Positive Velocity Gradient Research Articles

Tomographic study of upper-mantle structure of the South African continent, using wave-form inversion

Wave-form inversion has been applied to S wave and surface wave data from a broad-band portable array. The stations were located on the Kaapvaal Craton and adjacent Namaqua mobile zone. Using 10 modes, the vertical-component wave-forms were fitted in the frequency window from 0.05 to 0.02 Hz. Two events located roughly on the array's great circle were processed both forwards and in reverse. The background model for these inversions was the Preliminary Reference Earth Model (PREM). The southern part of the profile is located mainly on the mobile zone, and this has an S wave velocity-depth relation similar to that of the reference model. The velocity-depth distribution of the profile which is entirely within the Kaapvaal Craton has an S wave velocity of about 4.3 km s −1 down to a depth of 80 km; this is followed by a positive velocity gradient and a correspondingly higher velocity of about 4.6 km s −1 between 110 and 220 km. Published results of the structure of cratons within the African continent indicate positive velocity gradients down to depths of 110–120 km and negative gradients down to 200 km depth.

Shear-velocity structure of the crust and upper mantle beneath the Tibetan Plateau and southeastern China

This paper addresses the velocity structure of the crust and upper mantle beneath southern China with special emphasis on the Tibet region. Waveform data from 48 earthquakes as recorded on the WWSSN and GDSN are used in this detailed forward modelling study. Constraints on the upper crustal section are derived from modelling local Love waves in the time domain applying the mode-sum modelling technique. Lower crustal constraints are derived by modelling the P_(nl)-wavetrain with the reflectivity method. An average crustal thickness of 70 km is obtained beneath the Tibetan Plateau with a modest increase of velocity with depth. The lithospheric and upper mantle structure is deduced from modelling S and SS triplication waveform data and relative traveltimes by applying a combination of WKBJ and generalized ray methods. S-SS seismograms chosen with bounce-points directly under Tibet allow remote sensing of this inaccessible region. The resulting model is an averaged 1-D model where corrections for lateral variation have been applied. We conclude that the upper mantle structure in the entire region is basically shield-like below 200 km (SNA). However, the velocity of the lithosphere is abnormally slow, roughly 5 per cent beneath Tibet. The model for Tibet derived does not have a distinct lid, and has a positive velocity gradient in the crust, suggesting crustal shortening. A preliminary velocity model for southeastern China is also suggested.

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.

Crustal structure of the Northwestern Basin and Range Province from the 1986 Program for Array Seismic Studies of the Continental Lithosphere Seismic Experiment

