bluuebunny/crossref_metadata_2025 huggingface dataset:
Estimation of seismic moment and slip distribution of the April 1, 1946, Aleutian tsunami earthquake
Authors: Johnson Jean M. , Satake Kenji
Published: February 2004

The 1946 Aleutian earthquake produced a tsunami of tsunami magnitude Mt=9.3, but the surface wave magnitude is only Ms=7.4, making it a tsunami earthquake. The discrepancy between the apparent size of the earthquake based on the seismic data and the size of the tsunami has been explained by several mechanisms, including a landslide and a slow earthquake, but there are few seismic data available to determine the correct mechanism. We study the generating mechanism of the tsunami using tsunami waveforms recorded on tide gauges. We have modeled the source of the 1946 Aleutian tsunami as the result of an underthrusting earthquake. We performed both forward and inverse modeling of the data using a finite difference calculation to compute synthetic tsunamis. We include both vertical and horizontal deformation of the ocean bottom due to faulting in the computation. The results of the inversion of the tsunami waveforms show that the slip on the fault is mainly concentrated in the shallow section of the fault near the epicenter. There is little slip in the area of the aftershocks, indicating that the aftershocks may represent an area of afterslip or triggered seismicity. The moment estimate of the earthquake is 23×1020 N m, or Mw=8.2. The majority of the waveforms are well‐explained by this fault model and slip distribution, including the Honolulu waveform, but the Hawaiian data also suggest that other effects, such as a landslide, may be necessary for explaining the abnormal tsunami amplitudes in Hawaii. The 1946 earthquake is similar to other tsunami earthquakes, such as the 1896 Sanriku and 1992 Nicaragua earthquakes, indicating that slow, shallow rupture may explain the disproportionally large tsunamis.
Scale lengths of shear velocity heterogeneity at the base of the mantle from S wave differential travel times
Authors: Lay Thorne , Garnero Edward J. , Young Christopher J. , Gaherty James B.
Published: February 2004

Resolving the seismic velocity heterogeneity in the lowermost mantle is essential for understanding the chemical and dynamical processes operating in the boundary layer between the core and the mantle. Several regions of the lowermost mantle appear to have abrupt increases in shear velocity several hundred kilometers above the core‐mantle boundary, at the top of the D” layer. This structure is revealed by seismic wave energy refracted by the velocity increase, resulting in an extra arrival (Scd) in teleseismic S wavetrains at distances from 65° to 95°. Anomalies in differential travel times of the extra arrival relative to direct S (which turns at shallower depths in the midmantle) or ScS (which reflects from the core‐mantle boundary below the velocity increase) provide probes of the velocity heterogeneity in the lowermost mantle with better vertical resolution than provided by conventional ScS‐S measurements. We explore the spatial patterns in Scd‐S, ScS‐Scd, and ScS‐S differential time residuals for paths through the lower mantle beneath Alaska, Eurasia, and India (all being regions with coherent Scd phases) to place constraints on the strength and scale lengths of shear velocity heterogeneity and/or variable topography of the lower mantle discontinuity in these regions. The observed patterns are poorly predicted by existing global tomographic models. Significant small‐scale heterogeneity, with lateral length scales of 200–500 km or less, exists even within regions that display a relatively uniform D” discontinuity structure over scale lengths of 1500–2000 km. The strongest travel time variations are associated with structure above the D” region, in contrast to common assumptions. Rapid lateral fluctuations in ScS‐Scd differential times suggest that the anomalies accumulate within a relatively thin zone, less than the overall thickness of D”. Correlations among the differential time anomalies indicate that Scd variations, ranging over about 4 s, are more pronounced than ScS variations and that the travel time fluctuations accumulate near the top of D” due to shear velocity heterogeneity of ±4% within a 50 km thick region and/or discontinuity topography of ±50 km, with scale lengths of 200–500 km. It appears that the upper portion of D” is much more heterogeneous than the deeper region. Heterogeneity within the layer appears to be decoupled from that at the top of the layer or in the overlying mantle.
Influence of deformation on melt topology in peridotites
Authors: Daines M. J. , Kohlstedt D. L.
Published: February 2004

Melt microstructures in partially molten fine‐grained olivine‐basalt aggregates have been characterized using image processing methods to quantify the effects of deformation on the geometry of the melt network. Olivine‐basalt aggregates with melt fractions of 0.018 to 0.165 and mean grain size between 10 and 25 μm were deformed at differential stresses between 0 and 250 MPa. For samples with grain sizes of ∼10 to 25 μm, deformation at differential stresses greater than ∼100 MPa results in a preferred orientation of melt such that a larger fraction of melt is in melt pockets which have their long axis at 15–20° to σ1. The melt preferred orientation becomes more well developed with increasing differential stress and is related to the minimum in mean pressure that occurs at the ends of melt pockets that are inclined to σ1. As a result of this minimum in mean pressure, melt flows to the ends of melt pockets causing them to extend. Hydrostatic annealing of deformed samples results in the redistribution of melt into melt pockets oriented more perpendicular to the original σ1; this melt distribution is consistent with the preferred wetting of (010) planes of olivine which are oriented subperpendicular to σ1, after deformation. The development of an anisotropic melt distribution in response to stress during deformation could be important for the focusing of melt to mid‐ocean ridges.
P and PcP travel time tomography for the core‐mantle boundary
Authors: Obayashi Masayuki , Fukao Yoshio
Published: February 2004

