2003 Miyagi earthquakes
Two major earthquakes struck Miyagi Prefecture in Japan in 2003. The first event measured 7.1, had a near-intermediate depth of and struck the town of Kesennuma, near the border with Iwate Prefecture, injuring 174 people in five prefectures, while the second event, despite having a lower magnitude of 6.4, was much more destructive and intense due to occurring much closer to the surface at, injuring nearly 680 people across multiple towns near Sendai and Ishinomaki in Miyagi. Both events caused moderate damage to infrastructure, triggered landslides, soil liquefaction and other ground effects, and damaged or destroyed 18,488 homes, 5,108 of them severely.
Tectonic setting
The Pacific plate, made of oceanic lithosphere, subducts beneath the Okhotsk Sea plate along a convergent boundary located off the east coast of the northern half of Japan. It runs from the Boso triple junction and ends near Hokkaido, where it joins the Kuril–Kamchatka Trench. At this location, the Pacific plate moves approximately westward relative to the North American plate at a velocity of /yr, subducting beneath Japan at the Japan Trench. This subduction zone is capable of producing megathrust earthquakes with magnitudes greater than 8.5, evident in the historical records. It was on the subduction interface where the 2011 Tōhoku earthquake nucleated. That event involved a x rupture area on the subduction zone.Earthquakes
May 26
With an epicenter in Kesennuma, Miyagi Prefecture, the earthquake occurred as the result of oblique-reverse faulting at a depth of. The United States Geological Survey reported a magnitude of 7.0, while the Japan Meteorological Agency measured the earthquake at 7.1. According to GPS observations, horizontal crustal deformation of up to approximately was observed west of the epicenter, consistent with the earthquake's focal mechanism. The earthquake was followed by 27 aftershocks that struck throughout the remainder of 2003, most of which occurred in Kesennuma, Miyagi and Rikuzentakata, Iwate; the largest aftershock measured 4.8.The earthquake had a maximum intensity of Shindo 6- on Japan Meteorological Agency seismic intensity scale, which was observed in both Miyagi and Iwate Prefectures. A USGS seismic installation at Hanamaki, Iwate recorded 0.297 g in ground acceleration and in ground velocity ; the station data corresponded to a Modified Mercalli intensity of IX ; another station in Ishinomaki recorded a pga of 1.134 g. MMI IX shaking was also estimated in Kesennuma, based on damage to a road near the epicenter in the town. Additionally, a map created by the National Research Institute for Earth Science and Disaster Resilience estimated that shaking exceeding 1,000 gal had likely been observed in much of northeastern Miyagi and southeastern Iwate.
| Intensity | Prefecture | Locations |
| 6− | Miyagi | Ishinomaki, Wakuya, Kurihara |
| 6− | Iwate | Ichinoseki, Ōfunato, Hiraizumi, Ōshū |
| 5+ | Miyagi | Kesennuma, Minamisanriku, Tome, Higashimatsushima, Misato, Ōsaki, Shikama, Kami |
| 5+ | Iwate | Rikuzentakata, Morioka, Kanegasaki, Sumita, Kamaishi, Tōno, Hanamaki, Yahaba, Hirono, Ninohe |
| 5+ | Akita | Daisen |
| 5+ | Aomori | Hashikami |
| 5+ | Yamagata | Yamgata |
| 5− | Miyagi | Ōhira, Ōsato, Taiwa, Sendai, Natori, Kawasaki, Zaō, Ōgawara, Watari |
| 5− | Iwate | Kitakami, Nishiwaga, Shiwa, Takizawa, Ōtsuchi, Yamada, Miyako, Iwaizumi, Fudai, Fudai, Kuji, Hachimantai |
| 5− | Akita | Yuzawa, Ugo, Yokote, Daisen, Akita |
| 5− | Aomori | Hachinohe, Nanbu, Gonohe |
| 5− | Yamagata | Murayama |
| 5− | Fukushima | Sōma, Minamisōma, Tamura, Tomioka |
July 26
At 07:13:29 JST on 26 July, a 6.0 or 6.4 event struck Ōsato, Miyagi, at a depth of and occurring as a result of reverse faulting. It was preceded by a 5.5 or 5.6 foreshock at 00:13:07 JST, which had a maximum intensity of Shindo 6- at Higashimatsushima. The mainshock had a maximum intensity of Shindo 6+ in Higashimatsushima and Misato, Miyagi. It also had a Modified Mercalli intensity of VIII, and a pga of 2,037 gal was recorded in Naruse, the highest ever recorded at the time. The mainshock occurred on the previously-unknown Asahiyama flexure, an north-south trending fault in Higashimatsushima. Aftershocks from the event were distributed in a 15 km x 15 km area, occurring beneath the surface.| Intensity | Prefecture | Locations |
| 6+ | Miyagi | Higashimatsushima, Misato |
| 6− | Miyagi | Ishinomaki, Wakuya, Ōsaki |
| 5+ | Miyagi | Tome |
| 5− | Miyagi | Kurihara, Ōsato, Sendai |
| 4 | Miyagi | Kawasaki, Ōgawara, Watari, Natori, Tagajō, Shichigahama, Rifu, Matsushima, Ōhira, Shikama, Minamisanriku, Kesennuma |
| 4 | Iwate | Rikuzentakata, Ichinoseki, Hiraizumi, Ōshū, Hanamaki, Ōtsuchi, Yahaba, Morioka, Fudai, Fudai, Ninohe |
| 4 | Akita | Daisen |
| 4 | Yamagata | Murayama, Mogami, Shinjō, Nakayama |
| 4 | Fukushima | Shinchi, Iwaki |