Showing papers by "Domenico Giardini published in 2003"

Dynamics of retreating slabs : 2. Insights from three-dimensional laboratory experiments

Abstract: [1] A laboratory analogue of a three-layer linear viscous slab-upper mantle-lower mantle system is established in a silicone putty, honey and crystallized honey tank experiment. The same setup as in the numerical investigation (part 1) is used. We focus on the interaction of the slab with the induced passive mantle flow by widely varying the mantle volume flux boundary conditions. In our numerical experiments the lateral volume flux was set to zero. In interpreting the results relative to the real Earth, the base of the box is taken as the bottom of the mantle convection system, while the lateral boundaries may be associated with the presence of other nearby slabs. Dynamic force equilibrium, assessed on the basis of an analytical review of forces, is described for four different phases: (1) the subduction initiation instability, (2) the accelerating dynamic free fall phase of the slab, (3) the dynamic interaction with the 660-km discontinuity, and (4) a final phase of steady state trench retreat. Phase 3 is an important feature not observed in the numerical experiments. This highly dynamic phase of interrupted trench retreat can therefore be attributed to boundary conditions on mantle volume flux. On the basis of integration constants of force equilibrium in phases 2 and 4 we identify two different classes of volume flux: one in which the lateral boundary can be considered open and the other class where it is “closed.” Closed boundary condition cases are obtained if any of the lateral box boundaries are 600 km away from the slab. Assuming a one-to-one relation between trench retreat and back arc spreading, enigmatic observations of episodic opening of back arc basins can be explained by our experimental observations.

Inversion of local S-wave velocity structures from average H/V ratios, and their use for the estimation of site-effects

Abstract: H/V spectral ratios from microtremors areused to retrieve the S-velocity structurefrom a single ambient vibration record, byusing its relation to the ellipticity ofthe fundamental mode Rayleigh wave and theamplitude of observed H/V ratio.Constraints are needed in order to restrictthe possible range of solutions, and theinversion is applied to sites where thethickness of the unconsolidated sedimentsis approximately known from boreholeinformation. Within the uncertainty, theinverted structures agree well with theresults from other S-wave measuringtechniques such as downhole and cross-holemeasurements, and the analysis of ambientvibrations measured on an array.The influence of the inversion uncertaintyon site-amplification estimates forearthquakes is then investigated. For allinverted models, site response is computedfor a large number of events, which allowsto define the uncertainty by the aprioriunknown source position and mechanism of afuture earthquake. In most cases thevariability between the results obtainedfor the different models is much smallerthan the variability introduced by theunknown source position. The accuracy withwhich S-wave velocity structures can beretrieved from observed H/V ratios istherefore sufficient for an application ofthe method in seismic hazard analysis for aspecific site.

Seismic evidence for water deep in Earth's upper mantle.

Abstract: Water in the deep upper mantle can influence the properties of seismic discontinuities in the mantle transition zone. Observations of converted seismic waves provide evidence of a 20- to 35-kilometer-thick discontinuity near a depth of 410 kilometers, most likely explained by as much as 700 parts per million of water by weight.

Probabilistic earthquake location in complex three‐dimensional velocity models: Application to Switzerland

Abstract: [1] We present a new approach to determine precise and reliable hypocenter locations in the tectonically complex region of Switzerland. A three-dimensional (3-D) P wave velocity model to be used for earthquake relocation is obtained by simultaneously inverting arrival times of local earthquakes for hypocenter locations and 3-D P wave velocity structure. A 3-D P wave velocity model derived from controlled source seismology (CSS) is used as an initial reference model. The final 3-D model thus combines all available information from both CSS and local earthquake data. The probabilistic, nonlinear formulation of the earthquake location problem includes a complete description of location uncertainties and can be used with any kind of velocity model. In particular, the combination of nonlinear, global search algorithms, such as the Oct-Tree Importance Sampling, with probabilistic earthquake location provides a fast and reliable tool for earthquake location. The comparison of hypocenter locations obtained routinely by the Swiss Seismological Service (SED) to those relocated in the new 3-D velocity model using a probabilistic approach reveals no systematic shifts but does exhibit large individual shifts in some epicenter locations and focal depths. We can attribute these large shifts in part to large uncertainties in the hypocenter location. Events with a low number of observations (<8) and no observation within the critical focal depth distance typically show large location uncertainties. Improved hypocentral locations, particularly for mine blasts and earthquakes whose routine hypocenter locations had been questionable, confirm that improved velocity model and probabilistic earthquake location yield more precise and reliable hypocenter locations and associated location uncertainties for Switzerland.

