342 research outputs found
Deep view of the Subduction-Transform Edge Propagator (STEP) fault in the Calabrian Subduction Zone
The Calabrian Subduction Zone plays a key role in the evolution of the central Mediterranean in the framework of
the convergence between Africa and Europe.
Here, the remnants of the World’s oldest oceanic crust form a narrow NW-dipping slab passively subducting
beneath the Calabrian Arc.
Recently published high-resolution seismic profiles and bathymetric data of the western Ionian Sea highlight
the presence of a NNW-SSE faulting system connected with a series of Plio-Pleistocene syn-tectonic basins.
These features are correlated with the recent activity of a major NNW-SSE deformation zone confining the active
subduction to the SW and interpreted as a Subduction-Transform Edge Propagator (STEP) fault.
The goal of this work is to jointly reconstruct the geometry of the STEP fault and the subduction interface in its
surroundings.We use multichannel seismic profiles acquired in the southwestern part of the Calabrian accretionary
wedge to focus on the STEP fault geometry at depth and to analyse its relationships with shallow deformation
features.
The quantitative analysis and enhancement of seismic data provided an accurate image of the internal structure
of the accretionary wedge at various depths, showing growth strata in the Plio-Pleistocene succession and major
discontinuities in the lower crust.
Our results depict a main subvertical, slightly east-dipping, lithospheric fault cutting the oceanic crust down to
the Moho, and a rich set of associated secondary synthetic and antithetic faults. This picture also provides new
insights on the STEP fault propagation mechanism.
In addition, the tridimensional correlation of the STEP fault occurrences in various seismic profiles provides
a preliminary scheme of its segmentation and highlights the relationships of this master fault with other main
structural elements of the Calabrian Arc and Eastern Sicily, including some of the faults deemed to be responsible
for major historical earthquakes in the area
Deriving thrust fault slip rates from geological modeling: examples from the Marche coastal and offshore contraction belt, Northern Apennines, Italy.
We present a reconstruction of the central Marche thrust system in the central-northern Adriatic domain aimed at constraining the geometry of the active faults deemed to be potential sources of moderate to large earthquakes in this region and at evaluating their long-term slip rates. This system of contractional structures is associated with fault-propagation folds outcropping along the coast or buried in the offshore that have been active at least since about 3Myr. The ongoing deformation of the coastal and offshore Marche thrust system is associated with moderate historical and instrumental seismicity and recorded in sedimentary and geomorphic features. In this study, we use subsurface data coming from both published and original sources. These comprise cross-sections, seismic lines, subsurface maps and borehole data to constrain geometrically coherent local 3D geological models, with particular focus on the Pliocene and Pleistocene units. Two sections crossing five main faults and correlative anticlines are extracted to calculate slip rates on the driving thrust faults. Our slip rate calculation procedure includes a) the assessment of the onset time which is based on the sedimentary and structural architecture, b) the decompaction of clastic units where necessary, and c) the restoration of the slip on the fault planes. The assessment of the differential compaction history of clastic rocks eliminates the effects of compaction-induced subsidence which determine unwanted overestimation of slip rates. To restore the displacement along the analyzed structures, we use two different methods on the basis of the deformation style: the fault parallel flow algorithm for faulted horizons and the trishear algorithm for fault-propagation folds. The time of fault onset ranges between 5.3-2.2 Myr; overall the average slip rates of the various thrusts are in the range of 0.26-1.35 mm/yr
Variability Modes of Blazars from Intensive Optical Monitoring
We report the main results of our six year long intensive optical monitoring
on blazars ON 231 (W Com), BL Lac, and 3C 273. Intensive optical monitoring is
an indispensable tool to obtain well sampled light curves and thus to
understand the correlation with the variability in other bands and to
discriminate among the proposed emission models. The curves based on our work
show different variability modes: intermittent mode, quasi-regular mode and
mixed mode. It is not clear if different variability modes can be present, at
various times, in the life of an object suggesting that their occurrence can be
related to the evolution of blazars. An optimized and nearly continuous optical
monitoring is the only way to know the activity status of the sources and to
retrieve useful information on their physical dynamics. Also, the rapid
availability of information about the luminosity of a source is very important
to trigger space based observations and to activate large multifrequency
collaborations. We stress that a significant sample of blazars can be observed
with small size telescopes (0.35-0.80 cm) equipped with CCD cameras. A
world-wide network of several instruments in different countries, is very
useful to increase the time coverage and to reduce the number of nights lost
for bad meteorological conditions.Comment: Paper submitted to the AIP Conference Proceedings "High Energy
Gamma-Ray Astronomy" - Conf. Proc. of the gamma-2000 symposium held in
Heidelberg. 4 pages, doc format source (AIP Proc.), 1 gif figur
The Intra-Night Optical Variability of the bright BL Lac object S5 0716+714
We address the topic of the Intra-Night Optical Variability of the BL Lac
object S5 0716+714. To this purpose a long term observational campaign was
performed, from 1996 to 2003, which allowed the collection of a very large data
set, containing 10,675 photometric measurements obtained in 102 nights. The
source brightness varied in a range of about 2 mag, although the majority of
observations were performed when it was in the range 13.0 < R < 13.75.
