Dynamics of a velocity strengthening fault region: Implications for slow earthquakes and postseismic slip (original) (raw)
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Gradual Fault Weakening with Seismic Slip: Inferences from the Seismic Sequences of L
We estimate seismological fracture energies from two subsets of events selected from the seismic sequences of L' Aquila (2009), and Northridge (1994): 57 and 16 selected events, respectively, including the main shocks. Following ABERCROMBIE and RICE (Geophys J Int 162: 406-424, 2005), we postulate that fracture energy (G) represents the post-failure integral of the dynamic weakening curve, which is described by the evolution of shear traction as a function of slip. Following a direct-wave approach, we compute mainshock-/aftershock-source spectral ratios, and analyze them using the approach proposed by MALAG-NINI et al. (Pure Appl. Geophys., this issue, 2014) to infer corner frequencies and seismic moment. Our estimates of source parameters (including fracture energies) are based on best-fit gridsearches performed over empirical source spectral ratios. We quantify the source scaling of spectra from small and large earthquakes by using the MDAC formulation of WALTER and TAYLOR (A revised Magnitude and Distance Amplitude Correction (MDAC2) procedure for regional seismic discriminants, 2001). The source parameters presented in this paper must be considered as pointsource estimates representing averages calculated over specific ruptured portions of the fault area. In order to constrain the scaling of fracture energy with coseismic slip, we investigate two different slip-weakening functions to model the shear traction as a function of slip: (i) a power law, as suggested by ABERCROMBIE and RICE (Geophys J Int 162: 406-424, 2005), and (ii) an exponential decay. Our results show that the exponential decay of stress on the fault allows a good fit between measured and predicted fracture energies, both for the main events and for their aftershocks, regardless of the significant differences in the energy budgets between the large (main) and small earthquakes (aftershocks). Using the power-law slip-weakening function would lead us to a very different situation: in our two investigated sequences, if the aftershock scaling is extrapolated to events with large slips, a power law (a la Abercrombie and Rice) would predict unrealistically large stress drops for large, main earthquakes. We conclude that the exponential stress evolution law has the advantage of avoiding unrealistic stress drops and unbounded fracture energies at large slip values, while still describing the abrupt shear-stress degradation observed in high-velocity laboratory experiments (e.g., DI TORO et al., Fault lubrication during earthquakes, Nature 2011).
Evidence for gradual weakening in earthquake fault dynamics
2008
A number of recent studies suggest that dynamic slip on earthquake faults may trigger consistent frictional weakening or lubrication, a feature enhanced at relatively high slip rates (of the order of 1 m/s). Here we present the first clear seismological evidence of a progressive fault weakening under dynamic earthquake slip. The weakening increases with the estimated amount of heat rate (and resulting temperature increase) generated on the fault by frictional heating, indicating the presence of some thermally-activated weakening processes. The observed effect seems stronger for less mature slip systems, suggesting that confinement of heat or fluids is less effective on faults which possess a wider damage zone.
We estimate the critical slip-weakening distance on earthquake faults by using a new approach, which is independent of the estimate of fracture energy or radiated seismic energy. The approach is to find a physically based relation between the breakdown time of shear stress T b , the time of peak slip-velocity T pv , and the slip-weakening distance D c , from the time histories of shear stress, slip, and slip velocity at each point on the fault, which can be obtained from dynamic rupture calculations using a simple slip-weakening friction law. Numerical calculations are carried out for a dynamic shear crack propagating either spontaneously or at a fixed rupture velocity on a vertical fault located in a 3D half-space and a more realistic horizontally layered structure, with finite-difference schemes. The results show that T pv is well correlated with T b for faults even with a heterogeneous stress-drop distribution, except at locations near strong barriers and the fault edges. We also investigate this relation for different types of slip-weakening behavior.
2011
We explore experimentally and theoretically how fault edges may affect earthquake and slip dynamics, as faults are intrinsically heterogeneous with common occurrences of jogs, edges and steps. In the presented experiments and accompanying theoretical model, shear loads are applied to the edge of one of two flat blocks in frictional contact that form a fault analog. We show that slip occurs via a sequence of rapid rupture events that initiate from the loading edge and are arrested after propagating a finite distance. This event succession extends the slip size, transfers the applied shear across the block, and causes progressively larger changes of the contact area along the contact surface. This sequence of events dynamically forms a hard asperity near the loading edge and largely reduces the contact area beyond. These sequences of rapid events culminate in slow slip events that precede a major, unarrested slip event along the entire contact surface. We show that the 1998 M5.0 Sendai and 1995 Off-Etorofu Earthquake sequences may correspond to this scenario. Our work demonstrates, qualitatively, how a simple deviation from uniform shear loading can significantly affect both earthquake nucleation processes and how fault complexity develops.
