formation of coastline features by large scale instabilities induced by high angle waves along shore sediment transport that is driven by waves is generally assumed to smooth a coastline this assumption is valid for small angles between the wave crest lines and the shore as has been demonstrated in shoreline models but when the angle between the waves and the shoreline is sufficiently large small perturbations to a straight shoreline will grow here we use a numerical model to investigate the implications of this instability mechanism for large scale morphology over long timescales our simulations show growth of coastline perturbations that interact with each other to produce large scale features that resemble various kinds of natural landforms including the capes and cuspate forelands observed along the carolina coast of southeastern north america wind and wave data from this area support our hypothesis that such an instability mechanism could be responsible for the formation of shoreline features at spatial scales up to hundreds of kilometres and temporal scales up to millennia

the great tumaco colombia earthquake of 12 december 1979 southwestern colombia and northern ecuador were shaken by a shal low focus earthquake on 12 december 1979 the magnitude 8 shock located near tumaco colombia was the largest in northwestern south america since 1942 and had been forecast to fill a seismic gap thrust faulting occurred on a 280 by 130 kilometer rectangular patch of a subduction zone that dips east beneath the pacific coast of colombia a 200 kilometer stretch of the coast tectonically subsided as much as 1 6 meters uplift occurred offshore on the continental slope a tsunami swept inland immediately after the earthquake ground shaking intensity vi to ix caused many buildings to collapse and generated liquefaction in sand fills and in holocene beach lagoonal and fluvial deposits

a shallow crustal earthquake doublet from the trans mexican volcanic belt central mexico the trans mexican volcanic belt is an active volcanic arc related to subduction along the middle america trench and characterized by shallow seismicity and synvolcanic to postvolcanic extensional arc parallel faulting the mezquital graben is a major intra arc basin of the central trans mexican volcanic belt a doublet of moderate shallow shocks occurred in march and october 1976 in the region of this graben these earthquakes were recorded by the mexican national seismological network in particular by the bosch omori seismograph t 18 sec at the tacubaya observatory in mexico city we have carefully relocated the two main shocks and their major aftershocks by reading the original records and using a modified crustal velocity model for this region a difference of 50 km is observed between the locations reported by the mexican seismological service and those obtained in this study which are additionally supported by the damage distribution of these earthquakes a first motion analysis based on regional and teleseismic records defines for the march and october shocks normal fault mechanisms characterized by east west striking fault planes which coincides with the orientation of the master faults of the mezquital graben after calculating the instrumental response the source parameters were obtained from the bosch omori seismograph records by body wave modeling for the march earthquake we estimate a seismic moment of 4 5 1023 dyne cm equivalent to m w 5 0 and a stress drop of 0 7 mpa assuming a circular rupture model radius 3 km given the poor quality of the bosch omori record for the october earthquake we used comparison with the march event of long period t 20 sec teleseismic records at two stations to obtain corresponding source parameters for the october event by assuming a similar stress drop as for the march event we obtain a m of 5 6 1023 dyne cm and m w 5 1 with a rupture length of 6 5 km according to gravity data the regional east west faults are longer than reported in particular our detailed measurements indicate that the master fault of the mezquital graben continue westward in the subsurface below the basin fill in a more regional context additional magnetic data suggest that the el mezquital graben and aljibes half graben might be connected at their southern limits by part of the same fault system this is the first earthquake doublet reported from central mexico and the biggest seismic event in that zone during the past 50 years from a physical point of view it is possible that the seismic energy released by the march event was partially transferred toward the east triggering the october shock further east on the same fault system the presence of moderate seismicity in this zone should therefore be taken into account when assessing its seismic hazard

delta distributary dynamics in the skagit river delta washington usa extending testing and applying avulsion theory in a tidal system article i nfo analysis of historical aerial photos shows that skagit delta washington usa distributary dynamics are consistent with the slingerland and smith model of avulsion dynamics where the ratio of the water surface slopes of the two branches of a bifurcation predicts avulsion stability this model was extended to predict distributary inlet upstream width and bankfull cross sectional area the water surface gradient ratio for a bifurcation pair predicted distributary width well the lowest r 2 was 0 61 for the 1937 data points but r 2 ranged from 0 83 to 0 90 for other year specific regression lines gradient ratios were not constant over the historical record from 1937 to 1972 the mainstem river channel lengthened by 1250 m in the course of marsh progradation while distributary lengthening was comparatively negligible consequently the gradient advantage of the distributaries increased and their channels widened after the mainstem river terminus stabilized from 1972 to the present the distributaries continued to lengthen with marsh progradation so that distributary gradient advantage steadily declined and the distributaries narrowed while distributary cross sections were not available for the historical period they were surveyed in 2007 near the distributary inlets gradient ratio was more closely related to distributary inlet bankfull cross sectional area r 2 0 95 than to minimum distributary width for any photo year examined applying this form of analysis to skagit delta distributaries that have been dammed in the course of agricultural development suggests that their restoration to stabilize eroding marshes at their outlets and recover salmon migration pathways would be feasible without significant risk of full river avulsion

subsidence and strike slip tectonism of the upper continental slope off manzanillo mexico the direction of convergence between the rivera and north american plates becomes progressively more oblique in a counter clockwise sense as measured relative to the trench normal direction northwestward along the jalisco subduction zone by analogy to other subduction zones the forces resulting from this distribution of convergence directions are expected to produce a nw moving fore arc sliver and a nw se stretching of the fore arc area also a series of roughly arc parallel strike slip faults may form in the fore arc area both onshore and offshore as is observed in the aleutian arc in the jalisco subduction zone the jalisco block has been proposed to represent such a fore arc sliver however this proposal has encountered one major problem namely right lateral strike slip faulting within the fore arc sliver and between the fore arc sliver and the north american plate should be observed however evidence for the expected right lateral strike slip faulting is sparse some evidence for right lateral strike slip faulting along the jalisco block north american plate boundary the tepic zacoalco rift system has been reported although some disagreement exists right lateral strike slip faulting has also been reported within the interior of the jalisco block and in the southern colima rift which forms the se boundary of the jalisco block threefold multi channel seismic reflection data were collected in the offshore area of the jalisco subduction zone off manzanillo in april 2002 during the famex campaign of the n o l atalante these data provide additional evidence for recent strike slip motion within the fore arc region of the jalisco subduction zone this faulting offsets right laterally a prominent horst block within the southern colima rift from which we conclude that the sense of motion along the faulting is dextral these data also provide additional evidence for recent subsidence within the area offshore of manzanillo as has been proposed

low frequency sound speed and attenuation in sandy seabottom from long range broadband acoustic measurements a joint china u s underwater acoustics experiment was conducted in the yellow sea with a very flat bottom and a strong and sharp thermocline broadband explosive sources were deployed both above and below the thermocline along two radial lines up to 57 2 km and a quarter circle with a radius of 34 km two inversion schemes are used to obtain the seabottom sound speed one is based on extracting normal mode depth functions from the cross spectral density matrix the other is based on the best match between the calculated and measured modal arrival times for different frequencies the inverted seabottom sound speed is used as a constraint condition to extract the seabottom sound attenuation by three methods the first method involves measuring the attenuation coefficients of normal modes in the second method the seabottom sound attenuation is estimated by minimizing the difference between the theoretical and measured modal amplitude ratios the third method is based on finding the best match between the

miocene spreading centre south of isla guadalupe the juan de fuca rivera cocos and nazca plates in the eastern pacific are small fragments remaining from the fragmentation and partial subduction of the long narrow farallon plate which began as early as 55 myr ago1 this fragmentation apparently resulted from interaction of the very irregular pacific farallon ridge with the uneven coastlines of the american plate1 4 and involved the formation of ridge trench transforms5 creation of new spreading centres6 ridge jumps1 7 8 deactivation of subduction zones2 and other phenomena1 we present here magnetic evidence that a fifth small fragment of the farallon plate is preserved in the eastern pacific off the coast of baja california its preservation is the result of an eastward jump of the pacific farallon ridge 12 5 myr ago which was preceded by asymmetric spreading and perhaps pivotal subduction1 menard1 first suggested this idea and showed that anomaly 5b is symmetric and parallel to a linear trough trending southwards from guadalupe island he named the small fragment of the farallon plate preserved between this trough and the margin of baja california to the east the guadalupe plate1 we show that both anomalies 5b and 5c symmetrically flank the guadalupe trough our results and conclusions are not based on new magnetic data but rather on reinterpretation of aeromagnetic profiles published by taylor et al 9

