Propagation of seismic waves through and beneath the lithosphere that descends under the Tonga Island Arc

Abstract

This study is concerned with the propagation of shear waves inside and beneath the descending and horizontal parts of suboceanic lithosphere near the Tonga island arc. We present observations that lead to the following results: First, the descending and suboceanic parts of the lithosphere are continuous and form a wave guide for high-frequency seismic shear waves. Second, the northern edge of the zone of intermediate and deep earthquakes coincides with the edge of the lithospheric (high-Q) slab. Third, the observations also indicate the presence of low-velocity, high-attenuation asthenosphere beneath the suboceanic lithosphere. These results are based on analysis of seismograms from two stations of the WWSSN, Rarotonga (RAR) in the Cook Islands and Afiamalu (AFI) in Western Samoa. At RAR, 14° from the Tonga trench toward the oceanic side, two distinct S-wave arrivals are recorded from earthquakes in the Tonga inclined seismic zone. The predominant frequencies of the first arrival are significantly lower than those of the later arrival. The time between the two phases increases with focal depth from about 45 sec for 350-km depth to 95 sec for 650-km depth. We interpret the low-frequency S-phase as the ‘normal’ S-wave that travels along the path appropriate for a laterally uniform mantle and traverses the low-Q asthenospheric layer en route. The high-frequency S-phase travels up the high-Q, high-velocity material near the inclined seismic zone and then travels horizontally as Sn through the high-Q uppermost mantle to RAR. Intermediate and deep earthquakes in the northern end of the Tonga seismic zone produce, without exception, low-frequency S-waves at AFI, a station about 100 km from the northern end of the Tonga arc. This is evidence that the high-Q, high-velocity descending lithosphere does not extend beyond the northernmost limit of the deep seismic zone, and further supports the plate model of island arcs.