bluuebunny/crossref_metadata_2025 huggingface dataset:
Electrical resistivity image of the upper crust within the Taupo Volcanic Zone, New Zealand
Authors: Bibby H. M. , Caldwell T. G. , Risk G. F.
Published: February 2004

The Taupo Volcanic Zone (TVZ) forms a region of back‐arc extension within the continental crust of New Zealand and is notable for its caldera volcanism, high rate of magmatic production, and high heat flow. On the eastern side of the TVZ, volcanism terminates abruptly along a linear margin. We present the interpretation of three multiple‐source bipole‐dipole resistivity surveys across this margin. The major features of the measured data can be reproduced by using two‐dimensional models of the resistivity in the upper 10 km of the crust. Along the margin the high‐resistivity graywacke basement (300 Ωm increasing to 1300 Ωm) is downfaulted to the west across a zone about 6 km wide, with a total vertical offset of approximately 2 km. The easternmost fault is overlain by an unbroken 0.24 Ma ignimbrite sheet, suggesting the fault has been inactive for this period. Further to the west, at depths greater than 2 km, a conductive zone about 15 km wide and of total conductance 180–280 S has been identified. This conductive zone can be modeled by using a range of resistivities (40–3 Ωm) and thicknesses (8–2 km). Models incorporating resistivities less than 15 Ωm require the low‐resistivity zone to be wedge shaped, so that conductance increases to the west. The low resistivity is believed to be caused by the presence of clay alteration within volcanoclastic rocks. The large volume of volcanic rocks at depth can be linked to a series of caldera‐forming eruptions that have taken place along the eastern side of the TVZ.
A deep seismic crustal transect from the NE Iberian Peninsula to the western Mediterranean
Authors: Vidal Neus , Gallart Josep , Dañobeitia Juan José
Published: February 2004

The crustal structure of the transition from the NE Iberian Peninsula to the western Mediterranean is characterized by large‐scale variations related to its successive compressional and extensional Cenozoic tectonics. The Spanish ESCI (Spanish acronym for seismic studies of the Iberian crust) Program recorded 450 km of deep vertical reflection profiles on land and at sea, to complete a first crustal transect from the Pyrenees to the South Balearic basin. Highly energetic, north dipping reflections imaged by combining the Etude Continentale et Océanique par Reflexion et Refraction Sismique (ECORS) and ESCI land profiles mark a crustal thickening toward the Pyrenees. The reflective band becomes horizontal beneath the Catalan coastal ranges, and the Moho is found at 11s. No evidence of crustal thinning affecting the Iberian mainland is observed. The transition to the Mediterranean basin is constrained by complementary onshore‐offshore recordings. Forward modeling combined with multichannel processing of the wide‐angle data set shows that a strong thinning of the continental crust by a factor of 2 over a distance of 50 km affects both the Catalan and Balearic flanks of the Valencia trough. This thinning affects mainly the deep crust, characterized by low velocities of 6.4–6.5 km/s, suggesting a removal of part of the lower crust during the rifting processes. Velocities of 7.8–7.9 km/s are interpreted in the uppermost mantle with no evidence of underplating. The Moho appears as a marked discontinuity throughout the transect. The NW part of the Balearic promontory is characterized by a thick and reflective crust from 12 to 30 km depth. An intensely fracturated evaporitic sequence is seen for 100 km along the south Balearic basin. The seismic images of the continent‐ocean transition are disturbed by these prominent sediments. However, the deep reflectivity suggests a crustal thinning (Moho at 6–7 s) to be related to the Middle Miocene‐present extensional tectonics.
Testing hypotheses on plate‐driving mechanisms with global lithosphere models including topography, thermal structure, and faults
Authors: Bird Peter
Published: February 2004

A popular concept of plate tectonics is that the most important density anomalies are in oceanic plates, which descend from rise to trench while subducting slabs act as velocity regulators and the drag on the base of the lithosphere is resistive. This hypothesis has been shown to be consistent with velocities of rigid plates and to approximately predict stress directions in a global elastic shell. Here I test whether the hypothesis leads to correct plate velocities and stress directions in models of laterally heterogeneous plates of nonlinear rheology separated by faults with low friction. All models have bottom boundary conditions based on simple shear in an olivine asthenosphere extending to the transition zone, where various patterns of lower mantle flow are assumed. These models show that topography alone is not sufficient to drive plate motion at actual rates over a static or sluggish transition zone. If the forward component of velocity of each subducting slab is also imposed, then velocities are improved, but errors in stress direction become unacceptable. Better models are found by assuming that at least some parts of the transition zone have velocities greater than surface velocities, leading to active or forward basal drag. In the most plausible model, this forward drag acts only on continents, while oceanic lithosphere experiences negligible basal shear tractions. Probably the dense descending slabs of oceanic lithosphere not only pull the oceanic plates, but also stir the more viscous lower mantle, and this in turn helps to drive the slower drift of continents.
Three‐dimensional modeling of the 1983–1984 eruption at Piton de la Fournaise Volcano, Réunion Island
Authors: Cayol Valérie , Cornet François H.
Published: February 2004

