Mantle Earthquakes Research Articles

Identification of Continental Mantle Earthquakes Using Regional Waves Propagating Into a Thinned Continental Crust

AbstractWe evaluate three identifiers of continental mantle earthquakes (CMEs) motivated by surface‐wave normal‐mode theory: the amplitude ratio of Sn to Lg, and the frequency content of Sn and of Lg, after wave propagation through continental crustal thinning. These flexible and easily applicable methods allow for potential new discoveries of CMEs. They rely on guided waves whose propagation is dependent on the uniformity of their waveguides. For a range of Moho models, we perform 2.5D axisymmetric simulations that reach the conventional distance and frequency ranges of observational studies; we compare results from four different source depths straddling the Moho. Our synthetics, and six south‐Tibet earthquakes recorded by an array in Bangladesh, show our Sn/Lg identifier is robust in the presence of crustal thinning, but the identifying frequency contents of Sn and Lg are easily obscured. These results strengthen the utility of Sn/Lg methods for global studies of CMEs.

Relocation of Uppermost Mantle Earthquakes in the Atlas Mountains, Morocco

Relocation of Uppermost Mantle Earthquakes in the Atlas Mountains, Morocco

Ground motion prediction equations for the horizontal component from shallow crustal and upper mantle earthquakes in Japan using site period as site parameter

Ground motion prediction equations for the horizontal component from shallow crustal and upper mantle earthquakes in Japan using site period as site parameter

Effect of a micro-scale dislocation pileup on the atomic-scale multi-variant phase transformation and twinning

In this paper, we perform concurrent atomistic–continuum (CAC) simulations to assess the contribution of the internal stress induced by the microscale dislocation pileup at an atomically structured interface to the atomic-scale phase transformations (PTs), reverse PTs, and twinning. The main novelty of this work is to unify the atomistic description of the interface and the coarse-grained (CG) description of the lagging dislocations away from the interface within one single framework. Our major findings are: (a) the interface dynamically responds to a pileup by forming steps/ledges, the height of which is proportional to the number of dislocations arriving at the interface; (b) the pileup-induced internal stress concentration profile follows neither the classical Eshelby model nor the super-dislocation model alone, but a combination of them; (c) when the pre-sheared sample is compressed, a direct square-to-hexagonal PT occurs ahead of the pileup tip and eventually grows into a wedge shape. The two variants of the hexagonal phases form a twin with respect to each other; (d) upon a further increase of the loading, part of the newly formed hexagonal phase transforms back to the square phase. The square product phase resulting from this reverse PT forms a twin with respect to the initial square phase. All phase boundaries (PBs) and twin boundaries (TBs) are stationary and correspond to zero thermodynamic Eshelby driving forces; and (e) the microscale dislocation pileup-induced internal shear stress and the structural change at the atomic-scale interface reduces the stress required for initiating a PT by a factor of 5.5, comparing with that in the sample containing no dislocations. This work is the first characterization of the behavior of PTs/twinning resulting from the reaction between a microscale dislocation slip and an atomically structured interface. The gained knowledge will advance our understanding of how the multi-phase material behaves in many complex physical processes, such as the synthesis of multi-phase high-entropy alloys or superhard ceramics under high-pressure torsion, deep mantle earthquakes in geophysics, and so on, which all involve dislocation slip, PTs, twinning, and their interactions across from the atomistic to the microscale and beyond.

New ground-motion prediction equations for significant duration of shallow crustal and upper mantle earthquakes in Japan and intra-event spatial correlation

Several models have been proposed and developed to formulate prediction equations for the significant duration (DS) of shallow crustal and subduction earthquakes. However, no equation for DS and a corresponding spatial correlation model have been proposed for upper mantle earthquakes. To address records are selected from the Kyoshin network (K-net) and Kiban Kyoshin network (KiK-net) databases in this study for shallow crustal and upper mantle earthquake with moment magnitudes of 4 ≤ Mw ≤ 7.3, rupture distances Rrup ≤ 300 km and peak ground accelerations (PGAs) ≥ 0.01 g. A new prediction equation is established for DS of shallow crustal and upper mantle earthquakes in Japan by considering the influences of Mw, Rrup, the time-averaged shear-wave velocity to 30 m VS30, the difference between the measured and predicted values of the overburden thickness for a shear wave velocity of 1000 m/s δZ1, and the depth to the top of the rupture Ztor on the DS of ground motion. The rationality and reliability of the proposed prediction model is confirmed using residual analysis and by comparison with existing prediction models. Based on intra-event residuals and single-station intra-event residuals, a semivariogram function and exponential model are used to analyze the spatial correlation of DS, which provides a reference basis for the development of GMPEs based on the nonergodic hypothesis, and regional seismic risk analysis and loss assessment.

