A three-dimensional (3D) P-wave seismic velocity (Vp) model of the crust at the northern South China Sea margin drilled by IODP Expeditions 367/368/368X has been obtained with first-arrival travel-time tomography using wide-angle seismic data from a network of 49 OBSs and 11 air-gun shot lines. The 3D Vp distribution constrains the extent, structure and nature of the continental, continent to ocean transition (COT), and oceanic domains. Continental crust laterally ranges in thickness from ~8 to 20 km, a ~20 km-width COT contains no evidence of exhumed mantle, and crust with clear oceanic seismic structure ranges in thickness from ~4.5 to 9 km. A high-velocity (7.0-7.5 km/s) lower crust (HVLC) ranges in thickness from ~1 to 9 km across the continental and COT domains, which is interpreted as a proxy of syn-rift and syn-breakup magma associated to underplating and/or intrusions. Continental crust thinning style is abrupter in the NE segment and gradual in the SW segment. Abrupter continental thinning exhibits thicker HVLC at stretching factor (β) <~3, whereas gentler thinning associates to thinner HVLC at β>~4. Opening of the NE segment thus occurred by comparatively increased magmatism, whereas tectonic extension was more important in the SW segment. The Vp distribution shows the changes in deformation and magmatism are abrupt along the strike of the margin, with the segments possibly bounded by a transfer fault system. No conventional model explains the structure and segmentation of tectonic and magmatic processes. Local inherited lithospheric heterogeneities during rifting may have modulated the contrasting opening styles.

As high-resolution global coverage cannot easily be achieved by direct bathymetry, the use of gravity data is an alternative method to predict seafloor topography. Currently, the commonly used algorithms for predicting seafloor topography are mainly based on the approximate linear relationship between topography and gravity anomaly. In actual application, it is also necessary to process the corresponding data according to some empirical methods, which can cause uncertainty in predicting topography. In this paper, we established analytical observation equations between the gravity anomaly and topography, and obtained the corresponding iterative solving method based on the least square method after linearizing the equations. Furthermore, the regularization method and piecewise bilinear interpolation function are introduced into the observation equations to effectively suppress the high-frequency effect of the boundary sea region and the low-frequency effect of the far sea region. Finally, the seafloor topography beneath a sea region (117.25°-118.25° E, 13.85°-14.85° N) in the South China Sea is predicted as an actual application, where gravity anomaly data of the study area with a resolution of 1′×1′ is from the DTU17 model. Comparing the prediction results with the data of ship soundings from the National Geophysical Data Center (NGDC), the root-mean-square (RMS) error and relative error can be up to 127.4 m and approximately 3.4%, respectively.

Volcanic glass and its mixture with smectite are commonly observed in shallow parts of subduction zones. As volcanic glass layers often act as a glide plane to induce mass transportation such as submarine landslides, and because its alteration product, smectite, is one of the frictionally weakest geological materials, the frictional characteristics of volcanic glass-smectite mixtures are important for fault slip behavior in shallow parts of subduction zones. We performed a series of friction experiments on volcanic glass-smectite mixtures with different smectite contents at various velocity conditions from 10 μm/s to 1 m/s under an effective normal stress of 5 MPa and pore pressure of 10 MPa. In general, friction coefficients negatively depend on the smectite content at any velocity tested. We found that samples with smectite contents of 15-30 % showed a drastic slip-weakening behavior at intermediate velocities of 1-3 mm/s with a characteristic slip displacement of ~0.1 m. Finite element method modeling shows that thermal pressurization does not contribute to the observed weakening behavior. We propose that gouge fluidization or compaction-induced pore pressure increase may be the cause of the weakening. The slip-weakening behavior at intermediate velocities enlarges a critical nucleation length for frictional instability to 1-30 km, or prevent acceleration to seismic slip velocities. Therefore, gouges with minor amount of clay, such as subducting volcanic ash layers, may contribute to the occurrence of the at shallow depths in subduction zones.

