The Fiji Region Includes Three Back-Arc/Marginal Basins Known As South Fiji Basin, Lau Basin, and North Fiji Basin (Fig
J. Phys. Earth, 34, 407-426, 1986
UPPER MANTLE VELOCITY OF FIJI REGION FROM SURFACE WAVE DISPERSION
Kandiah SUNDARALINGAM
Department of Physics, The University of the South Pacific, Suva, Fiji (Received May 12, 1986; Revised October 17, 1986)
Single station group velocities over the period range 15-100 s are computed for nine fundamental mode Rayleigh wave propagation paths that cross tectonic provinces of the Fiji region. These computations are based on recordings of the earthquakes around the Fiji Islands by the WWSSN (World Wide Standard Seismic Network) stations AFI (Afiamalu), HNR (Honiara), and WEL (Wellington). The inversion of the derived dispersion curves for shear velocity-depth structure shows that uppermost mantle velocity beneath South Fiji basin, Lau basin, and North Fiji basin is slightly lower than that derived for the Pacific ocean region of similar age by Yu and MITCHELL(1979). The uppermost mantle lid is almost absent beneath these marginal basins, and the shear velocity is between 4.0-4.3 km/s in the depth range from Moho down to 220 km.
1. Introduction The Fiji region includes three back-arc/marginal basins known as South Fiji basin, Lau basin, and North Fiji basin (Fig. 1). The South Fiji basin is one of a series of marginal basins west of the Tonga- Kermadec Island arc-trench system (KARIG, 1970). It lies to the north of New Zealand and is bounded by the Lau-Colville ridge to the east, the Hunter fracture zone to the north and the Loyalty rise and Three Kings rise to the west. The basin is underlain by oceanic crust (SH©Ret al., 1971) and was considered by KARIG(1970) to be formed by crustal extension in Oligocene times, with the Three Kings rise and Loyalty rise being interpreted as remanent arcs. Magnetic anomalies had been studied by WEISSELand WATTS(1975) and WATTSet al. (1977). The anomalies 7A through 12 (corresponding age of 26-33 m.y.) had been identified in the northern part of the basin. The Lau basin is surrounded by two island chains, the Tonga Island arc to the east and the Lau ridge to the west. It is elongated in the N-S direction and resembles an inverted triangle. Investigations carried out so far show that the Lau basin was formed by rifting apart of the former Lau-Tonga ridge between the fore-arc and the volcanic arc in the early Pliocene (PACKHAM,1978). It is a region of high heat flow (SCLATERet al., 1972) and the uppermost mantle beneath has low Q (BARAZANG1et al., 1974).
407 408 K. SUNDARALINGAM
The North Fiji basin is bordered by the Vanuatu Islands, the Fiji Islands, the Vitiaz trench, and the Hunter fracture zone. It is one of the only sites where multiple back-arc basin spreading centres have been identified directly. Furthermore, the North Fiji basin is characterized by an extraordinarily high level of shallow seismicity. MALAHOFFet al. (1982) proposed that during the Eocene and Oligocene, Fiji and the Vanuatu were parts of a continuous island-arc system developing near a major plate boundary-the Vitiaz-Samoan lineament. During the early Miocene, sea floor spreading began behind the Fiji-Vanuatu arc similar to that presently occurring in the Lau basin behind the Tonga arc. Spreading caused the arc to migrate southward and brought the Fiji segment into collision with the Lau-Tonga ridge, resulting in closure and failure of the entrapped part of the subduction zone. Continued spreading progressively displaced the Vanuatu arc southwest with clockwise rotation about a pole located approximately at 8S, 167E. The Vanuatu segment, separated from the Fiji segment by transform faults-the Fiji fracture zone and the Hunter fracture zone-subsequently migrated southwestward to its present position. The heat flow beneath most of the North Fiji basin is very high (MACDONALDet al., 1973). Previous seismic velocity studies in these marginal basins are restricted to crust and sub-Moho structure (e.g., OFFICER,1955; HUNKINSand Kuo, 1965; DuBois, 1971; DUBOISet al., 1973). The upper mantle velocity structure in the Tonga- Kermadec Island arc region has been studied by KAILAand KRISHNA(1978) using P- and S-waves travel times. As