K-NET Networks Research Articles

SOME CHARACTERISTICS OF THE EARTHQUAKE CATALOG AND THE SEISMIC PROCESS ACCORDING TO THE KNET NETWORK

Some characteristics of the earthquake catalog and seismic process are considered. The catalog is obtained according to the KNET network (KN-code in FSDN, International Federation of Digital Seismograph Networks, that is operated by Research Station of RAS, RS RAS) and contains more than 10000 earthquake parameters from 1994 to 2020. The catalog and seismicity characteristics were determined the whole catalog as well as the catalog that is limited by the coordinates of the Bishkek geodynamic polygon (BGP). Statistical analysis of arrival times of direct P- and S-waves registered at KNET network stations was carried out. The maximum P- and S-waves were registered at AAK station, the minimum – at ULHL station. The territory of the BGP is covered by great P-traces density. We analyzed earthquake localization errors, i.e. the difference between the observed and calculated arrival times of P-and S-wave fronts (RMS), horizontal error (ERH, epicenter) and vertical error (ERZ, depth). Occurred on the BGP territory earthquakes have minimum values of the considered errors. The representative sample is defined: for the whole catalog it includes K≥7.2 earthquakes and for the limited by BGP coordinates catalog – K≥6.7. Statistical characteristics of the representative part of the catalog in terms of time and depth are determined. The temporal distribution of earthquakes by energy classes is constructed and the absence of positive or negative trends in the number of events is noted. Spatial distribution of earthquakes by depth – 0–5, 5–10, 10–15 and more than 15 km is constructed. The smallest errors of earthquake depth determination have the events that occurred on the BGP territory. During the study 46 moderate earthquakes with K≥12 occurred. The most part of these events happened in the North Tien Shan seismgenic zone. 22 events were determined with K≥10 followed by aftershock sequences and give some aftershock characteristics. The most events with aftershocks occurred in the eastern part of the Kyrgyz ridge. The distribution of earthquake numbers and STD intensity is constructed. Zones of seismic activity and maximum intense Earth’s crust deformation are identified. The Gutenberg – Richter law and provisions of nonextensive statistical physics were used to describe the energy distribution function of earthquakes.

Инструментальная мера сейсмической интенсивности по данным сети K-Net

Relevance. Traditionally, seismic impacts in the practice of seismic construction are estimated by the magnitude of the peak horizontal ground acceleration. This approach, being simple and convenient for practical use, is reflected in building codes for seismic design and construction in seismically hazardous areas in most countries of the world. At the same time, studies of instrumental records show that often the value of the dynamic gain does not depend on the values of peak accelerations. In recent years, attempts have been made in the calculations of buildings and structures for seismic effects to use the values of ground vibration velocities, which, according to engineers, allow you to directly take into account the kinetic, acting and absorbed energy of the object under study. When analyzing the consequences of strong and destructive earthquakes, it is necessary to take into account the multifactorial influence of the earthquake process on buildings. In this regard, it becomes unclear what determines the seismic resistance of a structure or which impact parameter determines its seismic effect and is the most important for an adequate assessment of seismic resistance. The aim of the work is to establish the relationship between macroseismic intensity and instrumental indicators of seismic event records. Research method. The analysis was carried out according to the K-net network with a selection of records obtained in areas with different soil conditions (parameter Vs). The recording parameters, in this case, were calculated according to the methods proposed by various authors: Arias intensity, cumulative absolute velocity, Ang intensity, Hausner intensity, Fajfar measure, total energy measure, real and normalized spectrum areas, as well as weighted average frequency, predominant period and duration. The connection of each of the parameters was estimated by the method of Pearson’s correlation, Fisher’s criterion and mutual information. Research results. As a result, the parameters for a number of criteria that best describe the seismic intensity were identified. The article presents the most suitable parameters for «macroseismic intensity» instrumental measure. The highest correlation of which with the intensity, based on the use of generally accepted methodology, was: the Anga intensity (0.76), the area of the so-called. real spectrum Sr (0.73), cumulative absolute velocity CAV (0.73), Arias intensity (0.7), rms acceleration (0.68), peak horizontal displacement PHD (0.51). It should be noted that the correlation for the Hausner intensity unexpectedly amounted to only 0.38. When using machine learning (Lasso), the parameters are narrowed down. The most informative features are the peak horizontal velocity (PHV), the cumulative square of accelerations - the Arias intensity, the maximum value of the Fourier amplitude spectrum, the Anga intensity and the Fajfar measure. At the same time, the correlation for the area of the real oscillation spectrum is also quite significant.

