V. M. Dem’yanovich

Stress-strain state of the lithosphere in the southern Baikal region and northern Mongolia from data on seismic moments of earthquakes

Investigation and understanding of the present-day geodynamic situation are of key importance for the elucidation of the laws and evolution of the seismic process in a seismically active region. In this work, seismic moments of nearly 26000 earthquakes with Kp ≥ 7 (MLH ≥ 2) that occurred in the southern Baikal region and northern Mongolia (SBNM) (48°–54°N, 96°–108°E) from 1968 through 1994 are determined from amplitudes and periods of maximum displacements in transverse body waves. The resulting set of seismic moments is used for spatial-temporal analysis of the stress-strain state of the SBNM lithosphere. The stress fields of the Baikal rift and the India-Asia collision zone are supposed to interact in the region studied. Since the seismic moment of a tectonic earthquake depends on the type of motion in the source, seismic moments and focal mechanisms of earthquakes belonging to four long-term aftershock and swarm clusters of shocks in the Baikal region were used to “calibrate” average seismic moments in accordance with the source faulting type. The study showed that the stress-strain state of the SBNM lithosphere is spatially inhomogeneous and nonstationary. A space-time discrepancy is observed in the formation of faulting types in sources of weak (Kp = 7 and 8) and stronger (Kp ≥ 9) earthquakes. This discrepancy is interpreted in terms of rock fracture at various hierarchical levels of ruptures on differently oriented general, regional, and local faults. A gradual increase and an abrupt, nearly pulsed, decrease in the vertical component of the stress field Sv is a characteristic feature of time variations. The zones where the stress Sv prevails are localized at “singular points” of the lithosphere. Shocks of various energy classes in these zones are dominated by the normal-fault slip mechanism. For earthquakes with Kp = 9, the source faulting changes with depth from the strike-slip type to the normal-strike-slip and normal types, suggesting an increase in Sv. On the whole, the results of this study are well consistent with the synergism of open unstable dissipative systems and are usable for interpreting the main observable variations in the stress-strain state of the lithosphere in terms of spatiotemporal variations in the vertical component of the stress field Sv. This suggests the influence of rifting on the present-day geodynamic processes in the SBNM lithosphere.

Estimating the energy of seismotectonic deformations in the lithosphere of the Baikal Rift Zone

Estimation and comparison of the energy of seismotectonic deformations in the lithosphere of the Baikal Rift Zone (BRZ) based on observations of large (M ≥ 6) earthquakes for the period of instrumental recording (1950–2002), for a historical period lasting 210 years (1740–1949), and inferred from palaeo-seismological materials for the past 2000 years, all indicate that the hypothesis of a stationary seismic process is appropriate for the region. The locations of maxima of the density of seismotectonic strain energy released during the time intervals under investigation show that most of the failures in the lithosphere occurred approximately in the same areas, which may be interpreted as stress concentrators. The isolines of increased density for the energy of seismotectonic deformations align themselves along the rift features from southwest to northeast in the Baikal region and this allows one to treat the BRZ lithosphere as an extended zone of enhanced, inhomogeneous, energy release of endogenous geotectonic processes. We assessed the power of the seismotectonic processes that reflect the release of endogenous energy through earthquakes. Identification of areas with deficits in the energy of seismotectonic deformations (“energy gaps”) is an important step toward long-term solution of seismic-safety problems for the Baikal region.

Baikal Rift Zone: An Area of Higher Energy of Seismotectonic Deformations in the Lithosphere

Analysis and comparison of the energy of seismo� tectonic deformation in the lithosphere of the Baikal Region, determined by data on strong earthquakes with magnitude M ≥ 6 in the period of instrumental observations (1950–2002), in the historical period of 210 years, and by palaeoseismogeologic data of the last two thousand years, prove the adequacy of the hypoth� esis of stationary seismic processes. A close spatial location of maximums of released energy of seismo� tectonic deformations in the studied time periods tes� tifies that the main brittle destructions of the lithos� phere took place in nearly the same areas, which can be considered as stress concentrators. Isolines of a higher energy level cover rift structures, outlining the seismotectonic deformations in the lithosphere of the region as an entire prolonged area of energetic dis� charge of endogenic processes that is the Baikal rift zone (BRZ).

