An earthquake is an intense shaking of Earth's surface, and scientists use an instrument called a seismometer to measure the shaking waves. Each whole number up in magnitude means a tenfold increase in measured wave size and about 32 times more energy released. Seismic moment equals the strength of the rock along the fault times the area of the fault that slipped times the distance the fault moved. 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. Parts of the San Andreas Fault creep steadily in many tiny shocks, but where creep is not constant, strain can build up for hundreds of years and produce great 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.
An earthquake under the ocean can push the water above it and start a tsunami, and a volcano in the ocean can start one too. The tsunami hazard is greatest near subduction zones, where large earthquakes and landslides can produce damaging waves. The most likely tsunami sources on the U.S. East and Gulf Coasts are underwater landslides and meteotsunamis, because those coasts are not near subduction zones. DART systems were developed by NOAA for early detection, measurement and real-time reporting of tsunamis in the open ocean. Once a tsunami is detected, warning centers use forecast models to forecast wave height and arrival times, the location and amount of flooding, and how long it will last. For a tsunami close to a coast there may not be time for a detailed forecast, so people should recognize natural warnings and be ready to respond.
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. 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 tsunami is not a tidal wave: tides come from the pull of the Sun and Moon, and tsunamis are not related to them. If the water pulls far back from the shore after an earthquake, a big tsunami may be on its way. The word tsunami is Japanese, and it means harbor wave.
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, and a mesocyclone, the large rotating updraft in a supercell, is usually 2-6 miles in diameter, much larger than the tornado that may develop within it. Most tornadoes form in supercell thunderstorms, but not all supercells produce tornadoes. Scientists say that they do not fully understand how tornadoes form. Studies of tornado formation suggest it can happen without temperature patterns at the edge of the downdraft; very little temperature variation was seen near some of the most destructive tornadoes, on 3 May 1999.
NSSL researchers discovered the Tornado Vortex Signature, a Doppler radar velocity pattern that shows a region of intense concentrated rotation. A Tornado Vortex Signature does not guarantee a tornado, but it strongly increases the probability of one. A hook echo is a pattern in radar images that looks like a hook on the right-rear part of a storm; it is often linked to a mesocyclone and favorable conditions for tornado formation. Dual-polarization radar can detect random shaped and sized targets like leaves and insulation, helping meteorologists confirm a damaging tornado is on the ground, especially at night. Experts rate a tornado by studying the damage it caused, and the Enhanced Fujita Scale has six ratings, EF0 to EF5. EF Scale wind speeds are estimates based on damage, not measurements. 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.
A tornado watch means tornadoes are possible in and near the area, and a tornado warning means a tornado is near and it is time to move to a safe place. During a tornado warning, go to a small room with no windows on the lowest floor of a sturdy building. A safe room built to FEMA criteria or a storm shelter built to ICC 500 standards is the best tornado protection; the next best is a small interior windowless room on the lowest level. Highway overpasses offer no extra protection in a tornado, and opening windows to equalize pressure does not help in a tornado, 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.
Now turn from the land to the ocean, where hurricanes begin. A hurricane is a big storm that forms over warm ocean water. Scientists call hurricanes and their relatives tropical cyclones, and a tropical cyclone is a hurricane when its winds reach 74 miles per hour. 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. Hurricanes are not just a coastal problem: rain and wind can reach far inland. Hurricane names come from six lists that rotate, so each list is used again every six years, and hurricanes rated Category 3 and higher are called major hurricanes.
Storm surge is sea water pushed onto land by the winds of a storm, and 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. A wide, gently sloping continental shelf allows a higher storm surge than a narrow, steeply sloping one. The Gulf Coast, especially Louisiana and Mississippi, is especially open to storm surge because the ocean floor deepens gradually offshore, while Florida's southeast coast has a shelf that drops off quickly. The National Weather Service SLOSH model lets forecasters customize storm surge predictions for particular storms and coasts. 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, and floods can come from heavy rain, from fast-melting snow, or when ocean waves come on shore. Melting snow or warm rain can break up river ice, and chunks of ice can jam against bridges and make the water rise quickly behind the jam. Flash floods can happen within minutes of heavy rain, and where it rains it can flood, even if a place is not near a river or on a flood map. In 2005, flooding hit New Orleans days after Hurricane Katrina came onshore because the levees designed to protect the city failed. 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.
Words to know
- seismic: about earthquakes
- subduction: plate sinking
- mesocyclone: rotating updraft
- meteotsunamis: weather-made waves
- polarization: wave direction
- levees: flood walls
