Data sources & methodology

Sources

Earthquake data: USGS Earthquake Catalog / ANSS ComCat and USGS real-time feeds — US federal public domain data.
Places and timezones: GeoNames (CC BY 4.0), populated places with 1,000+ residents.

Each place page covers earthquakes within 100 km of the place center.
A place gets its own page only if it passes an activity threshold: at least 3 recorded M3.0+ earthquakes within 100 km in the last 10 years, or at least one M5.0+ within 50 years. Places below the threshold are covered by state pages instead — this keeps thin pages off the site.
Recent windows (24 h / 7 d / 30 d / 365 d) count recorded events of magnitude 1.5 and above. The 10-year layer is based on M3.0+; the history layer on M4.0+ since 1980. Smaller events below M1.5 appear in the latest-events table when USGS reports them, but are not included in the window counts.
Magnitude and depth bins are half-open intervals — [1.5, 2.0), [2.0, 3.0) and so on — so an event of exactly M2.0 is counted in the 2.0–3.0 bin and never in two bins at once.
All times on place pages are shown in the place's own timezone. Event times are revised by USGS as solutions improve; records here are updated accordingly.

Catalog completeness varies: small earthquakes were under-recorded in earlier decades and in regions with sparse monitoring. Absence of a record is not proof an earthquake never happened.
USGS place descriptions ("24 km NNW of …") are USGS's own reference points, which may differ from the place a page is about; every event on a place page also shows its computed distance from that place.
Depths are relative to sea level and can be slightly negative for very shallow events in elevated terrain.
Predictions are not offered: earthquakes cannot be predicted, and this site does not try.