Gradual Increase In Velocity Research Articles

Deciphering the Whisper of Volcanoes: Monitoring Velocity Changes at Kamchatka's Klyuchevskoy Group With Fluctuating Noise Fields

AbstractVolcanic inflation and deflation often precede eruptions and can lead to seismic velocity changes () in the subsurface. Recently, interferometry on the coda of ambient noise‐cross‐correlation functions yielded encouraging results in detecting these changes at active volcanoes. Here, we analyze seismic data recorded at the Klyuchevskoy Volcanic Group in Kamchatka, Russia, between summer of 2015 and summer of 2016 to study signals related to volcanic activity. However, ubiquitous volcanic tremors introduce distortions in the noise wavefield that cause artifacts in the estimates masking the impact of physical mechanisms. To avoid such instabilities, we propose a new technique called time‐segmented passive image interferometry. In this technique, we employ a hierarchical clustering algorithm to find periods in which the wavefield can be considered stationary. For these periods, we perform separate noise interferometry studies. To further increase the temporal resolution of our results, we use an AI‐driven approach to find stations with similar responses and apply a spatial stack. The impacts of snow load and precipitation dominate the resulting time series, as we demonstrate with the help of a simple model. In February 2016, we observe an abrupt velocity drop due to the M7.2 Zhupanov earthquake. Shortly after, we register a gradual velocity increase of about 0.3% at Bezymianny Volcano coinciding with surface deformation observed using remote sensing techniques. We suggest that the inflation of a shallow reservoir related to the beginning of Bezymianny's 2016/2017 eruptive cycle could have caused this local velocity increase and a decorrelation of the correlation function coda.

The Inclusion of a Complementary Running Progression Program in the Rehabilitation of Acute Hamstring Injuries: A Critically Appraised Topic

Introduction/Clinical Scenario: As many hamstring injuries occur when the hamstrings are in a lengthened state during the deceleration phase of running when the muscle is eccentrically contracting to slow the body down this functional aspect needs to be addressed. Thus, a rehabilitation program with a focus on progressive targeted eccentric hamstring exercises by gradually placing and exposing the muscle to eccentric force in a lengthened state supplemented with progressive running drills should be evaluated. Focused Clinical Question: Does the inclusion of a complementary running progression program for the rehabilitation of an acute hamstring injury reduce the time to safe return to sport with less hamstring reinjury occurrence for an athletic population? Summary of Key Findings: Three studies assessed the inclusion of a progressive running program with several types of running progression parameters addressed. Progressive running drills will load the hamstring in a functional manner, with a gradual increase in velocity of movement and lengthening of the muscle. Clinical Bottom Line: It appears that a complementary progressive running program within an acute hamstring rehabilitation program should be included as it caused no further harm and does not tend to increase hamstring reinjury occurrence. Strength of Recommendation: There is grade B evidence to include a complementary running progression program within an acute hamstring rehabilitation program.

How distinctive are flood-triggered turbidity currents?

ABSTRACT Turbidity currents triggered at river mouths form an important highway for sediment, organic carbon, and nutrients to the deep sea. Consequently, it has been proposed that the deposits of these flood-triggered turbidity currents provide important long-term records of past river floods, continental erosion, and climate. Various depositional models have been suggested to identify river-flood-triggered turbidite deposits, which are largely based on the assumption that a characteristic velocity structure of the flood-triggered turbidity current is preserved as a recognizable vertical grain size trend in their deposits. Four criteria have been proposed for the velocity structure of flood-triggered turbidity currents: prolonged flow duration; a gradual increase in velocity; cyclicity of velocity magnitude; and a low peak velocity. However, very few direct observations of flood-triggered turbidity currents exist to test these proposed velocity structures. Here we present direct measurements from the Var Canyon, offshore Nice in the Mediterranean Sea. An acoustic Doppler current profiler was located 6 km offshore from the river mouth, and provided detailed velocity measurements that can be directly linked to the state of the river. Another mooring, positioned 16 km offshore, showed how this velocity structure evolved down-canyon. Three turbidity currents were measured at these moorings, two of which are associated with river floods. The third event was not linked to a river flood and was most likely triggered by a seabed slope failure. The multi-pulsed and prolonged velocity structure of all three (flood- and landslide-triggered) events is similar at the first mooring, suggesting that it may not be diagnostic of flood triggering. Indeed, the event that was most likely triggered by a slope failure matched the four flood-triggered criteria best, as it had prolonged duration, cyclicity, low velocity, and a gradual onset. Hence, previously assumed velocity-structure criteria used to identify flood-triggered turbidity currents may be produced by other triggers. Next, this study shows how the proximal multi-pulsed velocity structure reorganizes down-canyon to produce a single velocity pulse. Such rapid-onset, single-pulse velocity structure has previously been linked to landslide-triggered events. Flows recorded in this study show amalgamation of multiple velocity pulses leading to shredding of the flood signal, so that the original initiation mechanism is no longer discernible at just 16 km from the river mouth. Recognizing flood-triggered turbidity currents and their deposits may thus be challenging, as similar velocity structures can be formed by different triggers, and this proximal velocity structure can rapidly be lost due to self-organization of the turbidity current.

