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At least 1,153 records · Page 64Linked to original sources
USGS2012

Strain accumulation in western Washington

The Juan de Fuca plate is subducted beneath the North American plate off the coast of Washington at a rate of about 40 mm/yr N68°E. The average principal strain rates (extension reckoned positive) measured in northwestern Washington are as follows: Olympic peninsula 25 km south of Port Angeles from 1982 through 1990, and and near Seattle from 1972 through 1985, and . Both strain measurements are consistent with uniaxial contraction in the direction of plate convergence. Uplift rates inferred from tide gage recordings are about 4 mm/yr on the Pacific coast and near 0 mm/yr farther inland near Seattle. These deformation rates are consistent with a model of the Cascadia subduction zone in which the plate interface beneath the continental slope and outer continental shelf is locked but free to slip farther landward. The limited downdip extent of the locked segment of the plate interface is consistent with a shallow depth (∼20 km) of the isotherm (∼450°C) that defines the brittle‐ductile transition. Small thrust events diagnostic of seismic subduction should then occur only offshore and at shallow depths. The principal strain rates measured from 1972 through 1983 in the back arc region near Richland, Washington, are and .

Washington↗
USGS2012

Strain accumulation along the Denali Fault at the Nenana River and Delta River Crossings, Alaska

Surveys of trilateration networks across the Denali fault at the Nenana River in 1982, 1984, and 1988 and at the Delta River in 1975, 1979, 1982, and 1984 indicate a minor (0.10±0.04 μstrain/yr) northeastward uniaxial extension. The component of right‐lateral shear‐strain accumulation across the fault is not significant at the two‐standard‐deviation level. At the Delta River network the strain accumulation rate decreases rapidly with distance from the fault, but evidence for a similar decrease with distance from the fault is lacking at the Nenana River network. The strain accumulation rates inferred from trilateration are consistent with the very long baseline interferometry (VLBI) measurement reported by Ma et al. (1990) and support their contention that significant right‐lateral shear is not accumulating along the Denali fault at the present time. Savage et al. (1981) had earlier concluded erroneously that preliminary geodetic measurements at the Delta River network demonstrated right‐lateral shear strain accumulation. The absence of significant right‐lateral deformation across the Denali fault in the 1975–1988 interval is in marked contrast with the abundant geomorphic evidence for Holocene right‐lateral secular slip at the rate of 10–20 mm/yr on the Denali fault in this sector.

Alaska↗
USGS2012

Multicycle slip distribution along a laboratory fault

Slip distribution along a laboratory fault, which consists of eight spring-connected blocks that are elastically driven to slide on a frictional surface, has been examined for a “long” sequence of slip events to test the applicability of some conceptual models proposed recently in the literature. The distributions of large slip events are found to be quite variable and do not fit the uniform slip or characteristic earthquake models. The rupture initiation points are usually not near the corresponding maximum slip points, in contrast to observations by Thatcher (1990) and by Fukao and Kikuchi (1987) that earthquake hypocenters are commonly near corresponding regions of maximum slip in the fault planes. This contrast may suggest that either the present observations or theirs are not representative or the teleseismically determined hypocenters may not always be true rupture initiation points as usually assumed. Large slip events are also found to be a stress-roughening process. They are triggered by some small events after the stresses have been adjusted by some earlier small-to-moderate events to be near the critical levels at most locations along the fault. This suggests that earthquake prediction monitoring efforts should not be limited to a small region near an asperity but should be spread out to cover the entire fault segment in a seismic gap in order to detect the condition of simultaneous strain buildup.

Journal of Geophysical Research↗
USGS2012

Observations constraining near-source ground motion estimated from locally recorded seismograms

