bluuebunny/crossref_metadata_2025 huggingface dataset:
Slopes of P‐ to S‐wave spectral ratios—A broadband regional seismic discriminant and a physical model
Authors: Goldstein Peter
Published: February 2004

Concerns about nuclear proliferation and negotiations toward a Comprehensive Test Ban Treaty (CTBT) emphasize the need for robust seismic monitoring and regional discrimination capabilities, especially at low magnitudes. Standard regional discrimination techniques have many shortcomings. For example, they cannot always discriminate shallow, low‐magnitude earthquakes or mining‐related events. I show that slopes of P‐ to S‐wave spectral ratios can be used to discriminate shallow‐ and normal‐depth earthquakes from explosions and possibly to discriminate explosions from mining‐related rockburst or collapse events. P‐ to S‐wave slopes and conventional, high‐frequency P‐ to S‐wave spectral amplitude ratios discriminate even better in combination than alone. A comparison of Pn and Lg spectra from Nevada Test Site (NTS) explosions and earthquakes shows that this method discriminates concentrated explosions from earthquakes and other shallow seismic events such as mine collapses because S waves from explosions decay more rapidly at high frequencies than do S waves from earthquakes. Spectra of one‐dimensional reflectivity synthetic seismograms show that frequency‐dependent attenuation of explosion‐generated P‐ to S‐wave converted phases, in the near‐surface layers, may be responsible for the differences in Lg (S‐wave) spectral falloff. These simulations also show that absolute amplitudes of earthquake P‐ to S‐wave spectral ratios are very sensitive to mechanism and depth, which may help explain the variable performance of conventional methods.
Radio wave scattering in the outer heliosphere: Preliminary calculations
Authors: Cairns Iver H.
Published: February 2004

Detailed first estimates are presented of angular broadening in the outer heliosphere due to scattering of radio waves by density irregularities. The application is to the 2–3 kHz radiation observed by Voyager. Two plausible turbulence models, which account very well for scattering within 1 AU, are extrapolated beyond 10 AU. Both models predict significant angular broadening in the outer heliosphere, accounting semi‐quantitatively alone for the source sizes inferred from roll modulation data. Predictions are presented for radial variations in the apparent source size if scattering is important. Comparisons with available data argue that scattering is important (and indeed is the dominant contributor to the apparent source size) and that the radiation source is located in the outer heliosphere. Other evidence that scattering is important, such as the fluctuations in apparent source direction and intensity, are also identified. The effects of scattering should be included in future analyses of the 2–3 kHz emissions.
First results from the meteor radar at South Pole: A large 12‐hour oscillation with zonal wavenumber one
Authors: Forbes J. M. , Makarov N. A. , Portnyagin Yu. I.
Published: February 2004

The first mesopause‐region (ca. 92±5 km) wind measurements from the meteor radar at Amundsen‐Scott Station at South Pole are described. Measurements are made along four orthogonal azimuth directions approximately 2° from the geographic South Pole. A large (±20 ms−1) oscillation in the northward wind is observed, with 12‐hour period and zonal wavenumber one. A similar wave was observed during August 1–13, 1992 at South Pole by Hernandez et al. (1993) using optical methods. The predominant semidiurnal tide in the atmosphere is migrating with the apparent motion of the sun, with s=2. The s=1 oscillation is interpreted here to result from the nonlinear interaction between the migrating semidiurnal tide and a stationary wave with s=1. The present mechanism represents an alternative to the gravity‐wave driven ‘pseudotide’ theory put forth by Walterscheid et al. (1986) to explain the occurrence of unexpectedly large semidiurnal tidal oscillations at high latitudes.
Excitation of plasma sheet instabilities by ionospheric O+ ions
Authors: Lakhina G. S.
Published: February 2004

It is shown that the presence of ionospheric O+ ions in the inner central plasma sheet (ICPS) during growth phase of the substorms can excite nonresonant low‐frequency, long wavelength RH mode instability under certain conditions. For the ICPS parameters during enhanced convection events, the excited modes have real frequencies of (0.1–5.0) mHz, and the typical maximum growth rates of ∼ (0.1–4.0) mHz with corresponding wavelengths of ∼ (1–30) RE (RE being the Earth's radius). The excited low‐frequency modes are expected to saturate to the maximum level of δB²/B0² ≈ (2.5 × 10−3−0.25), where δB and δB0 are the fluctuating magnetic field associated with the unstable modes, and the equilibrium magnetic field respectively. The large amplitude magnetic field fluctuations due to the nonresonant low‐frequency instability driven by O+ ions could twist the ICPS magnetic field into flux ropes, and also produce localised minima, in the magnetotail magnetic field near the neutral axis, which can be the potential sites for the excitation of the tearing modes leading to substorm onset.
Tailward stretching of geomagnetic field lines in the presence of an enhanced ionospheric convection electric field
Authors: Cai H. J. , Lee L. C. , Zhang L.
Published: February 2004

