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1 SEISMICITY AND STRUCTURE OF THE EARTH S CRUST IN AREAS OF COMPLEX GEOLOGY AND ENVIRONMENT. THE GEOPHYSICAL EXPLORATION OF THE DEEP EARTH S CRUST IN CENTRAL MEDITERRANEAN AREA. Roberto Cassinis

2 SEISMICITY AND STRUCTURE OF THE EARTH S CRUST IN AREAS OF COMPLEX GEOLOGY AND ENVIRONMENT. THE GEOPHYSICAL EXPLORATION OF THE DEEP EARTH S CRUST IN CENTRAL MEDITERRANEAN AREA. Roberto Cassinis Published by Mistral Service sas Via U. Bonino, 3, Messina (Italy) This book is distributed as an Open Access work. All users can download copy and use the present volume as long as the author and the publisher are properly cited. The content of this manuscript has been revised by our international Editorial Board members. Important Notice The publisher does not assume any responsibility for any damage or injury to property or persons arising out of the use of any materials, instructions, methods or ideas contained in this book. Opinions and statements expressed in this book are these of the authors and not those of the publisher. Furthermore, the published does not take any responsibility for the accuracy of information contained in the present volume. First published: July, 2014 Assembled in Italy Roberto Cassinis A free online copy of this book is available at Hard copies can be order from mistral.messina@gmail.com SEISMICITY AND STRUCTURE OF THE EARTH S CRUST IN AREAS OF COMPLEX GEOLOGY AND ENVIRONMENT. THE GEOPHYSICAL EXPLORATION OF THE DEEP EARTH S CRUST IN CENTRAL MEDITERRANEAN AREA. Roberto Cassinis ISBN:

3 Abstract An outline is given of the geophysical exploration in the central Mediterranean region and of its contribution to the knowledge of the complex structure of the Earth s crust and to the study of seismicity. The results of the deep seismic surveys on the Italian Peninsula are described and the employed techniques discussed. 3

4 Introduction Seismicity occurs more frequently in areas characterized by structural geological complexity; however, the type of the environment is also a basic factor to determine the seismic risk. The Mediterranean area, and particularly the Italian peninsula as well as the eastern Mediterranean, is one of the seismic areas of the Earth where the earthquakes are producing the most devastating effects; this is due to the complexity both of geological structure and of surface morphology and topography. However, an additional, basic factor responsible for the amplification of the effects, is the type of the human environment, shaped by a long historical evolution. The map of figure 1, published by the EMSC (European Mediterranean Seismological Centre) shows the epicenters of the significant earthquakes (magnitude M >3,5) recorded worldwide during about two weeks; this document is continuously updated. It is clear that the central-eastern Mediterranean is the area on Earth where the seismic activity reaches a peak, but not the one where the highest magnitudes occur. It must be said that this type of information was made available only recently in Europe: the EMSC collects and processes in quasi real time the data recorded by several national seismological networks. The seismicity in the Euro-Mediterranean area. The map of figure 2 (also due to EMSC) shows the epicenters, magnitude and class of depth of significant earthquakes recorded in and around the area during the period This figure very clearly outlines the correlation of seismic activity with the major crustal structures (the Mid-Atlantic Ridge, the Alpine and Apennine chains and the Middle East mountain ranges, from the Taurus and Caucasus to the Hindu Kush and Pamir). While the correlation of seismicity with the crustal structure appears very clearly, the far origin of the earthquakes lies in the deep interior of the Earth; figure 3 is a sketch showing the unhomogeneities in the Mantle (and also in the Core) that are the far origin of the unstabilities of the Crust. In the sketch of figure 4 the different factors of instability are outlined, with both deep and shallow origin. 4

5 Figure 1: A global map showing the epicenters, magnitude and class of depth of significant earthquakes occurring worldwide in about two weeks. The map is continuously updated (EMSC, Euro-Mediterranean Seismological Center). 5

6 Figure 2: The earthquakes in the Euro-Mediterranean, north-atlantic and near East areas during the period (about 3500 events are plotted with magnitudes from 3 to 7) (EMSC). 6

