evidence for widespread nonlinear strong ground motion in the mw 6 9 loma prieta earthquake we exploit 55 repeating microearthquake sequences on the san andreas fault just south of the rupture zone of the 1989 m w 6 9 loma prieta earthquake to search for time dependent properties of the earth s crust using moving window waveform cross correlation we identify clear and systematic delays as large as 20 msec for the direct s wave and exceeding 50 msec in the early s wave coda following the loma prieta mainshock others have also identified phase delays velocity reductions associated with damaging earthquakes and they have suggested a myriad of possible causal mechanisms here we present new evidence for a mechanism to produce velocity reductions correlated in time and space with an earthquake a strong correlation between the spatial patterns of s delays and the intensity of strong ground motion in the loma prieta earthquake suggests that physical damage the formation or growth of cracks during strong ground motion to the earth s shallow crust is responsible for the observed velocity reductions the strong spatial variability in s delays over short distances and the strong correlation of the magnitude of delays with surface geology indicate that the phase delays accumulate primarily near the receiver the effect is stronger at stations on young soft rocks than at stations on old hard rock disproportionately larger s coda delays than p coda delays suggest that the cracks formed by the strong shaking are fluid filled in the 10 years after loma prieta the initial increase in travel times reduces logarithmically with respect to time often back to the premainshock levels we attribute this behavior to the same slow dynamic healing observed in laboratory studies of the recovery of materials from transient nonlinear strain manuscript received 14 january 2004

temporal changes of shallow seismic velocity around the karadere duzce branch of the north anatolian fault and strong ground motion we analyze temporal variations of seismic velocity along the karadere duzce branch of the north anatolian fault using seismograms generated by repeating earthquake clusters in the aftershock zones of the 1999 mw7 4 izmit and mw7 1 duzce earthquakes the analysis employs 36 sets of highly repeating earthquakes each containing 4 18 events the events in each cluster are relocated by detailed multi step analysis and are likely to rupture approximately the same fault patch at different times the decay rates of the repeating events in individual clusters are compatible with the omori s law for the decay rate of regional aftershocks a sliding window waveform cross correlation technique is used to measure travel time differences and evolving decorrelation in waveforms generated by each set of the repeating events we find clear step like delays in the direct s and early s coda waves sharp seismic velocity reduction immediately after the duzce main shock followed by gradual logarithmic type recoveries a gradual increase of seismic velocities is also observed before the duzce main shock probably reflecting post seismic recovery from the earlier izmit main shock the temporal behavior is similar at each station for clusters at various source locations indicating that the temporal changes of material properties occur in the top most portion of the crust the effects are most prominent at stations situated in the immediate vicinity of the recently ruptured fault zones and generally decrease with normal distance from the fault a strong correlation between the co seismic delays and intensities of the strong ground motion generated by the duzce main shock implies that the radiated seismic waves produced the velocity reductions in the shallow material

seismic guided waves trapped in the fault zone of the landers california earthquake of 1992 a mobile seismic array of seven stations was deployed at 11 sites along the fault trace of the m7 4 landers earthquake of june 28 1992 with a maximum offset of 1 km from the trace we found a distinct wave train with a relatively long period following the s waves that shows up only when both the stations and the events are close to the fault trace this wave train is interpreted as a seismic guided wave trapped in a low velocity fault zone to study the distribution of amplitude of the guided waves with distance from the fault trace and also their attenuation with travel distance along the fault zone we eliminated source and recording site effects by the coda normalization method the normalized amplitudes of guided waves show a spectral peak at 3 4 hz which decays sharply with distance from the fault trace spectral amplitudes at high frequencies 8 15 hz show an opposite trend increasing with distance from the fault trace the normalized amplitudes of guided waves at 3 4 hz also show a systematic decrease with hypocentral distance along the fault zone from which we infer an apparent q of 50 in order to confirm the existence of the guided waves a dense array of 31 stations was deployed at one of the 11 sites the resultant records revealed unequivocal evidence for the existence of guided waves associated with the fault zone by modeling the waveforms as s waves trapped in a low velocity waveguide sandwiched between two homogeneous half spaces with velocity vs 3 0 km s we infer a waveguide width of about 180 m a shear velocity of 2 0 2 2 km s and a q of 50 hypocenters of aftershocks with clear guided waves show a systematic distribution both laterally and with depth delineating the extent of the low velocity fault zone in three dimensions we find that the zone extends to a depth of at least 10 km this zone apparently continues to the south across the pinto mountain fault because guided waves are observed at stations north of the pinto mountain fault for earthquakes with epicenters south of it on the other hand the zone appears to be discontinuous at the fault bend located about 20 km north of the mainshock epicenter guided waves were observed for stations and epicenters which are located on the same sides of the fault bend but not for those on the opposite sides

