High-velocity Zone Research Articles

Integrated Analysis of Well Logs and Seismic Data to Deduce the Possible Distribution in Depth of Gas Hydrate in Shenhu Area, South China Sea

AbstractThe available geological and geophysical information indicates the presence of gas hydrate at Shenhu Area of Northern South China Sea. The drilling cores recovered in 2007 testify the expectation and reveal that the gas hydrate disseminates evenly within the hydrate stable zone (HSZ), but its regional distribution is inhomogeneous. However, the logging record length and coring depth coverage are limited, which causes the depth distribution of gas hydrate to be indeterminate. Logs at well site SH7 were used to link the information of physical properties of sediments to the coincident seismic data acquired in this area. According to the relation between physical properties of sediments and seismic reflection characteristics, three primary formations were calibrated and the pseudo‐well constrained impedance inversion was then applied on post‐stack seismic data. The results showed a high velocity zone below the interval bearing gas hydrate. To determine whether this high velocity zone is related to the occurrence of gas hydrate or not, the technique of AVO forward modeling was further used to analyze the reflecting characteristic of pre‐stack seismic data. The comparison between synthetic seismic models and real seismic data indicates that the HSZ was composed of thin interbeds of gas hydrate‐bearing sediments and water‐saturated sediments, which suggests that another gas hydrate deposit exists most likely below the depth where gas hydrate‐bearing sediments were cored in the hole.

Application of the homogeneous function and modeling methods for reconstruction of a Tibetan geological section from a travel-time inversion

New structures were revealed in Central Tibet after reinterpretation of the first arrivals of refracted seismic waves along the INDEPTH III profile. The method that was used for travel-time inversion is the numerical generalization of the Herglotz-Wiechert formula for the case of a 2D inhomogeneous medium (the homogeneous-function method). The new section shows that the Bangong-Nujiang Suture (BNS) is a contact in the upper crust, while its continuation in the middle crust is a listric thrust; at the top of the lower crust, which has risen to a 32-km depth in the northern part of the section, this thrust become gentler. Below the middle crust top, a layer with a lower velocity of seismic waves is found. The middle crust is deformed and forms oblique southward listric thrusts. In the upper crust south of the BNS, higher-velocity zones (probable zones of magmatism) are distinguished. North of the BNS, undisturbed outlines of the structures are observed.

Structural control and genesis of the Oligocene Zhenyuan orogenic gold deposit, SW China

The Zhenyuan gold deposit with >50t gold reserve is located within the Jinshajiang–Ailaoshan suture zone that formed through the closure of Paleo-Tethyan ocean in late-Permian. This suture separates the Simao and South China blocks. A west-dipping high velocity zone down to ca. 250km beneath the Simao block in the seismic tomography suggests that the South China lithosphere was underthrust westward. The NNW-trending Ailaoshan–Red River shear zone is situated to the east of Ailaoshan suture. The shearing in this belt, generally following a regional mantle upwelling and crust extension event, was initiated at 32Ma and culminated at ca. 27Ma. This shearing was induced by the lateral extrusion and large clockwise rotation of the Simao block during the India–Eurasia continental collision and the underthrusting of South China block. The Zhenyuan ore deposit consists of the Donggualin and Laowangzhai orebody clusters. The Donggualin orebodies are controlled by the NW-striking shear faults, whereas most orebodies in the Laowangzhai are dominated by NE- and ENE-striking transcompressional faults. It is observed that the NE- and ENE-striking faults are crosscut by the NW-striking faults. It is deduced that the former were primarily formed as nearly NS-trending faults during the regional extension or initial shearing stage at ca. 32Ma. The primary faults were then re-oriented to the current strike concomitant with the rotation of the Simao block, and further cut by the NW-trending shear faults, which are subsidiary to the Ailaoshan–Red River shear zone. The rock types in the ore deposit including slate, meta-sandstone, limestone, lamprophyre, and meta-mafic to ultramafic units, which were all mineralized by the infiltration of ore-bearing fluids into extensional fractures. The low temperature ore mineral assemblage (pyrite–stibnite–arsenopyrite), enriched element association, and ranges of S and Pb isotopic compositions in the two ore clusters are broadly similar, indicating that the mineralization is contemporary despite the different formation times of ore-controlling structures. The 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb values of hydrothermal pyrite are close to those of the lower crust. Pyrite δ34S values show a wide range with a peak near 0 per mil. The S and Pb isotopic compositions, together with previously published H–O–He–Ar isotopic data, suggest that the ore fluid is mainly metamorphic. The data also suggest the input of mantle volatiles, consistent with mantle upwelling. The control of the shear zones on mineralization favors the interpretation that the Ar–Ar isochron age ~27Ma of phlogopite in the mineralized lamprophyre represents the mineralization age. This mineralization age implies that the South China block had started westward underthrusting before ~27Ma. The Zhenyuan ore deposit is considered to be an orogenic type, formed in a transitional stage from mantle upwelling to continental underthrust. The continental underthrusting drove the release of gold-charged metamorphic fluids and formation of the Zhenyuan ore deposit.

