bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of moisture and carbon dioxide on concentrations of helium in soils and soil gases
Authors: Hinkle Margaret E. , Ryder Jean L.
Published: February 2008

Helium is often measured in exploration programs, either in interstitial soil gases collected by hollow probe or in gases desorbed from pore spaces of soil samples. However, measured helium concentrations are affected by various parameters that must be considered when interpreting data, including the variation in concentration of other soil gases. The effects of moisture and carbon dioxide in the soil are particularly important. This study compares helium concentrations with soil moisture and carbon dioxide contents of soil and soil gas samples collected in several areas. Where soil moisture was below about 4% by weight, helium concentrations in soil gases collected by hollow probe correlated well with helium concentrations desorbed from soil at the same site. Measured helium concentrations were significantly reduced, however, in areas where soil moisture was greater than 10% and were below the concentration of helium in air in areas where soil moisture exceeded 18%. Carbon dioxide content of the samples also affects helium concentrations. Helium concentrations were significantly reduced in samples where the carbon dioxide content exceeded 1% by volume of the gas sample.
Multitaper spectral analysis of high‐frequency seismograms
Authors: Park Jeffrey , Lindberg Craig R. , Vernon Frank L.
Published: February 2008

Spectral estimation procedures which employ several prolate spheroidal sequences as tapers have been shown to yield better results than standard single‐taper spectral analysis when used on a variety of engineering data. We apply the adaptive multitaper spectral estimation method of Thomson (1982) to a number of high‐resolution digital seismic records and compare the results to those obtained using standard single‐taper spectral estimates. Single‐taper smoothed‐spectrum estimates are plagued by a trade‐off between the variance of the estimate and the bias caused by spectral leakage. Applying a taper to reduce bias discards data, increasing the variance of the estimate. Using a taper also unevenly samples the record. Throwing out data from the ends of the record can result in a spectral estimate which does not adequately represent the character of the spectrum of nonstationary processes like seismic waveforms. For example, a discrete Fourier transform of an untapered record (i.e., using a boxcar taper) produces a reasonable spectral estimate of the large‐amplitude portion of the seismic source spectrum but cannot be trusted to provide a good estimate of the high‐frequency roll‐off. A discrete Fourier transform of the record multiplied by a more severe taper (like the Hann taper) which is resistant to spectral leakage leads to a reliable estimate of high‐frequency spectral roll‐off, but this estimate weights the analyzed data unequally. Therefore single‐taper estimators which are less affected by leakage not only have increased variance but also can misrepresent the spectra of nonstationary data. The adaptive multitaper algorithm automatically adjusts between these extremes. We demonstrate its advantages using 16‐bit seismic data recorded by instruments in the Anza Telemetered Seismic Network. We also present an analysis demonstrating the superiority of the multitaper algorithm in providing low‐variance spectral estimates with good leakage resistance which do not overemphasize the central portion of the record.
Inversion of borehole guided wave amplitudes for formation shear wave attenuation values
Authors: Burns D. R. , Cheng C. H.
Published: February 2008

Formation shear wave attenuation (Qβ−1) and borehole fluid attenuation (Qƒ−1) values are estimated from the guided wave arrivals in full waveform acoustic logs. The use of guided waves avoids the problem of head wave geometrical spreading losses. Attenuation estimates are obtained from the inversion of guided wave spectral ratios using analytic partition coefficient expressions. The guided wave amplitude ratios are measured at uniformly spaced frequencies within a selected frequency window containing the pseudo‐Rayleigh and Stoneley waves. Inversion of synthetic data from an open hole geometry results in Q estimates within 2–3% of the actual values. Inversion of cased hole synthetic data gives fluid Q and cement layer Q values within 2–3% of the actual values, but the formation shear wave Q estimate is in error by about 25% due to difficulty in identifying the pseudo‐Rayleigh wave cutoff frequency from the amplitude spectra. Synthetic data results also indicate that the Q estimates are very sensitive to errors in any of the parameters that affect the partition coefficient calculations, such as the borehole fluid velocity or borehole radius. Application of the method to real data recorded in open boreholes provides reasonable results, but noise increases the variance of the Q estimates.
Fitting degradation of shoreline scarps by a nonlinear diffusion model
Authors: Andrews D. J. , Bucknam Robert C.
Published: February 2008

The diffusion model of degradation of topographic features is a promising means by which vertical offsets on Holocene faults might be dated. In order to calibrate the method, we have examined present‐day profiles of wave‐cut shoreline scarps of late Pleistocene lakes Bonneville and Lahontan. It may be assumed that these scarps were initially at least as steep as the angle of repose. Offsets range from 1 to 12 m, and present slope angles range from 9° to 29°. A parameter called apparent diffusion age, defined as half the mean square horizontal extent of the slope function of each profile, is plotted as a function of scarp offset. The points show a clear trend of apparent age increasing nearly linearly with offset. If linear diffusion held and scarps were initially vertical, apparent diffusion age would be the same for all the shoreline profiles. The increasing trend can only partly be explained by nonvertical initial scarp slope, and therefore the rate of transport of material downslope must increase significantly faster than a linear law in the range of slopes spanned by the data. The transport law must become linear at small slope to reduce scatter between profiles with varying ambient slopes. The transport law adopted for the purpose of dating is k0(1 + 5s2), where s is local slope. The transport coefficient k0 is correlated inversely with fan slope, suggesting that there is a dependence on the particle size distribution. A table is included that allows easy application of the model to scarps with simple initial shape.
Vertical surface displacements at Yellowstone Caldera, Wyoming, 1976–1986
Authors: Dzurisin Daniel , Yamashita Kenneth M.
Published: February 2008

