flow energy and channel adjustments in rills developed in loamy sand and sandy loam soils the storms usually associated with rill development in nature are seldom prolonged so development is often interrupted by interstorm disturbances e g weathering or tillage in laboratory simulated rainfall experiments active rill development can be prolonged and under these conditions typically passes through a period of intense incision channel extension and bifurcation before reaching quasi stable conditions in which little form change occurs this paper presents laboratory experiments with coarse textured soils under simulated rainfall which show how channel adjustment processes contribute to the evolution of quasi stability newly incised rills were stabilized for detailed study of links between rill configuration and flow energy on a loamy sand adjustment towards equilibrium occurred due to channel widening and meandering whereas on a sandy loam mobile knickpoints and chutes pulsations in flow width and flow depth and changes in stream power and sediment discharge occurred as the channel adjusted towards equilibrium the tendency of rill systems towards quasi stability is shown by changes in stream power values which show short lived minima differences in energy dissipation in stabilized rills indicate that minimization of energy dissipation was reached locally between knickpoints and at the downstream ends of rills in the absence of energy gradients in knickpoints and chutes stabilized rill sections tended toward equilibrium by establishing uniform energy expenditure the study confirmed that energy dissipation increased with flow aspect ratio in stabilized rills flow acceleration reduced energy dissipation on the loamy sand but not on the sandy loam on both soils flow deceleration tended to increase energy dissipation understanding how rill systems evolve towards stability is essential in order to predict how interruptions between storms may affect long term rill dynamics this is essential if event based physical models are to become effective in predicting sediment transport on rilled hillslopes under changing weather and climatic conditions copyright 2008 john wiley and sons ltd

measurements of the strain field associated with episodic creep events on the san andreas fault at san juan bautista california over the past 6 years repeated strain steps recorded on a tensor strainmeter installed close to the san andreas fault trace at san juan bautista in california correlated closely with episodic creep events registered on a nearby creepmeter the strain events were remarkably similar in character were of about 1 hour duration and were followed within hours to days by creep events of 2 6 days duration the episodic slip of a source region from 200 m to 500 m in depth and at most a few kilometers in length is consistent with the observed strain and creep data we postulate that the additional stress transferred to this region by the loma prieta earthquake is giving rise to an increased rate of accumulation of shear strain around a locked patch beneath the creep source region thus accounting for the increased frequency of the creep strain events following the loma prieta earthquake

structure and geodynamic evolution of the central bransfield basin nw antarctica from seismic reflection data abstract the bransfield basin is a young active rift basin located between the northern margin of the antarctic peninsula and the south shetland islands margin deception and bridgeman islands divide the bransfield basin in three subbasins the western central and eastern specific morpho tectonic features and sediment fill differentiate each subbasin the structure and geodynamic evolution of the central bransfield basin which is in a stage of incipient seafloor spreading have been investigated in detail from a dense grid of single channel seismic reflection data the central bransfield basin is dominated by two families of normal faults which are oriented northeast and northwest the ne trending faults define three graben systems that are roughly parallel to the basin axis in an across basin direction the mean trend of this family of faults ranges from n71 the graben system nearest to the antarctic peninsula over n64 the intermediate graben system to n53 the graben system nearest to the south shetland islands the nw trending family of faults is responsible for the deepening of the basin from southwest to northeast both families of faults define the overall central bransfield basin structure resulting in a complex division of the basin floor additionally tens of volcanic edifices are located on the basin floor the larger ones being associated to the nw trending faults interaction of tectonics and sedimentation give place to the differentiation of three tectonostratigraphic units tu1 tu2 and tu3 from oldest to youngest tu1 occupies the southernmost graben system and it is affected by the ne trending bounding normal faults tu2 extends further northwestwards than tu1 and essentially fills the intermediate graben system tu3 represents a further extension of the sediment infill over most of the central bransfield basin and marks the initiation of the infill of the northernmost graben system faults bounding this graben also determine the straight and abrupt morphology of the south shetland islands margin the observed arrangement of the graben systems and the filling tectonostratigraphic units reveal a migration of extensional tectonics and associated depocentres from the antarctic peninsula margin to the south shetland island margin the left lateral rotation from n71 to n53 in the mean trend of the three successive graben systems could have been produced by the oblique subduction of the phoenix plate and the effect of the sinistral strike slip movement of the south scotia ridge 200 km northeastwards of the central bransfield basin