Characteristics of the catalogue according to the KNET network (Northern Tian Shan)

Various characteristics of the catalogue obtained from the data of the KNET network, containing the parameters of more than 10,000 earthquakes that occurred from 1994 to 2020, are considered. A statistical analysis of the number of P- and S-waves registered by each station of the network is carried out. The activity of the stations during the localization of earthquakes has been determined. The most active station is AAK. The minimum number of P- and S-waves is recorded at the ULHL station. Statistical analysis of localization errors has been performed. The boundaries of the territory for which the minimum values of errors are observed are determined. The statistical characteristics of earthquakes in terms of magnitude, time, depth are presented. The distribution of the number of earthquakes by different energy classes in time and the spatial distribution of earthquakes by depths: 0-5 km, 5-10 km, 10-15 km and more than 15 km are constructed. The Gutenberg Richter law was used to describe the energy distribution function of earthquakes. A representative sample of the catalogue was determined, which includes events with M ≥ 1.8. The b-value for the directory is 0.88.

О некоторых аспектах статьи «О сброшенных напряжениях в очагах землетрясений Северной Евразии и приведенной сейсмической энергии»

In the article by N.A. Sycheva and L.M. Bogomolov, the authors proposed to combine the interrelated data on the stress drop in the earthquake sources, ∆σ, and reduced seismic energy, ePR, to analyze the dependence of these parameters on earthquake scale along with expansion of the measurement statistics (assessments). The dependence of these parameters of a source on the seismic moment or on the earthquake magnitude within 2.2 ≤ М ≤ 4.0 magnitude range has been determined using the example of the Northern Tien Shan (Bishkek geodynamic polygon with the KNET network). The author of the letter to the editor notes the article conclusions to be limited, because such relationship is only manifested within the more or less narrow range of the magnitudes. Attention is also drawn to the semantic difference between the ∆σ and ePR parameters. It is the reduced seismic energy that reflects the mean strain in the source area, and its appliance to the analysis of scale dependences of earthquake sources is more informative.

Prediction of V s30 in Datong Basin and Amplification Effect of Site Ground Motion Based on Typical Geological Characteristics Analysis

Shear wave velocity of soil layer is an important dynamic parameter, which is of great significance for site classification, determination of ground motion parameters and prediction of ground motion. Based on the analysis of shear wave velocity data of 1429 geological boreholes in Datong Basin, this paper studies the relationship between V S and geological characteristics such as soil type, geological unit type, SPT blow number and compactness degree. Based on the correlation analysis of shear wave velocity ratio of soil, soil compactness coefficient and thickness of Quaternary upper overburden, this paper presents a soil V S30 prediction model. The goodness of fit R 2 > 0.90 and the prediction accuracy is very high under the overall sample condition. Based on the abundant strong earthquake records and drilling information of KiK-net and K-net networks in Japan, V S30 and site period T S are selected as the two quantitative indexes of the amplification effect of site ground motion. The correlation between V S30 and PGA amplification coefficient and site period T S is established. The distribution characteristics of PGA amplification coefficient and site period T S based on V S30 distribution are given. The results show that the maximum values of PGA amplification coefficient and TS distribution characteristics of site period are consistent with the geological location of the earthquake swarm with M6.1 in 1989 in Datong Basin. It shows that the characteristics of the site ground motion effect estimated by V S30 can indicate the occurrence of earthquakes. The occurrence of earthquakes is affected by active faults and local conditions of the site at the same time, which fully proves the feasibility of this research method and the accuracy of the research results. The shear wave velocity ratio and the soil compactness coefficient are proposed for the first time, and they are applied to the prediction of V S30 with the thickness of the upper overburden of the quaternary system. This paper provides a good research idea for V S30 prediction of non-shear wave velocity test site, study of amplification effect of site ground motion, study and judgment of earthquake location. The research results can provide important technical support for urban earthquake prevention and mitigation planning, large earthquake scenario construction, earthquake risk analysis, earthquake damage prediction and regional earthquake safety evaluation in Taiyuan Basin.

