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At least 1,783 records · Page 99Linked to original sources
USGS1998

Paleomagnetism, paleogeographic origins, and uplift history of the Coast Range ophiolite at Mount Diablo, California

Divergent paleogeographic origins have been proposed for the Coast Range ophiolite of western California which are testable using paleomagnetic methods. Paleomagnetic data for Middle Jurassic pillow lavas and diabase sills of the Coast Range ophiolite at Mount Diablo, northern California, indicate that they contain two components of remanent magnetization. The characteristic directions have normal and reversed polarities and apparently are carried by Ti-poor magnetite. This magnetization is inferred to have been acquired during emplacement and seafloor alteration at an ancient spreading ridge. The paleolatitude calculated from its structurally corrected mean direction is 20°N ± 9° and agrees with the expected direction for stable North America; this result is also consistent with the concordant paleolatitude (32°N ± 8°) recently determined for Upper Jurassic Coast Range ophiolite at Stanley Mountain in southern California. In addition, clockwise vertical axis rotation of Mount Diablo (143° ± 11°) is indicated by the characteristic magnetization direction. An overprint component is inferred to have been acquired during uplift of Mount Diablo since the Miocene.

Journal of Geophysical Research B: Solid Earth↗
USGS1998

A generalized law for brittle deformation of Westerly granite

A semiempirical constitutive law is presented for the brittle deformation of intact Westerly granite. The law can be extended to larger displacements, dominated by localized deformation, by including a displacement-weakening break-down region terminating in a frictional sliding regime often described by a rate- and state-dependent constitutive law. The intact deformation law, based on an Arrhenius type rate equation, relates inelastic strain rate to confining pressure P c , differential stress σ Δ , inelastic strain ε i and temperature T . The basic form of the law for deformation prior to fault nucleation is where σ o and ε o are normalization constants (dependent on confining pressure), a is rate sensitivity of stress, and α is a shape parameter. At room temperature, eight experimentally determined coefficients are needed to fully describe the stress-strain-strain rate response for Westerly granite from initial loading to failure. Temperature dependence requires apparent activation energy ( E * ∼ 90 kJ/mol) and one additional experimentally determined coefficient. The similarity between the prefailure constitutive law for intact rock and the rate- and state-dependent friction laws for frictional sliding on fracture surfaces suggests a close connection between these brittle phenomena.

Journal of Geophysical Research B: Solid Earth↗
USGS1998

Detrital zircon U-Pb geochronology of Cambrian to Triassic miogeoclinal and eugeoclinal strata of Sonora, Mexico

One hundred and eighty two individual detrital zircon grains from Cambrian through Permian miogeoclinal strata, Ordovician eugeoclinal rocks, and Triassic post-orogenic sediments in northwestern Sonora have been analyzed. During Cambrian, Devonian, Permian, and Triassic time, most zircons accumulating along this part of the Cordilleran margin were shed from 1.40–1.45 and 1.62–1.78 Ga igneous rocks that are widespread in the southwestern United States and northwestern Mexico. Zircons with ages of approximately 1.11 Ga are common in Cambrian strata and were apparently shed from granite bodies near the sample site. The sources of 225–280 Ma zircons in our Triassic sample are more problematic, as few igneous rocks of these ages are recognized in northwestern Mexico. Such sources may be present but unrecognized, or the grains could have been derived from igneous rocks of the appropriate ages to the northwest in the Mojave Desert region, to the east in Chihuahua and Coahuila, or to the south in accreted(?) arc-type terranes. Because the zircon grains in our Cambrian and Devonian to Triassic samples could have accumulated in proximity to basement rocks near their present position or in the Death Valley region of southern California, our data do not support or refute the existence of the Mojave-Sonora megashear. Ordovician strata of both miogeoclinal and eugeoclinal affinity are dominated by >1.77 Ga detrital zircons, which are considerably older than most basement rocks in the region. Zircon grains in the miogeoclinal sample were apparently derived from the Peace River arch area of northwestern Canada and transported southward by longshore currents. The eugeoclinal grains may also have come from the Peace River arch region, with southward transport by either sedimentary or tectonic processes, or they may have been shed from off-shelf slivers of continents (perhaps Antarctica?) removed from the Cordilleran margin during Neoproterozoic rifting. It is also possible that the Ordovician eugeoclinal strata are far traveled and exotic to North America.

