bluuebunny/crossref_metadata_2025 huggingface dataset:
Multiscale analysis of waves reflected by complex interfaces: Basic principles and experiments
Authors: Le Gonidec Yves , Gibert Dominique , Proust Jean‐Noël
Published: October 2002

This paper considers the reflection of waves by multiscale interfaces in the framework of the wavelet transform. First, we show how the wavelet transform is efficient to detect and characterize abrupt changes present in a signal. Locally homogeneous abrupt changes have conspicuous cone‐like signatures in the wavelet transform from which their regularity may be obtained. Multiscale clusters of nearby singularities produce a hierarchical arrangement of conical patterns where the multiscale structure of the cluster may be identified. Second, the wavelet response is introduced as a natural extension of the wavelet transform when the signal to be analyzed (i.e., the velocity structure of the medium) can only be remotely probed by propagating wavelets into the medium instead of being directly convolved as in the wavelet transform. The reflected waves produced by the incident wavelets onto the reflectors present in the medium constitute the wavelet response. We show that both transforms are equivalent when multiple scattering is neglected and that cone‐like features and ridge functions can be recognized in the wavelet response as well. Experimental applications of the acoustical wavelet response show how useful information can be obtained about remote multiscale reflectors. A first experiment implements the synthetic cases discussed before and concerns the characterization of planar reflectors with finite thicknesses. Another experiment concerns the multiscale characterization of a complex interface constituted by the surface of a layer of monodisperse glass beads immersed in water.
Correlation between volcanic and tectonic segmentation of fast‐spreading ridges: Evidence from volcanic structures and lava flow morphology on the East Pacific Rise at 9°–10°N
Authors: White Scott M. , Haymon Rachel M. , Fornari Daniel J. , Perfit Michael R. , Macdonald Ken C.
Published: October 2002

Combined analyses of volcanic features in DSL‐120 sonar data and Argo I images along the ridge crest of the East Pacific Rise, 9°09′–54′N reveal a consistent decrease in inferred lava effusion rate toward the ends of third‐order segments. The correlation of tectonic segmentation and volcanic style suggests that third‐order segmentation corresponds to the volcanic segmentation of the ridge. Along‐axis changes in volcanic structures (from collapse troughs to basaltic lava domes) and lava morphology (from sheet to pillow flows) coincide with the boundaries of morphologically defined third‐order tectonic segments of the ridge crest visible in shipboard multibeam bathymetry. Pillow lava flows cover 25% of the surveyed area of the ridge crest and are closely associated with small lava domes that occur primarily at third‐order segment ends. An additional 25% of the surveyed area of the ridge crest is covered by sheet lava flows found in close association with an axial collapse trough. The remaining terrain consists of lobate lava flows. We interpret the spatial correlations of morphologic, structural, seismic, and petrologic data as evidence that individual volcanic plumbing systems are organized at ∼20 km spacing along the ridge axis (third‐order segment scale) in agreement with the hypothesis that volcanic and tectonic segmentations are correlated. For fast spreading ridges, we estimate that the longevity of volcanic segments is ∼104–105 years, 1–3 orders of magnitude longer than fourth‐order segments (∼102–103 years). This implies the present pattern of hydrothermal activity may reorganize tens or hundreds of times while volcanic segmentation remains fairly stable.
Anatomy of apparent seasonal variations from GPS‐derived site position time series
Authors: Dong D. , Fang P. , Bock Y. , Cheng M. K. , Miyazaki S.
Published: October 2002

Apparent seasonal site position variations are derived from 4.5 years of global continuous GPS time series and are explored through the “peering” approach. Peering is a way to depict the contributions of the comparatively well‐known seasonal sources to garner insight into the relatively poorly known contributors. Contributions from pole tide effects, ocean tide loading, atmospheric loading, nontidal oceanic mass, and groundwater loading are evaluated. Our results show that ∼40% of the power of the observed annual vertical variations in site positions can be explained by the joint contribution of these seasonal surface mass redistributions. After removing these seasonal effects from the observations the potential contributions from unmodeled wet troposphere effects, bedrock thermal expansion, errors in phase center variation models, and errors in orbital modeling are also investigated. A scaled sensitivity matrix analysis is proposed to assess the contributions from highly correlated parameters. The effects of employing different analysis strategies are investigated by comparing the solutions from different GPS data analysis centers. Comparison results indicate that current solutions of several analysis centers are able to detect the seasonal signals but that the differences among these solutions are the main cause for residual seasonal effects. Potential implications for modeling seasonal variations in global site positions are explored, in particular, as a way to improve the stability of the terrestrial reference frame on seasonal timescales.
Growth and lateral propagation of fault‐related folds in the Siwaliks of western Nepal: Rates, mechanisms, and geomorphic signature
Authors: Champel Bénédicte , van der Beek Peter , Mugnier Jean‐Louis , Leturmy Pascale
Published: October 2002

