bluuebunny/crossref_metadata_2025 huggingface dataset:
Whistler waves, Langmuir waves and single loss cone electron distributions inside a magnetic cloud: Observations
Authors: Moullard O. , Burgess D. , Salem C. , Mangeney A. , Larson D. E. , Bale S. D.
Published: February 2004

Whistler waves propagating along the ambient magnetic field are observed within a coronal mass ejection (on January 10, 1997) associated in time with Langmuir waves and electron distributions of a single loss cone type. In addition, background observations are made on the plasma wave activity in the sheath and foreshock regions that precede the magnetic cloud, on the observed radio emissions (including a type II radio burst) and on the geometry of the cloud. All the data comes from the WIND spacecraft. The whistler waves are identified using full magnetic waveforms while possible evidence of coexisting parallel, and antiparallel propagating Langmuir modes are found in the waveform and spectral wave data from the WAVES experiment. A few hundred low energy electron distributions from the Three‐Dimensional Plasma (3DP) experiment are investigated. Finally, we tentatively suggest that this type of plasma wave particle activity is linked to the type II emission observed, i.e., that the emission mechanisms are proceeding and taking place within the magnetic cloud instead of at the shock region as usually thought. The extra suprathermal electrons could source from electrons accelerated at reconnection sites between the magnetic cloud and the ambient interplanetary magnetic field. A linear instability study using observed properties of the electron distributions is to be presented in a following paper.
Vertical wind observations with two Fabry‐Perot interferometers at Poker Flat, Alaska
Authors: Ishii M. , Conde M. , Smith R. W. , Krynicki M. , Sagawa E. , Watari S.
Published: February 2004

Characteristics of vertical winds in the polar thermospheric region were examined using data sets generated with two types of Fabry‐Perot interferometers at Poker Flat, Alaska (65.11°N, 147.42°W). The Communications Research Laboratory Fabry‐Perot Interferometer (CRLFPI) simultaneously observed the O I 557.7 nm and O I 630.0 nm emissions, whereas the Geophysical Institute Scanning Doppler‐Imaging Interferometer (GI‐SDI) observed the O I 630.0 nm emission. The height of the O I 557.7 nm and O I 630.0 nm emissions were 100–140 and 200–240 km, respectively. The data were obtained from October 1998 to February 1999, and our findings were as follows: (1) Observations of the O I 630.0 nm emission showed that upward (downward) vertical winds were often present when bright aurora existed equatorward (poleward) of the observatory. This is consistent with previous studies [Crickmore et al., 1991; Innis et al., 1996, 1997]. (2) Comparison of vertical winds estimated from two different wavelengths (557.7 and 630.0 nm) showed that vertical winds were often observed simultaneously at both wavelengths, as reported by Price et al. [1995]. However, the vertical winds observed at different heights sometimes had different features when thin but bright aurora passed over the observatory. A similar observation was reported by Ishii et al. [1999]. (3) Vertical winds were often observed along with divergence and rotation of the horizontal wind field. Some vertical winds not associated with active aurora may be driven by the divergence in the horizontal wind.
Rapid flux transport and plasma sheet reconfiguration
Authors: Schödel R. , Nakamura R. , Baumjohann W. , Mukai T.
Published: February 2004

On the basis of 3 1/2 years of Geotail data we examine typical plasma sheet reconfigurations that are observed during rapid flux transport events (RFTs) in the central plasma sheet. RFTs are bursts of rapid earthward or tailward plasma flow with a large flux transport rate, EC = [(VXBZ)2 + (VYBZ)2]1/2 > 2 mV m−1. A superposed epoch analysis shows that earthward RFTs are related to nonadiabatic heating, dipolarization, and thickening of the plasma sheet, features typically seen during substorm expansion phase. The average earthward velocity component of the RFTs decreases toward Earth, whereas the average convection electric field, VX BZ, is practically independent of radial distance. Earthward RFTs show characteristics of bubbles, i.e., flux tubes with lower ion density and slightly higher magnetic field strength than the surrounding medium. Tailward RFTs beyond a radial distance of ∼20 RE can be associated either with a northward or a southward magnetic field, and their signatures show that they are probably related to the leading and trailing edges of tailward ejected plasmoids. Inside of 20 RE, yet another type of tailward RFTs with BZ > 0 can be observed. These events are possibly signatures of vortices or rebouncing flows in the near‐Earth plasma sheet.
Identification of nonlinear processes in the magnetospheric dynamics and forecasting of Dst index
Authors: Boaghe O. M. , Balikhin M. A. , Billings S. A. , Alleyne H.
Published: February 2004

