bluuebunny/crossref_metadata_2025 huggingface dataset:
Scattering of high‐frequency surface waves in Scotland
Authors: MacBeth Colin , Snieder Roel
Published: February 2008

High‐frequency (≤5 Hz) coda waves for velocities of arrival less than 3 km/s, recorded on vertical component instruments and generated from a local earthquake in Scotland, are analyzed to ascertain their cause. The adaption of existing velocity models and scattering from near‐surface irregularities in Scotland such as mountains and lochs are considered as possible causes of the observed behavior. The former mechanism is not feasible, as it implies a significant alteration of the velocities in the upper 2 km crust, contradicting previous seismic surveys in the area. An analysis of the effects of scattering is performed using a formalism derived from the Born approximation. The scattered wave field is computed for interactions between first six Rayleigh and Love modes. The general character of the synthetic seismograms for these scattered waves agrees with the observations on a qualitative basis. The apparent absence of the fundamental mode energy from the records is also explained by the synthetic seismograms. The calculations imply that scatterers with a scale length of less than 300 m are applicable to these data from the northernmost stations but around 2 km for the more southern areas. It is thought that the scale length relates to the size of a region on the slopes of the mountains or lochs where there is a sharp gradient. This study emphasises the effectiveness of linear scattering theory in accounting, on a qualitative basis for many of the observed features of the apparently complex coda waves.
Decomposition of magnetotelluric impedance tensors in the presence of local three‐dimensional galvanic distortion
Authors: Groom Ross W. , Bailey Richard C.
Published: February 2008

There are many occasions on which the magnetotelluric impedance tensor is affected by local galvanic distortion (channelling) of electric currents arising from induction in a conductive structure which is approximately two‐dimensional (2‐D) on a regional scale. Even though the inductive behavior is 2‐D, the resulting impedance tensor can be shown to have three‐dimensional (3‐D) behavior. Conventional procedures for rotating the impedance tensor such as minimizing the mean square modulus of the diagonal elements do not in general recover the principal axes of induction and thus do not recover the correct principal impedances but rather linear combinations of them. This paper presents a decomposition of the impedance tensor which separates the effects of 3‐D channeling from those of 2‐D induction. Where the impedance tensor is actually the result of regional 1‐D or 2‐D induction coupled with local frequency independent telluric distortion, the method correctly recovers the principal axes of induction and, except for a static shift (multiplication by a frequency independent real constant), the two principal impedances. Also obtained are two parameters (twist and shear), which partially describe the effects of telluric distortion. It is shown that the tensor operator which describes the telluric distortions can always be factored into the product of three tensor sub‐operators (twist, shear, local anisotropy) and a scalar. This product factorization allows assimilation of local anisotropy, if present, into the regional anisotropy. The method of decomposition is given in the paper along with a discussion of the improvements obtained over the conventional method and an example with real data.
Scattering of teleseismic body waves under Pasadena, California
Authors: Langston Charles A.
Published: February 2008

Teleseismic receiver functions for structure under Pasadena, California (PAS) are derived from azimuthally distributed teleseismic P waves recorded on Benioff 1–90 instrumentation. The broadband three‐component Benioff 1–90 system is peaked at a 1‐s period and allows resolution of major crustal interfaces from large Ps conversions seen in the receiver function data. The observed body wave data are quite complex, showing exceptionally large Ps conversions and scattered waves on horizontal components. Radial and tangential motions are of equal magnitude and show major off‐azimuth converted Ps waves, suggesting large‐scale crustal heterogeneity beneath the station. Stochastic simulations of one‐dimensional plane layered structure show that geologically unreasonable one‐dimensional models are required to fit the data. The observed coda decay yields a scattering Q estimate of 239 at a 2‐s period using an energy flux model for a propagating plane wave interacting with a scattering layer over a homogeneous half‐space. Observed and synthetic coda decay follows the theoretical exponential decay predicted by the model and is due entirely to diffusion of coda energy out of the layer into the half‐space. PAS coda is compared to coda from deep teleseisms recorded at State College, Pennsylvania, and it is seen that scattering is more severe at PAS, as reflected in higher coda levels and slower decay rate. Consideration of energy partitioning and coda amplitude suggests that much of the coda consists of scattered surface waves. Analysis of a major Ps conversion arriving 3 s after direct P indicates that a major crustal discontinuity at about 20 km depth dips at moderate angles to the north under the San Gabriel Mountains. This interface probably represents the crustal tectonic boundary between the Transverse Ranges and the Los Angeles Basin.
Buried topography of Utopia, Mars: Persistence of a giant impact depression
Authors: McGill George E.
Published: February 2008

Knobs, partially buried craters, ring fractures, and some mesas permit a qualitative determination of the topography buried beneath younger northern plains materials. These features are widely distributed in the Utopia area but are absent in a large, roughly circular region centered at about 48°N, 240°W. This implies the existence of a circular depression about 3300 km in diameter buried beneath Utopia Planitia that is here interpreted to represent the central part of a very large impact basin. The presence of buried curved massifs around part of this depression, and a roughly coincident mascon, lend further support. Present topography, areal geology, and paleotopography of buried surfaces all point to the persistence of this major depression for almost the entire history of Mars.
Seismic reflection across a continental transform: An example from a convergent segment of the Dead Sea rift
Authors: Rotstein Y. , Bartov Y.
Published: February 2008

