bluuebunny/crossref_metadata_2025 huggingface dataset:
Evidence from geoid data of a hotspot origin for the southern Mascarene Plateau and Mascarene Islands (Indian Ocean)
Authors: Bonneville Alain , Barriot Jean Pierre , Bayer Roger
Published: February 2008

A high‐resolution geoid map of the Madagascar and Mascarene basins is constructed from GEOS 3 and Seasat satellite altimetric data with an accuracy of 0.7 m. Large positive anomalies are due to topographic highs and their regional compensation. The deflection of the lithosphere under volcanic loads and its effect on the geoid are modeled for the Southern Mascarene Plateau, Mauritius, Reunion, and Rodriguez islands. Assuming a purely elastic lithospheric model, the effective flexural rigidity is estimated for the four zones. Rigidity varies from 1×10+21 to 2×10+23 N m, increasing from north to south. The highest rigidity occurs under Reunion Island and suggests a recent origin for the volcanism (3–5 Ma). The Southern Mascarene Plateau shows the lowest rigidity, suggesting a near‐ridge origin; Rodriguez Island shows the same trend but in another tectonic framework. The topographic and geoid swell is studied as an effect of a thermal mantle plume. Using a conductive model of lithospheric heating, we confirm the assumption of “hotspot migration” for the origin of the Southern Mascarene Plateau and the Mascarene Islands.
Deformation in the Yakataga Seismic Gap, southern Alaska, 1980–1986
Authors: Savage J. C. , Lisowski M.
Published: February 2008

A 60‐by‐40‐km trilateration network in the Yakataga seismic gap was surveyed in 1980, 1982, 1984, and 1986 with precise electro‐optical distance‐measuring equipment to measure strain accumulation. The overall deformation is roughly approximated by a 0.24±0.03 µstrain/yr N32°W±2.4° uniaxial contraction that is uniform in time. However, the spatial distribution of deformation shows some concentration of convergence in the neighborhood of the Chugach‐St. Elias fault and of right‐lateral shear across the Contact fault. A simple dislocation model of the plate interaction in the Yakataga gap fits the observed deformation reasonably well but seems to require that the motion of the Pacific plate relative to the North American plate be directed more nearly N36°W than N15°W, the generally accepted direction of relative motion for this location. However, the direction of plate motion inferred from the dislocation model depends upon details of the interaction at the plate boundary that may not have been modeled accurately. A nearby but smaller trilateration network at Icy Bay was surveyed in 1982, 1984, and 1986. This network spans the southwest corner of the rupture zone of the 1979 St. Elias earthquake. The deformation at Icy Bay consists of left‐lateral shear across a northeast trending zone. The relation of this deformation to strain accumulation in the Yakataga gap, postseismic relaxation associated with the 1979 earthquake, or rebound from the unloading associated with the rapid recession of the Guyot glacier is not understood.
Depths of large earthquakes determined from long‐period Rayleigh waves
Authors: Zhang Jiajun , Kanamori Hiroo
Published: February 2008

The depths and source mechanisms of nine large shallow earthquakes were determined from long‐period (150 to 300 s) Rayleigh waves recorded by the Global Digital Seismograph Network (GDSN) and International Deployment of Accelerometers (IDA) networks. We inverted the data set of complex source spectra for a moment tensor (linear) or a double couple (nonlinear). By solving a least squares problem, we obtained the centroid depth or the extent of the distributed source for each earthquake. The depths and source mechanisms of large shallow earthquakes determined from long‐period Rayleigh waves depend on the models of source finiteness, wave propagation, and excitation. We tested various models of source finiteness, Q, group velocity, and excitation in the determination of earthquake depths. In order to determine the depth of large earthquakes from long‐period surface waves, source‐finiteness effects must be corrected using adequate models. The depth estimates obtained using the Q model of Dziewonski and Steim (1982) and the excitation functions computed for the average ocean model of Regan and Anderson (1984) are considered most reasonable. Dziewonski and Steim's Q model represents a good global average of Q determined over a period range of the Rayleigh waves used in this study. Since most of the earthquakes studied here occurred in subduction zones, Regan and Anderson's average ocean model is considered most appropriate. Our depth estimates are in general consistent with the Harvard centroid‐moment tensor (CMT) solutions. The centroid depths and their 90% confidence intervals (numbers in parentheses) determined by the Student's t test are Colombia‐Ecuador earthquake (December 12, 1979), d = 11 km (9, 24 km); Santa Cruz Island earthquake (July 17, 1980), d = 36 km (18, 46 km); Samoa earthquake (September 1, 1981), d = 15 km (9, 26 km); Playa Azul, Mexico, earthquake (October 25, 1981), d = 41 km (28, 49 km); El Salvador earthquake (June 19, 1982), d = 49 km (41, 55 km); New Ireland earthquake (March 18, 1983), d = 75 km (72, 79 km); Chagos Bank earthquake (November 30, 1983), d = 31 km (16, 41 km); Valparaiso, Chile, earthquake (March 3, 1985), d = 44 km (15, 54 km); Michoacan, Mexico, earthquake (September 19, 1985), d = 24 km (12, 34 km).
Neogene rhyolites of the northern Jemez volcanic field, New Mexico
Authors: Loeffler Bruce M. , Vaniman David T. , Baldridge W. Scott , Shafiqullah Muhammed
Published: February 2008

