Showing papers by "Domenico Giardini published in 1991"

The Deep Seismicity of the Tyrrhenian Sea

Abstract: The deep seismicity of the Tyrrhenian Sea is analysed using data from a new instrumental catalogue of the seismicity of the Italian area. We use algorithms for the determination of absolute and relative hypocentral locations and for the evaluation of the geometry and coherence of the state of stress within the subducting slab. The structure of the Benioff zone, although simpler than previously indicated, reveals anomalous traits both in the seismicity distribution and in the stress geometry, confirming that standard subduction models cannot be applied in the Tyrrhenian region. The velocity anomaly and the location of few isolated events indicate that the subducted slabs extend to the north along the Apenninic chain approximately to the latitude of the Irpinia region, in Central Italy (˜ 42°N).

Large‐scale three‐dimensional even‐degree structure of the Earth from splitting of long‐period normal modes

Abstract: Spectra of the Earth's free oscillations, which depart significantly from those predicted for spherically symmetric Earth models, contain important information on the large-scale aspherical structure of the Earth. In this paper we present theory, techniques, and numerical results for retrieval of Earth models (including mantle heterogeneity, topography of the core-mantle boundary (CMB), and inner core anisotropy), using such data. The inversions for Earth models in this study are based upon spectral splitting of 33 isolated multiplets, observed in long-period accelerograms of 10 large events recorded by the International Deployment of Accelerometers network. Approximately 1000 spectra are involved. Although the data set is insufficient to yield independent results for perturbations in P velocity, S velocity, and density, it is demonstrated that it is possible to obtain large-scale (spherical harmonic degrees s = 0, 2, 4) models of mantle heterogeneity from such a data set under the constraint that aspherical perturbations in seismic velocities and density are proportional to one another. The mantle models developed from modal data are remarkably similar to preexisting models based upon other kinds of seismic data, demonstrating that heterogeneity in seismic velocities is, at most, weakly dependent on frequency. The pattern of the inferred CMB topography is consistent with geodynamic predictions and agrees to a fair extent with results based on travel time anomalies of PcP and PKP, indicating that modal data can add independent constraints on CMB topography. The anomalous splitting of core modes is attributed to inner core anisotropy which is assumed to possess cylindrical symmetry about the Earth's rotation axis. We consider a relatively simple model which varies smoothly with radius (only with radially constant terms and terms varying with r2). Theoretically, 14 parameters are required to describe such an anisotropic tensor field if it is restricted to spherical harmonic degrees 2 and 4 and if analyticity of the field is required. The inversion for such an anisotropic inner core yields a model which can explain the splitting of anomalously split modes, without violating the constraints imposed by PKIKP travel time information. We solve the inverse problem by following two approaches: (1) using splitting function coefficients as data, we derive Earth models by solving linear inverse problems for each spherical harmonic degree and order; and (2) we directly solve the nonlinear inverse problem in which the data are observed modal spectra and the unknowns are the structural parameters. The second procedure has advantages in the case that there are insufficient data to obtain stable results for the splitting functions of some modes. The mantle models generated in these two ways are essentially identical, verifying that splitting functions can serve as a very convenient intermediate stage in modeling Earth structure using the split spectra of free oscillations.

The relative amplitudes of mantle heterogeneity in P velocity, S velocity and density from free-oscillation data

Abstract: Splitting functions retrieved from spectra of the free oscillations are sensitive to the lateral variations in P velocity (α), S velocity (β), and density (ρ) simultaneously. In this study they are used to constrain the values of the ratios d ln α/d ln β and d ln ρ/d ln α for the lower mantle. Assuming that the upper mantle structure is obtainable from model M84A (this is not a crucial assumption as experiments indicate), the optimal value of d ln α/d ln β inferred from the modal data is 0.44 and d ln α/d ln β lies in the interval (0.39, 0.60) with 75 per cent confidence, strongly discriminating against the value (0.8) often used. The constraints on density structure of the current data are insufficient to yield new definitive results. The analysis demonstrates, however, that the value of d ln ρ/d ln α could be estimated from a larger set of modal data.

MedNet : the very broad-band seismic network for the Mediterranean

Abstract: MedNet is the very broad-band seismographic network installed by the Istituto Nazionale di Geofisica (ING) in countries of the Mediterranean area, with a final goal of 12–15 stations and a spacing of about 1000 km between stations. The project started in 1987 and will be completed within 1992. MedNet is motivated both by research interest and by seismic hazard monitoring; it will allow to define the structure of the Mediterranean region to a high detail, to study properties of the seismic source for intermediate and large events, and to apply this knowledge to procedures of civil protection. To reach its goals, the network has been designed following the highest technical standards: STS-1/VBB sensors, Quanterra 24 bits A/D converters with 140 dB dynamic range, real-time telemetry. Five sites are now operational in Italy (L'Aquila, Bardonecchia and Villasalto) and in northern African countries (Midelt, Morocco; Gafsa, Tunisia); other sites are under construction in Pakistan (Islamabad), Irak (Rutba) and Egypt (Kottamya), while locations are examined for stations in Greece, Jugoslavia and Algeria. The centre of the MedNet network is the Data Center (MDC) in Rome; its tasks include data collection, verification, quality control, archival and dissemination, monitoring of station performance, event detection, routine determination of source parameters. Data distribution will follow the guidelines set by FDSN, and will be coordinated with other international network projects.