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EARTHQUAKE PREDICTION:  STUDIES AND EXPERIMENTS1999-2002

ZHANG Xiaodong, FU Zhengxiang,

ZHANG Yongxian, NIU Anfu, HUANG Fuqiong, PENG Keyin and LU Jun

Center for Analysis and Prediction, China Seismological Bureau, Beijing 100036, China

 

In China the earthquake prediction is usually divided into four kinds: long-term (few years to tens years), intermediate-term (few months to few years ), short-term (few weeks to few months ) and imminent-term (few days to few weeks ). The present report will give some research results on earthquake prediction during the period from 1999 to 2002. The methods of earthquake prediction are based on mainly the precursory observation of seismicity, active fault, crust deformation, electromagnetism, underground water, and so on.

 

I. RESEARCHES ON LONG-TERM EARTHQUAKE PREDICTION

The long-term earthquake prediction amounts to determining the recurrence time of earthquakes on a certain fault segment or a certain seismic zone and predicting the approximate time of the next earthquake from the known time of the previous one. Long-term earthquake prediction has extensively used earthquake rates for non-precursory information to estimate the rate of earthquakes and the probability of future event occurrence from seismological, geological and geodetic information. At the same time, the long-term precursory seismicity patterns (for example “seismic gap”) are also used in the long-term earthquake prediction.

(1) The estimation of annual seismic moment rate for a seismic zone or seismic area is necessary in the time-magnitude predictable model (Papazachos, 1989). Shao et al. (1999) proposed a method of estimating annual seismic moment rate by using different seismic data including different precision and completeness, even the paleo-earthquake; at the same time, Jin et al. (1999) used fault slip information to estimate of annual seismic moment rate. And their research results have applied to the time-magnitude predictable model for long-term earthquake prediction of North and Southwest China seismic area. Yi and Wen (2000) have analyzed the recurrence behavior of historical strong events for 39 seismogenic sources in North-South seismic zone of Western China and preliminarily constructed a statistically time-magnitude predictable model. The result shows that within the coming 30 years, 5 of the 39 seismogenic sources have higher conditional probabilities (0.4) for strong earthquake recurrence.

Fu et al. (2001) analyzed the time process of historic and recent seismicity and the empirical Gutenberg-Richter relation of magnitude-frequency along the Zhangjiakou-Penglai (Z-P) fault zone in the northern part of North China, and calculated the conditional probabilities of strong earthquake occurrence for eastern and western segment of the Z-P fault zone from the time of the last earthquake to 2010 using the Poison model.

(2) Based on the original stress release model of seismicity proposed by Vere-Jone (1978), Liu et al. (1999) have developed a stochastic coupled stress release model of time-dependent seismicity, which considers the earthquake interaction and stress transfer between different seismic subregions and is applied to a statistical analysis of the historical earthquake catalog with magnitude M6.0 during the period from 1480 to 1996 in North China. According to the Akaike information criterion (AIC), the results show that the coupled stress release model is better than the original model, which demonstrates the existence of long-range correlation between different seismic subregions.

(3) Huang and Yu (2000) have investigated the changes in time-space on seismic apparent strain (Wyss, 1970) of middle events, before and after 31 strong earthquake occurred in China since 1955. The result shows that there is a rather well corresponding relationship between the anomaly region of seismic apparent strain and the zone of strong event occurrence within the time range of one to about five years. Huang and Wu (2000) studied the relation between occurrence time and cumulative frequency of strong earthquakes on five subregions of South-North seismic zone of China, which obey an exponential distribution or power function and the time interval of strong earthquake occurrence had changes ranging from few years to tens years.

(4) To investigate the recurrence behaviors of segment-rupturing earthquakes on active faults of the Chinese mainland, Wen (1999a, b) analyzes quantitatively earthquake history of 19 fault segments based on earthquake data segments having behaviors of quasi-periodic earthquake recurrence at previous location and 12 segments with characteristics of time-predictable earthquake recurrence. And the empirical distributions for earthquake recurrence interval for the above-mentioned two earthquake recurrence behaviors have been established, both of them obey the normalized recurrence interval very well, which are no significant difference to that on the Circum-Pacific plate boundaries (NB model). Wen's researches might be important for long-term probabilistic earthquake prediction of active faults of Chinese mainland.

