Search USGS⌕ Search

SEARCH · Search USGS

Results for “Journal of Geophysical Research - Planets”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,261 records · Page 70Linked to original sources
USGS2012

Geomagnetic local and regional harmonic analyses

Procedures are developed for using rectangular and cylindrical harmonic analyses in local and regional areas. Both the linear least squares analysis, applicable when component data are available, and the nonlinear least squares analysis, applicable when only total field data are available, are treated. When component data are available, it is advantageous to work with residual fields obtained by subtracting components derived from a harmonic potential from the observed components. When only total field intensity data are available, they must be used directly. Residual values cannot be used. Cylindrical harmonic analyses are indicated when fields tend toward cylindrical symmetry; otherwise, rectangular harmonic analyses will be more advantageous. Examples illustrating each type of analysis are given.

Journal of Geophysical Research Solid Earth↗
USGS2012

The Yellowstone-Snake River Plain seismic profiling experiment: Crustal structure of the Eastern Snake River Plain

Seismic refraction profiles recorded along the eastern Snake River Plain (ESRP) in southeastern Idaho during the 1978 Yellowstone-Snake River Plain cooperative seismic profiling experiment are interpreted to infer the crustal velocity and attenuation (Q-1) structure of the ESRP. Travel-time and synthetic seismogram modeling of a 250 km reversed refraction profile as well as a 100 km detailed profile indicate that the crust of the ESRP is highly anomalous. Approximately 3 to 6 km of volcanic rocks (with some interbedded sediments) overlie an upper-crustal layer (compressional velocity ≅6.1 km/s) which thins southwestward along the ESRP from a thickness of 10 km near Island Park Caldera to 2 to 3 km beneath the central and southwestern portions of the ESRP. An intermediate-velocity (≅6.5 km/s) layer extends from ≅10 to ≅20 km depth. A thick (≅22 km) lower crust of compressional velocity 6.8 km/s, a total crustal thickness of ≅42 km, and a P n velocity of ≅7.9 km/s is observed in the ESRP, similar to the western Snake River Plain and the Rocky Mountains Provinces. High attenuation is evident on the amplitude corrected seismic data due to low-Q values in the volcanic rocks (Q p = 20 to 200) and throughout the crust (Q p = 160 to 300). Based on these characteristics of the crustal structure and volcanic-age progression data, it is suggested that the ESRP has resulted from an intensive period of intrusion of mantle-derived basaltic magma into the upper crust generating explosive silicic volcanism and associated regional uplift and caldera collapse. This activity began about 15 m.y. ago in southwestern Idaho and has migrated northeast to its present position at Yellowstone. Subsequent cooling of the intruded upper crust results in the 6.5 km/s velocity intermediate layer. Crustal subsidence and periodic basaltic volcanism as represented by the ESRP complete the sequence of crustal evolution.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Terranes and suture zones in east central Alaska

East central Alaska, with its 17 terranes, forms a part of the mosaic of allochthonous terranes that join the North American and Siberian plates. These terranes range from continental and continental margin, such as the Tatonduk with its thick well-bedded succession of marine shelf rocks, to seamount, arc, and ocean floor terranes. The Yukon crystalline terrane, the largest described here, is a composite of at least four subterranes juxtaposed across the Tintina fault with the Tatonduk terrane, a northwestern extension of the North American plate in Alaska. Inboard of the Yukon crystalline terrane are packets of closely appressed microterranes separated from the Tatonduk and other terranes belonging to North America by major suture zones. These microterranes lie between North America and the mosaic of accretionary terranes that form the more southerly part of Alaska. The most obviously allochthonous microterranes within the suture zones are the Woodchopper Canyon, an Early Devonian basaltic seamount, and the White Mountains, an Ordovician volcanic arc terrane capped by Silurian and Devonian carbonate bank deposits. The nearest counterpart of these terranes is the Alexander terrane in southeastern Alaska. The Tintina fault of Mesozoic and Cenozoic age, like the Denali fault, primarily follows old suture zones that separate terranes. Strike slip faulting developed after collision in places where further convergence was oblique to the terrane margins. Where terranes met head-on, their leading edges lie along a multiple set of high-angle faults that outline microterranes in accretion zones.

