the attenuation of seismic shear waves in quaternary alluvium in santa clara valley california we used shear waves generated by an air powered source at the ground surface and recorded in a borehole to estimate the shear wave quality factor at strong motion station gilroy no 2 we find similar values of q using both the decay of the spectra with depth and the slope of the spectral ratio at two depths we find no evidence of a frequency dependence of q the mean value of q over the depth range 10 to 115 m is close to 10 the use of this value over the depth of the borehole and the observed travel time of 0 358 sec gives a cumulative attenuation factor t of 0 036 sec for the upper 180 m of the quaternary alluvium this is comparable to the differential decay between gilroy no 2 and a rock site 1 9 km away gilroy no 1 as measured from the decay of the high frequency spectra of accelerograms from large earthquakes plotted on a log linear scale t 0 05 0 04 and 0 03 sec for the 1979 coyote lake 1984 morgan hill and 1989 loma prieta earthquakes respectively the similarity between the attenuations measured from the low strain surface source and those from the larger amplitude earthquake sources suggests that increases of damping due to nonlinear wave propagation effects are limited

nonlinear soil site effects in probabilistic seismic hazard analysis this study presents effective probabilistic procedures for evaluating ground motion hazard at the free field surface of a nonlinear soil deposit located at a specific site ground motion at the surface or at any depth of interest within the soil formation e g at the structure foundation level is defined here in terms either of a suite of oscillator frequency dependent hazard curves for spectral acceleration or of one or more spectral acceleration uniform hazard spectra each associated s s f a with a given mean return period it is presumed that similar information is available for the rock outcrop input the effects of uncertainty in soil properties are directly included this methodology incorporates the amplification of the local soil deposit into the framework of probabilistic seismic hazard analysis psha the soil amplification is characterized by a frequency dependent amplification function af f where f is a generic oscillator frequency af f is defined as the ratio of to the spectral s s f a acceleration at the bedrock level the estimates of the statistics of the ampli s s f a fication function are obtained by a limited number of nonlinear dynamic analyses of the soil column with uncertain properties as discussed in a companion article in this issue bazzurro and cornell 2004 the hazard at the soil surface or at any desired depth is computed by convolving the site specific hazard curve at the bedrock level with the probability distribution of the amplification function the approach presented here provides more precise surface ground motion hazard estimates than those found by means of standard attenuation laws for generic soil conditions the use of generic ground motion predictive equations may in fact lead to inaccurate results especially for soft clay soil sites where considerable amplifi cation is expected at long periods and for saturated sandy sites where high intensity ground shaking may cause loss of shear strength owing to liquefaction or to cyclic mobility both such cases are considered in this article in addition to the proposed procedure two alternative easier to implement but approximate techniques for obtaining hazard estimates at the soil surface are also briefly discussed one is based on running a conventional psha with a rock attenuation relationship modified to include the soil response whereas the other consists of using a simple analytical closed form solution that appropriately mod ifies the hazard results at the rock level

influence of focal mechanism in probabilistic seismic hazard analysis an extension of probabilistic seismic hazard analysis is proposed to introduce a priori information about seismic source parameters in particular faulting style is taken into account with a theoretical corrective coefficient applied to the attenuation law the validity of this correction is assessed through a comparison with observed data attenuation law predictions corrected and not corrected and the results of attenuation laws containing faulting style parameters the probabilistic nature of the analysis is maintained introducing into the classical hazard formulation a 2d probability density function describing the most probable focal mechanisms associated with each seismic source zone this new expression may also be used in the framework of deaggregation analysis thus the design earthquake resulting from the deaggregation is characterized by a focal mechanism an application to a site located in the southern apennines italy is shown the result of the analysis emphasizes the importance of strike slip events in the seismic hazard context compared with normal faulting seismic activity in this region

seismic velocities in the uppermost oceanic crust age dependence and the fate of layer 2a it has been widely accepted that upper crustal seismic velocities gradually increase from low values in the youngest crust to typical crustal velocities 5 2 km s 1 over a period of 30 40 million years with a corresponding decrease in the observed heat flow deficiency caused by hydrothermal circulation some recent seismic studies however suggest that the seismic velocities actually rise rapidly nearly doubling in 5 km s 1 and velocities 3 km s 1 are not observed in crust older than 5 ma there is no detectable systematic change in the seismic properties of the uppermost oceanic crust after 10 million years median and average velocities increase from 2 3 to 4 4 km s 1 by 7 5 ma and remain unchanged thereafter if layer 2a is an interval capping the oceanic crust in which velocities are significantly lower than velocities that are typical of the underlying rocks layer 2b 5 2 km s 1 it follows that layer 2a does not disappear but is a persistent feature of the oceanic crust these results are consistent with geochemical evidence that most hydrothermal alteration occurs within 3 million years after the formation of the oceanic crust and suggest that there is little correlation between seismic velocities and ongoing hydrothermal circulation

site specific probabilistic seismic hazard assessment direct amplitude based approach conventional probabilistic seismic hazard assessment psha is difficult to apply in regions lacking sufficient information concerning geological setting active faults and so forth also for a site specific psha site effects arising from both crustal rock and overlying soil sediments are generally not assessed rigorously this is of particular importance for those metropolitan cities having a significant proportion of reclaimed land because the site to site variability of such site effects can be very large the objective of this article is to demonstrate an alternative procedure for assessing seismic hazard developed from the conventional source based cornell mcguire psha approach based on considering an infinite number of sources the proposed new procedure is termed the direct amplitude based dab approach the major advantage of the proposed dab approach is that it is not necessary to characterize any seismic sources moreover if a site specific and event specific ground motion attenuation model is available a more accurate site specific psha could be performed also a generic analytical solution for the proposed procedure has been derived to avoid the need for a lengthy integration process using the proposed approach peak ground velocities have been computed at different return periods to form a seismic hazard curve citing hong kong as a case study

deep structure of the northern kerguelen plateau and hotspot related activity summary seismic refraction profiles were carried out in 1983 and 1987 throughout the kerguelen isles southern indian ocean terres australes antarctiques franqaises taaf and thereafter at sea on the kerguelen heard plateau during the md66 keobs cruise in 1991 these profiles substantiate the existence of oceanic type crust beneath the kerguelen heard plateau stretching from 46 s to 55 s including the archipelago seismic velocities within both structures are in the range of those encountered in standard oceanic crust however the kerguelen isles and the kerguelen heard plateau differ strikingly in their velocity depth structure unlike the kerguelen isles the thickening of the crust below the kerguelen heard plateau is caused by a 17 km thick layer 3 velocities of 7 4 km s 1 or so within the transition to mantle zone below the kerguelen isles are ascribed to the lower crust intruded and or underplated by upper mantle material the crust mantle boundary below the kerguelen heard plateau is abrupt and devoid of any underplated material the difference in structure between the northern edge of the kerguelen plateau including the archipelago and the kerguelen heard plateau may be related to variability of the time dependent hotspot activity the kerguelen heard plateau was emplaced during the cretaceous time 110 ma when the volcanic output rate of the kerguelen plateau and the ninetyeast ridge was high as well as high potential temperature the northernmost kerguelen plateau and the archipelago were emplaced during tertiary time 40 45 ma as the volcanic output rate reduced furthermore intraplate volcanism continued in the kerguelen archipelago for at least 40 ma the isostatic compensation of the kerguelen isles and the kerguelen heard plateau is achieved by low density mantle material as shown by refraction and geoid studies the velocity depth structure below the kerguelen isles is similar to that found below intraplate oceanic islands such as hawaii despite the differences in age the crust below iceland 0ma and the kerguelen plateau 100 120 ma are strikingly akin the similarity between the kerguelen heard plateau and iceland then strongly supports a similar origin for both structures the kerguelen heard plateau being a fossil equivalent of present day iceland crustal thickening beneath the kerguelen heard plateau which may result from an iceland type setting i e an active spreading centre over a hotspot is mostly produced by thickening of layer 3 layer 2 representing 25 per cent only of the thickness of the igneous crust the kerguelen isles despite the initial volcanism near the active southeast indian ridge behave as a midplate volcanic island and are definitely not representative of the whole kerguelen plateau structure

friction of mineralogically controlled serpentinites and implications for fault weakness serpentines are common minerals in several major tectonic faults in a variety of geodynamic settings and have variable frictional strength and complex deformation processes here we present friction experiments carried out on a suite of serpentine samples that include veins of antigorite lizardite and fibrous serpentine chrysotile and polygonal serpentine together with massive samples of retrograde lizardite and chrysotile rich and prograde antigorite rich serpentinites these samples were characterized from the hand specimen down to the nanoscale to precisely constrain their mineralogical composition and are interpreted to represent typical fault rocks and host rocks in serpentine bearing shear zones respectively experiments were performed at effective normal stress from 5 to 120 mpa at temperatures of 25 c and 170 c and water saturated i e under the faulting conditions of the brittle upper lithosphere friction of antigorite samples either massive or vein is relatively high 0 53 retrograde massive serpentinites constituted primarily of lizardite and fibrous serpentines are frictionally weak 0 30 end members lizardite and fibrous serpentines are even weaker 0 15 0 19 and this weakness is unchanged at high temperature we document deformation of lizardite and fibrous serpentines occurring predominantly via mode ii cracking crystal fiber folding and frictional sliding which account for the observed mechanical weakness when combined with frictional reactivation analysis our data provide mechanical evidence for fault weakness inferred from earthquake dip distributions at oceanic outer rises and low angle normal faults beneath rifted continental margins and at slow ultraslow spreading mid ocean ridges