(5) Guo et al. (2001) illustrate the gradient of vertical deformation rates, seismotectonic and seimicity in China continent, as well as their relationship, and get the gradients of vertical deformation rates on a GIS platform based on the map of vertical deformation rate of China continent (1951-1990). The variation of the gradients may reflect the magnitude of vertical crustal shear strain. In the period from 1990 to 1997, some of the strong earthquakes occurred along some high gradient zone, such as 1990 Gonghe Earthquake (M=7.1), 1996 Lijiang earthquake (M=7.0), 1996 Baotou earthquake (M=6.4) and 1998 Zhangbei earthquake (M=6.2). So the analysis of relation between the gradient of vertical crustal deformation rates will apply to the long- term earthquake prediction (Guo and Xie, 2001).

(6) Some investigators (e.g., Utsu, 1977); Aki, 1981; Kenji and Akio, 1990) have investigated the temporal probabilistic synthesis models of earthquake prediction. Wang et al. (2000) investigated a spatial and temporal synthesized probability gain model for the long-term earthquake prediction, which considers not only the relation between strong earthquake occurrence and abnormal temporal process of long-term precursor (seismicity , crust deformation and so on ), but also the abnormal geological structure, active fault behaviors and geophysical fields etc in space. The model has applied to the long-term earthquake prediction for North China.

 

II. RESERCHES ON MEDIUM-TERM AND SHORT-TERM EARTHQUAKE PREDICTION

1. Seismicity and Seismicity Triggering

Liu (1999) and Wang et al. (2001) use the seismicity data of moderate or small earthquake to space scanning of seismic inhomogenous degree (GL-value) before some strong earthquakes in North China. The result shows that there are obviously anomaly zones of GL-value in the neighbourhood of epicenter in general during the mid-term of 1-3 years before moderately strong earthquakes in China.

Yang and Qu (1999) calculated b value in the relation between magnitude and frequency by using robust regression of M estimation and choosing function as normal density function, and discussed the difference between the results obtained by the methods of robust.

Zhou and Zhu (1999) deduced a method for estimating strong earthquake recurrence period by the fractal based on the dynamic equation of the seismic energy accumulation.

Peng et al. (1999) deduced a formula of effective shear stress for the slip fault, the thrust fault and the strike fault based on the characteristics of regional fault activity and the result of tectonic stress field, and proposed a new Loading/Unloading distinction criterion. Chang et al. (1999) Studied the variation characteristics of the Loading/Unloading response ratio with time before the Baotou Ms6.4 earthquake. The result shows that there is obvious anomaly before the 1996 Baotou earthquake.

Shao et al. (2000) used the Morlet wavelet translation method to analyze the dynamic period of seismic energy in some area of North China. The results show that there exist both comparatively stable periods and some variable periods with time.

Jiao and Ding (2000) analyzed 24 earthquake cases with M6 occurred in China. The research results show that about 80% of the cases, earthquake activity increased within the epicentral area and its surroundings before the mainshock.

Zhao et al. (2001) applied the Morishita index Iδ to analyze the features of the day-night and seasonal distribution of earthquakes in China. The statistical results indicated that the occurrence time of strong earthquakes in China's inland is mainly concentrated at night.

Pei et al. (2002) introduced the semi-variogram analysis to study the seismic relativity, and detected the distinct difference of relativity between foreshocks and aftershocks of Songpan earthquake.

Cai et al. (2002) calculated the multi-fractal spectrums of the temporal distribution of earthquakes occurred in China from 1960 to 1982 and analyzed aftershock sequences of the four great earthquakes. The result may be useful for earthquake prediction.

Zhou et al. (1999) provided a method to distinguish the pre-and after-shocks of a strong shock from earthquake catalogue and then to form a sequence based on the analysis of SLC (Single- Link- Cluster).

Zhou et al. (2000) studied the dynamic changes of stress field of small earthquakes occurred before four moderate-strong earthquakes in North China by using the data of comprehensive mechanism solution of small earthquakes and using the initial P wave recorded by seismic recording. They also generalized preliminarily the dynamic evolution characteristics of stress field in the focal region and its adjacent area before moderate and strong earthquakes.

To understand the statistical characteristics of the time process of earthquakeoccurrence, Kagan and Jackson (1991) have defined a coefficient of variation (Cv) of earthquake interoccurrence time (T) as a ratio of the standard deviation (s) to the average time (Ta): Cv = s / Ta. Cao et al. (2000) analyzed the variation of Cv before the 6 strong earthquakes along North-south earthquake belt in China from 1970. The result shows that several months or one year before the main-shock, the coefficient of variation drops slowly, ranging from 0.5-1.3. Among six strong earthquakes, the variation of Cv drops in long and intermediate-term before four earthquakes. The result indicates the coefficient of Cv reflect some complexity of seismic precursors before strong earthquakes.

