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

• Main Authors: J. OLIVER, M. BARAZANGI, B. ISACKS
• Format: Article
• Language: English
• Published: Istituto Nazionale di Geofisica e Vulcanologia (INGV) 1977-06-01
• Series: Annals of Geophysics
• Online Access: http://www.annalsofgeophysics.eu/index.php/annals/article/view/4829

SUMMARY. - Seismic activity associated with the collision of the continental<br />part of the Australian plate with the oceanic Melanesian arcs along Papua New<br />Guinea and the Banda arc provides an unusual opportunity to study the relative<br />excitation of the seismic shear waves Sn and Lg. These waves are produced by<br />earthquakes located along the arcs in the upper 200 km of the earth and are<br />recorded by the Australian WWSSN Stations at Charters Towers (CTA) and Alice<br />Springs (ASP). The paths to these stations are predominantly continental. The data<br />clearly show that for events located at crustal depths, Lg is the predominant phase<br />on the records and Sn is either absent or very weak. For events deeper than about<br />50-70 km, Sn becomes the predominant phase on the records. These observations<br />arc in qualitative agreement with the explanations of Sn and Lg as higher<br />modes of surface waves, for the particle displacement amplitudes are maximum<br />within the crust for Lg and maximum within the lid of the lithospheric mantle<br />for Sn. The data suggest that either the crustal wave guide for Lg is more<br />efficient than that for Sn, or that Lg is more easily excited than Sn. No clear<br />Lg is observed from shallow earthquakes when the length of the segment of the<br />path crossing oceanic structure is greater than about 200 km. Also, widespread<br />Quaternary volcanism within the « stable » area of central Papua New Guinea<br />to the south of the mobile belt does not seem to affect the efficient transmission<br />of high-frequency (1 Hz) shear energy.<br />The paths from events located along the New Hebrides, Solomon, and New<br />Britain arcs to Australia traverse oceanic structure, and no Lg is observed from<br />these paths. The inefficient propagation of Sn along these paths from both<br />shallow and intermediate-depth events can be explained as follows: 1) For<br />the New Hebrides case, the inefficiency of Sn propagation for paths exceeding<br />about 20° distance is related to the relatively young age (Lower to Middle<br />Eocene) of the suboceanic lithosphere of the Coral Sea. As proposed by CHINN,<br />ISACKS and BARAZANGI (1979), such relatively thin lithosphere is probably not an<br />efficient wave guide for shear energy of about 1 Hz 2). The inefficiency of Sn<br />propagation from events located along the northern Solomon and the New Britainarcs is probably due to anomalous attenuation in the uppermost mantle beneath<br />the Woodlark basin and beneath southeastern Papua New Guinea. 3) The<br />inefficiency of Sn propagation from events deeper than about 150 km located<br />in the westernmost part of the northerly-dipping Benioff zone of the New<br />Britain arc could be due either to structural isolation of the nearly vertical<br />descending segment of the plate in which the events occur, or could be due to<br />structural complexity of the plates in the region which interrupts the wave<br />guide for Sn to the Australian stations. This structural feature could be the<br />result of the collision of the Australian plate and the New Britain arc.