Seismoactive Regions Research Articles

Heterogeneities of the field of short-period shear wave attenuation in the lithosphere of Central Asia and their relationship with seismicity

The characteristics of the short-period shear wave attenuation field in the lithosphere of the Turanian Plate, West Tien Shan, Pamir, and Hindu Kush have been studied. The method based on analysis of the logarithm of the ratio between maximal amplitudes of Sn and Pn waves (Sn/Pn parameter) has been applied. More than 400 records of earthquakes, obtained at distances of ∼400–1000 km from the AAK digital station, have been processed. It has been found that relatively weak attenuation is observed in the regions of the West Tien Shan and Pamir. The largest area of strong attenuation is located in the region of the Afghan-Tadjik Depression adjacent to Hindu Kush. A wide band of low Sn/Pn parameter values, stretched northeastwards, has been distinguished. Along with the analogous band of strong attenuation, distinguished before in the regions of Central Tien Shan and Dzungaria, it is the continuation of the largest Chaman Fault, which stretches 850 km along the boundary of the Indian Plate. Source zones of strong earthquakes with M ≥ 7.0 that occurred in the first half of 20th century correspond to relatively weak attenuation. Areas of high attenuation, where strong seismic events have not occurred for the last 110 years, are outlined. Analogously to other seismoactive regions, it is supposed that these areas are related to preparation of strong earthquakes.

Study of active faults: Theoretical and applied implications

Theoretical and applied implications of the study of active faults and their identification and parametrization are discussed. The term active fault is defined as a fault with displacements that occurred in the late Pleistocene and Holocene and are expected to resume in the future. The displaced young landforms, late Quaternary sediments, and man-made constructions are the main reference marks to recognize active faults and estimate their kinematics and intensity. Since the structural pattern and parameters of all active faults are referred to the same, geologically short time interval, they are important for the study of recent geodynamics and young tectogenesis on the global, regional, and local scales. The opportunities that are opened for such investigations are illustrated by verification of the real existence of the Okhotsk and Bering minor plates. With allowance for active faults, it is possible to make tectonic and geodynamic reconstructions of the past events more plausible. Natural hazards, primarily, earthquakes, are related to active faults. The geological and geomorphic assessment of the seismic potential of active fault zones is discussed with emphasis on the maximum possible magnitude of earthquakes (Mmax) estimated from the segmentation of the active zones, the length of particular segments (L), the empirical relationships Mmax/L for strong recent earthquakes, as well as the resumption periods of strong earthquakes and measurement of particular seismogenic offsets by trenching and other techniques, including archeoseismological methods. A question is posed about possible perennial variations in the stress-and-strain state of active zones, which are expressed on the scale of large seismoactive regions in oscillations of released seismic energy and should be taken into account by assessment of the seismic hazard.

Connection of large earthquakes occurring moment with the movement of the Sun and the Moon and with the Earth crust tectonic stress character

Abstract. It is acceptable that earthquakes certain exogenous (cosmic) triggering factors may exist in every seismoactive (s/a) region and in Caucasus among them. They have to correct earthquake occurring moment or play the triggering role in case when the region is at the limit of the critical value of the geological medium of course. Our aim is to reveal some exogenous factors possible to initiate earthquakes, on example of Caucasus s/a region, taking into account that the region is very complex by the point of view of the tectonic stress distribution. The compression stress directed from North to South (and vice versa) and the spread stress directed from East to West (and vice versa) are the main stresses acted in Caucasus region. No doubt that action of the smallest external stress may "work" as earthquakes triggering factor. In the presented work the Moon and the Sun perturbations are revealed as initiative agents of earthquakes when the directions of corresponding exogenous forces coincide with the directions of the compression stress or the spreading tectonic stress in the region.

Application of polarization ellipse technique for analysis of ULF magnetic fields from two distant stations in Koyna-Warna seismoactive region, West India

