a new gps acoustic method for measuring ocean floor crustal deformation application to the nankai trough 1 we developed a new geodetic method of monitoring crustal deformation on the ocean floor the measurements were conducted at two sites beneath the pacific ocean near the nankai trough where the philippine sea plate subducts into the pacific plate the first site was at a depth of 700 m in suruga bay and the second one was at a depth of 2000 m in the kumano basin at each survey site three seafloor transponders whose positions were repetitively measured were deployed to define a benchmark unit an ultrasonic signal was generated from a surface vessel drifting over the benchmark unit that was transmitting and receiving the signal and the replies from the transponders both sound speed structure and the benchmark unit positions were determined simultaneously using a tomographic technique we repeatedly carried out measurements over the two sites for the kumano basin we made six measurements from july 2004 to august 2005 the benchmark unit location s horizontal precision was 5 cm and its vertical precision was 10 cm for suruga bay we made five measurements and the positions were less accurate with horizontal variation of 7 cm and vertical variation of 16 cm we investigated the reason for the decreased precision in suruga bay with a numerical experiment and the results revealed that the geometry of the measurement that is the vessel s path is one of the most important variables in our method at the kumano basin a 21 5 cm southward displacement of the benchmark unit was detected just below the site before and after a large earthquake mw 7 5 our observation system therefore proved itself capable of detecting seafloor crustal deformation associated with crustal activities in offshore areas

out of sequence thrust faults developed in the coseismic slip zone of the 1946 nankai earthquake mw 8 2 off shikoku southwest japan a multi channel seismic mcs reflection survey was conducted to study the subsurface structure of the coseismic slip zone of the 1946 nankai earthquake mw 8 2 off shikoku island in 1997 we could identify a splay fault system consisting of several sigmoid out of sequence thrust ost faults dipping landward with slope 10 25 on the poststack depth migrated mcs profiles most of the osts are apparently developed from the subducting oceanic basement to the seafloor in the forearc region cutting both underthrust sediments and the overriding accretionary prism in addition most of the osts are within the coseismic zone of the 1946 nankai earthquake and the interseismic locked zone the osts are considered to be related to large interplate earthquakes including the 1946 nankai earthquake these osts may be responsible for tsunami generation following deformation of forearc accretionary wedge

precise gps acoustic positioning of seafloor reference points for tectonic studies abstract global networks for crustal strain measurement provide important constraints for studies of tectonic plate motion and deformation to date crustal strain measurements have been possible only in terrestrial settings on continental plates and island sites within oceanic plates we report the development of technology for horizontal crustal motion determination at seafloor sites allowing oceanic plates to be monitored where islands are not available seafloor crustal monitoring is an important component of global strain measurement because about 70 of the earth s surface is covered by water and this region contains most of the tectonic plate boundaries and zones of crustal deformation using the global positioning system gps satellites and underwater acoustics we have established a geodetic reference site on the juan de fuca plate at 2 6 km depth approximately 150 km off the northwest coast of north america we measure the baselines between this site and two terrestrial gps stations on vancouver island british columbia the juan de fuca plate site is an appropriate setting to develop seafloor observation methods since it is a well studied area easily accessible from west coast canadian and united states ports determination of seafloor motion at this site addresses questions related to convergence between the juan de fuca and north american plates across the cascadia subduction zone at the juan de fuca seafloor geodetic reference site we installed precision acoustic transponders on the seafloor and measured ranges to them from a sound source at a surface platform ship or buoy the platform is equipped with a set of three gps antennas allowing determination of the sound source position at times of signal transmission and reception merging the satellite and acoustic data allows determination of the transponder network location in global reference frame coordinates data processing to date suggests repeatabilities of 0 8 cm north and 3 9 cm east in the seafloor transponder network position relative to reference points on vancouver island

evidence of a laterally variable lower mantle structure from p and s waves summary examples of grafenberg array data showing anomalous p waves which typically arrive 3 5s after the direct p wave and which have a slowness 0 7 0 8 sdeg smaller than direct p are presented this additional phase is most frequently observed for events located in the ne portion of the southern kurile island subduction zone 73 80 from grafenberg but systematically disappears for events in the sw portion of this zone because of the magnitude of the slowness difference these observations cannot be attributed to a complex source rupture process nor to multipathing through the descending slab likewise they may not be accounted for by near receiver structure because these phases are not seen for all kurile events if present they appear at all stations of the array but they follow direct p too closely to be a multiple from the moho therefore we conclude they are very likely caused by lower mantle velocity structure the most likely explanation is the presence of a p velocity jump of about 3 per cent approximately 290 km above the core mantle boundary since such a reflector in the lowermost mantle not only gives a good fit of traveltimes and slowness but is also able to model the waveform and the amplitudes of this additional p phase the distribution of bounce points on this reflector for the kurile events indicates a lateral extension of this velocity anomaly under northern siberia of about 150 km by at least 200 km the best fitting s wave model has a reflector in the same depth but the velocity contrast seems to be only about 2 per cent suggesting a different behaviour of the p and s velocity in d few events from other regions in this distance range are suitable for a definitive analysis of this kind from among this group some observations indicate a lower mantle anomaly under the lomonosow ridge and under northern greenland but since the lower mantle under western siberia northern novaya zemlya the azores islands region and the ussr afghanistan border region does not produce an additional phase in the grafenberg recordings it is very unlikely that such a velocity anomaly in the lowermost mantle is a global feature

