mechanisms and depths of atlantic transform earthquakes we have studied 40 earthquakes on seven major transforms along the mid atlantic ridge the majority show the expected transform parallel strike slip motion on steeply dipping fault planes some anomalous mechanisms suggesting either unusually shallow dip or a component of dip slip motion occur near ridge transform intersections the vertical tectonism suggested by transform morphology may thus be restricted to intersections where thermal contrast across the transform is greatest and stress orientations can be oblique to both ridge and transform centroid depths determined by waveform inversion are limited to a shallow zone corresponding approximately to the area above the 400 c isotherm of the thermal structure expected from lithospheric cooling the depth extent of faulting predicted by estimating fault area from seismic moments extends approximately to the predicted 600 c isotherm assuming that the areas above the 400 c or 600 c isotherms fail seismically moment release for six of the transforms accounts for varying fractions of the slip predicted by plate motion suggesting that some aseismic slip occurs if the time period sampled is representative in contrast on the vema transform the inferred seismic slip is greater than predicted unless faulting extends to the 650 c isotherm alternatively the vema may have an unusually long recurrence time or be mechanically anomalous due to its complex history in contrast to transform events centroids for oceanic intraplate earthquakes extend to approximately the 750 c isotherm this result is intriguing if depths reflect lithospheric strength as a function of depth transforms have much higher strain rates than intraplate areas so if both had the same rheology the limiting isotherm for transforms should be higher alternatively if a weaker rheology were appropriate the higher strain rate should cancel the weakening and yield a limiting isotherm similar to that for intra plate regions transforms may thus be either weaker and or have higher temperatures than expected in the latter model the required high temperatures perhaps due to intrusion at depth predict measurable heat flow anomalies the second possibility unusual rheological weakness is reminiscent of the unexplained weakness of the san andreas fault where heat flow data exclude anomalously high temperatures

scour below pipeline in river crossings an experimental research on the scour produced under a pipe exposed to a stream is reported experiments were performed in a rectangular laboratory channel whose bed was composed of loose granular particles of two different diameters pipes of four different sizes were located either on top of the bed or at varying distances above it the accelerations and higher water velocities generated by the presence of the pipe increased shear velocity thus generating a scour hole the results obtained showed that the reynolds number of the pipe exerts a negligible influence on the scour process by contrast the froude number was found to be an important parameter in the definition of the scour process good correlations between the froude number of the flow and the most important characteristic dimensions of the scour hole were obtained and different general relationships that allow the prediction of the characteristic dimensions of the scour hole are thus proposed

vector constraints on western u s deformation from space geodesy neotectonics and plate motions the rate of slip vector on the san andreas fault in central california estimated from geodetic and holocene geological data 34 3 mm yr n41 w 2 is inconsistent with the prediction of rigid plate models such as rm2 56 3 mm yr n35 w 2 this well known san andreas discrepancy is diagnostic of plate deformation distributed both east of the fault in the basin and range and west of the fault along the california continental margin we construct constraints on the integrated deformation rates across these two regions consistent with 1 the kinematical boundary conditions imposed by the rigid plate model 2 neotectonic and paleoseismic estimates of deformation rates 3 ground based geodetic measurements and 4 rates of change observed by very long baseline interferometry along seven baselines to western u s sites the space geodetic data on basin and range extension taken over a 4 year interval are compatible with geological observations averaged over the holocene the best estimate of its integrated deformation rate provided by the joint inversion of both data types is 9 7 2 1 mm yr n56 w 10 too small and in the wrong direction to account entirely for the san andreas discrepancy we therefore attribute much of it to deformation west of the sierra nevada great valley block accommodated for example on faults such as the san gregorio hosgri system of coastal california the integral of this deformation estimated by subtracting the basin and range contribution from the discrepancy vector requires significant right lateral shear parallel to the san andreas 13 5 mm yr and some compression perpendicular to it 9 3 mm yr

location of moderate sized earthquakes recorded by the nars baja array in the gulf of california region between 2002 and 2006 we relocated the hypocentral coordinates of small to moderate sized earthquakes reported by the national earthquake information center neic between april 2002 and august 2006 in the gulf of california region and recorded by the broadband stations of the network of autonomously recording seismographs nars baja array the nars baja array consists of 19 stations installed in the baja california peninsula sonora and sinaloa mexico the events reported by the preliminary determinations of epicenters pde catalog within the period of interest have moment magnitudes m w ranging between 1 1 and 6 7 we estimated the hypocentral location of these events using p and s wave arrivals recorded by the regional broadband stations of the nars baja and the resban red sismologica de banda ancha arrays and using a standard location procedure with the hypocenter code lienert and havskov in seism res lett 66 26 36 1995 as a preliminary step to refine the location of the initial hypocenters we used the shrinking box source specific station term method of lin and shearer j geophys res 110 b04304 2005 we found that most of the seismicity is distributed in the nw se direction along the axis of the gulf of california following a linear trend that from north to south steps southward near the main basins wagner delfin guaymas carmen farallon pescadero and alarcon and spreading centers we compared the epicentral locations reported in the pde with the locations obtained using regional arrival times and we found that earthquakes with magnitudes in the range 3 2 5 0 mb differ on the average by as much as 43 km for the m w magnitude range between 5 and 6 7 the discrepancy is less differing on the average by about 25 km we found that the relocated epicenters correlate well with the main bathymetric features of the gulf

an extensive region of off ridge normal faulting earthquakes in the southern indian ocean an unusually large number of off ridge earthquakes have occurred within a broad region astride the southeast indian ridge the source mechanisms of nine of these earthquakes in lithosphere from 4 to 27 m y old have been determined with a formal inversion technique based on matching observed and synthetic p and sh waves all events occur in the oceanic mantle from 8 to 22 km below the seafloor the seismic moment and duration of faulting obtained from the inversion constrain the stress drop to lie between a few tens of bars and a few hundred bars for most of the events all of the off ridge earthquakes are characterized by normal faulting mechanisms with the t axes nearly parallel to the nearby ridge axis in sharp contrast to typical mid ocean ridge or intraplate earthquakes a variety of possible tectonic processes which might produce normal faulting in young oceanic lithosphere are considered as possible causes including bending stresses thermal stresses of various origins differential subsidence at a fracture zone off ridge volcanism latitudinal plate motions on a nonspherical earth basal shear stress from small scale convection and plate wide perturbations of the stress field due to the continental collision between india and asia no single mechanism is adequate to explain all the observed off ridge normal faulting earthquakes an apparent asymmetry in the observed seismicity with respect to the ridge suggests that the earthquakes on the antarctic and indian plates are caused by different processes the antarctic plate events can be satisfactorily attributed to the release of thermal and bending stresses associated with a residual depth anomaly between the kerguelen plateau and the southeast indian ridge a region which has been suggested as the site of a horizontal pipe for flow in the asthenosphere from the kerguelen hot spot to the vicinity of amsterdam and st paul islands on the southeast indian ridge the off ridge earthquakes in the indian plate are speculatively attributed to part of the plate wide response of the lithosphere to the ongoing continental collision

geometry and mechanism of faulting of the 1980 el asnam algeria earthquake from inversion of teleseismic body waves and comparison with field observations the el asnam earthquake of october 10 1980 ms 7 3 provided a wealth of geological and seismological data and is an ideal event for comparing geologically and seismologically derived models the event produced extensive surface faulting in addition to the main tectonic deformation which is clearly a thrust widespread secondary normal faulting was observed at the surface in the southwest region the surface break of a thrust fault could be traced for 24 km in the northeast part of the fault zone normal faults with throws of several meters were observed although no clear thrust type surface breaks were observed in this region geodetic measurements and aftershocks indicate that thrusting was nonetheless the main tectonic mode of deformation in this paper the rupture process of the el asnam earthquake is investigated by inversion of teleseismic p and sh waves the effort is concentrated on the late part of the waveforms which carries information about the faulting in the least understood north eastern segment of the fault zone during the earthquake rupture initiated at the southern terminus of the southwestern fault segment and propagated northward as indicated by the epicentral location and the observable azimuthal directivity of the body wave shapes and amplitudes the seismogenic faulting in the southwestern segment has a thrust mechanism with the following average parameters seismic moment 3 9 1019 n m depth 6 km strike 220 dip 46 and rake 72 the duration of rupture on this fault segment was approximately 10 s which together with the observed fault length of 24 km indicates a rupture velocity of 2 4 km s unconstrained inversions suggest normal faulting in the northeast part of the source region that began immediately following the arrival of the rupture to the northeast region the normal faulting mechanism is in agreement with that observed for the large surface fault scarps in this region guided by the evidence from aftershocks and geodetic measurements a solution with a thrust type mechanism was also sought using a priori constraints based on the field observations the observed strike and slip angle we obtain the following thrust source mechanism for the northeast segment seismic moment 3 6 1019 n m depth 6 km strike 230 dip 20 and rake 91 this solution requires that the thrusting in the northeast be initiated 10 s after the arrival of the rupture from the southwest suggesting that some time is required to overcome the geometric barrier marked by the abrupt change in the dip and azimuth inversions are nonunique with respect to the seismic moment associated with normal faulting the ambiguity being caused by the limited bandwidth of the observed body waves the moment estimated from the surface waves at periods about 300 s however favors thrust faulting to be dominant in the northeast region

