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2 km

Figure 7.37 (a) Distance of outcrop of thrust plane from position where the pulse stress of 20 kb is oriented parallel to the greatest, horizontal 'ambient' stress. (b) As in (a), except that the pulse stress is parallel to the least, horizontal 'ambient' stress.

If the ratio of 1:3 for the radii of the impact craters to the corresponding peripheral thrusts were to hold for all the arcuate trenches mentioned above, the impact crater diameter would range from 400 km diameter for the Mariana Trench down to as little as 150 km for the Banda Arc.

In reaching these conclusions, however, we have assumed that the impacting body was a solid boloid, rather than a comet. As we have seen, a 100 km crater, caused by a solid, stony meteorite, with 5 km s-1 impact velocity, would require this body to have a diameter of 16.7 km. Such an impacting body is likely to hit a continental target at more than 15 km s-1. An ocean splash-down, where the water is 5 km deep, would cause only a relatively small degree of retardation in velocity of the meteoritic impacting body.

We have also noted that it is now generally accepted that major impact events (i.e. giving rise to craters with diameters greater than 100 km) result from comets. It has been indicated that a simple, solid near-spherical asteroid can be likened to a high-velocity rifle bullet, whereas a comet is more likely to approximate to the effects of a 'super' shotgun. We have also noted that a single body that splashes down into the ocean must have a diameter that exceeds 1/15th to 1/20th the depth of water to the ocean floor, if it is to have the capability of generating even a small crater.

Readers who have seen films, shot by underwater cameras, in which a person, usually a man, is seen swimming at a depth of as little as 1.0 m beneath the surface, will have observed that he is completely safe from being wounded by bullets fired at him. The bullets very rapidly decelerate in the water and, within a depth of 0.5 m or less, begin to sink at their terminal velocity. The actual depth of penetration of a bullet depends upon its calibre, shape and muzzle velocity, the distance between rifle and target, and the angle at which it enters the water. Hence, the problem is a complex one. Nevertheless, such scenes are testimony to the general accuracy of the predictions of Ahrens and O'Keefe (1977) regarding the minimum diameter of impacting

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