bluuebunny/crossref_metadata_2025 huggingface dataset:
Correlated errors in geodetic time series: Implications for time‐dependent deformation
Authors: Langbein John , Johnson Hadley
Published: February 2004

Analysis of frequent trilateration observations from the two‐color electronic distance measuring networks in California demonstrate that the noise power spectra are dominated by white noise at higher frequencies and power law behavior at lower frequencies. In contrast, Earth scientists typically have assumed that only white noise is present in a geodetic time series, since a combination of infrequent measurements and low precision usually preclude identifying the time‐correlated signature in such data. After removing a linear trend from the two‐color data, it becomes evident that there are primarily two recognizable types of time‐correlated noise present in the residuals. The first type is a seasonal variation in displacement which is probably a result of measuring to shallow surface monuments installed in clayey soil which responds to seasonally occurring rainfall; this noise is significant only for a small fraction of the sites analyzed. The second type of correlated noise becomes evident only after spectral analysis of line length changes and shows a functional relation at long periods between power and frequency of 1/ƒα, where ƒ is frequency and α≈2. With α=2, this type of correlated noise is termed random‐walk noise, and its source is mainly thought to be small random motions of geodetic monuments with respect to the Earth's crust, though other sources are possible. Because the line length changes in the two‐color networks are measured at irregular intervals, power spectral techniques cannot reliably estimate the level of 1/ƒαnoise. Rather, we also use here a maximum likelihood estimation technique which assumes that there are only two sources of noise in the residual time series (white noise and random‐walk noise) and estimates the amount of each. From this analysis we find that the random‐walk noise level averages about 1.3 mm/√yr and that our estimates of the white noise component confirm theoretical limitations of the measurement technique. In addition, the seasonal noise can be as large as 3 mm in amplitude but typically is less than 0.5 mm. Because of the presence of random‐walk noise in these time series, modeling and interpretation of the geodetic data must account for this source of error. By way of example we show that estimating the time‐varying strain tensor (a form of spatial averaging) from geodetic data having both random‐walk and white noise error components results in seemingly significant variations in the rate of strain accumulation; spatial averaging does reduce the size of both noise components but not their relative influence on the resulting strain accumulation model.
Statistical relations between the parameters of aftershocks in time, space, and magnitude
Authors: Guo Zhenqi , Ogata Yosihiko
Published: February 2004

The correlation between statistical parameters of seismicity, such as the b value of the Gutenberg and Richter [1954] relation, the p value of the modified Omori formula [Utsu, 1961], the p and α value of the Epidemic Type Aftershock Sequence (ETAS) model [Ogata, 1992], and the fractal dimension D of the hypocenter distribution, is analyzed for 34 aftershock sequences in Japan from 1971 to 1995. All the parameters are estimated using maximum likelihood methods along with their error assessments. For the majority of the aftershock sequences, the ETAS model fits statistically better than the modified Omori formula, which suggests existence of clusters within the aftershock sequence. Most of the scatterplots between the estimates of the seismicity parameters in time, space, and magnitude distributions are clearly seen to be either positively or negatively correlated. The contrasting correlation patterns are revealed between the estimated parameters for the intraplate and interplate earthquakes, except for the two pairs (b,D) and (α,p) in which similar correlation patterns are found. We focus our attention on these patterns as a source of interesting contrasts between the two earthquake groups. In particular, the significant dependence of these parameters on the depth appears to be a key to understanding the correlation pattern for interplate aftershocks, while a different interpretation is made for intraplate aftershocks because no significant dependence on depth is observed.
Relative motions of the Pacific, Rivera, North American, and Cocos plates since 0.78 Ma
Authors: DeMets Charles , Wilson Douglas S.
Published: February 2004

We use magnetic anomaly and fracture zone crossings from 3°N–24°N along the East Pacific Rise to define the boundaries and relative velocities of the rigid Pacific, Rivera, North American, and Cocos plates since 0.78 Ma and to test several hypotheses regarding the nature of deformation between these plates. Crossings of the 0.78 Ma isochron from the Pacific‐Rivera rise north of 22.0°N are significantly better fit by the 0–0.78 Ma Pacific‐North America rotation than by a Pacific‐Rivera rotation, thereby supporting a recently proposed model for accretion of the northern part of the Rivera plate to North America. A significant misfit of the Pacific‐Cocos rotation to crossings of anomaly ln from the East Pacific Rise north of 16.4°N suggests that seafloor north of ∼16°N moves relative to the rigid Pacific or Cocos plates. We also find that nine crossings of the eastern Rivera fracture zone and eight crossings of anomaly ln flanking the Manzanillo spreading segment are fit well by a Pacific‐Rivera rotation. The Rivera‐Cocos angular velocity derived from the best fitting Pacific‐Cocos and Pacific‐Rivera angular velocities predicts 19±3 mm yr−1 (95% confidence limit) of ∼N‐S convergence near the center of the Rivera‐Cocos boundary. Nearby left‐lateral, strike‐slip earthquakes with north trending fault planes suggest that convergence is accommodated via sinistral shear along a north trending zone. The unambiguous kinematic and seismologic evidence for significant Rivera‐Cocos motion indicates that the absence of a well‐defined Rivera‐Cocos plate boundary cannot be interpreted as evidence that Rivera‐Cocos motion is slow or zero, as is postulated in several previous studies. The Rivera‐Cocos angular velocity predicts oblique Rivera‐Cocos convergence at the El Gordo graben and beneath the Manzanillo trough and southern Colima graben. Any extension across these features since 0.78 Ma thus cannot have resulted from divergence between the subducting Rivera and Cococs plates.
Postseismic strain following the 1989 Loma Prieta earthquake from GPS and leveling measurements
Authors: Bürgmann Roland , Segall Paul , Lisowski Mike , Svarc Jerry
Published: February 2004

