Megathrust earthquakes are strongly influenced by the elastic properties
of rocks surrounding the fault. However, these properties are often
overestimated in numerical simulations, particularly in the shallow
megathrust. Here we explore the influence that realistic depth-varying
upper-plate elastic properties along the megathrust have on earthquake
rupture dynamics and tsunamigenesis using 3D dynamic rupture and tsunami
simulations. We compare results from three subduction zone scenarios
with homogeneous and heterogeneous elastic media, and bimaterial fault.
Elastic properties in the heterogeneous model follow a realistic
depth-distribution derived from controlled-source tomography models of
subduction zones. We assume the same friction properties for all
scenarios. Simulations in the heterogeneous and homogeneous models show
that rigidity variation of the country rock determines the depth-varying
behavior of slip, slip rate, frequency content, and rupture time. Fault
friction may provide additional constraints, but to a lesser extent. The
depth-varying behavior of slip, frequency content, and rupture duration
quantitatively agree with previous predictions based on worldwide data
compilations, explaining the main depth-dependent traits of tsunami
earthquakes and large shallow megathrust earthquakes. Large slip, slow
rupture and slip rate amplification in bimaterial simulations are
largely controlled by the elastic rock properties of the most compliant
side of the fault, which in subduction zones is the upper plate. Large
shallow slip and trenchward increasing upper-plate compliance of the
heterogeneous model lead to the largest co-seismic seafloor deformation
and tsunami amplitude. This highlights the importance of considering
realistic variations in upper-plate rigidity to properly assess the
tsunamigenic potential of megathrust earthquakes.