. Compendium of meteorology. Meteorology. 1308 MICROSEISMS ances near the surface (but not from explosions) the surface waves are usually more prominent than the body waves. The first attempt to find the type of movement which prevails in microseismic waves was made by Geussen- hainer [10]. He found that in Gottingen the period of the vertical component of the microseisms normally changed gradually with time while in the horizontal component periods of 6, 73^, and 9 sec occurred more often than periods with intermediate values. If waves with one of these preferred periods existed in the ver- t
. Compendium of meteorology. Meteorology. 1308 MICROSEISMS ances near the surface (but not from explosions) the surface waves are usually more prominent than the body waves. The first attempt to find the type of movement which prevails in microseismic waves was made by Geussen- hainer [10]. He found that in Gottingen the period of the vertical component of the microseisms normally changed gradually with time while in the horizontal component periods of 6, 73^, and 9 sec occurred more often than periods with intermediate values. If waves with one of these preferred periods existed in the ver- tical component, the corresponding waves had espe- cially large amplitudes in the horizontal components; if the period of the microseisms in the vertical com- ponent was between two fundamental periods of the horizontal, both fundamental periods, sometimes changing repeatedly from one to the other, were re- corded in the horizontal components. Geussenhainer believed that the vertical pendulums record mainly free vibrations of the earth's crust. Such free vibra- tions would require additional theoretical considera- tions since the resulting movements may be very dif- ferent from those calculated for forced vibrations, especially if resonance phenomena are involved. The observed velocities of microseisms are usually between about 2 and 4 1cm sec~^; that is, in the range of velocities of surface waves with such periods. It is generally believed that the microseisms consist of sur- face waves. Unfortunately, measurements of the ampli- tude of microseisms as a function of depth in order to find the expected decrease are inconclusive [15, 19, 20]. The few_ authors who have studied the question of the wave type in microseisms agree that Rayleigh waves prevail [16, 22, 32]. On the other hand, waves of the Love type are also involved, although their amplitudes are relatively smaller. Increase in Period with Distance. It has been found that the period of elastic waves usually increases as
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