bluuebunny/crossref_metadata_2025 huggingface dataset:
Control of folding by gravity and matrix thickness: Implications for large‐scale folding
Authors: Schmalholz S. M. , Podladchikov Y. Y. , Burg J.‐P.
Published: October 2002

We show that folding of a non‐Newtonian layer resting on a homogeneous Newtonian matrix with finite thickness under influence of gravity can occur by three modes: (1) matrix‐controlled folding, dependent on the effective viscosity contrast between layer and matrix, (2) gravity‐controlled folding, dependent on the Argand number (the ratio of the stress caused by gravity to the stress caused by shortening), and (3) detachment folding, dependent on the ratio of matrix thickness to layer thickness. We construct a phase diagram that defines the transitions between each of the three folding modes. Our priority is transparency of the analytical derivations (e.g., thin‐plate versus thick‐plate approximations), which permits complete classification of the folding modes involving a minimum number of dimensionless parameters. Accuracy and sensitivity of the analytical results to model assumptions are investigated. In particular, depth dependence of matrix rheology is only important for folding over a narrow range of material parameters. In contrast, strong depth dependence of the viscosity of the folding layer limits applicability of ductile rheology and leads to a viscoelastic transition. Our theory is applied to estimate the effective thickness of the folded central Asian upper crust using the ratio of topographic wavelength to Moho depth. Phase diagrams based on geometrical parameters show that gravity does not significantly control folding in the Jura and the Zagros Mountains but does control folding in central Asia. Applicability conditions of viscous and thin sheet models for large‐scale lithospheric deformation, derived in terms of the Argand number, have implications for the plate‐like style of planetary tectonics.
Seismic body wave constraint on mechanisms of intermediate‐depth earthquakes
Authors: Tibi Rigobert , Bock Guenter , Estabrook Charles H.
Published: October 2002

Source parameters of six large earthquakes (7.1 ≤ Mw ≤ 7.7) in the depth range between 112 and 277 km from five different subduction zones are derived from teleseismic body waves recorded by global seismic networks. Except for the 21 October 1995 Mexican event, which is consistent with the model of simple rupture on a planar surface, the investigated earthquakes show complex rupture processes that can be explained by irregularities in the focal region as is the case of shallow events. The inferred velocity of the moment release varies between 2.5 and 4 km/s, corresponding to 40–90% of the shear velocity at the focus. Four of the six earthquakes show stress drops in the range observed for shallow events (1–10 MPa). Ruptures extend mainly parallel to the strike of subducting slabs, with dimensions reaching up to 50 km. The extent of rupture perpendicular to slab dip of generally less than ∼15 km is comparatively small. This suggests that the rupture during large intermediate‐depth earthquakes propagates predominantly parallel to the strike of the subducting plate and that the seismogenic zone perpendicular to the dip direction is limited to a narrow width. The relatively small rupture size perpendicular to slab dip suggests that the rupture process does not extend into regions of significantly different temperatures. The spatial extent of the investigated earthquakes is consistent with the orientation of trench‐parallel faults that are generally observed in near‐trench/outer rise regions, suggesting that these events may result from reactivation of such preexisting faults at intermediate depths through dehydration of hydrous minerals concentrated in these zones of weakness. Depths of most of the earthquakes are consistent with the dehydration embrittlement of serpentine; the Bolivian earthquake of 23 January 1997 seems to be too deep (277 km) for such an explanation.
Grain‐scale channelization of pores due to gradients in temperature or composition of intergranular fluid or melt
Authors: Wark David A. , Watson E. Bruce
Published: October 2002

Pores in rocks synthesized at deep crustal and upper mantle conditions (1.0–1.5 GPa and 800°–1400°C) become aligned parallel to imposed temperature gradients. The extent of pore “channelization” varies with lithology and is strongest for calcite + quartz + wollastonite with a mixed H2O‐CO2 fluid. Channelization of H2O fluids is strong but decreases in quartz, fluorite, and then olivine, and is weak for calcite + H2O. Melt‐filled pores in dunite also become aligned with an imposed gradient in temperature. Channelization of pores may be rare in most natural settings because of the relatively low temperature gradients. Pores may, however, become aligned in rocks subjected to gradients in fluid composition alone (e.g., XCO2). Compared with rocks characterized by uniform pore geometry, channelized rocks, whether experimental or natural, may be subject to directionalized flow and to establishment of fluid connectivity at a relatively low fluid or melt fraction.
Influence of water saturation on the nonlinear elastic mesoscopic response in Earth materials and the implications to the mechanism of nonlinearity
Authors: Van Den Abeele K. E.‐A. , Carmeliet J. , Johnson P. A. , Zinszner B.
Published: October 2002

