G. A. Sobolev

Microseismic Impulses as Earthquake Precursors

Records of the IRIS broadband stations in Petropavlovsk-Kamchatski, Yuzhno-Sakhalinsk, Magadan, Yakutsk, Arti, and Obninsk obtained before the Kronotskii (Kamchatka Peninsula) M = 7.7 earthquake of December 5, 1997, and the Neftegorsk (Sakhalin Island) M = 7.0 earthquake of May 27, 1995, are investigated with the use of the Spectra Analyzer interactive program, designed for the analysis of properties of scalar time series. It is found that, 5 to 10 days before the shocks, stations nearest to the sources of these earthquakes recorded pulsed vibrations a few minutes long that were separated by intervals of a few tens of minutes. The shape asymmetry of the pulses characterized by different amplitudes of positive and negative polarity phases increased toward the earthquake onset time, as did the frequency and regularity of the pulse sequence. It is assumed that the nature of this phenomenon is related to self-organization properties of the seismic process in a metastable lithosphere and to the synchronization of vibrations in the inner and outer shells of the Earth, including chaotic and quasi-periodic components.

Microseismic Anomalies before the Sumatra Earthquake of December 26, 2004

Microseismic records from five broadband IRIS stations located at distances of 1000–2000 km from the earthquake source are studied. Unordinary programs are used to extract hidden periodicities, determine signal coherence at different stations, and reveal asymmetry in wave amplitudes. The records obtained at a few stations 60 h before the Sumatra earthquake include periodic oscillations in the range of periods from 20 to 60 min that arose after the McQuary earthquake and continued for about 24 h. Synchronization of waves recorded at all stations commenced 53 h before the Sumatra earthquake and continued up to the time of the earthquake, with the predominant period gradually increasing from a few minutes to tens of minutes.

Multifractal Measures of Synchronization of Microseismic Oscillations in a Minute Range of Periods

The paper considers synchronous continuous records of microseismic background obtained within a month before the Kronotskii (Kamchatka) December 5, 1997, earthquake (M = 7.8) at six IRIS broadband stations that are located in a large region extending from central European Russia (the town of Obninsk) to the Far East (Kamchatka and Sakhalin). By averaging and downsampling, initial records were discretized at an interval of 30 s and the microseismic background was examined in the range of periods from 1 min to 2.4 h, after scale-dependent trends due to the effects of tides and temperature variations had been removed. Microseismic fluctuations were analyzed with the help of estimates of the evolution of their multifractal singularity spectra in a moving time window 12 h wide. As the criterion characterizing the background properties in a current time window, we took the values of the generalized Hurst exponent α* realizing the maximum of the singularity spectrum. Hidden synchronization effects of a microseismic field preceding a seismic event are identified by estimating the evolution of the spectral measure of coherent behavior of α* variations in a moving time window 5 days long for various combinations of jointly analyzed stations.

On some properties in the emergence and evolution of the oscillations of the Earth after earthquakes

The records from wideband seismic stations are analyzed for studying the oscillations of the Earth that emerged after the earthquakes in Sumatra on December 26, 2004 (M = 9.2), Chile on February 27, 2010 (M = 8.8), and after the Tohoku megaearthquake on March 11, 2011 (M = 9.0). Attention is focused on the band with a period of 20.46 min, which includes the free radial mode 0S0. It is established that the emergence of oscillations in the frequency interval corresponding to the free oscillations of the Earth is delayed by a lag, which increases with increasing period. Pulsations of the 20.46-min band, which appear in the interval from 5 to 7 days after the earthquake and have a period of 127–129 min, are revealed. The patterns of the amplitude attenuation of the 20.46-min band are different at stations located in zones with different tectonic activity. These features manifest themselves in the search through different stations and through different earthquakes.

Structural changes in the surface of a heterogeneous nanocrystalline body (sandstone) under the friction

The structure of a ~30 nm thick surface layer of a heterogeneous nanocrystalline solid body (sandstone) before and after the friction was investigated using photoluminescence and Raman spectroscopy. Before the friction, this layer contained nanocrystals of quartz, anatase, feldspar, and montmorillonite. The friction caused a sharp decrease in the concentration of nanocrystals of quartz and feldspar.