Displacement vectors on 125 points over a 6‐km2 area were calculated [Zlotnicki et al., 1990] for Piton de la Fournaise volcano, Réunion Island, from photogrammetric surveys conducted in September 1981 and again in September 1984. An eruptive crisis with two episodes occurred during the 3 years between the surveys. We modeled the displacements from the photogrammetric surveys using a three‐dimensional mixed boundary element method. The model shows that both eruptive episodes were fed by a single dike oriented 165° from north. As the dike approached the ground surface, it divided into segments that rotated into an en echelon pattern. The model that best fits all observations suggests that the stress state within the volcano is quasi‐isotropic such that horizontal stresses equal the overburden. These high horizontal stresses are incompatible with an elastic edifice loaded merely by gravitational forces. However, they may be attributed to the cumulative effect of repeated dike intrusions.
Growth and destruction of Gran Canaria deduced from seismic reflection and bathymetric data
Authors: Funck Thomas , Schmincke Hans‐Ulrich
Published: February 2004

The morphology and structure of the submarine flanks of Gran Canaria have been mapped using Hydrosweep swath bathymetry and high‐resolution reflection seismic data. The growth and destruction of the island has been characterized previously by three major periods of volcanic activity (16–9 Ma, 4.5–3.5 Ma, younger than 3 Ma) separatedby erosional intervals. Two major sector collapses along the west coast, inferred from the coastal morphology, are believed to have formed at the end of the shield‐building phase. One is characterized by a 19‐km‐wide reentrant along the northwestern coast that may have formed synchronously with the formation of the 20‐km‐diameterMiocene Tejeda Caldera. High sedimentation rates around Gran Canaria (>50 m/Myr) tend to cover and bury major landslide blocks. SSW off the island, several canyons continue seaward into a major sediment fan. A 9.5‐km‐wide volcaniclastic ridge in this fan is interpreted to represent deposits of the Pliocene subaerial Roque Nublo debris avalanche. We tentatively interpret the slope break at a depth of 600–800 m as the transition between subaerial and subaqueous chilled lavas at the end of the shield‐building phase. The subsidence caused by the volcanic load (30,000 km3) on the lithosphere may thus amount to no more than 800 m. Several canyons on the island can be traced down the submarine flanks to a depth of 3.5 km, indicating that at least deeper portions below the level of subsidence were eroded by mass flows continuing seaward from the subaerial canyons. Four submarine volcanoes were identified west and northeast of the island.
Quantifying the interaction of an ocean bottom seismometer with the seabed
Authors: Osler John C. , Chapman David M. F.
Published: February 2004

A theory is presented for the coupling between an ocean bottom seismometer (OBS), the sediments upon which it rests, and the surrounding water. Assuming that rotational and tilt effects are negligible (or have been made negligible through instrument design), the response of the OBS to forced harmonic motion is considered in both horizontal and vertical directions. Under these conditions it is concluded that the measured ratio of velocities when the OBS is on the seabed and when it is freely suspended in water (for an identical force) completely characterizes the OBS/seabed interaction. This enables the velocity transfer functions to be directly calculated without recourse to a detailed model of sediment/structure interaction. An OBS was designed and constructed with good coupling to seabed motion and reduction of rocking effects as principal design criteria, including an onboard shaker to conduct in situ coupling experiments. The amplitude and phase of coupling data collected on a clay seabed provide transfer functions due to horizontal and vertical seabed motion and horizontal water motion. In addition, a simple mass‐spring‐dashpot model of OBS/seabed interaction permits the analysis of amplitude‐only coupling data. Good vertical coupling can be achieved over a wide bandwidth by designing the sensor package to have a large hydrodynamic added mass in the vertical. Good coupling to horizontal seabed motion is more difficult to achieve but is possible within a limited bandwidth, even on very soft seabeds. Finally, an example of seismoacoustic noise is presented. Hydrophone signals are compared with horizontal geophone signals received from sources within the ocean and within the seabed, and the differences are explained in terms of the coupling transfer functions.
Numerical simulations of degassing scenarios for CO2‐rich Lake Nyos, Cameroon
Authors: McCord Stephen A. , Schladow S. Geoffrey
Published: February 2004

A catastrophic outgassing of carbon dioxide (CO2) from Lake Nyos, Cameroon, occurred in 1986. Concentrations of dissolved CO2 have been increasing in the bottom waters since 1986, prompting an international effort to engineer controlled degassing that would maintain lake stability by reducing the gas concentration. A new model, based on the one‐dimensional Dynamic Reservoir Simulation Model has been used to simulate the evolution and distribution of CO2 in Lake Nyos. Initial conditions for the simulations are taken from published observations and integrated forward with synthesized, daily meteorological data for 5 years. After model results were found to be in agreement with observed trends, a hypolimnetic piped withdrawal system for degassing the lake was then modeled to simulate its effect on the CO2 concentration profile over time. In the standard degassing operation, gas‐rich bottom water continuously flows up through pipes, discharging as a fountain at the lake surface. Simulation results indicate that the lake density structure will most likely remain stable and CO2 concentrations will decrease at all depths above the pipe intake. Seven alternative operational strategies for the degassing system were also investigated, including a submerged outlet, an intake raised above the lake bottom, higher flow rate, a disrupted operation schedule, employment of a stabilization pond, multiple intake depths, and release of the withdrawn water from the lake. Multiple intake depths best maintain lake stability. A tentative conclusion is that degassing may need to be carried out indefinitely, albeit intermittently or at a reduced flow rate.
An alternative, dynamic model of the axial topographic high at fast spreading ridges
Authors: Eberle Michael A. , Forsyth Donald W.
Published: February 2004

