. The Earth beneath the sea : History . 20 30 40 50 Period (sec) 500 1000 Fig. 5. Suramary of Love-wave group-velocity observations. Regional differences within continents and oceans are associated with observed departures from these average curves out to periods probably as great as 100 sec. The relatively high group velocity of the oceanic first mode is due to the shallow depth of high-velocity rock beneath the oceans. The absence of observations of the oceanic second mode is probably due to attenuation of the very short period waves which would necessarily prevail in this mode. curves in th
. The Earth beneath the sea : History . 20 30 40 50 Period (sec) 500 1000 Fig. 5. Suramary of Love-wave group-velocity observations. Regional differences within continents and oceans are associated with observed departures from these average curves out to periods probably as great as 100 sec. The relatively high group velocity of the oceanic first mode is due to the shallow depth of high-velocity rock beneath the oceans. The absence of observations of the oceanic second mode is probably due to attenuation of the very short period waves which would necessarily prevail in this mode. curves in these figures will be discussed in more detail later. Consider the observations of waves of the Rayleigh type first (see Ewing and Press, 1956). For periods greater than about 75 sec, for waves whose lengths are greater than about 300 km, differences between continents and oceans are very small, and the waves are represented by a single line in Fig. 4. The minimum value of the group velocity at a period of about 225 sec results primarily from the effect of the velocity increase at depths of the order of hundreds of kilometers in the earth's mantle. For periods less than 75 sec, the curve splits and the distinction between the continental and oceanic types of crust becomes readily apparent. In the period range 70 to 17 sec, the ocean curve falls well above the continental curve. This
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