Nucleation and early seismic propagation of small and large events in a crustal earthquake model (original) (raw)
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Elastodynamic analysis of earthquake sequences on slowly loaded faults with rate and state friction
2000
Lapusta et al., 2000(2) have developed an efficient and rigorous numerical pro- cedure for elastodynamic analysis of earthquake sequences on slowly loaded faults. This is done for a general class of rate and state friction laws with positive direct velocity effect. We use the procedure to study the response of a 2-D strike-slip fault model with depth-variable properties. We find the fol- lowing (as partially reported by Lapusta et al., 2000(2)): Small events appear in increasing numbers for decreasing values of the characteristic slip distance of the friction law. The nucleation phase of small and large events is very similar. For a large event that is preceded by a small event (and hence hetero- geneous stress distribution), moment acceleration in the beginning of dynamic propagation exhibits "slow-downs" and subsequent "speed-ups", consistently with some observations. Insufficient time and space discretization qualita- tively changes the results. Incorporating ...
Earthquake nucleation on dip-slip faults
Journal of Geophysical Research: Solid Earth, 2004
The nucleation of unstable slip on a fault is of key importance in our understanding of the seismic cycle. We investigate how the asymmetric geometry of dip-slip faults affects the nucleation of unstable slip on such faults. Previous researchers have devoted much effort to understanding this nucleation process on geometrically simple faults, using a variety of frictional parameterizations. However, there are many reasons to believe that earthquake nucleation may be affected by fault complexity. The breakdown of symmetry on a nonvertical dip-slip fault means that normal stress is not constant during the slip process and that the hanging wall and footwall may not necessarily move equally. Using a slip-strengthening and-weakening friction law in a two-dimensional quasi-static model based on a variational boundary integral method, we show that nucleation on dip-slip faults is affected by both the dip angle and the direction of slip (normal versus thrust/ reverse). Under otherwise identical conditions in a homogeneous half-space (i.e., neglecting depth-dependent frictional or material properties), thrust faults nucleate closer to the Earth's surface than normal faults and take less time to do so. These differences decrease as the dip angle of the fault increases to 90°. The amount of preseismic surface slip is a much more complicated function of dip angle. The results show that fault geometry may have an important role in the nucleation process as well as the process of dynamic rupture. Further research is needed to combine rigorous nucleation models with full rupture dynamics.
Earthquake activity related to seismic cycles in a model for a heterogeneous strike-slip fault
Tectonophysics, 2006
We investigate the evolution of seismicity within large earthquake cycles in a model of a discrete strike-slip fault in elastic solid. The model dynamics is governed by realistic boundary conditions consisting of constant velocity motion of regions around the fault, static/kinetic friction and dislocation creep along the fault, and 3D elastic stress transfer. The fault consists of brittle parts which fail during earthquakes and undergo small creep deformation between events, and aseismic creep cells which are characterized by high ongoing creep motion. This mixture of brittle and creep cells is found to generate realistic aftershock sequences which follow the modified Omori law and scale with the mainshock size. Furthermore, we find that the distribution of interevent times of the simulated earthquakes is in good agreement with observations. The temporal occurrence, however, is magnitude-dependent; in particular, the small events are clustered in time, whereas the largest earthquakes occur quasiperiodically. Averaging the seismicity before several large earthquakes, we observe an increase of activity and a broadening scaling range of magnitudes when the time of the next mainshock is approached. These results are characteristics of a critical point behavior. The presence of critical point dynamics is further supported by the evolution of the stress field in the model, which is compatible with the observation of accelerating moment release in natural fault systems.
