A study about the aftershock sequence of 27 December 2003 in Loyalty Islands

Investigating both aftershock behavior and a wide spectrum of parameters may find the key to better explain the mechanism of seismicity as a whole. In particular, the purpose of this work is to highlight some methodological aspects related to the observation of possible anomalies in the temporal decay of aftershock sequence. We have focalized our work mainly on the analysis of the Loyalty Islands seismic sequence of 27 December 2003 by means of some seismic parameters that showed remarkable variations during the aftershocks decay. The magnitude of the mainshock was M = 7.3. We also made a comparison with the sequence that happened in New Guinea on 29 April 1996, with mainshock magnitude M = 7.5. The daily temporal series of the shocks observed can be considered a sum of a deterministic contribution and of a stochastic contribution. If the decay can be modeled as a non-stationary Poissonian process where the intensity function is equal to n(t) = K⋅ (t+c)-p+K1, the number of aftershocks in a small time interval Δt is the mean value n(t)⋅Δt, with a standard deviation . We observe that, before the occurrence of large aftershocks, there are some Δ/σ values that can be considered as seismic anomalies. The data concerning the temporal series, checked according to completeness criteria, come from the web site of NEIC-USGS data bank.