Abstract. A new approach is developed to find the source azimuth of the ultra low frequency (ULF) electromagnetic (EM) signals believed to be emanating from well defined seismic zone. The method is test applied on magnetic data procured from the seismoactive region of Koyna-Warna, known for prolonged reservoir triggered seismicity. Extremely low-noise, high-sensitivity LEMI-30 search coil magnetometers were used to measure simultaneously the vector magnetic field in the frequency range 0.001–32 Hz at two stations, the one located within and another ~100 km away from the seismic active zone. During the observation campaign extending from 15 March to 30 June 2006 two earthquakes (EQs) of magnitude (ML>4) occurred, which are searched for the presence of precursory EM signals. Comparison of polarization ellipses (PE) parameters formed by the magnetic field components at the measurement stations, in select frequency bands, allows discrimination of seismo-EM signals from the natural background ULF signals of magnetospheric/ionospheric origin. The magnetic field components corresponding to spectral bands dominated by seismo-EM fields define the PE plane which at any instant contains the source of the EM fields. Intersection lines of such defined PE planes for distant observation stations clutter in to the source region. Approximating the magnetic-dipole configuration for the source, the magnetic field components along the intersection lines suggest that azimuth of the EM source align in the NNW-SSE direction. This direction well coincides with the orientation of nodal plane of normal fault plane mechanism for the two largest EQs recorded during the campaign. More significantly the correspondence of this direction with the tectonic controlled trend in local seismicity, it has been surmised that high pressure fluid flow along the fault that facilitate EQs in the region may also be the source mechanism for EM fields by electrokinetic effect.

High-voltage rectifier of the Energy-2 generator for electromagnetic sounding and monitoring of earthquake source zones

The high-voltage rectifier is described developed for the Energy-2 generator, with a capacity of 200 kW in which a step-up power transformer is used as a converter. The Energy-2 generator is intended for solving problems of precision deep electromagnetic monitoring of seismoactive regions of the Earth’s crust to find earthquake precursors. Theoretical investigation and numerical simulation of the high-voltage rectifier are carried out and parameters of its elemental base are optimized. All the high-voltage rectifier components, heat sink, and forced cooling system were manufactured on the basis of the developed documentation. The high-voltage rectifier was successfully tested as part of the“Energy-2 generator in August 2009 during the “FENIKS-2009” experiment.

Relationship of high-frequency geoacoustic emission and electric field in the atmosphere in seismotectonic process

In July–October 2006 and 2007, combined measurements of geoacoustic emission in the range of 2.0–6.5 kHz, the electric field in the atmosphere near the ground, and meteorological values were carried out in Kamchatka. Using the nonparametric method of Spearman’s correlation analysis, the relationship between their average hourly values was examined. After excluding results of bad weather (rain, strong and moderate wind, low atmospheric pressure), a highly important negative relation between disturbances in geoacoustic emission and the electric field were detected. Most probably, it was caused by amplification of the strain of near-surface sedimentary rocks at the observation point during a seismotectonic process. The revealed relation is evidence for another manifestation of the lithosphere’s influence on surface atmosphere in a seismoactive region.

Investigation of the ULFelectromagnetic phenomena related toearthquakes: contemporary achievementsand the perspectives

The results of ULF electromagnetic signal observations in seismoactive regions prior to earthquakes are presented and discussed. The new differential measurement technique developed in SPbF IZMIRAN for location of the ULF emission sources of space and lithosphere origin is described. The MVC-2DS geophysical instrumentation is introduced as a promising tool for registration of ULF signals related to earthquakes (both seismic and electromagnetic ones). Methods are proposed for ULF data processing to investigate the preparation processes in the earthquake source regions and to distinguish seismogenic signals on the background of space pulsations. Some examples of application of those methods for study of the earthquake precursory signatures are presented. Perspectives of seismo-electromagnetic tomography experiments in seismoactive regions, using MVC-2DS technique, are discussed in relation to the development of earthquake prediction methods.

A set-up converter and energy transfer system of an “Energy-2” generator for electromagnetic sounding and monitoring of zones of earthquake sources

A boost converter and a system of energy transfer at a high voltage (up to 1100 V) developed for an “Energy-2” generator 200 kW in power are described. This generator is intended for solving the problems of precision monitoring of the electromagnetic depth of the Earth’s crust at seismoactive regions with an aim to search for earthquake forerunners. High voltage allows taking off the total generator’s power on transmitting antennas with high resistance (7 Ω and higher) within a wide frequency range (0.1–200 Hz) as well as on low-resistance antennas (lower than 0.7 Ω) at high frequencies (greater than 20 Hz). Modern Russian and foreign literature on the problem of developing transformers and rectifiers of the generator’s power circuit, responsible for generation of powerful low-frequency electromagnetic signals, is reviewed. Theoretical research and numerical simulation of the “Energy-2” generator’s power circuit are performed and the element base of the latter is optimized. Based on the developed documentation of all components, radiators and a system of forced ventilation have been made. The proposed generator-measuring complex has no analogs in world practice and can be used for solving a wide range of fundamental and applied problems.