Three seismic reflection lines in the Dead Sea rift in northern Israel cross the transform plate boundary between Arabia and the Sinai block of the African plate. The Arabian plate edge consists of the Meshoshim tilted block, which we suggest to be partly a result of prerift activity. In the Sinai block the elevated Rosh Pinna Saddle is faulted over the Meshoshim block with a vertical separation of 2.5–3‐km. Much of this separation is due to young reverse faulting which is related to the oblique plate boundary in this area. The main fault zone transfers the motion from the western margin of the Kinneret‐Zemah graben to the eastern margin of the Hula graben. Immediately north of Lake Kinneret, the main fault is a 1‐ to 2‐km‐wide zone of extensive deformation and a discrete main fault trace is hard to define. Further to the north the deformation is less extensive and is concentrated in a narrow fault zone. Deformation across the plate boundary is asymmetrical and, while the Meshoshim block is quite coherent, the Rosh Pinna Saddle is a highly faulted and deformed positive flower structure.
Isotopic and geochemical provinces of the western Indian Ocean Spreading Centers
Authors: Mahoney J. J. , Natland J. H. , White W. M. , Poreda R. , Bloomer S. H. , Fisher R. L. , Baxter A. N.
Published: February 2008

Basalt glasses from the Central Indian Ridge are distinct isotopically from mid‐ocean ridge basalts (MORB) of the Indian Ocean triple junction and western few hundred kilometers of the Southeast Indian Ridge. In particular, very low 206Pb/204Pb and high 87Sr/86Sr signatures, which characterize the latter region, are absent over most of the Central Indian Ridge. In turn, lavas from the unusually deep eastern 1100–1500 km of the Southwest Indian Ridge are different chemically and isotopically from those of the above areas. A rather abrupt eastern boundary to Southwest Indian Ridge‐type compositions occurs at or very near the geographic triple junction. This provinciality in western Indian Ocean ridge basalts partly mirrors fundamental regional differences in the underlying mantle but, at least between the eastern Southwest Indian Ridge and the western Southeast Indian Ridge and triple junction, also may reflect variations in extent and depth of melting in a vertically zoned upper mantle. A pronounced low εNd, high 206Pb/204Pb, high 87Sr/86Sr anomaly exists on the Central Indian Ridge at the Marie Celeste Fracture Zone and on the adjacent ridge segment to the south. Despite the great distance (>1100 km) of Réunion Island from the ridge, this zone appears to demark a region of mantle containing substantial Réunion hotspotlike material. Several old (35–60 m.y.) Deep Sea Drilling Project basalts which erupted on the ancestral Central Indian Ridge also record a significant Réunion hotspotlike influence, whereas a 46‐m.y.‐old sample that formed farther from the presumed locus of the hotspot possesses isotopic values identical to many present (non‐Marie Celeste area) Central Indian Ridge MORB. The variably expressed and/or heterogeneous low 206Pb/204Pb material partly responsible for the isotopic distinctiveness of Indian Ocean ridge basalts may have entered into the Indian MORB mantle as a result of continental lithospheric remobilization preceding the breakup of Gondwana, particularly from the portion that would eventually become Greater India.
Large‐scale ice flow over a viscous basal sediment: Theory and application to ice stream B, Antarctica
Authors: MacAyeal Douglas R.
Published: February 2008

Recent seismic studies of ice stream B, Antarctica and field analysis of mid‐latitude glacial deposits suggest that deformable basal sediments (e.g., water‐saturated till) are important in determining ice sheet flow. If the ratio of till viscosity to effective ice viscosity is small, vertical shear associated with horizontal flow is confined to the deforming bed alone. Ice flow over a deformable bed is thus akin to that of floating ice shelves, because ice shelves flow over inviscid seawater. For some Antarctic ice streams, and possibly for portions of the late Wisconsin ice sheets in North America and Eurasia, basal drag associated with deforming basal sediment does not induce significant vertical gradients of horizontal velocity. Instead, basal drag affects the flow as if it were a horizontal body force balanced by longitudinal and transverse deviatoric stress gradients. Comparison of the observed flow of ice stream B to finite element simulations incorporating a viscous basal till suggest that a simple till rheology is sufficient to explain the current velocity profile. These simulations also highlight the importance of horizontal deviatoric stress in regions where driving stress and basal stress do not locally balance. While bed deformation is critical to ice stream existence, sensitivity tests suggest that ice shelf back pressure is still a crucial control affecting ice stream response to atmospheric and oceanic climate.
Mm: A variable‐period mantle magnitude
Authors: Okal Emile A. , Talandier Jacques
Published: February 2008