a slow earthquake sequence on the san andreas fault earthquakes typically release stored strain energy on timescales of the order of seconds limited by the velocity of sound in rock over the past 20 years observations1 13 and laboratory experiments14 have indicated that rupture can also occur more slowly with durations up to hours such events may be important in earthquake nucleation15 and in accounting for the excess of plate convergence over seismic slip in subduction zones the detection of events with larger timescales requires near field deformation measurements in december 1992 two borehole strainmeters close to the san andreas fault in california recorded a slow strain event of about a week in duration and we show here that the strain changes were produced by a slow earthquake sequence equivalent magnitude 4 8 with complexity similar to that of regular earthquakes the largest earthquakes associated with these slow events were small local magnitude 3 7 and contributed negligible strain release the importance of slow earthquakes in the seismogenic process remains an open question but these observations extend the observed timescale for slow events by two orders of magnitude

tectonic development of the bransfield basin and its prolongation to the south scotia ridge northern antarctic peninsula abstract the bransfield basin is located off the northern tip of the antarctic peninsula the analysis of multichannel seismic reflection profiles allows the shallow structure of the bransfield basin and its eastwards prolongation through the scotia sea to be imaged the bransfield basin is asymmetrical with most of the sediment input coming from the passive antarctic peninsula margin which is deformed by normal faults generally dipping less than 45 northwestwards much less sediment comes from the conjugate south shetland islands margin which is dominated by high angle normal faults generally dipping between 45 and 60 constituting the inner boundary of the south shetland block tectonic horst the detachment and moving apart of the south shetland block from the antarctic peninsula led to the opening of the bransfield basin where incipient oceanic spreading has been reported previously the south shetland block outer boundary to the northwest is made of the south shetland trench which to the east passes into the sinistral transpressive scotia antarctic plate boundary marked by the prominent south scotia ridge following cessation of spreading in the phoenix antarctic ridge northwest of the south shetland block during middle pliocene roll back played a major role in the development of the bransfield basin with maximum extension in the central subbasin in addition the eastern bransfield basin accommodates most of the sinistral transtensional deformation along the south scotia ridge normal and reverse faults were simultaneously active along the inner and outer boundaries respectively of the continental crust fragment constituting the south shetland block the development of the bransfield basin offers a good example of the tectonic processes involved in continental fragmentation associated with backarc basin development

recent plate motions and crustal deformation reports by u s workers on geodetic measurements of recent plate motions or crustal deformation published in 1987 1990 are reviewed the review begins with global plate motions proceeds through plate boundaries in california alaska and the pacific northwest and finishes with volcanic phenomena monument stability and longevity and gps relative position measurements 184 refs

postseismic strain following the 1989 loma prieta earthquake from gps and leveling measurements postseismic deformation in the 5 years following the 1989 loma prieta earthquake has been measured with the global positioning system and precise leveling postearthquake velocities at distances greater than 20 km from the coseismic rupture are not significantly different from those observed in the 20 years prior to the earthquake however velocities at stations within 20 km of the rupture exceed preearthquake rates and exhibit unanticipated contraction normal to the strike of the san andreas fault system a combination of forward modeling and nonlinear optimization suggests that the observed postseismic deformations were caused by aseismic oblique reverse slip averaging 2 9 cm yr on the san andreas fault and or the loma prieta rupture zone and 2 4 cm yr reverse slip along a buried fault within the foothills thrust belt the best fitting sources of postseismic deformation are all located at depths of less than 15 km we find no evidence for accelerated flow or shear below the loma prieta rupture in the first 5 years following the earthquake the inferred postseismic slip is likely to have been caused by the coseismic stress change updip of the 1989 rupture

