bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamics of retreating slabs: 2. Insights from three‐dimensional laboratory experiments
Authors: Funiciello Francesca , Faccenna Claudio , Giardini Domenico , Regenauer‐Lieb Klaus
Published: May 2003

A laboratory analogue of a three‐layer linear viscous slab‐upper mantle‐lower mantle system is established in a silicone putty, honey and crystallized honey tank experiment. The same setup as in the numerical investigation (part 1) is used. We focus on the interaction of the slab with the induced passive mantle flow by widely varying the mantle volume flux boundary conditions. In our numerical experiments the lateral volume flux was set to zero. In interpreting the results relative to the real Earth, the base of the box is taken as the bottom of the mantle convection system, while the lateral boundaries may be associated with the presence of other nearby slabs. Dynamic force equilibrium, assessed on the basis of an analytical review of forces, is described for four different phases: (1) the subduction initiation instability, (2) the accelerating dynamic free fall phase of the slab, (3) the dynamic interaction with the 660‐km discontinuity, and (4) a final phase of steady state trench retreat. Phase 3 is an important feature not observed in the numerical experiments. This highly dynamic phase of interrupted trench retreat can therefore be attributed to boundary conditions on mantle volume flux. On the basis of integration constants of force equilibrium in phases 2 and 4 we identify two different classes of volume flux: one in which the lateral boundary can be considered open and the other class where it is “closed.” Closed boundary condition cases are obtained if any of the lateral box boundaries are 600 km away from the slab. Assuming a one‐to‐one relation between trench retreat and back arc spreading, enigmatic observations of episodic opening of back arc basins can be explained by our experimental observations.
Structure and seismotectonics of the Mendocino Triple Junction, California
Authors: Henstock Timothy J. , Levander Alan
Published: May 2003

We analyze and interpret two marine multichannel seismic reflection/onshore‐offshore/ocean bottom seismometer (OBS) seismic profiles that cross in the region of the Mendocino Triple Junction over the fault plane of the 1992 Petrolia, California, Ms = 7.1 earthquake. From the offshore reflection data and the wide‐angle constraints provided by the onshore‐offshore and OBS data we have determined a rough three‐dimensional view of crustal and uppermost mantle structure beneath the triple junction region. This structure is also consistent with gravity data. The upper and mid crust has seismic velocities and densities consistent with accretionary complex rocks of the Franciscan terrane; a ubiquitous lower crustal layer with velocities of 6.5–7.1 km s−1 is 5–7 km thick throughout most of the region. Over an area extending at least 25 km to the south and 60 km to the west of the triple junction the lower crustal layer is up to 12 km thick; this area is overlain by undisturbed pre‐11 Ma sediments, indicating little deformation since that date. The thick lower crust extends beneath the coast, suggesting that the deep boundary between Pacific and North American terranes lies onshore. The northeastern edge of the thick lower crustal layer is marked by concentrated seismicity, with the hypocenter of the 1992 Petrolia earthquake lying close to the NE corner. We speculate that at depth the Pacific lower crust acts an asperity acting to concentrate stress, from which the Petrolia earthquake ruptured.
Transient elastic P and SV wave edge diffraction by a semi‐infinite perfect‐slip fracture in an isotropic dispersive solid: A canonical problem
Authors: de Hoop Adrianus T. , de Hoop Maarten V.
Published: May 2003

The canonical problem of edge diffraction of transient P or SV waves by a perfect‐slip (fluid‐filled) fracture is investigated. Closed‐form analytic expressions for the time domain particle velocities of the reflected, transmitted, and converted waves on either side of the fracture, for the cylindrical edge‐diffracted waves, and for the excited Rayleigh surface waves along the plane of the fracture are obtained. The hosting solid is taken to be homogeneous, isotropic, and dispersive. As to the seismic loss mechanism, two cases are considered: the frictional force/bulk viscosity mechanism and the standard linear solid or Zener mechanism. Numerical illustrations of the different wave constituents are presented, including those applying to postcritical incidence of SV waves. They deal with the case of a well‐developed, stationary fracture; the significance of the results to the theory of fracture mechanics with a view to the problem of a transient load at a crack face is briefly indicated.
Easter microplate dynamics
Authors: Neves M. C. , Searle R. C. , Bott M. H. P.
Published: May 2003

We use two‐dimensional elastic finite element analysis, supplemented by strength estimates, to investigate the driving mechanism of the Easter microplate. Modeled stresses are compared with the stress indicators compiled from earthquake focal mechanisms and structural observations. The objective is to constrain the tectonic forces that govern the Easter microplate rotation and to test the microplate driving hypothesis proposed by Schouten et al. [1993]. We infer that the mantle basal drag cannot drive the microplate rotation but opposes it, and that the asthenospheric viscosity is no more than about 1 × 1018 Pa s. At most, the basal drag comprises 20% of the force resisting microplate rotation. The outward pull of the main plates can drive the rotation by shear drag applied along the northern and southern boundaries of the microplate. However, we propose an additional driving force which arises from the strong variation of the ridge resistance force along the east and west rifts, so that the main driving torques come from the pull of the major plates acting across the narrowing and slowing rifts. This requires the strength to increase substantially toward the rift tips due to thickening of the brittle lithosphere as the spreading rate slows.
Introduction of thermal activation in forward modeling of hysteresis loops for single‐domain magnetic particles and implications for the interpretation of the Day diagram
Authors: Lanci Luca , Kent Dennis V.
Published: May 2003