An approach based on the Nonlinear Autoregressive Moving Average Model With Exogenous Inputs (NARMAX) is used to analyze simultaneous measurements of the geomagnetic Dst index and VBs, the merging rate of the interplanetary magnetic field and the geomagnetic field. A nonlinear model which describes the dynamics of the Dst index driven by VBs is identified directly from experimental measurements. The identified model is then used to forecast the evolution of the Dst index and to compute generalized frequency response functions (GFRFs). Models were also identified on different intervals of data to check the stability of the main properties of the GFRFs. Analysis of these functions provides information about the types of nonlinearities involved in the energy storage process in the magnetosphere; in particular, the importance of coupling processes between various spectral components in the input which transfer energy to the summation frequency in the Dst index is illustrated. It is shown that the GFRFs derived from the data can be used to validate theoretical models which have been proposed to describe the evolution of the magnetosphere. For example, a coupling between various spectral components in the input indicates that linear models which relate VBs to Dst are inadequate to represent real physical processes involved in the Dst evolution.
Global geoid and sea level changes due to present‐day ice mass fluctuations
Authors: Tamisiea M. E. , Mitrovica J. X. , Milne G. A. , Davis J. L.
Published: February 2004

We predict gravitationally self‐consistent global geoid and relative sea level (RSL) perturbations due to present‐day melting of ice complexes, including the Antarctic and Greenland ice sheets and a suite of mountain glaciers and ice sheets. Classic analyses of sea level change indicate that these perturbations will depart significantly from eustatic (i.e., geographically uniform) trends [e.g., Woodward, 1888; Farrell and Clark, 1976], although this result has not always been appreciated in modern analyses. Mass flux of individual ice reservoirs will produce unique geometries of sea level change, and this distinctiveness admits the possibility of using global geoid, sea surface, and RSL signatures of recent climate change to infer the ongoing mass balance of each reservoir rather than simply the net mass flux. As an example, we show that perturbations to the geoid arising from noneustatic water loads associated with each ice reservoir are sufficiently large (at low degrees) to be theoretically measurable within 5 years by the GRACE satellite mission. We complete the study by reanalyzing tide gauge data at 23 sites selected by Douglas [1997] in a recent analysis of global RSL rise. Traditionally, estimates of global sea level rise are generated by taking the mean of a set of secular tide gauge trends that have been corrected for the influence of ongoing glacial isostatic adjustment (GIA) related to the late Pleistocene glacial cycles. The common assumption in such studies is that the geographic scatter in the residual, GIA‐corrected trends is due to errors in the GIA model or unmodeled processes (e.g., tectonics). We consider a large suite of GIA model predictions and apply a least squares approach to the GIA‐corrected tide gauge trends to estimate the weighting of various present‐day sea level signatures. We find that the fit to the residual RSL trends is significantly improved and that the procedure is able to resolve a long‐standing observation of anomalously low sea level rates in Europe. This preliminary analysis, which is relatively insensitive to changes in the assumed geometry of the present‐day mass balance, assumes that ocean thermal expansion is globally uniform. However, the procedure can be easily extended to incorporate a realistic steric contribution once the geometry of the process is sufficiently well constrained.
A forward model for earthquake generation on interacting faults including tectonics, fluids, and stress transfer
Authors: Fitzenz Delphine D. , Miller Stephen A.
Published: February 2004

We present a forward model of interacting faults for systems of any geometry. The model generalizes that of Miller et al. [1996, 1999] to a fully three‐dimensional model where faults of any strike and geometry interact through an elastic matrix using the general solutions of Okada [1992]. The model includes large‐scale plate motion loading and increasing pore pressures from a source term, undrained poroelastic effects, large coseismic hydraulic property changes, and porosity creation through dilatant slip. To illustrate the basic behavior and utility of the model, results are presented of the long‐term evolution (≈9300 years) for a generic case of a blind, dipping fault and a subvertical strike‐slip fault in a transpressional environment. We show the stress state evolution along both faults, seismicity time lines, quasi‐static rupture propagation including rake angle changes, local and regional stress buildup and rotations, static and dynamic fault interactions, and ΔCFS (changes in Coulomb Failure Stress) within the fault system. Large compartments of varying overpressure result on both faults from coseismic pore pressure changes and contribute to the complexity of the stress state. For the considered case, we find that the poroelastic effects on the receiver fault are about twice the change in the shear stress, providing a significant contribution to the ΔCFS. Regional stress rotations in response to the model seismicity indicate that further model developments must include dynamic generation of new faults in response to the evolving tectonic regime.
Earthquake triggering in the Hawke's Bay, New Zealand, region from 1931 to 1934 as inferred from elastic dislocation and static stress modeling
Authors: McGinty Peter , Darby Desmond , Haines John
Published: February 2004

During the 1930s the Hawke's Bay region of New Zealand experienced four large earthquakes, Napier (MW 7.6) and Hawke Bay (MW 7.3) in 1931, Wairoa (MW 6.9) in 1932, and Pahiatua (MW 7.4) in 1934. We address the question of whether these comprise a triggered sequence of events. There are significant difficulties in dealing with earthquakes that were recorded 70 years ago as fault parameters are difficult to obtain. With the exception of the Pahiatua earthquake, no primary surface fault ruptures were identified, and locations for the other three events may be in error by tens of kilometers. However, geodetic data were collected before and after the Napier and Wairoa earthquakes, and regions of uplift and subsidence from the former have been mapped from low‐order leveling data. This information helps to constrain the fault parameters for the first of these events through elastostatic modeling. Results from recent teleseismic body wave modeling have been used to determine fault parameters for the Hawke Bay event. Our analysis of the induced static stresses with the Coulomb failure criterion shows that the Napier earthquake triggered both the Hawke Bay and Wairoa earthquakes but that the Hawke Bay earthquake probably delayed the Wairoa earthquake. We also conclude that these three events did not trigger the Pahiatua earthquake.
Multiparameter two‐dimensional inversion of scattered teleseismic body waves 3. Application to the Cascadia 1993 data set
Authors: Rondenay S. , Bostock M. G. , Shragge J.
Published: February 2004

