surface wave dispersion analysis in mexico city abstract in this article we present an observational investigation of ground motion at mexico city focused on surface waves our purpose is 2 fold first to understand incident ground motion during the great michoacan earthquake of 19 september 1985 and second to characterize surface waves propagating in the lake bed zone to this end we analyze the strong motion records obtained at mexico city for the large m s 8 1 earthquake of 19 september 1985 it is shown that in the low frequency range we observe the rayleigh fundamental mode in both the vertical and the radial components and the love fundamental mode in the transverse component at all the strong motion stations the vertical component also shows the first higher mode of rayleigh waves we use a very broadband record obtained at station cu for the smaller m s 6 7 earthquake of 14 may 1993 to verify that the dispersion computed from the model of campillo et al 1989 represents well the average surface wave propagation between the coast and mexico city in the 7 to 10 sec period range we use this result to assign absolute times to the strong motion records of the michoacan event this allowed us to identify additional wave trains that propagate laterally in directions other than great circle in the 3 to 5 sec period range these wave trains are identified as love waves in a second analysis we study a set of refraction data obtained during a small scale 250 m experiment on the virgin clay of the lake bed zone phase velocity dispersion curves for several modes of rayleigh waves are identified in the refraction data and inverted to obtain an s wave velocity profile this profile is used as the uppermost layering in a 2d model of mexico city valley the results of numerical simulation show that surface waves generated by lateral finiteness of the clay layer suffer large dispersion and attenuation we conclude that surface waves generated by the lateral heterogeneity of the upper most stratigraphy very significantly affect ground motion near the edge of the valley but their importance is negligible for distances larger than 1 5 km from the edge thus locally generated surface waves propagating through the clay layer cannot explain late arrivals observed for the 1985 event we suggest that the long duration of strong motion is due to the interaction between lateral propagation of waves guided by deep layers 1 to 4 km and the surficial clay layer this interaction is possible by the coincidence of the dominant frequency of the uppermost layers and the frequency of the deeply guided waves

natural deformation fabrics of plagioclase implications for slip systems and seismic anisotropy abstract the optical and tem microstructures and fabrics of twelve naturally deformed plagioclase containing rocks of compositions from an0 an80 have been studied with a view to determining the deformation mechanisms the optical indicatrix fabrics have been recorded in all samples with respect to the foliation and lineation in general and form a girdle in the foliation plane with a high concentration of normal to the foliation in some cases the orientation of 010 and 001 cleavages or albite and pericline twins has allowed the construction of pole figures in all cases 010 has been shown to be the dominant slip plane with either 001 or 100 the dominant slip direction tem shows that the dominant slip system is 010 001 with secondary 010 100 the seismic p wave anisotropy has been calculated for the first time from the lpo data of plagioclase the anisotropy coefficient is high at around 15 the minimum p velocity is normal to the lineation in the foliation plane and the maximum is sub parallel to the foliation normal the combined information on deformation kinematics and seismic anisotropy is valuable for interpretations of deep crustal seismic sections

why is the stability of the agulhas current geographically bi modal 1 the agulhas current the largest western boundary current of the southern hemisphere becomes fully constituted along the eastern coast of south africa at about 25 s from where is flows poleward along this coast south of the continent it retroflects with most of its water being returned to the indian ocean in the agulhas return current whereas the rest is injected into the south atlantic by the shedding of agulhas rings at the retroflection point of the current the flow of the agulhas current between its initiation and retroflection points can be divided into two distinct segments by trajectorial characteristics a northern and a southern agulhas current the path of the northern current is exceptionally stable whereas the southern current meanders in a way more characteristic of western boundary currents by applying a two layer linear stability theory that specifically considers the effect of a lateral boundary i e the shelf break on the stability of an adjacent current flowing parallel to it we show that the stability of the northern current is attributable to its proximity to the african land mass which is located only one tenth of the internal radius of deformation west of the current by comparison the southern current on average flows at a distance of over nine tenths of the internal radius of deformation from the shelf break where according to the theory the instability is over twice that of the northern current

deep water measurements of container ship radiated noise signatures and directionality underwater radiated noise from merchant ships was measured opportunistically from multiple spatial aspects to estimate signature source levels and directionality transiting ships were tracked via the automatic identification system in a shipping lane while acoustic pressure was measured at the ships keel and beam aspects port and starboard beam aspects were 15 30 and 45 in compliance with ship noise measurements standards ansi asa s12 64 2009 and iso 17208 1 2016 additional recordings were made at a 10 starboard aspect source levels were derived with a spherical propagation surface affected or a modified lloyd s mirror model to account for interference from surface reflections surface corrected ship source depths were estimated from spectral differences between measurements at different beam aspects results were exemplified with a 4870 and a 10 036 twenty foot equivalent unit container ship at 40 56 and 87 of service speeds respectively for the larger ship opportunistic ansi i

generation of submesoscale frontal eddies in the agulhas current copyright 2019 brill due to copyright restrictions the attached pdf file contains the abstract of the full text item for access to the full text item please consult the publisher s website https agupubs onlinelibrary wiley com doi full 10 1029 2019jc015229

phytoplankton and nutrient dynamics of six south west indian ocean seamounts abstract a survey of six seamounts and two transects through the subtropical convergence zone scz in the south indian ocean in november and december 2009 showed a strong latitudinal gradient from the subtropics to the sub antarctic front concentrations of oxygen nitrate nitrite soluble reactive phosphorous as well as phytoplankton biomass measured as chlorophyll a increased while salinity and temperature decreased with an increase in latitude these differences resulted in significant differences between seamounts the chlorophyll a maximum became shallower at higher latitudes changing from a depth of 85 m in the subtropics to 35 m over the seamounts and in the scz the mixed layer depth also increased from 50 m in the subtropics to 100 m at higher latitude stations the n p and n si ratio indicated that no 3 was limiting at all the seamounts except one at which sio 4 was the limiting nutrient the phytoplankton community also showed a latitudinal gradient with decreasing diversity and a change in dominance from dinoflagellates in the tropics to diatoms towards the scz the dominant diatom genus of the survey 50 of the cell counts was pseudo nitzschia nutrients exhibited an inverse linear relationship with temperature and salinity the oligotrophic subtropical areas differed from the mesotrophic seamounts in temperature while waters over seamounts north and south of the agulhas return current arc differed in salinity the phytoplankton 148 taxa responded to these differences showing three communities subtropical seamount phytoplankton atlantis seamount walters seamount and off mount samples phytoplankton of the waters north of the arc melville bank sapmer bank middle of what seamount and phytoplankton south of the arc coral seamount scz1 characterised by a bloom of phaeocystis antarctica the environmental drivers most strongly linked to these observed differences were nitrate temperature and oxygen these environmental drivers displayed a clear latitudinal gradient unaffected by mesoscale variability of the arc eddy field and allowing the three phytoplankton communities to persist phytoplankton biomass was enhanced in the shallow