sinoprobe deep reflection profile reveals a neo proterozoic subduction zone beneath sichuan basin abstract a new multichannel seismic reflection profile collected across the sichuan basin in southern china by sinoprobe images prominent reflectors that originate within the lower crust and penetrate well into the underlying mantle the geometry of these mantle reflectors is very similar to those observed on other deep reflection profiles that have been interpreted as relicts of ancient subduction considering the geological history of the basement beneath and surrounding sichuan basin and ages of granites encountered in nearby wells we propose that these newly revealed reflectors are the remnants of neo proterozoic subduction that occurred along the nw margin of the yangtze craton this interpretation is consistent with geochemical studies from a gneissic complex at the west margin of the sichuan basin moreover preservation of these reflectors supports the idea that the sichuan lithosphere served as a consolidated tectonic buttress against which the tibetan plateau has impinged to produce the longmenshan orogenic belt

delamination of eclogitized lower crust control on the crust mantle boundary in the central fennoscandian shield abstract the crustal structure of the central part of the fennoscandian shield is dominated by a high velocity lower crust hvlc vp 7 3 7 5 km s at the depth of 40 63 km in an area of ca 400 km 600 km formed in the palaeoproterozoic svecofennian orogen at 1 91 1 87 ga in the area of the hvlc the moho is deeper than 50 km and attains depths of up to 63 km in eastern finland at the archaean proterozoic boundary zone we suggest that the present moho dates back to late svecofennian time and was determined by delamination of high density eclogitized lower crust after tectonic collisional thickening of crust densities deduced from seismic and gravity data suggest a very small or completely lacking density contrast between lower crust and uppermost mantle new geological evidence from kimberlite hosted garnet xenocrysts supports the presence of a garnet population with compositions in agreement with crustal eclogitic garnets in addition small clinopyroxene garnet mineral aggregates microxenoliths show omphacitic clinopyroxene compositions therefore the presence of eclogites in the high velocity lower crust is implied although the eclogite proportion as estimated from seismic velocity data is only a few percent in the present hvlc with results on near incidence and wide angle seismic studies and geological and geochemical data on heat producing elements we modeled the svecofennian collision and the thermal evolution of the crust to provide temperature constraints for the delamination model the constructed thermal model simulates the genesis of svecofennian synkinematic at 1 89 1 88 ga and postkinematic 1 88 1 87 ga granitoids as well as the general metamorphic grade of the surface rocks and lower crustal xenoliths moreover the narrow distribution of the ages of the svecofennian mafic ultramafic magmatism ca 1 89 1 87 ga dispersed around the hvlc area is explained by the homogeneous warming of the lithosphere the conditions of lower crustal delamination were estimated with a rayleigh taylor convective instability model constrained with lower crustal and upper mantle rheological data delamination of a lower crustal eclogite layer 5 30 km thick takes place in about 7 50 ma if the rock dynamic viscosity is 1 1021 pa s it requires temperatures of 1000 c in the lower crust and upper mantle in the svecofennian crust such temperatures were attained at about 1860 1850 ma i e after the orogenic peak we conclude that the moho of the hvlc area was controlled by delamination of a ca 5 10 km thick layer of high density lowermost crust into the mantle such a density controlled process may have controlled the moho depth also in other proterozoic shield areas

relict subduction zones in canada 1 relict subduction zones that are interpreted on 10 lithoprobe deep seismic reflection profiles in canada provide key observational evidence that lithosphere plate interactions have operated with essentially the same resultant geometry for at least 2 7 gyr subduction zone reflections are generally found beneath known collisional belts and their orientations are often consistent with the locations of exposed arcs sutures and subduction complexes they project from the lower crust into the upper mantle and provide key geometric relationships between the crust moho and upper mantle the interpretation of subduction zone reflections as being syntectonic establishes a minimum age for the overlying lower crust and mantle wedge and precludes inferred formation of the moho and upper mantle by postorogenic delamination in these areas the orientation of subduction zone reflections may be synthetic or antithetic to exposed sutures indicating juxtaposition by subcretion or by delamination respectively the mantle wedge above the subduction zone reflections may exhibit sparse reflectivity subduction zone reflections commonly dip toward the craton indicating a preferred dip of subduction during the last stages of convergence within an orogen or else a higher likelihood of preservation beneath older domains interpreted ages of the canadian examples range from recent to neoarchean 2 7 ga providing observational evidence that plate tectonics or similar processes have been active since at least the neoarchean the locations and geometries of these relict subduction zones provide a snapshot of accretion boundaries in the mantle and establish a framework for the neoarchean recent accretionary architecture of the north american lithosphere

nonlinearity liquefaction and velocity variation of soft soil layers in port island kobe during the hyogo ken nanbu earthquake clear nonlinear behavior is analyzed from the acceleration records of the 1995 hyogo ken nanbu earthquake at port island kobe from four triaxial in struments placed at four different depths the surficial effects during strong ground motions were compared with those during weak motions before and after the main shock we used a spectral ratio technique and a nonlinear inversion for velocity structure to analyze the data from the spectral analysis we observed a large variation of the spectral ratios between the surface and different depths during the strong ground motions and during the liquefied state the spectral ratios after the mainshock i e after the liquefied state are different from those before the mainshock the peak frequencies in the spectral ratios after the mainshock are shifted to lower fre quencies with respect to those in the spectral ratios before the mainshock we in verted the s wave velocities using a genetic algorithm technique to determine the velocity structure before during and after the mainshock the s wave velocity struc ture before and after the mainshock was found to be different specifically the s wave velocity of the second layer 5 m to 16 m depth after the mainshock was 20 lower than before our analysis shows that the liquefied state remains at least 3 hr after the mainshock but no more than 24 hr the rigidity of the soil decreased close to zero when liquefaction happened and later increases gradually following a trend that resembles a consolidation curve the strong influence of nonlinearity during the mainshock yielded a big reduction of the horizontal surface ground motions so that the observed horizontal peak acceleration was only about 25 of the peak acceler ation expected from the linear theory however the nonlinear effects in the vertical peak acceleration were not significant

possible upper mantle reflection fabric on seismic profiles from the tethyan himalaya identification and tectonic interpretation upper mantle structures of the himalaya collision system are not well defined yet are integral to our understanding of the lithospheric scale processes which have built the tibetan plateau a 100 km long deep seismic reflection profile tib 1 and a short cross line tib 2 were recorded within the tethyan himalaya to help constrain the geometry of the crust and mantle at long travel times the returned seismic energy is especially weak compared to the noise therefore the data were reprocessed with signal to noise enhancing techniques to identify coherent events at mantle travel times visual comparisons of shot gathers recordings of the ambient noise common midpoint stacks and synthetic sections suggest that a dipping reflection fabric is present in the upper mantle on the basis of geometrical and velocity constraints the fabric is not likely related to sideswipe p to s phase conversions multiples or surface waves the fabric is most likely within the underthrust indian lithosphere beneath the peaks of the high himalaya the fabric strikes n63 e significantly oblique to the e w strike of crustal deformation however the fabric dip direction 44 n27 w is essentially the same as the fast direction of polarization inferred from a previous sks shear wave splitting study the crust versus mantle strike discordance suggests that the fabric may predate the collision of india with tibet and mark previous deformation of the indian lithosphere e g rifting episodes that formed the tethyan ocean conversely if the fabric is the result of collision the discordance would suggest that mantle deformation is decoupled from crustal deformation within the himalayas

variations in thickness of layer 3 dominate oceanic crustal structure abstract seismic refraction studies have determined that while much of the crust underlying the world s oceans is about 7 km thick it also occurs in lesser and considerably greater thicknesses here we show that vastly different thicknesses of oceanic crust formed at ridges by seafloor spreading are generally characterized by the two gradient velocity structure recognized as typical of 6 7 km thick oceanic crust adopting a standard we study ninety published seismic velocity structures of oceanic crust ranging in thickness from 2 to 37 km several structures formed in midplate settings ocean islands and seamounts are also considered for comparison regardless of origin ridge or midplate we find that the percentage of the whole crust formed by layer 2 systematically decreases with increasing total crustal thickness while the average velocity of layer 2 varies widely for all crust the average velocity of layer 3 increases systematically with increases in the average whole crustal thickness and velocity crust formed in midplate settings differs in that the thickness of layer 2 velocity crust is generally two to three times that of crust formed at ridges residual depths were calculated for oceanic crust by removing the effects of sediment loading and subsidence due to lithospheric cooling most of the data fall close to a model whereby different crustal thicknesses result from different extents of partial melting of oceanic upper mantle the crustal sections were restored to zero age assuming the average depth of emplacement for 7 km thick oceanic crust is 2 5 km below sea level from this restoration it appears that the depth to the top of layer 3 at the time it was formed is 4 25 1 25 km below sea level regardless of whole crustal thickness or spreading rate these results suggest that the mechanism of construction of oceanic crust is in many ways remarkably uniform despite very large changes in the total thickness of the crust produced very basic questions remain it is for instance not at all clear why layer 2 and layer 3 exist in the proportions they do while neutral buoyancy or melt trapping at the base of a brittle lid may account for local properties of the layer 2 3 boundary they do not seem adequate to account for the global phenomenon