Alaska↗
USGS2012

Post 12 m.y. rotation of southwest Washington

Paleomagnetic field directions from the basalt of Pack Sack Lookout are compared to those from the Pomona Member of the Saddle Mountains Basalt of the Columbia River Basalt Group. The Pomona crops out over a wide region on the Columbia Plateau east of the Cascade Range, and the basalt of Pack Sack Lookout crops out well to the west of the Cascades about 30 to 60 km east of the Washington coast. Our paleomagnetic results support existing geologic and geochemical evidence that both these units are part of a single 12‐m.y.‐old flow that erupted in western Idaho and flowed to the Washington coast. The magnetic data further indicate that southwest Washington has undergone clockwise rotation with respect to the Columbia Plateau and stable North America. The data suggest that both a large‐scale regional rotation of ∼16° and locally complex small‐scale rotations exist, the two being present to different degrees in the eastern and western regions of southwest Washington. The Cascade Range appears to coincide with the tectonic boundary, separating rotated and unrotated regions of Washington state. Rotation of southwest Washington appears to have been associated with the rotation of large pieces of western Oregon and southern Washington, possibly as a result of either a ridge‐push force from the Basin and Range province or shear along the Pacific‐North America plate boundary.

Washington↗
USGS2012

The emplacement of ophiolites by collision

Ophiolites, recognized in most of the world's orogenic belts, are generally interpreted to be oceanic crust and upper mantle (lithosphere) fragments that have been incorporated into continental margins at consuming plate boundaries. We suggest that the mechanism for ophiolite emplacement is the same in both the Alpine and Andean-type orogenes. In both geological settings, obduction of oceanic lithosphere onto the continental lithosphere is caused by the convergence of light, buoyant bodies such as oceanic plateaus, continental slivers, island arcs, or old hot spot traces. For example, the Troodos ophiolite complex, previously interpreted by some workers as resulting from continental collision, may have been emplaced by the collision of Cyprus with the Eratosthenes Plateau embedded in the oceanic eastern Mediterranean crust. On the other hand, the Upper Jurassic Coast Range Ophiolites of California, previously interpreted as resulting from typical oceanic subduction, may be the result of a continuous injection of thick nonsubductable packages of light, continentally derived sedimentary rocks, seamounts, and plateaus into the subduction zones. Many other ophiolite complexes may be similarly related to accreted terranes.

Journal of Geophysical Research↗
USGS2012

Source parameters of the 1980 Mammoth Lakes, California, earthquake sequence

From the more than 1500 Mammoth Lakes earthquakes recorded on three-component digital seismographs (Spudich et al., 1981), 150 were used in an analysis of the locations, mechanism, and source parameters. A composite fault plane solution of nine earthquakes 3.9 ≤ M ≤ 5.1 defines a right-lateral strike slip mechanism on a steeply dipping nearly east-west plane striking S75°E or left-lateral strike slip on a nearly north-south plane striking N10°E. Vertical cross sections of well-located aftershocks indicate possibly three east-west planes that coincide with the locations of the four largest earthquakes with M L ≥ 6.0. Using the spectral analysis of S waves (Brune, 1970), source parameters for 67 earthquakes were determined. Forty-eight had magnitudes greater than or equal to 3.0. Seismic moments ranged from 9.20×10 18 dyn cm to 2.33×10 24 dyn cm. Earthquakes with seismic moment greater than about 1.0×10 21 dyn cm had nearly constant stress drops (≃ 50 bars); earthquakes with seismic moment less than about 1.0×10 21 dyn cm had stress drops that apparently decrease as seismic moment decreases.