We study the controls on drainage development in tectonically active regions using a numerical tectonic surface processes model combining tectonic uplift caused by fault‐related folding with erosion by fluvial incision, hillslope diffusion, and landsliding. Our model shows the fundamental control exerted by the dip of the detachment underlying the folds on drainage evolution. When the detachment is horizontal, the relative rates of tectonic uplift and fluvial incision control the evolution. For a nonzero dip, in contrast, the lateral displacement gradient associated with fold propagation sets up a lateral slope behind the active structure, which deflects the stream network. We also demonstrate the importance of landsliding for the attainment of realistic and steady state topography. In contrast, discontinuous tectonic movements do not appear to influence the system. We apply our model to the Dundwa fault‐related fold ridge (Siwalik, Nepal). We estimate a remarkably low mean propagation rate for different segments of the structure. This finding, together with the structure and morphology of the ridge, leads us to propose that the ridge developed by linkage of several component segments. The drainage evolution predicted when modeling this scenario compares favorably with field observations. Our models provide insights into the dynamics of fault‐related fold propagation: In particular, as a result of fault segment linkage, estimates of propagation rate of these structures may be strongly scale‐dependent, and the observed morphology and drainage patterns on fault‐related fold ridges may be controlled by fault geometry rather than by the relative rates of tectonics and surface processes.
Strain accumulation and rotation in western Nevada, 1993–2000
Authors: Svarc J. L. , Savage J. C. , Prescott W. H. , Ramelli A. R.
Published: October 2002

The positions of 44 GPS monuments in an array extending from the Sierra Nevada at the latitude of Reno to near Austin, Nevada, have been measured several times in the 1993–2000 interval. The western half of the array spans the Walker Lane belt, whereas the eastern half spans the central Nevada seismic zone (CNSZ). The principal strain rates in the Walker Lane belt are 29.6 ± 5.3 nstrain yr−1 N88.4°E ± 5.4° and −12.8 ± 6.0 nanostrain yr−1 N01.6°W ± 5.4°, extension reckoned positive, and the clockwise (as seen from above the Earth) rotation rate about a vertical axis is 13.6 ± 4.0 nrad yr−1. The quoted uncertainties are standard deviations. The motion in the Walker Lane belt can then be represented by a zone striking N35°W subject to 16.8 ± 4.9 nstrain yr−1 extension perpendicular to it and 19.5 ± 4.0 nstrain yr−1 right‐lateral, simple shear across it. The N35°W strike of the zone is the same as the direction of the local tangent to the small circle drawn about the Pacific‐North America pole of rotation. The principal strain rates for the CNSZ are 46.2 ± 11.0 nstrain yr−1 N49.9°W ± 6.0° and −13.6 ± 6.1 nstrain yr−1 N40.1°E ± 6.0°, and the clockwise rotation rate about a vertical axis is 20.3 ± 6.3 nrad yr−1. The motion across the CNSZ can then be represented by a zone striking N12°E subject to 32.6 ± 11.0 nstrain yr−1 extension perpendicular to it and 25.1 ± 6.3 nstrain yr−1 right‐lateral, simple shear across it. The N12°E strike of the zone is similar to the strikes of the faults (Rainbow Mountain, Fairview Peak, and Dixie Valley) within it.
Plume‐driven plumbing and crustal formation in Iceland
Authors: Allen Richard M. , Nolet Guust , Morgan W. Jason , Vogfjörd Kristín , Nettles Meredith , Ekström Göran , Bergsson Bergur H. , Erlendsson Pálmi , Foulger G. R. , Jakobsdóttir Steinunn , Julian Bruce R. , Pritchard Matt , Ragnarsson Sturla , Stefánsson Ragnar
Published: October 2002

Through combination of surface wave and body wave constraints we derive a three‐dimensional (3‐D) crustalSvelocity model and Moho map for Iceland. It reveals a vast plumbing system feeding mantle plume melt into upper crustal magma chambers where crustal formation takes place. The method is based on the partitioned waveform inversion to which we add additional observations. Love waves from six local events recorded on the HOTSPOT‐ SIL networks are fitted,Sntravel times from the same events measured, previous observations of crustal thickness are added, and all three sets of constraints simultaneously inverted for our 3‐D model. In the upper crust (0–15 km) an elongated low‐velocity region extends along the length of the Northern, Eastern and Western Neovolcanic Zones. The lowest velocities (−7%) are found at 5–10 km below the two most active volcanic complexes: Hekla and Bárdarbunga‐Grímsvötn. In the lower crust (>15 km) the low‐velocity region can be represented as a vertical cylinder beneath central Iceland. The low‐velocity structure is interpreted as the thermal halo of pipe work which connects the region of melt generation in the uppermost mantle beneath central Iceland to active volcanoes along the neovolcanic zones. Crustal thickness in Iceland varies from 15–20 km beneath the Reykjanes Peninsula, Krafla and the extinct Snæfellsnes rift zone, to 46 km beneath central Iceland. The average crustal thickness is 29 km. The variations in thickness can be explained in terms of the temporal variation in plume productivity over the last ∼20 Myr, the Snæfellsnes rift zone being active during a minimum in plume productivity. Variations in crustal thickness do not depart significantly from an isostatically predicted crustal thickness. The best fit linear isostatic relation implies an average density jump of 4% across the Moho. Rare earth element inversions of basalt compositions on Iceland suggest a melt thickness (i.e., crustal thickness) of 15–20 km, given passive upwelling. The observed crustal thickness of up to 46 km implies active fluxing of source material through the melt zone by the mantle plume at up to 3 times the passive rate.
Seismic evidence for fluid migration accompanying subsidence of the Yellowstone caldera
Authors: Waite Gregory P. , Smith Robert B.
Published: October 2002