Journal of Geophysical Research, 2009
We investigate the coseismic and postseismic deformation due to the M w 6.0 2004 Parkfield, California, earthquake. We produce coseismic and postseismic slip models by inverting data from an array of 14 continuous GPS stations from the SCIGN network. Kinematic inversions of postseismic GPS data over a time period of 3 years show that afterslip occurred in areas of low seismicity and low coseismic slip, predominantly at a depth of $5 km. Inversions suggest that coseismic stress increases were relaxed by predominantly aseismic afterslip on a fault plane. The kinetics of afterslip is consistent with a velocity-strengthening friction generalized to include the case of infinitesimal velocities. We performed simulations of stress-driven creep using a numerical model that evaluates the time-dependent deformation due to coseismic stress changes in a viscoelastoplastic half-space. Starting with a coseismic slip distribution, we compute the time-dependent evolution of afterslip on a fault plane and the associated displacements at the GPS stations. Data are best explained by a rate-strengthening model with frictional parameter (a À b) = 7 Â 10 À3 , at a high end of values observed in laboratory experiments. We also find that the geodetic moment due to creep is a factor of 100 greater than the cumulative seismic moment of aftershocks. The rate of aftershocks in the top 10 km of the seismogenic zone mirrors the kinetics of afterslip, suggesting that postearthquake seismicity is governed by loading from the nearby aseismic creep. The San Andreas fault around Parkfield is deduced to have large along-strike variations in rate-and-state frictional properties. Velocity strengthening areas may be responsible for the separation of the coseismic slip in two distinct asperities and for the ongoing aseismic creep occurring between the velocity-weakening patches after the 2004 rupture.
Can observations of earthquake scaling constrain slip weakening?
Geophysical Journal International, 2005
We use observations of earthquake source parameters over a wide magnitude range (M W ∼ 0-7) to place constraints on constitutive fault weakening. The data suggest a scale dependence of apparent stress and stress drop; both may increase slightly with earthquake size. We show that this scale dependence need not imply any difference in fault zone properties for different sized earthquakes. We select 30 earthquakes well-recorded at 2.5 km depth at Cajon Pass, California. We use individual and empirical Green's function spectral analysis to improve the resolution of source parameters, including static stress drop (σ) and total slip (S). We also measure radiated energy E S. We compare the Cajon Pass results with those from larger California earthquakes including aftershocks of the 1994 Northridge earthquake and confirm the results of Abercrombie (1995): µE S /M 0 σ (where µ = rigidity) and both E S /M 0 and σ increase as M 0 (and S) increases. Uncertainties remain large due to model assumptions and variations between possible models, and earthquake scale independence is possible within the resolution. Assuming that the average trends are real, we define a quantity G = (σ − 2µE S /M 0)S/2 which is the total energy dissipation in friction and fracture minus σ 1 S, where σ 1 is the final static stress. If σ 1 = σ d , the dynamic shear strength during the last increments of seismic slip, then G = G, the fracture energy in a slip-weakening interpretation of dissipation. We find that G increases with S, from ∼10 3 J m −2 at S = 1 mm (M1 earthquakes) to 10 6-10 7 J m −2 at S = 1 m (M6). We tentatively interpret these results within slip-weakening theory, assuming G ≈ G. We consider the common assumption of a linear decrease of strength from the yield stress (σ p) with slip (s), up to a slip D c. In this case, if either D c , or more generally (σ p − σ d) D c , increases with the final slip S we can match the observations, but this implies the unlikely result that the early weakening behaviour of the fault depends on the ultimate slip that the fault will sustain. We also find that a single slip-weakening function σ F (s) is able to match the observations, requiring no such correlation. Fitting G over S = 0.2 mm to 0.2 m with G ∝ S 1+n , we find n ∼ 0.3, implying a strength drop from peak σ p − σ F (S) ∝ S n. This model also implies that slip weakening continues beyond the final slip S of typical earthquakes smaller than ∼ M6, and that the total strength drop σ p − σ d for large earthquakes is typically >20 MPa, larger than σ. The latter suggests that on average a fault is initially stressed below the peak strength, requiring stress concentration at the rupture front to propagate slipping.
Triggered aseismic fault slip from nearby earthquakes, static or dynamic effect?