geodetic measurements of plate motions across the central gulf of california 1982 1986 a trilateration network was set up in early 1982 by a team of french and mexican institutions across the central part of the gulf of california in order to study the plate boundary related movements in this transition area between the san andreas fault system and the east pacific rise the reobservation of this network in march 1986 provides a first set of data on the present day deformations in this area both surveys used aga8 laser geodimeter measurements between 11 stations located on elevated points of baja california and sonora and on the islands between the peninsula and mainland coasts deformation patterns during the 1982 1986 interval obtained through three different methods indicate mainly a right lateral shear movement in the gulf axis direction n46 w between baja california peninsula and angel de la guarda island 17 4 cm of dextral slip occurred between the coast of sonora and the central islands of the gulf the mean displacement amount to about 23 12 cm in the southwestern part of the network weaker movements seem to have occurred and are smaller than estimated errors this may indicate either that the boundary is locked in this part or is deflected towards the ese these results which give an estimation of the relative plate velocity of 8 3 cm a are consistent with the generally accepted relative movement between north american and pacific plates about 6 cm a

intraplate seismicity and stresses in young oceanic lithosphere we have conducted a systematic study of oceanic intraplate seismicity in the period 1964 1983 occurring in lithosphere younger than 35 million years focal mechanisms determined for 26 events using first motions p wave modeling and surface wave amplitude radiation patterns along with 13 previously determined mechanisms indicate a diversity of faulting styles and stress orientations in young oceanic lithosphere there is no evidence for a worldwide transition from extension to compression at a specific lithospheric age tensional axes of normal faulting events are oriented oblique to the spreading direction and do not show extension in the spreading direction congressional axes of thrust events show a weak preferred orientation in the spreading direction presumably due to ridge push stresses however stress axes of thrust events in very young lithosphere age 9 ma do not show a correlation with the spreading direction suggesting that the ridge push force is small near the ridge these observations indicate that tensional stress in the spreading direction is concentrated in a very narrow zone at the ridge axis in agreement with models which assume that the axial region has very little tensile strength normal faulting events occur at greater depths and temperatures than thrust events possibly because rocks under tensional stress have strength maxima at higher temperatures alternatively the depths of the normal faulting events may be a result of the higher thermal gradients at depth in young lithosphere if thermoelastic stresses are important in generating this seismicity differential thermal contraction along fracture zones does not seem to be a significant source of near ridge intraplate seismicity high seismicity rates in young lithosphere noted previously on a global scale seem characteristic of all three major ocean basins additionally most of the total oceanic intraplate moment release takes place in young lithosphere however most of the events and more than 96 of the total moment release of the near ridge intraplate seismicity are contained in five areas two zones in the indian ocean one near chagos bank and the other in the amsterdam st paul region indicate tension parallel to the trend of the plate boundary a region of high seismicity near the east pacific rise between 3 and 7 n shows both normal and thrust faulting while another region on the cocos plate west of the panama fracture zone is characterized by strike slip faulting thrust strike slip and normal faulting events are found in a small seismically active zone of the south central pacific the extremely uneven distribution of seismicity if not a result of the relatively short time period studied 20 years indicates local perturbations strongly affect near ridge seismicity

transform fault earthquakes in the north atlantic source mechanisms and depth of faulting we have determined the centroid depths and source mechanisms of 12 large earthquakes on transform faults of the northern mid atlantic ridge from an inversion of long period body waveforms the earthquakes occurred on the gibbs oceanographer hayes kane 15 20 and vema transforms we have also estimated the depth extent of faulting during each earthquake from the centroid depth and the fault width for five of the transforms earthquake centroid depths lie in the range 7 10 km beneath the seafloor and the maximum depth of seismic faulting is 14 20 km on the basis of a comparison with a simple thermal model for transform faults this maximum depth of seismic behavior corresponds to a nominal temperature of 900 100 c in contrast the nominal temperature limiting the maximum depth of faulting during oceanic intraplate earthquakes with strike slip mechanisms is 700 100 c the difference in these limiting temperatures may be attributed to the different strain rates characterizing intraplate and transform fault environments three large earthquakes on the 15 20 transform have shallower centroid depths of 4 5 km and a maximum depth of seismic faulting of 10 km corresponding to a limiting temperature of 600 c the shallower extent of seismic behavior along the 15 20 transform may be related to a recent episode of extension across the transform associated with the northward migration of the triple junction among north american south american and african plates to its present position near the transform the source mechanisms for all events in this study display the strike slip motion expected for transform fault earthquakes slip vector azimuths agree to within 2 3 of the local strike of the zone of active faulting the only anomalies in mechanism were for two earthquakes near the western end of the vema transform which occurred on significantly nonvertical fault planes secondary faulting occurring either precursory to or near the end of the main episode of strike slip rupture was observed for five of the 12 earthquakes for three events the secondary faulting was characterized by reverse motion on fault planes striking oblique to the trend of the transform in all three cases the site of secondary reverse faulting is near a compressional jog in the current trace of the active transform fault zone we find no evidence to support the conclusions of engeln wiens and stein that oceanic transform faults in general are either hotter than expected from simple thermal models or weaker than normal oceanic lithosphere

active crustal deformation in the area of san carlos baja california sur mexico as shown by data of local earthquake sequences we analyzed earthquakes of sequences that occurred at different times near san carlos a town of approximately 5000 inhabitants the seismic sequences happened during march april 1989 october 2000 june 2001 and 5 15 february 2004 at about 200 km west of the pacific north america plate boundary the strong shaking from initial earthquakes of the first two sequences prompted the installation of temporary seismic stations in the area with data recorded by these stations we found an earthquake distribution that is consistent with the northwest segment of the santa margarita fault both the focal depth that seemed to increase in e ne direction and a composite fault plane solution obtained from polarity data of the small earthquakes were also consistent with the main characteristics of that fault we also found that our normal faulting mechanism east side down was quite similar to centroid moment tensor solutions for earthquakes with mw 5 4 and 5 3 that occurred in the area in february 2004 it is likely then that these larger earthquakes also occurred along the santa margarita fault to get some insight into the regional stress pattern we compared the above mechanisms with mechanisms reported for other earthquakes of the pacific margin of baja california sur and the gulf of california regions we observed that focal mechanisms of the two regions have t axes of stress that plunge sub horizontally in e ne average direction the corresponding p axes have n nw average trend but for the pacific earthquakes these axes plunge at angles that are 35 larger than those for the gulf earthquakes these more vertically inclined p axes of compressive stress mean substantial oblique fault motions the mixture of oblique and strike slip components of fault motions as the focal mechanisms show confirms a transtensional stress regime for the region before this research we knew little about the seismicity and styles of faulting in the area now we know that earthquakes can occur along the coastline of baja california at 60 km east of the tosco abreojos fault system we conclude that transtensional deformation is taking place across a wide zone of the pacific margin of baja california finally we point out that although the studied earthquakes were of small magnitude they might serve as a reminder of the danger that future larger events pose to san carlos

source modeling of the hsingtai china earthquakes of march 1966 abstract the hsingtai china earthquakes of march 1966 were a series of destructive earthquakes associated with the shu lu graben five strong shocks of ms 6 occurred within a period of less than a month the largest of which was ms 7 2 body and surface waves over the period range from several to 100 s have been modeled for the four largest events using synthetic seismograms in the time domain and spectral analysis in the frequency domain data from ground deformation local geology regional seismic network and teleseismic joint epicenter determination have also been used to constrain the source model and the rupture process the fault mechanism of the hsingtai sequence was mainly strike slip with a small component of normal dip slip the strikes of the four largest shocks range from n26 to 30 e approximately along strike of the major faults of the shu lu graben and the aftershock distribution the source mechanisms can be explained with a nnw sse extensional stress and a nee sww compressional stress acting in the area the major shocks all had focal depths 10 km the four largest shocks in the sequence were characterized by a relatively simple and smooth dislocation time history the durations of the far field source time functions ranged from 3 5 to 5 s while the rise times were all 1 s the seismic moments of the four largest earthquakes ranged from 1 43 1025 to 1 51 1026 dyne cm 1 the fault sizes of the four events were very close assuming circular faults the diameters of the four events were determined to be between 10 and 14 km stress drops varied from 52 to 194 bars a trend of increasing stress drop with earthquake size was observed a survey of stress drop determinations for 15 major intraplate earthquakes shows that on the average the magnitude of stress drop of oceanic intraplate earthquakes and passive continental margin events is higher 200 to several hundred bars than that of continental intraplate earthquakes 100 bars or less

space geodetic observation of the deformation cycle across the ballenas transform gulf of california the gulf of california mexico accommodates 90 of north america pacific plate relative motion while most of this motion occurs on marine transform faults and spreading centers several fault segments in the central gulf come close to peninsular baja california here we present global positioning system and interferometric synthetic aperture radar data near the ballenas transform fault separating the peninsula from angel de la guarda island we observe interseismic motion between june 2004 and may 2009 and displacements associated with the 3 august 2009 mw 6 9 earthquake from the interseismic data we estimate a locking depth of 9 12 5 km and a slip rate of 44 9 48 1 mm yr indicating that faults east of angel de la guarda deform at negligible rates and that the ballenas transform accommodates virtually all of the relative motion between the north american plate and the baja california microplate our preferred model for coseismic slip on a finite rectangular fault plane suggests 1 3 m of strike slip displacement along a vertical rupture plane that is 60 km long and extends from the surface to a depth of 13 km in the eastern ballenas channel striking parallel to baja california north america relative plate motion these estimates agree with the seismic moment tensor and the location of the major foreshock and aftershocks and are compatible with the fault location identified from high resolution bathymetric mapping the geodetic moment is 33 higher than the seismic moment in part because some afterslip and viscous flow in the first month after the earthquake are included in the geodetic estimate coulomb stress changes for adjacent faults in the gulf are consistent with the location of smaller aftershocks following the 2009 main shock and suggest potential triggering of the 12 april 2012 mw 6 9 guaymas earthquake

