extension subsidence and thermal evolution of the los angeles basin a two dimensional model abstract the los angeles basin is a narrow deep basin containing large oil fields its formation began in the middle miocene and its subsidence has been extraordinarily rapid we suggest that although the basin is in a predominantly strike slip tectonic environment the subsidence is largely a result of crustal extension simple one dimensional extension models ignoring the lateral flow of heat are inadequate to explain the rapidity of the subsidence we use a two dimensional numerical method to model the thermal evolution of the basin based on our model studies the central southwest northeast and palos verdes blocks are inferred to have been extended by 178 69 20 and 27 respectively the average crustal extension for the whole basin is 66 the lateral flow of heat out of the central block into adjacent cooler blocks accounts for its rapid subsidence during the early stage the deepest sediments are predicted to have been heated to about 210 c hydrocarbon maturity calculations indicate that the upper miocene sediments reached oil maturity a little over 4 m y b p no sediment is predicted to have achieved natural gas maturity

localized seismic deformation in the upper mantle revealed by dense seismic arrays seismicity along continental transform faults is usually confined to the upper half of the crust but the newport inglewood fault nif a major fault traversing the los angeles basin is seismically active down to the upper mantle we use seismic array analysis to illuminate the seismogenic root of the nif beneath long beach california and identify seismicity in an actively deforming localized zone penetrating the lithospheric mantle deep earthquakes which are spatially correlated with geochemical evidence of a fluid pathway from the mantle as well as with a sharp vertical offset in the lithosphere asthenosphere boundary exhibit narrow size distribution and weak temporal clustering we attribute these characteristics to a transition from strong to weak interaction regimes in a system of seismic asperities embedded in a ductile fault zone matrix

surface wave tomography of the western united states from ambient seismic noise rayleigh and love wave phase velocity maps summary we present the results of rayleigh wave and love wave phase velocity tomography in the western united states using ambient seismic noise observed at over 250 broad band stations from the earthscope usarray transportable array and regional networks all available threecomponent time series for the 12 month span between 2005 november 1 and 2006 october 31 have been cross correlated to yield estimated empirical rayleigh and love wave green s functions the love wave signals were observed with higher average signal to noise ratio snr than rayleigh wave signals and hence cannot be fully explained by the scattering of rayleigh waves phase velocity dispersion curves for both rayleigh and love waves between 5 and 40 speriod were measured for each interstation path by applying frequency time analysis the average uncertainty and systematic bias of the measurements are estimated using a method based on analysing thousands of nearly linearly aligned station triplets we find that empirical green s functions can be estimated accurately from the negative time derivative of the symmetric component ambient noise cross correlation without explicit knowledge of the source distribution the average traveltime uncertainty is less than 1 s at periods shorter than 24 s we present rayleigh and love wave phase speed maps at periods of 8 12 16 and 20 s the maps show clear correlations with major geological structures and qualitative agreement with previous results based on rayleigh wave group speeds

tomographic images of the upper crust from the los angeles basin to the mojave desert california results from the los angeles region seismic experiment we apply inversion methods to first arriving p waves from explosive source seismic data collected along line 1 of the los angeles region seismic experiment larse extending northeastward from seal beach california to the mojave desert in order to determine a seismic model of the upper crust along the profile we use resolution information to quantify the extent of blurring in the larse images and to smooth a damped least squares dls image by postinversion filtering pif most of the original data fit is preserved while minimizing model artifacts we compare dls pif and smoothing constraint inversion images using both real and synthetic data a preferred pif image includes larger scale features in the smoothing constraint inversion image and finer scale features in the dls inversion image that are consistent with geologic information we interpret principal model features in terms of geology including faulting the maximum depth of low velocity sedimentary and volcanic rocks in the los angeles basin is 8 9 km and in the san gabriel valley is 4 5 5 km a horst like uplift of basement rocks occurs between these basins the northeastern boundary of the san gabriel valley is imaged as a tabular moderately north dipping low velocity zone that projects to the surface at the southernmost trace of the sierra madre fault system in the central and southern san gabriel mountains velocity depth profiles are consistent with intermediate velocity mylonites overlying lower velocity pelona schist along a shallowly southwest dipping vincent thrust fault tomography does not provide a definitive dip for the san andreas fault but combined with other larse results is consistent with a vertical to steep northeast dip

anisotropy of schists contribution of crustal anisotropy to active source seismic experiments and shear wave splitting observations we have made sets of five independent compressional and shear wave velocity measurements which with density allow us to completely characterize the transverse isotropy of samples from five metamorphic belts the haast schist terrane south island new zealand poultney slate chugach phyllite coldfoot schist and pelona schist united states these velocity measurements include compressional wave velocities for propagation parallel perpendicular and at 45 to the symmetry axis shear wave velocity for propagation and particle motion perpendicular to the symmetry axis and shear wave velocity for propagation parallel to the symmetry axis velocity measurements were made up to pressures of 1 gpa 35 km depth where microcracks are closed and anisotropy is due to preferred mineral orientation our samples exhibit compressional wave anisotropy of 9 20 as well as significant shear wave splitting metamorphic terranes that are anisotropic to ultrasonic waves may also be anisotropic at the scale of active and passive seismic experiments our data suggest that a significant thickness 10 20 km of appropriately oriented steeply dipping foliation schist in the crust could contribute as much as 45 of observed shear wave splitting our data set can also be used to model the effects of crustal anisotropy for active source seismic experiments in order to determine if the anisotropy of the terrane is significant and needs to be taken into account during processing and modeling of the data

site amplification in the los angeles basin from three dimensional modeling of ground motion it is well established that sedimentary basins can significantly amplify earthquake ground motion however the amplification at any given site can vary with earthquake location to account for basin response in probabilistic seismic hazard analysis therefore we need to know the average amplification and intrinsic variability standard deviation at each site given all earthquakes of concern in the region due to a dearth of empirical ground motion observations theoretical simu lations constitute our best hope of addressing this issue here 0 0 5 hz finite difference finite fault simulations are used to estimate the three dimensional 3d response of the los angeles basin to nine different earthquake scenarios amplifi cation is quantified as the peak velocity obtained from the 3d simulation divided by that predicted using a regional one dimensional 1d crustal model average ampli fication factors are up to a factor of 4 with the values from individual scenarios typically differing by as much as a factor of 2 5 the average amplification correlates with basin depth with values near unity at sites above sediments with thickness less than 2 km and up to factors near 6 above the deepest 9 km and steepest dipping parts of the basin there is also some indication that amplification factors are greater for events located farther from the basin edge if the 3d amplification factors are divided by the 1d vertical sh wave amplification below each site they are lowered by up to a factor of 1 7 the duration of shaking in the 3d model is found to be longer by up to more than 60 seconds relative to the 1d basin response the simu lation of the 1994 northridge earthquake reproduces recorded 0 0 5 hz particle velocities relatively well in particular at near source stations the synthetic and observed peak velocities agree within a factor of two and the log standard deviation of the residuals is 0 36 this is a reduction of 54 and 51 compared to the values obtained for the regional 1d model and a 1d model defined by the velocity and density profile below a site in the middle of the basin dow respectively this result suggests that long period ground motion estimation can be improved consid erably by including the 3d basin structure however there are uncertainties con cerning accuracy of the basin model model resolution the omission of material with shear velocities lower than 1 km s and the fact that only nine scenarios have been considered therefore the amplification factors reported here should be used with caution until they can be further tested against observations however the results do serve as a guide to what should be expected particularly with respect to increased amplification factors at sites located above the deeper parts of the basin