Journal of Geophysical Research B: Solid Earth↗
USGS1998

Observations on basaltic lava streams in tubes from Kilauea Volcano, island of Hawai'i

From 1986 to 1997, the Pu'u 'O'o-Kupaianaha eruption of Kilauea produced a vast pahoehoe flow field fed by lava tubes that extended 10–12 km from vents on the volcano's east rift zone to the ocean. Within a kilometer of the vent, tubes were as much as 20 m high and 10–25 m wide. On steep slopes (4–10°) a little farther away from the vent, some tubes formed by roofing over of lava channels. Lava streams were typically 1–2 m deep flowing within a tube that here was typically 5 m high and 3 m wide. On the coastal plain (<1°), tubes within inflated sheet flows were completely filled, typically 1–2 m high, and several tens of meters wide. Tubes develop as a flow's crust grows on the top, bottom, and sides of the tubes, restricting the size of the fluid core. The tubes start out with nearly elliptical cross-sectional shapes, many times wider than high. Broad, flat sheet flows evolve into elongate tumuli with an axial crack as the flanks of the original flow were progressively buried by breakouts. Temperature measurements and the presence of stalactites in active tubes confirmed that the tube walls were above the solidus and subject to melting. Sometimes, the tubes began downcutting. Progressive downcutting was frequently observed through skylights; a rate of 10 cm/d was measured at one skylight for nearly 2 months.

Journal of Geophysical Research B: Solid Earth↗
USGS1997

Large-scale variability of wind erosion mass flux rates at Owens Lake 1. Vertical profiles of horizontal mass fluxes of wind-eroded particles with diameter greater than 50 μm

A field experiment at Owens (dry) Lake, California, tested whether and how the relative profiles of airborne horizontal mass fluxes for >50-&mu;m wind-eroded particles changed with friction velocity. The horizontal mass flux at almost all measured heights increased proportionally to the cube of friction velocity above an apparent threshold friction velocity for all sediment tested and increased with height except at one coarse-sand site where the relative horizontal mass flux profile did not change with friction velocity. Size distributions for long-time-averaged horizontal mass flux samples showed a saltation layer from the surface to a height between 30 and 50 cm, above which suspended particles dominate. Measurements from a large dust source area on a line parallel to the wind showed that even though the saltation flux reached equilibrium &sim;650 m downwind of the starting point of erosion, weakly suspended particles were still input into the atmosphere 1567 m downwind of the starting point; thus the saltating fraction of the total mass flux decreased after 650 m. The scale length difference and ratio of 70/30 suspended mass flux to saltation mass flux at the farthest down wind sampling site confirm that suspended particles are very important for mass budgets in large source areas and that saltation mass flux can be a variable fraction of total horizontal mass flux for soils with a substantial fraction of <100-&mu;m particles.

Journal of Geophysical Research D: Atmospheres↗
USGS1997

Winter CO2 fluxes in a boreal forest

We measured soil respiration during two winters in three different ecotypes of the BOREAS northern study area. The production of CO 2 was continuous throughout the winter and, when totaled for the winter of 1994–1995, was equivalent to the release of ∼40–55 g C/m 2 from the soil surface. As soils cooled in the early winter, the CO 2 production rate decreased in a manner that appeared to be exponentially related to shallow soil temperatures. This exponential relationship was not observed when soils began to warm, possibly indicating that there may be additional or different processes responsible for increased CO 2 production during winter warming events. We also measured CO 2 concentrations in soil gas and the Δ 14 C of the soil CO 2 . These measurements show that the CO 2 produced in winter is not simply the return to the atmosphere of the carbon fixed during the previous growing season. We suggest that the wintertime production of CO 2 originates, at least in part, from the decomposition of old organic carbon stored at depth in the soil.

Journal of Geophysical Research D: Atmospheres↗
USGS1997

Comparing nocturnal eddy covariance measurements to estimates of ecosystem respiration made by scaling chamber measurements at six coniferous boreal sites