The Basin and Range province of the western United States is characterized by active east‐west extension at strain rates of ∼1 cm yr−1, high regional heat flow of ∼90 mW m−2, and widespread late Cenozoic bimodal rhyolitic‐basaltic magmatism. The 1986 Program for Array Seismic Studies of the Continental Lithosphere Basin and Range seismic experiment imaged a portion of northwestern Nevada to determine the crustal structure and to assess reported differences between refraction versus reflection determinations of Moho depth and how the crustal composition and structure has been influenced by volcanic and extension mechanisms. Our interpretation of the refraction/wide‐angle reflection data suggests that the crust is fairly uniform in thickness and varies by less than 5 km over the 280 km east‐west profile and 3 km over its 220 km north‐south length. We characterize the velocity structure by five layers: (1) an upper most crust, composed of sedimentary rocks and basement that has an average velocity of 5.7 km s−1, (2) a middle crust that extends to a depth of 18–22 with an average velocity of 6.1 km s−1, (3) a 10–12 km thick lower crust with an average velocity of 6.6 km s−1, (4) a 2–5 km thick transitional crust‐mantle boundary defined by a 7.6 km s−1 velocity, and (5) an upper mantle with an average Pn velocity of 7.9–8.0 km s−1. These data show a uniformly thick crust of 28 km±5 km beneath northwestern Nevada and a lack of evidence for significant low‐velocity crustal layers. In general, the entire crustal column exhibits uniform positive velocity gradients of 0.02–0.04 s−1. The midcrustal cliscontinuity is interpreted as a small velocity increase of 0.2 km s−1 at ∼18 km depth. The absence of a significant midcrustal reflection and the presence of large velocity gradients throughout the crust arises either from (1) a systematic increase in mafic composition with depth, or (2) possibly high lower crustal pore pressure that acts to decrease the velocity contrast across the midcrustal boundary. Our interpreted seismic velocity structure is consistent with a broad range of bulk compositions for the crust. Upper crustal velocities are consistent with bulk compositions that range from metasediments and granites at shallow depth to diorites at midcrustal levels, while lower crustal velocities suggest bulk compositions that range from diorites and amphibolities, at midcrustal depths, to mafic granulites and gabbros at the base of the crust. The uniform crustal thickness of 30–34 km in northwestern Nevada is close to the crustal average for the Basin and Range province but is thin when compared to surrounding regions, such as the Sierra Nevada and Colorado Plateau where crustal thickness is greater than 40 km. Similarly, northwestern Nevada Pn velocities of 7.9–8.0 km s−1 are near the province‐wide average of 7.9 km s−1. Temperature‐corrected Pn velocities, from a 90 mW m−2 geotherm for the Basin and Range to 60 mW m−2 for the stable interior of the United States, are within one standard deviation of the continental average of 8.05 km s−1 and suggest a uniform upper mantle composition across the Basin and Range. In addition to these observations, the homogeneity of the velocity structure beneath the western Basin and Range argues for a youthful Moho and crust that has been reworked by province‐wide late Cenozoic extension, episodic magmatism, and underplating.

The complex wind of the central star of the planetary nebula Abell 78

The authors have observed with the high-dispersion camera of the IUE the broad P Cygni profiles of C IV, N V, and O V that arise from Abell 78's nucleus. They find a terminal velocity to the wind that is close to 3670±30 km s<SUP>-1</SUP>, a positive velocity gradient, and an approximate mass loss rate of ≡4×10<SUP>-9</SUP>M_sun;yr<SUP>-1</SUP>. Superposed upon these high-velocity features are low-velocity absorption lines that may arise from a density enhancement in the wind as it brakes and builds up against the inner edge of the nebula.

A three-dimensional crustal model of southwest Germany derived from seismic refraction data

From densely covered seismic refraction data obtained in 1978 (Urach experiment) and 1984 (“Schwarzer Zollern-Wald” experiment) and from seismic reflection data and results from previous refraction investigations, a three-dimensional crustal model of southwest Germany was derived. Travel-time and amplitude information of seismic refraction data were interpreted with two-dimensional forward modeling (ray tracing) to calculate two crustal cross sections in southwest Germany. These results fill a gap in the existing data and enabled the construction of a detailed three-dimensional crustal model. While seismically the upper crust is laterally homogeneous (5.9–6.0 km/s) throughout the area, the middle and lower crust show pronounced lateral variations in thickness, velocity, and reflectivity. The Moho is a flat surface at a relatively shallow depth (25–26 km). We classify the middle and lower crust of southwest Germany into two characteristic crustal types. Type I consists of a mid-crustal low-velocity zone (5.4–5.8 km/s) overlying a thick (> 10 km), high-velocity (6.6–6.8 km/s) lower crust. Type II has no prominent mid-crustal low-velocity zone, and a thin (< 10 km), low-velocity (6.3–6.4 km/s) lower crust. The crustal types correlate with the major geologic units exposed in the area: Type I is present beneath the Black Forest, forming the eastern flank of the Rhinegraben and beneath the Swabian Jura, while Type II is present beneath the intervening Triassic sediments. Beneath the South German Molasse Basin, a low-velocity zone is also present in the upper middle-crust. Seismic reflection investigations have shown that the lower crust in southwest Germany comprises a stack of layers of alternating high- and low-velocities. The lateral variation of the reflectivity of this laminated lower crust has been recognized even on refraction data. We found that high-reflectivity of the lower crust correlates to high average velocity (6.7–6.8 km/s) in the lower crust (Type I). Thus, the average velocity of the lower crust in southwest Germany seems to be an indicator of the intensity of its lamination. The uppermost mantle has a velocity of 8.3 km/s in the area and a strong, positive velocity gradient.

