Surface wave dispersion and Earth structure in south -east em China

Transcription

1 Geophys. J. R. astr. Soc. (1982) 69, 3341 Surface wave dispersion and Earth structure in south east em China Stuart Wkr Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado INOAA, Boulder, Colorado 80309, USA Received 1981 July 20; in original form 1980 July 22 Summary. A reconnaissance study of crust and mantle structure in southeastern China was made using surface waves confined to that region from recent earthquakes. Data from the WWSSN stations ANF' and SEO, and from the digital stations TATO and MAT, were used to measure fundamentalmode group velocities of Love and Rayleigh waves over nine paths in southeastern China, an area which has been tectonically quiet since the early Cenozoic. Crustal structure in this region is typical of stable continents, but shearwave velocities in the uppermost mantle are low for a continent, 4.45 km s' or less. Other seismological data support this observation. Introduction Regional studies of earth structure and tectonics in China have recently become available (York, Cardwell & Ni 1976; Bird & Toksoz 1977; Tapponnier & Molnar 1977; Huang 1980; Pines et al. 1980; Romanowicz 1981; Chen 1981), but little is known of the geophysical characteristics of China east of the Tibet Plateau and south of the North China Plain. Active seismicity in central China and seismograph stations in Taiwan, Korea, and Japan permit a reconnaissance study of crust and upper mantle structure in southeastern China using surface wave dispersion. Data and methods of analysis The Formosa Strait lying between Taiwan and mainland China is shallow and underlain by continental crust. Seismograms of Chinese earthquakes recorded at the SRO station TATO in Taipei show surface waves with dispersion entirely influenced by continental structure. Two signals recorded at TATO from earthquakes in China during 1976 were suitable for analysis of surface waves. Seismograms from the WWSSN station ANP near TATO were also analysed. Surface waves from Chinese earthquakes recorded at the WWSSN station SEO in Seoul, Korea also provide information on Chinese crustal structure, since continental crust appears to be continuous from China under the Yellow Sea to Korea. Geophysical measurements show the continuation of crustal structures in China under the Yellow Sea to Korea L

2 34 S. Wier Figure 1. Map of the study area, showing the coastline of China, the 200 m contour below sealevel, the 2000 m contour above sealevel, epicentres of earthquakes used in this study (open circles), and locations of seismograph stations (filled circles with station codes). Station MAT is located ENE of SEO off the map. Solid lines show epicentrestation paths for which either Rayleigh or Lovewave fundamentalmode group velocity was measured. Double lines denote paths for which both Love and Rayleighwave group velocity was measured. Numbered epicentres refer to Table 1. (Wageman, Hilde & Emery 1970). Data from the HGLP station at MAT in Matsushiro also can be used to study Chinese crustal structure if the influence of the intervening nonchinese path segment can be removed. Fig. 1 shows the location of seismograph observatories and earthquakes used in this study. The earthquakes used as sources in this study are listed in Table 1. Source data were taken from the monthly listing of the Preliminary Determination of Epicenters published by the National Earthquake Information Service of the US Geological Survey. Shallow earthquakes whose depths were poorly determined are assigned a depth of 33 km. The SRO and the HGLP instruments record data digitally on magnetic tape, and have a larger dynamic range than WWSSN instruments. SRO stations have been described by Table 1. Earthquake source data Depth Date Time Latitude Longitude (km) hfb Ms 1966 Feb Sep July Apr Aug Jan May May May July Aug Aug Aug Oct Nov 6 15:12: : 00: 2 1.O 10:45: :32: :15: : 34: :23: :08: :35 : : 10: :49: :49: :30: :01: :04: N 27.53N N N N N N N N N N N N N N E E E E E E E E E E E E E E E