a shallow fault zone structure illuminated by trapped waves in the karadere duzce branch of the north anatolian fault western turkey summary we discuss the subsurface structure of the karadere duzce branch of the north anatolian fault based on analysis of a large seismic data set recorded by a local passcal network in the 6 months following the mw 7 4 1999 izmit earthquake seismograms observed at stations located in the immediate vicinity of the rupture zone show motion amplification and long period oscillations in both p and s wave trains that do not exist in nearby off fault stations examination of thousands of waveforms reveals that these characteristics are commonly generated by events that are well outside the fault zone the anomalous features in fault zone seismograms produced by events not necessarily in the fault may be referred to generally as fault zone related site effects the oscillatory shear wave trains after the direct s arrival in these seismograms are analysed as trapped waves propagating in a low velocity fault zone layer the time difference between the s arrival and trapped waves group does not grow systematically with increasing source receiver separation along the fault these observations imply that the trapping of seismic energy in the karadere duzce rupture zone is generated by a shallow fault zone layer traveltime analysis and synthetic waveform modelling indicate that the depth of the trapping structure is approximately 3 4 km the synthetic waveform modelling indicates further that the shallow trapping structure has effective waveguide properties consisting of thickness of the order of 100 m a velocity decrease relative to the surrounding rock of approximately 50 per cent and an s wave quality factor of 10 15 the results are supported by large 2 d and 3 d parameter space studies and are compatible with recent analyses of trapped waves in a number of other faults and rupture zones the inferred shallow trapping structure is likely to be a common structural element of fault zones and may correspond to the top part of a flower type structure the motion amplification associated with fault zone related site effects increases the seismic shaking hazard near fault zone structures the effect may be significant since the volume of sources capable of generating motion amplification in shallow trapping structures is large

reconstruction of 7500 years of coastal environmental change impacting seagrass ecosystem dynamics in oyster harbour sw australia abstract seagrass ecosystems which have important functions such as coastal protection and blue carbon sequestration are threatened by anthropogenic pressure including climate change long term data series from seagrass sedimentary archives mats can be used to understand natural cycles of environmental change and answer key questions related to contemporary management a 7500 yr sediment record from posidonia australis meadows in oyster harbour albany sw australia was subjected to multiproxy reconstruction by means of pigment analysis uhplc analytical pyrolysis py gc ms carbonate content 13c and 15n stable isotope ratios organic c corg content corg n ratio and glomalin related soil proteins grsp the study revealed a brackish lagoon 7500 7000 cal yr bp that was transformed in an open marine environment 7000 4100 cal yr bp due to holocene transgression earliest evidence of seagrass establishment was detected around 4500 cal yr bp and meadow extension accelerated between 4100 and 3700 cal yr bp the meadow environment was surprisingly resistant against environmental perturbations as the mat composed of p australis seagrass fibres embedded within a siliciclastic mineral matrix containing biogenic carbonates continued to develop steadily until 190 cal yr bp 1830 ce then shifts in several proxies pigments grsp showed evidence of terrestrial runoff triggered eutrophication turbidity likely driven by forest clearance and agricultural activities after european settlement but the seagrass showed resilience no decline of the proportion of seagrass derived corg by contrast since 1930 ce seagrass retreat is evident in the biogeochemical record lighter 13c values lower lignin abundance and shifts in pigment abundance and types affecting the balance between seagrass inputs and alternative sources as was observed in previous studies of the area the findings show that pigment proxies are useful early indicators of shifts in seagrass ecosystem condition while lignocellulose and other pyrolysis products are useful proxies of more profound ecosystem alterations that influence seagrass abundance the record indicates that the climax seagrass ecosystem condition which prevailed for several millennia had been impacted over the last century management of seagrass and coastal ecosystems should aim to avoid crossing ecological thresholds and diminish local impacts aggravating those of global change

