How can i tell if my tap water is safe to drink after the earthquake?

I think you: would have to rely on notification from the government/public health authorities. It would be obvious, I would think, if your water supply has been affected by an earthquake. If it seems likely wait for official notification before using tap water. This is why bottled water is recommended for disaster preparedness. Good wishes:)

Filter: I would use bottle water or RO water until authorities perform test and make an official announcement that tap water is safe. Hope this helps.