PcP arrival times reported to the International Seismological Centre (ISC) are inverted for vertical travel time anomaly at the core‐mantle boundary (CMB). We first invert about 2×106 ISC first P arrival data for the three‐dimensional (3‐D) structure of the mantle and hypocenter locations iteratively. The PcP data are corrected for the relocated hypocenters and the 3‐D structure. This correction has reduced the observed PcP residuals. The corrected PcP data are then inverted for the vertical travel time anomalies at the CMB, which are due either to velocity anomaly or topographic anomaly. The spherical average of the inverted structure requires either a velocity reduction in the boundary layer just above the CMB (by ∼3% if the layer is 20 km thick) or a lowering of the CMB by 3 km. The heterogeneity pattern of the CMB is drastically different from that of the overlying D′′ layer. Negative vertical travel time anomalies appear in the northern high‐latitude region, and positive anomalies appear under Southeast Asia and Middle to South America. The lateral dimensions of these anomalies are of the order of 40°. The anomalies can be interpreted as either a velocity perturbation in the boundary layer (±∼7% if the layer is 20 km thick) or an undulation of the CMB (±∼7 km). Various tests including the error and resolution estimations indicate that these three anomalies are the significant, resolvable features of the CMB. These anomalies are also corroborated by the result of the joint inversion of P and PcP for both the 3‐D mantle and the 2‐D CMB. Assuming that the layer is laterally heterogeneous in iron content and deforms at the base in isostatic equilibrium with the outer core, the PcP data are also inverted for the isostatic figure of the boundary layer. The velocity perturbation in this case is 5∼6% if the layer is 20 km thick, and the CMB undulation is only ±1∼2 km. Regardless whether the CMB heterogeneity is due to velocity and/or topographic perturbations, it is dominant in the zonal component of degree 2, while the degree 2 pattern in the lower mantle, including the D′′, is dominant in sectorial component.
Effect of anisotropy on oceanic upper mantle temperatures, structure, and dynamics
Authors: Hearn Elizabeth Harding , Humphreys Eugene D. , Chai Mu , Brown J. Michael
Published: February 2004

Olivine and orthopyroxene crystals composing the oceanic upper mantle align under progressive simple shear strain. Because the thermal diffusivities (κ) and viscosities of these minerals are anisotropic, mineral alignment affects vertical heat flow and upper mantle dynamics. The vertical thermal diffusivity of upper mantle peridotite decreases with progressive simple shear strain, leading to higher temperatures in the shallow upper mantle than predicted by an isotropic half‐space cooling model. This, in turn, causes higher surface heat flow, shallower ocean basins, weaker asthenosphere, and slightly thinner lithosphere. Viscosity associated with an oriented simple shear strain (ηsh), such as that caused by plate motion, also evolves with progressive strain, though ηsh at high strains has not been characterized. Regardless of the evolution of ηsh with strain, the effects of thermal diffusivity anisotropy on upper mantle temperatures, surface heat flow, lithosphere thickness, asthenosphere viscosity and shear stress at the plate bottom remain evident. Shear heating elevates the geotherm by more than κ anisotropy except in the youngest ocean and in cases with significant shear weakening or a slowly moving plate (less than ∼3 cm/yr). The magnitude of shear heating effects depends on how ηsh increases or decreases at high strains. For models in which ηsh increases as a function of strain, shear heating elevates temperatures beyond reasonable upper mantle estimates (i.e., above 1400°C), suggesting that such strain induced viscosity increases are not likely. We present a model for ocean upper mantle cooling and deformation that includes upper mantle thermal diffusivity anisotropy and shear heating. This model predicts heat flow, shear wave split times, lithosphere thicknesses, and basin depths which are consistent with observations but suggests that basal tractions on ocean plates are lower than previously thought.
On thermoelasticity and silica precipitation in hydrothermal systems: Numerical modeling of laboratory experiments
Authors: Martin J. T. , Lowell R. P.
Published: February 2004

