[The Scientist]

Could someone kindly enlighten me with an answer? Please elaborate upon explanation and provide sources if possible. Thank you.

[Don_1]

I'm not sure I understand your question.

The earthquake in itself has had no effect on nuclear meltdown. What it effected were the systems which control the reactor, leading to a failure of the systems which caused the reactor to overheat. The inability to regain control of the systems which cool the reactor lead to the meltdown.

[CliffordK]

While there may have been some primary damage due to the earthquake, the reactor itself was designed to be earthquake tolerant.

The main issue was that the earthquake caused a secondary tsunami.

The Earthquake + Tsunami effectively destroyed or shut down the power distribution network from the reactor to "the grid", effectively isolating the reactor from "grid power", and also eliminating "load".

Backup power generation systems were flooded by the tsunami, effectively damaging or destroying them.

Under normal shutdown procedures it would take a couple of days for the reactor to shut down, and cool down to moderately safe levels.  Had the shutdown procedure been started after the March 9^th foreshock, they likely could have had the power plant mostly shutdown before the March 11^th earthquake.  However, on March 9^th, it would not have been apparent that the quake was actually a foreshock, or that one of the top 5 earthquakes globally in the last century, or the strongest in Japan in over 1000 years was imminent.

It is unclear what control systems were damaged, or why the reactor was unable to self-power, but after the tsunami, it lost power and was unable to effectively pump coolant into the system.  The loss of coolant caused the system to overheat.  Pressures increased inside the reactor vessel, making pumping cooling water in more difficult, and caused the formation of Hydrogen Gas.  This combination caused explosions further damaging the system, and caused the meltdown or partial meltdown.

There apparently have been breaches in containment systems that occurred either secondary to the overheating and various explosions, or secondary to the earthquake.  However, these likely would not have caused a meltdown alone without additionally loosing cooling and control systems.