Between first‐order leveling surveys in 1976 and 1986, the east central floor of Yellowstone caldera rose 15.4 ± 1.4 mm/yr and the west central floor rose 17.0 ± 2.0 mm/yr with respect to points on the south caldera rim. The same pattern has prevailed since an initial survey in 1923, except that the western caldera rose 30% faster during 1976–1986 than during 1923–1976. The 1976–1986 uplift extends beyond the northwest caldera rim near the adjacent Hebgen Lake fault zone, site of a magnitude 7.3 earthquake in August 1959. Marginally significant displacements in the eastern caldera suggest that uplift continued during 1983–1984, stopped during 1984–1985, and reversed to subsidence during 1985–1986; thus long‐term uplift may accumulate episodically. Historical uplift is especially noteworthy because regional crustal extension and thermal contraction during cooling would cause the caldera floor to subside if other processes were not acting to warp it upward. The pattern and dimensions of the uplift indicate a source in the upper crust beneath Yellowstone caldera, probably a reservoir of rhyolite magma within a cooling granitic pluton. A simple point source model would suggest a source 10–15 km deep, but the reservoir is probably lens‐shaped and closer to the surface. Uplift may be caused by basaltic intrusions near the base of the reservoir or accumulation of magmatic fluids during cooling and crystallization within the reservoir. Subsidence might be a response to regional tectonic extension or release of trapped magmatic fluids. Magmatic inflation, volatile accumulation, and crustal extension may culminate eventually in renewed eruptive activity, but there is no evidence of an imminent volcanic hazard.
Kiyoo Wadati and early research on deep focus earthquakes: Introduction to Special Section on Deep and Intermediate Focus Earthquakes
Authors: Frohlich Cliff
Published: February 2008

This special section of the Journal of Geophysical Research is dedicated to Kiyoo Wadati. This paper evaluates his contribution to our knowledge of geophysics and, especially, deep earthquake phenomena. In at least three areas, Wadati wrote several papers before 1936 which profoundly influenced modern geophysics. First, his work provided the first convincing evidence that deep earthquakes existed. Second, he was a leader in the construction of travel time tables and the determination of mantle velocity structure. Finally, he published the first accurate description of the inclined planar zone of deep earthquakes which extends from trenches beneath volcanic island arcs. Wadati's work strongly influenced research of many other scientists both before and after World War II, particularly, Jeffreys, Gutenberg, and Benioff. Several of the questions raised by the research of Wadati and others before 1940 are still unanswered today.
Stress distribution due to olivine‐spinel phase transition in descending plate and deep focus earthquakes
Authors: Goto Kazuhiko , Suzuki Ziro , Hamaguchi Hiroyuki
Published: February 2008

The stress field due to olivine‐spinel phase transition in and around a descending plate is studied based on the calculations for an equilibrium phase transition with a constant Clapeyron's slope and for a nonequilibrium transition taking a kinetic effect into consideration. The material concerned is assumed to be a viscoelastic body whose physical coefficients are functions of temperature and pressures. The calculated stress distribution for the equilibrium transition is characterized by a compressional stress near the upper and/or lower surfaces of the plate and by a tensional stress at the central part in the depth range from 200 to 550 km, the magnitude of principal stress being greater than 0.5 GPa. However, for the nonequilibrium transition a tensional stress predominates near the upper and lower surfaces, and the central part is compressional in the depth range from 300 to 600 km, unlike the case of equilibrium transition. The maximum principal stress in the latter case is greater than 2 GPa, which is much more than that for the equilibrium transition. The principal axes in the high stressed region are oriented almost parallel to the descending direction in both cases. Comparison of the calculated results to the actually observed seismicity‐depth relation and to focal mechanisms of deep focus earthquakes in descending plates beneath island arcs shows that the nonequilibrium phase transition agrees better with the observations.
A high‐resolution local network study of the Nazca Plate Wadati‐Benioff Zone under western Argentina
Authors: Smalley Robert F. , Isacks Bryan L.
Published: February 2008

