A lithospheric velocity anomaly beneath the Shagan river test site. Part 1. Detection and location with network magnitude residuals

1992 ◽  
Vol 82 (2) ◽  
pp. 980-998
Author(s):  
B. W. Barker ◽  
J. R. Murphy
Keyword(s):  
Test Site ◽  
P Wave ◽  
Depth Range ◽  
Back Projection ◽  
Lateral Variation ◽  
Velocity Anomaly ◽  
Large Samples ◽  
P Wave Velocity ◽  
Close Proximity ◽  
River Test

Abstract Large samples of teleseismic P-wave data recorded from Shagan River under-ground explosions have been systematically analyzed in an attempt to understand the causes of mb variability observed for these explosions. Results of these analyses indicate that large (>0.5 magnitude unit) differences in station-corrected mb residuals between explosions in close proximity are associated with changes in the near-source P-wave propagation paths to teleseismic distances. Back projection of the mb residuals from seismic stations in continental Europe into the P-wave initiation area near the source reveals the existence of an anomalous volume of material located about 50 km northwest of the test site, apparently at a depth of about 100 km. This anomalous volume defocuses energy out of paths to certain stations and redirects this energy into the paths to other stations and is associated with no detectable travel-time anomaly. This anomalous volume most likely corresponds to some relatively sharp lateral variation in physical properties, in particular P-wave velocity, at this depth. The estimated lateral dimension of the anomaly is only on the order of 5 km, which would make it one of the smallest deterministic features ever isolated at this depth range.

A lithospheric velocity anomaly beneath the Shagan river Test Site. Part 2. Imaging and inversion with amplitude transmission tomography

1992 ◽  
Vol 82 (2) ◽  
pp. 999-1017
Author(s):  
K. L. McLaughlin ◽  
J. R. Murphy ◽  
B. W. Barker
Keyword(s):  
Fourier Transforms ◽  
Test Site ◽  
Small Scale ◽  
Projection Operators ◽  
Order Partial Differential Equation ◽  
Linear Inversion ◽  
Discrete Fourier Transforms ◽  
Velocity Anomaly ◽  
Inversion Procedure ◽  
River Test

Abstract A linear inversion procedure is introduced that images weak velocity anomalies using amplitudes of transmitted seismic waves. Using projection operators from geometrical ray theory, an image of an anomaly is constructed from amplitudes recorded at arrays of receivers using arrays of sources. The image is related to the velocity anomaly by a second-order partial-differential equation that is inverted using 2-D discrete Fourier transforms. As an example of the inversion procedure, magnitude residuals for European stations recording Shagan River explosions are used to image the deep lithospheric anomaly beneath the Shagan River test site described in Part 1. This formal inversion analysis confirms the existence of a small-scale lateral heterogeneity located 50 km west-northwest of the test site at a probable depth between 80 and 100 km and indicates that it is consistent with a deterministic 1.5% peak-to-peak (or 0.5% rms) velocity anomaly with a scale length of about 3 km. 3-D dynamic raytracing is then used to verify that the inferred laterally varying structure produces amplitude fluctuations consistent with observations.

The Australian Plate and underlying mantle from waveform tomography with massive datasets

2021 ◽  
Author(s):  
Janneke de Laat ◽  
Sergei Lebedev ◽  
Bruna Chagas de Melo ◽  
Nicolas Celli ◽  
Raffaele Bonadio
Keyword(s):  
Structural Information ◽  
Azimuthal Anisotropy ◽  
P Wave ◽  
Depth Range ◽  
S Wave ◽  
Crust And Upper Mantle ◽  
Southern Boundary ◽  
Velocity Anomaly ◽  
Australian Plate ◽  
Australian Continent

