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Local Earthquake Tomography in the Junction Between South-Eastern Alps and External Dinarides Using the Seismic Data of the CE3RN Network L

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Local Earthquake Tomography in the Junction Between South-Eastern Alps and External Dinarides Using the Seismic Data of the CE3RN Network L GNGTS 2015 SESSIONE 1.2 LOCAL EARTHQUAKE TOMOGRAPHY IN THE JUNCTION BETWEEN SOUTH-EASTERN ALPS AND EXTERNAL DINARIDES USING THE SEISMIC DATA OF THE CE3RN NETWORK L. Colavitti1, L. Tiberi1, G. Böhm2, G. Costa1, A. Gallo1 1 University of Trieste, Department of Mathematics and Geosciences, Seismological Research and Monitoring Group, Trieste, Italy 2 National Institute of Oceanography and Experimental Geophysics, Section of Geophysics, Trieste, Italy Introduction. The main force responsible of tectonic activity in south-eastern Alps is given by the collision between Adriatic microplate and Eurasian plate. This motion has a fundamental role in geodynamic evolution due to the convergence rate between Adria and Eurasian plate estimated greater than 2 mm/yr (Platt et al., 1989). The largest historical earthquakes occurred in the region between Italy and Slovenia are reported in Burrato et al. (2008) and Galadini et al. (2005). Several large earthquakes hit the studied region in historical times, most of them were distributed in the Friuli Venezia Giulia region, as two events recorded in Tramonti in 1776 and 1794, but the most important earthquake that spread its effects in wide region was the 1511 Idrija event that represents the most destructive event that occurred so far in the region of the junction between south-eastern Alps and the External Dinarides, as reported in the catologues (Ribarič, 1982). In general, the area of Friuli Venezia Giulia region is characterized by a moderate seismicity, in fact the most important instrumental seismic event recorded in the area was the destructive 1976 Friuli earthquake with Ms6.5 (Aoudia et al., 2000), that was widely studied thanks to the large amount of collected seismometric data. In Veneto region, seismicity normally decreases and the only destructive earthquake instrumentally recorded was the Cansiglio event with Mw5.9 (Sirovich and Pettenati, 2004). Evidences that the collision is still active nowadays and the main seismogenic structures are located in the proximity of the political boundaries between Italy and Slovenia is also shown by the main shock of Bovec in April 1998 with Ms5.7 (Bajc et al., 2001) and, later, in July 2004, with the event of Kobarid (Ms4.9), which generated a sequence that lasted until the end of November 2004 (Bressan et al., 2009). It is evident that, in an area with complex seismogenic structures, the use of 1-D velocity model might not be sufficient, so defining a 3-D velocity model represents a better solution to understand the inner structure of the Earth. For this reason, we used the travel-time tomography technique to obtain an accurate 3-D velocity model: in this work, we used a Cat 3-D software to perform the travel time tomography and a non-linear location tool to define the position of the events. Data set. For the input parameters of our tomographic analysis it is fundamental to have a complete data set of available and accurate events in the area where potentially destructive events are located in the closeness of the political boundary between Italy and Slovenia. For this reason, seismic data were recorded by the Transfrontier network (Costa et al., 2010), which is called Central Eastern European Earthquake and Research Network (CE3RN). This network was born thanks to a prolific cooperation of the governmental institutions of Italy, Austria, Slovenia and recently, Croatia, which decide to share in real time their seismological data for scientific purposes and for Civil Defense. The main importance of the CE3RN is to ensure a very good recording of the seismic events that occur very close to the political borders and that otherwise would be recorded in non-homogeneous way only by national networks; in this sense, the example of recent strong earthquakes demonstrated that this integration of services is essential for a rapid and efficient intervention (Bragatoet al., 2010). In Fig. 1a, the black rectangle shows the area of our investigation (longitude east between 12.5 and 14.5; latitude north between 46 and 46.5) while, red triangles represent the seismic stations which belong to the CE3RN network. For this study, we used 52 seismic stations and analyzed 180 earthquakes, including the events with magnitude more than 3 between 2004 and 67 GNGTS 2015 SESSIONE 1.2 Fig. 1 – a: In the black rectangle the zone of our investigation is represented. The red triangles indicate the stations of the CE3RN, the blue lines the sections extrapolated from our model. b: Zoom of the study area: rays’s coverage used for the tomographic inversion: with the red triangles are represented the stations included in our study area, while with the black dots are shown the event located using Non-Linear Location technique. 