bluuebunny/crossref_metadata_2025 huggingface dataset:
Seismic waves velocities and anisotropy in serpentinized peridotites from xigaze ophiolite: Abundance of serpentine in slow spreading ridge
Authors: Horen H. , Zamora M. , Dubuisson G.
Published: February 2004

The effect of serpentinization on seismic wave velocity and anisotropy has been analysed in 6 peridotite samples of the Xigaze ophiolite, having harzburgitic composition and a degree of serpentinization ranging from 3% to 70%. We found: i) P‐ and S‐ wave velocities are linearly correlated with serpentine content; ii) anisotropy of P‐ and S‐waves decreases with increasing serpentinization (while in fresh peridotites anisotropy is of the order of 6%, for 70% of serpentinization it drops to 1% for P‐waves, and to 2%, for S‐waves). Comparison of published laboratory velocity data in oceanic lithologies, like gabbros and dolerites, with serpentinized peridotite data shows the difficulty of distinguishing between gabbros and peridotites having a percentage of serpentine between 10 and 40% by seismic waves velocities only. This comparison suggests that seismic velocity data are consistent with the existence of partialy serpentinized peridotites (up to 40% of serpentinization) in the oceanic crust formed at slow‐spreading ridges in accordance with geological observation.
LF band terrestrial radio bursts observed by WIND/WAVES
Authors: Kaiser M. L. , Desch M. D. , Farrell W. M. , Steinberg J.‐L. , Reiner M. J.
Published: February 2004

We report on a constituent of Earth's radio spectrum that shows remarkable similarities to Jovian “type III” or quasi‐periodic bursts. These terrestrial bursts lie in the radio LF band, down to the in situ plasma frequency and are made of two spectral components. The lower frequency component exhibits relatively rapid negative frequency drifts, similar to type III solar bursts but on a much shorter time scale. It appears to emanate from a large source region and its characteristics are the same as those of isotropic terrestrial kilometric radiation described by other authors. The higher frequency component does not drift much in frequency, only lasts about 1 to 5 minutes and sometimes extends up to 500 kHz. It appears to emanate from a discrete source. This high frequency component was never reported before because it is often hidden by AKR events. It might be produced by a mechanism which differs from AKR. These LF bursts may belong to a more common class of radio bursts representing a previously unappreciated segment of Earth's radio spectrum.
A connectionist approach for automatic labeling of regional seismic phases using a single vertical component seismogram
Authors: Mousset E. , Cansi Y. , Crusem R. , Souchet Y.
Published: February 2004

We present an automatic system for regional seismic phase identification from monocomponent single station records. It is based on a neural network study of the spectrogram. A large dataset of regional events checked by experts has been used for the training step. A sophisticated neural network design allows the system to take into account the variability of the different regional seismic phases in a wide magnitude and distance range. On the training and test sets respectively, more than 85% and 70% of the data are correctly classified.
Hydrothermal venting and geothermal heating in Cascadia Basin
Authors: Thomson Richard E. , Davis Earl E. , Burd Brenda J.
Published: February 2004

Observations in Cascadia Basin on the eastern flank of the northern Juan de Fuca Ridge reveal significant bottom water modification as a result of regional conductive heating and local hydrothermal venting. Seafloor conductive heating occurs throughout the sedimented basin while hydrothermal fluid discharge is confined to small (∼1 km2) isolated igneous basement outcrops. In the northern sector of the plateaulike basin, the vertical fluxes of heat and mass associated with these seafloor processes lead to the formation of a 250‐ to 350‐m‐thick “geothermal boundary layer” characterized by anomalously high temperature, reduced vertical stability, and high dissolved silicate concentration. Using a basinwide average lithospheric heat flux of 0.3 W m−2 and the observed thermal anomaly structure of the water column, we obtain a mean residence time of 1 to 2 years for the deep water over Cascadia Basin. Detailed water property data collected in 1992 and 1993 within the immediate vicinity of three isolated igneous basement outcrops in the north‐central sector of the basin indicate that local bottom‐water heating arises from low‐temperature venting through the summit and flanks of the outcrops. Near the smallest edifice, especially well‐defined layers of anomalously warm, particle‐laden water were found within ±20 m of the outcrop summit depth of 2610 m. Maximum anomalies of temperature, light attenuation coefficient, and dissolved silicate concentration in the layers were 0.040°C, 0.015 m−1, and 5 μmol L−1, respectively. We estimate the local heat flux, Fo′ from the smallest outcrop to be (2.4±0.8)U × 109 W, where U (m s−1) is the mean horizontal current at the venting depth. For reasonable mean currents in the range 10−3 to 10−2 m s−1, we find Fo′ ≈ 0.2 to 2.4 × 107 W. Assuming that the depressed conductive heat flow of −0.05 W m−2 observed through the sedimented seafloor surrounding the smallest outcrop reflects the advective loss of heat through the outcrop, the radial distance over which crustal fluids must collect heat and converge on the outcrop is about 10 km.
Geochemical evolution of Phanerozoic magmatism in Transbaikalia, East Asia: A key constraint on the origin of K‐rich silicic magmas and the process of cratonization
Authors: Wickham Stephen M. , Litvinovsky Boris A. , Zanvilevich Ada N. , Bindeman Ilya N.
Published: February 2004

