bluuebunny/crossref_metadata_2025 huggingface dataset:
Crustal deformation measurements in central Japan determined by a Global Positioning System Fixed‐Point Network
Authors: Shimada Seiichi , Bock Yehuda
Published: February 2008

A Global Positioning System (GPS) fixed‐point network has been operating in the Kanto and Tokai districts of central Japan since April 1988 to detect crustal deformation associated with the convergence of the Eurasian, Pacific, North American, and Philippine Sea plates and to monitor the deformation cycles of frequent large interplate and intraplate earthquakes. This 10‐station network established by the National Research Institute for Earth Science and Disaster Prevention (NIED) is the first continuously monitoring network of its kind. We determine deformation within the network using two consecutive days of data every 2 weeks for the first 17 months of operations. We use a station and orbit relaxation method which relies exclusively on data collected within the NIED network, except for 1 week of global GPS tracking data which is used to determine initial station positions with respect to the global reference frame. We detect, relative to a station on the Eurasian plate in central Japan, significant westward motion of 28 mm/yr of the northern tip of the Philippine Sea plate, which is subducting beneath the Eurasian plate at the Suruga trough. Our results support finite element models of collision of the Izu Block with the Eurasian plate based on earthquake focal mechanisms and plate block motions of the Japanese archipelago determined from conventional geodetic measurements over the last century. We detect southwestward motion of 18 mm/yr of the southeastern tip of the Eurasian plate, confirming expected surface extension of the subducted plate parallel to the Suruga trough axis. Significant vertical uplift with a velocity of 20 mm/yr is suggested at a site inland of the Tokai district located in the Akaishi uplift zone and at a site on Hatsushima Island in Sagami Bay. The general tendency of vertical movements of the other sites agrees with vertical velocities obtained from 70 years of geodetic leveling and with Quaternary vertical displacements determined from geomorphological and other geological evidence. We detect no significant crustal motion across the Fossa Magna tectonic zone in central Japan (often considered a plate boundary), across the Tokyo metropolitan area, nor across the Sagami trough associated with the subduction of the Philippine Sea plate beneath northeast Japan. Our results demonstrate the power of regionally based, continuously monitoring GPS networks for obtaining temporally dense measurements of small horizontal and vertical crustal movements across plate boundary zones.
Seismic moment summation for historical earthquakes in Italy: Tectonic implications
Authors: Westaway Rob
Published: February 2008

Tectonic deformation rates in and around the Apennine mountains of Italy are studied using seismic moments estimated from macroseismic effects of historical earthquakes. Northeastward extension in the northern Apennines (north of ∼42.5°N) accompanies shortening along their northeast flank. Since the seventeenth century, the sparse seismicity in these two zones has included no earthquake with magnitude >6.5 or seismic moment above ∼6×1018 N m. Their spatially averaged deformation rates are only ∼0.3 mm yr−1, but are equal, such that the extension and shortening balance with no relative motion between their external surroundings. In contrast, the numerous historical earthquakes in the central and southern Apennines with magnitude ∼7 and seismic moment ∼20×1018 N m require northeastward relative velocity across the deforming zone up to ∼5 mm yr−1, matching the expected relative motion of their surroundings. This relative velocity is taken up in part by extension and in part by strike‐slip, depending on the trend of the deforming zone, and is not accompanied by en echelon shortening elsewhere in Italy. The northern Apennines thus show different senses and rates of deformation from localities farther south, and are thus technically distinct at present, in contrast with previous interpretations.
Gravity map of Brazil: 1. Representation of free‐air and Bouguer Anomalies
Authors: de Sá Nelsi Côgo , Ussami Naomi , Molina Eder Cassola
Published: February 2008

Gravity data of Brazil were processed to obtain continental scale free‐air and Bouguer anomaly maps for a large segment of the South American plate. Gravity measurements are referred to the International Gravity Standardization Net 1971 and the anomalies are calculated using Geodetic Reference System 1967 and 2.67 g cm−3 for Bouguer reduction. In the calculation of the gravity anomalies, information used included geopotential model, topography, geoidal heights, and astrogeodetic deviation of the vertical. Depending on the availability and distribution of land gravity data, various regions have been classified as complete, incomplete, or empty. In complete regions, the gravity field is represented by 0.5° × 0.5° mean free‐air and Bouguer anomalies which were computed from observed gravity data only. In incomplete regions, mean anomalies in empty 0.5° × 0.5° blocks were computed from nearest known mean anomalies, by least squares collocation. Regions totally devoided of gravity data had the gravity field calculated by least squares collocation using all available geodetic data. The gravity map obtained using this approach allows to identify the major gravity anomalies with dimensions greater than 100 km. Such anomalies are mainly caused by intracrustal density variations of geological and tectonic interest.
Upper Cenozoic volcanic rocks in the Mariana Forearc recovered from drilling at Ocean Drilling Program Site 781: Implications for forearc magmatism
Authors: Marlow Michael S. , Johnson Lynn E. , Pearce Julian A. , Fryer Patricia B. , Pickthorn Leda Beth G. , Murton Bramley J.
Published: February 2008

