bluuebunny/crossref_metadata_2025 huggingface dataset:
Active deformation of the Corinth rift, Greece: Results from repeated Global Positioning System surveys between 1990 and 1995
Authors: Briole P. , Rigo A. , Lyon‐Caen H. , Ruegg J. C. , Papazissi K. , Mitsakaki C. , Balodimou A. , Veis G. , Hatzfeld D. , Deschamps A.
Published: February 2004

Between 1990 and 1995, we carried out seven Global Positioning System (GPS) campaigns in the Corinth rift area in order to constrain the spatial and temporal crustal deformation of this active zone. The network, 193 points over ∼10,000 km2, samples most of the active faults. In order to estimate the deformation over a longer period, 159 of those points are also Greek triangulation pillars previously measured between 1966 and 1972. Two earthquakes of magnitude 6.2 and 5.9 have occurred in the network since it was installed. The extension rate deduced from the analysis of the different GPS data sets is 14±2 mm/yr oriented N9° in the west, 13±3 mm/yr oriented S‐N in the center, and 10±4 mm/yr oriented N19°W in the east of the gulf. The comparison between GPS and triangulation gives higher rates and less angular divergence (25±7 mm/yr, N4°E; 22±7 mm/yr, S‐N; 20±7 mm/yr, N15°W, respectively). Both sets of data indicate that the deforming zone is very narrow (10–15 km) in the west, might be wider in the center (15–20 km), and is more diffuse in the east. The analysis of the displacements observed after the Ms = 6.2, June 15, 1995, and the Ms = S.9, November 18, 1992, earthquakes, both located in the west of the gulf, together with seismological and tectonic observations shows that these two earthquakes occurred on low‐angle (≤35°) north dipping normal faults located between 4.5 and 10 km depth in the inner part of the rift. Assuming that the deformation is concentrated in relatively narrow deforming zones, we use a simple model of a dislocation in an elastic half‐space to study the implication of the localization. Using the geometry of the known seismogenic faults, our observations imply continuous aseismic deformation in the uppermost crust of the inner rift. This model predicts geodetic strain rates close to seismic strain rates in opposition to previous estimates. This is because our model takes into account the activity on low‐angle normal faults in the inner rift and an effective seismogenic layer of 6–7 km, about half that usually assumed.
A study on the mechanism of coseismic groundwater changes: Interpretation by a groundwater model composed of multiple aquifers with different strain responses
Authors: Kitagawa Yuuichi , Koizumi Naoji
Published: February 2004

In order to research the coseismic and postseismic groundwater changes for a groundwater system consisting of multiple aquifers we studied the mechanism of coseismic groundwater changes at Yudani hot spring, which is an artesian well supplied by at least three aquifers, in Tottori Prefecture, Japan. We observed the changes in discharge and water temperature at this site in response to earthquakes, tides, and atmospheric pressure changes. For the groundwater system of Yudani hot spring a realistic model composed of three aquifers having different frequency dependencies of groundwater responses to crustal strain and atmospheric pressure was adopted. The tidal and barometric responses of the discharges from the three aquifers, which were cyclic responses over a limited frequency band, were calculated, and the physical parameters of the three aquifers were estimated by fitting a theoretical model to the observed responses. Then, using the strain response over the whole frequency determined by the estimated physical parameters, the theoretical responses of the discharges from the three aquifers to step‐like volumetric strain changes were calculated. As a result, the three‐aquifer model with different frequency dependencies quantitatively explained the observed groundwater changes at Yudani hot spring due to earthquakes. In addition, the values of the coseismic volumetric strain steps were estimated based on groundwater changes at Yudani hot spring. These values did not agree with the theoretical coseismic volumetric strain changes at Yudani hot spring due to the earthquake fault slips, which were calculated under the assumption that the Earth's crust is a homogeneous elastic half‐space medium. It is possible that a local strain release, such as aseismic slip at a fault near Yudani hot spring, might have caused the large strain changes estimated from the coseismic groundwater changes.
Submarine hydrogeology of the Hawaiian archipelagic apron: 1. Heat flow patterns north of Oahu and Maro Reef
Authors: Harris Robert N. , Von Herzen Richard P. , McNutt Marcia K. , Garven Grant , Jordahl Kelsey
Published: February 2004

