scholarly journals Insights on the seismogenic layer thickness from the upper crust structure of the Umbria-Marche Apennines (central Italy)

Tectonics ◽  
2008 ◽  
Vol 27 (1) ◽  
pp. n/a-n/a ◽  
Author(s):  
F. Mirabella ◽  
M. Barchi ◽  
A. Lupattelli ◽  
E. Stucchi ◽  
M. G. Ciaccio
Keyword(s):  
Layer Thickness ◽  
Central Italy ◽  
Upper Crust ◽  
Crust Structure ◽  
Seismogenic Layer

Lithological and structural control on the seismicity distribution in Central Italy

2020 ◽  
Author(s):  
Massimiliano R. Barchi ◽  
Lauro Chiaraluce ◽  
Cristiano Collettini
Keyword(s):  
Regional Scale ◽  
Central Italy ◽  
Shear Zones ◽  
Geological Structure ◽  
Rock Properties ◽  
Mountain Range ◽  
Upper Crust ◽  
Seismogenic Layer ◽  
Brittle Ductile Transition ◽  
Seismicity Distribution

<p>In the seismically active region of Central Italy, national (permanent) and local (not-permanent) seismic networks provided very accurate location of the seismicity recorded during the major seismic sequences occurred in the last 25 years (e.g. 1997-1998; 2009; 2016-2017), as well as of the background seismicity registered in the intervening periods.  In the same region, a network of seismic reflection profiles, originally acquired for oil exploration purposes, is also available, effectively imaging the geological structure at depth, to be compared with the seismicity distribution. </p><p>This comparison reveals that, if the position of the brittle/ductile transition exerts a role at regional scale for the occurrence of crustal seismicity, at a more local scale the depth and thickness of the seismogenic layer is mostly controlled by the contrasting rheological properties of the different lithological groups involved in the upper crust. </p><p>The upper crust stratigraphy, including the sedimentary cover and the uppermost part of the basement, consists of alternated strong (rigid, e.g. carbonates and dolostones) end weak (not-rigid, e.g. shales, sandstones, and phyllites) layers. This mechanically complex multilayer is involved in a belt of imbricated thrusts (Late Miocene-Early Pliocene), displaced by subsequent extensional (normal) faults (Late Pliocene-present), responsible for the observed regional seismicity. The top of the basement s.l. (composed of clastic sedimentary and slightly metamorphosed rocks) is involved in major thrusts.  For these different lithological units, combined field and lab studies of fault rock properties have documented localized and potentially unstable deformation occurring in granular mineral phases (carbonates) and distributed and stable slip within phyllosilicate-rich shear zones (shales and phyllites).</p><p>By comparing the geological structure with the seismicity distribution, we observed that:</p><p>-     The seismicity cut-off (i.e. the bottom of the seismogenic layer) is structurally (not thermally) controlled, and grossly corresponds to the top basement; the upper boundary of the seismogenic layer corresponds to the top of carbonates.</p><p>-     Most seismicity occurs within the rigid layers (carbonates and evaporites), and do not penetrate the turbidites and basements rocks.</p><p>-      Close to the axial region of the mountain range, where the larger amount of shortening is observed, the presence thrust sheets from the previous compressional phase, significantly affect the seismicity distribution and propagation.</p><p>-     Major east-dipping extensional detachments, recognized in the seismic profiles, are also marked by distinctive seismicity alignments.</p>

scholarly journals From fault creep to slow and fast earthquakes in carbonates

Geology ◽  
2019 ◽  
Vol 47 (8) ◽  
pp. 744-748 ◽  
Author(s):  
Franҫois X. Passelègue ◽  
Jérôme Aubry ◽  
Aurélien Nicolas ◽  
Michele Fondriest ◽  
Damien Deldicque ◽  
...  
Keyword(s):  
Seismic Waves ◽  
Central Italy ◽  
Confining Pressure ◽  
Mediterranean Area ◽  
Rupture Propagation ◽  
Fault Creep ◽  
Seismogenic Layer ◽  
Elastic Loading ◽  
High Frequency Seismic Waves ◽  
Hypocentral Location

Abstract A major part of the seismicity striking the Mediterranean area and other regions worldwide is hosted in carbonate rocks. Recent examples are the destructive earthquakes of L’Aquila (Mw 6.1) in 2009 and Norcia (Mw 6.5) in 2016 in central Italy. Surprisingly, within this region, fast (≈3 km/s) and destructive seismic ruptures coexist with slow (≤10 m/s) and nondestructive rupture phenomena. Despite its relevance for seismic hazard studies, the transition from fault creep to slow and fast seismic rupture propagation is still poorly constrained by seismological and laboratory observations. Here, we reproduced in the laboratory the complete spectrum of natural faulting on samples of dolostones representative of the seismogenic layer in the region. The transitions from fault creep to slow ruptures and from slow to fast ruptures were obtained by increasing both confining pressure (P) and temperature (T) up to conditions encountered at 3–5 km depth (i.e., P = 100 MPa and T = 100 °C), which corresponds to the hypocentral location of slow earthquake swarms and the onset of seismicity in central Italy. The transition from slow to fast rupture is explained by an increase in the ambient temperature, which enhances the elastic loading stiffness of the fault, i.e., the slip velocities during nucleation, allowing flash weakening and, in turn, the propagation of fast ruptures radiating intense high-frequency seismic waves.

