bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermal histories of convective Earth models and constraints on radiogenic heat production in the Earth
Authors: Davies Geoffrey F.
Published: February 2008

Thermal histories have been calculated for simple models of the earth which assume that heat is transported by convection throughout the interior. The application of independent constraints to these solutions limits the acceptable range of the ratio of present radiogenic heat production in the earth to the present surface heat flux. The models use an empirical relation between the rate of convective heat transport and the temperature difference across a converting fluid. This is combined with an approximate proportionality between effective mantle viscosity and T−n, where T is temperature and it is argued that n is about 30 throughout the mantle. The large value of n causes T to be strongly buffered against changes in the earth's energy budget and shortens by an order of magnitude the response time of surface heat flux to changes in energy budget as compared to less temperature‐dependent heat transport mechanisms. Nevertheless, response times with n = 30 are still as long as 1 or 2 b.y. Assuming that the present heat flux is entirely primordial (i.e., nonradiogenic) in a convective model leads back to unrealistically high temperatures about 1.7 b.y. ago. Inclusion of exponentially decaying (i.e., radiogenic) heat sources moves the high temperatures further into the past and leads to a transition from ‘hot’ to ‘cool’ calculated thermal histories for the case when the present rate of heat production is near 50% of the present rate of heat loss. Requiring the calculated histories to satisfy minimal geological constraints limits the present heat production/heat loss ratio to between about 0.3 and 0.85. Plausible stronger constraints narrow this range to between 0.45 and 0.65. These results are compatible with estimated radiogenic heat production rates in some meteorites and terrestrial rocks, with a whole‐earth K/U ratio of 1–2×104 giving optimal agreement.
A model of hydrothermal convection in an aquifer
Authors: Torrance K. E. , Chan V. W. C. , Turcotte D. L.
Published: February 2008

Numerical calculations have been carried out to determine the temperature field due to the steady state hydrothermal flow of water through a smoothly curved cylindrical aquifer. It is found that the outflow water temperature relative to the ambient temperature at the base of the aquifer has a maximum at intermediate flow rates. If the flow velocity is low, the water cools by conduction to the wall rock before it reaches the surface; if the flow velocity is high, the water does not have time to be heated by conduction from the wall rock. The transition from laminar to turbulent flow reduces the flow through the aquifer and the temperature of the outflow water. It is found that the outflow temperatures are in general too low to explain continental hot springs. Our results show that shallow laminar hydrothermal flows are more effective in transporting heat to the surface than deeper turbulent flows. We suggest that very shallow (1–100 m) hydrothermal flows of seawater in the oceanic crust may be responsible for the reduction and variability of the measured near‐surface geothermal gradient.
Gradient analysis of geomagnetic fluctuations in the Adirondacks
Authors: Connerney J. E. P. , Kuckes A. F.
Published: February 2008

Geomagnetic fluctuations, recorded simultaneously at several sites located in the Adirondack region of northern New York State, have been analyzed using the gradient method of Kuckes and Schmucker. Four three‐axis flux gate magnetometers were deployed across the Adirondack Dome in a square array, separated by approximately 60 km. The complex inductive scale length, defined as the ratio of the vertical magnetic field and the horizontal gradient, has been estimated for fluctuations in the period range 10 to 200 s. These data indicate that the Adirondack Dome is laterally uniform and characterized by a conducting layer in the lower crust. The conductance of this layer is approximately 400 mhos.
Comparison of geodetic and radio interferometric measurements of the Haystack‐Westford base line vector
Authors: Carter William E. , Rogers Alan E. E. , Counselman Charles C. , Shapiro Irwin I.
Published: February 2008

A three‐dimensional geodetic measurement of the 1.24‐km Haystack‐Westford base line vector was performed to verify the accuracy of previously published radio interferometric measurements. The differences between the geodetic and the very long base line interferometry (VLBI) measurements of the base line length, the horizontal X and Y components, and the vertical Z component were −4, 2, 4, and −19 mm, respectively. After a correction was applied to the VLBI determination of the vertical component to account for a recently measured, repeatable gravitational flexure of the Haystack antenna, the discrepancy in this component was reduced to −6 mm. Other possible repeatable flexures in the two antennas, which have not yet been measured, could either further reduce or increase this discrepancy.
Radon anomalies and microearthquakes at Lake Jocassee, South Carolina
Authors: Talwani Pradeep , Moore W. S. , Chiang Jin
Published: February 2008

Continuous low‐level seismic activity was found to occur at Lake Jocassee after the impoundment of the reservoir in 1975. In addition to monitoring seismic activity we began (in January 1976) a series of geochemical measurements in an effort to search for precursors to ML ∼ 2–2.5 events. Discrete water samples were collected from wells and a spring, and their radon content was determined by scintillation counting. These data were comparable to those obtained from a continuous Rn monitor in the spring using an ionization chamber. Soil Rn measurements in track etch cups were started in 1977. The results of the data collected so far indicate (1) that long‐term fluctuations occur in the radon concentrations in the spring water with a period of ∼44 weeks (the radon content being 50–100% lower in summer and fall), (2) that anomalous changes in the radon concentrations have occurred both before and after earthquakes, the timing of the anomaly being dependent on the distance of the epicenter to the radon sample site, (3) that both high and low anomalous radon values occur, and (4) that the soil randon method using track etch cups is useful in determining areas of high and low Rn concentration but not for a study of short‐term temporal changes.
Three‐dimensional modeling of a magnetic reversal boundary from inversion of deep‐tow measurements
Authors: Macdonald K. C. , Miller S. P. , Huestis S. P. , Spiess F. N.
Published: February 2008

