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Bulletin of the Seismological Society of America 2011 Feb; 101(1)
The effectiveness of a distant accelerometer array to compute seismic source parameters: the April 2009 L'Aquila earthquake case history
he Mw 6.3 L'Aquila earthquake, central Italy, on April 6, 2009 has been recorded by the Irpinia Seismic Network (ISNet) about 250 km south-east of the epicenter. Up to 19 three-component accelerometer stations could be used to infer the main source p... expand abstractarameters with different seismological methods. We obtained an approximate location of the event from arrival times and array-based backazimuth measurements and estimated the local magnitude (6.1) from an attenuation relation for southern Italy. Assuming an omega-square spectral model we inverted S-wave displacement spectra for moment magnitude (6.3), corner frequency (0.33 Hz), stress drop (2.5 MPa) and apparent stress (1.6 MPa). Waveform modeling using a point source and an extended source model provided consistent moment tensors with a centroid depth around 6 km and a prevalently normal fault plane solution with a dominant directivity toward south-east. The relatively high corner frequency and an overestimated moment magnitude of 6.4 from moment tensor inversions are attributed to the rupture directivity effect. To image the rupture geometry we implemented a beamforming technique that back-projects the recorded direct P-wave amplitudes into the earthquake source region. A NW-SE striking rupture of 17 km length is imaged, propagating with an average velocity up to 3 km/s. This value is significantly higher than our estimate of 2.2 km/s from S-wave spectra. Our case study demonstrates that the use of array techniques and a dense accelerometer network can provide quick and robust estimates of source parameters of moderate-size earthquakes located outside the network. collapse abstract
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Geophysical Journal International 2009 Nov; 179(3)
Estimation of elastic contrasts in a layered model from seismic PStoPP amplitude ratios
We present a method to estimate seismic velocity and density contrasts at a given interface in a 1-D layered model using PS-to-PP reflection amplitude ratios. The velocity structure above the reflector is constrained by traveltime modelling, and the ... expand abstractamplitude ratios are determined using the same source–receiver pair for the measured PP and PS amplitudes (common-offset geometry). Thereby, source and receiver site effects are cancelled, and the remaining propagation effects are included in the ray theoretical forward modelling of theoretical PS-to-PP ratios. A minimization of the least-squares misfit between observed and modelled ratios provides the remaining elastic parameters below the reflector of interest (P velocity, P-to-S velocity ratio, density). 1-D examples and a 2-D synthetic case study with a dipping reflector and a laterally varying overburden demonstrate the possibilities and limitations of the method. An application of the method to a 0.6 km deep reflector below the Campi Flegrei caldera, Italy, reveals a strong contrast with a P-velocity increase from less than 2 to 3.5 km s−1 and a decrease of the P-to-S velocity ratio from 3.6 to 1.75. The proposed PS-to-PP amplitude ratio analysis is applicable for wide-angle seismic reflection data, especially when strong elastic parameter contrasts are expected and when source amplitudes or site effects are poorly known. collapse abstract
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Natural Hazards 2009 Feb; 52(1)
Rapid estimation of ground-shaking maps for seismic emergency management in the Campania Region of southern Italy
Strong ground-shaking mapping soon after a moderate-to-large earthquake is crucial to recognize the areas that have suffered the largest damage and losses. These maps have a fundamental role for emergency services, loss estimation and planning of eme... expand abstractrgency actions by the Civil Protection Authorities. This is particularly important for areas with high seismic risk levels, such as the Campania-Lucania Region in southern Italy. Taking advantage of the Irpinia Seismic Network (ISNet), a recently installed dense and wide dynamic seismic network, we have developed a procedure for rapid estimation of ground-shaking maps after moderate-to-large earthquakes (GRSmap). This uses an optimal data gridding scheme designed to account for bi-dimensional features of strong ground-motion fields, such as directivity, radiation patterns and focal mechanisms, to which most damage can be correlated. The basis of the mapping technique is a triangulation procedure to locally correct predicted data at the triangle barycentres where their vertices correspond to seismic stations. The method has been tested off-line using a simulated M 6.6 earthquake located at the centre of ISNet and applied to data of the 23 November 1980 Irpina M 6.9 earthquake recorded by a sparse network. This has highlighted its ability to predict peak ground-motion parameters of large magnitude earthquakes with respect to the attenuation relationships. Keywords Ground-shaking maps - Triangulation scheme - Seismic source - Seismic network - Seismic hazard collapse abstract