3 Surface wave dispersion in China 35 Peterson el al. (1976), and WWSSN stations by Oliver & Murphy (1961). HGLP instrumentation was described by Savino et al. (1972). Data at the SRO and HGLP stations are automatically digitized once per second at the seismograph observatory and recorded on magnetic tape. For each earthquake, time series of 3630 points were selected from the SRO and HGLP data tapes, mean values and linear trends were removed, and horizontal components were rotated to separate radial and transverse ground motion of the surface waves. A S7S9 filter (Kanamori 1971) was applied to the data, severely reducing the amplitude of signals of period less than 10 s, without introducing phase shifts. SRO longperiod and HGLP instruments are not designed to record significant ground motion of periods less than los, so little meaningful signal is lost by this technique. A time series of 512 points centred on the surface wave of interest was then constructed from every fifth point of the filtered data. Seismograms made at the WWSSN stations by photographic recording were digitized at 1 s intervals and then mean values and linear trends were removed. The horizontal components were rotated to separate radial and transverse ground motions, and a time series of 5 12 points centred on the surface wave of interest was selected. No filtering was applied to the WWSSN data. No tapering was applied to any of the data sets since the time series always started before the arrival of the fundamentalmode surface waves and extended past the time of large surface wave amplitudes. The multiple filter technique of Dziewonski, Bloch & Landisman (1969) was used to determine group arrival times for a range of periods. A system of Gaussian filters with constant Q yielded constant resolution on a longperiod scale. Analysis of SRO and HGLP data extended from 10 to 166s, and analysis of WWSSN data extended from 8 to 135s. Instrumental response was removed using the technique of Luh (1977). The multiple filter technique has a resolution of km s'. Errors in group velocity of km s' or less due to event mislocation and O.OlOkms' or less due to station timing are possible. These errors would uniformly shift the dispersion curve to higher or lower velocities at all frequencies. The paths from the Chinese earthquakes listed in Table 1 to the stations in Taipei and Seoul are confined to continental geological provinces. More than 20 per cent of the path from these events to the station at Matsushiro are in the Sea of Japan and Honshu. It would normally be impractical to try to estimate surface wave dispersion in China from data recorded at Matsushiro. However, the occurrence of an earthquake in the Yellow Sea on 1976 October 6 (event 14, Table 1) allows removal of the influence of the path between the Yellow Sea and Matsushiro. The event of 1976 October 6 lies slightly south of the path from Matsushiro to the northern source group (events 5, 11, 12 and 13; Table 1) north of the path from Matsushiro to the southern source group (events 7,8,9 and 10, Table I), and very close to the path from Matsushiro to the earthquake of 1976 November 6 (event 15, Table 1). The earthquake of 1976 October 6 is considered in a good position to provide a path correction for all of these sources. The group velocity of the surface waves in China recorded at Matsushiro is given by (A1 A*> U(w) = [tl(w) tz(w)l where A, and tl(w) are the distance and group travel time (at a given frequency a) for a Chinese event recorded at Matsushiro, and A, and t2(o) are the same values for the event of 1976 October 6, recorded at Matsushiro. This calculation has the advantage of identically removing all influences of instrumental group delay or instrumental group delay corrections (which are included in the terms t, and t2), since both waves are recorded by the same instrument. This technique will introduce error if the dispersion of the two signals between the