We present two profiles of collocated single‐channel seismic reflection and heat flow determinations across the Hawaiian flexural moat: one north of Oahu and the other north of Maro Reef. Seismic reflection data are used as an aid in determining depth to basement and interpreting moat stratigraphy. Moat sediments are locally up to 2 km thick and result mostly from slumps, debris avalanches and turbidites from volcanoes capping the Hawaiian Ridge. Each heat flow profile is ∼200 km long and consists of paired thermal gradient and conductivity measurements made at an interval of 1–2 km. The mean heat flow, unconnected for the cooling effect of sedimentation, along the Oahu profile is 63.9 mW m−2 (s.d. 11.9 mW m−2) and along the Maro Reef profile is 59.8 mW m−2 (s.d. 5.0 mW m−2). Our preferred sedimentation correction model gives a corrected heat flow mean along the Oahu profile of 74.2 mW m−2 (s.d. 14.6 mW m−2) and for the Maro Reef profile of 64.0 mW m−2 (s.d. 4.9 mW m−2). These values are not significantly different despite the fact that oceanic crust at Maro Reef is 19 Myr older than at Oahu. The mean heat flow along the Oahu profile is within one standard deviation of the global mean value for oceanic crust of the same age, while the mean heat flow along the Maro Reef profile differs by greater than one standard deviation for the global mean. However, the mean heat flow along the Maro Reef profile is within the uncertainty of previous heat flow determinations made offswell at Maro Reef [Von Herzen et al, 1989]. Variability along each profile is significantly greater than the measurement uncertainty (measurement uncertainty is 3 and 2 mW m−2 for the Oahu and Maro Reef profiles, respectively) and is significantly greater along the Oahu profile than the Maro Reef profile. While this variability could result from fluid flow within the archipelagic apron or underlying crust, simple moat‐wide circulation models do not appear to explain the dominant variability in heat flow. Heat flow means and variability of both profiles are inconsistent with simple conductive models of lithospheric reheating.
Deformation and seismicity in the Coso geothermal area, Inyo County, California: Observations and modeling using satellite radar interferometry
Authors: Fialko Yuri , Simons Mark
Published: February 2004

Interferometric synthetic aperture radar (InSAR) data collected in the Coso geothermal area, eastern California, during 1993–1999 indicate ground subsidence over a ∼50 km2 region that approximately coincides with the production area of the Coso geothermal plant. The maximum subsidence rate in the peak of the anomaly is ∼3.5 cm yr−1, and the average volumetric rate of subsidence is of the order of 106 m3 yr−1. The radar interferograms reveal a complex deformation pattern, with at least two irregular subsidence peaks in the northern part of the anomaly and a region of relative uplift on the south. We invert the InSAR displacement data for the positions, geometry, and relative strengths of the deformation sources at depth using a nonlinear least squares minimization algorithm. We use elastic solutions for a prolate uniformly pressurized spheroidal cavity in a semi‐infinite body as basis functions for our inversions. Source depths inferred from our simulations range from 1 to 3 km, which corresponds to the production depths of the Coso geothermal plant. Underpressures in the geothermal reservoir inferred from the inversion are of the order of 0.1–1 MPa (except a few abnormally high underpressures that are apparently biased toward the small source dimensions). Analysis of the InSAR data covering consecutive time intervals indicates that the depths and/or horizontal extent of the deformation sources may increase with time. This increase presumably reflects increasing volumes of the subsurface reservoir affected by the geothermal exploitation. We show that clusters of microearthquakes associated with the geothermal power operation may result from perturbations in the pore fluid pressure, as well as normal and shear stresses caused by the deflation of the geothermal reservoir.
Impact of atmospheric water vapor on the thermal infrared remote sensing of volcanic sulfur dioxide emissions: A case study from the Pu'u ‘O’ vent of Kilauea Volcano, Hawaii
Authors: Realmuto Vincent J. , Worden Helen M.
Published: February 2004

The December 18, 1999, launch of NASA's Terra satellite put two multispectral thermal infrared imaging instruments into Earth orbit. Experiments with airborne instruments have demonstrated that the data from such instruments can be used to detect volcanic SO2 plumes and clouds. However, one of the greatest challenges that will confront efforts to monitor volcanic SO2 emissions from space is the need to characterize the local atmosphere. In this paper we evaluate the sensitivity of the SO2 retrieval procedure to our knowledge of the local atmospheric conditions. We compare SO2 retrievals obtained with distant (radiosonde) and local (Fourier transform infrared (FTIR) soundings) atmospheric measurements and find that the relative difference is typically ±25%. For ground temperature retrievals the relative difference is ±1.5%. These results lead us to conclude that while local measurements of atmospheric conditions are preferable, useful retrievals can be obtained using atmospheric measurements from distant sites. In addition, we find very good agreement between SO2 and ground temperature retrievals obtained from thermal infrared imagery and FTIR soundings.
True triaxial strength and deformability of the German Continental Deep Drilling Program (KTB) deep hole amphibolite
Authors: Chang Chandong , Haimson Bezalel
Published: February 2004

We designed and fabricated a true triaxial loading system and used it to determine deformational and strength characteristics of the amphibolite penetrated by the superdeep hole drilled in the Bohemian massif of southeastern Germany under the German Continental Deep Drilling Program (KTB). Amphibolite is found between 3200 and 7300 m and thus the dominant rock in this 9100‐m boring. Our loading system enables the application of three unequal principal stresses to a rectangular prismatic rock specimen. During a test we maintained the least principal (σ3) and the intermediate (σ2) stresses constant and increased the major principal stress (σ1) until brittle failure occurred, in the form of a fracture steeply dipping in the σ3 direction. Typically, for the same σ3 level the amphibolite compressive strength increased substantially with the magnitude of σ2, demonstrating the inadequacy of Mohr‐like failure criteria that ignore the effect of the intermediate principal stress on rock strength. We found that a general criterion for the amphibolite could be expressed in the form of a power function relating the octahedral shear stress at failure to the mean normal stress acting on the plane containing the fracture. With respect to deformation, we established that for the same σ3 the onset of dilatancy increases significantly with the magnitude of σ2. Thus the intermediate principal stress appears to extend the elastic range of the stress‐strain behavior for a given σ3 and hence to retard the onset of the failure process. Scanning electron microscopy observations of the failure process reveal that microcracks develop mainly parallel to σ2 direction, as the intermediate stress grows beyond σ3, localizing in close proximity of the eventual main fracture.
Conditions for compaction bands in porous rock
Authors: Issen K. A. , Rudnicki J. W.
Published: February 2004