We develop a numerical model to understand the evolution of fracture permeability in hydrothermal upflow zones resulting from the combined effects of thermoelastic stresses and precipitation of silica as high‐temperature, reactive fluid traverses temperature and pressure gradients. Because we test the model by comparing the results with those from previously published laboratory experiments on cylindrical granite cores, we solve the problem of radial flow under an applied pressure difference of a silica‐saturated fluid through vertical, initially parallel‐walled cracks distributed evenly about the core sample. We emplace a steady state logarithmic temperature profile and assume that silica precipitation occurs so that the silica concentration in the fluid remains at equilibrium with local temperature and pressure along the flow path. The model results show a rapid initial decrease in permeability resulting from thermoelastic stresses, followed by a further decrease resulting from silica precipitation. The greater the initial temperature gradient, initial permeability, and/or initial crack width (at a given permeability), the greater the permeability decrease resulting from thermoelastic stresses. As a result of silica precipitation, the permeability eventually declines as t−3/2. The model results agree with the general trends in the laboratory data, thus confirming that silica precipitation is the main cause of the observed decrease in permeability during the experiments. Disagreement between the model and laboratory data in detail suggests that complications such as reaction kinetics, precipitation of other minerals and nonhomogeneous crack distributions need to be considered in the model. Thermoelastic stresses, though not important at the laboratory scale, may be important at the field scale.
Petrogenesis of axial lavas from the southern Chile Ridge: Major element constraints
Authors: Sherman Sarah B. , Karsten Jill L. , Klein Emily M.
Published: February 2004

We present major element glass data for 163 rock samples collected from four ridge segments of the southern Chile Ridge between the Chiloe Fracture Zone and the Chile Margin Triple Junction, including the segment currently being subducted at the Chile Trench (segment 1). The subridge mantle is heterogeneous at small spatial scales. Normal mid‐ocean ridge basalts (N‐MORB), recovered from all four ridge segments, have experienced variable extents of low pressure fractionation but have been generated by relatively uniform extents (F) and initial pressures (Po) of melting of a slightly heterogeneous depleted source. Type 1 E‐MORB, found only on segment 4, have trace element affinities to some ocean island basalts, display a large range of major element variations at constant and high MgO, and are spatially associated with N‐MORB. Type 2 E‐MORB have trace element affinities with suprasubduction zone settings. They are found at two segment 1 sites and along most of segment 3. In order to minimize fractionation and source heterogeneity effects and assess melting conditions, E‐MORB compositions were double‐backtracked to 8 wt % MgO and a K/Ti ratio of 0.1. Although the magnitudes of F and Po are model‐dependent, we find that N‐MORB and both types of E‐MORB were generated under similar melting conditions. These observations indicate that spreading rate and mantle temperature exert primary control on the southern Chile Ridge thermal regime. We see no influence of ridge subduction on the major element systematics and melting conditions of segments closest to the trench.
A model for the formation of transient event plumes above mid‐ocean ridge hydrothermal systems
Authors: Wilcock William S. D.
Published: February 2004

Event plumes (or megaplumes) are ephemeral bodies of anomalously warm water which form in the water column above mid‐ocean ridges as the result of the catastrophic release of large volumes of hydrothermal fluid. During the formation of the largest event plumes, heat fluxes increase to at least 100 times those of a typical high‐temperature vent field. Recent observations reveal a strong association between event plumes and diking‐eruptive events. It has previously been suggested that event plumes are the result of increased fluxes throughout hydrothermal systems following a large increase in the permeability of a critical region. In this paper, an alternative model is explored whereby the volume of fluid expelled is not balanced by increased down‐flow but rather by the expansion of fluids near the base of the system. For most geologically plausible patterns of hydrothermal circulation, the fracturing and faulting associated with a diking event will result in a much larger decrease in the flow resistance of the upflow zone than the down‐flow zone. To balance fluxes, the pressure must decrease throughout the system. There is considerable evidence that the fluids near the base of mid‐ocean ridge hydrothermal convection cells lie near the two‐phase boundary for seawater and thus are very compressible. Simple calculations show that fluid expansion in a depressurized reaction zone can match the observed formation times and sizes of large event plumes only if the permeability of the upflow zone increases to very high values (∼10−9–10−10 m2) and if the plume is underlain by a sizable reaction zone containing at least 0.01–0.2 km3 of fluid at temperatures near the two‐phase curve. Fluid expansion in a smaller reaction zone is a plausible mechanism to incorporate a component of mature hydrothermal fluid into event plumes whose heat content comes primarily from cooling lava flows. It may also be responsible for smaller event plumes or temporal fluctuations in chronic plumes which are not linked to volcanic eruptions but which follow technically or magmatically induced increases in permeability.
Lateral flow and ponding of starting plume material
Authors: Sleep Norman H.
Published: February 2004

Many mantle plumes started with voluminous heads of hot material that ascended as diapirs through the mantle. Flood basalts, radial dike swarms, and frequently continental breakup are associated with the impingement of starting plume heads on the base of the lithosphere. The buoyant plume head material ponds at the base of the lithosphere and then flows laterally. Relief on the base of the lithosphere acts as an upside‐down drainage pattern with enclosed catchments for this flow. That is, plume material preferentially collects beneath regions where the lithosphere is locally thin, such as thermally subsiding sedimentary basins. An example is uplift associated with the Iceland starting plume which inverted the Mesozoic Irish Sea basin. Additional relief at the base of the lithosphere is associated with rifting and seafloor spreading. In the case of the northern North Sea, material from the Iceland starting plume ponded beneath a basin with locally thin lithosphere. Rifting breached the enclosed catchment allowing the ponded plume material to drain across the new passive margin toward very thin lithosphere produced by seafloor spreading. Rifting also creates a conduit for the flow of plume material away from the region underlain by the starting plume head. An example is the volcanic passive margin off Nova Scotia which is 2000 km from the center of the Bahama starting plume head. Numerical and analytical calculations indicate that lateral flow of plume material consistent with this observation is expected if its viscosity is around 1018 Pa s.
Three‐dimensional tomographic velocity structure of upper crust, Coaxial segment, Juan de Fuca Ridge: Implications for on‐axis evolution and hydrothermal circulation
Authors: Sohn Robert A. , Webb Spahr C. , Hildebrand John A. , Cornuelle Bruce D.
Published: February 2004