Postseismic deformation in the 5 years following the 1989 Loma Prieta earthquake has been measured with the Global Positioning System and precise leveling. Postearthquake velocities at distances greater than ∼20 km from the coseismic rupture are not significantly different from those observed in the 20 years prior to the earthquake. However, velocities at stations within ∼20 km of the rupture exceed preearthquake rates and exhibit unanticipated contraction normal to the strike of the San Andreas fault system. A combination of forward modeling and nonlinear optimization suggests that the observed postseismic deformations were caused by aseismic oblique reverse slip averaging 2.9 cm/yr on the San Andreas fault and/or the Loma Prieta rupture zone and 2.4 cm/yr reverse slip along a buried fault within the Foothills thrust belt. The best fitting sources of postseismic deformation are all located at depths of less than 15 km. We find no evidence for accelerated flow or shear below the Loma Prieta rupture in the first 5 years following the earthquake. The inferred postseismic slip is likely to have been caused by the coseismic stress change updip of the 1989 rupture.
Cenozoic tectonic history of the North America‐Caribbean plate boundary zone in western Cuba
Authors: Gordon Mark B. , Mann Paul , Cáceres Dámaso , Flores Raúl
Published: February 2004

Structural studies of well‐dated Jurassic to lower Miocene rocks in western Cuba constrain the sequence of structural events affecting this oblique collisional zone between the late Cretaceous island arc and the Jurassic‐Cretaceous North America passive margin in the southeastern Gulf of Mexico and Straits of Florida. Results of detailed mapping and collection of fault slip data at 34 sites define a regionally consistent, five phase tectonic model for the period from the late Paleocene to the post‐early Miocene. During the late Paleocene to the early Eocene, the Cuban island arc collided with the North American passive margin (Bahamas Platform). Northwest‐ward overthrusting during the collision defines tectonic phase I. A NNE‐SSW compression concurrent with early Eocene left‐lateral strike‐slip faulting along the Pinar fault zone defines phase II. This result is consistent with structural mapping showing sinistral shear within the 065° striking Pinar fault zone. An ENE‐WSW to E‐W compression defining phase III overprinted phase II faults in the lower Eocene and older rocks. Post‐early Miocene normal faulting characterizes phase IV. Inversion of fault slip data indicates two contemporaneous directions of tension of 120 and 170. Strike‐slip faults that overprint phase IV normal faults yield a 120 compression (phase V). The direction of compression associated with the arc/continent collision rotates clockwise from NW‐SE in the late Paleocene/early Eocene (phase I), to NNE‐SSW (phase II) and to ENE‐WSW by the middle Eocene (phase III). The rotation in the compression direction occurred because the arc turned toward an oceanic area in the present‐day area of central and eastern Cuba. Progressive collision led to complete subduction of the remnant oceanic crust by middle to late Eocene time.
Permeability and porosity of the Illinois UPH 3 drillhole granite and a comparison with other deep drillhole rocks
Authors: Morrow C. A. , Lockner D. A.
Published: February 2004

Permeability, porosity, and volumetric strain measurements were conducted on granite cores obtained at depths of 0.7 to 1.6 km from the Illinois UPH 3 drillhole at effective confining pressures from 5 to 100 MPa. Initial permeabilities were in the range of 10−17 to 10−19 m2 and dropped rapidly with applied pressure to values between 10−20 and 10−24 m2 at 100 MPa, typical of other deep granite core samples. These values are several decades lower than equivalent weathered surface granites at comparable effective confining pressures, where weathering products in cracks and pores inhibit crack closure with applied pressure. Permeabilities of the Illinois cores were inversely related to sample depth, suggesting that stress relief and thermal microfractures induced during core retrieval dominated the fluid flow. Thus these samples provide an upper bound on in situ matrix permeability values. A comparison of core permeability from UPH 3 and other deep drillholes shows that stress relief damage can often dominate laboratory permeability measurements. We conclude that it may be difficult to make meaningful estimates of in situ permeability based on either borehole samples (possible damage during retrieval) or surface‐derived analogs (altered by weathering). Volumetric strain determined from porosity measurements was compared with differential strain analysis (DSA) data reported by other investigators on samples from the same depths in the drillhole. Our strain measurements (0.002 to 0.005 at 100 MPa) were nearly twice as large as the DSA values, probably because of the crack‐enhancing effects of fluids present in our samples that are absent in the dry DSA cores, as well as other time‐dependent deformation effects. This difference in observed strain magnitudes between the two measurement methods may be an important consideration if strain and/or porosity data from deep core samples are used in models of stress, fluid circulation, and excess fluid pressure generation in the midcrust.
Microstructures and iron partitioning in (Mg,Fe)SiO3 perovskite‐(Mg,Fe)O magnesiowüstite assemblages: An analytical transmission electron microscopy study
Authors: Martinez Isabelle , Wang Yanbin , Guyot Francois , Liebermann Robert C. , Doukhan Jean‐Claude
Published: February 2004