When the interplanetary magnetic field turns southward, dayside reconnection leads to an increase of open magnetic flux and an increase of the electric potential drop across the polar cap. The enhanced electrostatic field in the ionosphere can be mapped to the closed field region of magnetotail, contributing to a dawn‐to‐dusk electrostatic component of electric field in the plasma sheet. In the presence of this driving electric field, an adiabatic evolution of the plasma sheet is formulated in A‐space, where A is the magnetic flux function. It is found that the adiabatic evolution leads to the observed enhancement of the entropy content in the near‐earth plasma sheet and to a tailward stretching of geomagnetic field lines. As a result, the cross‐tail current in the near‐earth plasma sheet is strongly enhanced.
On the occurrence of polar mesosphere summer echoes
Authors: Hall C. M.
Published: February 2004

Polar Mesospheric Summer Echoes (PMSE) have become a well known characteristic of radar backscatter from the summer mesopause regions in polar regions. The phenomenon has been observed at frequencies from 50 MHz to almost 1 GHz. In this paper we attempt to identify a space‐time manifold for the occurrence of PMSE. The resulting manifold, defined only by temperature, agrees well with observations and furthermore indicates when and where we might expect to observe PMSE in the future.
VLF signatures of ionospheric disturbances associated with sprites
Authors: Inan U. S. , Bell T. F. , Pasko V. P. , Sentman D. D. , Wescott E. M. , Lyons W. A.
Published: February 2004

VLF perturbations on signals propagating along great‐circle‐paths (GCP) through electrically active midwest thunderstorms are associated with luminous high altitude glows (referred to as sprites) observed from aircraft or ground. The data constitutes the first evidence that the physical processes leading to sprites also alter the conductivity of the lower ionosphere.
A false alarm based on electrical activity recorded at a VAN‐Station in northern Greece in December 1990
Authors: Drakopoulos John , Stavrakakis George N.
Published: February 2004

A VAN‐prediction was announced on January 6, 1991, through the French newspaper “Le Monde” and on January 8–10, 1991, through Greek newspapers and TV stations. We evaluate this prediction on the basis of a letter which was sent by Prof. Varotsos (without date) to the Greek Minister of Public Works, and by considering previous VAN‐publications as well as recent seismological data for the candidate regions. We conclude that what was observed at ASS station (northern Greece) on December 31, 1990, was not SES‐activity but another disturbance or noise.
Probability of chance correlations of earthquakes with predictions in areas of heterogeneous seismicity rate: The VAN Case
Authors: Wyss M. , Allmann A.
Published: February 2004

Evaluations of 22 claims of successful earthquake predictions in Greece by Varotsos and Lazaridou [1991] were performed using the Ms (surface wave) as well as the ML (local) magnitude scales. If we assume that the predicted magnitudes were Ms (the scale was not specified in the prediction telegrams), and use the Preliminary Determinations of Epicenters (PDE) to estimate the seismicity rate expected at random, we find that 74% were false (they did not correlate with an earthquake as specified), 9% correlated by chance (probability, p, of one or more earthquakes at random >.50), and 17% are uncertain (0.15 < p < 0.40). There were no successes above the 85% confidence level. If we assume that the magnitude scale for predictions and seismicity rate was M(VAN)=ML(ATH)+0.5, and use the Monthly Bulletin of the Seismological Institute, National Observatory Athens (SI‐NOA), for estimating the seismicity rate, we find that 12 of 23 claims were false and that the rest, except for one, most likely correlated by chance with earthquakes. The missed earthquakes eligible for prediction numbered 35 and 456, respectively, using the two data sets. The probability that the observed correlation of predictions with earthquakes (6 and 11 respectively, out of 23 attempts) was due to chance is estimated as 69% and 96%, respectively, by summing the products of the individual probabilities for success and all the permutations of m successes out of n attempts. The probabilities of individual predictions to be fulfilled by chance were estimated based on the declustered catalogs for each prediction area separately, except for predictions during aftershock periods. For these the average seismicity rate during one month after the mainshock was used. We conclude that the various hypotheses of Varotsos et al. [1981a,b,c, 1982a,b, 1983, 1993a,b, 1996] and Varotsos and Lazaridou [1991] (these references will be referred to as VAN) relating seismic electrical signals (SES) to earthquakes, are invalid.
Changes in surface ozone amount and its diurnal and seasonal patterns, from 1954–55 to 1991–93, measured at Ahmedabad (23 N), India
Authors: Naja Manish , Lal Shyam
Published: February 2004

A comparative study of surface ozone observed at Ahmedabad during 1954–55 and 1991–93 periods has been made. The 1954–55 measurements were made at 0600, 1000, 1400, 1800, and 2100 hours (IST) by an electro‐chemical method, whereas 1991–93 measurements are made at every 15 minutes interval by an UV absorption technique. Higher ozone concentrations including larger amplitudes of diurnal and seasonal variations have been observed during 1991–93 as compared to the measurements made during 1954–55. The observed seasonal maximum during 1954–55 is in spring months whereas during 1991–93, it is in winter months. Increase in the concentration of ozone from 0600 hrs in the morning to 1400 hrs in the noon time is higher in summer and October–November months for both the periods and lowest during monsoon season. Furthermore, this change is higher during 1991–93 period as compared to 1954–55 suggesting higher levels of ozone precursors. The average ozone concentration increased from 14.7 ppbv during 1954–55 to 25.3 ppbv during 1991–93, resulting in a linear increase of 1.45% per year. The increase in background ozone concentrations, for 2100 and 0600 hrs during monsoon months when there is low photochemical ozone production even during day time, is 0.49% per year.
The 1990–1995 El Niño‐Southern Oscillation Event: Longest on Record
Authors: Trenberth Kevin E. , Hoar Timothy J.
Published: February 2004