it is found that the thickness and seismic velocity of the lid above the low velocity layer (LVL) and the LVL are a good indicator of the tectonic regime of the region, the present study aims to work out the above parameters, especially for the three marginal basin areas, using surface wave dispersion. Unfortunately there are not many long-period seismographs in operation in the Southwest Pacific and therefore our study is restricted to the records from the WWSSN stations AFI, HNR, and WEL. Further as none of the interstation paths cross the basins, the data for basin structural studies are confined to single station group velocities. Similar studies have been carried out by a number of scientists elsewhere to determine crust and upper mantle structures (e.g., CHAN and MITCHELL,1985; DORBATHand MONTAGNER,1983; CLARK,1983; KAWASAKIand KON'NO, 1984). KNOPOFFand CHANG(1977) suggested that inversion of group velocities with small uncertainty bounds is able to provide acceptable resolution in shear velocity-depth models. Furthermore an attempt was also made to compute Rayleigh wave inter- station velocities between the stations AFI-WEL with the objective of studying the effect of subduction on surface wave dispersion.
2. Procedure
Ten shallow focus earthquakes in the magnitude range 5.0-6.0 (ML) were selected for interstation and single station analysis (see Table 1). Figure 2 shows the Upper Mantle Velocity of Fiji Region 409
Fig. 1. Tectonic setting of the basins in South West Pacific. Large arrows indicate direction of relative plate convergence . Contour line shows 2-km isobath. Holocene volcanoes are indicated by open triangles . The figure is from HAMBURGERand ISACKS(1985). location of the earthquakes, except No. 3, as determined by the International Seismological Centre and the location of the seismic stations used in this study . The seismograms recorded on the LPz components at the WWSSN stations were photographically enlarged from the film chips to enable them to be digitized at intervals of less than 2 s. A few of the seismograms used in this study are shown in Fig. 3. The analysis of the data is based on the programs EXSPEC , FILTR and SURFACE described by HERRMANN(1978) and is restricted to the fundamental mode Rayleigh waves. 410 K. SUNDARALINGAM
Fig. 2. Map of the South West Pacific region showing locations of WWSSN stations (three letter codes) and the epicentre of the earthquakes used in the present study. Propagation paths of Rayleigh waves are shown by curves joining epicentres to the stations.
2.1 Group velocity computation The first program EXSPEC performs the Fourier spectral analysis on the digitized time series. It removes DC and linear trends as well as the response of the seismographs. The program FILTR uses a narrow band-pass filtering technique to determine the spectral amplitudes of individual frequencies making up the surface wave signal, using the output from EXSPEC. The spectral amplitudes are contour- ed in period/group velocity space to give an indication of whether the group veloc- ities obtained are at those periods which have energy above the noise level. This was confirmed by studying the amplitude spectrum from EXSPEC program. Furthermore, a clear trend in the contour was sought, where the contours were too Upper Mantle Velocity of Fiji Region
Table 1. Earthquakes used in this411 study.
Fig. 3. A few seismograms used in the present study. broad readings in those periods were assumed to be unreliable and not considered in this study. Group velocities corresponding to the maximum energy arrival at given periods are shown by `square' notation and the values were printed out. A few of the group velocity contours are shown in Fig. 4. 412 K. SUNDARALINGAM Fig. 4. Group velocity contour from multiple filtering technique analysis of a few seismograms. Upper Mantle Velocity of Fiji Region 413
2.2 Phase velocity computation For event 3, interstation phase velocities in the period range 25-100 s were computed from the phase angle of arrivals at the stations WEL and AFI of different periods (see KNOPOFF,1983). Figure 2 shows this interstation path. The azimuth difference between the propagation paths from the event 3 to the two stations is almost zero.
