Pn Waves Research Articles

A laboratory study of teleseismic Pn waves

In this paper, the influences of mechanical properties of low velocity layers and thickness of high velocity layers in the lower lithosphere on the horizontal propagaties of seismic waves are discussed by means of altrasonic seismic model experiments; It is shown that the structure of the lower lithosphere consisting of soft low velocity layers and thin high velocity layers is favourable for the horizontal propagation of seismic energy, therefore, it is a very possible structure pattern of the lower lithosphere responsible for Pn and later high velocity arrivals observed in long range distances; Such pattern is also consistent with studies of rheological stratification of the lower lithosphere beneath mid-ocean-ridges and continental rifts.

The Pn wave velocities and the relief of Moho in the Tibetan Plateau

Eleven PASSCAL broadband digital seismic stations were employed in the Tibetan Plateau by the Sino-US team from September, 1991 to June, 1992. Seven of them were distributed along the Qinghai-Tibet highway, others in Maqin and Yushu of Qinghai Province, Linzhi and Xigatze of Tibet. The data included 31 local earthquakes recorded by these stations from July, 1991 to January, 1992. Considering the characters of digital data, we identified the seismic phases carefully in several methods using SAC softwares (Seismic Analysis Code) in SUN workstation. We compared the seismic phases after converting the seismograms of the single stations to the seismic profiles; analyzed the first arrivals of P waves in the incident planes by rotating 3 component seismic records; identified the seismic phases from the particle motion pictures. The Pn apparent velocities were calculated in the distance range of 230–1200 km from Linzhi earthquakes, western Changtang earthquakes, Xitieshan and Gonghe earthquakes and the earthquakes in India. The results show that the Pn velocities change slightly in the Tibetan Plateau (8.0–8.1 km/s). These values near the velocities at the uppermost mantle of the normal continents. The Moho undulation in the Tibetan Plateau was also studied by using Pn data by the time-term method. The primary results indicate that the Moho beneath the Tibetan Plateau is flat, and its depths are 67–70 km.

Discrimination using spectral slopes at frequencies up to 50 hz

Abstract The spectral characteristics of regional Pn waves recorded at the High Frequency Seismic Element (HFSE) at the NORESS array in southern Norway are investigated to determine if spectral slope can be used to discriminate between earthquakes and chemical explosions. Displacement spectra are calculated for 134 events in southern Norway that include 36 known earthquakes and 36 known chemical explosions. Events range in distance from 230 to 620 km, and most have signal above the noise at frequencies over 30 Hz. The average frequency at the peak signal-to-noise ratio (SNR) is 15.5 Hz. Spectral slopes are measured on a displacement spectrum calculated with a 2-sec window. The slopes range between ω−1.8 and ω−5.1, and there is no strong evidence that the slope steepens with distance for this data set. According to one theory, earthquakes should have steeper slopes than explosions, but when this data set is separated into earthquakes and explosions we do not see separation between sources with respect to the slope of the spectrum. Both source types have events with the same range of spectral slopes, and the mean slopes and standard deviations are similar. Spectra of events at the same locations are often similar. However, explosions from the Blåsjø Mine in western Norway show variability in the spectral slope. Since the path and distance to HFSE are the same for these events, the variability must be due to differences in the source mechanism. For a set of small earthquakes, however, the observed variability in spectral slopes appears to be more related to distance and path effects. Therefore, we observe as much variation in spectral slope for one source type as there is between source types. The time window over which the spectrum is calculated is varied from 1 to 8 sec to determine if a time window consisting of a pure Pn wave was required to discriminate between source types. Spectral slope usually decreases as the window extends deeper into the coda and the energy content of the signal changes. The slopes also become more stable and more reliable as longer windows are used. However, source discrimination is not possible with any time window.

Geometrical spreading and Q of Pn waves: An investigative study in eastern Canada