The tendency for more frequent El Niño events and fewer La Niña events since the late 1970's has been linked to decadal changes in climate throughout the Pacific basin. Aspects of the most recent warming in the tropical Pacific from 1990 to 1995, which are connected to but not synonymous with El Niño, are unprecedented in the climate record of the past 113 years. There is a distinction between El Niño (EN), the Southern Oscillation (SO) in the atmosphere, and ENSO, where the two are strongly linked, that emerges clearly on decadal time scales. In the traditional El Niño region, sea surface temperature anomalies (SSTAs) have waxed and waned, while SSTAs in the central equatorial Pacific, which are better linked to the SO, remained positive from 1990 to June 1995. We carry out several statistical tests to assess the likelihood that the recent behavior of the SO is part of a natural decadal‐timescale variation. One test fits an autoregressive‐moving average (ARMA) model to a measure of the SO given by the first hundred years of the pressures at Darwin, Australia, beginning in 1882. Both the recent trend for more ENSO events since 1976 and the prolonged 1990–1995 ENSO event are unexpected given the previous record, with a probability of occurrence about once in 2,000 years. This opens up the possibility that the ENSO changes may be partly caused by the observed increases in greenhouse gases.
Infrasonic waves and volcanic tremor at Stromboli
Authors: Ripepe M. , Poggi P. , Braun T. , Gordeev E.
Published: February 2004

The origin of the volcanic tremor is still under debate. Many theories have been proposed in the last years, but none has yet been completely accepted. In 1993, highly sensitive pressure sensors (2.175 Pa/Volt) used to monitor the explosive activity at Stromboli have revealed unexpected correlation between small spike‐shaped pressure signals (1–2 Pa) and volcanic tremor. These pressure pulses repeat regularly in time with a recurrent period of ca. 1 s. Video camera images allowed us to correlate the pressure pulses with small gas bursts occurring at one of the active vents. The striking correlation (0.971) between infrasonic and seismic energy fluctuations is particularly meaningful in the frequency domain. Infrasonic and seismic signal share the same spectral content (3 Hz) for every station within a range of 700 m around the craters. Correlations in time and frequency domain remained unaltered during the 1994 field experiments. Moreover, during 1994, the increased degassing activity has been followed by an increase in pressure release (7–8 Pa) and by a shift towards higher frequencies (8 Hz) both in the infrasonic and seismic records. Infrasonic waves and volcanic tremor show similar energy fluctuations and frequency contents, appearing therefore to be produced by the same dynamical process. On this basis, we claim that volcanic tremor at Stromboli originates by continuous outbursting of small gas bubbles in the upper part of the magmatic column.
A rupture model for the 1967 Mudurnu Valley, Turkey Earthquake and its implication for seismotectonics in the western part of the North Anatolian Fault Zone
Authors: Pinar A. , Honkura Y. , Kikuchi M.
Published: February 2004

Six destructive earthquakes had successively occurred from 1939 to 1967 along the North Anatolian Fault Zone. The July 22, 1967 Mudurnu Valley earthquake (Ms=7.1) was the last of this series, with its western neighbour remaining as a seismic gap. Surface ruptures associated with this earthquake appeared within a broad shear zone, extending to a distance of 80 km. The generated teleseismic body waves were so complex that they were different even at stations in the same azimuth. In this study we inverted the observed teleseismic body waves to investigate such a presumably complicated rupture process of the Mudurnu Valley earthquake. We could identify six subevents within 2 minutes with their total seismic moment amounting to 1.1 × 1020 Nm. Although right‐lateral strike‐slip faulting was found to be dominant, two minor subevents had a dip‐slip mechanism; one showed a reverse faulting mechanism and the other a normal faulting mechanism.
In‐flight measurements of cruise altitude nitric oxide emission indices of commercial jet aircraft
Authors: Schulte P. , Schlager H.
Published: February 2004

Simultaneous in‐situ NO and CO2 measurements on board the DLR Falcon research aircraft in the exhaust plumes of commercial short to medium range jet aircraft are used to determine lower limits for the NOx emission indices EI(NOx) for cruising conditions. Concentration enhancements for NO and CO2 of 9 to 33 ppbv and 4 to 14 ppmv, respectively, relative to ambient background concentrations were observed in the exhaust trails 40 s to 130 s after emission. The derived EI(NOx)‐limits range between 6.4 to 11.7 g/kg. Though the NO2 fraction in the exhaust plumes has not been measured during these pilot investigations, arguments are given that the derived lower limits represent a close approximation to the EI(NOx) values. Within the present uncertainties they are in agreement with predictions based on ground‐based engine test data.
Neutrally buoyant diapirs: A model for Venus coronae
Authors: Koch Dorothy M. , Manga Michael
Published: February 2004

