Receiver functions from a temporary deployment of 25 broadband stations along a north–south transect through Central Australia are used to retrieve crustal and uppermost mantle structural constraints ...from a combination of different methods. Using H–K stacking as well as receiver function inversion, overall thick crust with significant thickness variation along the profile (40 to ≥55km) is found. Bulk crustal vp/vs values are largely in the felsic to intermediate range, with the southernmost stations on the Gawler Craton exhibiting higher values in excess of 1.8. A common conversion point (CCP) stacking profile shows three major discontinuities of the crust–mantle boundary: (1) a two-sided Moho downwarp beneath the Musgrave Province, which has previously been associated with the Neoproterozoic to early Cambrian Petermann Orogeny, (2) a Moho offset along the Redbank Shear Zone further north attributed to the Middle to Late Paleozoic Alice Springs Orogeny, and (3) another Moho offset further north, located at the boundary between the Davenport and Warramunga Provinces, which has not been imaged before. In all cases, the difference in crustal thickness between the two sides of the offset is >8–10km. Unlike the two southern Moho offsets, the northernmost one does not coincide with a prominent gravity anomaly. Its location and the absence of known reactivation events in the region make it likely that it belongs to a Proterozoic suture zone that marks a previously unknown block boundary within the North Australian Craton.
•Receiver function analysis along a 1000km long N–S transect through Central Australia was performed.•Three Moho discontinuities were retrieved, the northernmost of which has been previously unknown.•High vp/vs values obtained for the Gawler Craton, which could hint at a mafic lower crust
Two major earthquakes of Mw7.8 and Mw7.5 ruptured the Southern East Anatolian Fault (SEAF) and the Savrun‐Çardak‐Sürgü fault (SCSF), devastating southeast Türkiye and northwest Syria on 6 February ...2023. We adopt innovative nonlinear and linear approaches to analyze the coseismic ground displacements and estimate the complex slip geometry. Unlike conventional analytical solutions that simplify crust heterogeneity, finite‐element fault models invert the displacement data and simulate the dual‐fault geometry with non‐uniformly distributed shallow crustal materials. Our results suggest the west‐dipping SEAF and north‐dipping SCSF accommodate earthquake slips of >10 m. Their respective slip distributions and proximal aftershocks correlate spatially with local seismic velocity anomalies (i.e., ΔVp and ΔVs), which implies differences in structural control along these two faults and provides insights into assessing the seismic hazard of mixed incipient‐mature fault systems.
Plain Language Summary
In early February of 2023, southeast Türkiye and northwest Syria experienced two devastating earthquakes that claimed more than 50,000 lives and caused substantial economic loss. The epicenters are located near the active tectonic boundary between the Anatolian and Arabian Plate. The complex seismic rupture occurred over two faults, the Southern East Anatolian Fault (SEAF) and the Savrun‐Çardak‐Sürgü Fault (SCSF). Satellite interferometric images and global positioning networks capture large‐scale ground movements. Our study adopts a novel approach to analyze the slip pattern and explore the fault orientation and slip distribution. The result highlights that the fault rupture, aftershock, and local velocity anomalies are correlated, but the correlation pattern differs between the SEAF and SCSF. This improves our understanding of the earthquake hazard along the plate boundaries that host faults of different maturity levels.
Key Points
Coseismic slip distribution of the 2023 Kahramanmaraş Earthquake is estimated from the ground displacements observed by SAR and global navigation satellite system data
Innovative nonlinear‐crossover‐linear inversion method combined with finite element elastic models simulating rock heterogeneity
Earthquake slips and aftershocks correlating spatially with velocity anomalies imply structural controls and different fault maturity
Seismotectonics of the Pamir Schurr, Bernd; Ratschbacher, Lothar; Sippl, Christian ...