observations of body wave multipathing from broadband seismograms evidence for lower mantle slab penetration beneath the sea of okhotsk we present observations of anomalously broad and complex sh waveforms from two deep focus earthquakes in the southern kurils to digital world wide standard seismograph network and regional seismic test network stations in north america by making use of broadband three component seismograms and relying on a variety of phases one can limit the class of possible explanations for these phases in particular it is shown that the anomalous arrivals are not due to source complexity near receiver structure a triplication from a lower mantle velocity gradient a low q zone in the mid lower mantle or shear wave splitting we show that these arrivals correspond to relatively fast paths and have enhanced amplitudes indicating that they are most probably caused by multipathing due to interaction with a high velocity lower mantle feature below the source region we examine the hypothesis that this feature may be a lower mantle extension of the kuril slab ray tracing experiments show that multipathing is indeed expected along the azimuths corresponding to the north american stations used in this study for a realistic slab model observations of reversed polarity arrivals are consistent with such an explanation where it appears that two rays comprising the arrival emerge from different points on the focal sphere one test of this explanation is that slab induced multipathing should not be seen along back of slab oceanic azimuths examinations of seven events for which the stations are in this azimuth range do not exhibit the same broadening and complexity of the sh waveforms identification of slab induced multipathing associated with deep focus events has two important consequences first it appears to require slab penetration beyond the 650 km discontinuity at least 300 400 km into the lower mantle under the sea of okhotsk second these phases may act as a contaminant when attempting to study other seismic phenomena such as discontinuities low q zones near receiver structure or source complexity

magnetic model of the subduction zone in the northeast japan arc abstract magnetic modeling of the forearc side of the tohoku northeastern japan arc was carried out using spectral analysis of shipborne magnetic data the original data show anomalies caused by volcanic rocks on the western shallow side and linear marine magnetic anomalies on the eastern deeper side throughout the forearc region as calculated from temperature gradient data of heat flow measurements and deep drilling the curie depth is deeper than the moho on the eastern deeper side this suggests that the moho may correspond to the base of the magnetic layer because analysis of upper mantle xenolith suites indicates that the upper mantle should be non magnetic in addition the conrad discontinuity may correspond to the apparent top of magnetic layer because the lower crustal layer would be more mafic than the upper layer it was estimated by 2d spectral analysis that the magnetic base and surface lie at about 20 km and 10 km respectively this suggests that the moho and the conrad may roughly correspond to the magnetic base and surface the magnetic anomalies in the pacific basin are traced towards and over the inner side of the trench and show a gradual decrease in amplitude disappearing eventually about 100 km west of the trench this reflects the descent of the oceanic slab the 1d spectral analysis indicates that the marine magnetic layer is descending at an angle of about 4 in the japan trench this angle increasing to 20 degrees below the coast where the marine magnetic layer reaches a depth of more than 30 km this agrees with the seismic reflection data from the trench and because the magnetic layer can be traced down to a depth of 30 km it may be suggested that the curie isotherm deepens to more than 30 km just east of the coast

first measurement of the displacement rate of the pacific plate near the japan trench after the 2011 tohoku oki earthquake using gps acoustic technique the subduction rate of an oceanic plate may accelerate after large earthquakes rupture the interplate coupling between the oceanic and overriding continental plates to better understand postseismic deformation processes in an incoming oceanic plate we directly measured the displacement rate of the pacific plate near the japan trench after the 2011 tohoku oki earthquake using a gps acoustic technique over a period of 2 years september 2012 to september 2014 the displacement rate was measured to be 18 0 4 5 cm yr 1 n302 0 e relative to the north american plate which is almost twice as fast as the predicted interseismic plate motion because the sum of steady plate motion and viscoelastic response to the tohoku oki earthquake roughly accounts for the observed displacement rate we conclude that viscoelastic relaxation is the primary mechanism responsible for postseismic deformation of the pacific plate and that significant subduction acceleration did not occur at least not during the observation period

suboceanic geodetic measurements this paper reviews the needs for geodetic ties among seafloor points and between such points and others on land as well as the environmental constraints on potential systems for making such measurements underwater acoustic techniques provide the best opportunities and an essential element on which to build acoustic systems a precision transponder system capable of making travel time measurements over ranges of several kilometers with an accuracy of a few microseconds is described finally three particular systems are discussed one uses direct transmission between transponders to achieve one part in 105 accuracy over ranges from a few meters to about a kilometer the second using a larger transponder network and an intermediate vehicle can achieve similar accuracy over ranges up to about 10 km the third is a composite acoustic global positioning system gps which should be able to achieve subdecimeter accuracy over ranges of a few hundred kilometers

the crustal structure in iberia inferred from p wave coda abstract the crustal velocity structure at four locations in the iberian peninsula is inferred from p wave coda recorded by broad band stations the crustal responses of the stations are derived from the p wave coda of deep teleseismic events and the most pronounced features of the p and s velocity structure are obtained by waveform modeling of the vertical and radial component of these responses the method is applied to data of nars stations in toledo alicante and granada in spain and manteigas in portugal the optimum models of the waveform inversions are carefully evaluated and compared to results of deep seismic sounding experiments in general the depths of large velocity gradients are well resolved but the actual values of the seismic velocities are less constrained

