A. A. Dobrynina

Activation of Rifting Processes in the Northern Cis-Baikal Region: A Case Study of the Kichera Earthquake Sequence of 1999

The paper addresses the spatiotemporal development of the Kichera sequence of earthquakes of 1999 (more than 6000 events over the year) within the Kichera depression, terminating on land in the Northern Baikal basin; the series was the most intense of all earthquake sequences recorded in the Northern Cis-Baikal region (NCBR) since 1960. The spatial coordinates of earthquakes showed that the source rupture, originating in the area of the Kichera-Upper Angara interbasin mountainous isthmus, propagated in the SW direction toward Lake Baikal. Stresses in the sources of the two strongest shocks (Mw = 6.0 and 5.6) of the sequence were released along fault planes striking NE (normal type) and E-W (normal-strike-slip type). Focal mechanisms of aftershocks revealed the presence of differently oriented faults motions on which were controlled by a large rifting fault striking NE. The Kichera earthquakes are shown to have occurred under seismotectonic conditions dominated by NW-SE extension and to have been accompanied by active normal faulting promoting longitudinal growth of the Upper Angara depression and deepening of the Kichera depression. The seismotectonic strain rates calculated for the NCBR before and after 1999 were of the order of (0.1–1.0) × 10−10 yr−1, whereas their values were two to three orders larger during 1999. Thus, the Kichera earthquakes confirmed the high seismic potential of the NCBR and showed that this rift segment developed through growth of depressions and destruction of interbasin mountainous isthmuses.

Source parameters of the earthquakes of the Baikal rift system

The dynamic parameters of the earthquake source—the seismic moment, the moment magnitude, the source radius, the stress drop, and the amplitude of displacement—are determined by the amplitude Fourier spectra of the body shear waves (S-waves) for 62 earthquakes of the Baikal rift system with the energy class of KP = 9.1–15.7. In the calculations I used the classical Brune model. The seismic moment of the earthquakes being investigated changes from 3.65 × 1011 N m to 1.35 × 1018 N m, and the radii of earthquake sources vary from 390 m to 1.84 km. The values of the drop in stress Δσ grow with an increase in the seismic moment up to 1.7 × 108 Pa. For the group of weak earthquakes (Mw = 1.7–3.3), extremely low values of the drop in stress 103–104 Pa are observed. The maximum amplitude of displacement in the source amounts to 5.95 m. The empirical equations between the seismic moment and the other dynamic parameters of the source are determined. The regional dependence of the seismic moment and energy class is obtained: log M0 ± 0.60 = 1.03KP + 3.17. The character of the relationship between the seismic moment and the corner frequency indicates that the classical scaling law of the seismic spectrum for the earthquakes in question is not fulfilled. The obtained estimates of the dynamic parameters are in satisfactory agreement with the published data concerning the analogous parameters of the other rift zones, which reflects the general regular patterns of the destruction of the lithosphere and the seismicity in the extension zones of the lithosphere.

Contemporary horizontal movements and seismicity of the south Baikal Basin (Baikal rift system)

The contemporary horizontal movements and deformations in the central and southern parts of the Baikal depression are analyzed, and their relationship with contemporary seismicity is studied. Based on the long-term measurements by the Baikal geodynamical GPS monitoring network, the refined estimate is obtained for the velocity of the divergence of the Siberian and Transbaikalian blocks, which is found to occur in the southeastward direction (130°) at 3.4 ± 0.7 mm per annum. This agrees with the parameters of the long-term extension component estimated from the geological data and with the direction of extension determined from the seismic data. The distribution of the displacement velocity across the strike of the rift, which gradually increases from one block to another, suggests a nonrigid behavior of the continental lithospheric plates at the divergent boundary. About 30% (1.0–1.5 mm per annum) of the total increase in the velocity is accommodated by the Baikal Basin. The strain rate within the trough reaches 3.1 × 10−8 yr−1 and decreases on either side across the structure. The character of distribution of the horizontal displacement velocities on the Baikal divergent boundary between the Eurasian and Amurian plates favors the model of passive rifting. The zones of highly contrasting topography and increased seismicity are localized within the area of contemporary deformations, and the seismic moment release rate directly depends on the strain rate. Here, the rate of the seismic moment release rate makes up a few percent of the geodetic moment accumulation rate calculated by the approach suggested by Anderson (1979). Based on the coherence between the graphs of the rates of geodetic moment accumulation and seismic moment release rate by the earthquakes with M ≥ 5.0 during the historical and instrumental observation periods, the contemporary seismic hazard for the South Baikal Basin could be assessed at a level of seismic event with M = 7.5–7.6.