A near‐bottom magnetic survey was conducted over the Brunhes/Matuyama reversal boundary near the East Pacific Rise crest at 21° N. Magnetic measurements were made on a level plane approximately 200 m above the sea floor using the Marine Physical Laboratory's deep‐tow vehicle, with precise transponder navigation. Track density was high both parallel and perpendicular to the magnetic lineations in order to study fine scale deviations from two‐dimensionality. The magnetic field on a gridded map was inverted to obtain the crustal magnetization distribution (including the effects of topography) by extension of the Fourier technique of Parker and Huestis [1974]. Linearity of sources parallel to the spreading center was not assumed, nor was upward continuation necessary in this treatment. We found that the polarity transition boundary is extremely straight and sharp and is very close to two‐dimensional even on a scale of hundreds of meters. Deviations from two‐dimensionality, however, occur within the magnetized blocks away from the transition zone. The polarity transition width is narrow, only 1000 m to 1400 m throughout the study area. This suggests a zone of crustal emplacement which is only 600–1000 m wide at the spreading center, which is in excellent agreement with geologic observations in the area. Comparisons are made with a two‐dimensional treatment of the same data from profiles (i.e., assuming linearity of sources). These studies also document a long‐wavelength (≈ 60 km) sinuosity in the trend of the magnetic anomalies. This sinuosity is the result of offsets of the spreading center which are not transform faults but which involve a component of strike slip motion subparallel to the spreading center.
Estimation of the seismic moment tensor from teleseismic body wave data with applications to intraplate and mantle earthquakes
Authors: Fitch Thomas J. , McCowan Douglas W. , Shields Michael W.
Published: February 2008

Amplitude data from direct and near‐source reflected phases are inverted to obtain point‐source moment tensors. The inversion scheme is computationally efficient, and the results can be interpreted without the uniqueness problems that plague many geophysical inversion schemes. This follows from the linear relationship between the moment tensor components and the recorded waveforms. The L1 norm is used as an optimum solution criterion, thereby allowing first motions to be included in the data set. A mixed data set is warranted when only a small number of amplitude measurements are available. Displacement amplitudes at the recording stations are estimated by seismogram modeling in the case of the shallow earthquake and by the application of an optimum lag inverse filter in the case of the deep earthquake. The inverse filter is designed to remove the combined effects of the recording system and signal distortion owing to anelasticity. Long‐period P waves from an intraplate earthquake located between the Caribbean arc and the mid‐Atlantic ridge at a depth of 25 km reveal a source with a moment time function in the far field that has rise and fall times of 2±1 s. By implication the duration of faulting was short in comparison with shallow earthquakes of similar size at active plate margins. Approximately 89% of the total moment of 0.8×1025 dyn cm pertains to a change in deviatoric stress, which is represented almost totally by a double couple. A 20% increase in the double couple component was achieved by a systematic steepening by 5°–8° of takeoff angles for ray paths to teleseismic distances computed from the Herrin travel times. A submoho source depth is assumed, consistent with generally accepted models of oceanic lithosphere. The double couple component from the moment tensor is similar to the first motion solution but is dominated by a strike‐slip rather than a dip‐slip radiation pattern. Amplitudes and first motion polarities from a deep earthquake beneath the Bonin arc yield a moment tensor that is 72% double couple and 19% compensated linear vector dipole. A similar steepening of the ray paths in this case consistent with a 7% reduction in the compressional velocity at the source depth results in a double couple component of more than 90%. Lateral heterogeneity in the source region precludes a simple interpretation of this apparent velocity reduction. Our results demonstrate that the inversion of body wave amplitude data for the unconstrained moment tensor can yield essentially pure double couples.
Time‐dependent gravity in southern California, May 1974 to April 1979
Authors: Whitcomb James H. , Franzen Wolfgang O. , Given Jeffrey W. , Pechmann James C. , Ruff Larry J.
Published: February 2008

The Southern California gravity monitoring project, begun in May 1974, is intended to coordinate gravity measurements with the long‐baseline three‐dimensional geodetic measurements of the Astronomical Radio Interferometric Earth Surveying (Aries) project, which uses radio interferometry with extragalactic radio sources. Gravity data from 28 of the stations, monitored on an approximately 1‐ to 2‐month basis, have a single‐reading standard deviation of 11 μGal that gives a relative single determination between stations a standard deviation of 16 μGal. The averaging of data reduces the uncertainty, and if gravity does not change during the averaging time, it appears that gravity at a station relative to the base can be determined with a standard error of 2 to 3 μGal. Where stations could not be placed on lowporosity bedrock, the effects of variable groundwater levels must be considered. The largest gravity variation observed, 80 μGal, correlates with nearby water well variations and with smoothed rainfall. Smoothed rainfall data appear to be a good indicator of the qualitative response of gravity to changing groundwater levels at other suprasediment stations, but frequent measurement of gravity at a station is essential until the quantitative calibration of the station's response to groundwater variations is accomplished. The largest earthquake to occur during the survey time near the gravity network was the August 13, 1978, Santa Barbara Channel event (ML = 5.1–5.7, MS = 5.6). The closest gravity station to this earthquake, 67 km east of the epicenter, also exhibits the network's largest gravity change that cannot be related to factors other than tectonic distortion. This change is a 50‐μGal low occurring from mid‐1975 to mid‐1977. It is not known if the occurrence of the gravity anomaly is coincidental or related to the process of preparation for the earthquake.
The origin of lavas from the Ninetyeast Ridge, eastern Indian Ocean: An evaluation of fractional crystallization models
Authors: Ludden John N. , Thompson Geoffrey , Bryan Wilfred B. , Frey Frederick A.
Published: February 2008