This paper presents a precise determination of the fine structure of a Wadati‐Benioff zone based on analysis of analog and digital seismic data recorded by a local network in the San Juan Province of western Argentina. The network is located above one of the subhorizontal segments of the subducted Nazca plate. Without the complicating effects of a dipping slab on the locations of earthquakes, the nearly flat geometry of the subducted plate provides an excellent opportunity to determine the thickness and structure of the Wadati‐Benioff zone. We found that the depth of the Wadati‐Benioff zone beneath the network is 107 ± 5 km. For a set of digitally recorded events, the distance between the shallowest and the deepest event gives a thickness of 20km, with 90% of the data concentrated in a zone 12 km thick. The spatial distribution of hypocenters shows no evidence of complex internal structure such as a double Wadati‐Benioff zone. The local network data also suggest that in the region near 32° S where the dip of the Wadati‐Benioff zone changes from flat to steep the Nazca plate flexes rather than tears at depths less than at least 125 km. The intermediate‐depth seismicity is separated from an active zone of crustal seismicity by an aseismic region between depths of about 40 and 95 km. The upper surface of the subducted plate, inferred to be at about 90 km, provides an upper limit to the thickness of the South American plate in this region. An apparent westward dip of the Wadati‐Benioff zone as determined by the local network data can be accounted for by a 6° westward dip of the Moho beneath the San Juan region, a result that is in agreement with regional gravity and topographic data.
Aftershocks and temporal clustering of deep earthquakes
Authors: Frohlich Cliff
Published: February 2008

This paper evaluates how the focal depth, the geographic location, and magnitude affect the incidence of aftershocks and the occurrence of temporal clustering of nearby events for selected deep earthquakes (h> 70 km) in the catalog of the International Seismological Centre. For comparison, we also evaluate shallow (h< 70 km) and deep earthquakes for which locations and moment tensors are reported by the Harvard group. The primary conclusions are as follows: 1) The incidence of aftershocks depends strongly on focal depth, with aftershocks being less common for events between 100‐ and 450‐km depth than for shallower or deeper events. Events with focal depth shallower than 40 km have a higher incidence of aftershocks and more pronounced temporal clustering than do deeper events. 2) For the data available at present, the incidence of aftershocks and amount of temporal clustering for deep events are independent of geographic region, except for dependence caused by regional differences in the focal depths of events. 3) Deep earthquakes with large magnitudes are more likely to have aftershocks than are smaller magnitude events, in accordance with models where the number of aftershocks is a Poisson random variable that is a function of magnitude. However, several large events are anomalous in that they have many more aftershocks, or many fewer aftershocks, in the catalog than predicted by the model.
SEISMICITY OF THE GALAPAGOS 95.5°W PROPAGATING RIFT
Authors: Cooper Patricia A. , Milholland Phillip D. , Duennebier Frederick K.
Published: February 2008

An array of ocean bottom seismometers was deployed for 21 days about the 95.5°W Galapagos propagating rift tip to obtain seismic evidence for the propagating rift hypothesis and to investigate the dynamic effects of rift propagation. Seismicity was fairly constant (10–17 events per day) with no indication of swarm activity. Hypocenters were determined for 304 earthquakes, the largest of which is estimated to be mb = 2.3, with a seismic moment of 2.9 × 1020 dyn cm. The cumulative seismic moment released by all local earthquakes is estimated to be 6.6 × 1020 dyn cm. A b value of 0.77 ± 0.08 was estimated for the region, implying a stress regime dominated by a broad tectonic stress field. The distribution of earthquakes is consistent with the propagating rift hypothesis. Most earthquakes occurred south of the propagating rift within the De Steiguer Deep. Seismicity within the transform zone is clustered in such a way as to suggest that two or three discrete fault segments may be active at the same time. The seismicity observed along the propagating rift is diffuse. Earthquakes were observed at the propagating rift tip, but not in front of it. Almost all earthquakes occur south of the northern pseudofault and its extension west of the rift tip; that is, earthquakes occur almost exclusively in lithosphere that has been or is being overtaken by the propagating rift. Six composite focal mechanisms are consistent with the propagating rift hypothesis. First motions from earthquakes located on the propagating rift indicate extension, whereas motions associated with the failing rift area indicate compression. A compressional regime is also indicated west of the transform zone. Twelve earthquakes from the transform zone produce a poorly constrained focal mechanism consistent with right‐lateral strike‐slip motion along a fault plane that is roughly parallel to the isochrons of the transferred lithosphere.
Dynamic recrystallization and associated exsolution in perthites: Evidence of deep crustal thrusting
Authors: White Joseph Clancy , Mawer Christopher K.
Published: February 2008

Mylonitic gneisses from Parry Sound, Ontario, are intensely deformed and occur within a major deep crustal shear zone developed during thrusting of granulite facies rocks (800°C, 900–1100 MPa) northward onto an amphibolite facies terrane. We have examined, by optical and electron microscopy, microstructures in perthites from these mylonites that preserve the deformation‐recrystallization cycle which operated during high‐ductility deformation of the crystals. The exsolution and recrystallization microstructures are intimately associated. Exsolution in the perthites occurred in two stages. A calcic plagioclase (oligoclase) exsolved first and subsequently coarsened and deformed during the single‐crystal ductility phase of shear zone deformation. A progressive increase in size and boundary incoherence of oligoclase domains during deformation led to recrystallization of the host perthite crystals as discrete orthoclase and oligoclase grains. The principal mode of recrystallization was subgrain rotation, with a component of grain growth that followed the formation of mobile high‐angle boundaries. Misorientation of subgrains to form grains at the recrystallization front is abrupt and occurred over one or two subgrain diameters. The second exsolution stage postdated perthite deformation and saw formation of fine albite lamellae in both host and recrystallized orthoclase. Temperatures inferred from the exsolution textures for independently determined confining pressures indicate that this deformation occurred under at least granulite facies conditions. The perthites thus record deformation from crustal depths of the order of 34–42 km. Continental megathrusts, such as the Parry Sound shear zone, provide mechanisms for both the generation of large strains observed in otherwise nonductile minerals and the preservation of ensuing microstructures.
Three‐dimensional fine velocity structure and hypocentral distribution of earthquakes beneath the Kanto‐Tokai District, Japan
Authors: Ishida Mizuho , Hasemi Akiko Horie
Published: February 2008

