bluuebunny/crossref_metadata_2025 huggingface dataset:
Geophysical survey of a mud volcano seaward of the Barbados Ridge Accretionary Complex
Authors: Langseth Marcus G. , Westbrook Graham K. , Hobart Michael A.
Published: February 2008

A detailed geophysical investigation that included multibeam bathymetry, 3.5‐kHz‐channel and single‐channel seismic profiles, 20 heat flow measurements and four piston cores was made of a mud volcano located about 8 km seaward of the toe of the Barbados Ridge Accretionary Complex in the northern Guiana Basin. The material erupted from the volcano covers an oval area about 3×9 km with its long axis oriented E–W, roughly perpendicular to the deformation front. A flat‐topped mound on the western side of the feature is the currently active center. Acoustic stratigraphy suggests that mud volcanism began at the site about 200,000 Ma. A piston core taken on the flank of the active mound sampled deposits erupted 20,000–30,000 Ma that contain late Miocene nannofossils. Heat flow on the active mound is 4 times that in the surrounding basin. One‐dimensional modeling of this anomaly supports a low‐level continuing flux of mud and water at rates of 2–5 cm/yr. The mud volcanism appears to have arisen from the combined action of overpressured water that comes from beneath the accretionary prism and existing overpressures in the oceanic section. Large east‐west trending basement escarpments underlie the two known mud volcanoes seaward of the Barbados Ridge Accretionary Complex, and they probably play an important role in developing vertical pathways for fluid to escape and in determining the location where mud volcanism starts.
Three‐dimensional Vp/Vs variations for the Coso Region, California
Authors: Walck Marianne C.
Published: February 2008

Recent seismological studies of the Coso region of southeastern California document both low P wave velocities and abnormal SV attenuation in Indian Wells Valley, south of the Pleistocene volcanics of the Coso Range. In order to learn more about the physical nature of these colocated anomalies, a tomographic inversion for the three‐dimensional variations of Vp/Vs the ratio of compressional to shear velocity was performed. Iterative back projection of 2966 shear and compressional wave travel time residuals from local earthquakes recorded on vertical instruments reveals that Vp/Vs is generally high at the surface and decreases systematically to 10 km depth. Superimposed on this trend are several large anomalies. Near Devil's Kitchen in the Coso Geothermal Area, Vp/Vs values are very low near the surface, consistent with measured values for steam‐dominated geothermal systems. Abnormally high values of Vp/Vs are observed in portions of Indian Wells Valley and also below the Cactus Peak rhyolite dome from 2 to 5 km in depth. In Indian Wells Valley the co‐occurrence of low P velocities, low S velocities, high Vp/Vs, and anomalous SV attenuation are indicative of subsurface partial melt. Because the inversion is based on vertical data only, however, the results cannot be considered conclusive, only suggestive.
Geography and history of the Louisville Hotspot Chain in the southwest Pacific
Authors: Lonsdale Peter
Published: February 2008

The Louisville “Ridge” is a 4300‐km‐long Late Cretaceous and Cenozoic hotspot chain, the South Pacific equivalent of the Hawaiian‐Emperor chain. Its northwestern end is being consumed by the Tonga‐Kermadec subduction zone, but Seabeam and magnetic mapping shows that the chain still includes more than 60 major volcanoes, of both normal and reversed polarity, distributed along a 75‐km‐wide band that can be fitted to small circles about three successive poles of Pacific plate/hotspot rotation. This band obliquely crosses fracture zones of the Eltanin system, but there is little interaction and no evidence of any genetic connection between the two structures. Forty of the Louisville volcanoes grew above sea level and are preserved as high‐latitude (i.e., coral‐free) guyots. They are spaced less than 100 km apart along most of the chain, but there are none within 750 km of the inferred present location of the hotspot, beneath a swell at the southeast end of the chain. The rate of volcano building by the Louisville hotspot declined sharply about 20 m.y. ago, after being remarkably constant at 3–4×103 km3/m.y. for the previous 50 m.y., and none of the Louisville volcanoes built during the past 10–12 m.y. (the time of most profuse Hawaiian volcanism) has reached sea level. However, a seamount from which Pleistocene lavas were dredged rises to within 540 m of the sea surface from the crest of the hotspot swell at 50.5°S, 139.2°W. Guyot heights demonstrate that a hotspot swell several hundred meters high has persisted throughout the known life of the chain, and sometimes had an isostatic depression on its loaded crest. The depths of guyot shelf breaks increase systematically to the northwest (from 530 to 2100 m) proportional to the square root of volcano age, with some scatter and tilting caused by plate boundary tectonism and local isostatic subsidence and uplift.
Using surface displacement and strain observations to determine deformation at depth, with an application to Long Valley Caldera, California
Authors: Vasco D. W. , Johnson L. R. , Goldstein N. E.
Published: February 2008

