Seismic Crustal Structure Research Articles

Seismic crustal structure between the Transylvanian Basin and the Black Sea, Romania

In order to study the lithospheric structure in Romania a 450 km long WNW–ESE trending seismic refraction project was carried out in August/September 2001. It runs from the Transylvanian Basin across the East Carpathian Orogen and the Vrancea seismic region to the foreland areas with the very deep Neogene Focsani Basin and the North Dobrogea Orogen on the Black Sea. A total of ten shots with charge sizes 300–1500 kg were recorded by over 700 geophones. The data quality of the experiment was variable, depending primarily on charge size but also on local geological conditions. The data interpretation indicates a multi-layered structure with variable thicknesses and velocities. The sedimentary stack comprises up to 7 layers with seismic velocities of 2.0–5.9 km/s. It reaches a maximum thickness of about 22 km within the Focsani Basin area. The sedimentary succession is composed of (1) the Carpathian nappe pile, (2) the post-collisional Neogene Transylvanian Basin, which covers the local Late Cretaceous to Paleogene Tarnava Basin, (3) the Neogene Focsani Basin in the foredeep area, which covers autochthonous Mesozoic and Palaeozoic sedimentary rocks as well as a probably Permo-Triassic graben structure of the Moesian Platform, and (4) the Palaeozoic and Mesozoic rocks of the North Dobrogea Orogen. The underlying crystalline crust shows considerable thickness variations in total as well as in its individual subdivisions, which correlate well with the Tisza-Dacia, Moesian and North Dobrogea crustal blocks. The lateral velocity structure of these blocks along the seismic line remains constant with about 6.0 km/s along the basement top and 7.0 km/s above the Moho. The Tisza-Dacia block is about 33 to 37 km thick and shows low velocity zones in its uppermost 15 km, which are presumably due to basement thrusts imbricated with sedimentary successions related to the Carpathian Orogen. The crystalline crust of Moesia does not exceed 25 km and is covered by up to 22 km of sedimentary rocks. The North Dobrogea crust reaches a thickness of about 44 km and is probably composed of thick Eastern European crust overthrusted by a thin 1–2 km thick wedge of the North Dobrogea Orogen.

The crustal structure under Sanjiang and its dynamic implications: Revealed by seismic reflection/refraction profile between Zhefang and Binchuan, Yunnan

The fault belts in Sanjiang mainly include Jinshajiang-Honghe fault, Lancangjiang fault and Nujiang fault (called Sanjiang faults) in western Yunnan Province, China. By interpreting the wide-angle seismic reflection/refraction profile between Zhefang and Binchuan, which crosses Tengchong and Baoshan blocks in Dianxi (western Yunnan) tectonic zone, we reconstruct the crustal structure with seismic traveltime tomography for crustal P-wave velocity and the seismic scattering image for crustal seismic reflection structure. In this paper, we firstly present the crustal structure images of P-wave velocity and seismic reflection under the wide-angle seismic profile. These results demonstrate that, the crustal velocity structure and seismic reflection structure along the profile can be divided into 3 segments, and there is an obvious difference of crustal structure among the eastern, the western and the middle segment. Generally, crustal P-wave velocities in the Baoshan segment are 0.1―0.2 km/s slower and seismic reflection amplitudes from Moho discontinuity are stronger than the other 2 segments. In the studied area, crustal thickness is about 40 km, and shows the thickening tendency from west to east along the profile. Additionally, it can be seen that there is one strong-amplitude seismic reflection event as bright points at the depths of 8―10 km, along the segment of 80―115 km of the profile (southward of Tengchong); and seismic reflection wave-field from Moho discontinuity varies obviously along the lateral direction. Finally, we make some discussions on the crustal thickening pattern in the Sanjiang fault belt, structural environment of earthquake development and the contact rela-tionship between the Tengchong block, Banshan block and Luxi trough.

