Off-fault Deformation Research Articles

Off-fault deformation feedback and strain localization precursor during laboratory earthquakes

Recent large-scale seismological observations have shown that off-fault strain localization and foreshock migration could serve as an early warning of an impending earthquake. However, this process is still largely unknown. In this study, state-of-the-art friction experiments were conducted in a oil-confined biaxial shear apparatus to investigate the link between stick-slip nucleation and off-fault deformation. Our findings indicate that there is a direct link between stick-slip nucleation and off-fault deformation, provided that the fault is conditionally unstable (a − b < 0). Inelastic off-fault deformation may trigger unstable slip by decreasing the stiffness of the surrounding rock volume, which favors earthquake nucleation. Additionally, the study presents laboratory observation of precursory strain localization around a fault during stick-slip cycles. These findings suggest that volumetric deformation processes could be a main factor in the nucleation of large ruptures and strain localization could be a reliable harbinger of large earthquakes.

In-situ rock shattering and strain localization along a seismogenic fault in dolostones (Monte Marine fault, Italian Central Apennines)

In-situ shattered rocks are often associated with seismogenic fault zones, but their mechanism of formation is still matter of debate, partly because of the limited number of field studies. Here we describe the characteristics of in-situ shattered rocks distribution along the NW-SE-striking seismogenic Monte Marine Fault (MMF) in the Italian Central Apennines. In the studied area, the MMF cuts through Mesozoic carbonates, is exhumed from <3 km depth and consists of two >5 km-long major hard-linked segments with normal kinematics. The linkage between the two fault segments occurs along a ∼2 km-long step-over zone with E-W trending faults and oblique-slip kinematics. To the northwest, fault-related shear deformation is localized in a ∼5 m-thick cataclastic fault core and off-fault deformation is dominated by in-situ shattered rocks up to ∼40 m-thick. Instead, in the step-over zone to the southeast, the in-situ shattered rocks are up to ∼500 m thick, particularly where MMF crosscuts older low-angle thrust faults.We integrated detailed field structural surveys with microstructural and grain size distribution analyses of the fault rocks to assess the mechanism of (1) formation of in-situ shattered rocks and, (2) progressive localization of shear deformation along the MMF. The obtained results, after the viability of several formation mechanisms (mechanical models) have been reviewed, support the hypothesis that the formation of in-situ shattered rocks was associated with the propagation of (multiple) seismic ruptures (mainshocks and aftershock sequences) within a mechanically heterogeneous fault zone. Heterogeneity is due to the occurrence of preexisting damage related to previous earthquakes, but also inherited from the older low-angle thrust faults. Therefore, we suggest that the origin of these shattered rocks is more compatible with seismic related processes than only with quasi-static fault growth models. On the other hand, the cataclastic fault core derived from the progressive accommodation of shear deformation within the in-situ shattered rock volumes during several seismic cycles. We conclude that the large volumes of in-situ shattered rocks are the result of seismic-related dissipative processes in a geometrically and mechanically heterogeneous fault zone. In this scenario, large volumes of in-situ shattered rocks are compliant low velocity zones which can influence the propagation of earthquake ruptures.

Discontinuous Surface Ruptures and Slip Distributions in the Epicentral Region of the 2021 Mw7.4 Maduo Earthquake, China

Discontinuous Surface Ruptures and Slip Distributions in the Epicentral Region of the 2021 Mw7.4 Maduo Earthquake, China

Review of Geodetic and Geologic Deformation Models for 2023 U.S. National Seismic Hazard Model