strong motion in central mexico a model based on data analysis and simple modeling predictions of ground motion for mexico city during future large earthquakes are currently based on statistical analysis of the large quantity of data recorded by the mexico city accelerograph network mcan in spite of the robustness of these predictions large uncertanties come from our lack of a physical model that is able to account for the observations in this article we present such a model based on analysis of strong motion records obtained during the 25 april 1989 guerrero earthquake and on numerical modeling data analysis suggests that the large amplitudes and very long duration of ground motion observed in the lake bed zone in mexico city result from the interaction of diffracted wave trains of rayleigh waves and the very soft clay layer that covers the ancient lake of mexico city numerical modeling supports this hypothesis and indicates that this interaction is possible in spite of the largely different scales kilometers for the incident surface waves and tens of meters for the clay layer lateral heterogeneities split incoming wave trains into different modes each is strongly amplified and its duration is increased by the soft clay layer at its resonant frequency resulting in the very long monochromatic ground motion that has been the subject of a large number of articles since the 1985 michoacan earthquake manuscript received 23 august 2001

the natal pulse an extreme transient on the agulhas current an analysis of satellite data collected over a period of 9 years as well as historic hydrographic data show that the northern agulhas current is subject to large intermittent and solitary meanders these transient events collectively named the natal pulse progress downstream at consistent rates of 21 cm s upon reaching the area where the shelf broadens their rate of progression slackens to 5 cm s they are present in some stages of development at least 17 of the time extend offshore by about 170 km on average and show a continuous lateral growth on moving downstream with few exceptions the pulses are spawned as cold core cyclonic trapped lee eddies in the natal bight they are held responsible for the intermittent coastal counter currents observed inshore of the agulhas current along the southern african coast and may play a crucial role in sediment distribution on the shelf

mexico 1985 the case for gravity waves summary the argument for the existence of gravity waves in sedimentary layers during strong earthquakes such as the 1985 mexico earthquake is based on a theory which predicts a discontinuity in surface wave propagation at a finite strain value b effects of stress strain non linearity in sediments and c observations suggesting the presence of ultra short wavelengths in soils during the 1985 mexico earthquake

shear wave velocity in marine sediments on young oceanic crust constraints from dispersion analysis of scholte waves summary an experiment with a newly developed implosive source located about 1 m above the sea oor at 3665 m depth revealed a slow interface wave this wave is attributed to s waves in a soft sediment layer covering the hard rock sub bottom dispersion analysis by means of the multiple lter technique yields the group velocity as a function of frequency model calculations showed that the observed dispersion curve can be reproduced by considering a steep gradient of s velocity within the sediment layer nearly perfect agreement of experimental and model data could be achieved with a sediment layer thickness of 21 5 m an s speed of 225 m s 1 at the water sediment interface and an increase by 23 s 1 within the layer these values are relatively high compared to data from the literature however previous estimates of in situ s wave velocity have been obtained on old oceanic crust in the vicinity of continents or islands while our experiment was carried out on young paci c crust therefore we suggest two mechanisms which could support a relatively high s speed in sediments 1 the input of hydrothermally generated metalliferous sediments from the adjacent spreading axis and 2 post depositional diagenesis which has accelerated the induration of sediments

the seismic response of two dimensional sedimentary deposits with large vertical velocity gradients an extension of the aki larner technique to vertically inhomogeneous media is presented so that vertical velocity gradients may be taken into account in two dimensional models only sh waves are considered here but the theory is valid for p and sv waves as well this method is applied to investigate the seismic response of two dimensional sedimentary deposits with large velocity gradients three different cases are considered a shallow high contrast valley a deep high contrast valley and a deep low contrast valley in each case a comparison is performed with the results of a two dimensional model taking into account only homogeneous sediments and a one dimensional model taking into account the vertical inhomogeneity of sediments the presence of a large velocity gradient does not change a lot the qualitative behavior of a two dimensional deposit local surface waves and or two dimensional resonance patterns are observed as in the case of homogeneous sediments nevertheless the surface waves are much more dispersive with lower phase and group velocities and larger amplitudes only very deep valleys give rise to the development of two dimensional resonance on the other hand the amplifications obtained in such deposits may reach much larger values than those predicted with either two dimensional homogeneous models or one dimensional inhomogeneous models since these results have been obtained for realistic values of valley geometrical and mechanical considerations they should find some application in earthquake engineering or seismic microzonation studies

natural deformation and recrystallization of some intermediate plagioclase feldspars abstract intermediate an25 an48 plagioclase feldspars from the nordre stromfjord shear zone central west greenland have undergone extensive ductile deformation by intracrystalline slip and by twinning the plagioclases were deformed under low granulite facies conditions optical substructures include undulatory extinction deformation bands subgrains and deformational twins mainly on the albite law dynamic recrystallization occurred primarily by subgrain rotation with bulge nucleation migration of a high angle boundary as a secondary mechanism in some cases recrystallization is accompanied by a small decrease in the an content which is thought to indicate prograde metamorphic conditions the plagioclases exhibit a good crystallographic preferred orientation which is compatible with 010 001 being the principal slip system quartz has in the same rocks undergone more plastic deformation and more extensive recovery than plagioclase this is at least in part due to the fact that most dislocations in plagioclase are dissociated

a reusable implosive seismic source for midwater or seafloor use we have developed a new implosive seismic or acoustic source for seafloor or midwater use the fact that this device does not use pyrotechnics simplifies logistic and permitting problems it produces relatively little high frequency output so it is wildlife friendly this device enables us to place the source nearer to the image target compared to surface sources which thus increases resolution the simple 20 l version we have constructed must be reset after each use by bringing it to the sea surface we present measurements of seafloor shear velocity at a depth of about 1 km in the san diego trough there the surficial shear velocity is 16 m s and the gradientisabout10 s 1

