Upper Mantle Structure in Southern Saskatchewan and Western Manitoba from Project Edzoe

1975 ◽  
Vol 12 (12) ◽  
pp. 2134-2144 ◽  
Author(s):  
Allan Bates ◽  
D. H. Hall
Keyword(s):  
Upper Mantle ◽  
Linear Increase ◽  
P Wave ◽  
Mantle Structure ◽  
Low Velocity ◽  
Wave Velocities ◽  
P Wave Velocity ◽  
Upper Mantle Structure ◽  
Chemical Explosions ◽  
Velocity Zone

A line of eight recording sites in southern Saskatchewan and Manitoba, with ranges from 793 to 1284 km, recorded a series of chemical explosions in Greenbush Lake, British Columbia, as part of Project Edzoe in 1969.It is found that travel-times fall into two branches. The first branch is interpreted as representing the effect of a linear increase of P wave velocity, increasing from 8.10 km/s at the top of the mantle, with a gradient of 0.0017 s−1. The second branch indicates a rapid increase of gradient occurring somewhere between depths of 120 km and 150 km. Amplitude studies suggest, in the absence of complete triplication, a zone of low velocity gradient beneath the rapid increase. The presence or absence of a low velocity zone was not indicated in the data.Previous long-range refraction surveys indicate that a similar gradient in velocity also occurs beneath the Superior province of the Canadian shield, and that P-wave velocities are lowest at its center, reaching higher values at its edges.

UPPER MANTLE STRUCTURE IN CANADA FROM SEISMIC OBSERVATIONS USING CHEMICAL EXPLOSIONS

1967 ◽  
Vol 4 (5) ◽  
pp. 961-975 ◽  
Author(s):  
K. G. Barr
Keyword(s):  
Wave Velocity ◽  
Upper Mantle ◽  
Velocity Structure ◽  
P Wave ◽  
Hudson Bay ◽  
S Wave ◽  
Low Velocity ◽  
P Wave Velocity ◽  
Upper Mantle Structure ◽  
Chemical Explosions

Long-range seismic observations at the standard Canadian seismic stations, from chemical explosions in Hudson Bay and Lake Superior, are used to derive a P-wave velocity structure for the upper mantle. The coordinates of observed cusps are used to define the structural discontinuities. These discontinuities are at depths of 126 and 366 km, which agree closely with the depths of the S-wave velocity discontinuities deduced from surface-wave observations. The observations do not require a low velocity layer in the upper mantle.

scholarly journals Note on the variational and homogeneous layer approximations for the computation of Rayleigh-wave dispersion

1960 ◽  
Vol 50 (1) ◽  
pp. 81-85
Author(s):  
Frank Press ◽  
Hitoshi Takeuchi
Keyword(s):  
Upper Mantle ◽  
Wave Dispersion ◽  
Dispersion Curves ◽  
Homogeneous Layer ◽  
Mantle Structure ◽  
Low Velocity ◽  
Electronic Digital Computer ◽  
Upper Mantle Structure ◽  
Rayleigh Wave Dispersion ◽  
Velocity Zone

ABSTRACT An earlier study of upper mantle structure using a variational method is repeated using the homogeneous layer approximation programmed for an electronic digital computer to obtain dispersion curves. The dispersion curves computed by the two methods differ significantly but systematically so as to yield the same conclusion about the presence of the Gutenberg low-velocity zone in the upper mantle.

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.

P-wave amplitudes and magnitudes between 10° and 35°

1974 ◽  
Vol 64 (6) ◽  
pp. 1887-1899
Author(s):  
George A. McMechan ◽  
Warren G. Workman
Keyword(s):  
Upper Mantle ◽  
Epicentral Distance ◽  
Maximum Amplitude ◽  
Body Waves ◽  
P Wave ◽  
Ray Theory ◽  
Mantle Structure ◽  
Upper Mantle Structure ◽  
Wave Trains ◽  
Theory Technique

abstract The observed behavior of P-wave relative amplitudes, as a function of epicentral distance, between 10° and 35°, is controlled primarily by the velocity-depth structure of the upper mantle. P-wave synthetic seismograms calculated by the new quantized ray theory technique are used to determine theoretical log (A/T) versus log Δ curves from a number of upper mantle models. Maximum amplitude arrivals show less model dependence than the first arrivals in the same wave trains, and hence are more consistent magnitude indicators for regions where the upper mantle structure is poorly known. Log (A/T) versus log Δ curves vary considerably, but predictably, from model to model. This model-dependent variation can account for a major part of the large standard deviations usually associated with the calculation of magnitudes from body waves.

