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The Whittier Narrows earthquake sequence (local magnitude, M(L) = 5.9), which caused over $358-million damage, indicates that assessments of earthquake hazards in the Los Angeles metropolitan area may be underestimated. The sequence ruptured a previously unidentified thrust fault that may be part of a large system of thrust faults that extends across the entire east-west length of the northern margin of the Los Angeles basin. Peak horizontal accelerations from the main shock, which were measured at ground level and in structures, were as high as 0.6g (where g is the acceleration of gravity at sea level) within 50 kilometers of the epicenter. The distribution of the modified Mercalli intensity VII reflects a broad north-south elongated zone of damage that is approximately centered on the main shock epicenter.

The seismicity of the Indian Ocean is primarily confined to the mid‐ocean ridge system and its associated rift valley; however, a broad NW‐SE band of 29 epicenters is confirmed in the northeastern Indian Ocean—11 of the 29 earthquakes of magnitude 6 or higher. The location of seismic activity on fractures that cut the ridge is shown by the epicenters along the Owen, Prince Edward, and Amsterdam fracture zones and by the epicenters on the fracture at 41°S, 81°E. The seismic activity of the mid‐ocean ridge, as shown by the rate of energy release, has been relatively constant since 1922, except for the years 1942 and 1951 when large earthquakes occurred. A solution of the equation Log N = a + b(8 − M), based on data from 76 earthquakes over a period of thirty years, resulted in the values a = −1.78 and b = 0.91 for the mid‐ocean ridge system. A table of frequency of occurrence based on this solution shows the frequency of earthquakes to range from fifty years for a magnitude of 8 to one year for a magnitude of 6–6¼.

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