ABSTRACT We review five deformation models generated for the 2023 update to the U.S. National Seismic Hazard Model (NSHM), which provide input fault-slip rates that drive the rate of earthquake moment release. Four of the deformation models use the Global Positioning System-derived surface velocity field and geologic slip-rate data to derive slip-rate estimates (Evans, Pollitz, Shen-Bird, and Zeng), and one model uses geologic data (the “geologic model”). The correlation between the geologic model preferred slip rates and geodetically derived slip rates is high for the Pollitz, Zeng, and Shen-Bird models, and the median of all slip-rate models has correlation coefficient of 0.88. The median geodetic model slip rates are systematically lower than the preferred geologic model rates for faults with slip rates exceeding 10 mm/yr and systematically higher on faults with slip rates less than 0.1 mm/yr. Geodetically derived slip rates tend to the low end of the geologic model range along sections of the San Andreas fault and the Garlock fault, whereas they tend to be higher across north coast California faults. The total on-fault moment rates agree well across models with all rates within 18% of the median. Estimated off-fault strain rate orientations and styles vary considerably across models and off-fault moment rates vary more than on-fault moment rates. Path integrals across the western U.S. accounting for fault-slip rate and off-fault deformation are generally consistent with Pacific-North America plate motion with the median deformation rates recovering about 98% of the plate motion with about 20% of the total plate motion accommodated by off-fault strain rate. The geologic model, which has no off-fault deformation, accounts for about 82% of plate motion with fault slip. Finally, we make a recommendation for relative weighting of the models for the NSHM as well as several recommendations for future NSHM deformation model development.

Interseismic Coupling–Based Stochastic Slip Modeling of the 1920 Ms 8.5 Haiyuan Earthquake

Abstract The 1920 Ms 8.5 Haiyuan earthquake was the largest rupture in China in the twentieth century; however, the coseismic slip characteristics that provide insight into fault kinematics and future seismic hazards are unknown. In this study, we employed stochastic slip modeling to explore plausible slip distributions for this earthquake, incorporating different geodetic fault coupling models as prior constraints. Results demonstrate that fault coupling constraints have both positive and negative effects on stochastic models generating slip scenarios to fit the field-measured geomorphic offset observations. Notably, a Gauss coupling model helps to generate surface slip with higher probabilities to fit geomorphic offsets, exhibiting a closer resemblance to the slip distribution of the Haiyuan earthquake. Integrating 41 slip scenarios of 6000 that reasonably fit the geomorphic offsets, we find that the Haiyuan earthquake likely involved multiple asperities and occurred as a cascading rupture event. The western and eastern fault segments may involve shallow slip deficits, implying potential distributed and/or off-fault deformation during the earthquake, with implications for geomorphic offset–based interpretations of long-term fault behavior. The modeling not only provides insight into the kinematics of the Haiyuan earthquake but also offers a plausible scheme for investigating historical earthquakes.

On the importance of setting 3-D stress field in simulations of on- and off-fault deformation

SUMMARYDuring the last decades, many numerical models have been developed to investigate the conditions for seismic and aseismic slip. Those models explore the behaviour of frictional faults, embedded in either elastic or inelastic media and submitted to a far field loading (seismic cycle models), or initial stresses (single dynamic rupture models). Those initial conditions impact both on-fault and off-fault dynamics. Because of the sparsity of direct measurements of fault stresses, modellers have to make assumptions about these initial conditions. To this day, Anderson’s theory is the only framework that can be used to link fault generation and reactivation to the 3-D stress field. In this work, we look at the role of the 3-D stress field in modelling a 2-D strike-slip fault under plane-strain conditions. We show that setting up an incorrect initial stress field, based on Anderson’s theory, can lead to underestimation of the damage zone width by up to a factor of six, for the studied cases. Moreover, because of the interactions between fault slip and off-fault deformation, initial stress field influences the rupture propagation. Our study emphasizes the need to set up the correct initial 3-D stress field, even in 2-D numerical simulations.

Mechanical Analysis of Fault Slip Rate Sites within the San Gorgonio Pass Region, Southern California USA

