comparison of different site response estimation techniques using aftershocks of the 1999 izmit earthquake on 17 august 1999 the m w 7 4 izmit earthquake occurred in northwestern turkey a temporary seismic network was installed to improve the geometry of a previously installed network in this study 262 aftershocks of the izmit earthquake with magnitude m l ranging between 0 4 and 4 5 were analyzed using digital recordings from 17 stations of these networks s wave and p wave spectral records corrected for path attenuation function were inverted by means of the generalized inversion technique git the git site responses were compared with those calculated by horizontal to vertical h v ratios applied to both earthquake data and to pre event noise in most cases the h v of the earthquake data provided site responses with shapes consistent with those obtained using git however the level of amplification was occasionally found to be different in general these discrepancies can be explained either by the amplification affecting the vertical component of the ground motion that we detected by git or by the waves having a propagation in almost the vertical direction using the h v ratio applied to noise for most cases we obtained not only the peaks for the fundamental resonance frequencies but site responses with shapes similar to those obtained by git the detected level of amplifications using the h v ratio showed a tendency not to exceed those calculated by the inversion finally amplifications up to a factor 5 are found for stations placed over complex topography for these sites amplifications are not always correctly identified by the h v technique using either earthquakes or noise

3 d rotation of double couple earthquake sources summary we discuss 3 d rotations by which one double couple earthquake source can be rotated into another arbitrary double couple due to the symmetry of double couple sources there are four such rotations an algorithm is obtained in analytical form which is also available as a computer program solving the inverse problem of 3 d rotation of double couple earthquake sources i e for each pair of focal mechanisms or seismic moment tensor solutions the program finds all four rotations which rotate one mechanism into another this algorithm may be used in a wide variety of studies of stress field causing earthquakes investigations of the relationship between the focal mechanisms and the tectonic features of a seismogenic region etc the same inversion algorithm can be used to study the 3 d rotation of any symmetric second rank tensor such as the stress or strain tensor

the volcanic and geochemical development of sao nicolau cape verde islands constraints from field and 40ar 39ar evidence abstract we present 34 new age results from 40ar 39ar incremental heating analyses of groundmass separates from volcanic rocks from sao nicolau cape verde combining the age results with field observations we show that the volcanic activity that formed the island occurred in four separate stages 1 6 2 5 7 ma 2 4 7 2 6 ma 3 1 7 1 ma and 4 8 wt basic to ultrabasic ranging from nephelinites through basanites to picrobasalts evolved rocks range to phonolites during all four volcanic stages predominantly high mgo rocks were erupted with 8 19 wt mgo olivine microprobe data from eight highly porphyritic of these rocks had fo86 88 demonstrating that even crystal enriched rocks represent primitive melts and in general it can be concluded that mantle melts reached the surface after only little modification during much of the development of sao nicolau only during stage 2 magma chambers were established and highly developed magmas erupted at this time the major part of the island was formed the new results constrain the evolution of sao nicolau to have taken place during a later time interval than previously assumed with more ages of only around 50 ka the youngest volcanic rocks found on the island indicate that stage 4 may still be active the cyclicity of sao nicolau volcanism suggests that mantle plume material arrived under sao nicolau in batches separated by nonfertile material

seismic radiation from regions sustaining material damage we discuss analytical results for seismic radiation during rapid episodes of inelastic brittle deformation that include in addition to the standard moment term a damage related term stemming from changes of elastic moduli in the source region the radiation from the damage related term is associated with products of the changes of elastic moduli and the total elastic strain components in the source region order of magnitude estimates suggest that the damage related contribution to the motion in the surrounding elastic solid which is neglected in standard calculations can have appreciable amplitude that may in some cases be comparable to or larger than the moment contribution a decomposition analysis shows that the damage related source term has an isotropic component that can be larger than its double couple component

distribution of seismic velocities and attenuation in the crust beneath the north anatolian fault turkey from local earthquake tomography we investigate the crustal structure beneath the western part of the north anatolian fault zone nafz an area where at least five damaging earthquakes occurred during the twentieth century this study is based on local earthquake tomography using the data from aftershocks of the izmit event 17 august 1999 m 7 4 recorded by stations of permanent and temporary networks we derive the distribution of v p v s and the v p v s ratio based on the iterative inversion for both v p v s and v p v p v s using the lotos code innovatively in this study we perform an inversion for frequency dependent s wave attenuation 1 q s the reliability of the results is assessed through synthetic tests the distributions of the resulting seismic parameters v p v s v p v s and q s highlight important geodynamical features in the study area the low velocity and high attenuation patterns mostly correlate with the fracturing zones of the nafz low velocities are also observed beneath the main sedimentary basins e g adapazari duzce and kuzuluk high velocity and low attenuation patterns correlate with blocks presumed to be rigid kocaeli armutlu and almacik blocks the rupture traces of the largest earthquakes in this area pass generally in the transition areas between high and low velocities while moderate and weak seismicity is mostly concentrated in low velocity areas based on these results we propose and discuss the role that the almacik block could have played in producing the largest earthquakes in the study area in the twentieth century

