bluuebunny/crossref_metadata_2025 huggingface dataset:
Nature of the recent vertical ground movements inferred from high‐precision leveling data in an intraplate setting: NE Ardenne, Belgium
Authors: Demoulin A. , Collignon A.
Published: September 2002

Yearly high‐precision leveling surveys have been performed from 1993 to 1998 along a 48‐km‐long local network centered on Gileppe Lake, at the foot of the Ardenne massif, Belgium. Institut Géographique Militaire and Institut Géographique National (IGM‐IGN) levelings of 1948 and 1974 yielded additional information. The data were corrected for collimation and thermal errors. Other sources of error (Earth tides, atmospheric refraction, rod miscalibration) were estimated, and corrections were made when necessary. Random errors of individual surveys range from 0.15 to 0.74 mm/√km. The recorded elevation changes indicate the existence of tilted segments some 5 to 10 km in length, with yearly tilt values amounting up to 2 μrads. Fault movements of a few mm/yr are also demonstrated. The segment limits are spatially stable from one year to the next, but very frequent sense reversals of the vertical motions are observed. This explains why the total movement over a 20‐ to 30‐year period is barely higher than the yearly displacements. Various nontectonic causes of vertical ground movement (e.g., monument instability, varying water loading of the lake, sediment compaction, etc.) have been considered, yielding little satisfactory explanation of the patterns of observed elevation changes. We propose that the recorded motions, which we call reversible fault creep and reversible tilt, respectively, are a true tectonic signal, reflecting the “tectonic noise” which accompanies the long‐term deformation of uplifting areas in intraplate settings. Such movements are characteristic of fault segments which may occasionally generate earthquakes.
Geologic factors controlling patterns of small‐volume basaltic volcanism: Application to a volcanic hazards assessment at Yucca Mountain, Nevada
Authors: Connor Charles B. , Stamatakos John A. , Ferrill David A. , Hill Brittain E. , Ofoegbu Goodluck I. , Conway F. Michael , Sagar Budhi , Trapp John
Published: September 2002

The proposed high‐level radioactive waste repository at Yucca Mountain, Nevada, is located within an active volcanic field. Probabilistic volcanic hazard models for future eruptions through the proposed repository depend heavily on our understanding of the spatial controls on volcano distribution at a variety of scales. On regional scales, Pliocene‐Quaternary volcano clusters are located east of the Bare Mountain fault. Extension has resulted in large‐scale crustal density contrast across the fault, and vents are restricted to low‐density areas of the hanging wall. Finite element modeling indicates that this crustal density contrast can result in transient pressure changes of up to 7 MPa at 40 km depth, providing a mechanism to generate partial melts in areas where mantle rocks are already close to their solidus. On subregional scales, vent alignments, including one alignment newly recognized by ground magnetic mapping, parallel the trends of high‐dilation tendency faults in the Yucca Mountain region (YMR). Forty percent of vents in the YMR are part of vent alignments that vary in length from 2 to 16 km. Locally, new geological and geophysical data show that individual vents and short vent alignments occur along and adjacent to faults, particularly at fault intersections, and left‐stepping en echelon fault segments adjacent to Yucca Mountain. Conditions which formed these structures persist in the YMR today, indicating that volcanism will likely continue in the region and that the proposed repository site is within an area where future volcanism may occur. On the basis of these data the probability of volcanic disruptions of the proposed repository is estimated between 10−8/yr and 10−7/yr.
Rate of diffuse carbon dioxide Earth degassing estimated from carbon balance of regional aquifers: The case of central Apennine, Italy
Authors: Chiodini G. , Frondini F. , Cardellini C. , Parello F. , Peruzzi L.
Published: September 2002

Central Italy is characterized by an anomalous flux of deeply derived CO2. In the western Tyrrhenian sector of central Italy, CO2 degassing occurs mainly from focused emissions (vents and strong diffuse degassing) and thermal springs, whereas in the eastern Apennine area, deep CO2 is dissolved in “cold” groundwaters of regional aquifers hosted by Mesozoic carbonate‐evaporite formations. Influx of deep CO2 into 12 carbonate aquifers (12,500 km2) of the central Apennine is computed through a carbon mass balance that couples aquifer geochemistry with isotopic and hydrogeological data. Mass balance calculations estimate that 6.5×1010 mol yr−1 of inorganic carbon are dissolved in the studied aquifers. Approximately 23% of this amount derives from biological sources active during the infiltration of the recharge waters, 36% comes from carbonate dissolution, while 41% is representative of deep carbon sources characterized by a common isotopic signature (δ13C ≅ −3‰). The calculated deep CO2 influx rate ranges from 105 to 107 mol yr−1 km−2, increasing regionally from east to west in the study area.
Active deformation in eastern Indonesia and the Philippines from GPS and seismicity data
Authors: Kreemer Corné , Holt William E. , Goes Saskia , Govers Rob
Published: September 2002

