a new estimate for present day cocos caribbean plate motion implications for slip along the central american volcanic arc velocities from 153 continuously operatinggps sites on the caribbean north american and pacic plates arecombinedwith61newlyestimatedpacic cocosseafloor spreading rates and additional marine geophysical data to deriveanewestimate of present daycocos caribbean plate motion acomparisonofthepredictedcocos caribbean di rectiontoslipdirectionsofnumerousshallow thrustsubduc tion earthquakes from the middle america trench between costa rica and guatemala shows the slip directions to be deflected 10 clockwise from the plate convergence direc tion supporting thehypothesisthatfrequentdextralstrike slip earthquakes along the central american volcanic arc result from partitioning of oblique cocos caribbean plate convergence linear velocity analysis for forearc locations in nicaragua and guatemala predicts 14 2m m yr 1 of northwestward trench parallel slip of the forearc relative to thecaribbean plate possibly decreasing inmagnitude inel salvador and guatemala where extension east of the vol canic arc complicates the tectonic setting

stress evolution in southern california and triggering of moderate small and micro size earthquakes we calculate the evolution of stresses in southern california extending the study of deng and sykes 1997 by increasing from 6 to 36 the number of earthquakes for which coseismic changes in stress are computed and by expanding from m 6 to m 1 8 the range of magnitudes m of events whose focal mechanism solutions are examined in the context of the evolving stress field the cumulative stress on a given date is calculated with respect to an arbitrary zero baseline just before the 1812 wrightwood earthquake by taking into account the long term stress loading associated with 98 fault segments and coseismic stress changes for 36 significant earthquakes our calculations indicate that more than 85 of m 5 earthquakes from 1932 1995 occurred in regions of positive change in coulomb failure function cff most of the remaining about 15 earthquakes that occurred in areas of negative cff fall very close to boundaries between positive and negative cff some of which are sensitive to the less well controlled slip distributions of the earliest historic events calculations also show that from 1981 until just before the 1992 landers earthquake more than 85 of small m 3 and micro size m 1 8 shocks in the seeber and armbruster 1995 catalog with mechanisms involving either nw trending right lateral or ne trending left lateral strike slip faulting occurred in regions of positive cff the ratio of encouraged to all small and micro size events reaches a high value of about 88 if an apparent coefficient of friction between 0 0 and 0 6 is used the highest percentage of earthquakes occurred in areas where stress is about 1 mpa above the 1812 baseline most 66 events occurred in regions of cff between 0 0 and 2 0 mpa the upper limit indicates that the approximate range of stress variation in the earthquake cycle is of the order of 2 0 mpa the fact that the locations of most moderate small and micro size earthquakes are still related to stress changes remaining from large historical events might be used to constrain slip distribution of some of those earthquakes and to constrain the locations of future significant events

active strike slip faulting in el salvador central america several major earthquakes have affected el salvador central america during the past 100 yr as a consequence of oblique subduction of the cocos plate under the caribbean plate which is partitioned between trench orthogonal compression and strike slip deformation parallel to the volcanic arc focal mechanisms and the distribution of the most destructive earthquakes together with geomorphologic evidence suggest that this transcurrent component of motion may be accommodated by a major strike slip fault el salvador fault zone we present field geological structural and geomorphological data collected in central el salvador that allow the constraint of the kinematics and the quaternary activity of this major seismogenic strike slip fault system data suggest that the el salvador fault zone consists of at least two main e w fault segments san vicente and berlin segments with associated secondary synthetic wnw ese and antithetic nnw sse riedel shears and nw se tensional structures the two main fault segments overlap in a dextral en echelon style with the formation of an intervening pull apart basin our original geological and geomorphologic data suggest a late pleistocene holocene slip rate of 11 mm yr along the berlin segment in contrast with low historical seismicity the kinematics and rates of deformation suggested by our new data are consistent with models involving slip partitioning during oblique subduction and support the notion that a trench parallel component of motion between the caribbean and cocos plates is concentrated along e w dextral strike slip faults parallel to the volcanic arc

geological and seismological analysis of the 13 february 2001 mw 6 6 el salvador earthquake evidence for surface rupture and implications for seismic hazard the el salvador earthquake of 13 february 2001 mw 6 6 caused tectonic rupture on the el salvador fault zone esfz right lateral strike slip surface rupture of the east west trending fault zone had a maximum surface displacement of 0 60 m no vertical component was observed the earthquake resulted in widespread landslides in the epicentral area where bedrock is composed of volcanic sediments tephra and weak ignimbrites in the aftermath of the earthquake widespread damage to houses and roads and the hazards posed by landslides captured the attention of responding agencies and scientists and the presence of surface fault rupture was overlooked additionally the tectonic context in which the earthquake took place had not been clear until mapping of the esfz was completed for the present study we identified several fault segments the distribution of surface ruptures the aftershock pattern and fault rupture scaling considerations that indicate the 21 km long san vicente segment ruptured in the 2001 event static coulomb stress transfer models for the san vicente rupture are consistent with both aftershock activity of the 2001 sequence and ongoing background seismicity in the region at mw 6 6 the 2001 earthquake was of only moderate magnitude yet there was significant damage to the country s infrastructure including buildings and roads and numerous deaths and injuries thus earthquake hazard and risk in the vicinity of the esfz which straddles the city of san salvador with a population of 2 million is high because even moderate magnitude events can result in major damage deaths and injuries in the region

estimating exposure around san miguel volcano el salvador abstract san miguel one of the most active volcanoes in el salvador is surrounded by several villages and cantons which could be seriously affected in case of a new eruption this study presents an exposure analysis of the san miguel volcano area by considering population distribution land use private houses official buildings hospitals schools etc and communication infrastructure we use spatial analysis geographic information system tools to estimate exposure for different volcanic hazards air fallout lava flows pyroclastic density currents lahars moreover in the particular case of private houses and official buildings we estimate a vulnerability index for the hazardous areas applying the physical vulnerability methodology based on the characterization of the type of construction materials of walls and roofs we construct different exposure maps for 1 population 2 land use 3 road networks 4 schools and 5 health centers this approach identifies the elements at risk according to each potential hazard thus providing the authorities with a comprehensive tool to better understand the problem and to define emergency plans to minimize risk