A horst block was drilled in the center of the Mariana forearc near 20°N during leg 125 of the Ocean Drilling Program. At this site 781, the drill penetrated a Pleistocene vesicular, porphyritic basalt at 72 m below the seafloor, and the top of the basalt corresponds to a high‐amplitude reflection on seismic reflection profiles across the site. The thickness of the basalt unit can only be estimated to be between 13 and 25 m because of poor core recovery (28% to 55%). The presence of an upper glassy chilled zone and a lower, fine‐grained margin suggest that the basalt unit is either a single lava flow or a near‐surface sill. The basalt is an island‐arc tholeiite (IAT) enriched in large‐ion‐lithophile elements relative to high‐field‐strength elements, similar to the submarine lavas of the southern Mariana Arc seamounts. The basalt layer, the youngest in situ igneous layer reported from the Izu‐Bonin and Mariana forearcs, is enigmatic because of its location more than 100 km from the active volcanic arc. The sediment layers above and below the basalt unit are late Pliocene (about 2.5 Ma) and normally magnetized. The basalt has schlieren ‐ like structures, reverse magnetization, and a K‐Ar age of 1.68 ± 0.37 Ma. Thus, the basalt layer is probably a sill fed by magma intruded along a fault zone bounding the horst and graben in the forearc. The geochemistry of the basalt is consistent with a magma source similar to that of rocks from the magmatic axis (or volcanic front) of the island arc, and derived from a mantle source above the subducting Pacific plate.
Ultrahigh resolution marine magnetic anomaly profiles: A record of continuous paleointensity variations?
Authors: Cande Steven C. , Kent Dennis V.
Published: February 2008

A distinctive pattern of small‐scale marine magnetic anomalies (25–100 nT amplitude, 8–25 km wavelength: tiny wiggles) is superimposed on the more generally recognized seafloor spreading pattern between anomalies 24 and 27 in the Indian Ocean. By normalizing and stacking multiple profiles, it is demonstrated that this pattern of tiny wiggles is a high‐resolution recording of paleodipole field behavior between chrons C24 and C27. The pattern of tiny wiggles between anomalies 26 and 27 is compared to an ultrafast spreading (82 mm/yr half rate) profile from the southeast Pacific where a similar signal is observed, confirming the paleodipole field origin of the anomalies. Two basic models are considered in which the tiny wiggles are attributed either to short polarity intervals or to paleointensity fluctuations. We conclude that tiny wiggles are most likely caused by paleointensity fluctuations of the dipole field and are a ubiquitous background signal to most fast spreading magnetic profiles. The implications of this study are that (1) tiny wiggles may provide information on the temporal evolution of the geomagnetic dynamo; (2) the small‐scale anomalies observed in the Jurassic quiet zones may be due to paleointensity fluctuations; (3) tiny wiggles are potential time markers in large regions of uniform crustal polarity such as the Cretaceous quiet zones; and (4) much of the variance in anomaly profiles normally attributed to crustal emplacement processes, particularly at fast and ultrafast spreading rates, is actually due to intensity variations in the paleomagnetic field.
Model for intermediate‐term precursory clustering of earthquakes
Authors: Yamashita T. , Knopoff L.
Published: February 2008

A causal sequence of an earthquake swarm, extended quiescence, foreshocks, and main shock can be understood in terms of an aggregation model of crack growth and ultimate fusion on a multiplanar earthquake fault system. The rheological model is one of accelerated crack growth under conditions of slip weakening, coupled with a slow recovery of static friction after fracture. Once a slip feature has been generated that spans a large part of the available geometry, a stress shadow is cast on other, neighboring cracks that strongly retards and inhibits crack growth, thus initiating the quiescent phase abruptly. The success of the model depends on the assumption that nonlinear rheology regulates the rate of slip and that earthquake faults are not simply connected surfaces but instead have many strands.
Thermal stresses and the bimodal distribution of elastic thickness estimates of the oceanic lithosphere
Authors: Wessel Pål
Published: February 2008

Observations of plate flexure beneath seamounts and at deep‐sea trenches suggest that the mechanical strength of the oceanic lithosphere increases with age. The mechanical thickness (H) seems to follow the depth to a particular isotherm in the cooling plate model. A detailed review of available estimates of elastic thickness (Te) shows that the strength deduced from studies at seamounts is significantly different from the strength inferred at trenches and fracture zones. Whereas Te beneath seamounts approximately follows the 400°C isotherm, H at trenches follows the depth to the 700°–800°C isotherms. The cause of this bimodal distribution is not fully understood. Thermal bending stresses due to cooling of the lithosphere have been suggested to play an important role in the flexural deformation at transform faults and fracture zones, and their distribution with depth is consistent with intraplate seismicity. This paper evaluates the effects of thermal stresses on deformation beneath seamounts and at trenches and how their interaction with flexural stresses may modify plate strength. The results indicate that thermal bending stresses will bias the moment‐curvature relationship such that a plate of a certain strength will appear relatively weaker beneath a seamount (positive curvature) but relatively stronger at a trench (negative curvature). This systematic bias appears to be in qualitative agreement with published Te estimates. For old lithosphere the theory predicts a diminishing amount of bias. The general lack of sufficient Te determinations from old ocean basins and the uncertainties associated with modeling of trenches prohibit a detailed analysis of this part of the Te versus age relationship. I conclude that the scatter in the elastic thickness estimates appears to be caused by a combination of lithospheric reheating during seamount emplacement, bias introduced by thermal stresses, and variable amount of yielding of the plate beneath seamounts in proportion to seamount size and lithospheric age at the time of loading.
Paleomagnetism of Mesozoic rocks from the central Andes of southern Peru: Importance of rotations in the development of the Bolivian orocline
Authors: Roperch Pierrick , Carlier Gabriel
Published: February 2008