lithospheric extension from rifting to continental breakup at magma poor margins rheology serpentinisation and symmetry the symmetry or asymmetry of the process of continental breakup has been much debated over the last 20 years with various authors proposing asymmetric simple shear models others advocating more symmetric pure shear models and some combinations of the two the unroofing of vast expanses of sub continental mantle at non volcanic margins has led some authors to argue in favour of simple shear models but supporting evidence is lacking subsidence evidence from conjugate margin pairs is equivocal and the detailed crustal and lithospheric structure of such pairs not generally well enough known to draw firm conclusions in the porcupine basin where the final stages of break up are preserved the development of structural asymmetry is demonstrable and apparently related to late stage coupling of the crust to the mantle following the complete embrittlement of the crust this agrees with theoretical modelling results which predict that asymmetric models can develop only on a lithospheric scale when the crust and mantle are tightly coupled however whether such asymmetry is maintained during continued exhumation of the mantle is unclear

migration of widespread long lived volcanism across the galapagos volcanic province evidence for a broad hotspot melting anomaly abstract the well studied galapagos archipelago is a small part of the much larger galapagos volcanic province gvp consisting of the cocos carnegie coiba and malpelo aseismic ridges and related seamount provinces although these aseismic ridges and seamounts dominate the morphology of the region little is known about their origin due to a lack of direct age and geochemical information in order to establish how well the gvp fits with the predictions of the standard fixed hotspot and mantle plume hypotheses we conducted a first reconnaissance dredge grab sampling of submerged regions of the gvp we present here 40ar 39ar ages for many of these new basement samples and evaluate their implications for the various models put forward to explain the origin of the gvp correlating new and published sample site ages with distance from the western side of the galapagos islands show that volcanism has not progressed in narrow time progressive lines of seamounts and ridges as predicted by the conventional fixed hotspot and mantle plume hypothesis rather volcanism apparently migrated time progressively across the gvp in broad regions of long lived and possibly concurrent volcanism we propose that the most viable explanation for these observations is that the gvp is the product of cocos and nazca plate motions across a broad hotspot melt anomaly the complex spreading history of the cocos nazca spreading centre likely controlled the relative distribution of gvp volcanism between the cocos and nazca plates while creating lithosphere of variable age thickness across the region while the notion of a broad galapagos hotspot melting anomaly linked to a complex regional tectonic history requires significant testing it nevertheless highlights the need to test alternative mantle upwelling shapes and sizes compared to the widely accepted notion of a narrow continuous long lived galapagos mantle plume conduit defined by the size and location of a galapagos island

the polarity of deep seismic reflections from the lithospheric mantle evidence for a relict subduction zone abstract birps offshore deep seismic reflection profiles to the north of scotland have revealed two bright continuous reflectors in the continental lithospheric upper mantle the dipping flannan and sub horizontal w reflectors the polarity of reflections from the flannan has been determined from normal incidence reflection data using the far field source wavelet as a starting model the effective far field wavelet was calculated by adding a receiver ghost and the effects of recording filters attenuation during transmission and sea bottom multiples to the source wavelet reflection polarity was determined in a blind test by comparing the modelled wavelet with stacks of the flannan reflection in eight out of ten stacks the reflection was picked as positive polarity two out of ten were unknown to verify this result the test was repeated for moho reflections in this case nine out of ten stacks were picked as positive the modelling also demonstrated that the flannan reflection is apparently from a simple interface complex interlayering is not required to explain the waveforms and they are not consistent with reflections from a single thin layer seismic data acquired at wide angle across the w mantle reflector show sub crustal high amplitude arrivals which can only be explained as post critical reflections only a high velocity eclogitic layer contained within normal mantle with a sharp upper boundary and a diffuse base can explain all our observations we suggest that the flannan reflector represents the top of a relict oceanic and eclogitic component of a pre caledonian subduction zone within the lithospheric mantle

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

the malaguide los reales nappe an example of crustal thinning related to the emplacement of the ronda peridotites betic cordillera abstract the malaga region in southern spain represents a cross section through the continental crust and subcontinental mantle of the westernmost portion of the internal zone of the betic cordillera recent petrological and structural studies carried out in this region show a link between the malaguide complex and the underlying los reales nappe where the ronda peridotites were incorporated as allochthonous slabs the ronda peridotites are covered by the metamorphic sequence of the los reales nappe which consists of acidic granulites migmatites and schists the tectonometamorphic evolution of these granulite facies rocks involves a first high temperature 800 c high pressure 10 kbar metamorphism replaced by high temperature 750 c low pressure 3 5 kbar conditions this strong thermal event which is also recognized in biotite bearing schists of the los reales nappe and even in the overlying malaguide complex developed during an extensional deformation related to the diapiric stage of the ronda peridotite emplacement stretching lineations from the mylonites related to this extensional event are n 110 e in orientation and kinematic criteria show that low grade metamorphic rocks malaguide complex move down on to higher grade ones los reales nappe from west to east consequently we interpret that the malaguide complex and los reales nappe form a single domain of continental crust which was thinned during the emplacement of the ronda peridotites stretching lineations are subparallel to the trend of the betic cordillera and can be integrated in a model of lithospheric transcurrent thinning

crustal thinning mantle exhumation and serpentinization in the porcupine basin offshore ireland evidence from wide angle seismic data new wide angle seismic data were gathered along a 230 km long profile that runs east west across a deep structural feature in the porcupine basin offshore ireland known as the porcupine arch ocean bottom seismometers were deployed at 3 4 km intervals and seismic sources fired every 120 m along it prominent primary and secondary arrivals indicate that the continental crust is extremely thin locally less than 2 km across the basin centre the sedimentary succession is up to 12 km thick and comprises three distinctive seismic layers the two uppermost layers are interpreted as mostly a post rift succession of cretaceous and cenozoic strata the lowest layer thins rapidly towards the basin centre and is interpreted as a succession of predominantly jurassic synrift sediments a strong asymmetry in both the geometry of the crust and the sedimentary layers is probably related to a simple shear mode of extension and the subsidence that it induced crustal thinning is far greater than in the adjacent rockall basin and local exhumation of continental mantle lithosphere may have occurred in parts of the porcupine basin low p n velocities beneath the porcupine arch are compatible with larger amounts of mantle serpentinization than in the rockall basin

lithospheric shear zones and mantle crust connections abstract a crustal scale ductile shear zone network in the precambrian granulite facies crust of madagascar is examined to determine the nature of the connections between the mantle and lower crust based on three independent data sets field and satellite mapping c and o isotope geochemistry and gravimetry this crust is divided into three zones 1 outside of shear zones 2 minor shear zones that are 350 km long up to 1000 km and 20 35 km wide the mantle is uplifted by about 10 km beneath the major shear zones the major shear zones are rooted in and are inferred to be controlled by the mantle they directly tapped mantle derived co 2 the small scale minor shear zones were controlled by crustal processes and focused crustally derived h 2 o rich co 2 fluids the regular distribution of the shear zones on a crustal scale is in agreement with models of buckling of the continental lithosphere in a compressional context the propagation of these mechanical instabilities promoted and channelled fluid flow these major pan african shear zones thinned the crust and were reactivated during the subsequent drifting of madagascar and opening of the indian ocean during jurassic to cretaceous times they also controlled many of the brittle fault zones in the overlying sedimentary basins mantle rooted large scale shear zones are inferred to be a general feature of cratonic areas reactivated by shear zone systems

nature of the crust below the southern granulite terrain sgt of peninsular india across the bavali shear zone based on analysis of gravity data abstract the bavali shear zone is a major proterozoic structure in the southern granulite terrain sgt of peninsular india a detailed gravity survey has been carried out in order to delineate the crustal structure and tectonics of the shear zone and adjacent areas the bouguer anomaly map of the region shows a number of features i an e w gravity gradient zone with an average gradient of 3 0 mgal km in the west and 0 4 mgal km in the east across the shear ii a broad gravity low with a relief of 20 mgal in the wynad plateau se of the shear iii a series of gravity lows in the southern part of the shear between kannur and vayittiri iv a broad gravity high of 30 40 mgal encircling the charnockite province in the nw part of the shear and v a sharp localised gravity high closure of 40 50 mgal towards coast at nileswaram spatially filtered gravity anomaly maps clearly exhibit contrasting gravity signatures on either side of the shear zone with relatively higher gravity values within the northern block crustal thickness estimates in general vary from 36 to 41 km with a thinning of 2 to 3 km towards the north below the shear zone using two dimensional crustal models determined from gravity anomaly profiles across the bavali shear zone it is interpreted that the shear zone extends downward close to a plane separating blocks of different crustal densities the shear zone dips steeply towards the south and its downward extension meets with a slight moho rise suggesting that the bavali shear zone is a crustal scale structure eastwards below the wynad plateau however the shear zone becomes less prominent suggesting local variations in its geometry the models also point out the presence of a thick high density body 2 98 g cc in the lower part of the crust below the northern charnockite province which narrows towards the wynad plateau region in the east the presence of such a high density body could be due to factors such as evolution together with the protoliths of overlying charnockitic rocks a dense lower crustal residual fraction during anorthosite genesis or emplacement during pan african later anorogenic events the models also indicate that the wynad plateau region is characterised by a relatively homogeneous crust