Li et al. (2001) investigated the co-relationship between seismisity in the Taiwan area and its vicinity, finding that optimal angles for frequency over double standard deviations are about 50° of offset after quadratures and full moon. Li (2000) analyzed the triggering of the seasonal changes in earth rotation rate and tide for strong earthquakes occurring in North China, and suggested that earth rotation and tide force can trigger strong earthquakes. Zhang et al. (2001) analyzed the relationship between the Ms7.0 events in the Chinese mainland and the modulation and triggering from the earth tides.

Fu et al. (1999) investigated the interaction between parallel strike-slip faults and its effect on seismicity in visco-elastic model. The slipping along a strike-slip fault would lead to shear stress reduction on parallel fault planes, and delay the slip occurrence along the parallel faults.

2. Crustal Deformation

On the crust movement and the stress-strain filed of China mainland

Based on the regional GPS observations, the principal movement trend of China main blocks have been given out. Liu and Shao (1999), Zhou and Guo (2000), and Zhou and Wu (2001) have explained the characteristics of crust movement in Fujian coast which may be related to the Taiwan 7.8 earthquake of 21 Dec, 1999; Jiang and Zhang (2000a, 2000b, 2001) have studied the recent movement of Qing-Zang block and western China, Wang and Ding (2000) have explored the crust deformation in Xinjiang region.

The GPS data from China Crust Movement Observation Network have attracted many authors to study the characteristics of China mainland. Sui and Wu (1999), Gu (2001) have draw the images of movement of China mainland from 1998 to 1999, and explained the characteristics of crust movement of China mainland.

Except of the movement of block based on the GPS, the study on the stress-strain field is also an important research subject. Huang and Song (1999) have given the mechanism of horizontal deformation based on the GPS observation in northern China, He and Niu (2000) have simulated the stress field change in Northern China based on the regional tectonic data, Chen and Nie (2001) have simulated the distribution of deformation and stress field for a elastic body model, Chen and Zhang (2001) have studied the evolution of stress status and fault activity in Northern China, Jiang and Zhang (1999, 2000a, 2000b, 2001) and Chen and Jiang (2000) have discussed the method for solving the strain and calculated the stress status and block movement by using the non-continuous deformation method.

Study on the earthquake precursor and preparation stages

Based on the continuous deformation observations, for example, tilt tide, strain tide, gravity tide, leveling height and baseline of cross fault and so on, which sample period is usually less than one month, many scholars have studied the main characteristics of deformation before some earthquakes, and furthermore to understand the earthquake preparation process. Main achievements reflected in researches on the data processing, precursor features and identification method, earthquake preparation and prediction methods.

Zhang and Wang (1999,2001) have studied the tilt and gravity anomaly before Lijiang earthquake (M=7.0) in 1996 and Baotou earthquake (M=6.4) in 1996 by using the wavelet method. Zhang and Jiang (2001) have proposed the synthetic anomaly index SAD for deformation anomaly identification, Zhang and Wang (1999) have discussed the change of ratio of loading and unloading by using the body strain. Luo and Gu (2000) have discussed the stress change calculated from bore hole-strain tide change.

Deformation process before earthquake is an important way to realize the earthquake preparation stages. Che and Xie (1999) ,Gao and Huang (1999), Xing and Wang (1999,2000) have studied the evolution of deformation fields before Zhangbei earthquake (M=6.2) in 1998 and Jingyang earthquake (M=5.2) in 1998.  From the view of gravity observation, Zhang and Sun (2001), Zhu and Hu (1999) have discussed the images of gravity field before the Zhangbei earthquake (M=6.2) in 1998 and Yongdeng earthquake (M=5.8) in 1995, respectively. Hao and Hu (2001) have explored the relationships of high frequency fluctuation of gravity and earthquake.

The relationships between fault activity and earthquake had been deeply explored. A lot of scholars have made available works. Yang and Wang (1999, 2000) and Yang and Xie (2001) analyzed the recent active status of main tectonic blocks and boundaries in Northern China, Bo and Guo (2001) discussed the images of fault activity level around the Beijing area and some deformation features of block boundaries. Wang and Jiang (2001) and Guo and Xie (2001) have studied the anomaly characteristics of fault deformation and deformation velocity before Jingtai earthquake (M=5.9) in 2000 and Zhangbei earthquake (M=6.2) in 1998. Sun and Jia (2001) have given the gravity characteristics along the Xianshuihe fault that located in Yunnan province. Niu and Wang (2000) have studied the optimum method for calculating the fault activity parameters and for estimating the future earthquake risk.