Ferrobasalts from DSDP sites 214 and 216 on the Ninetyeast Ridge are characterized by high absolute iron (FeO>12.9 wt %), FeO/MgO>1.9, and TiO2>2.0 wt %. Their trace element abundances indicate a tholeiitic affinity; however, they are distinct from midocean ridge incompatible element‐depleted tholeiites owing to higher contents of Ba, Zr, and Sr and flat to slightly light‐REE‐enriched, chondritenormalized REE patterns. Calculations using major and trace element abundances and phase compositions are generally consistent with a model relating most major elements and phase compositions in site 214 and 216 ferrobasalts by fractionation of clinopyroxene and plagioclase. However, some incompatible element abundances for site 216 basalts are not consistent with the fractional crystallization models. Basalts from site 214 can be related to andesitic rocks from the same site by fractionating clinopyroxene, plagioclase and titanomagnetite. Site 254 basalts, at the southern end of the Ninetyeast Ridge, and island tholeiites in the southern Indian Ocean (Amsterdam‐St. Paul or Kerguelen‐Heard volcanic provinces) possibly represent the most recent activity associated with a hot spot forming the Ninetyeast Ridge. These incompatible‐element‐enriched tholeiites have major element compositions consistent with those expected for a parental liquid for the site 214 and 216 ferrobasalts. However, differences in the trace element contents of the basalts from the Ninetyeast Ridge sites are not consistent with simple fractional crystallization derivation but require either a complex melting model or a heterogeneous mantle source.
The mixed discrete‐continuous inverse problem: Application to the simultaneous determination of earthquake hypocenters and velocity structure
Authors: Pavlis Gary L. , Booker John R.
Published: February 2008

Some inverse problems are characterized by a model consisting of a piecewise continuous function and a set of discrete parameters. For linear problems of this general type, which we call mixed, we show that when the number of data d is greater than the number of parameters p, it is always possible to construct a set of a least d ‐ p equations that are independent of the values of the discrete part of the model. These equations, which we call the annulled data set, can be used to estimate the continuous part of the model. The discrete part of the model can be estimated from a second set of p equations that relate the discrete and continuous parts of the model. The linearization of the nonlinear travel time functional that enter in the hypocenter location problem leads to a mixed inverse problem. The splitting procedure is natural to this problem if the hypocenters are estimated initially by conventional nonlinear least squares by using travel times calculated from some initial estimate of the velocity model. The annulled data are a set of linear combinations of the residuals that are unbiased by that initial location, and as a result, they can be used directly to estimate a perturbation to the velocity model by a Backus‐Gilbert procedure. This makes an iterative algorithm possible that consists of a conventional hypocenter location followed by estimating a perturbation of the velocity model from the annulled data set. The uniqueness of the final velocity model is assessed via the linear resolution analysis of Backus and Gilbert (1968, 1970). We also construct a set of Frechet derivatives that relate perturbations of each hypocenter component to perturbations of the velocity model. These kernels are used to assess the possible error of the hypocenters due to inadequate knowledge of the velocity structure by an application of the generalized prediction approach of Backus (1970a). Good results are obtained when the procedure is applied to a simple synthetic data set.
Variations between Q values estimated from damped terrestrial eigenvibrations
Authors: Hansen Roger A. , Bolt Bruce A.
Published: February 2008

A general probabilistic model is used to investigate the variations in Q values as a function of time for two large earthquakes and a regional network of stations. The time series is assumed to be a linear combination of exponentially damped harmonics in the presence of noise. The analysis is by complex demodulation, which provides instantaneous amplitude and phase at each eigenfrequency as a function of time. Parameter estimation is also made by nonlinear regression for the same statistical model. Q values and their variances have been calculated using records from the ultra‐long‐period instruments at Berkeley (three components), Jamestown (vertical component), and Whiskeytown (vertical component). Time series following the earthquakes of August 1976 (Philippines) and August 1977 (Indonesia) provide estimates of Q for a number of well‐resolved fundamental spheroidal and torsional vibrations. Representative Q values and standard errors from well‐resolved torsional modes for the 1977 Indonesia data at Berkeley for 0T11, 0T12, and 0T35 are 230±81, 213±47, and 244±33. In a number of cases, intercomparison of Q values shows differences for the same mode greater than twice the formal standard error. For example, Q values for the sharp peak 0S16 after the Philippine earthquake are 326±64 at Berkeley and 244±24 at Whiskeytown. In contrast, the mode 0S21 is well excited on the periodograms for Berkeley from both the Indonesia and the Philippine data; Q values are, however, 204±39 and 354±70, respectively, an unacceptable difference. Partial explanation of these differences is helped by the complex demodulates which show varying lengths of coherency of phase angle with time probably due to nonstationary noise and varying interference from neighboring modes. These results suggest that estimates of Q values from eigenspectra should not be accepted at full weight unless specified coherency tests are satisfied.
Acoustic stratigraphy and biostratigraphy of neogene carbonate horizons in the north equatorial Pacific
Authors: Embley Robert W. , Johnson David A.
Published: February 2008