seismic imaging of hotspot related crustal underplating beneath the marquesas islands volcanism in active continental rift zones1 2or on rifted continental margins3is frequently associated with unusually high lower crustal seismic velocities indicating the presence of large igneous intrusions at the base of the crust the only comprehensive investigation of crustal underplating at an oceanic hotspot beneath hawaii4 6 has yielded controversial results7 here we report the results of seismic refraction experiments across the marquesas islands hotspot trace which show that the island chain is underlain by crust 15 17 km thick and that a large lower crustal region 275 km wide and 2 8 km thick has seismic velocities 7 3 7 75 km s 1 indicative of crustal underplating wide angle reflections occur near the base of the normal lower crustal velocities and at the base of the underplating complex we interpret these reflectors as the relict pre hotspot moho and the current post hotspot moho respectively the high velocity material may be purely intrusive or may consist of a mixture of intrusive and preexisting rocks the volume of the high velocity body is impressive amounting to nearly twice the volume of volcanics erupted at the surface

evidence for extensional shear zones in the mantle offshore britain and their implications for the extension of the continental lithosphere deep seismic profiles recorded in the seas around the british isles have revealed bands of seismic reflections in the uppermost mantle on the basis of their appearance geophysical characteristics and association with major crustal faults these features are best interpreted as mantle shear zones furthermore in the north sea the mantle reflectors appear to follow an extensional trend and when interpreted as extensional shear zones help explain the regional stratigraphy of the overlying basin the crosscutting relationships between mantle reflections to the west of scotland and a major strike slip fault the great glen fault ggf suggest that the mantle structure postdates the ggf and hence is probably related to subsequent extension furthermore kinematic analysis of lithospheric deformation to the north of scotland is consistent with the structure observed on the seismic data if the flannan mantle reflection is an extensional shear zone reconstruction of the lithosphere prior to permian extension restores the flannan to the edge of the rockall trough a rift probably initiated in the carboniferous thus the flannan fault may have developed during carboniferous rifting it is concluded that most of the mantle reflections are some form of extensional shear zone in the mantle and the role of such structures during lithospheric extension is considered

palaeozoic terranes and their lithospheric boundaries within the trans european suture zone tesz a review the trans european suture zone tesz is a broad and complex zone of terrane accretion separating ancient lithosphere of the baltic shield and east european craton eec from the younger lithosphere of western and southern europe there is debate about the number of terranes involved and their origins the most significant terrane boundaries originally oceanic sutures are poorly exposed and are defined using faunal provinciality prominent geophysical lineaments may represent sub terrane boundaries within composite terranes the avalonia composite terrane was amalgamated to baltica in latest ician time the provenance of the malopolska and moravo silesian terranes whose neoproterozoic basement is suspect with respect to baltica but with which they nonetheless share faunal affinities is still debated the extent of the variscide rhenohercynian zone defines the southern limit of the early palaeozoic accreted terranes terranes within the bohemian massif exhibit hp metamorphism recording subduction driven collision prior to incorporation in the tesz collage itself associated with a hp event in romania the saxothuringian zone and rheic suture are directly juxtaposed with the eec and the crustal structure of the tesz has been much modified by post palaeozoic tectonism deep seismic reflection data where available show that the oceanic sutures are frequently associated with inclined reflectivity zones irz in the lithospheric mantle here inferred to mark relict subduction zones interpreting the age of subduction is complicated by offset of originally coplanar mantle and crustal segments of the irz resulting from early orogenic lithospheric delamination at the moho interface and or post orogenic reordering processes the mantle irz appear to indicate a n directed polarity for early palaeozoic subduction apparently incompatible with the geological evidence which indicates subduction of the tornquist sea beneath avalonia and a large amount of underthrusting of avalonia by baltica possible explanations for these apparently incompatible observations are evaluated

mantle influx compensates crustal thinning beneath the cathaysia block south china evidence from sinoprobe reflection profiling abstract the crustal architecture of the cathaysia block south china is a key element in understanding the late mesozoic tectono magmatic history of east asia and includes a spectacular 260 000 km2 basin and igneous province that forms a natural laboratory for unravelling the interplay of extension magmatism and crust mantle interaction here we analyze the whole crustal architecture of east cathaysia through a 300 km long sinoprobe deep seismic reflection profile this profile reveals that east cathaysia is underlain by two seismically distinct terranes i e the wuyishan and coastal terranes separated by a low angle northwest dipping set of mantle penetrating reflectors interpreted as the zhenghe dapu fault zone which documents neoproterozoic terrane amalgamation and cretaceous extensional reactivation the seismic image of the coastal terrane shows large scale low reflectivity truncated by arrays of high reflective subhorizontal bands of layered reflectors corresponding to an extension related crustal architecture dominated by extensive cretaceous magmatism the layered reflectors are interpreted to represent mantle derived mafic sills which document significant magmatic underplating associated with asthenospheric upwelling beneath the terrane this interpretation accounts for the localized distribution of cretaceous magmatic rocks in the coastal terrane we suggest that the magmatic underplating added large volumes of mantle derived material to the crust compensating for the crustal thinning such a magma compensating process might have formed an integral part of crustal reworking and growth in creating the magma dominated crust and the broadly flat moho of the coastal terrane during the cretaceous

dating san andreas fault earthquakes with lichenometry regionalrockfalleventsinthesierranevadaofcaliforniaare caused by distant earthquakes on the northern and southern san andreasfaultaswellasbylocalearthquakes lichenometricdating of synchronous pulses of rockfalls that are presumed to be caused by five historical earthquakes suggests an accuracy of 2 2 3 5 yr 95 confidence level for dating young events comparison of lichenometric ages for prehistorical rockfalls with precise radiocarbon ages of surface rupture times at the pallett creek and wrightwood paleoseismology sites indicates that earthquakes on the southern san andreas fault caused rockfalls 200 to 400 km to the north a major rockfall event at about a d 1690 supports the fumal et al 1993 model for a san andreas fault earthquake that was not detected at pallett creek the a d 1690 event in the chronology indicates that the earthquake recurrence interval was shorter and that the degree of clustering of times of earthquakes was less on the mojave segment of the san andreas fault than previously thought

self consistent modeling of crustal thickness at chagos laccadive ridge from bathymetry and gravity data the chagos laccadive ridge clr is an aseismic ridge in the indian ocean that formed by the action of the reunion mantle plume we use modeling of bathymetry and gravity data to constrain the crustal thickness variations at three locations from 0 s to 6 s where the plume was likely interacting directly with the carlsberg spreading center ambiguities in the modeling were reduced by requiring both the density and thickness of the crust to be consistent with a single set of mantle melting conditions in this case using a passive upwelling model in which mantle potential temperature is the controlling factor mean crustal thicknesses beneath the ridge at the three locations predicted from the gravity modeling were 16 6 12 5 and 27 0 km these thicknesses are up to 50 greater than would be obtained had we not required self consistency in the modeling if the thick crust is produced entirely by elevated potential temperatures in the melting region mantle temperatures would have to be 100 250 c higher than normal abrupt decreases in crustal thickness at a large offset fracture zone are consistent with previous models for plume spreading center interactions

hydrothermal activity and the evolution of the seismic properties of upper oceanic crust in order to investigate the impact of off axis hydrothermal circulation on changes of the seismic properties of upper oceanic crust layer 2a we performed an extensive geophysical survey on the eastern flank of the east pacific rise at 14 s seismic refraction and heat flow data were obtained along a 720 km long and 25 to 40 km wide corridor covering thinly sedimented seafloor created since 8 5 ma the seismic data yield a seismic velocity of 2 9 km s at the top of 0 5 m y old basement rocks within about 8 m y the velocity increases gradually to a value of mature oceanic crust 4 3 km s heat flow data derived from 43 in situ thermal conductivity and 86 geothermal gradient measurements suggest that an open hydrothermal circulation system persists for at least 6 7 m y in crust older than 7 ma regional heat flow is close to values predicted by plate cooling models suggesting that hydrothermal circulation is going to cease considering published dating of alteration minerals it appears that the permeability of uppermost oceanic crust has decreased to values insufficient to promote a vigorous hydrothermal circulation within 10 15 m y this idea may explain why seismic velocities in the pacific ocean have not changed significantly in igneous crust older than 8 10 ma in regions where juvenile and consistently hot crust is buried rapidly by sediments the evolution of the seismic properties is quite different velocities increase rapidly and reach values of mature oceanic crust within 1 2 m y we therefore favor a model where basement temperature is governing the evolution of the seismic properties of upper oceanic crust stephen and harding 1983 rohr 1994

