neotectonic faulting and forearc sliver motion along the atirro rio sucio fault system costa rica central america the atirro rio sucio fault system forms a major northwest trending strike slip fault zone in east central costa rica we examined the kinematics and temporal evolution of this fault system through geomorphic structural and seismologic analysis this 150 km long strike slip fault zone traverses the northern flank of the paleovolcanic cordillera de talamanca and extends northwestward into the active cordillera volcanica central historical seismicity includes frequent minor swarms and occasional moderate magnitude m 5 0 6 5 damaging earthquakes field geomorphic evidence fault kinematic data and earthquake focal mechanisms are consistent in showing dextral slip along the mapped traces of northwest striking faults continuity with other transcurrent faults in northwest costa rica indicates that the atirro rio sucio fault system may form the southeastern end of a regional network of northwest trending dextral faults that accommodate margin parallel displacement of the central american forearc sliver the atirro rio sucio fault system originates within the central costa rica deformed belt inboard of the indenting cocos ridge we infer that ridge collision drives lateral escape of crustal fragments northwestward along an array of dextral central costa rica deformed belt faults including the major structures of the atirro rio sucio fault system this zone of arc parallel extrusion thus represents the root of the central american forearc sliver consistent with recent geodynamic models we propose that northwestward sliver escape along the atirro rio sucio faults is driven by rigid indentation of the aseismic cocos ridge into southern costa rica

earthquake locations by 3 d finite difference travel times abstract we present a new method for locating earthquakes in a region with arbitrarily complex three dimensional velocity structure called quake3d our method searches a gridded volume and finds the global minimum travel time residual location within the volume any minimization criterion may be employed the l1 criterion which minimizes the sum of the absolute values of travel time residuals is especially useful when the station coverage is sparse and is more robust than the l2 criterion which minimizes the rms sum employed by most earthquake location programs on a unix workstation with 8 mbytes memory travel time grids of size 150 by 150 by 50 are reasonably employed with the actual geographic coverage dependent on the grid spacing location precision is finer than the grid spacing earthquake recordings at six stations in bear valley are located as an example using various layered and laterally varying velocity models locations with quake3d are nearly identical to hypoinverse locations when the same flat layered velocity model is used for the examples presented the computation time per event is approximately 4 times slower than hypoinverse but the computation time for quake3d is dependent only on the grid size and number of stations and independent of the velocity model complexity using quake3d with a laterally varying velocity model results in locations that are physically more plausible and statistically more precise compared to flat layered solutions the earthquakes are more closely aligned with the surface fault trace are more uniform in depth distribution and the event and station travel time residuals are much smaller hypocentral error bars computed by quake3d are more realistic in that the trade off of depth versus origin time is implicit in our error estimation but ignored by hypoinverse

a new look at evidence for a wadati benioff zone and active convergence at the north panama deformed belt the nature of the northern boundary of the panama microplate often referred to as the north panama deformed belt npdb has been the subject of much speculation previous studies have used a variety of data including teleseismic gravity bathymetric marine magnetic and field studies of uplift and tsunami deposits as well as modified mercalli intensity distributions from historic earthquakes to reveal the nature of the npdb data have been collected for over 30 yrs and yet the character of the npdb remains unclear current analyses and interpretation provide a number of mutually exclusive options and much controversy in this article we examine local and regional seismicity combined with teleseismic observations and historic earthquake data to present an alternative analysis for the npdb using small earthquakes recorded by a local network we image a well defined wadati benioff zone dipping southward beneath the panama microplate these data provide new evidence regarding the nature of panama microplate caribbean plate boundary kinematics and demonstrate the existence of an active subduction zone a more complete understanding of the nature of the seismicity and plate interactions along the npdb offshore panama including a subduction zone capable of producing earthquakes of m 7 should be included in future earthquake hazard assessments

gps measurements of crustal deformation associated with the 22 april 1991 valle de la estrella costa rica earthquake we present analysis results for global positioning system gps measurements made at sites in costa rica during two campaigns in february and july 1991 gps solutions for 5 sites limon liberia bratsi etcg and vueltas reveal significant horizontal and vertical displacements relative to their february positions horizontal displacements relative to liberia measured 244 7 0 8 89 2 0 9 12 4 1 3 and 1 9 0 9 cm at limon bratsi vueltas and etcg respectively vertical displacements relative to liberia measured 163 2 1 15 3 3 0 10 5 4 4 and 0 6 2 1 cm at limon bratsi vueltas and etcg respectively we find differences in the gps derived vertical and horizontal displacements compared to other types of geodetic measurements of uplift in the coastal region and their associated models to address these differences we compute a dislocation model which fits the gps measured displacements a simple uniform planar slip model can not reconcile the differences between the coastal uplift data or the seismic moment suggesting considerable complexity of the earthquake source

