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  • 1
    ISSN: 1573-157X
    Keywords: Normal faulting ; Gulf of Corinth
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract We present the results of a multidisciplinary study of the Ms = 6.2, 1995, June 15, Aigion earthquake (Gulf of Corinth, Greece). In order to constrain the rupture geometry, we used all available data from seismology (local, regional and teleseismic records of the mainshock and of aftershocks), geodesy (GPS and SAR interferometry), and tectonics. Part of these data were obtained during a postseismic field study consisting of the surveying of 24 GPS points, the temporary installation of 20 digital seismometers, and a detailed field investigation for surface fault break. The Aigion fault was the only fault onland which showed detectable breaks (< 4 cm). We relocated the mainshock hypocenter at 10 km in depth, 38 ° 21.7 ′ N, 22 ° 12.0 ′ E, about 15 km NNE to the damaged city of Aigion. The modeling of teleseismic P and SH waves provides a seismic moment Mo = 3.4 1018 N.m, a well constrained focal mechanism (strike 277 °, dip 33 °, rake − 77°), at a centroidal depth of 7.2 km, consistent with the NEIC and the revised Harvard determinations. It thus involved almost pure normal faulting in agreement with the tectonics of the Gulf. The horizontal GPS displacements corrected for the opening of the gulf (1.5 cm/year) show a well-resolved 7 cm northward motion above the hypocenter, which eliminates the possibility of a steep, south-dipping fault plane. Fitting the S-wave polarization at SERG, 10 km from the epicenter, with a 33° northward dipping plane implies a hypocentral depth greater than 10 km. The north dipping fault plane provides a poor fit to the GPS data at the southern points when a homogeneous elastic half-space is considered: the best fit geodetic model is obtained for a fault shallower by 2 km, assuming the same dip. We show with a two-dimensional model that this depth difference is probably due to the distorting effect of the shallow, low-rigidity sediments of the gulf and of its edges. The best-fit fault model, with dimensions 9 km E–W and 15 km along dip, and a 0.87 m uniform slip, fits InSAR data covering the time of the earthquake. The fault is located about 10 km east-northeast to the Aigion fault, whose surface breaks thus appears as secondary features. The rupture lasted 4 to 5 s, propagating southward and upward on a fault probably outcropping offshore, near the southern edge of the gulf. In the shallowest 4 km, the slip – if any – has not exceeded about 30 cm. This geometry implies a large directivity effect in Aigion, in agreement with the accelerogram aig which shows a short duration (2 s) and a large amplitude (0.5 g) of the direct S acceleration. This unusual low-angle normal faulting may have been favoured by a low-friction, high pore pressure fault zone, or by a rotation of the stress directions due to the possible dip towards the south of the brittle-ductile transition zone. This fault cannot be responsible for the long term topography of the rift, which is controlled by larger normal faults with larger dip angles, implying either a seldom, or a more recently started activity of such low angle faults in the central part of the rift.
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Journal of seismology 1 (1997), S. 301-302 
    ISSN: 1573-157X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Journal of seismology 1 (1997), S. 289-299 
    ISSN: 1573-157X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract In this paper we assess the size and effects of the earthquakes of 12 May 1866, and 24 January 1916 in Anatolia (Turkey). We show that these events had a magnitude Ms 7.2 and that the former was associated with a 45-km long surface fault break along the north-east part of the East Anatolian Fault Zone. These two earthquakes are chosen among others in order to demonstrate how easy it is to miss out large earthquakes of the historical, even of the early instrumental period, and to draw the incompleteness of many existing catalogues to the attention of those who use them for the estimation of slip rates and the assessment of seismic hazard. Of the two earthquakes studied here, the former was only vaguely known and the latter is not included in Gutenberg and Richter's catalogue.
