1987 Chiba earthquake
{{Short description|Earthquake near Tokyo, Japan}}
{{Infobox earthquake
| title = 1987 Chiba earthquake
| native_name = 千葉県東方沖地震
| native_name_lang = ja
| image =
| local-date = {{start-date|December 17, 1987}}
| local-time = 11:08
| timestamp = 1987-12-17 02:08:19
| anss-url = usp0003bbq
| isc-event = 450774
| map2 = {{Location map | Japan Chiba Prefecture#Japan
| relief = yes
| label =
| lat = 35.372
| long = 140.519
| mark = Bullseye1.png
| marksize = 50
| position = top
| width = 260
| float = center
| caption = }}
| magnitude = 7.3 {{M|e|link=y}}{{Cite web|title=M 6.7 - 10 km SW of Mobara, Japan - Technical Summary|url=https://earthquake.usgs.gov/earthquakes/eventpage/usp0003bbq/technical|publisher=United States Geological Survey}}
6.7 {{M|w|link=y}}{{Cite web|title=M 6.7 - 10 km SW of Mobara, Japan|url=https://earthquake.usgs.gov/earthquakes/eventpage/usp0003bbq/executive|publisher=United States Geological Survey}}
| depth = {{Convert|62.9|km|abbr=on}}
| aftershocks =
| location = {{coord|35.372|140.519}}{{Cite journal |last1=Okada |first1=Y. |last2=Kasahara |first2=K. |date=1990 |title=Earthquake of 1987, off Chiba, central Japan and possible triggering of eastern Tokyo earthquake of 1988 |journal=Tectonophysics |volume=172 |issue=3–4 |pages=351–364 |doi=10.1016/0040-1951(90)90041-6|bibcode=1990Tectp.172..351O }}
| intensity = {{JMA|5-}}{{Cite web|url=https://www.data.jma.go.jp/svd/eqdb/data/shindo/index.html#19871217110816|title=震度データベース検索|accessdate=2022-04-11|publisher=気象庁}}
{{MMI|VII}}
| countries affected =
| damage =
| landslide =
| type = Strike-slip
| casualties = 2 dead, 144 injured
}}
The 1987 Chiba Prefecture offshore earthquake (Japanese: 千葉県東方沖地震) occurred off the east coast of the Boso Peninsula in Chiba Prefecture at 11:08:01 JST on December 17, 1987.
Tectonic setting
The crustal structure in the southern Kantō 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. 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.{{Cite journal |last1=Nakajima |first1=J. |last2=Hasegawa |first2=A. |date=2010 |title=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=Journal of Geophysical Research: Solid Earth |volume=115 |issue=B4 |doi=10.1029/2009JB006863|bibcode=2010JGRB..115.4301N |doi-access=free }}
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. The earthquake is interpreted to have ruptured part of the boundary between the two parts of the Philippine Sea plate mantle wedge.
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.{{Cite web|url=http://www.hiroi.iii.u-tokyo.ac.jp/index-houkokusho-rist-tibaken.pdf|title=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|trans-title=1987 Transmission of disaster information off the east coast of Chiba Prefecture and response of municipalities and residents|location=Hiroi Laboratory, Institute of Social Information, University of Tokyo|access-date=2022-04-11|archive-date=2016-03-05|archive-url=https://web.archive.org/web/20160305005417/http://www.hiroi.iii.u-tokyo.ac.jp/index-houkokusho-rist-tibaken.pdf|url-status=dead}}[https://doi.org/10.3313/jls1964.26.41 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}} 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.[https://doi.org/10.3313/jls1964.24.4_39 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}}{{Cite web|url=https://www.upi.com/amp/Archives/1987/12/17/Earthquake-in-Japan-kills-two-injures-53/6188566715600/|title=Earthquake in Japan kills two, injures 53|last=Butts|first=David|date=December 17, 1987|access-date=April 12, 2022|publisher=United Press International}}{{Cite web|url=https://typhoon.yahoo.co.jp/weather/calendar/127/|title=千葉県東方沖地震|trans-title=Chiba Prefecture East Offshore Earthquake|access-date=April 12, 2022|website=typhoon.yahoo.co.jp|language=ja}}
See also
References
{{Reflist}}
{{Earthquakes in Japan}}
{{Earthquakes in 1987}}
Category: Earthquakes in Japan