Relative excitation of the seismic shear waves Sn and Lg as a function of source depth and their propagation from Melanesia and Banda arcs to Australia

Abstract

SUMMARY. - Seismic activity associated with the collision of the continentalpart of the Australian plate with the oceanic Melanesian arcs along Papua NewGuinea and the Banda arc provides an unusual opportunity to study the relativeexcitation of the seismic shear waves Sn and Lg. These waves are produced byearthquakes located along the arcs in the upper 200 km of the earth and arerecorded by the Australian WWSSN Stations at Charters Towers (CTA) and AliceSprings (ASP). The paths to these stations are predominantly continental. The dataclearly show that for events located at crustal depths, Lg is the predominant phaseon the records and Sn is either absent or very weak. For events deeper than about50-70 km, Sn becomes the predominant phase on the records. These observationsarc in qualitative agreement with the explanations of Sn and Lg as highermodes of surface waves, for the particle displacement amplitudes are maximumwithin the crust for Lg and maximum within the lid of the lithospheric mantlefor Sn. The data suggest that either the crustal wave guide for Lg is moreefficient than that for Sn, or that Lg is more easily excited than Sn. No clearLg is observed from shallow earthquakes when the length of the segment of thepath crossing oceanic structure is greater than about 200 km. Also, widespreadQuaternary volcanism within the « stable » area of central Papua New Guineato the south of the mobile belt does not seem to affect the efficient transmissionof high-frequency (1 Hz) shear energy.The paths from events located along the New Hebrides, Solomon, and NewBritain arcs to Australia traverse oceanic structure, and no Lg is observed fromthese paths. The inefficient propagation of Sn along these paths from bothshallow and intermediate-depth events can be explained as follows: 1) Forthe New Hebrides case, the inefficiency of Sn propagation for paths exceedingabout 20° distance is related to the relatively young age (Lower to MiddleEocene) of the suboceanic lithosphere of the Coral Sea. As proposed by CHINN,ISACKS and BARAZANGI (1979), such relatively thin lithosphere is probably not anefficient wave guide for shear energy of about 1 Hz 2). The inefficiency of Snpropagation from events located along the northern Solomon and the New Britainarcs is probably due to anomalous attenuation in the uppermost mantle beneaththe Woodlark basin and beneath southeastern Papua New Guinea. 3) Theinefficiency of Sn propagation from events deeper than about 150 km locatedin the westernmost part of the northerly-dipping Benioff zone of the NewBritain arc could be due either to structural isolation of the nearly verticaldescending segment of the plate in which the events occur, or could be due tostructural complexity of the plates in the region which interrupts the waveguide for Sn to the Australian stations. This structural feature could be theresult of the collision of the Australian plate and the New Britain arc.