Volcanic centers previously mapped as the 2.0 Ma El Rechuelos Rhyolite in the northern Jemez volcanic field, New Mexico, include three distinct episodes of rhyolitic volcanism. An early (7.5 Ma) extrusive dome of flow‐banded biotite rhyolite and an intermediate (5.8 Ma) rhyolite, possibly a volcanic neck, correspond in age to rhyolites of the Keres Group in the southern Jemez volcanic field. Three other extrusive domes of aphyric, pumiceous rhyolite and obsidian comprise a late volcanic episode, dated at 2.0 Ma. We retain the name El Rechuelos Rhyolite only for these late centers. Another center, farther north than the others but previously mapped with the El Rechuelos Rhyolite, is a dacite pumice ring whose age (5.2 Ma), petrography, major‐ and trace‐element chemistry, and Sr initial ratio all suggest it should be included with rocks of the Tschicoma Formation. Nd and Sr isotopic ratios of the Neogene rhyolites of the northern Jemez volcanic field suggest that these rhyolites were not produced by partial melting of either upper or lower crust. Rather, they may have been generated from a mantle‐derived mafic magma, such as the nearby Lobato Basalt, by fractional crystallization with concomitant assimilation of small amounts (<6%) of lower crust. If the El Rechuelos is derived from a lower crustal magma chamber, as seems likely, then it is not related to the Bandelier magma system, even though it is part of a continuum of rhyolite volcanism ranging from 3.6 Ma to 130,000 years ago that includes the Bandelier and precursor rhyolitic units.
Linearized inversion for fault rupture behavior: Application to the 1984 Morgan Hill, California, earthquake
Authors: Beroza Gregory C. , Spudich Paul
Published: February 2008

We present a technique to infer the rupture history of an earthquake from near‐source records of ground motion. Unlike most previous studies, each point on the fault is assumed to slip only once, when the rupture front passes, with a spatially variable slip intensity. In this parameterization the data are linearly related to slip intensity but nonlinearly related to rupture time. We perform a linearized inversion for slip intensity and rupture time by iteratively perturbing an assumed starting model. The inverse problem for the model perturbation is solved using a tomographic back projection technique. Smoothing and inequality constraints are applied to ensure that the resulting solution is stable and feasible. Asymptotic ray theory is used to calculate theoretical seismograms and partial derivatives with respect to model parameters. In test cases with noisy synthetic data sets we found that it is possible to distinguish between rupture models having variable slip amplitude and models having variable rupture velocity, provided the station coverage is adequate. We apply the technique to recordings of the April 24, 1984, Morgan Hill earthquake. The results indicate that slip amplitude on the fault plane was extremely variable and that the rupture front did not propagate uniformly away from the hypocenter. In particular, rupture was delayed on a 12 km2section of the fault approximately 14 km to the southeast of the hypocenter. The rupture front surrounded the region, which subsequently failed with a component of rupture propagation back toward the hypocenter. Similar behavior has been observed in dynamic rupture models with stress or strength inhomogeneities. This segment of the fault ruptured with a large slip amplitude, releasing 12% of the total seismic moment from 4% of the total area of the aftershock zone. The surface trace of this section of the fault is characterized by a complex left step that could act to increase the normal stress acting across the fault. However, the distribution of aftershocks suggests that the fault at depth is simpler and that it may bend to the right. In either case, our rupture model suggests that this segment of the fault represents an asperity, which initially resisted rupture but eventually ruptured massively. We estimate the shear fracture energy for this earthquake to be 2×106J/m2.
Lunar composition: A geophysical and petrological synthesis
Authors: Mueller Steve , Taylor G. Jeffrey , Phillips Roger J.
Published: February 2008

Lunar compositional constraints are derived on the basis of geophysical data (in particular, the lunar seismic model as revised by Nakamura) and petrological arguments. Only in the case of extreme assumptions can critical aspects of bulk lunar composition be demonstrated to be equivalent to the present‐day terrestrial mantle; specifically, the Moon has an Mg number that is too low and an alumina abundance that is too high. Over a broad range of crustal densities the presence of a metallic core at least 150 km in radius is necessary to reconcile the upper mantle lunar seismic model with mass and moment of inertia constraints. A significant seismic discontinuity at 500 km depth may mark the lowest extent of early lunar differentiation, possibly representing a transition between highly fractionated upper mantle and less fractionated, perhaps even primordial, middle/lower mantle.
Small‐scale structure in the lithosphere and asthenosphere deduced from arrival time and amplitude fluctuations at NORSAR
Authors: Flatté Stanley M. , Wu Ru‐Shan
Published: February 2008

