1987 Chiba earthquake

The 1987 Chiba Prefecture offshore earthquake (Japanese: 1987 千葉県沖地震) occurred off the east coast of the Boso Peninsula in Chiba Prefecture at 11:08:01 JST on December 17, 1987.

1987 Chiba earthquake
千葉県沖地震
1987 Chiba earthquake is located in Chiba Prefecture
1987 Chiba earthquake
1987 Chiba earthquake is located in Japan
1987 Chiba earthquake
UTC time1987-12-17 02:08:19
ISC event450774
USGS-ANSSComCat
Local dateDecember 17, 1987 (1987-12-17)
Local time11:08
Magnitude7.3 Me[1]
6.7 Mw[2]
Depth62.9 km (39.1 mi)
Epicenter35.372°N 140.519°E / 35.372; 140.519[3]
TypeStrike-slip
Max. intensityJMA 5− [4]

VII (Very strong)
Casualties2 dead, 144 injured

Tectonic setting

The crustal structure in the southern Kanto region is complex, involving boundaries between three tectonic plates. The Philippine Sea Plate is subducting beneath the Okhotsk Plate along the line of the Sagami Trough and beneath the Eurasian Plate along the line of the Nankai Trough. Further to the east, the Pacific Plate is subducting beneath the Okhotsk and Philippine Sea plates along the line of the Japan Trench.[3] A velocity analysis of the region beneath Kanto suggests that the wedge of oceanic mantle lithosphere of the Philippine Sea Plate in contact with the Pacific Plate is affected by serpentinization as shown by the anomalously low seismic velocity. The boundary between the serpentinized and unmetamorphosed parts of the mantle wedge is interpreted to be vertical and NNW–SSE trending.[5]

Earthquake

The earthquake had a strike-slip focal mechanism, which with its depth, indicates that it was a result of faulting within the Philippine Sea Plate. The focal mechanism gives two possible fault planes, one west–east trending, dipping steeply to the south, and the other north–south trending, dipping steeply to the east. The distribution of aftershocks suggests a NNW–SSE trending rupture, consistent with the nearly north–south fault plane, indicating dextral (right-lateral) strike-slip faulting with a small component of reverse faulting.[3] The earthquake is interpreted to have ruptured part of the boundary between the two parts of the Philippine Sea Plate mantle wedge.[5]

Damage

According to the materials of the Fire and Disaster Prevention Division of the Chiba Prefecture General Affairs Department, two deaths (one in Mobara and another in Ichihara), 26 serious injuries and 118 minor injuries were reported. 16 buildings collapsed, 102 were severely damaged, and 63,692 suffered minor damage. Three fires, power outages, and train service disruptions were also reported.[6][7] Liquefaction was observed on landfills and riverbanks. Even though most of the damage occurred in Chiba Prefecture, damage and landslides were also reported in Kanagawa Prefecture.[8][9][10]

See also

References
  1. "M 6.7 - 10 km SW of Mobara, Japan - Technical Summary". earthquake.usgs.gov.
  2. "M 6.7 - 10 km SW of Mobara, Japan". earthquake.usgs.gov.
  3. Okada, Y.; Kasahara, K. (1990). "Earthquake of 1987, off Chiba, central Japan and possible triggering of eastern Tokyo earthquake of 1988". Tectonophysics. 172 (3–4): 351–364. doi:10.1016/0040-1951(90)90041-6.
  4. "震度データベース検索". 気象庁. Retrieved 2022-04-11.
  5. Nakajima, J.; Hasegawa, A. (2010). "Cause of M ~ 7 intraslab earthquakes beneath the Tokyo metropolitan area, Japan: Possible evidence for a vertical tear at the easternmost portion of the Philippine Sea slab". Journal of Geophysical Research: Solid Earth. 115 (B4). doi:10.1029/2009JB006863.
  6. "1987年千葉県東方沖地における災害情報の伝達と市町村・住民の対応] 東京大学社会情報研究所廣井研究室" [1987 Transmission of disaster information off the east coast of Chiba Prefecture and response of municipalities and residents] (PDF). Hiroi Laboratory, Institute of Social Information, University of Tokyo. Archived from the original (PDF) on 2016-03-05. Retrieved 2022-04-11.
  7. 千葉県東方沖地震による山地災害とその対応 -その後 地すべり Vol.26 (1989-1990) No.1 P.41-43, doi:10.3313/jls1964.26.41
  8. 久保 徹：千葉県東方沖地震に伴う三浦半島の被害状況 地すべり Vol.24 (1987-1988) No.4 P.39-40, doi:10.3313/jls1964.24.4_39
  9. Butts, David (December 17, 1987). "Earthquake in Japan kills two, injures 53". United Press International. Retrieved April 12, 2022.
  10. "千葉県東方沖地震" [Chiba Prefecture East Offshore Earthquake]. typhoon.yahoo.co.jp (in Japanese). Retrieved April 12, 2022.
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