If you have just heard the sirens and are wondering WTF! They are a tsunami warning for Numazu. There is an estimated 0.5m tsunami for the coast of Japan due to a 8.0 Magnitude earthquake in the Pacific Ocean near Indonesia.
Tsunami walls are several meters high, so there is little to worry about unless you are on the beaches in which case evacuate to higher ground beyond the tsunami wall immediately. Stay safe and follow the lead of the locals. Everything seems safe.