During the Phanerozoic, granitoid magmatism occurred on a vast scale in Transbaikalia and Mongolia in east central Asia. Within this huge and geologically complex region, many hundreds of individual plutons of syenite, quartz syenite, quartz monzonite, and K‐rich granite have been intruded within NE‐SW oriented belts 200–400 km wide and 2000–3000 km long. Five compositionally distinct, major stages of magmatism are distinguished: (1) Ordovician‐Silurian, (2) Devonian, (3) Early Permian I, (4) Early Permian II, and (5) Permo‐Triassic. Granites and syenites occur within each suite, but the proportion of syenite increases with time, as does the alkalinity of all magmas. Synplutonic mafic rocks are associated with plutons of all five suites and mafic/felsic magma‐mixing textures are common. Laser fluorination oxygen isotope analysis (Sharp, 1990) of titanite from granitoids indicates that within and among plutons of a given suite 18O/16O is fairly constant. This indicates that these magmas probably formed with a high degree of isotopic (and chemical) homogeneity. However, there is a progressive decrease in δ18O of titanite with time from about +6.5 in the earliest suite to about +1.5 in the youngest suite (corresponding to a decrease in magmatic δ18O from ∼+11 to ∼+6). The systematic evolution of geochemical and isotopic composition with time allows us to develop a model explaining the progressive increase in alkalinity (especially K2O) and decrease in δ18O over 250 m.y. of plutonism. These constraints may be satisfied by progressive hybridization of K‐rich (alkali) basalt magmas with crustally derived silicic melts, accompanied by fractional crystallization of K‐poor phases such as plagioclase and pyroxene (cf. Barker et al., 1975). Such a mechanism could form large volumes of syenitic residual liquids, having the appropriate isotopic composition. Younger suites were in part derived through remelting and hybridization of material formed or intruded during earlier magmatic episodes, leading to a progressive increase in overall alkalinity and decrease in δ18O. The repeated cycles of magmatism involved significant anorogenic crustal growth and a gradual cratonization of the continental crust of East Asia, caused by multiple melting and remelting events and associated large influxes of mantle‐derived alkalic mafic magma.
Mapping the fluid flow of the Mariana Mounds ridge flank hydrothermal system: Pore water chemical tracers
Authors: Wheat C. Geoffrey , McDuff Russell E.
Published: February 2004

We present a conceptual model of fluid circulation in a ridge flank hydrothermal system, the Mariana Mounds. The model is based on chemical data from pore waters extracted from piston cores and from push cores collected by deep‐sea research vessel Alvin in small, meter‐sized mounds situated on a local topographic high. These mounds are located within a region of heat flow exceeding that calculated from a conductive model and are zones of strong pore water upflow. We have interpreted the chemical data with time‐dependent transport‐reaction models to estimate pore water velocities. In the mounds themselves pore water velocities reach several meters per year to kilometers per year. Within about 100 m from these zones of focused upflow velocities decrease to several centimeters per year up to tens of centimeters per year. A larger area of low heat flow surrounds these heat flow and topographic highs, with upwelling pore water velocities less than 2 cm/yr. In some nearby cores, downwelling of bottom seawater is evident but at speeds less than 2 cm/yr. Downwelling through the sediments appears to be a minor source of seawater recharge to the basaltic basement. We conclude that the principal source of seawater recharge to basement is where basement outcrops exist, most likely a scarp about 2–4 km to the east and southeast of the study area.
Streaming potential and permeability of saturated sandstones under triaxial stress: Consequences for electrotelluric anomalies prior to earthquakes
Authors: Jouniaux Laurence , Pozzi Jean‐Pierre
Published: February 2004

The streaming potential, due to fluid circulation in rock, was measured on saturated sediments (Fontainebleau sandstones). The electrokinetic coupling coefficient, which is the ratio of the streaming potential and the excess pore pressure, is proportional to the fluid resistivity. Additionally, for a fluid conductivity of 10−3 S/m, the electrokinetic coupling coefficient varies from 10 to 6642 mV/0.1 MPa for sample permeability in the range of permeabilities from 0.15 × 10−15 to 1220 × 10−15 m2. The different values of the electrokinetic coupling coefficient have been explained by the effect of increasing surface conductivity which becomes nonnegligible compared to fluid conductivity for low permeability. When the sample is deformed under triaxial stress up to failure, the vertical permeability (along the principal stress) drops by about 0.20%/0.1 MPa when failure occurs. The typical variation of the electrokinetic coupling coefficient is a large increase beginning with the onset of the localization of the shear band at about 75% of the yield stress and stopping at the failure. This increase of the electrokinetic coupling coefficient is due to an increase of ζ potential in the shear zone when new surfaces are created and connected. Possible consequences of our results are given concerning the electrical fields which could appear during the preparation of an earthquake. It is shown that in some cases, self‐potential anomalies reported in the deformed zone preceding an earthquake occurrence could be due to an increase of the electrokinetic coupling coefficient from 75% of the yield stress to rupture in the vicinity of one of the electrodes. Any variation of fluid resistivity or permeability in the vicinity of one electrode could change the electrokinetic coupling coefficient, inducing a surface electrokinetic potential anomaly. In regard to the interpretation of the electrokinetic effect which occurs at large distance from the epicenter, a larger electrokinetic potential anomaly could be measured between electrodes situated along a vertical fluid flow, for instance, in a shallow borehole. An electrokinetic potential anomaly up to 30 mV, for a fluid conductivity of 0.01 S/m and a rock permeability of 10−12 m2, could be observed with a change of the underground water table level as slight as 50 cm (50 mbar). Moreover, if the permeability between the electrodes is increased by a factor of 8 × 103, the electrokinetic coupling coefficient could be enhanced by a factor up to 650.
An experimental deformation study of partially molten amphibolite: Application to low‐melt fraction segregation
Authors: Rushmer Tracy
Published: February 2004