implications of the north and south panama thrust belts for the origin of the panama orocline high resolution imaging of the north nptb and south panama thrust belts using the seamarc ii seafloor mapping system and digital seismic reflection profiling provides an excellent data base with which to determine the kinematics of oroclinal deformation of panama remarkable continuity of structural orientations over 300 km along the nptb argues for widespread uniformity in direction of thrusting between panama and the caribbean plate westward decrease in the width and height of the accretionary wedge indicates a westward decrease in the total amount of convergence two major cross faults cut the wedge in its eastern part forming two minor blocks with different relative motions with respect to the caribbean frontal thrust orientations change across these faults major faults have been proposed to cut the isthmus of panama but simple cross isthmus faulting alone cannot explain the orientations of the major thrusts of the nptb bending of a flexible beam can explain much of the observed structure both directions and rates though the scale of the orocline precludes considering this as an elastic phenomenon plastic deformation of panama may be a useful mechanical explanation at least for eastern panama but consideration of plastic slip lines must include the northern andes as well changing from more rigid blocklike offsets on faults in western panama to a continuum of strain in eastern panama may be the best explanation for observations at present

finite difference calculation of traveltimes in three dimensions the traveltimes of first arriving seismic rays through most velocity structures can be computed rapidly on a three dimensional numerical grid by finite difference extrapolation head waves are properly treated and shadow zones are filled by the appropriate diffractions differences of less than 0 11 percent are found between the results of this technique and ray tracing for a complex but smooth model this scheme has proven useful for earthquake location and shows promise as an inexpensive well behaved substitute for ray tracing in forward modeling and kirchhoff inversion applications

vegetation zones and diagnostic pollen profiles of a coastal peat swamp bocas del toro panama abstract a survey of the dominant vegetative cover of a large domed coastal swamp near changuinola in the province of bocas del toro panama has been undertaken as an initial step in reconstructing the holocene history of peat accumulation on this coast seven phasic communities of peat forming vegetation are defined and mapped 1 rhizophora mangle mangrove swamp 2 mixed back mangrove swamp 3 raphia taedigera palm swamp 4 mixed forest swamp 5 campnosperma panamensis forest swamp 6 sawgrass stunted forest swamp 7 myrica cyrilla bog plain pollen extracted from surface peat samples and collected from dominant vegetation at representative sites is used to prepare a pollen profile of each phasic community these profiles are then compared to pollen distribution in 2 peat cores one from the deep central part of the deposit and the second from a site near the marine margin in order to construct a history by floral succession of the 4000 year evolution of the deposit the changuinola mire originated as freshwater palm and hardwood forest swamps that developed in close proximity to both the changuinola river mouth probably behind a barrier bar and freshwater lagoon system adjacent to a low energy mangrove dominated bay the early swamp was likely drained to the southeast by brackish blackwater creeks much as it is today and formerly extended considerably farther in the direction of almirante bay the palm swamp was succeeded by hardwood forest swamp dominated by a very limited number of specialized species only one of which campnosperma panamensis is prone to forming monospecific stands increasing accumulation of woody peat promoted by the everwet climate impeded drainage of the mire leading to doming increased oligotrophy and establishment of bog plain conditions in a manner similar to that described by the anderson model of succession in the coastal swamps of malaysia and indonesia development of the changuinola mire did not require the initial mangrove phase which is common to the peat swamps of southeast asia as the palm raphia taedigera is able to colonise and institute peat accumulation in a variety of freshwater and brackish environments

geodetic and seismic constraints on some seismogenic zone processes in costa rica new seismic and geodetic data from costa rica provide insight into seismogenic zone processes in central america where the cocos and caribbean plates converge seismic data are from combined land and ocean bottom deployments in the nicoya peninsula in northern costa rica and near the osa peninsula in southern costa rica in nicoya inversion of gps data suggests two locked patches centered at 14 2 and 39 6 km depth interplate microseismicity is concentrated in the more freely slipping intermediate zone suggesting that small interseismic earthquakes may not accurately outline the updip limit of the seismogenic zone the rupture zone for future large earthquakes at least over the short 1 year observation period we also estimate northwest motion of a coastal sliver block at 8 3 mm yr probably related to oblique convergence in the osa region to the south convergence is orthogonal to the trench cocos caribbean relative motion is partitioned here with 8 cm yr on the cocos panama block boundary including a component of permanent shortening across the fila costena fold and thrust belt and 1 cm yr on the panama block caribbean boundary the gps data suggest that the cocos plate panama block boundary is completely locked from 10 50 km depth this large locked zone as well as associated forearc and back arc deformation may be related to subduction of the shallow cocos ridge and or younger lithosphere compared to nicoya with consequent higher coupling and compressive stress in the direction of plate convergence

seismicity and geometry of a 110 km long blind thrust fault 1 the 1985 kettleman hills california earthquake the august 4 1985 kettleman hills earthquake was the third in a sequence of moderate shocks to occur beneath the northern half of a 110 km long fold chain bounding the eastern california coast ranges the 1982 mw 5 4 new idria 1983 mw 6 5 coalinga and 1985 mw 6 1 kettleman hills events define a southward progression of seismic activity beneath the fold we use teleseismic waveforms geodetic modeling hypocenters relocated in a three dimensional velocity model and subsurface structural data to investigate the kettleman hills earthquake the main shock results from motion on a shallowly dipping thrust fault buried at 10 km depth aftershocks and coseismic fault slip extend 20 km along the fold axis nearly the full extent of the kettleman hills north dome anticline aftershocks occur primarily several kilometers in front of the fault tip and in the core of the anticline the main shock and several foreshocks occurred at a 2 km right step in the quaternary fold axis which also corresponds to the southern end of the 1983 coalinga and northern end of the 1985 kettleman hills aftershock zones from this we infer that the step in the fold is caused by an offset or tear in the underlying fault the scalar seismic moment is 1 6 1018 n m consistent with the geodetic deformation and the duration of rupture is 16 s 3 4 times greater than for the average earthquake with this scalar moment the slow rate of moment release provides an explanation for the low level of ground shaking and low local magnitude reported for the event the peak of the geodetic uplift is located 5 km perpendicular to the quaternary fold axis we argue that the fault is propagating northeast into the undeformed san joaquin valley sediments and that the overlying fold is growing at about 0 5 mm yr

