Earthquake model experiments in a viscoelastic fluid: A scaling of decreasing magnitudes of earthquakes with depth (original) (raw)
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Shallow earthquakes in a viscoelastic shear zone with depth���dependent friction and rheology
1986
Most crustal earthquakes of the world are observed to occur within a seismogenic layer which extends from the Earth's surface to a depth of a few tens of kilometres at most. A model is proposed in which the shear zone along a transcurrent plate margin is represented as a viscoelastic medium with depth-dependent power-law rheology. A frictional resistance linearly increasing with depth is assumed on a vertical transcurrent fault within the shear zone. Such a model is able to reproduce a continuous transition from the brittle behaviour of the upper crust to the ductile behaviour at depth. Assuming that the shear zone is subjected to a constant strain rate from the opposite motions of the two adjacent plates, it is found that there exists a maximum depth H below which tectonic stress can never reach the frictional threshold: this may be identified as the maximum depth of earthquake nucleation. The value o f H is consistent with observations for plausible values of the model parameters. The stress evolution in the shear zone is calculated in the linear approximation of the constitutive equation. A change in rigidity with depth, which is also introduced in the model, may reproduce the high vertical gradient of shear stress, which has been measured across the San Andreas fault, and the fact that most earthquakes are nucleated at some depth in the seismogenic layer. A crack which drops the ambient stress to the dynamic frictional level is then introduced in the model. To this aim, a crack solution is employed without a stress singularity at its edges, which is compatible with a frictional stress threshold criterion for fracture. A constraint on the vertical friction gradient is obtained if such cracks are assumed to be entirely confined within the seismogenic layer.
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.
Effects of Fluid Migration on the Evolution of Seismicity
Involvement of Fluids in Earthquake Ruptures, 2017
In this chapter, we study theoretically how high-pressure fluid affects seismic activity, specifically focusing on the sequence of seismic activity whose duration is much shorter than the interseismic periods, but much longer than duration times of dynamic fault slips. Typical examples of such activity will be aftershocks and earthquake swarm; seismological observations actually suggest, as described in Chap. 2, that high-pressure fluid affects the nucleation and evolution of these activities. We study here how the fluid flow and slip are coupled in aftershocks and earthquake swarm. Although stochastic modeling has also been made by some authors, it is beyond the scope of the present analysis and we focus on physical modeling. It will be useful, in the modeling, to distinguish the triggering mechanism from the driving one. Although it is clear that a mainshock triggers the aftershock sequence, several mechanisms have been proposed about the driving mechanism of aftershock sequence. For earthquake swarm, even the triggering mechanism is not very clear although indirect evidence of fluid effects is mentioned rather frequently. At the end of this chapter, we will mention a possibility to comprehensively understand the generation mechanisms of aftershocks, earthquake swarm, and slow slip event, in a single modeling framework, in terms of slip-induced dilatancy and fluid flow induced by the dilatancy. Here, we do not aim at quantitative understanding of observations; our focus is rather on the qualitative understanding based on simple models. Keywords Aftershocks Á Aseismic slip Á Dilatancy Á Earthquake swarm 6.1 Interactions Between Slip Evolution and Change of Hydromechanical Properties of Fault Zone Injection experiments of high-pressure fluid may have an advantage in understanding how the fluid pressurization and its flow induce seismic events because the fluid injection is controllable and dense observation network can be deployed in advance before the occurrence of seismic activity. Such experiments strongly
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
Viscoelastic-coupling model for the earthquake cycle driven from below
Journal of Geophysical Research, 2000
In a linear system the earthquake cycle can be represented as the sum of a solution which reproduces the earthquake cycle itself (viscoelastic-coupling model) and a solution that provides the driving force. We consider two cases, one in which the earthquake cycle is driven by stresses transmitted along the schizosphere and a second in which the cycle is driven from below by stresses transmitted along the upper mantle (i.e., the schizosphere and upper mantle, respectively, act as stress guides in the lithosphere). In both cases the driving stress is attributed to steady motion of the stress guide, and the upper crust is assumed to be elastic. The surface deformation that accumulates during the interseismic interval depends solely upon the earthquakecycle solution (viscoelastic-coupling model) not upon the drivingS source solution. Thus geodetic observations of interseismic deformation are insensitive to the source of the driving forces in a linear system. In particular, the suggestion of Bourne et al. [1998] that the deformation that accumulates across a transform fault system in the interseismic interval is a replica of the deformation that accumulates in the upper mantle during the same interval does not appear to be correct for linear systems.
