Disaster Terminology Glossary

Magnitude

A numerical measure of earthquake size based on seismic wave amplitude. Modern seismology uses the Moment Magnitude Scale (Mw) for all earthquakes because it accurately represents energy release across all size ranges. The older Richter scale is now obsolete for scientific use.

Moment Magnitude Scale (Mw)

The current standard for measuring earthquake size, based on the seismic moment (energy released). Unlike the Richter scale, Mw does not saturate at high magnitudes. Each whole number increase represents roughly 32 times more energy released.

• Mw < 3.0 — Micro earthquake, rarely felt
• Mw 3.0-3.9 — Minor, often felt but rarely causes damage
• Mw 4.0-4.9 — Light, noticeable shaking, minimal damage
• Mw 5.0-5.9 — Moderate, can cause damage to poorly constructed buildings
• Mw 6.0-6.9 — Strong, destructive in populated areas up to ~160 km across
• Mw 7.0-7.9 — Major, serious damage over larger regions
• Mw 8.0+ — Great, can cause catastrophic damage over hundreds of kilometers
• Mw 9.0+ — Rare megathrust events, devastating across vast regions (e.g., 2011 Tōhoku M9.1)

Epicenter

The point on the Earth's surface directly above the hypocenter (focus) where an earthquake rupture begins. Location is reported as latitude/longitude coordinates.

Hypocenter (Focus)

The actual underground location where the earthquake rupture initiates. Depth is measured in kilometers below the surface.

• Shallow — 0-70 km depth (most damaging to surface structures)
• Intermediate — 70-300 km depth
• Deep — 300-700 km depth (energy dissipates before reaching surface)

Aftershock

A smaller earthquake that occurs in the same region after a larger mainshock, caused by crustal adjustment to the stress changes. Can continue for weeks to years after the main event.

Foreshock

A smaller earthquake preceding a larger mainshock in the same area. Only identifiable in hindsight; not reliably predictive.

Tsunami Warning

Issued when a potential tsunami with significant inundation is imminent, expected, or occurring. Immediate evacuation to high ground is required.

Tsunami Watch

Issued to alert emergency management that a tsunami is possible based on seismic data. No immediate evacuation required, but readiness is advised.

Tsunami Advisory

Issued for areas where a tsunami capable of producing strong currents or waves dangerous to coastal activity is expected or occurring. Shoreline evacuation may be required.

Saffir-Simpson Hurricane Wind Scale

5-category scale rating hurricane intensity based on sustained wind speed. Used for Atlantic and Eastern Pacific tropical cyclones.

• Category 1 — 74-95 mph winds, some damage
• Category 2 — 96-110 mph winds, extensive damage
• Category 3 — 111-129 mph winds, devastating damage (major hurricane)
• Category 4 — 130-156 mph winds, catastrophic damage
• Category 5 — 157+ mph winds, catastrophic damage, total roof failure and wall collapse

Tropical Depression

Organized system of clouds and thunderstorms with defined circulation and maximum sustained winds of 38 mph or less.

Tropical Storm

Organized system with maximum sustained winds of 39-73 mph. Receives an official name from the regional meteorological authority.

Hurricane / Typhoon / Cyclone

Same phenomenon with different regional names. Sustained winds of 74+ mph (Category 1 or higher).

• Hurricane — Atlantic and Eastern Pacific
• Typhoon — Western Pacific
• Cyclone — Indian Ocean and South Pacific

Storm Surge

Abnormal rise of water generated by a storm, over and above normal astronomical tide levels. Often the greatest threat to life and property from a hurricane.

Active Fire Detection

Thermal anomaly detected by satellite sensors (MODIS, VIIRS) indicating current combustion. Updated multiple times daily.

Fire Radiative Power (FRP)

Measure of thermal energy output from a fire, expressed in megawatts (MW). Higher FRP indicates more intense burning. Useful for comparing fire intensity across different events.