nonlinear ground motion amplification by sediments during the 1994 northridge earthquake it has been known since at least 1898 ref 1 that sediments can amplify earthquake ground motion relative to bedrock for the weak ground motion accompanying small earthquakes the amplification due to sediments is well understood in terms of linear elasticity hooke s law 2 but there has been a long standing debate regarding the amplification associated with the strong ground motion produced by large earthquakes the view of geotechnical engineers based largely on laboratory studies is that hooke s law breaks down at larger strains causing a reduced nonlinear amplification seismologists on the other hand have tended to remain sceptical of this nonlinear effect mainly because the relatively few strong motion observations seemed to be consistent with linear elasticity although some recent earthquake studies have demonstrated nonlinear behaviour under certain circumstances3 4 the significance of nonlinearity for the type of stiff soil sites found in the greater los angeles region remains unresolved5 here we report that ground motion amplification due to sediments for the main shock of the 1994 northridge earthquake was up to a factor of two less than the amplification observed for its aftershocks these observations imply significant nonlinearity in such amplification and bring into question the use of measurements of weak ground motion to predict the strong ground motion at sedimentary sites

long term carbon storage and its recent loss in an estuarine posidonia australis meadow albany western australia abstract oyster harbour on the south coast of western australia supports 3 6 3 9 km 2 of seagrass meadows following the loss of approximately 2 8 3 1 km 2 in the 1980s this small area of prevailing meadows hold significant carbon stores accumulated over the past 3000 years in this study we sampled three sediment cores from a posidonia australis meadow and analysed organic matter om organic carbon oc and inorganic carbon ic contents in bulk sediments and 13 c signatures of om the om and oc contents mean se in the cores were 9 07 0 36 and 2 24 0 05 respectively the mean ic content was 3 16 0 17 13 c signatures of the sedimentary om ranged from 10 01 to 13 28 using a bayesian isotopic mixing model it is estimated that 57 67 of the om in the seagrass sediments was derived from p australis detrital matter the total carbon tc stores in 150 cm thick seagrass sediments averaged 27 92 kg tc m 2 10 79 kg oc m 2 and 17 13 kg ic m 2 based on radiocarbon dating the mean sediment accumulation rate was 0 0494 cm yr 1 which led to a long term tc accumulation rate of 8 92 g tc m 2 yr 1 3 45 g oc m 2 yr 1 and 5 47 g ic m 2 yr 1 based on historical seagrass cover 3 6 6 7 km 2 during the 1960s to 1980s the estimated tc stores in 150 cm thick seagrass sediments at oyster harbour would have been 101 187 gg tc the eutrophication driven loss of seagrasses during the 1980s resulted in the absence of oc accumulation capacity amounting to 280 310 mg oc over 29 years the loss of seagrass area could also have resulted in the release of 37 41 gg co 2 assuming that all of the oc in shallow sediment is remineralised after meadow disturbance these results exemplify the importance of seagrasses meadows as important carbon sinks and the potential for losses of carbon stores due to ecosystem degradation

observation and modelling of fault zone fracture seismic anisotropy i p sv and sh travel times summary three component vsp borehole seismograms taken in the vicinity of an active normal fault in california show strong systematic shear wave splitting that increases with proximity to the fault using cerven s method of characteristics for ray tracing in anisotropic heterogeneous media and hudson s formulation of elastic constants for media bearing aligned fractures we have fitted a suite of p sv and sh hanging wall and foot wall travel times with a simple model of aligned fractures flanking the fault zone the dominant fracture set is best modelled as parallel to the fault plane and increasing in density with approach to the fault the increase in fracture density is non uniform power law or gaussian with respect to distance to the fault although the hanging wall and the foot wall rock are petrologically the same unit the fracture halo is more intense and extensive in the hanging wall than in the foot wall upon approach to the fault plane the fracture density or fracture density gradient becomes too great for the seismic response to be computed by hudson cerven procedures the maximum fracture density that can be modelled is about 0 08 within this 25 m fracture domain it appears more useful to model the fault and near field fractures as a low velocity waveguide we observe production of trapped waves within the confines of the intense fracture interval