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  • 4
    ISSN: 1573-157X
    Keywords: moment tensor ; waveform inversion ; modal summation ; explosions ; monitoring ; lateral heterogeneity ; point source ; source time function
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The design of a monitoring system for detecting explosions is a very topical problem, both for routine data processing at seismological observatories as well as for the monitoring of a Comprehensive Test Ban Treaty. In this framework it is desirable to have the possibility to quantify the presence of the isotropic component in the seismic source. For this purpose a method is presented, which is based on waveform inversion for the full moment tensor retrieval. The method inverts either full waveforms or separate seismic phases and returns the mechanism and time history of a point source. Moreover, it allows to redefine the hypocentral depth of the event and, in a simplistic way, to optimize the structural model as well. In order to model strong laterally heterogeneous structures, different pairs of structural models can be used for each source-receiver path. The source is decomposed into a volumetric part (V), representing an explosive or implosive component, and into a deviatoric part, containing both the double couple (DC) and the compensated linear vector dipole (CLVD) components. The method is applied to an area in central Switzerland and to the network of the Swiss Seismological Service. The events of interest include both earthquakes and explosions. Despite some modelling inadequacies of the source-time function, the explosions can be well identified with the inverted isotropic component in the source, as long as the number of stations used for the inversion is larger than three. The results of the inversion are better for large epicenter-station distances of the order of 40–90 km.
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 1573-157X
    Keywords: Roermond ; source inversion ; regionalstructure
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The Mw = 5.4 Roermond earthquake of April 13, 1992, is used as a 'test' earthquake for the development of source inversion methods at a regional scale in Europe. We combine structural modelling of the European continent (Du et al., 1997) with two source inversion methods derived from Sileny et al. (1992), and Mao et al. (1994). We show that following this strategy, it is possible to fully analyze the inverse problem of the hypocentral relocation, source mechanism and rupture history. We define and discuss our methodology on the basis of the inverse problem and of the associated tools. The results of our application to the Roermond earthquake are discussed in the light of other previously published solutions. Such an approach appears to offer a promising tool for the global description of seismic sources in regions well studied from the structural point of view, through waveform inversion of a few regional records.
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  • 6
    ISSN: 1573-157X
    Keywords: regional amplification ; path effects ; coda-length ; magnituderesidual ; finite difference ; SH waves
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Seismic ground motion in central Mexico is amplified relative to ground motion observed at the same epicentral distance along the Pacific Coast in a frequency band that includes destructive ground motion at Mexico City. Although several hypothesis have been advanced, at present there is no generally accepted explanation of such amplification. We have analyzed coda-length magnitude data reported by Servicio Sismológico Nacional (SSN) for events recorded during 1993 to increase our understanding of the spatial distribution of this phenomenon. Our results indicate that regional amplification: (a) is detected by magnitude residual computed at each station, relative to the average of SSN network;and (b) is likely related to the crustal structure under the central portion of the Transmexican Volcanic Belt (TMVB). Finally, preliminary wave propagation modelling (using SH wave, finite difference method) suggests that crustal heterogeneity is a possible cause of regional amplification. However, if this is so, it is required that both geometry and velocity distribution vary between the coast and Mexico City.
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  • 7
    ISSN: 1573-157X
    Keywords: kinematic source model ; strong ground motion ; directivity effect ; source geometry effect
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract A mixed statistical-deterministic model of earthquake rupture is developed for evaluating the strong ground motion in the near source range (receiver distance comparable to the fault length). The source parametrization is based on the k-square model and the propagation is computed by asymptotic Green's functions. The method is applied to the case of 1976, Friuli earthquake (M = 6.5) in northern Italy which occurred on a low-dip thrusting fault. Acceleration records at 29 stations are computed for 100 simulations of rupture histories. The mean value map of peak ground accelerations shows clearly a maximum to the south due to the inner geometry and directivity of the source. The variation of the estimated PGA versus the epicentral distance is strongly dependent on azimuth and is not decreasing monotonically. The comparison of these curves with those predicted by empirical acceleration–distance relationships shows discrepancies in the near source distance range. This study shows the importance of considering the complexity of the source rupture process for strong motion estimate in the near source range.