Journal of Geophysical Research: Solid Earth, 2003
Observations show that an earthquake can affect aseismic slip behavior of nearby faults and produce ''triggered aseismic fault slip.'' Two types of stress changes are often examined by researchers as possible triggering sources. One is the static stress change associated with the faulting process and the other is the dynamic stress change or transient deformation generated by the passage of seismic waves. No consensus has been reached, however, regarding the mechanism(s) of triggered aseismic fault slip. We evaluate the possible triggering role of static stress changes by examining observations made after 10 large earthquakes in California. Most of the nearby fault segments that slipped aseismically were encouraged to move by the imposed positive changes in static Coulomb Failure Stress (CFS). Nonetheless, three discrepancies or failures with this model exist, which implies that static stress triggering either is or is not the sole mechanism causing the observed triggered slip. We then use a spring-slider system as a simplified fault model to study its slip behavior and the impact of transient (dynamic) loading on it. We show that a two-state-variable rate-dependent and state-dependent frictional law can generate creep events. Transient loads are then put into the system. Certain types of them can cause a large time advance of (or trigger) the next creep event. While our work examines triggered creep events near the surface, it may well have implications for the occurrence of similar events near the bottom of the seismogenic zone where a transition in frictional stability occurs.
Journal of Geophysical Research, 2009
We model ruptures on faults that weaken in response to flash heating of microscopic asperity contacts (within a rate-and-state framework) and thermal pressurization of pore fluid. These are arguably the primary weakening mechanisms on mature faults at coseismic slip rates, at least prior to large slip accumulation. Ruptures on strongly rate-weakening faults take the form of slip pulses or cracks, depending on the background stress. Self-sustaining slip pulses exist within a narrow range of stresses: below this range, artificially nucleated ruptures arrest; above this range, ruptures are crack-like. Natural earthquakes will occur as slip pulses if faults operate at the minimum stress required for propagation. Using laboratory-based flash heating parameters, propagation is permitted when the ratio of shear to effective normal stress on the fault is 0.2-0.3; this is mildly influenced by reasonable choices of hydrothermal properties. The San Andreas and other major faults are thought to operate at such stress levels. While the overall stress level is quite small, the peak stress at the rupture front is consistent with static friction coefficients of 0.6-0.9. Growing slip pulses have stress drops of 3MPa;slipandthelengthoftheslippulseincreaselinearlywithpropagationdistanceat3 MPa; slip and the length of the slip pulse increase linearly with propagation distance at 3MPa;slipandthelengthoftheslippulseincreaselinearlywithpropagationdistanceat0.14 and 30m/km,respectively.Thesevaluesareconsistentwithseismicandgeologicobservations.Incontrast,cracksonfaultsofthesamerheologyhavestressdropsexceeding20MPa,andslipatthehypocenterincreaseswithdistanceat30 m/km, respectively. These values are consistent with seismic and geologic observations. In contrast, cracks on faults of the same rheology have stress drops exceeding 20 MPa, and slip at the hypocenter increases with distance at 30m/km,respectively.Thesevaluesareconsistentwithseismicandgeologicobservations.Incontrast,cracksonfaultsofthesamerheologyhavestressdropsexceeding20MPa,andslipatthehypocenterincreaseswithdistanceat1 m/km.
Journal of Structural Geology, 2006
The spatial and temporal accumulation of slip from multiple earthquake cycles on active faults is poorly understood. Here, we describe a methodology that can determine the time period of observation necessary to reliably constrain fault behaviour, using a high-resolution longtimescale (the last 17 kyr) fault displacement dataset over the Rangitaiki Fault (Whakatane Graben, New Zealand). The fault linked at ca. 300 ka BP and analysis of time periods within the last 17 kyr gives insight into steady-state behaviour for time intervals as short as ca. 2 kyr. The maximum displacement rate observed on the Rangitaiki Fault is 3.6G1.1 mm yr K1 measured over 17 kyr. Displacement profiles of the last 9 ka of fault movement are similar to profiles showing the last 300 ka of fault movement. In contrast, profiles determined for short time intervals (2-3 kyr) are highly irregular and show points of zero displacement on the larger segments. This indicates temporal and spatial variability in incremental displacement associated with surface-rupturing slip events. There is spatial variability in slip rates along fault segments, with minima at locations of fault interaction or where fault linkage has occurred in the past. This evidence suggests that some earthquakes appear to have been confined to specific segments, whereas larger composite ruptures have involved the entire fault. The short-term variability in fault behaviour suggests that fault activity rates inferred from geodetic surveys or surface ruptures from a single earthquake may not adequately represent the longer-term activity nor reflect its future behaviour. Different magnitude events may occur along the same fault segment, with asperities preventing whole segment rupture for smaller magnitude events. q