Synthetic hysteresis loops were generated by numerically solving the classical Stoner‐Wohlfarth model and a thermally activated Stoner‐Wohlfarth model for a set of randomly oriented magnetic grains. Although computationally intensive this method allows forward modeling of hysteresis loops of single‐domain (SD) and viscous grains. In the classic Stoner‐Wohlfarth model the shape of the modeled loops can be modified by changing the distribution of the anisotropy energy but all the loops will all have similar hysteresis parameters Msr/Ms and Hcr/Hc corresponding to that of a theoretical assemblage of SD particles. The thermally activated Stoner‐Wohlfarth model, which allows the magnetic moment of each grain to switch between two energy minima according to Boltzmann statistics, extends the SD model toward superparamagnetic (SP) grains and introduces a volume dependency. Numerical simulation using the thermally activated model shows that the shapes of SD loops are modified by the effect of the thermal energy if the particles are sufficiently small. The major effect of the thermal disturbance is observed in highly viscous particles (smaller than approximately 0.03 μm in diameter, for magnetite) where it strongly reduces the coercivity and to a lesser extent the remanent magnetization. The effect on the hysteresis parameters is a large increase in Hcr/Hc and a decrease in Msr/Ms, by factors that vary with anisotropy distribution, grain volume and measurement time. For certain grain sizes, these result in hysteresis parameters that are similar to those attributed to pseudosingle‐domain (PSD) grains.
Horizontal coseismic deformation of the 1999 Chi‐Chi earthquake measured from SPOT satellite images: Implications for the seismic cycle along the western foothills of central Taiwan
Authors: Dominguez Stéphane , Avouac Jean‐Philippe , Michel Rémi
Published: May 2003

The 1999 Chi‐Chi earthquake, Mw = 7.6, broke a major thrust fault along the western foothills of the Central Range of Taiwan. We have measured the horizontal coseismic displacement field by correlating optical satellite images acquired before and after the earthquake. These data reveal the fault trace and a clockwise rotation of surface displacements toward the north with much larger displacements and strain in the hanging wall. At the surface, coseismic slip increases from 5–6 m near the epicenter to 10–11 m to the north. In the epicentral area, we observe a left‐lateral strike‐slip zone trending N125°E, and farther north, a fault zone trending N‐S with a right‐lateral component. The data were modeled using elastic dislocations. The fault geometry consists of a shallow 20–35° east dipping ramp, which soles out into a low dipping décollement at a depth of ∼6 to 8 km. Surface displacements can be satisfactorily modeled, assuming a constant slip azimuth on the main fault plane, close to the azimuth of plate convergence (N305°E ± 5°). At depth, slips along the fault plane evolve from 5–6 m in the south to 7 to 12 m to the north. Our model suggests that the deeper portion of the fault was not activated during the Chi‐Chi earthquake. This zone of slip deficit must break during large earthquakes or be activated during transient episodes of aseismic slip. On the basis of these observations, the western front of the central Taiwan should produce a M = 7 to 7.5 event, about every 150 to 250 years.
Whole mantle P wave tomography using P and PP‐P data
Authors: Fukao Yoshio , To Akiko , Obayashi Masayuki
Published: May 2003

We measured PP‐P differential travel times on broadband seismograms of the Incorporated Research Institutions for Seismology (IRIS) network. The measurement was made by cross‐correlating the observed PP waveform with the synthetic PP calculated from the observed P waveform. The effect of seawater reverberation on the measurement is taken into account if the PP bounce point is under ocean. The PP‐P times measured after low‐pass filtering is systematically shorter than those measured on raw broadband seismograms. We show that low‐pass filtering tends to merge the precursory and postcursory phases into the main PP phase so that the measured PP‐P times are artificially short. A cutoff frequency as high as 0.5 Hz is required to isolate the main PP. With such a high cutoff frequency, we obtained a total of about 7000 PP‐P times. The residuals relative to a reference P velocity model are consistent with those calculated for the aspherical model WEPP2 [Obayashi et al., 1997] when the PP bounce points are located in the region well resolved by WEPP2. The PP‐P times, combined with 2 million International Seismological Centre (ISC) first arrival times, were inverted for new model WEPP2′. This model renders a variance reduction of PP‐P residuals of 63% without compromising the fit to P residuals used originally in WEPP2. In WEPP2′, the low velocity anomaly beneath Hawaii is limited in depth down to the uppermost part of the lower mantle. The high velocity anomaly of the Russian continental lithosphere is underlain in the mantle transition zone by a body of low velocity anomaly.
The emplacement of low‐aspect ratio ignimbrites by turbulent parent flows
Authors: Dade W. B.
Published: May 2003