An amphibolite has been experimentally deformed under subsolidus and partially molten conditions to evaluate the influence of melt on the mechanical behavior of a natural mafic rock and to assess low‐melt fraction segregation processes. Experiments were performed under fluid‐absent conditions at 1.8 GPa, between temperatures of 650°C and 1000°C. These conditions are similar to those of thickened lower continental crust or intermediate depths in subducting oceanic lithosphere. At ≥850°C, melt is granitoid in composition, and its viscosity is that of “wet granite”, 103–105 Pa s. The results can be summarized as follows: (1) Under subsolidus conditions (tests at 650°C and 750°C the amphibolite is macroscopically ductile and deformation is homogeneously distributed throughout the sample. (2) At near solidus conditions (≥800°C, ∼ 0–5 vol % melt), fractures (∼1–10 μm in width) displace hornblende and plagioclase grains, and melt, formed in situ, is found in some of these cracks. The formation of a ductile shear zone in one sample is attributed to the presence of very fine grained reaction products from combined dehydration/hydration reactions that involve plagioclase. The dehydration reaction products have apparently changed the deformation mechanism and even overrode the melt‐embrittlement process, trapping melt in pockets of lower strain. With higher melt fractions (∼10–15 vol %), broad, melt‐bearing shear zones form and grains within these zones are brittlely deformed. (3) At ∼20 vol % melt (1000°C), additional weakening occurs but fractures are not observed and the overall deformation is by viscous flow. The results show that at low melt fractions (<15 vol %) fluid‐absent melting reactions can induce fracture in previously ductile rocks. This suggests that the percentage at which melt may escape by fracture is lower than the theoretical critical melt fraction or CMF of 26–40 vol %. In general, estimating the fraction of melt at the onset of segregation cannot be predicted by the CMF. Melt segregation models have to be adapted to variations in such major factors as pressure, temperature, type of melting reaction, rate of melting, and strain rate. Such models will greatly assist our understanding of continental growth and evolution during orogenesis.
Neogene paleomagnetism and oroclinal bending of the central Andes of Bolivia
Authors: MacFadden Bruce J. , Anaya Federico , Swisher Carl C.
Published: February 2004

Geophysical and tectonic evidence suggests that the principal vertical axis bending of the Bolivian Orocline is a late Cenozoic event related to subduction and Andean uplift. To further test this hypothesis, paleomagnetic data are presented from Miocene basins within the Altiplano and Cordillera Oriental of Bolivia, located in the presumed southern limb of the orocline. To factor out potential VGP data, samples were collected from sedimentary sections spanning at least 0.5 m.y. New paleomagnetic data are presented for two sites, i.e., Gerdas (21°S latitude, 16–15 Ma) and Quehua (20°S latitude, 13–7 Ma). In conjunction with previously published paleomagnetic results from middle to late Tertiary rocks from Bolivia and Peru spanning some 16° of latitude (6–22°S), a consistent pattern of oroclinal development is as follows: significant counterclockwise rotation (about 15–20°) in the northern limb, negligible rotation in the axis (at the latitude of the Arica Deflection), and significant clockwise rotation (about 15–20°) in the southern limb. Although these paleomagnetic data could represent independent, mesoscale (>102 to <104 km2) block rotations, the preferred interpretation is that they confirm the hypothesis that a significant portion (about 30–40°) of the oroclinal bending occurred during the late Cenozoic. Using the unit mean paleomagnetic declinations in the well‐dated sections, minimum rates of oroclinal bending during the late Cenozoic are calculated to range from 0.6°/m.y. to 1.7°/m.y. The remaining 15–20° of observed curvature of the orocline probably resulted from an earlier phase of Andean deformation or may represent the shape of the ancestral Andes.
Alaska crustal deformation: Finite element modeling constrained by geologic and very long baseline interferometry data
Authors: Lundgren Paul , Saucier François , Palmer Randy , Langon Marc
Published: February 2004