Reexamination of the results of Rudnicki and Rice for shear localization reveals that solutions for compaction bands are possible in a range of parameters typical of porous rock. Compaction bands are narrow planar zones of localized compressive deformation perpendicular to the maximum compressive stress, which have been observed in high‐porosity rocks in the laboratory and field. Solutions for compaction bands, as an alternative to homogenous deformation, are possible when the inelastic volume deformation is compactive and is associated with stress states on a yield surface “cap.” The cap implies that the shear stress required for further inelastic deformation decreases with increasing compressive mean stress. While the expressions for the critical hardening modulus for compaction and shear bands differ, in both cases, deviations from normality promote band formation. Inelastic compaction deformation associated with mean stress (suggested by Aydin and Johnson) promotes localization by decreasing the magnitude of the critical hardening modulus. Axisymmetric compression is the most favorable deviatoric stress state for formation of compaction bands. Predictions for compaction bands suggest that they could form on the “shelf” typically observed in axisymmetric compression stress strain curves of porous rock at high confining stress. Either shear or compaction bands may occur depending on the stress path and confining stress. If the increase in local density and decrease in grain size associated with compaction band formation result in strengthening rather than weakening of the band material, formation of a compaction band may not preclude later formation of a shear band.
Depth‐dependent earthquake focal mechanism orientation: Evidence for a weak zone in the lower crust
Authors: Bokelmann Götz H. R. , Beroza Gregory C.
Published: February 2004

The traction free boundary condition across the Earth's surface provides an opportunity for studying the relationship between stress orientation and earthquake focal mechanisms because it requires alignment of principal stress axes with vertical and horizontal orientations. A survey of earthquake focal mechanisms in northern California shows that their principal axes are also closely aligned with the vertical and the horizontal in the upper few kilometers of Earth's crust. Thus the signature of the free surface boundary condition on stress appears in focal mechanism orientations as well. The focal mechanism alignment can also be characterized by the relative magnitude of the off‐diagonal elements, Mxz and Myz, of the seismic moment tensor. We find significant and systematic depth variations in the “horizontal moment tensor element” ms, which relates to the shear traction acting on a horizontal plane for the special case of perfect alignment between principal stress and focal mechanism axes. Values of ms near Earth's surface are small but increase with depth to a maximum between 5 and 8 km. At greater depths, there is a gradual decrease, which suggests decreasing horizontal shear traction toward the base of the seismogenic zone. We interpret this tendency of axes to become oriented near the base of the seismogenic zone (and its expression in ms) as the signature of a weak zone in the lower crust. If correct, this observation would have important implications for the mechanics of lithospheric deformation.
Secular variation of the geocenter
Authors: Greff‐Lefftz Marianne
Published: February 2004

The degree 1 deformations of Earth induced by a surface load are computed, in a reference frame related to the center of mass of the planet, using a theoretical approach (Love number formalism), at secular timescale, where Earth has a viscoelastic behavior. The temporal variation of the coordinates of the geocenter induced by the Pleistocenic deglaciation is investigated for two loading histories (three ice sheets with the same time dependence or ICE‐3G model) testing different viscosity profiles within the mantle. The present‐day secular variation of Earth's geocenter induced by postglacial rebound is found to be from 0.2 up to 0.5 mm/yr, and consequently, we suggest that observations from Space Geodesy of this rate could be used to constrain the viscosity contrast between upper and lower mantle.
Improved remote sensing estimates of lava flow cooling: A case study of the 1991–1993 Mount Etna eruption
Authors: Wright Robert , Rothery David A. , Blake Stephen , Pieri David C.
Published: February 2004

Infrared radiance data acquired by the Landsat thematic mapper (TM) provide information regarding the thermal state of active lavas, which can be used to estimate flow surface temperatures and thus lava cooling. Saturation of one or both of TM's short‐wave infrared (SWIR) wavebands has meant, however, that previous studies of lava flow cooling have approximated what is almost certainly an n thermal component flow surface temperature distribution (where n > 1), with a single pixel‐integrated temperature (PIT). We show that the high sensitivity of TM's SWIR wavebands to the small amount of high‐temperature (e.g., ∼1000°C) material exposed in hot fractures within the flows cooler (e.g., ∼200°C) surface crust results in the calculation of PITs that are higher than the temperature of this crust, which is usually the dominant radiative component. This causes heat loss from such pixels to be overestimated. We present adaptations that allow more realistic heat loss estimates to be produced using TM data by assuming that a second high‐temperature component contributes to the radiance measured from saturated pixels and by defining an appropriate range for the temperature of the lava crust. By analyzing a suite of five TM images acquired during the 1991–1993 eruption of Mount Etna we present more realistic estimates of how this flow cooled, as well as results that show how the flow regime of the lava evolved over time. The methods we present will continue to be of use as, although the next generation of high spatial resolution sensors such as the Landsat 7 enhanced thematic mapper plus (ETM+) and the Terra advanced space‐borne thermal emission and reflection radiometer (ASTER) will be less susceptible to saturation than TM, the problem will still persist.
Deep structure of the Namibia continental margin as derived from integrated geophysical studies
Authors: Bauer Klaus , Neben Sönke , Schreckenberger Bernd , Emmermann Rolf , Hinz Karl , Fechner Notker , Gohl Karsten , Schulze Albrecht , Trumbull Robert B. , Weber Klaus
Published: February 2004