deformation and recrystallization of plagioclase along a temperature gradient an example from the bergell tonalite abstract syn to post mylonitic tilting of the bergell tonalite allows investigation of the deformation and recrystallization of plagioclase grains along a temperature gradient from the lower to the upper amphibolite facies at the lowest temperatures recrystallization occurs by nucleation and growth of new grains having a different composition from the old grains in contrast at the highest temperatures recrystallization occurs by subgrain rotation associated with grain boundary migration the temperature increase is inferred to induce this transition in two ways first at higher temperatures the anorthite content of the new grains is higher and thus the comositional difference between old and new grains is lower hence the chemical driving potential is small compared with recrystallization at lower temperatures second higher temperatures facilitate climb of dislocations and thus subgrain formation as well as grain boundary mobility the temperature increase is also associated with a transition in the dominant deformation mechanism intracrystalline plasticity dominates deformation at lower temperatures whereas at higher temperatures deformation occurs primarily by diffusion accommodated grain boundary sliding as indicated by the weakening of the indicatrix preferred orientation and the formation of a mixed plagioclase biotite matrix with increasing deformation therefore the strength of the mid to lower crust may be overestimated by dislocation creep flow laws for plagioclase lower crust rheology for polyphase rocks is better approximated by constitutive relationships involving diffusion accommodated grain boundary sliding

resonant response models for the valley of mexico i the elastic inclusion approach summary the model developed by bard and bouchon to study the earthquake response of open ended 2 d sediment filled valleys with simple geometries is generalized to describe completely enclosed basins with arbitrary geometries by solving the basin form that corresponds to the valley of mexico it is possible to understand via a two step process model several of the observed features of the 1985 september 19 earthquake that devastated downtown mexico city here a general approach that allows the treatment of the particular seismic behaviour of the valley of mexico which is a large sediment filled valley containing an extremely soft ground urban area with a very high water content using collective resonance methods that vary from elastic to acoustic models is established this will lead through a series of papers to a theory that takes into account the role played by any valley s collective modes for microzonation and seismic damage predictions

a systematic method for analyzing borehole recordings to estimate the wavefield in the lakebed zone of mexico city during the 1985 michoacan earthquake m s 8 1 large amplitude seismograms with extremely long duration hundreds of seconds were recorded in the lakebed zone of mexico city in spite of numerous studies the lakebed seismograms are not well understood in the present study we develop a systematic method for analyzing borehole recordings and apply it to recordings at depths of 0 and 102 m at the roma station to interpret the wavefield in the lakebed zone the target frequency period range is between 0 2 and 3 3 hz 0 3 and 5 0 sec we cross correlate the surface and borehole strong motion seismograms and identify body and surface wave portions in the main motions then we interpret the surface waves as fundamental mode love waves by calculating theoretical amplitude ratios for different scale structures using a statistical time series model with an information criterion we separate s and love wave portions from the surface recordings love waves are dominant and are recognized even in the early time section finally we attempt to explain s wave transfer functions between the surface and borehole recordings by considering subsidence of soft surficial deposits for several years on the basis of a three dimensional finite element technique this simulation demonstrates effects of the subsidence on amplification of s waves

waveform analysis of scholte modes in ocean sediment layers summary in an effort to determine the characteristics of seismic noise on the ocean bottom and its relationship to the structure of the sea floor we have adapted the method of nonlinear waveform fitting to accommodate multidimensional models shear velocity j and shear damping qs and have applied it to invert several records of interface waves scholte 1958 from the thumper experiment off southern california waveform fitting is a very powerful tool to determine the s velocity in the top few metres of the sediment starting from 30 m s l at the top clay layer the s velocity increases with a gradient of 2 8 m sk1 m over the first 150 m of sediment a theoretical estimation of the source strength gives coherent estimates of qs as a function of depth for distances between 400 and 1070m from the source the qs models are characterized by very low values 10 20 in the top three metres but by values in excess of 100 below that level the results confirm the identification of the noise as harmonics of interface waves in the area of this experiment the largest noise amplitudes belong to the fundamental mode and penetrate to a depth of about 20 m into the sediment the overtone energy can be appreciable too and is noticeable to about 80m depth the qs structure confirms the strong influence that the sea floor structure has on the noise spectrum the high attenuation at frequencies above 3 4 hz suppresses noise propagation and produces low noise at higher frequencies similarly high attenuation in the asthenosphere suppresses noise propagation below 0 1 hz

gravity waves in mexico city i gravity perturbed waves in an elastic solid abstract local site effects caused unprecedented damage at mexico city during the 19 september 1985 michoacan earthquake notwithstanding significant efforts observed long duration strong ground motion in the lake zone has not yet received a truly satisfactory explanation following the suggestions of gilbert 1967 and lomnitz 1988 1989 we explore in this paper the possibility of explaining this long duration in terms of gravity perturbed waves in an elastic solid after a brief review of gilbert9s model and of its limitations we use a linearization of the free surface boundary condition for nonnegligeable vertical displacements to introduce gravity in our equations we consider modifications due to gravity in rayleigh9s function and in free surface reflection coefficients and we find that rayleigh9s pole is significantly affected by gravity for very high poisson9s ratios finally we present results for the problem of ground motion simulation on the surface of an irregular thin layer of very soft clay our results show that gravity may affect rayleigh9s wave velocity by a factor larger than 2 only in the case of an extremely soft surface layer poisson9s ratio larger than 0 499999 but that the transition between rayleigh waves in solids and gravity waves in fluids suggested by lomnitz 1991 does not exist regarding mexico city we show that gravity perturbations in an elastic solid do not affect either the character of surface motion or amplitudes or duration of ground motion and thus do not provide an explanation for the long duration of strong motion in mexico city