Earthquake hazard assessments rely on observations from the field and geophysical data that provide fault slip rate estimates at specific sites and inform the geometry of active faults; however, uncertainty remains for both slip rate and geometry. Furthermore, incompatibilities between inferred fault geometry and geologic slip rates arise within crustal deformation models where model and geologic slip rates disagree. The impact of these incompatibilities may be local to sites or have wider effect on the fault system deformation. Here, we investigate the roles of structural position of sites and uncertainty of slip rates using three-dimensional mechanical models that simulate deformation across many earthquake cycles along southern San Andreas fault near the San Gorgonio Pass in California. Within the models, the impact of strike-slip rate sites on the fault system depends on their structural positions. Slip rates at sites along short and segmented faults has lesser impact on the slip along the fault system than either slip rates at sites along longer faults or at sites within fault branches. Consequently, inaccuracies in the slip rate estimates used for seismic hazard assessment may have differing impacts on the fault system depending on location and structural position of the slip rates. Fault branches along strike-slip faults warrant detailed investigation not only because these areas have high spatial variability of slip rate and accrue nearby off-fault deformation but also because changes in slip rates along branches has larger impact on deformation along fault system than other sites. Lack of data or large uncertainty in slip rate data from fault branches can affect our ability to accurately assess seismic hazard of the region.

Complex tsunamigenic near-trench seafloor deformation during the 2011 Tohoku–Oki earthquake

The near-trench coseismic rupture behaviour of the 2011 Tohoku–Oki earthquake remains poorly understood due to the scarcity of near-field observations. Differential bathymetry offers a unique approach to studying offshore coseismic seafloor deformation but has a limited horizontal resolution. Here we use differential bathymetry estimates with improved horizontal resolutions to investigate near-trench coseismic slip behaviours in the 2011 Tohoku–Oki earthquake. In the main rupture region, a velocity-strengthening behaviour in the shallow fault is observed. By contrast, the seafloor uplift decreases towards the trench, but the trend inverts near the backstop interface outcrop, revealing significant off-fault deformation features. Amongst various competing off-fault effects observed, we suggest that inelastic deformation plays a predominant role in near-trench tsunami excitation. Large trench-bleaching rupture is also observed immediately north of 39°, delimiting the northern extent of the main rupture region. Overall, striking spatial heterogeneity of the shallow rupture behaviour is revealed for the region.

New developments in onshore paleoseismic methods, and their impact on Quaternary tectonic studies

Since the publication of Paleoseismology (2nd Edition) in 2009, there has been no comprehensive survey of new trends in Quaternary tectonics. This paper seeks to remedy that situation, by describing the new technologies and interpretations that arose over the past decade. The major technological advances have been in remote sending, e.g., unpiloted aerial vehicles (drones); airborne laser scanning (lidar); terrestrial laser scanning; 3D topographic surveys from Structure-from-Motion; and satellite geodesy such as D-InSAR. Advances have also been made in dating Quaternary deposits, including single-grain luminescence dating (in the laboratory), and portable optically-stimulated luminescence dating (in the field). Geophysical surveys are now a common component of neotectonic investigations, permitting a more formal, 3D integration of subsurface data with surface data. These techniques have lowered the threshold of recognition to smaller and smaller earthquakes, and allowed detection of off-fault deformation such as distributed faulting and folding. We are now collecting so much data that quality control of coseismic field measurements has become an issue, especially when assembling data sets made of old and new data. Soon this data problem will force a reassessment of our time-honored interpretive paradigms, most of which originated in the 1970s and 80s in the early days of neotectonics.

Coseismic Surface Horizontal Deformation of the 2022 Mw 6.6 Menyuan, Qinghai, China, Earthquake from Optical Pixel Correlation of GF-7 Stereo Satellite Images

AbstractQuantifying surface deformation due to earthquake-related surface rupturing is a critical research focus. Localized offsets on the primary fault can be obtained via field measurements of dislocated landforms. However, effectively quantifying distributed deformation, which can extend for tens to hundreds of meters around the fault zone, has only become possible with the development of remote sensing technology and optical pixel correlation techniques. In this study, we correlated pre- and post-earthquake GaoFen (GF)-2 and -7 images that were ortho-rectified by a digital elevation model generated from GF-7 stereo images to obtain surface horizontal deformation of the 2022 Mw 6.6 Menyuan earthquake. The surface rupture had a total length of 28 km along two segments separated by a stepover; in this study, we focused on the northern segment (23.5 km), which was distributed along the Lenglongling fault (LLLF). The total surface offset measured by our study had the maximum value of 4.0 m and a mean value of 1.9 m. The mean offset measured by field observations captured just 50% of the mean offset from optical pixel correlation. Overall, 57% of off-fault deformation (OFD) occurred on the LLLF, which is a mature fault, owing to soft near-surface materials. Comparison of the surface offset measured by pixel correlation data in our study and near ground slip from joint inversion of Interferometric Synthetic Aperture Radar and pixel correlation data suggests that OFD played a significant role in accommodating the shallow slip. The results of this study offer new insight into the characteristics of surface deformation.