We analyze the pattern of phase and amplitude variations of seismic waves across the NORSAR array on a statistical basis in order to determine the statistical distribution of heterogeneities under NORSAR. Important observables that have been analyzed in the past are the phase (or travel time) and log amplitude variances and the transverse coherence functions (TCFs) of phase and amplitude fluctuations. We propose and develop the theory and methods of using other observables to reduce the degree of nouniqueness and increase the spatial resolution of the analysis. Most important are the angular coherence functions (ACFs), which characterize quantitatively the change in the pattern of fluctuations across the array from one incoming angle (or beam) to another and which have a different sensitivity to the depth distribution of heterogeneities than the TCFs. A combination of the ACFs and TCFs allows estimation of the power spectra of the P wave speed variations under the array as a function of depth. We use data for phase fluctuations from 104 incident beams and amplitude fluctuations from 185 beams with 2‐Hz center frequency at NORSAR to calculate the three ACFs and three TCFs (of phase, log amplitude, and their cross coherence). The measured rms travel time fluctuation is 0.135 s, and the rms log amplitude fluctuation is 0.41. The half‐coherence widths of the ACFs are 3° for log amplitude and 9° for phase. The half‐coherence widths of the TCFs are 18 km for phase and less than the minimum separation between the elements of the array for log amplitude. In order to account for these features of the data, we adopt a two‐overlapping‐layer model for lithospheric and asthenospheric heterogeneities underneath NORSAR, with spectra that are band‐limited between the wavelengths of 5.5 and 110 km. Our best model has an upper layer with a flat power spectrum extending from the surface to about 200 km, and a lower layer with a K−4 power spectrum extending from 15 to 250 km. The latter spectrum corresponds to an exponential correlation function with scale larger than the observation aperture (110 km). The rms P wave speed variations the in the range 1–4%. The small scale heterogeneities may be attributed to clustered cracks or intrusions; the larger‐scale wavespeed heterogeneities are temperature or compositional heterogeneities that may be related to chemical differentiation, or dynamical processes in the boundary layer of mantle convection.
Calving icebergs: A source of low‐frequency seismic signals from Columbia Glacier, Alaska
Authors: Qamar Anthony
Published: February 2008

Low‐frequency icequakes produced by Columbia Glacier, Alaska, are caused by large icebergs which break off the glacier terminus into Prince William Sound. Occasionally, many ice seracs fall in rapid succession, producing complex, low‐frequency icequake signals with durations sometimes exceeding 10 min. An empirical relationshipV= 4000Tbetween the volumeVof an iceberg in cubic meters and the durationTof the seismic signal in seconds, can be used to estimate the calving rate of Columbia Glacier from seismological data alone. During 1984 and 1985, a period of rapid retreat of the glacier, the calculated number of icequakes per month showed seasonal fluctuations that corresponded to variations in calving rate determined by photogrammetry. It is likely that most previously reported low‐frequency icequakes in southern Alaska are produced by calving, tidewater glaciers.
Metamorphic terranes, isotopic provinces, and implications for crustal growth of the western United States
Authors: Ernst W. G.
Published: February 2008

Simplified maps of the western Cordillera, indicating distribution of metamorphic facies and times of principal recrystallization have been compiled. Despite rearrangement by transcurrent shuffling, general trends are recognizable. Narrow belts of Phanerozoic high‐P blue schist, and serpentinized peridotite lie outboard from broad, penecontemporaneous high‐T metamorphosed calcalkaline magma‐intruded belts bordering the Pacific Ocean, and both are disposed approximately parallel to the present‐day North American margin; they represent recrystallized products of mid‐Paleozoic and younger subduction zones, Andean‐type margins, and exotic island arcs. In contrast, dominant metamorphic mineral parageneses within the continental interior are spatially associated with chiefly mesozonal granitic plutons. Glaucophane schists, eclogites, and large tracts of ophiolitic ultramafics are absent, in part due to selective overprinting of old accretionary margin lithotectonic assemblages by subsequent orogenies, high‐T recrystallization, and anatexis. For example, at depth within the California Coast Ranges, high‐P phases are currently being destroyed; farther inland, a general paucity of oceanic petrotectonic assemblages is evident, possibly reflecting the differential sinking of dense basaltic/peridotitic units during thermal softening. Preserved metamorphic field gradients reflect the magnitude of subsequent uplift and erosion as well as original PT conditions. Late Mesozoic to mid‐Cenozoic recrystallization accompanied earlier crustal thickening + ductile deformation, succeeded by later extension + brittle faulting + mylonitization; Mesozoic‐Cenozoic metamorphism involved cratonal rocks in the Mojave‐Sonoran Desert, but in Nevada and to the north, eugeoclinal sections were recrystallized. Metamorphic intensity decreases eastward toward the Phanerozoic hinge line. Basement rocks in the Rockies display the effects of shallow level crustal deformation but much less widespread evidence of post‐Proterozoic plutonism and regional metamorphism. Initial Pb, Nd, and Sr isotopic compositions and crystallization ages of igneous rocks, in comparison with times of intrusion, recrystallization, and metamorphic facies distributions are compatible with a geologic scenario involving gradual, proximal growth of the ancient continent chiefly southward from the Archean Wyoming craton during early and mid‐Proterozoic time, followed by late Proterozoic‐Phanerozoic westward development. Accretion involved the incorporation of some exotic, mostly oceanic, outboard terranes. However, continental growth in the western U.S. Cordillera during both Proterozoic and Phanerozoic time periods appears to have been due chiefly to partial fusion and magma ascent above subducting paleo‐Pacific lithospheric plates, combined with metamorphic (+ sedimentary) reworking in the forearc + trench + back arc setting, rather than resulting from the amalgamation of preexisting ancient continental fragments.
Focal mechanisms and the state of stress on the San Andreas Fault in southern California
Authors: Jones Lucile M.
Published: February 2008

