radiated energy from the mid niigata japan earthquake of october 23 2004 and its aftershocks 1 the mid niigata earthquake of october 23 2004 is a shallow reverse faulting event with mw 6 6 we analyze transverse component accelerograms of the main shock and four aftershocks mw 4 0 5 5 recorded at 29 strong motion observation stations the epicenters of the four aftershocks are located within 5 km from that of the main shock the stations are well distributed around the epicenters and their epicentral distances are between 15 and 60 km we calculate spectral ratio between records of the main shock and each of the aftershocks in order to obtain source spectral ratio by empirically removing the effects of wave propagation path and surface layers at the stations thus obtained source spectral ratio is inconsistent with that expected from the self similar omega square model fitting omega square source spectral ratio with variable corner frequency to the observed spectral ratio radiated energy and corner frequency are estimated radiated energy from the main shock is 3 2 1014 j although energy to moment ratios for the main shock and the aftershocks are all distributed within a narrow range from 8 10 6 to 4 10 5 they definitely increase with the event size this scale dependence is quite similar to that obtained previously for strike slip events in japan the relationship between the seismic moment m0 and the corner frequency f0 is m0 f0 3 3 0 2 these results suggest a break in self similarity

source fault of the 2007 chuetsu oki japan earthquake the 2007 chuetsu oki japan earthquake is the world s first major earthquake upon a source fault that extends beneath a nuclear power plant and is also characterized by difficulty determining the source fault plane centroid moment tensor solutions indicate an m w 6 6 reverse faulting crustal earthquake with conjugate fault planes dipping to the northwest and southeast early results of aftershock locations suggest that either northwest dipping plane or southeast dipping plane can be the source fault plane of this earthquake we carried out source inversions and empirical green s function simulations of observed seismograms however they resulted in similar waveform residuals for the two fault planes we then determined the relative locations of earthquake asperities to the hypocenter using travel time differences of strong motion pulses and relocated the aftershocks observed by ocean bottom seismometers deployed in the source region these results imply that slips mainly occurred on the southeast dipping fault plane this implication was later confirmed by results of reflection surveys during the earthquake the kashiwazaki kariwa nuclear power plant experienced stronger ground motions than those anticipated at the time of design the ground motions consist of three seismic pulses that correspond to three asperities the first and second pulses arose from rupture propagation to the plant while the compact asperity on the distant southeast dipping fault plane and its s wave radiation pattern are responsible for the significant third pulse

estimation of radiated energy using the kik net downhole records old method for modern data we use kik net nied down hole records to estimate the radiated energy er of 29 japanese inland earthquakes with a magnitude range from mw 5 6 to 7 0 the method is based on the work of gutenberg and richter in which the time integral of s wave ground motion velocity squared is measured as a basic metric of the radiated energy only stations within a distance of 100 km are used to minimize complex path and attenuation effects unlike the teleseismic method that uses mainly p waves the use of s waves which carry more than 95 per cent of the radiated energy allows us to obtain robust results we calibrate the method using synthetic seismograms to modernize and improve the gutenberg richter method we compute synthetic seismograms for a source model of each event with a given source function i e known er the actual mechanism and the source station geometry then we compare the given er with the computed energy metric to correct for the unknown effect of wave propagation and the mechanism the use of down hole records minimizes the uncertainty resulting from the site response our results suggest that the currently available estimates of er from teleseismic data are probably within a factor of 3 on average of the absolute value the scaled energy er er m0 is nearly constant at about 3 10 5 over a magnitude range from mw 5 6 to 7 0 with a slight increasing trend with mw we found no significant difference in er between dip slip and strike slip events

evolution of oceanic crust on the kolbeinsey ridge north of iceland over the past 22 myr the evolution of oceanic crust on the kolbeinsey ridge north of iceland is discussed on the basis of a crustal transect obtained by seismic experiment from the kolbeinsey ridge to the jan mayen basin the crustal model indicates a relatively uniform structure no significant lateral velocity variations are observed especially in the lower crust the uniform velocity structure suggests that the postulated extinct axis does not exist over the oceanic crust formed at the kolbeinsey ridge but supports a model of continuous spreading along the ridge after oceanic spreading started west of the jan mayen basin the oceanic crust formed at kolbeinsey ridge is 1 2 5 km thicker than normal oceanic crust due to hotter than normal mantle from the iceland mantle plume the observed generally uniform thickness throughout the transect might also indicate that the temperatures of the astheno spheric mantle ascending along the kolbeinsey ridge have not changed significantly since the age of magnetic anomaly 6b

microearthquakes beneath median valley of mid atlantic ridge near 23 n tomography and tectonics data from a microearthquake experiment in the median valley of the mid atlantic ridge near 23 n in 1982 are used to measure earthquake source parameters to determine the laterally heterogeneous seismic velocity structure across the inner floor and to develop a kinematic tectonic model for this portion of the median valley fifty three microearthquakes occurred over a 10 day period beneath the median valley inner floor and eastern rift mountains twenty of 23 well located inner floor epicenters define a line of activity about 17 km long having a strike of n25 e and located near an along axis depression some 300 400 m deeper than surrounding regions earthquakes with well resolved hypocenters generally have focal depths of 4 8 km beneath the seafloor of both the inner floor and the rift mountains the hypocentral locations are robust with respect to plausible lateral variations in seismic velocity structure composite fault plane solutions for inner floor events indicate normal faulting on planes dipping at angles near 45 normal faulting mechanisms although poorly constrained are also indicated for the rift mountain microearthquakes seismic moments approximate fault dimensions and average stress drops for the largest events recorded are 1019 1020 dyn cm 200 400 m and 1 70 bars respectively a twodimensional tomographic inversion of p wave travel time residuals from microearthquakes and local shots indicates a well resolved lateral heterogeneity in crustal velocity structure across the median valley inner floor p wave velocities at 1 5 km depth within a zone less than 10 km wide beneath the central inner floor are lower by several percent than in surrounding regions the most likely explanation for the low velocities is that the region is the site of the most recent local magmatic injection and remains pervasively fractured as a result of rapid hydrothermal quenching of the newly emplaced crustal column by this view the seismic velocity structure at the ridge axis evolves probably by the sealing of cracks and pores within the first few hundred thousand years of crustal accretion consideration of the detailed sea beam bathymetry in this region of the inner floor the characteristics of large earthquakes that the region has experienced during the past 25 years and the results of the microearthquake and tomography analysis suggests that this section of the median valley has been undergoing continued horizontal extension and block faulting without significant crustal injection of magma for at least the past 104 years

extent of oceanic crust in the labrador sea1 this paper examines the mode of formation of the northern margins of the labrador sea observations of faulted and rotated crustal blocks across the northwest margin of greenland had led to the suggestion that this region is underlain by extended continental crust a detailed analysis of all geophysical data from this region presented here suggests on the contrary the possibility that it could have been formed by an excessively slow seafloor spreading process and ridge axis propagation this gave rise to rotated fault blocks at a time when spreading was mainly confined to the greenland side prior to its shift to the west supporting evidence for such a reconfiguration of the spreading axis comes from gravity and magnetic observations of this and neighbouring regions modelling of magnetic anomalies in the area based on the average rate of spreading for the entire labrador sea shows that a gradual shift of the ridge axis to the west started after the formation of anomaly 32 because the earliest seafloor spreading anomalies are highly attenuated in amplitudes other models like extended and intruded continental crust are possible if one only considers the magnetic anomalies however an extended continental crustal model poses problems in accounting for other geophysical observations based on a slow spreading model it is shown that oceanic crust extends fairly close to the foot of the slope with a narrow belt of extended continental crust along the west greenland margin while a much wider zone of perhaps attenuated continental crust extends off labrador breakup of the continental landmass occurred much closer to the west greenland coast than to labrador the applicability of a slow spreading model to other passive margins where similar geophysical observations have been reported is discussed

lithospheric structure of the labrador sea from constrained 3 d gravity inversion regional inversions of free air gravity data constrained by bathymetric and sediment thickness information were undertaken over the labrador sea and its margins to generate 3 d density anomaly models to investigate broad scale crustal structural variations across the extinct spreading centre benchmarked against independent seismic moho depth constraints a density anomaly isosurface within the inverted volumes was selected as a moho proxy and regional maps of moho structure were developed inversions using two different sources for depth to basement constraints revealed similar moho structures with a depth to moho of 12 km beneath the labrador sea which deepens to 20 km and greater towards davis strait and beneath the offshore extension of the grenville province density anomaly slices through the models corresponding to seismic lines show good agreement between the inverted moho proxy and the seismic moho with the only exceptions occurring where a high velocity lower crustal zone or underplate has been modelled from wide angle reflection refraction profiling studies the inverted depth to moho estimates were combined with depth to basement constraints to investigate crustal thickness both for the full crust and for individual crustal layers revealing that the crust of the labrador sea is generally 5 10 km thick but thickens to 20 25 km towards davis strait and beneath the offshore extension of the grenville province not taking into account high density underplates or anomalously high density lower crust sediment and crustal thickness variations were investigated to compute stretching factors across the labrador sea and to identify zones which deviate from local isostatic compensation assuming both an initial unstretched crustal thickness of 35 km and using a variable unstretched crustal thickness model much of the labrador sea has experienced 70 90 per cent thinning the derived values suggest that embrittlement of the entire crust and serpentinization of the upper mantle are likely to have occurred for large portions of the central and southern labrador sea inboard of known oceanic crust isostatic considerations reveal sediment excess and deficiency on the labrador and greenland margins respectively possibly reflecting fundamental rheological asymmetry with the greenland margin appearing weaker than the labrador margin a strong gradient from sediment deficiency to excess exclusive to the labrador margin may reveal the southwestern limit of a regional graben bounding listric detachment or of a zone of distributed faults and detachments