inversion for parameters of tensile earthquakes the tensile source model generalizes the shear source model by assuming that the slip vector can be arbitrarily oriented with respect to the fault and is not constrained to lie within the fault plane the proposed inversion for the parameters of tensile sources is based on the evaluation of the isotropic iso compensated linear vector dipole clvd and double couple dc components in seismic moment tensors the most significant parameters inverted are the ratio at the fault denoted as the parameter and the inclination of the slip vector from the fault the parameter is significant for discriminating noisy moment tensors of shear earthquakes from those of tensile earthquakes the inclination can be accurately determined from the dc component in the moment tensor because the dc component rapidly decreases with increasing for example the inclination of 20 causes dc being 50 60 only the inversion is applied to earthquakes which occurred in january 1997 in west bohemia czech republic it is shown that some of these earthquakes display tensile faulting the parameter is 0 1 the inclination of the slip from the fault attains values of up to 20 this inclination is a result of tensile traction and reduced shear traction along the fault and high fluid pressure in the region

the surface rupture and slip distribution of the 17 august 1999 izmit earthquake m 7 4 north anatolian fault the 17 august 1999 izmit earthquake occurred on the northern strand of the north anatolian fault zone the earthquake is associated with a 145 km long surface rupture that extends from southwest of duzce in the east to west of hersek delta in the west detailed mapping of the surface rupture shows that it consists of five segments separated by releasing step overs herein named the hersek karamursel golcuk izmit sapanca lake sapanca akyazi and karadere segments from west to east respectively the hersek segment which cuts the tip of a large delta plain in the western end of the rupture zone has an orientation of n80 the n70 80 e trending karamursel golcuk segment extends along the linear southern coasts of the izmit gulf between karamursel and golcuk and produced the 470 cm maximum displacement in golcuk the northwest southeast striking golcuk normal fault between the karamursel golcuk and izmit sapanca segments has 2 3 m maximum vertical displacement the maximum dextral offset along the izmit sapanca lake segment was measured to be about 3 5 m and its trend varies between n80 e and east west the sapanca akyazi segment trends n75 85 w and expresses a maximum displacement of 5 2 m the karadere segment trends n65 e and produced up to 1 5 m maximum displacement the karadere and sapanca akyazi segments form fan shape or splaying ruptures near their eastern ends where the displacement also diminished

a volcanological and geochemical investigation of boa vista cape verde islands 40ar 39ar geochronology and field constraints abstract boa vista the easternmost island in the cape verde archipelago consists of volcanic products minor intrusions and a thin partial sedimentary cover the first 15 age results from 40ar 39ar incremental heating analysis of groundmass separates from volcanic and plutonic rocks from boa vista are presented the combination of age results and field observations demonstrates that the volcanic activity that formed the island occurred in three main stages 1 16 ma 2 15 12 5 ma and 3 9 5 4 5 ma the first stage restricted to the north eastern part of the island is dominated by ankaramitic lavas the second stage consisting of evolved lavas of phonolitic trachytic compositions and nepheline syenites makes up large central parts of the island the large volume of evolved rocks and the extended eruption period of several ma make stage 2 in boa vista unique to cape verde mainly basanites and nephelinites were erupted during the third stage initially dominated by eruption of subaerial mafic lavas around 9 ma pillow lavas are erupted around 7 ma whereupon dominantly subaerial mafic lavas were erupted stage 3 saw volcanism in many centres distributed mainly along the present coastline and with activity partly overlapping in time the volcanic evolution of boa vista constrains the initiation of volcanic activity in the cape verde archipelago to the eastern islands major and trace element geochemistry of 160 volcanic and plutonic rocks representing the entire exposed chronological sequence on boa vista is presented revealing an extremely well developed daly gap only a little was modified from the mafic magmas that rose in small batches from the mantle compositional variation distinguishes each volcanic complex and was to a large extent present in the mantle melts the highly evolved stage 2 phonolites and trachytes are related through the fractional crystallization of three compositionally distinct magmas two of these may have been derived by crystal fractionation of primitive boa vista melts whereas the third was not

sensitivity of coulomb stress change to the parameters of the coulomb failure model a case study using the 2008 mw 7 9 wenchuan earthquake the coulomb stress change has been widely employed to interpret mainshock mainshock and mainshock aftershock triggering as well as interactions amongst earthquake faults and volcanoes this quantitative index is computed based on the coulomb failure criterion and is a function of fault parameters including the source and receiver fault geometries the friction coefficient on the receiver fault and skempton s coefficient of the host rock thus for the robust determination of the coulomb stress change the sensitivity of the coulomb stress change to these model parameters should be thoroughly assessed however notwithstanding numerous case studies almost no systematic investigation of the sensitivity of the coulomb stress change has been performed here we present an error estimator for the coulomb stress change and then quantitatively investigate the sensitivity of the coulomb stress change to the fault model parameters for the 2008 mw 7 9 wenchuan earthquake our results indicate that for this case the coulomb stress change is the most sensitive to the uncertainty in the dip angle of the receiver fault while the influences of the uncertainties in the slip model of the source fault the strike and rake angles of the receiver fault and the friction and skempton s coefficients cannot be neglected accordingly it is crucial to perform a realistic estimate of the uncertainty in the coulomb stress change by performing such calculation future coulomb stress analyses such as the stress triggering of earthquake sequence and the likelihoods of potential earthquakes could be based on more robust coulomb stress change maps