7 Figure 3: A schematic illustration of the Earth interior showing the unhomogeneities in Mantle and Core. Figure 4: A sketch of the Earth (the deformations are exaggerated) to show the origin of the instability in our Planet. 7

8 Figure 5 is a map of the central Mediterranean area: crisscrossing the Italian peninsula, the surroundings seas and islands and the tip of north-africa the main profiles are shown where the structure of the Earth s Crust was interpreted and the depth of the Mohorovicic boundary (the Moho ) determined as it will be shown later. The Bouguer regional gravity anomalies are drawn, both inland and offshore: this information was the first, basic source to the knowledge of the deep structure of this very complex area (10). In figure 6 the historical seismicity (about 1000 years) is shown in central Mediterranean area (Alpine range, Italian Peninsula, Corsica and Sardinia, Sicily channel and Tunisia) (4,5,12). The epicenters (collected from the catalogue for magnitudes Ms>4.0 and for the period A.D.,) are plotted on the structural map of the region, interpreted according to the available geophysical and geological data (see the captions). In spite of the low accuracy of the historical earthquakes, the correlation is clear of the earthquakes with the structural model, especially along the Apenninic chain. Earth Crust and upper Mantle in Italy and in central Mediterranean The data leading to structural models like the one illustrated in figure 6 were collected especially during the last quarter of the 20th Century by a European group (European Seismological Commission). The group, jointly with the Italian CNR (National Research Council of Italy) and mainly under the supervision of the late Prof. Morelli (Univ. of Trieste) recorded a network of DSS (Deep Seismic Soundings), also identifyed as Wide Angle Reflection-Refraction profiles (WARR technique) (11). The interpretation of these profiles (both across the Peninsula and in the seas around it) was integrated by other types of information (gravity and magnetic anomalies, exploration boreholes, geological and topographic data). At the same time the national array of seismographic stations was extended and completely reorganized and several regional and local networks created for the detailed analysis of the seismic activity in particular areas. The new sets of data led to more detailed structural models that were compared with the updated maps of seismic events recorded both by the national and by several local seismographic networks. 8

9 Figure 5: The Bouguer gravity anomalies (mgal) and the main interpretative transects in the central Mediterranean area. b (3,8) 9

10 Figure 6: The historical epicenters (M>5) of the earthquakes occurring in the Italian and mid- Mediterranean region during the last 1000 years, plotted on a map showing the interpretation of the Earth s crustal structure according to the integrated sets of geophysical data (deep seismic profiles, gravity etc.). Captions: 1: European plate, 2: Afro-Adriatic plate, 3: Styrian and Pannonian basins, 4: Ligurian, Tuscan-Perityrrhenian transitional crust, 5: Oceanic-suboceanic crust, 6: Over-thrusting fronts: of the Moho boundary over the European plate (Alpine range); of the Ligurian, Tuscan, Perityrrhenian transitional crust over the European (Corsica); 7: fragmentation lines in the upper mantle; 8: Moho depth contour lines (km); 9: Moho depth contour lines (subducted). (2,5,14) 10