Tectonics,
08/2014, Letnik:
33, Številka:
8
Journal Article
Recenzirano
Odprti dostop
Based on a 2 year seismic record from a local network, we characterize the deformation of the seismogenic crust of the Pamir in the northwestern part of the India‐Asia collision zone. We located more ...than 6000 upper crustal earthquakes in a regional 3‐D velocity model. For 132 of these events, we determined source mechanisms, mostly through full waveform moment tensor inversion of locally and regionally recorded seismograms. We also produced a new and comprehensive neotectonic map of the Pamir, which we relate to the seismic deformation. Along Pamir's northern margin, where GPS measurements show significant shortening, we find thrust and dextral strike‐slip faulting along west to northwest trending planes, indicating slip partitioning between northward thrusting and westward extrusion. An active, north‐northeast trending, sinistral transtensional fault system dissects the Pamir's interior, connecting the lakes Karakul and Sarez, and extends by distributed faulting into the Hindu Kush of Afghanistan. East of this lineament, the Pamir moves northward en bloc, showing little seismicity and internal deformation. The western Pamir exhibits a higher amount of seismic deformation; sinistral strike‐slip faulting on northeast trending or conjugate planes and normal faulting indicate east‐west extension and north‐south shortening. We explain this deformation pattern by the gravitational collapse of the western Pamir Plateau margin and the lateral extrusion of Pamir rocks into the Tajik‐Afghan depression, where it causes thin‐skinned shortening of basin sediments above an evaporitic décollement. Superposition of Pamir's bulk northward movement and collapse and westward extrusion of its western flank causes the gradual change of surface velocity orientations from north‐northwest to due west observed by GPS geodesy. The distributed shear deformation of the western Pamir and the activation of the Sarez‐Karakul fault system may ultimately be caused by the northeastward propagation of India's western transform margin into Asia, thereby linking deformation in the Pamir all the way to the Chaman fault in the south in Afghanistan.
Key Points
New crustal seismicity, focal mechanism data from local network for the PamirNew comprehensive neotectonic map for the PamirSeismic deformation is dominated by N‐S shortening and westward extrusion
The b-value can be used to characterize the seismic activity for a given earthquake catalog and provide information on the stress level accumulated at active faults. Here we develop an algorithm to ...objectively estimate variations of b-value along one arbitrary dimension. To this end, we employ a Bayesian transdimensional approach where the seismic domains will be self-defined according to information in the seismic catalog. This makes it unnecessary to prescribe the location and extent of domains, as it is commonly done. We first show the algorithm's robustness by performing regressions from synthetic catalogs, recovering the target models with great accuracy. We also apply the algorithm to a microseismicity catalog for the Central Chile region. This segment is considered a seismic gap where the last major earthquake with shallow slip was in 1730. Our results illuminate the downdip limit of the seismogenic zone and the transition to intraslab seismicity. In the along-strike direction, low b-value coincides with the extent of locked asperities, suggesting a high-stress loading at the Central Chile seismic gap. Our results indicate the reliability of the Bayesian transdimensional method for capturing robust b-value variations, allowing us to characterize the mechanical behavior on the plate interface of subduction zones.
Abstract
Large subduction earthquakes induce complex postseismic deformation, primarily driven by afterslip and viscoelastic relaxation, in addition to interplate relocking processes. However, these ...signals are intricately intertwined, posing challenges in determining the timing and nature of relocking. Here, we use six years of continuous GNSS measurements (2015–2021) to study the spatiotemporal evolution of afterslip, seismicity and locking after the 2015 Illapel earthquake (
$$M_w$$
M
w
8.3). Afterslip is inverted from postseismic displacements corrected for nonlinear viscoelastic relaxation modeled using a power-law rheology, and the distribution of locking is obtained from the linear trend of GNSS stations. Our results show that afterslip is mainly concentrated in two zones surrounding the region of largest coseismic slip. The accumulated afterslip (corresponding to
$$M_w$$
M
w
7.8) exceeds 1.5 m, with aftershocks mainly occurring at the boundaries of the afterslip patches. Our results reveal that the region experiencing the largest coseismic slip undergoes rapid relocking, exhibiting the behavior of a persistent velocity weakening asperity, with no observed aftershocks or afterslip within this region during the observed period. The rapid relocking of this asperity may explain the almost regular recurrence time of earthquakes in this region, as similar events occurred in 1880 and 1943.
We deployed a dense geodetic and seismological network in the Atacama seismic gap in Chile. We derive a microseismicity catalog of >30,000 events, time series from 70 GNSS stations, and utilize a ...transdimensional Bayesian inversion to estimate interplate locking. We identify two highly locked regions of different sizes whose geometries appear to control seismicity patterns. Interface seismicity concentrates beneath the coastline, just downdip of the highest locking. A region with lower locking (27.5°S–27.7°S) coincides with higher seismicity levels, a high number of repeating earthquakes and events extending toward the trench. This area is situated where the Copiapó Ridge is subducted and has shown previous indications of both seismic and aseismic slip, including an earthquake sequence in 2020. While these findings suggest that the structure of the downgoing oceanic plate prescribes patterns of interplate locking and seismicity, we note that the Taltal Ridge further north lacks a similar signature.