rapid determination of near fault earthquake deformation using differential lidar improved near field measurements of earthquake slip and deformation patterns have the potential for expanding our understanding of fault behavior and the relationship of active faulting to topography current techniques for obtaining these measurements including field observation global navigation satellite systems displacement estimation and optical or radar remote sensing have limitations that can be mitigated by the inclusion of results from differential airborne light detection and ranging lidar analysis of the rupture zone the 2005 airborne lidar survey of the southern san andreas san jacinto and banning faults the b4 survey mapped 1100 km of the most seismically active fault systems in southern california for the purpose of providing a baseline for determining slip from a future earthquake we used the b4 survey to develop a processing algorithm that yields rapid estimates of near fault ground deformation using simultaneous cross correlation of both topography and backscatter intensity from pre earthquake and simulated postearthquake lidar datasets we show robust recovery of the direction and magnitude of an applied synthetic slip of 5 m in the horizontal and 0 5 m in the vertical within our area of study with clear discrimination between areas with and without applied slip we also successfully recovered more complex deformation from a modeled fault stepover in the same study area our results indicate that we should be able to recover slip to accuracies of better than 20 cm in the horizontal and 1 cm in the vertical at a spatial resolution of 15 m for lidar datasets with sample densities as low as 0 5 points m2

numerical simulation of fault zone guided waves accuracy and 3 d effects fault zones are thought to consist of regions with reduced seismic velocity when sources are located in or close to these low velocity zones guided seismic head and trapped waves are generated which may be indicative of the structure of fault zones at depth observations above several fault zones suggest that they are common features of near fault radiation yet their interpretation may be highly ambiguous analytical methods have been developed to calculate synthetic seismograms for sources in fault zones as well as at the material discontinuities these solutions can be used for accurate modeling of wave propagation in plane parallel layered fault zone structures however at present it is not clear how modest deviations from such simplified geometries affect the generation efficiency and observations of trapped wave motion as more complicated models cannot be solved by analytical means numerical methods must be employed in this paper we discuss 3 d finite difference calculations of waves in modestly irregular fault zone structures we investigate the accuracy of the numerical solutions for sources at material interfaces and discuss some dominant effects of 3 d structures we also show that simple mathematical operations on 2 d solutions generated with line sources allow accurate modeling of 3 d wave propagation produced by point sources the discussed simulations indicate that structural discontinuities of the fault zone e g fault offsets larger than the fault zone width affect significantly the trapping efficiency while vertical properly gradients fault zone narrowing with depth small scale structures and moderate geometrical variations do not the results also show that sources located with appropriate orientations outside and below a shallow fault zone layer can produce considerable guided wave energy in the overlying fault zone layer

properties of seismic fault zone waves and their utility for imaging low velocity structures a two dimensional solution for the scalar wave equation in a model of two vertical layers between two quarter spaces is used to study properties of seismic waves in a laterally heterogeneous low velocity structure the waves referred to as seismic fault zone waves include head waves internal fault zone reflections and trapped waves the analysis aims to clarify the dependency of the waves on media velocities media attenuation coefficients layer widths and source receiver geometry additional calculations with extreme low velocity layers provide examples that may be relevant for volcanic and geothermal domains the interference patterns controlling seismic fault zone waves change with the number of internal reflections in the low velocity structure this number increases with propagation distance along the structure decreases with fault zone width and increases for given length scales with the velocity contrast the relative lateral position of the source within the low velocity layer modifies die length scales associated with internal reflections and influences the resulting interference pattern low values of q affect considerably the dominant period and overall duration of the waves thus there are significant tradeoffs between propagation distance along the structure fault zone width velocity contrast source location within the fault zone and q the lateral and depth receiver coordinates determine the particular point where the interference pattern is sampled and observed motion is a strong function of both coordinates the zone connecting sources generating fault zone waves and observation points with appreciable wave amplitude can be over an order of magnitude larger than the fault zone width calculations for cases with layer p wave velocity of 200 m s 1 modeling a vertical dike or crack with fluid and gas show conspicuous persisting oscillations the results resemble aspects of seismic data in volcanic domains if these waves exist in observed records their explicit recognition and modeling will help to separate source and structural effects and aid in the interpretation of volcano seismology signals although the tradeoffs in parameters governing seismic fault zone waves are significant each variable has its own signature and the parameters may be constrained by additional geophysical data simultaneous modeling of many waveforms with an appropriate solution can resolve the various parameters and provide a high resolution structural image