California↗
USGS2012

Uranium-lead isotopic ages from the Sierra Nevada Batholith, California

This study provides new information on the timing and distribution of Mesozoic magmatic events in the Sierra Nevada batholithic complex chiefly between 36° and 37°N. latitude. U-Pb ages have been determined for 133 zircon and 7 sphene separates from 82 samples of granitoid rocks. Granitoid rocks in this area range in age from 217 to 80 m.y. Triassic intrusions are restricted to the east side of the batholith; Jurassic plutons occur south of the Triassic plutons east of the Sierra Nevada, as isolated masses within the Cretaceous batholith, and in the western foothills of the range; Cretaceous plutons form a continuous belt along the axis of the batholith and occur as isolated masses east of the Sierra Nevada. No granitic intrusions were emplaced for 37 m.y. east of the Sierra Nevada following the end of Jurassic plutonism. However, following emplacement of the eastern Jurassic granitoids, regional extension produced a fracture system at least 350 km long into which the dominantly mafic, calc-alkalic Independence dike swarm was intruded 148 m.y. ago. The dike fractures probably represents a period of regional crustal extension caused by a redistribution of the regional stress pattern accompanying the Nevadan orogeny. Intrusion of Cretaceous granitic plutons began in large volume about 120 m.y. ago in the western Sierra Nevada and migrated steadily eastward for 40 m.y. at a rate of 2.7 mm/y. This slow and constant migration indicates remarkably uniform conditions of subduction with perhaps downward migration of parent magma generation or a slight flattening of the subduction zone. Such steady conditions could be necessary for the production of large batholithic complexes such as the Sierra Nevada. The abrupt termination of plutonism 80 m.y. ago may have resulted from an increased rate of convergence of the American and eastern Pacific plates and dramatic flattening of the subduction zone. U-Pb ages of the Giant Forest-alaskite sequence in Sequoia National Park are all in the range 99±3 m.y., indicating a relatively short period of emplacement and cooling for this nested group of plutons. U-Pb ages of a mafic inclusion and its host granodiorite indicate that both were derived from a common source or that the mafic inclusion was totally equilibrated with the granodioritic magma. Comparison of isotopic ages determined by different methods such as zircon U-Pb, sphene U-Pb, hornblende K-Ar, and biotite K-Ar suggests that zircon U-Pb ages generally approximate the emplacement age of a pluton. However, some plutons probably contain inherited or entrained old zircons, and the zircons of some samples are disturbed by younger thermal and metamorphic events. The ages reported here are consistent with U-Pb age determinations previously made on granitic rocks to the north [Stern et al., 1981], The age distribution of granitic belts determined here is in general agreement with those established by K-Ar dating [Evernden and Kistler, 1970] but does not differentiate the five epochs of plutonism determined in their study.

California↗
USGS2012

Lead and strontium isotopes and related trace elements as genetic tracers in the Upper Cenozoic rhyolite-basalt association of the Yellowstone Plateau volcanic field

Supported by various field geologic and petrologic data, the contents of Pb, U, Th, Rb, and Sr and the isotopic compositions of Pb and Sr for upper Cenozoic volcanic rocks of the Yellowstone Plateau volcanic field are consistent with the hypothesis of derivation of the basaltic and rhyolitic magmas by partial melting of distinct source regions in the upper mantle and lower crust, respectively. All the basalt samples analyzed but one have systematically lower values of 207 Pb/ 204 Pb and 87 Sr/ 86 Sr than the rhyolites. The values of 206 Pb/ 204 Pb are smaller, and 87 Sr/ 86 Sr are mostly larger than known values in oceanic basalts. In all but one case, the values of 207 Pb/ 204 Pb are higher than expected from an extrapolation of known values in oceanic basalts to less radiogenic values of 206 Pb/ 204 Pb. Because there are no xenoliths, phenocrysts are only moderate to sparse in abundance, REE patterns are low and flat at the radiogenic end of lead isotopic compositions, several values of Rb/Sr are low, and 80% of the basalt samples form a well-developed secondary isochron separate from the rhyolites, we favor an interpretation for basalt genesis wherein isotopic signatures of most mafic magmas were attained in a continental ‘keel’ of mantlelike character about 2.6 b.y. old or somewhat older attached to the crust, and these signatures were unaltered by magma passage through the crust. At the very least, the current data continue to cast serious doubt as to the inevitability of crustal contamination for basaltic magma intruding the continental environment and postulate that much can be learned about the mantle under continents through the study of continental basalts. One basalt unit with an unusually low value of 207 Pb/ 204 Pb and an 87 Ar/ 86 Ar less than 0.704 may represent subcontinental ‘keel’-derived magma that rose unaltered to the surface. Our data also are not consistent with formation of this rhyolite-basalt association primarily by such processes as crystal fractionation, separation of immiscible silicate liquids from a common parental magma, or fractional melting of a homogeneous source. Rather as a conceptual model, we envision large mafic intrusions to have been injected into the lower crust resulting in rhyolite generation through partial anatexis of the adjacent wall rocks which probably had a 206 Pb/ 204 Pb < 17 and 87 Sr/ 86 Sr > 0.709; a model that has much in common with that proposed by Holmes (1931). All the other hypotheses listed have the necessary added complication that either the basalt or the rhyolite or both become contaminated after the two magma types separated, have problems accounting for the lack of igneous rocks of intermediate compositions or production of such large volumes of rhyolitic material (∼5000 km 3 ), and fail to explain why rhyolitic magma is not a more common occurrence in the ocean basin. We appeal to bouyancy of rhyolites to generate a barrier for basalt magma migration and account for the great preponderance of rhyolite relative to basalt at the surface. Furthermore, the complex isotopic picture in the rhyolites indicates that many of these magmas interacted with the upper crustal geologic units that they traversed. The interactions involved diverse processes, probably including reacton with hydrothermal fluids or hydrothermally altered rocks at high levels as well as by contamination with Phanerozoic sedimentary and Precambrian crystalline rocks at deeper levels. At the very least, we feel our study adds a cautionary note to the currently increasingly popular hypothesis that differentiation of basalt or gabbro magmas to rhyolite or granite (as distinct from tonalite or dacite) is a common occurrence and is therefore an important continential building process. Models for formation of rhyolite and granite predominantly by reworking of crust (anatexis) must still be considered. The primitive Archean mantle of the region was characterized by higher Rb/Sr, U/Pb, and Th/U values than are typical of modern suboceanic mantle. The mantle residuum within the continental subcrustal lithosperic ‘keel’ that resulted from the Archean crustal differentiation event probably was depleted in Rb/Sr and U/Pb, and the crust was correspondingly enriched in these ratios. The crust probably was further differentiated by an Archean high-grade metamorphism, during or after the primary event, into a granulitic lower crust depleted in U/Pb and Rb/Sr and a lower-grade upper crust enriched in these ratios.