short period body wave constraints on properties of the earth s inner core boundary in this study we model short period waveform data in an attempt to constrain the p and s velocity structure at the earth s inner core boundary icb the data set consists of recordings from 10 events in the south pacific and the data selection criteria as well as methods of analysis were designed to avoid problems with receiver response often associated with short period waveform data these data are modeled using a calculation technique that facilitates the consideration of a wide variety of models for the icb results suggest that q in the inner core has the frequency dependence proposed by doornbos 1983 the data determine the p velocities above and below the icb to within a trade off that is well constrained by the data for example with a p velocity structure above the icb given by prem the p velocity below the icb is 11 03 0 03 km s similarly for the data analyzed here the estimate of the s velocity at the top of the inner core trades off with the estimate of q but we use the q model of doornbos 1983 to estimate the jump in s velocity to be 3 1 km s

partitioned waveform inversion and two dimensional structure under the network of autonomously recording seismographs nonlinear optimization methods are very effective in finding models that predict optimal waveform fits to observed seismograms provided the number of model parameters is small to reduce the size of the large scale two or three dimensional seismic modelling problem path integrals over seismic velocities may be defined as model parameters instead of the seismic velocities themselves this partitions the large scale optimization problem into a number of independent small scale problems and reduces the computational effort by several orders of magnitude the path integral parameters are transformed such that their correlation matrix becomes diagonal a linear inversion performs the mapping from the space of path integral parameters into the model space of seismic velocities and makes backus gilbert resolution analysis possible the method has been applied to body wave and surface wave data from the broadband portable network of autonomously recording seismographs nars in western europe using a surface wave ray approximation to generate synthetics by mode summation waveforms were fitted for frequencies as high as 50 mhz variations in s velocity as large as 6 1 have been established with a vertical resolution of 100 150 km and a horizontal resolution as small as 400 km in parts of the model although a low velocity layer is present under the southern part of the nars array such layer is very weak or absent north of the paris basin horizontal velocity gradients are well resolved down to 400 km depth

a lower mantle s wave triplication and the shear velocity structure of d summary a lower mantle s wave triplication detected with short and longperiod wwssn and csn recordings indicates a substantial shear velocity discontinuity near 280 km above the core mantle boundary the triplication can be observed in rotated sh seismograms from intermediate and deep focus events throughout the distance range from 70 to 95 three distinct source region receiver array combinations that have been investigated in detail demonstrate consistent travel time and relative amplitude behaviour of the triplication with slight systematic shifts in the triplication indicating up to 40km variations in the depth of the discontinuity modelling of the observations with synthetic seismograms produced with the cagniard de hoop and reflectivity methods constrains the shear velocity increase to be 2 75 k 0 25 per cent comparable to upper mantle discontinuities short period observations indicate that the velocity increase may be a sharp first order discontinuity or may extend over a transition zone no more than 50 km thick the shear velocity gradient below the discontinuity within the d layer is not well constrained by the sh data but slightly positive or near zero velocity gradients are consistent with the long period amplitude ratios of scsh sh

moho depth and poisson s ratio in the western central alps from receiver functions summary current knowledge about deep crustal structure of the alpine orogen has mainly been derived from p wave velocity models obtained from active and passive seismic experiments a complementary s wave model to provide lithological constraints necessary for unique structural interpretation has been missing to date in this paper we present important new information on s wave velocity model in the alps we applied the receiver function method using 6 yr of high quality data from 61 permanent and temporary stations sampling the western central alps we determined first order crustal features moho depth h and average vp vs ratio with the h stacking technique that uses timing of direct and multiple p to s converted phases from the moho interface synthetic tests reveal a dipping moho interface expected beneath an orogen causes a systematic bias of h and potentially leading to misinterpretation we thus applied corrections determined from synthetic data to remove the bias providing better fit of recovered moho depths with active seismic estimates for each site we also obtained independent h and estimates based on the timing of the strong ps phase our results show a gently south southeast dipping european moho at a depth of 24 30 km beneath the northern alpine foreland steepening rapidly towards the europe africa suture zone to reach a maximum depth of 55 km south of the suture the moho of the adriatic crust promontory of the african plate is at 35 45 km depth in the previously ill constrained western alps we found the european moho at 30 km depth beneath the more external units dipping east northeast to reach 50 55 km in the inner core of the alps the poisson s ratio clearly correlates with the tectonic units that comprise the alps average crustal values in the european alpine foreland are close to 0 25 in the alps we observe low values 0 22 in the highly deformed nappes of the mesozoic helvetic and southern alps indicating a thickening of felsic upper crustal material in contrast the poisson s ratio is significantly higher 0 26 in the penninic and austroalpine units near the suture zone this rapid and significant change marks a clear rupture between the alpine forelands and the suture domain we assign this high poisson s ratio to doubling of mafic lower crust consistent with results from previous active seismic experiments a continuation of the lower crustal wedge into the central part of the western alps however seems unlikely based on low observed poisson s ratios