depth dependent structure of the landers fault zone from trapped waves generated by aftershocks we delineate the internal structure of the johnson valley and kickapoo faults landers southern rupture at seismogenic depth using fault zone trapped waves generated by aftershocks trapped waves recorded at the dense linear seismic arrays deployed across and along the surface breaks of the 1992 m7 5 landers earthquake show large amplitudes and dispersive wave trains following the s waves group velocities of trapped waves measured from multiple band pass filtered seismograms for aftershocks occurring at different depths between 1 8 km and 8 2 km show an increase in velocity with depth velocities range from 1 9 km s at 4 hz to 2 6 km s at 1 hz for shallow events while for deep events velocities range from 2 3 km s at 4 hz to 3 1 km s at 1 hz coda normalized amplitude spectra of trapped waves peak in amplitudes at 3 4 hz for stations located close to the fault trace the amplitude decays rapidly with the station offset from the fault zone normalized amplitudes also decrease with distance along the fault giving an apparent q of 30 for shallow events and 50 for deep events we evaluated depth dependent fault zone structure and its uncertainty from these measurements plus our previous results from near surface explosion excited trapped waves li et al 1999 in a systematic model parameter searching procedure using a three dimensional 3 d finite difference computer code graves 1996 our best model of the landers fault zone is 250 m wide at the surface tapering to 100 150 m at 8 2 km depth the shear velocity within the fault zone increases from 1 0 to 2 5 km s and q increases from 20 to 60 in this depth range fault zone shear velocities are reduced by 35 to 45 from those of the surrounding rock and also vary along the fault zone with an increase of 10 near ends of the southern rupture zone

source array analysis of coda waves near the 1989 loma prieta california mainshock implications for the mechanism of coseismic velocity changes coda waves are often considered to be generated by backscattering of primary waves from randomly distributed heterogeneities in the crust however modeling and experimental work indicates that much of the coda may be due to scattering near the receiver knowing where the coda is generated is important for several reasons coda waves have been used to characterize site amplification scattering and attenuation throughout the crust and velocity changes associated with large earthquakes however a lack of knowledge of where coda waves are generated can lead to ambiguous interpretations in this study we analyze 26 s of coda waves recorded at up to 78 stations using slowness stacking on two source arrays and find that at nearly all stations regardless of distance from the source nearly all the arrivals are within one hemisphere most are strongly clustered near the direct arrival and the clusters are in general symmetrically disposed about the array axis fault plane this finding suggests that the coda consists primarily of waves scattered near the stations rather than waves scattered throughout the crust this result holds even at stations very close to the sources where the amount of coda examined is about 6 times the direct s travel time and it suggests that much of the coseismic velocity decrease associated with the 1989 loma prieta earthquake ellsworth et al 1992 occurred in the shallow crust near the stations one possible mechanism for inducing such a change is a decrease in the mean stress from the mainshock slip however on the basis of static stress modeling we find that this mechanism would produce a velocity increase in the region where velocity was observed to decrease we conclude that the velocity decrease was more likely caused by crack opening or crack coalescence due to strong shaking from the mainshock