Three‐dimensional models of compressional velocity and azimuthal anisotropy from tomographic inversions using 23,564 ocean bottom seismometer P wave arrivals define systematic lateral variations in seismic structure of the CoAxial segment of the Juan de Fuca Ridge (JdFR). Over much of the segment the across‐axis structure is roughly axisymmetric, characterized by a progressive increase in dike velocities moving away from the ridge axis. This trend is most apparent in the basal dikes, where on‐axis velocities are about 800 m/s slower than those measured elsewhere within the rift valley. The on‐axis sheeted dikes also exhibit ridge‐oriented azimuthal anisotropy, with a peak‐to‐peak amplitude of about 600 m/s. Outboard of the rift valley, beneath ridge flanks with fault scarps, velocities in the upper 1500 m of crust are reduced. The maximum amplitude of this anomaly is about 700 m/s, located near the top of the sheeted dikes. Variations in the three‐dimensional velocity model are believed to reflect changes in crustal porosity, from which we infer an axisymmetric porosity model for seismic layer 2 of the CoAxial segment. As the crust ages, the evolution of layer 2 porosity could occur in the following way: (1) the porosity of zero‐age, on‐axis dikes is set at formation by the contraction of molten material, (2) hydrothermal alteration fills pore spaces as the dikes move away from the center of the axial valley, and (3) normal faulting on the ridge flank scarps opens fractures and increases porosity of the upper dikes as they move off‐axis. At the north end of the segment, dike velocities are several hundred meters per second slower, on average, and the across‐axis structure is lost. The transition from a coherent, aligned seismic structure to a less distinct pattern with reduced velocities may represent a shift from magmatic to amagmatic extension moving away from the Cobb hotspot on the ridge axis. The porosity structure we have derived for the CoAxial segment suggests an alternative to the usual hydrothermal circulation model of cross‐axis convection cells. A circulation model with along‐axis convection cells located entirely within the axial valley appears to be more compatible with our data.
Regional variations of seismic attenuation from coda andLgwaves in northern Baja California
Authors: Domínguez Tonatiuh , Rebollar Cecilio J.
Published: February 2004

Attenuation of coda andLgwaves was estimated from events recorded by the seismic network of northern Baja California, Red Sismica del Noroeste de Mexico (RESNOM), in the frequency band 1 to 12 Hz in order to find a regional variation ofQc(crustal quality factor forSwaves) in northern Baja California, Mexico. We used the single scattering model ofSato[1977] in the time domain to estimate quality factorQcand a spectral approach to relate acceleration spectra with distance to estimate the anelastic attenuation coefficient γ ofLgwaves. We calculatedQcfor short (<10 s) and long lapse times (<70 s) using local events and the events that originated at the main tectonic features of the area. Assuming a frequency dependence ofQcof the formQc(ƒ)=Qcƒαwe observed for short lapse times a regional variation ofQ0with low values from about 53 in the Inner Continental Borderland (ICB) province to about 115 in the Peninsular Ranges (PR) and 144 in the Gulf of California Depression (GCD) province. The α values range from 0.70 and 0.85 for events in the ICB and PR to 0.44 in the GCD region. For long lapse times and for events located in the PR we foundQ0values from 49 at stations within the ICB to 101 at stations within the GCD. Attenuation coefficient γ ofLgwaves between 1 and 6 Hz was also estimated using events located in southern California with source‐station distances up to 420 km. We found that frequency dependence of γ was higher for source‐receiver paths going from a typical continental structure to a pull‐apart basin (the GCD province) than for paths within the PR. Using all data in a linear inversion process and assuming a group velocity (VLg) of 3.4 km/s, we obtained a power law dependence of the formQLg(ƒ) = 288 ± 20ƒ0.32±.02
Expansion and quenching of vesicular magma fragments in Plinian eruptions
Authors: Kaminski Édouard , Jaupart Claude
Published: February 2004

