scholarly journals The Velocity Structure within the Subducted Slab Below the Lower North Island, New Zealand

2021 ◽  
Author(s):  
◽  
Pauline Maria Galea
Keyword(s):  
New Zealand ◽  
Upper Mantle ◽  
High Velocity ◽  
P Wave ◽  
Low Velocity ◽  
The North ◽  
The Pacific ◽  
Low Velocity Layer ◽  
High Velocity Layer ◽  
Velocity Layer

<p>A shallow aftershock sequence in the Hawkes Bay region of the North Island, New Zealand (May 1990) was recorded with high quality on an L-shaped, 7-station array of 3-component, short-period seismographs at Wellington, such that the seismic waves travelled almost along strike of the subducted Pacific plate in this region. The arrival times at the stations of the Pn wave pulse from a number of aftershocks could be picked sufficiently accurately for a least-squares inversion to be carried out for wavefront speed, c, and incident azimuth, [theta]. The results show a high apparent velocity, 8.7 [plus or minus] 0.2 km/s, and an azimuth which is shifted by 6.0 [plus or minus] 2.5 degrees east of the true epicentre - station azimuth. The azimuthal anomaly, [delta][theta], has been interpreted as due to lateral refraction of Pn off the subducted slab. The effect of different geometries of the slab on the Pn wavefront characteristics (c and [delta][theta]) at Wellington have been explored through both simple geometrical considerations (in the case of a plane or cylindrical slab) as well as through 3-dimensional ray tracing (in the case of irregular curvature of the slab). It has been shown that a plane or cylindrical slab would require P-wave velocities of about 9.0 km/s to exist within it in order to fit both c and [delta][theta], whereas a model of the slab which departs from a regular cylinder and has a small updip component along strike can fit the observations with P-wave velocities of 8.75 km/s in the high velocity medium. This model has been proposed by Ansell and Bannister (1991) after detailed consideration of the shallow seismicity that defines the slab surface in the lower North Island. Information about the nature of the high velocity medium has been obtained by modeling the waveforms through generation of synthetic seismograms by the reflectivity technique of Kennett (1983). The large number of aftershocks within a small source region, and the sampling of much the same wavepath, meant that a sufficient number of seismograms had very similar and characteristic features that could be modelled. The typical seismogram of the data set had a simple Pn wavepulse, followed immediately by a complex. high frequency (up to 15 Hz) phase (here referred to as Phf) and a high amplitude, lower frequency phase that dominated the P-wavetrain (here referred to as P). A velocity profile that contained a layer of 8.75 km/s material at least 4 km thick, underlying "normal" mantle material of P-velocity 8.2 [plus or minus] 0.2 km/s. and whose surface lies approximately 18 km below the slab surface reproduced the observed seismogram features well. The presence of velocity gradients above and below the layer is not excluded. A gradual decrease in velocity below the layer in fact gives a better fit of the Pn pulse shape. By breaking down the synthetic seismogram into simpler versions. using Kennett's wavefield approximation technique, it has been shown that the Pn wave propagates through the high velocity layer, the Phf phase through the overlying layers as a sequence of reflections and refractions, and the P group as a reverberatory phase in a crustal waveguide, with its energy mostly in the form of free surface reflections and S to P conversion. These results have also been confirmed by ray tracing. Waveform modelling has also clearly shown that a low velocity layer (representing subducted sediment) on the top of the subducted slab produces a highly characteristic imprint on the synthetic seismogram, in the form of an energetic, reverberatory, lower frequency signal late in the P-wavetrain. Wavefield approximations show that this is also a crustal waveguide effect, with a strong component of mode conversion at the free surface, but P - S conversion appears to be the dominant mechanism. Seismograms very similar to such synthetic ones have been observed for the Weber aftershocks recorded at stations along the northern East Coast. The presence of such a low velocity layer in the East Coast region is thus implied, consistently with previous proposals. The petrological implications of the high velocity layer in the subducted Pacific plate are discussed. The most likely explanation is that it represents the maximum P velocity of an anisotropic layer within the Pacific upper mantle. It is proposed that the conditions of stress orientation, pressure and temperature at approximately 36 - 50 km depth in this region induces a strong realignment of olivine crystals with their fast direction along strike of the slab, normal to the maximum compressive stress axis. The upper mantle of the segment of the Pacific ocean just east of the Tonga - Kermadec trench and the North Island has been shown in this study to possess P-wave anisotropy, with the P-velocity reaching a maximum of 8.37 km/s in a direction N60 degrees E. This result was obtained by analysing a large set of ISC travel times from earthquakes along the Tonga - Kermadec - New Zealand subduction zone recorded at stations Niue, Rarotonga and the Chatham Islands. It is suggested that an enhancement of this anisotropy, accompanied by some re-orientation, takes place as the upper mantle medium is subjected to the new stress conditions in the initial stages of subduction.</p>