breakup and early seafloor spreading between india and antarctica summary we present a tectonic interpretation of the breakup and early seafloor spreading between india and antarctica based on improved coverage of potential field and seismic data off the east antarctic margin between the gunnerus ridge and the bruce rise we have identified a series of ene trending mesozoic magnetic anomalies from chron m9o 130 2 ma to m2o 124 1 ma in the enderby basin and m9o to m4o 126 7 ma in the princess elizabeth trough and davis sea basin indicating that india antarctica and india australia breakups were roughly contemporaneous we present evidence for an abandoned spreading centre south of the elan bank microcontinent the estimated timing of its extinction corresponds to the early surface expression of the kerguelen plume at the southern kerguelen plateau around 120 ma we observe an increase in spreading rate from west to east between chron m9 and m4 38 54 mm yr 1 along the antarctic margin and suggest the tectono magmatic segmentation of oceanic crust has been influenced by inherited crustal structure the kinematics of gondwanaland breakup and the proximity to the kerguelen hotspot a high amplitude e w oriented magnetic lineation named the mac robertson coast anomaly mca coinciding with a landwards step down in basement observed in seismic reflection data is tentatively interpreted as the boundary between continental transitional zone and oceanic crust the exposure of lower crustal rocks along the coast suggests that this margin formed in a metamorphic core complex extension mode with a high strength ratio between upper and lower crust which typically occurs above anomalously hot mantle together with the existence of the mca zone this observation suggests that a mantle temperature anomaly predated the early surface outpouring steady state magmatic production of the kerguelen lip an alternative model suggests that the northward ridge jump was limited to the elan bank region whereas seafloor spreading continued in the west enderby basin and its sri lankan conjugate margin in this case the mca magnetic anomaly could be interpreted as the southern arm of a ridge propagator that stopped around 120 ma

heterogeneous stress state of island arc crust in northeastern japan affected by hot mantle fingers by considering a thermal structure based on dense geothermal observations we model the stress state of the crust beneath the northeastern japan island arc under a compressional tectonic regime using a finite element method with viscoelasticity and elastoplasticity we consider a three layer structure upper crust lower crust and uppermost mantle to define flow properties numerical results show that the brittle viscous transition becomes shallower beneath the ou backbone range compared with areas near the margins of the pacific ocean and the japan sea moreover several elongate regions with a shallow brittle viscous transition are oriented transverse to the arc and these regions correspond to hot fingers i e high temperature regions in the mantle wedge the stress level is low in these regions due to viscous deformation areas of seismicity roughly correspond to zones of stress accumulation where many intraplate earthquakes occur our model produces regions with high uplift rates that largely coincide with regions of high elevation e g the ou backbone range the stress state fault development and uplift around the ou backbone range can all be explained by our model the results also suggest the existence of low viscosity regions corresponding to hot fingers in the island arc crust these low viscosity regions have possibly affected viscous relaxation processes following the 2011 tohoku oki earthquake

crustal structure at the blake spur fracture zone from expanding spread profiles we present results from wkbj and reflectivity synthetic seismogram modeling of 10 reversed expanding spread profiles esps along flow lines parallel to the blake spur fracture zone across 140 ma atlantic crust these profiles provide detailed constraints on variations in crustal structure at the fracture zone seven profiles on either side of the fracture zone show a normal structure for old oceanic crust a 2 3 km thick upper layer with a steep velocity gradient is underlain by 4 5 km of crust with velocities of 6 5 7 2 km s and low gradient and a sharp transition to upper mantle velocities of 8 km s several velocity discontinuities were detected within the upper 2 3 km but these do not generally coincide with intracrustal reflectors detected by simultaneous normal incidence reflection profiles this structure shows little regional variation toward the fracture zone despite the small magnetic anomaly offset 12 km and the indistinct topographic signature of the fracture zone three esps within a 10 20 km wide ribbon centered on the fracture zone trough show clearly anomalous crustal structure relative to normal oceanic crust a 2 4 km thick high gradient upper layer is underlain by a thick prism of material with a velocity of 7 2 7 6 km s and high poisson s ratio probably at least 0 29 which is consistent with 15 30 serpentinization of upper mantle peridotites this interpretation requires the action of off axis hydrothermal circulation penetrating the cracked and relatively permeable fracture zone lithosphere to a depth of at least 7 km the original igneous crust in a narrow region close to the fracture zone is thus inferred to have been much thinner than adjacent normal crust which may imply a sharp reduction in the magma budget at the ends of the adjacent spreading segments

rupture process for micro earthquakes inferred from borehole seismic recordings we investigate the spatial extent of rupture and variability in fault slip for micro earthquakes by inverting seismic moment rate functions derived from empirical green s function deconvolution by using waveforms from an array of borehole seismometers we determine the spatial distributions of fault slip for m 3 earthquakes that occurred along the hayward fault in central california and identify a variety of slip behaviors including subevents directivity and high stress drop the 2013 m w 3 2 orinda earthquake exhibits a complex rupture process involving two subevents with northwest and up dip directivity the two subevents release 43 and 18 of the total seismic moment 6 7 1013 n m and their inferred peak stress drops are 18 and 8 mpa the 2011 m w 4 0 berkeley and 2012 m w 4 0 el cerrito earthquakes are marked by high stress drop the inferred peak and mean stress drops are about 100 130 and 40 mpa respectively which suggests that there are locally high levels of fault strength on the hayward fault our finite source modeling suggests that the radiation efficiency determined for these two earthquakes is very low 0 1 and implies that most energy is dissipated during the earthquake rupture process

origin and evolution of the kolbeinsey ridge and iceland plateau n atlantic variations in crustal structure along the 700 km long krise7 refraction reflection and gravity profile straddling 66 5 n across the iceland shelf iceland plateau and western norway basin confirm that extinct spreading centers coexisted with the now extinct aegir ridge prior to the initiation of the kolbeinsey ridge at 26 ma the western 300 km of the profile across the iceland shelf formed by rifting at the kolbeinsey ridge whereas the eastern 400 km across the iceland plateau and the western norway basin formed by earlier rifting possibly containing slivers of older oceanic or continental crust rifted off the central e greenland margin along with the jan mayen ridge crustal thickness increases gradually across the iceland shelf from 12 to 13 km near the kolbeinsey ridge to 24 28 km near the eastern shelf edge decreasing abruptly across the shelf edge to 12 13 km the iceland plateau has crustal thickness ranging from 12 to 15 km decreasing to 5 8 km across the western norway basin and 4 5 km at the aegir ridge we suggest that high velocity lower crustal domes and corresponding gravity highs across the iceland plateau mark the location of extinct rift axes that coexisted with the aegir ridge similar lower crustal domes are associated with the currently active rift segments within iceland and the kolbeinsey ridge

anomalous seismic crustal structure of oceanic fracture zones summary the seismic structure of crust found within fracture zones falls outside the range of velocity structures observed for normal oceanic crust in the north atlantic the crust in fracture zones is frequently very thin and is characterized by low crustal velocities and by the conspicuous absence of a refractor with a velocity typical of oceanic layer 3 anomalous crust is present in both large and small offset fracture zones since they are among the most common tectonic features in the ocean basins and are particularly closely spaced on slow spreading ridges fracture zones represent a major source of seismic crustal heterogeneity we interpret the anomalous crust as a thin intensely fractured faulted and hydrothermally altered basaltic and gabbroic section overlying ultramafics that in places are extensively serpentinized the unusually thin crust found within fracture zones and the gradual crustal thinning over a distance of several tens of kilometres on either side of the fracture zones can be explained by two main processes firstly the cold lithosphere edge opposite the spreading centre at the ridgetransform intersection modifies the normal intrusive and extrusive processes of the spreading centre leading to the accretion of an anomalous and thin igneous section and secondly each spreading ridge segment is fed from a separate subcrustal magma supply point so as the magma flows laterally down the spreading centre it generates a crustal section of decreasing thickness culminating in the very thin crust of the fracture zones at either end of the ridge segment