Coronae are typically circular features, 100–600 km in diameter, characterized by a deformed annular ring that is often topographically high. The central region may be raised or depressed relative to the ambient elevation. Previous studies have proposed an evolutionary progression beginning with dome‐shaped features which have radiating extensional rifts, followed by plateau‐shaped features which have both concentric deformation and radial extension, ending with the classic corona as described above. Previous dynamical calculations of a rising and spreading diapir simulated the evolution from dome to plateau‐shaped topography, but could not account for the raised rim or depressed central topography of some coronae. We demonstrate that these features can be modeled by a diapir that spreads laterally at a depth of neutral buoyancy.
Coalescence of fractures under shear stresses in experiments
Authors: Shen Baotang , Stephansson Ove , Einstein Herbert H. , Ghahreman Bidjan
Published: February 2004

A series of uniaxial compression tests were performed on gypsum specimens with preexisting fractures to study the failure mechanism of fractures and rock bridges in fractured rock masses. The coalescence mechanism of two parallel and offset fractures was investigated by monitoring the process of fracture initiation and propagation with a video camera. The tests showed that two inclined parallel fractures can coalesce by shear failure and/or tensile failure under a uniaxial load. The coalescence path and mechanism mainly depend on the relative position of the two fractures. For instance, when the two fractures are coplanar or slightly offset, coalescence is generated by shear failure; when they are overlapping in the loading direction, coalescence is generated by mixed shear and tensile failure. Two types of preexisting fractures, one without initial surface contact and hence frictionless and another with surface contact and friction, were used to study the influence of fracture contact conditions on the coalescence path and load. It was found that coalescence of fractures with surface contact and friction requires loads as much as 35% higher than that for coalescence of fractures without contact and friction. A stress analysis was conducted in this study to explain the different coalescence mechanisms. The analytical work indicated that different fracture geometries produce significantly different stress fields in the rock bridge area and hence result in different failure modes.
Pressure‐temperature phase diagram for the Allende meteorite
Authors: Agee C. B. , Li J. , Shannon M. C. , Circone S.
Published: February 2004

Piston cylinder and multianvil experiments from 1 to 27 GPa have been performed on the Allende CV3 meteorite to establish a pressure‐temperature phase diagram that includes major phase boundaries and the silicate‐oxide‐sulfide melting intervals. Olivine is the liquidus phase up to ∼14 GPa, followed by garnet up to ∼25 GPa. Near 26 GPa a cotectic exists where garnet and magnesiowüstite are liquidus phases. Magnesiowüstite is likely to be a lower mantle liquidus phase in both chondritic and peridotitic (see also Zhang and Herzberg, 1994) compositions. Hence element partitioning tests that neglect the role of liquidus magnesiowüstite may be incomplete for describing planetary differentiation at pressures >25 GPa. Allende shows immiscibility between (Fe,Ni)‐sulfide melt and FeO‐rich silicate melt. (Fe,Ni)‐sulfide is the lower temperature melt phase and is present at all experimental pressures and temperatures investigated. It is concluded that a terrestrial planet with a radius of ∼3000 km (maximum internal pressure of ∼30 GPa), and a bulk composition of carbonaceous chondrite, will upon magmatic differentiation form an FeO‐rich silicate mantle with an Fe‐Ni‐S core. The silicate fraction of Allende in our high‐pressure experiments is too rich in FeO to be a good match for the composition of peridotite xenoliths from Earth's upper mantle. However, the major elements of a peridotite upper mantle may be derived from an Allende‐like bulk Earth by a combination of lower mantle magnesiowüstite, perovskite, and sulfide fractionation and by upper mantle olivine flotation.
Magnetic and electric fields associated with changes in high pore pressure in fault zones: Application to the Loma Prieta ULF emissions
Authors: Fenoglio Mark A. , Johnston Malcom J. S. , Byerlee Jim D.
Published: February 2004

We determined the electric and magnetic fields generated during failure of faults containing sealed compartments with pore pressures ranging from hydrostatic to lithostatic levels. Exhumed fault studies and strain measurement data limit the possible size of these compartments to less than 1 km in extent. Rupture of seals between compartments produces rapid pore pressure changes and fluid flow and may create fractures that propagate away from the high‐pressure compartment, along the fault face. Nonuniform fluid flow results from pressure decrease in the fracture from crack‐generated dilatancy, partial blockage by silica deposition, and clearing as pressure increases. A direct consequence of this unsteady fluid flow may be associated transient magnetic signals caused by electrokinetic, piezomagnetic, and magnetohydrodynamic effects. Models of these processes for fault geometries with 1‐km‐high pressure compartments show that electrokinetic effects are several orders of magnitude larger than the other mechanisms. The electrokinetic signals produced by this unsteady flow are comparable in magnitude and frequency to the magnetic signals observed prior to the ML 7.1 Loma Prieta earthquake of October 18, 1989, provided fracture lengths are less than 200 m.
Superposed local and regional paleostresses: Fault‐slip analysis of Neogene extensional faulting near coeval caldera complexes, Yucca Flat, Nevada
Authors: Minor Scott A.
Published: February 2004