density of basalt core from hilo drill hole hawaii abstract density measurements of 1600 samples of core from 889 to 3097 m depth below sea level in the hawaii scientific drilling program hole near hilo hawaii show marked differences between the basaltic rock types and help define stratigraphy in the hole water saturated densities of subaerial lava flows occurring above 1079 m depth have the broadest range because of the large density variation within a single lava flow water saturated densities commonly range from 2 0 to 3 0 with an average of 2 55 0 24 g cc dikes and sills range from 2 8 to 3 1 g cc densities of hyaloclastite commonly range from 2 3 to 2 7 with an overall average of about 2 5 g cc the low density of most hyaloclastite is due primarily to palagonitization of abundant glass and presence of secondary minerals in the interstices between fragments four principal zones of pillow lava separated by hyaloclastite occur in the drill core the shallowest 1983 2136 m is paradoxically the densest averaging 3 01 0 10 g cc the second 2234 2470 m is decidedly the lightest averaging 2 67 0 13 g cc the third 2640 2790 m and fourth 2918 bottom at 3097 m are high averaging 2 89 0 17 and 2 97 0 08 g cc respectively the first pillow zone includes degassed pillows i e lava erupted on land that flowed into the sea these pillows are poor in vesicles because the subaerial one atmosphere vesicles were compressed when the flow descended to deeper water and higher pressure the second low density non degassed pillow zone is the most vesicle rich apparently because it was erupted subaqueously at a shallow depth the higher densities of the third and fourth zones result from a low vesicularity of only a few percent and an olivine content averaging more than 5 for the third zone and about 10 for the fourth zone the uppermost hyaloclastite extending about 400 m below the bottom of the subaerial basalt is poorly cemented and absorbs up to 6 wt of water when immersed progressing downward the hyaloclastite absorbs less water and becomes better cemented this change is apparently due to palagonitization of glass and addition of secondary minerals in the deeper older hyaloclastite a process favored by the increase of temperature with depth the cementation is largely complete at 1800 m depth where the temperature attains about 20 c the zone of freshest uncemented hyaloclastite represents the weakest rock in the drill hole and is a likely level for tectonic or landslide disruption

crustal structure of the sheared southwestern barents sea continental margin abstract eighteen expanding spread profiles from the southwestern barents sea continental margin were studied with a variety of seismic interpretation techniques and in conjunction with seismic reflection data the velocity depth profiles from the slope intercept herglotz wiechert and sum methods were compared and contrasted to the velocity depth curve calculated from the iterative forward modelling of the amplitudes the velocity depth profiles derived from the sum and forward modelling of the amplitudes were used to produce synthetics in both the x t and p domains to choose the technique that most accurately represented the data the results showed the oceanic crust at the senja fracture zone to be different from standard oceanic crust with a higher than average velocity for layer 2 of 5 7 6 4 km s 1 and slightly smaller crystalline crustal thickness the continent ocean transition at crustal depths was shown to be narrow in the range of 10 20 km our results were consistent with two different crustal provinces east of the senja fracture zone the tertiary marginal basin and the cretaceous basin region the velocity depth curves enabled us in a gross sense to suggest the depths to deep sedimentary interfaces and the top of the crystalline crust that were normally not observed in the reflection profiles depths down to 20 km were inferred for the base of the sedimentary section in the southwestern barents sea the crust thinned gently towards the continent ocean boundary at the senja fracture zone whereas the crustal thickness changed abruptly below the hornsund fault zone at the margin farther north we associated the change in crustal structure with cenozoic in the thick crust beneath the svalbard platform in the north the depths to mantle determined on the shelf were consistent with that observed on a deep multichannel seismic line that crosses the area

seismicity of the reykjanes peninsula 1971 1976 abstract the reykjanes peninsula oblique rift a segment of the mid atlantic plate boundary connects the reykjanes ridge to the rift zones of iceland the axis of the deformation zone of the plate boundary can be traced as a rather narrow epicentral zone of earthquakes that is highly oblique to the spreading direction of the north america plate with respect to the eurasia plate the seismicity of the area is episodic active periods with a duration of about a decade are separated by more quiet periods of one to two decades our observation period spans a good part of an episode that began in 1967 and ended in 1975 nine earthquake swarms illuminate a 50 km long segment of the plate boundary the epicentral zone cuts obliquely across the fissure swarms of the volcanic systems of the peninsula geothermal areas are preferentially located at the intersection of the fissure swarms with the seismic zone outside the intersections with the seismic zone the fissure swarms show very low seismicity the tectonic characteristics gradually change along the plate boundary segment from being similar to those of a spreading segment in the western part to being more like a transform zone in the eastern part this is seen in the type of earthquake sequences maximum magnitude and type of faulting

resistivity structure of the upper crust in iceland the results of numerous resistivity soundings from all parts of iceland are analyzed the resistivity and the p wave velocity of the uppermost kilometer in the crust show a strong correlation with the geological structure of iceland by estimating porosity from p wave velocity archie s law is found not to be valid in general the temperature dependence of nine water saturated basaltic rock samples was investigated in a laboratory showing stronger temperature dependence than expected from pore fluid conduction these two facts suggest that interface conduction is the most important conduction mechanism a relation between bulk resistivity fracture porosity pore fluid resistivity and temperature is established comparison of regional bulk resistivity with different zeolite zones shows that the resistivity is mainly a function of the maximum depth of burial of the lava pile emphasizing the importance of clay minerals and zeolites for the measured bulk resistivity

adjoint tomography of the southern california crust using an inversion strategy based on adjoint methods we developed a three dimensional seismological model of the southern california crust the resulting model involved 16 tomographic iterations which required 6800 wavefield simulations and a total of 0 8 million central processing unit hours the new crustal model reveals strong heterogeneity including local changes of 30 with respect to the initial three dimensional model provided by the southern california earthquake center the model illuminates shallow features such as sedimentary basins and compositional contrasts across faults it also reveals crustal features at depth that aid in the tectonic reconstruction of southern california such as subduction captured oceanic crustal fragments the new model enables more realistic and accurate assessments of seismic hazard

crustal structure and seismicity distribution adjacent to the pacific and north america plate boundary in southern california new three dimensional 3 d v p and v p v s models are determined for southern california using p and s p travel times from local earthquakes and controlled sources these models confirm existing tectonic interpretations and provide new insights into the configuration of geological structures at the pacific north america plate boundary the models extend from the u s mexico border in the south to the southernmost coast ranges and sierra nevada in the north and have a 15 km horizontal grid spacing and an average vertical grid spacing of 4 km down to 22 km depth the heterogeneity of the crustal structure as imaged by v p and v p v s models is larger within the pacific plate than the north american plate similarly the relocated seismicity deepens and shows more complex 3 d distribution in areas of the pacific plate exhibiting compressional tectonics the models reflect mapped changes in the lithology across major geological terranes such as the mojave desert the peninsular ranges and the transverse ranges the interface between the shallow mono of the continental borderland and the deep moho of onshore california forms a broad zone to the north beneath the western transverse ranges ventura basin and the los angeles basin and a narrow zone to the south along the peninsular ranges the near surface increase in velocity from the surface to up to 8 km depth is rapid and has a logarithmic shape for stable blocks and mountain ranges but is slow with a linear shape for sedimentary basins at midcrustal depths a rapid increase in v p is imaged beneath the sediments of the large sedimentary basins while beneath the adjacent mountain ranges the increase is small or absent

a detailed image of the continent borderland transition beneath long beach california new crustal images beneath long beach california show the region of the inner borderland to continent transition the cross sections are obtained from stacked autocorrelations of virtual sources generated from oil industry data recorded in the city of long beach ca they show that the moho is dipping at 65 and obliquely truncates an 10 km thick flat lying lower crustal fabric the moho appears to be fault controlled and an integral part of the extrusion of the catalina schist that underlays the inner borderland the basement interface has significant offsets of up to 2 km none of which correspond to the mapped trace of the newport inglewood fault