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Geophysical Prospecting 2008 Jun; 56(4)
Iterative tomographic analysis based on automatic refined picking
The ever-growing size of data sets for active and passive seismic imaging makes the availability of automatic procedures for rapid analysis more and more valuable. Such procedures are especially important for time-critical applications like emergency... expand abstract decisions or re-orienting of ongoing seismic surveys. In this paper a new, iterative scheme for 3D traveltime tomography is presented. The technique, based on a tool originally developed for earthquake data, uses cross-correlation to examine waveform similarity and to adjust arrival times on seismic sections. A preliminary set of reference arrival times is first corrected by the cross-correlation lag and then used to build an initial 3D tomographic velocity model through a standard inversion code; traveltimes calculated from this model are then taken as new reference arrivals and the process of pick adjustment is repeated. The result is a tomographic image, upgraded and refined at each iteration of the procedure. The test performed on the waveform data set recorded during the 2001 SERAPIS active seismic survey in the gulfs of Naples and Pozzuoli (Southern Italy) shows that the 3D iterative tomography scheme produces a velocity image of the structure of the Campi Flegrei caldera which is consistent with the results from previous studies, employing just a fraction of the time needed by a human analyst to identify first breaks. We believe that this technique can be effectively employed for rapid analysis of large data-sets within time-critical or time-dependent tasks and for automatic 4D tomographic investigations. collapse abstract
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Geophysical Prospecting 2008 Jun; 56(4)
Depth and morphology of reflectors from the nonlinear inversion of arrivaltime and waveform semblance data. Part I: method and applications to synthetic data
We propose a two-dimensional, non-linear method for the inversion of reflected/converted traveltimes and waveform semblance designed to obtain the location and morphology of seismic reflectors in a lateral heterogeneous medium and in any source-to-re... expand abstractceiver acquisition lay-out. This method uses a scheme of non-linear optimization for the determination of the interface parameters where the calculation of the traveltimes is carried out using a finite-difference solver of the Eikonal equation, assuming an a priori known background velocity model. For the search for the optimal interface model, we used a multiscale approach and the genetic algorithm global optimization technique. During the initial stages of inversion, we used the arrival times of the reflection phase to retrieve the interface model that is defined by a small number of parameters. In the successive steps, the inversion is based on the optimization of the semblance value determined along the calculated traveltime curves. Errors in the final model parameters and the criteria for the choice of the best-fit model are also estimated from the shape of the semblance function in the model parameter space. The method is tested and validated on a synthetic dataset that simulates the acquisition of reflection data in a complex volcanic structure. This study shows that the proposed inversion approach is a valid tool for geophysical investigations in complex geological environments, in order to obtain the morphology and positions of embedded discontinuities. collapse abstract
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Geophysical Prospecting 2008 Jun; 56(4)
Depth and morphology of reflectors from the nonlinear inversion of arrival times and waveform semblance data. Part II: modelling and interpretation of real data acquired in Southern Apennines, Italy
In order to retrieve a 2D background velocity model and to retrieve the geometry and depth of shallow crustal reflectors in the Southern Apennines thrust belt a separate inversion of first arrival traveltimes and reflected waveforms was performed. Da... expand abstractta were collected during an active seismic experiment in 1999 by Enterprise Oil Italiana and Eni-Agip using a global offset acquisition geometry. A total of 284 on-land shots were recorded by 201 receivers deployed on an 18 km line oriented SW–NE in the Val D'Agri region (Southern Apennines, Italy). collapse abstract
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Geophysical Prospecting 2008 Jun; 56(4)
Merging active and passive data sets in traveltime tomography: The case study of Campi Flegrei caldera (Southern Italy)