Why no anomaly is visible over most of the continent–ocean boundary in the global crustal magnetic field

It is generally believed that an anomaly should be expected over the continent–ocean (C–O) boundary in the global magnetic anomaly maps. However, such maps prepared at satellite altitude do not show an anomaly over most of the C–O boundary. We address this issue by a forward modelling technique. We use a Geographic Information System (GIS) technique to integrate information of the various geological units of continents and oceans, the seismic crustal structure, and standard susceptibility values of the rock types and compute a global vertically integrated susceptibility (VIS) model. In addition, a remanent magnetisation model is used for the oceanic crust. Combining the induced and remanent magnetisation models, the crustal field anomaly is predicted at a satellite altitude of 400 km for spherical harmonic degrees 16–80. The results show that an anomaly is indeed not predicted over the C–O boundaries where the oceanic crust is flanked by continental regions Phanerozoic in age. We demonstrate that, largely, a C–O anomaly over these regions is not even expected in the global magnetic anomaly map for degrees 1–80, confirming that it is not a long-wavelength feature masked by the core field. An anomaly, however, is predicted over some parts of the C–O boundary where the flanking geological provinces over the continents are Precambrian in age. We argue that the VIS values for the continents and oceans are largely comparable and therefore no anomaly is expected over most of the C–O boundary.

Microearthquake seismicity in relation to double convergence around the Solomon Islands arc by ocean-bottom seismometer observation

SUMMARY The Solomon Islands arc area is a complex plate convergence zone. At the North Solomon Trench on the northern side of the arc, it is believed that the Pacific Plate was subducting before coming into collision with the Ontong Java Plateau, the world's largest oceanic plateau. After the collision about 5 Ma, northeastward subduction initiated along the southern side of the arc at the San Cristobal Trench, another trench on the south side. GPS observations and crustal seismic structure surveys confirm that convergence occurs at both trenches. Without detailed and accurate seismicity, it is difficult to characterize the plate subduction to reveal the tectonics of such a complex zone where a key mechanism of continental growth may also exist. In 1994, an ocean-bottom seismometer (OBS) experiment was carried out for the first time in the area around the Solomon Islands arc to locate microearthquakes. Observations started in late August and continued until early September. Five digital recording OBSs were deployed around the Russell Islands west of Guadalcanal Island. OBS spacing was about 20 km. All the OBSs were recovered and yielded data with a good signal-to-noise ratio. 40 earthquakes, with magnitudes in the range 1.5–4.4 were located over 8 days. The seismicity clearly images the two subducting plates. Though the seismicity beneath the arc side slope of the San Cristobal Trench is relatively high, we can see the seismicity which is related to the subducting Pacific Plate beneath Santa Isabel Island. In addition, earthquakes occur within the crust beneath the southern part of the New Georgia Basin and the Russell Islands. An aseismic area extending 40 km inward from the San Cristobal trench axis implies initial aseismic slip of the India–Australia Plate at a small dip angle.

Upper crustal seismic structure of the Mazury complex and Mazowsze massif within East European Craton in NE Poland

The large-scale seismic experiment POLONAISE '97 ( POlish Lithospheric ONsets— An International Seismic Experiment) was carried out in May 1997 in Poland, Lithuania, and Germany. Its main purpose was to investigate the structure of the crust and the uppermost mantle in the region of the Trans European Suture Zone (TESZ) that lies between the East European Craton (EEC) and the Palaeozoic Platform. This paper covers the interpretation of seismic data along the NW–SE-trending, 180-km-long profile P5 located on the EEC. The recordings were of a high quality with seismic energy clearly visible along the whole profile. We have not found waves refracted below the upper crust in first arrivals. In the NW part of the profile, we have delineated a high-velocity body with the P-wave velocity in the range of 6.5–6.75 km/s in the upper crust. It corresponds to the Kętrzyn anorthosite massif within the Mazury complex. The Mazowsze massif is rather uniformly characterized by P-wave velocities 5.9–6.05 and 6.2–6.35 km/s in two layers, respectively. Sufficient S-wave data were available to estimate the V p/ V s ratio (as well as the Poisson ratio), being 1.80 (0.277) in the high-velocity body and 1.67 (0.220) in the upper crust. Apart from the 2-D model along the profile, results of 3-D modelling in the area of the P5 profile are presented. Using off-line recordings, we got P-wave velocity field up to 8 km/s below the P5 profile at the depth of about 40 km as well as horizontal extent of the high-velocity body.

