Large historical mega-thrust subduction earthquakes, such as the 2004 Aceh-Andaman, 2010 Maule, and 2011 Tohoku earthquakes, have triggered numerous aftershocks in subduction plate interfaces and continental crusts. The crustal seismicity occurs much closer to the population and buildings than the subduction earthquake which is likely to occur with a larger magnitude and at a greater distance. Therefore, the crustal earthquake can have a greater potential impact on seismic damage and loss than the subduction earthquake. Generally, times between major events may be too short to inspect and repair damaged buildings; in such situations, damage accumulation of buildings can be major issues. A new method for assessing spatiotemporal seismic hazard and risk due to a mega-thrust subduction earthquake that triggers both subduction and crustal aftershocks is developed. The Epidemic Type Aftershock Sequences (ETAS) model is used to generate synthetic earthquake catalogs and capture spatiotemporal earthquake clustering. The conventional isotropic ETAS simulation is extended to account for spatial anisotropic distribution of aftershocks by applying the scaling law of the rupture model, and implementing a 2D uniform distribution and a power law decay inside and outside of the rupture area, respectively. Moreover, to evaluate seismic hazard and risk, the ETAS model is convolved with ground motion prediction equations (GMPEs) and seismic fragility curves. A case study is set up for the 2011 Mw9 Tohoku event in Japan. By incorporating more realistic spatial anisotropy of aftershocks, we quantify how the spatiotemporal seismic hazard rate is changed by the triggered crustal and subduction aftershocks in comparison with long-term time-independent hazard rate. Furthermore, we propose to evaluate the impact of increased crustal and subduction aftershocks to seismic hazard and risk assessments for making various risk management decisions more effectively in the post-mainshock period.