This paper lays the theoretical groundwork for a variable period mantle magnitude, Mm, based on the measurement of the spectral amplitude X(ω) of very long period Rayleigh waves. We retain the concept of magnitude by restricting ourselves to single‐station measurements, ignoring the focal mechanism and the exact depth of the shallow earthquakes considered. Our measurements are made at a series of periods (in all cases greater than 40 s), and the largest value is retained. This procedure effectively avoids the well‐known interference effects leading to saturation of magnitude scales defined at a fixed period. Two corrections are used: a period‐dependent distance correction CD, and a source correction Cs, also period‐dependent, compensating for the variation of the excitation of Rayleigh waves with period. Both of these corrections are fully predictable on the basis of standard surface wave excitation and dispersion theory. The result is a formula of the type Mm = log10 X(ω) + CD + CS + C0 in which all coefficients, including the constant C0 are justifiable on sound theoretical grounds. The analysis of a data set of 256 records from the broadband seismograph at Papeete, Tahiti, the ultra‐long period system at Pasadena, and stations of the GEOSCOPE network, shows that the mean error in the estimation of the seismic moment is on the order of 0.1–0.2 units of magnitude, with the standard deviation at each station also on the order of 0.2 units of magnitude. No significant trend with either distance, period, or station can be identified. The method can also be transposed to the time domain, under some simple assumptions which are justifiable theoretically for typical teleseismic distances across the Pacific Basin. Both versions of the method lend themselves well to automation. Thus, by providing a real‐time estimate of the seismic moment of distant earthquakes, Mm has considerable potential for tsunami warning purposes. Its concept can easily be extended to Love waves and also to intermediate and deep earthquakes.
Bedrock geology and tectonic evolution of the Wrangellia, Peninsular, and Chugach Terranes along the Trans‐Alaska Crustal Transect in the Chugach Mountains and Southern Copper River Basin, Alaska
Authors: Plafker George , Nokleberg W. J. , Lull J. S.
Published: February 2008

The Trans‐Alaskan Crustal Transect in the southern Copper River Basin and Chugach Mountains traverses the margins of the Peninsular and Wrangellia terranes, and the adjacent accretionary oceanic units of the Chugach terrane to the south. The southern Wrangellia terrane margin consists of a polymetamorphosed magmatic arc complex at least in part of Pennsylvanian age (Strelna Metamorphics and metagranodiorite) and tonalitic metaplutonic rocks of the Late Jurassic Chitina magmatic arc. The southern Peninsular terrane margin is underlain by rocks of the Late Triassic (?) and Early Jurassic Talkeetna magmatic arc (Talkeetna Formation and Border Ranges ultra‐mafic‐mafic assemblage) on Permian or older basement rocks. The Peninsular and Wrangellia terranes are parts of a dominantly oceanic superterrane (composite Terrane II) that was amalgamated by Late Triassic time and was accreted to terranes of continental affinity north of the Denali fault system in the mid‐ to Late Cretaceous. The Chugach terrane in the transect area consists of three successively accreted units: (1) minor greenschist and intercalated blueschist, the schist of Liberty Creek, of unknown protolith age that was metamorphosed and probably accreted during the Early Jurassic, (2) the McHugh Complex (Late Triassic to mid‐Cretaceous protolith age), a melange of mixed oceanic, volcaniclastic, and olistostromal rocks that is metamorphosed to prehnite‐pumpellyite and lower greenschist facies that was accreted by middle Cretaceous time, and (3) the Upper Cretaceous Valdez Group, mainly magmatic arc‐derived flysch and lesser oceanic volcanic rocks of greenschist facies that was accreted by early Paleocene time. A regional thermal event that culminated in early middle Eocene time (48–52 Ma) resulted in widespread greenschist facies metamorphism and plutonism.
Examination of the deep levels of an island arc: Evidence from the Tonsina Ultramafic‐Mafic Assemblage, Tonsina, Alaska
Authors: DeBari Susan M. , Coleman R. G.
Published: February 2008

The Tonsina ultramafic and mafic assemblage, located in the northern Chugach Mountains along the Trans‐Alaska Crustal Transect (TACT), is interpreted to be the remains of a magma chamber that crystallized at the base of a mature interoceanic island arc. It is one of a number of isolated masses of peridotite and gabbro that comprise the basal part of the Peninsular terrane along the northern fringe of the Chugach Mountains in southern Alaska. These masses, the roots of a mid‐Jurassic island arc, have been uplifted along the north dipping Border Ranges fault system and crop out sporadically for 700 km from Kodiak Island to eastern Alaska. The Tonsina ultramafic‐mafic assemblage is a layered sequence whose basal rocks are dunite and harzburgite (1 km thick) overlain by a narrow zone of websterite and clinopyroxenite. Above these ultramafic rocks is a 5‐ to 6‐km thick layered gabbro that is garnet bearing at its base. Textural relationships and chemical trends throughout the series of dunite, pyroxenite, garnet bearing gabbro and spinel gabbronorite point to an origin by igneous accumulation from a hydrous, tholeiitic parent magma at deep levels, i.e., at the base of a mature island arc before being emplaced along the Border Ranges fault. The rocks were crystallized from a melt at pressures between 9.5 and 11 kbar (28‐ to 33‐km depth) and equilibrated at temperatures of 800° to 90O°C based on phase relationships in the cumulus assemblages, pyroxene thermobarometry, and olivine‐spinel thermometry. Transitional sequences from ultramafic to mafic rocks are characterized by increasingly AL‐rich pyroxenes and spinels (or garnet) and the absence of olivine + plagioclase assemblages. Deep‐level fractionation of Al‐poor phases such as olivine and pyroxene at moderately high pressures and temperatures resulted in concentration of Al in the evolving magma. As crystallization continued, the phases recorded this alumina concentration and the resultant gabbros are Al‐rich. This observed trend supports the theory of high‐alumina basalt generation by fractional crystallization of olivine and pyroxene. Comparison of the Tonsina rocks with Aleutian lower crustal xenoliths as well as other exposures of arc‐related lower crustal magma chambers in Japan, Pakistan, and western North America indicates that this process is a common mechanism for island arc magma evolution at depth.
Alkaline magmatism within the segment 38°–39°S of the Plio‐Quaternary Volcanic Belt of the southern South American Continental Margin
Authors: Muñoz Jorge B. , Stern Charles R.
Published: February 2008