Effect of initial conditions and loading path on earthquake nucleation
Journal of Geophysical Research, 2010
Previous studies of earthquake nucleation on faults with rate-and state-dependent friction show that nucleation process initially consists of a characteristic phase of localization to a limiting dimension, followed in some cases by a late-stage nucleation-zone expansion. Processes controlling the nucleation zone dimension are of interest for understanding scaling of the minimum earthquake dimensions and scaling of premonitory processes associated with earthquake nucleation. We examine the relationships among stressing history (loading path), initial conditions, and scaling of nucleation dimensions. Recent studies show that late-stage expansion depends strongly on the ratio of the rate-state parameters a and b. Increasing a/b to the limiting value of 1 favors expansion. We further find that initial conditions of frictional state and slip rate v strongly regulate the effect of a/b on nucleation-zone expansion. Significant nucleation-zone expansion is restricted to a narrow region around a critical point in the domain of initial conditions. The critical point marks a point of divergent paths of evolving slip rate and frictional state. Away from the critical point, nucleation-zone expansion decreases rapidly and becomes rather weak. We conclude that evolution of and v in natural earthquake cycles generally follows paths that significantly restrict nucleation-zone expansion, even for relatively large a/b values. This finding suggests that premonitory signals generated by a fast-slipping nucleation zone immediately before earthquakes may be small and difficult to detect.
Nucleation of characteristic earthquakes in simulated cycles involving deep huge slow slip events
Journal of Geophysical Research: Solid Earth, 2019
The 1944 Tonankai and 1946 Nankai megathrust earthquakes are likely to have been immediately preceded by a huge slow slip event (SSE) in the deeper brittle-to-ductile transition zone, which accumulates slip of a meter or more within several days. The present study investigates the interaction between such huge SSEs and the characteristic earthquakes through earthquake cycle simulations obeying the rate-and state-dependent friction. To simulate huge SSEs, we employ a cutoff velocity (V cx) above which the friction changes from velocity weakening to velocity strengthening. We assume that V cx decreases from 10 to 10 −9 m/s with depth over the transition zone located between 20-and 35-km depths. Depending on the characteristic slip distance L of rate and state friction, simulation results showed three patterns of earthquake nucleation. If L is small, an SSE in the transition zone directly grows into an earthquake without experiencing a separate nucleation at the brittle zone (brittle nucleation). If L is moderate, an SSE triggers brittle nucleation, which develops into an earthquake. If L is large, an SSE occurs but ceases without triggering an earthquake nor brittle nucleation, then slip deficit accumulates further to finally start brittle nucleation without the direct help of SSE, and an earthquake occurs without a large precursor. In addition, isolated huge SSEs, resembling precursory ones, occur in all cases, at most a few times per cycle.
We consider the effect of permanent stress changes on a velocity strengthening rate-and-state fault, through numerical simulations and analytical results on 1-D, 2-D, and 3-D models. We show that slip transients can be triggered by perturbations of size roughly larger than Lb = G dc/bs, where G is the shear modulus, dc and b are the characteristic slip distance and the coefficient of the evolution effect of rate-and-state friction, respectively, and s is the effective normal stress. Perturbations that increase the Coulomb stress lead to the strongest transients, but creep bursts can also be triggered by perturbations that decrease the Coulomb stress. In the latter case, peak slip velocity is attained long after the perturbation, so that it may be difficult in practice to identify their origin. The evolution of slip in a creep transient shares many features with the nucleation process of a rate-and-state weakening fault: slip initially localizes over a region of size not smaller than Lb and then accelerates transiently and finally expands as a quasi-static propagating crack. The characteristic size Lb implies a constraint on the grid resolution of numerical models, even on strengthening faults, that is more stringent than classical criteria. In the transition zone between the velocity weakening and strengthening regions, the peak slip velocity may be arbitrarily large and may approach seismic slip velocities. Postseismic slip may represent the response of the creeping parts of the fault to a stress perturbation of large scale (comparable to the extent of the main shock rupture) and high amplitude, while slow earthquakes may represent the response of the creeping zones to a more localized stress perturbation. Our results indicate that superficial afterslip, observed at usually seismogenic depths, is governed by a rate-strengthening rheology and is not likely to correspond to stable weakening zones. The predictions of the full rate-and-state framework reduce to a pure velocity strengthening law on a timescale longer than the duration of the acceleration transient, only when the triggering perturbation extends over length scales much larger than Lb.
