Crustal Deformation Before Strong Earthquakes and Utility of Earthquake Prediction by Geodetic Measurements: Extended Abstract

J. Phys. Earth, 34, Suppl., S111-S127, 1986

CRUSTAL DEFORMATION BEFORE STRONG EARTHQUAKES AND UTILITY OF EARTHQUAKE PREDICTION BY GEODETIC MEASUREMENTS: EXTENDED ABSTRACT

Zusheng ZHANG,Shaofen YING, and Yongjian SHEN

Geodetic Brigade, State Seismological Bureau, Tianjin, China (Received June 22, 1985)

1. Introduction

The preseismic deformation data of 10 major earthquakes that occurred in mainland China are collected and analyzed in this paper. We hope to clarify the characteristics of crustal deformation in the preparatory process of strong earthquakes and to evaluate the monitoring and predicting of earthquakes by available geodetic means. Earthquake studies show that preparatory process and occurrence of a major shallow earthquake always seem to be accompanied by crustal surface deformation. The deformation data obtained in large areas by available geodetic means have now been widely used in research on crustal motion and earthquake prediction. Therefore, the characteristics of crustal deformation prior to a large earthquake and whether an earthquake can be correctly detected and predicted with geodetic data, have become a matter of great interest. In this paper, the data of crustal deformation preceding 10 major Chinese earthquakes (Table 1 and Fig. 1; MA et al., 1982) have been collected and checked or reprocessed. We hope to obtain some characteristics of crustal deformation before an earthquake by analyzing these data and making an appropriate evaluation of the role played by crustal deformation measurements in earthquake prediction.

2. Preseismic Deformation 2.1 The Dongchuan earthquakes of February 5 and 13, 1966 (MS=6.5, 6.2) The Dongchuan earthquakes occurred in the Xincun Quaternary graben. On the west margin of the basin, the Xiaojiang deep fault outcrops discontinuously. The Cenozoic tectonic movement is very intense here and the earthquakes occurred right near the intersection of a few faults (Fig. 2). There is only a little preseismic releveling data, but they show that the Xincun basin subsided by 25 mm between 1958 and 1963 as compared with the surrounding area. It seems to coincide with the remarkable subsidence of the basin during the earthquake and reflects an inheritance of earlier movement. The triangulation data show dilatation near the epicenter and compression far away from the epicenter before the event (Fig. 3).

S111 S112 Z. ZHANG,S. YING, and Y. SHEN

Table 1. Ten major earthquakes in China.

Fig. 1. The map of distribution of the epicenters of 10 major Chinese earthquakes. Crustal Deformation before Strong Earthquakes S113

Fig. 2Fig. 3

Fig. 2. Simplified tectonic map of the Dongchuan region. Fig. 3. The distribution of strain before the earthquake.

Fig. 4. Simplified tectonic map of the Xingtai region. The locations of bench marks numbered 449 and 454 (see Fig. 5) are shown. The Fuyang River shown by the thin line flows from south to northeast.

2.2 The Xingtai earthquakes of March 8 and 22, 1966 (MS=6.8, 7.2) The Xingtai earthquakes occurred in the Shulu graben (Fig. 4). The preseismic releveling data show obvious subsidence in the Shulu graben (Fig. 5), which is consistent S114 Z. ZHANG, S. YING, and Y. SHEN

Fig. 5. Vertical deformation profile of the leveling line along the Fuyang River (GEODETICSURVEY BRIGADE FOR EARTHQUAKE RESEARCH, 1975 a). The right-hand- side of the figure shows the upriver side.

Fig. 7. Vertical deformation profile of the leveling line from Gaoyi to Datianzhuang (see Fig. 4). Elevation changes relative to the results for the survey in 1965 are shown. Crustal Deformation before Strong Earthquakes S115 with the movements during and after the earthquakes. In Fig. 6, the change of bench mark No. 449 represents the subsidence of the graben, but bench mark No. 454 shows the relative uplift of the Xingjiawan region. During the 10 years before the earthquake, the average rate of relative movements decreased quite significantly. In addition, leveling data of 3 epochs observed in the direction perpendicular to the fault indicate that in the region outside the graben, no obvious crustal deformation was detected from 1958 to just a few days before the M=6.8 earthquake in 1966 (Fig. 7). It suggests that significant preseismic crustal deformation was localized within the Shulu graben which stretches for 70 to 100 km in the NNE direction, and its width is not more than 20 to 30 km in the NWW direction.

