bluuebunny/crossref_metadata_2025 huggingface dataset:
Earthquake source scaling relationships from −1 to 5 ML using seismograms recorded at 2.5‐km depth
Authors: Abercrombie Rachel E.
Published: February 2004

The scaling relationships of earthquake sources less than about magnitude 3 have been the subject of considerable controversy over the last two decades. Studies of such events have shown a tendency for the constant stress drop, self similarity of larger earthquakes to breakdown at small magnitudes, and an apparent minimum source dimension of about 100 m has been observed. Other studies showed that this apparent breakdown in scaling could be an artifact of severe near‐surface attenuation, limiting the spatial resolution of surface data. In this study, source parameters are determined for over 100 nearby, tectonic earthquakes, from recordings at a depth of 2.5 km (in granite) in the Cajon Pass scientific drill hole, southern California. Comparison of the seismograms recorded at this depth with those at the wellhead clearly demonstrates the effect of the severe attenuation in the upper kilometers of the Earth's crust. Source parameters are calculated by spectral modeling of three‐component P and S waves, assuming four source models based on the Brune ω−2 (n = 2) model. In model l, n = 2 is fixed, and Q of P and S waves is determined to be 912 (581–1433) and 1078 (879–1323), respectively (the numbers in parentheses are ±1 standard deviation). In model 2, QP = QS = 1000 is assumed and n is allowed to vary. The ω−2 model is a good average for the data set, but there is some real scatter supported by the data. In model 3, QP = QS = 1000 is also assumed and ω−2 is constrained, and in model 4, attenuation is ignored and n is allowed to vary. Source dimensions of less than 10m are observed for all four models, 10 times smaller than the proposed “minimum”. No breakdown in constant stress drop scaling is seen in the downhole data (approximately ML‐1 to 5.5, M0 = 109 ‐ 1016 Nm). The ratio between radiated seismic energy (estimated by integrating the velocity squared spectra with adequate signal bandwidth) and seismic moment appears to decrease gradually with decreasing moment in the magnitude range −1 to 7. This is not incompatible with a constant stress drop but could result from errors in calculating energy. The ratio of the S wave energy to that radiated by the P waves is about 14, after correction for attenuation. This low value is consistent with the corner frequency shift of about 1.3. This corner frequency shift is observed for all four source models and therefore is interpreted as being source controlled.
The continental collision zone, South Island, New Zealand: Comparison of geodynamical models and observations
Authors: Beaumont Christopher , Kamp Peter J. J. , Hamilton Juliet , Fullsack Philippe
Published: February 2004

The South Island zone of oblique continent‐continent convergence occurs along a 400 km‐long section of the modern Australia‐Pacific plate boundary zone, across which about 50 km of shortening has been accommodated since about 10 Ma. The orogen comprises a central mountain range (Southern Alps) flanked on both sides by what are interpreted to be foreland basins. Two essential features that characterize the orogen are (1) the degree of denudation that accompanied deformation, and (2) a fundamental structural asymmetry. The architectural asymmetry of the orogen can be explained by plane strain, finite element models of continental convergence incorporating mantle subduction. Comparison of model and orogen polarity implies that Pacific plate mantle subducts. The models predict two crustal‐scale dipping shear zones that form above the point where the Pacific mantle subducts. The localized one more distant from the incoming plate (retro‐step‐up shear zone) corresponds to the Alpine fault, whereas its conjugate (pro‐step‐up shear zone) corresponds to the distributed strain and thrusting along the eastern margin of the mountain belt. Parameters that modify the model boundary conditions (top surface, degree of denudation; basal zone, subduction load, crust‐mantle velocity discontinuity, subduction of lower crust, mantle retreat, and distributed decrease in mantle velocity) and the internal strength of the crust (two‐layer crust with moderate coupling, temperature distribution, strain weakening) are varied in a series of numerical model calculations that establish the combination of material properties and boundary conditions that lead to different cross sectional architectures of the modeled collision zone. In turn, these are compared with observations about the South Island orogen. The calculations show how the style and extent of deformation across the whole orogen depend on the rheological properties of the crustal layer and on the balance between its internal strength and the combined effects of the boundary and gravitational stresses. North to south along‐strike differences in the width and two‐dimensional architecture of the orogen, simulated in the experiments by varying the model parameters, can be explained by a combination of southward increases in preconvergent crustal thickness, geothermal gradient, convergence, and potentially subduction retreat, with the added possibility of a southward decrease in the component of lower crustal subduction.
Dilatancy, compaction, and slip instability of a fluid‐infiltrated fault
Authors: Segall Paul , Rice James R.
Published: February 2004