San Carlos olivine and its synthetic ringwoodite polymorph have been transformed to (Mg,Fe)SiO3‐perovskite and magnesiowüstite at a pressure of 26 GPa in a 2000‐t uniaxial split‐sphere apparatus (USSA‐2000) for temperatures ranging from 700°C to 1600°C and run durations at peak temperatures of 0 min to 19 hours. The recovered samples were studied by analytical transmission electron microscopy to determine the evolution of the microstructures and the crystallographical relationships and iron partitioning between the coexisting phases in these assemblages. At 700°C, metastable olivine remained untransformed even after 19 hours. In runs performed at 1000°C and 1200°C, ringwoodite, in a topotactic relation with olivine, was identified even though olivine was used as starting material. Our results indicate that ringwoodite is an intermediate phase in the olivine → (Mg,Fe)SiO3‐perovskite + magnesiowüstite transformation in this temperature range. At or above 1300°C the transformation of olivine or ringwoodite (used as the starting material) into (Mg,Fe)SiO3‐perovskite + magnesiowüstite was complete in less than 10 min. The first microstructures that appear are eutectoid‐like as already described by previous authors. For longer run durations the microstructure consisted mostly of cylindrical magnesiowüstite crystals embedded within large, twinned (Mg,Fe)SiO3‐perovskite crystals. These observations suggest that magnesiowüstite grains are very unlikely to be interconnected for a wide range of possible bulk mantle compositions; magnesiowüstite will therefore play a relatively minor role in determining the transport properties of Earth's lower mantle. Analyses of (Mg,Fe)SiO3‐perovskite and magnesiowüstite crystals formed in the first steps of the transformation show that most of the iron‐magnesium partitioning is completed within the first minutes of the reaction and that subsequently only isochemical grain growth of the two phases occurs. The iron‐magnesium distribution between (Mg,Fe)SiO3‐perovskite (pv) and magnesiowüstite (mw), characterized by Kd‐Fe = (Fe/Mg)mw/(Fe/Mg)pv was precisely measured by analytical transmission electron microscopy in equilibrated runs and found to be Kd‐Fe = 3.8(3) at 1300°C and Kd‐Fe = 4.3(4) at 1600°C.
Changes in attenuation with depth in an ocean carbonate section: Ocean Drilling Program sites 806 and 807, Ontong Java Plateau
Authors: Frazer L. Neil , Sun Xinhua , Wilkens Roy H.
Published: February 2004

Changes of in situ seismic attenuation with depth are estimated from full waveform acoustic logs in a soft formation (rock with shear velocity less than borehole fluid velocity). The relative attenuation Qp−1 is computed from the variation with depth of the P wave amplitude spectrum of a single source receiver pair. The method is applicable when unknown source and receiver responses of the sonic tool make other methods difficult to apply. Tests with synthetic data generated by a full waveform method verify that in a soft formation the decay of the P wave amplitude spectrum depends mainly on attenuation. The method is applied to Ocean Drilling Program data from holes 806B and 807A on the Ontong Java Plateau in the western Pacific Ocean. Four attenuation logs were computed independently using data from four source receiver pairs. Agreement of the four logs at each site and agreement of results from the two sites suggest that the method is robust and practical. The attenuation logs show a maximum attenuation between 300 and 500 m below the seafloor. They agree well with compilations of high‐frequency attenuation versus porosity and frequency in marine sediments but are somewhat greater than results from seismic experiments, possibly owing to the presence of sedimentary microbeds and the three‐decade difference between the seismic and sonic frequency bands.
Three‐dimensional upper crustal velocity structure beneath San Francisco Peninsula, California
Authors: Parsons Tom , Zoback Mary Lou
Published: February 2004

This paper presents new seismic data from, and crustal models of the San Francisco Peninsula. In much of central California the San Andreas fault juxtaposes the Cretaceous granitic Salinian terrane on its west and the Late Mesozoic/Early Tertiary Franciscan Complex on its east. On San Francisco Peninsula, however, the present‐day San Andreas fault is completely within a Franciscan terrane, and the Pilarcitos fault, located southwest of the San Andreas, marks the Salinian‐Franciscan boundary. This circumstance has evoked two different explanations: either the Pilarcitos is a thrust fault that has pushed Franciscan rocks over Salinian rocks or the Pilarcitos is a transform fault that has accommodated significant right‐lateral slip. In an effort to better resolve the subsurface structure of the peninsula faults, we established a temporary network of 31 seismographs arrayed across the San Andreas fault and the subparallel Pilarcitos fault at ∼1–2 km spacings. These instruments were deployed during the first 6 months of 1995 and recorded local earthquakes, air gun sources set off in San Francisco Bay, and explosive sources. Travel times from these sources were used to augment earthquake arrival times recorded by the Northern California Seismic Network and were inverted for three‐dimensional velocity structure. Results show lateral velocity changes at depth (∼0.5–7 km) that correlate with downward vertical projections of the surface traces of the San Andreas and Pilarcitos faults. We thus interpret the faults as high‐angle to vertical features (constrained to a 70°–110° dip range). From this we conclude that the Pilarcitos fault is probably an important strike‐slip fault that accommodated much of the right‐lateral plate boundary strain on the peninsula prior to the initiation of the modern‐day San Andreas fault in this region sometime after about 3.0 m.y. ago.
Diffraction tomography using multimode surface waves
Authors: Meier T. , Lebedev S. , Nolet G. , Dahlen F. A.
Published: February 2004

