> Winter solstice and vernal equinox -- coincidence.
And both with a full moon...
I don't believe the solstice/equinox events would have much to do with pressure on the plates because those events are related to how far north or south of the equator the sun appears directly overhead at noon and are not involved in the magnitude of the tidal forces.
I could easily believe that a full moon or a new moon could have an effect on earthquake activity because in both cases, the sun/earth/moon system is lined up in a straight-ish line causing the tidal forces on the earth to be maximized in relation to that lineup. Additionally, I note that the quake happened within an hour of midnight at that site which is also interesting.
As I recall, though, seismologists have always maintained that there is no discernable correlation between moon phases and earthquake activity - which is surprising to me.