Distinctive acoustic reflectors have been identified and traced over hundreds of kilometers on both low‐frequency (air gun) and high‐frequency (3.5 kHz) seismic profiles from the region immediately to the north of the Clipperton Fracture Zone in the equatorial Pacific. The reflectors result from impedance contrasts between carbonate‐rich layers (up to 90% CaCO3) and siliceous clays, and they represent sharp decreases in porosity, increases in wet bulk density, and decreases in sound velocity. The reflectors have northern limits which increase with increasing age: reflector R0 (late Pleistocene) occurs only south of ∼5°N, R1 (upper Miocene) occurs south of ∼6°N, and R2 (middle to lower Miocene) extends to at least 10°N. Radiolarian biostratigraphy of the piston cores shows that the most extensive of the reflectors, R2, is clearly time transgressive within the middle and lower Miocene. The variable geographic extent and the time‐transgressive nature of individual acoustic reflectors are in part a consequence of the northward component of motion of the Pacific plate during the Neogene, with carbonate sedimentation generally restricted to a narrow equatorial region south of ∼5°N. There are no detectable unconformities within core intervals corresponding to the three major reflectors; hence the reflectors apparently do not represent significant erosional episodes. Superimposed on the effect of plate motion may be an abrupt narrowing of the equatorial productivity belt during the middle Miocene at 11–12 m.y. B.P. This abrupt transition from carbonate‐rich to carbonate‐poor sedimentation occurred over a widespread region (approximately 3° of latitude) and corresponds with reflector R2. Similar events may have been responsible for the shallower reflectors R0 and R1.
Distance corrections for single‐ and dual‐color lasers by ray tracing
Authors: Berg Eduard , Carter Jerry A.
Published: February 2008

The appendix is available with entire article on microfiche. Order from the American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document J80‐007; $1.00. Payment must accompany order.

The physical arc length difference between the red and blue beam of a dual‐color laser generates an error term in determining Δn (and thus n, the refractive index, and D, the line length). Numerical ray trace examples and theoretical approximations show the resulting relative error ΔD/D to increase as D2. The error reaches 1 part in 108 (107) at D of 12 (40) km, respectively, through an atmospheric structure of a line with Δh ≅ 2000 m. Error reduction by a factor of nearly 10 is possible by using the pressure, temperature, and humidity of both endpoints to calculate n or, to a lesser degree, by assuming a standard coefficient of refraction k. Two lines, approximately 30 km each, with Δh of 2000 and 3000 m, were sampled by helicopter on the island of Maui through an inversion layer. The two ray trace lengths were compared to standard reductions. Hypothetical shorter and longer lines through the same atmospheric structures and elevation difference Δh are also reduced by both methods. Length differences from the two methods appear at 10−7 level for lines longer than 60 km and depend on the sharpness of the temperature inversion. The dual zenith angle method of determining Δh would yield errors of the order of 60 cm for the real line.
Geochronology and origin of the Pratt‐Welker Seamount Chain, Gulf of Alaska: A new pole of rotation for the Pacific Plate
Authors: Turner D. L. , Jarrard R. D. , Forbes R. B.
Published: February 2008

40K‐40Ar and fission‐track dating of four seamounts near the southeast end of the Pratt‐Welker seamount chain in the Gulf of Alaska, in conjunction with previously published K‐Ar and fission‐track ages near the northwest end of the chain, documents the complex origin of this seamount chain. Transitional basalts from the adjacent guyots Hodgkins, Davidson, and Denson are dated as 14.3 to 18.2 m.y. These ages, only slightly younger than the ages of the underlying crust, indicate formation of these three seamounts at or very near a spreading center. In contrast, alkalic series lavas (alkali olivine basalts and trachytes) from Kodiak, Giacomini, Dickins, and Hodgkins fit a systematic linear age progression: 23.9±0.6 m.y., 20.9±0.4 m.y., 4.0±0.2 m.y., and 2.8±0.2 m.y., respectively. Hodgkins has apparently experienced two generically different episodes of volcanism, separated by about 12 m.y. The age progression among dated alkali basalts is consistent with the hot spot hypothesis and suggests that for the last 24 m.y. the Pacific plate has moved northwest at 4.4±0.4 cm/yr with respect to the Pratt‐Welker hot spot. This volcanic propagation rate, together with the rates from other parallel Neogene Pacific chains, allows an improved estimate of the pole and rate of rotation of the Pacific plate relative to hot spots: 70°N, 95°W, and 0.88°±0.10°/m.y. We conclude that no significant motion of the Pratt‐Welker hot spot with respect to other Pacific hot spots has yet been detected. However, the Pratt‐Welker age data may alternatively be explained by either the longitudinal roll or propagating crack hypothesis. New K‐Ar ages from Horton guyot, in the Cobb seamount chain, indicate alkalic volcanism 20.7± .0 m.y. ago, consistent with a predicted age of 20 m.y. based on the hot spot hypothesis. Guyot depths from Horton and the dated Pratt‐Welker seamounts are consistent with the K‐Ar ages and normal subsidence of oceanic crust.
The temperature profile of the upper mantle
Authors: Anderson Orson L.
Published: February 2008

