bluuebunny/crossref_metadata_2025 huggingface dataset:
Fluid kinematics, fluid residence times, and rock degassing in oceanic crust determined from noble gas contents of deep sea drilling project pore waters
Authors: Barnes Ross O. , Clarke W. Brian
Published: February 2008

The 3He, 4He, Ne, and Ar in sedimentary pore fluids, extracted in situ, were measured on samples from Deep Sea Drilling Project (DSDP) sites 398, 410, 419, 420, 424, 436, and 438. Earlier results from DSDP leg 15 are also discussed. Where regional sediment cover prevents direct penetration of seawater into basement rocks at sites 149, 436, and 438, He concentration profiles indicate slow vertical and/or horizontal advection of sedimentary pore fluids, suggesting that fluids can ventilate through the sediment cover either in diffuse, bulk flow or in “aquifers” confined to regions of relatively high permeability such as ash beds, sand/silt layers, or fracture channels in lithified sediments. At site 410, where basement outcrops are prevalent, no advection was detectable in the sediments; however, the basement was well flushed by seawater. On <2 m.y. crust near the East Pacific Rise and the Galapagos Spreading Center, the collection of only one sample per hole precluded Calculation of sedimentary fluid velocities. However, most juvenile helium is degassed from the crust in <6 × 105 years, except for the deep crust or upper mantle, where juvenile helium may be released locally to fluids advecting in deep fissures or faults. Fluid residence times in upper basement rocks increased from ∼5 × 103 years in 10 m.y. crust open to direct seawater penetration to ∼105–106 years in Cretaceous basement sealed by hundreds of meters of sediment, to >1–5 × 108 years (apparent age) in sediments on the active continental margin off NE Japan.
Heat flow measurement in lacustrine or oceanic sediments without recording bottom temperature variations
Authors: Wang K. , Beck A. E.
Published: February 2008

Water bottom temperature variations (BTV) introduce many uncertainties into heat flow density (HFD) values determined from the temperature‐depth (T‐z) plot from a one‐time probe insertion into the top few meters of sediments. The BTV have to be carefully monitored for a number of years up to the date of the probe measurements; this makes the method time consuming and economically unattractive. In this paper we present the theory for a promising new technique. The method does not require monitoring of the BTV but does require at least two T‐z profiles separated in time by only a few months. A generalized inversion of the BTV history using the separated T‐z data, and any other a priori information that is available, is then performed in the frequency domain to give the most probable Bayesian estimates of various parameters and their error bounds. Numerical examples show that the inversion can resolve not only the HFD but also the mean sediment surface temperature and gives information on the longer‐term trend as well as details of the BTV over the previous year; these, together with thermal properties structure, are the factors that contribute most to the shape of the time‐dependent T‐z profiles used. In a typical situation where a uniform half‐space with a background HFD = 60 mW m−2 has been perturbed by complex BTV with a maximum variation of ±1.8 K, using a misguessed HFD of 90 ± 100 mW m−2 in the inversion of two T‐z profiles taken 3 months apart over a 2‐ to 5‐m section results in a posteriori HFD of 58 ± 11 m W m−2. Although the technique gives an optimally determined mean HFD, its principal value lies in the ability to place reliable error bounds on the mean.
Multichannel seismic evidence for variations in crustal thickness across the Molokai Fracture Zone in the Mid‐Pacific
Authors: ten Brink Uri S. , Brocher Thomas M.
Published: February 2008

Coincident multichannel seismic reflection and refraction data from a N–S transect near Oahu, Hawaii, provide evidence for thickening of the Pacific crust by 1–2 ± 1 km south of the large‐offset (16 m.y.) Molokai Fracture Zone (FZ). Tau‐p stacks, tau‐sum inversions, and forward modeling of the refraction data indicate that the crustal thickening occurs primarily within the lower portion of seismic layer 2. Assuming isostatic balance, the differences in crustal thickness predict that seafloor having the same age will have different elevations across the FZ. Observations of sea‐floor depths across the FZ east of the Hawaiian Islands are consistent with this prediction implying that the processes which have generated the crustal differences have been stable for over 50 m.y. Previous correlations between the chemical composition of ridge crest basalts, crustal thickness, and ridge crest elevation have been attributed to variations in the thermal regime of the upper mantle under mid‐ocean spreading centers. In accord with this hypothesis, we propose that the observed differences in crustal structure across the Molokai FZ may have been produced by small (25°C) differences in the thermal regime of the upper mantle beneath the ancestral East Pacific Rise. Discontinuous intracrustal reflections located about 1.6 s below the sediment/basement interface are observed in migrated reflection data south of Oahu. These reflections are similar in character to the lower crustal “Horizon R” event observed in the western North Atlantic. Shallower intracrustal reflections, possibly from within seismic layer 2, are also observed. The observation of these intracrustal reflections in both the Atlantic and Pacific oceans suggests that they are a fundamental signature of the crustal accretion process at a variety of spreading rates and that they are mappable using modern seismic reflection/refraction methods.
Joint inversion of regional and teleseismic earthquake waveforms
Authors: Baker Mark R. , Doser Diane I.
Published: February 2008