The conditions of pumice generation in Plinian eruptions are studied. A physical model describes the behavior of gas bubbles in a magma fragment which is carried upward in a volcanic conduit and an atmospheric eruption column. The effects of pressure release and cooling are calculated for a range of eruption conditions. The magma fragment expands in the conduit and stops expanding soon after leaving the vent, when a thin viscous rind forms against the cold mixture of magmatic gas and air. This rind prevents further volume changes. Pumice vesicularity is a function of the decompression rate in the conduit, which depends on ascent velocity and fragmentation depth. It is also sensitive to the cooling rate in the atmospheric column, which depends on vent radius and mass discharge rate. Different fragments follow different trajectories in the column and are subjected to different cooling rates. This generates a range of vesicularities which reflects the eruptive conditions. All else being equal, pumice vesicularity increases as magma viscosity decreases. These predictions are consistent with observations. Pumices provide quantitative constraints on conduit flow conditions and mass discharge rate. These concepts are applied to two Plinian eruptions. Vesicularity values for the Bishop Tuff, Long Valley caldera, require a mass discharge rate between 108 and 109 kg s−1. Vesicularity variations during Plinian phase 1 of the Minoan eruption, Santorini, are explained by the conduit radius increasing from about 30 m to 120 m. Both cases require large decompression rates in the eruption conduit, suggesting that flow pressures were close to lithostatic values.
The state of stress on some faults of the San Andreas System as inferred from near‐field strong motion data
Authors: Bouchon Michel
Published: February 2004

We present a simple method to calculate the stress produced on an earthquake fault during rupture. This method allows the complete evaluation of the stress spatio‐temporal history over the fault. We apply this approach to study the changes in shear stress produced during four of the largest earthquakes which occurred along the San Andreas fault system over the last 20 years: the Imperial Valley earthquake of 1979, the Morgan Hill earthquake of 1984, the Loma Prieta earthquake of 1989, and the Northridge earthquake of 1994. We use for this study the tomographic models of the fault rupture obtained from the inversion of the near‐field seismic data recorded during these earthquakes. The results obtained show that the static and the dynamic stress drops vary greatly over the fault. The peak values obtained for the four earthquakes studied range from about 20 to 100 MPa. These high values imply that the initial shear stress level on the fault at the onset of the earthquake was high on at least a significant portion of the fault. The regions of the fault which break with a high stress drop are also the regions where slip is large. This suggests that most of the slip produced in a large earthquake takes place over the “strong” areas of the fault. Low slip regions tend to break with low stress drops. After the earthquake, the shear stress is increased over a significant portion of the fault, which corresponds to low slip regions. Aftershock activity tends to be concentrated in these areas of stress increase. The apparent strength of the fault before the earthquake (that is the local shear stress increase which is required for rupture) is also extremely heterogeneous. The rupture velocity seems to be inversely correlated with this apparent fault strength, the rupture accelerating over the “weak” areas of the fault and slowing down over the high strength areas.
The application of back‐propagation neural network to automatic picking seismic arrivals from single‐component recordings
Authors: Dai Hengchang , MacBeth Colin
Published: February 2004

An automatic approach is developed to pick P and S arrivals from single component (1‐C) recordings of local earthquake data. In this approach a back propagation neural network (BPNN) accepts a normalized segment (window of 40 samples) of absolute amplitudes from the 1‐C recordings as its input pattern, calculating two output values between 0 and 1. The outputs (0,1) or (1,0) correspond to the presence of an arrival or background noise within a moving window. The two outputs form a time series. The P and S arrivals are then retrieved from this series by using a threshold and a local maximum rule. The BPNN is trained by only 10 pairs of P arrivals and background noise segments from the vertical component (V‐C) recordings. It can also successfully pick seismic arrivals from the horizontal components (E‐W and N‐S). Its performance is different for each of the three components due to strong effects of ray path and source position on the seismic waveforms. For the data from two stations of TDP3 seismic network, the success rates are 93%, 89%, and 83% for P arrivals and 75%, 91%, and 87% for S arrivals from the V‐C, E‐W, and N‐S recordings, respectively. The accuracy of the onset times picked from each individual 1‐C recording is similar. Adding a constraint on the error to be 10 ms (one sample increment), 66%, 59% and 63% of the P arrivals and 53%, 61%, and 58% of the S arrivals are picked from the V‐C, E‐W and N‐S recordings respectively. Its performance is lower than a similar three‐component picking approach but higher than other 1‐C picking methods.
The most recent megalandslides of the Canary Islands: El Golfo debris avalanche and Canary debris flow, west El Hierro Island
Authors: Urgeles Roger , Canals Miquel , Baraza Jesús , Alonso Belén , Masson Doug
Published: February 2004

Two major landsliding events have been identified west of the island of El Hierro: The El Golfo debris avalanche and the Canary debris flow. These landslides were identified from swath bathymetry, seismic reflection, and TOpographic PArametric Sonar (TOPAS) data obtained in December 1994 during a cruise on board the Spanish R/VHesperides.The El Golfo debris avalanche originated subaerially on the western flank of the island of El Hierro and has an associated 150 km3rock debris deposit on the base of slope. The Canary debris flow, which dislocated some 400 km3of sediment, resulted from a different failure originated between 3200 and 3700 m depth at the base of slope of the island of El Hierro. According to the studied data set, its source area seems to have been covered by the El Golfo debris avalanche deposit. The triggering of the El Golfo debris avalanche (between 136 and 21 ka) is related to tensional stresses on the rift zones of the island. These rift zones control the emplacement and morphology of the landslide scar. In the Canary Islands, a relation between landslide ages and island ages can be established, indicating a link between subsidence history, age of shield phases, and giant landslides.
Long‐period ray parameters of the core diffraction Pdiff and mantle heterogeneity
Authors: Hock Silke , Roth Michael , Müller Gerhard
Published: February 2004