on the use of the norwegian geotechnical institute s prototype seabed coupled shear wave vibrator for shallow soil characterization i acquisition and processing of multimodal surface waves summary pure shear wave data are only very rarely acquired for offshore site investigations and exploration here we present details of a novel seabed coupled shear wave vibrator and field data recorded by a densely populated multicomponent ocean bottom cable to improve shallow soil characterization the prototype shear wave vibrator uses vibroseis technology adopted for marine environments through its instalment on top of a suction anchor assuring seabed coupling in combination with self weight penetration the prototype is depth rated to 1500 m water depth and can be rotated while installed in the seabed the philosophy is to acquire fully complementary seismic data to conventional p and p to s converted waves in particular for 2 d profiling vsp vertical seismic profiling or monitoring purposes thereby exploiting advantages of shear waves over compressional waves for determining for example anisotropy small strain shear modulus and excess pore pressures effective stress the source was primarily designed for reservoir depths however significant energy is emitted as surface waves which provide detailedgeotechnicalinformationthroughmappingofshearwavevelocitiesinpotentiallyhigh resolution of the upper soil units to fully utilize pure shear wave content a proper analysis of surface waves is paramount due to the proximity of surface wave propagation speed with shear wave velocities the experiment was carried out in the northern north sea in 364 m water depth cable dragging was necessary to obtain close receiver spacing 2 5 m effective spacing with total line length of 600 m frequency waveform transforms reveal both scholte and love waves up to six surface wave modes are identified that is fundamental mode and several higher surface wave modes the occurrence of these two different dispersive surface wave types with well resolved higher modes allows for a unique analysis and inversion scheme for highresolution mapping of physical properties in the shallow subsurface as well as anisotropy which is discussed in an accompanying paper the data presented in this paper are thus a unique long and densely populated receiver array allows for multimodal love and scholte surface waves from the marine environment but challenging marine operations marine data set

gravity waves in mexico city ii coupling between an anelastic solid and a fluid layer abstract this paper completes a previous study of the role of gravity during the 19 september 1985 michoacan earthquake at mexico city it is assumed as suggested by lomnitz 1989 1990 that nonlinear mechanisms could bring the rheology of the mexico city soft clay layer nearer to a fluid than to a solid in order to test whether gravity waves in a viscous fluid could provide an explanation of exceedingly long duration observed at mexico city we have studied the motion on the surface of an irregular viscous fluid layer overlaying an elastic half space our formulation takes into account vertical displacement at the free surface of the fluid layer but requires a linearization of the corresponding boundary condition a decomposition of diffracted fields in terms of plane waves allows the use of aki larner9s method excitation is given by vertically incident sv waves we present results both for nonviscous and viscous fluids and show that an adequate free boundary condition is essential for this problem lacking data we have performed several simulations for values of viscosity going from 300 to 30 000 pa s in the case of a nonviscous fluid gravity surface waves are generated efficiently by diffraction of elastic waves on the irregular interface but attenuate rapidly when including viscosity and are not significant for viscosities above 3000 pa s the results presented in our companion paper and here allow us to conclude that gravity is not a likely explanation to the long duration of ground motion observed at mexico city in a more general context our model may be useful to study the interaction of seismic waves with water reservoirs such as dams

a study on the response of a soft basin for incident s p and rayleigh waves with special reference to the long duration observed in mexico city abstract the responses of two types of soft basins for incident sh sv p and rayleigh waves in a 2 d elastic half space are investigated with special reference to the long duration of strong motions observed in mexico city during the michoacan mexico earthquake of 1985 first the difficulty for simple 1 d models to reproduce the later part of the accelerogram observed in mexico city is shown then the effect of a 2 d deep basin structure of a relatively large scale 10 km wide and 1 km deep and a moderate impedance ratio 2 5 is studied the results show that surface waves generated at the edges of the basin are clearly propagating inside the basin back and forth however the increase in duration by these surface waves is 20 sec at most introducing another much softer layer inside the basin we found that the calculated durations and envelop shapes become very similar to the observed ones because of the slowness of surface wave propagation these theoretical studies suggest that the exceptionally long duration observed in mexico city might be caused by a strong constructive interaction of soft surface layers with a deep basin structure beneath the city

attenuation mechanisms in sands laboratory versus theoretical biot data the velocity and attenuation of compressional q 1p vp respectively and shear waves q 1s vs respectively determined with the pulse transmission technique at a frequency of about 100 khz are compared with the grain size shape porosity density and static frame compressibility of dry and water saturated sands except for vs all the quantities vp q 1p and q 1s are dependent on grain size and are higher in coarser grains than in finer grains q 1s decreases significantly with increasing differential pressure in coarse grained sediments but the same sediments show an anomalous increase with differential pressure in q 1p at low pressures we have also modeled the vp vs q 1p and q 1s of these samples to understand the mechanisms governing the observed changes the contact radius model with surface force effects predicts both vp and vs to be dependent on grain size frictional losses in unconsolidated coarse grained sands must also be considered at small strains 10 7 velocity and losses measured

shear wave velocities and anisotropy in the barbados accretionary complex s wave velocities at the base of the barbados accretionary wedge were determined from mode converted s waves in two vertical seismic profiles in ocean drilling program boreholes at hole 949c the profile extends from 92 to 397 m below seafloor mbsf ending 17 m into the decollement zone separating the wedge from the underthrust sediment on the subducting plate beneath at hole 948d the profile extends from 105 to 471 mbsf ending 19 m above the decollement zone s wave velocities are 303 549 m s at hole 949c and 453 679 m s at hole 948d the profiles show that the lowest 70 m of the accreted sediment just above the decollement has lower s wave velocity 379 26 m s at hole 949c and 453 11 m s at hole 948d and higher ratio of p wave to s wave velocity vp vs 4 59 0 31 at hole 949c and 3 99 0 10 at hole 948d than in the overlying layer this coincides with high clay content that has a high proportion of smectite shear wave splitting observed in records from the lowest 100 m of the accretionary wedge at both sites indicates little or no anisotropy in the low velocity zone a single measurement at hole 949c of the horizontal component of polarization of the leading split s wave from an ocean bottom shot is n68 e 20 at hole 949c split shear wave delays in the downward propagating s wave originating at a fault at 275 mbsf indicate strong anisotropy in the range 275 306 mbsf and little or no anisotropy in the range 306 380 mbsf the rate of increase of delay between 275 and 306 mbsf 0 52 0 16 ms m can be modeled by either a hudson cracked medium with crack density 0 186 0 054 or preferably a hornby model of shale with 44 of clay particles aligned within 5 of the symmetry plane this strong anisotropy is attributed to scaly fabric alignment of clay particles by pervasive shear in the fault zone which is observed in core the low anisotropy in the remainder of the section shows that there is very little overall alignment of pore space in the lower part of the accreted sediments supporting the hypothesis that the sediments are undercompacted rather than hydrofractured the low s wave velocities and high vp vs ratios indicate undercompaction and low effective stress despite the tectonic burial of the lowermost accreted sediments