novel type of crust produced during continental rifting a normal oceanic crust cannot be generated after continental crust splitting of rifts filled by a thick pile of young sediments the magma released by the rising mantle is trapped mainly in these sediments as basalts sills and plugs the sediments are themselves metamorphosed by the heat flux from the underlying mantle a metasedimentary crust is formed in direct contact with an abnormal mantle vp 7 5 km s l produced by partial serpentinization

geodetic and tectonic analyses along an active plate boundary the central gulf of california the gulf of california is traversed by the shear plate boundary between pacific and north american plates and because of several islands in its central part offers the possibility of direct geodetic measurements of plate motion a geodetic network of 150 km aperture and comprising 11 stations was measured in 1982 and 1986 by laser trilateration methods the deformations deduced from the comparison of the two epochs indicate right lateral shear strain covering the entire gulf rather than localized movements in the eastern part of the network between the axial islands and the sonoran coast significant right lateral shear deformation occurs with a relative displacement of about 23 12 cm over 4 years in the northwestern region canal de ballenas a right lateral displacement of about 17 4 cm is observed whereas in the southwestern part of the network canal sal si puedes the deformation remains very weak this suggests that south of the canal de ballenas the plate boundary is locked a tectonic analysis of neogene and quaternary faults in baja california sonora and the central islands of the gulf permitted the reconstruction of the stress pattern evolution of this area these data also indicate the predominance of right lateral motion on a nw se trending zone within a regional framework characterized by an approximately n s compression and an e w extension the geodetic results are discussed in comparison with the neotectonic analysis and the seismic data available in the area the data suggest a broad strain accumulation zone covering the totality of the central gulf of california a nw se relative velocity of about 8 3 cm yr is found between the two sides of the gulf during the 1982 1986 interval

source processes of large earthquakes along the xianshuihe fault in southwestern china the xianshuihe fault is one of the most active faults in southwestern china recently three large earthquakes occurred along it in 1967 m s 6 1 1973 m s 7 5 and 1981 m s 6 8 the 1981 event occurred near the central portion of the fault zone modeling of the body and surface waves indicates pure left lateral strike slip motion on a vertical fault striking n40 w consistent with the surface trend of the xianshuihe fault two major ruptures are suggested for this source with a total moment of 1 3 10 26 dyne cm the 1973 event occurred about 65 km northwest of the 1981 event and ruptured about 90 km bilaterally along the fault the body wave synthetics indicate three main ruptures during faulting within 43 sec with a total moment of 1 8 10 27 dyne cm the mechanisms are similar to the 1981 event and the average slip is determined to be 3 8 m the largest aftershock m s 5 9 occurred 1 day after the main event with a normal fault mechanism striking almost perpendicular to the surface breakage the 1967 event occurred at the northwestern end of the fault zone with a strike of n65 e it had a nearly normalfault mechanism with a seismic moment of 4 5 10 25 dyne cm the largest aftershock m s 5 1 occurred 7 hr later with a similar focal mechanism the primary faulting along the xianshuihe fault is left lateral strike slip but the normal faulting with strike direction about perpendicular to the xianshuihe fault trace is common especially in the northwestern segment the faulting pattern in this region is consistent with the regional stress field caused by the india tibet collision the normal event which is not on the major fault seems to have more frequent foreshocks and aftershocks than those on the main fault

active deformation and shallow structure of the wagner consag and delfin basins northern gulf of california mexico oblique rifting began synchronously along the length of the gulf of california at 6 ma yet there is no evidence for the existence of oceanic crust or a spreading transform fault system in the northern gulf instead multichannel seismic data show a broad shallow depression 70 200 km marked by active distributed deformation and six 10 km wide segmented basins lacking well defined transform faults we present detailed images of faulting and magmatism based on the high resolution and quality of these data the northern gulf crust contains a dense up to 18 faults in 5 km complex network of mainly oblique normal faults with small offsets dips of 60 80 and strikes of n n30 e faults with seafloor offsets of tens of meters bound the lower and two upper delfin basins these subparallel basins developed along splays from a transtensional zone at the nw end of the ballenas transform fault twelve volcanic knolls were identified and are associated with the strands or horsetails from this zone a structural connection between the two upper delfin basins is evident in the switching of the center of extension along axis sonobuoy refraction data suggest that the basement consists of mixed igneous sedimentary material atypical of mid ocean ridges on the basis of the near surface manifestations of active faulting and magmatism seafloor spreading will likely first occur in the lower delfin basin we suggest the transition to seafloor spreading is delayed by the lack of strain partitioned and focused deformation as a consequence of shear in a broad zone beneath a thick sediment cover

a transform continental margin rich in hydrocarbons gulf of california conventional and near bottom geophysical surveys mapped shallow deformational structures and sediment accumulation patterns along part of the actively shearing continental margin of sonora including the intersection with a guaymas basin spreading center the principal strike slip faults occupy a zone 1 2 km 0 6 1 2 mi wide along the lower slope with a ridge of tightly folded sediments raised by uplift of sheared basement at the abrupt boundary between continental and oceanic crust this transform ridge and a continental rise on its seaward slope grows as the fault zone ages away from the spreading center intersection shoaling at a net rate of 500 m m y 1 600 ft m y despite erosion of its crest a clathrate horizon in the upper 80 m 260 ft of sediment is inferre from bottom simulating reflectors in the rise and lower slope hydrocarbon seeps occur at 1 600 m 1 mi below sea level along the crest of the transform ridge and patchy gas accumulation and seepage are recognized on a young marginal plateau

variation of seismic slip in the gulf of california and the possible effect on geodetic measurements of pacific north american plate motion the gulf of california is a unique tectonic transition zone which progresses from an oceanic ridge transform system at the mouth of the gulf to a predominantly continental transform system in southern california slip rates estimated from seismic moment data amount to about 17 to 30 of the accumulated tectonic slip in the gulf predicted by the nuvel 1 plate motion model even after accounting for the transition in crustal structure the seismic to tectonic slip ratios also appear to increase slightly from south to north this trend in the seismic moment release may reflect changes in the accommodation of pacific north american plate motion along the gulf which can be examined geodetically on the other hand geodetic measurements of the relative plate motion can be disturbed by short term effects associated with episodic seismicity to assess these effects the surface displacements due to typical transform events in the gulf are estimated using a simple dislocation model the results of this numerical calculation suggest that if a large transform event m0 1 5 1026 dyne cm were to occur within 100 to 200 km of a geodetic baseline for example one comprised in the geomex global positioning system network across the southern gulf the relative distance measurements could be affected by up to 15 mm this is marginally at the error level of a few millimeters plus 2 parts in 108 of baseline length for geomex measurements which thus are sensitive only to the far field displacement along the plate boundary over baselines of a few hundred kilometers in length

the 2006 bahia asuncion earthquake swarm seismic evidence of active deformation along the western margin of baja california sur mexico the study of the bahia asuncion earthquake swarm is important for two reasons first the earthquakes are clear evidence of present activity along the zone of deformation on the pacific margin of baja california the swarm with earthquakes of magnitude mw of up to 5 0 occurred on the coastline of the peninsula showing that the tosco abreojos zone of deformation is wider than previously thought second the larger earthquakes in the swarm caused some damage and much concern in bahia asuncion a small town located in the zone of epicenters we relocated the larger earthquakes with regional and or local seismic data our results put the earthquake sources below the urban area of bahia asuncion at 40 50 km to the north of the teleseismically determined epicenters in addition these new locations are in the area of epicenters of many smaller events that were located with data from local temporary stations this area trends in an e w direction and has dimensions of approximately 15 km by 10 km most earthquakes had sources at depths that are between 4 and 9 km a composite focal mechanism for the smaller earthquakes indicated right lateral strike slip motion and pure normal faulting occurred during this swarm interestingly the anss earthquake catalog of the united states geological survey usgs reported each one of these faulting styles for two large events of the swarm with one of these earthquakes occurring 2 days before the other one we associate the earthquake with strike slip mechanism with the san roque fault and the earthquake with the normal faulting style with the asuncion fault however there is need of further study to verify this possible relation between the faults and the earthquakes in addition we recorded peak accelerations of up to 0 63g with an accelerometer installed in bahia asuncion at this site an earthquake of mw 4 9 produced those high values at a distance of 4 1 km we also used the acceleration dataset from this site to estimate the linear response of sediments lying beneath the station the resulting average amplification function has a fundamental resonance frequency of about 5 hz and shows amplification factors of 2 4 for motions at frequencies in the range 2 8 hz a comparison of this amplification function with the response of soils to shaking from larger events shows that above 20 hz the amplification of motion in the larger events decreases relative to the weak motion response