Focal mechanisms have been determined from P wave first motion polarities for 138 small to moderate (2.6 ≤ M ≤ 4.3) earthquakes that occurred within 10 km of the surface trace of the San Andreas fault in southern California between 1978 and 1985. On the basis of these mechanisms the southern San Andreas fault has been divided into five segments with different stress regimes. Earthquakes in the Fort Tejon segment show oblique reverse sup on east‐west and northwest striking faults. The Mojave segment has earthquakes with oblique reverse and right‐lateral strikesup motion on northwest strikes. The San Bernardino segment has normal faulting earthquakes on north‐south striking planes, while the Banning segment has reverse, strike‐sup, and normal faulting events all occurring in the same area. The earthquakes in the Indio segment show strike‐slip and oblique normal faulting on northwest to north‐south striking planes. These focal mechanism data have been inverted to determine how the stresses acting on the San Andreas fault in southern California vary with position along strike of the fault. One of the principal stresses is vertical in all of the regions. The vertical stress is the minimum principal stress in Fort Tejon and Mojave, the intermediate principal stress in Banning and Indio, and the maximum principal stress in San Bernardino. The orientations of the horizontal principal stresses also vary between the regions. The trend of the maximum horizontal stress rotates over 35°, from N15°W at Fort Tejon to N20° at Indio. Except for the San Bernardino segment, the trend of the maximum horizontal stress is at a constant angle of about 65° to the local strike of the San Andreas fault, implying a weak fault. The largest change in the present stress state occurs at the end of the rupture zone of the 1857 Fort Tejon earthquake. It appears that the 1857 rupture ended when it propagated into an area of low stress amplitude, possibly caused by the 15° angle between the strikes of the San Jacinto and San Andreas faults. The strong correlation between present state of stress and segmentation in previous earthquakes suggests that the stress state is important in controlling rupture propagation.
Origin of cryptoperthites in the Bishop Tuff and their bearing in its thermal history
Authors: Snow Eleanour , Yund Richard A.
Published: February 2008

The Bishop Tuff is a large rhyolitic ash flow which erupted from Long Valley caldera in eastern California and contains abundant sanidine cryptoperthite phenocrysts, which can be used to constrain the time between the two major cooling units. The cryptoperthites (bulk composition Or66Ab33An1) have exsolved in two principal stages, followed by a third, minor stage. The earliest exsolution, which produced patches of relatively wide lamellae (100–800 nm) without wedge‐shaped ends (WSE), occurred by coherent nucleation and was probably related to a high flux of water associated with fumarolic activity in this area. The spacing of these lamellae shows no correlation with depth or thermal history. The second major stage of exsolution occurred by spinodal decomposition, which produced lamellae with WSE having spa ein g s of from 80 to 170 nm, and the spacing of these lamellae varies with depth in a pattern consistent with the two known cooling units. Heat flow calculations indicate that this pattern of lamellar spacings is consistent with an interval of about 1.0–2.0 years between the eruption of these two cooling units.
Overview of the structure and geothermal potential of Newberry Volcano, Oregon
Authors: Fitterman David V.
Published: February 2008

Evidence from a broad range of geological and geophysical investigations of Newberry Volcano in central Oregon suggests the presence of a small magma body beneath the summit caldera. Drilling has encountered temperatures indicative of a hydrothermal system above the hypothesized magma body. On the basis of hydrothermal mineralogy the temperature of the hydrothermal system is at or near its maximum value, indicating that the hydrothermal system is young. Basaltic underplating is postulated to replenish the magma body. The results of interdisciplinary studies at Newberry Volcano document that young, silicic volcanism in the Cascade Range can have geothermal potential. These results should be applied with caution to the rest of the Cascade Range because of the strong influence of Basin and Range extensional tectonics on Newberry Volcano's style of volcanism.
Electrical structure of Newberry Volcano, Oregon
Authors: Fitterman D. V. , Stanley W. D. , Bisdorf R. J.
Published: February 2008