We analyze the conditions for unstable slip of a fluid infiltrated fault using a rate and state dependent friction model including the effects of dilatancy and pore compaction. We postulate the existence of a steady state drained porosity of the fault gouge which depends on slip velocity as ϕss = ϕ0 + εln(v/v0) over the range considered, where v is sliding velocity and ε and v0 are constants. Porosity evolves toward steady state over the same distance scale, dc, as “state.” This constitutive model predicts changes in porosity upon step changes in sliding velocity that are consistent with the drained experiments of Marone et al. (1990). For undrained loading, the effect of dilatancy is to increase (strengthen) ∂τss/∂lnv by μssε/(σ – p)β where μss is steady state friction, σ and p are fault normal stress and pore pressure, and β is a combination of fluid and pore compressibilities. Assuming ε ∼ 1.7×10−4 from fitting the Marone et al. data, we find the “dilatancy strengthening” effect to be reasonably consistent with undrained tests conducted by Lockner and Byerlee (1994). Linearized perturbation analysis of a single degree of freedom model in steady sliding shows that unstable slip occurs if the spring stiffness is less than a critical value given by kcrit = (σ‐p)(b‐a)/dc ‐ εμssF(c*)/βdc where a and b are coefficients in the friction law and F(C*) is a function of the model hydraulic diffusivity c* (diffusivity/diffusion length2). In the limit c* →∞ F(c*) → 0, recovering the drained result of Ruina (1983). In the undrained limit, c* → 0, F(c*) → 1, so that for sufficiently large ε slip is always stable to small perturbations. Under undrained conditions (σ – p) must exceed εμss/β(b ‐ a) for instabilities to nucleate, even for arbitrarily reduced stiffness. This places constraints on how high the fault zone pore pressure can be, to rationalize the absence of a heat flow anomaly on the San Andreas fault, and still allow earthquakes to nucleate without concommitant fluid transport. For the dilatancy constitutive laws examined here, numerical simulations do not exhibit large interseismic increases in fault zone pore pressure. The simulations do, however, exhibit a wide range of interesting behavior including: sustained finite amplitude oscillations near steady state and repeating stick slip events in which the stress drop decreases with decreasing diffusivity, a result of dilatancy strengthening. For some parameter values we observe “aftershock” like events that follow the principal stick‐slip event. These aftershocks are noteworthy in that they involve rerupture of the surface due to the interaction of the dilatancy and slip weakening effects rather than to interaction with neighboring portions of the fault. This mechanism may explain aftershocks that appear to be located within zones of high mainshock slip, although poor resolution in mainshock slip distributions can not be ruled out.
Insights into the volcanic arc mantle wedge from magnesian lavas from the Kamchatka arc
Authors: Hochstaedter Alfred G. , Kepezhinskas Pavel , Defant Marc , Drummond Mark , Koloskov Alexander
Published: February 2004

Active volcanism in the Kamchatka arc occurs where the Pacific Plate subducts beneath the Kamchatka peninsula south of its junction with the Aleutian arc. Most volcanism occurs within the Central Kamchatka Depression (CKD), a large graben oriented parallel to the trench, and along the Eastern Volcanic Front (EVF), located south and east of the CKD and closer to the trench. Differentiation trends range from calc‐alkaline to tholeiitic. Fractionation of a mineral assemblage, including olivine, clinopyroxene, and orthopyroxene, produces the tholeiitic trend, whereas separation of amphibole and magnetite, along with possible crustal assimilation, produces the calc‐alkaline trend. A suite of near‐primitive high‐Mg basalts provides geochemical records of mantle sources and processes unobscured by differentiation. Rare earth element (REE) patterns range from slightly depleted ((Ce/Yb)n = 0.8–1.5) to slightly enriched ((Ce/Yb)n = 1.5–3.5). Rocks with the depleted REE patterns occur at the volcanic front in regions where a volcano or volcanic chain exists behind the volcanic front. Lavas with relatively enriched REE patterns occur behind the volcanic front and along portions of the volcanic front where behind‐the‐front volcanism is absent. Modeling of trace element abundances normalized to 10% MgO indicates that the rocks with the depleted REE patterns are derived from a more depleted source, inferred to represent refractory source material remaining after a previous generation of melt extraction within the arc. Mantle source material apparently convects into the mantle wedge from the rear, producing relatively enriched magmas when it melts for the first time. Relatively depleted magmas are produced if a second period of melting ensues as the mantle reaches the volcanic front.
Fracture propagation paths under mixed mode loading within rectangular blocks of polymethyl methacrylate
Authors: Cooke Michele L. , Pollard David D.
Published: February 2004

Mixed mode I + III loading of a fracture front results in out‐of‐plane propagation into echelon stepping fractures. Because a planar fracture geometry is the exception rather than the rule, and because the introduction of even a minor component of mode II or III loading is known to promote out‐of‐plane propagation, an understanding of mixed mode fracture growth is imperative to analyze fracture behavior. We have loaded cracks in mixed mode I + III within polymethyl methacrylate (PMMA or Plexiglas) rectangular blocks resembling conceptual fracture mechanics models of mixed mode loading and have analyzed the resulting geometries. The observed angle of twist of echelon fractures from the parent crack plane increases with the ratio KIII/KI and falls below theoretical predictions. Fracture propagation paths depend not only on the load ratio applied but also on sample geometry, loading configuration, and interaction among growing fractures. Sample geometry and loading configuration are approximately accounted for using analytical determinations of the stress intensity factors. We propose that interaction among growing fractures may contribute to the discrepancy between theoretically predicted twist angles and those observed in these and other mixed mode I + III experiments. Analysis of these experimental results has motivated the design of a new sample and loading configuration to test the propagation paths of uniformly loaded mixed mode I + III fractures.
Finite amplitude folding of a continuously viscosity‐stratified lithosphere
Authors: Zuber M. T. , Parmentier E. M.
Published: February 2004