Following the procedure outlined by Magnitsky [1971], thermal profiles of the upper mantle are computed by deriving the thermal gradient from the seismic data given as dvs/dρ used along with the values of (dvs/dT)P and (dvs/dP)T of selected minerals, measured at high temperature. The resulting values of dT/dZ are integrated from 380 km upward toward the surface, where the integrating constant is taken from Akagi and Akimoto's work, T = 1400°C at 380 km. The resulting geotherms for minerals are used to derive geotherms for an eclogite mantle and a lherzolite mantle, with and without partial melting in the low‐velocity zone. The geotherms are all subadiabatic, and some are virtually isothermal in the upper mantle. Some are characterized by a large thermal hump at the lithosphere boundary.
Phase transformations in Americium at high pressure: Relation to rare earth elements
Authors: Akella Jagannadham , Johnson Quintin , Schock R. N.
Published: February 2008

The crystal structure of243Am has been investigated up to 18.0 GPa, using a diamond anvil high‐pressure apparatus. The normal dhcp form of Am, stable at 1 atm and room temperature, undergoes three‐phase transformations below 18.0 GPa. The first transformation occurs at about 5.0±1.0 GPa, the second occurs at about 10.0±1.0 GPa, and the third occurs at about 15.0± .0 GPa. The structure of the Am phase at 5‐GPa pressure is fcc. However, we are unable to determine the structure of the new phases at about 10.0‐ and 15.0‐GPa pressure from our present data. The structural behavior of Am showing that Am undergoes a sequence of structural transformations somewhat similar to the lanthanide metals is discussed.
Exploratory models of the Earth's thermal regime during segregation of the core
Authors: Davies Geoffrey F.
Published: February 2008

Some simple exploratory theoretical models of the thermal effects of core segregation have been investigated, assuming an initially homogeneous earth and including convective heat transport through a ‘parameterized convection’ approximation. The results indicate that either (1) mantle temperatures 30% or more above present values may have resulted from the gravitational energy released during core segregation, (2) the earth retained very little of its accretional energy, (3) core segregation lasted for 1 b.y. or more, or (4) the earth accreted heterogeneously. Option 3 seems to be precluded by terrestrial lead isotope data, and the alternatives each raise substantial questions concerning the mechanics, chemistry, and petrology of the earth's early history. There is no recognized evidence for the early hot phase of option 1, and option 4 implies, among other things, an analogous early hot phase. Although it has not been favored, option 2 may be viable.
Tectonic model and seismic potential of the eastern Gulf of Alaska and Yakataga Seismic Gap
Authors: Perez Omar J. , Jacob Klaus H.
Published: February 2008

Based on 13 new fault plane solutions and published seismological, geological, and geophysical data, we interpret the deformation along the Pacific‐North American plate margin in the eastern Gulf of Alaska. Three major tectonic units can be distinguished: (1) the North American plate, (2) the Pacific plate, and (3) a belt of mobile borderland terranes. The Pacific plate moves in a NNW direction at rates of about 6 cm/yr in relation to the North American plate. That motion results in mostly right‐lateral strike slip at the Queen Charlotte‐Fairweather fault system, a well‐known observation. A new finding,however, is that a small component (∼1 cm/yr) of convergence may also be present which results in minor subduction of the oceanic plate beneath portions of the continental margin. Heretofore the Queen Charlotte‐Fairweather fault zone and associated continental margin was interpreted as a classical, pure transform boundary. The Yakutat block, a borderland terrane about 400 km long and 100 to 200 km wide, is carried passively by the Pacific plate except that the block slowly overrides this plate at about 1 cm/yr. This motion is taken up by almost pure thrust faulting in a southwesterly direction along a 400‐km long SE striking shelf edge structure. At its NW edge the Yakutat block is in turn being thrust beneath the North American plate along the Pamplona zone‐Icy Bay lineament. The underthrusting of the Yakutat block results in a major orogeny, crustal shortening and uplift of the Chugach‐St. Elias range. The effects of this collision may extend as far as 500 km inland and cause some deformation at the Denali fault in the central Alaska Range. Subduction of the Pacific plate beneath the colliding margin appears responsible for development of an active volcanic arc up to 300 km inland which trends SE from the Wrangell Mountains to Yukon Territory, Canada, and perhaps to Mt. Edgecumbe volcano in southeast Alaska. The tectonic model proposed implies a high seismic hazard for the Queen Charlotte, Fairweather, and Chugach‐St. Elias fault systems. At these fault zones we estimate recurrence times for great events of about 100 years, but they may vary between 50 and 200 years. A temporarily very high potential for a great earthquake has been determined for the ‘Yakataga seismic gap’ located between Icy Bay and Kayak Island. Large or great thrust earthquakes on the detachment fault underlying the entire Yakutat wedge also appear possible but may only occur infrequently. Their recurrence times are estimated to be several
Volcanism and aseismic slip in subduction zones
Authors: Acharya Hemendra
Published: February 2008