A paleomagnetic study has been undertaken in Jurassic volcanics and Cretaceous intrusions from coastal southern Peru. A secondary magnetization in the Jurassic volcanics was identified at 21 sites from three main localities along the southern coast of Peru (mean virtual geomagnetic pole: 59.5°N, 190.1°E, k=111, α95=3.0). This secondary magnetization was acquired during a tectonomagmatic event that may be related to the emplacement of the Peruvian coastal batholith and/or to the first Andean phase of compressional deformation in Late Cretaceous. If we assume that the remagnetization is of Late Cretaceous age, a regional counterclockwise rotation of about 20° is recorded by the forearc of southern Peru from 16°S to 18°S. Primary remanent magnetizations recorded by the volcanics and the batholith reveal a more complex tectonic history of block rotations. Counterclockwise rotations of 50° to 80° are documented near the localities of Chala (eight sites), Oconà (two sites) and Arequipa (four sites). Preliminary results based on three sites of Late Permian to Early Triassic sediments from northern Bolivia also show a large counterclockwise rotation. The observed differences between the primary and the secondary magnetization indicate that large block rotations occurred prior to the acquisition of the secondary magnetization. Perhaps, this first episode of deformation partially structured the Andean chain in the Central Andes and induced an incipient orocline. During the Cenozoic, a differential shortening along the chain gave the present‐day shape of the Bolivian deflection and resulted in a counterclockwise rotation of the forearc system by about 20° as indicated by the remagnetization.
A test of present‐day plate geometries for northeast Asia and Japan
Authors: DeMets Charles
Published: February 2008

Alternative geometries for the present‐day configuration of plate boundaries in northeast Asia and Japan are tested using NUVEL‐1 and 256 horizontal earthquake slip vectors from the Japan and northern Kuril trenches. Statistical analysis of the slip vectors is used to determine whether the North American, Eurasian, or Okhotsk plate overlies the trench. Along the northern Kuril trench, sup vectors are well‐fit by the NUVEL‐1 Pacific‐North America Euler pole, but are poorly fit by the Pacific‐Eurasia Euler pole. Results for the Japan trench are less conclusive, but suggest that much of Honshu and Hokkaido are also part of the North American plate. The simplest geometry consistent with the trench slip vectors is a geometry in which the North American plate extends south to 41°N, and possibly includes northern Honshu and southern Hokkaido. Although these results imply that the diffuse seismicity that connects the Lena River delta to Sakhalin Island and the eastern Sea of Japan records motion between Eurasia and North America, onshore geologic and seismic data define an additional belt of seismicity in Siberia that cannot be explained with this geometry. Assuming that these two seismic belts constitute evidence for an Okhotsk block, two published kinematic models for motion of the Okhotsk block are tested. The first model, which predicts motion of up to 15 mm yr−1 relative to North America, is rejected because Kuril and Japan trench slip vectors are fit more poorly than for the simpler geometry described above. The second model gives a good fit to the trench slip vectors, but only if Okhotsk‐North America motion is slower than 5 mm yr−1.
Downhole magnetostratigraphy in sediments: Comparison with the paleomagnetism of a core.
Authors: Pozzi Jean Pierre , Barthès Véronique , Thibal Jérôme , Pocachard Jacques , Lim Mutaek , Thomas Thierry , Pages Gilles
Published: February 2008

The magnetostratigraphy of weakly magnetized sediments is determined for the first time from direct downhole measurements. Experimental results were obtained in both scientific and oil‐exploration drillholes penetrating Jurassic sediments of the Paris basin, France. The magnetic polarity of the remanence is deduced from the total magnetic induction, combined with the magnetic susceptibility. The magnetic induction is measured in depth by a Proton Precession Magnetometer with Overhauser effect designed by the Laboratoire d'Electonique de Technologie et Instrumentation (Commissariat à I' Energie Atomique). This method gives a continuous record with an accuracy of 0.1 nT, and an excellent stability. In order to cancel out the transient component of the Earth's magnetic field, the magnetic induction is computed as the difference between the induction measured in the borehole and that obtained at a surface reference point located near the drillhole. The dipole‐dipole susceptibility probe gives a continuous record with an accuracy of a few 10−6. The four total magnetic induction logs recorded in oil exploration holes show a tight correlation, although distant from up to 80 km. This technique is therefore excellent for precise stratigraphic correlation. Data processing allows us to isolate the component of induction due to remanent magnetization and to find the magnetic polarity. A sequence of magnetic polarity is proposed for Kimmeridgian and Oxfordian stages. The variation of the sedimentation rate is analyzed with a very high resolution in time, and compared with the mean sedimentation rate deduced from a lithologic log. Cores were collected in the lowermost part of two holes. The correlation between the magnetostratigraphy deduced from paleomagnetic measurements performed on 240 standard plugs and that obtained by logging is good. A study of the components of magnetization found in the core has shown that the remagnetization occurred mainly during the coring operation itself or during the storage of the core. Consequently, downhole magnetostratigraphy proves to be reliable, besides providing the advantage of a continuous investigation performed over a large volume of rock scrutinized in situ.
Aluminum reactivity in hydrothermal plumes at the Mid‐Atlantic Ridge
Authors: Elderfield H. , Greaves M. J. , Rudnicki M. D. , Hydes D. J.
Published: February 2008

