129 research outputs found
Crustal and upper-mantle structure in the Eastern Mediterranean from the analysis of surface wave dispersion curves
The dispersive properties of surface waves are used to infer earth structure in the Eastern
Mediterranean region. Using group velocity maps for Rayleigh and Love waves from 7100
s, we
invert for the best 1D crust and uppermantle
structure at a regular series of points. Assembling the
results produces a 3D lithospheric model, along with corresponding maps of sediment and crustal
thickness. A comparison of our results to other studies finds the uncertainties of the Moho estimates to
be about 5 km. We find thick sediments beneath most of the Eastern Mediterranean basin, in the
Hellenic subduction zone and the Cyprus arc. The Ionian Sea is more characteristic of oceanic crust
than the rest of the Eastern Mediterranean region as demonstrated in particular by the crustal thickness.
We also find significant crustal thinning in the Aegean Sea portion of the backarc,
particularly towards
the south. Notably slower Swave
velocities are found in the uppermantle,
especially in the northern
Red Sea and Dead Sea Rift, central Turkey, and along the subduction zone. The low velocities in the
uppermantle
that span from North Africa to Crete, in the Libyan Sea, might be an indication of
serpentinized mantle from the subducting African lithosphere. We also find evidence of a strong
reverse correlation between sediment and crustal thickness which, while previously demonstrated for
extensional regions, also seems applicable for this convergence zone
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
Rayleigh wave dispersion measurements reveal low-velocity zones beneath the new crust in the Gulf of California
Rayleigh wave tomography provides images of the shallow mantle shear wave velocity structure
beneath the Gulf of California. Low-velocity zones (LVZs) are found on axis between 26 and 50 km depth
beneath the Guaymas Basin but mostly off axis under the other rift basins, with the largest feature underlying
the Ballenas Transform Fault. We interpret the broadly distributed LVZs as regions of partial melting in a solid
mantle matrix. The pathway for melt migration and focusing is more complex than an axis-centered source
aligned above a deeper region of mantle melt and likely reflects the magmatic evolution of rift segments.
We also consider the existence of solid lower continental crust in the Gulf north of the Guaymas Basin, where
the association of the LVZs with asthenospheric upwelling suggests lateral flow assisted by a heat source.
These results provide key constraints for numerical models of mantle upwelling and melt focusing in this
young oblique rift
Seismic structure beneath the Gulf of California: a contribution from group velocity measurements
Rayleigh wave group velocity dispersion measurements from local and regional earthquakes are used to interpret the lithospheric structure in the Gulf of California region. We compute group velocity maps for Rayleigh waves from 10 to 150 s using earthquakes recorded by broad-band stations of the Network of Autonomously Recording Seismographs in Baja California and Mexico mainland, UNM in Mexico, BOR, DPP and GOR in southern California and TUC in Arizona. The study area is gridded in 120 longitude cells by 180 latitude cells, with an equal spacing of 10 × 10 km. Assuming that each gridpoint is laterally homogeneous, for each period the tomographic maps are inverted to produce a 3-D lithospheric shear wave velocity model for the region.
Near the Gulf of California rift axis, we found three prominent low shear wave velocity regions, which are associated with mantle upwelling near the Cerro Prieto volcanic field, the Ballenas Transform Fault and the East Pacific Rise. Upwelling of the mantle at lithospheric and asthenospheric depths characterizes most of the Gulf. This more detailed finding is new when compared to previous surface wave studies in the region. A low-velocity zone in northcentral Baja at ∼28ºN which extends east–south–eastwards is interpreted as an asthenospheric window. In addition, we also identify a well-defined high-velocity zone in the upper mantle beneath central-western Baja California, which correlates with the previously interpreted location of the stalled Guadalupe and Magdalena microplates. We interpret locations of the fossil slab and slab window in light of the distribution of unique post-subduction volcanic rocks in the Gulf of California and Baja California. We also observe a high-velocity anomaly at 50-km depth extending down to ∼130 km near the southwestern Baja coastline and beneath Baja, which may represent another remnant of the Farallon slab
Normal faults and thrusts re-activated by deep fluids: the 6 April 2009 Mw 6.3 L’Aquila earthquake, central Italy.