Recent models that attempt to explain the formation of an axial topographic high at fast spreading ridges invoke the presence of a narrow, low‐density, low‐viscosity melt conduit to buoyantly support the high. Mantle up welling beneath mid‐ocean ridges must be highly focused to create the narrow melt zone required by these buoyancy models. We show that an alternative model in which extensional stresses in the upper crust are relieved by diking predicts uplift at the ridge and can match the shape and magnitude of the axial high. In this model, the extensional stresses deepen toward the ridge axis creating dynamic moments within the crust that are balanced by the axial topography. Along‐axis variations in the height and width of the axial high may be caused by changes in the depth and width of the axial magma chamber or flexural rigidity of the lithosphere. This model is similar to dynamic models of median valley formation at slow spreading ridges in that the axial topography is uncompensated and supported by extensional stresses in the lithosphere and asthenosphere. In addition to being able to explain the axial topography observed at fast spreading ridges, this model is consistent with either broad (passive) or narrow (dynamic) upwelling at mid‐ocean ridges. The change from median valley to axial high is caused by differences in strength of the axial lithosphere and rate of thickening of the plate.
Thermal effects on the location of ice stream margins
Authors: Jacobson H. P. , Raymond C. F.
Published: February 2004

West Antarctic ice streams move rapidly over beds that are thawed by heat generated by the motion. Intervening zones of slow‐moving ice are probably freezing at their bases, where heat generation is absent. Heat is also generated in the shear zone between the slow and fast moving ice. Through modeling of coupled heat and mass flow, we show that the position of the boundary between freezing and melting at the bed is unstable for an otherwise morphologically uniform bed. Above a threshold speed in the ice stream, heat generation in the shear zone allows melting beneath the slow ice and possible widening of the stream. Below this threshold, heat generation at the edge of the thawed zone is insufficient to maintain melting, leading to freezing and narrowing of the stream. The threshold speed appears to be a function of the temperature at the base of slow ice (relative to the melting point) and is also highly sensitive to the inflow of colder ice from the slow zone. For environmental conditions typical of West Antarctica, the threshold speed is about 100 m yr−1 in the absence of inflow and increases to typical ice stream speeds (500 m yr−1) for inflow speeds of a few meters per year. Thus the ice streams can be in a delicate thermal balance with the potential of melting outward or freezing inward. Rapidly moving ice streams could expand unless blocked by bed properties that would prevent fast motion even when there is melting at the bed.
The coseismic slip distributions of the 1940 and 1979 Imperial Valley, California, earthquakes and their implications
Authors: King N. E. , Thatcher W.
Published: February 2004

Geodetic arrays observed by the U.S. Coast and Geodetic Survey span the Imperial fault in southern California. For the 1940 M 7.1 Imperial Valley earthquake, a 1934–1941 triangulation network has sufficient resolution to allow inversion for the coseismic slip distribution on fault segments 5 to 25 km long extending from the surface to a depth of 9 km. The estimated right‐lateral slip is 0.8 to 1.7 m on the northern 30 km of the main trace of the Imperial fault, 4.8±0.2 m on a 10‐km‐long segment straddling the United States ‐ Mexico border, and 1.3±0.4 m on a southern 25‐km‐long segment in Mexico. Fixing this strike‐slip model and inverting 1940 leveling data only for dip slip yields 0.1 m of east‐side‐down dip slip. The seismic moment for this model is M0 = (3.2±0.3) ×1019 N m. The 1979 geodetic data set, mostly elevation changes from leveling routes, has insufficient resolution for inversion. However, it is possible to use this geodetic data set and results published by others to infer that the 1940 and 1979 earthquakes may be similar on the rupture zone common to both events. Our preferred 1940 model is similar to the 1979 geodetic results of Crook [1984] on the segments where both networks have good resolution. Elevation changes from 1940 and 1979 leveling data are very similar. Thus the geodetic data corroborate the surface slip evidence of Sharp [1982b] that the 1940 and 1979 slip distributions are examples of “characteristic slip” on the northern Imperial fault.
Hydrothermal circulation at the Cleft‐Vance overlapping spreading center: Results of a magnetometric resistivity survey
Authors: Evans Rob L. , Webb Spahr C. , Jegen Marion , Sananikone Khamla
Published: February 2004

We report on a magnetometric resistivity sounding carried out in the overlapping spreading center between the Cleft and Vance segments of the Juan de Fuca Ridge. The data collected reveal a strong three dimensionality in the crustal electrical resistivity structure on wavelengths of a few kilometers. Areas of reduced crustal electrical resistivities, with values approaching that of seawater, are seen beneath the neovolcanic zones of both active spreading centers. We interpret these reduced resistivities as evidence of active hydrothermal circulation within the uppermost 1 km of hot, young oceanic crust.
Spiess Ridge: An axial high on the slow spreading Southwest Indian Ridge
Authors: Mitchell Neil C. , Livermore Roy A.
Published: February 2004