A least squares joint inversion technique for regional and teleseismic waveforms is presented. The mean square error between seismograms and synthetics is minimized using true amplitudes. Matching true amplitudes in modeling requires meaningful estimates of modeling uncertainties and of seismogram signal‐to‐noise ratios. This also permits calculating linearized uncertainties on the solution based on accuracy and resolution. We use a priori estimates of earthquake parameters to stabilize unresolved parameters, and for comparison with a posteriori uncertainties. We verify the technique on synthetic data, and on the 1983 Borah Peak, Idaho (M = 7.3), earthquake. We demonstrate the inversion on the August 1954 Rainbow Mountain, Nevada (M = 6.8), earthquake and find parameters consistent with previous studies.
Introduction to Special Section on Subseafloor Metamorphism
Authors: Kimball Karen L. , Harper Gregory D.
Published: February 2008

Metamorphism at mid‐ocean ridges was indicated first by the presence of metabasites and peridotites in dredge hauls [e.g., Miyashiro et al., 1971]. Circulation of seawater through the oceanic crust was suggested as an explanation for the low average heat flow measurements at and near the mid‐ocean ridges [e.g., Lister, 1972]. Both dredged rocks and ophiolites display very similar types of alteration, and a generalized profile is recognized; zeolite facies pillow lavas grade downward into greenschist facies sheeted dikes which in turn grade into amphibolite facies gabbros. In Deep Sea Drilling Project (DSDP) hole 504B [Alt et al., 1986] the entire extrusive sequence and the upper part of a greenschist facies sheeted dike complex were penetrated, and a similar profile was sampled.
Microthermometry of fluid inclusions from the VC‐1 core hole in Valles Caldera, New Mexico
Authors: Sasada Masakatsu
Published: February 2008

Fluid inclusions in vein quartz and calcite from core samples of the VC‐1 hole were studied with microscope heating/freezing and crushing stages. All samples originate from hydrothermally altered Paleozoic rocks predating formation of the Jemez Mountains volcanic field and Valles caldera. Most homogenization temperatures (Th) of the liquid‐rich inclusions are above the present well temperature, but some Th of primary inclusions from 515 m and those of secondary inclusions from 723 m fit the present well temperature curve measured 10 months after completion of the well. The maximum temperature recorded by the primary inclusions is 275°C from hydrothermal quartz in the Sandia Formation at 811‐m depth. The total range of Th for samples from several depths (90°C) indicates cooling from the maximum temperature. The salinity of fluid inclusions in hydrothermal quartz and calcite is generally low, <1 wt % NaCl eq. High‐salinity fluid, up to 5 wt % NaCl eq, has been found in several calcite veins from the lower part of the Madera Limestone. The salinity decreases with decrease of Th of the secondary inclusions, and that with lowest Th at the lower part of the Madera Limestone is similar to those from the other depths. These data show that early hot water circulation system involved several types of fluid, whereas the later one was a homogeneous fluid. The salinity of fluid inclusions in detrital quartz (presumably inherited inclusions) is higher than that in hydrothermal minerals. Some of these inclusions show extraordinary low temperatures of final melting point of ice (about −40°C), suggesting that a CaCl2 component is present. CO2 contents in fluid inclusions were estimated by the bubble behavior on crushing. Crushing results indicate that CO2 content of the early fluid is ≧0.35 wt %, and that of the later fluid is ∼0.2–0.3 wt %. Geothermal fluid trapped in the fluid inclusions representing the present temperature regime is comparable in composition to those from the Baca geothermal field inside the caldera and to those from hot springs in San Diego Canyon.
Observed tidal braking in the Earth/Moon/Sun system
Authors: Christodoulidis D. C. , Smith D. E. , Williamson R. G. , Klosko S. M.
Published: February 2008

The low degree and order terms in the spherical harmonic model of the tidal potential have been observed through the perturbations which are induced on near‐Earth satellite orbital motions. This recovery, which is the most complete dynamic model ever obtained, has been achieved through evaluating tracking observations on 17 different, mostly laser, satellites. A new improved GEM‐Tl geopotential model, complete to degree and order 36, was estimated simultaneously with the 66 adjusted tidal coefficients. The gravitational and tidal models were developed using the J2000 Reference System with the adopted nutations of Wahr and the precession model of Lieske. The tidal recovery was made in the presence of an extended oceanographic model containing over 600 long‐wavelength coefficients from 32 major and minor tides. Since solid Earth tides have perturbing frequencies identical to those of the ocean tides, the solid Earth tidal model of Wahr was used as a basis for the recovery of the ocean tidal terms. This provided a complete description of the combined tidal potential sensed by these well‐tracked satellites. This tidal model (for all 32 adjusted and unadjusted tides) has then been used to calculate the secular change in the Moon's mean motion due to tidal dissipation and the tidal braking of the Earth's rotation. The secular change in the Moon's mean motion due to tidal dissipation is found to be −25.27±0.61 arc sec century−2. Our estimate of the lunar acceleration agrees well with that observed from lunar laser ranging techniques, which most recently found −24.9±1.0 arc sec century−2 (Newhall et al., 1986). The corresponding tidal braking of the Earth's rotation is −5.98 ± 0.22 × 10−22 rad s−2. If the nontidal braking of the Earth due to the observed secular change in the Earth's second zonal harmonic (Yoder et al., 1983) is considered, modern satellite techniques yield a total value of the secular change in the Earth's rotation rate of −4.69 ± 0.36 × 10−22rad s−2.
Contemporary seismicity and tectonics of the northern and central coast ranges‐Sierran Block Boundary Zone, California
Authors: Wong Ivan G. , Ely Richard W. , Kollmann Auriel C.
Published: February 2008

