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(a) Observed (blue) and modeled (red) interseismic GPS velocity fields for the best-fit viscosity structure highlighted in Figure 6b (Maxwell viscosity η M 10 19:0 Pa·s and a Kelvin viscosity η K 10 19:0 Pa·s). The observed velocity field shown here is the corrected interseismic velocity field v η M ; η K (equation 8). (b) Residual velocities for the case shown in (a). Note the different scale from (a).
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Along the North Anatolian fault (NAF), the surface deformation associated with tectonic block motions, elastic strain accumulation, and the viscoelastic response to past earthquakes has been geodetically observed over the last two decades. These observations include campaign-mode Global Positioning System (GPS) velocities from the decade prior to t...
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... ranging from 10 17:0 to 10 23:0 Pa·s. These block models also incorporate homogenous internal block strain (e.g., Meade and Loveless, 2009), and we enforce three tensile slip-rate constraints on the central and southern strands of the NAF in the Sea of Marmara to damp fault normal motion. We summarize the viscoelastic block model results (Figs. 7-10) in six major ...
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... Satellite geodetic data, such as Global Navigation Satellite System (GNSS) and Interferometric Synthetic Aperture Radar (InSAR), are regularly used to determine fault slip rates. A growing number of geodetic modeling approaches are being applied, including elastic block models (Evans, 2022;Hammond et al., 2024;McCaffrey, 2009;Meade & Loveless, 2009;Shen et al., 2015;Styron, 2022), deep dislocation models (Elston et al., 2024;Zeng, 2022;Zeng & Shen, 2014, NeoKinema-a kinematic finite element approach to estimate fault slip rates and off-fault strains (Bird, 2009;Shen & Bird, 2022), viscoelastic fault models (Pollitz, 2022;Pollitz et al., 2010;Pollitz & Evans, 2017), and viscoelastic earthquake cycle block models (Chuang & Johnson, 2011;DeVries et al., 2017;Johnson & Fukuda, 2010;Pollitz & Evans, 2017). Among these techniques, geodetically-derived surface velocity data are used as the primary constraints in models, alongside geologicallyderived slip rates typically applied as a prior constraint. ...
Fault slip rates estimated from geodetic data are being integrated into seismic hazard models. The standard approach requires modeling velocities and relative (micro‐)plate motions, which is challenging for fault‐based models. We present a new approach to directly invert strain rates to solve for slip rates and distributed strain simultaneously. We generate velocity and strain rate fields over the southeastern Tibetan Plateau, utilizing Sentinel‐1 Interferometric Synthetic Aperture Radar data spanning 2014–2023. We derive slip rates using block modeling and by inverting strain rates. Our results show a partitioning between localized strain on faults and distributed deformation. The direct inversion of strain rates matches the geodetic data best when incorporating distributed moment sources, accounting for a similar proportion to on‐fault sources. The direct strain methodology also aligns best with the independent geological slip rates, especially near fault tips. As high‐resolution strain rate fields become increasingly available, we recommend direct inversion as the preferred practice.
... In this sense, there is an apparent misalignment (or an incomplete knowledge transfer) between the seismic hazard community and the geodetic communities because the former has not yet entirely accepted the information that geodetic strain rates are not stationary. In contrast, the geodetic community is almost unequivocal on this point because many works have argued for long-term non-steady deformation by investigating the long-term effects of the viscoelastic response to past earthquakes [50][51][52] . Two competing (and possibly complementary) models can explain the transient spatial pattern of recent earthquakes: deep fault shear zone deformation in 2-D or viscoelastic relaxation of the crust and mantle in 3-D. ...
This study critically examines the use of geodetic strain rates for forecasting long-term earthquake rates in a slow-deforming region such as Italy, challenging the prevailing assumption of their temporal stationarity in interseismic stages for seismic hazard analyses. Typically, earthquake-rate models derived from geodesy assume stationary interseismic loading rates, with stress rates in the upper crust proportional to geodetic strain rates, leading to earthquake rates directly proportional to these strain rate tensors. However, our analysis unveils a pronounced correlation between the epicenters of earthquakes that occurred in the past 60–120 years and areas forecasted for higher future earthquake rates based on geodetic strain rates. This correlation appears weak and scattered in analyses of even older earthquakes. To corroborate our findings, we select the 2009 L’Aquila earthquake (mw = 6.3) to prove that its apparently marginal viscoelastic relaxation significantly alters the time series of adjacent benchmarks for the following ~ 30–60 years, explaining the high correlation between recent earthquakes and strain rate peaks. Our findings require a methodological shift in interpreting geodetic data for earthquake forecasting, emphasizing the two-component (plate-tectonics-driven stationary long-term deformation, and decadal transient viscoelastic relaxation after an earthquake) nature of crustal stress accumulation recorded in geodetic data. We underscore the potential of geodesy-derived forecasts to provide deeper insights into seismic hazards, stressing the importance of acknowledging the long-term temporal variability inherent in geodetic measurements.