New GPS constraints on the Africa‐Eurasia plate boundary zone in southern Italy

Abstract: [1] Southern Italy is a key area for understanding the tectonic processes in the Africa-Eurasia collision zone. We analyze new GPS measurements carried out between 1994 and 2001. The results are presented in terms of time series, trajectories and velocities of crustal motion, as well as a geodetic strain rate field. While central Italy, Corsica, Sardinia and the Tyrrhenian Sea move like the Eurasian plate, the overall motion of the Sicily Rift Zone region matches African plate motion. Unexpected are the north-northwest directed motions of northeastern Sicily and the Eolian islands. Most striking on the deformation field are a north-south oriented compression along the northern Sicilian coast, compression between Apulia and northwestern Greece as well as extension in the Sicily Rift Zone and the interior of Sicily.

Crustal structure beneath broad-band seismic stations in the Mediterranean region

Abstract: SUMMARY We have analysed receiver functions to derive simple models for crustal structure below 12 broad-band seismological stations from the MIDSEA project and 5 permanent broad-band stations in the Mediterranean region including northern Africa. To determine an accurate Moho depth we have reduced the trade-off between crustal velocities and discontinuity depth using a new grid search method, which is an extension of recently published methods to determine crustal thickness. In this method the best fitting synthetic receiver function, containing both the direct conversion and the reverberated phases, is identified on a model grid of varying Moho depth and varying Poisson's ratio. The values we found for Moho depth range from around 20 km for intra-oceanic islands and extended continental margins to near 45 km in regions where the Eurasian and African continents have collided. More detailed waveform modelling shows that all receiver functions can be well fit using a 2- or 3-layer model containing a sedimentary layer and/or a mid-crustal discontinuity. On comparing our results with Moho maps inferred from interpolated reflection and refraction data, we find that for some regions the agreement between our receiver function analysis and existing Moho maps is very good, while for other regions our observations deviate from the interpolated map values and extend beyond the geographic bounds of these maps.

Dynamics of retreating slabs: 1. Insights from two-dimensional numerical experiments

Abstract: from the effects of mantle flux (part 2). Therefore, in this paper, we apply forces to the slab using simple analytical functions related to buoyancy and viscous forces in order to isolate the role of rheology on slab dynamics. We analyze parameters for simplified elastic, viscous, and nonlinear viscoelastoplastic single-layer models of slabs and compare them with a stratified thermomechanical viscoelastoplastic slab embedded in a thermal solution. The near-surface behavior of slabs is summarized by assessing the amplitude and wavelength of forebulge uplift for each rheology. In the complete thermomechanical solutions, vastly contrasting styles of slab dynamics and force balance are observed at top and bottom bends. However, we find that slab subduction can be modeled using simplified rheologies characterized by a narrow range of selected benchmark parameters. The best fit linear viscosity ranges between 5 � 10 22 Pa s and 5 � 10 23 Pa s. The closeness of the numerical solution to nature can be characterized by a Deborah number >0.5, indicating that elasticity is an important ingredient in subduction. INDEX TERMS: 8120 Tectonophysics: Dynamics of lithosphere and mantle—general; 8159 Tectonophysics: Rheology—crust and lithosphere; 8160 Tectonophysics: Rheology—general; KEYWORDS: subduction, numerical models, lithospheric rheology

Joint inversion of local, regional and teleseismic data for crustal thickness in the Eurasia–Africa plate boundary region