We present a method for the inversion of surface displacement and strain to determine six independent combinations of the strain components at depth. These six combinations represent three mutually perpendicular double forces without moment and three pairs of double forces with moment. Furthermore, they may be directly related to a moment tensor density distribution for static strain. The method is linear and, unlike other methods, does not depend on an a priori source model. Due to the linearity of the formulation, methods such as generalized inversion or l1 residual norm minimization may be used to determine models of deformation at depth. The methods presented are applicable to a variety of available geodetic data sets. The technique is applied to repeat leveling data taken between 1982 and 1985 in Long Valley caldera. We determined a model of the magma intrusion suspected to have occurred beneath the caldera. The model derived minimized the l2 norm of the difference between the predicted and observed data subject to the constraint that only intrusion occur. The model indicates that most volume expansion took place in the caldera region above 8 km±2 km. In the 0–4 km depth range the expansion is widely distributed with a maximum value of 0.005 km3. Deeper, 4–8 km, there is a concentration of intrusion in the central portion of the caldera. The maximum volume increase here is 0.030 km3.
Sandstone fractal and Euclidean pore volume distributions
Authors: Krohn Christine E.
Published: February 2008

Geometrical measurements of pore microstructure indicate that the pore volume distributions for sandstones can be separated into two regimes: a short‐length regime with fractal behavior and a long‐length regime with no fractal behavior. The fractal part of the distribution is statistically described by a power law, and it appears to be governed by the growth of minerals and cements in the pore space. The nonfractal or Euclidean part of the distribution is dominated by a characteristic length, and it apparently reflects the original unaltered porosity. Two of the sandstones have both fractal and Euclidean regimes, and two of the sandstones have only the fractal regime. The latter two samples are more highly cemented than the former two. Fractal porosities calculated from the fractal parameters measured on fracture surfaces agree with the porosity in the fractal regime measured on thin sections. Criteria developed to determine the relative porosity associated with the fractal and Euclidean regimes could be used to characterize the amount of diagenetic alteration of the pore volume. Photographs are used to support conclusions derived from the geometrical measurements.
Fractal measurements of sandstones, shales, and carbonates
Authors: Krohn Christine E.
Published: February 2008

Measurements were made of the fractal properties of sandstones, shales, and carbonates using a statistical analysis of structural features on fracture surfaces. Fractal behavior is associated with power law behavior for the number of features as a function of the feature size on the pore‐rock interface. Only one sedimentary rock, a novaculite, was found not to have a fractal structure. The fractal dimensions range from 2.27 to 2.89, and the long‐length limits to the fractal regime range from 2 μm to over 50 μm. In all cases, the fractal behavior extends to less than 0.2 μm which is the measurement resolution. The porosity associated with the fractal pore‐rock interface can be calculated from the fractal parameters. Some of the samples have additional porosity not associated with power law behavior. Photographs and other evidence are used to show that the fractal structures are the result of diagenesis. Fractal diagenetic structures include euhedral quartz overgrowths, druse quartz, calcite, dolomite, clays, and chert.
Rupture initiation in shear fracture of rocks: An experimental study
Authors: Cox S. J. D. , Scholz C. H.
Published: February 2008

We used a torsional geometry to grow shear ruptures in two fine‐grained rock types. The sample has a slot that is loaded to produce a strong mode 3 shear stress concentration. The onset and growth of the damage zone is detected by monitoring the elastic stiffness of the samples. We find that the samples continue to harden after the onset of fracture. This is initially accompanied by oblique tensile cracks forming at the stress concentration. The peak in supported load apparently corresponds to the onset of strain localization, with the development of cross‐cutting shear fractures. We attempted to estimate the partitioning of energy dissipation between elastic and frictional components with a fracture mechanics analysis. The elastic energy increases with normal stress, but scatter in the inelastic data does not allow identification of convincing trends. The sample is too small to develop a complete transition to a shear rupture. The geometry of the fractures is used to explain the energy relations as well as other experimental observations
Deformation from inflation of a dipping finite prolate spheroid in an elastic half‐space as a model for volcanic stressing
Authors: Yang Xue‐Min , Davis Paul M. , Dieterich James H.
Published: February 2008

Exact analytic expressions are given for the deformation field resulting from inflation of a finite prolate spheroidal cavity in an infinite elastic medium. The field is equivalent to that generated by a parabolic distribution of double forces and centers of dilatation along the spheroid generator. Approximate, but quite accurate, solutions for a dipping spheroid in an elastic half‐space are found using the half‐space double force and center of dilatation solutions. We compare results of the surface deformation field with those generated by the point source ellipsoidal model of Davis (1986). In the far field both models give identical results. In the near field the finite model must be used to calculate displacements and stresses within the medium. We also test the limits of applicability of the finite model as it approaches the surface by comparing the surface displacement field from a vertical spheroid with that calculated from the finite element method. We find the model gives a satisfactory approximation to the finite element results when the minimum radius of curvature of the upper surface is less than or equal to its depth beneath the free surface. Comparison of surface displacements generated by the point and finite element models gives good agreement, provided this criterion is satisfied. We have used the finite model to invert deformation data from Kilauea volcano, Hawaii. The results, which compare favorably with those obtained from the point ellipsoid model, can be used to estimate the distribution of stresses within the volcano in the near field of the source.
The mechanism of intrusion of the Inyo Dike, Long Valley Caldera, California
Authors: Reches Ze'ev , Fink Jonathan
Published: February 2008

