scholarly journals Crustal velocity structure of the Apennines (Italy) from P-wave travel time tomography

1996 ◽  
Vol 39 (6) ◽  
Author(s):  
C. Chiarabba ◽  
A. Amato
Keyword(s):  
Velocity Structure ◽  
P Wave ◽  
Depth Interval ◽  
Low Velocity ◽  
Data Set ◽  
Arrival Times ◽  
Velocity Anomaly ◽  
Minimum Number ◽  
Velocity Images ◽  
Lower Crustal

In 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.

scholarly journals P-wave crustal tomography of Greece with use of an accurate two-point ray tracer

1997 ◽  
Vol 40 (1) ◽  
Author(s):  
G. Drakatos ◽  
G. Karantonis ◽  
G. N. Stavrakakis
Keyword(s):  
Large Scale ◽  
Velocity Structure ◽  
Three Dimensional ◽  
Seismic Energy ◽  
Aegean Sea ◽  
P Wave ◽  
Low Velocity ◽  
Aegean Area ◽  
Arrival Times ◽  
Horizontal Variations

The three-dimensional velocity structure of the crust in the Aegean sea and the surrounding regions (34.0º-42.OºN, 19.0ºE-29.0ºE) is investigated by inversion of about 10000 residuals of arrival times of P-wave from local events. The resulting velocity structure shows strong horizontal variations due to the complicated crustal structure and the variations of crustal thickness. The northern part of the region generally shows high velocities. In the inner part of the volcanic arc (Southern Aegean area), relatively low velocities are observed, suggesting a large-scale absorption of seismic energy as confirmed by the low seismicity of the region. A low velocity zone was observed along the subduction zone of the region, up to a depth of 4 km. The existence of such a zone could be due to granitic or other intrusions in the crust during the uplift of the region during Alpidic orogenesis.

Seismic detection of the low-velocity anomaly at the crust and uppermost mantle beneath the central Tien Shan

2020 ◽  
Author(s):  
Ran Cui ◽  
Yuanze Zhou
Keyword(s):  
Tarim Basin ◽  
Velocity Structure ◽  
Orogenic Belt ◽  
Tien Shan ◽  
P Wave ◽  
Uppermost Mantle ◽  
Low Velocity ◽  
Velocity Anomaly ◽  
Short Period ◽  
Seismic Detection

<p>As one of the most active intracontinental orogenic belts in the world, the Tien Shan orogenic belt originated in the Paleozoic and then experienced tectonic activities such as plate subduction and closure of the Paleo-Asian Ocean. Previous seismological and geodynamic studies have shown the observed the low-velocity anomaly (LVA) beneath the central Tien Shan at the uppermost mantle, which has a significant influence on the formation and modification of the crust and mantle lithosphere ( Lei et al, 2007). However, the distribution, morphology and physical property of the LVA are highly debatable.</p><p>We conduct 2-D forward waveform modeling based on spectral-element method (SEM) to investigate waveform distortions that were generated by the velocity contrast boundary of the LAV. The broadband P- and S- waves from three intermediate-depth earthquakes at Hindu Kush-Pamir were recorded by the Chinese Digital Seismograph Network (Zheng et al., 2010). We use these records to confirm the location, shape and velocity decrement of the LVA by fitting the observed records with the synthetics through SEM based on the 1D velocity structures (TSTB-B) of the central Tien Shan and northern Tarim basin (Gao et al., 2017). We find the LVA at 10~100 km beneath the eastern part of the central Tien Shan. And the northward under-thrusting of the Tarim Basin may trigger some mantle upwelling, contributing to the observed LVA.</p><p>Lei, J., Zhao, D. (2007). Teleseismic P-wave tomography and the upper mantle structure of the central Tien Shan orogenic belt.<em> Physics of the Earth and Planetary Interiors</em>, 162, 165-185, doi: 10.1016/j.pepi.200704010.</p><p>Zheng, X., Jiao, W., Zhang, C., et al. (2010). Short-Period Rayleigh-Wave Group Velocity Tomography through Ambient Noise Cross-Correlation in Xinjiang, Northwest China.<em> Bulletin of the Seismological Society of America</em>, 100(3): 1350-1355, doi: 10.1785/0120090225.</p><p>Gao, Y., Cui, Q., Zhou, Y. (2017). Seismic detection of P-wave velocity structure atop MTZ beneath the Central Tian Shan and Tarim Basin. <em>Chinese Journal of Geophysics ( in Chinese with English Abstract )</em>, 60 (1) : 98-111, doi: 10.6038 /cjg20170109.</p>

