a mass failure model for the initial degradation of fault scarps with application to the 1959 scarps at hebgen lake montana abstract calculation of earthquake scarp ages from scarp morphology usually assumes that scarp materials reach their angle of repose immediately after a rupture however observations of the 1959 hebgen lake montana earthquake scarp and similar features worldwide confirm that scarps require a finite period of mass failure to reach the initial conditions for hillslope diffusion so the age of features less than 1000 yr old cannot be accurately estimated with methods based only on the linear diffusion equation we apply a numerical model of this interval of mass failure degradation to vertical initial angle scarps from the 1959 rupture at hebgen lake montana the mass failure rate coefficient r m ranges from 1 0 10 2 to 1 2 10 1 m yr 1 for young scarps at hebgen lake and nine other locations and has little or no dependence on climate conditions such as annual temperature range or average rainfall including an interval of mass failure gives more accurate age estimates where a scarp age is of the same order as the characteristic mass failure relaxation time of 10 1000 yr

fault scarps and deformation rates in lazio abruzzo central italy comparison between geological fault slip rate and gps data abstract we compare new and literature data concerning the integrated deformation rate since 18 ka for the central apennines with deformation rate data derived from a 6 year gps campaign we constructed topographic profiles across the majority of the active faults in the area we derived deformation rate data from displaced post glacial sediments and slopes associated with the last major glacial retreat that occurred in the region about 18 kyr ago paleoseismic investigations in this region clearly show that offset of these features is the cumulative effect of repeated surface faulting earthquakes with magnitudes in the range 5 5 ms 7 0 a cumulative throw rate diagram incorporating both our values and the values extracted from the literature i e previously published trenching studies scarp profiles and offset terraces is presented providing a regional picture of the spatial deformation rate distribution in the central apennines during the latest pleistocene and holocene by converting cumulative throw rates into heave rates we calculate maximum extension rates of ca 2 mm year at the location of the same transect where 6 2 mm year has been measured with gps over 6 years this discrepancy between geodetic and geological fault slip rate data implies that we have to be cautious whenever we use gps data for seismic hazard assessment purposes or when attempting to reconstruct the tectonic processes in an area because geodetic data may be unrepresentative of the longer term deformation rates on the other hand this discrepancy may provide insights into the mechanics of the seismic cycle we discuss various scenarios that may explain the mis match between long term and short term measurements

mica deformation fabrics and the seismic properties of the continental crust abstract seismic anisotropy originating within the continental crust is used to determine kinematic flow lines within active mountain belts and is widely attributed to regionally aligned mica however naturally deformed micaceous rocks commonly show composite e g s c fabrics it is necessary therefore to understand how both varying mica content and differing intensities of multiple foliations impact on seismic interpretations in terms of deformation fields an outcrop analogue for granitic mid crustal deformed zones is used here to calibrate the seismic response against both parameters seismic responses are modelled using crystallographic preferred orientations for polymineralic micaceous granitic gneisses measured using electron back scatter diffraction the sample results are generalised by modelling the effects of variations in modal composition and the relative importance of deformation fabrics of variable orientation so called rock and fabric recipes the maximum p and s anisotropy are calculated at 16 6 and 23 9 for single foliation gneisses but for mixed i e s c foliation gneisses these values reduce to 5 8 and 7 5 respectively furthermore mixtures of multiple foliations generate significant variations in the geometry of the seismic anisotropy this effect coupled with the geographical orientation of fabrics in nature can generate substantial variations in the orientation and magnitude of seismic anisotropy especially for shear waves as measured for the continental crust using existing receiver function and teleseismic near vertical incidence methods thus maps of seismic anisotropy varying with depth in deforming continents need not imply necessarily depth varying deformation kinematics and tectonic decoupling

morphological analysis of the lake lahontan shoreline and beachfront fault scarps pershing county nevada abstract with the diffusion equation representation of landform evolution providing reference calculations quantitative comparisons between profiles of the high stand of lake lahontan presented here and profiles of the lake bonneville shoreline scarps analyzed elsewhere reveal that the two sets of scarps are nearly identical morphologically when care is exercised in the selection of profiles the lahontan high stand scarps are remarkably reproducible from one profile to the next highly suggestive evidence that the processes that have shaped and changed the shape of these landforms are correspondingly regular slope angle differences of only 2 are both discernible and interpretable features of these late quaternary landforms cut in weakly consolidated alluvium at a level of uncertainty of no more than 50 per cent the mass diffusivity of the lahontan high stand is the same as the lake bonneville shoreline 1 1 gkg 1 gkg 1 m 2 ka this result suggests that much can be learned about the ages of late quaternary fault scarps cut in weakly consolidated alluvium throughout the basin and range province from their morphology alone the beachfront fault scarps themselves truncated by lake lahontan in our study area are morphologically only slightly older 15 to 18 ka bp than the lahontan high stand 12 ka bp

morphological dating of cumulative reverse fault scarps examples from the gurvan bogd fault system mongolia morphological dating of cumulative reverse fault scarps

analysis of scarp profiles evaluation of errors in morphologic dating morphologic analysis of scarp degradation can be used quantitatively to determine relative ages of different scarps formed in cohesionless materials under the same climatic conditions scarps of tectonic origin as well as wavecut or rivercut terraces can be treated as topographic impulses that are attenuated by surface erosional processes this morphological evolution can be modelled as the convolution of the initial shape with erosion or degradation function whose width increases with time such modeling applies well to scarps less than 10m high formed in unconsolidated fanglomerates to a good approximation the degradation function is gaussian with a variance measuring the degree of rounding of the initial shape this geometric parameter can be called the degradation coefficient a synthetic experiment shows that the degradation coefficient can be obtained by least squares fitting of profiles levelled perpendicular to the scarp gravitational collapse of the free face is accounted for by assuming initial scarp slopes at the angle of repose of the cohesionless materials 30 35 uncertainties in the measured profiles result in an uncertainty in degradation coefficient that can be evaluated graphically because the degradation coefficient is sensitive to the regional slope and to three dimensional processes gullying loess accumulation stream incision etc a reliable and accurate determination of degradation coefficient requires several long profiles across the same scarp the linear diffusion model of scarp degradation is a gaussian model in which the degradation coefficient is proportional to numerical age in that case absolute dating requires only determination of the propotionality constant called the mass diffusivity constant for holocene scarps a few meters high in loose alluvium under arid climatic conditions mass diffusivity constants generally range between 1 and 6 m 2 kyr morphologic analysis is a reliable method to compare ages of different scarps in a given area and it can provide approximate absolute ages of holocene scarplike landforms

fabric related seismic anisotropy in upper mantle xenoliths evidence from measurements and calculations the seismic properties and their directional dependence have been measured in four mantle peridotites from picritic tuffs vitim baikal region russia geothermobarometric data suggest a source depth for the xenoliths corresponding to the spinel garnet transition zone 40 90km the experimental data include the measurement of p and s wave velocities and densities at confining pressures to 600 mpa and temperatures to 600 c and the determination of velocity anisotropy and shear wave splitting the measured velocities are compared with velocity calculations based on lattice preferred orientation lpo of olivine orthopyroxene the modal composition of the rocks and the single crystal stiffness coefficients macroscopic velocity anisotropy is strongly controlled by the lpo of olivine measured 600 mpa room temperature and calculated p wave anisotropies compare fairly well although the experimentally determined velocities are markedly lower than the calculated velocities the correspondence between measured and calculated shear wave splitting data is less pronounced this is attributed to grain boundary effects retrogressive reactions interstitial glass incomplete crack closure and to the fact that the lpo of the cpx fraction has not been included in the calculation the magnitude of p wave anisotropy corresponds to that observed in mantle peridotites exposed at the surface ivrea zone northern italy ronda southern spain