seismicity and geometry of a 110 km long blind thrust fault 2 synthesis of the 1982 1985 california earthquake sequence during 1982 1985 three 5 4 mw 6 5 earthquakes migrated 65 km along the north half of a string of quaternary folds at the east front of the california coast ranges there is no surface fault associated with these earthquakes but the fold chain oriented parallel to the san andreas fault 30 km to the west takes up contraction normal to the san andreas fault identification of active blind thrust faults is hindered by the absence of a fault trace but may be revealed by the presence of growing folds at the surface and by earthquakes at depth because of excellent seismic geodetic and geologic data this earthquake sequence illuminates the geometry of the blind faults and enables us to probe their mechanics we relocate seismicity and compute focal mechanisms in a three dimensional velocity space and model the vertical deformation associated with the 1983 m 6 5 coalinga earthquake the aftershock zones abut at echelon offsets in the fold axes and the mainshocks display reverse slip perpendicular to the axes suggesting that the folds conceal a contiguous segmented thrust fault background seismicity concentrates at bends and breaks in the fold chain sites that may correspond to tears and ramps in the thrust fault at depth seismic reflection profiles reveal thrust and reverse faults dipping toward the san andreas fault at depths of 5 10 km with several kilometers of cumulative slip and high angle reverse faults in the anticlines with several hundred meters cumulative slip coseismic fold uplift accompanied the coalinga and kettleman hills north dome earthquakes suggesting that fold growth is episodic and coupled closely to repeated earthquakes on the underlying thrusts the north half of the fold chain has been the site of several 6 m 6 5 earthquakes since 1885 thus kettleman hills middle dome the next fold segment to the south has an elevated seismic potential the aftershock zones of the three main shocks are diffuse and occupy a region much larger than the site of seismic slip aftershocks occur 5 7 km from the fault regions where on the basis of a boundary element model the shear strain increase caused by the fault slip exceeds 20 ppm equivalent to about 0 7 mpa we argue that the broad aftershock zone is a product of high sustained off fault stress caused by repeated displacement on faults that do not cut the earth s surface the fault tip stresses lead to the formation of secondary faults which can become sites of aftershocks and postseismic creep

comparison of depth dependent fault zone properties in the japan trench and middle america trench spatial variations in mechanical properties of the interplate thrust faults along the japan and middle america subduction zones are examined using teleseismic broadband earthquake recordings moment normalized source duration is used to probe rigidity variations along the interface we invert body waves to estimate source depth and source duration for 40 events in the japan subduction zone and 38 events in the middle america subduction zone for both areas there is a systematic decrease in source duration with increasing depth along the subduction zone interface this is most likely a result of variation in properties of sediments on the plate contact variations in source duration are greatly reduced at depths greater than 18 km in both regions enhanced spatial heterogeneity at shallow depth may reflect variations in plate roughness sediment distribution permeability of the fault zone and stress

earthquake source mechanism determination by use of body wave amplitudes an application to swedish earthquakes abstract a method for earthquake source studies based on a combined use of polarity and amplitude observations is proposed it makes use of the impulses of the observed body waves to avoid the introduction of additional source parameters besides the orientation angles and seismic moments a statistical model is proposed and the method is applied to five small earthquakes in sweden recorded by 3 to 5 stations in the distance range of 20 to 100 km the seismic moments of these earthquakes are in the range of 0 6 to 10 10 11 n m 1 n m 10 7 dyne cm the resulting mechanisms are statistically significant and well defined in spite of the fact that for four of the earthquakes only two or less polarities are used furthermore the solutions are close to the mechanism of the major m o 3 5 10 15 n m earthquake in the gulf of finland 1976 they all indicate horizontal principal compressive stresses in the sector e w to sse nnw the successful use of amplitude observations presented in this paper shows that source mechanism inversion can be made based on recordings at only a very few close stations

double sided onshore offshore seismic imaging of a plate boundary super gathers across south island new zealand abstract onshore offshore seismic refraction profiling allows for the determination of crustal and mantle structures in the transition between continental and oceanic environments islands and narrow landmasses have the unique geometry of allowing for double sided onshore offshore experiments that favor the construction of composite super gathers using the acquisition of onshore offshore and ocean bottom seismometer receiver gathers land explosion shot gathers and near vertical incidence multichannel seismic mcs profiling a number of sites at plate boundaries are amenable to the application of double sided onshore offshore imaging including the indo australian pacific transform boundary on south island new zealand by comparing the ratio of island width to mantle refraction pn maximum crossover distance using nondimensional distances we provide an indicator of raypath coverage for crustal illumination islands or narrow land masses whose widths are less than twice their maximum crossover distance are candidates for double sided onshore offshore experiments the sight south island geophysical investigation experiment in new zealand is located where the width of south island is sufficiently narrow with respect to its crustal thickness that a double sided onshore offshore experiment allows for complete crustal imaging of the associated plate boundary