Abstract Station site effects, uncertainties in seismic source spectrum, and instrument response errors are among the well-known frequency-dependent contaminating factors that limit the reliability of short-period Q measurements of regional phases. For the Pn wave, a regional phase of importance for both magnitude determination and nuclear test ban verification, the problem is made worse by the added uncertainty of its geometric spreading function. For realistic earth models, the Pn geometric spreading function is likely to depart drastically from that expected of canonical head waves. The extent of this departure is sensitively dependent upon the regional crust/mantle structure, making geometric spreading assumption a conspicuous source of disagreement among the published Pn attenuation (QPn) estimates. We describe a technique, referred to here as the extended reversed two-station method (RTSM), for simultaneous determination of QPn and geometrical spreading function. The formulation, being designed to bring about direct cancellation of the contaminating source, station and instrument effects, is a reliable tool for mapping the Pn propagation characteristics over continental paths, long and short. The extended RTSM has been tested using Pn spectral amplitude data derived from seismic records of the Eastern Canada Telemetered Network (ECTN). We find the spreading rate coefficient n in the power-law representation of the geometric spreading (d−n, d being epicentral distance) to be frequency dependent, increasing from 1.11 at 1 Hz to 1.77 at 20 Hz. Our QPn model in eastern Canada takes the form of QPn = 189f0.87. The results from eastern Canada suggest that: (a) there exists a significant positive velocity gradient in the uppermost mantle (≧ 0.0037 sec−1); (b) the regionally recorded Pn waves are dominated by the superposition of a series of interfering diving waves bent by the velocity gradient and internally reflected at the underside of the Moho discontinuity; and (c) the very strong frequency-dependence of QPn we found in this study region may not be unique among low-attenuating shield and platform regions.

Azimuth estimation capabilities of the NORESS regional seismic array

Abstract We have investigated the regional Pn backazimuth estimation capabilities of the NORESS seismic array as a function of element spacing, frequency band, and time window to determine which parameters are optimal for reducing azimuth errors. We used a broadband frequency-wavenumber estimator to calculate backazimuths from the Pn arrival for each of 274 regional events recorded at NORESS for 126 parameter combinations. The large data base provides a wide range of signal-to-noise ratio (SNR) (0 to 70 dB), distance (up to 10.5°), and azimuth characteristics, and includes identified earthquakes and explosions as well as “unknown” sources. Most of the errors in backazimuth are less than 20° when appropriate parameters are used, and mean backazimuth errors are close to zero. The best results are obtained using a 13-element array configuration that has a 1.4 km aperture and a maximum station spacing of about 600 m. With the 13-element configuration and the data filtered to include frequencies between 3 and 10 Hz, the mean errors for the 274 event data set are less than 1.4°, and standard deviations are as small as ± 11.1°. The entire array also produces good results for 3 to 10 Hz, and a 9-element configuration (two inner rings) performs well at high frequencies. The five-element B-ring (600 m aperture) appears to be important in obtaining good backazimuth estimates for regional Pn waves. Of the frequency bands considered in this study, the 3 to 6 Hz, 4 to 8 Hz, and 5 to 10 Hz bands yield the most reliable backazimuth estimates, even better than an “optimal” band that calculates the azimuth in the fixed frequency band that has the largest average SNR. The time-window length has little effect on the backazimuth estimates. Other factors investigated include SNR, source region, and phase type. We found that event backazimuth accuracy degrades if the SNR of a beamed Pn signal is less than 5 dB in the frequency band of interest. Conversely, the backazimuth estimation statistics improve if only events with 5 dB of SNR are included in the event set. These data sets yield near-zero mean errors and smaller standard deviations than the entire data set. For specific source regions, standard deviations are as low as 2° for some parameter combinations, but there can also be large biases in the backazimuth estimates. Events farther than 500 km from NORESS tend to have larger azimuth errors than the closer events, but combinations of small aperture configurations and middle (3 to 10 Hz) frequency bands work well for events over the entire distance range of 40 to 1200 km. The Pn arrival and RONAPP azimuths calculated from the Lg phase have similar accuracy statistics for a 220 event common data base, implying that the two phases work equally well for regional event backazimuth estimation. In fact, averaging of the Lg and Pn estimates provides the most accurate backazimuths relative to PDE reference information.