Numerous reduced stress tensors are computed by multiple inversions of 906 temporally and spatially partitioned fault‐slip data from the Yucca Flat region in the southwest Nevada volcanic field to constrain the Neogene paleostress and faulting history and to investigate how the regional tectonic stress field was affected by local caldera magmatism. Perturbed, shallow (<400 m), pre‐11 Ma paleostress configurations, determined west and northwest of present (post‐11 Ma) Yucca Flat basin, existed during mild extensional faulting and are attributed to superposition of transient caldera‐magmatic stresses on the regional stress field. Northwest of Yucca Flat a progressive shift in least principal stress (σ3) directions near known calderas located 5–15 km to the west occurred under a normal‐slip stress state during caldera development between about 15 and 13 Ma. A brief (∼0.5 m.y.) change to a strike‐slip stress state occurred at about 13 Ma and was accompanied by small‐offset, quasi‐conjugate strike‐slip faulting. This stress state was most distinct, relative to a normal‐slip state, near calderas where stress solutions and fault relations indicate closer affinities to a reverse‐slip state. Inferred 11.6–11.45 Ma paleostress tensors indicate radial tension associated with either initial caldera collapse or local post‐collapse topographic modification of the stress field. Post‐11 Ma normal‐slip stress tensors are associated with normal‐ and oblique‐slip faults that accommodated subsidence and eastward extension of Yucca Flat basin away from the caldera complexes. These tensors do not indicate stress modifications due to residual caldera‐related effects and thus were used to infer post‐11 Ma regional stress changes. The stress field has rotated as much as 65° clockwise since 11 Ma during extensional development of Yucca Flat basin, with most of the rotation and extension occurring before about 8.5 Ma. Results suggest that shallow magmatism and caldera development can strongly alter extensional tectonic stress fields, fault patterns, and slip directions in the uppermost crust out to distances of roughly two magma chamber radii away from a magma body.
Deflection of paleomagnetic directions due to magnetization of the underlying terrain
Authors: Baag C. , Helsley C. E. , Xu Shi‐zhe , Lienert B. R.
Published: February 2004

In the summers of 1989 and 1991 we made 344 near‐ground level measurements of the ambient geomagnetic field above recent basalts on the island of Hawaii using a three‐component fluxgate magnetometer. We studied 12 surface features, including a lava pond, lava channels, long tilted blocks, smooth sloping surfaces, two fissures, and a deep U‐shaped road cut. We observed substantial differences (up to 20°) between the observed and expected (International Geomagnetic Reference Field, IGRF) magnetic field directions over these features except those composed of shelly pahoehoe and a flat (horizontally) thin lava pond. We also observed inclinations that were systematically shallower than the IGRF field by up to 5°. We show that these shallower inclinations can be explained by the magnetization of the regionally sloping surface of the southern side of the island. We found that all of the observed inclination deflections can be explained by simple two‐dimensional models which assume uniform induced and remanent magnetization parameters in the local terrain. Our observations imply that the inclination deflections cannot be corrected without a complete knowledge of the preexisting terrain and the remanence in the underlying flows upon which the lavas cooled. Since this information is rarely available, it is difficult or impossible to discriminate between dispersion of paleomagnetic directions caused by the magnetic terrain effect and dispersion due to other factors such as paleosecular variation (PSV). We therefore conclude that PSV dispersion parameters cannot be accurately determined from paleomagnetic measurements on highly magnetic volcanic flows. We also suggest that some of the geomagnetic excursions inferred from measurements on volcanic rocks may be at least in part due to the magnetic terrain effect. It is unnecessary to invoke ad hoc mechanisms such as clastic, block, or crustal rotations, distortion of the top crust, or flow deformation to explain the large between‐site dispersions or inclination anomalies observed in many of the paleomagnetic data from volcanic rocks. Our observations also bring into question the general reliability of paleomagnetic pole positions inferred from volcanic rocks, as a systematic inclination deflection due to local and regional slopes and irregular terrain, such as those we observed, would lead to a corresponding error in. the inferred paleolatitude. The magnetic terrain effects also offer alternative explanations for anomalous paleomagnetically inferred plate motions.
Rift‐transform kinematics in south Iceland: Deformation from Global Positioning System measurements, 1986 to 1992
Authors: Sigmundsson Freysteinn , Einarsson Páll , Bilham Roger , Sturkell Erik
Published: February 2004