Vrancea intermediate-depth focal mechanism catalog: a useful instrument for local and regional stress field estimation

The Vrancea seismic zone, located in the bend region of the South-Eastern Carpathians, is a unique area with both crustal and intermediate-depth seismic activity and is known as one of the most active seismic area in Europe. Moderate crustal seismicity is recorded all over the Carpathian region, but the far more intense activity occurs in a small subcrustal seismogenic volume beneath the SE‐bend of the Carpathian arc with about 20 × 50 km lateral and 110 km vertical extent (70–180 km depth). A unique slab geometry, likely preserved until the present, causes stress localization due to the slab bending and subsequent stress release resulting in large mantle earthquakes in the region. The main focus of this study is to determine the focal mechanisms for events with a magnitude larger than 2.7, between 2005 and 2020 and evaluate the current stress field along the Vrancea subcrustal region, from the derived fault plane solutions. The main style of faulting for Vrancea subcrustal events presents a predominant reverse one, with two main earthquakes categories: the first one with the nodal planes, oriented NE–SW parallel with the Carpathian Arc and the second one with the nodal planes, oriented NW–SE perpendicular on the Carpathian Arc. The results of stress inversion indicate a dominant thrust faulting style, with an average stress regime index of 2.87. The stress pattern shows similar partitioning with vertical extension in the slab and no preferred orientation in the overlying crust, showing a transition regime from the extensional regime in the Moesian Platform to the compressional regime in the Vrancea subcrustal zone.

Error estimates for seismic body wave delay times in the ISC-EHB Bulletin

SUMMARYWe analyse the consistency of the delay time data in the most recent version of the ISC-EHB bulletin published by the International Seismological Centre covering the years 1964–2018. Considering that the delays are influenced by the lateral heterogeneity in the Earth’s mantle, we construct a tomographic matrix. We use singular value decomposition of the tomographic matrix for 19 707 dense clusters of earthquakes to compute objective estimates of the standard error from data that project into the null space and should be zero if there were no errors. Using a robust initial estimate of the standard deviation of the clustered delay times, we remove a small fraction of outliers before calculating the ultimate errors. We found that the errors depend on the type of body wave, depth of the earthquake (crust or mantle) and the number of decimals with which the arrival time was reported. Using these parameters, we distinguish 45 different classes of delay times for 11 different types of body waves. The errors of each class so divided generally follow a distribution that is approximately normal with a mean that ranges from 0.32 s for PKPbc waves from mantle earthquakes, to 2.82 s for S waves from shallow earthquakes bottoming in the upper mantle. The widths of the distributions of the errors themselves are small enough to serve in formal statistical quantification of the quality of fit in tomographic experiments.

Seismic Wave Attenuation Characteristics from the Ground Motion Spectral Analysis around the Kanto Basin

In order to study the seismic wave attenuation characteristics of complex plate tectonics in and around the Kanto Basin, based on the focal mechanism and Slab1.0 model, the research area is divided into four regions. The one-step non-parametric generalized inversion technique was used to analyze the seismic wave attenuation characteristics of each region separately. The results show that the seismic path attenuation of earthquakes occurring in the shallow crust (Reg.1) is weak, and the seismic wave refraction at the crust–mantle boundary leads to almost no attenuation over a long hypocentral distance (>60 km), the frequency–dependent inelastic attenuation is also weak with the 0.5–20 Hz quality factor Q = 92.33f1.87. The seismic path attenuation of the upper mantle earthquakes occurring in the Kanto Basin (Reg.2) is strong, and the attenuation curve decreases with the increase of hypocentral distance, which is approximately parallel to the geometric diffusion R−2.0, the frequency–dependent inelastic attenuation is stronger with the quality factor Q = 27.75f1.08. The seismic path attenuation of subduction zone earthquakes (Reg.3 and Reg.4) is more obvious in the high–frequency band and has a frequency correlation, indicating that the attenuation of subduction zone earthquakes includes more inelastic attenuation. The frequency–dependent inelastic attenuation Q of Reg.3 and Reg.4 are 52.58f0.95 and 58.07f0.89, respectively.