Maximal radius of the aftershock zone in earthquake networks

In this paper, several seismoactive regions were investigated (Japan, Southern California and two tectonically distinct Japanese subregions) and structural seismic constants were estimated for each region. Using the method for seismic clustering detection proposed by Baiesi and Paczuski [M. Baiesi, M. Paczuski, Phys. Rev. E 69 (2004) 066106; M. Baiesi, M. Paczuski, Nonlin. Proc. Geophys. (2005) 1607–7946], we obtained the equation of the aftershock zone (AZ). It was shown that the consideration of a finite velocity of seismic signal leads to the natural appearance of maximal possible radius of the AZ. We obtained the equation of maximal radius of the AZ as a function of the magnitude of the main event and estimated its values for each region.

Fluid dynamic effects of fault structures in the Moscow syneclise and applications to global geodeformation processes

The fundamental investigations of fluid dynamic aspects of fault structures in geosyncline regions [1, 2] that were carried out at the test sites of the former Soviet Union in the late 20th century established some systematic relations in the formation of geodeformation fields in a particular domain of the geologic medium under the influence of global variations in the stress‐ strain state of the lithosphere [3‐9]. In early 2002, we attempted to organize emanation seismic monitoring of geodeformation processes in the Moscow area [10]. It was intended that the results of monitoring would be used for the investigation of the nature of geodynamic phenomena observed in the structural units of the Moscow syneclise, primarily in those lineaments that were revealed by geophysical surveys within the city limits. As the methodic requirements for emanation monitoring of geodeformation processes were fulfilled at the selected observation site, it became possible to record processes occurring both in the Moscow region and far from it [11]. It was confirmed that the processes that create conditions in the geologic medium for the occurrence of catastrophic geodynamic events develop with the preliminary formation of locally unstable structures, both in seismoactive geosyncline regions and in platforms. The transition of the geologic medium into an unstable state was clearly indicated by changes in the spectral composition of measured parameters and the dynamics of their cross correlations. For instance, it was found that anomalous changes in the stress‐strain state of a rock massif are accompanied by almost synchronous changes in recorded parameters with a decrease in phase shift values. For instance, this was observed during the formation of global processes that triggered, in particular, strong seismic events in the Russian Far East and Altai in September 2003 [11].

Stressed state corresponding to the formation of large-scale brittle failure of rocks

Issue of relations between brittle failure and localized plastic flow, which are realized in the fault zones of the Earth’s crust, is one of the most important problems in geomechanics. New data on values of natural stresses in seismoactive regions allowed us to distinguish the regularities of their distribution, which correspond to the possibility of the appearance of large-scale brittle failure. It was found that areas of medium and low levels of confining pressure are the most hazardous regions in terms of a strong earthquake. Here, a small part of the released energy is consumed for negotiating friction forces. Therefore, brittle failure is more efficient. In the regions of high rock pressure, the dissipation of elastic energy is realized as creep along the fault or cataclastic flow due to a large number of weak earthquakes.

Unified scaling theory for distributions of temporal and spatial characteristics in seismology

The paper describes the unified scaling theory for distribution functions of temporal and spatial characteristics in seismology. It is based on the scaling of seismological characteristics calculated for various energy–spatial–temporal intervals. The common mathematical methods for the scaling of distribution functions are developed. The means to test possibility of such scaling are found as well. The relationship between the unified scaling theory and other present scaling approaches is determined. The theory is applied to two characteristics of different seismoactive regions. The first characteristic is the waiting time between earthquakes Δ T, the second one is a new space parameter Δ D min, which is the minimum distance of a current seismic event to the nearest (in space) neighbor in an energy–spatial–temporal interval. The distribution of the characteristics Δ T and Δ D min allows estimating the time interval to the next earthquake and the distance of the following earthquake from previous earthquakes. Thus, these characteristics are very important for seismic hazard estimations. Scaling of distributions functions is proven to be successful for Δ D min in all energy–spatial–temporal intervals and for Δ T with variations of energy/magnitude range. The distribution function of Δ T for various time domains was stable in only 60% of the cases, and near to unstable for spatial variations.

