Direction Of Maximum Velocity Research Articles

Measurements Combination of Elastic Anisotropy and Anisotropy of Magnetic Susceptibility on Case Study in Igneous Rock of Lava Flow Type from Ijen Volcanic Complex, East Java

Research about the anisotropy parameters of rock is important for geophysical investigations. This research was a preliminary study which aims to find the relationship between elastic anisotropy and anisotropy of magnetic susceptibility in igneous rock of lava flow type with MR1 code and to interpret preferred orientation in MR1 sample based on the integration of anisotropy measurement results with the supported results from micro-computed tomography (μCT) analysis, petrography analysis and geological data analysis. This research has never been done before. This study was divided into several main stages consist of elastic anisotropy measurements on a cube-shaped specimen to determine the direction of maximum velocity and elastic anisotropy parameter values, anisotropy of magnetic susceptibility measurements on six cylindrical specimens to determine the direction of maximum susceptibility (magnetic mineral orientation) and AMS parameter values, petrography analysis on the all sides of the cube in order to observe preferred orientation in mineral and μCT analysis in a cylindrical specimen to determine the pore trend of rock. This study showed the relationship between AMS and elastic anisotropy in MR1 sample which indicated by positive correlation between the direction of maximum susceptibility and maximum velocity. Based on the results, MR1 sample has elastic anisotropy degree ranges from 2,6 % - 6,6 % for P waves and 2,2 % - 4 % for S waves and anisotropy of magnetic susceptibility degree around 2,6 % - 5,4 %. The integration of all measurement results including the direction of maximum velocity, direction of maximum susceptibility, preferred orientation in mineral, pore trend, and dip direction of sheeting joint indicate preferred orientation in MR1 sample towards NE-SW. One possible cause of such preferred orientation is the direction of the lava flow. Further studies in this topic are needed to support this hypothesis.

YAKIN FAY YER HAREKETLERİNİN YÖN ETKİSİ

Performansa dayalı tasarımın temel amacı yer hareketine maruz yapılarda yapısal talebin belirlenebilmesidir. Yapısal talebin doğru belirlenebilmesi, yer hareketi özelliklerinin ve bunlarla yapısal talep arasındaki ilişkinin doğru tanımlanabilmesini gerektirmektedir. Son yıllarda faylara yakın bölgelerde elde edilen kayıtların diğer kayıtlara göre önemli farklılıklar içerdiği görülmüştür. İleri doğrultu ve ötelenme etkileri bu yer hareketlerini farklılaştıran ve hasar potansiyellerini arttıran temel özelliklerdir. İleri doğrultu etkisi fayın kırılma yüzeyine dik yönde meydana gelir ve bu yer hareketlerinin hasar potansiyelleri artırır. Ancak her kayıtın faya dik ve paralel yönlerini belirleyebilmek, fiziki imkansızlıklar nedeniyle, mümkün olmamaktadır. Bu nedenle etkili yönlerin belirlenebilmesi önemlidir. Literatürde maksimum hız yönü önerilen etkili yönlerden biridir. Bu çalışmada göreli kat ötelenmesi spektrum şiddeti ve spektral hızın maksimum yönlerinin de etkili yönlerden olduğu tespit edilmiştir. Maksimum hız yönünün etkinliği yer hareketinin atım periyodunun yapı periyodu oranına bağlı olduğu, bu oranın büyük olduğu durumlarda maksimum ivme yönü hasar potansiyelinin daha yüksek olduğu belirlenmiştir.

Estimation of azimuthal anisotropy in the NW Pacific from seismic ambient noise in seafloor records

We analysed background surface waves in seismic ambient noise by cross-correlating continuous records of eight ocean bottom seismometers and nine differential pressure gauges deployed in the northwestern Pacific Ocean by the PLATE project. After estimating the clock delay and instrumental phase responses of differential pressure gauges by using cross-correlation functions, we measured average phase velocities in the area of the array for the fundamental-, first higher- and second higher-mode Rayleigh waves, and the fundamental-mode Love waves at a period range of 3–40 s by waveform fitting. We then measured phase-velocity anomalies of fundamental-mode and first higher-mode Rayleigh waves for each pair of stations at a period range of 5–25 s, and corrected the effect of variation in water-depths. The seismic anomalies imply the presence of strong azimuthal anisotropy beneath the eastern part of array. The direction of maximum velocity is approximately N35°E in the fossil seafloor spreading direction perpendicular to magnetic lineations from the ancient triple junction at this location. The peak-to-peak intensity of shear-wave velocity anisotropy in the mantle is ∼7 per cent.

