EARTHQUAKE PREDICTION RESEARCH IN CHINA: STATUS AND PROSPECTS

Transcription

1 J. Phys. Earth, 34, Suppl., S1-S11, EARTHQUAKE PREDICTION RESEARCH IN CHINA: STATUS AND PROSPECTS Zhangli CHEN State Seismological Bureau, Beijing, China (Received June 22, 1985; Revised May 22, 1986) The initial understandings about time and space distribution of major earthquakes and earthquake precursors, and prediction achieved for the past 18 years in China are summarized. Two major aspects of the fundamental understandings are: 1) Before most of the strong shocks, particularly those with magnitude over 7 occurring within an area under the detectability of a network, varieties of anomalous phenomena are observed. 2) Because of the complicated continental crustal structures, the temporal and spatial distributions of the continental seismicity in China are rather complex, but there are some regularities, and some unique dynamic patterns such as block-like, stripe-like, and diversified distributions of earthquakes. The problems of earthquake prediction are identified. They are mainly caused by 1) non-uniqueness of the relation between anomalous variations and earthquake precursors, 2) complexity of the relation between local and regional anomalies, and 3) diversity of possible earthquake precursors. And how to improve the level of earthquake prediction, topics to be further studied, and the key work to be carried out are also discussed. Efforts should be directed to 1) systematic evaluation and in-depth research of various prediction methods, 2) setting up the optimum monitoring network and appropriate data processing systems, 3) profound fundamental research, and 4) comprehensive research on earthquake precursors. 1. Introduction The fairly comprehensive earthquake prediction research program in China has been formulated officially and put into operation gradually since the 1966 Xingtai earthquake. A number of great earthquakes which have occurred in Mainland China in the recent 18 years have provided a rare opportunity and very useful sites for both Chinese seismological workers and scientists from other countries to study the earthquake prediction problem and to make repeated experiments. Therefore, there have been great development in earthquake prediction research, and increased understanding of the complexity of earthquake predictions. In particular, the successful predictions of the Haicheng and other earthquakes, and the failure to predict the Tangshan and other shocks have made a great impact on, and fostered various opinions about, the future of earthquake prediction. The scientific assessments of the earthquake prediction research in China will be very helpful not only in working out a new earthquake prediction research program in China, but also in promoting further worldwide earthquake prediction studies. S1

2 S2 Z. CHEN 2. Extensive Studies and Fundamental Understandings Earthquake prediction research in China began with the prediction studies of aftershocks in the Xingtai epicentral area, and was gradually expanded toward. North China, Sichuan-Yunnan, and other main seismic areas in China. The prediction work in China is characterized by interdisciplinary comprehensive research in multiple areas, as well as by the joint efforts of experts and non-professionals, and by bilateral efforts towards basic research and practical earthquake prediction. The areas for earthquake prediction research include geology, geophysics, geodesy, geochemistry, biology, astronomy, meteorology, 'and mathematics. There are about 20 kinds of items for observational work, such as seismic activity, crustal deformation, underground water level, hydrochemical composition, earth resistivity, geomagnetism, gravity, in-situ stress, etc. With the various data accumulated, studies have been conducted on time-space-magnitude distributions of regional seismicity (DING, 1984; DENG et al., 1980; SUN et al., 1984; MA et al., 1980, 1982; XU, 1984), features of possible seismic precursors and corresponding prediction methods (MA et al., 1982; CI-IEN et al., 1981; Liu et al., 1984; Lu et al., 1984; ZHU et al., 1977; ZHEN and HU, 1978; FENG et al., 1976; MEI et al., 1982; JIANG, 1980), as well as experimental and theoretical work performed on the physical processes of an earthquake. Meanwhile, comprehensive research has been carried out on the basis of field geological investigations, deep seismic sounding results, geological interpretations of satellite images, and other techniques, to understand the geological conditions for an earthquake occurrence. In addition, there have been numerous discussions on the relation between Chinese seismicity and global seismicity, and on the relationships of abnormal meteorological changes, variations of the Earth's rotations, polar wandering, solar activity, and other environmental factors, to the seismic activity (LI et al., 1973; FU, 1981; LIU and LIU, 1982, XUE, 1982). The research results have been continuously applied to prediction experiments. Hence a comprehensive prediction method combined with multi-disciplinary approaches, integrated analyses with point and areal data, studies of tectonic background combined with geodynamic data, and physical pattern analysis integrated with statistics are gradually developing. From this 18-year process, the earthquake prediction work in China includes two developmental periods from 1966 to 1976, and from 1977 to the present. During the first period, the continental seismicity in China was featured by high frequency and large magnitude, and the professional and amateur monitoring networks were set up and expanded rapidly. In addition, the perceptual understandings were gained very quickly with the frequent occurrences of large earthquakes. During the second period, the seismicity seems to be relatively low. However, under such a new situation of seismic activity, the existing knowledge was challenged by the new problems encountered, and the scientific and technical advancement has further promoted the in-depth earthquake prediction research. On the one hand, a portion of the seismic stations have been upgraded technically, with construction of the fast data transmission and real time processing systems. As a result, 6 regional seismic data processing centers with telemetering via telephone line have been established in Beijing, Shanghai, Shenyang, Kunming, Chendu, and Lanzhou. The amateur earthquake prediction and observation networks have been