Coseismic deformation of the 1976Ms 7.3 Chaldiran earthquake in eastern Turkey measured by satellite imagery, in comparison with field measurements

SUMMARYLarge continental earthquakes are infrequent, but they are important for understanding active tectonics. Many pre-1990, magnitude greater than 7 earthquakes have not been well documented due to a lack of data. The declassification of KeyHole (KH) imagery provides opportunities for investigating those historical events. In this study, we present new geodetic observations of surface deformation from the 1976 November 24 Ms 7.3 Chaldiran earthquake in eastern Turkey. We use various historical KH imagery and modern Pleiades stereo imagery to determine the coseismic deformation, providing new constraints on the 1976 rupture. Based on the image measurements, we resolve the coseismic slip at depth. The inverted slip from a simple elastic dislocation model, ∼4.0 m at shallow depth of the Hidirmentes segment, is larger than the surface displacement, around 3 m from previous field surveys, suggesting that the coseismic deformation is a combination of slip on the primary fault with 30 ± 16 per cent off-fault deformation. We also measure cumulative offsets at two sites with well-preserved features, and find a constant ratio of horizontal to vertical offsets throughout the past few earthquake cycles. Both the observed constant slip ratio in this study and offset clusters from previous field measurements support that the Chaldiran fault exhibits a characteristic slip behaviour in the late Quaternary. By combining all the geodetic and geological observations in the broader region, we also present a brief analysis of the slip rate of the Chaldiran fault.

Evolution of the off-fault deformation of strike-slip faults in a sand-box experiment

Surface deformation associated with strike-slip faults can be distributed in space, with deformation located either along the primary fault strand or around it and referred to as off-fault deformation (OFD). Fault displacement hazard evaluation require to identify and estimate surface slip rates along active fault strands. We calculate the horizontal and vertical displacement of the analogue models surfaces with optical image correlation and photogrammetry, to investigate the OFD’s development with increasing cumulative deformation. The criterion uses the gradient of the horizontal displacement norm perpendicular to the basal fault. Below 0.005 (noise level), there is no deformation, up to 0.03, it is off-fault-deformation, above 0.03, it is on-fault. We confirm previous observations made on analogue models that the surface deformation starts with a broad diffuse deformation, then produces fault strands alternating with relay zones that may be abandoned and reactivated. OFD is located first between Riedels, then between synthetic shears, and finally takes place in the relay zones. We also show that the OFD initially accommodates 100% of the applied slip (no faults), then decreases abruptly during the Riedels stage down to 20 to 30% to finally remain stable for the rest of the experiment. The abandonment and reactivation of the relay zones has the consequence of maintaining the OFD ratio on a stable value. Our experiments show that, like the OFD ratio, the width of the fault zone decreases with cumulative displacement to reach a stable value. Consequently, the OFD is correlated with this fault zone width and its geometric complexities. The ratios of OFD observed in this study are also consistent with measurements of OFD made on seven natural faults that exhibit different cumulative displacements. Hence our models suggest that strike-slip faults will never reach a continuous, linear geometry, and will always maintain a minimum amount of OFD.

Off‐Fault Deformation in Regions of Complex Fault Geometries: The 2013, Mw7.7, Baluchistan Rupture (Pakistan)