seismicity and tectonics of the rivera plate and implications for the 1932 jalisco mexico earthquake seismicity on and near the boundaries of the rivera plate is examined in order to determine the subduction regime of the rivera plate and its influence on the earthquake behavior of the northernmost section of the mid america trench in particular we address the question of a rivera cocos boundary there have been several large historic earthquakes in the coastal areas of the mexican states colima and jalisco but the last large event was in june 1932 the 1932 jalisco earthquake ms 8 1 the present quiescence has lasted longer than the average recurrence interval for the mexican subduction zone and the jalisco area is now categorized as a seismic gap however if subduction in this area is dominated by the motion of the rivera plate which is subducting at half the rate of the cocos plate the deficiency of earthquake activity may not be unusual in an attempt to better delineate the rivera cocos boundary intermediate sized earthquakes along and near the rivera fracture zone and the mid america trench were relocated with the joint epicenter determination method we found that in general the catalog locations are fairly accurate with the average change in epicenter upon relocation being 12 km seismicity along the rivera fracture zone follows the physiographic trend of the fracture zone but east of its intersection with the east pacific rise seismicity is more diffuse two intermediate sized earthquakes in this diffuse area have focal mechanisms suggesting that they represent rivera cocos boundary motion reexamination of arrival time data for the 1932 jalisco earthquake yields an epicenter at 19 57 n 104 42 w close to the boundary zone inferred from these two earthquakes we conclude that the possibility that the 1932 jalisco earthquake broke the northernmost section of the cocos north american plate interface as opposed to the rivera north american plate interface cannot be ruled out

earthquake source mechanisms from body waveform inversion and intraplate tectonics in the northern indian ocean double couple point source parameters for 11 of the largest intraplate earthquakes in the northern indian ocean during the last 20 y were determined from a formal inversion of long period p and sh waveforms nine of the events have centroid depths at least 17 km below the seafloor well into the upper mantle two have centroid depths as great as 39 km using the source mechanisms of these earthquakes we distinguish two major intraplate tectonic provinces in the northern indian ocean to the west of the ninetyeast ridge in the southern bay of bengal intraplate earthquakes have thrust faulting mechanisms with p axes oriented n s the centroid depths of these earthquakes range from 27 to 39 km below the seafloor lithospheric shortening in this region is thus accomplished by thrust faulting in the strong core of the oceanic upper mantle while other geophysical evidence suggests that shallow sedimentary and crustal layers apparently deform predominantly by folding in the immediate vicinity of the ninetyeast ridge earthquakes display strike slip mechanisms with left lateral motion on planes parallel to the ridge this type of faulting occurs from at least 10 s to the northern end of the ninetyeast ridge near 10 n where the ridge meets the sunda arc seismic activity diminishes to the east of the ninetyeast ridge but is also characterized by strike slip faulting despite these variations in deformational style the inferred orientation of greatest compressive stress in the northern indian ocean displays a consistent long wavelength pattern over a large portion of the indian plate varying smoothly from nearly n s in the bay of bengal to nw se in the northeastern indian ocean this plate wide stress pattern and the high level of intraplate seismicity in the northern indian ocean are likely the results of substantial resistance along the himalayan continental collision zone to the continued northward motion of the western portion of the indian plate oceanic intraplate earthquakes in other regions where the level of deviatoric stress associated with the long wavelength part of the stress field is likely to be smaller need not be comparably reliable indicators of the plate wide stress field

rupture process of four medium sized earthquakes that occurred in the gulf of california 1 four medium sized earthquakes m w global cmt project 5 5 5 6 5 9 6 3 hereinafter named topolobampo angel de la guarda san lorenzo and loreto earthquakes respectively located in the gulf of california extensional province were studied to obtain their kinematic rupture processes a network of broadband seismic stations located around the gulf of california recorded the events network of autonomously recording seismographs baja and red sismica de banda ancha inversion of the seismic moment tensor and body waveform modeling were used to obtain the fault geometry and slip distribution on the fault plane respectively from these analyses we obtained source depths of the order of 5 5 0 5 km we found also that the source rupture processes of the topolobampo and angel de la guarda events have simple moment rate functions and source time durations of 5 0 1 2 and 4 2 1 2 s respectively the topolobampo event was a right lateral strike slip event and angel de la guarda was a normal event the san lorenzo and loreto shocks show a rather complex rupture with source time durations of 7 5 1 2 and 9 0 1 2 s respectively for these earthquakes we tested the resolution of numerical results performing an extra inversion with smoother waveforms the new inversions do not show the separated patches of slip as in the first analysis but the slip distribution has an elongated shape not characteristic of simple events we cannot therefore conclude rupture propagation for the san lorenzo event although the extent of the patch for the loreto earthquake agrees with aftershock locations estimates of source time durations for these earthquakes are at the upper limit of the values found for earthquakes elsewhere directions of p axes are in the same order of magnitude as the maximum horizontal stress obtained for the socalled gulf of california stress province from borehole elongations focal plane solutions and fault slip data

relative motion between the rivera and north american plates determined from the slip directions of earthquakes so far the direction and rate of relative motion between the rivera and the north american plates riv nam has been determined by the combination of two euler poles rivera riv with respect to pacific pac and pac with respect to north america here we estimate the relative motion of this plate pair riv nam assuming that the horizontal projection of the direction of slip of the earthquakes occurring on the riv nam boundaries reflect their relative plate motion a catalog of earthquakes for which focal mechanisms are reported since 1976 is used in the analysis earthquakes were considered in the three segments of the riv nam plate boundary the subduction zone of the rivera plate beneath the jalisco block the tres marias escarpment and the events associated with the tamayo fracture zone the best fitting euler pole is determined using a grid search of 64 potential poles the slip direction predicted for each grid point is compared to the slip direction of the focal mechanisms of the earthquakes on the plate boundary the best fitting euler pole determined in a root mean square sense rms is located at 21 8 n 107 6 w a rate of rotation of 5 3 year is estimated assuming the seismic earthquake cycle of the 1932 and 1995 great earthquakes represents a lower bound of the rate of plate motion in the subduction zone the best fitting euler pole shows that the subduction of the rivera plate takes place in a direction perpendicular to the trench with a relative velocity of 4 3 cm year offshore manzanillo the rate of relative motion riv nam decreases from se to nw north of approximately 21 n the subduction of the rivera plate becomes oblique to the trench and the relative velocity between the two plates decreases to an average of 1 9 cm year this slow rate of convergence may explain the rapid decrease of seismicity in the trench and the apparent absence of large earthquakes in this region in the tres marias escarpment our best fitting pole suggests that subduction stops giving way to high angle reverse faulting perpendicular to the tres marias escarpment in agreement with the reverse faulting earthquakes occurring here to the north of 22 5 n the slip predicted by the best fitting pole suggests right lateral faulting in a direction parallel to the tamayo fracture zone at a very low velocity 0 5 1 0 cm year the best fitting euler pole determined here lies very close to the riv nam plate boundary in the vicinity of the tamayo fracture zone this location of our best fitting euler pole explains the low relative plate velocity the relatively low level of seismic activity and the presence of a broad zone of deformation that accommodates the riv nam motion

episodic north america and pacific plate motions the north america plate motion for the period 20 0 m y is modeled by two absolute angular velocity vectors with a change occurring at about 9 m y the model incorporates new geologic constraints based on 1 the recognition of a discontinuity in north america plate motion at 9 m y 2 a reexamination of the iceland and yellowstone hotspot traces 3 improved delineation of the present north america pacific relative motion and 4 a revised history of pacific absolute motion the two stage model is consistent with several features of the late cenozoic geologic history of north america including geological observations relating to tectonism along a large portion of both the eastern and western boundaries of north america this plate model combined with a previously established pacific plate model makes specific predictions concerning the 15 0 m y history of the north america pacific plate boundary which compare favorably with the known history these models together resolve most of the discrepancies previously encountered between absolute and relative motion models of the two plates the implications of the north america plate model are the greatest for those events related to the 9 m y change in its motion among the geologic consequences of this change are rifting in the western united states intense compressional deformation along the northern caribbean plate boundary a hiatus in aleutian arc volcanic activity and a change in the orientation of the lesser antilles arc this picture reinforces the notion that the major plates behave rigidly and transmit stresses over great distances the predictive power of the model may help in interpreting the history of the north america plate boundary in those areas where clear observations are lacking an extra force needs to be applied to a plate to cause a change in plate motion by treating the plate as a rigid spherical shell overlying a uniformly viscous fluid we derive a simple class of forces sufficient to cause a given change in plate motion such force balance considerations indicate that an additional force applied in the northwest pacific region toward the northeast would result in the observed change in north america plate motion this force is surprisingly similar to that which apparently caused a large change in pacific plate motion about 5 m y later we propose that a common mantle process is responsible for both changes and is related to the ability of the northwest pacific subducted lithosphere to enter into the lower mantle

evolution of the margin of the gulf of california near loreto baja california peninsula mexico the gulf of california is a prime example of a young oblique divergent plate boundary this type of plate boundary is much less well understood than classic rifts and passive margins in divergent plate settings a complexity in the gulf of california is that the modern oblique rifting was preceded by a stage of orthogonal rifting we have used extensive mapping as well as stratigraphic geomorphic and structural analysis to determine the history of deformation near loreto on the southern baja california peninsula during formation of the gulf of california our data support the suggestion that middle to late miocene protogulf stage 12 to ca 6 ma orthogonal rifting was overprinted by transtensional structures during pliocene to quaternary time though we cannot closely define the time of change we further demonstrate that the plate margin was structurally segmented in the protogulf stage and then complexly overprinted by transtensional structures that suggest one type of process for initiating transform faults the loreto segment is 85 km long along the rift and is bounded on the west by a discontinuous series of aligned down tothe east extensional monoclines and normal faults these structures lie along or as far as 4 km in front east of a steep 1000 1600 m high main gulf escarpment that defines the western topographic margin of the rift zone the loreto segment is further defined by a rise in elevation of the main gulf escarpment of up to 1 km and an 500 800 m increase in structural relief of prerift strata in the escarpment from the segment boundaries to the center of the segment structural analysis of secondary faults of the loreto segment shows that fault populations that are known to be pliocene to quaternary in age are mixed normal and dextral normal faults with a bulk extension direction of west northwest eastsoutheast 2808 1008 in contrast fault populations that cut only prerift oligocene middle miocene volcanic and sedimentary rocks are mainly normal faults with northeast southwest to east northeast west southwest bulk extension directions that average 2458 658 these faults are interpreted to be late miocene in age and formed during protogulf orthogonal rifting thus the pliocene faults record a major change of extension direction of 358 from latest miocene to pliocene time compatible with the onset of oblique rifting in the gulf of california oblique divergent overprinting is mainly expressed as the loreto fault and loreto basin in the northern part of the segment the loreto fault and basin evolved with major fault reorganizations and partial basin inversion many observations suggest that the pliocene loreto fault was linked to two nascent transform faults in the gulf and that carmen island rotated 358 408 clockwise within the complex fault system