Characteristics of ejections in turbulent channel flow

Quantitative measurements of the structure of ejections from the wall region have been made using conditional-sampling techniques. Discrete ejections from a burst event were identified using fluorescent-dye flow visualization simultaneously with streamwise, u, and normal, v, velocity measurements. These velocity measurements, at y+ = 15, were conditionally sampled based on different phases of the ejection event. Features of the ejection which were educed from the conditional sampling were found to be very sensitive to the phase alignment. Results showed that ejections were characterized by a rapid deceleration at the leading edge followed by a strong positive velocity gradient at the trailing edge. An intense second-quadrant uv spike occurred immediately following the leading edge. This uv peak was highly correlated with a positive peak in the v velocity. The first ejection which occurred in a burst was found to be significantly more intense than the following ejections. Many characteristics of bursts which have been obtained from previous conditional-sampling studies were found to correspond to different phases of the ejection event.

A geologic interpretation of seismic-refraction results in northeastern California

In 1981, the U.S. Geological Survey conducted a seismic-refraction experiment in northeastern California designed to study the Klamath Mountains, Cascade Range, Modoc Plateau, and Basin and Range provinces. Key profiles include 135-km-long, north-south lines in the Klamath Mountains and Modoc Plateau provinces and a 260-km-long, east-west line crossing all of the provinces. The seismic-velocity models for the Klamath and Modoc lines are comparatively homogeneous laterally but are quite different from each other. The Klamath model is finely layered from the surface to at least 14-km depth, consisting of a series of high-velocity layers (6.1–6.7 km/s), ranging in thickness from 1 to 4 km, with alternating positive and negative velocity gradients. A layer with an unreversed velocity of 7.0 km/s extends from 14 km to an unknown depth. The Modoc model, in contrast, is relatively thickly layered and has lower velocities than does the Klamath model at all depths down to 25 km. An upper layer, 4.5 km thick, of low-velocity material (2.1–4.4 km/s) overlies a basement with a considerably higher velocity (6.2 km/s). Velocity increases slowly with depth, with a small velocity step (to 6.4 km/s) at 11 km and a 7.0-km/s layer beginning at 25-km depth. Moho is probably 38–45 km deep under the Modoc Plateau, but its depth is unknown under the Klamath Mountains. A combined velocity-density model for the east-west line consists of a western part similar in configuration to the Klamath velocity model, an eastern part similar to the Modoc velocity model, and laterally changing velocity-density structure in between, in the Cascade Range. Beneath its upper layer, the velocity model for the Modoc Plateau is similar to that determined by other researchers for the adjacent Sierra Nevada. The velocity model is unlike those for rift areas, to which the Modoc Plateau has been compared by some authors. We theorize that beneath a veneer of volcanic and sedimentary rocks (the upper layer), the Modoc Plateau is underlain by a basement of granitic and metamorphic rocks that, like rocks in the Sierra Nevada, are the roots of one or more magmatic arcs. The fine layering in the Klamath seismic-velocity model is consistent with the geologic structure of the Klamath Mountains, characterized by imbricate thrusting of oceanic rock layers of various compositions and ages. Independent modeling of aeromagnetic data indicates that the base of the Trinity ultramafic sheet, the second major rock layer down in the structural sequence, corresponds to a velocity step to 6.7 km/s at 7-km depth in our model. The 6.7-km/s layer beneath the Trinity ultramafic sheet apparently corresponds to rocks of the central metamorphic belt, which are mafic schists. Rock units structurally deeper than rocks of the central metamorphic belt can be correlated with velocity layers below the 6.7-km/s layer, but with less certainty. In the model for the east-west line, the region of laterally changing velocity structure beneath the Cascade Range includes a 10-km step down to the east in the top of the 7.0-km/s layer. This region of lateral velocity change we interpret to be a fault, fold, or intrusive contact (or some combination of the three) between the stack of oceanic rock layers that underlie the Klamath Mountains and the buried roots of magmatic arcs inferred to underlie the Modoc Plateau. Magmas forming the modern Cascade Range arc apparently rise through this region.

