ALBERT

Description: SUMMARY We study the scaling of spectral properties of a set of 68 aftershocks of the 2007 November 14 Tocopilla ( M 7.8) earthquake in northern Chile. These are all subduction events with similar reverse faulting focal mechanism that were recorded by a homogenous network of continuously recording strong motion instruments. The seismic moment and the corner frequency are obtained assuming that the aftershocks satisfy an inverse omega-square spectral decay; radiated energy is computed integrating the square velocity spectrum corrected for attenuation at high frequencies and for the finite bandwidth effect. Using a graphical approach, we test the scaling of seismic spectrum, and the scale invariance of the apparent stress drop with the earthquake size. To test whether the Tocopilla aftershocks scale with a single parameter, we introduce a non-dimensional number, , that should be constant if earthquakes are self-similar. For the Tocopilla aftershocks, C r varies by a factor of 2. More interestingly, C r for the aftershocks is close to 2, the value that is expected for events that are approximately modelled by a circular crack. Thus, in spite of obvious differences in waveforms, the aftershocks of the Tocopilla earthquake are self-similar. The main shock is different because its records contain large near-field waves. Finally, we investigate the scaling of energy release rate, G c , with the slip. We estimated G c from our previous estimates of the source parameters, assuming a simple circular crack model. We find that G c values scale with the slip, and are in good agreement with those found by Abercrombie and Rice for the Northridge aftershocks.