A quantitative model for a radially spreading, suspension‐driven gravity current is used to interpret parent flow conditions of a volumetrically wide range of pyroclastic deposits known from the recent and ancient geologic records of explosive volcanism. The analysis, which requires no freely adjustable parameters, yields values of the solids concentration, time of duration (or, equivalently, a minimal estimate of eruptive flux of solids) and average speed for the ground‐hugging, deposit‐forming gravity currents. A consistent picture emerges in which the initial volumetric concentration of pyroclastic solids in the parent flows is of the order 10−2 or less, and the expanded flows travel with average speeds in the range 25–250 m s−1 to reach their farthest extent in a matter of only minutes. Further application of the model suggests that depositional structures in a widespread and thin ignimbrite are likely to reflect the intensity of mass flux in the parent flow, and thus the intensity and overall volume of the parent eruption. Conditions in small‐magnitude pyroclastic currents of recent history can result in cross‐stratified deposits. Primary sedimentary structures (or their absence) in some regionally widespread and massive deposits known only in the geologic record, in contrast, were probably emplaced under conditions of intense sheet‐flow transport driven by the parent gravity currents.
Classification of lithology from seismic tomography: A case study from the Messum igneous complex, Namibia
Authors: Bauer K. , Schulze A. , Ryberg T. , Sobolev S. V. , Weber M. H.
Published: May 2003

Combined analysis of seismic P and S velocity information provides a reasonable and efficient basis for the petrologic interpretation of seismic cross sections. In this paper, a methodology is presented which allows extraction of prominent features related to well‐defined P velocities and Poisson's ratios from a tomographic velocity model using a classification approach. We used first‐arrival travel time data from a near‐vertical seismic experiment and independently determined P and S velocities by forward and inverse modeling. Resolution and uncertainties were estimated from inverting synthetic data. The classification procedure was carried out in two subsequent steps. First, prominent classes were identified in the parameter space spanned by Poisson's ratios and P wave velocities. For this purpose, a probability density function was calculated from the tomograms. A function measuring the topography of the probability density was then determined, and a histogram analysis was carried out to detect significant classes. In the second step, the results from principal component analysis for the identified classes were used to map their distribution along the seismic profile. We applied the method to the Messum intrusive complex of Namibia and identified three prominent classes. On the basis of the integration of petrophysical data and comparison with surface geology, we conclude that quartz‐syenite composition dominates the upper 800 m of the crust under the complex. Outside of the intrusion the upper crust has properties corresponding to felsic metasediments and granites which are abundant in the local basement. This material shows strong depth‐dependent changes of seismic properties which are ascribed to decreasing porosity and fluid saturation with depth.
Subduction factory 1. Theoretical mineralogy, densities, seismic wave speeds, and H2O contents
Authors: Hacker Bradley R. , Abers Geoffrey A. , Peacock Simon M.
Published: May 2003

We present a new compilation of physical properties of minerals relevant to subduction zones and new phase diagrams for mid‐ocean ridge basalt, lherzolite, depleted lherzolite, harzburgite, and serpentinite. We use these data to calculate H2O content, density and seismic wave speeds of subduction zone rocks. These calculations provide a new basis for evaluating the subduction factory, including (1) the presence of hydrous phases and the distribution of H2O within a subduction zone; (2) the densification of the subducting slab and resultant effects on measured gravity and slab shape; and (3) the variations in seismic wave speeds resulting from thermal and metamorphic processes at depth. In considering specific examples, we find that for ocean basins worldwide the lower oceanic crust is partially hydrated (<1.3 wt % H2O), and the uppermost mantle ranges from unhydrated to ∼20% serpentinized (∼2.4 wt % H2O). Anhydrous eclogite cannot be distinguished from harzburgite on the basis of wave speeds, but its ∼6% greater density may render it detectable through gravity measurements. Subducted hydrous crust in cold slabs can persist to several gigapascals at seismic velocities that are several percent slower than the surrounding mantle. Seismic velocities and VP/VS ratios indicate that mantle wedges locally reach 60–80% hydration.
Differential loading by prograding sedimentary wedges on continental margins: An arch‐forming mechanism
Authors: Kjeldstad A. , Skogseid J. , Langtangen H. P. , Bjørlykke K. , Høeg K.
Published: May 2003