... The difference in time-dependent loading between purely elastic fault models and those considering viscoelastic deformation suggests that viscoelastic interactions are an important ingredient for efforts aimed at modeling regional tectonics and multifault interactions, particularly given that the spatial footprint of this distributed deformation can be much larger than that of slip on individual faults (Figures 3 and 4). Viscoelastic stress interactions have been noted to be relevant to along-strike stress transfer and timing of a recent sequence of great earthquakes on the North Anatolian Fault (Devries & Meade, 2016;Devries et al., 2017), and Southern California (e.g., Freed & Lin, 2001). More generally, time-dependent loading alters the stress state on the fault preceding dynamic rupture. ...
Viscoelastic processes in the upper mantle redistribute seismically generated stresses and modulate crustal deformation throughout the earthquake cycle. Geodetic observations of these motions at the surface of the crust‐mantle system offer the possibility of constraining the rheology of the upper mantle. Parsimonious representations of viscoelastically modulated deformation through the aseismic phase of the earthquake cycle should simultaneously explain geodetic observations of (a) rapid postseismic deformation, (b) late in the earthquake cycle near‐fault strain localization. To understand how rheological formulations affect kinematics, we compare predictions from time‐dependent forward models of deformation over the entire earthquake cycle for an idealized vertical strike‐slip fault in a homogeneous elastic crust underlain by a homogeneous viscoelastic upper‐mantle. We explore three different rheologies as inferred from laboratory experiments: (a) linear Maxwell, (b) linear Burgers, (c) power‐law. The linear Burgers and power‐law rheologies are consistent with fast and slow deformation phenomenology over the entire earthquake cycle, while the single‐layer linear Maxwell model is not. The kinematic similarity of linear Burgers and power‐law models suggests that geodetic observations alone may be insufficient to distinguish between them, but indicate that one may serve as an effective proxy for the other. However, the power‐law rheology model displays a postseismic response that is non‐linearly dependent on earthquake magnitude, which may offer a partial explanation for observations of limited postseismic deformation near some magnitude 6.5–7.0 earthquakes. We discuss the role of mechanical coupling between frictional slip and viscous creep in controlling the time‐dependence of regional stress transfer following large earthquakes and how this may affect the seismic hazard and risk to communities living close to fault networks.
... One correction is for the transient deformation caused by postseismic deformation of large historic earthquakes, to meet the model assumption that GPS data reflect quasi-time-independent interseismic deformation. This transient correction is referenced to deformation averaged over many earthquake cycles and converts the strain rate pattern observed today to the hypothetical strain rate pattern observed over an average interseismic period (Hearn et al., 2013;Devries et al., 2017;Pollitz and Evans, 2017). The other correction is for effects of shallow creep on faults, such that the fault-locking effect can be uniformly modeled for the ultimate estimation of the long-term fault offset rates. ...
We develop a crustal deformation model of the western conterminous United States for the 2023 update of the National Seismic Hazard Model (NSHM). The kinematic finite-element code NeoKinema is used to describe crustal deformation, including long-term slip on faults and off-fault strains (both elastic and permanent). Three different data sets—Global Positioning System (GPS) velocities, geological fault offset rates, and crustal stress orientations—are used to constrain the model, and the plate tectonic rotation of Pacific relative to North America is also imposed on some boundaries. Compared to the last NSHM model update in 2014, the GPS and geological fault data are substantially updated, and new corrections are implemented in both the data and modeling approach, including the correction of the “ghost transient” effect due to postseismic deformation following large historic earthquakes, and correction for shallow creep on faults estimated from independent data. Based on these modeling results and a plate tectonic model of the Cascadia subduction zone, a long-term seismicity rate map is also computed for the western United States; this map is independent of the local seismic catalog and can, therefore, be tested retrospectively as well as prospectively. We find good success in most of the region, except in Cascadia, where the 45 yr instrumental seismicity record is much quieter than the forecast of our long-term model.