regional shortening followed by channel flow induced collapse a new mechanism for dome and keel geometries in neoarchaean granite greenstone terrains the lateral flow and extrusive exhumation of ductile migmatitic gneisses due to a horizontal gradient in lithostatic pressure a process termed channel flow or lateral protrusion has previously been proposed as an important tectonic process in large hot orogens centrifuge simulation of i layer parallel shortening followed by ii collapse of a cover sequence during ductile flow of underlying layers in an analogous manner to channel flow suggests a new mechanism for the development of structures within neoarchaean granite greenstone terrains the centrifuge model incorporates an upper package of silicone modelling clay microlaminates that simulate a greenstone sequence that overlies slightly less dense ductile silicone putties whose rheological properties simulate migmatitic felsic gneiss a low viscosity low density layer along half of the infrastructure superstructure interface simulates the presence of granitoid melt during initial layer parallel shortening upright folds form in the upper greenstone package and the interface with underlying ductile layers is folded in basal silicone layers recumbent to overturned non cylindrical and upright folds form with increasing distance from the moving end wall removal of material parallel to fold hinges at the foreland end of the model i e furthest from the ram used to shorten models in several stages simulates erosion and the difference in gravitational loading thus created induces ductile flow and lateral extrusion of basal silicone layers models aim to reproduce features comparable to those developed during extrusive channel flow and focused exhumation of basement migmatitic gneiss in nature early formed recumbent to inclined folds are then accentuated during simulated channel flow while new recumbent isoclinal folds in basal layers develop broad antiforms and tight synforms similar to the dome and keel geometry that typifies many archaean granite greenstone belts are produced as a late feature in the model channel flow and collapse of a fold thickened crust is therefore proposed as a potential alternative mechanism for the formation of structures in some neoarchaean granite greenstone terrains by analogy with other physical and numerical models channel flow in the neoarchaean may be enhanced by impingement of an upper mantle wedge into the base of the crust our results imply that both lithospheric shortening and non diapiric gravitational instabilities may be responsible for the formation of some archaean dome and keel structures and may account for the juxtaposition of some granite greenstone and high grade gneiss terrains in the archaean model results also show similarities to structures produced during lateral flow and withdrawal of salt in fold belts

on the changing nature of the regional connection between the north atlantic oscillation and sea surface temperature 1 evidence is presented that the correlation between the north atlantic oscillation nao in terms of the nao index and the north atlantic sea surface temperature sst is not stationary this is inferred from both reanalysis data from the u s national centers for environmental prediction national center for atmospheric research ncep ncar and the kaplan sea surface temperature data set two phases of winterly north atlantic atmosphere ocean covariability are identified by means of linear regression and correlation analysis during the recent decades since the late 1960s early 1970s and during the first 3 decades of the twentieth century the north atlantic sst is strongly correlated to the regional atmospheric circulation in the north atlantic sector i e the north atlantic oscillation during these periods the nao index defined as the difference of normalized sea level pressures on the azores and iceland is characterized by pronounced decadal variability and by mainly positive values in contrast the nao index is only weakly correlated to the north atlantic sst from the 1930s to the early 1960s when the nao index is characterized by weak decadal variability remote influences in particular from the tropical pacific region become important especially for the sst in the western tropical north atlantic

crustal structure flexure and subsidence history of the hawaiian islands seismic reflection profile data have been used to estimate the long term 106 years mechanical properties of the oceanic lithosphere underlying the hawaiian islands the data show prominent reflectors associated with the top of the oceanic crust reflector 2 the m discontinuity and the base of the crust reflector 3 the two way travel time to the reflectors has been depth converted using velocity depth functions derived from seismic refraction expanding spread profile data and compared to calculated depths based on simple three dimensional elastic plate models a good fit to the reflector depths was obtained for a model with an effective elastic thickness of the lithosphere te of 40 km the main discrepancies with the refraction data occur beneath oahu and molokai where the predicted depth of the base of the flexed crust is too shallow and beneath the southeast flank of hawaii where it is too deep we attribute the thickened crust beneath oahu to ponding of magma at the base of the crust following the last phases of tholeiitic shield building and the thinned crust beneath the flanks of hawaii to crustal melting beneath the youngest part of the chain the effective te of 40 km is higher than expected for 80 m y oceanic lithosphere possibly due to the effects of subsurface loads that act at or near the base of the flexed crust according to free air gravity anomaly and bathymetry data te does not change significantly along the island chain retaining a similar value for hawaii as it does for oahu we have used this result to estimate the pattern of subsidence and uplift that would be expected along the hawaiian islands because of flexural loading of hawaii the observed depth of the haleakala wave cut terrace northeast of maui can be generally explained by the flexure model the main discrepancies between observed and the predicted depths occur northwest of hawaii where they suggest a somewhat higher value of te possibly due to incomplete adjustment of the lithosphere we suggest that the addition of loads to the end of a volcanic chain generated as a plate moves over a mantle hotspot is accommodated by progressive flexing downstream causing subsidence of preexisting volcanos that lie in the flexural moat and uplift of islands on the flanking bulge the progressive flexure model predicts a characteristic pattern of offlap and onlap in the regions flanking the hawaiian islands that is generally similar to stratigraphic relationships observed along previously published seismic profiles of the moat a progressive flexure model may also apply to the case of a continental plate moving over a mantle hotspot and could explain the observations of 1 a clinoform pattern of stratigraphy in some flood basalt provinces and 2 a seaward dipping reflector sequence at some passive continental margins

influence of incoming plate relief on overriding plate deformation and earthquake nucleation cocos ridge subduction costa rica 18 pages 7 figures supporting information https doi org 10 1029 2019tc005586 data are available at the pangaea world data center https doi org 10 1594 pangaea 909047

crust generated above the iceland mantle plume from continental rift to oceanic spreading center since the north atlantic continental breakup in the early tertiary the process of rifting above a mantle plume has produced large thicknesses of igneous crust we report results of an integrated offshore onshore seismic study of the crust and upper mantle along a transect of the aseismic faroe iceland ridge between the continental fragment beneath the faroe islands and the present day spreading center in northeast iceland normal incidence seismic data provide an image of the uppermost crust which is complemented by a velocity model from streamer refraction analyses these data together image four sedimentary basins up to 400 m deep along the crest of the ridge the streamer refraction velocity model in turn forms the uppermost section of a full crustal velocity model derived from travel time modeling of air gun and explosive data the compressional to shear wave velocity ratio in the crust of the faroe iceland ridge is 1 77 0 02 similar to that of eastern iceland the moho beneath the faroe iceland ridge lies at a depth of 25 30 km crust of 25 30 km thickness requires an upper mantle potential temperature elevated 200 250 c above normal if formed by passive adiabatic decompression melting of the mantle or a somewhat less elevated temperature if as is likely there is a component of active convection in the underlying mantle plume core

faroe iceland ridge experiment 2 crustal structure of the krafla central volcano the seismic velocity structure of the krafla central volcano is characterized by large variations in compressional velocity a 40 km wide high velocity dome extends from the lower crust 11 14 km depth beneath the volcano narrowing upward a magma chamber sits at its top near 3 km depth it is defined by both 0 2 0 3 s compressional wave delays and shear wave shadowing to be 2 3 km n s 8 10 km e w and 0 7 1 8 km thick the near surface structure uppermost 2 5 km of the krafla caldera is approximately flat lying with only minor lateral heterogeneities the crust beneath the magma chamber has low shear wave attenuation and anomalously high compressional and shear wave velocities shear waves reflected from a 19 km deep moho are clearly visible for some paths through the crustal volume below the magma chamber even though the more shallow diving s waves are severely attenuated the midcrust beneath the shallow magma chamber cannot contain partial melt or even be at near solidus temperatures the krafla central volcano plays a major role in crustal genesis along the plate boundary the high velocity dome in our view represents crust generated in and around the magma chamber which has subsequently been advected to greater depths

re evaluation of the 1836 hayward fault and the 1838 san andreas fault earthquakes abstract current seismic hazard models include two major earthquakes m 7 in the san francisco bay area that are close in space and time an 1836 event on the northern hayward fault and an 1838 event on the peninsula section of the san andreas fault analysis and interpretation of the available historical accounts indicate that the 1836 event occurred east of monterey bay far from the hayward fault and was of m 6 also the 1838 event was not confined to the 60 km peninsula san andreas as current models indicate instead faulting probably extended from san francisco to san juan bautista 140 km indicating a significantly larger earthquake m 7 than previously thought damaging effects of the 1836 earthquake were reported only from santa clara to carmel and no contemporary effects were reported to the north of santa clara or near the hayward fault the illusion of an 1836 hayward earthquake evolved from a newspaper reminiscence published following the 1868 hayward earthquake stating that the 1868 effects in the east bay were similar to those of an 1836 event the article describes various strong effects in the east bay that differ completely from the effects recorded for the 1836 earthquake but are very similar to those documented for the major 1838 san andreas earthquake that caused extensive damage on both sides of san francisco bay based on this and other evidence we conclude that the reminiscence describes the destructive june 1838 effects but it erroneously indicates the date as june 1836 there is no evidence for any major historical earthquakes in the san francisco bay area before the 1838 earthquake back to the founding of mission san francisco dolores in 1776 during the 1838 san andreas fault earthquake the shaking intensity in monterey was as strong as or stronger than during the great 1906 san andreas fault earthquake this suggests that the 1838 san andreas fault rupture may have extended to san juan bautista as it did in 1906 numerous probable aftershocks were felt in the area south of san juan bautista these damaged carmel and santa cruz in 1840 and alisal 16 km west of the san andreas fault in 1841 the northern end of the 1838 faulting was previously assumed to be 25 km south of san francisco however mission san francisco dolores was damaged in 1838 but not in 1906 suggesting that the 1838 faulting extended to san francisco also the 1838 aftershocks were felt in oakland as frequently and violently as those following the major 1868 hayward earthquake suggesting that the 1838 faulting on the san andreas extended to the latitude of oakland the 1838 fault segment ruptured again 68 years later as part of the overlapping 1906 san andreas fault rupture this and similar evidence from southern california indicates that m 7 san andreas fault earthquakes can recur at intervals of 68 years or less when they are followed by m 8 earthquakes on overlapping segments of the fault