We propose a strategy for merging both active and passive data sets in linearized tomographic inversion. We illustrate this in the reconstruction of 3D images of a complex volcanic structure, the Campi Flegrei caldera, located in the vicinity of the ... expand abstractcity of Naples, southern Italy. The caldera is occasionally the site of significant unrests characterized by large ground uplifts and seismicity. The P and S velocity models of the caldera structure are obtained by a tomographic inversion based on travel times recorded during two distinct experiments. The first data set is composed of 606 earthquakes recorded in 1984 and the second set is composed of recordings for 1528 shots produced during the SERAPIS experiment in 2001. The tomographic inversion is performed using an improved method based on an accurate finite-difference traveltime computation and a simultaneous inversion of both velocity models and earthquake locations. In order to determine the adequate inversion parameters and relative data weighting factors, we perform massive synthetic simulations allowing one to merge the two types of data optimally. The proper merging provides high resolution velocity models, which allow one to reliably retrieve velocity anomalies over a large part of the tomography area. The obtained images confirm the presence of a high P velocity ring in the southern part of the bay of Pozzuoli and extends its trace inland as compared to previous results. This annular anomaly represents the buried trace of the rim of the Campi Flegrei caldera. Its shape at 1.5 km depth is in good agreement with the location of hydrothermalized lava inferred by gravimetric data modelling. The Vp/Vs model confirms the presence of two characteristic features. At about 1 km depth a very high Vp/Vs anomaly is observed below the town of Pozzuoli and is interpreted as due to the presence of rocks that contain fluids in the liquid phase. A low Vp/Vs body extending at about 3–4 km depth below a large part of the caldera is interpreted as the top of formations that are enriched in gas under supercritical conditions. collapse abstract
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Bulletin of Volcanology 2008 Jun; 71(3)
Seismic images and rock properties of the very shallow structure of Campi Flegrei caldera (southern Italy)
In September 2001, an extensive active-seismic investigation (Serapis experiment) was carried out in the Gulfs of Naples and Pozzuoli, with the aim of investigating and reconstructing the shallow crustal structure of the Campi Flegrei caldera, and po... expand abstractssibly identifying its feeding system at depth. The present study provides a joint analysis of the very shallow seismic reflection data and tomographic images based on the Serapis dataset. This is achieved by reflection seismic sections obtained by the 3D data gathering and through refined P-velocity images of the shallowest layer of Pozzuoli Gulf (z < 1,000 m). From the refined Vp model, the overall picture of the velocity distribution confirms the presence of a complex arc-shaped anomaly that borders the bay offshore. The deeper part of the anomaly (beneath 700 m, with Vp > 3,500 m/s) correlates with units made up of agglomerate tuffs and interbedded lava, which form the southern edge of the caldera, which was probably formed following the two large ignimbritic eruptions that marked the evolutionary history of the area under study. The upper part of the anomaly that tends to split into two parallel arcs is correlated with dikes, volcanic mounds and hydrothermal alteration zones noted in previous shallow reflection seismic analyses. The depth of the transition between the upper and lower parts of the anomaly is characterized by an abrupt Vp increase on the one-dimensional (1D) profiles extracted from the 3D tomographic model and by the presence of a strong reflector located at about 0.6/0.7 s Two Way Time (TWT) on Common Mid Point gathers. The move-out velocity analysis and stack of the P–P and P–S reflections at the layer bottom allowed to estimate relatively high Vp/Vs values (3.7 ± 0.9). This hypothesis has been tested by a theoretical rock physical modeling of the Vp/Vs ratio as a function of porosity suggesting that the shallow layer is likely formed by incoherent, water saturated, volcanic and marine sediments that filled Pozzuoli Bay during the post-caldera activity. Keywords Campi Flegrei caldera - Shallow structure - Seismic reflection Editorial responsibility: M. Ripepe. collapse abstract
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Geophysical Journal International 2006 Jul; 166(2)
Fault slip and rupture velocity inversion by isochrone backprojection
A new technique is proposed here for the retrieval of slip images from the backprojection of high-frequency displacement records. When direct S waves are seen to be dominant in the near-source data, Green functions can be approximated by the far-fiel... expand abstractd terms, as described by ray theory. Assuming that the slip rapidly reaches the final value (i.e. short slip duration), the measured displacement can be ascribed to the slip contributions lying on the corresponding isochrone on the fault plane. Here we use the far-field representation theorem to backproject on the fault plane the displacement amplitudes measured along the seismogram. Through the weighted stack of amplitude maps obtained from different stations we recover high slip zones on the fault. collapse abstract