General law of similarity for earthquakes: Evidence from the Baikal region

.We have already carried out analysis of the unifiedscaling law for earthquakes (1) (hereafter, USLE) bothfor global [3] and several regional earthquake cata-logues [4]. In this paper, we present results of an anal-ysis and mapping of coefficients of the USLE in theBaikal region, one of the most seismoactive regions ofRussian Federation with relatively high populationdensity.METHODSWe used the later version of an algorithm for evalua-tion of coefficients of the USLE, which allows us to sta-bilize the estimates of the parameters. The algorithm ingeneral and the later modifications are scrutinized in [4].log

On the origin of stable IR anomalies detected by satellites above seismo-active regions

A number of experimental evidences have recently been obtained for existence of stable anomalies of infra-red outgoing Earth radiation in seismo-active regions, basically by using NOAA-series satellite data. A theoretical model for temperature enhancement of the ground surface layer in the vicinity of fault system of the Earth’s crust is developed. It is assumed that a tectonic stress in this area results in a water squeezing-out from the higher depths towards the Earth’s surface. A convection mechanism of rock warming by groundwater upward filtrating is considered as a possible reason for stable temperature anomalies arising near the fault interception. According to this model the fluid filtrates upwards from the higher depth to the Earth’s surface to provide an increase in the ground temperature similar to that found in observations.

Seismogenic deformations in early holocene sediments of the Pechenga River Terrace (Kola Peninsula)

The Kola Peninsula, an element of the northwestern Baltic Shield, is not a high-seismicity region at present. This has not always been the case, however, as is evident from seismic paleodeformations reflecting strong earthquakes. The latter were discovered in the course of geologic‐geomorphologic works and the analysis of air photos of mainly crystalline rocks [1]. Morphologic patterns and genetic aspects of deformations in incoherent sediments of the Kola region have yet to be sufficiently studied despite the fact that many researchers have reported such structures. Their origin is usually attributed to glaciotectonic and cryoturbation processes. Indeed, distortions of such kind are widespread in Quaternary sediments, because the region under consideration was repeatedly subjected to glacial impact. At the same time, deformations in incoherent sediments were also caused by other phenomena—those related to the strong seismic impact. Deformations in incoherent sediments provoked by modern earthquakes are known not only in seismoactive regions, but also in territories of moderate seismicity, such as Canada, Fennoscandia, and Russia [2‐5]. Recently, special studies were carried out to define different types of deformations in Quaternary sediments exposed in quarries and natural outcrops along river banks in the northwestern Kola region (Fig. 1) and to discriminate them from those related to the glacial impact.

Relation between electricity structure of the crust and deformation of crustal blocks on the northeastern margin of Qinghai-Tibet Plateau

Study on the electricity structure along a magnetotelluric (MT) sounding profile on the northeastern margin of Qinghai-Tibet Plateau indicates that four crustal blocks can be de-termined from southwest to northeast, namely Bayan Har block (BH), Qin-Qi block (QQ), Hai-yuan block (HY) or the North-South seismotectonic belt and Ordos block (OD). The BH, QQ and OD blocks display a similar electricity structure of the crust. The upper crust represents a high- resistivity layer and the upper part of lower crust represents a low-resistivity layer with the resis-tivity increasing gradually with depth from the lower part of lower crust to the upper mantle. The electricity structure of the crust in these three blocks is similar to that in the complete blocks on the southern and eastern margins of the Qinghai-Tibet Plateau and belongs to normal electricity layering of the crust in slightly deformed or complete intracontinental blocks. The crust in HY block as a boundary zone has been significantly deformed, hence its electricity layering was de-stroyed and the structure was complex and the block became a recent tectonically active and great seismo-active region. The contact belts between the blocks on the northeastern margin of Qinghai-Tibet Plateau exhibit both upthrusting outward and strike-slip movement different from those on the southern and eastern margins of the plateau. The genesis of the low-resistivity layer in the crust is analyzed and the thickness of the lithosphere is estimated in the paper.

Geothermal Structure in a Seismoactive Region of Central India

— The geothermal structure beneath a seismoactive region of central India has been determined by using two independent geophysical parameters, i.e., P n velocity and Curie point depth, which are temperature dependent. P n velocity has been determined by reanalyzing deep seismic sounding (DSS) records obtained for this region. Surface heat flow has been computed using the P n velocity data from this region and found to be 55 ± 5 mW m−2. Curie point depths have been derived from the magnetic satellite data in this region, and reveal an identical value of heat flow of around 56 ± 4 mW m−2. We estimated temperatures of 600–700°C at a Moho depth of 39 km by using surface heat flow ranging from 50 to 60 mW m−2 and a simple 1-D geothermal model. The present study suggests the presence of a brittle–ductile transition (high strength) zone ranging in depth from 20 to 35 km, where the temperatures range from 350 to 550°C. We also suggest that the nucleation of earthquakes in this region took place in a relatively high strength zone. This is supported by the fact that the focal depths of all the recent earthquakes (from 1978 to 2000 of magnitude ≥ 3.5) lie in this zone of high strength.

