VOL. 77, NO. 32 JOURNAL OF GEOPHYSICAL RESEARCH NOVEMBER 10, 1972 P andS Traveltimes in the Tonga-Fiii Region. A Zone of Low Velocity in theUppermost Mantle behind theTonga Island Arc ¾ASH PAL AGGARWAL AND MUAWIA BARAZANGI 2 Lamont-Doherty GeologicalObservatory o] Columbia University, Palisades, New York 10964 BRYAN ISACKS Department o] Geological Sciences, Cornell University, Ithaca, New York 14850 The principal purposeof this study is to report the existence of a zone of extremely low compressional-wave velocities in the uppermost mantle beneath most of the Lau basin, an interarc basin located west of or behind the Tonga Island arc. Velocities beneath the basin appear to be as low as 7.1 kin/sec. In contrast,times of P and S waves traveling beneath and parallel to the Tonga-Kermadec ridge indicate velocities of 8.45and 4.75km/sec, respec- tively. Although the lateral boundaries of the zone of low velocity beneath the Lau basin are not well defined,they coincide approximately with the boundaries of the zone of high seismic-wave attenuation existing beneath the Lau basin. The large difference (up to 15%) between P wave velocities beneath the Lau basin and those in areas adjacent to it probably requires partial melting in the upper mantle beneaththe Lau basin.The P and $ velocities measured parallel and approximatelyperpendicular to the Tonga trench do not differ sig- nificantly and hence provideno evidence for anisotropy in the Pacificlithosphere. This paper reportsa detailed study of lateral variations of P and $ wave velocities in the uppermost mantle beneath the Tonga-Fiji- Kermadec region of the southwestPacific. Karig [1970] interprets the Lau basin as an interarc basin between the Tonga ridge (a frontal arc) and the Lau ridge (a third arc) (Figure 1). The Lau basinis a zoneof variable but generallyhigh heat flow [Sclater et al., 1972] and youngage [Karig, r970]. Barazangi and Isacks [1971] mapped in detail a zone of anomalously high seismic-wave attenuation un- der the Lau basin (Figure 1). High attenuation, or low Q, is often associated with low seismic- wave velocities, and togetherthey probably in- dicate anomaliesin temperature or degree of partial melting in the mantle. Quantitative esti- mates of these thermal anomalies are more re- liably determined from velocity data than from attenuation data, although observedeffects of x Lamont-Doherty Geological Observatory con- tribution 1877. 2 Now at Department of Geological Sciences, Cornell University, Ithaca, New York 14850. Copyright ¸ 1972 by the American Geophysical Union. attenuationare the most prominent.This study confirms the correlation of low velocities with high attenuation for the Lau basinand thereby provides quantitative data for estimating physi- cal properties of the anomalous zone. This study is basedon traveltimes of P and S phases from shallow earthquakes recorded at distances of about 1ø-15 ø. When the data are grouped according to whetherthe pathstraverse regions of high or low attenuation, well-defined linear traveltime plots are obtained. Theseplots allow us to determine the P and $ velocities in the uppermost mantle for each region. The distribution of shallow earthquakes along and behind the Tonga-Kermadecarc, as well as the locationof the three seismic stations of Afiamalu, westernSamoa (AFI), Niumate, Tonga (NIU), and Vunikawai, Fiji (VUN) (see Figure 1), makes it possible to obtain reversed velocity profiles along the Tonga-Kermadec ridge and across the Lau basin, and henceaccurate P and S velocities are obtained. Studies of other island arcs reveal similarly large lateral variations in seismic velocities (e.g., Fedotov and Slavina [1968] for the Kamchatka arc; Kanamori [1970] and Utsu [1971] for the Japanese arc; Dubds [1971] for the New Heb- 6427