2.3 The Hejian earthquake of March 27, 1967 (MS=6.3) The Hejian earthquake took place in the NNE-trending Litan graben in the North China plain (Fig. 8). The profile of vertical deformation along the Fuyang River is plotted in Fig. 9 and shows the subsidence of the Litan graben. It is similar to that of the Xingtai earthquake (Fig. 5), but is not as typical as the latter, because this area is covered by thick

Fig. 8. Simplified tectonic map of the Hejian region. The Fuyang River shown by the thin line flows from southwest to northeast.

Fig. 9. Vertical deformation profile of the leveling line along the Fuyang River. The right-hand side shows the upriver side. S 116Z. ZHANG, S. YING, and Y. SHEN soil and the contamination due to ground water movement cannot be neglected.

2.4 The Yangjiang earthquake of July 26, 1969 (MS=6.4) The tectonic background of the Yangjiang earthquake is the uplift of the NNE- trending Mt. Longgao-Mt. Luoqing region. There is only one leveling line running through the neighbourhood of the epicenter. The profile of the leveling line shows that the area near the epicenter has uplifted significantly (Figs. 10 and 11).

Fig. 10. Simplified tectonic map of the Yangjiang region.

Fig. 11. Vertical deformation profile of th e leveling line. Crustal Deformation before Strong Earthquakes S117

Fig. 12. Simplifie d tectonic map of the Tonghai region.

Fig. 13. Vertical deformation profile of the leveling line during 1958 to 1969 (from Daliyuan to Jianshui) (GEODETICSURVEY BRIGADE FOR EARTHQUAKE RESEARCH, 1975 b).

2.5 The Tonghai earthquake of January 5, 1970 (MS=7.7) The Tonghai earthquake occurred on the Qujiang fault in the southern part of the Diandong (east Yunnan) and Dianzhong (central Yunnan) fault blocks (Fig. 12). It is a tectonically complicated area where stress can be easily accumulated and released. S118 Z. ZHANG, S. YING, and Y. SHEN

Fig. 14. Vertical deformation profile of the leveling line Kunming to Yongwu). during1955 to 1958 (from

Fig. 15 Fig. 16 Fig.15. Simplified tectonic map of the Liaonan region. 16. Vertical deformation of the Liaonan regionFig. during 1958-1970. The numbers show uplift in milimeters (GEODETICSURVEY BRIGADE FOR EARTHQUAKE RESEARCH , 1977).

There are two leveling lines running through the epicentral region. The preseismic vertical deformations revealed by both lines are shown in Figs. 13 and 14. They show that a subsidence zone might exist along the fault and it is this zone that sank rapidly during this event.

2.6 The Haicheng earthquake of February 4, 1975 (Ms=7.3) The Haicheng earthquake occurred at a place where the Liaodong rise platform (east of Liao River flowing from northeast to southwest down to Yingkou) and the Mesozoic- Cenozoic down-faulting region of lower Liao River intersect. There are two principal faults here, one is the NE-striking Haicheng-Jinzhou fault and another is the NW-trending Xiaogushan-Xiuyan buried fault. The two faults form a T-shape near Haicheng. The Crustal Deformation before Strong Earthquakes S119

Fig. 17. The anomalous precursors detected by short-line leveling at the Jinxian station (see Fig. 15 for its location) (GEODETICSURVEY BRIGADE FOR EARTHQUAKE RESEARCH,1977). The upward motion indicates relative uplift of the NWW end of the short leveling line, which is 560 m long.

Fig. 18

Fig.Fig. 18. 19. Simplified tectonic map of the Longling region. Elevation variation of the leveling line (Tengchong—Mangshi).Fig. 19. earthquake occurred right on the latter and 20 km away from the former. It might be the rupture of the locked section of the latter fault (Fig. 15). This earthquake is well known, because it was successfully predicted and the anomalous deformation provided one of the bases for the prediction. Prior to the earthquake, the lower Liao River plain (the northwestern part of Fig. 16) kept subsiding and the mountain region in the east Liaoning Province kept uplifting (GEODETICSURVEY BRIGADEFOR EARTHQUAKE RESEARCH, 1977). The maximum rate of vertical uplift was S120 Z. ZHANG, S. YING, and Y. SHEN

3 mm per year from 1937 to 1958 and 5 mm per year from 1958 to 1971. And the earthquake occurred where contours are bent convex northward (Fig. 16). It should be noted that from 1973 to 1975, before the earthquake, deformation near the epicentral region was not great, while at the Jinxian station (see Fig. 15), on the south end of the Haicheng-Jinzhou fault 200 km away from the epicenter, the anomalous precursors detected by short-line leveling had lasted for 17 months (Fig. 17).