P-wave velocity structure of the Earth's crust beneath Vancouver Island

1983 ◽  
Vol 20 (5) ◽  
pp. 742-752 ◽  
Author(s):  
George A. McMechan ◽  
George D. Spence
Keyword(s):  
Velocity Structure ◽  
General Structure ◽  
Vertical Component ◽  
Vancouver Island ◽  
P Wave ◽  
Low Velocity ◽  
Velocity Anomaly ◽  
P Wave Velocity ◽  
Refraction Data ◽  
Velocity Zone

Refraction data were recorded from three shot points out to a maximum distance of ~330 km as part of the 1980 Vancouver Island Seismic Project (VISP80). These vertical component data are partially reversed and so can be interpreted in terms of two-dimensional structures by iterative modeling of P-wave travel times and amplitudes. The structure of the upper crust is the best constrained part of the model. It consists, generally, of a gradually increasing velocity from ~5.3 km/s at the surface to ~6.4 km/s at 2 km depth to ~6.75 km/s at 15.5 km depth, where the velocity increases sharply to ~7 km/s. Below ~20 km depth, the model becomes speculative because the data provide only indirect constraints on velocities at these depths. An interpretation that fits the observed times and amplitudes has a low velocity zone in the lower crust and a Moho at 37 km depth. The only significant departure from this general structure is beneath the central part of Vancouver Island where the 15.5 km boundary in the model attains a depth of ~23 km, below which there appears to be a local high velocity anomaly.

scholarly journals Tomographic image of the crust and uppermost mantle of the Ionian and Aegean regions

1997 ◽  
Vol 40 (1) ◽  
Author(s):  
B. Alessandrini ◽  
L. Beranzoli ◽  
G. Drakatos ◽  
C. Falcone ◽  
G. Karantonis ◽  
...  
Keyword(s):  
Wave Velocity ◽  
Mediterranean Area ◽  
P Wave ◽  
Moho Depth ◽  
Uppermost Mantle ◽  
Low Velocity ◽  
Central Mediterranean ◽  
Data Set ◽  
Velocity Anomaly ◽  
P Wave Velocity

We present a tomographic view of the crust and uppermost mantle beneath the Central Mediterranean area obtained from P-wave arrival times of regional earthquakes selected from the ISC bulletin. The P-wave velocity anomalies are obtained using Thurber's algorithm that jointly relocates earthquakes and computes velocity adjustments with respect to a starting model. A specific algorithm has been applied to achieve a distribution of epicentres as even as possible. A data set of 1009 events and 49072 Pg and Pn phases was selected. We find a low velocity belt in the crust, evident in the map view at 25 km of depth, beneath the Hellenic arc. A low velocity anomaly extends at 40 km of depth under the Aegean back arc basin. High velocities are present at Moho depth beneath the Ionian sea close to the Calabrian and Aegean arcs. The tomographic images suggest a close relationship between P-wave velocity pattern and the subduction systems of the studied area.

The Study of Crustal Velocity Structure Characteristics in Yangjiang Area of South China

2020 ◽  
Author(s):  
Xiaona Wang ◽  
Zhihui Deng ◽  
Xiuwei Ye ◽  
Liwei Wang
Keyword(s):  
South China ◽  
Seismic Data ◽  
Velocity Structure ◽  
Seismic Network ◽  
P Wave ◽  
Adjacent Area ◽  
Low Velocity ◽  
Arrival Times ◽  
P Wave Velocity ◽  
Buried Fault