2013 and the full seismicity of the year 2011. In overall, we identified 1960 phases (1350 P and 610 S) manually picked to avoid false detection that can be generate by the automatic STA/LTA trigger algorithm. All the data were recorded by Antelope®, a software which represents an integrated set of programs that are able to acquire, store and process seismic devices. Method. We applied the Local Earthquake Tomography (LET) technique that represents an important tool for the Earth’s investigation. In this study, we used earthquake location method and travel time tomography combined in an iterative procedure. For event location, we adopted the Non-Linear Location NLLOC (Lomax et al., 2009) that is a technique used to locate the events using the 3-D velocity model resulting from the tomographic inversion. This technique is formed by a set of programs for the creation of the grid location, the computation of travel times and the probabilistic, non-linear location of structures for 3-D data volume. As travel time tomography we adopted the software Cat3D, Computer Aided Tomography for 3-D models, developed by OGS (CAT 3D, User Manual, 2014) which computes the tomographic inversion of seismic waves travel times on 3-D models. In previous works, this software was mainly used for investigations in the field of exploration geophysics (Böhmet al., 2006, 2009), while in other recent investigation (Tiberi, 2014), it is applied on seismological data. The main difference between active seismic and seismological applications is represented by the positions of sources, which in the first case are well defined, while for seismological purposes, they represents the unknowns parameters with high uncertainty. As inversion algorithm for the velocity estimation, in this work we adopted the SIRT (Simultaneous Inversion Reconstruction Technique) technique; this method is part of series expansion methods, on which the investigated area can be divided on elements with constant velocity, called pixel in 2-D or voxel in 3-D. The ray which links the source (earthquake location) 68 GNGTS 2015 SESSIONE 1.2 with the receiver (station position) is composed by several segments, whose associated travel time ti is defined by: where dj represents the single straight line of the ray i which belong to the single pixel j, sj is the slowness of the same pixel and m is the total number of pixels in the model. To minimize time residuals, SIRT method uses an iterative procedure which leads to convergence. In formulas, i where ∆sj is the upgraded velocity of pixel j, ∆t is the time residual associated to ray i, N is the number of rays (travel times) and M is the number of pixels in the model. The computation of ray paths in the model follows the principle of minimum time, using an iterative algorithm based on Snell’s law (Böhm et al., 1999a). Initial model, data processing and reliability of the tomographic system. As we can see in Fig. 1b, the rays distribution is not homogeneous in the considered area. The greater ray density reflects the higher seismicity of the area, distributed in the center of the model, with the most intense activity concentrated along the Friuli region and in correspondence of the geologic junction between southern Alps and External Dinarides. We adopted the 1-D velocity structure computed by Costa et al. (1992) as initial velocity model. Futhermore, the Friuli plain constitutes an important part of our zone and obviously does not represent a seismic area, therefore we constrained, for the first layers, some voxels of the model with the velocity values coherent with real data considering the geological information by Slejko et al. (1987). The volume of our investigation, whose dimensions are 180 x 60 x 51 km, is discretized by 18 voxels along X direction, 6 voxels along Y direction and 60 layers in depth, obtaining a spatial resolution of 10 x 10 km in two horizontal directions and 0.85 km in the vertical component, which it coincides with the half of thinnest layer of the base model. The choice of this grid, which we called “base grid”, represents a compromise between two opposite requirements: the resolution and the reliability of the model. In this work, inside the tomographic process, we used the staggered grids method (Böhm et al., 1999b), which provides a high resolution velocity model by summing and averaging different inversions obtained from low resolution but well-constrained in a base grid that has been perturbed in the space. In our case, we applied two shifts in both horizontal directions (x, y) and one shift in vertical direction z. In tomographic inversion, the homogeneity of earthquake’s distribution is fundamental to obtain a reliable 3-D velocity model. The reliability of tomographic system (model discretization and ray geometry) can be computed in different ways. The ray density represents only a simple method to evaluate the reliability of the model, while the map of null space is a more complex and expensive in computation time.
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