How to read JMA earthquake reports
A guide to event and issue times, hypocentre, depth, Shindo, tsunami information, preliminary reports and corrections.
A JISHINDESU editorial guide based on the cited official and primary materials. It does not replace official safety information.
On this page
Start with the information type and timestamps
Information about one earthquake may arrive in stages. Early messages prioritise speed; later reports add or refine the hypocentre, magnitude, intensity observations and tsunami assessment.
The earthquake’s origin time is different from the bulletin issue time. JISHINDESU displays both when available and retains corrections as revisions of the same event.
Why does the information arrive in stages?
Speed matters immediately after an earthquake, so the first bulletin may contain only part of the picture. Later calculations use more stations to refine the location, depth and magnitude. Reports can then add Shindo observations from individual municipalities.
Not every earthquake follows the same sequence. A weak event may need only a short report. A strong or offshore earthquake can produce more updates, especially when JMA must assess tsunami risk.
Hypocentre, depth and named area
The hypocentre is the point inside Earth where rupture begins; the epicentre is its projection on the surface. The named area identifies the source region. It does not mean every place in that prefecture received maximum Shindo.
Reported depth is rounded and may be revised. JISHINDESU calculates surface distance to the epicentre and an approximate hypocentral distance using that reported depth.
Intensity areas and station observations
Maximum intensity is only the highest summary value. To understand one city, inspect the area and station observations. JISHINDESU places individual station readings on the event map for this reason.
No reading for a municipality does not automatically mean Shindo 0. The station may be absent from that bulletin, the data may still be importing, or the locality may not yet be linked in the station catalogue.
Example: the headline is not your local reading
Suppose a bulletin reports magnitude 5.4 and maximum Shindo 4 in Ibaraki Prefecture. If you care about Chiba, do not apply Shindo 4 to the whole city. Open the observations and look for a station in Chiba or a nearby municipality. It may report Shindo 2, Shindo 1, or not appear in the bulletin yet.
A source-region name such as “off Fukushima Prefecture” also describes where the earthquake occurred. It does not identify the places with the strongest shaking. For that, use the Shindo observations in the report.
Tsunami status and corrections
Read tsunami labels precisely: assessment, no threat, advisory, warning and major warning are different states. If a threat is possible, follow the official JMA page and local instructions directly.
Preliminary magnitude, depth and maximum Shindo can change as more data arrive. That correction is not a second earthquake. JISHINDESU joins those messages into one event and exposes its revision history.
Read the report without guessing
Begin with the origin time, then check the region, magnitude and depth. To understand what happened at a particular place, continue to the Shindo observations. If the report is only a few minutes old, check it again later because values and station lists may be updated.
Near the coast, after strong shaking or weak but prolonged swaying, immediately head to safe higher ground or a designated tsunami evacuation site. Do not wait for a warning, map or notification. Act on a tsunami warning or evacuation instruction even if you felt no shaking. During a tsunami advisory (津波注意報), leave the water and move away from the shore. Do not return to the water or shore while tsunami warnings or advisories remain in force; follow local instructions. Read the report only once you are safe.