A new method is described that makes it feasible to include scattered and converted surface waves into waveform inversions for the three‐dimensional (3‐D) structure of the Earth. The single scattering (Born) approximation forms the basis of the method. In order to minimize the amplitude of the scattered wave field, the background model is first adapted to correct for nonconverted, forward‐scattered wave energy. We then perform Born inversion of the difference between the measured and synthetic waveforms, including a suite of Love and Rayleigh modes. The Born approximation yields linear equations of the form Aδγ=δuBorn, which allow the determination of the three‐dimensional perturbations γ to the background model from the scattered wave field δuBorn. This procedure is followed separately for each source‐receiver pair to allow for optimized background models for each signal, as well as to minimize the computational burden. We winnow the data vector for each path by performing singular value decomposition using a diagonalization of AAT. In a realistic example we found that each vertical component seismogram yields 30–40 linear constraints on the 3‐D Earth, significantly more than with conventional pure‐path (WKBJ) inversions. In a synthetic test, one seismogram is shown to be able to image a simple model of a point scatterer off the great circle. As a spin‐off of the formulation of the multimode inverse scattering problem, we not only obtain a series of eigenvectors that rank the sensitivity of a seismogram to Earth structure in a series of geometrical patterns, we also can compute the surface wave equivalent of a Fresnel zone.
Detecting low‐dimensional chaos in geophysical time series
Authors: Marzocchi W. , Mulargia F. , Gonzato G.
Published: February 2004

Driven by the great appeal of the potential capability to reduce very complex and highly erratic phenomenologies to simple deterministic and predictable processes, much effort has been devoted to studying chaotic systems. Unfortunately, such studies have been essentially theoretical, and the problem of detecting chaos in real time series has so far received little attention. As a consequence, the available techniques are fairly inefficient and are often misused. Furthermore, if detecting chaos in real‐time data would, in any case, be important from a philosophical stand point, only low‐dimensional chaos is of practical interest, since it allows an effective short range predictability and could possibly also be modeled. A critical review of the available methods to detect chaos in a real series is presented together with a procedure which is efficient in the presence of experimental errors and with relatively small sets of data. An application to the series of geomagnetic inversions and to the eruptive activity of the Piton de la Fournaise volcano, for which a chaotic dynamics appeared best documented, does not lead to detection of any low‐dimensional chaos.
Crustal structure of Edoras Bank continental margin and mantle thermal anomalies beneath the North Atlantic
Authors: Barton A. J. , White R. S.
Published: February 2004

We discuss the structure of the continent‐ocean transition of the Edoras Bank area of the NE Atlantic margin and the influence of the Iceland mantle plume during continental breakup. Edoras Bank lies close to the present‐day limit of influence of the Iceland plume. Sequences of seaward dipping reflectors imaged on multichannel seismic profiles are well‐developed over much of the margin. They form aerially extensive, dipping sheets, with individual reflectors imaged on margin‐parallel profiles as laterally continuous and subhorizontal features. We interpret them as basalt flows extruded from linear fissure vents close to sea level at the time of continental breakup. A crustal velocity model for the margin was developed by travel time modeling of wide‐angle refractions and reflections recorded on digital ocean bottom hydrophones and the structure confirmed by gravity modeling. Oceanic crust formed immediately after breakup is 10 km thick and exhibits classic layer 2 and layer 3 seismic velocities. The crustal thickness increases to the SE reaching 17 km beneath the continental fragment of Edoras Bank. The continent‐ocean transition is dominated in the lower crust by a lens up to 8 km thick of high‐velocity material (7.2–7.5 km s−1), which exhibits a very low velocity gradient. It extends over a 60 km wide region beneath the seaward dipping reflectors. The high velocities and densities of the lower crust are probably caused by igneous rock with a high MgO content that underplated the margin or heavily intruded pre‐existing crustal material during continental breakup. Both the addition of large quantities of melt to the margin and the subsidence curves inferred from postrift sediments sampled by Deep Sea Drilling Project (DSDP) boreholes on the margin indicate the presence of abnormally hot mantle beneath the area at the time of continental breakup. Comparison of the structure and subsidence history of the Edoras Bank margin with other areas of the NW European margin allows us to deduce the spatial extent and temporal variation of the thermal anomalies caused by the Iceland mantle plume during and following continental breakup. We find that at the time of breakup a thin layer of anomalously hot asthenospheric mantle ∼150°C hotter than normal lay beneath the 2000 km long rift, extending from the Lofoten margin off Norway in the north to the Edoras Bank margin in the south. Over a period of 5–10 m.y. following the onset of rifting, the mantle thermal anomaly decayed rapidly to only 50°C or so above normal as the thin layer of abnormally hot mantle decompressed beneath the rift and produced a large pulse of magmatism.
Kinematic GPS as a source for airborne gravity reduction in the airborne gravity survey of Switzerland
Authors: Klingelé E. E. , Cocard M. , Kahle H.‐G. , Halliday M.
Published: February 2004

