Transform faults are a fundamental tenet of plate tectonics, connecting offset extensional segments of mid‐ocean ridges in ocean basins worldwide. The current consensus is that oceanic transform ...faults initiate after the onset of seafloor spreading. However, this inference has been difficult to test given the lack of direct observations of transform fault formation. Here we integrate evidence from surface faults, geodetic measurements, local seismicity, and numerical modeling of the subaerial Afar continental rift and show that a proto‐transform fault is initiating during the final stages of continental breakup. This is the first direct observation of proto‐transform fault initiation in a continental rift and sheds unprecedented light on their formation mechanisms. We demonstrate that they can initiate during late‐stage continental rifting, earlier in the rifting cycle than previously thought. Future studies of volcanic rifted margins cannot assume that oceanic transform faults initiated after the onset of seafloor spreading.
Key Points
We document the initiation of a proto‐transform fault during late‐stage continental rifting in Afar, Ethiopia
Surface faulting, seismicity, and geodetic observations reveal extensional transfer between magmatic segments
Numerical modeling predicts that the region between the offset magmatic segments will evolve to a stable oceanic transform fault
The 2008 Mw 7.9 Wenchuan earthquake, one of the largest continental intraplate events instrumentally recorded, struck the central part of Sichuan Province in southwestern China causing great ...destruction and loss of life but also providing a wealth of seismological data, geodetic measurements, and tectonic observations. The Wenchuan earthquake ruptured two northwest‐dipping imbricate oblique reverse faults along the middle segment of the Longmenshan fault zone—a northeast‐trending thrust belt located at the boundary between the Tibetan Plateau and the Sichuan Basin. In this study, a hybrid approach that combines deterministic modeling at low frequencies with stochastic modeling at high frequencies is used to simulate broadband ground motions at 52 strong‐motion stations and 506 geodetic sites in the vicinity of the causative fault. The low‐frequency components of the synthetic ground motion are simulated using an extended kinematic source model embedded in a layered medium, whereas the high‐frequency components are generated using a stochastic finite‐fault model. The two independently derived ground‐motion components are then combined using matched filtering at a crossover frequency of 0.8 Hz to generate broadband ground‐motion time histories and response spectra. The temporal and spectral characteristics of the synthetic and recorded ground motions at the 52 strong‐motion stations are compared and the effect of soil nonlinearity on the simulated ground motions is investigated through 1‐D nonlinear site response analysis. Finally, the simulated permanent ground displacements at the 506 geodetic sites are evaluated against geodetic observations and the peak amplitudes of the synthetic ground motions at the same locations are compared with predictions of empirical ground‐motion models.
Recent studies using satellite data have shown a growing interest in detecting and anticipating landslide failures. However, their value for an actual landslide prediction has shown variable results. ...Therefore, the use of satellite images for that purpose still requires additional attention. Here, we study the landslide of the Tunnel du Chambon in the French Alps that ruptured in July 2015, generating major impacts on economic activity and infrastructures. To evaluate the contribution of very high-resolution optical satellite images to characterize and potentially anticipate the landslide failure, we conduct here a retro analysis of its evolution. Two time periods are analyzed: September 2012 to September 2014, and May to July 2015. We combine Pléiades optical images analysis and geodetic measurements from in situ topographic monitoring. Satellite images were correlated to detect pre-failure motions, showing 1.4-m of displacement between September 2012 and September 2014. In situ geodetic measures were used to analyze motions during the main activity of the landslide in June and July 2015. Topographic measurements highlight different areas of deformations and two periods of strong activity, related to the last stage of the tertiary creep and to anthropic massive purges of unstable masses. The law of acceleration toward the rupture observed in June and July 2015 over the topographic targets also fits well the satellite observation between 2012 and 2014, showing that the landslide probably already entered into tertiary creep 2.5 years before its failure.