We use seismic ambient noise correlation and coda wave interferometry to estimate velocity variations at high temporal resolution, during the pre-eruptive period and the onset of the 2018 eruption of Kilauea volcano. A progressive velocity increase is observed from March to the end of April. It is followed by rapid decrease starting a few days before the onset of the East Rift Zone (ERZ) eruption and then by sharp velocity drop when the eruption started. The change of trend from velocity increase to decrease is progressively delayed by a few days from the summit caldera toward the ERZ. The location of the velocity perturbations shows a migration of the sources of velocity changes from the summit caldera toward the ERZ before the eruption. Using a model of pressure source, we show that the simultaneous caldera inflation and velocity increase probably result from an anisotropic distribution of fault and crack orientations. The velocity decrease could be due to damaging processes above the shallow reservoir and to plastic deformations around the caldera. We introduce a forward model of rock damage associated with the volcano-tectonic seismicity to calculate the velocity decrease. The good agreement between the calculated and the observed velocity variations shows that a large part of the velocity decrease results from damage of the medium. The delayed onsets of velocity decrease and the source migration of velocity perturbations are probably related to progressive fault openings in the Southern and Eastern parts of the caldera and to magma transfer toward the ERZ.

The Paleo-Tethys and Panthalassa are two major oceans that witnessed the end-Permian mass extinction, and they have been suggested to have distinct compositions, with the Paleo-Tethys Ocean euxinic, and the much larger Panthalassa Ocean being largely ventilated. Distinctions of these two once-connected oceans imply that interactions between them must have been restricted shortly before the end-Permian extinction. However, detailed geological processes for the disconnection between them along the eastern Paleo-Tethys Ocean due to the collision of North and South China, are still unclear. Previous geochronological studies on eclogite facies rocks in the Dabie–Sulu orogenic belt, which are the metamorphic products of the collision between North and South China, have yielded mainly Triassic metamorphic ages. Nonetheless, new Permian metamorphic ages are identified from southeastern North China, northern Dabie, and the Permo–Triassic intracontinental orogen of South China, which may collectively closely associate this major tectonic event with the end-Permian extinction. New age dating results, as well as a synthesis of recent studies on metamorphic rocks, show that the onset of the collisional orogenesis dates back to the Middle Permian (270–252 Ma). We thereby provide a new tectonic model for the major continents of East Asia, in which the initial collision between North and South China during the Middle Permian critically isolated the Paleo-Tethys Ocean from the Panthalassa Ocean, facilitating the oceanographic transition of the once fossiliferous Paleo-Tethys from a life-giving nutrient-rich ocean into a euxinic death trap, thereby serving as prelude to the end-Permian extinction.

We exploit nonlinear elastodynamic properties of fractured rock to probe the micro-scale mechanics of fractures and understand the relation between fluid transport and fracture aperture and area, stiffness proxy, under dynamic stressing. Experiments are conducted on rough, tensile-fractured Westerly granite specimen subject to triaxial stresses. Fracture permeability is measured from steady-state fluid flow with deionized water. Pore pressure oscillations are applied at amplitudes ranging from 0.2 to 1~MPa at 1~Hz frequency. During dynamic stressing we transmit acoustic signals through the fracture using an array of piezoelectric transducers (PZTs) to monitor the evolution of fracture interface properties. We examine the influence of fracture aperture and contact area by conducting measurements at effective normal stresses of 10, 12.5, 15, 17.5, and 20~MPa. Additionally, the evolution of contact area with stress is characterized using pressure sensitive film. These experiments are conducted separately with the same fracture and they map contact area at stresses from 9 to 21~MPa. The resulting ‘true’ area of contact measurements made for the entire fracture surface and within the calculated PZT sensor footprints, numerical modeling of Fresnel zone. We compare the elastodynamic response of the the fracture using the stress-induced changes ultrasonic wave velocities for a range of transmitter-receiver pairs to image spatial variations in contact properties, which is informed by fracture contact area measurements. These measurements of the nonlinear elasticity are related to the fluid-flow, permeability, in response to dynamic stressing and similar comparisons are made for the slow-dynamics, recovery, of the fracture interface following the stress perturbations.

Earthquakes are frequently accompanied by electromagnetic (EM) anomalies. These anomalies are thought to be caused by earthquakes but the generation mechanism is still unclear. The piezoelectric effect has been proposed as a possible mechanism but the EM responses to earthquakes due to such an effect has not been well understood. In this article, we study the EM signals generated by an earthquake source due to the piezoelectric effect. We develop a semi-analytical method to solve the seismic and EM fields in a 3D layered model and conduct numerical simulations to investigate the characteristics of the EM fields. The results show that the earthquake can generate two kinds of EM signals. One is the early-EM signal which arrives earlier than the seismic wave. The other is the co-seismic EM signal accompanying the seismic wave. For an earthquake the co-seismic electric field can reach ~10 μV/m and the magnetic field can reach ~10-4 nT. We also study the sensitivity of the co-seismic EM fields to the rock conductivity. The results show that the co-seismic EM fields are mainly affected by the conductivity of the shallow layer, and they are also affected by the conductivity of the deep layer when the top layer is thin.