the sklinnadjupet slide and its relation to the elsterian glaciation on the mid norwegian margin abstract the sklinnadjupet slide scar is a large buried feature on the continental slope southeast of the voring plateau the mass flow occurred ca 250 000 years ago on a slope angle of ca 0 5 and removed glacial debris from the up to 350 m thick naust s sequence on the upper slope this part of the mid norwegian margin appears to have been fairly stable throughout the last 2 8 million years naust s represents the third last glacial unit in the skjoldryggen region and was deposited during the elsterian ca 400 200 ka bp during this glaciation a wide depression was formed on the continental shelf between haltenbanken and traenabanken with the strongest glacial erosion in the west trending sklinnadjupet palaeo trough this trough leads directly towards the deep 90 km wide slide scar indicating that a local depocentre was formed here before the mass flow occurred the shallowest crest of the helland hansen arch is located below the southern part of the slide scar and two large craters were formed on the surface of this anticlinal structure our model suggests that the deposition of glacial sediments above the helland hansen arch caused high excess pore pressures along the shallowest crest of this structure and formed a high pore pressure gradient towards the sea floor on its western flank an earthquake possibly triggered the initial slide where the slope was steepest west of the crest most of the slide scar was already formed when the slide finally eroded into the anticline high excess fluid pressure subsequently mobilised the diatomaceous oozes in the helland hansen arch and formed the craters much of this material flowed down slope without being totally disintegrated and was deposited as levee ridges and mounds above slide deposits from the first phase of the mass flow they have previously been wrongly interpreted as diapirs

aseismic slip on the san andreas fault south of loma prieta two digital creepmeters installed within the san andreas fault zone after the 18 oct 1989 loma prieta main shock show less than 1 cm of post seismic right lateral slip in the four months following the earthquake at mt madonna road a 23 mm coseismic fracture slipped a further 3 mm after heavy rain and at nyland ranch near san juan bautista the fault slipped approximately 9 mm starting 42 days after the main shock if the current trend at nyland ranch persists more than 2 cm of post seismic slip will develop by 1991 at both sites minor left lateral displacements occurred which are attributed to near surface soil effects the abutments of the railroad bridge across the pajaro river at chittenden which were extended by the 1906 earthquake were not extended during the loma prieta event although they have evidently moved apart by more than 7 cm since bridge reconstruction in 1940 this corresponds to 10 cm of right lateral slip which could be related to m 5 events in mid century or could be due to aseismic slip at a mean rate of 2 1 mm a the absence of significant surface slip within the fault zone in the decades before and the months following the loma prieta event suggests either that near surface deformation is distributed over a wide zone or that a slip deficit remains several authors have proposed this region as a future location for m 5 events

geodetic measurement of crustal deformation on the san andreas hayward and calaveras faults near san francisco california analysis of a geodetic network of 115 lines crossing the san andreas hayward and calaveras faults in the vicinity of san francisco bay and measured repeatedly between 1970 and 1980 has revealed details about the accommodation of relative plate motion in this area the most striking result is that the deformation is not uniformly distributed across the area in the east bay along the hayward and calaveras faults all motion appears to take place as slip directly on the fault with no accumulation of strain in the adjacent crust on both the calaveras and the hayward faults the rate obtained for the 1970 to 1980 period agrees with geologic rates spanning a few million years and with creep rates spanning a few decades the hayward fault slip rate is 7 1 mm yr the calaveras fault slip rate is 7 1 mm yr with perhaps half of this slip distributed across a zone a few kilometers wide probably as inelastic deformation of weak near surface material the absence of strain accumulation in the east bay is surprising since the hayward and calaveras faults have been the site of large earthquakes in the past a block located east of the calaveras fault and south of the las positas fault has been rotating clockwise at a rate of 0 3 0 1 rad yr with very little internal deformation along the san francisco peninsula no detectable slip occurs less than 1 5 mm yr at the surface but appreciable strain is accumulating near fault shear strain rates are 0 6 0 1 strain yr engineering with direction n47 w 9 the slip rate near the san andreas fault is 12 2 3 9 mm yr distributed across a broad zone the relative motion across the whole region during the period 1970 1980 is 32 1 7 4 mm yr supplement available with entire article on microfiche order from the american geophysical union 2000 florida avenue n w washington d c 20009 document j81 011 1 00 payment must accompany order