Seismicity of the Yellowstone volcanic field, northwest Wyoming, is characterized by swarms of earthquakes (MC< 3) within the 0.64‐Myr‐old, 70 km by 40 km Yellowstone caldera and between the caldera and the eastern end of the 44‐km‐long rupture of theMS7.5 1959 Hebgen Lake, Montana, earthquake. Over 3000 earthquakes withMC< 5 were recorded during the largest historic swarm that spanned >3 months beginning in October 1985. The swarm had unusual characteristics indicative of interaction between seismicity and hydrothermal/magmatic activity: (1) the swarm followed the reversal of caldera‐wide uplift of up to 1 m from 1923 to 1984 to subsidence; (2) swarm hypocenters occupied a nearly vertical northwest trending zone, and during the first month of activity, the pattern of epicenters migrated laterally away from the caldera at an average rate of 150 m/d; (3) the dominant focal mechanisms of the swarm were oblique‐normal to strike‐slip contrasting with the normal‐faulting mechanisms typical of the region; and (4) the maximum principal stress axis averaged for the swarm events was rotated 90° from that of the normal background seismicity, from vertical to horizontal with a trend 30° from the strike of the plane defined by the swarm. We examined analytic models that best fit the focal mechanisms and the orientation of the plane defined by the swarm and found that the temporal shift of earthquake activity could be explained by the migration of hydrothermal fluids radially outward from the Yellowstone caldera following rupture of a sealed hydrothermal system within the caldera.
Modeling S wave amplitude patterns for events in the Kurile slab using three‐dimensional Gaussian beams
Authors: Pankow Kris L. , Lay Thorne
Published: October 2002

The shear velocity structure of the subducted Kurile slab is modeled using shear wave amplitude residual spheres. We measure long‐period SH and ScSH phase amplitudes for 10 intermediate and deep focus earthquakes recorded at World‐Wide Standardized Seismograph Network and Canadian Seismic Network stations from 1967 to 1986. After correction for source radiation pattern, geometric spreading, and empirical receiver/upper mantle effects, the data for intermediate depth Kurile slab events show smoothly varying amplitude residual sphere patterns with low amplitudes concentrated in the direction down the slab dip. The data are modeled using three‐dimensional dynamic ray tracing and a three‐dimensional Gaussian beam method. Starting models are taken from previous P and S wave arrival time residual sphere studies. Three‐dimensional ray tracing indicates that ray patterns and resulting amplitudes are very sensitive to the depth extent of the slab and to the position of the event relative to the across‐slab velocity gradient. This sensitivity allows us to place some constraints on the Kurile slab geometry, position of each event within the slab, and penetration depth of coherent slab structure. Our final slab model is defined by an asymmetric error function that peaks at 4% higher velocity than Preliminary Reference Earthquake Model 25 km from the top of the slab. The total width of the thermal anomaly averages 140 km. The slab is tabular and extends to 670 km in the northern half of the Kurile subduction zone. In the southern half of the subduction zone the slab flattens along the 670 km discontinuity and deflects laterally, persisting to 750 km depth.
Phase transitions in pyrolitic mantle around 670‐km depth: Implications for upwelling of plumes from the lower mantle
Authors: Hirose Kei
Published: October 2002

Phase relations in the natural pyrolitic mantle composition (KLB‐1) around 670‐km depth have been determined at 1600–2200°C by high‐pressure experiments using multianvil apparatus. A phase transition between majorite garnet and Al‐bearing Mg‐rich perovskite occurs at depths similar to the postspinel phase transition. The seismic discontinuity observed at this depth could be caused by a combination of both transitions. The majorite‐perovskite transition boundary has a positive Clausius‐Clapeyron slope (+0.0013 GPa/°C for majorite‐out curve), in contrast to the negative slope of the postspinel phase boundary (−0.0028 GPa/°C). Both transition boundaries cross each other at 1700–1800°C. Below this temperature, formation of Mg‐perovskite starts at the majorite‐perovskite or akimotoite‐perovskite transition coexisting with ringwoodite but is predominantly formed by the postspinel phase transition at higher pressures. On the other hand, the stability of majorite significantly expands relative to Mg‐perovskite at higher temperatures. Majorite becomes a dominant postspinel phase, and a majority of Mg‐perovskite is formed by the majorite‐perovskite transition with a positive Clapeyron slope. Phase transition within high‐temperature plumes (>1800°C at 670‐km depth) assists their upwelling from the lower mantle through the 670‐km boundary. The depth of the 670‐km seismic discontinuity becomes much less temperature‐sensitive and is greater in such a higher temperature region.
Electrical properties of zeolitized volcaniclastic materials
Authors: Revil A. , Hermitte D. , Spangenberg E. , Cochemé J. J.
Published: October 2002