During the Geophysical Measurements Across the Continental Margin of Namibia (MAMBA) experiments, offshore and onshore refraction and reflection seismic as well as magnetic data were collected. Together with the existing free‐air gravity data, these were used to derive two crustal sections across the ocean‐continent transition. The results show that the Early Cretaceous continental breakup and the separation of South Africa and South America were accompanied by excessive igneous activity offshore. Off Namibia we found a 150–200 km wide zone of igneous crust up to 25 km thick. The upper part of this zone consists of an extrusive section comprising three units of basaltic composition: two distinct wedges of seaward dipping reflectors (SDRs) separated by flat‐lying volcanic flows. The inner wedge of SDRs can be modeled as the source of a long‐wavelength magnetic anomaly that borders long parts of both South Atlantic margins (anomaly G). The crust underneath these extrusives is characterized by high‐velocity and high‐density material (average values 7 km s−1, 3×103 kg m−3). Free‐air gravity anomalies along both sides of the high‐density crust are interpreted as edge effects resulting from juxtaposition with normal oceanic and continental crust on either side. We define the abrupt landward termination of this zone as the continent‐ocean boundary, and consequently, the crust seaward is interpreted as exclusively igneous material and not intruded continental crust. Extrapolation of the interpreted geophysical features along the southwest African margin suggests a fast prograding narrow rift zone and sharp lithospheric rupture leading to the formation of a margin‐parallel magmatic belt south of the Walvis Ridge. The influence of the Tristan da Cunha mantle plume may explain the widening of this thick igneous crust near the Walvis Ridge.
Crustal complexity from regional waveform tomography: Aftershocks of the 1992 Landers earthquake, California
Authors: Helmberger D. V. , Song X. J. , Zhu L.
Published: February 2004

We construct a two‐dimensional velocity section sampling the Mojave crustal block in southern California by modeling shear wave (SH) seismograms. Our approach uses individual generalized rays computed from a layered model. The model is divided into blocks with variable velocity perturbations such that ray responses are allowed to shift relative to each other to maximize synthetic waveform fits to data. An efficient simulated annealing algorithm is employed in this search. The technique is applied to a collection of 25 aftershocks (Landers earthquake) as recorded at two stations, GSC and PFO, separated by ∼200 km, which bracket the event population along the Landers fault system. The events are assumed to have known mechanisms and epicenters, but both their depths and origin times are allowed to vary. The results indicate considerable variation, especially in the top layer (up to ±13%), which mirrors surface geology. Best fitting models contain a low‐velocity zone in the lower crust if we constrain the crustal thickness (29 km) from receiver function analysis. Reduced lower crustal velocities imply crustal weakening, which appears compatible with the shallow seismogenic zone found in the northern end of this section. There is also evidence for a lateral jump in velocity of several percent across the San Andreas with the faster velocities on the west.
Observation of diffusion processes in earthquake populations and implications for the predictability of seismicity systems
Authors: Marsan David , Bean Christopher J. , Steacy Sandy , McCloskey John
Published: February 2004

Scale invariance, either in space or in time, has been shown in many papers to characterize earthquake distributions. Unfortunately, little work has been dedicated to looking at the general space‐time scaling invariance of seismicity systems, even though a better understanding of how the two domains (spatial and temporal) link together could help the development of the stochastic dynamical modeling of earthquake populations. In this paper we report the observation of diffusion processes of temporally correlated seismic activity for three different data sets: a mine (Creighton Mine, Canada), the Long Valley Caldera in eastern California, and a 7‐year period of recorded seismic activity in southern California. The observed subdiffusion processes are indicative of the general space‐time scaling of the system, taking the form of a slow power law growthR(t) ∼tHof the mean distanceR(t) between the main event and the temporally correlated afterevents occuring after a delayt.His found on average to be small (0.1 for Creighton Mine, 0.22 for the Long Valley Caldera, and 0.22 for the southern California main events with magnitude ≥1.5) but fluctuates significantly from one main event to the other: the diffusion is found to be intermittent (non‐Gaussian) and multiscaling, and except for the Long Valley Caldera, a systematic correlation between the sizes of the main event and subsequent afterevents and the growth exponentHis observed. While classical viscous relaxation models (e.g., elastic listhosphere‐plastic asthenosphere coupling, or fluid flow triggered by sudden changes in pore pressure) have been proposed to characterize this relaxation by homogeneous (i.e., nonintermittent) normal (H= 0.5) diffusion processes, the direct implication of the reported results is that seismicity systems, at spatial scales from meters to hundreds of kilometers and small (microearthquakes in a mine) to intermediate magnitudes, relax spatiotemporally in a nonelastic way, revealing the stochastic space‐time scale‐invariant nature of such systems. Since these diffusion processes correspond to a loss of information with time on the location of the main event, they can be used to investigate the limits of predictability, at all spatial scales, of seismicity systems in terms of the spatiotemporal clustering of temporally correlated earthquakes.
Wide‐angle seismic imaging of a Mesoproterozoic anorthosite complex: The Nain Plutonic Suite in Labrador, Canada
Authors: Funck Thomas , Louden Keith E. , Reid Ian D.
Published: February 2004

