Recurrence interval/return period: The expected frequency of an event's occurrence (like earthquakes or floods). Smaller events happen more frequently, while bigger ones are less common.

Example: Minor tremors might occur multiple times in a month, while catastrophic earthquakes might only occur once in a hundred years.

• Magnitude refers to the size or energy of an event.
• Two common scales used for earthquakes are.
  • Richter Scale: Developed in 1935 by Charles Richter. It's a logarithmic scale, meaning a 5.0 earthquake is 10 times more powerful than a 4.0, and 100 times more powerful than a 3.0.
  • Moment Magnitude Scale (M): It measures the amount of energy released, with figures similar to the Richter scale. Each increase of 1 on the M scale equals over 30 times the energy released. Every increase of 0.2 doubles the energy.
  • Example: The 2011 Japan earthquake had a moment magnitude of 9.1, making it one of the most powerful earthquakes recorded.

• The Volcanic Explosivity Index (VEI) measures a volcano's strength. It's based on the amount of ejected material, the created cloud's height, and the damage caused. Anything above level 5 is considered large and violent.
• A VEI 8, or supervolcano, ejects more than 1,000 km^3 of material, which is 10 times more than a VEI 7.
• Example: The last VEI 8 eruption was from Toba, Indonesia, about 74,000 years ago, causing a global volcanic winter that lasted years.