Phase and group velocities and Q of mantle Love and Rayleigh waves of the first two modes and their azimuthal dependences for the 1963 Kurile Islands earthquake

Phase and group velocities and Q of mantle Love and Rayleigh waves from the 1963 Kurile Islands earthquake (M_w = 8.5) were determined over 37 great circle paths by a time variable filtering technique, in a period range 100–500 s for the fundamental modes and 100–275 s for the first higher modes. The preliminary reference Earth model (PREM) explains reasonably well the average dispersion results for the fundamental Love and Rayleigh waves. There exists a small, but significant inconsistency between the observation and the model for the first higher Love and Rayleigh waves. The Q structure of PREM is inconsistent with the observation for the fundamental Love waves, but explains other observations reasonably well. The dispersion of each mode shows a clear azimuthal dependence from which the four azimuthal windows were established. The phase and group velocity measurements for each window were, in general, shown to be mutually consistent. The azimuthal variations are largest for the first higher Rayleigh waves, indicating strong lateral heterogeneity in the structure of the low velocity zone. The first of the four windows is characterized by the largest fraction of Precambrian shields and the second window by the largest fraction of normal oceans. A comparison of these two windows may give some insight into deep lateral heterogeneity between continents and oceans. The observed phase and group velocities of the first window are systematically higher than those of the second window for the fundamental Love and Rayleigh waves at periods up to 400 s, and for the first higher Love and Rayleigh waves up to 175 s. Their differences are greatest for the first higher Rayleigh waves and least for the fundamental Rayleigh waves. Although the fundamental Rayleigh waves show the least velocity differences, their persistence up to a period of longer than 300 s is in striking contrast with some of the pure path phase velocities derived earlier for continents and oceans. A set of models for continents and oceans. PEM-C and PEM-O are not consistent with our observation. The third azimuthal window is characterized by trench-marginal seas and the fourth window by mountainous areas, typically the Asian high plateaus from northern China to the Middle East through Tibet. A comparison of these two windows gives some information about deep structural differences between subduction zones and continental collision zones, both belonging to plate convergence zones. For the fundamental and the first higher Love waves, the phase and group velocities for the third window are markedly low, whereas those for the fourth window are somewhat comparable to those for the second window. Slow Rayleigh waves are evident for two windows, with the fourth window apparently being the slowest for the fundamental Rayleigh above 200 s and for the first higher Rayleigh. For the fundamental Rayleigh waves, the third window is very slow below 200 s, but becomes progressively fast as the period increases and tends to be the fastest window around 400 s, suggesting a deep seated high velocity anomaly beneath trench-marginal seas. The dispersion characteristics of the fourth window indicate a thick high velocity lid with an extensive low velocity zone beneath it. The shield-like lithosphere, coupled with an extensive low velocity zone, may be a characteristic feature of continental collision zones. The particle motion of the fundamental Love waves was found not to be purely transverse to a great-circle connecting the epicenter to a station. The departure from the purely transverse motion is systematic among different periods, different G arrivals (G₂, G₃,…) and different stations, which may be interpreted as being due to lateral refraction.     

A surface-wave investigation of the rupture mechanism of the Gobi-Altai (December 4, 1957) earthquake

Long-period records of multiple Love waves from the 1957 earthquake in Mongolia (M_S = 8.0) at Pasadena are analysed and compared to synthetic seismograms, generated by the method of Kanamori. A fit in the time domain shows that the records are not consistent with the previous solution, achieved through a frequency-domain analysis of directivity by Ben-Menahem and Toksöz. The solution asks for a shorter rupture of 270 km at a velocity of 3.5 km/s. The focal parameters are constrained by updating all the reported first motion and are found to be: strike = 103°, dip = 53°, slip = 32°. A seismic moment of 1.8 · 10²⁸ dyn · cm is obtained. These figures are also consistent with a time-domain analysis of Love waves at Palisades and Strasbourg, and of Rayleigh waves at Pasadena, with a directivity study of Love waves at Pasadena, and with static deformation and isoseismal data. A discussion is given of the relation between moment, magnitude and rupture area, and a comparison is made with other events in the same region: it is concluded that this earthquake does not exhibit an “intra-plate” behavior, but rather compares better with “inter-plate” events, such as the great Assam earthquake.     

