1987 Chiba earthquake
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| 千葉県東方沖地震 | |
   | |
| UTC time | 1987-12-17 02:08:19 |
|---|---|
| ISC event | 450774 |
| USGS-ANSS | ComCat |
| Local date | December 17, 1987 |
| Local time | 11:08 |
| Magnitude | 7.3 Me[1] 6.7 Mw[2] |
| Depth | 62.9 km (39.1 mi) |
| Epicenter | 35°22′19″N 140°31′08″E / 35.372°N 140.519°E[3] |
| Type | Strike-slip |
| Max. intensity | JMA 5−[4] MMI VII (Very strong) |
| 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.
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.[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]