late cenozoic tectonic evolution of the los angeles basin and inner california borderland a model for core complex like crustal extension we propose that the los angeles basin inner california borderland lab ib and the southern borderland off baja california formed by large magnitude crustal extension beginning in latest oligocene earliest miocene time and that these regions represent major late cenozoic rifts the southern borderland is analogous to the gulf of california rift but the onset of rifting in this region may have preceded the opening of the gulf by as much as 15 to 20 m y the lab ib rift is more complex than the southern borderland because it formed in the wake of the rotating western transverse ranges wtr a large crustal block that has undergone more than 90 of neogene clockwise rotation total extension in both rifted regions is estimated to be in excess of 200 km or comparable to the magnitude of late cenozoic crustal extension now estimated for the basin and range province also like the basin and range the crustal floors of the basins in these highly extended domains are characterized by metamorphic core complexes and detachment faults voluminous miocene volcanic rocks and hanging wall fragments of pre rift strata and basement rocks stranded atop detachments during extension in support of this model we synthesize a wide variety of existing geologic data and present interpretations of new geophysical data seismic reflection profiles these profiles image features that we infer are detachment faults and overlying remnants of fragmented and rifted hanging wall rocks we also suggest that a similar but slightly earlier onset of crustal extension affected the santa maria basin and adjacent regions in central california and that the southern margin of this santa maria basin rift was overridden during neogene clockwise rotation of the wtr

crustal structure near the iceland research drilling project borehole from a seismic refraction survey a seismic refraction survey was conducted at reydarfjordur iceland as part of the iceland research drilling project in july 1978 an array of 15 seismometers was used to measure apparent velocities from 25 explosions along a 23 km refraction line the experiment yielded data used to determine the p and s wave velocity structures beneath the drill site and to obtain bounds on the depth to seismic layer 3 the observed apparent velocities vary continuously as a function of range with a marked increase in velocity at a range of 16 km this corresponds to an observed cross over in the travel time data the measurement of apparent velocities has yielded data of the form x p the distance to a measured ray parameter and p the delay time joint inversion of the x p and p data has been performed by linear programing layers of constant slowness gradient are used as the basis for the inversion process estimates of error in the data are used to produce extremal bounds on the family of earth models that satisfy the data when low velocity zones are assumed to be absent the resulting envelope of p wave velocity models indicates a continuous variation of velocity with depth two regions of high velocity gradient are observed one near the surface and the other at depths between 2 6 and 3 9 km associated with layer 3 in iceland these results suggest that modeling of iceland s upper crust with a few thick homogeneous constant velocity layers is an inadequate representation of the velocity structure some features observed in the seismic data suggest the presence of a low velocity zone with a lid between 1 6 and 2 5 km depth rough synthetic modeling places this lid at the shallower end of this range

crustal processes at spreading centers recent discoveries and research on mid ocean ridges mor s in the last four years have greatly increased our knowledge of this fundamental type of plate boundary the last iugg review of mor research was in 1975 aumento et al 1975 and its omission as a topic in 1978 may have reflected a feeling that spreading centers were reasonably well understood since that time however there has been a surfeit of important new observations and exciting research and there is now a turmoil of innovative ideas about the processes which create and leave their imprint on two thirds of the earth s crust new seismic measurements have been made and interpreted as evidence for the existence of shallow axial magma chambers amc beneath intermediate total opening rate 5 9 cm yr and fast 9 18 cm yr spreading centers fig 1 yet there is controversy as to whether they are steady state whether they persist beneath slow ridges as well in the steady state what the dimensions and shape of the chamber are its relation to oceanic crustal stratigraphy and its relation to observations in ophiolites black smoker hydrothermal vents have been discovered in the pacific spewing forth 350 c fluids blackened by polymetallic sulfide precipitates this discovery is embroiled in debate as to how long the vents last whether they occur at slow spreading rates as well whether the enormous heat flux of the vents freeze the magma chamber even at fast spreading rates their influence on geochemical balances in the ocean and the value of the sulfide deposits commercially it has been found that our initial view of mor s as two dimensional linear structures has served us well as an initial working model but must be significantly modified to account for variations in tectonic volcanic and hydrothermal activity along strike as well as for episodicity of these processes deep sea drilling project dsdp samples have raised fundamental questions about formation of the magnetic source layer hydrothermal circulation and fault mechanisms along the mor spreading centers which penetrate existing lithosphere along strike as propagating rifts have modified our views of how plate boundaries reorient themselves and how v shaped magnetic and topographic trends are formed we have also found that transform faults may modify spreading center processes for some distance away from their intersections with mor s and that rigid plate transform faults may fail to develop at fast spreading centers under certain conditions resulting in en echelon overlapping of spreading centers this review will focus on these developments and several others only example references to these topics will be given in the text and many important topics will not be discussed directly e g petrology and geochemistry mantle convection research on lithosphere older than 1 m y however i have tried to compile and have included an extensive bibliography for u s research on a broader range of mor topics for mid 1978 to mid 1982 which is the main purpose of this report pre 1978 references are occasionally included where appropriate abstracts are not included

viscosity structure beneath northeast iceland the dynamics of crustal rifting in iceland depend strongly on the lower crustal rheology which controls the intensity of upper crustal stress concentration and scale time of heat diffusion from the underlying mantle plume while magnetotelluric surveys suggest the presence of a pervasive hot and highly ductile lowermost crust with possibly high fraction of partial melt observations of low seismic attenuation and strong shear wave transmission suggest a much cooler lower crust and upper mantle since viscosity is also sensitive to the degree of partial melt present viscosity estimates for these regions could shed light on the factors responsible for these observations in this study we utilize horizontal and vertical displacement vectors determined in gps campaigns in northeast iceland since 1986 these are modeled in terms of steady state tectonic loading plus postseismic postdiking relaxation following the 1975 1984 krafla rifting episode as first proposed by foulger and others with the elastic part of the model fixed by external constraints these data have a high sensitivity to the viscosity structure beneath iceland lower crust and upper mantle viscosities of about 3 1019 pa s and 3 1018 pa s respectively yield the closest agreement with the data our lower crustal viscosity estimate is consistent with the low attenuation and low subsolidus temperature for the lower crust inferred in recent studies inversions for fissure opening during the krafla rifting episode yield about 7 m of opening centered on the krafla rift as is observed allowing for contemporaneous deep rifting on vertical faults along the askja segment partially accounts for the observed increase in separation across the rift during 1987 1992 but does not account for large displacements in the southeastern part of the network or the large relative subsidence around the askja rift during 1987 1990 recent deep normal faulting beneath the askja rift and further south might explain all of these remaining features

microcrack studies of basalts from the iceland research drilling project microcavities in basaltic core from the irdp borehole in reydarfjordur iceland can be characterized by using direct observations with the scanning electron microscope sem and inversion of velocity data in the iceland basalts neither method provides an accurate complete picture of the crack aspect ratio spectrum but by using both sem and inversion data a complete spectrum may be obtained sem observations provide information on high aspect ratio pores and cracks 0 005 that cannot be separated by velocity inversion theory while velocity inversion theory provides information on low aspect ratio cracks 0 005 that are too narrow to be seen with the sem alteration rims along cracks blunt ends and aspect ratios which imply closure pressures greater than the vertical stress all provide evidence that cracks are open in situ cracks coincident with grain boundaries are abundant in the basalt flows and suggest an origin early in the rock s history probably during cooling crack healing and alteration are observed to increase with depth causing a shift in crack shape distribution toward higher aspect ratios