Seismic imaging the D″ region beneath the Central Atlantic

The lowermost mantle beneath the central Atlantic is bounded by the western edge of African Large Low Shear Wave Province (LLSVP) and a high-velocity region beneath the eastern Americas, which is attributed to the accumulated subducted slab material at the core-mantle boundary (CMB). Data recorded at broadband seismic arrays in western Europe and Morocco allow us to investigate the detailed structure beneath the Central Atlantic, which provides an important constraint on how remnant slab material and LLSVP interact and reshape the D″ layer. Here, we exploit lower mantle triplicated S waveform data from deep earthquakes beneath South America. Our modelling results show that the D″ discontinuity is constantly observed from north to south beneath the Central Atlantic but at varied depths and with varied shear velocities. In the lowermost mantle beneath east of Caribbean, our results indicate a sharp 2.1% shear velocity increase at 340 km above the CMB, representing a possible bridgmanite dominant normal mantle D″ layer. Whereas, the models of northern and southern portions of Central Atlantic indicate a gradual velocity increase of 4.2% across 200 km starting from 360 km above the CMB downwards. We also find a distinct low velocity zone on top of the velocity jump, which can be interpreted as an upwelling produced by the dynamic response of ancient slab material reaching the CMB.

Growth of the lower continental crust via the relamination of arc magma

How does continental crust transition from basaltic mantle-derived magmas into an andesitic composition? The relamination hypothesis has been presented as an alternative dynamical mechanism to classical delamination theory to explain new crust generation and has been supported by petrological and geochemical studies as well as by thermomechanical numerical modeling. However, direct evidence of this process from detailed seismic velocity structures is lacking. Here, we imaged the three-dimensional (3D) velocity structures of the crust and uppermost mantle beneath the geologically stable Ordos terrane of the North China Craton (NCC). We identify a region of continental crust that exhibits extreme growth using teleseismic data and an imaging technique that models the Common Conversion Point (CCP) stacking profiles. Our results show an approximately 400×400km2 wide growth zone that underlies the primitive crust at depths of ~30–50km and exhibits a gradual increase of velocity with depth. The upper layer of the growth zone has a shear wave velocity of 3.6–3.9km/s (Vp=6.2–6.8km/s), indicating felsic material, and the lower layer has a shear wave velocity of 4.1–4.3km/s (Vp=7.2–7.5km/s), which corresponds to mafic material. We suggest that this vertical evolution of the layered structure could be created by relamination and that the keel structure formed by relamination may be the root of the supernormal stability of the ancient Ordos terrane.

Crustal structure of the Pannonian Basin: The AlCaPa and Tisza Terrains and the Mid-Hungarian Zone

The Pannonian Basin of Central Europe is one of the key examples of Miocene continental extension that is easily accessible to surface seismological investigation. It comprises two major crustal blocks: AlCaPa and Tisza which abut along a poorly understood structure referred to as the Mid-Hungarian Zone (MHZ), the whole being surrounded by the arc of the Carpathian Mountains, the Alps and the Dinarides. Using data from the CBP (Carpathian Basins Project) temporary broadband seismic array of 46 stations deployed across the western Pannonian Basin in 2006–2007, we calculated receiver functions that constrain the variation of crustal thickness across the basin and derive a map of Moho depth across a NW–SE oriented swath about 450km long and 75km wide. The measured Moho depths show no significant change in crustal thickness between AlCaPa and Tisza terrains, but the Moho is not or very weakly imaged along a ca. 40km wide strip centred on the MHZ. Moho depths within the Pannonian Basin are typically in the range 25–30km, and increase toward the periphery of the basin. Our measurements are generally consistent with earlier VP models from controlled-source seismic surveys and recent VS models determined by tomographic analysis of ambient noise signals. The lack of a sharp Moho image beneath the MHZ suggests that the crust–mantle boundary in that zone may consist of a gradual increase in velocity with depth. The relatively constant crustal thickness across the two domains of the Pannonian Basin suggests that thinning to the same final state is controlled thermally. This structural characteristic seems to be governed by a large-scale balance of gravitational potential energy that is insensitive to the separate prior histories of the two regions.