This is the third paper in a three‐part series that examines formal inversion of the teleseismic P wave coda for discontinuous, two‐dimensional (2‐D) variations in elastic properties beneath dense arrays of three‐component, broadband seismometers. In this paper, the method is applied to data from the Incorporated Research Institutions for Seismology‐Program for Array Seismic Studies of the Continental Lithosphere (IRIS‐PASSCAL) Cascadia 1993 experiment undertaken across central Oregon. Two major features are imaged in the resulting model. The continental Moho becomes evident ∼150 km from the coast beneath the Western Cascades and extends through the eastern end of the profile at 35–40 km depth. In the western portion of the model, oceanic crust of the subducting Juan de Fuca plate dips shallowly (12°) at the coast and more steeply (27°) below the Willamette Valley and is evident to depths of >100 km beneath the High Cascades. The abrupt increase in plate dip at ∼40 km depth coincides with an apparent thickening of the oceanic crust followed by a diminution in its signature. Building on previous work, we argue that these results are consistent with the consequences of prograde metamorphic reactions occurring within the oceanic crust. Progressive dehydration at lower‐grade facies conditions culminates in the transformation to eclogite, producing a pronounced increase in the seismic velocity, density and dip of the subducting plate, and structural complexity in the overlying wedge.
Fluid inclusions in pseudotachylytes from the Nojima fault, Japan
Authors: Boullier Anne‐Marie , Ohtani Tomoyuki , Fujimoto Koichiro , Ito Hisao , Dubois Michel
Published: February 2004

Pseudotachylytes (i.e., rocks formed by frictional melting) have been observed in the Nojima fault that was penetrated by the Hirabayashi borehole drilled 1 year after the 1995 Hyogo‐ken Nanbu (Kobe) earthquake. These rocks display millimeter‐scale banding defined by different pseudotachylyte layers. The nature of unmolten crystal fragments (K‐feldspar, albite, calcite, and/or quartz) allows us to infer a minimum melting temperature of 1200°C. The glass has a 8±3% volatile content and a higher CaO content than that of the parent granodiorite, thus suggesting that pseudotachylyte formation occurred in an already altered, calcite‐bearing and hydrated fault zone. Fluid inclusions have been observed in the glass and are filled with a dense low‐salinity (4.3±1.2 wt % eq. NaCl) CO2‐H2O fluid characterized by steep isochoric curves. The intersection between the isochores and the measured 24°C/km geothermal gradient indicates a minimum 15 km depth for the pseudotachylyte formation. Such a depth suggests that an important uplift has brought the studied pseudotachylytes to their present position and that these pseudotachylytes are probably pre‐Miocene in age. The calculated thermal evolution of a typical millimeter‐scale pseudotachylyte layer indicates that cooling does not last more than a few seconds. This indicates that related seismic processes, such as deceleration of fault movement and healing of the fault, were both very rapid. The geometry and thermal budget of the millimeter‐thick pseudotachylytes suggest that seismic events of magnitude 6 to 7 are responsible for their formation.
Seismic monitoring and modeling of an active volcano for prediction
Authors: Aki Keiiti , Ferrazzini Valérie
Published: February 2004

We describe seismic monitoring and modeling of an active volcano for the purpose of predicting the magma transport process in real time. We selected the Piton de la Fournaise, which has been monitored by an observatory since the early 1980s, and is active, with 28 eruptions from 1980 until August 1992. It erupted on March 9, 1998, after an unusually long quiet period. The eruption lasted until September 21, 1998. Before the March eruption, we completed the initial construction of a model of a magma system based primarily on long‐period (LP) events and coda localization. We found that LP events are associated with the lateral movement of magma to the rift zone and not with the vertical movement to the summit. We also found that the source of 2 Hz LP events is located below that of 1 Hz LP events. The coda localization is a newly discovered phenomenon that helped to locate a magma body. This model was used for interpreting the incoming information from the precursory seismic crisis and the eruption tremor during the 6 months of the eruption. The simultaneous monitoring and modeling of an active volcano led us to a quantitative simulation of the eruption process, computing excess pressure and flow rate in a system of reservoirs connected by channels.
On the nature of Pn
Authors: Tittgemeyer Marc , Wenzel Friedemann , Fuchs Karl
Published: February 2004