Velocity and Q structure of the Great Valley, California, based on synthetic seismogram modeling of seismic refraction data

The modified reflectivity method is used here to interpret seismic refraction data from the west side of the Great Valley, California, in terms of a onedimensional compressional wave velocity model of the crust. In addition, the apparent attenuation, Qp has been estimated for the 6-km-thick sedimentary section. The reflectivity method was used to calculate the prominent free-surface multiples that propagate in this sedimentary section. Modeling of the normalized amplitudes of these multiples yielded average Qp values of 100 from 0 to 2.83 km depth and 200 from 2.93 to 6.20 km depth. The velocity structure in the crust is complicated. It is characterized by positive velocity gradients within the sediment to a depth of 4.8 km and by a 1.4-kin-thick, low-velocity zone below this depth. The sediment overlies a basement with a velocity of 5.7 km/sec at its top, which increases to 6.3 km/sec at 12 km depth. Ambiguity of the data in critical regions leads to two equally valid interpretations of the lower crust at depths of 16 to 27 km: simple step increases or a more complex structure consisting of alternating high- and low-velocity zones.

Crustal structure of northeastern California

In 1981, the U.S. Geological Survey conducted a seismic refraction survey of northeastern California designed to characterize the structure in four geologic provinces: the Klamath Mountains, Cascade Range, Modoc Plateau, and Basin and Range provinces. The survey consisted of north‐south lines in the Klamath Mountains and Modoc Plateau provinces, northwest‐southeast lines centered on Mount Shasta and Medicine Lake volcano, and an east‐west line linking all the profiles. All lines except the east‐west line ranged in length from 125 to 140 km, contained three shot points, and were recorded by 100 instruments. The east‐west line was 260 km long, contained six shot points, and was recorded by 200 instruments. The Klamath and Modoc lines yielded the simplest models. The Klamath model is finely layered from the surface to at least 14‐km depth, consisting of a series of high‐velocity layers (6.1–6.7 km/s), ranging in thickness from 1 to 4 km, with alternating positive and negative velocity gradients. A layer with an unreversed velocity of 7.0 km/s extends from 14 km downward to an unknown depth. The Modoc model, in contrast, is thickly layered and has lower velocity at all depths down to 25 km. The uppermost layer, 4.5 km thick, consists of low‐velocity material (2–4.5 km/s). Velocity beneath this layer is much higher (6.2 km/s) and increases slowly with depth. A small velocity step (to 6.4 km/s) is seen at 11 km, and a larger step (to 7.0 km/s) is seen at 25 km depth. Moho is probably 38–45 km deep under the Modoc Plateau, but its depth is unknown under the Klamath Mountains. Models for the Shasta and Medicine Lake lines show special features including low velocity (less than 3.5 km/s) in the edifice of Mount Shasta but high velocity (5.6 km/s) at shallow depth (1–2 km) under the summit of Medicine Lake volcano. The model for the east‐west line consists of a western part similar to the Klamath model, an eastern part similar to the Modoc model, and laterally changing velocity structure in between, underlying the Cascade Range. This model was converted to a density model, and observed Bouguer gravity data were matched. A general decrease in Bouguer gravity values eastward may be explained by a general decrease in the density of crustal layers and does not require a change in crustal thickness. Beneath the 4.5‐km‐thick surficial layer, the velocity model for the Modoc Plateau is similar to that determined by other researchers for a refraction line in the Sierra Nevada. It is unlike velocity models for rift areas, to which the Modoc Plateau has been likened by some authors. We theorize that beneath its surficial volcanic and sedimentary rocks, the Modoc Plateau is underlain by a basement of granitic and metamorphic rocks that are the roots of ancient magmatic arc(s). The fine layering in the Klamath model is consistent with the imbricate structure of the Klamath Mountains. Independent modeling of aeromagnetic data indicates that the base of the Trinity ultramafic sheet corresponds to a velocity step from 6.5 to 6.7 km/s at 7‐km depth in our model. The 6.7 km/s layer beneath the Trinity ultramafic sheet apparently corresponds to rocks of the central metamorphic belt, mostly amphibole schists, which crop out west of the Trinity ultramafic sheet. Deeper velocity layers can likewise be correlated to terranes that crop out farther west. In our geologic cross section of northeastern California, derived from our velocity‐density model for the east‐west line, the Klamath Mountains are underlain by folded and thrust‐faulted slices of oceanic crust. The Modoc Plateau and westernmost Basin and Range province are underlain by a section of volcanic and sedimentary rocks overlying granitic and metamorphic rocks, all tilted westward between an inferred fault under Medicine Lake volcano and the Surprise Valley fault. In the Cascade Range, geologic units appear to be discontinuous, and structures include horsts, grabens, and a 10‐km step downward to the east in the 7 km/s layer. The latter step may represent a fault, fold, intrusion, or a combination of any of the three. Apparently, the Cascade Range, a modern magmatic arc, is developed across the suture region between the stack of oceanic rock layers underlying the Klamath Mountains and the inferred roots of magmatic arc(s) underlying the Modoc Plateau.

