The propagation velocity of seismic activity migrating along the directions of the geodynamic forces prevailing in the northeastern Baikal rift system, Russia

The recent tectonic stress field in the northeastern Baikal rift system (BRS) corresponds to the crustal deformation field. The stress-strain state of the Earth’s crust determines the fault network geometry and spatiotemporal structure of the epicentral field characterized by many earthquake swarms and earthquake migrations in the study area. In order to study the seismic process dynamics in different directions of the crustal deformation, the spatiotemporal analysis of earthquake time series has been made over the 1964–2015 instrumental period. To determine the relationship between crustal stress and spatiotemporal features of the epicentral field the seismic data were projected along horizontal stress tensor axes σ3 and σ2, consistent with major directions of the crustal deformation, a strike of major rifting structures, and a general azimuth of active fault groups. The NE-SW direction along the intermediate horizontal stress axes and main faulted arears exhibits slow earthquake migrations up to 60 km long, propagating with a modal velocity of about 30 kilometers per year. The NW-SE direction along the principal horizontal stress axes, orthogonal to the main faulted areas, is characterized by shorter migration sequences of less duration, propagating with a higher velocity than sequences registered in the NE-SW. The difference between the migration dynamics in mutually orthogonal directions can be attributed to the fault network configuration and the differences in the deformation process.

Neotectonics, Kinematics, and Evolution of the Vernon, Awatere, and Cloudy Faults of the Marlborough Fault System, New Zealand

<p>The coastal Awatere, Vernon, and Cloudy faults are bent and mutually intersecting, forming a complexly deforming dextral-oblique fault network. To try to explain the kinematic, paleoseismic and evolutionary complexities of this network, I present the results of an investigation into the rates, timing, and direction of slip on the faults within the network; which bifurcate eastwards from the central Awatere fault at the northeast end of the Marlborough Fault System. Displacements of dated and nondated late Quaternary features by the three faults were measured both onshore and offshore, constraining the kinematics of the fault network. The Vernon fault oddly maintains a dextral-reverse structure although it varies over 90° in strike and the Cloudy and coastal Awatere faults change from nearly pure strike slip to having a normal component eastwards. These data indicate that the fault-bounded blocks between the coastal Awatere, Vernon and Cloudy faults are rotating anticlockwise about a vertical axis relative to the block to the north of the fault system. Slip-rate data also indicate that of the 6 ± 1 mm/yr of slip on the central Awatere Fault, 1.1 ± 0.6 mm/yr has been partitioned ENE onto the coastal Awatere Fault and <4.9 mm/yr has been partitioned NNE onto the Vernon Fault. A slip-rate shortage in the splays of the Vernon Fault in the Vernon Hills is caused by a combination of unsighted faults and rotation of smaller splay-bounded blocks within the Vernon Hills. Paleoseismic records on the Vernon Fault were analysed onshore in a trench and offshore on seismic lines, with the records in good agreement. 3-5 earthquakes are recognised at different sites, with the last earthquake occurring 3.3 ka and a mean recurrence interval of 3-4 ka on the Vernon Fault. When combined with the paleseismic records from the Awatere and Cloudy faults I find that separate faults ruptured at similar times, suggesting a connectivity of the faults, as separate faults could mutually rupture during one earthquake or an earthquake could subsequently trigger an earthquake on a nearby fault. Finally I present the finite slip of geologic units and use these data as well as the late Quaternary slip data to describe the evolution of the fault network. I propose that the fault network at the NE end of the Awatere fault has stepped northwards into several splays, caused by clockwise rotation of the NE tips of the Marlborough faults.</p>

Neotectonics, Kinematics, and Evolution of the Vernon, Awatere, and Cloudy Faults of the Marlborough Fault System, New Zealand

<p>The coastal Awatere, Vernon, and Cloudy faults are bent and mutually intersecting, forming a complexly deforming dextral-oblique fault network. To try to explain the kinematic, paleoseismic and evolutionary complexities of this network, I present the results of an investigation into the rates, timing, and direction of slip on the faults within the network; which bifurcate eastwards from the central Awatere fault at the northeast end of the Marlborough Fault System. Displacements of dated and nondated late Quaternary features by the three faults were measured both onshore and offshore, constraining the kinematics of the fault network. The Vernon fault oddly maintains a dextral-reverse structure although it varies over 90° in strike and the Cloudy and coastal Awatere faults change from nearly pure strike slip to having a normal component eastwards. These data indicate that the fault-bounded blocks between the coastal Awatere, Vernon and Cloudy faults are rotating anticlockwise about a vertical axis relative to the block to the north of the fault system. Slip-rate data also indicate that of the 6 ± 1 mm/yr of slip on the central Awatere Fault, 1.1 ± 0.6 mm/yr has been partitioned ENE onto the coastal Awatere Fault and <4.9 mm/yr has been partitioned NNE onto the Vernon Fault. A slip-rate shortage in the splays of the Vernon Fault in the Vernon Hills is caused by a combination of unsighted faults and rotation of smaller splay-bounded blocks within the Vernon Hills. Paleoseismic records on the Vernon Fault were analysed onshore in a trench and offshore on seismic lines, with the records in good agreement. 3-5 earthquakes are recognised at different sites, with the last earthquake occurring 3.3 ka and a mean recurrence interval of 3-4 ka on the Vernon Fault. When combined with the paleseismic records from the Awatere and Cloudy faults I find that separate faults ruptured at similar times, suggesting a connectivity of the faults, as separate faults could mutually rupture during one earthquake or an earthquake could subsequently trigger an earthquake on a nearby fault. Finally I present the finite slip of geologic units and use these data as well as the late Quaternary slip data to describe the evolution of the fault network. I propose that the fault network at the NE end of the Awatere fault has stepped northwards into several splays, caused by clockwise rotation of the NE tips of the Marlborough faults.</p>