During the growing season, nighttime ecosystem respiration emits 30–100% of the daytime net photosynthetic uptake of carbon, and therefore measurements of rates and understanding of its control by the environment are important for understanding net ecosystem exchange. Ecosystem respiration can be measured at night by eddy covariance methods, but the data may not be reliable because of low turbulence or other methodological problems. We used relationships between woody tissue, foliage, and soil respiration rates and temperature, with temperature records collected on site to estimate ecosystem respiration rates at six coniferous BOREAS sites at half-hour or 1-hour intervals, and then compared these estimates to nocturnal measurements of CO 2 exchange by eddy covariance. Soil surface respiration was the largest source of CO 2 at all sites (48–71%), and foliar respiration made a large contribution to ecosystem respiration at all sites (25–43%). Woody tissue respiration contributed only 5–15% to ecosystem respiration. We estimated error for the scaled chamber predictions of ecosystem respiration by using the uncertainty associated with each respiration parameter and respiring biomass value. There was substantial uncertainty in estimates of foliar and soil respiration because of the spatial variability of specific respiration rates. In addition, more attention needs to be paid to estimating foliar respiration during the early part of the growing season, when new foliage is growing, and to determining seasonal trends of soil surface respiration. Nocturnal eddy covariance measurements were poorly correlated to scaled chamber estimates of ecosystem respiration ( r 2 =0.06–0.27) and were consistently lower than scaled chamber predictions (by 27% on average for the six sites). The bias in eddy covariance estimates of ecosystem respiration will alter estimates of gross assimilation in the light and of net ecosystem exchange rates over extended periods.

Journal of Geophysical Research D: Atmospheres↗
USGS1997

Land cover mapping, fire regeneration, and scaling studies in the Canadian boreal forest with 1 km AVHRR and Landsat TM data

A multitemporal 1 km advanced very high resolution radiometer (AVHRR) land cover analysis approach was used as the basis for regional land cover mapping, fire disturbance-regeneration, and multiresolution land cover scaling studies in the boreal forest ecosystem of central Canada. The land cover classification was developed by using regional field observations from ground and low-level aircraft transits to analyze spectral-temporal clusters that were derived from an unsupervised cluster analysis of monthly normalized difference vegetation index (NDVI) image composites (April-September 1992). Quantitative areal proportions of the major boreal forest components were determined for a 821 km ?? 619 km region, ranging from the southern grasslands-boreal forest ecotone to the northern boreal transitional forest. The boreal wetlands (mostly lowland black spruce, tamarack, mosses, fens, and bogs) occupied approximately 33% of the region, while lakes accounted for another 13%. Upland mixed coniferous-deciduous forests represented 23% of the ecosystem. A SW-NE productivity gradient across the region is manifested by three levels of tree stand density for both the boreal wetland conifer and the mixed forest classes, which are generally aligned with isopleths of regional growing degree days. Approximately 30% of the region was directly affected by fire disturbance within the preceding 30-35 years, especially in the Canadian Shield Zone where large fire-regeneration patterns contribute to the heterogeneous boreal landscape. Intercomparisons with land cover classifications derived from 30-m Landsat Thematic Mapper (TM) data provided important insights into the relative accuracy of the 1 km AVHRR land cover classification. Primarily due to the multitemporal NDVI image compositing process, the 1 km AVHRR land cover classes have an effective spatial resolution in the 3-4 km range; therefore fens, bogs, small water bodies, and small patches of dry jack pine cannot be resolved within the wet conifer mosaic. Major differences in the 1-km AVHRR and 30-m Landsat TM-derived land cover classes are most likely due to differences in the spatial resolution of the data sets. In general, the 1 km AVHRR land cover classes are vegetation mosaics consisting of mixed combinations of the Landsat classes. Detailed mapping of the global boreal forest with this approach will benefit from algorithms for cloud screening and to atmospherically correct reflectance data for both aerosol and water vapor effects. We believe that this 1 km AVHRR land cover analysis provides new and useful information for regional water, energy, carbon, and trace gases studies in BOREAS, especially given the significant spatial variability in land cover type and associated biophysical land cover parameters (e.g., albedo, leaf area index, FPAR, and surface roughness). Multiresolution land cover comparisons (30 m, 1 km, and 100 km grid cells) also illustrated how heterogeneous landscape patterns are represented in land cover maps with differing spatial scales and provided insights on the requirements and challenges for parameterizing landscape heterogeneity as part of land surface process research.

Journal of Geophysical Research D: Atmospheres↗
USGS1997

Factors controlling threshold friction velocity in semiarid and arid areas of the United States

A physical model was developed to explain threshold friction velocities u*t for particles of the size 60a??120 I?m lying on a rough surface in loose soils for semiarid and arid parts of the United States. The model corrected for the effect of momentum absorption by the nonerodible roughness. For loose or disturbed soils the most important parameter that controls u*t is the aerodynamic roughness height z 0. For physical crusts damaged by wind the size of erodible crust pieces is important along with the roughness. The presence of cyanobacteriallichen soil crusts roughens the surface, and the biological fibrous growth aggregates soil particles. Only undisturbed sandy soils and disturbed soils of all types would be expected to be erodible in normal wind storms. Therefore disturbance of soils by both cattle and humans is very important in predicting wind erosion as confirmed by our measurements.

