bluuebunny/crossref_metadata_2025 huggingface dataset:
Surficial offsets on the Central and Eastern Garlock Fault associated with prehistoric earthquakes
Authors: McGill Sally F. , Sieh Kerry
Published: February 2008

Geomorphic features offset along the central and eastern Garlock fault record the amount of surface slip associated with prehistoric earthquakes. Along the easternmost 90 km of the fault, the smallest offsets cluster around 2–3 m of left‐lateral slip, apparently associated with the most recent earthquake on this portion of the fault. Larger offsets along this part of the fault, especially in Pilot Knob Valley, cluster around values consistent with 2–4 m of slip in each of the past several events. Farther west, south of El Paso Mountains, offset geomorphic features suggest that each of the past two earthquakes on this stretch of the Garlock fault was produced by about 7 m of slip, whereas the third event back was produced by about 4 m of slip. Vertical displacements of geomorphic features range from 0% to 30% of the left‐lateral offsets. Within Pilot Knob Valley (along the southern side of the Slate Range), vertical displacements are consistently up on the northern side, whereas within the Avawatz Mountains both north‐ and south‐side‐up vertical displacements are present. On the basis of the geomorphic offsets, the geometry of the Garlock fault, and the precedents set by historical strike‐slip earthquakes elsewhere, a number of different rupture patterns are plausible. These range from rupture of the entire Garlock fault in a single event with a maximum magnitude of about Mw=7.8, to separate rupture of the western segment and of the central and eastern segments combined, with approximate magnitudes Mw≤1.1 and Mw=7.5, respectively, to separate rupture of even shorter segments, producing earthquakes of magnitudes Mw=6.6 to Mw=7.5. In conjunction with available slip rates for the Garlock fault, the geomorphic offsets suggest that average recurrence intervals are probably within the range of 600–1200 years south of El Paso Mountains, about 200–750 years in Searles Valley, about 200–1300 years in Pilot Knob Valley, and about 200–3000 years near Leach Lake and in the Avawatz Mountains.
Three‐dimensional velocity structure and hypocenters of earthquakes beneath the Hazara Arc, Pakistan: Geometry of the underthrusting Indian Plate
Authors: Ni James F. , Ibenbrahim Aomar , Roecker Steve W.
Published: February 2008

The three‐dimensional P and S wave velocity structures and hypocenters of 420 events beneath the western Hazara Arc are obtained simultaneously by inverting travel time data observed at fifteen Tarbela seismic stations. In general, the P and S wave velocity distribution of the top layer (0–6 km depth) correlates well with surface geology. Within this layer we find a low‐velocity region beneath the Hazara Thrust Zone (HTZ) corresponding to the underthrusted Murree Formation, and there are high‐velocity regions south of the Main Mantle Thrust (MMT) which are associated with the exposed Cambrian, late Paleozoic, and Tertiary granites. A low‐velocity zone immediately to the west of the Hazara‐Kashmir Syntaxis (HKS) indicates the existence of a Miocene foreland basin which is covered by late stage southeasterly directed thrusts along the Hazara Arc and is consistent with the idea that the HKS is detached from the lower crust. From the Salt Range to the HTZ, the Indian plate dips at a shallow angle, about 2°–3° to the northeast. North of the HTZ the underthrusting Indian plate dips gently to the northeast with an increased slope of 5° to 8° until it reaches the Indus‐Kohistan Seismic Zone (IKSZ). Along the NW trending IKSZ the Indian plate bends more steeply to the northeast beneath a seismically active midcrustal wedge directed to the southwest. The larger events in the IKSZ are interpreted as occurring on a major thrust zone that can be followed to a depth of 24 km. The IKSZ appears to consist of an upper seismic zone (from the surface to about 8 km) and a lower seismic zone (12 km to 24 km) separated by an aseismic region about 4 km thick. The lower IKSZ may represent the leading edge of a southwestward directed slab which has not yet ruptured the surface. Hypocenters of relocated earthquakes indicate that the HTZ is about 30 km wide with most of the larger microearthquakes occurring at 12–14 km. Seismicity along the HTZ suggests that the Panjal, and Murree thrusts are active.
Dike‐induced faulting and graben subsidence in volcanic rift zones
Authors: Rubin Allan M.
Published: February 2008