We analyze the intrusion of the 11‐km‐long Inyo Dike at the margins of Long Valley caldera, eastern California. The dike trends N07°W and is divided into at least three segments which are rotated by as much as 25° with respect to the main trend. The dike seems affected primarily by the regional stress field of right‐lateral shear of the western United States and by the local thermal conditions of the crust; the dike seems unaffected by the preexisting caldera margins and Sierra‐Nevada frontal fault system. The high heat flow in Long Valley caldera implies that crustal rocks below 3–4.5 km deform by steady state creep under tectonic strain rate and support low to vanishing tectonic shear stresses. The upper rocks, above 3–4.5 km, deform by frictional slip along fractures and may support tectonic shear stresses as high as 24 MPa. We demonstrate that depth variations of tectonic stresses may have a profound effect on the segmentation and rotation of dikes, both at Long Valley and in other areas of high heat flow. The analysis places constraints on several tectonic conditions. The Inyo Dike intruded under a tectonic stress state with a horizontal maximum compression oriented N07°W. The maximum extensional fracture strength of the host rocks is 1–2.5 MPa, and the pressure drop within the propagating Inyo Dike was about 0.55 MPa/km. The volatile overpressure in the magma chamber was about 15 MPa during eruption of rhyolitic lavas at the Inyo Domes.
Response of two‐phase fluids to fracture configurations within submarine hydrothermal systems
Authors: Goldfarb Marjorie S. , Delaney John R.
Published: February 2008

Submarine hydrothermal systems at mid‐ocean ridges evolve within a region of brittle deformation overlying a region of ductile deformation. The interplay between brittle and ductile modes of deformation and the geometry of fracture systems within the brittle zone can exert a significant influence on fluid flow, especially under conditions leading to aqueous two‐phase separation. Within the brittle region, fractures vary between large conduits directly open to the seafloor and small veins and microcracks with constricted or indirect connections to the seafloor. In the ductile region, fluids may be trapped under near‐lithostatic pressure, while fluids within throughgoing veins in the brittle region will experience hydrostatic pressure. In a narrow transition zone between these two extremes, a steep pore fluid pressure gradient must exist. In the brittle region itself, fluid pressures will vary between lithostatic and hydrostatic as fluid occupies interstices varying from isolated pores through tortuous microcracks to channels directly connected to the main upflow conduits. A hydrostatic gradient modified by the decreased density of hot fluid and the effects of the flow itself should be applicable in major conduits open to the seafloor. Fluctuating pressure conditions in the transition zone may induce phase changes in a fluid remaining at constant depth. As a result of the large volume increase that accompanies two‐phase separation, a two‐phase fluid must either accelerate upward or exert an increased pressure on its surroundings, possibly leading to enhanced fracturing. The fracture configuration will also affect the behavior of the mixed fluid after phase separation. A two phase‐fluid experiencing rapid turbulent flow in a large open conduit is more likely to remain a well‐mixed suspension of vapor and liquid, whereas a two‐phase mixture in a constricted, branching series of channels, moving more slowly in a laminar flow regime, is more likely to undergo physical isolation of the phases, leading to venting of a vapor phase and retention of a saline brine phase at depth. We propose a possible configuration for the fracture pattern within a submarine hydrothermal system consisting of a feeder zone comprised of anastomozing microcracks and veinlets, progressively increasing in diameter and decreasing in abundance, feeding into a main upflow/stockwork zone in which flow directly to the surface takes place in relatively large, unconstricted conduits. Fluid movement within a feeder zone may be modified by capillary flow, tortuous channelways, disconnected fractures, blocking of cracks by mineral deposition, and opening of new fractures.
Paleoseismicity and neotectonics of the Cordillera Blanca Fault Zone, northern Peruvian Andes
Authors: Schwartz David P.
Published: February 2008

The Cordillera Bianca fault zone is a major west dipping normal fault that bounds the west side of a 120‐ to 170‐km‐wide zone of active extension along the crest of the northern Peruvian Andes. The fault is approximately 210 km long and exhibits continuous geomorphic evidence of repeated late Pleistocene and Holocene displacements but has not been the source of historical or teleseismically recorded earthquakes. Trenching and mapping of fault scarps provide new information on earthquake recurrence, slip rate, timing of the most recent events, and Andean neotectonics. At Quebrada Queroccocha, 55 km from the south end of the zone, an 11,000‐ to 14,000‐year‐old moraine is displaced 12–15 m, and younger valley fill lacustrine and fluvial deposits are displaced 7.5–8 m. Scarp profiles, tectonic terraces, and trench exposures indicate five to seven scarp‐forming earthquakes of 2–3 m per event during the past 11,000–14,000 years at this location. Considering uncertainties in the number of events and in the age of the displaced moraine, the average recurrence interval is estimated to be 2440±1060 years. Detrital charcoal from colluviums that predate and postdate the most recent event yielded ages of 2480±65 and 750+80 14C years B.P., respectively, and stratigraphic relationships suggest that the event may be closer to the older date. The late Quaternary slip rate at Quebrada Queroccocha is 0.86–1.36 mm/yr and appears to be representative of much of the fault zone. At Pachma Bajo, located 30 km from the north end of the zone on a separate segment, displacement during each of the past two events was 2 to ≥ 3 m. The morphology of scarps in alluvial fan and debris flow deposits indicates recurrence intervals of about 1000–3000 years. Relationships between pre‐Inca walls and faulted deposits suggest that 1500–2000 years have elapsed since the most recent event. Fault behavior parameters, particularly displacement per event and fault segment length, show that the Cordillera Bianca fault zone is similar to seismogenic normal faults that nucleate M 7–7½ earthquakes at depths of approximately 15 km. The Quebrada Queroccocha slip rate and analysis of other normal faults in the region suggest cumulative late Quaternary extensional strain rates of about 1–4 mm/yr across the complete zone of Andean crestal extension.
Deformational models of rifting and folding on Venus
Authors: Banerdt W. Bruce , Golombek Matthew P.
Published: February 2008