crustal structure of the central nova scotia margin off eastern canada summary the central nova scotia margin off eastern canada is located at a transition from a volcanic margin in the south to a non volcanic margin in the north in order to study this transition a wide angle refraction seismic line with dense airgun shots was acquired across the central nova scotia margin the 500 km long transect is coincident with previous deep reflection profiles across the lahave platform and extending into the sohm abyssal plain a p wave velocity model was developed from forward and inverse modelling of the wide angle data from 21 ocean bottom seismometers and coincident normal incidence reflection profiles the velocity model shows that the continental crust is divided into three layers with velocities of 5 5 6 9 km s 1 the maximum thickness is 36 km a minor amount 5 km of thinning occurs beneath the outer shelf while the major thinning to a thickness of 8 km occurs over the slope region the seaward limit of the continental crust consists of 5 km thick highly faulted basement there is no evidence for magmatic underplating beneath the continental crust on the contrary a 4 km thick layer of partially serpentinized mantle 7 6 7 95 km s 1 begins beneath the highly faulted continental crust and extends 200 km seawards forming the lower part of the oceancontinent transition zone the upper part of the transition zone consists of the highly faulted continental crust and 4 to 5 km thick initial oceanic crust the continent ocean boundary is moved 50 km farther seawards compared to an earlier interpretation based only on reflection seismic data the oceanic crust in the transition zone consists of layer 2 and a high velocity lower crustal layer layer 2 is 1 3 km thick with velocities of 5 6 6 0 km s 1 the high velocity lower crustal layer is 1 2 km thick with velocities of 7 25 7 4 km s 1 suggesting a composite layer of serpentinized peridotite and gabbroic layer 3 oceanic crust with normal thickness of 5 7 km and more typical layer 3 with velocities of 6 95 7 3 km s 1 is observed at the seaward end of the profile

nearly magnitude invariant stress drops in simulated crack like earthquake sequences on rate and state faults with thermal pressurization of pore fluids stress drops inferred to be magnitude invariant are a key characteristic used to describe natural earthquakes theoretical studies and laboratory experiments indicate that enhanced dynamic weakening such as thermal pressurization of pore fluids may be present on natural faults at first glance magnitude invariance of stress drops and enhanced dynamic weakening seem incompatible since larger events may experience greater weakening and should thus have lower final stresses and higher stress drops we hypothesize that enhanced dynamic weakening can be reconciled with magnitude invariant stress drops due to larger events having lower average prestress when compared to smaller events we conduct numerical simulations of long term earthquake sequences in fault models with rate and state friction and thermal pressurization and in the parameter regime that results mostly in crack like ruptures we find that such models can explain both the observationally inferred stress drop invariance and increasing breakdown energy with event magnitude smaller events indeed have larger average initial stresses than medium sized events and we find nearly constant stress drops for events spanning up to two orders of magnitude in average slip comparable to approximately six orders of magnitude in seismic moment segment spanning events have more complex behavior which depends on the properties of the arresting velocity strengthening region at the edges of the faults

deep structure of the ocean continent transition in the southern iberia abyssal plain from seismic refraction profiles ocean drilling program legs 149 and 173 transect we present a wide angle seismic refraction study of an 80 40 km region of the southern iberia abyssal plain south of galicia bank an intersecting grid of two e w and four n s wide angle reflection refraction profiles is used to define variations of the basement velocity structure within this unusually wide ocean continent transition oct these structures can be systematically linked to variations in acoustic basement morphology and to results from ocean drilling program odp boreholes lateral changes in the velocity structure of the basement occur abruptly over distances of 20 km where complex variations may be found thinned upper continental crust 2 5 km thick with velocities of 5 0 6 6 km s is limited to a series of n s fault blocks immediately south of galicia bank this crust is underlain by a high velocity layer 7 3 7 9 km s of weakly serpentinized i e 0 25 peridotite which exists throughout the eastern part of the survey area basement within the oct appears to consist dominantly of a broad region of exposed upper mantle that has been serpentinized heterogeneously both vertically and horizontally in the southeast sector of our survey where basement topography deepens and becomes subdued continental fault blocks are absent instead basement contains an upper layer of more pervasively serpentinized i e 25 45 peridotite that is 2 km thick this layer is characterized by low velocity at the top of basement 4 2 km s that increases rapidly with depth and it probably corresponds to a seismically unreflective layer previously identified in reflection profiles to the south of our survey in the western section of our survey beneath a series of elevated basement ridges velocities are reduced within both the upper basement layer 3 5 6 0 km s and lower layer 6 4 7 5 km s these changes suggest that both upper and lower layers have become more highly serpentinized with values of 60 100 in the upper layer and 25 45 in the lower layer probably during the last stages of rifting and immediately before formation of oceanic crust a normal or slow spreading oceanic crustal structure is not found within the survey region thus it appears that the onset of seafloor spreading occurs in the region west of the peridotite ridge sampled at odp site 897 and east of the j magnetic anomaly

crustal structure of an extinct rift axis in the labrador sea preliminary results from a seismic refraction survey abstract a preliminary crustal structure for the extinct rift axis in the labrador sea is presented based on a one dimensional analysis of ocean bottom seismometer obs refraction profiles within the extinct rift low p wave velocities are observed for crustal 3 6 6 3 km s and upper mantle 7 3 8 1 km s refractors the velocity depth structures for obss sampling crust within the extinct rift suggest a marked thinning and variability in crustal thickness from 4 7 km to 2 6 km in comparison to typical thicknesses of 6 4 km observed in the adjacent flanks the crustal structures at the extinct rift resemble those observed at the active and inactive transforms of oceanic fracture zones following arguments for crustal thinning at fracture zones we speculate that the first order thinning of the lower crust is due to a reduction in the supply of partial melt to the accretionary axis at the extinct rift the reduced supply is attributed to changes in the thermal field and spreading rate as rifting ceased this is consistent with thermal models which predict that crustal thickness decreases appreciably for spreading rates less than 20 mm yr when plate spreading predominates over buoyancy in driving mantle upwelling

impact of bending related faulting on the seismic properties of the incoming oceanic plate offshore of nicaragua impact of bending related faulting on the seismic properties of the incoming oceanic plate offshore of nicaragua

teleseismic estimates of the energy radiated by shallow earthquakes the energy flux contained in the p wave groups p pp sp or the s wave groups s ps ss radiated by a shallow earthquake is modeled assuming that the energy flux in the direct and depth phases adds incoherently by defining generalized radiation patterns which incorporate this neutral interference the wave groups are analyzed as though they were comprised of a single phase measurements of the energy flux in the wave groups are corrected in the frequency domain for both the body wave attenuation and the frequency band of the recording the corrected measurements are then used to estimate the seismic energy radiated by the earthquake this analysis is applied to digital recordings of the teleseismic wave groups radiated by the may 2 1983 coalinga california earthquake and the october 28 1983 borah peak idaho earthquake for the coalinga earthquake an estimate of es 1 6 0 4 1021 dyn cm was determined from six p wave groups while the sh wave group recorded at station col returned an estimate of es 1 2 1021 dyn cm for the borah peak earthquake an estimate ofes 3 2 0 5 1021 dyn cm was determined from seven p wave groups the distribution of the isoseismals for the borah peak earthquake indicate that the energy radiated by this event was focussed to the northwest in the direction of rupture propagation correcting the teleseismic estimate for this focussing gives es 3 6 0 5 1021 dyn cm combining these estimates of the radiated energy with broad band estimates of the seismic moment yields estimates of the apparent stress of 17 and 7 bars for the coalinga and the borah peak earthquakes respectively

the oceanic crustal structure at the extinct slow to ultraslow labrador sea spreading center two seismic refraction lines were acquired along and across the extinct labrador sea spreading center during the seismic investigations off greenland newfoundland and labrador 2009 cruise we derived two p wave velocity models using both forward modeling rayinvr and traveltime tomography inversion tomo2d with good ray coverage down to the mantle slow spreading paleocene oceanic crust has a thickness of 5 km while the eocene crust created by ultraslow spreading is as thin as 3 5 km the upper crustal velocity is affected by fracturation due to a dominant tectonic extension during the waning stage of spreading with a velocity drop of 0 5 to 1 km s when compared to paleocene upper crustal velocities 5 2 6 0 km s the overall crustal structure is similar to active ultraslow spreading centers like the mohns ridge or the south west indian ridge with lower crustal velocities of 6 0 7 0 km s an oceanic core complex is imaged on a 50 km long segment of the ridge perpendicular line with serpentinized peridotites 7 3 7 9 km s found 1 5 km below the basement the second ridge parallel line also shows extremely thin crust in the extinct axial valley where 8 km s mantle velocity is imaged just 1 5 km below the basement this thin crust is interpreted as crust formed by ultraslow spreading which was thinned by tectonic extension

the frequency size scaling of non volcanic tremors beneath the san andreas fault at parkfield possible implications for seismic energy release abstract we analyse the frequency size distribution of non volcanic tremors observed along the parkfield section of the san andreas fault we suggest that these non volcanic tremors follow a power law scaling typical of scale invariant stick slip tectonic earthquakes but with an unusually high scaling exponent of more than 2 0 and a systematic depth dependency while each individual non volcanic tremor releases only a minuscule amount of energy and slip this is more than compensated by their sheer numbers consequently the integrated contribution of this largely invisible seismicity non volcanic tremors and nano earthquakes is non negligible and could potentially account in selected patches along the san andreas fault for up to 100 of the plate motion