neotectonic development of the el salvador fault zone and implications for deformation in the central america volcanic arc insights from 4 d analog modeling experiments the el salvador fault zone esfz is an active approximately 150 km long and 20 km wide segmented dextral strike slip fault zone within the central american volcanic arc striking n100 e although several studies have investigated the surface expression of the esfz little is known about its structure at depth and its kinematic evolution structural field data and mapping suggest a phase of extension at some stage during the evolution of the esfz this phase would explain dip slip movements on structures that are currently associated with the active dominantly strike slip and that do not fit with the current tectonic regime field observations suggest trenchward migration of the arc such an extension and trenchward migration of the volcanic arc could be related to slab rollback of the cocos plate beneath the chortis block during the miocene pliocene we carried out 4 d analog model experiments to test whether an early phase of extension is required to form the present day fault pattern in the esfz our experiments suggest that a two phase tectonic evolution best explains the esfz an early pure extensional phase linked to a segmented volcanic arc is necessary to form the main structures this extensional phase is followed by a strike slip dominated regime which results in intersegment areas with local transtension and segments with almost pure strike slip motion the results of our experiments combined with field data along the central american volcanic arc indicate that the slab rollback intensity beneath the chortis block is greater in nicaragua and decreases westward to guatemala

slab dip vs lithosphere age no direct function one paradigm of subduction relates the dip of the slab to the buoyancy of the downgoing lithosphere along subduction zones with the negative buoyancy proportional to the age of the oceanic lithosphere we measured the dip of the slab down to depths of 250 km along 164 sections crossing 13 subduction zones and compared it with the age of the subducting oceanic lithosphere both at the trench and at depth we show here that this relationship is far more irregular than previously suggested and that it is not possible to simply correlate the increase of the slab dip to the increasing age of the downgoing cooler lithosphere younger oceanic lithosphere may show steeper dip than older segments of slabs e g central america vs south america in contrast with predictions of models considering only slab pull the combination of slab age and subduction rate better accounts for slab dip however the correlation is not satisfactory correlation coefficient equal to 0 450 these results suggest that supplemental forces or constraints have to be accounted for such as thickness and shape of the hangingwall plate absolute plate velocity presence of lateral density variations in the hosting upper mantle effects of accretion erosion subduction of oceanic plateaus and slab deformation due to the motion of the mantle relative to the subducting plate d 2005 elsevier b v all rights reserved

neotectonic deformation within an extensional stepover in el salvador magmatic arc central america implication for the interaction of arc magmatism and deformation abstract dominantly westward movement of the el salvador forearc at rates of 11 mm yr is accommodated by a series of e w to wnw oriented dextral strike slip fault zones herein referred to as the el salvador fault system esfs the geometry of the esfs defines a series of extensional step overs along the arc basaltic volcanism in the stepovers is associated with nnw oriented normal faults whereas rhyolitic volcanism is associated with strike slip fault zones of the esfs on the esfs the san salvador extensional stepover sses is bound to the south by the san vicente fault zone where the rhyolitic ilopango caldera is located in the sses tephras from ilopango the tierra blanca tb sequence track long term elongation older tb units tb5 8 contain abundant normal faults lying unconformably above these older tb units younger tb members tbj tb2 4 are generally unfaulted analyses of faults in tb5 8 indicate ne to ene oriented elongation in the sses deformation occurred between deposition of the tb4 and tb5 units during quiescence of the ilopango eruptive center using this temporal constraint minimum elongation rates of 3 50 10 15 s 1 2 06 10 14 s 1 and 4 42 10 14 s 1 were calculated for three traverses from regional geodetic data and fault kinematics throughout el salvador we interpret the sses as part of a series of pull apart structures along the arc axis the calculated paleostress orientations are consistent with a pull apart geometry resulting from forearc movement the extensional deformation occurs during a 50 k y lull in rhyolitic activity suggesting an interplay between magmatism and deformation within the arc during significant rhyolitic volcanic activity only minor elongation is observed in the sses despite ongoing translation of the salvadoran forearc we speculate that rhyolitic magmatism along upper crustal faults may facilitate strike slip movement on the esfs rather than distributing deformation throughout the extensional stepover

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

radon signals related to seismic activity in ecuador march 1987 a radon concentration measurement survey was carried out in ecuador using the ssntd technique the data analysis shows a definite radon anomaly prior to march 6 1987 m 6 9 earthquake anomalies found before october 18 and october 28th mild earthquakes in 1986 could be correlated with these events other anomalies that we cannot correlate with any geophysical event were also found

cenozoic tectonics of the nicaraguan depression nicaragua and median trough el salvador based on seismic reflection profiling and remote sensing data lakes nicaragua and managua are the two largest lakes in central america and they cover a combined area of 9000 km 2 of the presently active nicaraguan depression and central america volcanic front as part of the subduction factory focus area of the u s national science margins program 1925 km of shallow geophysical data were acquired over lakes nicaragua and managua in may 2006 to establish their late quaternary structural and stratigraphic history and to better constrain regional models for active tectonics in western nicaragua the gulf of fonseca and the median trough in el salvador in order to investigate regional upper crustal deformation resulting from forearc sliver transport and or slab rollback of the cocos plate these new data were integrated with relocated earthquake epicenters earthquake focal mechanisms high resolution digital topography from the national aeronautics and space administration nasa shuttle radar topography mission srtm published global positioning system gps vectors onland geologic maps previous maps of lake bathymetry and bottom sediment types a previously unpublished regional aeromagnetic data set and multichannel seismic reflection profiles from the gulf of fonseca and pacific ocean these new data sets have improved bathymetric bottom sediment and recent fault maps for both nicaraguan lakes and can be used as new constraints on the regional geology and tectonics three regional structural cross sections across the nicaraguan depression indicate that the basin is a highly asymmetrical half graben bounded to the southwest by northeast dipping oblique slip normal faults late oligocene to holocene extension resulted in footwall uplift along the elevated and folded area of the nicaraguan isthmus and the area of greatest subsidence occurs on the footwall block underlying lake nicaragua in the southeast a similar but younger pattern of footwall uplift adjacent to the down thrown footwall block is present to the northeast beneath lake managua and the gulf of fonseca we interpret this structural pattern as a time transgressive rift opening where the oldest extension late oligocene early miocene began in the southeast and migrated to the northwest gps data indicate that this earlier phase of intra arc normal rifting is presently being superimposed by arc parallel right lateral shear related to the northwestward transport of the central america forearc sliver

fault kinematics in northern central america and coupling along the subduction interface of the cocos plate from gps data in chiapas mexico guatemala and el salvador new gps measurements in chiapas mexico guatemala and el salvador are used to constrain the fault kinematics in the north america na caribbean ca and cocos co plates triple junction area the regional gps velocity field is first analysed in terms of strain partitioning across the major volcano tectonic structures using elastic half space modelling then inverted through a block model we show the dominant role of the motagua fault with respect to the polochic fault in the accommodation of the present day deformation associated with the na and ca relative motion the na ca motion decreases from 18 22 mm yr 1 in eastern guatemala to 14 20 mm yr 1 in central guatemala assuming a uniform locking depth of 14 28 km down to a few millimetres per year in western guatemala as a consequence the western tip of the ca plate deforms internally with 9 mm yr 1 of east west extension 5 mm yr 1 across the guatemala city graben alone up to 15 mm yr 1 of dextral motion can be accommodated across the volcanic arc in el salvador and southeastern guatemala the arc seems to mark the northern boundary of an independent forearc sliver ar pinned to the na plate the inversion of the velocity field shows that a four block na ca co and ar model that combines relative block rotations with elastic deformation at the block boundaries can account for most of the gps observations and constrain the overall kinematics of the active structures this regional modelling also evidences lateral variations of coupling at the co subduction interface with a fairly high coupling 0 6 offshore chiapas and low coupling 0 25 offshore guatemala and el salvador