The study on deformation precursor and earthquake preparation is an important part of earthquake prediction based on deformation observations. Meng and Li (1999), Wang and Liu (1998) have studied the characteristics of deformation before the Zhangbei 6.2 earthquake, Niu (1999) has discussed the characteristics in different stages before Zhangbei earthquake and the experimental explanation. According to the characteristics of sudden change of deformation, Niu (2002) has explored the relationships of deformation anomaly and earthquake, and discussed the possible mechanism.

Except of the temporal process of earthquake preparation, a lot of scholars have paid much more attention to the distribution of deformation anomaly field characteristics. Niu and Liu  (1999) discussed the inhomogeneous distribution of deformation anomaly intensity before some strong earthquakes in China. Wang and Long (1999) have explored the preparation process of strong earthquake from the view of shear stress field of China.

Xiao and Zhu (2000), Niu (1999) and others have studied the mechanism of crust deformation in earthquake preparation process from the rock experiment and numerical modeling.

3. Electromagnetism

There are about 100 geoelectrical stations distributed over the major meizoseismal regions and the important cities in the China mainland. Zhao et al. (2001) noted that intermediate-term resistivity decreases of the 1976 Tangshan earthquake attributed to press-strain accumulation, which is supported by the shorten of base-line and decrease of groundwater level. Lu (1999) showed that resistivity decreases prior to the Tangshan earthquake cannot be caused by a groundwater decrease with the sensitivity analysis of the Schlumberger monitoring array. In other words, if we eliminate the effect of groundwater on apparent resistivity, the corrected value of apparent resistivity should be lower than the observed one.

The experiments for detecting crack omen and determining the main fracture spread direction of rock with dynamic rock resistivity change anisotropy have been made recently (Chen et al. 2000a, 2000b). Mao et al. (1999), Du et al. (2001), Ruan et al. (2000) and Feng et al. (2000) have discussed the relationship between the resistivity anisotropy and earthquakes. The intermediate- and short-term changes in apparent resistivity prior to earthquakes have been further studied (Wang et al., 1999a; Du and Tan, 2000; Du et al., 2000; Guo et al., 2002). Some cases from resistivity anomalies have been presented (Wang et al., 1999; Sun et al., 1999; Qiang, 1999; Zhang and Wu, 2000; Gao et al., 1999; Gao, 2000; Bai et al., 2001; Tian et al., 2002; Zhang and Wu, 2000; Zhang, 2002). The processing and evaluating of resistivity data have been improved (Zhao et al., 1999; Mao and Qian, 2001; Zhao et al., 1999). Liu et al. (1999), Gao et al. (1999) and Tian et al. (2000) give the analysis of the factors of disturbing resistivity observation.

The observations of electric field, electromagnetic emission and atmospheric electric field have progressed with a big step (Xi et al., 2002; Zhao and Ruan, 2002; Hao, 1999; Hao et al, 2000; Ling, 2000; Ruan and Zhao, 2000; Zheng et al., 2000; Tang, 1999; Chen et al., 1999; Zhao et al., 1999; Guan et al., 1999; Yang and Du, 2002).

4. Underground Water

It is useful for time prediction of strong earthquake using statistics that the temporal evolvement of minimum variation rate of radon content in underground water (Wang et al., 1999). The methods mentioned above are also used in the data of ground water table (Wang et al., 1999). Short-term anomalies of water table are trend increase or decrease changes over half year, or yearly cycle break changes, impending anomalies are fluctuations of the table (Cao et al., 2002 ).

It seemed that rapid and abnormal changes of the table is an important signal of the seismic preparation process approaching short-term stage. The area with rapid and anomaly changes of the table occurs in active segments of fault in seismogenic zone (Liu et al., 2000).

The water level system is a chaotic system, the freedom degree of which is within the range of 3-16, and the dimension and entropy of the system declined before earthquake occurrence (Li et al., 1999). Obviously, such a feature is of important significance of the earthquake prediction.

