the eastern termination of the zagros fold and thrust belt iran structures evolution and relationships between salt plugs folding and faulting 1 the zagros mountains result from the ongoing collision between the arabian and central iran plates the main features of the eastern zagros are 1 numerous emerged or buried salt diapirs made up of late precambrian hormuz salt and 2 the irregular along strike shape of the collision related detachment folds with frequent bending to understand this layout four geological cross sections have been constructed from the persian gulf foreland basin to the inner part of the zagros fold and thrust belt shortening in the deformed parts of the sections is less than 10 and is mainly accommodated by detachment folding we show that late cenozoic folding occurred in a region that was already punctuated by salt domes and diapirs in fact almost continuous halokinesis developed since the earlier paleozoic i e just short time after the deposition of the hormuz salt and continued up to the present these preexisting salt structures and their relevant local thickening strongly influenced both the localization and the direction of folds

broadband time domain modeling of earthquakes from friuli italy short period sp and long period lp seismograms written by the main shock and two principal aftershocks of the 1976 friuli italy earthquake sequence are modeled in the time domain using synthetic seismograms the main shock occurred on 6 may 1976 20h 00m m s 6 5 and both aftershocks on 15 september 1976 03h 15m m s 6 0 and 09h 21m m s 5 9 source models were determined initially by trial and error and then refined using a waveform inversion program two point sources of radiation are required to adequately model the aftershock short period records for the 09h 21m aftershock the model derived from short period records also produces good fits to the long period data the seismic moment of this earthquake is found to be 0 8 to 1 0 10 25 dyne cm the sp model for the 03h 15m aftershock on the other hand predicts long period synthetics which do not agree with the observations in particular the sp moment 0 37 10 25 dyne cm is about formula times smaller than the lp moment 1 10 25 dyne cm adding a long period component to the sp model considerably improves lp waveform and moment agreement in the case of the main shock a reasonable fit to the observed sp data is obtained using three point sources of radiation however lp synthetics computed using this model do not agree with the observations and the sp moment 0 65 10 25 dyne cm is a small fraction of the lp moment 3 to 5 10 25 dyne cm time function durations indicate that the individual events inferred from the sp records are radiated from patches of the fault having radii of 2 to 4 km and stress drops in the range 35 to 276 bars in comparison stress drops estimated from lp data are found to be 12 bars main shock and 24 bars 09h 21m aftershock these observations suggest that the short period instruments are sensitive to the high frequency radiation emitted from small high stress drop areas on the fault plane whereas the long period instruments record the overall motion during the earthquake

waveforms and spectra of preshocks and aftershocks of the 1979 imperial valley california earthquake evidence for fault heterogeneity we have compared digitally recorded waveforms of m l 2 0 2 8 earthquakes that occurred in two small areas along the imperial fault before and after it broke in the ml 6 6 imperial valley earthquake on october 15 1979 eight preshocks 1977 1979 from a 4 by 1 km area centered 4 km se of the mainshock epicenter have strikingly similar waveforms over the entire record length 30 s with an average peak cross correlation between seismograms of 0 74 the seismograms are well correlated at frequencies up to at least 4 hz this implies similar source mechanisms and hypocenters within of the 4 hz wavelengths i e 200 400 m five aftershocks from the same area show an average peak cross correlation between seismograms of only 0 23 any associated changes in mechanism must be small because they are not reflected in the first motion data analysis of frequency content of these events using bandpass filtering techniques showed no systematic temporal changes in spectral shape ten preshocks and 24 aftershocks from a 1 by 2 km source area centered along the fault 16 km nw of the mainshock epicenter were also studied first motion data suggest that all of the aftershocks and a swarm of six preshocks on december 7 9 1978 were associated with the main fault but that four earlier preshocks were not the six preshocks on december 7 9 1978 were tightly clustered as evidenced by the strong similarity of the waveforms most peak cross correlations 0 6 during this swarm the 8 to 16 hz spectral amplitude increased relative to the 1 to 2 hz spectral amplitude over the whole record length by about a factor of 3 suggesting a systematic increase in stress drop groups of like events are also present among the aftershocks in this data set the average peak correlation for pairs of aftershocks 0 43 is almost the same as that for pairs of preshocks 0 45 if all 10 preshocks are included however several sources appear to have been active simultaneously during the aftershock period so that no more than two to three consecutive aftershocks have maximum cross correlations 0 6 the highly localized sources characterized by waveform similarity may represent fault asperities or clusters of asperities our observations are consistent with a decrease in the number of these asperities as the weaker ones fail under increasing stress during the intervals between large earthquakes

teleseismically recorded seismicity before and after the may 7 1986 andreanof islands alaska earthquake the may 7 1986 andreanof islands earthquake mw 8 0 is the largest event to have occurred in that section of the aleutian arc since the march 9 1957 aleutian islands earthquake mw 8 6 teleseismically well recorded earthquakes in the region of the 1986 earthquake are relocated with a plate model and with careful attention to the focal depths the data set is nearly complete for mb 4 7 between longitudes 172 w and 179 w for the period 1964 through april 1987 and provides a detailed description of the space time history of moderate size earthquakes in the region for that period additional insight is provided by source parameters which have been systematically determined for mw 5 earthquakes that occurred in the region since 1977 and by a modeling study of the spatial distribution of moment release on the mainshock fault plane a technically significant component of oblique convergence in the central aleutian arc results in its breakup into clockwiserotating and westward translating blocks the western part of the andreanof block is distinct from and stronger than flanking regions the greater strength of this block segment and strong coupling along the main thrust zone result in the accumulation of high levels of shear stress which give rise to great earthquakes near its eastern boundary the occurrence of the 1986 rupture only 29 years after the 1957 earthquake may indicate that in the central aleutians m 7 earthquakes ordinarily do represent the predominant mode of strain release segmentation of the main thrust zone into upper and lower planes is supported by spatial and temporal patterns of seismicity and by focal mechanism data this and other lines of evidence indicate a downdip increase in fault strength and possibly in heterogeneity within the main thrust zone in subduction zone environments aftershocks of the 1986 earthquake were bounded to the west by the andreanof block boundary adak canyon and to the east by an aseismic segment of the main thrust zone near the subducted extension of the amlia fracture zone the aftershock distribution was bounded to the south by a forearc shear zone and to the north by the base of the main thrust zone in the hawley ridge segment and by the downdip edge of the upper plane of the main thrust zone in the eastern segment aftershocks which occurred near the volcanic line at shallow crustal depths in the upper plate were triggered by the mainshock and manifest a partial decoupling of oblique slip in this region along a west striking right lateral fault with low shear strength aftershock clustering along the main thrust zone was very similar to the distribution of prior seismicity suggesting a continuation of long term processes and the existence of areas with distinct mechanical properties interconnecting regions of low seismicity during both the premainshock and aftershock periods coincided with areas of major moment release during the mainshock seismicity data prior to the 1986 mainshock gave few clues about the location of the mainshock nucleation point the mainshock size and its time of occurrence however a large part of the mainshock moment release did coincide with a zone of seismic quiescence monitored by the adak local seismograph network