modelling the anisotropic seismic properties of partially molten rocks found at mid ocean ridges abstract the problem of modelling the seismic properties of mid ocean ridge rocks of the axial magma chamber amc and the low velocity triangle lvt immediately below it has been addressed using two samples from oman ophiolite as examples the specimens are a layered gabbro from the lower oceanic crustal sequence and a harzburgite from the upper most mantle section at the palaeo ridge axis the seismic properties have been simulated at a temperature of 1200 c and pressure of 200 mpa so that the basalt melt is above its solidus various effective medium methods are discussed in the perspective of modelling rocks that have a strong background elastic anisotropy due to crystal preferred orientation cpo and an introduced anisotropy due to oriented melt filled inclusions calculations using various methods show a wide range of predicted melt fractions for a given seismic velocity the self consistent scheme scs and differential effective medium dem are compared in some detail a tensorial model was developed using a poro elastic method of gassman at low seismic frequency and a standard dem with isolated basalt inclusions at high frequency at low frequency the basalt pore fluid is considered to be everywhere connected and the pressure uniform whereas at high frequency basalt inclusions are isolated assuming the medium to be a standard linear solid the low and high frequency velocities were used to calculate the anisotropy of attenuation calculations with spherical basalt inclusions show that the seismic velocities decrease and attenuation increase with increasing melt fraction the symmetry of the background anisotropy due to cpo is preserved but gradually reduced with increasing melt fraction for the seismic velocities observed for the axial magma chamber amc with v p of 3 5 to 4 7 km s the model predicts 50 70 melt for the low velocity triangle lvt below the amc with v p of 4 8 5 6 km s the model predicts 35 50 melt predictions from the observed attenuations q 1 at the amc of 0 05 0 02 and in the lvt of 0 02 0 01 are around 60 and 45 melt respectively seismic anisotropy has been modelled with ellipsoidal basalt pancake shaped inclusions with their circular sections in the xy foliation plane only a 2 3 of ellipsoidal basalt inclusions are required to over print the anisotropy of the background medium there is a rapid decrease in v p and increase of q 1 for propagation normal to the foliation z with increasing axial ratio of the inclusions with increasing axial ratio the model predicts decreasing amounts of melt for given velocity v p in the z direction for the lvt only 15 25 melt is predicted for a 10 1 inclusion instead of the 35 50 of the spherical inclusion

paleoseismicity and neotectonics of the cordillera blanca fault zone northern peruvian andes the cordillera bianca fault zone is a major west dipping normal fault that bounds the west side of a 120 to 170 km wide zone of active extension along the crest of the northern peruvian andes the fault is approximately 210 km long and exhibits continuous geomorphic evidence of repeated late pleistocene and holocene displacements but has not been the source of historical or teleseismically recorded earthquakes trenching and mapping of fault scarps provide new information on earthquake recurrence slip rate timing of the most recent events and andean neotectonics at quebrada queroccocha 55 km from the south end of the zone an 11 000 to 14 000 year old moraine is displaced 12 15 m and younger valley fill lacustrine and fluvial deposits are displaced 7 5 8 m scarp profiles tectonic terraces and trench exposures indicate five to seven scarp forming earthquakes of 2 3 m per event during the past 11 000 14 000 years at this location considering uncertainties in the number of events and in the age of the displaced moraine the average recurrence interval is estimated to be 2440 1060 years detrital charcoal from colluviums that predate and postdate the most recent event yielded ages of 2480 65 and 750 80 14c years b p respectively and stratigraphic relationships suggest that the event may be closer to the older date the late quaternary slip rate at quebrada queroccocha is 0 86 1 36 mm yr and appears to be representative of much of the fault zone at pachma bajo located 30 km from the north end of the zone on a separate segment displacement during each of the past two events was 2 to 3 m the morphology of scarps in alluvial fan and debris flow deposits indicates recurrence intervals of about 1000 3000 years relationships between pre inca walls and faulted deposits suggest that 1500 2000 years have elapsed since the most recent event fault behavior parameters particularly displacement per event and fault segment length show that the cordillera bianca fault zone is similar to seismogenic normal faults that nucleate m 7 7 earthquakes at depths of approximately 15 km the quebrada queroccocha slip rate and analysis of other normal faults in the region suggest cumulative late quaternary extensional strain rates of about 1 4 mm yr across the complete zone of andean crestal extension

the 3d quantitative lattice and shape preferred orientation of a mylonitised metagranite from monte rosa western alps combining neutron diffraction texture analysis and synchrotron x ray microtomography abstract two complementary 3d techniques neutron diffraction and synchrotron x ray microtomography sxr ct were used to compare the shape and lattice preferred orientations of a mylonitised metagranite from the monte rosa unit western alps italy the goal of using these techniques was to obtain two different orientation distribution functions although the two functions describe relatively independent characteristics of the rock fabric nonetheless they also exhibit close relationships to macroscopic fabrics and may be complementarily used to quantify rock fabrics and microstructures thereby highlighting 3d features that cannot be obtained with either technique if used independently we describe an approach that can be potentially useful in various disciplines e g structural geology rock mechanics tectonics and geophysics when a complete data set of preferred orientations and size distribution is needed micas display a strong orthorhombic symmetry between mesoscopic lineation and microscopic spo and lpo whereas quartz and feldspars are characterised by a monoclinic symmetry between mesoscopic lineation and lpo these observations suggest a rheological decoupling between the weak phase mica layers and the stronger quartz feldspar layers this mechanical decoupling occurred during the alpine subduction collision when the monte rosa unit was part of the insubric line system and accommodated large vertical strain

tomographic imaging of late quaternary faulting oquirrh mountains utah 1 seismic tomography can be used to image colluvial material in the subsurface by inverting first arrival travel times for velocity colluvial material deposited at the base of a fault scarp free face often appears as a low velocity zone lvz on a tomogram because it is generally less compacted and cemented than the surrounding alluvium a tomogram generated from a forward model of a synthetic velocity structure successfully images two lvzs stacked in the hanging wall of a normal fault the mercur fan oquirrh mountains utah provides an opportunity to look for stacked lvzs in a distributed fault zone three tomographic images across fault scarps on an intermediate age alluvial fan can be used to identify two stacked low velocity zones interpretation of a fourth tomogram is less conclusive these two low velocity zones are interpreted as colluvial packages separated by higher velocity alluvial material and suggest that tectonic activity is interspersed with pulses of fan building