Pn phases are observed along many refraction seismic profiles and are common in earthquake records. Their velocities usually range from 7.8 to 8.2 km s−1. Classical ray theory used to interpret these observations implies a positive upper mantle velocity gradient. However, a wide spread positive velocity gradient in the lithospheric mantle is not expected from petrological and petrophysical data. Laboratory velocity measurements at elevated temparatures and pressures suggest positive velocity gradients only for very low heat flow values (≤40 mW m−2). Higher heat flow causes negative gradients. Consequently, petrological models of the upper mantle would restrict Pn observations to Precambrian shields and old platforms, contrary to observations. We overcome this contradiction by considering media that contain random velocity fluctuations superimposed on positive or negative velocity gradients. In both cases, these structures generate Pn phases by wide‐angle scattered waves. Short‐wavelength random velocity fluctuations of only 0.5–1% superimposed on negative velocity gradients are sufficient for generating Pn phases. Consequently, this implies that an observed Pn wave does not necessitate a positive upper mantle velocity gradient. For a peridotitic upper mantle, fluctuations of this size can be explained by slightly varying the relative proportions of its mineralogical constituents. Anisotropy is likely to contribute to the inferred fluctuations.
Magnetic stratigraphy and tectonic rotation of the Eocene‐Oligocene Keasey Formation, northwest Oregon
Authors: Prothero Donald R. , Hankins Karina G.
Published: February 2004

The upper Eocene‐lower Oligocene Keasey Formation in northwestern Oregon consists of ∼700 m of deep marine forearc basin sediments exposed in stream beds and roadcuts. The abundant molluscs of this unit were the basis for the Keasey Stage of the Weaver et al. [1944] timescale, but the precise age of this fauna and formation have long been controversial. Analysis of over 130 magnetic samples from the Keasey in five sections yielded a stable remanence held mainly in magnetite which passed a reversal test. On the basis of calibration by benthic foraminifera and molluscs the Keasey Formation is correlative with Chrons C15r–C12r (35.0–33.0 Ma), so the unit spans the latest Eocene and early Oligocene. The magnetic directions suggest a tectonic rotation of 73.7°±4.7°, which is consistent with rotation of the nearest volcanic basement rock reported by Wells and Coe [1985].
The Zedong Window: A record of superposed Tertiary convergence in southeastern Tibet
Authors: Harrison T. Mark , Yin An , Grove Marty , Lovera Oscar M. , Ryerson F. J. , Zhou Xinhua
Published: February 2004

Determining the timing, magnitude, and location of deformation due to the Indo‐Asian collision is widely acknowledged as an important step in understanding how the lithosphere responds during continental collision. A puzzling result of geological investigations of the Lhasa Block over the past 2 decades has been the apparent lack of significant Tertiary deformation there. Perhaps the most important structural feature of the Lhasa Block is the south directed Gangdese Thrust System, which developed along its southern edge. The thrust system, which separates the Andean‐type batholith of southern Asia from rocks of Indian affinity, is obscured at most locations across southeastern Tibet by back thrusts of the younger, north directed Renbu Zedong Thrust System. The best documented site where both thrusts are exposed occurs near Zedong (Zedong Window). Systematic geochronologic analyses were conducted in this area. U‐Pb zircon dating of three samples of a synkinematically to postkinematically deformed hanging wall granitoid (the Yaja granodiorite) cut by the Gangdese Thrust indicates a crystallization age of 30.4±0.4 Ma (2σ), thus placing an upper bound on the initiation of the thrust. U‐Pb zircon dating of granitoid samples structurally higher in the Gangdese hanging wall yields emplacement ages of 42.5±1.0 Ma and 48.9±0.8 Ma (2σ), similar to other magmatic complexes within the Gangdese arc. Geochemical results are consistent with these plutons forming in the same environment as precollisional intrusions within the Gangdese batholith, suggesting a significant post‐collisional input of juvenile heat. The40Ar/39Ar thermochronology of samples from a vertical section through the hanging wall within the Yaja granodiorite, coupled with results of a numerical thermal model, indicate an average slip rate along the Gangdese Thrust of 7 mm/yr between 30 and 23 Ma and a minimum displacement of ∼50 km. Farther east in the Zedong Window, thermal effects produced by later north directed thrust sheets of the Renbu Zedong system appear to have obscured thermal history signatures in the Gangdese hanging wall related to earlier south directed thrusting. Thermochronological results from this region indicate that thermal overprinting related to the north directed thrusting occurred between 25 and 10 Ma, consistent with previous estimates.
Near‐ridge seamount chains in the northeastern Pacific Ocean
Authors: Clague David A. , Reynolds Jennifer R. , Davis Alicé S.
Published: February 2004