Journal of Geophysical Research Solid Earth↗
USGS2012

Northern East Pacific Rise: Magnetic anomaly and bathymetric framework

The oceanic crust in the eastern Pacific between 7°N and 30°N and east of 127°W contains a fairly complete history of the spreading centers associated with the East Pacific Rise since 25 m.y. B.P. (late Oligocene). In this paper, we have summarized the seafloor spreading magnetic-anomaly data and the bathymetric data that reflect the record of this tectonic history. The well-defined magnetic lineations north of the Clarion fracture zone, in the mouth of the Gulf of California, and on the east flank of the East Pacific Rise (EPR) are carefully examined and used to provide a guide for interpreting the spreading pattern between the Clarion and Clipperton fracture zones, southward of the Rivera fracture zone over the Mathematician Ridge, and over the entire EPR east of the Mathematician Ridge between the Rivera and Siqueiros fracture zones. The bathymetric data provide a trace of the fracture zone pattern in each of the above mentioned areas. The fracture zone bathymetry and the seafloor spreading magnetic lineations on the EPR south of the Rivera fracture zone have a distinctive fanning pattern caused by close poles of rotation and plate boundary reorganizations. All these data provide a good record of the plate reorganizations in the middle Miocene at magnetic anomaly 5 A time (12.5 to 11 m.y. B.P.), in the late Miocene at magnetic anomaly 3′−4 time (6.5 m.y. B.P.), and in the Pliocene at magnetic anomaly 2′−3 time (3.5 m.y. B.P.). Several abandoned spreading centers, including the Mathematician Ridge, were left behind as a result of these reorganizations. The Mathematician Ridge is shown to be a set of ridges and trough whose origin is related to the tectonic activity associated with each of the above mentioned reorganizations since anomaly 5A.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

The sea slope problem revisited

Discrepancies in sea surface topography based on comparisons between the results of steric leveling and repeated geodetic levelings have identified what is known as the ‘sea slope problem.’ This problem is actually twofold: (1) the sea surface relief based on steric leveling differs significantly from that based on geodetic leveling along several generally north-south coasts, and (2) successively propagated levelings between several widely separated tide stations indicate that the stationary sea slope seemingly has been changing with time, whereas differenced sea level means between these stations indicate that the sea surface relief has remained virtually invariant during the same intervals. Reexamination of the three reported discrepancies between geodetically and sterically determined sea slopes indicates that the Australian example is based on leveling of a quality inappropriate to the comparison. The discrepancy developed along the Atlantic coast of the United States is limited to the reach between Portsmouth–Hampton Roads, Virginia, and Key West, Florida, where the accuracy of steric leveling may be especially vulnerable owing to the dynamic effects of the Gulf Stream. Reconsideration of the example along the Pacific coast of the United States indicates that the various discrepancies are due chiefly to intrasurvey movement and resultant distortion of geodetically defined height differences between tide stations. Agreement between the results of steric and geodetic leveling along tectonically inactive north-south coasts devoid of strong boundary currents is generally good. This observation supports the conclusions (1) that any directionally dependent systematic error in geodetic leveling is measurably insignificant and (2) that where allowance is made for the possible effects of major boundary currents or intrasurvey movement during levelings between tide stations, the sea slope problem tends to vanish.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Earthquakes of Loihi submarine volcano and the Hawaiian hot spot

