bluuebunny/crossref_metadata_2025 huggingface dataset:
Generation of Pi2 pulsations by bursty bulk flows
Authors: Kepko Larry , Kivelson Margaret
Published: September 2002

We present the first observations directly relating bursty bulk flows (BBFs) to Pi2 pulsations. At ISEE 2, a large earthward flow was observed. The Institute of Geological Sciences (IGS) magnetometer chain located at the same local time as ISEE 2 recorded Pi2 pulsations, slightly delayed relative to flow onset, consisting of a long‐period (7–8 mHz) component associated with oscillations of the substorm current wedge. We demonstrate that the ionospheric current began to increase 90 s before the arrival of these Pi2 pulsations. We argue that this precursor was generated by earthward convection of plasma sheet plasma. Phase skips at midlatitudes on the nightside are associated with new flow bursts at ISEE 2. The Air Force Geophysics Laboratory (AFGL) magnetometer chain, which was located on the dusk flank, measured Pi2 pulsations that started ∼90 s after the flow bursts. We demonstrate that there is a one‐to‐one correlation between impulsive flow onset in the tail and Pi2 pulsations. We also show that oscillations at AFGL are directly related to temporal variations of the flow. Thus we suggest that the characteristic frequencies of low‐latitude Pi2 pulsations are established by the temporal structure of processes in the near‐Earth magnetotail.
A two‐dimensional simulation of the Kelvin‐Helmholtz instability with magnetic shear
Authors: Keller Kristi A. , Lysak Robert L.
Published: September 2002

The Kelvin‐Helmholtz instability is a mechanism for transport of mass and momentum from the solar wind to the magnetosphere. Using a two‐dimensional compressional MHD simulation, we explore the mass and momentum transport at the boundary for different magnetic field configurations corresponding to the flank low‐latitude boundary layer. We used data taken from Phan and Paschmann [1996] and Paschmann et al. [1993] to better approximate the density and magnetic field structure of the boundary layer for low‐ and high‐shear magnetic fields. We found that mass transport was the largest for the low‐shear magnetic field configuration where the magnetic field in the magnetosheath was almost parallel to the magnetospheric magnetic field. The largest momentum transport occurred for cases where the magnetic field in the magnetosheath was parallel or antiparallel to the magnetospheric magnetic field.
The relative recovery of galactic and anomalous cosmic rays in the distant heliosphere: Evidence for modulation in the heliosheath
Authors: McDonald F. B. , Heikkila B. , Lal N. , Stone E. C.
Published: September 2002

At Voyager 1 (46 AU, 33°N) the recovery of anomalous cosmic rays (ACR) is found to be very different from that of galactic cosmic rays (GCR) following the passage of the large interplanetary disturbances produced by the intensive solar activity of March/June 1991. If the modulation boundary for the GCR were at the termination shock, where anomalous cosmic rays are believed to originate, it would be expected that the intensity of the higher‐energy galactic cosmic rays would recover more rapidly than the relatively low energy anomalous component. On the contrary, we find that the time constant for the recovery of 265 MeV/nucleon GCR He is approximately twice as large as that of 43 MeV/nucleon ACR He+ and 13 MeV/nucleon O+. A regression plot of the ACR versus GCR intensity indicates a broad plateau in the ACR intensity over a period of several years while the GCR continues to increase. These differences in the relative recovery of the ACR and GCR strongly suggest that the combined interplanetary disturbances in the form of a global merged interaction region (GMIR) produced by the March/June 1991 solar activity remain an effective modulation agent for GCR after passing beyond the termination shock and into the region of the heliosheath. Some 0.37 years after the passage of the leading portion of the GMIR by Voyager 1, there is a large anisotropy in the ACR He+. One possible interpretation of this anisotropy is that it is produced by the initial flow of the ACR back into the heliosphere at the time that the leading portion of the interplanetary disturbance moves beyond the termination shock. If this interpretation is correct, then the inferred transit time between Voyager 1 and the termination shock of the GMIR along with an estimate of its velocity at 40 AU based on similar features in the Voyager 1 and Pioneer 11 energetic particle data give a value of the heliocentric distance to the termination shock of 88.5 ± 7 AU at ∼33°N in early 1992.
Magnetic field rotation and transition width in rotational discontinuities and Alfvén wave trains
Authors: Lin Y. , Lee L. C.
Published: September 2002

A hybrid simulation is carried out to study the evolution of magnetic field rotations in rotational discontinuities (RDs) and large‐amplitude Alfvén wave trains (AWTs) whose initial field rotation angle |Δф| > 180°. For a given initial half width D, the RDs or AWTs lose a multiple of ±360° field rotation at a critical time, until a smallest rotation angle |Δф| < 180° is reached. The critical time tc for losing the first 360° field rotation is found to be tc ≃ F(Δф, β1,θBn)D3.3±0.8, where the function F decreases linearly with Δф, decreases linearly with
for upstream ion beta β1 > 0.1, and decreases with decreasing shock normal angle θBn. At t < tc, however, the rotation angle Δф remains unchanged. A search is conducted for minimum widths required for RDs to preserve their |Δф| > 180° for a given transient convection time at the dayside magnetopause and minimum widths of AWTs per 360° field rotation in the solar wind.
Intense electron beams observed at Io with the Galileo spacecraft
Authors: Frank L. A. , Paterson W. R.
Published: September 2002