Much is known empirically about the qualitative and quantitative nonlinear response of rocks. Still, the mechanism of nonlinear response in porous media is only speculation. In this work we illustrate evidence from several types of experiments indicating that fluid plays an important role in the nonlinear response. For instance, measurements at low degrees of water saturation indicate that molecular layers of adsorbed moisture as well as condensation fluids significantly influence the linear and nonlinear response at dynamic strain levels due to the activation of internal molecular forces. In rock, we measure a significant increase in the nonlinear response, especially in the saturation range of 1–20%. This is consistent with observed changes in the linear response, but the extent of the variation is larger in the nonlinear measurements, especially in rocks containing small pore systems. These modifications can be attributed to an increased fluid‐solid interaction upon wetting causing the material to expand and to soften. Concurrently, the microscopic and mesoscopic hysteretic entities, which give rise to the macroscopically observed nonlinear response, are activated at saturation‐dependent opening and closing pressures. As a consequence of the moisture‐induced forces, the fraction of active hysteretic units increases with saturation. The fact that the nonlinear response increases with water saturation, especially in the low saturation range, implies that the presence of moisture plays a major role in the nonlinear mechanism or, more precisely, in the activation of that mechanism.
Draped aeromagnetic survey in Transantarctic Mountains over the area of the Butcher Ridge igneous complex showing extent of underlying mafic intrusion
Authors: Behrendt John C. , Damaske Detlef , Finn Carol A. , Kyle Philip , Wilson Terry J.
Published: October 2002

A draped aeromagnetic survey over the area surrounding the Butcher Ridge igneous complex (BRIC), Transantarctic Mountains, was acquired in 1997–1998 as part of a larger Transantarctic Mountains Aerogeophysical Research Activity survey. The BRIC is a sill‐like hypoabyssal intrusion ranging in composition from tholeiitic basalt to rhyolite. An 40Ar/39Ar age of 174 Ma and the chemical character of the basaltic rocks show the BRIC to be part of the widespread Jurassic Ferrar suite of continental tholeiitic rocks, that extends for 3500 km across Antarctica. The aeromagnetic survey shows a horseshoe‐shaped pattern of anomalies reaching amplitudes as great as 1900 nT generally associated with the bedrock topography where it is exposed. It is apparent that the high‐amplitude anomaly pattern is more extensive than the 10‐km‐long exposed outcrop, first crossed by a single 1990 aeromagnetic profile. The highest‐amplitude anomalies appear south of the profile acquired in 1990 and extend out of the survey area. The new aeromagnetic data allow determination of the extent of the interpreted Butcher mafic(?) intrusion beneath exposures of Beacon sedimentary rock and ice in the area covered, as well as beneath the small BRIC exposure. The magnetic anomalies show a minimum area of 3000 km2, a much greater extent than previously inferred. Magnetic models indicate a minimum thickness of ∼1–2 km for a horizontal intrusion. However, nonunique models with magnetic layers decreasing in apparent susceptibility with depth are consistent with of a 4‐ to 8‐km‐thick layered intrusion. These magnetic models indicate progressively deeper erosion of the interpreted mafic‐layered body from the south to north. The erosion has removed more magnetic upper layers that mask the magnetic effects of the lower less magnetic layers. The probable minimum volume of the intrusion in the area of the survey is ∼6000 km3. An alternate, but less likely, interpretation of a series of dikes can also fit the observed magnetic anomalies.
Loading and unloading of sedimentary basins: The effect of rheological hysteresis
Authors: Fowler A. C. , Yang Xin‐She
Published: October 2002

We present a model for the study of the effect of rheological hysteresis on compaction in sedimentary basins, when surface loading and unloading occurs. When compaction is slow (sedimentation rate is large and/or permeability is small), the hysteresis has little effect on the basal compaction layer, but in the more realistic case where compaction is fast (i.e., sedimentation is slow and/or permeability is large), surface unloading leads to downward propagation of a decompaction front, across which the vertical porosity gradient jumps, while subsequent surface reloading leads to downward propagation of a discontinuity in porosity, despite the fact that the porosity is governed by a diffusive equation of Richards type.
Sediment subduction, subduction erosion, and strain regime in the northern South Sandwich forearc
Authors: Vanneste Lieve E. , Larter Robert D.
Published: October 2002