The Mesoproterozoic Nain Plutonic Suite (NPS) of Labrador (Canada), one of the largest anorogenic plutonic terranes, was studied by a refraction/wide‐angle seismic experiment. Four ocean bottom seismometers and 18 land stations were deployed along a 330‐km profile and recorded air gun shots from the easternmost 160 km with the NPS located in the center of the line at the suture of the Nain and Churchill Provinces. P and S wave velocity models were developed by forward modeling of travel times and amplitudes. Upper and middle crustal P wave velocities outside and beneath the NPS range from 5.9 to 6.5 km/s, lower crustal P wave velocities range from 6.55 to 7.0 km/s. Within the anorthositic rocks, velocities are as high as 6.8 km/s, and reflections define the base of the NPS to be 8 km deep in the SE Churchill Province and 11 km in the Nain Province, a variation that may be the result of lateral density changes within the country rocks or the anorthosites. The total crustal thickness is 39 km west of the NPS but is only 32–34 km beneath the NPS, some 5 km less than Nain Province crust distal from the NPS. The inferred crustal thinning is possibly related to anatexis of the lowermost crust by a thermal plume that generated the plutonism. The Poisson's ratios are 0.275 within the anorthosite plutons, 0.27 in the upper and middle crust, and 0.285 in the lower crust. These values are some 0.03 higher than in the Archean Nain crust distal to the NPS, indicating a higher plagioclase content at all crustal levels as result of the plutonism. We postulate that a crustal root, similar to the root observed farther north in the Torngat Orogen, was completely removed by anatexis and the silicic and basic magmas probably ascended to midcrustal levels along preexisting zones of weakness at the Nain‐Churchill boundary.
Stress and deformation along wavy frictional faults
Authors: Chester Frederick M. , Chester Judith S.
Published: February 2004

Locally inhomogeneous stress states are expected along faults owing to slip on geometrically irregular fault surfaces. We use an analytical model of elastic deformation along a wavy frictional fault to evaluate the variation in local stress state as a function of surface roughness, elastic modulus, slip, coefficient of friction, and far‐field stress. The total stress state along the fault may be described by the sum of a basic stress component resulting from frictional slip on a planar fault surface and a perturbed stress component resulting from the presence of roughness. Roughness produces a variation in normal stress across the fault surface, and assuming roughness and modulus appropriate to crustal faults, the normal stress should be reduced to a near‐zero magnitude locally, such that separation of fault walls is likely. The large variation in normal stress along the fault surface resulting from fault roughness may be responsible, in part, for complexity in moment release during large earthquakes and for lateral variation in seismic coupling along faults. The variation in principal stress orientations and magnitudes along a fault increases with a decrease in the coefficient of friction of the fault. The location and size of regions with a high likelihood for brittle failure depend on the orientation of the far‐field principal stress and fault friction. The average orientation of the principal stresses in the region of likely failure is not the same as the far‐field principal stress orientation. Although inversion of earthquake and fabric data for stress orientation along a fault may be possible, the model results suggest that inversion results are insufficient to determine far‐field stress states and fault friction without additional independent data.
Earthquake stress drop and laboratory‐inferred interseismic strength recovery
Authors: Beeler N. M. , Hickman S. H. , Wong T.‐f.
Published: February 2004

We determine the scaling relationships between earthquake stress drop and recurrence interval tr that are implied by laboratory‐measured fault strength. We assume that repeating earthquakes can be simulated by stick‐slip sliding using a spring and slider block model. Simulations with static/kinetic strength, time‐dependent strength, and rate‐ and state‐variable‐dependent strength indicate that the relationship between loading velocity and recurrence interval can be adequately described by the power law VL∝trn where n≈−1. Deviations from n=−1 arise from second order effects on strength, with n>−1 corresponding to apparent time‐dependent strengthening and n<−1 corresponding to weakening. Simulations with rate and state‐variable equations show that dynamic shear stress drop Δτd scales with recurrence as dΔτd/dlntr≤σe(b‐a), where σe is the effective normal stress, μ=τ/σe, and (a‐b)=dμss/dlnV is the steady‐state slip rate dependence of strength. In addition, accounting for seismic energy radiation, we suggest that the static shear stress drop Δτs scales as dΔτs/dlntr≤σe(1 +ζ)(b‐a), where ζ is the fractional overshoot. The variation of Δτs with lntr for earthquake stress drops is somewhat larger than implied by room temperature laboratory values of ζ and b‐a. However, the uncertainty associated with the seismic data is large and the discrepancy between the seismic observations and the rate of strengthening predicted by room temperature experiments is less than an order of magnitude.
Compound rupture of the great 1998 Antarctic plate earthquake
Authors: Antolik Michael , Kaverina Asya , Dreger Douglas S.
Published: February 2004

