a self regulating model of bedrock river channel geometry 1 the evolution of many mountain landscapes is controlled by the incision of bedrock river channels while the rate of incision is set by channel shape through its mediation of flow the channel shape is itself set by the history of bedrock erosion this feedback between channel geometry and incision determines the speed of landscape response to tectonic or climatic forcing here a model for the dynamics of bedrock channel shape is derived from geometric arguments a normal flow approximation for channel flow and a threshold bed shear stress assumption for bedrock abrasion the model dynamics describe the competing effects of channel widening tilting bending and variable flow depth transient solutions suggest that channels may take 1 10 ky to adapt to changes in discharge implying that channel disequilibrium is commonplace if so landscape evolution models will need to include bedrock channel dynamics if they are to probe the effects of climate change

stabilization of a gravel channel by large streamside obstructions and bedrock bends jacoby creek northwestern california jacoby creek bed width 12 m bankfull discharge 32 6 m 3 s contains stationary gravel bars that have forms and positions controlled by numerous large streamside obstructions bedrock outcrops large woody debris and rooted bank projections and bedrock bends bank projection width and bar volume measured in 104 channel segments 1 bed width long are significantly cross correlated at lags of 1 3 and 4 indicating the tendency for large obstructions and bends to form bars 3 to 4 bed widths downstream and 1 bed width upstream all of the 18 bars downstream of large obstructions or bends in the study reach were along the obstruction side of the channel or outside bank of the bend most of the pools 85 were next to large obstructions or in bends conversely 92 of large obstructions or bends had pools comparison of the volume of four bars with volumetric bar changes and volume of bedload transported during four high flow events suggests that rates of sediment transport were sufficient to cause major changes in bars during bankfull events the only important channel changes observed in 4 yr however have been associated with the movement of large woody debris and with changes in the angle at which the flow approaches an obstruction a general model is proposed that large obstructions and nonalluvial bends stabilize the form and location of gravel bars bars are stabilized by two related mechanisms 1 large obstructions and bends cause intense quasi steady secondary circulation in scour holes that terminate upstream bars at fixed locations obstruction width channel deflection scour hole width and bed width were measured at 26 obstructions these data show that obstructions wider than approximately one third of the bed form pools spanning the entire channel and thus terminating bars smaller obstructions form scour holes contained within a single bar 2 bars are deposited upstream of large obstructions and sharp bends because of backwater reductions in stream power bars are deposited downstream because flow energy is expended around obstructions and bends and because the flow expands downstream of constrictions that result from large obstructions the formation of bars and pools inherent in many gravel channels can thus be enhanced and fixed in position by flow structures set up around large obstructions and bends formed of resistant materials

hydraulic geometry river sediment and the definition of bedrock channels a growing literature deals with bedrock channels erosion processes within them and their role in landscape evolution formal definitions currently in use classify channels as bedrock when alluvial cover is discontinuous or thin this is equated with a physical property of the flow using up to date erosion laws and descriptions of channel dynamics we show that existing definitions are difficult to apply and may lead to conflicting classifications we propose the definition a bedrock channel cannot substantially widen lower or shift its bed without eroding bedrock applied to channels in taiwan this leads to a different classification than previous definitions by analysing at a station hydraulic geometry of these channels we show that the classification exposes different controls of sediment effects on dynamics of channels classified as alluvial and bedrock

physically based modeling of bedrock incision by abrasion plucking and macroabrasion 1 many important insights into the dynamic coupling among climate erosion and tectonics in mountain areas have derived from several numerical models of the past few decades which include descriptions of bedrock incision however many questions regarding incision processes and morphology of bedrock streams still remain unanswered a more mechanistically based incision model is needed as a component to study landscape evolution major bedrock incision processes include among other mechanisms abrasion by bed load plucking and macroabrasion a process of fracturing of the bedrock into pluckable sizes mediated by particle impacts the purpose of this paper is to develop a physically based model of bedrock incision that includes all three processes mentioned above to build the model we start by developing a theory of abrasion plucking and macroabrasion mechanisms we then incorporate hydrology the evaluation of boundary shear stress capacity transport an entrainment relation for pluckable particles a routing model linking in stream sediment and hillslopes a formulation for alluvial channel coverage a channel width relation hack s law and exner equation into the model so that we can simulate the evolution of bedrock channels the model successfully simulates various features of bed elevation profiles of natural bedrock rivers under a variety of input or boundary conditions the results also illustrate that knickpoints found in bedrock rivers may be autogenic in addition to being driven by base level fall and lithologic changes this supports the concept that bedrock incision by knickpoint migration may be an integral part of normal incision processes the model is expected to improve the current understanding of the linkage among physically meaningful input parameters the physics of incision process and morphological changes in bedrock streams

interaction among alluvial cover bed roughness and incision rate in purely bedrock and alluvial bedrock channel a major control on bedrock incision is the interaction between alluvial cover and erosive mobile grains the extent of alluvial cover is typically predicted as a function of relative sediment flux sediment supply rate over bed load transport capacity qbs qbc yet little is known about how the bed roughness affects the alluvial cover we performed field experiments with various flow discharges sediment supply rates grain sizes and bed surface topographies we then developed physically based models for estimating the threshold of sediment movement and the extent of alluvial cover so as to include the effect of roughness change the results for the threshold of sediment movement and the extent of alluvial cover obtained from our models show reasonable agreement with the results of the field experiments we explored the sensitivity of the models to variations in sediment supply and bedrock relative roughness bedrock hydraulic roughness height over grain size ksb d the results suggest the following 1 a larger relative roughness yields a greater dimensionless critical shear stress required for initial sediment motion 2 at a given sediment supply rate the extent of alluvial cover is larger when the relative roughness is larger 3 when the sediment supply rate and the relative roughness are small throughput bed load moves over and can abrade a purely bedrock channel with no alluvial cover and 4 the critical value of sediment supply rate below which throughput bed load transport occurs increases with decreasing relative roughness the experimental results and analysis provide a framework for treating the a incisional morphodynamics of purely bedrock rivers by throughput bed load with no alluvial cover b incisional alluvial morphodynamics of mixed bedrock alluvial rivers and c purely alluvial morphodynamics as well as the transition between these states

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

aftershocks of the andreanof islands earthquake of june 10 1996 and local seismotectonics three questions have been addressed in connection with an mw 7 7 earthquake that occurred in the andreanof islands alaska on june 10 1996 first was there any variation in seismic activity rate during the previous five years that might have been diagnostic of the imminent occurrence of this event the answer based on the usgs pde catalog is no second though there were abundant pre event earthquakes and aftershocks on both sides of adak canyon why was there very little activity under the canyon itself a property that has been observed previously it is not possible to resolve the cause of the behavior with existing seismological data finally how does one explain the eastern group of aftershocks which fell within the zone of abundant aftershocks of the 1986 mw 8 andreanof islands earthquake when very little of the 1996 mainshock moment was released under or to the east of adak canyon one explanation of these aftershocks is that the recent aftershocks are late aftershocks of 1986 whose occurrence within a short time was induced by the stress pulse but not seismic slip from the 1996 earthquake though large earthquakes in the aleutians occur in distinct tectonic blocks the boundaries of these blocks may not always be defined reliably by aftershock zones

distribution of erosion across bedrock channels lateral erosion in bedrock rivers is an important control on the shape of channel crosssections and the coupling of channels and hillslopes recent observations link lateral erosionto the variability of flow we propose two mechanisms to explain this one is based on changing shear stress distributions within the channel with varying flood level the other on the competition between cover and tool effects in fluvial bedrock erosion we assess these processes for the liwu river taiwan and conclude that cover and tool effects dominate the partitioning of lateral and vertical erosion in this case

basin centered asperities in great subduction zone earthquakes a link between slip subsidence and subduction erosion 1 published areas of high coseismic slip or asperities for 29 of the largest circum pacific megathrust earthquakes are compared to forearc structure revealed by satellite free air gravity bathymetry and seismic profiling on average 71 of an earthquake s seismic moment and 79 of its asperity area occur beneath the prominent gravity low outlining the deep sea terrace 57 of an earthquake s asperity area on average occurs beneath the forearc basins that lie within the deep sea terrace in sw japan slip in the 1923 1944 1946 and 1968 earthquakes was largely centered beneath five forearc basins whose landward edge overlies the 350 c isotherm on the plate boundary the inferred downdip limit of the locked zone basin centered coseismic slip also occurred along the aleutian mexico peru and chile subduction zones but was ambiguous for the great 1964 alaska earthquake beneath intrabasin structural highs seismic slip tends to be lower possibly due to higher temperatures and fluid pressures kilometers of late cenozoic subsidence and crustal thinning above some of the source zones are indicated by seismic profiling and drilling and are thought to be caused by basal subduction erosion the deep sea terraces and basins may evolve not just by growth of the outer arc high but also by interseismic subsidence not recovered during earthquakes basin centered asperities could indicate a link between subsidence subduction erosion and seismogenesis whatever the cause forearc basins may be useful indicators of long term seismic moment release the source zone for cascadia s 1700 a d earthquake contains five large basin centered gravity lows that may indicate potential asperities at depth the gravity gradient marking the inferred downdip limit to large coseismic slip lies offshore except in northwestern washington where the low extends landward beneath the coast transverse gravity highs between the basins suggest that the margin is seismically segmented and could produce a variety of large earthquakes

ubiquitous weakening of faults due to thermal pressurization faults weaken during earthquakes analysis of the amount of energy released during earthquakes globally suggests that heat induced pressurization of pore fluids can weaken faults during earthquakes of all sizes laboratory simulations of earthquakes show that at high slip rates faults can weaken significantly aiding rupture1 2 3 various mechanisms such as thermal pressurization and flash heating have been proposed to cause this weakening during laboratory experiments1 4 5 6 yet the processes that aid fault slip in nature remain unknown measurements of seismic radiation during an earthquake can be used to estimate the frictional work associated with fault weakening known as an event s fracture energy7 8 9 here we compile new and existing8 9 measurements of fracture energy for earthquakes globally that vary in size from borehole microseismicity to great earthquakes we observe a distinct transition in how fracture energy scales with event size which implies that faults weaken differently during small and large earthquakes and earthquakes are not self similar we use an elastodynamic numerical model of earthquake rupture to explore possible mechanisms we find that thermal pressurization of pore fluid by the rapid shear heating of fault gouge can account for the observed scaling of fracture energy in small and large earthquakes over seven orders of fault slip magnitude we conclude that thermal pressurization is a widespread and prominent process for fault weakening