Aerodynamic performance of a vibrating piezoelectric fan under varied operational conditions

This paper experimentally examines the bulk aerodynamic performance of a vibrating fan operating in the first mode of vibration. The influence of operating condition on the local velocity field has also been investigated to understand the flow distribution at the exit region and determine the stalling condition for vibrating fans. Fan motion has been generated and controlled using a piezoelectric ceramic attached to a stainless steel cantilever. The frequency and amplitude at resonance were 109.4 Hz and 12.5 mm, respectively. A test facility has been developed to measure the pressure-flow characteristics of the vibrating fan and simultaneously conduct local velocity field measurements using particle image velocimetry. The results demonstrate the impact of system characteristics on the local velocity field. High momentum regions generated due to the oscillating motion exist with a component direction that is tangent to the blade at maximum displacement. These high velocity zones are significantly affected by increasing impedance while flow reversal is a dominant feature at maximum pressure rise. The findings outlined provide useful information for design of thermal management solutions that may incorporate this air cooling approach.

Tomographic evidence for a slab tear induced by fossil ridge subduction at Manila Trench, South China Sea

A tomographic travel-time inversion has been applied to trace the subducted slab of the South China Sea (SCS) beneath the Manila Trench. The dataset, taken from the International Seismological Centre (1960–2008), is composed of 13,087 P-wave arrival times from 1401 regional earthquakes and 8834 from 1350 teleseismic events. The results image the different morphology of the subducted SCS slab as a high-velocity zone. The subducting angle of the slab varies along the trench: at 16° N and 16.5° N, the slab dips at a low angle (24° ~ 32°) for 20–250 km depth and at a moderate angle (50°) for ~250–400 km depth. At 17° N, the slab dips at a low angle (32°) to near 400 km depth, and at 17.5° N and 18° N the slabs are near vertical from 70 ~ 700 km depth, while at 20° N the high-velocity anomalies exhibit features from horizontal abruptly to near vertical, extending to 500 km depth. The dramatic steepening of the slab between 17° N and 17.5° N may indicate a slab tear, which is coincident with the axis of a fossil ridge within the SCS slab at around 17° N. In addition, low-velocity zones in the three profiles above 300 km depth may represent the formation of the slab window, induced by ridge subduction and slab tear, initiating upward mantle flow and resulting in the partial melting of the edge of the slab. The slab tear could explain the volcanic gap and geochemical difference between the extinct Miocene and Quaternary volcanoes in the Luzon Arc, the much higher heat flow around the fossil ridge, and the distribution of most of the adakites and the related porphyry Cu-Au deposits in the Luzon area. Based on the geometry and morphology of the subducted slab and certain assumptions, we calculate the initial time of ridge subduction, which implies that ridge subduction and slab tear possibly started at ~8 Ma.