western nankai trough seismogenic zone results from a wide angle ocean bottom seismic survey the nankai trough southwestern japan is recognized as a vigorous seismogenic zone with well studied historic earthquakes this paper presents results of a wide angle ocean bottom seismographs obs study at the western nankai trough seismogenic zone the obs data used were acquired on a profile 250 km long across the presumed coseismic slip zone of the 1946 nankaido earthquake ms 8 2 the main purpose of the seismic study is to obtain an entire crustal cross section of the seismogenic zone for the 1946 earthquake the crustal model is characterized by a gentle sloping of subducting oceanic crust and thick overlying sedimentary wedge p wave seismic velocities of the subducting oceanic crust show normal oceanic crustal velocities vp 5 0 5 6 km s and 6 6 6 8 km s in oceanic layers 2 and 3 respectively the maximum thickness of the sedimentary wedge is 9 km at 70 km from the trough axis with vp 3 4 4 6 km s in the deeper part the subducting oceanic crust traced down to 25 km depth shows that the subduction angle becomes steeper landward 3 2 and 7 2 at 0 50 km and 50 100 km from the trough axis respectively the oceanic crust is smooth to the hypocenter zone down to 40 km depth beneath shikoku island our crustal model shows that the downdip limit of the coseismic slip area does not extend to the deep end of the oceanic crust island arc crust contact zone even though there is large uncertainty about the seaward limit of the coseismic slip zone the crustal model clearly indicates that the updip limit of the coseismic slip zone extends beneath the young accretionary prism

modelling of near surface seismic structure beneath hawaii using reverberations summary the reverberation in near surface layers contains valuable information about the near surface velocity and density structure however previously the reverberation was often regarded as noise to be eliminated this paper presents an approach for modelling the shallow velocity structure with the reverberation in the near surface layers the method was applied to three component waveform data recorded by seismic arrays during a passive seismic experiment arrays for lithosphere observations in hawaii aloha the near surface multiple reflections were enhanced by stacking the seismograms of local upper mantle earthquakes with near vertical incidence angles by matching synthetics and observed p and s multiple reflections we have successfully modelled the impedance structure for the top 2 3km of the crust beneath seven stations of aloha arrays in southern hawaii reverberative site effects are similar for stations in an array but vary significantly from one array to another reflecting the lateral heterogeneity of the near surface structure two short profiles of near surface p wave velocity structure were constructed across the ainapo and south flank areas in southern hawaii based on interpretation of the multiple reflections recorded by two of the aloha arrays the profiles reveal the detailed and variable shallow structure near two active volcanoes kilauea and mauna loa and provide basic information about site effects for further investigations of seismic hazard assessment earthquake source properties and deeper structure

slab remnants beneath the baja california peninsula seismic constraints and tectonic implications the formation of the gulf of california has been related to the cessation of subduction of the guadalupe and magdalena microplates various studies have identified features that point to the presence of a slab remnant beneath the baja california peninsula but its depth range and lateral extent remained unclear in this study we used surface wave phase velocity and receiver function data of nars baja stations around the gulf of california to better constrain the location of the slab for stations in central and southern baja california the shear velocity models show an upper mantle high velocity layer with its top in the depth range from 115 to 135km and a thickness varying between roughly 40 and 60km these high velocity anomalies are interpreted as subducted slab remnants in contrast the models for the northern peninsula show no slab signature this change directly correlates with the variation in relative motion between the baja california peninsula and the pacific plate as measured by gps data it is inferred that the stalled slab fragments beneath the peninsula produce strong coupling between baja california and the pacific plate the shear velocity models for stations on the mexican mainland show a layer of higher velocities above a low velocity upper mantle in the north the low velocity mantle starting at a depth of 40km is associated with upwelling as suggested by previous studies further south the transition from higher to lower mantle velocities occurs around 80km depth which is a typical value for the lithosphere asthenosphere boundary furthermore the models show strong crustal thinning towards the gulf both from the peninsula as well as from the mexican mainland

crustal thickness estimation using high frequency rayleigh waves the method of nonlinear waveform fitting was extended to include changes in the depth of the moho studies performed on synthetic data at 10 epicentral distance showed that it is theoretically possible to determine moho depth with an accuracy of less than a kilometer if the frequency range extends to 0 12 hz we applied this method on several broadband records from two earthquakes in western united states to invert for crustal thickness waveform fitting of rayleigh waves for epicentral distances between 3 and 10 for individual paths in this region show that the average moho depth can be resolved to 1 2 km it was possible to obtain good waveform fits for frequencies between 0 02 0 08 hz we conclude that broadband seismic data from local events allow us to determine moho depth independently from crustal shear wave velocity

scattering of teleseismic waves in the lower crust observations in the massif central france abstract high frequency coda signals consistently recorded by a temporary seismic network of 29 short period stations during a 6 month survey in the french massif central are concordant with independent evidence for a heterogeneous lower crust obtained from wide angle and near vertical reflection experiments in the same region the teleseismic recordings of 22 events have been analysed in record sections rather than as single station seismograms following the low frequency lf 0 5 1 5 hz first p arrival a high frequency hf coda 2 4 hz dominant frequency extends over several seconds duration the hf signals become clearly visible after bandpass filtering but can also be identified in the original seismograms in event sections the hf coda forms a consistent pattern of reverberations which is characterized by the following properties 1 the duration is typically longer on the radial 4 15 s than on the vertical 3 11 s component 2 the beginning of the hf coda referred to the picked lf first p arrivals varies between 0 2 5 s with a dominance around 1 5 2 s 3 the amplitude of the hf coda is about 3 10 of the primary lf p phase amplitude 4 the signals are incoherent between neighbouring stations and over the network 5 the hf coda arrives dominantly around the plane of incidence of the teleseismic wave in the first 2 s to maximum 4 s afterwards a widening of the particle motion to an elliptical shape is observed indicating arrivals off the sagittal plane the data parameters 1 to 5 point to the conclusion that the hf signals are generated by a scattering process wide angle and cdp reflection experiments in the same region measured a similar reverberating coda pattern and located the origin also in the lower crust these observations and the data parameters of the hf coda suggest that the hf teleseismic waves are scattered at heterogeneities in the lower crust in the massif central in this contribution we mainly concentrate on the presentation of the teleseismic recordings and conclude with a preliminary structural model which contains randomly distributed scatterers in the lower crust