Beneath the physiographic boundary between the Coast Ranges and the Great Valley, a fundamental tectonic boundary exists between the Coast Ranges province and the Sierran block. Recent geophysical studies have revealed the Coast Ranges‐Sierran block (CRSB) boundary zone to be a complex region of compressional tectonics wherein wedges of comparatively ductile Franciscan Complex of the Coast Ranges have overridden the competent Sierran block basement and peeled up the overlying sediments of the Great Valley Sequence. The boundary zone was the probable source of the two 1892 Winters earthquakes (ML 6–7) and the 1983 Coalinga earthquake (ML 6.7). We evaluated the seismicity within the northern and central portions of the zone from Red Bluff south to San Luis reservoir for the period 1969–1985. Seismicity was spatially diffuse; most of the earthquakes occurred in somewhat isolated concentrations within the upper crust. A low activity level, temporal clustering, and episodic behavior also characterized this seismicity. The largest earthquake in the boundary zone was a ML 4.2 event near Madison; 20 events exceeded ML 3.0. In a few cases the seismogenic sources appear to be eastward dipping, high‐angle reverse faults, although some right‐slip, north trending faults also appear to be seismogenic. In comparison, the Coalinga and 1985 Kettleman Hills main shocks in the southern portion of the zone appeared to involve gently westward dipping thrust faults, although aftershocks also occurred on moderately to steeply dipping reverse faults. Fault plane solutions along the zone exhibited a tectonic stress field of NE to E trending compression, normal to the boundary. Such compression may be a consequence of one or a combination of several influences: (1) the westward movement of the Sierran block against the Coast Ranges due to late Cenozoic crustal extension in the Great Basin, (2) slightly convergent motion between the Pacific and North American plates, and (3) fault‐normal crustal compression due to the low shear strength of the San Andreas fault. The tectonic deformation manifested by the Coalinga and recent moderate‐sized earthquakes in the southern zone appears to be occurring along the full extent of the boundary albeit at varying rates for different segments. Thus the potential for large earthquakes may exist along segments of the CRSB boundary and the 1892 main shock may represent a characteristic earthquake for that portion of the zone.
Stress transfer and seismic phenomena in coupled subduction zones during the earthquake cycle
Authors: Dmowska Renata , Rice James R. , Lovison Lucia C. , Josell Daniel
Published: February 2008

An elementary mechanical model of a coupled subduction zone is presented. It is used to study the transfer of extensional stresses along the underthrusting plate, and shear stress interactions between the plate itself and its thrust contact zone with the overriding plate, during the earthquake cycle. The model assumes an elastic plate which deforms one dimensionally, has shear drag interactions (treated approximately by the Elsasser procedure) with a Maxwellian viscoelastic mantle, and is subject to steady gravitational sinking forces. Great thrust earthquakes are simulated as sudden stress‐relieving displacements at the contact zone between the slab and overriding plate. The model allows analysis of the space‐ and time‐variable stresses in the main thrust zone, the adjacent oceanic plate, and the subducting slab over each earthquake cycle. Stress variations leading to outer‐rise events are discussed, as well as stress changes responsible for intraplate seismicity downdip from the main thrust zone, based on simulations of periodic great earthquakes. The results of simulated slow progression of deformation (slip) from below into the thrust contact zone are also presented. Such preslip, occurring over a time scale that is small in comparison with the whole seismic cycle (e.g., of the order of a tenth or so), is shown to transiently slow the rate of buildup of, or even relieve, extensional stresses in the slab. In the absence of preslip those stresses build up steadily in time during the latter part of the cycle. The results have implications for induction of sesimic quiescence as well as for explaining patterns of seismicity in relation to spatial position along the underthrusting plate and to time throughout the cycle. Correlations are reported between the model simulations and seismicity observations from coupled subduction zones around the world. The observations include tensional outer‐rise events after large subduction earthquakes and compressional ones in the latter parts of the cycle, normal (tensional) intraplate earthquakes preceding the main thrust events and positioned downdip from the thrust zones, and interlaced periods of seismic activity in the lower part of the main thrust zone and in the descending slab.
Linearized augmented plane wave electronic structure calculations for MgO and CaO
Authors: Mehl M. J. , Cohen R. E. , Krakauer H.
Published: February 2008

