bluuebunny/crossref_metadata_2025 huggingface dataset:
A theoretical comparison of buoyancy‐driven and compaction‐driven fluid flow in oceanic sedimentary basins
Authors: Christiansen Lizet B. , Garven Grant
Published: May 2003

Compaction‐driven and buoyancy‐driven fluid flow are the major driving forces in submarine basins; however, the impact of coupling these driving forces has yet to be fully evaluated. This is the first analysis to couple the transient interactions of compaction‐driven and buoyancy‐driven flow in oceanic sedimentary basins. We use finite element modeling to explore the effects of sediment compaction overlying a permeable oceanic crust layer where free convection occurs. Four numerical experiments examine the parameter space where each driving force dominates the flow field. In higher‐permeability crust (k > 10−13 m2) buoyancy‐driven flow dominates the fluid flow patterns in the basin, though compaction does cause minor alterations to fluid velocities and convection cell evolution. Compaction dominates the system when low permeability crust inhibits free convection (k < 10−15 m2) and can stimulate fluid flow in the oceanic crust by directing fluid from sediment pore spaces downward into the crust, possibly rejuvenating flow in older, lower‐permeability submarine environments. In mixed‐flow systems (k ∼ 10−15 to 10−13 m2) both forces are approximately equal in importance and both contribute to the development of flow patterns. Fluid from the sediment layer is expelled downward into the permeable crust, which modifies the fluid flowlines. Instead of convection cells, a flow‐through pattern forms in which all fluid is transported laterally across the basin. This increase in lateral transport could have significant implications for geochemical mass transport in submarine environments. Though flow rates are relatively small (∼mm/yr), low‐velocity fluid flow can be important in geochemical, mechanical, biological, and hydrogeologic systems.
Spatial and temporal distribution of seismicity along the northern Mid‐Atlantic Ridge (15°–35°N)
Authors: Smith Deborah K. , Escartin Javier , Cannat Mathilde , Tolstoy Maya , Fox Christopher G. , Bohnenstiehl DelWayne R. , Bazin Sara
Published: May 2003

A detailed investigation of the relationship between the spatial and temporal patterns of the seismic activity recorded by six autonomous hydrophones and the structure of the northern Mid‐Atlantic Ridge between 15° and 35°N is presented. Two years of monitoring yielded a total of 3485 hydroacoustically detected events within the array recorded by four or more hydrophones. The seismically active zone extends ∼20 km to either side of the ridge axis, consistent with earlier results from studies of fault morphology. Along the axis, hydrophone‐recorded activity shows important variations. Areas with intense and persistent seismic activity (stripes) stand in sharp contrast to areas that lack seismicity (gaps). The regions of persistent activity are a new observation at mid‐ocean ridges. In general, the patterns of seismically active/inactive regions are also recognized in the 28‐year teleseismic record, implying that these patterns are maintained at timescales between a few years and a few decades. We find no simple relationship between individual segment variables (e.g., length or trend of the segment, maximum offset of discontinuities, or along‐axis change in mantle Bouguer anomaly (MBA) and water depths) and number of hydrophone‐recorded events. There does appear to be a correlation between axial thermal structure and seismicity. Regions of low and high numbers of events would thus correspond to thinner (hotter) and thicker (colder) lithosphere, respectively. Seismicity may reflect thermal structure at short timescales (decadal or longer), while relief and inferred crustal thickness may integrate this structure over longer periods of time (order of 1 Myr).
Development of finite strain in the convecting lower mantle and its implications for seismic anisotropy
Authors: McNamara Allen K. , van Keken Peter E. , Karato Shun‐Ichiro
Published: May 2003

Seismological observations have revealed patches of seismic anisotropy in regions related to mantle upwelling and paleosubduction within an otherwise isotropic lower mantle. A combination of numerical modeling and mineral physics is used to constrain the source of anisotropy in these regions in an effort to better understand lower mantle dynamics and mineral physics. Specifically, it is investigated whether lattice‐preferred orientation (LPO) can explain the anisotropy observed in regions of paleosubduction. Since LPO is caused by dislocation creep and is destroyed by diffusion creep, we can develop deformation mechanism maps to determine which regions allow for the development of a mineral fabric. Strain is then calculated in these regions and is related to mineral physics experiments combined with high‐pressure elastic constants of lower mantle minerals in order to assess the predicted seismic anisotropy. Uncertainties in rheological parameters such as the transition stress between dislocation creep and diffusion creep necessitate a full evaluation of the parameter range. The effect of variations in transition stress, activations parameters, and strength of slabs on fabric development is investigated. It is shown that LPO is a likely candidate for the cause of lowermost mantle anisotropy in regions of paleosubduction.
Slip distribution of the 1952 Tokachi‐Oki earthquake (M 8.1) along the Kuril Trench deduced from tsunami waveform inversion
Authors: Hirata Kenji , Geist Eric , Satake Kenji , Tanioka Yuichiro , Yamaki Shigeru
Published: May 2003