Direct measurements of buoyant plume waters collected by submersible show that the behavior of Al is highly nonconservative during the early mixing of submarine hydrothermal fluids and seawaters at the TAG and MARK sites on the Mid‐Atlantic Ridge. Al is removed from the hydrothermal end‐members and is also stripped from the entrained seawater to produce levels of Al in the plume solutions about one fifth of normal seawater concentrations. The behavior of Al appears to be similar to that inferred for other anionic species (V, As, Cr, P) which exhibit constant element‐iron ratios in hydrothermal plume particles and suggests that such elements are removed from seawater during the early stage of formation of hydrothermal plumes.
The effect of block rotations on the global seismic moment tensor and the patterns of seismic P and T axes
Authors: Twiss Robert J. , Souter Barbara J. , Unruh Jeffrey R.
Published: February 2008

Distributed brittle deformation of the Earth's crust involving block rotations is comparable to the deformation of a granular material, with fault blocks acting like the grains. The deformation of a granular material is not adequately described using classical continuum mechanics because the individual grains within the material rotate in a manner that is not uniquely determined by the large‐scale average deformation. Thus a theoretical link has not existed between the kinematics of deformation involving block rotation and the associated effects on the seismic moment tensor and focal mechanism solutions. We establish this link using micropolar continuum theory (Eringen, 1964, 1966 a, b; Eringen and Suhubi, 1964) and the analysis of the effect of block rotations on fault slickenline patterns by Twiss et al. (1991). This theory takes into account two separate scales of motion: a large‐scale average motion of the material, the macromotion, composed of a macrodeformation rate (i.e., a macrostrain rate) and a macrospin, and a local motion, the microspin, that describes the average rotation rate of grains in the material. The micropolar kinematic theory allows us to predict the orientations of coseismic slip directions V on local shear planes of any orientation in a large‐scale shear zone. We define a local and a global asymmetric micropolar seismic moment tensor in terms of these slip directions. For a restricted kinematic model, the theory shows that two scalar parameters, D and W, determine the symmetry of the global micropolar seismic moment tensor and the pattern of seismic P (shortening) and T (lengthening) axes. The deformation rate parameter D is defined in terms of the principal values of the deformation rate tensor . The deformation is transtensional (constrictional) if 0 ≤ D ≤ 0.5, plane strain if D = 0.5, and transpressional (flattening) if 0.5 < D ≤ 1. The net vorticity parameter W is a normalized value of the difference between microspin and macrospin
. It is an objective variable. W = 0 implies that the global micropolar seismic moment tensor is symmetric and that P and T axis patterns have orthorhombic or higher symmetry. W ≠ implies that the global micropolar seismic moment tensor is asymmetric and that P and T axis patterns have monoclinic symmetry. The antisymmetric part of the global micropolar seismic moment tensor is associated with the net vorticity that characterizes the deformation. W has different values for different models of rigid block rotation and thus could serve to identify the rotation mechanism.
A geochemical study of metalliferous sediment from the TAG Hydrothermal Mound, 26°08′N, Mid‐Atlantic Ridge
Authors: German C. R. , Higgs N. C. , Thomson J. , Mills R. , Elderfield H. , Blusztajn J. , Fleer A. P. , Bacon M. P.
Published: February 2008

An Alvin push core collected near the base of the TAG hydrothermal mound, 26°08′N, Mid‐Atlantic Ridge, sampled a 7 cm‐thick layer of Fe‐rich red‐brown mud (20–40% Fe) overlying a 4 cm‐thick layer of carbonate ooze (5–70% CaCO3) which also contains up to 32%Fe. On the basis of chemical compositions, two separate layers can be identified within the red‐brown mud. The core position, X ray diffractometry, Pb isotope analyses, and elemental abundances all indicate that these two layers were derived from mass wasting of oxidized, originally sulphidic material from the slopes of the TAG hydrothermal mound. High concentrations (11–19 ppm) of seawater‐derived U occur in both Fe‐rich layers. These enrichments may derive from uptake of U linked to the oxidation of sulphide phases. Metal concentrations in the rich layer are lower than in the upper part of the core but higher than in open ocean pelagic sediments. The 230Thxs/Fe, 231Paxs/Fe, and rare earth element/Fe distributions indicate that the hydrothermal input to the carbonate‐rich layer is dominated by settling of suspended particulate material from the overlying hydrothermal plume. Such CaCO3‐rich sediments may record an important component of the flux of hydrothermal material to sediments of the Mid‐Atlantic Ridge rift valley.
Fault growth in brittle‐ductile experiments and the mechanics of continental collisions
Authors: Sornette Anne , Davy Philippe , Sornette Didier
Published: February 2008