Evidence of shear-wave anisotropy in the upper crust of central Italy

1989 ◽  
Vol 79 (6) ◽  
pp. 1905-1912
Author(s):  
G. Iannaccone ◽  
A. Deschamps
Keyword(s):  
Shear Wave ◽  
Cross Correlation ◽  
Seismic Station ◽  
Central Italy ◽  
Upper Crust ◽  
Wave Polarization ◽  
Time Separation ◽  
S Waves ◽  
Digital Network ◽  
Wave Splitting

Abstract Shear-wave polarization analysis has been performed on data from twelve earthquakes recorded in central Italy. These data are part of a set of high quality seismograms recorded by a three-component digital network installed in Abruzzo region after a Ms = 5.8 earthquake on 7 May 1984. Analysis performed on 2 to 6 Hz bandpass-filtered seismograms reveals shear-wave splitting. In order to determine the direction for which the time separation between the two split S waves is maximum, the horizontal traces are rotated in the range 10 to 90° with steps of 10°. At each step the cross-correlation between the two shear waves and the time separation is estimated. The azimuth of 50°N yields the highest correlation coefficient and the maximum time separation of about 0.09 sec. This direction and the orthogonal to it represent axes of minimum and maximum shear-wave velocity respectively. If the time delay is distributed over the whole hypocenter-seismic station path, the maximum variation in velocity is at least 1.4 per cent. The theoretical linear polarization of particle motion at the source is verified after correcting the seismograms for the anisotropic propagation effect. A distribution of vertical cracks aligned in the direction 140°N may explain the observed anisotropy. The stress field deduced from the focal mechanism of the Abruzzo earthquake is compatible with this hypothetical crack distribution.

scholarly journals Upper crust structure in the West of Iran

2017 ◽  
Vol 3 (1) ◽  
pp. 61-82
Author(s):  
Zohreh Sadat Riazi Rad ◽  
Keyword(s):  
Upper Crust ◽  
The West ◽  
Crust Structure

Gravity and Magnetic Modeling of Central Italy: Insights Into the Depth Extent of the Seismogenic Layer

2019 ◽  
Vol 20 (4) ◽  
pp. 2157-2172 ◽  
Author(s):  
P. Mancinelli ◽  
M. Porreca ◽  
C. Pauselli ◽  
G. Minelli ◽  
M.R. Barchi ◽  
...  
Keyword(s):  
Central Italy ◽  
Seismogenic Layer ◽  
Magnetic Modeling ◽  
Gravity And Magnetic ◽  
Depth Extent

scholarly journals Magnetic transfer function entropy and the 2009 <i>M</i><sub>w</sub> = 6.3 L'Aquila earthquake (Central Italy)

2012 ◽  
Vol 19 (4) ◽  
pp. 401-409 ◽  
Author(s):  
G. Cianchini ◽  
A. De Santis ◽  
D. R. Barraclough ◽  
L. X. Wu ◽  
K. Qin
Keyword(s):  
Transfer Function ◽  
Main Shock ◽  
Transfer Functions ◽  
Central Italy ◽  
Magnetic Observatory ◽  
Component Ratio ◽  
Seismic Sequence ◽  
Seismogenic Layer ◽  
Magnetic Transfer ◽  
Physical Phenomena

Abstract. With the aim of obtaining a deeper knowledge of the physical phenomena associated with the 2009 L'Aquila (Central Italy) seismic sequence, culminating with a Mw = 6.3 earthquake on 6 April 2009, and possibly of identifying some kind of earthquake-related magnetic or geoelectric anomaly, we analyse the geomagnetic field components measured at the magnetic observatory of L'Aquila and their variations in time. In particular, trends of magnetic transfer functions in the years 2006–2010 are inspected. They are calculated from the horizontal to vertical magnetic component ratio in the frequency domain, and are very sensitive to deep and lateral geoelectric characteristics of the measurement site. Entropy analysis, carried out from the transfer functions with the so called transfer function entropy, points out clear temporal burst regimes of a few distinct harmonics preceding the main shock of the seismic sequence. A possible explanation is that they could be related to deep fluid migrations and/or to variations in the micro-/meso-fracturing that affected significantly the conductivity (ordered/disordered) distribution in a large lithospheric volume under the seismogenic layer below L'Aquila area. This interpretation is also supported by the analysis of hypocentres depths before the main shock occurrence.

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