In an attempt to map lateral velocity variations in the D” zone of the lowermost mantle of the Earth, long‐period Pdiff ray parameters are determined from digital data of the Global Seismic Network. Twenty‐six earthquakes between 1981 and 1986 provided 37 profiles in North America and Europe. Average profile length is 18°, and the number of stations per profile is 3 or 4. For accurate travel time determination of the Pdiff wave group peaks, seismograms are equalized to the same instrument characteristic. Ray parameter interpretation in terms of the velocity along the D′′ path of each profile requires correction of Pdiff travel times for mantle heterogeneity above D′′ on the upgoing Pdiff paths. The finite difference method of Podvin and Lecomte [1991] in two dimensions is used to calculate Pdiff travel time curves for the global tomography model of Inoue et al, [1990] and for related models, having lateral heterogeneity only in D′′ or only above D′′. From these calculations, Pdiff ray parameter corrections of up to 1.5% are found. Maps with uncorrected and corrected ray parameters are shown. Conflicting ray parameters on close D′′ profiles continue to exist after correction, which points to insufficient correction. Global tomography models underestimate upper mantle heterogeneity, which, according to regional tomographic studies, has scale lengths of a few hundred kilometers and velocity variations of up to ±3%. Such upper mantle structures are modeled by Gaussian random media with correlation distances of 250–1000 km and standard deviations of 0.5–1% of their slowness fluctuations. Pdiff ray parameter corrections for such structures can reach 3%. The results of this study show that regionalization of D′′ with Pdiff requires detailed knowledge of mantle heterogeneity, in particular of heterogeneity in the upper mantle. Currently, this information is only rarely available.
Natural gas hydrates on the southeast U.S. margin: Constraints from full waveform and travel time inversions of wide‐angle seismic data
Authors: Korenaga J. , Holbrook W. S. , Singh S. C. , Minshull T. A.
Published: February 2004

Strong bottom‐simulating reflectors (BSR) have been mapped over a region of approximately 50,000 km2 on the southeastern U.S. margin and have been associated with possible abundance of natural gas hydrates. In June 1992, coincident single‐channel seismic and wide‐angle ocean bottom seismic data were acquired in the region, focusing on the Blake Ridge and the Carolina Rise. Wide‐angle reflections from the BSRs were clearly observed at offsets up to ∼6 km. Joint travel time inversion was conducted with wide‐angle and vertical‐incidence data in order to explore possible regional variation, and the resultant two‐dimensional average velocity models imply higher background velocities on the Carolina Rise. Full waveform inversion was then performed to determine the seismic origin of the BSRs. The best fit model shows a similar low velocity (∼1.4 km/s) beneath the BSR at both sites, indicating trapped free gas with low saturation (<10%). The inversion results also indicate that a thin, high‐velocity wedge, with a maximum velocity of ∼2.3 km/s, is present just above the Blake Ridge BSR. Sediment reflectivities were also calculated, and higher reflectivities are observed on the Carolina Rise. An increase in reflectivity below the BSR seems to correspond to the gas‐bearing zone at both sites. Concentration of hydrates were estimated based on these velocity models. Whereas average hydrate concentration of 3% of the total sediment volume is suggested for the lower half of the hydrate stability zone at the Blake Ridge, only a very low average concentration of hydrate can be expected at the Carolina Rise. The hydrates seem to be concentrated near the base of the hydrate stability zone, and the maximum hydrate concentration is estimated as ∼20% at the Blake Ridge and ∼7% at the Carolina Rise, both of which are too high to be explained by in situ biogenic activity only and require some secondary accumulation mechanism. It is suggested that hydrate recycling caused by the stability field migration may have effectively condensed hydrates at both sites. Additional enhancement by upward fluid expulsion may also be viable for the Blake Ridge.
Mapping the frequency‐magnitude distribution in asperities: An improved technique to calculate recurrence times?
Authors: Wiemer Stefan , Wyss Max
Published: February 2004