Field observations and geodetic data indicate that dike intrusion in volcanic rift zones typically generates normal faulting and graben subsidence at the Earth's surface. Elastic models indicate that two‐dimensional (infinite strike length) dikes do not lower the ground surface above the dike and that normal faults do not lower the surface significantly, more than one down‐dip fault length from the fault trace. Dikes of finite length produce subsidence above the dike, but not by an appreciable amount, for appropriate dike lengths. Therefore the observed graben subsidence can be achieved only if fault slip extends virtually to the dike plane at depth. Dike intrusion increases the horizontal compression adjacent to the dike and decreases the compression beyond the dike perimeter. Therefore fault slip extending to the dike plane is most likely to occur above or in front of the laterally propagating dike. Two data sets documenting the change in surface elevation accompanying dike intrusion in the Krafla rift zone, Iceland, were inverted to determine the dike and fault geometry at depth. Ten kilometers south of the Krafla caldera, subsidence of a graben 1.5 km wide was produced by fault slip to 1.5–2 km depth, essentially to the dike top. Forty kilometers north of the caldera, subsidence of a graben 6 km wide was produced by fault slip to 4–5 km depth, well within the zone of compression adjacent to the dike. In order to determine if fault slip in front of the dike could have produced the observed subsidence north of the caldera, a three‐dimensional boundary element model that computes fault slip during lateral dike propagation was developed. Results indicate that fault slip in front of the dike is capable of producing most of the subsidence observed. Additional subsidence could result from reasonable mechanical anisotropy of the rift zone. The lack of deep fault slip south of the caldera is attributed to a less favorable initial stress state. This is consistent with differences in the tectonic history of the two regions over the past several centuries.
A thermodynamic analysis of the conditions of equilibrium at nonhydrostatically stressed and curved solid‐fluid interfaces
Authors: Heidug Wolfgang K.
Published: February 2008

This work presents a complete thermodynamic analysis of the equilibrium and stability of solid‐fluid phase boundaries under stress when capillary effects are present. The approach is based on the standard thermodynamic extremum principle according to which the free energy assumes a minimum in stable thermodynamic equilibrium. The results obtained allow for anisotropic surface strain and extend in this respect earlier work by Gibbs. An equation quantifying the combined action of capillarity and nonhydrostatic stress on mineral solubility is derived and applied to determine the relative significance of these effects for pressure solution. It is demonstrated that the phase equilibrium at curved and stressed boundaries is metastable, strict thermostatic stability being attained only when the solid‐fluid phase boundary is plane and the stress state in the solid is hydrostatic at the phase boundary.
Stress deflection in a tectonic compressional field: A model for the northwestern Iberian Chain, Spain
Authors: Casas Antonio M. , Simon Jose L. , Seron Francisco J.
Published: February 2008

By means of fault population analysis (using the right dihedra, y‐R diagram and Etchecopar's methods), the states of stress in a number of sites of the Cameras Massif (Northwestern Iberian Chain, Spain) and nearby areas of the Ebro Basin have been determined. These states of stress have been used as a framework in order to infer a model of the actual stress field in that area during the main stage of the Alpine compression (Oligocene‐lower Miocene). The three main compression directions obtained (NW‐SE, NNE, NE‐SW) can be explained by a single compression field with horizontal σ1 trending NNE and vertical σ2, whose primary trajectories are deflected due to (1) large‐scale strike‐slip faults or lateral ramps of thrusts or (2) frontal ramps of thrusts oblique to the main compressive axis. The reliability of this scheme has been checked by means of a two‐dimensional mathematical model based upon the finite element method and applied on a sheet whose rheological properties are similar to those of a standard limestone. This model simulates the main features of the regional stress field, and it was constructed after determining the average parameters of the possible external or “primary” field and simulating the major faults that could deflect the stress trajectories.
Geologic evidence for earthquakes during the past 2000 years along the Copalis River, southern coastal Washington
Authors: Atwater Brian F.
Published: February 2008

Evidence for several prehistoric earthquakes is present in deposits less than 2000 years old that crop out at the Copalis River estuary (47°07.2′N, 124°09.7′W). The deposits show that land subsided decimeters into the intertidal zone on as many as three occasions and that sand vented while the land underwent little or no subsidence on another occasion. The evidence for earthquake‐induced subsidence consists of buried marsh and forest soils that evince sudden estuarine submergence attended on at least one occasion by a landward directed surge of sandy water. This combination of submergence and surge implies subsidence and tsunami. Exposed evidence for subsidence and tsunami along the Copalis River is strong for a time within a few decades of 300 years ago, moderate for a time 1400–1900 years ago, and weak (subsidence) or lacking (tsunami) for an intermediate time also about 1400–1900 years ago. The history of sudden submergence and tsunami in the past 2000 years at the Copalis River estuary resembles that shown by buried soils farther south in coastal Washington at Grays Harbor and Willapa Bay. The history can be explained most simply by great (Mw of 8 or 9) thrust earthquakes at the boundary between the Juan de Fuca and North America plates, with one or two earthquakes occurring 1400–1900 years ago and another close to 300 years ago. The evidence for earthquake‐induced venting along the Copalis River consists of sand intrusions and vented‐sand volcanoes. The sand rose through muddy estuarine deposits at least 3 m thick, entraining these deposits in fragments as much as 0.3 m long. Intrusion occurred at one or more times in the past 2000 years, and at least some of the venting occurred 900–1300 years ago. The sand bodies do not imply artesian flow of flood‐pressured groundwater because the sand vented forcefully and infrequently through a lowland that probably lacked natural levees. The earthquake implied by the venting of sand 900–1300 years ago could have come from a source below, above, or at the boundary between the Juan de Fuca and North America plates. If from the subducted Juan de Fuca plate the implied earthquake was farther west or larger than the largest earthquakes within that plate during the past 100 years. Evidence permitting a source within the North America plate includes a fault scarp, an uplifted tideflat, and landslides near Puget Sound dating from the interval 700–1700 years ago. A plate boundary source would explain buried soils whose estuarine submergence and burial imply coseismic subsidence for parts of coastal Washington in the interval 600–1300 years ago. However, stratigraphy shows that little or no coseismic subsidence occurred along the Copalis River during the venting 900–1300 years ago. If a plate boundary earthquake struck coastal Washington 900–1300 years ago, the earthquake produced a different distribution of land level change than did the plate boundary earthquakes inferred for 1400–1900 and 300 years ago.
Investigation of laterally heterogeneous shear velocity structure in D″ beneath Eurasia
Authors: Gaherty James B. , Lay Thorne
Published: February 2008