Containment Percentage

Portion of fire perimeter surrounded by control lines (natural barriers or constructed firebreaks). 100% containment does not mean the fire is extinguished, only that it is prevented from spreading beyond existing boundaries.

MODIS

Moderate Resolution Imaging Spectroradiometer — NASA satellite instrument aboard Terra and Aqua satellites. Provides 1 km resolution fire detection with twice-daily global coverage.

VIIRS

Visible Infrared Imaging Radiometer Suite — satellite instrument aboard Suomi NPP and NOAA-20. Provides 375 m resolution fire detection with more frequent coverage than MODIS.

Volcanic Explosivity Index (VEI)

Logarithmic scale from 0 to 8 measuring the explosiveness of volcanic eruptions based on volume of ejecta, eruption cloud height, and duration.

• VEI 0-1 — Non-explosive to small (e.g., Hawaiian lava flows)
• VEI 2 — Explosive, plume 1-5 km high
• VEI 3 — Severe, plume 3-15 km high (Catastrophic category)
• VEI 4 — Cataclysmic, plume 10-25 km high (Eyjafjallajökull 2010)
• VEI 5— Paroxysmal, plume >25 km (Mount St. Helens 1980)
• VEI 6 — Colossal (Krakatoa 1883, Pinatubo 1991)
• VEI 7 — Super-colossal (Tambora 1815)
• VEI 8 — Mega-colossal (Yellowstone ~640,000 years ago)

Pyroclastic Flow

Fast-moving current of hot gas, ash, and volcanic rock traveling down volcano slopes at speeds up to 700 km/h. One of the most deadly volcanic hazards.

Lahar

Destructive mudflow composed of volcanic material and water. Can occur during eruptions or years after when rainfall mobilizes loose volcanic deposits.

Ash Cloud

Suspension of volcanic ash particles in the atmosphere. Poses severe risk to aviation, can disrupt air traffic over continental distances.

GDACS Alert Levels

Global Disaster Alert and Coordination System severity classification for humanitarian impact:

• Green — Low humanitarian impact expected
• Orange — Medium humanitarian impact, response likely required
• Red — High humanitarian impact, major response required

PAGER (Prompt Assessment of Global Earthquakes for Response)

USGS system that rapidly estimates fatalities and economic losses following significant earthquakes worldwide. Provides color-coded alerts (green, yellow, orange, red) based on estimated impact.

NASA EONET

Earth Observatory Natural Event Tracker — NASA's catalog of natural events tracked from satellite and ground-based sources. Covers 13 event categories including wildfires, storms, volcanoes, earthquakes, floods, and more.

USGS

United States Geological Survey — US government agency providing authoritative earthquake data, including real-time feeds, shakemaps, and scientific analysis.

NOAA

National Oceanic and Atmospheric Administration — US agency responsible for weather forecasting, hurricane tracking, tsunami warnings, and atmospheric monitoring.

GDACS

Global Disaster Alert and Coordination System — cooperation framework providing disaster alerts and impact estimations for humanitarian response coordination.

FIRMS

Fire Information for Resource Management System — NASA program distributing near-real-time active fire data from MODIS and VIIRS satellite instruments.

Ring of Fire

Horseshoe-shaped zone of intense seismic and volcanic activity encircling the Pacific Ocean. Contains ~75% of the world's active volcanoes and ~90% of earthquakes.

Plate Boundary

Edge where two tectonic plates meet. Most earthquakes and volcanoes occur along plate boundaries.

• Convergent — Plates collide (subduction zones, mountain building)
• Divergent — Plates separate (mid-ocean ridges, rift valleys)
• Transform — Plates slide past each other (e.g., San Andreas Fault)

Subduction Zone

Region where one tectonic plate descends beneath another. Capable of producing the largest earthquakes (magnitude 9.0+) and tsunamis. Examples: Cascadia, Japan Trench, Chile.