quantitative analysis of seismic fault zone waves in the rupture zone of the 1992 landers california earthquake evidence for a shallow trapping structure summary we analyse quantitatively a waveform data set of 238 earthquakes recorded by a dense seismic array across and along the rupture zone of the 1992 landers earthquake a grid search method with station delay corrections is used to locate events that do not have catalogue locations the quality of fault zone trapped waves generated by each event is determined from the ratios of seismic energy in time windows corresponding to trapped waves and direct s waves at stations close to and off the fault zone approximately 70 per cent of the events with s p times of less than 2 s including many clearly off the fault produce considerable trapped wave energy this distribution is in marked contrast with previous claims that trapped waves are generated only by sources close to or inside the landers rupture zone the time difference between the s arrival and trapped waves group does not grow systematically with increasing hypocentral distance and depth the dispersion measured from the trapped waves is weak these results imply that the seismic trapping structure at the landers rupture zone is shallow and does not extend continuously along strike by more than a few kilometres synthetic waveform modelling indicates that the fault zone waveguide has depth of approximately 2 4 km a width of approximately 200 m an s wave velocity reduction relative to the host rock of approximately 30 40 per cent and an s wave attenuation coefficient of approximately 20 30 the fault zone waveguide north of the array appears to be shallower and weaker than that south of the array the waveform modelling also indicates that the seismic trapping structure below the array is centred approximately 100 m east of the surface break

factors controlling 13c contents of sedimentary organic compounds principles and evidence the carbon isotopic composition of any naturally synthesized organic compound depends on 1 the carbon source utilized 2 isotope effects associated with assimilation of carbon by the producing organism 3 isotope effects associated with metabolism and biosynthesis and 4 cellular carbon budgets these factors are reviewed and quantitative considerations summarized particularly with regard to active and passive modes of carbon assimilation by phytoplankton and the existence of discernible regularities in isotope effects associated with lipid biosynthesis it is concluded that n alkyl lipids are in general depleted in 13c by about 1 5 relative to polyisoprenoid lipids produced by the same organism the effects of biological reworking on isotopic compositions of organic carbon are examined and it is suggested that enrichment of 13c in sedimentary organic carbon may result from loss of ch4 from zooplanktonic gut communities isotopic methods for estimation of ancient co2 levels are considered and a hyperbolic form favored for the relationship between concentrations of dissolved co2 and isotopic fractionation

characterization of soils beneath a posidonia oceanica meadow the study of a 475 cm core spanning 4316 y bp sampled in a posidonia oceanica meadow portlligat bay nw mediterranean allowed us to make the first detailed description of the soil below this endemic seagrass the sediments under p oceanica often referred to as mat or matte are low density average bulk density of 0 69 g dw cm 3 marine soils mainly composed of siliciclastic 46 and biogenic carbonated 46 fine grained sediments particles 1 mm and finer organic matter 3 of som 1 mm the mat is heterogeneous and has a high total organic matter tom content in the upper layers average of 32 in the top 50 cm after ca 500 years of burial after which the layers become increasingly dominated by inorganic fractions ranging from 20 to 1 of tom in 50 475 cm the tom content in the mat decreases exponentially at an overall rate of 0 0005 y 1 p oceanica sediments have been found to contain the highest areal stocks of tom and organic c 194 kg dw tom m 2 and 79 kg corg m 2 respectively out of all seagrasses the average corg refractory burial rates were estimated to be 21 g corg m 2 y 1 carbon as carbonates accreted at a rate of 54 g ccarb m 2 y 1 for the mat thickness studied the two c fractions yielded a total stock that is among the highest ever recorded in terrestrial and marine soils 282 kg ctot m 2 the mat was also found to trap large amounts of sediment rate 899 g dw m 2 y 1 stock 3342 kg dw m 2 and in particular muddy silt and clay fractions rate 120 g dw m 2 y 1 stock 417 kg dw m 2 the results obtained in this study provide sound additional proof of the valuable role p oceanica plays in stabilizing coastal sediments and sequestering c and also of its potential as a holocene palaeorecord the p oceanica mats located at shallow depths 2 m can be tentatively classified as a limnic subaquatic histosols calcaric eutric world reference base for soil resources 2007

