1,721,443 research outputs found
Genesi e modalita' deposizionali delle piroclastiti della Media Valle dell'Aniene ("necks di Vicovaro" Auct.).
No full textI depositi piroclastici della Media Valle dell'Aniene: correlazioni stratigrafiche con i prodotti del Vulcano Laziale
No full textPore-pressure migration along a normal-fault system resolved by time-repeated seismic tomography
No full textIn this study, we outline the space and time variations of body-wave velocities during one of the best-documented series of normal-faulting earthquakes, the 1997 Umbria-Marche sequence in central Italy. We show the fi rst ever observations of rock fracturing and fl uid overpressure propagation along a fault system by using space-time resolved variations of Vp/Vs anomalies (four-dimensional variations) that accompany earthquake migration and precede large aftershocks. The Vp/Vs increase observed before the mid-October earthquakes was related to a pore-pressure increase on fl uid-fi lled cracks in the volume around the fault. We also document that such variations are measurable only by using S- and P-wave arrival times
DIFFERENT APPROACHES IN LOCAL EARTHQUAKE TOMOGRAPHY: AN APPLICATION ON THE ALBAN HILLS VOLCANO (CENTRAL ITALY)
No full textWe compare the results of two different approaches in Local Earthquake Tomography (LET) as applied to the Alban Hills volcano. The first approach is called the graded inversion approach in which we progressively focused towards the center of the volcano while decreasing the model grid spacing in subsequent inversions, optimizing gradually the velocity model. The second approach is called the normal approach in which the seismic arrival times were directly inverted using a 1D velocity model optimally representing the background structure. The target region, i.e. Alban Hills volcano (or Vulcano Laziale) is a Quaternary age volcano laying about 20 km southeast of Rome, Italy. A micro seismic network temporarily installed on and around the volcano during a quake swarm gathered the arrival time data that we used. In the normal approach, we used three different grid spacings (i.e. 2, 1.5, and 1 km) that provided detailed images of the volcano. The resolution analysis carefully performed on the model parameters allowed the determination of a more reliable final model that represented the best results for the velocity structure beneath the volcano. The inversion results attained with the graded approach were not satisfactory, as some small-scale heterogeneities were not properly detected as done by the inversion with the normal approach. The normal approach in fact revealed a horn shaped structure with a high velocity located beneath the volcano and a low velocity anomaly dominated the depths around 1-4 km in western side of the volcano. In general, the graded approach relative to the normal approach failed to detect the fine details of the volcano.Submittedope
Microseismic activity on a low angle normal fault (northern Apennines, Italy). European Geosciences Union.
No full textSeismic coupling for the Aegean - Anatolian region
Full text linkSeismic coupling helps define how large the earthquake potential of a region is, as well as the presence of asperities along plate zones. This work seeks to provide an improved picture of the seismic coupling for the Aegean-Anatolian region by taking advantage of extensive seismic and geodetic datasets. To estimate coupling, we compiled a series of by-products that are specific ingredients also for seismic hazard studies. With these by-products, we found that the seismogenic thickness is thinner (10–15 km) or thicker (20 to 30 km) to the east and to the west, respectively and even deeper along the Hellenic subduction zone. The b-value ranges between 0.9 and 1.1 for the entire area with high values concentrated at locations of Late Miocene to -recent volcanism whereas low b-values (<0.8) concentrate along most of the Northern Anatolian fault zone that may suggests stress accumulation. Seismic coupling is low (<35%) or intermediate (35% - 70%) in most of the area, while the Karliova triple junction, on a N-S-oriented belt along the boundary between western and central Anatolia, and the southeastern Peloponnese are fully coupled, suggesting a full seismic release of the entire deformation budget. An intermediate value of seismic coupling is observed for the eastern and central segments of the Northern and Eastern Anatolian Fault zones, for part of the Hellenic volcanic arc, the Kefalonia Transform Fault and the Corinth gulf active faults. Considering historical earthquake data, these intermediate coupling values indicate either aseismic deformation or catalog incompleteness. Furthermore, the time period since large magnitude earthquakes clearly raises the possibility of impending earthquakes on the Northern and Eastern Anatolian Fault zones. A broad seismic gap is evidenced along the Hellenic subduction zone, because of the reduced coupling and the absence of ~M8 earthquakes in the last 700 years, at least. We conclude that in most of the central Aegean Sea aseismic deformation prevails as suggested by the small value of coupling and the modest seismic release over the last millennium
Minimum 1D velocity models in Central and Southern Italy: a contribution to better constrain hypocentral determinations
No full textWe computed one-dimensional ( I D) velocity models and station corrections for Centrai and Southern Italy, in- verting re-picked P-wave alTival times recorded by the Istituto Nazionale di Geofisica seismic network. The re-picked data yield resolved P-wave velocity results and proved to be more suited than bulletin data for de- tailed tomographic studies. Using the improved velocity models, we relocated the most significant earthquakes which occurt.ed in the Apennines in the past 7 years, achieving constrained hypocentral determinations for events within most of the Apenninic belt. The interpretation of the obtained lD velocity models allows us to infer interesting features on the deep structure of the Apennines. Smooth velocity gradients with depth and low P-wave velocities are ob,'ierved beneath the Apennines. We believe that our results are effective to constrain hypocentral locations in Italy and may represent a first step towards more detailed seismotectonic analyses.JCR Journalope
