Earthquake swarm

A series of earthquakes occurring in a relatively small area, not including earthquakes with a magnitude significantly larger than other earthquakes. The epicenters are very shallow, and they tend to occur repeatedly in a specific location. They often occur in volcanic belts on mid-ocean ridges and island arcs. Earthquakes accompanying volcanic eruptions are also earthquake swarms.

Earthquake swarms are mainly composed of several groups with similar waveforms, and the earthquakes in each group occur within an extremely narrow area. These correspond to areas in the earth's crust where strength is particularly weak, and it is believed that stress concentrates there, causing repeated destruction. The earthquake swarms in the seas off Miyakejima, Niijima, and Kozushima in 2000 (Heisei 12) were the result of magma intruding in a vertical plate shape extending from the northwest to the southeast, and included five earthquakes of magnitude 6 or higher.

[Kunihiko Shimazaki]

"Masaaki Kimura, Volcanic Eruptions and Earthquakes: The Shaking Japanese Archipelago: Predicting Crustal Movement from Earthquake Clusters and Crater Floor Rising" (1992, Tokuma Shoten) " "Earthquakes in Hokkaido" by Hidenori Shimamura and Takeo Moriya (1994, Hokkaido University Press) " "The Story of Earthquakes" by Kiyoo Mogi (2001, Asakura Shoten)

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

比較的狭い地域に続発する地震活動で、マグニチュードが他の地震より際だって大きい地震を含まない活動。震源はきわめて浅く、特定の場所に繰り返しおこることが多い。中央海嶺(かいれい)や島弧（弧状列島）の火山帯に多く発生する。火山噴火に伴う地震も、群発地震である。

　群発地震は、主として波形がよく似たいくつものグループから構成されており、各グループの地震はきわめて狭い範囲内でおきている。これは、地殻内の、強度がとくに弱い所に相当し、ここに応力が集中して破壊が繰り返しおこるものと思われる。2000年（平成12）の三宅島(みやけじま)近海、新島(にいじま)・神津島(こうづしま)近海の群発地震は、マグマが北西から南東の方向に延びる垂直の板状に侵入した結果おこったもので、マグニチュード6以上の地震が5回発生した。

［島崎邦彦］

『木村政昭著『噴火と地震――揺れ動く日本列島　群発地震と火口底上昇で地殻変動を予測する』（1992・徳間書店）』▽『島村英紀・森谷武男著『北海道の地震』（1994・北海道大学図書刊行会）』▽『茂木清夫著『地震のはなし』（2001・朝倉書店）』

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