3 Earthquake Prediction Research in China S3 restructured and readjusted with the concentrated efforts of mass prediction and mass prevention focused in some key areas. On the other hand, the seismological research has been further strengthened. The similarity and dissimilarity of the possible precursors before the earthquakes, especially those before several large shocks of M (magnitude) over 7, have been reviewed; and the theoretical studies and laboratory experiments have been conducted on the environmental and dynamic conditions for an earthquake occurrence, the seismogenic process, and the precursory mechanisms, so that the existing understandings will be supplemented and further developed. In summary, the two main aspects of the fundamental understandings gained within the past 18 years are as follows: 1) Before most of the strong shocks, particularly those with M over 7, which occur within an area under the detectability of a network, there are varieties of anomalous phenomena. According to the time when the anomalies appear and their morphology, these phenomena can be split into 4 categories, and in turn, the time period for an earthquake preparation and prediction can also be divided into 4 corresponding intervals (Table 1). It should be noted that various anomalies listed in Table 1 are not observed before all earthquakes. Some of the anomalies (asterisks in the table) have been repeatedly observed. However, it is hard to say that these anomalies presage a large earthquake occurrence once they appear. Therefore, the observed anomalies before shocks regarded as possible precursors can be deemed as possible ones only. The time-space distributions of these possible precursors as a whole are different for different earthquakes, but there still exist some common characteristics as shown in Table 2. 2) Because of the complicated continental crustal structures, the temporal and spatial distributions of the continental seismicity in China are rather complex, but there are still some regularities, and some unique dynamic patterns featured with block-like, stripelike, and diversified distributions of earthquakes (see Table 3). In addition to the above basic understandings, there are still many problems encountered in the earthquake prediction research in the past 18 years. These key difficulties for earthquake prediction are as follows: 2.1 Non-uniqueness of the relation between the anomalous variations and earthquake precursors The variations of the observed values are due to various factors as given in Fig. 1. Decisive anomalies are by no means synonymous with earthquake precursors. Very often no large earthquake occurs after anomalies take place. In fact, the so-called preseismic precursory anomalies may include variations due to other factors. Therefore, it is important for earthquake prediction research to clarify the factors in the observational variations, and the roles played by various factors, and to identify the nature of the observational variations. There are some available studies about these aspects, but the knowledge is still very primitive and qualitative. Especially, there is no effective quantitative method to remove the disturbance anomalies caused by various possible factors. Furthermore, there are no efficient methods and criteria for distinguishing the earthquake precursors from other anomalies caused by normal crustal movement. So the extremely difficult problem in earthquake prediction is to recognize the anomalies, and this remains a

4

5 Earthquake Prediction Research in China S5 Table 2. Some common characteristics of possible earthquake precursors. key reason why an earthquake cannot be predicted accurately. 2.2 Complexity of the relation between local and regional anomalous variations Using the actual observational data, the methods to predict earthquakes involve searching for areas with concentrated local anomalies in which a large shock may occur, and judging the seismic stages through the analysis of temporal and spatial distributions of various anomalies obtained from a large regional range. In this sense, the relation between the local and regional variations of observational data is to be studied. However, the relation between the two is very perplexing; for instance, due to the heterogeneous continental crustal tectonics, there might exist multiple points prone to stress concentration in a large region, and the stress state of these points undergoes common evolutions under the unified regional stress field. But they also influence each other via their feedback response to the regional stress field. As the tectonic conditions, stress distributions, and medium status are heterogeneous, the evolutions may produce different results, and the stress accumulations do not necessarily lead to occurrences of large earthquakes. The seismicity and variations of other observational data from a large region

