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S-wave Velocity Models in the Scotia Sea Region, Antarctica, from Nonlinear Inversion of Rayleigh Waves Dispersion (1999)

Vuan, A. ; Cazzaro, R. ; Costa, G. ; [et al.]

Springer

Pure and applied geophysics 154 (1999), S. 121-139

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ISSN: 1420-9136

Keywords: Key words: Antarctica, Scotia Sea, lithosphere structure, surface waves.

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences , Physics

Notes: Abstract —More than 60 events recorded by four recently deployed seismic broadband stations around Scotia Sea, Antarctica, have been collected and processed to obtain a general overview of the crust and upper mantle seismic velocities.¶Group velocity of the fundamental mode of Rayleigh waves in the period between 10 s to 30–40 s is used to obtain the S-wave velocity versus depth along ten different paths crossing the Scotia Sea region. Data recorded by two IRIS (Incorporated Research Institutions for Seismology) stations (PMSA, EFI) and the two stations of the OGS-IAA (Osservatorio Geofisico Sperimentale—Instituto Antarctico Argentino) network (ESPZ, USHU) are used.¶The Frequency-Time Analysis (FTAN) technique is applied to the data set to measure the dispersion properties. A nonlinear inversion procedure, "Hedgehog," is performed to retrieve the S-wave velocity models consistent with the dispersion data.¶The average Moho depth variation on a section North to South is consistent with the topography, geological observations and Scotia Sea tectonic models.¶North Scotia Ridge and South Scotia Ridge models are characterised by similar S-wave velocities ranging between 2.0 km/s at the surface to 3.2 km/s to depths of 8 km/s. In the lower crust the S-wave velocity increases slowly to reach a value of 3.8 km/s. The average Moho depth is estimated between 17 km to 20 km and 16 km to 19 km, respectively, for the North Scotia Ridge and South Scotia Ridge, while the Scotia Sea, bounded by the two ridges, has a faster and thinner crust, with an average Moho depth between 9 km and 12 km.¶On other paths crossing from east to west the southern part of the Scotia plate and the Antarctic plate south of South Scotia Ridge, we observe an average Moho depth between 14 km and 18 km and a very fast upper crust, compared to that of the ridge. The S-wave velocity ranges between 3.0 and 3.6 km/s in the thin (9–13 km) and fast crust of the Drake Passage channel. In contrast the models for the tip of the Antarctic Peninsula consist of two layers with a large velocity gradient (2.3–3.0 km/s) in the upper crust (6-km thick) and a small velocity gradient (3.0–4.0) in the lower crust (14-km thick).

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/s000240050224

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Group Velocity Tomography in the Subantarctic Scotia Sea Region (2000)

Vuan, A. ; Russi, M. ; Panza, G. F.

Springer

Pure and applied geophysics 157 (2000), S. 1337-1357

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ISSN: 1420-9136

Keywords: Key Words: Antarctica, Scotia Sea, lithospheric structure, tomography, surface waves.

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences , Physics

Notes: Abstract —More than 150 events, recently recorded by seven seismic broadband stations (OGS-IAA, IRIS, GSETT3-IDC), have been collected and processed to obtain an overview of the crust and upper mantle shear wave velocities.¶Group velocities of the fundamental mode Rayleigh and Love waves, in the period range from 15 s to 50 s, are used to obtain tomographic maps of the Scotia Sea region, the tip of Antarctic Peninsula, and the tip of South America. Errors in the measurements, estimated on clusters, are larger for Love waves than for Rayleigh waves and their averages are 0.060–0.080 km/s and 0.030–0.040 km/s, respectively.¶From the regionalisation of the dispersion measurements, we obtain smoothed local dispersion curves in correspondence with the main geological and tectonic features, and from their nonlinear inversion, the shear wave velocity versus depth profiles.¶The correlation length of the heterogeneity, which can be resolved by Rayleigh waves, varies between 200 and 400 km in most parts of the studied area, but becomes greater near the periphery of the maps. The spatial resolution of Love waves (400–600 km) is poorer than that of Rayleigh waves, due to the deteriorated path coverage and to the larger errors in the group velocity measurements.¶Models of the shear wave velocity in the crust and upper mantle for the tip of South America, the Falkland Plateau, the Scotia Sea, the South Sandwich oceanic spreading ridge, the South Sandwich trench, the South Scotia ridge, the tip of Antarctic Peninsula, the Bransfield Strait and the Drake Passage are presented.¶Our regional models and the existing large-scale models (e.g., CRUST5.1), help to define a 3-D velocity model of the Scotia Sea region to be further investigated by waveform inversion.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/PL00001122

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A Hybrid Approach for Broadband Simulations of Strong Ground Motion: The Case of the 2008 Iwate-Miyagi Nairiku Earthquake (2015)

Moratto, L., Vuan, A., Sarao, A.

