Earthquakes in Japan: Recent Seismic Activity and Impact on Northern Ibaraki Prefecture

At approximately 20:40 on the 24th (Wednesday), an earthquake estimated at magnitude 5.1 and a depth of 55 km occurred with its epicenter in northern Ibaraki Prefecture. As a result of this earthquake, the highest seismic intensity of 4 was observed in Mito City, Hitachi City, Hitachinaka City, Hitachiomiya City, Ichikai Town, Tochigi Prefecture, Noda City, Chiba Prefecture, etc. in Ibaraki Prefecture , and seismic intensity 3 was observed in a wide area of ​​Kanto and parts of Fukushima Prefecture.

This is the first earthquake of magnitude 4 or higher in northern Ibaraki Prefecture since last March. The mechanism of the earthquake is studied as a type of reverse fault with a stress axis running northwest-southeast.

In addition, at approximately 13:46 on the 25th (Thursday), a magnitude 3.7 earthquake from almost the same epicenter, with a maximum seismic intensity of 3 was felt in Tokai City, Ibaraki Prefecture.

Earthquakes in northern Ibaraki Prefecture can be broadly divided into shallow earthquakes that occur near Fukushima Prefecture, and slightly deeper earthquakes that occur near the epicenter. – current land. In this region, at a depth of 50 to 60 km, there is a boundary where the Pacific plate enters the Earth’s plate. In 2016, an earthquake with a magnitude of 5.4 occurred at almost the same epicenter as this time, and the maximum seismic intensity was 5 lower.

Near the center of this earthquake, the Philippine Sea plate is subducting into the earth’s plate. The plate boundary is thought to be about 30km deep, and this earthquake occurred deeper than that.

Furthermore, the mechanism of the earthquake was a type of normal faulting, rather than the type of reverse faulting expected at plate boundaries, so it appears to have occurred along a- inside the Philippine Sea Plate.

The 1968 magnitude 6.6 earthquake was also estimated at a depth of 39km, and had similar mechanisms at this time.

The government’s Central Earthquake Research Promotion Office says that no changes were observed as a result of the earthquake on Wednesday the 17th that would have increased the Nankai Trough’s mega earthquake potential.

At approximately 17:36 on the 27th (Saturday), an earthquake estimated at magnitude 6.9 and a depth of approximately 540 km occurred with its epicenter off the west coast of the Ogasawara Islands. As a result of this earthquake, tremors with a seismic intensity of 3 were observed in Ogasawara City and Hahajima, Tokyo, and tremors with an intensity of 1 to 2 were observed in a wide area from Kanto to Tohoku.

This is the seismic intensity distribution of the “abnormal seismic zone” that occurs during deep earthquakes. In deep earthquakes, strong shaking is transmitted along the subducting plate, and the shaking can be stronger at a distance near the plate boundary. This is called an “unusual seismic zone”, and even if the shaking is small near the epicenter, strong shaking can be sent out far away, so care must be taken.

In 2015, off the west coast of the Ogasawara Islands, a deep earthquake with a magnitude of 8.1 and a depth of 682 km occurred, the largest deep earthquake around Japan since 1919, when detailed records still exist. At this time, the city of Ninomiya in Kanagawa Prefecture, which is far from the epicenter, experienced a seismic intensity of 5 or higher, 23 wards of Tokyo and other areas suffered a tremor of intensity 4, and 47 suffered prefectures seismic intensity of 1 or higher. . Even deep earthquakes can cause damage depending on the scale, so we cannot let our guard down.

At approximately 14:18 on the 9th (Saturday), an earthquake estimated at magnitude 4.5 and a depth of approximately 70 km occurred off the coast of Urakawa, Hokkaido. As a result of this earthquake, a maximum seismic intensity of 3 was observed in the cities of Atsuma, Abira, and Mukawa in Hokkaido, and a seismic intensity of 2 was observed in Sapporo, Chitose, Iwamizawa, Hakodate, Mutsu , Aomori, and other areas. .

This is the second earthquake this year with a seismic intensity of 3 or higher that occurred off the coast of Urakawa on the 28th of last month. The mechanism of the earthquake is studied as a type of reverse fault with a stress axis running northwest-southeast.

In this region, the Pacific plate subducts under the Earth’s plate, and large earthquakes often occur. More recently, in 2016, an earthquake occurred with a magnitude of 6.7 and a maximum seismic intensity of just under 5, and in 1982, an earthquake occurred with a magnitude of 7.1 and a maximum seismic intensity of 6 (class at the time).

According to a study by the US Geological Survey, there have been five earthquakes with a magnitude of 6 or higher. Two of these earthquakes had two epicenters near Taiwan.

At about 3:26 a.m. on the 23rd (Tuesday) Japan time, an earthquake with a magnitude of 6.1 and a depth of about 11 km occurred near Taiwan. The mechanism of the earthquake is studied as a type of reverse fault with a stress axis running northwest-southeast. Also, just 5 minutes later, a 6.0 magnitude earthquake occurred.

Aftershock activity from the magnitude 7.4 earthquake that occurred on April 3rd was temporarily reduced, but became active again around the 22nd (Monday). A magnitude 5.7 earthquake occurred in the early hours of the 27th (Saturday). It is possible that active activity will continue for some time, and there are concerns that the effect will spread in areas that were badly damaged by the main shock.

On the night of the 5th (Friday, Japan time), an earthquake that was estimated to have a magnitude of 4.8 and a depth of about 5 km occurred with its epicenter in New Jersey, near New York. The earthquake mechanism is considered to be strike-slip.

This earthquake is believed to have caused shaking of magnitude IV on the revised Mercalli intensity scale, even in central New York. Although a precise comparison is not possible, the shaking corresponds to a seismic intensity level of 1 to 2 on the Japanese seismic intensity scale.

Earthquakes are very rare in the northeastern United States, including the New York area, and only 40 earthquakes of magnitude 3 or greater have occurred within 250 km of the epicenter. -average since 1950. Among them, this size 4.8 is the largest.

In addition, a magnitude 5.8 earthquake that occurred in Virginia in August 2011 is known as an earthquake that caused tremors in New York and other areas.