Earthquakes in Switzerland: from a global to a local perspective

Transcription

1 Earthquakes in Switzerland: from a global to a local perspective N. Deichmann Swiss Seismological Service, Institute of Geophysics, ETH Hönggerberg, CH-8093 Zürich, Switzerland. deichmann@seismo.ifg.ethz.ch 1 May 2000 Key Words: Earthquakes, Focal Mechanisms, Plate Tectonics, Mediterranean, Switzerland. 1 Introduction The following lecture notes are meant to place the earthquakes of Switzerland into the larger context of global seismicity and of Alpine-Mediterranean tectonics. It is divided into three parts: Earthquakes and plate tectonics, with typical examples of the various styles of seismic deformation as a function of the types of plate boundary or intraplate setting. This material is mainly drawn from the textbook of Lay and Wallace (1995). Highlights of the seismotectonics of the Alpine-Mediterranean region. This is based largely on two review papers by Udias and Buforn (1994) and Jackson (1994). Seismicity of Switzerland, based on recent work of the Swiss Seismological Service and the Institute of Geophysics of the ETH-Zurich. A more detailed review can be found in Pavoni et al. (1997). Annual reports on the recent seismicity in Switzerland and surrounding regions are published regularly each year in the second issue of the Eclogae Geologicae Helvetiae (e.g. Baer et al. 1998), which includes also a more complete reference list. A macroseismic earthquake catalog of the last 1000 years and an instrumental catalog of earthquakes with M L > 3 since 1975 together with the corresponding epicenter maps can be found under under catalogs. Before beginning, we will briefly review the notion of earthquake focal mechanism and what we mean by seismotectonics (Figure 1). 1 Contribution to the Nachdiplomkurs in angewandten Erdwissenschaften, Naturgefahren - Erdbebenrisiko, 15-19/05/2000, ETH-Zürich und Volkshochschule Tiengen. 1

2 Figure 1: Fault types (1), their representation on gelogic maps (2) and the corresponding earthquake focal mechanisms (3). (A): Pure thrust fault, P-axis horizontal and T-axis vertical; (B): Pure normal fault, T-axis horizontal and P-axis vertical; (C): Pure strike-slip fault, P- and T- axes horizontal; (D): Oblique thrust. P = P-axis, direction of maximum compression; T-axis, direction of maximum extension. Arrows next to the focal mechanisms indicate directions of maximum horizontal compression and extension. (After Pavoni, NTB 84-45, Nagra, 1984) 2

3 Figure 2: Absolute plate motions based on NUVEL-1. The vectors point in the direction of plate motion relative to a fixed frame of reference (hot spots), and their length is proportional to the velocity in cm/year. (From Lay and Wallace, 1995, based on data from DeMets et al., GJI, 101, , 1990) 1.1 Seismotectonics and earthquake focal mechanisms Seismotectonics deals with the relation of the distribution and focal mechanisms of earthquakes to their tectonic setting and to the deformation that they induce. Only in the past 100 years has it become generally accepted that earthquakes occur as a consequence of sudden shear displacements on more or less planar faults at some depth inside the Earth. The elastic waves that are radiated from these rupture processes are what cause the Earth's surface to shake and what seismologists observe on their seismograms. The analysis of the azimuthal variations of the recorded seismograms allows the seismologist to reconstruct the sense of deformation that occurred at the earthquake source. A key concept in this context is the focal mechanism (also referred to as fault-plane solution or moment tensor) and its representation as a stereographic projection of the compressional and dilatational deformation deduced from the observed seismograms. Figure 1 shows the relation between the three main types of faulting (normal, thrust, strike-slip) and the corresponding focal mechanism representations. Thus, maps of focal mechanisms give us direct information about the style and sense of tectonic deformation that the Earth's crust is undergoing at a specific location. 3

4 Figure 3: Epicenters of earthquakes with focal depth less than 100 km, determined by the global networks for the years (From Lay and Wallace, 1995) 2 Earthquakes and plate tectonics On a global scale, the distribution of earthquakes can be understood best in the context of plate tectonics (Figure 2). As the lithospheric blocks that constitute the outermost shell of the Earth are forced apart, move past each other or collide, stress is accumulated, which is then released as earthquakes. Thus, the vast majority of earthquake epicenters are aligned along the major plate boundaries and are referred to as interplate earthquakes (Figure 3). However, earthquakes can also occur within plate interiors. Such intraplate earthquakes can be due either to stress applied at plate boundaries that propagates across the plate and is released along zones of weakness or to local tectonic processes that are not directly related to the interaction between two plates. 2.1 Interplate earthquakes We distinguish three types of plate boundaries: divergent transcurrent convergent Each of these boundary types is associated with distinctly different seismic behaviour. 4