asymmetric geophysical signatures in the greenland norwegian and southern labrador seas and the eurasia basin a thorough examination of geophysical data from the greenland norwegian sea eurasia basin and southern labrador sea shows significant asymmetry of several parameters basement topography adjusted for sediment loading free air gravity anomaly spreading half rate and seismicity with respect to crustal age 1 1 average zero age depth 0 57 m y b p depth of highest rift mountain summits and depth to magnetic basement 10 30 km from axis of mohns and knipovich ridges is less on the north american plate flanks the zero age depth asymmetry is 400 500 m for the eurasia basin 0 57 m y b p and for mohns ridge 57 22 m y b p and 150 200 m for younger mohns ridge crust 22 0 m y b p and for the extinct aegir ridge 57 27 m y b p there is little or no asymmetry in the labrador sea except near the extinct rift valley where the east flank is 150 300 m shallower magnetic depth to source computations provide an independent confirmation of basement asymmetry the belts 10 30 km from the axis of mohns and knipovich ridges are 100 150 m shallower on the west flank of these ridges the shallower ridge flank is topographically rougher so that average rift mountain summits are 300 m shallower on the west flanks of the mohns knipovich ridges a larger asymmetry than for average zero age depth the amount of topographic asymmetry is greatest near the mohns knipovich bend asymmetry appears to be greatest for ridges oriented normal to the spreading direction and less for oblique spreading 2 2 free air gravity anomaly asymmetries of 5 to 20 mgal sign indicates west flank is more positive are associated with topographic asymmetry at least within 10 15 m y of the axis of mohns and knipovich ridges gravity is reduced on the older flanks west of the extinct mid labrador ridge and east of mohns ridge asymmetric crustal layer thicknesses or densities provide one possible explanation although deep seated sources e g mantle convection unrelated to the crust cannot be excluded 3 3 spreading half rate was about 5 15 lower on the north american plate flanks of mohns ridge 57 35 m y and in the eurasia basin 0 57 m y thus the fast spreading flank tends to produce deeper smoother crust however topographic asymmetry cannot relate only to spreading rate asymmetry since for the young mohns ridge crust 9 m y b p faster spreading and higher topography are both associated with the west flank 4 4 mid plate seismicity is higher on the eurasia eastern flank of mohns and knipovich ridge but this effect may be unrelated to the other three the fluid dynamical model of stein et al correctly explains the sense of spreading rate asymmetry the north american plate moving faster over mantle is growing more slowly however the other asymmetries and their causal relationships remain theoretically unexplained

rupture processes of the 2004 chuetsu mid niigata prefecture earthquake japan a series of events in a complex fault system 1 we relocated the hypocenters of the 2004 chuetsu earthquake sequence niigata japan using the double difference method the distribution of aftershocks reveals a complex fault system consisting of five different fault planes inversions of strong motion records for the five major events within the sequence indicate that the mainshock mw 6 6 and the largest aftershock mw 6 3 occurred on parallel wnw dipping fault planes the zones of large slip asperities of these two events are located near the hypocenters in contrast the three other large earthquakes mw 5 9 5 7 and 5 9 occurred on east dipping fault planes oriented perpendicular to the fault plane of the mainshock we used slip distributions deduced from waveform inversions to estimate the coulomb failure stress changes that occurred immediately prior to each event cff values were positive at the hypocenters and within the asperities of the major aftershocks these results suggest that stress changes resulting from individual earthquakes led directly to the occurrence of subsequent earthquakes within the complex fault system

earthquake source scaling relationships from 1 to 5 ml using seismograms recorded at 2 5 km depth the scaling relationships of earthquake sources less than about magnitude 3 have been the subject of considerable controversy over the last two decades studies of such events have shown a tendency for the constant stress drop self similarity of larger earthquakes to breakdown at small magnitudes and an apparent minimum source dimension of about 100 m has been observed other studies showed that this apparent breakdown in scaling could be an artifact of severe near surface attenuation limiting the spatial resolution of surface data in this study source parameters are determined for over 100 nearby tectonic earthquakes from recordings at a depth of 2 5 km in granite in the cajon pass scientific drill hole southern california comparison of the seismograms recorded at this depth with those at the wellhead clearly demonstrates the effect of the severe attenuation in the upper kilometers of the earth s crust source parameters are calculated by spectral modeling of three component p and s waves assuming four source models based on the brune 2 n 2 model in model l n 2 is fixed and q of p and s waves is determined to be 912 581 1433 and 1078 879 1323 respectively the numbers in parentheses are 1 standard deviation in model 2 qp qs 1000 is assumed and n is allowed to vary the 2 model is a good average for the data set but there is some real scatter supported by the data in model 3 qp qs 1000 is also assumed and 2 is constrained and in model 4 attenuation is ignored and n is allowed to vary source dimensions of less than 10m are observed for all four models 10 times smaller than the proposed minimum no breakdown in constant stress drop scaling is seen in the downhole data approximately ml 1 to 5 5 m0 109 1016 nm the ratio between radiated seismic energy estimated by integrating the velocity squared spectra with adequate signal bandwidth and seismic moment appears to decrease gradually with decreasing moment in the magnitude range 1 to 7 this is not incompatible with a constant stress drop but could result from errors in calculating energy the ratio of the s wave energy to that radiated by the p waves is about 14 after correction for attenuation this low value is consistent with the corner frequency shift of about 1 3 this corner frequency shift is observed for all four source models and therefore is interpreted as being source controlled

the east pacific rise and the thermal model although the thermal mode of lithosphere cooling can be used to predict the east pacific rise topography reasonably well it does not adequately predict the observed gravity when the zero temperature boundary condition is at the seafloor the misfit arises primarily because this model predicts thermally lowered densities in the crust at the rise axis which counteract the gravitational effect of the topography generating a theoretical gravity anomaly smaller than that observed it is shown using data at 12 n as an example that improved agreement with the thermal model is obtained if to first order the crust is treated as an isothermal constant thickness constant density layer with variable depth this result is consistent with seismic data and observations of convective cooling of the crust by seawater circulation it implies that convective cooling of the whole crust is very rapid even at the axis of fast spreading ridges the oceanic moho may represent the depth to which cracking and convective cooling extends

northern east pacific rise magnetic anomaly and bathymetric framework the oceanic crust in the eastern pacific between 7on and 30on and east of 127ow contains a fairly complete history of the spreading centers associated with the east pacific rise since 25 m y b p late oligocene in this paper we have summarized the seafloor spreading magnetic anomaly data and the bathymetric data that reflect the record of this tectonic history the well defined magnetic lineations north of the clarion fracture zone in the mouth of the gulf of california and on the east flank of the east pacific rise epr are carefully examined and used to provide a guide for interpreting the spreading pattern between the clarion and clipperton fracture zones southward of the rivera fracture zone over the mathematician ridge and over the entire epr east of the mathematician ridge between the rivera and siqueiros fracture zones the bathymetric data provide a trace of the fracture zone pattern in each of the above mentioned areas the fracture zone bathymetry and the seafloor spreading magnetic lineations on the epr south of the rivera fracture zone have a distinctive fanning pattern caused by close poles of rotation and plate boundary reorganizations all these data provide a good record of the plate reorganizations in the middle miocene at magnetic anomaly 5a time 12 5 to 11 m y b p in the late miocene at magnetic anomaly 3 4 time 6 5 m y b p and in the pliocene at magnetic anomaly 2 3 time 3 5 m y b p several abandoned spreading centers including the mathematician ridge were left behind as a result of these reorganizations the mathematician ridge is shown to be a set of ridges and trough whose origin is related to the tectonic activity associated with each of the above mentioned reorganizations since anomaly 5a

multistage hydrothermal alteration of gabbroic rocks from the failed mathematician ridge oxygen isotopic mineralogical and fluid inclusion data are presented for fresh and hydrothermally altered gabbroic rocks from the failed mathematician ridge the mathematician ridge though initially a fast spreading ridge presently has a deep central rift valley more typical of slow spreading rifts the inner rift walls expose rare plutonic rocks uncomplicated by proximity to deep large offset fracture zones undeformed mathematician ridge gabbros preserve most of the original texture and phase compositions and have been only slightly mineralogically or isotopically modified by post crystallization hydrothermal alteration metagabbros have been subjected to a pervasive ductile deformation and recrystallization to foliated rocks composed of aluminous amphibole low calcium plagioclase and ilmenite oxygen isotope and phase chemical data suggest this event began at temperatures of up to 700 c water rock ratios of less than one and produced modified seawater fluids with isotopic compositions of 3 5 6 0 the amphibolites are cross cut by multiple generation quartz epidote veins forming a network replacement of the amphibole grade mineralogies by greenschist grade mineralogies fluid oxygen isotopic compositions of 2 4 and a two phase separation in the fluid captured in conjugate vapor fluid inclusions are all associated with the formation of this network fluid inclusion data for the quartz indicate initial temperatures of over 500 c these samples may represent rocks from the 1 2 km deep plumbing system of black smoker type venting