seismic attenuation from recordings of ambient noise abstractwe applied seismic interferometry to data from an ocean bottom survey offshore norway and found that ambient seismic noise can be used to constrain subsurface attenuation on a reservoir scale by crosscorrelating only a few days of recordings by broadband ocean bottom seismometers we were able to retrieve empirical green s functions associated with surface waves in the frequency range between 0 2 and 0 6 hz and acoustic waves traveling through the sea water between 1 0 and 2 5 hz we discovered that the decay of these surface waves cannot be explained by geometrical spreading alone and required an additional loss of energy with distance we quantified this observed attenuation in the frequency domain using a modified bessel function to describe the cross spectrum in a stationary field we averaged cross spectra of equally spaced station couples and sorted these azimuthally averaged cross spectra with distance we then obtained frequency dependent estimates of attenuation by minimizing the misfit

agulhas current meanders facilitate shelf slope exchange on the eastern agulhas bank large solitary meanders are arguably the dominant mode of variability in the agulhas current observational studies have shown that these large meanders are associated with strong upwelling velocities and affect the shelf circulation for over 100 days per year here 10 year time series from two ocean general circulation models are used to create a composite picture of the agulhas current and its interactions with the shelf circulation in meandering and nonmeandering modes both models show good agreement with the size propagation speed and frequency of observed meanders these composite meanders are then used to examine the response of shelf waters to the onset of large meanders with the use of model output enabling the dynamics at depth to be explored results show a composite mean warming of up to 3 c of depth averaged temperature along the shelf edge associated with an intrusion of the current jet onto the shelf driven by an intensification of the flow along the leading edge of large meanders however this intensification of flow results in cooling of bottom waters driving cold events at the shelf break of 10 c at 100 m thus the intensification of the current jet associated with large meander events appears to drive strong up and downwelling events across the inshore front of the agulhas current facilitating shelf slope exchange

prograde rayleigh wave particle motion summary fundamental mode rayleigh waves generally show retrograde particle motion at the surface of the earth if there exists a thick sedimentary layer however reversal of the sign of vertical eigenfunction occurs near the surface resulting in prograde rayleigh wave particle motion at the surface we show that for structures similar to those found in the los angeles basin with thickness up to 8 km surface prograde motion may occur within the frequency band 0 05 0 3 hz although it has been suggested that the effect of gravity on waves in unconsolidated surface layer may be important partitioning of energy between the elatic and gravitational energy shows that the gravitational energy is less than 1 per cent and thus is not important the phenomenon is caused by elastic effects mainly caused by extremely slow shallow seismic velocities observation of prograde elliptical particle motion may be difficult however because particle motion is largely horizontal and high microseismic noise exists in the same frequency band

linear instability of an anticyclonic vortex in a two layer ocean the linear stability of a zero potential vorticity circular lens i e warm core ring is investigated for the case where the ocean surrounding the lens is of finite depth the hydrostatic balance across the lens interface couples the pressure variations inside the lens with those in the surrounding ocean which produces the instability it is found that in contrast to lenses in an infinitely deep ocean which are always stable lenses in a finite depth ocean are unstable for a fixed lens thickness the growth rates instability exponents increase with decreasing total depth for each azimuthal wave number there exists a total ocean depth cutoff value above which the lens is stable and below which the lens is unstable this cutoff value increases with wave number in most cases instability would occur whenever the ocean lens depth ratio is less than 3 or 4 suggesting that actual rings in the ocean may become unstable as they approach the shore where the ocean depth is relatively small

the velocity and vorticity structure of the agulhas current at 32 s the full depth velocity field of the agulhas current the western boundary current of the southwest indian ocean has been directly measured using a lowered acoustic doppler current profiler ladcp fifteen combined conductivity temperature depth ctd and ladcp stations were occupied across the current at 32 s off the east coast of south africa in february march 1995 as part of the world ocean circulation experiment indian ocean program the deep velocity structure of the agulhas current was found to be very different than previously described using geostrophic estimates in particular ladcp results reveal a v shaped pattern for the level of no motion across the agulhas current and an agulhas undercurrent is observed flowing equatorward below 800 m depth directly beneath the surface core of the poleward flowing agulhas current comparisons of direct and geostrophic velocities suggest that the velocity structure of the agulhas current is essentially geostrophic below about 200 m depth where differences are generally less than estimated errors above these depths the ladcp measured shears and geostrophic shears exhibit differences and velocities can diverge shipboard adcp data support ladcp results near the surface indicating that both instruments are measuring real signals the geostrophic and ladcp measured shears are well matched away from the surface and a depth averaged fit below 200 m between them exhibits small standard deviations the geostrophic volume transport of the agulhas current as referenced to ladcp is 73 sv and is just 3 different from the direct ladcp transport estimate the most recent full depth estimate of agulhas current transport in the literature is given by toole and warren 1993 who estimate a geostrophic volume transport of 85 sv the baroclinic velocity structures from their section and from this work are similar indicating that the difference in the transport estimates is due to the choice of geostrophic reference level and ultimately to the presence of the previously unobserved agulhas undercurrent combined ctd ladcp data are very useful for potential vorticity analysis since velocity and density measurements are simultaneous and coincident the vorticity structure of the agulhas current suggests the presence of a mixing boundary at middepths similar to features observed in the gulf stream the feature appears to be associated with a sharp upturn in isopycnals coincident with the foot of the continental slope the boundary inhibits cross stream mixing of intermediate water masses and may help explain the appearance of a discrete filament of red sea water inshore of antarctic intermediate waters in the same density layer despite intense cross stream shears

modeling of long period ground motions in the nankai subduction zone model simulation using the accretionary prism derived from oceanfloor local s wave velocity structures the accretionary prism in the subduction zone which consists of thick low velocity oceanic sediments significantly affects the propagation of seismic waves for shallow offshore earthquakes including large interplate earthquakes in order to simulate long period 5 s ground motions in the nankai subduction zone we constructed a three dimensional 3d seismic velocity structure model of the accretionary prism by interpolation extrapolation of local s wave velocity structures beneath 46 oceanfloor seismic stations donet which are deployed just above the accretionary prism off the southern kii and eastern shikoku regions we modeled local s wave velocity structures using a simple two parameter depth varying velocity function to investigate the effects of the accretionary prism on ground and seafloor motions we conducted numerical simulations of seismic wave propagation for three local earthquakes that occurred in southwestern japan the simulations reasonably reproduced the observed seismograms not only for the period ranges of the moment tensor inversion 50 s but also for the strong long period ground motions in the sedimentary basins 5 s especially in the region where donet stations are densely deployed since depth varying local s wave structures significantly improve the reproducibility of long period ground motions our modeling procedure is useful for modeling long period ground motions of local and regional offshore subduction zone earthquakes