The first swath bathymetry and side‐scan sonar imagery from the South Sandwich forearc reveals detailed seafloor morphology, tectonic fabric, and sedimentary features in an area where plate convergence is approximately normal to the trench. Simultaneously collected seismic reflection, gravity, and magnetic data provide information about the structure and composition of the forearc crust. Interpretation of these data together with the marine magnetic record of seafloor spreading in the east Scotia Sea back arc basin since 15 Ma has enabled quantitative estimation of sediment subduction and subduction erosion rates. Any accretionary prism present is constrained to be very small, extending 6 km or less from the trench. The contrast between this result and a time‐integrated estimate of potential accretionary prism volume suggests that >95% of the sediment which has entered the trench since 15 Ma has been subducted. The position of the South Sandwich island arc in relation to back arc magnetic lineations implies that the arc has migrated ∼70 km westward relative to the Sandwich plate since 15 Ma. Taking account of published observations concerning the typical geometric evolution of arc‐trench systems, the average rate of forearc slope retreat during this interval is inferred to be 3.1–4.7 km Myr−1. Gravity modeling suggests that the forearc crust averages ≤10 km in thickness within 110 km of the trench. For 10‐km‐thick forearc crust the rate of slope retreat implies an average rate of subduction erosion of forearc crust of 31–47 km3 km−1 Myr−1. At the present convergence rate of 74 km Myr−1 this erosion rate requires a 420–635 m thick layer of subducting material derived from the forearc crust. A time‐integrated estimate of the potential volume of forearc‐derived trench fill sediments subducted since 15 Ma implies that frontal erosion can only account for <40% of total subduction erosion. Therefore basal erosion must significantly exceed frontal erosion. The new data also provide insights concerning forearc strain regime and forearc basin evolution in the area studied. Extensional faulting near the trench slope break is probably caused by gravitational instability of the steep lower forearc slope. A lack of extensional strain indicators in the upper forearc and arc may be a consequence of the fact that the east Scotia Sea is a mature back arc basin contributing significant “ridge push” in the vicinity of the arc. The data reveal no evidence of arc‐parallel extension or of large serpentinite seamounts, as found in the Mariana forearc, and a causal link between these two observations is proposed. Seismic profiles across the forearc basin suggest that the balance between sediment accumulation and erosion is sensitive to changes in the elevation of the trench slope break. It is hypothesized that the basin is a dynamic feature which goes through repeated cycles of growth and destruction controlled by cyclic uplift and collapse of the trench slope break.
Continuous GPS measurements of postglacial adjustment in Fennoscandia 1. Geodetic results
Authors: Johansson J. M. , Davis J. L. , Scherneck H.‐G. , Milne G. A. , Vermeer M. , Mitrovica J. X. , Bennett R. A. , Jonsson B. , Elgered G. , Elósegui P. , Koivula H. , Poutanen M. , Rönnäng B. O. , Shapiro I. I.
Published: October 2002

Project BIFROST (Baseline Inferences for Fennoscandian Rebound Observations, Sea‐level, and Tectonics) combines networks of continuously operating GPS receivers in Sweden and Finland to measure ongoing crustal deformation due to glacial isostatic adjustment (GIA). We present an analysis of data collected between August 1993 and May 2000. We compare the GPS determinations of three‐dimensional crustal motion to predictions calculated using the high‐resolution Fennoscandian deglaciation model recently proposed by Lambeck et al. [1998a, 1998b]. We find that the maximum observed uplift rate (∼10 mm yr−1) and the maximum predicted uplift rate agree to better than 1 mm yr−1. The patterns of uplift also agree quite well, although significant systematic differences are evident. The root‐mean‐square residual rate for a linear error model yields estimates of rate accuracy of 0.4 mm yr−1 for east, 0.3 mm yr−1 for north, and 1.3 mm yr−1 for up; these figures incorporate model errors, however. We have also compared the values for the observed radial deformation rates to those based on sea level rates from Baltic tide gauges. The observational error for the vertical GPS rates required to give a reduced χ2 of unity is 0.8 mm yr−1. The time series do exhibit temporal variations at seasonal frequencies, as well as apparent low‐frequency noise. An empirical orthogonal function analysis indicates that the temporal variations are highly correlated among the sites. The correlation appears to be regional and falls off only slightly with distance. Some of this correlated noise is associated with snow accumulation on the antennas or, for those antennas with radomes, on the radomes. This problem has caused us to modify the radomes used several times, leading to one of our more significant sources of uncertainty.
Detecting time variations in gravity associated with climate change
Authors: Leuliette Eric W. , Nerem R. Steven , Russell Gary L.
Published: October 2002

General circulation models offer a way to estimate the impact of mass redistributions on the Earth's time‐varying gravity field. In this way, the prospects for detecting climate signals in the gravity field by dedicated satellite gravity missions, such as Gravity Recovery and Climate Experiment (GRACE), can be assessed. Using monthly averaged fluid mass diagnostics from a coupled atmosphere‐ocean model developed at the Goddard Institute for Space Studies (GISS), we have estimated geoid variations from the fundamental model mass components. From these estimates the seasonal geoid signals from sea level, snow, soil moisture, water vapor, and atmospheric mass can be compared to the estimated errors for GRACE. All of these seasonal mass flows from the GISS model are well above the preliminary GRACE measurement errors. In addition, mass flows with significant secular trends attributable to the model's simulated increase of greenhouse gases would, in principle, be detectable by GRACE. However, the interannual variability of mass flows may require longer time series of gravity data, pattern analysis, or modeling improvements in order to detect trends.
Plate kinematics in the western Pacific derived from geodetic observations
Authors: Tregoning Paul
Published: October 2002

Plate kinematics have been derived from GPS site velocities spanning Australasia and the western Pacific and define a framework in which deformation fields at plate boundaries can be investigated. The Australian Plate appears to be rigid from Cocos Island in the west to Noumea in the east and incorporating all of the Australian continent. A revision of the angular velocity vector of the Pacific Plate using six site velocities has led to the detection of significant relative motion between the Pacific Plate and sites in northern Papua New Guinea. This provides strong evidence in support of a North Bismarck Plate between the South Bismarck and Pacific Plates and we model the motion of the North Bismarck Plate with the angular velocity vector (45.0°S, 126.4°E, 0.85°/Myr). These new angular velocity vectors model the rigid plate motions of a substantial part of the Australasian and western Pacific regions with an accuracy better than 3 mm/yr.
Carbon dioxide emission rate of Kīlauea Volcano: Implications for primary magma and the summit reservoir
Authors: Gerlach T. M. , McGee K. A. , Elias T. , Sutton A. J. , Doukas M. P.
Published: October 2002