damage to the shallow landers fault from the nearby hector mine earthquake crustal faults have long been identified as sites where localized sliding motion occurs during earthquakes which allows for the relative motion between adjacent crustal blocks although there is a growing awareness that we must understand the evolution of fault systems on many timescales to relate present day crustal stresses and fault motions to geological structures formed in the past fault zone damage and healing have been documented quantitatively in only a few cases we have been monitoring the healing of damage on the shallow johnson valley fault after its rupture in the 1992 magnitude 7 3 landers earthquake and here we report that this healing was interrupted in 1999 by the magnitude 7 1 hector mine earthquake rupture which occurred 20 30 km away the hector mine earthquake both strongly shook and permanently strained the johnson valley fault adding damage discernible as a temporary reversal of the healing process the fault has since resumed the trend of strength recovery that it showed after the landers earthquake these observations lead us to speculate that fault damage caused by strong seismic waves may help to explain earthquake clustering and seismicity triggering by shaking and may be involved in friction reduction during faulting

sediment deposition and production in se asia seagrass meadows abstract seagrass meadows play an important role in the trapping and binding of particles in coastal sediments yet seagrass may also contribute to sediment production directly through the deposition of detritus and also the deposition of the associated mineral particles this study aims at estimating the contribution of different seagrass species growing across an extensive range of deposition to inorganic carbonate and non carbonate and organic sediment production total daily deposition measured with sediment traps varied from 18 8 2 0 g dw m 2 d 1 in silaqui philippines to 681 1 102 g dw m 2 d 1 in bay tien vietnam these measurements correspond to a single sampling event and represent sedimentation conditions during the dry season in se asia coastal areas enhalus acoroides was the most common species in the seagrass meadows visited and together with thalassia hemprichii was present at sites from low to very high deposition halodule uninervis and cymodocea species were present in sites from low to medium deposition the mineral load in seagrass leaves increased with age and was high in e acoroides because it had the largest and long lived leaves up to 417 mg calcium carbonate per leaf and 507 mg non carbonate minerals per leaf and low in h uninervis with short lived leaves 4 mg calcium carbonate per leaf and 2 mg non carbonate minerals per leaf in se asia seagrass meadows non carbonate minerals accumulate at slower rates than the production of calcium carbonate by the epiphytic community consequently the final loads supported by fully grown leaves were as average lower than calcium carbonate loads our results show that organic and inorganic production of the seagrasses in se asia represents a small contribution maximum of 15 of the materials sedimented on a daily base by the water column during the sampling period the contribution of the carbonate fraction can be locally significant i e 34 in silaqui in areas where the depositional flux is low but is minor

a tomographic image of the shallow crustal structure in the eastern marmara 1 we present a 2 d tomographic seismic velocity image in eastern marmara region along a n s trending 120 km long seismic refraction profile which traverses the northern branches of north anatolian fault naf and tectonically active cinarcik basin in the sea of marmara the 2 d velocity model which is constrained down to upper crustal depth of 7 km shows significant heterogeneities in the upper crust displayed by well constrained velocity anomalies the seismicity observed following the 17 august 1999 izmit earthquake concentrates in three distinct zones and takes place near the inferred low velocity zones and below high velocity anomalies obtained from tomographic inversion this correlation is interpreted as a result of intensely sheared zones of deformation imposed by strike slip motion of the northern branches of the north anatolian fault

guided waves from sources outside faults an indication for shallow fault zone structure using 3 d numerical modeling of seismic wave propagation we investigate the possibility of generating fault zone fz trapped wave energy from sources well outside a fault the fz is represented by a o 200 m wide vertical low velocity layer in a half space we find that fz trapped waves can be excited from sources well outside the fault if 1 the low velocity structure extemds only to shallow depth and the source is located at greater depth or 2 the structure of the low velocity zone is such that only the shallow part of the fz traps energy fz trapped waves are not excited from sources well outside a fz continuous with depth the results support in conjunction with recent observational evidence a model for natural faults with shallow trapping structures rather than ones that span the entire seismogenic zone this may have implications for fault mechanics as well as for aspects of shaking hazard near faults