We present a neural network-based method to detect anomalies in time-dependent surface deformation fields given a set of geodetic images of displacements collected from multiple viewing geometries. ...The presented methodology is based on a supervised classification approach using combinations of line of sight multitemporal, multi-geometry interferometric synthetic aperture radar (InSAR) time series of displacements. We demonstrate this method with a set of 170 million time series of surface deformation generated for the entire Italian territory and derived from ERS, ENVISAT, and COSMO-SkyMed Synthetic Aperture Radar satellite constellations. We create a training dataset that has been compared with independently validated data and current state-of-the-art classification techniques. Compared to state-of-the-art algorithms, the presented framework provides increased detection accuracy, precision, recall, and reduced processing times for critical infrastructure and landslide monitoring. This study highlights how the proposed approach can accelerate the anomalous points identification step by up to 147 times compared to analytical and other artificial intelligence methods and can be theoretically extended to other geodetic measurements such as GPS, leveling data, or extensometers. Our results indicate that the proposed approach would make the anomaly identification post-processing times negligible when compared to the InSAR time-series processing.
The redistribution of past and present ice and ocean loading on Earth's surface causes solid Earth deformation and geoid changes, known as glacial isostatic adjustment. The deformation is controlled ...by elastic and viscous material parameters, which are inhomogeneous in the Earth. We present a new viscoelastic solid Earth deformation model in ASPECT (Advanced Solver for Problems in Earth's ConvecTion): a modern, massively parallel, open‐source finite element code originally designed to simulate convection in the Earth's mantle. We show the performance of solid Earth deformation in ASPECT and compare solutions to TABOO, a semianalytical code, and Abaqus, a commercial finite element code. The maximum deformation and deformation rates using ASPECT agree within 2.6% for the average percentage difference with TABOO and Abaqus on glacial cycle (∼100 kyr) and contemporary ice melt (∼100 years) timescales. This gives confidence in the performance of our new solid Earth deformation model. We also demonstrate the computational efficiency of using adaptively refined meshes, which is a great advantage for solid Earth deformation modeling. Furthermore, we demonstrate the model performance in the presence of lateral viscosity variations in the upper mantle and report on parallel scalability of the code. This benchmarked code can now be used to investigate regional solid Earth deformation rates from ice age and contemporary ice melt. This is especially interesting for low‐viscosity regions in the upper mantle beneath Antarctica and Greenland, where it is not fully understood how ice age and contemporary ice melting contribute to geodetic measurements of solid Earth deformation.
Plain Language Summary
Mass changes on the Earth's surface, for example, from melting ice sheets or sea level rise, cause deflections of Earth's surface as interior rocks deform and flow. Scientists have developed models of the interior deformation resulting from loads applied to Earth's surface. Such models depend on the viscous and elastic properties of interior rocks, which quantify their capacity to deform and flow. However, because the Earth is heterogeneous, its viscoelastic properties exhibit large lateral variations that have proven difficult to accommodate within a (numerical) model. Here, we present and benchmark a new application of the open‐source code in ASPECT (Advanced Solver for Problems in Earth's ConvecTion), which was originally designed to model mantle convection occurring on timescales of millions of years or longer. The ASPECT code makes use of modern numerical methods, such as adaptive mesh refinement and advanced solver techniques. In particular, we show that this code is accurate and useful for modeling solid Earth deformation occurring on timescales relevant to contemporary (in response to climate change) and ice age melting (from decades to millennia). This code is especially useful for studying regions with both past and present ice melt and a heterogeneous Earth structure, such as Greenland and Antarctica.
Key Points
The solid Earth is deforming in response to past and present ice loading changes at rates determined by elastic and viscous parameters
We benchmark a new viscoelastic solid Earth deformation model in the open‐source code ASPECT in combination with adaptive mesh refinement
This code can be used to study regional Earth deformation rates from ice age and contemporary ice melt on a laterally heterogeneous Earth
The ~400‐km‐long Talas‐Fergana Fault is one of a series of major right‐lateral strike‐slip faults that cross the Tien Shan Range. This fault has been recognized as active in the late Holocene and ...accommodates part of the deformation induced by the ongoing Indo‐Asian collision. The kinematics and the role of this strike‐slip fault are poorly understood with no large earthquakes reported in the instrumental or historical catalogs, and no well‐constrained geological slip‐rate estimates. Here we used high‐resolution satellite imagery to present a first detailed analysis of the fault segmentation. We identified nine geometric segments based on strike variations for the Talas‐Fergana Fault. Along the Kyldau segment, through morphological analyses of an offset alluvial fan and the application of multiple dating methods (10Be, 26Al, 36Cl, luminescence, and radiocarbon), we calculated a late Quaternary slip rate ranging from 2.2 to 6.3 mm/year. This rate is higher than the geodetic measurements, but the discrepancy can be partly explained if the Talas‐Fergana Fault accommodates shortening by counterclockwise rotation around a vertical axis. Paleoearthquakes identified by trenching indicate that at least two primary surface ruptures (and possibly a third) occurred in the past 3,800 years, and that no large earthquake has ruptured the Kyldau segment since at least 420 years B.P. (possibly within the last 2,700 years), making this fault segment a potential candidate to generate an earthquake with M > 7 in the near future.
