Fluid pressurisation and earthquake propagation in the Hikurangi subduction zone (original) (raw)
Related papers
Drilling reveals fluid control on architecture and rupture of the Alpine fault, New Zealand
Geology, 2012
Rock damage during earthquake slip affects fl uid migration within the fault core and the surrounding damage zone, and consequently coseismic and postseismic strength evolution. Results from the fi rst two boreholes (Deep Fault Drilling Project DFDP-1) drilled through the Alpine fault, New Zealand, which is late in its 200-400 yr earthquake cycle, reveal a >50-m-thick "alteration zone" formed by fl uid-rock interaction and mineralization above background regional levels. The alteration zone comprises cemented low-permeability cataclasite and ultramylonite dissected by clay-fi lled fractures, and obscures the boundary between the damage zone and fault core. The fault core contains a <0.5-m-thick principal slip zone (PSZ) of low electrical resistivity and high spontaneous potential within a 2-m-thick layer of gouge and ultracataclasite. A 0.53 MPa step in fl uid pressure measured across this zone confi rms a hydraulic seal, and is consistent with laboratory permeability measurements on the order of 10-20 m 2. Slug tests in the upper part of the boreholes yield a permeability within the distal damage zone of ~10-14 m 2 , implying a six-orders-of-magnitude reduction in permeability within the alteration zone. Low permeability within 20 m of the PSZ is confi rmed by a subhydrostatic pressure gradient, pressure relaxation times, and laboratory measurements. The low-permeability rocks suggest that dynamic pressurization likely promotes earthquake slip, and motivates the hypothesis that fault zones may be regional barriers to fl uid fl ow and sites of high fl uid pressure gradient. We suggest that hydrogeological processes within the alteration zone modify the permeability, strength, and seismic properties of major faults throughout their earthquake cycles.
The role of fluids in lower-crustal earthquakes near continental rifts
Nature, 2007
The occurrence of earthquakes in the lower crust near continental rifts has long been puzzling, as the lower crust is generally thought to be too hot for brittle failure to occur 1,2 . Such anomalous events have usually been explained in terms of the lower crust being cooler than normal . But if the lower crust is indeed cold enough to produce earthquakes, then the uppermost mantle beneath it should also be cold enough 2 , and yet uppermost mantle earthquakes are not observed 5 . Numerous lower-crustal earthquakes occur near the southwestern termination of the Taupo Volcanic Zone (TVZ), an active continental rift in New Zealand 6 . Here we present three-dimensional tomographic imaging of seismic velocities and seismic attenuation in this region using data from a dense seismograph deployment 7 . We find that crustal earthquakes accurately relocated with our three-dimensional seismic velocity model form a continuous band along the rift, deepening from mostly less than 10 km in the central TVZ to depths of 30-40 km in the lower crust, 30 km southwest of the termination of the volcanic zone. These earthquakes often occur in swarms, suggesting fluid movement in critically loaded fault zones 8 . Seismic velocities within the band are also consistent with the presence of fluids, and the deepening seismicity parallels the boundary between high seismic attenuation (interpreted as partial melt) within the central TVZ and low seismic attenuation in the crust to the southwest. This linking of upper and lower-crustal seismicity and crustal structure allows us to propose a common explanation for all the seismicity, involving the weakening of faults on the periphery of an otherwise dry, mafic crust by hot fluids, including those exsolved from underlying melt. Such fluids may generally be an important driver of lower-crustal seismicity near continental rifts.
As a result of the comminution that takes place over numerous earthquake cycles, mature faults are characterized by thick layers of pulverized gouge with finite porosity that is saturated with water at seismogenic depths. The heat generated during earthquakes raises the gouge temperature and thermal expansion of the pore fluid, and surrounding solids produce elevated pore pressures that cause fault strength to decrease in the process known as thermal pressurization. Building upon this framework, we describe a model that imposes a plane-strain configuration and shows that the stress variations caused by porothermoelasticity promote the Mohr-Coulomb failure of previously undeformed regions. Except in special cases where the friction is rate strengthening, we find that the frictional strength must vary throughout the post-failure region, which we identify in our model with the shear zone. We introduce a strain rate function that describes the overall influence of distributed slip on energy dissipation and fault strength as the shear zone thickness expands. Using typical fault parameters at 7 km depth, the shear zone reaches several millimeters of thickness after 1 s sliding at an overall rate of 1 m/s. The expansion of the shear zone limits the temperature rise to several hundred degrees Celsius, and the average fault strength falls to about a tenth of the static frictional strength.
