The Common Features of Deep Structures in Some Large-Earthquake Areas of the North China Plain and Their Implication for Earthquake Site Prediction

J. Phys. Earth, 34, Suppl., S223-S239, 1986

THE COMMON FEATURES OF DEEP STRUCTURES IN SOME LARGE-EARTHQUAKE AREAS OF THE NORTH CHINA PLAIN AND THEIR IMPLICATION FOR EARTHQUAKE SITE PREDICTION

Xuezhong SHAO,Jiru ZHANG,Qiyuan LIU, and Siya ZHANG

Institute of Geology, State Seismological Bureau, Beijing, China (Received June 22, 1985; Revised July 5, 1986)

This work deals with the deep structure of the earth's crust and the uppermost mantle as revealed by a method using P-SV converted waves from earthquakes in Beijing-Tianjin- Tangshan and some large-earthquake areas of the North China Plain. During the last ten years a series of seismic profiles have been completed in the studied region with a total length of about 2,000 km. The comparison of deep structure with the distribution of earthquakes indicates that most strong earthquakes are located at the boundary of fault- blocks and especially at the intersection of deep-seated fault zones where the depth of deep boundaries changes discontinuously. Some common features of the deep structure have been revealed in Tangshan and other large-earthquake areas of the North China Plain. These include shallow crystalline basement, deep Moho discontinuity, thick crystalline crust, and intersection of deep-seated faults with different trends. By way of structural comparison, some areas with deep structures similar to those of large-earthquake areas have been established in the northern part of the North China Plain, which could be considered the areas with potential risk for strong earthquakes.

1. Introduction

When the Tangshan earthquake occurred, many seismogeologists were surprised and wondered how a strong, catastrophic earthquake with magnitude of 7.8 could occur in a place with no evidence for large faults on the surface, no evidence for strong differential movement of neotectonics, and where predicted seismic intensity is only about VI degree in the Chinese scale (MEl et al., 1982). The usual principle of the intensity prediction turned out to be inappropriate and an important problem was raised: how can we predict accurately the dangerous places of occurrence of strong earthquakes? After conducting investigations of deep structures, we noticed that there are very complicated deep structures beneath the Tangshan earthquake area, as will be shown in this study. Thus occurrence of a strong earthquake in the Tangshan area has become more understandable. This coincidence of a major earthquake and a region of deep structural complexity suggests that the study of deep structures may play an important role in prediction of potential strong-earthquake areas. The problem is how to realize the earthquake site prediction with the aid of deep structures. In this respect, it seems S223 S224 X. SHAO et al. reasonable to use a "method of structural comparison." In other words, we should first investigate the characteristics of deep structures in the areas where strong earthquakes have occurred, and on the basis of these data, try to discover some common features of deep structures for these areas (if they exist), which may serve as the diagnostic criteria for identifying other potential areas of strong earthquake in the same seismotectonic region. Based on this idea, we have studied the common features of deep structures in three large-earthquake areas (Tangshan, Mafang, and Juxian) and applied the results to predict potential places of strong earthquakes in the North China Plain.

2. Method of Investigation and Interpretation of Data

For investigation of deep structure we have applied the method of PS converted waves, generated by distant earthquakes (ANDREEV, 1957; COOK et al., 1962; BULIN, 1971; POLSHKOV et al., 1973; POMERANCEVA and MOZZENKO, 1977; VINNIK, 1977; VINNIK et al.,

1983; BURDICK and LANGSTON, 1977; SHAO and ZHANG, 1978; OKADA, 1979). The advantages of this method are: low cost, great depth of investigation, and convenience for studying the configuration of the boundaries. The principle of the method is shown in Fig.