On December 7, 1995, the plasma instrumentation (PLS) on board the Galileo spacecraft detected intense low‐energy electron beams which were aligned along the magnetic field during the close approach of Jupiter's moon Io. The energies of these electrons were in the range of several hundred eV. An average field‐aligned current density of ∼0.05 nA/cm2 associated with the electron beams was measured during three consecutive samples of the electron velocity distributions. The total current in this beam is ∼1 × 106 A and in the range of the current magnitudes of 5 × 106 A previously reported at larger distances with the magnetometer on Voyager 1. Possible contributions to these currents from the thermal electrons and ions were not measured by the Galileo PLS. Estimates of the total energy influxes in the Io footprint in the Jovian atmosphere are in the range of 3 × 1010 W and similar to those obtained from observations of the Jovian far‐ultraviolet emissions with the Hubble Space Telescope. These electron beams are embedded in cool, dense thermal plasmas with kT ≃ 4 eV, a temperature which is similar to that observed in the adjacent torus plasmas which are corotating with Jupiter.
Comparison of global MHD simulations with AMIE simulations for the events of May 19–20, 1996
Authors: Slinker S. P. , Fedder J. A. , Emery B. A. , Baker K. B. , Lummerzheim D. , Lyon J. G. , Rich F. J.
Published: September 2002

Using WIND‐measured solar wind data, we have simulated the magnetosphere for the time between 1200 UT May 19 and 0200 UT May 20, 1996, with a three‐dimensional MHD model. This time period has been chosen as an International Solar‐Terrestrial Physics/Global Geospace Science event for community study, and there is a large set of data with which to compare. In this paper we will compare the simulation predictions with results from the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) analysis. We show comparisons for the convection, the auroral precipitation, the ionospheric conductances, the field‐aligned currents, and the Joule heating distribution. The results concentrate on four time periods when the two DMSP spacecraft, F12 and F13, and the POLAR spacecraft were passing over the northern (summer) polar region. The comparisons show excellent agreement with the F13 electric field measurements. The ionospheric convection patterns agree well between the simulation and the AMIE analysis with the cross polar potential drop somewhat higher in the MHD model. The auroral electron precipitation energy flux from the MHD model is too low, particularly in the late morning, when compared with the POLAR UVI data because of the lack of electron drift physics in the model. We show how the MHD auroral input can be improved by adjusting the parameters in the auroral precipitation model.
Evaluation of low‐latitude Pi2 pulsations as indicators of substorm onset using Polar ultraviolet imagery
Authors: Liou K. , Meng C.‐I. , Newell P. T. , Takahashi K. , Ohtani S.‐I. , Lui A. T. Y. , Brittnacher M. , Parks G.
Published: September 2002

Impulsive Pi2 pulsations have long been recognized as one of the key signatures of magnetic activity during substorm periods due to their wide observable range both in latitudes and longitudes. It is well documented that there is usually more than one Pi2 wave burst associated with a substorm and only one of them corresponds to the onset of the substorm. This observational fact poses obstacles to determining substorm onsets with Pi2 signals. Although the Pi2 have become one of the most popular indicators for substorm onsets, the reliability of using the Pi2 in this fashion has not been seriously investigated. In this paper we address this question with a statistical approach by using ∼650 auroral substorm onsets identified with Polar ultraviolet images for a time interval from April 1996 to May 1997. A comparison of the low‐latitude Pi2 pulsation onsets from Kakioka (L = 1.07) with the auroral breakups indicates that identifying substorm onset with the Pi2 alone is often ambiguous. Of a total of 119 isolated (defined as ∼10 min of quiet time preceding the onset) Pi2 bursts seen within ∼10 min from a magnetic positive bay, there were 65 events (∼55%) taking place within 3 min from breakups and 34 events (29%) indicating no sign of an auroral breakup within 10 min of the Pi2 burst. This result suggests that Pi2 may not be as a good indicator of the substorm onset as it was thought to be. Interestingly, it is always possible to associate Pi2 pulsations with some forms of auroral intensification. When compared to auroral breakups, Pi2 onsets are subject to a small delay of 1 – 3 min, with a peak around l min. Delays of Pi2 onsets are revealed to be a function of location relative to auroral breakup. This dependence is found to be consistent with the time of flight for a fast‐mode wave, in a plasmapause cavity mode model, propagating in the magnetosphere.
Temporal properties of magnetospheric line radiation
Authors: Rodger Craig J. , Clilverd Mark A. , Yearby Keith H. , Smith Andy J.
Published: September 2002