We inverted 13 tsunami waveforms recorded in Japan to estimate the slip distribution of the 1952 Tokachi‐Oki earthquake (M 8.1), which occurred southeast off Hokkaido along the southern Kuril subduction zone. The previously estimated source area determined from tsunami travel times [Hatori, 1973] did not coincide with the observed aftershock distribution. Our results show that a large amount of slip occurred in the aftershock area east of Hatori's tsunami source area, suggesting that a portion of the interplate thrust near the trench was ruptured by the main shock. We also found more than 5 m of slip along the deeper part of the seismogenic interface, just below the central part of Hatori's tsunami source area. This region, which also has the largest stress drop during the main shock, had few aftershocks. Large tsunami heights on the eastern Hokkaido coast are better explained by the heterogeneous slip model than previous uniform‐slip fault models. The total seismic moment is estimated to be 1.87 × 1021 N m, giving a moment magnitude of Mw = 8.1. The revised tsunami source area is estimated to be 25.2 × 103 km2, ∼3 times larger than the previous tsunami source area. Out of four large earthquakes with M ∼ 7 that subsequently occurred in and around the rupture area of the 1952 event, three were at the edges of regions with relatively small amount of slip. We also found that a subducted seamount near the edge of the rupture area possibly impeded slip along the plate interface.
Static compression of iron‐silicon alloys: Implications for silicon in the Earth's core
Authors: Lin Jung‐Fu , Campbell Andrew J. , Heinz Dion L. , Shen Guoyin
Published: May 2003

Three iron‐silicon alloys (Fe85Si15, Fe71Si29, and ε‐FeSi) have been studied in a diamond anvil cell at room temperature up to 55 GPa by in situ energy‐dispersive X‐ray diffraction techniques. A body centered cubic (bcc) to hexagonal close packed (hcp) phase transformation in Fe85Si15 began at 16 GPa and was completed by 36 GPa. No phase transformations were observed in either Fe71Si29 or ε‐FeSi at high pressures, even when laser‐heated to about 2000 K. The isothermal bulk modulus (K0T) of hcp‐Fe85Si15 is 141 (±10) GPa with K′0T = 5.70(±0.60) and V02 = 6.882(±0.031) cm3/mol (per molar atom). The K0T of Fe71Si29 is 199.0 (±5.3) GPa with K′0T = 5.66(±0.61) and V0 = 6.887(±0.014) cm3/mol, and the K0T of ε‐FeSi is 184.7 (±3.9) GPa with K′0T of 4.75 (±0.37) and V0 = 6.790(±0.007) cm3/mol. Our study indicates that the substitution of Si into iron would lower the density of iron, but significantly changes its compressibility neither in the bcc phase, nor at high pressures in the hcp phase. Upon comparison with the Preliminary Reference Earth Model, the calculated equations of state (EOS) of hcp‐Fe85Si15, using the Mie‐Grüneisen EOS, indicate that an outer core containing about 8–10 wt.% Si and inner core containing about 4 wt.% Si in iron would satisfy the seismological constraints. Addition of silicon into iron increases the bulk sound velocity of iron, consistent with silicon being a light element in the Earth's core.
Deciphering shallow paleomagnetic inclinations: 1. Implications from correlation of Albian volcanic rocks along the Insular/Intermontane Superterrane boundary in the southern Canadian Cordillera
Authors: Haskin M. L. , Enkin R. J. , Mahoney J. B. , Mustard P. S. , Baker J.
Published: May 2003

Geologic and paleomagnetic data lead to two contradictory hypotheses regarding the paleoposition of the Insular and Intermontane Superterranes that presently constitute the western Canadian Cordillera. Paleomagnetic data from the Insular and Intermontane superterranes suggest a southerly origin coinciding with the latitude of Mexico and the northwest United States, respectively, during the mid‐Cretaceous. Geologic evidence points to a northerly origin for these same tectonic entities during this period; both models cannot be correct. Geologic and paleomagnetic data from the Empire Valley–Churn Creek area in south central British Columbia (51.5°N, 122.5°W) are critical to resolving these contradictory hypotheses. Late Cretaceous rocks correlated to the Insular Superterrane with large paleomagnetic displacements unconformably overlie mid‐Cretaceous rocks correlative to the Spences Bridge Group of the Intermontane Superterrane. We provide paleomagnetic evidence of this correlation based on similar magnetic properties, opaque mineral assemblages, demagnetization behavior, fold test results, mean inclinations, clockwise vertical axes rotations, and statistically indistinguishable paleomagnetic poles and displacement estimates. This correlation and the observed geologic relationships in the Empire Valley–Churn Creek area indicate that the Insular and Intermontane Superterranes were linked by the mid‐Cretaceous. Sites from the two previous Spences Bridge Group studies are combined with their correlatives in the Empire Valley–Churn Creek area to give 81 sites that yield a paleomagnetic pole of 60.5°N, 304.5°E, dp = 3.7°, dm = 5.5° which corresponds to 1050 ± 450 km of displacement from the south. This new displacement estimate suggests that the Spences Bridge arc formed at the latitude of southern Oregon during the mid‐Cretaceous.
Deciphering shallow paleomagnetic inclinations: 2. Implications from Late Cretaceous strata overlapping the Insular/Intermontane Superterrane boundary in the southern Canadian Cordillera
Authors: Enkin R. J. , Mahoney J. B. , Baker J. , Riesterer J. , Haskin M. L.
Published: May 2003