Upper crustal seismic structure of the slow spreading Mid‐Atlantic Ridge, 35°N: Constraints on volcanic emplacement processes

The upper crustal seismic structure of the slow spreading Mid‐Atlantic Ridge is studied using a genetic algorithm‐based waveform inversion of multichannel streamer data. Four single‐ship multichannel profiles from 35°N are analyzed: one in the rift valley and three in the rift mountains along 0.7, 1.6, and 1.9 Ma crust. A layer 2A horizon is continuously imaged along three profiles and is associated with a sharp velocity increase from extrusives to dikes. Its depth and regularity in the rift valley indicate that most of the extrusive section is built on the inner valley floor through a pattern of deposition and fault‐bounded uplift into the rift mountains. Its variability along one line, however, shows that this process is disrupted during tectonically dominated periods. A thickening of layer 2A toward the Oceanographer fracture zone may be the result of along‐axis magma transport. The interval of rapid velocity increase at the base of layer 2A thins with age, a possible response to enhanced hydrothermal mineralization within the zone of mixed dikes and extrusives. Transition zone (∼200 m) and off‐axis layer 2A thicknesses (350–600 m) are similar to those at other spreading centers. This indicates that equivalent extrusive volumes are produced at all spreading rates along a relatively narrow zone of dike emplacement. However, differences in on‐axis layer 2A thickness between this area and fast spreading ridges suggest that the exact pattern of thickening varies between spreading regimes. Relative to fast spreading ridges, the moderate velocity increase with age recorded in the upper crust (from 2.3 to >2.7 km s−1 within ∼2 Myr) may be due to a less active hydrothermal system and hence slower porosity reduction.

Seismic crustal structure in the Gulf of Cadiz (SW Iberian Peninsula)

The sedimentary structure in the Gulf of Cadiz has been extensively studied by oil exploration companies. However, up to now little is known about its deep crustal structure. Moreover, the total thickness of the sedimentary layers remains unknown in large areas. The purpose of this paper is the crustal-scale interpretation of deep seismic near-vertical reflection and refraction/wide-angle reflection data obtained during the IAM (Iberian Atlantic Margins) project, carried out in 1993. Our results indicate that a continental type crust is underlying the entire Gulf of Cadiz, with progressive thinning from east to west. The sedimentary cover shows a great thickness, reaching 8 km in the center of the Gulf. Three main sedimentary units can be recognized: Jurassic-Cretaceous calcareous rocks, continuation of Algarve outcrops; the Allochthonous Units of Guadalquivir/Gulf of Cadiz, the offshore continuation of the inland Carmona nappe; and sub-horizontal post-Miocene marine sediments. The crystalline crust is divided into three main layers: the upper crust is characterized by P-wave velocity values of 5.7–6.1 km/s; the middle crust shows values of 6.3–6.4 km/s; the lower crust has a mean vertical velocity gradient of 0.02 km/s/km, with velocities between 6.9 to 7.1 km/s. The total crustal thickness varies from 27 km for the eastern part of the studied area, to 20 km for the westernmost part. The crustal thinning is more pronounced in a N-S direction than in an E-W direction. No major structures related with a defined Iberia-Africa plate boundary could be found.