5 2.1.1 Divergent boundaries The main divergent boundaries are the midoceanic ridges along which molten mantle material rises to the surface, where it solidifies to produce new oceanic crust. This process leads to rifting and thus to extensional deformation of the oceanic crust. The midoceanic ridges are faithfully imaged by a string of earthquake epicenters (Figure 3) and, as expected, their focal mechanisms show evidence of almost pure normal faulting (e.g. Figure 4). Figure 4: Focal mechanisms along a midoceanic ridge in the Indian Ocean. The T-axes are oriented NE-SW, parallel to the direction of spreading. (Huang and Solomon, JGR, 92, , 1987, from Lay and Wallace, 1995) Transcurrent boundaries Typical transcurrent plate boundaries along which two plates slide past each other horizontally can be found between offsets in the midoceanic ridges. However, transform faults can also be found between offsets of a combination of any type of plate boundary. Focal mechanisms associated with typical transform faults are pure strike-slip mechanisms (e.g. Figure 5). It is important to note, however, that the sense of motion implied by the focal mechanisms is opposite to the offset between the individual ridge segments. Earthquakes are in fact not the consequence of the relative motion between two ridge segments but of plates moving past each other, due to crustal spreading along the ridge segments whose positions are fixed relative to each other. The correct interpretation 5

6 of these focal mechanisms by T. Wilson in 1965 was one of the milestones in the process of validating plate tectonics as a theory. Transform faults are not restricted to the oceans. The famous San Andreas fault in California corresponds to a transform fault between segments of the Pacific ridge off Cape Mendocino in the North and off the west coast of Mexico, in the South. Another continental transform fault is the Alpine fault of New Zealand. A seismically active continental transcurrent fault, which however is not considered a transform fault, is the North Anatolian fault in Turkey. Figure 5: Seismicity and focal mechanisms along mid-atlantic transform faults near the equator (left) and sketch showing the sense of motion of the spreading plates that agrees with the focal mechanisms (right). (Engeln et al., JGR, 91, , 1987; from Lay and Wallace, 1995) Convergent boundaries Where two lithospheric plates collide with each other, we have a so called convergent plate boundary. The consequences of plate collisions depend on the plate types involved: subduction occurs where two oceanic plates or an oceanic and a continental plate meet, while crustal thickening and mountain ranges are formed where two continental plates collide. Subduction zones: Subduction zones constitute 90% of today's convergent boundaries. They are associated for the most part with the well-known deep oceanic trenches that line the Circumpacific. By far the largest number of earthquakes and the greatest individual events correspond to lithospheric subduction. Indeed, the two largest earthquakes in the past century, occurred as shallow subduction events: during the 1960 Chile event (Mw = 9.5), the Pacific plate moved an average of 24 m over a fault length of more than 1000 km beneath the South American continent, and in 1964 the great Alaskan earthquake (Mw = 9.2) was associated with an average slip of 14 m over a fault 700 km long and 180 km wide. The focal mechanisms of these events as well as of all other earthquakes associated with the shallow interaction between the subducting and overriding plate correspond to thrust faults (e.g. Figures 6 and 7). 6

7 Contrary to the uniformity observed along the contact between the two plates, focal mechanisms in the overriding plate at some distance from the trench show a greater variety. Ocean-ocean collisions give rise to volcanic island arcs and back-arc basins charactarized by extensional mechanisms. As exemplified by the Andean Cordillera and Altiplano in South America, huge mountain ranges can form inland from ocean-continent subduction zones. In accordance with the crustal shortening associated with the origin of such mountain ranges, earthquake faulting is characterized by thrust and strike-slip mechanisms. However in the highest parts, some normal faults are also present, which are probably related to topographic loading. Figure 6: Focal mechanisms along the Alaska Peninsula and Queen Charlotte Islands fault (QCF). The latter, extending from northern Vancouver Island (VI) to Cross Sound (CS), is mainly a strike-slip fault, but may also take up some convergence. Black dots show epicenters of earthquakes from 1963 to 1985 with focal depths shallower than 50 km. The seismicity that extends far inland from the trench suggests that convergence is taken up over a broad region. (DeMets et al., GJI, 101, , 1990) 7