Quaternary convergence at rates of ~10 mm/yr between the Arabian and Eurasian plates is largely accommodated by the Kura fold-thrust belt at the longitude of the Greater Caucasus Mountains in Azerbaijan and eastern Georgia. Here we present the results of the first paleoseismic study of the Kura fold-thrust belt in Azerbaijan. A single paleoseismic trench was excavated across a 2-m-high fault scarp near Agsu revealing evidence of two recent surface rupturing earthquakes. Radiocarbon dating of the faulted sediments places limits of earthquake timing of AD 1713-1895 and AD 1872-2003 for the two events. Allowing for uncertainties in radiocarbon dating, the two events likely correspond to historical destructive M~7 earthquakes near Shamakhi, Azerbaijan in AD 1668 and 1902. Holocene shortening and dip-slip rates for the Kura fold-thrust belt are 8 and 8.5 mm/yr, respectively, based on the depositional age of an abandoned uplifted strath terrace in a water gap to the west of Agsu. These rates should be treated as maxima, as they are ~100% of the previously determined structurally and geodetically measured shortening across the belt, and were measured from only one of two primary structures in this part of the belt. The lack of reported historical ruptures from the past 8 centuries to the west of Agsu, in contrast with the numerous recorded destructive earthquakes of the Shamakhi region, suggests that the central and western parts of the Kura fold-thrust belt produce less frequent, but more destructive earthquakes, and may have accumulated sufficient strain to produce a M>8 earthquake.

Continent-scale observations of seismic phenomena have provided multi-scale constraints of the Earth’s interior. Of those analysed, array-based observations of slowness vector properties (backazimuth and horizontal slowness) and multipathing have yet to be made on a continental scale. Slowness vector measurements give inferences on mantle heterogeneity properties such as velocity perturbation, velocity gradient strength and quantify their effect on the wavefield. Multipathing is a consequence of waves interacting with strong velocity gradients resulting in two arrivals that arrive with different slowness vector properties and times. The mantle structure beneath the contiguous Unites States has been thoroughly analysed by seismic studies and is data-rich, making it an excellent testing ground to analyse mantle structure with our approach and compare with other techniques. We apply an automated array-analysis technique to an SKS dataset to create the first continent-scale dataset of multipathing and slowness vector measurements. We analyse the divergence of the slowness vector deviation field to highlight seismically slow and fast regions in our data. Our results resolve several slow mantle anomalies beneath Yellowstone, the Appalachian mountains and fast anomalies throughout the mantle. Many of the anomalies cause multipathing in frequency bands 0.15–0.30 and 0.20–0.40 Hz which suggests velocity transitions over at most 500 km exist. Comparing our observations to synthetics created from tomography models, we find model NA13 \citeA{bedle_continental_2021} fits our data best but differences still remain. We therefore suggest slowness vector measurements should be used as an additional constraint in tomographic inversions and will lead to better-resolved models of the mantle.

Pigmentary hematite carries important signals in paleomagnetic and paleoenvironmental studies. However, weak magnetism and the assumption that it has high magnetic coercivity prevents prevents routine identification of the size distribution of pigmentary hematite, especially for fine particle sizes. We present a strategy for estimating joint hematite particle volume and microcoercivity (f (V, Hk0)) distributions from low-temperature demagnetization curves and thermal fluctuation tomography (TFT) of pigmentary hematite in bulk samples of Triassic-Jurassic Inuyama red chert, Japan. The coercivity of the pigmentary hematite increases exponentially with decreasing temperature, following a modified Kneller’s law, where microcoercivity has a wide but approximately symmetric distribution in logarithmic space from ~1 tesla to tens of tesla. All of the red chert samples contain stable single domain (SSD) hematite with 35 - 160 nm diameter; a significant superparamagnetic (SP) hematite population with sizes down to several nanometers also occurs in Jurassic samples. The SP/SSD threshold size is estimated to be 8 - 18 nm in these samples. The fine particle size of the pigmentary hematite is evident in its low median unblocking temperature (194 °C to 529 °C) and, thus, this hematite may contribute to all four paleomagnetic components identified in published thermal magnetization studies of the Inuyama red chert. In this work, uniaxial anisotropy and magnetization switching via coherent rotation are assumed. Uniaxial anisotropy is often dominant in fine-grained hematite, although the dominant anisotropy type should be evaluated before using TFT. This approach is applicable to studies that require knowledge of coercivity and size distributions of hematite pigments.