To estimate the seismic hazard to underground facilities or operations in the environs of a mining-induced tremor or a natural earthquake, it is useful to be able to relate locally recorded seismic waveforms to peak ground velocity and slip at the causative fault. For this purpose, far-field S wave pulses are analyzed to define the faulting slip D and near-fault peak ground velocity D /2 that give rise to the most significant ground motion. This most intense region of faulting, an assumed circular asperity, has radius r within a broader source zone of radius r 0 , which is traditionally calculated from the corner frequency of the S wave spectrum. In developing relationships between peak far-field velocity v and peak acceleration a, and the source processes of the asperity, D and D , as well as its radius r , the key model assumption is that r = k β/ω, where ω is the angular frequency of the sinusoidal velocity pulse of maximum amplitude, β is the sheaf wave speed, and k is a constant. Observations in deep-level gold mines of fault slip and slip velocity as well as laboratory observations of slip rate as a function of stress drop for stick-slip failure support a choice of about k = 2.34, the value commonly used for estimating r 0 using the Brune model. In particular, observations of fault slip up to 410 mm for mining-induced tremors in the moment magnitude range 4–5 are consistent with D = 8.1 R v/β, where R is hypocentral distance. Moreover, estimates based on underground damage of near-fault ground velocities ranging up to 3.5 m/s are in accord with D /2 = 1.28(β/μ) ρ R a, where μ is the modulus of rigidity and ρ is the density. Alternatively, the average slip velocity 〈 D 〉 can be expressed in terms of the stress drop Δσ a of the asperity as 〈 D 〉 = 0.51 β Δσ a /μ, and the agreement of this relationship with measurements made during stick-slip failure in the laboratory is good. To the extent that seismic slip exterior to the asperity is a consequence of preevent suppression of slip due to the asperity, the broader-scale( r 0 ) slip can be related to that of the asperity. Just as the asperity radius r can be estimated from r = 2.34 βv/a, an alternative estimate for r 0 is given by r 0 = ρ R a M 0 /[75.8ρμ( R v) 2 ], the results of which are generally in good agreement with estimates based on the spectral corner frequency method.

Journal of Geophysical Research↗
USGS2012

Crustal subsidence and extension and Medicine Lake volcano, northern California

The pattern of historical ground deformation, seismicity, and crustal structure near Medicine Lake volcano illustrates a close relation between magmatism and tectonism near the margin of the Cascade volcanic chain and the Basin and Range tectonic province. Between leveling surveys in 1954 and 1989 the summit of Medicine Lake volcano subsided 389±43 mm with respect to a reference bench mark 40 km to the southwest (average rate = 11.1±1.2 mm/yr). A smaller survey across the summit caldera in 1988 suggests that the subsidence rate was 15–28 mm/yr during 1988–1989. Swarms of shallow earthquakes ( M ≤ 4.6) occurred in the region during August 1978, January–February 1981, and September 1988. Except for the 1988 swarm, which occurred beneath Medicine Lake caldera, most historical earthquakes were located at least 25 km from the summit. The spatial relation between subsidence and seismicity indicates (1) radially symmetric downwarping of the volcano's summit and flanks centered near the caldera and (2) downfaulting of the entire edifice along regional faults located 25–30 km from the summit. We propose that contemporary subsidence, seismicity, and faulting are caused by (1) loading of the crust by more than 600 km 3 of erupted products plus a large volume of mafic intrusives; (2) east‐west extension in the western Basin and Range province; and, to a lesser extent, (3) crystallization or withdrawal of magma beneath the volcano. Thermal weakening of the subvolcanic crust by mafic intrusions facilitates subsidence and influences the distribution of earthquakes. Subsidence occurs mainly by aseismic creep within 25 km of the summit, where the crust has been heated and weakened by intrusions, and by normal faulting during episodic earthquake swarms in surrounding, cooler terrain.

California↗
USGS2012

Seismicity and shear strain in the southern Great Basin of Nevada and California

This study examines the relationship between the distribution of small earthquakes (M L ≤4.3) and mechanisms of strain accumulation and relaxation in an area with long repeat times between large events, the Southern Great Basin Seismic Network (SGBSN) region. The Great Basin is a unique continental extensional province characterized by normal and strike-slip faulting, high heat flow, crust of thin to normal thickness, and high elevations. The SGBSN is operated to provide data to address suitability issues pertaining to Yucca Mountain, Nevada which is being evaluated as a potential site for a national mined geologic nuclear waste repository. Suitability issues include estimation of the probability of occurrence of future damaging earthquakes, the characterization of the mechanisms that drive hydrologic flow, and the identification of fractures (faults) that might act as flow conduits or barriers. This study attempts to explain the distribution of small earthquakes in terms of spatial variations in the shear strain field; where strain concentrates there should be a greater number of small earthquakes. Strain field models are constructed under the assumption that long term fault behavior perturbs an otherwise uniform strain field. These strain field models are then interpreted with regard to the regional tectonics and site suitability issues. Modeling results provide one possible explanation of why earthquake clusters cover regions much larger than the surface projections of any of mapped major faults; clusters in a wide band along and extending northeast of the northern half of the Furnace Creek fault may correspond to elevated shear strains along the fault and a broad cluster in the Pahranagat Shear Zone may be associated with shear strain arising from a distribution of smaller localized faults. The relatively large number of small earthquakes in the southern and eastern portions of the Nevada Test Site is consistent with the strain field models. A minimum in shear strain at Yucca Mountain is predicted by all models consistent with an almost total lack of earthquakes observed there. The region to the west of the Death Valley/Furnace Creek fault system, the portion of the study area with the most active deformation but few small earthquakes, is an area of low shear strain. A possible reason for this is that the fault configuration in the area is optimal for accommodating regional deformation via large earthquakes or creep. While there is also a relative lack of earthquakes at Yucca Mountain, this may be indicative of a lack of accumulating strain energy and thus, a lower potential for a large earthquake.