crystallographic preferred orientation magnetic and seismic anisotropy in rocks from the finero peridotite ivrea verbano zone northern italy interplay of anisotropy contributions from different minerals abstract mineral alignment can provide valuable information on a rock s geological history many structural tectonic or geodynamic studies rely on determining the preferred orientation of rock forming minerals either by direct texture determination or using anisotropy measurements as proxies robust interpretations of anisotropy require detailed understanding of its relationship with texture this study investigates the texture magnetic and seismic anisotropies of ultramafic rocks from the ivrea verbano zone ivz northern italy with a special focus on the interplay between anisotropy contributions carried by different minerals texture was obtained from electron backscatter diffraction magnetic anisotropy from low and high field anisotropy of susceptibility measurements and seismic anisotropy from p and s wave velocities along the three principal directions of the macroscopic fabric using 22 mm diameter cores texture based models of magnetic anisotropy agree well with corresponding measurements larger variability is observed for seismic anisotropy possible explanations are 1 velocities were measured in three directions insufficient to determine the full elasticity tensor and 2 seismic anisotropy depends on grain boundaries pores and cracks in addition to crystallographic preferred orientation the contributions to magnetic and seismic anisotropy of each constituent mineral can interfere positively leading to a larger overall anisotropy or negatively resulting in a weaker anisotropy seismic anisotropy is mostly controlled by olivine whereas magnetic anisotropy can be dominated by olivine pyroxene or hornblende this study illustrates that although the relationship between texture and anisotropy can be complex it is possible to quantitatively predict magnetic and to a lesser degree seismic anisotropy from texture data even though both magnetic and seismic anisotropies are common proxies for texture they are not coaxial and no simple relationship between the degree of magnetic and seismic anisotropy was found these results underline the importance of understanding different minerals contributions to anisotropy in upper mantle rocks

modification of wave cut and faulting controlled landforms from a casual observation that the form of degraded fault scarps resembles the error function this investigation proceeds through an elementary diffusion equation representation of landform evolution to the application of the resulting equations to the modern topography of scarplike landforms the morphologic observations can be analyzed either in the form of one or more cross strike elevation profiles or in the form of the slope offset plot a point plot of maximum scarp slope versus scarp offset working with either or both of these data representations for nine geologic structures which range in age from 3 to 400 ka b p and in offset from 1 to 50 m we apply analytical solutions for the vertical initial value scarp the vertical continuous offset scarp and the finite slope initial value scarp the model calculations are intrinsically ambiguous yielding as the final answer only the product t in the case of the initial value problem or the product a 1 in the case of the repeated faulting problem here t is the age of a single scarp forming event 2a is the vertical slip rate and is the mass diffusivity a single profile across three sea cliffs along the santa cruz california coast is analyzed as three separate initial value problems a reasonably constrained age for the sea cliff standing above the highway 1 platform returns 11 gkg 1 gkg 1 m2 ka with this we can date the two older sea cliffs in fact we do the converse age estimates for these two older sea cliffs based on a uniform rate of uplift both yield the same as for the lower sea cliff we treat a single profile of the raymond fault in pasadena san marino in terms of the repeated faulting problem for it the uplift rate of r crook and others yields 16 gkg the very substantial preexisting offset across the raymond fault must have been buried leveled some 230 ka b p when the modern topography began to form our analysis of the lake bonneville shoreline scarps reveals a dependence of t on 2a suggestive of nonlinear modification processes this appearance is treated with the finite slope initial value scarp model to determine 1 1 gkg for the lake bonneville shoreline scarps the suggestion of m n machette that approximately 100 000 year old meter high scarps are unobservable in weakly consolidated alluvial terranes of the basin and range and rio grande rift valley provinces can be formulated as 1 gkg the coincidence between this inequality and the lake bonneville shoreline is striking and it suggests that the value of 1 gkg may be generally applicable as a good first approximation to the modification of alluvial terranes within the semiarid regions of the western united states the lake bonneville shoreline is the basis for dating four sets of fault scarps in west central utah the drum mountains fault scarps can be modeled in several different circumstances but the most likely interpretation is that these fault scarps formed as the result of a single episode of normal faulting 3 6 to 5 7 ka b p the younger age is associated with quite low initial slope angles 25 the other three sets of fault scarps show no evidence for finite initial value slopes fault scarps along the eastern base of the fish springs range are very young 3 ka b p we estimate the age of fault scarps along the western flank of the oquirrh mountains to be 32 ka b p which meets the weak geologic constraint that they be older than the lake bonneville shoreline fault scarps along the northeastern margin of the sheeprock mountains are even older 53 ka b p an intriguing consequence of our single event analysis of these scarps is that an 11 5 m offset occurred in a single earthquake

morphological dating of cumulative reverse fault scarps examples from the gurvan bogd fault system mongolia summary we relate reverse fault scarp morphology formed by several earthquake dislocations to the average deformation rate using a morphological dating model based on a diffusion analogue of erosion our scarp degradation model includes diffusive erosion during the interseismic period the gravitational collapse of the coseismic fault scarp just after formation and the variation of the surface rupture location interactions between thrusting and geomorphic processes acting on scarp morphology are analysed along the gurvan bogd range in mongolia four main processes acting on scarp morphology were distinguished i gravitational collapse of the frontal scarp resetting the diffusive scarp if fault offsets are big and faulting is localized 2 progressive erosion of the fault scarp during the interseismic period 3 folding associated with the frontal thrust and backthrusts 4 competing alluvial deposition on mountain piedmont slopes and abrasion of the fault scarp by wash processes the growth of cumulative reverse fault scarps is suppressed when they are located in the outwash of major drainage basins they can grow higher in distance from major catchment discharges the modelling suggests that the morphology of the scarp and its apparent degradation stage depend on the parameters controlling the amount of frontal collapse the magnitude of coseismic offsets the dip of the fault near the surface and the step distance between faults folding associated with thrusting creates a convexity on the upper part of the scarp and increases its height the comparison of different scarp profiles suggests that folding leads to an overestimate of the morphological age we estimate a diffusion coefficient at 3 3 1 7 m 2 ka l morphological ages calculated with our model confirm that slip rate along reverse faults of the gurvan bogd range has not been constant over the last 100 ka

seismic velocity modelling of the carboneras fault zone se spain abstract the carboneras fault zone forms part of a major strike slip fault system in se spain striking ne sw and accommodating up to 40 km displacement it affects basement metamorphic rocks and unconformably overlying upper miocene sediments and volcanic rocks high resolution shallow seismic tomographic sections were made across the fault zone in two localities from the same areas fault rocks and their wallrocks were collected for laboratory seismic velocity measurements the laboratory data were corrected for the substantial effects of near surface crack damage by combining these results with geological cross sections forward velocity models for the fault zone were constructed to compare with field seismic measurements and hence to ground truth the inferences made from them these velocity depth relationships matched moderately well with those extracted from the in situ tomography results aspects of the in situ seismic sections matched features on the forward modelled sections but the comparisons showed that it is important to have some degree of foreknowledge of the geology to be able successfully to interpret seismic tomography sections as an exploration tool

relating relative hydraulic and electrical conductivity in the unsaturated zone numerical modeling was used to generate pore scale structures with different structural properties they were partially saturated according to wetting and drainage regimes using morphological operations for a range of saturations the hydraulic and electrical conductivities of the resulting partially saturated grain packs were numerically computed to produce relative hydraulic conductivity versus saturation and relative electrical conductivity versus saturation curves the relative hydraulic conductivities were then compared to the relative electrical conductivities for the same saturations and it was found that relative hydraulic conductivity could be expressed as relative electrical conductivity to a power law exponent this exponent was not correlated to porosity specific surface area or tortuosity it did change according to whether the soil was wetting or draining however a value of 2 1 reproduced relative hydraulic conductivity from relative electrical conductivity with little added error the effects of surface conduction on the observed power law relationship due to either low fluid electrical conductivity or increased clay content were analyzed the relationship was found to hold for fluid conductivities typical of groundwater and for clay content of less than 5 if the clays were layered perpendicular to electrical flow the relationship breaks down for electrical flow parallel to clay layers which makes the choice of electrode arrangement important in cases where clay may be present this relationship can be used with secondary pressure or saturation data to characterize a soil s hydraulic conductivity curve