The spatial and temporal relationship of volcanism to the occurrence of large earthquakes and convergent plate motion is examined. The number of volcanic eruptions per year in a convergent zone is found to be linearly related to the aseismic slip component of plate motion. If the aseismic slip rate is low (coupling between converging plates is strong), then the primary manifestation of tectonic activity is the occurrence of large earthquakes with only infrequent volcanic activity. If, however, the aseismic slip rate is high (coupling is weak), then there are few large earthquakes, and volcanism is the principal manifestation of tectonic activity. This model is consistent with the spatial distribution of large earthquakes and active volcanoes in the circum‐Pacific area. It is tested by examining the extent of volcanic activity in the rupture zones of the 1952–1973 sequence of earthquakes in the Japan‐Kurile Islands area. The number of volcanic eruptions along these zones during the interval between large earthquakes is used to compute the aseismic slip rates for these segments, based on the relationship developed in this study. The aseismic slip rates so computed agree with those determined from the earthquake history of the area and rates of plate motion. The agreement suggests that in the interval between large earthquakes, the aseismic plate motion is manifested in a specific number of volcanic eruptions. Therefore in areas with adequate historical data it should be possible to use the model developed in this study to monitor volcanic eruptions for long‐term prediction of large earthquakes.
Seismic energy release of the Moon
Authors: Goins Neal R. , Dainty Anton M. , Toksöz M. Nafi
Published: February 2008

Previous estimates of the seismic energy released by lunar events have not properly accounted for instrument bandwidth, variations in corner frequency, and the effects of intense scattering. In this paper, equations are developed that include all of these effects and give realistic estimates for source parameters. These equations are applied to seismograms and displacement spectra from near‐surface and deep moonquakes to obtain M0 (seismic moment), Ēs (seismic energy release), Eyr (seismic energy released annually by each lunar event class), Δσ (stress drop), and mb (body wave magnitude). The calculations yield M0 ∼ 3 × 1021 dyn cm, Ēs ∼ 2 × 1017 ergs, Δσ ∼ 400 bars, and mb ∼ 5.0 for the largest shallow moonquakes; and M0 ∼ 5 × 1020 dyn cm, Ēs ∼ 1 × 1013 ergs, Δσ ∼ 0.1 bars, and mb ∼ 3.0 for large deep events. The average energy released annually is Eyr = 2 × 1017 erg/yr and Eyr = 8 × 1013 erg/yr by shallow and deep events, respectively; overall, lunar energy release is dominated by the shallow events. The energy released by the deep events may be accounted for by tidal dissipation, and deep event stress drops are comparable to and smaller than the calculated tidal stresses. A comparison of the above values with those observed terrestrially (Eyr ∼ 1025 ergs) and with the energy available from heat flow and tidal dissipation emphasizes the importance of tectonic style (e.g., plate tectonics) in determining the characteristics of planetary seismicity.
Creep mechanism in Mg2GeO4: Effects of a phase transition
Authors: Vaughan P. J. , Coe R. S.
Published: February 2008

We deformed polycrystalline Mg2GeO4 olivine and spinel in a Griggs‐type solid medium apparatus. Flow of Mg2GeO4 olivine can be represented by
= 6.5×l07 σ3.5 exp(−105(kcal/mol)/ RT) where σ (kbar) is the differential stress and
(s−1) is the natural strain rate. For Mg2GeO4 spinel the flow law is
= 2.6×104 σ2exp(−73(kcal/ mol)/RT). The low stress exponent and activation enthalpy coupled with fine grain size (3 μm) suggest that Mg2GeO4 spinel deformed by a superplastic mechanism. Flow parameters for the olivine phase suggest a dislocation creep mechanism. Comparison of theoretical superplastic flow laws for Mg2GeO4 olivine with the spinel phase data suggests that strain rates in Mg2GeO4 spinel are only about a factor of 3 lower than for Mg2GeO4 olivine of the same grain size. A similar estimate holds for dislocation creep of the two phases if it is controlled by diffusion. Transformation from Mg2GeO4 olivine to spinel reduced grain size to approximately 3 μm. Thus we might expect a similar reduction in grain size in the earth's transition zone which could result in superplastic deformation of the transformed phase and cause a weak ‘decoupling’ zone at the transition boundary in the mantle. Superplasticity brought about by transformation‐induced reduction in grain size may also provide a mechanism for deep focus earthquakes and an explanation for the correlation observed between their distribution with depth and the depths of phase transitions within the mantle.
Preseismic and coseismic deformation associated with the Coyote Lake, California, earthquake
Authors: King N. E. , Savage J. C. , Lisowski M. , Prescott W. H.
Published: February 2008