rivera ocean seismic experiment rose overview the rivera ocean seismic experiment rose was designed as a combined sea and land seismic program to utilize both explosive and earthquakes to study a number of features of the structure and evolution of a mid ocean ridge a major oceanic fracture zone and the transition region between ocean and continent the primary region selected for the experiment included the rivera fracture zone the crest and eastern flank of the east pacific north of the rivera and adjacent areas of baja california and mainland mexico the experiment included 1 study of the east pacific rise south of the orozco fracture zone primarily using ocean bottom recording and explosive sources 2 a seismicity program at the orozco and 3 a land based program of recording natural events along the coastal region of mexico a considerable amount of useful data was obtained in each of the three subprograms in the marine parts of the experiment we were able to address a variety of problems including structure and evolution of young oceanic crust and mantle structure and dynamics of the east pacific rise seismicity of the orozco fracture zone and partitioning of energy transmission between the ocean volume and the crust lithosphere on land the fortuitousmore occurrence of the petatlan m7 6 earthquake of march 14 1979 permitted the acquisition of an excellent data set of foreshocks and aftershocks of this large event which provide new insight into the filling of a major seismic gap in the region this overview describes the scientific rationale and the design of the experiments along with some general results less

kinematic evolution of the northern cocos plate as the east pacific rise approached and impinged upon the western margin of north america the farallon plate began to break into multiple smaller plates whose relative movements changed strongly from those before breakup movements of the northern cocos plate inferred from magnetic lineations show rapidly changing rates and pole positions which may be characteristic of other small remnants of the farallon plate finite difference poles and rates for cocos pacific relative motion during the time intervals 2 5 5 7 7 10 and 10 14 m y have been determined from the geometry of magnetic isochrons prior to about 2 m y the cocos pacific pole was 10 to 20 south of its present location and angular rates were 2 4 times the present rate from 10 7 m y we deduce a large degree of oblique convergence between the cocos and north american plates which may be an important constraint on the evolution of the structure of the continental margin of southwestern mexico the tehuantepec ridge originated as the ridge ridge transform which generated the clipperton fracture zone spreading rate estimates for the southern part of the cocos plate suggest the possibility that the tehuantepec ridge was reactivated as a ridge trench transform between the two cocos plate fragments prior to about 7 m y

the a d 1567 mw 7 2 ameca jalisco earthquake western trans mexican volcanic belt surface rupture parameters seismogeological effects and macroseismic intensities from historical sources the a d 1567 ameca jalisco earthquake west central mexico is the strongest known historical crustal earthquake in the trans mexican volcanic belt an active volcanic arc being deformed by an intra arc extensional fault network the location of the surface rupture at the base of the sierra de ameca mountain range its length 13 14 leagues 54 59 km and the scarp height 1 2 estados 1 67 3 34 m are inferred from the description of this earthquake in the relacion de ameca of 1579 which contains what is probably the earliest documentation of an earthquake surface rupture in the americas from scaling relations the moment magnitude of this earthquake can be estimated as 7 2 0 3 based on the rupture length the a d 1567 ameca earthquake is likely to have ruptured to the surface along two or all three major segments of the ameca san marcos normal fault system online material excerpts from five historical documents related to the 1567 ameca earthquake

geology of the northern colima and zacoalco grabens southwest mexico late cenozoic rifting in the mexican volcanic belt the south trending northern colima graben intersects two other grabens the northwest trending zacoalco graben and the east trending chapala graben to form a rift rift rift triple junction 50 km south southwest of guadalajara in southwest mexico the northern colima graben represents the northernmost extension of the 190 km long 20 to 65 km wide colima rift which overlies the projection of the subducting boundary of the rivera and cocos plates and represents a rift of probable pliocene pleistocene age of the east trending mexican volcanic belt mvb associated with the northernmost 90 km of the rift there are 4 6 m y old to holocene alkaline volcanic rocks rarely found in continental volcanic arc environments and which are more typically associated with either continental intraplate rifting or oceanic islands these alkaline lavas have erupted side by side with 10 m y old to holocene calc alkaline lavas that represent typical arc volcanism it is proposed that the grabens are associated with the initiation of a rifting of a portion of southwest mexico away from the mexican mainland similar to the rifting of baja california away from the mexican mainland the calc alkaline k ar dates are consistent with post mid miocene development of east trending mvb volcanism

quaternary intra arc extension in the central trans mexican volcanic belt the trans mexican volcanic belt is an active volcanic arc related to subduction along the middle america trench and characterized by shallow seismicity and synvolcanic to postvolcanic extensional arc parallel faulting major intra arc basins within the central part of the belt between long 998w and 1028w are from west to east the cuitzeo and acambay grabens the aljibes half graben and the mezquital graben in this region 100 eastwest2striking 2 km long steeply dipping normal faults expressed by pronounced multi event fault scarps have been mapped of the 100 faults 65 displace rocks of known quaternary age younger than 1 6 ma and 22 cut rocks with a documented age younger than 750 ka known historical surface ruptures are limited to faults of the acambay graben overall the faults cause north south to north northwest2south southeast2oriented extension of 1 km which is distributed over a 30250 km wide zone that has 5210 faults in cross section and a relative extension of 3 quaternary vertical slip rates estimated for 13 of the faults have a mean of 0 07 mm yr they are highest at the southern margin of the cuitzeo graben the northern margin of the acambay graben and in the aljibes half graben where they measure between 0 16 and 0 18 mm yr the north south to north northwest2south southeast2oriented quaternary bulk extension rate of the system is likely to be 0 2 6 0 05 mm yr this fault system is in an initial stage of coalescence its western and eastern parts consist mostly of isolated fault segments whereas in the central part deformation is localized onto a few through going faults the longest of these are the venta de bravo fault 45 km and the acambay tixmadeje fault 34 km the short fault traces have a simple structure whereas the longer ones are commonly composed of two or more segments there is no obvious migratory pattern of quaternary fault activity which suggests that the entire region is tectonically active

overview of recent coastal tectonic deformation in the mexican subduction zone holocene and pleistocene tectonic deformation of the coast in the mexico subudction margin is recorded by geomorphic and stratigraphic markers we document the spatial and temporal variability of active deformation on the coastal mexican subduction margin pleistocene uplift rates are estimated using wave cut platforms at ca 0 7 0 9 m ka on the jalisco block coast rivera north america tectonic plate boundary we examine reported measurements from marine notches and shoreline angle elevations in conjunction with their radiocarbon ages that indicate surface uplift rates increasing during the holocene up to ca 3 0 5 m ka in contrast steady rates of uplift ca 0 5 1 0 m ka in the pleistocene and holocene characterize the michoacan coastal sector south of el gordo graben and north of the orozco fracture zone ofz incorporated within the cocos north america plate boundary significantly higher rates of surface uplift ca 7 m ka across the ofz subduction may reflect the roughness of subducting plate absence of preserved marine terraces on the coastal sector across el gordo graben likely reflects slow uplift or coastal subsidence stratigraphic markers and their radiocarbon ages show late holocene ca last 6 ka bp coastal subsidence on the guerrero gap sector in agreement with a landscape barren of marine terraces and with archeological evidence of coastal subsidence temporal and spatial variability in recent deformation rates on the mexican pacific coast may be due to differences in tectonic regimes and to localized processes related to subduction such as crustal faults subduction erosion and underplating of subducted materials under the southern mexico continental margin

the dynamic history of the trans mexican volcanic belt and the mexico subduction zone abstract the trans mexican volcanic belt tmvb is a 1000 km long neogene continental arc showing a large variation in composition and volcanic style and an intra arc extensional tectonics it overlies the rivera and cocos slabs which display marked changes in geometry geophysical studies indicate that lithospheric mantle is very thin or absent beneath the forearc and arc the fluids from the slab are released in a 40 to 100 km wide belt beneath the frontal part of the arc and the lower crust beneath the arc is partially molten east of 101 w the tmvb is built on a precambrian to paleozoic crust with thickness of 50 55 km west of 101 w the tmvb is underlain by jurassic to cenozoic marine and continental arcs with a 35 40 km thick crust the evolution of the tmvb occurred in four stages 1 from 20 to 10 ma the initial andesitic arc moved inland showing progressively drier melting and eventually slab melting suggesting flattening of the subducted slab 2 since 11 ma a pulse of mafic volcanism migrated from west to east reaching the gulf of mexico by 7 ma this mafic lavas marks the lateral propagation of a slab tear triggered by cessation of subduction beneath baja california 3 thereafter the volcanic front started moving trenchward with a marked phase of silicic volcanism between 7 5 and 3 ma local emplacement of small volume intraplate like basalts since 5 ma and development of extensional faulting these features are related to slab rollback enhancing asthenophere flux into the mantle wedge and promoting partial melting of the crust 4 the modern arc consists of a frontal belt dominated by flux and slab melting and a rear belt characterized by more differentiated rocks or by mafic lavas with little or no evidence of subduction fluids but higher asthenosphere fingerprint