We compute crustal motions in Alaska by calculating the finite element solution for an elastic spherical shell problem. The method we use allows the finite element mesh to include faults and very long baseline interferometry (VLBI) baseline rates of change. Boundary conditions include Pacific‐North American (PA‐NA) plate motions. The solution is constrained by the oblique orientation of the Fairweather‐Queen Charlotte strike‐slip faults relative to the PA‐NA relative motion direction and the oblique orientation from normal convergence of the eastern Aleutian trench fault systems, as well as strike‐slip motion along the Denali and Totschunda fault systems. We explore the effects that a range of fault slip constraints and weighting of VLBI rates of change has on the solution. This allows us to test the motion on faults, such as the Denali fault, where there are conflicting reports on its present‐day slip rate. We find a pattern of displacements which produce fault motions generally consistent with geologic observations. The motion of the continuum has the general pattern of radial movement of crust to the NE away from the Fairweather‐Queen Charlotte fault systems in SE Alaska and Canada. This pattern of crustal motion is absorbed across the Mackenzie Mountains in NW Canada, with strike‐slip motion constrained along the Denali and Tintina fault systems. In south central Alaska and the Alaska forearc oblique convergence at the eastern Aleutian trench and the strike‐slip motion of the Denali fault system produce a counterclockwise pattern of motion which is partially absorbed along the Contact and related fault systems in southern Alaska and is partially extruded into the Bering Sea and into the forearc parallel the Aleutian trench from the Alaska Peninsula westward. Rates of motion and fault slip are small in western and northern Alaska, but the motions we compute are consistent with the senses of strike‐slip motion inferred geologically along the Kaltag, Kobuk Trench, and Thompson Creek faults and with the normal faulting observed in NW Alaska near Nome. The nonrigid behavior of our finite element solution produces patterns of motion that would not have been expected from rigid block models: strike‐slip faults can exist in a continuum that has motion mostly perpendicular to their strikes, and faults can exhibit along‐strike differences in magnitudes and directions.
Transition from interplate slip to double seismic zone along the Kuril‐Kamchatka arc
Authors: Kao Honn , Chen Wang‐Ping
Published: February 2004

During subduction, the downgoing slab interacts with a strong overriding plate at shallow depths and with the surrounding asthenosphere at intermediate depths. Thus the transition from earthquakes representing interplate slip to those reflecting intraplate deformation within subducting lithosphere is important for understanding how the mechanics of subduction varies with depth. To characterize this transition, we studied 44 large to moderate sized earthquakes that occurred between 1964 and 1990 in two regions along the central portion of the Kuril‐Kamchatka arc. Precise earthquake source parameters are determined by analyzing body waveforms recorded at teleseismic distances. At depths of 30–50 km, our results delineate a transitional thrust zone, characterized by earthquakes showing thrust faulting on moderately dipping nodal planes. The northwesterly dipping nodal plane dips approximately 40°, some 10°–15° steeper than the dip of low‐angle earthquake faulting along the interplate thrust zone at depths shallower than 30 km. For earthquakes along the transitional thrust zone, secondary features in observed waveforms suggest that either a thick layer of sediments is present in the forearc region or the sources are embedded in materials with crustal seismic wave speeds. In the former case, the transitional thrust zone is interpreted as the deep‐seated portion of the plate interface whose dip increases with depth. The alternative implies that the transitional thrust zone occurrs below the plate interface within subducted crust of the Pacific plate and that the occurrence of earthquakes along the transitional zone may indicate a locked plate interface between depths of 30 and 50 km. The inference of a locked plate interface is consistent with the occurrence of compressional earthquakes beneath the outer rise region. However, a locked interface does not seem to affect the state of strain in the double seismic zone. The upper, compressional layer of the double seismic zone abuts the transitional thrust zone at a depth of approximately 50 km. At this depth, the two layers of seismicity in the double seismic zone are separated by as much as 30–40 km, too large a distance to be accommodated by models of seismogenesis invoking either the gabbro‐eclogite transition or dehydration of subducted oceanic crust.
Interseismic uplift at the Nankai subduction zone, southwest Japan, 1951–1990
Authors: Savage J. C.
Published: February 2004

Uplift as a function of time from 1951 through 1990 has been deduced from annual mean sea level measured at 15 tide gages along the Nankai subduction zone, southwest Japan. The recurrence interval for rupture of the Nankai subduction zone is about 100 years, and the most recent rupture was in late 1946. Thus the 1951–1990 uplift record covers most of the first half of the earthquake cycle. The precision of the uplift record is better than could be obtained currently by annual Global Positioning System (GPS) measurements. The pre‐1960 uplift record shows rapid deformation that appears to terminate in 1959. After 1959 the uplift record is remarkably linear in time: Significant curvature in the uplift‐versus‐time plots is detected at only three of the 15 tide gages. The inferred uplift rates are not quantitatively consistent with the predictions of either the viscoelastic coupling or elastic half‐space models of subduction, but the agreement is sufficient to suggest that the causative physical mechanisms have been identified. The immediate postseismic response is controlled by the propagation of slip downward along the downdip extension of the coseismic rupture, and the interseismic response is caused by the accumulation of a slip deficit on the main thrust zone. Asthenosphere relaxation is not required to explain the observations.
The 1990 Rudbar‐Tarom Iranian earthquake sequence: Evidence for slip partitioning
Authors: Gao Liping , Wallace Terry C.
Published: February 2004

