Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007geoji.168..299p&link_type=abstract
Geophysical Journal International, Volume 168, Issue 6628, pp. 299-314.
Astronomy and Astrophysics
Astronomy
Guided Waves, Low-Velocity Layer, S Waves, Subduction Zone, Sumatra, Synthetic Waveforms
Scientific paper
S phases generated by shallow earthquakes in the Sumatra subduction zone and recorded at the Geoscope station PAF on the Kerguelen Isles in the southern Indian Ocean, are often followed by a long and particularly large wave train on the longitudinal component. The wave train has a dominant period of 25-30 s and lasts usually for more than 100 s. It shows slight normal dispersion, and has a prograde elliptical motion in the vertical propagation plane. These characteristics suggest that the observed waves are shear-coupled PL (SPL) waves. Similar wave trains are not observed for events at comparable distances in other source regions. A computational scheme, based on the reflectivity method and allowing for different crust and upper mantle structures beneath source and receiver, has been used to model the SPL waveforms for six of the events located beneath the accretionary domain and outer high of the Sumatra subduction zone. For the source area, a model representing the structure seaward of the trench has been used, comprising a 5.5 km water layer and 600 m of sediments. By placing the sources in the sedimentary layer, the SPL wave train on the longitudinal component is reproduced quite well. These findings suggest that wave energy is guided into the water and sedimentary layers seaward of the trench, where P and S wave reverberations are generated and leak as SV waves into the mantle. The guidance can be explained by trapping of wave energy within the wedge shaped accretionary domain and outer high, or in oceanic sediments which are subducted with the oceanic plate.
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