(1)The Meiji Sanriku Earthquake (June 15, 1896, M 8 1/2) and Sanriku Earthquake (March 3, 1933, M 8.1)

Both earthquakes generated the tsunami that caused enormous damage. Therefore, the tsunami are referred to separately as the Meiji Sanriku Earthquake Tsunami and the Sanriku Earthquake Tsunami. In this section, we will distinguish between the earthquakes by referring to them as the Meiji (1896) earthquake and the Showa (1933) earthquake.

Both the Meiji and the Showa earthquakes occurred near the Japan Trench, from which the Pacific Plate is beginning to subduct directly under the land plate. The results of previous research suggest that the Meiji (1896) earthquake was an interplate earthquake on a reverse fault, while the Showa (1933) earthquake occurred on a normal fault within the Pacific Plate. Both earthquakes were followed by large tsunami about 30 to 40 minutes after they were felt. These earthquakes were catastrophic, destroying countless houses and ships. The reported number of fatalities for the Meiji earthquake 26,360, and that for the Showa earthquake was 3,064, primarily on the Pacific Ocean coast (Fig.4-6, Fig.4-7).

The maximum height for both tsunami was observed at Ryori in Sanriku, Iwate Prefecture. The tsunami height of the Meiji earthquake was 38.2 m, the highest tsunami recorded near Japan since 1868 (Meiji period) (Fig.4-8). The Showa tsunami, also large, was observed at a height of 23.0 m (Fig.4-9). The ground motion of the Meiji earthquake was not substantial, and is estimated at seismic intensity of 4 in JMA scale at most (Fig.4-10). Its tsunami was extremely high, however, leading observers to believe this was a tsunami earthquake with a slower fault slip than that which occurs during normal earthquakes. The seismic ground motion for the Showa earthquake was stronger than that of the Meiji earthquake. It was observed to have a seismic intensity of 5 in JMA scale, primarily in the area along the Pacific Ocean coast. The resulting damage included cracked walls and landslide (Fig.4-11).

Two large aftershocks of M 7.5 occurred on June 16, the day following the main Meiji earthquake. These were the largest aftershocks associated with this earthquake (Fig.4-12). The largest aftershock of the Showa earthquake was measured at M 6.8 and occurred about 3 hours after the main shock. A total of 77 aftershocks with a magnitude greater than 5 were recorded within a six-month period after the main shock (Fig.4-13).

The tsunami accompanying the Showa earthquake was the first to occur after a modern system for observing and studying tsunami was put in place. Accordingly, many different studies have been conducted on this tsunami. The results indicate that tsunami became much larger in areas with a V-shaped bay, such as those in a rias coast. Later studies found that tsunami in general become larger in V-shaped bays when the earthquake occurs relatively close to shore. It was discovered that the tsunami generated by earthquakes occurring in distant locations, such as the one caused by the 1960 Chile Earthquake, grow large in the interior of a bay when the mouth of the bay is narrow and the interior of the bay is wide. This can be attributed to differences in the period of the tsunami.