Seismoactive Zone Research Articles

ГЕОДИНАМІКА

In the paper discusses the features of long-term seismicity (1961-2010) the territory of Transcarpathian depression and separate seismoactive zones (A, B and C). The relationship of these zones with the main tectonic elements of depression is set. It is proved that the boundary between zones A and B passes through Oash deep fault.

Regularities in discrete hierarchy seismo-acoustic mode in a geophysical field

Some regularities in seismo acoustic mode have been studied during the preparation and development of dynamic events generated during the deformation at different scales in a geophysical field. The time-space behavior of certain auto similarity parameters: the slope of the recurrence plot c, the fractal dimension of the hypocenters set D, the relationship D-3c and the crack concentration parameter Ksr in laboratory and field experiments in Algerian seismoactive zone have been analyzed as well. Precursory stage and local failure evolution in rock samples and in natural conditions were studied. It is shown that the regularities in the behavior of the parameters under study do not qualitatively depend on the dimension of the object being loaded. The quoted examples above speak of inhomogeneity and self-similarity of seismicity distribution in space-time. This suggests that the evolution of cracking process at different scales, from rock samples to Earth crust, is controlled by the same physical mechanism. The further development of these studies consolidates the physical basis of the prediction of the dynamic events.

Surface and subsurface structures of Kalabsha area, southern Egypt, from remote sensing, aeromagnetic and gravity data

Kalabsha area is known to be the most seismo-active zone at the southwestern region of the Aswan High Dam, in southern Egypt. The main purpose of this work is the analysis and interpretation of satellite imagery, aeromagnetic and ground gravity data of Kalabsha area in order to reveal the subsurface lithology and structure. The Landsat ETM+ data were produced in a false-color composite image (bands 1, 2 and 3 in RGB) at the same scale as the geological map in order to reveal some extra geological and structural features. An attempt has been made to analyse the complex nature of gravity and magnetic anomalies over the Kalabsha area to reveal their relationship with surface geology, structure and tectonics setting. These analyses include isolation of anomalies into regional and residual components using the band-pass filter, second vertical derivatives (SVD), upward continuation, and shadowgrams technique. It was noticed that, there is close correlation between gravity, magnetic and many of the major surface geological features of the region. The basement structural map of Kalabsha area has been prepared from the integration of SVD of regional component maps of both gravity and magnetic. The interpretation of the basement tectonic map of the area indicated the presence of two sets of faults NNW–SSE to N–S which is dissected by an E–W to WNW–ESE fault system. These two sets of fault systems as deduced from the gravity and aeromagnetic data were found to match well with that obtained from the Landsat image and geological map.

Series of asymmetric pulses in the low-frequency range (Periods of 1–300 min) of microseisms as indicators of a metastable state in seismically active zones

Analysis of microseismic oscillations in a period range of several minutes recorded before the Kronotsky (December 5, 1997), Simushir (November 15, 2006), Hokkaido (September 25, 2003), and Neftegorsk (May 27, 1995) earthquakes revealed series of asymmetric pulses (SAPs) arising a few days before the earthquakes. Records of IRIS stations located at various distances from epicenters of the earthquakes were analyzed. SAPs were recorded only by stations located in a seismoactive zone in which an earthquake occurred. The SAP onset times and amplitudes are uncorrelated with the microseismic noise level in a period range of several seconds caused by storm phenomena in the ocean and other meteorological factors. Some intervals between successive pulses had the same length, which indicated a periodicity of their occurrence. It is supposed that SAPs can be caused by elastoplastic displacements in a fault zone.