Journal of Geophysical Research, 2003
The record of physical processes that occur during seismic slip events is well preserved in fault rocks from the active Nojima fault in Japan. The fault rocks formed at about 3 km depth, and comprise thin alternating layers of very fine gouge and pseudotachylyte derived from granite. Each layer is thinner than a few millimeters, and corresponds to one seismic slip event. The very thin slip zone width suggests that some mechanisms of slip weakening operated, and our studies of the fault rocks suggest that fluidization and melting of gouge were particularly important. Fluidized and nonfluidized gouges were distinguished using the detection probability of fragmented counterparts method. It is known from granular material science that the phase transition from a grain friction regime to fluidization of granular materials can occur only by a very small decrease in volume fraction of solid grains. Once fluidization occurs, the frictional resistance decreases abruptly to nearly zero even b...
A geological fingerprint of low-viscosity fault fluids mobilized during an earthquake
Journal of Geophysical Research, 2009
1] The absolute value of stress on a fault during slip is a critical unknown quantity in earthquake physics. One of the reasons for the uncertainty is a lack of geological constraints in real faults. Here we calculate the slip rate and stress on an ancient fault in a new way based on rocks preserved in an unusual exposure. The study area consists of a fault core on Kodiak Island that has a series of asymmetrical intrusions of ultrafinegrained fault rock into the surrounding cataclasite. The intrusive structures have ductile textures and emanate upward from a low-density layer. We interpret the intrusions as products of a gravitational (Rayleigh-Taylor) instability where the spacing between intrusions reflects the preferred wavelength of the flow. The spacing between intrusions is 1.4 ± 0.5 times the thickness of the layer. This low spacing-to-thickness ratio cannot be explained by a low Reynolds number flow but can be generated by one with moderate Reynolds numbers. Using a range of density contrasts and the geometry of the outcrop as constraints, we find that the distance between intrusions is best explained by moderately inertial flow with fluid velocities on the order of 10 cm/s. The angle that the intrusions are bent over implies that the horizontal slip velocity was comparable to the vertical rise velocity, and therefore, the fault was slipping at a speed of order 10 cm/s during emplacement. These slip velocities are typical of an earthquake or its immediate afterslip and thus require a coseismic origin. The Reynolds number of the buoyant flow requires a low viscous stress of at most 20 Pa during an earthquake.
Mechanical effect of fluid migration on the complexity of seismicity
Journal of Geophysical Research, 1997
Spatio-temporal variation of earthquake activity is modeled assuming fluid migration in a narrow porous fault zone whose boundaries are impermeable. The duration of earthquake sequence is assumed to be much shorter than the recurrence period of characteristic events on the fault. Principle of the effective stress coupled to the Coulomb failure criterion introduces mechanical coupling between fault slip and pore fluid pressure. A linear relation is assumed in our simulations between the accumulated slip and fault zone width on the basis of laboratory and field observations. High complexity is observed in the rupture activity so long as an inhomogeneity is introduced in the spatial distribution of initial strength, which is defined as the fracture threshold stress before the intrusion of the fluid. Frequency-magnitude statistics of intermediate-size events obeys the Gutenberg-Richter relation for all the models in which spatial heterogeneity is introduced for the initial strength. The behavior of larger-size events seems to be rather model dependent. It is also observed that the rupture occurrence tends to be inactivated immediately before the occurrence of the largest event in a sequence. This never happens if a brittle rupture is assumed in an elastic medium with no mechanical effect of fluid. This inactivation will occur because it takes much time to build up fluid pressure to break a fault segment having high initial strength, whose rupture triggers the largest event in a sequence. Our calculations also show that a single predominant principal event cannot be observed in a sequence when both the variance and average value of the distributed initial strengths are large. This may explain a feature observed for earthquake swarm. tive stress coupled to the Coulomb failure criterion [Raleigh et al., 1976], which is thought to be applicable at least to the top few kilometers of the crust [Raleigh et al., 1976; Zoback and Hickman, 1982; Zoback and Healy, 1984; Hickman et al., 1995]. The role of pore fluid in reducing the effective value of the confining stress in bulk samples and the normal stress across frictional surfaces has been demonstrated in laboratory experiments [e.g., Brace and Martin, 1968; Byeflee and Brace, 1972]. Field evidence comes from earthquakes induced either through direct injection of fluids down boreholes or from the filling of large reservoirs with subsequent infiltration of water into the underlying rock mass [e.g., Healy et al., 1968; Raleigh et al., 1976]. Additional evidence of mechanical involvement of fluids in earthquake faulting comes from the substantial change in groundwater level, and surface discharge before and after some earthquakes in the shallow crust. For example, the variation in groundwater flow is observed at many locations before and after the 1995 Hyogoken-Nanbu (Kobe) earthquake; Tsunogai and Wakita [1995] report a steady increase in C1-and SO•-concentrations with time from
Earth and Planetary Science Letters, 2021
Pore-fluid pressure is an important parameter in controlling fault mechanics as it lowers the effective normal stress, allowing fault slip at lower shear stress. It is also thought to influence the nature of fault slip, particularly in subduction zones where areas of slow slip have been linked to regions of elevated pore-fluid pressure. Despite the importance of pore-fluid pressure on fault mechanics, its role on controlling fault stability, which is determined by the friction rate parameter (−), is poorly constrained, particularly for fault materials from subduction zones. In the winter of 2018-19 the accretionary complex overlying Nankai Trough subduction zone (SW Japan) was drilled as part of Integrated Ocean Drilling Program (IODP) Expedition 358. Here we test the frictional stability of the accretionary sediments recovered during the expedition by performing a series of velocity-stepping experiments on powdered samples (to simulate fault gouge) while systematically varying the pore-fluid pressure and effective normal stress conditions. The Nankai gouges, despite only containing 25% phyllosilicates, are strongly rate-strengthening and exhibit negative values for the rate-and-state parameter. We find that for experiments where the effective normal stress is held constant and the pore-fluid pressure is increased the Nankai gouges become more rate-strengthening, and thus more stable. In contrast, when the pore-fluid pressure is held constant and the effective normal stress is varied, there is minimal effect on the frictional stability of the gouge. The increase in frictional stability of the gouge at elevated pore-fluid pressure is caused by an evolution in the rate-and-state parameter , which becomes more negative at high pore-fluid pressure. These results have important implications for understanding the nature of slip in subduction zones and suggest the stabilizing effect of pore-fluid pressure could promote slow slip or aseismic creep on areas of the subduction interface that might otherwise experience earthquake rupture.