2.3 Anelasticity correction
Liu et al. (1976), Yu and MITCHELL (1979), and MITCHELL and Yu (1980) studied the effects of anelasticity on phase and group velocity dispersion. They
showed that a correction d C(T) to the phase velocity C(T), has to be used and is
given by:
AC(T)=-C(T)ln(TO/T)ƒÎQ(T)
where Tis the period at which the correction is made, To is the reference period, and
Q is the quality factor for Love or Rayleigh wave attenuation. The correction for group velocities U(T) can be calculated from ĢC(T) by
Using a reference period of TO = 50 s, as had been used by Mitchell and Yu in 1980 (we have taken 50 s, as it lies centrally within the period range of our data), and the Q(T) values given by CANAS and MITCHELL (1978) for the Pacific ocean region
of similar age (the Q(T) values for the marginal basins are not worked out yet) it is
found that ƒ¢C is of the order of 0.1 to 0.2% in the period range 15 to 90 s. This is well within the error range for the determination of phase velocity (•}0.03 km/s) and
therefore it is ignored. Similarly the group velocity anelastic correction is also found to be insignificant.
3. Interpretation The inversion procedure involved compares the observed phase and group velocity data with dispersion curves calculated for specific models, and the model parameters are varied until an acceptable fit is obtained. L_??_V_??_QUE(1980) showed how important the starting model is in determining the final model. Thus considerable care was taken to obtain a well-constrained starting model and the following steps were taken to set up the starting model: 1. The average thickness of the water layer along the profile was taken from a bathymetric map of the region. 2. The crustal structure was determined from controlled source seismic studies on or near the profile (Fig. 5). 3. The velocity of the LID was taken as that of the sub-Moho velocity, Pn, from controlled source seismic studies. 414 K. SUNDARALINGAM
CRUSTAL STRUCTURE: P-velocity (km s-1)
Fig. 5. Crustal structures, from controlled source investigations, of P-wave velocities. See text for references.
4. The thickness of the lid was estimated from the age of the region using the results of KNOPOFF(1983). 5. The LVL was assumed to extend from the bottom of the lid to a depth of 220 km (see ANDERSONand REGAN,1983). 6. The S-wave velocity of the LVL was taken as 4.3 km/s as obtained in several other studies (see for example, KNOPOFFet al., 1975). 7. The S-wave velocity for the depth range 220-400 km and that for the semi- infinite layer below 400 km was taken from the earth's structure 'PREM' (DZIEWONSKIand ANDERSON,1981). 8. Where it was necessary the propagation paths were decomposed into two parts according to the age and crustal structure of the lithosphere. For each part assumptions in 1-7 above were then applied. The theoretical dispersion curves for the full paths were then computed by adding the computed phase and group velocities of two parts from SURFACE, according to the formulae Upper Mantle Velocity of Fiji Region 415
1/U=r/U1+(1-r)/U2
and 1/C=r/C1+(1-r)/C2 where r is the portion of the path with structure of one type, and (1 —r) is the portion of the path with structure of the second type. As most of the data analysed was in the 15-90 s period range, the effect of the layers deeper than 220 km is small and it is justifiable to use standard values for these depths. In this study, values from `Preliminary reference Earth model' of DZIEWONSKIand ANDERSON(1981) were used. The theoretical dispersion curves for a given model were calculated using the program SURFACE, with the correction for sphericity (HERRMANN,1978; NORTHand DZIEWONSKI,1976). There are disadvantages in constructing a model with a large number of layers, each specified by several independent physical quantities, as the inversion becomes highly non-unique. Furthermore, if the model has an excessive number of layers over, any depth range, instability in the inversion process, which takes the form of large unreasonable variations in the parameters describing adjacent layers, occurs. As such the layers, lid and LVL are selected as they are with the option of introducing sublayers, if necessary, to improve the matching to satisfy our criteria for an acceptable model. The matching procedure consisted of first changing the S-wave velocities of the lid and the LVL in turn until a better fit was obtained for the observed dispersion curve at the longer period range (> 30 s). The boundary of the lid-LVL was also varied to improve the match. Having matched the longer periods, the thickness of the sedimentary layer and the crust were varied to obtain a better fit in the 15-30 s period range. This is justified on the grounds that the crustal structure from the explosion study is that of a small portion of the path and not that of the full path in a tectonic province. As stated above, wherever possible the starting model of the crustal structure was taken from published explosion seismic studies. These studies usually de- termined P-wave models only. For the necessary S-wave velocities the P/S ratio from the earth's structure 'PREM', and for densities NAFE-DRAKE(1957) curves were used.