Tectonic features on Venus with characteristic widths and/or spacings can be used to place quantitative constraints on the structure of the mechanical lithosphere through geophysical modeling. We develop theoretical models for both compressional (buckling) and extensional (rifting) deformation based on classical elastic plate models with characteristic spacings determined by the flexural parameter, modified to account for the effects of a realistic rheology. This rheology consists of a relatively weak crust overlying a more creep‐resistant mantle. In both layers the upper portions deform by frictional yielding on preexisting faults with a yield stress that increases linearly with depth, and the lower portions deform by non‐Newtonian temperature‐dependent creep. For extensional deformation a graben model is used in which the primary structure of the rift is formed in the strong mantle layer and secondary faulting occurs in the brittle crustal layer. The introduction of a weak zone in the lower crust that acts to decouple these two layers allows the formation of rifts with the large widths and depths observed on Venus. The compressional folding model utilizes an elastic buckling approach in which the non‐elastic yielding of the upper and lower portions of the layer modifies the elastic folding wave‐length. This approach avoids the unreasonably high stresses that are required for conventional buckling models. The two scales of deformation observed in radar images, 15–25 km and 100–300 km, can be related to the elastic flexural properties of the crust and mantle, respectively, which are separated by an intermediate weak zone in the lower crust. These results imply that for regions in which both scales of deformation are present the crust is 5–15 km thick and the thermal gradient is 10°–15°km−l. In regions such as Ishtar Terra, where only the smaller features are observed, the crust may be considerably thicker. The apparent structure seen in high‐resolution images of the Beta Regio rift, with the major faults located at the edges of an extended linear central depression, lends support to the assumption of a graben mechanism of rift formation.
Martian neutron leakage spectra
Authors: Drake D. M. , Feldman W. C. , Jakosky B. M.
Published: February 2008

Energy spectra of Martian leakage neutrons are calculated by a high‐energy nucleon‐meson transport code using a Monte Carlo technique and a one‐dimensional diffusion accelerated neutral‐particle transport code, which solves the Boltzmann equation. Four series of calculations were made to simulate (1) a uniform surface layer containing various amounts of H2O, (2) different burial depths of a 50% H2O layer underneath a 1% H2O layer, (3) changing atmospheric pressure, and (4) a thick CO2 ice sheet overlying a “dirty” water ice sheet. We found that all calculated spectra at energies less than about 1000 eV could be fitted by a superposition of thermal and epithermal functions having four free parameters. Two of these parameters, the thermal and epithermal amplitudes, were found to vary systematically over ranges exceeding 1 order of magnitude and to specify uniquely the configuration in each of the series. We conclude that measurements of leakage neutron spectra should allow determination of the hydrogen content of surface layers buried to depths up to about 100 g/cm2 and determination of the thickness of a polar dry ice cap up to thicknesses of about 250 g/cm2. Variations of these parameters were also shown to depend on the composition of the assumed surface layers through the average atomic mass and the macroscopic scattering and absorption cross sections.
Electric resistivity structure of the Siljan impact region
Authors: Zhang Ping , Rasmussen Thorkild M. , Pedersen Laust B.
Published: February 2008

Based on 65 magnetotelluric (MT) measuring stations, a cross section of the electric conductivity structure in the Siljan impact area, located in the central part of Sweden, is presented. Static shift effects due to local near‐surface inhomogeneities are present in nearly all of the MT soundings. In an attempt to correct for near‐surface distortions, average impedance data and distortion parameters were interpreted. Derived two‐dimensional models show some coincidence with seismic reflection results and support an impact origin of the Siljan area. Fluid migration through the fractures generated by the impact is one possible explanation for the anomalously high conductivity within the upper crust.
Measurements of coseismic deformation in southern California: 1972–1982
Authors: Wyatt Frank K.
Published: February 2008