scholarly journals The Velocity Structure within the Subducted Slab Below the Lower North Island, New Zealand

2021 ◽  
Author(s):  
◽  
Pauline Maria Galea
Keyword(s):  
New Zealand ◽  
Upper Mantle ◽  
High Velocity ◽  
P Wave ◽  
Low Velocity ◽  
The North ◽  
The Pacific ◽  
Low Velocity Layer ◽  
High Velocity Layer ◽  
Velocity Layer

<p>A shallow aftershock sequence in the Hawkes Bay region of the North Island, New Zealand (May 1990) was recorded with high quality on an L-shaped, 7-station array of 3-component, short-period seismographs at Wellington, such that the seismic waves travelled almost along strike of the subducted Pacific plate in this region. The arrival times at the stations of the Pn wave pulse from a number of aftershocks could be picked sufficiently accurately for a least-squares inversion to be carried out for wavefront speed, c, and incident azimuth, [theta]. The results show a high apparent velocity, 8.7 [plus or minus] 0.2 km/s, and an azimuth which is shifted by 6.0 [plus or minus] 2.5 degrees east of the true epicentre - station azimuth. The azimuthal anomaly, [delta][theta], has been interpreted as due to lateral refraction of Pn off the subducted slab. The effect of different geometries of the slab on the Pn wavefront characteristics (c and [delta][theta]) at Wellington have been explored through both simple geometrical considerations (in the case of a plane or cylindrical slab) as well as through 3-dimensional ray tracing (in the case of irregular curvature of the slab). It has been shown that a plane or cylindrical slab would require P-wave velocities of about 9.0 km/s to exist within it in order to fit both c and [delta][theta], whereas a model of the slab which departs from a regular cylinder and has a small updip component along strike can fit the observations with P-wave velocities of 8.75 km/s in the high velocity medium. This model has been proposed by Ansell and Bannister (1991) after detailed consideration of the shallow seismicity that defines the slab surface in the lower North Island. Information about the nature of the high velocity medium has been obtained by modeling the waveforms through generation of synthetic seismograms by the reflectivity technique of Kennett (1983). The large number of aftershocks within a small source region, and the sampling of much the same wavepath, meant that a sufficient number of seismograms had very similar and characteristic features that could be modelled. The typical seismogram of the data set had a simple Pn wavepulse, followed immediately by a complex. high frequency (up to 15 Hz) phase (here referred to as Phf) and a high amplitude, lower frequency phase that dominated the P-wavetrain (here referred to as P). A velocity profile that contained a layer of 8.75 km/s material at least 4 km thick, underlying "normal" mantle material of P-velocity 8.2 [plus or minus] 0.2 km/s. and whose surface lies approximately 18 km below the slab surface reproduced the observed seismogram features well. The presence of velocity gradients above and below the layer is not excluded. A gradual decrease in velocity below the layer in fact gives a better fit of the Pn pulse shape. By breaking down the synthetic seismogram into simpler versions. using Kennett's wavefield approximation technique, it has been shown that the Pn wave propagates through the high velocity layer, the Phf phase through the overlying layers as a sequence of reflections and refractions, and the P group as a reverberatory phase in a crustal waveguide, with its energy mostly in the form of free surface reflections and S to P conversion. These results have also been confirmed by ray tracing. Waveform modelling has also clearly shown that a low velocity layer (representing subducted sediment) on the top of the subducted slab produces a highly characteristic imprint on the synthetic seismogram, in the form of an energetic, reverberatory, lower frequency signal late in the P-wavetrain. Wavefield approximations show that this is also a crustal waveguide effect, with a strong component of mode conversion at the free surface, but P - S conversion appears to be the dominant mechanism. Seismograms very similar to such synthetic ones have been observed for the Weber aftershocks recorded at stations along the northern East Coast. The presence of such a low velocity layer in the East Coast region is thus implied, consistently with previous proposals. The petrological implications of the high velocity layer in the subducted Pacific plate are discussed. The most likely explanation is that it represents the maximum P velocity of an anisotropic layer within the Pacific upper mantle. It is proposed that the conditions of stress orientation, pressure and temperature at approximately 36 - 50 km depth in this region induces a strong realignment of olivine crystals with their fast direction along strike of the slab, normal to the maximum compressive stress axis. The upper mantle of the segment of the Pacific ocean just east of the Tonga - Kermadec trench and the North Island has been shown in this study to possess P-wave anisotropy, with the P-velocity reaching a maximum of 8.37 km/s in a direction N60 degrees E. This result was obtained by analysing a large set of ISC travel times from earthquakes along the Tonga - Kermadec - New Zealand subduction zone recorded at stations Niue, Rarotonga and the Chatham Islands. It is suggested that an enhancement of this anisotropy, accompanied by some re-orientation, takes place as the upper mantle medium is subjected to the new stress conditions in the initial stages of subduction.</p>