The Improvement of Structural Adequacy Against Large Demands of Near-Fault Ground Motions

Near-fault ground motions impose large demands on structures compared to ordinary ground motions. Recordings suggest that near-fault ground motions with ‘forward’ directivity are characterized by a large pulse, which is mostly orientated perpendicular to the fault. The maximum velocity direction (MVD) component is calculated by finding the angle that the maximum velocity resultant of the original longitudinal and transverse components will be occurred. The MVD component has larger PGV and PGD values compared to other components of near-fault ground motions. The spectral values of the MVD component are an envelope of the other decomposed components. The improvement of structural adequacy against large demands of near-fault ground motions has been the subject of research for the last decade. This study intends to provide quantitative knowledge on the near-fault ground motion demand and corresponding structural capacity requirements. The spectral quantities were computed through an attenuation relationship that is based on the near-fault ground motion records. New findings in this study are compared with the Uniform Building Code provisions and other attenuation relationships. The results demonstrate that for large magnitude events M w > 7, the UBC97 near-source factors (N a , N v ) have more conservative values compared to the study estimates. Hence, seismic coefficients of UBC97 have serious concern for estimating strength when short distances and large magnitude events are considered.

Modelling the sedimentary signature of long waves on coasts: implications for tsunami reconstruction

We describe a process-based mathematical model of suspended and bedload sediment transport under long, non-breaking waves on a plane beach, and we use this model to investigate the relationship between the hydrodynamics of run-up and run-down and the resulting erosive and depositional ‘signature’ which the wave leaves. In particular, we compare the results of our ‘forward’ model with several recently proposed methods for reconstructing tsunami hydrodynamics from their deposits. We find that sediment transport, both as bedload and in suspension, is strongly controlled by asymmetries in the direction of maximum velocity; in the latter case it is also affected by settling lag, especially around the point of maximum run-up. The combination of these effects appears to preclude simple methods of reconstructing tsunami hydrodynamics from the large-scale features of their signatures; however, our results suggest some promising avenues for further investigation.

Isotropic and anisotropicPnvelocity inversion of regional earthquake traveltimes underneath Germany

Summary This paper investigates the uppermost mantle velocity beneath Germany using regional earthquake traveltime data. 2149 Pn traveltimes corresponding to 220 events recorded at 70 stations covering the region of 47°N–52°N latitude, 5°E–15°E longitude result in a satisfactory ray-path distribution. Three methods with increasing degree of complexity are used to analyse the Pn traveltime data: a straight-line fit; the classical time-term method; and a modified time-term method including azimuthal anisotropy. First, from the straight-line fit to the data set, an average Pn velocity of 7.98 km s−1 is inferred. Second, the classical time-term method yields a mean uppermost mantle velocity of 7.99 km s−1. The most important feature in this analysis is the azimuth-dependent pattern of the residuals, indicating some evidence of velocity anisotropy in the upper mantle. The time-term method achieves about 55 per cent variance reduction relative to the straight-line fit. Third, two modified ‘anisotropic’ time-term methods provide an average Pn velocity of 8.09 km s−1, with a further data variance reduction of 64 and 20 per cent relative to the straight-line fit and the classical time-term method, respectively. The estimated anisotropy level is about 3.5–4 per cent, with maximum and minimum velocities of 8.24–8.27 km s−1 and 7.95 km s−1. Our estimated maximum velocity direction of ∼N25°E coincides with those of previous anisotropic studies on the uppermost mantle in this region based on seismic refraction data. The results from the present study thus support the idea that Pn-wave anisotropy is a large-scale lithospheric feature over much of central Europe.

SAW diagnostics of GaAs surface structure

Surface acoustic waves (SAW) are very sensitive to the properties of crystal surfaces, so they find an application in crystal surface diagnostics. In this work GaAs (1 1 1) semi-insulating samples had SAW transducers formed as Al thin-film interdigital structures that operated at a central frequency of 65 MHz. SAW parameters (amplitude and phase variations of the SAW output signals) were measured during irradiation of the GaAs surface from a light source. The results indicate that SAW propagation is sensitive not only to the state of the natural oxide layers covering the GaAs surface, including oxide composition, surface morphology etc., but also to the crystalline state of the GaAs surface itself, particularly to the presence of stress deformations within the GaAs. These deformations are most likely due to a modification of the oxide layer that changes the SAW maximum velocity direction, as indicated by changes in the SAW phase velocity.