Mantle earthquakes in the Crimea-Black Sea-Caucasus regions

The question of the existence of foci of deep earthquakes in the region of the Crimea-Black Sea-Caucasus is extremely important from the point of view of the geodynamics of the region. Previously it was thought that only crustal earthquakes could occur in this region. Recently, results have been obtained that show that earthquakes with depths of at least 300 km occur in this region. The article discusses the question of how plausible these results are and why they were not obtained earlier. Seven specific examples of the ambiguous determination of the depth of earthquake hypocenters in the Crimea-Black Sea-Caucasus region are considered. These examples clearly show that determining the coordinates of earthquake hypocenters using algorithms based on the Geiger method does not allow one to uniquely determine the depth of the hypocenters. The article gives an idea of the authors about the origin of mantle earthquakes in the Caucasian and Crimean-Black Sea regions. For the Caucasus region, mantle earthquakes are associated with two reasons: submersion of the lithospheric layer; in the asthenospheric layer, represented in the seismotomographic sections by a low-velocity anomaly, the nature of earthquake foci is associated with fluids formed during phase transition reactions. In the Crimean-Black Sea region, earthquake foci are located in the lithosphere layer, and the sliding of the lithosphere along the less viscous underlying layer of the upper mantle causes tectonic movements in the lithosphere accompanied by earthquakes. In addition, to determine the coordinates of the hypocenters of the Crimean and Caucasian earthquakes during routine processing, hodographs were used for depths not exceeding 35 km for the Crimea and 50 km for the Caucasus and 150 for the North Caucasus. This circumstance is the main reason why deep earthquakes could not be detected.

Numerical modelling of the stress in the Pamir-Hindu Kush region

The current stress state of the Earth's crust in Central Asia depends entirely on the interaction of the Eurasian plate with the Indian and Arabian plates. Moreover, an essential role in this action is played by the subduction process in the Pamir-Hind Kush zone. In this region, deep earthquakes and anomalies in seismic velocities indicate subduction of the Indian Plate. To determine the effect of earthquakes on the stress state of the Earth's crust in Central Asia, we analyzed the hypocenters according to the available data over a hundred years. Taking the envelope of the hypocenters as the surface of the subducted indenter, we analyzed its penetration into the mantle. Instead of the pushing force of the indenter, the velocity of the Indian plate is taken into consideration. A model of the stresses is constructed on the equations of creeping motion of a viscous incompressible fluid. The stress variation in the Earth's crust and mantle was calculated as the stress difference before and after the earthquake. The mechanism of earthquakes is modelled by an introduction of equivalent volume forces in the equilibrium equations corresponding to a couple dipoles without a moment. The equations of creeping motion are numerically solved using boundary element methods. Numerical experiments with different physical model parameters for the Earth's crust and mantle were conducted. To introduce the negative buoyancy of the subducting plate, partial eclogitization of the rocks in the collision zone was assumed. As a result, a certain combination of stiffness and density was determined for the lithosphere and mantle, which explained the peculiarities of this region. The influence of crustal and mantle earthquakes in the Pamir-Hindu Kush region on the change in background stresses in Central Asia was analyzed which are not higher than 2–3%.

Tectonic regimes and stress patterns in the Vrancea Seismic Zone: Insights into intermediate-depth earthquake nests in locked collisional settings