Journal of Geophysical Research↗
USGS1997

A comparison of six methods for measuring soil-surface carbon dioxide fluxes

Measurements of soil-surface CO 2 fluxes are important for characterizing the carbon budget of boreal forests because these fluxes can be the second largest component of the budget. Several methods for measuring soil-surface CO 2 fluxes are available: (1) closed-dynamic-chamber systems, (2) closed-static-chamber systems, (3) open-chamber systems, and (4) eddy covariance systems. This paper presents a field comparison of six individual systems for measuring soil-surface CO 2 fluxes with each of the four basic system types represented. A single system is used as a reference and compared to each of the other systems individually in black spruce (Picea mariana), jack pine (Pinus banksiana), or aspen (Populus tremuloides) forests. Fluxes vary from 1 to 10 μmol CO 2 m −2 s −1 . Adjustment factors to bring all of the systems into agreement vary from 0.93 to 1.45 with an uncertainty of about 10–15%.

Journal of Geophysical Research D: Atmospheres↗
USGS1997

Dust deposition downwind of Owens (dry) Lake, 1991–1994: Preliminary findings

Salt‐rich dust derived from the Owens Lake playa is deposited in significant quantities to distances of at least 40 km north and south of the playa. Semiannual measurements from 1991 to 1994 of dust deposition rates (dust flux) and composition 2 m above the ground at seven sites in Owens Valley show that (1) dust in Owens Valley is derived mainly from the playa, although areas closer to the sites can also be sources; (2) south of the playa, dust flux is higher in the winter than in the summer, but north of the playa, dust flux is about the same or slightly lower in the winter; (3) on the playa, interannual variation in dust flux is large (factors of 5–10 during the 3 years), but at downwind sites, the variation is much smaller; (4) the dust typically has total soluble salt content as high as 30%, generally much higher than that of dust elsewhere in southern Nevada and California; and (5) to a distance of at least 40 km south of the playa, soluble salt flux is significantly higher than regional rates. The dust flux measurements indicate that significant quantities of salt‐rich dust are probably being added to the soils in the region around Owens Lake playa; these dust additions may affect soil pH and vegetation.

California↗
USGS1997

Accumulation and turnover of carbon in organic and mineral soils of the BOREAS northern study area

Rates of input, accumulation, and turnover of C differ markedly within soil profiles and in soils with different drainage in the BOREAS northern study area. Soil C storage increases from ∼3 kg C m −2 in well-drained, sandy soils to greater than 100 kg C m −2 in wetlands. Two modes of C accumulation were observed in upland soil profiles. Large annual C inputs (0.06–0.1 kg C m −2 yr −1 ) and slow decomposition (turnover times of 6–250 years) lead to rapid C accumulation in regrowing surface moss and detrital layers following fire. Deep organic layers that have accumulated over the millennia since the initiation of soil development, and are located below the most recent charred horizon, show slower rates of input (0.015–0.03 kg C m −2 yr −1 ) and turnover (100–1600 years) and accumulate C about 10 times slower than surface detrital layers. Rates of C input to soils derived from C and 14 C data were in accord with net primary production estimates, with highest rates of input (0.14–0.6 kg C m −2 yr −1 ) in wetlands. Turnover times for C in surface detrital layers were 6–15 years for well-drained sand soils that showed highest soil temperatures in summer, 30–40 years for wetlands, and 36–250 years for uplands with thick moss cover and black spruce trees. Long (>100 years) turnover times in upland black spruce/clay soils most likely reflect the influence of woody debris incorporated into detrital layers. Turnover times for deep organic and mineral layer C were controlled by drainage, with fastest turnover (80–130 years) in well-drained sand soils and slowest turnover (>3000 years) in wetlands. Total C accumulation rates, which account for C losses from both deep organic and surface detrital layers, are close to zero for sand/jack pine soils, 0.003–0.01 kg C m −2 yr −1 for moderately to poorly drained sites in mature forest stands, and 0.03 kg C m −2 yr −1 for a productive fen. Decomposition of organic matter more than several decades old accounts for 9–22% of total heterotrophic respiration at these sites. The rates of C accumulation derived here are decadal averages for specific stands and will vary as stands age or undergo disturbance. Extrapolation to larger regions and longer timescales, where burning offsets C gains in moss layers, will yield smaller rates of C storage.