Significant discrepancies exist between paleomagnetic and geologic estimates of the timing and magnitude of terrane displacement in the southern Canadian Cordillera. Lithostratigraphic, palynological, geochronologic, geochemical, and structural data, to which we add paleomagnetic data, demonstrate that Upper Cretaceous strata in Churn Creek are laterally equivalent facies of the Silverquick/Powell Creek (SPC) succession. These rocks therefore comprise a single overlap sequence linking the Insular and Intermontane Superterranes by Late Albian‐Cenomanian time (∼95 Ma), contradicting previous assertions based solely on paleomagnetic interpretation that major (∼2000 km) latitudinal displacement separated the two regions. The Churn Creek succession retains primary magnetic remanence, as demonstrated by positive fold, conglomerate and contact tests. The sediments do not hold the anisotropy signature of compaction‐shallowed inclinations. The mean inclination of all SPC sites, from both Mount Tatlow and Churn Creek, is 55.6° ± 2.3° (N = 36 sites), corresponding to a paleolatitude of 36.1° ± 2.4°. This reflects northward translation of the composite Insular/Intermontane Superterrane of 3050 ± 450 km between 85 and 50 Ma. The Silverquick/Powell Creek succession at Churn Creek unconformably overlies the northern end of the Spences Bridge Group, a late Early Cretaceous volcanic arc assemblage on the Intermontane Superterrane. Paleomagnetic data in the companion paper place it 1050 ± 450 km south of its current position between 100 and 105 Ma. Integration of these new data sets require that the Insular and Intermontane Superterranes formed a single, enormous crustal block that underwent complex, episodic south‐north translation between circa 105 and 50 Ma. The timing, rate, and vectors of translation are problematic in terms of geologic constraints along the continental margin.
Spatiotemporal gravity changes at Miyakejima Volcano, Japan: Caldera collapse, explosive eruptions and magma movement
Authors: Furuya Masato , Okubo Shuhei , Sun Wenke , Tanaka Yoshiyuki , Oikawa Jun , Watanabe Hidefumi , Maekawa Tokumitsu
Published: May 2003

Calderas are ubiquitous topographic features of volcanos, yet caldera formation itself has not been recorded intensively by modern measurement techniques. Here we report the spatiotemporal gravity changes before and after caldera collapse at the Miyakejima volcano, Japan in 2000. A gravity decrease of as much as 145 μGal (1 μGal = 10−8 m/s2) at the summit area since June 1998 was detected 2 days prior to the collapse, interpreted as reflecting the formation of a large void beneath the volcano. Gravity changes detected after the initiation of collapse can mostly be corrected by the effect of collapsed topography, from which a rapid rate of collapse of more than 1.6 × 107 m3/d can be inferred. Correcting for the effect of topography change, we identified a temporal decrease in gravity from the middle of July to late August despite ground subsidence. The gravity decrease is interpreted as a reduction of the density in a cylindrical conduit, attributed to water inflow from an ambient aquifer that also promoted intensive magma‐water interaction and subsequent explosive eruptions. From September to at least November 2000, gravity values at all sites increased significantly by a degree that cannot be explained by ground displacement alone. We interpret this temporal evolution as primarily due to magma ascent and refilling of the conduit.
Mainshocks are aftershocks of conditional foreshocks: How do foreshock statistical properties emerge from aftershock laws
Authors: Helmstetter Agnès , Sornette Didier , Grasso Jean‐Robert
Published: May 2003

The inverse Omori law for foreshocks discovered in the 1970s states that the rate of earthquakes prior to a mainshock increases on average as a power law ∝ 1/(tc − t)p′ of the time to the mainshock occurring at tc. Here, we show that this law results from the direct Omori law for aftershocks describing the power law decay ∼ 1/(t − tc)p of seismicity after an earthquake, provided that any earthquake can trigger its suit of aftershocks. In this picture, the seismic activity at any time is the sum of the spontaneous tectonic loading and of the activity triggered by all preceding events weighted by their corresponding Omori law. The inverse Omori law then emerges as the expected (in a statistical sense) trajectory of seismicity, conditioned on the fact that it leads to the burst of seismic activity accompanying the mainshock. In particular, we predict and verify by numerical simulations on the epidemic‐type aftershock sequence (ETAS) model that p′ is always smaller than or equal to p and a function of p, of the b‐value of the Gutenberg–Richter law (GR), and of a parameter quantifying the number of direct aftershocks as a function of the magnitude of the mainshock. The often documented apparent decrease of the b‐value of the GR law at the approach to the mainshock results straightforwardly from the conditioning of the path of seismic activity culminating at the mainshock. However, we predict that the GR law is not modified simply by a change of b‐value but that a more accurate statement is that the GR law gets an additive (or deviatoric) power law contribution with exponent smaller than b and with an amplitude growing as a power law of the time to the mainshock. In the space domain, we predict that the phenomenon of aftershock diffusion must have its mirror process reflected into an inward migration of foreshocks toward the mainshock. In this model, foreshock sequences are special aftershock sequences, which are modified by the condition to end up in a burst of seismicity associated with the mainshock. Foreshocks are not just statistical creatures but are genuine forerunners of large shocks as shown by the large prediction gains obtained using several of their qualifiers.
Constraints on aspherical core structure from PKiKP‐PcP differential travel times
Authors: Koper Keith D. , Pyle Moira L. , Franks Jill M.
Published: May 2003

