SECULAR VARIATION OF THE ANNUAL MEAN SEA LEVEL 

 ALONG THE JAPANESE COASTS 



By Masamori Mivazaki 

 Kobe Marine Observatory, Japan 



I. Introduction 



Chief purposes of this manuscript are to analyse the time series of 

 the annual mean sea-level computed for 11 stations (Fig. 1) along the 

 Japanese coasts, and to investigate the origins of secular tide variations 

 by using these data. The source material consists of continuous records 

 (20-50 years) obtained at Aburatsubo, Kushimoto, Shimotsu, Osaka, 

 Kobe, Uwajima, Shimizu, Hosajima, Tonoura, Sakai, and Wajima. Most 

 of these data were already tabulated in the Tidal Record [Land Survey 

 (1), Geographical Survey Institute (2), and the Tidal Observations 

 Imperial Marine Observatory (3), CMO (4)]. 



The variations of annual mean sea-level at these stations are shown 

 in Figure 2, which suggest the existence of the progressive and periodic 

 variations. Especially, the progressive trends are clearly shown in these 

 figures, and are the order of a few millimeters per year at most sta- 

 tions. Besides them, the sudden fall of about 1.3 m. was observed at 

 Aburatsubo in 1923 accompanied with the Kwanto Earthquake, and 

 similar changes took place at Shimizu in 1946 by the Nankaido Earth- 

 quake. 



2. Progressive Variations 

 Several methods are now available to compute the secular trends 

 from the given time series. In this paper, the expansion in orthogonal 

 polynomes 



is used for this purpose, ^vhere /(^n) is the annual mean sea-level, t, the 

 time in year measured from the central instant, ji, the number of years, 

 a^, a^, a^ are constants. 



This method is very convenient since the significance can be 

 checked for each order term. Computations are made for these curves, 

 and we find that, excepting Kobe, the first and second order terms are 

 reliable at the significance level of 5%. The dashed curves in Figure 2 

 represent the progressive trends computed by this method. 



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