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  • 04. Solid Earth::04.06. Seismology::04.06.11. Seismic risk  (11)
  • Deutschland
  • Q11
  • Astronomy
  • J24
  • 2005-2009  (16)
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  • 1
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    Effects of Dust Growth and Settling in T Tauri Disks (2006)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: We presented self-consistent disk models of T Tauri stars that include a parameterized treatment of dust settling and grain growth, building on techniques developed in a series of papers by D'Alessio et al. The models incorporate depleted distributions of dust in upper disk layers along with larger sized particles near the disk midplane, which are expected theoretically and, as we suggested earlier, are necessary to account for millimeter-wave emission, SEDs, scattered light images, and silicate emission features simultaneously. By comparing the models with recent mid- and near-IR observations, we find that the dust-to-gas mass ratio of small grains at the upper layers should be less than 10% of the standard value. The grains that have disappeared from the upper layers increase the dust-to-gas mass ratio of the disk interior; if those grains grow to maximum sizes of the order of millimeters during the settling process, then both the millimeter-wave fluxes and spectral slopes can be consistently explained. Depletion and growth of grains can also enhance the ionization of upper layers, increasing the possibility of the magnetorotational instability for driving disk accretion.
    Keywords: Astronomy
    Type: The Astrophysical Journal; 638; 1; 314-335
    Format: text
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  • 2
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    Why Do T Tauri Disks Accrete? (2006)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: Observations of T Tauri stars and young brown dwarfs suggest that the accretion rates of their disks scales roughly with the square of the central stellar mass. No dependence of accretion rate on stellar mass is predicted by the simplest version of the Gammie layered disk model, in which nonthermal ionization of upper disk layers allows accretion to occur via the magnetorotational instability. We show that a minor modification of Gaminie's model to include heating by irradiation from the central star yields a modest dependence of accretion on the mass of the central star. A purely viscous disk model could provide a strong dependence of accretion rate on stellar mass if the initial disk radius (before much viscous evolution has occurred) has a strong dependence on stellar mass. However, it is far from clear that at least the most massive pre-main-sequence disks can be totally magnetically activated by X-rays or cosmic rays. We suggest that a combination of effects are responsible for the observed dependence, with the lowest mass stars having the lowest mass disks, which can be thoroughly magnetically active, while the higher mass stars have higher mass disks that have layered accret,ion and relatively inactive or "dead" central zones at some radii. In such dead zones, we suggest that gravitational instabilities may play a role in allowing accretion to proceed. In this connection, we emphasize the uncertainty in disk masses derived from dust emission and argue that T Tauri disk masses have been systematically underestimated by conventional analyses. Furtlier study of accretion rates, especially in the lowest mass stars, would help to clarify the mechanisms of accretion in T Tauri stars.
    Keywords: Astronomy
    Type: The Astrophysical Journal; 648; 1; 484-490
    Format: text
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  • 3
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    Disks in Transition in the Taurus Population: Spitzer IRS Spectra of GM Aurigae and DM Tauri (2005)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: We presented Spitzer Infrared Spectrograph (IRS) observations of two objects of the Taurus population that show unambiguous signs of clea,ring in their inner disks. In one of the objects, DM Tau, the outer disk is truncated at 3 AU; this object is akin to another recently reported in Taurus, CoKu Tau/4, in that the inner disk region is free of small dust. Unlike CoKu Tau/4, however, this star is still accreting, so optically thin gas should still remain in the inner disk region. The other object, GM Aur, also accreting, has about 0.02 lunar masses of small dust in the inner disk region within about 5 AU, consistent with previous reports. However, the IRS spectrum clearly shows that the optically thick outer disk has an inner truncation at a much larger radius than previously suggested, of order 24 AU. These observations provide strong evidence for the presence of gaps in protoplanetary disks.