three stage evolution of the los angeles basin southern california we propose that an episode of transtension dominated development of the los angeles basin area from 12 to 6 ma following middle miocene transrotation and prior to the modern transpressional regime transtension resulted from the releasing bend of the san gabriel chino hills cristianitos fault which acted as the primary transform boundary in southern california during this episode such an interpretation implies that significant transform motion did not occur on the southern san andreas fault prior to 6 ma and that the gulf of california has opened primarily since 6 ma we propose a three stage model for evolution of the los angeles basin and vicinity within the evolving transform fault system transrotation 18 12 ma transtension 12 6 ma and transpression 6 0 ma timing of these stages correlates with microplate capture events which occurred during conversion from a convergent margin to a transform margin

flat slab deformation caused by interplate suction force we image the structure at the southern end of the peruvian flat subduction zone using receiver function and surface wave methods the nazca slab subducts to 100 km depth and then remains flat for 300 km distance before it resumes the dipping subduction the flat slab closely follows the topography of the continental moho above indicating a strong suction force between the slab and the overriding plate a high velocity mantle wedge exists above the initial half of the flat slab and the velocity resumes to normal values before the slab steepens again indicating the resumption of dehydration and ecologitization two prominent midcrust structures are revealed in the 70 km thick crust under the central andes molten rocks beneath the western cordillera and the underthrusting brazilian shield beneath the eastern cordillera

crustal thickness variations in southern california from los angeles region seismic experiment passive phase teleseismic travel times the goal of the 1993 los angeles region seismic experiment larse93 passive phase was to collect waveform data from local and distant earthquakes to study lower crust and upper mantle structural features in southern california particularly under the san gabriel mountains and san andreas fault during larse93 approximately 88 stations were deployed in a 175 km long linear array across the los angeles basin san gabriel mountains and mojave desert northeast of los angeles during the four weeks of continuous recording teleseismic events recorded at each site provided a wide range of ray path backazimuths the teleseismic events included a number of intermediate magnitude earthquakes with epicenters in the aleutian island kamchatka kuril island mid atlantic ridge solomon island japan fiji island and chile regions this experiment was followed by larse94 which involved land refraction and deep crustal seismic reflection profiles from offshore and onshore explosion sources we have used the joint data sets in this study to distinguish upper crustal features from adjacent lower crustal and upper mantle structures p wave travel times were determined from 17 teleseisms and upper crustal residual signatures were removed by incorporating larse94 upper crustal velocity model results within each backazimuth range the resulting relative traveltime residuals increase from negative values 0 5 sec average recorded in the northern san gabriel valley southern san gabriel mountain foothills to positive values 0 2 sec average in the central and northern san gabriel mountains the residual patterns display variations for different backazimuths and incidence angles but show almost no lateral spatial shift of maximum or minimum residual along the array indicating that the dominant source of the residual pattern is shallow 50 km the patterns of residuals require a sharp lateral gradient in shallow velocities between the northern san gabriel valley located in the northernmost los angeles basin and the san gabriel mountains over a distance of less than 50 km most of the residual pattern can be explained by laterally varying crustal thickness and a high velocity anomaly in the upper mantle in our model of moho depth variations the northern san gabriel valley to the southwest of the array is underlain by thinned continental crust crustal thickness increases laterally by 12 km over a distance of less than 50 km into the san gabriel mountains this conclusion is supported by widespread surface evidence for rift style volcanism and crustal extension in southern california crustal rocks

simulations of ground motion in the los angeles basin based upon the spectral element method we use the spectral element method to simulate ground motion generated by two recent and well recorded small earthquakes in the los angeles basin simulations are performed using a new sedimentary basin model that is constrained by hundreds of petroleum industry well logs and more than 20 000 km of seismic reflection profiles the numerical simulations account for 3d variations of seismic wave speeds and density topography and bathymetry and attenuation simulations for the 9 september 2001 m w 4 2 hollywood earthquake and the 3 september 2002 m w 4 2 yorba linda earthquake demonstrate that the combination of a detailed sedimentary basin model and an accurate numerical technique facilitates the simulation of ground motion at periods of 2 sec and longer inside the basin model and 6 sec and longer in the regional model peak ground displacement velocity and acceleration maps illustrate that significant amplification occurs in the basin

deep crustal reflectors under reydarfjordur eastern iceland crustal accretion above the iceland mantle plume summary deep seismic reflection profiling on the faroe iceland ridge experiment fire imaged a sequence of dipping events in the upper to crust beneath the tertiary lava pile in reydarfjordur eastern iceland these events dip towards the spreading centre at which the crust was formed and continue down to 6 s two way time corresponding to depths down to 15 18 km similar crustal reflections have been observed previously within the crust in other areas of iceland although the reflective events located by previous surveys have not been interpreted rigorously in this study we calculate synthetic seismograms from modelled sequences of lava flows to examine the reflective character of a lava le the synthesized lava pile sections are based on field measurements made in the reydarfjordur area the 1 km vertical exposures on the sides of the fjord are composed of lava flows within stratigraphic units which have a range of compositions and some intercalated tuffaceous clastic horizons we compare the synthetic seismograms with our seismic reflection data to test the interpretation that the deep dipping reflections are caused by a gently dipping sequence of subaerially erupted lavas we conclude that the upper and mid crustal reflections that we observe can be interpreted as lava flow sequences similar to those seen at the surface with the main reflections resulting from packets of flows of similar thicknesses the reflectors that we have imaged beneath iceland are similar to those defining the seaward dipping reflectors on the volcanic continental margins and we therefore suggest that the seaward dipping reflections commonly result from packets of lavas with similar distributions of thicknesses the occurrence of lavas down to 15 18 km beneath reydarfjordur can be explained partly by multiple episodes of accretion of the upper crust although the volcanic processes forming the crust in iceland generally lead to advection of extrusive material to mid crustal levels the production of an average extrusive layer totalling 15 km thickness is within reasonable estimates of the current volcanic productivity the advection of frozen lava flows to mid crustal levels contributes to the maintenance of the subsolidus crust that is observed on iceland and the zeolite minerals within the extrusive sequence may be responsible for the crustal low resistivity zone that has been observed using magnetotelluric techniques

unified structural representation of the southern california crust and upper mantle we present a new 3d description of crust and upper mantle velocity structure in southern california implemented as a unified structural representation usr the usr is comprised of detailed basin velocity descriptions that are based on tens of thousands of direct velocity vp vs measurements and incorporates the locations and displacement of major fault zones that influence basin structure these basin descriptions were used to developed tomographic models of crust and upper mantle velocity and density structure which were subsequently iterated and improved using 3d waveform adjoint tomography a geotechnical layer gtl based on vs30 measurements and consistent with the underlying velocity descriptions was also developed as an optional model component the resulting model provides a detailed description of the structure of the southern california crust and upper mantle that reflects the complex tectonic history of the region the crust thickens eastward as moho depth varies from 10 to 40 km reflecting the transition from oceanic to continental crust deep sedimentary basins and underlying areas of thin crust reflect neogene extensional tectonics overprinted by transpressional deformation and rapid sediment deposition since the late pliocene to illustrate the impact of this complex structure on strong ground motion forecasting we simulate rupture of a proposed m 7 9 earthquake source in the western transverse ranges the results show distinct basin amplification and focusing of energy that reflects crustal structure described by the usr that is not captured by simpler velocity descriptions we anticipate that the usr will be useful for a broad range of simulation and modeling efforts including strong ground motion forecasting dynamic rupture simulations and fault system modeling the usr is available through the southern california earthquake center scec website http www scec org