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

the deformation of plagioclase investigated using electron backscatter diffraction crystallographic preferred orientation data abstract extensive plagioclase grain orientation datasets have been collected using the rapid and accurate electron backscatter diffraction sem technique the crystallographic preferred orientation cpo of a porphyroclastic rich plagioclase layer and an equigranular plagioclase layer in an amphibolite facies shear zone from harris scotland were investigated in order to study their cpos and deformation mechanisms the layers have random cpos together with other microstructures and misorientation distributions these suggest dynamic recrystallisation followed by dominant grain boundary diffusion creep this is surprising given the coarse 100 250 m recrystallised grain size of the layers plagioclase within tectonites of these grain sizes in the amphibolite facies are generally inferred to deform by dislocation creep from microstructures and observed strong cpos deformation mechanism maps show that this style of deformation is possible at very slow strain rates

coexistence of gravity waves and elastic waves in unconsolidated sediments based on a linear viscoelastic model we study the coexistence and transition of gravity waves and surface elastic waves in a half space of unconsolidated sediment the dispersion relation and surface response are calculated as a function of the period and rigidity of the system the dispersion relation shows two branches one branch corresponds to a rayleigh wave for short wave periods and it becomes a leaky surface wave as the period increases the second branch shows a linear dependence of the phase velocity with period as expected for gravity waves as the period increases this branch evolves into a leaky surface wave joining the first branch the surface response function which is related to the vertical displacement is also calculated two distinct peaks are present in the surface response function one corresponds to a classical rayleigh wave while the second is a gravity driven mode whose amplitude is strongly dependent on the wave period and soil rigidity

jasper seamount structure seafloor seismic refraction tomography the velocity structure of jasper seamount was modeled using one and three dimensional inversions of p wave travel times the results represent the first detailed seismic images of a submerged intraplate volcano two seismic refraction experiments were completed on jasper seamount incorporating ocean bottom seismometers and navigated seafloor shots the p wave travel times were first used to compute a one dimensional velocity profile which served as a starting model for a three dimensional tomographic inversion the seamount p velocities are significantly slower than those observed in typical oceanic crust at equivalent subbasement depths this suggests that jasper seamount is constructed predominantly of extrusive lavas with high average porosity the velocity models confirm morphological predictions jasper seamount is a shield volcano with rift zone development high seismic velocities were detected beneath the large radial ridges while low velocities characterize the shallow summit and flanks comparisons between p velocity models of jasper seamount and the island of hawaii reveal that these two shield volcanoes are not structurally proportional jasper seamount is far smaller than hawaii yet both volcanoes exhibit an outer extrusive layer of similar thickness this suggests that seamount size influences the intrusive extrusive proportions density equilibrium between melt and country rock may explain this behavior

determination of stress directions from plagioclase fabrics in high grade deformed rocks alem paraiba shear zone ribeira fold belt southeastern brazil abstract a new method to determine stress directions using the preferential orientation of plagioclase mechanical twins has been applied to high temperature mylonitic rocks from the alem paraiba shear zone ribeira fold belt southeastern brazil we have measured the lattice preferred orientation of plagioclase grains and calculated the orientation of the stress axes possible for the observed twin orientations the maximum compressive stress direction 1 determined for all studied samples is a function of the mechanical twin orientations of a number of distinct plagioclase populations the 1 direction is generally subperpendicular to the 010 plane the statistical treatment for most of the plagioclase grains examined for each sample shows that 1 is almost perpendicular to the foliation plane suggesting a significant coaxial component in the deformation process of these rocks

estimation of gas hydrate concentration from multi component seismic data at sites on the continental margins of nw svalbard and the storegga region of norway high resolution seismic experiments employing arrays of closely spaced four component ocean bottom seismic recorders were conducted at a site off western svalbard and a site on the northern margin of the storegga slide off norway to investigate how well seismic data can be used to determine the concentration of methane hydrate beneath the seabed data from p waves and from s waves generated by p s conversion on reflection were inverted for p and s wave velocity vp and vs using 3d travel time tomography 2d ray tracing inversion and 1d waveform inversion at the nw svalbard site positive vp anomalies above a sea bottom simulating reflector bsr indicate the presence of gas hydrate a zone containing free gas up to 150 m thick lying immediately beneath the bsr is indicated by a large reduction in vp without significant reduction in vs at the storegga site the lateral and vertical variation in vp and vs and the variation in amplitude and polarity of reflectors indicate a heterogeneous distribution of hydrate that is related to a stratigraphically mediated distribution of free gas beneath the bsr derivation of hydrate content from vp and vs was evaluated using different models for how hydrate affects the seismic properties of the sediment host and different approaches for estimating the background velocity of the sediment host the error in the average vp of an interval of 20 m thickness is about 2 5 at 95 confidence and yields a resolution of hydrate concentration of about 3 if hydrate forms a connected framework or about 7 if it is both pore filling and framework forming at nw svalbard in a zone about 90 m thick above the bsr a biot theory based method predicts hydrate concentrations of up to 11 of pore space and an effective medium based method predicts concentrations of up to 6 if hydrate forms a connected framework or 12 if hydrate is both pore filling and framework forming at storegga hydrate concentrations of up to 10 or 20 were predicted depending on the hydrate model in a zone about 120 m thick above a bsr with seismic techniques alone we can only estimate with any confidence the average hydrate content of broad intervals containing more than one layer not only because of the uncertainty in the layer by layer variation in lithology but also because of the negative correlation in the errors of estimation of velocity between adjacent layers in this investigation an interval of about 20 m thickness equivalent to between 2 and 5 layers in the model used for waveform inversion was the smallest within which one could sensibly estimate the hydrate content if lithological layering much thinner than 20 m thickness controls hydrate content then hydrate concentrations within layers could significantly exceed or fall below the average values derived from seismic data