3.1 Errors NAKANISHI and ANDERSON(1984) presented a very detailed analysis of errors involved in the computation of phase and group velocities of Rayleigh and Love waves. They identified the main sources of errors as origin time, epicentral locations, source depth and source mechanism. For single station Rayleigh wave group velocities it is estimated that the total rms error, from the factors considered above, introduced in the travel time is about 7.4 s and that for Rayleigh wave phase velocity is 6.8 s. By using the above error value in the travel time, the rms error in the 416 K. SUNDARALINGAM
group velocity of those profiles starting from (i) WEL station is estimated to be about 0.04km/s, (ii) AFT station is estimated to be about 0.08 km/s, and (iii) HNR station is estimated to be about 0.05 km/s. But the error at a given period could be as much as twice the above value. On this basis we ensured that the rms deviation between the mean data points and the best theoretical values are less than the above estimate of error.
4. Results
Figure 6 shows the dispersion curves computed and predicted from the models shown in Figs. 5 and 7. We will discuss that of each path in turn.
4.1 WEL-Loyalty events Events 1 and 2 were used to compute fundamental mode Rayleigh wave group velocity for this path. Figure 6(A) shows the result of the observations and the theoretical curve. The path was decomposed into two parts, 39% of the full path falling within New Zealand and the remainder falling within the South Fiji basin. The crustal structure for the New Zealand path is taken from DAVEY and BROADBENT(1980) and for South Fiji basin (SFB. 1, Fig. 5) is taken from SHORet al. (1971). The New Zealand upper mantle (see Fig. 7) was taken as similar to that beneath Lord Howe rise and Norfolk ridge (cf., SUNDARALINGAMand DENHAM, 1985). The best model obtained for this profile shows that beneath the South Fiji basin (SFB. 1, Fig. 7) the lid shear velocity is low, 4.35 km/s, and the LVL shear velocity is very low, 4.05 km/s. The mean of the residuals between the observed and predicted from the final model is 0.035 km/s and the largest difference between the two is 0.056 km/s, at 40 s period. A comparison of the observation points with that obtained for four regions of different age groups of Pacific ocean (Yu and MITCHELL,1979; see Fig. 8) shows that in the period range 40-70 s our observation points coincide with those of region 1 (0-20 m.y.). The upper mantle shear velocity values reported by Yu and Mitchell for region 1 are similar to those deduced for South Fiji basin.
4.2 WEL-Hunter fracture events Events 7 and 10 were used to compute the fundamental mode Rayleigh wave group velocity for this path. Figure 6(B) shows the observations and the theoretical curve from the best fitting model. The path was decomposed into two parts, 28% of the full path falling within New Zealand and the remainder falling within the South Fiji basin. The crustal models are taken from the same references as those of the above path. The best model obtained for the profile, as before, shows that beneath the South Fiji basin (SFB. 2, Fig. 7) the lid shear velocity is low, 4.30 km/s, and the LVL shear velocity is also low, 4.15 km/s. The mean of the residuals between the observed Upper Mantle Velocity of Fiji Region 417
A D
B E
C F Fig. 6. Rayleigh wave dispersion obtained for six different profiles and theoretical curves computed from the models deduced (refer: Figs. 5 and 7). and predicted from the final model is 0.036 km/s and the largest difference between the two is 0.065 km/s, at 30 s period. A comparison with Pacific regional values, as before, shows that in the period range 35-95 s our observation points lie between those of regions 1 and 2.