an 40ar 39ar study of the cape verde hot spot temporal evolution in a semistationary plate environment the 40 ar 39 ar analyses of 28 groundmass separates from volcanic rocks from the islands of santiago sal and sao vicente cape verde archipelago are presented the new age data record the volcanic evolution for santiago from 4 6 to 0 7 ma for sal from around 15 to 1 1 ma and for sao vicente from 6 6 to 0 3 ma the major submarine constructional phase of santiago was erupted within a few hundred thousand years interval around 4 6 ma most of the subaerial santiago volcanic rocks were erupted in a second episode from 3 3 to 2 2 ma and late volcanism occurred at 1 1 0 7 ma volcanism on sal evolved in five stages 1 poorly constrained early miocene activity 2 16 14 ma 3 12 8 ma 4 around 5 4 ma and 5 1 1 0 6 ma sao vicente was constructed during three active periods 1 6 6 5 9 ma 2 4 7 4 5 ma and 3 0 3 ma sr isotope analyses of carbonates from maio confirm an early cretaceous age for limestones deposited on the seafloor and later uplifted the cape verde rise is indicated to have fully formed in the early miocene around 22 ma accompanied by the initial alkaline volcanism considerable volcanism on sal boa vista and maio took place in the miocene and pliocene and extended over much larger areas than the present islands whereas volcanism of the southwestern and northwestern island groups developed mainly during the pliocene and pleistocene and was mostly confined to the present island areas the periods of volcanic activity may be broadly correlated between the northwestern and southwestern groups of islands young volcanism 0 3 0 1 ma throughout the northwestern group extends along a 150 km long nw se trending lineament a relatively moderate average melting rate for the hot spot over the 22 ma period is estimated at 0 026 km 3 a corresponding to a total volume of 570 x 103 km 3 of magma emplaced in the crust and a mantle volume flux of 28 m 3 s much lower than iceland or hawaii the archipelago is situated to the south and sw of the center of the mantle plume anomaly and ahead of its relative movement the timing and location of volcanism suggest that mantle melting takes place in three channels an eastern one that has been active for 22 ma and in southwestern and northwestern channels since late miocene

structure composition and age of the small islands of santa luzia branco and raso cape verde archipelago abstract the northern alignment of cape verde archipelago contains three small islands in its central sector santa luzia branco and raso on which there is very scarce geological petrological or geochronological information available the three islands along with the island of sao vicente arise on a continuous underwater relief less than 50 m deep and about 70 km long that formed from separate volcanic edifices the southernmost island raso is formed by accumulation of several tens of lava flows and some remaining strombolian cones an erosion surface covered by sediments allows the separation of two different episodes of growth the lower one older than 2 ma and the upper one between 0 9 and 1 2 ma the smaller island of branco is the remnant of a large stratovolcano about 6 km in radius 1000 m of altitude and between 5 and 6 ma in age santa luzia the northernmost and largest island of this particular group is essentially a remaining rift zone between 4 and 7 ma old at least 600 m high about 10 km long and 7 km wide the se extreme of the island is occupied by rocks of an older partially submarine stage of growth which also include gabbroic intrusions crossed by a dense network of dikes whose age may be estimated in 8 ma or more on the contrary the nw end shows the only remains of late post erosion activity that is of much less volumetric importance and it has been dated between 0 8 and 1 4 ma the three islands as a whole are formed by moderately or strongly alkaline basic and ultrabasic rocks similar to those found in the neighboring island of sao vicente ultraalkaline rocks only appear in raso and differentiate types as trachyandesites and phonolites are exclusively found in santa luzia the similar age found for the main stages of growth recorded in the mentioned four islands and even more in the other two islands santo antao and sao nicolau situated at the end of the barlovento alignment are not easily comprehensible with evolution models implying a simple spatial progression of the volcanic activity from east to west in this array of the archipelago on the other hand the temporal coincidence in periods of quiescence observed in the different islands seems to point to the existence of a cyclical activity for the ensemble

