V. I. Melnikova

Present‐day stress field changes along the Baikal rift and tectonic implications

Intraplate extension, in a frame of a global compressional stress field, seems linked to local lithospheric perturbations (lithospheric thinning or thickening) able to modify the resulting state of stress [Zoback, 1992]. The Baikal Rift Zone (BRZ), Siberia, is located north of the India-Asia collision zone and exhibits no direct communication with any oceanic domain. It can thus be fully considered as an area of continental extension, dominated by the “global compressional intraplate stress field” resulting from plate driving forces. In order to address the problem of its dynamics and kinematics and their links with the India-Asia collision, a comprehensive stress tensor analysis is presented, based on 319 focal mechanisms of earthquakes located along the whole Baikal rift. The stress field is varying at different scales of observation: when looking at central Asia (several thousands kilometers), the maximum horizontal stress SHmax directions remain rather constant (with a fan-shape geometry) when the tectonic regime goes from compressional (Himalayas) to extensional (Baikal). When observing the Baikal rift (about 1000 km long), clear variations of the stress regime are observed, from an extensional regime in the central part of the rift to wrench ones in its northern and southern ends. Finally, at the scale of 100 km, systematic SHmax reorientations occur close to major rift faults. We thus infer that the interaction between collisional processes and inherited structures may have a strong influence on rift dynamics. We then use computed stress tensors to predict slip vectors on major rift faults. Deformation patterns show two distinct parts of the rift: the South Baikal Rift (SBR) is characterized by a constant trending (around N100°E) slip vector, meanwhile the North Baikal Rift (NBR) exhibits a complex block rotation behavior involving at least three crustal blocks. We propose to interpret these surficial structures and motions as the result of an interaction between the regional compression coming from the India-Asia collision and the geometry of the hardly deformable Siberian platform. This particular setting can explain most of the surficial deformation patterns, which suggest a large-scale cracking of the lithosphere in the Baikal region. Other possible sources of stress could also be considered, like deep mantellic upwelling, or trench suction linked to the Pacific subduction.

Seismic moment tensor of Pribaikalye earthquakes from the surface-wave amplitude spectra

The method of surface-wave amplitude spectra inversion for the seismic moment tensor (SMT) is implemented and tested in the Pribaikalye region. The SMTs are calculated for 39 events with Mw = 4.4–6.3, which occurred in the region in 2000–2011. Based on the obtained data, the seismotectionic deformations of the crust are estimated in two seismically active areas-the Northern Pribaikalye and northeastern Baikal rift zone. It is found that on a level of moderate-magnitude events, the region is dominated by the regimes of subhorizontal northwestern extension and strike-slip faulting, which reflects the long-term trends in the stress field of the crust in these parts of the rift.

The M w 4.3 January 17, 2014, earthquake: very rare seismic event on the Siberian platform

We described the 2014 January 17 earthquake (Mw = 4.3) occurred on the Siberian platform in the area of sublongitudinal part of the Angara river in the zone of possible influence of two large reservoirs—Ust-Ilimsk and Boguchan. This is the first event of such magnitude recorded in this previously aseismic area during the whole period of instrumental observations. A seismic moment, a moment magnitude, a hypocentral depth, and a focal mechanism of the event were calculated on the basis of surface wave amplitude spectra. Analysis of the geological and geophysical data showed that the earthquake origin is connected with high-velocity gradient zone located at the border of the Late Proterozoic cover and the Precambrian basement of the Siberian platform. Some evidences for a natural character of the earthquake were considered.

