Seismotectonic re-evaluation of the 1976 Friuli, Italy, seismic sequence

The 1976 Friuli seismic sequence is known for itscomplexity, with several unusually large aftershocksoccurring in the epicentral area. The source region, where the northeastern part of the Alpine chain meetsthe northern Dinaric chain, is characterized by a complexcompressional tectonic regime. Previous studies have not clearly identified which of the two main tectonic systems was activated, in part due to the limited precision of derived earthquake parameters, such as hypocentral coordinates and focal mechanisms. We review the locations for the 10 largest events of the sequence, including theSeptember 16, 1977 and April 18, 1979 earthquakes andwe compute their centroid moment tensors. Source parameters are calculated using intermediate period surfacewaves and the modification of the Harvard centroid-moment tensoralgorithm proposed by Arvidsson and Ekström (1998). A summary of all available geological, geodetic and seismological data show that most of the earthquakes may be associated with the Periadriatic overthrust and other related thrust faults. Based on their locations and focal mechanisms only two ofthe early aftershocks (May 7, 1976 and May 9, 1976) appear instead to have occurred on Dinaric structures.     

Seismotectonics and seismic quietness of the Oranie region (Western Algeria): The Mascara earthquake of August 18th 1994, Mw = 5.7, Ms = 6.0

The plate dynamics in the central western Mediterranean region is characterised by a collision between the Eurasian and African plates. In response to this dynamics, many systems of faults and folds having a NE-SW and E-W trending have been generated along the Tellian Atlas of Algeria. The Oranie region (north western Algeria) has experienced some significant earthquakes in the last centuries, the most important one is that of Oran city on February 9^th 1790, I_o = XI which destroyed the town completely and caused the loss of many lives. Since 1790 no other event was so disastrous except that of August 18^th 1994, M_w = 5.7, which struck Mascara province (Algeria) at 01 h 13 mn GMT. Since the beginning of this century the region has been dominated by a seismic quietness. Thus, no event with magnitude larger than 5.5 have occurred in this area. In relation with this recent event, a seismotectonic framework summarising the tectonic, seismicity and focal solution results is presented. The Maximum Observed Intensities Map (MOI) made for Algeria (Bezzeghoud et al., 1996) is also used to show that the Mascara region is located in an VIII-X intensity zone, which explain partially the casualties caused by the 18/08/1994 (M_w = 5.7) earthquake. This earthquake is not anomalous compared to historical records but is unusual compared to recorded seismicity of this century. The seismotectonic map made in this study and also the review of the focal solutions given by the EMSC, Harvard, and other authors shows that our event is probably associated with a source belonging to a system of faults located in the vicinity of the village of Hacine where the maximum damage was observed.     

Crustal structure and dynamical analysis of seismogenesis in the lower Yangzi Valley — South Yellow Sea area

Seismotectonic characteristics and its relationships with crustal structure have been analyzed in the lower Yangzi—South Yellow Sea area. The finite element method is employed to evaluate the stress-strain status of the lithosphere, and the dynamic origin of earthquakes is discussed. The moderate-weak earthquake division is tectonically controlled by three first-degree faults in the Hangzhou bay—Qinling, Tancheng—Lujiang, and the Yangzi River mouth—Jizhou island. The earthquake frequency is higher than 70% in the major hypocentral layer of the crust at a depth of 5 to 15 km of which the highest frequency particularly emerges at a depth of 10 to 15 km, and generally, the depth ranges from 5 to 20 km in the predominant hypocentral layer. The hypocentral layer is superimposed in the crustal magnetosphere with the mid-crustal low density layer. The results from the finite element analysis show that the maximum horizontal displacements appear in the hypocentral layer of the crust, and that the radial hypocentral zones on the flanks of the anomalous mantle, accompanied by the faults or fractured zones, correspond to the gradient zones of the major principal stresses and strains varying within the lithosphere. Therefore, we could consider the mechanism of the decoupling-drift of the lithosphere and the crustal sliding or detachment, from the continent to the sea, as a main dynamogenesis of the earthquakes in the area studied. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,14, 164–171, 1992. This research was supported by the China National Post-Doctoral Research Foundation in part. Some contents on the aeromagnetic anomalies and their mathematical processes are deleted in revision.     