We have assembled a data set of 302 differential PKiKP‐PcP travel times that were recorded by seismic array stations of the International Monitoring System. This is an order of magnitude more data than all previous PKiKP‐PcP observations combined. The differential times were measured using a beam‐forming and cross‐correlation algorithm that has a precision of 1–2 sampling intervals (<0.1 s). The use of differential times mitigates the effects of hypocentral errors and heterogeneities in the crust and upper mantle, leaving the data sensitive to deep Earth structure. The PKiKP‐PcP times are especially sensitive to variations in the thickness of the liquid outer core. The data are well fit by PREM, with residuals having a mean of −0.29 s and a standard deviation of 0.44 s, and rule out lateral variations in outer core thickness of more than 4 km. Most of the geographically coherent residuals can be explained by lower mantle heterogeneities and ellipticity corrections however there remain at least three regions that seem to require aspherical core structure. The anomalous data are best explained by long‐wavelength topography on the core‐mantle boundary (CMB) with peak‐to‐peak amplitude of 3.5 km. The CMB topography is most likely maintained by isostatic compensation of thickness variations in a distinct layer at the base of the mantle.
Structure of the SE Greenland margin from seismic reflection and refraction data: Implications for nascent spreading center subsidence and asymmetric crustal accretion during North Atlantic opening
Authors: Hopper John R. , Dahl‐Jensen Trine , Holbrook W. Steven , Larsen Hans Christian , Lizarralde Dan , Korenaga Jun , Kent Graham M. , Kelemen Peter B.
Published: May 2003

Seismic reflection and refraction data from the SE Greenland margin provide a detailed view of a volcanic rifted margin from Archean continental crust to near‐to‐average oceanic crust over a spatial scale of 400 km. The SIGMA III transect, located ∼600 km south of the Greenland‐Iceland Ridge and the presumed track of the Iceland hot spot, shows that the continent‐ocean transition is abrupt and only a small amount of crustal thinning occurred prior to final breakup. Initially, 18.3 km thick crust accreted to the margin and the productivity decreased through time until a steady state ridge system was established that produced 8–10 km thick crust. Changes in the morphology of the basaltic extrusives provide evidence for vertical motions of the ridge system, which was close to sea level for at least 1 m.y. of subaerial spreading despite a reduction in productivity from 17 to 13.5 km thick crust over this time interval. This could be explained if a small component of active upwelling associated with thermal buoyancy from a modest thermal anomaly provided dynamic support to the rift system. The thermal anomaly must be exhaustible, consistent with recent suggestions that plume material was emplaced into a preexisting lithospheric thin spot as a thin sheet. Exhaustion of the thin sheet led to rapid subsidence of the spreading system and a change from subaerial, to shallow marine, and finally to deep marine extrusion in ∼2 m.y. is shown by the morphological changes. In addition, comparison to the conjugate Hatton Bank shows a clear asymmetry in the early accretion history of North Atlantic oceanic crust. Nearly double the volume of material was emplaced on the Greenland margin compared to Hatton Bank and may indicate east directed ridge migration during initial opening.
Triggered aseismic fault slip from nearby earthquakes, static or dynamic effect?
Authors: Du Wen‐xuan , Sykes Lynn R. , Shaw Bruce E. , Scholz Christopher H.
Published: May 2003

Observations show that an earthquake can affect aseismic slip behavior of nearby faults and produce “triggered aseismic fault slip.” Two types of stress changes are often examined by researchers as possible triggering sources. One is the static stress change associated with the faulting process and the other is the dynamic stress change or transient deformation generated by the passage of seismic waves. No consensus has been reached, however, regarding the mechanism(s) of triggered aseismic fault slip. We evaluate the possible triggering role of static stress changes by examining observations made after 10 large earthquakes in California. Most of the nearby fault segments that slipped aseismically were encouraged to move by the imposed positive changes in static Coulomb Failure Stress (CFS). Nonetheless, three discrepancies or failures with this model exist, which implies that static stress triggering either is or is not the sole mechanism causing the observed triggered slip. We then use a spring‐slider system as a simplified fault model to study its slip behavior and the impact of transient (dynamic) loading on it. We show that a two‐state‐variable rate‐dependent and state‐dependent frictional law can generate creep events. Transient loads are then put into the system. Certain types of them can cause a large time advance of (or trigger) the next creep event. While our work examines triggered creep events near the surface, it may well have implications for the occurrence of similar events near the bottom of the seismogenic zone where a transition in frictional stability occurs.
Fluid conduits in carbonate‐hosted seismogenic normal faults of central Italy
Authors: Agosta F. , Kirschner D. L.
Published: May 2003

We studied the structures and stable isotope geochemistry of carbonate fault rocks in four normal faults of central Italy. The faults juxtapose Meso‐Cenozoic carbonates of the footwalls against continental basins of the hanging walls. Footwall rocks exposed along fault scarps have been exhumed from depths of ∼1 km. The fault rocks are systematically arranged in each fault and can be separated into five distinct domains. Farthest from the main fault contact are undeformed host rock and fractured host rock (domain 1). Progressively closer to the fault contact, in the core of the fault, are gouge (domain 2), cataclasite (domain 3), and cement‐dominated horizons with planar slip surfaces (domain 4). Thin horizons of brecciated hanging wall sediments (domain 5) are adjacent, and locally accreted, to the footwall. Structural and stable isotope data are consistent with compartmentalization of fluid in the faults during exhumation. The data are most consistent with these fluids being predominantly evolved meteoric water rather than fluids from the mantle, crustal magmas, and/or devolatilizing carbonate rocks. Meteoric water infiltrated domains 4 and 5 of the faults, either from hanging wall sediments or directly at the land surface. The gouge and cataclasites of domains 2 and 3 were impermeable barriers to movement of meteoric water into domain 1 and undamaged host rocks of the footwall. The results of this study do not support models of earthquake nucleation and rupture that envision large volumes of deep‐seated fluids passing upward through shallow portions of these seismogenic faults.
Hybrid collapse mechanism at Piton de la Fournaise volcano, Reunion Island, Indian Ocean
Authors: Merle Olivier , Lénat Jean‐François
Published: May 2003