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Bulletin of Volcanology 2005 Jul; 47(2)
Space-time distribution of small earthquakes at Phlegraean Fields, south-central Italy
A preliminary statistical analysis of the space-time distribution of small seismic events in the volcanic area of Phlegraean Fields, south-central Italy, was done on the basis of a catalogue of earthquakes recorded by the local seismic stations in th... expand abstracte period January 1, December 31, 1983.The non-random character of the sequence has been tested by matching the observed time-dstribution of seismic events with the theoretical Poisson process. collapse abstract
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Bulletin of Volcanology 2005 Jul; 47(2)
Error analysis in hypocentral locations at Phlegraean Fields
The location reliability of the earthquakes occurred at Phlegraean Fields has been analyzed, and the theoretical errors, inferred from the diagonal elements of the covariance matrix, have been estimated. Using only first P-phase arrivals to the local... expand abstract network (22 stations) and assuming a reading error of 0.05 sec., the average error on the spatial coordinates is estimated to be of the order of 0.2 km.Shallow events (depth collapse abstract
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Bulletin of Volcanology 2005 Jul; 47(2)
Seismic coda Q and turbidity coefficient at the Phlegraean Fields volcanic area: Preliminary results
Digital recordings of microearthquake codas from shallow seismic events in the Phlegraean Fields region (south-central Italy) were used to calculate the attenuation factor Qc.A quite unusual frequency dependence was found for the coda attenuation com... expand abstractparable to Hawaii pattern of Qc. This is interpreted as due to the presence of magma that increases the amount of anelasticity. Amount of scattering at Phlegraean Fields was estimated through the « turbidity » coefficient (Dainty model), that shows a high degree of scattering due to inhomogeneities as compared to Hawaii. Probably this is due to the greater crustal thickness of Phlegraean Fields with respect to Hawaii that produces more scattering. collapse abstract
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Pure and Applied Geophysics 2004 Dec; 126(1)
Scaling of peak ground motions from digital recordings of small earthquakes at Campi Flegrei, southern Italy
The dependence of peak ground acceleration and velocity on seismic moment is studied for a set of small earthquakes (0.7ML-square spectral model with constant stress drop and ane–kf high frequency decay, fit well both the velocity and accelera... expand abstracttion data for an averagek=0.015. The observed ground motions in the 1–24 Hz frequency band appear to consist of radiation from simple sources modified only slightly by attenuation effects. Moreover, the scaling of peak values agrees closely with those determined in nonvolcanic areas, once the difference in stress drop is taken into account.Key Words scaling of peak ground motions - volcanic microearthquakes - source parameters - high frequency attenuation collapse abstract
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Pure and Applied Geophysics 2004 Dec; 125(6)
Seismic sources and attenuation properties at the Campi Flegrei volcanic area
Microearthquake digital data collected at Campi Flegrei during the recent (1982–1985) ground uplift episode have been analyzed in order to infer source and medium seismic properties. The main results obtained from these analyses are:1. Hy... expand abstractpocenter distribution and the size of the seismic zone do not change with time and do not depend on the ground uplift rate. Events occurred clustered in time with no simple causal relations between the cluster occurrences and their energy.2. Anelastic attenuation does not depend strongly on frequency, showing a constant pattern at high frequencies. The observed values of low and high frequency attenuation, due to the short source receiver distances, do not seriously affect the spectral content of signals radiated by the sources.3. A constant Brune stress drop pattern (4–5 bars) as a function of seismic moment is observed. This indicates that the manner of fracturing is almost independent on magnitude of earthquakes (hypothesis of self-similarity (Aki, 1967)). Seismic processes in a prefractured medium can explain the observed small stress drop values.4. Focal mechanisms from moment tensor estimates show that radiation patterns are mostly well interpreted in terms of double couple source models.5. The scaling of peak ground motion parameters (Amax andVmax vs seismic moment) can be explained by an 2 source model (constant stress drop) multiplied by an exponential function with a small decay parameter, which takes into account the measured attenuation.These results support the hypothesis of earthquakes generated by simple shear fractures along prefractured structures as a response to changes in the stress field due to the ground deformation. collapse abstract
