ALBERT

Description: We examine the frictional behavior of a range of lithified rocks used as analogs for fault rocks, cataclasites and ultracataclasites at seismogenic depths and compare them with gouge powders commonly used in experimental studies of faults. At normal stresses of ∼50 MPa, the frictional strength of lithified, isotropic hard rocks is generally higher than their powdered equivalents, whereas foliated phyllosilicate-rich fault rocks are generally weaker than powdered fault gouge, depending on foliation intensity. Most samples exhibit velocity-strengthening frictional behavior, in which sliding friction increases with slip velocity, with velocity weakening limited to phyllosilicate-poor samples. This suggests that lithification of phyllosilicate-rich fault gouge alone is insufficient to allow earthquake nucleation. Microstructural observations show prominent, throughgoing shear planes and grain comminution in the R1 Riedel orientation and some evidence of boundary shear in phyllosilicate-poor samples, while more complicated, anastomosing features at lower angles are common for phyllosilicate-rich samples. Comparison between powdered gouges of differing thicknesses shows that higher Riedel shear angles correlate with lower apparent coefficients of friction in thick fault zones. This suggests that the difference between the measured apparent friction and the true internal friction depends on the orientation of internal deformation structures, consistent with theoretical considerations of stress rotation.

Description: Published

Description: B08404

Description: JCR Journal

Description: restricted

Description: Narrow, highly-comminuted shear localization features in faults, known as principal slip zones (PSZs), are commonly associated with large-offset seismogenic faults. In this study, laboratory friction experiments were performed using shale and slate gouges where deformation was encouraged to localize at the gouge–wall-rock boundary. The slate gouges develop a black, narrow PSZ composed of densely packed submicron particles that appear sintered while the spectator gouge remains largely undeformed. These PSZs form at subseismic slip velocities of ~10 –5 m/s and with a calculated temperature rise of ~3 °C. Instances of velocity-weakening friction, which is necessary for unstable fault slip, are only observed for slate samples with a PSZ; shale gouges, however, do not develop a PSZ and exhibit only velocity-strengthening frictional behavior. The development of a PSZ may therefore be a prerequisite for future earthquake slip to occur, rather than unequivocal evidence of past earthquake slip.

Description: Seismicity patterns offshore Costa Rica (Central America) at the Middle America Trench have led to speculation that large (moment magnitude, M w ~7.0) earthquakes are associated with subducting topographic highs. In areas of high basement topography, a regionally extensive nannofossil chalk unit is exposed at the seafloor on the incoming plate, whereas in regions of low basement topography, hemipelagic clay-rich sediment is exposed. Because the entire sediment section is subducted at this margin, lithologic variation in the uppermost subducting sediments may control plate boundary fault behavior. Our laboratory experiments reveal that the chalk is frictionally strong (µ = 0.71–0.88) and characterized by velocity-weakening and stick-slip behavior, notably at elevated temperature. In contrast, the hemipelagic sediment is weak (µ = 0.22–0.35) and in many cases velocity strengthening. We suggest that the presence of frictionally unstable carbonates at bathymetric highs may play a key, previously unrecognized, role in governing earthquake nucleation.

Description: The near-surface areas of major faults commonly contain weak, phyllosilicate minerals, which, based on laboratory friction measurements, are assumed to creep stably. However, it is now known that shallow faults can experience tens of meters of earthquake slip and also host slow and transient slip events. Laboratory experiments are generally performed at least two orders of magnitude faster than plate tectonic speeds, which are the natural driving conditions for major faults; the absence of experimental data for natural driving rates represents a critical knowledge gap. We use laboratory friction experiments on natural fault zone samples at driving rates of centimeters per year to demonstrate that there is abundant evidence of unstable slip behavior that was not previously predicted. Specifically, weak clay-rich fault samples generate slow slip events (SSEs) and have frictional properties favorable for earthquake rupture. Our work explains growing field observations of shallow SSE and surface-breaking earthquake slip, and predicts that such phenomena should be more widely expected.

Description: The Hikurangi subduction zone hosts shallow slow-slip events, possibly extending to the seafloor. The mechanisms allowing for this behavior are poorly understood but are likely a function of the frictional properties of the downgoing seafloor sediments. We conducted friction experiments at a large range of effective stresses, temperatures, and velocities on incoming sediment to the Hikurangi subduction zone to explore the possible connection of frictional properties to slow-slip events. These experiments were conducted on multiple apparatuses, allowing us to access a wider range of deformation conditions than is available on any one machine. We find that the material frictionally weakens and becomes less velocity strengthening with increasing effective stress, whereas temperature has only a small effect on both friction and frictional stability. When driven at the plate convergence rate, the sediment exhibits velocity-weakening behavior. These results imply that the frictional properties of the sediment package subducting at Hikurangi could promote slow-slip events at the pressures, temperatures, and strain rates expected along the plate boundary thrust up to 10-km depth without requiring elevated pore fluid pressures. The transition to velocity-strengthening behavior at faster slip rates could provide a mechanism for limiting unstable slip to slow-sliding velocities, rather than accommodating deformation through ordinary earthquakes. ©2018. The Authors.

Description: Slow slip events (SSEs) at the northern Hikurangi subduction margin, New Zealand, are among the best-documented shallow SSEs on Earth. International Ocean Discovery Program Expeditions 372 and 375 were undertaken to investigate the processes and in situ conditions that underlie subduction zone SSEs at the northern Hikurangi Trough. We accomplished this goal by (1) coring and geophysical logging at four sites, including penetration of an active thrust fault (the Pāpaku fault) near the deformation front, the upper plate above the SSE source region, and the incoming sedimentary succession in the Hikurangi Trough and atop the Tūranganui Knoll seamount; and (2) installing borehole observatories in the Pāpaku fault and in the upper plate overlying the slow slip source region. Logging-while-drilling (LWD) data for this project were acquired as part of Expedition 372, and coring, wireline logging, and observatory installations were conducted during Expedition 375. Northern Hikurangi subduction margin SSEs recur every 1–2 y and thus provide an ideal opportunity to monitor deformation and associated changes in chemical and physical properties throughout the slow slip cycle. In situ measurements and sampling of material from the sedimentary section and oceanic basement of the subducting plate reveal the rock properties, composition, lithology, and structural character of material that is transported downdip into the SSE source region. A recent seafloor geodetic experiment raises the possibility that SSEs at northern Hikurangi may propagate to the trench, indicating that the shallow thrust fault (the Pāpaku fault) targeted during Expeditions 372 and 375 may also lie in the SSE rupture area and host a portion of the slip in these events. Hence, sampling and logging at this location provides insights into the composition, physical properties, and architecture of a shallow fault that may host slow slip. Expeditions 372 and 375 were designed to address three fundamental scientific objectives: Characterize the state and composition of the incoming plate and shallow fault near the trench, which comprise the protolith and initial conditions for fault zone rock at greater depth and which may itself host shallow slow slip; Characterize material properties, thermal regime, and stress conditions in the upper plate directly above the SSE source region; and Install observatories in the Pāpaku fault near the deformation front and in the upper plate above the SSE source to measure temporal variations in deformation, temperature, and fluid flow. The observatories will monitor volumetric strain (via pore pressure as a proxy) and the evolution of physical, hydrological, and chemical properties throughout the SSE cycle. Together, the coring, logging, and observatory data will test a suite of hypotheses about the fundamental mechanics and behavior of SSEs and their relationship to great earthquakes along the subduction interface.