the geological history of maio cape verde islands the oldest igneous rocks on maio are pillow lavas of mid ocean ridge pillow basalts character which have been tilted and uplifted about 4 km from the ocean floor to outcrop as a partial ring dipping steeply away from a central plutonic complex made up of pyroxenites essexites syenites and carbonatites the ocean floor volcanic rocks are overlain conformably by a stratigraphically continuous pelagic carbonate succession which demonstrates a shallowing depositional environment from the upper jurassic to upper cretaceous times when tuffaceous beds indicate renewed volcanism the tuffs are associated with rudites demonstrating the emergence of the island and amongst the clasts are plutonics indicating upper cretaceous magmatism and the unroofing of the volcano to a substantial depth deformation under compressive stress resulted in the folding and local repetition by thrusting of this sedimentary cover which together with the plutonic core had been intensively injected by major sills the mesozoic succession has been planed off and overlain with marked unconformity by a largely neogene sequence of volcanic and terrestrial sedimentary rocks there is a hiatus throughout the palaeogene and constructional activity appears to recommence with ankaramitic hyaloclastite and lava deltas and subaerial ankaramitic flows these are overlain by fluvial sediments and tuffs stratigraphically above these is an extensive plateau of silica undersaturated lavas olivine melilitites and nephelinites which rest on a planed and locally lateritized surface at topographically higher levels in the eastern part of the island there are thick ankaramitic lavas and pyroclasts which evidently flowed eastward through valleys cut down into the mesozoic strata and appear to be of pliocene age the subsequent history of the island appears to be non volcanic

vertical movements of ocean island volcanoes insights from a stationary plate environment abstract uplift reconstructions based on the cape verde s geological record provide a unique opportunity to study the long term isostatic movements associated with hotspot activity on a stationary plate environment the archipelago is considered stationary with respect to its melting source so the hotspot driven isostatic effects affecting the ocean islands are expected to be enhanced in this study ar ar and u th geochronology techniques were used to date a set of palaeo markers of sea level from santiago s and sao nicolau s edifices two of the main cape verde islands a comparison between relative sea level and eustatic sea level from a modern eustatic curve was established to extract the vertical displacement undergone by the markers and to reconstruct the uplift subsidence history of each island the resulting uplift reconstructions confirm that both santiago and sao nicolau experienced a general uplift trend over the last 6 ma seemingly synchronous with the vigorous volcanic activity that built their exposed edifices these islands however exhibit different uplift histories despite their common uplift trend several uplift mechanisms were tested and a local rather than regional mechanism is proposed as the main cause of uplift generally unrelated with far field effects of surface loading this mechanism is probably associated with magmatic additions at crustal level

40ar 39ar geochronology of the active phonolitic cadamosto seamount cape verde abstract cadamosto seamount is located in the sw of the cape verde archipelago in the central atlantic ocean off the west coast of africa many radiometric dates exist for the islands in the archipelago however no geochronological information has been obtained from the numerous seamounts the timescales for igneous processes in the submarine realm are thus poorly understood in this study we investigated five lavas that were sampled by dredging and rov remotely operated vehicle from the flanks and summit areas of the largely phonolitic cadamosto seamount during two different research cruises chemical zonation patterns of minerals were determined by electron microprobe and radiometric ages were obtained from single crystal total fusion and single multi grain step heating 40ar 39ar analyses of sanidine nepheline and sodalite group minerals our 40ar 39ar results reveal young sanidine eruption ages all the older 40ar 39ar ages of many sanidine and nepheline antecrysts also give us clues regarding older magmatic events at cadamosto seamount despite these grains having undergone resorption and phenocrystic rim overgrowths resulting in some radiogenic 40ar loss during entrainment in the subsequent magmas the antecrysts minimum ages extend back to 1 5215 0 0083 ma which supports the age progression of magmatism observed in the southern islands chain of the cape verde archipelago the youngest volcanic eruption period 21 14 0 62 ka occurred during the last glacial maximum a period of global sea level lowstands we suggest that the comparatively rapid crust unloading leading up to the lowstand may have reduced pressure conditions within the cadamosto seamount magma plumbing system and thus led to enhanced submarine eruption activity

resolution of non double couple components in the seismic moment tensor using regional networks i a synthetic case study summary we perform a detailed synthetic study on the resolution of non double couple non dc com ponents in the seismic moment tensors from short period data observed at regional networks designed typically for monitoring aftershock sequences of large earthquakes in addition we test two different inversion approaches a linear full moment tensor inversion and a non linear moment tensor inversion constrained to a shear tensile source model the inversions are applied to synthetic first motion p and s wave amplitudes which mimic seismic observa tions of aftershocks of the 1999 mw 7 4 izmit earthquake in northwestern turkey adopting a shear tensile source model to analyse the resolution capability for the obtained non dc components inverted we contaminate synthetic amplitudes with random noise and incorporate realistic uncertainties in the velocity model as well as in the hypocentre locations we find that the constrained moment tensor inversion yields significantly smaller errors in the non dc components than the full moment tensor inversion in particular the errors in the compensated linear vector dipole clvd component are reduced if the constrained inversion is applied furthermore we show that including the s wave amplitudes in addition to p wave amplitudes into the inversion helps to obtain reliable non dc components for the studied station config urations the resolution remains limited due to the lack of stations with epicentral distances less than 15km assuming realistic noise in waveform data and uncertainties in the velocity model the errors in the non dc components are as high as 15 percent for the isotropic and clvd components respectively thus being non negligible in most applications however the orientation of p and t axes is well determined even when errors in the modelling procedure are high

