  Large-scale earthquake sequence simulations using the boundary element method
(BEM) incur extreme computational costs through multiplying a dense matrix with
a slip rate vector. Hierarchical matrices (H-matrices) have often been used to
accelerate this multiplication. However, the complexity of the structures of
the H-matrices and the communication costs between processors limit their
scalability, and they therefore cannot be used efficiently in distributed
memory computer systems. Lattice H-matrices have recently been proposed as a
tool to improve the parallel scalability of H-matrices. In this study, we
developed a method for earthquake sequence simulations applicable to 3D
nonplanar faults with lattice H-matrices. We present a simulation example and
verify the mesh convergence of our method for a 3D nonplanar thrust fault using
rectangular and triangular elements. We also performed performance and
scalability analyses of our code. Our simulations, using over 10^5 degrees of
freedom, demonstrated a parallel acceleration beyond 10^4 MPI processors and a
>10-fold acceleration over the best performance when the normal H-matrices are
used. Using this code, we can perform unprecedented large-scale earthquake
sequence simulations on geometrically complex faults with supercomputers. The
software HBI is made an open-source and freely available.