The dynamics of the acoustic emission with water initiation

The long-term experiments on the biaxial compression of the models prepared from basalt sand, crushed limestone, and cement are carried out. The volume of the models exceeded 4 × 103 cm3. At different grades of load, water was infused into the model through the borehole. The volume of the infused water was less than 0.1% of the volume of the model itself. Water infusion resulted in the activation of the acoustic emission with a particular time lag. The initiation of the acoustic emission is connected with the local reduction in the strength and with the increase in the stresses near the metastable, which does not contradict the trigger mechanism. The hypocenters of the sources of acoustic signals initiated by water infusion lay in the vicinity of the boreholes. Within the time interval between weak events, pulses with higher energy appeared, which has an analogy with the swarm of earthquakes. The growth and extinction of the acoustic emission after the stepwise additional load and with the initiation by water infusion is significantly different. In the first case, the Omori law is obeyed. In the second case, the intensity of acoustic emission has a pronounced maximum. The equations of the kinetic concept of strength in solids make it possible to describe the dynamics of the acoustic emission caused by water infusion.

Study of nanocrystals in the dynamic slip zone

Mineral composition is studied and a search to detect nanocrystals is conducted in the surface layers of slickensides formed due to dynamic slip in arkose sandstone. The infrared and Raman spectroscopy show that the slickensided layer is composed of nanocrystals of montmorillonite and anatase measuring ≈15 nm and 3 nm, respectively. The crystalline lattice of the nanocrystals of montmorillonite is stretched by ≈2.5% while the lattice of the nanocrystals of anatase is compressed by ≈0.12%. Deeper than 3 mm below the slickenside surface, the sandstone contains nanocrystals of montmorillonite, beidellite and nontronite, quartz, plagioclase, and anatase. The nanocrystals of anatase have a linear size of ≈8 nm. Their crystalline lattice is compressed by ≈0.03%. It is supposed that montmorillonite in the slickensides was formed due to hydrolytic decomposition of silicates under friction of the fault planes sliding past each other.

Effects of high pressure and temperature on the properties of nanocrystals in rocks: Evidences from Raman spectroscopy

A search is conducted to detect nanocrystals in a sample of apogranitic pseudotachylite, which is a product of extremely strong crushing of granite in a seismogenic fault. Raman spectroscopy revealed nanocrystals of quartz measuring approximately 17 to 25 nm and low-temperature albite ranging from 8 to 30 nm. The crystallographic cell in the nanocrystals is deformed. The internal stresses which might have been responsible for these deformations vary from approximately −300 (compression) to +480 (tension) MPa. It is found that after having been exposed to high pressure (1 GPa) and temperature (470–500°C for 10 minutes and 550–600°C for 16 minutes), the nanocrystals of quartz reduced in size to ≈10 nm, and the nanocrystals of albite, to 13 nm. At the same time, the level of tension in the lattice spacing of quartz increased.

Dynamics of Interaction between Fields of Seismicity and Surface Deformations (Bishkek Geodynamic Test Area)

The spatial-temporal dynamics of surface crustal movements revealed from GPS data is compared with seismicity in the Bishkek geodynamic test area documented in the regional KNET catalog. The geological information system (GIS) GeoTaim 2.0 is substantially improved, which allowed variations in seismicity and deformation fields to be analyzed in the 3D raster. It is shown that seismicity and surface deformations are correlative in the test area. The periods with extreme values of contraction and the extension rates of the Earth’s surface areas are accompanied by enhanced seismicity and strong earthquakes. The increase in the spatial gradient of surface crustal movements coincides with changes in the azimuths of compression axes indicated by mechanisms of earthquakes that occurred at depths of up to 25 km. For a better geological—geophysical interpretation of interactions between deformation and seismicity fields in the Bishkek geodynamic test area, the spatial system GPS stations and measurement frequency need substantial improvement.

Dynamics of fluid-triggered fracturing in the models of a geological medium

Implications of infusion of small (compared to the pore space) amounts of water for the dynamics of fracturing are studied in the laboratory experiments on the models of a heterogeneous geological medium. Variations in the acoustic emission that precede macrofracture are analyzed, including as candidate precursors. In all experiments, the macrofracture had occurred not immediately after the mechanical loading or water infusion but with a time-delay after a period of acoustic quiescence and subsequent activation. The enhancement in acoustic activity preceding macrofracture is well reproduced by the exponential law; the correlation between the actual number of events (or the released energy) and the exponential approximation exceeds the 95% confidence level. The power law is slightly worse although also a confident approximation of the acoustic emission process. The facts of subsequent occurrence of quiescence and activation suggest that, in principle, this phenomenon can be used as a precursory signature in the prediction of macrofracturing.