flow behavior of fine grained synthetic dunite in the presence of 0 5 wt h2o the deformation behavior of synthetic dunite mg forsterite plus 2 5 vol enstatite with a grain size of 1 m has been investigated in combined constant displacement rate and temperature stepping experiments at temperatures of 850 to 1000 c strain rates of 10 7 to 10 5 s 1 and a confining pressure of 600 mpa nominally dry material behaved purely elastically in contrast samples with 0 5 wt added water flowed at stresses between 9 and 81 mpa reaching strains up to 12 fitting a dorn type power law to the wet data yielded stress exponents n of 1 7 0 4 and apparent activation energies in the range 302 22 kj mol 1 the wet deformed samples showed polygonal grains no crystallographic preferred orientation no subgrains low dislocation densities and evidence for grain boundary cavitation grain growth was minor it is concluded that the wet samples probably deformed by a grain size sensitive grain boundary sliding dominated mechanism though the nature of the accommodation process remains unclear and the possibility of transitional behavior between dislocation creep and diffusion creep cannot be eliminated application of the results to upper mantle shear zones supports previous speculation that the presence of such zones can lead to a significant weakening of the top 10 to 20 km of the upper mantle during extension of the continental lithosphere

implications of fault slip rates and earthquake recurrence models to probabilistic seismic hazard estimates abstract increasingly fault slip rates are being used to constrain earthquake recurrence relationships for site specific probabilistic seismic hazard ground motion assessments this paper shows the sensitivity of seismic hazard assessments to variations in recurrence models and parameters that incorporate fault slip rates two models are considered to describe the partitioning of the slip rate or seismic moment rate into various magnitude earthquakes an exponential magnitude distribution and a characteristic earthquake distribution assuming an exponential distribution the activity rate n m 0 is constrained by the upper bound magnitude m u the b value for the region and the fault slip rate s for a given s variations in m u and b value have significant effects on recurrence and computed hazard depending on whether the assumption is made that the seismicity rate is constant or the moment rate is constant there is increasing evidence that the characteristic earthquake model is more appropriate for individual faults than the exponential magnitude distribution based on seismicity data from areas having repeated large earthquakes a generalized recurrence density function is developed and the resulting recurrence relationship requires only m u b value and s a comparison of the recurrence relationships from this model with the historical seismicity and paleoseismicity data on the wasatch and san andreas faults shows a good match the computed hazard based on the characteristic earthquake model differs from that obtained for the exponential model as a function of the fault to site distance and the acceleration level one check on the recurrence models using slip rate is to compare over large regions activity rates based on seismicity data and slip rate data such a comparison in the western transverse ranges shows a reasonable match for both the exponential and characteristic earthquake models in the moderate to large magnitude range

waveform inversion of deep seismic reflection data the polarity of mantle reflections abstract the flannan and w reflectors are two prominent mantle features observed on seismic reflection data off the northwest coast of scotland they are the brightest most laterally extensive intra mantle reflectors identified on a deep seismic dataset anywhere in the world despite extensive study their physical origin is still the subject of speculation we present a scheme to determine the polarity of these mantle reflectors and constrain their upper structure using near normal incidence seismic reflection data the technique exploits the convolutional model of the earth we use a deterministic source signature deconvolution to invert the data we have explored the parameterization of the inversion by testing real and synthetic data we find that it is critical to the legitimacy of the reflectivity model that many traces are stacked prior to the inversion and that the data have a good signal to noise ratio furthermore an accurate estimate of the effective source wavelet is a fundamental requirement for obtaining a valid reflectivity model in particular we find the deconvolution results are most sensitive to the precise value of the water depth and reflection coefficient used in estimating the sea bed multiple train in the case of the flannan reflector the inversion shows unequivocally that it has a positive polarity modelling the w reflector is less straightforward as a result of reduced signal to noise ratio none the less the inversion suggests a positive polarity for the w reflector in agreement with observations of post critical reflections seen on wide angle seismic data the near normal incidence polarity measurements support the suggestion that both the flannan and w reflectors represent an eclogitic slab presumably a relict oceanic subduction zone preserved within the continental lithospheric mantle

extinct spreading on the cocos ridge using recently acquired marine magnetic data and existing magnetic and bathymetric data sets together with odp leg 170 age determinations we present a revised plate tectonic model for the southern cocos and northern nazca plate area according to this model the formation of the southern cocos plate was governed by spreading at different ridge axes with alternations between spreading ridges producing a complex magnetic anomaly pattern in the cocos and malpelo ridge area we have identified two precursors of the recently active cocos nacza spreading system which were active from 22 8 to 14 7 ma with a change in spreading direction from nw se to ene wsw at 19 5 ma the oceanic crust of these abandoned spreading systems was subsequently thickened and overprinted by hotspot volcanism that formed the cocos and malpelo ridges the centre of this hotspot volcanism is about 500 km away from but most probably related to the galapagos hotspot

lithospheric structure north of scotland ii poisson s ratios and waveform modelling summary a 200 by 50 km grid of near normal incidence seismic reflection data and a 350 km wide angle seismic line have been acquired across the flannan and w mantle reflectors north of scotland a number of geological models have been published to explain the origin of these reflections we examine these models by considering the physical properties of the lithosphere in this region we present p and s wave velocity models for the lithosphere obtained using ray based inversion schemes we show full wavefield synthetic p wave data generated using a 2 d finite difference code to confirm that our velocity model can reproduce the amplitude behaviour of the observed mantle arrivals p and s wave velocities beneath the moho give a poisson s ratio of 0 25t0 01 which together with evidence from local mantle xenoliths suggests a normal peridotitic upper mantle we observe both normal incidence and wide angle reflections from the mantle reflectors modelling and inversion of these reflection events show that the flannan and w reflectors represent at least a 20 per cent increase in acoustic impedance within the mantle and that the region of elevated acoustic impedance has a thickness of at least 3 km a mafic eclogite is the only common mantle rock type that can have an acoustic impedance significantly higher than peridotite the physical properties of the upper mantle north of scotland thus suggest that the flannan and w reflectors represent reflection events from the tops of slabs of eclogite a weak event is interpreted as a wideangle reflection from the base of one of these slabs a slab thickness of 6 0t1 0 km with a poisson s ratio of 0 29t0 02 was used to model this phase the high poisson s ratio within the slab gives further support to the interpretation that these slabs are eclogitic the lateral extent of these mantle reflection events and the thickness of the slab are most readily explained as oceanic crust that was metamorphosed to eclogite when subducted beneath the existing continent we conclude that the eclogitic slabs are relicts of oceanic crust that remained within the lithosphere when subduction halted

upper mantle reflectors modelling of seismic wavefield characteristics and tectonic implications summary deep seismic experiments on continental lithosphere generally reveal marked reflectivity from structures in the crust and a significant decrease in reflectivity from the upper mantle however reflected and refracted energy from mantle lithosphere are observed in both near normal incidence and wide angle data the origin of the reflective structures is a matter of debate hypotheses include remnant subduction zones shear zones fluids and seismic anisotropy through analytical and numerical modelling studies including full wavefield modelling we investigate seismic characteristic signatures generated from a variety of geologically plausible models we have found that both upper mantle shear zones of reduced velocity and density and remnant subduction slabs containing high density eclogites may contain sufficient seismic impedance contrasts to normal mantle peridotites to generate near normal incidence reflectivity wide angle energy originates from subduction slabs containing either high or lowvelocity eclogites whereas intermediate velocity eclogites are unlikely to produce significant wide angle phases in general energy of seismic phases originating from upper mantle zones of anomalous seismic velocities and densities is significantly increased if homogeneous zones are replaced by zones of inhomogeneous petrophysical properties resulting from constructive interference maximum wavefield anomalies are generated from sub moho dipping slabs of incomplete transformation of low velocity low density crustal material to high velocity highdensity eclogites localized shear zones generated in mantle peridotite generally do not produce significant wide angle energy only if highly inhomogeneous structures containing material of marked ca 10 per cent velocity and density reduction are present may shear zones be observed in wide angle data analyses of two specific deep seismic data sets mona lisa data from the north sea and babel data from the baltic sea show good agreement between observations and modelling results for dipping remnant subduction slabs containing small scale inhomogeneities associated with incomplete transformation our modelling results improve our possibilities of distinguishing between two often contrasting tectonic interpretations for dipping upper mantle seismic reflectors the remnant subduction and shear zone models