4.3 WEL-Lau event Event 5 was used to compute the fundamental mode Rayleigh wave group 418 K. SUNDARALINGAM
UPPER MANTLE STRUCTURE : S-Velocity (km s-1)
Fig. 7. Upper mantle shear velocity structure for various regions of South West fic, deduced in the present study. Pac velocity for this path. Figure 6(C) shows the result of the observations and the theoretical curve. The path was decomposed into two parts, 20% of the full path falling within New Zealand and the remainder falling within the Lau-Colville ridge. The crustal information for the Lau-Colville ridge comes from SHORet al. (1971). The best model obtained for the profile shows that beneath the Lau-Colville ridge the lid shear velocity is very low, 4.05 km/s, similar to that found underneath mid-ocean ridges. The LVL shear velocity is similar to that of above two profiles, 4.25 km/s. The mean of the residuals between the observed and predicted from the final model is 0.038 km/s and the largest difference between the two is 0.10 km/s, at 35 s period. Though this largest difference is little over the maximum difference, 0.08 km/s, allowed for this path it is occurring at this period 35 s only (sharp rise) and must be due to some interference. Comparison of the observation points with that of Pacific regional points shows that from 15-60 s our observation points are much lower than those of any of the Pacific region and between 60-80 s between those of regions 1 and 2. At present, earthquakes are occurring beneath Lau-Colville ridge at depths of 400-700 km. Upper Mantle Velocity of Fiji Region 419
4.4 WEL-AFI Event 3 was used to compute the interstation fundamental mode Rayleigh wave phase and group velocities for this path. Figure 6(D) shows the observations and the best-fitting theoretical curves that could be found. The observed group velocity dispersion looks different from that of the theoretical curve, whereas the phase velocity dispersion agrees closely. The mean phase velocity residuals between the observed and predicted from the final model is 0.021 km/s and the largest difference between the two is 0.045 km/s, at 95 s period. Examination of the group velocity contour (see Fig. 4) shows that group energy pattern was disturbed while passing from WEL to AFI. The phase spectrum from EXSPEC was almost smooth at both stations. The path was decomposed into two parts, 15% of the full path falling within New Zealand and the remainder falling within the Tonga-Kermadec ridge. The crustal information for the Tonga-Kermadec ridge comes from SHORet al. (1971). Because of the presence of the Pacific subduction layer underneath this profile at a shallow depth (earthquakes are occurring at a depth of about 70 km), doubt has been expressed about the validity of interpretation of the above dispersion in terms of plane parallel flat layers. Bearing this limitation in mind, the best model obtained for the profile shows that beneath the Tonga-Kermadec ridge the uppermost mantle shear wave velocity is higher than that of Pacific ocean region older than 100 m.y.
4.5 AFI—Yasawa events Events 6 and 9 were used to compute the fundamental mode Rayleigh wave group velocity for this path. Figure 6(E) shows the result of the observation and the theoretical curve. The path was decomposed into two parts, 20% of the full path falling within Pacific ocean (> 100 m.y.) and the remainder falling within the Lau basin. The crustal information for the Pacific ocean is taken from SHORet al. (1971) and for the Lau basin is taken from LARUEet al. (1982). The uppermost mantle of Pacific ocean (see Fig. 7) was taken as similar to that given by Yu and MITCHELL (1979) for region of age > 100m.y. The best model obtained for the profile shows that beneath the Lau basin the uppermost mantle shear velocity is very low, 4.0 km/s, from Moho down to a depth of 220 km. The mean of the residuals between the observed and predicted from the final model is 0.036 km/s and the largest difference between the two is 0.083 km/s, at 25 s period. Along this profile occurs a zone of shallow seismicity, identified with left-lateral shear zones (EGUCHI,1984) extending from north of the Fiji Islands to the northern end of the Lau basin.