Seismological Society of America (SSA)

In: Bulletin of the Seismological Society of America (BSSA)

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Publication Date: 2015-10-02

Description: We developed a hybridization code that merges the full-wave low-frequency (LF) signals with stochastic high-frequency (HF) synthetics to simulate broadband strong ground motion for engineering applications. The approach is applied to reproduce, within the known source and structure constraints, the Kiban–Kyoshin network (KiK-net) and Kyoshin net (K-NET) accelerometric data recorded during the 2008 Iwate–Miyagi Nairiku earthquake ( M w 7.0). The results show that our approach is able to simulate satisfactorily the observed waveforms and the related response spectra over the broadband frequency range. We estimated the effects of the hybridization in the peak and spectral parameters through the goodness-of-fit criteria: the overall fit improves (~15%–20%) when, instead of full-wave calculated seismograms, we use the low-pass filtered recordings, showing that a good match at LFs is fundamental to obtain reliable and consistent results even in the HF band. Peak ground velocities are less affected than accelerations by the inclusion of the HF contribution and are well reproduced by our method; peak ground accelerations show larger discrepancies due to the intrinsic characteristics of the stochastic model and the small-scale heterogeneity that affect the seismic radiation and propagation at HFs. Nevertheless, simulated signals reproduce the observed ones fairly well, and an acceptable match is also found in the comparison of the spectral responses. Online Material: Figures showing the COMPEX merging procedure, comparison of recorded and simulated response spectra, and comparison of recorded and synthetic broadband signals; and tables of source parameters, 1D velocity model, and matching frequencies.

Print ISSN: 0037-1106

Electronic ISSN: 1943-3573

Topics: Geosciences , Physics

Published by Seismological Society of America (SSA)

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Moho Reflection Effects in the Po Plain (Northern Italy) Observed from Instrumental and Intensity Data (2011)

Bragato, P. L. ; Sugan, M. ; Augliera, P. ; Massa, M. ; Vuan, A. ; Sarao, A.

Seismological Society of America (SSA)

In: Bulletin of the Seismological Society of America (BSSA) 101: 2142-2152.

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Publication Date: 2011-10-01

Description: Recent moderate-sized earthquakes (ML[≤]5.2) that occurred in northern Italy demonstrate that the currently available ground-motion prediction equations (GMPEs) largely underestimate shaking in the Po Plain region at hypocentral distances greater than 70 km. In order to investigate this phenomenon, we collected a set of peak ground acceleration (PGA) observations for weak and moderate earthquakes in the area. Nonparametric regression analysis of PGA observations as a function of magnitude and hypocentral distance shows that PGA is systematically enhanced for distances between 70 and 200 km. An indirect estimation through the analysis of the attenuation of macroseismic intensities suggests that the effect also applies to strong earthquakes (ML〉5.5). We performed numerical modeling experiments to investigate the cause of the phenomenon. The characteristics of the computed synthetic seismograms indicate that the enhancement of ground motion is mainly an effect of the reflection of S waves at the Moho (SmS phase). The analysis of both real and synthetic data shows that the Moho reflection effect is maximized at hypocentral distances between 90 and 150 km, where the PGA increases by a factor larger than 2.4.

Print ISSN: 0037-1106

Electronic ISSN: 1943-3573

Topics: Geosciences , Physics

Published by Seismological Society of America (SSA)

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Evaluating the Relevance of Moho Reflections in Accelerometric Data: Application to an Inland Japanese Earthquake (2012)

Sugan, M. ; Vuan, A.

Seismological Society of America (SSA)

In: Bulletin of the Seismological Society of America (BSSA) 102: 842-847.

