scholarly journals Project Tor: Deep lithospheric variation across the Sorgenfrei-Tornquist Zone, Southern Scandinavia

1999 ◽  
Vol 46 ◽  
pp. 13-24
Author(s):  
Trine Pedersen ◽  
Søren Gregersen
Keyword(s):  
Upper Mantle ◽  
Surface Expression ◽  
P Wave ◽  
Northern Germany ◽  
Rectangular Array ◽  
Teleseismic Tomography ◽  
North East ◽  
The North ◽  
P Wave Velocity ◽  
Southern Scandinavia

The Tor project makes use of teleseismic tomography across the Sorgenfrei-Tornquist Zone and has now revealed significant variations in the deep lithosphere under northern Germany, Denmark and southern Sweden. Here we present the first interpretations of P-wave traveltime anomalies from the Tor project. The project utilised 120 seismographs placed in a rectangular array, the largest seismic antenna so far used in Europe, for half a year in the period 1996–1997. The present investigation establishes a 3D crustal/upper mantle model of the P-wave velocity based on existing data. A picture of the crustal influence on the seismic P-wave rays is established by ray tracing through the model. When this is subtracted from that observed by the Tor array, a picture of the influence of the lower lithosphere/asthenosphere system emerges. For several earthquakes it is shown that the observed P-wave traveltime anomalies of nearly 2 seconds can be divided almost equally between known crustal effects and lower lithosphere/asthenosphere differences. The transition appears gradual from most directions but for rays coming from the north-east direction the transition appears sharper. This means that the broad scale deep lithosphere transition is gradual with the sharpest discontinuity plane dipping down steeply in a north-easterly direction from the Sorgenfrei-Tornquist Zone. Based on existing knowledge of the area we conclude that the transition from thin to thick lithosphere occurs within a short distance, and that the lithosphere/asthenosphere boundary dips steeply down from the surface expression of the Sorgenfrei-Tornquist Zone.

An amplitude constrained P-wave velocity profile for the upper mantle beneath the Eastern United States

1979 ◽  
Vol 69 (6) ◽  
pp. 1733-1744
Author(s):  
George A. McMechan
Keyword(s):  
United States ◽  
Velocity Profile ◽  
Wave Velocity ◽  
Upper Mantle ◽  
Epicentral Distance ◽  
P Wave ◽  
Eastern United States ◽  
Lateral Velocity ◽  
The North ◽  
P Wave Velocity

abstract A P-wave velocity profile for the upper mantle at depths between 200 and 800 km beneath Eastern United States has been constructed from a combination of data from natural and artificial sources. Data for this part of the upper mantle are scarce, particularly beyond 20° epicentral distance, because of the sparse distribution of relevant sources and stations. Nevertheless, this study is the first to use amplitude constraints in a model determination for this region, and the model that has been chosen can account for the main observed amplitude features as well as travel times. The resulting velocity profile is similar to those previously determined for the regions to the north and west, but has a broadening of velocity transitions relative to those in the western United States. Evidence is found for the existence of lateral velocity inhomogeneity within the mantle.

scholarly journals Insights from the P Wave Travel Time Tomography in the Upper Mantle Beneath the Central Philippines

2021 ◽  
Vol 13 (13) ◽  
pp. 2449
Author(s):  
Huiyan Shi ◽  
Tonglin Li ◽  
Rui Sun ◽  
Gongbo Zhang ◽  
Rongzhe Zhang ◽  
...  
Keyword(s):  
Travel Time ◽  
Upper Mantle ◽  
High Velocity ◽  
Velocity Structure ◽  
The Philippines ◽  
P Wave ◽  
The South ◽  
Teleseismic Tomography ◽  
The North ◽  
Velocity Anomalies

In this paper, we present a high resolution 3-D tomographic model of the upper mantle obtained from a large number of teleseismic travel time data from the ISC in the central Philippines. There are 2921 teleseismic events and 32,224 useful relative travel time residuals picked to compute the velocity structure in the upper mantle, which was recorded by 87 receivers and satisfied the requirements of teleseismic tomography. Crustal correction was conducted to these data before inversion. The fast-marching method (FMM) and a subspace method were adopted in the forward step and inversion step, respectively. The present tomographic model clearly images steeply subducting high velocity anomalies along the Manila trench in the South China Sea (SCS), which reveals a gradual changing of the subduction angle and a gradual shallowing of the subduction depth from the north to the south. It is speculated that the change in its subduction depth and angle indicates the cessation of the SCS spreading from the north to the south, which also implies that the northern part of the SCS opened earlier than the southern part. Subduction of the Philippine Sea (PS) plate is exhibited between 14° N and 9° N, with its subduction direction changing from westward to eastward near 13° N. In the range of 11° N–9° N, the subduction of the Sulu Sea (SS) lies on the west side of PS plate. It is notable that obvious high velocity anomalies are imaged in the mantle transition zone (MTZ) between 14° N and 9° N, which are identified as the proto-SCS (PSCS) slabs and paleo-Pacific (PP) plate. It extends the location of the paleo-suture of PSCS-PP eastward from Borneo to the Philippines, which should be considered in studying the mechanism of the SCS and the tectonic evolution in SE Asia.

