11:30 AM - 11:45 AM
[SSS10-04] Nature of surface fault earthquakes generated from the hierarchy patch source model
Keywords:Hierarchy patch source model, Dynamic source model, Scaling law, Surface fault earthquake
In simulating earthquake ground motion, the level of motion is basically set according to the magnitude scale. However, it has been pointed out that there is a difference in the frequency characteristics of the observed ground motions between earthquakes that involve surface faults (surface fault earthquakes) and those that do not (buried fault earthquakes) (Somerville, 2003). Attempt was made to explain this nature from a physical point of view, and it was shown that it can be naturally explained by the stress drop near the ground surface and the boundary conditions around the fault (Pitarka et al., 2009; Wada and Goto, 2012). The depth-dependent frequency characteristics of interpolate earthquakes can also be explained by the boundary condition of the free surface (Yin and Denolle, 2021). In this way, it is important to understand the characteristics of surface fault earthquakes, which are naturally explained by the physical condition of the existence of the free surface. The main shock of the 2016 Kumamoto earthquake generated long period pulse-like ground motions observed in the vicinity of the fault (Nishihara village), which attracted engineering attention to the characteristics of surface fault earthquakes in Japan.
In this study, we first generate a hypothetical earthquake (Ideal Quake) with little influence from the free surface. Then, earthquakes with the mechanical conditions of the free surface added to the Ideal Quake are generated. For the Ideal Quake in this study, we adopt a source model with a hierarchical patch structure (Ide and Aochi, 2005; Ide 2014). This model assumes that the rupture energy of earthquakes has a hierarchical structure, in which circular patches with different critical slip (Dc) are randomly arranged on the fault with a hierarchical structure. It is known that the scaling laws of earthquakes (G-R law, M-τ relation) is explained (Ide and Aochi, 2005), which can also explain the characteristics of slow earthquakes as a natural extension. In this study, we further confirmed that it can generate source spectra following the ω-2, and thus it is assumed that it has the requirements for Ideal Quake.
We analyze the effects of the surface on the Ideal Quake for vertical lateral faults. In the cases where the stress drop is constant on the fault and the stress drop is reduced at the shallow part in addition to the presence of surface, the source process is different from that of the Ideal Quake. However, there is no tendency for the rise time of the source time function to be significantly longer. A comparison of the slip rate function at the point of maximum slip in the fault and at the point of maximum slip at the free surface shows that the shape of the slip rate function at the ground surface is similar to that of the Kostrov type, where the maximum slip rate occurs immediately after failure and the slip rate gradually decreases. On the other hand, the slip rate function at the ground surface has a triangular symmetrical shape, and its frequency characteristics are different corresponding to the shape. This is consistent with the difference in the shape of the slip rate function between the shallow and deep sections of the main shock of the Kumamoto earthquake.
In this study, we first generate a hypothetical earthquake (Ideal Quake) with little influence from the free surface. Then, earthquakes with the mechanical conditions of the free surface added to the Ideal Quake are generated. For the Ideal Quake in this study, we adopt a source model with a hierarchical patch structure (Ide and Aochi, 2005; Ide 2014). This model assumes that the rupture energy of earthquakes has a hierarchical structure, in which circular patches with different critical slip (Dc) are randomly arranged on the fault with a hierarchical structure. It is known that the scaling laws of earthquakes (G-R law, M-τ relation) is explained (Ide and Aochi, 2005), which can also explain the characteristics of slow earthquakes as a natural extension. In this study, we further confirmed that it can generate source spectra following the ω-2, and thus it is assumed that it has the requirements for Ideal Quake.
We analyze the effects of the surface on the Ideal Quake for vertical lateral faults. In the cases where the stress drop is constant on the fault and the stress drop is reduced at the shallow part in addition to the presence of surface, the source process is different from that of the Ideal Quake. However, there is no tendency for the rise time of the source time function to be significantly longer. A comparison of the slip rate function at the point of maximum slip in the fault and at the point of maximum slip at the free surface shows that the shape of the slip rate function at the ground surface is similar to that of the Kostrov type, where the maximum slip rate occurs immediately after failure and the slip rate gradually decreases. On the other hand, the slip rate function at the ground surface has a triangular symmetrical shape, and its frequency characteristics are different corresponding to the shape. This is consistent with the difference in the shape of the slip rate function between the shallow and deep sections of the main shock of the Kumamoto earthquake.