Seismic Velocity and Q Structure of the Middle Eastern Crust and Upper Mantle from Surface-wave Dispersion and Attenuation

—Observed velocities and attenuation of fundamental-mode Rayleigh waves in the period range 7–82 sec were inverted for shear-wave velocity and shear-wave Q structure in the Middle East using a two-station method. Additional information on Q structure variation within each region was obtained by studying amplitude spectra of fundamental-mode and higher-mode Rayleigh waves. We obtained models for the Turkish and Iranian Plateaus (Region 1), areas surrounding and including the Black and Caspian Seas (Region 2), and the Arabian Peninsula (Region 3). The effect of continent-ocean boundaries and mixed paths in Region 2 may lead to unrealistic features in the models obtained there. At lower crustal and upper-mantle depths, shear velocities are similar in all three regions. Shear velocities vary significantly in the uppermost 10 km of the crust, being 3.21, 2.85, and 3.39 km/s for Regions 1, 2, and 3, respectively. Q models obtained from an inversion of interstation attenuation data show that crustal shear-wave Q is highest in Region 3 and lowest in Region 1. Q’s for the upper 10 km of the crust are 63, 71, and 201 for Regions 1, 2, and 3, respectively. Crustal Q’s at 30 km depth for the three regions are about 51, 71, and 134. The lower crustal Q values contrast sharply with results from stable continental regions where shear-wave Q may reach one thousand or more. These low values may indicate that fluids reside in faults, cracks, and permeable rock at lower crustal, as well as upper crustal depths due to convergence and intense deformation at all depths in the Middle Eastern crust.     

Shallow Velocity Structure at the Shagan River Test Site in Kazakhstan

—?During 1997 and 1998, twelve chemical explosions were detonated in boreholes at the former Soviet nuclear test site near the Shagan River (STS) in Kazakhstan. The depths of these explosions ranged from 2.5 to 550 m, while the explosive yield varied from 2 to 25 tons. The purpose of these explosions was for closure of the unused boreholes at STS, and each explosion was recorded at local distances by a network of seismometers operated by Los Alamos National Laboratory and the Institute of Geophysics for the National Nuclear Center (NNC). Short-period, fundamental-mode Rayleigh waves (Rg) were generated by these explosions and recorded at the local stations, resultingly the waves exhibited normal dispersion between 0.2 and 3 seconds. Dispersion curves were generated for each propagation path using the Multiple Filter Analysis and Phase Match Filtering techniques. Tomographic maps of Rg group velocity were constructed and show a zone of relatively high velocities for the southwestern (SW) region of the test site and slow propagation for the northeastern (NE) region. For 0.5?sec Rg, the regions are separated by the 2.1?km/sec contour, as propagation in the SW is greater than 2.1?km/sec and less in the NE region. At 1.0 sec period, the 2.3?km/sec contour separates the two regions. Finally, for 1.5 and 2.0 sec, the separation between the two regions is less distinct as velocities in the NE section begin to approach the SW except for a low velocity region (<2.1?km/sec) near the center of the test site. Local geologic structure may explain the different regions as the SW region is composed predominantly of crystalline intrusive rocks, while the NE region consists of alluvium, tuff deposits, and Paleozoic sedimentary rocks. Low velocities are also observed along the Shagan River as it passes through the SW region of the test site for shorter period Rg (0.5–1.0?sec). Iterative, least-squares inversions of the Rg group velocity dispersion curves show shear-wave velocities for the southwestern section that are on average 0.4?km/sec higher than the NE region. At depths greater than 1.5?km the statistical difference between the models is no longer significant. The observed group velocities and different velocity structures correlate with P-wave complexity and with spatial patterns of magnitude residuals observed from nuclear explosions at STS, and may help to evaluate the mechanisms behind those observations.     