The interpretation of electromagnetic anomalies associated with volcanic activity requires a good understanding of two rock properties, the electrical conductivity and the streaming potential coupling coefficient. We measured these properties on 22 consolidated tuff samples containing clays and zeolites typically found in hydrothermal systems or in other areas of high water‐rock interactions associated with active volcanic areas. These rocks exhibit unusually high surface conductivity and electrical cementation exponent (2.5–3.3). These features are explained by the highly complex texture of these rocks due to hydration/dissolution processes of the volcanic glass and the formation of clay minerals and zeolites as alteration products. At a pore fluid conductivity of 1.0 ± 0.2 S m−1, the streaming potential coupling coefficient ranges from −3.55 to −10.7 mV MPa−1. The zeta potential (a key electrochemical property of the pore water/mineral interface) determined from these measurements at T0 = 20°C and pH ∼6–7 and corrected for surface conductivity is ∼−28 ± 8 mV at 0.1 M NaCl. Two clay‐free samples exhibit a zeta potential ∼−16 ± 5 mV, a value associated with zeolites (clinoptilolite). The temperature dependence of the ζ potential is rather strong with ζ(T) = ζ(T0) [1 + νζ (T − T0)], where νζ ≈ 4.2 × 10−2 C−1 in the temperature range 20–60°C. These data are applied to the understanding of large‐scale self‐potential anomalies located at the ground surface of Miyake‐Jima volcano.
The 12 September 1999 Upper East Rift Zone dike intrusion at Kilauea Volcano, Hawaii
Authors: Cervelli P. , Segall P. , Amelung F. , Garbeil H. , Meertens C. , Owen S. , Miklius A. , Lisowski M.
Published: October 2002

Deformation associated with an earthquake swarm on 12 September 1999 in the Upper East Rift Zone of Kilauea Volcano was recorded by continuous GPS receivers and by borehole tiltmeters. Analyses of campaign GPS, leveling data, and interferometric synthetic aperture radar (InSAR) data from the ERS‐2 satellite also reveal significant deformation from the swarm. We interpret the swarm as resulting from a dike intrusion and model the deformation field using a constant pressure dike source. Nonlinear inversion was used to find the model that best fits the data. The optimal dike is located beneath and slightly to the west of Mauna Ulu, dips steeply toward the south, and strikes nearly east‐west. It is approximately 3 by 2 km across and was driven by a pressure of ∼15 MPa. The total volume of the dike was 3.3 × 106 m3. Tilt data indicate a west to east propagation direction. Lack of premonitory inflation of Kilauea's summit suggests a passive intrusion; that is, the immediate cause of the intrusion was probably tensile failure in the shallow crust of the Upper East Rift Zone brought about by persistent deep rifting and by continued seaward sliding of Kilauea's south flank.
Subsidence above a planar reservoir
Authors: Walsh J. B.
Published: October 2002

A tabular reservoir, circular in plan, is located at depth parallel to the surface. Compaction of the reservoir as fluid is withdrawn causes a change in the stress and displacement fields in the surrounding medium. A solution to this problem is obtained by determining the boundary conditions at the interface between the reservoir and the half‐space using Eshelby's [1957] cut‐and‐weld technique. The displacement field in the reservoir and the surrounding medium is then calculated using the reciprocal theorem. The displacement field is found to be the same as that calculated by Geertsma [1973a, 1973b] and by Segall [1992]Segall et al. [1994] using another technique.
Using shear wave amplitude patterns to detect metastable olivine in subducted slabs
Authors: Pankow Kris L. , Williams Quentin , Lay Thorne
Published: October 2002

Olivine within the lowest temperature regions of subducting oceanic lithosphere may persist metastably to depths well below the equilibrium phase boundary for transformation to β phase. This may produce a tapering low seismic velocity wedge within the slab in the transition zone; transformational faulting of such metastable olivine has been proposed as a possible explanation of deep earthquakes. Detecting seismic velocity structure associated with metastable olivine within a slab using seismic wave arrival time information alone has proven problematic. Motivated by previous two‐dimensional numerical calculations for upgoing and downgoing seismic waves, a three‐dimensional Gaussian beam method is used to assess whether the azimuthally varying teleseismic amplitude signature produced by a slab with an internal wedge of metastable olivine is distinguishable from that for a slab with equilibrium transitions. Five slab velocity structures are considered: a thermal model with horizontal phase transitions at the same depths as in ambient mantle, two models incorporating equilibrium transitions within the slab, and two models with tongues of metastable olivine. A depth‐ and temperature‐dependent thermal conductivity term and the effect of the latent heat of transformation of olivine to β phase are included in the thermal modeling. Three‐dimensional dynamic ray tracing and Gaussian beam summation are performed for events located at different depths and lateral positions for each of the slab models. For sources near 400 km depth the presence of low‐velocity metastable olivine produces relatively complex azimuthal shear wave amplitude patterns compared to thermal and equilibrium models. Comparison with observed amplitudes for four events in the Kurile slab indicates that allowing for internal slab velocity complexity in the transition zone provides better agreement with observations than simple thermal and equilibrium slab models. This indicates that careful modeling of teleseismic shear wave amplitude patterns may have the resolution to detect and characterize the presence of metastable olivine in deep slabs.
Experimental reassessment of the Shaw paleointensity method using laboratory‐induced thermal remanent magnetization
Authors: Pan Yongxin , Shaw John , Zhu Rixiang , Hill Mimi J.
Published: October 2002

