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The uppermost oceanic crust composes of Layers 2A and 2B, and the boundary between them is debated to be a lava/dike transition or a hydrothermal alteration boundary within the lava unit. Here, we present the analyses of ultralong multichannel seismic data along a 1,500 km long profile covering 0–75 Ma of the oceanic lithosphere on the African plate in the equatorial Atlantic Ocean. We find that the Layer 2A is observed along the whole profile, with its urn:x-wiley:ggge:media:ggge22094:ggge22094-math-0001 velocity increasing from 2.5 km/s near the ridge axis to urn:x-wiley:ggge:media:ggge22094:ggge22094-math-00024 km/s at urn:x-wiley:ggge:media:ggge22094:ggge22094-math-00034 Ma with slight variations thereafter. We also find that the sediment thickness increases rapidly from 0 m at the ridge axis to 170 m at 4 Ma, suggesting that there is a link between the high sedimentation rate and the increase in Layer 2A velocity. These observations indicate that crust younger than 4 Myr may be influenced by active hydrothermal circulation. The observed thickness of Layer 2A decreases from urn:x-wiley:ggge:media:ggge22094:ggge22094-math-0004850 m near the ridge axis to urn:x-wiley:ggge:media:ggge22094:ggge22094-math-0005600 m at 15 Ma with no significant changes beyond. We also find an increase in Layer 2B velocity from 5.1 km/s at 4 Myr to 5.5 km/s at 46 Myr, suggesting that passive hydrothermal circulation may extend deeper than Layer 2A/2B boundary. We propose Layer 2A/2B boundary to be a lava/dike transition at the ridge axis and a hydrothermal alteration boundary within the extrusive section away from the ridge axis. Published version

Sumatra subduction zone is one of the most seismically active zones on Earth. After having produced three Mw > 8.4 earthquakes and several Mw > 7.5 earthquakes, including the Mw = 7.8 2010 tsunami earthquake, the northern Mentawai segment is still locked and is capable of generating a great earthquake, possibly a disastrous tsunami. We analyzed ultralong offset seismic reflection data from this locked zone to characterize the nature of the accretionary prism and the plate interface using a combination of traveltime tomography, full waveform inversion, and prestack depth migration. In order to enhance the refractions, we downward extrapolate the streamer data to the seafloor, allowing the refraction arrivals to be observed from near‐zero offset up to far offset, and use a traveltime tomography to determine the background velocity in the upper sediments. Starting from these velocities, we perform a multiscale elastic full waveform inversion to determine the detailed P wave velocity structure of the subsurface. Based on this velocity, we perform a prestack depth migration to obtain seismic image in the depth domain and compute the porosity of the sediments to determine fluid content along faults. Our results show a low‐velocity subduction channel with high porosity at the plate interface that connects the likely active frontal thrusts at the toe of accretionary wedge, suggesting that the frontal section of the prism is seismogenic. We have also observed a low‐velocity layer in the middle of wedge separating old sediments below from new sediments above, defining the roots of bivergent thrust faults up to the seafloor, which can be interpreted as a psudo‐décollement. Published version

On the northern Cascadia subduction margin, the multichannel seismic amplitude-versus-offset (AVO) behavior of a bottom-simulating reflector (BSR) suggests a P wave velocity change from high-velocity hydrate-bearing sediment to lower velocity sediment containing a small amount of free gas. The observed nonlinear AVO behavior, constant or slightly decreasing amplitudes at near-to-mid offsets and a large-amplitude increase at far offsets, can be reproduced in the models if an S wave velocity enhancement is assumed as expected from hydrate cementation. The AVO behavior of the hydrate BSR is found not to be as useful as was earlier thought for determining the amount of free gas below the BSR. This is because Poisson's ratio change below the BSR due to gas is likely very small in high-porosity unconsolidated sediments. The uncertainty in the models is large, as there is no reliable S wave velocity information for the sediments containing hydrate or gas. AVO modeling alone is not sufficient to distinguish different velocity models across the BSR. Our interpretation of the BSR amplitude behavior is that a P wave velocity increase above the BSR is the main cause of the BSR reflection amplitude increase at large incidence angles. Caution must be taken in applying AVO analysis, as little is known about S wave velocities. However, subtle differences in BSR amplitude behavior and reflection waveform can provide constraints through very careful full waveform inversion. A well-defined reference velocity-depth profile is also required to represent water-saturated sediment unaffected by either hydrate or free gas. Using full waveform velocity inversion, a high-resolution velocity model for the hydrate BSR has been derived. The best fit model for the seismic data near the Ocean Drilling Program (ODP) sites 889/890 consists of a high-velocity zone above the BSR and a thin low-velocity layer below, in agreement with the ODP downhole velocity data.