Plain Language Summary
Deformation along plate boundaries can occur seismically (i.e. through earthquakes) as well as aseismically (i.e. slipping slowly), and it is important to understand where each of these modes is dominant. Along the Chilean subduction contact, North‐Central Chile is the only place where aseismic deformation episodes have been observed so far. In order to study these processes in detail, we deployed and operated dense geodetic and seismological networks in this region. Analyzing the data collected by these networks, we find notable relationships between seismic and aseismic processes. Thousands of small earthquakes are found at the boundaries of locked regions, whereas no small earthquakes are found at their interior. Thus, implying such regions are mechanically coupled, that is, currently accumulating elastic deformation energy that will 1 day be released during a large earthquake. Along the North‐Central Chilean plate boundary, there is one region (around 27.5°S) that shows many signs of aseismic deformation. It is located where a chain of seamounts is being subducted, which is likely responsible for the different behavior of this segment.
Key Points
Microseismicity catalog and map of interplate locking derived for the Atacama 1922 seismic gap in North‐Central Chile
Seismicity in vicinity of plate interface coincides with downdip edge of high coupling
Seismo‐geodetic signals due to the subduction of the Copiapó ridge are prominent but negligible for the subducting Taltal Ridge
Introduction:
The continental lithosphere of the Northeast China Plain (NECP) is probed using P–wave receiver function analysis of passive seismic data. The NECP is much–discussed as it includes ...different geological provinces of varying tecto–magmatic origin and dates back from Archean to Holocene. Quantifying the tectonic and magmatic influences on the structure and composition of the lithosphere puts important constraints on evolution of NECP. For this, we explore 75 sites across NECP using receiver function analysis.
Methods:
A recently developed technique of inverting for 1–D S–wave velocity profiles beneath seismic stations that is based on the principles of Bayesian statistics (hierarchical transdimensional Bayesian Inversion; HTBI) is applied to receiver functions from the NECP. In addition, an improved crustal thickness–compressional to shear wave velocity ratio (H–κ) analysis was conducted to retrieve the crustal thickness and V
p
/V
s
ratio of the region. These estimated point measurements are integrated and systematically studied for a regional view of the current crustal architecture.
Results and Discussion:
We observe a laterally varying and highly complex lithosphere beneath the NECP. A shallower crust–mantle transition (≤32 km) characterises the Precambrian North China craton and Late Mesozoic–Cenozoic Songliao Basin from the adjacent Central Asian Orogenic Belt and the Changbaishan Volcanic field (35–40 km). Beneath the latter, low V
p
/V
s
ratios (~1.65) are obtained, whereas all other regions feature ratios in excess of 1.75. Multiple velocity gradients are observed at crustal depths within the craton, in contrast to the adjacent orogen, which indicates a higher degree of crustal complexity of the former. The width of the crust–mantle transition across the NECP is found to be mainly intermediate (2–7 km) and occasionally sharp (≤2 km). From our observations, we infer that there is a substantial difference between the eastern North China Craton’s lithospheric architecture and the rest of the NECP, with most of the NECP exhibiting more complexity than previously reported.
Double seismic zones (DSZs) of intermediate-depth intraslab seismicity are observed in many subduction zones around the globe, and have been related to dehydration reactions in the downgoing crust ...and mantle lithosphere. These reactions occur at, to first order, constant temperatures, which explains the observed linear arrangements of seismicity that appear to follow isotherms of thermal models.
Intermediate-depth seismicity in Northern Chile, however, exhibits a pattern of intraslab seismicity that substantially deviates from a classical DSZ. Whereas two parallel seismicity planes are present in the updip part of the slab, these abruptly change into a 25–30 km thick, homogeneously seismogenic volume at a depth of ∼80–100 km. Seismicity rate and moment release significantly increase in this depth interval. In order to understand which processes evoke this configuration and what distinguishes the Northern Chile subduction zone from more conventional subduction zone settings (e.g. Japan), we performed a detailed seismological investigation of slab seismicity in Northern Chile using data from the IPOC permanent network. We determined >600 moment tensors of intraslab earthquakes, processed and evaluated location uncertainties for 8 years of high-resolution earthquake hypocenter data, and performed statistical analysis of the different seismicity populations.
We observe that earthquakes both in the highly active cluster and the DSZ above exhibit consistently downdip extensive source mechanisms that align with the dip angle and direction of the slab. This implies strong slab pull, which is also evident from slab steepening outlined by hypocenters towards the downdip termination of the highly active cluster. Moreover, events in the cluster show a very weak aftershock productivity and a high background event rate, which leads to a temporal distribution of seismicity that is close to a purely random process. We find that the position of the highly seismogenic volume spatially coincides with: 1) the disappearance of the velocity contrast between oceanic crust and the underlying mantle in receiver function images, 2) the transition from the “cold nose” (i.e. the stagnant part) to the hot part of the mantle wedge, as evidenced by seismic attenuation images, and 3) with an increase of the slab dip angle. Based on these different pieces of evidence, we speculate that high tensile stresses and heat input from above could lead to a sudden burst of kinetically delayed metamorphic reactions there that then enables the observed increased seismicity rates. Since these reactions have overall a negative volume change that leads to slab densification and hence further increases slab pull, the spatial pattern of seismicity we observe could result from a runaway-type process, which would explain its abrupt start and high moment release rates.