Based on the mechanism of subsurface fluid, the corresponding relation in time and space between subsurface fluid anomalies and seismicity parameter are systematically studied. The results show that there is obvious spatial relation among the medium-and short-term anomalies of subsurface fluid, seismic gap, weak earthquake belt and distribution of b value anomalies (Liu et al., 1999, 2000).

It is also found that the spatial distribution of abnormal mercury content in the underground water are correlated not only with the magnitude of the coming earthquakes, epicenter distance, but also the stress field and the observation site. The duration of the mercury content anomalies before earthquake occurrence in the North China are shorter comparing to that in Northwest and in Southwest China (Kang et al., 1999).

The far-field precursor has come to wide attention. The result of comprehensively evaluating observation network of ground fluid in Northwest China shows that medium-short term anomalies are observed at sensitive sites in the distant field and short impending anomalies are observed at sensitive sites in the near field (Zhang and Wang, 1999).

The experiment studies on the features of the transmission of hydro-chemical pulse information through aquifer show that the regularity of pulse hydro-chemical anomaly migration in the aquifer system is rather complicated. With increase of the migration distance, the information amplitude attenuates obviously, and even the anomaly forms are changed. These features of variations are also related to the stress state that the aquifer has been situated in (Yu et al., 2000).

The numerical simulation of time-spatial evolvement of subsurface fluid related to earthquake using asperity model of earthquake-generation, show that (1) at early phase of earthquake preparation, there is no prominent anomaly of subsurface fluid, (2) at the metaphase, the subsurface fluid anomaly occurs around focus zone, and (3) at medium-short term, the range of subsurface fluid anomaly enlarges, and the anomaly occurring around focus zone at metaphase continues. Those results prove that the ‘precursors in the source area and near field' emerges earlier than the ‘precursors in the distant field' (Zhang and Liang, 2000). The results of numerical simulation of hydrodynamic condition relating to precursory sensitivity of underground water level show that the confining condition of well-water bearing system has heavy influence on reflecting earthquake effect of water level change, and the observation quality is directly affected by the leaking level on the sides of aquifer and the height of water head. Moreover, for earthquake prediction purpose, in a good well-water bearing system the good penetrability is also on kind of the important factors to affect water level change (Zhao and Yin, 2000; Li et al., 2000).

The result of study on the characteristics of hydrothermal activity and earthquake activity in the southern Zhangjiakou indicates that the faulting strength will reduce owing to the effect of chemical and physical act of ground fluid on fault and its surrounding rocks. The effect will be intenser with the increase of temperature and circulation depth of fluid. It is suggested that the underground fluid will play a great role in the earthquake preparation and occurrence (Wang and Lin, 2000).

The relation between the heat annually released at hydrothermal spots and seismicity is analyzed through the calculation of heat quantity annually released in Shanxi province. The results show that in the region where the lower heat released, the more frequently the large earthquakes occur, but the more rarely the moderate and small earthquakes appear. On the other hand, the medium and small earthquakes occur frequently in the region where heat released higher, and large earthquakes occur rarely. The research also shows that the energy, accumulated by crustal activity in Shanxi region and released annually in the form of heat, is equal to the energy released by an earthquake with M7.3. It weakens the seismogenic process of strong earthquakes and breaks free from the recurrence interval of strong earthquake (Zhang et al., 2002).

III. PROGRESS OF COMPREHENSIVE EARTHQUAKE PREDICTION

1. Scientific Thoughts for Comprehensive Earthquake Prediction

Zhu (2002) and Zhang (2002) summarized the scientific thoughts of comprehensive earthquake prediction in China based on the earthquake prediction practice during the past thirty years or more. They pointed out that the normal dynamic variation of field stress could be measured through the monitoring of physical parameters in large range and long time. Then anomalous variation of field stress could be detected through these data. Earthquake prediction is involved in long term, medium term, short term and imminent term prediction. A short or imminent prediction might be based on a long and medium term prediction. The comprehensive earthquake prediction aims to analyze information from all aspects and distinguish the source precursors and field precursors the large amount of data and give out an approximate prediction.

2. Theories of Comprehensive Earthquake Prediction

Mei (1999) proposes a model of preparation of Xingtai earthquake (M=7.2) in 1966 based on the results of  artificial seismic exploration and natural seismic exploration. According to this model, the pattern of regional stress distribution and its development has been analyzed. Combined with characteristics of deep structure, the nucleation of Xingtai earthquake and mechanism of its main precursors (direct foreshocks, deformation, underground water level) are discussed. Wang et al. (1999) put forward a method of simulating the pregnant, generation and development process of earthquakes. From motion equation of particle moving in medium with linear rheological property, they derived the equation of 3D finite element which can simulate the whole dynamic state of earthquake's nature and to realize physical earthquake prediction. Zhang et al. (2000a2000b2000c2000d) supposed a possible mechanism of gravity anomaly before earthquake based on the up-down feature of gravity variation near the epicenter.

