The number of felt earthquakes observed in Japan over the past week was lower compared to the previous week. Earthquakes are evident in the Kanto region and on the Pacific side of northern Japan. There have been two earthquakes of magnitude 3 or above. (Statistics from 10 a.m. on August 19th to 25th)
At around 0:50 a.m. on Monday, the 19th, an earthquake with a magnitude of 5.1 and a depth of 8 kilometers occurred in the northern part of Ibaraki Prefecture. The earthquake caused the maximum intensity of Hitachi City in Ibaraki Prefecture to decrease by 5 levels, Takahagi City, Hitachi-Ota City, Kita Ibaraki City, Kasama City, Hitachi Naka City, Shirakawa City in Fukushima Prefecture, Furudo Town, Haga Town in Tochigi Prefecture, etc. The intensity of the earthquake in Kasu City, Saitama Prefecture, dropped to magnitude 3 in the prefecture where the earthquake magnitude decreased by 4 magnitude.
The observed intensity of the earthquake was magnitude 3.
This is the first earthquake in about two years since April 2022 with an epicenter in northern Ibaraki Prefecture and an intensity below magnitude 5 or above. In addition, this is the third time in August that Japan has observed an earthquake with temperatures below 5 degrees or above. The earthquake mechanism is analyzed as a normal fault type with the tension axis trending northeast-southwest.
Just before this earthquake occurred, at around 0:48, an earthquake with a magnitude of 4.8 on the Richter scale and a maximum intensity of 4 occurred at almost the same epicenter. The mechanism was almost the same, so it seemed to be a series of events.
The northern part of Ibaraki Prefecture is an area where earthquakes of magnitude 5 to 6 occur frequently due to the subduction of the Pacific Plate and the Philippine Sea Plate. Earthquakes accompanied by strong shaking mostly occur at shallow depths less than 10 kilometers and slightly deeper than 50 to 100 kilometers. In 2016, a magnitude 6.3 earthquake occurred near the epicenter, with the maximum earthquake intensity being 6 magnitudes lower.
The Japanese Government’s Earthquake Research Promotion Headquarters estimates that there is a 70-80% chance of an earthquake of around magnitude 7.1 occurring in Hyuga-nada within the next 30 years, and this earthquake appears to be comparable to that.
Because the epicenter was located in the Nankai Trough Extraordinary Earthquake Monitoring Area and the magnitude was above 6.8, an assessment and review meeting was held. Based on this, it was judged that the possibility of a large-scale earthquake in the Nankai Trough earthquake was higher than usual, and the Nankai Trough earthquake emergency information (major earthquake warning) was issued.
This is the first time this information has been announced since it began operations in 2019. Should
Those who are already prepared for an earthquake should double-check their preparedness, and those who are not fully prepared should also use this time to check disaster prevention supplies, evacuation routes, hazard maps, etc.
At around 9:33 on Saturday, the 17th, an earthquake with a magnitude of 4.5 and a depth of approximately 50 kilometers occurred near the coast of Miyagi Prefecture, the epicenter of the earthquake. The maximum intensity of this earthquake in Iwanuma and Ishinomaki cities in Miyagi Prefecture was magnitude 3.
This is the first magnitude 3 earthquake to occur along the coast of Miyagi Prefecture since the 23rd of last month. The source analysis is a reverse fault type, and the pressure cycle is from west-northwest to east-southeast.
Although the offshore area of Miyagi Prefecture is an earthquake-prone area, no earthquake of magnitude 3 or above has occurred since October last year. After the first magnitude 3 earthquake in 10 months was observed on the 21st of last month, another earthquake occurred on the 23rd. This was the first earthquake since then. The epicenter and depth are almost the same as those of the earthquake on the 21st.
Even after the Great East Japan Earthquake in 2011, earthquakes of magnitude 6 occurred frequently along the coast of Miyagi Prefecture. In 2021, two earthquakes with a maximum intensity of magnitude 5 or above occurred. Although the situation has remained relatively calm this year, it is not surprising that a major earthquake can occur at any time, and preparation is essential.
At around 12:29 on Saturday, the 10th, an earthquake with an epicenter located in the southern Sea of Okhotsk occurred. It was estimated to have a magnitude of 6.8 and a depth of approximately 490 kilometers. The magnitude of this earthquake was 3 in Hakodate City and Hachinohe City in Hokkaido, Mutsu City, Hashiage Town, Hiraiuchi Town in Aomori Prefecture, and Morioka City in Iwate Prefecture.
In this earthquake, there were very few places with seismic intensity above 1 degree on the Sea of Okhotsk side of Hokkaido close to the epicenter, while seismic intensity of 3 degrees was observed on the Northeast Pacific side, which was far away from the epicenter.
When a large-scale deep earthquake occurs, a seismic intensity distribution called an “abnormal seismic zone” will appear, and strong vibrations will be transmitted to long distances. Although it is called an anomaly, it is a phenomenon that often occurs in deep earthquakes.
In deep earthquakes, strong shaking propagates along the subducting plate, and the shaking becomes larger closer to the plate boundary. In this earthquake, the shaking is thought to have spread along the Pacific Plate to Hokkaido and the Pacific coast to the northeast, where seismic waves propagate easily.
Similar earthquakes have occurred in the Sea of Okhotsk in the past, including a 7.3-magnitude earthquake in 2012 that caused vibrations that even spread to the Kanto region. Large, deep earthquakes can cause strong shaking far away from the epicenter, so caution is required.
According to analysis by the U.S. Geological Survey, no earthquake of magnitude 6 or above has yet occurred. The largest earthquake occurred in the Philippines with a magnitude of 5.7.
This time we focus on the Hawaii earthquake. On the evening of the 22nd (Thursday), Japan time, an earthquake with an epicenter approximately 7 kilometers away and an estimated magnitude of 4.7 occurred in the southeastern part of Hawaii Island.
In the southeastern part of Hawaii Island, there are two types of earthquakes: those that occur directly beneath Kilauea volcano, and those that occur off the coast, in this case the latter. In 2018, the same type of 6.9-magnitude earthquake occurred offshore, triggering a tsunami of up to 40 centimeters.
Since around the 20th (Tuesday), seismic activity in the area has continued to be active. According to the Hawaii Earthquake Observatory, the number of earthquakes is decreasing slightly and there are no signs of imminent eruptions. Kilauea volcano’s alert level is “ADVISORY”, the second to last level, and monitoring continues.
On the evening of the 5th (Friday, Japan time), an earthquake with a magnitude of 4.8 and a depth of about 5 kilometers occurred in New Jersey, near New York. The earthquake mechanism is speculated to be a strike-slip earthquake.
The quake is thought to have caused a magnitude IV earthquake on the revised Mercalli intensity scale even in central New York. Although it is impossible to compare accurately, the earthquake was equivalent to a magnitude 1 to 2 earthquake on Japan’s seismic intensity scale.
Earthquakes are relatively rare in the northeastern United States, including the New York area. Since 1950, only 40 earthquakes of magnitude 3 or above have occurred within 250 kilometers of the epicenter. Among them, this earthquake with a magnitude of 4.8 is the largest.
In addition, the 5.8-magnitude earthquake that occurred in Virginia in August 2011 is known as the earthquake that caused tremors in New York and other areas.
Source/Reference
*Unless otherwise stated, information on Japan’s epicenter and intensity comes from the Japan Meteorological Agency. Unless otherwise stated, overseas epicenter information comes from the United States Geological Survey (USGS). The epicenter information may vary depending on the issuing agency.
