He is always the fearful god of the earthquake. In the Iliad he so shakes the earth that the mountains tremble and the horrible realm of the abyss threatens to burst open.
Otto establishes earthquake as the definitive, primordial attribute of Poseidon, inseparable from his identity as earth-consort and chthonic ruler.
, The Homeric Gods: The Spiritual Significance of Greek Religion, 1929thesis