An airborne gravity survey of Switzerland was performed in a joint project between the Swiss Federal Institute of Technology, Zurich and LaCoste and Romberg Gravity Meters, Inc. The survey was flown in a Twin‐Otter aircraft equipped by the Swiss Federal Directorate of Cadastral Surveying. The aircraft was outfitted with three Global Positioning System receivers, one for navigational purpose and two as sources of positioning, velocity, and vertical acceleration for airborne gravity reduction. Four receivers were installed on the ground as reference stations. The gravity data were recorded with a modified LaCoste and Romberg marine gravimeter at a sampling rate of 1 s. Modifications included anti‐alias filters and an absolute encoder for the measuring screw. The flights were performed at a barometric altitude of 5100 m above sea level. An airborne Bouguer anomaly map, computed at flight altitude with topographic corrections of up to 167 km and a density of 2670 kg/m3, is presented. Bouguer anomaly values at crossing points between lines flown at comparable altitudes (ΔH<10 m) show differences varying between 1.1 and 2.7 mGal (10−5 m/s2). Comparison of the ground Bouguer anomaly map, upward continued to 5100 m, and the one computed from airborne data shows a strong similarity both in amplitude and wavelength.
Quartz dislocation microstructure between 7000 m and 9100 m depth from the Continental Deep Drilling Program KTB
Authors: Dresen Georg , Duyster Johannes , Stöckhert Bernhard , Wirth Richard , Zulauf Gernold
Published: February 2004

We investigated the quartz microstructures from gneiss samples recovered from the German Continental Deep Drilling Program (KTB) main hole between 7000 m and the final depth of 9100 m. Optical microscopy and transmission electron microscopy (TEM) show similar microstructures for most of the studied profile. At the final depth, enhanced recovery is indicated by fewer dislocation tangles, fewer submicroscopic fluid inclusions, and well‐developed low‐angle grain boundaries. Between 7000 and 9100 m depth, the mean dislocation density is reduced from 4×109 cm−2 to 1×109 cm−2. Using dislocation density as a piezometer, the differential stress recorded in samples from 9100 m is estimated as approximately 140 MPa. Microstructures indicate that the drill hole reached the semibrittle transition zone and that strain is partitioned between brittle deformation, solution precipitation creep, and plastic flow. Differential stress estimates from in situ measurements extrapolated down to 9.1 km range from 170 to 220 MPa. Fluid injection induced microearthquakes do not seem to occur at a depth greater than 9 km, possibly indicating the absence of critically stressed brittle faults. Microstructural observations and differential stress estimates from entirely different techniques suggest that in situ differential stresses are not likely to increase with further depth. Stresses predicted from extrapolated quartz flow laws are mostly smaller for the low strain rates assumed for the KTB tectonic environment.
Crustal evolution of the KTB drill site: From oldest relics to the Late Hercynian granites
Authors: O'Brien Patrick J. , Duyster Johannes , Grauert Borwin , Schreyer Werner , Stöckhert Bernhard , Weber Klaus
Published: February 2004

The crustal unit penetrated during the German Continental Deep Drilling Program (KTB), the Zone of Erbendorf‐Vohenstrauss (ZEV), comprises an association of metabasites and paragneisses and orthogneisses that underwent high‐ and medium‐pressure metamorphic cycles during the early Palaeozoic. In this summary of the structural, petrological, geochemical and geochronological information from borehole and surface rocks, we show that geological models proposed prior to drilling have had to be significantly modified. Features of the ZEV, such as the dominant NW‐SE trending structures, Devonian (>370 Ma) medium‐pressure, amphibolite facies, metamorphism (0.6–0.8 GPa, ∼720°C), and earlier eclogite stage, are directly comparable with those of the nearby Bohemicum unit in western Bohemia. Intervening units, in contrast, exhibit NE‐SW trending structures and Carboniferous (315–325 Ma), low‐pressure, metamorphism: all units are cut by predominantly posttectonic granites (mostly <320 Ma). Earlier models to explain this contrast by means of a nappe emplacement of the ZEV, in the narrow time interval between the younger regional metamorphism and granite intrusion, could not be substantiated because, contrary to predictions, the same rock association was present in the whole of the drilled 9.1 km and therefore no nappe boundary was drilled. Instead, it is interpreted that shallow crustal levels reached by the ZEV, subsequent to the Devonian event, shielded most of it from the effects of the Carboniferous metamorphism. A lack of expected thermal and baric gradients within the drilled metamorphic pile is a result of repeated stacking of an original ∼3 km depth section in Mesozoic times. An important result of the geological investigations of the KTB project is the recognition that the present‐day geometry of the ZEV is a result of cold, shallow‐level, thin‐skinned tectonics that, in the absence of the view into the third dimension offered by the drill hole, could not have been accurately predicted from the fossil, high‐grade assemblages and structures seen at the surface.
Constraints on Macquarie Ridge tectonics provided by Harvard focal mechanisms and teleseismic earthquake locations
Authors: Frohlich Cliff , Coffin Millard F. , Massell Christina , Mann Paul , Schuur Catherine L. , Davis Scott D. , Jones Trevor , Karner Garry
Published: February 2004

