Generalized seismic properties of the lithosphere in the northwestern Pacific Basin

329

Te~t~~op~ysi~s, 201 W92) 329-340 Elsevier Science Publishes B.V., Amsterdam

Generalized seismic properties of the lithosphere in the northwestern Pacific Basin A.A. Ostrovsky I~s~j~~ie of ~~ea~~~~,

and A.A. Buravtsev

USSR Academy of Sciences, 23 ~~~o~a,

II 7218, Moscow, USSR

(Received December 30,1989; revised version accepted April 19,1991)

ABSTRACT Ostrovsky, A.A. and Buravtsev, A.A., 1992. Generalized seismic properties of the lithosphere in the northwestern Pacific Basin. In: H. Shimamura, A. Hirn and J. Makris (Editors), Detailed Structure and Processes of Active Margins. Tectonophysics, 201 (spec. sect.): 329-340. The results of seismological experiments by the USSR, Japan and USA for measuring seismic velocities in the northwestern Pacific Basin were combined. Data on more than 60 earthquakes recorded in drillholes and on the ocean bottom were induded. Average traveltime curves for P- and S-waves were obtained. The P- and S-wave velocities vary from 7.5 to 8.X and from 4.3 to 5.2 km/s respectively. The combined data show that, depending on the azimuth there are two different ways in which the seismic velocity changes with depth. This confirms previous results which revealed a ~nsiderable degree of anisotropy in the upper lithosphere of the region. The m~imum values of P- and S-wave velocities (8.6 and 5.1 km/s, respectively), were those for earthquake waves travelling in the north-south direction, while the minimum values (8.0 and 4.6 km/s, respectively) were those for the east-west direction. These data indicate that the wave-guide properties of the oceanic lithosphere depend on the Bzimuth.

Introduction Seismic methods are very important in the study of the structure of the earth’s lithosphere. Since the study of the deeper lithosphere requires seismic waves to be registered at a considerable distance from their source, such studies have used natural ea~hquakes instead of the traditional sources of seismic waves, such as explosions and airgun shots. The advantages and disadvantages of this technique are obvious: earthquakes, though effective and intense, remain uncontrolled sources of seismic waves. Nevertheless, a knowledge of the distribution of the regions of high seismic activity over the earth, sometimes allows the reliable registration of earthquakes to be expected during seismic experiments, even when ocean bottom. seismographs, which are known to have a limited operating time, are used. For the last 20 years, the northwestern Pacific Basin has been studied mainly by scientific teams from Japan, the USA and the

USSR. The main results of the studies of the deep seismic structure in this region have been reported by Shlue and Knopoff (1977), Shimamur and Asada (19761, Shimamura et al. (1983), Walker (19811, Walker and McCreery (19871, Rykunov et al. (19841, Duennebier et al. (1987). Despite the great practical difficulties involved, the registration of ea~hquakes by ocean bottom seismographs permits the study of the deep seismic structure of the lithosphere in regions which formerly were beyond reach. However, the method requires a great amount of other seismological information, which makes generalization of various results extremely important. ~nst~~ents

and data

The Institute of Oceanology of the USSR Academy of Sciences conducted many experiments using ocean bottom seismographs (OBS) in the northwestern Pacific Basin during cruises 21,

0040-1951/92/$0.5.00 0 1992 - Elsevier Science Publishers B.V. All rights reserved

330

A A. OSl’KOVSKY

23 and 29 of the R/V “Dmitriy Mendeleeu”. The prototype of the instrument used (the Moscow State University OBS) has been described by Rykunov and Sedov (1976). The OBS used on the above cruises has the same general features as its prototype but differs from it in respect of a few important improvements made in the Laboratory of Seismic Research. The seismic signal from a gimbal-suspended one- or three-orthogonal-component geophone (with natural frequency f = 5 Hz) is recorded directly by the analog magnetic tape recorder. The tape speed can be varied from 0.5 to 0.95 mm/s. Two recording channels with two levels of amplification for each component provide a 65 dB dynamic range. Two accessory channels are used for time marks and a pilot signal, which makes it possible to eliminate the instabilities of the speed of the record/play-back system when the OBS signals are digitized. The accuracy of the quartz time-mark generator of the prototype was lo-‘, but has been improved to lo-‘. Total OBS operating time is limited by tape cassette capacity and is up to 7 days. The OBS frequency band is from 4 to 25 Hz. Figure 1 shows the amplitude response function of the OBS including seismometer, electronics and amplifier of the record/playback systems up to the point where the signal was fed into the spectral analyser. The main features of the OBS deployment procedure currently used at the Laboratory of Seismic Research are shown in Fig. 2. At the start of a deployment, the buoy, with a 300 kg weight-carrying capacity, is put in the water, and

7

2

JllO t g100 8 2 p

i >

90

80 100

10

1 F-Y,

HZ

Fig. 1. Example of the OBS amplitude response function.