We study the development of finite amplitude folding of the lithosphere through the application of finite element models characterized by both discontinuous (layer over half‐space) and continuous (strength envelope) viscosity distributions. Both models include strongly strain rate‐dependent rheologies that approximate slip on pervasive faults. Folding with layers of uniform, contrasting viscosities is driven solely by the magnitudes of discontinuities in viscosity at layer interfaces; however, folds may also develop in a medium with a continuously varying viscosity distribution. The net driving force for folding in a region of continuous viscosity variation is the same as the driving force across a discontinuity with the same net viscosity contrast. Continuous and discontinuous viscosity variations give essentially identical fold growth rates if the depth over which the viscosity varies is small compared to the wavelength of folding or the depth of the viscosity variation. In the uniform viscosity layer model, flexural bending of the layer results in a state of extension on topographic highs and compression in topographic lows. In contrast, the strength envelope model is characterized by significant penetrative shortening near the surface that results in a state of compression everywhere along the fold. This model predicts layer thickening beneath fold troughs, as observed in the intraplate deformation zone in the central Indian Ocean. Buoyancy forces retard the rate of fold amplification but are inadequate to prevent folding for a range of realistic lithospheric rheological structures.
On the effect of continents on mantle convection
Authors: Guillou Laurent , Jaupart Claude
Published: February 2004

At the Earth's surface, continents and oceans impose different thermal boundary conditions at the top of the mantle. Laboratory experiments are used to investigate the consequences of this for mantle convection. The upper boundary of the experimental tank was made of copper plates enforcing a fixed temperature and had a conductive lid of finite width in the middle. Beneath this lid, the thermal boundary condition was of the “mixed” type, with a Biot number depending on the dimensions and thermal conductivity of the lid. Experimental values of the Biot number were scaled to Earth values. Experiments were run for a large range of Rayleigh numbers, from 104 to 107, and for several lid widths. The effects of temperature‐dependent viscosity and of the shape of the lid were investigated. At steady state, in all cases, there is an upwelling beneath the conductive lid, which feeds two symmetrical and elongated convective cells. Three different dynamic regimes were identified as a function of Rayleigh number, independently of the lid width. At Rayleigh numbers lower than 1.2 105, the upwelling is steady both in geometry and temperature structure. At Rayleigh numbers between 1.2 105 and 2 106, this central upwelling is intermittent. At larger values of the Rayleigh number, there is no longer a simple upwelling structure, but a set of small plumes rising together and distorted by a cellular circulation of large horizontal extent. Thus the conductive lid always imposes a large‐scale flow pattern. The length of these convective cells is a function of lid width. It is equal to the lid width at large values and decreases to the Rayleigh‐Bénard value as the lid width decreases to zero. A fluid loop model explains the most important features of this form of convection. The cell length is such that the upwelling temperature is minimized for a given Rayleigh number and lid width and is an increasing function of lid width and a decreasing function of Rayleigh number. Transient experiments demonstrate that the large‐scale flow structure develops rapidly with even small horizontal temperature differences. Implications for the Earth are that large‐scale convection cells exist in conditions which, in the absence of continents, would probably lead to a chaotic convection pattern dominated by plumes. At high Rayleigh number, continental breakup is effected by a large‐scale line upwelling structure which includes a number of individual plumes.
Numerical modeling of a highly explosive source in an elastic‐brittle rock mass
Authors: Donzé Frédéric , Magnier Sophie‐Adélaïde , Bouchez Jacques
Published: February 2004

A highly explosive source is input in an elastic‐brittle rock mass and the resulting near‐source complexities are considered. This study is undertaken with a three‐dimensional numerical model similar to the discrete element method. This model can simulate the creation of the damage zone with the associated block motions as well as the elastic wave propagation. Free‐field ground motion data are recorded to determine the reduced displacement potential (RDP). In a fully elastic numerical model, as expected, the RDP computation corresponds to the analytical one‐dimensional problem of a spherically symmetric explosive source acting in a homogeneous elastic medium. When fracturing occurs, during the creation of the damage zone, block motions are observed, and the RDP computations are affected. For an ideal system which conserves energy, the potential elastic energy is converted to kinetic energy during the fracturing process resulting in an apparent RDP which is larger than in the homogeneous elastic case. This three‐dimensional approach is undeniably advantageous where a global description of the heterogenous effects of block motions and the continuous aspects of wave propagation are considered simultaneously.
The 1964 Prince William Sound earthquake: Joint inversion of tsunami and geodetic data
Authors: Johnson Jean M. , Satake Kenji , Holdahl Sanford R. , Sauber Jeanne
Published: February 2004