Seismic waves are used to explore large‐scale radial structure and lateral heterogeneity of shear velocity in the D″ region, the chemical and thermal boundary layer at the base of the mantle. We analyze long‐period, horizontally polarized (SH) shear waveforms from earthquakes in the northwest Pacific recorded in Europe and the Middle East for evidence of proposed laterally varying shear velocity stratification in D″ beneath Eurasia. Specifically, our data sample the lower mantle extending from beneath the Arctic Ocean near Novaya Zemlya, USSR, to under northern India. Using a reflectivity technique to model the SH waveforms, we determine a locally stratified shear velocity model, SGLE, that provides a good average travel time and waveform fit to over 80% of the data sensitive to D″ structure. SGLE is characterized by a rapid 2.75% increase in shear velocity approximately 290 km above the core‐mantle boundary, similar to previously proposed models for this region. Unlike for other regions analyzed using similar waveform modeling techniques, the percentage of data inconsistent with this average radial model is fairly substantial, indicating that laterally heterogeneous D″ velocity structure is important in this region. Some waveforms lack the reflected arrival expected for a stratified model, while others show evidence of two arrivals, both possibly originating within D″. While our data are insufficient for direct inversion of the three‐dimensional structure, we utilize several simple parameterizations of the waveforms to gain some understanding of the region. Both the relative amplitude and differential travel time variations of the waveforms indicate that D″ beneath Eurasia is characterized by (1) broad regions in which the structure has large‐scale stratification giving coherent long‐period reflections, with superimposed velocity variations that scatter shorter‐wavelength waves; and (2) smaller regions which are sufficiently heterogeneous to scatter even the long‐period energy. The scale length of the lateral heterogeneity in D″ that produces these variations is inferred to be somewhat less than 500 km, which contrasts with weaker lateral heterogeneity that is observed in the central region of the lower mantle, where heterogeneity scale lengths of approximately 500–1000 km appear to dominate. These observations are consistent with a model of the lower mantle in which distributed heterogeneity is concentrated in D″, generating a locally stratified layer in some regions. The effects of a diffuse or topographically variable D″ discontinuity must be further explored using two‐ and three‐dimensional synthetic calculations before inversions for the detailed structure can be undertaken.
Importance of pressure solution and coble creep in the deformation of polymineralic rocks
Authors: Wheeler John
Published: February 2008

The theoretical framework for modeling pressure solution in polyphase rocks is presented. It is often assumed that pressure solution is a deformation mechanism of secondary importance deep in the lithosphere. It is shown here that the rheology of rock during pressure solution may be drastically reduced in a bimineralic and polymineralic rock relative to its monomineralic counterparts, and therefore the importance of this deformation mechanism importance is perhaps underestimated. This behavior contrasts with that of a polymineralic rock undergoing dislocation creep, where existing models show that the rheology will be some average of the rheologies of individual phases. For instance, the rheology of a dunite in pressure solution is governed by the diffusion of both MgO and SiO2; whichever is the slowest will control the viscosity of the rock. Conversely, in a harzburgite with a specific texture, flow will be governed by the fastest diffusing species; thus its strength may be as low as that of a quartz rock of comparable grain size. Given the comparative strength of many phases undergoing dislocation creep relative to quartz, this prediction implies a larger range of conditions under which pressure solution forms an important part of bulk rock deformation than inferred previously. Predictions on textural evolution of polymineralic aggregates are also made, and it is shown that some minerals grains may become elongate parallel to the maximum principal stress during deformation, even though the rock as a a whole shortens in this direction.
Fracture zones in the North Atlantic from combined Geosat and Seasat data
Authors: Müller R. Dietmar , Roest Walter R.
Published: February 2008