Global upper mantle tomography of seismic velocities and anisotropies

A data set of 2600 paths for Rayleigh waves and 2170 paths for Love waves enabled us to retrieve three‐dimensional distributions of different seismic parameters. Shallow layer corrections have been carefully performed on phase velocity data before regionalization and inversion at depth. The different seismic parameters include the five parameters of a radially anisotropic medium and the eight azimuthal anisotropic parameters as defined by Montagner and Nataf. It is found that the lateral heterogeneities of velocities and anisotropies in the upper mantle are dominated down to 250–30 km by plate tectonics with slow velocities below ridges, high velocities below continents and a velocity increasing with the age of the seafloor. Anisotropy is present in this whole depth range and the directions of maximum velocities are in good agreement with absolute plate velocities. Below 300 km, there is a sharp decreasing of the amplitude of lateral heterogeneities of seismic velocities and anisotropies. Below 450 km, lateral heterogeneities display a degree 2 and to a less extent a degree 6 pattern. Therefore, between 250 km and 450 km, there is a transition region where vertical circulation of matter is possible as shown by subducted slabs and “plumes” of slow velocities but which probably separates two types of convection. The first one is closely related to plate tectonics and to the distribution of continents. The second one dominates below 450 km and is characterized by two downgoing and two upgoing flows.

Global anisotropy in the upper mantle inferred from the regionalization of phase velocities

A data set of 2600 paths for Rayleigh waves and 2170 paths for Love waves was selected in order to resolve azimuthal anisotropy of surface waves in the period range 70–250 s. The epicenters were chosen so that the paths provide uniform spatial and azimuthal coverage of Earth. A synthetic seismogram is computed for each path and compared to the observed seismogram in order to infer the phase velocity along each path. To process such a large data set, a new tomographic technique is designed. A good correlation is found with surface tectonics in the whole period range for Love waves and at shorter periods for Rayleigh waves. The directions of maximum velocities are also well correlated with plate tectonics motions for the 2ψ azimuthal term of Rayleigh waves. When the power spectrum of heterogeneities is computed, it is found that for Love waves the even orders are usually larger than the odd orders. For Rayleigh waves the degrees 2 and 6 dominate only at long periods, suggesting a deep origin for these components.

Pnobservation and interpretation in Washington

abstractThe travel-time characteristics of Pn waves in the state of Washington were investigated using regional earthquakes as seismic sources and stations of the state-wide University of Washington telemetered network as receivers. There is a significant difference in upper mantle velocity between eastern Washington (8.20 ± 0.02 km/sec) and western Washington (7.79 ± 0.02 km/sec). Modifications of the time-term method were used to find the dependence of P velocity on distance and azimuth to the source. Pn velocity in eastern Washington depends on the distance to the source, implying a velocity gradient with depth in the upper mantle. No significant distance dependence was found for western Washington. Azimuthal anisotropy was found for Pn with a northwest direction of maximum velocity in eastern Washington and a north-northwest direction in western Washington. Assuming no lateral crustal velocity gradients, the Pn refracting interface dips to the west approximately 0.8° in eastern Washington and to the southeast approximately 3.2° in western Washington. Contour maps of the station time terms indicate significant variations in crustal structure across the state. The combination of low Pn velocity and small Moho dip beneath western Washington suggests that the subducting lithospheric slab underlying the continental margin lies below the level of transition to upper mantle velocity over much of this region.

On the lithospheric seismic structure of Reykjanes Ridge at 62.5°N

A long reversed refraction line on the southeast flank of Reykjanes Ridge subparallel to the axis is modeled with synthetic seismograms. These are calculated using the reflectivity method. The data have been recorded by two ocean bottom seismometers during the Reykjanes Ridge Iceland Seismic Program 1977 (RRISP77 experiment). The observed amplitudes are characterized by a low at around 80‐km range and a pronounced maximum at around 100‐km distance. Here the apparent velocity increases from about 8 km/s to at least 8.6 km/s in both directions. The modeling demonstrates that most of the observed features can be described by a laterally homogeneous structure except for layer 2 which thickens markedly toward Iceland. The structure of the lower crust seems to be similar to that of the Bay‐of‐Islands ophiolite, Newfoundland. There is an indication of a low‐velocity zone at the base of the crust which may be caused by partial serpentinization of the olivine component of the gabbros at that depth. The Moho seems to be modeled best by a discontinuity about 10 km below the water surface. In the uppermost mantle there is a very slight velocity increase from 8 to 8.25 km/s. It is underlain by a transition zone to velocities as high as 8.6–8.9 km/s at about 20 km depth. This could possibly be explained by velocity anisotropy, perhaps together with a petrological boundary. The direction of maximum velocity of the deep anisotropic layer should be parallel to the ridge axis. This could be caused by a pipe flow from the Iceland mantle plume which is possibly indicated by the shear wave structure at greater depth.