Magnetospheric line radiation (MLR) events are relatively narrowband VLF signals that sometimes drift in frequency and have been observed in both ground‐based and satellite data sets. We present the results of a survey undertaken on the basis of measurements made of MLR events observed at Halley, Antarctica (75°30′S, 26°54′W, L ≈ 4.3), in June, July, September, and December 1995, specifically looking at the temporal properties of Halley MLR events. We find that (1) single MLR lines described in previous papers tend to be comprised of up to 3 lines with widths of 5–10 Hz. (2) The multiple lines show highly variable spacings (e.g., < 10 to ∼100 Hz). (3) The rate at which MLR lines appears from the noise is ∼0.1 to 0.2 dB s−1, considerably smaller than previously reported for other VLF emissions. MLR exhibits slow growth, relative frequency stability, and long lifetimes in comparison with other coherent whistler mode emissions and thus may be generated by a separate mechanism. (4) The diurnal occurrence of MLR observed at Halley is twin‐peaked, indicating an association with a combination of chorus and midlatitude hiss. (5) MLR occurrence rates can vary greatly within a given month, but the proportion of MLR present as part of the overall Halley wave activity is roughly constant (10–13%) throughout the year. (6) The occurrence of MLR activity at Halley is weakly linked to geomagnetic activity but only 24–48 hours after very large storms (Kp > 6), which affects only 8% of the total MLR events in this study. For smaller storms, there is little effect, although MLR events tend not to occur when the geomagnetic activity has been quiet in the previous 48 hours. (7) There is no dependence of MLR occurrence rates upon the instantaneous levels of geomagnetic activity. (8) The average duration of a typical MLR event at Halley is ∼ 30 min, quite similar to previous reports.
Mountains versus valleys: Semiannual variation of geomagnetic activity
Authors: Cliver E. W. , Kamide Y. , Ling A. G.
Published: September 2002

The semiannual variation in geomagnetic activity is generally attributed to the Russell‐McPherron effect. In that picture, enhancements of southward field Bs, near the equinoxes account for the observed higher geomagnetic activity in March and September. In a contrary point of view, we argue that the bulk of the semiannual variation results from an equinoctial effect (based on the Ψ angle between the solar wind flow direction and Earth's dipole axis) that makes Bs, coupling less effective (by ∼25% on average) at the solstices. Thus the semiannual variation is not simply due to “mountain building” (creation of Bs) at the equinoxes but results primarily from “valley digging” (loss of coupling efficiency) at the solstices. We estimate that this latter effect, which clearly reveals itself in the diurnal variation of the am index, is responsible for ∼65% of the semiannual modulation. The characteristic imprint of the equinoctial hypothesis is also apparent in hourly/monthly averages of the time‐differentiated Dst index and the AE index.
On the source location of radiation belt relativistic electrons
Authors: Selesnick R. S. , Blake J. B.
Published: September 2002

Observations from the High Sensitivity Telescope (HIST) on Polar made around January and May 1998 are used to constrain the source location of outer radiation belt relativistic electrons. Phase space densities calculated as a function of the three adiabatic invariants show positive radial gradients for L < 4, suggestive of no source in that region. In particular, the peak intensity near L = 3 of a large enhancement beginning on May 4, 1998, appears to have been formed by inward transport over a period of several days. For L > 4, peaks in the radial dependence of the phase space density are suggestive of a local electron source that may be nonadiabatic acceleration or pitch angle scattering. However, discrepancies in the results obtained with different magnetic field models and at different local times make this a tentative conclusion.
Predictions of substorms following northward turnings of the interplanetary magnetic field
Authors: Blanchard G. T. , Lyons L. R. , Spann J.
Published: September 2002

Substorms are often observed to occur when the interplanetary magnetic field (IMF) turns northward at the end of intervals of southward IMF. Despite the significant correlation between northward turnings and substorms, a direct causal relationship between northward turnings and substorms has not been demonstrated. If such a causal relationship exists, then we should be able to predict that substorms will occur within a particular interval following the observation of a northward turning in the IMF. We have searched Wind IMF data and identified 16 strong northward turnings that following intervals of steady, southward IMF. To enhance the chances that the selected northward turnings impacted the magnetosphere, we required that each of them be also observed at either Geotail or IMP 8 while the separation of the second spacecraft from Wind was more that 10 RE in the y direction. Additionally, we required that either the second satellite was on the opposite side of the y = 0 plane from Wind or that one of the satellites was near y = 0. These two spacecraft observations also allowed us to more accurately predict the arrival times of the northward turnings at the Earth than we could by using data only from one spacecraft. Of the predicted substorms, 10 predictions were clearly successful to within ±8 min. Five predictions failed, but these failures reveal important new criteria that must be satisfied for a northward turning to cause a substorm. Three of the failures were caused by an increase in |ByGSM| that occurred simultaneously with the northward turning. This suggests that the triggering is associated with sharp reductions in the magnetospheric electric field, since increases in |ByGSM| negate the convection reduction associated with northward turnings. Two of the failures occurred for cases with the weakest southward IMF preceding the northward turning, which improves our criteria for the strength of the southward IMF that must precede a substorm. The final northward turning resulted in unusual auroral activity. These results establish the predictability of substorms following sharp northward turnings of the IMF and demonstrate that a direct causal relationship between northward turnings and substorm onset exists.
Local time and interplanetary magnetic field By dependence of field‐aligned currents at high altitudes
Authors: Zhou X.‐W. , Russell C. T. , Le G.
Published: September 2002