8 Figure 7: Focal mechanisms and epicenters along the Middle America Trench. (DeMets et al., GJI, 101, , 1990) A variety of focal mechanisms can also be found within the subducted oceanic plate, depending on the delicate balance between the various forces acting on it. In fact, the state of stress at any point within a slab is determined by the resultant of the push from the spreading centers, the pull due to the negative buoyancy of a cold slab sinking into warmer mantle material, the resistance to further sinking due to changes in mantle rheology with depth, the degree of frictional coupling between subducting and overiding plate, the stress associated with local bending or sagging of the plate. Whatever the actual focal mechanism might be, it is important to note that these earthquakes are a result not of the interaction between the sinking slab and the surrounding mantle material but of the internal deformation of the slab. Thus, strictly speeking, they could be considered intraplate events. The spatial distribution of the earthquake hypocenters associated with the subducting slab is known as a Wadati- Benioff zone and its determination has allowed seismologists to image the slab as it sinks into the mantle. Although the depth down to which seismicity has been observed varies greatly over the whole range of subduction zones, it seems that the maximum depth of the Wadati-Benioff zones is reached at the 670 km discontinuity in the mantle. The fate of the subducted slabs beyond that depth is controversial. Some of the deep earthquakes are known to attain magnitudes of 8 or more. However, because of their relatively small number and their large focal depth, they do not contribute significantly to the global earthquake hazard. 8

9 Figure 8: Tectonic map of the India-Eurasia collision zone. (From Lay and Wallace, 1995) Figure 9: Focal mechanisms of earthquakes along the India-Eurasia collision zone. (Ni and Barazangi, JGR, 89, , 1984, from Lay and Wallace, 1995) 9

10 Continental collision: Continents are in general too buoyant to subduct. Thus, when two continents come into collision, the convergence is taken up through lithospheric shortening and thickenning along major thrusts or through lateral expulsion of lithospheric material along strike-slip faults. The most dramatic example of continental convergence with consequent lithospheric thickenning is the collision between India and Eurasia that gave rise to the Himalaya and Tibetan Plateau. Most of the convergence is taken up along the Main Boundary Thrust and the Main Central Thrust and produces earthquakes with focal mechanisms corresponding to slip on low angle thrust faults (Figures 8 and 9). Earthquakes with magnitudes of 8.5 and more are known to occur on these faults and the total seismic activity associated with the India-Eurasia collision is estimated to amount to 15% of the yearly global seismic energy release. Some normal faulting events in the South, with T- axes perpendicular to the strike of the Himalayan mountain chain, are probably a consequence of lithospheric bending of the Indian Shield before being thrust beneath the Eurasian continent. The T-axes of the normal faulting mechanisms of the earthquakes beneath the Trans-Himalaya and Tibetan Plateau, on the other hand, are oriented parallel to the montain chain and are interpreted as evidence for E-W extension due to increased loading of an overthickenned lithosphere. Stresses associated with the India-Eurasia collision are thought to propagate far into the Asian continent where they give rise to large earthquakes (M > 8) along major strike-slip faults. Figure 10: India-Eurasia and Arabia-Eurasia linear velocities predicted by different models together with seismicity shallower than 40 km. Deformation associated with the Arabia-Eurasia collision extends northeast over 1000 km from the Zagros fold and thrust belt. MRF: Main Recent fault; ONF: Ornach-Nal fault; CF: Chaman fault. (DeMets et al., GJI, 101, , 1990) 10

11 Figure 11: Earthquake focal mechanisms associated with the Main Recent fault, the Zagros fold and thrust belt and with the Makran subduction zone. (DeMets et al., GJI, 101, , 1990) Figure 12: Major faults and earthquake focal mechanisms associated with the Arabia-Eurasia collision. GC: Greater Caucasus; D: Dagestan; LC: Lesser Caucasus; NAF: North Anatolian fault; EAF: East Anatolian fault; BKF: Borzhomi-Kazbeg fault zone; AF: Araxes River fault zone; MRF: Main Recent fault. (From Jackson, JGR, 97, , 1992) 11