We determine the three‐dimensional P wave velocity structure beneath the Kanto‐Tokai district, central Japan, and simultaneously relocated by inverting travel time data for 91 events that were observed at 61 stations operated by the National Research Center for Disaster Prevention. The variance of the travel time data is reduced by 75%, as compared to the variance for the routinely used laterally homogeneous model. In general, our velocity model is consistent with tectonic models of the Kanto‐Tokai district. Our velocity model is also in good agreement with seismic explosion data and Bouguer gravity anomalies. The uppermost layer (from 0 to 16 km depth) is characterized by lateral velocity differences of more than 20%. We find low‐velocity zones beneath Tokyo Bay, the middle of the Boso Peninsula, and Omae‐zaki, where there are thick sedimentary layers. A marked 30‐km‐thick high‐velocity zone extends to the northeast and the west of the Izu Peninsula. This zone dips gradually from the surface to a depth of 70 km in the eastern region and to a depth of 40 km in the western region and is interpreted as the Philippine Sea (PHS) plate, which is being subducted at the Sagami and the Suruga troughs. The data unquestionably require a high‐velocity zone beneath Mount Tsukuba that cannot be explained in terms of present tectonic models. We use our three‐dimensional velocity model to relocate about 23,000 earthquakes that took place from 1983 through 1985. The accuracy of the hypocentral parameters improved considerably. As a result, the double seismic zone within the subducting Pacific plate is more clearly delineated than in previous studies. The inclined seismic zone for the subducting PHS plate coincides fairly well with the high‐velocity zone of our three‐dimensional velocity model.
Fault stability changes induced beneath a reservoir with cyclic variations in water level
Authors: Roeloffs Evelyn A.
Published: February 2008

The stress and pore pressure changes produced by a steady periodic variation of water level on the surface of a uniform porous elastic half‐space are evaluated using the fully coupled (Biot) equations of elastic deformation and pore fluid flow. Diverse choices of material properties all give a coupled stress field differing from the elastic stress field by at most 0.035 po, where po is the water pressure at the bottom of the reservoir. Peak coupled pore pressure change can lag peak water level in the depth range 0 < (ω/2c)1/2z < π, where ω is frequency of the cyclic change in water level, c is diffusivity, and z is depth. The maximum lag increases as B decreases, where B is the ratio of pore pressure increase to mean compressive stress increase under undrained conditions. Directly beneath the reservoir, for example, peak pore pressure in an annual cycle can lag peak water level by at most 10 days if B = 0.80, but can lag by up to 122 days if B = 0.11. When cyclic water level changes are superimposed on the steady state reservoir level, the time during the cycle at which a fault is most destabilized depends on whether the weight of the reservoir stabilizes or destabilizes the fault, which, in turn, depends on its orientation and location relative to the reservoir. B and c also influence the timing of the greatest destabilization. If B and c are low, maximum destabilization at low water level is possible for faults that are stabilized by the weight of the reservoir; this mechanism may have operated at Lake Mead. The analysis suggests that induced seismic events should be separated into groups having a common focal mechanism and occurring in similar locations relative to the reservoir before studying the time at which the events occur relative to the water level. The fully coupled solution is compared with an uncoupled solution, with a solution that is coupled but which assumes incompressible solid and fluid constituents (consolidation) and with a decoupled solution in which the difference between the pore pressure field and B times the elastic mean compressive stress obeys a homogeneous diffusion equation. The uncoupled and consolidation solutions respectively underestimate and overestimate pore pressure during short‐term reservoir level fluctuations as well as at times short compared to that required to achieve steady state. In contrast, the decoupled solution agrees closely with the fully coupled solution for the problem studied here.
Age‐depth relationships and depth anomalies in the southeast Indian Ocean and south Atlantic Ocean
Authors: Hayes Dennis E.
Published: February 2008