The Rudbar‐Tarom Iran earthquake (June 20, 1990, MS= 7.7) occurred in the high Alborz mountains south of the Caspian Sea in northwest Iran. Although the structural style of the region is dominated by reverse faulting and folding, the focal mechanisms of the events indicate pure right‐lateral strike slip motion. We have studied the source parameters of the 11 largest aftershocks (MS > 4.6) using three different methodologies: (1) joint inversion of teleseismic and regional distance body waves; (2) single‐station moment tensor inversion (SSMT); and (3) comparison of entire wave trains to those of master events. The aftershocks show a clear bipolar distribution: thrusting earthquakes on NW‐SE trending planes and strike slip mechanisms with fault geometry similar to that of the mainshock. We interpret the faulting behavior of the 1990 earthquake sequence to indicate slip partitioning. In northwest Iran the Arabian and Eurasian plates are converging in a NE direction and the Alborz mountains are undergoing shortening and left‐lateral shear. This oblique motion results in dominantly thrusting earthquakes and infrequent strike slip earthquakes.
Crustal structure of the northern Yukon and Mackenzie Delta, northwestern Canada
Authors: O'Leary D. M. , Ellis R. M. , Stephenson R. A. , Lane L. S. , Zelt C. A.
Published: February 2004

Travel time inversion and amplitude forward modeling have been applied to two seismic refraction profiles from the northern Yukon‐Mackenzie Delta region of northwestern Canada. The two‐dimensional crustal P wave velocity models feature a near‐surface layer which is 1–7 km thick and has an average velocity of 4 km/s; this overlies three crustal units, each having an average thickness of 11–15 km and with average velocities of 5.9, 6.1, and 7.1 km/s. The Moho is at ∼37 km with little relief and overlies an upper mantle with a poorly constrained velocity. Tectonically, the study area lies between cratonic and Cordilleran North America and adjacent Mesozoic polar continental margin. The velocity models clearly illustrate a domainal crustal structure in the study area. A cratonic domain is characterized by a middle and lower crust with homogeneous velocities of 6.6–6.8 km/s. The other domain (“Yukon domain”) is characterized by midcrustal velocities near 6 km/s and a lower crustal layer with velocities near 7.1 km/s. The transition zone between these domains is well‐defined and is interpreted as a Proterozoic paleocontinental margin, supporting previous interpretations based on geological trends and potential field data. Lateral homogeneity of the crustal velocity structure within Yukon domain supports interpretations that Arctic Alaska was not emplaced into its present position on strike‐slip faults. Local variations in lower crustal thickness, together with clear wide‐angle Moho reflections, suggest a lower crustal and Moho signature possibly related to rifting, crustal extension, and magmatic intrusion and underplating during the Jura‐Cretaceous development of the Arctic Ocean and polar continental margin.
Décollement processes at the Nankai accretionary margin, southeast Japan: Propagation, deformation, and dewatering
Authors: Morgan Julia K. , Karig Daniel E.
Published: February 2004

The décollement zone, expressed on seismic profiles and observed in drill cores from the Nankai accretionary margin off the southeast coast of Japan, reveals several unique characteristics which appear to distinguish it from thrust faults identified in the same setting. Physical manifestations of these include evidence for the asymmetric distribution of deformation structures about the décollement, the extension of this fault zone well in front of the tectonic deformation front, and the absence of features indicative of precursory shear, for example, folded sediments, shear bands, and penetrative mineral fabrics. These characteristics suggest that the mode of formation and evolution of this décollement zone may be unique from that of most thrust faults. We propose that the décollement zone propagates not as a shear fracture controlled by tectonic stress conditions but rather as a subhorizontal tension fracture propagating under high pore pressures. To test this possibility, physical property measurements and clay mineral fabrics were obtained for several samples from the Nankai décollement zone using computed tomography and X ray texture methods. Our findings suggest that deformation within the décollement zone is partitioned into a volumetric component, preserved as reduced porosities within coherent fragments, and a localized shear component, evidenced by mineral preferred orientations along discrete slip surfaces. We suggest that the reduced porosities result from the destruction of “cementation” in the sediments during the early stages of deformation and may arise from cyclic fatiguing of the sediment induced by fluctuating pore pressures. The nonpenetrative shear fabrics probably develop as the tectonic deformation front migrates seaward, and the weakened protodécollement subsequently accommodates shear displacements along discrete fractures.
Regional extent of Great Valley basement west of the Great Valley, California: Implications for extensive tectonic wedging in the California Coast Ranges
Authors: Jachens Robert C. , Griscom Andrew , Roberts Carter W.
Published: February 2004