In this study we combine Global Positioning System (GPS) velocities with information on the style of regional seismicity to obtain a self‐consistent model velocity and strain rate field for the entire eastern Indonesia and Philippines region. In the process of interpolating 93 previously published GPS velocities, the style and direction of the seismic strain rate field, inferred from earthquakes with M0 < 1 × 1020 N m (from the Harvard centroid moment tensor catalog), are used as constraints on the style and direction of model strain rates within the plate boundary zones. The style and direction of the seismic strain rate field are found to be self‐similar for earthquakes up to M0 = 1 × 1020 N m (equivalent to Mw < 7.3). Our inversion result shows the following: The Java Trench, which lacks any significant (historic) seismicity, delineates the Australian plate (AU) ‐ Sunda block (Sunda) plate boundary west of the island of Sumba. East of Sumba, convergence is distributed over the back arc and Banda Sea, and there is no subduction at the Timor Trough, suggesting that the northern boundary of the AU plate runs north of this part of the Banda arc through the Banda Sea. In New Guinea most motion is taken up as strike‐slip deformation in the northern part of the island, delineating the Pacific plate (PA) ‐ AU boundary. However, some trench‐normal convergence is occurring at the New Guinea Trench, evidence that the strain is partitioned in order to accommodate oblique PA‐AU motion. PA‐AU motion is consistent with NUVEL‐1A direction, but ∼ 8 mm yr−1 slower than the NUVEL‐1A estimate for PA‐AU motion. The Sulawesi Trench and Molucca Sea delineate zones of high strain rates, consistent with high levels of active seismicity. The Sulawesi Trench may take up some of the AU‐Sunda motion. Philippine Sea plate motion is in a direction slightly northward of the NUVEL‐IA estimate and is partitioned in some strike‐slip strain rates along the Philippine Fault and relatively larger trench‐normal convergence along the Philippine Trench and on the Philippine mainland in the southern Philippines and along the Manila Trench in the northern Philippine islands. The high level of strain rate along the Manila Trench is not released by any significant (historic) seismic activity. For the entire eastern Indonesia‐Philippines region, seismicity since 1963 has taken up ∼40% of the total moment rate inferred from our model.
Modeling of radial microcracks at corners of inclusions in garnet using fracture mechanics
Authors: Whitney Donna L. , Cooke Michele L. , Du Frane S. Andrew
Published: September 2002

Radial microcracks around inclusions in garnet are important indicators of ultrahigh‐pressure metamorphism and have been used to interpret changes in pressure as a function of temperature during decompression of metamorphic terrains. We investigate the growth of microcracks from inclusion corners using a two‐dimensional fracture mechanics based boundary element method code and propose that microcracks emanating from inclusions are primarily controlled by inclusion shape and size and do not require high confining (lithostatic) pressure or significant decompression to initiate. Results suggest that inclusions (∼100 μm diameter) develop microcracks during modest decompression. The magnitude of decompression required to initiate microcracks decreases with increased degree of faceting and increased inclusion size. The potential for crack growth at each corner depends on the corner sharpness and the overall shape of the inclusion. We integrate model results with petrographic observations to understand mechanisms and conditions of garnet microcracking during regional metamorphism.
Crustal and upper mantle seismic structure beneath the rift mountains and across a nontransform offset at the Mid‐Atlantic Ridge (35°N)
Authors: Canales J. Pablo , Detrick Robert S. , Lin Jian , Collins John A. , Toomey Douglas R.
Published: September 2002

We present new results on the crustal and upper mantle structure beneath the rift mountains along two segments of the Mid‐Atlantic Ridge and across a nontransform offset (NTO). Our results were obtained from a combination of forward modeling and two‐dimensional tomographic inversion of wide‐angle seismic refraction data and gravity modeling. The study area includes two segments: OH‐1 between the Oceanographer fracture zone and the NTO‐1 at 34°35′N and OH‐2 between NTO‐1 and the NTO at 34°10′N. The center of OH‐1 is characterized by anomalously thick crust (∼8 km) with a thick Moho transition zone with Vp = 7.2–7.6 km/s. This transition zone, coincident with a gravity low, is probably composed of gabbro sills alternating with dunites, as observed in some ophiolites. OH‐1 has larger along‐axis crustal thickness variations than OH‐2, but average crustal thicknesses are similar (6.0±1.2 km at OH‐1, 6.1±0.7 at OH‐2). Thus we do not find significant differences in magma supply between these segments, in contrast to what has been inferred from morphological and gravity studies. At both segments the shoaling of the Moho is more rapid at the inside than at the outside corners, consistent with models in which the inside‐corner crust is technically modified. The structural differences between inside‐ and outside‐corner crust are more apparent at OH‐2, suggesting that the extrusive layer is thinner at the inside corner of OH‐2 than at the inside corner of OH‐1, probably due to differences in axial morphology and along‐axis magma transport. NTO‐1 is characterized by a nearly constant velocity gradient within the upper 5 km and low upper mantle velocities (7.4–7.8 km/s). The anomalous structure beneath NTO‐1 is interpreted as fractured mafic crust. The P wave velocities and densities required to match the gravity data suggest that serpentinites are common beneath the NTO‐1 and possibly beneath the inside corners. Serpentinization could be as much as 40% at ∼3.8 km below seafloor and probably does not occur at subseafloor depths greater than ∼6.2 km at the NTO‐1. Our results indicate that in a slow spreading environment where magmatism and tectonism are equally important, the seismic Moho cannot be correlated with an unique geological structure. At the center of a segment the seismic Moho may represent the lower boundary of an interlayered grabbro‐dunite transition zone, while beneath the inside corner and NTO where the crust is thinner, it may correspond to an alteration front.
Mobility of continental mantle: Evidence from postseismic geodetic observations following the 1992 Landers earthquake
Authors: Pollitz Fred F. , Peltzer Gilles , Bürgmann Roland
Published: September 2002