Deep velocity image of the north Zagros collision zone (Iran) from regional and teleseismic tomography

2019 ◽  
Vol 219 (3) ◽  
pp. 1729-1740 ◽  
Author(s):  
M Rahmani ◽  
K Motaghi ◽  
A Ghods ◽  
F Sobouti ◽  
M Talebian ◽  
...  
Keyword(s):  
Upper Mantle ◽  
High Velocity ◽  
Velocity Structure ◽  
Central Iran ◽  
P Wave ◽  
Arabian Plate ◽  
Zagros Mountains ◽  
Teleseismic Tomography ◽  
The North ◽  
Collision Zone

SUMMARY We inverted 3555 regional and teleseismic P-wave relative time residuals to resolve deep velocity structure beneath the NW part of the Zagros collision zone. The data were gathered by 46 seismic stations installed along a ∼520-km-long seismic profile crossing the Zagros Mountains, Central Iran and the western Alborz Mountains. The obtained tomogram reveals a high velocity lithospheric root beneath the Zagros Mountains and a low velocity wedge in the frontal edge of the Arabian Plate beneath the suture that might be interpreted as beginning of delamination of lower part of the Arabian mantle lithosphere from its upper part. A significant deep (depth >350 km) high velocity feature is observed in the lower part of the upper mantle to the north of the Zagros suture and beneath Central Iran. We interpret this feature as the remains of oceanic slab of the Neotethys lying in the lower portion of the upper mantle and the transition zone.

Preliminary results from teleseismic tomography of the upper mantle beneath northern Borneo

2020 ◽  
Author(s):  
Simone Pilia ◽  
Nick Rawlinson ◽  
Felix Tongkul ◽  
Amy Gilligan ◽  
Dave Cornwell
Keyword(s):  
Upper Mantle ◽  
Receiver Function ◽  
Function Analysis ◽  
P Wave ◽  
The West ◽  
Tomographic Images ◽  
Teleseismic Tomography ◽  
North East ◽  
Receiver Function Analysis ◽  
Entire Network

<p>We present preliminary P-wave tomographic images of the upper mantle beneath northern Borneo (Sabah) using teleseismic earthquake data. Sabah underwent diachronous double-polarity subduction, one dipping to the southeast (terminated in the early Miocene) and the other to the northwest (terminated 5-6 Ma). With the goal of better understanding post-subduction processes in Sabah, 24 permanent seismic stations of MetMalaysia were augmented by the deployment of 46 temporary stations of the nBOSS network, which ran from March 2018 to January 2020. Relative P-wave traveltime residuals from nearly a thousand teleseismic events have been extracted from the continuous records using an adaptive stacking technique, which uses the coherency of global phases across the entire network. Using a grid-based eikonal solver and a subspace inversion technique implemented in FMTOMO, relative arrival time residuals are mapped as 3-D P-wave perturbations.</p><p>The most intriguing feature of the final tomographic model is a north-east trending lithospheric structure running across northern Borneo and separating relatively low to high wavespeeds to the west and east, respectively. This structure possibly indicates the suture between pre-Cenozoic lithosphere to the east and the Cenozoic accreted material to the west.</p><p>Results from receiver function analysis (i.e., crustal thickness) and crustal velocities from ambient noise tomography will be in the future incorporated in the tomographic inversion in order to obtain an integrated view of the crust-mantle system beneath Sabah.</p>

P-wave travel times from shallow earthquakes recorded in India and inferred upper mantle structure

1968 ◽  
Vol 58 (6) ◽  
pp. 1879-1897
Author(s):  
K. L. Kaila ◽  
P. R. Reddy ◽  
Hari Narain
Keyword(s):  
Upper Mantle ◽  
P Wave ◽  
Mantle Structure ◽  
Travel Times ◽  
The North ◽  
Shallow Earthquakes ◽  
Upper Mantle Structure ◽  
Wave Travel ◽  
Excess Value ◽  
Velocity Changes

ABSTRACT P-wave travel times of 39 shallow earthquakes and three nuclear explosions with epicenters in the North in Himalayas, Tibet, China and USSR as recorded in Indian observatories have been analyzed statistically by the method of weighting observations. The travel times from Δ = 2° to 50° can be represented by four straight line segments indicating abrupt velocity changes around 19°, 22° and 33° respectively. The P-wave velocity at the top of the mantle has been found to be 8.31 ± 0.02 km/sec. Inferred upper mantle structure reveals three velocity discontinuities in the upper mantle at depths (below the crust) of 380 ± 20, 580 ± 50 and 1000 ± 120 km with velocities below the discontinuities as 9.47 ± 0.06, 10.15 ± 0.07 and 11.40 ± 0.08 km/sec respectively. The J-B residuals up to Δ = 19° are mostly negative varying from 1 to 10 seconds with a dependence on Δ values indicating a different upper mantle velocity in the Himalayan region as compared to that used by Jeffreys-Bullen in their tables (1940). Between 19° to 33° there is a reasonably good agreement between the J-B curve and the observation points. From Δ = 33° to 50° the J-B residuals are mostly positive with an average excess value of about 4 sec.