<p>This paper collects 43,225 absolute first arrival P wave arrival times and 422,956 high quality relative P arrival times of 6,390 events occurred in Yangjiang and its adjacent area from Jan, 1990 to Aug, 2019, these seismic data is recorded by 49 stations from Guangdong seismic network, Guangxi seismic network and Hainan seismic network. Based on the seismic data above, we simultaneously determine the crustal 3D P wave velocity structure and the hypocenter parameters of 6255 events in Yangjiang and its adjacent area by applying Double-Difference seismic tomography. The result shows that, shallow P wave velocity in Yangjiang area is higher due to the thinner sedimentary layer and widely exposed Yanshanian granite, Indosinian granite and Cambrian metamorphic rocks. There are obvious correspondences between the distribution of shallow velocity and fault structure as well as geological structure. A wide range of low velocity anomaly exists in 20km depth, which verifies the low velocity layer in the middle crust at Yangjiang area of South China continent. The velocity image from land to ocean in 30km depth shows low velocity in NW side and high velocity in SE side, which verifies the characteristic of crust thinning in South China coastal continent. The NEE seismic belt from Yangbianhai to Pinggang is speculated to locate in a buried fault of southwest segment of Pinggang fault. The buried thrust fault is a N78°E strike fault, dip to NW with a dip angle of 85 °. In addition, the buried fault locates in the abnormal junction of high velocity on the NW side and low velocity on the SE side, which reflects the tectonic activity characteristic of NW plate uplifting and SE plate declining from Miocene period. The characteristic of activity in the buried fault shows thrust movement with a small strike slip component, which is consistent with the focal mechanism of M4.9 earthquake occurred in 2004.</p>

Simultaneous inversion for earthquake location and velocity structure beneath central Costa Rica

1996 ◽  
Vol 86 (1A) ◽  
pp. 19-31 ◽  
Author(s):  
Marino Protti ◽  
Susan Y. Schwartz ◽  
George Zandt
Keyword(s):  
Costa Rica ◽  
Velocity Structure ◽  
P Wave ◽  
Inversion Method ◽  
Cocos Plate ◽  
Low Velocity ◽  
Arrival Times ◽  
Simultaneous Inversion ◽  
Late Tertiary ◽  
Shallow Crust

Abstract We have imaged the complex crustal and upper mantle structure beneath central Costa Rica using P-wave arrival times from locally recorded earthquakes. Thurber's (1983) iterative inversion method is used to simultaneously estimate velocities along a three-dimensional grid and hypocentral parameters of local earthquakes. Our data consist of over 12,000 arrival times from more than 1300 earthquakes recorded by stations of a permanent seismographic network in Costa Rica. Our resulting velocity model correlates well with mapped geologic units at very shallow depth and with tectonic features at greater depth. We find low velocities (4.0 to 4.8 km/sec) in the shallow crust (above 10 km) near the active volcanoes and associated with a NW-SE trending late Cretaceous to late Tertiary sedimentary basin southeast of Herradura peninsula. High velocities (5.4 to 5.7 km/sec) in the shallow crust correlate with outcrops of late Jurassic to early Tertiary ultramafic ophiolitic units and with basic Tertiary volcanic units. At depths between 20 and 30 km, high velocities (6.8 to 7.2 km/sec) are associated with the subducting Cocos plate under Costa Rica and two prominent low-velocity bodies (6.3 to 6.5 km/sec) are present about 30 km trenchward of the volcanic arc and along the projection of the aseismic Cocos Ridge as it subducts beneath Costa Rica. The thickened oceanic crust of the Cocos Ridge is most likely responsible for its low velocities. The deep low-velocity anomaly located trenchward of the axis of the volcanoes may indicate the presence of a low-density intrusive resulting from an earlier phase of magmatism, possibly the late Miocene episode that produced the Talamanca intrusive complex.

scholarly journals Teleseismic P-residual study in the Italian region - inferences on large scale anisotropic structure of the subcrustal lithosphere

1998 ◽  
Vol 41 (1) ◽  
Author(s):  
J. Plomerová ◽  
V. Babuska ◽  
R. Scarpa
Keyword(s):  
Travel Time ◽  
Upper Mantle ◽  
Large Scale ◽  
Velocity Structure ◽  
P Wave ◽  
Anisotropic Structure ◽  
Low Velocity ◽  
Arrival Times ◽  
Station Corrections ◽  
Lateral Variations