11 Some examples of crustal models obtained from the integration of different sets of data. Figure 7 (2,5,12) is a detail of figure 6 for southern Italy and Sicily, showing the South Tyrrhenian arc (also known as Calabrian or Aeolian ) where the hypocenters are dipping along a subducting slab in the Earth s Mantle, to the depth of about 250 km. The subduction is clearly observed in figure 8 (12)where the subducting foci are projected on a cross section bisecting the Calabrian Arc (9 9 in the figure). It must be remarked that less clear evidences of the crustal subduction are observed also in other areas of the Peninsula. Instead, sub-crustal foci are absent beneath the Alpine belt, except below the Maritime Alps. Figure 9 (4,5,6) describes an interpretative W-E transect starting in the western Mediterranean, crossing the Corsica and Elba islands, central Italy and reaching the Adriatic coast south of the city of Ancona. The absence of seismic activity beneath the Elba island must be interpreted as evidence that the subduction of the European crust is no more active. Instead, the active subduction of the Adriatic plate is clearly observed. Figure 10 (8,14) illustrates five interpretative transects across the western Alps (see the figure captions). While the interpretation of the central line (Grenoble-Asti) is based mainly on the results of deep seismic soundings, in the parallel transects only the data coming from passive tomography, earthquake foci and gravity anomalies (see the well known Ivrea positive anomaly) are available. One must also note that some sub-crustal foci are observed only in the southermost crosssection (Maritime Alps). Figure 11 (12,13) illustrates the interpretation of a seismic crustal transect across the eastern Alps (from Innsbruck to Trieste). Also here the gravity data are considered for the interpretation. The earthquakes are concentrated in the upper Crust, just south of the Peri-Adriatic line PL where the Earth s Crust reaches its greatest thickness (about 60 km ). Therefore the seismicity seems only indirectly related to the structure of the deep Crust and upper Mantle. Finally, in figure 12 a synthesis is attempted to outline the seismicity in the Italian region, projecting all the hypocenters on two cross-sections (N-S and E-W). While the focal depth of the majority of 11

12 Figure 7: A detail of the map of figure 6 showing the seismicity in the south Thyrrenian, Calabrian arc and Sicily plotted against the depth contour lines of the Moho boundary. Several deep foci are observed. The position of some interpretative crosssections is shown. (3,5,14) Figure 8: Cross-section 9 (see the location in fig. 7). The foci clearly show the slab of the Adriatic crust and upper mantle subducting the Thyrrhenian crust and upper mantle to the depth of about 300 km. ( Aeolian arc ) (3,5,14) 12

13 - Figure 9: Interpretative section from the western Mediterranean to Corsica and Elba islands, central Italy and Ancona on the Adriatic coast. (6,9,12); Velocity of P seismic waves=8.2km/s 13

14 Figure 10: Interpretative transects across the western Alps: the interpretation of the central line (from Grenoble to Turin and the Po valley) comes mainly from the deep seismic soundings (DSS), while in the other transects only the data of seismic passive tomography and gravity anomalies were available. Sub-crustal foci are observed only in the southernmost line. Vp = velocity of P seismic waves (7,8) 14

15 Figure 11: Interpretative transect across the eastern Alps (from Innsbruck to Trieste). P.L.:Periadriatic line. The earthquakes are concentrated only in the upper crust, in the transitional area between the European and the Adriatic crust (Friuli). (7,8,13) Figure 12: Synthetic view of the historical seismicity in the Italian region: the hypocenters are projected on two cross-sections (N-S and E-W); in four areas subcrustal foci are recorded; the arrow indicates the major subduction of the Aeolian Arc. 15

16 the events is less than 35 km (the average crustal thickness), in four areas (red circles) sub-crustal foci are observed; the red arrow points at the southernmost and most active one (the Aeolian arc ). By the end of last Century the field operations for the recording of DSS (the technique that can be considered as the basic geophysical method to obtain detailed models of the deep Crust and of the Moho boundary), came to an end: the stop had two origins: lack of funds and new national rules (the big shots are no more allowed for environmental reasons ). In some areas the geophysical exploration of the Crust and upper Mantle was continued through the international cooperation and with the help of petroleum industry (see the programme TRANSALP through the Alpine range) and also in the Peninsula (see the national programme CROP (Deep Crust). The seismic near-vertical reflection (NVR), the technique leader for the exploration of the hydrocarbons in the sediments, was modifyed and applied to the exploration of the deep Crust and upper Mantle. In some instances a better detail was obtained of the structure of the upper Crust. As an example, in figure 13 (12) the interpretation is shown of one of these profiles (CROP 03), crossing central Italy from the Tyrrhenian Sea coast and, through the geothermal province of south Tuscany, reaching the Adriatic coast near Ancona (thus approximately repeating the trace of the DSS line of figure 9). Three sets of data are compared: the profile of the Bouguer anomalies, the heat flow and, finally, the un-migrated and migrated reflections. The contrast between the thin and warm Tuscan, transitional crust, and the thicker, cold Adriatic crust, is clearly evidenced. In spite of these results it can be said that the DSS technique remains outstanding to define the deep boundaries (the lower crust and the Moho); however, to reach this goal, the distance sourcereceivers (the offset ) must reach more than 100 km (depending on the crustal thickness) and this requires large quantities of energy. The results of the integrated data sets collected over the Italian territory and around it can be considered as a homogeneous and reliable source of information on the complex structure of the central Mediterranean region and on the origin and character of the earthquakes. 16