From the interpretation of magnetotelluric, transient electromagnetic, and Schlumberger resistivity soundings, the electrical structure of Newberry Volcano in central Oregon is found to consist of four units. From the surface downward, the geoelectrical units are (1) very resistive, young, unaltered volcanic rock, (2) a conductive layer of older volcanic material composed of altered tuffs, (3) a thick resistive layer thought to be in part intrusive rocks, and (4) a lower‐crustal conductor. This model is similar to the regional geoelectrical structure found throughout the Cascade Range. Inside the caldera, the conductive second layer corresponds to the steep temperature gradient and alteration minerals observed in the USGS Newberry 2 test hole. Drill hole information on the south and north flanks of the volcano (test holes GEO N‐1 and GEO N‐3, respectively) indicates that outside the caldera the conductor is due to alteration minerals (primarily smectite) and not high‐temperature pore fluids. On the flanks of Newberry the conductor is generally deeper than inside the caldera, and it deepens with distance from the summit. A notable exception to this pattern is seen just west of the caldera rim, where the conductive zone is shallower than at other flank locations. The volcano sits atop a rise in the resistive layer, interpreted to be due to intrusive rocks. The intrusive material has served as a heat source to produce enhanced hydrothermal alteration and, perhaps in the case of the west‐flank anomaly, elevated fluid temperatures. While no public drill hole information is available to confirm this hypothesis, the west‐flank anomaly appears to be a good geothermal target. In addition to the possibility that a region on the west side of the volcano could be favorable for prospecting, part of the resistive structure under the center of the volcano could be due to a vapor‐dominated environment with temperatures above 300°C. In other parts of the Cascades, pervasive alteration has produced mixed layer clays and zeolites, resulting in low‐resistivity anomalies. Low resistivities cannot be assumed to indicate high‐temperature pore fluids. The use of electrical methods that measure resistivity as a function of excitation frequency, such as spectral induced polarization, may provide a way of obtaining information about the type and extent of alteration.
Evidence for a sharp 670‐km discontinuity as inferred from P‐to‐S converted waves
Authors: Paulssen Hanneke
Published: February 2008

The fine structure of the upper mantle discontinuities is investigated using observations of converted waves on short‐period and broadband seismograms. Using a stacking technique to analyze the P wave coda of teleseismic records, evidence is found for coherent near‐receiver P‐to‐S converted phases generated by the 400‐ and 670‐km discontinuity beneath a number of the stations used in this study. Variations in travel time, slowness, and amplitude of these phases as observed among the stacks for stations of the Regional Seismic Test Network in the United States and of the Network of Autonomously Recording Seismographs in western Europe are very likely the expression of upper mantle heterogeneity. Observations of coherent converted phases from the 400‐km discontinuity are fewer in number than of phases converted at the 670‐km discontinuity, suggesting that the latter is more pronounced. Some of the seismograms, especially of station RSCP, show extremely high‐amplitude P‐to‐S converted phases from the 670‐km discontinuity. These seismograms allow a detailed waveform comparison of the converted phase with the direct P phase and present evidence for a sharp 670‐km discontinuity.
Inclination anomalies from Indian Ocean sediments and the possibility of a standing nondipole field
Authors: Schneider David A. , Kent Dennis V.
Published: February 2008

We report magnetic inclinations measured in deep‐sea sediments of the equatorial Indian Ocean which record the behavior of a nondipole component of the timeaveraged geomagnetic field during the Plio‐Pleistocene (0–5 Ma). The magnitude of the nondipole effect recorded in these sediments appears to depend on polarity state, with inclinations showing departures from geocentric axial dipole directions which are small (2°) during normal polarity and larger (5°) during reverse polarity times. The overall nondipole effect observed here is consistent with prior spherical harmonic estimates of the paleomagnetic field; the polarity bias found agrees, in both sense and magnitude, with earlier reports of polarity asymmetry in the low‐degree zonal harmonic fields. The presence of this asymmetry supports previous suggestions of the existence of a standing component of the nondipole field which does not invert during reversals of the main field. We explore whether the standing field so indicated may have influenced paleomagnetic directions recorded during polarity transitions at other equatorial sites.
An interpretation of wide‐angle compressional and shear wave data in southwest Germany: Poisson's ratio and petrological implications
Authors: Holbrook W. Steven , Gajewski Dirk , Krammer Anton , Prodehl Claus
Published: February 2008

Shear wave velocity, especially when used with Compressional wave velocity, is an important constraint on estimates of the composition and physical state of the continental lithosphere. We present shear velocity (νs) models derived from high‐quality S wave data along three profiles in Southwest Germany, which combine with existing P wave velocity (νp) models to give Poisson's ratio (σ) in the crust and uppermost mantle. We then construct a laterally varying petrological model of the crust by comparing our νp and νs models with published laboratory‐measured rock velocities. In general, Poisson's ratio is 0.25 in the uppermost crust, 0.22–0.23 in the mid‐crust (but reaching a low of 0.15 in the Black Forest low‐νp zone), and varies laterally from 0.24 to 0.30 in the lower crust; the average Poisson's ratio of the whole crust is everywhere about 0.25. The laminated signature of the lower crust prominent on reflection and P wave refraction data is absent on the S wave refraction data, implying a strong high/low alternation of Poisson's ratio in the lower crust. This suggests that the lower‐crustal laminae represent mafic/silicic compositional layering (for example, basic sills in an originally more quartz‐rich lower crust) rather than alternating zones of high and low pore pressure. There is no evidence for a shear wave low‐velocity zone (LVZ) in the Black Forest mid‐crust, despite the presence of a strong P wave LVZ; this suggests high quartz content and the presence of pore fluids at low (not high) pore pressure. A clear P‐to‐S converted Moho reflection (Pm S) is visible on several shots, indicating that the Moho is, at least locally, a first‐order discontinuity. No shear wave refractions from the upper mantle are present, even where strong P wave refractions are observed, possibly indicating an increase of Poisson's ratio with depth in the upper mantle. The petrological modeling indicates that the upper and middle crust are probably of granitic composition, that the mid‐crustal low‐νp zones have high quartz content, and that the lower crust probably consists of granulites of laterally varying bulk composition, from granodioritic to gabbroic. The lower crust is more mafic in areas of high reflection density on seismic reflection profiles, suggesting that the laminations are caused by mafic intrusions into the lower crust, either prior to the Variscan orogeny or during (Tertiary?) volcanic activity.
Electronic distance measuring network monitoring during the Rabaul seismicity/deformational crisis of 1983–1985
Authors: Archbold Malcolm J. , Mckee Chris O. , Talai Benjamin , Mori Jim , De Saint Ours Patrice
Published: February 2008