Artificial thermoremanent magnetization imposed on natural basalt samples was used to check the reliability of Shaw's palaeointensity method [Shaw, 1974; Kono, 1978; Rolph and Shaw, 1985] and to give an indication of the accuracy of paleointensity determinations. First, thermal remanence was induced in all samples (50 in total) in a known field (50 ± 0.1μT). This remanence was regarded as the “original” natural remanent magnetization (“NRM”). The samples were divided in to three subsets (A, B, and C) and intensity experiments were performed. Each subset of samples used different thermal remanent magnetization (TRM) acquisition parameters. Rock‐magnetic properties on 14 representative subsamples were also measured both before and after the TRM acquisition steps to monitor possible magnetochemical alteration. It has been demonstrated that intensity estimates by the Shaw method give values close to the expected value (50 ± 5μT) in all three sets of basalt samples. The modified method with Rolph and Shaw [1985] correction, i.e., using ratios of anhysteretic remanent magnetization given before the TRM step (ARM1) and after (ARM2), is proved to be a powerful approach to correct the TRM capacity changes that result from magnetochemical alteration. This is especially so for sample set C, where the samples were heated up to 700°C and then were held at this temperature for 40 min during TRM acquisition. After correction, 45% of set C samples gave acceptable results. However, some samples with strong magnetochemical alteration sometimes invalidated the ARM1/ARM2 ratio correction and gave wrong intensity estimates. Monitoring rock magnetic properties is therefore very important when applying the Shaw method.
Cellular automaton model for recrystallization, fabric, and texture development in polar ice
Authors: Ktitarev Dmitri , Gödert Günter , Hutter Kolumban
Published: October 2002

A model for simulations of the ice microstructure evolution during its deformation is presented. Several competing processes are considered which lead to the development of anisotropic crystal c axes orientation distribution. Existing theories of the plastic deformation and recrystallization dynamics are combined in a discrete algorithm based on the cellular automata approach. We simulate the evolution of the fabric and texture, that is, orientations and sizes of single crystals of polycrystalline ice which experiences uniaxial compression, the loading condition close to that at the summit of the Greenland Ice Sheet. The results of our simulations are in a good qualitative agreement with the data obtained from the Greenland Ice Core Project ice core drilled at this location.
Strain accumulation and rotation in western Oregon and southwestern Washington
Authors: Svarc J. L. , Savage J. C. , Prescott W. H. , Murray M. H.
Published: October 2002

Velocities of 75 geodetic monuments in western Oregon and southwestern Washington extending from the coast to more than 300 km inland have been determined from GPS surveys over the interval 1992–2000. The average standard deviation in each of the horizontal velocity components is ∼1 mm yr−1. The observed velocity field is approximated by a combination of rigid rotation (Euler vector relative to interior North America: 43.40°N ± 0.14°, 119.33°W ± 0.28°, and 0.822 ± 0.057° Myr−1 clockwise; quoted uncertainties are standard deviations), uniform regional strain rate (εEE = −7.4 ± 1.8, εEN = −3.4 ± 1.0, and εNN = −5.0 ± 0.8 nstrain yr−1, extension reckoned positive), and a dislocation model representing subduction of the Juan de Fuca plate beneath North America. Subduction south of 44.5°N was represented by a 40‐km‐wide locked thrust and subduction north of 44.5°N by a 75‐km‐wide locked thrust.
Location and mechanism of the Little Skull Mountain earthquake as constrained by satellite radar interferometry and seismic waveform modeling
Authors: Lohman Rowena B. , Simons Mark , Savage Brian
Published: October 2002

We use interferometric synthetic aperture radar (InSAR) and broadband seismic waveform data to estimate source parameters of the 29 June 1992, Ms 5.4 Little Skull Mountain (LSM) earthquake. This event occurred within a geodetic network designed to measure the strain rate across the region around Yucca Mountain. The LSM earthquake complicates interpretation of the existing GPS and trilateration data, as the earthquake magnitude is sufficiently small that seismic data do not tightly constrain the epicenter but large enough to potentially affect the geodetic observations. We model the InSAR data using a finite dislocation in a layered elastic space. We also invert regional seismic waveforms both alone and jointly with the InSAR data. Because of limitations in the existing data set, InSAR data alone cannot determine the area of the fault plane independent of magnitude of slip nor the location of the fault plane independent of the earthquake mechanism. Our seismic waveform data tightly constrain the mechanism of the earthquake but not the location. Together, the two complementary data types can be used to determine the mechanism and location but cannot distinguish between the two potential conjugate fault planes. Our preferred model has a moment of ∼3.2 × 1017 N m (Mw 5.6) and predicts a line length change between the Wahomie and Mile geodetic benchmarks of ∼5 mm.
High‐resolution image of Calaveras Fault seismicity
Authors: Schaff David P. , Bokelmann Götz H. R. , Beroza Gregory C. , Waldhauser Felix , Ellsworth William L.
Published: October 2002

By measuring relative earthquake arrival times using waveform cross correlation and locating earthquakes using the double difference technique, we are able to reduce hypocentral errors by 1 to 2 orders of magnitude over routine locations for nearly 8000 events along a 35‐km section of the Calaveras Fault. This represents ∼92% of all seismicity since 1984 and includes the rupture zone of the M 6.2 1984 Morgan Hill, California, earthquake. The relocated seismicity forms highly organized structures that were previously obscured by location errors. There are abundant repeating earthquake sequences as well as linear clusters of earthquakes. Large voids in seismicity appear with dimensions of kilometers that have been aseismic over the 30‐year time interval, suggesting that these portions of the fault are either locked or creeping. The area of greatest slip in the Morgan Hill main shock coincides with the most prominent of these voids, suggesting that this part of the fault may be locked between large earthquakes. We find that the Calaveras Fault at depth is extremely thin, with an average upper bound on fault zone width of 75 m. Given the location error, however, this width is not resolvably different from zero. The relocations reveal active secondary faults, which we use to solve for the stress field in the immediate vicinity of the Calaveras Fault. We find that the maximum compressive stress is at a high angle, only 13° from the fault normal, supporting previous interpretations that this fault is weak.
Oblique convergence between India and Eurasia
Authors: Soofi Muhammad A. , King Scott D.
Published: October 2002