Crustal structure of Oaxaca, Mexico, from seismic refraction measurements

Seismic refraction and gravity data have been analyzed to obtain a model of the compressional-wave structure of the ocean-to-continent transition in the State of Oaxaca in southwestern Mexico. Crustal thickness on the continent at the latitude 18°N is 45 _ 4 km, based on reflected phases from the Moho discontinuity. The crust has been modeled with three layers, with velocities of 4.3 to 4.6, 5.0 to 5.7, and 6.85 to 7.0 km/sec, each with positive velocity gradient. The crust thins to 10 km at the coast near Pinotepa Nacional, where Precambrian metamorphic rocks are exposed 45 km from the mid-America trench. Offshore, the oceanic structure consists of an 8-km-thick crust with a normal crustal velocity structure (Spudich and Orcutt, 1980). The apparent dip of the subducting plate beneath western Mexico is 10 °, On the oceanic side, strong reflections suggest a minimum depth of 35 km for the lithosphere-asthenosphere boundary. The asthenosphere has a seismic velocity of 7.6 km/sec, and a thin lid in which the velocity is 8.6 km/sec.

A seismic refraction survey in the Kerguelen Isles, southern Indian Ocean

Two 65 km long on-land refraction lines, with shot detonated at sea, were implemented in the Courbet Peninsula, eastern region of the Kerguelen archipelago (Terres Australes et Antarctiques Françaises, southern Indian Ocean). The first profile P1, oriented NE—SW, runs from Cap Ratmanoff toward Mont Ross, and the second one, P2, from Pointe Suzanne, SE of the Courbet Peninsula, through the Val Studer. Data from both profiles were supplemented by using the Bouguer gravity anomaly map not previously studied. The data combined standard travel-time interpretation, wide-angle reflected wave study, synthetic seismograms and S-wave analysis. The velocity—depth behaviour shows that the mean crustal thickness ranges from 14 to 17 km only, varying with the locality. The structure of the crust beneath Kerguelen resembles those observed beneath aseismic ridges (i.e. the Crozet Rise and the Madagascar Ridge). Average velocity of 5.5 km s−1 on both lines is in the range of those determined for oceanic layer 2, which is there 8–9 km thick. Average velocity of 6.6 km s−1 is in the range of velocities within oceanic layer 3. The transition to mantle velocity, 7.95 km s−1 is best modelled by a positive velocity gradient within the crust. This feature is similar to that observed on structures generated near spreading centres. These new data combined with geological and geochemical investigations in the archipelago support an oceanic origin for the Kerguelen—Heard Ridge, and also Broken Ridge. Both structures were joined 40 Myr ago, as shown by the magnetic anomaly pattern. Refraction studies invalidate the assumption that isostatic compensation is achieved by a 23 km deep crustal root, but substantiate the contribution of a low velocity mantle. Gravity data are consistent with refraction studies. No crustal root appears to be present beneath Mont Ross. The best match between gravity and refraction data in the Val Studer is obtained by assuming the presence of a shallow intrusive body, related to the fault and dyke system, and Montagnes Vertes plutonic intrusive complex nearby.