High‐resolution bathymetry and side‐scan data of the Vance, President Jackson, and Taney near‐ridge seamount chains in the northeast Pacific were collected with a hull‐mounted 30‐kHz sonar. The central volcanoes in each chain consist of truncated cone‐shaped volcanoes with steep sides and nearly flat tops. Several areas are characterized by frequent small eruptions that result in disorganized volcanic regions with numerous small cones and volcanic ridges but no organized truncated conical structure. Several volcanoes are crosscut by ridge‐parallel faults, showing that they formed within 30–40 km of the ridge axis where ridge‐parallel faulting is still active. Magmas that built the volcanoes were probably transported through the crust along active ridge‐parallel faults. The volcanoes range in volume from 11 to 187 km3, and most have one or more multiple craters and calderas that modify their summits and flanks. The craters (<1 km diameter) and calderas (>1 km diameter) range from small pit craters to calderas as large as 6.5×8.5 km, although most are 2–4 km across. Crosscutting relationships commonly show a sequence of calderas stepping toward the ridge axis. The calderas overlie crustal magma chambers at least as large as those that underlie Kilauea and Mauna Loa Volcanoes in Hawaii, perhaps 4–5 km in diameter and ∼1–3 km below the surface. The nearly flat tops of many of the volcanoes have remnants of centrally located summit shields, suggesting that their flat tops did not form from eruptions along circumferential ring faults but instead form by filling and overflowing of earlier large calderas. The lavas retain their primitive character by residing in such chambers for only short time periods prior to eruption. Stored magmas are withdrawn, probably as dikes intruded into the adjacent ocean crust along active ridge‐parallel faults, triggering caldera collapse, or solidified before the next batch of magma is intruded into the volcano, probably 1000–10,000 years later. The chains are oriented parallel to subaxial asthenospheric flow rather than absolute or relative plate motion vectors. The subaxial asthenospheric flow model yields rates of volcanic migration of 3.4, 3.3 and 5.9 cm yr−1 for the Vance, President Jackson, and Taney Seamounts, respectively. The modeled lifespans of the individual volcanoes in the three chains vary from 75 to 95 kyr. These lifespans, coupled with the geologic observations based on the bathymetry, allow us to construct models of magma supply through time for the volcanoes in the three chains.
Slip behavior of simulated gouge‐bearing faults under conditions favoring pressure solution
Authors: Bos Bart , Peach Colin J. , Spiers Christopher J.
Published: February 2004

Geophysical observations as well as deformation experiments indicate that under hydrothermal conditions, crustal faults can be significantly weakened with respect to conventional brittle‐plastic strength envelopes. Pressure solution has long been proposed as a mechanism leading to fault weakness. However, pressure solution has also been proposed as contributing to interseismic fault healing, and the competition between the weakening and healing effects of pressure solution is unclear. To investigate this issue, we have conducted rotary shear experiments on synthetic faults containing granular halite (NaCl) gouge using NaCl‐saturated mixtures of water and methanol as pore fluid. The NaCl‐water‐methanol system was chosen as a rock analogue because pressure solution is known to be important in this system at ambient conditions. We explored the influence of varying pore fluid composition (hence pressure solution rate), gouge grain size, and wall rock surface roughness, as well as normal stress and sliding velocity on slip behavior. All experiments were done under drained conditions. An acoustic emission detection system allowed detection of brittle events in the gouge. The results show no evidence for steady state pressure solution‐controlled fault slip. Frictional, rate‐insensitive behavior was observed, whereas the microstructures and compaction behavior clearly demonstrated that pressure solution was active in the gouge. Our data show that fluid‐assisted healing effects dominated over weakening, causing fault strength to be controlled mainly by brittle‐frictional processes. Existing models describing pressure solution‐controlled fault creep may not be applicable to a porous gouge undergoing compaction as well as slip.
Identification of sources of potential fields with the continuous wavelet transform: Complex wavelets and application to aeromagnetic profiles in French Guiana
Authors: Sailhac Pascal , Galdeano Armand , Gibert Dominique , Moreau Frédérique , Delor Claude
Published: February 2004

A continuous wavelet technique has been recently introduced to analyze potential fields data. First, we summarize the theory, which primarily consists of interpreting potential fields via the properties of the upward continued derivative field. Using complex wavelets to analyze magnetic data gives an inverse scheme to find the depth and homogeneity degree of local homogeneous sources and the inclination of their magnetization vector. This is analytically applied on several local and extended synthetic magnetic sources. The application to other potential fields is also discussed. Then, profiles crossing dikes and faults are extracted from the recent high‐resolution aeromagnetic survey of French Guiana and analyzed using complex one dimensional wavelets. Maps of estimated depth to sources and their magnetization inclination and homogeneity degree are proposed for a region between Cayenne and Kourou.
Volcano‐wide fringes in ERS synthetic aperture radar interferograms of Etna (1992–1998): Deformation or tropospheric effect?
Authors: Beauducel François , Briole Pierre , Froger Jean‐Luc
Published: February 2004