The March 25, 1998, Antarctic plate earthquake ruptured a portion of the Antarctic plate more than 200 km west of its boundary with the Australian plate. The Harvard centroid moment tensor solution indicates that the earthquake was primarily a strike‐slip event, but the large non‐double‐couple component of the moment tensor suggests considerable complexity in the rupture process. We use a finite fault inversion method to determine details of the rupture process from teleseismic body waves recorded by the Global Seismic Network. The P waves are poorly fit by one or more subevents having only a strike‐slip mechanism. We find that the presence of a large oblique‐normal faulting subevent located to the east of the hypocenter is necessary to improve the fit. This subevent combines with a larger strike‐slip subevent to the west to comprise the main moment release in the earthquake and is the cause of the large non‐double‐couple component in the long‐period focal mechanism. The earthquake exhibited very high slip and high stress drop compared with most interplate strike‐slip events and the rupture was largely confined to the upper 15–20 km of the lithosphere. Both constituent focal mechanisms indicate that this part of the Antarctic plate is under NW‐SE oriented tension, although the origin of these stresses is unknown.
Heat flow in the Indian shield
Authors: Roy Sukanta , Rao R. U. M.
Published: February 2004

Seventy new heat flow values have been determined for the Indian shield. The new sites include 28 from the Archaean Dharwar craton, one from the Proterozoic Cuddapah basin of southern India, four from the southern part of the Proterozoic Aravalli mobile belt of northwestern India, and 37 from the southern part of the Cretaceous‐Eocene Deccan Volcanic Province (DVP) of central India. Existing and new data lead to the following inferences: (1) The Dharwar craton, in common with other Archaean cratons is characterized by low heat flow values generally ranging from 25 to 50 mW m−2. (2) The western part of the Cuddapah basin and the DVP to the south of the Son‐Narmada‐Tapti megalineament zone are characterized by low heat flow values in the range 31 to 50 mW m−2. (3) There is no evidence for thermal transients associated with the K‐T Deccan volcanism on the present‐day heat flow regime in the southern part of the DVP. (4) In contrast to the southern Indian shield, the Aravalli mobile belt is characterized by significantly higher heat flow from 56 to 96 mW m−2. (5) Observations in the Aravalli and Singbhum Proterozoic belts show that some parts of the Precambrian shield are characterized by high as well as variable heat flow. With no evidence for recent tectonothermal events the variations in heat flow in the Indian shield can be mostly explained on the basis of radioactive heat production of the crustal rocks.
Evolution of the geomagnetic reversal rate since 160 Ma: Is the process continuous?
Authors: McFadden P. L. , Merrill R. T.
Published: February 2004

Two main interpretations have been advanced for the evolution of the geomagnetic reversal rate λ since about 160 Ma. These interpretations have quite different implications for Earth history and associated events and processes. Our own interpretation is that λ gradually and continuously decreased from its value of about 41/2 per million years at about 160 Ma to zero at about 118 Ma, then at about 83 Ma λ began to recover, again gradually and continuously, until reaching its present value of about 41/2 per million years. The Cretaceous Superchron occurred from about 118 to about 83 Ma, when the reversal rate was zero. Recently, Gallet and Hulot [1997] suggested an alternative interpretation where μ (= 1/λ) was constant from 160 Ma to about 130 Ma, at which time the process was rapidly and strongly perturbed. This perturbation led to a discontinuous increase in the mean length μ of intervals and caused the Cretaceous Superchron. The process then gradually recovered from the strong perturbation with the mean length μ gradually decreasing until the reversal process again became stationary at about 25 Ma. This interpretation is predicated on μ being constant from 160 to 130 Ma, whereas our interpretation requires a gradual increase in μ over this time interval. Consequently, a new statistic is suggested to distinguish between the null hypothesis that the reversal process is stationary against the specific alternative that there is a trend in the mean length μ. Application of this statistic shows that the data require a trend toward larger values of μ in the reversal process going from 160 to 130 Ma and that this trend continues strongly to the beginning of the superchron. Thus the addition of a discontinuity at 130 Ma is neither required nor supported by the data. Furthermore, application of this statistic shows that the trend toward smaller values of μ from 83 to 25 Ma appears to continue from 83 Ma to at least 12 Ma. Certainly there is no requirement for a change in the style of process at 25 Ma. Overall, the Gallet and Hulot interpretation seems untenable with the available data. Despite this, the Gallet and Hulot interpretation has great value in that it is inherently testable and our ability to reject it has increased our knowledge about the reversal process.
Practicalities of extrapolating one‐dimensional fault and fracture size‐frequency distributions to higher‐dimensional samples
Authors: Borgos Hilde G. , Cowie Patience A. , Dawers Nancye H.
Published: February 2004