12 Another example of continental convergence is given by the collision between Arabia and Eurasia (Figure 10). Northeast of the Persian Gulf this convergence is taken up by thrust faulting along the Zagros fold and thrust belt and along the Main Recent fault (Figure 11). Also to the North between the Black and Caspian Seas, thrust mechanisms are evidence for crustal shortening in the Caucasus (Figure 12). On the other hand, the Anatolian Block (Turkey) in the Northwest is being squeezed out laterally towards the West along the East and North Anatolian faults and other lesser strike-slip faults (Figure 12). 2.2 Intraplate earthquakes Although the vast majority of earthquakes are found along plate boundaries, earthquakes can also occur within plate interiors. Intraplate earthquakes reflect internal deformations of a plate. They differ from interplate earthquakes by having much longer recurrence periods and higher stress drops. A typical example of intraplate seismicity is the New Madrid seismic zone in central United States. In 1811 and 1812, three earthquakes with magnitudes estimated at around 8 occurred along what is interpreted as an ancient rift-like structure. Microseismic activity is still strong today. Based on the predominantly thrust and strike-slip focal mechanisms, it is thought that this zone of weakness has been reactivated by the stresses caused by the ridge push from the North Atlantic propagating into the interior of the North American plate. Other examples of intraplate seismicity are the normal faulting events in the Basin and Range province of western United States, the seismic activity along the East African Rift and most of the diffuse seismicity in Central Europe. 12

13 Figure 13: Earthquake epicenters determined by the European-Mediterranean Seismological Center for Seismotectonics of the Mediterranean region The seismicity of the Mediterranean region is the westernmost expression of the long line of earthquakes that extends along the southern edge of the Eurasian continent (Figure 13). In the region of the Mediterranean the N-S convergence of Africa and Europe is confined laterally by the eastward drift due to the opening of the Atlantic in the West and by the northward motion of the wedge-shaped Arabian plate in the East (Figure 14). 3.1 Arabia, Anatolia and the Aegean As a result of the northward push of Arabia, the Anatolian block is expelled westward along the north Anatolian fault at an average rate of about 1-2 cm/year. A series of destructive earthquakes with right-lateral strike-slip mechanisms takes up this motion (Figure 15). The magnitude M s 7.4 earthquake of Izmit was the most recent of this series of events, that has been migrating steadily from East to West along the North Anatolian fault over the last century. 13

14 Figure 14: Seismotectonic framework of the plate boundary between Eurasia and Africa. Typical focal mechanisms are shown for different regions. (Udias and Buforn, PAGEOPH, 136, , 1991) The westward motion of Anatolia is resisted by the eastward convergence of the Adriatic and Apulian platforms along the western coast of Greece and Albania (Figure 15). This forces the Aegean Sea to the SW, where it overrides the Mediterranean oceanic crust (Figure 15). Both the hypocentral depths (Figure 16) and the thrust faulting focal mechanisms (Figure 15) along the Hellenic trench are evidence of the subduction of this part of the Mediterranean beneath the continental crust of the Aegean. The deformation of the continental blocks of Greece and SW-Anatolia, on the other hand, is dominated by normal faulting mechanisms, due to back-arc spreading (Figure 15). 3.2 Africa and the western Mediterranean The seismotectonics of the western Mediterranean is dominated by the convergence of Africa and Eurasia. The westernmost limit of the boundary between these two major lithospheric plates lies on the Mid-Atlantic ridge at the Azores triple junction. The plate boundary extending from the Azores into the Maghreb is marked by a long line of earthquakes, whose mechanisms change from normal faulting near the Mid-Atlantic ridge, to strike-slip along the oceanic part of the boundary and to thrust faulting south of Portugal and Gibraltar (Figure 17). Earthquakes with thrust mechanisms characterize the deformation also along most of the northern margin of the Maghreb, as witnessed by the destructive earthquake of El Asnam, Algeria, in The systematic change of the deformational style, indicated by the focal mechanisms along this eastern part of the Africa-Eurasia plate boundary, suggests a counter-clockwise rotation of Africa relative to Eurasia around a pole located west of the Canary Islands. 14

15 Figure 15: Top: Summary map of earthquake focal mechanismsm in the Aegean region. The strike-slip mechanism at the right margin of the Figure is typical of the earthquakes along the North Anatolian fault. Note the thrust mechanisms along coastal Albania and Yugoslavia. Bottom: Sketch map of active tectonics in the Aegean region. Faults are schematic. The shaded regions of the Mediterranean and Black Sea have water depths greater than 1800 m and correspond approximately to those regions underlain by oceanic crust. The stippled region of coastal Albania and Yugoslavia is where active continental shortening and thickening are occurring, caused by the collision with the Adriatic and Apulian platforms. (From Jackson, 1994) Seismicity of the Iberian peninsula is more moderate. It is concentrated in the South, where it represents the northward continuation of the Africa-Eurasian collision, and along the Pyrenees in the North. One of the more puzzling phenomena is the occurrence of intermediate depth (30 < z < 150 km) and very deep (z 630 km) earthquakes beneath southern Iberia. Whereas the intermediate depth events are more frequent and extend along a N-S trend from Granada and Malaga to the Alboran Sea, the deep events are rare and are concentrated in a small volume. No seismicity has been observed between 150 and 600 km depth, so that the processes causing the intermediate and deep seismicity are probably different from each other. 15