planar high angle faulting in the basin and range geodetic analysis of the 1983 borah peak idaho earthquake geodetic elevation changes record the broad scale deformation associated with the m 7 0 october 28 1983 borah peak idaho earthquake on the lost river fault the crest of the lost river range rose 0 2 m and adjacent thousand springs valley subsided 1 0 m in relation to reference points 45 km from the main shock epicenter the deformation was modeled by dislocations in an elastic half space a planar fault with uniform dip slip of 2 05 0 10 m dipping 47 o 2osw and extending to a vertical depth of 13 3 1 2 km fits the geodetic data best and is also consistent with the main shock hypocenter and fault plane solution the geodetic moment is 2 6 0 5 x 1019 n m 2 6 0 5 x 1026 dyn cm and the estimated static stress drop is 2 9 0 4 mpa 29 4 bars tests for coseismic slip on listric faults which flatten with depth and on detachments horizontal faults or shear zones showed fits to the geodetic data that are inferior to those for planar high angle faults no detectable coseismic slip occurred on the mesozoic white knob thrust fault although the low angle thrust sheet intersects the south end of lost river fault near the 1983 mainshock epicenter if the high angle lost river fault abuts a flat lying detachment fault or shear zone such a structure must lie at depths of 12 km near the brittle ductile transition where stick slip behavior gives way to creep the depth and geometry of faulting at borah peak is similar to that inferred from seismic and geodetic evidence for the 1954 m 7 2 fairview peak nevada and the 1959 m 7 3 hebgen lake montana events suggesting that if detachments are active at these localities they are deep and most likely slip by creep

pyroxene orientation within the upper mantle petrofabric analyses of orthopyroxene and olivine for a suite of rocks from the bay of islands ophiolite newfoundland show systematic relationships between the minerals9 fabrics the plane of the mohorovicic discontinuity determined from the contact of the gabbro and ultramafic sections and the spreading direction inferred from the attitude of the sheeted dikes the orthopyroxene c crystallographic axes maxima which are aligned subparallel to the spreading direction coincide with olivine a axes maxima the orthopyroxene b axes and olivine c axes also lie approximately within the plane of the mohorovicic discontinuity but parallel the ridge whereas the orthopyroxene a axes maxima and olivine b axes maxima are approximately perpendicular to the mohorovicic discontinuity using olivine and orthopyroxene elastic constants and their pressure derivatives velocities have been calculated from composite petrofabric data and contoured for varying proportions of olivine and pyroxene the results of these calculations show that within a vertical direction compressional wave velocities change only slightly with increasing pyroxene content whereas parallel to the mohorovicic discontinuity velocities decrease with increasing pyroxene content anisotropy decreases from 4 for a rock with 100 olivine to 2 6 for a rock with 50 olivine and 50 orthopyroxene also within the plane of mohorovicic discontinuity the maximum and minumum velocities occur at approximately the same azimuth for differing percentages of orthopyroxene a comparison of the calculated anisotropy and the observed seismic anisotropy off the coast of california shows a striking agreement from this comparison it is estimated that the percentage of orthopyroxene within the upper mantle of this region varies between 0 and 30

first results on the lpo derived seismic properties of iron ores from the quadrilatero ferrifero region southeastern brazil abstract determinations of the lpo derived seismic properties of iron ores were carried out in five samples with contrasting mineralogy hematite magnetite and quartz and deformed in different conditions all the samples are seismically quasi isotropic or weakly anisotropic which reflect i an absent or weak crystallographic preferred orientation cpo in some samples ii the high modal content of magnetite iii the relatively weak anisotropy of elastic stiffness of hematite single crystal such variables induce a low anisotropic seismic behavior even in high strained iron ores with strong preferred orientation of hematite a plane of seismic transversal isotropy parallel to the foliation of the aggregates is developed in hematite magnetite aggregates and in itabirites with strong cpo of hematites in these high strained aggregates some relationships between the crystallographic axes of hematite and the propagation velocities can be observed the magnitudes of p and s wave velocities derived of hematite cpo are lower than the values experimentally determined in iron ores such a difference probably reflects other microstructural variable which were not taken into account in the present contribution e g shape preferred orientation

an olivine fabric database an overview of upper mantle fabrics and seismic anisotropy we present a unique database of 110 olivine petrofabrics and their calculated seismic properties the samples come from a variety of the upper mantle geodynamic environments ophiolites subduction zones and kimberlites with a wide range of micro structures a phenomenological relationship is established between p and s wave seismic anisotropy and the degree of crystal alignment fabric strength seismic anisotropy increases rapidly at low fabric strength before reaching a near saturation level of 15 to 20 for p waves and 10 to 15 for s waves despite a large variation in the symmetry of fabric patterns the average seismic properties of the different fabric micro structural and geodynamic settings have similar anisotropies in both magnitude and symmetry hence it would seem possible to determine some measure of fabric strength from seismic anisotropy if the dimensions of the anisotropic region are known but not geodynamic environment or details of the petrofabric pattern a simple pattern of seismic anisotropy characterises the average sample of the database which has the following features the polarisation plane of the fastest s wave is parallel or sub parallel to the foliation plane the maximum shear wave splitting is parallel to the y structural direction in foliation plane and normal to the lineation the maximum of the p wave velocity is parallel to the high concentration of 100 axes which is sub parallel to the lineation the 100 orientation distribution has the greatest influence on the p wave seismic anisotropy the 100 and 001 orientation distributions have the greatest influence on the symmetry of s wave anisotropy although the magnitude of anisotropy is influenced by the distribution of all three principal axes although the database only contains olivine petrofabrics this statistical study clearly shows that seismic anisotropy can be used to deduce the orientation of the structural frame in the upper mantle 1998 elsevier science b v all rights reserved

when is a fault extinct existing regulatory definitions of time periods over which evidence should be searched to demonstrate whether a fault is or is not active may be unconservative in slowly deforming continental regions where recurrence intervals of surface faulting are in excess of 100 000 years self evidently no definition of an active fault can be satisfactory when founded on some extrinsic time period instead it must be derived from the properties of the system itself preferably through knowledge of fault movement recurrence intervals or alternatively and conservatively from an understanding of the time period over which the present conditions of regional stress and strain the current tectonic regime have prevailed borrowing from volcanological terminology out of the definition of the active fault emerges an antithesis the extinct fault time periods by which directly dated faults are designated as active or extinct have to be locally derived

the growth of geological structures by repeated earthquakes 2 field examples of continental dip slip faults a strong test of our understanding of the earthquake cycle is the ability to reproduce extant fault bounded geological structures such as basins and ranges which are built by repeated cycles of deformation along strike slip faults the coseismic and interseismic deformation can be nearly equal in magnitude and opposite in sign resulting in little permanent deformation except for the fault offset for dip slip faults portions of the crust are lifted and dropped and so buoyancy forces are exerted the seismic and interseismic deformations do not balance and structures grow and become subject to erosion and deposition we consider three examples for which the structure and fault geometry are well known the white wolf reverse fault in california site of the 1952 kern county m 7 3 earthquake the lost river normal fault in idaho site of the 1983 borah peak m 7 0 earthquake and the cricket mountain normal fault in utah site of quaternary slip events basin stratigraphy and seismic reflection records are used to profile the structure and coseismic deformation measured by leveling surveys is used to estimate the fault geometry to reproduce these structures we add the deformation associated with the earthquake cycle the coseismic slip and postseismic relaxation to the flexure caused by the observed sediment load treating the crust as a thin elastic plate overlying a fluid substrate the cumulative deformation is principally dependent on the elastic plate thickness modestly sensitive to the sediment substrate density difference and insensitive to the fluid viscosity for the 4 to 8 ma structures we deduce a longterm flexural rigidity of 2 15 1019 nm this is equivalent to an elastic plate thickness of 2 4 km for a young s modulus of 2 5 1010 nm 2 this value is found where independent estimates of the elastic thickness from the coherence between surface topography and gravity yield values of about 4 km but where coseismic fault slip extends to a depth of 10 15 km thus much of the seismogenic crust must weaken substantially during the life of active faults causing the fault bounded basins to narrow over time