Using laboratory models of continental collisions, we study the mechanisms responsible for large‐scale deformation and the nature of the penetrative deformation, localized or homogeneous, and its characteristic scales and structure. In order to focus the study, one of the most spectacular cases of continental collisions, namely the India‐Asia collision, is considered. Different models with varying rheologies are analyzed which attempt to respect the brittle‐ductile stratification corresponding to the crust and mantle structure in the Earth. Each experiment is quantified by studying the strain field and the fault pattern as a function of the position P of the indenter within the system. The strain field is characterized by (1) the second invariant of the two‐dimensional strain tensor and its variation with position for a fixed P, the evolution of its average as a function of P, (2) the evolution with P of the participation ratio S2, which quantifies the fraction of the system surface area which participates in the deformation, (3) the evolution of the “escape ratio,” calculated as the surface increase gained by the system during its eastward lateral escape divided by the surface covered by the indenter penetration. Qualitatively, we find that the deformation first spreads out but later on undergoes localization. Once the fault pattern is “mature” (the cumulative fault length does not increase anymore), we observe that the strain field is essentially controlled by the kinematics of the larger faults. Each fault pattern is quantified by studying (1) the histograms of fault orientations, (2) the “capacity” and barycenter fractal dimensions, (3) the “multifractal” generalized dimensions, and (4) the distribution of fault lengths. The fractal dimension Dƒ is found to almost constant within experimental uncertainty (Dƒ =1.7±0.1) and thus appears rather insensitive to the particular chosen rheology. We find a correlation between the generalized multifractal dimensions and two exponents, the barycenter fractal dimension “b” and the exponent “a” of the fault length distribution. This shows that the scaling properties of fault patterns can be characterized by the knowledge of only two exponents “b” and “a” of the spatial and length distributions of faults. Our main results are (1) observation of the growth of self‐similar fault patterns, (2) complete characterization of the fault patterns with two scaling exponents “a” and “b,” (3) wide distribution of undeteriorated domains and large heterogeneity of the deformation field, (4) maturation of the fault structure corresponding to a localization of the deformation on a few large faults in the late stage of the deformation.
A determination of the three‐dimensional seismic properties of anorthosite: Comparison between values calculated from the petrofabric and direct laboratory measurements
Authors: Seront Bernard , Mainprice David , Christensen Nikolas I.
Published: February 2008

We have undertaken a comparative study of the seismic properties of an anorthosite. The seismic properties have been calculated from the lattice preferred orientation (LPO) of plagioclase (90% by volume) and olivine (10% by volume), and the elastic constants of diese minerals. Laboratory measurements of Vp (24 directions) and Vs (6 directions) have been made on the same sample at room temperature and pressures up to 1.0 GPa. The two techniques agree with 0.1 km/s for all orientations studied when the Voigt averaging method is used. Hill and Reuss averaging methods give calculated velocities significantly lower than the experimental ones. A study of the difference between the velocity values given by the two methods reveals a systematically larger departure in orientations normal to the foliation. The orientation dependent difference between the two techniques can be explained if mere are small errors in the elastic constants for single crystal plagioclase used in the initial calculations. The axial LPO symmetry of the plagioclase with all the crystallographic b axes parallel to the specimen symmetry axis allowed us to calculate C22 (normal to (010)) directly from the laboratory measurement of Vp. Some constraints can be put on C22, C44, C66, C12 and C23. On average our data require 10% modification of the single crystal constants reported by Ryzhova (1964)
Dynamics of fault interaction: parallel strike‐slip faults
Authors: Harris Ruth A. , Day Steven M.
Published: February 2008

We use a two‐dimensional finite difference computer program to study the effect of fault steps on dynamic ruptures. Our results indicate that a strike‐slip earthquake is unlikely to jump a fault step wider than 5 km, in correlation with field observations of moderate to great‐sized earthquakes. We also find that dynamically propagating ruptures can jump both compressional and dilational fault steps, although wider dilational fault steps can be jumped. Dilational steps tend to delay the rupture for a longer time than compressional steps do. This delay leads to a slower apparent rupture velocity in the vicinity of dilational steps. These “dry” cases assumed hydrostatic or greater pore‐pressures but did not include the effects of changing pore pressures. In an additional study, we simulated the dynamic effects of a fault rupture on ‘undrained’ pore fluids to test Sibson's (1985, 1986) suggestion that “wet” dilational steps are a barrier to rupture propagation. Our numerical results validate Sibson's hypothesis by demonstrating that the effect of the rupture on the ‘undrained’ pore fluids is to inhibit the rupture from jumping dilational stepovers. The basis of our result differs from Sibson's hypothesis in that our model is purely elastic and does not necessitate the opening of extension fractures between the fault segments.
Thermal constraints on the zone of major thrust earthquake failure: The Cascadia Subduction Zone
Authors: Hyndman R. D. , Wang K.
Published: February 2008