We present full‐potential self‐consistent linearized augmented plane wave (LAPW) calculations for the electronic structure and total energy of MgO and CaO, within the local density approximation. Calculated equilibrium properties are in good agreement with experiment. We also consider the transition from the low‐pressure sodium chloride (B1) structure to the high‐pressure cesium chloride (B2) structure for MgO and CaO and the transition to the NiAs (B8) structures for MgO. In calcium oxide we find that the B1‐B2 transition occurs at 54 GPa, in agreement with the experiment. In magnesium oxide a transition from B1 to B2 is found at 510 GPa. This is a factor of 2 lower than found by a previous pseudopotential calculation. We find no transition to the B8 structure at any pressure. The calculated charge densities of MgO and CaO are very close to the charge densities calculated from the overlap of spherical ions used in the potential induced breathing (PIB) model at zero pressure. More significant deviations from overlapping ions are found at extreme pressures. Near equilibrium, the error in the PIB model (compared to the LAPW calculation) is of the same order as the error in the LAPW calculation (compared to the experiment).
Thermal model of oceanic transform faults
Authors: Chen Yongshun
Published: February 2008

A thennal model of the oceanic transform fault which includes effects of the lateral heat conduction across the fault and the shear heating along the fault has been developed. The results show that shear heating along the transform fault significantly raises the temperature along the fault. The depths to isotherms of the thermal model were compared to the depths of earthquakes along transform faults. By including the shear heating, the limiting isotherm of the focal depths of Atlantic transform earthquakes is 700°C (600°C if not including the shear heating), which is close to the limiting temperature of the intraplate oceanic earthquakes. If the transform earthquakes are limited by the 750°C isotherm, then the transform earthquakes should be shallow events as predicted by our thermal model. The model predicts the maximum depth of the 750°C isotherm increases with the length of the transform fault and decreases with the half‐slip rate. Both the effects of the lateral heat conduction across the transform fault and the shear heating along the fault generate topographic and geoid anomalies in a 30 to 60‐km‐wide zone at the fault with magnitudes of the order of a few hundred meters and tens of centimeters, respectively. Given the thermal structure of the transform fault, the shear traction along the fault can be estimated. The average shear traction per unit length of the transform fault is of the order of 1012 N/m which is comparable to the magnitude of the major plate‐driving forces.
Flexural modeling of the midcontinent rift
Authors: Nyquist Jonathan E. , Wang Herbert F.
Published: February 2008

A basement profile obtained from seismic reflection data has been used to constrain a two‐dimensional flexural model of basin formation for the Midcontinent Rift at a latitude of 45° 25′N. Model parameters included the thickness of the elastic plate, the basin width, and the maximum basin thickness. Modeling suggests that flexure produced a deep narrow basin along the rift axis and that the crust was thinned at the time of basin formation to an elastic thickness of 9.6 km for a plate ruptured by rifting, or 5.7 km for an unbroken plate, with corresponding maximum basin thicknesses of 14 km and 16 km respectively. The plate thickness depends most strongly on the basin width and is well constrained by the seismic data, although erosion may have narrowed the basin. The maximum basin thickness is poorly constrained because of the lack of seismic data for depths greater than about 10 km and because the strata at the center of the rift have been disturbed by a postrift compressional event which produced the St. Croix horst. Despite uncertainty about the basin thickness, the load required to flex the crust to produce the Midcontinent Rift basin is too large to be attributed to the weight of the central flood basalts unless the basin subsided into a fluid less dense than the solidified basalts. On the basis of seismic refraction data and by analogy with other rifts, we hypothesize that a magmatic “rift pillow” intruded in the lower crust. The basaltic pillow subsequently solidified to produce a large, high‐velocity region in the lower crust, centered under the rift axis, as determined from deep seismic refraction. This crystallization and cooling may be responsible for the “sag” phase of rift evolution, as evidenced by laterally widespread occurrence of post volcanic sediments.
Shallow borehole tilt: A reprise
Authors: Wyatt Frank K. , Morrissey Sean‐Thomas , Agnew Duncan Carr
Published: February 2008

We describe results from nearly a decade of tilt measurements produced by two arrays of shallow borehole tiltmeters: one in a semiarid environment at Piñon Flat Observatory (PFO), California (depth of burial 4.5 m) and the other in a maritime‐Arctic environment at Adak, Alaska (depth of burial 2 m). Although renovation and reinstallation of the instruments at the two sites reduced thermal noise, it did not change the secular records significantly. This implies that the large tilts observed reflect instability of the ground rather than the sensor, so that deeper installations should give better results. The PFO data show large rainfall‐related tilts (caused by near‐surface weathering) and also periodic temperature‐related tilts (from several thermoelastic effects). The Adak data are dominated by tilts from the annual temperature cycle (though by a smaller amount than at PFO), but at other frequencies they show essentially the same power levels as at PFO. Both data sets confirm earlier results that burial at such shallow depths, even in apparently stable material, is inadequate for the measurement of tectonic tilts.
Rheology of melts and magmatic suspensions: 1. Design and calibration of concentric cylinder viscometer with application to rhyolitic magma
Authors: Spera Frank J. , Borgia Andrea , Strimple James , Feigenson Mark
Published: February 2008