Journal of Geophysical Research D: Atmospheres↗
USGS1997

Moss and soil contributions to the annual net carbon flux of a maturing boreal forest

We used input and decomposition data from 14 C studies of soils to determine rates of vertical accumulation of moss combined with carbon storage inventories on a sequence of burns to model how carbon accumulates in soils and moss after a stand-killing fire. We used soil drainage—moss associations and soil drainage maps of the old black spruce (OBS) site at the BOREAS northern study area (NSA) to areally weight the contributions of each moderately well drained, feathermoss areas; poorly drained sphagnum—feathermoss areas; and very poorly drained brown moss areas to the carbon storage and flux at the OBS NSA site. On this very old (117 years) complex of black spruce, sphagnum bog veneer, and fen systems we conclude that these systems are likely sequestering 0.01–0.03 kg C m −2 yr −1 at OBS-NSA today. Soil drainage in boreal forests near Thompson, Manitoba, controls carbon storage and flux by controlling moss input and decomposition rates and by controlling through fire the amount and quality of carbon left after burning. On poorly drained soils rich in sphagnum moss, net accumulation and long-term storage of carbon is higher than on better drained soils colonized by feathermosses. The carbon flux of these contrasting ecosystems is best characterized by soil drainage class and stand age, where stands recently burned are net sources of CO 2 , and maturing stands become increasingly stronger sinks of atmospheric CO 2 . This approach to measuring carbon storage and flux presents a method of scaling to larger areas using soil drainage, moss cover, and stand age information.

Journal of Geophysical Research D: Atmospheres↗
USGS1997

Case study modeling of turbulent and mesoscale fluxes over the BOREAS region

Results from aircraft and surface observations provided evidence for the existence of mesoscale circulations over the Boreal Ecosystem-Atmosphere Study (BOREAS) domain. Using an integrated approach that included the use of analytical modeling, numerical modeling, and data analysis, we have found that there are substantial contributions to the total budgets of heat over the BOREAS domain generated by mesoscale circulations. This effect is largest when the synoptic flow is relatively weak, yet it is present under less favorable conditions, as shown by the case study presented here. While further analysis is warranted to document this effect, the existence of mesoscale flow is not surprising, since it is related to the presence of landscape patches, including lakes, which are of a size on the order of the local Rossby radius and which have spatial differences in maximum sensible heat flux of about 300 W m-2. We have also analyzed the vertical temperature profile simulated in our case study as well as high-resolution soundings and we have found vertical profiles of temperature change above the boundary layer height, which we attribute in part to mesoscale contributions. Our conclusion is that in regions with organized landscapes, such as BOREAS, even with relatively strong synoptic winds, dynamical scaling criteria should be used to assess whether mesoscale effects should be parameterized or explicitly resolved in numerical models of the atmosphere.

Journal of Geophysical Research D: Atmospheres↗
USGS1997

Martian drainage densities

Drainage densities on Mars range from zero over large areas of volcanic plains to 0.3–0.5 km −1 locally on some volcanoes. These values refer to geologic units, not to drainage basins, as is normal for terrestrial drainage densities. The highest values are close to the lowest terrestrial values derived by similar techniques. Drainage densities were determined for every geologic unit portrayed on the 1:15,000,000 geologic map of Mars. Except for volcanoes the geologic unit with the highest drainage density is the dissected Noachian plains with a drainage density of 0.0074 km −1 . The average drainage density for Noachian units is 0.0032 km −1 , for Hesperian units is 0.00047 km −1 , and for Amazonian units is 0.00007 km −1 , excluding the volcanoes. These values are 2–3 orders of magnitude lower than typical terrestrial densities as determined by similar techniques from Landsat images. The low drainage densities, despite a cumulative record that spans billions of years, indicate that compared with the Earth, the channel-forming processes have been very inefficient or have operated only rarely or that the surface is extremely permeable. The high drainage density on volcanoes is attributed to a local cause, such as hydrothermal activity, rather than to a global cause such as climate change.

