Earth-printshttps://www.earth-prints.orgThe DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.Sun, 21 Jul 2024 12:45:45 GMT2024-07-21T12:45:45Z5061- A new approach to construct 3-D crustal shear-wave velocity models: method description and application to the Central Alpshttp://hdl.handle.net/2122/16192Title: A new approach to construct 3-D crustal shear-wave velocity models: method description and application to the Central Alps
Authors: Colavitti, Leonardo; Hetényi, Gyorgy; AlpArray Working Group
Abstract: We develop a new inversion approach to construct a 3-D structural and shear-wave velocity model of the crust based on teleseismic P-to-S converted waves. The proposed approach does not require local earthquakes such as body wave tomography, nor a large aperture seismic network such as ambient noise tomography, but a three-component station network with spacing similar to the expected crustal thickness. The main features of the new method are: (1) a novel model parametrization with 3-D mesh nodes that are fixed in the horizontal directions but can flexibly vary vertically; (2) the implementation of both sharp velocity changes across discontinuities and smooth gradients; (3) an accurate ray propagator that respects Snell’s law in 3-D at any interface geometry. Model parameters are inverted using a stochastic method composed of simulated annealing followed by a pattern search algorithm. The first application is carried out over the Central Alps, where long-standing permanent and the temporary AlpArray Seismic Network stations provide an ideal coverage. For this study we invert 4 independent parameters, which are the Moho discontinuity depth, the Conrad discontinuity depth, the P-velocity change at the Conrad and the average Vp/Vs of the crust. The 3-D inversion results clearly image the roots of the Alpine orogen, including the Ivrea Geophysical Body. The lower crust's thickness appears fairly constant. Average crustal Vp/Vs ratios are relatively higher beneath the orogen, and a low-Vp/Vs area in the northern foreland seems to correlate with lower crustal earthquakes, which can be related to mechanical differences in rock properties, probably inherited. Our results are in agreement with those found by 3-D ambient noise tomography, though our method inherently performs better at localizing discontinuities. Future developments of this technique can incorporate joint inversions, as well as more efficient parameter space exploration.
Tue, 08 Nov 2022 00:00:00 GMThttp://hdl.handle.net/2122/161922022-11-08T00:00:00Z
- Physics-based parametrization of a FAS nonergodic ground motion model for Central Italyhttp://hdl.handle.net/2122/16905Title: Physics-based parametrization of a FAS nonergodic ground motion model for Central Italy
Authors: Sgobba, Sara; Lanzano, Giovanni; Colavitti, Leonardo; Morasca, Paola; D'Amico, Maria Clara; Spallarossa, Daniele
Abstract: We propose a new fully nonergodic ground motion model for Central Italy, which is one of the most sampled areas in the world after the occurrence of the last seismic sequences of 2009 and 2016–2017. The model predicts 69 ordinates of the Fourier Amplitude Spectrum in the magnitude range 3.2–6.5 and is constrained on a dense set of seismological and geophysical parameters (i.e. stress-drop
, shear-wave velocity in the uppermost 30 m, VS,30 and high-frequency attenuation parameter at source
and site
) made available from a non-parametric generalized inversion technique (GIT). The aim of this work is to capture the underlying physics of ground motion related to different source energy levels, as well as to the crustal and geological structure of the region, thus providing less uncertain predictions. Calibration is performed using a stepwise regression approach which has the advantage of taking a more complex functional form (advanced model) when all physical parameters are known while returning a simpler form (base model) when physical data are missing. As a result, the advanced model reproduces the reference rock motion of the region in case the site additional proxies are set to their average values (VS,30 = 1100 m/s,
=15 ms). We show that the inclusion of the set of physically-based explanatory variables in the regression has a beneficial effect in constraining the uncertainty, leading to a reduction of the high-frequency variability of about 70% on the between-event and 35% on the site-to-site. This reduction can be viewed as the result of the combination of a more effective physical description through the incorporation of the additional proxies and a calibration embedded in a completely nonergodic framework.
