We present a highly scalable 3D fully-coupled Earth & ocean model of earthquake rupture and tsunami generation and perform the first fully coupled simulation of an actual earthquake-tsunami event and a 3D benchmark problem of tsunami generation by a megathrust dynamic earthquake rupture. Multi-petascale simulations, with excellent performance demonstrated on three different platforms, allow high-resolution forward modeling. Our largest mesh has ≈261 billion degrees of freedom, resolving at least 15 Hz of the acoustic wave field. We self-consistently model seismic, acoustic and surface gravity wave propagation in elastic (Earth) and acoustic (ocean) materials sourced by physics-based non-linear earthquake dynamic rupture, thereby gaining insight into the tsunami generation process without relying on approximations that have previously been applied to permit solution of this challenging problem. Complicated geometries, including high-resolution bathymetry, coastlines and segmented earthquake faults are discretized by adaptive unstructured tetrahedral meshes. This inevitably leads to large differences in element sizes and wave speeds which can be mitigated by ADER local time-stepping and a Discontinuous Galerkin discretization yielding high-order accuracy in time and space.