We hypothesize that highly stressed asperities may be defined by mapping anomalously lowbvalues. Along the San Andreas fault near Parkfield the asperity under Middle Mountain, with itsb=0.46, can be distinguished from all other parts of the fault surface. Likewise, along the Calaveras fault the northern asperity of the Morgan Hill 1984 (M6.2) rupture can be identified by its lowbof 0.5 as a high stress patch along the fault. We add further evidence to the observations that thebvalue of the frequency‐magnitude relationship of earthquakes is inversely proportional to stress by showing that it decreases with depth in the Parkfield segment of the San Andreas and along the Calaveras fault. In both of these areas,bvalues above and below 5 km depth are ∼1.2 and 0.8, respectively. We propose that probabilistic recurrence timesTr, based on the seismicity parametersaandb, should be calculated from their values within asperities only, instead of from the values of the entire rupture area of the maximum expected earthquake. The strong patches on faults control the time of rupture because they are capable of accumulating larger stresses than the rest of the fault zone, which slips along passively when an asperity breaks. Therefore no information onTris contained in the passive fault segments, only in the asperities. At Parkfield the probabilistic estimates ofTrderived from the data in the whole rupture and in the asperity only are 72 (−18/+24) and 23 (−12/+18) years, respectively, compared to the historically observed repeat time of 22 years. At Morgan Hill theTrestimates are 122 (−46/+76) and 78 (−47/+110) years, respectively, compared to the observed repeat time of 72 years.
Listric normal faulting on the Cascadia continental margin
Authors: McNeill Lisa C. , Piper Kenneth A. , Goldfinger Chris , Kulm LaVerne D. , Yeats Robert S.
Published: February 2004

Analysis of multichannel seismic reflection profiles reveals that listric normal faulting is widespread on the northern Oregon and Washington continental shelf and upper slope, suggesting E‐W extension in this region. Fault activity began in the late Miocene and, in some cases, has continued into the Holocene. Most listric faults sole out into a subhorizontal décollement coincident with the upper contact of an Eocene to middle Miocene mélange and broken formation (MBF), known as the Hoh rock assemblage onshore, whereas other faults penetrate and offset the top of the MBF. The areal distribution of extensional faulting on the shelf and upper slope is similar to the subsurface distribution of the MBF. Evidence onshore and on the continental shelf suggests that the MBF is overpressured and mobile. For listric faults which become subhorizontal at depth, these elevated pore pressures may be sufficient to reduce effective stress and to allow downslope movement of the overlying stratigraphic section along a low‐angle (0.1°–2.5°) detachment coincident with the upper MBF contact. Mobilization, extension, and unconstrained westward movement of the MBF may also contribute to brittle extension of the overlying sediments. No Pliocene or Quaternary extensional faults have been identified off the central Oregon or northernmost Washington coast, where the shelf is underlain by the rigid basaltic basement of the Siletzia terrane. Quaternary extension of the shelf and upper slope is contemporaneous with active accretion and thrust faulting on the lower slope, suggesting that the shelf and upper slope are decoupled from subduction‐related compression.
Electric potential changes associated with slip failure of granite: Preseismic and coseismic signals
Authors: Yoshida Shingo , Uyeshima Makoto , Nakatani Masao
Published: February 2004

Electric potential changes were measured for stick‐slip events in granite samples with a three‐block direct shear arrangement at 8 MPa normal stress. Two electrodes were mounted on the left‐ and right‐hand blocks, and the electric potential difference between each electrode and the ground was measured with a high input impedance recording system of frequency range from DC to 100 Hz. As well as coseismic electric signals of about 1.5 V which appeared the moment of the dynamic slip event, preseismic signals were detected just before the slip event. The coseismic signal rises stepwise with opposite polarities at the two electrodes and exponentially decays with a time constant of ε/s, where ε is the permittivity and s is the conductivity of the rock sample. We conducted a simple test of rapid stress drop without slipping and observed almost the same electric signal as the coseismic signal. This suggests that the electric signal is generated by the piezoelectric effect. We proposed a generation model based on the piezoelectric effect and the resultant relaxation process and obtained a theoretical frequency response, which is in agreement with experimental data. The preseismic signal appears about 2–3 s before the dynamic event with an amplitude of about 50 mV. The local strains along two sliding surfaces were also measured to monitor the growth of the rupture nucleation zone. When the growth of the rupture nucleation zone occurred on the left sliding surface, a clear preseismic signal was detected at the electrode mounted on the left granite block. When the growth occurred on the right‐hand surface, a signal was detected at the electrode on the right block. This shows that the preseismic electric signal is caused by stress change in the rupture nucleation zone. These preseismic and coseismic signals were also detected with an antenna, which was placed away from the sample surface.
Study of the crustal thickness and the subducting lithosphere in Greece from gravity data
Authors: Tsokas Gregory N. , Hansen Richard O.
Published: February 2004

The area of Greece has experienced a complex tectonic history dominated by the subduction of the African plate beneath Eurasia. In this study the variations of the crustal thickness in the area of Greece were obtained by means of the multiple‐source Werner deconvolution (MSWD) method applied to gravity data. Thicknesses of 40–49 km are estimated beneath the Hellenides mountain belt to the west. Eastward thinning of the crust, to thicknesses ranging from 25 km in the north to 30 km in the south is seen in the Aegean region. These results are in good agreement with recent seismological results, demonstrating that the MSWD method successfully treated the problem. Using the crustal model we derived, we computed the gravity effect of the crust and extracted it from the Bouguer anomaly. We also extracted the gravity effect of the subducting lithosphere from the Bouguer anomaly, producing a residual map where most of the original gravity variation has been successfully removed. The remaining anomalies appear related to near‐surface features and an area of low‐velocity mantle in the central Aegean Sea.
Devolatilization‐generated fluid pressure and deformation‐propagated fluid flow during prograde regional metamorphism
Authors: Connolly J. A. D.
Published: February 2004