Journal of Geophysical Research↗
USGS2012

Historic creep rate and potential for seismic slip along the Hayward Fault, California

The Hayward fault is considered the most likely source of one or more major earthquakes in the San Francisco Bay area in the next few decades. Historically, at least one, and probably two, major earthquakes (about M 6.8) occurred along the Hayward fault, one in 1836 and another in 1868. Little is known about the 1836 event, but the 1868 earthquake was accompanied by a surface rupture that extended as much as 41 km along the southern part of the fault. Although the amount of surface slip in 1868 is uncertain, right slip (including afterslip) reached at least several centimeters, and possibly several decimeters in places. This paper documents the spatial variation of creep rate along the Hayward fault since the 1868 earthquake. Creep (aseismic fault slip) occurs over at least 66 km and may extend over the fault's entire 82-km length, of which about 13 km lies underwater. Creep rate seems nearly constant over decades, but short-term variations occur. We derive creep rate mainly from our own systematic surveying of offset cultural features (curbs, fences, and buildings). On each feature we solve directly for accumulated creep by using multiple linear regression. Creep rate mostly falls in the range of 3.5–6.5 mm/yr; but systematic variation occurs along strike. Fault segments with distinctly higher and lower rates generally correspond to parts of the fault most salient from the overall average alinement of the fault. Most distinctive is a 4-km-long section near the south end of the fault that creeps at about 9 mm/yr. Such a high rate has occurred there at least since the 1920s and probably since the 1868 earthquake, as indicated by an offset railroad track built in 1869. We suggest that this 9 mm/yr slip rate may approach the long-term or deep slip rate that controls average recurrence interval between major earthquakes. If so, assuming an elastic rebound model, the potential for slip in large earthquakes below the surficial creeping zone is now ∼1.1 m in the southern (1868) segment of the fault and ≥ 1.4 m in the northern (1836?) segment. Subtracting surface creep rates from a long-term slip rate of 9 mm/yr gives present potential for surface slip in large earthquakes of up to 0.8 m, with an average of 0.6 m in the northern segment and 0.4 m in the southern segment. We present a simple hypothesis for rupture potential that is compatible with historic creep rate, microseismicity distribution, and geodetic data. If seismic rupture occurs on segments 41 km long by 10 km deep (7 km fully locked, 3 km creeping), today's potential for seismic moment release is 1.4 × 10 19 and 1.1 × 10 19 N m for both 1836? and 1868 segments, respectively, and 2.5 × 10 19 N m for both segments jointly. Converting moment to magnitude gives M L 6.8 in the northern segment, M L 6.7 in the southern segment, and M L 7.0 for simultaneous rupture of both.

California↗
USGS2012

Seismicity and detection/location threshold in the southern Great Basin seismic network

A spatially varying model of the detection/location capabilities of the Southern Great Basin seismic network (SGBSN) has been derived that is based on simple empirical relations and statistics. This permits use of almost all the catalog data gathered; instead of ignoring data that are below the threshold of completeness, a spatially varying threshold model is developed so that subregions having lower completeness levels than the network as a whole can be outlined and the completeness level of each sub-region determined. Such a model is required to unambiguously identify regions that are aseismic due to natural processes rather than to limited detection and/or location capabilities. Accounting for spatial variations in detection/location threshold is also important for studies in which magnitude-frequency distributions are interpreted in terms of source scaling properties. The characteristics of the spatial distribution of earthquakes, where earthquake clusters and aseismic regions locate, appear to be stable at all magnitude levels so that inferences about where strain is being accommodated will be the same whether numbers of earthquakes or strain estimated from seismic moments are examined. For the southern Great Basin region these principal characteristics include clusters at the northern end of the Furnace Creek fault and in the Pahranagat Shear Zone, and a relatively large number of earthquakes in the northern and southeastern portions of the Nevada Test Site. These clusters cover regions much larger than the surface projections of any of the mapped faults. The extent to which seismicity is induced by nuclear testing is unclear. The predominantly aseismic regions include the area west of the Death Valley/Furnace Creek fault system and an almost complete absence of events at Yucca Mountain. Finally, a considerable number of isolated events in the SGBSN catalog cannot be correlated with mapped faults.