structural architecture of the colima volcanic complex 1 the colima volcanic complex cvc is currently the most active mexican volcano and is located in the western sector of the trans mexican volcanic belt inside the active colima rift a regional n s striking extensional structure the colima rift is filled by a 1 km thick sequence of quaternary lacustrine sediments alluvium and colluvium mostly underling the about 3000 m thick volcanic pile of the cvc in this work we present the results of a detailed morphostructural and field study of quaternary faults and fractures in the cvc and the surrounding area including the regional structures of the colima rift we also present a geometrical modeling of the faults inside the volcano and a numerical model of the gravity induced stress and strain fields of the cvc the study attempts to characterize the geometry kinematics and dynamics of the deformation features of the cvc and relate it with the volcano structure the geology of the substratum and the geodynamic setting of the region our model considers that the observed deformation of the cvc and the surroundings results from the interplay between the active n s trending regional extensional tectonics and the southward spreading of the volcano over its basement forming an e w oriented volcanotectonic graben the interaction between regional tectonics and previously unrecognized volcanic spreading can control magma migration and flank instability in an area where eruptions and sector failures represent a potential high risk for more than 500 000 people

self similar clustering of cinder cones and crust thickness in the michoacan guanajuato and sierra de chichinautzin volcanic fields trans mexican volcanic belt abstract the spatial clustering of basaltic vents in monogenetic volcanic fields has been used as a proxy for crustal thickness in extensional and back arc tectonic settings the basaltic vents have a fractal clustered distribution self similar clustering described by a power law the power law is defined over a range the size range of the distribution of values in this case the vents separation delimited by a lower and an upper cut offs here we apply the fractal clustering analysis to the two largest monogenetic volcanic fields of the trans mexican volcanic belt tmvb a continental arc built on different crustal terranes the michoacan guanajuato volcanic field mgvf located in the central western tmvb includes over 1000 vents of late pliocene to quaternary age built on attenuated crust of mesozoic to tertiary age the sierra de chichinautzin volcanic field scvf in the central eastern tmvb is composed of 220 late pleistocene to holocene vents laying above thicker crust of precambrian to tertiary age monogenetic vents in both volcanic fields show self similar clustering with fractal exponent d 1 67 in the range 1 3 38 km mgvf and d 1 56 in the range 1 5 32 km scvf the upper cut off uco for the power law distribution of the mgvf well fits the crustal thickness below the volcanic field as derived from independent geophysical data the uco value of scvf indicates a crust thickness of about 32 km this value is in agreement with new geophysical data that indicate magma underplating the crust beneath the volcanic field area

geoacoustic inversion using low frequency broadband acoustic measurements from l shaped arrays in the shallow water 2006 experiment when using geoacoustic inversion methods one objective function may not result in a unique solution of the inversion problem because of the ambiguity among the unknown parameters this paper utilizes acoustic normal mode dispersion curves mode shapes and modal based longitudinal horizontal coherence to define a three objective optimization problem for geoacoustic parameter estimation this inversion scheme is applied to long range combustive sound source data obtained from l shaped arrays deployed on the new jersey continental shelf in the summer of 2006 based on the sub bottom layering structure from the compressed high intensity radiated pulse reflection survey at the experimental site a two layer sand ridge overlaying a half space basement range independent sediment model is utilized the ambiguities of the sound speed density and depth of the sand ridge layer are partially removed by minimizing these objective functions the inverted seabed sound speed over a frequency range of 15 170 hz is comparable to the ones from direct measurements and other inversion methods in the same general area the inverted seabed attenuation shows a nonlinear frequency dependence expressed as b 0 26f1 55 db m from 50 to 500 hz or b 0 32f1 65 db m from 50 to 250 hz where f is in khz

active rifting in southwestern mexico manifestations of an incipient eastward spreading ridge jump a major ongoing episode of continental rifting has formed a system of three intersecting grabens in the western mexican volcanic belt these structures may mark the initial stage of an eastward spreading ridge jump beneath continental crust eastward ridge segment jumps have been propagating northward along this part of the east pacific rise for the past 12 m y but this is the first plate boundary reorganization event to involve continental crust since the separation of baja california in the miocene since pliocene time small volumes of unusual alkaline magmas have erupted in the graben areas in close association with the more abundant subduction related calc alkaline magmas erupted from andesitic stratovolcanoes the alkaline magmas are varieties found in zones of active rifting elsewhere in the world

geomorphic tectonic control of sediment discharge to the ocean the importance of small mountainous rivers analysis of data from 280 rivers discharging to the ocean indicates that sediment loads yields are a log linear function of basin area and maximum elevation of the river basin other factors controlling sediment discharge e g climate runoff appear to have secondary importance a notable exception is the influence of human activity climate and geology on the rivers draining southern asia and oceania sediment fluxes from small mountainous rivers many of which discharge directly onto active margins e g western south and north america and most high standing oceanic islands have been greatly underestimated in previous global sediment budgets perhaps by as much as a factor of three in contrast sediment fluxes to the ocean from large rivers nearly all of which discharge onto passive margins or marginal seas have been overestimated as some of the sediment load is subaerially sequestered in subsiding deltas before the proliferation of dam construction in the latter half of this century rivers

northern east pacific rise magnetic anomaly and bathymetric framework the oceanic crust in the eastern pacific between 7on and 30on and east of 127ow contains a fairly complete history of the spreading centers associated with the east pacific rise since 25 m y b p late oligocene in this paper we have summarized the seafloor spreading magnetic anomaly data and the bathymetric data that reflect the record of this tectonic history the well defined magnetic lineations north of the clarion fracture zone in the mouth of the gulf of california and on the east flank of the east pacific rise epr are carefully examined and used to provide a guide for interpreting the spreading pattern between the clarion and clipperton fracture zones southward of the rivera fracture zone over the mathematician ridge and over the entire epr east of the mathematician ridge between the rivera and siqueiros fracture zones the bathymetric data provide a trace of the fracture zone pattern in each of the above mentioned areas the fracture zone bathymetry and the seafloor spreading magnetic lineations on the epr south of the rivera fracture zone have a distinctive fanning pattern caused by close poles of rotation and plate boundary reorganizations all these data provide a good record of the plate reorganizations in the middle miocene at magnetic anomaly 5a time 12 5 to 11 m y b p in the late miocene at magnetic anomaly 3 4 time 6 5 m y b p and in the pliocene at magnetic anomaly 2 3 time 3 5 m y b p several abandoned spreading centers including the mathematician ridge were left behind as a result of these reorganizations the mathematician ridge is shown to be a set of ridges and trough whose origin is related to the tectonic activity associated with each of the above mentioned reorganizations since anomaly 5a

the colima mexico earthquake mw 5 3 of 7 march 2000 seismic activity along the southern colima rift the 7 march 2000 earthquake m w 5 3 located within the southern colima rift in southwestern mexico is the largest crustal earthquake ever recorded within the state of colima mexico the mainshock was located at 8 km depth and the aftershocks are distributed at depths between 5 and 10 km aftershock distribution follows a normal decay as measured by the modified omori s law with a p value of 0 96 both aftershock locations and focal mechanisms determined for the mainshock and some of the aftershocks delineate a fault dipping toward the northwest and striking west southwest this fault segment is parallel to the southern end of the tamazula fault which ends at manzanillo bay regional waveform modeling for the mainshock situates the earthquake on a normal fault with a significant left lateral strike slip component on a plane striking 260 and dipping toward the northwest at 46 we use a small aftershock with similar focal parameters as the mainshock to deconvolve p and s waves recorded at the permanent station coig from the deconvolution we obtained a source duration of 0 75 sec from the p waves and 0 9 sec from the s waves for the main event and a stress drop of 320 bars the aftershock area is much larger than the area measured from the apparent source time function from the aftershock area and the seismic moment obtained by harvard university centroid moment tensor catalog we compute a static stress drop of 20 bars fault plane solutions from the mainshock and some of the aftershocks indicate that the northern and larger segment of the tamazula fault is perpendicular to the extensional axis directed east southeast this study confirms the change in the direction of the extensional axis of the southern colima rift with respect to the northern colima rift extensional axis oriented almost east west here we show that the tamazula fault zone at least its eastern segment is active and a re evaluation of the seismic hazard posed by this fault to the state of colima and the cities of colima and manzanillo is urgent manuscript received 13 september 2002

a pliocene ignimbrite flare up along the tepic zacoalco rift evidence for the initial stages of rifting between the jalisco block mexico and north america the tepic zacoalco rift a nw trending corridor 50 250 km is one arm of a triple rift system in western mexico together with the colima rift and the middle america trench it bounds the jalisco block a portion of western mexico that may be moving independently of north america the predominant basement rock types in the tepiczacoalco rift are rhyolitic ash fl ow tuffs and lavas which were previously assumed to be oligocene miocene in age related to the sierra madre occidental volcanic province or older new 40 ar 39 ar dates on 41 volcanic samples reveal a previously unrecognized voluminous fl are up of rhyolitic ignimbrites between 5 and 3 ma throughout the entire corridor of the tepic zacoalco rift they are often associated with pliocene high ti basalts the eruption rate during this pliocene time period was an order of magnitude higher hundreds of m m y than that documented in the tepic zacoalco rift over the last 1 m y the pliocene ash fl ow tuffs have been faulted along nw trending lineaments producing vertical offsets up to at least 500 m the voluminous ignimbrite fl are up in the tepiczacoalco rift at 5 3 ma may refl ect the initial stages of rifting of the jalisco block away from north america analogous to what occurred in the proto gulf region at 12 6 ma prior to the transfer of baja california from north america to the pacifi c plate additionally new 40 ar 39 ar dates show that the sierra madre occidental volcanic province extends across the entire width of the tepic zacoalco rift and terminates abruptly at the northern boundary of the jalisco block near the rio ameca in contrast paleoceneeocene basement from the jalisco block extends northward into the tepic zacoalco rift where it is locally overlain by sierra madre occidental rhyolites

