bluuebunny/crossref_metadata_2025 huggingface dataset:
Occurrence of type I plasma waves in the equatorial electrojet during morning and evening hours
Authors: Ravindran Sudha , Krishna Murthy B. V.
Published: February 2004

Type I waves are generally observed under strong electrojet (EEJ) conditions because of the threshold condition on the electron drift velocity (with respect to ions) for their generation. In this paper we present observation of type I waves during morning and evening hours under weak (as revealed from the low values of mean drift velocity of type II waves and also from low ground magnetic field values) EEJ conditions. In these conditions, type I waves are observed when the plasma turbulence in the EEJ as revealed by the width of the Doppler spectra is high even though the electron drift velocity as given by the mean Doppler frequency is much smaller than the ion acoustic velocity. This suggests that there are regions within the volume probed by the radar wherein the electron drift velocity exceeds the ion acoustic velocity Cs (even though the mean electron drift velocity is very much less than Cs) due to the irregular electric fields of the primary irregularities.
Average terrestrial ring current derived from AMPTE/CCE‐CHEM measurements
Authors: De Michelis Paola , Daglis Ioannis A. , Consolini Giuseppe
Published: February 2004

The present study attempts to analyze the average ring current density and pressure in the magnetospheric equatorial plane (as a function of geocentric distance and local time) based on the AMPTE/CCE‐CHEM proton distributions according to two different AE ranges, corresponding to a low (AE<100 nT) and a moderate (100 nT<AE<600 nT) geomagnetic or auroral activity. In order to obtain a statistically significant database for a high spatial resolution, the data were averaged over 2 years. The results provide a trend of the ring current and show its general average features as a function of geomagnetic activity. In particular, the total ring current is dominated by the pressure gradient term, resulting in an eastward current in the inner magnetosphere (L<4.5) and an extended westward current at higher altitudes. The westward current is always stronger than the eastward current. Moreover, the ring current system is a function of magnetic local time, exhibiting a noon‐midnight asymmetry in general, as well as a function of geomagnetic activity exhibiting an enhancement of about 20% during active times (in midnight‐dusk region only). The location of the total current peak in the midnight sector is at about MLT=2300, and its changes with magnetic activity are interpreted as an effect of the particles injection from the tail during the storms/substorms. The derived current values are compared to previous model predictions and observations.
DPY currents in the cusp/cleft region: A crucial role of southward interplanetary magnetic field
Authors: Troshichev O. A. , Kotikov A. L. , Shishkina E. M. , Papitashvili V. O. , Clauer C. R. , Friis‐Christensen E.
Published: February 2004

Geomagnetic variations observed at the meridional chain of stations in Greenland during 1982–1992 are utilized for the study of relationships between occurrence of azimuthal currents in the dayside cusp/cleft region and changes in the interplanetary magnetic field (IMF) By and Bz components. Events were selected for analysis if (1) the Greenland West Coast stations were located near magnetic noon ∼ 1400 UT and (2) the absolute values of the IMF Bz were less than By. Sixteen events were selected under these conditions. The analysis shows that intensive azimuthal currents develop in the cusp/cleft region in association with the southward IMF despite the fact that the IMF By component had the same orientation during the few previous hours. The direction of these currents (westward or eastward) is specified by the sign of the By component (negative or positive, respectively). If the IMF By is not zero but Bz is mainly northward, no significant currents are observed in the cusp/cleft region at all. As a result, because magnetic substorms usually follow extended periods of southward Bz, the dayside cusp/cleft magnetic perturbations often precede substorm activity in the midnight sector of the auroral zone. This can simply be a connection between the dayside cusp/cleft and nighttime auroral electrojets and therefore could be used as the dayside substorm precursors.
Upstream whistlers generated by protons reflected from a quasi‐perpendicular shock
Authors: Hellinger P. , Mangeney A.
Published: February 2004

Numerical simulations of collisionless supercritical quasi‐perpendicular shocks suggest that the upstream whistlers may be generated near the shock by the reflected protons which gyrate back to the shock. We investigate in detail results of hybrid simulations and a linear theory for a gyrotropic gyrating beam in a Maxwellian plasma, and we compare their outcomes. We find good agreement between the linear theory and the simulations. We compare these theoretical issues with observations, and we discuss the results. We also discuss dimensionality effects on the simulations of upstream whistlers. The investigation of lower Mach shocks leads us to a condition for the existence of a so‐called shock rippling.
Ionospheric observations of the November 1993 storm
Authors: Bust Gary S. , Gaussiran Thomas L. , Coco David S.
Published: February 2004

Complementary measurements from three different electron density measurement techniques are presented for the time period of the November 4, 1993, storm. Computerized ionospheric tomography (CIT) data from an array of nine ground stations operating as part of the Mid‐America CIT Experiment (MACE 93) is presented along with data from a digital ionosonde operating near the midpoint of the CIT array. Corroborating data from the DMSP satellites are also presented. Taken together the data provide evidence of a strongly disturbed ionosphere with rapid variations of electron density structures in space and time. The CIT data show a deep equatorward surge of the the midlatitude trough to nearly 50° geomagnetic latitude, while the ionosonde shows dramatic variations in the virtual height of the ionosphere. DMSP data confirm the equatorward surge of the trough and also display a number of sharp latitudinal variations in vertical drift velocities. An interesting occurrence of spread F also occurred during this time period.
Thermal conductivity of corundum and periclase and implications for the lower mantle
Authors: Manga Michael , Jeanloz Raymond
Published: February 2004