the ilopango tierra blanca joven tbj eruption el salvador volcano stratigraphy and physical characterization of the major holocene event of central america abstract the ilopango caldera is the source of the large tierra blanca joven tbj eruption that occurred about 1 5 ka years ago between ca ad270 and ad535 the eruption dispersed volcanic ash over much of the present territory of el salvador and pyroclastic density currents pdcs extended 40 km from the volcano in this study we document the physical characteristics of the deposits from all over el salvador to further constrain the eruption processes and the intensity and magnitude of the different phases of the eruption the succession of deposits generated by the tbj eruption is made of 8 units the eruption started with pdcs of hydromagmatic origin unit a0 followed by fallout deposits units a and b that are

mesozoic and early cenozoic sediment influx and morphology of the mozambique basin abstract the mozambique basin is one of the oldest extensional sedimentary basins developed along the eastern african margin the basin hosts a continuous record of sediments since the jurassic separation of antarctica from africa the objectives of this study were to extend the regional stratigraphic framework north of the zambezi delta into the deep abyssal plains and review the early evolution of the mozambique basin using nine multi channel seismic reflection profiles we identify six major stratigraphic units that were deposited in jurassic early cretaceous late cretaceous paleogene neogene and quaternary times mesozoic sedimentation rates of 5 10 cm kyr and 1 3 cm kyr during the paleogene are calculated in the deeper basin the presence of shales in neighbouring wells on the shelf implies an euxinic environment in the rapidly subsiding basin until early cretaceous times the mesozoic sediments have a high seismic velocity that exceeds 4 5 km s except in a distinct early cretaceous low velocity 3 7 km s zone that may indicate the presence of undercompacted overpressured shales in spite of the fact that the zambezi catchment was much smaller in pre miocene times the high late cretaceous sedimentation rates can be attributed to rapid denudation of the african continent after a major tectonic uplift episode at approximately 90 ma increased sediment influx into the basin from the zambezi in late cretaceous times resulted in the formation of an elongated submarine fan lobe into the mozambique channel north of beira high strong north south bottom currents commenced within the channel in late cretaceous times forcing the aggradation of sediments on the southern flank of the lobe in addition we observe several current controlled sediment deposits in the deeper basin that are influenced by north south bottom currents low paleogene sedimentation rates are attributed to a sediment starved basin during a relative quiet tectonic phase onshore

present day crustal deformation along the el salvador fault zone from zfesnet gps network abstract this paper presents the results and conclusions obtained from new gps data compiled along the el salvador fault zone esfz we calculated a gps derived horizontal velocity field representing the present day crustal deformation rates in the esfz based on the analysis of 30 gps campaign stations of the zfesnet network measured over a 4 5 year period from 2007 to 2012 the velocity field and subsequent strain rate analysis clearly indicate dextral strike slip tectonics with extensional component throughout the esfz our results suggest that the boundary between the salvadoran forearc and caribbean blocks is a deformation zone which varies along the fault zone we estimate that the movement between the two blocks is at least 12 mm yr 1 from west to east this movement is variably distributed between faults or segments of the esfz we propose a kinematic model with three main blocks the western central and eastern blocks delimited by major faults for the first time we were able to provide a quantitative measure of the present day horizontal geodetic slip rate of the main segments of esfz ranging from 2 mm yr 1 in the east segment to 8 mm yr 1 in the west and central segments this study contributes new kinematic and slip rate data that should be used to update and improve the seismic hazard assessments in northern central america

triggering of earthquake aftershocks by dynamic stresses it is thought that small static stress changes due to permanent fault displacement can alter the likelihood of or trigger earthquakes on nearby faults many studies of triggering in the near field particularly of aftershocks rely on these static changes as the triggering agent and consider them only in terms of equivalent changes in the applied load on the fault here we report a comparison of the aftershock pattern of the moment magnitude mw 7 3 landers earthquake not only with static stress changes but also with transient oscillatory stress changes transmitted as seismic waves that is dynamic stresses dynamic stresses do not permanently change the applied load and thus can trigger earthquakes only by altering the mechanical state or properties of the fault zone these dynamically weakened faults may fail after the seismic waves have passed by and might even cause earthquakes that would not otherwise have occurred we find similar asymmetries in the aftershock and dynamic stress patterns the latter being due to rupture propagation whereas the static stress changes lack this asymmetry previous studies have shown that dynamic stresses can promote failure at remote distances but here we show that they can also do so nearby

gps constraints on deformation in northern central america from 1999 to 2017 part 1 time dependent modelling of large regional earthquakes and their post seismic effects we use continuous and campaign measurements from 215 gps sites in northern central america and southern mexico to estimate coseismic and afterslip solutions for the 2009 m w 7 3 swan islands fault strike slip earthquake and the 2012 m w 7 3 el salvador and m w 7 4 guatemala thrust faulting earthquakes on the middle america trench our simultaneous time dependent inversion of more than 350 000 daily gps site positions gives the first jointly consistent estimates of the coseismic slips for all three earthquakes their combined time dependent post seismic effects and secular station velocities corrected for both the co seismic and post seismic deformation our geodetic slip solutions for all three earthquakes agree with previous estimates that were derived via static coseismic offset modelling our time dependent model which attributes all transient post seismic deformation to earthquake afterslip fits nearly all of the continuous gps site position time series within their several millimetre position noise afterslip moments for the three earthquakes range from 35 to 140 per cent of the geodetic coseismic moments with the largest afterslip estimated for the 2012 el salvador earthquake along the weakly coupled el salvador trench segment forward modelling of viscoelastic deformation triggered by all three earthquakes for a range of assumed mantle and lower crustal viscosities suggests that it accounts for under 20 per cent of the observed post seismic deformation and possibly under 10 per cent our results thus point to afterslip as the primary and perhaps dominant mode of post seismic deformation for these c the author s

a case of correlation between rn anomalies and seismic activity on a volcano vulcano island southern tyrrhenian sea a factor of 10 increase in the rn concentration in a shallow aquifer forefunning a shallow seismic swarm was observed at the island of vulcano aeolian island arc the peak of rn anomaly preceded by about one month the seismic swarm which had a cumulative magnitude of 2 1 the time lag between the two phenomena is much longer than expected given the small energy released by the swarm the observed phenomena may not have a direct cause effect relationship but they both can be a consequence of volcanic phenomena