We report a CO2 emission rate of 8500 metric tons per day (t d−1) for the summit of Kīlauea Volcano, several times larger than previous estimates. It is based on three sets of measurements over 4 years of synchronous SO2 emission rates and volcanic CO2/SO2 concentration ratios for the summit correlation spectrometer (COSPEC) traverse. Volcanic CO2/SO2 for the traverse is representative of the global ratio for summit emissions. The summit CO2 emission rate is nearly constant, despite large temporal variations in summit CO2/SO2 and SO2 emission rates. Summit CO2 emissions comprise most of Kīlauea's total CO2 output (∼9000 t d−1). The bulk CO2 content of primary magma determined from CO2 emission and magma supply rate data is ∼0.70 wt %. Most of the CO2 is present as exsolved vapor at summit reservoir depths, making the primary magma strongly buoyant. Turbulent mixing with resident reservoir magma, however, prevents frequent eruptions of buoyant primary magma in the summit region. CO2 emissions confirm that the magma supply enters the edifice through the summit reservoir. A persistent several hundred parts per million CO2 anomaly arises from the entry of magma into the summit reservoir beneath a square kilometer area east of Halemaumau pit crater. Since most of the CO2 in primary magma is degassed in the summit, the summit CO2 emission rate is an effective proxy for the magma supply rate. Both scrubbing of SO2 and solubility controls on CO2 and S in basaltic melt cause high CO2/SO2 in summit emissions and spatially uncorrelated distributions of CO2 and SO2 in the summit plume.
Three‐dimensional elastic dislocation modeling of the postseismic response to the 1964 Alaska earthquake
Authors: Zweck Chris , Freymueller Jeffrey T. , Cohen Steven C.
Published: October 2002

GPS velocity observations from the Kenai Peninsula and Kodiak Island, Alaska, display a pattern of spatial variability suggesting the presence of multiple active processes on the underlying Pacific—North American plate interface. Velocities of sites on the eastern Kenai Peninsula and Kodiak Island are consistent with elastic strain accumulation at the plate interface, whereas velocities of sites on the western Kenai Peninsula and Cook Inlet are oriented in the opposite direction and suggest an ongoing postseismic response to the 1964 Alaska earthquake. We can reproduce the observed velocities using a three‐dimensional elastic dislocation model to estimate the variation in coupling between the Pacific and North American plates. Two different inversion schemes are examined to check the robustness of the results. We find that the GPS data can be satisfied by the presence of a locked area near southwest Prince William Sound, a locked area near southwest Kodiak Island, and an area of postseismic reverse slip beneath and north of the western Kenai Peninsula. The shallow plate interface trenchward of the western Kenai Peninsula does not appear to be locked. The locked areas correspond to the Prince William Sound and Kodiak Island asperities that ruptured in 1964, and the area of postseismic slip lies downdip of the 1964 rupture area. The correspondence between the present shallow coupling distribution and the 1964 slip distribution suggests that the locked regions repeat from one earthquake cycle to another.
Crustal thicknesses in SE Brazilian Shield by receiver function analysis: Implications for isostatic compensation
Authors: Assumpção Marcelo , James David , Snoke Arthur
Published: October 2002

The Brazilian Lithosphere Seismic Project (BLSP, a joint project by University of São Paulo and Carnegie Institution, 1992–1999) operated more than 20 temporary broadband stations in the southeastern Brazilian shield. The area, a transect ∼1000 km long and 300 km wide, covers different geological provinces: the Precambrian São Francisco craton, the adjacent Brasiliano (700–500 Ma) fold belts, and the Paraná basin of Paleozoic origin. Crustal thicknesses were estimated for 23 sites using receiver functions. For each station, receiver functions were stacked for different sets of earthquakes according to azimuth and distance. The P‐to‐S Moho converted phase was clearly identified at most sites. Crustal thicknesses were estimated using an average crustal P wave velocity of 6.5 km/s. Poisson's ratio of 0.23 (Vp/Vs = 1.70) was used for the São Francisco craton and adjacent fold belt (based on travel times from small, local earthquakes) and 0.25 was used for the Paraná basin and coastal belt. Crustal thicknesses ranged from 35–47 km. Although there is a clear inverse correlation between topography and Bouguer gravity anomalies in the study area, Moho depths show the opposite pattern from that expected: areas of low topography and less negative Bouguer anomalies, such as the Paraná basin, have thicker crust (40–47 km) compared with the high elevation areas of the craton and fold belt (37–43 km). Two hypothesis are proposed to explain the data: (1) A lower density, by 30–40 kg/m3, in the lithospheric mantle under the Archean block of the São Francisco craton relative to the Proterozoic lithosphere is responsible for maintaining the high elevations in the plateau area. Relatively low density and high P wave velocity are compatible with a depleted (low FeO) composition for the Archean lithosphere. (2) Alternatively, if the density contrasts between Archean and Proterozoic lithospheres are smaller than the values above, then the crust beneath the Paraná basin must be more dense than that of the craton. Higher crustal density and high Poisson's ratio would be consistent with magmatic underplating in the lower crust beneath the Paraná basin, as inferred from other studies.
Crustal structure across the coseismic rupture zone of the 1944 Tonankai earthquake, the central Nankai Trough seismogenic zone
Authors: Nakanishi Ayako , Takahashi Narumi , Park Jin‐Oh , Miura Seiichi , Kodaira Shuichi , Kaneda Yoshiyuki , Hirata Naoshi , Iwasaki Takaya , Nakamura Masao
Published: October 2002