Mount Etna (3300 m) is the volcano that has been first and most studied by differential synthetic aperture radar. Previous papers gave evidence for a large‐scale deformation of the entire edifice consistent with unrest episodes but with a poor fit with classical elastic models. Also, atmospheric effects on mountainous areas are known to be very significant. Accordingly, interferograms may reflect both deformation and tropospheric effects. We investigate here the possibility of evaluating and correcting tropospheric effects directly from the interferograms. From 38 ERS synthetic aperture radar ascending scenes taken from 1992 to 1999, we have computed 238 interferograms. The amount of data allows us to analyze how the coherence is maintained over long periods of time and thus how it reveals the location of permanent scatters. From a simple analysis of phase‐elevation relation, we give evidence for the clear difference in pattern between stratified tropospheric effects and large‐scale deformation produced by a source in a Threedimensional elastic body with topography. Using combination of inversion processes, we analyze both deformation source and tropospheric effects for the complete set of 238 interferograms. Finally time evolution of the parameters and modeling probability estimation are retrieved for each SAR image. Estimated relative change in tropospheric delay varies in time from −2.7 to +3.0 (±1.2) fringes. They are consistent with other tropospheric estimations at Etna based on ground met and Global Positioning System observations on the same period of time. Magmatic volume variations remain below error bars and thus cannot be estimated properly. Finally, some of the interferograms may be entirely explained by the difference in tropospheric conditions between the two acquisition periods, without the need to invoking a deformation of the volcano.
Measured and calculated seismic velocities and densities for granulites from xenolith occurrences and adjacent exposed lower crustal sections: A comparative study from the North China craton
Authors: Gao Shan , Kern Hartmut , Liu Yong‐Sheng , Jin Shu‐Yan , Popp Till , Jin Zhen‐Min , Feng Jia‐Lin , Sun Min , Zhao Zu‐Bin
Published: February 2004

Granulites from the Neogene xenolith‐bearing Hannuoba alkaline basalt and from the Manjinggou‐Wayaokou exposed lower crustal section in the Archean Huai 'an terrain, which occurs within and surrounds the Hannuoba basalt, provide a unique opportunity for a comparative study on petrophysical properties and composition of the lower crust represented by these two types of samples. P and S wave velocities and densities of 12 Hannuoba lower crustal xenoliths and one associated spinel Iherzolite xenolith as well as nine granulites and granulite‐facies metasedimentary rocks from the Archean Huai 'an terrain were measured in laboratory at pressures up to 600 MPa and temperatures up to 600°C. Calculations of P and S wave velocities were also made for the same suite of samples based on modal mineralogy and single‐crystal velocities whose variations with composition are considered by using microprobe analyses and velocities of end members. The measured and calculated Vp at room temperature and 600 MPa, where the microcrack effect is considered to be almost eliminated, agree within 4% for rocks from the Manjinggou‐Wayaokou section and the adjacent Wutai‐Jining upper crustal to upper lower crustal section. In contrast, the xenoliths show systematically lower measured Vp by up to 15% relative to calculated velocities, even if decompression‐induced products of kelyphite and glass are taken into account. The lower measured velocities for xenoliths are attributed to grain boundary alteration and residual porosity. This implies that although granulite xenoliths provide direct information about lower crustal constitution and chemical composition, they are not faithful samples for studying in situ seismic properties of the lower crust in terms of measured velocities due to alterations during their entrainment to the surface, which changes their physical properties significantly. In this respect, granulites from high‐grade terrains are better samples because they are not subjected to significant changes during their slow transport to the surface and because physical properties depend primarily on mineralogy in addition to pressure and temperature. On the other hand, calculated velocities for granulite xenoliths are consistent with velocities for granulites from terrains, suggesting that they can be also used to infer lower crust composition by correlating with results from seismic refraction studies.
Flexure and mechanical behavior of cratonic lithosphere: Gravity models of the East African and Baikal rifts
Authors: Petit Carole , Ebinger Cynthia
Published: February 2004

The effective elastic thickness of the lithosphere Te has been investigated by inverse and forward models based on the assumption that flexure of the surface and crust‐mantle boundaries produces gravity field variations which are commonly expressed in free air or Bouguer gravity anomalies. The aim of this study is to examine Te variations across the East African and Baikal rifts that have developed within or adjacent to early Proterozoic‐Archean cratons in order to investigate the possibility that the crust and upper mantle beneath the cratons retain considerable strength in extension. We use forward models of five 1800‐km‐long Bouguer gravity profiles in the East African rift and one profile previously modeled in the Baikal rift. The forward modeling method uses a multilayered lithospheric rheology to compute the mechanical plate deflection in response to vertical loads. We show that the origin of the rift topography may be highly variable: uplifted rift flanks are few in the Baikal rift, where mountain ranges have generally deep crustal roots, while flexural rebound accounts for most of the short‐wavelength topography encountered in the East African rift. Reasonable fits of models and observed Bouguer gravity anomalies can only be obtained if we allow large rheological contrasts between the cratons and surrounding areas; Te values reach 60 km beneath the cratons and range between 20 and 45 km beneath the surrounding orogenic belts. The major part of the rifts develops close to the craton margins, where rheological contrasts may cause stress concentrations. These results agree well with previous Bouguer coherence analyses but are significantly different from previous free‐air coherence models and forward models of topography alone, which yielded Te estimates <10 km beneath the rifts and the surrounding areas. We believe that these differences largely stem from model assumptions of compensation depth and lithospheric mechanical properties and from the fact that free‐air anomalies contain short‐wavelength noise that correlates with topography, which biases free‐air based methods toward low Te estimates.
Lithospheric instability beneath the Transverse Ranges of California
Authors: Houseman Gregory A. , Neil Emily A. , Kohler Monica D.
Published: February 2004