Magnetic field measurements from the Polar spacecraft reveal a field‐aligned current system in the high‐latitude, high‐altitude magnetosphere that is the extension of the traditional region 1 and region 2 field‐aligned current system observed at low altitudes. On average both the low‐ and high‐altitude observations show that the region 1 and region 2 currents flow in opposite senses on the morning and afternoon sides. Recent studies reveal that negative (dusk to dawn) interplanetary magnetic field By extends the upward flowing region 1 current onto the morningside and positive By extends the downward flowing region 1 onto the afternoonside in the northern hemisphere. We use Polar magnetic field data to study both the local time and interplanetary‐By orderings of the high‐latitude, high‐altitude, field‐aligned currents. We find that the interplanetary‐By component plays a very important role in controlling the flow sense of the field‐aligned current at high altitudes. Except for small clock angles, the effect of the negative (positive) interplanetary By is an upward (downward) current both in the afternoon and morning sectors.
Ionospheric electric field estimates from radar observations of the equatorial electrojet
Authors: Hysell D. L. , Burcham J. D.
Published: September 2002

Estimates of the zonal ionospheric electric field have been made on the basis of JULIA radar observations of the equatorial electrojet at Jicamarca. Two observing techniques were used and their merits compared. One technique (here referred to as the “oblique” technique) made use of a new, small antenna array with a broad beam in the equatorial plane. Electric fields were estimated from the Doppler shifts of type II radar echoes at low elevation angles as a function of range in the manner of Balsley [1969a]. For the other technique the main Jicamarca antenna array was used, and interferometry was employed to measure altitude profiles of the phase speeds of intermediate‐scale primary gradient drift waves in the electrojet directly overhead. Electric fields were inferred from these profiles following a modeling approach similar to the one described by Murthy and Ravindran [1994], Zonal ionospheric electric field measurements comparable to seasonal average incoherent scatter radar measurements were obtained using both methods. However, the interferometric observations reveal the presence of intense waves and jets in the zonal neutral wind similar to what has been seen in recent chemical release sounding rocket experiments. Zonal winds mainly control the zonal phase speeds of the primary gradient drift waves in the late afternoon and evening. Although the prereversal enhancement of the zonal electric field is evident in data from the oblique technique, there is no evidence of it in measurements derived from the interferometric technique.
Comparison of the generalized and bi‐Maxwellian multimoment multispecies approaches of the terrestrial polar wind
Authors: Leblanc François , Hubert Daniel , Blelly Pierre‐Louis
Published: September 2002

A comparison between two multimoment approaches is provided in the context of an application to the terrestrial polar wind. We compare the bi‐Maxwellian 16‐moment approach with the 16‐moment generalized model, which has been built in order to account for the suprathermal part of the velocity distribution function better than the previous multimoment approaches. This comparison has shown a general similarity between the two approaches and has shown that the better adapted closure assumption of the set of transport equations of the generalized model generates a higher acceleration. Moreover, the better determination of the collisional energy transfers generates a smaller increase of the temperature of the supersonic species that generally agrees with the adiabatic cooling assumption predicted by Monte Carlo or direct resolution of the Fokker Planck equation. The generalized model also provides the typical profiles of the velocity distribution function from the original collision‐dominated region to the collisionless region. These profiles are in good agreement with the Monte Carlo and collision kinetic resolution in the collision‐dominated region and in the lower part of the transition region.
A model for accelerated density enhancements emerging from coronal streamers in Large‐Angle and Spectrometric Coronagraph observations
Authors: Schmidt Joachim M. , Cargill Peter J.
Published: September 2002

Recent Large‐Angle and Spectrometric Coronagraph (LASCO) white light observations, ranging between 2 RS and 32 RS, trace accelerated density enhancements in the vicinity of the Sun which can be detected as enhancement in the intensity of the Thomson scattered light. One interpretation of these observed density enhancements is that they indicate moving plasmoids in the corona. Such plasmoids are accelerated away from the Sun by a Lorentz force due to the radial solar magnetic field. A model of accelerated plasmoids is presented which includes this magnetic acceleration, the Sun's gravitational attraction, and the aerodynamic drag acting on the plasmoid as it interacts with the solar wind. With reasonable choices of physical parameters the model produces velocity‐height profiles close to the observed ones. Profiles for external initial Alfvén velocities vAe0 = 80, 350, and 450 km/s (external initial magnetic field Be0 = 0.04, 0.16, and 0.21 Gauss) at r0 = 5 RS approximately match the lower, the middle, and the upper range of the observed velocity profiles, respectively. Choices for the ratio of the initial plasmoid number density ni0 and the initial solar wind number density ne0 of 2 < ni0/ne0 < 12 guarantee a slowly decreasing relative intensity enhancement in the range of observations. Only moderate line of sight intensity enhancements can be caused by a small and dense plasmoid.
A steeply dipping discontinuity in the lower mantle beneath Izu‐Bonin
Authors: Castle John C. , Creager Kenneth C.
Published: September 2002