partition coefficients of hf zr and ree between zircon apatite and liquid concentration ratios of hf zr and ree between zircon apatite and liquid were determined for three igneous compositions two andesites and a diorite the concentration ratios of these elements between zircon and corresponding liquid can approximate the partition coefficient although the concentration ratios between apatite and andesite groundmass can be considered as partition coefficients those for the apatite in the diorite may deviate from the partition coefficients the hree partition coefficients between zircon and liquid are very large 100 for er to 500 for lu and the hf partition coefficient is even larger the ree partition coefficients between apatite and liquid are convex upward and large d 10 100 whereas the hf and zr partition coefficients are less than 1 the large differences between partition coefficients of lu and hf for zircon liquid and for apatite liquid are confirmed these partition coefficients are useful for petrogenetic models involving zircon and apatite

volcanic evolution of sao vicente cape verde islands the praia grande landslide abstract the island of sao vicente has undergone continuous volcanic activity from pliocene to pleistocene times the earliest evidence of activity corresponds to some 9 million years ago when a submarine edifice of alkaline basaltic affinity had already started to develop the island resulted from the growth of a single major edifice the sao vicente edifice built up in several distinguishable growth stages the early main stage occurred at 6 5 to 4 5 ma giving rise to an edifice of about 10 12 km in radius and more than 2500 m in height whose centre was located south of the present city of mindelo although the edifice in its earlier phase showed typical characteristics of a shield volcano the mindelo formation it adopted later on those of an ordinary composite volcano the madeiral monte cara formation a giant landslide event the praia grande landslide destroyed the ne sector of the edifice and left a 10 12 km large depression that was successively refilled by nephelinitic lava flows and some associated carbonatites the monte verde formation 4 5 3 1 ma the volcanic activity ceased about 3 2 ma ago when the edifice started to be deeply eroded until disappearing about 90 of its total volume only in recent times 0 3 ma very scarce and local strombolian activity has been developed aside the sao vicente edifice in the eastern sector of the island

39 ar 40 ar ages and geochemistry of the basaltic shield stage of tenerife canary islands spain abstract we report the first 39 ar 40 ar ages from the three early basic shield like massifs of tenerife canary islands and couple these with detailed major and trace element chemistry to constrain the nature and timing of the mantle melting processes the massifs have chemically different sources and independent evolutionary histories the teno and roque del conde massifs appear chemically to represent the products of single mantle melting cycles with progressive decrease in mean melt fraction and increase in mean melting depth in younger rocks the teno massif nw was erupted in a short time period around 6 0 6 4 ma while at least the lower half of the roque del conde massif sw is older than 11 ma in contrast the anaga massif ne is polygenetic with 39 ar 40 ar ages ranging from 8 0 4 2 ma and no simple stratigraphic chemical progression these ages run counter to published suggestions of progressive younging of canary shield stages to the southwest basic rocks in all three massifs are the result of much deeper melting and smaller melt fractions than equivalent units in gran canaria but nevertheless the melting column must have extended significantly into the spinel facies requiring substantial disruption of the local lithosphere the age and melting relationships broadly support the mantle blob model for canary magmatism proposed by hoernle and schmincke hoernle k schminke h u 1993 the role of partial melting in the 15 ma geochemical evolution of gran canaria a blob model for the canary hotspot j petrol 34 599 626 in all three massifs extensive fractional crystallisation has taken place at crustal levels so that mean mgo contents are only some 6 7 the fractionation sequence is olivine clinopyroxene magnetite in basaltic compositions with the involvement of plagioclase amphibole and apatite only to generate the infrequent more evolved hawaiites to benmoreites despite the abundance of basanitic magmas in the tenerife older massifs these follow a differentiation trend towards weakly undersaturated benmoreite rather than to phonolite this probably reflects early crystallisation of magnetite perhaps resulting from somewhat high oxygen fugacity the chemical evidence for replenished magma chambers in tenerife described by neumann et al neumann e r wulff oedersen e simonsen s l pearson n j marti j mitjavila j 1999 evidence for fractional crystallisation of periodically refilled magma chambers in tenerife canary islands j petrol 40 1089 1123 is a consequence of treating as a single cogenetic suite the products of several magmatic systems that differ in parental melt fraction

intraplate seismicity across the cape verde swell a contribution from a temporary seismic network abstract we present an analysis and characterization of the regional seismicity recorded by a temporary broadband seismic network deployed in the cape verde archipelago between november 2007 and september 2008 the detection of earthquakes was based on spectrograms allowing the discrimination from low frequency volcanic signals resulting in 358 events of which 265 were located the magnitudes usually being smaller than 3 for the location a new 1 d p velocity model was derived for the region showing a crust consistent with an oceanic crustal structure the seismicity is located mostly offshore the westernmost and geologically youngest areas of the archipelago near the islands of santo antao and sao vicente in the nw and brava and fogo in the sw the sw cluster has a lower occurrence rate and corresponds to seismicity concentrated mainly along an alignment between brava and the cadamosto seamount presenting normal faulting mechanisms the existence of the nw cluster located offshore sw of santo antao was so far unknown and concentrates around a recently recognized submarine cone field this cluster presents focal depths extending from the crust to the upper mantle and suggests volcanic unrest no evident temporal behaviour could be perceived although the events tend to occur in bursts of activity lasting a few days in this recording period no significant activity was detected at fogo volcano the most active volcanic edifice in cape verde the seismicity characteristics point mainly to a volcanic origin the correlation of the recorded seismicity with active volcanic structures agrees with the tendency for a westward migration of volcanic activity in the archipelago as indicated by the geologic record