Differences in the coseismic rupture process between the 1944 Tonankai and the 1946 Nankaido earthquakes have been studied by many fault models. To understand what factors control coseismic rupture zones, it is important to investigate differences in deep crustal structures of the rupture zones between the 1944 and 1946 earthquakes. The previously published crustal structure of the rupture zone of the 1946 earthquake shows that the coseismic rupture extends to the Neogene‐Quaternary accretionary prism. However, little is known about the structure of the rupture zone of the 1944 earthquake. To obtain a complete image of the seismogenic zone of the 1944 earthquake, a wide‐angle seismic survey was performed across the presumed coseismic rupture zone of the 1944 earthquake from ocean to land. Our model for the crustal structure is based on ocean bottom seismographic data. The crustal structure appears characteristic for subducting oceanic crust and a Neogene‐Quaternary accretionary prism bounded by an island arc crust. The Neogene‐Quaternary accretionary prism reaches a maximum thickness of 7 km at 50 km distance landward from the deformation front. The subducting oceanic crust can be traced down to 35 km. The subduction angle becomes steeper landward, reaching up to 11° beneath the island arc crust. The depth of the top of subducting oceanic crust at the downdip limit of the rupture zone is 23 km, while the updip limit is located beneath the island arc upper crust. Similar structures of the updip and downdip limits are also published for several other subduction zones.
Cutoff scattering angles for random acoustic media
Authors: Kawahara Jun
Published: October 2002

The seismic scattering attenuation in randomly inhomogeneous media is successfully explained by a Born approximation‐based theory, established by R. S. Wu and H. Sato. The key is to eliminate the contribution of forward scattering within a cutoff scattering angle (CSA) when evaluating the scattering attenuation in order to avoid overestimating attenuation at high frequencies. The value of the CSA is, however, not objectively determined in the theory, and the choice of it remains an open question. We investigate the constraint by causality on the choice of the CSA for random acoustic media with constant densities. On the basis of the Kramers‐Krönig relation, we derive simple relations of the phase velocities in the high‐ and low‐frequency limits with the CSA. We further discuss the probable values of the CSA on the basis of a thought experiment. Surprisingly, they are independent of the details of inhomogeneities.
Source parameters of the 1908 Messina Straits, Italy, earthquake from geodetic and seismic data
Authors: Amoruso Antonella , Crescentini Luca , Scarpa Roberto
Published: October 2002

We carry out a nonlinear joint inversion of P wave first‐motion polarities and coseismic surface displacement data of the 1908 Messina earthquake. We model the earthquake using a single planar fault: Slip is at first assumed to be uniform across the whole fault, then independent in a small set of coplanar subfaults, and finally smoothly variable across the fault. The first two steps are accomplished using a global minimization technique. The main features of the retrieved model are very robust and independent of the seismic velocity profile, of the presence of questionable bench marks, and of the assumed residual distribution. The along‐strike component of slip is about half the along‐dip component, in agreement with the direction of the extensional stress axis retrieved from geological observations. Surface vertical displacement is consistent with tsunami data and with the morphology of the Messina Straits.
Mid‐ocean ridge magma chamber processes: Constraints from olivine zonation in lavas from the East Pacific Rise at 9°30′N and 10°30′N
Authors: Pan Yucheng , Batiza Rodey
Published: October 2002

Lavas from the northern East Pacific Rise with both robust (9°30′N) and nonrobust (10°30′N) ridge morphologies have compositionally diverse populations of plagioclase and/or olivine due to magma mixing. We interpret zoning in olivines to be primarily due to diffusion after mixing events, which allows us to calculate the residence times of individual crystals in the magma. For the four studied samples, olivine populations exhibit exponential distributions of calculated diffusion times, with short times exponentially more abundant than longer times. We model this distribution as resulting from mixing in an open‐system axial magma chamber. Magma residence times in axial chambers at East Pacific Rise 9°30′N and 10°30′N for the four studied samples are of the order of months. Surprisingly, we find that the differences in mineralogy and magma residence times between robust and nonrobust East Pacific Rise segments are not significant. Taken together, our observations and measurements suggest that the seismically imaged melt lens of the axial magma chamber does not play a significant role in controlling the crystal content and characters of erupted lavas.
Calculation of dike trajectories from volcanic centers
Authors: Mériaux Catherine , Lister John R.
Published: October 2002