16 Figure 16: Hypocenters of intermediate depth (z > 70 km) earthquakes along six profiles perpendicular to the Hellenic trench. (From Papazachos 1988, in Bonnin et al. 1988) 16

17 Figure 17: Epicenter map of shallow earthquakes between 1964 and 1987 along the African- Eurasian plate boundary, together with fault-plane solutions of 26 events with magnitudes greater than 5.5. (From Mueller and Kahle, 1993) 3.3. The Italian peninsula and the Alps The Italian peninsula and the Alps, caught in the middle of a three-way vice between the converging continental plates of Eurasia, Africa and Arabia, would be expected to feature predominantly thrust and strike-slip earthquake focal mechanisms. However, this is only partly the case (Figure 18). Sicily-Calabria: The arc-shaped distribution of seismicity in the Sicily-Calabria region is related to a complex Benioff zone reaching depths of 500 km, caused by the subduction of Ionian oceanic lithosphere beneath the Tyrrhenian Sea (Figure 19). The focal mechanisms of the earthquakes associated with this complex subduction zone show a great diversity (Figure 18). As exemplified by the 1908 earthquake of Messina, the shallow events in this region represent a serious hazard. Appennines and Adriatic: Structurally, the Apennines represent a collisional structure that developed through a process of crustal shortening and thickening. Major orogen parallel thrust faults are evidence for this. However, recent earthquakes with thrust faulting focal mechanisms are restricted to the Adriatic region NW of the Italian peninsula. These focal mechanisms are similar to those observed along the southeastern edge of the Balkans and are evidence for ongoing NE-SW directed crustal shortening. The many earthquakes that line the backbone of the Appennines, on the other hand, have almost exclusively normal faulting mechanisms, indicating that extension perpendicular to the strike of the orogen is active today (Figure 18). The 1980 magnitude 7 Irpinia earthquake in the southern Appennines as well as the extended Umbria-Marche earthquake sequence of 1997 in the central Appennines are eloquent examples of this process. Structural studies indicate that indeed the Appennines have formed as a consequence of the Italian peninsula having been thrust over the Adriatic crust, but that normal faulting is predominant in regions behind the thrust front today (Figure 19). 17

18 Figure 18: Sketch map of the main tectonic features of Italy and focal mechanisms for the large earthquakes between 1976 and (From Montone et al., JGR, 104, B11, 25'595-25'610, 1999) Alps: In the Alps, the classical example of a collisional mountain belt, one would also expect earthquakes with predominantly thrust and strike-slip faulting mechanisms. In the Friuli region, north of the Adriatic, this is indeed the case. The 1976 Friuli earthquakes occurred as slip on low-angle northward dipping thrust faults (Figure 18). Newer studies, however, show that in the internal nappes of the central and western Alps, where elevations are high, earthquakes with normal faulting mechanisms, corresponding to extension directed at large angles to the strike of the Alpine belt, predominate (Figures 20 and 21). Thus, once again, structures which were originally formed in a collision process are now undergoing extensional deformation. 18

19 Figure 19: Lithospheric cross-section through the Calabrian arc (top) and the southern Appennines (bottom). 1: continental crust; 2: thinned continental crust; 3: oceanic crust; 4: upper mantle. (From Philip, 1988, in Bonnin et al., 1988) Figure 20: Focal mechanisms projected onto the ECORS-CROP seismic reflection profiles across the western Alps, showing the normal faulting earthquakes along the front of the internal nappes. Note that in this figure the fault-plane solutions have been rotated into a vertical plane. (Sue et al., JGR, 104, B11, 25'611-25'622, 1999) 19