We present a new tectonic fabric map of the northern and central North Atlantic Ocean floor from 10°S to 66°N based on combined Geosat and Seasat altimetry data. Short wavelength vertical gravity along‐track was derived by Hilbert transforming and high‐pass filtering Geosat and Seasat deflection of the vertical profiles. The along‐track gravity data were used in conjunction with magnetic anomaly data to locate a large number of fracture zones as well as extinct spreading ridges and V‐shaped structures. We traced many fracture zones more continuously and closer to adjacent continental margins than was previously possible. We also mapped a number of unnamed fracture zones and fracture zone segments, many of which were previously uncharted. We have evaluated the agreement between the mapped fracture zones and synthetic flow lines as predicted from recent plate models for the opening of the North, central North and South Atlantic. Most fracture zones in this area correlate quite well with synthetic flow lines, supporting the proposition that Geosat/Seasat altimetry data serve as a useful tool to map fracture zones, where insufficient bathymetric data are available. Many fracture zones, including some small‐offset fracture zones in the central North Atlantic, have been stable and follow tectonic flow lines for tens of millions of years. However, many other fracture zones do not reflect a ″railroad track″ pattern of plate motions. The geometry of many small‐ and medium‐offset fracture zones indicates that the ridge axis discontinuities that produce them can migrate along the ridge, causing off‐axis traces that deviate from tectonic flow lines. Velocity‐space diagrams constructed using a recent relative and absolute present day plate motion model indicate that some low‐angle V‐shaped troughs and ridges are likely formed by migration of the plate boundary relative to a mesospheric reference frame. Other low‐angle V‐shaped structures reflect slow migrations/oscillations of small‐offset ridge axis discontinuities that are distinct from both tectonic flow lines and V‐shapes reflecting “absolute” plate motions. High‐angle V‐shaped structures are the result of fast migrating ridge axis discontinuities (propagating rifts).
Thermal conductivity measurements of depth hoar
Authors: Sturm Matthew , Johnson Jerome B.
Published: February 2008

The effective thermal conductivity of snow (keff), which includes latent heat transfer due to vapor diffusion, was measured during three winters in Fairbanks, Alaska. In 1986–1987, keff of several layers of snow was monitored in detail as the snow metamorphosed into depth hoar. Measurements were made using a needle probe with an estimated accuracy of ±8%; keff was found to decrease and then increase as the snow passed from new snow through several distinct stages of depth hoar. For depth hoar, keff ranged from 0.026 to 0.105, with an average value of 0.063 W m−1 K−1. This is one half to one fourth the value suggested by most studies for snow of similar density. For depth hoar of a given type, keff can be represented as a linear function of temperature between 0° and −20°C but requires a nonlinear function for the range from 0° to −196°C. At −196°C the thermal conductivity of depth hoar approached that of still air, suggesting that conduction through the ice skeleton of the snow was limited and that the increase in keff at temperatures near 0°C is the result of the strong temperature dependence of water vapor density. This conclusion is consistent with the nature of the ice bonds in depth hoar, which are thin and relatively few in number.
Upper crustal structure of the northeast Paraná Basin, Brazil, determined from integrated magnetotelluric and gravity measurements
Authors: Padilha Antonio L. , Trivedi Nalin B. , Vitorello Ícaro , da Costa José M.
Published: February 2008

Eight magnetotelluric (MT) soundings were performed as a continuation of an earlier audiomagnetotelluric (AMT) survey conducted in the northeastern border of the Paraná basin, a large intracratonic basin located in central eastern South America and constituted by Silurian to Jurassic sedimentary rocks with Lower Cretaceous sill‐type magmatic intrusions and overlying volcanics. Two of the MT stations were carried out near two deep petroleum wells. The remaining MT soundings were done on a profile traversing two important gravity features: a positive anomaly near the border of the states of São Paulo and Minas Gerais and a strong (trending NW‐SE) linear gradient. Major findings of an integrated interpretation of the MT survey and of available gravity data are as follows: (1) inhomogeneities and/or strong magnetization of the superficial volcanics and diabase intrusions in the Paleozoic sedimentary rocks appearing to distort the MT results; and (2) identification of important structural discontinuities, including a possible different crustal structure beneath the Paraná basin compared to the region on the north, a thickening of the crust toward the NE, and the probable existence of a trough (graben?) within the basin, characterized by a thick accumulation of sediments and basalts.
Paleomagnetic investigation of Upper Permian sediments in the South Junggar Basin, China
Authors: Sharps Robert , Li Yianping , McWilliams Michael , Li Yongan
Published: February 2008

Lacustrine organic‐rich mudstones of the Upper Permian Lucaogou Formation are a significant petroleum source bed throughout most of the Junggar basin in northwest China, and paleomagnetic data may provide insight into tectonic conditions prevalent during their deposition. Secondary magnetizations were removed from 180 samples from 29 sites within the South Junggar basin. Characteristic magnetizations of these samples were either revealed directly during thermal demagnetization or were calculated from great circle trajectories. The primary nature of the characteristic magnetization is attested to by a positive fold test, by exclusively reversed magnetizations which are consistent with the mid‐Carboniferous to Late Permian Kiaman reversed interval for the geomagnetic field, and by relatively low levels of thermal maturity found in the mudstones. Tectonically corrected remagnetization circles and direct observations combined give a steep direction (I = −61.9°, D = 163.0°, k = 17, n = 78 specimens). The steep mean inclination recorded within the lacustrine organic‐rich mudstones suggests that Junggar was situated near its present‐day latitude during deposition of the mudstones in Late Permian times, consistent with the marginal ocean basin origin for Junggar that has been proposed based upon stratigraphic relationships. Our data suggest that Junggar may always have been associated with the northern continents of Laurasia, initially as a pre‐Permian marginal ocean basin north of the Tethys and subsequently as a Permian, Mesozoic, and Cenozoic nonmarine foreland basin.
New Cambrian and Ordovician paleomagnetic poles for the North China Block and their paleogeographic implications
Authors: Zhao Xixi , Coe Robert S. , Liu Chun , Zhou Yaoxiu
Published: February 2008