Tectonic setting of kimberlites in the Vilyui-Markhinsky fault zone according to modern data

The lateral heterogeneity of the Vilyui-Markha fault zone was determined, the central and western subzones were identified. The high-grade diamondiferous Mir and Nakyn kimberlite fields are confined to the central subzone. The low-grade diamondiferous Syuldyukar kimberlite field is confined to the western subzone of the VilyuiMarkha zone. The analysis of the fault network density in the research area was carried out. It was found that the fault network density increases within the subzones, which characterizes them as increased permeability areas favorable for kimberlite melts uprising. This fact can be another tectonic criterion for setting up diamond prospecting operations.

The deformation history of southern England, and its implications for ground engineering in the London Basin

Structures in the basement beneath the London Basin affect the geology of relevance to geotechnical engineering within London. Unfortunately, the basement beneath London is covered by Cretaceous and Tertiary sediments. It is cut by major faults linked to the compressive phases of the Hercynian and Alpine Orogenies and to the regional extension that occurred during the Mesozoic between these compressive events. Evidence is presented that movement on basement fractures beneath London played a major role in the distribution and deformation of sediments within the Basin, causing local folding and faulting significant to engineering works. Basement rocks are exposed in SW England where the type and orientation of these fractures (faults and joints) can be examined in outcrop. This study, complemented by seismic sections in the southern UK, enable the architecture of this fault network within the basement to be determined. Understanding the fracture system in the basement provides a basis for (i), interpreting the lateral facies variations of sediments in the Basin and hence provides a means for predicting from a ground investigation the likely presence, activity or influence on site of such structures at depth and (ii), understanding the extent of local, steeply inclined and sub-horizontal planar zones of shearing when encountered on site.Thematic collection: This article is part of the Geology of London and its implications for ground engineering collection available at: https://www.lyellcollection.org/cc/london-basin

Tectonic Stress Distribution in the Song Tranh 2 Hydropower Reservoir: Implication for Induced Earthquake

The Song Tranh 2 hydropower reservoir was built in Tra My area, Quang Nam province, composing magmatic and high-grade metamorphic rocks of the northern part of the Kon Tum massif. Since the reservoir was put into operation, induced earthquakes have occurred in the Song Tranh 2 hydropower reservoir and its vicinity. Tectonically, the northwest-southeast to east-west striking faults developed strongly. Detailed analysis of slickensides and attitude of faults occurring in the studied area have shown that the northwest-southeast striking faults are reactivated as dextral ones during the Pliocene-Quaternary up to the present day. Based on the geometric distribution of the fault network, kinematic characteristics, and the youngest tectonic stress regime, we computed the distribution of tectonic stress in the studied area. Computation results show two positive anomalies of stress directly related to the northwest-southeast faults numbered 2, 10, 11a, 11b and sub-latitude striking fault numbered 1. These faults run in line with the local river channels and are likely to reactivate and generate induced earthquakes.

3D attenuation image of fluid storage and tectonic interactions across the Pollino fault network

Summary The Pollino range is a region of slow deformation where earthquakes generally nucleate on low-angle normal faults. Recent studies have mapped fault structures and identified fluid-related dynamics responsible for historical and recent seismicity in the area. Here, we apply the coda-normalization method at multiple frequencies and scales to image the 3D P-wave attenuation (QP) properties of its slowly-deforming fault network. The wide-scale average attenuation properties of the Pollino range are typical for a stable continental block, with a dependence of QP on frequency of $Q_P^{-1}=(0.0011\pm 0.0008) f^{(0.36\pm 0.32)}$. Using only waveforms comprised in the area of seismic swarms, the dependence of attenuation on frequency increases ($Q_P^{-1}=(0.0373\pm 0.0011) f^{(-0.59\pm 0.01)}$), as expected when targeting seismically-active faults. A shallow very-low-attenuation anomaly (max depth of 4-5 km) caps the seismicity recorded within the western cluster 1 of the Pollino seismic sequence (2012, maximum magnitude MW = 5.1). High-attenuation volumes below this anomaly are likely related to fluid storage and comprise the western and northern portions of cluster 1 and the Mercure basin. These anomalies are constrained to the NW by a sharp low-attenuation interface, corresponding to the transition towards the eastern unit of the Apennine Platform under the Lauria mountains. The low-seismicity volume between cluster 1 and cluster 2 (maximum magnitude MW = 4.3, east of the primary) shows diffuse low-to-average attenuation features. There is no clear indication of fluid-filled pathways between the two clusters resolvable at our resolution. In this volume, the attenuation values are anyway lower than in recognized low-attenuation blocks, like the Lauria Mountain and Pollino Range. As the volume develops in a region marked at surface by small-scale cross-faulting, it suggests no actual barrier between clusters, more likely a system of small locked fault patches that can break in the future. Our model loses resolution at depth, but it can still resolve a 5-to-15-km-deep high-attenuation anomaly that underlies the Castrovillari basin. This anomaly is an ideal deep source for the SE-to-NW migration of historical seismicity. Our novel deep structural maps support the hypothesis that the Pollino sequence has been caused by a mechanism of deep and lateral fluid-induced migration.