Soil Hardening and Generation of Abnormally High PGA During Large Earthquakes with Extended Sources

During past strong earthquakes, observations of abnormally high accelerations exceeding 1 g were usually attributed to disturbances in recordings, effects of recording piers, falling objects, impacts, etc., or interferences with nearby weak earthquakes, landslides, or similar possible causes. However, during the 2011 Tohoku earthquake, 19 K-NET and KiK-net stations reliably recorded PGA higher than 1 g, the highest ~ 3 g. Generation of abnormally high PGA in the near-fault zones of large earthquakes may be associated with the effects of directivity of seismic wave radiation from the sources and their limiting case, shock waves. Since the 1980s, seismologists have been studying the effects of radiation directivity of extended earthquake sources. Archuleta and Hartzell (1981) discuss “the complications that arise when one is no longer in the far-field but situated close to a finite rupture”, when the radiation originates over some area, and the receiver distance, radiation pattern, and arrival times of P- and S-waves are ill defined. The ground motion cannot be interpreted using the same approach that was appropriate for the far-field. By means of numerical simulation they revealed a strong influence of directivity effects on the accelerations in the near-source zones of the 1979 Imperial Valley earthquake, where PGA ~ 1195 cm/s2 were recorded. Directivity effects occur when the propagation of rupture toward a site at a velocity that is almost as large as the shear wave velocity causes most of the seismic energy from the rupture to arrive in a single large pulse of motion, which represents the cumulative effect of almost all of the seismic radiation from the fault (Sommerville et al. 1997). Recent years with developing strong-motion networks, seismologists more often observe directivity effects, and publications appear where they discuss the generation of shock waves during strong earthquakes due to supershear crack propagation (Bouchon et al. 2001; Bouchon and Vallee 2003; Aagaard and Heaton 2004; Ellsworth et al. 2004; Robinson et al. 2006; Dunham and Bhat 2008; Vallee and Dunham 2012; Zhan et al. 2013, etc.). In this chapter, we discuss soil behavior and possible mechanisms of generation of high PGA during two large earthquakes with extended sources, such as, the 2011 Tohoku earthquake (Mw ~ 9.0) and 2003 Tokachi-oki earthquake (Mw ~ 8.3). The distributions of PGA in the near-fault zones show mosaic patterns, and the highest accelerations were recorded close to the epicenter, as well as at remote sites. Models of soil behavior during these earthquakes indicate nonlinearity of soft soil response only at sites located just near the fault plane. Instead, a rather widespread soil hardening was observed, most expressed at remote sites recorded the highest PGA. To explain the observed features of soil response, two possible mechanisms are suggested, that could act together, such as, (1) shock wave fronts generated by rupture propagation along the fault plane induce soil hardening and high PGA; (2) soil compaction and hardening represent soil response to long-lasting dynamic loading during strong motion. Obviously, we may expect similar effects of soil hardening and generation of high PGA during other mega-thrust earthquakes in future.