Sun, 01 Jan 2023 00:00:00 GMThttp://hdl.handle.net/2122/169052023-01-01T00:00:00Z
- Empirical correlations between a FAS non-ergodic ground motion model and a GIT derived model for Central Italyhttp://hdl.handle.net/2122/15808Title: Empirical correlations between a FAS non-ergodic ground motion model and a GIT derived model for Central Italy
Authors: Morasca, Paola; D'Amico, Maria; Sgobba, Sara; Lanzano, Giovanni; Colavitti, Leonardo; Pacor, Francesca; Spallarossa, Daniele
Abstract: In this study we investigate the correlation between the residuals of a neGMM (non-ergodic Ground Motion Model) and the physics-based parameters obtained using a non-parametric GIT approach (Generalized Inversion Technique) to lay the groundwork for the implementation of an ad-hoc FAS (Fourier Amplitude Spectra) neGMM for the Central Italy region.
This region is particularly suitable for data-driven methodologies as those applied in this work because of the large amount of available data due to the recent multiple mainshock-aftershock sequences occurred in this area.
Both neGMM and GIT models are developed for Fourier spectra in the frequency range between 0.5 and 25 Hz and using the same reference sites. The comparison of the non-ergodic terms with the source, path and site spectral parameters provides interesting results. First, we find a strong correlation between the source parameters, stress drop Δσ and decay ksource, and the source neGMM corrective terms (the combination of the between-event δBe and the location-to-location terms δL2L). This correlation is frequency dependent and, at high frequency, is remarkably positive for Δσ and negative for ksource. Concerning the attenuation terms, the path-to-path residuals (δP2P) are clearly associated with the deviations from the regional Q estimates obtained from GIT analysis. This indicates that the neGMM properly captures the properties of the anelastic attenuation and that the corrective terms δP2P can be used to account for differences in travel paths across different crustal domains.
Finally, adopting the same reference sites for neGMM and non-parametric GIT, we observe that the systematic site terms (δS2Ss) and the GIT-derived amplification functions are in good agreement. The next step for an appropriate modeling is to identify the physical parameters (for example VS,30 and k0) describing the empirical amplification curves to be introduced as explanatory variables in the ground motion model.
Description: This article has been accepted for publication in Geophysical Journal International ©:The Author(s) 2022. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved. Uploaded in accordance with the publisher's self-archiving policy.
Sun, 01 Jan 2023 00:00:00 GMThttp://hdl.handle.net/2122/158082023-01-01T00:00:00Z
- Empirical Evidence of Frequency‐Dependent Directivity Effects From Small‐To‐Moderate Normal Fault Earthquakes in Central Italyhttp://hdl.handle.net/2122/15845Title: Empirical Evidence of Frequency‐Dependent Directivity Effects From Small‐To‐Moderate Normal Fault Earthquakes in Central Italy
Authors: Colavitti, Leonardo; Lanzano, Giovanni; Sgobba, Sara; Pacor, Francesca; Gallovič, František
Abstract: Rupture directivity and its potential frequency dependence is an open issue within the seismological community, especially for small‐to‐moderate events. Here, we provide a statistical overview based on empirical evidence of seismological observations, thanks to the large amount of high‐quality seismic recordings (more than 30,000 waveforms) from Central Italy, which represents an excellent and almost unique natural laboratory of normal faulting earthquakes in the magnitude range between 3.4 and 6.5 within the time frame 2008–2018. In order to detect an anisotropic distribution of ground motion amplitudes due to the rupture directivity, we fit the smoothed Fourier Amplitude Spectra (FAS) cleared of source‐, site‐ and path‐effects. According to our criteria, about 36% of the analyzed events (162 out of 456) are directive and the distribution of rupture direction is aligned with the strikes of the major faults of the Central Apennines. We find that the directivity is a band‐limited phenomenon whose width may extend up to five times the corner frequency. The results of this research provide useful insights to parameterize directivity, to be explicitly implemented in future ground motion modeling and scenario predictions.
