Peter Suhadolc

Complete Synthetic Seismograms for High-Frequency Multimode SH -waves

We present an efficient scheme to compute high-frequency seismograms (up to 10 Hz) forSH-waves in a horizontally stratified medium with the mode summation method. The formalism which permits the computation of eigenvalues, eigenfunctions and related integral quantities is discussed in detail. Anelasticity is included in the model by using the variational method. Phase velocity, group velocity, energy integral and attenuation spectra of a structure enable the computation of complete strong motion seismograms, which are the basic tool for the interpretation of near-source broad-band data.Different examples computed for continental structures are discussed, where one example is the comparison between the observed transversal displacement recorded at station IVC for the November 4, Brawley 1976 earthquake and synthetic signals. In the case of a magnitudeML=5.7 earthquake in the Friuli seismic area we apply the mode summation method to infer from waveform modeling of all three components of motion of observed data some characteristics of the source.

A new method for the realistic estimation of seismic ground motion in megacities: the case of Rome

A hybrid technique, based on mode summation and finite differences, is used to simulate the ground motion induced in the city of Rome by the January 13, 1915, Fucino (Italy) earthquake (M L =6.8). The technique allows us to take into consideration source, path, and local soil effects. The results of the numerical simulations are used for a comparison between the observed distribution of damage in Rome, and the computed peak ground acceleration, the maximum response of simple oscillators, and the so-called “total energy of ground motion”. The total energy of ground motion is in good agreement with the observed distribution of damage. From the computation of spectral ratios, it has been recognized that the presence of a near-surface layer of rigid material is not sufficient to classify a location as a “hard-rock site” when the rigid material has a sedimentary complex below it. This is because the underlying sedimentary complex causes amplifications due to resonances. Within sedimentary basins, incident energy in certain frequency bands can also be shifted from the vertical, into the radial component of motion. This phenomenon is very localized, both in frequency and space, and closely neighboring sites can be characterized by large differences in the seismic response.

Seismogenic potential and earthquake hazard assessment in the Tell Atlas of Algeria

Seismotectonic zonation studies in the Tell Atlas of Algeria, a branch of the Africa-Eurasia plate boundary, provide a valuable input for deterministic seismic hazard calculations. We delineate a number of seismogenic zones from causal relationships established between geological structures and earthquakes and compile a working seismic catalogue mainly from readily available sources. To this catalogue, for a most rational and best-justified hazard analysis, we add estimates of earthquake size translated from active faulting characteristics. We assess the regional seismic hazard using a deterministic procedure based on the computation of complete synthetic seismograms (up to 1 Hz) by the modal summation technique. As a result, we generate seismic hazard maps of maximum velocity, maximum displacement, and design ground acceleration that blend information from geology, historical seismicity and observational seismology, leading to better estimates of the earthquake hazard throughout northern Algeria. Our analysis and the resulting maps illustrate how different the estimate of seismic hazard is based primarily on combined geologic and seismological data with respect to the one for which only information from earthquake catalogues has been used.

Zoning of the Italian territory in terms of expected peak ground acceleration derived from complete synthetic seismograms

Abstract An automatic procedure for the seismic zonation of a territory is presented. The results consist of deterministic computation of acceleration time series distributed on a regular grid over the territory. For the estimation of the accelerations, complete synthetic seismograms are computed by the modal summation technique. A first rough zonation can be accomplished by considering a map showing the distribution of peak ground acceleration. In this work the new procedure has been applied to the Italian territory. The structural and source models necessary to compute the synthetic signals have been fixed after an extensive bibliographic research. Seismogenic areas have been defined in the framework of the GNDT (Gruppo Nazionale per la Difesa dai Terremoti of the Consiglio Nazionale delle Ricerche, Rome) research activities dedicated to the definition of the kinematic model of Italy. Information on historical and recent seismicity has been taken from the most updated Italian earthquake catalogues. The estimated peak ground accelerations have been found to be compatible with available data, both in terms of intensity (historical earthquakes) and accelerations (recent earthquakes).

Seismic input modelling for zoning and microzoning

The strong influence of lateral heterogeneities and of source properties on the spatial distribution of ground motion indicates that the traditional methods require an alternative when earthquake records are not available. The computation of broadband synthetic seismograms makes it possible, as required by a realistic modelling, to take source and propagation effects into account, fully utilizing the large amount of geological, geophysical and geotechnical data, already available. For recent earthquakes, where strong motion observations are available, it is possible to validate the modelling by comparing the synthetic seismograms with the experimental records. The realistic modelling of the seismic input has been applied to a first-order seismic zoning of the whole territory of several countries. Even though it falls in the domain of the deterministic approaches, the method is suitable to be used in new integrated procedures which combine probabilistic and deterministic approaches and allow us to minimize the present drawbacks which characterise them when they are considered separately. Detailed modelling of the ground motion for realistic heterogeneous media (up to 10 Hz) can be immediately used in the design of new seismo-resistant constructions and in the reinforcement of existing buildings, without having to wait for a strong earthquake to occur. The discrepancies between the ground responses computed with standard methods and the results of our detailed modelling cannot be ignored when formulating building codes and retrofitting the built environment.