shumagin seismic gap alaska peninsula history of great earthquakes tectonic setting and evidence for high seismic potential the shumagin seismic gap a segment of the plate boundary along the eastern aleutian arc has not ruptured during a great earthquake since at least 1899 1903 because at least 77 years have elapsed since the shumagin gap last ruptured in a great earthquake and repeat times for the 1938 rupture zone and part of the shumagin gap are estimated to be 50 to 90 years a high probability exists for a great earthquake to occur within the shumagin gap during the next one to two decades reconsideration of the rupture zones of the aleutian earthquakes of 1938 1946 and 1948 suggests that those events did not break the interplate boundary beneath the shumagin islands thus the shumagin seismic gap extends from the western end of the 1938 rupture zone to the eastern end of that of 1946 these boundaries also coincide with transverse structural features at least the eastern half of the shumagin gap broke in great earthquakes in 1788 and 1847 and possibly in 1898 1903 the shumagin gap is probably not the result of aseismic slip rather plate motion is accommodated there seismically and episodically and can be expected to produce large earthquakes in the future although there is no definitive evidence of long term precursors of a possible future earthquake several observations suggest that the shumagin gap is in an advanced stage of the earthquake cycle both teleseismic and local network data indicate a near absence of seismic activity m 2 above a depth of 30 km along the main thrust zone within the gap this is in strong contrast to adjacent portions of the arc where seismic activity is scattered across most of the main thrust zone two earthquakes with high stress drops 600 900 bars which occurred at the base of the main thrust zone may indicate the accumulation of a considerable amount of strain energy within the gap a possible seismic gap at the eastern end of the aftershock zone of the aleutian earthquake of 1957 has been identified near unalaska island an earthquake that nucleates in the shumagin gap could also rupture the possible unalaska gap to the west the 1938 aftershock zone to the east or both with resultant magnitude up to mw 9 0 alternatively the shumagin gap alone or in one of the above combinations could rupture in a series of very large earthquakes instead of a single great shock past alaska aleutian earthquakes including those of 1788 1938 1946 1957 1964 and 1965 have generated very large tsunamis future large earthquakes in the shumagin islands region could generate wave heights of several tens of meters along shorelines near the rupture areas the shumagin gap is one of two major gaps along the united states portion of the alaska aleutian plate boundary and is one of the few areas in the united states where processes leading to a great earthquake are likely to be observed within a reasonable span of time

strong motion modeling of the imperial valley earthquake of 1979 twelve three component strong motion displacement records are modeled for the 1979 imperial valley earthquake to recover the distribution of slip on the imperial fault plane the final model for which point source responses are calculated by a discrete wavenumber finite element technique uses a structure with gradients in material properties rather than layers the effects of a velocity gradient are investigated by comparing synthetics with a layer over a half space model using generalized rays it is shown that a uniform fault rupture model on a rectangular fault plane does not explain the data the preferred fault model has slip concentrated below 5 km in the basement material and between the epicenter 5 km south of the international border and highway 80 within this region there appears to be two localized areas of larger dislocations one just north of the border near bonds corner and a second under interstate 8 at meloland overpass a major arrival associated with large amplitude vertical accelerations up to 1 7 g is identified in the el centro array records this arrival has an s p time of approximately 2 3 sec at many of the array stations and is modeled as originating from a localized source 8 km to the south of the array the moment is estimated to be 5 0 10 25 dyne cm from the strong motion records which is consistent with teleseismic body wave estimates the preferred fault model is strike slip with a 90 dip the average strike is 143 however to explain vertical waveforms near the fault trace a corrugated or wiggly fault plane is introduced the average rupture velocity is in the range 2 5 to 2 7 km sec 0 8 to 0 9 times the basement shear wave velocity the preferred model has unilateral rupture propagation to the north although the data would allow a small amount of propagation to the south the estimated stress drop for the entire fault plane is only 5 to 10 bars however the stress drop over the more localized sources is about 200 bars the fault model is consistent with the pattern of seismicity and observations of aseismic creep in the imperial valley and suggests that the southern half of the imperial fault acts as a locked section which breaks periodically

geologic implications of great interplate earthquakes along the aleutian arc the aleutian arc is one of the most seismically active regions of the world it consists of a number of segments or blocks that are rotating clockwise and translating westward along the north american pacific plate boundary two of the largest recorded earthquakes have occurred along the interplate boundary south of the andreanof islands in 1957 and the rat islands in 1965 the large magnitudes of these earthquakes implied that the probability of the recurrence of a large magnitude event at these same locations was low however the occurrence of a great earthquake south of the andreanof islands in 1986 less than 30 years after the 1957 event has prompted a reevaluation of the seismic potential of the arc we present new marine geophysical observations and synthesize previous geologic interpretations of the aleutian arc to show that the epicenters of these great thrust type earthquakes coincide with upper plate segments of the arc characterized by a coherent forearc structural fabric these segments which are associated with large plutonic masses are mechanically stronger and are related to the location of asperities positioned at depth along the main thrust zone they appear to be more strongly attached to the north american plate than adjacent blocks the tectonically controlled locations of these asperities thus appear to influence the seismicity along the arc the andreanof and rat segments of the arc are separated by the delarof segment which is considered to have a high probability of a large magnitude earthquake in the near future based on probabilistic estimates of seismic potential nishenko 1991 the delarof segment of the arc however is highly disrupted by arc transverse faults this segment is structurally weak and does not appear to be capable of accumulating the interplate strain necessary to generate a great subduction zone earthquake we propose that variations in upper plate structural strength and mobility affect the mechanical properties of the interplate thrust zone and need to be considered in localizing interplate asperities forearc tectonic segmentation associated with the partitioning of strike slip and thrust motions may exert long term controls on the rates of seismic moment release

paleoseismology of the mount narryer fault zone western australia a multistrand intraplate fault system our paleoseismological study of faults and fault related folds comprising the mount narryer fault zone reveals a mid to late quaternary history of repeated morphogenic earthquakes that have influenced the planform and course of the murchison roderick and sanford rivers western australia the dominant style of deformation involves folding of near surface sediments overlying discrete basement faults carbon 14 optically stimulated luminescence and in situ produced 10 be constrain the timing of the events and late quaternary slip rates associated with fault propagation folds in tectonically uplifted and deformed alluvial channel deposits a flight of five inset fluvial terraces is preserved where the murchison river flows across the roderick river fault these terraces which we infer to be coseismic are consistent with at least four late quaternary seismic events on the order of moment magnitude m w 7 1 within the last 240 k y secondary shears expressed on the folds indicate a component of dextral strike slip displacement quaternary slip rates on the underlying faults range from 0 01 to 0 07 mm yr 1 with a total slip rate for the zone between 0 04 and 0 11 mm yr 1 these rates are intermediate to those in the adjacent mesozoic basin 0 1 mm yr 1 and precambrian craton 1 and so provide insight into how tectonic strain is partitioned and transferred across a craton margin

erosion on a line abstract an erosion model is proposed to calculate erosion rates for plane strain models in which the earth s surface is represented on a line the fundamentals of river erosion networks are captured by two principles hack s law which describes the drainage area structure of river network and a stream power erosion law which describes the rate of incision of a river for a simple morphology of parallel transverse rivers with rectangular drainage basins this allows the earth s surface to be parameterized by two heights the trunk stream channel height and the interfluvial ridge height the resulting expressions are solved for the simple cases of constant uplift rate and a constant mean slope as occurs in critical wedge problems in the latter case the uplift rate is variable and changes in space so that the trunk channel elevation and the interfluvial ridge elevation average to maintain a constant mean slope a general numerical solution is presented for application to any numerical model with arbitrary surface velocity variable rock erodibility and precipitation this algorithm is coupled to a plane strain plastic deformation model to demonstrate the utility of the model

