Layout 6 19/06/18 11:41 Pagina 1
Total Page：16
File Type：pdf, Size：1020Kb
siino.qxp_Layout 6 19/06/18 11:41 Pagina 1 ANNALS OF GEOPHYSICS, 61, 2, SE228, 2018; doi: 10.441/ag-7711 MULTISCALE PROCESSES TO DESCRIBE THE EASTERN SICILY SEISMIC S“EQUENCES „ 1 2 1,2, Marianna Siino , Antonino D’Alessandro , Giada Adelfio *, Salvatore Scudero 2 and Marcello Chiodi 1,2 (1) Dipartimento di Scienze Economiche, Aziendali e Statistiche, Università degli Studi di Palermo, Palermo, Italy (2) Istituto Nazionale di Geofisica e Vulcanologia, Centro Nazionale Terremoti, Rome, Italy Article history Receveid November 26, 2016; accepted September 5, 2017. Subject classification: Earthquakes; Point process; Hybrid of Gibbs process; Residual G-function; Spatial raw residuals ABSTRACT In this paper, a version of hybrid of Gibbs point process models is proposed as method to characterise the multiscale interaction struc - ture of several seismic sequences occurred in the Eastern Sicily in the last decade. Seismic sequences were identified by a clustering tech - nique based on space-time distance criterion and hierarchical clustering. We focus our analysis on five small seismic sequences, showing that two of these are described by an inhomogeneous Poisson process (not significant interaction among events) while the other three clusters are described by a Hybrid-Geyer process (mutiscale interaction between events). The proposed method, although it still needs extensive testing on a larger catalogue, seems to be a promising tool for the characterization of seismogenic sources through the anal - ysis of induced seismicity. 1. INTRODUCTION and clustering (where points tend to be close together). To describe real point process phenomena, like seis - Given an area observed during a time interval, earth - mic events, more complex models than the stationary quakes can be considered as a realization of a marked Poisson that assumes statistical independence of events space-time point process, assuming that each event, are defined. Indeed, seismic data represent an interest - with a given magnitude (the mark), is identified by a ing situation where the study and the interpretation of point in space (by its geographical coordinates) and features like self-similarity, long-range dependence and time (fixing its time of occurrence). fractal dimension are crucial. Generally in the point process theory, the attention Moreover, the distribution of epicentres generally is focused on two complementary aspects that can be shows diverse interaction structures at different spatial untangled with summary statistics and model formula - and spatio-temporal scales varying according to the lo - tions [Diggle, 2013; Baddeley et al., 2015]. The first as - cation of earthquake sources in the study area. pect is the description of the first-order characteristics Therefore, the study of the second-order properties of of the point process estimating the intensity function; a point process has a relevant role in the comprehension whilst the second aspect concerns the description of the of the process and its realization. events dependence, and so of the second-order proper - Earthquakes are typically observed as clusters in ties of the process. In a given pattern, the events may space and time. In space, earthquakes epicentres are typ - exhibit regularity (where points tend to avoid each ically highly concentrated along the plate boundaries other), independence (complete spatial randomness), and, in general, along active faults [Baddeley et al., 1 siino.qxp_Layout 6 19/06/18 11:41 Pagina 2 SIINO ET AL. 2015]. On the other hand, clustering in time can be seen tion 4. The results of the analysis and the estimated mod - as a significant increase of seismic activity immediately els are presented in Section 5. after large earthquakes leading to aftershock sequences. The last section is devoted to conclusions and final Currently in the seismological context when events remarks. are clustered together, the modelling formulation is usually based on self-exiting point processes assuming that the occurrence of an event increases the probabil - 2. SETTING: THE STUDY AREA ity of occurrence of other events in time and space (such as the Hawkes model, Hawkes and Adamopoulos [1973] East Sicily and South Calabria is the area with greater or the Epidemic Type Aftershock-Sequences (ETAS) deformation rate and seismic strain release in Italy [Ser - model, Ogata [1988]). pelloni et al., 2010; Palano, 2015]. In this area, the Nubia Although the models cited above represent a wide va - and Eurasia plates collide, giving birth to a complex tec - riety of solutions to describe different types of spatio and tonic puzzle dominated by a NNW-SSE-oriented conver - spatio-temporal patterns, more complex models need to ge nce where also crustal extension and strike-slip zones account for environmental heterogeneity while detecting coexist in a relative small region [Hollenstein et al., 2003; interactions at several spatial scales (multiscale depen - Serpelloni et al., 2007; Caporali et al., 2009], see Figure 1. dences). For this reason, it is attractive and