tomography of southern california via bayesian joint inversion of rayleigh wave ellipticity and phase velocity from ambient noise cross correlations the data used in this work are archived and distributed by the southern california earthquake data center scedc http scedc caltech edu research tools waveform html and the iris earthquake data center https ds iris edu ds nodes dmc both the scec community velocity models used in this work cvms 4 26 and cvmh 15 1 0 are available from the southern california earthquake center https github com sceccode ucvmc this study was supported by the southern california earthquake center award 17195 the national science foundation grants cybersees 1442665 and ear 162061 and the king abdullah university of science and technology award ocrf 2014 crg3 2300 we also thank the reviewers of this paper an anonymous individual and jan dettmer for their valuable insight and constructive reviews of the manuscript the 3 d shear wave velocity model will be available from iris data services products earth model collaboration https ds iris edu ds products emc

the iceland research drilling project synthesis of results and implications for the nature of icelandic and oceanic crust study of a 3 km thick section of tertiary basalts in eastern iceland has resulted in the formulation of a model for crustal construction based on accumulation of lava flows on the flanks of large central volcanoes and has led to the development of alteration and physical property profiles in a crustal segment formed at the mid atlantic ridge deep drilling at reydarfjordur revealed the existence of subaerial lava flows and volcaniclastic rocks to a crustal depth of 3 5 km three major stratigraphic units composed largely of basalts are recognized each characterized by distinctive major and trace element patterns each of these units is believed to be the product of a separate episode of mantle melting and fractionation and probably reflects accumulation on the flanks of a central volcano extensive fractionation in shallow magma chambers led to intermediate and silicic lavas which mark the cores of the central volcanic complexes dike density does not increase downward below about 1 5 km crustal depth and the sampled dikes appear to have been intruded laterally along rifts on the flanks of the central volcanoes secondary mineral assemblages are dominated by smectite and zeolites near the top of the section and by epidote and quartz near the base in general the temperature of alteration increases downward but the distribution of individual minerals is irregular locally earlier secondary mineral assemblages are overprinted by contact metamorphism along dike margins the upper surface of greenschist facies metamorphism crops out at the surface in thingmuli volcano but it lies at an estimated crustal depth of about 4 km in the reydarfjordur borehole located only 3 km east of the edge of thingmuli thus the metamorphic zones clearly crosscut the lava stratigraphy physical property profiles correlate well with variations in secondary mineralogy and the top of layer 3 velocities corresponds closely with the projected upper boundary of amphibolite facies rocks at about 6 km crustal depth magnetic susceptibility decreases with depth and is extrapolated to zero at about 4 3 km crustal depth close to the estimated upper limit of greenschist facies metabasalts a much greater crustal thickness leading to the emplacement of high level magma chambers within the extrusive part of the crust is thought to account for many of the differences in crustal structure and evolution between iceland and the rest of the mid atlantic ridge

four dimensional analysis of the inversion of a half graben to form the whittier fold fault system of the los angeles basin abstract analysis of structural and stratigraphic data from hundreds of oil wells surface geologic maps and earthquake focal mechanisms reveals a three phase structural evolution of the whittier fold fault system that evolution progressed from an extensional phase 16 ma accommodated by the formation of a half graben through an episode of graben filling 14 8 ma to lithospheric shortening which began ca 8 ma with contractional reactivation of the graben forming fault as a blind reverse fault and accompanying anticlinal uplift about 1 ma the blind reverse fault broke through onto the sea floor initiating the activity of the present whittier fault system horizontal contraction estimated from reconstructed cross sections ranges from 1 4 to 3 2 km and has been at an average rate of 0 3 mm year in the p direction n s of the 1987 whittier narrows earthquake for the past ca 8 my that rate may have accelerated after breakthrough of the blind reverse fault to as much as 1 0 mm year the ratio of strike slip to dip slip displacement on the whittier fault over the past ca 8 my is about 0 4

microplate capture rotation of the western transverse ranges and initiation of the san andreas transform as a low angle fault system tectonic rotation of the western transverse ranges block is explained by capture of the partially subducted monterey microplate by the pacific plate at about anomaly 6 time ca 20 ma as pacific monterey spreading slowed and eventually ceased the slip vector along the gently northeast dipping subduction interface beneath the california margin changed from slightly oblique subduction to transtensional dextral transform motion this change in slip vector and a shift of pacific plate motion eastward along the already subducted monterey plate interface imply that the san andreas transform began as a system of low angle faults that locally subjected the overriding continental margin to distributed basal shear and crustal extension this basal shear produced the rotated western transverse ranges this model helps explain the timing of initial rotation and basin formation the sudden appearance of widely distributed transform motion well inland of the margin in early miocene time why the western transverse ranges uniquely rotated as a large coherent crustal block and several fundamental structural characteristics of central and southern california the model also provides major constraints on the amount of pacific north america strike slip motion the position through time of offshore oceanic plates relative to onshore geology and a general explanation for what may happen as a spreading ridge approaches a trench and the subduction zone evolves into a transform system

shear wave structure of a transect of the los angeles basin from multimode surface waves and h v spectral ratio analysis we use broad band stations of the los angeles syncline seismic interferometry experiment lassie to perform a joint inversion of the horizontal to vertical spectral ratios h v and multimode dispersion curves phase and group velocity for both rayleigh and love waves at each station of a dense line of sensors the h v of the autocorrelated signal at a seismic station is proportional to the ratio of the imaginary parts of the green s function the presence of low frequency peaks 0 2 hz in h v allows us to constrain the structure of the basin with high confidence to a depth of 6 km the velocity models we obtain are broadly consistent with the scec cvm h community model and agree well with known geological features because our approach differs substantially from previous modelling of crustal velocities in southern california this research validates both the utility of the diffuse field h v measurements for deep structural characterization and the predictive value of the cvm h community velocity model in the los angeles region we also analyse a lower frequency peak 0 03 hz in h v and suggest it could be the signature of the moho finally we show that the independent comparison of the h and v components with their corresponding theoretical counterparts gives information about the degree of diffusivity of the ambient seismic field

low frequency variability in the equatorial atlantic abstract low frequency v and u component oscillations in the eastern equatorial atlantic are addressed using lime series of up to 1 5 years duration from the lower portion of the main thermocline a distinct frequency separation is observed with characteristic time scales of 1 month for v and 4 8 months for u the principal v component oscillations correspond to seasonally modulated linear equatorially trapped rossby gravity waves apparently generated at the surface in the central equatorial atlantic by shear instability between the south equatorial current and the north equatorial countercurrent their length and time scales are well defined and they agree with theoretical predictions the principal u component oscillations correspond to the deep zonal jets observed by recent profiling experiments in the indian and pacific oceans the vertical scales found here agree with the results from the other oceans however insufficient data are available anywhere to ascertain zonal scales linear theory would

wind generated equatorial kelvin waves observed across the pacific ocean abstract sea level fluctuations during 1978 80 at equatorial pacific islands separated by as much as one quarter of the earth s circumference are coherent at periods of 1 6 weeks with phases implying eastward propagation eastward speeds are 16 7 5 higher than expected for a linear first baroclinic mode kelvin wave based upon hydrography zonal winds in the western pacific exhibit variation on meridional scales comparable to those of equatorial ocean baroclinic motions roughly one quarter of sea level variance in the 1 6 week period range can be explained by local zonal wind alone the observed admittance magnitude o 0 1 cm sea level per m s 1 2 zonal wind pseudo stress and phase lag a few days sea level lagging wind can be accounted for in a linear model of baroclinic equatorial kelvin waves generated by a crudely idealized wind patch of 1000 km zonal scale zonal winds at the equator excite among other things low mode kelvin waves which are recognizable o 10000 km to the east of the for