2.7 The Longling earthquakes of May 29, 1976 (Ms=7.3, 7.4) The Longling earthquakes took place within a granite body surrounded by the faults of Nu River (striking NNW), Wanding (striking EW), and Longling-Ruili (striking NE) in Southwest Yunnan. It is a quasi-uniform massive rock body in which there are no large faults, but only some small NNW- and NEE-trending fracture zones (Fig. 18). The data of Tengchong-Mangshi leveling line running through the neighborhood of the epicenter show that there is a subsidence in the section crossing the Longling-Ruili fault (CHEN,1979). The maximum subsidence occurred within the Longling basin on the margin of the granite rock body, indicating movement of the granite body relative to the surrounding areas (Fig. 19). The post-seismic data show that the granite body has uplifted as compared with the Longling, Luxi, and Zhen'an basins on the margin of the body.

2.8 The Tangshan earthquakes of July 28, 1976 (Ms=7.8, 7.1) The Tangshan earthquakes occurred at the intersection of the E-W-trending Yanshan fault and fold zone (extending from the left-hand side of Fig. 20) in the northern part of North China and the extension of the NNE-striking Cangdong fault zone (extending from the lower left corner of Fig. 20) in the eastern part of the North China plain. The epicentral

Fig. Fig. 2020. Simplified tectonic map of the Tangshan region. Fig. 2121. Elevation changes of the bench marks Fig. before the Tangshan earthquake (ZHANGet al., 1981). The changes show relative movement with respect to a bench mark in Changli. Crustal Deformation before Strong Earthquakes S121 region lies on the Tangshan fault within the Tangshan rhombic block (Fig. 20). Since 1954, precise levelings have been carried out many times in the Tangshan region and abundant preseismic data are available (ZHANGet al., 1981). It can be seen from the vertical displacements of the bench marks within the epicentral region (Fig. 21) that before 1967 the variation was not large, but it increased rapidly from 1967 to 1972, with large amplitudes of uplift at the beginning and with large subsidence after the Bohai earthquake of July 18, 1969 (Ms=7.4). Around 1972, the bench marks began to rise. After 1973 until the occurrence of the earthquake in 1976, the epicentral region was relatively "quiet," but at the same time a series of anomalies appeared in the surrounding areas.

2.9 The Songpan earthquakes of August 16 and 23, 1976 (MS=7.2, 7.2) The Songpan earthquakes took place on the NS-trending Huya fault on the east side of the Xuebaoding uplift zone elongating in a NS direction in West Sichuan. It is thought to be the result of the uplift of this zone under compression in the EW direction (Fig. 22). Although the leveling line lies a long way from the epicenter, it still can be seen that before the earthquake, changes are fairly large near the epicenter (Fig. 23). The data reflect the continuous rising of the Xuebaoding uplift zone with possible slow variations prior to the earthquake.

2.10 The Daofu earthquake of January 24, 1981 (Ms=6.9) The Daofu earthquake in 1981 occurred at the Xianshui River (or Xianshui He) fault zone which is well-known in West Sichuan (Fig. 24). This is a deep and large fault with left lateral strike-slip movement. The tectonic movement along the Xianshui River fault is

Fig.

Fig. 2222. Simplified tectonic map of the Songpan region. Fig. 2323. Vertical deformation profile of the levelingFig. line (1960-1975). S122 Z. ZHANG, S. YING, and Y. SHEN