High-frequencyPnattenuation in the Canadian shield

AbstractFrequency-dependent spatial attenuation of Pn waves, incorporating the combined effects of geometrical spreading and anelastic dissipation, is investigated in eastern Canada between 3 and 15 Hz. Measurement of this total attenuation, instead of the two separate effects, circumvents the usual need for making a priori assumptions regarding the Pn geometrical spreading rate. The data consist of 77 Pn spectra, all amplitude-normalized using a set of previously measured spectral ratios of eastern Canadian earthquake sources. The normalization procedure eliminates the need for explicit source spectral assumptions, another common source of error in Pn attenuation measurements. An unconventional technique is introduced to account for differences in observed Pn spectra, which arise from geological site effects and instrument response error, both of which are frequency-dependent phenomena. We conclude that, at each frequency, the Pn amplitude falls off with distance according to: Δ−n, where Δ is epicentral distance. The exponent n is weakly frequency-dependent and takes the form: 2.2 + 0.02f. A unique interpretation of the behavior of the Pn spectral amplitude decay is unattainable, owing to the difficulty in separating the effects due to anelastic dissipation and the velocity structure in the uppermost mantle. It is interesting to note, however, that the spatial decay of pure elastic head waves follows the classical Δ−1/2L−3/2 relation, where L is the distance the waves travel in the mantle refractor. For the distance range of our interest (260-1087 km), this amounts to Δ−2.2. This indicates that towards low frequencies our n approaches a value which is consistent with the spatial decay rate expected of pure elastic head waves.

Seismic model of the Earth's crust and upper mantle for the east European platform

A model is presented of the P-wave velocity distribution for the east European platform from the sedimentary layer through the crystalline complex of the Earth 's crust, the lower lithosphere and the asthenosphere, down to a depth of 900 km in the upper mantle. The structure of the Earth 's crust for the east European platform is complex and heterogeneous. The crystalline basement usually lies at a depth of 2 -3 kin, outcropping in the area of the Baltic and Ukrainian shields. A characteristic of the marginal zones of the Precambrian Platform is the existence of large sedimentary basins, where the thickness of the sediments exceeds 10 km (e.g. P o l i s h G e r m a n D a n i s h ' trough, Donbass basin, Pre-Caspian foredeep, Pechora basin and Barents Sea depression. For the sedimentary cover we have determined the dependences of the velocities on depth from the results obtained from 261 boreholes in the Polish part of the east European platform. In the sediments, a distinct increase in velocity with increasing depth is observed and there is also a large velocity differentiation, from about 1.8 km s for the Tertiary and Quaternary to about 5.0-5.5 km s a for Permian, Devonian and Cambrian formations. The j ump in velocity at the boundary of the Cenozoic-Mesozoic cover and the Permian exceeds 1.0 km s i , and consequently it is the most distinct seismic discontinuity in the sedimentary layer. For the Cenozoic-Mesozoic cover and for the whole sedimentary complex, we determined the dependences of the mean velocities on thickness. To determine the dependence of the mean velocity on the thickness of the whole sedimentary layer, we used data from 71 boreholes which reached the crystalline basement. For these data, up to a depth of 5.6 km we obtained the linear dependence Vsed(z) = 2.13 + 0.34 z. For a greater depth, for which there are no data from direct measurements in boreholes, we used the results of interpretation and modelling of shallow refraction seismic data. With the formula obtained from these data, Vsed(z) = 2.35~/1 + 0.227 z , we can estimate the mean velocity of sediments for thicknesses up to 10 km. In the Polish part of the east European platform the thickness of the Earth 's crust is 42-47 kin, and the boundary velocity of Pn waves at the Moho is 8.1-8.3 km s -1. The crystalline complex of the crust consists of three parts, with velocities of 6.1-6.3, 6.7-6.9 and 7.2-7.3 km s -a (Grad, 1986). In the other regions of the platform, the Earth 's crust thickness varies over a wide range from 35 to 55 km. The mean crust thickness is 40-45 kin. In the crystalline complex we can distinguish three different parts in the layer velocity and the velocity gradient. The first, upper part is characterized by velocities of 5.8-6.4 km s -a , corresponding to acid rocks. In this part there are zones of low velocities, horizontal heterogeneities and also fractures and tectonic disturbances. The second, transition part, is characterized by velocities of 6.3-6.6 km s -1, corresponding to basic rocks with an admixture of acid rocks. The lower part with velocities of 6.8-7.5 km s-1 is built from basic rocks with the addition of ultrabasic rocks (Pavlenkova, 1984). The mean velocity of the whole crystalline complex is 6.4-6.5 km s -1, and the velocity beneath the Moho is 8.1-8.3 km S -1 Over the last decade, within the east European platform, a number of long-range seismic profiles were made (Fig. 1). Their purpose was to study the Earth 's crust and the lower lithosphere. In this study, we have determined one-dimensional seismic models of the lithosphere for two travel-time branches. For the shot-point P4 on the Baltic Sea-Black Sea profile, we used UkrainiaJa and Polish recordings from the distance intervals 0-400 and 600-900 kin, respectively. Refracted Pn waves from the Moho occur as the first arrivals, beginning at a distance of 220 km, and are