The crust around the rupture zone of the 1992 Landers earthquake has continued to deform in the years following the earthquake at rates ∼3 times greater than pre‐earthquake rates. We use a combination of Global Positioning System (GPS) and synthetic aperture radar (InSAR) data collected during a ∼3‐year epoch following the earthquake in order to investigate postseismic mechanisms responsible for the high transient velocities. In order to maximize the potential signal from viscoelastic relaxation we evaluate and model postseismic relaxation following the first few months of documented accelerated deformation. The combination of GPS and InSAR data allows us to establish viscoelastic relaxation of the lower crust and upper mantle as the dominant postseismic process and to discriminate among possible viscoelastic models. The data particularly require the presence of a highly ductile uppermost mantle beneath the central Mojave Domain, with temperature between the wet and dry basalt solidus. This is consistent with independent seismic and geochemical inferences of a regionally warm uppermost mantle. Further consideration of seismic velocity variations in conjunction with faulting patterns within the Mojave Desert suggests that the primary faulting characteristics of the Mojave Desert, namely, the pervasive late Cenozoic deformation within the Eastern California Shear Zone versus the near absence of faults in the Western Mojave Domain, are controlled by the rheology of the uppermost mantle.
Comparison of the rupture and radiation characteristics of intermediate and deep earthquakes
Authors: Campus Paola , Das Shamita
Published: September 2002

We study the depth dependence of the rupture and radiation characteristics of earthquakes below 70 km depth by investigating the source properties of intermediate (70–300 km depth) and deep (>300 km depth) earthquakes along the Fiji and Japan subducting slabs. The source properties are explored by retrieval of the moment rate functions and by spectral analysis, using P waves. Thirty‐two earthquakes in the period 1986–1997 with seismic moment >1.× 1018 N m are analyzed. The main conclusions are (1) the dependence of the source duration on the scalar seismic moment M0 follows the usual one‐third trend, on average; (2) this behavior is still visible when the source durations are adjusted for the shear wave velocity and the modulus of rigidity at the hypocentral depth; (3) the effective stress drop and the effective strain drop for subcrustal earthquakes in Fiji and Japan tend to be larger than those for the crustal ones; (4) the duration and shape of the moment rate functions do not show any systematic difference between intermediate and deep events; and (5) no obvious difference in the spectral properties of intermediate and deep events is seen.
Quasi‐static fault growth and cracking in homogeneous brittle rock under triaxial compression using acoustic emission monitoring
Authors: Lei Xinglin , Kusunose Kinichiro , Rao M. V. M. S. , Nishizawa Osamu , Satoh Takashi
Published: September 2002

This paper describes the localization of deformation acceleration in the period prior to dynamic failure in hornblende schist rock under triaxial compression using acoustic emission (AE) monitoring. Rather than stabilize the failure process by controlling axial stress to maintain a constant rate of AE (for monitoring AE hypocenters) as in previous works [e.g., Lockner et al, 1991], we have instead developed a rapid multichannel data collection system. This enables us to elucidate the dynamics of fault nucleation under condition of constant stress (creep) loading, which is a better approximation to low strain rate condition in the Earth and allows both quasi‐static and dynamic crack growth to occur. The waveforms of more than 8000 AE events which occurred mainly during a 15 s period were recorded on 32 channels, with a sampling rate of 50 ns and mask time of 200 μs. Hypocentral locations of AE sources revealed that the fault initiated at one end of the core and then propagated into the unfaulted rock with a process zone (fault front) of intense cracking. We found that there were two different processes operating during the quasi‐static nucleation of a shear fault, namely, a process zone in front of the fault tip and a “wake” of damage zone following the process zone. The process zone had the following features: (1) major tensile cracking, (2) low b value and fewer larger events, and (3) strong self‐excitation. The mechanism of crack interaction and fault growth was, therefore, a mutual enhancement on dilatation due to tensile cracking. On the other hand, the damage zone was characterized by (1) major shear cracking, (2) low b value and more larger events, and (3) weak self‐excitation, indicating that in the damage zone, following the development of a shear fault, linkage between cracks became the major mechanism of crack interaction and fault development. The mutual changes of b value and self‐exciting strength observed in our experiments seem to occur as a result of the hierarchy of fault growth, which was not observed under slowed down loading conditions. Therefore our experimental results, under a realistic approximation of the dynamic condition of the Earth, are meaningful for the problems of earthquakes as well as rock bursts.
Factors controlling the Alpine evolution of the central Pyrenees inferred from a comparison of observations and geodynamical models
Authors: Beaumont Christopher , Muñoz Josep Anton , Hamilton Juliet , Fullsack Philippe
Published: September 2002