evolution of a strongly differentiated suite of phonolites from the klinghardt mountains namibia phonolites of tertiary age occur as eroded tholoids lava flows ignimbrites and coulees in the klinghardt mountains of southern namibia sixty samples have been analyzed for major and trace elements and fourteen of these for 87sr86sr the phonolites lie close to the low pressure cotectics in q ne ks in keeping with their petrography which indicates that most samples have phenocrysts of both nepheline and sanidine na has been variably lost from the rocks during crystallization and devitrification alteration of hypocrystalline specimens the concentrations of other elements except perhaps lree remain unaffected the composition variation within the phonolites is consistent with fractional crystallization of nepheline sanidine and minor clinopyroxene to produce a number of near parallel lineages from similar but not identical parent compositions fractional crystallization results in increasing sio2 na2o zr nb ce rb th y zn pb and ree but decreasing k2o tio2 cao mgo p2o5 ba and sr with differentiation enrichment factors for zr and nb indicate that the most evolved phonolites represent 20 residual liquids a result which is consistent with major element modelling in the evolution of the most differentiated phonolites fractional crystallization was accompanied or followed by volatile transfer causing depletion of th y lree zn and pb relative to zr and nb initial 87sr86sr for most of the phonolites lie in the range 0 7050 0 7057 rising to 0 7080 in the low sr samples this is suggestive of some interaction with crustal rocks but this interaction has no detectable influence on the fractionation controlled compositional evolution of the suite the phonolites are associated on a regional scale with nephelinite and melilitite rocks but there is no favourable evidence for an evolutionary link between these and phonolite alternatively the primitive phonolites may have originated through anatexis of metasomatized nepheline normative crustal rocks

reply to comment by marques et al on the insular shelves of the faial pico ridge azores archipelago a morphological record of its evolution

on dating the magmatism of maio cape verde islands conventional k ar and40ar 39ar studies of mesozoic ocean floor basalts and tertiary plutonic and volcanic rocks from maio cape verde islands have been determined to elucidate the magmatic evolution of this ocean island pillow lavas of the basement complex yield a minimum age of 113 8 ma though thermal overprinting of their formation age by the younger central intrusive complex cic and subsequent sheet intrusions is in some cases almost total activity in the cic began before 20 ma and plutons continued to develop until about 8 ma the youngest ages possibly indicating a cooling history of more than 2 ma for these bodies relative to their volcanic counterparts sheet intrusion occurred throughout the period 20 to 9 ma though the peak of this activity probably occurred 11 ma ago field relations allow the time of thrusting s on the monte branco thrust to be bracketed between 9 and 7 ma volcanic activity began in the tertiary probably before 12 ma and culminated in the development of a stratovolcano at 7 ma

characterization of aftershock fault plane orientations of the 1999 izmit turkey earthquake using high resolution aftershock locations 1 joint inversion for hypocentral parameters and the velocity field is nowadays a state of the art tool to obtain high resolution images of seismically active regions in this study we focus on the location accuracy of aftershocks of the 1999 mw 7 4 izmit nw turkey earthquake we obtained a new velocity model for the region and depicted its improvement on absolute locations in terms of uncertainty and misfit two well developed aftershock clusters located in the akyazi area and karadere duzce region were analyzed in detail based on a waveform cross correlation approach that allowed improving the location accuracy by a factor of 6 relocation results reveal that hypocenters form narrow planes of activity that can be correlated with focal mechanisms of the larger aftershocks as well as nearby clouds of activity with no internal structure down to the resolved scale of 300 m

40ar 39ar geochronology of santo antao cape verde islands santo antao the northernmost island of the cape verde archipelago consists entirely of silica undersaturated volcanic products and minor intrusions 40ar 39ar incremental heating experiments have been carried out on 24 samples that cover the entire exposed chronological sequence the oldest lavas 7 57 0 56 ma representing an older volcanic basement are exposed about 620 m above mean sea level after an interval of quiescence of up to 4 3 ma the volcanic activity resumed and continued at low eruption rates the older basement is unconformably overlain by a ca 810 m thick lava sequence that spans an age range from 2 93 0 03 to 1 18 0 01 ma this sequence is cut by many dykes and sills simultaneous volcanic activity occurred in the northeastern central and eastern part of the island a phonolitic pumice deposit that forms a noteworthy feature over most of the island has an estimated age of 0 20 ma this predates volcanic activity that formed the highest point of the island tope de coroa which has an age of 0 17 0 02 ma the most recent eruption on the island formed nephelinitic lavas in the porto novo region at 0 09 0 03 ma the oldest volcanism exposed on santo antao which took place about 7 6 ma ago was simultaneous with waning activity on maio at the eastern end of the cape verde archipelago