DIFFERENT APPROACHES IN LOCAL EARTHQUAKE TOMOGRAPHY: AN APPLICATION ON THE ALBAN HILLS VOLCANO (CENTRAL ITALY)
No full textWe compare the results of two different approaches in Local Earthquake Tomography (LET) as applied to the Alban Hills volcano. The first approach is called the graded inversion approach in which we progressively focused towards the center of the volcano while decreasing the model grid spacing in subsequent inversions, optimizing gradually the velocity model. The second approach is called the normal approach in which the seismic arrival times were directly inverted using a 1D velocity model optimally representing the background structure. The target region, i.e. Alban Hills volcano (or Vulcano Laziale) is a Quaternary age volcano laying about 20 km southeast of Rome, Italy. A micro seismic network temporarily installed on and around the volcano during a quake swarm gathered the arrival time data that we used. In the normal approach, we used three different grid spacings (i.e. 2, 1.5, and 1 km) that provided detailed images of the volcano. The resolution analysis carefully performed on the model parameters allowed the determination of a more reliable final model that represented the best results for the velocity structure beneath the volcano. The inversion results attained with the graded approach were not satisfactory, as some small-scale heterogeneities were not properly detected as done by the inversion with the normal approach. The normal approach in fact revealed a horn shaped structure with a high velocity located beneath the volcano and a low velocity anomaly dominated the depths around 1-4 km in western side of the volcano. In general, the graded approach relative to the normal approach failed to detect the fine details of the volcano.Submittedope
Crustal velocity structure of the Apennines (Italy) from P-wave travel time tomography
No full textIn this paper we provide P-wave velocity images of the crust underneath the Apennines (Italy), focusing on the lower crustal structure and the Moho topography. We inverted P-wave arrival times of earthquakes which occurred from 1986 to 1993 within the Apenninic area. To overcome inversion instabilities due to noisy data (we used bulletin data) we decided to resolve a minimum number of velocity parameters, inverting for only two layers in the crust and one in the uppermost mantle underneath the Moho. A partial inversion of only 55% of the overall dataset yields velocity images similar to those obtained with the whole data set, indicating that the depicted tomograms are stable and fairly insensitive to the number of data used. We find a low-velocity anomaly in the lower crust extending underneath the whole Apenninic belt. This feature is segmented by a relative high-velocity zone in correspondence with the Ortona-Roccamonfina line, that separates the northern from the southern Apenninic arcs. The Moho has a variable depth in the study area, and is deeper (more than 37 km) in the Adriatic side of the Northern Apennines with respect to the Tyrrhenian side, where it is found in the depth interval 22-34 km.JCR Journalope
From tomographic images to fault heterogeneities
No full textLocal Earthquake Tomography (LET) is a useful tool for imaging lateral heterogeneities in the upper crust. The pattern of P- and S-wave velocity anomalies, in relation to the seismicity distribution along active fault zones. can shed light on the existence of discrete seismogenic patches. Recent tomographic studies in well monitored seismic areas have shown that the regions with large seismic moment release generally correspond to high velocity zones (HVZ's). In this paper, we discuss the relationship between the seismogenic behavior of faults and the velocity structure of fault zones as inferred from seismic tomography. First, we review some recent tomographic studies in active strike-slip faults. We show examples from different segments of the San Andreas fault system (Parkfield, Loma Prieta), where detailed studies have been carried out in recent years. We also show two applications of LET to thrust faults (Coalinga, Friuli). Then, we focus on the Irpinia normal fault zone (South-Central Italy), where a Ms = 6.9 earthquake occurred in 1980 and many thousands of attershock travel time data are available. We find that earthquake hypocenters concentrate in HVZ's, whereas low velocity zones (LVZ’ s) appear to be relatively aseismic. The main HVZ's along which the mainshock rupture bas propagated may correspond to velocity weakening fault regions, whereas the LVZ's are probably related to weak materials undergoing stable slip (velocity strengthening). A correlation exists between this HVZ and the area with larger coseismic slip along the fault, according to both surface evidence (a fault scarp as high as 1 m) and strong ground motion waveform modeling. Smaller wave-length, low-velocity anomalies detected along the fault may be the expression of velocity strengthening sections, where aseismic slip occurs. According to our results, the rupture at the nucleation depth (~ 10-12 km) is continuous for the whole fault lenoth (~ 30 km), whereas at shallow depth, different fault segments are activated due to lateral heterogeneities in the sedimentary cover. This finding confirms that the rupture process is controlled by lithologic and structural discontinuities in the upper crust, and emphasizes the contribution that LET can make to the study of fault mechanics.JCR Journalope
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