When P wave from a distant earthquake meets a velocity discontinuity, it gives rise to a converted PS wave besides refracted P wave, which can be recorded by three-component seismograph on the surface. The time delay of PS wave with respect to the direct P wave

(ƒ¢tps) depends on the .depth of conversion boundary (h), the average velocities in medium above the conversion boundary (V-p and K-=V-p/V-s), and the epicentral distance (or apparent velocity of P waves) by the formula:

Fig. 1. The principle of the method of PS converted waves generated by distant

earthquakes. S1, S2 are the seismic stations on the surface; VP, VS are the average

velocities of P and S waves; ƒ¢tPS is the time delay of PS wave with respect to the P

wave; hi (i=1, 2) is the depth of the conversion boundary. Deep Structures in Some Large-Earthquake Areas S225

(1) where c=1/VP* is the reciprocal of apparent velocity of P wave. When the dipping

boundaries are present, a correction term should be added to the right-hand side of formula

(1) (POMERANCEVA and MOZZENKO, 1977). But theoretical calculation has shown that if dip angle ƒÓ_??_10•‹-15•‹, the effect of 9 on the dips will be small, and the formula (1) is still usable. So, if we set a series of seismic stations along a profile, and if we can identify the PS converted waves from different boundaries on the seismograms and determine their time

delays ƒ¢tps, then we will be able to calculate the depth of all the conversion boundaries using formula (1) and construct a cross section of deep structure along the profile, on condition that the velocity parameters of the medium are known.

We use about the first 10 s of three-component records from distant earthquakes with foci within the mantle for analyzing PS waves from boundaries in the crust and the upper-

most mantle. The PS waves are picked out only on horizontal components of the records. In

principle, this part of teleseismograms may contain the following phases: 1) PS converted waves from boundaries of the earth's crust and uppermost mantle; 2) Multiple converted and refracted waves; 3) Multiple reflected and converted waves inside the sediment layer and between the earth's surface and the sharp boundaries; 4) Multiple reflected and converted waves inside the crystalline crust; 5) Horizontal components of P waves; 6)

Surface waves; and 7) Laterally reflected and diffracted waves from the fault plane. But actually, not all these phases can be seen on the seismograms.

The theoretical study for several crustal models show that PS converted waves are so strong that their amplitudes may reach to within a factor of 0.2-0.5 or more of the corresponding P wave (LIU and SHAO, 1985). Calculations also show (POMERANCEVA and

MOZZENKO, 1977) that, taking into account the absorption of medium, the amplitude of multiple converted and refracted waves and multiple reflected and converted waves is much lower than that of PS waves.

Usually the surface waves are very weak at the beginning part of teleseismograms recorded with short-period equipment as used in Our field observations. As regards to the P and lateral waves, their horizontal components could be large in some special condition,

but their polarization patterns are quite different from those of PS converted waves. The method of polarization filtering is an effective means for eliminating the interference of these waves (MONTALBETTI and KANASEWICH, 1970; ZHANG et al., 1982).

From both theoretical and practical considerations, we find that by choosing suitable teleseismograms (usually from deep focus events) and by properly processing them, the PS converted waves have dominant energy on the first 10 s horizontal-component records in

comparison with other possible seismic phases. This important result has laid a basis for us to recognize PS converted waves correctly on the teleseismograms.

A data processing procedure mainly includes a rotation of the coordinate system and the polarization analyzing and filtering to get the SV, SH, and P components and enhance the signal/noise ratio of PS waves (ZHANG et al., 1982).

For horizontally layered and isotropic medium the PS waves are of SV type, and have only SV components, but for medium with dipping boundaries or anisotropic properties S226 X. SHAO et al. the SH components of PS waves will appear besides SV components (EGORKINA,1975). So we should analyze PS waves in a plane perpendicular to the motion direction of P wave (Fig. 2), containing both SV and SH components of the PS waves (GALPERIN,1977). The major diagnostic criteria to identify the PS waves have been summarized as follows (SHAGand ZHANG,1978): 1) The PS waves coming from boundaries in the crust and uppermost mantle always follow the direct P wave very closely with the time delay of about 6-8 s, which are quite

Fig. 2. P component and seismograms with converted waves in the plane per-

pendicular to the motion direction of P wave. ƒ¿ is the azimuth of projection in the

plane. SV and SH components correspond to a =90•‹ and 0•‹, respectively. The observed station is Xiaweidian, west of Beijing. The parameters of the used

explosion is: 8h59m 56.5S (origin time in GMT) October 18, 1975, 70.84•‹N, 53.53•‹E,