evaluation and analysis of surface deformation in west chapala basin central mexico vertical deformation was measured at 14 benchmarks within the urban area of jocotepec mexico using first order leveling methods and then spatially analyzed in relation to land subsidence and soil discontinuity patterns the study area is located within the western portion of the chapala basin middle west mexico observations of vertical surficial deformation were made at each benchmark september and november 2012 relative to a fixed station and compared to an initial survey of each benchmark april 2012 results indicate that a maximum subsidence of 7 16 cm over the 8 month measurement interval occurs near downtown coincident with the largest levels of drawdown and translates to a maximum subsidence rate of 0 89 cm month for the sampling period two benchmarks located northwest and southeast of the urban area exhibited uplift of 2 8 and 0 76 cm respectively suggesting a complex mechanical response between the sedimentary soil units and the factors causing deformation a potential spatial relationship exists between subsidence patterns and soil discontinuities four separate cones of groundwater depression were observed with two being coincident with subsidence bowls downtown and south of the urban area however there is no clear relationship between drawdown and subsidence in the remaining areas hydrogeologic reconstructions reveal alternating sequences of alluvial aquifers and highly deformable lacustrine aquitards an analysis of the soil discontinuities reveals that they are directly aligned with the patterns of vertical deformation

morphodynamics of a high discharge tropical delta san juan river pacific coast of colombia abstract the san juan river has one of the most extensive and best developed deltas on the pacific coast of south america measuring 800 km 2 the river drainage basin measures 16 465 km 2 and is located in one of the areas with the highest precipitation in the western hemisphere the annual rainfall varies from 7000 to 11 000 mm and as a result the san juan river has the highest water discharge 2550 m 3 s 1 sediment load 16 10 6 t yr 1 and basin wide sediment yield 1150 t km 2 yr 1 on the west coast of south america the san juan delta growth began approximately 5000 years bp the structure of the delta is determined by the interactions between fluvial deposition and the effect of 1 7 m significant swells mostly from the sw and strong tidal currents analysis of delta progradation indicates that during 1848 1992 the morphology of the delta was characterized by beach ridge accretion spit growth narrowing of inlets and a general advance of the delta shoreline during the past decade processes such as rapid erosion of the delta shore narrowing of barrier islands and breaching of a new inlet are the result of a long term relative sea level rise of 2 6 mm yr 1 due to tectonically induced subsidence coupled with a eustatic rise of sea level the delta also experiences strong oceanographic manifestations associated with the el nino la nina cycle causing regional sea level elevation of 20 30 cm during el nino years recent coastal subsidence in the delta is evidenced by 1 increased occurrence of non storm washover events 2 increased erosion of barrier islands with average loss of 11 m yr 1 during 1993 1997 and 3 a relative sea level rise of 3 4 mm yr 1 during 1991 1999 the morphology and recent evolution of the san juan delta are unique when compared to other deltas of south america because of the singular combination of extreme climatic geologic and oceanographic conditions under which the delta has formed and the absence of human induced impact in the drainage basin

high bedrock incision rates in the atenguillo river valley jalisco western mexico lava flows from three basaltic shield volcanoes preserve pliocene pleistocene river levels in the atenguillo river basin western mexico k ar dates of these basalt flows together with present and palaeoriver levels allow calculation of bedrock incision rates at three points along the length of the atenguillo river at volcan la laja dated at 0 65 ma incision rates are 25 cm ka 1 at volcan la cienega 2 2 ma incision rates are 23 cm ka 1 and at volcan el vigia 2 7 ma incision rates are 23 cm ka 1 these high incision rates as well as two distinct knickpoints along the profile of the atenguillo river are related to a base level c hange at the northern end of the basin the dynamics of this river basin are controlled by the ongoing process of continental fragmentation associated with the opening of the gulf of california 1997 by john wiley sons ltd

sediment yield along the andes continental budget regional variations and comparisons with other basins from orogenic mountain belts abstract we assess the sediment yield at 119 gauging stations distributed from colombia to patagonia covering the different morphotectonic and morphoclimatic settings of the andes the most productive areas are the meta river basin within the northern andes and the bolivian and northern argentina chaco systems which produce an average of 3345 4909 and 2654 t km2 y 1 of sediment respectively the rivers of the northern and central andes excluding the pacific watersheds of peru northern chile and central argentina have a weighted mean sediment yield of 2045 t km 2 y 1 and produce 2 25 gty 1 of total sediment a major constraint estimating the andean continental budget of sediment yield lies in the lack of gauging data for the peruvian region using the available gauge stations the regional sediment yield appears underestimated assuming a higher value of sediment yield for the peruvian andes the total budget for the whole central andes could range between 2 57 gt y 1 and 3 44 gt y 1 a minimum of 0 55 gt y 1 and a probable maximum of 1 74 gt y 1 of sediment are deposited in the intramontane and surrounding proximal sedimentary basins the magnitude of sediment yield in the andes is comparable to other rivers draining orogenic belts around the world

normal mode measurements and remote sensing of sea bottom sound velocity and attenuation in shallow water the dispersion analysis of explosive signals and measurements of individual mode attenuation in the yellow sea is reported by adjusting input parameters of a normal mode computer program and making the calculated results coincide with field data the compressional wave velocity and attenuation in the sediment for the frequency range of 80 800 hz is simultaneously deduced the sea bottom velocity attenuation and its frequency dependence probed from two sea areas with a different depth and sound velocity profile in the summer and the winter are very close resultant sea bottom attenuation approaches the results of earlier workers at the upper end of the frequency band and joins with the data obtained by others at the lower end and the absorption appears to have a nonlinear frequency dependence 0 37 f1 84 db m khz the errors due to the change of environmental parameters such as water depth water sound velocity profile sediment density and seafloor roughness are discussed

channel incision in the rio atenguillo jalisco mexico defined by 36cl measurements of bedrock abstract the jalisco block of western mexico has undergone uplift in the tertiary in response to subduction tectonics the atenguillo river interior to the jalisco block has incised the bedrock units of the jalisco block including cretaceous ash flow tuff granite as well as younger volcanic lava flows to study incision rates knickpoint propagation and the uplift rate of the region samples from nine different points along the atenguillo channel were collected for 36 cl exposure age dating exposure ages along the length of the channel are high near the head of the basin where the river meanders in a broad plain ages are younger at the knickpoints and downstream where the river cuts deeply and forms a canyon incision rates measured at different points along the channel also correlate with channel slope and age low incision rates are measured in the oldest part of the channel near the head station b 2 5 mm year and just above the knickpoints station c 0 6 mm year and station g 1 4 mm year the incision rates are highest just below the knickpoints station f 2 4 mm year and farther downstream stations h and i 5 6 and 2 9 mm year respectively the high incision rates can be attributed to subduction related uplift of the jalisco block other features attributable to uplift are emergent paleoshorelines and uplifted marine sediments along the coast and relatively lower incision rates were measured north of the jalisco block

near global validation of the srtm dem using satellite radar altimetry abstract this paper reports the results of a near global validation of the srtm dem dataset using a unique database of completely independent height measurements derived from satellite altimeter echoes primarily gathered by ers 1 these heights are obtained using a rule based expert system which identifies each echo as 1 of 11 different characteristic shapes and selects the optimal retracking algorithm to obtain best range to surface the results of this comparison which includes over 54 million altimeter derived heights show generally very good agreement with the srtm data with global statistics for mean difference of 3 m and a standard deviation of 16 m quantitative validation results are given for each continent and are summarised here mean difference m standard deviation of differences m africa 1 86 15 62 australia 1 09 11 49 eurasia 2 54 16 09 north america 3 15 15 18 south america 12 22 18 51 global 3 60 16 16 a range of examples are included to illustrate key findings of this work and to indicate synergistic applications of these two datasets currently being studied including the potential for forest canopy measurement

water discharge and sediment load from the western slopes of the colombian andes with focus on rio san juan abstract small rivers draining high rainfall basins and mountainous terrain west of the cordilleras in south america have disproportionately high water discharge and sediment load fifteen rivers in western colombia discharge a combined 254 km3 yr 1 or 8020 m3 s 1 of water into the pacific sediment yield is strongly correlated with basin area documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape r 2 0 97 end document and sediment load is correlated with water discharge documentclass aastex usepackage amsbsy usepackag