Elastic wave propagation along layers in two-dimensional models

1963 ◽  
Vol 53 (3) ◽  
pp. 593-618
Author(s):  
D. K. Chowdhury ◽  
Peter Dehlinger
Keyword(s):  
Layer Thickness ◽  
High Velocity ◽  
Wave Length ◽  
Two Dimensional ◽  
Layer Model ◽  
Low Velocity ◽  
Long Period ◽  
Low Velocity Layer ◽  
High Velocity Layer ◽  
Velocity Layer

Abstract Propagation of direct waves and dispersive long-period waves along a layered system was investigated experimentally by means of two-dimensional ultrasonic models. Velocities of direct and head waves were measured within layers or in a medium adjacent to layers as functions of layer thickness to wave length or source-from-interface distance to wave length. Amplitudes of direct longitudinal, direct shear, and long-period waves were measured on three profiles, each perpendicular to the layers. Three models were used: the first consisted of a low-velocity layer between two thick sheets; the second of a high-velocity layer between two sheets; the third of six alternating high- and low-velocity layers between two sheets. The source was a wave train, simulating a wave from a seismic explosion. The frequency was varied so as to obtain different ratios of layer thickness to wave length. In the single low-velocity layer model the direct longitudinal wave contained a larger amplitude than the dispersive long-period wave in the layer at offset distance of 6 to 10 times the layer thickness. In the single high-velocity layer model the direct longitudinal wave was attenuated rapidly and the amplitudes of the long-period waves were negligigble. In the multilayered model, direct waves had negligible amplitudes at the corresponding distances; nearly all of the energy was in the dispersive long-period waves. In this model the low-velocity layer carried 1 1/2 to 3 times the amplitude observed in the high-velocity layers, whether the source was located in the high- or low-velocity layers. Dispersion of the long-period waves in the multilayered model was pronounced within the low-velocity layers and weak in the high-velocity layers, when the source was either in a high- or low-velocity layer. Dispersion was anomalous when the source was in a low-velocity layer and normal when in a high-velocity layer.