4.6 HNR-Uasawa events Events 4 and 8 were used to compute the fundamental mode Rayleigh wave group velocity for this path. Figure 6(F) shows the result of the observations and the theoretical curve. As the Rayleigh waves propagated from the epicentres of events 4 and 8 to the HNR station they would have passed through the North Fiji basin, 420 K. SUNDARALINGAM
Vanuatu Islands, Vanuatu trench, Solomon trench and finally the Solomon Islands. Therefore one should expect some complication in the energy propagation. This was not obvious when we examined the group velocity energy contour (see Fig. 4) and the seismograms (see Fig. 3). Multiple arrivals were present in the seismograms, but they are only in the short period range. The path was decomposed into two parts, 32% of the full path falling within Solomon arc and the remainder falling within the North Fiji basin. The crustal information for the Solomon arc was taken from COOPERet al. (1984) and for the North Fiji basin was taken from SUTTONet al. (1971). The uppermost mantle of the Solomon arc (see Fig. 7) was taken to be similar to that given by KRISHNAand KAILA(1984). The best model obtained for the profile shows that beneath the North Fiji basin the uppermost mantle shear velocity is very low, 4.0 km/s, from the Moho down to a depth of 220 km. The mean of the residuals between the observed and predicted from the final model is 0.038 km/s and the largest difference between the two is 0.08 km/s, at 80 s period. At periods longer than 80 s period there was not much en- ergy in the spectrum, therefore the observed data at these longest periods are not very reliable. The limitation on this model comes about from the simple structure assumed for the Solomon arc portion of the path.
5. Discussion One must appreciate the problem of non-uniquenessinvolved in the inversion of surface wave dispersion (BACKUSand GILBERT,1968, 1970). The problem is tackled in this paper by putting constraints on a number of parameters using elastic wave velocitiesand layer thicknessesobtained by other geophysicalstudies of the area, as well as by putting bounds on the rest of the parameters based on the `extreme-results' obtained by geophysical investigations of tectonic areas of the world. The fine structure or the continuous variation in properties with depth can- not be resolved.However, we have worked out a shear velocity for each layer by inverting observed data, which must be taken as an average velocity over a depth range covered by the layer as well as over lateral distance covered by the propagation path. Furthermore the presence of regional boundaries, especially the continental boundary within the propagation path can cause refraction and multiple pathing (MCGARR,1969; CAPON, 1971) but there would not be any change of wavetype as there is with the Love wave. Fundamental mode Rayleigh waves incident at a continental boundary at periods of about 30 s are transmitted almost entirely as fundamental mode Rayleighwave (LYSMERand DRAKE,1971; DRAKE, 1972). In the case of a continental boundary the period of the wave that begins to suffer such refraction effectsis about 25 s. However, by using filtering and windowing tech- niques one can remove multiple arrivals provided they are physically separated. This method involves examination of initial print-out from multiple filtering techniques and determination of the periods within which wave energy is higher Upper Mantle Velocity of Fiji Region 421
Fig. 8. Rayleigh wave group dispersion deduced for four different age group c ocean by Yu and MITCHELL(1979). regions of the Pcifi
than the noise level. If there was more than one arrival of packet-of-energy at any given periods, such late-arrivals are removed by selecting the trace-length accord- ingly. One other source of errors at shorter periods of 20-30 s is source process of fault rupturing. This effectively delays arrival of wave energy propagation. A calculation of change in the total propagation path length from the great circle path due to refraction, at a boundary separating two tectonic provinces, was carried out using fundamental mode Rayleigh wave velocities of typical tectonic provinces of this region, in the period range of this study. It shows that the effect on the observed velocity values is to lower it by a small amount, about 0.5%, which is well within the limits of experimental error. As mentioned by SouRIAU (1985), the most reliable data are those that are as pure as possible, that is, those that oversample single tectonic region without oversampling the other ones, because the great circle surface waves introduced a coupling between those propagating the different tectonic regions. In this study the tectonic provinces under investigation had larger fractions of the total paths from the epicentres to the stations. We have interpreted the dispersion curves using a plane parallel horizontally layered earth model, allowing only a few parameters to vary; each layer is assumed to be homogeneous. In the presence of polarization anisotropy this can lead to considerable errors as shown by DZIEWONSKIand ANDERSON(1981); however, this is not so much when dealing with oceanic structure (CRAMPIN,1976; SCHULEand KNOPOFF,1978). The use of Rayleigh wave data only does not allow us to resolve polarization anisotropy. 422 K. SLINDARALINGAM
A
B
Fig. 9. Rayleigh wave group dispersion of different propagation paths are plotted on the same graph for comparison.
