Propagation of distinct Love-wave pulses from regional to teleseismic distances in continental and oceanic environments

Surface waves are usually dispersive with long wave trains and steady decay of amplitude with distance. However, if the group velocity is nearly constant fora span of periods a strong pulse is produced that retains its amplitude for large distances. This situation arises for the fundamental mode of Love waves in the period band 40-500 s for crust and mantle structures with a positive gradient of S wave speed in the uppermost mantle. Such a distinct Love-wave pulse with limited dispersion observed at teleseismic distance is termed the G wave in honour of Gutenberg. The long-period G-wave pulse caused by large earthquakes carries a large amount of energy to substantial distances, with significant effects across the globe, for example event triggering. A similar G-type Love-wave pulse with a much shorter-period of 10-20 s is generated for crustal structures without thick sediment. Such pulses produce anomalously large ground displacement at near-regional distances with, for example an overestimate of surface wave magnitude. We investigate the generation and propagation mechanism of the G-type Love-wave pulses in the crust and upper-mantle with the analysis of observed strong motion records from the M-w 6.2 2016 Central Tottori earthquake and the M-w 9.0 2011 Off Tohoku earthquake in Japan, in conjunction with 3-D finite-difference simulation of seismic wave propagation and analysis of dispersion curves.