Abstract: SUMMARY A new map for the Moho discontinuity (EAM02) in the Eurasia‐Africa plate boundary region is presented. Reliable results have also been obtained for the southern and eastern Mediterranean Basin, the northern African coasts and the eastern Atlantic Ocean, regions only occasionally considered in studies on the Mediterranean region. The Moho topography model is derived from two independent sets of constraints. Information contained in the fundamental and highermode Rayleigh waves obtained from waveform modelling is used to constrain the Moho depth between estimates of crustal thickness taken from published reflection and refraction surveys, gravity studies and receiver function analysis. Strong lateral variations in the Moho topography have been observed in the Mediterranean Sea, confirming the complex evolution of this plate boundary region. In the west, the Moho discontinuity has been found at 15‐20 km depth, suggesting extended and, at least in some locations, oceanic crust, while in the east the crust is on average 25‐30 km thick. There it is interpreted either as Mesozoic oceanic or thinned Precambrian continental crust covered by thick sedimentary deposits. Standard continental crust (30‐35 km) is observed along the eastern part of the northern African coast, while to the west a rapid change from a relatively deep Moho (down to 42 km) below the Atlas Mountain Range to the thin crust of the southwestern Mediterranean Sea has been found. The crust beneath the eastern North Atlantic Ocean can be up to 5 km thicker compared with standard oceanic crust (6 km). The crust has been interpreted to be heterogeneous as a consequence of irregular magma supply at the Mid-Atlantic ridge. In addition, serpentinization of the subMoho mantle could contribute to the imaging of apparently anomalous thick oceanic crust. In Europe, the presence of crustal roots (>45 km) beneath the major mountain belts has been confirmed, while thin crust (<25 km) has been found beneath extensional basins. Comparing the obtained Moho topography and Moho depth computed assuming isostatic compensation at 60 km depth shows that most of the Mediterranean and eastern Atlantic region appears to be in isostatic equilibrium. The large positive residuals observed for the eastern Mediterranean are likely to be due to overestimating crustal thickness, owing to the thick sediment deposits present.

74 - The GSHAP Global Seismic Hazard Map

Abstract: This chapter summarizes the development and achievements of the GSHAP and the compilation of the Global Seismic Hazard Map. The Global Seismic Hazard Assessment Program (GSHAP) was designed to provide a useful global seismic hazard framework and to serve as a resource for national and regional agencies, by coordinating national efforts in multi-national regional-scale projects, by reaching a consensus on the scientific methodologies for seismic hazard evaluation and by ensuring that the most advanced methodologies are available worldwide through technology transfer and educational programs. Seismic hazard is defined as a probabilistic measure of earthquake ground shaking at a given location. The assessment of seismic hazard is the first step in the evaluation of the seismic risk, obtained by combining the seismic hazard with local site effects (anomalous amplifications tied to soil conditions, local geology, and topography) and with the vulnerability factors (type, value, and age of buildings and infrastructures, population density, land use, date and time of the day). The GSHAP Global Seismic Hazard Map is the first reference map for seismic hazard on a global scale, expressing the probability of ground shaking in a parameter of engineering interest (PGA), and the first obtained by the close collaboration of the scientists responsible for national seismic hazard zonations.

Spectral Shear-Wave Ground-Motion Scaling in Switzerland

Abstract: Three-component recordings of local and regional earthquakes and explosions are used to assess the spectral characteristics of attenuation, excitation, and duration of ground motion in Switzerland. The data set consists of 292 events in the magnitude range from 2.0 to 5.2, with a total of 2958 waveforms recorded in Switzerland and the German border region from 1984 to 2000. Distance ranges from 5 to 350 km. Stations are located on National Earthquake Hazard Reduction Program 1994 site classes A and B outcrops. Empirical excitation, site, and attenuation terms are derived for the Fourier spectra and peak ground velocities of the ground motion in the frequency range 1-15 Hz by applying an iterative damped least-squares regression. These results are used to calibrate effective theoretical attenuation and excitation models. A grid search through the parameter space is then applied to obtain the quality factor Q ( f ) and a piecewise linear geometrical spreading function G ( r ), allowing complex behavior of attenuation. Optimum results are obtained for Q ( f ) = 270 f 0.50, G ( r ) = r -1.1 for 0-50 km, r -0.6 for 50-70 km, r +0.2 for 70-100 km, and r -0.5 for distances greater than 100 km. The increased amplitudes in the distance range 70-100 km can be explained by the reflection of shear waves at the Moho. This reflected wave energy also leads to an increased duration of ground motion for the same range. The inverted excitation terms are modeled, based on Brune's source spectrum, applying an effective stress parameter of 5-10 bars and a high-frequency roll off (e- πκ fk) κ = 0.015. The small values of κ agree with the mean site condition. A regional site difference between the Alps and the Alpine Foreland site is obtained with a factor of two higher amplitudes in the latter. Manuscript received 25 August 2001.