The distributions of crustal depths as a function of their age have been analyzed for the Southeast Indian Ocean and South Atlantic regions. Average depth, sediment thickness, and age data, compiled and digitized for 1°×1° areas, are used to establish the relationships of adjusted crustal depths to crustal age and to map the planform of residual depth anomalies as referred to a new global, empirical standard. Both the Southeast Indian Ocean and the South Atlantic are characterized by systematic, intermediate wavelength (∼500–1500 km) depth anomalies that serve to define “tectonic corridors” trending parallel to spreading directions. The exact boundaries of these corridors have not yet been determined. Within each corridor, crustal depth increases linearly with age½ as predicted by simple cooling models. The crustal subsidence curves can vary significantly from corridor to corridor and with regard to the adopted global standard. Furthermore, the presence of asymmetric subsidence of oceanic crust within certain corridors is not in accord with the conventionally accepted theory describing crustal depth as a function of crustal age. Corridors exhibiting asymmetries coexist adjacent to corridors exhibiting no asymmetries, thus indicating that the mechanisms controlling these depth distributions can change fairly abruptly along strike of the midocean ridge. A qualitative model is presented that indicates that very small regional differences in the density of the asthenosphere (of the order of ±0.01 g/cm3) can give rise to ∓15% differences in the isostatically controlled equilibrium depth of the top of the lithosphere. Such density differences could easily exist if lateral variations of ∼100°C persisted in the upper asthenosphere. Since the lithosphere is generally perceived as a thermal boundary layer, this is equivalent to predicting small but systematic differences in the thickness of the lithosphere and hence the depth to the top of the lithosphere. An alternative dynamic model involving small‐scale convection (e.g., Buck and Parmentier, 1986) can also account for the observed depth‐age relationships. Both of the above explanations imply the existence of lateral temperature variations in the upper asthenosphere.
Multicomponent paleomagnetic data from the Nosoni Formation, eastern Klamath Mountains, California: Cratonic Permian primary directions with Jurassic overprints
Authors: Renne Paul R. , Scott Gary R. , Bazard David R.
Published: February 2008

Paleomagnetic data from the Guadelupian (Upper Permian) Nosoni Formation of the Redding Section (RS) consistently exhibit two directional components of magnetization in addition to a late Quaternary normal weathering overprint. Six sites in ash flow tuffs and tuffaceous sedimentary rocks, spanning 200 m of section, yield data that allow precise characterization of all components. Two additional sites in calcareous mudstones were too heavily overprinted to yield stable paleomagnetic directions. Thermal demagnetization of the tuffaceous samples in 16–18 steps between natural remanent magnetization and 660°C provides a detailed data base from which to calculate directional components. Analysis of demagnetization trends identifies a prefolding reversed direction carried by both magnetite and hematite and a postfolding normal direction carried by hematite only. Petrologic considerations suggest paragenesis of these oxides in the Permian (primary magnetite and hematite) and Early‐Middle Jurassic (metamorphic hematite). Least squares analysis of progressively demagnetized vector endpoint trends between 540° and 600°C yields a formation mean direction in magnetite havingD=156.4°,I=−29.7°,k= 101, α95=7.6°. This result is in close agreement with the composite prefolding direction (D= 160.3°,I=−31.5°), carried by both magnetite and hematite, determined by the intersection of remagnetization circles. The postfolding hematite‐carried overprint direction calculated by the latter technique hasD=352.3°,I=35.0°. Paleopoles corresponding to these two directions are located at 57.6°N, 103.0°E (magnetite formation mean) and 67.0°N, 104.1°E (hematite overprint), coinciding within error to Upper Permian and Early Jurassic (respectively) reference poles from the North American craton. These data indicate no significant rotation or latitudinal translation of the sample sites with respect to cratonic North America since the Permian. This conclusion contrasts with seemingly reliable paleomagnetic results from elsewhere in the RS, which indicate major post‐Permian clockwise rotations. These rotations are evidently specific to individual structural domains and are probably manifestations of oroclinal bending about vertical axes.
Solidus curves, mantle plumes, and magma generation beneath Hawaii
Authors: Wyllie Peter J.
Published: February 2008

The eruption of nepheline‐normative lavas in the early and late stages of formation of the large Hawaiian tholeiite shields is well established, as is the conclusion that volatile components are involved in the genesis of these alkaline lavas. For magmas to be generated, the source materials must be transported across their solidus curves. Solidus curves for volatile‐free peridotite and for peridotite‐C‐H‐O provide the depth‐temperature framework for the sites of magma generation. The assumption (controversial) that garnet remains in the source material locates the major melting region in plume material at depths of about 80 km, with isotherms in plume center exceeding 1500°C. The plume carries traces of volatile components from depths greater than 300 km. These dissolve in a trace of interstitial melt as the plume crosses the solidus for peridotite‐C‐H‐O at depths decreasing from 350 km to about 150 km with distance from the plume axis. The volatile‐charged melt, enriched in incompatible elements, is swamped by the picrites generated in the major melting region. From the outer portions of the plume, the volatile‐rich melt enters the lithosphere at 80–90 km depth, where the change in rheology retards its upward percolation. This magma (remaining in equilibrium with peridotite) is carried toward the solidus for peridotite‐C‐H‐O, changing composition toward nephelinite; evolution of vapor as magma approaches the solidus may facilitate intermittent crack propagation, releasing the nephelinitic magmas for eruption from depths of 75–85 km. Movement of the lithosphere plate over the rising plume establishes asymmetry. Eruption of nephelinitic magmas on the upstream side of the plume (early volcanism) may be suppressed or very close in time and space to eruption of alkaline lavas and tholeiites. On the downstream side (late volcanism), eruption of nephelinites is delayed by lateral transport away from the main melting region.
The mechanics of ground deformation precursory to dome‐building extrusions at Mount St. Helens 1981–1982
Authors: Chadwick William W. , Archuleta Ralph J. , Swanson Donald A.
Published: February 2008