수신함수 역산 및 H-κ 중합법을 이용한 제주도 하부의 S파 지각 속도

제주도 한라산 주위에 위치한 JJU와 JJB 관측소 하부의 S파 속도구조를 규명하기 위하여, 이 관측소에서 2007년 이후 기록한 <TEX>$M_W$</TEX> 5.5 이상인 원격 지진자료 중 150개 수신함수를 이용하여 역산 및 H-<TEX>${\kappa}$</TEX> 중합법에 적용하였다. 모호면에서 변환된 <TEX>$P_S$</TEX>파는 JJU 관측소의 북서쪽(후방위각 <TEX>$207{\sim}409^{\circ}$</TEX>, 평균 <TEX>$308^{\circ}$</TEX>)과 JJB 관측소의 남동쪽(후방위각 <TEX>$119{\sim}207^{\circ}$</TEX>, 평균 <TEX>$163^{\circ}$</TEX>) 방향으로 뚜렷하지 않게 나타났다. 이것은 아마도 모호면의 점이적인 속도변화나 지각 내의 속도 불균질층 때문일 수 있다. 수신함수 역산으로부터 계산된 S파 속도모델은 지각 내의 저속도층을 뚜렷이 보여주며, 30 ~ 40 km 깊이에서 점이적으로 증가하는 양상을 보인다. JJB 관측소 반경 18 km 이내에서 저속도층(<TEX>$v_s{\leq}3.5km/s$</TEX>)은 14 ~ 26 km에 있고, <TEX>$v_s{\geq}4.3km/s$</TEX>으로 정의한 '모호면'은 34 km 깊이에 있는 것으로 분석되었다. 서쪽으로 약 10 km 떨어진 곳에 위치한 JJU 관측소의 반경 16 km 이내에서는 저속도층과 '모호면'이 14 ~ 24 km와 30 km에 각각 존재하여 JJB 관측소에 비해 다소 얕은 깊이에서 나타난다. JJU와 JJB 관측소에 대한 H-<TEX>${\kappa}$</TEX> 분석결과는 지각 두께가 29 km와 33 km이며, 종파/횡파 속도비(<TEX>$v_p/v_s$</TEX>)가 1.64과 1.75임을 각각 나타내어 화산 정상에 가까운 곳에서 상대적으로 낮은 <TEX>$v_p/v_s$</TEX>가 관찰되었다. Shear-wave velocity (<TEX>$v_s$</TEX>) structures beneath two seismic stations, JJU and JJB on the flanks of the volcano Halla on Jeju island, Korea, were estimated by receiver-function inversion and H-<TEX>${\kappa}$</TEX> stacking applied to 150 teleseismic events (<TEX>$M_W{\geq}5.5$</TEX>) recorded since 2007. <TEX>$P_S$</TEX> waves converted at the Moho discontinuity does not appear clearly for northwesterly back-azimuths (<TEX>$207{\sim}409^{\circ}$</TEX>, average <TEX>$308^{\circ}$</TEX>) at station JJU and southeasterly back-azimuths (<TEX>$119{\sim}207^{\circ}C$</TEX>, average <TEX>$163^{\circ}$</TEX>) at station JJB. This may be due to a gradual velocity increase at Moho or heterogeneity within the crust. The <TEX>$v_s$</TEX> models derived by inversion of receiver functions indicate a distinct low velocity layer (<TEX>$v_s{\leq}3.5km/s$</TEX>; LVL) within the crust and a gradual increase in <TEX>$v_s$</TEX> in the depth interval of 30 to 40 km. Within the radius of 18 km beneath station JJB, the LVL occurs at depths of 14 ~ 26 km and the 'Moho' (<TEX>$v_s{\geq}4.3km/s$</TEX>) is at 34 km depth. Ten kilometers to the west, within the radius of 16 km beneath station JJU, both the LVL and the Moho are significantly shallower, at depths of 14 to 24 km and 30 km, respectively. H-<TEX>${\kappa}$</TEX> analyses for stations JJU and JJB yield estimated crustal thickness of 29 and 33 km and <TEX>$v_p/v_s$</TEX> ratios of 1.64 and 1.75, respectively. The lesser <TEX>$v_p/v_s$</TEX> ratio was derived for rocks nearest to th peak of the volcano.