observational constraints on the fracture energy of subduction zone earthquakes we relate seismologically observable parameters such as radiated energy seismic moment rupture area and rupture speed to the dynamics of faulting to achieve this objective we computed the radiated energy for 23 subduction zone earthquakes recorded between 1992 and 2001 most of these earthquakes have a magnitude m w 7 5 but we also included some smaller m w 6 7 well studied subduction zone earthquakes and six crustal earthquakes we compiled static stress drop estimates for these 29 earthquakes from literature and used a slip weakening model to determine the radiation efficiency of these earthquakes we also determined the rupture speed of these earthquakes from literature from fracture mechanics fracture energy and hence radiation efficiency can be related to the rupture speed the radiation efficiencies estimated from the partitioning of energy as given by the slip weakening model are consistent with the rupture speed estimated for these earthquakes most earthquakes have radiation efficiencies between 0 25 and 1 and are hence efficient in generating seismic waves but tsunami earthquakes and two deep earthquakes the 1994 bolivia and the 1999 russia china border earthquakes have very small radiation efficiencies 0 25 and hence dissipate a large amount of energy during faulting we suggest that differences in the radiation efficiencies of different types of earthquakes could be due to fundamental differences in their rupture mechanics in deep events the energy is probably dissipated in thermal processes in the fault zone while it is possible that the morphology of the trench causes large energy dissipation during the rupture process of tsunami earthquakes

sources of tsunami and tsunamigenic earthquakes in subduction zones we classified tsunamigenic earthquakes in subduction zones into three types earth quakes at the plate interface typical interplate events earthquakes at the outer rise within the subducting slab or overlying crust intraplate events and tsunami earthquakes that generate considerably larger tsunamis than expected from seismic waves the depth range of a typical interplate earthquake source is 10 40km controlled by temperature and other geological parameters the slip distribution varies both with depth and along strike recent examples show very different temporal change of slip distribution in the aleutians and the japan trench the tsunamigenic coseismic slip of the 1957 aleutian earthquake was concentrated on an asperity located in the western half of an aftershock zone 1200km long this asperity ruptured again in the 1986 andreanof islands and 1996 delarof islands earthquakes by contrast the source of the 1994 sanriku oki earthquake corresponds to the low slip region of the previous interplate event the 1968 tokachi oki earthquake tsunamis from intraplate earthquakes within the subducting slab can be at least as large as those from interplate earthquakes tsunami hazard assessments must include such events similarity in macroseismic data from two southern kuril earthquakes illustrates difficulty in distinguishing interplate and slab events on the basis of historical data such as felt reports and tsunami heights most moment release of tsunami earthquakes occurs in a narrow region near the trench and the concentrated slip is responsible for the large tsunami numerical modeling of the 1996 peru earthquake confirms this model which has been proposed for other tsunami earthquakes including 1896 sanriku 1946 aleutian and 1992 nicaragua

the effect of spatial heterogeneities on geomorphic expression in a model of basin evolution spatial variability is a basic component of all natural systems the system being studied here is the topography of an evolving drainage basin spatially heterogeneous erosivity within the drainage basin influences the structure of the drainage network the roughness of small scale features and the overall appearance of the simulated topography the presence of heterogeneities does not simply blur and smear analytical relationships the heterogeneities themselves introduce new structure to the organization of the drainage basin three measures the slope area relationship the cumulative area relationship and the hypsometric distribution are all systematically affected by spatial heterogeneity we will demonstrate how and why these relationships are affected and also how different structures in the heterogeneities themselves are manifested in the resulting simulated topography

reduction of long term bedrock incision efficiency by short term alluvial cover intermittency fluctuations of the sediment volume stored in mountain channels are driven by stochastic variations of discharge and sediment supply and can inhibit bedrock incision if sediment thickness is too large here i study how this short term stochasticity propagates into the long term reduction of bedrock incision efficiency the cover effect at geological time scales i introduce a new numerical model that resolves sediment transport and bedrock incision at daily time scales and is run for thousands of years it incorporates 1 a transport threshold and daily stochastic variations in water discharge and sediment supply 2 a freely evolving channel width and slope and 3 an explicit treatment of alluvial thickness variations and corresponding bed incision reduction for typical mountain river conditions the model predicts that alluvial cover oscillates between complete and negligible incision reduction in this intermittent regime the long term cover effect is mainly set by the fraction of time spent in full cover and the present day extent of alluvial cover is not representative of long term dynamics the long term integrated cover effect law differs strongly from proposed theoretical and experimental models and it is controlled by sediment supply stochasticity rather than the details of cover development at the hydraulic time scale model results also suggest that steady state channel configuration always depends on sediment supply rate while being never limited by transport capacity or strictly detachment limited these results point out that discharge and sediment supply stochasticity should not be considered less important than the intricate details of incision laws to model long term bedrock channel dynamics

numerical simulations of bedrock valley evolution by meandering rivers with variable bank material bedrock river valleys are fundamental components of many landscapes and their morphologies from slot canyons with incised meanders to wide valleys with strath terraces may record environmental history several formation mechanisms for particular valley types have been proposed that involve changes in climatic and tectonic forcing but the uniqueness of valley evolution pathways and the long term stability of valley morphology under constant forcing are unknown and are not predicted in existing numerical models for vertically incising rivers because rivers often migrate more rapidly through alluvium than through bedrock we explore the hypothesis that the distribution of bank materials strongly influences river meandering kinematics and can explain the diversity of bedrock river valley morphology simulations using a numerical model of river meandering with vector based bank material tracking indicate that channel lateral erosion rate in sediment and bedrock vertical erosion rate and initial alluvial belt width explain first order differences in bedrock valley type that bedrock bound channels can evolve under steady forcing from alluvial states and that weak bedrock and low vertical incision rates favor wide shallow valleys while resistant bedrock and high vertical incision rates favor narrow deep valleys during vertical incision sustained planation of the valley floor is favored when bedrock boundaries restrict channel migration to a zone of thin sediment fill the inherent unsteadiness of river meandering in space and time is enhanced by evolving spatial contrasts in bank strength between sediment and bedrock and can account for several valley features including strath terraces and underfit valleys commonly ascribed to external drivers

flow in bedrock canyons a survey along the fraser canyon in canada reveals complex flow dynamics involving velocity inversions and upwelling which suggests ways to improve flow and bedrock incision modelling canyons carved into the bedrock in areas of actively uplifting terrain are important for our understanding of the link between tectonics climate and topography fluid flow and sediment transport are processes associated with canyon bedrock incision in models but to date there are no field observations of the flow structure in bedrock canyons that can be used to test the model components that represent fluid flow this paper describes a survey along the fraser canyon in canada that provides detailed measurements of flow in a narrow bedrock canyon the data reveal a more complex flow structure than currently assumed in flow models implying that a revision in our understanding of bedrock canyon incision is required bedrock erosion in rivers sets the pace of landscape evolution influences the evolution of orogens and determines the size shape and relief of mountains1 2 a variety of models link fluid flow and sediment transport processes to bedrock incision in canyons the model components that represent sediment transport processes are increasingly well developed3 4 5 in contrast the model components being used to represent fluid flow are largely untested because there are no observations of the flow structure in bedrock canyons here we present a 524 kilometre continuous centreline acoustic doppler current profiler survey of the fraser canyon in western canada which includes 42 individual bedrock canyons our observations of three dimensional flow structure reveal that as water enters the canyons a high velocity core follows the bed surface causing a velocity inversion high velocities near the bed and low velocities at the surface the plunging water then upwells along the canyon walls resulting in counter rotating along stream coherent flow structures that diverge near the bed the resulting flow structure promotes deep scour in the bedrock channel floor and undercutting of the canyon walls this provides a mechanism for channel widening and ensures that the base of the walls is swept clear of the debris that is often deposited there keeping the walls nearly vertical these observations reveal that the flow structure in bedrock canyons is more complex than assumed in the models presently used fluid flow models that capture the essence of the three dimensional flow field using simple phenomenological rules that are computationally tractable are required to capture the dynamic coupling between flow bedrock erosion and solid earth dynamics

quantitative testing of model of bedrock channel incision by plucking and macroabrasion river incision can play a role in societally relevant or human timescale which requires careful engineering planning and design physically based incision models can help us perform the engineering computations to prevent erosion damage to engineering structures such as bridges and channel bank protection works this paper presents a field application of our previously published work in which we developed a mechanistically based model of bedrock incision by plucking and macroabrasion a process of fracturing of the bedrock into pluckable sizes mediated by particle impacts from bedload the plucking macroabrasion incision model is tested quantitatively with unnamed drainage 1 in southern indiana which showed obvious channel incision over the past 19 years destroying a highway bridge foundation the field data of incision rate hydraulic conditions rainfall grain size characteristics and morphological conditions were collected in the field or obtained from previous studies and the national oceanic

multiple mantle metasomatism in central southern italy geochemical effects timing and geodynamic implications the regional petrological and geochemical variations observed in the recent magmatism along the italian peninsula are interpreted to testify to the coexistence of distinct sectors of upper mantle these are suggested to result from at least three compositionally and temporally distinct metasomatic events that affected petrologically different premetasomatic mantle sources geological and geochemical evidence suggests that metasomatism in the northern sector occurred during the alpine subduction by introduction of a composition similar to the deeply subducted dora maira metagranites into a residual lithospheric mantle of upper crustal material metasomatic events beneath the roman and neapolitan areas are younger these are related to the addition of melts and fluids into a fertile asthenospheric mantle during the latest stage of the west directed apennine subduction of the adria plate and during the current northwest subduction of the ionian sea floor

