March 1, 2024
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* Since February 26 at about 23 o’clock (JST, hereinafter the same time), there continues a seismic activity centering on offshore east of Chiba Prefecture. Till March 1 at 13 o’clock there were 16 earthquakes with seismic intensity 1 or over (maximum seismic intensity: number of earthquakes, 4: 2, 3: 3), the largest scale earthquake being a magnitude (M) 5.2 (preliminary) earthquake occurring on March 1 at 05:43. The focal mechanisms of these earthquakes showed a reverse fault type with a compression axis generally in an NNW-SSE direction. These events occurred at the boundary between the Philippine Sea and the continental plates.
* According to the GNSS and tilt data, since about February 26, slight crustal deformations were observed in the Boso Peninsula. The crustal deformations detected by GNSS is about 1 cm at maximum. These phenomena are considered as due to slow slip at the boundary between the Philippine Sea and the continental plates.
* In the neighborhood of this activity, similar one with slow slip was observed at the frequency once per several years in the past. Hitherto in 1996, 2002, 2007, 2011, 2014, and 2018, the seismic activity continued for 1 week or several months. Also, in 2007 maximum seismic intensity 5 Lower was observed.
* Based on the past seismic activity, the activity continues, and it is possible that strong shaking as seismic intensity 5 Lower will be observed. We should pay attention to strong shaking. Note that the scale of the earthquakes in the activity is M5 or so at maximum, and is smaller than that of earthquake accompanying the subduction of the plate evaluated in "Long-Term Evaluation of the Seismic Activity Along the Sagami Trough (2nd ed.) (published on April 25, 2014)."
* In the southern Kanto region where the recent event occurred, the Pacific plate is subducting from the Japan Trench in the east under the Philippine Sea plate which is subducting from the south, and there have occurred many earthquakes of M7 or so. In the "Long-Term Evaluation of the Seismic Activity Along the Sagami Trough (2nd ed.)" the probability of the occurrence of an earthquake accompanying the subduction of the plate, of M7 or so (M6.7~M7.3) within 30 years was evaluated as rank III and, of M8 class (M7.9~M8.6) was evaluated as rank II (#), classified as the group of the high and rather high probability, respectively, in the subduction zone earthquakes.
#: Rank of the probability of occurrence of a subduction zone earthquake within 30 years is expressed as follows; rank: probability, III: greater than 26%, II: 3% ~ 26%, I: smaller than 3%, X: unknown (an immediate occurrence of an earthquake cannot be denied).
Note: GNSS is a general name of satellite positioning system such as GPS.