Rapid deposition of prograding sedimentary wedges on continental margins will cause excess pore pressure, fluid flow, and compaction as they load the substratum. They may also cause faulting and structural deformation in the sedimentary succession below and in front of the load. Large depositional units are in addition associated with pronounced isostatic subsidence. The magnitude and effects each component has on the basin as a whole is often debated. To address this problem, we use a quantitative approach with a coupled hydromechanical mathematical model based on linear isotropic elasticity. The equations are solved by the finite element method (FEM). In the modeling, a wedge‐shaped load emplaced successively over a period of 1.5 Myr is used to simulate the progradation of a thick sedimentary wedge. Results show that the differential load generates excess pressure below the prograding depocenter with lateral fluid flow in front and a tail of draining pore pressure behind. The sediments are pushed downward and laterally in front of the wedge and vertical and horizontal compressional and shear stresses are generated below the wedge. The formation of the Helland Hansen Arch, located in the Vøring Basin off mid‐Norway, is genetically associated with differential loading. Our results indicate that the whole eastern flank of the arch is a result of differential loading, whereas the western flank is mainly related to thermal subsidence since Paleogene times. Thus, alternative explanations like intraplate stress seem to be irrelevant as a genetic mechanism.
Roof assimilation at fast spreading ridges: An investigation combining geophysical, geochemical, and field evidence
Authors: Coogan Laurence A. , Mitchell Neil C. , O'Hara Michael J.
Published: May 2003

The role of assimilation at the roof of the axial magma chamber (AMC) at fast spreading mid‐ocean ridges is investigated using field, geophysical, and geochemical data. Field observations from ophiolites indicate that roof assimilation is a common process, but provide little constraint on the amount of material assimilated. Arguments based on geophysical data that include the subsidence of the layer 2A/2B boundary across the width of the AMC, and the depth of large axial summit troughs, require at least ∼50–100 m of assimilation. However, significant variations in the depth of the AMC on the East Pacific Rise (EPR), over short along‐axis distances, suggest up to ∼500 m fluctuations in the depth of the AMC may be common on short timescales. Geochemical modeling of the over‐enrichment of chlorine in EPR basalts suggests that ∼20% of the oceanic crust may go through a cycle of crystallization, alteration, then assimilation. This is far more than can be accounted for by roof subsidence and suggests that fluctuations in the level of the roof must account for the majority of the assimilation. Assimilation most likely occurs on a number of different timescales ranging from decadal, due to perturbations in the thermal regime related to diking, to longer‐period fluctuations controlled by magma supply from the mantle. Large‐scale roof assimilation has implications for the accretion of the lower crust due to heat losses associated with assimilation, and for the interpretation of the geochemistry of the oceanic crust.
Morphology and tectonics of the Mid‐Atlantic Ridge, 7°–12°S
Authors: Bruguier N. J. , Minshull T. A. , Brozena J. M.
Published: May 2003

We present swath bathymetric, gravity, and magnetic data from the Mid‐Atlantic Ridge between the Ascension and the Bode Verde fracture zones, where significant ridge–hot spot interaction has been inferred. The ridge axis in this region may be divided into four segments. The central two segments exhibit rifted axial highs, while the northernmost and southernmost segments have deep rift valleys typical of slow‐spreading mid‐ocean ridges. Bathymetric and magnetic data indicate that both central segments have experienced ridge jumps since ∼1 Ma. Mantle Bouguer anomalies (MBAs) derived from shipboard free air gravity and swath bathymetric data show deep subcircular lows centered on the new ridge axes, suggesting that mantle flow has been established beneath the new spreading centers for at least ∼1 Myr. Inversion of gravity data indicates that crustal thicknesses vary by ∼4 km along axis, with the thickest crust occurring beneath a large axial volcanic edifice. Once the effects of lithospheric aging have been removed, a model in which gravity variations are attributed entirely to crustal thickness variations is more consistent with data from an axis‐parallel seismic line than a model that includes additional along‐axis variations in mantle temperature. Both geophysical and geochemical data from the region may be explained by the melting of small (<200 km) mantle chemical heterogeneities rather than elevated temperatures. Therefore, there may be no Ascension/Circe plume.
Role of back arc spreading in circulation and melting at subduction zones
Authors: Kincaid Chris , Hall Paul S.
Published: May 2003

We present results from two‐dimensional (2‐D) numerical experiments on mantle flow in subduction zones and the thermal and compositional evolution of the mantle wedge. Experiments build upon previous modeling efforts by including melting processes and back arc plate motion relative to the trench. The thermal structure of the wedge is calculated for various plate motion conditions, and a particle technique is used to track melt production and extraction in the back arc mantle and the arc‐wedge corner. Numerical experiments test models developed from geochemical data that suggest the mantle is partially melted, or preconditioned, beneath the back arc prior to entering the arc‐wedge corner. Results show wedge temperatures increase with increasing back arc plate rates and higher degrees of retrograde subduction. Decompression melting is recorded for material in the wedge. Mean degrees of melting are generally smaller than beneath ridges (1–7%) and depend on back arc plate rate and, in particular, the choice of ambient mantle potential temperature (Tm). Results show that preconditioning of mantle by partial melting at the back arc is optimized for intermediate back arc plate rates. Implications of these models include (1) arc melts should have components with characteristics of a low degree, decompression melting process, (2) arc melts may be depleted in incompatible elements, or preconditioned, prior to entering the wedge, and (3) models for slab‐influenced arc magmagenesis within the wedge should consider the melt history of material just above the slab‐wedge interface.
Tests on the validity of atmospheric torques on Earth computed from atmospheric model outputs
Authors: de Viron O. , Dehant V.
Published: May 2003