paleoseismology of the 1912 acambay earthquake and the acambay tixmadeje fault trans mexican volcanic belt the acambay graben is a major intra arc basin of the trans mexican volcanic belt trenching studies at four sites along the acambay tixmadeje fault reveal evidence for the 1912 acambay earthquake and at least four late pleistocene and holocene ground rupturing earthquakes with similar displacements to the 1912 event evidence for multiple events was recognized by a combination of upward terminating faults fissure fills erosionally truncated fissures colluvial wedge fills and downward increasing separation of stratigraphic units sites along the central part of the ese wnw trending rangefront fault indicates large dip slip separations across a multi event scarp in holocene colluvium with a penultimate event that occurred since 5230 calibrated years before present cal yr b p on the huapango plain near the eastern end of the 1912 rupture where slip was oblique the penultimate earthquake occurred after 4700 cal yr b p and there is evidence for at least four more earthquakes in the last 34 000 cal yr b p three dimensional excavation of lacustrine and channel deposits at las lomas yields a left normal slip vector of s40 5 e with 105 10 cm of slip for the last three events there this slip vector is consistent with nne oriented extension across the graben the amount and style of slip documented for prehistoric ruptures are also consistent with the 1912 rupture with a mean of 60 cm event of dip slip on the rangefront and a mean of 35 cm event of oblique slip on the n48 w trending huapango plain assuming these values represent a characteristic behavior yields an average recurrence interval of 3600 years for the last four large earthquakes since 11 570 cal yr b p and a slip rate of 0 17 mm yr across the rangefront empirical earthquake parameters calculated from our paleoseismic data support instrumental measures indicating that the 1912 acambay earthquake was of m 6 8 7 0 the 1912 event is representative of ground rupturing earthquakes on the acambay tixmadeje fault and demonstrates that large seismic events m 7 can be generated by faults in the trans mexican volcanic belt and in other intra arc settings

geomorphology survive or subside deltas are among the most valuable coastal ecosystems but they are very dynamic and the factors that influence their health are complex the rate of compaction of underlying sediments might be a more significant factor than was thought

land subsidence by groundwater over exploitation from aquifers in tectonic valleys of central mexico a review abstract structurally controlled differential subsidence scds is the gradual sinking of the ground characterized by the development of a damage band terrain discontinuities and collapses aligned according to the strike of a controlling geological structure scds has been reported since the 1980s in several cities settled on tectonic valleys in central mexico although groundwater abstraction is the main trigger recent research efforts also point out a tectonic component as a driving force the monitoring and quantification of scds has been done through a variety of techniques such as extensometry gps and insar furthermore the associated hazards endangering the population are floods aquifer pollution cracking and housing collapse this paper presents a comprehensive review of the current state of scds allowing for the first time the standardization of its definition mechanisms and triggering factors additionally this helps to avoid misinterpretation in the cases of sinking produced by the mexico city subsidence type mcst and thus provides the elements for proper methodological study of scds finally the review includes future research directions that need to be improved in order to reduce the impact of the phenomenon

reply to comment on active crustal deformation in the trans mexican volcanic belt as evidenced by historical earthquakes during the last 450 years by g suarez et al

statistical examination of the existence and relative motion of the jalisco and southern mexico blocks the continental lithosphere of mexico south of the trans mexican volcanic belt may consist of two lithospheric blocks the jalisco and southern mexico blocks the existence of these two blocks can be examined employing the statistical f test formulated to test for the presence of significant misclosure around a plate circuit to plate motion data derived from marine magnetic anomaly lineations earthquake slip vectors and transform fault azimuths the result of the f test applied to the pacific cocos north american plate circuit indicates that there is no significant at the 99 confidence level misclosure around this plate circuit therefore if the southern mexico block exists its motion relative to the surrounding lithospheric plates is too small to be resolved from these data in contrast the result of the f test applied to the rivera pacific north american plate circuit indicates the presence of a significant misclosure about the rivera pacific north american plate circuit the cause of which is uncertain one interpretation of the misclosure around the rivera pacific north american plate circuit is that it is due to the presence of an independent jalisco block however it is more likely that this misclosure is instead primarily due to the effects of recent changes in the relative motion between the pacific and rivera plates assuming the second explanation is correct a new rivera north american euler pole location 21 8 n 110 4 w is determinable solely from earthquake slip vectors located along the rivera north american boundary

the memory of the accreting plate boundary and the continuity of fracture zones a detailed aeromagnetic anomaly map of the mesozoic seafloor spreading lineations southwest of bermuda reveals the dominant magnetic grain of the oceanic crust and the character of the accreting boundary at the time of crustal formation the magnetic anomaly pattern is that of a series of elongate lobes perpendicular to the fracture zone flowline trends the linear sets of magnetic anomaly peaks and troughs have narrow regions of reduced amplitude anomalies associated with the fracture zones during the period of mesozoic geomagnetic polarity reversals when 1200 km of central north atlantic seafloor formed the atlantic accreting boundary consisted of stationary elongate spreading center cells that maintained their independence even though sometimes only minor spatial offsets existed between cells normal oceanic crustal structure was formed in the spreading center cells but structural anomalies and discontinuities characteristic of fracture zones were formed at their boundaries which parallel flowlines of mesozoic relative plate motion in the central north atlantic we suggest that the memory for a stationary pattern of independent spreading center cells resides in the young brittle lithosphere at the accreting boundary where the lithosphere is weakest here each spreading center cell independently goes through its cylce of stress buildup stress release and crustal accretion after which its memory is refreshed the temporal offset between the peaks of the accretionary activity that takes place within each cell may provide the mechanism for maintaining the independence of adjacent spreading center cells through times when no spatial offset between the cells exists

assessing the effect of sea level change and human activities on a major delta on the pacific coast of northern south america the patia river abstract this paper presents the main physical and human induced stresses that have shaped the recent evolution of the patia river delta the largest and best developed delta on the western margin of south america during the holocene the patia delta moved southward and the northern part became an estuarine system characterized by large extensions of mangrove ecosystems however a major human induced water diversion starting in 1972 diverted the patia flow to the sanguianga river and shifted the active delta plain back to its former holocene location this discharge diversion has led to sediment starvation of the southern delta lobe and changed the northern estuarine system into an active delta plain in addition coastal areas of the patia delta subsided as a result of a devastating tsunami in 1979 morphological changes along the delta coast are evidenced by 1 coastal retreat along the whole delta front during the period 1986 2001 2 coastal retreat along the abandoned delta lobe for the period 2001 2008 56 of the southern delta shoreline is retreating and only 4 of the coast shows signs of accretion 3 progradation of the northern delta region during the period 2001 2008 the discharge diversion of the patia river to the sanquianga has apparently balanced the observed trends in coastal erosion and sea level rise 5 1 mm yr 1 for the period 1984 2006 after the 1979 tsunami 4 formation of transgressive barrier islands with exposed peat soils in the surf zone and 5 abandonment of former active distributaries in the southern delta plain with associated inlet closure in the northern delta lobe major geomorphic changes include 1 distributary channel accretion by morphological processes such as sedimentation also in crevasses overbank flow increasing width of levees inter distributary channel fill and colonization of pioneer mangrove 2 freshening conditions in the sanguianga distributary channel a hydrologic change that has shifted the upper estuarine region salinity max q min 3 spatial switch of delta distributaries related to tectonic movements and subsidence 4 a relative sea level rise of 5 1 mm yr 1 after the occurrence of the 1979 tsunami 5 episodes of sea level rise associated with the enso cycle and 6 human induced discharge diversion the information presented here is valuable evidence for understanding the role of extreme events versus normal conditions in creating and shaping deltas

an inverse solution for the depth and frequency dependent sediment attenuation of low frequency 20 320 hz compressional waves the mechanisms that control low frequency acoustic propagation and that are unique to the shallow water environment are resident in the seabed a method of determining the sediment attenuation of low frequency compressional waves as a function of frequency and sediment depth is examined the method utilizes the backus gilbert geophysical inverse theory and the emphasis is placed on assessing the resolution and error associated with the inferred structure and the experimental geometry rather than merely obtaining a solution that fits the simulated data a laterally homogeneous geoacoustic test model is used to generate artificial data for input to the inverse procedures the results provide insights regarding resolution and error for generically similar sediments and experimental geometries

