Magnitude
A single number for the size of an earthquake at the source. USGS says each whole-number increase is a tenfold increase in measured amplitude. Read the page.
Moment magnitude
The magnitude USGS uses for all but the smallest earthquakes. It comes from seismic moment: rigidity times fault area times slip. Read the page.
Intensity
The strength of shaking at a place. One earthquake has many intensities. In the United States the scale is Modified Mercalli, written in Roman numerals. Read the page.
Epicenter
The point on the ground surface above where the earthquake starts. The USGS largest-earthquake table publishes an epicenter for each event. Read the page.
Hypocenter
The place at depth where the rupture starts. Depth is why Nisqually and Northridge felt different despite similar magnitudes. Read the page.
Aftershock
An earthquake that follows a larger mainshock in the same region as the crust adjusts. Rates usually fall off with time. That pattern is not a clock for one city. Read the page.
Swarm
A cluster of earthquakes without one obvious mainshock. Seeing a swarm is not a prediction of a larger earthquake. Read the page.
Subduction
One plate being forced beneath another. Cascadia, the Japan Trench, Chile, and the Aleutians are subduction margins in the pages on this site. Read the page.
Transform boundary
Plates sliding past each other. The San Andreas system is the USGS example used here. Read the page.
Megathrust
The plate-interface fault at a subduction zone. The 26 January 1700 Cascadia earthquake was a megathrust earthquake. Read the page.
Intraplate
Inside a tectonic plate, not on its edge. New Madrid and the Hawaiian hotspot are the two intraplate examples on this site. Read the page.
VEI
Volcanic Explosivity Index. A comparison of explosive eruptions based mainly on ash volume, from Newhall and Self (1982), as summarized by USGS. Read the page.
Holocene catalog
The Smithsonian list of volcanoes with eruptions in the Holocene, stored here as a snapshot fetched 19 September 2026. Not a live alert feed. Read the page.
Ring of Fire rectangle
One of the lat/lon boxes on the About page. A point inside the union of the boxes is counted as Ring of Fire on this site. The boxes are not plate polygons. Read the page.
Tsunami warning
An NWS alert: a tsunami that may cause widespread flooding is expected or occurring. Evacuation is recommended. Read the page.
Tsunami advisory
An NWS alert for strong currents or waves dangerous to people in or very near the water. Stay off beaches. Read the page.
DART buoy
A deep-ocean tsunami-monitoring station. On this map, DART markers are observations from an NDBC snapshot, not a forecast. Read the page.
Scenario
A what-if written under stated assumptions. On this site a scenario is never a prediction of time or place. Read the page.