The Azambuja fault: An active structure located in an intraplate basin with significant seismicity (Lower Tagus Valley, Portugal)

The Azambuja fault is a NNE trending structure located 50 km NE of Lisbon, in an area of important historical seismicity. It is sited in the Lower Tagus Basin, a compressive foredeep basin related to tectonic inversion of the Mesozoic Lusitanian Basin in the Miocene. The fault is evident in commercial seismic reflection data, where it shows steep thrust geometry downthrowing the Cenozoic sediments to the east. It has also a clear morphological signature, presenting a NNE-SSW trending, east facing, 15 km long scarp, reaching a maximum height of 80 m. The fault scarp is the geomorphic appearance of a flexure expressed as a zone of distributed deformation, where Miocene and Pliocene sediments are tilted eastwards and are cut by steeply dipping meso-scale faults presenting reverse and normal offsets, with a net downthrow to the east. This pattern at the surface is compatible with a steep fault in the basement that tilts and branches through the overlying Cenozoic sedimentary cover. In order to constrain the neotectonic activity of this structure, detailed geological studies were conducted. Morphotectonics was studied through aerial photo interpretation, analysis of topographic maps and digital mapping. Those studies indicate Quaternary slip on the fault in the ranges of 0.05–0.06 mm per year. Seismogenic behaviour was assumed for the Azambuja fault based on the evidence of Quaternary tectonic activity and its location in an area of significant historical seismicity. M _w 6.4–6.7 maximum earthquakes, with recurrence intervals of 10000–25000 years, were estimated based upon the displaced morphological references, cumulative offsets and fault length.     

Rheology and seismotectonic regime in the northern central Mediterranean

The connection between thermal field and mechanical properties is analysed in the northern central Mediterranean region, extending from the Ligurian-Provençal basin to the Adriatic foredeep. As the thermal regime is still far from equilibrium in most of the tectonic units, transient thermal models are used. The temperature-depth distribution is estimated in four areas affected by the volcanic activity, which from the Neogene to the Present shifted from Corsica to the Apenninic arc. In the Adriatic foredeep, the thermal effects of the recent thrust-faulting phase in the buried sectors of the northern Apennines are taken into account.

The general context consists of convergence involving westward subduction of the Adriatic plate. This process caused anti-clockwise rotation of Corsica and Sardinia, which led to formation of the Ligurian marginal basin, and also resulted in crustal doubling and overthrusting in the northern Apennines and rifting in the northern Tyrrhenian.

Seismic activity is focused in the internal and external zones of the Apenninic arc, where low surface heat flux is observed, and in the western margin of the Ligurian-Provençal basin. This is a consequence not only of lateral variations in the thermal field but also of the different tectonic settings. Regional extensional patterns in the shallow crust, with minimum principal stress axes trending N60°E and E-W, are observed in the northern and in the southern sectors of the Apenninic arc, respectively. A compressional regime at depths greater than 30 km is observed below the northern sector of the arc, while to the south a change in the structure of the lithosphere is marked by a decrease in deeper seismic activity. Thrust faults and strike-slip faults with a thrust component support a compressional regime along the western margin of the Ligurian basin with maximum principal stress axis oriented N120°E.

Two lithospheric cross-sections across the study region are constructed, based on structural, thermal, gravity, rheological and seismic data. There is clear evidence of the presence of the subducting slab of the Adriatic plate, corresponding to a thickening of the uppermost brittle layer. The crustal seismicity cut-off corresponds to temperatures of 320–390°C. A brittle layer of considerable thickness is present in the uppermost mantle beneath Variscan Corsica and the Adriatic foredeep, with estimated seismic cut-off temperature of about 550 ± 50°C.     

Microearthquake swarms: scaling and lacunarity

The continental collision between the African and Eurasian plates in the Iberian peninsula is solved with a NW-SE compression with an extensional basin, the Alboran Sea. To explain this paradox, several models have been proposed, including the subduction process of a lithospheric plate. In this study a relocation of the seismicity of the area using a JHD algorithm for the period 1950–1995 has been performed, which shows a specific hypocentre pattern that is not compatible with plate subduction. In addition, the direction of the stresses acting in the area has been determined through a comprehensive study of all existing focal mechanisms, together with calculations for 19 new earthquakes with sufficient data to determine the focal solution. After considering all these data, the absence of intermediate shocks in the central part of the Alboran basin and the predominantly thrusting mechanisms at the borders of the basin, with pressure axes perpendicular to the front, we consider a delamination process as a possible mechanism acting in the area     

The milos island earthquake of March 20, 1992 and its tectonic significance

An earthquake sequence took place on March 20, 1992, in Milos island (Greece) and lasted for about ten days. The main shock registered a magnitudeM _s =5.3 and a depth of 9.6 km. The majority of the events were shallower than 5 km. Theb value of the sequence (b=0.96) is characteristic for tectonic rather than volcanic activity. Geological, tectonic and seismological observations show that in the island of Milos the seismic energy is mainly released along fault zones. Minor swarm activity was also detected.Recent seismic activity is due to the reactivation of the tectonic graben which traverses the central part of the island in NW-SE trend.