    Keywords: Astronomy
    Type: The Astrophysical Journal; 630; 2; L185 - L188
    Format: text
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  • 4
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    The Accretion Disk of the Lithium-Depleted Young Binary St 34 (2005)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: We presented the infrared spectrum of the young binary system St 34 obtained with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope. The IRS spectrum clearly shows excess dust emission, consistent with the suggestion of White & Hillenbrand that St 34 is accreting from a circumbinary disk. The disk emission of St 34 is low in comparison with the levels observed in typical T Tauri stars; silicate features at 10 and 20 microns are much weaker than typically seen in T Tauri stars; and excess emission is nearly absent at the shortest wavelengths observed. These features of the infrared spectrum suggest substantial grain growth (to eliminate silicate features) and possible settling of dust to the disk midplane (to reduce the continuum excess emission levels), along with a relatively evacuated inner disk, as expected due to gravitational perturbations by the binary system. Although the position of St 34 in the H-R diagram suggests an age of 8f Myr, assuming that it lies at the distance of the Taurus-Auriga molecular clouds, White & Hillenbrand could not detect any Li I absorption, which would indicate a Li depletion age of roughly 25 Myr or more. We suggest that St 34 is closer than the Taurus clouds by about 30-40 pc and has an age roughly consistent with Li depletion models. Such an advanced age would make St 34 the oldest known low-mass pre-main-sequence object with a dusty accretion disk. The persistence of optically thick dust emission well outside the binary orbit may indicate a failure to make giant planets that could effectively remove dust particles.
    Keywords: Astronomy
    Type: The Astrophysical Journal; 628; 2; L147 - L150
    Format: text
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  • 5
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    Disk Evolution in Cep OB2: Results from the Spitzer Space Telescope (2006)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: We presented the results of an infrared imaging survey of Tr 37 and NGC 7160 using the IRAC and MIPS instruments on board the Spitzer Space Telescope. Our observations cover the wavelength range from 3.6 to 24 microns, allowing us to detect disk emission over a typical range of radii 0.1 to 20 AU from the central star. In Tr 37, with an age of about 4 Myr, about 48% of the low-mass stars exhibit detectable disk emission in the IRAC bands. Roughly 10% of the stars with disks may be "transition" objects, with essentially photospheric fluxes at wavelengths i 4.5 microns but with excesses at longer wavelengths, indicating an optically thin inner disk. The median optically thick disk emission in Tr 37 is lower than the corresponding median for stars in the younger Taurus region; the decrease in infrared excess is larger at 6-8 microns than at 24 microns, suggesting that grain growth and/or dust settling has proceeded faster at smaller disk radii, as expected on general theoretical grounds. Only about 4% of the low-mass stars in the 10 Myr old cluster NGC 7160 show detectable infrared disk emission. We also find evidence for 24 micron excesses around a few intermediate-mass stars, which may represent so-called "debris disk" systems. Our observations provided new constraints on disk evolution through an important age range.
    Keywords: Astronomy
    Type: The Astrophysical Journal; 638; 897-919
    Format: text
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  • 6
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    First studies of probabilistic seismic hazard assessment in the volcanic region of Mt. Etna (southern Italy) by means of macroseismic intensities (2008)
    Istituto Nazionale di Oceanografia e di Geofisica Sperimentale
    Details
    Publication Date: 2021-03-01
    Description: Results of probabilistic seismic hazard assessment (PSHA), in terms of macroseismic intensity applied to the Mt. Etna region, are presented. PSHA has been performed using a numerical procedure based on the extensive use of local macroseismic information, as an alternative to the usual Cornell-McGuire methods. The large amount of intensity data available for this area - coming from the Italian intensity database DBMI04 for the regional earthquakes, and from the Etna catalogue for the ‘local’ events - has provided fairly exhaustive seismic site histories (i.e. the data set of macroseismic observations available for a given locality) to estimate the seismic hazard for 402 localities on the volcano. In order to improve the completeness of the site catalogue when historical information is missing, observed intensity data have been integrated with values calculated from epicentral information obtained by using an attenuation law specific for the Etna region. Using a probability distribution considering the completeness of the input database and the uncertainty of intensity data, the hazard in terms of maximum intensity (Iexp) characterised by a 10% probability of exceedance in an exposure time of 50 years, has been computed. The highest values ( Iexp = IX or X) are found in the south-eastern flank of Mt. Etna while the rest of the volcano is exposed to a lower hazard (Iexp = VIII). Despite the low energy (M≤4.8) compared with that of the large regional earthquakes affecting the area (6.6≤M≤7.4), the local events strongly influence the pattern of the hazard in the eastern sector of Mt. Etna, representing a significant, and sole, source of hazard when a shorter exposure time (e.g. 30 years) is considered.