Journal of Geophysical Research↗
USGS2012

Comparison of Vibroseis and explosive source methods for deep crustal seismic reflection profiling in the Basin and Range province

Direct comparison of low-fold, high-energy explosive and high-fold, lower-energy Vibroseis methods for acquiring deep crustal seismic reflection data in the Basin and Range Province suggests that the high-fold common midpoint (CMP) method there does not provide the best possible image of lower crustal structure. During the recent acquisition of a Vibroseis profile in the Basin and Range Province we fired single deep shot holes to obtain a coincident single-fold explosive section. Within the upper crust (upper 3 s) the explosive source and Vibroseis records are nearly equivalent. For record times below 3 s, however, comparison of the explosive source gathers and the coincident final 60-fold Vibroseis section demonstrates that low-fold explosive profiling provides a higher-quality image of the midcrust to lower crust (3–10 s). The higher record quality of the explosive sources results primarily from the larger seismic energy levels produced by the explosives, making them less sensitive to common noise sources. Whereas deeper than 4–5 s the Vibroseis energy levels on individual source efforts fall to that of ambient noise levels, the explosions provide signal-generated energy exceeding ambient noise levels down to 18–19 s. Although individual reflections can be correlated on explosion and Vibroseis shot gathers, reflection events on the 60-fold Vibroseis stack do not correlate to those on the single-fold explosion profile, suggesting that the high-fold CMP method in our study did not maintain the integrity of the weak lower crustal reflected arrivals. Reasons why the high-fold CMP method apparently failed include complex, even time-varying, statics, nonhyperbolic moveout at long offsets, and the difficulty in resolving stacking velocities with data having low signal-to-noise ratios. Reflections on the explosion section are longer and imply a greater degree of layering than one would infer from the lower-energy Vibroseis section.

Journal of Geophysical Research↗
USGS2012

Visible and near-infrared (0.4-2.5 μm) reflectance spectra of playa evaporite minerals

Visible and near-infrared (VNIR; 0.4–2.4 μm) reflectance spectra were recorded for 35 saline minerals that represent the wide range of mineral and brine chemical compositions found in playa evaporite settings. The spectra show that many of the saline minerals exhibit diagnostic near-infrared absorption bands, chiefly attributable to vibrations of hydrogen-bonded structural water molecules. VNIR reflectance spectra can be used to detect minor hydrate phases present in mixtures dominated by anhydrous halite or thenardite, and therefore will be useful in combination with X ray diffraction data for characterizing natural saline mineral assemblages. In addition, VNIR reflectance spectra are sensitive to differences in sample hydration state and should facilitate in situ studies of minerals that occur as fragile, transitory dehydration products in natural salt crusts. The use of spectral reflectance measurements in playa studies should aid in mapping evaporite mineral distributions and may provide insight into the geochemical and hydrological controls on playa mineral and brine development.

Journal of Geophysical Research↗
USGS2012

Chemical bonding in the outer core: high-pressure electronic structures of oxygen and sulfur in metallic iron

From its density the outer core is believed to be an alloy of iron and a light element such as sulfur or oxygen. The nature of the light element in the core is an important constraint for theories of the Earth's formation. In this paper the electronic structures of oxygen and sulfur impurities in metallic iron are investigated to determine if pressure, temperature, and composition-induced changes in bonding might affect phase equilibria along the Fe-FeS and Fe-FeO binaries. The electronic structure of sulfur in metallic iron is consistent with the miscibility between Fe and FeS liquids. Volume compression strengthens the Fe-S bond, and it is expected that at sufficiently high pressure, sulfur can substitute for Fe and give solid solution behavior between Fe and FeS. In contrast, the electronic structure of oxygen in metallic iron shows that oxygen cannot act as a substitutional impurity (replacing Fe). This explains the observed miscibility gap on the Fe-FeO binary at 1 atm pressure. Volume compression does not greatly change the electronic structure if oxygen substitutes for iron in bcc and fcc iron. Iron-oxygen bonding does occur, however, if oxygen occupies interstitial sites. Insofar as the molar volume of FeO incorporated as interstitial oxygen in metallic iron is smaller than that of pure FeO, the incorporation of oxygen into metallic iron may be favored under the pressures of the Earth's core.