The 1964 Prince William Sound (Alaska) earthquake, Mw = 9.2, ruptured a large area beneath the continental margin of Alaska from Prince William Sound to Kodiak Island. A joint inversion of tsunami waveforms and geodetic data, consisting of vertical displacements and horizontal vectors, gives a detailed slip distribution. Two areas of high slip correspond to seismologically determined areas of high moment release: the Prince William Sound asperity with average slip of 18 m and the Kodiak asperity with average slip of 10 m. The average slip on the fault is 8.6 m and the seismic moment is estimated as 6.3 × 1022 N m, or over 75% of the seismic moment determined from long‐period surface waves.
Inversion of three‐dimensional wide‐angle seismic data from the southwestern Canadian Cordillera
Authors: Zelt B. C. , Ellis R. M. , Clowes R. M. , Hole J. A.
Published: February 2004

Seismic refraction/wide‐angle reflection data were recorded on a triangular array in southwestern British Columbia centered on the boundary between the Coast Belt to the southwest and the Intermontane Belt to the northeast. The experiment, part of the Lithoprobe Southern Cordillera transect, enabled determination of the three‐dimensional (3‐D) velocity structure of the crust and upper mantle. An algorithm for the inversion of wide‐angle seismic data to determine 3‐D velocity structure and depth to reflecting interfaces is developed. The algorithm is based on existing procedures for the inversion and forward modeling of first arrival travel times and forward modeling of reflection travel times, including (1) forward modeling using a 3‐D finite difference algorithm; and (2) a simple velocity model parameterization for the inversion which eliminates the need to solve a large system of equations. The existing procedure is extended to allow (1) the inversion of reflection times to solve for depth to a reflecting interface and/or velocity structure; (2) the inversion of first arrival travel times to solve for depth to a refracting interface; and (3) layer stripping. Application of the algorithm to southern Cordillera data uses Pg to constrain upper crustal velocity structure, PmP to constrain lower crustal velocity structure and depth to Moho, and Pn to constrain upper mantle velocities and depth to Moho. The 3‐D velocity model for the southwestern Canadian Cordillera is characterized by (1) significant lateral velocity variations at all depths that do not, in general, correlate with surface geological features or gravity data; (2) a relatively high velocity middle and lower crust in the southwestern part of the study area which correlates with a strong relative gravity high and outlines the eastern extent of lower Wrangellia, an accreted terrane forming the Insular Belt to the west; (3) a narrow zone of slower velocity in the lower crust and change in crustal thickness associated with the Fraser Fault system, lending additional support to the view that it is a crustal penetrating fault; (4) an average upper mantle velocity of 7.85 km/s; and (5) a depth to Moho of 33–36 km in the Intermontane Belt and 36–38 km throughout most of the Coast Belt, decreasing in the west to 33 km near the Insular‐Coast contact. Horizontal velocity structure slices and an interpreted cross section based on these and other results show the complexity of crustal structure in the region.
Trench investigations of the 1915 Fucino earthquake fault scarps (Abruzzo, central Italy): Geological evidence of large historical events
Authors: Michetti Alessandro M. , Brunamonte Fabio , Serva Leonello , Vittori Eutizio
Published: February 2004

Earthquake hazard analyses in Italy are mainly based on historical seismic catalogs. However, the completeness of the Italian catalog, which spans more than 2000 years, progressively decreases going back in time. In the Fucino basin, historical research has not produced evidence of large earthquakes before the January 13, 1915, event (Ms = 7.0; 33,000 people killed). This event generated surface faulting along at least two parallel Quaternary normal faults bounding the eastern side of the basin (the Celano‐Gioia and Parasano‐Cerchio faults), over an end‐to‐end length of about 23 km, with a downthrow of 30–90 cm toward southwest. To improve geological aspects of the seismic history of this area, we trenched one of the coseismic scarps along the Celano‐Gioia fault. At a trench site, near the village of San Benedetto de' Marsi, the fault traverses the area formerly covered by the Fucino Lake, drained for farming in 1875 A.D. The evidence collected in the trenches suggests at least two large events in historical times before 1915. Radiocarbon dates constrain the age of event B between the 6th and the 9th centuries A.D., possibly coinciding with the event felt in Rome in 801 A.D. [Molin and Guidoboni, 1989]. Event A probably occurred after the 10th century A.D. and almost certainly before the 1349 A.D. earthquake, because historical studies show (1) that event A was not produced during the earthquake sequence that occurred in central Italy in 1349 A.D. [Molin and Guidoboni, 1989] and (2) that the post‐1349 seismic catalog can be regarded as complete for surface faulting earthquakes. Surface displacements determined for events A and B, compared to the 1915 surface faulting, indicate that M > 6.5 earthquakes were associated with each rupture. These observations suggest that the return period for large earthquakes in the Fucino basin is shorter than that indicated by the historical record alone. The Fucino example clearly shows that the exceptionally long historical record in Italy is still inadequate for a comprehensive seismic hazard characterization.
The interaction of mantle plumes with surface thermal and chemical boundary layers: Applications to hotspots on Venus
Authors: Smrekar Suzanne E. , Parmentier E. Marc
Published: February 2004