Shallow crustal radial anisotropy beneath the Tehran basin of Iran from seismic ambient noise tomography

We studied the shear wave velocity structure and radial anisotropy beneath the Tehran basin by analyzing the Rayleigh wave and Love wave empirical Green’s functions obtained from cross-correlation of seismic ambient noise. Approximately 199 inter-station Rayleigh and Love wave empirical Green’s functions with sufficient signal-to-noise ratios extracted from 30 stations with various sensor types were used for phase velocity dispersion analysis of periods ranging from 1 to 7s using an image transformation analysis technique. Dispersion curves extracted from the phase velocity maps were inverted based on non-linear damped least squares inversion method to obtain a quasi-3D model of crustal shear wave velocities. The data used in this study provide an unprecedented opportunity to resolve the spatial distribution of radial anisotropy within the uppermost crust beneath the Tehran basin. The quasi-3D shear wave velocity model obtained in this analysis delineates several distinct low- and high-velocity zones that are generally separated by geological boundaries. High-shear-velocity zones are located primarily around the mountain ranges and extend to depths of 2.0km, while the low-shear-velocity zone is located near regions with sedimentary layers. In the shallow subsurface, our results indicate strong radial anisotropy with negative magnitude (VSV>VSH) primarily associated with thick sedimentary deposits, reflecting vertical alignment of cracks. With increasing depth, the magnitude of the radial anisotropy shifts from predominantly negative (less than −10%) to predominantly positive (greater than 5%). Our results show a distinct change in radial anisotropy between the uppermost sedimentary layer and the bedrock.

Crustal shear-wave velocity structure beneath northeast India from teleseismic receiver function analysis

We investigated the seismic shear-wave velocity structure of the crust beneath nine broadband seismological stations of the Shillong–Mikir plateau and its adjoining region using teleseismic P-wave receiver function analysis. The inverted shear wave velocity models show ∼34–38km thick crust beneath the Shillong Plateau which increases to ∼37–38km beneath the Brahmaputra valley and ∼46–48km beneath the Himalayan foredeep region. The gradual increase of crustal thickness from the Shillong Plateau to Himalayan foredeep region is consistent with the underthrusting of Indian Plate beyond the surface collision boundary. A strong azimuthal variation is observed beneath SHL station. The modeling of receiver functions of teleseismic earthquakes arriving the SHL station from NE backazimuth (BAZ) shows a high velocity zone within depth range 2–8km along with a low velocity zone within ∼8–13km. In contrast, inversion of receiver functions from SE BAZ shows high velocity zone in the upper crust within depth range ∼10–18km and low velocity zone within ∼18–36km. The critical examination of ray piercing points at the depth of Moho shows that the rays from SE BAZ pierce mostly the southeast part of the plateau near Dauki fault zone. This observation suggests the effect of underthrusting Bengal sediments and the underlying oceanic crust in the south of the plateau facilitated by the EW-NE striking Dauki fault dipping 300 toward northwest.

A Velocity–Dissipation Lagrangian Stochastic Model for Turbulent Dispersion in Atmospheric Boundary-Layer and Canopy Flows

An extended Lagrangian stochastic dispersion model that includes time variations of the turbulent kinetic energy dissipation rate is proposed. The instantaneous dissipation rate is described by a log-normal distribution to account for rare and intense bursts of dissipation occurring over short durations. This behaviour of the instantaneous dissipation rate is consistent with field measurements inside a pine forest and with published dissipation rate measurements in the atmospheric surface layer. The extended model is also shown to satisfy the well-mixed condition even for the highly inhomogeneous case of canopy flow. Application of this model to atmospheric boundary-layer and canopy flows reveals two types of motion that cannot be predicted by conventional dispersion models: a strong sweeping motion of particles towards the ground, and strong intermittent ejections of particles from the surface or canopy layer, which allows these particles to escape low-velocity regions to a high-velocity zone in the free air above. This ejective phenomenon increases the probability of marked fluid particles to reach far regions, creating a heavy tail in the mean concentration far from the scalar source.