rates of continental breakup magmatism and seafloor spreading in the norway basin iceland plume interaction 1 in year 2000 an ocean bottom seismometer obs profile was acquired across the more margin to the aegir ridge an extinct seafloor spreading axis the margin is an early eocene volcanic passive margin located between the faeroe iceland ridge fir and the east jan mayen fracture zone ejmfz the p wave data were modeled by ray tracing to give a crustal transect showing a 10 11 km thick igneous crust created by breakup magmatism tapering off to magma starved seafloor spreading by c23 time 51 4 ma the location of the ejmfz was reinterpreted from a satellite derived gravity map and spreading direction in the norway basin reevaluated no other fracture zones were confirmed and both thin oceanic crust 4 5 km and lack of fracture zones resemble ultraslow spreading on the arctic gakkel ridge magnetic seafloor spreading anomalies were identified from the magnetic track recorded with the obs profile and half spreading rates were derived early seafloor spreading was slow 15 32 mm yr 1 approaching ultraslow 6 8 mm yr 1 by c20 time 42 7 ma a v shaped pattern seen in the gravity field located only around the northern part of the aegir ridge corresponds to increased crustal thickness in the seismic model recording northeast transport 3 6 mm yr 1 of more melt fertile asthenosphere zones the magma starved character of the norwegian basin seen also during slow seafloor spreading may be the result of depletion of the asthenosphere when the iceland plume constructed the fir to the south as the asthenosphere is subsequently transported into the norway basin

rifting of old oceanic lithosphere geophysical data from five regions in the pacific and indian oceans reveal that long distance 400 km spreading center jumps have occurred in the past the present day seafloor morphology is used to develop a scenario for a spreading center jump the major events are 1 thinning and weakening of the lithosphere at the future rifting site 2 rifting of the weakened lithosphere during rifting the crack is filled from above by normal faulting and wedge subsidence viscous upwelling fills the crack from below 3 spreading at the rift site results in a ridge bounded by two troughs spreading ceases at the dying spreading center resulting in a deep central graben surrounded by flexural ridges periods of slow spreading at both spreading centers produce rough topography 4 ageing and cooling that produce a general deepening of the abandoned spreading ridge and also reduce the thermal contrast across the fossil rifting site the new spreading center develops into a normal spreading rift the major topographic expressions apparent in the seafloor today are the deep trough of the abandoned spreading center and the proximal and distal troughs which formed when the emerging spreading center bisected the fossil rifting site the proximal trough nearer the new spreading ridge and the distal trough farther from the new ridge are first order topographic features 100 1000 km long and 300 km wide resembling fracture zones with which they are often confused they share with fracture zones the characteristic of bringing together fragments of lithosphere of different ages but unlike fracture zones they are generally parallel to magnetic lineations

variability in earthquake stress drop and apparent stress 1 we apply empirical green s function coda based analysis to four earthquake sequences in japan that span a magnitude range of 1 8 to 6 9 to measure radiated energy corner frequency and stress drop we find no systematic dependence of apparent stress or stress drop on seismic moment for these sequences and find they both are log normally distributed however we identify several anomalous events both energetic and enervated that show sharply different spectral signatures from the rest of the population these events indicate that much of the variation in apparent stress and stress drop is statistically significant which may have important implications for seismic hazard analysis

a deep seismic investigation of the flemish cap margin implications for the origin of deep reflectivity and evidence for asymmetric break up between newfoundland and iberia summary seismic reflection and refraction data were acquired along the southeast margin of flemish cap at a position conjugate to drilling and geophysical surveys across the galicia bank margin the data document first order asymmetry during final break up between newfoundland and iberia an abrupt necking profile of continental crust observed off flemish cap contrasts strongly with gradual tapering on the conjugate margin there is no evidence beneath flemish cap for a final phase of continental extension that resulted in thin continental crust underlain by a strong s like reflection which indicates that this mode of extension occurred only on the galicia bank margin compelling evidence for a broad zone of exhumed mantle or for peridotite ridges is also lacking along the flemish cap margin instead anomalously thin 3 4 km thick oceanic crust is observed this crust is highly tectonized and broken up by high angle normal faulting the thin crust and rift structures that resemble the abandoned spreading centre in the labrador sea suggest that initial seafloor spreading was affected by processes observed in present day ultra slow spreading environments landwards flemish cap is underlain by a highly reflective lower crust the reflectivity most likely originates from older palaeozoic orogenic structures that are unrelated to extension and break up tectonics

the central basin spreading center in the philippine sea structure of an extinct spreading center and implications for marginal basin formation 1 newly acquired marine geophysical data along the central basin spreading center cbsc the extinct spreading axis of the west philippine basin wpb display along axis variations of spreading style we have described and analyzed the tectonic spreading fabric and segmentation patterns along a 1000 km long section of the fossil ridge between 126 00 and 133 30 e slow spreading features deep rift valleys and nodal basins rough abyssal hills on the ridge flanks and mantle bouguer anomaly lows beneath segment centers are observed in the eastern cbsc in contrast fast spreading features overlapping spreading centers volcanic axial ridges and smooth abyssal hill fabric are identified along the western cbsc we attribute the large morphological and geophysical variations along the cbsc to higher melt supply in the western region caused by high mantle temperature and or mantle heterogeneity which may be related to a relatively small scale mantle plume forming the oceanic plateaus located in the wpb another prominent feature of the cbsc is the development of a deep valley oblique to the spreading fabric reconstructions of the plate boundary geometry through time using abyssal hill geometry as well as other measurements provide evidence for a later stage of amagmatic extension i e reactivation along the cbsc after the formation of the main basin this stage is dominated by tectonic deformation with minor amounts of volcanism mainly located in eastern segments resulting in the observed surface brittle deformation distributed within a broad zone of ductile deformation

apparent break in earthquake scaling due to path and site effects on deep borehole recordings 1 we reexamine the scaling of stress drop and apparent stress rigidity times the ratio between seismically radiated energy to seismic moment with earthquake size for a set of microearthquakes recorded in a deep borehole in long valley california in the first set of calculations we assume a constant q and solve for the corner frequency and seismic moment in the second set of calculations we model the spectral ratio of nearby events to determine the same quantities we find that the spectral ratio technique which can account for path and site effects or nonconstant q yields higher stress drops particularly for the smaller events in the data set the measurements determined from spectral ratios indicate no departure from constant stress drop scaling down to the smallest events in our data set mw 0 8 our results indicate that propagation effects can contaminate measurements of source parameters even in the relatively clean recording environment of a deep borehole just as they do at the earth s surface the scaling of source properties of microearthquakes made from deep borehole recordings may need to be reevaluated

the structure of mid ocean ridges recent research results on the structure of midocean ridges are reviewed the new view of ridge axis crustal structure obtained from high resolution seismology is reviewed emphasizing the variation of that structure with spreading rate and along axis at a given spreading rate recent results on upper mantle structure beneath ridges are examined including variations with seafloor age indications from anisotropy for directions of mantle flow and long wavelength along axis variations in structure and their implications for lateral heterogeneity in mantle temperature and composition

large nucleation before large earthquakes is sometimes skipped due to cascade up implications from a rate and state simulation of faults with hierarchical asperities 1 does a large earthquake have a large quasi static preparation a hierarchical asperity model in which a large tough patch patch l contains smaller fragile patches patch s enables a large earthquake to start with only small preparation because of cascade up rupture growth we realized such a model in a rate and state framework by heterogeneous distributions of the state evolution distance and its consequences are investigated by earthquake sequence simulations focus is put on elementary processes the interaction between two scales one patch l containing one patch s is simulated with their size ratio as a parameter if patch s is larger than the nucleation size of patch l the system falls into a limit cycle consisting of only one earthquake that starts with patch s nucleation and grows dynamically to rupture entire patch l if patch s is considerably smaller than the nucleation size of patch l small earthquakes never dynamically cascade up and the large earthquakes are initiated by large quasi static nucleation in between large earthquakes start in various ways by large nucleation dynamic cascade up or delayed cascade up in the final stage of quasi static nucleation the preseismic moment release rate increases roughly inversely proportional to the time to failure tf with its amplitude depending on the nucleation size for a patch s rupture to cascade up strength in the adjacent region must have been reduced manifested by a higher creep velocity before the patch s nucleation starts following 1 tf acceleration large nucleation sometimes has a precursory small earthquake characterized by larger afterslip than nonprecursory ones

