Evidence for postseismic deformation of the lower crust following the 2004 Mw6.0 Parkfield earthquake

Previous studies have shown that postseismic relaxation following the 2004 Mw6.0 Parkfield, CA, earthquake is dominated by afterslip. However, we show that some fraction of the afterslip inferred from kinematic inversion to have occurred immediately below the seismically ruptured area may in fact be a substitute for viscous postseismic deformation of the lower crust. Using continuous GPS and synthetic aperture radar interferometry, we estimate the relative contribution of shallow afterslip (at depth less than 20km) and deeper seated deformation required to account for observed postseismic surface displacements. Exploiting the possible separation in space and time of the time series of displacements predicted from viscoelastic relaxation, we devise a linear inversion scheme that allows inverting jointly for the contribution of afterslip and viscoelastic flow as a function of time. We find that a wide range of models involving variable amounts of viscoelastic deformation can fit the observations equally well provided that they allow some fraction of deep-seated deformation (at depth larger than ∼20 km). These models require that the moment released by postseismic relaxation over 5 years following the earthquake reached nearly as much as 200% of the coseismic moment. All the models show a remarkable complementarity of coseismic and shallow afterslip distributions. Some significant deformation at lower crustal depth (20–26 km) is required to fit the geodetic data. The condition that postseismic deformation cannot exceed complete relaxation places a constraint on the amount of deep seated deformation. The analysis requires an effective viscosity of at least ~10^(18) Pa s of the lower crust (assuming a semi-infinite homogeneous viscous domain). This deep-seated deformation is consistent with the depth range of tremors which also show a transient postseismic response and could explain as much as 50% of the total postseismic geodetic moment (the remaining fraction being due to afterslip at depth shallower than 20 km). Lower crustal postseismic deformation could reflect a combination of localized ductile deformation and aseismic frictional sliding

Modeling afterslip and aftershocks following the 1992 Landers earthquake

One way to probe the rheology of the lithosphere and fault zones is to analyze the temporal evolution of deformation following a large earthquake. In such a case, the lithosphere responds to a known stress change that can be assessed from earthquake slip models constrained from seismology and geodesy. Here, we model the postseismic response of a fault zone that is assumed to obey a rate-strengthening rheology, where the frictional stress varies as aσ ln(ε), ε being the deformation rate and aσ > 0 a rheological parameter. The model is simple enough that these parameters can be estimated by inversion of postseismic geodetic data. We apply this approach to the analysis of geodetic displacements following the M_w 7.3, 1992, Landers earthquake. The model adjusts well the measured displacements and implies aσ ≈ 0.47–0.53 MPa. In addition, we show that aftershocks and afterslip follow the same temporal evolution and that the spatiotemporal distribution of aftershocks is consistent with the idea that they are driven by reloading of the seismogenic zone resulting from frictional afterslip

An aseismic slip pulse in northern Chile and along-strike variations in seismogenic behavior

We use interferometric synthetic aperture radar, GPS, and seismic observations spanning 5 to 18 years to reveal a detailed kinematic picture of the spatiotemporal evolution of fault slip in a region corresponding to the 30 July 1995 M_w 8.1 subduction zone megathrust earthquake in northern Chile. In a single area, we document a complex mosaic of phenomena including large earthquakes, postseismic afterslip with a spatial distribution that appears to be tied to variations in coastal morphology, and a completely aseismic pulse that may have triggered a M_w 7.1 earthquake on 30 January 1998. In contrast to simple models of fault slip behavior, this spatial heterogeneity indicates that frictional parameters on the fault do not have a systematic transition with depth and also vary rapidly along strike. The low amount of afterslip from the M_w 8.1 earthquake relative to other similar events suggests that postseismic behavior may be modulated by the amount of sediment subducted

Constraints on fault and lithosphere rheology from the coseismic slip and postseismic afterslip of the 2006 M_w 7.0 Mozambique earthquake

The 2006 M_w 7.0 Mozambique (Machaze) normal-faulting earthquake ruptured an unusually steeply dipping fault plane (~75°). The amount of slip in the earthquake decreased from depths of ~10 km toward the surface, and this shallow slip deficit was at least partly recovered by postseismic afterslip on the shallow part of the fault plane. An adjacent normal fault segment slipped postseismically (and possibly also co-seismically) at shallow depths with a large strike-slip component, in response to the stresses generated by slip on the main earthquake fault plane. Our observations suggest that the fault zone behaves in a stick-slip manner in the crystalline basement, and that where it cuts the sedimentary layer the coseismic rupture was partially arrested and there was significant postseismic creep. We discuss the effects of such behavior on the large-scale tectonics of continental regions, and on the assessment of seismic hazard on similar fault systems. The steep dip of the fault suggests the re-activation of a preexisting structure with a coefficient of friction at least ~25–45% lower than that on optimally oriented planes, and analysis of the deformation following an aftershock indicates that the value of the parameter ‘a’ that describes the rate-dependence of fault friction lies in the range 1 × 10^(−3)–2 × 10^(−2). The lack of long-wavelength postseismic relaxation suggests viscosities in the ductile lithosphere of greater than ~2 × 10^(19) Pa s, and an examination of the tectonic geomorphology in the region identifies ways in which similar fault systems can be identified before they rupture in future earthquakes