The pressure and temperature dependent thermal conductivity k of Al2O3 and MgO is determined for mantle and core conditions. We model the pressure dependence by relating k to compression, either linearly or with a power law relationship; we assume k is inversely proportional to temperature as expected in nonmetals at temperatures greater than a few hundred Kelvin. In order to determine model parameters, we use experimental data collected in the laser‐heated diamond‐anvil cell at pressures of 58 and 125 GPa, combined with a model for heat transfer in the diamond cell. The thermal conductivities predicted by the two models differ by less than 30% and are in good agreement with results obtained from independent shock experiments. Throughout the lower mantle we find that k∼5–12 W/mK, consistent with geodynamical constraints and previous theoretical estimates, and is nearly constant along an adiabat.
A comparison of geoid undulations for west central Greenland
Authors: Roman Daniel R. , Csathó Beata , Jezek Kenneth C. , Thomas Robert H. , Krabill William B. , von Frese Ralph R. B. , Forsberg Rene
Published: February 2004

The accuracy of a new local gravity field model, GEOID94A, is examined at a site on the western Greenland ice sheet. The model, developed by the Danish National Survey and Cadastre, incorporates several new gravity data sets including an extensive amount of airborne gravity data. Model‐derived geoid undulations were compared to independently determined undulations found by differencing the elevations from Global Positioning System controlled airborne laser altimetry and optical leveling surveys. Differences between the two sets of undulations were less than +/−6 cm RMS. The comparison improved (+/−5 cm RMS) when GEOID94A undulations were adjusted by local gravity observations also acquired at the site. Our comparisons demonstrate that GEOID94A adequately models the long to intermediate wavelengths of the gravity field. We conclude that GEOID94A constitutes a reliable reference model for studies of Greenland's gravity field.
The 1936, 1945–1947, and 1950 earthquake sequences near Lassen Peak, California
Authors: Norris R. D. , Meagher K. L. , Weaver C. S.
Published: February 2004

Three vigorous earthquake sequences occurred near Lassen Peak in 1936, between 1945 and 1947, and in 1950; the latter two sequences included mainshocks of magnitude 5.0 and 5.5, respectively, and thousands of smaller events. No comparable earthquake sequences have occurred near Lassen Peak since 1950. The epicentral area lies within 20 km of the southern boundary of Lassen Volcanic National Park, in a northwest striking seismic zone that extends from Lake Tahoe to the vicinity of Mount Shasta. In comparing their time history and magnitude distribution with other earthquake sequences that have occurred in regions of Cenozoic volcanism within and east of the Cascade Range and the Sierra Nevada, we find that the Lassen earthquake sequences show similar characteristics to two earthquake sequences that occurred on Basin and Range faults near Herlong, California, and Klamath Falls, Oregon. We interpret this similarity as evidence that the Lassen earthquakes were caused by Basin and Range extension and may have occurred on one or more Basin and Range faults in the Lassen region. However, the limitations of the data do not allow other possible sources, such as magmatic injection, to be ruled out. The most important implication of the Lassen earthquake sequences is that earthquakes ofM5 or greater may occur in the Lassen region, perhaps quite close to Lassen Peak or other volcanoes. The record of Holocene volcanism and fault displacements in the region indicates that earthquake sequences driven by either tectonic or magmatic processes may occur near Lassen Peak, and any significant earthquake sequence should be carefully monitored to assess its nature.
Seismicity and stress field in the vicinity of the KTB location
Authors: Dahlheim H.‐A. , Gebrande H. , Schmedes E. , Soffel H.
Published: February 2004

A local, broadband, seismic network of four observatory‐quality stations (KTB‐NET) was operated during the drilling of the KTB hole, within the framework of the interdisciplinary German Continental Deep Drilling Program (KTB). The aim was to investigate the seismic activity with regard to the tectonic stress field and to compare it with data from in situ measurements in the 9.1‐km deep borehole sections (bottom temperature of 260°C and heat flow of 82–85 mW/m2). From October 1990 to November 1995, over 80 local microearthquakes with magnitudes from 0.2 to 2.8 ML were recorded: eight small events by the KTB‐NET only and four earthquake swarms with 73 events by the KTB‐NET and stations of the Vogtland/Western Bohemia networks. Six of the small events are located within or close to the KTB‐NET. The swarm events occurred at the southwestern extension of the Ohre rift, in an area 20 km north of the drill site, which is revealed to be part of the Vogtland/Western Bohemia seismotectonic unit, characterized by swarm activity. The hypocenters are limited to the upper 13 km of the crust, with a distinct concentration between 10 and 12 km. All types of fault plane solutions are found, but at depths greater than 8 km, reversed faulting mechanisms predominate. P axes are very uniformly oriented in a NNW‐SSE direction, corresponding to the well‐known regional stress orientation in central Europe and in agreement with the special in situ stress measurements of the KTB program. The focal mechanism of a ML=1.2 event induced by a fluid injection experiment fits into the results obtained from the natural events. Possible indications for the brittle‐ductile transition are discussed in view of the observed earthquake depth and focal mechanism distributions.
Crustal deformation across the Imperial Fault: Results from kinematic GPS surveys and trilateration of a densely spaced, small‐aperture network
Authors: Genrich Joachim F. , Bock Yehuda , Mason Ronald G.
Published: February 2004