The shape and infill of the Basadingen overdeepened glacial valley from P-wave seismic reflections

2021 ◽  
Author(s):  
Anna-Catharina Brandt ◽  
David C. Tanner ◽  
Hermann Buness ◽  
Thomas Burschil ◽  
Gerald Gabriel
Keyword(s):  
P Wave ◽  
Borehole Data ◽  
North East ◽  
The North ◽  
Glacier Valley ◽  
Sedimentary System ◽  
Climatic History ◽  
The Alps ◽  
Seismic Reflections ◽  
Seismic Images

<p><span>Overdeepened valleys in the Alps allow to probe the glacial sedimentation record, which in turn can illuminate the climatic history. In particular, seismic reflections can be used to extend punctual borehole data (for instance a number of boreholes are to be drilled into Alpine glacial overdeepened valleys as part of the DOVE ICDP project) in the second dimension or even survey a region before drilling begins. Thus, we use detailed, 2-D seismic P-wave profiles to reveal the shape and infill of an overdeepened Rhine glacier valley in the area of Basadingen, near to the German/Swiss border. We acquired two profiles nearly perpendicular to the valley strike, approximately 500 m apart. The first profile was 1246 m long, and consisted of a single spread of 624 geophones. The second profile was 1120 m long and was acquired using 200 3-component geophones using a roll-along method. For both profiles we used a vibro-source with a 12 s linear sweep of 20-240 Hz at every second geophone (two metre spacing), which produced a high fold.</span></p><p><span>Both seismic images reveal that the overdeepened basin at this location is asymmetrical and circa 260 m deep, although the deepest part (220</span><span> </span><span>m wide) covers only a small portion of the broader main valley. The infill is characterised by at least three unconformities and distinct onlap and erosive boundaries between the sedimentary units. We interpret the infill to represent a highly dynamic sedimentary system. The lower part, within the deepest part of the basin is filled with chaotic sediments and slumping. Above a major unconformity, the upper part contains strongly-dipping reflectors that probably represent a prograding point-bar in a glacio-fluviatile environment that migrated toward the north-east. Beneath the deepest part of the basin we see evidence for faults in the Tertiary Molasse basement, which correlate with known faults at the surface. The faults most likely caused the valley to be sited at this location and they were probably also the cause of the ‘valley in valley’ shape.</span></p><p><span>A new DOVE research borehole will be drilled in the centre of the valley in 2021. This will bring more light on the sedimentary history and OSL-dating of the material will bracket the timing of the infill. </span></p>

Formation of the North–South Seismic Zone and Emeishan Large Igneous Province in Central China: Insights from P-Wave Teleseismic Tomography

2020 ◽  
Vol 110 (6) ◽  
pp. 3064-3076
Author(s):  
Chuansong He ◽  
M. Santosh
Keyword(s):  
High Velocity ◽  
Large Scale ◽  
P Wave ◽  
Large Igneous Province ◽  
Seismic Zone ◽  
Teleseismic Tomography ◽  
Emeishan Large Igneous Province ◽  
The North ◽  
Igneous Province ◽  
Stress Accumulation

ABSTRACT The geodynamic features of the north–south seismic zone (NSSZ) and the formation of the Emeishan large igneous province (ELIP) in China remain controversial. In this study, we conducted detailed P-wave teleseismic tomography studies in the NSSZ and adjacent regions. The results revealed large high-velocity anomalies beneath the Songpan–Ganzi Block and the South China Block, possibly representing large-scale lithospheric delamination. We further identified low-velocity structures at 50–200 km depths in the western and southern parts of the NSSZ, suggesting an upwelling asthenosphere induced by delamination and the absence of a rigid lithosphere. Two high-velocity structures beneath the Sichuan basin and the Alashan block were also revealed, which may represent the lithospheric roots of these structures. These rigid lithospheric roots may have obstructed the eastward extrusion of the Tibetan plateau and led to stress accumulation and release (triggering earthquakes) in the Longmenshan Orogenic Belt and the northern part of the NSSZ. Because of this obstruction, the eastward extrusion was redirected southeastward to Yunnan in the southern part of the NSSZ, which led to stress accumulation and release causing earthquakes along the Honghe and Xiaojiang faults. The results from this study reveal a high-velocity structure with a subducted slab-like appearance that may represent vestiges of the Paleo-Tethys oceanic lithosphere, which subducted beneath the ELIP and initiating large-scale mantle return flow or mantle upwelling, contributing to the formation of the ELIP.

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