Seismic P-wave anisotropy in the subcrustal lithosphere of north-west Australia

Summary. Reduced P,, travel times from the Archaean Pilbara Craton of north-west Australia show a strong correlation with azimuth, which could be used as evidence of anisotropy. However, the azimuthal correlation could also be explained by a southerly dip of between 1 and on the crustmantle boundary, although the models from several reversed seismic profiles across the craton suggest a smaller dip. A time-term analysis of the P,, data yielded several models. The preferred solution, in which the dip on the crust-mantle boundary is similar to that in the models from the reversed profiles, has approximately 2 per cent anisotropy in the uppermost mantle, with the direction of maximum velocity 30 east of north. One possible cause of the anisotropy is that olivine crystals were aligned by syntectonic recrystallization and/or power law creep in the tensional environment caused at the base of the lithosphere by flexure during loading of the lithosphere by the strata of the Hamersley Basin which overlies the Pilbara Craton. A seismic discontinuity occurs about 15 km below the crust-mantle boundary under the craton. A qualitative analysis of all available seismic data suggests that the velocity below the boundary is probably also anisotropic, with the direction of maximum velocity between north and 40 west of north. The direction of minimum velocity below the sub-Moho boundary correlates loosely with the direction of basement lineaments in the Proterozoic Capricorn Orogenic Belt to the south of the craton, suggesting that the anisotropy under the boundary may be younger than that immediately under the crustlmantle boundary. This is consistent with the notion that the Aichaean lithosphere was thinner than the present lithosphere.

Seismic anisotropy of the Pacific Ocean inferred from long-period surface waves dispersion

The dispersion of surface (Rayleigh and Love) waves in the period range 40–300 s along a large number of paths, allows the estimation of both the azimuthal anisotropy and the shear-wave polarization anisotropy. The regional dispersion is determined, taking into account simultaneously its dependence with age and an azimuthal factor. The Pacific Ocean has been divided into 5 regions for Rayleigh waves and into 3 regions for Love waves. This partition discriminates the regions of extreme age which show a fast variation of dispersion with age, from the regions of intermediate age where the variation is weak. A variation of ∼ 2% of Rayleigh-wave group velocity with the azimuth of the path, measured with respect to the direction of spreading is displayed, up to very long-period. On the contrary, the azimuthal anisotropy for Love waves is difficult to resolve. For Rayleigh waves, the present-day direction of plate motion seems to agree best with the direction of maximum velocity. On the other hand, the isotropic inversion of the regional dispersion curves indicates, except for young regions, a discrepancy between Rayleigh-wave and Love-wave models. With this hypothesis, SH-velocities are higher than SV-velocities for the regions older than 23 Ma, down to a depth of 300 km, which is indicative of the presence of polarization anisotropy. The latter, very weak for the young part of the ocean, increases with age and reaches 7%, for the oldest region.

Longshot experiments to study velocity anisotropy in the oceanic lithosphere of the northwestern Pacific

Several long-range explosion seismology experiments have been conducted in the northwestern Pacific basin, where one of the oldest oceanic lithospheres is postulated to exist. The experiments were conducted from 1974 to 1980. Highly sensitive ocean-bottom seismographs which had been developed for longshot experiments were used. The lengths of the profiles ranged from 1000 to 1800 km, and the directions were chosen to provide wide azimuthal coverage. One of the aims of this series of experiments was to test the existence of velocity anisotropy on a large, regional scale. The results show that the oceanic lithosphere has anisotropy wherein the velocity changes by 4–7%. The anisotropy extends from a depth of at least 40 to 140 km beneath the sea bottom; however, the magnitude of the anisotropy may vary with depth. The azimuth of the maximum velocity is 150–160° clockwise from north, and coincides with the “fossil” direction of spreading of the Pacific plate, whereas it differs from the present direction of plate motion by ∼ 30°. The azimuth does not seem to depend on depth. In the direction of maximum velocity, the lithosphere is basically two-layered: 8.0–8.2 and 8.6 km s −1. The depth of the interface is 50–60 km beneath the sea floor.