faulting geometry and slip from co seismic elevation changes the 18 october 1989 loma prieta california earthquake abstract leveling surveys conducted before and after the loma prieta earthquake provide observations of the co seismic elevation changes these data are used to determine the faulting geometry and distribution of slip considering planar listric and negatively listric fault shapes both the planar and nonplanar models produce elevation changes consistent with the observations most of the observed elevation changes can be modeled with a rupture surface that extends from 4 to 15 km depth if the rupture surface is planar the observations require 2 4 m of right lateral strike slip and 1 7 m of reverse slip on a 34 km long plane that dips 60 sw the best fitting model faults lie above and to the southwest of the aftershock zone a significantly better fit to the observations is obtained when these fault geometries are allowed to have two rake values with a larger thrust component northwest of the epicenter and a larger strike slip component southeast of the epicenter when a low modulus layer over a half space is used for consistency with the seismic p wave velocity structure the fault deepens coming within 3 km of the hypocenter but still locates several kilometers southwest of most aftershocks

mass wasting processes in the western wilkes land margin possible implications for east antarctic glacial history abstract multichannel seismic data collected off western wilkes land east antarctica reveal widespread sediment mass movement events mainly in the form of mega debris flows deposited on the continental rise in places they have a seaward extent of more than 200 km and a thickness of up to 400 m debris flow units are commonly separated by thin deposits of well stratified facies interpreted as glaciomarine mixed contouritic and distal turbidite deposits possibly with some hemipelagic sediments these units due to a dominant contouritic component represent weakened layers and could then play a major role in the slope instability based on stratigraphic correlation with dsdp site 268 it is suggested that slope failures in the western wilkes land occurred mostly during the miocene inferred high sedimentation rates due to large amounts of sediment delivered from a temperate wet based ice sheet would have constituted a key factor in the sediment failures the distribution of mass wasting related features appears also strongly related to the amount of meltwater discharge the main trigger mechanism would probably have been earthquakes enhanced by isostatic rebounds following major ice sheet retreats

use of active fault data versus seismicity data in the evaluation of seismic hazard in the granada basin southern spain in this article we evaluate the seismic hazard in the granada basin southern spain using for the first time the slip rate of known active faults our study as an attempt to compute seismic hazard using active fault data in low to moderate seismicity regions relies on a complete database of these faults containing information relevant to their seismic potential we obtain peak ground acceleration values above 0 4g for a return period of 475 years this result is compared with previous evaluations carried out on the basis of the historical seismicity of the area and the application of the well known theorem of total probability in these cases maximum values of 0 2g are obtained we explain the discrepancies found between the slip rate derived and seismicity derived estimates of seismic hazard as owing to the different strikes of the faults in relation to the directions of the main stresses affecting the granada basin in the context of the betic cordilleras some of them with evidence of aseismic slip

surface slip subsequent to the 24 november 1987 superstition hills california earthquake monitored by digital creepmeters three creepmeters of new design were operated on the superstition hills fault following the 24 november 1987 earthquake each creepmeter consists of a digital displacement sensor interrogated periodically by a computer at the end of each hour the computer determines a recording rate appropriate to the velocity of the slip on the fault in that hour no data are recorded if the fault is inactive data are recorded at a maximum rate of one sample per minute if slip on the fault is detected the resolution and range of the creepmeters are 10 m and 15 cm respectively fault activity is observed to be confined to periods of episodic slip creep events with mean amplitudes 3 to 6 mm and durations less than 4 hr a total of 80 creep events were recorded during the first week following the earthquake creep events occurred several times each day after 6 months events occurred every 3 weeks this increase in the mean time interval between creep events results in the observed decay in afterslip rate between creep events the fault is inactive although more than 200 microstrain of fault zone deformation may accumulate before each event although creep events were observed to propagate unilaterally in both directions along the fault the most impulsive events appeared to propagate bilaterally from a dilatant offset separating the northern and southern halves of the 24 km long fault zone maximum propagation velocities of 30 m sec were observed aftershocks 2 m 4 7 were observed to occur near the creepmeters following 6 to 9 cm of surface slip on the fault a future aftershock in this region is anticipated in mid 1989 based on afterslip rates in the year following the main shock