Loihi is an active submarine volcano located 35 km south of the island of Hawaii and may eventually grow to be the next and southernmost island in the Hawaiian chain. The Hawaiian Volcano Observatory recorded two major earthquake swarms located there in 1971–1972 and 1975 which were probably associated with submarine eruptions or intrusions. The swarms were located very close to Loihi's bathymetric summit, except for earthquakes during the second stage of the 1971–1972 swarm, which occurred well onto Loihi's southwest flank. The flank earthquakes appear to have been triggered by the preceding activity and possible rifting along Loihi's long axis, similar to the rift-flank relationship at Kilauea volcano. Other changes accompanied the shift in locations from Loihi's summit to its flank, including a shift from burst to continuous seismicity, a rise in maximum magnitude, a change from small earthquake clusters to a larger elongated zone, a drop in b value, and a presumed shift from concentrated volcanic stresses to a more diffuse tectonic stress on Loihi's flank. The 1971–1972 swarm began at depths of 20–50 km about 1 month before the shallow swarm started and suggests an upward migration of seismicity as the first stage of the 13-month swarm. The seismic ‘root’ of Kilauea volcano is well defined by earthquakes which plunge to the south and southwest to a depth of 50–60 km, terminating in a region in which earthquakes are associated with deep harmonic tremor and magma. The zone of these deeper earthquakes and tremor lies between Kilauea, Loihi, and Mauna Loa volcanoes, may feed magma to all three volcanoes, and probably locates the Hawaiian hot spot.

Journal of Geophysical Research Solid Earth↗
USGS2012

Ice sculpture in the Martian outflow channels

Many landforms in Martian outflow channels have characteristics that suggest sculpture by glaciers, ice streams, or ice sheets. Viking Orbiter and terrestrial satellite images were examined at similar resolution to compare features of the Martian outflow channels to features produced by the movement of ice on earth. Many resemblances were found. They include the anastomoses, sinuosities, and U‐shaped cross profiles of valleys; hanging valleys; and linear scour marks on valley walls, grooves and ridges on valley floors, and streamlining of bedrock highs. The question of whether ice could have moved in the Martian environment is investigated. Since gradients on Martian channel floors are extremely low or even reversed, a certain thickness of ice is required to initiate glacial motion. If ice filled the Martian channels to the level of interior plateaus, flow could have occurred, even under present climatic conditions, although warmer climates in the past would have been more favorable. Additionally, if the channel fluids were brines, ice might have been wet based and would have moved readily by slipping over the ground. It is envisioned that springs or small catastrophic outbursts discharged fluids from structural outlets or chaotic terrains. The fluids built icings that may have grown into substantial masses and eventually flowed like glaciers down preexisting valleys. Alternatively, the fluids may have formed rivers or floods that formed ice jams and consolidated into icy masses in places where obstacles blocked their flow. These masses of ice or slush were probably episodic and temporary, but where they occurred within the channels, they may have produced landforms that are characteristic of glacial sculpture on earth.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

A teleseismic analysis of the New Brunswick earthquake of January 9, 1982

The analysis of the New Brunswick earthquake of January 9, 1982, has important implications for the evaluation of seismic hazards in eastern North America. Although moderate in size ( m b 5.7), it was well-recorded teleseismically. Source characteristics of this earthquake have been determined from analysis of data that were digitally recorded by the Global Digital Seismograph Network. From broadband displacement and velocity records of P waves, we have obtained a dynamic description of the rupture process as well as conventional static properties of the source. The depth of the hypocenter is estimated to be 9 km from depth phases. The focal mechanism determined from the broadband data corresponds to predominantly thrust faulting. From the variation in the waveforms the direction of slip is inferred to be updip on a west dipping NNE striking fault plane. The steep dip of the inferred fault plane suggests that the earthquake occurred on a preexisting fault that was at one time a normal fault. From an inversion of bodywave pulse durations, the estimated rupture length is 5.5 km. Average properties of the rupture process were examined by a moment tensor analysis of long-period P and SH body waves. The long-period moment of this earthquake was 5.3 × 10 24 dyne cm. The static and dynamic stress drops are 41 and 65 bars, respectively, similar to those of many earthquakes with similar moment in regions that are more seismically active. The joint epicenter determination algorithm was used to locate, relative to the mainshock, the three teleseismically recorded aftershocks that occurred through March 31, 1982. The relocated hypocenters of the aftershocks are significantly different from each other and from that of the mainshock; they provide additional support for the source dimensions inferred from the waveform analysis.