AND

A./\.

HC!KAV’I

St\’

Fig. 2. Procedure for OBS deployment: (a) a buoy (2) is put in the water and the buoy-rope (1) is reeled out from the spool (3); (b) the OBS (5) connected to the anchor (4) by the rope (6) weighted by an iron chain (7) is let into the water when the rope is reeled out of a winch; (c) the anchor is dropped into the water; Cd)the OBS rests on the bottom.

the nylon buoy-rope is reeled out from the spool when the vessel moves off (Fig. 2a). After the necessary length of buoy-rope (lo-15%/ longer than the water depth at a site) has been reeled out, the vessel stops, and the OBS, connected to the anchor by a 70-NO-m-long rope with a heavy iron chain in its middle, is reeled out into the water from a winch (Fig. 2b). Then the anchor is thrown into the water and drops down to the bottom with an average speed of 1.4 m/s (Fig. 2c). While dropping down, the OBS and the anchor maintain an almost (but not exactly) vertical orientation with each other, so that when the OBS rests on the bottom, the rope between it and the anchor is not tight (Fig. 2d). The oB6 has the shape of a cylinder (90 cm length, 10 cm radius), its weight is around 120 kg in air and 90 kg in water. The iron anchor weighs appmtitely 100 h; the chain 30 kg.

GENERALIZED

331

SEISMIC PROPERTIES OF THE LITHOSPHERE, NW PACIFIC BASIN

170"

160"

50"

40"

30"

140"

150"

170"

160"

Fig. 3. Seismological observations in the northwestern Pacific Basin. 1-6 = earthquakes with hypocenter depths of: O-50, 50-100, 100-150, 150-200, 250-300, and 300-350 km, respectively (the numbers are from the table); 7 = the DSS lines: l-3 made during cruises 21,23 and 29 of the R/V “DmitriyMendeleeu”, respectively, 4-5 data of Japan scientists.

TABLE 1 Parameters of the OBSs deployments Latitude

OBS No.

Longitude

Data recording interval

Time Beginning

End

Water depth (m>

Cruise 21

Ctie

158” 369E 158 020 162 360 161 149 160 290 159 535

20-26.09.78 22-27.09.78 07-17.10.78 08-16.10.78 08-16.10.78 08-14.10.78

14-00 14-30 OS-10 02-00 15-40 21-20

11-00 21-35 02-00 16-16 09-30 21-00

5100 5630 56.50 4170 4800 4800

41282 41080

153 134 153 435

23-26.07.79 24-27.07.79

20-00 02-30

21-00 02-30

5400 5400

43 458 43 392 43 312

158 464 I58 189 157 550

04-10.09.82 04-10.09.82 04-09.09.82

06-35 15-00 18-40

14.10 09-44 19-40

5500 5650 5540

37’”318N 38 483 34 326 35 317 36 135 36 350

3 1B 6 1 2 11 23

3 8 Cnrise 29 1 6 11

A.

65”

OBS-6,

EARTHQUAKE -17, S-P : 73 s

450

60

s

400

350 i 55”

Fig. 4. The detailed layout of OBS deployment during cruises 21 (circles), 23 (squares) and 29 (triangles) of the R/V

B.

0BS-6.

EARTHCMJAKE- 9, S-P : 95 s

“ Dmitriy Mendeleec “.

