G. F. Panza

Seismic wave propagation in laterally heterogeneous anelastic media: Theory and applications to seismic zonation

Publisher Summary The ability to estimate accurately seismic hazard at very low probability of exceedance may be important in protecting special objects in the built environment against rare earthquakes. The deterministic approach, based upon the assumption that several earthquakes can occur within a predefined seismic zone, represents a conservative definition of seismic hazard for preevent localized planning for disaster mitigation, over a broad range of periods. Computation of realistic synthetic seismograms, using methods that take into account source, propagation, and site effects, and utilizing the huge amount of available geological, geophysical, and geotechnical data provides a powerful and economically valid scientific tool for seismic zonation and microzanation. First-order zonations can be made at regional scale, considering average structural models and a set of sources with damaging potential distributed within the identified seismogenic areas. Seismic microzonations of urban areas can be performed even more accurately when the required geotechnical data are available, so that local site effects can be effectively modeled.

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.

Characterization of Seismogenic Zones of Romania

Abstract—Although the time and magnitude range covered by available seismological data is limited, several significant regional trends are outlined in the seismogenic zones of Romania. Vrancea region, which is by far the most seismically active area, has a persistent rate of occurrence of intermediate-depth earthquakes, clustered in a very confined focal volume, and a clear compressive stress regime. The deformation field, as deduced from the available fault plane solutions, is drastically reduced in the crust, where the maximum magnitude is below 6.5 (except Shabla zone, in Bulgaria). The system of major faults developed in a NW–SE direction in the Carpathians foredeep area is certainly linked to the subduction process in Vrancea, although they seem not to play a significantly active role, as could be expected for an active subduction process. The existing data indicate an extensional deformation regime over the foredeep area and Southern Carpathians, while a predominant compressive regime is outlined at the contact between the eastern margin of the Pannonian Depression and Carpathians orogen, in agreement with the bending tendency of the maximum horizontal compression orientation of the crustal stress field from NE–SW, in western and central Europe, to E–W, in the intra-Carpathian region (Grünthal and Strohmeyer, 1992).

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 hazard of Romania : Deterministic approach

Abstract—The seismic hazard of Romania is estimated in terms of peak-ground motion values—displacement, velocity, design ground acceleration (DGA)—computing complete synthetic seismograms, which are considered to be representative of the different seismogenic and structural zones of the country. The deterministic method addresses issues largely neglected in probabilistic hazard analysis, e.g., how crustal properties affect attenuation, since the ground motion parameters are not derived from overly simplified attenuation “functions,” but rather from synthetic time histories. The synthesis of the hazard is divided into two parts, one that of shallow-focus earthquakes, and the other, that of intermediate-focus events of the Vrancea region.¶The previous hazard maps of Romania completely ignore the seismic activity in the southeastern part of the country (due to the seismic source of Shabla zone). For the Vrancea intermediate-depth earthquakes, which control the seismic hazard level over most of the territory, the comparison of the numerical results with the historically-based intensity map show significant differences. They could be due to possible structural or source properties not captured by our modeling, or to differences in the distribution of damageable buildings over the territory (meaning that future earthquakes can be more spectacularly damaging in regions other than those regions experiencing damage in the past). Since the deterministic modeling is highly sensitive to the source and path effects, it can be used to improve the seismological parameters of the historical events.

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.

DETERMINISTIC SEISMIC HAZARD ASSESSMENT

Using the available information about regional structural models, past seismicity, and the seismotectonic regime in Italy, we have generated a set of synthetic seismograms covering the whole Italian territory on a 0.2° × 0.2° grid. Peak values of ground motion (displacement, DMAX, and velocity, VMAX) and Design Ground Acceleration (DGA) based on Eurocode 8 (EC8, 1993), extracted from the synthetic signals, have been compared with the macroseismic intensities felt in Italy. The correlation relations that we have obtained are in a good agreement with empirical relationships given by other authors and compare quite well with the few observations available in the Italian territory.

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.

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.

Asymmetric ocean basins

While the superficial expression of oceanic ridges is generally symmetric, their deeper roots may be asymmetric. Based on a surface wave tomographic three-dimensional model of the Earths upper 300 km, we construct a global cross section parallel to the equator of the net rotation of the lithosphere, the so-called tectonic equator. Shear wave velocities indicate a difference between the western and eastern flanks of the three major oceanic rift basins (Pacific, Atlantic, and Indian ridges). In general, the western limbs have a faster velocity and thicker lithosphere relative to the eastern or northeastern one, whereas the upper asthenosphere is faster in the eastern limb than in the western limb. We interpret the difference between the two flanks as the combination of mantle depletion along the oceanic rifts and of the westward migration of the ridges and the lithosphere relative to the mantle. The low-velocity layer in the upper asthenosphere at the depth of 120–200 km is assumed to represent the decoupling between the lithosphere and the underlying mantle. It is also well defined by the distribution of radial anisotropy that reaches minimum values close to the rifts, but with an eastward offset. These results could be explained in the frame of the westward drift of the lithosphere relative to the underlying mantle.