the climatic signature of incised river meanders climate controls landscape evolution but quantitative signatures of climatic drivers have yet to be found in topography on a broad scale here we describe how a topographic signature of typhoon rainfall is recorded in the meandering of incising mountain rivers in the western north pacific spatially averaged river sinuosity generated from digital elevation data peaks in the typhoon dominated subtropics where extreme rainfall and flood events are common and decreases toward the equatorial tropics and mid latitudes where such extremes are rare once climatic trends are removed the primary control on sinuosity is rock weakness our results indicate that the weakness of bedrock channel walls and their weakening by heavy rainfall together modulate rates of meander propagation and sinuosity development in incising rivers

a theoretical framework for the morphodynamics of bedrock channels 1 bedrock rivers are characterized by conditions of insufficient sediment supply wherein the amount of sediment supplied to the channel is less than the channel s sediment transport capacity consequently bedrock channels generally exhibit discontinuous sediment cover which current approaches to the morphodynamics of alluvial rivers are unable to handle to fill this gap we present a theoretical framework in which local sediment transport rates are proportional to the areal concentration of sediment available for transport on the bed and the sediment continuity equation is reformulated to account for temporal changes in areal concentration of sediment on nonalluviated surfaces we then perform a linear stability analysis that shows that an initially uniform distribution of the areal concentration of sediment on the bed is unstable to small perturbations this suggests that the sediment cover in bedrock channels tends to concentrate into regions which may eventually be alluviated in general agreement with experimental observations

bedrock channel incision along piccaninny creek australia piccaninny creek in northwestern australia drains approximately 60 km 2 of high relief sandstone and conglomerate terrain the drainage basin lies within the seasonal tropics and is characterized by ephemeral high magnitude flows flood flows of 50 to 100 m 3 s 1 are probably primarily responsible for the spectacular erosional features along the bedrock channel segments of the creek these erosional features occur in two basic forms as parallel longitudinal grooves and as a downstream sequence that begins with shallow linear depressions parallel to flow and ends in a deep narrow inner channel the longitudinal grooves are assumed to result from longitudinal vortices while the erosional sequences which closely resemble von karman vortex streets are interpreted to result from turbulent vortices shed off irregularities of the channel bed the longitudinal grooves and the inner channel sequence alternate with each other and with gravel floored depositional reaches downstream along the channe

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

hot fingers in the mantle wedge new insights into magma genesis in subduction zones abstract quaternary volcanoes in the northeast japan arc can be grouped into 10 volcanic clusters striking transverse to the arc these have an average width of 50 km and are separated by parallel gaps 30 75 km wide this clustering of volcanic centres topographic profiles low velocity regions in the mantle wedge and local negative bouguer gravity anomalies along the japan sea side of the volcanic arc are closely correlated all these observations may be related to locally developed hot regions within the mantle wedge that have the form of inclined 50 km wide fingers each of the 10 fingers recognised extends from deep mantle 150 km below the back arc region towards the shallower mantle 50 km beneath the volcanic front quaternary volcanoes are built immediately above the hot mantle fingers the volcanic basement along the fingers has been uplifted by repeated injection of magmas into the crust accompanied by quaternary volcanic activity at the surface although volcanic activity is rare along the japan sea coast tomographic results show that hot low velocity mantle fingers exist within the mantle wedge the negative bouguer anomalies at the rear of the volcanic arc could be caused by magmas supplied from the hot mantle fingers these have not yet been erupted but have accumulated at the moho discontinuity

a surface roughness model for predicting alluvial cover and bed load transport rate in bedrock channels partial alluvial cover in bedrock channels influences downcutting rates and mountain river morphodynamics flume experiments have demonstrated that a wide range of stable cover fractions are possible for a given ratio of sediment supply to transport capacity existing cover models impose different unique relationships between cover fraction and reach scale transport variables e g slope discharge sediment supply and grain size individually these models cannot predict the range of cover behaviors observed experimentally suggesting that additional variables may be important i propose a 1 d model in which bedrock surface roughness is a control on partial alluvial cover in the model surface roughness affects both shear stresses and thresholds of grain motion and therefore sediment transport capacity the roughness of the combined bedrock alluvial bed surface varies with the cover fraction model trends are reasonably consistent with previous experiments over ranges of roughness slope and shear stress when bedrock roughness is much greater than grain size cover increases approximately linearly with the ratio of sediment supply to transport capacity when bedrock roughness is much less than the sediment diameter the model tends to predict abrupt shifts between complete bedrock exposure and alluvial cover this runaway alluviation occurs for model solutions that are unstable to perturbations in supply which is the case when transport capacity decreases as alluvial cover increases in addition a model variant in which sediment supply is controlled by the amount of sediment on the bed suggests that a linear cover relation may be sufficient for modeling sediment flux dependent erosion at landscape scales

noncharacteristic behavior and complex recurrence of large subduction zone earthquakes the last few years have been remarkable with respect to the number of large underthrusting earthquakes in subduction zones that reruptured plate boundary segments that failed in previous great events availability of modern seismic data for two consecutive large earthquakes rupturing the same portion of the plate interface provides the opportunity to compare the spatial distribution of moment release for both events such comparisons have been made for the plate boundary segments that failed in 1 the 1957 mw 8 6 1986 mw 8 0 and 1996 mw 7 9 aleutian islands earthquakes 2 the 1963 mw 8 5 and 1995 mw 7 9 kuril islands earthquakes 3 the 1971 mw 8 0 and 1995 mw 7 7 solomon islands earthquakes and 4 the 1968 mw 8 2 and 1994 mw 7 7 northern honshu earthquakes the spatial distribution of moment release for all four of the initial great earthquakes and two of the repeat events has been determined in previous studies here slip distributions for the three most recent events are determined from inversion of source time functions derived by empirical greens function analysis of long period surface waves and broadband body waves comparisons of the spatial distribution of moment release for sequential earthquake ruptures reveal considerable differences in the pattern of recurrent fault slip the 1994 northern honshu and 1995 solomon islands earthquakes primarily fill in areas of slip deficit left by their preceding events rather than rerupture identical asperities the 1995 kuril islands and the 1996 aleutian islands earthquakes both rerupture portions of an asperity distribution defined by preceding events but with variable amounts of slip this study provides the first direct evidence that recurrence of large circum pacific plate boundary events is more complex than repeat rupture of dominant asperities recurrent fault slip for the four plate boundary segments studied does not support characteristic slip models either where failure on an entire fault segment occurs repeatedly in events with nearly identical rupture lengths locations and slip magnitudes or where failure of individual asperities occurs with identical slip functions through consecutive earthquake cycles these sequential slip patterns are not consistent with physical models of earthquake rupture where slip complexity is exclusively controlled by invariant geometric and or material heterogeneity but suggest that dynamic considerations are also important

seismic cycles along the aleutian arc analysis of seismicity from 1957 through 1991 we catalog and relocate aleutian arc seismicity between 1957 and 1991 two great earthquakes ruptured the same 250 km long portion of the central aleutian arc the 1957 aleutian islands earthquake and the 1986 andreanof islands earthquake because accurate estimates of the moment distribution of the 1957 earthquake are not available the spatial distribution of aftershocks for each of these events is compared and tested against models describing the modes of occurrence of great subduction zone earthquakes earthquake relocations are based on p wave arrival times published in the international seismological summary the bureau central international seismologique and the international seismological centre bulletins and include corrections for the near source velocity structure associated with the down going slab magnitude estimates are extracted from bulletins and prior to 1964 are estimated by us from microfilmed records our catalog is complete above magnitude 5 5 aftershocks associated with the 1957 and 1986 earthquakes appear to occur in different areas east of the main shock epicenters aftershock locations are anticorrelated west of the main shock epicenter aftershocks of the 1986 earthquake tended to concentrate along the updip edge of aftershock clusters associated with the 1957 earthquake if we assume aftershocks rim the distribution of seismic moment release associated with each event these observations imply that the moment distribution of the 1986 earthquake was different from that of the 1957 earthquake this suggests that we should use caution in identifying mechanically strong portions of a fault asperities by simply mapping the moment distribution of a single great earthquake a fundamental tenet of the asperity model that rupture always occurs on the strongest portions of the fault with weaker portions rupturing either aseismically or dynamically as a result of rupture on a strong fault patch may in the case of the central aleutian arc not be correct thus observing the moment distribution from a single great earthquake may tell us little about what the distribution of moment release will look like during the next earthquake

linear and nonlinear computations of the 1992 nicaragua earthquake tsunami numerical computations of tsunamis are made for the 1992 nicaragua earthquake using different governing equations bottom frictional values and bathymetry data the results are compared with each other as well as with the observations both tide gauge records and runup heights comparison of the observed and computed tsunami waveforms indicates that the use of detailed bathymetry data with a small grid size is more effective than to include nonlinear terms in tsunami computation linear computation overestimates the amplitude for the later phase than the first arrival particularly when the amplitude becomes large the computed amplitudes along the coast from nonlinear computation are much smaller than the observed tsunami runup heights the average ratio or the amplification factor is estimated to be 3 in the present case when the grid size of 1 minute is used the factor however may depend on the grid size for the computation

crustal material recycling induced by subduction erosion and subduction channel exhumation a case study of central tibet western china based on p t t paths of the eclogite bearing baqing metamorphic complex

river gorge eradication by downstream sweep erosion narrow river gorges are often short lived features images of a bedrock gorge in taiwan which was carved after 1999 reveal rapid widening where the upstream floodplain meets the gorge an erosional front that propagates downstream as the gorge is erased

the aleutian earthquake of june 10 1996 mw 7 9 ruptured parts of both the andreanof and delarof segments the epicenter of the aleutian earthquake of june 10 1996 is located in the delarof segment where a large subduction earthquake is expected however the aftershock area of the earthquake is not only in the delarof segment but also in the andreanof segment where the 1986 andreanof earthquake occurred previously we estimate the fault parameters of the 1996 earthquake by numerically computing the tsunami and comparing the waveforms with that recorded at adak the fault width is 30 km or less and the average slip is about 4 m the observed far field tsunamis around the pacific are also explained by the synthetic tsunami computed using the above fault parameters the results indicate that the 1996 earthquake ruptured parts of both the delarof and andreanof segments however the 1996 earthquake did not rupture the region of the largest moment release by the 1986 earthquake

