Learn
Plain-language explainers so the live map makes sense. These pages teach concepts — they do not forecast earthquakes for any city or time.
Earthquake swarms
Clusters of quakes without a clear single mainshock — how they differ from aftershock sequences.
Aftershocks
Why shaking can continue for days or months after a large event — Omori’s law in plain English.
Magnitude & intensity
What M numbers mean vs what people feel (intensity), and why depth matters.
Scenario / what-if desks
Hypothetical impact briefs — shake intensity language, who might feel strong shake, tsunami if coastal. Not forecasts.
Faults and trenches
Cascadia, San Andreas, Japan, Chile, Alaska-Aleutian, and New Madrid as a contrast.
Tsunami alert words
Warning, advisory, watch, and information statement, from the National Weather Service.
VEI scale
How USGS compares explosive eruptions, with St. Helens and Pinatubo as the named examples.
Plate boundaries
Transform, divergent, and convergent margins, and which Ring of Fire pages match each.
Earthquake depth
Why Northridge shook harder than a slightly larger, deeper Nisqually earthquake.
During an earthquake
Drop, cover, and hold on, in the words USGS uses for indoors, outdoors, and driving.
If a tsunami is possible
Natural warnings at the coast, then the official alert level if one arrives.
Volcano alerts
How to tell this catalog from an observatory notice.
Glossary
Magnitude, megathrust, VEI, DART, and the other words the pages keep using.
FAQ
Hawaii, New Madrid, forecasts, and the largest earthquake since 1900.
Energy comparison
Two magnitudes in, amplitude ratio and energy ratio out.
Distance to references
Kilometers to USGS epicenters, catalog volcanoes, and corridor boxes.
VEI explorer
VEI 0 through 8, limited to the USGS examples.
Largest since 1900
The USGS list of 20, with each Pacific event linked to one region page.
Sumatra-Andaman
The 2004 M9.1 earthquake and the other Sunda rows on that USGS list.
Magnitude scales
Why Mw, not the Richter scale, is the size USGS uses for the largest earthquakes.
Foreshocks
A foreshock can be named only after a larger earthquake has already happened.
Liquefaction
Water-logged sediment that loses strength in strong shaking.
ShakeAlert
Early warning after a quake has started. Not a prediction.
How quakes are located
P waves, S waves, and why one station is not a location.
DART buoys
Seafloor pressure recorders that report a tsunami already in the water.
Volcano alert words
Normal, Advisory, Watch, Warning, and Unassigned.
Ring of Fire corridors
How we approximate the Pacific Rim belt with auditable lat/lon rectangles.