Patterns of dike swarms around volcanic centers or above mantle plumes are interpreted by a mechanical analysis of regional and dike‐induced stresses, in which dike emplacement is controlled and guided by the stress state. Comparisons of dike patterns with patterns of principal‐stress trajectories caused by a source and a regional stress system are commonly used to infer paleostresses. However, dike trajectories are determined by a complex interaction between dike‐induced stresses and the source/regional stress system. We present numerical calculations based on a novel boundary‐integral formulation, which examines the simultaneous effects of regional stresses, magma pressure, and dike injection on the local stress state around a continuously curving dike. Dike paths are calculated from the condition that dikes propagate by mode I failure. Our results suggest that the magnitude of the regional stresses would be 2–5 times higher than previous estimates based on principal‐stress trajectory analysis.
Grain‐size‐dependent paleointensity results from very recent mid‐oceanic ridge basalts
Authors: Carlut J. , Kent D. V.
Published: October 2002

We report paleointensity analyses on a suite of four samples from two axial zero‐age mid‐oceanic ridge basalt flows from the East Pacific Rise. Paleointensity experiments have been performed on several profiles, each consisting of a batch of small (millimeter scale) subsamples going from the rapidly cooled glassy margin to the interior. The Coe version of the Thellier double‐heating procedure was used with back checks performed after every other heating step. Most of the samples show very good behavior, i.e., constant ratio between the lost and acquired magnetization through a large temperature range (quality factor usually above 10) and positive checks, which lead to unambiguous paleointensity determinations. Paleointensities obtained on glasses reported here and in a previously published study are very consistent and reproducible and in agreement with expected in situ values of Earth's magnetic field intensity. However, results found within the crystalline part of the samples show values that are considerably (up to 50%) higher than expected. These variations seem to be correlated with the cooling history of the samples and appear to be universal since all samples exhibit the same intriguing pattern. An extensive study of magnetic properties allows us to link this incongruent behavior to (1) the presence of multidomain effects and (2) the cooling rate difference between laboratory experiments and in situ cooling.
Coseismic and postseismic stress changes in a subducting plate: Possible stress interactions between large interplate thrust and intraplate normal‐faulting earthquakes
Authors: Mikumo Takeshi , Yagi Yuji , Singh Shri Krishna , Santoyo Miguel A.
Published: October 2002

A large intraplate, normal‐faulting earthquake (Mw= 7.5) occurred in 1999 in the subducting Cocos plate below the downdip edge of the ruptured thrust fault of the 1978 Oaxaca, Mexico, earthquake (Mw= 7.8). This situation is similar to the previous case of the 1997 normal‐faulting event (Mw= 7.1) that occurred beneath the rupture area of the 1985 Michoacan, Mexico, earthquake (Mw= 8.1). We investigate the possibility of any stress interactions between the preceding 1978 thrust and the following 1999 normal‐faulting earthquakes. For this purpose, we estimate the temporal change of the stress state in the subducting Cocos plate by calculating the slip distribution during the 1978 earthquake through teleseismic waveform inversion, the dynamic rupture process, and the resultant coseismic stress change, together with the postseismic stress variations due to plate convergence and the viscoelastic relaxation process. To do this, we calculate the coseismic and postseismic changes of all stress components in a three‐dimensional space, incorporating the subducting slab, the overlying crust and uppermost mantle, and the asthenosphere. For the coseismic stress change we solve elastodynamic equations, incorporating the kinematic fault slip as an observational constraint under appropriate boundary conditions. To estimate postseismic stress accumulations due to plate convergence, a virtual backward slip is imposed to lock the main thrust zone. The effects of viscoelastic stress relaxations of the coseismic change and the back slip are also included. The maximum coseismic increase in the shear stress and the Coulomb failure stress below the downdip edge of the 1978 thrust fault is estimated to be in the range between 0.5 and 1.5 MPa, and the 1999 normal‐faulting earthquake was found to take place in this zone of stress increase. The postseismic variations during the 21 years after the 1978 event modify the magnitude and patterns of the coseismic stress change to some extent but are not large enough to overcome the coseismic change. These results suggest that the coseismic stress increase due to the 1978 thrust earthquake may have enhanced the chance of occurrence of the 1999 normal‐faulting event in the subducting plate. If this is the case, one of the possible mechanisms could be static fatigue of rock materials around preexisting weak planes involved in the subducting plate, and it is speculated that that one of these planes might have been reactivated and fractured because of stress corrosion cracking under the applied stress there for 21 years.
Initiation of shear instability in three‐dimensional elastodynamics
Authors: Favreau Pascal , Campillo Michel , Ionescu Ioan R.
Published: October 2002