3D seismic survey at the Millennium uranium deposit, Saskatchewan, Canada: Mapping depth to basement and imaging post-Athabasca structure near the orebody

Three-dimensional seismic reflection measurements have been used to assist mine planning at the Millennium uranium deposit, Canada. The deposit is located within the crystalline basement, separated from the overlying Athabasca Basin sediments by an unconformity potentially associated with significant fluid flow. The primary objective of the [Formula: see text] survey was to image the unconformity and possible post-Athabasca deformation structures in and around the deposit. Clear unconformity reflections are observed within most of the survey area, although there are amplitude variations due to complex geology, including intense hydrothermal clay alteration around the deposit. Finite-difference modeling indicates that the wide-angle character of the unconformity reflections is due to a gradual velocity increase at the unconformity. The reflections are obscured by large time delays, due to Quaternary sediments covering the area, making refraction static corrections crucial. The seismic interpretation shows large variations in the unconformity depth (from approximately 430 to 650 m), indicating a pronounced basement depression that coincides with a gravity low. Reflections from the unconformity are vague within the depression, especially in the vicinity of the deposit. Although the orebody is not directly visible in the seismic image, there is a lack of reflectivity coincident with the alteration surrounding the mineralization. We also observed reflections which likely originate at the contact between the altered and fresh basement rock located beneath the deposit. The seismic data further indicate post-Athabasca faults in the vicinity of the orebody. Based on the initial seismic interpretation, the depth of the crown pillar was adjusted and the mine infrastructure moved away from areas interpreted to be affected by the intense hydrothermal alteration surrounding the deposit. The capability to image the unconformity, post-Athabasca structure, and hydrothermal alteration also highlights the potential use of seismic surveys in uranium exploration.

Fundamental studies on free stream acceleration effect on drag force in bluff bodies

This paper describes fundamental studies on free stream acceleration effect on drag force in bluff bodies. The flow with gradual velocity increase assumed an accelerated flow. The wind tunnel tests were conducted in order to investigate the difference of aerodynamic characteristics between non-accelerated flow and accelerated flow. The experimental models were a circular cylinder and a square cylinder. In an accelerated flow, the condition of free stream was an acceleration of about 3.6m/s2. Experimental Reynolds number varied between form 4.0×104 to 1.64×105. The pressure distributions and the aerodynamic force were measured in both case of nonaccelerated flow and accelerated flow. In case of a circular cylinder, the drag of accelerated flow is lower than that of non-accelerated flow in the low Reynolds number regime. Then, it becomes higher than that of non-accelerated flow in the high Reynolds number regime. On the other hand, in case of a square cylinder, the drag of accelerated flow is higher than that of non-accelerated flow in the whole Reynolds number regime. If a separation point can be movable such as the circular cylinder, the additional momentum due to flow acceleration leads to delay separation and to decrease the drag in the low Reynolds number regime. If a separation point is nearly fixed such as a square cylinder, the additional momentum due to acceleration always affects to increase the drag than that of non-accelerated flow. Based on this research, it is expected that the roof shape of high speed train where the separation point is movable has the advantages to reduce the effect of accelerated flow such as strong crosswind/gust.

Shear-wave velocity structure of the western part of the Mediterranean Sea from Rayleigh-wave analysis

The lithospheric structure of the western part of the Mediterranean Sea is shown by means of S-velocity maps, for depths ranging from 0 to 35 km, determined from Rayleigh-wave analysis. The traces of 55 earthquakes, which occurred from 2001 to 2003 in and around the study area have been used to obtain Rayleigh-wave dispersion. These earthquakes were registered by 10 broadband stations located on Iberia and the Balearic Islands. The dispersion curves were obtained for periods between 1 and 45 s, by digital filtering with a combination of MFT and TVF filtering techniques. After that, all seismic events were grouped in source zones to obtain a dispersion curve for each source-station path. These dispersion curves were regionalized and after inverted according to the generalized inversion theory, to obtain shear-wave velocity models for rectangular blocks with a size of 1° × 1°. The shear velocity structure obtained through this procedure is shown in the S-velocity maps plotted for several depths. These maps show the existence of lateral and vertical heterogeneity. In these maps is possible to distinguish several types of crust with an average S-wave velocity ranging from 2.6 to 3.9 km/s. The South Balearic Basin (SBB) is more characteristic of oceanic crust than the rest of the western Mediterranean region, as it is demonstrated by the crustal thickness. We also find a similar S-wave velocity (ranging from 2.6 km/s at the surface to 3.2 km/s at 10 km depth) for the Iberian Peninsula coast to Ibiza Island, the North Balearic Basin (NBB) and Mallorca Island. In the lower crust, the shear velocity reaches a value of 3.9 km/s. The base of the Moho is estimated from 15 to 20 km under Iberian Peninsula coast to Ibiza Island, continues towards NBB and increases to 20–25 km beneath Mallorca Island. While, the SBB is characterized by a thinner crust that ranges from 10 to 15 km, and a faster velocity. A gradual increase in velocity from the north to the south (especially in the upper 25 km) is obtained for the western part of the Mediterranean Sea. The base of the crust has a shear-wave velocity value around of 3.9 km/s for the western Mediterranean Sea area. This area is characterized by a thin crust in comparison with the crustal thickness of the eastern Mediterranean Sea area. This thin crust is related with the distensive tectonics that exists in this area. The low S-wave velocities obtained in the upper mantle might be an indication of a serpentinized mantle. The obtained results agree well with the geology and other geophysical results previously obtained. The shear velocity generally increases with depth for all paths analyzed in the study area.