steep subduction geometry of the rivera plate beneath the jalisco block in western mexico the morphology of the rivera plate subducted beneath the jalisco block in western mexico is determined from accurately located hypocenters of locally recorded microearthquakes and from earthquakes with magnitude mb 4 5 recorded at teleseismic distances the hypocenters of these latter earthquakes are relocated and for five of them the focal depth is constrained by the inversion of long period body waves the wadati benioff zone inferred from these data indicates a steep subduction of the rivera plate that resembles the geometry of subduction of the cocos plate beneath central america it is however very different from the shallower and almost subhorizontal subduction of the cocos plate observed in southern mexico southeast of this region the rivera plate is comparable to the juan de fuca plate in terms of the small areal extent young seafloor age low relative velocity and low teleseismic activity in the subduction zone this study shows that the dip of both the juan de fuca and rivera plates are similar once they are decoupled from the overriding continental crust the downgoing rivera plate initially starts with a dip of 10 down to a depth of 20 km and then increases gradually to a constant dip of 50 below a depth of 40 km intermediate depth seismicity is low in this zone associated with the subduction of the slow 2 cm yr and young 9 m y rivera plate the maximum depth extent of earthquakes observed in the rivera subduction zone is about 130 km the andesitic calc alkaline colima volcano appears to be directly related to the subduction of the rivera plate to the nw of this volcano the observed quaternary volcanism in the jalisco block which is parallel to the trench may also be explained by the subduction of the rivera plate

sediment flux from a mountain belt derived by landslide mapping in humid uplands landsliding is the dominant mass wasting process in the western southern alps of new zealand landslides are scale invariant and have a power law magnitude frequency distribution independent studies from other regions suggest that this is a general property of landsliding this observation is of critical importance to the evaluation of the impact of events of different length scales over different time intervals on landscape evolution it is particularly useful when estimating regional geomorphic rates because it constrains the frequency and overall significance of extreme events which cannot otherwise be evaluated by integrating the complete response of the system we estimate the regional denudation rate due to landsliding to be 9 4 mm yr 1 sediment discharge from the western southern alps is dominated by landslide derived material

human induced discharge diversion in a tropical delta and its environmental implications the patia river colombia summary the patia river the number one in terms of sediment yield 1500 t km 2 yr 1 draining the western south america has the most extensive and well developed delta on the pacific coast measuring 1700 km 2 during the holocene nature forced the patia delta to the south however a major water diversion starting in 1972 diverted the patia flow to the sanguianga river the latter a small stream draining internal lakes from the pacific lowlands this human induced discharge diversion shifted the active delta plain back to the north and changed the northern estuarine system into an active delta plain overall major environmental consequences of this discharge diversion in terms of morphological changes along the delta coast and distributary channels are evidenced by 1 coastal retreat along the abandoned delta lobe 63 of the southern shoreline is retreating at maximum rates of 7 m yr 1 with a corresponding coastal land loss of 106 m yr 1 2 transgressive barrier islands with exposed peat soils in the surf zone 3 abandonment of former active distributaries in the southern delta plain with associated closing of inlets and formation of ebb tidal deltas 4 breaching events on barrier islands and 5 distributary channel accretion in the northern delta plain by morphological processes such as sedimentation also in crevasses overbank flow increasing width of levees interdistributary channel fill and colonization of pioneer mangrove the sanguianga mangrove national park smnp the largest mangrove reserve in colombia measuring 800 km 2 lies in this former estuary where major hydrologic and sedimentation changes are occurring observed environmental changes in the smnp include 1 seaward advance of the sub aqueous delta front at the sanquianga inlet evidenced by an increase in tidal flat area from 5 4 mm 2 in 1986 to 14 mm 2 in 2001 2 freshening conditions in the sanguianga distributary channel a hydrologic change that has shifted the upper estuarine region salinity 1 for the 1984 2006 yr period after the devastating tsunami of 1979 in the patia catchment erosion rates have been more pronounced during the 1970 1980 and 1990 2000 decades as a result of land degradation and deforestation preliminary results indicate that relative resent anthropogenic influences on the patia river drainage basin have altered the deltaic environment and beyond significantly

active crustal deformation in the jalisco block mexico evidence for a great historical earthquake in the 16th century abstract on december 27th 1568 a large earthquake occurred to the southwest of guadalajara mexico near the northeastern corner of the jalisco block in an area where no great earthquakes have been reported before it caused heavy damage in the region where the colima and tepic zacoalco grabens intersect many churches houses and convents in the neighboring towns collapsed and severe deformation of the ground was observed in the area landslides apparently dammed the ameca river for several days and the opening of large cracks was reported in the lowlands the flow of natural springs and the level of lake zacoalco changed dramatically after the earthquake all of these reports strongly suggest that a local fault was the source of this large and destructive earthquake based on the intensity data inferred from the historical reports the 1568 event is perhaps the largest earthquake to date in the trans mexican volcanic belt compared to other well documented large earthquakes that occurred in the volcanic belt in 1875 1912 and 1920 the magnitude appears to be greater than 7 0 m w the jalisco block is presumed to be rifting away from the north american plate the colima and tepic zacoalco grabens which bound the jalisco block to the east and north respectively are apparently the boundaries where rifting is taking place in a complex and highly faulted environment based on the data available it is impossible to identify the specific fault ruptured during the earthquake unequivocally however the occurrence of this large event in 1568 confirms that active deformation is still taking place in the jalisco block the apparently long recurrence times of these large events suggest that tectonic deformation is slow

the acambay graben active intraarc extension in the trans mexican volcanic belt mexico the trans mexican volcanic belt is an active volcanic arc related to subduction along the middle america trench the central part of the belt is being deformed by the chapala tula fault zone an approximately 450 km long and 50 km wide zone of active extension the volcanic arc and the arc parallel chapala tula fault zone are superposed nearly perpendicularly on the preexisting stress and deformation province of the mexican basin and range the acambay graben about 40 km long and 15 km wide is located approximately 100 km northwest of mexico city and is one of the major troughs within the chapala tula fault zone the border faults of the acambay graben acambay tixmadeje in the north and pastores in the south are separated in the west by stepovers from range bounding faults of similar orientation epitacio huerta in the north and venta de bravo in the south the stepovers occur at the intersection of these faults with an older system of basin and range faults an early stage right lateral component of motion along the venta de bravo and pastores faults is inferred on a map scale from a left stepping en echelon array of normal fault segments the divergence of the en echelon segments from the general fault trend decreases gradually from west to east suggesting that the early extension was rotational the present relative displacement along the southern margin of the system on the other hand results in a left lateral strike slip component this is documented on a map scale from extension structures at left stepovers and on an outcrop scale from fault striations indicating left oblique slip the striations measured at the northern system margin indicate nearly pure extensional dip slip without a consistent lateral displacement component this is supported on a map scale by the structure of the right stepover between the acambay tixmadeje and epitacio huerta faults which shows no evidence of local extension or shortening the divergence between the present directions of motion at the southern and northern margins of the extended zone can be explained by a minor rotational deformation component with the pole of rotation being located to the east of the zone of deformation this could explain why no active extension has been observed to the east of the chapala tula fault zone in the eastern part of the trans mexican volcanic belt during the ms 6 9 acambay earthquake of november 19 1912 surface rupture occurred along both margins of the graben at the base of multiple event scarps along the acambay tixmadeje fault the coseismic rupture is 41 km long the vertical offset increases gradually from the eastern end of the surface rupture to its center where it is with 50 cm at a maximum furthermore the change in the vertical surface offset along the fault is approximately proportional to the change in height of the acambay tixmadeje multiple event fault scarp the easternmost part of the ground rupture passes through a plain and not at the base of a multiple event scarp as farther west it may therefore correspond partly to an increase in length of the acambay tixmadeje fault during the 1912 earthquake the slip rate along the southern border of the acambay graben can be estimated from the displacement and age of a basalt flow for which we have obtained a 40ar 39ar age of 0 4 0 1 ma this basalt may be displaced up to 15 m by the pastores fault which indicates a middle late quaternary slip rate of 0 04 mm yr furthermore based on a coseismic surface rupture of approximately 20 cm along this fault in the 1912 earthquake we estimate a recurrence interval of 5000 years for major earthquakes along the faults of the acambay graben

source characteristics of the 22 january 2003 m w 7 5 tecoman mexico earthquake new insights aftershock locations source parameters and slip distribution in the coupling zone between the overriding north american and subducted rivera and cocos plates were calculated for the 22 january 2003 tecoman earthquake aftershock locations lie north of the el gordo graben with a northwest southeast trend along the coast and superimposed on the rupture areas of the 1932 m w 8 2 and 1995 m w 8 0 earthquakes the tecoman earthquake ruptured the northwest sector of the colima gap however half of the gap remains unbroken the aftershock area has a rectangular shape of 42 2 by 56 2 km with a shallow dip of roughly 12 of the wadati benioff zone fault geometry calculated with the nab lek 1984 inversion procedure is strike dip rake 277 27 78 from the teleseimic body wave spectra and assuming a circular fault model we estimated source duration of 20 2 s a stress drop of 5 4 2 5 mpa and a seismic moment of 2 7 7 1020 nm the spatial slip distribution on the fault plane was estimated using new additional near field strong motion data 54 km from the epicenter we confirm their main conclusions however we found four zones of seismic moment release clearly separated one of them not well defined before is located toward the coast down dip this observation is the result of adding new data in the inversion we calculated a maximum slip of 3 2 m a source duration of 30 s and a seismic moment of 1 88 1020 nm

