What Is Fling Step? – Its Physics, Theoretical Simulation Method, and Applications to Strong Ground Motion Near Surface Fault

Date: 4/25/2019

Time: 08:45 AM

Room: Pine

Fling Step is displacement waveform like a step function accompanied by a permanent offset appearing in the very vicinity of the surface faulting. Despite its importance in Seismological and Engineering field, its physics and theory is not clear, and has been interpreted variously. For example, Dreger et al. (BSSA, 2011) interpreted it as the intermediate term of the elastodynamic equations of motion of Aki and Richards (2002). On the other hand, Koketsu et al. (Nature / Scientific Report 2016) interpreted it as the near field term. Here, Aki and Richards (2002) is a theoretical solution of a seismic point source in a homogeneous infinite medium (Eq. (4.32)), where it is separated into the near-, intermediate- and far-field terms.

However, the above interpretations of Fling Step are incorrect in the following points.

1. The static solution by a point source is contributed of both the near- and intermediate-terms, as presented in the equation (4.34) in Aki and Richards.

2. Since the actual ground medium is a complicated stratified half-spaces, it is not appropriated to use the solution of Aki and Richards to explain Fling Step.

3. Fling Step is the result of the elastic rebound near rupturing fault, it cannot be expressed by a point source, and it needs to be evaluated by considering the effect of the fault plane. In this case, even if the observation point approaches the fault plane, its amplitude does not diverge like a point source, but converges to a fault slip itself (Hisada and Bielak, 2003).

Hisada and Bielak (BSSA, 2003) defined Fling Step as "The contribution of static Green function in the representation theorem", which is valid in any ground media. Based on this definition, we proposed an efficient theoretical simulation method in layered half-space, and have released Fortran code for public use (http://kouzou.cc.kogakuin.ac.jp/Open/Green/). Here, we show the validity of the definition and show various simulation examples including the strong ground motions during the 2016 Kumamoto earthquake.

Presenting Author: Yoshiaki Hisada

Authors