Constraints on the seismogenic portion of the subduction thrust zone along the Cascadia margin are provided by the thermal regime. The zone of stick‐slip “locked” behavior where earthquakes can nucleate may be limited downdip by a temperature of about 350°C, and the transition stable sliding zone into which coseismic displacement can extend by a temperature of approximately 450°C. The seaward limit of the stick‐slip zone may be associated with the dehydration of stable sliding clays at 100 to 150°C and dissipation of high pore pressures in the area of the deformation front. Temperatures on the thrust have been estimated by numerically modelling the thermal regimes along three profiles crossing the margin with constraints provided by surface heat flow and detailed structural information, particularly at southern Vancouver Island. The models that best fit the heat flow data have negligible shear strain heating. The Cascadia subduction margin is unusually hot as a consequence of the very young plate age and the thick insulating sediment section on the incoming plate; the temperature at the top of the oceanic crust at the deformation front is about 250°C. As a result, the modelled zone of stick‐slip seismogenic behaviour is restricted to a narrow zone beneath the continental slope and outer shelf, with the transition zone extending to the inner shelf. The seismogenic zone is wider off the Olympic Peninsula compared to off southern Vancouver Island because of the much shallower thrust dip angle and the slightly older incoming plate. The profile off Oregon is found to have intermediate width zones. An important assumption, well justified only off southern Vancouver Island, is that the thrust detachment is located at the top of the downgoing oceanic crust. The same modelling technique shows that more typical subduction zones with older incoming oceanic lithosphere such as central Chile have thermally restricted seismogenic zones that are much wider, commonly extending well beneath the coast. Support for the position of the Cascadia locked zone from the thermal results is provided by a comparison of the horizontal and vertical interseismic deformation predicted by simple dislocation models with the observed rates from tide gauge and geodetic surveys on adjacent coastal regions. The general agreement indicates that any seismic “locked zone” must be located offshore where the subduction thrust fault is less than about 20 km deep and where the contact is between the oceanic crust and the accreted sedimentary wedge, not between the oceanic and continental crusts. The restriction to an offshore zone provides an important limit to the maximum magnitude and to the ground motion and seismic hazard from subduction megathrust earthquakes in southwestern British Columbia, Washington, and Oregon.
The La olivina pyroxenite suite and the isotopic compositions of mantle basalts parental to the Mid‐Cenozoic arc volcanism of northern Mexico
Authors: Nimz Gregory J. , Cameron Kenneth L. , Niemeyer Sidney
Published: February 2008

Two chemically and isotopically distinct groups of pyroxenite xenoliths occur in ∼2 Ma basanites at the La Olivina xenolith locality within the Camargo basalt field, Chihuahua, Mexico. Both groups occur in composite mantle‐peridotite/pyroxenite xenoliths, indicating that they formed in the upper mantle. One group, which has igneouslike textures and chemical and isotopic compositions similar to the host basanite, is assumed to have formed from the Camargo magmas and is thus late Cenozoic in age. The second group has equigranular, recrystallized textures and is unlike the Camargo basanites in isotopic and trace element characteristics. This group appears to have formed in a manner analogous to the igneous‐textured group but from magmas related to mid‐Cenozoic arc‐related basalts that occur in the La Olivina vicinity. The equigranular pyroxenites have Sr‐Nd‐Pb isotopic compositions similar to these mid‐Cenozoic basalts, and magmas calculated to be in equilibrium with the pyroxenites have trace element concentrations similar to these basalts. The recrystallized textures of the equigranular pyroxenites reflect their longer mantle residence time. The equigranular pyroxenites provide a unique opportunity to assess the relative contributions of crust and mantle components to the mid‐Cenozoic continental arc‐related basalts of northern Mexico. They provide evidence that the isotopic compositions of the mid‐Cenozoic basalts (e.g.,87Sr/86Sr = ∼0.7043; ϵNd= ∼ + 1;206Pb/204Pb = ∼18.7) are subcrustal in origin. It appears, therefore, that MASH processes are not responsible for the Sr‐Nd isotopic uniformity that is observed in the basalts, nor for their “bulk earth” Sr‐Nd isotopic values. The pyroxenites and basalts have elevated207Pb/204Pb with respect to206Pb/204Pb compared to oceanic basalts. The subcrustal origin of this signature suggests that it is the result of incorporation of a sedimentary component from the subducted slab. The Sr‐Nd‐Pb isotopic compositions of the equigranular pyroxenites are similar to those present in “enriched” lithospheric mantle in southwestern North America. This enrichment has been observed both directly in metasomatized peridotite xenoliths and indirectly in late Cenozoic basalts of the Rio Grande Rift and Southern Transition Zone of the Colorado Plateau. For the latter, the enrichment has been thought to be Proterozoic in age, but the evidence from the equigranular pyroxenites suggests that the event responsible for the observed isotopic compositions may have instead occurred during the mid‐Cenozoic.
Aeromagnetic study of the Island of Hawaii
Authors: Hildenbrand Thomas G. , Rosenbaum Joseph G. , Kauahikaua James P.
Published: February 2008

