July 11, 2022
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
The Earthquake Research Committee on today (July 11, 2022) discussed synthetically on the seismic activity and crustal deformation in the Noto region, Ishikawa prefecture where a high seismic activity continues, collecting and analyzing observed results and researched results so far of relevant administrative organizations and universities.
In the Noto region, Ishikawa prefecture, the seismic activity has become high since December 2020. On June 19, 2022, there occurred the largest M5.4 earthquake in the series of the seismic activity, which caused maximum seismic intensity 6 Lower observed. Beside this, there occurred several earthquakes with observed strong shaking, such as an M5.1 earthquake on September 16, 2021 with maximum seismic intensity 5 Lower and an M5.0 earthquake on June 20, 2022 with maximum seismic intensity 5 Upper. The series of the seismic activity still continues to be high showing no sign of decline at present.
Therefore, considering the necessity of intensifying transmission of information from the Earthquake Research Committee, in addition to the traditional evaluation of the seismic activity, we have decided to announce "View of the Earthquake Research Committee Chairman" in order to transmit the related information.
[About observed data so far and analyzed results]
The seismic activity is generally classified into 2 types: after a large scale earthquake, smaller scale earthquakes occur (main shock-aftershocks type) and earthquakes of the same order scale occur consecutively for a long period as the recent seismic activity in the Noto region, Ishikawa prefecture. In the former case, the seismic activity become high because of the direct influence of the initial large scale earthquake (main shock). On the other hand, in the latter case, there is a possibility that some external forces, which produce a high seismic activity, is operating on the seismic activity area: By analyzing the seismic activity statistically (*1), we can estimate the rate of such external operation (rate of back ground seismic activity). According to this analysis, in the recent series of seismic activity in general still continues the state of high rate of back ground seismic activity.
Based on the observation and analyzed result of the crustal deformation, under the ground of the surroundings of the seismic activity area, some cause, which produces the recent crustal deformation and seismic activity, is considered to exist. Spherical inflation source (*2), tensile fault (*2), fault slip (*2) are possible causes. But the present observed data and analyzed result cannot reject any cause and it is difficult to identify the 1 cause.
Similar to the recent one, the seismic activity, in which earthquakes of the same order scale occur consecutively for a long period, have sometimes been observed in Japan. Of these some continued for more than 1 year. Also, in some, a fluid involvement is pointed out as the cause of the high seismic activity.
From the distribution of electrical conductivity (*3) of the surroundings of the seismic activity area, we can infer that there exist more conductive regions in the seismic activity area and its deeper ground. Also, by the seismic wave analysis, there exist regions reflecting seismic wave near the deepest place of the southern seismic activity area. The analysis of the hot spring water of the northern Noto Peninsula indicates the possibility of involvement of some fluid.
Based on these results and the detailed hypocenter distribution of the recent seismic activity, and the researched results on the seismic activity which continue for a long period in Japan, there is a possibility of fluid involvement to the recent seismic activity and crustal deformation. But from the present observed data and analyzed results, we cannot say how the fluid is involved.
In the surroundings of the recent seismic activity, there is no active fault which is the aim of the long-term evaluation by the Earthquake Research Committee. But in the ocean bottom offshore northern coast of the Noto Peninsula, several reverse faults dipping southeast extending in an NE-SW direction are known to be active faults. These active faults slipped repeatedly in the past and will slip in the future. Although the effect to these active faults by the recent seismic activity is not clear, we should pay attention to the existence of active faults.
Note that "the 2007 Noto-Hanto Earthquake" is judged as the result of the activity of the westernmost region of these ocean bottom active faults.
Earthquake Research Committee announced "Long-Term Evaluation of Offshore Active Faults in the Southwestern Sea of Japan" in March 2022. Now the long-term evaluation of active faults in Kinki region is in progress. Active faults in other regions will be evaluated consecutively in the future.
In the northern Noto Peninsula volcanic activities in Oligocene~Miocene (*4) are known, but Quaternary volcanos (*5) are not known. Also, the analyzed result of the hot spring water of the area doesn't supply a positive evidence of magma involvement.
As regards the cause generating the seismic activity and the crustal deformation, we can count several possibilities from observed data and analyzed result so far, but we cannot identify the definite cause. Also, at present the relation with active faults and so on is not clear. For these it is necessary to think about, basing upon the future researched and observed results.
[The points to pay attention for disaster prevention]
In the surroundings of the Noto Peninsula, there have been large scale earthquakes of M6 or over which cause damage such as "the 2007 Noto Hanto Earthquake."
In the surrounding of the recent seismic activity area, there are areas with shaky grounds in the coasts and river-sides. There are shaky grounds in the surrounding of the observing points with seismic intensity 6 Lower observed at the time of M5.4 earthquake on June 19, 2022. You can check the shakability of the grounds where you live on the "National Seismic Hazard Maps for Japan (2020)" published on March 2021 by the Earthquake Research Committee.
Judging comprehensively, based on the situation of the seismic activity
which does not show decay tendency and the active state continues, and
the crustal deformation still continues, the series of the seismic activity
will continue for the time being. Depending to the scale of earthquakes
and the area where you live, especially at the area where strong shaking
was felt due to the earthquakes so far, you should pay attention for a
strong shaking in the future. Note that in the case of a large scale earthquake
occurring at the ocean bottom, you should pay attention to tsunami. Again,
it is important to confirm the everyday preparation for an earthquake.
*1: Analysis using the statistical model (non-stationary ETAS model) which quantifies the seismic activity. This model assumes the parameters corresponding to the rate of back ground seismic activity and the triggering strength of aftershocks vary with time, and can be used in the evaluation of the seismic activity which continues for a long period.
*2: Source models which explain the observed values of crustal deformation. Spherical inflation source is a model in which pressure varies isotropically (spherically) with accumulation of water and so on. Tensile fault is a model in which pressure varies as the fault opens in the shape of plate by the entering water and so on. Fault slip here is slip at the fault plane and the same as the usual earthquake, but the slip velocity is extremely small and doesn't emit seismic waves.
*3: Electrical conductivity is an index which represents the ease of flow of the electric current. Generally, in the presence of water and so on, electric current flows easily.
*4: The interval of about 34 million years BP to about 5 million years BP.
*5: The volcano active in the interval of about 2.6 million years BP to present.