Lg attenuation and source studies using 1982 Miramichi data

Abstract Using data from earthquakes in the 1982 Miramichi earthquake source zone, spectral excitation and attenuation of the Lg phase are studied. With data in the distance range of 135 to 994 km, interpretation is complicated by the presence of high-frequency Sn and Pn waves which interfere with the Lg wave. At the larger distances, the signal at frequencies above 7 Hz is completely dominated by the non-Lg arrivals. A frequency-dependent Lg-Q is determined which rises from 300 at 0.5 Hz to about 1400 at 10 Hz, at which it flattens at higher frequency. The Sn coda apparent Q rises above 3000 at frequencies higher than 10 Hz. Seismic moment and corner frequency estimates are made using Lg-Q corrected spectra. The moment estimates compare well with those obtained from long-period surface waves and short distance spectral estimates. The Lg corner frequency estimates are substantially lower than the short distance estimates. This discrepancy is the subject of discussion, but the Lg moment-corner frequency estimates do model observed data well using a Brune (1970) source model and the derived attenuation relation.

Long-range propagation of short-period transversal and longitudinal seismic waves in the Baikal zone

Summary In June—October 1979–84 seismographs, recording waves with the prevailing frequency of 8 Hz, have been installed temporarily on the bottom and coasts of Lake Baikal. On quiet days the level of background noise varied from 3 to 4 nm on the west coast and on the deep bottom of the lake to 10 nm on the Ol’khon Island and to 15 nm on the east coast of the lake. The 3–4 nm noise level allowed magnification of the instruments to be kept around 1 million. Seismographs recorded earthquakes with magnitude Ms from -0.3 to +4.5. On average, two and a half seismic events were registered per day. The strongest ones were observed at distances up to 600 km. Compilation of amplitude curves and their comparison with the data available from the literature has shown that in the Baikal zone waves of 8 Hz frequency attenuate with distance not quicker than waves of 2 Hz frequency. Comparison of empirical amplitude curves with theoretical ones, calculated for the adopted velocity sections of the Baikal zone, has resulted in estimates of the quality of the medium equal to Q= 300 for 2 Hz frequency and Q= 1800-2000 for 8 Hz frequency. This phenomenon has some common features with the long-range propagation of Pn and Sn waves on continents and oceans. In the Baikal zone the long-range propagation of high-frequency P- and S-waves can be attributed to the existence of a high-quality wave-guide in the middle part of the Earth's crust, not with low Q, as is usually supposed.

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.

近畿地方北部の地殻構造

The crustal structure in the northern Kinki district is examined using apparent velocities of P first arrivals from crustal earthquakes. Data used were mainly obtained by the seismic network of the Regional Center for Earthquake Prediction of Kyoto University. It is most characteristic in this study that no sharp discontinuity in the seismic wave velocity is found at the depth of the Conrad discontinuity. This is suggested by the fact that first arrivals from the 1984 Yamasaki Fault earthquake of about 20km depth show an apparent velocity of about 6.45km/s and their plots against epicentral distances do not suggest the head wave propagating along the Conrad discontinuity. Furthermore, it is examined that the Moho discontinuity is about 36km in depth and the Pn wave about 7.8km/s in velocity. A constant velocity near 6.0km/s is feasible for the P wave in the upper crust of a depth range from 4 to 13km in which the most part of crustal earthquakes occur.

Synthesis of realistic oceanic Pn wave trains

The synthesis of realistic oceanic Pn phases was accomplished using wave number integration. Wave trains with durations of the order of 100 s following the initial compressional wave arrival were generated for a laterally homogeneous, vertically inhomogeneous oceanic lithosphere model. The excitation of these long wave trains is primarily a result of near‐receiver water and sediment reverberations and does not involve scattering. A late arriving wave train with a group velocity of 1.6 km/s having an emergent onset and substantial duration, characteristic of oceanic abyssal T phases, was also generated even with phase velocities restricted to be greater than 5.5 km/s. The synthetic Pn wave train was compared to data collected in the southwest Pacific during the Ngendei Seismic Experiment. Many of the spectral characteristics associated with sediment and water reverberation observed in the synthetics were also observed in the data. First, it was found that the amplitude falloff rate of the Pn wave train is a strongly increasing function of frequency, in direct contrast to results predicted for forward scattered coda. This leads to a predominance of low frequencies late in the wave train which is easily interpreted in terms of the constructive interference of different modes of reverberation. Second, the presence of spectral peaks at frequencies associated with both sediment and water reverberations was observed. The predicted frequencies were computed using values of the water depth, sediment thickness, and sediment velocity obtained in an independent study. Strong alignment of the spectral peaks with the predicted frequencies illustrates the reverberating nature of the Pn wave train. The presence of substantial power in excess of 15 Hz can be attributed to the confinement of Pn to the oceanic lithosphere. The oceanic Pn phase is thus characterized as a refraction from the lower lithosphere with multiple reverberations in the oceanic water column and sediment layer comprising the subsequent coda. Complicated oceanic lithosphere models are not needed to explain the gross characteristics of Pn propagation.