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The Coyote Lake earthquake (ML = 5.9; August 6, 1979; epicenter about 100 km southeast of San Francisco) occurred on the Calaveras fault within a geodetic network that had been surveyed annually since 1972 to monitor strain accumulation. The rupture surface as defined by aftershocks is a vertical rectangle 20 km in length extending from a depth of 4 km to about 12 km. The observed deformation of the geodetic network constrains the average slip to be about 0.33 ± 0.05 m right lateral. Although the geodetic data furnished an exceptionally detailed picture of the preearthquake deformation, no significant premonitory anomaly associated with the Coyote Lake earthquake can be identified.
The Galapagos Spreading Center at 86°W: A detailed geothermal field study
Authors: Green Kenneth E. , Von Herzen Richard P. , Williams David L.
Published: February 2008

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We report here measurements of the heat flow field of the Galapagos Spreading Center on crust of age less than 1.0 m.y. The 443 measurements in an area of about 570 km2 reveal the general planform of the geothermal flux and permit the first truly areal estimate of the near‐axis conductive heat flux. The intrusion process and associated hydrothermal circulation dominate the surface heat flow pattern, with circulation apparently continuing beyond the limits of our survey. The areal average of the conductive heat flux is 7.1 ± 0.8 HFU (295 ± 33 mW/m2), about one‐third the heat flux predicted by plate models. The remaining heat is apparently removed by venting of hydrothermal waters at the spreading axis and through basalt outcrops and hydrothermal mounds off axis. The pattern of surface heat flux is lineated parallel to the axis and the strongly lineated topography. Sharp lateral gradients in the heat flow, greater than 10 HFU/km near escarpments and commonly expressed as high heat flow at the tops of the scarps and lower heat flow in the valleys, may indicate a local concentration of the circulation by surface fault systems and/or variable sediment thickness.
San Luis Obispo Transform Fault and Middle Miocene rotation of the Western Transverse Ranges, California
Authors: Hall Clarence A.
Published: February 2008

A fault‐disrupted, linear belt of lower middle Miocene pyroclastic and volcanic rocks was erupted 15–17 m.y. B.P. along a trend 140 to 200 km long in the southern Coast Ranges and along the northern flank of the western Transverse Ranges of California. Offset segments of the belt are coincident with the Oceanic‐West Huasna, Santa Maria River‐Little Pine, and Lompoc‐Solvang fault zones. The volcanic rocks thin eastward and also vary in thickness along strike, the latter reflecting the undulation of ‘porpoising’ highs and lows of basins. To account for the linear distribution of over 3000 km2 of bimodal volcanic rock types, it is proposed that they were intruded, extruded, and ejected into a continental submarine margin environment along a feature here named the San Luis Obispo transform (SLOT). The fault was near the mid‐Miocene continental margin and evidently joined the early mid‐Miocene Mendocino triple junction with the crest of the East Pacific Rise. Subduction of the Farallon Plate had stopped along the adjacent continental margin before Miocene magmatism in this region. The inferred position of the East Pacific Rise crest was too far to the south, and the inferred position of the Mendocino triple junction was too far to the north between 15 and 20 m.y. B.P. to have been directly related to the Miocene volcanic event near 35°N latitude at that time. A part of the western Transverse Ranges was transported northwestward immediately following Miocene volcanism. Earlier movement is possible, but evidence is inconclusive. Microplate transport was concurrent with the development of pull‐apart structures and the ≃12‐ to ≃16‐m.y.‐old period of volcanism along the southern margin of the microplate within the Los Angeles basin. Variable paleomagnetic directions that display clockwise deflections in the Oligocene (e.g., Morro Rock‐Islay Hill Complex) and Miocene igneous rocks (e.g., western Transverse Ranges) in the vicinity of SLOT can be accounted for by extension and rotational tectonics during pull‐apart development, rotation of blocks between faults during the northwestward transport of the western Transverse Ranges along SLOT, and by rotation and possible northward translation of a part of the western Transverse Ranges.
Effects of fault interaction on moment, stress drop, and strain energy release
Authors: Rudnicki J. W. , Kanamori H.
Published: February 2008

Solutions for collinear shear cracks are used to examine quantitatively the effects of fault slip zone interaction on determinations of moment, stress drop, and static energy release. Two models, the barrier model and the asperity model, are considered. In the asperity model, the actual distribution of strengths on a fault plane is idealized as a combination of two limiting cases: areas which slip freely at a uniform value of a residual friction stress and unbroken ligaments or ‘asperities’ across which slip occurs only at the time of a seismic event. In the barrier model, slip zones separated by unbroken ligaments (barriers) are introduced into a uniformly stressed medium to approximate the nonuniform fault propagation proposed by Das and Aki. The strain energy change due to introducing collinear slip zones or due to breaking the asperities between them is shown to be given by the usual formula for an isolated slip zone with the stress drop replaced by the effective stress. Significant interaction between slip zones occurs only if the length of the asperity is less than half the length of the slip zones. For the case of two collinear slip zones, fracture of the asperity between them is shown to cause a large moment primarily because of the additional displacement which is induced on the adjacent slip zones. For example, if the asperity length is 0.05l, where l is the length of each adjacent slip zone, then fracture of the asperity causes a moment almost 1.8 times the moment caused by introducing a slip zone of length l. For two collinear slip zones, the local stress drop due to fracture of the separating asperity is shown to become unbounded as the asperity length goes to zero, but in the same limit the stress drop averaged over the entire fault length is approximately equal to the apparent stress drop inferred for an isolated fault of the same moment and total fault length. This apparent stress drop is approximately equal (within a factor of 2 or 3) to the effective stress and hence can be used in the usual formula to give a good estimate of the strain energy change. For the barrier model, numerical results are given for the ratio of the stress drop calculated on the assumption of an isolated slip zone to the true stress drop. For example, in the case of two collinear slip zones of length l separated by a barrier of length 0.2l, this ratio is 0.5, whereas for a barrier length equal to that of the adjacent slip zones, the ratio is 0.24. Stress drop estimates become worse with increasing number of fault segments.
Lead isotopic studies of the Samail Ophiolite, Oman
Authors: Chen James H. , Pallister John S.
Published: February 2008

