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  • 1
    ISSN: 1573-157X
    Keywords: peak ground acceleration map ; seismic hazard ; seismic energy ; spatially smoothed activity ; Slovenia
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract A progress report on the mapping effort for construction of a peak ground acceleration (PGA) map of Slovenia for 475-year return period for rock and firm soil is presented. The methodology is similar to that recently applied in Central and Eastern United States. It is based on historical seismicity spatially smoothed to different length scales. The procedure is described by Frankel (1995). He uses the characteristic earthquake recurrence relationship and in his firt version four different seismicity models. We also use four seismicity models. But instead of characteristic earthquake recurrence, we use the doubly truncated exponential magnitude-frequency relationship; no evidence of characteristic earthquakes in Slovenia has been found yet. Three of our models are similar to Frankel's first three models. Model 1 uses spatially smoothed activity rate based on magnitude 3.7 and above events since 1880. Model 2 deals with spatially smoothed activity rate based on magnitude 5.0 abd above events since 1690. Model 3 smoothes the observed activity over the entire region; it represents a uniform seismicity zone. Frankel skipped this model in his latest version (Frankel et al. 1996). In model 4, we introduce a new approach of calculating seismic activity rate taking into account released seismic energy. The ground motion attenuation model of Pugliese and Sabetta (1989) is used for all models. PGA maps for models 1, 2, 3 and 4 have been calculated, and a weighted mean map derived from them. A map of model 1 has been compared with the corresponding source zone map; the two maps do not differ significantly. A worst-case map derived from all four models has also been produced.
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  • 2
    ISSN: 1573-157X
    Keywords: seismicity ; source mechanism ; intermediate and deep earthquakes ; South Spain ; subduction zone ; delamination process
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Focal mechanisms of 10 intermediate-depth earthquakes (30〈h〈150 km) and one very deep (h 650 km) which occurred in southern Spain and the Alboran Sea are studied in this paper. Distribution of epicenters with foci at intermediate depth shows a N–S alignment with a geometry parallel to the east of the Arc of Gibraltar. Focal mechanisms have been determined from first motion of P-wave and modeling wave forms of direct P arrivals. Most of the intermediate depth events present single source time functions with small time duration (smaller than 0.3 s) and only for the event of March 27, 1987 a complex source time function has been found. The very deep earthquake of March 8, 1990 has a complex focal mechanism with a long source time function (1.2 s) and two different fault-plane orientations. Scalar seismic moments and dimensions have been obtained from modeling and spectral analysis. The results are interpreted in terms of the seismotectonic framework of the region and suggest the existence of a vertical slab extending from 50 km to 150 km with strike N–S produced by a lithospheric delamination process. The existence of the very deep activity at 650 km of depth may be explained in terms of a block of lithospheric material that still cold and rigid, generates the very deep earthquakes.
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  • 3
    ISSN: 1573-157X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Approximately one thousand microearthquakes with body-wave magnitude mb have been located in northern Venezuela and the southern Caribbean region (9–12° N; 64–70° W) since the installation in 1980 of the Venezuelan Seismological Array, together with forty events of mb ≤ 4, one of them with surface-wave magnitude Ms ∼ 6. Focal depths are in the range of 0 to 〈15 km. This geologically complex region is part of the boundary between the Caribbean and the South American Plates. Epicentral locations indicate that this E–W oriented portion of the boundary is formed by two ∼400 km long subparallel fault zones: San Sebastián fault zone (SSF), ∼20 km north of Caracas along the coast; and La Victoria fault zone (LVF), ∼25 km south of the city. They are clearly delineated by the microseismicity. New composite focal mechanism solutions (CFMS) along these faults show right-lateral strike-slip (RLSS) motion on nearly E–W oriented fault planes. NW-striking subsidiary active faults occur in the region and intercept the two main E–W fault zones. These interceptions show high levels of microearthquake activity and seismic moment release when compared to other portions of both, the main and subsidiary faults. New CFMS at those fault crossing sites show NW-striking RLSS motion and normal faulting, in an en-echelon-like structural behavior. Geological data and quantitative comparisons with other transcurrent plate boundaries in the world suggest that the rate of plate motion in this area is on the order of 20 mm/y. Several moderate and large shocks have occurred along the SSF and LVF since ∼1640, including an Ms ∼ 7.6 event in 1900 on SSF. Although the region may be relatively far from a repeat of this earthquake, seismicity data indicate that strong shocks could take place along segments of the seismically active faults identified in this study.