Stream Interactions and Interplanetary Coronal Mass Ejections at 5.3 AU near the Solar Ecliptic Plane

We have performed a survey of the characteristics of two types of large spatial-scale solar-wind structures, stream interaction regions (SIRs), and interplanetary coronal mass ejections (ICMEs), near 5.3 AU, using solar-wind observations from Ulysses. Our study is confined to the three aphelion passes of Ulysses, and also within ± 10° of the solar ecliptic plane, covering a part of 1992, 1997 – 1998, and 2003 – 2005, representing three slices of different phases of the solar activity cycle. Overall, there are 54 SIRs and 60 ICMEs in the survey. Many are merged in hybrid events, suggesting that they have undergone multiple interactions prior to reaching Jovian orbit. About 91% of SIRs occur with shocks, with 47% of such shocks being forward – reverse shock pairs. The solar-wind velocity sometimes stays constant or even decreases within the interaction region near 5.3 AU, in contrast with the gradual velocity increase during SIRs at 1 AU. Shocks are driven by 58% of ICMEs, with 94% of them being forward shocks. Some ICMEs seem to have multiple small flux ropes with different scales and properties. We quantitatively compare various properties of SIRs and ICMEs at 5.3 AU, and study their statistical distributions and variations with solar activity. The width, maximum dynamic pressure, and peak perpendicular pressure of SIRs all become larger than ICMEs. Dynamic pressure (Pdyn) is expected to be important for Jovian magnetospheric activity. We have examined the distributions of Pdyn of SIRs, ICMEs, and general solar wind, but these cannot explain the observed bimodal distribution of the location of the Jovian magnetopause. By comparing the properties of SIRs and ICMEs at 0.72, 1, and 5.3 AU, we find that the ICME expansion slows down significantly between 1 and 5.3 AU. Some transient and small streams in the inner heliosphere have merged into a single interaction region.

Using seismic traveltime tomography in geoarchaeological exploration: an application at the site of Chatby cemeteries in Alexandria, Egypt

We apply seismic traveltime tomography in an investigation of the subsurface of a geoarchaeological site at Chatby cemeteries (Alexandria, Egypt). We are interested in obtaining information about the geological structure and the remains of man‐made structures. Energy sources are placed at the surface and recorders are distributed vertically in an underground location. Traveltime data are inverted with a three‐dimensional tomographic algorithm. Inversion results are evaluated and interpreted with the aid of independent geophysical and archaeological information, and various tests are carried out with different initial velocity models and synthetic traveltimes.The inversion results show good correlation with the estimated geological structure, in particular at deeper levels, as suggested by independent geophysical and archaeological information and later confirmed by excavation. The depth and variations of the bedrock surface are satisfactorily imaged. Indications of possible man‐made structures are presented and discussed. Shallow velocity features are evaluated and interpreted using tests with synthetic traveltimes. The main goal of imaging the deeper structure was successful.As inversion tests with different initial velocity models indicate, the subsurface structure and the complexity of data information in this particular case are better handled when using a single‐layer model than when using an average two‐layer model. The single‐layer model allows for a more versatile velocity variation and raypath distribution, while the two‐layer model leads to an inversion result that is considerably influenced by the initial velocity distribution. Enhancement of particular aspects of the single‐layer inversion result can be derived with the use of an initial velocity model, that permits a gradual increase of velocity with depth. The gradient model improves the raypath distribution and limits velocity artefacts.The resulting images can assist in the assessment of the archaeological potential of a site, particularly in excavation design, in a non‐destructive manner. They can be also of great value in other applications, for example in engineering works, when detailed information on the subsurface is sought in areas of restricted access.

