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Significant P wave conversions from upgoing S waves generated by very deep earthquakes around Japan
B. L. N. Kennett , T. Furumura
Deep earthquakes, S to P conversion, sP phases, Shear-coupled PL wave, Guided waves in subducting plate
Upgoing S wave from deep earthquakes generate sP and sS converted phase at the free surface, sSPmP phase traveling in the crust by wide-angle reflections, sPn wave propagating along Moho. The strength of these converted phases are strongly influenced by the thickness of the crust at the reflection point. The sSPmp wave traveling in the crust makes coupling with upgoing S wave, generating s-PL wave (see the bottom of figure). Left figure: The hypocenter of recent three deep earthquakes in the area around Japan (2012 Off Sakhalin, 2010 Vladivostok, and 2015 West Ogasawara earthquakes) and the reflection zone of the sPn (dark lings) and the sSPmp phase (light lings). Right figure: Rcord section of radial-component ground velocity recorded at the F-net stations in Japanese from Hokkaido to Ishigaki Island.
An important component of the seismic wavefield at moderate epicentral distances from deep earthquakes comes from seismic waves that are radiated upwards from the source. For very deep events, there is a range of distances at which upgoing S can convert into P waves that travel in the crust or in the upper mantle as the sPn phase. For a 600-km-deep event, sPn becomes a precursor to S from about 8°, and can have significant amplitude if the source radiation pattern is favourable. These conversions to crustal P have a very similar travel time property to S, and interfere strongly with S to produce complex wavetrains on both vertical and radial components. Where the locus of conversion falls on thicker continental crust, S waves can be coupled into partially trapped P waves in the crust that produce a long-period shear-coupled PL (s-PL) wave. Such longer period phases generated by large, very deep earthquakes can make a major contribution to sustaining large ground motion for considerable distances from the source. Observations of three very deep (> 575 km) events around Japan demonstrate the range of propagation effects associated with S to P wave (sP) conversion that plays an important role in shaping the later part of the recorded seismograms. The influence of sP conversion on the observed seismograms and the development of the s-PL wave depend on the variation of crustal thickness along the path and epicentral distance, and particularly on the locus of the conversion zone and the properties of the crust at that location.