Recent studies have shown that hydrothermal alteration in the core of volcanoes may weaken the edifice and trigger sector collapse [Lopez and Williams, 1993; Day, 1996; Voight and Elsworth, 1997; van Wyk de Vries et al., 2000; Reid et al., 2001]. Alteration of volcanic products must take place over long periods of times before collapse can occur, so this process is considered to be less frequent than volcanic failure generated by magmatic intrusion and flank spreading. However, acceleration of this slow process may occur along one direction as a result of a weak lateral confinement following outward gravitational sliding of a flank of the volcano. We present here evidence that the Enclos‐Fouqué depression of Piton de la Fournaise volcano results from such a hybrid mechanism. Seaward sliding of the Grand Brûlé trough on top of an ancient hypovolcanic complex led to a lateral unconfined boundary along which collapse of the altered core of the edifice took place. The result is a caldera‐like structure breached on the Grand Brûlé slide. Scaled experiments to simulate this process of pseudocaldera formation reveal tectonic features remarkably similar to those encountered in the field and demonstrate the mechanical consistency of the model. Similar processes may have played a role in the evolution of other volcanoes.
Fossil hot spot‐ridge interaction in the Musicians Seamount Province: Geophysical investigations of hot spot volcanism at volcanic elongated ridges
Authors: Kopp H. , Kopp C. , Phipps Morgan J. , Flueh E. R. , Weinrebe W. , Morgan W. J.
Published: May 2003

The Musicians Seamount Province is a group of volcanic elongated ridges (VERs) and single seamounts located north of the Hawaiian Chain. A 327° trending seamount chain defines the western part of the province and has been interpreted as the expression of a Cretaceous hot spot beneath the northward moving Pacific Plate. To the east, elongated E‐W striking ridges dominate the morphology. In 1999, wide‐angle seismic data were collected across two 400 km long VERs. We present tomographic images of the volcanic edifices, which indicate that crustal thickening occurs in oceanic layer 2 rather than in layer 3. This extrusive style of volcanism appears to strongly contrast with the formation processes of aseismic ridges, where crustal thickening is mostly accommodated by intrusive underplating. High‐resolution bathymetry was also collected, which yields a detailed image of the morphology of the VERs. From the occurrence of flat‐top guyots and from the unique geomorphologic setting, two independent age constraints for the Pacific crust during the Cretaceous “quiet” zone are obtained, allowing a tectonic reconstruction for the formation of the Musicians VERs. Hot spot‐ridge interaction leads to asthenosphere channeling from the plume to the nearby spreading center over a maximum distance of 400 km. The Musicians VERs were formed by mainly extrusive volcanism on top of this melt‐generating channel. The proposed formation model may be applicable to a number of observed volcanic ridges in the Pacific, including the Tuamotu Isles, the eastern portion of the Foundation chain, and the western termination of the Salas y Gomez seamount chain.
Identification of sources of potential fields with the continuous wavelet transform: Two‐dimensional wavelets and multipolar approximations
Authors: Sailhac Pascal , Gibert Dominique
Published: May 2003

We show new continuous wavelet‐based transformation techniques of potential field maps and propose interpretation schemes for characterizing three‐dimensional (3‐D) sources having some finite extent. As in previous studies, we use wavelets derived from the Poisson kernel which generate a position‐altitude representation of potential field data initially measured at one level above the ground. We use first‐order or second‐order wavelets to obtain wavelet transform parameters related to the 3‐D analytic signals or 3 × 3 tensors of the data. We show how such parameters can be used to characterize the sources, first, by using the assumption of their local homogeneity and, second, by using multipolar expansions to estimate the sizes of the horizontal or vertical extent of the 3‐D source. Thus we generalize to 3‐D the Taylor expansion of upward continued 2‐D analytic signals previously shown for the interpretation of profiles transformed using complex wavelets. Such a 3‐D interpretation scheme based upon position‐altitude representations has been developed to interpret the maps of scalar potential field anomalies (e.g., magnetic total field anomalies or vertical gravity anomalies) but can be also used for upward continued maps of vector or tensor of potential field anomalies as obtained by new surveys of the full tensor of gravity; this also concerns the interpretation of derived potentials considered in poroelasticity and electrokinetic in porous materials of the Earth. We illustrate the technique on synthetic data; in addition, a first application on aeromagnetic data from French Guyana shows the potential of the technique.
Origin of the International Terrestrial Reference Frame
Authors: Dong D. , Yunck T. , Heflin M.
Published: May 2003