relationships between block quarrying bed shear stress and stream power a physical model of block quarrying of a jointed bedrock channel we examine interactions among substrate characteristics hydraulic forces and erosive processes acting on a jointed resistant substrate in a flume experiencing supercritical flow we conducted six experimental runs with no sediment in transport constant slope varying discharge varying channel width and varying geometry of concrete blocks forming the channel bed the initial downstream boundary was a vertical knickpoint lip perpendicular to flow observed changes in bed morphology occurred primarily by episodic horizontal knickpoint retreat the knickpoint lip can develop a curve that reflects interlocking of the irregular boundaries between otherwise physically disjointed blocks during rotation of the blocks sometimes a block acts like a keystone in an arch and mobilization of the key block facilitates mobilization of the linked blocks on either side resulting in a large magnitude event in contrast to previous studies where blocks were vertically plucked blocks were removed by sliding from the knickpoint building on previous work we develop a theoretical force threshold for sliding block mobilization which suggests that increased block side length parallel to flow increases block stability whereas increased side length perpendicular to flow decreases block stability stream power was also linked to observed erosion rates as a function of block length consistent with the idea that block loosening rates play a key role in controlling average erosion rates inferred amplification of bed shear stress and pressure at the knickpoint lip stream power and joint spacing correlate with the rate of block erosion the results although limited in scope and scale suggest a linear relationship between bed shear stress and stream power with the rate of block erosion block sliding especially at near vertical knickpoints can play a key or even dominant role in the morphologic dynamics of the river channel

extreme oxygen isotope zoning in garnet and zircon from a metachert block in melange reveals metasomatism at the peak of subduction metamorphism

the role of sediment in controlling steady state bedrock channel slope implications of the saltation abrasion incision model abstract the saltation abrasion model is a mechanistic model for river incision into bedrock by saltating bedload which we have previously derived and used experimental data to constrain all parameter values here we develop a method for applying the saltation abrasion model at a landscape scale and use the model as a reference for evaluating the behavior of a wide range of alternative incision models in order to consider the implications of the saltation abrasion model as well as other models for predicting topographic steady state channel slope to determine the single valued discharge that best represents the effects of the full discharge distribution in transporting sediment and wearing bedrock we assume all runoff can be partitioned between a low flow and a high flow discharge in which all bedload sediment transport occurs during high flow we then use the gauged discharge record and measurements of channel characteristics at a reference field site and find that the optimum discharge has a moderate magnitude and frequency due to the constraints of the threshold of grain motion and bed alluviation by high relative sediment supply incision models can be classified according to which of the effects of sediment on bedrock incision are accounted for using the predictions of the saltation abrasion model as a reference we find that the threshold of motion is the most important effect that should be represented explicitly followed in order of decreasing importance by the cover effect the tools effect and the threshold of suspension effect models that lack the threshold of motion over predict incision rate for low shear stresses and under predict the steady state channel slope for low to moderate rock uplift rates and rock strengths models that lack the cover effect over predict incision rate for high sediment supply rates and fail to represent the degree of freedom in slope adjustment provided by partial bed coverage models that lack the tools effect over predict incision rate for low sediment supply rates and do not allow for the possibility that incision rate can decline for increases in shear stress above a peak value overall the saltation abrasion model predicts that steady state channel slope is most sensitive to changes in grain size such that the effect of variations in rock uplift rate and rock strength may affect slope indirectly through their possible but as yet poorly understood influence on the size distribution of sediments delivered to channel networks by hillslopes

implications of the saltation abrasion bedrock incision model for steady state river longitudinal profile relief and concavity the saltation abrasion model predicts rates of river incision into bedrock as an explicit function of sediment supply grain size boundary shear stress and rock strength here we use this experimentally calibrated model to explore the controls on river longitudinal profile concavity and relief for the simple but illustrative case of steady state topography over a wide range of rock uplift rates we find a characteristic downstream trend in which upstream reaches are close to the threshold of sediment motion with large extents of bedrock exposure in the channel bed while downstream reaches have higher excess shear stresses and lesser extents of bedrock exposure profile concavity is most sensitive to spatial gradients in runoff and the rate of downstream sediment fining concavity is also sensitive to the supply rate of coarse sediment which varies with rock uplift rate and with the fraction of the total sediment load in the bedload size class variations in rock strength have little influence on profile concavity profile relief is most sensitive to grain size and amount of runoff rock uplift rate and rock strength influence relief most strongly for high rates of rock uplift analysis of potential covariation of grain size with rock uplift rate and rock strength suggests that the influence of these variables on profile form could occur in large part through their influence on grain size similarly covariation between grain size and the fraction of sediment load in the bedload size class provides another indirect avenue for rock uplift and strength to influence profile form copyright 2008 john wiley sons ltd

a potassic volcanic front in western mexico the lamprophyric and related lavas of san sebastian the superposition of two tectonic regimes rifting and subduction in western mexico has led to the development of a lamprophyric volcanic front 70 km closer to the middle america trench than is the main axis of andesitic volcanism the san sebastian cindercone field represents the northernmost extent of lamprophyric activity in western mexico and lies 40 km trenchward of volcan ceboruco a historically active andesitic stratovolcano the predominant country rocks are cretaceous silicic ash flows although marine sedimentary rocks emplaced between 45 and 115 ma also occur minette absarokite and kersantite lavas were erupted between 0 48 and 0 26 ma in the san sebastian field in close association with flows of basaltic andesite and hornblende andesite there is a gradual transition observed between the two intermediate lava types basaltic andesite and kersantite as plagioclase is suppressed and hornblende and phlogopite are stabilized in melts of intermediate composition this transition reflects the increasing influence of water during crystallization and documents a wide range of hydrous conditions examination of the primary lamprophyric lava types absarokite and minette reveals a remarkable range in their oxidation states if these lavas reflect their mantle source conditions then the redox state of western mexico9s sub arc mantle spans the interval between the ni nio and magnetite hematite buffers a variation in fo 2 of five log units the oxidation and hydrous enrichment of mexico9s sub arc mantle probably formed in response to subduction the eruption of lamprophyric lavas in front of the andesitic suite of the main axis however may have depended upon rifting 1 to trigger melting of the modified mantle and 2 to provide an easy passage to the surface through crustal thinning and block faulting

a comparison of the spatio temporal distribution of moment release for the 1986 andreanof islands earthquake with relocated seismicity we compare relocated teleseismically recorded seismicity before and after the may 7 1986 andreanof islands earthquake mw 8 to the spatial and temporal distribution of seismic moment release during the mainshock relocated seismicity from 1964 through april 1987 clusters strongly along the main thrust zone in similar locations for the pre and post mainshock periods we performed simultaneous iterative inversions of broadband teleseismic gdsn and nars p waveforms using the method of kikuchi and fukao 1987 fault plane parameters e g strike dip depth and extent of fault plane consistent with regional tectonics and aftershock locations were specified the results of the inversions are spatial and temporal maps of moment release on the fault plane most of the moment found by the inversion is localized in several regions of the fault plane resolution of the location and timing of the major subevents is good the rupture was bilateral with a highly variable rupture velocity a shallow updip region of the fault plane ruptured toward the end of the mainshock process the mainshock moment release tended to occur in regions of the fault plane in which no or few aftershocks or preshocks are located this tendency suggests that regions of high moment release in the 1986 andreanof islands earthquake resulted from rupture of mechanically strong regions i e asperities on the fault plane which exhibited little seismicity before or after the mainshock that ruptured them we infer that the asperity regions were locked and quiescent before the mainshock while surrounding seismically active areas did not store as much strain and slipped mostly aseismically

plate coupling variation and block translation in the andreanof segment of the aleutian arc determined by subduction zone modeling using gps data 1 we use gps measurements in the first geodetic study of plate coupling on the andreanof segment aleutian subduction zone convergence of 7 3 cm yr at the subduction zone results in northwestward displacements greater than 1 cm yr at stations located on the west end of the andreanof region but velocities at the east end are smaller in magnitude and oriented west southwest these velocity variations are caused by differences in plate coupling along the subduction zone which correlate with the rupture zones of the 1986 and 1996 mw8 earthquakes we construct a dislocation model to estimate the velocity of the arc in the andreanof region relative to north america and fault plane coupling coefficients our best model shows a southwestward velocity of 7 8 mm yr a high degree of coupling in the main thrust zone at the west end of the subduction zone and little to no coupling at the east end

modeling the effects of bed topography on fluvial bedrock erosion by saltating bed load abrasion by saltation is an important mechanism of fluvial incision into bedrock sklar and dietrich 2004 introduced an abrasion model in which the erosion rate of an approximately planar bed is linearly dependent on the kinetic energy transferred by the vertical velocity of saltating grains however most bedrock floored channels exhibit topographic variations that yield deviations from a planar surface referred to as bed topography observations show that bed topography affects erosion here the saltation abrasion model is extended for a nonplanar bed a several centimeter high bump transverse to the flow is repeated every 50 cm the kinetic energy of grain impacts is calculated in two ways 1 impact velocity normal to bed topography and 2 vertical impact velocity by comparing the latter case with the planar model it is possible to isolate the effects of topography on the interception of saltation trajectories incorporating bed topography changes erosion in three ways first erosion is 10 to 1000 times faster depending upon transport stage and grain size enhanced erosion results from both the interception of grains by topography and the increased kinetic energy transfer associated with high angle impacts on the stoss side of bumps second erosion increases monotonically with transport stage whereas maximum erosion occurs at low to intermediate transport stage with a planar bed third erosion decreases monotonically with grain size whereas maximum erosion occurs with intermediate sized grains with a planar bed although the model is highly simplified results show that bed topography should be considered when simulating erosion of bedrock

melt segregation and depletion during ascent of buoyant diapirs in subduction zones author posting american geophysical union 2020 this article is posted here by permission of american geophysical union for personal use not for redistribution the definitive version was published in journal of geophysical research solid earth 125 2 2020 e2019jb018203 doi 10 1029 2019jb018203