Dynamic failure in the extensive plastic-strain range

Here we consider failure in thin-walled cylindrical shells made of a material showing not only viscosity but also work hardening. A differential equation is derived with fuller incorporation of expanding-shell motion. The breakup of a jet* of continuous material having a velocity gradient (another example of failure in the deep plastic region) can also be described by tlhe differential equation and solution derived for a shell. Formulation and Solution. Consider the motion of a thin ring of unit width having radius r 0 and thickness 6 o conceptually cut from the cylindrical shell. An observer moving radially from the center with a particle of the material with speed v sees an adjacent particle on another radius at a distance@ r moving with a velocity proportional to the distance. In :fact, in dt the adjacent particle moves a distance (r + vdt)@ with velocity @v. Therefore, :from the observer's viewpoint there is a velocity gradient v/r in the tangential direction. We consider simultaneously a freely moving jet having a constant positive velocity gradient u/m in the direction of motion (u is the velocity difference between the head and tail of the jet and m is the length). The gradients v/r and u/m cause the ring and jet to stretch and thin out. To simplify the discussion, we assume that v and u are constant. This assumption is equivalent to the energy dissipated in plastic flow being small by comparison with the total kinetic energy of the ring or the kinetic energy of the jet material relative to its center of mass.# For the~jet, the speed of the center of mass is independent of u, while in the ring the energy dissipation in plastic flow retards the basic radial motion. This ,difference i:n the energy sources in plastic-flow dissipation must be borne in mind in determining the limitations of the final solution. The velocity gradient in the ring is perpendicular to the radial motion, while that in the jet is parallel to it, but this has no essential importance. The main kinematic difference is that all the phases of motion and failure in the sectors are synchronous for the ring, while they are sequential for the jet, beginning with the head parts. However, even this difference can be avoided with certain simplifying assumptions. As the ring (cylindrical shell) [6] and jet split up into a large number n of fragments (with the failure independent), without loss of generality it is sufficient to consider the ewolution of a ring sector or part of the jet of size Zi/n of the whole on the basis that all the processes are synchronous in such a part and the diameter is dependent only on *The first such attempt was made by S. V. Serikov at the Third All-Union Seminar on Explosion Physics (June 25-29, 1984, Krasnoyarsk). '~The assumption made in [2] for a ring is not too strong also for a jet such as a cumulative one if one bears in mind the reduction in the yield point of the material due to the initial heating to several hundred degrees, since the material undergoes shock compression as the jet is formed and then isentropic unloading.

The utility of regional Chinese seismograms for source and path studies in central Asia

Broadband seismograms from the National Seismic Network of the People's Republic of China (PRC) have recently become available through a data exchange programme between NOAA and the State Seismological Bureau of the PRC. In this study, regional surface waves recorded at the Urumchi station located about 700 km north of the Tibetan Plateau in the Sinkiang Province are used to study East Kazakh explosions and wave propagation in central Asia. The data consist of broadband (flat to displacement between 0.1 and 10 Hz), photographic records from an SK Kirnos galvanometric system. Simultaneous inversion of Rayleigh wave phase and group velocities for the path from East Kazakh through the Dzhungarian Basin yields a crustal model dominated by the presence of very low velocities and a strong positive velocity gradient above 15 km depth. Velocities below 15 km depth are not significantly different from other continental structures underlain by Palaeozoic or Precambrian basement. Seismic moments were estimated for seven East Kazakh explosions using models of explosion sources with associated tectonic strain release. The largest explosion studied occurred on 1980 September 14 and had an mb of 6.2 and a seismic moment of 2.7 × 1023 dyn cm. The observed amplitude spectra of Rayleigh waves are richer in high frequencies than spectra calculated from our models. This could be caused by a path effect involving seismic wave focusing by the Dzhungarian Basin, although source medium effects cannot be ruled out.