Asperities and barriers on the seismogenic zone in North Chile: state-of-the-art after the 2007 Mw 7.7 Tocopilla earthquake inferred by GPS and InSAR data

The Mw 7.7 2007 November 14 earthquake had an epicentre located close to the city of Tocopilla, at the southern end of a known seismic gap in North Chile. Through modelling of Global Positioning System (GPS) and radar interferometry (InSAR) data, we show that this event ruptured the deeper part of the seismogenic interface (30–50 km) and did not reach the surface. The earthquake initiated at the hypocentre and was arrested ~150 km south, beneath the Mejillones Peninsula, an area already identified as an important structural barrier between two segments of the Peru–Chile subduction zone. Our preferred models for the Tocopilla main shock show slip concentrated in two main asperities, consistent with previous inversions of seismological data. Slip appears to have propagated towards relatively shallow depths at its southern extremity, under the Mejillones Peninsula. Our analysis of post-seismic deformation suggests that small but still significant post-seismic slip occurred within the first 10 d after the main shock, and that it was mostly concentrated at the southern end of the rupture. The post-seismic deformation occurring in this period represents ~12–19 per cent of the coseismic deformation, of which ~30–55 per cent has been released aseismically. Post-seismic slip appears to concentrate within regions that exhibit low coseismic slip, suggesting that the afterslip distribution during the first month of the post-seismic interval complements the coseismic slip. The 2007 Tocopilla earthquake released only ~2.5 per cent of the moment deficit accumulated on the interface during the past 130 yr and may be regarded as a possible precursor of a larger subduction earthquake rupturing partially or completely the 500-km-long North Chile seismic gap

Geodetic displacements and aftershocks following the 2001 M_w = 8.4 Peru earthquake: Implications for the mechanics of the earthquake cycle along subduction zones

We analyzed aftershocks and postseismic deformation recorded by the continuous GPS station AREQ following the M_w = 8.4, 23 June 2001 Peru earthquake. This station moved by 50 cm trenchward, in a N235°E direction during the coseismic phase, and continued to move in the same direction for an additional 15 cm over the next 2 years. We compare observations with the prediction of a simple one-dimensional (1-D) system of springs, sliders, and dashpot loaded by a constant force, meant to simulate stress transfer during the seismic cycle. The model incorporates a seismogenic fault zone, obeying rate-weakening friction, a zone of deep afterslip, the brittle creep fault zone (BCFZ) obeying rate-strengthening friction, and a zone of viscous flow at depth, the ductile fault zone (DFZ). This simple model captures the main features of the temporal evolution of seismicity and deformation. Our results imply that crustal strain associated with stress accumulation during the interseismic period is probably not stationary over most of the interseismic period. The BCFZ appears to control the early postseismic response (afterslip and aftershocks), although an immediate increase, by a factor of about 1.77, of ductile shear rate is required, placing constraints on the effective viscosity of the DFZ. Following a large subduction earthquake, displacement of inland sites is trenchward in the early phase of the seismic cycle and reverse to landward after a time t i for which an analytical expression is given. This study adds support to the view that the decay rate of aftershocks may be controlled by reloading due to deep afterslip. Given the ratio of preseismic to postseismic viscous creep, we deduce that frictional stresses along the subduction interface account for probably 70% of the force transmitted along the plate interface