Geodynamical numerical modeling has been combined with crustal structural restoration of the central Pyrenees in order to gain insight into fundamental processes that control the evolution of collisional orogens. Models are based on deformation of the crust by stresses transmitted upward from kinematic basal boundary conditions corresponding to the subduction of part of the lithosphere. The influence of inherited crustal heterogeneities, denudation, subcrustal loads, and crustal mechanical properties, consistent with well‐constrained crustal partial restored cross sections of the central Pyrenees, is investigated by progressively incorporating them into model experiments. The primary result inferred from the modeling is that the asymmetry of the central Pyrenees double‐wedge, seen as strain partitioning and in the morphological evolution, is a consequence of the asymmetric distribution of inherited crustal heterogeneity. The tectonic style of the central Pyrenees is the result of the inversion of the Early Cretaceous extensional fault system, during the early stages of the collision, and the reactivation of Hercynian heterogeneities during the late stages. Most of the upper crustal mass that entered the orogen during the calculated 165 km of convergence was accommodated by an increase of upper crustal cross sectional area or lost by denudation. To explain the upper crustal mass partitioning, as well as the geometry of the foreland basins and the preservation of synorogenic deposits in piggyback basins, a subduction load has to be applied to the models. Lower crust and mantle lithosphere were consumed by the mantle.
Plumes in a convecting mantle: Models and observations for individual hotspots
Authors: Steinberger Bernhard
Published: September 2002

The motion of hotspots and the deformation of their underlying plume conduits as calculated within models of global mantle flow are presented. A new list of 44 possible hotspots with associated tracks has been compiled. For all of them, calculations have been performed under consideration of individual age and anomalous mass flux for three different models of plume buoyancy and mantle flow. Plume source depth has usually been assumed to be the top of D”, but an alternative source depth at the 670‐km discontinuity has also been considered. Using models of relative plate motions and boundaries, hotspot tracks on plates have been calculated and compared with age data, ocean floor topography, and distribution of volcanics on continents. Absolute plate motions have been redetermined under consideration of hotspot motion, using a new least squares method. For the Hawaiian and Yellowstone hotspots, source locations and hotspot motion have been computed for a total of up to 23 different models. The results show plume conduits being tilted, with source regions at the D” moving in the lowermost mantle flow, generally toward large‐scale upwellings under southern Africa and the south central Pacific. Hotspot surface motion often represents the horizontal component of midmantle flow, which is frequently opposite to plate motion, toward ridges and away from subduction zones. In particular, almost all models tested predict southward motion of the Hawaii and Kerguelen hotspots and westward motion of the Iceland hotspot. For models including hotspot motion the agreement between calculated and observed hotspot tracks is frequently about as good as, or better than, for the fixed hotspot model, but sometimes fixed hotspots give the best fit. In some cases where the track ends at a subduction zone, e.g., for the Bowie hotspot, results can give indications about the otherwise unknown age of the hotspot. In other cases, especially for the Tahiti hotspot, results suggest an origin shallower than D”, and in yet other cases, particularly East Africa, the failure of the hotspot models used supports other evidence indicating the presence of comparatively broad upwellings rather than localized plumes.
Geothermal reservoir characterization: Application of microseismicity and seismic wave properties at Olkaria, Kenya rift
Authors: Simiyu Silas M.
Published: September 2002

A seismic network comprising 18 stations was established to study the seismicity associated with the Olkaria geothermal field. During the period of network recording, 4765 local earthquakes originating within the study area (ts‐tp < 3 s) as well as 162 earthquakes at regional distances (5 s < ts‐tp < 40 s) were recorded. Also, 60 rock blasts from a nearby quarry and 25 calibration shots were recorded. P and S waves from 2613 well‐located microearthquakes, 45 quarry blasts, and 25 calibration shots were used in this study. Average velocities for the upper crust are estimated to be 6.4±0.04 and 3.74±0.03 km/s for P and S waves, respectively. For the four uppermost layers the thicknesses and average velocities are estimated to be as follows: for layer 1, 0.6 km. Vp = 2.8 km/s, and Vs = 1.65 km/s; for layer 2, 1.5 km, Vp = 3.9 km/s, and Vs = 2.28 km/s; for layer 3, 4.0 km, Vp = 4.7 km/s, and Vs = 2.66 km/s; for layer 4, >6 km, Vp = 6.4 km/s, and Vs = 3.74 km/s. The magnitude threshold is Mc = 1.4 and the observed b value is 1.05. Vp/Vs ratios were determined for the six individual fields composing the study area and were found to be low (1.58) in the east production field (EPF) and high (1.82) in the Olkaria central field (OCF). Hypocenter analysis show that the main geothermal heat source for the area is at a depth of at least 6 km. Poisson's ratio (σ) is low (0.16) in the EPF and Olkaria west field (OWF) but high (0.26) in the central (OCF and Olkaria south field (OSF)) N‐S zone parallel to the Ololbutot fault zone. The low σ corresponds to a decrease in P wave velocity in areas with low reservoir pressure, high heat flow, and steam/gas saturation. Evaluation of σ and Vp/Vs with regard to directly measured reservoir parameters such as pressure, temperature, and fluid chemistry give consistent and comparable results. This shows that σ and Vp/Vs values are indicators of reservoir characteristics and tools for monitoring natural or induced changes.
Transpression between two warm mafic plates: The Queen Charlotte Fault revisited
Authors: Rohr Kristin M. M. , Scheidhauer Maren , Trehu Anne M.
Published: September 2002

