There’s also bulk triboelectric effects where the mechanical forces, and the subsequent physical movement of large blocks of material can build up static charges due to having different electron affinity, like a rubber balloon and a wool sweater… by rubbing one against the other a static charge builds up… one theory for earthquake lights is ground to sky discharge of triboelectric charge buildup due to different electron affinity between the two sides of a fault plane under stress… a large enough area can make a small enough difference add up, so even if it isn’t earthquake lights, the triboelectric effect could be contributing to what was measured in this case… fault “surfaces” are going to put square kilometres of differing materials against each other… a non zero effect per square meter, can add up quite a bit when applied to several square kilometres.