone and siltstone rocks that comprise the Torlesse Supergroup. In this study, mapping was performed in the Central Southern Alps on the Liebig and Ostler-Great Groove faults, and samples of Rakaia Terrane sandstone, siltstone, and cataclasite rocks were collected for geophysical characterisation. Combined with previously collected Rakaia Terrane greywacke samples, a set of 67 samples were studied. Samples were classified as representative of three phases of the seismic cycle: the intact pre-seismic, fractured coseismic and post-seismic, and mineral sealed interseismic. Elastic wave speeds for the samples were measured using a bench top ultrasonic oscilloscope at room temperature and atmospheric pressure. Results show that intact sandstone has a Vp of 5.37 ± 0.07 km/s and a Vs of 3.26 ± 0.04 km/s, while intact siltstone has a Vp of 5.34 ± 0.10 km/s and a Vs of 3.13 ± 0.08 km/s. Fractured sandstone has a Vp of 4.40 ± 0.16 km/s and a Vs of 2.71 ±0.08 km/s, fractured siltstone has a Vp of 4.73 ± 0.14 km/s and a Vs of 2.73 ± 0.07 km/s and cataclasite has a Vp of 4.59 ± 0.16 km/s and a Vs of 2.80 ± 0.11 km/s. Sealed sandstone has a Vp of 5.26 ± 0.06 and Vs of 3.17 ± 0.02 km/s, while siltstone has a Vp of 5.35 ± 0.04 km/s and a Vs of 3.09 ± 0.05 km/s. Five samples showed intrinsic P-wave anisotropy of 16% to 33% and S-wave anisotropy of 9% to 17%. Laboratory measurements of P- and S-wave velocities, combined with grain densities and porosities determined using a nitrogen pycnometer and the water submersion method, were used to calculate the elastic moduli. Comparing elastic moduli across three phases of the seismic cycle reveals important trends. Intact samples, indicative of preseismic rocks that have not experienced brittle deformation, have a bulk modulus of 39.4 ± 3.82 GPa and shear modulus of 26.43 ± 1.5 GPa. Fractured rocks, having experienced brittle coseismic deformation, exhibit a reduction in elastic moduli, with a bulk modulus of 28.7 ± 6.78 GPa and shear modulus of 19.37 ± 3.07 GPa. Interseismically, when vein-forming minerals precipitate, sealing open fractures, the elastic moduli increase to intact values. Sealed rocks have a bulk modulus of 39.03 ± 3.72 GPa and shear modulus of 27.73 ± 2.08 GPa. Using the elastic wave velocity and density results, three numerical models were constructed to calculate the seismic anisotropy exhibited by: (1) vertically dipping isotropic layers of intact sandstone and siltstone; (2) vertically dipping intact sandstone interlayered with sandstone layers containing randomly oriented fractures; and (3) a vertically dipping simulated fault zone comprising a central fault core composed of sandstone containing randomly oriented fractures surrounded on both sides by layers of sealed sandstone and intact sandstone Model results show that alternating sandstone and siltstone layers produce minimal anisotropy (0.01%). In contrast, alternating layers of intact and fractured rock exhibit a 2.5% P-wave anisotropy and a 2.2% S-wave anisotropy. The fault zone model produces a maximum P-wave anisotropy of 17.3% and a S-wave anisotropy of 9.5%.</strong></p><p>This study highlights the roles that seismic cycle processes, specifically coseismic fracturing and interseismic sealing through mineral precipitation from fluids, play in controlling greywacke sandstone and siltstone elastic moduli. The findings also have implications for the interpretation of geophysical data, showing that fractured rocks interlayered with intact and/or sealed rocks can produce seismic wave anisotropy. Moreover, the work underscores the importance of integrating field and laboratory research to understand rock properties and their applications.</p>