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Pure and Applied Geophysics 2004 Dec; 123(5)
Statistical analysis of earthquake activity at Etna volcano (March 1981 eruption)
Seismic activity that preceded, accompanied, and followed the 17–23 March 1981 Etnean eruption has been statistically analyzed.On the grounds of both time evolution of seismicity and catalogue completeness, three time intervals have been defin... expand abstracted (12 February–2 March, 12–17 March, 19–31 March) and for each of these periods both the b coefficient of theGutenberg-Richter''s (1956) relationship and the E parameter of the cluster size (Shlien andToksoz, 1970) have been calculated. collapse abstract
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Journal of Seismology 2004 Nov; 1(4)
Modelling of ground acceleration in the near source range: the case of 1976, Friuli earthquake (M 6.5), northern Italy
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 ... expand abstractand 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.kinematic source model - strong ground motion - directivity effect - source geometry effect collapse abstract
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Journal of Seismology 2004 Nov; 3(3)
High frequency strong ground motion modelling in the Catania area associated with the Ibleo-Maltese fault system
A hybrid statistical-deterministic approach has been applied to estimate strong ground motion parameters (PGA, spectral ordinates) in South-Eastern Sicily for a M = 7 earthquake. A number of 100 different rupture processes have been simulated along a... expand abstract composite fault system representing two segments of the Ibleo-Maltese fault scarp. Map at regional scale of mean PGA in the 0.5–20 Hz frequency band shows highest values (0.4–0.5 g) nearby and North of Catania, due to a dominant directivity effect. The COV parameter, which expresses the variability of PGA values as a function of source complexity, is higher in the region nearby and South of the town of Augusta, where, depending on the rupture history, rather large PGA values can be observed (>0.4 g). PGA attenuation curves suggest that an azimuthal variation could be related to the source extent and directivity. The response and pseudo acceleration spectra are computed for different sites in the town of Catania including an approximate 1D site response. Ground motion amplification effects at high frequency (5–20 Hz) are produced by thin shallow layer of soft clay, loose pyroclastites and fill. We observe small amplification effects, in the frequency ranges 2–3 Hz and 5–10 Hz, in sites where recent alluvia reach a thickness of some tens of meters. Otherwise, sites located on outcrops of massive lavas show moderate attenuation.complex source model - site effect - strong motion collapse abstract
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Journal of Seismology 2004 Nov; 4(4)
Rupture mechanism and source parameters of Umbria-Marche mainshocks from strong motion data
A long sequence of earthquakes causing few casualties and considerable damage in a wide zone struck Central Italy starting on September 26, 1997. Theearthquakes are characterized by normal faulting mechanism, with a NE-SW(anti-Apenninic direction) te... expand abstractnsion axis. In this paper we analyze the accelerometric recordings collected by the accelerograph stations belonging to the National Accelerograph Network. About 10 stations were triggered by the mainshocks of the sequence. In particular, a small size foreshock and the two mainshocks that occurred on September,26 (00:33(GMT) MW = 5.7 and 09:40 MW = 6.0) have been recorded by two digital 3-C accelerometers located at near source distances (within 30 km from the faults). These records are relevant to investigate the detail of therupture kinematics, due to the close epicentral distance and azimuthallocation relative to the fault orientation and geometry. Using a trial and error approach we modeled the source mechanism through the fit of the arrival times, the apparent source time duration, the main polarization features and the entire waveforms of the recorded signals, in order to get some insight on the rupture evolution, the location of the fracture origin point and the fault geometry. Based on this fault kinematic model, inferences on fault slip distribution are obtained by modeling the S acceleration waveform, comparing the ray theory synthetics with 1–5 Hz band filtered ground velocity records.The final model shows that the seismic ruptures occurred along two adjacent,sub-parallel, low angle dipping normal faults. Ruptures bothnucleated from the fault bottom and propagated up-dip, showing differentrupture velocity and length. The presence of a transfer zone (barrier)can be suggested by the mainshocks rupture evolution. This transfer zonehas probably controlled the amplitude increase of local stressreleased by the first rupture at its NW edge which triggered about 9 hourslater the second rupture. The inferred model was used to compute the predictedground acceleration in the near source range, using a hybridstatistical-deterministic approach.A similar trial and error method has been also applied to the October 14, 199715:23 earthquake (MW = 5.6). The inferred kinematic model indicates a rupture nucleating from the faultbottom and propagating up-dip, toward the SE direction. Thus the three mainshocks ruptured distinct fault segments, adjacent and slightly offsetfrom one to another.directivity - isochrone - rupture kinematics - strong motion data collapse abstract