Coherence of the ULF fields in the seismoactive zone of Japan

The work is devoted to a numerical analysis specifically adapted for multi-point detection of geomagnetic events, which precede or accompany earthquakes. Both numerically simulated data and geomagnetic perturbations during real earthquakes were investigated. The objective of this paper is to study the correlation properties of the ULF fields on small distances in seismoactive zone of Japan. By applying a combination of the Fourier, Hilbert and wavelet transforms to a single component and to their combinations we developed five correlation techniques and analyzed the geomagnetic variations at five observation points for two-year permanent observations. Our study was in two main directions: correlation properties of the field before an earthquake, and a coherency of geomagnetic pulsations launched by the earthquake already taken place. Five different coherency criteria have been examined: (1) the cross correlation function; (2) semblance; (3) in-phase; (4) σ; and (5) frequency stability criterion. We found that the signal detection times determined by the cross correlation, semblance and in-phase criteria are much more stable compared to that found with the help of the sigma correlation technique which allows detecting when the signal to noise ratio is about ∼3. The study revealed anomaly signals in the period range from 15 min to 1.5 h independently at all observatories. The signature of the anomaly signal is a wave train usually consisting of 2–3 of 0.5–1 h pulses which appears 3–4 h and 8 h prior to the earthquake time if we study correlations within 90 km radius. Intensity of the signals is around 0.5–1 nT.

Correlation analysis technique revealing ionospheric precursors of earthquakes

Abstract. In this paper we focus on the variability of electron concentration in the ionosphere measured by ground based ionosondes and GPS receivers around the time of strong earthquakes. It has been detected and statistically proven that several days before the seismic shock the level of this variability increases at the station closest to the epicenter, a fact which can be regarded as precursory phenomenon. More precisely the localness of this specific kind of ionospheric variability is used for the correlation analysis of data of several observation points. The similarity of geographical location of the observation points leads to the similarity of ionospheric variations registered at these sites during both quiet and disturbed geomagnetic conditions, except in the case of those located at the seismoactive zone. As a rule, the local anomalies in the F2 layer and TEC accompanying the preparation of strong earthquakes show themselves in the breaking of the mutual correlation of the critical frequencies foF2 or TEC between stations situated in and outside the seismic zone. The precursory phenomenon appears 1 to 7 days before the time of the seismic shock.

Active EM sounding for early warning of earthquakes and volcanic eruptions

Strong earthquakes (EQs) and dramatic volcanic eruptions (VEs) have a great impact on social and industrial communities. In spite of long-term studies, the problem of reliable predictions of EQs or VEs has not yet been solved. Proposed here is an active method for electromagnetic (EM) sounding of the Earth crust in ULF band. It is based on the use of a source of EM field such as a long linear cable, and a magnetometric monitoring part that can be placed in a seismoactive zone or volcanic area. After initial compensation of the primary field, the small detected deviations of conductivity and structure in crustal region caused by stresses before EQs or magma movements before VEs change the picture of the magnetic field observed at the surface. The main parameters of the active EM sounding are estimated and it is shown that the method has good perspective for EQs and VEs prediction.

The anomalous daily dynamics of local geophysical and geochemical fields (ADF) effect study in the connection with earthquake preparation and occurrence

The present paper represents theoretical concepts and experimental data on ADF effect observed in seismically active zones during the strong seismic event preparation period. Based on the obtained results, the author concludes that the ADF effect, preceeding strong seismic events, is observed in geophysical, geochemical, hydrogeodynamic and other fields only in certain, highly sensitive energy-active points.The technology for revealing of energy-active points based on the developed software packages “DF” and “Expert” is presented. For registration and using of the ADF effect aimed at seismic hazard assessment, a permanent monitoring on seismoactive zone through multiparameter observation network, the sites of which are mainly located directly in previously revealed energy-active points, is proposed.