Energy of Seismotectonic Deformations in the Lithosphere of Mongolia

The information about strong earthquakes of Mongolia for the instrumental period of observations has been generalized in a sketch of the energy of seismotectonic deformations in the lithosphere. This sketch has been correlated with the map of general seismic zoning of Mongolia. The derived results characterize the energy of seismotectonic deformations in the lithosphere in terms of absolute units, which is favorable for projecting and building activities. The approach to energy estimation being developed is recommended for further development of modern engineering and building technologies in the framework of the regional seismic safety problem.

Faults and Source Parameters of Earthquakes in the Baikal Rift Zone: Dip Angles of Fault Planes

Analysis of gentle, intermediate, and steep dip angles of fault planes in earthquake sources of the Baikal region is carried out with the slips in seismic sources being taken into account. The areas where normal faulting slips at steep, intermediate, and gentle angles took place are distinguished, and this can be interpreted as transformation of a steep dip to gentle one. It is found that the increase in energy class is accompanied by the number of earthquakes with steep dip angles, whereas the proportion of earthquakes with intermediate angles decreases, and the fraction of earthquakes with gentle angles remains quasi-stable and small.

Diagnostics of the stress state of the lithosphere in Mongolia based on seismic source data

The obtained results indicate that earthquakes of Mongolia form under the conditions of an alternating field of regional stresses, which is related to pulsed activations in the BRS and superimposed on the quasi-stationary field of global stresses of the Earth.

The 3D Seismotectonic Flow of Geological Masses in the Lithosphere of the Baikal Rift Zone

This study of displacements in the lithosphere of the Baikal Rift Zone (BRZ), which was based on data on dynamic earthquake-source parameters for events with energy class KP ≥ 7 for the 1968–1994 period revealed the most conspicuous patterns in the 3D-seismotectonic flow of geological masses in the region. It is shown that the main contribution to the total absolute rock displacement is due to numerous small earthquakes, while the greatest displacements of geological material occur in the zones of long-continued aftershock and swarm sequences. About half of the entire seismotectonic displacements occurred in the northeastern flank of the BRZ, while the total displacements of geological material due to earthquakes that occurred in the southwest and in the middle of the region are approximately equal. When small shocks with energy class KP = 7 are considered, it appears that the total relative displacements of the geological medium are largely due to normal faulting events, while when the KP = 10 earthquakes are in question, the total displacements due to normal mechanisms are considerably below those due to reverse and strike-slip movements. The resulting images of 3D seismotectonic flow of geological masses in the BRZ lithosphere indicate good agreement between negative vertical displacements and rift basins, while the positive movements of geological material are coincident with inter-basin mountain junctions. These local features in the seismotectonic flow of geological material reflect patterns in the 3D inhomogeneous deformation of the BRZ lithosphere and are due to stress rearrangements within the lithosphere of the region among three rifting structures, with compression and tension areas alternating.

Energy structure of seismicity at the southwestern flank of the Baikal rift system

The energy structure of seismicity at the southwestern flank of the Baikal rift system (BRS) is analyzed on the data of the annual and total numbers of earthquakes. It is characterized by three parameters of the earthquake occurrence plot: the maximum energy class Kmax, the slope γ, and the level A10. The parameters change most significantly after strong earthquakes and the series of aftershocks. With time and the increase in number of earthquakes used, they attain the limiting values that characterize the long-term distribution of shocks by the scale of energy classes. In spite of the unusual modern geodynamics of the Baikal rift system, the energy structure of seismicity of the southwestern flank corresponds to the model of the stationary seismic process.