THE EARTHQUAKE FIELD GUIDE
Understand the shake.
A few numbers tell a much richer story when you know what they measure. Here’s how to read an earthquake report, follow its updates and make notifications useful.
MAKE IT TANGIBLE
Step inside the earthquake lab.
Explore an Earth cutaway, compare magnitudes and discover what local intensity can feel like in an illustrated room.
Explore magnitude & shaking in 3D01 / SIZE AND SHAKING
One magnitude. Many intensities.
Magnitude measures an earthquake’s size. Intensity describes shaking at a particular location, influenced by distance, depth and local ground conditions. Two people can experience the same earthquake very differently.
The magnitude scale is logarithmic: a one-unit increase represents ten times the measured wave amplitude and roughly 32 times the energy. The type label, such as ml or mww, identifies the method used to calculate the magnitude.
USGS ShakeMap and community intensity values describe shaking, not another magnitude. A summary intensity is not a personalized estimate for your home. Read the USGS explanation of magnitude and intensity.
02 / BELOW THE SURFACE
Find the epicenter. Then look deeper.
The map marker shows the reported epicenter: the point on the surface above the earthquake’s origin. Depth measures how far below the reference surface that origin lies. Location descriptions such as “10 km west of…” are landmarks to help you orient yourself.
Compare depth alongside magnitude and distance; none of these values alone tells you whether a building was damaged. The event’s USGS page is the place to look for detailed maps and impact products. USGS defines these event fields here.
03 / A DEVELOPING PICTURE
Why a report changes
Our explorer uses the same stored USGS feed reports that support Forecast Bar’s earthquake features. We check the daily feed every two minutes and preserve event pages so a shared link still works later.
Initial reports can be incomplete. A missing magnitude or intensity means the value is unavailable, not zero. “Reviewed” describes the reporting network’s review status; it does not mean that later revisions are impossible.
The default view covers the past 24 hours. Seven- and thirty-day views use the reports collected here; they are not a complete historical USGS catalog. A separate background check collects older revisions and withdrawals every ten minutes. Catch-up can take longer after interruptions; event pages show the last information collected here. Use the source link for the latest revision and the USGS catalog for historical research.
A tsunami flag is a prompt to check official information. It does not itself indicate that a warning has been issued. Check official tsunami messages and follow instructions from local authorities.
04 / MAKE IT PERSONAL
Choose the reports that matter to you.
In Forecast Bar’s earthquake alert settings, manual controls let you set a minimum magnitude and a maximum distance. Auto mode uses a shaking estimate to help decide which events are relevant. Earthquake notifications require a qualifying Pro or Gold subscription.
Notifications are based on detected, reported events. They are not earthquake predictions or early warnings before shaking arrives. Your device, network connection, permissions and notification settings also affect delivery.
Open the event from an app card or notification to see its details, source report and nearby events. Share the page with someone else; they can read it in a browser.
Explore earthquake alerts05 / BEFORE THE NEXT REPORT
Know what to do when you feel shaking.
If you are indoors, drop, cover and hold on. Keep clear of windows and objects that could fall. Follow local emergency guidance; a map or notification should never delay protective action. Read USGS guidance for different situations.
Use the CDC’s after-earthquake guidance to prepare for aftershocks and hazards. For coastal areas, learn the local tsunami evacuation route and follow official tsunami information.