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Journal of Seismology 2004 Oct; 7(1)
Modeling of Rayleigh waves dispersion in the Sannio region (Southern Italy) from seismic active refraction data
Short period surface waves, recorded during a seismic refractionsurvey in the Sannio region (Southern Italy), have been modeled to infera shallow velocity model for the area. Based on the decrease of resolutionwith depth, due to the bias on group vel... expand abstractocity estimates arising frominterference of the Rayleigh waves with higher modes, we carried out aprocedure of fitting, with synthetic seismograms, of selected filtered traceswith a gaussian filter, having a width at half height equal to 1 Hz and acentral frequency lying in the range [1,4] Hz. We estimated the likelihoodbetween synthetic and observed seismograms by measuring their semblance.In this way we were able to infer a more refined local velocity modelcharacterized by a high Vp and Vs vertical gradient in the sedimentarycover. Two ad hoc resolution studies, based on group velocity andamplitude data respectively, indicate that the local velocity model is a goodvelocity model also for the entire studied area. The increase in the numberof available data when using amplitude information allows us to make amore selective choice in the model parameter space (Vp and Vs of eachlayer) and to solve for the Vp/Vs ratio. The inferred Vp velocity in thehalf-space is equal to 2.8 km/s. This value is in excellent agreement withthat inferred by other authors (3 km/s) by modeling P-wave travel timevs. distance. The best-fit model furnish low Vp/Vs for the sedimentarycover so indicating a high degree of the sediment's compaction in thestudied area. The inferred shallow high-velocity gradient indicates thatthe shallow sedimentary layer in the area could trap and focus the energytraveling into it.complete wave field - dispersion - group velocity - Rayleigh waves - Sannio - semblance - synthetic seismograms collapse abstract
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Geophysical Journal International 2003 Oct; 155(2)
Size and geometry of microearthquake seismic ruptures from P and S pulse width data
We propose a method to estimate the radius, dip and strike of a circular seismic rupture through the inversion of first P- and S-wave pulse widths measured on microearthquake records.
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Geophysical Journal International 2003 Jan; 152(2)
Locating and quantifying the seismic discontinuities in a complex medium through the migration and AVA analysis of reflected and converted waves: an application to the Mt Vesuvius volcano
In this paper, we show how the migration of active seismic data can be used to identify and position a seismic discontinuity in a complex medium, and how the amplitude variations of the converted P to S waves can be interpreted to constrain the seism... expand abstractic velocities below the interface. For the application we turn our attention on Mt Vesuvius, an active volcano threatening a densely populated area. To better define its plumbing system, we investigate a mid-crust seismic discontinuity first identified by Zollo et al. (1996, Science,274, 592–594) and assumed to be the top of a layer containing low-velocity material. We deduce a reference velocity model from previous works on first arrival times, and use it to migrate PP reflected and PS converted waves. In the migration image, the interface extends at least 20 000 m NE and 20 000 m SW of the volcano, at the depth of 8000 m, and with a mean dip less than 3 per cent. The migration of finite-difference synthetics exclude the fact that the migrated phases interpreted as 8000 m deep reflections or conversions might be multiples in the shallow layers. collapse abstract
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Geophysical Journal International 2002 Oct; 151(2)
Bayesian estimation of 2D Pvelocity models from active seismic arrival time data: imaging of the shallow structure of Mt Vesuvius (Southern Italy)