An attempt was made to show that the inelasticity correction to our velocity values, in the period range of this study, is insignificant if we are to use 50 s as the reference period. But if we would have used 1 s as the reference period (similar period as body-waves) all our observed phase and group velocity values would have gone up by 0.5 to 1.3% in the period range 20-100 s. This means that for all our final models the shear velocity of lid and LVL must be raised by about 0.05 km/s, keeping the lid-LVL boundary fixed. The effect of trade-off between parameters are studied around the final model Upper Mantle Velocity of Fiji Region 423
obtained, by varying a number of combinations of variable parameters. It shows
that our shear velocity values for the lid and LVL is correct within about 0.1 km/s. •} Figure 9(A) shows the Rayleigh wave dispersion at WEL station for paths
crossing the South Fiji basin. The propagation path of Loyalty events to WEL
station is almost perpendicular to the magnetic lineament whereas the propagation
path of Hunter Fracture events to WEL station is almost parallel to the magnetic lineament and closer to the magnetic anomaly, 7A of WEISSEL and WATTS (1975). The comparison of the dispersion curves of the above two paths shows that they are
almost the same, within the limits of error of the two data sets. Therefore the very slight differences between the models obtained for the two paths may not be significant. A thorough examination shows that the low lid shear velocity obtained
for the South Fiji basin must be real, and cannot be a trade-off with another
parameter. I have given two more additional group velocity dispersions data for the
paths WEL-Yasawa/Fiji Islands events in Fig. 9(A). These two profiles include about 19% of south of Fiji Islands and north of Hunter Fracture lithosphere path.
The similarity of all four dispersion values are obvious. In this study no attempt was made to work out a separate model for the lithosphere beneath Fiji Islands. The propagation path from the Yasawa events to the AFI station passes closer
to the northern end of the Tonga trench system and follows the transform fault line
running to the north of the Fiji Islands. The low uppermost mantle shear velocity deduced for the northern end of the Lau basin must be interpreted as the absence of lid, and LVL starting at the Moho. This interpretation agrees with the model
obtained for the Lau basin from gravity data by LARUE et al. (1982). Figure 9(B) shows the Rayleigh wave dispersion at HNR station for paths
crossing the North Fiji basin. The propagation path of the Fiji Islands event to HNR station passes through the North Fiji basin, the northern end of Vanuatu
trench and then the eastern end of the Solomon trench. Whereas the propagation
path of the Hunter Fracture event to HNR station passes through the North Fiji basin, centre of Vanuatu trench and the Solomon trench. The striking similarity of
the two dispersion curves is difficult to explain unless we assume that adverse effects such as refraction and scattering are negligible (see Fig. 4). If that is the case then the low uppermost mantle shear velocity deduced for the North Fiji basin must be
interpreted as absence of lid, and LVL starting at the Moho. This conclusion is independent of the type of structure used for the Solomon arc and agrees with the model obtained for the North Fiji basin from gravity data by LARUE et al. (1982).
Most of this research work has been done at the Bureau of Mineral Resources (Geology and Geophysics) of the Australian government in Canberra. I wish to thank the Director of the B. M. R. for the facilities to work there. I would also like to thank David Denham (BMR) for his valuable help. Finally I thank M. Hamburger and I. Everingham for supplying some of the records, and L. Drake for critically reading the manuscript. 424 K. SUNDARALINGAM
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