energy and heat fluxes due to vertically propagating yanai waves observed in the equatorial indian ocean shipboard current measurements in the equatorial indian ocean in october and november of 2011 revealed oscillations in the meridional velocity with amplitude 0 10m s these were clearest in a layer extending from 300 to 600 m depth and had periods near 3 weeks phase propagation was upward measurements from two sequential time series at the equator four meridional transects and one zonal transect are used to identify the oscillation as a yanai wave packet and to establish its dominant frequency and vertical wavelength the doppler shift is accounted for so that measured wave properties are translated into the reference frame of the mean zonal flow we take advantage of the fact that in the depth range where the wave signal was clearest the time averaged current and buoyancy frequency were nearly uniform with depth allowing application of the classical theoretical representation of vertically propagating plane waves using the theory we estimate wave properties that are not directly measured such as the group velocity and the zonal wavelength and phase speed the theory predicts a vertical energy flux that is comparable to that carried by midlatitude near inertial waves we also quantify the wave driven meridional heat flux and the stokes drift

seismic properties density and composition of the icelandic crust near reydarfjordur compressional and shear wave velocities densities and porosities have been measured to confining pressures of 6 kbar for 135 samples from a deep borehole in eastern iceland the measurements are from a random sampling of dikes and sills through the 2 km hole velocities within the flow units are relatively low at the tops reach maximum values within central portions and decrease near the bases this systematic variation correlates well with variations in porosity and bulk density the dike units on the other hand have relatively uniform velocities which are approximately equivalent to maximum velocities measured within flow units below 700 m the dike and flow velocities show systematic increases with depth which are related to changes in mineralogy and independent of porosity the velocity gradient dvp dz t within this region is well defined by the laboratory velocities and approximates 0 6 s 1 in good agreement with field refraction measurements high velocities near the base of the hole are related to increasing alteration with depth and in particular an increase in epidote content the abundance of epidote in the rocks recovered from the lower portion of the drillhole supports a metamorphic origin for the layer 2 layer 3 boundary in this region

seismic structure of basalt flows from surface seismic data borehole measurements and synthetic seismogram modeling we use the wide angle wavefield to constrain estimates of the seismic velocity and thickness of basalt flows overlying sediments wide angle means the seismic wavefield recorded at offsets beyond the emergence of the direct wave this wide angle wavefield contains arrivals that are returned from within and below the basalt flows including the diving wave through the basalts as the first arrival and p wave reflections from the base of the basalts and from subbasalt structures the velocity structure of basalt flows can be determined to first order from traveltime information by ray tracing the basalt turning rays and the wide angle base basalt reflection this can be refined by using the amplitude variation with offset avo of the basalt diving wave synthetic seismogram models with varying flow thicknesses and velocity gradients demonstrate the sensitivity to the velocity structure of the basalt diving wave and of reflections from the base of the basalt layer and below the diving wave amplitudes of the

forecasting the path of a laterally propagating dike an important aspect of eruption forecasting is predicting the path of propagating dikes we show how lateral dike propagation can be forecast using the minimum potential energy principle we compare theory to observed propagation paths of dikes originating at the bar arbunga volcano iceland in 2014 and 1996 by developing a probability distribution for the most likely propagation path the observed propagation paths agree well with the model prediction we nd that topography is very important for the model and our preferred forecasting model considers its in uence on the potential energy change of the crust and magma we tested the in uence of topography by running the model assuming no topography and found that the path of the 2014 dike could not be hindcasted the results suggest that lateral dike propagation is governed not only by deviatoric stresses but also by pressure gradients and gravitational potential energy furthermore the model predicts the formation of curved dikes around cone shaped structures without the assumption of a local deviatoric stress eld we suggest that a likely eruption site for a laterally propagating dike is in topographic lows the method presented here is simple and computationally feasible our results indicate that this kind of a model can be applied to mitigate volcanic hazards in regions where the tectonic setting promotes formation of laterally propagating vertical intrusive sheets

distinct 17 and 33 day tropical instability waves in subsurface observations abstract tropical instability waves tiws within a half degree of the equator in the pacific ocean have been consistently observed in meridional velocity with periods of around 20 days on the other hand near 5 n tiws have been observed in sea surface height ssh thermocline depth and velocity to have periods near 30 days tropical atmosphere ocean tao project moored equatorial velocity and temperature time series are used to investigate the spatial and temporal structure of tiws during 3 years of la nina conditions from 1998 through 2001 along 140 w where the tiw temperature and velocity variabilities are at their maxima these variabilities include two distinct tiws with periods of 17 and 33 days rather than one broadbanded process as predicted by modeling studies the 17 day tiw variability is shown to occur not only in meridional velocity at the equator but also in subsurface temperature at 2 n and 2 s while the 33 day tiw variability is observed primarily in subsurface temperature at 5

observations of the dispersion characteristics and meridional sea level structure of equatorial waves in the pacific ocean spectral techniques applied to altimetry data are used to examine the dispersion relation and meridional sea level structure of wavelike variability with periods of about 20 to 200 days in the equatorial pacific ocean zonal wavenumber frequency power spectra of sea surface height when averaged over about 7 s 7 n exhibit spectral peaks near the theoretical dispersion curves of first baroclinic mode equatorial kelvin and rossby waves there are distinct statistically significant ridges of power near the first and second meridional mode rossby wave dispersion curves sea level variability near the theoretical kelvin wave and first meridional mode rossby wave dispersion curves is dominantly but not perfectly symmetric about the equator while variability near the theoretical second meridional mode rossby wave dispersion curve is dominantly antisymmetric these results are qualitatively consistent with expectations from classical or shear modified theories of equatorial waves the meridional structures of these modes resemble the meridional modes of equatorial wave theory but there are some robust features of the meridional profiles that were not anticipated the meridional sea level structure in the intraseasonal kelvin wave band closely resembles the theoretically expected gaussian profile but sea level variability coherent with that at the equator is detected as far away as 11 75 s possibly as a result of the forced nature of these kelvin waves both first and second meridional mode rossby waves have larger amplitude in the northern hemisphere the meridional sea level structure of tropical instability waves closely resembles that predicted by lyman et al using a model linearized about a realistic equatorial zonal current system

an estimate of equatorial wave energy flux at 9 to 90 day periods in the central pacific deep fluctuations in current along the equator in the central pacific are dominated by coherent structures which correspond closely to narrow band in wave number propagating equatorial waves currents were measured roughly at 1500 and 3000 m depths at 5 moorings between 144 and 148 w from january 1981 to march 1983 as part of the pacific equatorial ocean dynamics pequod program in each frequency band resolved a single complex empirical orthogonal function accounts for half to three quarters of the observed variance in either zonal or meridional current dispersion for equatorial first meridional rossby and rossby gravity waves is consistent with the observed vertical zonal coherence structure the observations indicate that energy flux is westward and downward in long first meridional mode rossby waves at periods 45 days and longer and eastward and downward in short first meridional mode rossby waves and rossby gravity waves at periods 30 days and shorter a local minimum in energy flux occurs at periods corresponding to a maximum in upper ocean meridional current energy contributed by tropical instability waves total vertical flux energy transport per unit zonal distance across the 9 to 90 day period range is 2 5 kw m