Crustal deformation in the plate boundary regions in south Iceland is estimated from repeated Global Positioning System (GPS) geodetic measurements in the period 1986–1992. We compare coordinate solutions for the 1986 and 1989 surveys with the results from the most recent survey in 1992. Horizontal position uncertainty is about 2 cm for the 1986 and 1989 coordinates and about 4 mm for the 1992 coordinates. Little internal deformation is observed in the area west of the western volcanic zone (within the North American plate) and at the southern tip of the eastern volcanic zone (within the Eurasian plate). The observed relative velocity of these two areas is 2.1±0.4 cm/yr in direction N117±11°E (1σ uncertainties), compatible with the 1.94 cm/yr widening in direction N 104°E across south Iceland predicted from the NUVEL‐1 global plate motion model. Left‐lateral shear strain is developing across the intervening transform zone, the E‐W trending south Iceland seismic zone (SISZ). Strain is concentrated within a 20‐ to 30‐km‐wide zone that correlates with seismic activity in the SISZ. About 85±15% of the relative plate motion is accommodated by this zone in such a way that the area south of the SISZ is moving toward the east with the Eurasian plate and the area north of the SISZ is moving toward the west with the North American plate. Accordingly, north of the SISZ the western rift zone can accommodate a maximum of 15±15% of the relative plate motion. Within the SISZ the shear strain results in an anticlockwise rotation of lineaments oriented north‐south, at a rate of 0.5–1 μrad/yr. The shear strain accumulation can be accommodated by “bookshelf faulting” on mapped recent north‐south faults in the SISZ. If the deformation is accommodated by an array of N‐S faults spaced 1–5 km apart, an average slip rate of about 0.5–5 mm/yr is required on each fault. The rate of geometric moment release due to earthquakes averaged over centuries,
, expected from such an array is about the same as expected from a simple transform fault, 2vAD, where 2v is the relative plate motion, A is the total length of the seismic zone, and D is the thickness of the brittle crust. If 2v = (0.85±0.15) × 1.94 cm/yr, D = 10–15 km, ana A = 75–85 km, then
= 1.0–2.5 × 107 m3/yr, comparable to the rate of geometric moment release in large historical earthquakes in the SISZ for the last several centuries. We conclude that historical seismicity in the SISZ can be attributed to left‐lateral shear accumulation across the seismic zone at a similar rate throughout historical times as during the years 1986–1992.
Seismic velocity structure and composition of the continental crust: A global view
Authors: Christensen Nikolas I. , Mooney Walter D.
Published: February 2004

Seismic techniques provide the highest‐resolution measurements of the structure of the crust and have been conducted on a worldwide basis. We summarize the structure of the continental crust based on the results of seismic refraction profiles and infer crustal composition as a function of depth by comparing these results with high‐pressure laboratory measurements of seismic velocity for a wide range of rocks that are commonly found in the crust. The thickness and velocity structure of the crust are well correlated with tectonic province, with extended crust showing an average thickness of 30.5 km and orogens an average of 46.3 km. Shields and platforms have an average crustal thickness nearly equal to the global average. We have corrected for the nonuniform geographical distribution of seismic refraction profiles by estimating the global area of each major crustal type. The weighted average crustal thickness based on these values is 41.1 km. This value is 10% to 20% greater than previous estimates which underrepresented shields, platforms, and orogens. The average compressional wave velocity of the crust is 6.45 km/s, and the average velocity of the uppermost mantle (Pn velocity) is 8.09 km/s. We summarize the velocity structure of the crust at 5‐km depth intervals, both in the form of histograms and as an average velocity‐depth curve, and compare these determinations with new measurements of compressional wave velocities and densities of over 3000 igneous and metamorphic rock cores made to confining pressures of 1 GPa. On the basis of petrographic studies and chemical analyses, the rocks have been classified into 29 groups. Average velocities, densities, and standard deviations are presented for each group at 5‐km depth intervals to crustal depths of 50 km along three different geotherms. This allows us to develop a model for the composition of the continental crust. Velocities in the upper continental crust are matched by velocities of a large number of lithologies, including many low‐grade metamorphic rocks and relatively silicic gneisses of amphibolite facies grade. In midcrustal regions, velocity gradients appear to originate from an increase in metamorphic grade, as well as a decrease in silica content. Tonalitic gneiss, granitic gneiss, and amphibolite are abundant midcrustal lithologies. Anisotropy due to preferred mineral orientation is likely to be significant in upper and midcrustal regions. The bulk of the lower continental crust is chemically equivalent to gabbro, with velocities in agreement with laboratory measurements of mafic granulite. Garnet becomes increasingly abundant with depth, and mafic garnet granulite is the dominant rock type immediately above the Mohorovicic discontinuity. Average compressional wave velocities of common crustal rock types show excellent correlations with density. The mean crustal density calculated from our model is 2830 kg/m3, and the average SiO2 content is 61.8%.
Compressive and tensile failure of inclined well bores and determination of in situ stress and rock strength
Authors: Peška Pavel , Zoback Mark D.
Published: February 2004

In this paper we investigate the occurrence of compressive and tensile failures of arbitrarily inclined well bores under a wide variety of stress conditions. The principal assumptions in this analysis are that the rock is isotropic and that it deforms elastically to the point of failure. As has been shown by previous investigators, for a given stress state and well bore orientation, it is straightforward to predict the orientation of the failures around the well bore as well as whether failure is likely to occur depending on such parameters as rock strength and borehole fluid pressure. However, as the stress state is almost never known in situ, we demonstrate how observations of compressive and tensile wall failures in inclined holes can be used to constrain in situ stress orientations and magnitudes if there are independent data on the magnitude of the least principal stress from either leak‐off or microfrac tests and on the formation pore pressure. We further demonstrate how once the stress state is determined, it is possible to assess both an upper bound on the effective in situ rock strength and the degree to which increasing the borehole fluid pressure (or mud weight) can reduce the likelihood of borehole failure. Through application of this methodology to an inclined well bore in an area of complex faulting in the Gulf of Mexico, we illustrate how it is possible to utilize observations of borehole failures to determine the magnitude and orientation of the stress tensor in areas such as offshore sedimentary basins where drilling inclined well bores is quite common.
Melt segregation in migmatites
Authors: Brown Michael , Averkin Yuri A. , McLellan Eileen L. , Sawyer Edward W.
Published: February 2004

