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Caltech Authors (82.845 recursos)
Repository of works by Caltech published authors.

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Mostrando recursos 1 - 20 de 32.148

1. Homogeneous vs heterogeneous subduction zone models: Coseismic and postseismic deformation - Masterlark, T.; DeMets, C.; Wang, H. F.; Sánchez, O.; Stock, J.
A finite-element model (FEM) incorporating geologic properties characteristic of a subduction zone is compared with FEMs approximating homogeneous elastic half-spaces (HEHS)s to investigate the effect of heterogeneity on coseismic and postseismic deformation predictions for the 1995 Colima-Jalisco M_w =8.0 earthquake. The FEMs are used to compute a coefficient matrix relating displacements at observation points due to unit dislocations of contact-node pairs on the fault surface. The Green's function responses are used to solve the inverse problem of estimating dislocation distributions from coseismic GPS displacements. Predictions from the FEM with heterogeneous material properties, loaded with either of the HEHS dislocation distributions,...

2. Slip kinematics and dynamics during and after the 1995 October 9 M_w=8.0 Colima–Jalisco earthquake, Mexico, from GPS geodetic constraints - Hutton, W.; DeMets, C.; Sánchez, O.; Suárez, G.; Stock, J.
We use horizontal and vertical crustal displacements derived from GPS measurements at 26 sites in western Mexico to study the coseismic and post-seismic kinematics and dynamics of the 1995 October 9 (M_w=8.0) Colima–Jalisco earthquake along the Middle America Trench. The measurements bracket the entire landward edge of the approximately 150 km long rupture zone and span a 4 yr period for most sites. We solve for the temporal evolution of slip along the subduction interface by inverting GPS displacements for the coseismic and four post-seismic intervals (March 1995–March 1999), subject to the assumption that the crust responds elastically to slip...

3. Lead concentrations in the northeast Pacific: evidence for global anthropogenic perturbations - Schaule, Bernhard K.; Patterson, Clair C.
Lead concentrations were determined by isotope dilution mass spectrometry in 34 surface- and deep-water samples collected in the northeast Pacific between Hawaii and California and off the California coast using a deep-water sampler protected against fouling by contamination from the ship and hydrowire. Measured concentrations lie more than one order of magnitude below previously published open ocean values and they show that in most cases 90% or more of the total lead is in a dissolved form. Lead concentrations are about 10-fold higher in surface and thermocline waters than in deep waters; values drop as low as 1 ng/kg (5...

4. Winter Frost at Viking Lander 2 Site - Svitek, Thomas; Murray, Bruce
A key question in the study of Mars is water exchange between atmosphere and surface on daily, seasonal, and astronomical timescales. We believe that small-scale processes are a key for enhanced understanding of the global water behavior of Mars. The principal data for this study of small scale properties of the Martian surface were collected by the second Viking lander (VL 2) and by both Viking orbiters. The annual deposition and retreat of the frost layer were observed in situ by VL 2. The frost is inferred to be H_2O frost but with some properties suggesting a much thicker layer...

5. Postseismic Rebound in Fault Step-Overs Caused by Pore Fluid Flow - Peltzer, Gilles; Rosen, Paul; Rogez, Francois; Hudnut, Ken
Near-field strain induced by large crustal earthquakes results in changes in pore fluid pressure that dissipate with time and produce surface deformation. Synthetic aperture radar (SAR) interferometry revealed several centimeters of postseismic uplift in pull-apart structures and subsidence in a compressive jog along the Landers, California, 1992 earthquake surface rupture, with a relaxation time of 270 ± 45 days. Such a postseismic rebound may be explained by the transition of the Poisson's ratio of the deformed volumes of rock from undrained to drained conditions as pore fluid flow allows pore pressure to return to hydrostatic equilibrium.

6. Rapid localization of Pacific–North America plate motion in the Gulf of California - Oskin, Michael; Stock, Joann; Martín-Barajas, Arturo
Correlation of late Miocene volcaniclastic strata across the northern Gulf of California shows that the Pacific–North America plate boundary localized east of the Baja California peninsula ca. 6 Ma. Dextral offset of the 12.6 Ma Tuff of San Felipe and a pair of overlying ca. 6.3 Ma pyroclastic flows indicate at least 255 ± 10 km of displacement along an azimuth of 310°. Isopach and facies trends of the Tuff of San Felipe support no more than a few tens of kilometers of additional dextral displacement between 12.6 and 6.3 Ma. These constraints indicate that nearly all of the dextral...