In the segment 38°–39°S of the southern South American volcanic belt, the subalkaline central‐vent volcanoes that form the late Quaternary volcanic front of the Andean orogenic arc are located in the Main Andean Cordillera 30–80 km west of the centers that formed the Pliocene/early Pleistocene volcanic front. The Pliocene/early Pleistocene volcanic front was located along a precordilleran uplift that diverges southeast from the Main Andean Cordillera. During the Pliocene the centers along this uplift erupted subalkaline magmas similar to those currently being erupted along the late Quaternary front. In the early Pleistocene the centers at the southeast end of this uplift began to erupt alkaline magmas. By the late Pleistocene, magmatic activity in these centers had diminished, and the volcanic front had migrated westward to its current location, but alkaline back arc basaltic volcanism continued in the late Quaternary within the valleys flanking the precordilleran uplift on which the Pliocene/early Pleistocene arc had been situated. The early Pleistocene alkaline orogenic arc lavas and the late Quaternary alkaline back arc basalts have La/Yb higher than, but Ba/La and 87Sr/86Sr similar to, the subalkaline magmas erupted from centers of both the Pliocene and late Quaternary volcanic fronts. Their TiO2 and La/Nb are intermediate between subalkaline magmas erupted from both the Pliocene and late Quaternary volcanic fronts and alkaline basalts of the Patagonian plateau lavas south of 39°S which have intraplate oceanic island petrochemical characteristics. Compared with the subalkaline lavas of the current volcanic front, Quaternary alkaline arc and back arc lavas between 38° and 39°S formed by lower degrees of partial melting of a similar mantle source. This mantle had been modified by source region contamination processes related to dehydration of subducted oceanic lithosphere such as occur below the current volcanic front in the southern Andes.
The volcanism of southern Italy: Role of subduction and the relationship between potassic and sodic alkaline magmatism
Authors: Ellam R. M. , Hawkesworth C. J. , Menzies M. A. , Rogers N. W.
Published: February 2008

The ultrapotassic magmatism of southern Italy (the Roman province) is well known, and recently these highly unusual lavas have been explained in terms of subduction‐related processes. Less well studied are the coeval calc‐alkaline to potassic rocks of the nearby Aeolian Islands, which are situated above a Benioff zone and are therefore demonstrably related to recently active subduction. On a number of geochemical diagrams the Roman and Aeolian provinces define continuous trends, which may be accommodated in a single petrogenetic model involving mixing of three isotopically and elementally distinct components. Two of these are subduction‐related: first, a high Sr/Nd, high Th/Ta component derived largely from basaltic ocean crust and, second, a component with extremely high Th/Ta, but relatively low Sr/Nd derived largely from subducted sediments. These are mixed with mantle wedge material which, prior to subduction, was characterised by highly radiogenic Pb isotope ratios, and is therefore comparable to the mantle source of Mount Etna volcanism. Thus it would appear that midplate tholeiitic to Na‐alkalic magmatism and continental margin calc‐alkaline to ultrapotassic magmas were derived from mantle sources which, prior to subduction, had similar isotopic signatures. This observation has important implications for the potential involvement of trace element and isotope enriched (OIB‐like) mantle in the genesis of subduction‐related volcanism.
Influence of fault plane heterogeneity on the seismic behavior in the Southern Kurile Islands Arc
Authors: Schwartz Susan Y. , Dewey James W. , Lay Thorne
Published: February 2008

The seismic behavior in the southern Kurile Islands Arc is strongly influenced by spatial heterogeneity in the mechanical properties of the plate interface. Maps showing sites of maximum seismic moment release for the great underthrusting earthquakes of 1958, 1963, 1969, and 1973 provide a first‐order image of the major regions of enhanced strength, or asperities. Hypocenters of these and an additional 83 smaller earthquakes with mb ≥ 5.3 and known underthrusting mechanisms were recomputed using the method of joint hypocenter determination in order to assess better the influence that the strength distribution exerts on the interplate seismicity patterns. Nearly all of the smaller‐magnitude earthquakes locate outside of the principal asperities of the great earthquakes. This observation is consistent with substantial release of accumulated strain energy in asperity regions during the great earthquakes and a redistribution of stress in adjacent areas following their rupture. An along‐dip, depth dependent component of strength heterogeneity appears to influence the seismicity patterns. The asperities of the 1958 and 1973 great earthquakes and hypocenters of most of the Ms ≥ 7 earthquakes occur near the downdip edge of the 100‐km‐wide seismically active plate interface. This would be expected if shear resistance on the interface increases monotonically with lithostatic pressure up to the brittle‐ductile or stick slip‐stable slip transition that defines the base of the seismogenic zone. The asperities of the 1963 and 1969 earthquakes, however, appear to be located well above the downdip edge of the coupled plate interface. The occurrence of large asperities in middepth ranges of the seismogenic interface suggests the influence of along‐dip changes in other parameters, besides lithostatic pressure, that control shear resistance. Along‐strike segmentation of the Kurile Islands thrust zone may be a consequence of along‐strike alternation in the location of the largest asperities between the deep interface and the midinterface.
Seismic structure of the crust and upper mantle in the Peace River Arch Region, Canada
Authors: Zelt C. A. , Ellis R. M.
Published: February 2008