In this study we evaluate teleseismically determined focal mechanisms and epicenters for earthquakes along the Macquarie Ridge Complex (MRC) from 45°S to 61°S and 155°E to 168°E, a region characterized by some previous investigators as undergoing subduction initiation. From 65 centroid moment tensors reported by Harvard, we develop statistical guidelines for choosing 26 which represent better determined, more reliable focal mechanisms for tectonic analysis. Although thrust mechanisms occur in the north, near Fiordland, elsewhere along the MRC the better determined mechanisms virtually all indicate that present‐day motion along most of the MRC is strike‐slip. This is consistent with sidescan sonar and multichannel reflection data collected between 50°S and 57°S on 1994 and 1996 cruises; the active plate boundary zone appears to be quite narrow (<5 km wide), and no active compressional features can be observed on the seafloor. If we determine a rotation pole for plate boundary motion using only slip vectors from better determined Harvard mechanisms along the MRC, the best fitting “instantaneous” pole is at 57.4°S, 179.4°E, about 2.5° north of the NUVEL‐I Australian‐Pacific pole, which averages motion over the last 3.0 my. If the MRC pole was formerly farther south than at present, this could explain the existence of relict features associated with crustal shortening, such as bathymetric highs and troughs; yet, the absence of active features such as thrust faults, etc., suggests ongoing compression or subduction initiation. We also carefully read arrival times for P phases for 53 earthquakes at 16 teleseismic stations, selected to represent a range of azimuths surrounding the earthquakes; we relocated these earthquakes using standard joint epicentral determination (JED) methods. While most of the better quality relocations lie on or very close to the Australia‐Pacific boundary as determined on the 1994 cruise, a few epicenters occur well away from the boundary, apparently on Cenozoic fracture zones. Thus, on the Macquarie Ridge Complex and other major strike‐slip boundaries (e.g., in California), it appears that the very largest earthquakes occur on the principal plate boundary fault but that other earthquakes, some quite large, may occur away from the boundary along zones of preexisting weakness.
Cutting of the European continental lithosphere: Plasticity theory applied to the present Alpine collision
Authors: Regenauer‐Lieb Klaus , Petit Jean‐Pierre
Published: February 2004

Presently active European Alpine collision has a history of more than 100 Myr convergence. We investigate the collisional contact problem of an apparently rigid African promontory (currently the Italy/Adria block (IAB)) steadily penetrating at 1 cm/yr, over at least the past 45 Myr, into the deformable European plate. We assume that neither continental subduction nor delamination occurs presently in the 300‐km‐wide contact zone enabling use of the indenter‐indentee approach. A key parameter is the aspect ratio h/a, defined by the width of the deformable foreland over the width of the indenter itself. This parameter controls the activation of two fundamentally different modes of deformation: (1) classical indentation mode for h/a≥4.4, described by previous authors, and having effect only in a portion of the deformable plate in the vicinity of the indenter and (2) cutting mode for h/a<4.4 giving plane strain longitudinal splitting of the entire deformable plate and plastic separation of the two halves. We use a matrix operator method to solve for slip lines in the mixed boundary value problem (obliquity, velocity, friction, stress) of the European Alpine case. Implications are as follows: (1) plane stress, vertical lithospheric thickening is the first indentation mode of continental collision; and (2) for the present ratio of h/a≈3 deformation is in the cutting mode and slow plane strain intraplate extension (rifting) is predicted. This mode explains the intraplate stress field obtained from borehole and earthquake data. Considering that the paleoindenter probably has been larger than the IAB it is inferred that h/a has been below 4.4 since at least the end of the Eocene and that consequently the intraplate graben system (Lower Rhine, Rhine, Bresse, Limagne) is due to the cutting of the European plate by Alpine collision.
Application of a unified rate and state friction theory to the mechanics of fault zones with strain localization
Authors: Sleep Norman H.
Published: February 2004

The formalism for rate and state friction is extended to represent fault zones where temperature, porosity, effective normal traction, and strain rate are functions of position (measured across the fault zone). A traditional form for the instantaneous coefficient of friction is retained,where μ0is the steady state coefficient of friction at shear strain rate,aandbare small constants,is the shear strain rate, ψ is a state variable that represents damage, and ψ0a normalizing factor. Percolation theory of cracked solids is used to justify a relation between porosity and the state variable of ψ=exp[(ϕ0‐f)/Cϵ], wherecϵis a dimensionless constant, and ϕ0is the porosity at a reference steady state strain rate ϵ0and at a reference temperature where Δ≡0 and a reference effective normal traction ΔP0. These relationships and percolation theory imply an evolution law for porosity of the (normalized) formwheretis time, ΔPis the effective normal traction,Cηis a material property (related to compaction viscosity) that depends on the temperature difference ΔTfrom reference condition, 0 indicates reference conditions, and η is a power law rheology exponent. The first term represents creation of porosity by frictional dilatancy while the second term represents closure of porosity by compaction. The effects of transient changes of pressure and temperature on the coefficient of friction are represented when the normalizing factor ψ0is ΔPnCη(0)/ΔPn0Cη(ΔT). The theory is complete in the sense that the complete earthquake cycle is represented and that there are no unmeasurable state parameters. The theory was applied to investigate earthquake quenching by fluid pressure decreases associated with frictional dilatancy and the related topic of strain localization and delocalization within fault zones. It was found that strain localization will occur whenb>a. Such strain localization tends to destabilize sliding within drained faults by reducing the effective value of the critical displacement. Fault zones that are hydraulically sealed from the country rock but internally hydraulically connected are also destabilized because strain localization reduces the fluid pressure decrease from frictional dilatancy. Two mechanisms that delocalize strain once sliding is well underway were investigated. Strain rate strengthening at high‐strain rates leads to a high strain zone that gradually broadens throughout an earthquake. An increase in the coefficient of friction with temperature leads to a high strain rate zone that moves through the fault zone from hot regions created by frictional heating to cold regions.
Late Mesozoic and Cenozoic thermotectonic evolution of the central Brooks Range and adjacent North Slope foreland basin, Alaska: Including fission track results from the Trans‐Alaska Crustal Transect (TACT)
Authors: O'Sullivan Paul B. , Murphy John M. , Blythe Ann E.
Published: February 2004