The obstruction to fluid flow formed by the rocks overlying a metamorphic devolatilization front causes the fluid pressure gradient in the reacting rocks to diverge from lithostatic. This drives deformation in tandem with the fluid pressure anomaly generated by the volume change of the reaction. Numerical simulations show that once the vertical extent of the reacted rocks is comparable to the compaction length, compaction processes caused by the difference between confining and fluid pressure gradients generate a positive fluid pressure anomaly (effective pressure <0) above the reaction front, irrespective of the reaction volume change. Consequent dilational deformation propagates the anomaly upward, leading to underpressuring and densification at the reaction front and detachment of a wave of anomalous fluid pressure and porosity. Creep is a viable mechanism for such wave propagation for crustal viscosities <1015 MPa s. Continuous upward strengthening of the crust increases the wavelength and amplitude of the fluid pressure waves and thereby the likelihood of hydrofracture. Order of magnitude strength contrasts are adequate to arrest wave propagation, forming water sills that become increasingly stable in the absence of deviatoric stress. Although the fluid pressure gradient within a wave may be near hydrostatic, Rayleigh convection is unlikely. Thus, in the absence of lateral perturbations, fluid flow is upward and episodic, despite continuity of devolatilization. Porosity waves provide a mechanism for temporal focusing of metamorphic fluid fluxes with the potential to increase the efficacy of heat and mass transport.
Stochastic analysis of seafloor morphology on the flank of the Southeast Indian Ridge: The influence of ridge morphology on the formation of abyssal hills
Authors: Goff John A. , Ma Ying , Shah Anjana , Cochran James R. , Sempéré Jean‐Christophe
Published: February 2004

In this study we estimate the statistical properties of abyssal hill morphology adjacent to the Southeast Indian Ridge in a region where the axial morphology changes from axial high to axial valley without a corresponding change in spreading rate. We explore the influence of axial morphology on abyssal hills and place these results within the context of response to spreading rate. Two cruises aboard the R/V Melville collected Sea Beam 2000 multibeam data along the Southeast Indian Ridge, providing continuous multibeam coverage of the axis from ∼89°W to ∼118°W, and ∼100% coverage within four survey regions extending out to ∼45 km (∼1.2 Ma) from the axis [Sempéré et al., 1997; Cochran et al., 1997]. We apply the statistical modeling method of Goff and Jordan [1988] to gridded data from the four survey areas, examining in particular estimates of abyssal hill rms height, characteristic width and length, aspect ratio, and skewness. Two analyses are performed: (1) comparison of the along‐axis variation in abyssal hill characteristics to ridge segmentation, and (2) a calculation of population statistics within axial high, intermediate, and axial valley data populations of this study, and comparison of these results to population statistics derived from studies adjacent to the Mid‐Atlantic Ridge and East Pacific Rise. We find that abyssal hills generated along axial high mid‐ocean ridges are very different from those generated along axial valley mid‐ocean ridges, not only with respect to size and shape, but also in their response to such factors as spreading rate and segmentation.
Shallow dips of normal faults during rapid extension: Earthquakes in the Woodlark‐D'Entrecasteaux rift system, Papua New Guinea
Authors: Abers Geoffrey A. , Mutter Carolyn Z. , Fang Jia
Published: February 2004

Earthquakes in the Woodlark‐D'Entrecasteaux rift system reveal changes in fault geometry along strike, as extension progresses from continental to oceanic modes. In one segment, evidence is found for earthquakes on shallow‐dipping normal faults. To examine these trends, locations of 147 earthquakes are redetermined, and focal mechanisms for 21 events are derived from waveform inversion. Where seafloor spreading occurs, east of 152°E, most earthquakes illuminate a NE‐SW strike‐slip fault north of the oceanic rift system along the Woodlark Rise. Elsewhere, earthquakes occur along a well‐defined rift zone, and focal mechanisms reveal normal faulting with north‐south T axes and depths less than 10 km. Most normal‐faulting earthquakes show high‐angle fault planes (dips 40°–50°), but several are consistent with slip on moderately low‐angle planes dipping 15°–35°. These low‐angle nodal planes are only found in a region between 150.5°E and 152.5°E, where the transition occurs from seafloor spreading to continental rifting. A seismic reflection profile near 151°E reveals a major normal fault that dips to the north at 25°–35° just landward of the oceanic rift tip. That fault and parallel structures are excellent candidates for the fault that slipped in at least two of the earthquakes studied. Hence normal faults dipping ∼30° are likely to be seismically active in at least one part of this rift system. In this region just landward from the rift tip the highest extension rates are expected (∼40 mm/yr) and strain appears localized on a single system of normal faults. Because shallow dips for normal faults are seen near the rift tip, but not elsewhere along strike, high strain rates may be an important factor in allowing normal faults to slip at unusually shallow dips. Perhaps high shear traction at the base of the upper crust, localized by nearby spreading, sufficiently alters the local stress regime.