Basic Parameters and Characteristics of Seismicity in Mongolia

Much of the territory of Mongolia is located in highly seismic areas of Central- Asian seismic belt and is subjected to frequent and large earthquakes. Its western half is particularly seismically dangerous. Only in twentieth century more than 60 earthquakes with M>5.5 (7 to 11-12 - intensity) occurred here, among which tens of earthquakes were responsible for severe destruction of the earth surface, and seismic disasters of 1905, 1931 and 1957 were accompanied by seismotectonic deformations to several hundred kilometers in length. Judging from the retained traces of ancient earthquakes and the chronicles, similar and probably larger seismic events occurred here in the recent past.DOI: http://dx.doi.org/10.5564/pmas.v0i4.39Proceedings of the Mongolian Academy of Sciences 2007 No 4 pp.36-4

Generation of infrasonic signals during earthquakes under Lake Hovsgool (Northern Mongolia) on December 5, 2014

The paper discusses the results of the detection of seismic and infrasonic waves generated by a major earthquake and its aftershock (the moment magnitude MW=4.9 and MW=4.2 respectively), which occurred in northern Mongolia under Lake Hovsgool on December 5, 2014. The joint analysis of waveforms of seismic and infrasonic oscillations has shown that the signal recorded by the infrasound station of the Geophysical Observatory of the Institute of Solar-Terrestrial Physics SB RAS (ISTP SB RAS) is formed from sources of three generation types: local, secondary, and epicentral. This analysis enables us to propose a hypothesis of generation of epicentral infrasonic signal by flexural waves in an elastic ice membrane on the surface of Lake Hovsgool, which appear during the passage of seismic wave packets. This hypothesis explains the similarity between seismic and epicentral infrasonic signals, negative initial phase of epicentral infrasonic waves, and detection of a weak signal after a small-magnitude aftershock

Lithospheric stress in Mongolia, from earthquake source data

Lithospheric stress in Mongolia has been studied using mechanisms of 84 MLH ≥ 4 earthquakes that occurred in the 20th century and instrumental seismic moments of 17,375 MLH ≥ 2.5 events recorded between 1970 and 2000. The MLH ≥ 3.5 earthquakes mostly have strike-slip mechanisms in southern and central Mongolia, with frequent reverse-slip motions in the west and normal slip in the north, especially, in the area of Lake Hovsgol. The principal stresses are, respectively, SH>Sv>Sh in the center and in the south; high horizontal compression with SH>Sh>Sv in the west; and a heterogeneous stress pattern with Sv>SH>Sh in the north. According to seismic moments of MLH = 2.5 events, oblique slip generally predominates over the territory, at Sv≈SH>>Sh, while frequent strike slip motions in the west record high horizontal compression (SH>Sv>Sh). Earthquake mechanisms show the principal horizontal compression SH to be directed W–E in the east, NE–SW in the central and Gobi-Altay regions, and approximately N–S in the west of Mongolia. The patterns of principal lithospheric stresses in the territory of Mongolia have undergone three events of dramatic change for a few recent decades, and these events were synchronous with three similar events in the Baikal rift system (BRS): in the latest 1960s, latest 1970s to earliest 1980s, and in the latest 1980s to earliest 1990s. The seismicity of Mongolia has been controlled by superposition of variable stresses associated with rifting activity pulses in the neighbor BRS on the background of quasi-stationary super-regional compression

Coupled large earthquakes in the Baikal rift system: Response to bifurcations in nonlinear resonance hysteresis

The current lithospheric geodynamics and tectonophysics in the Baikal rift are discussed in terms of a nonlinear oscillator with dissipation. The nonlinear oscillator model is applicable to the area because stress change shows up as quasi-periodic inharmonic oscillations at rifting attractor structures (RAS). The model is consistent with the space-time patterns of regional seismicity in which coupled large earthquakes, proximal in time but distant in space, may be a response to bifurcations in nonlinear resonance hysteresis in a system of three oscillators corresponding to the rifting attractors. The space-time distribution of coupled MLH > 5.5 events has been stable for the period of instrumental seismicity, with the largest events occurring in pairs, one shortly after another, on two ends of the rift system and with couples of smaller events in the central part of the rift. The event couples appear as peaks of earthquake ‘migration’ rate with an approximately decadal periodicity. Thus the energy accumulated at RAS is released in coupled large events by the mechanism of nonlinear oscillators with dissipation. The new knowledge, with special focus on space-time rifting attractors and bifurcations in a system of nonlinear resonance hysteresis, may be of theoretical and practical value for earthquake prediction issues. Extrapolation of the results into the nearest future indicates the probability of such a bifurcation in the region, i.e., there is growing risk of a pending M ≈ 7 coupled event to happen within a few years