Interpretation and modeling of the magnetic field of central California indicate that the magnetic basement of the forearc deposits of the Great Valley sequence extends westward beneath the coeval subduction‐related rocks of the Franciscan Complex. The basement surface slopes gently to the west, reaching midcrustal depths (15–19 km) at distances of 50–100 km west of the Great Valley. This magnetic basement is disrupted by the Hayward‐Rodgers Creek Fault system and is cut by the San Andreas Fault at the south end of the Great Valley and possibly throughout much of central California. The widespread presence of the Great Valley basement beneath rocks of the Franciscan Complex implies that the basement is more extensive than proposed in earlier interpretations based on seismic studies near the Franciscan Complex‐Great Valley sequence contact. This result forces major modifications to ideas concerning this fossil subduction complex and other subduction zones. The eastern boundary fault of the Franciscan Complex (Coast Range Fault) is not (and never was) a subduction zone thrust fault but rather was originally a roof thrust (wedge‐roof fault) formed above the eastward wedging mass of Franciscan Complex intruded along the top of the basement beneath the Great Valley deposits. This tectonic interpretation offers a solution for the question of how high‐pressure metamorphic rocks of the Franciscan Complex were juxtaposed at the Coast Range Fault against low‐pressure metamorphic rocks of the Great Valley sequence. This interpretation also implies an older flat‐lying thrust fault (wedge‐floor fault) that forms the top of magnetic basement between the active San Andreas and Hayward Faults at depths of 15–17 km. This older thrust fault may today transfer strain between the two young strike‐slip faults, possibly explaining the apparent coupling of major nineteenth century earthquakes on these two faults. The former east dipping subduction zone along which the rocks of the Franciscan Complex accumulated must lie west of the western limit of the Great Valley magnetic basement.
Early Mesozoic rift stage half graben formation beneath the DeSoto Canyon salt basin, northeastern Gulf of Mexico
Authors: MacRae Grant , Watkins Joel S.
Published: February 2004

Multifold seismic reflection data provide the basis for recognition of an offshore Late Triassic‐Early Jurassic half graben complex beneath the DeSoto Canyon salt basin (DSCSB) along the northeastern Gulf of Mexico margin. The base of salt or equivalent (BSE) surface is a prominent unconformity recognized throughout the DSCSB that is mostly overlain and onlapped by extensive Middle Jurassic (Callovian age?) premarine evaporites (Louann salt and equivalents) and younger sedimentary rocks. Widespread dipping subsalt reflectors, truncated by the BSE, are the seismic expression of an interpreted thick section of synrift strata deposited within a half graben. The half graben probably overlies older prerift Paleozoic (and Precambrian?) sedimentary, igneous, and metamorphic rocks and widens basinward about a NE‐SW axis. Stratigraphic onlap relationships of subsalt reflectors close to the BSE surface suggest a lacustrine sequence may be present in the uppermost section of the rift fill; similar deposits occur within synrift strata below Aptian salt along the rifted West African margin. A NE‐ENE striking normal slip boundary fault beneath the Mississippi‐Alabama‐Rorida (MAFLA) shelf area and the inferred NW‐SE oriented Florida‐Bahamas transfer fault along the eastern margin of the DSCSB approximate the updip limit of rift fill. Trends of these structural lineaments and overall half graben morphology are similar to those of the South Georgia basin, a buried onshore early Mesozoic graben complex in northern Florida and southern Georgia, and the trend of exposed Triassic‐Jurassic continental rift basins along eastern North America. Structural architecture of the DSCSB half graben is consistent with NW‐SE rift phase extension during the early Mesozoic opening of the Gulf of Mexico.
Evidence for a role of the downgoing slab in earthquake slip partitioning at oblique subduction zones
Authors: Liu Xinping , McNally Karen C. , Shen Zheng‐Kang
Published: February 2004

A new model, incorporating shear deformation within a subducting slab, is proposed to explain slip partitioning for oblique plate motion at subduction zones. On the basis of investigation of 450 interplate earthquakes at 24 subduction zone segments we find that the degree of slip partitioning is largely correlated with the calculated slab pull force. Such correlation suggests that other than the upper plate deformation, the slab pull force plays an important role in controlling oblique subduction. Our model proposes that the force balance between the slab pull force, the interplate coupling resistance, and the viscous mantle drag (the latter two are passive forces to the former one) produces a lateral shear within the slab, which causes the slab to deform and change its motion direction gradually toward trench normal as it subducts. The amount of direction change, which would be observed as the slip partitioning during earthquakes, therefore is closely related to the major plate driving force at the subduction zone, which is the slab pull force in our model.
The transfer of slip between two en echelon strike‐slip faults: A case study from the 1992 Landers earthquake, southern California
Authors: Zachariasen Judith , Sieh Kerry
Published: February 2004

Detailed mapping of the fresh surficial ruptures in the right step between the Homestead Valley Fault and the Emerson Fault, two right‐lateral en echelon faults that broke in the 1992 Landers earthquake, indicates that the transfer of dextral slip between the faults is accommodated primarily by a series of obliquely striking right‐lateral strike‐slip cross faults. Right‐lateral slip on the cross faults and counterclockwise rotation of the interior blocks are sufficient to transfer virtually 100% of the dextral slip and to accommodate the extension across the jog. Asymmetry of the dextral slip curves along the cross faults indicates that some of them may have been induced to fail by slip on the Homestead Valley Fault, while others were induced to fail by slip on the Emerson Fault. Comparison of the magnitude of slip in 1992 to bedrock and geomorphic offsets suggests that the Homestead Valley Fault and several secondary faults have experienced about 100 nominal Landers events and that the stepover structure originated at the same time as the Homestead Valley Fault, about 1 m.y. ago. Because the Emerson Fault has an order of magnitude greater total offset but only slightly greater surficial slip in 1992, we conclude that it is a significantly older fault. Repetition of 1992‐like events should eventually lead to a merging of the Homestead Valley Fault with the Emerson.
A global tomographic model of shear attenuation in the upper mantle
Authors: Romanowicz B.
Published: February 2004