Four intersecting ∼300‐km‐long reversed refraction lines within northwestern Alberta and northeastern British Columbia have been interpreted for crustal and upper mantle seismic velocity structure in the Peace River Arch (PRA) region of the Western Canada Sedimentary Basin. The data have been modeled using a two‐dimensional ray trace forward modeling approach based on asymptotic ray theory to match travel times and amplitudes of first and coherent later arrivals. In addition, an inversion of first arrival travel times along a fan shot profile has been performed to constrain crustal thickness northwest of the arch in a region not sampled by the reversed profiles. The major features of the structural models are (1) weak to moderate lateral variations in crustal structure with no evidence of significant layering or thick low‐velocity zones within the crust, (2) an average sub‐basement RMS crustal velocity of 6.6 km/s, average upper mantle velocity of 8.25 km/s and average crustal thickness of 40 km, (3) regional variations in structure which appear related to the dominant N‐S trending cratonic structure, including crustal thickness, upper crustal and upper mantle velocities and PmP character, and (4) subtle variations in structure that may be associated with the E‐W trending Devonian axis of the PRA, including a shallowing of high lower crustal velocities, thickening of the crust, and an anisotropic PmP character beneath the arch and a weak trend in the dip of intracrustal reflectors away from the center of the PRA region. Evidence from the refraction and other geophysical data for the presence of a local crustal expression of the PRA is weak, but suggests a thermal as opposed to flexural origin for its anomalous vertical movements.
A study of mantle layering beneath the western Pacific
Authors: Revenaugh Justin , Jordan Thomas H.
Published: February 2008

The reverberative interval of the seismogram, defined as the portion following the surface wave train propagating along the minor arc and ending with the first body wave arrivals from the major arc, provides an excellent window in which to observe subcritical reflections from mantle discontinuities. On an SH‐polarized seismogram the intervals between ScSn and sScSn wave groups (zeroth‐order reverberations) are approximately 15 min long and consist almost entirely of waves reflected one or more times from mantle discontinuities (first‐ and higher‐order reverberations). We have developed signal enhancement and waveform inversion schemes to progressively extract information pertaining to mantle layering from the waveforms of zeroth‐ and first‐order reverberations and have applied them to a data set of long‐period digital seismograms drawn from a number of tectonic regions in and around the western Pacific. The results place important constraints on the nature of the transition zone discontinuities. Measurements of the two‐way, vertical shear wave travel time from the free surface to the 400‐km and 650‐km discontinuities (τ400 and τ650 respectively) yield estimates of transition zone travel time, τTZ, largely uncontaminated by outside heterogeneity. Variations in the path‐averaged estimates of τTZ are of the order of 5.4 s, indicating significant lateral heterogeneity in the transition zone. Correlations of τTZ with τ400 and τLM (travel time through the lower mantle) are consistent with, but do not require, the thermal deflection of an exothermic phase transition near 400 km and an endothermic transition at 650 km, with ∼12 km of long‐wavelength (500–5000 km) topography on each. The amount of topography estimated for the 650‐km discontinuity is significantly less than predicted for a rigorously stratified mantle (>100 km). In a search for other mantle discontinuities, we find evidence for a strong reflector possibly associated with the onset of partial melting in the upper mantle (average depth of 86 km). Due to ambiguity inherent to the method, this feature can also be interpreted as an impedance increase in the D″ region of the lower mantle (70–170 km above the core‐mantle boundary). No evidence is found for a 220‐km discontinuity or the D″ discontinuity proposed by Lay and Helmberger (1983), suggesting that if such features are present in the western Pacific, they are either small (normal incidence, SH reflection coefficient R < 1.5%) or intermittent or have lateral depth variations well in excess of 50 km.
How preexisting weaknesses control the style of continental breakup
Authors: Dunbar John A. , Sawyer Dale S.
Published: February 2008