crustal structure and tectonics from the los angeles basin to the mojave desert southern california a seismic refraction and low fold reflection survey known as the los angeles region seismic experiment larse was conducted along a transect line 1 extending from seal beach california to the mojave desert crossing the los angeles and san gabriel valley basins and san gabriel mountains the chief result of this survey is an interpreted cross section that addresses a number of questions regarding the crustal structure and tectonics of southern california that have been debated for decades and have important implications for earthquake hazard assessment the results or constraints are as follows 1 the maximum depth of the los angeles basin along line 1 is 8 9 km 2 the deep structure of the sierra madre fault zone in the northern san gabriel valley is as follows the duarte branch of the sierra madre fault zone forms a buried 2 5 km high moderately north dipping buttress between the sedimentary and volcanic rocks of the san gabriel valley and the igneous and metamorphic rocks of the san gabriel mountains for deeper structure see following 3 there are active crustal decollements in southern california at middle crustal depths the sierra madre fault zone appears to sole into a master decollement that terminates northward at the san andreas fault and projects southward beneath the san gabriel valley to the puente hills blind thrust fault 4 the dip and depth extent of the san andreas fault along line 1 dips steeply 83 northward and extends to at least the moho 5 the subsurface lateral extent of the pelona schist in southern california is as follows along line 1 the pelona schist underlies much if not all of the san gabriel mountains south of the san andreas fault to middle crustal depths north of the san andreas fault it is apparently not present along the transect

the iceland research drilling project crustal section variation of magnetic properties with depth in icelandic type oceanic crust an attempt has been made to identify the processes that give rise to a number of depth trends in the magnetization of a 3 1 km vertical section of icelandic type oceanic crust and to assess the possibility that similar processes act and depth trends occur in typical oceanic crust the depth trends in the icelandic section consist of a general increase in saturation and induced magnetization to 2 km crustal depth below which flow magnetization decreases while dike magnetization remains constant and of large changes in flow magnetization that occur on a scale of a few hundred metres below 3 km crustal depth increase in saturation and induced magnetization with depth in the upper 2 km is thought to be the result of two processes a decrease in low temperature oxidation from the original lava surface to 700 800 m crustal depth thence an increase in hydrothermal alteration with depth this interpretation is based on oxide petrography and curie temperatures which show a weakly defined minimum in the 700 8

deep currents and their interpretation as equatorial waves in the western pacific ocean abstract vertical profiles of current and density made within 5 latitude of the equator along longitudes 168 and 179 e the vicinity of the gilbert islands reveal multiple deep current reversals which are confined to the equator these current jets have amplitudes of roughly 5 20 cm s 1 vertical scales of order hundreds of meters and time scales longer than the one month of measurements during the cruise spectral analysis of profiles indicates that 1 meridional trapping varies roughly as the square root of the vertical scale 2 both east and north currents are coherent over meridional separations of 0 45 within roughly 1 30 of the equator and 3 zonal current lags vertical displacement by 4 in depth within 0 45 of the equator zonal coherence scales are smaller than the smallest separation of 3 30 along the equator kinetic energy decreases near the island chain current meter records taken over nearly two years near the islands suggest strong quasi annual variability in deep currents near t

p wave seismic velocity structure derived from sonic logs and industry reflection data in the los angeles basin california 1 we present a new three dimensional seismic p wave velocity model of the los angeles basin that is based on more than 150 sonic logs and 7000 stacking velocities from industry reflection profiles the model was developed using kriging techniques implemented in gocad a commercial application specializing in the construction of surfaces the analysis of spatially distributed data and the modeling or simulation of properties and is characterized by a heterogeneous spatial varying velocity gradient with maximum velocities of 5000 m s in the sedimentary section variability of the observed p wave velocities increases with depth reflecting complex structural and sedimentological controls the model contains a new representation of the basin shape as defined by the sediment basement interface which was mapped using well control and seismic reflection profiles our model differs substantially in approach and outcome from previous representations of crustal velocity structure in southern california

structure of the ecuadorian forearc from the joint inversion of receiver functions and ambient noise surface waves the ecuadorian forearc is a complex region of accreted terranes with a history of largemegathrust earthquakes most recently amw7 8 megathrust earthquake ruptured the plateboundary offshore of pedernales ecuador on 16 april 2016 following this event an inter national collaboration arranged by the instituto geofisico at the escuela polit ecnica nacionalmobilized a rapid deployment of 65 seismic instruments along the ecuadorian forearc wecombine this new seismic data set with 14 permanent stations from the ecuadorian nationalnetwork to better understand how variations in crustal structure relate to regional seismichazards along the margin here we present receiver function adaptive common conversionpoint stacks and a shear velocity model derived from the joint inversion of receiver functionsand surface wave dispersion data obtained through ambient noise cross correlations for theupper 50 km of the forearc beneath the forearc crust we observe an eastward dipping slowvelocity anomaly we interpret as subducting oceanic crust which shallows near the projectedcentre of the subducting carnegie ridge we also observe a strong shallow positive conversionin the ecuadorian forearc near the borbon basin indicating a major discontinuity at a depth of 7 km this conversion is not ubiquitous and may be the top of the accreted terranes we alsoobserve significant north south changes in shear wave velocity the velocity changes indicatevariations in the accreted terranes and may indicate an increased amount of hydration beneaththe manab i basin this change in structure also correlates geographically with the southernrupture limit of multiple high magnitude megathrust earthquakes the earthquake record alongthe ecuadorian trench shows that no event with amw 7 4 has ruptured south of 0 5 sinsouthern ecuador or northern peru our observations along with previous studies suggestthat variations in the forearc crustal structure and subducting oceanic crust may influance theoccurrence and spatial distribution of high magnitude seismicity in the region

interpretation of seismic reflection gravity and magnetic data across middle proterozoic mid continent rift system northwestern wisconsin eastern minnesota and central iowa seismic reflection gravity and magnetic data were used along four profiles to study the mid continent rift system between northwestern wisconsin and central iowa mafic lavas in medial horsts are characterized by strong laterally continuous reflections that indicate a stratigraphic package as much as 10 km thick mafic roots extending beneath the reflective sequence to mid crustal depths were defined from gravity modeling in northwestern wisconsin southeastern minnesota and central iowa a strong normally directed remanent magnetization was modeled for most lavas although some lavas deep in the section of northwestern wisconsin and central iowa were modeled as magnetically reversed the horsts are apparently bounded by faults that are vertical or dip moderately inwar several of these faults originated as growth faults in fault bounded grabens but these faults were transposed into reverse faults by a late stage compressional event the medial horsts are flanked by several half graben basins that contain as much as 5 km of sedimentary material inferred to be lithologically akin to the oronto and bayfield groups of michigan and wisconsin several synclinal basins characterized by 2 4 km of sedimentary rocks are developed atop the medial horsts when viewed regionally the four interpretive profiles show a marked asymmetry with regard to the off axis placement of the deep mafic roots and the positions of postulated growth faults this asymmetry is consistent with the rift propagating as a series of northeast striking en echelon half grabens separated by northwest striking scissor faults or accommodation zones

crustal structure across the san andreas fault southern california from teleseismic converted waves crustal structure along a 140 km long profile across the san andreas fault saf in southern california was imaged by stacking teleseismic p s converted phases recorded by a dense short period seismic array the crust mantle discontinuity moho is visible as a continuous feature at a depth around 30 km but is offset 6 to 8 km beneath the saf a small moho disruption can also be seen under the eastern california shear zone ecsz these results suggest that the saf and ecsz extend through the entire crust the moho upwarp under the san gabriel mountains indicates that the mountain ranges were lifted en masse as a result of crustal buckling under horizontal compression

