> It's not some phenomenon,

I would say probably

depending on the kind of earthquake and sediment/ground/stone it can involve a bunch of friction not just in the crust which could create electric charge which could lead to ground to sky/cloud lightning

which would make it a (normal non mystical) natural phenomena

if you look at this random googled photo of ground to sky lightning (https://www.pinterest.com/pin/567172146806422809/) it color and form wise can "close enough" match the observed behavior