Detailed monitoring at Mount St. Helens since 1980 has enabled prediction of the intermittent eruptive activity (mostly dome growth) with unprecedented success. During 1981 and 1982, accelerating deformation of the crater floor around the vent (including radial cracks, thrust faults, and ground tilt) was the earliest indicator of impending activity. Numerical experiments using the finite element method to model the mechanics of the crater floor show that all deformation features can be explained by a uniform shear‐stress boundary condition along the conduit wall. The magnitude of the shear stress required to match observed displacements (1–7 MPa) is inversely proportional to the conduit diameter (estimated to be 25–100 m). The most probable source of this shear stress is the flow of viscous magma up the conduit and into the lava dome. We propose a model in which the accelerating deformation, beginning as much as 4 weeks before extrusions, is caused by the increasing velocity of ascending magma in the conduit. This model is examined by using deformation data of the dome before four extrusions in 1981 and 1982 to estimate the volumetric flow rate through the conduit. This flow rate and an estimate of the effective viscosity of the magma enable calculation of an ascent velocity and an applied shear stress that, again, depend on the conduit diameter. The results of these calculations are consistent with the finite element experiments and show that the proposed model is feasible. In light of this model, events observed just before or near the time extrusions began, such as reversals of ground tilt direction from outward to inward and the sudden decrease in the number of shallow earthquakes, may indicate an abrupt decrease of shear stress in the conduit. This could be explained by a decrease in either the ascent velocity, or the effective viscosity of the magma ascending through the shallow conduit, or both, near the time of extrusion. Precursory deformation like that measured at Mount St. Helens should be observable at similar volcanoes elsewhere because it is caused by the fundamental process of magma ascent.
Petrology of gabbroic xenoliths from Mauna Kea Volcano, Hawaii
Authors: Fodor R. V. , Vandermeyden H. J.
Published: February 2008

Gabbroic xenoliths from the hawaiite summit cone of Mauna Kea volcano, Hawaii, are up to 15 cm in size and form two groups: olivine (+pl+cpx) gabbros and opaque‐oxide (+pl+cpx) gabbros. Both types have members that are laminated and probably cumulate in origin, and each type has interstitial glass of hawaiitic‐mugearitic composition. Olivine gabbros have MgO 12–25 wt.%, FeO* 4–12%, and TiO2 0.5–1%, whereas opaque‐oxide gabbros have MgO about 6 wt.%, FeO* 12–18%, and TiO2 5–7% (due to 11–27 vol.% Fe‐Ti oxides). Cli‐nopyroxene and plagioclase compositions in both gabbro types (Fs7–14 Wo41–47; Al2O3 3–4%) (An52–89 Or0.5–3) indicate that these xenoliths are products of Hawaiian magmatism and not N‐type mid‐ocean ridge basalts. These phases plus olivine compositions (Fo71–84), paragenesis, and mixing models using representative Mauna Kea basaltic rocks indicate that the gabbros are crystalline products from mafic alkalic parent basalt (e.g., ∼8 wt.% MgO). Modeling shows that residual liquid after 25–30% crystallization of ol+pl+cpx (the xenoliths) may be basalt/hawaiite of the Hamakua Volcanics on Mauna Kea. Alternatively, 60% crystallization of ol+pl+ cpx+mt+il feasibly produced liquid represented by the hawaiite of Laupahoehoe Volcanics; interstitial glass may represent liquid after more than 60% crystallization. Crystallization depth was shallow if daughter magmas were Hamakua, but was at least at 8‐kbar pressure if Laupahoehoe hawaiites were produced (due to absence of liquidus cpx in Laupahoehoe rocks). Laupahoehoe hawaiite transported the xenoliths to the surface.
The 1986 Eruption of Mount St. Augustine: Field Test of a Hazard Evaluation
Authors: Swanson Samuel E. , Kienle Juergen
Published: February 2008