We analyze the coda of teleseismic P waves to deterministically map seismic scatterers in a 1000 km on‐a‐side cube beneath the Izu‐Bonin trench by migrating and stacking waveforms. This method was applied to several hundred short‐period vertical‐component western United States seismometer recordings of 17 deep‐focus Izu‐Bonin earthquakes. Except for isolated arrivals, the midmantle appears devoid of sharp discontinuities. S‐to‐P conversions at the 660‐km discontinuity generated the largest signals; the 410‐km discontinuity occasionally generated signals. Horizontal discontinuities poorly explain other signals; however, S‐to‐P conversions generated at a nearly north‐south trending, steeply dipping discontinuity at 30°N, 145°E, and 1000 km beneath and parallel to the Izu‐Bonin trench explain most additional signals. Observed coherent S‐to‐P conversions of 2‐s period waves limit the width of a gradient zone to <7 km. Compressional and shear tomographic models [Grand et al., 1997; Widiyantoro, 1997] image a fast velocity structure at a similar depth and orientation extending southward from the middle of the Izu‐Bonin trench to the Mariana trench. We interpret the sharp scattering surface as a subducted crust and the tomographically imaged structure as the cold thermal anomaly associated with an ancient slab. We observe neither the deep scatterer nor high‐seismic wavespeeds north of 32°N. This feature is as much as 800 km east of recent deep earthquakes near 30°N and is at a latitude where the slab appears to extend horizontally on the 660‐km discontinuity to the west of the seismicity. We suggest that this deep slab fragment was carried north 500–1000 km owing to oblique subduction. In this scenario, it would be associated with the near vertical subduction at the Mariana subduction zone.
Paleomagnetic and geochronological identification of the Réunion subchron in Ethiopian Afar
Authors: Kidane Tesfaye , Carlut J. , Courtillot V. , Gallet Y. , Quidelleur X. , Gillot P. Y. , Haile Tigistu
Published: September 2002

This paper reports the paleomagnetic and geochronological analysis of a suite of samples from a basaltic fault‐related scarp of Pliocene age in the central part of the Afar depression (Ethiopia). Paleomagnetic work uncovers a clear characteristic remanent magnetization carried by (titano‐) magnetites, with high unblocking temperatures, close to the Curie temperature of pure magnetite. The characteristic direction, based on 27 non transitional flows, is D=8.1°, I=10.1° (α95=4.1°). This is consistent with earlier determinations of the clockwise tectonic rotation, due to rift propagation and overlap, of the block to which the section belongs. Magnetic stratigraphy consists of a succession from bottom to top of 16 reversed, 7 normal, and 4 reversed flows. One flow at the upper normal to reverse transition demonstrates complex behavior upon thermal and alternating field (AF) demagnetization, which is due to remagnetization by the overlying flow. K/Ar dating of five samples provides consistent determinations, averaging 2.07 ± 0.05 Ma. All ages agree with this average, given their rather large individual uncertainties (from 0.04 to 0.08 Ma at the 1σ level). Comparison with recent reference geomagnetic polarity timescales (GPTS) indicates that the normal subchron recorded in the Gamarri section must be linked with (one of) the Réunion event(s). The normal subchron and an earlier episode of large secular variation could provide the best volcanic record of a double Réunion event. Although uncertainties in ages are too large to allow unequivocal inferences, our age determination for the Réunion event is in better agreement with earlier determinations of McDougall and Watkins [1973] than with a more recent value proposed by Baksi et al. [1993] and used in the GPTS.
Formation of large summit troughs along the East Pacific Rise as collapse calderas: An evolutionary model
Authors: Lagabrielle Yves , Cormier Marie‐Hélène
Published: September 2002

Summit troughs wider than 500 m and 30–110 m deep are present along 15–20% of the fast spreading East Pacific Rise (EPR). They occur only along ridge segments with large cross‐sectional areas, indicative of a time‐averaged robust magma supply. Where available, seismic data confirm that these troughs are underlain by an axial magma chamber (AMC) 1.2–1.6 km below the seafloor. Furthermore, detailed investigation of the large summit troughs which notch the EPR between 17°56′ and 18°35′S indicates that the vertical relief of the troughs tends to be maximum where the AMC is shallowest. Both of these observations are inconsistent with the predictions from a model in which large summit troughs form by rifting of the brittle upper crust during phases of amagmatic extension. Rather, we propose that they represent elongated collapsed calderas that form when the melt supply to formerly inflated AMCs wanes or nearly ceases. Because seismic studies constrain the melt lens along the EPR to be only 30–80 m thick, the proposed existence of collapsed calderas 40–110 m deep implies that the entire magma reservoir comprising the melt lens and the underlying crystal mush zone deforms and compacts during periods of waning magma supply. In particular, we suggest that a voluminous crystal mush zone will stretch in response to steady state seafloor spreading when the magma supply temporarily decreases. The resulting caldera will further widen with each subsequent dike intrusion. When an abundant melt supply finally resumes, the associated tumescence of the neovolcanic zone and profuse lava flows will combine to smooth out the caldera.
Evolution of the Louisiade triple junction
Authors: Gaina Carmen , Müller R. Dietmar , Royer Jean‐Yves , Symonds Phil
Published: September 2002

