2000 Izu Islands earthquakes

What Happened

On 26 June 2000, a swarm of earthquakes began beneath the northern Izu Islands, a chain of volcanic islands lying south of Tokyo Bay. The activity clustered around three islands in particular: Miyake-jima (三宅島), Kōzu-shima (神津島) and Nii-jima (新島). A swarm of this kind — many earthquakes of similar size in the same area over a short period, with no single dominant mainshock — is called a gunpatsu jishin (群発地震) in Japanese.

The largest earthquake in the sequence reached moment magnitude 6.5. Six earthquakes in the swarm registered a maximum seismic intensity of Shindo 6- (震度6弱) on the Japan Meteorological Agency's scale, the second-highest tier below the top of the JMA ladder, and between VII and IX on the Modified Mercalli intensity scale. The activity continued through the year 2000.

Background

The Izu Islands sit on the northern edge of the Philippine Sea Plate, where it meets the Pacific Plate and is forced beneath the Okhotsk Plate along the Sagami Trough and the Izu-Bonin arc. The result is one of the most seismically and volcanically active stretches of Japan: a line of volcanic islands running south from the Izu Peninsula, with Miyake-jima, Kōzu-shima and Nii-jima among the most restless.

The year 2000 was already a period of heightened activity in the region before 26 June. Miyake-jima, the southernmost of the three islands involved, would go on to produce a major volcanic eruption later in the same year, and the seismic unrest that began in late June is part of that broader episode of crustal disturbance beneath the northern Izu Islands.

The events fell in the Heisei era (平成), the reign period of Emperor Akihito, which ran from 1989 to 2019. In the Japanese year-counting system, 2000 corresponds to Heisei 12.

Why It Mattered

A swarm of this scale — with six shocks at Shindo 6- or above and a peak of Mw 6.5 — is a significant seismic event in its own right, and it struck a part of Japan that is administratively within Tokyo Prefecture. The Izu Islands are governed as part of Tokyo (東京都), even though they lie well out in the Pacific, which meant the earthquakes were a Tokyo metropolitan concern as well as a local one.

The swarm also mattered because of what it signalled. Sustained earthquake swarms beneath volcanic islands are often a precursor to, or a companion of, volcanic unrest, and the 2000 activity around Miyake-jima, Kōzu-shima and Nii-jima is studied in that context: as a case of seismic and volcanic processes interacting beneath a densely monitored island arc close to the capital.

The Name in Japanese

The Japanese name is 伊豆諸島北部群発地震, read in kana as いずしょとうほくぶぐんぱつじしん and romanised as Izu-shotō hokubu gunpatsu jishin.

Breaking it down character by character:

• 伊 (i) — the first character of the old province name Izu. It is used here purely as part of the place name; on its own it carries the sense of "that one" in classical Chinese, but in this word it has no independent meaning.
• 豆 (zu) — "bean". Again, here it is only the second half of the place name Izu (伊豆), the peninsula and island region southwest of Tokyo.
• 諸 (sho) — "various", "many", "all together". It signals a group or collection of things.
• 島 (tō) — "island". Together, 諸島 (shotō) means "islands" or "archipelago" — literally "the various islands". So 伊豆諸島 is the Izu Islands.
• 北 (hoku) — "north".
• 部 (bu) — "part", "section". Together, 北部 (hokubu) means "the northern part" — here, the northern portion of the Izu Islands.
• 群 (gun) — "group", "flock", "cluster".
• 発 (patsu) — "to occur", "to break out", "to be emitted". Together, 群発 (gunpatsu) describes something that breaks out in clusters or repeated bursts — the standard term for a swarm.
• 地 (ji) — "earth", "ground".
• 震 (shin) — "to shake", "tremor". Together, 地震 (jishin) is "earthquake".

Read as a whole, the name means "the northern Izu Islands earthquake swarm" — a description of the phenomenon rather than a single named quake, which is exactly what a swarm is.

Key Figures

• 26 June 2000 — the date the swarm began.
• Mw 6.5 — the moment magnitude of the largest earthquake in the sequence.
• Six — the number of earthquakes that reached a maximum JMA seismic intensity of Shindo 6-.
• VII–IX — the range on the Modified Mercalli intensity scale recorded during the swarm.
• Three islands — Miyake-jima (三宅島), Kōzu-shima (神津島) and Nii-jima (新島), the islands around which the activity was concentrated.
• Heisei 12 — the Japanese era year corresponding to 2000.