inversion of seabed attenuation using time warping of close range data an inversion scheme based on time warping is presented for estimating seabed sound attenuation from modal dispersion of close range single hydrophone data the dispersion information is extracted directly from the warped signal spectrum seabed sound speed and density are inverted from the modal group velocity curves and the attenuation is inverted from the normalized modal amplitudes the method is applied to experimental data collected in the yellow sea of china during the winter of 2002 the inverted sound speed and density are consistent with the sand silt clay sediment at the site and the attenuation is nonlinear over the frequency band from 125 500 hz

human impacts on headwater fluvial systems in the northern and central andes abstract south america delivers more freshwater runoff to the ocean per km2 land area than any other continent and much of that water enters the fluvial system from headwaters in the andes mountains this paper reviews ways in which human occupation of high mountain landscapes in the andes have affected the delivery of water and sediment to headwater river channels at local to regional scales for millennia and provides special focus on the vulnerability of paramo soils to human impact people have intentionally altered the fluvial system by damming rivers at a few strategic locations and more widely by withdrawing surface water primarily for irrigation unintended changes brought about by human activities are even more widespread and include forest clearance agriculture grazing road construction and urbanization which increase rates of rainfall runoff and accelerate processes of water erosion some excavations deliver more sediment to river channels by destabilizing slopes and triggering processes of mass movement the northern and central andes are more affected by human activity than most high mountain regions the wetter northern andes are also unusual for the very high water retention characteristics of paramo high elevation grass and shrub soils which cover most of the land above 3000 m paramo soils are important regulators of headwater hydrology but human activities that promote vegetation loss and drying cause them to lose water storage capacity new data from a case study in southern ecuador show very low bulk densities median 0 26 g cm 3 high organic matter contents median 43 and high water holding capacities 12 to 86 volumetrically these data document wetter soils under grass than under tree cover effects of human activity on the fluvial system are evident at local scales but difficult to discern at broader scales in the regional context of geomorphic adjustment to tectonic and volcanic processes

estimation of the epicentral area of the 1912 acambay earthquake m 6 9 mexico determined from the earthquake archaeological effects eae and the esi07 macroseismic scale abstract earthquake environmental effects describe the geological features of an earthquake including the surface faulting ground cracks soil liquefaction and landslides among others historical large earthquakes could be classified according to the eee macroseismic geological scale esi07 with the advantage that it is possible to compare different earthquakes which have occurred in different places and estimate the parameterization of their seismogenic sources hence we have determined the esi07 scale by the eee description of the geological effects observed during the 1912 acambay earthquake which took place in the central basin of mexico additionally we have estimated the epicentral area of this earthquake the total rupture length and the potential seismicity of the acambay tixmadeje fault zone furthermore we have estimated the earthquake archaeological effects eaes of the oriented damage to do this we have modelled the s wave travelling by assuming different epicentre locations at several active fault segments and thus we have compared with the oriented damage mapped by urbina and camacho 1913 at different villages therefore we propose a model of rupture of 1912 acambay earthquake releasing a m 7 earthquake and with the epicentre located at tixmadeje this rupture model implies earthquake directivity towards east and the s wave travelling towards south the total rupture length was estimated 33 km and the maximum coseismic offset 52 cm

seabed acoustics of a sand ridge on the new jersey continental shelf acoustic measurements were made on a sand ridge on the new jersey continental shelf data collected on two l arrays separated by 20km from a single multi frequency tow suggest small horizontal environmental variability values for the sound speed structure of the seabed are extracted by first applying a geo acoustic inversion method to broadband and narrowband acoustic data from short range sources then a parabolic equation algorithm is used to properly include the bathymetry and sub bottom layering finally the frequency dependence of the seabed attenuation is inferred by optimizing the model fit to long range transmission loss data in the 50 3000hz band

a geodynamical perspective on the subduction of cocos and rivera plates beneath mexico and central america abstract the middle america subduction zone masz is one of the world most complex convergent margins as it involves the subduction of the rivera and cocos young oceanic plates beneath the north american and caribbean plates and is bounded by the gulf of california rift and the panama slab window characterized by contorted and unusual slab geometry irregularly distributed seismicity and volcanism exceptionally large slow slip events sse and non volcanic tremors nvt this subduction system represents a great natural laboratory for better understanding geodynamic processes at a fundamental level based on a solid observational foundation and incorporating the latest experimental results into a coherent geodynamical framework we shed light on the main processes controlling the subduction system evolution in this region the tectonics volcanism slab geometry and segmentation along the margin are reviewed from a geodynamical perspective we proposed and discussed a series of evolutionary scenarios for the mexican and central american subduction zones providing a coherent starting base for future geodynamical modeling studies tailored to this active margin we discuss comparatively the recently discovered sses and nvts along the masz and try to differentiate among the proposed mechanisms responsible for these observations finally we discuss the recent seismic anisotropy observations in a geodynamic context offering an integrated view of mantle flow pattern along the entire active margin although the masz as a whole may be considered a fairly complicated region with many unusual features and sometimes controversial interpretations its complexity and unusual characteristics can improve our knowledge about the linkage between deep and surface processes associated with subduction zone dynamics

the sanfandila earthquake sequence of 1998 queretaro mexico activation of an undocumented fault in the northern edge of central trans mexican volcanic belt abstract a sequence of small earthquakes occurred in central mexico at the northern edge of the trans mexican volcanic belt tmvb in the state of queretaro during the first 3 months of 1998 medium to large events in the continental regime of central mexico are not common but the seismic history of the region demonstrates that faults there are capable of generating destructive events the sequence was analyzed using data from a temporary network with the goals of identifying the causative fault and its relation to regional tectonics employing a waveform inversion scheme adapted from a method used for regional studies we found that the source mechanisms conform to the style of faulting i e extension in the e w direction representative of the taxco san miguel allende fault system this system has been proposed as the southernmost extension of the basin and range br province the spatial distribution of hypocenters and source mechanisms indicate that the seismogenic segment was a fault with an azimuth of approximately 334 with almost pure dip slip since events which occurred just south from this region show features which are consistent with tmvb tectonics i e extension in an n s direction the sequence may mark the boundary between the tmvb and br stress domains

wave influence in the construction shaping and destruction of river deltas a review abstract waves are an important agent in the construction shaping and destruction of river deltas notwithstanding the commonality of waves in oceans and seas wave influence on deltas varies considerably depending on the coastal morphology and nearshore bathymetry although there have been advances in understanding the way waves approaching a delta shape its shoreline much still remains to be known of the interactions between waves and river deltas deltas are built essentially from sediments supplied by rivers sand sized and coarser sediments may also be derived from nearby abandoned delta lobes or from older relict nearshore deposits transported by wave reworking and longshore currents alternatively delta erosion by waves can also release sediment that is redistributed alongshore or that accumulates offshore the extent to which bedload is supplied to and sequestered in or lost by deltas through waves and longshore transport strongly depends on interactions between waves and fluvial discharge at the river mouth these interactions and the mutual adjustments they engender are not only important in the overall balance between delta retreat progradation or aggradation but also in processes such as avulsion and channel switching as well as in the eventual survival of a delta in the face of sea level rise where waves are important fluvial liquid discharge is high and sediment supply is rich in bedload two important aspects are the blocking of waves and longshore currents by strong river discharge and the formation of bars at the river mouth field studies of the complex interactive processes prevailing where river flows encounter waves are however non existent and numerical modelling though promising hampered by scale constraints and the difficulty of replicating them and generating mouth bars in the presence of longshore currents this interaction influences the seaward extent of the delta mouth protuberance and its stability this protuberance then forming the regional shoreline template to which waves and longshore currents adjust longshore currents can redistribute wave reworked mouth bar deposits emplaced during strong river flow transport may be either divergent from the mouth or may be regionally unidirectional but wherein the symmetry of some deltas probably rare may be maintained by a strong river blocking effect on transport from the updrift flank the mouth protuberance may be such as to foster transport reversal counter drift at the delta margins that contributes to sediment sequestering within the delta these interactions largely contribute in shaping delta shorelines and together with the abundance of sediment supply and grain size determine the resultant wave formed shoreline barrier types which include spits more or less closely spaced beach ridges and barrier islands and cheniers in situations of punctuated progradation or retreat where several distributary mouths occur pronounced longshore variability in wave processes and wave induced sediment transport may ensue resulting in multiple drift cells that assure the retention of sand and coarse grained sediments within the delta waves can also be an important agent in the reworking and retreat of mud rich deltas that generally conform in morphology to the river dominated such as the mississippi or tide dominated such as the ganges brahmaputra or chao phraya types resulting in the episodic formation of sandy cheniers and beach ridges although sea level rise is likely to lead to enhanced wave reworking of deltas the possible prevalence of aggradation in lieu of progradation channel switching and avulsion and washover processes may contribute to the disorganization of waves and longshore transport fostering deltaic sequestering of sand and coarser sized sediment and delta survival the weakening of river discharges resulting from human activities will invariably lead however to enhanced wave reworking of deltas and to deltaic sediment redistribution by longshore currents the massive swing towards significant reductions in fluvial sediment supply today may signify the ultimate demise of many deltas in the coming decades through a process of delta shoreline straightening by waves in addition to accelerated sinking these various foregoing aspects of the relationship between waves and river deltas are reviewed here across a range of timescales and new interaction concepts proposed using numerous examples of deltas in the world and on the basis of case studies conceptual studies and numerical modelling studies in the literature spanning more than forty years