p wave travel times stability and change within the source volume of a m 7 2 earthquake the dense seismograph network on the south flank of kilauea afforded a unique opportunity to study travel time changes as a function of space and time within the source volume of the hawaii earthquake ms 7 2 of 1975 careful analysis of more than 600 teleseismic p arrivals from deep fiji tonga earthquakes revealed the following out of eight stations studied six showed constant travel times to within 0 03 s for up to 10 years with the exception of some small changes of about 0 03 s at the time of the main shock one station ahu showed a highly significant and unique travel time decrease by 0 13 s during 1971 1972 at the beginning of this velocity anomaly a southwest rift intrusion caused closure of surface cracks associated with normal faults located near the station ahu also geodetic measurements revealed that between august and october 1971 compressive strain of 4 10 5 was accumulated perpendicular to the southwest rift in the area of ahu we conclude that these data show for the first time that in situ velocity increases occur due to the closure of cracks by tectonic forces the ahu residuals returned to normal approximately at the time of a major earthquake swarm on the fault zone near ahu during which surface cracks were observed to have opened again this travel time decrease was corroborated by residuals from novaya zemlya explosions but highfrequency local signals did not show it assuming that the stress sensitivity of the velocity was similar to that found by reasenberg and aki 1974 we have interpreted the teleseismic residual change at ahu as due to a p velocity increase of about 10 in the top 3 5 1 5 km of the crust the only station wha which showed a large 0 2 s and extended 1972 to 1975 travel time increase was located only 4 km from the main shock epicenter we interpret this velocity decrease as a precursor to the 1975 main shock and we hypothesize that a process reverse from that at ahu caused this anomaly by first opening and then closing cracks in the crust below wha both of these velocity anomalies were extremely localized in crustal volumes of dimensions of about 5 3 km

precursors to the kalapana m 7 2 earthquake the kalapana hawaii earthquake of november 1975 had a rupture length of 40 to 50 km and was located on the south flank of the active volcano kilauea the source mechanism was dip slip normal faulting on a plane dipping 20 degrees to the se with the greatest principal stress oriented in that direction and accumulated by volcanic intrusions into kilauea s rifts the source area of the 1975 earthquake was subject to intensive geological and geophysical research for many years before this earthquake because of its proximity to the volcano we studied the distribution of epicenters for small earthquakes from 1962 to 1975 seven seismographs located within 3 km of the aftershock area had been in operation for 3 to 10 years before the mainshock and geodetic triangulations and trilaterations in the source area had been carried out repeatedly since 1914 we found that precursory changes occurred throughout most of the rupture area but in two distinctly different patterns in the larger outer anomalous area the seismicity rate was decreased by 50 during the 3 8 years before the mainshock in addition several geodetic lines indicated anomalous strain release during this time within two inner areas the seismicity remained high then increased shortly before the mainshock in one of the inner areas a p wave travel time delay of 0 2 s could be detected which began about 3 5 years before the mainshock within the other inner anomalous area geodetic strain was accumulating until the first half of 1975 when 3 5 10 4 strain 35 bars was released aseismically by contrast the outer anomalous volume was experiencing strain softening from 1970 1971 on we interpret these observations as indicating that strain softening by fault creep in the outer anomalous area transferred stress into two major asperities locked portions of the fault a velocity decrease and foreshocks were observed in one asperity and high stress accumulation in the other implying that dilatancy of the crust probably occurred our model is qualitatively supported by the independent evidence of strong motion records which show that the kalapana earthquake was a complex multiple rupture we conclude that the kalapana earthquake was preceded by a preparatory process which lasted 3 8 0 3 years and which had dimensions of 45 10 km covering approximately the aftershock area

effects of fault interaction on moment stress drop and strain energy release solutions for collinear shear cracks are used to examine quantitatively the effects of fault slip zone interaction on determinations of moment stress drop and static energy release two models the barrier model and the asperity model are considered in the asperity model the actual distribution of strengths on a fault plane is idealized as a combination of two limiting cases areas which slip freely at a uniform value of a residual friction stress and unbroken ligaments or asperities across which slip occurs only at the time of a seismic event in the barrier model slip zones separated by unbroken ligaments barriers are introduced into a uniformly stressed medium to approximate the nonuniform fault propagation proposed by das and aki the strain energy change due to introducing collinear slip zones or due to breaking the asperities between them is shown to be given by the usual formula for an isolated slip zone with the stress drop replaced by the effective stress significant interaction between slip zones occurs only if the length of the asperity is less than half the length of the slip zones for the case of two collinear slip zones fracture of the asperity between them is shown to cause a large moment primarily because of the additional displacement which is induced on the adjacent slip zones for example if the asperity length is 0 05l where l is the length of each adjacent slip zone then fracture of the asperity causes a moment almost 1 8 times the moment caused by introducing a slip zone of length l for two collinear slip zones the local stress drop due to fracture of the separating asperity is shown to become unbounded as the asperity length goes to zero but in the same limit the stress drop averaged over the entire fault length is approximately equal to the apparent stress drop inferred for an isolated fault of the same moment and total fault length this apparent stress drop is approximately equal within a factor of 2 or 3 to the effective stress and hence can be used in the usual formula to give a good estimate of the strain energy change for the barrier model numerical results are given for the ratio of the stress drop calculated on the assumption of an isolated slip zone to the true stress drop for example in the case of two collinear slip zones of length l separated by a barrier of length 0 2l this ratio is 0 5 whereas for a barrier length equal to that of the adjacent slip zones the ratio is 0 24 stress drop estimates become worse with increasing number of fault segments

dynamic interactions between erosion deposition and three dimensional deformation in compressional fold belt settings 1 this study investigates whether the three dimensionality of erosion and deposition can influence three dimensional folding even when compression is purely convergent this problem is studied by employing a three dimensional mathematical model incorporating dynamic thin plate deformation coupled with fluvial and hillslope sediment transport the approach taken is to impose several fundamentally different sediment routing patterns by changing the initial regional topographic slope and the ratio of the characteristic timescales for deformation to fluvial surface processes r and to investigate how deformation responds numerical results indicate that when and or r are small i e initial topography is regionally flat and or rates of surface processes are slow compared to rate of imposed deformation folding is two dimensional and unaffected by surface processes since loads created by surface processes are imposed after deformation this situation favors development of small scale drainage networks transporting sediment from anticlines to adjacent basins when and or r are relatively large the development of a large scale transverse drainage network which can be maintained during folding is able to strongly amplify and localize folding this leads to interesting features where the largest transverse rivers coincide with the greatest structural and topographic amplitude of doubly plunging folds which is caused by erosion enhanced deformation for intermediate parameter values folding and surface processes interact in a complex manner giving rise to three dimensional en echelon structures that migrate in a transient manner this study demonstrates that under some circumstances one can anticipate major two way interaction between surface processes and three dimensional deformation

channel changes in badlands stripping of the vegetation and soil from a 13 hectare site in virginia underlain by coastal plain sediments created a rapidly evolving badland topography two types of channels developed 1 sand bed alluvial channels were graded to transport the bed material load supplied from slope erosion with available runoff but they also generally eroded their beds slowly and 2 steeper bedrock floored channels incised rapidly in bedrock channels the erosion rate was proportional to the 4 9ths power of drainage area and the 2 3rds power of gradient these exponents are consistent with a model in which the erosion rate is proportional to the bed shear during high flows due to rapid mass wasting and reduced runoff the alluvial channels became as much as 50 steeper during the winter than the summer with an attendant yearly cycle of winter aggradation and summer entrenchment the gradients their seasonal variability and their downstream hydraulic geometry were consistent with the predictions of total load transport formulas for sand beds and high loads the hydraulic geometry of alluvial channels in the virginia badlands were similar to that on the morrison formation in the western united states