An indirect seismic method for determining the thickness of a low‐velocity layer underlying a high‐velocity layer

Geophysics ◽  
1981 ◽  
Vol 46 (7) ◽  
pp. 1003-1008 ◽  
Author(s):  
K. L. Kaila ◽  
H. C. Tewari ◽  
V. G. Krishna
Keyword(s):  
High Velocity ◽  
Low Velocity ◽  
Field Case ◽  
Reflection Data ◽  
First Arrival ◽  
Low Velocity Layer ◽  
Velocity Of Propagation ◽  
High Velocity Layer ◽  
Refraction Data ◽  
Velocity Layer

We present an indirect method for determining the thickness of a low‐velocity layer (LVL) underlying a high‐velocity layer (HVL) in seismic prospecting. Comparison of the average velocity‐depth function determined from the first arrival refraction data with that obtained from reflection data in the same region, especially below the LVL, makes it possible to recognize the presence of the LVL and to estimate its probable thickness. The applicability of the method has been demonstrated in a field case where the presence of an LVL is indicated by geologic evidence. It has been shown that thickness estimates of an LVL and an HVL can be made reliably in situations where the velocity in the LVL can be accurately estimated from nearby exposures or in a drilled well. For the field case analyzed, a thickness of 0.75 km was estimated for an LVL (probably Mesozoic sediments) underlying a 0.25 km thick HVL (probably basalt). The velocity of propagation in the LVL was taken from seismic data on nearby exposed Mesozoics as 4.0 km/sec, and the velocity of the HVL is 5.4 km/sec, based on the refraction data. In areas where the velocity in the LVL cannot be inferred accurately, an upper limit of this velocity can be obtained which permits estimation of the maximum possible thickness of the LVL. In the field example presented, we show that the velocity in the LVL cannot exceed 4.17 km/sec.

scholarly journals TWO‐DIMENSIONAL MODEL SEISMOLOGY

Geophysics ◽  
1954 ◽  
Vol 19 (2) ◽  
pp. 202-219 ◽  
Author(s):  
Jack Oliver ◽  
Frank Press ◽  
Maurice Ewing
Keyword(s):  
High Velocity ◽  
Rayleigh Waves ◽  
Low Cost ◽  
Shear Velocity ◽  
Small Scale ◽  
Two Dimensional ◽  
Low Velocity ◽  
Low Velocity Layer ◽  
High Velocity Layer ◽  
Velocity Layer

The solutions of many problems in seismology may be obtained by means of ultrasonic pulses propagating in small scale models. Thin sheets, serving as two dimensional models, are particularly advantageous because of their low cost, availability, ease of fabrication into various configurations, lower energy requirements, and appropriate dilatational‐to‐shear‐velocity ratios. Four examples are presented: flexural waves in a sheet, Rayleigh waves in a low velocity layer overlying a semi‐infinite high velocity layer, Rayleigh waves in a high velocity layer overlying a semi‐infinite low velocity layer, and body and surface waves in a disk.

Sea-floor evolution: rare-earth evidence

1971 ◽  
Vol 268 (1192) ◽  
pp. 663-706 ◽  
Keyword(s):  
Rare Earth ◽  
Mantle Source ◽  
Upper Mantle ◽  
Gulf Of Aden ◽  
Low Velocity ◽  
East Pacific ◽  
Mid Ocean Ridge ◽  
Low Velocity Layer ◽  
Ocean Ridge ◽  
Velocity Layer