Modern fault formation in the Earth’s crust of the Baikal rift system according to the data on the mechanisms of earthquake sources

The spatial characteristics of seismotectonic deformations and the most likely fracture planes in the earthquake sources of the Baikal rift system (BRS) are determined using the method of cataclastic analysis of fractures [1]. It is shown that extension conditions with a strike of modern fractures parallel to the rift-controlling faults are dominant in the central zone and in most of the NE flank of the BRS. The flat average dip of fractures in the earthquake sources of the main fault zones for some rift depressions allow a suggestion about the flattening of faults in the middle crust. The antithetic faults are steeper. The BRS flanks are characterized by dominant shear deformations and more diverse morphogenetic faults in the earthquake sources (strike-slip faults, reversed faults, and normal faults). The modern faults at the BRS flanks weakly inherit the neotectonic structure.

Active Faulting in the Earth’s Crust of the Baikal Rift System Based on the Earthquake Focal Mechanisms

The destruction of the lithosphere with the formation of fault zones is one of the leading geological processes determining the structure of the continents, both in the past and at the present stage. Seismicity providing information on the structure and dynamics of formation of large fault zones in real time reflects the modern fault formation in the crust. For its study, both the epicentral field of earthquakes (see, for example, Sherman 2009) and the data on the position of their hypocenters are actively used (see, for example, Kaven and Polland 2013). To determine the orientation of modern faults of various orders, one can also use data on the earthquake focal mechanism solutions preliminarily distinguishing the true fault planes in the source. In the case of strong earthquakes, the geological data (the outcrop of the fault on the surface, the existing of faults with similar geometry, etc.), the data on the orientation of the aftershock field, the shape of the first isoseits, and other data are indirect features that help to choose one plane or another as the true fault plane. These approaches are inapplicable in study of weak earthquakes (magnitude M ≤ 4.0) and the only information available on them is concerned with their waveforms.

Seismoacoustic effects of the Hovsgol earthquake (M w = 4.9) of December 5, 2014

The results of study of the Hovsgol earthquake with Mw = 4.9, which occurred on December 5, 2014, in the northern part of Hovsgol Lake in Mongolia, are presented. An infrasonic signal of ~140 s long was recorded by the Tory infrasonic station for the first time for the Baikal Rift System. On the basis of the source parameters and focal mechanism of this earthquake determined, displacements in the epicentral zone of the Hovsgol earthquake are modeled. It is shown that they could not have produced an infrasonic sound. The use of acceptable values of group velocities of infrasonic waves (0.28–0.35 km/s) demonstrates that the signal source was located approximately midway between the Tory station and the epicenter of the Hovsgol earthquake, indicating it was a secondary source. Based on the data on the azimuth and arrival time of the acoustic wave at the Tory station, the location of this secondary source is determined to have been on the northern slopes of the Khamar Daban Range. The infrasonic signal formed most likely by interaction between seismic waves from the earthquake and the mountain relief.

New data on seismic wave attenuation in the lithosphere and upper mantle of the northeastern flank of the Baikal rift system

The investigation data on seismic wave attenuation in the lithosphere and upper mantle of the northeastern flank of the Baikal rift system obtained with a seismic coda envelope and sliding window are considered. Eleven local districts were described by one-dimensional attenuation models characterized by alternation of high and low attenuation layers, which are consistent with the results obtained previously by Yu.F. Kopnichev for the southwestern flank of the Baikal rift system [9]. The subcrust of the lithosphere contains a thin layer with high attenuation of seismic waves likely related to higher heterogeneity (fragmentation) and occurrence of fluids. The lithosphere basement depth varies from 100–120 km in the west within the Baikal folded area to 120–140 km in the east within the Siberian Platform. It is concluded that there are two asthenosphere layers. Based on specific features of the lithosphere and upper mantle structure, it can be assumed that they were subject to gradual modification involving fluidization processes and partial melting in the Late Cenozoic extension under the influence of distant tectogenesis sources.