Journal of Geophysical Research Solid Earth↗
USGS2012

Statistical averaging of marine magnetic anomalies and the aging of oceanic crust

Visual comparison of Mesozoic and Cenozoic magnetic anomalies in the North Pacific suggests that older anomalies contain less short-wavelength information than younger anomalies in this area. To test this observation, magnetic profiles from the North Pacific are examined from crust of three ages: 0–2.1, 29.3–33.1, and 64.9–70.3 m.y, B.P. For each time period, at least nine profiles were analyzed by (1) calculating the power density spectrum of each profile, (2) averaging the spectra together, and (3) computing a ‘recording filter’ for each time period by assuming a hypothetical seafloor model. The model assumes that the top of the source is acoustic basement, the source thickness is 0.5 km, and the time scale of geomagnetic reversals is according to Ness et al. (1980). The calculated power density spectra of the three recording filters are complex in shape but show an increase of attenuation of short-wavelength information as the crust ages. These results are interpreted using a multilayer model for marine magnetic anomalies in which the upper layer, corresponding to pillow basalt of seismic layer 2A, acts as a source of noise to the magnetic anomalies. As the ocean crust ages, this noisy contribution by the pillow basalts becomes less significant to the anomalies. Consequently, magnetic sources below layer 2A must be faithful recorders of geomagnetic reversals.

Journal of Geophysical Research Solid Earth↗
USGS2012

In situ studies of velocity in fractured crystalline rocks

A study of the effects of macroscopic fractures on P and S wave velocities has been conducted in four wells drilled in granitic rock to depths between 0.6 and 1.2 km. The effect of macroscopic fractures is to decrease both V p and V s and increase V p / V s . In wells with a relatively low density of macroscopic fractures, the in situ velocity is similar to that of saturated core samples under confining pressure in the laboratory, and there is a clear correlation between zones with macroscopic fractures and anomalously low velocities. In wells with numerous macroscopic fractures, the in situ velocity is lower than that of intact samples under pressure, and there is a correlation between the rate at which in situ velocity increases with depth and the rate at which the velocity of laboratory samples increases with pressure. Differences in in situ P wave velocity between wells cannot be explained solely by differences in the degree of macroscopic fracturing, thus emphasizing the importance of composition and microcracks on velocity. In one highly fractured well the in situ P wave velocity is essentially the same for frequencies ranging from 10 Hz to 20 kHz; this suggests that the macrofractures affect velocity similarly over a broad frequency range. Chemical alteration of rock adjacent to macroscopic fractures appears to play an important role in reducing in situ velocities. Synthetic reflection seismograms generated from the velocity logs suggest that fracture zones are one possible source of deep-crustal reflectors observed on seismic reflection profiles.

Journal of Geophysical Research Solid Earth↗
USGS2012

Investigation of internal friction in fused quartz, steel, Plexiglass, and Westerly granite from 0.01 to 1.00 Hertz at 10-8 to 10-7 strain amplitude

A detailed evaluation on the method of internal friction measurement by the stress-strain hysteresis loop method from 0.01 to 1 Hz at 10 −8 to 10 −7 strain amplitude and 23.9°C is presented. Significant systematic errors in relative phase measurement can result from convex end surfaces of the sample and stress sensor and from end surface irregularities such as nicks and asperities. Preparation of concave end surfaces polished to optical smoothness having a radius of curvature >3.6×10 4 cm reduces the systematic error in relative phase measurements to <(5.5±2.2)×10 −4 radians. The values of Q E −1 (internal friction under uniaxial compression) determined from the relative phase measurements are | Q E −1 – Q s −1 |< 2.8×10 −3 for the tool steel sample and | Q E −1 – Q s −1 |< 2.2×10 −3 for the Westerly granite sample, where Q s −1 is the internal friction of the fused quartz stress sensor under uniaxial compression. These values are consistent with those inferred from the relative modulus dispersion data also presented in this paper. The polymethyl methacrylate (PMM, trade name Plexiglass) sample shows high values of internal friction ( Q E −1 ≅5×10 −2 ) with strong frequency dependence and with a maximum in Q E −1 at ≅0.4 Hz.