Vertically Inhomogeneous Models of the Upper Crustal Structure in the West-Bohemian Seismoactive Region Inferred from the Celebration 2000 Refraction Data

As part of the international refraction measurements in Central Europe in the year 2000, three profiles traversed the region of earthquake swarms in West-Bohemia/Vogtland. The shots were also recorded at the permanent stations of the local seismic networks. The travel times of P-waves, observed in the West-Bohemian region, are discussed and interpreted in the present paper. In general, significantly lower P-wave velocities were found in the Saxothuringian (northern) part of the studied area than in the adjacent southern parts. The observed travel times are interpreted separately for the individual geological units, in particular for the plutons, crystallinicum, and the Marianske Lazně (Marienbad) Complex. After smoothing the selected data using rational approximations, the Wiechert-Herglotz method was used to compute vertically inhomogeneous velocity models. The characteristic features of the derived models are relatively low P-wave velocities at the surface and prominent velocity increases within the uppermost crust down to a depth of about one kilometre.

Use of wavelet analysis for detection of seismogenic ULF emissions

Wavelet analysis is applied to high‐resolution magnetic ULF data in a seismoactive region to determine whether there is evidence of ULF electromagnetic emissions that precede or accompany earthquakes. We have developed an algorithm that is specially adapted to the single‐station wavelet detection of geomagnetic events. For this purpose we have constructed wavelet‐based magnetic signatures of certain earthquakes. Namely, we have used the distribution of energies among blocks consisting of coefficients of wavelet packet transforms. Our computer experiments have shown that common features preceding two strong earthquakes appear in geomagnetic fields recorded close (around 20 km) to the epicenter of the earthquakes. The anomalies occupy a wide range of periods (from 10 s to 250 s). Evidence was also found on the presence of short period seismogenic pulses associated with a strong earthquake (M = 5.6) on March 26, 1997, in Kyushu, Japan. A comparison of extracted geomagnetic variations at two observatories located in its epicentral zone has indicated that seismogenic geomagnetic disturbances occurred 6 to 7 hours prior to the earthquake.

Space-time distribution of earthquake swarms in the principal focal zone of the NW Bohemia/Vogtland seismoactive region: period 1985–2001

The NW Bohemia/Vogtland region situated at the western part of the Bohemian Massif is characteristic in a frequent reoccurrence of earthquake and micro-earthquake swarms. We present a comprehensive, integrated pattern of the space and time distribution of seismic energy release in the principal NK (Nový Kostel) focal zone for the period 1991–2001 and for the intensive 1985/1986 swarm. More than 3000 earthquakes, recorded by the WEBNET, the KRASLICE net and by temporary stations VAC, TIS and OLV operating during the 1985/1986 swarm, were located or re-located using the master event technique. Swarm-like sequences were identified and discriminated from solitary events by detecting local minima of the inter-event time using a standard short-time/long-time average (STA/LTA) detection algorithm. Most of the seismic energy in the NK zone was released during the two intensive 1985/1986 and 2000 swarms and in the course of the weaker January 1997 swarm. Further 27 swarm-like sequences (micro-swarms) and many solitary micro-earthquakes (background activity) were identified in the NK zone for the period 1991–2001 by the inter-event time analysis. Relative location revealed a pronounced planar character of the NK focal zone. Most of the events, including those of the intensive 1985/1986 and 2000 swarms, were located at the main focal plane (MFP) striking 169° N and dipping 80° westward at depths between 6 and 11 km. A singularity was the January 1997 swarm together with a micro-swarm that were both located across the MFP. The position and geometry of the MFP match quite well the Nový Kostel-Počátky-Zwota tectonic line. The space distribution patterns of larger events and of micro-swarms at the MFP differ: larger events predominantly grouped in planar clusters while the micro-swarms lined up along two parallel seismogenic lines. The temporal behaviour was examined from two aspects: (a) migration and (b) recurrence of the seismic activity. It was found that (a) the seismic activity in the time span 1991–2001 migrated in an area of about 12×4 km and (b) several segments of the MFP were liable to reactivation. The activity before, during and after the 2000 swarm took place in different parts of the MFP.