We present a global three‐dimensional model of shear attenuation in the upper mantle, based on the measurement of amplitudes of low‐frequency (100–300s) Rayleigh waves observed at stations of the Geoscope and Iris networks. Attenuation coefficients are measured on R1 and R2 paths using a method which minimizes the effects of focussing due to propagation in a three‐dimensional elastic Earth. Through a series of tests which, in particular, involve the computation of synthetic models of attenuation and focussing, we demonstrate that long wavelength lateral variations in attenuation in the first 400–500 km of the mantle can indeed be resolved. The model is obtained in a two‐step procedure. The first step consists in the computation of maps of Rayleigh wave attenuation at different periods, using an inversion method without a priori parametrisation, which involves the introduction of a correlation length, chosen here at 3000 km to optimize the trade‐off between resolution and variance in the model. In the second step, after corrections for shallow structure, an inversion with depth is performed, assuming lateral heterogeneity is confined to depths between 80 and 650 km. The resulting model presents lateral variations in Qβ that are correlated with tectonic features, in particular ridges and shields in the first 250 km of the upper mantle. Below that depth the pattern shifts and becomes correlated with the hotspot distribution, particularly so if the buoyancy strength of hotspots is taken into account. Two major low‐velocity zones appear to be located in the central pacific and beneath northern Africa, in the depth range 300–500 km. This pattern seems to continue at greater depth, but resolution becomes insufficient below 500 km to draw definitive conclusions. The smooth lateral variations retrieved are on the order of ±50% down to 400 km. We propose an interpretation in terms of plume/lithosphere/ridge interaction in the upper mantle, arguing for deflection of the bulk of hot upwelling material from plumes towards ridges, which may be occurring between 200 and 300 km depth.
Seismic studies of a bottom simulating reflection related to gas hydrate beneath the continental margin of the Beaufort Sea
Authors: Andreassen K. , Hart P. E. , Grantz A.
Published: February 2004

The upper continental margin of the Beaufort Sea, north of Alaska, is underlain by a strong bottom simulating reflection (BSR) that lies 300 to 700 m beneath the seafloor and corresponds to the phase boundary between interstitial water and natural gas below and solid gas hydrate above. BSRs of similar origin are common worldwide, where they are usually interpreted to mark the base of gas hydrate‐bearing clastic sediment, with or without underlying free gas in the sediment. Surprisingly little is known about the origin of these strong reflections. In this paper we analyze the contrasting physical properties which produce BSRs by comparing synthetic BSR amplitudes and waveforms for varying source‐receiver offsets with multichannel seismic reflection data across the well‐developed BSR of the Beaufort Sea. In order to discriminate whether free gas is present under the BSR or not, it was necessary to supplement near‐vertical incidence data with prestack offset data. The amplitude‐versus‐offset (AVO) analysis indicates that the BSR is produced mainly by the existence of free gas in the clastic sediments beneath the BSR. The zone of free gas is, based on vertical incidence synthetics, estimated to be thinner than 11–16 m. It is possibly thicker than 16 m if the gas concentration decreases with depth. Saturation of gas hydrate in the sediment above the BSR is tentatively estimated from the AVO modeling to be less than 10% of the pore volume.
A three‐dimensional gravity study of the Pacific‐Antarctic East Pacific Rise/Menard transform intersection
Authors: Enriquez Kelly D. , Chen Y. John
Published: February 2004

Three‐dimensional gravity analysis has been carried out over the eastern intersection of the Menard transform with the Pacific‐Antarctic East Pacific Rise at 49.8°S. Typical of the East Pacific Rise (EPR), the residual mantle Bouguer anomalies are nearly constant (varying less than 5 mGal) along the 60 km of ridge axis surveyed. This uniform character of the residual mantle Bouguer anomalies suggests that this part of the EPR axis has a fairly uniform crustal/upper mantle structure. Although this observation is less constrained by the only three crossings along the ridge axis, it is consistent with observations elsewhere along the EPR. Significant features such as thinner crust are not suggested by the residual mantle Bouguer anomalies (RMBA) at the ridge‐transform intersection or along the eastern 75 km of the Menard transform which is in marked contrast to what has been reported for the large gravity signatures at the slow spreading Mid‐Atlantic Ridge. The lack of the positive RMBA at the intersection could be due to the dual effects of a relatively small thermal contrast across the intersection and the excess lavas from the overshot ridge. At the ridge‐transform intersection, fresh lavas from the observed overshot ridge have filled in the transform valley and have subsequently thickened the crust from above, possibly compensating the effect of any thinner crust there. A large active seamount is present near the inside corner (about 40 km west of the ridge axis), and fresh lavas were observed surrounding the seafloor and between the seamount and the ridge. A large circular low in the residual mantle Bouguer anomalies surrounds the seamount, suggesting that both a thicker crust due to the active volcanism and hotter, less dense material are the source of the gravity low.
The heat flow constraint on mantle tomography‐based convection models: Towards a geodynamically self‐consistent inference of mantle viscosity
Authors: Pari Giovanni , Peltier W. Richard
Published: February 2004