centroid depths and mechanisms of mid ocean ridge earthquakes in the indian ocean gulf of aden and red sea we have determined the centroid depths and source mechanisms of 14 earthquakes along spreading centers in the indian ocean gulf of aden and red sea the earthquakes are all characterized by normal faulting on planes dipping at about 45 and generally striking parallel to the local trend of the spreading center for the ridge axis earthquakes in the indian ocean gulf of aden and central red sea seismic moments range from 3 to 9 1024 dyn cm source time functions are all of simple form and centroid depths of events with adequate azimuthal coverage are all very shallow between 1 and 4 km beneath the seafloor if the centroid depth typically marks the mean depth of fault slip then earthquake faulting extended to 2 8 km beneath the seafloor during these earthquakes the centroid depths for two earth quakes near the mouth of the gulf of suez support the view that seismic faulting accompanying continental rifting generally extends to greater depth than does seismic faulting along the axes of oceanic spreading centers when these findings are combined with our earlier results from the mid atlantic ridge the greatest centroid depths of earthquakes with well constrained source mechanisms tend to shoal with increasing spreading rate this result supports a general decrease with spreading rate in the maximum thickness of the zone of brittle behavior for slow spreading ridges

active continental rifting in southern baja california mexico implications for plate motion partitioning and the transition to seafloor spreading in the gulf of california three major earthquake series have occurred during the past 30 years on fault systems that are spatially and kinematically distinct from oceanic spreading centers and transform faults in the southern gulf of california focal mechanism solutions for earthquakes of the april 1969 and june 1995 series indicate nearly pure normal displacement across nnw striking planes this seismicity falls along different strands of a major system of normal faults that extends at least 300 km along strike and defines the western limit of the gulf extensional province the dominant normal faults control distribution of neogene basins and have been active since middle to late miocene time epicenters of the august 1969 earthquake series lie south of the baja california peninsula and focal mechanism solutions indicate strike slip displacement this seismic activity is likely related to active wnw striking faults in the continental borderland which isolate baja california as a microplate within the pacific north american plate boundary therefore modern pacific north america plate motion is partitioned into at least three distinct fault systems oceanic spreading centers and transforms in the gulf of california normal faults along the western margin of the gulf of california and faults along the pacific margin of the baja california peninsula kinematic partitioning of plate motion is required to accommodate the strongly three dimensional strain of transtensional rifting continental rifting and seafloor spreading have overlapped in time for the past 3 6 myr which also is likely attributed to the highly oblique kinematics of rifting

focal depths and mechanisms of large earthquakes on the arctic mid ocean ridge system as part of a global study of the source characteristics and tectonic implications of large earthquakes on mid ocean ridges we report on the focal depths and mechanisms of the six largest earthquakes that have occurred in the last 20 years on the arctic mid ocean ridge system for each earthquake we invert the long period p and sh waveforms to estimate the parameters of the best fitting point source including seismic moment centroid depth double couple source orientation and source time function three of the earthquakes occurred on the oceanic spreading center in the eurasian basin along ridge segments spreading at half rates of 4 6 mm yr these events have mechanisms very similar to those of ridge crest earthquakes on the mid atlantic ridge almost pure normal faulting on planes that dip at approximately 45 and strike parallel to the rift axis moments of 4 5 1024 dyn cm centroid depths of 1 2 km beneath the seafloor and water depths inferred from the predominant period of water column reverberations appropriate to epicentral locations within the median valley the remaining three earthquakes also characterized by normal faulting are associated with the continuation of the divergent plate boundary 2 3 mm yr half rate onto the continental shelf of the laptev sea where the crust becomes transitional in nature one of the largest known spreading center earthquakes august 25 1964 m0 1 1026 dyn cm occurred where the oceanic ridge intersects the outer edge of the continental slope waveform inversion for this event can resolve unilateral rupture from north to south landward along a fault at least 30 km in length the preferred centroid depth is 5 km beneath the seafloor in crustal material with an unusually low shear velocity but a centroid depth as great as 15 km cannot be ruled out two earthquakes beneath the continental shelf have significantly greater centroid depths 10 20 km than mid ocean ridge earthquakes indicating a thicker brittle regime and a cooler thermal structure than are typical of oceanic spreading centers the tectonic environment of these events is more representative of rifted continental lithosphere than of a mid ocean ridge

heat flow hydrothermal circulation and basalt intrusions in the guaymas basin gulf of california abstract the guaymas basin is part of an en echelon series of short seafloor spreading segments in the gulf of california the basin is separated from adjacent deeps by long northwest southeast striking transform faults and divided into subparallel northern and southern troughs which trend northeast to southwest rapid sedimentation 1 to 5 m ka 1 results in a striking morphological contrast between seafloor spreading in the guaymas basin and that in more mature mid ocean settings basalt does not extrude onto the seafloor in the basin as flows and pillows but instead intrudes into a thick sediment layer the presence of relatively unconsolidated sediments lining the floor of the basin make this setting ideal for the study of axial heat flow and the thermal budget of oceanic crustal formation thermal data delineate several modes of heat transfer in both troughs of the basin with negligible heat flow in some areas and values greater than 3 w m 2 in others broad heat flow highs several square kilometers in area result from a combination of deep conductive heat flow slow intergranular fluid circulation and vigorous hydrothermal venting areas of low heat flow between heat flow highs and near normal faults which bound the basin may be indicative of hydrothermal recharge the geometry of faults shallow sills and broad northeast southwest structural trends in the basin largely seems to govern the distribution of heat flow the highest values are clustered within a few isolated zones in each trough with a total area of about 30 km 2 variations in heat flow of two orders of magnitude occur over distances as small as a few hundred meters fourteen hydrothermal vents in the southern trough of the guaymas basin have been sampled directly and found to have temperatures in excess of 250 c locally conductive heat flow at the seafloor is significantly raised in the vicinity of these vents but the general pattern appears to require more deeply seated conductive heat sources the mean heat flow in the southern trough is 453 342 s d mw m 2 a simple analysis of the heat budget in the southern trough suggests that the total intruded section between the top of the magma chamber and the seafloor may be unusually thin perhaps only several hundred meters in thickness

seismicity and state of stress in the la paz los cabos region baja california sur mexico we present the results of an analysis of seismic data collected for seven years in the la paz los cabos region the data set includes earthquakes with magnitudes of up to 3 6 and focal depths mostly between 2 and 14 km the results show that some located epicenters correlated with known faults of the region but others did not suggesting faults that have not yet been recognized in addition even though earthquakes occurred all over the study area zones of stress concentration were identified in such zones the seismicity often developed as earthquake swarms composite fault plane solutions were prepared by using data of those areas the resulting mechanisms for events of the northern part of the study area indicated normal faulting east side down in such case the p axis had a mean vertical angle of 55 in the n12 w direction whereas the t axis was nearly horizontal and with a n68 e average trend this part of the study area is found to be under a predominant tensional stress regime to the southwest of the area the focal mechanisms showed predominant components of strike slip faulting there the t axes were still subhorizontal but the p axes were more horizontal resulting in a mean plunge of about 20 in the n38 w direction on the other hand highly consistent directions of p and t axes from 13 representative events of the gulf of california fault system led to average axes that are close approximations to the principal stresses that drive such a fault system a comparison of these axes with average p and t axes of our region shows that the latter are about 16 to 23 degrees more westerly than the former we propose then either that the earthquakes in the study area occur more in response to local tectonic forces or that our inferred stress axes are only approximations to the principal stresses the complex tectonic situation of the region makes the first possibility feasible but the fact that the earthquakes occurred on pre existing faults favors the second one future seismic data of the region will help to elucidate this uncertainty finally the microearthquakes studied were not generated by the gulf of california fault system that forms the boundary between the pacific and north america plates our results then are evidence of a wide zone of deformation across which subsidiary faulting accommodates part of the relative plates motion