post rift sedimentary evolution of the gebra debris valley a submarine slope failure system in the central bransfield basin antarctica abstract the combined analysis of paleomorphology stratigraphy and seismic facies allows us to present new insights into the formation and evolution of the gebra valley located on the lower continental slope of the central bransfield basin ne antarctic peninsula repeated large scale slope failure events were responsible for the cut and fill features forming the gebra valley this study revealed a mid pleistocene gebra paleovalley that was progressively and completely infilled during the last glacial cycle the infilled paleovalley was reoccupied forming the present day gebra valley both valley incisions are genetically related to large scale failures associated with high energy gravity flows or mass flows the infilling of the valley involved channelized mass flows of various dimensions and channelized and unchannelized turbidity currents alternating erosive periods during which the valley evacuated sediment from the slope toward the basin and depositional periods during which it was fully infilled allows it to be defined as the gebra debris valley taking into account the presence of faults controlling the headwall locations and the stratigraphic correlation established with glacial periods the genesis of the gebra debris valley could have been controlled by the interplay of both the tectonic history of the bransfield basin and the glacial cycles that allowed grounding events to reach the upper continental slope

dynamics of fault creep observations along the surface traces of active faults in california and elsewhere indicate that tectonic displacement can occur as seismic slip or as aseismic fault creep fault creep occurs as secular displacement and as displacement episodes called creep events which last a few hours to days and include from a millimeter or less to a few tens of millimeters of displacement instrumental measurements of displacement versus time during creep events show that many events have a characteristic simple form including a steep beginning followed by a gradual decay in the rate of displacement some creep events display small amounts of precursory slip and many seem to be composed of several discrete simple events propagation of creep events along the san andreas calaveras and imperial faults in california has been inferred from the nearly simultaneous observation of creep events at nearby creepmeters and from kinematic modeling of continuous measurements of strain during creep events fault creep can be viewed as arising from the interplay of three factors stress applied to the fault from external sources stress caused by the geometry and distribution of displacement on the fault arising from the elastic response of the surrounding medium to the displacements within the anelastic fault zone itself and the constitutive relations characterizing the resistance to slip on the fault analytic solutions are presented for a simple rectangular dislocation quasi static model of nonpropagating fault creep assuming viscous and quasi plastic power law creep fault zone rheology and for a multi element composed of several rectangular or striplike dislocation surfaces quasi static model assuming a viscous rheology viscous rheology implies solutions composed of exponentials that decay with time the simple one element quasi plastic model is in agreement with the form of creep events observed at melendy ranch california a matrix formulation is presented to calculate dislocation stresses on a fault zone in a half space or plate the formulation involves the division of the dislocation surface into strips or rectangles and the use of published solutions to calculate the stress resulting from displacement on each individual element results from the matrix method agree with analytic results for equilibrium displacements on stress free cracks the matrix formulation is then used as the basis for a numerical method to simulate one and two dimensional propagating creep assuming a quasi plastic rheology on the fault zone results from this method agree reasonably well with observations of afterslip from the 1966 parkfield 1975 oroville and 1979 imperial valley california earthquakes and of a propagating creep event observed along the southern calaveras fault central california during july 1977 this method of analysis and the assumption that creep at each point on the fault zone follows a power law rheology after the applied stress reaches a spatially varying yield stress can explain the secular and episodic nature of fault creep afterslip the simple shape of some individual events and the multiple composite shape of others and the propagation of creep events on the southern calaveras fault observations of propagating creep indicate a yield stress near zero at the surface and throughout much of the fault zone however they also indicate the presence of a zone at a depth of about 0 5 km and about 5 km long with a yield stress of about 15 bars 1 5 mpa the methods of solution presented here should be applicable to many problems in the dynamics of faulting

