Seismic Compressional Wave Velocity Research Articles

Continuous evolution of oceanic crustal structure following an eruption at Axial Seamount, Juan de Fuca Ridge

Abstract We present the first continuous observations of the temporal evolution of oceanic crustal shear velocity beneath Axial Seamount, a submarine volcano on the Juan de Fuca Ridge (offshore northwestern North America). Weekly values of seafloor compliance, the periodic deformation of the seafloor under ocean waves, were estimated over the time period between December 2014 and May 2018 using data from two cabled broadband ocean-bottom seismometers with collocated absolute pressure sensors. We inverted these measurements for shear-wave velocity within the volcano beneath the two stations as a function of depth and time. Our results, combined with estimates of seismic compressional wave velocity, suggest that the shallow melt reservoir and the lower crust beneath the central caldera contain melt fractions of 14% and at least 4%, respectively. The eruption of April 2015 induced a dramatic drop in shear velocities beneath the central station, primarily in the lower crust, which could have been caused by an increase in melt fraction, a change in small-scale melt geometry, or both. The absence of such a change beneath the eastern flank of the caldera indicates that there is a lower-crustal conduit beneath the caldera center, which is much narrower in cross section (<1 km2) than the overlying melt reservoir (≥42 km2). Our study demonstrates the promise of using continuous data to understand submarine volcanism and crustal accretionary processes.

Using Shallow Seismic Method for Evaluation of the Foundation Beds of New Cairo Extension (Andalus Area), Egypt.

Shallow Seismic Refraction Method (SSR) in engineering investigation is growing interest after the October 1992 earthquake in Egypt. In the present study, eight shallow seismic refraction (SSR) profiles were acquired along the Al-Andalus area, which located at the southeastern part of new Cairo extension, Egypt, in order to identify the shallow subsurface geological features of the near-surface section. Both Primary (VP)-wave and Secondary (VS)-wave velocities that were picking and computed during processing and interpretation. The seismic compressional wave velocity (Vp) distribution indicates that there are three geoseismic layers ranging between (420 and 680 m/s), (1040 and 1680 m/s) and (2170 and 3100 m/s), for first, second and third layers, respectively. The first shear wave velocity (VS) is ranging between (240 – 390 m/s), the second VS layer ranging between (570 - 980 m/s), and the third VS layer is ranging between (1240 - 1800 m /s). The thickness of the first layer varies from 1.8 to 2.6 m and the second layer ranges between 7.6 and 10.5 m. The geoseismic cross sections are interpreted geologically as the first low velocity values which indicate weathered sediments fine sands change to sandy gravels sometimes. However, the second geoseismic layer is represented by consolidated sands with clay intercalations and the third layer reflected sandstones. The geotechnical parameters (elastic modulus, material competence and bearing capacities) are estimated. The distribution of these parameters noticed that the areas can be carried heavy loads at the central, east central and north central parts of the study area.

Three-dimensional seismic velocity structure across the 2008 Wenchuan Ms 8.0 earthquake, Sichuan, China

We present three-dimensional (3D) seismic compressional wave velocity ( V p), shear wave velocity ( V s) and V p/ V s models around the Longmen Shan fault, Sichuan, China region, using aftershocks associated with the 2008 Wenchuan Ms 8.0 earthquake. The velocity and ratio models are obtained using a new version of the double-difference seismic tomography code tomoDDPS (Zhang, 2003) to simultaneously solve for V p, V s, V p/ V s and event locations. The data used in inversion include 73,013 P arrival times, 62,287 S arrival times and 61,823 S–P travel times recorded by 63 stations from both permanent and temporary networks in a region 400 km northeast-southwest by 200 km northwest-southeast. The velocity model shows structure heterogeneity both along and across the fault zone. Generally, the velocity model is consistent with the local geology, with older rocks having high velocity and younger rocks having low velocity. The Longmen Shan fault zone is a clear boundary in V p, V s and V p/ V s ratio. Down to the depth around 15 km, higher V p and V s and lower V p/ V s ratio exist to the west of the fault, corresponding to the Songpan–Ganze Fold System, while lower V p and V s and higher V p/ V s ratio exist to the east, corresponding to the Sichuan Basin. Along the fault zone, the velocity structure is generally consistent with various rupture slip models, with two high velocity bodies corresponding well to the two large slip patches. This shows the structure control on the slip distribution along the fault plane.