We have applied full waveform inversion to wide-aperture seismic reflection data from the southern East Pacific Rise near 14°S. The data contain clear compressional wave and doubly converted shear wave arrivals that provide good constraints on the P and S-wave velocities (Vp, Vs, and hence Poisson's ratio σ) and seismic attenuation (Qp, Qs) structure of seismic layer 2. Layer 2A is highly attenuating (Qp = 18–30 and Qs = 8–15) and layer 2B is moderately attenuating (Qp = 30–50 and Qs = 20–25). Our results show high σ at the seafloor and in layer 2A (σ = 0.48). Across the top of the 2A/B transition the rapid increase in Vp is accompanied by a sharp drop in σ to 0.25 within just 200 m of the seafloor. We perform simple calculations to gain an insight into the porosity and crack distribution with depth. These calculations suggest that porosity is in excess of 30% in layer 2A but reduces to 6–7% at the top of the 2A/B transition and to about 5% at a depth of 600 m below seafloor within layer 2B. Our results suggest that there is an increase in the average aspect ratio with depth across the 2A/B transition. The most likely explanation is that numerous thin cracks either mechanically close or are infilled at depth. Our results show there to be an abrupt change in the pore structure across the 2A/B transition which is consistent with a lithologic transition from extrusives to dykes but is equally consistent with a transition (mechanical or hydrothermal) within the extrusive pile.

We have applied waveform inversion to multichannel seismic reflection data collected at the East Pacific Rise at 9°40'N in order to determine the precise velocity structure of the magma body causing the axial magma chamber reflection. Our analysis supports the idea of a molten sill as previously suggested from forward modeling of seismic data from this location. Our inverted solution has a 30-m-thick sill with a P wave seismic velocity of 2.6 km s -1 . Although not well constrained by the data we believe that the S wave velocity in the sill is not significantly different from 0.0 km s -1 . The low P- and S- wave velocities in the sill imply that it contains less than 30% crystals. The molten sill is underlain by a velocity gradient in which the P wave velocity increases from 2.6 to 3.5 km s -1 over a vertical distance of 50-m. The shape of our velocity-depth profile implies that accretion of material to the roof of the sill is minor compared to accretion to the floor. The underlying velocity gradient zone may represent crystal settling under gravity. We suggest that only material from the 30-m-thick layer can erupt.

Seismic reflection profiles across many continental margins have imaged bottom simulating reflectors (BSRs), which have been interpreted as being formed at the base of a methane hydrate stability field. Such reflectors might arise either from an impedance contrast between high-velocity, partially hydrated sediments and water-saturated sediments or from a contrast with partially gas-saturated sediments. These alternatives may be hard to distinguish by conventional amplitude-versus-offset or waveform modeling approaches. Here we investigate the origin of a high amplitude BSR in the accretionary wedge offshore of western Colombia by seismic waveform inversion. The inversion procedure consists of three steps: firstly, determination of root-mean-square velocities and hence estimates of the interval velocities between major reflectors by a global grid search for maximum normalized energy along elliptical trajectories in the intercept time-slowness domain; secondly, determination of accurate interval velocities between these reflectors by a Monte Carlo search for maximum energy; and thirdly, a waveform fit in the frequency-slowness domain, using differential reflectivity seismograms and a conjugate-gradient optimization algorithm to minimize the sample-by-sample waveform misfit between data and synthetic. At two locations, near a structural high, we find a ∼30-m thick low-velocity zone beneath the BSR, with the properties of a partially gas-saturated zone, while at a third location, where the BSR amplitude is lower, we find no evidence for anomalously low velocities. The preferential development of the BSR in structures that would tend to intercept fluid flow or migrating gas and the presence of free gas beneath the BSR indicate a mechanism of BSR formation in which free methane gas migrates upward into the hydrate stability field or is carried there in advecting pore water.

