Weekly Earthquake Update: High Number of Felt Earthquakes in Japan, M5.2 Southern Chiba Earthquake and Updates on Noto Peninsula Earthquake Trends

At around 4:16 on the 11th (Thursday), an earthquake estimated to have a magnitude of 5.2 and a depth of 40 km occurred with an epicenter in southern Chiba Prefecture. In this earthquake, strong shaking with a maximum seismic intensity of upper 5 was observed in Kisarazu City, Chiba Prefecture, and a seismic intensity of lower 5 was observed in Kimitsu City, Chiba Prefecture. In addition, seismic intensity 4 was observed in Chiyoda Ward, Tokyo, Chiba City in Chiba Prefecture, Kamogawa City in Chiba Prefecture, and Yokohama City in Kanagawa Prefecture, and seismic intensity 3 was observed in a wide area of ​​Kanto.

The mechanism of the earthquake is a type with a tension axis in the northwest-southeast direction, and is analyzed as a type close to a normal fault with a slight strike-slip component. It is believed that the earthquake was caused by the rupture of the interior of the Philippine Sea plate.

It was the first earthquake of magnitude 5 or higher centered in southern Chiba Prefecture since July 2012, when it had a magnitude of 5.2. It was the first time in 11 years that an earthquake with a seismic intensity of upper 5 was observed in Chiba Prefecture since the March 2012 earthquake off the east coast of Chiba Prefecture.

Underneath the southern part of Chiba Prefecture, the North American plate, the Philippine Sea plate, and the Pacific plate intertwine, creating a complex structure. Looking at the distribution of hypocenters since 2001, we can see that the areas with many earthquakes are roughly divided into areas with a depth of 10 to 20 km, around 40 km, which is the same as this time, and around 70 km.

The 5.2-magnitude earthquake that occurred in July 2012 occurred near the southern end of the Boso Peninsula, and was of a slightly deeper type with a depth of 88 km. Magnitude 5.2 in August 2007 is a shallow type with a depth of 20 km.

In this area, there are cases where strong earthquakes occur in the sea areas off the southeast of Chiba Prefecture and off the east of Chiba Prefecture, rather than land areas, so it is essential to be prepared on a regular basis.

Since the earthquake with a magnitude of 6.5 and a maximum seismic intensity of upper 6 on the 5th (Friday), as of 10:00 on the 15th (Sunday), the number of felt earthquakes with a seismic intensity of 1 or higher has reached 100 times, of which 5 earthquakes with a seismic intensity of 3 or higher have occurred. A total of 15 tremors were observed, including a seismic intensity of upper 6 on Sunday (Friday).

Although seismic activity has continued to be more active than before the 5th (Friday), an earthquake with a seismic intensity of 3 or more has not occurred since the night of the 10th (Wednesday), and the number of felt earthquakes has also increased. It is declining, and the activity is gradually calming down.

On the 5th (Friday), the epicenter that was conspicuous near the northern coast of the Noto Peninsula expanded offshore, and the magnitude 5.0 earthquake that occurred on the 10th (Wednesday) was centered near the northern end of a series of seismic activity. . Although the depth is different, the region is similar to the 1993 magnitude 6.6 earthquake.

Earthquakes in the Noto Peninsula have continued for more than two years, and even if immediate activity subsides, there will still be concerns about strong tremors. We are also seeing changes in the epicenter area, so please continue to pay attention to earthquake trends and be well prepared.

There has been one earthquake with a magnitude of 6 or higher according to US Geological Survey analysis. The largest one was magnitude 7.6, which occurred off the coast of Tonga.

An earthquake estimated to have a magnitude of 7.6 and a depth of about 210 km occurred early in the morning of the 11th (Monday) Japan time, with an epicenter near Tonga in the South Pacific.

Large earthquakes often occur in the seas near Tonga and Fiji because the Pacific plate and the Australian plate are intricately intertwined.

While deep-focus earthquakes with depths of around 200 km and even deeper 500 to 600 km occur like this time, there are also earthquakes with a magnitude of around 8 in relatively shallow areas.

Shallow earthquakes can generate tsunamis, and the 2009 magnitude 8.1 earthquake sent 4-6m tsunamis to Samoa and Tonga, causing major damage. A tsunami with a height of around 30 cm reached Japan in this earthquake.

*Information on earthquake sources and seismic intensity in Japan is from the Japan Meteorological Agency unless otherwise specified. Overseas epicenter information is from the United States Geological Survey (USGS) unless otherwise specified. There may be differences in hypocenter information depending on the publishing organization.