Great-circle Rayleigh wave attenuation and group velocity,Part I: Observations for periods between 150 and 600 seconds for seven great-circle paths

Rayleigh wave attenuation coefficients and group velocities have been estimated for seven great-circle paths. The attenuation coefficient measurements cover the period range from 100 to 500 s, and group velocities the range from 100 to 600 s. Global average group velocities and attenuation coefficients have also been estimated for these period ranges. The spread of the individual path group velocities for 20-s averaging windows centred at 290, 250, 210, 180 and 150 s is less than 0.034, 0.028, 0.024, 0.048 and 0.071 km/s, respectively. Global average attenuation coefficients, when combined with global average group velocities, show that Q for Rayleigh waves has an approximately constant value of about 145 for periods between 150 and 220 s and slowly increases to a value of about 200 at a period of 400 s.     

Seismic Precursors to Space Shuttle Shock Fronts

—?Seismic precursors to space shuttle re-entry shock fronts are detected at TXAR in Southwest Texas when the ground track of the orbiter vehicle passes within ～150–200?km of the observatory. These precursors have been termed “shuttle-quakes” because their seismograms superficially mimic the seismograms of small earthquakes from shallow sources. Analysis of the “shuttle-quake” seismograms, however, reveals one important difference. Unlike ordinary earthquakes, the propagation azimuths and horizontal phase velocities of the individual phases of the “shuttle-quakes” are functionally related. From a theoretical model developed to account for the origin of these precursors it is found that the seismic phases of “shuttle-quakes” are “bow” waves. A “bow” wave originates at the advancing tip of the shock front trace (i.e., intersection of the re-entry shock front with the surface of the earth) when the ground speed of the orbiter vehicle exceeds the horizontal phase velocity of a particular seismic phase. “Bow” waves are shown to differ in two important respects from the ordinary seismic phases. They vanish ahead of the advancing tip of the shock front trace and their propagation azimuths and horizontal phase velocities are functionally related. The ground speed of the orbiter vehicle exceeds the horizontal phase velocities of crustal seismic phase over much of the re-entry flight profile. As a result, P,S, and R _g “bow” waves will be seen as precursors to the re-entry shock front at stations located within a few hundred km of its ground track.     

40Ar-39Ar age determinations on the Owyhee basalt of the Columbia Plateau

⁴⁰Ar/³⁹Ar step-heating analyses have been performed on 11 samples of basalt from sites near Owyhee Reservoir of southeastern Oregon, U.S.A. These rocks were extruded during the great flood basalt episode of the Pacific Northwest. The whole-rock points are highly correlated on a plot of⁴⁰Ar/³⁶Ar versus³⁹Ar/³⁶Ar, corresponding to a common age of the samples of 14.3 ± 0.3 m.y. Inspite of this, individual “plateau” plots of the age versus fraction of³⁹Ar released do not give good plateaux. These age spectra exhibit to varying degrees a common structure in which lower age values are found at higher temperatures. This pattern may result from a closed-system redistribution of the argon isotopes. The usefulness of grinding the basalts in removing a loosely held atmospheric argon component is confirmed.     

Contrasted turbulence intensities in the Indonesian Throughflow: a challenge for parameterizing energy dissipation rate

Microstructure measurements were performed along two sections through the Halmahera Sea and the Ombai Strait and at a station in the deep Banda Sea. Contrasting dissipation rates (??) and vertical eddy diffusivities (K _z ) were obtained with depth-averaged ranges of \(\sim [9 \times 10^{-10}-10^{-5}]\) W kg^??1 and of \(\sim [1 \times 10^{-5}-2 \times 10^{-3}]\) m² s^??1, respectively. Similarly, turbulence intensity, \(I={\epsilon }/(\nu N^{2})\) with ν the kinematic viscosity and N the buoyancy frequency, was found to vary seven orders of magnitude with values up to \(10^{7}\). These large ranges of variations were correlated with the internal tide energy level, which highlights the contrast between regions close and far from internal tide generations. Finescale parameterizations of ?? induced by the breaking of weakly nonlinear internal waves were only relevant in regions located far from any generation area (“far field”), at the deep Banda Sea station. Closer to generation areas, at the “intermediate field” station of the Halmahera Sea, a modified formulation of MacKinnon and Gregg (2005) was validated for moderately turbulent regimes with 100 < I < 1000. Near generation areas marked by strong turbulent regimes such as “near field” stations within strait and passages, ?? is most adequately inferred from horizontal velocities provided that part of the inertial subrange is resolved, according to Kolmogorov scaling.     

