Stress estimates for South American shallow and deep earthquakes

Amplitude spectra of long-period mantle and body waves were used to obtain seismic moments for 37 earthquakes associated with the South American trench system. Corresponding seismic energies were estimated from the spectral densities of short-period P waves and also from the Gutenberg energy-magnitude relation. The apparent average stress (efficiency times average stress) in the source region was obtained from the ratio of energy to moment. The apparent average stress is a lower bound for the actual average stress. Near the surface a mean value for the apparent average stress is 18 bars. The mean value for depths between 45 and 150 km is 270 bars. Around 600 km depth the mean value is very similar to that at the surface, 21 bars. Differences in apparent average stress most likely reflect differences in strength of the material in the source region. The pattern of strength versus depth can be explained by ocean floor consumption. As lithospheric material plunges underneath oceanic trenches, it reaches higher pressures but is heated very slowly and thus will have relatively high strength until the temperature rises enough to weaken it. Comparison of the apparent average stress with estimates of the stress drop indicates an upper bound of about 0.1 for the seismic efficiency of deep and intermediate earthquakes.