6 S6 Z. CHEN Table 3. Some features of time and space distribution of seismicity in China Mainland. * References: 1, DING (1984); 2, DENG et al. (1980); 3, SUN et al. (1984); 4, MA et al. (1980); 5, MA et al. (1982); 6, LI et al. (1973); 7, FU (1981); 8, LIU and LIU (1982); 9, XUE (1982); 10, XU (1984); 11, CHEN et al. (1981); 12, MEI et al. (1982); 13, LIU et al. (1984). Fig. 1. Possible factors caused variations of the observed quantity. might be regarded as a complicated and variable chess game, and the seismic activity and other anomalies are sometimes concentrated, but sometimes scattered. During a seismogenic process of a strong shock, areas with concentrated anomalies in various items may appear around a hypocentral region, but there are also some areas with concentrated local anomalies and some points with sporadic anomalies in a large region. The variations of

7 Earthquake Prediction Research in China S7 anomalies outside the source region are sometimes even greater than those in the epicentral area. Therefore predicting the location of a future earthquake becomes increasingly difficult. The perplexing relation between the local and regional anomalies, etc., is another prominent difficulty encountered very often in earthquake prediction practice. 2.3 Diversity of possible earthquake precursors The above-mentioned common features of possible earthquake precursors are summed up based on the most distinct and typical phenomena observed before some large shocks. In reality, the possible precursors before different large shocks behave very discrepantly, probably due to different conditions such as epicentral and regional tectonic environments, types of earthquakes, history of seismic areas during stress accumulation, etc. For example, the duration of potential anomalies before some strong earthquakes with M over 7 is 3 or 4 years, whereas the anomaly lasts for about 1 year for some earthquakes. Before most strong earthquakes with M over 7, shocks with M over 4 in a large area may migrate toward the epicenter of mainshock, or may be distributed in stripes. However, this is not the case for some strong earthquakes. Several months before some strong shocks, the regional seismicity may spread outward from the mainshock epicenter, while no such phenomena are observed before other strong shocks. The macroscopic phenomena may only appear one or two days prior to the mainshock for some strong shocks; but for some strong shocks, the phenomena may occur very intensely one or two months before the mainshock. Still, for instance, before some moderate shocks, there are obvious precursors; but before some other moderate shocks, particularly in the epicentral regions of moderate earthquakes which occurred in the past several or tens of years, there are less obvious precursory phenomena. The diversity of possible earthquake precursors very often. constitutes some difficulties in applying the above-mentioned common characteristics to predicting earthquakes. While describing our considerable efforts in earthquake prediction research, by reviewing its 18-year history, we are fully aware that earthquake prediction still remains a difficult scientific research problem, and the false alarms and failure predictions will be unavoidable in the long run. However, it is possible to make a certain prediction, even a successful one for a few large earthquakes, especially for those with M over 7 under some advantageous conditions. 3. Future Work and Designs From the current state of earthquake prediction research in perspective, the following four main aspects for future research work can be outlined in order to improve the existing accuracy of earthquake prediction. 3.1 Systematic sorting out and in-depth research of various prediction methods The difficulty and the current state of earthquake prediction studies have implied that the predictions will be basically empirical in the fairly long distant future. Therefore, it is very important to sort out systematically the available various prediction methods, and on such a basis, to conduct profound studies on the existing main problems, so as to