Kinematic Global Positioning System (GPS) surveys in 1991 and 1993 of a 66‐station, small‐aperture network across a 12‐km‐long segment of the Imperial Fault near El Centro, California, reveal a fairly uniform secular displacement field consistent with right‐lateral slip along a single, vertical dislocation surface. Total relative motion across the array equals 20±3 mm/yr at N49±7°W for sites 5–6 km away from the fault trace on opposite sides. These rates agree with simple dislocation models that assume 10 mm/yr of creep along the fault trace from the surface to a locking depth of 9 km and a plate boundary rate of 45 mm/yr oriented N40°W at depth. Model fit does not significantly improve if creep rates are permitted to vary along strike by several millimeters per year. A comparison of 1991/1993 GPS with 1987 high‐precision electronic distance measurements validates the dislocation models but suggests a smoother deformation field than that defined by the 1991/1993 velocities. The difference can be attributed to the poorer 1991 kinematic positioning accuracy rather than to temporal variations in strain. There is evidence for 20 mm of sympathetic coseismic slip from the 1987 Superstition Hills earthquake, although aseismic slip along both faults appears to be completely decoupled.
Fragmentation of basaltic melt in the course of explosive volcanism
Authors: Zimanowski Bernd , Büttner Ralf , Lorenz Volker , Häfele Hans‐Georg
Published: February 2004

With the aim to enhance interpretation of fragmentation mechanisms during explosive volcanism from size and shape characteristics of pyroclasts experimental studies have been conducted using remelted volcanic rock (olivine‐melilitite). The melt was fragmented and ejected from a crucible by the controlled release of pressurized air volumes (method 1) or by controlled generation of phreatomagmatic explosions (Molten Fuel Coolant Interaction (MFCI); method 2). Both methods were adjusted so that the ejection history of the melt was identical in both cases. The experiments demonstrate that exclusively during MFCI, angular particles in the grain size interval 32 to 130 μm are generated that show surface textures dominated by cracks and pitting. The physical process of their generation is described as a brittle process acting at cooling rates of >106 K/s, at stress rates well above 3 GPa/m2, and during ∼700 μs. In this time period the emission of intense shock waves in the megahertz range was detected, releasing kinetic energy of >1000 J. By both experimental methods, three more types of particles were produced in addition, which could be identified and related to the acceleration and ejection history of the melt: spherical particles, elongated particles, and Pele's hair. Abundance and grain size distribution of these particles were found to be proportional to the rate of acceleration and the speed of ejection but were not influenced by the experimental method used. Pele's hair occurred at ejection speeds of >75 m/s.
Seismic skeletonization: A new approach to interpretation of seismic reflection data
Authors: Li Qing , Vasudevan Kris , Cook Frederick A.
Published: February 2004

Automatic identification and isolation of events on seismic reflection data provides the basis for a new approach to seismic interpretation. The approach uses a syntactic pattern recognition method in which an objective function accommodates near‐neighbor and non‐neighbor trace cycle correlation and in which correlations between three traces (triplets), two traces (doublets), and single traces (singlets) are all included in configurations. The method yields a file that includes a listing of all identified events and their characteristics such as length (number of traces), dip, frequency, and amplitude. Thus, for the first time, these data can be statistically analyzed according to the relational attributes of seismic events.
P wave velocity tomography of the Venezuelan region from local arrival times
Authors: Bosch Miguel
Published: February 2004

Arrival times from the local seismological network of Venezuela were used to estimate a three‐dimensional P wave velocity model for the region between longitude 60°–74° W and latitude 6°–14° N to a depth of more than 80 km. The inversion was carried out by damped least squares, describing the media by homogeneous velocity blocks. The resolved lateral velocity variations in the first layer (0–30 km depth) showed a correlation with the main stratigraphic features of the area, while second layer (30–50 km depth) showed the influence of Moho depth variations through the region, generating a pattern well correlated to the Bouguer Anomaly Map. Lithospheric seismic velocities below the Moho appear to be influenced by the major crustal fault systems. An important low‐velocity zone is present below the triple junction of the fault systems of Oca, Boconó, and Morón in northwestern Venezuela. Farther south, a similar low‐velocity zone is present below the junction of the Boconó and the Santa Marta fault systems. Those are the two continental corners of the triangular Maracaibo Block. Below 80 km depth (the fourth layer) the low velocity zones show a connected pattern that follows or is adjacent to the crustal fault zones. The presence of subducted Atlantic lithosphere below the Eastern Venezuelan Basin could explain the high‐velocity zone at this location. A similar interpretation emerges from the tectonic wedging model, previously proposed to explain the pronounced minimum of the gravity anomaly.
Control of magma volatile content and chamber depth on the mass erupted during explosive volcanic eruptions
Authors: Bower Sally M. , Woods Andrew W.
Published: February 2004