We present a detailed study of the advective contribution to the radial flow of heat in the mantle as deduced using a compressible internal loading theory in which the flow is assumed to be driven by the density heterogeneities implied by recent global seismic tomographic models. We calculate the radial flow velocity response of a viscous mantle and find that for reasonable values of the parameters which enter the theory, the heat flow correlation integral delivers the correct area‐integrated value of the heat flow observed at Earth's surface. This result is unlikely to be significantly affected by the low‐degree truncation of the tomographic models we employ as in both the upper mantle and lower part of the lower mantle, heat is primarily transported by the degree 2 components of the flow. We propose that the radial profile of heat advection is a particularly useful diagnostic with which to “prospect” for the existence of thermal boundary layers in the mantle. For split mantle tomographic models, we find a sharp drop in advected heat at a depth of 670 km. We investigate in detail the rheological consequences of a circulation that is layered at this depth by the action of the endothermic phase transformation of spinel to a mixture of perovskite and magnesiowustite. On physical grounds this is expected to lead to the development of a dipolar viscosity structure centered on the internal thermal boundary layer. We investigate the impact that such a structure has on the predicted aspherical geoid. A sequence of forward modeling calculations of this geophysical observable demonstrates that a viscosity profile which includes a dipolar structure centered at 670 km depth and a significant increase of viscosity below mantle depths of 1200–1500 km optimally reconciles the long‐wavelength GEM‐T2 observations. The increase in viscosity in the lower part of the lower mantle is also required by the heat flow data whereas the introduction of the dipolar structure in the viscosity profile allows the upper mantle value of the viscosity which is close to that inferred by Haskell of 1021 Pa s to be continued to a depth of 1200–1500 km in accord with the requirements of recent postglacial rebound inferences. In the context of a whole mantle model of the circulation, we are also able to accommodate the constraints imposed by the data by introducing only a low‐viscosity notch at 670 km depth rather than the dipolar structure. The viscosity models derived herein therefore provide a fully self‐consistent reconciliation of these distinctly different geodynamic data and would appear to resolve a previously unresolved conflict between them.
Observations of the slow initial phase generated by microearthquakes: Implications for earthquake nucleation and propagation
Authors: Iio Yoshihisa
Published: February 2004

It is found that the initial rise of far‐field P wave velocity pulses generated by microearthquakes is not well represented by a ramp function but exhibits a gradual increase according to the function tn (2 < n < 4), where t is the time measured from the onset of the P wave. This slow rise, termed the slow initial phase, is detected in all 69 earthquakes analyzed here. Their seismic moments range between 108 and 1013 N m. No ramplike onsets are observed, suggesting that the slow rise is not an anomalous feature but is always generated by earthquakes of this size. The slow initial phase is not due to the transient response of the recording system. Analyses and simulations suggest that the slow initial phase is also not likely to result from an inhomogenuous velocity structure but that it may be partly due to anelastic attenuation. The slow initial phase likely results from the source process of the microearthquakes, especially those instances of the phase with a longer duration. The slow initial phase can not be explained by theoretical source models which assume constant dynamic friction and rupture velocity in an expanding fault. This is because these models predict a ramplike behavior in the initial rise of the far‐field P wave velocity pulse. Only the observed ramplike waveform following the slow initial phase can be explained by these models. The slow initial phase can be explained by models which predict slow slip velocities and/or rupture velocities immediately after rupture initiation, such as the slip‐weakening crack model. The duration of the slow initial phase is proportional to the rise time of the P wave velocity pulse. This dependence implies that longer, slow initial phases are generated by larger earthquakes, the duration of the slow initial phase being a measure of the earthquake size.
Nonlinear characteristics of paleoseismicity in China
Authors: Xu Xiwei , Deng Qidong
Published: February 2004

Detailed studies of historical seismicity and paleoseismological data for major late Quaternary faults in China reveal spatial and temporal patterns in paleoseismic activity associated with these faults. Analysis of space‐time distributions of late Quaternary faults and consideration of new models for deformation of the Asian continent provide a basis for proposing a division of China and surrounding areas into five active tectonic provinces. Slip rates for active faults are highest in the Himalayan collision zone between the Indian and Eurasian plates and gradually decrease away from the collision zone into the adjacent active tectonic provinces. Paleoearthquake recurrence intervals in the active tectonic provinces of China reflect seismogenic properties that are closely related to the position of these provinces relative to the converging Indian and Eurasian plates. Average recurrence intervals are <2000 years to as little as 310–900 years for active faults in the Qinghai‐Tibetan active tectonic province, whereas recurrence intervals are 2000–4500 years in the Xinjiang active tectonic province and longer than 2000 years to as much as 7250 years in the North China active tectonic province. Although average recurrence intervals are closely related to the slip rates of late Quaternary faults, no simple linear relationship exists between recurrence interval and slip rate. Further, analysis of paleoseismicity on major active faults shows that there are temporal variations in recurrence intervals. The case studies also indicate that for some provinces, faults, or fault segments there are three paleoseismic patterns: linear, logarithmic, and stepped. In the logarithmic and stepped patterns, the earthquake temporal sequence is characterized by active periods (earthquake clusters with relatively short recurrence intervals) and quiet periods (lack of clustering and relatively long recurrence intervals).