We relocated interplate and intraslab earthquakes beneath the Tohoku district of NE Japan using the Double-Difference location method [Waldhauser and Ellsworth (2000)]. Relative earthquake arrival times were determined by waveform cross-spectrum analysis and catalog-picking data. Our relocations show that between 50 and 150 km in depth, most of the upper-plane seismicity occurs in the slab crust, and most of repeating earthquakes seem to occur along the upper boundary of the Pacific plate, where the intraslab seismicity is active. Obtained result shows that both the lower-plane and upper-plane seismicity are distributed unevenly in space. Moreover, there is a spatial correlation between clusters of upper-and lower-plane seismicity. If intermediate-depth earthquakes are caused by dehydration and/or CO2-bearing devolatilization of hydrated minerals [Kirby et al. (1996), Kirby et al. (2004), Peacock (2001), Seno and Yamanaka (1996), Yamasaki and Seno (2003)], the present result suggests that hydrated and/or carbonate minerals are distributed unevenly but in common in both the crust and deeper mantle of the slab. In the southern part of Tohoku district, where some seamounts subduct, seismicity between upper and lower seismic planes is active. If the hydrated and carbonate minerals are abundantly distributed in the slab crust and the slab mantle due to the volcanic activities of the seamount formation, dehydration and devolatization of these minerals may cause intraslab earthquakes.
A short-term slow slip event (SSE) synchronizing with an activity of low frequency earthquakes occurred beneath Aichi prefecture on July 20-22, 2005. Characteristic strain changes caused by the short-term SSE were recorded by the strainmeters of Japan Meteorological Agency. We examined the strain data of the Tokai area in detail since July 1984. As a result, eleven strain changes were detected in about fifteen years from July 1984 to August 1999, during which only the data of volumetric strainmeters were available. Twenty strain changes were detected in six years from September 1999 to August 2005, during which the data of multi-component strainmeters and information of low frequency earthquakes were also available. The detected strain changes were classified into three types. The three types correspond to different active areas of low frequency earthquakes. The three patterns of the observed stain changes were explained by assuming rectangular faults in the active areas of low frequency earthquakes. It was found that the short-term SSE occurred more frequently in the periods of 1987-1989 and 2003-2004. These active periods roughly agree with that of the long-term SSE in the Tokai region.
We investigated changes in the depth distribution of aftershocks of three large inland earthquakes in Japan, the 2000 western Tottori Prefecture Earthquake of M 7.3, the 2003 northern Miyagi Prefecture earthquake of M 6.4, and the 2004 mid Niigata Prefecture earthquake of M 6.8. For all of them we found that the seismogenic layer extended to the deeper and shallower zones after occurrence of the main shock. The shallow and deep activities decayed faster than the activity in the intermediate depth. This suggests that when stress is built up very fast, brittle fractures can occur in the zone where increase of stress is usually relaxed through a ductile process. Our results also show that a fault motion could extend to the outside of the seismogenic zone estimated from the background seismicity, which should be taken into consideration in the hazard assessment.
Measurements of seiche were conducted using a portable pressure gauge at heads of bays and ports in the Pacific coast of Shikoku, Japan, and the sea levels at seventeen points were detected from the records of six hours. The dominant periods derived from the spectra show a distribution having a median of 21 minutes with a variation from 8 minutes to 93 minutes. For the 1946 Nankai tsunami the maximum height ratio is defined as a ratio of the maximum height at a bay head to that at an open coast nearest to the bay. The values estimated from old documents at the corresponding bays are plotted as a function of dominant period of seiche. The distribution shows the maximum at a period of 19 minutes. The amplification is explained as a resonance of tsunami having a period of 19 minutes. The value almost agrees with results obtained from tide gauge records of Uchiura and Hosozima, eyewitness information, and tsunami source size. The height ratio of 2 at the resonance period suggests that the Nankai tsunami consists of wave train with a crest and a trough.
JMA revised the information framework concerning the prediction of the Tokai earthquake in January, 2004, incorporating the most updated knowledge on the rupture nucleation process immediately prior to the earthquake occurrence. The new information framework enables early commencement of provisional actions for the disaster mitigation in advance of the warning statement issuance. In this article, JMA’s strategy for short-term prediction of the Tokai earthquake, details of the information framework, its scientific background, and future plans are briefly introduced, including a historical review.
Despite its extreme importance and decades of efforts, practical short-term earthquake prediction still remains to be achieved in future. However, the electromagnetic research has been demonstrating some promises. This paper briefly reviews the recent progress of what we call “seismo-electromagnetics”, mainly referring to Japanese studies by the observational point of view. We demonstrate some results of observations, in varied frequency ranges, on the anomalous telluric current, ULF geomagnetic transient change, VLF-HF natural emissions and anomalous transmission of VLF and VHF band radio waves. We also summarize proposed physical mechanisms of these phenomena, including the notion called “Lithosphere-Atmosphere-Ionosphere (LAI) Coupling”. We believe that electromagnetic studies will play an important role in not only earthquake prediction but also in understanding physical processes of earthquake generation.