4 36 S. Wier Yellow Sea and MAT is not identically the same. Some error is therefore expected when the paths do not overlap. The earthquake of 1976 October 6 was assigned bodywave and surfacewave magnitudes of 5.2. The high quality of the data recorded at Matsushiro allowed determination of the Rayleigh wave group velocity for periods from 11 to 67s for this earthquake. As a result Rayleigh wave group velocity in China was estimated in this period range at Matsushiro from four events at three separate source regions. The data from event 15 is expected to have the best correction since the path from that event nearly overlaps the path from the reference event used for correction. The Love wave from the event of 1976 October 6, was not as well developed as the Rayleigh wave and could not be used as a source of surface wave traveltime corrections. Since no other earthquakes in the Yellow Sea were recorded at Matsushiro, other ways to estimate Love wave group velocity across Korea, the Sea of Japan, and Honshu were considered. The Love wave from the event of 1968 July 1 (event 3, Table 1) recorded at Seoul was used. This event is lookm from Matsushiro and the path to Seoul is close to the path from Matsushiro to the northern source area. The same technique was used to estimate group velocities for the Love waves as for the Rayleigh waves, subtracting travel times and distances. In this case the effects of instrumental group delay are not eliminated as is the case when both waves are recorded at the same station. The correction of Love wave group velocities recorded at Matsushiro is considered less satisfactory than the correction of Rayleigh waves. All the usable Love waves recorded at MAT were from the southern source area whereas the path of the waves used for correction was near the path to the northern source area. Furthermore, the path of the waves used for correction crossed 500km of the Sea of Japan deeper than looom, while the path to the southern source area parallels the edge of the shelf west of Japan. Surface waves on the southern path travelled farther through the shelf before reaching MAT. The depth of seawater has no effect on Love wave dispersion, but Love waves are typically faster in oceanic crust than in areas of thicker crust such as the shelf. If the group velocity of the Love waves used for correction is faster than the group velocity of the Love waves on the nonchinese path segments, then the apparent group velocity of the Love waves in China will be slower than the true value. The corrected Love wave group velocities measured at MAT are slower than others measured in this study, and this may be a sign that the correction for Love waves is unsatisfactory. The results of the surface wave group velocity measurements are summarized in Tables 2 and 3, and plotted in Figs 2 and 3. The values listed for MAT are corrected to show dispersion in China and the Yellow Sea only. The fastest Rayleigh waves were recorded at ANP from event 6 and similar values were measured at ANP from event 5 and at MAT from events 12 and 13. The slowest Rayleigh waves were recorded at MAT from event 7. Other Rayleigh wave group velocity measurements were intermediate between these extremes. The fastest Love waves were also recorded at ANP from event 6, and the slowest from three events in the southern source group recorded at MAT. Love wave group velocity values measured at SEO, and at ANP from event 4, are intermediate. The increase in group velocity with decreasing period below 20 s observed at MAT is unlike other Lovewave observations and may be an artifact of the correction procedure. A previous study (Sung, He & Xu 1965) determined Love wave group velocities in the period range 718s for a path across southeastern China. The path is virtually identical to that shown in Fig. 1 as a double line from event 9 towards MAT. The path ends at Nanking near the sea coast. The group velocities parallel the dispersion curve measured in this study for the profile from event 6 to ANP, and are slightly slower. The values do not resemble the

5 Surface wave dispersion in China 37 Table 2. Observed Rayleighwave group velocities (km s') versus period T(s) for the paths indicated by event number (from Table 1) and recording station. Data at MAT have been corrected for a path segment outside of China. Period (S) ANP 6ANP 2SEO 7MAT 12MAT 13MAT 15MAT 9TAT0 11TAT , ~ s Table 3. Observed Lovewave group velocities (km s') versus period T(s) for the paths indicated by event number (from Table 1) and recording station. Data at MAT have been corrected for a path segment outside of China. Period (s) 4ANP ANP SEO 8MAT 9MAT 1 0M AT

6 ~~ ~ ~ 38 S. Wier I I I I I E t. k 3.4 V s 3.3 W ' 3.2 a a " ~ RAYLEIGH WAVES 0 6ANP 0 2SEO A?MAT ZHl ZH2 008 AAA 1 1 I UJ PERIOD (sec) Figure 2. Selected fundamentalmode Rayleighwave group velocities for eastern China determined in this study. Values shown span the range of most measured Rayleighwave group velocities. Epicentrestation pairs are denoted by event number (Table 1) and station code. Rayleighwave group velocities of models ZH1 and ZH2 used in this study are also shown. I I I I I 4.0 h C 0 0 6ANP 0 I SEO A 9MAT ZHl ZH2 I I I I I I PERIOD (sec) Figure 3. Selected fundamentalmode Lovewave group velocities for eastern China determined in this study. Values shown span the range of most of the measured Lovewave velocities. Epicentrestation pairs are denoted by event number (Table 1) and station code. Lovewave group velocities of models ZH1 and ZH2 used in this study are also shown. dispersion curve derived from data recorded at MAT, suggesting the Lovewave dispersion curves derived from MAT data may be incorrect. Inversion for earth structure Simultaneous inversion of both Rayleigh and Lovewave dispersion measured on the same path provides a better constraint on earth structure than inversion of either one alone. In this data set both Rayleigh and Lovewave fundamental mode group velocities are available