h=0 km, Novaya Zemlya, Mb= 6.6. Deep Structures in Some Large-Earthquake Areas S227 stable with the change in epicentral distance. 2) The amplitude of PS waves can reach to 10-80%, mostly to 10-40% of that of P wave. 3) The waveform of PS waves is quite similar to that of P wave. 4) The polarization of PS waves is linear and their vectors of particle displacement are distributed mainly in the plane perpendicular to the motion direction of P waves. In some special conditions the multiple reflected waves of PPS and PSS types between the surface and the sharp boundaries in the crust may form the major interferences for identification of PS waves on the horizontal component after filtering. The following criteria are applied to discriminate the PPS and PSS waves from the PS waves (Fan and Shao, in preparation, 1985). 1) The time delays of PPS, PSS waves increase with the epicentral distance while the time delays of PS waves decreases. 2) Amplitude of PPS and PSS waves is usually smaller than PS waves from the same

Fig. 3. The typical P and PS wave-groups observed at station Chuangku in the west of

Beijing. Records of DD-1 seismograph show the dominant converted wave-groups

PSg and PSm from the top and bottom of the crystalline crust, respectively. The ISC

parameters of used earthquakes or explosions are: 1, 8h 59m 56.5S (origin time in GMT), October 18, 1975, 70.84•‹N, 53.53•‹E, h=0 km, Novaya Zemlya, Mb=6.6; 2,

1h41m32.1s, October 30, 1975, 42.05•‹N, 142.66•‹E, h=62 km, Hokkaido region,

Mb=5.9; 3, 14h35m15.3S, October 11, 1975, 24.91•‹S, 175.16•‹E, h=11 km, south of

Tonga, Mb=6.4; 4, 11h59m57.7S, October 21, 1975, 73.32•‹N, 54.93•‹E, h=0 km,

Novaya Zemlya, Mh=6.6. S228 X. SHAO et al. boundary, but more stable with the change in epicentral distance. 3) The form of the PPS and PSS waves is similar to that of the PS wave, but with polarity inversion and the period of these waves should be slightly longer than PS waves. 4) The polarization of PPS and PSS waves should be close to that of the PS waves from the same boundary. By comparing the seismograms received at one station from several earthquakes we can make phase correlations to reliably distinguish various groups of PS waves (Fig. 3): The PS waves from the top and the bottom of the crystalline crust (PSG and PSM) are usually distinguished by their dominant amplitude and stability, but the PS waves from the boundaries within the crystalline crust or from the uppermost mantle are usually weak and unstable. If the distances between stations are not large (about 5 km or less), the phase correlation of PS waves along the profile can be also carried out by comparing the seismograms recorded at all stations of the profile from the same earthquake (Fig. 4). As a result, the cross section of time delay of PS waves is constructed (Fig. 5), which is similar to that obtained by the seismic reflection method. In such a cross section of time delay, the main features of deep structure are expressed in a general way. The interpretation of the data of the PS waves is based on a model of horizontally

Fig. 4. Phase correlation of the PS convertedwaves along a profile in the northwest of Beijing using the SV component records for the earthquake of October 18, 1975. Th e stations are: 1, Erdaoguan; 2, Heishanzhai;3 , Laojuntang; 4, Kanglingyuan;5, Chuangku; 6, Xiguang; 7, Longquansi; and 8, Xiaweidian. For the parameters of the earthquake, see the caption of Fig. 3. Deep Structures in Some Large-Earthquake Areas S229

Fig. 5. Cross section of time delays of PS waves along a profile in the west of Beijing. 1, Observation stations; 2, time delays of PS waves from different boundaries. PSm is th PS wave-group from Moho discontinuity; PSg, PSc, and Pc1 are the PS wave- groups from the boundaries inside the crust; PSm1,and PSm2are the PS wave-groups from the boundaries of the uppermost mantle.

layered medium. The depths of the conversion boundaries are calculated in accordance with the formula (1), where the time delays ƒ¢tPS are determined from teleseismograms. The

average velocities of P wave VP(h) are taken from the results of the DSS, which have

previously been carried out in the studied region (TENG et al., 1974, 1975; ZENG, 1984; LIU and YANG, 1982; ZENG et al., 1985; Zeng, in preparation, 1979). We can determine the

parameter K by using empirical relation curve between VP and K, when the parameter VP is known (SHAO and ZHANG, 1978). We can also determine the parameter K by interpretation of the data from explosions or local earthquakes (POMERANCEVA and MOZZENKO, 1977;

TAKANO, 1978). The parameter c is calculated from the J-B table of travel time for P waves.