the ilopango caldera complex el salvador stratigraphic revision of the complete eruptive sequence and recurrence of large explosive eruptions abstract ilopango caldera erupted episodically at least 13 tuff forming eruptions with a minimum estimate volume of 1 5 km3 dre per eruption reaching up to 150 km3 dre for the first caldera forming eruption all tuffs are of dacitic rhyolitic composition the complete pyroclastic sequence spans a range in time from 1 785 to 0 0015 ma and based on stratigraphy and geochronology constraints can be divided into three formations the comalapa altavista and tierras blancas formations in this work we focus on the members of the newly described altavista formation middle part of ilopango caldera volcanic sequence which consist of six consolidated pyroclastic deposits or tuffs each tuff corresponds to a specific eruption followed by a period of quiescence during which soil beds were developed on the deposits the ages of the altavista formation ranges from 918 to 257 ka based on new 40ar 39ar u pb zircon and u th zircon analyses the tuffs of this formation show similar characteristics in mineralogy and composition they are calcalkaline rhyodacitic tuffs with plagioclase clinopyroxene and hornblende from field mapping and descriptions of the deposits we have inferred the eruptive styles that include pumice fallouts pyroclastic density currents and also hydromagmatic explosions the common vent in all tuffs was the ilopango caldera and each member of the altavista formation could correspond to a caldera collapse event except for one of the six eruptions the volume of each member was estimated to be 30 km3 dre which is the same order of magnitude than that estimated for the tierra blanca joven tbj eruption at about 1 500 b p and smaller than those of the ignimbrites of the comalapa formation the first three members of the ilopango caldera reported previously the tuffs of the altavista formation are visible up to 15 20 km away from the caldera s topographic margin the recurrence interval of large explosive events at the ilopango caldera was established by integrating the stratigraphic and geochronologic data of all 13 ignimbrites and pumice fallouts erupted from ilopango caldera since the first one at 1 78 ma to the last explosive event tbj

the southern mozambique basin the most promising hydrocarbon province offshore east africa recent offshore acquisition of approximately 12 800 km 7 950 mi of seismic reflection data with some gravity and magnetic profiles encompassing the southern half of the mozambique basin reveals new facets of the subsurface geology integrated interpretation of these new geophysical data with old well information allows the development of depositional and tectonic models that enhance the basin s hydrocarbon potential previous drilling was sparse however predating modern seismic technology and exploration philosophy and left the area classified as a frontier province despite some encouraging hydrocarbon shows previous operators considered the entire basin gas prone and immature recent interpretation suggests that the basin s geothermal history must have been more favorable than generally was inferred from present day gradients and oil prone source rocks may be buried below the sequences drilled during wildcatting also marine oil prone source rocks may be present furthermore the liquid hydrocarbon sourcing potential of the karroo coal measures should not be underestimated wildcatters have not in retrospect optimally positioned exploratory wells in the vicinity of ample structural closures nor have they drilled to sufficient depth owing to poorly resolved outdated seismic data and to erroneous age assignments to lava flows therefore the viability of prospects mapped along a major offshore extension of the east african rift system delineated by this new survey has not been adequately assessed finally a well remains to be drilled within the rift to assess the presence of reservoirs and the maturity richness and volume of source rock sequences not necessarily represented over the eroded structures drilled to date end page 399

destructive upper crustal earthquakes of central america since 1900 we have compiled a catalog of 51 destructive upper crustal earthquakes in central america since 1900 an event is included if it caused casualties or heavy damage of modified mercalli mm intensity vii all events larger than magnitude ms 5 7 are found to be destructive we believe the catalog is complete for events of ms 6 along the volcanic front the catalog includes estimates of epicentral coordinates depth magnitude and casualties for all events and presents mm intensity vii contours for most events data in this catalog place severe constraints on the spatial temporal and magnitude distribution of destructive upper crustal earthquakes in central america the catalog contains 30 events of ms 6 depths of well constrained mainshock hypocenters range from 5 to 15 km the most striking feature of the catalog is the spatial alignment along the volcanic front of 23 of the 30 events volcanic front earthquake source mechanisms for such events show east west tension and are compatible with a model of the volcanic front as a zone of right lateral strike slip faulting volcanic front earthquake magnitudes do not exceed ms 6 5 except in southeastern guatemala where they reach ms 6 9 maximum earthquake magnitudes are apparently related to the distance between adjacent holocene volcanic centers destructive events occur at offsets in the volcanic chain as isolated mainshock aftershock sequences however within linear segments destructive earthquakes have a strong tendency to cluster and are often preceded by significant foreshock activity although the volcanic front is the locus of both destructive earthquakes and volcanism we find no temporal correlation between the two of the seven events of ms 6 that did not originate near the volcanic front six originated near the transcurrent caribbean north american plate boundary including the ms 7 5 guatemala earthquake of 1976 along the volcanic front destructive upper crustal earthquakes have occurred at a rate of about one event every 2 1 2 years during this century much more frequently than along either the caribbean north american plate boundary or the subduction zone in central america since 1900 about 17 000 people have died from volcanic front earthquakes but only about 2000 people are known to have died from subduction zone earthquakes although subduction zone earthquakes can have larger magnitudes and produce more widespread damage destructive volcanic front earthquakes pose the greater hazard because they are more frequent and originate much closer to the bulk of central america s population which is concentrated along the volcanic front

the separation of madagascar and africa identification of a sequence of east west trending magnetic anomalies of mesozoic age in the western somali basin helps define the position of madagascar in the gondwana reconstruction the anomalies are symmetric about ancient ridge segments and are flanked to the north and south by the jurassic magnetic quiet zone the motion of madagascar relative to africa was from the north and began in the middle jurassic about the same time as the initial breakup of gondwanaland sea floor spreading ceased when madagascar assumed its present position in the early cretaceous

the 13 january 2001 el salvador earthquake a multidata analysis 1 on 13 january 2001 a large normal faulting intermediate depth event mw 7 7 occurred 40 km off the el salvadorian coast central america we analyze this earthquake using teleseismic regional and local data we first build a kinematic source model by simultaneously inverting p and sh displacement waveforms and source time functions derived from surface waves using an empirical green s function analysis in an attempt to discriminate between the two nodal planes 30 trenchward dipping and 60 landward dipping we perform identical inversions using both possible fault planes after relocating the hypocentral depth at 54 km we retrieve the kinematic features of the rupture using a combination of the neighborhood algorithm of sambridge 1999 and the simplex method allowing for variable rupture velocity and slip we find updip rupture propagation yielding a centroid depth around 47 km for both assumed fault planes with a larger variance reduction obtained using the 60 landward dipping nodal plane we test the two possible fault models using regional broadband data and near field accelerograms provided by universidad centro americana 2001 near field data confirm that the steeper landward dipping nodal plane is preferred rupture propagated mostly updip and to the northwest resulting in a main moment release zone of approximately 25 km 50 km with an average slip of 3 5 m the large slip occurs near the interplate interface at a location where the slab steepens dip significantly the occurrence of this event is well explained by bending of the subducting plate

