A magnitude 5.5 earthquake struck 5 kilometers east-southeast of Asahi, on the Pacific side of Chiba Prefecture, at 15:07 UTC on October 28, which was seven minutes past midnight on Thursday in Japan. Within hours, 50 people had filed felt reports with the U.S. Geological Survey. No tsunami flag was set for the event, and nothing in the early record points to damage.
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Fifty reports is a small number for a quake east of Tokyo Bay, and it is also a number that keeps changing. How it is collected explains most of what it can and cannot say about the shaking.
Fifty volunteers and a 3.9 intensity
The USGS record for the quake lists 50 felt reports, a tally from the agency’s felt-report questionnaire. program, which collects information from people who felt an earthquake through an online questionnaire. The agency turns those answers into a community intensity, and for this event the figure stands at 3.9 on the Modified Mercalli scale. The ShakeMap estimate, which blends instrument records with ground-motion models rather than human testimony, comes out higher at 4.8.
The gap between the two is ordinary. Respondents are self-selected, and they answer from wherever they happened to be: a person on the ground floor of a low house reports something different from a person on the tenth floor of an apartment block. A midnight origin time also thins the pool, since the people most likely to fill out a form are those awake and holding a phone.
The count tends to climb for a day or two after an event as stragglers find the form, which is why a total quoted on the morning after is a floor and not a final figure. The query run for this article, last updated by USGS at 00:20 UTC on October 29, still showed 50.
Set against the rest of the world, the response stands out. The USGS feed of magnitude 5.0 and larger events since October 26 contains 26 earthquakes, and the Chiba quake drew by far the most felt reports. The next highest tallies belong to the pair of events near Balangonan in the Philippines, with 7 and 5, while the magnitude 6.0 earthquake off Vanuatu, the largest in the window, drew a single report. Magnitude alone does not produce a felt count; proximity to people does, and the Boso Peninsula puts a moderate quake within reach of a large population. Every one of the 26 carries a tsunami flag of zero, including the Chiba event.
Lower 5 on the JMA scale, and the overnight revisions
Japan’s own agency measures shaking differently. The Japan Meteorological Agency’s earthquake bulletin logged the event at 00:07 JST on October 29 and placed it in northeastern Chiba Prefecture. The first reports gave a magnitude of 5.4 at a depth of 50 kilometers. A notice issued at 02:10 JST updated the source parameters to magnitude 5.5 and 52 kilometers, bringing the agency’s magnitude in line with the USGS value. The USGS depth, 41.4 kilometers, stays about 10 kilometers shallower than JMA’s final figure, a routine spread between networks that use different station sets and velocity models.
The strongest recorded shaking reached 5 lower on the shindo scale, registered in a single municipality. Seven more recorded 4, and 26 recorded 3.
The JMA’s published scale describes lower 5 in terms that match a quake with no headline damage: “Many people are frightened and feel the need to hold onto something stable.” At that level the agency lists slight cracks in the walls of weakly built wooden houses, dishes and bookshelf contents falling indoors, and, in some cases, windows breaking and falling outdoors. Level 4, by comparison, startles most people and sets lamps swinging, with no building damage listed.
Two plates under the Boso Peninsula
Chiba sits where the Pacific plate and the Philippine Sea plate both dive beneath central Japan, and the government’s Headquarters for Earthquake Research Promotion says the Kanto region is “one of those in Japan with an extreme amount of seismic activity.” The Pacific plate arrives at roughly 8 centimeters a year and produces earthquakes to about 200 kilometers depth. The Philippine Sea plate moves more slowly, at about 3 to 4 centimeters a year. Beneath southern Kanto the boundary between them lies just 10 to 30 kilometers below the surface, a geometry the headquarters flags as capable of great earthquakes under urban areas.
The same document names the 1987 earthquake off the east coast of Chiba Prefecture, a magnitude 6.7 event, among the notable episodes around the peninsula, and observes that no earthquake of magnitude 7 or greater has occurred near the Sagami Trough in several decades. Against that record, a magnitude 5.5 at 41 to 52 kilometers is a moderate, expected event.
Which structure moved this time has not been published in the material available on the morning after. The headquarters figure of 10 to 30 kilometers applies to southern Kanto, and the new hypocentre lies deeper and farther northeast. Whether the rupture occurred on the plate interface or inside one of the slabs is a question the agencies’ source mechanisms will answer. Until then, the record rests on two numbers that are still open to revision: the 50 felt reports on the latest USGS update, and the 52 kilometers at which JMA now places the hypocentre.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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