moored observations of deep low frequency motions in the central pacific ocean vertical structure and interpretation as equatorial waves abstract two year time series of current and temperature collected in the deep equatorial central pacific 0cean as part of the pacific equatorial ocean dynamics pequod project indicate that motions with vertical scales comparable to those of low baroclinic modes dominate motions from annual to fortnightly frequencies properties of equatorial waves are consistent with many aspects of the observed spectra and coherences in particular the hypothesis of a spectrum of long rossby and kelvin waves at periods longer than about 40 days and rossby mixed rossby gravity and kelvin waves at periods between about 14 and 40 days is consistent with the observations neither vertically propagating rays nor randomly phased baroclinic modes can explain coherence phases uniformly but forced phase locked baroclinic modes provide a possible explanation sea level in the central pacific is coherent with deep motions at periods of months or shorter in particular the quasi annual sea level signal associated with the 19

tropical instability wave kinematics observations from the tropical instability wave experiment the kinematics of planetary waves originating from instability of the nearsurface equatorial currents are reported on using velocity measurements from an array of acoustic doppler current profilers deployed in the equatorial pacific during the tropical instability wave experiment a distinctive wave season was observed from august to december 1990 with wave energy confined primarily above the core of the equatorial undercurrent particle motions in the horizontal plane are described by eccentric ellipses oriented toward the north but tilting into the cyclonic shear of the south equatorial current the tilt is maximum near the surface just north of the equator and decreases to the south and with depth the distribution of wave variance is narrowband in both frequency and zonal wavenumber with central period zonal wavelength and westward directed phase propagation estimated to be 500 hours 1060 km and 59 cm s 1 respectively neither the meridional nor the vertical wavenumber component is statistically different from zero these results generally agree with previous findings on tropical instability waves from the atlantic and pacific oceans and in the undersampled arena of geophysical measurements they provide an example where statistical inference is supported by an ensemble of independent measurements

the scec southern california reference three dimensional seismic velocity model version 2 we describe version 2 of the three dimensional 3d seismic velocity model of southern california developed by the southern california earthquake center and designed to serve as a reference model for multidisciplinary research activities in the area the model consists of detailed rule based representations of the major southern california basins los angeles basin ventura basin san gabriel valley san fernando valley chino basin san bernardino valley and the salton trough embedded in a 3d crust over a variable depth moho outside of the basins the model crust is based on regional tomographic results the model moho is represented by a surface with the depths determined by the receiver function technique shallow basin sediment velocities are constrained by geotechnical data the model is implemented in a computer code that generates any specified 3d mesh of seismic velocity and density values this parameterization is convenient to store transfer and update as new information and verification results become available

abyssal mixing through critical reflection of equatorially trapped waves off smooth topography abstractthe inferred diapycnal upwelling in the abyssal meridional overturning circulation amoc is intensified near the equator but little is known as to why this is so in this study it is sho

constraints on crustal structure in eastern iceland based on extremal inversions of refraction data in the summer of 1978 the iceland research drilling project undertook the drilling of a deep crustal hole near reydarfjorder in eastern iceland as a part of this project the scripps institution of oceanography and the university of washington undertook a small scale seismic refraction experiment near the drill site in an attempt to compare surface geophysical measurements with observations of samples from and logging in the hole using recent advances in the methods of extremal inversion of seismic data we have determined an approximate one dimensional velocity structure for the drill site this structure indicates that the 1 9 km hole failed to penetrate the layer 2 layer 3 transition which was at some 3 0 4 5 km beneath the drill site the transition appears to be rather abrupt unlike that beneath the ocean with velocity increasing from 5 2 5 5 km s in the upper crust to about 6 7 km s in layer 3 we observe a steep eastward dip and a shallow westward dip in the lower crust away from the nearby thingmuli and reydarfjordur volcanic centers respectively in agreement with previous work associating shallow depths to layer 3 with tertiary volcanic centers as a result of increasing metamorphic grade and increased dyke swarm intensity

structure of the los angeles basin from ambient noise and receiver functions a velocity v s and structure model is derived for the los angeles basin california based on ambient noise surface wave and receiver function analysis using data from a low cost short duration dense broad band survey lassie deployed across the basin the shear wave velocities show lateral variations at the compton los alamitos and the whittier faults the basement beneath the puente hills san gabriel valley shows an unusually high velocity 4 0 km s 1 and indicates the presence of schist the structure of the model shows that the basin is a maximum of 8 km deep along the profile and that the moho rises to a depth of 17 km under the basin the basin has a stretch factor of 2 6 in the centre grading to 1 3 at the edges and is in approximate isostatic equilibrium

full 3 d tomography for crustal structure in southern california based on the scattering integral and the adjoint wavefield methods we have successfully applied full 3 d tomography f3dt based on a combination of the scattering integral method si f3dt and the adjoint wavefield method aw f3dt to iteratively improve a 3 d starting model the southern california earthquake center scec community velocity model version 4 0 cvm s4 in f3dt the sensitivity frechet kernels are computed using numerical solutions of the 3 d elastodynamic equation and the nonlinearity of the structural inversion problem is accounted for through an iterative tomographic navigation process more than half a million misfit measurements made on about 38 000 earthquake seismograms and 12 000 ambient noise correlagrams have been assimilated into our inversion after 26 f3dt iterations synthetic seismograms computed using our latest model cvm s4 26 show substantially better fit to observed seismograms at frequencies below 0 2 hz than those computed using our 3 d starting model cvm s4 and the other scec cvm cvm h11 9 which was improved through 16 iterations of aw f3dt cvm s4 26 has revealed strong crustal heterogeneities throughout southern california some of which are completely missing in cvm s4 and cvm h11 9 but exist in models obtained from previous crustal scale 2 d active source refraction tomography models at shallow depths our model shows strong correlation with sedimentary basins and reveals velocity contrasts across major mapped strike slip and dip slip faults at middle to lower crustal depths structural features in our model may provide new insights into regional tectonics when combined with physics based seismic hazard analysis tools we expect our model to provide more accurate estimates of seismic hazards in southern california

a geophysical profile of the southern margin of the midcontinent rift system in western lake superior a 70 km long seismic reflection profile in western michigan provides new insight into the nature distribution and structure of the keweenawan supergroup volcanic and overlying sedimentary rocks and the controversial keweenaw fault along the southern boundary of the midcontinent rift system in the lake superior basin interpretation of the 5 s reflection data constrained by surface geology magnetics and gravity modeling shows that volcanic rocks which cropout north of the keweenaw fault dip northerly to depths of the order of 17 km south of the fault volcanic rocks overlain by 2 km of clastic sedimentary rocks thin gradually to the south as they dip upward at a shallow angle to the outcrop in the south range the volcanic pile within the basin thickens rapidly to the north of the keweenaw fault suggesting that the volcanics were deposted in an extensional fault bounded basin clear evidence of normal faulting is not present in the seismic reflection data because of a later compressional event the thickness of the volcanic filled basin implies that the upper crust was almost completely broken during the rifting event the available evidence is interpreted to show the keweenaw fault as a moderate to high angle reverse fault that occurs within the volcanic pile and breaks through to the surface along the abrupt change in thickness of the volcanic sequence there is no evidence from the seismic profiling for major faulting except for the keweenaw fault intrusions or folding of the keweenawan supergroup in this region

evidence for seafloor intensified mixing by surface generated equatorial waves little is known about mixing in the abyssal equatorial oceans in spite of its inferred importance for upwelling dense water here we present full depth microstructure turbulence profiles obtained in the equatorial pacific that show evidence for intense wind generated abyssal mixing mixing was intensified over the bottom 700 m where the diffusivity reached 10 3 m2 s 1 of similar intensity to mixing driven by tidal flow over rough topography however here the mixing was found over smooth topography we suggest that the intense mixing could have been driven by surface generated equatorial waves through two possible mechanisms 1 near bottom wave trapping as a result of the horizontal component of the earth s rotation and 2 inertial instability the generation of lee waves over smooth topography at low latitudes and their subsequent breaking is another viable mechanism for the mixing