Multicomponent magnetization has been revealed in several Cambrian and Ordovician sedimentary units from localities in Hebei and Shandong provinces of the North China Block (NCB). A well‐defined component with lower unblocking temperature is characterized by northerly declination and moderate to steep inclination with both polarities, and postdates the Late Cretaceous folding. The pole position derived from the overprinted directions is close to both the Cretaceous pole for the NCB as well as the present North Pole. The overprint is not a simple thermal remanent magnetization, but is probably a chemical remanent magnetization imposed at moderately elevated temperature during the major episode of regional deformation in the Late Mesozoic. The higher unblocking temperature characteristic components pass reversal, baked contact, and fold tests, and are internally consistent with rocks of different lithologies from widely distributed localities. Moreover, the data exhibit an overall similarity in the magnetic polarity pattern in the stratigraphic sequence at sampled localities. Hence the high unblocking temperature characteristic component was probably acquired at or near the time of deposition. The Cambrian and Ordovician paleopoles are somewhat different from the other coeval poles for the NCB, which are based on much less data and do not pass as many reliability tests, but all imply that the NCB was in the equatorial zone in the Early Paleozoic.
Paleomagnetism of Cretaceous rocks from Eastern Qiangtang Terrane of Tibet
Authors: Huang Kainian , Opdyke Neil D. , Li Jiguang , Peng Xingjie
Published: February 2008

Thermal demagnetization has successfully isolated high unblocking temperature characteristic directions from paleomagnetic samples of Lower and Middle Cretaceous redbed formations from eastern Qiangtang Terrane, Tibet. The tilt‐corrected mean direction for the Lower Cretaceous is D/I = 57.7°/50.2° with α95 = 10.9° and N = 12 sites, and that for the Middle Cretaceous is D/I = 39.7°/51.6° with α95 = 8.5° and N = 11 sites. Both formations pass the fold test, and the latter possesses two polarities. The magnetization predates the Eocene and was probably acquired during Cretaceous time. Comparison of the mean directions with expected directions from the stable interior of Eurasia indicates significant clockwise rotation but little paleolatitudinal change for the sampling area. Deformation and rotation of these rocks are presumably related to the collision of India with Asia and its subsequent northward indentation into Asia. Existing paleomagnetic data from Tibet and its environs seems to be compatible with a crustal shortening/thickening model (Dewey et al., 1988) in that the continental crust directly north of the Indian indenter was substantially shortened north‐south, whereas in the vicinity of the Western and Eastern Syntaxes, latitudinal displacement was minimal but tectonic rotation was very important. More data are needed to further differentiate between this model and the lateral eastward crustal extrusion model (Molnar and Tapponier, 1975).
Rapid static and kinematic global positioning system solutions using the ambiguity function technique
Authors: Mader Gerald L.
Published: February 2008

This paper describes an ambiguity function technique by which phase data from the Global Positioning System (GPS) maybe used to obtain survey positions for monuments occupied using a suitable GPS receiver. The principal benefit of the technique is the speed with which it may be used and its immunity to cycle slips or other discontinuities in the phase data that commonly occur. The technique uses phase measurements from satellites at both L1and L2frequencies from various directions in the sky. When sufficient measurements are available, the signals will destructively interfere at all but the correct location. Depending on the satellite geometry, the number of satellites in view, and the a priori knowledge of the station's position, a sufficient number of measurements may be obtained from a single monument occupation. The data collected over an occupation lasting only a few minutes maybe averaged to a single value at a particular epoch. Equivalently, the data collected at only a single epoch may also be used. The ability of the technique to work with data from a single epoch allows the integer ambiguities on moving platforms to also be determined. This will greatly enhance the practical operation of kinematic GPS for a variety of remote sensing applications. A computer program employing this ambiguity function technique has been written and used to process static as well as kinematic data. The results indicate horizontal precisions and accuracies of 1 cm or better and vertical precisions and accuracies of 1 to 3 cm. The data processing requires no data editing, is easy to use, and can be operated without any specialized knowledge of kinematic GPS techniques. The correction for the effects of the ionosphere are also demonstrated at distances up to 250 km. The use of the ambiguity function for geophysical monitoring and kinematic remote sensing applications appears limited in distance only by the accuracy of the GPS orbits. The utility of the ambiguity function for rapid static surveying and on‐the‐fly bias fixing out to distances of several hundred kilometers appears practical.
Seismicity and geometry of a 110‐km‐long blind thrust fault 1. The 1985 Kettleman Hills, California, earthquake
Authors: Ekström Göran , Stein Ross S. , Eaton J. P. , Eberhart‐Phillips D.
Published: February 2008