Age dependent porosity of young upper oceanic crust: Insights from seafloor gravity studies of recent volcanic eruptions

An October 1997 ALVIN program collected a series of seafloor gravity stations over the February, 1996, volcanic eruption site on the northern Gorda Ridge (46°41′N, 126°47′W). These measurements yielded estimates of the density and porosity of mid‐ocean ridge crust in the first few decades following formation. The estimated crustal density for this pillow eruption of 2270 ± 260 kg/m3 is very low, but in good agreement with the estimated densities of other recent seafloor volcanic flows. Rock matrix densities, measured from hand samples retrieved from the study area, give a bulk porosity for this flow of 34% ± 16%. This data, when included with similar estimates of upper crustal porosity, argue for a rapid evolution of upper crustal porosities from very high values (≥30%) for recent eruptions to much lower values (∼10–15%) where the seafloor is older than 0.5 million years. These variations of density and porosity are of a magnitude consistent with previously observed changes in seismic compressional wave velocity and permeability of ocean crust.

Relationships Among Formation Resistivity Factor, Compressional Wave Velocity, and Porosity for Reservoirs Saturated with Multiphase Fluids

The electric formation resistivity factor (F) and the seismic compressional wave velocity (v p) are powerful parameters in understanding the electric and elastic behavior of porous media and in identifying the type of fluid saturating the pore spaces. The formation resistivity factor is a function of various influences, including pore and grain properties; saturation, salinity, and viscosity of pore water; formation and pore-water resistivities; cation-exchange capacity; and clay content. The compressional wave velocity is a function of bulk (grain and fluid) density; type of saturant and degree of saturation; and various elastic moduli, including bulk (pore, fluid, and grain) compressibility. Both parameters (F and v p) are significantly affected by variations of porosity ( φ ), pressure, and temperature. The three parameters (F, v p, φ) were obtained from well log measurements for complex, heterogeneous, and consolidated shaly sandstone reservoirs, saturated with multiphase fluids, offshore of the eastern coast of Canada. Relationships among the three parameters, having coefficients of correlation ranging from 0.75 to 0.92, were obtained. Both F and v p are correlated inversely to φ and directly to each other.

Relationships Among Formation Resistivity Factor, Compressional Wave Velocity, and Porosity for Reservoirs Saturated with Multiphase Fluids

The electric formation resistivity factor (F) and the seismic compressional wave velocity (v p) are powerful parameters in understanding the electric and elastic behavior of porous media and in identifying the type of fluid saturating the pore spaces. The formation resistivity factor is a function of various influences, including pore and grain properties; saturation, salinity, and viscosity of pore water; formation and pore-water resistivities; cation-exchange capacity; and clay content. The compressional wave velocity is a function of bulk (grain and fluid) density; type of saturant and degree of saturation; and various elastic moduli, including bulk (pore, fluid, and grain) compressibility. Both parameters (F and v p) are significantly affected by variations of porosity ( φ ), pressure, and temperature. The three parameters (F, v p, φ) were obtained from well log measurements for complex, heterogeneous, and consolidated shaly sandstone reservoirs, saturated with multiphase fluids, offshore of the eastern coast of Canada. Relationships among the three parameters, having coefficients of correlation ranging from 0.75 to 0.92, were obtained. Both F and v p are correlated inversely to φ and directly to each other.

Computer modelling of a pressure induced phase change in clinoenstatite pyroxenes