the seismic source parameters of the 1991 costa rica aftershock sequence evidence for a transcurrent plate boundary the april 22 1991 valle de la estrella costa rica earthquake ms 7 6 was a back arc thrusting event associated with the underthrusting of the caribbean plate beneath central america a network of three passcal type portable instruments was deployed to monitor the aftershock activity in southern costa rica 2 to 6 weeks after the main shock the waveforms recorded on three component midperiod seismometers were used to recover source information for 15 small aftershocks magnitudes between 3 2 and 4 4 with a linear moment tensor inversion method we conducted several tests to investigate the effects of unknown structure and event mislocation on source parameter recovery the longer period waveforms in general are less sensitive to the effects of the structural details so that the essential source information can be successfully extracted from the waveform data the earlier part of the seismic wave forms has proven to be the most important carrier of the source information a gross crustal model can be used to describe the structure for the source study the small changes in the waveform character resulting from the mislocation of the events or inexact green s functions generated from the oversimplified crustal model do not prohibit us from the recovery of the source orientation at local distances in contrast the determination of the focal depth is subject to uncertainty because of the lack of detailed structural information our focal mechanisms are generally in good agreement with p wave first motion fault plane solutions determined from a local short period network the aftershocks show a clear spatial segmentation based on focal mechanism type most aftershocks near or southeast of the main shock were thrusting events with focal mechanisms similar to the main shock in contrast a cluster of aftershocks northwest of the main shock showed dominantly left lateral strike slip motion on a northeasterly striking nodal plane this suggests that a diffuse deformation zone exists in central costa rica and is characterized by left lateral strike slip motion this diffuse transcurrent deformation zone coincides with several geologic and geophysical features and perhaps is a result of the slower subduction rate of the buoyant cocos ridge than its adjacent segments along the middle america trench mat the diffuse transcurrent boundary may intersect with the north panama deformed belt npdb near limon costa rica and is very likely a plate boundary for the proposed panama block

kinematic 3 d retro modeling of an orogenic bend in the south limon fold and thrust belt eastern costa rica prediction of the incremental internal strain distribution the south limon fold and thrust belt in the back arc area of southern costa rica is characterized by a 90 curvature of the strike of the thrust planes and is therefore a natural laboratory for the analysis of curved orogens the analysis of curved fold and thrust belts is a challenge because of the varying structural orientations within the belt based on seismic reflection lines we created a 3 d subsurface model containing three major thrust faults and three stratigraphic horizons 3 d kinematic retro deformation modeling was carried out to analyze the spatial evolution of the fold and thrust belt the maximum amount of displacement on each of the faults is from hinterland to foreland thrust 1 800 m thrust 2 600 m thrust 3 250 m the model was restored sequentially to its pre deformational state the strain history of the stratigraphic horizons in the model was calculated at every step this shows that the internal strain pattern has an abrupt change at the orogenic bend contractional strain occurs in the forelimbs of the hanging wall anticlines while a zone of dilative strain spreads from the anticline crests to the backlimbs the modeling shows that a nne directed transport direction best explains the structural evolution of the bend this would require a left lateral strike slip zone in the north to compensate for the movement and thereby decoupling the south limon fold and thrust belt from northern costa rica therefore our modeling supports the presence of the trans isthmic fault system at least during the plio pleistocene

centroid moment tensor solutions for october december 1991 centroid moment tensor solutions are presented for 160 earthquakes that occurred during the fourth quarter of 1991 the solutions are obtained using corrections for aspherical earth structure represented by a whole mantle shear velocity model sh8 u4l8

mechanics of back arc deformation in costa rica evidence from an aftershock study of the april 22 1991 valle de la estrella costa rica earthquake mw 7 7 the april 22 1991 mw 7 7 valle de la estrella costa rica earthquake represents back arc thrusting of the caribbean plate beneath the panama block along the north panama thrust belt large back arc thrusting events are quite rare occurring in only two other locations along the sunda arc and japan sea to better understand the mechanics of back arc thrusting we constrain the faulting geometry associated with the 1991 costa rica earthquake using aftershock locations and focal mechanisms obtained from a three component portable digital network deployed in and around the aftershock area following the mainshock the spatial distribution of aftershocks reveals a complicated faulting geometry in the rupture area focal mechanisms determined from inversion of p wave and tangentially and radially polarized s wave sh and sv respectively amplitudes recorded by this temporary network confirm fault complexity and indicate active thrust normal and strike slip faults in the back arc of costa rica most of the thrust events are confined to the southern portion of the aftershock zone in the vicinity of the mainshock their distribution suggests the existence of a near horizontal basal fault plane at a depth of about 15 km with many imbricate faults having steeper dips extending from the basal plane toward the surface events with strike slip mechanisms locate northwest of the thrust events and define a sw ne trending left lateral strike slip fault zone that represents the nw termination of the mainshock rupture and possibly the maximum nw extension of the panama block the superposition of the aftershock locations on a geologic map of the region shows that aftershocks are restricted to occur in the older more competent rock units volcanic and volcaniclastic rocks interbedded with carbonates of the back arc sedimentary basin shallow events depth 5 km occur only where these oldest units are exposed at the surface this suggests that 1 exposure of the lower units results from repeated earthquake slip on the shallow crustal faults imaged by the aftershocks and 2 as much as 7 km of basin fill material overlying the lower units northeast of the mainshock does not deform seismically but instead folds and possibly faults aseismically