While the deep seismic sounding (DSS) method was used for the study of the lithosphere’s seismic structure, many natural earthquakes were recorded by ocean bottom seismographs. Some of these events were included in the bulletins of the World Seismic Network, which made it possible to use the information on the epicentral coordinates and the times of the earthquakes registered. An analysis of earthquake records obtained during cruises 21 and 23 of the R/V “Dmirriy Mendeleeu” has been given by Rykunov et al. (1984). Seismological data obtained during cruise 29 of the same R/V are presented here for the first time. The epicenters of the earthquakes taken for analysis as well as the array schemes of the seismological experiments during the aforementioned three cruises of the R/V “Dmihy Mendkieeu” are shown in Figures 3 and 4. Figure 3 shows a general map of the region and the epicenters of the earthquakes used for the generalization. Figure 4 shows the details of the OBSs arrays. The main parameters of the OBS deployments during the three cruises are given in Table 1. The earthquake records obtained during the 29th cruise were registered by a linear array of three ocean bottom seismographs located 200 km to the west of drillhole 581 of the DSDP (Fig. 4). OBSs were operating on the bottom from 4 to 10 September 1982. The water depth at the deploy-

Fig. 5. Examples of two earthquake records obtained during cruise 29: (A) earthquake no. 17 (see Table 2): (B) earthquake no. 9 (see Table 2).

ment points was around 5500 m. Fourteen earthquakes with clear P- and S-arrivals, registered by at least two OBSs, were selected for analysis. The range of epicentral distances was about 600-2100 km or 5- 19”. Examples of two earthquake records, illustrating the quality of the data are given in Figure 5.

To determine the average velocities of P- and S-waves we used the data of cruise 29 of the R/V “Dmitriy Mendekev” and also material of cruises 21 and 23 of the same R/V, as well as the data given by Duennebier et al. (1987). General data for the parameter of the various earthquakes are listed in Table 2. It is well known that the average velocity is defined by V= X/T, where X is the epicentral distance, and T is the time required

GENERALIZED

333

SEISMIC PROPERTIES OF THE LITHOSPHERE, NW PACIFIC BASIN

for the seismic wave to be propagated from the source to the seismometer. The analysis of the average velocities shows that they vary within a

rather wide range, namely from 7.5 to 8.8 km/s for P-waves and from 4.3 to 5.2 km/s for S-waves. The results obtained are shown as a plot of

TABLE 2 The parameters of the earthquakes, used for the analysis Number

Date

Time (GMT)

Lat. N

Long. E

(h/mitt/s) R/V

” Dm.

Depth

M

&rn)

Mendeleev ” cruise 21

1

20

23/09

05/13/00.7

54.50

161.50

30

2

23

23/09

18/06/14.9

43.34

142.79

80

3

24

23/09

22/43/06.6

38.60

142.99

30

4

27

25/09

01/50/51.0

44.70

150.08

90

5

29

25/09

21/39/09.9

50.45

156.75

40

4.7 _

6

37

08/10

19/09/21.8

36.05

137.33

260

5.7

7

58

ll/lO

01/49/01.1

33.41

140.76

60

5.7

8

61

ll/lO

10/26/16.1

44.20

149.27

50

9

62

11/12

11/38/40.0

44.50

149.30

40

5.2 _

4.7

10

67

ll/lO

15/35/05.5

44.70

149.00

30

11

75

12/10

02/20/38.0

48.20

154.60

40

12

78

12/10

18/15/23.7

36.68

141.38

40

4.2

13

81

13/10

08/49/57.0

35.82

140.88

30

4.4

100

R/V

“Dm. Mendeleev” cruise 23

14

5

23/07

22/15/40.0

46.40

152.10

15

7

24/07

03/08/51.0

29.43

142.83

90

5.4

16

9

24/07

05/29/42.5

36.00

140.87

50

4.1

17

10

24/07

07/11/07.0

36.18

140.80

50

3.1

18

15

24/07

12/19/45.0

35.43

139.58

100

19

18

24/07

22/53/59.0

44.50

148.20

30

3.7 -.

20

19

25/07

02/45/24.0

44.20

149.50

30

-.

21

20

25/07

04/20/16.0

36.83

141.70

50

4.0

22

21

25/U

05/14/41.5

41.12

142.53

30

3.4

23

22

25/01

05/23/21.5

36.82

141.80

40

24

30

25/01

16.31/26.0

44.40

146.70

100

4.2 _.

25

31

25/01

16/53/11.0

38.48

142.23

40

3.9

26

42

25/Ol

23/D/39.4

40.18

145.03

40

5.1

27

48

26/07

05/26/14.0

40.27

142.23

40

3.5

_.

R / V “Dm. Mendeleev” cruise 29 28

1

@t/Q9

01/59/58.0

43.60

148.80

40

29

6

05/09

08/05/53.6

50.42

157.16

40

_.

