How can elderly people, especially those who live alone, be best protected when natural disasters like flooding, landslides and earthquakes occur?

Get back to a real community life, where people know and talk with their neighbours, and support each other.

Agree with wanderlust, build your communities. Create a network among your neighbors and identify who would need the most help and who can lend a hand in times of emergencies. Another thing I'd recommend is for landlords or the local government (ward office and emergency services) to identify all the elderly that live alone within their area of responsibility, that way i'd be easier to take account of them should there be a need to.

I can't speak for cities, but inaka has very strong communities already.

Given that prevention is cheaper than cure, perhaps it would be wise for the government to subsidize reinforcement of private homes to make them more disaster resistant. Any house that floats in a flood is not on a proper foundation. Any house that collapses spectacularly in an earthquake must have little to no bracing. Doing this advance will be much cheaper than rehousing people post disaster.

Everyone will hate it, but the most effective way with flooding will be pro-active compulsory evacuation for all vulnerable people. More able-bodied people get advisories and warnings, but the vulnerable get rounded up for their own safety. This will not be popular, many "false alarms", but it would be effective.

Keep in touch with your neighbors and community. Don't be afraid to reach out.

My city has a pretty strong system in place as well. The are neighborhood monitors who check up on all the older people in our neighborhood. Not much else one can do. Forcing elderly people to leave their homes, unless disaster in imminent, is not a good idea. More elderly people die from living in evacuation centers than from the actual disasters in many cases.

first they all need to be identified, then the government needs to support them. in Japan, we get the sense that if you are not active, in one way or another, you are forgotten and nobody cares about you. this happens with old people because hey they,re old, right? but if the government / local community supports them, things can be different. it needs to start with their own families too. events, going out with other (older) people, etc. once they,re "out there", it all becomes easier, with possible warnings and evacuations before something bad happens. the thing is, Japan needs to care about those who need to be taken care of.

Precautionary evacuations would only be for a day or two and only in the case of big rains that threaten floods. No other form of natural disaster can be predicted that precisely. If you could predict a landslide or earthquake, everyone should evacuate because even able-bodied can't outrun them. Jisuberi (slippage) type landslides can be predicted (wires and measuring equipment), but everyone has to evacuate.

Old people who die at evacuation centers are usually victims of DVT and weakened immune systems that build up over extended stays, sometimes due to overcrowding.

Connections. All people should stay connected in some way or form with their neighbors and, as some have posted before, identify who is most in need, identify vulnerabilities, and formulate plans to best assist them during various natural disaster scenarios. This is much much easier said than done, however.