Earthquake nests are anomalous clusters of seismicity located far from active collisional systems in intraplate, locked suture zones, or the deep part of relic subducted slabs, challenging classic earthquake generation mechanism theories. The Vrancea Seismic Zone in Romania is such an upper-mantle seismic nest located in the SE Carpathians, releasing the largest strain in continental Europe. To better understand earthquake generation and the relationship with lithospheric deformation, we estimate earthquake source parameters in Vrancea and surrounding regions between 2014 and 2020, and determine the stress field via focal mechanism inversion and unsupervised machine learning. In the crustal domain, maximum horizontal stress is in agreement with surface fault kinematics and GPS-derived S-SE trending horizontal plate velocities relative to Eurasia, implying that tectonic stress is vertically coherent on a crustal scale. The stress regime changes from transpression beneath the orogen to transtension towards the foreland where movement is accommodated along major crustal faults, and tension further away from the epicentre, in the Moesian Platform and the North Dobrogea Orogen. Inside the seismogenic body vertical tension and an overall compressive regime dominates, implying that vertical elongation may be the driving mechanism for brittle failure and that stress is transmitted along the sinking slab to the surface. However, the retrieved stress ratios are low: ~0.2 for mantle earthquakes Mw>4 and ~0.4 for Mw<4, challenging the brittle failure assumption. Increased pore fluid pressure has been shown to lower stress ratios, implying that dehydration embrittlement may contribute to generating intermediate-depth seismicity in the Vrancea slab. Comparisons with seismic tomography and anisotropy studies show excellent correlations between maximum horizontal stress directions, possible slab strike orientation, and seismic anisotropy, especially below ~130km depth, suggesting ambient mantle flow may also promote in-slab stress build-up and seismic potential.

Statistical Analysis of Pre‐earthquake Electromagnetic Anomalies in the ULF Range

AbstractAssessing the statistical significance of electromagnetic anomalies in the ultralow frequency (ULF) range observed prior to earthquakes is a necessary step toward determining whether these perturbations constitute actual earthquake precursors. A statistical epoch analysis (SEA) was recently performed by Han et al. (2014, https://doi.org/10.1002/2014JA019789) to analyze earthquakes happening between 2001 and 2010 near the geomagnetic observatory of Kakioka, Japan; the authors found a significant number of anomalies 6 to 15 days prior to the earthquake day within 100 km from Kakioka, while no significant pre‐earthquake activity was observed for the farther region 100 to 216 km from the observatory. In this current paper, we describe the application of our independent software implementation of their method. Despite using a different outlier rejection scheme, we manage to approximate their results. Upon validation of our program, we conduct multiple sensitivity studies. First, we explore how different outlier rejection schemes impact the results. We then restrict the analysis to only mantle earthquakes, highlighting a marginally significant number of anomalies prior to the earthquake day. Next, we test a higher band‐pass filter than the one initially used but find no anomalous pre‐earthquake activity in this higher‐frequency band. We then use a different catalog to establish the list of qualifying “earthquake days” which also leads the anomalous pre‐earthquake episode to vanish, thus raising concerns about the robustness of the results. Finally, we apply the SEA to another time window, ranging from 2013 to 2018: No significant pre‐earthquake episode can be observed for this interval. We conclude our study by providing guidelines for upcoming work.

Determination of the crustal structure and seismicity of the Linfen rift with S-wave velocity mapping

The Linfen rift is a Cenozoic extensional rift with significant seismicity and seismic hazards. Studies of this rift shed light on deep dynamic processes and seismogenic mechanisms relevant to crustal structure and seismic activity. We first conducted a joint inversion of receiver functions and surface wave dispersion on waveform data collected from 27 broadband seismic stations to image the crustal S-wave velocity in the Linfen rift and its surroundings. We then relocated the source parameters for 10 earthquake events with depths>20 km and studied the relationship between crustal S-wave velocity and seismicity. The results show that low-velocity zones of different scales exist in the middle-lower crust, and that the depth of the seismogenic layer gradually increases from ~25 km in the south to ~34 km in the north, roughly corresponding to the bottom of the low-velocity zone.We found that most of the relocated earthquakes occurred in the low-velocity zone at depths of 18 km to 34 km, with the deepest at 32 km. Two of the greatest historic earthquakes, Linfen (Ms 7.75) in 1695 and Hongtong (Ms 8.0) in 1303, occurred at the bottom of the high-velocity zone at depths of 12 km to 18 km. Our results, combined with previous studies, suggest that the upwelling mantle material below the rift did not remarkably affect the velocity structure from the bottom of the seismogenic layer down to the uppermost mantle nor heat the crust. It is likely that neither crustal-scale faults nor mantle earthquakes exist in the Linfen rift.