<p>We present a new S-wave velocity tomographic model of the Australian Plate, Aus21.  It is constrained by waveforms of 0.9 million seismograms with both the corresponding sources and stations located within the half-hemisphere centred at the Australian continent. Waveform inversion extracts structural information from surface, S- and multiple S-waves on the seismograms in the form of a set of linear equations. These equations are then combined in a large linear system and inverted jointly to obtain a tomographic model of S- and P-wave speeds and S-wave azimuthal anisotropy of the crust and upper mantle. The model has been validated by resolution tests and, for particular locations in Australia with notable differences with previous models, by independent inter-station measurements of surface-wave phase velocities, which we performed using available array data. </p><p> </p><p>Aus21 offers new insights into the structure and evolution of the Australian Plate and its boundaries. The Australian cratonic lithosphere occupies nearly all of the western and central Australia but shows substantial lateral heterogeneity. It extends up to the northern edge of the plate, where it is colliding with island arcs, without subducting. The rugged eastern boundary of the cratonic lithosphere provides a lithospheric definition of the Tasman Line. The thin, warm lithosphere below the eastern part of the continent, east of the Tasman Line, underlies the Cenozoic volcanism locations in the area. The lithosphere is also thin and warm below much of the Tasman Sea, underlying the Lord Howe hotspot and the submerged part of western Zealandia. A low velocity anomaly that may indicate the single source of the Lord Howe and Tasmanid hotspots is observed in the transition zone offshore the Australian continent, possibly also sourcing the East Australia hotspot. Another potential hotspot source is identified below the Kermadec Trench, causing an apparent slab gap in the overlying slab and possibly related to the Samoa Hotspot to the north. Below a portion of the South East Indian Ridge (the southern boundary of the Australian Plate) a pronounced high velocity anomaly is present in the 200-400 km depth range just east of the Australian-Antarctic Discordance (AAD), probably linked to the evolution of this chaotic ridge system.</p>

scholarly journals Rapid lateral variation of P-wave velocity at the base of the mantle near the edge of the Large-Low Shear Velocity Province beneath the western Pacific

2014 ◽  
Vol 200 (2) ◽  
pp. 1052-1065 ◽  
Author(s):  
Satoru Tanaka ◽  
Hitoshi Kawakatsu ◽  
Masayuki Obayashi ◽  
Y. John Chen ◽  
Jieyuan Ning ◽  
...  
Keyword(s):  
Wave Velocity ◽  
Shear Velocity ◽  
Western Pacific ◽  
P Wave ◽  
Lateral Variation ◽  
P Wave Velocity ◽  
The Western Pacific ◽  
Low Shear

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.

P-Wave Residuals in Western Canada

1975 ◽  
Vol 12 (2) ◽  
pp. 174-181
Author(s):  
E. J. Roebroek ◽  
E. Nyland
Keyword(s):  
United States ◽  
Wave Velocity ◽  
Upper Mantle ◽  
Velocity Structure ◽  
The United States ◽  
P Wave ◽  
Western Canada ◽  
Lateral Variation ◽  
P Wave Velocity ◽  
P Wave Velocity Structure

A study of over 1500 P-wave phases which pass, for the most part, through the upper mantle beneath Western Canada, shows that there is no significant lateral variation in P-wave velocity structure beneath Western Canada, and that the Wiggins and Helmberger model for the Western regions of the United States does not fit the data for Western Canada.One possible interpretation of the data for Western Canada is closer to the classical Herrin model of P-wave velocity as a function of depth than it is to the Wiggins and Helmberger A model.

Seismological studies in a rock body at Chalk River, Ontario and their relation to fractures

1982 ◽  
Vol 19 (8) ◽  
pp. 1535-1547 ◽  
Author(s):  
C. Wright
Keyword(s):  
Wave Velocity ◽  
Test Site ◽  
P Wave ◽  
Seismic Velocities ◽  
S Wave ◽  
P Waves ◽  
Rock Body ◽  
Near Surface ◽  
Wave Velocities ◽  
P Wave Velocity

Seismological experiments have been undertaken at a test site near Chalk River, Ontario that consists of crystalline rocks covered by glacial sediments. Near-surface P and S wave velocity and amplitude variations have been measured along profiles less than 2 km in length. The P and S wave velocities were generally in the range 4.5–5.6 and 2.9–3.2 km/s, respectively. These results are consistent with propagation through fractured gneiss and monzonite, which form the bulk of the rock body. The P wave velocity falls below 5.0 km/s in a region where there is a major fault and in an area of high electrical conductivity; such velocity minima are therefore associated with fracture systems. For some paths, the P and 5 wave velocities were in the ranges 6.2–6.6 and 3.7–4.1 km/s, respectively, showing the presence of thin sheets of gabbro. Temporal changes in P travel times of up to 1.4% over a 12 h period were observed where the sediment cover was thickest. The cause may be changes in the water table. The absence of polarized SH arrivals from specially designed shear wave sources indicates the inhomogeneity of the test site. A Q value of 243 ± 53 for P waves was derived over one relatively homogeneous profile of about 600 m length. P wave velocity minima measured between depths of 25 and 250 m in a borehole correlate well with the distribution of fractures inferred from optical examination of borehole cores, laboratory measurements of seismic velocities, and tube wave studies.

Sign in / Sign up

Export Citation Format

Share Document