Strong small‐scale seismic scatters (<10 km) have been recently observed at 660 km depth, but their origin remains uncertain. We systematically conduct both high‐resolution 2‐D geodynamic computations that include realistic thermodynamic properties, synthetic seismic waveforms, and insight from shallow seismic observations to explore their origin. We demonstrate that neither short‐term subduction, nor long‐term mechanical mantle mixing processes can produce sufficiently strong heterogeneities to explain the origin of such small‐scale seismic scatters. Instead, the intrinsic heterogeneities inside the oceanic lithosphere which subducts into the mantle transition zone and the uppermost lower mantle can explain the observed short‐wavelength scatter waves.Plain Language Summary