The Queen Charlotte Fault is a transpressive transform plate boundary between the Pacific and North American plates offshore western Canada. Previous models for the accommodation of transpression include internal deformation of both plates adjacent to the plate boundary or oblique subduction of the oceanic plate; the latter has been the preferred model. Both plates are warm and mafic and have similar mechanical structures. New multichannel seismic reflection data show a near‐vertical Queen Charlotte Fault down to the first water bottom multiple, significant subsidence east of the Queen Charlotte Fault, a large melange where the fault is in a compressive left step, and faulting which involves oceanic basement. Gravity modeling of profiles indicates that Moho varies fairly smoothly across the plate boundary. Isostatic anomalies indicate that the Pacific plate is flexed downward adjacent to the Queen Charlotte Fault. Upward flexure of North America along with crust thickened relative to crust in the adjacent basin creates topography known as the Queen Charlotte Islands. Combined with other regional studies, these observations suggest that the plate boundary is a vertical strike‐slip fault and that transpression is taken up within each plate.
Viscoplastic self‐consistent and equilibrium‐based modeling of olivine lattice preferred orientations: Implications for the upper mantle seismic anisotropy
Authors: Tommasi Andréa , Mainprice David , Canova Gilles , Chastel Yvan
Published: September 2002

Anisotropy of upper mantle physical properties results from lattice preferred orientation (LPO) of upper mantle minerals, in particular olivine. We use an anisotropic viscoplastic self‐consistent (VPSC) and an equilibrium‐based model to simulate the development of olivine LPO and, hence, of seismic anisotropy during deformation. Comparison of model predictions with olivine LPO of naturally and experimentally deformed peridotites shows that the best fit is obtained for VPSC models with relaxed strain compatibility. Slight differences between modeled and measured LPO may be ascribed to activation of dynamic recrystallization during experimental and natural deformation. In simple shear, for instance, experimental results suggest that dynamic re‐crystallization results in further reorientation of the LPO leading to parallelism between the main (010)[100] slip system and the macroscopic shear. Thus modeled simple shear LPOs are slightly misoriented relative to LPOs measured in natural and experimentally sheared peridotites. This misorientation is higher for equilibrium‐based models. Yet seismic properties calculated using LPO simulated using either anisotropic VPSC or equilibrium‐based models are similar to those of naturally deformed peridotites; errors in the prediction of the polarization direction of the fast S wave and of the fast propagation direction for P waves are usually <15°. Moreover, overestimation of LPO intensities in equilibrium‐based and VPSC simulations at high strains does not affect seismic anisotropy estimates, because these latter are weakly dependent on the LPO intensity once a distinct LPO pattern has been developed. Thus both methods yield good predictions of development of upper mantle seismic anisotropy in response to plastic flow. Two notes of caution have nevertheless to be observed in using these results: (1) the dilution effect of other upper mantle mineral phases, in particular enstatite, has to be taken into account in quantitative predictions of upper mantle seismic anisotropy, and (2) LPO patterns from a few naturally deformed peridotites cannot be reproduced in simulations. These abnormal LPOs represent a small percent of the measured natural LPOs, but the present sampling may not be representative of their abundance in the Earth's upper mantle.
Subduction‐induced strain in the upper mantle east of the Mendocino triple junction, California
Authors: Hartog Renate , Schwartz Susan Y.
Published: September 2002

We observe splitting of teleseismic shear waves at five stations of the Berkeley Digital Seismic Network located east of the Mendocino triple junction in northeastern California that is dependent on the arrival direction of the seismic phases. The observed variations with back azimuth cannot be explained with laterally varying anisotropy with a horizontal symmetry axis and are attributed to the presence of fabric with an inclined symmetry axis. We assume that the anisotropy is caused by the preferred alignment of olivine crystals. A grid search over possible orientations of the olivine a axes reveals that south of the Mendocino triple junction they dip to the east, whereas north of the triple junction they dip to the northeast. On the basis of a comparison of the ray paths of our data to the spatial distribution of fast and slow P wave velocity anomalies in the upper mantle, we conclude that the anisotropy is located within seismically slow regions and that the directions are controlled by the geometry of a steeply dipping fast P wave velocity anomaly. Assuming that the fast P velocity anomaly represents subducted slab material, we conclude that the fabric beneath the stations north of the triple junction is most likely caused by the differential motion between this rigid, strong down going plate and the surrounding mantle. South of the triple junction the fabric may have developed while subduction of the Farallon plate was still ongoing in this region (prior to 6 Ma). However, we prefer to attribute the observations to more recent asthenospheric flow associated with the opening of a slabless window beneath the North American lithosphere. The flow is modulated by the presence of rigid lithosphere to the north and east.
Local sharpness and shear wave speed jump across the 660‐km discontinuity
Authors: Castle John C. , Creager Kenneth C.
Published: September 2002