The 1986 eruption of Mount St. Augustine, an island volcano in Cook Inlet of southcentral Alaska, provided a good test of previously prepared volcanic hazard models for that volcano. The 1986 eruption was generally similar to other historic eruptions. An initial vent‐clearing explosive eruptive phase in late March of 1986 removed part of the dome emplaced at the end of the last eruption (1976). Pyroclastic flows reached the sea during this phase, and airborne ash spread over much of Cook Inlet, including Anchorage. The initial explosive phase was followed by dome growth which continued over much of the spring and late summer of 1986. Pyroclastic flows, much less intense than those of March 1986, accompanied dome growth. Volcanic hazards from the 1986 eruption were generally within the limits set by previous hazard studies. Pyroclastic flows were confined to a very high hazard zone on and immediately surrounding Augustine Island. Airborne ash and volcanic gases presented a moderate hazard over much of Cook Inlet and presented a serious hazard to aircraft encountering the ash plume. The potential for a large debris avalanche that could enter Cook Inlet and result in a tsunami existed during the 1986 eruption. Previous work on Mount St. Augustine underestimated the landslide‐related hazard, and more work is needed to assess fully the potential of this hazard. These new data, when incorporated into existing hazard assessments, will improve an already successful hazard evaluation for Mount St. Augustine. Assuming that Augustine Island will never be developed, far‐field hazards associated with ashfalls and tsunamis are the most serious. Real‐time satellite and radar imagery can potentially provide adequate warnings about ash plumes, but the warning time for tsunami hazards is short. Improvements in the tsunamis warning system are needed to reduce the hazard.
Petrology and oxygen isotope geochemistry of a fossil seawater hydrothermal system within the Solea graben, northern Troodos ophiolite, Cyprus
Authors: Schiffman Peter , Smith Brian M.
Published: February 2008

Hydrothermal mineral zonations and O isotope patterns of the northern Troodos complex do not parallel the ophiolite pseudostratigraphy, but reflect the convective geometry of an Upper Cretaceous seawater hydrothermal system. Large areas of the sheeted intrusive complex (SIC), including the subaxial region of the Solea graben, are composed of 18O‐rich, subgreenschist mineral assemblages and may represent regions of diffuse seawater recharge. Other areas of the SIC are recrystallized to distinctive epidosite rocks: granular, high‐variance assemblages of epidote + quartz ± chlorite that are depleted in 18O, Al2O3, Na2O, K2O, Zr, Cu, and Zn and are enriched in CaO and Sr compared with other mafic volcanic and dike rocks of the Solea graben. Epidosite alteration occurred at temperatures of ∼310–370°C and involved fluids with δ18O values and salinities similar to those of Upper Cretaceous seawater. The epidosite zones are discordant with earlier, mineral/O isotope zonations and with the axis of spreading in the Solea graben, suggesting a postspreading, off‐axis origin. The seawater hydrothermal system responsible for Solea graben massive sulfide deposits was probably driven by hypabyssal intrusions (not exposed), emplaced in a terminal, failed spreading episode. The geometries of O isotope surfaces within the Solea graben imply that the epidosites formed in fossil upflow and deep recharge conduits. Depletions in base metals show that epidosite alteration liberated Cu and Zn to mineralizing fluids within the fossil upflow zone.
A physical model for earthquakes: 1. Fluctuations and interactions
Authors: Rundle John B.
Published: February 2008

This is the first of a series of papers whose purpose is to develop the apparatus needed to understand the problem of earthquake occurrence in a more physical context than has often been the case. To begin, it is necessary to introduce the idea that earthquakes represent a fluctuation about the long‐term motion of the plates. This idea is made mathematically explicit by the introduction of a concept called the fluctuation hypothesis. Under this hypothesis, all physical quantities which pertain to the occurrence of earthquakes are required to depend on a physically meaningful quantity called the offset phase, the difference between the present state of slip on the fault and its long‐term average. For the mathematical treatment of the fluctuation problem it is most convenient to introduce a spatial averaging, or “coarse‐graining” operation, dividing the fault plane into a lattice of N patches. In this way, integrals are replaced by sums, and differential equations are replaced by algebraic equations. As a result of these operations, the physics of earthquake occurrence can be stated in terms of a physically meaningful energy functional: an “external potential” WE. WE is a functional potential for the stress on the fault plane acting from the external medium and characterizes the energy contained within the medium external to the fault plane which is available to produce earthquakes. A simple example is discussed which involves the dynamics of a one‐dimensional fault model. To gain some understanding, a simple friction law and a failure algorithm are assumed. It is shown that under certain circumstances the model fault dynamics undergo a sudden transition from a spatially ordered, temporally disordered state to a spatially disordered, temporally ordered state. It is found that the latter states are stable for long intervals of time. For long enough faults the dynamics are evidently chaotic.
Geothermal gradients in the conterminous United States
Authors: Nathenson Manuel , Guffanti Marianne
Published: February 2008

Geothermal gradients from published temperature/depth measurements in drill holes generally deeper than 600 m are used to construct a temperature gradient map of the conterminous United States. The broadly contoured map displays 284 temperature gradients that are applicable to a depth of 2 km. In terms of the number of contoured areas and the fraction of data points having a value not within a contour interval (outliers), the temperature gradient data and associated deep heat flow data have similar measures of contourability. Areally, most of the United States is contoured from 15° to 35°C/km. The eastern United States is generally cooler (average 25°C/km) than the western United States (average 34°C/km), in accordance with broad heat flow trends. Differences between the temperature gradient and heat flow maps are caused by areal differences in rock thermal conductivities. The effect of conductivity on gradients is particularly apparent in the eastern United States where heat flow is relatively constant over large areas. Gradients are elevated where thick, low‐conductivity, sedimentary deposits occur such as in the Atlantic Coastal Plain province and in basins in the Allegheny Plateau and the Great Plains provinces. No clear gradient pattern emerges where both heat flow and conductivity vary widely, such as in the northern Basin and Range and Rocky Mountain provinces. Using the temperature gradients determined in this study and associated heat flow values, derived thermal conductivities are calculated for the depth range of a few hundred meters to 2 km; the average conductivity is 2.5±0.8 W/m °C. Some areas show little variation in derived thermal conductivity, while others show a wide range. The Atlantic Coastal Plain, Appalachian Plateaus, Superior Upland, Gulf Coastal Plain, Columbia Plateaus and Cascade Mountains, and Sierra Nevada provinces have restricted ranges of conductivities, which supports the concept of regional conductivities.
A numerical model for simulation of tephra transport and deposition: Applications to May 18, 1980, Mount St. Helens eruption
Authors: Armienti P. , Macedonio G. , Pareschi M. T.
Published: February 2008