Recent high‐resolution seismic experiments reveal that the crust beneath the San Gabriel Mountains portion of the Transverse Ranges thickens by 10–15 km (contrary to earlier studies). Associated with the Transverse Ranges, there is an anomalous ridge of seismically fast upper mantle material extending at least 200 km into the mantle. This high‐velocity anomaly has previously been interpreted as a lithospheric downwelling. Both lithospheric downwelling and crustal thickening are associated with the oblique convergence of Pacific and North America plates across the San Andreas Fault, though it seems likely that the lithospheric downwelling is driven at least partly by gravitational instability of the cold lithospheric mantle. We show by means of numerical experiment that the balance between buoyancy forces that drive deformation and viscous stresses that resist deformation determines the geometry of crustal thickening and mantle downwelling. We use a simple two‐layered lithospheric model in which dense lithospheric mantle overlies relatively inviscid and less dense asthenosphere and is overlain by buoyant crust. External plate motion drives convergence, which is constrained by boundary conditions to occur within a central convergent zone of specified width. A fundamental transition in the geometry of downwelling is revealed by our experiments. For slow convergence, or low crustal viscosity, downwelling occurs as multiple sheets on the margins of the convergent zone. For fast convergence or crust that is stronger than mantle lithosphere a single downwelling occurs beneath the center of the convergent zone. This complexity in the evolution of the system is attributed to the interaction of crustal buoyancy with the evolving gravitational instability. In order for a narrow downwelling slab to have formed beneath the Transverse Ranges within the last 5 Myr, the effective lithospheric viscosity of the convergent region is at most about 1020 Pa s.
Acoustic properties of partially molten media studied on a simple binary system with a controllable dihedral angle
Authors: Takei Yasuko
Published: February 2004

Borneol‐diphenylamine, a binary eutectic system of the organic compounds, provides an appropriate analogue of melting in the Earth's mantle. Eutectic temperature is just above room temperature (43°C), and at this temperature the dihedral angle is about 40°. As the temperature increases, the dihedral angle gradually decreases at a rate of about 1.5° per 1°C, and becomes nearly zero at 70°C. Melt fraction change is small at this temperature range; this system is therefore appropriate in investigating a systematic effect of dihedral angle. Using this system, reduction of the shear and longitudinal wave velocities caused by grain boundary melt, having nearly equilibrium textures, was measured accurately as functions of both melt fraction and dihedral angle. The results demonstrate the significant effect of equilibrium melt geometry on shear wave velocities, while also showing that the effects of melting and dihedral angles are much smaller on the longitudinal waves. The quantitative effects of the melt fraction and dihedral angles on the acoustic wave velocities can be predicted theoretically using the elasticities of granular media derived as functions of grain‐boundary contiguity [Takei, 1998]. The present experimental results described in this paper agree well with the theoretical predictions and demonstrate the validity of the theory. Clarifying the analogy and difference between the present organic system and the Earth's materials, the shear and longitudinal wave velocities of the partially molten rocks in the Earth were predicted as functions of melt fraction, dihedral angle, and the compressibility ratio between solid and melt.
High‐temperature plastic deformation of quartz‐plagioclase multilayers by layer‐normal compression
Authors: Ji Shaocheng , Wirth Richard , Rybacki Erik , Jiang Zhenting
Published: February 2004

It is a common practice in rheological modeling to take the extrapolated compressive flow strengths of quartzite and plagioclase rock as the bulk flow strengths of the upper and lower continental crusts, respectively. Such a practice implies that the bulk flow strength of polyphase rocks is identical to that of the pure weak phase. To test this assumption, we performed deformation experiments on synthetic layered and particulate quartz‐anorthite (50∶50) composites and aggregates of the end‐members in a Paterson gas‐medium apparatus at a confining pressure of 300 MPa, a constant strain rate of 10−5 s−1 and temperatures from 1273 to 1473 K. Under these conditions, anorthite deforms by recrystallization‐accommodated dislocation creep, while quartz is semibrittle. Electron backscatter diffraction measurements show a strong lattice‐preferred orientation (LPO) of anorthite developed in deformed specimens, and the LPO pattern is interpreted as a result of dislocation slip on the (010)[100] system. Our experimental results show that the bulk flow strength of layered composites increases with decreasing thickness of the layers. Thinlayered composites are significantly stronger than pure anorthite aggregates and a homogeneous quartz‐anorthite mixture (i.e., particulate composites) with the same modal composition but weaker than pure quartz aggregates. Our experimental results together with the theoretical overview presented previously in the materials science literature demonstrate that (1) compressive flow strength of a polyphase rock cannot be represented by that of the weak phase and (2) thin‐layered rocks compressed normal to the layering are rheologically stronger than homogeneous, isotropic mixtures under the same deformation conditions. Thus weak‐phase‐based rheology will lead to an underestimation of the bulk flow strength of the continental crust in which polyphase rocks dominate.
Tectonic features of the Japan Trench convergent margin off Sanriku, northeastern Japan, revealed by multichannel seismic reflection data
Authors: Tsuru Tetsuro , Park Jin‐Oh , Takahashi Narumi , Kodaira Shuichi , Kido Yukari , Kaneda Yoshiyuki , Kono Yoshiteru
Published: February 2004