Lithological interpretation of crustal composition in the Fennoscandian Shield with seismic velocity data

In this study, we report the results of an investigation of lithological interpretation of the crust in the central Fennoscandian Shield (in Finland) using seismic wide-angle velocity models and laboratory measurements on P- and S-wave velocities of different rock types. The velocities adopted from wide-angle velocity models were compared with laboratory velocities of different rock types corrected for the crustal PT conditions in the study area. The wide-angle velocity models indicate that the P-wave velocity does not only increase step-wise at boundaries of major crustal layers, but there is also gradual increase of velocity within the layers. On the other hand, the laboratory measurements of velocities indicate that no single rock type is able to provide the gradual downward increasing trends. Thus, there must be gradual vertical changes in rock composition. The downward increase of velocities indicates that the composition of the crust becomes gradually more mafic with increasing depth. We have calculated vertical velocity profiles for a range of possible crustal lithological compositions. The Finnish crustal velocity profiles require a more mafic composition than an average global continental model would suggest. For instance, on the SVEKA'81 transect, the calculated models suggest that the crustal velocity profiles can be simulated with rock type mixtures where the upper crust consists of felsic gneisses and granitic–granodioritic rocks with a minor contribution of amphibolite and diabase. In the middle crust, the amphibolite proportion increases. The lower crust consists of tonalitic gneiss, mafic garnet granulite, hornblendite, pyroxenite and minor mafic eclogite. Assuming that these rock types are present in sufficiently extensive and thick layers, they would also have sufficiently high acoustic reflection coefficients for generating the generally well-developed reflectivity in the crust in the central part of the shield. Density profiles calculated from the lithological models suggest that there is practically no density contrast at Moho in areas of the high-velocity lower crust. Comparison of reflectors from FIRE-1 and FIRE-3 transects and the velocity model from SVEKA'81 wide-angle transect indicated that the reflectors correlate with velocity layering, but the three-dimensional structures of the crust complicate such comparisons.

Investigations of andesitic volcanic debris terrains: Part I — Geophysical

This article contains the findings of geophysical investigations conducted at the site of the proposed international airport at Melville Hall, Commonwealth of Dominica, West Indies. The main geophysical method used was seismic refraction; additionally, ground-penetrating radar, resistivity sounding, and resistivity tomographic imaging were performed at some of the sites, but only the results of the seismic refraction and electrical resistivity sounding contributed to the development of the model presented. Analysis of the seismic data shows a gradual increase in velocity with depth, for which a model has been determined. Ancillary models or predictions of porosity, density, and natural compaction with depth are given, based on the basic seismic model. The seismic velocity model allows an engineer with a topographic map of the area to predict the volume of rock/soil in each rippability class and to project the rate of ripping. The results of the 1D resistivity soundings strongly suggest a nearly homogeneous material with resistivity variations that can be explained by moisture and low-level lithological variations. Therefore, based on integrated seismic and resistivity results, our model is a thick ash/boulder conglomerate with velocity increasing with depth, mainly from overburden stress compaction. In other words, the material at depth is not well-cemented rock. As the overburden stress is removed, the material essentially expands and loosens to produce a lower seismic velocity, similar to present surface conditions. The soil/rock can therefore be removed with little or no ripping or blasting.

Upper mantle structure of Arctic Canada from Rayleigh wave dispersion

Models of upper mantle structure across the Canadian High Arctic region, from 62° North to 82° North, and 134° West to 56° West, are presented in this study. Teleseismic Rayleigh waves were recorded by an array of temporary and permanent broadband seismographs across the region. Fundamental-mode phase velocity dispersion curves were calculated for 84 two-station paths. The period range for the majority of the dispersion curves was ∼ 25–170 s, allowing the resolution of mantle structure to a depth of ∼ 250–300 km. A tomographic inversion was used to combine dispersion data from 61 stable paths, to give a set of phase velocity maps for the region. The maps show a persistent high-velocity anomaly across the central–northern mainland, strongest in the period range 50–110 s, and a low-velocity anomaly along the Arctic continental margin at periods ≤ 90 s. A number of dispersion curves from two-station paths were chosen for further analysis, together with localised dispersion curves derived from the phase velocity maps, for areas with good path coverage. Analysis of these dispersion curves was carried out, using a Monte-Carlo method, to obtain 1-D models for shear wave velocity structure across the region, assuming an isotropic upper mantle. The average velocity of the upper 100–200 km of the mantle is compared to that of the iasp91 global reference model, and the variations in velocity perturbation are examined. The highest velocity perturbations, ∼ 4–6% above iasp91, occur in the southern part of the region and are, for the most part, associated with the Canadian Shield and southern Arctic Platform. In contrast, velocity perturbations of only 1–3% above iasp91 characterise the northwestern Arctic islands, with the lowest velocities occurring beneath the Sverdrup Basin. The characteristics of the upper mantle velocity–depth sections also vary significantly. For paths crossing the central–northern mainland and Baffin Island, the Monte-Carlo models give near-constant velocity to at least 240 km depth in the upper mantle. The best match to the dispersion curves can be obtained by the inclusion of a shallower (∼ 200 km depth) low-velocity zone, but this feature is not required to match the data within physically realistic error bounds. In the central Arctic islands, the models show a ‘lid’ of relatively high velocities in the upper section of the mantle model, underlain by a low-velocity zone. This velocity pattern is consistent with previous interpretations of the character of the seismological lithosphere and asthenosphere. The base of the lid is difficult to identify, but negative velocity gradients are modelled over the ∼ 100–200 km depth range. Along the Sverdrup Basin margins, where the velocity anomalies are smallest, the models are similar in character to the iasp91 reference profile, showing a gradual increase in velocity with depth.