The August 4, 1985, Kettleman Hills earthquake was the third in a sequence of moderate shocks to occur beneath the northern half of a 110‐km‐long fold chain bounding the eastern California Coast Ranges. The 1982MW=5.4 New Idria, 1983MW=6.5 Coalinga, and 1985MW=6.1 Kettleman Hills events define a southward progression of seismic activity beneath the fold. We use teleseismic waveforms, geodetic modeling, hypocenters relocated in a three‐dimensional velocity model, and subsurface structural data to investigate the Kettleman Hills earthquake. The main shock results from motion on a shallowly dipping thrust fault buried at ∼10 km depth. Aftershocks and coseismic fault slip extend 20 km along the fold axis, nearly the full extent of the Kettleman Hills North Dome anticline. Aftershocks occur primarily several kilometers in front of the fault tip and in the core of the anticline. The main shock and several foreshocks occurred at a 2‐km right step in the Quaternary fold axis, which also corresponds to the southern end of the 1983 Coalinga and northern end of the 1985 Kettleman Hills aftershock zones. From this we infer that the step in the fold is caused by an offset or tear in the underlying fault. The scalar seismic moment is 1.6 × 1018N m, consistent with the geodetic deformation, and the duration of rupture is 16 s, 3–4 times greater than for the average earthquake with this scalar moment. The slow rate of moment release provides an explanation for the low level of ground shaking and low local magnitude reported for the event. The peak of the geodetic uplift is located 5 km perpendicular to the Quaternary fold axis. We argue that the fault is propagating northeast into the undeformed San Joaquin Valley sediments and that the overlying fold is growing at about 0.5 mm/yr.
Electric resistivity in the Gravberg‐1 Deep Well, Sweden
Authors: Pedersen L. B. , Juhlin C. , Rasmussen T. M.
Published: February 2008

The Siljan impact structure located in central Sweden has been the object of intense geophysical and geological studies for several decades. When The Swedish State Power Board decided to drill a deep borehole in the area, investigations increased dramatically around and in the hole. In this paper we present the results of electrical resistivity measurements on the surface, in the hole, and on core samples from shallow coreholes distributed over the whole impact structure. Magnetotelluric data constrain the central pan of the structure to be essentially onedimensional with an upper crustal unit of about 10000 Ωm followed by a less resistive impact related unit of 1000 Ωm from a depth of about 6 km to a depth of 20 km. The lower crust is a homogeneous unit of about 300 Ωm extending down to about 40 km, where the upper mantle is marked by an increase in resistivity of about 1000 Ωm. The borehole resistivity log extends down to about 6100 m, i.e., the expected depth where the resistivity drops by an order of magnitude. Major fracture zones, some of which coincide with the horizontal dolente sills, known to extend over tens of kilometers, are probably the main carriers of current. Fracture zones are typically 10m thick surrounded by more intact rock masses, which show a general tendency of increasing resistivity with depth. In fracture zones the electrical resistivity lies between 1000 and 5000 Ωm, with the interesting trend that below 4000 m depth the resistivity is smaller than in the fracture zone in the depth interval 2000–4000 m. Saline fluids recovered during testing below 5.4‐km depth and the magnetotelluric results lead us to conclude that pore fluids are highly saline below this depth even on a regional scale, and the zone of decreased resistivity found in the upper crust within the Siljan ring structure is interpreted as a zone of increased porosity relative to outside the ring.
Rock magnetism of Late Neogene and Pleistocene deep‐sea sediments: Relationship to sediment source, diagenetic processes, and sediment lithology
Authors: Bloemendal J. , King J. W. , Hall F. R. , Doh S.‐J.
Published: February 2008

We use bivariate scatter plots to illustrate variations in selected rock magnetic properties (low‐field susceptibility, anhysteretic and isothermal remanence) of late Neogene and Pleistocene deep‐sea sediments from 16 sites in the Arctic Ocean, North Atlantic, equatorial Atlantic and North Pacific Ocean, and the Arabian Sea. Our intention is to examine the ability of the rock magnetic properties to differentiate the sediments according to factors such as lithology, geographical area, and the dominant mode of terrigenous sedimentation, which at these sites is via ice‐rafting, via bottom currents, or via eolian processes. Overall, correlations between sediment magnetic properties and gross lithology is poor, and factors such as the source and transport path of terrigenous sediment (and detrital magnetic minerals), together with the action of reductive diagenetic processes, are the major controls on the magnetic properties. On the bivariate scatter plots, sites with major ice‐rafted contributions tend to have high sedimentary ferrimagnetic mineral concentrations, relatively coarse ferrimagnetic grain‐sizes, and scattered sample point distributions; in contrast, sites where we infer significant bottom‐current supply of terrigenous material have tightly grouped sample point distributions. Carbonate sediments in which the terrigenous component is supplied by eolian processes tend to have a broad range of magnetic mineral concentration, caused by glacial‐interglacial fluctuations in carbonate accumulation and eolian activity. Sediments containing significant volcanogenic material have high concentrations of relatively coarse‐grained ferrimagnetic material. Reductive diagenesis is a significant determinant of sediment magnetic properties in high‐productivity areas and has the effect of preferentially removing the fine‐grained ferrimagnetic fraction, causing a coarsening of the ferrimagnetic grain‐size distribution and a rise in the antiferromagnetic:ferrimagnetic ratio.
Optical properties of the Earth's surface and long‐term perturbations of LAGEOS's semimajor axis
Authors: Lucchesi David , Farinella Paolo
Published: February 2008