rockbed wear at a flow convergence zone in fifteen mile creek niagara peninsula ontario sculpted bedforms developed in rock channels must be considered within the context of temporally and spatially persistent flow dynamics a shallow meandering channel 14 m long 30 50 cm wide 3 4 cm deep with raised margins has been eroded into resistant irondequoit limestone forming the streambed of fifteen mile creek ontario potholes 18 cm deep 25 cm wide 35 cm long within the channel increase in size downstream subaerial solution of the limestone and fracture patterns within the irondequoit limestone are not responsible for this large dolostone blocks within the channel stabilize patterns of vorticity that form in the lee of the blocks during highflows when flow is about 1 3 m deep vortices migrating downstream along a major flow junction have eroded the distinctive channel small irregularities in the bed may enlarge to potholes by attracting vortices and undergoing erosional wear as they pass the feature may have developed within several hundred to several thousand years but likely d

modeling the effects of weathering on bedrock floored channel geometry 1 field and modeling studies suggest that bedrock channels equilibrate to base level change through geometry and slope adjustment to imposed discharge sediment supply and substrate erodibility conditions in this study we model the influence of bedrock weathering on channel geometry and slope as mean peak discharge qm and uplift rate u vary we find that channels in which weathering is allowed to increase erodibility are wider deeper and less steep than nonweathering channels with the same initial conditions while fixed erodibility channels maintain similar width depth ratios regardless of qm or u the width depth ratio of weathering channels is sensitive to uplift rate at low uplift rates weathering outpaces erosion and channels obtain similar width depth ratios but are wider and less steep than fixed erodibility channels with equal initial conditions at high uplift rates erosion outpaces weathering and erodibility remains near the unweathered value with channel shape and slope nearly identical to a fixed erodibility channel with equal initial conditions weathering channels differ most from fixed erodibility channels at intermediate uplift rates with greater width depth ratios and lower slopes than fixed erodibility channels with the same initial conditions our results support the hypothesis that cross channel variations in erodibility created by weathering may be an important control on channel geometry and provide guidance for further testing of this hypothesis in natural systems

alkaline and calc alkaline lavas near los volcanes jalisco mexico geochemical diversity and its significance in volcanic arcs a remarkably diverse suite of lavas erupted during the late pliocene at the volcanic front of the western mexican volcanic belt near the town of los volcanes jalisco this region is much closer to the middle america trench than the main axis of quaternary andesite dacite stratovolcanoes and volcanism occurred in a complex tectonic regime involving both subduction of the young rivera plate and transverse crustal extension of the jalisco structural block the variety of lava types covers a wide spectrum from highly potassic minettes and leucitites to calc alkaline basalts and andesites which are compositionally similar to those erupted elsewhere in the mexican volcanic belt other alkaline varieties intermediate between these extremes include absarokites trachybasalts and trachyandesites phlogopite amphibole and apatite are common phenocryst phases whole rock compositions show a wide range of alkali contents e g k2o of 1 0 8 6 wt and typically contain 5 wt mgo mgo ni v and cr show little systematic decrease with increasing sio2 suggesting that these lavas have evolved from primitive mantle derived magmas with a wide range of sio2 contents strong enrichments in incompatible trace elements are observed in all of the lavas sr 700 5100 ppm ba 470 4800 ce 22 325 ppm zr 90 700 ppm as is the relative enrichment of large ion lithophile ba sr rb and light rare earth elements la ce over the high field strength elements ti zr which is typical of magmas in volcanic arcs this enrichment pattern suggests that these magmas come from source regions which contain incompatible element rich phases such as phlogopite amphibole and apatite the petrological and geochemical features of the lavas which occur in the los volcanes region provide direct evidence of the extreme heterogeneity which may exist in magma source regions at convergent margins the complex tectonic regime in western mexico further suggests that rifting and crustal extension play an important role in the generation and successful ascent of melts from enriched regions of the sub arc mantle

the great 2006 and 2007 kuril earthquakes forearc segmentation and seismic activity of the central kuril islands region we present a structural study of the central kuril islands forearc region where the great megathrust tsunamigenic earthquake m w 8 3 occurred on november 15 2006 based on new bathymetry and seismic profiles obtained during two research cruises of r v akademik lavrentiev in 2005 and 2006 ten crustal segments with along arc length ranging from 30 to 100 km separated by ns and nw trending transcurrent faults were identified within the forearc region the transcurrent faults may serve as barriers impeding stress transfer between the neighboring segments so that stress accumulated within separate forearc segments is usually released by earthquakes of moderate to strong magnitudes however the great november 15 2006 earthquake ruptured seven of the crustal segments probably following a 226 year gap since the last great earthquake in 1780 the geographic extent of earthquake rupture zones aftershock areas and earthquake clusters correlate well with forearc crustal segments identified using the geophysical data based on segmented structure of the central kuril islands forearc region we consider and discuss three scenarios of a great earthquake occurrence within this area although the margin is segmented we suggest that a rupture could occupy the entire seismic gap with a total length of about 500 km in such a case the earthquake magnitude m w might exceed 8 5 and such an event might generate tsunami waves significantly exceeding in height to those produced by the great 2006 2007 kuril earthquakes

observations on the role of lithology in strath terrace formation and bedrock channel width field observations from western washington and eastern tibet indicate a strong lithologic influence on strath terrace formation and highlight the response of bedrock channel width to spatial gradients in bedrock erodibility and or rock uplift rates measurements of local bedrock bed and bank erosion rates together with observations of the role of weathering processes on erosion of siltstones and sandstones illustrate a mechanism that underlies a conceptual model that predicts a strong lithological control on strath terrace formation direct measurements of bedrock erosion rates in lithologies susceptible to accelerated erosion upon sub aerial exposure show that lateral rates of bedrock bank erosion can substantially exceed vertical incision below the perennial flow level due to an asymmetry in erodibility between perennially submerged rock and rock exposed to cyclic wetting and drying on bedrock channel walls under such conditions the rapid weathering mediated lowering of bedrock exposed above baseflow promotes development of a beveled bedrock platform that could become a strath terrace upon abandonment in contrast to previous studies channel widths measured across a substantial gradient in lithology slope and long term erosion rates in eastern tibet do not follow conventional relations where channel width scales as a power law function of drainage area instead as they flow across a zone of rapid uplift these channels systematically narrow relative to the predicted width extrapolated from traditional power law relations field studies from a wide range of geological settings and lithologies support the interpretation that in general strath terraces tend to be more extensive in rivers flowing over weak sedimentary rock and tend to be poorly developed and or preserved where rivers flow over hard erosion resistant rock

evaluating the controls of shear stress sediment supply alluvial cover and channel morphology on experimental bedrock incision rate 1 we explored the dependence of experimental bedrock erosion rate on shear stress bed load sediment flux alluvial bed cover and evolving channel morphology we isolated these variables experimentally by systematically varying gravel sediment flux qs and water discharge qw in a laboratory flume gradually abrading weak concrete bedrock all else held constant we found that 1 erosion rate was insensitive to flume averaged shear stress 2 erosion rate increased linearly with sediment flux 3 erosion rate decreased linearly with the extent of alluvial bed cover and 4 the spatial distribution of bed cover was sensitive to local bed topography but the extent of cover increased with qs qt where qt is flume averaged transport capacity once critical values of bed roughness and sediment flux were exceeded starting from a planar geometry erosion increased bed roughness due to feedbacks between preferential sediment transport through interconnected topographic lows focused erosion along these zones of preferential bed load transport and local shear stresses that depended on the evolving bed morphology finally continued growth of bed roughness was inhibited by imposed variability in discharge and sediment flux due to changes in spatial patterns of alluvial deposition and impact wear erosion was preferentially focused at lower bed elevations when the bed was cover free but was focused at higher bed elevations when static alluvial cover filled topographic lows natural variations in discharge and sediment flux may thus stabilize and limit the growth of roughness in bedrock channels due to the effects of partial bed cover

climate driven bedrock incision in an active mountain belt measurements of fluvial bedrock incision were made with submillimeter precision in the east central range of taiwan where long term exhumation rates and precipitation driven river discharge are independently known they indicate that valley lowering is driven by relatively frequent flows of moderate intensity abrasion by suspended sediment is an important fluvial wear process and channel bed geometry and the presence of widely spaced planes of weakness in the rock mass influence erosion rate and style

experimental study of bedrock channel alluviation under varied sediment supply and hydraulic conditions 1 channel incision into bedrock plays a critical role in mountain landscape evolution a bed that is completely alluviated cannot undergo incision as a result incision driven by the collision of bedload grains and bedrock requires transport at below capacity conditions for example where the bed is partially covered by alluvium in a mechanistic model of bedrock incision it is important to have a formula describing the relationship between the bedload transport rate and the degree to which the bed is covered by sediment the purpose of this work is to understand the mechanisms underlying the degree of bed exposure in bedrock rivers by means of an experimental study a number of experiments with various hydraulic and morphological bed conditions were performed in order to characterize this model here we focus on planar bedrock beds with some roughness the results suggest that the sediment supply channel slope hydraulic bed roughness the degree of areal coverage and thickness of antecedent gravel in the channel as well as the presence of boulders in the channel are major factors controlling bedrock exposure for sufficiently low ratios of sediment supply to transport capacity it was found that bedrock roughness can play a role in determining the degree of bedrock exposure for higher sediment ratios on the other hand the composite roughness associated with grain roughness bars and or antidunes dominates so that the underlying bedrock roughness no longer affects the degree of exposure of the bed for lower bed slopes i e less than 0 015 based on our experimental setting bedrock exposure decreases more or less linearly with increasing values of the ratio of sediment supply rate to capacity rate for higher bed slopes i e more than 0 015 there is a range of lower values of the ratio of sediment supply rate to capacity rate where a bedrock surface becomes fully exposed without any alluvial deposit for a given bedrock roughness this range expands solely with increasing slope regardless of shear stress within the upper range of values of the ratio of sediment supply to capacity rate a linear relationship between the degree of bedrock exposure and this supply to capacity ratio still prevails the addition of model boulders can be viewed as very high hydraulic bed roughness to the channel has been found to suppress the overexposure of bedrock and so restore a linear relation between the degree of exposure and the ratio of sediment supply rate to capacity rate formulations for estimating bedrock exposure as a function of sediment supply to capacity ratio under different river channel characteristics are proposed a linear relation between the degree of alluvial cover and the ratio of sediment supply rate to capacity rate is a previously proposed model the present study expands the result for other cases including runaway alluviation some landscape evolution models assume an abrupt shift between fully exposed bedrock and complete alluviation whereas others assume the linear relation the present study shows that both formulations are valid under different settings of river characteristics