Continental breakup is a highly variable process. Differences occur in the relative timing and extent of volcanism, uplift, and graben formation as well as in the mode and amount of continental extension before breakup. We propose a model which reconciles this variability with the previously recognized tendency for breakup to occur along preexisting weak trends. Continental lithosphere is viewed as a composite material composed of two strong layers, one in the upper mantle and one in the middle crust. Finite element simulation indicates that extensional failure at weaknesses in the mantle causes concentrated extension in the mantle and diffuse extension in the crust. This leads to early melt segregation and volcanism, margin uplift during the late stages of the extension process, and relatively narrow symmetrical extended margins. In contrast, failure at weaknesses in the crustal strong zone causes focused extension in the crust and diffuse extension in the mantle. This produces initial graben formation, cooling in the lower crust and upper mantle, and broad asymmetrical extended margins. Volcanism only occurs late in the process. Failure at laterally offset weaknesses within both strong layers leads to a deformation pattern dominated simple shear. Thus differences in the prerift configuration of the continental lithosphere can control the overall style of continental breakup.
Compaction of basin sediments: Modeling based on time‐temperature history
Authors: Schmoker James W. , Gautier Donald L.
Published: February 2008

Porosity decrease of sandstones, carbonates, and shales during burial is modeled here in terms of time‐temperature exposure rather than in terms of depth. Loss of porosity (ϕ) in the subsurface is represented by a power function, ϕ = A(M)B, where A and B are constants and M is a measure of integrated time‐temperature history. Regression lines of carbonate and sandstone porosity upon Lopatin's time‐temperature index of thermal maturity (TTI) generally fall within a rather narrow envelope whose axis is approximated by ϕ = 30(TTI)−0.33. This equation is useful for regional modeling of carbonate and sandstone compaction. Dependence of porosity upon integrated time‐temperature history implies that basin sediments are not in equilibrium but compact through geologic time. Calculations show that subsidence resulting from the loss of porosity with increasing time‐temperature exposure (as opposed to deeper burial) can produce a second‐stage, passively formed basin containing many hundreds of meters of sediments.
A mechanical model for the inside corner uplift at a ridge‐transform intersection
Authors: Chen Yongshun
Published: February 2008

At least part of the inside corner uplift observed at slow ridge‐transform intersections is shown to be a consequence of the flexure of an elastic plate caused by a twisting moment exerted along the transform fault. The frictional drag exerted along the transform increases with depth (overburden) down to the depth where plastic flow predominates; this depth‐dependent drag results in a moment applied to the edge of the plate. An uplift of an inside corner of several hundred meters can be obtained by this mechanism. This can be increased to roughly 1 km (the observed uplift at some intersections) if we assume the elastic thickness of the plate is effectively thinner because of relaxation by long‐term creep of the stresses resisting plate flexure. Simple models formulated to test this hypothesis show that the observed inside comer uplift of the Vema and Oceanographer transforms can be explained if the “effective elastic thickness” of the plate is about half of the thickness over which drag increases with depth along the transform fault boundary.
Mechanics of fault junctions
Authors: Andrews D. J.
Published: February 2008

A sharp bend in a fault must be part of a triple junction, and slip on the three fault segments at the junction must have a vector sum of zero, in order to avoid an unphysical 1/r stress singularity. Accompanying the slip, a volume change occurs at the junction; either a void opens or intense local deformation is required to avoid material overlap. The energy absorbed due to the volume change is proportional to the slip increment times the total past slip accumulated at the junction. At a new junction the energy absorbed is a small fraction of the energy released by slip on the fault system, but after a number of earthquakes (proportional to shear modulus over confining stress) the junction becomes a strong barrier to further slip. Although slip occurs more easily on old rupture surfaces than on fresh fractures, the growing barrier strength of junctions requires that there be some fresh fracture in earthquakes. Perhaps a small fraction of the surface that slips in any earthquake is fresh fracture, which could provide the instability needed to explain earthquakes. A numerical model in two‐dimensional static plane strain shows, even without accounting for energy absorbed at the junction, that a bend in a fault acts as a barrier if slip is impeded on the associated fault spur. The stress concentration at the bend will tend to induce slip on the spur. Slip occurring on the spur unstably (with a drop in the coefficient of friction) can induce increased slip on the main fault segments with no change in the coefficient of friction. A fault junction provides a natural realization of barrier and asperity models without appealing to arbitrary variations of fault strength. The location of the fresh fracture that occurs after a junction becomes a strong barrier remains an unanswered question. It is likely to be initiated near the stress concentration at the old junction. A model simulating the effect of fresh fracture near old junctions might explain earthquakes without appealing to an unstable friction law anywhere.
The 1987 Whittier Narrows earthquake sequence in Los Angeles, southern California: Seismological and tectonic analysis
Authors: Hauksson Egill , Jones Lucile M.
Published: February 2008