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 p wave upper mantle structure beneath an active spreading centre the gulf of california summary detailed analysis of short period travel time dt d and wave form data reveals the upper mantle structure beneath an oceanic ridge to depths of 900 km more than 1400 digital seismograms from earthquakes in mexico and central america recorded at scarlet yield 1753 travel times and 58 direct measurements of dt d as well as high quality stable waveforms the 29 events combine to form a continuous record section from 9 to 40 with an average station spacing of less than 5 km first the travel times are inverted using the tau method of bessonova et al the resultant model is adjusted to agree with the experimental p values further constraints arise from the observed relative amplitudes of mantle phases which are modelled by trial and error using wkbj synthetic seismograms chapman wiggins model gca which is appropriate for western mexico north of 20 latitude is similar to existing upper mantle models for shield tectonic continental and arc trench regimes below 400 km but differs significantly in the upper 350 km gca velocities are very low in this depth range the model catches up with the others with a very large velocity gradient from 225 to 390 km this well resolved feature is consistent with the shear wave model tna for western north america of grand helmberger the abundant data from 20 to 30 control the detailed shape of the 660 km discontinuity very large velocity gradients lie both above 620 660 km and below 661 680km a 2 8 per cent velocity change

late miocene to recent transtensional tectonics in the sierra san fermin northeastern baja california mexico basins and ranges within part of the gulf of california extensional province mexico have experienced complex distributed deformation including normal and strike slip faulting and block rotations linked to dextral shear at the pacific north america plate boundary in the sierra san fermin and southern sierra san felipe northeastern baja california normal faulting began between 12 5 and 6 ma although most extension occurred between about 6 and 3 ma strongly influencing thickness and distribution of ash flow tuffs and sedimentary deposits extension is generally 10 in 6 ma rocks and somewhat more in 12 5 ma rocks inversion of kinematic data interpreted together with published paleomagnetic data suggests that the axis of least principal stress was oriented between w e and sw ne in late miocene time our data indicate an important change in the amount of dextral shear but not necessarily the least principal stress direction wsw ene at about 3 ma structural constraints limit significant sinistral strike slip faulting conjugate to the dextral plate boundary to the last 3 my progressive changes in the geometry of faulting through time are consistent with regional strain partitioning within the pacific north america plate boundary zone and are predicted by physical and analytical models of oblique divergence as the orientation of the stretching vector changes to lower and lower values

translation of terranes lessons from central baja california mexico baja california is one of the best modern examples of a continental block or terrane that is translating hundreds of kilometers along a highly obliquely divergent plate the central domain of baja california is particularly appropriate as a modern analogue for terrane translation because it has a relatively simple late neogene history and is a buoyant continental block that is likely to be preserved in any future accretion to the north american continent in contrast the deep dense oceanic crust of the gulf of california where the main transform faults of the plate boundary lie has little preservation potential it is surprising that there is little strike slip faulting in the central domain of baja california even though the plate boundary has a rift angle angle between rift trend and azimuth of plate motion alpha of only 20 recent field data confirm modeling studies of oblique rifting and show that there is a predictable change in fault type fault orientation and extension direction with changing rift angle that is from secondary structures alone one can predict the approximate obliquity of rifting and the orientation of the plate motion with rift angles of 0 20 strike slip faults at low angles to the rift trend will dominate the secondary structures with rift angles from 20 to 35 there are strike slip faults subparallel to the rift trend and normal faults 20 40 clockwise from the rift trend in dextral shear above 35 normal faults are dominant and there are few or no strike slip faults as secondary structures

oceanic transform earthquakes with unusual mechanisms or locations relation to fault geometry and state of stress in the adjacent lithosphere on oceanic transforms most earthquakes are expected to occur on the principal transform displacement zone ptdz and to have strike slip mechanisms consistent with transform parallel motion we conducted a search for transform earthquakes departing from this pattern on the basis of source mechanisms and locations taken from the harvard centroid moment tensor catalogue and the bulletin of the international seismological centre respectively events with unusual mechanisms occur on several transforms we have determined the source mechanisms and centroid depths of 10 such earthquakes on the st paul s marathon owen heezen tharp menard and rivera transforms from inversions of long period body waveforms relative locations of earthquakes along these transforms have been determined with a multiple event relocation technique much of the anomalous earthquake activity on oceanic transforms is associated with complexities in the geometry of the ptdz or the presence of large structural features that may influence slip on the fault reverse faulting earthquakes occur at a compressional bend in the owen transform in the area of mount error and at the st paul s transform near st peter s and st paul s rocks a normal faulting earthquake on the heezen transform is located at the edge of a pull apart basin marking an extensional offset of the fault normal faulting earthquakes along the tharp menard and rivera transforms may also be related to extensional offsets some events with unusual mechanisms occur outside of the transform fault zone however and do not appear to be related to fault zone geometry for instance earthquakes with mechanisms indicating reverse faulting on ridge parallel fault planes are located near the ridge transform intersections of the st paul s and the marathon transforms possible additional contributors to the occurrence of anomalous earthquakes include recent changes in plate motion differential lithospheric cooling and the development of a zone of weakness along the fault zone but we do not find strong evidence to confirm the influence of these processes

p wave complexity and fault asperities the borrego mountain california earthquake of 1968 results from a synthetic seismogram analysis of the short period p waves from the borrego mountain earthquake of 9 april 1968 m l 6 4 are used to model the strong motion recording at el centro a short period long period deconvolution analysis of the teleseismic p waves suggested that a two source model would fit the data much better than the single source model presented by burdick and mellman 1976 forward and inverse modeling of the data demonstrated that two sources each of less than 2 sec duration the second occurring 2 2 sec after the first and both being at about 8 km depth best fit the short period waveforms from this model long period synthetics were generated which were found to be quite compatible with the data this source model was also used to synthesize the strong motion sh displacement velocity and acceleration records from el centro california the close match of synthetics and data is used to argue that short period waveforms contain much information about asperities which play a crucial role in the near source strong motions from an earthquake the borrego mountain event probably began with the failure of a fault asperity the evidence for this is the several hundred bars stress drops of the two short period sources and the probable location of these sources in a place where there was almost no aftershock activity or postseismic creep on the fault

the influence of the lithospheric thickness on bilateral slip summary a 2 d model of slip formation and dynamic propagation in an elastic lithospheric plate coupled to the asthenosphere and having a fault plane is investigated using a finite element method slip is nucleated at a weak portion of the fault plane and propagates bilaterally at the edges slip is assumed to develop according to a critical shear stress criterion combined with a slip weakening law except for the weak portion the slip resistance is constant along the fault plane the influence of resistive stresses from the asthenosphere at the bottom of the plate is taken into account the results indicate that bilateral slip lengths are limited to about the lithospheric plate thickness or somewhat larger such lengths will only be reached for sufficiently high tectonic loading after maximum slip length has been reached the slipping region is split into two regions of unidirectional slip moving in opposite directions the results thus illustrate one mechanism which limits the maximum amount of slip during an earthquake even in the absence of strong barriers

deformation of mexico from continuous gps from 1993 to 2008 1 we combine the velocities of 13 continuous global positioning system stations from mexico and 448 north american plate stations to better understand deformation and earthquake cycle effects in mexico velocities estimated at the mexican sites from high quality gps data collected since 2003 show no evidence for a previously reported eastward bias at sites in and near the yucatan peninsula the new velocities are compared to the predictions of two models one in which all motion in mexico is attributed to north american plate motion and the second of which attributes site motions to a combination of plate motion and the elastic effects of frictional coupling along the mexican subduction zone and faults in the gulf of california the second model fits the velocities within their estimated uncertainties mainland mexico thus moves with the north american plate to within 1 mm per year and undergoes elastic interseismic deformation far into its interior two stations inland from the guerrero and oaxaca segments of the mexican subduction zone have alternated between several year long periods of landward motion and several month long periods of trenchward motion frequently since 1993 consistent with previously described repeating transient slip events along the subduction interface the motions of two stations inland from the rivera plate subduction zone are dominated by the coseismic and postseismic effects of the m 8 0 9 october 1995 colima jalisco earthquake and m 7 5 22 january 2003 tecoman earthquake offshore from western mexico

centroid depths of mid ocean ridge earthquakes dependence on spreading rate from inversions of teleseismic p and sh waveforms we have determined the source parameters of 50 large earthquakes that occurred during 1962 1983 on slowly spreading mid ocean ridges all events are characterized by predominantly normal faulting on planes that dip at approximately 45 and strike parallel to the local trend of the ridge axis centroid depths range from 1 to 6 km beneath the seafloor the p waves from these earthquakes show strong water column reverberations suggesting that fault rupture extended to the seafloor under the assumption that the centroid depth marks the mean depth of fault slip earthquake faulting extended to a depth of 2 10 km for these earthquakes the water depths inferred from the predominant periods of the water column reverberations constrain the epicenters of these earthquakes to lie within the relatively deep inner floor of the median valley the maximum centroid depths of ridge crest earthquakes decrease with increasing spreading rate and the maximum seismic moment may also decrease with increasing spreading rate these results indicate a general decrease with spreading rate in maximum thickness of the mechanically strong layer beneath the ridge axis region the concentration of large earthquakes within the deepest parts of the median valley and the depth extent of seismic faulting support lithospheric necking models for the origin of the median valley