defect microdynamics in minerals and solid state mechanisms of seismic wave attenuation and velocity dispersion in the mantle the propagation of seismic waves in the earth s mantle can be significantly affected by relaxation processes causing attenuation and velocity dispersion reduction this paper reviews the solid state mechanisms of relaxation processes based on the theory of defect microdynamics in solids together with some experimental observations on defects in minerals particularly in olivine for a given mechanism to have a significant effect on seismic wave propagation both the density and the mobility of the defects must be in an appropriate range the examination of the densities and geometry and mobilities of defects in olivine shows that dislocation and or grain boundary mechanisms can have a significant effect on seismic wave propagation although wide distributions of geometrical factors such as spacing of pinning points and of mobilities are required to explain all available data point defect mechanisms however are unlikely to be important because their densities are too small and or their mobilities are too large since the dislocation density and or grain size are determined in most cases by the long term tectonic stress seismic wave attenuation and velocity dispersion reduction involving these defects are likely to depend on the magnitude of the tectonic stress as well as the temperature theoretical considerations suggest a wide range of dependence of seismic wave attentuation and velocity dispersion on the long term tectonic stress this is particularly the case for dislocation mechanisms and warrants careful experimental investigation dislocations and or grain boundaries cause anelastic behavior relaxation peaks when they are pinned or blocked at some points pinning or blocking becomes ineffective at high temperatures and or low frequencies causing a transition to viscoelastic behavior both laboratory and seismological observations of internal friction are dominated by the high temperature background where internal friction increases monotonically with temperature which can be interpreted as a gradual transition to viscoelastic behavior or to a wide distribution of relaxation times however in most experimental studies to date the dislocation densities or the grain sizes were not well controlled making it difficult to identify the attenuation mechanisms and preventing any quantitative applications to earth the need for better characterization of defect microstructures in experimental specimens is emphasized

coupled seismoacoustic modes on the seafloor 1 wave to wave coupling arises when an acoustic pulse selects a rayleigh mode of the same speed and both travel together swapping energy across an interface ewing et al 1957 a distinctive signal is observed at the hawaii 2 observatory for purely oceanic paths from earthquakes on the blanco and mendocino fracture zones off the coast of north america the signal appears to be a composite of undispersed higher rayleigh modes propagating along the ocean floor both in the sediments and in the water the new coupled modes are identified by their frequency composition and their phase and group velocities seismoacoustic coupling at the seafloor is conditioned on a the presence of a low velocity interface at the ocean floor and b the wavelength of the rayleigh component being shorter than the depth of the water layer

plagioclase and quartz preferred orientations in a low grade schist the roles of primary growth and plastic deformation albite in a lower greenschist facies haast schist from new zealand has a maximum of poles to 001 parallel to the elongation direction 010 forms a girdle about grain length plastic deformation with slip dominantly on 010 001 is rejected as the orienting mechanism because 1 the relationship of crystal axes to grain length would require multiple slip systems incompatible with the lack of rotation of albite twin planes 2 euhedral terminations and other textural features indicate primary crystallization 3 albites in quartzose layers are less elongate than those in sheet silicate rich layers the albite fabric is ascribed to growth anisotropy and competition in response to stress driven solution and growth during formation of metamorphic segregation layering this explanation is compatible with the common observation that plagioclase is brittle at low metamorphic grades associated weak quartz c axis fabrics are ascribed to plastic deformation and this deformation enhanced a dimensional preferred orientation already established by primary growth

t phase observations in northern california acoustic to seismic coupling at a weakly elastic boundary plans for a hydroacoustic network intended to monitor compliance with the ctbt call for the inclusion of five t phase stations situated at optimal locations for the detection of seismic phases converted from ocean borne t phases we examine factors affecting the sensitivity of land based stations to the seismic t phase the acoustic to seismic coupling phenomenon is described by upslope propagation of an acoustic ray impinging at a sloping elastic wedge we examine acoustic to seismic coupling characteristics for two cases the first in which the shear velocity of the bottom is greater than the compressional velocity of the fluid i e vp vs vw the second is a weakly elastic solid in which vs vw vw we show that acoustic energy couples primarily to shear wave energy except at very high slope angles we show that the weakly elastic solid i e vs vw behaves nearly like a fluid bottom with acoustic energy coupling to both p and s waves even at low slope angles

p m and g tectonites a classification based on origin of mineral preferred orientations abstract the mechanisms responsible for mineral preferred orientations can be used as the basis for a classification of tectonites p tectonites develop mineral preferred orientations by plastic deformation m tectonites by mechanical reorientation of inequidimensional grains and g tectonites by various growth processes intermediate tectonite types can be defined with the aid of a triangular diagram examples discussed include marble and quartzite p types slates m or mg types certain hornblende or feldspar rich schists g types granite derived gneisses and mylonites pm types and common schists mg or pmg types the classification is designed to promote a holistic approach to fabric studies in addition the pmg triangle provides a framework for tracking the sequential fabric development of tectonites

dynamic recrystallization of feldspar a mechanism for ductile shear zone formation feldspar aggregates experimentally deformed in the dislocation creep regime undergo dynamic recrystallization because recovery is difficult due to the limited climb of dislocations recrystallizing aggregates have a lower strength because of the cyclic production of small strain free grains and they develop a strong preferred orientation consistent with that observed in mylonites thus recrystallization accommodated dislocation creep may be responsible for the grain size reduction and strain softening that lead to the formation of many mylonites and ductile shear zones

rupture dimensions and rupture processes of fluid induced microcracks in salt rock abstract we developed and applied a simple empirical green function method to study induced microcracks observed during hydraulic fracturing experiments in salt rock either unidirectional ruptures on rectangular faults or allround ruptures on elliptical faults are tested to explain the observed directivity effects in body wave amplitude spectra mostly the rectangular rupture model and horizontal fault planes are favored the average rupture lengths are between 15 and 27 mm the average rupture durations between 14 and 26 s small average rupture velocities of 30 of the s wave velocity of the rock are indicated the dispersive low frequency coda waves present in the data look similar to coda waves observed during other hydraulic fracturing experiments and to long period events from some volcanoes which have been explained by the resonance of a fluid filled crack the radiation pattern of first motion amplitudes of most events is dominated by a dip slip double couple indicating slip on horizontal or vertical planes we cannot distinguish whether the observed low frequency coda waves are influenced by a source effect or by a possible sensor borehole coupling however a simple method using gauss filter at different center frequencies and relocation is tested to analyze the low frequency coda waves in terms of source models

modeling explosion generated scholte waves in sandy sediments with power law dependent shear wave speed experimental measurements of scholte waves from underwater explosions collected off the coast of virginia beach va in shallow water are presented it is shown here that the dispersion of these explosion generated scholte waves traveling in the sandy seabed can be modeled using a power law dependent shear wave speed profile and an empirical source model that determines the pressure time series at 1 m from the source as a function of tnt equivalent charge weight