We derived new finite rotations for the opening of the Coral Sea using revised magnetic anomaly interpretations and fracture zone data from a gravity anomaly grid based on from satellite altimetry. These rotations differ from the finite rotations that describe the opening of the Tasman Sea; this confirms the existence of a triple junction between the Australian Plate, the Mellish Rise, and the Louisiade Plateau active during the opening of the Coral Sea (62 to 52 Ma). Magnetic anomalies, fracture zones visible on the gravity grid, and strike‐slip faults indicate that extension occurred between the Mellish Rise and the Louisiade Plateau, and extensional and transform motion occurred between Australia and the Mellish Rise (attached to the Chesterfield and Kenn Plateaus). The configuration of the triple junction from chron 27 to 26 was either ridge‐ridge‐ridge (RRR) or ridge‐fault‐fault (RFF). At chron 26 (58 Ma) the triple junction had a RFF configuration and migrated southward as the relative motion between the Louisiade Plateau and the Mellish Rise was transferred to the boundary between the Mellish Rise and the Kenn Plateau. The gravity low between the Kenn Plateau and the Mellish Rise is interpreted as a strike‐slip fault active from about 57 to 52 Ma. This configuration lasted until seafloor spreading ceased in the Coral and Tasman seas at about 52 Ma. Our model implies extension in the Osprey Embayment that might explain small areas of oceanic crust west of the Coral Sea Basin. The western boundary of the Coral Sea was a NE‐SW strike‐slip fault, active between 58 and 52 Ma.
A detailed record of paleomagnetic field change from Searles Lake, California: 1. Long‐term secular variation bounding the Gauss/Matuyama polarity reversal
Authors: Glen Jonathan M. G. , Liddicoat Joseph C. , Coe Robert S.
Published: September 2002

More than 33 m of 2.5 Ma sediment from Searles Lake, California was studied in order to construct a record of secular variation (SV) across the Gauss/Matuyama (G/M) normal‐to‐reverse polarity transition. The behavior of the field preceding and following the reversal is considered here, while in a companion paper [Glen et al., this issue] the details of the transition are discussed. The record encompasses an interval of roughly 183,000 years beginning 50 kyr (9 m) before and extending more than 128 kyr (23 m) beyond the transition, while the main phase of the transition lasts for nearly 5 kyr (1 m). Because the core was rotary drilled, and declinations lost, SV was characterized by the inclination and its angular dispersion. Inclination‐only statistics reveal that (1) the record displays overall higher than expected values of angular dispersion (normal S∼20°; reverse S∼19°; expected S∼15.5°), suggesting that the field proximal to transitions may be more noisy than the distal field. In addition, normal data from immediately before the transition display higher S than reverse data immediately following it, implying that the postransitional field is more stable than the pretransitional field. One of the most prominent features of this record is an excursion of the field occurring roughly 4 kyr prior to the onset of the reversal. A record of the G/M transition from Chinese loess (R. Zhu et al., submitted manuscript, 1999) displays a similar event (also occurring roughly 4 kyr before the transition). This and the fact that the event is associated with anomalously low intensities suggest that the disturbance may be global in nature. The fact that comparable features are associated with other transitions [Hartl and Tauxe, 1996; Clement, 1992] intimates that the field may commonly show signs of early instability. This precursory event is actually one of a sequence of oscillations (in inclination and intensity) preceding the transition. That these fluctuations occur at roughly 4 kyr intervals leading up to the reversal (which also appears at this same interval) strongly suggests that an oscillatory disturbance in the core, active over at least 15 kyr prior to the transition, had eventually triggered the reversal. In addition, that these waveforms are absent from the postransitional record suggests the reversal process actively rejuvenates and stabilizes the field.
In situ measurements of hydraulic properties of a shear zone in northwestern South Carolina
Authors: Talwani Pradeep , Cobb Jason S. , Schaeffer Malcolm F.
Published: September 2002

Subsequent to the initial impoundment of the Bad Creek Reservoir in northwest South Carolina in January 1991, lake level fluctuations (up to 33 m d−1) caused delayed (98 hours) correlative water level changes in an observation well (OW3) 250 m away. The bottom of the well is connected to the bottom of the reservoir by a shallowly dipping, 1 m wide shear zone. The amplitude of water level changes in OW3 was 0.19 of that in the reservoir. The amplitude ratio, R, of the water level changes in OW3 to those in the reservoir is a measure of the pore pressure transmitted through the shear zone. Assuming one‐dimensional diffusion of pore pressure in the shear zone between the reservoir and OW3, these values of time lag and amplitude ratio of the water level fluctuations changes yield estimates of 0.05 m2 s−1 for hydraulic diffusivity and 1.1 × 10−15 m2 for permeability of the shear zone. In subsequent years the time lag decreased to 72 hours and R increased to 0.23, suggesting a nearly 60% increase in the two hydraulic parameters. We interpret the increase to be owing to flushing of fines in the shear zone due to the pumping action of lake level fluctuations or due to dissolution. After 1994, there were no further changes in these parameters suggesting steady state conditions. Our analysis demonstrated that the fluid pressure flow is restricted to the shear zone and the pore pressure transmitted through the shear zone showed a lognormal relationship with the frequency of lake level fluctuations, R = 0.5 exp (−14.93/T), where T (days) is the period of lake level changes. Spectral analyses of the data were used to obtain a frequency independent hydraulic diffusivity (∼0.076 m2 s−1) of the shear zone after it had reached steady state conditions. Using the undrained response of the reservoir, an in situ estimate value 0.66 was obtained for the Skempton's coefficient. The hydraulic diffusivity ∼0.1 m2 s−1 was found to be lower than that encountered at other locations of induced seismicity in the region.
Geodynamic models of crustal‐scale episodic tectonic accretion and underplating in subduction zones
Authors: Ellis Susan , Beaumont Christopher , Pfiffner O. Adrian
Published: September 2002