30

7

04/09

08/28/04.0

44.70

145.50

180

4.3

31

9

05/09

10/01/36.0

42.90

145.80

50

32

10

05/09

11/17/04,9

51,58

158,98

33

4.1 __

33

17

O6/09

00/37/56.0

43.50

148.70

40

4.2

34

18

06/09

01/47/03,2

2940

140,43

179

35

19

06/09

06/04/57.0

43.50

148.80

40

6.4 ._

36

25

07/09

14/59/cKl,o

45,20

151,lO

37

29

07/09

23/17/28.0

52.66

159.72

35

38

30

08/09

01/06/07.2

53.01

159.67

103

39

32

08/09

06/47/37.0

43.40

146.40

40

39

08/09

14/18/42.0

45.10

150.30

40

41

42

08/09

19/37/24.0

44.50

148.20

30

._

_

334

A.A.

OSTKOVSKY

AND

A..4

IHIKAV

I‘S’l~‘i

TABLE 2 (continued) Number Due~~b~er et oi. (19871 42 9 43 10 44 11 45 12 46 14 47 15 4x 16 49 17 50 18 51 19 52 20 53 21 54 22 55 23 56 24 57 25 58 26 59 27 60 28 61 29 62 30 63 31 64 32

Date

13/w 14/09 26/09 26,/09 26,‘09 03/10 04/10 07/10 16/10 17/10 20/10 20/w 21/10 22/10 23/10 28,‘lO 30,/10 04/11 04/l 1 10/l 1 ll/ll 04/11 04/11

Time (GMT) fh/min/s)

Iat. N

18/03/35.X

37.544 43.476 50.053 47.015 48.059 42.038 s1.435 32.190 34.532 49.632 36.596 33.613 52.732 43.512 47.340 46.307 Sl.SlO 44.045 38.568 44.208 44.255 44.190 44.181

11,‘37,‘22.4 01/09/28.5 04/46/37.x 20/03/14.07 23/29/27.9 07/46/52.8 21/57/03.6 03/02/55.9 18/ 12,W.O 11/23/04.0 19/23/ 10.0 i l/10/34.7 02/28/38.0 11/40/57.6 18/30/30.7 16/25/10.3 09/29/53.2 H/54/13.0 07/36/23.0 01/55/.X.8 02/01/13.4 02/52/08.6

average velocity V,, Vs against apicentral distance (Fig. 6a, b). These results were also compared with the data given by Walker and McCreery (1987). Figure 6a also shows the average velocities which we calculated from the data on apparent velocities obtained by Shimamura and Asada (1976). The accuracy of the average velocity estimates is about 4% for X = 4” and 1% for X = 20”. Indeed the lo-* stability of the quartz generators used in the OBSs for the time marks permits the P- and S-wave onset times to be measured with an accuracy of 0.1 and 0.3 s, respectively. Thus the main error in the time measurements comes from the inaccuracy in measuring the time at which the earthquake beg& This error is about 1 s, that is about 1.5%. Thus the maximum error for the estimated T is 1.5% at X = 4”. The accuracy of the X measurements depends on the accuracy of dete~ation of the epicenter coordinates and that of the OBS location. The accuracy with which the epicenters can be located

Long. E

141.335 140.153 158.798 152.289 153.769 140.818 176.620 142.327 139.602 155.892 141.195 140.949 172.132 146.840 153.360 144.100 157.366 148.040 143.401 149.482 149.472 149.523 149.481

Depth (km) 5x 707 44 112 33 124 38 33 121 47 53 57 33 36 33 307 33 39 33 33 33 33 33

M

-

~5.1 “.t 5.5 5.6 3.4 4.7 5.5 4.8 4.9 5.S 4.7 4.9 4.9 5.0 4.7 4.4 4.9 5.7 5.4 5.0 5.4 5.0 5.3

in the Kurill Island region is an average I10 km (Gusev, 1974); that is about 2% for X = 4”. The accuracy with which the OBS locations are known depends on the accuracy of navigation of the vessel (about 1 km - 0.2% for X = 4” 1, and the distance that the buoy may be shifted horiaxmtahy from the OBS (see Fig. 21, which is also about 1 km. Thus, the distance is measured with an accuracy of about 2.5%. Hence, the rn~~ error in the estimate of V is about 4% for X- 4” and 1% for 28’; ~~~~ndi~ to about 0.3 km/s and 0.1 km/s in the velocity estimates, respectiveiy. This accuracy is sin&u to that estimated by Wafker and McCreety (1987). The data obtained make it possibIe to supplement considerably the set of average velocities within the epicentral distance range of about 4” to 15”(Fig. 6a,b). It is clear from these plots, that within this range the average veiocities of P- and S-waves (7.5-8.8 and 4.3-5.2 km/s) show a considerablc scatter. Then, up to a distance of about 35”, both velocity dispersions decrease and con-

