Dependence of Microseism Variation Periods upon the Cyclone Propagation Velocity and Direction

摘要

Time variations in periods of microseism types I and II were experimentally revealed by many researchers. The present work demonstrates that the increase and decrease in microseism periods are caused not only by mechanisms of progressive wave development and dispersion but also by propagations of barometric depressions related to the Doppler effect. This work is focused on the analysis of microseis-mic-range variations based on processing of 2-yr-long data of a 52.5-m laser deformograph (its operation principle is described in [1]). The analysis of a significant number of spectrograms has shown that periods of microseism types Ⅰ and Ⅱ change with time according to the following scenarios (Fig. 1): (1) increase, (2) decrease, and (3) successive increase and decrease (repeated alternations of increase and decrease are possible). Let us consider the probable causes of such a behavior of microseism periods. A slight increase in wind wave periods can be caused by wave progression and transition to the swell wave phase. Under certain conditions, a progressive marine wave can form a stationary wave. Stationary and progressive marine waves generate microseisms Ⅰ and Ⅱ, respectively. In this case, spectrograms recorded by a laser deformograph exhibit an increase in periods of microseisms Ⅰ and Ⅱ (hereafter, periods Ⅰ and Ⅱ). It should be noted that variations in periods Ⅰ and Ⅱ correlate with each other in almost all the data processed. At first glance, the decrease in the periods Ⅰ and Ⅱ may predominantly be related to dispersion of marine waves. However, the successive increase and decrease (or decrease and increase) in microseisms Ⅰ and Ⅱ is difficult to explain by the effects mentioned above.