the crustal structure from teleseismic p wave coda ii application to data of the nars array in western europe and comparison with deep seismic sounding data summary the crustal structure beneath a seismic station has a large influence on the p wave coda recorded by that station in this study we employ the vertical and radial component of the crustal receiver response to determine the most important features of the crustal velocity structure beneath stations of the nars array the receiver response is estimated from the p wave coda of teleseismic events by deconvolution with a source wavelet and by stacking responses of different events the crustal velocity structure at the station is derived from these data by non linear waveform inversion the responses of some of the nars stations show anomalous features such as an apparent delay of the first arrival on the radial component relative to the onset on the vertical component this appears to be a combined effect of very low velocities in the top layer of the model and a strong velocity discontinuity in the uppermost part of the crust a high amplitude coda on the radial component is observed for stations on a structure with strong s velocity gradients in the upper crust the receiver responses of the nars stations are generally well modelled by the synthetics of the final models of the inversions the method provides an adequate procedure to estimate the dominant effects of the crustal structure at the site with the models representing the most significant velocity gradients of the crustal structure

large scale waveform inversions of surface waves for lateral heterogeneity 2 application to surface waves in europe and the mediterranean linear surface wave scattering theory is used to reconstruct the lateral heterogeneity under europe and the mediterranean using surface wave data recorded with the network of autonomously recording seismographs nars the waveform inversion of the phase and the amplitude of the direct surface wave leads to a variance reduction of approximately 40 and results in phase velocity maps in the period ranges 30 40 s 40 60 s and 60 100 s a resolution analysis is performed in order to establish the lateral resolution of these inversions using the phase velocity perturbations of the three period bands a two layer model for the s velocity under europe and the mediterranean is constructed the s velocity perturbations in the deepest layer 100 200 km are much more pronounced than in the top layer 0 100 km which confirms that the low velocity zone exhibits pronounced lateral variations in both layers the s velocity is low under the western mediterranean while the s velocity is high under the scandinavian shield in the deepest layer a high s velocity region extends from greece under the adriatic to northern italy several interesting smaller features such as the massif central are reconstructed one of the spectacular features of the reconstructed models is a sharp transition in the layer between 100 and 200 km near the tornquist tesseyre zone this would indicate that there is a sharp transition at depth between central europe and the east european platform the waveform inversion of the surface wave coda leads to good waveform fits but the reconstructed models are chaotic this is due both to a lack of sufficient data for a good imaging of the surface wave energy on the heterogeneities and to an appreciable noise component in the surface wave coda

shallow structure effects on broadband teleseismic p waveforms the first 5 sec of source equalized p wave receiver functions from an array of broadband portable instruments located about 50 km from the coast in western washington are strongly influenced by very shallow structure depths less than 3 km these data display behavior associated with a dipping high velocity contrast interface at shallow depths the direct p wave and the p to s converted phase vary strongly with backazimuth producing an apparent delay in first arriving energy at western backazimuths these patterns are attributed to a shallow contact between low velocity sandstones at the surface and an underlying high velocity volcanic unit this shallow structure influences even the original unprocessed teleseismic p waves suggesting that such shallow structure could affect other studies such as seismic source analysis utilizing broadband data

automatic 1 d waveform inversion of marine seismic refraction data summary full waveform synthetic seismograms are now used as standard in the interpretation of marine refraction data but only with a laborious trial and error fitting procedure we show how the matching of observed waveforms with wkbj synthetic seismograms can be efficiently and automatically performed for 1 d earth models automation of the inversion is difficult because of the multi modal irregular form of the misfit of data and synthetics a sequence of three steps is required for a complete inversion location of the deepest valley of the misfit function descent to the global minimum and description of the neighbourhood of the minimum for error analysis the first step is accomplished with a monte carlo search through a very large model space defined by poor prior knowledge of velocities and gradients of the solution weak traveltime constraints are used to eliminate poorly fitting models from waveform calculations a comparison of the traveltime and waveform misfits of the monte carlo models clearly illustrates that waveforms are providing more information than traveltimes alone bayesian statistics are used to construct marginal probability distributions and the covariance matrix which give a rough preliminary error analysis in the second step damped least squares linearized inversion makes small adjustments to the best fitting monte carlo model as it descends to the global minimum finally the immediate neighbourhood of the global minimum is explored with constrained least squares inversion realistic error bounds on each parameter are defined from the resulting slices through the misfit function these bounds are much narrower than traveltime inversion provides correlations between parameters are obtained from the covariance matrix constructed from models examined during this error analysis the inversion methods are illustrated on the ff2 refraction data set of the scripps institution of oceanography the monte carlo search successfully locates the valley of the global minimum as well as a nearby secondary minimum the error analysis puts realistic error bounds on the detailed inversion of this data set by spudich orcutt

offshore postseismic deformation of the 2011 tohoku earthquake revisited application of an improved gps acoustic positioning method considering horizontal gradient of sound speed structure