Geophysical inversion with a neighbourhood algorithm—I. Searching a parameter space

SummaryThis paper presents a new derivative-free search method for finding models of acceptable data fit in a multidimensional parameter space. It falls into the same class of method as simulated annealing and genetic algorithms, which are commonly used for global optimization problems. The objective here is to find an ensemble of models that preferentially sample the good data-fitting regions of parameter space, rather than seeking a single optimal model. (A related paper deals with the quantitative appraisal of the ensemble.)The new search algorithm makes use of the geometrical constructs known as Voronoi cells to derive the search in parameter space. These are nearest neighbour regions defined under a suitable distance norm. The algorithm is conceptually simple, requires just two ‘tuning parameters’, and makes use of only the rank of a data fit criterion rather than the numerical value. In this way all difficulties associated with the scaling of a data misfit function are avoided, and any combination of data fit criteria can be used. It is also shown how Voronoi cells can be used to enhance any existing direct search algorithm, by intermittently replacing the forward modelling calculations with nearest neighbour calculations.The new direct search algorithm is illustrated with an application to a synthetic problem involving the inversion of receiver functions for crustal seismic structure. This is known to be a non-linear problem, where linearized inversion techniques suffer from a strong dependence on the starting solution. It is shown that the new algorithm produces a sophisticated type of ‘self-adaptive’ search behaviour, which to our knowledge has not been demonstrated in any previous technique of this kind.

Synthesis of Crustal Seismic Structure and Implications for the Concept of a Slab Gap beneath Coastal California

Compilation of seismic transects across the central and northern California Coast Ranges provides evidence for the widespread tectonic emplacement beneath the margin of a slab of partially subducted oceanic lithosphere. The oceanic crust of this lithosphere can be traced landward from the former convergent margin (fossil trench), beneath the Coast Ranges, to at least as far east as the Coast Range/Great Valley boundary. Comparison of measured shear and compressional wave velocities in the middle crust beneath the Hayward fault with laboratory measurements suggests that the middle crust is a diabase (oceanic crust). Both of these observations are consistent with recent models of the high heat flow and age progression of Neogene volcanism along the Coast Ranges based on tectonic emplacement (stalling) of young, hot oceanic lithosphere beneath the margin, but appear to contradict the major predictions of the slab-gap or asthenospheric-window model. Finally, the Neogene volcanism and major strike-slip faults in the Coast Ranges occur within the thickest regions (>14 km thick) of the forearc, suggesting that the locations of Cenozoic volcanism and faulting along the margin are structurally controlled by the forearc thickness rather than being determined by the location of a broad slab gap.

Seismic crustal structure in the southwest of the Iberian Peninsula and the Gulf of Cadiz

The crust under the southwestern Iberian Peninsula and the Gulf of Cadiz has been sampled by 1200 km of deep seismic refraction/wide-angle reflection profiles, together with many seismic reflection lines and bore-holes. Wide-angle seismic data were collected during the last three decades. Commercial multichannel data provide a detailed image of the uppermost crust, improving the confidence about the models of the deeper structures. P-wave velocities within the thick column of sediments in the Gulf of Cadiz range from 2.0 to 3.8 km/s, while the Algarve and the Sines areas have higher velocities of 4.3 to 4.8 km/s. The top of the Palaeozoic basement rises to the northwest, outcropping in the South Portuguese zone of the Iberian Massif, and is characterized by P-wave velocities of 5.7–5.9 km/s. High velocities of 6.4 km/s have been found at shallow depths of 7 to 10 km in the South Portuguese zone, that could be related to the mafic and ultramafic rocks in the Beja-Acebuches zone. Lower crustal velocities are in the range of 6.7–6.9 km/s. The crustal thickness shows important lateral changes from 29 km beneath the Guadalquivir Basin/Iberian Massif contact to 35 km in the southeastern part of the South Portuguese zone. From the interpretation of these seismic data, a geodynamic model of the evolution of the crust in Southwestern Iberia and the Gulf of Cadiz is proposed. The Guadalquivir Basin, and its continuation at sea, the Gulf of Cadiz, is a flexure area of the crust that could be related to the overloading due to the overthrusting of the Alboran Domain over the Iberian plate.