Oblique convergence is the general rule along convergent boundaries. Such convergence produces asymmetric deformation (topography) along the collision boundary. To quantify this asymmetry, we develop a method that allows us to calculate the asymmetry in topography with respect to a reference line drawn perpendicular to the collision boundary. The asymmetry in topography is presented as a number, or series of numbers, called here the asymmetry index. A positive asymmetry index indicates that there is more area with topography greater or equal to certain value to the east (right) of the reference line than to the west (left). The asymmetry index is negative for the opposite topography distribution. We use oblique convergent boundary conditions with a thin viscous sheet model to investigate the observed topography along the India‐Eurasia collision boundary. The goal of our study is to reproduce the observed asymmetry in the topography (i.e., the high topography in the Tibetan Plateau is shifted to the east). We consider a laterally varying velocity (both magnitude and angle, which we call the angle of obliquity) along this boundary and use a homogeneous lithosphere for Eurasia. With boundary conditions alone we are unable to account for the degree of asymmetry present in the observed topography. The asymmetry in the calculated topography is enhanced by considering a rheologically stronger anomalous zone representing the Tarim basin. The use of a constant velocity (both magnitude and angle) boundary condition, as in earlier studies, produces topographic asymmetry in the opposite direction to the observed topography. This indicates that in the study of obliquely converging boundaries it is important to consider the variation of velocity (magnitude and angle) in the collisional boundary.
Hydrothermal venting along Earth's fastest spreading center: East Pacific Rise, 27.5°–32.3°
Authors: Baker E. T. , Hey R. N. , Lupton J. E. , Resing J. A. , Feely R. A. , Gharib J. J. , Massoth G. J. , Sansone F. J. , Kleinrock M. , Martinez F. , Naar D. F. , Rodrigo C. , Bohnenstiehl D. , Pardee D.
Published: October 2002

During March/April 1998 we conducted detailed mapping and sampling of hydrothermal plumes along six segments of Earth's fasting spreading mid‐ocean ridge, 27.5°–32.3°S on the East Pacific Rise. We compared the distribution and chemistry of hydrothermal plumes to geological indicators of long‐term (spreading rate) and moderate‐term (ridge inflation) variations in magmatic budget. In this large‐offset, propagating rift setting, these geological indices span virtually the entire range found along fast spreading ridges worldwide. Hydrothermal plumes overlaid ∼60% of the length of superfast (>130 km/Myr) spreading axis surveyed and defined at least 14 separate vent fields. We observed no plumes over the slower spreading propagating segments. Finer‐scale variations in the magmatic budget also correlated with hydrothermal activity, as the location of the five most intense plumes corresponded to subsegment peaks in ridge inflation. Along the entire ridge crest, the more inflated a ridge location the more likely it was to be overlain by a hydrothermal plume. Plume chemistry mostly reflected discharge from mature vent fields apparently unperturbed by magmatic activity within the last few years. Plume samples with high volatile/metal ratios, generally indicating recent seafloor volcanism, were scarce. Along‐axis trends in both volatile (3He; CH4; ΔpH, a proxy for CO2; and particulate S) and nonvolatile (Fe, Mn) species showed a first‐order agreement with the trend of ridge inflation. Nevertheless, a broad correspondence between the concentration of volatile species in plumes and geological proxies of magma supply identifies a pervasive magmatic imprint on this superfast spreading group of ridge segments.
High‐pressure structure of ScAlO3 perovskite
Authors: Magyari‐Köpe Blanka , Vitos L. , Johansson B. , Kollár J.
Published: October 2002

Total energy calculations performed for ScAlO3 perovskite, using the global parametrization method (GPM) in conjunction with the exact muffin‐tin orbitals theory, show that the orthorhombic Pbnm structure remains stable relative to the cubic structure for pressures up to 150 GPa. The slightly larger compressibility of AlO6 octahedra relative to the ScO12 polyhedra is reflected in increasing polyhedral volume ratio, VSc/VAl and in a moderate reduction of the orthorhombic distortion with hydrostatic pressure. This observation indicates limitations in the structural analogy between ScAlO3 and the geophysically important MgSiO3 perovskite.
Seismic reflections from the crystalline crust below the Continental Deep Drilling Site KTB: Modeling and inference on reflector properties
Authors: Zillmer M. , Müller G. , Stiller M.
Published: October 2002

