Natural Hazards

Hazard, risk and disaster

In the earth sciences, a hazard is an event that has the potential to cause harm.^[1] Risk is the statistical chance that a hazard will actually lead to loss of life or property, and a disaster is what happens when it does.^[1] The US Geological Survey (USGS) gives an example: two towns beside the same fault share the same earthquake hazard, but not the same risk, if only one has buildings made to withstand shaking.^[1]

Most earthquakes and eruptions do not strike at random but cluster in particular places, such as along plate boundaries.^[4]

Earthquakes: stuck faults that suddenly slip

An earthquake is caused by a sudden slip on a fault.^[2] Tectonic plates are always moving slowly, but friction makes them stick at their edges; when the stress overcomes the friction, the stored energy is released as waves that travel through the crust and cause the shaking.^[2] A stuck fault may not move for tens to hundreds of years; the earthquake relieves the stress, the fault locks again, and the cycle begins anew.^[3]

Not every fault stays locked. Some move in a slow, steady motion called creep, and an area undergoing creep rarely has an earthquake above magnitude 6.0, because its stress is continually relieved.^[4]

The point underground where the rupture starts is the focus, and the point on the surface directly above it is the epicenter.^[4] Scientists locate an earthquake from the times its waves reach a network of seismometers, adjusting a guessed place, depth and time until predicted and observed arrivals match.^[5]

Magnitude and intensity

Magnitude is the size of an earthquake, and each earthquake has a single magnitude.^{[6, 7]} Intensity is the strength of shaking at one place, which varies with distance from the rupture, the type of ground and other factors.^{[6, 7]} Intensities are written in Roman numerals, and the scale used in the United States is the Modified Mercalli Intensity Scale; other countries use other scales.^[6]

Magnitude scales are logarithmic: each whole number up means ten times the wave amplitude on a seismogram and about 30 times more energy.^{[6, 7]} USGS says the familiar Richter scale is now rarely used except for small local earthquakes, and that the moment magnitude scale measures size more accurately.^[6]

The math (optional)

USGS defines the seismic moment as M₀ = rigidity × area × slip: the strength of the rock, the area of fault that slipped, and how far it moved.^[6] The moment magnitude is then Mw = ⅔ (log₁₀ M₀ − 9.1).^[6]

Why earthquakes are not predicted

USGS states that neither it nor any other scientists have ever predicted a major earthquake.^[8] Claimed warning signs, such as swarms of small earthquakes or unusual animal behaviour, often occur with no earthquake following.^[8] What USGS can do is calculate the probability of a significant earthquake in an area within a certain number of years.^[8]

An older USGS text shows why: after the Parkfield segment of the San Andreas had magnitude 6.0 earthquakes about every 22 years on average since 1881, USGS gave a 95 percent chance of one before 1993, and it did not come.^[4]

Volcanoes: rising magma and trapped gas

Molten rock underground is magma, a mix of molten rock, crystals and dissolved gas; once it erupts it is called lava.^[10] Magma is lighter than the solid rock around it, so buoyancy and gas pressure drive it upward.^{[10, 11]} As it rises and the pressure drops, its gases come out: fluid lava lets them escape easily, while thick, pasty lava traps them until they burst out explosively.^[11]

That difference shapes volcanoes. Shield volcanoes, such as those of Hawaii, are built almost entirely of fluid lava flows and tend to erupt non-explosively.^{[10, 4]}

Eruptions harm in several ways:

• Pyroclastic flows are dense mixes of hot lava blocks, pumice, ash and gas that race down slopes, typically along valleys, often faster than 80 km/h and at 200–700 °C.^[12]
• Lahars are hot or cold mixtures of water and rock fragments flowing down a volcano; eruptions can trigger them by melting snow and ice, and in steep areas they can exceed 200 km/h.^[13]
• Volcanic ash, particles smaller than 2 mm, can be carried very long distances; ashfall rarely endangers lives but can disrupt buildings, transport, water and power, and ash is a major hazard to aviation.^[14]

Rising magma typically sets off swarms of earthquakes and releases gas, and a volcano tends to swell as magma fills its reservoir.^{[17, 11]} By tracking such signals together, the National Park Service notes, scientists are sometimes able to anticipate an eruption days to weeks ahead.^{[17, 16]} USGS also cautions that geoscientists still do not fully understand how volcanoes work.^[11]

Tsunamis: when the seafloor moves

Tsunamis are seismic sea waves caused by earthquakes, underwater landslides and, infrequently, eruptions of island volcanoes.^[4] A major earthquake can shift the seafloor by several meters, suddenly moving a huge volume of water.^[4] Out at sea a tsunami barely lifts the surface, but it grows taller as it reaches shallower water near the coast.^{[4, 18]} USGS explains that large earthquakes that release their energy mainly as thrust motion, which moves the seafloor up and down, generate large tsunamis, while large strike-slip earthquakes, which move it mostly sideways, are not efficient tsunami makers.^[18]

Hurricanes and storm surge

NOAA (the US National Oceanic and Atmospheric Administration) defines a tropical cyclone as a rotating, organized system of clouds and thunderstorms that forms over tropical or subtropical waters.^[19] It becomes a hurricane when its sustained winds reach 74 mph; the same storms are called typhoons in the western North Pacific and cyclones in the Indian Ocean and South Pacific.^[19] On the Saffir-Simpson Hurricane Wind Scale, Categories 3 to 5 (111 mph or more) count as major hurricanes.^[19] Storm surge is an abnormal rise of water, above the predicted tide, generated by a storm.^[20] It is caused mainly by a hurricane’s strong onshore winds: near the coast, the shallowing seabed blocks the water’s circulation and pushes it up and inland.^[20] Stronger or larger storms raise it; a landfall roughly perpendicular to the coast makes a higher surge more likely, and a wide, gently sloping continental shelf allows a higher surge.^[20] For the United States, the National Hurricane Center calls storm surge the leading cause of hurricane deaths.^[20]

Drought, landslides and wildfire, briefly

NOAA describes a drought as a prolonged period of less-than-average rain or snow in a region, worsened when heat dries the soil.^[21] USGS notes that earthquakes can trigger landslides, and that landslides blocking streams can cause flooding when those dams break.^[22] It calls wildfire unusual because it threatens life and property immediately and also sets up later debris flows, since burned ground sheds more water and soil.^{[23, 24]}

Common misconceptions

“A big earthquake can swallow a city or drop California into the sea.” USGS says the Earth will not open up, and that the San Andreas fault zone reaches only about 15 km deep.^[4]

“A bigger magnitude always means harder shaking.” Depth and distance matter too: the shallower M6.7 Northridge quake shook harder and wider than the slightly larger, deeper M6.8 Nisqually quake.^[6]

“Hot and dry means drought.” NOAA says droughts occur only when an area is abnormally dry for that place.^[21]

What this page does not cover

This page explains mechanisms only. It does not cover floods, warning and early-warning systems, preparedness or safety steps, how hurricanes form, or long-term climate trends, and it gives wildfire and landslides only a few lines. For current hazard information and local guidance, turn to official agencies such as USGS and NOAA’s National Hurricane Center, or their national equivalents.

Going further

See Geology for plates and faults, Oceans for the water that tsunamis and storm surge move, Weather & Climate for the atmosphere behind storms, and Landscapes for how land is shaped over time.