Implications from the seismic crustal structure of the northern Izu-Bonin arc

Implications from the seismic crustal structure of the northern Izu-Bonin arc

Heterogeneous crustal structure across a seismic block boundary along the Nankai Trough

The Nankai trough, which runs along the western Japanese islands, is widely known for its systematic activity of large earthquakes with the Magnitudes greater than 7. According to the fault‐plane studies of these earthquakes, a major seismic block boundary is supposed to run off the Kii Peninsula almost perpendicular to the trough axis. A seismic experiment was conducted across this boundary region in order to determine seismic crustal structure. P‐wave travel time analyses were applied to the data from the OBSs deployed along the trough axis, including travel time nonlinear inversion. The result shows an abrupt change in thickness of the subducting Philippine Sea Plate's crust at the expected location of the seismic block boundary. The crust is thicker in the east by about two kilometers, and the interface between the overriding and subducting plates runs shallower in the east. These structural features may affect the mode of earthquake occurrence.

Implications from the seismic crustal structure of the northern Izu–Bonin arc

The results of a controlled source seismic reflection–refraction experiment carried out in 1992 reveal the following characteristics of the northern Izu–Bonin (Ogasawara) oceanic island arc–trench system. (1) The crust rapidly thickens from the Shikoku back‐arc basin to the arc, is thickest beneath the active rifts, and then gradually thins to the forearc. The thickness of the crust beneath the arc rift zone and the back‐arc basin are ∼ 20 km and 8 km, respectively. (2) The Moho vanishes beneath the forearc. Velocities rapidly decrease eastwards beneath the inner trench wall. (3) The velocity of the lower crust of the arc and the back‐arc basin is 7.1–7.3 km/s. This velocity is higher than the typical oceanic lower crust whose velocity is ∼ 6.7 km/s. (4) The velocity of the middle crust of the arc is ∼ 6 km/s. This layer does not exist beneath the back‐arc basin. (5) A slight difference in the velocity gradient of the middle crust exists between the arc rift zone and the forearc. Based on these findings and previous studies, it is inferred that: (i) the middle crust is probably granitic rock and formed in more than two episodes; (ii) the lower crust formed by igneous underplating which may also have affected part of the back‐arc basin; and (iii) the root of the serpentinite diapir on the inner trench wall is a low‐velocity mantle wedge that was probably caused by large amounts of water released from the subducting Pacific plate at depths shallower than 30 km.

Seismic evidence for stretched continental crust in the Japan Sea

The Japan Sea is a major marginal sea in the western Pacific comprised of basins and bathymetric highs. Technically, these features were created by multiple‐rifting and spreading processes, as exemplified in the creation of the Japan Basin and Yamato Basin. However, there are smaller scale features for which seismic crustal models have been inadequate to infer or verify their origin and evolution. Recent advances in marine seismological surveys enable us to obtain heterogeneous seismic crustal structures in sufficient detail to characterize these features. In September of 1992, a seismic experiment was conducted to obtain the detailed seismic crustal structure in the southwestern part of the Japan Sea, including the basin and bank areas. Seventeen ocean bottom seismometers (OBSs) were deployed on two lines. Explosives and airguns were fired as controlled seismic sources. The model of the crust obtained in the northern Tsushima Basin is about 13 km thick including a 1.5‐km‐thick sedimentary layer. Beneath the bank area the crust is 22 km thick and the upper crust is relatively homogeneous with a P‐wave velocity of about 6 km/s. The bank is interpreted to be a stretched continental crust fragment.

Three-dimensional seismic crustal structure of the Monashee hinterland core complex on the Lithoprobe Southern Canadian Cordillera Transect

Three-dimensional seismic coverage in an approximately 12 km × 12 km area of the southern Monashee metamorphic complex in the south-central portion of the Canadian Cordillera reveals a complex geometry to the Mesozoic–early Tertiary contractional Monashee décollement. Data were acquired as part of the Lithoprobe Southern Canadian Cordillera Transect where two approximately perpendicular lines intersected on the south flank of the Monashee mountains in the hinterland of the Cordillera. Stacks of traces within 100 m × 100 m bins are nominally 6-fold, but range from zero to 108-fold due to the crooked nature of the lines. Both migrated and unmigrated data have been examined for interpretation, but the highly variable data quality and discontinuous reflectivity cause excessive added noise during the migration process.