GENERAUZED

335

SEISMIC PROPERTIES OF THE LITHOSPHERE, NW PACIFK BASIN

8. 8-

.8-

.6-

25

30

35

Fig. 6. The variation of average veto&y with epicentral distance: (A) for P-waves; (b) for S-waves. I = the average velocity data obtained during cruises 21,23 and 29 of the R/V “Dmitriy Mendeleeu” and drillhole data (Duennebier et al., 1987); 2 = data from Shimamura and Asada (1976); 3 = data from Walker and McCreery (1987); 4 = the generalized velocities.

verge to average vaIues of 8.2 f 0.1 and 4.7 f 0.05 km/s, respectively. Thus, a certain depth interval exists, within which a considerabiy greater scatter of average velocities is observed. In Figure 6a,b this interval is indicated by vertical lines. Within this range, two velocity groups may be distinguished; with relatively higher and lower values. Averaged lines give the following values for the velocities of P- and S-waves for each group: 8.6 f 0.17; 8.0 f 0.17 and 5.1+ 0.07; 4.6 f 0.13

km/s, respectively. It should be noted that no group with relatively high velocities has been found in previous investigations, and that the difference between the averaged values of the two groups is much higher than the accuracy of the measurements. Travel-time curves of first arrivals of P- and S-waves were plotted against the epicentral distance in the range from 500 to 2200 km to determine apparent wave velocities. The travel-time

336

curves were supplemented with the data given by Rykunov et al. (1984) and Duennebier et al. (19871. The composite travel-time curves are shown in Figure 7. The apparent velocities for both types of wave within the distance ranges of 400 to 800, 800 to 1600 and 1600 to 2300 km are 8.0 and 4.55; 8.3 and 4.72; 8.75 and 4.9 km/s, respectively. Average apparent velocities for Pand S-waves are 8.3 and 4.7 km/s, respectively. The Jeffries-Bullen travel-time curve for P-waves, as shown on the graph, lies 4 to 8 set above the composite one. Thus, we can infer a relatively high-velocity upper mantle under the northwestern Pacific Basin as compared with the velocities of the generalized model. This observation was also made by Rykunov et al. (1984). A relatively high-velocity upper mantle (down to a depth of about 150 km) was detected also from the data of seismic tomography (Woodhouse and Dziewonski, 1984). hlysis of seismic veIocity anisotropy in the li~~~he~ It was m~nti~ed earlier that the highest scatter of average velocities (7.5-8.8 and 4.3-5.2

km/s) falls within the distance range from 400 to 1500 km, which corresponds to depths of between about 40 and 130 km (Shimamura et al.. 19751. The high scatter of the average velocities in the upper mantle under the northwestern Pacific may be due to a variety of causes. These include for instance, large horizontal and vertical inhomogeneities, different states of stress within the moving Iithospheric plate, or its age. To study the effect of azimuthal anisotropy in the upper mantle we plotted the diagram shown in Figure 8. It illustrates the dependence between the average velocities of the P- and S-waves and the direction of their propagation. The data from Table 2 were used in constructing this diagram. Maximum values for compressional and shear wave velocities (averages of 8.6 and 5.0 km/s, respectively) are found in the NNW direction; that is, perpendicular to the Mesozoic magnetic lineations and parallel to the system of crustaf faults. ~i~imurn velocity values (averages of 7.8 and 4.5 km/s, respectively) were found in the WNW direction. Inte~ediate velocities (averages of 8.2 and 4.6 km/s, respectively) correspond to the SW and NE (few data) directions. The results show that the lithosphere has a considerable anisotropy of

Fii. 7. Travel-time curves for P- and S-waves: I - cruises 21 and 23 of R/V “Dmitriy hacndekev”; 2 = Cruise 29 of R/V “LWtriy Mendefeeu”; 3 = drillhole data (D~ancbicr et al., 1987); 4 - Jeffrics-B&n travel-time CUIVC.