We present a three-dimensional upper crustal model of the 9degrees03'N overlapping spreading center (OSC) on the East Pacific Rise that assists in understanding the relationship between melt sills and upper crustal structure at a ridge discontinuity with enhanced melt supply at crustal levels. Our P wave velocity model obtained from tomographic inversion of similar to 70,000 crustal first arrival travel times suggests that the geometry of extrusive emplacement are significantly different beneath the overlapping spreading limbs. Extrusive volcanic rocks above the western melt sill are inferred to be thin ( similar to 250 m). More extensive accumulation of extrusives is inferred to the west than to the east of the western melt sill. The extrusive layer inferred above the eastern melt sill thickens from similar to 350 ( at the neovolcanic axis) to 550 m ( to the west of the melt sill). Volcanic construction is likely to be significant in the formation of ridge crest morphology at the OSC, particularly at the tip of the eastern limb. On the basis of our interpretation of the velocity model, we propose that enhanced magma supply at crustal levels at the OSC may provide an effective mechanism for the migration of ridge discontinuities. This "dynamic magma supply model'' may explain the commonly observed nonsteady migration pattern of ridge discontinuities by attributing this to the temporal fluctuations in melt availability to the overlapping spreading limbs

Overlapping spreading centers (OSCs) play a key role in models of magma distribution at fast spreading ridges. To investigate the relationship between ridge-axis discontinuities and magma supply, we conducted a three-dimensional seismic reflection and tomography experiment at the 9°03′N OSC along the East Pacific Rise. Tomographic analysis imaged a broad mid-crustal low velocity zone (LVZ) beneath parts of the overlapper and the associated overlap basin, demonstrating that it is magmatically robust. The complementary datasets reveal a complex storage and tapping of melt: the LVZ and melt sill at either end of the overlap basin are not simply centered beneath the rise crest but are skewed inwards. The subsequent focussing of the LVZ and sill beneath the axis of the eastern limb appears to be due to melt migration toward the tip. The OSC western limb is less magmatically robust and may be in the process of dying.

The first three-dimensional (3-D) seismic reflection survey of a mid-ocean ridge was shot in 1997 and, while it is still too early for firm interpretations of the data, it can be confirmed that significant crustal melt bodies have been located and one widely accepted model does not seem to apply to the presence of a robust magma supply there. The survey the Anatomy of a Ridge-Axis Discontinuity (ARAD) experiment, was centered over an overlapping spreading center (OSC) system that offsets the ridge axis at 9°03′ north latitude on the East Pacific Rise (EPR) (Figure 1). It was conducted aboard the R/V Maurice Ewing during September and October and included a coincident 3-D crustal seismic tomography experiment.

The theory of a direct inversion method to obtain the P and S wave seismic velocities and the density of a horizontally, finely layered elastic medium from vertical and horizontal component plane wave seismograms in the tau-p (slant stacked) domain has been developed. The method is based on the downward continuation method of Bube and Burridge (1983), but plane wave seismograms with a range of ray parameters are used simultaneously to obtain the velocity and density profiles. A corrected acoustic approximation of the elastic case has also been developed with which one may obtain a model of the P wave velocity and density from vertical component seismic recordings. However, it requires the S wave velocity to be estimated a priori, for instance as a fixed fraction of the P wave velocity. Many direct inverse methods, including the Bube and Burridge procedure followed here, may lack stability when implemented numerically, specially when they are applied to real data which necessarily are band limited at both low and high frequencies. The possibility of a stable implementation of our methods has been investigated, firstly by using synthetic data for layered acoustic media containing a full range of low frequencies. In this case, the inverted velocity and density profiles are in good agreement with the test models. Secondly, the indeterminacy associated with the lack, in practice, of low frequencies has been studied, and a method has been devised to estimate the missing low-frequency data from travel time information. Finally, the inversion algorithm has then been applied to high-quality marine seismic reflection data obtained by the North Atlantic Transect (NAT) group in the deep ocean using an expanding spread profile (ESP). In this test, only the P wave velocity profile was estimated independently, because the move-out data cannot provide low-frequency information about density. Also, the “source wavelet” of the seismic system had to be estimated from the seismic recordings. A stable and plausible estimate of the velocity variation was obtained. Unfortunately, no independent measurement of this velocity structure is available with which to verify the method's accuracy.