Deep-Focus Mantle Earthquakes in the Eastern Part of the Caucasian Isthmus

Deep-Focus Mantle Earthquakes in the Eastern Part of the Caucasian Isthmus

Deep Earthquakes in the Caucasus: Recalculation Results

The question of the existence of deep earthquake sources in the Caucasus is extremely important both for solving geodynamics problems, assessing seismic hazard, and seismic zoning of the region. It was previously believed that earthquakes with depths not exceeding 150 km can occur in the Caucasus. However, in recent years, this has become argumentative, reflected in a number of publications. This article shows that in the Caucasus, in addition to crustal earthquakes, rather deep mantle earthquakes also occur. This, in addition to direct determinations, is confirmed by data published in Caucasian earthquake catalogs and bulletins. The presence of deep earthquake sources in the Caucasus significantly changes our views on the regional structure and geotectonics. Obviously, crustal earthquakes dominate in the region’s seismicity, but it is clear that mantle earthquakes also make a significant contribution. The fact that they are fewer than crustal earthquakes in the total number may indicate that the crust is harder and more brittle than the upper mantle. Keywords: earthquakes, hypocenters, catalogs, bulletins

In situ evidence of earthquakes near the crust mantle boundary initiated by mantle CO2 fluxing and reaction-driven strain softening

This study aims to understand the process behind the worldwide connection between deep crustal/upper mantle earthquakes and CO2 emissions along faults in rift zones. We do this by studying CO2-induced mineral reactions that facilitate strain localization in peridotites from an ancient rift zone in the Seiland Igneous Province (SIP), North Norway.Strain localization in association with hydration processes is well documented in all types of tectonic settings and has major implications for rheological behavior in active plate margin processes. The implications of CO2-bearing fluids are less studied, though experiments have shown how CO2 can influence the flow laws of olivine by imposing a brittle and more localized type of deformation.This study documents narrow shear zones observed within ultramafic rocks from the Seiland Igneous Province (SIP) comprising large volumes (>20,000 km3) of mafic, ultramafic, silicic and alkaline melts that were emplaced into the lower continental crust (25–30 km) between 570 and 560 Ma under an extensional regime. The extensional shear zones are mm cm-scale and contain extremely fine-grained material with a distinct shape preferred orientation (SPO), but weak to absent crystallographic preferred orientation. The shear zones offset dykes across numerous micro-faults that are documented in areas close to a major fault zone cutting through the area. Within the shear zones, olivine and clinopyroxene react to form orthopyroxene and dolomite at approximately 11 kb and 850°C according to the reaction:2 Olivine + Clinopyroxene + 2 CO2 = Dolomite + 2 OrthopyroxeneThis reaction formed coronas of orthopyroxene and dolomite between olivine and clinopyroxene in the shear zones. In addition, large olivine grains proximal to the shear zones show a microfabric with subgrain walls decorated by rounded grains of dolomite and more irregular and elongated grains of orthopyroxene. Clinopyroxene grains are separated from the enstatite and dolomite by at least hundreds of microns, suggesting material transport within the shear zone. The shear zones thus provide a unique insight into the interplay between CO2-metasomatism and reaction accommodated strain softening. Carbonation-driven cracking and mineral reaction also serves to reduce grain size, making grain boundary sliding an efficient process, further enhancing the rheological contrast between the shear zone and the host rock. The sudden decrease in rock strength could lead to rapid deformation and triggered pseudotachylite formation during earthquake events in the near proximity of the micro-shear zones. Our observations match the relations between CO2 emissions and earthquakes observed in present rift environments such as the East African rift and in New Zealand, and underline the importance of active shear zones as fluid conduits in the lower crust and upper mantle.

Revisiting Intraslab Earthquakes Beneath Kyushu, Japan: Effect of Ridge Subduction on Seismogenesis

AbstractThis study investigates intraslab seismicity of the Philippine Sea Plate beneath Kyushu, Japan, where the Kyushu‐Palau Ridge (KPR) is being subducted and the age of the incoming plate varies across the ridge. Intraslab earthquakes at depths of 30–270 km are first relocated, and focal mechanism solutions of 181 earthquakes are determined. The relocated hypocenters show marked along‐arc variations in their depth limits and distributions. The geometry of the slab Moho is delineated by considering observed focal mechanisms, precisely relocated hypocenters, and the depth extent of the hydrated slab crust previously estimated by receiver function analysis. Intraslab earthquakes are classified into crustal and mantle earthquakes based on the inferred Moho geometry. The number of crustal earthquakes decreases monotonically with depth, and little seismicity occurs beyond 100‐km depth, but mantle earthquakes become increasingly active with increasing depth and reach a peak at a depth of ~150 km. An area of locally thickened crustal seismicity (up to 12 km thick) in offshore central Kyushu corresponds to the subducted KPR, suggesting that the KPR influences crustal seismicity to a depth of at least ~60 km. The double seismic zone is clearly observed at depths of 80–150 km in southern Kyushu, where the old Philippine Sea Plate has continued to subduct over at least &gt;5 Myr. Our observations suggest that the genesis of intraslab earthquakes is locally affected by the subduction of the KPR and regionally governed by the along‐arc variation in the formation age of the incoming plate.