Before the depth determination the effect of the sediment cover was removed by subtracting from ƒ¢tPS the time delay of PS wave caused by the sediment cover, and when we construct the cross section of deep structure, we use the basement as the initial surface for the construction.

The principles used to infer the fault zones are (SHAO and ZHANG, 1979): 1) Sudden change of the ƒ¢tPS;

2) Anomalous changes of the amplitude, the waveform, and the polarization characteristics of PS waves;

3) Sudden change in depth of the conversion boundaries or in thickness of the layer.

For a limited region the parameters VP and K can be considered as laterally invariant S230 X. SHAO et al,

Fig. 6. A comparison between the results of the DSS method and the method of the PS

converted waves along a profile in Xingtai earthquake area of the province of Hebei.

1, Observation stations; 2, crustal structure given by the DSS method (TENG et al.,

1974, 1975); 3, PS conversion points: •œ, reliable data with high S/N ratio of PS

waves; •›, less reliable data with low S/N ratio of PS waves.

in the first approximation. Thus when we correctly identify the PS phases, the relative error in determination of boundary configuration depends only upon the relative error of the ƒ¢

tPS, and is less than 5-10%. But the relative error of the absolute depth will depend upon the relative error of d ƒ¢tPS, K, and VP, and will be 10-15% or less (SHAG and ZHANG, 1979; SHAO et al., 1986).

Figure 6 shows the comparison between the results from the method of DSS and the method of PS waves in the Xingtai earthquake area of Hebei Province. The agreement of the results from the two methods is good. The method of PS waves can give clearly four boundaries (B, G, C, and M). The difference between the two methods in depths is about 1-2 km or less, on condition that VP values taken from DSS are used. The PS converted

waves from the other boundaries within the crust or from the uppermost mantle (G1, Cl, and M1) are weak and unstable with low signal/noise ratio. This comparison together with others has shown that the method of PS converted waves is effective for studying the

configuration of some major boundaries in the crust and uppermost mantle (BULIN, 1971; POMERANCEVA and MOZZENKO, 1977; SHAO and ZHANG, 1978; TAKANO, 1978).

3. Results of the Investigations in Large-Earthquake Areas

North China Plain is one of the seismically active regions in China. During the past two thousand years about 20 strong earthquakes with magnitude greater than 7 have Deep Structures in Some Large-Earthquake Areas S231

Fig. 7. Distribution of the profiles and the epicenters of the largest earthquakes in North China Plain. 1, Observation profiles; 2, depression region of the Cenozoic era; 3, uplift zone of the Cenozoic era; 4, boundary between the plain and the mountains; 5, epicenter of earthquake with M=7-7.9; 6, epicenter of earthquake with M_??_8.0(after MA et al., 1982).

occurred here. The three largest of these are: the Juxian earthquake with magnitude 8.5 in

1668; the Mafang earthquake with magnitude 8.0 in 1679; the Tangshan earthquake with magnitude 7.8 in 1976. In each of three large-earthquake areas we have completed two profiles (Fig. 7). Recording stations are spaced at intervals of 5-10 km along the main profiles and about 10-30 km along supplementary profiles. Mobile observations were made along the profiles using 10 three-component short-period seismographs of type DD-1 with frequency bandwidth of 1-10 Hz. During the period of 1-3 months of observation, the number of distant earthquakes was sufficient to provide adequate data for analysis of PS converted waves. Most of the available teleseisms come from earthquake zones of West Pacific Ocean with epicentral distances of 20•‹-90•‹ and with Ms magnitude of 5-6.5. Large underground nuclear explosions can also give good PS waves. The results of our investigations are illustrated in Figs. 8-10. For each area only one cross section of deep structure along the main profile is presented here. Seven to eight boundaries of conversion have been determined in the crust and upper mantle. Among them the top and the bottom of the crystalline crust (boundaries G and M, respectively) are most conspicuous. We find evidence for anomalous deep structure beneath the large earthquake areas. Here the layers in the crust and uppermost mantle are deformed substantially, reflecting complicated deep processes. Some of the faults extending to the

Moho discontinuity divide the crust into several blocks. The distribution of the hypocenters in the crust show that most hypocenters of strong earthquakes are located in the layer between boundaries G and C. Mainshock hypocenters are usually near the boundary S232 X. SHAO et al.