large magnitude crustal seismic sources in el salvador and deterministic hazard scenarios abstract during the last years several crustal seismic sources have been proposed in el salvador however the actual destructive potential of these proposals has not been revealed yet here we present several seismic scenarios related with the main crustal faults in the country we have characterized the corresponding hazard scenarios in a deterministic way estimating the peak ground accelerations pga including local site effects in addition we present a list of 29 sources that can be considered potentially dangerous with their main features when they are available and four seismic scenarios that we consider to be more probable we have resorted to the february 13th 2001 mw 6 6 destructive earthquake the only earthquake occurred in a known crustal source in el salvador and recorded by the strong motion network in order to test the methodological approach and the reliability of the pga results predicted by two available ground motion prediction equations gmpes including the site effect the consistence between the values given by the models and the ones recorded during the event provides reliability to our results this approach allowed us to conclude that faults within the volcanic arc can produce accelerations up to 1 g in the most conservator case mw 7 0 while accelerations up to 0 6 g are usual results related with specific faults within the salvadoran volcanic arc besides the reported damage distribution of the 8th june 1917 earthquakes and its similarity with one of the proposed scenarios allowed us to infer the possible source of this event the guaycume fault

the crustal structure of the central mozambique continental margin wide angle seismic gravity and magnetic study in the mozambique channel eastern africa abstract the continental margin of mozambique formed during the initial dispersal of gondwana about 180 ma due to the lack of deep seismic and dense potential field data many details of the timing and geometry of the early breakup in this region remained unknown to date to close this gap a research project mobamasis mozambique basin marine seismic survey with the french research vessel r v marion dufresne ii was conducted in 2007 this paper presents the results of p wave magnetic and 2d gravity modelling along two parallel seismic refraction profiles between 37 and 41 e crossing the mozambique rifted margin the crust shows the characteristics of normal to slightly thickened oceanic crust a lower crustal high velocity body with p wave velocities of 7 0 7 5 km s is observed along both profiles its origin is discussed in the context of upper mantle convection and thermal properties the existing magnetic anomaly identifications have been extended to older ages we postulate that the oldest oceanic crust near the central mozambique continental margin has been formed around m41n 166 ma closer to the coast a pronounced negative magnetic anomaly exists that we interpret to coincide with the continent ocean transition this implies that the position of the continent ocean transition is located significantly closer to the shoreline than proposed before

volcan el chichon mexico pre 1982 s rich eruptive activity abstract mapping and sampling of the interior of el chichon volcano was made possible by the exposures of the 1982 explosion crater the 1 km diameter 200 m deep crater exposes rocks produced during several eruptive episodes volcanic domes talus breccias plinian airfall deposits and pyroclastic flow deposits of compositionally similar alkalic hornblende trachyandesites with phenocrysts of anhydrite all of the rocks except those leached of anhydrite have extremely high s contents el chichon consists of a volcanic dome complex airfall debris and pyroclastic outflow sheets prior to 1982 its last major activity was about 650 100 yr b p

somali basin chain ridge and origin of the northern somali basin gravity and geoid low the northern somali basin located between chain ridge and the horn of africa north of 4 n is characterized by a distinct 5 m geoid low and by large negative gravity anomalies the boundaries of the basin are marked by steep gradients in both gravity and geoid basement in northern somali basin is 1 2 km deeper than on the carlsberg ridge flank to the southeast or on the sheba ridge flank to the north with a sharp discontinuity across the boundary of the basin the western somali basin to the south was created in the late jurassic and early cretaceous by the movement of madagascar away from africa reinterpretation of the magnetic anomalies in the western somali basin shows that they record both limbs of a mid ocean ridge that was active by m22 time kimmeridgian and died soon after mo aptian magnetic and gravity data allow the relict ridge crest to be traced from davie ridge near the african coast to the dhow vlcc ars fracture zone complex at 50 e davie ridge is a transform fault connecting the western somali basin spreading center with a similar age spreading center in the mozambique basin the dhow vlcc ars complex can be shown to continue north of the 4 n bathymetric high separating the northern and western somali basins and to intersect the african margin near 7 n the northern somali basin thus appears to be the third of a series of oceanic basins separated by long transform faults created during movement between east and west gondwanaland the original northern somali basin was split apart by the northward motion of india in the late cretaceous and chain ridge formed along the new boundary thermal and gravity modeling shows that the flexure resulting from differential subsidence across chain ridge combined with the difference in lithospheric thermal structure late jurassic vs early tertiary on either side of it explains well the amplitude and shape of the observed geoid step and gravity anomalies across chain ridge the geoid step up from the basin up to the african coast can be modeled as an edge effect between the juxtaposed jurassic oceanic and african continental crust thus the geoid and gravity low over the northern somali basin results from the superposition of a continental edge effect anomaly and the fracture zone edge effect anomaly

triggering of destructive earthquakes in el salvador we investigate the existence of a mechanism of static stress triggering driven by the interaction of normal faults in the middle american subduction zone and strike slip faults in the el salvador volcanic arc the local geology points to a large strike slip fault zone the el salvador fault zone as the source of several destructive earthquakes in el salvador along the volcanic arc we modeled the coulomb failure stress cfs change produced by the june 1982 and january 2001 subduction events on planes parallel to the el salvador fault zone the results have broad implications for future risk management in the region as they suggest a causative relationship between the position of the normal slip events in the subduction zone and the strike slip events in the volcanic arc after the february 2001 event an important area of the el salvador fault zone was loaded with a positive change in coulomb failure stress 0 15 mpa this scenario must be considered in the seismic hazard assessment studies that will be carried out in this area

radon emanometry in soil gases and activity in ashes from el chichon volcano radon 222em emanation measurements in soil gases are reported in connection with the 1982 eruptions of el chichon volcano 222em detection is performed with lr 115 cellulose nitrate track detectors results show a general decreasing pattern of222em concentration in soil with time after the eruptions activity measurements of radon daughters in ashes show that a large amount of222em and220em was released from magma degassing during the eruptions and that only a fraction of the degassed magma was erupted an important fluid interaction between the magma chamber and the surrounding sedimentary rock is also suggested