Propagation of PL and implications for the structure of Tibet

PL is a long‐period (20 s or more) wave train beginning just after the P arrival in seismograms and continuing until the S arrival or the Rayleigh wave. The wave train can be observed at epicentral distances of about 5°–20° with instruments sensitive to these low frequencies. PL propagates as a partially trapped P‐SV wave in the crust; S wave energy is lost to the mantle during propagation making PL a “leaky mode.” We study PL propagation for a variety of earth models using the synthetic seismogram algorithm wave number integration and find that the vertical travel time in the crust is the most important parameter controlling PL's oscillation period. This period can vary by more than a factor of two between oceanic and continental paths. P‐SV leaky mode propagation includes many different modes; the low‐frequency motion termed “PL” is only the first, or fundamental mode, in this family. A second, higher‐frequency mode roughly equal in amplitude to the fundamental appears for models without a net positive velocity gradient in the crust. We use these results to match an observed PL wave train whose propagation path consisted almost entirely of the Tibetan Plateau. By considering first and second PL mode behavior we find that the Tibetan crust is about 85 km thick, with an uncertainty of about 20 km, and possesses a significant (≥0.01 s−1) positive velocity gradient. The presence of a large, low‐velocity zone in the lower Tibetan crust thus seems unlikely. Much higher‐frequency PL modes also appear in the Green's functions for layer over half space models and appear to be responsible for the high‐frequency Pg phase, making PL and Pg different members of the same type of leaky mode propagation.

A lower mantle S-wave triplication and the shear velocity structure of D''

Summary. A lower mantle S-wave triplication detected with short- and longperiod WWSSN and CSN recordings indicates a substantial shear velocity discontinuity near 280 km above the core-mantle boundary. The triplication can be observed in rotated SH seismograms from intermediate and deep focus events throughout the distance range from 70 to 95. Three distinct source region-receiver array combinations that have been investigated in detail demonstrate consistent travel time and relative amplitude behaviour of the triplication, with slight systematic shifts in the triplication indicating up to 40km variations in the depth of the discontinuity. Modelling of the observations with synthetic seismograms produced with the Cagniard de Hoop and reflectivity methods constrains the shear velocity increase to be 2.75 k 0.25 per cent, comparable to upper mantle discontinuities. Short-period observations indicate that the velocity increase may be a sharp first-order discontinuity, or may extend over a transition zone no more than 50 km thick. The shear velocity gradient below the discontinuity, within the D layer, is not well-constrained by the SH data, but slightly positive or near zero velocity gradients are consistent with the long-period amplitude ratios of ScSH/SH.

The shear‐wave velocity gradient at the base of the mantle

The relative amplitudes and travel times of ScS and S phases are utilized to place constraints on the shear‐wave velocity gradient above the core‐mantle boundary. A previously reported long‐period ScSH/SH amplitude ratio minimum in the distance range 65° to 70° is shown to be a localized feature, apparently produced by an amplitude anomaly in the direct S phase, and therefore need not reflect the velocity gradient at the base of the mantle. The amplitude ratios that are free of this anomaly are consistent with calculations for the JB model or models with mild positive or negative velocity gradients in the lowermost 200 km of the mantle. ScSV arrivals are particularly sensitive to the shear velocity structure just above the core‐mantle boundary. The apparent arrival time of the peak of ScSV is as much as 4 s greater than that of ScSH in the distance range 75° to 80° for Sea of Okhotsk events recorded in North America. This can be explained by interference effects produced by a localized high velocity layer or strong positive S wave velocity gradient in the lowermost 20 km of the mantle. A velocity increase of about 5% is required to explain the observed shift between ScSV and ScSH. This thin, high velocity layer varies laterally, as it is not observed in similar data from Argentine events. Refined estimates of the outermost core P velocity structure are obtained by modeling SKS signals in the distance range 75° to 85°.