Anatectic stromatic migmatites have a symmetrical layered structure with a low ratio of thickness to length and a periodicity, features that have not been explained satisfactorily but which are related to physical processes of melt segregation. We evaluate the compaction model for segregation as it applies to migmatites and develop models for melt segregation based upon convection driven by volume change and upon advection down pressure gradients that result from applied differential stress acting on an anisotropic multilayer protolith. Compaction by gravity‐driven two‐phase flow results in asymmetric separation, yields calculated segregation times that are slow or geologically unreasonable, and yields relative volumes that are not consistent with the abundance of leucosome seen in natural migmatites. Although calculated segregation times for “very wet” granite might allow segregation by compaction, some driving force in addition to gravity is needed to cause widespread melt segregation in the crust. For segregation at low volume fraction of melt, we develop a two‐dimensional two‐phase (matrix and fluid) viscous flow cell model (length much greater than thickness) with phase changes which predicts either expansion or contraction convection (depending on whether volume change, ΔVr, is positive or negative). Convection leads to melt segregation to form a stromatic structure in a geologically reasonable timescale. At moderate volume fraction of melt, segregation may occur by filter pressing in compositionally layered rocks in response to applied differential stress. Melt migration is by porous medium flow driven by differences in mean normal stress between the layers, as a consequence of differences in rheology, and shear‐enhanced matrix collapse. Calculated segregation rates are fast, and the model yields adequate volume of leucosome. The positive ΔVr for volatile phase‐absent melting reactions under crustal pressures promotes melt escape, unless extensional deformation facilitates melt accumulation. If the rate of melt production exceeds the rate of melt escape, then the increase in melt pressure may lead to melt‐enhanced embrittlement and fracture, and melt may migrate out of the system. In contrast, water‐rich volatile phase‐present melt‐producing reactions have a negative ΔVr, which promotes melt retention after segregation, unless deformation generates melt escape pathways. Evolution of stromatic structure through anatectic erosion of mesosome by increasing the melt fraction in situ may lead to breakdown of the solid matrix and will lead to instability due to buoyancy, and magma may become mobile and entrain residual material (“restite”).
On the analysis of paleomagnetic secular variation
Authors: Gubbins David , Kelly Peter
Published: February 2004

Modern methods of geomagnetic field analysis can be applied to paleomagnetic data provided account is taken of the relatively sparse and inaccurate nature of paleomagnetic results. Paleomagnetism tends to produce data sets dominated by inclination measurements, which do not provide a unique field model. As an example, two magnetic fields are found with closely similar inclinations but completely different declinations and strengths. Directional measurements are difficult to deal with because they are nonlinearly related to the field model. The equations are simplified by linearizing the field about an axial dipole; the approximation is adequate for paleomagnetic data and can be used to show that the VGP scatter function is not an appropriate estimate of the non‐axial‐dipole field even when averaged over longitude. Four assertions made in the paleomagnetic literature are shown to be correct within this linearization but are not true in general. In particular, equatorial symmetry of the VGP scatter functions for the separate symmetric (“quadrupole”) field holds within the approximation. The virtual geomagnetic pole (VGP) scatter functions for the modern geomagnetic field has strong north‐south asymmetry, but historical field models show it to be a relatively recent feature. The VGP scatter function for the symmetric part of the field, Sq, is dominated by the equatorial dipole and is approximately equal to the angular distance between the geomagnetic and geographic poles; that for the antisymmetric part of the field, Sd, is dominated by geomagnetic coefficients of degree 2 and order 1. Surprisingly, the axisymmetric quadrupole and octupole terms contribute very little, highlighting the dangers of modeling the paleomagnetic field without regard to departures from axial symmetry.
Deducing patterns of fluid flow and mixing within the TAG active hydrothermal mound using mineralogical and geochemical data
Authors: Tivey Margaret Kingston , Humphris Susan E. , Thompson Geoffrey , Hannington Mark D. , Rona Peter A.
Published: February 2004

The TAG active hydrothermal mound, located 2.4 km east of the neovolcanic zone at 26°N, Mid‐Atlantic Ridge, is −200 m in diameter, exhibits 50 m of relief, and is covered entirely by hydrothermal precipitates. Eight different types of vent solids were recovered from the mound by the submersibles Alvin and Mir in 1986, 1990, and 1991. Detailed petrographic and geochemical studies of samples and their distribution are used to deduce patterns of fluid flow and seawater/hydrothermal fluid interaction. Geochemical modeling calculations using fluid composition data corroborate these interpretations. Current activity includes highly focused flow of 363°C fluid from a chimney cluster on the top of the mound and deposition of a high ƒS2‐ƒO2 mineral assemblage that reflects low concentrations of H2S in black smoker fluid. Slow percolation of black smoker fluid pooled beneath the black smoker cluster and entrainment of seawater result in formation of massive sulfide crusts and massive anhydrite. These three sample types are enriched in Co and Se. Blocks of sulfide and white smoker chimneys, enriched in Zn, Au, Ag, Sb, Cd, and Pb, are forming on the surface of the mound from black smoker fluid that has been modified by mixing with entrained seawater, precipitation of sulfides and anhydrite, and dissolution of sphalerite within the mound. This is the first time that on‐going remobilization, zone refinement, and significant modification of high‐temperature fluid in the near surface has been documented in a seafloor hydrothermal system. Deposits of ocherous material and massive sulfide with outer oxidized layers that formed during previous hydrothermal episodes are exposed on the steep outer walls of the mound. Studies of the full range of samples demonstrate that highly focused fluid flow, consequent seawater entrainment, and mixing within the mound can result in formation of a large seafloor hydrothermal deposit exhibiting sample types similar to those observed in Cyprus‐type ore bodies.
Propagating rift west of the Fiji Archipelago (North Fiji Basin, SW Pacific)
Authors: Auzende Jean‐Marie , Hey Richard N. , Pelletier Bernard , Rouland Daniel , Lafoy Yves , Gracia Eulalia , Huchon Philippe
Published: February 2004