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.     

Velocities,elastic moduli and weathering-age relations for pacific layer 2 basalts

Compressional (V_p) and shear (V_s) wave velocities have been measured to 10 kb in 32 cores of basalt from 14 Pacific sites of the Deep Sea Drilling Project. Both V_pandV_s show wide ranges (3.70to6.38km/sec forV_pand1.77to3.40km/sec forV_sat0.5kb) which are linearly related to density and sea floor age, confirming earlier findings by Christensen and Salisbury of decreasing velocity with progressive submarine weathering based on studies of basalts from five sites in the Atlantic. Combined Pacific and Atlantic data give rates of decreasing velocity of ?1.89and?1.35km/sec per100my forV_pandV_s respectively. New analyses of oceanic seismic refraction data indicate a decrease in layer 2 velocities with age similar to that observed in the laboratory, suggesting that weathering penetrates to several hundred meters in many regions and is largely responsible for the extreme range and variability of layer 2 refraction velocities.     

Great-circle Rayleigh wave attenuation and group velocity,Part IV: Regionalization and pure-path models for shear velocity and attenuation

Pure-path averages for group velocities and specific attenuation have been calculated from individual observations and from path averages for two regionalizations; one original to this study and the other previously devised by Wu. Both are based on four upper-mantle provinces: ocean basin, continent, island arc and mid-ocean ridge. Pure-path group velocities and specific attenuation have also been calculated for combinations of regions and provide well separated regional measurements for such composite regions.Shear-velocity models for pure and combined regions have been derived by a controlled Monte Carlo inversion procedure and indicates that a low-velocity zone is required beneath the oceans, but is not required beneath continents. Models have been produced for pure and combined ocean, ocean-ridge, continent and continent-arc provinces.Q^?1_R determined from pure-path average group velocities and attenuation coefficients has been regionalized successfully for 2- and 3-region combinations. The resulting pure-path Q^?1_R for continents is much lower than that for ocean basins and ocean-ridge provinces. Inversion of Q^?1_R for ocean-ridge provinces shows that the average Q_β for the upper 200 km of these regions is between 85 and 100.     

East Pacific Rise at latitude 21°N: isotopic composition and origin of the hydrothermal sulphur

The sulphur isotope composition of 16 pyrite and chalcopyrite samples from recent sulphide deposits (“Cyana”—project RITA) and active sulphide mineralisation (“Alvin”—project RISE) associated with hydrothermal sources at 380±30°C on the East Pacific Rise at latitude 21°N have been measured. The³⁴S/³²S ratios are relatively uniform and essentially identical for both sites: δ³⁴S=+1.4to3.0%. (CDT), mean +2.1‰. The sulphides were analysed after the majority of the very numerous micro-inclusions of anhydrite had been removed.Two independent physico-chemical analyses of the data demonstrate that about 90% of the sulphur was leached from the basaltic host rocks by the circulating seawater-hydrothermal fluids.     

Attenuation models of the earth

Free oscillation and body wave data are used to construct average Q models for the earth. The data set includes fundamental and overtone observations of the radial, spheroidal and toroidal modes, ScS observations and amplitudes of body waves as a function of distance. The preferred model includes a low-Q zone at both the top and the bottom of the mantle. In these regions the seismic velocities are likely to be frequency dependent in the “seismic” band. Absorption in the mantle is predominantly due to losses in shear. Compressional absorption may be important in the inner core.A grain-boundary relaxation model is proposed that explains the dominance of shear over compressional dissipation, the roughly frequency independent average values for Q and the variation of Q with depth. In the high-Q regions, the lithosphere and the midmantle (200–2000 km), Q is predicted to be frequency dependent. However, the low-Q regions of the earth, where Q is roughly frequency independent, dominate the observations of attenuation.     

