Broadband strong ground motion modeling using planar dynamic rupture model with fractal parameters

Dynamic rupture modeling represents a preferable physics-based approach to strong ground motion simulations. However, its application in a broad frequency range (0-10Hz), interesting for engineering studies, is challenging. The main reason is that relatively simple models with smooth distributions of initial stress and frictional parameters on planar faults result in ground motions with depleted high-frequency content. Several studies suggested that nonplanar rupture surfaces can solve this issue. Nevertheless, fully accounting for rough ruptures typically requires supercomputers, preventing widespread use.

Here we test an efficient approach for the linear slip-weakening friction model on planar fault, based on the Ide and Aochi (2005) multiscale model, with a small-scale fractal distribution of the slip-weakening distance Dc. To intensify the incoherence of the rupture propagation, we also include a variation of the strength and initial stress correlated with Dc. We propose a way to combine the fractal variations of the dynamic parameters with a large-scale dynamic model. The planar fault assumption permits the use of the computationally very fast code FD3D_TSN (Premus et al., 2020). 

We illustrate the approach on a canonical elliptical model with linearly increasing fracture energy (i.e., constant rupture velocity) and the 2016 Mw6.2 Amatrice earthquake smooth rupture model from the dynamic source inversion by Gallovič et al. (2019). We demonstrate that the addition of the small-scale fractal properties results in sustained high-frequency radiation during the rupture propagation and omega-square (apparent) source time functions. The model improves the fit of the recordings of the Amatrice earthquake in the frequency range of 0-10Hz and generates synthetics agreeing with ground motion prediction equations up to 5Hz.

Our FD3D_TSN takes about 5 minutes to simulate the Mw6.2 rupture propagation on a single GPU. Nevertheless, the fractal dynamic model can be easily implemented in any dynamic rupture propagation code. This makes the proposed approach readily applicable in physics-based ground motion predictions for scenario earthquakes in seismic hazard assessment.