We examine vertical component short‐period teleseismic seismograms from earthquakes in the Izu‐Bonin subduction zone recorded by networks in the western United States for phases associated with conversions from mantle discontinuities. The dominant phase in the stacked P coda is the result of near source S‐to‐P conversions from a subhorizontal discontinuity at a depth ranging from 650 km to 745 km. We previously used 88 timings of this phase, called S660P to determine the topography of the 660‐km discontinuity. Employing the 17 best recorded S660P phases, we modeled the S660P amplitude accounting for attenuation and correcting for three‐dimensional discontinuity topography. Just to the east of the Izu‐Bonin subduction zone, the 660‐km discontinuity is sharp (<10 km) and the 5‐wave velocity contrast across the discontinuity is 0.60±0.11 km s−1. This value is 60% larger than the preliminary reference Earth model (PREM) value (70% larger than in the iasp91 and ak135 models) and, unlike estimates from reflected SH waves, is independent of the estimated density contrast at 660 km. The large 5‐wave velocity contrast inferred here is consistent with recent mineral physics experiments and extrapolations if the mantle at 660 km depth and a few hundred kilometers east of the slab is at near normal mantle temperatures and contains between 3% and 5% cation aluminum, as expected in a pyrolitic mantle.
Further evidence for the compound nature of slow earthquakes: The Prince Edward Island earthquake of April 28, 1997
Authors: McGuire Jeffrey J. , Jordan Thomas H.
Published: September 2002

Seismograms of the April 28, 1997, Prince Edward island transform fault earthquake (Mw 6.8) show clear evidence for the superposition of two types of rupture, a normal earthquake and a slow component which primarily radiated energy at low frequencies. We combined low‐frequency spectral data with P waveform data to invert for a source time function that satisfied data from 0.001 to 2.5 Hz. The results show that the earthquake started with a slow event which began ∼15 s before the fast mainshock and was part of a slow earthquake which lasted over 30 s. The smoothness of the initial event is inferred from the low amplitude of high‐frequency energy at regional stations during the initial portion of the P wave arrival. Directivity analysis shows that the two components of the rupture occurred on different faults. The slow event occurred on the main transform, and the fast event occurred on a parallel fault 40 km to the east. A similar compound sequence, involving both slow and fast components and rupture on multiple faults, occurred on the Romanche transform in 1994. These and other results for slow earthquakes suggest that many large events on oceanic transform faults are compound events.
Synoptic density and velocity observations of slope waters in the Chukchi and East‐Siberian seas
Authors: Münchow Andreas , Carmack Eddy C. , Huntley David A.
Published: September 2002

Synoptic velocity and water mass observations obtained in 1993 provide a snapshot of the circulation over the slope of the Chukchi and East Siberian Seas. We describe flow and density fields between the 100‐m and 2000‐m isobaths >350 km to the north of Siberia near the date line. Strong property gradients on sloping isopycnal surfaces reflect sharp boundaries between upper and lower halocline, as well as Atlantic layer waters. Concurrent velocity observations from a towed acoustic Doppler current profiler reveal an unexpected, 5–10 cm/s strong generally westward sub‐surface flow between the 200‐m and 350‐m isobaths. The flow is most intense 50 m to 150 m below the surface and is embedded in a tidal current with amplitudes of similar magnitude. Subtidal velocity and water mass observations from sections 250 km apart suggest a spatially continuous westward slope current. Variability associated with submesoscale vortices is shown by a geostrophically balanced anticyclonic eddy which carries anomalous warm shelf water of Pacific origin. The eddy is nearly spun down: Its Rossby number ∼0.1 is an order of magnitude smaller than those of eddies observed in the Beaufort Sea.
Processes controlling the advance and retreat of sea ice in the Sea of Okhotsk
Authors: Kimura Noriaki , Wakatsuchi Masaaki
Published: September 2002

Using Defense Meteorological Satellite Program (DMSP) special sensor microwave imager (SSM/I) sea‐ice data and meteorological data, we investigate processes controlling the advance and retreat of sea ice in the Sea of Okhotsk. For this investigation we apply a new algorithm to the SSM/I data, which is capable of calculating ice concentration and distinguishing sea‐ice types; this algorithm is confirmed by comparison with National Oceanic and Atmospheric Administration (NOAA) advanced very high resolution radiometer (AVHRR) imagery. Our investigations show that there exists a locality for the areas of new‐ice formation, and the ice formation near the ice edge always occurs as a recovery after sea ice has retreated suddenly. We also reveal that the speed of ice‐edge advance fairly coincides with that predicted by 2% of the geostrophic wind. Furthermore, there exists a good correspondence between the times when the overall advance and retreat of ice cover begin and the decrease and increase of air temperature begin. We conclude that the advance of the ice cover in the Sea of Okhotsk occurs primarily through ice advection by wind and that the interannual variation in the maximum sea‐ice extent can be interpreted by that in wind and air temperature conditions.
Coupled sea surface‐atmosphere model: 2. Spectrum of short wind waves
Authors: Kudryavtsev V. N. , Makin V. K. , Chapron B.
Published: September 2002