Previously published theory, which extrapolates fault and fracture population statistics observed in a one‐dimensional sample to two‐ and three‐dimensional populations, is found to be of limited value in practical applications. We demonstrate how significant the discrepancies may be and how they arise. There are two main sources for the discrepancies: (1) deviations from ideal spatial uniformity (spatial Poisson process) of a fault or fracture pattern and (2) non‐power law scaling of the size frequency distributions of the population. We show that even small fluctuations in spatial density, combined with variance in the estimator of population statistics, can lead to considerable deviations from the theoretical predictions. Ambiguity about power law scaling or otherwise of the underlying population is a typical characteristic of natural data sets, and we demonstrate how this can affect the extrapolation of one‐dimensional data to higher dimensions. In addition, we present new theoretical approaches to the problem of extrapolation when clustering of faults and fractures is explicitly considered. Clustering is commonly observed in the field as en echelon arrays of fault or fracture segments and we show how this property of natural patterns can be quantified and included in the theory. These results are relevant to building more realistic three‐dimensional models of the physical properties of fractured rocks, such as fracture permeability and seismic anisotropy.
Extensional faulting and caldera collapse in the axial region of fast spreading ridges: Analog modeling
Authors: Lagabrielle Yves , Garel Erwan , Dauteuil Olivier , Cormier Marie‐Hélène
Published: February 2004

The axial high of the East Pacific Rise (EPR) is bounded by ridge‐parallel lateral grabens that develop 2–8 km off‐axis. These troughs appear to lengthen away from the ridge crest, suggesting that tectonics is active at least 10 km away from the axis. Along 15–20% of the length of the ridge the axial high is notched by a summit trough 500 to 1800 m wide. These large axial troughs represent elongated collapse calderas that form when the EPR magma reservoir (comprising the melt lens and the underlying crystal mush zone) deforms and compacts during periods of waning magma supply under continuous stretching. We report the results of analog experiments performed in order to constrain the tectonic‐magmatic evolution of the crestal region of fast spreading ridges and more particularly the possible link that may exist between the development of axial caldera and the creation of lateral grabens along the crestal region. We used inflatable elongated balloons filled with water as an analog of the magma reservoir. The balloon is capped with a silicone layer representing hot rocks below the brittle‐ductile transition and is covered by a sand layer representing the brittle crust. The sand surface was given a dome shape that approximates the morphology of a fast spreading ridge. Mobile walls activated by a stepping motor allowed us to conduct the deflation experiments during continuous extension. Combination of deflation and extension leads to the creation of two lateral depressions and one axial trough. The lateral depressions are controlled by normal faults, while the central trough is delineated by reverse faults. During balloon deflation, tectonic extension is accommodated away from the axis due to the presence of the ductile silicone layer, whereas the deflation of the axial reservoir is accommodated by the collapse of the overlaying brittle crust. This accounts (1) for the development of axial troughs as collapse calderas, and (2) for the ubiquitous formation of lateral grabens of the flanking tectonic province in response to the deformation of the crystal mush.
Paleomagnetism of Triassic and Jurassic red bedded chert of the Inuyama area, central Japan
Authors: Oda Hirokuni , Suzuki Hisashi
Published: February 2004

A paleomagnetic study was conducted on Triassic and Jurassic red bedded chert from the Inuyama area, central Japan. Paleomagnetic specimens were collected from single chert beds at nine sites within mesoscopic (1∼5 m) folds, whose ages were determined by radiolarian fossils. Progressive thermal demagnetization experiments were conducted, and three of the four magnetization components provided tectonic implications. Component D (unblocking temperature of 560°–680°C), from six sites, passes the fold test with the mesoscopic folds. Paleolatitude was estimated as 12.3°±15.6°N or S, 19.0°±10.5°N or S, and 25.7°±16.7°N or S for 237 Ma, 223.5 Ma, and 210 Ma, respectively. In the case of northern latitude, a northward migration of 5.6± .9 cm/yr is estimated between 237 Ma and 210 Ma, with no significant latitudinal migration from 210 Ma to 160 Ma. Component C (unblocking temperature of 350°–530°C) has normal polarity and is thought to have been acquired as a chemical remanent magnetization carried by magnetite, which was acquired as synfolding magnetization with the Sakahogi Synform. The magnetization is considered to have been acquired through heating in the accretionary wedge during the Cretaceous Normal Superchron (120.5–83.5 Ma). Component B (unblocking temperature of 200°–350°C) has reversed polarity and is thought to have been acquired as a thermoviscous remanent magnetization after the bending of the Sakahogi Synform. The most probable period of the magnetization acquisition is between C27r and C24r (58–63 Ma) after the emplacement of Late Cretaceous granitic batholiths.
Investigating the frequency dependence of mantle Q by stacking P and PP spectra
Authors: Warren Linda M. , Shearer Peter M.
Published: February 2004