relationship between spreading rate and the seismic structure of mid ocean ridges different parts of the global mid ocean ridge system create oceanic crust at rates that differ by more than a factor of ten the rates of supply of heat to these different systems must also vary substantially and it is this heat supply in the form of hot mantle material rising beneath the ridge axis that determines the nature of the magmatic tectonic and hydrothermal processes that control the production of new oceanic crust here we present evidence suggesting that a systematic relationship exists between the depth to zones of elevated temperature and partial melt beneath mid ocean ridges and their spreading rate recent seismic measurements provide robust determinations at six different locations of the depth to the top of the axial low seismic velocity zone which suggest that as full spreading rate decreases from 150 to 20 mm yr 1 the depth below the sea floor to the top of the lvz increases from 1 to 4 km although this relationship might arise in a number of ways our preferred explanation is that the major faults that extend to greater depths on slower spreading ridges1 allow a vigorous circulation of cooling sea water to reach greater depths blocking the upward migration of large melt bodies

structure and seismotectonics of the vema fracture zone atlantic ocean the structure and seismo tectonics of the vema fracture zone and the adjacent regions of its transverse ridge to the south and the mid atlantic ridge spreading centre to the north is discussed based on an analysis of seismic refraction profiles micro earthquake activity and gravity anomalies crustal structures were determined by analysis of wide angle seismic data from explosive and airgun sources recorded on arrays of ocean bottom seismometers micro earthquakes were recorded by six instruments for a period of 14 days in an array to the north of the ridge transform intersection immature crust similar to that reported by fowler 1976 from the famous area lies beneath the spreading centre the lower crust thins and normal mantle appears as the fracture zone is approached the thinnest crust lies beneath the northern end of the topographic deep at the fracture zone intersection this is also the locus of current microearthquake activity at sub bottom depths of 3 4 km which a composite fault plane solution suggests may originate on normal faults that parallel the trend of the local topography on the eastern wall of the median valley within the central region of the fracture zone valley the crust thickens but still retains an anomalous velocity structure seismic layer 3 is absent and crustal velocities are low this is probably the consequence of attenuated crustal accretion of faulting and cracking and of hydrothermal alteration the pattern of residual gravity anomalies after correction for topography for sediment thickness and for mantle temperature variations is consistent with the crustal structure defined by the seismic experiments the gravity measurements indicate that there is a broad thinning of the crust over 20 40 km in the vicinity of the fracture zone on which are superimposed localized regions of low and high densities to the south of the fracture zone the crust thickens and assumes a normal velocity structure including a mature seismic layer 3 the seismic moho is warped upwards under the transverse ridge suggesting a tectonic origin from buckling rather than uplift by serpentinization

seismic energy release in mexican subduction zone earthquakes we estimated radiated seismic energy es from 18 shallow thrust mexican subduction zone earthquakes 4 1022 m 1 1 1028 dyne cm 11 h 37 km using digital accelerograms from the guerrero accelerograph array and neglecting stations with large site effects es is computed by integrating squared velocity spectra after applying geometrical spreading and q corrections we discarded epicentral recordings for the largest michoacan event we find that log es m 4 152 0 275 which gives a median value of es m and apparent stress a of 7 1 10 5 and 24 bars respectively the median es m value is in accordance with gutenberg and richter s g r 1956 formula for es in which es m 5 10 5 is implicit worldwide es m data where es is computed from local records mostly fall between 5 10 5 and 5 10 4 for events with m 1022 dyne cm on the other hand es m values generally lie between 5 10 6 and 5 10 5 if es is estimated from teleseismic records especially anomalous are the es m data from kikuchi and fukao 1988 for large and great earthquakes which fall near 5 10 6 thus while es from local data suggests that the g r relation seldom overestimates seismic energy release the teleseismic data point to the contrary the cause of this discrepancy may lie in the difficulty of resolving incoherent radiation from the fault and inappropriate choice of t in the analysis of teleseismic data

long term survival of the axial valley morphology at abandoned slow spreading centers results of a lithospheric stretching model of mid ocean ridges suggest that the axial valley topography created at active slow spreading centers may be preserved for tens of millions of years after cessation of spreading we simulate the evolution of a slow spreading center using a finite element model that incorporates a temperature dependent rheology and discrete faulting model results show that significant valley morphology persists after regional extension ceases because the high lithospheric viscosities associated with a slow spreading center prevent ductile relaxation and because no effective mechanism exists to reverse rift bounding normal faults removal of the axial valley can only be accomplished by reversal of the regional extensional strains immediately upon cessation of active spreading however such a process does not seem likely these results suggest that the persistence of axial valleys at extinct spreading centers is consistent with a lithospheric stretching model for active slow spreading ridges

high resolution mean sea surface computed with altimeter data of ers 1 geodetic mission and topex poseidon the remote sensing satellite ers 1 launched in 1991 to study the earth s environment was placed on a geodetic 168 day repeat orbit between 1994 april and 1995 march to map through altimetric measurements the gravity field over the whole oceanic domain with a resolution of 8 km at the equator in both along track and cross track directions we have analysed the precise altimeter data of the geodetic mission and by also using one year of topex poseidon altimeter data we have computed a global high resolution mean sea surface the various steps involved in pre processing the ers 1 data consisted of correcting the data for environmental factors editing and reducing through crossover analyses the radial orbit error which directly affects sea surface height measurements for this purpose we adjusted sinusoids at 1 and 2 cycle rev 1 along the ers 1 profiles in order to minimize crossover differences between ers 1 and yearly averaged topex poseidon profiles in effect the orbit of topex poseidon is very accurately determined within 2 3 cm for the radial component so topex poseidon altimeter profiles can serve as a reference to reduce the ers 1 radial orbit error the ers 1 residual orbit error was further reduced through a second crossover analysis between all ascending and descending profiles of the geodetic mission the along track ers 1 and topex poseidon data were then interpolated over the whole oceanic domain on a regular grid of 1 16 1 16 size the mapping of the gridded sea surface heights reveals the very fine structure of the marine geoid up until now unknown at a global scale this new data set will be most useful for marine geophysical and tectonic investigations

no evidence from multichannel reflection data for a crustal magma chamber in the mark area on the mid atlantic ridge the role of crustal magma chambers in the construction of oceanic crust along the slowly spreading mid atlantic ridge mar is still highly disputed1 some investigators have argued for small short lived crustal magma bodies2 whereas others have postulated the presence of permanent reservoirs centred between ridge offsets which expand and contract in response to a fluctuating magma supply3 thermal modelling4 5 detailed seismic refraction experiments at several different locations along the mar6 9 and micro earthquake10 11 and teleseismic source mechanism studies12 preclude the existence of large steady state magma chambers under the entire mar rift valley however smaller localized crustal magma bodies could be present at least intermittently here we use multichannel seismic reflection profiles to look for evidence of a crustal magma chamber along the mar south of the kane fracture zone in one of the most magmatically robust and hydro thermally active areas presently known along the mar although some subbasement reflectors are imaged we do not see a shallow large amplitude intracrustal reflector that might be associated with a crustal magma body if a crustal magma chamber was associated with the most recent volcanic episode along this part of the mar our results suggest that it was extremely short lived

radiated seismic energy from coda measurements and no scaling in apparent stress with seismic moment 1 the seismic coda consists of scattered waves that leave the earthquake source in a variety of directions the averaging of source radiation that results leads to stable ground motion spectra that we use as the basis for a robust measurement of radiated wave energy we apply an empirical green s function egf method to the seismic coda in order to investigate scaling of the radiated seismic energy we correct for path effects in the spectra of earthquakes by using a stack of closely located small earthquakes as an egf we apply this approach to four earthquake sequences in western north america that span a magnitude range from mw 3 0 mw 7 1 our estimates of scaled energy are consistent with independent measurements where available we find no dependence in individual seismic energy estimates on source station distance which validates the egf approximation we find that a constant scaled energy of 3 5 10 5 provides a reasonable fit to the data with no dependence of the scaled energy on seismic moment

three dimensional lithospheric structure of the western continental margin of india constrained from gravity modelling implication for tectonic evolution this paper describes a 3 d lithospheric density model of the western continental margin of india wcmi based on forward modelling of gravity data derived from satellite altimetry over the ocean and surface measurements on the indian peninsula the model covers the north eastern arabian sea and the western part of the indian peninsula and incorporates constraints from a wide variety of geophysical and geological information salient features of the density model include 1 the moho depth varying from 13 km below the oceanic crust to 46 km below the continental interior 2 the lithosphere asthenosphere boundary lab located at depths between 70 km in the southwestern corner under oceanic crust and about 165 km below the continental region 3 thickening of the crust under the chagos laccadive and laxmi ridges and 4 a revised definition of the continent ocean boundary the 3 d density structure of the region enables us to propose an evolutionary model of the wcmi that revisits earlier views of passive rifting the first stage of continental scale rifting of madagascar from india at about 90 ma is marked by relatively small amounts of magmatism a second episode of rifting and large scale magmatism was possibly initiated around 70 ma with the opening of the gop rift subsequently at around 68 ma the drifting away of the seychelles and formation of the laxmi ridge was a consequence of the down faulting of the northern margin during this second episode of rifting the northern part of the wcmi witnessed massive volcanism attributed to interaction with the reunion hotspot at around 65 ma subsequent stretching of the transitional crust between about 65 and 62 ma formed the laxmi basin the southward extension of the failed gop rift as the interaction between plume and lithosphere continued the chagos laccadive ridge was emplaced on the edge of the nascent oceanic crust rifted continental margin in the south as the indian plate was moving northwards