Accurate rheological data for silicate melts and crystal‐melt, melt‐vapor, and crystal‐melt‐vapor suspensions of both natural compositions and simple binary and ternary systems are useful in constraining theories of momentum transport in magmatic systems and theories of melt structure. Data addressing these problems are presented in a series of papers of which this is the first. A high‐torque wide‐gap concentric cylinder viscometer has been constructed and used for characterization of the rheometric properties of rhyolitic magma under conditions of varying temperature (1100°–1350°C) and shear rate (0.05–13 s−1) at 105Pa total pressure. Two natural rhyolitic starting materials were used in these experiments. Four independent temperature‐shear rate cycles were performed on composition RlA (∼76% SiO2) and two independent cycles on composition R2A (∼74% SiO2). Each run had a small (3–8 vol %) fraction of vapor present during viscometric testing. A power law model of the form τ = m (T)n(T)was found to fit the data. The temperature dependencies of m and n are m (т = a exp (b/T) and n (T) = c + dT, where a, b, c, and d are constants for fixed melt composition and bubble content. The viscosity function is given by η = a exp (b/T)c−1+ d T. Representative values of the constants for composition R2A are a = 5.579 ± 0.034 × 10−7Pa s−n, b = 41491 ± 9.5 K, c = 3.039 ± 0.003 and d = −1.516 ± 0.002 K−1. The power law index coefficient n increases as temperature increases in the range 1100°–1350°C due to both volumetric expansion of vapor bubbles with increasing temperature and because of the effects of high shear rate on vapor bubble shapes. The Capillary number (Ca =ηa/ϕ), the ratio of stress tending to distort bubbles and stress tending to maintain sphericity, is high for these experiments, indicating substantial deviations from bubble sphericity. The activation energy for viscous flow, defined according to ∂1nn/∂(1/T) = b ‐ dT21nϒ, depends strongly and monotonically on shear rate. The Weissenberg effect, a phenomenon indicative of the presence of nonzero normal stresses has been observed. Melt climbs along the inner rotating rotor due to a positive first normal stress difference defined according to τΘΘ‐ τrr= ‐Ψ1rΘ2, where ‐Ψ1is the first normal stress coefficient. Based on a plot of climb height Chversus rotor angular frequency ω, ‐Ψ1is found to be ∼2.5 × 103Pa s2. Viscoelastic effects may be important in certain high shear rate flows of petrologic significance in which the Deborah number, defined according to De = Ψ1/2η, is of order unity. To our knowledge, this is the first reported measurement of a normal stress coefficient for magma.
Paleolatitude of the Butano Sandstone, California, and its implications for the kinematic histories of the Salinian terrane and the San Andreas Fault
Authors: Kanter Lisa R.
Published: February 2008

The lower to middle Eocene Butano Sandstone of the Santa Cruz Mountains, California, is part of the central portion of the Salinian terrane, a slivered granitic fragment west of the San Andreas fault. The Butano Sandstone has been correlated with the Point of Rocks Sandstone across the San Andreas fault, approximately 315 km to the southeast. The magnitude of this offset is the same as the amount of Neogene San Andreas fault offset documented in other studies. Thus, the correlation of these two Eocene units implies that no pre‐San Andreas system motion of the Butano Sandstone has occurred. Paleomagnetic studies indicating large northward translations of Cretaceous and Paleocene rocks of coastal California, however, raise the possibility of continued northward motion of the Salinian terrane during or after deposition of the Butano Sandstone. To better our understanding of the Eocene position of the Salinian terrane, the paleolatitude of the Butano Sandstone was determined by paleomagnetic analysis of azimuthally unoriented cores from exploratory oil wells. Specimens from 30 core intervals from the top to the middle or lower parts of the formation were subjected to alternating field and thermal demagnetization. The resulting paleolatitudes group better when bedding is horizontal than in their drill hole attitudes, providing a positive fold test. Several magnetic polarity zones are present. The mean paleolatitude, 34.6°±2.5°, is significantly lower than that expected if the rocks were deposited in their present‐day location on an underformed North American craton. When Neogene slip on the San Andreas and on smaller, inboard faults is restored, however, the measured paleolatitude does not differ significantly from that expected. This result supports the correlation of the Butano and Point of Rocks Sandstones and implies that the Salinian terrane experienced no significant motion between deposition of the Butano Sandstone and establishment of the Neogene San Andreas dextral fault system.
Late Quaternary slip rate on the Oak Ridge fault, Transverse Ranges, California: Implications for seismic risk
Authors: Yeats Robert S.
Published: February 2008