Swave velocity structure in southwest China from surface wave tomography and receiver functions

Using the surface wave records of 504 teleseismic events at 50 temporary and 92 permanent seismic stations in southwest China, we obtain phase velocity maps at 10, 15, 25, 40, 60, and 75 s at a grid spacing of 0.5° × 0.5° from the interstation correlation method and surface wave tomography. We also obtain the S wave velocity structures beneath three profiles using the joint inversion of receiver functions and surface waves. At short periods (10 and 15 s), high-velocity zones (HVZs) are found in the Panzhihua-Emeishan region, the Sichuan basin and the Weixi-Lijiang region surrounding the low-velocity zones (LVZs) from Songpan-Ganzi block to the east of Lijiang, where the elevations are significantly higher. At long periods (40, 60, and 75 s), HVZs are found in the Weixi-Lijiang region, the Panzhihua-Chuxiong basin,and the Kunming-Tonghai region, which form a belt in the center part of the study area. The fast polarization directions on both sides of the belt defined by the shear wave splitting of teleseismic SKS waves vary significantly and indicate that the flow of material from the plateau is blocked in two different depth intervals and leads to different horizontal extents. The long-period maps and the structures along the three profiles show that LVZs are present in the upper mantle beneath rapidly slipping fault zones, such as the Anninghe-Zemuhe-Xiaojiang fault zone, the Red River fault zone, and the Xiaojinhe fault zone, implying that these faults penetrate deep into the mantle.

New Insights from Seismic Imaging Over the Youanmi Terrane, Yilgarn Craton,Western Australia

The general crustal structure of the Youanmi Terrane – a tectonic unit within the Neoarchean Yilgarn Craton – and its relationship to ore deposits are of major interest in Western Australia. To image geological structures, the Geological Survey of Western Australia in collaboration with Geoscience Australia acquired three deep crustal 2D seismic profiles over the region in 2010. In this study, we have reprocessed a 120km long section of one of the seismic profiles, line 10GA-YU1. We reprocessed the data with focus on shallow reflections down to 2 s (about 6km depth). A cross-dip analysis was performed to investigate the dip of the reflections in cross-line direction. The cross-dip analysis showed that there is no significant cross-dip influence, confirming that a 2D interpretation is a valid assumption for this dataset. The seismic image of the reflections was improved after applying dip-moveout correction. Finally, the reflections were migrated to their “true” locations in the subsurface. To better understand the physical properties of the subsurface, we also tried First Arrival Travel-Time Tomography. The tomography results show that low velocities correlate with shear zones and that high velocity zones are common where mineral deposits are located close to the surface.

Crustal structures of the Weihe graben and its surroundings from receiver functions

We use 15 seismic stations, crossing the Qinling orogen (QO), Weihe graben (WG) and Ordos block (OB), to study the crustal structures by receiver functions (RFs) methods. The results show quite a difference in crustal structures and materials of three tectonic units (orogenic belt, extentional basin and stable craton). The average crustal thickness in the northern QO is 37.8 km, and Poisson ratio is 0.247, which indicates the increase of felsic materials in QO. In the southern OB, the average crustal thickness is 39.2 km and Poisson ratio is 0.265. Comparatively high value of Poisson ratio is related with old crystallized base in the lower crust and shallow sediments. The artificial RFs reveal that low-velocity and thick sediments have a significant effect on phases of the Mohorovicic discontinuity (Moho). As a result, the Moho phases in WG are tangled. S-wave velocity (VS) inversion shows that there are shallow sediment layers with 4-8 km’s thickness and high velocity zones in the middle-lower crust in WG. Complex Moho structure and high velocity zone may have been induced by the activities of the Weihe faults series.

Scandinavia: A former Tibet?