orogeny and orography the effects of erosion on the structure of mountain belts a numerical model of the coupled processes of tectonic deformation and surface erosion in convergent orogens is developed to investigate the nature of the interaction between these processes crustal deformation is calculated by a two dimensional finite element model of deformation in response to subduction and accretion of continental crust erosion operates on the uplifted surface of this model through fluvial incision which is taken to be proportional to stream power the relative importance of the tectonic and erosion processes is given by a dimensionless erosion number relating convergence velocity rock erodibility and precipitation rate this number determines the time required for a system to reach steady state and the final topographic shape and size of a mountain belt fundamental characteristics of the model orogens include asymmetric topography with shallower slopes facing the subducting plate and an asymmetric pattern of exhumation with the deepest levels of exhumation opposite to subduction these characteristics are modified when the regional climate exhibits a dominant wind direction and orographically enhanced precipitation on one side of the mountain belt the two possible cases are dominant wind in the direction of motion of the subducting plate and dominant wind direction in the opposite direction of the subducting plate velocity models of the former case predict a broad zone of exhumation with maximum exhumation in the orogen interior models of the latter case predict a focused zone of exhumation at the margin of the orogen and at high erosion number a reversal in the topographic asymmetry natural examples of these two cases are presented the southern alps of new zealand exhibits the climate and exhumation asymmetry characteristic of wind in the direction opposite to motion of the subducting plate the asymmetry of topography suggests that erosion is not efficient enough to have reversed the topographic asymmetry the contrasting example of dominant wind in the direction of subduction motion is provided by the olympic mountains of washington state in this case exhumation of deep levels of the cascadia accretionary wedge shows a broad domal pattern consistent with the observed orographic precipitation

surface expression of eastern mediterranean slab dynamics neogene topographic and structural evolution of the southwest margin of the central anatolian plateau turkey 1 the southwest margin of the central anatolian plateau has experienced multiple phases of topographic growth including the formation of localized highs prior to the late miocene that were later affected by wholesale uplift of the plateau margin our new biostratigraphic data limit the age of uplifted marine sediments at the southwest plateau margin at 1 5 km elevation to 7 17 ma and regional lithostratigraphic correlations imply that the age is 6 7 ma single grain ca tims u pb zircon analyses from a reworked ash within the marine sediments yield dates as young as 10 6 ma indicating a maximum age that is consistent with the biostratigraphy our structural measurements within the uplifted region and fault inversion modeling agree with previous findings in surrounding regions with early contraction followed by strike slip and extensional deformation during uplift focal mechanisms from shallow earthquakes show that the extensional phase has continued to the present broad similarities in the change in the tectonic stress regime after 8 ma and the onset of surface uplift after 7 ma imply that deep seated process es caused post 7 ma uplift the geometry of lithospheric slabs beneath the plateau margin pliocene to recent alkaline volcanism and the uplift pattern with accompanying normal faulting point toward slab tearing and localized heating at the base of the lithosphere as a probable mechanism for post 7 ma uplift of the southwest margin considering previous work in the region there appears to be an important link between slab dynamics and surface uplift throughout the anatolian plateau s southern margin

fluvial erosion transport equation of landscape evolution models revisited we present a mesoscale erosion deposition model which differs from previous landscape evolution models equations by taking explicitly into account a mass balance equation for the streamflow the geological and hydrological complexity is lumped into two basic fluxes erosion and deposition and two averaged parameters unit width discharge q and stream slope s the model couples the dynamics of streamflow and topography through a sediment transport length function x q which is the average travel distance of a particle in the flow before being trapped on topography this property reflects a time lag between erosion and deposition which allows the streamflow not to be instantaneously at capacity the so called x q model may reduce either to transport limited or to detachment limited erosion modes depending on x but it also may not we show in particular how it does or does not for steady state topographies long term evolution and high frequency base level perturbations apart from the unit width discharge and the settling velocity the x q function depends on a dimensionless number encompassing the way sediment is transported within the streamflow using models of concentration profile through the water column we show the dependency of this dimensionless coefficient on the rouse number we discuss how consistent the x q model framework is with bed load scaling expressions and einstein s conception of sediment motion

numerical investigation of deformation mechanics in fold and thrust belts influence of rheology of single and multiple decollements 1 thin skinned fold and thrust belts related to convergence tectonics develop by scraping off a rock sequence along a weaker basal decollement often formed by water saturated shale layers or low viscosity salt horizons a two dimensional finite element model with a viscoelastoplastic rheology is used to investigate the structural evolution of fold and thrust belts overlying different types of decollements in addition the influence of multiple weak layers in the stratigraphic column is studied model shale decollements are frictional with lower friction angles as the cover sequence model salt layers behave linear viscous due to a lower viscosity as the cover sequence or with a power law rheology single viscous decollement simulations have been compared to an analytical solution concerning faulting versus folding results show that fold and thrust belts with a single frictional basal decollement generate thrust systems ramping from the decollement to the surface spacing between thrust ramps depends on the thickness of the cover sequence the structural evolution of simulations with an additional low frictional layer depends on the strength relationship between the basal and the intersequential decollement tectonic underplating and antiformal stacking occur if the within sequence decollement is weaker in the frontal part of models deformation is restricted to the upper part and imbrication occurs with a wavelength depending on the depth of the intermediate weak layer salt decollement with a viscosity of 10 18 pa s leads to isolated box folds detachment folds multiple salt layers 10 18 pa s result in long wavelength folding our results for both frictional and viscous decollements are in bulk agreement with the mohr coulomb type critical wedge theory citation ruh j b b j p kaus and j p burg 2012 numerical investigation of deformation mechanics in fold and thrust belts influence of rheology of single and multiple decollements tectonics 31 tc3005 doi 10 1029 2011tc003047

exhuming norwegian ultrahigh pressure rocks overprinting extensional structures and the role of the nordfjord sogn detachment zone 1 the nordfjord sogn detachment zone nsdz is widely cited as one of the primary structures responsible for the exhumation of norwegian ultra high pressure uhp rocks here we review data from the considerable volume of research describing this shear zone and compile a strikeparallel cross section along the nsdz from the solund basin in the south to the soroyane uhp domain in the north this cross section highlights several previously unrecognized patterns revealing a shear zone with top to the west asymmetric fabrics that 1 initiated at amphibolite facies 2 overprints metamorphic breaks and tectonostratigraphic contacts and 3 has a gradational continuum of muscovite cooling ages these patterns constrain the kinematic evolution of the nsdz and suggest a new three step model for the exhumation of norwegian u hp rocks the initial stages of exhumation were characterized by the rise of crustal rocks from u hp depths to the base of the crust by buoyancydriven mechanisms not specified in this paper mantle exhumation was followed by top to thewest normal sense displacement within a broad noncoaxial ductile shear zone near the base of the crust that overprinted tectonostratigraphic contacts formed previously during mantle exhumation in the final stages of crustal exhumation top w brittleductile detachments soled into and partially excised this ductile shear zone dropping the devonian basins into contact with rocks of varying tectonostratigraphic levels this new interpretation of the nsdz is significant as it accounts for the extreme crustal excision observed in western norway using three sequentially overprinting structures active at different stages of uhp rock exhumation citation johnston s m b r hacker and t b andersen 2007 exhuming norwegian ultrahigh pressure rocks overprinting extensional structures and the role of the nordfjord sogn detachment zone tectonics 26 tc5001

