THE 20~(th) SEPTEMBER 1999 CHI-CHI EARTHQUAKE (TAIWAN):A CASE OF STUDY FOR ITS AFTERSHOCK SEISMIC SEQUENCE

摘要

The purpose of this work is to highlight some methodological aspects related to the observation of possible anom-alies in the temporal decay of aftershocks temporal series following a mainshock with magnitude M ≥7.0. In this paper we present the results for the Taiwan seismic sequence started on 20 September 1999 (M = 7.7) by tuning some seismic parameters that show considerable variations during the aftershock decay process. In here we also present the results obtained using a fractal approach for the seismic sequence. Earthquakes belong to a class of phe-nomena known as multifractals. In general it is important to define the fractal dimension D, but sometimes is not useful if we are describing a natural phenomenon; so it is necessary to define D_0 called box-counting dimension and D_2 called correlation dimension, usually D_0≥D_2. In the elaborations of the fractal dimensions, for this sequence, we have obtained values lesser than 1, with a greater tendency of aftershocks to clusterize in time before a large aftershock. This is coherent with the possible existence of seismic anomalies, that could occur before the large aftershock. We also report the results obtained by using the delta/sigma method described firstly in [Caccamo et al., 2005] and later applied to different seismic sequence. The observed temporal series of the aftershocks per day can be considered as a sum of a deterministic and a stochastic contribution. If the decay can be modeled as a non-stationary Poissonian process, the number of aftershocks in a small time interval Δt is the mean value n(t) Δt, with a standard deviation ( σ= n(t)Δt~(1/2)). Investigating both aftershock behavior and a wide spectrum of parameters may fmd the key to explain better the mechanism of seismicity as a whole.