deep seismic reflectivity beneath an intracratonic basin insights into the behavior of the uppermost mantle beneath the illinois basin abstract reflectivity images of the lower crust and uppermost mantle beneath the paleozoic illinois basin have been derived from reprocessing of several hundred kilometers of industry seismic reflection data using extended vibroseis recorrelation the recorrelation was based on extending an originally 4 s correlated record acquired with a 16 s sweep from 14 to 126 hz to the absolute limit of the full 20 s 70 km listening traveltime sub moho reflections are recorded to 18 s two way traveltime unmigrated and are observed on intersecting profiles apparently dipping to the southwest and striking northwest southeast occasional moho reflections are also observed across the profiles 12 s or 38 km while reflectivity in the lower crust is generally marked by intermittent horizontal packages and short gently dipping reflections and diffraction segments the presence of newly observed mantle reflectivity beneath the illinois basin indicates significant uppermost mantle heterogeneity relative to other parts of the usa studied using reflection methods the mantle reflectivity effectively defines an area of anomalous mantle that rests beneath the depocenter of the early paleozoic illinois basin which is itself superimposed over a deeper proterozoic basinal depocenter filtering and inversion of geopotential field data reveal that this anomalous mantle also partially correlates with a positive density anomaly in the upper mantle these correlations suggest that the anomalous mantle zone exerted an influence on the early paleozoic subsidence of the basin perhaps as a buried load due to eclogitization of former lower crust the relatively isolated occurrence of sub moho reflections beneath the basin makes it difficult to uniquely infer their origin however available geologic and geophysical constraints especially from geochemical and geochronological studies of drilled and exposed basement rocks limit the possibilities to 1 remnants or scars of sub crustal processes associated with lithospheric extension or delamination related to the melting of the proterozoic crust that led to the emplacement of the granite rhyolite province that underlies much of usa midcontinent or 2 deformation caused by plate subduction associated with the hypothetical accretion of a juvenile island arc to the pre 1 6 ga southern margin of the laurentian continent

nonlinear soil response a reality geotechnical models consistently indicate that the stress strain relationship of soils is nonlinear and hysteretic especially at shear strains larger than 10 5 to 10 4 nonlinear effects such as an increase in damping and reduction in shear wave velocity as excitation strength increases are commonly recognized in the dynamic loading of soils on the other hand these effects are usually ignored in seismological models of ground motion prediction because of the lack of compelling corroborative evidence from strong motion observations the situation is being changed by recently obtained data explicit evidence of strong motion deamplification accompanied by changes in resonant frequencies are found in the data from the 1985 michoacan mexico and the 1989 loma prieta california earthquakes the events recorded by the vertical and surface accelerograph arrays in taiwan as well as a number of other events throughout the world evidence of nonlinear behavior becomes apparent beyond a threshold acceleration of 100 to 200 gal nonlinearity is considerable in cohesionless soil but may be negligible in stiff soils the findings of recent years indicate that nonlinear site effects are more common than previously recognized in strong motion seismology

probabilistic seismic hazard analysis considering nonlinear site effect a modified method is proposed for the probabilistic seismic hazard analysis considering nonlinear site effect a case study is presented to show the hazard curves that consider the nonlinearity of the soil layers at gilroy 2 source parameters and an earthquake occurrence model are assigned to the caleveras fault which is assumed the dominant seismic source to the site a relationship between ground surface and bedrock accelerations at the site is established by recordings from three earthquake events using the program nonli3 which calculates the nonlinear responses of the soil layers to strong motions various values of standard errors of the estimated surface pga given the bedrock motions are assumed to perform a sensitivity analysis an alternative nonlinear relation is included in the case study in addition to the nonlinear models a linear response model is also performed using the same soil profile at gilroy 2 the results demonstrate that the hazard curves are sensitive to the standard error of the estimated surface pga values at the site the hazard curve may be severely distorted if the soil condition of a nonlinear site is treated inadequately the alternative nonlinear model obtained empirically from regression analysis and the linear response model are both unsuitable for the hazard assessment considering nonlinear site effect

irregular recurrence of large earthquakes along the san andreas fault evidence from trees old trees growing along the san andreas fault near wrightwood california record in their annual ring width patterns the effects of a major earthquake in the fall or winter of 1812 to 1813 paleoseismic data and historical information indicate that this event was the san juan capistrano earthquake of 8 december 1812 with a magnitude of 7 5 the discovery that at least 12 kilometers of the mojave segment of the san andreas fault ruptured in 1812 only 44 years before the great january 1857 rupture demonstrates that intervals between large earthquakes on this part of the fault are highly variable this variability increases the uncertainty of forecasting destructive earthquakes on the basis of past behavior and accentuates the need for a more fundamental knowledge of san andreas fault dynamics

ridge subduction at an erosive margin the collision zone of the nazca ridge in southern peru the 1 5 km high obliquely subducting nazca ridge and its collision zone with the peruvian margin have been imaged by wide angle and reflection seismic profiles swath bathymetry and gravity surveying these data reveal that the crust of the ridge at its northeastern tip is 17 km thick and exhibits seismic velocities and densities similar to layers 2 and 3 of typical oceanic crust the lowermost layer contributes 10 12 km to the total crustal thickness of the ridge the sedimentary cover is 300 400 m thick on most parts of the ridge but less than 100 m thick on seamounts and small volcanic ridges at the collision zone of ridge and margin the following observations indicate intense tectonic erosion related to the passage of the ridge the thin sediment layer on the ridge is completely subducted the lower continental slope is steep dipping at 9 and the continental wedge has a high taper of 18 tentative correlation of model layers with stratigraphy derived from ocean drilling program leg 112 cores suggests the presence of eocene shelf deposits near the trench continental basement is located 15 km landward of the trench normal faults on the upper slope and shelf indicate extension a comparison with the peruvian and northern chilean forearc systems currently not affected by ridge subduction suggests that the passage of the nazca ridge along the continental margin induces a temporarily limited phase of enhanced tectonic erosion superposed on a long term erosive regime

fault behavior and characteristic earthquakes examples from the wasatch and san andreas fault zones paleoseismological data for the wasatch and san andreas fault zones have led to the formulation of the characteristic earthquake model which postulates that individual faults and fault segments tend to generate essentially same size or characteristic earthquakes having a relatively narrow range of magnitudes near the maximum analysis of scarp derived colluvium in trench exposures across the wasatch fault provides estimates of the timing and displacement associated with individual surface faulting earthquakes at all of the sites studied the displacement per event has been consistently large measured values range from 1 6 to 2 6 m and the average is about 2 m on the basis of variability in the timing of individual events as well as changes in scarp morphology and fault geometry six major segments are recognized along the wasatch fault on the basis of the most likely number of surface faulting events 18 that have occurred on segments of the wasatch fault zone during the past 8000 years an average recurrence interval of 400 666 years with a preferred average of 444 years is calculated for the entire zone geologic data on the distribution of slip associated with prehistoric earthquakes and slip rates along the south central segment of the san andreas fault suggest that the m 8 1857 earthquake is a characteristic earthquake for this segment comparisons of earthquake recurrence relationships on both the wasatch and san andreas faults based on historical seismicity data and geologic data show that a linear constant b value extrapolation of the cumulative recurrence curve from the smaller magnitudes leads to gross underestimates of the frequency of occurrence of the large or characteristic earthquakes only by assuming a low b value in the moderate magnitude range can the seismicity data on small earthquakes be reconciled with geologic data on large earthquakes the characteristic earthquake appears to be a fundamental aspect of the behavior of the wasatch and san andreas faults and may apply to many other faults as well

slip rate earthquake recurrence and seismogenic potential of the rodgers creek fault zone northern california initial results instrumental seismicity defines a seismic gap along the rodgers creek fault zone rcfz between santa rose and san pablo bay results of a paleoseismicity study within the gap using offset channels in late holocene alluvial deposits as piercing points indicate a minimum slip rate of 2 1 to 5 8 mm yr for the past 1 300 years a preferred range for the maximum recurrence interval of 248 to 679 years and a surface offset of 2 0 3 minus 0 2 m during the most recent event the rcfz has produced past m7 earthquakes and historical seismicity data indicate a minimum elapsed time of 182 years since the most recent earthquake of this size

spatial distribution of hotspot material added to the lithosphere under la reunion from wide angle seismic data wide angle seismic lines recorded by ocean bottom and land seismometers provide a pseudo three dimensional investigation of the crust and upper mantle structure around the volcanically active hotspot island of la reunion the submarine part of the edifice has fairly low seismic velocities without evidence for intrusives an upper unit with a velocity depth gradient is interpreted as made of material erupted subaerially then transported and compacted downslope between this unit and the top of the oceanic plate imaged by normal incidence seismic reflection a more homogeneous unit indicated by shadow zones on several wide angle sections may correspond to lavas of a different nature extruded underwater in the earlier phase of volcanism coincident wide angle and normal incidence reflections document that the oceanic plate is not generally downwarping toward the island but doming instead toward its southeastern part with limited evidence for some intracrustal intrusion deeper in the lithosphere the presence of a layer of intermediate velocity between the crust and mantle is firmly established it is interpreted as resulting from the advection of hotspot magmatic products possibly partially molten and of a composition for which the crust is a density barrier the extensive wide angle coverage constrains the extent of this body it does not show the elongated shape expected from plate drift above a steady hotspot supply alternative propositions can hence be considered for example that la reunion is caused by a solitary wave of hotspot material or by a young hotspot the size of the underplate 140 km wide and up to 3 km thick corresponds to less than half the volume of the edifice on top of the plate