Following a dramatic increase in the seismicity and rate of ground deformation at Rabaul Caldera in September 1983, an electronic distance measuring (EDM) network was established in November–December 1983 to provide additional surveillance of the developing volcanic hazard. The network and operational procedure were designed to monitor frequently a large area and to obtain results rapidly. Initially, data reduction was carried out using the conventional refractive index technique to correct for observed atmospheric variables, but the principle of line ratios using modeled atmospheric variables showed promise for accelerating and simplifying data reduction by obviating meteorological considerations. A standard error of ±1.31 ppm using the line ratio reduction technique favorably compares with results obtained by other researchers. Horizontal strain in the east central part of the caldera increased at monthly rates of 10–80 ppm, reaching a peak in April 1984. This peak coincided with the highest monthly rate of earthquake occurrence (13,800) and the largest monthly rate of ground uplift (approximately 80 mm). The greatest amount of horizontal ground deformation recorded during the period 1983–1985 was about 400 mm. After April 1984 the strain rates declined gradually and, in relation to other components of deformation, probably resumed a precrisis level by mid‐1985. Horizontal deformation in the southern part of the caldera was relatively large in comparison to the amount of ground tilt and uplift recorded there. Because of the relatively small number of reflector stations, modeling of the EDM data is somewhat inconclusive. However, a Mogi point source model provides a reasonable fit to the observed deformation in the northern part of the caldera. The point source model and simple axisymmetric models do not provide a good match to the deformation recorded in the southern part of the caldera. The dominance of horizontal deformation there could be explained by a dyke intrusion.
Fluid Inclusions in Salton Sea Scientific Drilling Project Core: Preliminary Results
Authors: Roedder Edwin , Howard Kevin W.
Published: February 2008

Fluid inclusions (191) in calcite, quartz, K‐feldspar, and epidote from ≈l‐mm veinlets in cores and well cuttings from 604–2560 m homogenize from 217° to >500°C and vary widely in salinity, suggesting a complex history of fluids surrounding these samples. No daughter minerals were seen, and no clathrates were recognized on freezing. Vapor‐rich inclusions under pressure, presumably containing CO2 and/or CH4, were found from a wide range of depths, suggesting that effervescence has occurred. Low‐salinity fluids (1.2 –4.0 wt % NaCl eq) were present as deep as 1939 m. The data can be explained by a combination of processes such as thermal metamorphism of evaporites and other sediments and mixing of water from metamorphic dehydration reactions with partly evaporated Colorado River water.
Source Process of the Great 1977 Sumba Earthquake
Authors: Lynnes Christopher S. , Lay Thorne
Published: February 2008

In order to investigate the applicability of the asperity model to intraplate earthquakes we have studied the rupture process of the great 1977 Sumba normal‐faulting earthquake (Mw=8.2–8.3), one of the largest earthquakes since 1963. This event has been variously interpreted as a plate detachment event (i.e., rupture through the entire lithosphere) or a shallow plate‐bending event. We have analyzed long‐period body waves in order to determine the spatiotemporal rupture characteristics, including the depth extent of the rupture area. Visible depth phases in the initial stage of weak radiation beginning about 15 s before the main pulse suggest that the earthquake nucleated near 29 km depth, which corresponds to the maximum depth of the aftershock sequence. The excitation of a strong tsunami implies that rupture extends to the surface, while deconvolutions of long‐period P waves suggest a relatively shallow (≤ 50 km) maximum depth extent. However, the existence of slip below this depth cannot be ruled out, particularly if it preferentially radiates frequencies below the passband of the body waves used (< 0.02 Hz). For undiffracted phases the source time function of the principal rupture, which initiated 30 km west of the epicenter of the precursor, comprises a dominant, initial short‐duration (24 s) triangular pulse followed by several smaller pulses. However, slightly diffracted phases indicate a simple basic (underlying) character for the rupture, consisting of the initial 24‐s triangle superimposed on a longer‐duration (42–44 s) trapezoid. The absence of observable directivity in the deconvolved source time functions suggests a bilateral rupture mode. Although the body waves do not require a fault length greater than 80 km, the aftershock area and surface wave results of Zhang and Kanamori [1988] imply a total length of at least 200 km. The body wave source functions are well modeled by a 200‐km‐long fault, extending to 30–50 km depth, on which slip nucleates at the center of the fault and has variable displacement along strike. The peak moment release of the deconvolved source functions indicates higher displacement in the hypocentral area than in other areas of the fault. The spatial relationship of the precursor and the main pulse is suggestive of rupture initiating at and propagating along the edge of an asperity prior to the rupture of the asperity itself. The large size of the event may be due to an unusual stress environment arising from strong lateral gradients in seismic interplate coupling along the Java‐Timor arc.
Relaxation of impact basins on icy satellites
Authors: Thomas Paul J. , Squyres Steven W.
Published: February 2008