The slip rate on the Oak Ridge fault at South Mountain in the densely populated lower Santa Clara Valley, California, is estimated as 5.9–12.5 mm/yr since the end of Saugus deposition at 0.4–0.2 Ma. A displacement of 3 m per event, assumed on the basis of surface rupture on the 1952 Kern County, 1971 San Fernando, and 1978 Tabas‐e‐Golshan earthquakes, gives an average recurrence interval of 250–500 years, with the major uncertainty being the age of the top of the Saugus. Displacement of 2375–2490 m of the top of the Saugus at South Mountain includes piercing‐point displacement and distributed displacement from drag folds near the fault; both are the near‐surface expression of faulting at potential main shock depths beneath well control. Slip rates for the nearby San Cayetano and Red Mountain faults are less well constrained because post‐Miocene strata are largely absent in their hanging‐wall blocks, but available evidence suggests that their slip rates are in the same range as that for the Oak Ridge fault. The lower Santa Clara Valley has not had a large, damaging earthquake in 200 years of record keeping, although the December 21, 1812, tsunami‐producing earthquake could have occurred on the offshore continuation of the Oak Ridge or Red Mountain fault. A trench on the Harmon alluvial fan near the Ventura County Government Center revealed evidence of cracks filled with sediments from below, suggesting liquefaction during a Holocene earthquake. Earthquake repeat times measured in hundreds of years have been reported by others for the 1971 San Fernando fault and a small fault near the Red Mountain fault, and comparable repeat times are suggested for a normal fault within the hanging‐wall block of the Oak Ridge fault. These are consistent with the average recurrence interval calculated for the Oak Ridge fault, suggesting that a destructive earthquake may strike the lower Santa Clara Valley in the near future.
Deformations and Inferred Stress Field for Ellipsoidal Sources at Long Valley, California, 1975–1982
Authors: Wu M. , Wang H. F.
Published: February 2008

Ground deformation and seismicity observed from 1975 to the present support the existence of magma beneath the central part of Long Valley caldera, California. Constraints on the source geometry are obtained from deformation modeling of uplift and trilateration data over the period 1975–1982. The deformation data are modeled with an elastic half‐space incorporating an ellipsoidally shaped magma chamber under uniform pressure, the Hilton Creek fault, and the South Moat fault. The best fit to the deformation data indicates a nearly spherically shaped source beneath the central part of the caldera at a depth of 9.5 ± 1 km. However, significant deviations of some horizontal displacement data suggest that movements along the faults are probably more complicated than the assumed dislocations. The predicted normal stress on the vertical South Moat fault plane varies from weak tension above 4 km depth to strong compression below that depth and the shear stress is right‐lateral in the eastern half and left‐lateral in the western half of the fault. It is unlikely that such a stress field could trigger slip movement on the fault. Tangential stress at the surface of the magma chamber has a maximum tension near its intersection with the South Moat fault. It is possible that the 1983 earthquake swarms in the South Moat were triggered by shallow magma intrusions beneath the South Moat of the caldera. A narrow prolate ellipsoidal source, which produces about the same uplift as the spherical source model, leads to a horizontal tensile stress that is about 1 order of magnitude higher in the region directly above the source. However, the crack opening location for magma injection is at the top of the source in this case and would not be related to the seismicity in 1983.
LARGE EARTHQUAKES IN THE TONGA REGION ASSOCIATED WITH SUBDUCTION OF THE LOUISVILLE RIDGE
Authors: Christensen Douglas H. , Lay Thorne
Published: February 2008

Subduction of the Louisville Ridge influences both interplate and intraplate seismicity in the southern Tonga region (22°S–26°S). Five earthquakes with diverse mechanisms and seismic moments greater than 1 × 1027 dyn cm have occurred in this region since 1975. These include two outer rise, one underthrusting, and two intermediate depth events, all of which occurred in close proximity to the subducting ridge. In order to understand better the nature of these events and to determine the role played by the Louisville Ridge in the subduction process, detailed rupture models have been determined for each earthquake by body and surface wave analysis. Three events occurred at shallow depths near the oblique intersection of the Louisville Ridge and the Tonga are. These are the compressional (thrust) outer rise event of October 11, 1975 (Mw = 7.4); the tensional (normal) outer rise event of October 10, 1977 (Mw = 7.4); and the subsequent underthrusting event on December 19, 1982 (Mw = 7.5), the latter being the largest documented interplate event in Tonga. The intermediate depth events of June 22, 1977 (d = 70 km, Mw = 8.1) and April 13, 1980 (d = 160 km, Mw = 7.6) both have normal fault mechanisms; however; the 1977 event has a downdip tension axis, whereas the 1980 event has a downdip compression axis. The 1977 event is unusual due to its large size and the fact that most intermediate depth activity in Tonga involves downdip compression. The body wave radiation for the 1977 event indicates concentrated rupture in the depth range 70–90 km, near the downdip projection of the Louisville Ridge. It appears that the Louisville Ridge perturbs the regional stress regime at both intermediate and shallow depths in the slab. The ridge appears to be a buoyant feature that produces locally strong interplate coupling to the north of its junction with the Tonga trench, which accounts for the 1982 thrust event and the preceding 1975 compressional outer rise event located seaward of the thrust rupture zone. South of the ridge intersection the plate interface appears to be less strongly coupled: tensional stresses due to slab pull are communicated to shallow depths, resulting in the October 10, 1977 (Mw = 7.4), tensional outer rise event. The locally enhanced interplate coupling, along with a downdip stress barrier to the intraplate compressional stresses that arise from deep resistance to the slab subduction, are two effects of the ridge that appear to explain the accumulation of in‐plate tension in the region of the June 22, 1977, event, the April 13, 1980, downdip compressional event reflects a return to in‐plate compression following the great 1977 rupture which relaxed the tensional stress. The unusual stress regime of the subducting ridge and its oblique trajectory down the plate are thus responsible for the diversity of large earthquake occurrence in the southern Tonga arc.
Excitation of the Earth's Polar Motion due to Mass Variations in Major Hydrological Reservoirs
Authors: Chao B. Fong
Published: February 2008