sediment structure at grf from polarization analysis of p waves of nuclear explosions near vertical incidence angles and quasi elliptical particle motions characterize broadband p waves from nuclear explosions recorded at the three 3 component stations of the grafenberg array grf in southern germany which is situated on the jurassic sediments of the franconian jura comparison with data from the broadband stations khc ka perske hory czech republic and ksp ksiaz poland located on gneiss in the bohemian massif and precambrian sediments respectively show that the observed effect is caused by local receiver conditions polarization analysis shows that p to sv wave conversions at the basement sediment boundary and multiples in the sedimentary layers are mainly responsible for these observations at the grf array borehole and refraction data confirm low p wave velocities in the sediments under the grf stations by modeling the broadband particle motions of the p waves of nuclear explosions average models for the sedimentary layers below the grf stations are derived the main modifications to the model of aichele 1976 are one grf c1 and two low velocity layers grf b1 and grf a1 between the out cropping limestone and the basement the first low velocity layer has an s wave velocity of about 1 0 km sec and a thickness of about 200 m below the three 3 component stations and a p wave velocity about 2 8 km sec which is not well constrained the second low velocity layer underlying the first low velocity layer at the stations grf b1 and grf a1 has a lower p wave velocity of about 2 4 km sec but a higher s wave velocity of about 1 6 km sec together with the surficial limestone layer about 200 m thick the total thickness of the sediment column increases from about 400 m below grf c1 in the south to about 750 m below grf b1 and to about 1150 m below grf a1 in the north slow sediment layers near the surface are important for the polarization of teleseismic p waves in turn polarization analysis of broadband recordings can be used to analyze the structure of shallow sediment layers

shear wave statics using receiver functions summary we extend the seismological receiver function method to the exploration seismic domain to solve the problem of shear wave statics for multicomponent data the method relies on crosscorrelation or deconvolution of pressure with radial component traces in the common receiver domain followed by a stacking step this procedure is repeated for each receiver resulting in a profile of high resolution stacked receiver functions events corresponding to shallow modeconverted waves constrain the shear wave static the method is applied to a line of commercial multicomponent seabed seismic data and gives results in good agreement with a conventional statics method although results along some other lines require additional interpretation in contrast to seismological receiver function analysis the main converted wave energy in the receiver functions originates from reflection at shallow interfaces rather than conversion of upgoing wavefields

regional variation of magnetization of oceanic crust subducting beneath the nankai trough 1 we present the characteristics of marine magnetic anomalies in the nankai trough and the northern shikoku basin the marine magnetic anomalies are clearly visible over the shikoku basin but gradually disappear across the nankai trough the expected reason for the decrease in the marine magnetic anomaly amplitudes across the trough is the increase in depth of the magnetic source the subducting oceanic crust forward and inverse magnetic modeling results which take into account the slab depth distribution show variations in magnetization that cannot be explained solely by increasing depth of the slab we identify five zones along the trough axis on the basis of the pattern of the magnetic amplitude profile two ridges a seamount chain and two basement zones the magnetic inversion analysis indicates cyclic intensity variations along the ridge zones the largest contrast in magnetization along the n s lineation before the deformation front occurs off ashizuri as low temperature chemical oxidation effects are most likely the main cause of the reduction in magnetization there are considerable differences in the original carrier of magnetic intensity in the as and kii peninsula kp zones and the original carrier in as is characterized by much higher original magnetic intensities this indicates several possibilities the oceanic crust originated from asymmetric spreading there was magma injection during the late stage volcanism or the as zone was affected by an anomalous heat source it would be concluded from the current data that the basement in as originally had a much stronger magnetic carrier and suffered stronger low temperature oxidation effects than the eastern part of the shikoku basin the regional differences in intensity strongly suggest a variety of magnetic minerals and phase changes associated with temperature and mechanical dislocation

trench normal variation in observed seafloor displacements associated with the 2011 tohoku oki earthquake 1 seafloor geodetic measurements using the gps acoustic technique just above the rupture area of the 2011 tohoku oki earthquake revealed strong trench normal variation in the horizontal crustal displacement associated with the earthquake one set of observations at a site only 50 km away from the trench yielded the most trench ward data ever reported with an accuracy better than 1 m it showed a coseismic displacement of up to 31 m whereas another set of observations 100 km away from the trench showed a 15 m displacement these horizontal data as well as vertical data from pressure gauges and tsunami observations strongly indicate that the seismic slip reached the trench they are also likely to contribute quantitatively to further investigations that reveal the coseismic behavior of the frontal wedge of the overriding plate in the source region along the japan trench

viscoelastic relaxation following subduction earthquakes and its effects on afterslip determination afterslip is commonly thought to be the controlling process in postseismic deformation immediately following a great megathrust earthquake and is usually inferred from geodetic observations using purely elastic models however observed motion reversal of the near trench area right after the 2011 mw 9 tohoku oki earthquake demonstrates the dominance of viscoelastic relaxation of coseismically induced stresses to understand the importance of incorporating viscoelasticity in afterslip determination we employ biviscous burgers mantle rheology and use finite element models to explore how viscoelastic relaxation in short term postseismic deformation is controlled by various geometrical and rheological factors our results indicate that immediately after large megathrust earthquakes mw 8 0 viscoelastic deformation should always cause opposing motion of inland and trench areas and subsidence around landward termination of the rupture although the rate of such postseismic motion depends on local conditions such as the age and hence thickness of the slab and transient mantle viscosity values while elastic models may be adequate for afterslip estimation for earthquakes of mw 7 5 neglect of viscoelasticity for larger events leads to overestimate of afterslip downdip of the rupture and underestimate of afterslip at shallower depths reassessing shallow afterslip following the 2005 mw 8 7 nias earthquake using 2 d viscoelastic models suggests that the actual afterslip may be greater than that estimated using an elastic model by more than 50 similarly interpreting trenchward motion of some seafloor gps sites following the tohoku oki earthquake using a viscoelastic model suggests large shallow afterslip outside of the main rupture area

