An earthquake is an intense shaking of Earth's surface. Scientists use an instrument called a seismometer to measure the shaking waves. In 1935, Charles F. Richter of the California Institute of Technology introduced the idea of earthquake magnitude. Each whole number up in magnitude means a tenfold increase in measured wave size and about 32 times more energy released. For most earthquakes the moment magnitude scale is a more accurate measure of size than the Richter scale, which is now used mainly for small local earthquakes. An earthquake has a single magnitude, but the shaking it causes varies from place to place with distance, type of surface material and other factors.

If the ground shakes, drop down, cover your head and neck, and hold on until the shaking stops. Aftershocks are smaller earthquakes that follow a larger earthquake in the same area. As of November 2026, the USGS developed the ShakeAlert Earthquake Early Warning system together with many partner organizations, including universities and state agencies. An earthquake under the ocean can push the water above it and start a tsunami. A tsunami can also be started by a volcano in the ocean.

In deep water a tsunami moves as fast as a jet plane, but near land it slows to about the speed of a car, 20 to 30 miles per hour. A tsunami wave grows taller as it moves into shallow water near the shore. If the water pulls far back from the shore after an earthquake, a big tsunami may be on its way. Of 754 confirmed tsunamis from 1900 to 2015, about 78% happened in the Pacific Ocean around the Ring of Fire. On December 26, 2004, a NASA satellite instrument called MISR took pictures over six minutes that show tsunami waves breaking on the southeast coast of India.

A tornado is a spinning column of air that reaches from a storm cloud down to the ground. Winds moving in different speeds and directions at different heights make the rising air in a storm start to spin. A supercell is a long-lived, highly organized storm with a tilted, rotating updraft. Most tornadoes form in supercell thunderstorms, but not all supercells produce tornadoes. Scientists do not fully understand how tornadoes form.

Experts rate a tornado by studying the damage it caused. The Enhanced Fujita Scale has six ratings, EF0 to EF5. Over two-thirds of all tornadoes are weak, rated EF0 or EF1. Official tornado records only go back to 1950, so the true average number of tornadoes each year is not known. The widest tornado path on record, near El Reno, Oklahoma on 31 May 2013, was officially 2.6 miles wide. Tornadoes happen all over the United States, and violent tornadoes happen outside "Tornado Alley" every year.

A tornado watch means tornadoes are possible in and near the area. A tornado warning means a tornado is near and it is time to move to a safe place. Go to a small room with no windows on the lowest floor of a sturdy building. Highway overpasses offer no extra protection in a tornado, and opening windows to equalize pressure does not help, because the tornado will blast open the windows anyway. The "safe southwest corner" of a basement is a myth, because walls and furniture can fall into any corner.

A hurricane is a big storm that forms over warm ocean water. When a storm's winds reach 74 miles per hour, it is called a hurricane. Hurricane conditions occur in the tropics, typically between five and 20 degrees of latitude north and south of the equator. Because of the Coriolis effect, hurricanes rotate counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. The Saffir-Simpson scale rates hurricanes from 1 to 5 by their wind speed, and those rated Category 3 and higher are called major hurricanes. Hurricane names come from six lists that rotate, so each list is used again every six years.

Storm surge is sea water pushed onto land by the winds of a storm. It is primarily caused by the strong onshore winds of a hurricane or tropical storm. Storm surge height depends on a storm's intensity, forward speed, size, angle of approach, and the shape of the coastline. When high tide happens at the same time as a storm surge, the combination is called a storm tide, and the water can be 20 feet or more above normal.

A flood is water that covers land that is usually dry. Floods can come from heavy rain, from fast-melting snow, or when ocean waves come on shore. Flash floods can happen within minutes of heavy rain. Where it rains it can flood, even if a place is not near a river or on a flood map. As of November 2026, homeowner's insurance policies do not cover flooding, and a flood insurance policy typically takes up to 30 days to go into effect. Never walk, swim or drive through flood water.

Weather forecasters send out warnings so people have time to get to a safe place. If officials say to evacuate before a hurricane, go right away to a safe place. A family can make a plan together for what to do in a disaster, and they can pick a place to meet if they are ever apart.

Words to know

• magnitude: earthquake size
• supercell: rotating thunderstorm
• latitude: distance from equator
• surge: pushed water
• evacuate: leave for safety