Spectral Scaling of the aftershocks of the Tocopilla 2007 earthquake in Northern Chile
Résumé
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, Cr = ŒEs M2 o ²3f 3c , that should be constant if earthquakes are selfsimilar. For the Tocopilla aftershocks, Cr varies by a factor of 2. More interestingly, Cr 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, Gc, with the
slip. We estimated Gc from our previous estimates of the source parameters, assuming a simple
circular crack model. We find that Gc values scale with the slip, and are in good agreement
with those found by Abercrombie and Rice for the Northridge aftershocks.