Seismic crustal structure along the Axarfjörður trough at the eastern margin of the Tjörnes fracture zone, N-Iceland

Seismic crustal structure along the Axarfjörður trough at the eastern margin of the Tjörnes fracture zone, N-Iceland

Seismic crustal structure in Iceland and surrounding area

The basaltic crust of Iceland and surrounding areas can be divided into two parts, the upper crust and the lower crust. The upper crust is characterized by increasing P-wave velocity with depth. At the surface of the basaltic lava pile, the velocity can be as low as 2.0–3.0 km/s in fresh highly porous lava in the volcanic rift zones of Iceland and at the crest of the adjacent Mid-Atlantic Ridge (MAR). Elsewhere, the P-wave velocity at the surface is in the range of 3.0–5.0 km/s, mainly depending on the degree of secondary mineralization. From the surface the velocity of the upper crust increases more or less continuously down to the top of the lower crust where the velocity is nearly constant at 6.3–6.7 km/s. The continuous increase in velocity with depth in the upper crust is considered to be due to closure of fractures caused by lithostatic pressure and to an increasing content of secondary minerals which reduces the porosity. In addition to the general increase in velocity with depth, thin high-velocity layers are observed within the upper crust. These are interpreted as relatively dense parts of the lava pile with a low content of scoriaceous material. The thickness of the upper crust is in the range 3–6 km in Iceland, except in southeastern Iceland where it exceeds 10 km. On the Greenland-Iceland-Faeroe Rise it is in the range of 3–8 km but on the Reykjanes Ridge it is considerably less, close to 2 km. This implies that the structure of the Icelandic upper crust is more similar to that observed on the Greenland-Iceland-Faeroe Rise than that on the MAR. The lower crust is very homogeneous in terms of P-wave velocity. It is most likely also composed of basaltic lava and intrusive rocks of almost zero porosity and with considerable amounts of heavy secondary minerals such as epidote In some cases there seems to be a low velocity layer at the base of the lower crust indicating partially melted material. There are no signs of any sudden changes in the velocity structure of the crust between Iceland and the adjacent oceans, but the crustal layers in Iceland are generally thicker than on the Reykjanes Ridge. The crustal thickness in Iceland is rather poorly mapped but seems to be of the order of 10–15 km. This thickness reaches as much as 30–35 km on the Iceland-Faeroe Rise and probably 25 km on the Greenland-Iceland Rise. The subcrustal material has a P-wave velocity of 7.0–7.4 km/s below Iceland and at the crest of the MAR, 7.8–7.9 km/s below the Iceland-Faeroe Rise and 8.4 km/s below the flank of the Reykjanes Ridge. There seems to be a sharp change in the subcrustal velocity structure at the edge of the Icelandic shelf. Multichannel reflection surveys in the Tjörnes Fracture Zone north of Iceland reveal a sedimentary basin with up to 4 km of sediments. The western part of the basin is a prominent graben with numerous listric normal faults. This graben is an apparent continuation of the spreading axis on the Kolbeinsey Ridge.

The three-dimensional seismic velocity structure of the East Pacific Rise near latitude 9° 30′ N

Three-dimensional images of crustal seismic structure beneath the East Pacific Rise show pronounced axial heterogeneity over distances of a few kilometres. A linear high-velocity anomaly, approximately 1–2 km in width and restricted to the uppermost 1 km of the crust, is centred on the rise axis. An axial low-velocity anomaly at depths of 1–3 km varies in amplitude along the axis, consistent with a zone of higher crustal temperatures midway between two discontinuities in the morphology of the rise axis. This apparent thermal segmentation along axis is consistent with injection of mantle-derived melt midway along a locally linear, 12-km-long segment of the rise.