7. Lead in Albacore: Guide to Lead Pollution in Americans - Settle, Dorothy M.; Patterson, Clair C.
Lead contamination in canned tuna, exceeding natural concentrations 10,000-fold, went undiscovered for decades because of analytical error. The magnitude of this pollution effects helps explain the difference between the lead concentration in the diets of present-day. Americans (0.2 part per million) and in the diets of prehistoric peoples (estimated to be less than 0.002 part per million). It also explain how skeletal concentrations of lead in typical Americans became elevated 500-fold above the natural concentrations measured in bones of Peruvian's who lived in a unpolluted environment 1800 years ago. It has been tacitly assumed that natural biochemical effects of lead...

8. Compression directions in southern California (from Santa Barbara to Los Angeles Basin) obtained from borehole breakouts - Wilde, Melita; Stock, Joann
Borehole elongation in 71 drill holes was used to infer breakout orientation and directions of maximum horizontal principal stress S_H for six areas west of the San Andreas fault in southern California: Santa Barbara, Ojai, Central Ventura Basin, East Ventura Basin, West Los Angeles Basin, and East Los Angeles Basin. Breakouts were determined from analysis of oriented four-arm caliper data. The breakouts form at the position of the maximum compressive stress on the borehole wall; if the borehole is vertical and parallel to one of the principal stress directions, the breakouts will form parallel to the minimum horizontal principal stress...

9. Thermal studies of Martian channels and valleys using Termoskan data - Betts, Bruce H.; Murray, Bruce C.
The Termoskan instrument on board the Phobos '88 spacecraft acquired the highest spatial resolution thermal infrared emission data ever obtained for Mars. Included in the thermal images are 2 km/pixel, midday observations of several major channel and valley systems including significant portions of Shalbatana, Ravi, Al-Qahira, and Ma'adim Valles, the channel connecting Valles Marineris with Hydraotes Chaos, and channel material in Eos Chasma. Termoskan also observed small portions of the southern beginnings of Simud, Tiu, and Ares Valles and some channel material in Gangis Chasma. Simultaneous broadband visible reflectance data were obtained for all but Ma'adim Vallis. We find that...

10. REE Variations Across the Peninsular Ranges Batholith: Implications for Batholithic Petrogenesis and Crustal Growth in Magmatic Arcs - Gromet, L. Peter; Silver, Leon T.
Rare earth element (REE) patterns of plutonic rocks across the Cretaceous Peninsular Ranges batholith vary systematically west to east, transverse to its long axis and structural trends and generally parallel to asymmetries in petrologic, geochronologic and isotopic properties. The batholith can be divided into three distinct parallel longitudinal regions, each defined by distinct REE pattern types. An abrupt transition occurs between rocks with slightly fractionated REE patterns in the western (coastal) region and rocks with middle to heavy REE fractionated and depleted patterns in the central region. Further to the east a second transition to strongly light REE enriched rocks...

11. Evidence That the Reactivity of the Martian Soil Is Due to Superoxide Ions - Yen, A. S.; Kim, S. S.; Hecht, M. H.; Frant, M. S.; Murray, B.
The Viking Landers were unable to detect evidence of life on Mars but, instead, found a chemically reactive soil capable of decomposing organic molecules. This reactivity was attributed to the presence of one or more as-yet-unidentified inorganic superoxides or peroxides in the martian soil. Using electron paramagnetic resonance spectroscopy, we show that superoxide radical ions (O_(2)–) form directly on Mars-analog mineral surfaces exposed to ultraviolet radiation under a simulated martian atmosphere. These oxygen radicals can explain the reactive nature of the soil and the apparent absence of organic material at the martian surface.

12. Behavior of the Superstition Hills fault during the past 330 years - Hudnut, K. W.; Sieh, K. E.
We have investigated the recent prehistoric behavior of the Superstition Hills fault by examining its effect on the beach deposits of ancient Lake Cahuilla. Excavation of these sediments in three dimensions where they are cut by the fault has enabled determination of total offset since the latest highstand of the lake, about 330 years ago. As of 3 March 1988, total dextral offset was 1106 ± 50 mm. About 609 mm of this amount can be attributed to one or more slip events before 1987. The remaining slip occurred during the moderate earthquake of 24 November 1987 and as subsequent...

13. Evidence for prehistoric earthquakes on the Superstition Hills fault from offset geomorphic features - Lindvall, Scott C.; Rockwell, Thomas K.; Hudnut, Kenneth W.
Offset geomorphic features along the Superstition Hills fault show evidence for at least one slip event prior to the 1987 surface rupture, and possibly as many as four to five earlier prehistoric earthquakes. We documented several geomorphic features that appeared offset by multiple events by making detailed topographic maps. Offset features were abundant along reaches of the fault with high topographic relief and large displacement. Slip distribution for the penultimate event, as recorded by offset rills, streams, and shrub-coppice dunes, is very similar to the slip distribution from the 1987 earthquake through April 1988. This similarity may prove to be...