a deep strong reflector in the nankai accretionary wedge from multichannel seismic data implications for underplating and interseismic shear stress release 1 we present the results of structural and stratigraphic interpretations for the multichannel seismic profiles to reveal the structure of the nankai accretionary wedge off shikoku island on the basis of reflection characteristics we identify three major seismic units that correspond to a unit consisting of both oligocene miocene accretionary prism and the sedimentary cover unit a a unit of underthrust miocene pliocene hemipelagic sediments unit b and a unit of subducting pre miocene oceanic crustal rocks unit c according to relative strength of the topmost reflector of unit b the nankai accretionary wedge can be divided into three different structural zones along the cross strike direction apparent decollement reflector zone low amplitude reflector lar zone and deep strong reflector dsr zone despite the velocity uncertainty involved in the depth conversion and therefore thickness estimation error the decollement step down and the possible duplex structures in the lar and dsr zones support that the landward thickness variation of unit b is caused by the structural thickening such as the underplating process structural features and reverse polarity suggest that the dsr as an upper interface of the underplated layer unit b corresponds to a shear plane or a fault zone with high pore fluid pressures we found that most of the dsr was observed in a region which has not suffered from the 1946 coseismic dislocation even though it was locked during the interseismic period the dsr as a shear plane or a fault zone constrains a regional variety of the coseismic dislocation around the updip limit of the nankai seismogenic zone considering all of the dsr characteristics we propose a hypothesis for the dsr s development and role the dsr may play an important role as a possible releaser of the shear stress energy stored at the plate boundary around the updip limit of the nankai seismogenic zone a good correlation between the theoretical vertical displacement and seafloor topographic feature support the possible slip along the dsr as a shear plane

seismic evidence for an ancient rift beneath the cumberland plateau tennessee a detailed analysis of broadband teleseismic p waveforms broadband receiver functions developed from teleseismic p waveforms recorded on the midperiod passband of regional seismic test network station rscp are inverted for vertical velocity structure beneath the cumberland plateau tennessee the detailed broadband receiver functions are obtained by stacking source equalizd horizontal components of teleseismic p waveforms the resulting receiver functions are most sensitive to the shear velocity structure near the station a time domain inversion routine utilizes the radial receiver function to determine this structure assuming a crustal model parameterized by many thin flat lying homogeneous layers lateral changes in structure are identified by examining azimuthal variations in the vertical structure the results reveal significant rapid lateral changes in the midcrustal structure beneath the station that are interpreted in relation to the origin of the east continent gravity high located northeast of rscp the results from events arriving from the northeast show a high velocity midcrustal layer not present in results from the southeast azimuth this velocity structure can be shown to support the idea that this feature is part of a keweenawan rift system another interesting feature of the derived velocity models is the indication that the crust mantle boundary beneath the cumberland plateau is a thick probably laminated transition zonemore between the depths of 40 and 55 km a result consistent with interpretations of early refraction work in the area less

comprehensive analysis of traveltime data collected through gps acoustic observation of seafloor crustal movements we have developed a comprehensive inversion scheme to analyze traveltime data collected through gps acoustic observations on a campaign basis our method uses a quantity called the nadir total delay ntd which is analogous to the zenith total delay ztd in gnss analyses to represent the time variation of the sound speed the observation equation using the ntd is an approximated formulation we examine its applicable scope using numerical experiments and demonstrate that the approximation holds well enough in practice traveltime data from all our observation campaigns were utilized together for inversion calculation to determine 1 the positions of individual precision acoustic transponders pxps at the time of a particular reference campaign 2 the displacement from these positions of a rigid array at the time of each campaign and 3 the time variation of the ntd during each campaign we applied this method to actual data to fix the array geometry more precisely also we have made a first step in detecting vertical motions as well as horizontal motions though further enhancement of the accuracy will be required in performing linear regression of array displacements obtained from the inversion their error covariance has a significant effect on the resulting velocities when the covariance is taken into account nearly the same velocities are obtained no matter what campaign is taken as reference in the inversion

effect of sinusoidal interfaces on teleseismic p wave receiver functions summary using the t matrix or extended boundary condition method we model seismic wave scattering from earth models with irregular interfaces this is motivated by numerous observations that surface and subsurface topography is responsible for the amplification and de amplification of seismic waves and the generation of coda waves we demonstrate that irregular moho and free surface scattering significantly affects teleseismic p wave receiver function waveforms receiver functions are routinely used to obtain detailed crustal and upper mantle structure for a layer over halfspace model we perform a sensitivity study on the effects of ray parameter interface roughness and wavelength receiver position and incident wave frequency content the free surface responses vary significantly with receiver position interface wavelength and with incident wave frequency content using frequency wavenumber analysis the coda is shown to contain scattered rg waves and s waves receiver functions are formed from the free surface responses by deconvolution and these are then inverted as synthetic data for plane layered velocity structure to show how scattered energy maps onto the final velocity models spurious structure models result with the amplitude of the spurious features for example low velocity zones decreasing with interface roughness the resulting velocity models also vary appreciably with receiver position