macrofracturing of oceanic lithosphere in complex large earthquake sequences major earthquakes in oceanic lithosphere seaward of the subduction zone outer trench slope are relatively uncommon but several recent occurrences have involved very complex sequences rupturing multiple nonaligned faults and or having high aftershock productivity with diffuse distribution this includes the 21 december 2010 m w 7 4 ogasawara bonin 11 april 2012 m w 8 6 indo australia 23 january 2018 m w 7 9 off kodiak island and 20 december 2018 m w 7 3 nikol skoye northwest pacific earthquakes major oceanic intraplate event sequences farther from plate boundaries do not tend to be as complex in faulting or aftershocks outer trench slope extensional faulting can involve complex distributed sequences particularly when activated by great megathrust ruptures such as 11 march 2011 m w 9 1 tohoku and 15 november 2006 m w 8 3 kuril islands intense faulting sequences also occur near subduction zone corners with many fault geometries being activated including some in nearby oceanic lithosphere as for the 29 september 2009 m w 8 1 samoa 6 february 2013 m w 8 0 santa cruz islands and 16 november 2000 m w 8 0 new ireland earthquakes the laterally varying plate boundary stresses from heterogeneous locking recent earthquakes or boundary geometry influence the specific faulting geometries activated in nearby major intraplate ruptures in oceanic lithosphere preexisting lithospheric structures and fabrics exert secondary influences on the faulting intraplate stress release in oceanic lithosphere near subduction zones favors distributed macrofracturing of near critical fault systems rather than localized single fault failures both under transient loading induced by plate boundary ruptures and under slow loading by tectonic motions and slab pull

deep structure of the ocean continent transition in the southern iberia abyssal plain from seismic refraction profiles the iam 9 transect at 40 20 n we present a crust and mantle velocity structure for the west iberia passive continental margin derived from a 320 km long wide angle seismic profile acquired in the southern iberia abyssal plain we observe a 170 km wide ocean continent transition zone which includes a pair of overlapping peridotite ridges and is bounded by oceanic crust and landward by fault bounded blocks of continental crust the profile lies 40 km south of the transect sampled by ocean drilling program odp legs 149 and 173 the transition zone structure can be divided into an upper layer 2 4 km thick with velocities of between 4 5 and 7 0 km s 1 and generally a high velocity gradient 1 s 1 and a lower layer up to 4 km thick with a velocity of 7 6 km s 1 and a low velocity gradient a weak moho reflection in this zone was seen only on wide angle profiles at an offset of 30 km the upper layer has a distinctly lower velocity than thinned continental crust adjacent to the continental slope conversely the lower layer has too high a velocity to be magmatically intruded or underplated lower continental crust on the coincident seismic reflection profile fault bounded crustal blocks identified in unequivocal extended continental crust are not observed in the transition zone the upper layer has velocity bounds and gradient similar to oceanic layer 2 observed west of the peridotite ridges but no oceanic layer 3 velocity structure is present while magnetic anomalies have been identified within the transition zone they have not been modeled successfully as seafloor spreading magnetic anomalies nor do they generally form long linear margin parallel features finally odp boreholes 40 km north of our profile and within the interpreted transition zone have recovered up to 140 m thick sections of serpentinite and serpentinized peridotites with little evidence of mafic igneous material we conclude that the transition zone cannot be dominantly composed of either extended continental crust or oceanic crust although current melting models predict a considerably thicker crust of decompression melt products we interpret this region as exposed upper mantle peridotite with little or no synrift extrusive material and limited amounts of synrift material intruded within the serpentinized peridotite

crustal structure of the mid atlantic ridge at 23 n from seismic refraction studies an explosive seismic refraction experiment using four ocean bottom hydrophone receivers was carried out along a 120 km long section of the mid atlantic ridge mar median valley at latitude 23 n immediately south of its intersection with the kane fracture zone the data are interpreted in terms of laterally homogeneous horizontally layered structures using travel time inversion and synthetic seismogram modeling techniques these solutions are combined using two dimensional ray tracing to produce a single model defining the major along axis structural changes occurring beneath this section of the median valley with the exception of slightly low layer 3 velocities the crust beneath several tens of kilometers of the mar median valley has all the characteristics of simple mature ocean crust including a well defined moho transition zone 8 km s upper mantle velocities and a total crustal thickness of 6 7 km no evidence is found for the presence of a steady state axial magma chamber in the crust or upper mantle and the excellent propagation characteristics and orderly amplitude versus range relationships imply the existence of a considerable degree of lateral homogeneity beneath the median valley over distances of several tens of kilometers along axis we infer from these observations that the basic seismic characteristics of the ocean crust are frozen in in a time period less than that separating the major volcanic injection events 10 000 50 000 years by hydrothermal circulation penetrating to the base of the crust along the 100 km long ridge segment studied here two major along axis structural changes occur the first is a 10 to 15 km wide along axis zone of lowered velocities in the lower crust centered beneath a major along axis topographic high this is interpreted to be the remnants of the most recent phase of injection that has temporarily left behind it a region of elevated temperatures and pervasive cracking and thus reduced velocities the second occurs abruptly at latitude 23 15 n and is accompanied by both a major change in rift valley and crestal mountain morphology and the apparent noncoincidence of the median valley and central magnetic anomaly north of this boundary the 30 km long ridge segment that abuts the kane fracture zone has a crustal thickness of 4 5 km and no distinctive layering to produce the characteristic amplitude patterns that typify mature oceanic crust interpretation of the cause of this structural change is uncertain but may be related to local tectonic events perhaps a recent 10 to 20 km ridge jump to the east

serpentinization in the trench outer rise region offshore of nicaragua constraints from seismic refraction and wide angle data summary recent seismic evidence suggested that most oceanic plate hydration is associated with trenchouter rise faulting prior to subduction hydration at trenches may have a significant impact on the subduction zone water cycle previous seismic experiments conducted to the northwest of nicoya peninsula northern costa rica have shown that the subducting cocos lithosphere is pervasively altered which was interpreted to be due to both hydration serpentinization and fracturing of the crustal and upper mantle rocks new seismic wide angle reflection and refraction data were collected along two profiles running parallel to the middle american trench axis offshore of central nicaragua revealing lateral changes of the seismic properties of the subducting lithosphere seismic structure along both profiles is characterized by low velocities both in the crust and upper mantle velocities in the uppermost mantle are found to be in the range 7 3 7 5 km s 1 thus are 8 10 per cent lower than velocities typical for unaltered peridotites and hence confirm the assumption that serpentinization is a common process at the trench outer rise area offshore of nicaragua in addition a prominent velocity anomaly occurred within the crust beneath two seamounts here velocity reduction may indicate increased porosity and perhaps permeability supporting the idea that seamounts serve as sites for water percolation and circulation

segmentation of the mid atlantic ridge between 24 n and 30 40 n the mid atlantic ridge mar extends for over 12 000 km from iceland to the bouvet triple junction in the south atlantic most of our knowledge of the geology of this slow spreading centre comes either from widely spaced low resolution echo sounding lines or from detailed studies of small areas using deep towed instruments or submarines high resolution multibeam bathy metry1 2 has been insufficient to define the variability of accretionary processes along this plate boundary here we present the results of a recent investigation of a 800 km long section of the mar carried out using the multibeam echo sounder seabeam3 analysis of these data reveals the scale and nature of the segmentation of the mar between the kane fracture zone 24 n and latitude 30 40 n