sediment characterization at the equatorial mid atlantic ridge from p to s teleseismic phase conversions recorded on the pi lab experiment accurate marine sediment characteristics for example thickness and seismic velocity are important for constraining sedimentation rates with implications for climate variations and for seismic imaging of deeper structures using ocean bottom seismic deployments we analyze p to s seismic phase conversions from the sediment crust boundary recorded by the passive imaging of the lithosphere asthenosphere boundary pi lab experiment to infer the sediment thickness across the mid atlantic ridge covering 0 to 80 myr old seafloor we find p d s p delay times of 0 04 0 37 s or 5 to 82 m thickness sediment thickness increases with age thickness agrees with global estimates for young 15 20 myr seafloor but is thinner on older lithosphere our result may represent a lower limit on sediment thickness given that several of our stations are on topographic highs the sedimentation rate decrease observed from 5 to 1 2 mm kyr at 10 myr suggests a recent increase in productivity related to climate change eolian dust fluxes and or biogenic marine activity

why are the meanders of the north atlantic current stable and stationary a simple 1 5 layer reduced gravity model of an f plane jet with a symmetric cross section is employed to explain the stability and stasis of meanders in the north atlantic current the main factors affecting the current s stability are the highly contrasting water masses on both its sides and the current s vertical density structure the stability and stasis of the nac results from 1 the near surface shoaling of the deep isopycnals inshore of the subpolar front and 2 a small internal radius of deformation which is due mainly to the small upper to lower layer density difference in the absence of unstable modes the dominant wavelength of the meanders is determined by the bathymetry so the nac simply follows those contours

determining paleo flow directions from groundmass fabrics in the lyttelton radial dykes new zealand abstract the poles to 010 of groundmass feldspar in radial dykes cutting the dissected miocene volcano lyttelton banks peninsula form generally oblique girdles about the paleo flow direction the obliquity is a response to velocity gradients of magma within the dykes and the sense of obliquity relative to each dyke wall provides a unique determination of the sense of flow the fabrics give support to the model of blanchard et al 1979 for the movement of rigid particles in flowing magmas this method of flow determination overcomes the problem inherent in most others which provide a flow line but no sense of movement along that line methods of sampling and examples of the feldspar fabrics are described together with the relationship of these fabrics to other dyke features such as secondary shears and stretched vesicles

p and s wave delays beneath intraplate volcanic ridges and gravity lineations near the east pacific rise the gravity lineations intraplate melting petrology and seismic expedition glimpse explored the origin of two intraplate non hot spot ridge systems associated with gravity lineations in the south central pacific using the 11 month ocean bottom seismometer obs deployment of the glimpse experiment we determined the average p and s station delays for the region across the array station delays varied by 1 22 s for p waves and 2 13 s for s waves the presence of ultraslow shear velocity

tectonic implications of exposure of lower continental crust beneath the iberia abyssal plain northeast atlantic ocean geophysical evidence we present new seismic data from a basement high beneath the southern iberia abyssal plain that appears from the latest ocean drilling program odp results and indirectly from various seismic observations to consist of lower continental crustal material this is a unique opportunity to use seismic profiles and other geophysical evidence to investigate the tectonic process which led to the exhumation of these rocks we infer that the lower crust was exhumed at the seafloor and then uplifted toward the end of rifting our results lead to the following new observations low angle detachment faults previously reported along the east west lusigal 12 profile across the southern iberia abyssal plain penetrate both the 3 10 km thick continental crust and unusually the uppermost mantle processing of new multichannel seismic reflection profiles across the basement high on which odp sites 900 1067 and 1068 are located reveals that the high is bounded by a previously unsuspected steep landward dipping normal fault on its east flank basement cores from the above odp sites also reveal that the high is not capped by the previously predicted early syn rift sediment but by rocks from the lower continental crust these unexpected observations are incorporated into a new tectonic model of the development of this part of the west iberia margin which is also consistent with other geophysical observations a novel feature of this model is the proposal that the seaward edge of the continental crust is thinned to 3 6 km by a currently poorly understood process the model implies three stages of deformation 1 lithospheric extension principally by symmetric pure shear which leads to a 10 km thick crust in which the lower crust is largely absent over the axial zone and the crust mantle boundary forms a shear zone 2 further thinning and then dissection of the most distal continental crust by seaward dipping low angle normal faults 3 inception of a high angle landward dipping normal fault that offsets the tectonized crust mantle boundary and uplifts the lower crust to the crest of the basement high on which odp sites 900 1067 and 1068 were drilled

regional path effects on seismic wave propagation in central mexico we present results of the analysis of a set of seismograms recorded in a temporary array of seven broadband seismic stations located in central mexico our purpose was to investigate path effects on ground motion in central mexico for earthquakes occurring in the subduction zone along the pacific coast all our stations were located on rock nineteen earthquakes 3 6 m c 4 9 recorded by all seven stations were analyzed the results show that path effects result in significant amplification and an increase of the duration of the seismic motion at two stations located in the central part of the transmexican volcanic belt tvb analysis of the records for periods larger than 5 sec shows that the 3d crustal structure acts as an efficient wave guide for surface waves coming from distances greater than 200 km only for paths oriented north south or east west in the band of 2 6 sec we observe large amplification and an increase in the duration of ground motion associated to the regional propagation of higher modes of surface waves after correlating our results with recent geological and gravimetric studies we suggest that this amplification could be due to the impedance contrast of low velocity volcanic rocks of the tvb overlying higher velocity limestones if we are right the increase of ground motion duration would result from mode conversion at the southern boundary of the tvb these path effects in the 2 to 6 sec period band are very important for ground motion in mexico city because the very soft surficial deposits in that basin amplify greatly ground motion in this same period band

a comparison between p wave and s wave propagation characteristics in the southern part of the gulf of california mexico abstract we performed a regional study of p seismic wave attenuation and source excitation from moderate size earthquakes recorded at distances ranging from 23 to 120 km off the coast of la paz baja california sur from a remarkably productive earthquake sequence that occurred in 2007 data were recorded from four strong motion seismic stations using the la paz seismic network and two broadband stations located close to the bay of la paz we compared our results with a previously reported study in the same region with virtually the same method from peninsular earthquakes using s lg seismic waves the regional attenuation was carried out by measuring the maximum amplitude of the p waves after separating the excitation and site terms from the propagation term d r f our best model of the p quality factor is q p f 53 f 0 32 in addition offshore s and scholte modes in ocean sediment layers are discussed in this article