The isotopic composition of Pb and the concentrations of U, Th, and Pb have been determined for samples from various lithologic units and massive sulfides of the Samail ophiolite. The observed 206Pb/ 204Pb ratios range from 17.90 to 19.06, 207Pb/204Pb ratios from 15.43 to 15.63, and 208Pb/204Pb from 37.66 to 38.78. In Pb isotopic evolution diagrams, the initial Pb isotopic compositions of most of the samples from the Samail ophiolite plot within the field of oceanic basalt, clearly distinct from island arc data, and define some of the least radiogenic Pb observed from oceanic rocks. Lead data from the Samail are compatible with a model involving magma generation from an oceanic mantle source and formation of the ophiolite at an oceanic spreading center. U‐Th‐Pb isotopic systematics demonstrate that vertical heterogeneity in the oceanic crust can be created through differential concentration of U, Th, and Pb during crystal fractionation and alteration at, or near, the spreading ridge. Calcite from amygdules in the ophiolite basalt has similar Pb isotopic composition to the igneous rocks, suggesting precipitation of the calcite from seawater which contained Pb derived mostly from the oceanic crust. Lead isotopic data on Fe‐Cu sulfides are also similar to the results from the igneous suite suggesting that the source of the sulfides is predominently from the oceanic crust. Lead data from serpentinized peridotite and a galena sample from below the ophiolite suggest that part of the serpentinization process and the formation of galena could involve addition of radiogenic Pb from either a continental source or from oceanic sediments.
Earthquake focal mechanisms and tectonic processes along the southern boundary of the Caribbean Plate
Authors: Kafka Alan L. , Weidner Donald J.
Published: February 2008

The focal mechanisms of five small earthquakes (4.9 ≤ mb ≤ 5.6) that occurred in northwestern Venezuela have been determined using teleseismically recorded Rayleigh waves. The Rayleigh wave data were also used to constrain the depths of the five events studied. These results suggest that the southern boundary of the Caribbean plate is presently characterized by a broad zone of internal deformation where the eastward displacement of the Caribbean plate with respect to the South American plate is being accommodated along variously oriented strike slip faults. This strike slip faulting appears to occur along pre‐existing zones of weakness such as the Bocono fault, and the earthquakes studied here are all consistent with an east‐west trending compressive stress field being released along these zones of weakness. These focal mechanisms, therefore, reflect the tectonic stress field generated by the eastward motion of the Caribbean plate as opposed to relative plate motion per se. Thus, the southern boundary of this plate cannot be classified as a simple plate boundary (i.e., transform fault, spreading center, or subduction zone). Instead, relative plate motion in this region appears to be accommodated by internal deformation within a broad zone extending from the interior of the Caribbean plate into northern South America.
Time‐dependent deformation and stress relaxation after strike slip earthquakes
Authors: Yang Mai , Toksöz M. Nafi
Published: February 2008

The quasi static time‐dependent behavior after strike slip earthquakes is studied using three‐dimensional finite‐element models. The effects of lateral heterogeneities, finite fault, and nonuniform fault slip are included. Models of a great earthquake and a medium size earthquake are constructed using a set of possible viscosity structures. Deformation change after an earthquake is found to be sensitive to the viscosity structure located beneath the fault zone. The presence of low viscosity beneath the fault concentrates the horizontal displacement near the fault zone and completely changes the character of vertical deformation. Thus leveling, in addition to horizontal geodetic measurements after a strike slip event, is important in determining the viscosity structure near fault zones. The shear stress in front of the fault tip increases significantly within several decades after the event for models with a shallow low‐viscosity zone beneath the fault. This accelerated stress accumulation is consistent with the observed phenomenon of earthquake migration along the North Anatolian fault zone.
Bidirectional reflectance spectroscopy: 2. Experiments and observations
Authors: Hapke Bruce , Wells Eddie
Published: February 2008

Certain aspects of the theoretical bidirectional reflectance expression derived by Hapke [this issue] were studied experimentally. The theoretical expression satisfactorily describes the measured angular scattering and spectral properties of a cobalt glass powder whose absorptance and reflectance vary greatly with wavelength. The theoretical expression for the spectral single‐scattering efficiency for irregular particles derived in paper I is shown to describe the measured efficiency of the cobalt glass particles. The predicted approximately linear dependence of the bidirectional espat‐function W on the effective particle absorption optical thickness αDe is shown experimentally to be valid for αDe at least as high as 3. The theoretical reflectance function also describes measured distributions of intensity across the surfaces of planets, except near the sunlit limb, where the macroscopic roughness influences the brightness.