7 Surface wave dispersion in China 39 for two paths across eastern China: from event 6 to ANP, and from a group of adjacent events (7,8,9 and 10) to MAT. Since the group velocities on these two paths span essentially ail the observed group velocities, inversion of these two data sets should give an indication of the range of possible earth structures in eastern China. The path to ANP was chosen as the starting point since the special corrections used to process the data recorded at MAT were not required at ANP. Six flat homogeneous layers were used to represent the crust. The top two layers in the crust represent the influence of possible low velocity sediments above a wellconsolidated basement. Starting values of shearwave velocity were derived from previous studies of continental structure using surface waves (Brune & Dorman 1963; Biswas & Knopoff 1974). Considerable effort was made to incorporate the most recent features of seismic and chemical studies of continental crust (Smithson & Decker 1974; Berry & Mair 1977; Mueller 1977; Prodehl 1977; Smithson & Brown 1977; Smithson 1978). These new crustal models include such details as low velocity zones at depths near lokm, low velocity zones near the base of the crust, and rapidly increasing compressionalwave velocity with depth in the lower crust, in contrast to previous models of one or two homogeneous layers for the lower crust. Such finescale features are not resolved by the data used here, but these studies indicate that compressionalwave velocity in the crust has great range and complexity, and provide the starting values for compressionalwave velocity in the crustal model ZH1. Several estimates of crustal thickness are available for eastern China (Ting 1965; Rodriguez 1969; Chinese Academy of Sciences 1974). There is considerable variation in detail between these publications but they are broadly consistent. The crustal thickness near the western epicentres used in this study is near 70 km, rapidly dropping to 50 km near 104" E. Most of China east of Tibet has crust gradually thinning from 50 km in the west to 30 km under the eastern continental shelf and Yellow Sea. A miform crustal thickness of 40 km was chosen as the starting value for the models. The effective group velocity observed across a crust of uniformly decreasing thickness is the group velocity of the same crust at the average thickness. Surface wave dispersion in the period range observed will be little influenced by thin layering in velocity structure in the upper mantle, so only two layers were used between the crust and 220 km depth. The top represents the 'lid' or upper mantle in the lithosphere, and the lower layer represents the asthenosphere or 'low velocity zone'. Below 220km depth 11 homogeneous flat layers represented the remainder of the mantle. Velocity structure in this depth range has a comparatively small effect on the group velocity dispersion in the period range observed. The layer parameters were derived from the mantle of model PEMC (Dziewonski, Hales & Lapwood 1975). Differences between the layered model of the mantle derived from PEMC and PEMC itself (which has smoothly varying parameters with depth) cause insignificant variations (less than 0.01 kms') in calculated group velocities at the periods observed. This representation of mantle structure below the asthenosphere was retained unchanged for all models in this study. Compressionalwave velocities and densities in the lid and lowvelocity zone were also derived from model PEFW. The surfacewave dispersion parameters measured in this study are very insensitive to uppermantle compressionalwave velocities. Reducing the lid's cornpressionalwave velocity from to 7.80 km s' changes Rayleighwave group velocity by kms' or less, depending on period. The lid compressionalwave velocity used in modelling, 8.15 kms', could be reduced to 7.80 km' or less with no effect on the conclusions of this study. Uppermost mantle compressionalwave velocities less than 8.00 km sl are more compatible with the low shearwave velocities found in this study than is 8.15 km.s'> judging from results in other regions.