The October 1, 1987, Whittier Narrows earthquake (ML = 5.9) was located at 34°2.96′N, 118°4.86′W, at a depth of 14.6±0.5 km in the northeastern Los Angeles basin. The focal mechanism of the mainshock derived from first motion polarities shows pure thrust motion on west striking nodal planes with dips of 25°±5° and 65°±5°, respectively. The aftershocks define an approximately circular surface that dips gently to the north, centered at the hypocenter of the mainshock with a diameter of 4–6 km. Hence the spatial distribution of the mainshock and aftershocks as well as the focal mechanisms of the mainshock indicate that the causative fault was a 25° north dipping thrust fault striking west and is confined to depths from 10 to 16 km. Although most of the 59 aftershock focal mechanisms presented here document a complex sequence of faulting, they are consistent with deformation of the hanging wall caused by the thrust faulting observed in the mainshock. A cluster of reverse faulting events on north striking planes occurred within hours after the mainshock, 2 km to the west of the mainshock. The largest aftershock (ML = 5.3) occurred on October 4 and showed mostly right‐lateral faulting on the same north‐northwest striking plane within the hanging wall. Similarly, several left‐lateral focal mechanisms are observed near the eastern edge of the mainshock rupture. The earthquake and calibration blast arrival time data were inverted to obtain two refined crustal velocity models and a set of station delays. When relocating the blast using the new models and delays, the absolute hypocentral location bias is less than 0.5 km. The mainshock was followed by nearly 500 locatable aftershocks, which is a small number of aftershocks for this magnitude mainshock. The decay rate of aftershock occurrences with time was fast, while the b value was low (0.67±0.05) for a Los Angeles basin sequence.
Near‐surface expression of the eastern part of the San Cayetano Fault: A potentially active thrust fault in the California Transverse Ranges
Authors: Çemen Ibrahim
Published: February 2008

Examined in this study are the near‐surface geometry, late Cenozoic geologic history, and seismic potential of the eastern part of the San Cayetano fault, a major north dipping thrust fault of the Transverse Ranges, California. The fault generally dips with a low angle close to the surface, but the dip increases up to 40° at about 5 km below sea level. The fault may become nearly horizontal at depth and root into a midcrustal detachment surface of the Transverse Ranges. The late Cenozoic reverse movement along the San Cayetano fault may have begun during the deposition of the lower part of the Saugus Formation. The fault has also moved after the deposition of the Saugus Formation within the last 0.2 m.y. The San Cayetano fault should be considered, at least, potentially active and may be capable of producing damaging earthquakes. This conclusion is based collectively on (1) the presence of several surficial geological features suggestive of recent movement along the fault, (2) the presence of microearthquakes on the fault plane, and (3) the structural similarity of the fault to seismically active north dipping thrust faults of the region.
An observational search for carbonates on Mars
Authors: Blaney Diana L. , McCord Thomas B.
Published: February 2008

A search for carbonates and other salts exposed on the surface of Mars was undertaken by observing areas of the surface in the 2.6‐μm to 4.2‐μm spectral region using the University of Hawaii 2.2‐m telescope at the Mauna Kea Observatory, Hawaii. Observations of four Mars areas (Syrtis Major, Hellas, Arabia, and an area northeast of Hellas) were made on the night of September 19, 1986. The reflectance spectra for two partially overlapping areas show a weak absorption band in the 3.8‐μm to 3.9‐μm region, which is in the spectral region where carbonate absorptions appear: 3.8‐μm to 4.0‐μm. The Mars area of overlap contains Syrtis Planum. The spectral feature (from about 3.76 μm to 3.95 μm) is clearly shortward of and broader than the calcium carbonate band (about 3.9 μm to 4.0 μm) and is at a slightly shorter wavelength position than magnesium carbonate (magnesite) bands (about 3.8 μm to 3.96 μm) as measured in this study. Positive identification of this band cannot be made by us at this time. Measurements for two other Mars areas show no evidence of carbonate or other bands. If the band is due to absorptions in the Mars surface material spectrum and if it is due to carbonates, then the results imply about 3 to 5 wt % of a carbonate such as magnesite on average in the upper few millimeters of the Mars surface material over the two areas where the weak spectral feature appears. We can place an upper limit of 3 to 5 wt % calcium carbonate and about 5 wt % magnesite for all areas observed. The interpretations are based on laboratory measurements of the reflectance spectra of calcite and magnesite and their mixtures with palagonite. Effects of thermal emission on the detection of carbonates were also explored for a wide range of temperatures. Thermal effects are not sufficient to mask the presence of calcite in the 3 to 5 wt % range. Theoretical reasoning strongly supporting the presence of carbonates as a component of the Mars surface is reviewed. These and earlier observations do not rule out carbonates below the surface in other locations, as small outcrops, or in concentrations of less than 3 to 5 % of the areas observed.
Effects of deviations from hydrostatic equilibrium on atmospheric corrections to satellite and lunar laser range measurements
Authors: Hauser James P.
Published: February 2008

The accuracy of the atmospheric correction to laser range measurements is one of the main limiting factors for distance measurements to retroreflectors on artificial satellites and on the Moon. The main part of this correction is currently obtained by using the local atmospheric pressure and zenith angle to determine the integrated atmospheric density along the observing path. One of the factors affecting this method of correction is possible deviations of the atmosphere from hydrostatic equilibrium for observation sites in mountainous areas. This article examines the integrated equations of motion for the atmosphere and estimates the effect of such deviations on the overall range error. It is found that the expected error is less than 1 cm most of the time, even for elevation angles as low as 20°.
Seismic strain rates in regions of distributed continental deformation
Authors: Ekström Göran , England Philip
Published: February 2008