A systematic survey of rare-earth (r.e.) abundances in submarine tholeiitic basalts along mid-oceanic ridges has been made by neutron activation analysis. The r.e. fractionation patterns are remarkably uniform along each mid-oceanic ridge and from one ridge to another (Juan de Fuca Ridge, East Pacific and Chile Rise, Pacific-Antarctic, Mid-Indian and Carlsberg Ridge, Gulf of Aden, Red Sea Trough and Reykjanes Ridge). The patterns are all depleted in light r.e. except for three samples (Gulf of Aden and Mid-Indian Ridge) which are unfractionated relative to chondrites. They contrast markedly with tholeiitic plateau basalt which are shown to be related to the early volcanic phases associated with continental drift. Tholeiitic plateau basalts are light r.e. enriched as are most continental rocks. Mid-ocean ridge basalts are also distinguishable from spatially related oceanic shield volcanoes of tholeiitic composition (Red Sea Trough-Jebel Teir Is., East Pacific Rise-Culpepper Island). Thus on a r.e. basis there are tholeiites within tholeiites. The r.e. difference between mid-ocean ridge tholeiites and tholeiitic plateau basalts can be related to distinct thermal and tectonic régimes and consequently magmatic modes and rates of intrusions from the low velocity layer in the upper mantle. The difference between continental and oceanic volcanism appears to be triggered by: (1) presence or absence of a moving continental lithosphere over the low velocity layer, and (2) whether or not major rifts tap the low velocity layer through the lithosphere. Fractional crystallization during ascent of melts before eruption at the ridge crest does not affect appreciably the relative r.e. patterns. R.e. in mid-ocean ridge basalts appear to intrinsically reflect their distribution in the upper mantle source, i.e. the low velocity layer. Based on secondary order r.e. variation of mid-ocean ridge basalts: (1) If fractional crystallization is invoked for the small r.e. variations, up to approximately 50 % extraction of olivine and Ca-poor orthopyroxene in various combinations can be tolerated. However, only limited amount of plagioclase or Ca-rich clinopyroxene can be extracted, the former because of its effect on the abundance of Eu abundance and the latter because of its effect on the [La/Sm] e.f. ratio, alternatively. (2) If partial melting during ascent is invoked, and a minimum of 10% melting is assumed, the permissible degree of melting of originally a lherzolite upper mantle may vary between 10 and 30% . It is not possible to establish readily to what extent these two processes have been operative as they cannot be distinguished on the basis of r.e. data only. However, there is evidence indicating that both have been operative and are responsible for the small r.e. variations observed in mid-ocean ridge basalts. An attempt to correlate second order r.e. variations along or across mid-oceanic ridges with spreading rate, age, or distance from ridge crests has been made but the results are inconclusive. No r.e. secular variation of the oceanic crust is apparent. R.e. average ridge to ridge variations are attributed to small lateral inhomogeneities of the source of basalts in the low velocity layer, and to a certain extent, to its past history. The remarkable r.e. uniformity of mid-oceanic ridge tholeiites requires a unique and simple volcanic process to be operative. It calls for upward migration of melt or slush from a relatively homogeneous source in the mantle—the low velocity layer, followed by further partial melting during ascent. The model, although consistent with geophysics, may have to be reconciled with some evidence from experimental petrology. Models for r.e. composition of the upper mantle source of ridge basalt, formation of layers 2 and 3, and the moho-discontinuity, are also presented.

Crust-mantle velocity structure in Shanxi rift, Central North China Craton

2020 ◽  
Author(s):  
Yan Cai ◽  
Jianping Wu
Keyword(s):  
Upper Mantle ◽  
High Velocity ◽  
North China Craton ◽  
Lower Crust ◽  
North China ◽  
Velocity Structure ◽  
Low Velocity ◽  
Crust And Upper Mantle ◽  
The North ◽  
Shanxi Rift

&lt;p&gt;North China Craton is the oldest craton in the world. It contains the eastern, central and western part. Shanxi rift and Taihang mountain contribute the central part. With strong tectonic deformation and intense seismic activity, its crust-mantle deformation and deep structure have always been highly concerned. In recent years, China Earthquake Administration has deployed a dense temporary seismic array in North China. With the permanent and temporary stations, we obtained the crust-mantle S-wave velocity structure in the central North China Craton by using the joint inversion of receiver function and surface wave dispersion. The results show that the crustal thickness is thick in the north of the Shanxi rift (42km) and thin in the south (35km). Datong basin, located in the north of the rift, exhibits large-scale low-velocity anomalies in the middle-lower crust and upper mantle; the Taiyuan basin and Linfen basin, located in the central part, have high velocities in the lower crust and upper mantle; the Yuncheng basin, in the southern part, has low velocities in the lower crust and upper mantle velocities, but has a high-velocity layer below 80 km. We speculate that an upwelling channel beneath the west of the Datong basin caused the low velocity anomalies there. In the central part of the Shanxi rift, magmatic bottom intrusion occurred before the tension rifting, so that the heated lithosphere has enough time to cool down to form high velocity. Its current lithosphere with high temperature may indicate the future deformation and damage. There may be a hot lithospheric uplift in the south of the Shanxi rift, heating the crust and the lithospheric mantle. The high-velocity layer in its upper mantle suggests that the bottom of the lithosphere after the intrusion of the magma began to cool down.&lt;/p&gt;

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