Key Points
The 400‐km‐long Talas‐Fergana Fault (TFF) can be subdivided into nine geometric segments
A first geological slip rate of 2.2–6.3 mm/year is constrained by geochronological dating of an alluvial fan
The TFF may be associated with counterclockwise rotations around a vertical axis. This model reconciles geodetic and geologic slip rates
Increased rates of glacier retreat and thinning need
accurate local estimates of glacier elevation change to predict future
changes in glacier runoff and their contribution to sea level rise. Glacier
...elevation change is typically derived from digital elevation models (DEMs)
tied to surface change analysis from satellite imagery. Yet, the rugged
topography in mountain regions can cast shadows onto glacier surfaces,
making it difficult to detect local glacier elevation changes in remote
areas. A rather untapped resource comprises precise, time-stamped metadata on
the solar position and angle in satellite images. These data are useful for
simulating shadows from a given DEM. Accordingly, any differences in shadow
length between simulated and mapped shadows in satellite images could
indicate a change in glacier elevation relative to the acquisition date of
the DEM. We tested this hypothesis at five selected glaciers with long-term
monitoring programmes. For each glacier, we projected cast shadows onto the
glacier surface from freely available DEMs and compared simulated shadows to
cast shadows mapped from ∼40 years of Landsat images. We
validated the relative differences with geodetic measurements of glacier
elevation change where these shadows occurred. We find that shadow-derived
glacier elevation changes are consistent with independent photogrammetric
and geodetic surveys in shaded areas. Accordingly, a shadow cast on Baltoro
Glacier (the Karakoram, Pakistan) suggests no changes in elevation between 1987
and 2020, while shadows on Great Aletsch Glacier (Switzerland) point to
negative thinning rates of about 1 m yr−1 in our sample. Our estimates
of glacier elevation change are tied to occurrence of mountain shadows and
may help complement field campaigns in regions that are difficult to access.
This information can be vital to quantify possibly varying
elevation-dependent changes in the accumulation or ablation zone of a given
glacier. Shadow-based retrieval of glacier elevation changes hinges on the
precision of the DEM as the geometry of ridges and peaks constrains the
shadow that we cast on the glacier surface. Future generations of DEMs with
higher resolution and accuracy will improve our method, enriching the
toolbox for tracking historical glacier mass balances from satellite and
aerial images.
Velocity variations obtained from ambient seismic noise are sensitive to many factors. We aimed to disentangle these processes in a 10‐year‐long recording of seismic noise from a single station in ...the Pollino region, in southern Italy. This region is characterized by aquifers and by a relatively short period of high seismic activity that included slow slip events and a MW5.0 earthquake that occurred on October 25, 2012. We apply two models that estimate the water level inside an aquifer, which show a good correlation with the measured δv/v, showing that the velocity variations are inversely proportional to the pore pressure inside the aquifer. Our interpretation is further confirmed by geodetic measurements that show that in a direction parallel to the strike angle of the fault rupture, the expansion‐contraction displacement of the zone follows the same patterns observed in the models and in the velocity variations, as a result of the pressure generated by the water on its interior. Going one step further, we analyze the nature of the small discrepancies between the measured and modeled velocity variations. These correlate well with the rainfall and with the vertical geodetic measures, which indicates an elastic response of the zone to the loading generated by the rainwater. Comparisons between these variables allow us to clearly identify the period of the seismic activity in the zone, which is represented by the characteristic drop in the seismic velocity in the period from the beginning of 2012 to mid‐2013.
