5,114 research outputs found
The ScS precursors for the study of the lowermost mantle
The exploration of the lowermost-mantle structures by means of body waveform modeling allows the small-scale detection of heterogeneity and anomalous layers. In some regions the D00 layer presents a discontinuity at its top that seems to
be a local feature. This anomalous reflector may be recognized by the detection of a small core-reflected phases precursor. These studies may present different order of problems. The main difficulties, are connected to the identification of the precursor and its association to the D00 region. Misunderstandings often result because of phases
produced by heterogeneity and anisotropy along and in the vicinity of the ray paths, in the crust and mantle structures. These complexities are increased when large dataset and recording arrays, which may facilitate the waveform analysis, are not available. In this paper we discuss the body waveform modeling of lower-mantle phases for the study of the D00 with particular focus on the case of sparse data with only few events and stations available
What sets the magnetic field strength and cycle period in solar-type stars?
Two fundamental properties of stellar magnetic fields have been determined by
observations for solar-like stars with different Rossby numbers (Ro), namely,
the magnetic field strength and the magnetic cycle period. The field strength
exhibits two regimes: 1) for fast rotation it is independent of Ro, 2) for slow
rotation it decays with Ro following a power law. For the magnetic cycle period
two regimes of activity, the active and inactive branches, also have been
identified. For both of them, the longer the rotation period, the longer the
activity cycle. Using global dynamo simulations of solar like stars with Rossby
numbers between ~0.4 and ~2, this paper explores the relevance of rotational
shear layers in determining these observational properties. Our results,
consistent with non-linear alpha^2-Omega dynamos, show that the total magnetic
field strength is independent of the rotation period. Yet at surface levels,
the origin of the magnetic field is determined by Ro. While for Ro<1 it is
generated in the convection zone, for Ro>1 strong toroidal fields are generated
at the tachocline and rapidly emerge towards the surface. In agreement with the
observations, the magnetic cycle period increases with the rotational period.
However, a bifurcation is observed for Ro~1, separating a regime where
oscillatory dynamos operate mainly in the convection zone, from the regime
where the tachocline has a predominant role. In the latter the cycles are
believed to result from the periodic energy exchange between the dynamo and the
magneto-shear instabilities developing in the tachocline and the radiative
interior.Comment: 43 pages, 14 figures, accepted for publication in The Astrophysical
Journa
Elementary seismological analysis applied to the April 6, 2009 L'Aquila mainshock and its larger aftershock
To understand the source complexity of the April 6, 2009 L’Aquila earthquake (MW =
6.3), a quick seismological analysis is done on the waveforms of the mainshock and
the larger aftershock that occurred on April 7, 2009. We prove that a simple waveform
analysis gives useful insights into the source complexity, as soon as the seismograms
are available after the earthquake occurrence, whereas the reconstruction of the
rupture dynamics through the application of sophisticated techniques requires a
definitely longer time. We analyzed the seismograms recorded at broadband and
strong motion stations and provided firm constraints on rupture kinematics, slip
distribution, and static surface deformation, also discriminating the actual fault plane.
We found that two distinct rupture patches associated with different fracture
propagation directions and possibly occurring on distinct rupture planes, characterized
the source kinematics of the April 6 events. An initial updip propagation successively
proceeds toward SE, possibly on a different plane. We also show that the same
processing, applied to the April 7, 2009 aftershock (MW = 5.6), allows us to obtain
useful information also in the case of lower magnitude events. Smaller events with
similar location and source mechanism as the mainshock, to be used as Green’s
empirical function, occur in the days before or within tens of minutes to a few hours
after the mainshock. These quick, preliminary analyses can provide useful constraints
for more refined studies, such as inversion of data for imaging the rupture evolution
and the slip distribution on the fault plane. We suggest implementing these analyses
for real, automatic or semi-automatic, investigations
Comment on the paper by Barreca et al.: “The Strait of Messina: Seismotectonics and the source of the 1908 earthquake” (Earth-Science Reviews 218, 2021, 103685)
We discuss the new causative source model for the 1908 Messina Straits earthquake recently proposed by Barreca et al. (2021), where an aseismic slip of 1.13 m along a low-angle discontinuity, preceding the 1908 earthquake, have mechanically destabilized a set of overlying faults, therefore leading them to the rupture. The lack of significant variations of the relative sea level in the Messina harbor area, in the time period relevant for the levelling data (1907–1908) analyzed by Barreca et al., and at least for the decade preceding the event proves the inconsistency of the assumed pre-earthquake aseismic slip. A careful interpretation of crustal earthquake distribution in the Strait does not support the presence of the low-angle discontinuity. The modelled horizontal coseismic pattern reveals a scenario that is not supported by any other independent geological and geophysical observation. We conclude that the source model proposed by Barreca et al. for the 1908 Messina Straits earthquake can not be considered as a viable hypothesis for the causative fault
The 28 December 1908 Messina Straits Earthquake (Mw 7.1): A Great Earthquake throughout a Century of Seismology
Early in the morning on 28 December 1908, just a few days after Christmas, a severe earthquake struck the Messina Straits, a rather narrow sound that separates Calabria, in southern Italy, from Sicily. The shaking was distinctly felt in Albania, Montenegro, and the Greek Ionian islands, about 400 km to the east and northeast of the Straits; in Malta, about 250 km to the south; and as far as Ustica Island, about 220 km to the west. The earthquake was catastrophic in the epicentral area and
was immediately followed by fires and a large tsunami. Messina (Sicily) and Reggio Calabria (Calabria), two significant cities located less than 10 km apart on the two facing shores of the straits, were almost completely destroyed, and buildings were severely damaged over an area in excess of 6,000 km2
Source complexity of the May 20, 2012, MW 5.9, Ferrara (Italy) event
A Mw 3.9 foreshock on May 19, 2012, at 23:13 UTC, was followed at 02:03 on May 20, 2012, by a Mw 5.9 earthquake that hit a densely populated area in the Po Plain, west of the city of Ferrara, Italy (Figure 1). Over the subsequent 13 days, six Mw >5 events occurred; of these, the most energetic was a Mw 5.8 earthquake on May 29, 2012, 12 km WSW of the main shock. The tragic balance of this sequence was 17 casualties, hundreds of injured, and severe damage to the historical and cultural heritage of the area. From a seismological point of view, the 2012 earthquake was not an outstanding event in its regional context. The same area was hit in 1996 by a Mw 5.4 earthquake [Selvaggi et al. 2001], and previously in 1986 and in 1967 (DBMI11) [Locati et al. 2011]. The most destructive historical event was the 1570, Imax 8 event, which struck the town of Ferrara [Guidoboni et al. 2007, Rovida et al. 2011]. The 2012 seismic sequence lasted for several weeks and probably developed on a well-known buried thrust fault [Basili et al. 2008, Toscani et al. 2009, DISS Working Group 2010], at depths between 2 km and 10-12 km
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