Variability time scales were estimated from the rates of magnitude variation,
which were found to have a distribution function well fitted by an exponential
law with a mean value of 0.027 mag/h, corresponding to an e-folding time scale
of the flux tau_F = 37.6 h. The highest rates of magnitude variation were
around 0.10--0.12 mag/h and lasted less than 2 h. These rates were observed
only when the source had an R magnitude < 13.4, but this finding cannot be
considered significant because of the low statistical occurrence. The
distribution of tau_F has a well defined modal value at 19 h. Assuming the
recent estimate of the beaming factor delta about 20, we derived a typical size
of the emitting region of about 5 times 10^{16}/(1 + z) cm. The possibility to
search for a possible correlation between the mean magnitude variation rate and
the long term changes of the velocity of superluminal components in the jet is
discussed.Comment: 12 pages, 10 figures, accepted for Astronomy and Astrophysic
The Long Term Optical Variability of the BL Lac object S5 0716+714: Evidence for a Precessing Jet
We present the historic light curve of the BL Lac object S5 0716+714,
spanning the time interval from 1953 to 2003, built using Asiago archive plates
and our recent CCD observations, together with literature data. The source
shows an evident long term variability, over which well known short term
variations are superposed. In particular, in the period from 1961 to 1983 the
mean brightness of S5 0716+714 remained significantly fainter than that
observed after 1994. Assuming a constant variation rate of the mean magnitude
we can estimate a value of about 0.11 magnitude/year. The simultaneous
occurrence of decreasing ejection velocities of superluminal moving components
in the jet reported by Bach et al. (2005) suggests that both phenomena are
related to the change of the direction of the jet to the line of sight from
about 5 to 0.7 degrees for an approximately constant bulk Lorentz factor of
about 12. A simple explanation is that of a precessing relativistic jet, which
should presently be close to the smallest orientation angle. One can therefore
expect in the next ten years a decrease of the mean brightness of about 1
magnitude.Comment: to appear on The Astronomical Journal, 17 pages, 7 figures. Fig.2 is
given as a separated jpg fil
Optical and Radio monitoring of S5 1803+74
The optical (BVRI) and radio (8.4 GHz) light curves of S5 1803+784 on a time
span of nearly 6 years are presented and discussed. The optical light curve
showed an overall variation greater than 3 mag, and the largest changes occured
in three strong flares. No periodicity was found in the light curve on time
scales up to a year. The variability in the radio band is very different, and
shows moderate oscillations around an average constant flux density rather than
relevant flares, with a maximum amplitude of 30%, without a simultaneous
correspondence between optical and radio luminosity. The optical spectral
energy distribution was always well fitted by a power law. The spectral index
shows small variations and there is indication of a positive correlation with
the source luminosity. Possible explanations of the source behaviour are
discussed in the framework of current models.Comment: 25 pages, 12 figure
Testing Different Tectonic Models for the Source of the M w 6.5, 30 October 2016, Norcia Earthquake (Central Italy): A Youthful Normal Fault, or Negative Inversion of an Old Thrust?
We adopted a multidisciplinary approach to investigate the seismotectonic scenario of the 30
October 2016, Mw 6.5, Norcia earthquake, the largest shock of the 2016\u20132017 central Italy earthquake
sequence. First, we used seismological and geodetic data to infer the dip of the main slip patch of the
seismogenic fault that turned out to be rather low\u2010angle (~37\ub0). To evaluate whether this is an acceptable dip
for the main seismogenic source, we modeled earthquake deformation using single\u2010 and multiple\u2010fault
models deduced from aftershock pattern analyses. These models show that the coseismic deformation
generated by the Norcia earthquake is coherent with slip along a rather shallow\u2010dipping plane. To
understand the geological significance of this solution, we reconstructed the subsurface architecture of the
epicentral area. As the available data are not robust enough to converge on a single fault model, we built
three different models encompassing all major geological evidence and the associated uncertainties,
including the tectonic style and the location of major d\ue9collement levels. In all models the structures
derived from the contractional phase play a significant role: from controlling segmentation to partially
reusing inherited faults, to fully reactivating in extension a regional thrust, geometrically compatible
with the source of the Norcia earthquake. Based on our conclusions, some additional seismogenic sources
falling in the eastern, external portions of the Apennines may coincide with inherited structures. This
may be a common occurrence in this region of the chain, where the inception of extension is as recent as
Middle\u2010Upper Pleistocene
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