aeromagnetic study of the island of hawaii an aeromagnetic study of the island of hawaii provides new insight on magnetic properties of subsurface rock and geologic structure on a regional scale spectral depth analysis delineates two shallow magnetic zones each roughly 1 5 km thick lying at a depth of 1 km one zone of unknown origin lies in the center of the island and correlates with a regional magnetic high the other zone coincides with pronounced magnetic lows paralleling kilauea s active east rift zone these magnetic lows probably depict rocks chemically altered by hydrothermal fluids in which titanomagnetite has been destroyed analysis of magnetic terrain effects indicates that magnetization also decreases with depth within mauna kea and mauna loa we estimate that magnetization is reduced by about half at a depth of 1 km the magnetic method is particularly useful in delineating the lateral extent of local shield structures such as rifts summit calderas pit craters and vent fissures rifts possess characteristic magnetic patterns primarily long wavelength linear magnetic low zones we propose alteration processes reduce magnetizations along the flanks of rifts on the other hand along young rifts e g kilauea s east rift zone short wavelength magnetic anomalies probably reflect slowly cooled unaltered intrusions altered rock may also produce magnetic lows that help define buried summit calderas

scotia sea regional tectonic evolution implications for mantle flow and palaeocirculation abstract the scotia sea and surrounding scotia arc have evolved over the past 40 ma by extension behind an east migrating subduction zone at the boundary between the south american sam and antarctic ant plates the considerable data set now available regional geology and geophysics earthquake seismology satellite altimetry global plate analyses suggest why east migrating subduction began what has been the driving force that has sustained it and what other processes have controlled the mode of back arc extension in the scotia sea a suite of six reconstructions has been developed based on this data set the reconstruction to 40 ma creates a compact cuspate continental connection between south america and the antarctic peninsula at the subducting pacific margin with fragments now dispersed around the scotia arc occupying positions within it compatible with their known geology the driving force has been subduction of south american ocean floor which began as a result of southward migration of the pole of south american antarctic plate rotation and a key modulator of back arc extension has been collision of ridge crest sections of the south american antarctic plate boundary with the east advancing trench cenozoic regional tectonic evolution has two other likely consequences which greatly increase its importance firstly this region saw the tectonic disruption of the final barrier to complete circum antarctic deep water flow that may have had a profound effect on palaeoclimate secondly it is possible that the rapid roll back of the hinge of subduction is related to shallow eastward flow in the sub lithospheric mantle both of these consequences are explored the reconstructions show that rapid roll back of the subduction hinge averaging 50 mm a over the last 40 ma with respect to the south american plate has been a feature of all of scotia sea evolution and provide a history of motion of several oceanic microplates most of which are now welded together within the scotia sea this will guide the location of seismometers and or dredge hauls to test the hypothesis of shallow mantle flow and help interpret the results the reconstructions also allow an assessment of the creation of deep water pathways that would have permitted the development of the present day antarctic circumpolar current acc an early miocene onset within the period 22 17 ma seems likely for the acc depending on the structure and palaeo elevation of davis bank and aurora bank sections of the north scotia ridge however the study shows there was a delay of one or more million years between initial provision of a deep water pathway and the major mid miocene change in global climate involving the general level of antarctic glaciation that may have been related if these changes were related then the delay suggests that other factors possibly rough elevated ocean floor but also non tectonic factors such as atmospheric co2 were important in determining palaeoclimate

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

occurrence of a predicted earthquake on the san andreas fault in may 1985 we predicted1 that an earthquake would occur on the san andreas fault near stone canyon california within a year the prediction was based on the observation of seismic quiescence defined as a significant decrease in the average occurrence rate of earthquakes within the source volume of the future mainshock a mainshock of magnitude ml 4 6 occurred on 31 may 1986 rupturing exactly the specified segment of the fault this is the first successful prediction of an earthquake along the san andreas fault and the probability to have come true by chance is 5 although the prediction addressed only a small main shock its success was significant because the location size and occurrence time were correctly specified for an earthquake in a populated area larger earthquakes will undoubtedly be successfully predicted by the same method in the future but the major segments of the san andreas fault near san francisco and los angeles have such a low background seismicity rate that the method will probably not be applicable there