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Laboratory evidence of strength recovery of a healed fault: implications for a mechanism responsible for creating wide fault zones
© Masuda. 2015
Received: 12 August 2015
Accepted: 17 December 2015
Published: 24 December 2015
Fault zones consist of a high-strain fault core and a surrounding damage zone of highly fractured rock. The close, reciprocal relationship between fault zones and earthquake rupture evolution demands better understanding of the processes that create and modify damage zones. This study modeled the evolution of a damage zone in the laboratory by monitoring seismic signals (acoustic emissions) in a specimen of ultramylonite stressed to failure. The result provided evidence supporting the strength recovery of parts of the healed surface. A new fault initiated in an area of heterogeneous structure a short distance from the preexisting fault plane. Repeated cycles of fracture and healing may be one mechanism responsible for wide fault zones with multiple fault cores and damage zones.
The structure and development of fault zones are closely related to the evolution of earthquake ruptures. Although fault zones occupy a small volume of the crust, they strongly affect crustal dynamic processes such as earthquakes and fluid flow in the crust (e.g., Faulkner et al. 2010). Fault zones consist of a core, in which strain is high and localized, surrounded by a highly fractured region called the damage zone (e.g., Chester and Logan 1986; Chester et al. 1993). Damage zones also influence fluid flow properties and the frictional strength of faults through their effects on pore pressure (Caine et al. 1996).
The dynamic processes responsible for creating off-fault damage have been modeled mostly on the basis of data obtained in field surveys of well-exposed damage zones (Blenkinsop 2008; Mitchell and Faulkner 2009; Fukuchi et al. 2014). Despite a wealth of field evidence and models, however, our understanding of these dynamic processes is incomplete. Recently, Valoroso et al. (2014) used high-resolution earthquake distributions to argue that during the coseismic phase of the seismic cycle, most of an earthquake’s energy, which produces the damage zone, is concentrated along the main fault plane. At laboratory scale, Lockner et al. (1991) and Lockner (1993) investigated the evolution of acoustic emissions (AEs) in time and space and showed that AE clusters characterize the development of fault zones in intact rocks. Many earlier works have studied the effect of heterogeneity in a rock sample on failure processes (e.g., Lei et al. 2004; Jouniaux et al. 2001). These previous works show that precursory anomalies related to rock fracture are strongly dependent on the heterogeneity of the rock sample. In this study, I examined the strength recovery of a healed plane by comparing the locations of a preexisting healed plane and a new fracture plane. Fault zones are typically characterized by the presence of many healed surfaces, the strength of which is unknown. If a healed fault recovers its strength such that its strength is equal to or greater than that of the intact host rock, then repeated cycles of fracture and healing may be one mechanism producing wide fault zones with multiple fault cores and damage zones. In this paper, I present laboratory evidence supporting the strength recovery of at least some parts of a healed fault surface, obtained by using state-of-the-art AE monitoring and X-ray computed tomography (CT) imaging techniques. Because of the well-established scale similarity of fault zones (Faulkner et al. 2010), loading experiments in the laboratory allow earthquakes and the evolution of fault zones to be studied under well-controlled conditions (e.g., Masuda et al. 2012).
A conventional triaxial compression test of the ultramylonite cylinder was conducted using a constant loading rate of 31.7 kPa s–1 under 50 MPa of confining pressure, which prevented initial cracks in the sample from having any effect. Macroscopic failure of the sample occurred when the applied differential stress reached 439 MPa, close to the strength under similar conditions of the intact granite host rock (e.g., Jouniaux et al. 2001). The transducers recorded AEs during the loading experiment. Velocities of P waves that propagated perpendicular to the loading axis were measured intermittently by the pulse transmission method and used to calculate the hypocenters of AE events. The space and time variations of AE source locations, which included the effects of heterogeneity and anisotropy of wave speed associated with accumulated damage in the rock sample during the deformation, were examined. Hypocenters were determined by automatically picking the first arrivals of P waves using the technique of Lei et al. (2004). Hypocenter locations with probability errors smaller than 2 mm (Lei et al. 2004) were used in this study.
The internal structure of the rock sample was imaged by X-ray CT before and after the loading experiment, using the method of Jouniaux et al. (2001). I used a conventional medical X-ray CT scanning system to obtain images of a scanned volume 160 mm in diameter in slices perpendicular to the sample axis; each slice was 1.0 mm thick and divided into a 512 × 512 voxel grid. The resolution of the resulting images was about 0.3 mm, and light-colored regions in the images corresponded to areas of high density. Density contrasts, which probably reflect the presence of pores and cracks in the internal structure, generally reflect the strength distribution in the sample rock. The slices were digitally combined into three-dimensional models displaying the shapes and locations of the preexisting and new fracture planes.
Discussion and conclusion
The structure of fault zones has been studied in field surveys of exhumed fault zones (Faulkner et al. 2003; Shigematsu et al. 2009). High-resolution earthquake locations determined in a field survey have shown a close relationship between seismicity and fault plane features (Valoroso et al. 2014), and high-resolution hypocenter determinations of natural seismicity have shown that active fault zones are narrow features (Hauksson 2010; Powers and Jordan 2010). Valoroso et al. (2014) speculated that coseismic deformation is concentrated along the main fault plane and is responsible for producing the observed damage zone structure. The loading experiment in this study corroborates these earlier observations.
Tomoaki Tomita contributed to the experimental part of this study and data analysis as a part of his master’s thesis at the University of Tsukuba. Y. Kobayashi, O. Nishizawa, and X. Lei contributed to the early stages of this work. Constructive comments by the Editor and reviewers were very helpful to improve this manuscript. A part of this work was supported by JSPS KAKENHI Grant Number 15K06221.
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