We have reproduced the numerical model of Rubincam et al. (1987) for estimating the maximum along‐track orbit‐averaged perturbative acceleration 〈T〉 on LAGEOS's orbit due to radiation pressure from sunlight anisotropically reflected by the oceans. For the two optical models discussed by Rubincam et al. we have obtained 〈T〉 = 1.17 and 2.79 × 10−12 m/s2. The latter value is about 3 times larger than the corresponding result of Rubincam et al., who made an error, and shows that a reasonable reflection model can give acceleration peaks of the same order as those observed in the residuals of LAGEOS's orbit. Changing the parameters appearing in the ocean reflection law (in particular, the width of the cone about the specular reflection direction which contains a significant amount of radiation) results into peak values of 〈T〉 ranging between 0 and ≈5 × 10−12 m/s2. The “extreme” assumptions of Rubincam et al. (polar orbit, cloudless Earth surface with a diffusive continent in the northern hemisphere, and a partially reflective ocean in the southern one) are found to overestimate 〈T〉 with respect to more realistic models by a factor of the order of, but probably smaller than, 2. The numerical error due to the finite number of Earth surface elements used to compute the radiation force does not exceed a few percents, provided ≈100 uniformly distributed elements are used; a comparable numerical error is involved in the integral over the orbital anomaly if a step of 5° is adopted. Since the largest observed fluctuations in the anomalous semimajor axis decay of LAGEOS correspond to values of 〈T〉 ≈ 3 × 10−12 m/s2, we conclude that radiation pressure from Earth‐reflected sunlight cannot be ruled out as an important contributor to them. However, a reliable quantitative model of this effect appears to require a rather detailed knowledge of the optical behavior of the oceans. At present, this problem somewhat degrades our capability to predict (or model in a deterministic way) LAGEOS's orbital evolution over long spans of time.
The Santa Monica Basin tracer experiment: A study of diapycnal and isopycnal mixing
Authors: Ledwell James R. , Watson Andrew J.
Published: February 2008

We have studied cross isopycnal (diapycnal) and lateral mixing and stirring below the sill of Santa Monica Basin by releasing two tracers, sulfur hexafluoride and perfluorodecalin, as close as possible to an isopycnal surface and measuring their subsequent dispersion. The target for the release was a potential temperature surface at about 790 m depth, roughly 100 m above the bottom and 50 m below the sill. Three surveys, performed immediately after, about 7 weeks after, and about 6 months after the release, showed that the time scales for lateral stirring and mixing in the basin were between 2 and 5 months. The diapycnal diffusivity for the whole period was found to be 0.29±0.06cm2/s near the injection surface, where the buoyancy frequency was about 1.1 cph. This estimate may include some mixing in the turbulent boundary layer near the walls of the basin. Our best estimate for the diapycnal diffusivity in the basin interior is 0.25±0.08 cm2/s.
Generation and spreading of a turbulent mixed layer in a rotating, stratified fluid
Authors: Davies Peter A. , Fernando Harindra J. S. , Besley Phillip , Simpson Richard J.
Published: February 2008

Results are presented from an experimental investigation into the generation and growth of a turbulent mixed layer above and below an oscillating horizontal grid in a rotating stratified fluid. The (square) grid was located at middepth at one end of a long rectangular container, and was confined on three of its four sides by the vertical walls of the container. Flow visualization data are presented to show (1) the rapid initial vertical growth of the mixed layer above and below the grid, (2) the collapse of the mixed layer and the generation of an intrusive flow into the initially undisturbed interior of the fluid, (3) the deflection of the intrusive flow toward the right side wall, and (4) the formation of a boundary current, which circulates around the container before returning mixed fluid to the source region at the grid. Examples of stable and unstable boundary currents are illustrated. Quantitative data are presented to show that the thickness of the mixed layer increases with either decreasing buoyancy frequency, and/or increasing grid frequency, throughout all phases of its development. The influence of background rotation upon the vertical growth of the mixed layer is shown to be negligible during the rapid initial vertical expansion of the layer, though this is not the case in the longer‐term behavior. For this latter phase, the layer thickness increases with increasing rotation rate of the system, under otherwise identical conditions. Measurements of the initial equilibrium thicknesshcof the layer show that it scales with the parameter (K/N)½, whereKis the grid action, andNis the buoyancy frequency of the stratified fluid. Values of the dimensionless parameter [hc/(K/N)½] show no significant dependence upon the ratioN/2Ω of the buoyancy and inertial frequencies, for a range of values of the latter between 0.4 and 29.7. This behavior is shown to be compatible with the result (Hopfinger, 1987) that shear‐free turbulence is dominated by background rotation only if the Rossby number based on the rms velocity and integrated length scale is less than 0.2.
New vertical modes for dissipative stratified circulations: With application to coastal upwelling
Authors: Gardiner‐Garden Robert S.
Published: February 2008

The method of vertical mode expansions for inviscid time‐dependent motions is simple and well known (see Gill, 1982). The method simplifies the description of three‐dimensional stratified circulation by representing each circulation variable as a series expansion of orthogonal functions ofz(the vertical coordinate). Several papers, for example, McCreary (1981) and McCreary and Chao (1985), have represented wind‐forced coastal upwelling in terms of such inviscid modes, even when the circulation is viscous. By using the inviscid modes, however, these papers were unable to represent the sea surface density anomaly produced by the wind stress. Instead of persisting with the vertical modes of the inviscid system, this paper derives new viscous vertical modes which have the advantage of allowing nonzero surface density anomalies, produced by the vertical diffusion of density anomalies below the surface.
Waves in frazil and pancake ice and their detection in Seasat synthetic aperture radar imagery
Authors: Wadhams Peter , Holt Benjamin
Published: February 2008