8 S8 Z. CHEN supplement and develop the experience and understandings already acquired. The sortingout work began last year, and will continue for a period of time. The profound research on such a basis will be a major task in the days to come. It is hoped that the scientific analysis and evaluations will be made on the station environments, observational conditions, instrumental performance, and other factors which might affect the observational reliability for all seismic stations from their initial observation data. Based on this, some fundamental facts will be checked, including the characteristics of normal variations, the repetition of some anomalies accompanying or not accompanying large shocks, and their mutual discriminations. In turn, practical assessments will be made for the predictability of various methods, so as to further clarify the possible precursory features and chief prediction methods used at the different stages during a seismogenic process, and to revise and formulate the corresponding trial prediction criteria. 3.2 The optimum monitoring network and appropriate data processing systems to be set up The present earthquake mornitoring networks in China were established mostly during the seismically active period from 1966 to In recent years, there have been some adjustments and technical upgradings for these networks. However, in general, performances of many instruments, and observational conditions at numerous seismic stations have failed to meet the actual prediction needs; the networks themselves have not been distributed very reasonably because of technical limitations and insufficient experience at that time. Therefore, the future several years' main tasks are to readjust the existing networks as soon as possible, concentrate efforts on technical upgrading of the existing seismic stations in some key areas, so as to establish the optimum monitoring networks. There are many earthquake-prone areas in China, but the earthquake risks and seismic hazards differ. Thus, the earthquake monitoring work will be carried out in some key areas for more effective utilizations of limited resources and manpower. In recent years, while the above-mentioned various prediction methods were being sorted out, the work force has been organized to research and to identify some risk areas with potential great earthquakes in the next several years, a decade, or an even much longer period. Based on the identifications of risk areas and their corresponding forecasts of seismic hazards, some areas must be chosen to be observation-intensified areas. Then, based on the results of the sorting out work, some appropriate observational items and network distributions should be selected in these observation-intensified areas, so as to build the optimum monitoring networks which will be featured by comprehensive observational items, strategical positions, regional uniqueness, advanced instruments, good observational conditions, and reasonable distributions. In addition, the fast data acquisition and real time data processing systems should be established gradually in these critical areas to meet the needs of impending-earthquake prediction. Meanwhile, the West Yunnan earthquake prediction test site should be completely developed as soon as possible in order to create more chances for testing the capability of the current prediction methods and to experiment with new methods as well. Through all these works, it is expected that earthquake prediction will be based on the more reliable and suitable observation data.

9 Earthquake Prediction Research in China S9 3.3 Profound fundamental research The basic studies on the environmental and dynamic conditions of an earthquake, and on the physical process of earthquake preparation and its corresponding precursory mechanisms will not only provide guiding significance for the current prediction experiments and network constructions, but also in the long run, remain as a key job for enabling the present empirical predictions to become reliable theoretical ones. In the past 18 years, extensive research in these aspects has been conducted in China, with some important results gained, some physical models and hypotheses for seismogenic process advanced, and some interpretations for the characteristics of seismicity and precursors made. For the new earthquake prediction program, the basic research will be further strengthened, particularly the studies on the relation among great earthquakes, active faults, and deep crustal environments, and theoretical research and laboratory simulations of seismogenic physical process. It is expected that on the basis of these studies and more reliable observational data, there will be new seismogenic theories and the corresponding precursory models to suit different tectonic environments and different types of earthquakes in China Mainland. This will enhance the scientific credibility of earthquake prediction. 3.4 Intensified comprehensive research on earthquake prediction Comprehensive research is a vital approach in establishing accurate earthquake prediction. Since every kind of possible precursor may be the outcome of a seismogenic process at a certain stage or on a certain aspect, an individual precursor can hardly be used to predict the exact occurrence time, place, and magnitude of an earthquake. Therefore, predictions have been very often made based on the comprehensive analysis of various observational data. The comprehensive earthquake predictions are based on the reductions, analysis, and integrations of either independent or correlated individual observational data, and the relations between the individual observation data and research results given by the available theories and experience on earthquake process. In accordance with the abovementioned basic understandings obtained and the main problems encountered in the past 18 years, the integrated earthquake prediction research covers rather extensive areas. However, in the sense of reduction and integration, the following aspects should be clarified emphatically: 1, the relation between slow crustal movement and rapid crustal movement; 2, the correlation between the deep and shallow crustal stresses and variations of medium status; 3, the relation between crustal movement and fluid. migration; 4, the relation of local and regional crustal movements to the corresponding accompanying phenomena; 5, the relation among characteristics of possible earthquake precursors, terrain environments, and types of earthquakes; 6, the distinctions between the anomalies caused by normal crustal movement and earthquake precursors. For many years, studies on these aspects have been performed. Some results have been obtained, but the understandings are still rather incomplete. The present integrated analysis still remains as a simple collection and preliminary analysis of various phenomena. In the new earthquake prediction research program, the studies and relevant laboratory experiments on these basic problems shall be continued. It is hoped that the physical basis for the comprehensive