evaluating hillslope diffusion and terrace riser degradation in new zealand and idaho 1 the relationship of sediment flux to surface slope is a predominant control on the evolution of transport limited hillslopes although both linear and nonlinear sediment flux models have been proposed and widely debated this functional relationship remains one of the outstanding questions in geomorphology we examine degradation of fluvial terrace risers in new zealand and idaho in order to test the linear diffusion model and to better understand the relationship between sediment flux and slope we implement three techniques based on the linear diffusion model as well as a geometric scaling method that uses proportional relationships in riser geometry rather than a specific transport equation model to assess rates of degradation we show that techniques that utilize the riser midpoint slope as the primary metric of degradation as opposed to the full scarp profile offer the simplest and most reliable assessment of riser degradation our findings suggest that the linear diffusion model does not accurately describe terrace riser degradation except for low angle risers for which linear and nonlinear forms are indistinguishable results from the geometric scaling method show that degradation rates in both regions differ by a factor of two between equator and pole facing risers and that diffusion ages of idaho risers are approximately 2 times those of new zealand risers similar geometric trends in riser evolution from both regions reveal that terrace riser degradation is governed by a widely applicable relationship between sediment flux and slope

using neutron diffraction to investigate the elastic properties of anisotropic rocks results from an olivine orthopyroxene mylonite 1 the penetrating power of neutrons means that neutron diffraction may be employed to determine the lattice parameters of mineral phases within samples over 1 cm in size we have exploited this fact to obtain high resolution measurements of the elastic strains experienced by olivine and orthopyroxene during room temperature uniaxial deformation experiments performed in a neutron beam line on a mylonite from the oman ophiolite specimens were loaded in three orthogonal directions with respect to the macroscopic fabric of the rock in each case the average olivine and orthopyroxene strains were the same even though the elastic stiffnesses of the two phases were different in comparison with the strains expected from the single crystal elastic stiffness tensors the average strains in the different lattice directions for each phase were found to be greater in the stiffer directions and lesser in the more compliant directions the overall effect of this tendency toward strain homogenization was for the measured elastic anisotropy of the rock to be significantly lesser than that given by voigt reuss averaging of the single crystal elastic properties according to the lattice preferred orientations of each phase the technique used to derive these conclusions potentially provides an important experimental method for the quantitative examination of nonfracture related controls on the elastic anisotropy of geological materials

viscoplastic self consistent and equilibrium based modeling of olivine lattice preferred orientations implications for the upper mantle seismic anisotropy anisotropy of upper mantle physical properties results from lattice preferred orientation lpo of upper mantle minerals in particular olivine we use an anisotropic viscoplastic self consistent vpsc and an equilibrium based model to simulate the development of olivine lpo and hence of seismic anisotropy during deformation comparison of model predictions with olivine lpo of naturally and experimentally deformed peridotites shows that the best fit is obtained for vpsc models with relaxed strain compatibility slight differences between modeled and measured lpo may be ascribed to activation of dynamic recrystallization during experimental and natural deformation in simple shear for instance experimental results suggest that dynamic re crystallization results in further reorientation of the lpo leading to parallelism between the main 010 100 slip system and the macroscopic shear thus modeled simple shear lpos are slightly misoriented relative to lpos measured in natural and experimentally sheared peridotites this misorientation is higher for equilibrium based models yet seismic properties calculated using lpo simulated using either anisotropic vpsc or equilibrium based models are similar to those of naturally deformed peridotites errors in the prediction of the polarization direction of the fast s wave and of the fast propagation direction for p waves are usually 15 moreover overestimation of lpo intensities in equilibrium based and vpsc simulations at high strains does not affect seismic anisotropy estimates because these latter are weakly dependent on the lpo intensity once a distinct lpo pattern has been developed thus both methods yield good predictions of development of upper mantle seismic anisotropy in response to plastic flow two notes of caution have nevertheless to be observed in using these results 1 the dilution effect of other upper mantle mineral phases in particular enstatite has to be taken into account in quantitative predictions of upper mantle seismic anisotropy and 2 lpo patterns from a few naturally deformed peridotites cannot be reproduced in simulations these abnormal lpos represent a small percent of the measured natural lpos but the present sampling may not be representative of their abundance in the earth s upper mantle

petrofabric derived seismic properties of a mylonitic quartz simple shear zone implications for seismic reflection profiling the link between petrofabric lpo and seismic properties of an amphibolite facies quartzofeldspathic shear zone is explored using sem ebsd the shear zone lpo develops by a combination of slip systems and dauphine twinning with a maximum parallel to lineation x and c maximum normal to mylonitic foliation xy the lpo are used to predict elastic parameters from which the three dimensional seismic properties of different shear zone regions are derived results suggest that lpo evolution is reflected in the seismic properties but the precise impact is not simple in general the p wave velocity vp minimum is parallel to the a axis maximum the direction of maximum shear wave splitting avs is parallel to mylonitic foliation and the vp maximum and avs minimum are parallel to the c axis maximum the seismic anisotropy predicted is significant and increases from shear zone wall rock to mature mylonite p wave anisotropy ranges from 11 13 fast and slow shear waves anisotropy range from 6 15 and the magnitude of shear wave splitting ranges from 9 16 nevertheless such anisotropy requires a considerable thickness of rock with this lpo before it becomes seismically visible i e 100 s m for local earthquakes 10 s m for controlled source experiments however reflections and mode conversions provide much better resolution particularly across tectonic boundaries the low symmetry and strong anisotropy due to the lpo suggest that multi azimuth wide angle reflection data may be useful in the determination of the deformation characteristics of deep shear zones a number of factors may produce elastic anisotropy in rocks including variations in the spatial distribution of mineral phases layering grain size and shape fabrics grain boundary properties and the presence of oriented pores or fractures e g kern wenk 1985 mainprice et al 2003 in addition because elastic properties vary with respect to direction in single crystals the majority of rock forming minerals are elastically anisotropic e g christensen 1966 thus deformation processes such as dislocation creep which produce strong crystal lattice preferred orientations lpo in anisotropic minerals may also induce bulk elastic anisotropy it is no surprise therefore that petrophysical properties such as seismic velocities are often highly anisotropic in bulk rock aggregates e g babuska cara 1991 understanding of the impact of microstructural variables on elastic anisotropy has been obtained typically from experimental measurements of elastic properties of minerals or theoretical models that incorporate microstructural variables e g burlini kern 1994 wendt et al 2003 experimental studies have focused primarily on the influence of fractures or lpo on the elastic properties of minerals and rocks in particular the seismic properties can be measured via direct laboratory methods at ambient or elevated temperatures and pressures e g kern 1982 although it is often difficult or impossible to obtain the complete three dimensional property orientation distribution from a single experiment however see pros et al 2003 furthermore experimental investigation of seismic properties usually involves rocks in which all microstructural fabric elements contribute effectively masking any lpo component an alternative approach is to calculate the seismic properties of a rock aggregate from individual crystal orientation measurements incorporating the single crystal elastic constants and the lpo for each mineral weighted according to its modal content e g mainprice humbert 1994 the seismic phase velocities and anisotropies then can be calculated in every direction e g mainprice silver 1993 recently the advent of electron backscattered diffraction ebsd in the scanning electron microscope sem has made it possible to measure lpo in statistically viable numbers from different minerals within a rock aggregate whilst maintaining a oneto one relationship with other microstructural elements e g mainprice et al 1993 lloyd 2000 mauler et al 2000 bascou et al 2001 in this contribution the seismic properties of an amphibolite facies mylonitic quartz shear zone are predicted by averaging the single crystal elastic properties according to the sem ebsd derived lpo in order to study the relationship between microstructure petrofabric and seismic properties this approach illustrates the usefulness of integrating geological studies of rock microstructures and particularly petrofabrics with geophysical studies of seismic properties to obtain a fuller understanding of geodynamic processes