8 40 S. Wier The technique of Schwab & Knopoff (1972) was programmed and used in this study to calculate surface wave dispersion parameters from the flatlayered earth models. Corrections for sphericity are incorporated in Lovewave dispersion calculations. Corrections of Rayleighwave group velocity for sphericity may be neglected for the period range used here (North & hiewonski 1976). In order to find the family of acceptable earth models which satisfy any given dispersion curve, five of the most significant parameters of the starting model were systematically vaned through a range of plausible values. Models were considered acceptable if the resulting dispersion vaned from the observed values by less than km s' at each point, and if the rms deviation between the observed and calculated dispersion was less than km s'. This technique for finding acceptable models was first used by Biswas & Knopoff (1974). The five variable parameters are the thickness of the lowest crustal layer (thereby controlling total thickness of the crust), shearwave velocity of the lowest crustal layer, thickness of the lid, shearwave velocity of the lid, and shearwave velocity of the lowvelocity zone. Lid thickness also determines lowvelocity zone thickness, since the bottom of the lowvelocity zone is held at 220 km. The shearwave velocity in the lid is allowed to be less than that in the lowvelocity zone. A starting model for the data on the path from event 6 to ANP was found by adjusting crustal layer thickness, density, and shearwave velocity to match the observed Lovewave dispersion. Small changes were then made in compressional velocity to produce a model which was also consistent with Rayleighwave dispersion. The starting model, denoted ZH1, and the upper mantle model, are listed in Table 4. The results of the fiveparameter search for acceptable models for the path from event 6 to ANP are shown in Fig. 4. Each filled circle indicates a combination of the five parameter values which produce an earth model matching both Love and Rayleighwave dispersion to Table 4. Crust and mantle model ZH1 used for inversion of Rayleigh and Lovewave group velocities of waves crossing southeastern China to station ANP. Values in parentheses are changed in inversion to find a range of models fitting the observed data. Values shown are for the particular model which best fits both the Rayleigh and Lovewave data. Thickness (km) 1.o O (5.0) (35.0) (1) 50;O Density (kg m Pvelocity (km s') Svelocity (kms') ( 3.850) (4.350) (4.500) , Depth (km) 0.o 1.o O

9 5 4.7 H LID h (km) LID h H E 4.5H H Surface wave dispersion in China 41 LID h (Km) HHHH HHHH HEI3EBEB HHDEE3 ~ m m ~ 4.45 LID h (Km) E~EE~HH HHHB EElHBH 4 7 H H H H HHHH LID B (Km/Sec) lower crust Figure 4. Fivedimensional model space of acceptable solutions for inversion of Rayleigh and Lovewave group velocities on the profile to station ANP from event 6 across southeastern China. the required degree. These results suggest that shearwave velocity in the upper mantle under southeastern China is fairly low for a stable continental regime. In all cases but one the region of lowest shearwave velocity in the mantle is immediately under the crust. Both Love and Rayleighwave dispersion were measured on the path to MAT from the adjacent events 7, 8, 9 and 10. It was not possible to find an earth model which simultaneously matched both Love and Rayleighwave dispersion. The explanation likely involves the special corrections required for the data recorded at MAT, especially the rather unsatisfactory Lovewave correction. The corrected Love and Rayleighwave velocities may contain fairly small systematic errors prohibiting a good fit to an earth model, but as they stand they do not contradict the suggestion of low shearwave velocity in the upper mantle. The group velocities measured at MAT are the slowest in this study. Both Love and Rayleighwave group velocity were measured at SEO for paths across southeastern China, but from different events. Even though the separate paths suggest that distinct geological structures are involved, a search for average earth models with dispersion fitting both wave types was attempted. A starting model was derived from model ZH1 using partial derivatives of Rayleighwave group velocity with respect to the layer parameters. The

10 42 S. Wier Table 5. Earth model ZH2 used for inversion of Rayleigh and Lovegroup velocities of waves crossing eastern China to station SEO. Values in parentheses are changed in inversion to find range of models fitting the observed data. Values shown are for the particular model which best fits both the Rayleighand Lovewave data. The two top layers of the mantle are also shown. Thickness Density Pvelocity Svelocity Depth (km) (kg m3) (km s') (kms') (km) O (10.0) (95.0) (85.0) (4.000) (4.350) (4.200) O LID h (km) H 3 2! 4.3B 83 H LID h (Km) HBBEB E 4. 4 m R LID h (Km) &lbhw LID B (Km/Sec) lower crust BBHH 4.55 c B (Km /Set) Figure 5. Fivedimensional model space of acceptable solutions for inversion of Rayleigh and Lovewave group velocities on the profiles to station SEO across eastern China. Open circles denote earth models fitting Rayleighwave data alone, and filed circles show acceptable earth models fitting both Love and Rayleighwave data.