holocene eruptive activity of el chichon volcano chiapas mexico geologic and radiometric age data indicate that el chichon was frequently and violently active during the holocene including eruptive episodes about 600 1250 and 1700 years ago and several undated older eruptions these episodes involving explosive eruptions of sulfur rich magma and associated dome growth processes were apparently separated by intervals of approximately 350 to 650 years some of el chichon s eruptions may correlate with unusual atmospheric phenomena around a d 1300 and possibly a d 623

reconstruction of madagascar and africa evidence from the davie fracture zone and western somali basin new seismic reflection gravity and magnetic data from offshore east africa allow the davie fracture zone to be traced from 11 s to its intersection with the kenyan coast at 2 s constraining the relative motion of madagascar and africa seasat derived free air gravity anomalies and slope rise positive magnetic anomalies observed in shipboard data help to locate the continent ocean boundaries cob off the shore of east africa and madagascar seismic reflection data further document a diapirprovince off madagascar presumably conjugate to that observed off kenya and somalia the dhow and very large crude carrier vlcc basement ridges are complex features and do not appear to be simple fracture zones owing their existence entirely to the separation of madagascar and africa from these data we determine a predrift fit of madagascar and africa involving a 14 2 rotation of madagascar to africa about a pole at 10 n 150 e the geometry of the reconstruction adheres to seismic and potential field data indicating the oceanic nature and extent of the comoros basin and of the somali basin between kenya and the seychelles and it does not conflict with onshore or offshore stratigraphy timing of the opening of the western somali basin is constrained by mesozoic marine magnetic anomalies and extrapolation to the interpreted cob and occurred between approximately 165 and 130 ma

crater lake and post eruption hydrothermal activity el chichon volcano mexico abstract explosive eruptions of volcan el chichon in chiapas mexico on march 28 and april 3 4 1982 removed 0 2 km3 of rock to form a 1 km wide 300 m deep summit crater by late april 1982 a lake had begun to form on the crater floor and by november 1982 it attained a maximum surface area of 1 4 105 m2 and a volume of 5 106 m3 accumulation of 4 5 m of rainfall between july and october 1982 largely formed the lake in january 1983 temperatures of fumaroles on the crater floor and lower crater walls ranged from 98 to 115 c by october 1983 the maximum temperature of fumarole emissions was 99 c in january 1983 fumarole gas emissions were greater than 99 vol h2o with traces of co2 so2 and h2s the water of the lake was a hot t 52 58 c acidic ph 0 5 dilute solution 34 046 mg l 1 dissolved solids cl s 20 5 sediment from the lake contains the same silicate minerals as the rocks of the 1982 pyroclastic deposits together with less than 1 of elemental sulfur the composition and temperature of the lake water is attributed to 1 solution of fumarole emissions 2 reaction of lake water with hot rocks beneath the lake level 3 sediments washed into the lake from the crater walls 4 hydrothermal fluids leaching sediments and formational waters in sedimentary rocks of the basement 5 evaporation and 6 precipitation

is there continental crust underneath the northern natal valley and the mozambique coastal plains 1 to draw conclusions about the crustal nature and history of the natal valley and the mozambique ridge systematic potential field data were obtained during the aistek iii cruise with r v pelagia in 2009 this paper presents and interprets the results of that expedition the new magnetic data reveal a pattern of linear magnetic spreading anomalies nw se trending in the southwestern part of the mozambique ridge and e w trending on its central part the ariel graben which separates the mozambique ridge from the northern natal valley is represented by a pronounced negative anomaly in the magnetic as well as the free air gravity field the northern natal valley bears a complicated pattern of anomalies with mainly sw ne trends in the northern natal valley no indications for a continent ocean boundary between continental crust in the north and oceanic crust in the south exist either in the free air gravity or in the magnetic field the magnetic wavelengths of the mozambique coastal plains are similar to those of the northern natal valley and the mozambique ridge particularly in the gravity data the mozambique coastal plains the northern natal valley and the mozambique ridge appear as one continuous geological province we interpret the region from the mateke sabi monocline in the north to the southwestern tip of the mozambique ridge as mainly being floored by thickened oceanic crust

development of the mozambique and ruvuma sedimentary basins offshore mozambique two major sedimentary basins have been identified on the continental margin of mozambique mozambique basin and ruvuma basin the formation of the basins is related to the break up of gondwana and opening of the western part of the indian ocean the basins are relatively young having developed discordantly to the structural plan of gondwana sedimentary basins the history of the formation of the east african continental margin sedimentary basins within mozambique has been studied on the basis of the present day concept of gondwana break up and madagascar s drift with respect to africa two stages in the history of the east african basins can be recognized late gondwana and post gondwana the late gondwana stage 303 157 ma is typified by sedimentary and volcanogenic rocks of the karoo group the post gondwana stage 157 0 ma corresponds to the period of active gondwana break up and the formation of indian ocean marginal basins the mozambique basin occupies both central and southern parts of the coastal plain of mozambique extending onto the continental shelf and slope the sedimentary fill is composed of upper jurassic cretaceous and cenozoic rocks which discordantly overly the karoo basalts the upper jurassic occurs as continental red beds mostly distributed within buried grabens cretaceous rocks occur as terrigenous sediments of continental and marine genesis cenozoic deposits are of predominantly marine and deltaic origin the ruvuma basin situated in the north of mozambique is part of an extended east african marginal basin which includes parts of the coastal plains and continental margins of tanzania and kenya the basement of the basin is composed of crystalline and metamorphic rocks of pre cambrian age the sedimentary fill is represented by continental terrigenous karoo sediments marine and lagoonal jurassic deposits and marine and deltaic rocks of cretaceous and cenozoic age the main difference between the sedimentary fill of the mozambique and ruvuma basins is related to the changing sedimentary environment in the period of gondwana break up jurassic and early cretaceous when the drift of madagascar and sea floor spreading took place and the marine basin gradually extended southwards between the east african margin and madagascar from the late cretaceous on the sedimentary environment within both basins was similar the mozambique and ruvuma basins forming a single system on the east african continental margin the main stages of the basins development are illustrated by a series of palaeogeographic reconstruction maps

constraints from finite element modeling on the active tectonics of northern central america and the middle america trench 1 we have developed an elastic finite element model in order to study the role of the different forces acting on the northwestern part of the central american volcanic arc and the chortis block we present synthetic focal mechanisms maps of tectonic regime and strain crosses to analyze the results the models show that to achieve the observed state of stress on the volcanic arc the arc must be modeled as a lithospheric weak zone also the forces related to the eastward drift of the caribbean plate must be higher than those related to the subduction of the cocos plate the coupling on the subduction interface must be low with or without slip partitioning due to the obliquity of the subduction at the trench at guatemala the western edge of the chortis block is pinned against north america even with low trench normal forces making the triple junction between the cocos north american and caribbean plates a zone of diffuse deformation the extension in the western part of the chortis block from guatemala to the honduras depression is explained by the geometry of the north american caribbean plate boundary and the direction of motion of the caribbean plate with respect to north america the direction of extension in the chortis block is always e w regardless of the magnitude of the applied forces and the main part of the deformation is absorbed between the ipala graben and the honduras depression both features being consistent with our models