from crystal to crustal petrofabric derived seismic modelling of regional tectonics this collection of papers presents recent advances in the study of deformation mechanisms and rheology and their applications to tectonics many of the contributions exploit new petrofabric techniques particularly electron backscatter diffraction to help understand the evolution of rock microstructure and mechanical properties

comparison of morphological dating models for cumulative reverse fault scarps 1 degradation morphology of scarps can be used to estimate the age of an episode of uplift linear diffusion models for slope degradation provide analytical solutions relying on simplified geometry and kinematics in this paper we evaluate to what extent such models can be applied to date cumulative reverse fault scarps by comparing their predicted degradation coefficients with those of a more realistic numerical model two analytical models with increasing complexity have been considered the cu and iu models represent the morphological evolution of a single vertical fault at a constant rate of uplift and that of a vertical fault shifting incrementally respectively synthetic data are generated by a numerical model accounting for reverse faulting linear diffusion and gravity controlled collapse we show that for cumulative reverse faulting without folding approximated models neglecting fault dip and gravitational collapse lead to valid estimates of the degradation coefficient in a lot of cases because gravitational collapse and reverse faulting are competitive geomorphic processes we provide estimates of the shifts expected on the degradation coefficient according to neglected processes

effect of height and orientation microclimate on geomorphic degradation rates and processes late glacial terrace scarps in central idaho terrace scarps can serve as a nearly ideal natural laboratory for the study of the evolution of slopes this paper examines the effects of scarp size height and orientation microclimate by keeping constant variables such as age lithology and regional climate if a scarp degrades as a closed system and downslope movement is directly proportional to surface gradient the evolution of the scarp is modeled by the diffusion equation for a group of scarps of same age and known starting angle the diffusion equation model predicts the relation between maximum scarp angle and scarp height h late pleistocene terrace scarps now as steep as 33 25 as well as measured angles of repose for sand and gravel require a starting angle as steep as 33 5 for latest pleistocene idaho and utah scarps as h increases is gentler more degraded than modeled by the diffusion equation with a constant rate coefficient the degradation rate coefficient c increases tenfold with scarp height it should not change with scarp height if downslope movement is solely determined by surface gradient to the first power soil wash appears to be responsible for this departure from the diffusion equation model for transport rate by soil wash is a function of scarp size height south facing scarps are less vegetated and more degraded than north facing scarps for scarps 2 m high the degradation rate c on s facing scarps is 2 times that on n facing scarps for 10 m scarps it is 5 times the observed dependence of the rate coefficient c on scarp height can be removed by normalizing c to values for west facing scarps of the same height the residual c values calculated by this method correlate well with differences in incident solar radiation resulting from the different scarp orientations and maximum gradients this correlation demonstrates the importance of orientation on slope processes and their rates through the differences in freeze thaw cycles soil moisture and vegetative cover scarp morphology may be used to estimate age if one accounts for the effects of climate and for scarp height orientation and lithology for example using the dated bonneville shoreline scarps for calibration and comparing only scarps of equal height we estimate the drum mountains fault scarps to be 9 000 yr old this age is about twice that produced by previous diffusion equation calculations that have not accounted for the height as we have here but it is the same as independent geologic estimates of their age

fault behavior and characteristic earthquakes examples from the wasatch and san andreas fault zones paleoseismological data for the wasatch and san andreas fault zones have led to the formulation of the characteristic earthquake model which postulates that individual faults and fault segments tend to generate essentially same size or characteristic earthquakes having a relatively narrow range of magnitudes near the maximum analysis of scarp derived colluvium in trench exposures across the wasatch fault provides estimates of the timing and displacement associated with individual surface faulting earthquakes at all of the sites studied the displacement per event has been consistently large measured values range from 1 6 to 2 6 m and the average is about 2 m on the basis of variability in the timing of individual events as well as changes in scarp morphology and fault geometry six major segments are recognized along the wasatch fault on the basis of the most likely number of surface faulting events 18 that have occurred on segments of the wasatch fault zone during the past 8000 years an average recurrence interval of 400 666 years with a preferred average of 444 years is calculated for the entire zone geologic data on the distribution of slip associated with prehistoric earthquakes and slip rates along the south central segment of the san andreas fault suggest that the m 8 1857 earthquake is a characteristic earthquake for this segment comparisons of earthquake recurrence relationships on both the wasatch and san andreas faults based on historical seismicity data and geologic data show that a linear constant b value extrapolation of the cumulative recurrence curve from the smaller magnitudes leads to gross underestimates of the frequency of occurrence of the large or characteristic earthquakes only by assuming a low b value in the moderate magnitude range can the seismicity data on small earthquakes be reconciled with geologic data on large earthquakes the characteristic earthquake appears to be a fundamental aspect of the behavior of the wasatch and san andreas faults and may apply to many other faults as well

fault slip rates and initiation age based on diffusion equation modeling wasatch fault zone and eastern great basin models of the evolution of fault scarp morphology provide time elapsed since slip initiated on a faulted surface and may therefore provide more accurate estimates of slip rate than the rate calculated by dividing scarp offset by the age of the ruptured surface to accomplish this task linear and nonlinear models of sediment transport are calibrated from the morphology of lake bonneville shoreline scarps and fault scarps formed by multiple surface rupturing earthquakes along the wasatch fault zone wfz profile modeling of scarps formed by several events distributed through time is done using a constant slip rate csr solution and yields a value of a 1 2 slip rate diffusivity time elapsed since slip initiated on a fault is determined by establishing a value for and measuring total scarp offset csr nonlinear modeling 0 2 8 1 1 m2 kyr wfz of faults along the west slope of the oquirrh mountains indicates a slip rate of 0 1 mm yr since 50 to 65 ka which is corroborated by cosmogenic dating 10be 26al age 75 ka the slip rate along the west flank of the stansbury mountains varies from 0 04 to 0 2 mm yr for time frames of 10 to 100 ka with the most recent rupture on the northern portion of the fault zone 10 ka scarp analysis of the southern end of the nephi segment wfz suggests either temporal clustering or variable slip rate as indicated by differences in the short term 1 3 mm yr for 4 3 ka versus long term 0 4 mm yr for 70 ka slip rates