A multi-2-D non linear traveltime tomography of the shallow (3–4 km deep) structure of Mt Vesuvius volcano was performed. Data have been collected during two recent active seismic experiments using a total of 17 on-land shots and about 140 thre... expand abstracte-component digital seismographs. A newly developed technique for imaging the volcano velocity structure has been applied, based on an adaptive model space investigation where the number of grid nodes is progressively increased (multi-scale approach). The optimal model parametrization is chosen according to the minimum of the Akaike Information Criteria (AIC) parameter. This corresponds to finding the best compromise between the data misfit and simplicity of the model. The model parameter estimate is performed through the computation of an a posteriori probability density function (pdf), defined following the Bayesian approach. The maximum likelihood model is searched by an optimization technique which combines the genetic and simplex algorithms. The evaluation of the a posteriori pdf is based on traveltime computations using ray tracing techniques. Constraints on the model parameters are inserted in the form of prior pdf and error maps are inferred from cross-sections of the posterior probability around the found best fit solution. The retrieved images of Mt Vesuvius volcano show variable P-velocities in the range 1700–5800 m s−1. A fairly detailed image of the top of the Mesozoic carbonate rocks forming the basement of the volcanic area is obtained. A 9 km long, 1 km deep depression was detected at the N side of the volcano. The presence of a shallow high velocity body is evidenced underneath the Mt Somma caldera and it can be interpreted as a sub- or palaeovolcanic structure. collapse abstract
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Geophysical Journal International 2002 Sep; 151(1)
Seismic imaging of complex structures by nonlinear traveltime inversion of dense wideangle data: application to a thrust belt
We have used a dense wide-angle data set to test a two-step procedure for the separate inversion of first-arrival and reflection traveltimes. Data were collected in a complex thrust belt environment (southern Italy) along a 14-km line, with closely s... expand abstractpaced sources (60 m) and receivers (90 m). We have applied a fully non-linear tomographic technique, specially designed to image complex structures, to over 6400 first-arrival traveltimes in order to determine a detailed velocity model. A bi-cubic spline velocity model parametrization is used. The inversion strategy follows a multiscale approach, and employs a non-linear velocity optimization scheme. The tomographic velocity model is adopted as the background reference medium for a subsequent interface inversion aimed at imaging a target upper-crust reflector. The interface inversion method is also based on a multiscale approach and uses a non-linear technique for model parameters (interface position nodes) optimization. We have applied the interface inversion method to over 1600 reflection traveltimes of a target event picked both in the near- and in the wide-angle offset range. The retrieved interface is well resolved in the central part of the model, where ray coverage mainly includes clear post-critical reflections and the background velocity model is accurate in depth thanks to large offset deep turning rays. The velocity and interface models thus determined are consistent with Vertical Seismic Profiling data and correlate well with the geometry of known geological structures. This study shows that the used inversion approach is efficient for target-orientated investigations in complex geological environments. collapse abstract
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Science (New York, N.Y.) 2001 Nov; 294(5546)
Seismic evidence of an extended magmatic sill under Mt. Vesuvius.
Mt. Vesuvius is a small volcano associated with an elevated risk. Seismic data were used to better define its magmatic system. We found evidence of an extended (at least 400 square kilometers) low-velocity layer at about 8-kilometer depth. The inferr... expand abstracted S-wave (approximately 0.6 to 1.0 kilometer per second) and P-wave velocities (approximately 2.0 kilometer per second) as well as other evidence indicate an extended sill with magma interspersed in a solid matrix. collapse abstract
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Geophysical Journal International 2001 Jul; 146(2)
Precise, absolute earthquake location under Somma–Vesuvius volcano using a new threedimensional velocity model
The Somma–Vesuvius volcanic complex and surroundings are characterized by topographic relief of over 1000 m and strong 3-D structural variations. This complexity has to be taken into account when monitoring the background volcano... expand abstract seismicity in order to obtain reliable estimates of the absolute epicentres, depths and focal mechanisms for events beneath the volcano. We have developed a 3-D P-wave velocity model for Vesuvius by interpolation of 2-D velocity sections obtained from non-linear tomographic inversion of the Tomoves 1994 and 1996 active seismic experiment data. The comparison of predicted and observed 3-D traveltime data from active and passive seismic data validate the 3-D interpolated model. We have relocated about 400 natural seismic events from 1989 to 1998 under Vesuvius using the new interpolated 3-D model with two different VP/VS ratios and a global search, 3-D location method. The solution quality, station residuals and hypocentre distribution for these 3-D locations have been compared with those for a representative layered model. collapse abstract
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