Jeffreys-Bullen (absolute) and relative P-wave travel-time residuals were analyzed over Italy and its surrounding using P arrival times from the ISC bulletins supplemented by the data from local observatories. We analyzed the travel-time station corrections by two independent methods to obtain information on lateral variations of the velocity structure over the area and a view of possible upper mantle anisotropy. In the first method, the station corrections are computed as a constant and two cosine terms with appropriate phase shifts. Besides a static term, the second method allows us to study the relative residuals in dependence both on azimuths and incidence angles and thus to investigate their spatial variations and to map lateral variations of anisotropic structure of the subcrustal lithosphere. The high and low-velocity directions inferred from the spatial distribution of the relative residuals as well as the high- and low-velocity upper mantle heterogeneities reflect the geodynamic development of the region, governed by the collision between the African and Eurasian plates

Structure beneath Queen Charlotte Sound from seismic-refraction and gravity interpretations

1992 ◽  
Vol 29 (7) ◽  
pp. 1509-1529 ◽  
Author(s):  
Tianson Yuan ◽  
G. D. Spence ◽  
R. D. Hyndman
Keyword(s):  
Crustal Structure ◽  
Velocity Structure ◽  
Sedimentary Basin ◽  
Single Channel ◽  
Velocity Variation ◽  
Moho Depth ◽  
High Porosity ◽  
Upper Crust ◽  
Low Velocity ◽  
Data Set

A combined multichannel seismic reflection and refraction survey was carried out in July 1988 to study the Tertiary sedimentary basin architecture and formation and to define the crustal structure and associated plate interactions in the Queen Charlotte Islands region. Simultaneously with the collection of the multichannel reflection data, refractions and wide-angle reflections from the airgun array shots were recorded on single-channel seismographs distributed on land around Hecate Strait and Queen Charlotte Sound. For this paper a subset of the resulting data set was chosen to study the crustal structure in Queen Charlotte Sound and the nearby subduction zone.Two-dimensional ray tracing and synthetic seismogram modelling produced a velocity structure model in Queen Charlotte Sound. On a margin-parallel line, Moho depth was modelled at 27 km off southern Moresby Island but only 23 km north of Vancouver Island. Excluding the approximately 5 km of the Tertiary sediments, the crust in the latter area is only about 18 km thick, suggesting substantial crustal thinning in Queen Charlotte Sound. Such thinning of the crust supports an extensional mechanism for the origin of the sedimentary basin. Deep crustal layers with velocities of more than 7 km/s were interpreted in the southern portion of Queen Charlotte Sound and beneath the continental margin. They could represent high-velocity material emplaced in the crust from earlier subduction episodes or mafic intrusion associated with the Tertiary volcanics.Seismic velocities of both sediment and upper crust layers are lower in the southern part of Queen Charlotte Sound than in the region near Moresby Island. Well velocity logs indicate a similar velocity variation. Gravity modelling along the survey line parallel to the margin provides additional constraints on the structure. The data require lower densities in the sediment and upper crust of southern Queen Charlotte Sound. The low-velocity, low-density sediments in the south correspond to high-porosity marine sediments found in wells in that region and contrast with lower porosity nonmarine sediments in wells farther north.

Three-dimensional seismic structure of the lithosphere under Montana Lasa

1976 ◽  
Vol 66 (2) ◽  
pp. 501-524
Author(s):  
Keiiti Aki ◽  
Anders Christoffersson ◽  
Eystein S. Husebye
Keyword(s):  
Generalized Inverse ◽  
Random Medium ◽  
Three Dimensional ◽  
P Wave ◽  
Seismic Structure ◽  
Low Velocity ◽  
Medium Theory ◽  
Velocity Anomaly ◽  
Inversion Techniques ◽  
Teleseismic Events

abstract Using P-wave residuals for teleseismic events observed at the Montana Large Aperture Seismic Array (LASA), we have determined the three-dimensional seismic structure of the lithosphere under the array to a depth of 140 km. The root-mean-square velocity fluctuation was found to be at least 3.2 per cent which may be compared to estimate of ca. 2 per cent based on the Chernov random medium theory. The solutions are given by both the generalized inverse and stochastic inverse methods in order to demonstrate the relative merit of different inversion techniques. The most conspicuous feature of the lithosphere under LASA is a low-velocity anomaly in the central and northeast part of the array siting area with the N60°E trend and persisting from the upper crust to depths greater than 100 km. We interpret this low-velocity anomaly as a zone of weakness caused by faulting and shearing associated with the building of the Rocky Mountains.

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