Using seismograms from globally distributed, shallow earthquakes between 1988 and 1998, we compute spectra for P arrivals from epicentral distances of 40° to 80° and PP arrivals from 80° to 160°. Selecting records with estimated signal‐to‐noise ratios greater than 2, we find 17,836 P and 14,721 PP spectra. We correct each spectrum for the known instrument response and for an ω−2 source model that accounts for varying event sizes. Next, we stack the logarithms of the P and PP spectra in bins of similar source‐receiver range. The stacked log spectra, denoted as log(DP′) and log(DPP′), appear stable between about 0.16 and 0.86 Hz, with noise and/or bias affecting the results at higher frequencies. Assuming that source spectral differences are randomly distributed, then for shallow events, when the PP range is twice the P range, the average residual source spectrum may be estimated as 2 log(DP′)–log(DPP′), and the average P wave attenuation spectrum may be estimated as log(DPP′) – log(DP′). The residual source spectral estimates exhibit a smooth additional falloff as ω−0.15±0.05 between 0.16 and 0.86 Hz, indicating that ω−2.15±0.05 is an appropriate average source model for shallow events. The attenuation spectra show little distance dependence over this band and have a P wave
value of ∼0.5 s. We use measurements from individual P and PP spectra to invert for a frequency‐independent Q model and find that the upper mantle is nearly 5 times as attenuating as the lower mantle. Frequency dependence in Qα is difficult to resolve directly in these data but, as previous researchers have noted, is required to reconcile these values with long‐period Q estimates. Using Q model QL6 [Durek and Ekström, 1996] as a long‐period constraint, we experiment with fitting our stacked log spectra with an absorption band model. We find that the upper corner frequency f2 in the absorption band must be depth‐dependent to account for the lack of a strong distance dependence in our observed values. In particular, our results indicate that f2 is higher in the top 220 km of the mantle than at greater depths; the lower layer is about twice as attenuating at 1 Hz than at 0.1 Hz, whereas the upper mantle attenuation is relatively constant across this band.
Causality constraint for fractures with linear slip weakening
Authors: Knopoff L. , Landoni J. A. , Abinante M. S.
Published: February 2004

We consider the problem of dynamic fracture growth characterized by linear slip‐weakening properties. If the problem is attacked by standard linear spectral methods, we show that the elastic wave radiation from the slip in the weakening zone has noncausal terms. The nonphysical behavior is specifically attributable to the application of a weak causality constraint in standard linear spectral analysis to a problem with an active boundary condition. We give the correct analytic solution, derived from linear spectral analysis applied to a boundary integral procedure.
Anisotropy of schists: Contribution of crustal anisotropy to active source seismic experiments and shear wave splitting observations
Authors: Godfrey Nicola J. , Christensen Nikolas I. , Okaya David A.
Published: February 2004

We have made sets of five independent compressional and shear wave velocity measurements, which with density, allow us to completely characterize the transverse isotropy of samples from five metamorphic belts: the Haast schist terrane (South Island, New Zealand), Poultney slate, Chugach phyllite, Coldfoot schist, and Pelona schist (United States). These velocity measurements include compressional wave velocities for propagation parallel, perpendicular, and at 45° to the symmetry axis, shear wave velocity for propagation and particle motion perpendicular to the symmetry axis, and shear wave velocity for propagation parallel to the symmetry axis. Velocity measurements were made up to pressures of 1 GPa (∼35‐km depth) where microcracks are closed and anisotropy is due to preferred mineral orientation. Our samples exhibit compressional wave anisotropy of 9–20% as well as significant shear wave splitting. Metamorphic terranes that are anisotropic to ultrasonic waves may also be anisotropic at the scale of active and passive seismic experiments. Our data suggest that a significant thickness (10–20 km) of appropriately oriented (steeply dipping foliation) schist in the crust could contribute as much as 45% of observed shear wave splitting. Our data set can also be used to model the effects of crustal anisotropy for active source seismic experiments in order to determine if the anisotropy of the terrane is significant and needs to be taken into account during processing and modeling of the data.
President Jackson Seamounts, northern Gorda Ridge: Tectonomagmatic relationship between on‐ and off‐axis volcanism
Authors: Davis Alicé S. , Clague David A.
Published: February 2004

Basaltic lavas and hyaloclastites dredged from the President Jackson Seamounts, a 65‐km‐long, linear volcanic chain starting near latitude 42°20′N west of the northern Gorda Ridge, are normal mid‐ocean ridge basalt (MORB), with compositions ranging from highly depleted (0.07% K2O, 0.54 La/SmN) to moderately enriched in incompatible elements (0.24% K2O, 0.86 La/SmN). The seamount lavas are similar in many respects to those erupted at the adjacent ridge but have some important geochemical differences. Seamount lavas are concentrated toward primitive compositions with higher MgO contents (to 9.3%, Mg number 69.5) than most of the nearby ridge lavas. They also have systematically lower TiO2 and FeO and higher CaO, Na2O, and Sr at comparable MgO content than adjacent ridge basalts. Seamount basalts contain phenocrysts in equilibrium with the melt and lack compositionally diverse glass inclusions and compositional zoning common in phenociysts of most ridge basalt. Minor and trace element variability at a given MgO content indicates complex petrogenetic processes. Radiogenic isotopic ratios indicate somewhat less depleted sources than for basalt from the adjacent ridge axis. The higher Pb isotopic ratios of some lavas trend toward enriched MORB. The seamounts typically have multiple, nested calderas or pit craters, stepping downward toward the ridge axis, indicating formation in the active, near‐ridge, extensional environment. The predominantly primitive nature of the lavas suggests that they pass through crustal magma reservoirs, presumably underlying the calderas, very rapidly. The lack of evidence for magma mixing suggests that batches of magma are delivered to the seamounts episodically and either solidify or are drained into ridge‐parallel faults before the next batch arrives. In contrast, lavas from the ridge axis show evidence for magma mixing involving more fractionated melts that show evidence for clinopyroxene fractionation. Despite a lack of seismic evidence for magma chambers under slow spreading centers, continuous melt zones must be present under the Gorda Ridge axis to give the ubiquitous imprint of magma mixing.