February 26, 2010 Earthquake: Tori Shima, Okinawa, Japan

Felt (V) on Okinawa. Also felt at Taipei and T'ao-yuan, Taiwan. Recorded (5L JMA) on Okinawa; (4 JMA) in the Kerama-retto and on Kitadaito and Yoron-jima; (3 JMA) on Aguni-jima, Amami-oshima, Ie-jima, Iheya-shima, Izena-shima, Kikaiga-shima, Kume-jima, Minamidaito-jima, Okino-erabu-shima, Tokuno-shima, Tonaki-jima, Uke-jima and Yoro-jima; (2 JMA) on Akuseki-jima, Kareroma-jima, Miyako-jima and Suwanose-jima; (1 JMA) on Iriomote-jima, Ishigaki-jima, Tanega-shima, Tarama-shima and Yaku-shima. Also recorded (1 JMA) in Kagoshima, Kumamoto and Miyazaki, Kyushu.

The February 26, 2010, M 7.9 Ryukyu Islands earthquake occurred as the result of shallow strike-slip faulting near the boundary that accommodates most of the relative motion between the Philippine Sea and Eurasia tectonic plates. Focal mechanism solutions indicate that rupture occurred on either a left-lateral east-striking fault or a right-lateral north-striking fault. In the region of the earthquake, the Philippine Sea plate moves west-northwest with respect to the Eurasia plate at a velocity of about 60 mm/yr. The Philippine Sea plate subducts beneath the Eurasia plate at the Ryukyu Trench and is seismically active to depths of about 250 km. The initial estimates of the earthquake’s epicenter, focal depth, and focal mechanism solutions imply that the shock occurred as an intraplate event, either within the subducting Philippine Sea plate or within the overlying Eurasia plate, rather than on the thrust fault plate interface that separates the two, but preliminarily data do not clearly discriminate between these two possibilities. Slip on a fault aligned with either nodal plane of the focal mechanism solution is consistent with intraplate rupture in either tectonic plate.

The largest, instrumentally recorded, shallow-focus earthquakes from the region of the central Ryukyu Trench have had magnitudes in the M 7.1–7.4 range. The two closest and most recent events of M 7+ relative to the February 26th event are a M 7.7 earthquake in September 1998 east of Taiwan and a M 7.1 earthquake in October 1995 that was also within the Ryukyu Islands, about 200 km to the northeast. Both of these events were also shallow-focus earthquakes, but with dissimilar focal mechanism solutions to the February 26th event.

Hayes et al. (2016) Tectonic summaries of magnitude 7 and greater earthquakes from 2000 to 2015, USGS Open-File Report 2016-1192. (5.2 MB PDF)

Felt (V) on Okinawa. Also felt at Taipei and T'ao-yuan, Taiwan. Recorded (5L JMA) on Okinawa; (4 JMA) in the Kerama-retto and on Kitadaito and Yoron-jima; (3 JMA) on Aguni-jima, Amami-oshima, Ie-jima, Iheya-shima, Izena-shima, Kikaiga-shima, Kume-jima, Minamidaito-jima, Okino-erabu-shima, Tokuno-shima, Tonaki-jima, Uke-jima and Yoro-jima; (2 JMA) on Akuseki-jima, Kareroma-jima, Miyako-jima and Suwanose-jima; (1 JMA) on Iriomote-jima, Ishigaki-jima, Tanega-shima, Tarama-shima and Yaku-shima. Also recorded (1 JMA) in Kagoshima, Kumamoto and Miyazaki, Kyushu. (reference #1053)

Two people aged 66 and 74 were injured as they fell following the earthquake, but neither was seriously hurt, local officials said. Water pipelines were ruptured at more than 20 places, while a tree fell over in central Okinawa. (reference #7060)