Using lattice dynamic modelling of pure MgSiO3 clinopyroxenes, we have be able to simulate the properties of both the low-clino (P21/c) and a high-density-clino (C2/c) phases and our results are comparable with the high pressure (HP) X-ray study of these phases (Angel et al. 1992). The transition between the two phases is predicted to occur at 6GPa. The volume variation with pressure for both phases is described by a third-order Birch-Murnaghan equation of state with the parameters V 0 low=31.122 cm3·mol−1, K T0 low= 107.42 GPa, K′ T0 low=5.96, V 0 high=30.142 cm3·mol–1, K T0 high102.54 GPa and K′ T0 high=8.21. The change in entropy between the two modelled phases at 6GPa is ΔS 6 Gpa=−1.335 J·mol−1·K−1 and the equivalent change in volume is ΔV 6 GPa=−0.92 cm3·mol−1, from which the gradient of the phase boundary δP/δT is 0.0014 GPa·K−1. The variation of the bulk modulus with pressure was also determined from the modelled elastic constants and compares very well with the EOS data. The reported Lehmann discontinuity, ∼220 km depth and pressure of 7.11Gpa, has an increase in the seismic compressional wave velocity, v p , of 7.14% using the data given for PREM (Anderson 1989). At a pressure of 7GPa any phase transition of MgSiO3 pyroxene would be between ortho (Pbca) and high-clino. We find the value of v p at 7GPa, for modelled orthoenstatite (Pbca), is 8.41 km·sec−1 and that for the modelled high-clino phase at 7GPa is 8.93 km·sec−1, giving a dv p /v p of 6.18%.

Increase of seismic velocities in upper oceanic crust: The “superfast” Spreading East Pacific Rise at 14°14'S

One of the most important observations of oceanic crustal evolution is that seismic compressional wave velocities in the upper igneous crust, layer 2A, increase as the crust ages. The most viable explanation for this phenomenon is decreasing porosity due to filling of open void spaces with hydrothermally generated minerals. But the idea that seismic velocity could be a function of plate age, however, depends primarily on airgun/sonobuoy profiles collected more than 20 years ago [Houtz and Ewing, 1976]. Therefore, in late 1995 new seismic refraction measurements were obtained on the eastern flank of the “superfast” spreading East Pacific Rise south of the Garrett transform. We found that velocities rapidly increase by about 45–50% within only 0.5–1 m.y., double within 5 m.y. or less and remain constant thereafter.

Modification of lower crust by continental rift magmatism

We document a systematic inverse relationship between the volume of erupted volcanic rocks associated with continental rifts and the seismic compressional wave velocity contrast across the Moho. This relationship is interpreted as evidence that magmatism significantly modifies the composition(s) of the lower crust (and upper mantle) during rifting processes. The relationship is not influenced by the current thermal state of the lithosphere, being observed for both modern rifts (Rio Grande, Kenya, Rhine) and paleorifts (Oslo), and may also characterize continental flood basalt provinces (Columbia River and Snake River plain). The trend is reconciled by igneous and metamorphic processes that collectively modify the lower crust to more mafic bulk compositions, resulting in higher compressional velocities, and that decrease the density of the residual mantle, resulting in lower compressional velocities. These results support the view that most mantle‐derived magma in continental rift settings is trapped in the crust.

Recent crustal movements in Switzerland and their geophysical interpretation

In this paper a review is presented on the most recent results of geodetically determined recent crustal movements in Switzerland. These data are compared with the new Bouguer gravity map of Switzerland, and an attempt is made to discuss the geodetic and gravity results in connection with the crustal structure of the Swiss Alps in terms of seismic compressional wave-velocities and depths to the crust—mantle boundary.

Heat flow, heat production and crustal dynamics in the Central Alps, Switzerland

Interrelations between temperature field, crustal structure and crustal dynamics (vertical crustal movements) have been investigated along a selected profile: the Swiss Geotraverse which cross-sects in a NW-SE direction the following tectonic units: Rhine-Graben, Jura Mountains, Molasse Basin, Helvetic Nappes, Central Massifs with autochthonous cover, Penninic units, basement and sedimentary units of the Southern Alps (total length: 220 km). The corrected heat flow is slightly elevated along or close to the traverse (≈ 75 mW/m 2). Thermal effects of Alpine overthrusting and metamorphism on the surface gradient are negligible today. For steady-state calculations of the temperature field heat production has been determined experimentally for surface samples; for deep crustal rocks it was inferred from an empirical relationship between heat production and seismic compressional wave velocity or density. The temperature field shows downwarped isotherms where a pronounced inversion of seismic velocity and density occurs in the upper crust. The same area of the Central Alps (Lepontine gneiss region) exhibits the strongest recent crustal movements (vertical uplift ≈ 1 mm/yr). The Mohorovićič discontinuity is clearly not an isothermal surface; its asymmetric shape found by seismic and gravimetric measurements is likely to be a result of the early Alpine subduction tectonics.