GENERALIZED

140"

SEISMIC

PROPERTIES

160"

OF THE

LITHOSPHERE,

NW

180"

50"

50"

40"

40"

30"

30" 140"

160"

180"

Fig. 8. Generalized map illustrating the azimthal anizotropy of seismic velocities in the upper mantle of the northwestern Pacific Basin. 1= data of cruises 21, 23, and 29 of the R/V “Dmitriy Mendeleeu”; 2 = major crustal faults; 3 = boundaries of the Pacific plate; 4 = main tectonic elements; 5 = linear magnetic anomalies.

PACIFIC

337

BASIN

Two ranges of velocity variations for P- and S-waves were obtained in the present study on the basis of a statistical analysis of the suite of average velocity data points, namely 8.0 k 0.17 and8.6 k 0.17and4.6 k 0.13and5.1 k O.O7km/s, respectively. The authors suggest that these values probably represent two distinct ranges of propagation velocities of P- and S-waves in the lithosphere of the region under study. The assumption of the presence of a low-velocity wave-guide is an important feature of many models of the upper mantle structure. The hypothesis of the existence of such a wave-guide has been considered in many papers devoted to the problem of the anomalous propagation of highfrequency P,- and &,-waves over large distances (Sutton and Walker, 1972; Walker, 1977). Recently, new papers have been published which

P - VELOCITY, 7.5

compressional and shear velocities (7% and lo%, respectively throughout a wide depth range. Japanse data also indicate that the anisotropy of the apparent velocities in the northwestern Pacific reaches 4-7% and extends down to a depth of about 140 km (Shimamura et al., 1983). Maximum values for the apparent velocities of p-waves (8.4-8.8 km/s) were obtained by the cited authors in a general north-south direction (near the Japanese Islands), whereas minimum values (7.9 km/s) were obtained in a general east-west direction. Comparison of models Various models of the upper mantle structure of the analysed region have been simulated on the basis of seismological observations, as well as from the experimental data of explosion seismology. These models differ considerably. The models of Odegard and Sutton (19721, Schlue and Knopoff (19771, Shimamura and Asada (19761, Shimamura et al. (1983), Rykunov et al. (1984), and Neprochnov et al. (1989) are shown in Figure 9. It is clear that the models differ most markedly within the depth range of 30-130 km.

8.0

a.5

I

I

km/s 9.0 9.5 I

I

50

1oa

250

300

350

Fig. 9. Velocity models based on data from different authors; I = Neprochnov et al., 1989; 2 = Rykunov et al., 1984; 4 = Odegard and Sutton, 1972; 5 = Schlue and Knopoff, 1977; 6 = Shimamura and Asada, 1976; 7 = Shimamura et al., 1983.

A.A.

OSTKOVSKY

AND AA

!iIJiIAV’I‘S~,\’

8.8 8.6 t a

8.4 ”

c

8.2

zi d 8.0 > 0: 7.8> 7.6 ’ I 5

7.41

I 15

I

10 A

Q) 5.22 Y 5.0 i

4.8-

is ,iii

4.6-

I 20

I 25

I 30

1 35

(DeelrpEs)

__-----,........................................ . . . _----_--

c

._---I---. . . . . . . ..*...-..I..“...-...---... _----_--

CiJ 4.44.21

I 5

I 10

t

I

I 25

I 30

I 35

Fig. 10. Velocity models for the upper mantle, obtained by analysis of the average velocities of P_(a) and S-(b) waves. See description in the text.

explain the existing data while avoiding the hypothesis of such a wave-guide (Serene and Orcutt, 1987). Piires 10a,b show various models obtained from the analysis of the average veiocities of P- and S-waves: Walker (1981) - model 1, Sereno and Orcutt (1987) - model 2, and also models A,A’, B,B’, and C,C’ basedon the data of the present paper. It is clear from these figures, that model 1 shows two discontmuities in the curves of the average velocities against diatanoe; in the near (less than 12”) and remote (over 300)

zones, which, as Walker (1981) explains, are caused by the wave-guide prop&es of the Iithosphere in the ~o~hw~~~rn Pacific. Model 2 (Serene and Orcutt, 1987) displays only a single discontinuity (at 3OV, which can be expi&ed by the shadow effect of the asthenospkere with a low Q-factor. The 1node18 proposed in the present paper explain the upper-mantle structure in a different way. An an&& of the new data presented here and the recu&ion of a strong azimutal velocity anisotropy in the lithosphere of