Sat, 01 Jan 2022 00:00:00 GMThttp://hdl.handle.net/2122/158452022-01-01T00:00:00Z
- Temporal Variation of the Spectral Decay Parameter Kappa Detected before and after the 2016 Main Earthquakes of Central Italyhttp://hdl.handle.net/2122/15844Title: Temporal Variation of the Spectral Decay Parameter Kappa Detected before and after the 2016 Main Earthquakes of Central Italy
Authors: Castro, Raúl R; Spallarossa, Daniele; Pacor, Francesca; Colavitti, Leonardo; Lanzano, Giovanni; Vidales-Basurto, Claudia A; Sgobba, Sara
Abstract: We investigated the temporal variation of the spectral decay parameter κ before and after two main earthquakes that occurred in the central Italy region, namely the Amatrice (Mw 6.0) of 24 August 2016 and the Norcia (Mw 6.5) of 30 October 2016 earthquakes. For this analysis, we used seismograms from the central Italy dense seismic array stations, and earthquakes located at hypocenter distances r < 80 km, having magnitudes Mw 3.4–6.5. The dataset consists of 393 events recorded at 92 stations. We estimated, for both earthquake sequences, average functions κ˜(r) that describe the distance dependence of κ along the S-wave source-station paths using acceleration spectra from foreshocks, mainshock, and aftershocks. We observed that there was a regional attenuation drop within approximately two months after the Amatrice earthquake. Then, κ˜(r) tends to return toward the attenuation values observed before the occurrence of the main event, namely to the values of κ˜(r) obtained from the foreshocks, when the earthquake cycle is probably completed. We also estimated the near-source kappa (κs) using aftershocks from 24 August 2016 to 3 September 2016. The results show that the values of κs are lower than those from aftershocks located to the north near the epicenter of the Amatrice earthquake, suggesting that the tectonic stress was probably high near the rupture zone, and that there may be a likely fluid flow of crustal fluids. κ˜(r) obtained from the foreshocks of the Norcia earthquake is like that calculated with the records of the Amatrice aftershocks. Then, κ˜(r) drops to lower attenuation values during the Norcia main event and tends to increase again during the aftershocks. From the analysis of these two earthquake sequences that occurred in a short-time interval in central Italy, we conclude that the temporal variation of κ˜(r) could be a valuable indicator to monitor the earthquake cycle.
Sat, 01 Jan 2022 00:00:00 GMThttp://hdl.handle.net/2122/158442022-01-01T00:00:00Z
- Near-Source Attenuation and Spatial Variability of the Spectral Decay Parameter Kappa in Central Italyhttp://hdl.handle.net/2122/16015Title: Near-Source Attenuation and Spatial Variability of the Spectral Decay Parameter Kappa in Central Italy
Authors: Castro, Raúl R; Colavitti, Leonardo; Vidales-Basurto, Claudia A; Pacor, Francesca; Sgobba, Sara; Lanzano, Giovanni
Abstract: We study the spectral decay parameter κ using S-wave recordings from the central Italy dense regional array. The data set used consists of 266 earthquakes, 353 stations, and 13,952 observations of κ with a mean value of 0.0412 ± 0.0177 within the distance range of 7.1–168.8 km. We model the variation of κ with hypocenter distance r as κ r κ0 κs ̃κ r , in which κ0 and κs represent the near-site and the near-source decay parameters, respectively, and ̃κ r the average κ along the S-wave source-station paths. We first determine ̃κ r with a nonparametric inversion approach and then we solved for κ0 and κs with a second inversion. We found that ̃κ r increases with distance within the whole distance range analyzed (9.2–80.6 km). The near-source decay parameter takes values in the range 0:0 < κs ≤ 0:026 with a mean value of 0.003 ± 0.006, which represents 7.52% of the mean value of the observed κ. The values of the near-site decay parameter vary in the range 0:0035 ≤ κ0 ≤ 0:0823 with a mean value of 0.0298 ± 0.0133, that is, 72.28% of the mean value of the κ observed. We conclude that most of the high- frequency attenuation takes place near the site, because ̃κ r contributes with only 20.2% of the spectral decay. We also investigate the spatial variability of κ by determining ̃κ r within four quadrants that divide the studied region taking as a reference axis the Apennines chain orientation. We found higher values of ̃κ r in the southern quad- rants, where seismicity and faulting are more active, and less attenuation in the more stable northeast quadrant.
Sat, 01 Jan 2022 00:00:00 GMThttp://hdl.handle.net/2122/160152022-01-01T00:00:00Z