Which term refers to fires caused by natural events, such as lightning or earthquakes?

The term that refers to fires caused by natural events, such as lightning or earthquakes, is "Natural fire." This is because natural fires are specifically initiated by naturally occurring phenomena, distinguishing them from human-induced fires. For example, lightning striking a dry area can ignite vegetation, leading to a wildfire. Similarly, earthquakes might cause landslides or other events that can inadvertently set off a fire.

Undetermined fires, on the other hand, are those where the cause cannot be easily identified or established after an investigation. Mechanical fires typically arise from malfunctions of machinery or equipment, while accidental fires are usually caused by human carelessness or negligence, such as leaving a candle unattended or improperly handling flammable materials. Therefore, "Natural fire" is the most appropriate term for fires that occur due to natural causes.