With recent improvements in space geodesy, the Earth's center of mass (CM) and center of figure (CF) are no longer indistinguishable. The current origin of the International Terrestrial Reference Frame (ITRF) is defined as the CM, which shows measured seasonal variations of several millimeters to 1 cm with respect to true CM. As scientists study Earth's dynamic deformations on seasonal and shorter timescales and begin to compare observed geocenter motion with predictions from geophysical models, the reference frame origin presents significant error due to missing the geocenter motion. This paper discusses the nature of the origin of the ITRF and explores the sensitivity of GPS measurements to geocenter motion. We find that since the values of nonlinear geocenter motion are not included in the positions of the ITRF sites, the behavior of the current ITRF origin reflects CM on secular timescale but reflects CF on seasonal and short timescales. The nature of the ITRF origin depends on both the adopted kinematic model and unmodeled network motion. The realized ITRF origin should be defined by a new nomenclature to reflect its nature accurately. By the new nomenclature, the origin will maintain its current long‐term stability, while improving its stability on seasonal timescales to the submillimeter level. With the degree‐1 deformation approach, GPS measurements are able to provide potentially valuable information on geocenter variations on seasonal and short timescales.
Evidence for small‐scale mantle convection in the upper mantle beneath the Baikal rift zone
Authors: Gao Stephen S. , Liu Kelly H. , Davis Paul M. , Slack Phillip D. , Zorin Yuliy A. , Mordvinova Valentina V. , Kozhevnikov Vladimir M.
Published: May 2003

Inversion of teleseismic P wave travel time residuals collected along a 1280‐km‐long profile traversing the Baikal rift zone (BRZ) reveals the existence of an upwarped lithosphere/asthenosphere interface, which causes a travel time delay of about 1 s at the rift axis (“central high”). An area with early arrivals relative to the stable Siberian platform of up to 0.5 s is observed on each side of the rift, about 200 km from the rift axis (“flank lows”). While the location of the central high is approximately fixed in the vicinity of the rift axis, those of the flank lows vary as much as 200 km with the azimuth of the arriving rays. We use three techniques to invert the travel time residuals for velocity anomalies beneath the profile. Two of the techniques assume an isotropic velocity structure, and one of them considers a transversely isotropic velocity model with a vertical axis of symmetry. We use independent geophysical observations such as gravity, active source seismic exploration, and crustal thickness measurements to compare the applicability of the models. Other types of geophysical measurements suggest that the model involving transverse isotropy is a plausible one, which suggests that the central high and flank lows are caused by the combined effects of an upwarped asthenosphere with a 2.5% lateral velocity reduction, and a velocity increase due to transverse isotropy with a vertical axis of symmetry. We consider the anisotropy to be the result of the vertical component of a lithosphere/asthenosphere small‐scale mantle convection system that is associated with the rifting.
A comparison of tidal ocean loading models using superconducting gravimeter data
Authors: Boy J.‐P. , Llubes M. , Hinderer Jacques , Florsch Nicolas
Published: May 2003

Errors in global ocean models have been shown to be larger than the accuracy of surface gravity measurements [Baker and Bos, 2001] and also new space gravity missions [Ray et al., 2001]. In this paper, we compare the tidal loading estimated using 10 different ocean tide models to the surface gravity variations observed with 12 superconducting gravimeters (SG) belonging to the Global Geodynamics Project [Crossley et al., 1999] worldwide network. Ocean tidal gravity variations are shown to be mostly dependent on tidal sea height variations within a radius of ∼5000 km. Precise surface gravity measurements are therefore another tool for validating ocean tidal models from short to long wavelengths in addition, for example, to tide gauge data. In this study we also compute the tidal loading using several different available computer codes. As for ocean tidal models, we show that the differences between tidal loading programs are frequently larger than the error estimates of SG measurements. Compared to previous models, both recent hydrodynamical and TOPEX‐Poseidon derived tidal models allow a significant reduction of discrepancies between ocean tidal observations using SG and tidal loading estimates. There still remains, however, a nonnegligible in‐phase discrepancy between ocean tidal loading estimates and surface gravity observations, whereas the out‐of‐phase component is much smaller. We have shown that the in‐phase disagreement does not seem to be caused by an inaccurate determination of the SG calibration.
Postseismic crustal deformation following the 1993 Hokkaido Nansei‐oki earthquake, northern Japan: Evidence for a low‐viscosity zone in the uppermost mantle
Authors: Ueda Hideki , Ohtake Masakazu , Sato Haruo
Published: May 2003

Postseismic crustal deformation following the 1993 Hokkaido Nansei‐oki earthquake (M = 7.8), northern Japan, was observed by GPS, tide gauge, and leveling measurements in southwestern Hokkaido. As a result of comprehensively analyzing these three kinds of geodetic data, we found the chief cause of the postseismic deformation to be viscoelastic relaxation of the coseismic stress change in the uppermost mantle. Afterslip on the main shock fault and its extension, by contrast, cannot explain the deformation without unrealistic assumptions. The viscoelastic structure, which we estimated from the postseismic deformation, consists of three layers: an elastic first layer with thickness of 40 km, a viscoelastic second layer with thickness of 50 km and viscosity of 4 × 1018 Pa s, and an elastic half‐space. This model suggests the presence of an anomalously low‐viscosity zone in the uppermost mantle. The depth of the viscoelastic layer roughly agrees with that of the high‐temperature portion and the low‐Vp zones of the mantle wedge beneath the coast of the Japan Sea in northern Honshu. These correlations suggest that the low‐viscosity primarily results from high temperature of the mantle material combined with partial melt and the presence of H2O. These correlations also suggest that the low‐viscosity zone in the uppermost mantle is widely distributed in the back arc side of the volcanic front and that postseismic deformation induced by viscoelastic relaxation may be frequently observed after large earthquakes in this area.
Isotopic evidence (3He/4He, of fluid‐triggered intraplate seismicity
Authors: Bräuer Karin , Kämpf Horst , Strauch Gerhard , Weise Stephan M.
Published: May 2003