The three‐dimensional (3‐D) reflection‐seismic data set ISO‐89 3D was recorded near the deep borehole KTB in southeastern Germany. Reflections from the SE1 reflector and from the top of the Erbendorf body (EB) in the upper crystalline crust can be identified in 5–10% of the single‐shot sections. The reflectors have been first identified in previous studies of stacked and migrated seismic data. In this paper the velocity and density variations of these two structures are estimated in a new way using true amplitude single‐shot (vibroseis) data. The method uses the direct wave Pg as a reference phase and models the amplitude ratios of the SE1 and EB reflections to Pg. Modeling in this paper uses a combination of ray theory and the reflectivity method, and the SE1 and the top of the EB are assumed to be obliquely oriented 1‐D structures. Pg modeling shows that a depth‐dependent velocity function within the uppermost crystalline basement explains the amplitudes and travel times of this phase with sufficient accuracy. The largest observed amplitude ratios SE1/Pg and EB/Pg are explained by laminated models with strong velocity contrasts and with reflection coefficients of magnitude 0.1–0.2 (SE1) and 0.05–0.15 (EB). The total thickness of the reflecting zones is less than ∼300 m. Pg amplitude modeling requires low Qp factors (<100) to a depth of ∼1 km, whereas at larger depths, values of several hundred are necessary to keep the SE1 and EB velocity contrasts in realistic ranges. Both reflectors can be interpreted as cataclastic zones. For the SE1 this interpretation agrees with the view that it is a steeply dipping thrust fault which continues the tectonic Franconian Lineament into the upper crust. We assume that the EB is the fractured top of a high‐velocity zone at depths below ∼10 km, known from earlier wide‐angle measurements. Both reflectors have large weakly reflecting or nonreflecting parts. The SE1 is nonreflecting at the intersection with the KTB borehole.
Post‐1906 stress recovery of the San Andreas fault system calculated from three‐dimensional finite element analysis
Authors: Parsons Tom
Published: October 2002

The M = 7.8 1906 San Francisco earthquake cast a stress shadow across the San Andreas fault system, inhibiting other large earthquakes for at least 75 years. The duration of the stress shadow is a key question in San Francisco Bay area seismic hazard assessment. This study presents a three‐dimensional (3‐D) finite element simulation of post‐1906 stress recovery. The model reproduces observed geologic slip rates on major strike‐slip faults and produces surface velocity vectors comparable to geodetic measurements. Fault stressing rates calculated with the finite element model are evaluated against numbers calculated using deep dislocation slip. In the finite element model, tectonic stressing is distributed throughout the crust and upper mantle, whereas tectonic stressing calculated with dislocations is focused mostly on faults. In addition, the finite element model incorporates postseismic effects such as deep afterslip and viscoelastic relaxation in the upper mantle. More distributed stressing and postseismic effects in the finite element model lead to lower calculated tectonic stressing rates and longer stress shadow durations (17–74 years compared with 7–54 years). All models considered indicate that the 1906 stress shadow was completely erased by tectonic loading no later than 1980. However, the stress shadow still affects present‐day earthquake probability. Use of stressing rate parameters calculated with the finite element model yields a 7–12% reduction in 30‐year probability caused by the 1906 stress shadow as compared with calculations not incorporating interactions. The aggregate interaction‐based probability on selected segments (not including the ruptured San Andreas fault) is 53–70% versus the noninteraction range of 65–77%.
Anhydrite precipitation and the extent of hydrothermal recharge zones at ocean ridge crests
Authors: Lowell Robert P. , Yao Yufeng
Published: October 2002

We develop mathematical models to investigate anhydrite precipitation in hydrothermal recharge zones at ocean ridge axes. We first consider constant flow during hydrothermal recharge to estimate the depth, the depth interval over which anhydrite precipitation would occur, and the initial rate of porosity reduction. We find that the smaller the downward mass flux, the shallower and more broadly distributed the region of precipitation. For values of downward mass flux similar to that for high‐temperature black smoker systems, the porosity would be clogged significantly in a matter of months to years depending upon the initial porosity. We then consider anhydrite precipitation in a buoyancy‐driven single‐pass model in which the reduction in permeability resulting from anhydrite precipitation exerts a feedback on the flow rate. This model gives results similar to the constant flow model for the initial depth, depth interval, and rate of porosity reduction. The effect of the permeability clogging on the flow rate, however, is twofold. First, the zone of anhydrite precipitation shallows and broadens as the flow rate decreases, thus allowing deeply deposited anhydrite to be preserved temporarily. Second, if the recharge area of the same size as the discharge area, anhydrite precipitation rapidly reduces the flow rate and the heat output of the hydrothermal system. If the initial porosity of the recharge zone is 1%, the area of the recharge zone must be ∼10–100 times that of the discharge zone to maintain near steady state conditions for time periods of decades to hundreds of years. This result implies that recharge zones may be quite large and extend off axis as well as along axis.
Revised magnetic power spectrum of the oceanic crust
Authors: Korte M. , Constable C. G. , Parker R. L.
Published: October 2002

The magnetic field originating within the Earth can be divided into core and crustal components, which can be characterized by the geomagnetic power spectrum. While the core spectrum is determined quite well by satellite studies, models of the shorter wavelength crustal spectrum disagree considerably. We reexamine aeromagnetic data used by O'Brien et al. [1999] to obtain a new, improved estimate of the crustal geomagnetic power spectrum. O'Brien et al.'s model somewhat failed to give a satisfactory connection between the longer‐wavelength satellite studies and a reliable crustal model. We show that this was caused by an inadequate processing step that aimed to remove external variations from the data. We moreover attempt to bound the long‐wavelength part of the spectrum using constraints of monotonicity in the correlation of the magnetization. However, this proves to be a weak constraint. Reversing the process, though, we are able to evaluate the correlation function using the reliable part of our geomagnetic spectrum. Thus we can obtain a sensible estimate for the long‐wavelength part of the spectrum that is not well constrained by the data. Our new model shows better agreement with earlier satellite studies and can be considered reliable in the spherical harmonic degree range l = 30 to 1200.
Fine‐scale seismic structure of young upper crust at 17°20′S on the fast spreading East Pacific Rise
Authors: Hussenoeder Stefan A. , Detrick Robert S. , Kent Graham M. , Schouten Hans , Harding Alistair J.
Published: October 2002