motivating the Two main zones of crustal contraction are located in definition and estimation of models that account for the Tyrrhenian Sea, and at the front of the Calabrian Arc both attractive and repulsive dependences, such as the in the Ionian Sea [Billi et al., 2007; Lavecchia et al., Hybrid of Gibbs models [Baddeley et al., 2013], while also 2007; Giunta et al., 2009; Polonia et al., 2011, 2012] including the effect of observed covariates in order to bet - characterized by compressional directions roughly ter describe the probability of occurrence of earthquakes transversal to the chain axis. They all are limited by a in fixed regions. Siino et al. [2017] provide a novel con - strike-slip transfer zone extending from the Aeolian Is - tribution to the literature on point processes and seismo - lands to the Malta escarpment [Palano et al., 2012; Polo - logical applications using a hybrid approach and the nia et al., 2012; Gallais et al., 2013; Orecchio et al., 2014]. distance to seismic sources as covariates for the analysis Eastern Sicily and southern Calabria are affected by of earthquakes in the Hellenic area. crustal extension mainly WNWESE oriented whose ori - In this paper, we try to characterize the interdepen - gin has been related to different tectonic processes [Tor - dence of events occurred, at several spatial scales, in the torici et al., 1995; Monaco et al., 1997; Catalano et al., East Sicily area. Indeed, an accurate study of the moder - 2008; De Guidi et al., 2012]. All these domains are well ate and small seismicity of the Eastern Sicily could be documented at local scales by the occurrence of active useful to better understand the seismogenic sources of fault segments, marine terracing, growing folds and ac - this area. For this reason, starting from a seismic cata - tive basins [Catalano and De Guidi, 2003; Bianca et al., logue over the last decade, we identify some sequences 1999, 2011; Catalano et al., 2011; Sulli et al., 2013; and we analyze their spatial distribution with a point pro - Argnani et al., 2013; De Guidi et al., 2013]. cess approach. The area is therefore characterized by a high variabil - In particular, we describe the spatial seismicity of the ity of tectonic regimes and trajectories of the tectonic selected clusters using an advanced spatial point process stress field [Montone et al., 2012; Pierdominici and Hei - model formulation based on the hybrid of Gibbs pro - dbach, 2012; De Guidi et al., 2013; Presti et al., 2013; Scarf cesses [Baddeley et al., 2013; Siino et al., 2017]. The main et al., 2013; Palano et al., 2015]. Such complexity is also characteristics of Hybrid of Gibbs models is that it is pos - testified by recent tomographies: large Vp anomalies in sible to describe the multiscale interaction between the the crust structure are well depicted and reflect the artic - points in presence of larger scale inhomogeneity. There - ulated crustal structure [Neri et al., 2012; Presti et al., fore, in this paper, after describing each sequence in terms 2013; Musumeci et al., 2014; Palano et al., 2015; Scarf et of the previous model, the results are related to the un - al., 2016; D ╒Alessandro et al., 2016]. derlying geological information of the studied areas and Moderate and small crustal earthquakes are di used some comparisons are done. throughout the whole area, well marking the active zones The paper is organized as follows. Section 2 presents aforementioned and with coherent seismogenic stress ori - a review of the main geological and seismic characteris - entation [Neri et al., 2005; Scarf et al., 2013; Presti et al., tics of the Eastern Sicily. The methodological methods 2013]. Deep seismicity marks the eastern Aeolian Island used to select clusters are explained in Section 3. The sector and the Tyrrhenian o shore of the Calabria; here model formulation used in this paper is reported in Sec - deep seismicity (down to 450 km) is reported and is in - 2 siino.qxp_Layout 6 19/06/18 11:41 Pagina 3 MULTISCALE PROCESSES FOR SICILIAN SEISMICITY terpreted as the subducing Ionian slab under the Cal - Rigano, 2001], and a subduction fault plane in the Io - abrian Arc [Selvaggi and Chiarabba, 1995; Chiarabba et nian basin [Gutscher et al., 2006]. al., 2005]. The seismogenic source of the Messina earthquake is debated although the epicentral area, and the focal mechanism are somehow well constrained [Pino et al., 2000, 2009]. The different fault plane solutions proposed in the literature are somehow coherent each other, sug - gesting a roughly NNE-SSW to SSE-NNW trending structure with prevailing extensional motion. However, the dip directions are sometime opposite and angles variable; in fact some authors propose various west-dip - ping faults in the Calabrian side of Messina Straits [Schick, 1977; Bottari et al., 1986; Catalano et al., 2008; Aloisi et al., 2013], while some others propose east-dip - ping structures in the Sicilian side [Capuano et al., 1988; Boschi et al., 1989; De Natale and Pingue, 1991; Valen - sise and Pantosti, 1992; Amoruso et al., 2002; Neri et al., 2004; Valensise et al., 2008; Bonini et al., 2011].