spatiotemporal variations of crustal anisotropy from similar events in aftershocks of the 1999 m7 4 izmit and m7 1 duzce turkey earthquake sequences summary we analyse spatiotemporal variations of crustal anisotropy along the karadere duzce branch of the north anatolian fault from similar earthquakes in the aftershock regions of the 1999 mw7 4izmit and mw7 1 duzce earthquakes the similar earthquake clusters are identified by performing cross correlation on waveforms generated by 18 000 earthquakes depending on the applied similarity criterion about 4 60 per cent of the events belong to similar event clusters splitting parameters averaged within each cluster show significant variations for slightly different ray paths indicating strong spatial variations of crustal anisotropy in this area we also find clear changes in the spatiotemporal seismicity patterns following the duzce main shock large apparent co seismic changes up to 30 per cent of shear wave splitting delay times are observed across the time of the duzce main shock at stations near the epicentral region however the changes can be mostly explained by the spatial variations of ray paths due to the changing seismicity rather than changes in the properties of the anisotropic medium splitting parameters measured within similar earthquake clusters indicate at most 2 per cent changes in delay times associated with the occurrence of the duzce main shock the results do not show systematic precursory changes before the duzce main shock

spatial variability and non linearity of strong ground motion near a fault summary we present observations of ground accelerations recorded at a small array close to the fault during the duzce earthquake and its early aftershocks the records show the strong spatial variability of ground acceleration over distances of only a few hundred metres during the main shock the peak horizontal acceleration values ranged from 0 3 to about 1 0 g at stations distant of 1 5 km only we attribute this spatial variability to a fault zone site effect as peak ground acceleration steadily increases as the distance to the fault trace decreases the spectral ratio between the ground motion recorded near the fault and the one outside the fault zone shows a shift of the spectral peak to lower frequencies with increasing peak accelerations such an observation suggests a non linear behaviour of the fault zone due to the strong ground shaking as much as a 45 per cent reduction in the shear wave velocity is necessary for the observed shifts the opening of pre existing cracks throughout the fault zone is the proposed mechanism to account for the observed shear wave reductions the observation that elastic fault zone properties are soon recovered following episodes of large strains shows that cracks and fissures close rapidly after the strong shaking is over

can seismic waves be trapped inside an inactive fault zone the case study of nocera umbra central italy in march 1998 a small aperture 350 m linear array of five broadband stations was installed across a pliocene strike slip fault crossing the town of nocera umbra central italy three component seismograms of local earthquakes recorded during the umbria marche seismic sequence have shown that the degradation of mechanical properties in damaged rock causes a significant amplification of ground motion within the fault zone marra et al 2000 during the operation of the array an m w 5 3 subcrustal earthquake occurred at a depth of 48 km and an epicentral distance of 10 km from nocera umbra the near vertical incidence of this event offers a unique opportunity for constraining the 2d geometry of the laterally heterogeneous low velocity structure responsible for the large amplification across the fault based on the observed symmetry of the amplification pattern a 2d wedge shaped model of the fault zone is used for the computation of antiplane sh synthetic waveforms to be compared with observations along the array two parameters of the wedge model can be constrained by independent geophysical and geological data 1 shear wave velocity in pristine rock outside the fault zone v s 2200 m sec and 2 the lateral extent of the damaged rock zone l 160 m the other two model parameters namely the wedge depth h and shear wave velocity of the fault zone v f s are varied computer simulations show that trapped waves which have been observed so far for earthquakes occurring inside the fault zone can develop inside a 2d wedge model when struck by external vertically incident seismic radiation results of numerical modeling indicate that in spite of its extreme simplicity such an idealized model is successful in reproducing many of the essential characters of ground motion within the fault zone of nocera umbra even though the best fit solution is not unique and the model parameters trade off each other a study of parameter sensitivity comparing synthetics with recorded data indicates that the variability of h and v f s can be restricted to a limited range of values the quality of fit is optimal when the shear velocity reduction lies in the range of 40 50 and the depth of the trapping zone is comprised between 1 and 2 km