... Although the classical elastic block model can generally retrieve the surface GPS observations around the EHS, we should note that the continued viscoelastic relaxation following great earthquakes could be an important factor that may significantly disturb the surface deformation (Devries et al., 2017;Wang et al., 2012;Li et al., 2021). Here we focused on the postseismic viscoelastic relaxation in the lower crust and upper mantle caused by the great 1950 Assam Mw 8.7 earthquake endures as the largest intracontinental earthquake ever recorded. ...
The Eastern Himalayan Syntaxis (EHS) played important role in accommodating the intracontinental deformation and material flow in southeastern Tibet. Quantifying the regional kinematics around the EHS is critical for understanding the deformation mechanism of the Tibetan Plateau. In this study, we compiled all available GPS measurements and constructed an elastic block model to analyze the fault kinematics around the EHS. The results suggest that the deformation characteristics around the EHS can be well explained by a linear spherical block model that considers the intra-block homogeneous strain. The fault slip rates from the block model agree well with the late Quaternary slip rates, suggesting that the GPS-derived slip rates can generally represent the long-term slip rates. However, some relatively large residuals, especially in the frontal EHS region, are still detectable, suggesting that various other factors, such as the postseismic viscoelastic relaxation, may also influence the deformation. We simulated the postseismic viscoelastic relaxation deformation caused by the 1950 Mw 8.7 Assam earthquake endured as the largest intracontinental earthquake ever recorded. The results indicate significant postseismic viscoelastic deformation due to the Assam earthquake even after 65 years. The interseismic convergence rates across the Mishmi thrust and the Arunachal Himalaya derived from pure elastic solution may be overestimated if ignoring the viscoelastic relaxation effects. The spatial variations of convergence rates across the central and eastern Himalayas could be partly related to the transient deformation following great Himalayan earthquakes.
... Perturbation style approaches have also been advanced to model the viscoelastic effects throughout the earthquake cycle in three-dimensions (Devries et al., 2017;Hearn et al., 2013;Pollitz and Evans, 2017). These methods rely on the assumption of linear rheologies so that the viscoelastic effects can be superimposed independently. ...
... This represents the expected interseismic deformation in the case where there is no time-dependent viscoelastic deformation. The remaining perturbation to the steady-state velocity expectation, v p , can be interpreted as a contribution from exclusively viscoelastic effects (Devries et al., 2017;Hearn et al., 2013;Pollitz and Evans, 2017). It is this component of the total velocity field that we represent with a set of effective displacements at the base of the elastic crust to represent the kinematic effects of the fully coupled mechanical model. ...
Viscoelastic deformation below the Earth’s elastic crust is modulated by stresses generated by both plate tectonic and earthquake cycle processes. Rapid near-fault deformation following large earthquakes has been interpreted as the signature of viscoelastic stress diffusion in the upper mantle and earthquake cycle models have been developed that integrated this effect throughout the duration of the earthquake cycle. Here we develop a surrogate method to approximate the upper crustal kinematics associated with time-dependent viscoelastic deformation that does not require knowledge of nor provide constraints on subcrustal rheology. To do this we show how the effects of deformation in the viscoelastic upper mantle can be emulated by introducing a set of effective dislocations at crust-mantle interface. The approach is shown to approximate both linear and power-law viscoelastic rheologies and offers a way to accurately represent viscoelastic kinematics even in the case where upper mantle rheology is poorly constrained.
... The Anatolian region (McKenzie 1972), a prominent example of microplate, has been the focus of numerous studies of seismicity, crustal strain accumulation and associated hazard (e.g. England et al. 2016;DeVries et al. 2017). Its tectonic structure features a very low seismicity core of continental lithosphere bounded by the North Anatolian Fault (NAF) to the north (Barka 1992), the East Anatolian Fault (EAF) to the east (Bulut et al. 2012), the Cyprus Arc to the south (Wdowinski et al. 2006) and the Western Anatolian Extensional Province (WAEP) to the west (Fig. 1). ...
... Most of the seismicity in AT occurs along the NAF, an active right-lateral strike-slip fault that separates AT from EU, and that staged over 15 earthquakes of magnitude M W ≥ 6.5 since the beginning of the last century (DeVries et al. 2017;Barka 1992;Stein et al. 1997). The most recent of these is the 2014 M W = 6.9 Samothraki-Gökçeada event along the North Aegean Trough (Saltogianni et al. 2015). ...