Radon and metallic aerosols emanation before strong earthquakes and their role in atmosphere and ionosphere modification

One of the widely used techniques for earthquakes prediction research is radon concentration measurements within the seismo-active area. Radon concentration gradually grows during several months before a strong earthquake with a sharp increase few days before and abrupt drop few hours before the shock. This phenomenon usually is restricted to the region where the ground water trapped in the vicinity of the region of the earthquake, hollows out due to stress and escapes through the cracks developed. At the same time electromagnetic precursors exist showing the ionosphere modification before the earthquakes. One of them is the phase variations of the VLF signals passing over the region of anticipated earthquake. As it was shown by theoretical calculations this effect is connected with the changing of the conductivity of the “Earth-ionosphere” waveguide. The second effect is the ionosphere peak electron density variations. Scaled for particular local times (3 h, 6 h, 18 h LT) they show very high level of anticorrelation with the radon variations behavior. The observed variations imply existence of a coupling between them and radon concentration in the ionosphere. This coupling is interpreted in terms of modification of electrodynamic properties of atmosphere-ionosphere system before the earthquakes over the seismo-active zone. The atmosphere conductivity changes itself cannot explain the observed variations. The submicron metal aerosols emanated from earth together with radon have great influence on the modification processes. The same combination of radiation and dust occurred during Chernobyl atomic plant catastrophe and similar phase variations of VLF transmitters signal were observed when the signal passed over the Chernobyl plant during active emanation of the radioactive dust from the exploded reactor. Radon and metallic aerosols monitoring over the seismo-active regions could be used for strong earthquake prediction, and their effects on the electrodynamics of atmosphere-ionosphere system can explain the observed variations within the ionosphere.

Input parameters for estimating seismic loading

The estimation of seismic loading requires both the featuring of the seismotectonic environment as well as a prognosis of site-specific strong ground-motion parameters. Due to the long recurrence times of larger events, there is a lack of high quality strong motion recordings at many sites. The use of seismograms obtained from different seismotectonic regions, additionally recorded under different subsoil conditions, poses several questions as to the significance of the assessment of seismic loading. A way to overcome these problems is the generation of synthetic seismograms, e.g. by the use of Boore's stochastic procedure. Subsoil conditions may be explicitly included in the simulation as proposed by Kunze et al. In this context, the seismotectonic environment forms the deterministic frame of any stochastic strong ground-motion simulation. In terms of Boore's approach, seismic moment, global stress drop, high frequency cut off, and focal depth act as controlling parameters. In order to achieve a more realistic assessment of possible seismic loading, a classification of seismoactive zones should account for parameter vectors as described above rather than for single parameters like magnitudes or intensities. A tool to attack this task may be found in ‘cluster analysis’, which is a type of pattern recognition based on statistical considerations. Crucial to this technique, however, is the use of properly estimated parameters, which still cause discussion in the seismological community. Problems and chances of the approach are discussed with examples from the Swabian Jura and the Rhine graben.

Tectonic stress orientation and the seismic regime of a single fault

The seismic regime of tectonic faults was studied by the method of physical modelling. This method is based on the assumption that the physical processes in actual seismo-active fault zones can be scaled in laboratory conditions. The results of our laboratory experiments on plate models proved the great importance of tectonic stress orientation on the seismic regime of tectonic faults. Changes in the fault inclination led to distinct changes both in the recurrence time intervals between individual seismic events and in the amount of seismic energy released. For some fault inclinations the seismic activity of faults vanished altogether. Only a limited zone in the fault plane was found to participate in the process of seismic energy release. The extent of this so-called seismo-active zone depends not only on the contact conditions, but also on the value of the acting load and on the fault inclination; this presumption is supported by the fact that for each inclination there exists an upper limit to the seismic energy that can be released in one seismic event.

The structure of the Baltic Shield region on the basis of DSS and earthquake data

The lithospheric models obtained for the Baltic Shield by using deep seismic soundings are discussed and results from different parts of the shield are compared with models achieved by the investigation of surface waves and of P to S converted waves. The results are found to agree rather well with each other particularly with regard to the first interface (at a depth of 10–15 km) and the whole thickness of the crust (c. 40 km). The macroseismic focal depth determinations of regional earthquakes are correlated with lithospheric structure. The main maximum in the focal depth distributions of Finnish earthquakes occur at a depth of 10–12 km. The geographical distribution of the earthquake epicentres suggests several seismo-active zones in the northern Baltic Shield. One new finding in this field concerns the Lapland zone, which runs in a north-westerly direction.