We compare the rates and configurations of seismic deformation determined from moment tensor mechanisms of earthquakes in five continental regions with the rates and configurations expected if the seismicity reflected the relative plate motions in the regions. The five regions (the Aegean, Iran, California, Western New Guinea and Central Asia) are characterized by high levels of seismicity distributed over hundreds to thousands of kilometers from the accepted plate boundaries. The earthquake mechanisms are compiled primarily from the catalog of centroid moment tensor solutions, which is nearly complete over the interval 1977–1987 for earthquakes greater than MS ≈ 5. The thickness of the seismogenic layer in each region is constrained by the results of modeling broadband body wave phases and of previous earthquake studies. In the Aegean, California, and Western New Guinea, the observed configurations of deformation agree with a simple model of distributed accommodation of relative plate motion. In Iran and Central Asia, the moment tensors indicate that there is appreciable strain parallel to the strikes of the belts. We augment our 10‐year estimates of seismic deformation with the available historical moment and magnitude estimates for the five regions and find that moderate and large earthquakes account for between 25 and 70% of the deformation of the seismogenic layer.
Crack aperture statistics and pore space fractal geometry of westerly granite and rutland quartzite: Implications for an elastic contact model of rock compressibility
Authors: Wong Teng‐Fong , Fredrich Joanne T. , Gwanmesia Gabriel D.
Published: February 2008

Micromechanical models based on the elastic contact of nominally flat pore surfaces covered with asperities have been formulated by other workers to analyze the effect of microcracks on the elastic and transport properties of rock. In this study we employ quantitative stereology techniques to measure the crack surface area per unit volume as a function of microcrack aperture and then test quantitatively the Hertzian contact model developed by Walsh and Grosenbaugh (1979). Although Westerly granite and Rutland quartzite have similar pressure‐volumetric strain curves and crack porosity, the pore microstructures differ significantly and have features characteristic of igneous and metamorphic rocks, respectively. Our data show that the crack surface area decreases as a function of crack aperture and therefore imply that the exponential distribution used previously by Walsh and Grosenbaugh is not appropriate. However, the Hertzian contact model describes adequately the pressure dependence of compressibility when the appropriate crack aperture statistics are used. The crack aperture statistics data are also used to infer the fractal dimension of pore space roughness. The aperture statistics for both rocks can be fitted well with a power law, and fractal dimensions of 2.843 and 2.804 can be assigned to 85% and 65% of the crack surface of Westerly granite and Rutland quartzite, respectively.
Microstress and microcoercivity in multidomain grains
Authors: Xu Song , Merrill Ronald T.
Published: February 2008

Although microstress is well‐recognized as playing a very important, and sometimes dominant, role in “pinning” domain walls in minerals of interest to palaeomagnetists, it has often been treated in rock magnetism in a grossly oversimplified and incorrect manner. This paper attempts to review and extend the knowledge of how microstress in general and dislocations in particular affect the motion of domain walls. It is found that only such unrealistic cases as a large grain containing a single dislocation is the microcoercivity, hc, independent of domain wall thickness, w. In a first order approximation, hc is linearly proportional to (λwm)/Ms, where Ms is the saturation magnetization, λ is some appropriate magnetostriction constant, and m is a constant determined by the distribution of microstress. A reasonable upper limit for m for magnetite based on experimental studies is found to be 0.5. Both positive and negative values for m are allowed. Microstress acts like a narrow‐band‐pass filter in the “pinning” of domain walls. Domain walls that have thicknesses roughly 1/5 of the microstress wavelength are far more effectively impeded by the stress than are larger or smaller walls. This implies that scientists can reduce their focus to a narrow range of stress wavelengths that effectively “pin” domain walls and that it is inappropriate to use sinusoidal ranging stress fields to model microstress. Moreover, because domain wall thicknesses usually vary with temperature, the most effective pinning sites are also temperature dependent, even when defects are completely immobile.
Postspinel transformations in the system Mg2SiO4‐Fe2SiO4 and some geophysical implications
Authors: Ito Eiji , Takahashi Eiichi
Published: February 2008

The high‐pressure transformation in MgSiO3 and those in the spinel phases of compositions from Mg2SiO4 to (Mg0.5Fe0.5)2SiO4 in the Mg2SiO4‐Fe2SiO4 system were investigated using a uniaxial split‐sphere apparatus. The phase boundaries between ilmenite‐perovskite in MgSiO3 and between Mg2SiO4 spinel and the assemblage of MgSiO3 perovskite and MgO periclase were determined to be P(GPa) = 26.8–0.0025T(°C) and P(GPa) = 27.6–0.0028T(°C), respectively, in the temperature range 1000–1600°C. The pseudobinary diagrams for the system Mg2SiO4‐Fe2SiO4 were determined at temperatures of 1100°C and 1600°C. It was demonstrated that the magnesian spinel (with Fe/Mg + Fe < 0.22 at 1100°C and <0.26 at 1600°C) dissociates into perovskite and magnesiowüstite within an extremely narrow pressure interval (<0.15 GPa at 1600°C). The dissociation pressure was found to be almost independent of iron content and to coincide to that at 670 km depth within experimental uncertainties. These experimental results indicate that the sharpness of the 670‐km discontinuity may indeed be due to this dissociation in a peridotitic or pyrolitic mantle. The current status of our understanding of deep mantle mineralogy and chemistry is discussed based on recent high‐pressure and high‐temperature experiments.