    Description: Published
    Description: 77-91
    Description: N/A or not JCR
    Description: reserved
    Keywords: probabilistic sesmic hazard ; macroseismic intentity ; Mt. Etna ; 04. Solid Earth::04.06. Seismology::04.06.11. Seismic risk
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: article
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  • 7
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    Valutazioni di pericolosità sismica in termini di intensità macrosismica utilizzando metodi di sito (2007)
    Details
    Publication Date: 2021-03-01
    Description: Vengono presentate le stime di pericolosità sismica, in termini di intensità macrosismica, ottenute mediante l’approccio probabilistico proposto da Albarello e Mucciarelli (2002), basato sull’impiego dei dati documentari relativi agli effetti locali prodotti dai terremoti passati (storie sismiche di sito). I risultati forniti da questa procedura (approccio “di sito”), in termini di minimo valore di intensità caratterizzato da una probabilità di eccedenza inferiore al 10% in 50 anni (Iref), sono confrontati con quelli ottenuti sul territorio italiano tramite la metodologia “standard” di Cornell-McGuire. Per meglio comprendere i motivi delle differenze osservate tra le due stime e il ruolo di differenti basi informative, sono state prodotte diverse mappe di pericolosità. Stime di pericolosità a scala locale sono state inoltre effettuate nell’area dell’Etna dove, grazie a numerosi studi macrosismici di dettaglio, sono disponibili storie sismiche di sito particolarmente ricche.
    Description: Istituto Nazionale di Geofisica e Vulcanologia
    Description: Published
    Description: 4.2. TTC - Scenari e mappe di pericolosità sismica
    Description: open
    Keywords: seismic hazard estimates ; intensity data ; Italy and Etna region ; 04. Solid Earth::04.06. Seismology::04.06.11. Seismic risk
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: report
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  • 8
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    The earthquake of 6 September 2002 and the seismic history of Palermo (Northern Sicily, Italy): Implications for the seismic hazard assessment of the city (2005)
    Springer
    Details
    Publication Date: 2021-03-01
    Description: On September 6, 2002, aML =5.6 earthquake, occurring some tens of kilometres offshore from the Northern Sicilian coast (Southern Tyrrhenian Sea), slightly damaged the city of Palermo and surroundings (degree 6 in the European Macroseismic Scale 1998). The macroseismic investigation of the shock and a detailed study of effects of the main earthquakes which affected Palermo in the past have been performed in order to evaluate the seismic response of the city. Moreover, the comparison of the recent event, which is instrumentally constrained, with historical earthquakes allows us to infer new insights on the seismogenic sources of the area, that seem located offshore in the Tyrrhenian sea. In the last 500 years, Palermo has never been completely destroyed but has suffered effects estimated between intensities 6 and 8 EMS-98 many times (1693, 1726, 1751, 1823, 1940, 1968, 2002). The damage scenarios of the analysed events have shown that damage distribution is strongly conditioned by soil response in the different parts of the city and by a high building vulnerability, mainly in the historical centre and in the south-eastern zone of the modern city. As a matter of fact, Palermo has always suffered greater effects than those reported for other nearby localities. The hazard assessment obtained using observed site intensities has shown that the probability of occurrence for intensity 8 (the strongest intensity observed in Palermo) exceeds 99% for 550 years, while the estimated mean return period is 152 ± 40 years. These results, in connection with building vulnerability due to the urban expansion before the introduction of seismic code, suggest that the city is exposed to a relatively high seismic risk.