A Sea Beam survey of an area located west of the Fiji Islands in the North Fiji Basin, between 17°10′S and 18°S and 175°40′E and 176°40′E, shows a complex structure characterized by two deep grabens bounding a central plateau. In previous papers, we interpreted this area as an intraoceanic strike‐slip deformation zone, rejecting the possibility of oceanic spreading in the area. By comparison with the structures observed in the Galapagos 95.5° W area, we now reinterpret the area as a southward propagating spreading system. The analysis of structural and magnetic data and earthquake focal mechanisms confirms this new interpretation. The full spreading rate for the North Fiji Basin ridge, deduced from magnetic lineations, varies from 50 to 80 mm/yr. For the West Fiji spreading ridge, the magnetic anomaly analysis gives a spreading rate of about 50 mm/yr for the northern segment and slightly slower (40 mm/yr) for the southeastern segment. Using a 50 mm/yr spreading rate, the angles between the pseudofaults allow us to calculate the propagation velocity. For an angle of 75° north of 17°35′S, the propagation velocity is 33 mm/yr. The tip of the propagator is characterized by a 20° angle between the pseudofaults, which implies a recent increase of propagation velocity to 141 mm/yr. The initiation of this propagation could be related to the emplacement of the North Fiji fracture zone 1 to 1.5 m.y. ago, resulting in the formation of the 16°50′S triple junction. The main kinematic implication of the existence of a propagating rift west of the Fiji Islands is that in the North Fiji Basin the present‐day accretion is distributed on two parallel active spreading ridges, one in the central part of the basin, the other one in its southeastern quadrant. Such a phenomenon, unusual on mid‐oceanic ridges, except for microplates such as Galapagos, Easter and Juan Fernandez, may be more common in marginal basins because of their peculiar geodynamic setting.
Response of Long Valley Caldera to theMw= 7.3 Landers, California, Earthquake
Authors: Hill David P. , Johnston Malcolm J. S. , Langbein John O. , Bilham Roger
Published: February 2004

Of the many sites in the western United States responding to the June 28, 1992, Landers earthquake (Mw= 7.3) with remotely triggered seismicity, only Long Valley caldera is monitored by both seismic and continuous deformation networks. A transient strain pulse and surge in seismicity recorded by these networks began within tens of seconds following arrival of the shear pulse from Landers. The cumulative strain and number of triggered earthquakes followed the same exponentially decaying growth rate (time constant 1.8 days) during the first 6 days following Landers. The strain transient, which was recorded on a borehole dilatometer at the west margin of the caldera and a long‐base tiltmeter 20 km to the east, peaked on the sixth day at ≈0.25 ppm and gradually decayed over the next 15–20 days. The absence of a clear strain signal exceeding 0.4 ppm in data from the two‐color geodimeter deformation lines, which span the central section of the caldera, indicates that the strain transient cannot be due solely to pressure changes in the concentrated pressure source 7 km beneath the central part of the caldera that accounts for most of the uplift of the resurgent dome since 1980. The triggered seismicity occupied the entire seismogenic volume beneath the caldera. The focal mechanisms, the frequency‐magnitude distribution, and the spatial distribution of the triggered earthquakes are typical of other swarms in Long Valley caldera. The cumulative seismic moment of the triggered earthquakes through the first 2 weeks after the Landers earthquake corresponds to a single M = 3.8 earthquake, which is too small by nearly 2 orders of magnitude to account for the 0.25‐ppm peak amplitude of the observed strain transients. Evidently, the strain transient represents the dominant response mode, which precludes direct triggering of local earthquakes by the large dynamic stresses from Landers as the dominant process. Conditionally viable models for the triggering process beneath the caldera include (1) the transient pressurization of magma bodies beneath the resurgent dome and Mammoth Mountain by the advective overpressure of rising bubbles, (2) a surge in fluid pressure within the seismogenic zone due to upward cascading failure of isolated compartments containing superhydrostatic pore fluids, (3) relaxation (fluidization) of a partially crystallized magma body or dike intrusion in the deep crustal roots of Long Valley magmatic system, or (4) aseismic slip on midcrustal faults. Either the deep, relaxing‐magma body or lower crustal dike intrusion satisfy all the strain observations with a single deformation source. The latter model admits the possibility that large, regional earthquakes can trigger the episodic recharge of the deep roots of crustal magmatic systems.