An aeromagnetic study of the Island of Hawaii provides new insight on magnetic properties of subsurface rock and geologic structure. On a regional scale, spectral‐depth analysis delineates two shallow magnetic zones, each roughly 1.5 km thick, lying at a depth of 1 km. One zone (of unknown origin) lies in the center of the island and correlates with a regional magnetic high. The other zone coincides with pronounced magnetic lows paralleling Kilauea's active east rift zone. These magnetic lows probably depict rocks chemically altered by hydrothermal fluids, in which titanomagnetite has been destroyed. Analysis of magnetic terrain effects indicates that magnetization also decreases with depth within Mauna Kea and Mauna Loa. We estimate that magnetization is reduced by about half at a depth of 1 km. The magnetic method is particularly useful in delineating the lateral extent of local shield structures, such as rifts, summit calderas, pit craters, and vent fissures. Rifts possess characteristic magnetic patterns, primarily long‐wavelength linear magnetic low zones. We propose alteration processes reduce magnetizations along the flanks of rifts. On the other hand, along young rifts (e.g., Kilauea's east rift zone), short‐wavelength magnetic anomalies probably reflect slowly cooled, unaltered intrusions. Altered rock may also produce magnetic lows that help define buried summit calderas.
Tectonic deformation in the New Madrid Seismic Zone: Inferences from map view and cross‐sectional boundary element models
Authors: Gomberg Joan
Published: February 2008

The lack of instrumental recordings and obvious fault scarps associated with the 1811–1812 New Madrid earthquakes necessitates examination of more subtle indicators of the geometry and type of faulting associated with these events. Morphologic and geologic features and the distribution of modern seismicity are used to infer the slip distribution and type of faulting (strike‐ or dip‐slip), number, strike, length, and width of the major faults in the New Madrid seismic zone (NMSZ). This is accomplished through two‐dimensional boundary element modeling of the strain field arising from slip on hypothetical faults that is driven by either coseismic or uniform regional strains. Tectonic deformation is reflected in (1) the seismicity, (2) the Lake County uplift, (3) Reelfoot Lake, (4) the fractured rocks of the Blytheville arch, and (5) the St. Francis Sunk Lands. Many of these features can be qualitatively explained as resulting from tectonic deformation due to slip on two left‐stepping right‐lateral strike‐slip faults that are coincident with the northeast trending zones of seismicity and the Blytheville arch. The morphology appears to be, at least in part, a consequence of major earthquakes that rupture these faults. The locations of the 1811–1812 and largest post‐1812 earthquakes and the models are consistent with a process in which the 1811–1812 earthquakes relieved accumulated regional shear strain causing the greatest post‐1812 shear strains to exist at the ends of the fault zone. Modeling results also suggest that the numerous small earthquakes in the NMSZ are not aftershocks of the 1811–1812 earthquakes but instead represent continuous localized adjustments to a uniform regional strain field. The Bootheel lineament does not appear to be significant in shaping the morphology, geologic structure, and pattern of seismicity in the NMSZ. The absence of fault scarps and coherence of shallow reflectors overlying the Blytheville arch suggests that the major NMSZ faults may not have ruptured to the surface during the 1811–1812 earthquakes. The inferred lengths of the 1811–1812 earthquake ruptures also suggest that their sizes may have been overestimated. Model‐predicted subsidence within the St. Francis Sunk Lands indicates that tectonic deformation also may have influenced alluvial processes in the NMSZ.
Petrophysical model of deep electrical conductors: Graphite lining as a source and its disconnection due to uplift
Authors: Katsube T. J. , Mareschal M.
Published: February 2008

Petrophysical information, including stress release data, recently made available for low‐porosity (0.1–0.4%) crystalline rocks has been used to develop a “broken conductor model” that may explain the origin of enhanced electrical conductivity of the intermediate to lower continental crusts of shields and its disappearance when the rocks are uplifted. Average pore size and pore density of their fluid flow paths are 0.1–0.4 μm and 1–400/m, respectively. The model assumes that these paths are filled or lined with interconnected electrical conducting minerals, such as graphite, resulting in the rocks having resistivities as low as 40 to 800 Ω m in situ. Graphite film thicknesses required to produce these resistivities are 5–20 nm (50–200 Å). The model also assumes that electrical conducting paths break up during uplift due to stress release, resulting in a poorly conducting rock. Petrophysical data of granites and observations from deep drill holes suggest a porosity increase of the order of 3% from an uplift of 10 km or more. However, the newly formed pore space would fill with deposits or intruded material so that surface rock porosities would not be much larger than at depth. Electrical resistivities of rock samples containing disconnected grain boundary graphite presently exposed by the Kapuskasing uplift of the Canadian Shield are in the range of 3000–12000 Ω m for the direction of foliation. These values are 10–100 times larger than the estimates for similar rocks containing interconnected graphite but are 10–100 times smaller than usually observed in the Canadian Shield for similar rocks absent of grain boundary graphite. These observations are in good agreement with the results predicted by the broken conductor model for an uplift of 10 km or more if a gross linear relationship is assumed between crack porosity and uplift for the upper 10 km of the crust.
Seismotectonics of the San Andreas Fault System between Point Arena and Cape Mendocino in northern California: Implications for the development and evolution of a young transform
Authors: Castillo David A. , Ellsworth William L.
Published: February 2008