Journal of Geophysical Research↗
USGS2012

Magnetic susceptibility and relation to initial 87Sr/86Sr for granitoids of the central Sierra Nevada, California

Measurement of the magnetic susceptibility of more than 6000 samples of granitic rock from the Mariposa 1° by 2° quadrangle, which crosses the central part of the Sierra Nevada batholith between 37° and 38°N latitude, shows that magnetic susceptibility values are above 10 −2 SI units in the east and central parts of the batholith and drop abruptly to less than 10 −3 SI units in the western foothills. In a narrow transitional zone, intermediate values (10 −3 to 10 −2 ) prevail. Magnetic susceptibility appears to decrease slightly westward within the zones of both high and low values. Magnetic susceptibility in plutonic rocks is chiefly a function of the abundance of magnetite, which depends, in turn, on the total iron content of the rocks and their oxidation ratio. Lower magnetic susceptibilities of felsic members of Sierran intrusive suites and of some felsic rock units relative to adjacent mafic rock units commonly reflect differences in total iron content, but the differences of magnetic susceptibility that define the regional pattern generally are much larger and are determined chiefly by the oxidation ratios of the rocks. The relatively unaltered condition of the samples and restriction of δ 18 O to the range of +7 to +10.3‰ indicate that neither hydrothermal fluids nor subsolidus alteration were important in modifying oxidation ratios. Correlations of magnetic susceptibility with initial 87 Sr/ 86 Sr suggest that oxidation ratios have been inherited from the source regions for the magmas from which the rocks crystallized. Reduction of Fe 3+ to Fe 2+ by organic carbon or other reducing substances may also have affected magnetic susceptibility.

Journal of Geophysical Research↗
USGS2012

Strain accumulation across the Wasatch Fault near Ogden, Utah

Deformation of a 70 by 40‐km trilateration network spanning the north trending Wasatch fault near Ogden, Utah, has been monitored from 1972 through 1990. All but nine of the 200 measurements are consistent with deformation that is linear in time. We presume that those nine observations are contaminated by some blunder in making the measurements and that deformation is linear in time. The strain rate over the 1972–1990 interval across a 40‐km‐wide zone west of the Wasatch fault (hanging wall block) is adequately approximated by a 0.04 ± 0.01 μstrain/yr N85°E±5° uniaxial extension. Adequate coverage of the footwall block is available only over the 1981–1990 interval, and the strain rate across a 30‐km‐wide zone there is approximated by a 0.07 ± 0.03 μstrain/yr N20°E±12° uniaxial extension. The observed extension of the hanging wall block in the direction normal to fault strike suggests that the Wasatch fault may be listric; a planar fault would produce a zone of contraction normal to the fault at the surface of the hanging wall block. The component of extension parallel to fault strike observed on the footwall block is unexplained.

Utah↗
USGS2012

Seismicity and geometry of a 110‐km‐long blind thrust fault 1. The 1985 Kettleman Hills, California, earthquake

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 1982 M W =5.4 New Idria, 1983 M W =6.5 Coalinga, and 1985 M W =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 × 10 18 N 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.