numerical simulations of the 1991 limon tsunami costa rica caribbean coast the second largest recorded tsunami along the caribbean margin of central america occurred 25 years ago on april 22nd 1991 an earthquake with magnitude mw 7 6 ruptured along the thrust faults that form the north panama deformed belt npdb the earthquake triggered a tsunami that affected the caribbean coast of costa rica and panama within few minutes generating two casualties these are the only deaths caused by a tsunami in costa rica coseismic uplift up to 1 6 m and runup values larger than 2 m were measured along some coastal sites here we consider three solutions for the seismic source as initial conditions to model the tsunami each considering a single rupture plane we performed numerical modeling of the tsunami propagation and runup using neowave numerical model yamazaki et al in int j numer methods fluids 67 2081 2107 2010 doi 10 1002 fld 2485 on a system of nested grids from the entire caribbean sea to limon city the modeled surface deformation and tsunami runup agreed with the measured data along most of the coastal sites with one preferred model that fits the field data the model results are useful to determine how the 1991 tsunami could have affected regions where tsunami records were not preserved and to simulate the effects of the coastal surface deformations as buffer to tsunami we also performed tsunami modeling to simulate the consequences if a similar event with larger magnitude mw 7 9 occurs offshore the southern costa rican caribbean coast such event would generate maximum wave heights of more than 5 m showing that limon and northwestern panama coastal areas are exposed to moderate to large tsunamis these simulations considering historical events and maximum credible scenarios can be useful for hazard assessment and also as part of studies leading to tsunami evacuation maps and mitigation plans even when that is not the scope of this paper

the relative amplitude method applied to 19 march 1984 uzbekistan earthquake and its aftershocks abstract the presumed double couple source mechanisms of the 19 march 1984 uzbekistan earthquake and six teleseismically recorded aftershocks down to m b 4 4 pde are determined using the relative amplitude method it is found that a well constrained solution for the main earthquake can be obtained when world wide long period p wave first motions are combined with constraints on s wave polarisation angles determined from the relative polarities of three component seismograms solutions for the aftershocks are obtained from between two and four short period medium aperture array observations of p pp and sp the main shock and all but one of the aftershocks give thrust type solutions in broad agreement with each other their fault planes trending northeast southwest where the available data provide a constraint on the strike the remaining aftershock gives a vertical strike slip solution though this conclusion is in need of corroborative evidence results for the main earthquake are similar to those obtained by other workers but it appears that the predominant strike directions of the 1984 aftershocks are different from those previously computed for aftershocks of the 1976 earthquakes which gave a predominance of northwest southeast striking fault planes

an analysis of the 1985 nahanni earthquakes abstract two large magnitude earthquakes occurred in 1985 near the north nahanni river in the northeast cordillera northwest territories canada on 5 october m s 6 6 and 23 december m s 6 9 these events were unprecedented for this region of the cordillera and the december event is the largest earthquake ever known in the eastern part of the canadian cordillera this report presents detailed studies of the nahanni earthquakes their source parameters macroseismic effects aftershock sequences triggered rock avalanches their relationship to the geology of the epicentral area and their seismic hazard implications parameters calculated here for these two major events are parentheses indicate other studies the earthquakes occurred within a small relatively undeformed plateau the mackenzie plain in the foreland fold belt created along the northeastern cordillera during the columbian or laramide orogeny that fold belt is now subjected to northeast southwest horizontal compressive stresses possibly very similar to the original laramide stress regime and the fault plane implied by the seismic mechanisms is very similar to the laramide faults in the area however no surface fault break has been found and none of the mapped laramide faults in the epicentral area appears to be involved in the aftershock activity it is thus believed that the nahanni earthquakes occurred on a blind laramide thrust fault under the mackenzie plain the large magnitudes of the nahanni earthquakes show that the present seismic zoning for building code purposes in the eastern cordillera north of 60 n needs reassessment and suggest that the zoning in the eastern cordillera in british columbia and alberta may also need revision the felt areas of the earthquakes show that regional anelastic attenuation in the eastern canadian cordillera is significantly anisotropic and much lower along strike than across strike the felt areas also appear to scale with seismic moment rather than m b magnitude the thrust mechanisms regional stress regime shallow focal depth and high velocity sedimentary rocks in the focal region suggest that the nahanni earthquakes and particularly the near field strong ground motion recorded for the second event are ideal engineering design data for critical facilities in eastern north america