evolution of the santa cruz mountains california through tectonic growth and geomorphic decay i describe a strategy for modeling a specific large scale topographic feature that recognizes the discrepancy between tectonic and geomorphic scales and allows for important feedbacks between these processes in the geomorphic model the largest channel in each 4 4 km cell is explicitly treated the channel incision being driven by a stream power rule hillslopes internal to each cell respond rapidly to incision of the local channel prompting their treatment in the model as steady state forms i present analytic expressions for the local relief which is controlled by the local channel incision rate and rate constants associated with the dominant hillslope process either diffusive or landsliding initial conditions include any preexisting topography and a lightly etched a channel network the model is illustrated using the santa cruz mountains california i show that an earlier hypothesis that the santa cruz mountains are due to advection of topography past an uplift source related to a restraining bend in the san andreas fault remains viable when more specific geomorphic processes are considered the local tectonics are represented by crustal thickening necessitated by conservation of mass in the bend right lateral translation of crust with respect to the bend and flexural response to the distributed load the model produces reasonable geographic and statistical distributions of topography using rates of tectonic and geomorphic processes that are within the range of those measured locally this argues that this restraining bend is a long lived feature of the san andreas fault the slightly higher crest and shorter length of the southern santa cruz mountains may potentially be explained by slightly faster slip of the crust west of the fault with respect to the bend than of that to the east deflection is minimal beneath these relatively small mountains disallowing the interesting feedback of erosionally driven uplift the predicted pattern of exhumation could be used to guide the search for fission track sites that could aid in the testing of the model

erosional dynamics flexural isostasy and long lived escarpments a numerical modeling study erosional escarpments are common features of high elevation rifted continents fission track data suggest that these escarpments form by base level lowering and or marginal uplift during rifting followed by lateral retreat of an erosion front across tens to hundreds of kilometers previous modeling studies have shown that this characteristic pattern of denudation can have a profound impact upon marginal isostatic uplift and the evolution of offshore sedimentary basins yet at present there is only a rudimentary understanding of the geomorphic mechanisms capable of driving such prolonged escarpment retreat in this study we present a nonlinear two dimensional landscape evolution model that is used to assess the necessary and sufficient conditions for long term retreat of a rift generated escarpment the model represents topography as a grid of cells with drainage networks evolving as water flows across the grid in the direction of steepest descent the model accounts for sediment production by weathering fluvial sediment transport bedrock channel erosion and hillslope sediment transport by diffusive mechanisms and by mass failure numerical experiments presented explore the effects of different combinations of erosion processes and of dynamic coupling between denudation and flexural isostatic uplift model results suggest that the necessary and sufficient conditions for long term escarpment retreat are 1 incising bedrock channels in which the erosion rate increases with increasing drainage area so that the channels steepen and propagate headward 2 a low rate of sediment production relative to sediment transport efficiency which promotes relief generating processes over diffusive ones 3 high continental elevation which allows greater freedom for fluvial dissection and 4 any process including flexural isostatic uplift that helps to maintain a drainage divide near an escarpment crest flexural isostatic uplift also facilitates escarpment retreat by elevating topography in the vicinity of an eroding escarpment thereby increasing channel gradients and accelerating erosion which in turn generates additional isostatic uplift of all the above conditions high continental elevation is common to most rift margin escarpments and may ultimately be the most important factor

numerical modeling of fluvial strath terrace formation in response to oscillating climate many river systems in western north america retain a fluvial strath terrace rec ord of discontinuous downcutting into bedrock through the quaternary their importance lies in their use to interpret climatic events in the headwaters and to determine long term incision rates terrace formation has been ascribed to changes in sediment supply and or water discharge produced by late quaternary climatic fluctuations we use a one dimensional channel evolution model to explore whether temporal variations in sediment and water discharge can generate terrace sequences the model includes sediment transport vertical bedrock erosion limited by alluvial cover and lateral valley wall erosion we set limits on our modeling by using data collected from the terraced wind river basin two types of experiments were performed constant period sinusoidal input histories and variable period inputs scaled by the marine 18 o rec ord our simulations indicate that strath terrace formation requires input variability that produces a changing ratio of vertical to lateral erosion rates straths are cut when the channel floor is protected from erosion by sediment and are abandoned and terraces formed when incision can resume following sediment cover thinning high sediment supply promotes wide valley floors that are abandoned as sediment supply decreases in contrast wide valleys are promoted by low effective water discharge and are abandoned as discharge increases widening of the valley floors that become terraces occurs over many thousands of years the transition from valley widening to downcutting and terrace creation occurs in response to subtle input changes affecting local divergence of sediment transport capacity formation of terraces lags by several thousand years the input changes that cause their formation our results suggest that use of terrace ages to set limits on the timing of a specific event must be done with the knowledge that the system can take thousands of years to respond to a perturbation the incision rate calculated in the field from the lowest terrace in these systems will likely be higher than the rate calculated by using older terraces because the most recent fluvial response in the field is commonly downcutting associated with declining sediment input since the last glacial maximum this apparent increase in incision rates is observed in many river systems and should not necessarily be interpreted as a response to an increase in rock uplift rate

lifespan of mountain ranges scaled by feedbacks between landsliding and erosion by rivers an important challenge in geomorphology is the reconciliation of the high fluvial incision rates observed in tectonically active mountain ranges with the long term preservation of significant mountain range relief in ancient tectonically inactive orogenic belts river bedrock erosion and sediment transport are widely recognized to be the principal controls on the lifespan of mountain ranges but the factors controlling the rate of erosion and the reasons why they seem to vary significantly as a function of tectonic activity remain controversial here we use computational simulations to show that the key to understanding variations in the rate of erosion between tectonically active and inactive mountain ranges may relate to a bidirectional coupling between bedrock river incision and landslides whereas fluvial incision steepens surrounding hillslopes and increases landslide frequency landsliding affects fluvial erosion rates in two fundamentally distinct ways on the one hand large landslides overwhelm the river transport capacity and cause upstream build up of sediment that protects the river bed from further erosion on the other hand in delivering abrasive agents to the streams landslides help accelerate fluvial erosion our models illustrate how this coupling has fundamentally different implications for rates of fluvial incision in active and inactive mountain ranges the coupling therefore provides a plausible physical explanation for the preservation of significant mountain range relief in old orogenic belts up to several hundred million years after tectonic activity has effectively ceased

a bimodal hybrid model for time dependent probabilistic seismic hazard analysis the evaluation of evidence provided by geological studies and historical catalogs indicates that in some seismic regions and faults multiple large earthquakes occur in cluster then the occurrences of large earthquakes confront with quiescence and only the small to moderate earthquakes take place clustering of large earthquakes is the most distinguishable departure from the assumption of constant hazard of random occurrence of earthquakes in conventional seismic hazard analysis in the present study a time dependent recurrence model is proposed to consider a series of large earthquakes that occurs in clusters the model is flexible enough to better reflect the quasi periodic behavior of large earthquakes with long term clustering which can be used in time dependent probabilistic seismic hazard analysis with engineering purposes in this model the time dependent hazard results are estimated by a hazard function which comprises three parts a decreasing hazard of last large earthquake cluster and an increasing hazard of the next large earthquake cluster along with a constant hazard of random occurrence of small to moderate earthquakes in the final part of the paper the time dependent seismic hazard of the new madrid seismic zone at different time intervals has been calculated for illustrative purpose

controls on the spacing of first order valleys 1 many landscapes are composed of ridges and valleys that are uniformly spaced even where valley locations are not controlled by bedrock structure models of long term landscape evolution have reproduced this phenomenon yet the process by which uniformly spaced valleys develop is not well understood and there is no quantitative framework for predicting valley spacing here we use a numerical landscape evolution model to investigate the development of uniform valley spacing we find that evenly spaced valleys arise from a competition between adjacent drainage basins for drainage area a proxy for water flux and that the spacing becomes more uniform as the landscape approaches a topographic equilibrium valley spacing is most sensitive to the relative rates of advective erosion processes such as stream incision and diffusion like mass transport such as soil creep and less sensitive to the magnitude of a threshold that limits the spatial extent of stream incision analysis of a large number of numerical solutions reveals that valley spacing scales with a ratio of characteristic diffusion and advection timescales that is analogous to a peclet number we use this result to derive expressions for equilibrium valley spacing and drainage basin relief as a function of the rates of advective and diffusive processes and the spatial extent of the landscape the observed scaling relationships also provide insight into the cause of transitions from rill like drainage networks to branching networks the spatial scale of first order drainage basins the contributing area at which hillslopes transition into valleys and the narrow range of width to length ratios of first order basins

cover effect in bedrock abrasion a new derivation and its implications for the modeling of bedrock channel morphology the sediment load of a bedrock river plays an important role in the fluvial incision process by providing tools for abrasion the tools effect and by covering and thereby protecting the bed the cover effect we derive a new formulation for the cover effect in which the fraction of exposed bed area falls exponentially with increasing sediment flux or decreasing transport capacity and explore its consequences for the model of bedrock abrasion by saltating bed load erosion rates predicted by the model are higher than those predicted by earlier models in a closed system the maximum erosion rate is predicted to occur when sediment supply is equal to transport capacity for a flat bed by optimizing the channel geometry to minimize the potential energy of the stream and using representative values for both discharge and grain size we derive equations for the geometry of a bedrock river and explore how predictions for width slope and bed cover vary as functions of drainage area rock uplift rate and rock strength the equations predict a dependence of channel width on drainage area similar to the relations using a simple shear stress incision law the slope area relationship is predicted to be concave up in a log log regime with a curvature dependent on uplift rate however this curvature does not deviate sufficiently from a straight line to allow discrimination between models using empirical data dependence of channel width and slope on rock uplift rate can be separated into two domains for low uplift rates channel geometry is largely insensitive to uplift rate due to a threshold effect at high uplift rates there is a power law dependence bed cover is predicted to increase progressively downstream and to increase with increasing uplift rate in our model the width to depth ratio is a function of both tectonic and climatic forcing this indicates that the scaling between channel width and bed slope is neither a unique indicator of tectonic forcing at steady state nor a signature of transience or steady state we conclude that sediment effects need to be taken into account when modeling bedrock channel morphology