Large volcanic swells on Venus are believed to be a manifestation of mantle upwelling, or hotspots. The study of these regions provides important information on the interior of the planet. Numerical experiments are carried out to examine the interaction of mantle plumes with the thermal lithosphere and a layer of depleted mantle, a product of pressure‐release melting, using an axisymmetric finite element and finite difference code that incorporates temperature‐dependent viscosity and pressure‐release melting. The lithosphere is defined as a high‐viscosity lid; plumes are initiated and maintained by a prescribed temperature at the base of the computational domain. The topographic uplift, the geoid‐to‐topography ratio, and the volume of pressure‐release melt are compared to estimated values for possible hotspots on Venus to constrain the properties of plumes and the lithosphere. The effects of lithospheric thickness, depleted layer viscosity and thickness, mantle temperature, and plume temperature and duration on the surface observables are predicted. Models with a thermal lithospheric thickness of approximately 100–150 km are consistent with observations, assuming a mantle temperature of 1300°C, a maximum plume temperature of approximately 1500°C, and mantle plume durations of 150–250 m.y. To be consistent with estimated volumes of volcanics at Venusian hotspots, a significantly thinner thermal lithosphere requires a much cooler mantle; a thicker lithosphere requires a hotter mantle. Models with a depleted layer thickness of 100–250 km, in addition to a 100‐km thick thermal lithosphere, predict the range of parameters found on Venus. Interpretation of large volcanic swells on Venus based on the evolutionary sequence predicted here implies that Beta, Atla, Western Eistla, and Imdr Regiones overlie either active or recently active mantle plumes. The geoid‐to‐topography ratio at Beta Regio is much larger than values found at other hotspots and may indicate that it is in an early stage of evolution or that the chemical or thermal boundary layers are significantly thicker. Bell, Dione, and Themis Regiones appear to represent very late stage, possibly now extinct, hotspots. An estimate of the total plume buoyancy flux for Venus is much lower than the value obtained for Earth. The small areal distribution of volcanism is consistent with a low level of ongoing resurfacing at a few active hotspots.
Magnetostratigraphic dating of the uplifted atoll of Maré: Geodynamics of the Loyalty Ridge, SW Pacific
Authors: Guyomard T. S. , Aïssaoui D. M. , McNeill D. F.
Published: February 2004

The Loyalty Islands (SW Pacific) are uplifted as they are progressively affected by the lithospheric flexure of the Australian plate, before its subduction under the New Hebrides Arc. These geodynamic changes are constrained by magnetostratigraphically dating two sections from Maré Island, where mineral extractions coupled with rock‐magnetic experiments suggest that the magnetic remanence is mostly carried by a mixture of single‐domain to multidomain magnetite/maghemite. With the help of faunal determinations and radiometric dating, the sequences of polarity reversals, correlated to the geomagnetic polarity timescale, range from the top of Chron C4n (late Miocene) to the top of the Gauss Chron (late Pliocene). This new chronostratigraphy refines the timing of two distinctive carbonate units (rhodolith platform/coral reefs) separated by a hardground whose transition is known to coincide approximately with a regional event around the Miocene/Pliocene boundary. The magnetostratigraphic dating indicates that the hardground represents about a 1.9 m.y. hiatus and suggests variable sedimentation rates ranging from 4.7 to 65.4 m/m.y. during the atoll construction. The lithospheric bulge seems to have influenced the evolution of Maré Island some 3.1 m.y. ago, leading to a diachronous emersion of the northeast and southwest rim of the atoll with a mean uplift rate of the order of 4 cm/kyr.
Nonlinear elasticity and stress‐induced anisotropy in rock
Authors: Johnson P. A. , Rasolofosaon P. N. J.
Published: February 2004

The elastic nonlinear behavior of rocks as evidenced by deviations from Hooke's law in stress‐strain measurements, and attributable to the presence of mechanical defects (compliant features such as cracks, microfractures, grain joints), is a well‐established observation. The purpose of this paper is to make the connection between the elastic nonlinearity and stress‐induced effects on waves, in this case uniaxial‐stress‐induced transverse isotropy. The linear and nonlinear elastic coefficients constitute the most condensed manner in which to characterize the elastic behavior of the rock. We present both the second‐ and the third‐order nonlinear elastic constants obtained from experimental data on rock samples assumed homogeneous and isotropic when unstressed. As is normally the case, the third‐order (nonlinear) constants are found to be much larger than the second‐order (linear) elastic constants. Contrary to results from intact homogeneous solids (materials without mechanical defects), rocks exhibit weak to strong nonlinearity and always in the same manner (i.e., an increase of the moduli with pressure). As a consequence the stress‐induced P wave anisotropy and S wave birefringence can be large. The stress‐induced P wave anisotropy appears to be much larger than the S wave birefringence. The fast direction is parallel to the stress direction, and the anisotropy goes as sin2 θ, θ being the angle between the propagation direction and the stress direction. Experiments on rocks indicate that at low applied stresses, the proportionality of the stress and the induced S birefringence and P anisotropy predicted by theory is well corroborated.
Modeling of the Acoustic Reverberation Special Research Program deep ocean seafloor scattering experiments using a hybrid wave propagation simulation technique
Authors: Robertsson Johan O. A. , Levander Alan , Holliger Klaus
Published: February 2004