A theoretical model of waves propagating into an ice cover composed of frazil and pancake ice is developed and compared with measurements of wavelength and direction derived from synthetic aperture radar (SAR) imagery obtained from Seasat in October 1978. The theoretical model is based on the concept that ice of these types, which consists of small crystals or cakes, has only a mass‐loading effect on the water surface. We derive the dispersion relation for phase and group velocities, finding that there is an upper frequency limit for propagation into the ice. From the reflection coefficient at the ice edge we derive the wave radiation pressure exerted on the ice, showing that it will cause a slick of frazil ice backed by thicker floes to become more dense or thick with increasing penetration. The implications for radar scattering enabling detection on SAR are that the Bragg resonant wavelength corresponds to waves above the frequency limit for propagation, so that a frazil slick appears dark on an SAR image. When the frazil ice becomes transformed into pancake ice, through slick compression or other means, the raised edges of the pancakes cause the ice to appear bright despite the fact that there are no waves present at the Bragg wavelength. These results are applied to a Seasat SAR image obtained from the Chukchi Sea. The appearance of the ice in the image corresponds to what we expect for frazil ice gradually transforming itself into pancake ice, backed by thicker floes. We derive directional wave number spectra outside and inside the ice cover by digital Fourier analysis of image subscenes, and we find that the change of wavelength and angle of refraction of the dominant wave entering the ice field are both characteristic of the dispersion relation derived theoretically. Mean ice thicknesses extracted from the theory correspond to thicknesses expected for such slicks. The technique offers a possible means of extracting the thickness of fields of frazil and pancake ice from SAR imagery; this may be of considerable utility when ERS 1 SAR is used to study the advancing winter ice edge in the Antarctic, which consists of vast areas of these ice types.
Small air bubbles in reagent grade water and seawater: 2. Dissolution of 20‐ to 500‐μm‐diameter bubbles at atmospheric pressure
Authors: Harris Isaac A. , Detsch Richard M.
Published: February 2008

The dissolution rates for 20‐ to 500‐μm‐diameter air bubbles in water and seawater at 50% and 100% air saturation are presented. The data indicate a dissolution rate that is independent of diameter for bubbles larger than ∼70 μm but is strongly dependent on diameter for smaller bubbles. Rise velocity data from a companion study were used in conjunction with Levich's “dirty bubble” formula to predict dissolution rates for the bubbles. There is good agreement with theory for bubbles in saturated liquids and for bubbles with diameters larger than 70 μm. Agreement is fair for smaller bubbles in 50% saturated water and seawater.
Laboratory studies of radar sea spikes at low grazing angles
Authors: Trizna D. B. , Hansen J. P. , Hwang Paul , Wu Jin
Published: February 2008

We report on measurements of low grazing angle radar sea spikes in a laboratory wave tank. Using anXband radar with 10‐cm range resolution and multiple‐range bin capability, the sea spikes were observed to have a horizontal/vertical polarization ratio much larger than unity. This is similar to observations in earlier field experiments, but is in contrast to a ratio of unity recently observed for moderate grazing angles (Jessup et al., 1990). Our experiment was designed to validate the low grazing field results and to establish design criteria for spatial‐temporal measurement instrumentation; single‐point measurements currently used as surface truth cannot provide water surface profiles needed for modeling of radar scatter from deterministic surface features. Surface scattering features were generated using a paddle‐generated wave, combined with wind waves for a series of wind speeds. The surface feature responsible for the sea spike echo was visually identified as a short wave crest, occurring on the windward side of the paddle wave, driven to extreme steepness by the passage of the faster paddle wave crest. Long decorrelation times observed in the field are also seen in these studies, of the order of 500 ms for horizontal polarization and less than 5 ms for vertical. We suggest that rather than Bragg scatter, the occurrence of similar ubiquitous features on the ocean are responsible for horizontally polarized low grazing angle sea scatter at theXband for low to moderate winds, and perhaps at other frequencies as well.
Ice motions forced by boundary layer turbulence
Authors: DiMarco R. L. , Dugan J. P. , Martin W. W.
Published: February 2008

The motions of ice on frozen oceans in the frequency band from 0.05 to 10.0 s−1 have typically been associated with surface gravity waves which were generated by distant storms in the open ocean and which then propagate into the ice. Evidence relating these motions to local wind forcing has been less direct. Data on ice motions have been obtained with tilt meters on a land‐locked, frozen lake, and the motions are shown to be directly related to forcing by the local wind. The variance of ice surface tilt increased by more than 3 orders of magnitude when the mean wind speed increased by a factor of less than 2, even when the wind speed remained below the minimum phase speed for freely propagating waves. A model is presented in which ice motions result from an interaction between turbulent eddies in the atmosphere and the ice surface. Model predictions are shown to be consistent with the lake observations.