The northernmost and relatively youthful segment of the San Andreas fault system is situated within a 100+ km wide zone of northwest trending strike‐slip faults that includes, from west to east, the San Andreas, Maacama, and Bartlett Springs faults. Although the San Andreas fault is the principal strike‐slip fault in this system, it has been virtually aseismic since the 1906 earthquake. Moderate levels of seismicity locate to the east along the Maacama fault and, to a lesser extent, the Bartlett Springs fault at focal depths typical of other strike‐slip faults within the San Andreas fault system in central California. North of the San Andreas fault system, within the Cape Mendocino area, earthquakes occur at depths of up to 40 km and primarily reflect internal deformation of the subducting Gorda slab, and slip along the Mendocino Fracture Zone. Seismicity along the Maacama and Bartlett Springs faults is dominated by right‐lateral to oblique‐reverse slip along fault planes that dip 50°–75° to the northeast. The northern extent of seismicity along these faults terminates near the surface projection of the southern edge of the Gorda slab. The onset of seismicity along these faults may be related to the abrupt change in the elastic thickness of the North American plate as it enters the asthenospheric window. The Maacama and Bartlett Springs faults are strike‐parallel with active reverse faults within the forearc region of the Cascadia subduction zone. This preexisting structural fabric of northwest trending reverse faults in the forearc area appears to have strongly influenced the initial slip and complexity of these faults. Continuation of the moderately dipping Maacama fault to the southeast along the steeply dipping Healdsburg and Rodgers Creek fault zones and the near‐vertical Hayward and Calaveras fault zones in the San Francisco Bay area suggests that these faults evolve toward a more vertical dip to minimize the shear stresses that tend to resist plate motion.
A comprehensive comparison between satellite‐measured skin and multichannel sea surface temperature
Authors: Wick Gary A. , Emery William J. , Schluessel Peter
Published: February 2008

At present, the most widely used algorithms for the calculation of sea surface temperatures (SSTs) from infrared satellite data are based on the regression of infrared radiances with in situ buoy SST measurements (the multichannel SST, MCSST, and the cross‐product SST, CPSST). Drifting buoys measure the SST anywhere from 0.5 to 1.0 m below the surface, thus missing the important radiative skin of the ocean. It is the temperature of this skin layer that is active in the air‐sea heat exchange. To determine the differences between the algorithms for the satellite‐measured skin SST (or satellite‐measured surface skin temperature, SMSST), the MCSST and the CPSST, historical infrared imagery was used to compute coincident maps of each product from which the temperature differences were calculated. The MCSST ‐ SMSST differences were computed for five consecutive seasons. CPSST ‐ SMSST comparisons were computed with two different CPSST algorithms for periods from two different years. Also, a global map of SMSST, using both AVHRR and HIRS data, was computed for one period and compared with the AVHRR‐only SMSST, MCSST, and CPSST maps. Histograms of all differences, stratified by zonal band, clearly suggest the mean and rms differences between the SMSST, MCSST, and CPSST The MCSST has a mean offset from the SMSST of 0.45 K, while the CPSST and SMSST have a mean offset of 0.1 K. The MCSST has its greatest rms differences (∼0.7 K) with the SMSST at low latitudes with much lower rms differences at mid and polar latitudes. The CPSST and SMSST, however, exhibit larger rms differences (∼0.6 K) at mid and higher latitudes where smaller differences would be expected. The AVHRR/HIRS SMSST has a small mean offset from the AVHRR‐only SMSST (0.20 K) and CPSST (0.19 K) but shows a large amount of scatter with both algorithms (rms difference of 0.59 K with SMSST, 0.65 K with CPSST). Owing to the lack of in situ skin SST measurements, it is difficult to say which of the algorithms produces the most accurate results; only the differences may be discussed.
The double silica maximum in the North Pacific
Authors: Talley Lynne D. , Joyce Terrence M.
Published: February 2008

The North Pacific has two vertical silica maxima. The well‐known intermediate maximum occurs between 2000 and 2500 m with a potential density relative to 2000 dbar of 36.90 in the northeastern Pacific. The deep maximum, which has not been observed extensively before, is found at or near the ocean bottom in the northern North Pacific in a narrow latitude range. Maps of silica on isopycnals which intersect the intermediate and bottom maxima show that the lowest silica is found in the western tropical North Pacific, suggesting a route for the spread of South Pacific water into the deep North Pacific. Low‐silica water is found along the western boundary of the North Pacific, with a separate broad tongue south of Hawaii. The highest silica on both isopycnals is in the northeast Pacific. A bottom maximum in the Cascadia Basin in the northeastern Pacific can be differentiated from both open‐ocean maxima. Four sources for the vertical maxima are considered: in situ dissolution of sinking particles, bottom sediment dissolution, hydrothermal venting, and upslope advection in the northeastern Pacific. Because not enough is known about any of these sources, only rough estimates of their contributions can be made. The bottom maximum is most likely to result from bottom sediment dissolution but requires a flux larger than some current direct estimates. The Cascadia Basin bottom maximum may result from both bottom sediment dissolution and hydrothermal venting. The intermediate maximum is likely to result primarily from dissolution of sinking particles. There is no quantitative estimate of the effect of possible upslope advection or enhancement of bottom fluxes due to the Columbia River outflow.