AbstractObservations of recent earthquake surface ruptures show that ground deformations include a localized component occurring on faults, and an off‐fault component affecting the surrounding medium. This second component is also referred to as off‐fault deformation (OFD). The localized component generally occurs on complex networks of faults that connect at depth onto a unique fault plane, whereas OFD consists of distributed fracturing and diffuse deformation of the bulk volume, and occurs over scales of hundreds of meters to kilometers around the faults. High‐resolution optical image correlation presents a unique potential to characterize the complexity of the surface displacements, including on‐fault displacements and OFDs. In this study, we used sub‐pixel correlation of 0.5‐m resolution optical images to measure the surface displacement field with a &lt;20 cm accuracy for a 30‐km long section of the 2013 Mw7.7 Baluchistan, Pakistan, rupture. Our results document significant variability in the fault displacements, associated with large proportions of OFD in regions of fault geometrical complexity. Conversely, in regions where the fault geometry is simple, surface deformation is entirely accommodated by the primary faults with 0% OFD. When combining the localized deformation on faults with the OFD, we show that the total surface displacement budget is constant along the strike of the rupture, despite strong variations observed in the rupture geometry. Based on this analysis, we propose an idealized scenario of earthquake surface deformation as a function of the rupture geometrical variations.

Extremely Large Off-Fault Deformation during the 2021 Mw 7.4 Maduo, Tibetan Plateau, Earthquake

AbstractThe 21 May 2021 Mw 7.4 Maduo earthquake ruptured a 170 km long immature strike-slip fault within the eastern Tibetan plateau. Based on pixel correlation of pre- and postevent Sentinel-2 optical satellite images (band 8; pixel = 10 m), we determine the coseismic horizontal displacement and deformation-zone width at the surface. These results, compared with the on-fault slip from geological measurements, document that &amp;lt;20% of the fault displacement is localized on the fault plane, whereas as high as &amp;gt;80% of the fault displacement occurred as off-fault deformation (OFD), over a mean deformation-zone width of 835 m (ranging from 60 to 1670 m). The OFD% of the Maduo earthquake is significantly larger than the OFD% (28%–64%) of all other (eight in total) previously documented earthquakes occurring on immature strike-slip faults, amongst which five earthquake faults have a structural maturity (cumulative displacement) even lower than the Maduo earthquake fault. These observations may be explained by that (1) the fault maturity is not the only factor controlling the behavior of OFD or the degree of strain localization during an earthquake, or that (2) the calculated OFD includes some elastic deformation due to fault slip reduction in the shallow depth. Our results have an implication that the seismic hazard assessment of immature strike-slip faults is more challenging than previously thought.

Dynamic off-fault failure and tsunamigenesis at strike-slip restraining bends: Fully-coupled models of dynamic rupture, ocean acoustic waves, and tsunami in a shallow bay

Inelastic off-fault deformation can lead to large tsunamigenesis in different tectonic settings. Here a mechanism for tsunami generation by strike-slip earthquakes that involves dynamic off-fault failure at restraining bends is presented. Dynamic rupture on a vertical strike-slip fault is modeled with undrained inelastic off-fault response incorporated by the Drucker-Prager yield criterion. I show that in a local transpressive stress regime dynamic off-fault failure at restraining bends produces significantly larger and more localized surface uplift than is produced by purely elastic dislocation models, resulting in a positive flower and coseismic pop-up structure. The larger uplift is due to frictional sliding with a thrust component on conjugate microfractures, modeled by inelastic deformation. The short-wavelength inelastic uplift, largely controlled by bend size, is shown to generate localized tsunami efficiently in a shallow bay by fully coupling dynamic rupture, ocean acoustic waves, and tsunami. Fully-coupled models also indicate that supershear rupture on a vertical planar strike-slip fault does not generate large tsunami as the large kinetic energy of supershear rupture is carried away by ocean acoustic waves. Dynamic off-fault failure at fault complexities, such as restraining bends and compressional stepovers, may need be urgently incorporated in the current tsunami hazard assessments in strike-slip environments.

How Land Deformation Occurs When Fault Sections Creep

Using a physical experiment, researchers show how off-fault deformation occurs along strike-slip faults with different types of motion.