Identifying 2D Resonance in Microtremor Wave Fields

Abstract: To predict local site effects during earthquakes, we must know the geometrical and mechanical properties of local sedimentary structures and their resonance behavior. We present a concept to identify resonance modes for 2D structures. This is done for synthetic as well as measured microtremor wave fields. The microtremor data were investigated by using the reference station and horizontal-to-vertical (H/V) methods. Analyzing the synthetic microtremor data in profiles across 2D structures, the reference station method (RSM) turned out to be much more suitable for the detection of 2D resonances than the H/V method. We therefore focused on RSM when analyzing the measurement data from two test sites in Switzerland, the St. Jakob–Tullingen trough near Basel in the north and a site in the Rhone valley near the village Vetroz in the south. The results of these two test sites led to different conclusions. Whereas the spectral peaks in data of the St. Jakob–Tullingen trough were not well developed and could only tentatively be interpreted, those from the Rhone valley clearly exhibited 2D resonance effects: the SH 00 and the SV -fundamental mode could be successfully identified at 0.32 ± 0.03 and 0.35 ± 0.03 Hz, respectively.

Joint inversion of broadband teleseismic and interferometric synthetic aperture radar (InSAR) data for the slip history of the Mw = 7.7, Nazca ridge (Peru) earthquake of 12 November 1996

Abstract: [1] The slip distribution of the 12 November 1996, Mw = 7.7, Peru earthquake is determined using broadband teleseismic waveforms, a differential SAR interferogram (interferometric synthetic aperture radar [InSAR]), and a fault parametrization allowing slip and rupture velocity to vary along the rupture plane. Both data sets are inverted jointly to limit the trade-off between the space and time aspects of the rupture. The earthquake fault plane is located at the subduction interface; it strikes parallel to the trench and dips 30° NE. By inverting synthetic data, we show how the InSAR and teleseismic data are complementary and how the joint inversion produces a gain in the spatial and temporal resolution of the slip model, even with a SAR interferogram that covers only part of the coseismic deformation. The rupture of the 1996 Peru event initiated on the southern flank of the subducted Nazca ridge and propagated unilaterally toward the southeast (along strike) for more than 100 km at a depth between 20 and 40 km. The area of maximum slip (6–7 m) is located 50 km southeast of the hypocenter. The total seismic moment is 4.4 × 1020 N m (our joint inversion). The source time function is approximately 60 s long and presents three major pulses of moment release. The dominant one, which occurred between 30 and 45 s, does not correspond to the area of largest slip but to the rupture of a wide zone located about 100 km away from the hypocenter where slip reaches only 2–3 m. Computed coseismic coastal uplift correlates well with the location of raised marine terraces and with the topography of the coastal cordillera, suggesting that these features may be related to the repetition of 1996-type events at the interface between the Nazca ridge and the South American plate.

The earthquake in Unterwalden on September 18, 1601: A historico-critical macroseismic evaluation

Abstract: The present study is part of the new Earthquake Catalogue of Switzerland (ECOS) It combines historical information clearly categorized according to its quality, with the aim of assessing the intensity and magnitude of the 1601 earthquake in Unterwalden On the basis of eyewitness testimonies, subsidized by copies of lost sources, the epicenter can be located in Unterwalden, with an epicentral intensity of VIII and a moment magnitude of 62 Consequently, the 1601 earthquake is the strongest known event in central Switzerland of the post millenium and among the seven strongest events in Switzerland during the last millennium

Shallow Anisotropy in the Mediterranean Mantle From Surface Waves

Abstract: [1] We present new evidence for the existence of the Love-Rayleigh discrepancy in the Mediterranean region and constrain the average polarization anisotropic structure of the Mediterranean mantle. We analysed regional Rayleigh and Love waveforms recorded at 3-component broadband seismic stations. None of the 3-component seismograms could be fit with a single 1D isotropic and smooth velocity model. However, satisfactory fits for individual Rayleigh and Love waveforms could often be obtained using realistic 1D velocity models. We used these 1D path-average velocity structures to derive 3D S-velocity models for the Mediterranean region. Our results show that Love waves require higher velocities (about 200 m/s) compared to Rayleigh waves between 30 and 120 km depth. We relate the observed anisotropy to lattice-preferred orientation of crystallographic axes of elastically anisotropic minerals such as olivine.