Velocities and propagation characteristics of Pn and Sn waves beneath the Himalayan arc and Tibetan plateau: possible evidence for underthrusting of Indian continental lithosphere beneath tibet

Velocities and propagation characteristics of Pn and Sn waves beneath the Himalayan arc and Tibetan plateau: possible evidence for underthrusting of Indian continental lithosphere beneath tibet

Some analyses of P- and Rayleigh wave amplitudes observed at North American stations

Summary P- and Rayleigh wave amplitudes observed by the Long Range Seismic Measurements network of stations in North America are analysed into source, distance and station effects. The main results of the P-wave analyses are: (a) there is a significant correlation of station effect and αn the Pn wave speed in the vicinity of the recording station; (b) there are no contributions to the station effect from differences in the noise levels at the recording stations; (c) the short-period (SP) station effects are weakly dependent on period, but the correlation may be partially masked by systematic errors in the measurement of period; (d) the variation in crustal structure makes little or no contribution to the SP and long-period (LP) station effects. The main results of the Rayleigh wave analyses are: (a)the gradient h of the decay of amplitude with distance A is nearly unity. This is in agreement with earlier estimates obtained assuming the decay has the form h log Δ+ constant; (b)the attenuation coefficient k as a function of period in the range 20 to 37.5 s in agreement with other estimates for paths across North America and for various paths from epicentres in China. The study confirms that the surface wave decay across North America is much slower than that implied by the standard calibration functions used for computing surface wave magnitude. Combined with other published estimates the values of Qγ-1 suggest that the average Qγ-1 is lower (Qγ is higher) for North America than on many other paths.

The P wave velocity of the uppermost mantle of the Rio Grande Rift region of north central New Mexico

A network of seismograph stations has operated in north‐central New Mexico since 1975. The network is approximately 200 by 300 km in size and encompasses the Rio Grande rift there. Several seismic refraction experiments have been reported in the literature for the region of the network and adjacent areas. Because all of the seismic refraction lines are unreversed, Pn velocities reported were mainly of the inverse travel time slope for the direction of the corresponding line. The values of the inverse slope for those studies range from 7.6 to 8.2 km/s. The purpose of our study is to estimate the P wave velocity of the uppermost mantle by using the time term method. First, we timed the Pn waves of strong signals from five explosions and eight shallow earthquakes recorded by the network. The main data set, which contains 87 time‐distance pairs, was processed by using the time term method. The Pn velocity estimated by this method is 8.0 ± 0.1 km/s. To corroborate this estimate, we then processed 10 subsets of the main data set in the same way. Almost all of the solutions show velocities of 7.9–8.1 km/s, in agreement with the velocity determined for the main data set. The station time terms of the main data set also are substantiated, and they suggest that the base of the crust dips northward by a few degrees in the region of the survey. The smallest value reported by other investigators for the inverse slope (7.6 km/s) appears to be related to the dip. The normal P wave velocity of the uppermost mantle of north‐central New Mexico places restrictions on thermal models of the rift. For instance, the results exclude the likelihood of a wide zone of asthenosphere at the base of the crust beneath the rift, but they do not exclude a narrow such zone.

Crust‐mantle whispering gallery phases: A deterministic model of teleseismic Pn wave propagation

Teleseismic Pn waves are modeled as a sum of whispering galley waves, which propagate in a waveguide composed of a simple high velocity mantle lid underlain by a low velocity zone. This model is able to account for those Pn wave propagation properties that are not dominated by scattering, i.e., their apparent velocity and lack of long period energy. Pn apparent velocity data constrain the P wave velocity gradient in the upper 100 km of mantle to be low; dvp/dz<0.001 s−1. The whispering gallery rays are shown to have significant amplitude (when compared to the direct P wave) and to have spectra that rapidly fall off at long periods. Yet this model cannot account for certain details of the observed Pn amplitude spectra. Most important among these is the spectral ratio of Pn to P, which the model underestimates by a factor of 10. Nevertheless, the model presents a useful framework for understanding some characteristics of Pn wave propagation and provides an estimate of the distribution of energy with depth in the Pn waves.