Fig. 24

Fig. 24 . Simplified tectonic map of the Daofu region. Fig. 2525. Elevation variation Fig. of the leveling line along the Xianshuihe fault (pre- earthquake). significant and earthquakes are frequent. In 1973 a strong shock of magnitude 7.9 occurred in Luhuo on this fault. There is only one leveling line with preseismic data near the epicenter. It runs in the Xianshui River valley, wandering along the fault. On the section from Xialatuo to Daofu, there were some variations associated with the 1973 Luhuo earthquake, but the data of 1980 and 1974 showed no significant variations (Fig. 25). This might suggest that the Xianshui River fault produces only pure horizontal offset and considerable subsidence may occur coseismically only in the sedimentary basin. These are major earthquakes with recorded preseismic deformation data. Generally speaking, the data are not sufficient and most of them are vertical deformation data. The patterns of the preseismic deformation revealed by these data are not very clear. For some earthquakes the data are rather abundant and cover the epicentral region, for example the Tangshan earthquake. But for most earthquakes the data are rather limited in quantity. Preseismic data of only 2 or 3 epochs are available, e.g., the Haicheng, Tonghai, Xingtai, and other earthquakes. A number of leveling lines are far away from the epicenter, e.g., the Yangjiang and Songpan earthquakes. According to the variations observed before the earthquakes, the data can be divided into three types. In the first type, the preseismic variations in the epicentral region were detected and are quite reliable, e.g., the Tangshan, Haicheng, Xingtai, and Tonghai earthquakes. In the second type, the preseismic variations were not detected, although the leveling line passes through the epicenter, e.g., the Daofu earthquake. In the third type, certain deformations were detected, but their reliability is uncertain, e.g., the Dongchuan, Crustal Deformation before Strong Earthquakes S123

Longling, Yangjiang, and Songpan earthquakes. Particularly, due to the large distance of the leveling lines to the epicenters of the last two earthquakes, the observed deformation might not directly reflect the movement of the seismogenic zone.

3. Some Characteristics of the Preseismic Deformation

The area with anomalous deformation occurring before a large earthquake is connected with the tectonic elements and its shape is closely related to the shallow tectonic feature, e.g., the deformation preceding the Tangshan earthquake occurred in the Tangshan rhombic block and the surrounding area; the epicenter of the Haicheng earthquake is situated on the margin of the Liaodong rise platform and the epicenter of the Xingtai earthquake is in the Shulu graben. Some distance away from these tectonic elements, especially from earthquake faults, the deformation seems to decay rapidly. The preseismic deformation shows relative movements among tectonic units or between different parts in individual units. This feature of preseismic deformation agrees with that of coseismic deformation. For some earthquakes, no significant deformation was detected in the epicentral region before their occurrence, e.g., the Daofu earthquake. For some earthquakes, the deformation appeared on the margin of the focal region instead of on the earthquake fault, e.g., the Tonghai and Longling earthquakes. Probably it suggests that before the earthquake, the earthquake fault , is locked and the relative movement between the two blocks on the both sides of the fault was rather small, while the preseismic deformation is the movement of the whole tectonic unit relative to the surrounding area. The Songpan and Yangjiang earthquakes appear to indicate that before an earthquake, significant deformation sometimes occurs at places far away from the epicenter. This deformation might not be directly associated with the movement of the seismogenic body, but it might reflect the change of the regional stress field. That is to say, certain precursory phenomena related to the preparation of earthquakes may be detected by deformation measurements in some "sensitive" parts in the area surrounding the focal region. The amplitude, velocity and gradient of deformation before each earthquake are quite different. Generally speaking, for earthquakes in North China, the value was rather smaller than that in Southwest China. At present it is too difficult to give more quantitative discussion because of the limited number of data. The anomalies can only be judged through comparison with data in the surrounding areas and the past history of vertical movements. The preseismic deformation shows a pattern of continued movement consistent with the regional Cenozoic tectonics and displays different phases in its development. The development phases have been discussed in many papers from China and else- where. It can be seen from the Tangshan and Haicheng earthquakes, which have more complete data than that provided by other earthquakes in China, that the two phases of slow deformation and rapid change preceding the earthquakes were quite clear. Of course, for the given data, the phases can only be roughly outlined, and the epochs that mark the S124 Z. ZHANG, S. YING, and Y. SHEN

Fig. 26. Elevation variation of the leveling point in epicentral region before a strong earthquake. The data for the 1964 Niigata earthquake are taken from DAMBARA (1973).