Densities and porosities in the oceanic crust and their variations with depth and age

We have used drilling and downhole logging results (from Deep Sea Drilling Project holes 395A and 418A), measured properties of core samples from the seafloor and from ophiolites, and seismic velocity structure models of the oceanic crust to constrain empirical (statistical) relationships between in situ densities, porosities, and seismic velocities and to estimate densities and porosities in the oceanic crust, with the following principal results. Our review of drilling results and studies of ophiolites suggests that the velocity structure of the oceanic crust is more closely related to changes in porosity and alteration (or metamorphic grade) than to its igneous structure: the transition from layer 2a (3.7±0.4 km s−1) to 2b (5.2±0.4 km s−1) occurs within the extrusive pile; the top of layer 2c (6.2+0.2 km s−1) is associated with the top of the sheeted dike complex and with the first appearance of greenschist facies alteration products; ophiolite studies have shown that the top of layer 3 occurs within the sheeted dikes where the metamorphic grade of alteration products increases from greenschist to amphibolite facies. A comparison of reprocessed and corrected downhole logs from holes 395A and 418A indicates that in situ densities and fractional porosities differ by 0.2 Mg m−3 and 0.07, respectively. These differences are consistent with prevailing views of the progressive alteration of the upper oceanic crust. Statistical relationships between in situ density, porosity and slowness are highly linear, and indicate that the density/slowness relation estimated from laboratory samples [pb = (3.81±0.02)–(6.0±0.1)S] applies to in situ properties as well. We obtained a similar empirical relationship for porosities from the hole 418A log data [Фf = −(0.35±0.03) + (2.37±0.15) S; r2 = 0.84 S.E. = 0.015]. Applying these empirical models to seismic velocity structure models yields the following crustal densities and porosities: layer 2, 2.62–2.69 Mg m−3, 0.10–0.12 and layer 3, 2.92–2.97 Mg m−3. The estimated average density of the oceanic crust is 2.86±0.03 Mg m−3. Densities and porosities estimated from a seismic crustal structure model for old Atlantic crust agree very well with the logging data from hole 418A, suggesting that in situ densities and porosities can be estimated from seismic velocities. High density and porosity gradients are characteristic of the upper oceanic crust; increasing densities reflect both a progressive decrease of porosity and increasing grain density with depth. Based on the seismic properties of layer 2a, we estimate that the layer thins at an approximate rate 20 to 45 m m.y.−1 while the velocity increases by 0.040 km s−1 m.y.−1. The corresponding rates of change of density and porosity are 0.016 ± 0.009 Mg m−3 m.y.−1 and −0.005±0.003 m.y.−1, respectively.

Pore pressure and oceanic crustal seismic structure

Marine refraction studies during the past decade have found considerable lateral variability in the seismic properties of the upper basaltic regions of the oceanic crust. In many localities, compressional and shear-wave velocities are quite low at the top of the basalt section and velocities increase rapidly with depth. It is concluded that pore pressure may be at least in part responsible for the low upper crustal velocities and contribute significantly to the lateral variability. This is supported by compressional and shear-wave velocity measurements as functions of confining pressure and pore pressure for basalt from the Juan de Fuca ridge and dolerite from the Samail ophiolite, Oman. Within the oceanic crust, regions of overpressure and underpressure will possess anomalous velocities, the magnitude of which will depend upon the porosity and the deviation of the pore pressure from hydrostatic. The influence of pore pressure on velocities is expected to diminish with depth and is unlikely to be significant at lower crustal depths where porosity is extremely low. Of significance, Poisson's ratio is shown to be dependent on pore pressure as well as confining pressure. At constant confining pressure, Poisson's ratio increases with increasing pore pressure. Thus, overpressured regions within the upper oceanic crust are likely to have relatively high Poisson's ratios as well as low compressional- and shear-wave velocities.