topographic evolution and morphology of surfaces evolving in response to coupled fluvial and hillslope sediment transport 1 this paper quantifies how the ratio of sediment transport on hillslopes to sediment transport in channels influences surface and channel network morphologies and the dynamics of topographic evolution this problem is investigated by development and investigation of a simple deterministic model incorporating mass balance of sediment and runoff coupled with a law combining dispersive and concentrative sediment transport processes our analysis includes the identification of a new nondimensional parameter de that is a function of rainfall system size rock type and hydraulic regime and that is a measure of the relative importance of fluvial and hillslope sediment transport we show that de has an important influence on the surface morphology e g total exposed surface area and interface width which reflects surface roughness and relief channel network form e g channel sinuosity channel spacing and timescale of surface evolution surface and channel network morphologies are also strongly influenced by the overall surface slope relative to the magnitude of initial topographic roughness topographic evolution occurs in distinct phases of relief growth and decay the transition between which is controlled by a saturation phenomenon related to the growth of spatial correlations the scaling behavior of simulated topography with respect to both time and space is obtained and is shown to be independent of de roughness exponents are found to be independent of de but dependent on the magnitude of initial roughness interface width is shown to grow and decay as a logarithm of time

influence of syntectonic sedimentation on thrust wedges in analogue models scaled analogue sandbox models have been used to simulate the growth of coulomb thrust wedges that include syntectonic sedimentation during thrusting increases in the syncontractional sedimentation rate dramatically change the shape of the coulomb thrust wedge decreasing the number of thrust slices the internal shortening and the critical taper of the wedge thrust trajectories change from ramp flat to sigmoidal with increased syncontractional sedimentation the analogue models show many similarities to thrust fault geometries in accretionary prisms and foreland fold and thrust belts in which thrust slices prograded into basins with active sedimentation

a very efficient o n implicit and parallel method to solve the stream power equation governing fluvial incision and landscape evolution abstract we present a new algorithm to solve the basic stream power equation which governs channel incision and landscape evolution in many geomorphic settings the algorithm is highly efficient because computation time increases linearly with the number of points used to discretize the landscape and is ideally suited to parallelization it is also unconditionally stable because it uses an implicit scheme for the time integration of the landscape evolution equation which means that large time steps can be used without sacrificing accuracy in this paper we describe the algorithm and present results that demonstrate its efficiency and accuracy

influences of surface processes on fold growth during 3 d detachment folding in order to understand the interactions between surface processes and multilayer folding systems we here present fully coupled three dimensional numerical simulations the mechanical model represents a sedimentary cover with internal weak layers detached over a much weaker basal layer representing salt or evaporites applying compression in one direction results in a series of three dimensional buckle folds of which the topographic expression consists of anticlines and synclines this topography is modified through time by mass redistribution which is achieved by a combination of fluvial and hillslope erosion as well as deposition and which can in return influence the subsequent deformation model results show that surface processes do not have a significant influence on folding patterns and aspect ratio of the folds nevertheless erosion reduces the amount of shortening required to initiate folding and increases the exhumation rates increased sedimentation in the synclines contributes to this effect by amplifying the fold growth rate by gravity the main contribution of surface processes is rather due to their ability to strongly modify the initial topography and hence the initial random noise prior to deformation if larger initial random noise is present folds amplify faster which is consistent with previous detachment folding theory variations in thickness of the sedimentary cover in one or two directions also have a significant influence on the folding pattern resulting in linear large aspect ratio folds our simulation results can be applied to folding dominated fold and thrust belt systems detached over weak basal layers such as the zagros folded belt

control of folding by gravity and matrix thickness implications for large scale folding 1 we show that folding of a non newtonian layer resting on a homogeneous newtonian matrix with finite thickness under influence of gravity can occur by three modes 1 matrix controlled folding dependent on the effective viscosity contrast between layer and matrix 2 gravity controlled folding dependent on the argand number the ratio of the stress caused by gravity to the stress caused by shortening and 3 detachment folding dependent on the ratio of matrix thickness to layer thickness we construct a phase diagram that defines the transitions between each of the three folding modes our priority is transparency of the analytical derivations e g thin plate versus thick plate approximations which permits complete classification of the folding modes involving a minimum number of dimensionless parameters accuracy and sensitivity of the analytical results to model assumptions are investigated in particular depth dependence of matrix rheology is only important for folding over a narrow range of material parameters in contrast strong depth dependence of the viscosity of the folding layer limits applicability of ductile rheology and leads to a viscoelastic transition our theory is applied to estimate the effective thickness of the folded central asian upper crust using the ratio of topographic wavelength to moho depth phase diagrams based on geometrical parameters show that gravity does not significantly control folding in the jura and the zagros mountains but does control folding in central asia applicability conditions of viscous and thin sheet models for large scale lithospheric deformation derived in terms of the argand number have implications for the plate like style of planetary tectonics

erosion and exhumation in accretionary orogens experimental and geological approaches 1 this study examines the role of erosion on exhumation and deformation history of accretionary orogens through experimental and geological methods fault propagation kinematics and exhumation rates in an eroded thrust wedge are analyzed through scaled sandbox simulations by testing different parameters friction rheologic layering and sediment input output ratio a model coulomb wedge is submitted to erosion under flux steady state conditions that is the volume of eroded material remains equal to the volume of newly accreted material maintaining a constant surface slope during shortening results show that the way of exhumation depends on the internal dynamics of thrust wedges and conversely on how this dynamics is modified by erosion in the eroded thrust wedges the diversity of exhumation patterns is controlled by the mode of fault propagation depending on the basal friction high or low the vertical component of exhumation is generally higher for the wedges with high basal friction than for low friction wedges the uplift of material occurs along a cluster of subvertical thrusts in the middle part of the eroded thrust wedge with low basal friction the material is exhumed along a series of inclined 20 50 thrusts in the rear of the high friction wedge the zone of maximum exhumation is generally localized in the middle part of the thrust wedge and migrates toward the backstop with continued shortening the vertical exhumation rate increases with time and the material accreted later is rapidly transferred to the main exhumation zone compared to the material accreted during the early stages the exhumation occurs at rates 2 times faster in the wedges with half the thickness than in full thickness wedges the total quantity of eroded material at the end of convergence constitutes 36 50 of initial model area for the thrust wedges with low basal friction and 20 40 for the high basal friction wedges the extent of basal underplating increases with total shortening the area of basal underplated material constitutes up to 30 and 40 of the eroded thrust wedge area for the models with low and high basal friction respectively presence of decollements in the accreted series allows underplating of thrust units developing an anticlinal stack whose growth and location is favored by erosion our results are compared to different present day active and ancient convergent orogens himalaya taiwan southern alps of new zealand and cascadia orogen allowing the different specific exhumation processes to be characterized

spatial seismicity variations and asperities in the new hebrides seismic zone the term asperity has been used in many recent studies to describe sections of faults with higher strength than the surrounding zones the level of background seismicity expected in such zones is unclear they may appear as quiet zones because of high strength or active zones because of high stress in this work the problem of seismicity levels in asperities is addressed by identifying regions of anomalously high or low seismicity and comparing the locations of these regions with the locations of asperities as determined from other observations the regions of anomalously high or low seismicity are identified using a quantitative technique which relies on spatial rate comparisons done with the z test for a difference between two means this technique allows the determination of the significance of spatial rate differences and the uniqueness of zones of outstanding seismicity four active zones which make up 10 and eight quiet zones which make up 16 of the length of the new hebrides seismic zone were recognized using this technique asperities in the new hebrides seismic zone were identified using characteristics which include large event initiation and stopping clusters of aftershocks and moderate events foreshock activity and earthquake stress drops eight regions which showed two or more of these characteristics were identified they make up 34 of the length of the seismic zone the overlap between the asperities and the active and quiet regions provides a test of possible relationships between these zones thirty four percent of the asperity area is active as opposed to the 10 expected and 6 of the asperity area is quiet as opposed to the 16 expected this suggests that in the asperities in the new hebrides seismic zone stress concentration overcomes increased strength and that phenomena other than increased strength can cause seismic quiescence nearly all of the asperities in the new hebrides seismic zone show high background seismicity or high stress drops which suggests that likely zones of mainshock initiation can be identified using background seismicity