Earthquake doublets in the Solomon Islands

Large, shallow, thrust earthquakes in the Solomon Islands region tend to occur in closely related pairs. Two recent sequences are July 14, 1971 (M_S = 7.9) and July 26, 1971 M(_S = 7.9) and 14^h37^m, July 20, 1975 (M_S = 7.9) and 19^h54^m, July 20, 1975 (M_S = 7.7). The mechanism of these seismic doublets has important bearing on the triggering mechanism of earthquakes in subduction zones. Detailed analysis of the seismic body waves and surface waves were performed on the 1971, 1974, and 1975 doublets, providing a better understanding of: (1) the mechanics of seismic triggering, (2) the state of stress on the fault plane, and (3) the nature of subduction between the Pacific and Indian plates. The results indicate that although the geometry of the subduction zone in the Solomon Islands is complicated by the presence of several sub-plates, the slip direction of the Indian plate with respect to the Pacific plate is relatively uniform over the entire region. The large seismic moments of the 1971 sequence (1.2 · 10²⁸ and 1.8 · 10²⁸ dyne cm) indicate that these events directly represent the underthrusting of the Indian and Solomon plates beneath the Pacific plate. The body waves from these doublets, recorded on the WWSSN long-period seismograms, are remarkably impulsive and simple compared with those from events of comparable seismic moment in other subduction zones. In addition, the source dimensions of the body waves are 30–70 km in length, substantially smaller than the overall rupture surfaces radiating the surface waves which are 100–300 km in length. These facts suggest the existence of relatively large, isolated high-stress zones on the fault plane. This type of stress distribution is distinct from other regions which have more heterogeneous stress distribution on the fault plane, and this is proposed as the principal characteristic of this region responsible for the occurrence of the doublets and for the apparent efficiency of triggering in the Solomon trench. Prior to the 1971 sequence, similar sequences have occurred in the same area in 1919–1920 and 1945–1946. From the amount of slip (1.3 m) determined for the 1971 sequence and the apparent recurrence interval of 25 years, a seismic slip rate of 5 cm yr^?1 is determined. This value is a significant portion of the convergence rate between the Indian and Pacific plates indicating that the plate motion here is taken up largely by seismic slip.     

Sources of microseismicP waves

Summary Origin ofP waves detected earlier in microseisms of very quiet locations in the USSR is discussed in detail. It appears that the most pronounced sources ofP waves are tropical cyclones over the Pacific. The amplitude of the force in the source for a medium power typhoon is found to be of the order of 10¹⁶ dynes. The effective source area is estimated as 10⁴–10⁵ km² approximately. The shape of the amplitude spectrum ofP wave corrected for the absorption in the mantle does not contradict with the standing wave theory of microseisms generation. Results of observations at various epicentral distances give strong evidences of the predominant attenuation of the fundamental Rayleigh mode as compared with higher Rayleigh modes andP waves in the frequency band of 0.3–0.15 cps.     

Anisotropic models of the upper mantle

Long period Rayleigh wave and Love wave dispersion data, particularly for oceanic areas, have not been simultaneously satisfied by an isotropic structure. In this paper available phase and group velocity data are inverted by a procedure which includes the effects of transverse anisotropy, anelastic dispersion, sphericity, and gravity. We assume that the surface wave data represents an azimuthal average of actual velocities. Thus, we can treat the mantle as transversely isotropic. The resulting models for average Earth, average ocean, and oceanic regions divided according to the age of the ocean floor, are quite different from previous results which ignore the above effects. The models show a low-velocity zone with age dependent anisotropy and velocities higher than derived in previous surface wave studies. The correspondence between the anisotropy variation with age and a physical model based on flow aligned olivine is suggestive. For most of the Earth SH > SV in the vicinity of the low-velocity zone. Neat the East Pacific Rise, however, SV > SH at depth, consistent with ascending flow. Anisotropy is as important as temperature in causing radial and lateral variations in velocity. The models have a high velocity nearly isotropic layer at the top of the mantle that thickens with age. This layer defines the LID, or seismic lithosphere. In the Pacific, the LID thickens with age to a maximum thickness of ～50 km. This thickness is comparable to the thickness of the elastic lithosphere. The LID thickness is thinner than derived using isotropic or pseudo-isotropic procedures. A new model for average Earth is obtained which includes a thin LID. This model extends the fit of a PREM, type model to shorter period surface waves.     

