Abstract
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.
Highlights
Most shallow earthquakes have a concentrated zone of strong ground motion in the immediate vicinity of the epicentre, but deeper events spread their effects more widely at moderate epicentral distances (> 1000 km)
Observations of very deep events across the Japanese Islands We examine the patterns of observed seismograms for the three events shown in Fig. 1, recorded at a common set of stations from northern Hokkaido through to the southern Ryukyu islands
In the left-hand panel of Fig. 5, we indicate for each source the surface locations of the bands where conversion can occur from upgoing S to upper-mantle and crustal P arrivals
Summary
Most shallow earthquakes have a concentrated zone of strong ground motion in the immediate vicinity of the epicentre, but deeper events spread their effects more widely at moderate epicentral distances (> 1000 km). We will examine the properties of the longer-period P converted phases generated by upward travelling S wave from deep earthquakes, based on analysis of the F-net broadband, strong motion records across Japanese islands, and complemented by numerical simulation of seismic wave propagation.
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