20 Figure 21: Two seismic sections through the southern Valais, showing the normal faulting mechanisms in the Penninic nappes above the Basal Penninic thrust (BPT). Note that the faultplane solutions are shown as the usual stereographic projections into the lower hemisphere. (Maurer at al., Terra Nova, 9, 91-94, 1997) 4 Seismotectonics of Switzerland Epicentral distribution: Historical information about earthquakes in Switzerland is available for about the past 1000 years. Although seismic activity in Switzerland is moderate compared to other areas of the Alpine-Mediterranean region, we know that destructive earthquakes have occurred in the past. The event that in 1356 destroyed large parts of Basel and many castles in the surroundings is thought to be the strongest historically known earthquake in central Europe, and the area at the southern end of the Rhinegraben has been seismically active for several centuries. Earthquakes that caused severe damages are also known to have occurred in the Valais (e.g. 1755, 1855 and 1946), in Central Switzerland and along the Rhine Valley in Eastern Switzerland (Figure 22). Although first seismographs were already in operation in Switzerland at the beginning of the 20th century, a modern nationwide station network was installed only after By 1982 it consisted of more than 20 permanent stations. Additional local station networks have been in operation in different parts of the country for varying periods of time to monitor in more detail the local seismicity. Since 1975, the Swiss Seismological Service has recorded almost 6000 earthquakes in Switzerland and bordering regions as well as more than 600 quarry blasts and several landslides and rock avalanches. Magnitudes range from less than 1 to 5.2. Although epicenters are not uniformly distributed and earthquakes often tend to occurr clustered in both space and time, they can not be associated with obvious tectonic features visible at the Earth's surface. With a few noteworthy exceptions (Central Switzerland and the Upper Valais), the epicentral distribution of the last 25 years agrees quite well with the known locations of historical earthquakes of the past 700 years (Figure 22). 20

21 Figure 22: Epicenters of historical earthquakes (I 0 > V) and of instrumentally recorded earthquakes (M L > 2.5). 21

22 Figure 23: Focal depths of reliably located earthquakes projected onto a cross-section from Basel to Locarno (Deichmann et al., unpublished). Figure 24: Focal mechanisms of earthquakes in and around Switzerland (Kastrup et al., unpublished). 22

23 Focal depths: The reliably determined focal depths show that in the northern Alpine foreland seismicity extends over the whole crust down to depths of about 30 km, whereas under the Alps, where the crust thickens to about 60 km, seismicity seems to be restricted to the uppermost 15 km (Figure 23). Focal mechanisms: Over 130 well constrained fault-plane solutions are available (Figure 24). In the northern Alpine foreland, focal mechanisms are strike-slip and normal-faulting with a uniform ENE-WSW to NE-SW orientation of the T-axes, in agreement with the known regional western European stress field. In the external Helvetic nappes at the northern front of the Alps, strike-slip and normal-faulting mechanisms coexist in close proximity with some well constrained thrust mechnisms, and thus the deformational style is not uniquely defined. In the internal Penninic nappes under the highest elevations of the Swiss Alps, focal mechanisms are almost exclusively normal-faulting, with T-axes consistently oriented at high angles to the strike of the Alpine mountain range, contrary to what would be expected from its tectonic expression (Figure 25). Figure 25: Tectonic sketch map of Switzerland with predominat styles of deformation as deduced from earthquake focal mechanisms. (From Eva et al., J.Geodyn., 26/1, 27-43, 1988) 23

24 References Baer, M., Deichmann, N., Ballarin Dolfin, D., Bay, F., Delouis, B., Fäh, D., Giardini, D., Kastrup, U., Kind, F., Kradolfer, U., Künzle, W., Röthlisberger, S., Schler, T., Sellami, S., Smit, P., & Spühler, E. (1999): Earthquakes in Switzerland and surrounding regions during Eclogae Geol. Helv., 92/2, Bonnin, J., Cara, M., Cisternas, A., Fantechi, R., eds. (1988): Seismic Hazard in the Mediterranean Regions. Kluwer Academic Publishers, Dordrecht, 399 pp. Jackson, J. (1994): Active Tectonics in the Mediterranean Region. Annual Rev. Earth Planet. Sci., Lay, T., Wallace, T. C. (1995): Modern Global Seismology. Academic Press, 521 pp. McKenzie, D. (1972): Active tectonics of the Mediterranean region. Geophys. J. R. Astr. Soc., 30, Mueller, S., Kahle, H. (1993): Crust-Mantle Evolution, Structure and Dynamics of the Mediterranean-Alpine Region. In: Contributions of Space Geodesy to Geodynamics: Crustal Dynamics, ed. by D. E. Smith and D. L. Turcotte, Geodynamics Series, Vol. 23, AGU, Washington, Pavoni, N., Maurer, H., Roth, P., Deichmann, N. (1997): Seismicity and seismotectonics of the Swiss Alps. In: Deep structure of the Swiss Alps, results of NRP 20, Birkhäuser, Basel, Udias A., Buforn, E. (1994): Seismotectonics of the Mediterranean Region. Advances in Geophysics, 36,