Shallow distributed faulting in the Imperial Valley, California, USA

Abstract In the tectonically complex Imperial Valley, California (USA), the Imperial fault (IF) is often considered to be the primary fault at the U.S.-Mexico border; however, its strain partitioning and interactions with other faults are not well understood. Despite inferred evidence of other major faults (e.g., seismicity), it is difficult to obtain a holistic view of this system due to anthropogenic surface modifications. To better define the structural configuration of the plate-boundary strain in this region, we collected high-resolution shallow seismic imaging data in the All American Canal, crossing the Imperial, Dixieland, and Michoacán faults. These data image shallow (&amp;lt;25 m) structures on and near the mapped trace of the Imperial fault, as well as the Michoacán fault and adjacent stepover. Integration of our data with nearby terrestrial cores provides age constraints on Imperial fault deformation. These data suggest that the Michoacán fault, unmapped in the United States, is active and likely produces dynamic or off-fault deformation within its stepover to the Dixieland fault. Together, these data support more strain partitioning than previously documented in this region.

Surface ruptures and off-fault deformation of the October 2016 central Italy earthquakes from DInSAR data

Large magnitude earthquakes produce complex surface deformations, which are typically mapped by field geologists within the months following the mainshock. We present detailed maps of the surface deformation pattern produced by the M. Vettore Fault System during the October 2016 earthquakes in central Italy, derived from ALOS-2 SAR data, via DInSAR technique. On these maps, we trace a set of cross-sections to analyse the coseismic vertical displacement, essential to identify both surface fault ruptures and off-fault deformations. At a local scale, we identify a large number of surface ruptures, in agreement with those observed in the field. At a larger scale, the inferred coseismic deformation shows a typical long-wavelength convex curvature of the subsiding block, not directly recognizable in the field. The detection of deformation patterns from DInSAR technique can furnish important constraints on the activated fault segments, their spatial distribution and interaction soon after the seismic events. Thanks to the large availability of satellite SAR acquisitions, the proposed methodological approach can be potentially applied to worldwide earthquakes (according to the environmental characteristics of the sensed scene) to provide a wider and faster picture of surface ruptures. Thus, the derived information can be crucial for emergency management by civil protection and helpful to drive and support the geological field surveys during an ongoing seismic crisis.

Automated near-field deformation detection from mobile laser scanning for the 2014 Mw 6.0 South Napa earthquake

Abstract Quantifying off-fault deformation in the near field remains a challenge for earthquake monitoring using geodetic observations. We propose an automated change detection strategy using geometric primitives generated using a deep neural network, random sample consensus and least squares adjustment. Using mobile laser scanning point clouds of vineyards acquired after the magnitude 6.0 2014 South Napa earthquake, our results reveal centimeter-level horizontal ground deformation over three kilometers along a segment of the West Napa Fault. A fault trace is detected from rows of vineyards modeled as planar primitives from the accumulated coseismic response, and the postseismic surface displacement field is revealed by tracking displacements of vineyard posts modeled as cylindrical primitives. Interpreted from the detected changes, we summarized distributions of deformation versus off-fault distances and found evidence of off-fault deformation. The proposed framework using geometric primitives is shown to be accurate and practical for detection of near-field off-fault deformation.

The influence of off-fault deformation zones on the near-fault distribution of coseismic landslides

AbstractCoseismic landslides are observed in higher concentrations around surface-rupturing faults. This observation has been attributed to a combination of stronger ground motions and increased rock mass damage closer to faults. Past work has shown it is difficult to separate the influences of rock mass damage from strong ground motions on landslide occurrence. We measured coseismic off-fault deformation (OFD) zone widths (treating them as a proxy for areas of more intense rock mass damage) using high-resolution, three-dimensional surface displacements from the 2016 Mw 7.8 Kaikōura earthquake in New Zealand. OFD zones vary in width from ~50 m to 1500 m over the ~180 km length of ruptures analyzed. Using landslide densities from a database of 29,557 Kaikōura landslides, we demonstrate that our OFD zone captures a higher density of coseismic landslide incidence than generic “distance to fault rupture” within ~650 m of surface fault ruptures. This result suggests that the effects of rock mass damage within OFD zones (including ground motions from trapped and amplified seismic waves) may contribute to near-fault coseismic landslide occurrence in addition to the influence of regional ground motions, which attenuate with distance from the fault. The OFD zone represents a new path toward understanding, and planning for, the distribution of coseismic landslides around surface fault ruptures. Inclusion of estimates of fault zone width may improve landslide susceptibility models and decrease landslide risk.