Since 1972, a number of static strain changes have been measured at Piñon Flat Observatory, southern California, as a result of regional faulting. As recorded by a collection of long‐baselength strainmeters and tiltmeters, these signals are in good quantitative agreement with the deformation calculated for a dislocation in an elastic half‐space. In contrast, colocated short‐baselength sensors are found to produce spurious results whose size is much larger than expected theoretically and well correlated with peak site acceleration. Calculations show that the root‐mean‐square (rms) of the coseismic signals from several different instruments at one site, or from several sites at known hypocentral distances, provide a good estimate of the seismic moment, a result confirmed by the observations. Static moment estimates for the four largest events in this period are Imperial Valley 1979, M0 = 9.0 × 1018Nm; Mexicali Valley 1980, M0 = 6.1 × 1018Nm; Westmorland 1981, M0 = 5.3 × 1017Nm; and Brawley 1979, M0 = 2.3 × 1017 Nm. These estimated moments generally exceed those determined seismically by about 25%, which is as expected since they include some postseismic deformation. Results from the Mexicali Valley earthquake are particularly interesting, indicating an average fault strike approximately 9° different from the historic fault trace. Several small to moderate earthquakes (ML 3.7–5.2) were also well enough recorded to estimate source parameters. For the only other detectable events in the period 1972–1982 (a total of three others for which the strain change Δεrms was greater than 0.4 nε), high accelerations caused most of the instruments at the site to malfunction. No precursory signals were evident for any of the earthquakes recorded at this site.
Sedimentary and crustal structure of the northwestern Gulf of Mexico
Authors: Ebeniro Joseph O. , Nakamura Yosio , Sawyer Dale S. , O'Brien William P.
Published: February 2008

We conducted a large‐offset seismic experiment in the northwestern Gulf of Mexico using largecapacity air guns and digital ocean‐bottom seismographs to determine the velocity structure of the sediments, crust, and upper mantle. Five multiple‐detector split‐profile lines were shot over an area extending from the shallow midshelf south of Galveston to the continental rise just south of the Sigsbee escarpment. The data were of adequate spatial density and quality to allow combined use of near‐vertical reflections, wide‐angle reflections, and refractions for interpretation. Several techniques were used to obtain velocity‐depth functions: (1) conventional constant‐velocity‐layer analysis, (2) interval velocities from moveouts of the precritical reflections, (3) analysis in the tau‐p domain to determine extremal depth bounds, (4) estimation of the thickness of the allochthonous salt from the limiting distance of salt refractions, and (5) forward modeling using two‐dimensional (2‐D) ray tracing. A sedimentary sequence of nearly constant thickness (13 to 15 km) covers the study area. Various mobilized salt features exist within the sediments in the slope area, ranging from deeply buried layers and diapirs under the inner slope to shallow, thin, allochthonous bodies under the outer slope. In contrast to the sediments, the crust shows considerable variation in thickness, from normal oceanic crustal thickness beneath the continental rise to nearly normal continental thickness beneath the shelf. The transition under the slope, however, is not monotonie but includes thinning to nearly oceanic thickness under the midslope, possibly suggesting an incipient rift that failed to materialize during the opening of the gulf.
Crustal structure of east central Oregon: Relation between Newberry Volcano and regional crustal structure
Authors: Catchings R. D. , Mooney W. D.
Published: February 2008

A 180‐km‐long seismic refraction transect from the eastern High Cascades, across Newberry Volcano, to the eastern High Lava Plains is used to investigate the subvolcanic crustal and upper mantle velocity structure there. Near‐surface volcanic flows and sedimentary debris (1.6–4.7 km/s), ranging from 3 to 5 km in thickness, overlie subvolcanic Basin and Range structures. East and west of Newberry Volcano, the subvolcanic basement (5.6 km/s) has been downwarped, producing 5‐km‐deep basins. The midcrust (8‐ to 28‐km depth) is characterized by velocities ranging from 6.1 to 6.5 km/s and varies laterally in thicknesses. The lower crust is characterized by art unusually high velocity (about 7.4 km/s), and its geometry mirrors the subvolcanic basement geometry. The Moho is located at a depth of 37 km and represents a transition to an upper mantle velocity of 8.1 km/s. The shallow subsurface (1.2 km) beneath Newberry Volcano is characterized by high‐velocity (5.6 km/s, versus 4.1 km/s for the surrounding area) intrusions and appears to be located on a basement high. Beneath the seismic refraction array at Newberry Volcano, an absence of low‐velocity anomalies suggests that large silicic magma chambers do not exist in the upper crust, but apparent high attenuation of the seismic wave field may be consistent with either partial melts in small volumes, elevated crustal temperatures, and/or poor geophonerecording site coupling. The east central Oregon velocity structure is nearly identical to that of the northwestern Nevada Basin and Range and the Modoc Plateau of northeastern California, and variations in the deep crustal structure about Newberry Volcano are consistent with tectonism involving crustal underplating, melting, and extension. If paleomagnetic estimates of extension in the east central Oregon area are correct, the North American continent experienced significant growth through extension, underplating, and volcanism in east central Oregon. Comparison of east central Oregon with other continental back arc areas shows similarities in crustal thickness, velocity, and heat flow.
The hydrothermal system at Newberry Volcano, Oregon
Authors: Sammel E. A. , Ingebritsen S. E. , Mariner R. H.
Published: February 2008

