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

M. BARAZANGI, J. OLIVER, B. ISACKS

Abstract


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

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References


DOI: https://doi.org/10.4401/ag-4829
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Published by INGV, Istituto Nazionale di Geofisica e Vulcanologia - ISSN: 2037-416X