the mechanism of the 1983 japan sea earthquake as inferred from long period surface waves and tsunamis abstract the source process of the japan sea earthquake of may 26 1983 is studied by using the long period surface waves and tsunamis the moment tensor inversion of rayleigh waves which consider the lateral heterogeneity of the earth is developed and applied to the ida records of this event the moment tensor solution and first motion data indicate that the mechanism is dip slip and the seismic moment is 7 6 1027 dyne cm the fault is estimated to be 120 km in length 40 km in width and dips eastward with a dip angle of 30 from the aftershock distribution tsunami simulation for an actual topography is made to restrain the slip on the fault the aftershock area and the tsunami records suggest that the fault is divided into two segments the northern part strikes nnw and slips 4 m while the southern part strikes nne and slips 5 m the fault geometry the aspect ratio and the stress drop of this event are similar to those of the 1964 niigata earthquake but differ from those of the earthquakes in the pacific coast of japan

seamount ridge and transform subduction in southern central america understanding the factors that control subduction zone processes is a first order goal in the study of convergent margins in southern central america a growing body of research reveals strong links between the character of the subducting slab and the mechanics of important processes that include subduction erosion fluid flow deformation and seismogenesis in this paper i evaluate the role that seamount ridge and transform subduction have in the development of upper plate deformation and volcanism by summarizing previous work across a 500 km long region of central america where each of these three scenarios are present along strike the data show that the subduction of short wavelength bathymetry e g seamounts and faults on the seafloor produces short wavelength deformation that persists for relatively short timescales 104 105 years whereas the subduction of longer wavelength bathymetry e g the aseismic cocos ridge results in longer wavelength deformation that endures over a longer time scale 106 years the timing and distribution of upper plate deformation are consistent with subhorizontal cocos ridge subduction driving upper plate deformation and the increased crustal thickness 20 km of the subducting cocos ridge is likely one of the most important factors in the production of upper plate contraction and crustal thickening the data illustrate a fundamental connection between lower plate properties and upper plate deformation and highlight the profound influence that bathymetry and crustal thickness have in the localization and kinematics of upper plate strain and volcanism in middle america

moment tensor estimation using regional seismograms from a tibetan plateau portable network deployment the authors present the results of moment tensor inversions using regional seismograms recorded during the 1991 1992 passcal passive seismic experiment on the tibetan plateau using data recorded over a one year period they estimate the source mechanisms for 38 earthquakes within and near the tibetan plateau with moment magnitudes m sub w ranging from 3 8 to 5 0 with only one exception all the events within the plateau but away from the plateau boundary indicate either strike slip or extensional faulting a result consistent with previous studies three fourths of all the events occurred at depths less than 10 km and all but two at depths less than 15 km several events located near the southeastern section of the plateau exhibit an unusual m sub b m sub w difference of approximately one magnitude unit 19 refs 3 figs

the april 22 1991 valle de la estrella costa rica mw 7 7 earthquake and its tectonic implications a broadband seismic study the rapture process of a large back arc thrusting earthquake the april 22 1991 valle de la estrella costa rica mw 7 7 earthquake is investigated using broadband body waves and long period surface waves we find the source process to be relatively simple with the source models separately obtained from body and surface waves being very consistent the event occurred on a shallow southwest dipping rapture plane on which most energy is released updip of the hypocentral location 10 20 km deep high frequency radiation appears to have been released over a relatively small source area our preferred model has a focal mechanism with strike 102 10 dip 17 14 and rake 63 17 a seismic moment of 3 8 1 5 1020 n m and a total rupture duration of 40 6 s the earthquake appears to be associated with the north panama deformed belt npdb a thrust and fold complex that has accommodated the oroclinal deformation of panama this event along with previous large events north of panama in 1882 and 1916 indicates that there is substantial convergence along the npdb marking the npdb as a probable emerging plate boundary it remains difficult to gauge the earthquake hazard in the region because of the tectonic complexity

new fossil sea turtle trackway morphotypes from the pleistocene of south africa highlight role of ichnology in turtle paleobiology more than 130 late pleistocene trackway sites from the coastal eolianites and beach deposits of the cape south coast south africa have previously mostly yielded tracks of large mammals and birds however two sites east of still bay and a third near garden route national park yield distinctive trackways of hatchling sea turtles made during the short posthatching postemergence interval when the trackmakers headed for the sea one assemblage of approximately parallel trackways indicates smaller loggerhead turtle hatchlings with alternating gaits and contrasts with a wider trackway indicating a leatherback turtle hatchling these are the world s first reports of fossil traces that document this brief run for the sea phenomenon they help delineate late pleistocene sea turtle breeding ranges and indicate climatic conditions along the cape south coast ichnotaxonomically defined swim tracks of large adult sea turtles are known from a few mesozoic sites likewise walking and swim traces of terrestrial freshwater turtles are also known from the mesozoic and cenozoic however as no ichnotaxonomy exists for these diagnostic hatchling trails we assign the trackways of the inferred loggerheads to the new ichnotaxon australochelichnus agulhasii ichnogen et ichnosp nov and the inferred leatherback trackway to marinerichnus latus ichnogen et ichnosp nov