The detailed upper crustal structure of the East Pacific Rise (EPR) at 17°20′S is examined by applying a genetic algorithm‐based waveform inversion to five multichannel seismic lines: one on‐axis and two to either side along 42‐ and 85‐kyr‐old crust. On‐axis, a double‐stepped velocity pattern is recorded beneath 70–100 m of low‐velocity extrusives (2.1–2.4 km s−1). We define the upper velocity contrast as the base of seismic layer 2A due to its severity and continuity along and across axis. The more subdued and intermittent lower‐velocity step is not observed off‐axis. Material between the two high‐gradient intervals is proposed to represent the pillow/dike transition, bounded above by a sharp increase in dike fraction with depth and below by an abrupt change in rheology and/or deformation. Extrusive velocities increase quite rapidly in this area, with velocities ∼3 km s−1 common in crust ≤85 kyr old. This, plus a rapid (300–400 m) thickening of layer 2A observed within 1–4 km of the rise axis, indicates that this segment is undergoing focused melt delivery (<500‐m‐wide dike intrusion zone) and elevated hydrothermal activity. These findings demonstrate the ability of single‐ship multichannel data to record detailed information on the reflectivity and velocity of the upper crust and the ability of the genetic algorithm to efficiently construct accurate seismic models based on this information.
Upper crustal seismic structure of the slow spreading Mid‐Atlantic Ridge, 35°N: Constraints on volcanic emplacement processes
Authors: Hussenoeder Stefan A. , Kent Graham M. , Detrick Robert S.
Published: October 2002

The upper crustal seismic structure of the slow spreading Mid‐Atlantic Ridge is studied using a genetic algorithm‐based waveform inversion of multichannel streamer data. Four single‐ship multichannel profiles from 35°N are analyzed: one in the rift valley and three in the rift mountains along 0.7, 1.6, and 1.9 Ma crust. A layer 2A horizon is continuously imaged along three profiles and is associated with a sharp velocity increase from extrusives to dikes. Its depth and regularity in the rift valley indicate that most of the extrusive section is built on the inner valley floor through a pattern of deposition and fault‐bounded uplift into the rift mountains. Its variability along one line, however, shows that this process is disrupted during tectonically dominated periods. A thickening of layer 2A toward the Oceanographer fracture zone may be the result of along‐axis magma transport. The interval of rapid velocity increase at the base of layer 2A thins with age, a possible response to enhanced hydrothermal mineralization within the zone of mixed dikes and extrusives. Transition zone (∼200 m) and off‐axis layer 2A thicknesses (350–600 m) are similar to those at other spreading centers. This indicates that equivalent extrusive volumes are produced at all spreading rates along a relatively narrow zone of dike emplacement. However, differences in on‐axis layer 2A thickness between this area and fast spreading ridges suggest that the exact pattern of thickening varies between spreading regimes. Relative to fast spreading ridges, the moderate velocity increase with age recorded in the upper crust (from 2.3 to >2.7 km s−1 within ∼2 Myr) may be due to a less active hydrothermal system and hence slower porosity reduction.
A numerical investigation of slanted convection and subsurface anticyclone generation in an Arctic baroclinic current system
Authors: Chao Shenn‐Yu , Shaw Ping‐Tung
Published: October 2002

A nonhydrostatic three‐dimensional numerical model is used to investigate convection along slanted isopycnal surfaces and subsurface anticyclone generation in a baroclinic jet system appropriate for the Beaufort Sea environment. The current is initially bounded to the left by a coastal wall and subsequently subject to surface friction exerted by sea ice and cooling at the sea surface. Despite spatially uniform cooling rates used in the model, convection favors limited areas with high cyclonic vorticity. In a meandering current system, rigorous convection occurs primarily in meander troughs where the current turns cyclonically. The cyclonic vorticity enhances convection, which in turn enhances cyclonic vorticity. The positive feedback mechanism, previously known in atmospheric literature as convective instability of the second kind, often strengthens convection and the consequent subsurface anticyclones by as much as 1 order of magnitude. Subsequent seaward detrainment in subsurface depths of interest moves anticyclones from the cyclonic side of the jet to the anticyclonic side. The subsurface anticyclones may be strengthened for the second time because of favorable changes of ambient vorticity. Thus downward convection along slanted isopycnal surfaces of a left‐bounded baroclinic jet (similar to segments of the Beaufort Sea anticyclonic gyre) is an efficient way to produce subsurface anticylcones and propel them seaward afterward.
A numerical study of barotropicly forced intrusion in DeSoto Canyon
Authors: Yuan Dongliang
Published: October 2002

The intrusion of open ocean water across the head of DeSoto Canyon offshore of Pensacola, Florida, is investigated using a version of the Bryan‐Cox model of the entire Gulf of Mexico. The Loop Current‐forced model circulation in DeSoto Canyon features an eastward, nearly along‐isobath, current with a weak cross‐isobath velocity component. In contrast, the southward wind‐driven currents over the west Florida shelf in response to northerly winds induce strong cross‐isobath flow at the head of DeSoto Canyon. The flow is diagnosed to be the bottom Ekman flow associated with the strong along‐isobath currents. In addition, transient responses of sea levels to the northerly wind bursts give rise to pressure gradient‐forced currents up the canyon during the collapse phase of the winds. A case study in November 1997 is conducted, and the above arguments are substantiated with the simulated and the observed currents.