Apatite fission track data are used to evaluate the thermal and tectonic history of the central Brooks Range and the North Slope foreland basin in northern Alaska along the northern leg of the Trans‐Alaska Crustal Transect (TACT). Fission track analyses of the detrital apatite grains in most sedimentary units resolve the timing of structures and denudation within the Brooks Range, ranging in scale from the entire mountain range to relatively small‐scale folds and faults. Interpretation of the results indicates that rocks exposed within the central Brooks Range cooled rapidly from paleotemperatures 110° to 50°C during discrete episodes at ∼ 100±5 Ma, ∼60±4 Ma, and ∼24±3 Ma, probably in response to kilometer‐scale denudation. North of the mountain front, rocks in the southern half of the foreland basin were exposed to maximum paleotemperatures 110°C in the Late Cretaceous to early Paleocene as a result of burial by Upper Jurassic and Cretaceous sedimentary rocks. Rapid cooling from these elevated paleotemperatures also occurred due to distinct episodes of kilometer‐scale denudation at ∼60±4 Ma, 46±3 Ma, 35±2 Ma, and ∼24±3 Ma. Combined, the apatite analyses indicate that rocks exposed along the TACT line through the central Brooks Range and foreland basin experienced episodic rapid cooling throughout the Late Cretaceous and Cenozoic in response to at least three distinct kilometer‐scale denudation events. Future models explaining orogenic events in northern Alaska must consider these new constraints from fission track thermochronology.
The permeable crust: Geohydraulic properties down to 9101 m depth
Authors: Huenges E. , Erzinger J. , Kück J. , Engeser B. , Kessels W.
Published: February 2004

Both boreholes of the German Continental Deep Drilling Program encountered a fluid‐conducting crust over a 9101 m vertical profile. Low matrix permeability was observed on the laboratory scale (geometric mean value 7×10−20 m2 with something more than one decade standard deviation) and in situ higher values of the order of 5×10−18 to 3×10−16 m2. With respect to the scatter of measured matrix permeability there was no lithology dependence observed, whereas permeability parallel to the foliation (geometric mean value 3×10−19 m2) is significant higher than those perpendicular to the foliation (2×10−20 m2). With increasing depth the in situ permeability varies within a few orders of magnitude, showing, however, no clear depth dependence. The in situ permeability increases over 3 orders of magnitude while the effective pressure decreases by 50 MPa, whereas in contrast, the permeability of laboratory core specimens changes by 1 order of magnitude. The pilot drill hole “Vorboh‐rung” and the main drill hole “Hauptbohrung” communicate through a network of conductive fractures at the bottom hole level of the Vorbohrung as well as between both bottom hole sections. The formation pressure increases with a mean gradient of 11.5 MPa km−1 from surface to 103 ± 3 MPa to 9101 m depth. Overhydrostatic increasing pore pressure and so a strong stress reduction can be excluded as a reason for the low permeability decrease observed at the bottom of the hole. Formation pressure remains hydrostatic with respect to probable increasing salinity.
The thermal regime of the crystalline continental crust: Implications from the KTB
Authors: Clauser Christoph , Giese Peter , Huenges Ernst , Kohl Thomas , Lehmann Holger , Rybach Ladislaus , Šafanda Jan , Wilhelm Helmut , Windloff Karla , Zoth Gustav
Published: February 2004

