Dynamic pore pressure changes in the overriding wedge above a shallow-dipping plate interface significantly affect the rupture dynamics of shallow subduction zone earthquakes and their tsunamigenesis. For a wedge on the verge of Coulomb failure everywhere including the basal fault, the dynamic pore pressure increase due to up-dip rupture propagation leads to widespread yielding within the wedge, which is greatly enhanced by the shallow dip of the fault. The widespread yielding reduces the stress drop, slip velocity, slip, and rupture velocity, giving rise to prolonged rupture duration, thus explaining many anomalous features of shallow subduction zone earthquakes. Significant inelastic seafloor uplift occurs in the case of a shallow fault dip, with the largest uplift located landward from the trench. Integrating this physical mechanism with existing seismic, geodetic, and tsunami observations can provide new insights into earthquake dynamics and deformation processes in shallow subduction zones.