California↗
USGS2012

Application of continuum models to deformation of the Aleutian Island Arc

Continuum models were constructed to describe large‐scale deformation of the Aleutian Island Arc over the past 5 m.y. These models consider the island arc as a continuum in the horizontal plane with the velocity boundary condition at the Pacific edge stated as a fraction of Pacific plate convergence transferred to the arc. First, a simple model of uniformly distributed strain is formulated to illustrate the mechanics of continuous deformation. Lineaments along the arc massif rotated about a vertical axis are matched by small‐element rotation calculated from the model. However, this model does not predict across‐arc variations in deformation and produces an unrealistic amount of crustal thickening after 5 m.y. A physically more meaningful model of deformation is the thin viscous sheet model based on averages of stress and rheology throughout the lithosphere. The amount of motion transferred from the Pacific plate to the arc is constrained by the rotated lineaments, while the effective stress‐strain exponent ( n ) and the ability the lithosphere has to sustain crustal thickness contrasts (the Argand number) are independent variables. Primarily, bathymetry, earthquake focal mechanisms, and styles of faulting are used to evaluate the models. The preferred model is one where the amount of motion transferred from the Pacific plate is greater in an arc‐parallel direction than in an arc‐normal direction, producing stresses consistent with strike‐slip faulting at the far western end of the arc and tensional stresses consistent with transverse normal faulting elsewhere in the arc massif. This model agrees with observations of slip vectors by Ekström and Engdahl (1989), who conclude that a portion of the arc‐parallel component of relative plate motion is taken up in the overriding plate. This model implies that compressive stress transferred to the arc is small in comparison to along‐arc shear stress and that stresses conducive to strikeslip faulting are prevalent throughout the arc.

Alaska↗
USGS2012

Use of weather types to disaggregate general circulation model predictions

General circulation models (GCMs) simulate climatic conditions with a grid cell resolution on the order of 100,000 km 2 . This resolution is inadequate to assess the effects of climatic change on water resources at a regional scale. A method has been developed that uses weather-type analysis as a tool to spatially disaggregate GCM predictions to make them useful for water resource studies. The method has been applied to the Delaware River basin to predict the effects of doubling atmospheric carbon dioxide on precipitation patterns in the region. An application of the technique to the Delaware River basin indicates that future climatic conditions will show minimal changes in weather-type frequency, implying that air circulation patterns will remain unchanged. Results of this study indicate that changes in regional precipitation patterns under a doubling of atmospheric carbon dioxide will be a result of within-type changes in weather characteristics.

Journal of Geophysical Research↗
USGS2012

Limitations of quantitative analysis of deep crustal seismic reflection data: Examples from GLIMPCE

Amplitude preservation in seismic reflection data can be obtained by a relative true amplitude (RTA) processing technique in which the relative strength of reflection amplitudes is preserved vertically as well as horizontally, after compensating for amplitude distortion by near-surface effects and propagation effects. Quantitative analysis of relative true amplitudes of the Great Lakes International Multidisciplinary Program on Crustal Evolution seismic data is hampered by large uncertainties in estimates of the water bottom reflection coefficient and the vertical amplitude correction and by inadequate noise suppression. Processing techniques such as deconvolution, F-K filtering, and migration significantly change the overall shape of amplitude curves and hence calculation of reflection coefficients and average reflectance. Thus lithological interpretation of deep crustal seismic data based on the absolute value of estimated reflection strength alone is meaningless. The relative strength of individual events, however, is preserved on curves generated at different stages in the processing. We suggest that qualitative comparisons of relative strength, if used carefully, provide a meaningful measure of variations in reflectivity. Simple theoretical models indicate that peg-leg multiples rather than water bottom multiples are the most severe source of noise contamination. These multiples are extremely difficult to remove when the water bottom reflection coefficient is large (>0.6), a condition that exists beneath parts of Lake Superior and most of Lake Huron.

Great Lakes↗
USGS2012

Interseismic deformation at the Nankai Trough, Japan, subduction zone

Uplift along the coast of southwestern Japan following the 1944 Tonankai ( M s = 8.0) and 1946 Nankaido ( M s = 8.2) earthquakes has been inferred from the 1950–1985 tide gage records. Although uplift rates vary with geographic position, the temporal dependence at each site can be described as the superposition of an exponentially decaying (time constant ∼5 years) transient, significant only in the first decade postseismic, and a steady interseismic rate. The steady interseismic uplift rate is defined only over the 1956–1985 interval, but the linearity in time ofthat uplift record is such that uplift over the remainder of the interseismic cycle (∼80 years) might reasonably be expected to occur at the same rate. Moreover, the spatial pattern and magnitude of the steady uplift rate are reasonably close to those predicted by a simple dislocation model of subduction. However, the currently observed steady uplift rate is not consistent with the uplift rate inferred from geodetic leveling for the final half of the previous earthquake cycle. Thus, either the hypothesis of a steady interseismic uplift rate after the first decade postseismic is wrong or the strain accumulation pattern for one earthquake need not be the same as for the preceding earthquake on the same segment.

Journal of Geophysical Research B: Solid Earth↗