Previous studies on the hydrological excitation of the Earth's polar motion have neglected the effect of water mass variations of hydrological reservoirs that have time scales much longer than a month. To remedy this, the present paper calculates the polar motion excitation caused by variations in major natural lakes, the impoundment in artificial reservoirs, and the depletion of a major groundwater aquifer. It is found that (1) the annual water mass variation in natural lakes contributes a significant fraction in the total hydrological excitation of the annual wobble, (2) the hydrological reservoirs have been negligible in the Chandler wobble excitation, and (3) the contribution of hydrological reservoirs to the total secular polar drift has been negligible in this century due to fortuitous cancellation among reservoirs.
Theoretical investigation of the oceanic inverted barometer response
Authors: Dickman S. R.
Published: February 2008

Dynamic tidal theory is generalized in order to predict the oceanic response to atmospheric pressure variations. The oceans are found to exhibit distinct dynamic behavior when forced at periods less than 1 week; depending on the harmonic type of forcing, the amplitude of the response can differ by ∼ 20% or more from the static response. Even at roughly 2 months the traditional “inverted barometer” may not be sufficiently accurate for applications in which detection of long‐term sea level trends or vertical crustal motion is the goal. For forcing at periods of order 1 year the oceanic response differs by at most a few percent from the static (not the inverted barometer) response; thus calculations of meteorological excitation of the annual and Chandler wobbles can employ the static approximation without reservation.
Source parameters of earthquakes in the Nevada Seismic Zone, 1915–1943
Authors: Doser Diane I.
Published: February 2008

The Nevada Seismic Zone has historically been one of the most active seismic regions of the intermountain west. Although all large (M > 7.0) earthquakes located within this zone occurred prior to the establishment of a worldwide seismograph network, most have been well recorded instrumentally. This study uses available instrumental data to determine the source parameters for some of the earliest earthquakes occurring within the region, namely, the 1915 (M = 7.6) Pleasant Valley and 1932 (M = 7.2) Cedar Mountain earthquakes, and several smaller events of 5.5 ≤ M ≤ 6.5 occurring within the Cedar Mountain region between 1934 and 1943. Results of body waveform inversions suggest that the Pleasant Valley earthquake, at the northern end of the Nevada Seismic Zone, is a complex event best fit by a multiple source‐time function. Waveform and first‐motion information for the Cedar Mountain earthquake, located within the Walker Lane, suggest the earthquake was composed of two events, each having significant components of right‐lateral strike‐slip motion (rakes of −176° to −179°). Waveforms of the M = 6.5 Excelsior Mountain earthquake, located only 40 km southwest of Cedar Mountain, suggest the earthquake was a simple rupture along a normal fault (rake of −90° ± 20°), although surface faulting indicates that the event had a component of left‐lateral strike‐slip motion.
Active high‐resolution seismic tomography of compressional wave velocity and attenuation structure at Medicine Lake Volcano, Northern California Cascade Range
Authors: Evans John R. , Zucca John J.
Published: February 2008

We determine compressional wave velocity and attenuation structures for the upper crust beneath Medicine Lake volcano in northeast California using a high‐resolution active source seismic tomography method. Medicine Lake volcano is a basalt through rhyolite shield volcano of the Cascade Range, lying east of the range axis. The Pg wave from eight explosive sources which has traveled upward through the target volume to a dense array of 140 seismographs provides 1‐ to 2‐km resolution in the upper 5 to 7 km of the crust beneath the volcano. The experiment tests the hypothesis that Cascade Range volcanoes of this type are underlain only by small silicic magma chambers. We image a low‐velocity low‐Q region not larger than a few tens of cubic kilometers in volume beneath the summit caldera, supporting the hypothesis. A shallower high‐velocity high‐density feature, previously known to be present, is imaged for the first time in full plan view; it is east‐west elongate, paralleling a topographic lineament between Medicine Lake volcano and Mount Shasta. This lineament is interpreted to be the result of an old crustal weakness now affecting the emplacement of magma, both on direct ascent from the lower crust and mantle and in migration from the shallow silicic chamber to summit vents. Differences between this high‐velocity feature and the equivalent feature at Newbeny volcano, a volcano in central Oregon resembling Medicine Lake volcano, may partly explain the scarcity of surface hydrothermal features at Medicine Lake volcano. A major low‐velocity low‐Q feature beneath the southeast flank of the volcano, in an area with no Holocene vents, is interpreted as tephra, flows, and sediments from the volcano deeply ponded on the downthrown side of the Gillem fault, a normal fault mapped at the surface north of the volcano. A high‐Q normal‐velocity feature beneath the north rim of the summit caldera may be a small, possibly hot, subsolidus intrusion. A high‐velocity low‐Q region beneath the eastern caldera may be an area of boiling water between the magma chamber and the ponded east flank material. These structural data are useful both for understanding Cascade Range volcanism and for geothermal development in progress on the volcano.
Mechanical behavior of the oceanic lithosphere at a corner area defined by an axial valley and a large offset transform fault: Case of the oceanographer transform fault
Authors: Lago B.
Published: February 2008