11 Surface wave dispersion in China 43 new starting model ZH2 is listed in Table 5. The same five parameters were varied as before, and satisfactory parameter combinations are plotted in Fig. 5. Each open circle indicates a combination of parameters or earth model which matches Rayleighwave dispersion alone. The filled circles indicate models which match both Love and Rayleighwave dispersion. There are a large number of models (not shown) which satisfy Lovewave data but not Rayleigh waves. There is very little overlap between the family of models which match the two wave types, probably due to the fact that the waves passed through different parts of the crust of China. This is in contrast to the (single path) data recorded at ANP, where every model that matched Rayleighwave dispersion also matched Love waves. The models which matched Rayleigh waves recorded at SEO are shown simply to indicate the family of models which are permitted on one path without further restriction. Low uppermantle shearwave velocities are indicated by the data recorded at SEO. It is possible to find separate starting models of the crust which fit Love or Rayleighwave dispersion at SEO alone better than a single model fitting both simultaneously. In that case the crustal structure indicated by Rayleigh waves has a thicker surface layer and lower shearwave velocities in the crust than the structure for Love waves. This is consistent with the fact that Rayleigh waves travel on a path which is closer to the centre of the sedimentary basins of Sichuan and the south part of the North China Plain (Terman 1973). One model can be found that satisfies the requirements of both Rayleigh and Lovewave dispersion because the compressionalwave velocity structure can be used to modify Rayleighwave dispersion without influencing Love waves. In this case low compressionalwave velocities reduce the Rayleighwave group velocities that would otherwise be too fast if values from model ZH1 are used with the shearwave structure satisfying Lovewave dispersion. The shearwave structure has a much larger influence on both Rayleigh and Love waves in any case, and should be regarded as a sound estimate of average shearwave velocity in the regions traversed by these waves. The shearwave velocities in model ZH2 are slightly slower than in model ZH 1, and the thicker surface layer reflects the presence of sedimentary basins crossed in Sichuan, the North China Plain, and the Yellow Sea. Discussion The models of the crust determined for southeastern China in this study are similar to models of continental structure elsewhere on the Earth. The crustal model ZH1 is most similar to the Gutenberg model (Alterman, Jarosch & Pekeris 1961), and the crustal shearwave velocity structure also resembles model R6 of Biswas & Knopoff (1974), recognizing that small variations in the thickness and shearwave velocity of the lowest crustal layer ZH 1 were permitted in finding a family of acceptable earth models. The Gutenberg model represents an average continental structure, and R6 was constructed for the central United States, a stable continental platform. The shearwave velocities in the crust of model ZH1 are slower than in model CANSD (Brune & Dorman 1963), representing the Canadian shield. The compressionalwave crustal velocity structure of model ZH1 is faster than previous continental models in most cases, reflecting the higher compressionalwave velocities found in recent studies of continental crust which were used to assist inversion. Model ZH2 is slower than ZH1 and also is a reasonable continental structure. An average crustal thickness of km is indicated by both crustal models. The remarkable feature of all the acceptable models of earth structure in southeastern China is the low shearwave velocities in the 'lid' or uppermost mantle. Values of 4.45 km s' or less occur in all acceptable models except one, where a very thin (5 km) lid of 4.65 km s' overlies a thick 4.40 km s' lowvelocity zone (Figs 4,5 and 6).