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.

On the composition of the oceanic lithosphere

The composition and elastic properties of the earth's lithosphere beneath the sea floor are fundamental to our understanding of the mechanism of sea floor spreading and continental drift. New experimental information on the elastic properties, as a function of temperature, pressure, and petrology, of eclogite and peridotite is presented. The density and seismic wave velocities in peridotite simulated in the laboratory for the oceanic lithosphere of the first 15–20 km of depth match results of recent seismic investigations very well. The elastic properties of olivine eclogite describe the seismic structure of the remaining lithosphere. The present study favors the idea of a chemical change within the lithosphere, and our laboratory results tend to favor m¯ ∼22 for the oceanic lithosphere. In modeling the structure in terms of temperature and pressure coefficients the surface wave studies introduce two complications: First, since partial melting is required by the very low shear wave velocities of the asthenosphere, some consistent and sensible way of treating velocities in mush must be used. Second, the mantle is anisotropic; therefore a systematic crystal orientation has to be considered rather than just the average Vp and Vs for the mantle constituents. The seismic anomalies of azimuth-dependent fluctuations in the velocity of Pn waves along the base of the oceanic crust are as much as 8%; the high velocity tends to be in the direction of sea floor spreading. A consistent explanation of this effect would be the presence of a sustained extensional strain rate in the spreading direction, applied at the base of the oceanic lithosphere. It would appear that the anisotropy vanishes near the surface of the oceanic plate.

Some results for the middle western Mediterranean basin from the study of Pn waves

abstract The most important contributions to our understanding of the structure of the deep layers of the crust beneath the western Mediterranean Sea come from surface-wave dispersion studies. The results obtained in this region from studies of body waves are few, sometimes discordant, and generally applicable only to areas of moderate areal extent. In this paper, the results of a study of Pn waves are reported. A first solution, based on arrival-time data at Mediterranean coastal stations, gives, for the distance interval 1.78° to 10.70°, the relation T = 8.21 ( ± 0.24 ) + Δ / ( 8.04 ± 0.02 ) where Δ is expressed in kilometers. Subsequently, by analysis of the residuals and by taking account of the uncertainty in the value of the apparent velocity, as shown in the previous equation, a further approximation can be attempted in the form of two linear equations. The first one, which applies to the interval 1.78° to 5.59°, takes the form T 9 = 6.99 ( ± 0.45 ) + Δ / ( 7.82 ± 0.07 ) , giving for the apparent Pn velocity the value of 7.82 km/sec, which is in agreement with the results of a seismic refraction study carried out by Fahlquist in the northeastern area of the Balearic Islands. The second equation, for the interval of 5.59° to 10.70°, indicates a further refracting horizon, 25 km deeper than the previous one, with a velocity of 8.19 km/sec. This equation takes the form T = 10.14 ( ± 1.04 ) + Δ / ( 8.19 ± 0.08 ) . . These equations, even with some reservations because of insufficient observational data at about 9° to 10°, confirm the validity of the hypothesis formulated by Berry and Knopoff regarding the longitudinal wave velocity at depths between 29 and 49 km. They suggest a crust of oceanic type in the upper part and of transition type in the deeper part (epicontinental crust) of the western Mediterranean basin.

A Seismic Investigation of the Crust and Moho on a Line Perpendicular to the Grenville Front: Reply

A major structural feature 50 to 75 km in width has been outlined along the Moho in an area just south of the Grenville Front from a delay-time analysis of Pn waves arriving from a number of different azimuths. The crust in this anomalous region was found to be from 5–10 km thicker than in the surrounding area. The low frequency portion of the gravity low in central Quebec can be attributed to this feature. The fact that the front is deep-seated suggests that it may mark the position of an ancient fault connected tectonically in origin to the mid-continent gravity high and Lake Superior syncline.During the summer of 1968, the University of Western Ontario participated with the Dominion Observatory of Canada and other research groups in a crustal survey of eastern Quebec. Shots of various sizes were fired by the Dominion Observatory at six different locations in central Quebec. Two of the locations were in the Superior (structural) Province, two in the Grenville (structural) Province, and two in the vicini...