Earthquake nucleation may begin with a stable quasi‐static localization of fault slip. This long‐term process, which conditions the fault for the rupture, is followed by a dynamic and unstable phase that we call initiation and ends with the onset of rupture propagation. In this paper, we investigate the three‐dimensional elastodynamics of this unstable initiation phase both analytically and numerically. The fault model consists of two symmetric semi‐infinite elastic bodies in frictional contact across a flat interface with a slip‐weakening law and loaded until its sliding threshold is reached. On the basis of our previous studies in two dimensions, we generalize the linear stability and the spectral analysis and then solve analytically the "dominant part" approximation for the homogeneous infinite fault. We then derive the slip pattern on the fault, the decay of the displacement amplitude in the media, and its growth in time. The model also gives a theoretical approximation for the time of initiation. To test the dominant part approximation, we compare it with the numerical solution in the case of a concentrated perturbation. Finally, we find that unlike the infinite fault, finite faults of a given geometry allow a domain of stability that depends on the fault size. When a finite fault becomes unstable and rupture initiates, the slip growth is determined by a single dominant eigenmode. The duration of the initiation phase varies strongly with the size and geometry of the fault, especially when it is close to the stability limit.
Quantifying iron oxide coatings on dune sands using spectrometric measurements: An example from the Simpson‐Strzelecki Desert, Australia
Authors: Bullard Joanna E. , White Kevin
Published: October 2002

Red soils and sediments are common in arid and semiarid areas. The red color, caused by iron oxides, has been used by researchers to infer information about sediment provenance, age, and environmental history. However, the objective description and quantification of color is problematic. This paper tests a rapid technique for quantifying “redness” in the field using visible reflectance spectrometry. The reflectance ratings obtained from the field are converted to redness values and are compared with similar measurements taken under controlled conditions. The results show that the color of dune sands is determined by the variation in intensity of the UV conduction band. We demonstrate that estimates of spectral redness offer much greater precision in differentiating sand samples by color than conventional estimates based on color charts. The relationship between increasing redness and increasing concentrations of iron oxide in the sediments is strong and positive. This suggests that, in certain circumstances, simple field spectral measurements are able to provide reliable estimates of iron oxide concentrations and redness, therefore negating the need for more detailed controlled laboratory measurements.
Subsonic and intersonic shear rupture of weak planes with a velocity weakening cohesive zone
Authors: Samudrala O. , Huang Y. , Rosakis A. J.
Published: October 2002

A substantial effort has been devoted in the past toward modeling earthquake source mechanisms as dynamically extending shear cracks. Most of the attention was focused on the subsonic crack speed regime. Recently, a number of reports have appeared in the seismological literature citing evidence of intersonic rupture speeds during shallow crustal earthquakes. In the first part of this paper, we discuss direct experimental observations of intersonic in‐plane shear crack growth along a weak plane joining two homogeneous, isotropic, linear elastic plates. Associated with the primary intersonic crack and at locations behind the propagating shear crack tip, a series of secondary tensile cracks, at a steep angle to the shear crack plane, were also observed. Motivated by these observations, subsonic and intersonic mode II crack propagation with a velocity weakening cohesive zone is analyzed in the main body of the paper. A cohesive law is assumed wherein the cohesive shear traction is either a constant or decreases linearly with the local slip rate, the rate of decrease governed by a slip rate weakening parameter. The cohesive shear traction is assumed to vanish when the crack tip sliding displacement reaches a characteristic breakdown slip. It is shown that a positive energy flux into the rupture front is possible in the entire intersonic regime. The influence of shear strength and of the weakening parameter on the crack propagation behavior is investigated. Crack tip stability issues are also addressed, and favorable speed regimes are identified. Estimates of the slip rate weakening parameter are obtained by using the theoretical model to predict the angle of the secondary cracks. The rest of the parameters are subsequently estimated by comparing the isochromatic fringe patterns (contours of maximum in‐plane shear stress) predicted by the solution with those recorded experimentally.
A unified description of localization for application to large‐scale tectonics
Authors: Montési Laurent G. J. , Zuber Maria T.
Published: October 2002

Localized regions of deformation such as faults and shear zones are ubiquitous in the Earth's lithosphere. However, we lack a simple unified framework of localization that is independent of the mechanism or scale of localization. We address this issue by introducing the effective stress exponent,ne, a parameter that describes how a material responds to a local perturbation of an internal variable being tested for localization. The value ofneis based on micromechanics. A localizing regime has a negativene, indicating a weakening behavior, and localization is stronger for more negative 1/ne. We present expressions for the effective stress exponent associated with several mechanisms that trigger localization at large scale: brittle failure with loss of cohesion, elastoplasticity, rate‐ and state‐dependent friction, shear heating, and grain‐size feedback in ductile rocks. In most cases, localization does not arise solely from the relation between stress and deformation but instead requires a positive feedback between the rheology and internal variables. Brittle mechanisms (failure and friction) are generally described byneof the order of −100. Shear heating requires an already localized forcing, which could be provided by a brittle fault at shallower levels of the lithosphere. Grain size reduction, combined with a transition from dislocation to diffusion creep, leads to localization only if the grain size departs significantly from its equilibrium value, because either large‐scale flow moves rocks through different thermodynamic environments or new grains are nucleated. When shear heating or grain‐size feedback produce localization, 1/necan be extremely negative and can control lithospheric‐scale localization.
Satellite interferometric observations of displacements associated with seasonal groundwater in the Los Angeles basin
Authors: Watson Karen M. , Bock Yehuda , Sandwell David T.
Published: October 2002