the tangshan earthquake sequence and its implications for the evolution of the north china basin the 1976 tangshan earthquake sequence that occurred on the northern margin of the north china sedimentary basin is one of the strongest and most destructive intracontinental earthquake sequences ever recorded by worldwide long period seismograph networks we have studied the source process of the six largest events of this earthquake sequence by formally inverting long period p and sh wave seismograms from the world wide standardized seismographic network our analysis shows that the main shock was caused by motion on at least three fault segments each with a different orientation the initial and dominant faulting occurred on two segments of a north northeast trending right lateral strike slip fault system the last stage of faulting had a significant component of thrusting and occurred on an east northeast trending subsidiary fault at the southern end of the main fault the largest aftershock occurred at the northeastern end of the main fault and had a nearly pure normal faulting mechanism with east west striking nodal planes this event occurred in a pull apart region where the north northeast trending main strike slip fault system steps to the right this region subsided by more than 1 m during the earthquake sequence to the north of this step the main fault is delineated by numerous aftershocks whose mechanisms are by and large consistent with right lateral slip on this segment the large event in 1945 m 6 3 probably occurred on this part of the fault a region of substantial subsidence up to 1 5 m associated with the earthquake sequence was also observed in the south where the main fault system takes another step to the right two of the large aftershocks occurred nearby and had strike slip mechanisms the overall faulting process of the tangshan sequence is well characterized by right lateral slip on a set of right stepping faults the mechanisms and the points of nucleation of the strongest shocks and the extent of the ruptures appear to be closely tied to preexisting fault geometries a compilation of the mechanisms of large earthquake sequences in the north china basin strongly suggests that the present deformation of the entire basin is dominated by motions on right lateral strike slip systems trending north northeast the basin seems to have formed and evolved as a large transtensional structure at an en echelon step of a large scale strike slip system the deformation within the basin is both strike slip and extensional and appears to occur on smaller scales in an approximately selfsimilar fashion as evidenced by the deformation during the tangshan sequence this interpretation provides an explanation for the variable rate of subsidence in both space and time and the scatter in measured heat flow values as well as the overall shape of the north china basin it also provides an explanation for occasional thrust faulting earthquakes that occur apparently as a result of geometrical complexities of the strike slip faults transpression these observations are otherwise difficult to fit into a simple model of regional crustal extension and thermal subsidence

structural controls on the continent ocean transition in the northern gulf of california in the gulf of california the pacific north america plate boundary changes character from an oceanic type spreading center and transform fault system to the south to a region of diffuse continental deformation to the north the presence of spreading centers commonly inferred in the northernmost gulf is not supported by bathymetric heat flow gravity or seismic data which indicate significant differences north and south of latitude 30 n we suggest instead that north of 30 n a continent ocean transition begins which we name the wagner transition zone wtz diffuse deformation characterizes the wtz where slip occurs along reactivated north to nnw striking normal faults developed during late miocene or pliocene ene directed extension transtensional deformation varies from ene directed extension along dip slip faults in the west to dextral shear along the coast to dextral oblique slip along inferred north to nnw striking faults submerged in the northern gulf by accounting for rotational and extensional plate motion deformation in northeastern baja california vector constraints require that submerged structures accommodate 30 mm yr of slip in a direction slightly clockwise of the relative plate motion direction the juxtaposition of the discrete spreading center system in the central gulf with the diffuse wtz appears to have been a stable configuration since 4 6 ma perhaps controlling the evolution of spreading center jumps between upper and lower tiburon and delfin basins due to the juxtaposition of kinematically partitioned structural domains different histories of prerift extension and subduction related arc magmatism along the length of the gulf partly related to the migration of the rivera triple junction may explain the location of the continent ocean transition

age dependence of oceanic intraplate seismicity and implications for lithospheric evolution we have determined depths for 16 oceanic intraplate earthquakes using body wave modeling a data set composed of these and 11 other well constrained depths shows that the maximum depth of seismicity deepens with increasing lithospheric age and appears to be bounded by a 700 800 c isotherm the maximum depth to which intraplate earthquakes occur is approximately equal to the flexural elastic thickness but much less than the seismic thickness from surface wave dispersion the maximum faulting depth is consistent with the predictions of rapid weakening at high temperatures for dry olivine rheologies determined by experimental rock mechanics oceanic intraplate earthquakes can occur at greater depths and temperatures than has previously been reported for continental crustal events presumably because oceanic lithosphere is stronger at high temperatures both the number of earthquakes and the cumulative seismic moment decrease with lithospheric age the cause of this trend is unclear possibilities include nonrandom location of preexisting weak zones with respect to the age provinces fault strengthening with age or a decrease in the stress level a decrease in intraplate stress with age is opposite that expected from ridge push stresses which increase with age but may result either from thermoelastic stresses which are greatest in young lithosphere or near ridge stress concentrations due to local heterogeneities in the spreading process

the mechanisms of earthquakes and faulting in the southern gulf of california abstract accurate earthquake locations and their focal mechanisms can illuminate the distribution and mode of deformation at rifted continental margins the pacific north america pa na plate boundary within the gulf of california goc provides an excellent opportunity to explore the evolution and kinematics of rifting as continental extension in the north transitions to seafloor spreading in the south from october 2005 to october 2006 an array of eight four component ocean bottom seismographs deployed in the goc recorded seismicity as part of the sea of cortez ocean bottom array scooba experiment by combining the data with those from the onshore network of autonomously recording seismographs nars baja array we detect and locate 700 earthquakes m w 2 2 6 6 mainly along the northwest southeast striking transform faults that delineate the plate boundary for 36 events m w 3 5 6 6 with high signal to noise ratio in a long period 10 20 s band we determine deviatoric moment tensors and associated double couple focal mechanisms by regional waveform inversion many focal mechanisms are consistent with right lateral strike slip faulting along the pa na transform fault system which suggests that the transform faults primarily accommodate seismic deformation within the gulf in addition we capture a swarm of events on baja california along the right lateral northwest southeast striking las viboras el azufre fault which may be related to ongoing geothermal activity and volcanic deformation within the peninsula the combination of high resolution earthquake locations and focal mechanisms improves our understanding of the distribution of seismic deformation within the greater extensional zone in the southern goc online material earthquake catalog of the 695 events detected and located during our combined on offshore deployment of seismometers

tectonic elements of the southern part of the gulf of california closely spaced seismic and bathymetric surveys in the southern province of the gulf of california reveal a tectonically active and structurally complex plate boundary while heat flow measurements seismicity large sediment accumulations on continental margins and the presence of oceanic crust in deep basins strongly suggest divergent plate motion magnetic anomaly patterns are conspicuously absent in the gulf as a whole the rate of sediment accumulation range 6 1 000 m m y however provides an alternative for the differentiation of two types of fault traces and suggests a process by which recently formed plate boundary zones evolve active faults manifest themselves as steep elongate scarps delineating the active plate boundary zone these fault traces appear to converge and diverge for example in the pescadero basin complex in a manner similar to the southern california borderland the floors of the intervening basins are flat and relatively sediment free relict faults also elongate fracture zones are distinguished from active traces in that they are completely filled with hemipelagic muds the relict faults are located along both sides of the active plate boundary zone and subparallel to it these features infer that plate motion is relatively rapid thus explaining the paucity of sediment accumulation in actively spreading basins in spite of high sedimentation rates substantial plate boundary readjustment in short time periods is suggested by the periodic abandoment and subsequent infilling and attachment of formerly active transform faults to the more stable margins of the plates involved the gulf of california therefore represents a working model of the evolution of a plate boundary and its sedimentological consequences in the early tectonic developmental stage of a passive margin ocean basin such as the atlantic

three dimensional temperature structure of a ridge transform ridge system abstract we have solved the problem of three dimensional heat transfer by conduction and material transport within a ridge transform ridge system the boundary conditions are those of a finite plate with temperatures fixed at the top and bottom we do not solve for the pattern of flow so must arbitrarily describe the material transport within triangular regions beneath the spreading centers flow is assumed to be vertical and at constant velocity with the volume of upwelling matching the volume of horizontal transport by the plates the steady state solution is approached by repeated finite steps in time each involving two stages first to represent material transport temperatures in the grid of points are shifted horizontally and or vertically to the point at which the associated particle would arrive given the time period and assumed velocity field second a three dimensional fourier transform technique is used to calculate the cooling which would occur in the static case during the same period we find that the geometry of flow in the upwelling region here parameterized by velocity of upwelling or width of the upwelling zone is more important than spreading rate in determining the temperature structure the results indicate that conductive cooling cannot alone account for deepening of the median valley towards the fracture zone for a 10 m y offset on the transform fault conductive cooling across the fracture zone to the older lithosphere affects the upwelling material only to a distance of about 10 km from the fault in contrast to the deepening of the median valley over several tens of kilometers observed on the mid atlantic ridge

source functions and path effects from earthquakes in the farallon transform fault region gulf of california mexico that occurred on october 2013 we determined source spectral functions q and site effects using regional records of body waves from the october 19 2013 m w 6 6 earthquake and eight aftershocks located 90 km east of loreto baja california sur mexico we also analyzed records from a foreshock with magnitude 3 3 that occurred 47 days before the mainshock the epicenters of this sequence are located in the south central region of the gulf of california goc near and on the farallon transform fault this is one of the most active regions of the goc where most of the large earthquakes have strike slip mechanisms based on the distribution of the aftershocks the rupture propagated northwest with a rupture length of approximately 27 km we calculated 3 component p and s wave spectra from ten events recorded by eleven stations of the broadband seismological network of the goc resban these stations are located around the goc and provide good azimuthal coverage the average station gap is 39 the spectral records were corrected for site effects which were estimated calculating average spectral ratios between horizontal and vertical components hvsr method the site corrected spectra were then inverted to determine the source functions and to estimate the attenuation quality factor q the values of q resulting from the spectral inversion can be approximated by the relations q text p 48 1 pm 1 1 f 0 88 pm 0 04 and q text s 135 4 pm 1 1 f 0 58 pm 0 03 and are consistent with previous estimates reported by vidales basurto et al bull seism soc am 104 2027 2042 2014 for the south central goc the stress drop estimates obtained using the 2 model are below 1 7 mpa with the highest stress drops determined for the mainshock and the aftershocks located in the ridge zone we used the values of q obtained to recalculate source and site effects with a different spectral inversion scheme we found that sites with low s wave amplification also tend to have low p wave amplification except for stations bahb guyb and sfqb located on igneous rocks where the p wave site amplification is higher