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

global variations of stress drop for moderate to large earthquakes 1 we investigate the global variation of earthquake stress drops using spectra of about 2000 events of mb 5 5 between 1990 and 2007 we use an iterative least squares method to isolate source displacement spectra from travel path and receiver contributions based on a convolutional model the observed p wave source spectra are corrected with a globally averaged empirical correction spectrum and estimates of near source attenuation assuming a brune type source model we estimate corner frequencies and compute stress drops stress drop estimates for individual earthquakes range from about 0 3 to 50 mpa but the median stress drop of about 4 mpa does not vary with moment implying earthquake self similarity over the mw 5 2 to 8 3 range of our data a comparison of our results with previous studies confirms this observation over most of the instrumentally observable magnitude range while the absolute values of our estimated stress drops depend upon the assumed source model we identify relative regional variations of stress drop that are robust with respect to the processing parameters and modeling assumptions which includes an inherent assumption of constant rupture velocity we find a dependence of median stress drop on focal mechanism with a factor of 3 5 times higher stress drops for strike slip earthquakes and also find a factor of 2 times higher stress drops for intraplate earthquakes compared to interplate earthquakes

source analysis of post blasting events recorded in deep copper mine poland seismic and rock burst hazard is a very important factor that has to be considered in seismically active mining areas several different methods are used to decrease seismic activity in mines among others an active prevention method known as destress blasting is considered the most effective technique the aim of this work is to find possible influences of such active prevention on observed seismic activity recorded in the established waiting time after blasts using technological knowledge and seismic data recorded by an underground seismic network we estimated several parameters that characterize sources provoked by blasting works and compared them with the parameters obtained for spontaneous non provoked mining tremors according to our studies the source mechanisms of post blasting seismicity are characterized by the similar non dc part of the full moment tensor whereas the source mechanisms of spontaneous events are not characterized by any specific features on the other hand non provoked seismicity presents lowest values of apparent stress also es to ep ratio suggest some differences between events occurred immediately after blasting up to 30 s and the rest dataset we believe that these parameters could be considered valuable tools supporting established safety time limitations after active prevention

evidence from gravity data for focused magmatic accretion along the mid atlantic ridge gravity data from the mid atlantic ridge between latitudes 27 50 n and 30 40 n show that the accretion of magma at the ridge is focused at discrete centres along the spreading axis large positive gravity anomalies equivalent to reductions of almost 50 in crustal thickness are observed over non transform discontinuities bounding spreading segments

seismic evidence for hydrothermally altered mantle beneath old crust in the tydeman fracture zone abstract to study the structure of old atlantic fracture zones seismic refraction profiles were obtained using ocean bottom seismometers and two ship multichannel expanding spread experiments on two inactive sections of the tydeman fracture zone the results indicate that a crustal seismic velocities are low compared with normal oceanic crust b there is a marked absence of an identifiable layer 3 c the depth to the moho is shallow though not exceptionally so and d anomalously low seismic velocities of 7 2 7 5 km s 1 are observed over ranges at which upper mantle velocities are normally detected s wave arrivals corresponding to both crustal and mantle refractions are also observed which enable the first estimates of poisson s ratio within a fracture zone to be made using these results and those from other surveys we show that the seismic velocity in the upper mantle beneath fracture zones decreases systematically with increasing age these findings suggest that hydrothermal circulation which can lower the upper mantle velocity by serpentinization persists under fracture zones for at least 70 ma and occurs at greater depths than for normal oceanic crust the serpentinization process is not sufficiently rapid to obscure the location of the original crust mantle boundary

reconciling teleseismic and regional estimates of seismic energy estimates of the radiated seismic energy based on teleseismic and regional data often differ by up to an order of magnitude with a tendency for regional estimates to be larger than teleseismic estimates for the same event in this study we compare the velocity spectrum determined from teleseismic data after correction for radiation pattern and propagation effects with the velocity spectrum determined from regional data after the corresponding corrections for nine earthquakes in the middle america subduction zone of mexico this comparison of the corrected spectra is used to identify and reduce the sources of the regional versus teleseismic energy discrepancy which is about an order of magnitude for these events we find that the teleseismic attenuation operator needs to be calibrated in our case for the tectonic environment of the mexican subduction zone we need a teleseismic attenuation operator that is stronger at high frequencies than the global average a larger factor however is the correction needed to account for site amplification this correction has an impact on both regional and teleseismic data but it has a larger influence on the regional estimates because the angle of incidence for teleseismic waves is steep and the stations are located on more competent rock by modifying the teleseismic attenuation operator and applying site corrections based on a generic site model we essentially eliminate the order of magnitude discrepancy between teleseismic and regional estimates of the radiated seismic energy for these events

variation with spreading rate of oceanic crustal thickness and geochemistry abstract away from the influence of fracture zones hotspots and marginal basins oceanic crust exhibits remarkably uniform crustal thickness rare earth element concentrations and bulk composition at all but the most slowly spreading ridges at full spreading rates below 15 mm a however all these observed parameters show marked and abrupt changes the seismically determined crustal thicknesses decrease sharply from their average of 7 1 km as do melt thicknesses inferred from rare earth element inversions of basalts bulk element compositions of basalts show an increase in the percentage of na2o and decreases in the feo content and cao al2o3 ratio as the full spreading rate drops below 15 mm a we present a model of the melting beneath oceanic spreading centres which reproduces the observed variations with spreading rate of crustal thickness and the main geochemical changes observed in basalts changes in the volume and composition of melt generated at very slow spreading rates are caused by conductive heat loss from the mantle welling up beneath rifts we model the observed behaviour with normal mantle potential temperatures of 1300 20 c greater crustal thicknesses are generated at the north atlantic spreading centre adjacent to the iceland mantle plume suggesting mantle temperatures about 80 c hotter than normal in this area

source parameters and radiation efficiency for intermediate depth earthquakes in northeast japan we estimated source parameters of 216 intermediate depth 65 150 km earthquakes mw 4 0 7 0 in the pacific slab beneath japan along using hi net data we made determinations of static stress drop radiated energy and radiation efficiency along with estimates of the whole path attenuation to study the source scaling as a function of earthquake size and depth our results show that there is a small increase in the values of the ratio of radiated energy to seismicmoment as a function of seismic moment which is due to an associated slight increase of static stress drop with earthquake size we also estimated the radiation efficiency for these events using the static stress drops and radiated energies the values of radiation efficiency are slightly lower compared to shallow crustal earthquakes these results indicate that dissipative energy processes may be relatively more important for intermediate depth earthquakes

rupture directivity effect and stress heterogeneity of the 2013 nantou blind thrust earthquakes taiwan abstract we investigated broadband evidence of the rupture directivity effect and estimated stress drop for the 2013 nantou blind thrust earthquakes the 0327 and 0602 crustal events from three individual analyses using empirical green s functions deconvolution source time function analysis source spectral ratio analysis and broadband waveform modeling of a strong motion generation area although these two adjacent events have similar focal mechanisms the 0327 event shows only up dip directivity to the west with an estimated stress drop of 9 0 mpa and the 0602 event displays significant rupture directivity to the southwest with an estimated stress drop of 14 2 mpa the obvious variance in the rupture behavior and stress drop of these two nearby events which might be related to immature blind faulting suggests that this region requires additional study of the stress heterogeneity to better assess the seismic hazard

passive and active seismological study of bending related faulting and mantle serpentinization at the middle america trench water transported within the subducting oceanic lithosphere into the earth s interior affects a wealth of subduction zone processes including intraslab earthquakes and arc magmatism in recent years growing evidence suggests that much of the hydration of oceanic plates occurs at the trench ocean slope right before subduction here normal faults are created while the rigid lithosphere bends into the trench offshore of middle america multi channel seismic reflection imaging suggests that bending related faults cut into the uppermost mantle providing a mechanism for hydration and transformation of mantle peridotites into serpentinites seismic wide angle reflection and refraction data were collected coincident with one of the seismic profiles where the faults have been imaged travel time inversion provides evidence that both crustal and uppermost mantle velocities are reduced with respect to the velocity structure found in mature oceanic crust away from deep sea trenches if mantle velocity reduction is solely produced by hydration velocities indicate 10 15 of serpentinization in the uppermost 3 km of the mantle where seismic data provide enough resolution a small network of ocean bottom hydrophones deployed for about a month detected 3 local micro earthquakes per day earthquake epicentres align with fault scarps at the seafloor and continuous earthquake activity might be an important process to facilitate the percolation of seawater into the upper mantle

earthquake scaling characteristics and the scale in dependence of seismic energy to moment ratio insights from kik net data in japan 1 we investigate earthquake source characteristics and scaling properties using the results of a spectral inversion of more than 29 000 accelerometric borehole recordings from 1 826 earthquakes mjma 2 7 8 throughout japan we find that the calculated source spectra can be well characterized by the omega square model and show on average self similar scaling over the entire magnitude range with median stress drops of 1 1 and 9 2 mpa for crustal and subcrustal events respectively the seismic energy to moment ratio as theoretically expected if the omega square model is valid shows a strong dependency on stress drop only which in conjunction with data selection practice in some studies to cope with limited recording bandwidth can explain the often observed apparent scale dependence our observations suggest that there is no significant deviation from similarity of the energy radiation in the investigated magnitude range and that the observed scatter is mainly related to the scatter in stress drop

extinct spreading center in the labrador sea crustal structure from a two dimensional seismic refraction velocity model the labrador sea contains a rare example of an abandoned mid ocean ridge where active accretion of oceanic crust ceased due to a change in the spreading geometry of lithospheric plates seismic refraction data were collected along a line which transversely crosses the extinct spreading center two dimensional analyses of the refraction data using ray tracing and synthetic seismogram techniques reveal major variations in crustal thickness and velocity in relation to the axis of the extinct spreading center in the extinct spreading center a crustal thickness of approximately 4 km is determined compared with 5 5 km for the flanks substantial lateral variations in p wave velocities of the upper and lower crust are observed with a marked decrease within the extinct spreading center low velocities are also observed in the uppermost mantle underlying the extinct spreading center and are interpreted as being the result of hydrothermal alteration the anomalously low crustal velocities and crustal thinning are attributed to a decreasing supply of partial melt and increasing degree of tectonism at the slow spreading rates preceding extinction