•Downdip transition from double seismic zone to 25-30 km thick seismically active volume observed in Northern Chile•Such behavior cannot be explained with metamorphic dehydration reactions that occur at constant temperature conditions•We analyze moment tensors and statistical properties of seismicity and compare to existing imaging evidence•Seismicity transition coincides with transition cold to warm mantle wedge above as well as sudden slab steepening•Cascade-like metamorphic reactions of kinetically delayed material due to T input and slab pull could cause these features
In this study, we present high‐resolution seismicity images of the northern Chile subduction zone forearc. We used 8 years of continuous seismic waveform data from the Integrated Plate Boundary ...Observatory Chile network and auxiliary stations to produce an extensive earthquake catalog containing 101,601 double‐difference relocated earthquake hypocenters using automatic event detection and phase picking routines. The minimum magnitude of retrieved events is <2, and the catalog is estimated to be complete at magnitudes above ∼2.8. Intraslab seismicity makes up the majority of detected earthquakes. Where the seismogenic zone of the megathrust is active, a clear separation of seismicity into three distinct planes can be observed. The uppermost plane corresponds to the plate interface, which is observed to terminate downdip at a depth of 50–55 km. The other two planes, located ∼7 and ∼26 km below the slab surface, dip at a constant angle of about 20° until they are absorbed by a 25 km thick highly active cluster of intermediate‐depth seismicity at depths of 80–120 km. Downdip of this cluster, the slab steepens and lower plate seismicity is considerably sparser, even absent in the northern part of the study area. Upper plate seismicity is also considerable, with a segment between 21 and 21.6°S standing out for featuring pervasive activity occurring all the way down to the plate interface. Here the seismicity resembles a wedge in west‐east profile view and occurs where the upper plate crust is coldest based on thermal models.
Key Points
Eight years of IPOC network seismic data were harvested for microseismicity, obtained catalog features 101k events
Majority of seismicity is at intermediate depths, where a usual double seismic zone grades into a highly active, 25 km thick cluster
Pervasive upper plate seismicity is observed in an area around 21 degrees S, forming a wedge‐like structure extending down to the interface
We study the spatial variability of the crustal stress in northern Chile. A margin‐parallel compressive crustal stress regime is inferred along the coastal region between 19° and 23.5°S, similar to ...stress observations in Cascadia and Japan. The Andean Precordillera shows a distinct stress field associated with a strike‐slip faulting regime around 21°S. These results are constrained by over a decade of observations, for which earthquake catalogs report thousands of events in the continental crust. We present focal mechanisms for 817 of these crustal earthquakes, including mechanism qualities. The best mechanisms were grouped and inverted to infer the stress‐field variability. We interpret the margin‐parallel compression to be caused by the concave shape of the margin and the locking of the plate interface. The inferred strike‐slip regime in the Andes agrees with previous studies and has been proposed to be mostly caused by local stresses imposed by a thicker crust.
Plain Language Summary
New observations of thousands of earthquakes occurring within the continental crust (depths <60 km) in northern Chile provide an opportunity to study the tectonic forces acting in this region of the South American continent. We obtain fault orientations and slip directions of 817 crustal earthquakes. The orientations are used to understand the stresses that cause deformation of the crust. With hundreds of earthquakes studied, we can resolve differences in the stress between coastal and inland regions: The coastal region experiences a compression along an approximate north‐south direction. Further east, near the Andes mountains, compression is nearly east‐west, almost parallel to the collision direction of the tectonic plates. This could be mostly due to local stresses acting in higher topography regions. Here, earthquakes occur mostly in nearly vertical faults with slip in the horizontal direction. Conversely, the compression near the coast is likely due to the bending of this region along the coastline, in combination with the locking on the plate interface between the Nazca and South American tectonic plates. The results are remarkably similar to western North America and Japan, where the shape of plate boundaries cause similar stresses.
Key Points
We resolve spatial variability of the regional crustal stress field in northern Chile based on focal mechanisms of crustal earthquakes
Margin‐parallel compressional crustal stress is observed along the coast and may be due to the concave margin and friction on the interface
A strike‐slip regime is observed toward the Andean Precordillera at 21°S, where the elevated topography could affect the local stress
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