Results of recent geological and geophysical studies at Newberry Volcano have been incorporated into conceptual and numerical models of a magma‐based hydrothermal system. Numerical simulations begin with emplacement of a small magma body, the presumed source of silicic eruptions at Newberry that began about 10,000 B.P., into a thermal regime representing 100,000 years of cooling of a large underlying intrusion. Simulated flow patterns and thermal histories for three sets of hypothetical permeability values are compatible with data from four geothermal drill holes on the volcano. Meteoric recharge cools the caldera‐fill deposits, but thermal water moving up a central conduit representing a permeable volcanic vent produces temperatures close to those observed in drill holes within the caldera. Meteoric recharge from the caldera moves down the flanks and creates a near‐isothermal zone that extends several hundred meters below the water table, producing temperature profiles similar to those observed in drill holes on the flanks. The temperatures observed in drill holes on the flanks are not influenced by the postulated Holocene magma body. The elevated temperature gradients measured in the lower portions of these holes may be related to the cumulative effect of older intrusions. The models also indicate that meteoric recharge to the deep hydrothermal system probably originates within or near the caldera. Relatively low fluid velocities at depth suggest that at least a significant fraction of the thermal fluid may be very old.
Core hole drilling and the “rain curtain” phenomenon at Newberry Volcano, Oregon
Authors: Swanberg Chandler A. , Walkey William C. , Combs Jim
Published: February 2008

Two core holes have been completed on the flanks of Newberry Volcano, Oregon. Core hole GEO N‐1 has a heat flow of 180 mW m−2, reflecting subsurface temperatures, sufficient for commercial exploitation of geothermally generated electricity. GEO N‐3, which has a heat flow of 86 mW m−2, is less encouraging. We emphasize the “rain curtain” effect with the hope that a detailed discussion of this phenomenon at two distinct localities will lead to a better understanding of the physical processes in operation. Core hole GEO N‐1 was cored to a depth of 1387 m at a site located 9.3 km south of the center of the volcano. Core hole GEO N‐3 was cored to a depth of 1220 m at a site located 12.6 km north of the center of the volcano. Both core holes penetrated interbedded pyroclastic lava flows and lithic tuffs ranging in composition from basalt to rhyolite, basaltic andésite being the most common rock type. Potassium‐argon age dates range up to 2 Ma. Caving and sloughing were encountered in both core holes at depths near the regional water table. Both core holes penetrate three distinct thermal regimes. The uppermost regime is isothermal at mean air temperature down to about 900–1000 m (the rain curtain). A thermally conductive regime exists near the bottom of each core hole. Separating these two thermal regimes is a transition zone exhibiting considerable hydrologic disturbance. The uppermost isothermal regime is characterized by (1) a low and fairly uniform response on the gamma ray log (N‐1, N‐3) and the electrical conductivity log (N‐1), (2) temperatures below surface ambient measured downhole with a maximum recording thermometer (MRT) during periodic pauses in drilling operations (N‐1, N‐3), and (3) drilling fluids whose chemistry does not reflect an influx of geothermal fluids (N‐3). In contrast, the thermally conductive regime is characterized by (1) a high and variable response on the gamma ray log (N‐1, N‐3) and on the electrical conductivity log (N‐1), (2) temperatures (MRT) measured downhole during pauses in drilling which are above ambient and which track in situ conditions (N‐1, N‐3) and (3) drilling fluids whose chemistry clearly reveals a geothermal component (N‐3). The transition zone is characterized by major washouts in the caliper log (N‐1, N‐3), a major anomaly in the mercury content of the rocks (N‐1), an extremely strong response on the gamma ray log (N‐1, N‐3) and electrical conductivity log (N‐1), and a major self‐potential anomaly (N‐1). Smectite alteration, which seems to control the results of surface geoelectrical studies, begins in the isothermal regime close to and perhaps associated with the regional water table.
Igneous processes and closed system evolution of the Tharsis region of Mars
Authors: Finnerty A. A. , Phillips R. J. , Banerdt W. B.
Published: February 2008

A quantitative petrologic model for the formation of the Tharsis rise of Mars has been developed that is consistent with global gravity and topography data. Partial melting of the mantle underlying the Tharsis plateau results in the depletion of basaltic chemical components in the source region. Iron is partitioned preferentially into the basaltic liquid, so that the density of the source region is lowered relative to surrounding unmodified mantle. Basalt is erupted to the surface and/or intruded at shallow levels in the crust, where it solidifies to low‐pressure mineral assemblages or perhaps glass. These low‐pressure materials are less dense than their mantle equivalents; hence the net volume of the Tharsis crust‐mantle system is increased, resulting in the prominent topographic rise with no net gain or loss of mass. The distribution of densities with depth is quantitatively consistent with the isostatic models of Sleep and Phillips [1979, 1985]. Typical values of parameters that are constrained by the gravity and topography data are 150‐km crustal thickness, 250‐km elastic mantle thickness, 26% partial melting in the mantle source region, and 32% addition of intrusive material to the crust. This closed‐system petrologic process is strictly constructional in nature, but shallow intrusion of magma into the crust can cause uplift of overlying strata, as advocated by R. J. Phillips and N. H. Sleep [submitted manuscript, 1987].
Earthquakes in the Davie Ridge‐Madagascar region and the southern Nubian‐Somalian plate boundary
Authors: Grimison Nina L. , Chen Wang‐Ping
Published: February 2008