Quantitative modeling of bottom‐interacting ocean acoustic waves is complicated by the long propagation ranges and by the complexity of the scattering targets. We employ a two‐dimensional (2‐D) hybrid technique combining Gaussian beam, finite difference, and Kirchhoff integral solutions of the wave equation to simulate ocean acoustic experiments within half of a convergence zone in the SOFAR channel. The 2‐D modeling approach is reasonable due to the one‐dimensional (1‐D) velocity distribution in the water column and the strong lineation of the seafloor morphology parallel to the mid‐ocean ridges. Full‐waveform modeling of ocean acoustic data requires that the topography and the material properties of the seafloor are available at scales that are several orders of magnitude smaller than typical bathymetric sampling rates. We have therefore investigated the effects on the ocean acoustic response of a stochastic interpolation scheme used to generate seafloor models. For typical grazing angles of the incident wave field (approximately 5°–20°), we found that different stochastic realizations of the same seafloor segment (sampled at 200 m) yield an intrinsic uncertainty of the order of 3–8 dB in amplitude and 0.1–0.3 s in time for individual prominent events in the reverberant acoustic field. Hybrid simulations are compared to beam‐formed ocean acoustic data collected during the Acoustic Reverberation Special Research Program (ARSRP) cruises. Side lobe noise in the observed acoustic data is simulated by adding band‐limited white noise at −30 dB relative to the maximum intensity in the synthetic data. Numerical simulations can be limited to the response of only one of the mirror azimuth beams provided that the experimental geometry is suitably chosen. For the 2‐D approximation to be valid, the cross‐range resolution of the observed data must be smaller than the characteristic scale of seafloor lineations, and the beams of interest must be approximately perpendicular to the dominant structural grain. Under these premises, the arrival time and maximum intensity of the observed backscattered acoustic events can be modeled within 0.3 s and 5–10 dB, respectively. The mismatch in arrival time is interpreted to be due predominantly to intrinsic uncertainties in the stochastic interpolation of the seafloor profiles, whereas the fact that the intensity of the backscattered events is systematically overestimated is attributed to 3‐D effects within the “footprint” of the beam and/or to underestimated noise levels.
Geometry of continental normal faults: Seismological constraints
Authors: Braunmiller Jochen , Nábělek John
Published: February 2004

Teleseismic body waves from large earthquakes are used to study the downdip geometry of continental normal faults in the Aegean. Waveform modeling techniques together with rigorous statistical tests are applied to put firm bounds on the amount of downdip curvature of these faults and the role of coseismic slip on a basal detachment. Synthetic modeling shows that good azimuthal station coverage and inclusion of SH waves are necessary to resolve fault curvature. The data indicate ruptures of the Aegean events occurred on planar faults extending across the entire brittle portion of the crust. No seismogenic low‐angle detachment faulting at the base of the upper crust was detected for these events. Decoupling of the brittle upper crust from the plastic lower crust probably occurs aseismically in a ductile fashion.
A simple discrete element model for large multiplet earthquakes
Authors: Nomanbhoy Nazli , Ruff Larry J.
Published: February 2004

Adjacent segments of the plate boundary may fail separately as earthquakes that occur very close in time. When the time between events is a small fraction of the recurrence interval, then the sequence is referred to as “doublets” or “multiplets”. In the first part of this paper, we determine the frequency of occurrence of large multiplet earthquakes. We count the number of multiplet earthquakes present in the Abe catalogue with aftershocks removed (MS ≥ 7.1) using a series of space, time and magnitude filters. We compare this number to the number of multiplet earthquakes present in the Abe catalogue where origin times of earthquakes are randomized and epicentral locations and magnitudes are kept constant. The difference between the observed and random number of multiplets gives us the percentage of multiplets that do not occur randomly. We find that this percentage ranges from 2% to 7% of all large earthquakes based on the best choice of space, time and magnitude filters. In the second part of this paper, we model multiplet earthquakes using three simple frictional slider models. The models consist of slider blocks resting on a frictional “conveyer belt” and interconnected by springs. The seismogenic portion of the plate interface is represented by blocks that exhibit stick‐slip behavior (asperities). Aseismic creep within the seismogenic zone is represented by “creepers”. The “creepers” are blocks that obey a simple linear creep law. The three models differ in that the positions of the creeping blocks within the system of asperities and creepers are different. We find that model 3 produces a complex sequence of events consisting of single, double and multiplet earthquakes. These synthetic sequences of events are comparable to the earthquake cycle observed along Nankai trough. Model 3 produces a high percentage of multiplet earthquakes when the creeper is allowed to relax within a small fraction of the maximum recurrence time of the event sequence. This implies that the details of the frictional law invoked for the creeper blocks does not influence the model results significantly. The time evolution of model 3 is compared to the postseismic phase of the loading cycle at Muroto Point, Japan. We suggest that the rapid postseismic deformation that occurs immediately after the Nankaido earthquake is due to a rapid creeping zone that is in this region.
Natural long‐term annealing of the zircon fission track system around a granitic pluton
Authors: Tagami Takahiro , Shimada Chica
Published: February 2004