scholarly journals A new model of the upper mantle structure beneath the western rim of the East European Craton

2014 ◽  
Vol 6 (1) ◽  
pp. 559-598
Author(s):  
M. Dec ◽  
M. Malinowski ◽  
E. Perchuc
Keyword(s):  
Upper Mantle ◽  
Seismic Velocity ◽  
Seismic Station ◽  
Structural Features ◽  
East European Craton ◽  
P Wave ◽  
Mantle Structure ◽  
Reflected Waves ◽  
East European ◽  
Upper Mantle Structure

Abstract. In this article we present a new 1-D P wave seismic velocity model (called MP1-SUW) of the upper mantle structure beneath the western rim of the East European Craton (EEC) based on the analysis of the earthquakes recorded at the Suwałki (SUW) seismic station located in NE Poland which belongs to the Polish Seismological Network (PLSN). This analysis was carried out due to the fact that in the wavefield recorded at this station we observed a group of reflected waves after expected P410P at epicentral distances 2300–2800 km from SUW station. Although the existing global models represent the first arrivals, they do not represent the full wavefield with all reflected waves because they do not take into account the structural features occurring regionally such as 300 km discontinuity. We perform P wave traveltime analysis using 1-D forward ray-tracing modelling for the distances up to 3000 km. We analysed 249 natural seismic events that were divided into four azimuthal spans with epicentres in the western Mediterranean Sea region (WMSR), the Greece and Turkey region (GTR), the Caucasus region (CR) and the part of the North Atlantic Ridge near the January Mayen Island (JMR). Events from each group were sorted into four seismic sections respectively. The MP1-SUW model documents bottom of the asthenospheric low velocity zone (LVZ) at the depth of 220 km, 335 km discontinuity and the zone with the reduction of P wave velocity atop 410 km discontinuity which is depressed to 440 km depth. The nature of a regionally occurring 300 km boundary here we explained by tracing the ancient subduction regime related to the closure of the Iapetus Ocean, the Rheic Ocean and the Tornquist Sea.

scholarly journals The April 29, 1965, Puget Sound earthquake and the crustal and upper mantle structure of western Washington

1977 ◽  
Vol 67 (3) ◽  
pp. 693-711 ◽  
Author(s):  
Charles A. Langston ◽  
David E. Blum
Keyword(s):  
Upper Mantle ◽  
Source Parameters ◽  
Puget Sound ◽  
P Wave ◽  
Low Velocity ◽  
Crust And Upper Mantle ◽  
Low Velocity Zone ◽  
Crustal Model ◽  
Short Period ◽  
Velocity Zone

abstract Simultaneous modeling of source parameters and local layered earth structure for the April 29, 1965, Puget Sound earthquake was done using both ray and layer matrix formulations for point dislocations imbedded in layered media. The source parameters obtained are: dip 70° to the east, strike 344°, rake −75°, 63 km depth, average moment of 1.4 ± 0.6 × 1026 dyne-cm, and a triangular time function with a rise time of 0.5 sec and falloff of 2.5 sec. An upper mantle and crustal model for southern Puget Sound was determined from inferred reflections from interfaces above the source. The main features of the model include a distinct 15-km-thick low-velocity zone with a 2.5-km/sec P-wave-velocity contrast lower boundary situated at approximately 56-km depth. Ray calculations which allow for sources in dipping structure indicate that the inferred high contrast value can trade off significantly with interface dip provided the structure dips eastward. The effective crustal model is less than 15 km thick with a substantial sediment section near the surface. A stacking technique using the instantaneous amplitude of the analytic signal is developed for interpreting short-period teleseismic observations. The inferred reflection from the base of the low-velocity zone is recovered from short-period P and S waves. An apparent attenuation is also observed for pP from comparisons between the short- and long-period data sets. This correlates with the local surface structure of Puget Sound and yields an effective Q of approximately 65 for the crust and upper mantle.