We investigate the dynamics of crustal‐scale episodic accretion at convergent margins using a finite element model of subduction. The numerical model incorporates effects from flexure, subduction loading, and large‐scale variations in material properties. The model is used to assess the influence of these parameters on the discrete transfer of units from the lower to the upper plate during subduction. The primary controls investigated are spatial variations in density, material strength, and thickness of material layers. These controls correspond to natural examples where episodic accretion results from the thermal structure of subducting lithosphere, giving rise to areas of thermally activated ductile creep; attempted subduction of small crustal blocks (density, frictional and/or ductile strength variations); interaction between well‐attached basement blocks and overlying, detachable sediments; and spatial changes in thickness of sediment and crust. In the experiments, accretion of small continental terranes within a model subduction zone can cause crustal‐scale fold nappes and shear zones to develop, with accompanying tectonic underplating and/or frontal accretion. The model results may provide insight into real cases of episodic accretion, for example, the emplacement of the upper Penninic nappe stack of the Central Alps.
Mechanism of the 1975 Kalapana, Hawaii, earthquake inferred from tsunami data
Authors: Ma Kuo‐Fong , Kanamori Hiroo , Satake Kenji
Published: September 2002

We investigated the source mechanism of the 1975 Kalapana, Hawaii, earthquake (MS= 7.2) by modeling the tsunamis observed at three tide‐gauge stations, Hilo, Kahului, and Honolulu. We computed synthetic tsunamis for various fault models. The arrival times and the amplitudes of the synthetic tsunamis computed for Ando's fault model (fault length = 40 km, fault width = 20 km, strike = N70°E, dip = 20°SE, rake = −90°, fault depth = 10 km, and slip = 5.6 m) are ∼10 min earlier and 5 times smaller than those of the observed, respectively. We tested fault models with different dip angles and depths. Models with a northwest dip direction yield larger tsunami amplitudes than those with a southeast dip direction. Models with shallower fault depths produce later first arrivals than deeper models. We also considered the effects of the Hilina fault system, but its contribution to tsunami excitation is insignificant. This suggests that another mechanism is required to explain the tsunamis. One plausible model is a propagating slump model with a 1 m subsidence along the coast and a l m uplift offshore. This model can explain the arrival times and the amplitudes of the observed tsunamis satisfactorily. An alternative model is a wider fault model that dips 10°NW, with its fault plane extending 25 km offshore, well beyond the aftershock area of the Kalapana earthquake. These two models produce a similar uplift pattern offshore and, kinematically, are indistinguishable as far as tsunami excitation is concerned. The total volume of displaced water is estimated to be ∼2.5 km3. From the comparison of slump model and the single‐force model suggested earlier from seismological data we prefer a combination of faulting and large‐scale slumping on the south flank of Kilauea volcano as the most appropriate model for the 1975 Kalapana earthquake. Two basic mechanisms have been presented for explaining the deformation of the south flank of Kilauea: (1) pressure and density variation along the rift zone caused by magma injection and (2) gravitational instability due to the steep topography of the south flank of Kilauea. In either mechanism, large displacements on the south flank are involved that are responsible for the observed large tsunamis.
Miocene clockwise rotation of southwest Japan and formation of curvature of the Median Tectonic Line: Paleomagnetic implications
Authors: Otofuji Yo‐ichiro , Enami Ryo , Yokoyama Masahiko , Kamiya Kazumasa , Kuma Saori , Saito Hiromi , Matsuda Takaaki
Published: September 2002

The origin of the curvature of the Median Tectonic Line (MTL) in southwest Japan is still controversial. In order to investigate its formation, we carried out a paleomagnetic study in the Nohi area, north of the curved part of the MTL in the eastern part of southwest Japan. More than 420 paleomagnetic samples were collected at 35 sites from Cretaceous to Paleogene welded tuffs in the Kasagatake Rhyolites, Oamamiyama Group, and Nohi Rhyolite. Characteristic directions with high unblocking temperature component above 560°C were isolated from 32 sites. Paleomagnetic directions of the Kasagatake Rhyolites and Oamamiyama Group (55–66 Ma) have a clockwise deflected declination (D = 51.7°, I = 52.6°, α95 = 8.6°) which is consistent with the characteristic Paleogene paleomagnetic direction of the central part of southwest Japan. A similarly deflected declination is also observed in the peripheral part of the Nohi Rhyolite region, far away from the Atera fault system. Paleomagnetic data indicate that the Nohi area experienced a clockwise rotation through more than 45° with respect to the Asian continent as a part of southwest Japan but that the amount of rotation of the Nohi area is 23° smaller than that of the central part of southwest Japan. A new model is developed to explain the tectonic rotation. At about 15 Ma during the latest stage of the clockwise rotation of southwest Japan, the eastern end moved southward by more than 60 km along a left‐lateral strike‐slip fault which comprised the present north‐south segment of the MTL and the Akaishi Tectonic Line. We conclude that the curvature of the MTL is a composite of the 15 Ma strike‐slip fault system and a small bend of the MTL associated with the small differential rotation of the Nohi area with respect to the central part of southwest Japan.
Fine details of the Wadati‐Benioff zone under Indonesia and its geodynamic implications
Authors: Schöffel Hans‐Joachim , Das Shamita
Published: September 2002