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  • 4
    ISSN: 1573-157X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract We use near-field accelerograms recorded by the very broadband seismographic station of PNIG to locate the Pinotepa Nacional earthquake of 27 March, 1996 (Mw = 5.4) and to determine its source parameters. The data from PNIG on P and S arrival times, the azimuth of the arrival of P wave, and the angle of incidence of the P wave at the free surface permit the determination of the location (16.365° N, 98.303° W, depth = 18 km) and the origin time (12:34;48.35) of the earthquake. The displacement seismograms of the earthquake clearly shows contribution from the near-field terms. We compute a suite of synthetic seismograms for local mechanisms in the vicinity of the mechanism reported by the U.S. Geological Survey (USGS) and compare them with the observed seismograms at PNIG. The point whose synthetics fit the observed records well has the following parameters: seismic moment, M0 = 1.2 × 1024 dyne-cm; source time function: a triangular pulse of 0.9 sec duration; fault plane: strike ϕ = 291°, dip δ = 10°, and rake λ = 80°. The location and the source parameters obtained from the analysis of PNIG records differ significantly from those reported by the USGS. This demonstrates again, what has been shown by some previous researchers, that high-quality recordings from a single near-field station can considerably improve the estimation of the source parameters of an earthquake. The main event was preceded by a subevent which occurred ∼0.18 sec before and whose seismic moment was ∼1% of the main event. It is possible that even this subevent was preceded by a couple of smaller subevents. This earthquake supports the body of evidence showing that an earthquake begins with a sequence of smaller subevents, cascading in the occurrence of the main event.
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  • 5
    ISSN: 1573-157X
    Keywords: microzonation ; seismic ambient noise ; numerical modelling ; strong ground motion ; historical earth-quakes ; site effects ; Basel ; Rhinegraben ; lateral heterogeneity
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract During the past centuries, the city of Basel has suffered damage caused by earthquakes. One extraordinary event described in historical documents is the strong earthquake which occurred in 1356. The 1356 event, one of the strongest earthquakes in northwest-Europe, was obviously much stronger than the low-magnitude earthquakes observed in the area during this century. Even though the present seismicity in the Basel area is low, strong earthquakes have to be expected due to the city's geographical location close to the northern boundary of the African-European convergence zone, at the southern end of the Rhinegraben. A crucial step towards preparedness for future events and mitigation of earthquake risk involves a microzonation study of the city. The study is carried out in three steps: (1) a detailed mapping of the geology and geotechnical properties of the area, (2) measurement, interpretation and modelling of ambient noise data, and (3) numerical modelling of expected ground motions during earthquakes. A qualitative microzonation of the centre of Basel is presented, and it is discussed by comparing it to the historically reported damage of the 1356 earthquake.
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  • 6
    ISSN: 1573-157X
    Keywords: historical earthquakes ; historical seismology ; Crete ; Mediterranean ; tsunami
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract By conducting a historical review of this large seismic event in the Mediterranean, it has been possible to identify both the epicentral area and the area in which its effects were principally felt. Ever since the nineteenth century, the seismological tradition has offered a variety of partial interpretations of the earthquake, depending on whether the main sources used were Arabic, Greek or Latin texts. Our systematic research has involved the analysis not only of Arab, Byzantine and Italian chronicle sources, but also and in particular of a large number of never previously used official and public authority documents, preserved in Venice in the State Archive, in the Marciana National Library and in the Library of the Museo Civico Correr. As a result, it has been possible to establish not only chronological parameters for the earthquake (they were previously uncertain) but also its overall effects (epicentral area in Crete, Imax XI MCS). Sources containing information in 41 affected localities and areas were identified. The earthquake also gave rise to a large tsunami, which scholars have seen as having certain interesting elements in common with that of 21 July 365, whose epicentre was also in Crete. As regards methodology, this research made it clear that knowledge of large historical earthquakes in the Mediterranean is dependent upon developing specialised research and going beyond the territorial limits of current national catalogues.
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  • 7
    ISSN: 1573-157X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Reservoir induced earthquakes began to occur in the vicinity of Shivajisagar Lake formed by Koyna Dam in Maharashtra state, western India, soon after its filling started in 1962. Induced earthquakes have continued to occur for the past 34 years in the vicinity of this reservoir, and so far a total of 10 earthquakes of M ≥ 5.0, over 100 of M ≥ 4 and about 100,000 of M ≥ 0.0 have occurred. Every year, following the rainy season, the water level in the reservoir rises and induced earthquakes occur. Seismic activity during 1967–68 was most intense when globally, the largest reservoir induced earthquake occurred on 10 December, 1967. Other years of intense seismic activity are 1973 and 1980. During 1986 another reservoir, Warna, some 20 km south of Koyna, began to be filled. The recent burst of seismic activity in Koyna-Warna region began in August, 1993, and was monitored with a close network of digital and analog seismographs. During August, 1993–December, 1995, 1,272 shocks of magnitude ≥ 2 were located, including two earthquakes of M 5.0 and M 5.4 on 8 December, 1993 and 1 February, 1994, respectively. Two parallel epicentral trends in NNE-SSW direction, one passing through Koyna and the other through Warna reservoir are delineated. The 1993 increase in seismicity has followed a loading of 44.15 m in Warna reservoir during 11 June 11, 1993 through August 4, 1993, with a maximum rate of filling being 16 m/week. The larger shocks have been found to be preceded by a precursory nucleation process.