The Himalaya and the Tibetan Plateau are uplifted by the ongoing northward underthrusting of the Indian continental lithosphere below Tibet resulting in lithospheric stacking. The layered structure of the Tibetan upper mantle is imaged by seismic methods, most detailed with the receiver function method. Tibet is considered as a place where the development of a future craton is currently under way. Here we study the upper mantle from Germany to northern Sweden with seismic S receiver functions and compare the structure below Scandinavia with that below Tibet. Below Proterozoic Scandinavia, we found two low‐velocity zones on top of each other, separated by a high‐velocity zone. The top of the upper low‐velocity zone at about 100 km depth extends from Germany to Archaean northern Sweden. It agrees with the lithosphere‐asthenosphere boundary (LAB) below Germany and Denmark. Below Sweden it is known as the 8°discontinuity, or as a mid‐lithospheric discontinuity (MLD), similar to observations in North America. Seismic tomography places the LAB near 200 km in Scandinavia, which is close to the top of our deeper low‐velocity zone. We also observed the bottom of the asthenosphere (the Lehmann discontinuity) deepening from 180 km in Germany to 260 km below Sweden. Remnants of old subduction in the upper about 100 km below Scandinavia and Finland are known from controlled source seismic experiments and local earthquake studies. Recent tomographic studies indicate delamination of the lithosphere below southern Scandinavia and northern Germany. We are suggesting that the large‐scale layered structure in the Scandinavian upper mantle may be caused by processes similar to the ongoing lithospheric stacking in Tibet.

Crustal velocity and Vp/Vs structures beneath central Anatolia from local seismic tomography

We applied a seismic tomography technique to arrival time data generated by local crustal earthquakes in central Anatolia in order to study the three-dimensional velocity and Vp/Vs structures and their relation with the complex tectonic processes and seismic activity occurring in the study region. The relatively equal and large number of both P- and S-wave arrival times comprising a total of 51,650 arrivals and the relatively uniform distribution of the recording stations imply that the obtained velocity anomalies are reliable features down to a depth of 40 km. This is also evident from the results of the checkerboard resolution test, hit count, and the ray-path coverage. The inversion results indicate the existence of strong lateral heterogeneities in the crust and uppermost mantle beneath central Anatolia. Prominent low-velocity anomalies are clearly imaged at all layers especially beneath existing volcanoes and the active fault segments. Higher-than-average Vp/Vs ratios are widely distributed, indicating the possible existence of over-pressurized fluids that may be responsible for the triggering of the large crustal earthquakes along the north and east Anatolian fault zones. We noticed that the seismic activity occurs mainly at the low-velocity areas and to a lesser extent in some high-velocity zones, perhaps because of the complex tectonics and geological structures. These observations imply that all the zones with velocity anomalies—either low or high—are potential sites for strain energy accumulation and subsequent release. The obtained velocity and Vp/Vs models are consistent with previous geophysical measurements conducted beneath central Anatolia and give much deeper understanding of the current seismotectonic processes occurring in the region.

Turbulent Characteristics of the Stratified Flow in the Braided River with Two Symmetrical Anabranches

The turbulent characteristics of stratified shear flow in braided rivers have not been known until now. The physical model experiment was conducted to study the turbulence kinetic energy distribution of stratified shear flow in the typical braided rivers with a mid-bar between two symmetrical anabranches. Firstly, the distributions of depth-averaged turbulence intensity components in different sections of the anabranch were analyzed. The turbulent intensity peak occurs at the boundary of the separation zone near outer bank and the high velocity zone near inner bank in the entrance region of the anabranch. In the outlet section of the anabranch, turbulent intensity increased at the inner bank. Secondarily, Lateral-averaged turbulence intensity components were compared between different sections. There was obvious high turbulence appearing at the interface of hot and cold water in the inlet section of the anabranch. Finally, the distributions of total turbulence intensity were analyzed by the sectional contours.

P-wave anisotropic tomography in Southeast Tibet: New insight into the lower crustal flow and seismotectonics

We determined the first 3-D P-wave anisotropy tomography beneath Southeast Tibet and adjacent regions using 63,773 P-wave arrivals from 2866 local earthquakes and 55,457 arrivals from 2802 teleseismic events. A remarkable low-velocity layer with a thickness of about 20km is revealed in the lower crust, which may reflect a mechanically weak zone capable of flow on a geological timescale. Our seismic anisotropy results suggest that the flow direction changes when it encounters the mechanically strong Sichuan basin. Most of the large earthquakes including the 2008 Wenchuan earthquake (M 8.0) and the 2013 Lushan earthquake (M 7.0) occurred at the margin of the ductile flow in the lower crust, suggesting that the seismogenesis is controlled by the deep dynamic processes. In the upper mantle, the subducting Indian plate is imaged clearly as a high-velocity zone which has reached near the Jinsha River suture. In addition, our results show significant variations of seismic anisotropy with depth, implying that the upper crust and the lithospheric mantle deform separately beneath most parts of the study region.

