Why do seismic waves travel more slowly near oceanic ridge crests?

Seismic waves travel more slowly near the crests of oceanic ridges primarily due to the presence of magma chambers located beneath these spreading centres.

Geologic research indicates that partially molten or molten rock significantly slows the velocity of seismic waves as they pass through the subsurface. These magma chambers, which are essential to the formation of new oceanic crust, are typically found at depths of approximately 2 km (1.2 miles) below the seafloor at ridges like the East Pacific Rise.12

Additionally, the high heat flow at these crests contributes to the thermal expansion of the lithosphere, creating a warmer, less rigid environment compared to the cooler, denser plates found further away from the ridge axis. This elevated thermal state, combined with the presence of molten material, is the fundamental reason for the observed reduction in seismic wave speed in these regions.231