calibration of an ml scale in northwestern turkey from 1999 izmit aftershocks a local magnitude scale is derived for northwestern turkey using data collected by a temporary network installed by the german task force for earthquakes after the 1999 izmit earthquake m w 7 4 and the permanent sapanca bolu network we computed wood anderson seismograms for over 5353 arrivals at 27 three component stations from 530 earthquakes the hypocentral distances considered range from 5 to 140 km with the best represented range being from 5 to 70 km we inverted the measured amplitudes following both the nonparametric richter s 1958 and parametric bakun and joyner 1984 approaches these methods yield consistent magnitude values and station corrections however the calculated nonparametric distance correction log a implies that ground motion attenuation is higher than what is accounted for by the equation calibrated in central california in the range 5 62 km the best fit is provided by log a log r 17 0 00960 r 17 2 this equation is obtained constraining the geometrical spreading parameter n to 1 the calculated value of k 0 00960 confirms that in this range of distance seismic waves could propagate through a low q volume station corrections which allow for a significant reduction of m l residuals range between 0 5 magnitude units suggesting a strong influence of local site effects on the amplitude of ground motion in accord with the obtained attenuation and station corrections the magnitudes of the considered events range from 0 4 to 4 8

non double couple earthquakes 1 theory historically most quantitative seismological analyses have been based on the assumption that earth quakes are caused by shear faulting for which the equiv alent force system in an isotropic medium is a pair of force couples with no net torque a double couple or dc observations of increasing quality and coverage however now resolve departures from the dc model for many earthquakes and find some earthquakes especially in volcanic and geothermal areas that have strongly non dc mechanisms understanding non dc earth quakes is important both for studying the process of faulting in detail and for identifying nonshear faulting processes that apparently occur in some earthquakes this paper summarizes the theory of moment tensor expansions of equivalent force systems and analyzes many possible physical non dc earthquake processes contrary to long standing assumption sources within the earth can sometimes have net force and torque components described by first rank and asymmetric sec ond rank moment tensors which must be included in analyses of landslides and some volcanic phenomena non dc processes that lead to conventional symmetric second rank moment tensors include geometrically complex shear faulting tensile faulting shear faulting in an anisotropic medium shear faulting in a heteroge neous region e g near an interface and polymorphic phase transformations undoubtedly many non dc earthquake processes remain to be discovered progress will be facilitated by experimental studies that use wave amplitudes amplitude ratios and complete waveforms in addition to wave polarities and thus avoid arbitrary assumptions such as the absence of volume changes or the temporal similarity of different moment tensor com ponents

a new felsic cone sheet swarm in the central atlantic islands the cone sheet swarm of boa vista cape verde abstract the island of boa vista is one of the oceanic islands with the largest amount of felsic rocks reported in the world 50 of the total outcropping igneous rocks of boa vista the felsic rocks are trachytes and phonolites generated within the second main stage of the volcanic activity that lasted about four million years three consecutive felsic episodes are distinguished the first occurred between 14 5 ma and 13 5 ma the second between 13 6 ma and 12 0 ma and the third and last between 12 0 ma and 10 4 ma their geochemical composition reveals the presence of crystal fractionation processes with participation of feldspars and amphibole as well as different accessory phases involved in each episode the boa vista felsic cone sheet swarm the youngest third episode is formed by several hundreds of peralkaline ne phonolitic sheets distributed within a circular surface about 11 km in radius the cone sheets conform a single general pattern characterized by a decreasing dip outward the structure together with a constant dip of each individual sheet the average inclination of sheets is around 40 in the sectors next to the centre of the structure whereas it is only about 30 next to its periphery the magmatic focus of the sheets is located over 3 km deep almost coinciding with the convergence centre of a contemporary basic radial dike swarm

the middle triassic evolution of the bangong nujiang tethyan ocean evidence from analyses of oib type basalts and oib derived phonolites in northern tibet in this paper we present new major and trace element chemical data for the basalts and phonolites of the nare ocean island fragment naoi as well as zircon u pb age data and hf isotope compositions for the naoi phonolites in the middle segment of the bangong nujiang suture zone northern tibet our aim is to assess the genesis of these rocks and to reconstruct the middle triassic evolution of the bangong nujiang tethyan ocean bnto the naoi retains an ocean island type double layered structure comprising a basaltic basement and an oceanic sedimentary cover sequence conglomerate and limestone the latter accompanied by layers of erupted phonolite near the top of the sequence the basalts in the naoi are enriched in light rare earth elements and high field strength elements nb ta zr hf and ti and they exhibit chondrite normalized ree patterns and primitive mantle normalized trace element patterns similar to those of ocean island basalts taking into consideration their high dy yb sm yb and la sm ratios we conclude that the naoi basalts were derived from the partial melting of garnet peridotite in the mantle the naoi phonolites have lree enriched chondrite normalized ree patterns with negative eu anomalies eu eu 0 41 0 43 and primitive mantle normalized trace element patterns with enrichments in nb ta zr and hf and depletions in ba u sr p and ti given the high contents of nb 172 256 ppm ta 11 8 16 0 ppm zr 927 1117 ppm and hf 20 8 26 9 ppm and the very low contents of mgo 0 11 0 25 wt the very low mg values 5 10 and the near zero contents of cr 1 27 7 59 ppm ni 0 43 7 19 ppm and co 0 11 0 38 ppm and the small and homogeneously positive e hf t values 4 9 to 9 5 we infer that the naoi phonolites were formed by the fractional crystallization of an oib derived mafic parent magma the phonolites of the naoi contain zircons that yielded u pb ages of 239 and 242 ma indicating that the naoi formed during the middle triassic these data combined with data from modern ocean islands e g canary islands cape verde fernando de noronha tristan da cunha and gough in the atlantic ocean and society and austral cook in the pacific ocean lead us to infer that the bnto was open for a long time before the middle triassic and that the ocean had already developed into a mature ocean with a thick oceanic lithosphere by at least the middle triassic