building the zagros collisional orogen timing strain distribution and the dynamics of arabia eurasia plate convergence he zagros mountains are the result of the arabia eurasia collision initiated at 35 ma as the rifted arabian lithosphere was underthrusted beneath the iranian plate due to its negative buoyancy the onset of crustal thickening started at 25 ma as recorded by the hinterland exhumation and foreland clastic deposition deformation throughout the arabia eurasia collision zone and the uplift of the iranian plateau occurred after 15 12 ma as a result of shortening thickening of the thin iranian crust we emphasize that only 42 of the post 35 ma convergence is partitioned by shortening within central iran tomographic constraints show ongoing slab steepening or breakoff in the nw zagros whereas underthrusting of the arabian plate is observed beneath central zagros the current subduction dynamics can be explained by the original lateral difference in the buoyancy of the distal margin that promoted slab sinking in nw zagros and underthrusting in central zagros critical wedge approach applied to the zagros favors the hypothesis of strong brittle crust detached above a viscous lower crust in contrast the weak sedimentary cover deforms by buckling of a thick multilayered cover thrust faulting associated with folding occurs in the competent layers and is responsible for most of the earthquakes there is evidence that the role of the slab pull force in driving the arabian plate motion was reduced after 12 ma large scale mantle flow induced by mantle upwelling at the afar plume appears to be the main driver of the arabia plate motion we stress that the main kinematic change in the zagros region occurred at 15 12 ma as the zagros uplifted before the arabian slab detached the zagros appears key to investigate coupling between continental rheology plate driving forces and mountain building in which the role of rift inheritance appears to be central

single link cluster analysis of earthquake aftershocks decay laws and regional variations using single link cluster analysis the authors investigate how various properties of aftershock sequences depend on their tectonic regime and focal depth for international seismological centre earthquakes of m sub b ge 4 8 they find that earthquakes deeper than 70 km have the fewest and smallest aftershock sequences even after accounting for differences in detectability and maximum magnitude they find that ridge transform earthquakes have smaller aftershock sequences that shallow subduction zone earthquakes among different subduction zones they find that zones with high moment release rates possess larger aftershock sequences comparing ridge transform zones they find those with slower spreading rates possess larger aftershock sequences by transposing origin times of several different aftershock sequences as if all had main shocks occurring at time zero they are able to study the properties of aftershock sequences which individually have too few aftershocks to study by other means secondary events determined by single link cluster analysis follow a modified omori s power law decay for time separations of 0 1 day to 20 days from the parent event with p values ranging from 0 539 plus minus 0 022 intermediate and deep focus earthquakes to 0 928 plus minus 0 024 ridge transform earthquakes they find that earthquake foreshocks and multiplets also follow a modified omori s law atmore greater times from the main shock the decay is steeper than a power law decay more like an exponential decay aftershocks in the adak catalog m sub b ge 2 0 show a marked decrease in activity between 40 and 50 km depth they speculate that the observed differences in number of aftershocks and p values may be caused by variations in fault heterogeneity or in fluid pressures less

seismicity in subduction zones from local and regional network observations the data provided by local and regional seismograph networks are essential for the solution of many problems of subduction zone seismology the capabilities of such networks are limited by the instrumentation currently in common use and the unfavorable source station geometry often imposed by the regional geography nevertheless important contributions have come from the data gathered in many of the earth s subduction zones the accuracy of hypocenter locations based on regional data is affected by the complex velocity structures characteristic of subduction zones but the problems are now well understood examples of numerous studies of the spatial configurations of the seismicity in subduction zones and consequent interpretations of seismogenesis and subduction processes are reviewed studies of the distributions of earthquakes in time and with magnitude for events down to the microearthquake level have the potential for clarifying the earthquake generating processes and possibly a basis for earthquake prediction other uses of local and regional network data have been for investigations of coda s and the identification of asperities in subduction zones

influence of syntectonic sedimentation on thrust geometry field examples from the iberian chain spain and analogue modelling abstract steep thrusts are usually interpreted as oblique slip thrusts or inverted normal faults however recent analogical and numerical models have emphasised the influence of surface mass transfer phenomena on the structural evolution of compressive systems this research points to sedimentation and erosion during deformation as an additional explanation for the origin of steeply dipping thrusts the present study uses both field observations and analogue modelling to attempt to isolate critical parameters of syntectonic sedimentation that might control the geometry of the upper part of thrust systems a field study of thrust systems bounding two compressive intermountain tertiary basins of the iberian chain is first briefly presented we describe the surface geometry of thrusts surrounding the montalban basin and the alto tajo syncline at the vicinity of deposits of oligocene early miocene alluvial fans at the footwall of faults field observations suggest that synthrusting sedimentation should influence the structure of thrusts indeed the faults are steeper and splitted at the edge of the syntectonic deposits effects of sedimentation rate on footwall of thrusts and of its change along fault strike are further investigated on two layer brittle ductile analogue models submitted to compression and syntectonic sediment supply two series of experiments were made corresponding to two end members of depositional geometries in the first series the sedimentation was homogeneously distributed on both sides of the relief developed above the thrust front in the second series deposits were localised on a particular area of the footwall of thrust front in all experiments the sedimentation rate controls the number and the dip of faults for low sedimentation rates a single low angle thrust develops whereas for high sedimentation rates a series of steeper dipping thrust is observed in experiments with changing sedimentation rate along fault strike the thrust geometry varies behind the areas with the thickest sediment pile

slip along the san andreas fault associated with the great 1857 earthquake historical records indicate that several meters of lateral slip along the san andreas fault accompanied the great 1857 earthquake in central and southern california these records together with dendrochronological evidence suggest that the rupture occurred along 360 to 400 km of the fault including several tens of kilometers of the currently creeping reach in central california geomorphic expressions of late holocene right lateral offsets are abundant along the 1857 rupture along 300 kilometers of the 1857 rupture between cholame and wrightwood the youngest discernible offset ranges from 3 to 9 1 2 meters dormancy of the fault since 1857 almost certainly indicates that this latest offset was created in 1857 fault slip apparently associated with the 1857 earthquake varies in a broadly systematic way along the trace of the fault it is relatively uniform along each of several long segments but changes rather abruptly in value between these segments this nonuniform displacement pattern may imply that some segments of the fault rupture more frequently or experience a slower long term slip rate than others the 1857 offsets indicate a seismic moment m o between 5 3 and 8 7 10 27 dyne cm assuming a 10 to 15 km depth of rupture and relatively uniform slip as a function of depth a comparison with the rupture length average slip value and tectonic setting of the california earthquake of 1906 m s 8 1 4 indicates a value of m 8 1 4 for the 1857 event