P-wave tomography and dynamics of the crust and upper mantle beneath western Tibet

The continental collision between the Indian and Asian plates plays a key role in the geologic and tectonic evolution of the Tibetan plateau. In this article we present high-resolution tomographic images of the crust and upper mantle derived from a large number of high-quality seismic data from the ANTILOPE project in western Tibet. Both local and distant earthquakes were used in this study and 35,115 P-wave arrival times were manually picked from the original seismograms. Geological and geochemical results suggested that the subducting Indian plate has reached northward to the Lhasa terrane, whereas our new tomography shows that the Indian plate is currently sub-horizontal and underthrusting to the Jinsha river suture at depths of ~100 to ~250km, suggesting that the subduction process has evolved over time. The Asian plate is also imaged clearly from the surface to a depth of ~100km by our tomography, and it is located under the Tarim Basin north of the Altyn Tagh Fault. There is no obvious evidence to show that the Asian plate has subducted beneath western Tibet. The Indian and Asian plates are separated by a prominent low-velocity zone under northern Tibet. We attribute the low-velocity zone to mantle upwelling, which may account for the warm crust and upper mantle beneath that region, and thus explain the different features of magmatism between southern and northern Tibet. But the upwelling may not penetrate through the whole crust. We propose a revised geodynamic model and suggest that the high-velocity zones under Lhasa terrane may reflect a cold crust which has interrupted the crustal flow under the westernmost Tibetan plateau.

Finite difference tomography of the crustal velocity structure in Tengchong, Yunnan province

This paper applies the finite difference tomography method to invert for the P-wave velocity structure in the Tengchong area using earthquake data from a temporary network and other permanent stations, and analyzes the magmatic activity of the Tengchong volcanoes and the deep tectonic genesis of Longling M7.3/M7.4 earthquakes. The results show the mean velocity in the crust with clear heterogeneity is a little lower. The lower velocities in the shallow layers are caused by unconsolidated basin deposits, Cenozoic volcanic deposits, fluid fractures and hot springs. The high-velocity zone at 5 similar to 15 km depth probably represents the cooled and solidified magmatic intrusions or the less volatile ultramafic remnants. The low-velocity structure in the deep indicates the molten or semi-molten magma. It is probable that the magmatic activity beneath the volcanoes and heat flow intrusions in the deep crust of Longling earthquake zone are from the same magmatic hearth area, which is the main area of the present magmatic activities. A strong lateral heterogeneity is observed in Longling earthquake zone. The east of the Nujiang fault belt and the west of the Long ling fault belt have the characters of high-velocity with the high strength of rock, which are the main bearing body of stress accumulation. However, the low-velocity anomaly between the two faults extends to lower crust, probably caused by magmatic intrusions in the deep crust. The magmatic activity in this area is controlled clearly by the Long ling fault and the Nujiang fault. The Long ling M7.3 and M7.4 earthquakes occurred near the velocity boundary beneath the faults. The main reasons of the strong earthquakes in the Long ling area contain the heterogeneity of stress accumulation caused by the lateral variation of the crustal medium strength, the deep magmatic aggregation and dynamic effects.