evolution of undercut slopes on abandoned incised meanders in the eastern highland rim of tennessee usa abstract field surveys location for time reasoning and computer modeling were used to study the evolution of slopes on valley walls of abandoned bedrock meanders on the eastern highland rim tennessee hillslopes on the undercut slopes of cutoff incised meanders were ordered as to relative age by the height of their meander floors above the modern stream level the assumption is that the undercut slope is actively eroded by the stream until abandonment of the meander at which time the slope begins to evolve to a different form more advanced stages of evolution occur on walls of higher meanders that were abandoned earlier the most rapid change in this initial form is the elimination of a free face which occurs soon after the meander is abandoned in addition the hillslopes associated with even the lowest youngest cutoff meanders show somewhat gentler overall gradients than the actively undercut slopes hillslopes associated with meanders 3 to about 20 m above modern stream level maintain straight segments with angles showing only a slight decrease from the 36 38 associated with the lowest cutoffs overall angles decrease however as the straight segment becomes shorter the oldest slopes those on cutoffs 30 m or greater above modern stream level have developed into convex concave slopes with maximum slopes of 15 a hillslope evolution model based on previously published algorithms was used to simulate the transition of actively undercut hillslopes into hillslopes on abandoned meanders hillslope modeling is particularly useful in this setting if the valley incision rate is known an age can be estimated for the cutoff and hence for the hillslope alternatively if hillslope process rates are known a model age obtained for the hillslope can be used to estimate an incision rate even where both incision rates and hillslope process rates are poorly constrained as in the present setting modeling allows assumptions about specific rates to be evaluated by determining their implications for other rates for example for three cutoff meanders along one stream best fit criteria were used to select process rates for the model model ages of hillslopes were then obtained and compared with those calculated from a valley incision rate measured elsewhere in the same physiographic province for two of the hillslopes model ages were found to be much younger than those calculated from the incision rate in order to make the two ages agree unreasonably low process rates had to be used in the model thus implying that the incision rate probably underestimates the actual incision rate in this valley experimentation with heights of initial profiles again using best fit criteria suggests that since abandonment of the highest cutoff the plateau has been downwasting at a rate about one fourth that of the valley incision rate a finding in agreement with published rates of chemical denudation in the area

effect of far field slope on morphologic dating of scarplike landforms the principal finding of this paper is that the far field slope has a first order effect on model age determinations of scarplike landforms in weakly consolidated terrains observationally this can be demonstrated in two ways using the lake bonneville and lahontan shoreline scarps as separate and combined data sets use of the reduced scarp slope tan theta sub s minus b where theta sub s is the maximum scarp angle and b is the far field or fan slope instead of tan theta sub s alone as the measure of scarp slope measurably reduces separation between the two data sets induced by different average fan slopes for the two data sets and significantly reduces scatter in the slope offset plot for both the separate and combined data sets theoretically the argument can be put even more strongly at least within the range of linear and nonlinear diffusion models that the authors consider here together with a mathematical transformation of the empirical approach of r c bucknam and r e anderson when one correctly takes into account the far field slope one will basically get the same age determination no matter which of these models one uses conversely without accounting properly for the effect of far field slope one is virtuallymore guaranteed to get an erroneous age determination no matter which model is used less

the influence of crenulation cleavage development on the bulk elastic and seismic properties of phyllosilicate rich rocks abstract the anisotropy of seismic wave propagation is strongly influenced by the mineralogy and microstructure of rocks phyllosilicates are elastically highly anisotropic and are therefore thought to be important contributors to seismic anisotropy in the continental crust crenulation cleavage is one of the most common microstructural fabrics found in multiply deformed phyllosilicate rich crustal rocks we calculated the bulk elastic properties and resulting wave velocities for rock samples that preserved three different stages of crenulation cleavage development an initial planar foliation a moderately developed crenulation cleavage and a well developed crenulation cleavage mineral orientation maps were obtained using electron backscatter diffraction and calculations were made using asymptotic expansion homogenization combined with the finite element method the difficulties involved with sample preparation and data acquisition of phyllosilicate rich rock samples are also discussed we compare our results to more conventional methods for calculating an aggregate stiffness matrix from a mineral orientation map namely voigt and reuss averages these averages do not account for grain scale interactions and therefore deviate from the results calculated using asymptotic expansion homogenization our results show that the rocks characterized by a planar foliation and a moderately developed crenulation cleavage are highly anisotropic with p wave anisotropies up to 30 9 and s wave anisotropies up to 34 2 whereas the rock characterized by a well developed crenulation cleavage is only mildly anisotropic with a p wave anisotropy of 15 5 and s wave anisotropy of 10 7 progressive development of the fabric also causes the orientations of p and s wave velocity maxima and s wave polarization directions to change markedly despite the high anisotropy imparted by a planar schistosity the variety of folds and fabrics typically found in phyllosilicate rich rocks within larger scale crustal volumes will tend to mute the anisotropy possibly to the point of appearing nearly isotropic

holocene scarp on the sawtooth fault central idaho usa documented through lidar topographic analysis high resolution lidar data reveal a prominent latest pleistocene holocene scarp on the sawtooth fault central idaho united states the fault scarp marks 55 65 km of the range front and may comprise two segments the scarp is 4 9 m high in latest pleistocene glacial landforms 11 14 ka and 2 3 m high in holocene alluvial landforms implying 2 3 postglacial rupture events patterns of fault scarp continuity coupled with existing gravity data suggest that active faulting may have migrated northward during pleistocene time detailed comparisons of raw lidar digital elevation models dems bare earth lidar dems and field surveys indicate that the bare earth lidar data document the fault scarp morphology accurately and allow for detailed fault analysis where field evaluation is difficult the documentation of holocene motion on the sawtooth fault demonstrates that ene directed extension extends across central idaho and that the fault contributes to seismic hazards

morphologic dating of the pre 1983 fault scarp on the lost river fault at doublespring pass road custer county idaho abstract the focus of this study is the age of the earthquake antecedent to the 1983 borah peak earthquake in the vicinity of doublespring pass road where the faulting signature of both events has been studied in detail through trenching operations in 1976 and again in 1984 we sample the morphologic expression of the pre 1983 event with six profiles along a 100 m segment of the fault scarps adjacent to and just north of the trench site where both the 1983 and pre 1983 scarps are remarkably linear and regular diffusion equation modeling of the profiles suggest that the age of the pre 1983 event is 6000 to 8000 yr before present exclusive of the uncertainties in applying the lakes bonneville and lahontan mass diffusivity to this central idaho setting

paleoseismology and neotectonics of the shivwits section of the hurricane fault northwestern arizona the shivwits section of the hurricane fault in northwestern arizona has been largely ignored in evaluating the seismic hazard posed to the rapidly growing populations of southwestern utah to assess this hazard we conducted studies along the shivwits section using field observations and geomorphic modeling to understand the quaternary tectonism of this portion of the hurricane fault we have found evidence that it ruptured with up to 2 to 3 m of vertical displacement per event and likely produced m 7 earthquakes our results suggest that the slip rate along the southern hurricane fault has not decreased during the quaternary the moriah knoll basalt offset 150 to 200 m along the hurricane fault yielded a maximum long term slip rate of 0 15 to 0 25 mm yr estimated using a new 40ar 39ar age of 0 85 0 06 ma the late quaternary slip rates on alluvial fan surfaces offset 2 to 7 4 m were estimated using pedogenic carbonate rind thickness as a calibrated proxy for age these observations yielded slip rates of 0 05 to 0 3 mm yr paleoseismic trench investigations showed that two surface rupturing events occurred in the past 15 78 k y a radiocarbon sample from a most recent event mre fissure yielded a calibrated age of 8900 10 400 years b p the penultimate event was likely 10 k y before the most recent event slip rate estimates using the moriah knoll basalt 0 15 0 24 mm yr 850 ka surface offset 0 05 0 3 mm yr 100 ka morphologic modeling 0 06 0 21 mm yr 100 ka and observations from the trench 0 06 0 34 mm yr 15 75 ka suggest that there has been no detectable change in slip rate in the past 1 million years or so this implies a constant deformation rate for this portion of the colorado plateau margin therefore basin and range extension is actively encroaching manuscript received 24 november 2003