Influence of overpressure on formation velocity evaluation of Neogene strata from the eastern Bengal Basin, Bangladesh

Interpretation of sonic log data of anticlinal structures from eastern Bangladesh reveals significant variations of acoustic velocity of subsurface strata. The amount of variation in velocity is 32% from Miocene to Pliocene stratigraphic units in Titas and Bakhrabad structure, whereas 21% in Rashidpur structure. Velocity fluctuations are influenced by the presence of gas-bearing horizons, with velocities of gas-producing strata 3–7% lower than laterally equivalent strata at similar depth. Average velocities of Miocene Boka Bil and Bhuban formations are, respectively, 2630 and 3480 m/s at Titas structure; 2820 and 3750 m/s at Bakhrabad; and 3430 and 3843 m/s at the Rashidpur structure. From the overall velocity–depth distribution for a common depth range of 915–3000 m, the Titas, Bakhrabad and Rashidpur structures show a gradual increase in velocity with depth. In contrast, the Sitakund anticline in SE Bangladesh reveals a decrease in velocity with depth from 3000 to 4000 m, probably due to the presence of overpressured mudrocks of the Bhuban Formation. Tectonic compression, associated with the Indo-Burmese plate convergence likely contributed the most toward formation of subsurface overpressure in the Sitakund structure situated in the Chittagong–Tripura Fold Belt of the eastern Bengal basin, Bangladesh.

Simultaneous determination of hypocenter location and crustal structure in the Virginia seismic region

Simultaneous inversions for one‐ (1‐D) and three‐dimensional (3‐D) seismic structure and hypocenters (SSH) for both the central and western Virginia seismic regions are performed, using direct and refracted phases from about 60 earthquakes. Optimal inversion models are computed by employing regularization techniques of Tikhonov and of Backus and Gilbert. Improvements for the residual sum squared, of up to 70% are obtained for the 3‐D models and somewhat less for the 1‐D models. For the central Virginia seismic region the SSH 1‐D velocity models show a strong negative bias in the deduced Pn velocity which could only be alleviated after a ray‐tracing travel time correction for the Pn phases due to the east‐west downdipping Moho has been applied. A 4° Moho dip turns out to be optimal for best explaining the observed travel time residuals and returning a realistic Pn velocity of 8.2–8.3 km/s. However, the deduced velocity in the lower crust is only ∼ 6.35 km/s, which is lower than that predicted from gravity modeling but can be explained in terms of a lower crustal petrology of anorthosite‐rich granulite. A more detailed analysis of the lower crust shows that the hypothesized high‐velocity layer at the bottom of the crust (Pratt et al., 1988) is not clearly identifiable by the present travel time data and so can neither be corroborated nor refuted. Many of the 1‐D crustal results are further substantiated by the 3‐D model, which also provides a reduced lower crustal velocity under the Goochland Terrane. Positive velocity anomalies, which are found in the upper and middle crust under the Blue Ridge/Piedmont boundary, correlate qualitatively with the high‐density, thrust‐faulted slab deduced by Pratt et al. (1988) from gravity modeling. The vertical movements of the relocated hypocenters in the central region are of O(2–5 km) and are such as to further widen the aseismic gap under the Goochland Terrane. In the eastern section of the seismic zone (at the Piedmont/Atlantic Coastal Plain boundary) the downward focal shifts at the lower seismic boundary place events well into the Grenville basement, indicating a lowering of the brittle‐ductile transition boundary. For the western Virginia seismic region the SSH 1‐D velocity models result in a gradual increase in velocity with depth. Significant shifts in the hypocentral relocations are obtained which indicate a further deepening of the brittle‐ductile transition. This and results of thermomechanical creep models favor a quartz‐poor, diorite rich, middle and lower crust in the western region. The 3‐D velocity model of the western region shows a variety of features which can be related to the geology and the tectonics of the Valley and Ridge province. In the surficial layer a velocity low in the Mississippian‐Permian molasse of the West‐Virginian Valley and Ridge province and a velocity high in the thicker Cambrian‐Ordovician carbonate thrust and fault sheets of the Giles county area of the Valley and Ridge are obtained. In the lower crust, higher velocities are detected in the western Valley and Ridge province, which correlates well with the thickening of the crust in the western section of the Virginian Appalachian.