The largest earthquake sequence (mb up to 6.4 and M0 up to 5 × 1018 Nm) to have struck eastern Africa within the past 60 years occurred in May and June of 1985 off the coast of the Tanzania‐Mozambique border. The epicenters are located on the northern extension of the Davie Ridge, a prominent north‐south trending bathymétrie feature accompanied by distinct gravity anomalies. A joint hypocenter location of the 14 largest events in this sequence appears as a cluster of epicenters with no apparent lineation. Al teleseismic distances, P and SH waves generated by the two largest events of this sequence have nearly identical waveforms on the long‐period records of the WorldWide Standard Seismograph Network. A formal inversion of the data shows a pure normal faulting mechanism on nodal planes striking slightly west of north. At least two point sources are necessary to model each event, one at a depth of 40 ± 10 km and the other at about 20 ± 5 km. On a regional scale, normal faulting seems to characterize the scattered seismicity throughout the region between Madagascar and the eastern arm of the East African rift zone, even though oceanic lithosphere dominates part of the region and no topographic expression of a rift system can be identified. Since normal faulting events of up to 25 ± 5 km deep also characterize the scattered seismicity west of the topographic expression of the western arm of the rift zone at about the same latitude, we propose that the southern termination of the Nubian‐Somalian plate boundary consists of a diffuse zone of east‐west extension up to 2000 km wide.
Near‐source ground motion from a steady state dislocation model in a layered half‐space
Authors: Mendez A. J. , Luco J. E.
Published: February 2008

A method to calculate the response on the surface of a multilayered half‐space for a fault of finite width and infinite length is presented. The model involves a piecewise‐rectilinear and continuous rupture front propagating at a constant rupture velocity along the length of a fault of arbitrary dip angle. The motion produced by this steady state dislocation model corresponds to the passage of the rupture front phase, which is a predominant phase in the near‐source region away from the ends of a finite fault. The model gives an efficient way to synthesize a ubiquitous, intermediate frequency, high‐amplitude pulse observed in many near‐source records. A series of validation tests, using both three‐ and two‐dimensional kinematic fault models, and a limited set of parametric studies clarifying the mechanisms involved in the generation of high amplitudes, are presented. Finally, it is shown that the distribution of peak horizontal velocities in the near‐source region calculated by use of a steady state model in a layered medium compare favorably with the regression results of Joyner and Boore (1981).
Large‐scale waveform inversions of surface waves for lateral heterogeneity: 1. Theory and numerical examples
Authors: Snieder Roel
Published: February 2008

Surface wave scattering theory is presented as a new method for analyzing teleseismic surface wave data. Using surface wave scattering integrals the effect of lateral heterogeneity both on the surface wave coda generation and on the direct surface wave is described. Since the employed scattering theory for the forward problem is linear, the inverse problem can conveniently be solved in the least squares sense using an iterative matrix solver. For waveform inversions of the direct surface wave, only near forward scattering contributes. For this case the isotropic approximation is introduced, which makes it possible to retrieve phase velocity information from scattering theory. It is shown that for practical waveform inversions the resulting system of linear equations is extremely large and how row action methods can be used conveniently for carrying out the inversion on moderate size computers. The performance of the inversions is illustrated with two numerical examples. In the first example the surface wave coda generated by one point scatterer is inverted. It is shown that the reconstruction in this case is similar to Kirchhoff migration methods as used in exploration seismics. In the second example, ray geometrical effects (focusing and phase shifting) are obtained from the linear inversion with scattering theory. It follows from this example that linear waveform inversion can simultaneously fit the amplitude and the phase of surface wave data.
LITHOSTRATIGRAPHY OF THE STATE 2–14 BOREHOLE: SALTON SEA SCIENTIFIC DRILLING PROJECT
Authors: Herzig Charles T. , Mehegan James M. , Stelting Charles E.
Published: February 2008

Sedimentary rocks from the (California) State 2–14 research borehole of the Salton Sea Scientific Drilling Project, located in the Imperial Valley of southern California, were deposited in the continental basin of the Salton Trough. Lacustrine shale and siltstone are the dominant lithologies. Sandstones were deposited in lake margin, meander channel fill, and lacustrine delta environments. Framework modal clast compositions of the sandstones indicate a dominant Colorado River provenance. Rare pebbly mudstones were deposited as storm‐induced debris flows. Primary depositional structures in the sediments are commonly disrupted by seismic disturbances. Nonmarine gastropods and ostrocods are relatively uncommon, and occurrences of stratabound sulfides suggest periods of brackish water conditions. Bedded anhydrite and centimeter‐scale mudcracks record intervals of dessication. The State 2–14 sedimentary rocks are interpreted as the basinward facies of the Borrego and Brawley formations which range from Pliocene to Pleistocene in age.
Analysis of Geophysical Well Logs Obtained in the State 2–14 Borehole, Salton Sea Geothermal Area, California
Authors: Paillet Frederick L. , Morin Roger H.
Published: February 2008