17 Figure 13: Interpretation of the deep reflection line CROP 03 from the Thyrrhenian coast to the city of Ancona on the Adriatic coast (approx. the same position of the DS transect of fig. 9). From top to bottom: traces of Bouguer anomaly and of the heath flow, topographic profile, un-migrated reflections (time), migrated reflections (depth), interpretation: typology of reflections. T.M.: Tuscan Moho, A.M.: Adriatic Moho. (6,12). 17

18 References: Buness,H. and Giese,P., A crustal section through the Northwestern Adriatic Plate. In: R. Freeman, P. Giese and St. Mueller (eds.), the European geotraverse: integrative studies. Europ. Science Foundation, Strasbourg, pp Cassinis R. and Ranzoni A.: 1987: Contribution of controlled source seismology to the study of seismogenesis: examples from the Italian transitional area. Tectonophysics, 140, Cassinis R., Mazzoni P. and Ranzoni A., 1985: Active seismic layers and crustal structure in some Italian regions. Journal of Geophysics, 56, Cassinis, R., Helbig, K. and Panza, G.F.,(editors), 1993: Geophysical exploration in areas of complex geology, Journal of Applied Geophysics, special issue, Vol. 29 n.3/4 Cassinis, R. and Solarino, S., 2006: Seismicity and crustal structure in the Italian region: a new review using a synthesis of DSS results and updated catalogues of earthquakes, Boll. di Geof. Teor. e Appl., Vol. 47, n.3, p Cassinis, R. & Pialli, G.P., 1998; Seismic response and crustal characters of the CROP 03 transect, Mem. Soc.Geol. It., 52, ) Deichmann, D., Ansorge, J. and Mueller, St., Crustal structure of the southern Alps beneath the intersection with the European Geotraverse, The European Geotraverse, Part 1, Tectonophysics, 126: Giese, P. and Prodehl, C., 1976: Main features of crustal structures of the Alps. In: Giese P., Prodehl C. and Stein A. (eds.), Explosion seismology in central Europe, Deutsch. Geophys. Gesell., Sprinter, pp Laubscher, H., Biella G.C., Cassinis R., Gelati R., Lozej A., Scarascia S. and Tabacco, I., The collisional knot in Liguria. Geol. Rundsch., 81 (2): Morelli, C., Giese, P., Carrozzo, M.T., Colombi, B., Guerra, I., Hirn, A., Letz H., Nicolich, R., Prodehl, C., Reichert, C., Rower, P., Sapin, M., Scarascia, S. and Wigger, P., Crustal and Upper Mantle structure of the Northern Apennines, the Ligurian sea and Corsica, derived from seismic and gravimetric data, Boll. Geof. Teorica e Applicata, Morelli, C., 2000: The themes of crustal research in Italy and the role of DSS-WA seismics. Boll. Soc. Geol. It., 119,

19 Pialli, G., Barchi, M. & Minelli G. (eds.), 1998: Results of the CROP 03 seismic profile. Mem. Soc. Geol. It. 52, 647 Scarascia, S. and Cassinis, R., 1997: Crustal structures in the central-eastern Alpine sector: a revision of the available DSS data. Tectonophysics, 271, Solarino, S. and Cassinis R.: 2007: Seismicity of the upper Lithosphere and its relationships with the crust in the Italian region. Boll. Geof. Appl., 48,

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