The extent and style of viscous relaxation of the largest crater basins on the icy satellites of Saturn can be used as a probe of the viscosity of the deep interiors of these bodies. In particular, some constraints on the size of a silicate core and the magnitude of the internal viscosity gradient can be determined if the relaxation process is sufficiently well understood. We discuss the application of a Unite element model incorporating spherical symmetry to the general problem of relaxation of large craters on spherical bodies. We examine the influence of core size, geothermal gradient, and rheologic law on relaxation flow and consider the implications for the internal structures of Mimas and Tethys. Our model results show that relaxation of crater topography on a spherical satellite is qualitatively and quantitatively different from relaxation on a planar surface once the crater diameter becomes a significant fraction of the satellite radius. For small‐to moderate‐sized basins, relaxation flow is bidirectional, with flow both from rim to bowl and rim to surrounds. This is the same style of flow observed for craters on planar surfaces. For large enough basins, however, the spherical geometry of the problem leads to flow that is unidirectional, toward the crater bowl throughout the satellite. The transition from bidirectional to unidirectional flow appears to be very sensitive to the basin diameter. The critical basin diameter for transition increases with core size, thermal gradient, rim height, and rheologie law stress exponent. Unidirectional and bidirectional flow produce fundamentally different stress fields in the regions surrounding the relaxing crater. Relaxation velocities for large craters on spherical bodies can be appreciably more rapid than those for equivalent craters on planar surfaces, again because of the geometric effects of relaxation on a sphere. The difference in degree of relaxation between the large basins Odysseus on Tethys (which has undergone significant modification) and Herschel on Mimas (which appears essentially unmodified) can be explained by different thermal histories for Tethys and Mimas, by the effects of stress‐dependent rheology, or by a combination of the two. The influence of a core (at sizes consistent with the observed densities of the satellites) on relaxation time scales is unimportant. If the Ithaca Chasma graben system on Tethys is the result of stresses generated by relaxation of the Odysseus basin, then Tethys's core must be small enough that it allows unidirectional relaxation flow for a basin of Odysseus's size. If this assumption it correct, we estimate an upper limit of roughly 20% of Tethys's radius for the radius of the satellite's core.
Subduction of the Juan de Fuca Plate: Thermal consequences
Authors: Lewis T. J. , Bentkowski W. H. , Davis E. E. , Hyndman R. D. , Souther J. G. , Wright J. A.
Published: February 2008

Terrestrial heat flux was measured in fjords, in boreholes, and in offshore wells at sites across the convergent margin of southwestern British Columbia from the continental shelf landward to the Garibaldi Volcanic Belt. Temperatures in the offshore wells were corrected for drilling disturbances, and formation thermal conductivities were modeled using measurements on cuttings and downhole geophysical logs. Marine measurements in the fjords were corrected for the large effects of refraction as well as aperiodic temperature variations in the bottom waters. There was excellent agreement between marine measurements and those from nearby onshore boreholes. The heat flux above the subducting Juan de Fuca plate steadily decreases landward from over 50 mW m−2 on the shelf to 25 mW m−2 seaward of the Garibaldi Volcanic Belt. An abrupt increase to 80 mW m−2 over a distance of 20 km is centered 30 km seaward of the volcanic zone. Very large variations in heat flux occur locally within the Pleistocene volcanic area, the result of advective cooling of intrusive magmas. The measured heat generation of crustal samples along the entire profile is low, 0.6–0.8 μW m−3. A landward dipping, seismically reflective zone above the subducting oceanic plate beneath Vancouver Island appears to be nearly profile is low, 0.6–0.8 μW m−3. A landward dipping, seismically reflective zone above the subducting oceanic plate beneath Vancouver Island appears to be nearly isothermal. It is postulated that dehydration of the subducting oceanic crust at and above approximately 450°C absorbs heat and produces water which flows updip along this zone in the overlying subduction complex, effectively redistributing the heat seaward to where the water is reabsorbed in hydration processes. A relatively cool crustal wedge lies above the deeper subducting oceanic crust, and at its thick, landward side the abrupt increase in surficial heat flux must be caused by a shallow (10 km depth) heat source produced by ascending magma.
Heat flow and thermotectonic problems of the central Ventura Basin, southern California
Authors: De Rito Robert F. , Lachenbruch A. H. , Moses T. H. , Munroe R. J.
Published: February 2008

The Ventura Basin, southern California, is located near the Big Bend area of the San Andreas fault system, within the Transverse Ranges physiographic province. Continuous equilibrium temperature logs were measured in 12 idle oil wells located within the onshore Ventura Avenue, San Miguelito, Filmore, Oxnard, and West Montalvo fields to an average depth of about 3100 m (10,200 feet). Thermal conductivities were measured on all available samples. Heat flows were calculated with the aid of a thermostratigraphic scheme based on correlative gradient intervals and average thermal conductivity for the appropriate units. Negative curvature of the Ventura Avenue temperature profiles may be explained by an increase in thermal conductivity associated with tectonic compaction of the underlying Pliocene clastic sequence. Temperature profiles at Fillmore are enigmatic but suggest highly unusual geotectonic conditions. Basinwide, heat flow averages about 48 mW/m2, a value which is low relative to most of southern California. As heat flow does not vary systematically to the maximum measured depth of about 4 km, this anomaly is not easily explained in terms of hydrologic effects or recent uplift and erosion. However, a diminution of heat flow is an expectable consequence of the accumulation of cold sediments (up to 12 km) since Eocene time. If 70 mW/m2 is accepted as the background heat flow, then the sedimentation effect is probably sufficient to explain the anomaly.
Full waveform inversion of reflection data
Authors: Singh S. C. , West G. F. , Bregman N. D. , Chapman C. H.
Published: February 2008