The effect of the atmosphere on the Earth rotation is usually computed using the angular momentum budget equation. In particular, the interaction torque between the solid Earth/ocean and the atmosphere can be computed from the output of global circulation models. This torque is composed of three parts: a mountain torque due to the pressure action on the topography, a gravitational torque due to the interaction between the mass inside the solid Earth and inside the atmosphere, and a friction torque. The purpose of the paper is to test the torque computed from the output of different atmospheric global circulation models (GEOS‐1, National Centers for Environmental Protection reanalysis, and ERA‐15) in order to see to what extent they are reliable in the frame of Earth rotation studies. The test has been performed by comparing each part of the torque computed from the different models, as well as by verifying of the angular momentum budget equation for the atmosphere.
Implications of rate‐and‐state friction for properties of aftershock sequence: Quasi‐static inherently discrete simulations
Authors: Ziv A. , Rubin A. M.
Published: May 2003

Dieterich [1994] modeled the response to a stress step of a population of faults governed by rate‐ and state‐dependent friction. This model assumes that aftershocks nucleate over areas on the fault that at the time of the main shock are already accelerating toward failure and disregards the effect of interactions among aftershocks. The main objective of this study is to examine consequences of relaxing these underlying assumptions. Aftershock activity is simulated using an inherently discrete earthquake fault model, with a fault surface governed by an approximate constitutive friction law similar to the one used by Dieterich. We find that the governing equations in nondimensional form are a function of three main parameters and explore the effect of these parameters on the simulated catalogs. We derive a simple expression for the time‐dependent seismicity response to a stress step that approximates the effect of multiple interactions among aftershocks as a time‐dependent stressing rate. Close match is found between the simulated seismicity response to a stress step and that predicted analytically. However, the numerical simulations show that the effect of the main shock is not only to raise the local seismicity rate but also to systematically modify the earthquake size distribution. As a result, the actual seismicity rate change early during the aftershock sequence may be higher than that predicted, whereas seismicity rate late in the sequence may be lower than that predicted. Such a modification of the earthquake size distribution can explain observations of lower b values immediately following a stress step.
Subsurface geophysics of the Phlegrean Fields: New insights from downhole measurements
Authors: Rabaute Alain , Yven Béatrice , Chelini Walter , Zamora Maria
Published: May 2003

The volcanic complex of Phlegrean Fields, located northwest of Naples (Italy), has been the site of deep geothermal exploration in the 1980s. Several wells were drilled by an Agip‐Enel joint venture, with downhole continuous physical properties acquired in each well. The main purpose of this study is to map and describe the spatial variations of the volcanic deposits, in terms of their nature, thickness and in situ physical properties. We apply fuzzy k means classification of the well‐logging data available in the wells cored in the San Vito area. Fuzzy set theory provides an approach that quantitatively assigns individuals to physically continuous classes. The optimal number of classes is found by minimization of three mathematical functions, thus reducing subjectivity. The resulting classifications are found (1) to reflect the main lithologies in the San Vito plain, (2) to provide more detail on the geological stratigraphy, allowing a more precise volcanic history, and (3) to differentiate between transition zones and interbedded well‐defined deposits. In addition to discriminating between lithologies that have different physical properties, this study gives information on the degree of homogeneity of each lithologic unit, and the range of variation for the measured properties. Finally, using the physical classification of the deposits, we are able to propose a detailed two‐dimensional compressional acoustic velocity structure of the studied area.
Streaming potential of a sand column in partial saturation conditions
Authors: Guichet Xavier , Jouniaux Laurence , Pozzi Jean‐Pierre
Published: May 2003

The understanding of the streaming potential in partial water saturation conditions in porous media is of great interest for the interpretation of spontaneous polarization observations. We built a device which allows us to quantify the streaming potential at various saturation conditions using a sand column of 1‐m height and 8‐cm diameter. This is the first time that such a quantification has been performed. Different gases such as argon, nitrogen, and carbon dioxide are injected into the sand to decrease its water saturation, and to make the fluid flow within the sand. The measured electrokinetic coupling coefficient in partial saturation is either constant or decreases by a factor ∼3 with decreasing water saturation from 100 to 40%, whereas the sand electrical resistivity is enhanced by a factor of ∼5.
Ridge‐like lower mantle structure beneath South Africa
Authors: Ni Sidao , Helmberger Don V.
Published: May 2003