The variation of cosmogenic Kr and Xe in a core from Estherville mesosiderite: direct evidence that the lunar131Xe anomaly is a depth effect

Kr and Xe were measured by a stepwise heating technique in three samples of a drill core in the “Minnesota” fragment of the Estherville mesosiderite. The cosmogenic⁷⁸Kr/⁸³Kr decreased from the “top” sample to the “bottom” sample(“top” = 0.163 ± 0.005, “bottom” = 0.151 ± 0.005) while the cosmogenic¹³¹Xe/¹²⁶Xe ratio increased(“top” = 5.58 ± 0.35, “bottom” = 6.92 ± 0.17). Cosmic-ray track studies have shown that the “top” sample was indeed closer to the preatmospheric surface than the “bottom” sample by ～ 10 cm. This is the first direct evidence, in a sample of known geometry, that the cosmogenic¹³¹Xe/¹²⁶Xe ratio increases as a function of depth, and as such, confirms the hypothesis that the lunar¹³¹Xe anomaly is a bona fide depth effect due to resonance neutron capture in¹³⁰Ba.     

Electromagnetic induction in the earth due to stationary and moving sources

A new approach to the theory of electromagnetic induction is developed that is applicable to moving as well as stationary sources. The source field is considered to be a standing wave generated by two waves travelling in opposite directions along the surface of the earth. For a stationary source the incident waves have velocities of the same magnitude, however for a moving source the velocities of the two incident waves are respectively increased and decreased by the velocity of the source. Electromagnetic induction in the earth is then considered as refraction of these waves and gives, for both stationary and moving sources, the magnetotelluric relation: $$\frac{{ - E_y }}{{H_x }} = \left( {\frac{{i\omega \mu }}{\sigma }} \right)^{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}} \left( {1 - i\frac{{v^2 }}{{\omega \mu \sigma }}} \right)^{ - {1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}} $$ where ν is the wavenumber of the source, μ is the permeability (4π·10^?7) and σ is the conductivity of the earth. ω is the angular frequency of the variation observed on the earth. For a stationary source the observed frequency is the same as the source frequency, however the effect of moving a time-varying source is to make the observed frequency different from the frequency of the source. Failure to recognise this in previous studies led to some erroneous conclusions. This study shows that a moving source isnot “electromagnetically broader” than a stationary source as had been suggested.     

Studies of mantle structure of U.S.S.R. territory on long-range seismic profiles

Long-range seismic sounding carried out during the last few years on the territory of the U.S.S.R. has shown a basic inhomogeneity of the uppermost mantle, as well as evidence of regularities in the distribution of its seismic parameters. The following data were used: times and apparent velocities of P- and S-waves for investigation of mantle velocities, converted waves for seismic discontinuity model studies and wave attenuation for Q-factor estimation. Strong regularities were distinguished in the distribution of average seismic velocities for the uppermost mantle, in their dependence on the age and type of geostructure and on their position relative to the central part of the continent. Old platforms and the inner part of the continent are marked by velocities under the Mohorovi?i? discontinuity of more than 8.2–8.3 km s^?1, young platforms and outer parts of the continent by 8.0–8.2 km s^?1, and orogenic and rift zones by 7.8–8.0 km s^?1. The difference becomes more pronounced at a depth of about 100–200 km: for the old platform mantle velocities of 8.5–8.6 km s^?1 are typical; beneath the orogenic and rift areas, inversion zones with velocities less than 7.8 km s^?1 are observed.The converted waves show fine inhomogeneities of the crust and uppermost mantle, the presence of many discontinuities with positive and negative changes of velocity, and anisotropy of seismic waves in some of the layers. Wave attenuation allowed the determination of the Q-factor in the mantle. It varied from one region to another but a close relation between Q and P-wave velocity is the main cause of its variation.     

Morphology of seamounts in the western Pacific and Philippine Basin from multi-beam sonar data

New multi-beam bathymetric data from the Philippine Sea and northwest Pacific Basin reveal linear chains of small (less than 40 km³) domed-shaped volcanoes (Philippine) and coned-shaped volcanoes (Pacific) rising 100–1000 m above the 6 km deep ocean floor. Some appear to have well-developed collapsed calderas and spines. Their morphology suggest recent formation in supposedly stable mid-plate regions and their occurrence in linear chains approximately parallel to plate motion may suggest an origin by extrusion from “mini-hot spot” plumes.