    Description: Published
    Description: 525-543
    Description: partially_open
    Keywords: intensity ; damage ; earthquakes ; Italy ; macroseismics ; Palermo ; seismic hazard ; vulnerability ; 04. Solid Earth::04.06. Seismology::04.06.11. Seismic risk
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: article
    Format: 513 bytes
    Format: 1263995 bytes
    Format: text/html
    Format: application/pdf
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  • 9
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    Seismic hazard assessment from intensity data (2008)
    Details
    Publication Date: 2012-02-03
    Description: A new approach has been developed aiming at the probabilistic estimate of seismic hazard from the local seismic history. This is expressed in terms of seismic effects documented at the site during past earthquakes (macroseismic intensity) and thus a specific statistical treatment is required to take into account the peculiar character of macroseismic data that are discrete, ordinal and range-limited. The proposed procedure also allows the correct treatment of relevant uncertainty (ill-defined intensity values, local catalogue completeness, etc.) in the frame of a coherent distribution-free statistical approach. This methodology has been implemented in a free computer program (SASHA: Site Approach to Seismic Hazard Assessment) and applied to the seismic hazard estimation at the 8100 municipalities in Italy. The program also implements a probabilistic procedure to “translate” hazard estimates provided in terms of Intensity to other ground-shaking parameters (e.g. PGA): this “rescaling” procedure allows to com-pare the results coming from this approach with those provided by standard PSHA procedures. This comparison results of great importance at the sites where a relatively rich seismic history is available and it could provide a benchmark for different approaches to PSHA.
    Description: Unpublished
    Description: Hersonissos, Crete, Greece
    Description: 4.2. TTC - Scenari e mappe di pericolosità sismica
    Description: open
    Keywords: probabilistic seismic hazard assessment ; intensity data ; Italy ; 04. Solid Earth::04.06. Seismology::04.06.11. Seismic risk ; 05. General::05.01. Computational geophysics::05.01.04. Statistical analysis
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: Poster session
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  • 10
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    A microtremor survey in the area shocked by the ml 5.2 Salò earthquake (north Italy): an empirical approach to determine the effects of ground motions (2008)
    Details
    Publication Date: 2017-04-04
    Description: In this work the results of a quick microtremor survey performed in the municipalities situated in the epicentre area of the Ml 5.2 2004 Salò earthquake (North Italy) are presented. The aim of this study is to understand if the large amount of damage caused by the event (about 215 millions of euros only in the areas near to the epicenter) is correlated more to the local surface geology conditions or to the vulnerability of ancient Italian historical centers. A preliminary seismic zonation was carried out in 5 villages including about 30 measurements of microtremors analysed by the Nakamura technique (hereinafter HVNR). The points of measurement were carefully selected considering sites located both near damaged buildings and over different local geology conditions (alluvium deposits, fluvial-glacial deposits, debris fans and rock). In order to strengthen the HVNR results and to evaluate the reliability of the Nakamura analysis, a comparison with spectral ratios calculated on earthquakes (hereinafter HVSR) recorded at the strong motion station of Vobarno was made. In general, the outcomes of the survey highlight a possible correlation between local geology conditions and ground motion amplification for different frequency bands. In order to check if this evidence is linked with the damage, a series of macroseismic intensities values were collected for different zones of the investigated area, and a non parametric correlation approach was used to establish a possible correlation between damage and ground motion amplification for selected frequency bands. The results show, from a statistical point of view, that in the area surrounding the epicenter of the 24 November 2004 mainshock, the damage pattern is not strongly dependent upon the local surface geology but more correlated to the low quality of the civil structures present in the area, including old buildings of the last century.
    Description: Submitted
    Description: 12-36
    Description: 1.1. TTC - Monitoraggio sismico del territorio nazionale
    Description: JCR Journal
    Description: open
    Keywords: local surface geology ; HV spectral ratios ; non parametric correlation technique ; Salò earthquake ; 04. Solid Earth::04.06. Seismology::04.06.04. Ground motion ; 04. Solid Earth::04.06. Seismology::04.06.11. Seismic risk
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: manuscript
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