Explosive volcanic eruptions are triggered when magma is sufficiently overpressured to open up a fracture to the surface and the eruption continues until this overpressure is relieved. Here we show that the mass erupted from the chamber during the eruption depends critically on whether the magma becomes saturated in its volatiles. Unsaturated liquid magma may only deform elastically and is therefore relatively incompressible. Thus the eruption of relatively small fractions, 0.001–0.01, of the magma in the chamber is sufficient to relieve the chamber overpressure. In contrast, if the magma is saturated and contains exsolved bubbles of vapor, then the mixture becomes much more compressible and the chamber overpressure is only relieved when a fraction of order 0.01–0.1 of the initial mass in the chamber has erupted. In such cases the mass of exsolved bubbles of vapor in the magma as well as the depth and vertical extent of the chamber have a dominant control on the mass erupted. It is also shown that for a given total volatile content, since the mass of volatiles exsolved as gas increases with the crystal content of the magma, the mass erupted also tends to increase with magmatic crystal content. In a layered chamber the volatile content of each layer and the depth and vertical extent of the chamber again exert the main controls on the mass erupted. We use our model to interpret chamber sizes in a number of well‐documented historic eruptions.
Lithospheric structure of the Chaco and Paraná Basins of South America from surface‐wave inversion
Authors: Snoke J. Arthur , James David E.
Published: February 2004

Surface‐wave data from a portable broadband array have been used to invert for the velocity structure of the crust and upper mantle beneath the Chaco and Paraná Basins of central South America. The upper‐mantle velocity structure beneath the Paraná Basin is cratonic in character, whereas that beneath the Chaco Basin is tectonic or asthenospheric in character. The surface‐wave analysis used broadband recordings from a subset of a 14‐station array deployed in a roughly east‐west sawtooth arrangement along 20°S latitude, with a total E‐W aperture of ∼1,400 km. Results from receiver‐function analysis, as well as direct P‐wave regional travel‐time data, were used in the inversions to help constrain Moho depths and crust and upper‐mantle velocities. S‐wave structure for the intracratonic Paraná Basin was determined using interstation phase and group velocities for Rayleigh waves (fundamental and first higher mode) and Love waves (fundamental mode only) based on seven events with paths which traverse the eastern Paraná Basin and one event with a path across the western Paraná Basin. The average Moho depth in the eastern Paraná Basin is ∼42 km. The high‐velocity upper‐mantle lid has a maximum S‐wave velocity of 4.7 km/s, with no resolvable low‐velocity zone to at least 200 km depth. This cratonic velocity structure indicates the presence of a lithospheric root beneath the Paraná Basin despite emplacement of the Paraná plume. The limited data from the western Paraná Basin are consistent with a homogeneous upper‐mantle structure throughout the Paraná Basin. Waveform inversion of fundamental‐mode and first‐higher‐mode Rayleigh waves from a single subandean event was used to obtain estimates for pure‐path dispersion along propagation paths through the Chaco Basin and the western half of the Paraná Basin. The data were partitioned to isolate the partial‐path contribution of the phase and group velocities for the Chaco Basin. The phase and group velocities from this somewhat sparse data set were inverted to obtain a velocity‐depth model for the Chaco Basin. The distinguishing features of the Chaco model consist of a rather shallow Moho depth, 32 km, and low (“asthenospheric”) upper‐mantle S‐wave velocities, about 4.2 km/s, with velocity increasing only slightly to about 4.3km/s at 150 km depth.
Very low frequency (0.2–10.0 Hz) seismoacoustic noise below the seafloor
Authors: Bradley C. R. , Stephen R. A. , Dorman L. M. , Orcutt J. A.
Published: February 2004

The sources and propagation of VLF (0.2 → 10 Hz) ambient noise on and within the deep ocean crust at Deep Sea Drilling Project (DSDP) Hole 534B in the Blake Bahama Basin are shown to be related to the surface sea state and local lithology. This study represents the first experiment where ambient noise is measured simultaneously at several depths below the seafloor. The low‐frequency microseism power spectral density (PSD) peak at 0.3 Hz is nearly invariant with depth between 0 and 100 m below the seafloor. PSD levels of the peak are 65 and 75 dB (rel 1 (nm/s2)2/Hz) for the vertical and horizontal components, respectively, and both horizontal and vertical components of the ocean bottom seismometer and borehole array compare favorably. Above 0.5 Hz the noise levels decrease with depth and increasing frequency. At 1.0 Hz, 100 m below the seafloor the noise level is 10 and 20 dB below the levels observed at the seafloor for vertical and horizontal components, respectively. There is evidence that amplification in some frequency bands may make deeper sites noisier than shallower sites in the same well. Temporal variation of the noise shows nonlinear interaction of local water‐borne gravity waves to be the dominant source mechanism and that the passing of a local storm generates interface waves and increases the noise level (∼10 dB) from 0.3 to 1.5 Hz and 5 to 64 Hz. Between 1.5 and 5 Hz the spectrum is not strongly affected by the passing storm, indicating that the ocean wave spectrum may be saturated.
Timing of detachment faulting in the Bullfrog Hills and Bare Mountain area, southwest Nevada: Inferences from40Ar/39Ar, K‐Ar, U‐Pb, and fission track thermochronology
Authors: Hoisch Thomas D. , Heizler Matthew T. , Zartman Robert E.
Published: February 2004