beginning of the two phases and the changing from one phase to another are far from accurate. Special attention should be paid to phenomenon in the epicentral region. There might be a phase of "quiescence" between the active preseismic phase and the occurrence of the earthquake. Several examples follow: Before the Tangshan earthquake, the deformation in the earthquake region was relatively "stable" from 1973 to 1976; The rate of deformation slowed down prior to the Xingtai earthquake; The deformation retardation might also exist before the Haicheng earthquake; And similar phenomenon can also be found in the precursory variations related to the Niigata earthquake (DAMBARA,1973) (Fig. 26). So this "quiescence" seems to be an important signal of an impending strong earthquake , indicating that the seismogenic structure is in "locked" or "rigid" state immediately before an earthquake. Before an earthquake, certain precursory anomalies may be observed at observatories and by other geophysical means in the area surrounding the epicenter. These anomalies may appear approximately within 2-3 years before the earthquake, that is in the relative "quiescence" phase of deformation in the epicentral region . It can be clearly seen in the data of the Tangshan earthquake, and similar phenomenon also appears for the Haicheng earthquake. It suggests that while the epicentral area is in the relatively "quiescent" state Crustal Deformation before Strong Earthquakes S125 preceding an earthquake, the surrounding region is in the phase of highly active deformation. Since most of the preseismic deformation data are vertical deformation data, it is very difficult to explain the deformation mechanism. Based on the data now available, a preliminary conclusion can be made that in a large tectonic area the evolution of crustal deformation could be reasonably explained by a stress field with multipoint concentrations (MA, 1980), and the variations in the epicentral region may be explained as a process of elastic rebound with stick-slip. It can illustrate the deformation phenomena exposed by the leveling lines running through the epicenter, the deformation observed on the lines far away from the epicenter, and some precursor phenomena appearing in a region extending as wide as several hundred kilometers around the epicenter. It can be inferred from some earthquakes that dilatation might occur in the epicentral region prior to an earthquake. Examples are the horizontal deformation preceding the Dongchuan earthquake and the regional dilatational uplift appearing from 1969 to 1970 in the epicentral region of the Tangshan earthquake. Of course, more data are needed to confirm whether it is occasional or general in the preparatory process of an earthquake.

4. Discussion

Whether the data obtained by geodetic means (mainly by leveling) can reflect the real crustal movement is affected by many factors. Firstly, it is affected by the precision of observations. At present, the precision of the geodetic measurement is about 10-5 to 10-6, but the preseismic deformation is often less than 10-5-10-6 or even less as shown in many papers and data obtained. Secondly, it is affected by the density of control survey marks and the frequency of measurements. It has been shown in some data that the strain accumulation before an earthquake might concentrate in a narrow zone on both sides of a seismogenic structure. Usually the control points are sparse; the interval between two epochs is rather long and the survey work is heavy. Therefore, if the network is not properly designed and the measurements are not carried out in suitable epochs, it would be very difficult to detect the real strain accumulation. Thirdly, it depends on the stability of the bench marks. Under some conditions, particularly in the area with thick alluvial cover, the displacements of bench marks does not represent the real tectonic movement of the lithosphere due to some contaminations. They are the effects of temperature, rainfall, surface water, ground water, variation of load on ground, etc. More work has been done on the contamination of leveling data. They are in the order of tens of millimeters. Obviously they seriously affect the reliability and the representativeness of the deformation data. Fourthly, a satisfactory theoretical model has not been established at present. Deducing the process of an earthquake at depth from the observations made on the earth surface and predicting the danger of earthquake occurrence still remains a matter of guesswork. S126 Z. ZHANG, S. YING, and Y. SHEN

The data of the above mentioned earthquakes and other earthquakes indicate that the maps of crustal deformation in a large area show the pattern of regional tectonic movements. Though the patterns of preseismic deformations may vary, the deformations in and near the earthquake region are more intense than those in the surrounding regions and in the past years. They have some regularities and certain phases of development which make possible the determination of areas of variable earthquake risk. It is important that only the deformation caused by tectonic movements is related to the occurrence of earthquakes. Therefore, the seismogeological background of the crustal deformation should be considered in the determination of areas of earthquake risk. Theoretically, the occurrence time of an earthquake can be approximately estimated from the evolution of the crustal deformation. But it is very difficult to do so in practice. Because the qualitative and quantitative indices of deformation phases are immature in theory and the data are insufficient, it is not easy to identify the characteristics in each phase and the moment that marks the change from one phase to another. Therefore the prediction of the occurrence time of an earthquake should be based on preseismic anomalies of various kinds. In general the precursory anomalies may appear in the area of earthquake risk or the surrounding area. They could last for several months or several years. But during this period the deformation in the epicentral region might be stable, following the phase of rapid change. In short, it is believed that the data of crustal deformation in large areas can mainly give a general picture of regional crustal motion. These data can be used and can only be used to provide the background of crustal motion for the occurrence of large earthquakes (MS>7). It is difficult to make short-term and impending earthquake predictions with these data alone. At present, the problem of earthquake prediction by means of crustal deformation measurement is still at the stage of data accumulation. Although quite a few valuable results have been obtained, the problem is still far from being truly solved and there is still a long way to go.

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