Negative abnormal changes of atmospheric electric field have been monitored many times before earthquakes, especially before large earthquakes. However, its mechanism has not been clearly concluded yet. Based on the comparative analysis of the positive anomaly characteristics of atmospheric electric field before lightning and geoelectric field before earthquakes in earthquake areas, this paper concludes that the negative anomalies of atmospheric electric field before earthquakes are caused by the rise of the geoelectric potential in earthquake areas, and the areas with the abnormal rise of the geoelectric potential are potential earthquake areas. Based on the inference above , this paper further proposes a comprehensive earthquake early warning method with station network layout by geoelectric field instruments in combination with the atmospheric electric field data monitored by Electric Field Mills (EFMs). The research results will provide a possible reference for exploring the mechanism of earthquake generation, and will be help to explore earthquake early warning.

Tectonic tremors occurring on subducting plate boundaries are known to migrate at various timescales and migration speeds. Spatiotemporal patterns of tremor migration are key to investigating the rupture growth of slow earthquakes. However, spatiotemporal patterns are not sufficiently simple to visually define tremor migrations. This study developed a space-time Hough transform to objectively extract tremor migrations. The space-time Hough transform enables the extraction of multiple tremor migrations with various durations, migration directions, and migration speeds. We applied this method to a catalog of tremors for the period from 2012 to 2014, which was determined from the data analysis of a dense seismic array deployed in the Kii Peninsula, Southwest Japan. We successfully extracted 1,010 tremor migrations with durations ranging from 10 min to 24 h. Along-strike migrations propagating southwestward were predominant in the northeastern part of the Kii Peninsula, whereas those propagating northeastward were principal in the southwestern part. Regarding the along-dip direction, tremor migrations propagating in the up-dip directions were predominant in the deep part, and those propagating in the down-dip directions were principal in the shallow part. The patterns of along-strike migrations were related to the distribution of tremor energies, suggesting that tremor migrations may be controlled by heterogeneous structures of frictional properties on the plate interface. We further found that the migration speed is proportional to the inverse of the square root of the duration. This relation implies that a diffusion process controls the growth of fault ruptures behind tremor migrations.

A document by Hugh Harper. Click on the document to view its contents.

Microseismicity is expected in potash mining due to the associated rock-mass response. This phenomenon is known, but not fully understood. To assess the safety and effciency of mining operations, producers must quantitatively discern between normal and abnormal seismic activity. In this work, statistical aspects and clustering of microseismicity from a Saskatchewan, Canada, potash mine are analyzed and quantified. Specifically, the frequency-magnitude statistics display a rich behavior that deviates from the standard Gutenberg-Richter scaling for small magnitudes. To model the magnitude distribution, we consider two additional models, i.e., the tapered Pareto distribution and a mixture of the tapered Pareto and Pareto distributions to fit the bi-modal catalog data. To study the clustering aspects of the observed microseismicity, the nearest-neighbor distance (NND) method is applied. This allowed the identification of potential cluster characteristics in time, space, and magnitude domains. The implemented modeling approaches and obtained results will be used to further advance strategies and protocols for the safe and effcient operation of potash mines.  

Injection or production of fluids from subsurface reservoirs lead to stress changes affecting both reservoir and surrounding rocks. For low-permeable caprocks overlying such reservoirs, the movement of pore fluids to or from the formation is restricted and the immediate and short-term response to changes in the stress field will be undrained. Consequently, stress changes transfer partly into pore pressure changes. The aim of the current study is to investigate theoretical means of forecasting the undrained pore pressure generation in the Draupne Formation shale and to compare predictions with experimental results. Predictions are based on measurements from a single undrained triaxial test on a sample with known orientation, and a combination of Skempton's classical formulation and anisotropic poroelastic theory. The predicted pore pressures are compared to measured pore pressures from a series of triaxial tests on samples with various orientations exposed to different total stress paths. First, it is confirmed that the normalized undrained pore pressure measured is linearly connected to the total stress path. Then it is demonstrated that a tensorial pore pressure parameter can be used to accurately predict the influence of stress orientation on generated pore pressure. Lastly, it is experimentally confirmed that the two predictions can be combined to predict the pore pressure arising from stress changes along any compressional stress path and orientation. The observations herein may contribute significantly to the understanding of induced pore pressure in low-permeable materials and provide valuable input to geomechanical modeling of various field operations.

A document by Robert Lewis Walker. Click on the document to view its contents.