Crustal structure of the western Arafura Sea from ocean bottom seismograph data

Abstract Refraction data taken from ocean bottom seismograph recordings in the western Arafura Sea indicate a continental‐type structure for the region. This structure is characterised by a thin column (2 km) of sediments, with velocities ranging from about to 2 to 4 km s‐1, overlying an essentially two layer crust. The compressional wave velocities in the upper and lower crust are 5.97 and 6.52 km s‐1, respectively, with the boundary between the layers at a depth of 11 km. Very weak mantle‐refracted arrivals with a velocity of about 8.0 km s‐1 were recorded. Large‐amplitude, later arrivals, beginning at distances near 100 and 150 km, have been interpreted to be part of the retrograde branches from the 8.0 and 7.33 km s‐1 layers, respectively. Model studies indicate that a small positive velocity gradient is required between 17 and 30 km, and that the Moho is at a depth of 34 km. A third set of large amplitude, later arrivals starting at a distance near 250 km has been interpreted as most probably multipl...

The Vancouver Island Seismic Project: a co-CRUST onshore–offshore study of a convergent margin

The seismic structure of the British Columbia continental margin has been investigated using four reversed refraction profiles. The profile across strike extended 350 km from the volcanic arc on the continent to the deep ocean of the Juan de Fuca Plate; the three profiles along strike were located on Vancouver Island, on the continental shelf, and in the deep ocean on the Juan de Fuca Plate. Interpretation of the profile along Vancouver Island yields a well constrained model for the upper crust with velocity increasing from ~5.3 km/s at the surface to ~6.4 km/s at 2 km depth to ~6.75 km/s at 15.5 km depth where the velocity increases sharply to ~7 km/s. The velocity structure of the deep crust and the crustal thickness are poorly constrained. Four possible velocity functions, based on ambiguous first arrivals and (or) secondary phases interpreted as Moho reflections, are presented. The preferred one includes a deep crustal low velocity zone with a crustal thickness of 37 km; models with a constant 7.1 km/s deep crust require thicknesses of 52 km. Preliminary results from the profile across strike show the dip of the basement towards the continent steepens from approximately 1.4° immediately west of the continental rise to ~4° beneath the rise. Sediment velocities increase as the sedimentary layer thickens towards the shelf. The Moho, with velocity near 8 km/s, appears to dip at similar angles in this region; the dip is ~6° from the edge of the shelf to the central portion of Vancouver Island; here there is an abrupt thickening of the continental crust by about 10 km with a flat-lying Moho to the east. This suggests a contact between subducting oceanic Moho and continental Moho. A small positive velocity gradient is required in the mantle.Two short reflection lines, one using explosives and the other a large air gun fired in an inlet, were recorded on a land-based multichannel reflection system. These were run to test the feasibility of obtaining coherent reflections to upper mantle depths in this complex geological environment, and of acquiring deep reflection data in coastal areas with an air-gun source. The preliminary explosion section showed reflections near 4.4 and 6.8 s. The depths of these reflections correspond closely to the 15.5 km crustal refractor and the top of the subducting oceanic lithosphere, respectively. Dip on the deeper reflector is close to that estimated from the refraction profile. Without stacking or velocity filtering, the air-gun recordings on a line adjacent to the explosion profile show arrivals of energy at similar times.

Constraints on the structure of the East Pacific Rise from seismic refraction data

University of Washington seismic data taken during ROSE phase 1 have been used to study the structure of the East Pacific Rise in the vicinity of 12°N. Off axis but within 50 km of the axis we find that the oceanic crust can be modeled with a rapid change in velocity in the top 2.5 km, a zone of low but uniform velocity gradient from 2.5 to about 5.5 km and a transitional zone about 1 km thick to mantle material. There is also evidence for substantial anisotropy in mantle velocity gradients, with a positive velocity gradient in the spreading direction and a negative velocity gradient parallel to the rise. Under the rise axis the data allow at most a very narrow zone of low velocity (less than about 2 km wide) in the crust.