A numerical model for the computation of tephra fall accumulation resulting from Plinian or sub‐Plinian eruptions is presented. Mass accumulation at the ground level is found by solving a continuity equation which describes the transport of tephra in the atmosphere. The treatment incorporates horizontal advection due to wind, vertical gravitational settling, and dispersion due to atmospheric turbulence. Aspects of the parameters which enter the model, such as settling velocity, diffusion coefficients, deposition velocity, and source shape, are considered and discussed. Particular attention is devoted to settling velocity dependence on particle size and density, to deposition velocity behavior in weak and strong wind fields, and to eddy diffusion coefficient dependence on atmospheric conditions, particle size, and density. The results for some theoretical cases are presented to illustrate the behavior of the model. The validity of the model is tested by comparing observed and calculated deposits for the May 1980 Mount St. Helens eruption. The model predicts a double thickness maximum, the first near the volcano, the second at a distance of about 300 km, as was observed in the actual deposit.
Effect of wet tropospheric path delays on estimation of geodetic baselines in the Gulf of California using the Global Positioning System
Authors: Tralli David M. , Dixon Timothy H. , Stephens Scott A.
Published: February 2008

Geodetic baseline measurements using the Global Positioning System (GPS) were acquired in the Gulf of California between Loreto and Cabo San Lucas in Baja California and Mazatlan on the mainland of Mexico. Tropospheric water vapor content was high during the experiment, typically yielding wet path delays in excess of 20 cm at zenith. Surface meteorological (SM) and water vapor radiometer (WVR) measurements were recorded at each site, providing independent means of calibrating the GPS signal for the wet tropospheric path delay. Residual wet delays at zenith are estimated as constants and also as first‐order exponentially correlated stochastic processes. In addition, the entire wet zenith delay is estimated stochastically without prior calibration. The results of these approaches are compared in terms of day‐to‐day baseline repeatability and other system performance discriminants. Calibration with WVR data yields the best repeatabilities, of the order of 1–7.5 parts in 108in the horizontal components of 350‐ to 650‐km baselines with carrier phase data. Further improvement in these results occurs if combined carrier phase and pseudorange data are used. WVR data are important for direct characterization of the wet tropospheric path delays in humid regions. SM measurements, if used with a simple atmospheric model and estimation of residual zenith delays as constants, can introduce significant errors in baseline solutions. However, SM calibration and stochastic estimation of residual zenith wet delays may be adequate for precise estimation of GPS baselines, with a deterioration in repeatability of less than 1–2 cm compared to WVR calibration. Stochastic estimation of the entire zenith wet delay yields comparable repeatabilities, particularly if both carrier phase and pseudorange data are used. Similar analyses of the Owens Valley Radio Observatory‐Mojave baseline in California, where zenith wet delays are factors of 3–5 less, show no significant differences among the various tropospheric calibrations.
Dynamically supported geoid highs over hotspots: Observation and theory
Authors: Richards Mark A. , Hager Bradford H. , Sleep Norman H.
Published: February 2008

Hotspots are associated with long‐wavelength geoid highs, an association that is even stronger when the geoid highs associated with subduction zones are removed. We quantify these associations by expanding the hotspot distribution in spherical harmonics and calculating correlation coefficients as a function of harmonic degree. The hotspot distribution spectrum is nearly white, except for peaks at degrees 2 and 6. It is correlated positively with the slab residual geoid for degrees 2–6, with low seismic velocity in the lower mantle at degree 2, and with low seismic velocity in the upper mantle at degree 6. We test fluid mechanical models for hotspots, including lithospheric delamination and hot plumes, by calculating their predicted dynamic geoid responses and comparing them to the observations. These models include the effects of temperature dependent rheology. Our preferred hotspot model, based on observations of the geoid and seismic tomography, has plumes preferentially occurring in regions of large‐scale background temperature highs in a mantle which has a substantial viscosity increase with depth, although other models are possible. Alternatively, strong plumes in the lower mantle may neck down and attenuate in the upper mantle due to changes in plume viscosity. The major low‐density anomalies causing the geoid highs associated with plumes appear to be in the lower mantle. We hypothesize that cold lower mantle underlying the complementary geoid lows is due to deeply subducted oceanic lithosphere and that hotspot plumes tend to be excluded from these cold regions of the mantle.