An extensive geothermal research program within the German Continental Deep Drilling Program (KTB) covered an almost complete spectrum of experimental and theoretical aspects. The main results and conclusions are as follows: (1) Equilibrium temperature is 118.6°C at 4000 m in the KTB pilot borehole (KTB‐VB) and will be around 260°C at 9100 m in the KTB main borehole (KTB‐HB). Time required for thermal equilibration of the KTB‐HB to within 1% of the initial perturbation will be about 13–16 years. (2) The failure to predict temperature correctly for the KTB was mainly due to an unaccounted perturbation by a transient ground surface temperature history. (3) Pleistocene surface temperature variations affect the present‐day crustal temperature between 1.3 to 2.7 K up to a depth of 4000 m. Accordingly, present‐day heat flow density is systematically too low down to approximately 1500 m. Model simulations indicate that groundwater flow does not eliminate paleoclimatic signals, even though it may translate them to a depth incompatible with both their diffusive age and their amplitude. These results emphasize the importance of an adequate consideration of paleoclimatic effects for the interpretation of thermal data. (4) Lateral heat transport is significant when steep inclination and folding of the rock formations coincide with contrasts in thermal conductivity. This is indicated by typical variations in the vertical components of temperature gradient and heat flow density, such as in the KTB‐HB. In contrast, thermally relevant advection of heat is confined to the top 500–1000 m. In the vincinity of the KTB, this is about twice the maximum difference in elevation. (5) In the deeper crust, free convection systems require permeabilities greater than 10−17 m2 for large rock volumes, but simple numerical models indicate that the associated temperature regimes are imcompatible with KTB borehole data. (6) Heat production rate shows no systematic variation with depth and is related to lithology at the KTB as in other deep boreholes in crystalline rock. Numerical models, using heat production rate derived from seismic velocities, yield temperatures compatible with KTB borehole data.
Permeability of partially molten upper mantle rocks from experiments and percolation theory
Authors: Faul Ulrich H.
Published: February 2004

Experiments with olivine and a basaltic melt were conducted to analyze the melt distribution in partially molten aggregates at low melt fractions. Grain size and melt distribution at the start of an experiment are transient; to reach steady state conditions requires run durations from 2 to 3 weeks at 1 GPa and 1300° to 1400°C. Quantitative analysis of the melt distribution from backscattered electron images of postrun samples shows that most of the melt resides in low aspect ratio, disk‐shaped inclusions on two‐grain boundaries at melt contents from 0.8 to 3.3 vol. %. Tubules along three‐grain edges most likely interconnect the melt at all melt fractions, but their shape and size varies even at the lowest melt fractions due to the effects of anisotropy and nonuniform grain sizes. These triple junction tubules can therefore not be approximated by uniform, equally spaced cylinders with near constant cross‐sectional area for the purpose of calculating the permeability of the matrix. Moreover, the tubules are much smaller than the disk‐shaped inclusions; tubule‐like geometries contain only ∼10% of the total melt content in each experiment. The permeability of the triple junction tubule network only is therefore low (k ∼ 10−17 m2), and segregation velocities are less than 1 mm yr−1 The permeability of the aggregate increases substantially only after the disk‐shaped inclusions become interconnected. This melt fraction is calculated from percolation theory, using parameters of the melt inclusions obtained from the backscattered electron images. The threshold melt content, as calculated from the experimental melt distribution, lies between 2 and 3 vol. %.
Effect of Ca2+ and Fe2+ on the equation of state of MgSiO3 orthopyroxene
Authors: Hugh‐Jones D. A. , Angel R. J.
Published: February 2004

Room temperature equations of state (EOSs) have been determined for synthetic and natural orthopyroxenes on and near the (Mg,Fe)SiO3 join by single‐crystal X‐ray diffraction to a maximum pressure of 11 GPa. The values of the room pressure bulk moduli, K0,T, and its pressure derivative, K0′, are shown to be independent of the form of the equation of state used to fit the data. Variation in the Fe2+ content of the orthopyroxene has very little effect on either K0,T or K0′, with the EOS parameters of synthetic FeSiO3 orthoferrosilite being K0,T=95.1 (3.0) GPa, and K0′=10.6 (1.6), both within one combined experimental estimated standard deviation of those previously determined for synthetic MgSiO3 orthoenstatite. However, substitution of even very small amounts of Ca2+ (and Al3+) to the orthopyroxene structure causes an increase in K0,T by some 14%. The density difference between the synthetic MgSiO3 and natural orthoenstatites (calculated from the equations of state at 10 GPa) is of the order of 3.4%, with a corresponding unit‐cell volume difference of the order of ∼0.3%. It is concluded that pure MgSiO3 orthoenstatite is not a good representation of the pyroxene component of the Earth's upper mantle.
Resolution of fault slip along the 470‐km‐long rupture of the great 1906 San Francisco earthquake and its implications
Authors: Thatcher Wayne , Marshall Grant , Lisowski Michael
Published: February 2004

Data from all available triangulation networks affected by the 1906 earthquake have been combined to assess the trade‐off between slip resolution and its uncertainty and to construct a conservative image of coseismic slip along the rupture. Because of varying network aperture and station density, slip resolution is very uneven. Although slip is determined within uncertainties of ±1.0 m along 60% of the fault, constraints are poor on the remaining, mostly offshore portions of the rupture. Slip decreases from maxima of 8.6 and 7.5 m at Shelter Cove and Tomales Bay to 4.5 m near Mount Tamalpais and 2.7 m at Loma Prieta. The geodetically derived slip distribution is in poor agreement with estimates based on analysis of S wave seismograms, probably because these waves register only 20–30% of the total seismic moment obtained from longer‐period surface waves. Consideration of a range of fault geometries for 1906 slip near Loma Prieta indicates right‐lateral motions lie between 2.3 and 3.1 m. These values are considerably greater than the 1.5 m of measured surface slip on which several assessments of high earthquake hazard for this fault segment were based. This factor, along with the absence of 1989 slippage where 1906 surface slip was used to make the forecasts, casts doubt on some claims of success in predicting the 1989 M=6.9 Loma Prieta earthquake.