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.     

Dynamic estimate of seismic danger based on multifractal properties of low-frequency seismic noise

A new method of dynamic estimate of seismic danger is presented which is based on estimating multifractal properties of low-frequency seismic noise. The efficiency of the method is illustrated by the analysis of seismic noise from broadband seismic network F-net in Japan. The analysis of multifractal properties of low-frequency seismic noise from Japan seismic network F-net since the beginning of 1997 allowed a hypothesis about approaching Japan Islands to a future seismic catastrophe to be formulated at the middle of 2008. The base for such a hypothesis was statistically significant decreasing of multi-fractal singularity spectrum support width mean value. The peculiarities of correlation coefficient estimate within 1 year time window between median values of singularity spectra support width and generalized Hurst exponent allowed to make a decision that starting from July 2010, Japan come to the state of waiting strong earthquake. This prediction of Tohoku mega-earthquake, initially with estimate of lower magnitude as 8.3 only (at the middle of 2008) and further on with estimate of the time beginning of waiting earthquake (from the middle of 2010), was published in advance in a number of scientific articles and abstracts on international conferences. The analysis of seismic noise data after Tohoku mega-earthquake indicates increasing probability of the 2nd strong earthquake within the region where the north part of Philippine Sea plate is approaching island Honshu (Nankai Trough). This region is characterized by relatively low values of singularity spectrum support width which is an indicator of seismic danger. In one paper (Sobolev in Izv Phys Solid Earth 47:1034–1044, 2011), the low-frequency seismic noise at the same range of periods was investigated retrospectively using data from the stations of broadband network IRIS which are located around the epicenter of Tohoku mega-earthquake with a distance up to 1,200 km. It was shown that the variance of the noise and the number of high-amplitude asymmetric impulses were grown dramatically before the event for stations which are located within the radius up to 500 km from the epicenter.     

Global coherence of GPS-measured high-frequency surface tremor motions

The traditional approach of GPS investigations is determining trends which are connected with the motion of tectonic plates. At the same time, a global GPS network provides the possibility of investigating statistical properties of high-frequency earth surface tremor in different parts of the world. Based on the results of coherence and correlation analysis of noise components of daily three-component GPS time series, representing measurements of earth surface displacements at 1097 stations, we have found that, during 2010–2011, there was a significant increase in the average level of noise coherence or correlation with dominant periods 7–9 days of surface tremor in nine regions of the earth, and in some of these regions, the average level of coherence or correlation is still high and does not return to the previous level. The increase of the average level of coherence and correlation could be detected on the graphs purely visually, while the middle time point of the time interval in which the ascending occurred is detected more precisely by a formal method based on the use of the Fisher’s ratio.     

Application of a Bayesian Approach for Estimation of Seismic Hazard Parameters in Some Regions of the Circum-Pacific Belt

—?The maximum possible (regional) magnitude M_max and other seismic hazard parameters like β which is the slope of Gutenberg-Richter law, and λ which is the intensity (rate) of seismic activity are estimated in eight seismic regions of the west side of the circum-Pacific belt. The Bayesian approach, as described by (Pisarenko et?al., 1996; Pisarenko and Lyubushin, 1997, 1999) is a straightforward technique of estimating the seismic hazard. The main assumptions for the method applied are a Poissonian character of seismic events flow, a frequency-magnitude law of Gutenberg-Richter's type with cutoff maximum value for the estimated parameter and a seismic catalog, which have a rather sizeable number of events. We also estimated the quantiles of the probabilistic distribution of the “apparent” M_max for future given time-length intervals.     

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.     