solid sediment load history of the zambezi delta abstract we calculate the solid sediment load history of the zambezi river for the last 120 ma i e middle cretaceous recent times our starting point is a detailed grid of two dimensional seismic reflection profiles calibrated by well log information after conversion from two way travel time to depth we made simple assumptions about the compaction history in order to construct isochore maps of solid sediment load and yield as a function of geological time our results show that solid sediment load has varied by 1 order of magnitude over the last 120 ma there have been three periods of elevated flux the first period occurred in late cretaceous times 90 65 ma and was synchronous with the rapid denudation of southern africa recognised by for example apatite fission track modelling the second period occurred in oligocene times 34 24 ma during proposed rapid regional uplift of southern africa the youngest phase of elevated flux started in late miocene times 10 ma and has continued to increase rapidly through to the present day a large proportion of this increase can be attributed to a doubling of the size of the zambezi catchment during the pliocene at other times we suggest that load variations can be used to bound the uplift history of the region which encompasses the catchment finally changes in the shape of the zambezi delta through time suggest that the mozambique current which sweeps through the mozambique channel from northeast to southwest initiated at the start of the miocene

a model of plate kinematics in gondwana breakup summary an accurate model of relative plate motions in gondwana breakup is based on visual fitting of seafloor isochrons and fracture zones fz from the riiser larsen sea and mozambique basin used predictively the model precisely locates kinematic markers in the west somali basin which allows the conclusion that the spreading centres in the west somali and mozambique basins and the riiser larsen sea formed parts of the boundary between the same two plates the locations of fz and less well defined isochrons from neighbouring regions are also consistent with their formation on other lengths of this same boundary and with its relocation from the west somali basin and northern natal valley to the west enderby basin and lazarev sea during chron m10n small independently moving plates thus played no role in the breakup of this core part of gondwana in an inversion procedure the data from these areas yield more precise finite rotations that describe the history of the two plates separation breakup is most simply interpreted to have occurred in coincidence with karoo volcanism and a reconstruction based on the rotations shows the lebombo and mateke sabi monoclines and the mozambique and astrid ridges as two sets of conjugate volcanic margins madagascar s pre drift position can be used as a constraint to reassess the positions of india and sri lanka in the supercontinent

geology basin analysis and hydrocarbon potential of mozambique and the mozambique channel abstract the phanerozoic structural and stratigraphic history of mozambique can be separated into a karoo and a post karoo phase outcrops of karoo rocks limited to the zambezi valley and small outliers in the mozambique belt to the north show an eastward facies change marked by an increased argillaceous content and the appearance of carbonates the wide coastal plain and immediate offshore of southern mozambique is marked by extensional horsts and grabens with a mesozoic fill covered by cenozoic sediments little is known of lower stratigraphic horizons in this coastal province there is a facies transition from continental clastics in the northwest and west to more lacustrine and marine deposits to the southeast and east upwards the succession passes into the cenozoic without a noticeable break in the offshore in the north seismic lines help define two basins the zambezi delta basin and the mozambique channel basin separated by the beira high in the south several offshore grabens have been defined one of which the south mozambique graben has been examined here by an integrated basin analysis technique the technique which uses available depositional structural and thermal data provides information on maturation and the timing of hydrocarbon maturation the analysis showed variation in the extension rate with time and two phases of rifting the greatest rate of extension accumulation and subsidence between 96 and 76 ma depends upon interpretational assumptions during this time interval fault blocks were systematically tilted domino fashion this event broadly coincides with the rifting between madagascar and antarctica a period of plate reorganization the earlier rifting phase was probably related to the strike slip separation of madagascar from africa modelling heat flow suggests that the later event is the more important from the maturation hydrocarbon standpoint the offshore and potentially the deeper onshore post karoo basins have traps reservoirs and seals with the presumed source rock within lower cretaceous or jurassic strata there is at the present time no means to evaluate the potential of the karoo rocks

constraints for the recent tectonics of the el salvador fault zone central america volcanic arc from morphotectonic analysis abstract we have used hypsometric analysis to improve our understanding of the current tectonic deformation and structure of el salvador fault zone a n90 e oriented strike slip fault zone that extends 150 km through el salvador central america our results indicate an important amount of transtensive strain along this fault zone providing new data to understand the tectonic evolution of the salvadorian volcanic arc we have defined kilometric scale tectonic blocks and its relative vertical movements length of segments with homogeneous vertical motions and lateral relay of active structures we have identified and quantified slip rate variations along strike of the el triunfo fault within el salvador fault zone ranging from 4 6 mm year in its central parts to 1 mm year towards the tips of the fault this study supports the hypothesis of a recent rotation in the maximum shortening direction and the accommodation of the current deformation through the reactivation of pre existing structures inherited from a previous tectonic regime

a geotectonic history of the northern mozambique basin including the beira high a contribution for the understanding of its development abstract the interpretation of a comprehensive two dimensional 2d seismic reflection data set discloses several rifting events for a typical passive rifted margin two major phases have been identified rift i phase is in agreement with the initial rifting of gondwana postulated for the early jurassic this rift resulted in break up stage 1 governed by a north eastern motion of antarctica along the reactivated pebane shear zone indications for lava flows were identified in the seismic profiles along the offshore zambezi depression they form a link between early rifting the initial break up and early sea floor spreading the nature of these lava flows may be associated with the emplacement of thick volcanic dykes during post rift magmatism that occurred when the antarctica plate with the beira high drifted dextrally from the west to east until the mid jurassic times it is more likely that the break up in stage 1 resulted from high tension due to strain relief of rift i phase above all the extensional deformation occurred in a narrow rift mode therefore the v shape of the offshore zambezi depression suggests a possible rift failure structure this is best explained by the reeves and de wit model 2000 this model postulates that the motion of antarctica changed to southward direction at around 170 ma consequently a rift jump from the offshore zambezi depression in the northwest to the south eastern edge of the beira high occurred and rift ii phase may have started leading to the break up in stage 2 during this stage rift ii phase the extension migrated towards the east thus thinning the crust and exhuming the sub continental mantle in the continental ocean transition zone the rift ii phase shows a sequence of half graben morphologies confining the syn rift infill that is subdivided in three units syn rift i syn rift ii and rift sag all three units appear to have developed under minor extensional regimes in the crust evidenced by gently dipping low angle detachment faults with respect to the presence of the two rift phases it is deduced that break up and sea floor spreading are diachronous within rift i and rift ii segments

