Statistical analysis of triggered seismicity in Kresna region (SW Bulgaria, 1904) and Umbria-Marche region (Central Italy, 1997)

Gospodinov, D. and Rotondi, R. (2004) Statistical analysis of triggered seismicity in Kresna region (SW Bulgaria, 1904) and Umbria-Marche region (Central Italy, 1997). Technical Report. Consiglio Nazionale delle Ricerche. Istituto di Matematica Applicata e Tecnologie Informatiche (IMATI). Sezione di Milano, Milano, IT.

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A version of the restricted trigger model is used to analyse the temporal behaviour of some aftershock sequences. The conditional intensity function of the model is similar to the one of the Epidemic Type Aftershock-Sequence (ETAS) model with the restriction that only the aftershocks with magnitude bigger than or equal to some threshold M_tr can trigger secondary events. For this reason we have named the model restricted epidemic type aftershock-sequence (RETAS) model. Varying the triggering threshold we examine the variants of the RETAS model which range from the Modified Omori Formula (MOF) to the ETAS model, including such models as limit cases. In this way we have a quite large set of models in which to seek the model that fits best an aftershock sequence bringing out the specific features of the seismotectonic region hit by the crisis. We have applied the RETAS model to the analysis of two aftershock sequences: the first is formed by the events which followed the strong earthquake of M = 7.8 occurred in Kresna, SW Bulgaria, in 1904, the second includes three main shocks and a large swarm of minor shocks following the quake of 26 September 1997 in Umbria-Marche region, central Italy. The modified Omori formula (MOF) provides the best fit to the sequence in Kresna; that leads to think that just the stress field changes due to the very strong main shock generate the whole sequence. On the contrary the complex behaviour of the seismic sequence in Umbria-Marche appears when we make the magnitude threshold vary. Setting the cut-off magnitude M_cut = 2.9 the best fitting is provided by the ETAS model, while if we arise the magnitude threshold M_cut = 3.6 and set M_tr = 5.0, the RETAS model turns out to be the best model. In fact observing the time distribution of this reduced data set it appears more evident that each strong secondary event is followed by a cluster of aftershocks.

Item Type: Monograph (Technical Report)
Uncontrolled Keywords: Epidemic-type models; Modified Omori law; Trigger model; Thinning simulation; Triggering magnitude
Subjects: 500 Scienze naturali e Matematica > 510 Matematica
Depositing User: biblioteca 3
Date Deposited: 15 Mar 2006
Last Modified: 20 May 2010 12:00

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