3. Approaches for Comprehensive Earthquake Prediction

Song et al. (1999) studied the periodic spectral characteristics of earthquake activity in the seismic strengthening areas of 24 earthquakes with M6.0 in China by the maximum entropy spectral method whose superiority is tested. The follow results have been obtained: The periodic spectra of seismic activity in seismic strengthening areas are different in different stage in earthquake-generating processes. Long periodic spectral and short ones coexist in normal stage, while only short ones (on average, 43% of long ones) exist and long ones disappear prior to earthquakes. The appearing time of short period before earthquakes has some relations with magnitude. The result shows that decades or even one hundred years is the common value for a great earthquake of M=8.0, 30 years for one with magnitude about 7 and 20~30 years for a strong quake of M=6.0. For the same magnitude in different regions the appearing time is also different. For example, it is longer in North China than that in the western part of China. Then the characteristics are preliminarily explained applying the strong body earthquake-generating model. Applying the maximum entropy spectral method, the idea of tendency prediction for prediction for strong and great earthquakes is suggested and used into practice, for example, the tendency predictions of the Wuding earthquake with M=6.5 and the Lijiang earthquake of M=7.0 in 1995 in Yunnan Province effects. So a new method of tendency prediction of M=7.0 in Yunnan Province got some positive effects. So a new method of tendency prediction of M6.0 earthquakes is offered.

Ke et al. (1999) took the seismogenic process of a large earthquake as a phase transition process of percolation by making use of modern nonlinear physics theory. The percolation probability exponent and correlative length exponent for the critical system can be calculated under the fixed point as which in the renormalization transformation infinite correlative length in percolation phase transition is taken. The percolation phase transition process of two large earthquakes, which are Haicheng and Tangshan events occurred in 1975 and 1976 respectively, has been discussed by means of seismicity data before and after two shocks.

Li et al. (2000) analyzed the micro-earthquakes activities, the anomalous crustal deformation, underground water, hydrochemistry and earth resistivity before some medium and strong earthquakes in Hebei province and its vicinity from 1970 to 1988 . Based on the above study, they extracted the variation character of 9 precursory anomaly factors, established the probability prediction model with each factor, and calculated the probability of earthquake during different stage after the precursors occurred.

Wang et al. (2000) used the BP neural network in intermediate-term earthquake prediction. Some usual prediction parameters of seismicity are used as input units of neural networks. And the output units of neural networks are called as the characteristic parameter W1 describing enhancement of seismicity. It is used for spatial scanning in North China. The result shows that the mid-term anomaly zone of W1-value usually appears obviously around the epicenter about 1~3 years before future earthquakes with good prediction effect.

In order to search comprehensive anomalous area and prediction future moderate earthquake, space concentration C, which describes time, space and strength characteristics of earthquake distribution, seismic risk D and strength factor Mf are intersected for north China, which has epicenter accuracy of 1.5 radiusWang et al. 2000). The stability of comprehensive anomalous area and the future moderate earthquake are illustrated using examples. Through comparative analysis for the application of various area of north China, it is considered that different efficiency is caused by data detail and defect.

Based on the projection pursuit regression (PPR) principle, the PPR model has been set up with 15 independent variables such as b value, Cb value, Ab value etc., which are selected from seismometry parameters and one dependent variable representing maximum magnitude of earthquake that would occur in coming several months. By using the model, maximum magnitude of coming earthquake and probability of earthquake to occur in the future can be predicted. The PPR model of four mainly seismic regions in Xinjiang and their ridge functions are shown. The prediction effects of reservation test and practical test are satisfactory (Zhao et al., 1999).

Zhang and Qin (2000) defined modulating blocks ratio on the basis of microseismic modulating ratio's concept. It can reflect variation of earthquake field. Taking some earthquakes as example, they conclude that modulating blocks ratio is a medium-term anomaly occurring 1-2 year before strong earthquakes. Anomaly mechanics of modulating blocks ratio is related with local strain field concentrated and strengthened. They can be easily modulated by solid tide.

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