on the retrieval of moment tensors from borehole data the complete moment tensors of seismic sources in homogeneous or vertically inhomogeneous isotropic structures cannot be retrieved using receivers deployed in one vertical borehole the complete moment tensors can be retrieved from amplitudes of p waves provided that receivers are deployed in at least three boreholes using amplitudes of p and s waves two boreholes are in principle sufficient similar rules also apply to transversely isotropic media with a vertical axis of symmetry in the case of limited observations the inversion can be stabilized by imposing the zero trace constraint on the moment tensors however this constraint is valid only if applied to observations of shear faulting on planar faults in isotropic media which produces double couple mechanisms for shear faulting on non planar faults for tensile faulting and for shear faulting in anisotropic media the zero trace constraint is no longer valid and can distort the retrieved moment tensor and bias the fault plane solution numerical modelling simulating the inversion of the double couple mechanism from real data reveals that the errors in the double couple and non double couple percentages of the moment tensors rapidly decrease with increase in the number of boreholes used for noisy p and s wave amplitudes with noise of 15 of the top amplitude at each channel and for a velocity model biased by 10 the errors in the double couple percentage attain 25 13 and 6 when inverting for the double couple mechanism from one two and three boreholes

investigating collapse structures in oceanic islands using magnetotelluric surveys the case of fogo island in cape verde abstract one of the most remarkable natural events on earth are the large lateral flank collapses of oceanic volcanoes involving volumes of rock exceeding tens of km3 these collapses are relatively frequent in recent geological times as supported by evidence found in the geomorphology of volcanic island edifices and associated debris flows deposited on the proximal ocean floor the island of fogo in the cape verde archipelago is one of the most active and prominent oceanic volcanoes on earth the island has an average diameter of 25 km and reaches a maximum elevation of 2829 m above sea level m a s l at pico do fogo a young stratovolcano located within a summit depression open eastward due to a large lateral flank collapse the sudden collapse of the eastern flank of fogo island produced a megatsunami 73 ky ago the limits of the flank collapse were deduced as well from geomorphologic markers within the island the headwall of the collapse scar is interpreted as either being located beneath the post collapse volcanic infill of the summit depression or located further west corresponding to the bordeira wall that partially surrounds it the magnetotelluric mt method provides a depth distribution of the ground resistivity obtained by the simultaneous measurement of the natural variations of the electric and magnetic field of the earth two n s magnetotelluric profiles were acquired across the collapsed area to determine its geometry and boundaries the acquired mt data allowed the determination of the limits of the collapsed area more accurately as well as its morphology at depth and thickness of the post collapse infill according to the newly obtained mt data and the bathymetry of the eastern submarine flank of fogo the volume involved in the flank collapse is estimated in 110 km3 this volume the first calculated onshore stands between the previously published more conservative and excessive calculations offshore that were exclusively based in geomorphic evidence the model for the summit depression proposing two caldera collapses preceding the collapse of the eastern flank of fogo is supported by the mt data

tsunami deposits in santiago island cape verde archipelago as possible evidence of a massive flank failure of fogos volcano abstract massive flank failures of volcanic edifices generate tsunami waves these low frequency but high magnitude hazards remain poorly documented because of the scarcity of observations offshore deposits are studied only by geophysical surveys and the failure rheologies are poorly constrained marine conglomerates found at unusually high elevations in hawaii and in the canary islands were previously interpreted as being the result of giant tsunami waves generated by massive flank failures this study focuses on the search for and interpretation of sedimentary evidence of a tsunami around the coastline of santiago island which is located 55 km east of the active fogo volcano the bordeira caldera in fogo island which opens to the east was formed by the monte amarelo flank collapse and the active edifice is now nested in the failure s scar the only evidence of a tsunami was found north of tarrafal bay the deposits display many similarities with the tsunami conglomerate described in hawaii and the canary islands erosive contact with the substratum rip up clasts of paleo soil marine bioclasts never found in growth or live position internal organisation into different facies with distinct characteristics of clast size up to boulders sorting from moderately to very poorly sorted and clast fabrics landward or seaward orientations this organisation suggests that the tsunami consisted of two main waves the absence of tsunami deposits elsewhere on santiago island coupled with the relatively low elevation of the conglomerate in tarrafal