In the current plate tectonics paradigm, relative plate motions are assumed to remain unperturbed by temporal stress changes occurring during the seismic cycle, whereby stress slowly built up along tectonic plate boundaries is suddenly released by rapid fault slip during earthquakes. However, direct observations that could challenge such a tenet have not been identified so far. Here we show that the rigid motion of the whole Anatolian microplate, measured using space geodetic techniques, was altered by the stress released during the 1999 Izmit–Düzce earthquakes, which ruptured along the North Anatolian Fault. This kinematic change requires a torque change that is in agreement with the torque change imparted upon the Anatolian microplate by the Izmit–Düzce coseismic stress release. This inference holds across realistic ranges of data noise and controlling parameters, and is not hindered by active deformation in western Anatolia. These results suggest the existence of a whole–plate kinematic signal associated with the stress released by large earthquakes.
... This fault system consists, over much of its length, of a single, isolated, structurally mature fault that extends from near its eastern end westward for ∼870 km to the Mudurnu Valley at 31°W (Figure 2d). Rate data from the central NAF indicate that the fault accommodates almost all of the ∼25 mm/yr of relative plate motion in this region (DeVries et al., 2017;Kozaci et al., 2007Kozaci et al., , 2009Reilinger et al., 2006). Although other faults occur in this section of the plate boundary, they exhibit relatively slow slip rates. ...
... Slip-rate studies along the central part of the NAF at a range of Holocene-late Pleistocene time scales suggest that the average rate is ∼18-21 mm/yr Kozaci et al., 2007Kozaci et al., , 2009). These rates are generally similar to, but slightly slower than, decadal geodetic slip-deficit rates of 20-25 mm/yr (DeVries et al., 2017;McClusky et al., 2000;Reilinger et al., 2006). The similarity in geodetic slip-deficit and geologic fault slip rates indicates that almost all (80%-95%) of the relative plate motion in north-central Turkey is accommodated along the NAF itself, demonstrating that this section of the NAF is one of the most structurally isolated major strike-slip faults in the world. ...
A comparison of published incremental fault slip rates from four major strike‐slip faults with the proximity and number of other active faults in the surrounding plate boundary systems shows that the behavior of the primary fault is correlated with the structural complexity of its tectonic setting. To do this, we characterize the relative structural complexity of the fault network surrounding a fault location of interest by defining the coefficient of complexity, which quantifies the density and displacement rates of the faults in the plate‐boundary network at specified radii around the site of interest. We show that the relative constancy of incremental slip rates measured along primary faults of the Alpine, San Andreas, North Anatolian, and Dead Sea fault systems reflects the proximity, number, and activity of their close neighbors. Specifically, faults that extend through more structurally complex plate boundary fault systems exhibit more irregular slip behavior than faults that pass through simpler settings. We suggest that these behaviors are likely a response to more complex stress interactions within more structurally complicated regional fault systems, as well as possible temporal changes in fault strength and/or kinematic interactions amongst mechanically complementary faults within a system that collectively accommodates overall relative plate motion. Our results provide a potential means for better evaluating the future behavior of large plate‐boundary faults in the absence of well‐documented incremental slip‐rate behavior, and may help improve the use of geological slip‐rate data in probabilistic seismic hazard assessments.
... Deformation in most of the eastern Mediterranean can also be described by the motion of vertical dislocations (Lundgren et al. 1998;Armijo et al. 2004;Flerit et al. 2004), but the prolongation of localized faults in the lower crust and upper mantle has little physical justification. Patterns of interseismic strain accumulation (Özeren & Holt 2010) and post-seismic deformation following recent large earthquakes along the North Anatolian Fault including the 1999 Düzce and Izmit earthquakes indicate the presence of a ductile layer below mid-crustal depths and a deep, low-viscosity shear zone following the trace of the North Anatolian Fault (Hearn et al. 2002(Hearn et al. , 2009Yamasaki et al. 2014;DeVries et al. 2017), implying a transition to more distributed deformation at depth. ...