Stress transfer and strain rate variations during the seismic cycle

The balance of forces implies stress transfers during the seismic cycle between the elastobrittle upper crust and the viscoelastic lower crust. This could induce observable time variations of crustal straining in the interseismic period. We simulate these variations using a one-dimensional system of springs, sliders, and dashpot loaded by a constant force. The seismogenic zone and the zone of afterslip below are modeled from rate-and-state friction. The ductile deeper fault zone is modeled from a viscous slider with Newtonian viscosity ν. The force per unit length, F, must exceed a critical value F_c to overcome friction resistance of the fault system. This simple system produces periodic earthquakes. The recurrence period, T_(cycle), and the duration of the postseismic relaxation phase, which is driven dominantly by afterslip, then both scale linearly with ν. Between two earthquakes, interseismic strain buildup across the whole system is nonstationary with the convergence rates V_i, just after each earthquake, being systematically higher than the value V_f at the end of the interseismic period. We show that V_i/V_f is an exponential function of α = T_(cycle)/T_M ∝ Δτ/(F – F_c ) ∝ Δτ/(νV_ 0), where Δτ is the coseismic stress drop and V_0 is the long-term fault slip rate. It follows that departure from stationary strain buildup is higher if the contribution of viscous forces to the force balance is small compared to the coseismic stress drop (due to a low viscosity or low convergence rate, for example). This simple model is meant to show that the far-field deformation rate in the interseismic period, which can be determined from geodetic measurements, might not necessarily be uniform and equal to the long-term geologic rate

Spectral-element simulations of long-term fault slip: Effect of low-rigidity layers on earthquake-cycle dynamics

We develop a spectral element method for the simulation of long-term histories of spontaneous seismic and aseismic slip on faults subjected to tectonic loading. Our approach reproduces all stages of earthquake cycles: nucleation and propagation of earthquake rupture, postseismic slip and interseismic creep. We apply the developed methodology to study the effects of low-rigidity layers on the dynamics of the earthquake cycle in 2-D. We consider two cases: small (M ~ 1) earthquakes on a fault surrounded by a damaged fault zone and large (M ~ 7) earthquakes on a vertical strike-slip fault that cuts through shallow low-rigidity layers. Our results indicate how the source properties of repeating earthquakes are affected by the presence of a damaged fault zone with low rigidity. Compared to faults in homogeneous media, we find (1) reduction in the earthquake nucleation size, (2) amplification of slip rates during dynamic rupture propagation, (3) larger recurrence interval, and (4) smaller amount of aseismic slip. Based on linear stability analysis, we derive a theoretical estimate of the nucleation size as a function of the width and rigidity reduction of the fault zone layer, which is in good agreement with simulated nucleation sizes. We further examine the effects of vertically-stratified layers (e.g., sedimentary basins) on the nature of shallow coseismic slip deficit. Our results suggest that low-rigidity shallow layers alone do not lead to coseismic slip deficit. While the low-rigidity layers result in lower interseismic stress accumulation, they also cause dynamic amplification of slip rates, with the net effect on slip being nearly zero

Interseismic coupling and seismic potential along the Central Andes subduction zone

We use about two decades of geodetic measurements to characterize interseismic strain build up along the Central Andes subduction zone from Lima, Peru, to Antofagasta, Chile. These measurements are modeled assuming a 3-plate model (Nazca, Andean sliver and South America Craton) and spatially varying interseismic coupling (ISC) on the Nazca megathrust interface. We also determine slip models of the 1996 M_w = 7.7 Nazca, the 2001 M_w = 8.4 Arequipa, the 2007 M_w = 8.0 Pisco and the M_w = 7.7 Tocopilla earthquakes. We find that the data require a highly heterogeneous ISC pattern and that, overall, areas with large seismic slip coincide with areas which remain locked in the interseismic period (with high ISC). Offshore Lima where the ISC is high, a M_w∼8.6–8.8 earthquake occurred in 1746. This area ruptured again in a sequence of four M_w∼8.0 earthquakes in 1940, 1966, 1974 and 2007 but these events released only a small fraction of the elastic strain which has built up since 1746 so that enough elastic strain might be available there to generate a M_w > 8.5 earthquake. The region where the Nazca ridge subducts appears to be mostly creeping aseismically in the interseismic period (low ISC) and seems to act as a permanent barrier as no large earthquake ruptured through it in the last 500 years. In southern Peru, ISC is relatively high and the deficit of moment accumulated since the M_w∼8.8 earthquake of 1868 is equivalent to a magnitude M_w∼8.4 earthquake. Two asperities separated by a subtle aseismic creeping patch are revealed there. This aseismic patch may arrest some rupture as happened during the 2001 Arequipa earthquake, but the larger earthquakes of 1604 and 1868 were able to rupture through it. In northern Chile, ISC is very high and the rupture of the 2007 Tocopilla earthquake has released only 4% of the elastic strain that has accumulated since 1877. The deficit of moment which has accumulated there is equivalent to a magnitude M_w∼8.7 earthquake. This study thus provides elements to assess the location, size and magnitude of future large megathurst earthquakes in the Central Andes subduction zone. Caveats of this study are that interseismic strain of the forearc is assumed time invariant and entirely elastic. Also a major source of uncertainty is due to fact that the available data place very little constraints on interseismic coupling at shallow depth near the trench, except offshore Lima where sea bottom geodetic measurements have been collected suggesting strong coupling