Crustal extension in the Bullfrog Hills and Bare Mountain area of southwest Nevada is associated with movement along a regional detachment fault. Normal faulting in the upper plate and rapid cooling (denudation) of the lower plate were coeval with Miocene silicic volcanism and with west‐northwest transport along the detachment fault. A west‐northwest progression of tilting along upper plate normal faults is indicated by ages of the volcanic rocks in relation to angular unconformities. Near the breakaway, tilting in the upper plate occurred between 12.7 and 11.6 Ma, continued less strongly past 10.7 Ma, and was over by 8.2 Ma. Ten to 20 km west of the breakaway, tilting occurred between 10.7 and 10.33 Ma, continued less strongly after 10.33 Ma, and was over by 8.1 Ma. The cooling histories of the lower plate metamorphic rocks were determined by thermochronologic dating methods: K‐Ar and40Ar/39Ar on muscovite, biotite, and hornblende,40Ar/39Ar on K‐feldspar, U‐Pb on apatite, zircon, and sphene, and fission track on apatite, zircon, and sphene. Lower plate rocks 10 km west of the breakaway cooled slowly from Early Cretaceous lower‐amphibolite facies conditions through 350±50° to 300±50°C between 57 and 38 Ma, then cooled rapidly from 205±50° to 120±50°C between 12.6±1.6 and 11.1±1.9 Ma. Lower plate rocks 20 km west of the breakaway cooled slowly from Early Cretaceous upper‐amphibolite facies conditions through 500±50°C at 78–67 Ma, passed through 350±50° to 300±50°C between 16.3±0.4 and 10.5±0.3 Ma, then cooled rapidly from 285±50° to 120±50°C between 10.2 and 8.6 Ma. Upper plate tilting and rapid cooling (denudation) of the lower plate occurred simultaneously in the respective areas. The early slow‐cooling part of the lower plate thermal histories was probably related to erosion at the Earth's surface, which stripped off about 9 km of material in 50 to 100 m.y. The results indicate an initial fault dip ≥30° and a 12 mm yr−1west‐northwest migration of the locus of rapid tilting in the upper plate.
A constitutive model for olivine‐spinel aggregates and its application to deep earthquake nucleation
Authors: Liu Ming
Published: February 2004

A constitutive model is developed to describe the creep behavior of olivine‐spinel aggregates, and conditions for transformation shear instability and deep earthquake nucleation are predicted. Using the Eshelby‐Mori‐Tanaka theory [Eshelby, 1957; Mori and Tanaka, 1973], a piecewise incremental method is employed to calculate the stress‐strain relation for the olivine‐spinel aggregate. The creep strain and strain rate depend on differential stress, the aspect ratio of spinel inclusions, latent heat of the phase transformation, the relaxation of the volumetric phase transformation strain εphKK, and the creep law parameters of spinel. A constant strain rate calculation suggests that above a critical percentage transformation ξc an earthquake may nucleate owing to the system's potential for a shear stress drop, provided εphKK of ∼−8% is not relaxed and spinel grain size dsp remains small. A normalized critical stiffness, calculated from the potential shear stress drop Δτ and the critical shear strain γc needed to change from one steady state creep condition to another, suggests that a plate‐like, partially transformed region is required for deep earthquake nucleation. Such a region may be formed by localized deformation or preexisting heterogeneities in a subducting rock.
Deepening of the axial magma chamber on the southern East Pacific Rise toward the Garrett Fracture Zone
Authors: Tolstoy Maya , Harding Alistair J. , Orcutt John A.
Published: February 2004

A wide‐aperture profile along the ridge axis from 14°29′S to 13°39′S, 120 km to 30 km south of the Garrett Fracture Zone, is analyzed to constrain the thickness of layer 2a and the depth to the axial magma chamber reflector. Five areas along the 90 km line are examined in detail, with several consecutive gathers being analyzed for each area to establish the degree of consistency within each area. A genetic algorithm code is used to find a best fit model from a comparison of the data and WKBJ synthetic seismograms. One hundred starting models are generated using a predefined set of velocity nodes, with a fixed window of allowable depth variations between nodes. An evolutionary process favors the better fitting models in each generation, and a satisfactory misfit is usually obtained within 40 generations. Within individual areas the models were in good agreement with the depth of a given velocity node, generally varying by not more than 20 m, the depth discretization interval for the models. A consistent deepening trend of the axial magma chamber (AMC) is observed across the five areas as the Garrett Fracture Zone is approached. The depth varies from 0.99 km at area 1, which is approximately 100 km south of the Garrett, to 1.23 km at area 5, which is approximately 40 km south of the Garrett. The depth to the axial magma chamber is highly sensitive to any ship wander off axis since layer 2a thickens rapidly off axis with age. For the areas examined here, layer 2a is observed to be relatively constant in thickness along the axis, although it is about 40 m thicker over area 5, where the axial magma chamber is deepest. This variation is within the scatter of previously detailed layer 2a measurements at 13°N on the East Pacific Rise, where an effectively constant thickness is observed. This implies that layer 2a thickening is not a significant factor along this profile and that the AMC deepening is real rather than apparent. Theoretical modeling suggests that the depth to the lid of the axial magma chamber is related to the rate of heat supply at a given location. Thus the gradual consistent deepening of the axial magma chamber can be taken as an indication of a slightly reduced magma supply toward the Garrett Fracture Zone, which marks a major interruption of hundreds of kilometers of continuous ridge axis. The deepening may also be interpreted as a downward limb from a central injection point; however, there is no indication of a similar downward trend in the other (southern) direction. Furthermore, there is no accompanying systematic variation in axial depth or axial volume, both of which are proposed to be indicators of central injection and along‐axis flow.
Reflections from a mylonitized zone in central Sweden
Authors: Law Adam , Snyder David B.
Published: February 2004