long term perspectives on giant earthquakes and tsunamis at subduction histories of earthquakes and tsunamis inferred from geological evidence aid in anticipating future catastrophes this natural warning system now influences building codes and tsunami planning in the united states canada and japan particularly where geology demonstrates the past occurrence of earthquakes and tsunamis larger than those known from written and instrumental records under favorable circumstances paleoseismology can thus provide long term advisories of unusually large tsunamis the extraordinary indian ocean tsunami of 2004 resulted from a fault rupture more than 1000 km in length that included and dwarfed fault patches that had broken historically during lesser shocks such variation in rupture mode known from written history at a few subduction zones is also characteristic of earthquake histories inferred from geology on the pacific rim

discharge discharge variability and the bedrock channel profile long term bedrock incision is driven by daily discharge events of variable magnitude and frequency with ineffective events below an incision threshold we explore theoretically how this short term stochastic behavior controls long term steady state incision rates and bedrock channel profiles combining a realistic frequency magnitude distribution of discharge with a deterministic detachment limited incision model in which incision rate is a power function of basal shear stress above a critical shear stress our model predicts a power law relationship between steady state slope and drainage area consistent with observations the exponent of this power law is independent of discharge mean and variability while the amplitude factor which controls mountain belt relief is a power law function of mean runoff with an exponent of 0 5 and a complex function of runoff variability in accordance with evidence that incision occurs between 6 and 20 of time in rapidly incising rivers 1 mm yr our model predicts that channel steepness is virtually insensitive to runoff variability runoff variability can only decrease channel steepness for very slow incision rates and or weak lithologies the relationship between channel steepness and incision rate is always a power law whose exponent depends on the channel cross sectional geometry and runoff variability this contradicts models neglecting discharge stochasticity in which the steepness incision scaling is set by the incision law exponent our results suggest that changes in climate variability cannot explain an increase in bedrock incision rates during the late cenozoic within the context of a detachment limited model

circum pacific seismic potential 1989 1999 the seismic potential for 96 segments of simple plate boundaries around the circum pacific region is presented in terms of the conditional probability for the occurrence of either large or great interplate earthquakes during the next 5 10 and 20 years i e 1989 1994 1989 1999 and 1989 2009 this study represents the first probabilistic summary of seismic potential on this scale and involves the comparison of plate boundary segments that exhibit varying recurrence times magnitudes and tectonic regimes presenting these data in a probabilistic framework provides a basis for the uniform comparison of seismic hazard between these differing fault segments as well as accounting for individual variations in recurrence time along a specific fault segment and uncertainties in the determination of the average recurrence time

the origin of summit basins of the aleutian ridge implications for block rotation of an arc massif it is proposed that many summit basins along the aleutian arc form from the clockwise rotation of blocks of the arc massif summit basins are arc parallel grabens or half grabens formed within the arc massif and are commonly located near or along the axis of late cenozoic volcanism geomorphically the aleutian arc appears to consist of contiguous rhombic blocks of varying size tens to hundreds of kilometers in length the boundaries between adjacent blocks are delineated by fault controlled canyons that cut the southern slope of the arc transverse to its regional trend evidence that these blocks have rotated clockwise is provided by the triangular shaped summit basins bordering the blocks to the north oblique physiographic trends offsets in the summit platform and broad deflections in the southern slope of the arc we present a model for block rotation that involves translation of blocks parallel to an arc it is suggested that block rotation which appears to have accelerated in late cenozoic time is linked to 1 a shift in the euler pole for the pacific plate 2 the consequential start up of late cenozoic volcanism 3 improved interplate coupling instigated by sediment flooding of the aleutian trench and 4 westward subduction of northeast striking segments of the inactive kula pacific ridge

worldwide doublets of large shallow earthquakes abstract we investigated all the pairs of mw 7 5 shallow earthquakes in the harvard catalog that occurred at a centroid distance of less than 100 km we showed that most of these pairs have similar focal mechanisms because these earthquakes generally should have focal regions in excess of 100 km diameter their rupture zones apparently intersect for all these pairs the time interval is significantly less than the time span needed for plate motion to accumulate the strain released by the first event these observations conflict strongly with quasi periodic recurrence models on which the seismic gap hypothesis is based power law recurrence fits these earthquake ob servations much better introduction many models used to forecast long term seismic hazard are based on the quasi periodic or seismic cycle hypothesis of large earthquake occurrence this hypothesis assumes that the probability of a large earthquake is low after signifi cant release of seismic energy and that danger increases only when tectonic plate motion accumulates strain sufficient for a new strong event several models have been formulated to apply this idea to long term earthquake prediction mccann

seismicity trends and potential for large earthquakes in the alaska aleutian region the high likelihood of a gap filling thrust earthquake in the alaska subduction zone within this decade is indicated by two independent methods analysis of historic earthquake recurrence data and time to failure analysis applied to recent decades of instrumental data recent may 1993 earthquake activity in the shumagin islands gap is consistent with previous projections of increases in seismic release indicating that this segment along with the alaska peninsula segment is approaching failure based on this pattern of accelerating seismic release we project the occurrence of one or morem 7 3 earthquakes in the shumagin alaska peninsula region during 1994 1996 different segments of the alaska aleutian seismic zone behave differently in the decade or two preceding great earthquakes some showing acceleration of seismic release type a zones while others show deceleration type d zones the largest alaska aleutian earthquakes in 1957 1964 and 1965 originated in zones that exhibit type d behavior type a zones currently showing accelerating release are the shumagin alaska peninsula delarof and kommandorski segments time to failure analysis suggests that the large earthquakes could occur in these latter zones within the next few years

seismic potential of the queen charlotte alaska aleutian seismic zone the 5000 km long queen charlotte alaska aleutian seismic zone is subdivided into 17 unequally sized segments their boundaries are delineated based on the prior distribution of large and great earthquakes the 17 segments are chosen to represent areas likely to be ruptured by characteristic earthquakes this term usually implies repeated breakage of a plate boundary segment by either a large or great earthquake whose source dimensions remain consistent from cycle to cycle this definition does not exclude the possibility that occasionally adjacent characteristic earthquake segments may break together in a single giant event that is larger than the characteristic size outlined conversely a segment can also sometimes break in a series of smaller ruptures formal computations of the conditional probabilities for future large and great earthquakes in the 17 segments of the queen charlotte alaska aleutian seismic zone are based on the following data sets and findings 1 recurrence intervals from historic and geologic data 2 direct recurrence time estimates based on rates of relative plate motion and the size or displacement of the most recent characteristic event in each segment and 3 the application of a lognormal distribution of recurrence times for large and great earthquakes results of these computations indicate seven areas that have high i e 60 conditional probabilities for the recurrence of either large or great earthquakes within the next 20 years 1988 2008 these areas include cape st james yakataga the shumagin islands unimak island and the fox delarof and near islands segments of the aleutian arc when a shorter time interval is considered 1988 1998 those segments more likely to rupture in large ms 7 7 7 rather than great earthquakes have a high conditional probability these areas include the unimak fox and delarof islands segments the largest uncertainties in these forecasts stem from the short historic record providing a single recurrence time estimate for some segments or widely varying estimates for others from the unknown importance of aseismic slip and from a vague definition of characteristic earthquake size in fact characteristic earthquake size may not be a time invariant quantity

analysis of the far field tsunamis generated by the 1998 papua new guinea earthquake the tsunami generated by the 1998 papua new guinea earthquake was observed at several tide gauges and ocean bottom pressure gauges in japan the fault model of the 1998 papua new guinea earthquake was estimated using those tsunami waveforms observed in japan and coseismic subsidence observed along the coast of papua new guinea near the source region the numerical simulation of the tsunami using the linear boussinesq equation was carried out the tsunami waveforms in japan are explained by the fault model which is consistent with the harvard cmt solution including the seismic moment of 3 7 1019 nm the additional tsunami source such as a sediment slump is not needed to explain the observed tsunami in japan the mechanism of the earthquake was likely to be a steeply dipping reverse fault the 1998 papua new guinea earthquake was likely to occur within a small accretionary prism near the new guinea trench

the 1957 great aleutian earthquake the 9 march 1957 aleutian earthquake has been estimated as the third largest earthquake this century and has the longest aftershock zone of any earthquake ever recorded 1200 km however due to a lack of high quality seismic data the actual source parameters for this earthquake have been poorly determined we have examined all the available waveform data to determine the seismic moment rupture area and slip distribution these data include body surface and tsunami waves using body waves we have estimated the duration of significant moment release as 4 min from surface wave analysis we have determined that significant moment release occurred only in the western half of the aftershock zone and that the best estimate for the seismic moment is 50 100 1020 nm using the tsunami waveforms we estimated the source area of the 1957 tsunami by backward propagation the tsunami source area is smaller than the aftershock zone and is about 850 km long this does not include the unalaska island area in the eastern end of the aftershock zone making this area a possible seismic gap and a possible site of a future large or great earthquake we also inverted the tsunami waveforms for the slip distribution slip on the 1957 rupture zone was highest in the western half near the epicenter little slip occurred in the eastern half the moment is estimated as 88 1020 nm or m w 8 6 making it the seventh largest earthquake during the period 1900 to 1993 we also compare the 1957 earthquake to the 1986 andreanof islands earthquake which occurred within a segment of the 1957 rupture area the 1986 earthquake represents a rerupturing of the major 1957 asperity