14. Slip on the Superstition Hills fault and on nearby faults associated with the 24 November 1987 Elmore Ranch and Superstition Hills earthquakes, southern California - McGill, Sally F.; Allen, Clarence R.; Hudnut, Kenneth W.; Johnson, David C.; Miller, Wayne F.; Sieh, Kerry E.
Alignment arrays and creepmeters spanning several faults in southern California recorded slip associated with the 24 November 1987 Elmore Ranch and Superstition Hills earthquakes. No precursory slip had occurred on the Superstition Hills fault up to 27 October 1987, when the last measurement before the earthquakes was made. About 23 days before the earthquake, dextral creep events of about 13 mm and 0.5 mm may have occurred simultaneously on the Imperial and southern San Andreas faults, respectively, but the tectonic origin of the smaller event is questionable. Within 12 hr after the Superstition Hills earthquake, 20.9 cm of dextral slip occurred...

15. Geodesy tracks plate motion - Hudnut, Kenneth W.
Our current understanding of the large scale movements of the Earth's great crustal plates is based mainly upon offsets of old geological features on land as well as on the ocean floor.

16. Cenozoic motion between East and West Antarctica - Cande, Steven C.; Stock, Joann M.; Müller, R. Dietmar; Ishihara, Takemi
The West Antarctic rift system is the result of late Mesozoic and Cenozoic extension between East and West Antarctica, and represents one of the largest active continental rift systems on Earth. But the timing and magnitude of the plate motions leading to the development of this rift system remain poorly known, because of a lack of magnetic anomaly and fracture zone constraints on seafloor spreading. Here we report on magnetic data, gravity data and swath bathymetry collected in several areas of the south Tasman Sea and northern Ross Sea. These results enable us to calculate mid-Cenozoic rotation parameters for East...

17. Improved techniques for studies of mass balances and fractionations among families of metals within terrestrial ecosystems - Elias, Robert W.; Hinkley, Todd K.; Hirao, Yoshimitsu; Patterson, Clair C.
From a remote, pristine study area the authors collected and analyzed samples for K, Rb, Cs, Ca, Sr, Ba and Pb. The results show that heavy metals move through natural ecosystems in a predictable manner relative to lighter elements of the same periodic family. Four developments which refine these measurements and enhance the interpretation of the results are: (1) a technique for collecting a soil moisture film; (2) a device for the collection of dry deposition of aerosols; (3) a procedure for the removal of foliar deposits from the surface of plant leaves; (4) a method of collecting a snow sample representative of the...

18. Arc-rift transition volcanism in the Puertecitos Volcanic Province, northeastern Baja California, Mexico - Martín-Barajas, Arturo; Stock, Joann M.; Layer, Paul; Hausback, Brian; Renne, Paul; López-Martínez, Margarita
The Neogene Puertecitos Volcanic Province of northeastern Baja California records a transition from arc-related volcanic activity to rift volcanism associated with opening of the Gulf of California. The eastern Puertecitos Volcanic Province is divided into three volcanic sequences based on mapping, petrology, and ^(40)Ar/^(39)Ar geochronology. The lowest sequence comprises early to middle Miocene (20–16 Ma) arc-related andesitic lava flows, volcanic necks, and proximal pyroclastic and epiclastic deposits up to 400 m in thickness, with minor basaltic lava flows. Following the initiation of crustal extension in the region (11–6 Ma), synrift volcanism produced two rhyolitic sequences that discordantly overlie the arc-related...

19. Discussion: Salmon on Explanatory Relevance - Hitchcock, Christopher Read
One of the motivations for Salmon's (1984) causal theory of explanation was the explanatory irrelevance exhibited by many arguments conforming to Hempel's covering-law models of explanation. However, the nexus of causal processes and interactions characterized by Salmon is not rich enough to supply the necessary conception of explanatory relevance. Salmon's (1994) revised theory, which is briefly criticized on independent grounds, fares no better. There is some possibility that the two-tiered structure of explanation described by Salmon (1984) may be pressed into service, but more work would have to be done. Ironically, Salmon's difficulties are similar to those suffered by his seventeenth-century predecessors.

20. Causal Explanation and Scientific Realism - Hitchcock, Christopher Read
It is widely believed that many of the competing accounts of scientific explanation have ramifications which are relevant to the scientific realism debate. I claim that the two issues are orthogonal. For definiteness, I consider Cartwright's argument that causal explanations secure belief in theoretical entities. In Section I, van Fraassen's anti-realism is reviewed; I argue that this anti-realism is, prima facie, consistent with a causal account of explanation. Section II reviews Cartwright's arguments. In Section III, it is argued that causal explanations do not license the sort of inferences to theoretical entities that would embarass the anti-realist. Section IV examines...

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