10 S10 Z. CHEN predictions will be strengthened and the prediction methods will be notably improved. In reviewing the history and the current state of earthquake prediction research, we see the progress achieved in the past 18 years, present problems for earthquake prediction research, and the direction for future work. We believe the earthquake prediction will still be in an exploratory stage for a considerably long time in the future. It is unlikely there will be an immediate panacea for accurately predicting earthquakes. But improved accuracy will be constantly achieved with the in-depth studies from multiple scientific fields and the technical advancements. REFERENCES CHEN, Z., P. LIU, D. HUANG, D. ZHENG, F. XUE, and Z. WANG, Characteristic of regional seismicity before great earthquake, in Selecting Papers of International Symposium on Earthquake Prediction of 1979, pp , Seismological Press, Beijing, 1981 (in Chinese). DENG, Q., Y. ZHANG, W. HUAN, H. ZHANG, G. XU, Y. LIu, R. DENG, Q. LI, F. FAN, and T. YANG, Principles and methods of composing the seismic zoning map of China, Acta Seismol. Sin., 2, , (in Chinese). DING, G., Active faults of China, in A Collection of Papers of International Symposium on Continental Seismicity and Earthquake Prediction, pp , Seismological Press, Beijing, FENG, D., S. ZHENG, G. CHENG, Z. FU, S. GAO, R. LUO, and B. Li, Preliminary study of the velocity anomalies of seismic waves before and after some strong and moderate earthquakes in western China (I). The velocity ratio anomalies, Acta Geophys. Sin., 19, , 1976 (in Chinese). FU, Z., The shallow source large earthquake and the secular variation of the angular velocity of the Earth, Sci. Res. Earthq., 3, 7-12, 1981 (in Chinese). JIANG, J., Animal abnormal behavior is an earthquake short-term precursor, Acta Seismol. Sin., 2, , 1980 (in Chinese). LI, Q., X. XIAO, and Z. LI, A preliminary study on the relation between the great earthquakes of China and the secular variation of the angular velocity of relation of the Earth, Acta Geophys. Sin., 16, 71-80, 1973 (in Chinese). LIU, D. and L. LIU, The evidence of the earthquake affected by solar activity, Sci. Res. Earthq., 1, 38-44, 1982 (in Chinese). LIU, P., D. HUANG, L. WANG, Z. WANG, D. ZHENG, and H. FENG, Seismicity pattern over the preparatory process of strong earthquakes, in A Collection of Papers of International Symposium on Continental Seismicity and Earthquake Prediction, pp , Seismological Press, Beijing, LU, Y., J. SHEN, and W. WANG, Seismic gaps in the main-land of China, in A Collection of Papers of International Symposium on Continental Seismicity and Earthquake Prediction, pp , Seismological Press, Beijing, MA, Z., Z. CHEN, Z. FU, and F. XUE, Seismotectonics of Asiatic-European seismic system, Sci. Sin., 9, , 1980 (in Chinese). MA, Z., Z. FU, Y. ZHANG, C. WANG, G. ZHANG, and D. LIU, Nine Earthquakes in China , Seismological Press, Beijing, 1982 (in Chinese). MEI, S., C. HU, C. ZHU, J. MA, Z. ZHANG, and M. YANG (ed.), 1976 Tangshan Earthquake, Seismological Press, Beijing, 1982 (in Chinese). SUN, W., C. LIU, Z. ZHU, L. ZHANG, and X. ZHANG, A preliminary study on the relationship between the continental earthquake and the deep crustal structures of North China, in A Collection of

11 Earthquake Prediction Research in China S11 Papers of International Symposium on Continental Seismicity and Earthquake Prediction, pp , Seismological Press, Beijing, XU, S., A review of seismicity patterns, in A Collection of Papers of International Symposium on Continental Seismicity and Earthquake Prediction, pp , Seismological Press, Beijing, XUE, F., The relationship between Chinese earthquake and the global earthquake, Sci. Res. Earthq., 1, 19-23, 1982 (in Chinese). ZHEN, Z. and Z. HU, Time and spatial variations of the average dislocation before and after large earthquakes, Acta Geophys. Sin., 21, 58-66, 1978 (in Chinese). ZXU, C., C. FU, Z. JUNG, and S. LUO, Source parameter for small earthquakes and the quality factor of the medium before and after the Haicheng earthquake, Acta Geophys..Sin., 20, ,1977 (in Chinese).