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  • 8
    Electronic Resource
    Electronic Resource
    Springer
    Journal of seismology 1 (1997), S. 181-201 
    ISSN: 1573-157X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract A catalogue of 356 macroseimic maps which are available for the Balkan area was compiled, including information on the source parameters of the corresponding earthquakes, the macroseismic parameters of their strength and their macroseismic field. The data analysis of this catalogue yields new empirical relations for attenuation, which can be applied for the calibration of historical events, modelling of isoseismals and seismic hazard assessment. An appropriate analysis allowed the separation and estimation of the average values of the geometrical spreading, n, and anelastic attenuation factor, c, for the examined area which were found equal to −3.227 ± 0.112 and −0.0033 ± 0.0010. Scaling relations for the focal macroseismic intensity, If, and the epicentral intensity I0, versus the earthquake moment magnitude were also determined for each Balkan country. A gradual decrease of the order of 0.5 to 1 intensity unit is demonstrated for recent (after 1970) earthquakes in Greece. Finally the depths of the examined earthquakes as they robustly determined (error 〈5 km) on the basis of macroseismic data were found to have small values (∼ 10 km). However large magnitude earthquakes show higher focal depths (∼ 25 km), in accordance with an increase of the seismic fault dimensions for such events.
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  • 9
    ISSN: 1573-157X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract An earthquake with local magnitude (ML) 5.2 occurred February 18, 1996 in the eastern Pyrenees (France) near the town of Saint-Paul de Fenouillet. This event is the first of this magnitude in France to be well recorded instrumentally. Less than 24 hours after the main shock, we installed a temporary network of 30 seismological stations in the epicentral area to record the aftershock sequence. In this paper, we analyse the main shock and present the 37 largest aftershocks (1.8 ≤ Ml ≤ 3.4) in the two months following the main shock. These events are located using data from the permanent Pyrenean seismological network and the temporary network when available. We also determined eight fault plane solutions using the P-wave first motions. The main shock and the aftershocks are located inside the small Agly massif. This Hercynian structure sits some 8 km north of the North Pyrenean Fault, which is usually considered to be the suture between the Iberian and Eurasian plates. The mechanism of the main shock is a left-lateral strike-slip on an E–W trending fault. The fault plane solutions of the aftershocks are mostly E–W striking reverse faults, in agreement with the general north-south shortening of the Pyrenees. The aftershocks located down to 11 km depth, indicating that the Agly massif is deeply fractured. The main interpretations of these results are: (i) The main shock involved an E–W trending fault inside the highly fractured Agly massif, relaying the North Pyrenean Fault which had, at least in the last 35 years, a poor seismic activity along this segment; (ii) The Saint-Paul de Fenouillet syncline to the north and the North Pyrenean Fault to the south delimit a ∼15 km wide senestral shear zone. Such a structure is also suggested by the highly fractured pattern of the Agly massif and by small en echelon faults and secondary folds in the Saint-Paul de Fenouillet syncline; (iii) we suggest that the North Pyrenean Frontal Thrust, located less than 10 km north of the Agly massif, has a ramp geometry at depth below the Agly massif.
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  • 10
    ISSN: 1573-157X
    Keywords: block structure dynamics ; Gutenberg-Richter law ; boundarym movements ; structure separateness
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract A seismically active region is modelled as a system of absolutely rigid blocks separated by infinitely thin plane faults. The interaction of the blocks along the fault planes and with the underlying medium is viscous-elastic. The system of blocks moves as a consequence of prescribed motion of the boundary blocks and of the underlying medium. When for some part of a fault plane the ratio of the stress to the pressure exceeds a certain strength level a stress-drop (‘a failure’) occurs (in accordance with the dry friction model), and it can cause a failure in other parts of the fault planes. In the model the failures represent earthquakes. As a result of the numerical simulation a synthetic earthquake catalog is produced. The numerical modelling was carried out for three types of structures with increasing of the structure separateness inside of each type and for two types of boundary movements. A synthetic earthquake flow is characterised by several features including the frequency-magnitude relation (the Gutenberg-Richter curve). When the structure separateness increases the slope of the curve changes monotonously in the same direction for all considered types of structures if the boundary movement is the same. The directions of changing of the slope are opposite for two considered boundary movements.
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