the arce tephra two subsequent paroxysmal plinian eruptions from coatepeque caldera el salvador abstract the coatepeque volcanic complex in el salvador produced at least four plinian eruptions within the last 80 kyr the eruption of the 72 ka old arce tephra formed the coatepeque caldera and was one of the most powerful explosive eruptions in el salvador hitherto it was thought that the arce tephra had been emplaced only by one mostly plinian eruptive event that ended with the deposition of a thick ignimbrite however our stratigraphic geochemical and zircon data reveal a temporally closely spaced double eruption separated by a gap of only a couple of hundred years and we therefore distinguish lower and upper arce tephras both eruptions produced in the beginning a series of fallout units generated from fluctuating eruption columns and turning wind directions the final phase of the upper arce eruption produced surge deposits by several eruption column collapses before the terminal phase of catastrophic ignimbrite eruption and caldera collapse mapping of the individual tephra units including the occurrences of distal marine and lacustrine ash layers in the pacific ocean the guatemalan lowlands and the caribbean sea result in 25 6 km3 tephra volume areal distribution of 4 105 km2 and eruption column heights between 20 33 km for the lower arce eruption and 40 5 km3 tephra volume including 10 km3 for the ignimbrite distributed across 6 105 km2 and eruption column heights of 23 28 km for the upper arce eruption these values and the detailed eruptive sequence emphasize the great hazard potential of possible future highly explosive eruptions at coatepeque caldera especially for this kind of double eruption

gps constraints on deformation in northern central america from 1999 to 2017 part 2 block rotations and fault slip rates fault locking and distributed deformation

enhancement of radon signals in geophysical studies with the track technique the existence of a suspected geological fault has been confirmed using solid state nuclear track detectors ssntds by measuring radon concentration variations in the upper soil above its inferred position the results obtained prompted us to increase the natural radon signal in the soil using an additional radon source this enhancement technique has been experimentally checked with ssntd detectors

corrigendum to a geotectonic history of the northern mozambique basin including the beira high a contribution for the understanding of its development mar pet geol 36 2012 1 12

the jurassic history of the africa antarctica corridor new constraints from magnetic data on the conjugate continental margins abstract finding the best fit for east and west gondwana requires a detailed knowledge of the initial jurassic movements between africa and antarctica this study presents results of systematic and densely spaced aeromagnetic measurements which have been conducted in 2009 2010 across the astrid ridge antarctica and in the western riiser larsen sea to provide constraints for the early seafloor spreading history between both continents the data reveal different magnetic signatures of the northern and southern parts of the astrid ridge which are separated by the astrid fracture zone the southern part is weakly magnetised corresponding to the low amplitude anomaly field of the southwestern riiser larsen sea in contrast the northern astrid ridge bears strong anomalies of positive value furthermore several sets of trends are visible in the data in the mozambique channel we extended the existing magnetic spreading anomaly identifications combined with the spreading anomalies in the conjugate riiser larsen sea they were used to establish a new model of the relative movements of africa and antarctica after the breakup of gondwana in jurassic times a detailed model for the emplacement of the mozambique ridge is now incorporated the model postulates a tight fit between africa and antarctica and two stages of breakup the first of which lasting until 159 ma m33n during this stage antarctica rotated anticlockwise with respect to africa the grunehogna craton cleared the coastal plains of mozambique to a position east of the mozambique fracture zone the southern astrid ridge is interpreted to consist of oceanic crust also formed during this first stage prior to the riiser larsen sea during the second stage antarctica moved southward with respect to africa forming the mozambique basin and the conjugate riiser larsen sea the mozambique ridge and the northern natal valley were formed at different spreading centres being active subsequently

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

gps derived coupling estimates for the central america subduction zone and volcanic arc faults el salvador honduras and nicaragua summary we invert gps velocities from 32 sites in el salvador honduras and nicaragua to estimate the rate of long term forearc motion and distributions of interseismic coupling across the middle america subduction zone offshore from these countries and faults in the salvadoran and nicaraguan volcanic arcs a 3 d finite element model is used to approximate the geometries of the subduction interface and strike slip faults in the volcanic arc and determine the elastic response to coupling across these faults the gps velocities are best fit by a model in which the forearc moves 14 16 mm yr 1 and has coupling of 85 100 per cent across faults in the volcanic arc in agreement with the high level of historic and recent earthquake activity in the volcanic arc our velocity inversion indicates that coupling across the potentially seismogenic areas of the subduction interface is remarkably weak averaging no more than 3 per cent of the plate convergence rate and with only two poorly resolved patches where coupling might be higher along the 550 km long segment we modelled our geodetic evidence for weak subduction coupling disagrees with a seismically derived coupling estimate of 60 10 per cent from a published analysis of earthquake damage back to 1690 but agrees with three other seismologic studies that infer weak subduction coupling from 20th century earthquakes most large historical earthquakes offshore from el salvador and western nicaragua may therefore have been intraslab normal faulting events similar to the m w 7 3 1982 and m w 7 7 2001 earthquakes offshore from el salvador alternatively the degree of coupling might vary with time the evidence for weak coupling indirectly supports a recently published hypothesis that much of the middle american forearc is escaping to the west or northwest away from the cocos ridge collision zone in costa rica such a hypothesis is particularly attractive for el salvador where there is little or no convergence obliquity to drive the observed trench parallel forearc motion

the zambezi river tectonism climatic change and drainage evolution abstract the longitudinal profile of the zambezi river forms two concave upwards sections with their boundary at the victoria falls this form has been ascribed to the process of pediplantation and the victoria falls identified as one of several knick points that have traversed the river since the breakup of gondwanaland an alternative model explains the river s long profile by suggesting that the upper and middle zambezi evolved as entirely separate river systems which only joined together in comparatively recent times the alluvial sequence of the middle zambezi is described and interpreted in terms of the latter hypothesis the river capture event caused a change in grade and is marked by a deposit believed to record a cataclysmic flood capture is inferred to have resulted from overtopping of the lake that formed the end sink of the proto upper zambezi rather than from headward erosion of the proto middle zambezi this event is dated from archaeological evidence to the peak of the last interglacial at the end of the middle pleistocene subsequent drainage diversions between the middle zambezi and the kalahari are interpreted as the product of rifting of the chobe graben and aggradation of the chobe swamps river capture by overtopping implies high rainfall over central southern africa at the peak of the last interglacial which is contrary to predictions that africa s rain belts then lay north of their modern mean positions this anomaly is resolved by postulating a southern polar warming episode at that time and agrees with suggestions that the last interglacial was marked by large scale ablation of the west antarctic ice sheet