structural evolution of a three dimensional finite width crustal wedge abstract we present the results of three dimensional numerical experiments designed to study the response of a layer of crustal material subjected to convergence through an imposed basal velocity discontinuity and to surface processes erosion sedimentation we focus on the three dimensional response of the system arising from the finite width of the imposed velocity discontinuity in particular we describe the complex structures that develop around the wedge and their interactions with the loading unloading produced by the surface processes we show that the pro and retro shear zones that develop in a doubly vergent two dimensional wedge curve around the end of the velocity line discontinuity to merge into the strike slip structure that naturally develop i e as a consequence of the imposed boundary conditions along the edge of the wedge along the retro shear zone the stress orientation rotates along a vertical axis which implies that the retro shear zone is a pure thrust along all of its curved length whereas along the pro shear zone stresses rotate along a horizontal axis which in turn implies that the pro shear zone progressively evolves towards an oblique thrust in its curved section furthermore the outward motion i e perpendicular to the direction of imposed shortening along the curved section of the retro shear zone is accommodated by oblique extension along a secondary kinked structure antithetic to the retro shear zone we also show the complex evolution of the wedge when ductile flow and ductile strain softening is activated by increasing the imposed basal temperature in this situation the wedge is broader as structures develop at finite distances on either side of the line discontinuity and its dynamics resembles more that of a vise like orogen at the surface a flat plateau forms that accumulates sediment from the surrounding actively deforming mountain ranges until a channel breaks through one of the sides and flushes the inward draining basin of its sedimentary content the complex numerical models provide a detailed insight into the geometry and complex three dimensional structures within and at the margins of an orogen and how they can be affected by erosional processes at the surface

fold interaction and wavelength selection in 3d models of multilayer detachment folding abstract many fold and thrust belts are dominated by folding and exhibit a fairly regular fold spacing yet in map view the aspect ratio of doubly plunging anticlines varies considerably from very elongated and sometimes slightly curved cylindrical folds to nearly circular dome like structures in addition the fold spacing often varies significantly around an average value so far it remains unclear whether these features are consistent with a folding instability therefore we here study the dynamics of multilayer detachment folding process by which shortening can be accommodated in thin skinned fold and thrust belts we start by analysing the physics of this process by using both a semi analytical thick plate theory and numerical simulations results show that several different folding modes occur about half of which are affected by gravity and have a wavelength that depends on the background deformation rate non dimensional expressions are derived that predict the dominant wavelength and growth rate of each of these folding modes and mechanical phase diagrams are presented that illustrate the applicability of each of the modes next we perform 3d simulations and compare the results with those of 2d models and analytical theory both 2d and 3d numerical simulations have wavelengths that are in good agreement with the analytical predictions in the high resolution 3d simulations the lateral growth of folds is studied in particular with respect to fold segment interactions and evolution of fold width length aspect ratio the numerical simulations show a number of similarities with the fars region of the zagros fold and thrust belt including a large range of fold aspect ratio and a normally distributed fold wavelength around a dominant one

a test of foreshock occurrence in the central aleutian island arc abstract a statistical method for estimating the occurrence of foreshocks to larger earthquakes using local seismicity data is applied to the catalog of earthquakes from a local seismograph network near adak alaska foreshocks are defined in a restricted sense as earthquakes preceding larger earthquakes within a critical time calculated from the mean time between successive earthquakes in the volume surrounding the target event the threshold of completeness of the earthquake catalog is estimated as duration magnitude m d 2 2 a low magnitude cutoff on the catalog of m d 2 5 was used to minimize potential bias from variations in weather and instrumentation condition about 6 to 10 per cent of 51 main shocks of m b 4 5 in the adak thrust zone were preceded by detectable earthquakes satisfying this statistical definition for foreshocks as many as 14 percent of m b 5 0 earthquakes may have been preceded by detectable earthquakes satisfying this definition an estimate of foreshock occurrence using as background the seismicity rate in the 4 months preceding larger earthquakes indicates a slightly higher rate of occurrence the volume around the target events which showed foreshock activity is used to infer the size of a zone of preparation for larger earthquakes in the adak area the data are consistent with a zone of preparation whose size is a linear function of target event magnitude the size of the zone of preparation ranges from 20 to 45 km in radius for body wave magnitudes 4 0 to 5 7

an experiment in earthquake prediction and the 7 may 1986 andreanof islands earthquake abstract the occurrence of the 7 may 1986 andreanof islands earthquake provides the opportunity to evaluate the earthquake forecast and specific prediction that had been formulated and promulgated starting more than 2 yr before the prediction experiment was based on the observation of seismic quiescence in the part of the aleutian subduction zone monitored by the central aleutians adak seismic network the magnitude estimate m s 7 to 7 5 was based on the size of the quiescent zone and the seismic history of the adak region since 1900 the time of occurrence was predicted as a test of the discovery by habermann of a quiescence lasting about 3 yr prior to the 2 may 1971 adak canyon event and the hypothesis that a characteristic precursor time is a valid concept the place was originally forecast as near adak island then specified with epicenter at the eastern margin of adak canyon rupturing to the west under the canyon the 7 may 1986 earthquake had magnitude m s 7 7 m w 8 0 with epicenter 140 km east of the predicted place rupturing east of adak canyon and stopping in the area predicted as the nucleation point the time of this event was 6 months later than predicted thus the specifically predicted event has not happened the largest part of the rupture did go through the segment that had been most active before the onset of quiescence and which experienced the sharpest and most persistent quiescence a cause and effect relation between preparation for this earthquake and the quiescence is still to be firmly established but the distribution of the pattern of quiescence relative to the epicentral location and the site of the greatest moment release suggest a physical connection between earthquake and seismicity patterns the predictions of location and time were acknowledged throughout as highly speculative no firm basis exists for quantitative estimation of the uncertainties to be assigned to the elements of an intermediate term prediction this experiment lends support to the validity of quiescence as a precursor at least for some earthquakes and demonstrates the need for local network data to provide the high resolution of seismicity required for prediction efforts based on seismicity patterns

three dimensional folding of an embedded viscous layer in pure shear a thick plate analysis is carried out to first order in interface slope for three dimensional folding of a single linear viscous layer of thickness h and viscosity embedded in a uniform viscous medium with viscosity 1 layer and medium are subject to a basic state of homogeneous pure shear with principal directions of strain rate x and y lying in the plane of the layer the solution is explicitly carried out for a fold perturbation in interface shape that is symmetric with respect to the principal strain rate axes f cos lx cos my but it is shown to apply to an arbitrary perturbation in the x y plane the growth rate of the perturbation is found to be are the principal strain rates of the basic state the wavenumber of the most rapidly growing perturbation d is the same as that obtained for a cylindrical perturbation m 0 in a basic state of plane strain yy 0 for maximum rate of shortening parallel to x the cylindrical fold form with axis normal to x m l 0 grows most rapidly for any ratio of yy xx 1 if yy xx all fold forms grow at the same rate and in particular an egg carton fold form is not preterentially amplified the velocity field for the perturbing flow consists of a poloidal field which solely determines the growth of the fold form and a toroidal field with non zero component of vorticity about the axis normal to the layer the latter is required to satisfy both of the two independent shear traction continuity conditions at interfaces there is no coupling between the two fields three dimensional fold forms in the appalachian plateau province in western pennsylvania are described