GENERALIZED

SEISMlC PROPERTIES OF THE LITHOSPHERE. NW PACIFIC BASIN

the northwestern Pacific Basin allows us to suggest that a low-velocity wave-guide exists only for some “high-velocity” azimuths while it is absent for other “low-velocity” azimuths. This approach may resolve the contradictions in the above-mentioned models on the structure of the upper mantle. Conclusions The seismological data, obtained during cruise 29 of the R/V “Dmitriy Mendeleeu”, their comparison with the data of cruises 21 and 23 of the same R/V, and data from other sources, shed new light on the structure of the upper mantle of the northwestern Pacific Basin and on the velocity anisotropy within it. Some 60 near and distant earthquakes were analysed, with their epicentral distances ranging from 400 to 2500 km. An analysis of the average velocities of P- and S-waves showed that their values exhibit considerable variation; that is from 7.5 to 8.8 and from 4.3 to 5.2 km/s, respectively. The greatest dispersion of the velocity values analysed falls within the distance range of about 400 to 1500 km. The data presented here show that relatively high velocities are present in this distance range. The new data also indicate that the lower lithosphere boundary lies at a depth of about 130 km. The generalized apparent velocities of P- and S-waves are 8.3 and 4.7 km/s, respectively. An analysis of the anisotropy in the azimuthal velocity in the upper mantle (near the Kuril Islands) showed that the P- and S-waves exhibit m~mum velocities (8.4-8.8 and 4.8-5.2 km/s) in a general north-south direction, whereas minimum velocities (7.6-8.0 and 4.4-4.6 km/s) were measured in a general east-west direction. The anisotropy in the velocities of P- and S-waves reaches respectively 7 and 10% over a wide range of epicentral distances from about 400 to 1500 km. The occurrence of a strong velocity anisotropy in the lithosphere allows us to suggest that a low-velocity wave-guide exists under it only for some “high-velocity” azimuths, while such a wave-guide is absent for “low-velocity” azimuths. This approach makes it possible to resolve con-

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tradictions in some earlier models of the structure of the upper mantle in the investigated region. Acknowledgements The authors wish to thank Yu.P. Neprochnov for making available the new OBS data obtained during the 29th cruise of R/V “Dmitriy Mendeleev”. OBS records were obtained with the collaboration of V.V. Sedov, A.A. Pokryshkin, B.V. Kholopov, N.N. Kichin. References Duennebier, F.K., Lienert, B., Cessaro, R., et al., 1987. Controlled-source seismic experiment at hole 581. Init. Repts. DSDP, 91, Washington (U.S. Govt. Printing Office) pp. 10.5-125. Gusev, A.A., 1974. The errors in the dete~ination of the parameters of the Kamchatka earthquakes Sources. In: Seismicity and Seismic Forecast, upper mantle properties and its connection with the volkanism of Kamchatka. Science, Novosibirsk, pp. 66-81 (in Russian). Neprochnov, Ju.P., Sedov, V.V. and Buravtsev, A.A., 1989. Results of new seismic lithosphere experiment in the North-Western Basin of the Pacific ocean. Oceanologiya, 29: 599-607 (in Russian). Odegard, M.E. and Sutton, G., 1972. Preliminary report on the Cannikin Airborne Seismic Experiment. Paper presented at the annual meeting of the Seismological Society of America. Hawaii Inst. Geophys., pp. 2-8. Rykunov, L.N. and Sedov, V.V. 1967. Bottom Seismograph, Izv. AN SSSR. Fiz. Zemli (Solid Earth) 8: 83-87 (in Russian). Rykunov, L.N., Neprochnov, Ju.P., Sedov, V.V., et al., 1984. Structure of the lithosphere according to the data of seismological investigations on the geotraverse the South Kuril Islands-Shatsky Rise. In: Structure of the Bottom of the Northwestern Part of the Pacific Ocean. Nauka, pp. 106-114 (in Russian). Schlue, J.W. and Knopoff, L., 1977. Shear-wave polariztion anisotropy in the Pacific basin. Geophys. J. R. Astron. Sot., 49: 145-165. Shimamura, H. and Asada, T., 1976, Apparent velocity measurements on an oceanic lithosphere. Phys. Earth Planet. Inter., 13: 15-22. Shimamura, H., Tomoda, Y. and Asada, T., 1975. Seismograph c observation at the bottom of the Central Basin Fault of the Philippine Sea. Nature, 253: 177-179. Shimamura, H., Inatani, H., Asada, T., et al.,, 1983. Long-shot experiments to study velocity anisotropy in the oceanic lithosphere of the northwest Pacific. Phys. Earth Planet Inter., 31: 348-362.

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