Analysis of the seismic process of the Bishkek geodynamic test site (Northern Tien Shan)

Various characteristics of seismic process on the territory of Bishkek geodynamic proving ground are considered. Seismicity analysis is performed on the basis of earthquake catalogue, according to KNET network data. The catalog includes more than 10,000 seismic events that occurred from 1994 to 2019. To describe the energy distribution function of earthquakes, the Gutenberg - Richter law was used, as well as the position of nonequilibrium thermodynamics with the use of Tsallis statistics, which generalizes classical thermodynamics to the case of nonextensive systems. It is shown that the flow of earthquakes on the territory of the Bishkek geodynamic proving ground is a system with memory and long-range spatial correlations, and the study region itself is characterized as seismically active. The following seismic process characteristics are distributed in space and in time:KSRseismogenic fracture parameter, seismotectonic deformation intensity, seismic activity, Tsallis parameterqand constantsa, as well as a parameter characterizing the slope of the repeatability graph (b-value).

Analysis of seismic noise level at high-mountain stations of the KNET network

In this work we estimate level of seismic noise at Kyrgyz Network’s (KNET) high-mountain stations for 19 years of the time this network operated in real-time mode (1999-2017). To do that for each channel (E – west-east, N – south-north and Z – vertical, all with 100 Hz sample rate) of each station we made fifteen-minute (900 seconds) segments of seismic noise. We took such segments for different seasons (March, June, September, December) and for various times of day (00, 06, 12 and 18 hours UTC) for the whole 19 years. We analyzed probabilistic power spectral density heatmaps and found out some features of KNET’s high-mountain station’s operation. These features depend on season-wise weather changes. The worth month for all stations is September. In general, when weather is favorable these stations can be characterized as low-noise ones and their data may be used in various researches.

Detailed Structure of Attenuation Field in the Western Tien Shan Based on Short-Period Coda Waves

The seismic coda waves attenuation characteristic in the western Tien Shan region were studied. The short-period coda waves of the local earthquakes, recorded by the frequency-selective seismic stations in 1960–1984, and digital seismogram data of 150 earthquakes that had been occurring in the western Tien Shan region since 2005 until 2017, recorded at the AAK station (KNET network), were analyzed. The attenuation field is represented by the zones (blocks) having high Q-factor and being close to isometric in plan and by the linear zones of strong attenuation (weakened zones), which coincide with faults. It was revealed that the structure of attenuation field agrees with the structure of S-wave velocity field and the velocity anomalies of P-waves: low-velocity anomalies correspond to low-Q zones. The most expressed conducting zones, revealed from magnetotelluric data, coincided with the low-Q zones, indicating their significant fluid saturation. Sources of the strongest earthquakes with K > 16.0 were confined to the zones of maximum attenuation contrast, to the boundaries of blocks and weakened zones, where the mobility of blocks was determined by reduced viscosity of low-Q layers and small thickness of high-Q ones. The relationship between the features of seismic processes and spatial inhomogeneities of the attenuation field in the background of variations in the Earth’s rotation speed was also considered.

Collisional Processes in the Crust of the Northern Tien Shan Inferred From Velocity and Attenuation Tomography Studies

AbstractBased on seismic data from existing seismic networks in Kyrgyzstan, we have constructed new crustal models of seismic velocity and attenuation for P and S wave s beneath the Kyrgyz Tien Shan. With data from more than 6,000 events recorded by the international KNET network, the most detailed structures were detected in the central‐northern part of the study region, where the Kazakh Shield collides with the northern Tien Shan. The independently computed 3‐D distributions of P and S wave attenuation show features that are consistent with the main structural elements. The high‐attenuation areas correspond to folded areas of the northern Tien Shan, whereas the partitions of the stable Kazakh Shield and the Issyk Kul block match with the low‐attenuation areas. The velocity model reveals some structures that help to determine the details of the collision processes in the northern Tien Shan. In the upper crust, we observe the alternation of several higher‐ and lower‐velocity anomalies that likely represent the interaction of brittle and ductile crustal layers of the collided Kazakh and Tien Shan plates. In deeper sections, both P and S wave velocity models show a prominent low‐velocity anomaly just beneath the northern boundary of Tien Shan. We propose that this anomaly represents an anomalous crustal thickening at the point of underthrusting of the Kazakh Plate beneath Tien Shan.