scholarly journals Investigation of the 1727 Newbury, Massachusetts, USA, earthquake using LiDAR imagery and P-wave velocity tomography

2018 ◽  
pp. 267-283
Author(s):  
Ronald T. Marple ◽  
James D. Hurd, Jr. ◽  
Lanbo Liu ◽  
Seth Travis ◽  
Robert J. Altamura
Keyword(s):  
Wave Velocity ◽  
New Hampshire ◽  
Seismic Refraction ◽  
Surface Expression ◽  
P Wave ◽  
Light Detection And Ranging ◽  
Low Velocity ◽  
P Wave Velocity ◽  
Velocity Tomography ◽  
Velocity Zone

High-resolution LiDAR (light detection and ranging) images of northeastern Massachusetts and southeastern New Hampshire reveal a 10-km-long, NW-SE-oriented topographic lineament in northeastern Massachusetts that we interpret to be the surface expression of a SW-dipping thrust fault along which the 1727 Newbury, Massachusetts, earthquake occurred. The Newburyport lineament coincides with the northeast edge of a 10-kmlong, NW-SE-oriented ridge, herein named Merrimack ridge, that parallels the NW-SE-trending segment of the Merrimack River downstream from where it bends 90° to the southeast. The northwestern end of the Newburyport lineament coincides with a 1-km-long, ~7- to 15-m-high, NE-facing Newburyport scarp that is located just south of the bend in the river. The Newburyport lineament also parallels the NW-SE-oriented nodal planes of the focal mechanism that was generated for the 1999 Amesbury, Massachusetts, earthquake. A P-wave velocity tomographic model generated from a seismic-refraction profile across the Newburyport scarp shows a ~40-m-wide low-velocity zone dipping ~41° SW. Velocities along this zone decrease 15–50%, which suggests that the Newburyport lineament is associated with the surface expression of a SW-dipping brittle fault zone. The LiDAR images also revealed three other NW-SE-trending lineaments in the study area.

scholarly journals A new model of the upper mantle structure beneath the western rim of the East European Craton

Solid Earth ◽  
2014 ◽  
Vol 5 (1) ◽  
pp. 523-535
Author(s):  
M. Dec ◽  
M. Malinowski ◽  
E. Perchuc
Keyword(s):  
Upper Mantle ◽  
Seismic Velocity ◽  
Seismic Station ◽  
Velocity Model ◽  
East European Craton ◽  
P Wave ◽  
Mantle Structure ◽  
Reflected Waves ◽  
East European ◽  
Upper Mantle Structure

Abstract. We present a new 1-D P wave seismic velocity model (called MP1-SUW) of the upper mantle structure beneath the western rim of the East European Craton (EEC) based on the analysis of the earthquakes recorded at the Suwałki (SUW) seismic station located in NE Poland which belongs to the Polish Seismological Network (PLSN). Motivation for this study arises from the observation of a group of reflected waves after expected P410P at epicentral distances 2300–2800 km from the SUW station. Although the existing global models represent the first-arrival traveltimes, they do not represent the full wavefield with all reflected waves because they do not take into account the structural features occurring regionally such as 300 km discontinuity. We perform P wave traveltime analysis using 1-D and 2-D forward ray-tracing modelling for the distances of up to 3000 km. We analysed 249 natural seismic events from four azimuthal spans with epicentres in the western Mediterranean Sea region (WMSR), the Greece and Turkey region (GTR), the Caucasus region (CR) and the part of the northern Mid-Atlantic Ridge near the Jan Mayen Island (JMR). For all chosen regions, except the JMR group for which 2-D modelling was performed, we estimate a 1-D average velocity model which will characterize the main seismic discontinuities. It appears that a single 1-D model (MP1-SUW model) explains well the observed traveltimes for the analysed groups of events. Differences resulting from the different azimuth range of earthquakes are close to the assumed picking uncertainty. The MP1-SUW model documents the bottom of the asthenospheric low-velocity zone (LVZ) at the depth of 220 km, 335 km discontinuity and the zone with the reduction of P wave velocity atop 410 km discontinuity which is depressed to 440 km depth. The nature of the regionally occurring 300 km boundary is explained here by tracing the ancient subduction regime related to the closure of the Iapetus Ocean, the Rheic Ocean and the Tornquist Sea.

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.

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