active tectonic studies in the united states 1987 1990 the techniques and instrumentation used in active tectonic studies are discussed and recent results are reviewed it is suggested that a critical mass of data on several particular regions has been accumulated making possible critical debates and attempts to assess earthquake hazards particular attention is given to studies of the pacific northwest region basin and range deformation studies and distributed deformation and hidden earthquake sources also included is a comprehensive bibliography for the period

lattice preferred orientation and seismic anisotropy in sedimentary rocks summary although it is well known that sedimentary rocks can be seismically anisotropic there have been few detailed investigations of the underlying cause of such anisotropy here we investigate anisotropy due to the preferred orientation of minerals or lattice preferred orientation lpo in a suite of sedimentary rocks seismic properties are predicted by averaging single crystal elastic constants of minerals according to their crystal orientation and modal volume fraction in the rock aggregate both electron backscattered diffraction ebsd and x ray texture goniometry xtg are tested as quantitative techniques for measuring the lpo of sedimentary rocks although ebsd has promise for future lpo measurements in polymineralic sedimentary rocks problems currently remain in measuring low symmetry phases e g feldspars and very small clay or mica particles however the lpo of very fine grained phyllosilicates can be measured using xtg and the lpo of low symmetry minerals can be measured using manual ebsd pattern analysis here we use such a hybrid approach to estimate lpo in a suite of sedimentary samples the seismic properties calculated from lpo data show anisotropy values for p waves ranging from 1 5 per cent in sandstones to over 3 5 per cent in a siltstone to 12 per cent in a shale the effect of thin multilayering on long wavelength propagation in a siltstone is predicted by applying backus type averaging the layering does not enhance the anisotropy because of the small differences in density and in elasticity between the two layer types the lpo of phyllosilicates and to a lesser extent dolomite and siderite seem to contribute significantly to the seismic anisotropy of phyllosilicate rich rocks siltstones and shales the weak lpo of quartz in sandstones causes a few per cent anisotropy cumulatively our results suggest that field observations of seismic anisotropy have the potential to be used as an indicator of rock lithology

quaternary tectonic setting of the 1983 borah peak earthquake central idaho the 1983 borah peak earthquake was accompanied by extensive surface faulting along a part of the lost river fault that has abundant evidence of latest quaternary last 15 000 yr offset this fault and two similar range front normal faults along the lemhi range and beaverhead mountains lie in an area of basinand range structure in central idaho that is part of a roughly v shaped belt of latest quaternary surface faulting that extends from the wasatch fault through the yellowstone area to the lost river fault the position of this belt may be related to the outward migration of a thermal front associated with the northeastward progression of late cenozoic silicic volcanism along the yellowstonesnake river plain axis the central segments of the lost river lemhi and beaverhead faults have been active more recently and probably more active throughout quaternary time than the southern and northern segments the main 1983 surface faulting occurred in an area of high structural relief along a central segment of the lost river fault that has ruptured in latest quaternary time which suggests that comparable areas along other range fronts in the area should be regarded as likely sites of future surface faulting other perspective of fault behavior suggest additional possible sites and all segments of the range front faults are regarded as capable of surface faulting

crustal radial anisotropy across eastern tibet and the western yangtze craton 1 phase velocities across eastern tibet and surrounding regions are mapped using rayleigh 8 65 s and love 8 44 s wave ambient noise tomography based on data from more than 400 program for array seismic studies of the continental lithosphere and chinese earthquake array stations a bayesian monte carlo inversion method is applied to generate 3 d distributions of vsh and vsv in the crust and uppermost mantle from which radial anisotropy and isotropic vs are estimated each distribution is summarized with a mean and standard deviation but is also used to identify highly probable structural attributes which include 1 positive midcrustal radial anisotropy vsh vsv across eastern tibet spatial average 4 8 1 4 that terminates abruptly near the border of the high plateau 2 weaker 1 0 1 4 negative radial anisotropy vsh vsv in the shallow crust mostly in the songpan ganzi terrane 3 negative midcrustal anisotropy 2 8 0 9 in the longmenshan region 4 positive midcrustal radial anisotropy 5 4 1 4 beneath the sichuan basin and 5 low vs in the middle crust 3 427 0 050 km s of eastern tibet midcrustal vs 3 4 km s perhaps consistent with partial melt is highly probable only for three distinct regions the northern songpan ganzi the northern chuandian and part of the qiangtang terranes midcrustal anisotropy provides evidence for sheet silicates micas aligned by deformation with a shallowly dipping foliation plane beneath tibet and the sichuan basin and a steeply dipping or subvertical foliation plane in the longmenshan region near vertical cracks or faults are believed to cause the negative anisotropy in the shallow crust underlying tibet

an experimental and numerical study of the microstructural parameters contributing to the seismic anisotropy of rocks 1 the elastic properties of rocks are influenced by several microstructural variables including the lattice preferred orientation and grain shape fabric of the mineral phases variations in the spatial distribution of the mineral phases the properties of the grain boundaries and the presence of porosity fractures consequently in principle directional variations in these variables can be inferred from seismic velocity anisotropy observations experimental seismic velocity measurements made on rocks of well characterized microstructure may be used to improve the quality of such interferences however since most rocks are microstructurally complex in order to interpret the measurements fully theoretical analyses which can accommodate all the relevant microstructural variables are required theoretical analyses of the requisite sophistication have only recently been developed we have tested one of these due to ponte castaneda and willis by using it to calculate the elastic properties of an upper mantle harzburgite and by then comparing the results with experimental velocity measurements to determine if the values of those microstructural variables which are difficult to quantify grain boundary properties fracture shape are physically realistic we find that they are and conclude that the ponte castaneda and willis analysis provides a powerful method for a more detailed assessment of the causes of elastic property anistropy in rocks than has previously been possible

evidence for nonlinear diffusive sediment transport on hillslopes and implications for landscape morphology steep soil mantled hillslopes evolve through the downslope movement of soil driven largely by slope dependent transport processes most landscape evolution models represent hillslope transport by linear diffusion in which rates of sediment transport are proportional to slope such that equilibrium hillslopes should have constant curvature between divides and channels on many soil mantled hillslopes however curvature appears to vary systematically such that slopes are typically convex near the divide and become increasingly planar downslope this suggests that linear diffusion is not an adequate model to describe the entire morphology of soil mantled hillslopes here we show that the interaction between local disturbances such as rainsplash and biogenic activity and frictional and gravitational forces results in a diffusive transport law that depends nonlinearly on hillslope gradient our proposed transport law 1 approximates linear diffusion at low gradients and 2 indicates that sediment flux increases rapidly as gradient approaches a critical value we calibrated and tested this transport law using high resolution topographic data from the oregon coast range these data obtained by airborne laser altimetry allow us to characterize hillslope morphology at 2 m scale at five small basins in our study area hillslope curvature approaches zero with increasing gradient consistent with our proposed nonlinear diffusive transport law hillslope gradients tend to cluster near values for which sediment flux increases rapidly with slope such that large changes in erosion rate will correspond to small changes in gradient therefore average hillslope gradient is unlikely to be a reliable indicator of rates of tectonic forcing or baselevel lowering where hillslope erosion is dominated by nonlinear diffusion rates of tectonic forcing will be more reliably reflected in hillslope curvature near the divide rather than average hillslope gradient