12 44 S. Wier Southeastern China has traditionally been regarded as a stable continental platform, and some new geophysical information supports that view. Tapponnier & Molnar (1977) found no evidence for folding or faulting in southeastern China during the Cenozoic. The same area is presently free of earthquake activity (York et al. 1976). In fact southeastern China is geologically heterogeneous, and describing it as a stable platform, perhaps analogous to the interior craton of North America, is oversimplified. The uppermantle shear velocities measured in this study are unusually low for a stable continental platform. The area produced volcanism in the Cretaceous, especially along the southeastern coast (Terman 1973; Jahn 1974). Vigorous earthquake activity and extensional tectonics are found north of the Yellow River, outside of the region of this study pork et al 1976, Tapponnier & Molnar 1977), and there is evidence for limited extensional tectonics in the area between the Yellow River and the Yangtze River (Tapponnier & Molnar 1977). The earth models with the lowest shearwave velocities found in this study are for paths crossing this area to station SEO. The complexity of geological structure and the late Mesozoic volcanism and tectonism of southeastern China suggest a rough similarity to western Europe, and mantle shearwave velocities found in this study are similar to earth models for western Europe derived from surface wave dispersion (Cara, Nercessian & Nolet 1980; Fig. 6). Other current seismological studies indicate low shearwave velocities under southeastern China. Pines et al. (1980) derived a model of earth structure along the eastern coast of China from surface wave dispersion analysis. The model shows upper mantle velocities of 4.5 km s' or less. Timing multiple ScS waves gives a measure of the Swave travel time in the upper mantle under surface reflection points (Sipkin & Jordan 1980a). Sipkin & Jordan found that the twoway travel time for Swaves under eastern China is about 1.0s longer than for the reference JeffreysBullen model. Other continental reflection points gave a median twoway time about 1.O s less than the JeffreysBullen model. The twoway Swave travel time for the Figure 6. Shearwave velocity structure of the earth model ZH1 which best fits the data of this study compared to the earth models CANSD (Brune & Dorman 1963), PEMC (Dziewonskiet al. 1979, and a modei for western Europe by Cara et al. (1980), denoted CNN.

13 Surface wave dispersion in China 45 JeffreysBullen model above 175 km is 82.0 s. The models of shearwave velocity found in this study also give delayed S times compared to the JeffreysBullen model. In a separate study Sipkin & Jordan studied attenuation of ScS waves and found a strong correlation between degree of attenuation and tectonic setting of surface reflection points. Attenuation under southeastern China was found to be high compared to stable cratons (Sipkin & Jordan 1980b). The seismic phase S, travels in the crust and upper mantle, and its velocity is a direct measure of the shear velocity in the upper mantle. S, is well observed on most continents and in old ocean basins, but its absence indicates enhanced shearwave attenuation as in active tectonic regions, ocean spreading centres, and backarc basins (Molnar & Oliver 1969). S,, is inefficiently propagated through southeastern China, and there is some evidence for low uppermantle S velocities (Wier 1981). Ruzaikin et al. (1977) found that the crustal phase Lg propagates less efficiently in eastern China than in other stable regions of Asia, such as the Indian shield, the Eurasian platform, and the Tarim Basin. AU these studies indicate low shearwave velocities in the upper mantle under China, compared to many continental areas, and enhanced shearwave attentuation. There is a possibility that the asthenosphere extends virtually to the base of the crust, and that the mantle lithosphere is weak or missing. Tapponnier & Molnar (1977) suggest that the entire southeastern China stable block has been pushed eastward over the Philippine Sea, as a result of the collision of India with Asia which started about 40Myr ago. The mantle under eastern China may have been modified by this event. Alternately, we note that some Chinese scientists believe that mantle upwelling is responsible for the present active extensional tectonics under the North China Plain (Ding Goyu 1980, private communication). Possibly a similar process at a weaker intensity is active under the remainder of southeastern China. Conclusions Simultaneous inversion of Rayleigh and Lovewave dispersion suggest that the crust of southeastern China is typical of continents, but the upper mantle has an unusually low shearwavevelocity. The shearwavevelocity of the uppermost mantle may be as low as 4.35 km s. Other seismological data support this observation and also indicate that attenuation of shear waves under southeastern China is enhanced compared to other stable continental areas. Acknowledgments Data analysis and generation of earth models was supported by the Cooperative Institute for Research in Environmental Sciences, University of Colorado/NOAA, Boulder, Colorado 80309, while the author was a Visiting Fellow. Data were supplied by the Seismic Data Analysis Center, Teledyne Geotech, Alexandria, Virginia, and by the Environmental Data Service, National Oceanic and Atmospheric Administration, Boulder, Colorado. The suggestions of James W. Dewey and an anonymous reviewer are appreciated. References Alterman, Z., Jarosch, H. & Pekeris, C. L., Propagation of Rayleigh waves in the Earth, Ceophys. J. R. astr. SOC., 4, Berry, M. J. & Mair, J. A., The nature of the Earth s crust in Canada, in The Earth s Oust, ed. Heacock, J. G., American Geophysical Union, Washington, Dc. Bird, P. & Toksoz, M. N., Strong attenuation of Rayleigh waves in Tibet,Nature, 266,

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