structure and ageing of oceanic crust at 14 s on the east pacific rise a wide angle reflection refraction survey was conducted along a flow line on the eastern flank of the east pacific rise at 14 s the data were acquired with a single 32 litre airgun and ocean bottom hydrophones obh and consist of six profiles on 0 5 8 3 myr old seafloor in total 17 obhs provided seismic records of excellent quality to generate 2 d velocity models traveltimes and amplitudes have been calculated using asymptotic ray theory a comparison of the traveltimes and amplitudes of p wave arrivals on all lines revealed important similarities that correlate well with the classical model of oceanic crust the uppermost crust is composed of a 0 4 0 7 km thick low velocity 2 9 4 6 km s 1 surficial layer layer 2a and is bounded at its base by a sharp velocity increase to 5 5 km s 1 at the top of layer 2b overall layer 2 is 1 4 1 8 km thick small seamounts frequently superposed on fast spreading crust are characterized by velocities lower than within the adjacent upper crust velocities 6 0 km s 1 exist throughout the edifices suggesting that the seamounts are dominantly extrusive decreasing amplitudes beyond 8 10 km offset constrain a moderate gradient lower crust layer 3 with velocities of 6 8 7 2 km s 1 crustal thickness is found to be remarkably uniform at 6 11 0 14 km nevertheless the seismic data provide evidence for age dependent variations in both the low velocity surficial layer and the upper mantle we find that velocities in layer 2a increase from 2 9 to 4 3 km s 1 velocities in the middle and lower crust show uniform values while velocities within the uppermost mantle increase gradually from 7 5 km s 1 to 8 0 km s 1 the most viable explanation for increasing velocities in the upper crust is that off axis hydrothermal circulation was accompanied by the precipitation of secondary minerals in open void spaces thereby decreasing the porosity and increasing the velocity low mantle velocities beneath young crust are suggested to be a temperature controlled phenomenon and may indicate the presence of partial melt in the upper mantle

the effect of quaternary alluvium on strong ground motion in the coyote lake california earthquake of 1979 abstract the effect of alluvium on strong ground motion can be seen by comparing two strong motion records of the coyote lake california earthquake of 6 august 1979 m l 5 9 one record at a site on franciscan bedrock had a peak horizontal acceleration of 0 13 g and a peak horizontal velocity of 10 cm sec the other at a site 2 km distant on 180 meters of quaternary alluvium overlying franciscan had values of 0 26 g and 32 cm sec amplifications by factors of 2 and 3 horizontal motions computed at the alluvial site for a linear plane layered model based on measured p and s velocities show reasonably good agreement in shape with the observed motions but the observed peak amplitudes are greater by a factor of about 1 25 in acceleration and 1 8 in velocity about 15 per cent of the discrepancy in acceleration and 20 per cent in velocity can be attributed to the difference in source distance the remainder may represent focusing by refraction at a bedrock surface concave upward there is no clear evidence of nonlinear soil response fourier spectral ratios between motions observed on bedrock and alluvium show good agreement with ratios predicted from the linear model in particular the observed frequency of the fundamental peak in the amplification spectrum agrees with the computed value indicating that no significant nonlinearity occurs in the secant shear modulus computations show that nonlinear models are compatible with the data if values of the coefficient of dynamic shear strength in terms of vertical effective stress are in the range of 0 5 to 1 0 or greater the data illustrate that site amplification may be less a matter of resonance involving reinforcing multiple reflections and more the simple effect of the low near surface velocity application of traditional seismological theory leads to the conclusion that the site amplification for peak horizontal velocity is approximately proportional to the reciprocal of the square root of the product of density and shear wave velocity

the crustal structure of the cocos ridge off costa rica 1 the submarine cocos ridge in the northwestern panama basin a bathymetric feature more than 1000 km long and 250 500 km broad is about 2 km shallower than the adjacent basin it is generally interpreted as the trace of the galapagos hot spot two 127 and 260 km long seismic wide angle sections were recorded along and across this ridge offshore the osa peninsula costa rica crustal thickening is seen everywhere along the sections on the northwestern outer ridge flank increased thickness is exclusively attributed to the upper crust and expressed by 2 km thick flow basalts the quepos plateau caps the upper crust in this area toward the center of the cocos ridge the moho deepens from 11 12 to 21 km depth and crustal thickening is almost entirely attributed to the lower crust which makes up 80 of the crust and is three times the thickness of normal oceanic lower crust it is homogeneously structured and the velocities which range from 6 5 km s at the top to 7 35 km s at the base are comparable to normal lower crust under these depth conditions and suggest no differences to a gabbroic rock composition similarities to the crustal velocity structure of iceland central kerguelen plateau and broken ridge are consistent with a formation of this 13 15 ma old cocos ridge segment by excessive magmatism in a near plate boundary setting

mantle shear zones and the evolution of the northern north sea basin mantle reflections observed on deep seismic profiles across the northern north sea may be tentatively interpreted as extensional shear zones modeling them as such explains many of the features of the basin in particular the broad regions of postrift onlap onto the basin flanks consequently it seems likely that the mantle reflections represent mantle shear zones which control the extension of the subcrustal lithosphere

numerical modelling of strain localisation during extension of the continental lithosphere dipping reflectors in the upper mantle are observed on reflection seismic data and in some cases have been interpreted to represent mantle shear zones it is not clear however how these shear zones develop and under what circumstances previous attempts to model strain localisation by strain softening in the ductile regime have failed to show the development of mantle shear zones we present here results from a numerical model of lithospheric extension the model uses an elasto visco plastic rheology dependent on temperature pressure deviatoric stress and composition strain softening is incorporated into the mechanical model via a simple ad hoc strain viscosity relationship the results demonstrate that strain localisation can dominate lithospheric deformation in an extensional regime we illustrate the effects of varying the parameters which define the strain viscosity relationship on the geometry of lithospheric scale shear zones and the rift basin we also investigate the effects on shear zone development of introducing a variety of pre extension weak zones in the crust and or mantle our results suggest that if dipping reflectors observed on deep seismic data around the world are evidence of the existence of mantle shear zones there is a need to review our conceptual models of strain strength evolution during the development of continental rifts

crustal structure of the cocos ridge northeast of cocos island panama basin 1 the submarine cocos ridge in the northwestern panama basin pacific ocean is generally interpreted as the trace of the galapagos hotspot a 278 km long seismic wide angle transect was carried out across a comparatively narrow ridge segment 150 km northeast of cocos island the results indicate a huge thickened crust where crustal thickening is mainly achieved by the lower crust which makes up 75 of the crustal thickness and is tripled compared to normal oceanic lower crust the velocities are comparable to normal lower crust and suggest no differences to a gabbroic composition the moho deepens from 10 to 18 km depth below the ridge similarities to other hotspot related oceanic ridges and plateaus with massive lower crusts and velocities below 7 4 km s suggest the formation of this cocos ridge segment near or at the plate boundary

re evaluation of several large historic earthquakes in the vicinity of the loma prieta and peninsular segments of the san andreas fault california abstract the size and location of several of the largest 19th century earthquakes in the san francisco bay area including the june 1838 8 october 1865 21 october 1868 and 24 april 1890 events are re evaluated by comparing these historic events to several modern events in particular the 1989 loma prieta earthquake similar high intensities in the monterey bay area during both the 1989 and 1838 events indicate that the 1838 shock probably ruptured the loma prieta segment in addition to the peninsular segment of the san andreas fault if so the resulting earthquake would have been of moment magnitude m w 7 2 thus like segments of many other plate boundaries the loma prieta segment exhibits heterogeneous rupture in that sometimes it ruptures with the peninsular segment as in 1906 and probably in 1838 and at other times it ruptures alone as in 1989 in contrast the area of high intensities generated by the 1865 earthquake was centered in the santa clara valley and the eastern foothills of the santa cruz mountains indicating that the 1865 shock probably was located northeast of the loma prieta rupture and not on the san andreas fault in addition the felt areas taken to be one of the best measures of relative source size of modified mercalli intensities mmi v and vi for the 1865 shock are considerably smaller than that of the 1989 event relationships between felt areas of intensities v and vi and moment magnitude are developed for the san francisco bay area based on these relationships the average computed moment magnitudes for the 1865 1868 and 1890 earthquakes are 6 51 0 18 6 84 0 20 and 6 32 0 16 respectively if these results are correct then the loma prieta segment taken to include all faults within the san andreas fault zone i e within a zone up to a few kilometers wide appears to rupture with or without the peninsular segment every 76 11 years these data on repeat times may be the most accurate information now available for ascertaining the date of future rupture of that segment and for testing recurrence models the smaller size of the 1865 earthquake would be consistent with it being part of the long term seismic build up to the 1868 event rather than being paired with it as events of similar size if the 1865 earthquake did occur northeast of the loma prieta segment then a future source of that type located closer to san jose and the santa clara valley could pose a considerable hazard to the southern san francisco bay area on going searches of letters and diaries of missionaries and early settlers could yield new information that should help to further constrain the locations and magnitudes of these and other historic earthquakes