We report recent mapping of Spiess Ridge with the Hawaii‐MR1 sidescan sonar. Spiess Ridge is an unusual elongate 90 by 50 km volcanic feature on the slow spreading Southwest Indian Ridge near the Bouvet hotspot. The northwest half of the ridge has a narrow ∼7‐km neovolcanic zone in MR1 sonar images and a simple magnetic anomaly sequence including possible anomalies C1n and C2n. In contrast, the southeast half shows extrusive volcanism over >40 km with distributed eruption sites, a broadened central magnetic anomaly with no anomaly C2n, and a volcanic ridge radiating from the summit of Spiess Ridge, oblique to the spreading orthogonal trend. The images show no evidence for large‐offset normal faults or an axial rift valley typical of slow spreading ridges. Overall, Spiess Ridge has an appearance very unlike that of either fast or slow spreading ridges and more transitional between a spreading ridge and a seamount. We compare the morphology of Spiess Ridge to other large volcanic structures in oceanic rifts. Spiess Ridge and Bouvet Island represent localized zones of excess melting along the Southwest Indian Ridge and we discuss their origin in the light of current ideas on ridge‐hotspot interaction.
Some physical requirements for the emplacement of long basaltic lava flows
Authors: Keszthelyi L. , Self S.
Published: February 2004

Long basaltic lava flows (over 100 km in length) require specific emplacement conditions to prevent the lava from freezing as it is transported to the flow front. The minimum dimensions of the lava transport systems (tubes, channels, or sheets) require that the flow have a volume greater than several cubic kilometers. Long lava flows are emplaced on slopes less than 10% (∼5°) and the lava being transported must cool at a rate less than 0.5°C/km. We show that there are two modes by which thermally efficient, long distance lava transport can be achieved: (1) “rapid” emplacement in which the lava flows so quickly that it does not cool excessively despite large heat losses and (2) “insulated” emplacement in which heat loss is minimized. We here estimate cooling in the rapid mode using a modified version of a previously published thermal model for aa flows and find that, for a range of inputs appropriate for subaerial terrestrial condition, effusion rates of at least 3100 to 11000 m3/s, channel flow velocities in excess of 4–12 m/s, and minimum channel depths of 3–17 m are required for basaltic flows >100 km in length. For emplacement in the insulated mode, we construct a very simple heat balance model for roofed sheet flows which shows that extremely long sheet‐fed flows are possible with velocities as low as 0.2–1.4 m/s, flow thickness of 6–23 m, and minimum effusion rates of the order of 50–7100 m3/s. Also, earlier work has suggested that tube‐fed flows more than 100 km long can be produced at effusion rates as low as several tens of m3/s and with tube diameters of a few tens of meters. We argue that flows emplaced in the rapid mode should be morphologically similar to channel‐fed aa flows while those emplaced in the insulated mode should be similar to tube‐fed or sheet‐like inflated pahoehoe flows. This leads to several field criteria for distinguishing these two modes of emplacement in ancient lava sequences. Additional constraints on the emplacement of long lava flows are expected from the continued study of the formation and evolution of lava channels, tubes, and sheets.
Oceanic basement structure, sediment thickness, and heat flow near Hole 504B
Authors: Swift Stephen A. , Kent Graham M. , Detrick Robert S. , Collins John A. , Stephen Ralph A.
Published: February 2004

A new seismic reflection survey around Hole 504B, the deepest borehole in ocean crust, reveals active faulting, possible volcanic centers, and a lateral change in the relationship of heat flow and basement structure near the borehole. Migration of single channel and multichannel seismic profiles collected in a 25 by 25 km grid with a 1 km line spacing significantly improved the resolution of basement structure and sediment thickness. West of Hole 504B, heat flow is high above east‐west lineated basement ridges, whereas heat flow to the east is normal above ridges and high above two buried basement knolls. The difference is probably due to lateral variations in sediment thickness. Small, buried basement knolls are common and may have been point sources for lava flows. Hole 504B lies in a flat‐floored basin that slopes gently upward to the west. A recently active fault 1.1–1.2 km south of Hole 504B is indicated by sediment reflector discontinuities that extend up to the seafloor. The fault strikes east‐west and crosses a buried volcanic knoll where Holes 678B and 896A were drilled. Regionally, basement relief north of Hole 504B is 100 to 150 m lower than to the south, which we attribute to an increased spreading rate obtained from dating published local magnetic anomaly patterns with a recent timescale. We find at least five graben structures resembling failed rifts which may have formed in response to asymmetric spreading or to the change in tectonic stress accompanying the spreading rate change. South facing scarps on basement ridges are as common as north facing scarps. Sediment thickness is highly correlated to basement depth due to preferential deposition in topographic lows when the crust was 1–2 Ma old and to later winnowing.
Spatial aftershock distribution: Effect of normal stress
Authors: Kagan Yan Y. , Jackson David D.
Published: February 2004

