Cyclic Properties of Seismic Noise and the Problem of Predictability of the Strongest Earthquakes in Japanese Islands

Аннотация

The results of an analysis of the properties of low-frequency seismic noise in the Japanese Islands from early 1997 to March 2018 are presented. The time interval under consideration includes one of the largest seismic disasters of recent times, the Tohoku earthquake of March 11, 2011. The presence of a dense network of seismic observations provides a unique opportunity to investigate how the preparation of a strong earthquake is reflected in changes in the properties of seismic noise in time and space. An analysis of the clustering of the daily multifractal and entropic properties of seismic noise in a 1-year moving time window averaged over all stations of the network made it possible to find a 2.5-year periodicity established since the beginning of 2003. This periodicity correlates with the occurrence of strong earthquakes in Japan. Studying features of the spatial distribution of seismic-noise properties allows us to put forward a hypothesis about the increased danger of the next megaearthquake in Japan in the area of the contact between the northern boundary of the Philippine Sea plate and Honshu Island, in the Nankai deepwater trough area, not far from Tokyo. In the Japanese Islands, in addition to the network of seismic observations, there is a dense network of fixed GPS points for which observations are available from the beginning of March 2015 with a time step of 5 min. The availability of such measurements makes it possible to complement the analysis of seismic-noise properties and to calculate the degree of correlation of GPS data at any point from measurements at neighboring stations. An analysis and processing of measurement results show that the most intense spot of increased correlation of the Earth’s surface tremor, measured using GPS, is located in the Nankai Trough, with the center at the point of 34° N and 138° E.