Oh my gosh yes there are natural disasters in Managua, Nicaragua! In fact, as I write this, we just had two days where we had several good-sized earthquakes. Managua is especially known for its earthquakes. On December 23, 1972 there was a horrific earthquake that ruined the old downtown Managua. Thousands of people were made homeless and the loss of life was tragic.
The "Ring of Fire" is very apparent in Nicaragua with its collection of volcanoes and the seismic shifts that occur as a result of the Cocos Plate in the Pacific colliding with the Caribbean Plate in the Caribbean.
Speaking of fire, yes Managua can have fires just like anywhere in the urban areas. I would consider the fire risk to be negligible as fire prevention, extinguishers, firemen, and equipment are in fairly good supply.
The next most frequent natural disaster after earthquakes would have to be flooding. Nearly every rainy season, there are times when a drainage canal overflows, or low-lying land becomes flooded when a few days of unrelenting rain accumulate and seek their lowest point by flowing toward their natural drainage. Unfortunately, there are cases where the drainage to the lake passes through residential communities, usually of the poorest construction quality.
For the typical visitor to Nicaragua, floods should not be an issue when staying in Managua.
Sometimes when there is substantial rain causing flooding, it comes from a hurricane which has come in from the Caribbean coast and then loses its circulation, causing it to sit still and just drop torrential amounts of rain in a stationary position. This is what happened with Hurricane Mitch years ago. It is very rare, almost unheard of, for a hurricane to form in the Pacific side of Nicaragua, so damages from wind is normally not a problem.
I hope this article helped you to know a little more about natural disasters in Managua!