Vertical-to-horizontal response spectral ratio for offshore ground motions: Analysis and simplified design equation

In order to study the differences in vertical component between onshore and offshore motions, the vertical-to-horizontal peak ground acceleration ratio (V/H PGA ratio) and vertical-to-horizontal response spectral ratio (V/H) were investigated using the ground motion recordings from the K-NET network and the seafloor earthquake measuring system (SEMS). The results indicate that the vertical component of offshore motions is lower than that of onshore motions. The V/H PGA ratio of acceleration time histories at offshore stations is about 50% of the ratio at onshore stations. The V/H for offshore ground motions is lower than that for onshore motions, especially for periods less than 0.8 s. Furthermore, based on the results in statistical analysis for offshore recordings in the K-NET, the simplified V/H design equations for offshore motions in minor and moderate earthquakes are proposed for seismic analysis of offshore structures.

The stress state of the northern Tien Shan crust based on the KNET seismic network data

In this study we present a detailed analysis of natural stresses in the Northern Tien Shan crust averaged in a window of 10-15 km obtained from seismological data of the local KNET network. The transformation of focal mechanism data into the parameters of the stress tensor was based on the method of cataclastic analysis of rupture displacement elaborated by Yu.L. Rebetsky (Institute of Physic of the Earth, Moscow). The results, including the orientation of the principal stress axes and the reduced stresses, are presented for four depth layers. It was shown that the central part of the study area is dominated by horizontal compression, while multiple domains characterized by horizontal shear and superimposed compression or pure horizontal shear are also present (uppermost layers in the eastern part of the Chuya depression, Suusamyr depression and adjoining regions, in the central part of the Kyrgyz Range). There are also several large domains of high and low effective confining pressure, which defines the corresponding deviator stress, according to the Coulomb-Mohr law. It was shown that relatively strong earthquakes are correlated with zones with low levels of effective pressure where the ruptures are characterized by lower resistance to brittle fracturing, i.e., Coulomb friction stresses. It was also shown that a distinct segment of the ~60 km E-W striking fault on the northern slope of the Kyrgyz Range generates a uniform distribution of stresses, corresponding to a dextral slip along of its edges.

Stress drop in the sources of intermediate-magnitude earthquakes in northern Tien Shan

The paper is devoted to estimating the dynamical parameters of 14 earthquakes with intermediate magnitudes (energy class 11 to 14), which occurred in the Northern Tien Shan. For obtaining the estimates of these parameters, including the stress drop, which could be then applied in crustal stress reconstruction by the technique suggested by Yu.L. Rebetsky (Schmidt Institute of Physics of the Earth, Russian Academy of Sciences), we have improved the algorithms and programs for calculating the spectra of the seismograms. The updated products allow for the site responses and spectral transformations during the propagation of seismic waves through the medium (the effect of finite Q-factor). By applying the new approach to the analysis of seismograms recorded by the seismic KNET network, we calculated the radii of the sources (Brune radius), scalar seismic moment, and stress drop (release) for the studied 14 earthquakes. The analysis revealed a scatter in the source radii and stress drop even among the earthquakes that have almost identical energy classes. The stress drop by different earthquakes ranges from one to 75 bar. We have also determined the focal mechanisms and stress regime of the Earth’s crust. It is worth noting that during the considered period, strong seismic events with energy class above 14 were absent within the segment covered by the KNET stations.

Site-Response Estimation for the 2003 Miyagi-Oki Earthquake Sequence Considering Nonlinear Site Response

The M w 7.0 Miyagi-Oki earthquake, which occurred on 26 May 2003, was well recorded by the KiK-net and K-net networks. A large number of stations recorded very high peak ground accelerations above 0.5g and large peak ground velocities above 0.5 m/sec. These high ground-motion values are thought to come from a combination of the effect of shallow sediment layers of the upper couple of meters and the enhanced high-frequency ground-motion content associated with this intraslab earthquake. The objective of this study is to examine the effect of sediment amplification at network stations with peak ground acceleration ! 0.3g. Linear site response is first estimated from observed weak motion (aftershock) records. In this case, we use a spectral inversion method, without reference stations, to separate the source, path, and site-response effects. The resulting weak motion analysis for the source, path, and site response agree with other previous studies. The mainshock site response is obtained separately using the same spectral inversion technique with the addition of a frequency-dependent radiation pattern. The comparison of the site am- plification from aftershocks with the mainshock indicates the possibility of nonlinear site response at many stations during the Mw 7.0 event. The results also suggest a correlation between lownear-surface materialvelocity and thedegreeofnonlinearity.