The 1976 Friuli (NE Italy) thrust faulting earthquake: A reappraisal 23 years later

We revisit the 1976 Friuli earthquake sequence by combining hypocenters relocation, long period surface wave inversion, field geology and strong motion modelling. We show that fault-related folding is the main active deformation by which the seismic energy was released during the main shock (Ms=6.5) and that some of the surface effects reported in 1976 correspond to widespread bedding planes displacements induced by flexural-slip folding. The fault evolved from blind to semi-blind along strike showing the control of the inherited structural geology on the fault surface break and rupture arrest. Our fault model produces waveforms that fit the accelerograms recorded in the area.

Upper mantle properties of the Tuscan-Tyrrhenian area: a framework for its recent tectonic evolution

Abstract Within the Alpine-Mediterranean deformational system, several interesting features in the physical properties of the upper mantle have been recognized through the regional analysis of surface-wave dispersion. A large upper mantle anomaly (UMA), here called the Tuscan UMA, has been identified in the area northeast of Corsica, which includes part of the northern Tyrrhenian Sea and part of the North-Central Apennines. This anomalous upper mantle area together with the upper mantle characteristics of the Tyrrhenian Sea, located further to the south, provides a natural framework for comprehensive interpretation of the large data set of geological and geophysical observations, such as land and marine geology, volcanism, gravity, magnetic data, seismic data and seismology, deep temperatures, and surface heat flow.

Physical properties of the lithosphere—asthenosphere system in Europe☆

Abstract All the geophysical fields considered evidence a clear structural difference between the Adriatic Sea-Ionian Sea basin on one side and the Tyrrhenian Sea-Western Mediterranean basin on the other. The former displays continental type characteristics, while the latter shows features definitely related to young ocean basins. Lateral variations in the thickness of the high-velocity sub-crustal layer, the “lid”, are clearly evidenced in the whole European area. The lateral changes in the shear-wave velocities of the lithosphere-asthenosphere system are quite pronounced. Gravity data imply a positive density contrast (0.03–0.07 g/cm 3 ) in the upper mantle related to almost aseismic high-velocity lithospheric roots, which seem to characterize many orogenic belts. The analysis of a transect along the longitudinal axis of the Alps seems to indicate that relevant lateral variations in density are present in the lithospheric part of the mantle.

The 1998 Bovec-Krn mountain (Slovenia) earthquake sequence

We study the 1998 Bovec-Krn mountain (Slovenia) earthquake sequence by combining hypocenters relocation, strong motion inversion, digital elevation modelling and field geology. The main shock (Ms=5.7), a 12 km right lateral strike-slip event on the Dinaric fault system, occurred on a sub-vertical fault plane. The rupture, confined between 3 and 9 km depth, with no evidence of surface faulting, propagated bilaterally within two structural barriers. The northwestern barrier is at the junction between Dinaric and Alpine structures where there is a sharp change in the geometry of faulting. The southeastern barrier is within the Dinaric system and its surface expression corresponds to the Tolminka-spring perched basin, a 1 km restraining step-over. At this site, the Bovec-Krn earthquake-fault overlaps with a 30 km strike-slip fault segment that is free of aftershocks and could be undergoing an increase of stress. This fault system represents the northern branch of the Idrija right-lateral fault.

Rayleigh wave group velocity tomography in the Aegean area

Data from a large-scale experiment which took place in Greece during the period January–July 1997 have been used to investigate the structure of the Aegean area using surface waves. During this experiment, 30 seismic broadband instruments were deployed throughout the whole Greek area. Additional data during the period 1996–2000 from other temporary networks have been included in the dataset. One hundred eighty-five events with magnitudes 4.0VMwV5.5 recorded by these stations have been collected and processed. The individual dispersion curves of the group velocity of Rayleigh waves for each source-station path have been calculated, producing more than 700 paths covering the studied region. These curves have been used to determine Rayleigh group velocity maps using a 2D-tomography method. On the basis of a regionalization of the dispersion measurements, local averaged dispersion curves have been obtained and non-linearly inverted to obtain models of shear-wave velocity versus depth. Since the dispersion curves in the period range 5 sVTV30 s are mostly affected by the crustal structure, the model velocities are estimated down to a depth of approximately 35–45 km. The results from the non-linear Hedhehog inversion as applied to a few local dispersion curves show a crustal thickness of approximately 32 km for the Northern Aegean Sea, and a relatively thin crust of approximately 22–24 km for the Southern Aegean Sea. D 2002 Elsevier Science B.V. All rights reserved.