the 1987 whittier narrows earthquake sequence in los angeles southern california seismological and tectonic analysis the october 1 1987 whittier narrows earthquake m l 5 9 was located at 34 2 96 n 118 4 86 w at a depth of 14 6 0 5 km in the northeastern los angeles basin the focal mechanism of the mainshock derived from first motion polarities shows pure thrust motion on west striking nodal planes with dips of 25 5 and 65 5 respectively the aftershocks define an approximately circular surface that dips gently to the north centered at the hypocenter of the mainshock with a diameter of 4 6 km hence the spatial distribution of the mainshock and aftershocks as well as the focal mechanisms of the mainshock indicate that the causative fault was a 25 north dipping thrust fault striking west and is confined to depths from 10 to 16 km although most of the 59 aftershock focal mechanisms presented here document a complex sequence of faulting they are consistent with deformation of the hanging wall caused by the thrust faulting observed in the mainshock a cluster of reverse faulting events on north striking planes occurred within hours after the mainshock 2 km to the west of the mainshock the largest aftershock m l 5 3 occurred on october 4 and showed mostly right lateral faulting on the same north northwest striking plane within the hanging wall similarly several left lateral focal mechanisms are observed near the eastern edge of the mainshock rupture the earthquake and calibration blast arrival time data were inverted to obtain two refined crustal velocity models and a set of station delays when relocating the blast using the new models and delays the absolute hypocentral location bias is less than 0 5 km the mainshock was followed by nearly 500 locatable aftershocks which is a small number of aftershocks for this magnitude mainshock the decay rate of aftershock occurrences with time was fast while the b value was low 0 67 0 05 for a los angeles basin sequence

backarc thrust faulting and tectonic uplift along the caribbean sea coast during the april 22 1991 costa rica earthquake surface deformation and a tsunami accompanied the destructive april 22 1991 costa rica panama earthquake ms 7 5 along a 135 km stretch of caribbean coast coseismic uplift was measured between the lower and upper limits of sessile intertidal organisms stranded on coral reefs the preearthquake and postearthquake high tide levels located from driftwood lines on beaches and the preearthquake and postearthquake tide levels as pointed out by local residents the nature and distribution of offshore vertical displacements were further constrained from analysis of measured run up heights and reported arrival times of the tsunami uplift detected along the coast jumped within 4 km from zero to 157 cm near limon and generally decreased over a distance of 70 km southward to the border with panama these data map an axis of uplift that intersects the coastal beach ridge just north of the port of moin and runs offshore to the east and south roughly parallel to the coast no surface faulting was found the earthquake and tsunami were generated by backarc thrusting along faults that bound the north panama deformed belt and dip from the caribbean sea beneath costa rica and northern panama combined geodetic and seismological data indicate that the main rupture dips landward at an angle of about 30 and is approximately 40 km wide and 80 km long dislocation models suggest 2 2 m of slip on a causative thrust fault striking between 105 and 120 we estimate that the repeat time for this type of earthquake is 200 to 1100 years the historical record and new isotopic data favor the middle of the range

the geometry of back arc thrusting along the eastern sunda arc indonesia constraints from earthquake and gravity data the flores earthquake of december 23 1978 represents the first seismological evidence for active back arc thrusting behind the eastern sunda arc this mb 5 8 earthquake occurred north of flores island where seismic reflection profiling has revealed back arc thrusting interpreted to be a reaction to compression of the arc following collision with the australian continent we investigate the source mechanism and depth of the flores earthquake by inversion of long period p waveforms and relocate the earthquake s epicenter incorporating arrival time data from local stations we find that this event occurred beneath the accretionary prism south of the back arc thrust zone at a depth of 11 5 km 7 km below the seafloor the best fitting fault plane solution is consistent with active southward thrusting of the floor of the flores basin beneath the volcanic arc along a 30 dipping fault plane the morphology of the thrust zone and the free air gravity profile over the flores basin in the epicentral region resemble those of oceanic trenches and the observed gravity field is best interpreted if the crust of the flores basin dips smoothly below the accreted wedge at an average angle of about 6 the position of the earthquake hypocenter south of the flores thrust and the fault plane solution suggest that the 1978 earthquake is analogous to thrust events at subduction zones and represents slip between the subducting and overriding plates the flores thrust is thus the surface expression of a deep seated thrust zone and may represent the initial stage of polarity reversal of the eastern sunda arc

the limon costa rica earthquake of april 22 1991 back arc thrusting and collisional tectonics in a subduction environment on april 22 1991 a large earthquake mw 7 7 occurred along the caribbean coast of costa rica and western panama the rupture area of the fault mapped from the aftershocks is 45 85 km2 the distribution of aftershocks and the local geological record suggest that faulting occurred on a blind thrust sheet that shallows toward the northeast uplift of the caribbean coast ranging from 1 5 m near puerto limon and decreasing gradually toward the southeast was observed along the caribbean northwest of puerto limon no significant coastal uplift was observed this observation agrees with the aftershock data suggesting the rupture did not extend to the northwest of this location the limon earthquake also triggered aftershocks on secondary faults in the crust these events are apparently associated with a family of imbricate thrust and strike slip faults that lie in the eastern piedmont of the talamanca cordillera the historical seismicity indicates that the caribbean coast has been the site of several historical earthquakes with magnitudes greater than 7 0 on april 26 1916 another earthquake ms 6 9 took place in the same region summing the scalar seismic moment release along the caribbean coast the average rate of slip is approximately 0 8 cm yr compared with a value of 0 4 to 0 8 cm yr along the pacific subduction zone depending on the estimated width of the seismogenic zone therefore a large fraction of the relative plate motion between the cocos and caribbean plates 9 8 cm yr appears to be taken up by crustal deformation in the back arc the tectonic regime in the area appears to be dominated by the collision of the buoyant cocos ridge with the subduction zone the absence of a wadati benioff zone where the cocos ridge collides with the trench suggests the slab does not subduct beneath the osa peninsula this is supported by the pliocene gap of volcanism present in costa rica thus the predicted relative motion between the cocos and caribbean plates appears to be absorbed by a low rate of seismic moment release in the forearc and by a broad zone of active crustal shortening and underthrusting in the back arc this type of tectonic deformation resembles more a collisional regime than a typical subduction zone environment