response of a landscape to tectonics using channel steepness indices ksn and osl a case of study from the jalisco block western mexico abstract rapid tectonic uplift is known to increase river incision rates and to trigger the propagation of knickpoints along a river profile augmenting local relief thus the topography of mountainous landscapes records long term interactions between tectonics and climate we evaluate the geomorphic response to differential rock uplift with topographic analysis of 16 river basins including the normalized channel steepness index k sn our study focuses on rivers incising the northern and western sectors of the jalisco block western mexico a crustal unit bounded by plio quaternary extensional faulting we use osl signals from 17 riverbed samples to assess erosion rates the results indicate that basins in the northern sector have higher rates of rock uplift and erosion mean k sn 111 18 m 0 9 than those in the southwest mean k sn 77 23 m 0 9 in spite of the plio quaternary lavas that cap large areas of the northern sector the development of plio quaternary grabens and half grabens along the northern boundary of the jalisco block has increased the erosion rates augmenting the local relief by contrast the topography of the basins in the southwestern sector has resulted from a long erosional history controlled by the migration of old tectonic signals farther upstream in this region the landscape seems to have reached the steady state and is currently experiencing the propagation of knickpoints due to the increase in the rate of rock uplift in the coast of jalisco during the holocene

characteristics of seismicity in the coast and north of jalisco block mexico abstract several studies of local seismicity were carried out between 1996 and 1998 on the coast and north of the jalisco block jb using a portable seismic network consisting of five lennartz m88 seismographs with le 3d 1 hz seismometers the data from these studies complemented with data from resco colima seismic network is used to characterize three seismogenic zones in the region one seismicity zone located east of the middle america trench mat under the continental plate is associated with the interaction of the rivera and north america plates and can be characterized as a double seismic zone dsz with geometry suggesting a bending plate and probably oblique subduction this pattern is clearer south of the 20 n parallel a second zone of continental intraplate seismicity is located between ameca and amatlan de canas in the northeastern part of the study region where the source depths vary from very shallow to about 35 km the third zone is bahia de banderas where the stress pattern seems to be very complex here we observe different types of seismic events the first type consists of shallow events located in the southern part of the bay and a under continental area that seems to be associated with the interplate rivera north america activity a second event type with depths between 10 and 30 km seems to be related to features that cross the bay in an ew direction in this zone there is also a high shallow continental seismicity which agrees with continental topographic features

constraints on jalisco block motion and tectonics of the guadalajara triple junction from 1998 2001 campaign gps data a gps campaign network in the state of jalisco was occupied for 36 h per station most years between 1995 and 2005 we use data from 1998 2001 to investigate tectonic motion and interseismic deformation in the jalisco area with respect to the north america plate the twelve stations used in this analysis provide coverage of the jalisco block and adjacent north america plate and show a pattern of motion that implies some contribution to jalisco block boundary deformation from both tectonic motion and interseismic deformation due to the offshore 1995 earthquake the consistent direction and magnitude of station motion on the jalisco block with respect to the north america reference frame 2 mm year to the southwest 95 confidence level perhaps can be attributed to tectonic motion however some station velocities within and across the boundaries of the jalisco block are also non zero 95 confidence level and the overall pattern of station velocities indicates both viscoelastic response to the 1995 earthquake and partial coupling of the subduction interface together termed interseismic deformation our results show motion across the northern colima rift the eastern boundary of the jalisco block which is likely to be sinistral oblique extension rather than pure extension we constrain extension across both the colima rift and the northeastern boundary of the jalisco block the tepic zacoalco rift to 8 mm year 95 confidence level slow compared to relative rates of motion at nearby plate boundaries

inversion for sediment geoacoustic properties at the new england bight this article discusses inversions for bottom geoacoustic properties using broadband acoustic signals obtained from explosive sources two different inversion schemes for estimating the compressional wave speeds and attenuation are presented in this paper in addition to these sediment parameters source receiver range is also estimated using the arrival time data the experimental data used for the inversions are sus charge explosions acquired on a vertical hydrophone array during the shelf break primer experiment conducted south of new england in the middle atlantic bight in august 1996 the modal arrival times are extracted using a wavelet analysis in the first inversion scheme arrival times corresponding to various modes and frequencies from 10 to 200 hz are used for the inversion of compressional wave speeds a hybrid inversion scheme based on a genetic algorithm ga is used for the inversion in an earlier study potty et al j acoust soc am 108 3 973 986 2000 have used this hybrid scheme in a range independent environment in the present study results of range dependent inversions are presented the sound speeds in the water column and bathymetry are assumed range dependent whereas the sediment compressional wave speeds are assumed range independent the variations in the sound speeds in the water column are represented using empirical orthogonal functions eofs the replica fields corresponding to the unknown parameters were constructed using adiabatic theory in the second inversion scheme modal attenuation coefficients are calculated using modal amplitude ratios the ratios of the modal amplitudes are also calculated using time frequency diagrams a ga based inversion scheme is used for this search finally as a cross check the computed compressional wave speeds along with the modal arrival times were used to estimate the source receiver range the inverted sediment properties and ranges are seen to compare well with in situ measurements and historical data

hydrodynamic controls on sediment retention in an emerging diversion fed delta abstract the morphodynamics of river dominated deltas are largely controlled by the supply and retention of sediment within deltaic wetlands and the rate of relative sea level rise yet sediment budgets for deltas are often poorly constrained in the mississippi river delta a system rapidly losing land due to natural and anthropogenic causes restoration efforts seek to build new land through the use of river diversions at the davis pond freshwater diversion a new crevasse splay has emerged since construction was completed in 2002 here we use beryllium 7 activity in sediment cores and usgs measurements of discharge and turbidity to calculate seasonal sediment input deposition and retention within the vegetated davis pond receiving basin in winter spring 2015 which included an experimental period of high discharge through the diversion davis pond received 106 800 metric tons of sediment 44 of which was retained within the basin during this time mean flow velocity was 0 21 m s 1 and mean turbidity was 56 formazin nephelometric units fnu in summer fall 2015 the davis pond basin received 35 900 metric tons of sediment 81 of which was retained mean flow velocity in summer fall was 0 10 m s 1 and mean turbidity was 55 fnu the increase in sediment retention from winter spring 2015 to summer fall 2015 may be due in part to the corresponding drop in water flow velocity which allowed more sediment to settle out of suspension although high water discharge increases sediment input and deposition increased turbulence associated with higher current velocity appears to increase sediment throughput and thereby decrease the sediment trapping efficiency sediment retention in davis pond is on the high end of the range seen in deltaic wetlands perhaps due to the enclosed geometry of the receiving basin future diversion design and operation should target moderate water discharge and flow velocities in order to jointly maximize sediment deposition and retention and provide optimal conditions for delta growth

subduction of the eastern panama basin and seismotectonics of northwestern south america spreading along the cocos nazca plate boundary since the breakup of the farallon plate in the miocene has resulted in the formation of the panama basin and a complex interaction of plates in and near northwestern south america current plate boundaries have been defined and segments of subducted lithosphere identified through selection of hypocentral locations of earthquakes considering only well located events and through focal mechanism determinations the existence of relict plate boundaries bathymetric features and the panamanian isthmus has affected the subduction process of the nazca plate beneath south america and determined the present day configuration of the subducting lithospheric plate there is no single triple junction separating the caribbean south american and nazca plates instead the panamanian isthmus and surrounding areas are accommodating east west compression and a lesser degree of north south compression along a series of thrust faults striking nw to ne and the andean ranges of ecuador colombia and venezeula are moving as a block nne relative to the rest of the south american plates along a system of faults following the front of the eastern cordillera the subducted portions of the panama basin and old farallon plate have become segmented into three pieces recognized in this study from north to south they are 1 a bucaramanga segment continuous with the caribbean seafloor northwest of colombia 2 a cauca segment continuous with oceanic crust nazca plate currently being subducted beneath south america at the colombia ecuador trench and 3 an ecuador segment at the northern end of the subducted lithospheric plate which is dipping at a small angle to the east beneath northern peru the segmentation of the subducted plate can be explained by the buoyancy of bathymetric features which have been partially subducted