Finite Difference Tomography of Crustal Velocity Structure Beneath Tengchong, Yunnan Province

Abstract:This work applies the finite difference tomographic method to invert P‐wave velocity structure beneath the Tengchong area using earthquake data from a temporary network and other permanent stations, and analyzes the magmatic activity of Tengchong volcanoes and the deep structural genesis of the Longling M7.3/M7.4 earthquakes. The results show the velocity structure of the crust in this area has a pronounced heterogeneity. The lower velocities in the shallow subsurface are caused by unconsolidated basin deposits, Cenozoic volcanic deposits, fluid fractures and hot springs. The high‐velocity zone at 5∼15 km depth probably represents cooled and solidified magmatic intrusions or less volatile ultramafic remnants. The low‐velocity structure in the deep crust indicates molten or semi‐molten magma. It is probable that the magmatic activity beneath volcanoes and magma intrusions in the deep crust of the Longling earthquake zone are from the same magmatic hearth area, which is the main area of the present magmatic activities. A strongly lateral heterogeneity is observed in the Longling earthquake zone. The east of the Nujiang fault belt and the west of the Longling fault belt are characterized by high velocity, implying large strain strength of rocks, which are the main carriers of stress accumulation. The low‐velocity anomalies between the two faults extend to lower crust, probably caused by magmatic intrusions in the deep crust. The magmatic activity in this area is controlled clearly by the Longling fault and the Nujiang fault. The Longling M7.3 and M7.4 earthquakes occurred near the velocity boundary beneath the faults. The main reasons for the strong earthquakes in the Longling area inlcude the heterogeneity of stress accumulation caused by the lateral variation of the crustal medium strength, the deep magmatic aggregation and dynamic actions.

Distributed lock-in drives broadband vortex-induced vibrations of a long flexible cylinder in shear flow

AbstractA slender flexible body immersed in sheared cross-flow may exhibit vortex-induced vibrations (VIVs) involving a wide range of excited frequencies and structural wavenumbers. The mechanisms of broadband VIVs of a cylindrical tensioned beam of length-to-diameter aspect ratio 200 placed in shear flow, with an exponentially varying profile along the span, are investigated by means of direct numerical simulation. The Reynolds number is equal to 330 based on the maximum velocity, for comparison with previous work on narrowband vibrations in linear shear flow. The flow is found to excite the structure at a number of different locations under a condition of wake–body synchronization, or lock-in. Broadband responses are associated with a distributed occurrence of the lock-in condition along the span, as opposed to the localized lock-in regions limited to the high inflow velocity zone, reported for narrowband vibrations in sheared current. Despite the instantaneously multi-frequency nature of broadband responses, the lock-in phenomenon remains a locally mono-frequency event, since the vortex formation is generally synchronized with a single vibration frequency at a given location. The spanwise distribution of the excitation zones induces travelling structural waves moving in both directions; this contrasts with the narrowband case where the direction of propagation toward decreasing inflow velocity is preferred. A generalization of the mechanism of phase-locking between the in-line and cross-flow responses is proposed for broadband VIVs under the lock-in condition. A spanwise drift of the in-line/cross-flow phase difference is identified for the high-wavenumber vibration components; this drift is related to the strong travelling wave character of the corresponding structural waves.

Crustal thickness and velocity structure beneath Singapore’s seismic network

Abstract We estimated the crustal thickness and velocity structure beneath the five stations comprising the Republic of Singapore’s seismic network. Our data set was composed of 697 teleseismic receiver functions and 7 months of broad-band data that was cross-correlated to produce inter-station Green’s functions. Surface wave group velocities were extracted from the Green’s functions to obtain dispersion data for a path from central Sumatra to Singapore in order to provide a complimentary data set to the receiver functions. Crustal thickness was estimated via an H − k stacking technique, and high-resolution 1D P -wave velocity profiles were generated beneath each station by jointly inverting receiver function stacks and the group velocity data using a linearised time-domain inversion scheme. Crustal thickness beneath four stations was found to be between 28.0 km and 32.0 km, while one station in the northeast of Singapore indicates 24.0 km thick crust. This implies a significant crustal thinning beneath Singapore over the lateral extent of 50.0 km. Inversion results exhibit several crustal features that are observable in the derived models at all five stations, indicating that they exist across Singapore as a whole. There appears to be an upper-crustal high-velocity zone beneath Singapore, underlain by a velocity inversion. Station NTU shows slower near-surface velocities than the other stations, consistent with its situation above the sedimentary Jurong formation. These results expand the available global velocity data set, as well as being useful for assessing the seismic hazard in Singapore.