Fig. 8. Cross section of deep structure along the NNE observation profile in Juxian

earthquake area of the province of Shandong. 1, Observation points; 2, conversion

points and reliability of PS waves: •œ, high S/N ratio; _??_, intermediate S/N ratio; •›, low S/N ratio. 3, conversion boundaries; 4, inferred faults; 5, hypocenter of the M=

8.5 Juxian earthquake. The assumed average velocities are: VP=5.25 km/s and K=

/VS=1.87 for boundary B, V-P=5.55 km/s and K= 1.83 for G, V-p = 5.75 km/s V-P and K-=1.81 for C, V-P=6.05 km/s and K=1.80 for M, V-P=6.25 km/s and K-=1.79 for

M1, V-P=6.4km/s and K-=1.77 for M2, and V-P=6.6km/s and K-=1.76 for M3.

C where it intersects deep faults. Carefully comparing the results presented in Figs. 8-10, we have found some similarities between the structural sections. The common features of deep structures beneath all three large-earthquake areas are the following: 1) Presence of deep faults or intersection of them with vertical offset of about 3-5 km; 2) Presence of obvious local shallowing of the boundary G in the upper crust with amplitude more than 5 km above the hypocenters of large earthquakes; 3) Considerable thickening of the middle and the lower layers of the crust by 5-7 km immediately beneath the hypocenters. As a result, the thickness of the crystalline crust is greatly increased; 4) Thickening of the layer between boundaries M and M1 in the uppermost mantle by 4-6 km, forming a kind of "root" under the hypocenters; 5) Presence of a sharp vertical offset in the depths of the boundaries M, M1, etc. with amplitude of about 5-10 km beneath the hypocenters. We believe that the common features of deep structures in large-earthquake areas must be closely related to each other. The existence of common features suggests similarities in earthquake-generating processes in the North-China Plain. Thus, we can use Deep Structures in Some Large-Earthquake Areas S233

Fig. 9. Cross section of deep structure along the NWW observation profile in Mafang earthquake area, northeast of Beijing. 5, Sediment; 6, middle layer of the crust; 7, hypocenter of the M=8.0 Mafang earthquake. Other legends are the same with those in Fig. 8. The assumed average velocities are: V-P=5.3 km/s and K-=1.87 for boundary G, V-P=5.8 km/s and K-=1.82 for C, V-P=5.95 km/s and K-=1.81 for C1, =6.2km/s and K-=1.79 for M, V-P=6.4km/s and K-=1.78 for M1, V-P=6.6km/sV-P and K=1.77 for M2, and V-P=6.8 km/s and K-=1.76 for M3.

these common features as diagnostic criteria of deep occurrence areas of strong earthquakes in this region.structures for determining possible

4. Predictionof PossiblePlaces of Potential Earthquakes

One of the most important regions to be protected from earthquake hazard in China is the northern part of the North China Plain, including Beijing, Tianjin, and other large cities. During the last ten years a series of seismic profiles of investigation have been completed by means of the PS converted waves in this region (Fig. 11). Based on the cross sections along the profiles, we have constructed contour maps of depths of the boundaries G and M and the map of thickness of the crystalline crust for the region in Figs. 12-14. The contour map of boundary G (Fig. 12) represents in general the shape of the crystalline basement in the North China Plain. It is evident on the map that epicenters of Tangshan and Mafang earthquakes are located near the locally and strongly uplifted zones of boundary G. The depths of these locally uplifted boundary G are only about 3-4 km. In terms of this characteristic we indicate some other uplifted zones of boundary G in this region as shown in Fig. 12. The depth of these locally uplifted zones, however, is 1-3 km deeper than those of Tangshan and Mafang areas. As can be seen on the contour map of boundary M in Fig. 13, the studied region is divided into several fault-blocks with thick (thickness: h=36-40 km) or thin (h=30-34 km) S234 X. SHAO et al.