The Newport‐Inglewood fault zone (NIFZ) displays interferometric synthetic aperture radar (SAR) phase features along most of its length having amplitudes of up to 60 mm. However, interpretation in terms of right‐lateral, shallow slip along the fault fails to match the range of geologic estimates of slip. Recently, Bawden et al. [2001] proposed that these phase features, as well as a broader deformation pattern in the Los Angeles basin, are due to vertical motion related to annual variations in the elevation of the water table. We confirm this hypothesis through the analysis of a longer span of data consisting of 26 SAR images collected by the ERS‐1 and ERS‐2 spacecraft between June 1992 and June 2000. Moreover, we use continuous GPS measurements from 1995 to the present to establish the amplitude and phase of the vertical deformation. The Los Angeles basin becomes most inflated one quarter of the way through the year, which is consistent with water table measurements as well as with the end of the rainy season when the aquifer should be at a maximum. The spatial pattern of the amplitude of the annual signal derived from continuous GPS measurements is consistent with the shape of the interferometric fringes. GPS sites both near the NIFZ and in a 20 by 40 km zone within the basin also show significant N‐S annual variations that may be related to the differential expansion across the fault. Since these horizontal signals have peak‐to‐trough amplitudes of 6 mm, they mask the smaller tectonic signals and need to be taken into account when interpreting GPS time series of site position. Moreover, since the groundwater signal appears to have a long‐term vertical trend which varies in sign depending on location, it will be difficult to distinguish interseismic tectonic slip along the NIFZ and within the affected areas in the basin.
Magmatic inflation at a dormant stratovolcano: 1996–1998 activity at Mount Peulik volcano, Alaska, revealed by satellite radar interferometry
Authors: Lu Zhong , Wicks Charles , Dzurisin Daniel , Power John A. , Moran Seth C. , Thatcher Wayne
Published: October 2002

A series of ERS radar interferograms that collectively span the time interval from July 1992 to August 2000 reveal that a presumed magma body located 6.6 ± 0.5 km beneath the southwest flank of the Mount Peulik volcano inflated 0.051 ± 0.005 km3between October 1996 and September 1998. Peulik has been active only twice during historical time, in 1814 and 1852, and the volcano was otherwise quiescent during the 1990s. The inflation episode spanned at least several months because separate interferograms show that the associated ground deformation was progressive. The average inflation rate of the magma body was ∼0.003 km3/month from October 1996 to September 1997, peaked at 0.005 km3/month from 26 June to 9 October 1997, and dropped to ∼0.001 km3/month from October 1997 to September 1998. An intense earthquake swarm, including threeML4.8–5.2 events, began on 8 May 1998 near Becharof Lake, ∼30 km northwest of Peulik. More than 400 earthquakes with a cumulative moment of 7.15 × 1017N m were recorded in the area through 19 October 1998. Although the inflation and earthquake swarm occurred at about the same time, the static stress changes that we calculated in the epicentral area due to inflation beneath Peulik appear too small to provide a causal link. The 1996–1998 inflation episode at Peulik confirms that satellite radar interferometry can be used to detect magma accumulation beneath dormant volcanoes at least several months before other signs of unrest are apparent. This application represents a first step toward understanding the eruption cycle at Peulik and other stratovolcanoes with characteristically long repose periods.
Late surge glacial conditions on Bakaninbreen, Svalbard, and implications for surge termination
Authors: Smith A. M. , Murray T. , Davison B. M. , Clough A. F. , Woodward J. , Jiskoot H.
Published: October 2002

Bakaninbreen is a polythermal glacier in southern Spitsbergen, Svalbard, that last surged between 1985 and 1995. Seismic reflection data were acquired during early quiescence in spring 1998, just upstream of the surge front. The results were combined with complementary ground‐penetrating radar data to investigate the glacial structure and basal conditions. We find no difference between the ice thickness values determined from the seismic and radar methods, suggesting that any layer of basal ice cannot be greater than ∼5 m thick. Interpretation of the amplitude of the seismic reflections indicates the presence of permafrost close to the glacier base. A thin layer of thawed deforming sediment separates the glacier from this underlying permafrost. In an area just upstream of the surge front the permafrost becomes discontinuous and may even be absent, the ice being underlain by 10–15 m of thawed sediments overlying deeper bedrock. High‐pressure water is believed to have been required to maintain the propagation of the surge, and this area of thawed sediment is interpreted as a route for that water to escape from the basal system. When the surge front passed over this thawed bed, the escaping water reduced the pressure in the subglacial hydraulic system, initiating the termination of the surge. Surge termination was therefore primarily controlled by the presurge permafrost distribution beneath the glacier, rather than any feature of the surge itself. This termination mechanism is probably limited to surges in polythermal glaciers, but the techniques used may have wider glaciological applications.