Recent (ScS‐S) results from probing the deep structure beneath southern Africa display strong delays of up to 10 s at distances beyond 90°. Such delays could be explained by long‐period tomographic models containing smooth (weak) features with the addition of rough (strong) D″ structure (3–9% drops in shear velocities). However, these structures cannot explain the (SKS‐S) differentials sampling the same region. To explain the (SKS‐S) and (ScS‐S) data sets simultaneously requires instead a large‐scale ridge‐like structure with a relatively uniform 3% reduction in shear velocity. The structure is about 1000 km wide and extends at least 1200 km above D″. It is orientated roughly NW‐SE and leans toward the east at latitudes from 15° to about 30°. It proves difficult to explain such sharp features with thermal effects alone and, thus, the importance of high‐resolution waveform modeling to establish their existence. To derive the above results, we developed a special algorithm by matching simulated synthetics to observed broadband waveforms. This is achieved by computing the various arrivals separately using generalized ray theory for a reference model and allowing the arrivals to shift in relative times to match the data. Tomographic models can then be constructed, or existing tomographic models can be altered, to match these data, and new 2‐D synthetics can be constructed as well to better fit the waveform data. These updated synthetics can again be decomposed and reassembled, and the process can be repeated. This algorithm is applied to a combination of analog and digital data along a corridor from South America, producing the high‐resolution 2‐D model described above.
Geomorphic evidence for an emergent active thrust along the edge of the Po Plain: The Broni‐Stradella fault
Authors: Benedetti Lucilla C. , Tapponnier Paul , Gaudemer Yves , Manighetti Isabelle , Van der Woerd Jerome
Published: May 2003

We present here geomorphic evidence for an emergent active thrust along the northwestern Apennine front about 50 km south of Milano. Fieldwork combined with SPOT image analysis attests to the presence of a ∼35‐km‐long, en échelon, cumulative fault scarp, that cuts E‐W across Quaternary surface deposits from Casteggio to Sarmato. The scarp offsets vertically alluvial fans and terraces emplaced by tributaries of the Po River that flow northward from the Apennines. Incision by such tributaries occurs only south of the scarp. Valleys of smaller streams stand in hanging position. To the northeast, the buried, east dipping Pavia lateral ramp bounds the San Colombano anticline. We interpret the Montebello‐Sarmato scarp to result from the emergence of an active thrust fault dipping south, the Stradella thrust, which splays eastward from that ramp. Total station profiles leveled perpendicular to the Stradella thrust show variable cumulative surface throws ranging from 2 to 25 m. Scarp degradation analysis indicates fairly recent offset of the terrace surfaces (10–100 kyr) suggesting a minimum uplift rate of 0.3 mm/yr. The underlying fault might thus be one of the most active thrust ramps along this segment of the Apennines. N‐S to N15°W shortening is consistent with both the E‐W strike of this thrust ramp and with coeval oblique sinistral motion on the NNE‐SSW trending Pavia lateral ramp. The Vogherese‐Bobbiese earthquake of 1828 (I ≈ IX) south of Casteggio and Broni may have activated one segment of this lateral ramp rather than the Stradella thrust. Other historical events (I > 7) that caused damage in Pavia, Piacenza, Lodi, and Milano in the last 2000 years (290 A.D., 1249 A.D., 1276 A.D., 1473 A.D.), may have occurred on the Broni‐Stradella thrust or on the blind San Colombano and Lodi thrusts farther north. This makes detailed paleoseismological studies and trenching across its trace mandatory.
Reassessment of pore shapes in microstructurally equilibrated rocks, with implications for permeability of the upper mantle
Authors: Wark David A. , Williams Charles A. , Watson E. Bruce , Price Jonathan D.
Published: May 2003

With the goal of constraining fluid and melt transport rates in the upper mantle and lower crust, various permeability–porosity relations have been proposed for deep‐seated, microstructurally equilibrated rocks. Two relations that are in close agreement include one that is based on numerical modeling for an idealized rock and another that is based on direct measurement of permeability on synthetic quartzite analogs. Each relation describes a continuous increase in permeability as a function of porosity, raised to some power. The empirical relation is displaced to lower permeabilities, reflecting the more complex (nonuniform) pore geometry of the quartzites, which resembles that expected in natural, deep‐seated rocks. Despite differences, the numerical models assume, and the quartzites display, a similar pore geometry that is consistent with thermodynamic equilibria: grain boundaries are “dry,” with pores largely confined to joins of three or more grains. In contrast, a fundamentally different pore microstructure, in which most of the melt occupies grain boundaries, has been proposed for partially molten dunite. This has led to the suggestion of a unique, noncontinuous, or “threshold” permeability relation for upper mantle melts. A more critical analysis of pore microstructure in dunite indicates, however, that pore shapes had been misinterpreted: very little melt is present along grain boundaries. Melt instead occupies a triple‐junction network closely resembling that of the quartzite analog. Consequently, a relation similar to that for synthetic quartzites can describe upper mantle grain‐scale permeabilities, because permeability is a function of pore geometry alone.
Three‐dimensional, prestack, plane wave migration of teleseismic P‐to‐S converted phases: 2. Stacking multiple events
Authors: Poppeliers Christian , Pavlis Gary L.
Published: May 2003