the slip distribution of the 1946 nankai earthquake estimated from tsunami inversion using a new plate model abstract a new model for the shape of the philippine sea plate psp subducting beneath southwest japan was constructed by combining marine seismic survey results and seismicity data this resulted in a plate model that is generally 3 5 km shallower than previous estimates and contains new details of plate boundary structure near the nankai trough axis we estimated the slip distribution of the 1946 nankai earthquake by inverting tsunami waveforms using the new plate boundary model and found that the large slip off cape muroto and seismic moment were reduced by about 10 from the previous estimate of tanioka and satake 2001a however we also found reason to suspect the tide gauge closest to the epicenter at shimotsu and this resulted in a much more substantial reduction in the amount of slip in the eastern segment of the rupture area we also find that our new rupture model correlates well with features of the subducting plate boundary such as a large subducting seamount off cape muroto kodaira et al 2000a this suggests that such features have an important effect on either earthquake rupture propagation or the interseismic accumulation of elastic strain energy or both

the influence of non regularly decaying coda wave envelopes upon the estimation of coda q the single isotropic scattering model proposed for the origin of seismic coda predicts a regularly decaying coda wave envelope in real data however this regular decay is often interrupted by non random arrivals e g reflections coming later in the coda this additional energy may be due to clustered scatterers and represents a departure from the inherent assumption in the theory that the scatterers are uniformly randomly distributed we have simulated the effect of clustered scatterers in synthetic seismograms in order to investigate their influence upon the measured coda q qc we show that the position and amplitude of such arrivals within the otherwise regularly decaying coda give rise to elevated qc estimates often this additional energy in the coda is difficult to detect on a single isolated seismogram as evidenced by an example from the 1994 northridge california earthquake we therefore recommended that where possible the use of seismic arrays commonly reserved for p wave coda studies be extended to s wave coda studies to aid in the detection of clustered scatterers prior to the calculation of qc

the crustal structure from teleseismic p wave coda i method summary the crustal structure beneath a broad band station has a strong influence on the recorded body wave coda conversely it is evident that crustal receiver effects can be employed to obtain information about the crustal structure at the receiver site in this paper we present a method to estimate the crustal receiver response and we invert the response to obtain a model of the most distinct features of the crustal structure beneath the station the crustal receiver response of a station is determined from the p wave coda of deep teleseisnic events by deconvolution with an empirically determined source wavelet this source wavelet is assumed to incorporate source attenutation and instrument effects responses obtained from clusters of events are stacked to enhance the signal to noise ratio of the data and to obtain an estimate of the variance of the data the crustal p and s velocity structure at the station is determined by non linear waveform inversion of its crustal responses an inversion strategy was developed to search efficiently for the global minimum in the misfit between data and synthetics the inversion appears to be most sensitive to large velocity constrasts in the velocity structure which give rise to high amplitude reflected and converted phases the method is illustrated by the inversion of the crustal responses of nars station ne05 it is shown that a thin layer 1 2 km of unconsolidated sediments and a strong mid crustal discontinuity at 11 km depth constitute the most prominent features of the crustal structure at this station they cause an apparent delay of the first arrival on the radial component of the seismograms and produce high amplitude sv multiples in the p wave coda

crustal structure beneath broad band seismic stations in the mediterranean region summary we have analysed receiver functions to derive simple models for crustal structure below 12 broad band seismological stations from the midsea project and 5 permanent broad band stations in the mediterranean region including northern africa to determine an accurate moho depth we have reduced the trade off between crustal velocities and discontinuity depth using a new grid search method which is an extension of recently published methods to determine crustal thickness in this method the best fitting synthetic receiver function containing both the direct conversion and the reverberated phases is identified on a model grid of varying moho depth and varying poisson s ratio the values we found for moho depth range from around 20 km for intra oceanic islands and extended continental margins to near 45 km in regions where the eurasian and african continents have collided more detailed waveform modelling shows that all receiver functions can be well fit using a 2 or 3 layer model containing a sedimentary layer and or a mid crustal discontinuity on comparing our results with moho maps inferred from interpolated reflection and refraction data we find that for some regions the agreement between our receiver function analysis and existing moho maps is very good while for other regions our observations deviate from the interpolated map values and extend beyond the geographic bounds of these maps

geodetic data inversion using a bayesian information criterion for spatial distribution of fault slip summary we developed a new inversion method to reconstruct static images of seismic sources from geodetic data using akaike s bayesian information criterion abic coseismic surface displacements are generally related with a slip distribution on a fault surface by linear integral equations parametric expansion of the fault slip distribution by a finite number of known basis functions yields a set of observation equations expressed in a simple vector form incorporating prior constraints on the smoothness of slip distribution with the observation equations we construct a bayesian model with unknown hyperparameters the optimal values of the hyperparameters which control the structure of the bayesian model are objectively determined from observed data by using abic once the values of hyperparameters are determined we can use the maximum likelihood method to find the optimal distribution of fault slip we examined the validity of this method through a numerical experiment using theoretical data with random noise we analysed geodetic data associated with the 1946 nankaido earthquake ms 8 2 by using this method the result shows that the fault slip distribution of this earthquake has two main peaks of 4 and 6 m located off kii peninsula and muroto promontory these two high slip areas are clearly separated by a low slip zone extending along kii strait such a slip distribution corresponds with the fact that the rupture process of this earthquake in the western part is notably different from that in the eastern part