We study the spatial clustering of shallow aftershock hypocenters with respect to focal mechanisms of mainshocks. We use the Harvard centroid moment tensor (CMT) global catalog, the Preliminary Determination of Epicenters (PDE) earthquake list, the California Institute of Technology/U.S. Geological Survey catalog of earthquakes in southern California, and a catalog of focal mechanisms for all earthquakes since 1850 in southern California with magnitude larger than 6. We need to account for possible systematic bias in hypocenter distribution due to the geometry of seismogenic zones, especially that of subduction zones. We also select only strike‐slip earthquakes from the catalogs to investigate aftershock clustering in circumstances more favorable for direct observation. We compare the spatial distribution of hypocenters before each strong earthquake with the distribution during the first 250 days after the earthquake and for the time interval extending beyond 250 days. If the friction coefficient in the Coulomb criterion is positive one expects that after a strong earthquake, aftershocks and other earthquakes would concentrate in the direction of the P axis (dilatational quadrant) rather than in the direction of the T axis (compression quadrant). Such correlations have been pointed out previously for selected earthquakes sequences, but is such correlation a general feature of earthquake occurrence? We study spatial earthquake distributions before and after each event for several choices of focal sphere partition, cutoff magnitude, focal mechanisms of large events, time periods, distance from a mainshock, etc. Although some earthquake distributions agree with a nonzero friction coefficient, others produce the opposite pattern, suggesting that the concentration of events along the P and T axes is due to random effects. This result implies that the friction coefficient in the Coulomb law is close to zero.
Hugoniot equation of state and high‐pressure transformation of jadeite
Authors: Takazawa E. , Sekine T. , Kobayashi T. , Zhu Y.
Published: February 2004

The Hugoniot equation of state of jadeite (NaAlSi2O6) was investigated with a two‐stage light gas gun extending the range of shock pressure up to 150 GPa. From the present results coupled with previous data a high‐pressure phase transformation was observed in jadeite over 60 GPa. Moreover, the Hugoniot elastic limit was found to be ∼7 GPa. Fitting the theoretical Hugoniot equation of state to the high‐pressure phase results in a bulk modulus at ambient condition of 280 ± 10 GPa for a zero‐pressure density of 4.26 ± 0.05 g cm−3. These physical properties for high‐pressure NaAlSi2O6 are consistent with the perovskite‐type structure at pressure above 120 GPa. Because its density is close to MgSiO3 and CaSiO3 perovskites, NaAlSi2O6 perovskite is a candidate to be a host for sodium and aluminum near the core‐mantle boundary.
The effect of strain energy on growth rates during the olivine‐spinel transformation and implications for olivine metastability in subducting slabs
Authors: Liu Ming , Kerschhofer Ljuba , Mosenfelder Jed L. , Rubie David C.
Published: February 2004

We have studied growth kinetics during the transformation of Mg1.8Fe0.2SiO4 San Carlos olivine to its high‐pressure polymorphs wadsleyite (β phase) and ringwoodite (γ phase) at 800°–1200°C and nominal pressures of 16–20 GPa. In experiments in which a large (500–600 μm) olivine single crystal (contained in a matrix of either fine‐grained olivine or NaCl) was transformed, reaction rims of wadsleyite/ringwoodite form on the margins of the single crystal by incoherent grain‐boundary nucleation and interface‐controlled growth. Contrary to theoretical expectations, the growth rate of these reaction rims decreases sharply as a function of time; for instance, at 1100°C and 18 GPa, growth ceases on an experimental timescale after the rim width reaches 20–25 μm. In order to explain this observation, we develop an elastic model based on the theory of a misfitting inclusion. Comparing the results of this model with the experimental data suggests that elastic strain energy, which develops because of the large volume decrease associated with the transformation, is responsible for the decreasing growth rates. On the other hand, experimental and theoretical results suggest that elastic strain energy is relatively unimportant when grains of the product phase are randomly dispersed and distinct reaction rims do not form; this is the case when the nucleation rate is low, the growth rate is fast, and the reactant olivine is fine grained. On a geological timescale in subducting lithosphere, where the grain size of the olivine is large, the growth rates of grain‐boundary nucleated reaction rims are likely to be controlled by viscoelastic relaxation. Therefore current kinetic models of olivine metastability in subducting slabs, which are based on simple extrapolations of experimental data and on the assumption that the growth rate is constant at fixed temperature and pressure, need to be reevaluated.
The process zone: A microstructural view of fault growth
Authors: Vermilye Jan M. , Scholz Christopher H.
Published: February 2004

The deformation associated with several small, brittle faults was investigated on both microscopic and macroscopic scales. While the dominant macroscopic structures are solution cleavage planes and secondary shear fractures, the dominant microscopic deformation structures are healed tensile microfractures. The fault‐related microfractures display densities and orientations distinct from the background microfracture population. These densities and orientations are consistent with formation within the altered stress fields of propagating shear fracture tips. This microfracture population is used to define the fault process zone associated with growth of the macroscopic fault plane. Process zone microfractures show logarithmic density increases with proximity to the fault, a constant maximum density that is independent of fault length, and orientations which can be used to infer the direction of propagation of the fault plane. The width of the process zone scales linearly with fault length with a proportionality constant of the order of 10−2.
Barkhausen jumps in large versus small platelets of natural hematite
Authors: Halgedahl Susan L.
Published: February 2004

