In this section, we will examine the location and distribution of recent earthquakes in and around the Japanese archipelago.

Fig.2-2 shows the distribution of earthquakes that have occurred in and around the Japanese archipelago over the past 70 years or so with M 5 or greater and a depth of 100 km or less. Fig.2-3 shows the distribution of earthquakes that have occurred in the past 110 years or so with M 6 or greater and a depth of 100 km or less. Most of the earthquakes shown are ranked as either M 5 or M 6. At a minimum, this indicates that earthquakes of this magnitude have occurred throughout Japan over the past about 100 years. It is also apparent that several areas have an extremely high rate of occurrence of earthquakes, particularly the area in the offshore area of the Pacific Ocean of the Tohoku region. We can also see that on land in the Japanese archipelago, earthquakes occur at depths of 20 km or less. Further earthquakes that occur in these shallow locations-even M 5 or M 6 earthquakes-sometimes cause extensive damage locally. Finally, when viewed from a nationwide perspective, earthquakes of M 6 or greater occur at a generally uniform frequency over the long term (Fig.2-4).

Earthquakes off the Pacific coast occur at increasingly deeper locations the closer they come to the Japanese archipelago. The tendency continues to the land area and areas deep under the Sea of Japan (Japan Sea). Using the earthquakes in Japan's Tohoku region as an example, the depth of these earthquakes extends from 0-50 km in the open sea, 50-200 km in the area from the coastal area to land, and 400-600 km in the area from the western part of the Sea of Japan to the coast of Russia (Fig.2-5).

Next, we'll look at the distribution of larger earthquakes. Fig.2-6 shows the distribution of earthquakes of M 7 or greater that have occurred in the past 110 years or so and at a depth of 100 km or less. A comparison with Fig.2-2 and Fig.2-3 shows that the frequency of occurrence significantly declines as the magnitude increases, and the areas of occurrence are rather limited. In such case, a rule has been found that the frequency of earthquake occurrence drops to about 1/10 with each increase of one degree in magnitude. Fig.2-6 shows there is an extremely high rate of occurrence of earthquakes with M 7 or greater in the open sea of the Pacific Ocean from Hokkaido to the Kanto region. There is also a high frequency of earthquakes in the belts ranging from the open sea off the Boso Peninsula (Boso Hanto) to Sagami Bay (Sagami Wan), and the area in the open sea in the Pacific Ocean from Kyushu to the Nansei Islands (Nansei Syoto). Deep ocean trenches known as "troughs" run parallel to these zones (Fig.2-7). In this region, few earthquakes occur along the Nankai Trough, which extends from Suruga Bay (Suruga Wan) to the open sea off Shikoku, as shown in Fig.2-6. It is known, however, that a series of great M 8-level earthquakes occurred here in the past. There is also a zone running from north to south in the Sea of Japan from Hokkaido to the Tohoku region in which earthquakes of M 7 or greater frequently occur. While there are comparatively fewer earthquakes in the land area than in the sea regions, particularly in the areas where there are active faults (which are described later), earthquakes of M 7 or greater do occur in shallow locations on land.

Many destructive earthquakes are known based on report contained in historical documents. Fig.2-8 shows the locations of the primary destructive earthquakes of the past that are known based on historical documents. There are periods in the past in which a concentration of earthquakes of M 7 or greater occurred. For example, there were many M7 - M8 destructive earthquakes in the 1850s during the Ansei Era (*1).

Thus, we can estimate the location and scale of earthquakes that occurred before 1884, when modern earthquake observation began, from the conditions and extent of the damage described in these historical accounts. Therefore, there are larger errors for the earlier earthquakes than for those for which modern observation equipment was available. Also, the precision with which the location and the scale of earthquakes is determined by observation equipment improves every year due to advances in observation equipment, greater numbers of monitoring areas, and improved methods for determining the hypocenter of an earthquake.

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*1

The 1853 Odawara Earthquake (M 6.7), the 1854 Iga-Ueno Earthquake (M 7 1/4), the 1854 Ansei Tokai Earthquake (M 8.4), the 1854 Ansei Nankai Earthquake (M 8.4), the 1854 earthquake near Ehime and Oita Prefectures (M 7.3-7.5), the 1855 Ansei Edo Earthquake (M 6.9), the 1856 earthquake off the east coast of Aomori Prefecture (M 7.5), the 1857 Earthquake at Aki-nada Sea (M 7 1/4), the 1858 Hietsu Earthquake (M 7.0-7.1), and the 1858 earthquake off the east coast of Aomori Prefecture (M 7-7.5)