Journal of Geophysical Research Solid Earth↗
USGS2012

Wandering terranes in southern Alaska: The Aleutia Microplate and implications for the Bering Sea

Paleomagnetic and geological data suggest that much of southern Alaska is a collage of tectonostratigraphic terranes which originated in Mesozoic time at paleolatitudes far south of their present position. The time of ‘docking’ of the terranes against cratonic Alaska is critical to defining their amalgamated size and extent during their northward motion as well as their role in the evolution of the Bering Sea. One of the largest of the tectonostratigraphic terranes, the Peninsular terrane of south central and southwestern Alaska, extends offshore along the outer Bering Sea continental margin (Beringia). Paleomagnetic data suggest that this terrane has moved northward through all of Cenozoic time, but geologic data imply that the terrane had accreted to Alaska by the end of the Mesozoic. In early Cenozoic time the eastern part of the Aleutian arc appears to have been superimposed on the Peninsular terrane, and postulated northward Cenozoic motion of the terrane would therefore have required northward motion of the arc. Two accretion models, based on docking times for terranes in Alaska, are proposed, and they illustrate that large areas of the abyssal Bering Sea, the Alaska Peninsula, the Aleutian arc, and the Beringian continental margin may be part of a superterrane or microplate called Aleutia (microplate as defined by Beck et al. (1980), i.e., a microplate is a displaced segment of lithosphere that has crustal roots, whereas a superterrane is an amalgamation of terranes which may or may not be rootless). Model A implies that the Aleutian arc developed in situ on the southern edge of Aleutia after the microplate had docked. In model B, the final docking time of the Peninsular terrane is late Cenozoic, which implies that the Aleutia microplate encompasses a mammoth area that includes parts of southern Alaska, the Alaska Peninsula, the southern Beringian margin, the abyssal Bering Sea (Kula plate), and the Aleutian arc. If model A is correct, the docking time of the Peninsular terrane is late Mesozoic or earliest Tertiary. The Aleutia microplate in this model is made up solely of the abyssal Bering Sea (Kula plate), which presumably docked at the same time or slightly after the Peninsular terrane accreted against Alaska. If model B is correct, that is, if the Aleutia collided with nuclear Alaska during the Cenozoic, then a late Cenozoic suture zone, the vestige of a large open sea that must have closed between Aleutia and Alaska, must exist in south central and southwest Alaska. Either evidence for Cenozoic closure and suturing has been obliterated in Alaska or the inferences of Cenozoic terrane motion derived from paleomagnetic data are suspect.

Alaska↗
USGS2012

A dislocation model of strain accumulation and release at a subduction zone

Strain accumulation and release at a subduction zone are attributed to stick slip on the main thrust zone and steady aseismic slip on the remainder of the plate interface. This process can be described as a superposition of steady state subduction and a repetitive cycle of slip on the main thrust zone, consisting of steady normal slip at the plate convergence rate plus occasional thrust events that recover the accumulated normal slip. Because steady state subduction does not contribute to the deformation at the free surface, deformation observed there is completely equivalent to that produced by the slip cycle alone. The response to that slip is simply the response of a particular earth model to embedded dislocations. For a purely elastic earth model, the deformation cycle consists of a coseismic offset followed by a linear‐in‐time recovery to the initial value during the interval between earthquakes. For an elastic‐viscoelastic earth model (elastic lithosphere over a viscoelastic asthenosphere), the postearthquake recovery is not linear in time. Records of local uplift as a function of time indicate that the long‐term postseismic recovery is approximately linear, suggesting that elastic earth models are adequate to describe the deformation cycle. However, the deformation predicted for a simple elastic half‐space earth model does not reproduce the deformation observed along the subduction zones in Japan at all well if stick slip is restricted to the main thrust zone. As recognized earlier by Shimazaki, Seno, and Kato, the uplift profiles could be explained if stick slip were postulated to extend along the plate interface beyond the main thrust zone to a depth of perhaps 100 km, but independent evidence suggests that stick slip at such depths is unlikely.

Journal of Geophysical Research B: Solid Earth↗