Do flexural stresses explain the mantle fault zone beneath Kilauea volcano?
Geophysical Journal International, 2007
Recent relocation and focal mechanism analyses of deep earthquakes beneath Kilauea volcano, Hawaii indicate that seismicity is concentrated on a horizontal fault zone at a depth of 30 km, with seaward slip of the upper block on a low-angle plane. We discuss whether the observed localization of the earthquakes can be explained primarily by stresses induced by flexure of the Pacific Plate beneath the Hawaiian load. We find that flexural stresses are consistent with the observed fault plane orientation, and the direction and rate of slip. The mechanisms of mantle earthquakes in other regions of Hawaii are also consistent with the flexural calculation. However, the model has four shortcomings: (1) the fault zone is displaced 15-20 km to the NW of the region of predicted maximum shear stress; (2) the maximum shear stress on preferred fault planes in the vicinity of the fault zone seems too low to overcome Coulomb friction (by about a factor of 2, assuming hydrostatic pore pressure); (3) the fault zone is much more localized laterally than is the region of large flexural stresses and stressing rates and (4) the fault zone is more localized vertically than might be inferred from the calculation as well. Simple and plausible extensions of the plate flexure model that account for spatial variations in the location of pore fluids, and/or the possible existence of a passive low shear stress magma transport system can overcome most of these shortcomings. Several magma pipes would be necessary to explain the observed earthquake locations, and simple thermal arguments indicate that such pipes could be conduits for porous flow if they are a few kilometres in radius.
Bulletin of the Seismological Society of America
Coseismic rupture of the 19-km-long north-striking and west-dipping sinistral reverse Papatea fault and nearby structures and uplift/translation of the Papatea block are two of the exceptional components of the 14 November 2016 M w 7.8 Kaikōura earthquake. The dual-stranded Papatea fault, comprising main (sinistral reverse) and western (dip-slip) strands, ruptured onshore and offshore from south of Waipapa Bay to George Stream in the north, bounding the eastern side of the Papatea block. Fault rupture mapping was aided by the acquisition of multibeam bathymetry, light detection and ranging (lidar) topography and other imagery, as well as differential lidar (D-lidar) from along the coast and Clarence River valley. On land, vertical throw and sinistral offset on the Papatea fault was assessed across an aperture of 100 m using uncorrected D-lidar and field data to develop preliminary slip distributions. The maximum up-to-the-west throw on the main strand is ∼9:5 0:5 m, and the mean throw across the Papatea fault is ∼4:5 0:3 m. The maximum sinistral offset, measured near the coast on the main strand, is ∼6:1 0:5 m. From these data, and considering fault dip, we calculate a maximum net slip of 11:5 2 m and an average net slip of 6:4 0:2 m for the Papatea fault surface rupture in 2016. Large sinistral reverse displacement on the Papatea fault is consistent with uplift and southward escape of the Papatea block as observed from Interferometric Synthetic Aperture Radar (InSAR) and optical image correlation datasets. The throw and net slip are exceedingly high for the length of the Papatea fault; such large movements likely only occur during multifault Kaikōura-type earthquakes that conceivably have recurrence times of ≥ 5000-12; 000 yrs. The role of the Papatea fault in the Kaikōura earthquake has significant implications for characterizing complex fault sources in seismic hazard models. Electronic Supplement: Rupture descriptions for minor faults, figures of detailed bathymetry and vertical throw, far-field profiles of deformation, and surface rupture photographs, and table of site localities with fault and slip information.