Mapping of low P wave velocity structures in the subducting plate of the central New Hebrides, southwest Pacific

Arrival times of compressional (P) and shear (S) waves generated by earthquakes located in the New Hebrides subduction zone and recorded by local and regional arrays of seismographs are used to determine large‐scale one‐ and three‐dimensional elastic wave velocity structures of the subduction zone between 15° and 20°S and from the surface to about 250 km depth. The results obtained from inverting the locally and regionally recorded arrival times individually corroborate each other, and they are inverted jointly in order to improve the resolution of shallow to intermediate depth structures. The results for one‐dimensional structure indicate a gradual increase of velocity with depth until a 9% reversal appears between 60 and 100 km depth. The three‐dimensional structure determined from the joint inversion shows that these low velocities lie within a sizable seismic gap in the descending Benioff zone. Taken with other observations such as the attenuation of high‐frequency shear waves travelling across this gap and the locations of active volcanoes at the surface, we infer that the low‐velocity region represents a thermal anomaly of about 750°C which alters the physical properties of the descending plate in this region. At shallower depths there is evidence of low‐velocity structures included in the descending plate: one beneath north Malekula island and another between Malekula and Efate islands corresponding to the large embayment of the leading edge of the upper plate. The locations of these structures combined with previous investigations of the area lead us to infer that these low velocities are due to the subduction of small‐scale features such as seamounts and accretionary wedges.

Flow Velocity Acceleration in the Left Ventricle: A Useful Doppler Echocardiographic Sign of Hemodynamically Significant Mitral Regurgitation

Doppler echocardiography is a sensitive method to detect mitral regurgitation in patients with both native and prosthetic valves. However, estimates of the amount of mitral regurgitation remain semiquantitative, and even severe mitral regurgitation may be underestimated in the presence of markedly eccentric regurgitant jets or acoustic shadowing of the left atrium by mitral or aortic prostheses. This report describes the Doppler findings in 10 patients with severe native valve mitral regurgitation (angiographic grade III or IV) and in 15 patients with severe bioprosthetic mitral regurgitation that required valve replacement. An increase in peak mitral flow velocity above normal values was seen in eight of 10 patients with severe native valve mitral regurgitation (greater than or equal to 130 cm per second) and 11 of 15 patients with severe prosthetic valve mitral regurgitation (greater than or equal to 210 cm per second). One of 10 patients with a native valve and four of 15 patients with a bioprosthetic valve appeared to have only a localized left atrial systolic flow disturbance, incorrectly suggesting that the mitral regurgitation was mild. However, in all patients with severe mitral regurgitation, a low velocity (less than 100 cm per second) flow signal could be recorded in the left ventricle that was directed toward the mitral valve in systole. This flow signal showed a gradual increase in velocity as the sample volume was moved toward the mitral valve, with an abrupt further increase on entry into the left atrium. This signal was continuous with antegrade mitral flow and had the same orientation as mitral regurgitation recorded by continuous wave technique from the apex. A similar flow signal was not recorded in the left ventricle of any individual in a control group of 30 patients who had no mitral regurgitation or who had angiographic grade I or II mitral regurgitation. These findings suggest that acceleration of left ventricle flow toward the mitral valve in systole is only recorded when there is hemodynamically significant mitral regurgitation that is approximately equal to angiographic grade III or IV. Recognition of this Doppler finding may help in the estimation of mitral regurgitation severity, especially in difficult diagnostic situations.

The crust and upper mantle of the Aegean region from deep seismic soundings

Deep seismic soundings performed in Greece in the years 1971–1974 revealed that the area of Greece consists of continental crust of variable thickness. The crust of the west Hellenic chains is 46 km thick and covered by sediments which locally exceed 10 km. That of the Aegean Sea is attenuated from north to south having 32 km below north Evia, 30 below south Evia and 28 km below Mikonos. The Cretan Sea shows an east-west elongated dome with maximum crustal attenuation at the central trough zone, with only 20 km thickness. The crust of Crete increases again to 32 km. Good Pg, Pn and PmP seismic phases were recorded at nearly all observed sections. They showed that the basement has a true velocity of 6 km/sec and that the lower crust is controlled by a layer of gradual velocity increase from about 6.2 to 6.8 km/sec. The Pn velocities have smaller values than the normal upper mantle has, ranging between 7.6 and 7.8 km/sec and indicating that below the Hellenides and the Aegean Sea a low velocity zone exists in the uppermost part of the mantle, limited in depth by a high velocity layer of approx. 8.5 km/sec. The geometry of the crust—mantle boundary across the coast of Peloponnesos showed that the crust nearly doubles in thickness from 26 to 46 km in only 10 km of horizontal distance.