Using about 5200 handpicked P, S, pP, sP, PcP, and ScP phases from digitally recorded seismograms, together with International Seismological Centre reported phases, we obtain improved hypocentral locations for ∼1790 earthquakes, ∼983 of them having 90% confidence limits <30 km, at depths >50 km, in the period 1962 to September 1996, along the Indonesian subduction zone. We use an improved joint hypocenter determination method in which the solution is more stable than in the original one. We show that for deep earthquakes, use of the core‐reflected phases is as good as use of depth phases, and since such core phases often have larger amplitudes and sharper onsets than the depth phases, their arrival times can be read very accurately. The positions of the relocated hypocenters show that (1) a portion of the Indonesian arc between ∼110°E and 123°E longitude, and deeper than ∼500 km, is dipping southward at an angle of ∼75°, that is, in a direction opposite to the upper part of the north dipping slab, and (2) east of about 108°E, the seismic zone is wider near 670 km than near the 500 km depth. The first suggests southward lateral flow in the mantle, relative to the plate motion vector here. From the contortion of the seismic zone along the eastern portion of the Indonesian arc, we can estimate the average lateral shear strain rate in the 300 to 670 km depth range to be of the order of 10−16s−1, over the last 10–20 Myr.
Numerical simulation of electrokinetic potentials associated with subsurface fluid flow
Authors: Ishido Tsuneo , Pritchett John W.
Published: September 2002

A postprocessor has been developed to calculate space/time distributions of electrokinetic potentials resulting from histories of underground conditions (pressure, temperature, vapor saturation, concentrations of dissolved species, flow rate, etc.) computed by unsteady multidimensional geothermal reservoir simulations. Electrokinetic coupling coefficients are computed by the postprocessor using formulations based on experimental work reported by Ishido and Mizutani [1981]. The postprocessor was applied to both numerical modeling of natural self‐potential (SP) anomalies in geothermal fields and production‐induced SP changes. The essential features of the SP anomalies and SP changes which have actually been observed in real geothermal fields are reproduced reasonably well in these calculations for both single‐phase (liquid) and two‐phase (vapor/liquid) geothermal reservoirs. The postprocessor was also applied to predicting the magnitude of electrical earthquake precursory signals caused by dilatant strains taking place within a narrow vertical fault zone. If the rate of porosity increase is high enough to induce substantial pore pressure decrease, two‐phase flow develops in the fault zone and produces observable signals at the ground surface if a high conductivity channel connects the near‐surface region and the deep fault zone.
Measured acoustic wave velocities of R11 (CCl3F) hydrate samples with and without sand as a function of hydrate concentration
Authors: Berge Lars Inge , Jacobsen Kjell Arne , Solstad Arne
Published: September 2002

We report acoustic laboratory measurements on initially unconsolidated sand packs with hydrate formation taking place in the pore space. Both P and S wave velocities were measured. Hydrate was formed from water and the refrigerant R11 (CCl3F) at 2°C and atmospheric pressure. Measurements were performed on two types of initially unconsolidated sand packs with average grain size 100 μm and 280 μm, respectively. P wave velocities varied from about 1700 m/s at low hydrate content to 3810 m/s at a calculated hydrate fraction in the pore space equal to 0.52. S wave velocities were only detectable at hydrate fractions larger than 0.35, where S wave velocities up to 2230 m/s were measured. Our measurements indicate that acoustic measurements are not sensitive to small amounts of hydrate in a system of initially unconsolidated sand. The data show a very distinct change in behavior when the hydrate fraction exceeds 0.35, the concentration at which hydrate cementation of the sand grains is believed to occur. Measurements for samples without sand (samples containing water, R11, and hydrate) are also reported. For hydrate without sand, P wave velocities ranged from about 1400 m/s at very low hydrate content to about 2500 m/s at a measured hydrate fraction equal to 0.68. For these samples, reliable S wave data were not detected.
Detection of fluid flow variations at the Nankai Trough by electric and magnetic measurements in boreholes or at the seafloor
Authors: Jouniaux Laurence , Pozzi Jean‐Pierre , Berthier Jean , Massé Philippe
Published: September 2002

Detection of changes in the flow rate of expelled fluids in accretionary prisms by monitoring of electric and magnetic fields is discussed. A numerical model of the electric and magnetic fields associated with fluid flow variations at the Nankai Trough is presented which gives a numerical solution of the coupled system of electric convection currents and conduction currents that directly determines the magnetic anomaly itself. Measurements in a borehole located between two vents are shown to be well adapted to detection of fluid flow variations using the vertical gradient of the electric potential and the horizontal magnetic field. The vertical electric field is about 10 mV/km up to 500 m depth where there is a lithologic reflector and about 50 mV/km below this reflector. The horizontal gradient of the magnetic field is 2 nT/km at the seafloor. Modelization with a lower fault conductivity and a larger décollement thickness has also been modeled. The vertical gradient of the horizontal magnetic field is ∼5 to 15 nT/km. A variation of 3 mV and 1.5 to 3 nT at 600 m depth in a borehole could reveal a fluid flow rate variation of 20%, which is a reasonable fluid flow change based on some observations at short‐scale time. Since a 1.5 to 3 nT anomaly seems easier to detect than a 3 mV anomaly, it is likely that the variation of the magnetic field would more sensitively reveal fluid flow variations. When monitoring the magnetic field at the seafloor, a change of 0.4 nT/km in the horizontal gradient could reveal a fluid flow rate variation of 20%.