To better understand the physical links among hysteresis properties, defect distributions, and grain size, Barkhausen jumps have been studied in individual platelets of natural hematite from Elba, Italy during hysteresis. Both Bitter patterns and hysteresis curves have been investigated in two very different groups of platelets: large, ∼1‐mm‐sized platelets with coercive forces (Hc) in the range of ∼20–50 Oe (2–5 mT), and much smaller, ∼100 μm‐sized platelets, with Hc in the 100–140 Oe range (10–14 mT). Single platelets were cycled through both major and minor hysteresis, in order to determine (1) the changes of magnetic moment caused by Barkhausen jumps and (2) how these changes may depend on grain size and the critical field Hcrit required to unpin a wall from a defect. Results of these experiments lead to the following conclusions. First, the steepest part of the major hysteresis loop is dominated by large‐scale wall motion through that part of the grain where the combination of wall nucleation and wall propulsion across defects requires the lowest fields anywhere in the particle. In the large platelets, this “soft” region (e.g., where Hcrit≤∼50 Oe, or 5 mT) can amount to as much as 25% of the grain's volume; within this region, a wall can be driven well over 100 μ by fields commensurate with Hc. In the large platelets, the defects within this “soft” region appear to have quite variable volume densities. By contrast, in the small platelets, defects with comparably soft wall‐pinning strengths appear to be distributed much more uniformly. Second, in both platelet groups, most of the relatively “hard” defects (e.g., Hcrit≥∼80 Oe) which have the greatest impact on wall pinning during hysteresis generally appear to be concentrated within a localized portion of the particle, rather than being distributed randomly throughout the volume. This “hard” region can amount to several tens of percent of a grain's volume, but within it, the “hard” defects appear to be distributed rather homogeneously. It is through this “hard” region that the wall is forced, as fields stronger than Hc drive the platelet toward saturation. Consequently, these results lead to a very different picture of defect distributions than envisioned by previous models, based on random defects. We propose that this nonrandom spatial distribution of “hard” versus “soft” defects could originate from internal strain generated when these platelets cleaved naturally from their much larger, parent crystals.
Axial magnetic anomaly amplitude along the Mid‐Atlantic Ridge between 20°N and 40°N
Authors: Ravilly Morgane , Dyment Jérôme , Gente Pascal , Thibaud Rémy
Published: February 2004

After reduction to correct for the topography, spreading rate, latitude, and lineation azimuth effects, axial magnetic anomalies amplitudes are compared with bathymetric, gravity, seismological, and geochemical data along a 2600‐km‐long section of the Mid‐Atlantic Ridge axis between 20°N and 40°N. Significant results are obtained at two different scales, a regional one (>100 km) related to sublithospheric thermal perturbations such as those associated to hotspots and a local one (<100 km) related to ridge segmentation. At a regional scale, two long‐wavelength highs of the axial magnetic anomaly amplitudes are observed between 37.5°N and 40°N and between 27°N and 30°N. The first high, associated with marked bathymetric, gravity, geoid, seismic velocity, and geochemical anomalies, is related to the Azores hotspot and is interpreted as reflecting the existence of a thick and/or Fe‐Ti enriched magnetic source layer. A second long‐wavelength high centered at 28.5°N corresponds to a weaker bathymetric and gravity signature; it is associated with a large low seismic velocity anomaly and a marked intermediate‐wavelength positive geoid high which suggest higher sublithospheric temperatures, deeper and more extensive partial melting, and a thicker and/or Fe‐Ti enriched magnetic source layer. At a local scale, all but one of the 25 ridge segments investigated show that axial magnetic anomaly amplitude is higher at segment ends by a factor of 2 compared to segment centers. A clear correlation is observed between the range of axial magnetic anomaly amplitude and the range of the mantle Bouguer anomaly (ΔMBA) within each segment. More complex relationships exist between median amplitude, amplitude at segment centers and at segments ends, and ΔMBA. Separation of segments in two groups (“hotter” and “colder”) based on ΔMBA shows different types of variations among segment, with grouped amplitudes at segment centers and scattered amplitudes at segment ends for hotter segments and the opposite for colder segments. These observations support shallow fractionation and the presence of serpentinized bodies in the vicinity of discontinuities as the major processes which control the axial magnetic anomaly amplitude variations.
Rupture history of the January 1, 1996, Ms 6.6 volcanic earthquake preceding the simultaneous eruption of Karymsky and Akademia Nauk volcanoes in Kamchatka, Russia
Authors: Zobin Vyacheslav M. , Levina Valeria I.
Published: February 2004