The seismic moment tensor of earthquakes in the Pribaikalye Region based on surface waves

The most complete characterization of the stress– strain state of the lithosphere in seismoactive regions based on seismological data can be obtained by calcu� lation of seismic moment tensors (SMT) and their fur� ther analysis. In the Pribaikalye Region, the percent� age of experimentally determined SMT, which is an important characteristic of a source, is generally small, about 1% (CMT catalog, NEIC, USGS [1–4]), because the routine estimation of focal mechanisms for most earthquakes is regularly implemented based on the polarities of first arrivals of body waves,

The August 27, 2008, M w = 6.3 Kultuk earthquake (South Baikal): The stress-strain state of the source area from the aftershock data

We consider the results of reconstructing the stress-strain state of the Earth’s crust in South Baikal from the focal mechanism data for the Kultuk earthquake of August 27, 2008 (Mw = 6.3) and its aftershocks. The source parameters of the main shock were determined by calculating the seismic moment tensor. The focal mechanism solutions of 32 aftershocks (Mw ≥ 2.3) were obtained through the deployment of a local seismic network at South Baikal. It is found that the main shock and first aftershocks (August–September) gave rise to the activation of latitudinal fragments of the segmented near-edge fault, and the sources of the consequent aftershocks were dominated by the NW-striking planes related to the small intrabasin structures. The calculations of seismotectonic deformations based on the data on the focal mechanisms of the earthquakes show that the area of activation is dominated by the transtension regime (with deformation in the form of extension with shear). The epicentral and hypocentral fields of the aftershocks and the mechanisms of their sources reflect the complex tectonic structure of the source zone of the Kultuk earthquake, which exhibits a clear subvertical zonality of the local seismically active volume and a wedge-shaped area of crustal destruction.

Seismotectonic Crustal Strains of the Mongol-Baikal Seismic Belt from Seismological Data

Focal parameters of the earthquakes occurred within the Mongol-Baikal seismic belt have been considered from seismological and geological literature data for 9 strongest instrumentally recorded seismic events (1905–1967) and calculated for 89 medium regional earthquakes (2000–2016) on the basis of the data on surface wave amplitude spectra. The obtained seismic moment tensor solutions significantly complete the existing dataset on reliable focal parameters of medium earthquakes and allow us to carry out the seismotectonic reconstruction more precisely. It has been shown that the regions of recent mountain building in southern Siberia and western Mongolia are characterized by transpression and strike-slip regimes. Normal-fault deformation is observed in the central segment of the Baikal rift and in the major part of its northeastern flank. Transpression and strike-slip regimes dominate at the southeastern rift flank and in the junction of the rift and the Olekma-Stanovoi zone.

New data on earthquake focal mechanisms in the Laptev Sea region of the Arctic-Asian seismic belt

We consider 16 earthquakes with Mw = 4.2–5.2 that occurred in the south-eastern part of the Laptev Sea shelf, Lena River Delta, and North Verkhoyanye (Russia) in 1990–2014. Focal mechanisms, scalar seismic moments, moment magnitudes, and hypocentral depths of the seismic events have been calculated from the data on amplitude spectra of surface waves and P wave first-motion polarities. The obtained results sufficiently implement the existing dataset on reliable earthquake source parameters for the study region and prove the change of the stress-strain state of the crust from extension on the Laptev Sea shelf to compression on the continent providing finer spatial details of the deformation field in the transition zones such as Buor-Khaya Bay and the Lena River Delta.

Relationship between seismicity in the northern Pribaikalye and the block structure of the crust

The character of seismic activity in the northern Pribaikalye is considered in connection with the block structure of the crust. It is supposed that presence of structural elements of different scale levels and density heterogeneities in the crust gives ground for earthquake clustering and prevents large (M > 6.0) seismic events. The representativeness of clusters and the character of stress discharge in sources change gradually as you move away from the conditional axis of the rift zone to the S-E demonstrating a weak influence of rift processes in the area of the Ikat Range.

Seismotectonic destruction of the Earth’s crust in the northeastern flank of the Baikal Rift Zone

481 The peculiarities of the geodynamic development and the mechanism of formation of morphotectonic structures in the northeastern flank of the Baikal Rift Zone (BRZ) remain poorly studied due to the lack of actual data on the fracture structure, fields of tectonic stresses, and geophysical characteristics of the medium. This study is aimed to distinguish the mod ern structural tectonic map and types of seismotec tonic destruction in the most active seismic segment of the BRZ (its northeastern flank) and the dynamically conjugated system of the seismic generating structures of the West Stanovoi tectonic block.