2021 Chiba earthquake

What Happened

At 22:41 Japan Standard Time on 7 October 2021, an earthquake struck the northwestern part of Chiba Prefecture (千葉県, Chiba-ken), east of central Tokyo. The epicentre lay roughly 4 kilometres south-west of the city of Chiba, at coordinates 35.577°N, 140.070°E. The rupture occurred at a depth of between about 62 and 75 kilometres, and the earthquake registered a magnitude of 5.9.

The Japan Meteorological Agency (気象庁, Kishōchō) assigned the event a maximum seismic intensity of Shindo 5+ (震度5強, shindo go-kyō). Shindo (震度) is Japan's own intensity scale, which measures the strength of shaking at a given place rather than the energy released at the source; 5+ is the second-highest of the three grades in the "5" band and corresponds to VI (Strong) on the Modified Mercalli scale. Because the hypocentre was deep, the shaking spread widely across the Kantō region rather than concentrating near the epicentre.

Background

Japan sits at the junction of four tectonic plates, and the Kantō plain around Tokyo is underlain by the Philippine Sea Plate diving beneath the Okhotsk and Pacific plates. Earthquakes at depths of 60 to 80 kilometres in this zone are common: they occur within the subducting slab rather than at the surface, which is why a magnitude 5.9 event can shake a wide area without producing the violent local damage of a shallow rupture.

The date fell in the third year of the Reiwa era (令和, Reiwa), the era name adopted on 1 May 2019 on the accession of Emperor Naruhito. Under the nengō (年号) system, years are counted afresh from the start of each imperial era, so 7 October 2021 is written Reiwa 3 in official Japanese dating.

Why It Mattered

The 2021 Chiba earthquake was the strongest earthquake to strike the Tokyo area since the 2011 Tōhoku earthquake (東北地方太平洋沖地震, Tōhoku-chihō Taiheiyō-oki Jishin) of 11 March 2011. That comparison gave the event its weight: for residents of the Kantō region, it was the sharpest reminder in a decade that significant shaking can arrive without warning directly beneath the capital region.

The earthquake also illustrated the character of deep-focus events in the Kantō basin. A magnitude 5.9 at 62 to 75 kilometres' depth produces long, rolling shaking across a broad footprint rather than a tight zone of destruction, and the Shindo 5+ maximum reflected that pattern. The event caused no tsunami, since a rupture at that depth does not displace the sea floor.

The Name in Japanese

The Japanese name is 千葉県北西部地震, read in kana as ちばけんほくせいぶじしん and romanised as Chiba-ken Hokuseibu Jishin. The pattern is the standard one for Japanese earthquake names: the place, then the direction within it, then the word for earthquake.

The final two characters, 地震 (じしん, jishin), mean "earthquake" and close almost every Japanese seismic event name. The distinctive part is everything before them.

千 (ち, chi) — "thousand". The first character of the place name.

葉 (ば, ba) — "leaf". Together, 千葉 (ちば, Chiba) is written with the characters for "thousand" and "leaf". The combination is a jukujikun (熟字訓), a compound whose reading cannot be predicted from the individual characters; the name is simply Chiba.

県 (けん, ken) — "prefecture". One of the standard suffixes for Japan's 47 prefectures, here marking Chiba Prefecture as the unit of geography.

北 (ほく, hoku) — "north". The first element of the directional compound.

西 (せい, sei) — "west". With 北 it forms 北西 (ほくせい, hokusei), "north-west". Japanese compass compounds follow the order north/south first, then east/west, so "north-west" is literally "north-west" in that sequence.

部 (ぶ, bu) — "part, section". Added to a direction, it gives "the … part", so 北西部 (ほくせいぶ, hokuseibu) means "the north-western part" — here, of Chiba Prefecture.

地 (じ, ji) — "ground, earth".

震 (しん, shin) — "shake, quake". With 地 it forms 地震 (じしん, jishin), "earthquake".

Read as a whole, the name means "the earthquake of the north-western part of Chiba Prefecture". It is a descriptive label rather than a poetic one, and it tells a Japanese reader exactly where the event occurred before they reach the word "earthquake".

Key Figures

| Item | Detail | |---|---| | Date and time | 7 October 2021, 22:41 JST (13:41 UTC) | | Japanese name | 千葉県北西部地震 (Chiba-ken Hokuseibu Jishin) | | Epicentre | 35.577°N, 140.070°E — about 4 km south-west of Chiba | | Depth | Approximately 62–75 km | | Magnitude | 5.9 | | Maximum intensity | Shindo 5+ (震度5強), equivalent to VI (Strong) on the Mercalli scale | | Era | Reiwa 3 (令和3年) | | Prefecture | Chiba Prefecture (千葉県) |