Upper crustal structure of the Mount Hood, Oregon, region as revealed by time term analysis

Seismic refraction data with a dense areal distribution were collected to study the seismic structure of Mount Hood and the surrounding region. This area is typical of Cascade volcanoes and is geologically quite complex. The prime goals of this project were to search for velocity variations in the upper crustal rocks and to determine if the velocity of these rocks is anisotropic. A new system, including 100 remote recording units, was developed to facilitate the collection of data in this type of survey. The data collected in this study reveal a large variation in velocity and thickness of the uppermost crustal rocks that is probably typical of the High Cascade province. A regional structural pattern surrounding Mount Hood, where there is a marked thinning of low‐velocity near‐surface rocks, suggests that the present edifice of Mount Hood lies on top of a much larger structure, possibly the roof of a large batholith that was emplaced prior to the eruption of the volcanic rocks that form the modern mountain. A straightforward time term analysis of the data reveals this structure. When the time terms are used to correct the travel time observations for the variation in structure under each station, the remaining set of residuals indicates a variation of travel time with azimuth. While a systematic error in the time term reduction cannot be ruled out, this variation is probably the result of anisotropic velocity structure in the underlying refractor. The alignment of the direction of maximum velocity with the direction of maximum principal stress and the decrease in anisotropy with depth suggest that the anisotropic velocity structure is related to the opening and closing of fractures in the present stress field. The direction of maximum velocity is about N25°W, and the degree of anisotropy appears to vary with depth from about 3% near the surface to zero at depths below 8 km.

Propagation of high frequency Pn waves at great distances in the central and South Pacific and its implications for the structure of the lower lithosphere

Using 17 seismic stations in French Polynesia, we study the propagation of high‐frequency Pn waves in the Central and South‐Central Pacific. This study is prompted by the presence of striking Pn wave phases on the seismograms of the Hawaii earthquakes recorded in the Tuamotu and Society islands (Tahiti), almost 40° away. The Pn velocity inferred from these observations (7.95±0.03 km/s) is in good agreement with the values obtained in the North Pacific by Walker (1977a, b) and shows the existence over most of the Pacific basin of a remarkably efficient Pn wave guide of relatively low velocity. Compared to Pn, Sn waves are much less efficiently transmitted. The Pn velocities measured from intraplate earthquakes in the South‐Central Pacific suggest the presence of a 2–3% anisotropy in the wave guide, the direction of maximum velocity being parallel to the trend of the island chains. All these observations are interpreted as evidence of the presence of minimum velocity channels within the subcrustal lithosphere and a discussion of the models which can account for these features is presented.

Regionalized shear velocity models of the Pacific upper mantle from observed Love and Rayleigh wave dispersion

Summary. Group and phase velocities for periods between 16 and 110 s have been determined using the single-station method along 33 Rayleigh-and 30 Love-wave paths across the Pacific. These dispersion data were used to detect variations in upper mantle structure beneath the Pacific. Taking both regional changes and azimuthal anisotropy into consideration, regionalized group and phase velocities were derived for four regions of differing age. The boundaries of the regions were chosen to correspond to isochrons of 20, 50 and 100 Myr, as inferred from geomagnetic lineations. A small, but significant degree of azimuthal anisotropy (less than 0.8 per cent) was found to occur throughout the Pacific if the degree and direction of maximum velocity were assumed to be uniform everywhere. The average direction of maximum velocity was found to be 85°± 6° as measured clockwise from north. Regionalized group and phase velocities for Rayleigh waves at the larger periods increase systematically with increasing age of the ocean floor. For Love waves, departures from a systematic increase occur in the youngest (0–20 Myr) and oldest (> 100 Myr) portions of the Pacific. The dispersion data were corrected for the effects of anelasticity and were inverted to obtain shear velocity models for each of the four regions using the stochastic inversion method. A comparison of the shear wave models derived from Rayleigh waves in the four regions indicates that average shear velocities at depths between 30 and 110 km increase rapidly with age. This increase occurs mainly because the depth to the base of the lithosphere increases; however, the shear velocities of the lithosphere also increase for the regions out to 100 Myr in age. Slightly lower velocities in the upper part of the lithosphere for greater ages may be due to a large influx of mid-plate volcanism during the Cretaceous. Separate inversions of Love-and Rayleigh-wave velocities indicate that polarization anisotropy can be resolved for the lithosphere. Polarization anisotropy cannot, be resolved for the low-velocity zone beneath the Pacific using these data, except possibly in the youngest regions, where lateral complexities of the East Pacific Rise are apt to affect the data adversely. Assuming that we can adequately invert our Love-and Rayleigh-wave data separately to obtain information on anisotropy, we find that average SV velocities in the lithosphere increase from about 4.3 to 4.6 km/s, and average SH velocities increase from about 4.5 to 4.7 km/s, from the youngest to oldest regions. Average velocities in the upper portion of the low-velocity zone increase from about 4.1 to about 4.25 km/s for the same regions. The existence of anisotropy in the lithosphere is constant with the results of seismic refraction experiments in the Pacific and supports peroxide as the predominant upper mantle material. The occurrence of anisotropy through-out the entire lithosphere, suggests a situation in which periodite crystallizes at the base of the lithosphere from a partially molten low-velocity zone and olivine crystals become aligned in response to the stress field within the Pacific plate.