A physical model of the short wind wave spectrum in the wavelength range from a few millimeters to few meters is proposed. The spectrum shape results from the solution of the energy spectral density balance equation. Special attention is paid to the description of the capillary range of the short wave spectrum. It is assumed that in this range the spectrum shape is determined mainly by the mechanism of generation of parasitic capillaries. This is described as the cascade energy transfer from the gravity to the capillary waves. Thus the capillary wave spectrum results through the balance between generation of capillaries and their viscous dissipation. The short gravity wave spectrum results through the balance between wind input and dissipation due to wave breaking. A parameterization of wind input is obtained in part 1 of the present paper. To describe the dissipation due to wave breaking, the approach developed by Phillips [1985] is used. The spectral rate of energy dissipation is presented in the form of a power dependence of the ratio of the saturation spectrum to some threshold level. It is further shown that the threshold level depends on the drift current shift in the water viscous sublayer, which affects the energy losses by wave breaking. To obtain a short wave spectrum which is valid in the whole wavenumber domain, the capillary and the short gravity wave spectra are patched in the vicinity of the wavenumber corresponding to the minimum phase velocity. This short wave spectrum is incorporated into the wind over waves coupled model developed in part 1 of the present paper. The measured statistical properties of the sea surface related to the short waves, such as the spectral shape of omnidirectional and up‐wind spectra, their wind speed dependence and angular spreading, and the wind speed dependence of integral mean square slope and skewness parameters, are well reproduced by the model. Also the model well reproduces the measured wind speed dependence of the drag coefficient and of the coupling parameter.
Sensitivity of an oceanic general circulation model forced by satellite wind stress fields
Authors: Grima Nicolas , Bentamy Abderrahim , Katsaros Kristina , Quilfen Yves , Delecluse Pascale , Levy Claire
Published: September 2002

Satellite wind and wind stress fields at the sea surface, derived from the scatterometers on European Remote Sensing satellites 1 and 2 (ERS‐1 and ERS‐2) are used to drive the ocean general circulation model (OGCM) “OPA” in the tropical oceans. The results of the impact of ERS winds are discussed in terms of the resulting thermocline, current structures, and sea level anomalies. Their adequacy is evaluated on the one hand by comparison with simulations forced by the Arpege‐Climat model and on the other hand by comparison with measurements of the Tropical Atmosphere‐Ocean (TAO) buoy network and of the TOPEX/Poseidon altimeter. Regarding annual mean values, the thermal and current responses of the OGCM forced by ERS winds are in good agreement with the TAO buoy observations, especially in the central and eastern Pacific Ocean. In these regions the South Equatorial Current, the Equatorial Undercurrent, and the thermocline features simulated by the OGCM forced by scatterometer wind fields are described. The impact of the ERS‐1 winds is particularly significant to the description of the main oceanic variability. Compared to the TAO buoy observations, the high‐frequency (a few weeks) and the low‐frequency of the thermocline and zonal current variations are described. The correlation coefficients between the time series of the thermocline simulated by ERS winds and that observed by the TAO buoy network are highly significant; their mean value is 0.73, over the whole basin width, while it is 0.58 between Arpege model simulation and buoy observations. At the equator the time series of the zonal current simulated by the ERS winds, at three locations (110°W, 140°W, and 165°E) and at two depths, are compared to the TAO current meter and acoustic Doppler current profiler (ADCP) measurements. The mean value of the significant correlation coefficients computed with the in situ measurements is 0.72 for ERS, while it is 0.51 for the Arpege‐Climat model. Thus ERS wind fields through the OGCM generate more realistic current variations than those obtained with Arpege climate winds, and they are particularly efficient in capturing abrupt changes (“wind bursts”) which may be important regarding ocean dynamics.
Crest‐length effects in nearshore tsunami run‐up around islands
Authors: Cho Yong‐Sik , Liu Philip L.‐F.
Published: September 2002

The large run‐up heights observed on the lee side of a circular island are investigated, using the shallow‐water equations, to understand the devastating run‐up heights reported in the lees of both Babi and Okushiri Islands during the Flores 1992 and the Hokkaido 1993 tsunamis. Specifically, the effects of the crest length of incident waves are examined both numerically and experimentally. It is found that the run‐up heights around a circular island are strongly dependent on the crest length of incident waves when the crest length is less than the base diameter of the island but are almost insensitive if the crest length is greater than twice the base diameter. This observation implies that it may be possible to calculate a worst case scenario of run‐up on an island from nearshore events.
Estimation of salinity profiles in the upper ocean
Authors: Hansen Donald V. , Thacker W. Carlisle
Published: September 2002

A new algorithm is presented for estimating salinity profiles in the upper ocean from measurements of temperature profiles and surface salinity. In application to the eastern tropical Pacific the method replicates a large fraction of the variability of salinity in the upper few tens of meters and provides modest to substantial improvement at nearly all levels. Estimated salinity profiles are able to characterize barrier layers, regions formed by a halocline within the thermal mixed layer. The rms error of geopotential height calculations based on estimated salinity profiles is reduced more than 50% by this method relative to methods not using surface salinity. Even without the surface salinity measurement some reduction of error in geopotential heights can be obtained relative to previous methods.
Interstation prediction of ocean water levels using methods of nonlinear dynamics
Authors: Frison Ted W. , Earle Marshall D. , Abanel Henry D. I. , Scherer Wolfgang D.
Published: September 2002