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

source parameters of aftershocks of the 1991 costa rica and 1992 cape mendocino california earthquakes from inversion of local amplitude ratios and broadband waveforms abstract source parameters of aftershocks of the 22 april 1991 m w 7 7 costa rica and the 25 april 1992 m w 7 1 cape mendocino california earthquakes are determined using a grid search inversion of p sh and sv amplitude ratios recorded by sparse local networks of three component broadband and short period stations the inversion procedure consists of computing synthetic seismograms for three fundamental fault orientations for all source receiver pairs over a range of source depths calculating the complex envelopes of the observed and synthetic seismograms to determine peak amplitudes of p sh and sv waves combining the fundamental fault amplitudes for all possible values of strike dip and rake at 10 increments and determining the best fault orientation and depth as the one that yields the smallest misfit between observed and synthetic p sh p sv and sv sh amplitude ratios the ambiguity in the sense of motion on the nodal planes arising due to the use of amplitude ratios is resolved by examining p wave polarities the sensitivity of source parameters to uncertainties in earthquake location and crustal structure is explored for events with good station coverage focal mechanism determinations are stable for a wide range of assumed values of crustal structure earthquake location and depth source parameters for many of the largest events m 3 4 are also determined by inversion of broadband displacement waveforms using a similar grid search technique comparable results were obtained using both broadband waveforms and amplitude ratios focal mechanism solutions for 20 aftershocks of the costa rica earthquake reveal a complicated faulting geometry indicating active thrust normal and strike slip faults in the back arc of costa rica the 1992 cape mendocino earthquake occurred at the intersection of the north american gorda and pacific plates while the mainshock was associated with underthrusting of the gorda plate beneath the north american plate fault plane solutions for 70 of the 38 largest aftershocks indicate that these events result from either motion between the gorda and pacific plates or from internal deformation within the gorda plate

central costa rica deformed belt kinematics of diffuse faulting across the western panama block fault kinematics seismicity and geodetic data across central costa rica reveal a diffuse fault zone here named the central costa rica deformed belt ccrdb the ccrdb defines the western margin of the panama block and links the north panama deformed belt npdb along the caribbean coast with the middle america trench mat along the pacific coast the junction of the ccrdb and the mat coincides with an abrupt transition from smooth to rough crust on the subducting cocos plate rough smooth boundary shallow subduction of rough thickened oceanic crust associated with the cocos ridge shifts active shortening into the volcanic arc along faults of the ccrdb variable fault kinematics along this zone may reflect three combined deformation mechanisms horizontal shortening and shear from oceanic ridge indentation basal traction from shallow subduction and localized block uplift from subducting seamount roughness within the forearc domain 1 mesoscale faults express transtension where steep ne striking regional scale faults intersect the pacific coast across the volcanic arc domain 2 mesoscale faults exhibit mostly sinistral and dextral slip on ne and nw striking conjugate faults respectively approaching the npdb in the back arc domain 3 transcurrent faulting is modified by transpression and crustal thickening fault kinematics are consistent with earthquake focal mechanisms and global positioning system gps measurements radiometric age constraints confirm that faulting postdates the late neogene onset of shallow subduction the ensuing deformation front has propagated northward into the volcanic arc to its present position along the seismically active ccrdb within the forearc the effect of shallow subduction is overprinted by local uplift related to underthrusting seamounts

lithospheric structure of the costa rican isthmus effects of subduction zone magmatism on an oceanic plateau we present the results of a multidisciplinary geophysical study conducted to investigate the lithospheric structure of the costa rican isthmus the physical properties of the lithosphere are resolved by three dimensional 3 d simultaneous inversion of velocity anomalies and hypocenter parameters using local earthquakes and 2 d forward modeling of onshore and offshore seismic refraction and gravity data according to our results the northern half of the costa rican isthmus is constituted by a 40 km thick crust with a 6 to 7 km thick oceanic crust subducting under it the uppermost level of the basement and most of the marginal wedge show intermediate velocities and high densities in good agreement with those described for flood basalts the midlevel shows velocities and densities representative of oceanic crust the bottommost level 20 40 km shows high velocities and densities typical of mafic rocks and the upper mantle displays anomalously low densities and velocities intracrustal heterogeneities at intermediate wavelengths are indicated by prominent velocity anomalies these results are consistent with a basement beneath the costa rican isthmus being part of the caribbean plateau originated at 85 90 ma with the onset of the galapagos hotspot the upper level corresponds to the flood basalts extruded during this phase and it includes most of the marginal wedge the second level represents the preexisting oceanic crust the mafic lower crust intracrustal heterogeneities and anomalous upper mantle are interpreted to be built up by underplating intrusion and crystallization of basaltic melts formed under the influence of subducting lithosphere dehydration