The aim of this study is to find a link between the occurrence of mantle‐derived CO2 exhalations and earthquake swarms beneath the western Eger rift, central Europe. We monitored the gas and isotope composition of the free gas of the Eisenquelle mineral spring and the Bublak mofette weekly for 2 years. These time series include periods before, during, and after an earthquake swarm event. Shifts in δ13C and 3He/4He were found for several months after the occurrence of the earthquake swarm on 4 and 5 December 1994. The isotopic shifts were explained by the admixture of crustal fluids released at the hypocenter to the “permanent” mantle volatile flux. From the beginning of the earthquake swarm, as starting point for the gas release from the hypocenter and the moment of arriving fluid with changed isotopic signature at the locations, we estimated that the fluid transport velocity in the upper crust ranges between 400 m/day near a center of CO2 emanation (Bublak mofette) and 50 m/day in the periphery (Eisenquelle mineral spring). Fault rupture induced by the buildup of fluid overpressure in the brittle crust (associated with sealing processes surrounding the ascending channels of the permanent mantle flux) seems to be the main reason for periodic/episodic earthquake swarm activity in this area.
Geomagnetic field strength during late Miocene: First paleointensity results from Baja California
Authors: Goguitchaichvili Avto , Morales Juan , Cañon‐Tapia Edgardo , Negrete Raquel
Published: May 2003

Thellier paleointensity experiments were carried out on 73 selected samples from the late Miocene Baja California volcanic flows for the first time. The selected samples showed a stable remanent magnetization observed upon thermal treatment and reasonably reversible continuous susceptibility curves, compatible with Ti‐poor titanomagnetites. Judging from the ratios of hysteresis parameters, it seems that the samples fall in the small pseudosingle‐domain grain size region. Thirty‐three samples from 10 individual flows yielded acceptable paleointensity estimates. The natural remanent magnetization (NRM) fractions used for paleointensity determination range from 37 to 81% and the quality factor varies between 4.5 and 24.5, being normally >5. The mean paleointensity values per flow obtained in this study range from 18.1 ± 3.1 to 45.6 ± 4.3 μT and the corresponding virtual dipole moments (VDMs) range from 3.8 ± 0.6 to 8.8 ± 0.8 (1022A m2). These values correspond to mean value of 6.6 ± 1.7 × 1022A m2, which is ∼85% of the present geomagnetic axial dipole. The average VDM value for late Miocene, according to all available, selected data, is 6.1 ± 1.5 (1022A m2), which is higher than average paleofield for the period 5–160 Ma (4.2 ± 2.3 × 1022A m2afterJuarez and Tauxe[2000]). Given the large dispersion and the very poor distribution of reliable absolute intensity data, it is hard to draw any firm conclusions regarding the time evolution of the geomagnetic field. More high‐quality determinations are strongly needed.
Magnetic fabric and implications for pyroclastic flow and lahar emplacement, Albano maar, Italy
Authors: Porreca M. , Mattei M. , Giordano G. , De Rita D. , Funiciello R.
Published: May 2003

The Albano polygenetic maar is the youngest eruptive center of the quiescent Colli Albani volcano, located near the city of Rome. The most recent activity of the Albano maar extends from ∼23 ka into the Holocene and produced the small volume, basic, phreatomagmatic Peperino Albano (PA) ignimbrite, and, more recently, phreatomagmatic surge and lahar deposits related to the overspill of the Albano maar lake. We have performed an anisotropy of magnetic susceptibility (AMS) study, in order to define the relationships between the magnetic fabric and the flow mechanisms of the PA and of the phreatomagmatic deposits and lahars. AMS results indicate different transport and/or depositional systems for the veneer and valley pond facies in the PA ignimbrite and for the lahar deposits. AMS also demonstrates that flow directions are mainly controlled by the paleotopography. The paleotopographic control has been interpreted in terms of talweg sedimentation even at proximal locations where deposition occurs from dilute pyroclastic flows. Furthermore, AMS results clearly evidence a southward provenance for Holocene post‐PA ignimbrite units, cropping out in the Ciampino plain, and confirm their origin from Albano maar lake overspill. We demonstrate that AMS is a reliable marker to determine paleoflow directions also in small volume phreatomagmatic ignimbrites and in syneruptive lahar deposits and can be successfully used to define their depositional systems.
Transmission of basal variability to a glacier surface
Authors: Gudmundsson G. Hilmar
Published: May 2003