Journal of Geophysical Research, 1984
Vertical displacements due to periodic reverse faulting events in an elastic plate overlying a viscoelastic (Maxwell) half space are obtained and compared with the observed deformation cycle (coseismic strain release, postseismic transients, interseismic strain accumulation) from Japan. The viscoelastic effects, including the influence of buoyant restoring forces, are obtained using the method developed by Rundle, and plate convergence and strain accumulation are incorporated following the procedure suggested by Savage. The resulting deformation cycle is compared with that of an analogous elastic half-space dislocation model in which postearthquake effects are due to transient aseismic slip below the coseismic fault. Cyclic deformation is similar but not identical for the two models, and observations from southwest Japan suggest the superiority of the viscoelastic coupling model. In particular, inclusion of the effects of steady state flow in the asthenosphere overcomes a defect of the elastic half-space model and results in agreement with the observed interseismic movement pattern. Several aspects of the postseismic deformation, its landward migration, and its transition to the interseismic phase of the cycle are explained as well, but the short duration of near-trench transients relative to those observed farther inland is not matched. The success of a buried slip model in explaining early postseismic near-trench movements and asthenospheric flow in accounting for cumulative postearthquake transient motions suggests the existence of a transition zone between lithosphere and asthenosphere whose behavior is brittle/elastic in the short term and ductile for longer-term deformation, and such a modification may reconcile remaining discordant observations. However, reasonable variations in coupling model parameters cannot account for observed differences in the deformation cycle in other parts of Japan, and these regional differences remain unexplained.
Global postseismic deformation: Deep earthquakes
Journal of Geophysical Research, 2000
We study the global viscoelastic deformations associated with a shear dislocation on a fault embedded in a viscoelastic mantle. To address this problem, we extend the theory of the quasi-static deformations of a Maxwell, spherical, N-layer incompressible Earth, previously limited to the modeling of the effects of earthquakes occurring only within the elastic lithosphere. Permanent postseismic deformations, which can be generated by lithospheric sources, cannot be sustained if the source region is viscoelastic; however, the transient response of the mantle strongly depends on the viscosity of the source region. We use the technique developed here to investigate thoroughly the quasi-static surface deformations induced by seismic events of variable depth. We show that owing to the combined effect of sphericity and viscoelastic mantle relaxation, the surface displacements do not systematically decrease a,s the depth of the source increases. Instead, with increasing source depth we predict an increasing efficiency of mantle relaxation in triggering postseismic deformations of large size. Most of our results are based on a simple four-layer model, with a 100-km-thick lithosphere, upper and lower mantle separated by the 670-km discontinuity, and a fluid core; the last part of this work is devoted to a study of the effects of a low-viscosity asthenosphere on the rates of deformation detected at the Earth's surface. The findings reported here may be useful for the interpretation of the transient motions of the Earth's surface in response to deep-focus earthquakes. 1. Introduction When an earthquake occurs, the Earth reacts elastically and deforms instantaneously. This coseismic deformation tends to be permanent, since a seismic event occurring within an elastic portion of the Earth acts as a permanent distribution of body forces. Owing to its long-term fluid behavior, a viscoelastic mantle cannot sustain the elastic shear stresses generated by the earthquake. Therefore it will deform quasi-statically until a new state of equilibrium is reached, where the shear stresses within the mantle vanish. In analogy with the effects of a surface mass load, we expect that the coseismic deformations be followed by a, large-scale delayed viscoelastic readjustment. The timescales which characterize this relaxation, as well a.s their amplitude, depend on the rheological profile of the lithosphere and
Viscoelastic Triggering Between Large Earthquakes along the East Kunlun Fault System
Chinese Journal of Geophysics, 2003
We study stress transfer and triggering of large earthquakes along the East Kunlun fault system, northern Tibetan Plateau. Five M ≥7 earthquakes occurred along the fault zone during the past 70 years are considered: the 1937 M 7.5 Huashi Canyon, the 1963 MS7.1 Dulan, the 1973 MS7.3 Manyi, the 1997 MS7.5 Manyi, and the 2001 MW7.8 Kokoxili earthquakes.
Fluid pressurisation and earthquake propagation in the Hikurangi subduction zone
Nature Communications, 2021
In subduction zones, seismic slip at shallow crustal depths can lead to the generation of tsunamis. Large slip displacements during tsunamogenic earthquakes are attributed to the low coseismic shear strength of the fluid-saturated and non-lithified clay-rich fault rocks. However, because of experimental challenges in confining these materials, the physical processes responsible for the coseismic reduction in fault shear strength are poorly understood. Using a novel experimental setup, we measured pore fluid pressure during simulated seismic slip in clay-rich materials sampled from the deep oceanic drilling of the Pāpaku thrust (Hikurangi subduction zone, New Zealand). Here, we show that at seismic velocity, shear-induced dilatancy is followed by pressurisation of fluids. The thermal and mechanical pressurisation of fluids, enhanced by the low permeability of the fault, reduces the energy required to propagate earthquake rupture. We suggest that fluid-saturated clay-rich sediments, o...
Geophysical Research Letters, 1999
Following the Mw 6.7 Northridge earthquake, significant postseismic displacements were resolved with GPS. Using a three-dimensional viscoelastic model, we suggest that this deformation is mainly driven by viscous flow in the lower crust. Such flow can transfer stress to the upper crust and load the rupture zone of the main shock at a decaying rate. Most aftershocks within the rupture zone, especially those that occurred after the first several weeks of the main shock, may have been triggered by continuous stress loading from viscous flow. The long-term decay time of aftershocks (about 2 years) approximately matches the decay of viscoelastic loading, and thus is controlled by the viscosity of the lower crust. Our model provides a physical interpretation of the observed correlation between aftershock decay rate and surface heat flow.