The intensive foreshock‐mainshock‐aftershock sequence of earthquakes preceded and accompanied the January 2, 1996, simultaneous eruption of the active Karymsky and the dormant Akademia Nauk volcanoes in Kamchatka, Russia. The finite fault, broad band teleseismic P waveform inversion was applied to the Ms 6.6 mainshock of this sequence which occurred on January 1, 1996, about 14 hours before the beginning of eruption. It was one of the strongest volcano‐tectonic earthquakes in this century after the 1912–1914 great Ms 7.0 volcanic earthquakes related to the Katmai, Alaska, and Sakurajima, Japan, volcanic eruptions. The main feature of the rupture process inferred from the inversion was the breaking of a sequence of four asperities with displacement greater than 200 cm which were situated at depths from 0 to 35 km just beneath the Akademia Nauk volcano. There were no broken asperities beneath the Karymsky volcano. The majority of small earthquake foci were located out of maximum asperities and filled a space between two zones of asperities at depths from 12 to 20 km. The position of broken asperities allows us to suggest that during the large earthquake rupturing an ancient magmatic column beneath the volcano was destroyed. Small earthquakes which were recorded mainly from 5 to 20 km depth were able only to destroy the intermediate‐depth part of this column between 12 and 20 km. The main work for opening a way to move magma to the surface was made by a large shock which had broken the upper and deep parts of the ancient magma column.
Crustal thickness map of the western United States by partitioned waveform inversion
Authors: Das Triparna , Nolet Guust
Published: February 2004

The partitioned waveform inversion method [Nolet, 1990] was used to obtain a new crustal thickness map of the western United States. We fitted 200 seismograms containing higher‐ and fundamental mode Rayleigh waves for events and stations in the western United States, which provided average crustal thickness constraints between each source‐receiver pair. All these constraints were combined in a linear inversion to obtain a crustal thickness map. Local crustal thickness estimates from published seismic reflection/refraction work were used as additional constraints to overcome deficiencies of path coverage in some areas. The resulting crustal thickness map shows thicker (∼45 km) crust beneath the Cascades, thick (∼40 km) crust beneath the Sierra Nevada, thin (∼30 km) crust beneath the Basin and Range, and thick (greater than 45 km) crust beneath the Colorado Plateau. The boundary between the Colorado Plateau and the southern leg of the Basin and Range is a prominent feature of the model. Thick crust is seen at the bend of the San Andreas fault near Los Angeles. Our model is broadly consistent with the crustal thickness map of Mooney and Weaver [1989]. A quantitative comparison with the crustal thickness model 3SMAC [Nataf and Ricard, 1996] was performed by inverting for a model that deviates least from it. Results show that we are able to retain most of the features of that model while maintaining a good fit to the surface wave and point constraint data.
Seismic structure and crustal magmatism at the Mid‐Atlantic Ridge, 35°N
Authors: Barclay Andrew H. , Toomey Douglas R. , Solomon Sean C.
Published: February 2004

We have determined the three‐dimensional P wave velocity structure and azimuthal anisotropy within the shallow crust of the inner valley at the Mid‐Atlantic Ridge (MAR) near 35°N. The best fitting one‐dimensional model is ∼0.8 km/s lower in velocity at a given depth than for fast spreading ridges, while the isotropic three‐dimensional velocity structure shows a strong lateral heterogeneity (up to 1.4 km/s peak‐to‐peak variation at 1 km depth) in both the along‐ and cross‐axis directions. An anomalous region, with velocities as much as 0.8 km/s lower than average and 3–5 km in lateral extent, is located beneath a near‐axis seamount. We interpret this feature as a region of near‐solidus temperatures with possibly a small degree of partial melt. From the variation of travel time residuals with azimuth we determine a best fitting P wave anisotropy model that has 4% anisotropy from 500 m to 1 km depth, 2% anisotropy in the depth range 1–1.5 km, and no anisotropy at greater depth. This finding is consistent with vertical cracks aligned parallel to the axial valley. We find no evidence for the shallow, high‐velocity, ridge‐parallel feature commonly observed at the axis of fast spreading ridges and attributed to high‐velocity dikes beneath a thin extrusive layer. The absence of this anomaly at the MAR implies that the axis of crustal accretion is not at a stable location with respect to the inner valley walls on timescales greater than about 25,000 years. An unstable axis of accretion will result in a thicker transition from extrusive rocks to dikes, which may account for the generally lower velocities beneath the MAR compared with the East Pacific Rise, and a lateral variation in upper crustal lithology.
Kinematics of the southern Walker Lane Belt and motion of the Sierra Nevada block, California
Authors: Hearn Elizabeth Harding , Humphreys Eugene D.
Published: February 2004

Deformation in the southern Walker Lane Belt region of the southwestern Great Basin accommodates significant portions of both Pacific‐North America transform motion and Basin and Range extension. Apparent kinematic inconsistencies between geodetic and fault slip data in this region have made it difficult to understand the nature of the interaction between these deformation processes and to infer the regional kinematics. We model the kinematics of this region in a manner that enforces kinematic consistency and includes fault geometry and slip rate data, Global Positioning System (GPS) survey data, and very long baseline interferometry/very long baseline array (VLBI/VLBA) site velocity data. A model is found that is consistent with the set of available data, and this model differs significantly from prior models for the region. Our model has the Sierra Nevada block, which bounds the western margin of the Great Basin, moving N50°W±5° at 12.7±1.5 mm/yr, with only minor amounts of counterclockwise rotation. Left‐lateral slip on the western Garlock Fault is a consequence of the relatively great westerly component of Sierra Nevada velocity. We also find that the Basin and Range extension east of our study area occurs at a rate of about 2.5 mm/yr, which is slower than the rate in the central Great Basin.
Shear wave anisotropy in the upper mantle beneath the Nazca Plate in northern Chile
Authors: Bock G. , Kind R. , Rudloff A. , Asch G.
Published: February 2004