Fig. 10. Cross section of deep structure along the NNW profile in Tangshan earthquake area (SHAOet al., 1986). 7, Hypocenter of the Tangshan earthquake with M=7.8; 8, hypocenter of aftershocks with M=5-6. The other legends are the same with those in Fig. 9. The assumed average velocities are: V-P=5.1 km/s and K= /PS=1.9 for boundary G, V-P=5.6 km/s and K-=1.84 for C, V-P=5.7 km/s V-P and K-=1.83 for C1, V-P=6.1km/s and K-=1.79 for M, V-P=6.35 km/s and K-=1.78 for M1, V-P=6.6 km/s and K-=1.77 for M2, and V-P=6.9 km/s and K-=1.76 for M3. crust by NNE- and NWW-trending deep-seated faults. The epicenters of the large earthquakes are situated near the fault-blocks with thick crust and in the largest gradient zones of depth variation of boundary M, where the amplitude of variation of the depth of boundary M reaches 7-9 km. Besides Tangshan and Mafang areas we can also note some other thick-crust blocks and zones with large gradient of depth variation of the boundary M as indicated in Fig. 13. If we subtract the depths of the boundary G from the depths of the boundary M, we will get the thickness map of the crystalline crust for the region (Fig. 14). It is clear from this map that the epicenters of Tangshan and Mafang earthquakes are situated just inside the areas where the thickness of the crystalline crust becomes locally maximum. In terms of this specific relation, we also indicate on the map in Fig. 14 some other areas with relatively thick crystalline crust. The thickness in these areas, however, is 2-3 km thinner than in Tangshan or Mafang areas. Finally, we put together the zones of shallow boundary G, the zones of thick crystalline crust and the high-gradient belts of M-discontinuity, and compare them with the epicenters of strong earthquakes (Fig. 15). It is interesting that all strong earthquakes in studied region with magnitude MS_??_6are situated on the high-gradient belts of M- discontinuity, near the zones of shallow boundary G and on or near the zones of thick crystalline crust. Deep Structures in Some Large-Earthquake Areas S235

Fig. 11. The distribution of observation points and profiles for investigations by method of PS converted waves in the northern part of North China Plain (SHAOet al., 1982). 1, Observation points and profiles; 2, the coast line.

Table 1. Comparison of deep structural parameters between large-earthquake areas and areas with potential risk for strong earthquakes identified in this study. S236 X. SHAO el al.

Fig. 12. Contour map of the depth of the boundary G. 1, Observed profiles; 2, contour lines of the depth in km; 3, inferred faults; 4, uplifted zone; 5, subsided zone; 6, epicenter of the M=7.8 Tangshan earthquake; 7, epicenter of the M=8.0 Mafang earthauake.

Fig. 13. Contour map of the depth of the boundary M . The legends are the same with those in Fig. 12. Deep Structures in Some Large-Earthquake Areas S237

Fig. 14. Contour map of the thickness of the crystalline crust. 4, Thickened areas of the crystalline crust; 5, epicenter of the M=7.8 Tangshan earthquake; 6, epicenter of the M=8.0 Mafang earthquake. Other legends are the same with those in Fig. 12.

Fig. 15. Comparison of the characteristics of deep structure with the epicenters of strong earthquakes in the northern part of North China Plain. 1, Zone of shallow boundary G; 2, zone of thick crystalline crust; 3, high gradient belt of M- discontinuity. S238 X. SHAO et al.

On the basis of structural comparison with large-earthquake areas as described above, we conclude that in the northern part of the North China Plain four other areas with anomalous deep structures, quite similar to those of Tangshan and Mafang area, can be distinguished. They are 1) Baodi area, 2) Jinghai area, 3) Zhuoxian area, and 4) Longquansi area; each could be considered a potential area for strong earthquakes when the tectonic stress of the region is increased. The specific parameters of the deep structural anomalies in these four areas together with Tangshan and Mafang areas are listed in Table 1. Because the amplitude of deep-structural anomalies in these four areas is smaller than that in large-earthquake areas, the magnitude of earthquakes, if they occur in furture, will also probably be smaller than those of the Tangshan and Mafang earthquakes. High- density survey networks should be established for continuous observation of precursory phenomena of earthquakes in these areas. The data of deep structures provided by our investigations are applicable to various other research, but discussion of these possibilities was not the aim of this paper.

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