Elastic behavior of lithospheric plates is able to account for the return to the isostatic equilibrium level, observed at the corner where a mid‐ocean ridge axial valley and a large offset transform fault intersect. The formalism is a generalization in two dimensions of the one‐dimensional one, due to Kuo et al. [1986]. Good agreement of depth profiles of the oceanic platform can be obtained between models and data from the two corner areas, associated with the Oceanographer Transform Fault (OTF); a band 10 to 20 km wide, around the transform fault affected by shear faulting, has to be excluded from the elastic behavior and possibly also a band 20 to 30 km wide around the ridge axis, affected by normal faulting. A major result concerns the evaluation of the amount of the static contribution, due to anomalous crust, in the deepening of the crustal valley in the vicinity of the OTF, to a value in the range 50 to 70%; the residual 30 to 50% is attributed to an increase of the dynamical effect of the asthenospheric upwelling, feeding the lithospheric plates.
Continuation of a deep borehole stress measurement profile near the San Andreas Fault: 1. Hydraulic fracturing stress measurements at Hi Vista, Mojave Desert, California
Authors: Hickman Stephen H. , Zoback Mark D. , Healy John H.
Published: February 2008

Hydraulic fracturing stress measurements were made in a 592‐m‐deep well at Hi Vista, California, 32 km from the San Andreas fault in the western Mojave Desert. The relative magnitudes of the horizontal principal stresses and the calculated overburden stress indicate that the stress regime at this site is transitional between thrust faulting and strike‐slip faulting. The azimuths of the induced hydraulic fractures at Hi Vista exhibit considerable scatter, and the indicated direction of the maximum horizontal principal stress ranges from north‐northeast to northwest. The measured magnitudes of the horizontal principal stresses and the horizontal deviatoric stress in this well are less than or equal to those measured in a nearby well of comparable depth 4 km from the San Andreas fault. This result contrasts with the increase in these stress components with distance from the San Andreas fault that was observed in a shallower borehole profile in the same area. Marked fluctuations in both stress magnitudes and orientations with depth in the Hi Vista well, however, may result from a localized perturbation to the regional stress regime. No correlation was found to exist in this well between stress magnitudes and either P wave velocities or natural fracture densities, although the low stresses measured at a depth of about 540 m may reflect proximity to an intensely fractured and permeable zone at the bottom of the well.
Thermal stresses due to cooling of a viscoelastic oceanic lithosphere
Authors: Denlinger Roger P. , Savage William Z.
Published: February 2008

Theories based upon thermal contraction of cooling oceanic lithosphere provide a successful basis for correlating seafloor bathymetry and heat flow. The horizontal components of the contraction of the lithosphere as it cools potentially give rise to large thermal stresses. Current methods to calculate these stresses assume that on the time scales of cooling, the lithosphere initially behaves as an inviscid fluid and instantly freezes into an elastic solid at some critical temperature. These instant‐freezing methods inaccurately predict transient thermal stresses in rapidly cooling silicate glass plates because of the temperature dependent rheology of the material. The temperature dependent rheology of the lithosphere may affect the transient thermal stress distribution in a similar way, and for this reason we use a thermoviscoelastic model to estimate thermal stresses in young oceanic lithosphere. This theory is formulated here for linear creep processes that have an Arrhenius rate dependence on temperature. Our results show that the stress differences between instant freezing and linear thermoviscoelastic theory are most pronounced at early times (0–20 m.y.) when the instant freezing stresses may be twice as large. The solutions for the two methods asymptotically approach the same solution with time. A comparison with intraplate seismicity shows that both methods underestimate the depth of compressional stresses inferred from the seismicity in a systematic way.
Determination of earthquake moment tensors from teleseismic relative amplitude observations
Authors: Pearce R. G. , Rogers R. M.
Published: February 2008

A method is described which uses the relative amplitudes of related seismic phases to impose constraints on the form of the P and S wave radiation patterns, from which a moment tensor determination is obtained complete with a well‐founded measure of its precision. Two types of teleseismic relative amplitude measurements may be employed: those relating P, pP, and sP and those relating the three components of the direct S wave. Examples are given which apply the method to (1) smaller shallow earthquakes (mb ≲ 5.5) recorded at short period, (2) larger intermediate‐depth and deep earthquakes (mb ≳ 5.8) using long‐period seismograms, and (3) large mb shallow earthquakes, which require both P and S wave data at long period. The relative amplitude moment tensor program (RAMP) is an advance on the relative amplitude method of Pearce [1977, 1980] in that the a priori double couple assumption is now relaxed. Results are compared with those of other moment tensor methods where available. RAMP is found to be a sensitive indicator of volumetric source component even for small data sets, for which it is less good at discriminating between different constant‐volume source types, notably the double couple and the compensated linear vector dipole. Since current inverse methods presume a constant‐volume source, our method alone is a reliable indicator of volumetric component, which is important for the analysis of source processes and in earthquake‐explosion discrimination.