A Bayesian Approach to Seismic Hazard Estmation: Maximum Values of Magnitudes and Peak Ground Accelerations

Earthquake Researeh in Ch一na461 .METHODLet R be some value measured or estimated as a sequenee ina’‘Past”time interval(一丁,O)(I)万‘月,=(RI,…,R。),R,之R。,R=nlaX l二f匕11(RI,…,R,,) Values(l)eould have an arbitrary Physieal nature.BelowweshalleonsiderEq.(l)asearthquakemagnitudes in a given seismic aetive region or logarithms of seismie Peak ground aeeelerations at习given site.Ro isa而nimum eutoff value;it 15 defined by Possibilities of registration systems or wasehosen as the …     

Variations in the Parameters of Background Seismic Noise during the Preparation Stages of Strong Earthquakes in the Kamchatka Region

The results of the long (2011–2016) investigation of background seismic noise (BSN) in Kamchatka by the method suggested by Doct. Sci. (Phys.-Math.) A.A. Lyubushin with the use of the data from the network of broadband seismic stations of the Geophysical Survey of the Russian Academy of Sciences are presented. For characterizing the BSN field and its variability, continuous time series of the statistical parameters of the multifractal singularity spectra and wavelet expansion calculated from the records at each station are used. These parameters include the generalized Hurst exponent α*, singularity spectrum support width Δα, wavelet spectral exponent β, minimal normalized entropy of wavelet coefficients En, and spectral measure of their coherent behavior. The peculiarities in the spatiotemporal distribution of the BSN parameters as a probable response to the earthquakes with М_w = 6.8–8.3 that occurred in Kamchatka in 2013 and 2016 are considered. It is established that these seismic events were preceded by regular variations in the BSN parameters, which lasted for a few months and consisted in the reduction of the median and mean α*, Δα, and β values estimated over all the stations and in the increase of the En values. Based on the increase in the spectral measure of the coherent behavior of the four-variate time series of the median and mean values of the considered statistics, the effect of the enhancement of the synchronism in the joint (collective) behavior of these parameters during a certain period prior to the mantle earthquake in the Sea of Okhotsk (May 24, 2013, M_w = 8.3) is diagnosed. The procedures for revealing the precursory effects in the variations of the BSN parameters are described and the examples of these effects are presented.     

Assessment of seismic hazard of the Japanese islands based on fractal analysis of GPS time series

Based on the fractal analysis of the time series of the Earth’s surface vertical displacements in the region of the Japanese Archipelago, the maps of the estimates of seismic activity in the region over 2015 are constructed. The analysis of the maps revealed several segments of the territory which are prone to the emergence of significant earthquakes. The characteristic peculiarity is noted in the change of the behavior of the geophysical dynamic system—the Earth’s crust—before the occurrence of seismic events: the mechanism of transition to the critical state demonstrates the energy preservation of the low frequencies with the simultaneous energy decay of the middle and high frequencies, which differs from the behavior of the other dynamical systems.     

The statistics of the time segments of low-frequency microseisms: Trends and synchronization

The problem of identifying the effects of synchronization in the parameters of low-frequency microseismic noise from the data of 77 stations belonging to the F-net broadband seismograph network in Japan for the period from the beginning of 1997 through August, 2009 is considered. The vertical components measured initially with a sampling rate of one second and subsequently converted into the signals sampled at 1 minute intervals by means of averaging and decimation are used in the analysis. Six statistics are taken as the parameters: the support width of the multifractal singularity spectrum; the generalized Hurst exponent; the asymmetry coefficient of the spectrum of singularity; the logarithmic variance; the spectral exponent; and the linear predictability index. These parameters are calculated from the realizations contained within consecutive daily time intervals. When using the moving time window with a width of one year for evaluating the multiple correlation, the daily variations in the median values of the statistics of the noise measured at five spatial clusters of stations exhibit a stable increase in the synchronization not long before the Hokkaido earthquake (September 25, 2003; M = 8.3), subsequently passing to the new level of high synchronization. Based on the analysis of the trends in the index of linear predictability it turned out possible to estimate the beginning of the enhancement in the synchronization with rather high accuracy as the middle of 2002. The effect revealed for the variations in the different parameters of microseisms is an independent argument for the earlier conclusion about the synchronization in the field of the microseismic noise on the Japan Islands.