We describe prediction of ocean water levels between geographically separated locations by using a method derived from studies of chaotic dynamical systems. This interstation predictor requires only previously observed water‐level data collected simultaneously from the target and baseline water‐level measuring stations. The current observations at the baseline station are then used for making the predictions. The method is demonstrated using data from seven “tide” stations with different water level characteristics operated by the U.S. government along the U.S. southeast coast. The data are averaged over 3 min at the sensor to filter out high‐frequency motions and are reported at 6‐min intervals. Thus the recorded water levels are all ocean surface motions that occur on timescales greater than a few minutes. The predictor forms the reconstructed attractor for both stations using previously observed data. For each new observation at the baseline station, it places the corresponding state‐space vector onto the attractor for that station. A map is then derived that associates the neighborhood around that point to the corresponding temporal neighborhood of past observations at the target station. The current observation at the baseline station is then mapped to the appropriate neighborhood for the target station. This is the estimate of the water level at the target station. This method is attractive because the data requirements are simple, the computation burden is low, and there are few decisions about the parameters needed by the algorithm. The state‐space predictor compares favorably to traditional methods including statistical correlation, cross‐spectral analysis, harmonic analysis, and response analysis. Interstation predictions are important for marine navigation and other applications. The state‐space predictor can also provide an objective way of locating tide stations, quantifying the spatial variability of ocean water levels, and identifying regions where ocean water level dynamics are similar.
Experiments on mixing due to internal solitary waves breaking on uniform slopes
Authors: Michallet H. , Ivey G. N.
Published: September 2002

The shoaling and breaking of an internal solitary wave of depression on a uniform slope were studied experimentally. The waves were generated with as large an amplitude as possible while minimizing mixing at the generation site, thus maximizing the amount of energy propagating onto the slope in the experiment. Various bottom slopes, fluid layer thickness ratios, and density ratios were investigated. The mechanism leading to breaking was examined with flow visualization and particle image velocimetry. Since the layer thickness ratio primarily controls the length (LW) of the solitary wave (for a given amplitude a), it is found that the ratio of LW and the characteristic length of the slope LS determines the amount of energy reflected from the slope. The mixing efficiency of the breaking event, defined as the ratio of the increase of potential energy divided by the amount of wave energy lost at the slope, peaks at a maximum of 25% when LW/LS = 0.5, with a decrease in efficiency for points on either side of this peak value.
Response of the deep ocean internal wave field to traveling midlatitude storms as observed in long‐term current measurements
Authors: Niwa Yoshihiro , Hibiya Toshiyuki
Published: September 2002

It has been demonstrated in a recent numerical experiment that double‐inertial frequency internal waves may play a crucial role in diapycnal mixing processes in the deep ocean, with the energy effectively transferred across the internal wave spectrum down to small dissipation scales by nonlinear wave‐wave interactions [Hibiya et al., 1998]. To examine whether or not such double‐inertial frequency waves are actually generated in the real deep ocean, current meter data from long‐term moorings in the northwest Pacific basin are analyzed together with global sea surface wind data. By incorporating the wind data into a simple damped slab model, predominant inertial currents are shown to be excited in the mixed layer in the northwest Pacific basin by traveling midlatitude storms during fall and winter. The multiple filter analysis demonstrates that double‐inertial frequency waves as well as near‐inertial frequency waves are significantly amplified in the deep ocean internal wave field during the periods strong inertial currents are excited in the mixed layer. This suggests that in addition to near‐inertial frequency waves, double‐inertial frequency waves are actually excited by strong atmospheric disturbances through nonlinear effects as demonstrated in the numerical experiment by Niwa and Hibiya [1997]. Double‐inertial frequency waves thus excited seem to propagate over horizontal distances of the order of 1000 km from their source region while feeding their energy to the local internal wave field, consistent with the theoretical prediction based on the magnitudes of group velocity and nonlinear interaction time [Olbers, 1983].
Modeling the seasonal variability of hydrography and circulation in the Kara Sea
Authors: Harms I. H. , Karcher M. J.
Published: September 2002

In the frame of a project on transport of contaminants in the Arctic, the Hamburg shelf ocean model (HAMSOM) is applied to the Kara Sea. The HAMSOM system consists of a three‐dimensional, baroclinic circulation model coupled to a thermodynamic and dynamic sea ice model. The Kara Sea model is forced with climatological winds, atmospheric heat fluxes, river runoff, and tides. The obtained results reveal no typical Kara Sea circulation that prevails throughout the year. Instead, the model showed a strong seasonal variability in circulation and hydrography due to winds, freshwater runoff, and ice formation. The circulation is weakest in spring when the wind speeds are low and horizontal density gradients are small. Fresh water from the rivers spreads toward the north and northwest rather than forming a coastally trapped current that flows to the east. In autumn, the circulation is significantly enhanced because of increasing wind speeds and strong horizontal density gradients. Good agreement was found between model results and recent observations. The “classical” cyclonic current pattern in the southern Kara Sea, however, was not reproduced by the model.