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Publication Date: 2012-04-01

Description: The relevance of SmS seismic phase or multiple reflected SmS on the ground motion away from the earthquake source has been discussed in many studies during the past 30 years. The SmS and multiples are normally recognized by analyzing travel time arrivals on the basis of a priori well-known crustal models. Moho reflections in borehole and surface accelerometric data of the KiK-net network for an inland Japanese earthquake are investigated. When reflected phases are transmitted efficiently, both amplitude and frequency content can be retrieved by using a simple time-frequency analysis of accelerometric records. For the 15 April 2007 M??5.4 Kinki crustal earthquake, Moho reflections are found at epicentral distances from 70 to 180 km. Borehole data show that SmS and multiples can be easily found in the frequency range from 0.25 to 2 s and in the group velocity window from 2.8 to 3.5??km/s. The coupling of borehole and surface stations of KiK-net data allows for the separate evaluation of the propagation effect along the source-receiver path and of the transfer function due to soft soils for these specific phases. An amplitude–period scaling relationship is found between the National Earthquake Hazards Reduction Program site classification and the observed transfer function for SmS phase and multiples.Online Material: Figures showing the effect of the source-time function duration on the multiple filter technique (MFT) diagrams, an MFT analysis of surface and borehole accelerograms, period–distance domain spectral amplitudes, and the average period–velocity transfer function for different NEHRP soil classes.

Print ISSN: 0037-1106

Electronic ISSN: 1943-3573

Topics: Geosciences , Physics

Published by Seismological Society of America (SSA)

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Seismic Evidence of an Early Afterslip During the 2012 Sequence in Emilia (Italy) (2019)

M. Sugan, A. Vuan, A. Kato, M. Massa, G. Amati

Wiley

In: Geophysical Research Letters

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Publication Date: 2019

Description: Abstract We apply a matched‐filter‐technique to augment the detected events in the time window between the two mainshocks of the 2012 Emilia seismic sequence (Italy), 20 (Mw 6.1) and 29 (Mw 6.0) May. Using well‐located templates, we increase the number of detections from 1,727 to 7,616. This greater detail allows evidencing migrations of seismic events from the nucleation point of the first shock to the second. Repeating earthquakes are also found between the two mainshocks. The seismicity pattern suggests a transient slip acting immediately after the shallow first event, weakening and loading the volume around the deep nucleation point of the 29 May Mw 6.0 earthquake. We interpret migrations and repeaters as the fingerprint of an early afterslip triggered by the first mainshock. By taking into account the earliest repeater we estimate a cumulative slip of approximately 27 cm (Mw 5.7). This value could represent the lower bound of the total released afterslip.

Print ISSN: 0094-8276

Electronic ISSN: 1944-8007

Topics: Geosciences , Physics

Published by Wiley on behalf of American Geophysical Union (AGU).

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Local Site Response in an Alpine Valley from Recorded Earthquakes and Ambient Noise Analysis (2012)

Barnaba, C., Vuan, A.

Seismological Society of America (SSA)

In: Bulletin of the Seismological Society of America (BSSA)

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Publication Date: 2012-10-01

Description: Local response is estimated within the sediment fill of the Tagliamento River Valley (THV), northeastern Italy, using about 90 weak-to-medium local and regional earthquakes. Ground motion is recorded at six stations deployed in a linear array across the valley. Site amplification factors are determined using the standard spectral ratio, the generalized inversion technique, and evaluations of receiver functions. Results obtained from earthquake data are then compared with ambient noise analysis. Azimuthal effects in the seismic response are observed locally for waves coming from the east-southeast, aligned with the longitudinal axis of the valley. The seismic responses evaluated at the mid-valley and at the valley edge are then convolved with the 1976 M w 6.4 Friuli mainshock recorded at accelerometric station TLM1, located close to the array. The local response at the valley edge, in the town of Cavazzo Carnico, is responsible for an enhancement of the input motion of an average factor of about 4 in the frequency range from 2 to 8 Hz. The elastic demand from the current national regulation for a 475-year return period and specific for Cavazzo Carnico is overcome by the 5%-damped pseudoacceleration response spectrum estimated at the valley edge. The underestimation of the Italian code is mostly due to a 2D amplification effect at the valley edge rather than a simple inadequacy of the V S 30 site classification. Online Material: Figures showing locations of historical earthquakes, distribution of fundamental frequency of vibration, waveforms of the M D 5.1 Kobarid earthquake, S -wave velocity versus depth, and GIT and RF amplification median values and spectra; tables of seismic station locations and soil type, and of local and regional earthquakes.

Print ISSN: 0037-1106

Electronic ISSN: 1943-3573

Topics: Geosciences , Physics

Published by Seismological Society of America (SSA)

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