Assuming three kinds of materials, 100% olivine ((Mg0.89Fe0.11)2SiO4), pyrolite, and piclogite as composition of the upper mantle, we constructed pressure--temperature dependent seismic velocity structure models in the upper mantle beneath a subduction zone. For this purpose, we employed values of physical properties of composite minerals newly determined by using a genetic algorithm (GA) and obtained the thermal structure beneath the subduction zone as a numerical solution of a two-dimensional (2-D) thermal-fluid coupled problem. Furthermore, we investigated the effects of a metastable olivine(α) wedge in subducted slabs on theoretical seismic waveforms of deep earthquakes, which propagate through the velocity structure by solving the P-SV problem numerically. The effects of center frequency of the excited waves at hypocenters, focal mechanism, location of hypocenter, upper mantle compositions, values of physical properties, a dip angle of subducting slabs, anelastic attenuation, subducting velocity, and depth of deep earthquakes on seismic waveforms were investigated in detail. Theoretical seismograms at the Earth's surface indicate that coda waves with small amplitudes of duration time of 20--30 s appear following direct P and S waves for models with the metastable olivine(α) wedge inside the slabs. The coda waves can be explained by the excitation by reflections and conversions at a large seismic velocity discontinuity between the metastable olivine(α) wedge and the surrounding slab. For models with center frequency of 0.25 Hz, purer olivine composition, a faster subducting velocity, and focal depth of 500 km, waves with larger amplitudes were excited at the discontinuity. The results of this study may give important clues to detect the existence of the metastable olivine(α) wedge from seismological observations.