the 4 january 2006 mw 6 6 san pedro martir earthquake example of an earthquake for calibrating excitation and attenuation studiesthe mw 6 6 san pedro martir earthquake for calibrating excitation and attenuation studies

revealing the source of the 27 august 1810 loreto baja california tsunami from historical evidence and numerical modelling historical documents revealed that on 27 august 1810 an earthquake affected the loreto town and the surrounding region along 200 km of the coast of baja california mexico and damage was considerable due to strong ground motions however the 1810 loreto earthquake unlike other earthquakes of the nineteenth and twentieth centuries produced an extensive tsunami flooding on the eastern coast of baja california from loreto to la paz we report here the historical account of this event and analyse the source of the related tsunami based on the historical record and the tectonic setting we infer that this earthquake was produced along a transform fault right lateral strike slip fault farallon fault on the baja california gulf however the associated tsunami was disproportionally large if compared with the earthquake intensity ix and approximate magnitude 7 4 mw fault style and the relatively late tsunami arrival i e an hour after the earthquake suggesting that a submarine landslide triggered by this earthquake was the suspect source for the loreto tsunami we present here historical data and a numerical model of the loreto 1810 tsunami to elucidate the source using both the 1810 earthquake and a submarine landslide and confirm that a submarine landslide is most likely the tsunami triggering mechanism that followed the 1810 earthquake

earthquake source mechanisms and transform fault tectonics in the gulf of california we have determined the source parameters of 19 earthquakes in the gulf of california from an inversion of long period p and sh waveforms fifteen of the earthquakes on transforms between the cerro prieto fault and the tamayo fracture zone are characterized by right lateral strike slip faulting and all but one of these likely reflect the relative motion between the north american and pacific plates slip vectors from these events constitute an improved data set for determination of north american pacific motion most centroid depths are poorly resolved because of trade offs between depth and source time function both the ranges in acceptable centroid depths and the average fault width estimated from cumulative moment release during this century are compatible with the hypothesis that seismic slip does not extend below about 10 km depth or the approximate depth of the nominal 800 c isotherm along the principal oceanic transforms of the gulf two normal faulting earthquakes during a swarm in the northern gulf have centroid depths of 6 3 and 3 2 km consistent with earthquake centroid depths in other settings arctic northern red sea where plate separation has carried continental rifting nearly to completion a thrust faulting earthquake near the tres marias escarpment has a centroid depth of 11 2 km and probably reflects relative motion between the rivera and north american plates the transition from oceanic transform faulting to continental transform faulting in the northern gulf is marked by a broadening of the zone of deformation and a 10 change in the strike of the principal fault accommodating plate motion fault kinematics in the transition zone may be understood in terms of an unstable fff triple junction with the agua bianca and san miguel faults acting to transfer a portion of the plate motion to faults west of the san andreas system and with the delfin and wagner basins forming as a direct result of slip on the three fault systems meeting in the junction region

a revised estimate of pacific north america motion and implications for western north america plate boundary zone tectonics for the past 20 years much effort has been directed to determining the present day relative motion of the pacific and north american plates using two independent approaches one uses geologic observations and geodetic measurements along the san andreas fault and other faults in the plate boundary zone the other is based on plate motion models that incorporate spreading rates from marine magnetic anomalies transform azimuths and earthquake slip vectors geologic and geodetic studies find two principal elements of deformation slip along the san andreas of 34 mm yr directed n41 w and extension across the basin and range province of about 10 mm yr directed n56 w in contrast plate motion studies find 56 60 mm yr directed n35 w the discrepancy between these estimates a vector of about 15 mm yr oriented nearly due north is often attributed to a combination of slip along faults parallel to the san andreas and shortening normal to it here we revise the estimate of pacific north america motion by analyzing marine magnetic profiles from the gulf of california since 3 ma spreading has averaged 48 mm yr 10 mm yr slower than estimated before and consistent with the 49 mm yr spreading predicted by a new global plate motion model derived without any data along the pacific north america boundary the discrepancy with geodetic and geologic estimates is thus reduced to only 5 mm yr parallel to the san andreas 60 less than estimated before and 7 mm yr of shortening across the san andreas similar to prior estimates these results suggest that strike slip motion on faults west of the san andreas is less than thought before a conclusion consistent with geodetic seismological and other geologic observations

oxygen isotopic composition of lacustrine carbonates since 130 ka bp from a tianshuihai lake core tibet an overall increasing δ 18 o trend and its implications abstract the oxygen isotopic composition of lacustrine authigenic carbonates from a core taken from lake tianshuihai in western qinghai tibet plateau shows an overall increasing 18o trend oit from about 130 to 15 ka bp based on the argument by wei and gasse 1999 that the 18o of lacustrine carbonates from lakes in west china is controlled mainly by the 18o of the local precipitation we propose that this oit may have resulted from a similar oit of 18o of precipitation over the drainage area of the tianshuihai lake this oit suggests that from about 130 to 15 ka bp variations of 18o of precipitation on the qinghai tibet plateau were controlled mainly by changes not in temperature but in moisture sources especially in the ratio of precipitation of moisture from marine sources mms to that from continental evaporation mce the changes in the sources of the moisture and in the mms mce ratio may be mainly a function of the uplift of the qinghai tibet plateau

source mechanisms of earthquakes near mid ocean ridges from body waveform inversion implications for the early evolution of oceanic lithosphere to investigate the early tectonic evolution of oceanic lithosphere we present a synthesis of the characteristics of near ridge earthquakes i e earthquakes which occur off the axis of major mid ocean spreading centers but in lithosphere less than 35 m y old near ridge seismicity for the past two decades has been rather evenly distributed along the major ridge systems with the exception of the central indian ocean and the eastern part of the cocos plate which have experienced a high rate of earthquake activity during that time for 32 near ridge earthquakes we determined the double couple orientation seismic moment centroid depth and source time function using a formal inversion technique based on matching synthetic and observed p and sh waveforms all types of faulting styles are observed seismogenic deformation in young oceanic lithosphere is concentrated in the first 15 m y and occurs almost entirely in the uppermost mantle from just below the moho to depths at which the temperature estimated from standard thermal models exceeds 800 c thrust faulting is not observed at depths greater than 10 km below the seafloor the deepest near ridge earthquakes are generally characterized by normal faulting focal mechanisms with the t axis oblique or perpendicular to the local spreading direction most of these earthquakes occur in the central indian ocean the transition from mechanisms characteristic of ridge axes to those characteristic of the near ridge environment occurs within the first few million years for lithosphere less than 35 m y old horizontal compressive stress associated with the cooling and subsidence of the lithosphere ridge push does not appear to be the dominant source of stress released in earthquakes this result may be used to place a lower bound of several hundred bars on the typical stress difference in young oceanic lithosphere this bound together with the observed depth of faulting implies that the strain rate associated with seismic deformation is locally at least 10 15 s 1 we propose that thermoelastic stresses associated with plate cooling play an important role in near ridge earthquakes this source of stress can account for the mechanisms and depths of many near ridge earthquakes and for the concentration of activity in very young lithosphere the comparative aseismicity of the oceanic crust may reflect the concentration of thermoelastic stress below the moho due to different material properties for the crust and upper mantle or the early release of thermoelastic stresses through hydrothermal circulation and cracking throughout the crust at or near the ridge axis the higher level of near ridge seismicity in the indian ocean may be the result of modification or enhancement of the more typical near ridge stress state by such local processes as the continental collision between india and asia or active secondary convection beneath the young lithosphere of the indian plate

relative motions of the pacific rivera north american and cocos plates since 0 78 ma we use magnetic anomaly and fracture zone crossings from 3 n 24 n along the east pacific rise to define the boundaries and relative velocities of the rigid pacific rivera north american and cocos plates since 0 78 ma and to test several hypotheses regarding the nature of deformation between these plates crossings of the 0 78 ma isochron from the pacific rivera rise north of 22 0 n are significantly better fit by the 0 0 78 ma pacific north america rotation than by a pacific rivera rotation thereby supporting a recently proposed model for accretion of the northern part of the rivera plate to north america a significant misfit of the pacific cocos rotation to crossings of anomaly ln from the east pacific rise north of 16 4 n suggests that seafloor north of 16 n moves relative to the rigid pacific or cocos plates we also find that nine crossings of the eastern rivera fracture zone and eight crossings of anomaly ln flanking the manzanillo spreading segment are fit well by a pacific rivera rotation the rivera cocos angular velocity derived from the best fitting pacific cocos and pacific rivera angular velocities predicts 19 3 mm yr 1 95 confidence limit of n s convergence near the center of the rivera cocos boundary nearby left lateral strike slip earthquakes with north trending fault planes suggest that convergence is accommodated via sinistral shear along a north trending zone the unambiguous kinematic and seismologic evidence for significant rivera cocos motion indicates that the absence of a well defined rivera cocos plate boundary cannot be interpreted as evidence that rivera cocos motion is slow or zero as is postulated in several previous studies the rivera cocos angular velocity predicts oblique rivera cocos convergence at the el gordo graben and beneath the manzanillo trough and southern colima graben any extension across these features since 0 78 ma thus cannot have resulted from divergence between the subducting rivera and cococs plates