Dipping reflections imaged on a deep seismic reflection profile acquired within the Baltic Shield can be indirectly linked to strain‐induced acoustic impedance contrasts within a regional brittle‐ductile shear zone: the Singö shear zone. Measurements made at various scales and frequencies estimate rock velocity within the shear zone and correlate well with the ductile component of strain previously determined by structural analysis. Velocity measurements include some made on cored field samples at confining pressures of ≤ 0.4 GPa in a laboratory (10 MHz sensor frequencies) and some made in situ using ultrasonic (54 kHz) sensors on exposures within the shear zone. Laboratory measurements show a 10% change in seismic anisotropy and a contrast of >0.5 km s−1 in bulk rock velocity between granitoid protolith and highly strained ultramylonite. The in situ velocity measurements demonstrate that the velocity variations persist over considerable volumes, including zones where strain and associated volume loss of silica remain elevated without mylonitization. Synthetic seismograms generated using a one‐dimensional velocity model of the shear zone show that velocity variations observed in the field were large enough in magnitude and of sufficient scale to produce reflections observable on the deep seismic reflection profile collected using 6–25 Hz frequencies. Ray trace, two‐dimensional forward modeling of a high‐density, wide‐angle reflection section demonstrates that deep dipping reflections could result from strain‐induced acoustic impedance contrasts within the Singö or similar, nearby shear zones.
A chronology of the 1991 to 1993 Mount Etna eruption using advanced very high resolution radiometer data: Implications for real‐time thermal volcano monitoring
Authors: Harris Andrew J. L. , Blake Stephen , Rothery David A. , Stevens Nicki F.
Published: February 2004

Between December 1991 and March 1993 a continuous effusive eruption at Mount Etna built a 7.6 km2 lava flow field. Flows extended to within 1 km of the town of Zafferana before a successful artificial diversion was carried out higher up the volcano. During this eruption the spaceborne advanced very high resolution radiometer (AVHRR) acquired 308 images on which the activity could be detected. Since these data can be freely and directly available, such coverage potentially allows regular, real‐time monitoring. Ground observations and a flow map that we produced using a SPOT image and electronic distance measurement allowed us to develop and test data extraction techniques. AVHRR radiance maps were consistent with known locations of surface activity. These documented the transition from channel to tube fed phases, the changing threat to Zafferana, and flow diversion. Quantitative analysis of the AVHRR data enabled estimation of active lava area, thermal flux, effusion rates, and total flow field volume. Our estimates for eruption rate and total flow field volume, 5.6 to 7.6 m3/s and 220×106 to 300×106 m3, respectively, are in agreement with published ground‐based estimates of 5.8 m3/s and 235×106 m3. These correlations demonstrate the high degree of confidence that can now be placed in interpretations of AVHRR time series for eruptions where ground‐based data are scanty.
Tectonic reconstruction of the Clipperton and Siqueiros Fracture Zones: Evidence and consequences of plate motion change for the last 3 Myr
Authors: Pockalny Robert A. , Fox Paul J. , Fornari Daniel J. , Macdonald Ken C. , Perfit Michael R.
Published: February 2004

Bathymetry, side‐looking sonar and magnetics data from the northern East Pacific Rise have been analyzed to determine the tectonic history of the Clipperton Fracture Zone (CFZ) and the Siqueiros Fracture Zone (SFZ) over the last 2–3 m.y. Results of tectonic reconstructions indicate a series of counterclockwise changes in spreading direction at ∼2.5 Ma (1–2°), ∼1.5 Ma (1–2°), and ∼0.5 Ma (4–5°). Along the right‐stepping Clipperton Transform, the most recent change in spreading direction has resulted in fracture zone normal compression, which we propose has created a median ridge and transform‐parallel troughs along the active transform fault. Volcanic intersection highs located near the ridge‐transform intersections (RTI) of the CFZ are interpreted to be the result of fracture‐zone‐normal extension that has created pathways for magma emplacement into/onto the older lithosphère across from the RTI. Along the left‐stepping SFZ, we propose the changes in spreading direction have generated extension across the transform and have resulted in the formation of intratransform spreading centers and flexural transverse ridges. Tectonic reconstructions indicate a single Euler pole is unable to describe the motion of the Pacific‐Cocos plate boundary between the CFZ and SFZ during periods of spreading direction change since ∼3 Ma. Transtensional transforms will adjust to a new spreading direction almost immediately, while transpressional transforms will experience compression for a period of time comparable to half the age offset of the compressional transform.
The influence of fluid flow in fault zones on patterns of seismicity: A numerical investigation
Authors: Henderson Jeremy R. , Maillot Bertrand
Published: February 2004