Mantle earthquakes in the Himalayan collision zone

AbstractEarthquakes are known to occur beneath southern Tibet at depths up to ∼95 km. Whether these earthquakes occur within the lower crust thickened in the Himalayan collision or in the mantle is a matter of current debate. Here we compare vertical travel paths expressed as delay times between S and P arrivals for local events to delay times of P-to-S conversions from the Moho in receiver functions. The method removes most of the uncertainty introduced in standard analysis from using velocity models for depth location and migration. We show that deep seismicity in southern Tibet is unequivocally located beneath the Moho in the mantle. Deep seismicity in continental lithosphere occurs under normally ductile conditions and has therefore garnered interest in whether its occurrence is due to particularly cold temperatures or whether other factors are causing embrittlement of ductile material. Eclogitization in the subducting Indian crust has been proposed as a cause for the deep seismicity in this area. Our observation of seismicity in the mantle, falling below rather than within the crustal layer with proposed eclogitization, requires revisiting this concept and favors other embrittlement mechanisms that operate within mantle material.

P and S wave travel time tomography of the SE Asia-Australia collision zone

The southeast (SE) Asia - Australia collision zone is one of the most tectonically active and seismogenic regions in the world. Here, we present new 3-D P- and S-wave velocity models of the crust and upper mantle by applying regional earthquake travel-time tomography to global catalogue data. We first re-locate earthquakes provided by the standard ISC-Reviewed and ISC-EHB catalogues using a non-linear oct-tree scheme. A machine learning algorithm that clusters earthquakes depending on their spatiotemporal density was then applied to significantly improve the consistency of travel-time picks. We used the Fast Marching Tomography software package to retrieve 3-D velocity and interface structures from starting 1-D velocity and Moho models. Synthetic resolution and sensitivity tests demonstrate that the final models are robust, with P-wave speed variations (~130 km horizontal resolution) generally recovered more robustly than S-wave speed variations (~220 km horizontal resolution). The retrieved crust and mantle anomalies offer a new perspective on the broad-scale tectonic setting and underlying mantle architecture of SE Asia. While we observe clear evidence of subducted slabs as high velocity anomalies penetrating into the mantle along the Sunda arc, Banda arc and Halmahera arc, we also see evidence for slab gaps or holes in the vicinity of east Java. In the Banda arc, we image the slab as a single curved subduction zone. Furthermore, a high-velocity region in the mantle lithosphere connects northern Australia with Timor and West Papua. The S-wave model shows broad-scale features similar to those of the P-wave model, with mantle earthquakes generally distributed within high-velocity slabs. The high velocity mantle connection between northern Australia and the eastern margin of the Sunda arc is also present in the S-wave model. While the S-wave model has a lower resolution than the P-wave model due to the availability of fewer paths, it nonetheless provides new and complementary insights into the structure of the upper mantle beneath southeast Asia.

Controlling Factors of Seismicity and Geometry in Double Seismic Zones

AbstractDouble seismic zones are ubiquitous features of subduction zones, where seismicity is distributed along two layers separated by a region with significantly less seismic activity. Dehydration embrittlement is thought to be responsible for earthquakes in the subducting crust (upper layer), but the case for it in the lithospheric mantle (lower layer) is less clear. We apply a recently developed relative relocation technique to characterize seismicity in 32 slab segments. The high‐precision hypocentral depths allow us to assign events to either the upper or lower layer and to separately estimate frequency size distributions for each plane. We find consistently larger b values, correlating with slab age, for the upper layer and roughly constant values for the lower. We also show that thermal parameter and plate age are the key controls on double seismic zone geometry. Our results point to a relatively dry lower layer and suggest a fundamentally different mechanism for lithospheric mantle earthquakes.