A complete suite of conventional geophysical well logs was obtained in the upper part of a 3220‐m‐ deep borehole drilled into geothermally altered alluvial sediments on the southeastern edge of the Salton Sea. In situ temperatures greater than 300°C and an inability to cool parts of the borehole by circulation limited the suite of logs run below 2000 m in depth to deep induction, spontaneous potential, un‐calibrated neutron, natural gamma, and temperature. Bottom‐hole temperature trends given by repeat temperature logs were extrapolated to undisturbed temperatures approaching 355°C at a depth of 3220 m. Geophysical logs obtained in the State 2–14 borehole indicate that neutron porosity, gamma‐gamma, and deep‐induction logs provide useful information on lithologic trends with depth. The natural gamma log contains almost continuous, high‐frequency fluctuations that obscure lithologic trends and that may be related to recent radioisotope redistribution and departure from radiometric equilibrium. Acoustic transit time logs give unrealistically low in situ compressional velocities ranging from 1.8 to 3.0 km/s, whereas acoustic waveform logs indicate that sediment compressional velocities range from less than 3.0 km/s shallower than 1000 m in depth to almost 5.0 km/s at depths greater than 2000 m. Analyses indicate that most log values lie between two lithologic end points: an electrically conductive claystone with moderate neutron porosity, but no effective porosity, and an electrically nonconductive, fully cemented siltstone that has small but finite porosity. A limited number of clean sandstones depart from this trend; geophysical logs from these sandstones indicate an effective porosity ranging from 5 to 14%, and saturation with brines having equivalent NaCl concentrations greater than 100,000 mg/L. Depth‐ averaged trends in neutron porosity and deep‐induction logs, along with trends in acoustic velocity determined from acoustic waveform logs, demonstrate that major changes in the properties of alluvial sediments occur within the depth range from 1200 to 1800 m. Although caliper logs were not obtained deeper than 2000 m, resistivity values less than 10 ohm m at those depths probably correspond to borehole enlargements in production zones rather than local increases in effective porosity. The transition in sediment properties indicated by the geophysical logs in the depth interval from 1200 to 2000 m apparently represents a detailed vertical profile of the transition from relatively unaltered clay minerals in alluvial sediments to electrically nonconductive alteration products such as epidote and feldspar.
"Nature of local magma storage zones and geometry of conduit systems below basaltic eruption sites: Pu'u 'O 'o, Kilauea East Rift, Hawaii, Example""
Authors: Wilson Lionel , Head James W.
Published: February 2008

The fluid dynamics of the well‐documented eruptive episodes at Pu'u 'O 'o, Kilauea (Wolfe et al., 1987) are used to investigate quantitatively the size and shape of the shallow conduit system beneath the vent. Cooling calculations are employed to study the long‐term survival of conduits and the consequences of multiple dike injection events. We find that the subvent conduit must have a planar geometry at depths greater than a few tens to at most a few hundreds of meters, with a width of less than a few meters, a length of the order of 100 m, and a height of the order of 1 km. This structure is clearly the residue of the preemption dikes. Although such a feature can be widened somewhat by repeated dike emplacement events, there is no evidence to suggest that a larger and much more equant magma reservoir should develop at shallow depth. The extensive degassing often occurring after single eruptions or between repeated eruptions, previously thought to imply the presence of a large equidimensional shallow magma chamber, can be readily explained by the volume of magma in a dikelike subvent planar magma storage zone. On the basis of these observations, which we infer to apply commonly to basaltic eruption sites, we suggest that the terms “chamber” or “reservoir” be used with caution, because they most often connote a relatively large equant magma body, distinct from the feeder dike, lying just below the vent. We find that the subvent region has the same basic geometric characteristics as the parental dike, and we propose that it be referred to as a “planar magma storage zone.”
A numerical simulation of the Plinian Fall Phase of 79 A.D. eruption of Vesuvius
Authors: Macedonio G. , Pareschi M. T. , Santacroce R.
Published: February 2008

The tephra transport and fallout during the Plinian phase of the 79 A.D. eruption of Vesuvius has been numerically simulated by using an advection‐diffusion model based on a continuity equation for the mass concentration in which wind field, atmospheric diffusion, and gravity settling are considered. The solution of the equation has been obtained on a nonuniform three‐dimensional grid to optimize the computation time and to reduce the errors. The input data are (1) the total erupted mass, (2) the variation with time of the column height, (3) the particle population in terms of the settling velocity, and (4) the wind field. The vertical mass distribution of the particles which leave the column is given by an empirical formula (Suzuki, 1983), whose parameters are obtained from data of Carey and Sigurdsson (1987) and best fitting with field data. Two distinct phases have been distinguished in the eruption: (1) related to white phonolitic pumice fallout and (2) dominated by the emission of gray tephritic‐phonolite pumice. According to the literature the total duration of the pumice fallout was about 19 hours with column height ranging between about 14 and 32 km. The “white phase” has been estimated to have had 9 hours duration, while the “gray phase” lasted 10 hours. Magma discharge rate has been calculated assuming a fourth power relationship with column height. Total erupted volumes of 1.0 and 2.6 km3 of dense rock equivalent products have been assumed for the white and the gray phases, respectively (Sigurdsson et al., 1985). The particle population of the cloud has been assumed to correspond to that of a pyroclastic flow deposit related to a nearly total column collapse. The wind field results from a 60° clockwise rotation of the presently most frequent wind distribution field during the summer reduced in speed module of about 40%. The simulated ground thicknesses generally agree with the actual ones. The computed granulometric spectra at specific localities are in excellent agreement with the measured ones. As a whole, the simulation was successful. This fact emphasizes the significant consequences that the used model with a suitable choice of the input parameters can have for the establishment of a truly probabilistic air fall hazard map of Vesuvius.