Data from the Projecto de Investigacion Sismologica de la Cordillera Occidental (PISCO) seismic network and from six broadband seismographs that were operating in northern Chile were used to investigate the mantle in the convergent boundary zone between Nazca plate and the South American continent for the presence of anisotropy. Broadband data as well as long‐period filtered data of teleseismic SKS and PKS phases were analyzed for the presence of shear wave splitting as a possible indicator for seismic anisotropy in the mantle beneath the PISCO network. Measurable shear wave splitting was observed with maximum delay times between the slow and fast split wave of the order of 1 s. Splitting of S waves from intermediate‐depth events located directly beneath the PISCO network in the descending Nazca plate is generally associated with small delay times of the order of 0.1 s, a value typical for the continental crust. Near‐vertical ScS reflections from two deep earthquakes in Argentina and one nearby intermediate‐depth earthquake have similar splitting parameters as the SKS phases. This means that the anisotropic zone causing the splitting of the core phases can be constrained to the Pacific mantle underlying the subducting Nazca plate. It probably does not extend deeper than about 260 km. The majority of the anisotropy directions inferred from the core phases are parallel to the absolute plate motion (APM) direction of the Nazca plate, which is about N80°E. At some stations, however, the fast polarization direction is pointing N160°E, nearly parallel to the strike of the trench and the Andes which would be compatible with the trench‐parallel flow model for South America proposed by Russo and Silver [1994]. This direction is observed over an approximately 100‐km‐wide band to the west of the active volcanic zone. It may represent either a second anisotropy regime in the mantle, a small‐scale diversion of slab‐entrained mantle flow, or a relatively small area where slab entrainment of mantle flow is reduced or ceases to exist. The large number of observed APM‐parallel fast directions suggests, however, that the mantle beneath the descending Nazca plate in northern Chile deforms mainly as the result of slab‐entrained mantle flow. The large variations of anisotropy directions in the Andean subduction zone indicate that asthenospheric flow in the Pacific mantle has a complex pattern which may vary over scale lengths of a few hundred kilometers and which may be governed by slab morphology.
Stress coupling between earthquakes in northwest Turkey and the north Aegean Sea
Authors: Nalbant Süleyman S. , Hubert Aurélia , King Geoffrey C. P.
Published: February 2004

We have investigated the Coulomb stress interactions of 29 earthquakes (Ms≥6.0) that have occurred in the region of northwest Turkey and north Aegean Sea since 1912. Of these events, 23 may be related to earlier events, and 16 are clearly related to earlier events. All events after 1967 are related to previous events. Events in the early part of our time interval that show no correlation could be related to historical events as yet unidentified. In some cases, faults that have received a stress reduction from earlier events are prepared for an event by an earthquake occurring a few years before that creates a local Coulomb stress rise. Thus regions of Coulomb stress shadow can become regions where a damaging earthquake may occur. The relation between smaller events and the Coulomb stress distribution is less clear, but may be related to poor data quality and practical limitations of our modeling technique. Nonetheless, there are 4 times as many events per unit area in regions of enhanced stress than where stress is reduced. We discuss the contemporary distribution of Coulomb stress and argue that it is possible to identify the likely locations of future damaging earthquakes including identifying the most likely candidate faults.
Detailed mapping of a mantle diapir below a paleo‐spreading center in the Oman ophiolite
Authors: Jousselin David , Nicolas Adolphe , Boudier Françoise
Published: February 2004

Detailed mapping of the well‐exposed Maqsad area in the Oman ophiolite addresses the question of accretion mechanisms at oceanic ridges. Steep peridotite lineations are interpreted as a three‐dimensional mantle diapir below a paleo‐fast spreading ridge. The uppermost part of the diapir reveals three heads covering 100 km2 in area. The main head (30 km2) displays warped vertical foliation trajectories. Cross sections show that the vertical flow rotated to the horizontal below the Moho within a 500‐m‐thick melt‐impregnated transition zone. Penetrative structures dip outward from the diapir. and the Moho bulges upward several hundred meters above this area. The ridge axis is ascribed to a 1‐ to 3‐km wide‐corridor of lineations parallel to the sheeted dike trend, which crosses the main diapir head. Mass balance calculations show that this diapir could account for the formation of a 25‐km‐long crustal segment, provided that the upward flow velocity is ≈10 times the ridge spreading rate. The narrow space left for the horizontal flow at the outskirts of the diapir implies active divergent flow from the diapir center with velocities 1 order of magnitude faster than the spreading rate. Accordingly, the horizontal foliations display the strongest fabrics. The calculations predict that the diverging flow should become passive 10 km past the diapir limits, as suggested by field data. The mechanisms making possible the sharp flow rotation at the top of the diapir are discussed.