STRESS DROPS, WAVE SPECTRA AND RECURRENCE INTERVALS OF GREAT EARTHQUAKES - IMPLICATIONS OF THE ETOROFU EARTHQUAKE OF 1958 NOVEMBER 6

Summary. The great Etorofu earthquake of 1958 November 6 is characterized by a relatively small aftershock area (70 × 150 km2) and an extremely large felt area. The felt area is more extensive than those of any other large earthquakes which have occurred in the southern Kurile to northern Japan arc since the beginning of this century. The mechanism is a pure thrust fault typical of most great earthquakes in island arcs. A body wave magnitude of mb= 8.2 is obtained at periods around 6 s using more than 40 observations, although an mb value of only 7.6–7.7 would be expected empirically from the observed surface wave magnitude of Ms= 8.1–8.2. Both an unusually large felt area and a high mb indicate a dominance of high‐frequency components in the seismic waves. A seismic moment of Mo= 4.4 × 1028 dyne cm is determined from long‐period surface waves from which a high stress drop of Δσ= 78 bar is obtained using a relatively small aftershock area. Historic data indicate an anomalously long time interval between the 1958 event and any earlier great earthquake from the same source region. The observed high stress drop can be interpreted as a consequence of this long intervening period through which strain built up. The dominance of the high‐frequency seismic waves can then be interpreted as a result of this high stress drop. Stress drops, seismic wave spectra and recurrence intervals of great earthquakes are in this way closely related to each other. The 1958 event may represent a high strength extreme of stochastic fluctuation of fracture strength relevant to great earthquakes. Copyright © 1979, Wiley Blackwell. All rights reserved