We present a coupled two‐dimensional model of the fluid flow within a tabular fault zone and frictional failure of the fault, incorporating the effects of compaction and dilatancy. The model fault zone is loaded externally, resulting in a constant shear traction along the fault and a constant normal stress τn across it. Nonuniform compaction of the fault material leads to fluid pressure gradients and fluid flow. Frictional failure of element i is triggered when the fluid pressure is sufficiently high that the shear stress exceeds a critical level τci = μi(τn ‐ Pfi), where μi is the frictional coefficient and Pfi is the fluid pressure. Failure of a fault element results in an increase in element porosity and a decrease in fluid pressure. We model the diffusion of fluid pressure using a lattice Bhatnagar‐Gross‐Krook technique, and frictional failure is simulated by a simple cellular automaton type model. We show that the failure history of the fault is critically dependent on the ratio of the fluid diffusivity of the fault material to the compaction rate. For high diffusivities a power law distribution of failure sizes is observed, but at low diffusivities a non‐power law distribution results. High diffusivities promote a cyclic failure history, whereas at low diffusivities the failure occurs at a constant level. Clusters of failed fault elements may be identified with seismic events, and therefore our results place bounds on the range of fault parameters which are permitted in a situation in which seismicity is observed to follow a Gutenberg‐Richter distribution.
Farewell
Authors: Hinze William J.
Published: February 2004

I am pleased to welcome Paul M. Davis, Professor of Geophysics at the University of California, Los Angeles, as the new Senior Editor of the Journal of Geophysical Research‐Solid Earth for a 4‐year term commencing January 1,1997. Dr. Davis is a world renowned seismologist who brings a broad geophysical background to his new duties. I am especially pleased because I am convinced that he is dedicated to improving the quality and readability of our journal and in maximizing its service to the geophysical research community.

My 5 years at the helm of JGR‐Solid Earth have been much more enjoyable and rewarding than I anticipated. The task has been eased by an extremely competent, dedicated, and forgiving office staff, Deborah Gray, Melanie King, and Lois Gast; by an encouraging group of colleagues and the Department of Earth and Atmospheric Sciences at Purdue University; by cooperative co workers in the AGU office; and by a superb cadre of referees and Associate Editors. The Associate Editors have done an outstanding job of maintaining the quality of our journal. The tasks they perform are time‐consuming and thus have taken valuable professional time that could have been spent on more professionally visible activities. I thank all those who served, but I especially want to recognize the following geophysicists who have acted as Associate Editors in 1996. I thank them for the time devoted to the journal and for the uniformly high quality of editing.
Distinguishing between random and nonrandom patterns in the energy release of great earthquakes
Authors: Johnson Stephen M. , Sheridan Judith M.
Published: February 2004

Romanowicz [1993] suggested that great earthquakes alternate in a predictable fashion between strike‐slip and dip‐slip mechanisms on a 20–30 year cycle. We investigate whether or not the observed pattern of shifting predominance between strike slip and dip slip is capable of frequent reproduction by random simulation. To accomplish this, we randomly drew strike‐slip and dip‐slip events in the same proportion as in the historic record and determined how frequently a pattern of predominance similar to the historic record is generated. Out of 1000 simulations a pattern similar to the historic record was observed 183 times. This throws considerable doubt on Romanowicz's [1993] conslusion that a mechanism is at work producing the distribution she observed of great earthquakes. The difficulty in coming to conclusions from data of the type used by Romanowicz [1993] is explored, along with the psychological literature on the difficulty of detecting random and nonrandom patterns from relatively small amounts of information.
Thermal and dynamical evolution of the upper mantle in subduction zones
Authors: Kincaid Chris , Sacks I. Selwyn
Published: February 2004

We present results from two‐dimensional (2‐D) numerical experiments on the thermal and dynamical evolution of the subducting slab and of the overlying mantle wedge for a range in subduction parameters. These include subduction rate and the age and rheology of both subducting and overriding plates. Experiments also consider the influence of slab forcing conditions (from purely kinematic to purely dynamic) on the evolution of both the slab and mantle wedge. One goal is to determine how different parameters control thermal evolution of the slab‐wedge interface, from just after subduction initiation up through roughly 500–600 km of subduction, where temperatures are approaching steady state. An additional goal is to define optimal conditions for the melting of slab sediments and crust. Results show slab surface temperatures (SSTs) depend strongly on subduction velocity, plate thermal structure, and upper mantle (or wedge) viscosity structure. Fast subduction beneath a thick (>70 km) overriding plate results in the coolest SSTs. Maximum SSTs are recorded as an early transient event for cases of slow subduction (<3 cm/yr) beneath young, thin lithosphere (<45 km). The latter result supports a model for melting of slab sediments, and possibly crust, early on in cases where young plates subduct beneath thin lithosphere, such as in the Cascades. Maximum wedge temperatures are recorded at higher subduction rates and are found to be strongly dependent on factors influencing return flow into the wedge, such as age of the overriding plate and the ratio of retrograde to longitudinal slab motion. Assuming a model for arc magma genesis driven by fluids migrating into the wedge, these results predict higher‐temperature, Mg‐rich melts coming up beneath subduction zones with fast, steep slabs and young overriding plates, such as in Japan. The influence of variable viscosity is most pronounced in the slab‐wedge corner, which tends to stagnate, or freeze out, with time. Moreover, a region of highly viscous mantle develops above the slab at intermediate depths (>100 km) which deflects the zone of maximum shear away from slab‐wedge interface.
Hydrothermal regime and constraints on reservoir depth of the Jade site in the Mid‐Okinawa Trough inferred from heat flow measurements
Authors: Kinoshita Masataka , Yamano Makoto
Published: February 2004

