After the disastrous 1995 Kobe earthquake, a new national project has started to drastically improve seismic observation system in Japan. A large number of strong-motion, high-sensitivity, and broadband seismographs were installed to construct dense and uniform networks covering the whole of Japan. The new high-sensitivity seismograph network consisting of 696 stations is called Hi-net, while the broadband seismograph network consisting of 71 stations is called F-net. At most of Hi-net stations strong-motion seismographs are also equipped both at depth and the ground surface. The network of these 659 stations with an uphole/downhole pair of strong-motion seismographs is called KiK-net, while another network consisting of 1034 strong-motion seismographs installed at the ground surface is called K-NET. Here, all the station numbers are as of April 2003. High-sensitivity data from Hi-net and pre-existing seismic networks operated by various institutions have been transmitted to and processed by the Japan Meteorological Agency since October 1997 to monitor the seismic activity in and around Japan. The same data are shared to university group in real time using satellite communication for their research work. The data are also archived at the National Research Institute for Earth Science and Disaster Prevention and stored in their database system for public use under a fully open policy.

Episodic slow slip events have been recognized by means of tilt changes in the western Shikoku area, southwest Japan. The crustal tilt deformation was observed repeatedly with a recurrence interval of approximately six months coincident with the occurrences of major non‐volcanic deep tremor activities in this area. Observed tilt changes can be explained by slow slip events occurring around the source area of tremors. In each episode, the source of the slow slip event and tremor migrate simultaneously. The spatial and temporal coincidence of tremors and slow slip events indicates that they both may be coupling phenomena reflecting the stress accumulation process at the subducting plate.

Seismic observations to retrieve various information from the Earth are the basis of seismology. A seismic observation system requires various technologies for vibration sensors, analog-and-digital measurement, data transmission, and computing for mass data analysis, for example. New developments in technology are adopted whenever possible in the construction of seismic observation systems. In Japan, after the disastrous Kobe Earthquake in 1995, a high-density and high-sensitivity seismograph network was constructed. The seismic network, called the National Research Institute for Earth Science and Disaster Prevention (NIED) Hi-net, uniformly covers the Japanese Islands with a spacing of 20–30km. As a result, the detection capability for microearthquakes has been greatly improved, and various research using Hi-net data has indicated that this seismic network has a great potential to resolve the underground structure and various geophysical phenomena as a radar-array oriented toward the Earth. Equipped with modern standard techniques, the Hi-net system is designed to be able to be smoothly upgradeable in the future.

The Gutenberg‐Richter frequency‐magnitude distribution of earthquakes has become well established in seismology. The slope of the relation between frequency and magnitude (b value) is typically 1, but it often shows variations around 1. Based on an analysis of seismicity prior to the 2011 Tohoku and 2004 Sumatra earthquakes (both in magnitude (M) 9 class), we show that the pronounced decade‐scale decrease inbvalue was a common precursor to both mega‐quakes around their hypocenters. This is the first report onM9‐class quakes to confirm a change inb value, which has been predicted based on the results of laboratory experiments. We propose that the b value is an important indicator of an impending great earthquake, and has great potential in terms of predicting a future large quake off the Pacific coast of Hokkaido, Japan.