We measured 24 ages and 14 length distributions of fission tracks in zircons separated from basement sandstones around a granitic pluton to investigate natural long‐term annealing behaviors of the zircon fission track (ZFT) system. Samples were collected from three Cretaceous coherent units of the Shimanto accretionary complex, Shikoku, Japan, along an eastward traverse at distances of between 1.6 and 10.6 km from the intrusion contact of 15 Ma Takatsukiyama Granite exposing about 5 km in diameter. Sampling was carried out around the expected position of the transition zone of track retention, i.e., the fossil partial annealing zone (FPAZ), based on previous regional ZFT age distributions. Approaching the contact, original ZFT ages of ∼100 Ma show a continuous reduction to ∼15 Ma at ∼3 km distance. Their single‐grain age distributions accordingly change from widespread multimodal patterns reflecting provenance ages to unimodal ones characterizing the secondary heating by the granite intrusion. Mean and distributions of ZFT lengths also show notable variations within the FPAZ, as a result of mixing both preexistent tracks that were shortened by the Miocene heating and unannealed ones having original, long lengths that have been formed and accumulated after the cooling of the pluton. These facts, along with observed track segmentation and annealing anisotropy, suggest that the shortening patterns of tracks in nature (a heating duration of 105–106 years, estimated from thermal modeling) are indistinguishable from those previously demonstrated at laboratory timescales. This basically validates the conventional approach of extrapolating laboratory‐based kinetic parameters to geologic timescales. The observed FPAZ and thermal modeling results, in conjunction with laboratory heating data previously reported and recent constraints from an ultradeep borehole, favor a relatively narrow temperature range for the partial annealing zone, i.e., ∼230° to ∼330°C for a heating duration of ∼106 years.
Spectral characteristics of the excitation and propagation of Lg from underground nuclear explosions in central Asia
Authors: Xie Jiakang , Cong Lianli , Mitchell B. J.
Published: February 2004

The nonlinear inverse method of Xie [1993] is applied to analyze Lg spectra from many recent underground nuclear explosions in central Asia. The analysis simultaneously determines Lg seismic moments (M0) and corner frequencies (fc) of events, as well as path‐variable Lg Q values and their frequency dependences at 1 Hz. Lg Q at 1 Hz estimated for many paths from the Lop Nor and Balapan test sites to various broadband Incorporated Institution for Seismology, Chinese Digital Seismic Network, and Kyrghizstan Network stations agree well with path‐averaged Lg coda Q values from previously obtained tomographic maps for Eurasia. This suggests that the Lg Q values obtained in this inversion have not been significantly affected by focusing/defocusing or other effects of three‐dimensional structural complexities. The logarithms of Lg M0 values obtained in this study correlate linearly, with a slope of about 1.2, with International Seismological Centre (ISC) body wave magnitudes (Mb). The linear relationship is consistent with a previous empirical relationship found between the logarithm of P wave seismic moments and ISC Mb values. The slope of the linear relationship also suggests that the logarithm of Lg M0 is a more direct measure of event size than is Mb, which, for larger events, is affected by high‐frequency amplitude falloff. Another important finding of this study is that the M0 values for the explosions scale with fc−4, rather than with fc−3, as would be expected if stress drops were constant. This is consistent with previous scaling relations derived using Lg from earthquake sources and suggests that the difference between Lg and local S excitation by the same source types (explosion or earthquake) may overwhelm that between the excitation of Lg by different source types. A comparison between Lg M0 and fc values obtained using an explosion source model with those obtained using an earthquake source model suggests that the M0 values estimated using the explosion source model are systematically lower (by a factor of 0.27) than those estimated using the earthquake source model. This occurs even though the data used are the same in the inversions with the two types of source model. The factor of 0.27 is therefore due to the source model dependence of the M0, fc estimates. The overshoot and lower spectral levels at lower frequencies in the explosion source model tend to reduce both the estimated M0 and fc values. This suggests that in any nuclear monitoring program, the same type of source model (either an earthquake or explosion) should always be used to discriminate explosions using Lg. The source model dependence also suggests that more effort should be made to estimate the amount of overshoot in the Lg excitation caused by various source types.
Precision of terrestrial exposure ages and erosion rates estimated from analysis of cosmogenic isotopes produced in situ
Authors: Gillespie Alan R. , Bierman Paul R.
Published: February 2004

The concentration of cosmogenic isotopes produced within mineral grains varies with both the exposure age and erosion rate of the rock surface. In principle therefore, exposure age and erosion rate may be determined by analyzing two cosmogenic isotopes from the same sample, provided the erosion rate is constant. It is also possible to find either age or erosion rate from one isotope if the other parameter can be determined independently. Simple mathematical models predict the precision and accuracy of the exposure ages and erosion rates. The results provide insight into the use of cosmogenic isotopes and a framework to optimize dating experiments and predict tractable geologic questions. The precision and accuracy of the exposure ages and erosion rates depend on the precision of the measured isotope concentrations, the half‐lives of the isotopes, and the age and erosion rate of the sampled rock surface. They also vary with the analytic strategy. Exposure age estimates from some isotope pairs approach measurement precision for late‐Pleistocene surfaces eroding at <1 cm kyr−1. Uncertainties in erosion rate for the same surfaces may be better than ±0.2 cm kyr−1. For older surfaces the upper limits for both age and erosion rate estimates become infinite. Pairing of isotopes with different half‐lives may give misleading results unless the erosion rate is constant because they record different erosion histories. Analyzing a single isotope removes this source of error but is appropriate only if either the age or erosion rate can be well determined independently. If the erosion rate is poorly constrained, ages for some realistic situations may be inaccurate by 50% or more even though their precision may approach the measurement precision. Although it is important to understand the limitations of cosmogenic dating, it is equally important not to lose sight of the potential of this powerful tool for quantitative geomorphologic studies.
Late Quaternary geomagnetic secular variation from historical and 14C‐dated lava flows on Hawaii
Authors: Hagstrum Jonathan T. , Champion Duane E.
Published: February 2004