Near the Japan Trench convergent plate margin the seaward edge of the continental plate is deformed by subduction of the oceanic plate. We report the results of a multichannel seismic survey in the northern Japan Trench in which this deformed zone is demarcated from the rigid continental framework by a pronounced landward dipping reflector. The oceanic plate also undergoes deformation as the two plates interact in the subduction processes, resulting in a progressive deformation or destruction of a hörst structure along the top of the subducting oceanic crust. This may eventually lead to the formation of a smooth plate boundary at the greater depth. More than 45 km landward from the trench axis, a smooth reflector suggesting a stable slip plane is visible along and above the oceanic crust. Our data indicate that the deformed zone pinches out landward ∼60 km from the axis at 13 km depth and the slip plane becomes less obvious there. Seismicity of interplate earthquakes rapidly increases landward from this location, leading us to speculate that this is where coupling at the plate boundary becomes strong enough for earthquakes to occur. We conclude that the updip limit of the seismogenic zone of interplate earthquakes in the study area is characterized by the tectonic feature of a pinchout of the deformed sediments which overlie the subducting oceanic crust.
Seismic evidence for ultralow‐velocity zones beneath Africa and eastern Atlantic
Authors: Helmberger Don , Ni Sidao , Wen Lianxing , Ritsema Jeroen
Published: February 2004

SKS waveforms recorded at distances of about 110° are extremely useful to constrain seismic velocity structure at the base of the mantle. SKS waves near this distance develop a complicated interference pattern with the phases SPdKS and SKPdS. We report anomalous behavior of this interference in a number of recordings of deep earthquakes beneath South America from stations in Europe and Africa. We model these data with two‐dimensional dome‐like structures at the base of the mantle which extend laterally by a few hundred kilometers and in which the shear velocity is up to 30% lower than in the Preliminary Reference Earth Model (PREM). The spatial extent of these structures, their position with respect to the SKS core exit points, and their seismic characteristics can not be uniquely determined. However, the presence of a dipping or a concaved upper interface is a key attribute of successful models. Models that invoke flat layers are insufficiently complex to explain the most erratic waveform behavior. The most anomalous data correspond to sampling regions at the base of the mantle beneath the East African Rift and beneath the Iceland, where possibly, whole mantle upwellings form.
Surface displacements and energy release rates for constant stress drop slip zones in joined elastic quarter spaces
Authors: Rodgers Michael J. , Wen Shengmin , Keer Leon M.
Published: February 2004

A three‐dimensional quasi‐static model of faulting in an elastic half‐space with a horizontal change of material properties (i.e., joined elastic quarter spaces) is considered. A boundary element method is used with a stress drop slip zone approach so that the fault surface relative displacements as well as the free surface displacements are approximated in elements over their respective domains. Stress intensity factors and free surface displacements are calculated for a variety of cases to show the phenomenological behavior of faulting in such a medium. These calculations showed that the behavior could be distinguished from a uniform half‐space. Slip in a stiffer material increases, while slip in a softer material decreases the energy release rate and the free surface displacements. Also, the 1989 Kalapana earthquake was located on the basis of a series of forward searches using this method and leveling data. The located depth is 8 km, which is the closer to the seismically inferred depth than that determined from other models. Finally, the energy release rate, which can be used as a fracture criterion for fracture at this depth, is calculated to be 11.1×106 J m−2.
Dynamic versus static stress triggering and friction parameters: Inferences from the November 23, 1980, Irpinia earthquake
Authors: Voisin C. , Campillo M. , Ionescu I. R. , Cotton F. , Scotti O.
Published: February 2004

This paper concentrates on the problem of fault interaction and earthquake triggering through the 1980 Irpinia, Italy, sequence. More specifically, this paper deals with the problem of the triggering of the second subevent by the mainshock. The interaction between the two segments is modeled through a dynamic Coulomb failure function. The aims of this paper are, first, to discriminate between the dynamic and the static stress effects on the triggering, if these effects exist, second, to estimate the fault strength relative to the initial state of stress, third, to determine the parameters of a slip‐dependent friction law that lead to the observed delay of 20 s. Numerical simulations show that the critical slip Dc may range from 0.03 m up to 1.7 m, and that the initial slope of the friction law μ′(0) must be lower than 0.04 m−1. We show that the relative magnitude of the fault strength and the initial state of stress govern the existence and value of a Dc lower threshold under which the fault always ruptures before 13 s. A close to failure fault is not consistent with a critical slip Dc less than 0.8 m, whereas small values of Dc, typically 0.03 m, imply a far from failure fault. General results concern the effect of a dynamic stress pulse. We show that an event can be triggered by a transient stress pulse and that in this case the event can have an initiation duration much longer than the pulse duration. We show that it is possible to explain both the triggering and the time delay only with the effect of the transient stress pulse. This may explain aftershock triggering even in regions of negative Coulomb failure function or long distance triggering of earthquakes by propagating waves.