The theory of a direct inversion method to obtain the P and S wave seismic velocities and the density of a horizontally, finely layered elastic medium from vertical and horizontal component plane wave seismograms in the tau‐p (slant stacked) domain has been developed. The method is based on the downward continuation method of Bube and Burridge (1983), but plane wave seismograms with a range of ray parameters are used simultaneously to obtain the velocity and density profiles. A corrected acoustic approximation of the elastic case has also been developed with which one may obtain a model of the P wave velocity and density from vertical component seismic recordings. However, it requires the S wave velocity to be estimated a priori, for instance as a fixed fraction of the P wave velocity. Many direct inverse methods, including the Bube and Burridge procedure followed here, may lack stability when implemented numerically, specially when they are applied to real data which necessarily are band limited at both low and high frequencies. The possibility of a stable implementation of our methods has been investigated, firstly by using synthetic data for layered acoustic media containing a full range of low frequencies. In this case, the inverted velocity and density profiles are in good agreement with the test models. Secondly, the indeterminacy associated with the lack, in practice, of low frequencies has been studied, and a method has been devised to estimate the missing low‐frequency data from travel time information. Finally, the inversion algorithm has then been applied to high‐quality marine seismic reflection data obtained by the North Atlantic Transect (NAT) group in the deep ocean using an expanding spread profile (ESP). In this test, only the P wave velocity profile was estimated independently, because the move‐out data cannot provide low‐frequency information about density. Also, the “source wavelet” of the seismic system had to be estimated from the seismic recordings. A stable and plausible estimate of the velocity variation was obtained. Unfortunately, no independent measurement of this velocity structure is available with which to verify the method's accuracy.
Determination of bedrock topography beneath the Greenland ice sheet by three‐dimensional imaging of radar sounding data
Authors: Fisher Elizabeth , McMechan George A. , Gorman Michael R. , Cooper A. Paul R. , Aiken Carlos L. V. , Ander Mark E. , Zumberge Mark A.
Published: February 2008

In the summer of 1987, approximately 42,600 radar reflections were obtained along 124 radial lines, 5 km long, centered at Dye 3 in southern Greenland. Processing of these data using a three‐dimensional kinematic migration algorithm produces a high‐resolution image of the rock surface topography beneath the ice sheet. Estimated uncertainties in the position of the rock surface increase where sampling is less dense, such as toward the edges of the survey, but are less than 5 m over most of the survey area. The main structure revealed is a northwest‐southeast trending valley in the bedrock that crosses the westward regional dip of the rock surface. Ice thickness increases from approximately 1800 m in the east to approximately 2100 m in the west.
Stress origins and earthquake potentials in Cascadia
Authors: Spence William
Published: February 2008

Focal mechanism solutions for shallow earthquakes throughout the Cascadia plate system indicate that the primary regional stress is northerly compression, even though the Juan de Fuca plate generally is thought to be subducting N50°E. This compressional stress is pervasive throughout the Gorda‐Juan de Fuca‐Explorer plate system and much of the adjoining section of North American plate. Modeling, using a discrete element code, shows that this stress primarily is due to the Pacific plate being driven into the Gorda block and Juan de Fuca plate (at the Mendocino and Blanco fracture zones). This plate collision causes compression of the offshore plate system northward through the 45°W trending Vancouver Island and leads to transfer of northerly compression into the overriding plate. The northerly compression in the Cascadia plate system has caused large, crustal earthquakes in the regions of Vancouver and northern California and may be capable of causing large earthquakes also in and offshore of Washington and Oregon. Several independent lines of evidence confirm that the Cascadia subduction interface is coupled strongly and that subduction primarily must be accompanied by significant earthquakes, rather than occurring aseismically. For numerous earthquakes within the subducted plate, focal mechanisms indicate extensional stresses that trend downdip. These earthquakes reflect plate extension that is due to sinking of the more deeply subducted plate and to plate motion being resisted at the shallow subduction interface. The slab pull force of the sinking plate also can lead to earthquakes at the shallow subduction interface and the potential exists for M 7.5–8.0 subduction earthquakes to occur at segments of the subduction boundary at Washington and Vancouver and less frequently so at segments of the subduction boundary at Oregon. The in‐plate driving forces of the offshore Cascadia plate system do not appear to contribute significantly to regional stresses. Stresses and earthquakes at the Cascadia province primarily result from the action of the Pacific plate on the Gorda‐Juan de Fuca‐Explorer plate system, a transfer of this action into the overriding North American plate, and the independent sinking and slab pull force of the subducted oceanic lithosphere.