A paleomagnetic record of geomagnetic paleosecular variation (PSV) is constructed for the last 4400 years based on 191 sites in historical and 14C‐dated lava flows from Mauna Loa, Kilauea, and Hualalai Volcanoes on the island of Hawaii. The features of this new record are similar to those recorded by sediments from Lake Waiau near the summit of Mauna Kea Volcano, but overall mean inclinations for the lava flows (31° to 33°, depending on window size) are nearer the expected dipole‐field value (35°) than is that for the sediments (27°). Divergence of the inclination records with increasing age suggests that the Lake Waiau values at depths below 2 m have been affected by compaction‐related inclination shallowing, although magnetic terrain effects cannot be ruled out. The rate of PSV indicated by the record presented here is highly variable (<0.5°/century to >20°/century), and a pronounced shift in inclination from 25° to 40° occurred between ∼1030 and ∼975 years B.P. Paleomagnetic directions from undated materials can be correlated with our calibrated curve, but the resolution is largely dependent on the PSV rate and data densities for both the reference and unknown directions. The upper part of the Puna Basalt (18 lava flows), previously sampled for paleomagnetism along the northern wall of Kilauea's caldera (Uwekahuna Bluff), was likely deposited sometime between 1030 and 750 years B.P., but the lowest two flows beneath the Uwekahuna Ash (∼2100 years B.P.) are correlated with an age of ∼3034 years B.P. Paleomagnetic data for 54 lava flows of the Ka'u Basalt, exposed in the northwest wall of Mauna Loa's summit caldera (Mokuaweoweo), indicate that they probably accumulated over a relatively short time interval (∼200+ years) and are assigned to a 1000 to 1199 year B.P. time window. The mean of ages within this window is ∼1030 years B.P., but mapping and other 14C dates indicate that these summit overflows are probably closer to ∼1200 years B.P. in age.
Spatial and temporal power spectra of the geomagnetic field
Authors: McLeod Malcolm G.
Published: February 2004

Spatial and temporal power spectra of the geomagnetic field are useful for stochastic inversion and optimal prediction of the geomagnetic field and its secular variation. Spherical harmonic coefficients for the field and its first time derivative were determined from magnetic observatory annual means and first differences of annual means for each year in the time interval 1910–1983. Internal spherical harmonic coefficients through degree 9 and external zonal coefficients in geomagnetic coordinates for degrees 1, 3, 5, and 7 were included in the analysis. Autocovariance functions for spherical harmonic coefficients of the geomagnetic field were estimated from these coefficients. Characteristic time constants for the estimated internal autocovariance functions depend upon both spherical harmonic degree and whether the autocovariance function is for the field or its first derivative. The characteristic time constant for the field is approximately 10,000 years divided by the square of (n + 1/2), where n is the spherical harmonic degree. The characteristic time constant for the first derivative is about a factor of 10 smaller. The first time constant is thought to be related to magnetic diffusion, and the second time constant is thought to be related to core‐fluid velocity correlation time. There is also a third time constant of approximately 5 years associated with the second time derivative. External autocovariance functions for degree 1 and degree 3 both show clear evidence of a solar cycle component. A simple analytic expression for the spatial and temporal power spectra of the geomagnetic field was fitted to these data and agrees well with the experimental results.
Paleoremanence dispersal across a transpressed Archean terrain: Deflection by anisotropy or by late compression?
Authors: Werner Tomasz , Borradaile Graham J.
Published: February 2004

The Quetico Belt is an Archean metasedimentary terrain sandwiched between greenstone subprovinces. A single regional schistosity is subparallel to magnetic foliation defined by anisotropy of low‐field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanence (AARM). Aligned micas are the primary cause of AMS. Aligned pyrrhotite is the sole cause of AARM. Each magnetic subfabric is due to minerals that grew in overlapping intervals in the strain history. Their slight obliquity may be due to crystallization during noncoaxial strain. Magnetic fabric ellipsoids suggest varying severity of flattening: very strong flattening on the northern boundary, less severe on the southern boundary, and least severe in the interior. The interior and the southern boundary of the belt stretched horizontally and the shortening direction varies gradually, from NS in the north to NNW in the south. The paleomagnetic record carried by pyrrhotite shows characteristic remanences dispersed around a great circle subparallel to the regional schistosity and magnetic foliations. Late stress effects rotated paleoremanences into this pattern; we discount deflection of the paleofield by anisotropy. The pattern of dispersal indicates uniaxial oblate flattening of the region with NNW‐SSE shortening, late in the metamorphic cooling history (<300°C). This postdates schistosity, AMS and AARM fabrics but it is controlled by the same kinematic pattern.