escarpment evolution on high elevation rifted margins insights derived from a surface processes model that combines diffusion advection and reaction experiments with a surface processes model of large scale 1 1000 km long term 1 100 m y erosional denudation are used to establish the controls on the evolution of a model escarpment that is related to the rifting of a continent the model describes changes in topographic form as a result of simultaneous short and long range mass transport representing hillslope diffusive processes and fluvial transport advection respectively fluvial entrainment is modeled as a first order kinetic reaction which reflects the credibility of the substrate and therefore the fluvial system is not necessarily carrying at capacity one dimensional and planform models demonstrate that the principal controls on the evolution of an initially steep model escarpment are 1 antecedent topography drainage 2 the timescale or equivalently a length scale in the fluvial entrainment reaction 3 the flexural response of the lithosphere to denudation and 4 the relative efficiencies of the short and long range transport processes when rainfall and substrate lithology are uniform a significant amount of discharge draining over the escarpment top causes it to degrade only when the top of the model escarpment coincides with a drainage divide can escarpment retreat occur for these conditions an additional requirement for retreat of a model escarpment without decline is a long reaction time scale for fluvial entrainment this corresponds to a substrate that is hard to detach by fluvial erosion and therefore to fluvial erosion that is not transport limited continuous backtilting of an escarpment due to flexural isostatic uplift in response to denudational unloading helps maintain the scarp top as a divide it is essential if the escarpment gradient is to be preserved during retreat in a uniform lithology low flexural rigidities promote steep and slowly retreating escarpments for given rainfall and substrate conditions the morphology of a retreating model escarpment is determined by the ratio of the short range diffusive and long range advective transport efficiencies a low ratio which is interpreted to correspond to a relatively arid climate and weathering limited conditions promotes steep sharp topped escarpments with straight main slopes and escarpment retreat occurs over a wide range of height scales a high ratio interpreted to correspond to a more humid temperate climate produces a convex upper slope and concave lower slope morphology and only major escarpments are predicted to preserve a high scarp gradient lithological contrasts in the model produce more complex morphologies and predict the formation of scarps crowned by an erosionally resistant caprock however resistant caprocks are not an essential requirement for model scarps to retreat we conclude that the inferred controls and model behavior are both consistent with the present day morphology of rifted continental margins and with modern conceptual models of landscape evolution

seismicity associated with a cluster of earthquakes of mb 4 5 near adak alaska evidence for an asperity abstract an area of the main thrust zone south of adak alaska in the central aleutian island arc is characterized by clustering of hypocenters of earthquakes of m b 4 5 and by low b value relative to the rest of the thrust zone the location and dimension of two of these clusters has been constant during 15 yr of teleseismic and 9 yr of local monitoring six earthquakes of m b 4 5 have occurred in one of the cluster areas in the past 3 yr in contrast to two earthquakes of m b 4 1 in the previous 6 yr the concentration of moderate sized earthquakes and low b value may reflect a stress concentration at an asperity on the interface between the pacific and north american plates the recent activation of this asperity may indicate preparation for a larger earthquake a change of local sv p z amplitude ratios of small earthquakes before two earthquakes of m b 4 5 may indicate a change of focal mechanism reflecting a reorientation of stress around the asperity prior to larger earthquakes

interplay between tectonics climate and fluvial transport during the cenozoic evolution of the ebro basin ne iberia 1 three dimensional modeling that integrates fluvial sediment transport crustal scale tectonic deformation and lithospheric flexural subsidence is carried out to simulate the landscape and drainage evolution of the ebro sedimentary basin ne iberia the ebro basin underwent a long period of closed intramountain drainage as a result of tectonic topography generation at the pyrenees the iberian range and the catalan coastal range in the late oligocene the catalan coastal range underwent extension leading to the formation of the valencia trough nw mediterranean but the ebro basin remained closed for nearly 15 myr more before the ebro river cut through the remnants of the topographic barrier this drainage opening caused widespread basin incision that shaped spectacular outcrops of the syntectonic and posttectonic infill here we investigate the processes controlling these major drainage changes the modeling results constrained by a large data set on the tectonic and transport evolution of the area predict a closed phase characterized by a large lake in the central eastern ebro basin dry climatic conditions probably lowered the lake level and contributed together with rift flank uplift to prolong this endorheic basin stage the age and amount of reworked sediment after the opening are consistent with an onset of basin incision between 13 and 8 5 ma as a result of lake capture by escarpment erosion and lake level rise associated with sediment accumulation and wetter climatic conditions sea level changes in the mediterranean had no major impact in the large scale drainage evolution of the ebro basin index terms 1625 global change geomorphology and weathering 1824 1886 1815 hydrology erosion and sedimentation 3210 mathematical geophysics modeling 3344 meteorology and atmospheric dynamics paleoclimatology 8102 tectonophysics continental contractional orogenic belts keywords pyrenees drainage evolution lake sediment budget erosion flexure

a comparison of seismicity rates near adak island alaska september 1988 through may 1990 with rates before the 1982 to 1986 apparent quiescence abstract the rates of seismicity in the part of the aleutian arc monitored by the central aleutians seismic network and in subregions of that zone during september 1988 through june 1990 have been compared with the rates during 1977 through 1981 1981 through 1982 and 1983 through 1985 the purpose of the study was to determine whether the rate of earthquake occurrence had returned to the level observed before 1982 when a prolonged decrease in activity began this decrease had been interpreted as seismic quiescence and was used as the basis of a prediction of a strong earthquake in this area an m w 8 0 earthquake did occur on 7 may 1986 the comparison with more recent activity is intended as one test of whether the observed decrease in activity was a real physical phenomenon or had been artificially produced by changes in network instrumentation or data processing procedures the only change in instrumentation since 1986 was the installation of a new digitizer and tests were made to confirm that this new unit and the installation of new software for picking arrival times did not alter the completeness of detections or the magnitude values the test cannot be completely definitive because aftershocks of the 1986 event were still in progress according to the modified omori fit to the data the interval used is the latest possible because the network has been terminated the 1988 to 1990 rates are similar to those in the prequiescence interval but higher by an amount that is in qualitative agreement with the expected rate of continuing aftershocks the rates in most of the zone are significantly higher than the rates during the period of decreased activity rates in the sites near the 1986 mainshock epicenter and the western termination of the 1986 rupture are higher than the pre 1982 rates and indicate a concentration of the late aftershocks in these places the results of the test support the conclusion that the 1982 to 1985 decrease in cataloged activity represents a real drop in the rate of seismicity prior to the 1986 earthquake examination of many more cases of seismicity changes before and following a strong earthquake is needed as the basis for judging the validity of quiescence as a true precursor another observation from recent seismicity is that an intense swarm in january 1989 and a magnitude m b 6 1 event in march 1990 with a well developed aftershock sequence have filled a spatial gap in the aftershocks of the 1986 event near its epicenter

role of river incision in enhancing deformation many active compressional belts contain examples of transverse rivers intersecting anticlines at their highest structural and topographic position this unusual association between rivers and doubly plunging anticlines at relatively small scales is usually explained by some form of antecedence or superposition in this article i suggest an alternative explanation i e that river erosion unloads the crust and that this process causes local deformation to be enhanced in the vicinity of rivers i show the plausibility of this hypothesis with the aid of a coupled three dimensional mechanical surface process mathematical model the conclusion is that river incision can have a major influence on deformation of the surrounding crust if incision occurs at the same time that the crust is deforming plastically in response to regional compression in this case incision amplifies background deformation at a relatively small scale leading to the formation of noncylindrical folds with culminations coinciding with river incision in the absence of regional deformation the response of the crust to river incision is small and of long wavelength because it is governed by flexural isostasy of relatively rigid elastic crust thus whether rivers exert an influence on local deformation depends critically on the timing between river incision and regional deformation this point may explain why in some cases rivers appear to have had no significant influence on local deformation whereas in other cases they have