In part 1 we developed the theoretical foundations of a prestack migration procedure to image forward scattered P to S (PdS) converted waves in the coda of teleseismic P waves. This paper addresses the issue of how to optimally stack data from multiple events migrated by this procedure. We apply matrix perturbation theory to develop an objective way to quantify noise in deconvolved PdS data. Application of the theory demonstrates that an optimal stack requires weighting the migrated data from each event by a signal‐to‐noise ratio criterion. We also find that the migrated PdS images have to be binned by back azimuth and balanced prior to the final stack. This is necessary to mitigate coherent noise that results from aliased microseism noise that is enhanced by our processing method. We processed 23 events recorded by the Lodore array in northwestern Colorado with our procedure. The results indicate the presence of a major, lithospheric scale discontinuity defined by a south dipping boundary within the crust that we interpret as the subsurface expression of the Cheyenne Belt. The suture is also marked by a transition in crustal thickness from 35 km on the Archean side to over 40 km on the Colorado Plateau side. We also observe a strong difference in the lithospheric mantle PdS conversion signature on opposite sides of the suture that suggests delamination and northward convergence of the Colorado lithosphere beneath the Wyoming province.
Helium isotopes in paleofluids and present‐day fluids of the Larderello geothermal field: Constraints on the heat source
Authors: Magro Gabriella , Ruggieri Giovanni , Gianelli Giovanni , Bellani Stefano , Scandiffio Giovanni
Published: May 2003

The He isotope composition of paleofluids entrapped in fluid inclusions of hydrothermal minerals is compared with the present‐day fluid composition of the Larderello geothermal field. Almost constant values of (3He/4He)m/(3He/4He)air (=R/Ra) over time indicate that no important changes have occurred in the deep source of gases, at least during the last 3.8 million years. On a regional scale, a correlation has been found between the R/Ra spatial distribution, heat flow, and Bouguer gravity anomaly. High values of R/Ra and heat flow, and low Bouguer anomaly values indicate that the Larderello field is an area of preferential escape for mantle‐derived fluids. A positive correlation has also been found between the R/Ra spatial distribution and a major seismic reflector named the “K horizon.” A deep magma source, refilled by periodic gas input from the mantle, is the most likely source of 3He‐enriched fluids and the anomalously high heat flow. The nearly constant value of R/Ra clearly indicates that input of fresh mantle material has occurred up to recent times. Clear evidence of mixing between mantle and crustal fluids indicates that the high R/Ra is the lower limit of the actual mantle value, which is suggested to be similar to the subcontinental European mantle. The decrease of R/Ra over time in the peripheral part of the Larderello field indicates that important changes in the feeding fracture system and/or cooling rate have occurred in these areas.
Mixing of magmatic volatiles with groundwater and interaction with basalt on the summit of Kilauea Volcano, Hawaii
Authors: Hurwitz Shaul , Goff Fraser , Janik Cathy J. , Evans William C. , Counce Dale A. , Sorey Michael L. , Ingebritsen Steven E.
Published: May 2003

We interpret new chemical and isotopic data from samples collected between October 1998 and March 2002 from the NSF well (also called the Keller well), the only deep well on the summit of Kilauea Volcano, Hawaii. Sample collection followed cleaning of the well, which renewed access to the hydrothermal system very close to the loci of magmatic and fumarolic activity. The chemical and isotopic compositions of the new samples differ remarkably from data published previously. On the basis of the S/Cl ratio and carbon and helium isotopes we conclude that the thermal fluids formed by condensation of magmatic gas into shallow meteoric groundwater. Gas condensation was followed by a complex pattern of basalt dissolution accompanied by an increase of fluid pH and precipitation of secondary minerals. Geochemical modeling and geothermometry imply that the fluids equilibrated with an assemblage of secondary minerals at temperatures between 90 and 140°C. The significantly different chemical composition of the NSF well fluids from that of springs along the southern coast of the island indicates that mass transport from the summit region toward the lower flanks of the volcano is limited.
Paleogene clockwise tectonic rotations in the forearc of central Andes, Antofagasta region, northern Chile
Authors: Arriagada César , Roperch Pierrick , Mpodozis Constantino , Dupont‐Nivet Guillaume , Cobbold Peter R. , Chauvin Annick , Cortés Joaquin
Published: May 2003

For the Central Valley of northern Chile (Antofagasta region), a paleomagnetic analysis of data from 108 sites, mainly in Mesozoic and Paleogene volcanic rocks, has yielded stable remanent magnetization directions for 86 sites. From these data, we infer clockwise tectonic rotations of up to 65° within the forearc domain of the central Andes. The apparent relationship between tectonic rotations and structural trends suggests that rotations occurred mainly during the Incaic orogenic event of Eocene–early Oligocene age. A few paleomagnetic results obtained in Neogene rocks do not show evidence of clockwise rotations. Hence the development of the Bolivian orocline during late Neogene time cannot be explained by simple bending of the whole margin. These results demonstrate that tectonic rotations within the forearc and pre‐Cordillera are key elements of early Andean deformation, which should be taken into account by kinematic models of mountain building in the central Andes.
Evaluation of “effective angular momentum function” formulations with respect to core‐mantle coupling
Authors: Dickman S. R.
Published: May 2003

