Recurrence Times of Great Earthquakes in the Seismotectonic Areas along the Philippine Sea Side Coast of Southwest Japan and South Kanto District Tetsuzo SEND Geophysical Institute, Faculty of Science, University of Tokyo (Received December 16, 1976) The seismic crustal movement on the south Kanto district and the Philippine Sea coast of southwest Japan is characterised by the chronic (in the interseismic period) and acute (at the earthquake) tiltings with reverse sense. The accumulation of these tiltings in geological time scale explains the topographic features on the coast such as tilted terraces. The value "d", which is defined as the ratio of the subsidence in the interseismic period to the coseismic upheaval, was expressed by two independent linear functions of recurrence interval T of series of great earthquakes. The coefficients of these functions are determined by the data of precise levellings and the heights of coastal terraces in the late Quaternary period. Then, the d and T can be determined on the cross point of the two lines which represent the above two functions. This scheme (called d-t diagram) was applied to the Seismotectonic areas of the southern Kanto and the outer arc of the southwest Japan. The results of T obtained are almost in concordance with the intervals of great earthquakes inferred from the historical documents and the rates of convergence of the Philippine Sea plate under the southwest Japan. It was also shown that the d-t diagram can be used as a tool of earthquake prediction taking an example of the Tokai region.
Fig. 1. Schematic diagram of the vertical movement in the seismic crustal movement area. VA: coseismic upheaval, VR: recovery during the postseismic stage, VS: subsidence during the interseismic period, T: recurrence interval, Tp: duration of the precursory crustal movement, vup: rate of residual upheaval, vsub: rate of subsidence.
Fig. 2. Representation of d by two linear functions of T.
Fig. 3. Seismic crustal movement areas and sites for calculation in the text.
A B Aburatsubo VA=1.0m -J35.1 Fig. 4A. Secular change of the height of the bench mark at the Aburatsubo tidal station relative to J 35.1 after 1923 Kanto earthquake [after Dambara (1971)]. Fig. 4B. d-t diagram for the vertical movement of Aburatsubo referring to J35.1. Heavy lines show the case r=0.2, and light lines the case r= 0. The area shadowed represents the area in which the cross point (d, T) can exist.
A o obscrved by G. SJ O bscrvrd by E. RJ Fig. 5A. Secular change of the height of Muroto point referring to Aki before and after Nankaido earthquake [after O. S. I. (1972)]. Fig. 5B. Vertical distribution of height of former shore lines on the S and H Terraces (lower graph) and vertical change of bench marks surveyed by precise levelling (upper graph) along the southeast coast of Shikoku [after Ota (1975) revised from Yoshikawa et al. (1964)].
Muroto-Aki VA=1.22m Fig. 6. d-t diagram for the vertical movement of Muroto point referring to Aki.
Muroto Promontory VA=5.76" (tilting) Unit:x10yr Fig. 7, d-t diagram for the tilting of the Muroto promontory.
Kushimoto -Esumi VA 0.55m Fig. 8A. Secular change of height of Kushimoto (9221) referring to Esumi (9208) before and after Tonanki and Nankaido earthquakes. Kushimoto- VA 0.55m Esumi Fig. 8B, d-t diagram for the vertical movement of Kushimoto referring to Esumi.
Shimizu Ya=1.5m Fig. 9A. d-t diagram for the vertical movement of Shimizu (Udo hill) referring to the mean sea level.
Omaezaki VA=1.2m Fig. 9B. d-t diagram for the vertical movement of Omaezaki referring to the mean sea level.
Table 1. d and recurrence time T at various sites along the Philippine Sea coast of Japan.
Fig. 10. d-t diagram used for earthquake prediction.
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