On April 6 2009, a Mw=6.3 earthquake occurred in the central Apennines (Italy) damaging L’Aquila city and the surrounding country. We relocate the October 2008-April 6 2009 foreshocks and about 2000 aftershocks occurred between April 6 and April 30 2009, by applying a double-difference technique and determine the stress field from focal mechanisms. The events concentrate in the upper 15 km of the crust. Three main NW-SE to NNW-SSE striking, 30°-45° and 80°-90° dipping faults activate during the seismic sequence. Among these, a normal fault and a thrust were re-activated with dip-slip movements in response to NE-SW extension. The structural maturity of the seismogenic fault system is lower than that displayed by other systems in southern Apennines, because of the lower strain rate of the central sector of the chain with respect to the southern one. VP/VS increases progressively from October 2008 to the April 6 2009 mainshock occurrence along a NW-SE strike due to an increment in pore fluid pressure along the fault planes. Pore pressure diffusion controls the space-time evolution of aftershocks. A hydraulic diffusivity of 80 m2/s and a seismogenic permeability of about 10-12 m2 suggest the involvement of gas-rich (CO2) fluids within a highly fractured medium. Suprahydrostatic, high fluid pressure (about 200 MPa at 10 km of depth) within overpressurized traps, bounded by pre-existing structural and/or lithological discontinuities at the lower-upper crust boundary, are required to activate the April 2009 sequence. Traps are the storage zone of CO2-rich fluids uprising from the underlying, about 20 km deep, metasomatized mantle wedge. These traps easily occur in extensional regimes like in the axial sector of Apennines, but are difficult to form in strike-slip regimes, where sub-vertical faults may cross the entire crust. In the Apennines, fluids may activate faults responsible for earthquakes up to Mw=5-6. Deep fluids more than tectonic stress may control the seismotectogenesis of accretionary wedges
Mantle wedge dynamics vs crustal seismicity in the Apennines (Italy)
In the Apennines subduction (Italy), earthquakes mainly occur within overriding
plate, along the chain axis. The events concentrate in the upper 15 km of the crust above the
mantle wedge and focal solutions indicate normal faulting. In the foreland, the seismogenic
volume affects the upper 35 km of the crust. Focal solutions indicate prevailing reverse faulting
in the northern foreland and strike-slip faulting in the southern one. The deepening of the
seismogenic volume from the chain axis to the foreland follows the deepening of the Moho and
isotherms. The seismicity above the mantle wedge is associated with uplift of the chain axial
zone, volcanism, high CO2 flux, and extension. The upward pushing of the asthenospheric mantle
and the mantle-derived, CO2-rich fluids trapped within the crust below the chain axis causes this
seismicity. All these features indicate that the axial zone of Apennines is affected by early rifting
processes. In northern Italy, the widespread and deeper seismicity in the foreland reflects active
accretion processes. In the southern foreland, the observed dextral strike-slip faulting and the lack
of reverse focal solutions suggest that accretion processes are not active at present. In our
interpretation of the Apennines subduction, the shallower seismicity of the overriding plate is due
to the dynamics (uprising and eastward migration) of the asthenospheric wedge
The Partial Visibility Representation Extension Problem
For a graph , a function is called a \emph{bar visibility
representation} of when for each vertex , is a
horizontal line segment (\emph{bar}) and iff there is an
unobstructed, vertical, -wide line of sight between and
. Graphs admitting such representations are well understood (via
simple characterizations) and recognizable in linear time. For a directed graph
, a bar visibility representation of , additionally, puts the bar
strictly below the bar for each directed edge of
. We study a generalization of the recognition problem where a function
defined on a subset of is given and the question is whether
there is a bar visibility representation of with for every . We show that for undirected graphs this problem
together with closely related problems are \NP-complete, but for certain cases
involving directed graphs it is solvable in polynomial time.Comment: Appears in the Proceedings of the 24th International Symposium on
Graph Drawing and Network Visualization (GD 2016
Results from the seismological component of CAT/SCAN, the Calabria-Apennine Tyrrhenian/Subduction-Collision-Accretion-Network
The Calabrian Arc is the final remnant of a Western Mediterranean microplate driven by rollback. The Calabrian-Apennine-Tyrrhenian/Subduction-Collision-Accretion Seismic Network (CAT/SCAN) was a passive seismic experiment to study of the Calabrian Arc and its transition to the southern
Apennines. The follow up Calabrian Arc project added a multidisciplinary (seismology, geology, geomorphology,
geochronology, GPS, etc.) approach to better understand the tectonics of southern Italy imaged by the CAT/SCAN experiment. Here we focus on the seismological results of the
two projects.
The CAT/SCAN land deployment consisted of three phases. The initial phase included an array of 39 broadband seismometers onshore, deployed during the winter of 2003/4.
In September 2004, the array was reduced and in April 2005, the array was reduced once again. The field deployment was completed in October 2005. Offshore, 12 broadband Ocean
Bottom Seismometers (OBSs) were deployed in the beginning of October 2004. However, only 1 was recovered normally while several others were recovered after being disturbed by trawling. The experiment goal was to determine the structure of the Calabrian subduction and southern Apennine collision
systems and the structure of the transition from oceanic subduction in Calabria to continental collision in the southern
Apennines.Published7922T. Tettonica attivaN/A or not JCRrestricte
Ambient Seismic Noise Image of the Structurally Controlled Heat and Fluid Feeder Pathway at Campi Flegrei Caldera
The TIDES-COST Action (STSM-ES1401-34011) provided a travel grant to framework the research project. The Japan Society for the Promotion of Science - Short-Term Fellowship (JSPS/OF215/022) financed the work, undertaken at Tohoku University and concluded at the University of Aberdeen. We thank Giuseppe Vilardo and Eliana Bellucci Sessa for providing the geomorphological maps, and Simona Petrosino and Paola Cusano for the P- and S-wave pickings used to localise the seismicity. Informal revisions from Guido Ventura, Nick Rawlinson and Chris Kilburn helped us improving the analyses and interpretation, respectively. We acknowledge the help of Naveed Khan in parallelising the codes and two anonymous reviewers who improved the analysis, interpretation, and readibility with their comments. All data to reproduce the maps can be downloaded from the World Data Center PANGAEA data repository, permanent link https://doi.pangaea.de/10.1594/PANGAEA.890238.Peer reviewedPublisher PD
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