primitive calc alkaline and alkaline rock types from the western mexican volcanic belt since the early pliocene small volumes of alkaline magmas have erupted in the western mexican volcanic belt mvb in close association with the volumetrically dominant calc alkaline magmas both suites include relatively rare primitive types characterized by high mg cr and ni contents primitive hypersthene normative basalts parental to the calc alkaline suite are found along the volcanic front but are absent at greater distances from the trench the volcanic front calc alkaline suites are typically hornblende bearing indicating relatively high water contents associated primitive alkaline magmas at volcanic front locations also contain hydrous minerals these nepheline normative suites include basanites phlogopite bearing minettes and other hornblende bearing lamprophyres these are probably the youngest and freshest lamprophyres yet discovered on earth and ideal samples for addressing the origin of this exotic class of rocks on extended morb normalized elemental plots all lamprophyres show patterns similar to the calc alkaline basalts but have overall enrichments of 2 to 25x in ti k p ba sr light rare earth elements and other incompatible elements the lamprophyres show the same strong relative enrichments of ba k and sr and depletions in ti and nb that characterize the calc alkaline rocks of western mexico and all subduction zones these similarities in relative element abundances support a common source region for all primitive magmas of the western mvb the primitive calc alkaline and alkaline rocks of western mexico show narrow overlapping ranges in sr nd and pb isotopic ratios also consistent with derivation of all magmas from a common source the presence of the relatively cold subducted cocos and rivera plates below the volcanic front demands that all primitive magmas originated in the depth range 30 to 75 km these magmas are enriched in ba k sr and other elements probably transported by hydrous fluids rising from the subducted slab several lines of evidence support the presence of phlogopite in the source region of western mvb magmas probably in the form of phlogopite rich veins cutting asthenospheric peridotite the first liquids formed upon partial melting of this veined source region will concentrate the vein component and generate the lamprophyres these lamprophyric melts rarely erupt in arc settings more commonly forming dikes in the arc crust eruption of lamprophyric magmas in the western mvb is favored by through crustal extensional fracture systems related to active rifting of the jalisco block from the n american plate larger volumes of calc alkaline basalt are generated by melting of the same source which dilutes the vein component with larger proportions of peridotitic wall rock

mount st helens potential example of the partial melting of the subducted lithosphere in a volcanic arc mount st helens 50 km to the west of mount adams and the main cascade volcanic chain is only 80 km above the subducting oceanic lithosphere the elevated temperatures off the subducting slab because of the close proximity of the juan de fuca ridge to the trench may induce slab melting at a depth of 80 km dacites from mount st helens have geochemical compositions off magmas that are derived by direct partial melting of metamorphosed basalts at high pressure i e relatively high ai al2o3 15 at 70 sio2 low y and yb because of garnet and amphibole stability in the source low sc and high sr and eu trace element modeling of the partial melting of mid oceanic ridge basalt morb from the juan de fuca ridge that yields a hornblende eclogite residue can reproduce the mount st helens data results off the model are quite distinct from data derived from the mount adams volcanic rocks in contrast mount adams is 135 km above the subducting slab and is associated with normal arc magmatism believed to be derived from the mantle above the subducting plate the cascade are has been active in its present locality because of oblique subduction for the past 7 m y the major volcanoes along the arc have existed for at least 500 ka but mount st helens has existed for 40 ka we suggest that the subducting plate may have reached elevated temperatures because of the approach of north america to the juan de fuca ridge at 40 ka which initiated melting of the slab

global omori law decay of triggered earthquakes large aftershocks outside the classical aftershock zone 1 triggered earthquakes can be large damaging and lethal as evidenced by the1999 shocks in turkey and the 2001 earthquakes in el salvador in this study earthquakes with ms 7 0 from the harvard centroid moment tensor cmt catalog are modeled as dislocations to calculate shear stress changes on subsequent earthquake rupture planes near enough to be affected about 61 of earthquakes that occurred near defined as having shear stress change 0 01 mpa the ms 7 0 shocks are associated with calculated shear stress increases while 39 are associated with shear stress decreases if earthquakes associated with calculated shear stress increases are interpreted as triggered then such events make up at least 8 of the cmt catalog globally these triggered earthquakes obey an omori law rate decay that lasts between 7 11 years after the main shock earthquakes associated with calculated shear stress increases occur at higher rates than background up to 240 km away from the main shock centroid omori s law is one of the few time predictable patterns evident in the global occurrence of earthquakes if large triggered earthquakes habitually obey omori s law then their hazard can be more readily assessed the characteristic rate change with time and spatial distribution can be used to rapidly assess the likelihood of triggered earthquakes following events of ms 7 0 i show an example application to the m 7 7 13 january 2001 el salvador earthquake where use of global statistics appears to provide a better rapid hazard estimate than coulomb stress change calculations

dynamic adjustments in channel width in response to a forced diversion gower gulch death valley national park california we studied the 1941 diversion of furnace creek wash drainage area 439 km 2 into gower gulch 5 8 km 2 as an experiment in the transient response of a channel to a large change in water and sediment discharge we measured sequential changes in valley width using a time series of aerial photographs 1948 1995 airborne laser elevation data from 2005 and a field survey we found that the response of the system varied depending on the prediversion channel morphology and bedrock geology in two steep knickzone segments narrowing knickpoint retreat and bedrock incision dominated indicating a detachment limited response in the relatively low gradient main part of gower gulch widening dominated as the coarse postdiversion sediment load covered the channel bed this transport limited part of the system has undergone only modest incision and adjustments in gradient over long periods the lowering rate of gower gulch probably depends on knickpoint retreat but the present day response of this non steady state system is a hybrid of incision and narrowing in detachment limited reaches and widening in transport limited reaches this system demonstrates the importance of evolving channel geometry in setting the transient response of rivers to changes in forcing parameters

response of bedrock channel width to tectonic forcing insights from a numerical model theoretical considerations and comparison with field data the morphology of bedrock river channels is controlled by climatic and tectonic conditions and substrate properties knowledge of tectonic controls remains scarce this is partly due to slow tectonic rates and long response times of natural channels and partly due to the difficulty in isolating and constraining tectonic forcing conditions in the field to study the effect of tectonic forcing on channel geometry we have developed a numerical model of the cross sectional evolution of a detachment limited channel its predictions are matched by an analytical model based on the assumption of the minimization of potential energy expenditure using these models we illustrate how local tectonics can alter the observed width discharge scaling and discuss published field data in light of our findings except for one case the models fail to correctly describe field observations of well constrained cases this implies that the shear stress stream power family of models is too simple to describe the behavior of natural channels additional complexities such as sediment effects and discharge variability exert a strong control on channel morphology and need to be taken into account in the modeling of channel dynamics and steady state

fluvial erosion of physically modeled abrasion dominated slot canyons abstract abrasion dominated fluvial erosion generates slot canyons in massive bedrock with intricately undulating walls flows in slot canyons are unusual in that the walls comprise a significant portion of the wetted perimeter of the flow during geomorphically effective floods in wire pass utah the upper paria river incises through massive crossbedded navajo sandstone incision in wire pass and related slots occurs only during flash floods paleoflood debris indicates that the width depth ratios of these flows are at times as low as 1 1 submeter resolution field mapping of a 20 m length of wire pass shows that the wall morphology is a complicated combination of in phase meander like and out of phase pinch and swell undulations in order to investigate evolution of slot canyons and the influence of their wall shapes on flow dynamics we recorded the evolution of four distinct canyon wall morphologies in a 2 4 m flume box at the st anthony falls laboratory in a substrate consisting of 3 2 mixtures of f110 sand and plaster of paris we molded canyons with in phase and out of phase undulations and wide 6 5 cm and narrow 4 cm straight initial wall profiles discharges ranged from 1 4 l s to 2 9 l s and wall and bed morphology were measured at 5 h intervals at 0 5 cm resolution results show efficient back eddy erosion in the undulating canyon walls and related erosional bedforms in all channels created by vortices in the flow maximum filaments of velocity are depressed and asymmetric and the implied shear stress distribution varied in space and time on the channel beds flow width depth ratios strongly influence the flow structure and distribution of shear stress in a slot and appear to be a factor in dictating whether a bedrock channel widens its walls or incises its bed

experimental evidence for fluvial bedrock incision by suspended and bedload sediment fluvial bedrock incision sets the pace of landscape evolution and can be dominated by abrasion from impacting particles existing bedrock incision models diverge on the ability of sediment to erode within the suspension regime leading to competing predictions of lowland river erosion rates knickpoint formation and evolution and the transient response of orogens to external forcing we present controlled abrasion mill experiments designed to test fluvial incision models in the bedload and suspension regimes by varying sediment size while holding fixed hydraulics sediment load and substrate strength measurable erosion occurred within the suspension regime and erosion rates agree with a mechanistic incision theory for erosion by mixed suspended and bedload sediment our experimental results indicate that suspension regime erosion can dominate channel incision during large floods and in steep channels with significant implications for the pace of landscape evolution

the sanriku oki japan earthquake of december 28 1994 mw 7 7 rupture of a different asperity from a previous earthquake fault geometry depth and slip distribution of the sanriku oki earthquake of december 28 1994 ms 7 5 are estimated from seismic waveforms geodetic measurements and tsunami waveforms and compared with those of the 1968 tokachi oki earthquake mw 8 2 the most recent large earthquake in the epicentral region seismic wave inversions indicate a shallowly dipping thrust type mechanism and the focal depth of 22 28 km representing an underthrust event at the subduction interface the source time function has an initial stage of 20 s followed by a main pulse of 30 s duration a shape similar to that of the 1968 tokachi oki earthquake joint inversion of geodetic and tsunami data shows that the region of the largest slip 1 7 m corresponds to a region of relatively small slip for the 1968 tokachi oki earthquake furthermore the joint inversion shows that the region of the largest asperity of the 1968 earthquake had essentially zero slip during the 1994 sanriku oki earthquake these results indicate that the dominant asperities ruptured by the 1994 sanriku and 1968 tokachi oki earthquakes are different the result of the joint inversion also shows that the aseismic zone near the japan trench was not ruptured by the 1994 earthquake