Physics of Near-Source Strong Ground Motions

Date: 4/25/2019

Time: 08:30 AM

Room: Pine

Some theories have been proposed for the cause of near-source strong ground motion. The rupture directivity effect is one that has been long studied and widely accepted, and its physics is clear. The fling effect was also proposed for the 1999 Chi-Chi, 2016 Kumamoto, and 2016 Norcia earthquakes, but its physics is unclear. Bolt and Abrahamson (2003) wrote “the other (cause) is due to the movement of the ground associated with the permanent offset of the ground.” However, since both the permanent offset and strong ground motion are results of earthquake faulting, one of them cannot be a cause of the other.

Then, what is the physical entity of the fling effect? Hisada and Bielak (2003) mentioned that the fling effect “corresponds to mostly the static term.” In their formulation, the static term is the time-independent part of the Green’s function. Since ground motion is a convolution of the Green’s function and slip time function, ground motion by the static term is proportional to slip time function. Among ground motions radiated from an earthquake, one proportional to slip time function is called “intermediate-field term.” Dreger et al. (2011) already wrote so and “it is physically the sudden elastic rebound of the crust around the rupturing fault, which is called fling in the earthquake engineering community.”

They also concluded from ground motion simulations for an Mw 6.5 earthquake: The fling effect “is only sensitive to the slip on the immediate fault surface and is very sensitive to the depth of burial of the fault” but “there are no observations in this very near-fault distance range (<100 m).” This means that there exists the fling effect for large slips shallower than 100m. For the 2016 Kumamoto earthquake, if a very shallow large slip exists, its fling effect and the rupture directivity effect by the upward rupture propagation, which all the source inversion results indicated, should produce extraordinarily large ground motions. But such ground motions have not yet been found.

Presenting Author: Kazuki Koketsu

Authors