Seismic anisotropy of the subcrustal lithosphere as evidence for dynamical processes in the upper mantle

Summary. Anisotropy of seismic waves in the uppermost mantle has not only been observed in the oceanic but recently also in the continental lithosphere. Laboratory experiments on the formation of preferred orientation of olivine crystals suggest plastic flow ‘as the most likely mechanism for the genesis of anisotropy in the upper mantle. Since the direction of maximum velocity correlates in the ocean and on the continent with a number of tectonic features, a causal connection between anisotropy and dynamical processes related to plate motions must be suspected.

Far-field body waves generated by slip faulting: Case of an elliptically expanding rupture.

The far-field body waves generated by a slip dislocation with an elliptically expanding rupture are analysed in the time domain. It is assumed that the ratio of the lengths of the axes of an expanding ellipse is an arbitrary constant α. The radiated body waves are described by the product of two factors. The first one due to the double couple without moment is excluded from the consideration. The second factor Tj (j stands for the P- and S- motions), which includes the time dependence of the seismic impulse and which reflects the character of the rupture propagation, is analysed for three models with different values of the parameter α. The graphic representation of the Tj pattern consists of three components: 1) the spatial distribution of the slope of the initial part of the impulse Tj, 2) the graphs of the impulse Tj as a function of time for the chosen directions, 3) the spatial distribution of the maximum amplitude of the impulse Tj. The slope of the initial part of the displacement Tj is the greatest in the rupture plane and the smallest in the direction perpendicular to the fault. If the angle between the rupture plane and the observer is kept constant, then the slope is the greatest for the direction whose projection on the rupture plane lies along the direction of maximum velocity of the rupture propagation. If the boundary of the faulting area is rectilinear or concave, then some impulses can exhibit two displacement maxima, at the times of the first and the second stopping phases, respectively. The maximum amplitude of the displacement Tj depends to a high degree on the ultimate boundary of faulting in relation to the initial point of rupture, as well as on the form of the rupture propagation. It is found that if the fault surface is not elongated and the starting point of the rupture is approximately in the center of the fault, then the maximum amplitude of the displacement Tj should be the greatest, in general, for the directions whose projection on the rupture plane lies along the direction of minimum and not of maximum velocity of the rupture propagation.

Preliminary Analysis of Geophysical Measurements North of Juan de Fuca Ridge

Preliminary analyses of gravity, magnetic, seismic reflection and refraction, dredging, and heat flow measurements off Queen Charlotte Islands and Vancouver Island are presented. Seismic reflection and gravity measurements show the presence of a sedimentary basin at the foot of the continental slope in which the sediments are progressively more intensely deformed from north to south, indicating the interactions between the lithospheric plates. Heat flow values in the northern part of Explorer Ridge, and recovery of fresh basalts with little mineral coating in this region suggest that Explorer Ridge is a presently active spreading segment of East Pacific Rise. Seismic Refraction results in the deep ocean basin west of Queen Charlotte Islands show a marked anisotropic mantle P wave velocity, the direction of maximum velocity being 107° east of north and the maximum change in velocity being about 0.6 km/s.