Seismic Anisotropy and Its Impact on Imaging the Shallow Alpine Fault: An Experimental and Modeling Perspective

Abstract

The transpressional Alpine Fault in New Zealand has created a thick shear zone with associated highly anisotropic rocks. Low seismic velocity zones and high seismic reflectivity are recorded in the Alpine Fault Zone, but no study has explored the underlying physical rock parameters of the shallow crust that control these observations. Protomylonites are the volumetrically dominant lithology of the fault zone. Here we combine experimental measurements of P‐wave speeds with numerical models of elastic wave anisotropy of protomylonite samples to explore how the fault zone can be seismically imaged. Numerical models that account for the porosity‐free real samples' fabric elastic tensors from electron backscatter diffraction (EBSD) are calculated by MTEX and a finite element model (FEM), while microfractures are modeled with differential effective medium (DEM) theory. At effective pressures representative of the Alpine Fault brittle zone, experimental wave speeds are lower than those predicted by MTEX/FEM. A possible DEM model suggests that a combination of random and aligned microfractures with aspect ratios increasing with pressure can explain the experimental wave speeds for pressures <70 MPa. Such microporosity in the form of foliation‐ and mica basal plane‐parallel microfractures and grain boundaries is validated with synchrotron X‐ray microtomography and transmission electron microscopy (TEM) images. Finally, by modeling anisotropy of seismic reflection coefficients with angle of incidence, we demonstrate that the high reflectivity and low‐velocity zone (LVZ) observed at the Alpine Fault can only be explained if this microporosity is accounted for throughout the brittle fault zone, even at depths of 7–10 km.

Fracture Shape and Orientation Contributions to P-Wave Velocity and Anisotropy of Alpine Fault Mylonites

“…Moreover, grain size insensitive creep-accommodated ductile shear resulted in a CPO of quartz and plagioclase as well as their SPO (Toy et al, 2008). Velocity measurements under confining pressure also show directionally, non-uniform changes in P-wave velocities with depth which implies that aligned fractures are present (Adam et al, 2020;Simpson et al, 2020).…”

“…Previous studies using seismic data (Savage et al, 2007;Karalliyadda and Savage, 2013), core measurements (Okaya et al, 1995;Godfrey et al, 2000;Christensen and Okaya, 2007;Allen et al, 2017;Simpson et al, 2019;Adam et al, 2020;Jeppson and Tobin, 2020) and numerical modeling (Godfrey et al, 2002;Dempsey et al, 2011;Adam et al, 2020;Simpson et al, 2020) demonstrate that the rock in the vicinity of the Alpine Fault is strongly anisotropic. The anisotropy is attributed to many factors such as mineral crystallographic preferred orientation (CPO), grain shape preferred orientation (SPO), foliation and fractures.…”

“…These factors should be incorporated into seismic velocity modeling. Adam et al (2020) showed that open grain boundaries and microfractures can enhance the seismic wave anisotropy at all depths of the seismogenic zone in the Alpine Fault. Simpson et al (2020) predict the P-wave anisotropy of shallow Alpine Fault rocks.…”

“…Within the first 10 km depth, the P-wave velocity of the low-velocity zone is 5.5-6.0 km/s compared to the velocity outside the zone of 6.0-6.5 km/s. This low-velocity zone is commonly interpreted to be due to the presence of fluid-filled microfractures, but experimental and model data needed to validate such a claim are rare (Allen et al, 2017;Adam et al, 2020). The fault damage zone has been investigated using fault guided (trapped) seismic wave observation and modeling (Eccles et al, 2015;Kelly et al, 2017).…”

“…The anisotropy is attributed to many factors such as mineral crystallographic preferred orientation (CPO), grain shape preferred orientation (SPO), foliation and fractures. The Alpine Fault rocks contain significant (>15 vol%) mica with preferred orientation (Dempsey et al, 2011;Adam et al, 2020). Dempsey et al (2011) has shown how the volume of aligned mica influences P-wave anisotropy in mylonite from the Alpine Fault.…”