Key Points
Noise‐based velocity variation measurements are explained by modeled water levels inside an aquifer
Geodetic evidence of elastic and poroelastic response of the zone to the rainfall
Identification of the effect of seismic active period in the velocity variations
Climate shifts are particularly conspicuous in glaciated areas. Satellite and terrestrial observations show significant increases in the melting and breakup of tidewater glaciers and their influence ...on sea level rise and ocean mixing. Increasing melt rates are creating an urgency to better understand the link between atmospheric and oceanic conditions and glacier frontal ablation through iceberg calving and melting. Elucidating this link requires a combination of short- and long-timescale measurements of terminus activity. Recent work has demonstrated the potential of using underwater sound to quantify the time and scale of calving events to yield integrated estimates of ice mass loss (Glowacki and Deane, 2020). Here, we present estimates of subaerial calving flux using underwater sound recorded at Hansbreen, Svalbard, in September 2013 combined with an algorithm for the automatic detection of calving events. The method is compared with ice calving volumes estimated from geodetic measurements of the movement of the glacier terminus and an analysis of satellite images. The total volume of above-water calving during the 26 d of acoustical observation is estimated to be 1.7±0.7×107 m3, whereas the subaerial calving flux estimated by traditional methods is 7±2×106 m3. The results suggest that passive cryoacoustics is a viable technique for long-term monitoring of mass loss from marine-terminating glaciers.
Block Kinematics in North China From GPS Measurements Wang, Wei; Zhao, Bin; Qiao, Xuejun ...
Geochemistry, geophysics, geosystems,
March 2022, 2022-03-00, 20220301, 2022-03-01, Letnik:
23, Številka:
3
Journal Article
Recenzirano
Odprti dostop
The North China Craton (NCC) is an important tectonic element in China, which is characterized by intense seismic activity and complex tectonic setting. Thus quantifying its deformation is essential ...for studying the tectonic processes and deformation mechanisms in this region. Here we use a combination of dense GPS data sets to analyze the crustal movement and present updated estimates of fault motions based on elastic block models. We uncover spatial variations in deformation patterns, indicating that different mechanisms may dominate the tectonic processes for different parts of the NCC. In the western part, the rifts around the Ordos block manifest strike‐slip and extensional deformation, and the strike‐slip motion is more significant than the extensional component; whereas in the east, the Tanlu fault experiences right‐lateral and shortening slips, consistent with the features from historical seismic activity and regional tectonics. Our analysis indicates that the deformation in the eastern NCC is principally influenced by the Pacific subduction, whereas the deformation in the western NCC is mainly related to the India‐Eurasia collision. Moreover, our estimate of motion along the Tanlu fault supports a relatively long seismic interval of ∼10 ka, comparable to those inferred from recent paleoseismic research on this fault and other slow‐slip faults in the NCC. Our study indicates that the faults in the NCC even with very slow motions may still have the potential to produce strong earthquakes. Thus seismic risks across these slow‐slip faults cannot be ignored and should not be determined solely based on the rate of fault motions.
Plain Language Summary
North China is situated at the eastern part of the Chinese continent, which is characterized by intense seismic activity and dense population. Numerous strong earthquakes have occurred in this region that caused huge casualties during the past decades. Thus quantifying the fault slip rates is essential for understanding the deformation regimes and seismic risk assessments in this region. Here we use dense space‐geodetic measurements to analyze the crustal movement and fault motion in the study area. We reveal that the fault motion derived in our study exhibits distinct features of deformation from west to east of the region, implying that different regimes may dominate the tectonic processes in different parts of North China. Moreover, our analysis suggests that the faults even with very slow motions in this region may still have the potential to produce strong earthquakes. Thus, seismic hazards across these slow‐slip faults cannot be ignored and should not be determined solely based on the rate of fault motions.
Key Points
We use dense GPS measurements and elastic block models to quantify the faulting kinematics and slip partitioning in North China
We find spatial variations of kinematics, implying that different regimes may dominate tectonic processes for different parts of North China
Seismic risks along slow‐slip faults cannot be ignored and should not be determined solely based on the rate of fault motions in North China
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