Journal of Geophysical Research E: Planets↗
USGS1997

The Phanerozoic impact cratering rate: Evidence from the farside of the Moon

The relatively recent (<1 b.y.) flux of asteroids and comets forming large craters on the Earth and Moon may be accurately recorded by craters with bright rays on the Moon's farside. Many previously unknown farside rayed craters are clearly distinguished in the low-phase-angle images returned by the Clementine spacecraft. Some large rayed craters on the lunar nearside are probably significantly older than 1 Ga; rays remain visible over the maria due to compositional contrasts long after soils have reached optical maturity. Most of the farside crust has a more homogeneous composition and only immature rays are visible. The size-frequency distribution of farside rayed craters is similar to that measured for Eratosthenian craters (up to 3.2 b.y.) at diameters larger than 15 km. The areal density of farside rayed craters matches that of a corrected tabulation of nearside Copernican craters. Hence the presence of bright rays due to immature soils around large craters provides a consistent time-stratigraphic basis for defining the base of the Copernican System. The density of large craters less than ∼3.2 b.y. old is ∼3.2 times higher than that of large farside rayed craters alone. This observation can be interpreted in two ways: (1) the average cratering rate has been constant over the past 3.2 b.y. and the base of the Copernican is ∼1 Ga, or (2) the cratering rate has increased in recent geologic time and the base of the Copernican is less than 1 Ga. We favor the latter interpretation because the rays of Copernicus (800–850 m.y. old) appear to be very close to optical maturity, suggesting that the average Copernican cratering rate was ∼35% higher than the average Eratosthenian rate. Other lines of evidence for an increase in the Phanerozoic (545 Ga) cratering rate are (1) the densities of small craters superimposed on Copernicus and Apollo landing sites, (2) the rates estimated from well-dated terrestrial craters (≤120 m.y.) and from present-day astronomical observations, and (3) the Proterozoic rate suggested by the crater record of Australia. The hypothesis most consistent with several key observations is that the cratering rate has increased by ∼2x during the past ∼300 m.y‥

Journal of Geophysical Research E: Planets↗
USGS1997

Creep of water ices at planetary conditions: A compilation

Many constitutive laws for the flow of ice have been published since the advent of the Voyager explorations of the outer solar system. Conflicting data have occasionally come from different laboratories, and refinement of experimental techniques has led to the publication of laws that supersede earlier ones. In addition, there are unpublished data from ongoing research that also amend the constitutive laws. Here we compile the most current laboratory-derived flow laws for water ice phases I, II, III, V, and VI, and ice I mixtures with hard particulates. The rheology of interest is mainly that of steady state, and the conditions reviewed are the pressures and temperatures applicable to the surfaces and interiors of icy moons of the outer solar system. Advances in grain-size-dependent creep in ices I and II as well as in phase transformations and metastability under differential stress are also included in this compilation. At laboratory strain rates the several ice polymorphs are rheologically distinct in terms of their stress, temperature, and pressure dependencies but, with the exception of ice III, have fairly similar strengths. Hard particulates strengthen ice I significantly only at high particulate volume fractions. Ice III has the potential for significantly affecting mantle dynamics because it is much weaker than the other polymorphs and its region of stability, which may extend metastably well into what is nominally the ice II field, is located near likely geotherms of large icy moons.

Journal of Geophysical Research E: Planets↗
USGS1998

Viscoelastic coupling model of the San Andreas fault along the Big Bend, southern California

The big bend segment of the San Andreas fault is the 300-km-long segment in southern California that strikes about N65°W, roughly 25° counterclockwise from the local tangent to the small circle about the Pacific-North America pole of rotation. The broad distribution of deformation of trilateration networks along this segment implies a locking depth of at least 25 km as interpreted by the conventional model of strain accumulation (continuous slip on the fault below the locking depth at the rate of relative plate motion), whereas the observed seismicity and laboratory data on fault strength suggest that the locking depth should be no greater than 10 to 15 km. The discrepancy is explained by the viscoelastic coupling model which accounts for the viscoelastic response of the lower crust. Thus the broad distribution of deformation observed across the big bend segment can be largely associated with the San Andreas fault itself, not subsidiary faults distributed throughout the region. The Working Group on California Earthquake Probabilities [1995] in using geodetic data to estimate the seismic risk in southern California has assumed that strain accumulated off the San Andreas fault is released by earthquakes located off the San Andreas fault. Thus they count the San Andreas contribution to total seismic moment accumulation more than once, leading to an overestimate of the seismicity for magnitude 6 and greater earthquakes in their Type C zones.

Journal of Geophysical Research B: Solid Earth↗