Flames associated with combustion are a gaseous mixture of incandescent small particles often of carbon, and rapidly moving and fluorescing molecules and ions. The smoke associated with fire is comprised of cooling gases and small ash particles. By naturally occurring fires we mean ones that are not 'man-made'. About 90 percent of wildfires are started by humans. Lightning and hot lava are intrinsically hot enough to start fires.
Friction produces heat that in some cases can be sufficient to start a fire.
Biological activity can sometimes create enough heat to start a fire as, for example, in a damp haystack where microorganisms growing and reproducing can produce sufficient heat to set the haystack burning. The earthquake was a natural force and human action was not responsible for the fire. However, the fire only happened because of the presence of humans and their dwellings. Caused by lightning, a disastrous fire in 1910 swept across three million acres of Idaho for several weeks. Caused by an earthquake, a ferocious fire in 1923 destroyed more than half of Japan.