Transmission of basal variability to a glacier surface is investigated using analytical models for a linearly viscous medium. The three‐dimensional transient response of the surface to both bedrock undulations and spatial variations in basal slipperiness for perturbations of arbitrary wavelengths is determined using perturbation methods. Both information transfer toward the surface and lateral transmission of horizontal stresses are strongly affected by the slip ratio, that is, the ratio of basal sliding to deformational velocity. For any mean bedrock slope, and above a minimum value of slip ratio, the amplitude transfer of bedrock undulations toward the surface has a local maximum at undulation span corresponding to about 3–8 times the mean ice thickness. The transmission of basal variability to a glacier surface increases quite significantly with increasing slip ratio. This explains why the surfaces of fast flowing ice streams are more undulating than the slower moving bordering areas. At slip ratios higher than about 100, the flow of glaciers and ice sheets becomes nonlocal in the sense that surface velocities and buildup and propagation of surface undulations cannot be calculated accurately on the basis of local thickness and slope. Using linearized long‐wave theories at these slip ratios, instead of the more accurate arbitrary wavelength theory, gives estimates of decay times that are an order‐of‐magnitude too small and phase velocities several times too large. The problem of the propagation and decay of small‐amplitude surface undulations on glaciers in three dimensions is solved. Small‐amplitude surface waves on glaciers are strongly diffusive and dispersive. Redistribution of mass on ice sheets and glaciers is a diffusion process, and it is misleading, albeit not mathematically incorrect, to describe the reaction of glaciers to surface perturbations in terms of a wave propagation.
The C27n‐C26r geomagnetic polarity reversal recorded in the west Greenland flood basalt province: How complex is the transitional field?
Authors: Riisager Janna , Riisager Peter , Pedersen Asger Ken
Published: May 2003

We present a paleomagnetic record of the C27n‐C26r (∼60.9 Ma) geomagnetic polarity reversal (GPR) sampled in two separate lava sequences in the Vaigat Formation, west Greenland flood basalt province. The two profiles, distanced 16 km apart, consist of 45 and 35 sampled lava flows of which 31 and 23, respectively, record transitional field directions. A prominent feature of each profile is the presence of successive flows with similar paleomagnetic directions, which based on geological, geochemical, and paleomagnetic evidences are interpreted to represent flow fields (i.e., lavas erupted in a short period of time) rather than periods when the geomagnetic field stood still. Multimodel photogrammetry shows that the typical flow length of Vaigat Formation picritic lavas is <2 km, which explains why flow‐to‐flow correlation of paleomagnetic directions between the two profiles (16 km apart) is not possible. The two profiles therefore record different and independent time series of the same GPR, allowing us to cast light on some of the questions that have arisen from single records. Particularly noteworthy is the fact that the two high‐resolution paleomagnetic records share no general trends, clearly indicating that the C27n‐C26r transitional field was more complex and faster changing than can be resolved by either record. In addition, we obtained stable polarity paleomagnetic data from both pretransitional C27n and posttransitional C26r yielding a paleomagnetic pole latitude 64.8°N, longitude 141.5°E (N = 14, K = 19.8, A95 = 9.2°) that passes reversal test.
Surface temperature trends in Russia over the past five centuries reconstructed from borehole temperatures
Authors: Pollack Henry N. , Demezhko Dmitriy Y. , Duchkov Albert D. , Golovanova Inessa V. , Huang Shaopeng , Shchapov Vladislav A. , Smerdon Jason E.
Published: May 2003

We analyze borehole temperature logs from 101 sites in Russia and nearby areas to reconstruct the ground surface temperature history (GSTH) over the past five centuries. The data are drawn principally from three regions: the Urals, southwest Siberia, and northeast Siberia. We derive GSTHs for each region individually, and a composite “all‐Russia” GSTH from the full ensemble of sites. The results show that over the past 500 years, the investigated areas have on average warmed ∼1 K, with more than half of the warming occurring in the 20th century alone, and 70–80% in the 19th and 20th centuries taken together. The 16th through 18th centuries in the Urals and southwest Siberia were on average 0.1–0.2 K cooler than at the beginning of the 19th century, but northeast Siberia was more moderate in the 16th through 19th centuries, relative to the present‐day, than the Urals or southwest Siberia. A wide variety of instrumental, proxy, and indirect evidence support these geothermal results.
A seismic reflection profile study of lithospheric flexure in the vicinity of the Cape Verde Islands
Authors: Ali M. Y. , Watts A. B. , Hill I.
Published: May 2003

Seismic reflection profile data are used to determine the stratigraphic “architecture” of the flexural moat that flanks the Cape Verde Islands. The moat region is characterized by upward of 1–2 km of poorly to well‐stratified material. The two lowermost units thicken from west to east and are attributed to sediment loading and flexure at the nearby West Africa continental margin during Early Cretaceous to early Miocene. The two uppermost units, in contrast, thicken concentrically around the islands and are attributed to the infilling of a flexural moat that formed by volcanic loading since the early Miocene. Flexure modeling shows that the thickness of the moat infill cannot be explained only by surface loading and requires that the downward flexure due to surface loads is opposed by an upward acting subsurface load. The best fit combined surface and buried loading model is for an elastic thickness, Te, of 29 km, a load and infill density of 2700 kg m−3, and a ratio of surface to subsurface loads, f, of 0.2. These results are in accord with spectral studies of free‐air gravity anomaly and topography data based on the admittance technique. The subsurface loads are spatially limited to the islands and their submarine flanks. Nevertheless, they are associated with a broad regional uplift of up to ∼400 m. The uplift is large enough to explain why the moat infill is generally tilted away from, rather than toward, the islands. The uplift is too small, however, to account for the height of the swell, upon which the Cape Verde Islands are superimposed. The origin of the Cape Verde swell is not known. However, a normal Te and a modest heat flow anomaly suggest that the swell cannot be fully explained by uplift due to thermal reheating of the lithosphere by an underlying “hot spot” and that other, deep‐seated, mantle processes must be involved.