The Eastern Mediterranean is the most seismically active region in Europe due to the complex interactions of the Arabian, African and Eurasian tectonic plates. Deformation is achieved by faulting in the brittle crust, distributed flow in the viscoelastic lower-crust and mantle, and Hellenic subduction but the long-term partitioning of these mechanisms is still unknown. We exploit an extensive suite of geodetic observations to build a kinematic model connecting strike-slip deformation, extension, subduction, and shear localization across Anatolia and the Aegean Sea by mapping the distribution of slip and strain accumulation on major active geologic structures. We find that tectonic escape is facilitated by a plate-boundary-like, trans-lithospheric shear zone extending from the Gulf of Evia to the Turkish-Iranian Plateau that underlies the surface trace of the North Anatolian Fault. Additional deformation in Anatolia is taken up by a series of smaller-scale conjugate shear zones that reach the upper mantle, the largest of which is located beneath the East Anatolian Fault. Rapid north-south extension in the western part of the system, driven primarily by Hellenic Trench retreat, is accommodated by rotation and broadening of the North Anatolian mantle shear zone from the Sea of Marmara across the north Aegean Sea, and by a system of distributed transform faults and rifts including the rapidly extending Gulf of Corinth in central Greece and the active grabens of western Turkey. Africa-Eurasia convergence along the Hellenic Arc occurs at a median rate of 49.8 mm/yr in a largely trench-normal direction except near eastern Crete where variably-oriented slip on the megathrust coincides with mixed-mode and strike-slip deformation in the overlying accretionary wedge near the Ptolemy-Pliny-Strabo trenches. Our kinematic model illustrates the competing roles the North Anatolian mantle shear zone, Hellenic Trench, overlying mantle wedge, and active crustal faults play in accommodating tectonic indentation, slab rollback, and associated Aegean extension. Viscoelastic flow in the lower crust and upper mantle dominate the surface velocity field across much of Anatolia and a clear transition to megathrust-related slab pull occurs in western Turkey, the Aegean Sea, and Greece. Crustal scale faults and the Hellenic wedge contribute only a minor amount to the large-scale, regional pattern of Eastern Mediterranean interseismic surface deformation.
... Inversion of geodetic data for the spatial distribution of slip on a fault is also subject to fundamental limitations, notably due to the St. Venant principle that implies a decreasing resolution with increasing distance between source and observations. However, the deployment of increasingly large and dense geodetic observatories, the development of better analytic standards in inverse theory (Aster et al., 2012;Funning et al., 2014;Fukahata & Wright, 2008;Hang et al., 2020;Nocquet, 2018;Yabuki & Matsu'ura, 1992), and the joint inversion of complementary data sets, both geodetic and seismological, has increased the accuracy of slip distributions (Atzori & Antonioli, 2011;Amey et al., 2018;Barbot et al., 2013;Duputel et al., 2014;DeVries et al., 2017;Evans & Meade, 2012;Gombert et al., 2017Gombert et al., , 2018McGuire & Segall, 2003;Minson et al., 2014;Sathiakumar et al., 2017). For example, the large uncertainties associated with shallow slip near the trench during the 2011 Mw = 9.1 Tohoku, Japan, earthquake were largely reduced by considering tsunami data (e.g., Bletery et al., 2014;Jiang & Simons, 2016;Yamazaki et al., 2011). ...
The source characteristics of slow and fast earthquakes provide a window into the mechanical properties of faults. In particular, the average stress drop controls the evolution of friction, fault slip, and event magnitude. However, this important source property is typically inferred from the analysis of seismic waves and is subject to many epistemic uncertainties. Here, we investigate the source properties of 53 earthquakes and 17 slow‐slip events on thrust and strike‐slip faults in various tectonic settings using slip distributions constrained by geodesy in combination with other data. We determine the width, potency, and potency density of slow and fast earthquake sources based on static slip distributions. The potency density, defined conceptually as the ratio of average slip to rupture radius, is a measure of anelastic deformation with limited bias from rigidity differences across depths and tectonic settings. Strike‐slip earthquakes have the highest potency density, varying from 20 to 500 microstrain. The potency density is on average lower on continental thrust faults and megathrusts, from 10 to 200 microstrain, with an algebraic decrease with centroid depth, indicative of systematic changes in dominant rupture processes with depth. Slow slip events represent an end‐member style of rupture with low potency density and large rupture width. Significant variability in potency density of slow‐slip events affects their moment‐duration scaling. The variations of source properties across tectonic settings, depth, and rupture styles can be used to better constrain numerical simulations of seismicity and to assess the source characteristics of future earthquakes and slow slip events.
