The energy release in great earthquakes

Abstract

The conventional magnitude scale M suffers saturation when the rupture dimension of the earthquake exceeds the wavelength of the seismic waves used for the magnitude determination (usually 5–50 km). This saturation leads to an inaccurate estimate of energy released in great earthquakes. To circumvent this problem the strain energy drop W (difference in strain energy before and after an earthquake) in great earthquakes is estimated from the seismic moment M0. If the stress drop Δσ is complete, W = W0 = (Δσ/2μ)M0 ∼ M0/(2×104), where μ is the rigidity; if it is partial, W0 gives the minimum estimate of the strain energy drop. Furthermore, if Orowan's condition, i.e., that frictional stress equal final stress, is met, W0 represents the seismic wave energy. A new magnitude scale Mw is defined in terms of W0 through the standard energy-magnitude relation log W0 = 1.5Mw + 11.8. Mw is as large as 9.5 for the 1960 Chilean earthquake and connects smoothly to Ms (surface wave magnitude) for earthquakes with a rupture dimension of about 100 km or less. The Mw scale does not suffer saturation and is a more adequate magnitude scale for great earthquakes. The seismic energy release curve defined by W0 is entirely different from that previously estimated from Ms. During the 15-year period from 1950 to 1965 the annual average of W0 is more than 1 order of magnitude larger than that during the periods from 1920 to 1950 and from 1965 to 1976. The temporal variation of the amplitude of the Chandler wobble correlates very well with the variation of W0, with a slight indication of the former preceding the latter. In contrast, the number N of moderate to large earthquakes increased very sharply as the Chandler wobble amplitude increased but decreased very sharply during the period from 1945 to 1965, when W0 was largest. One possible explanation for these correlations is that the increase in the wobble amplitude triggers worldwide seismic activity and accelerates plate motion which eventually leads to great decoupling earthquakes. This decoupling causes the decline of moderate to large earthquake activity. Changes in the rotation rate of the earth may be an important element in this mechanism.