Detailed heat flow measurements revealed an enormous heat flow variation (102 to 105 mW m−2) in the Jade hydrothermal field (27°16′N, 127°05′E and water depth 1350 m) located on the slope of the Izena Hole in the Mid‐Okinawa Trough. Within the Jade site, heat flow is higher than 1000 mW m−2 and decreases to ∼100 mW m−2 with a 1 km horizontal scale. Near the Jade black smoker, heat flow varies from >30,000 mW m−2 at a hydrothermally altered area to 1700 mW m−2 less than 100 m from it. A large‐scale heat flow variation suggests that the base of the escarpment near the Jade site serves as a recharge area for the Jade site. Linear geotherms indicate that the upward Darcian flow within sediments is slower than ∼3 cm yr−1. Thus the fluid circulation pathways would basically be restricted in permeable channels. Estimated conductive heat output rate from the Jade site is 4–7 MW, which is comparable to heat output by a single black smoker vent (∼8 MW). One‐dimensional thermal modeling predicts the existence of a boiling zone at ∼200 m beneath the Jade site, which may be overlain by a fractured impermeable layer. The normal chloride content of venting fluids from the black smoker may be explained either by upwelling of fluid which boiled at shallow depth but suffered no phase segregation, or by upwelling of fluid above the boiling interface.
Paleomagnetism of the Joban Seamount Chain: Its origin and tectonic implications for the Pacific plate
Authors: Masalu Desiderius C. P. , Tamaki Kensaku , Sager William W.
Published: February 2004

The paleomagnetism of 10 seamounts from the Joban Seamount Chain (northwestern Pacific) were studied using a method that calculates mean magnetization parameters by an inversion of magnetic anomaly and edifice bathymetry. Of the 10 seamounts, eight gave results consistent with other paleomagnetic studies of Pacific seamounts. Joban seamounts appear to have formed at two different mean paleolatitudes, contrary to what would be expected for a single hotspot origin. Furthermore, six of the consistent poles plot along the 129 to 82 Ma portion of the Pacific plate apparent polar wander path (APWP), implying the seamounts formed mainly during the mid‐ to Late Cretaceous. Two other poles, from Iwaki and Hitachi seamounts, are located northwest of the older end of the established Pacific APWP, possibly indicating Early Cretaceous ages. Because Iwaki and Hitachi seamounts are located in the middle of the chain, age does not progress along the chain, arguing against a single‐hotspot origin. Perhaps the chain formed by recurrent volcanism along a line of weakness or by another mechanism. Iwaki and Hitachi seamounts display smaller northward drift compared to the others, consistent with the Pacific plate drifting southward from Late Jurassic or Early Cretaceous to mid‐Cretaceous time.
Intraplate stress distribution induced by topography and crustal density heterogeneities beneath the Killari, India, region
Authors: Mandal P. , Manglik A. , Singh R. N.
Published: February 2004

The Deccan Traps covered region of the Indian peninsular shield experienced a devastating earthquake of magnitude 6.3(mb&) on September 29, 1993, with the epicenter (18.07°N, 76.45°E) near Killari(Latur). The event had a focal depth of 7 km, and its teleseismic moment tensor analysis suggests reverse faulting characterized by 120°strike, 46°dip, and 100°rake. The fault plane solution of the rnainshock also correlates the reverse faulting with NW‐SE trending nodal plane. In order to gain more insight into understanding the causative factors, the three‐dimensional intraplate lithospheric stresses associated with the topography and subsurface mass hetrogeneities are estimated up to a depth of 15 km beneath a region surrounding the epicenter and lying between 17°–20°N and 75°–78°E. A two‐layered lithospheric model with irregular interface of small amplitude at the Moho has been used for elastostatic stress calculations. The plate boundary force induced stresses are superimposed on the computed stress regime to assess their contribution vis‐a‐vis local causes. The estimated local stresses alone show a subdued northeasterly extensional regime at hypocentral depth. However, the superimposition of an assumed 30 MPa ridge push trending N10°E leads to an enhanced concentration of total stresses beneath the region near the hypocenter. Its maximum value at Killari is 31 MPa at 2 km depth, and it reduces to 29 MPa at 15 km depth. The total deviatoric stresses are also found maximum near Killari. Analysis of computed stresses indicates that the orientation of total principal stresses favors a strike‐slip movement at the focal depth with a NE trending maximum compression. The shear component of these stresses when resolved on the fault plane of the Killari mainshock [U.S. Geological Survey, 1993], however, suggests a thrust movement at 7 km depth beneath the Killari region. This study thus suggests that concentration of total stresses along some kind of crustal weak zones beneath the epicentral region could be an important factor for the reactivation of faults and therefore the Killari earthquake.