In this work, various formulations describing the excitation of Earth rotation, originating with the effective angular momentum functions of Barnes et al. [1983], are assessed in the context of zero or complete coupling between the core and the mantle. Popular formulations have assumed zero coupling for the length of day component but are ambiguous for polar motion. Versions are developed here for the case of arbitrary core‐mantle coupling, and employed to derive consistent formulations for both extremes of coupling. Even for complete decoupling, all of the formulations can differ from each other noticeably, depending on the treatment of the oceans and the parameter values employed. It is recommended that agencies responsible for archiving atmospheric and other excitation data consider distributing “bare bones,” parameter‐free time series. A possible inconsistency in all formulations to date is the role of the degree 2 solid earth Love number. If the Chandler wobble takes place with a (near) absence of core‐mantle coupling, then prior estimates of the Love number have in reality been measuring its mantle‐only component. By implication the mantle may be much more anelastic at a 14‐month period than had previously been recognized.
An application of seismic tomography to basin focusing of seismic waves and Northridge earthquake damage
Authors: Baher Shirley A. , Davis Paul M.
Published: May 2003

Damage during the 1994 Northridge earthquake was anomalously concentrated in Santa Monica, California. Aftershock studies suggest that the basin substructure caused localized amplification of seismic waves by focusing the energy. However, the geometry and velocities of the geological structure are uncertain, relying on extrapolation from widely separated boreholes. We present results of a tomographic study which includes inverting over 30,000 travel times from a seismic array experiment on a 194 station array and a 10‐km‐long Vibroseis survey. We find that the damage zone is located above a subbasin several kilometers deep where low‐velocity sediments of the northeastern Los Angeles basin are separated from higher‐velocity rocks of the Santa Monica Mountains by the steeply dipping Santa Monica‐Potrero Canyon fault zone. Three‐dimensional ray tracing through this structure shows that focusing of rays from the Northridge earthquake epicenter occurs in the region where damage was greatest and where aftershock amplitudes are magnified. We also find that near‐surface P velocities, while low in the Santa Monica basin, do not correlate with the zone of damage. In a related study [Baher et al., 2002], site effects determined from coda waves are better correlated with damage, suggesting that compliant site effects were also important in localizing the damage distribution. We conclude that the anomalous damage was caused by a combination of site and basin focusing effects. Such geometries may be quite common in earthquake zones worldwide where similar results have been observed.
Large‐scale flank collapse events during the activity of Montagne Pelée, Martinique, Lesser Antilles
Authors: Le Friant Anne , Boudon Georges , Deplus Christine , Villemant Benoît
Published: May 2003

A horseshoe‐shaped structure already identified on the southwestern flank of Montagne Pelée (Martinique, Lesser Antilles arc) was previously interpreted as resulting of a flank collapse event, but no debris avalanche deposits were observed at the time. New offshore high‐resolution bathymetry and geophysical data (Aguadomar cruise; December 1998 to January 1999; R/V L'Atalante) lead us to identify three debris avalanche deposits on the submarine western flank of Montagne Pelée extending down to the Grenada Basin. They display morphological fronts and hummocky morphology on bathymetric data, speckled pattern on backscatter data and hyperbolic facies on 3.5 kHz and seismic profiles. New on‐land geological studies lead us to identify two other horseshoe‐shaped structures on the same flank of the volcano. The three submarine deposits have been traced back to the structures identified on land, which confirms the occurrence of repeated flank collapse events during the evolution of Montagne Pelée. The ages of the last two events are estimated at ∼9 ka and ∼25 ka on the basis of 14C and 238U/230Th dates. Every flank collapse produced debris avalanches which flowed down to the Caribbean Sea. We propose that the repeated instabilities are due to the large asymmetry of the island with western aerial and submarine slopes steeper than the eastern slopes. The asymmetry results from progressive loading by accumulation of volcanic products on the western slopes of the volcano and development of long‐term gravitational instabilities. Meteoric and hydrothermal fluid circulation on the floor of the second flank collapse structure also creates a weakened hydrothermalized area, which favors the recurrence of flank collapses.
Pwave seismic velocity structure derived from sonic logs and industry reflection data in the Los Angeles basin, California
Authors: Süss M. Peter , Shaw John H.
Published: May 2003

We present a new three‐dimensional seismicPwave velocity model of the Los Angeles basin that is based on more than 150 sonic logs and 7000 stacking velocities from industry reflection profiles. The model was developed using kriging techniques implemented in GoCAD (a commercial application specializing in the construction of surfaces, the analysis of spatially distributed data, and the modeling or simulation of properties) and is characterized by a heterogeneous, spatial varying velocity gradient with maximum velocities of ∼5000 m/s in the sedimentary section. Variability of the observedPwave velocities increases with depth, reflecting complex structural and sedimentological controls. The model contains a new representation of the basin shape as defined by the sediment‐basement interface, which was mapped using well control and seismic reflection profiles. Our model differs substantially in approach and outcome from previous representations of crustal velocity structure in southern California.