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  • 8
    ISSN: 1573-157X
    Keywords: Seismic noise ; signal-to-noise ratio ; site selection ; German Regional Seismic Network
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The German Regional Seismic Network (GRSN) comprizes now 16 digital broadband stations equipped with Wieland-Streckeisen STS-2 seismometers, 24-bit dataloggers and a seismological data center at Erlangen. It covers the whole territory of Germany with station-spacings between 80 km to 240 km. The stations are sited in very different environments ranging from near shore at the Baltic Sea coast up to distances of about 700 km from the coast, both within cities and up to about 10 km away from any major settlement, industry or traffic roads. The underground varies from outcropping hard rocks in Hercynian mountain areas, sedimentary rocks in areas of Mesozoic platform cover to up to 1.5 km unconsolidated Quarternary and Tertiary subsoil. Accordingly, seismic background noise varies in a wide range between the upper and lower bounds of the new global noise model. The noise conditions at the GRSN have been investigated systematically by means of displacement power spectral analysis within the frequency range 10-2 <f < 40 Hz. Smoothed power spectra have been calculated by applying the 'average segment method' using record intervals between 4 and 45 min long and between 6 and 25 overlapping segments. Representative samples were taken at different times of the day and the year in order to quantify for all sites the level and degree of variability of seismic background noise. The worst stations of the original GRSN were Berlin (BRLN), Hamburg (HAM) and Liddow (LID), all placed on unconsolidated soft-soil cover, between 50 m (at LID) and about 1.5 km thick (at HAM). But no spectral noise peaks due to wave resonance in this soft-soil cover could be identified because the noise spectra at all these sites are clearly dominated by strong ambient noise sources (traffic, industry and/or coastal surf sea-noise). For all three sites better locations had to be found at not too large distance from the original sites so as to preserve the good overall GRSN network configuration. Suitable alternatives were found at Rüdersdorf (RUE), Bad Segeberg (BSEG) and west of the village of Neunkirchen, Island of Rügen (RGN). RUE and BSEG were placed on locally outcropping sedimentary rock on top of a salt dome and within the cap of a diapir, respectively. The new station RGN was installed only 2.8 km away from the former LID in a huge soil-covered army bunker which provided much better thermal shielding and a more stable basement platform for long-period recordings. The noise power at RUE and BSEG as compared to BRLN and HAM is reduced by about 10 to 50 dB between 0.4 Hz <f < 50 Hz. This corresponds to 1 to 5 orders of magnitude in power spectral density or a factor of 3 to 300 in displacement amplitudes. For some selected, both near and teleseismic events improvements of the spectral signal-to-noise ratio > 5 for RUE and > 10 for BSEG have been confirmed for frequencies between about 0.6 Hz <f < 5 Hz. Thus BSEG has become now a station only somewhat inferior to the good hard rock sites in the central and southern part of Germany while RUE near Berlin is almost comparable with FUR near Munich. RGN, still being inappropriate in the teleseismic detection window around 1 s, is now almost as good as other fine GRSN stations in the long-period range between about 10 s <t < 50 s, comparable with FUR around f = 2 Hz and even better for f > 3 Hz. Strong lateral velocity and impedance contrasts between the outcropping Triassic/Permian sedimentary rocks and the surrounding unconsolidated Quarternary/Tertiary sediments are shown to be the main cause for the strong noise reduction and signal-to-noise ratio improvement at RUE and can account for about 50% of the noise reduction at BSEG.
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  • 9
    ISSN: 1573-157X
    Keywords: earthquake location ; fault plane solutions ; seismicity ; tectonics ; Alaska
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract A subset of 2660 shallow earthquakes (0–50 km) that occurred from 1988 to 1996 in south central Alaska between 155°W and 145°W and 59°N and 63°N was relocated using the joint hypocenter determination (JHD) method. Both P- and S-wave observations recorded by the regional seismic network were used. Events were relocated in twenty different groups based on their geographic location and depth using two velocity models. As a result of the relocation, the majority of the hypocenters shifted downward, while the epicenter locations did not change significantly. The distribution of the shallow subduction zone earthquakes indicates the existence of two seismically independent blocks, with one block occupying the northeastern part and the other occupying the central and western parts of the study area. The boundary between the blocks is marked by a 15 to 20 km wide seismicity gap to the southeast of 149.5°W and 62°N. The analysis of the fault plane solutions for shallow subduction zone earthquakes shows that an overwhelming majority of the solutions represent normal, oblique-normal or strike-slip faulting with predominant WNW-ESE orientation of T-axes. This indicates a down-dip extensional regime for the subducting slab at shallow depths. Very few earthquakes yielded fault plane solutions consistent with thrusting on a contact zone between the overriding and subducting plates. This result may be an indication that currently either the strain energy is not released at the contact zone or it is associated with aseismic motion.
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  • 10
    Electronic Resource
    Electronic Resource
    Springer
    Journal of seismology 2 (1998), S. 103-103 
    ISSN: 1573-157X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Type of Medium: Electronic Resource
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