Influence of friction and fault geometry on earthquake rupture
Journal of Geophysical Research, 2000
We investigate the impact of variations in the friction and geometry on models of fault dynamics. We focus primarily on a three-dimensional continuum model with scalar displacements. Slip occurs on an embedded two-dimensional planar interface. Friction is characterized by a two-parameter rate and state law, incorporating a characteristic length for weakening, a characteristic time for healing, and a velocity-weakening steady state. As the friction parameters are varied, there is a crossover from narrow, self-healing slip pulses to crack-like solutions that heal in response to edge effects. For repeated ruptures the crack-like regime exhibits periodic or aperiodic systemwide events. The self-healing regime exhibits dynamical complexity and a broad distribution of rupture areas. The behavior can also change from periodicity or quasi-periodicity to dynamical complexity as the total fault size or the length-to-width ratio is increased. Our results for the continuum model agree qualitatively with analogous results obtained for a one-dimensional Burridõe-Knopoff model in which radiation effects are approximated by viscous dissipation. context of a three-dimensional continuum model and a one-dimensional Burridge-Knopoff model. In our studies, dynamical complexity refers to observations of a
Models of recurrent strike-slip earthquake cycles and the state of crustal stress
Journal of Geophysical Research, 1991
Numerical models of the strike-slip earthquake cycle, assuming a viscoelastic asthenosphere coupling model, are examined. The time-dependent simulations incorporate a stress-driven fault, which leads to tectonic stress fields and earthquake recurrence histories that are mutually consistent. Single-fault simulations with constant far-field plate motion lead to a nearly periodic earthquake cycle and a distinctive spatial distribution of crustal shear stress. The predicted stress distribution includes a local minimum in stress at depths less than typical seismogenic depths. The width of this stress "trough" depends on the magnitude of crustal stress relative to asthenospheric drag stresses. The models further predict a local near-fault stress maximum at greater depths, sustained by the cyclic transfer of strain from the elastic crust to the ductile asthenosphere. Models incorporating both low-stress and high-stress fault strength assumptions are examined, under Newtonian and non-Newtonian rheology assumptions. Model results suggest a preference for low-stress a shear stress level of---10 MPa) fault models, in agreement with previous estimates based on heat flow measurements and other stress indicators. 10 7 years and the episodic near-fault motions that are variable on time scales as short as years or months. This transition is largely mediated by the crustal stress field, which in concert with deeper mantle convection drives both the brittle frictional behavior of faulted rock and the broadscale movement of the plates.
Seismic Fault Rheology and Earthquake Dynamics
2006
As preparation for a workshop on "The Dynamics of Fault Zones" (95 th Dahlem Workshop, Berlin, January 2005), specifically on the sub-topic "Rheology of Fault Rocks and Their Surroundings", we addressed critical research issues for understanding the seismic response of fault zones in terms of the constitutive response of fault materials. That requires new concepts and a host of new observations and experiments to document material response, to understand the shear localization process and the inception of earthquake instability, and especially to understand the mechanisms of fault weakening and dynamics of rupture tip propagation and arrest during rapid, possibly large, slip in natural events. We examine in turn the geological structure of fault zones and its relation to earthquake dynamics, the description of rate and state friction at slow rates appropriate to the interseismic period and earthquake nucleation, and the dynamics of fault weakening during rapid sl...
Dynamics of a creep-slip model of earthquake faults
Physica A: Statistical Mechanics and its Applications, 1998
Starting o from the relationship between time-dependent friction and velocity softening we present a generalization of the continuous, one-dimensional homogeneous Burridge-Knopo (BK) model by allowing for displacements by plastic creep and rigid sliding. The evolution equations describe the coupled dynamics of an order parameter-like ÿeld variable (the sliding rate) and a control parameter ÿeld (the driving force). In addition to the velocity-softening instability and deterministic chaos known from the BK model, the model exhibits a velocitystrengthening regime at low displacement rates which is characterized by anomalous di usion and which may be interpreted as a continuum analogue of self-organized criticality (SOC). The governing evolution equations for both regimes (a generalized time-dependent Ginzburg-Landau equation and a non-linear di usion equation, respectively) are derived and implications with regard to fault dynamics and power-law scaling of event-size distributions are discussed. Since the model accounts for memory friction and since it combines features of deterministic chaos and SOC it displays interesting implications as to (i) material aspects of fault friction, (ii) the origin of scaling, (iii) questions related to precursor events, aftershocks and afterslip, and (iv) the problem of earthquake predictability. Moreover, by appropriate re-interpretation of the dynamical variables the model applies to other SOC systems, e.g. sandpiles.