what is diatomite different types of biogenic remains ranging from siliceous algae to carbonate precipitates accumulate in the sediments of lakes and other aquatic ecosystems unicellular algae called diatoms which form a siliceous test or frustule are an ecologically and biogeochemically important group of organisms in aquatic environments and are often preserved in lake or marine sediments when diatoms accumulate in large numbers in sediments the fossilized remains can form diatomite in sedimentological literature diatomite is defined as a friable light coloured sedimentary rock with a diatom content of at least 50 however in the quaternary science literature diatomite is commonly used as a description of a sediment type that contains a large quantity of diatom frustules without a precise description of diatom abundance here we pose the question what is diatomite what quantity of diatoms define a sediment as diatomite is it an uncompacted sediment or a compacted sediment we provide a short overview of prior practices and suggest that sediment with more than 50 of sediment weight comprised of diatom sio2 and having high 70 porosity is diatomaceous ooze if unconsolidated and diatomite if consolidated greater burial depth and higher temperatures result in porosity loss and recrystallization into porcelanite chert and pure quartz less

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

spatial and temporal variations in coda attenuation associated with the 2011 off the pacific coast of tohoku japan mw 9 earthquake the spatial and temporal variations of coda attenuation graphic 1 were studied in the source region of the 2011 off the pacific coast of tohoku japan m w 9 earthquake the graphic 2 values were determined from the amplitude decay rate of the s wave coda in narrower overlapping frequency bands in the range f 1 0 24 hz based on a single isotropic scattering model for more than 400 earthquakes m jma 3 6 5 in the region recorded in a period from january 2005 to august 2011 including pre and postseismic period our estimates of the spatially averaged graphic 3 value graphic 4 in f 1 0 24 hz is almost stable with small variations increases by about 10 16 after the 2011 tohoku event in f 1 25 3 5 hz in some stations of northern japan which is confirmed by a statistical t test at 99 9 confidence level the change in graphic 6 is spatially limited to the rupture zone while other paths remain nearly unaffected suggesting local changes of scattering properties in the vicinity of the tohoku oki source volume this change may be attributed to an increase in the density of open microcracks in the mainshock source volume such as due to increase in stress induced by the 2011 event and probably the fluid content in fractures in the rocks a model of heterogeneity due to coseismically opened cracks dominant scale length of a 0 6 1 8 km in f 1 25 3 5 hz enhanced by increased stress change possibly controls the increased graphic 7 after the 2011 event in the source rupture region 1 embed inline graphic 1 gif 2 embed inline graphic 2 gif 3 embed inline graphic 3 gif 4 embed inline graphic 4 gif 5 embed inline graphic 5 gif 6 embed inline graphic 6 gif 7 embed inline graphic 7 gif

application of fourier transform infrared spectroscopy to silica diagenesis the opal a to opal ct transformation abstract an important goal in silica diagenesis research is to understand the kinetics of opal transformation from noncrystalline opal a to the disordered silica polymorph opal ct because the conventional technique for monitoring the transformation powder x ray diffraction xrd is applicable only to phases with long range order we used fourier transform infrared spectroscopy ftir to monitor the transformation we applied this technique combined with xrd and tem to experimental run products and natural opals from the monterey formation and from siliceous deposits in the western pacific ocean using a ratio of two infrared absorption intensities i472 cm 1 i500 cm 1 the relative prop rtions of opal a and opal ct can be determined the progress of the transformation is marked by changes in slope of vs depth or time when a sufficient stratigraphic profile is available there are three stages in the opal a to opal ct reaction 1 opal a dissolution 2 opal ct precipitation whose end point is marked by completion of opal a dissolution and 3 opal ct ordering during which tridymite stacking is eliminated in favor of cristobalite stacking

the occurrence and significance of biogenic opal in the regolith abstract biogenic opal produced by vascular plants diatoms and siliceous sponges have been found in soils and terrestrial sediments of all continents except antarctica since the middle of the 19th century the opal particles range in size from fine silt to fine sand almost all soils contain detectable opal up to levels of 2 3 and a significant number contain values in excess of 5 even higher values have been found from soils and sediments of all continents in a wide range of soil types the most important factor is poor soil drainage and seasonal to permanent water logging this encourages the proliferation of silica producing organisms such conditions have been found in the soils and aquatic sediments of the monsoonal tropics tropical rain forests temperate forests tropical savanna tropical islands semi arid grasslands and savanna and temperate woodland and grassland the presence of a volcanic substrate also appears favourable in some cases but is not necessary biogenic opal preferentially collects in the a horizon of soils and to a lesser extent in the b horizon this preferential distribution facilitates identification of palaeosols in stacked soil sequences biogenic opal is also a component of windblown dust even in arid environments biogenic opal is significant to regolith processes in a number of ways firstly as in the case in marine environments it is likely to be important in silica cycling and storage because of its greater lability compared to quartz secondly dissolution and reprecipitation of opal a as opal ct or micro quartz may play a role in cementation and silicification of regolith to form silica hardpans and silcrete thirdly the organisms that form biogenic opal can have considerable palaeoenvironmental significance and be valuable in reconstructing regolith evolution finally some forms of biogenic silica in particular sponge spicules can present a health hazard their high abundance in some soils and sediments needs to be considered when assessing the health implications of airborne dust

temporal change in coda q before the tangshan earthquake of 1976 and the haicheng earthquake of 1975 a new earthquake precursor called coda q is demonstrated for the tangshan earthquake of 1976 and for the haicheng earthquake of 1975 the precursor is based on the measurement of coda decay rate for small local earthquakes in the general area that is preparing for a major earthquake during the 3 year period preceding the tangshan earthquake the coda q was about 3 times lower than it was before or after this period a comparable change also occured at the time of the haicheng earthquake the observed change cannot be due to the change in the source parameters such as the focal depth or the source spectrum the coda q precursor appears to detect the same physical process detected by other precursors such as b value quiescence radon water quality vp vs and animal behavior but has advantages over them in its clearer physical mechanism easier sampling and better defined anomalous region

anoxic events during sedimentation of a palaeogene diatomite in denmark a diatomite about 60 m thick of late palaeocene early eocene age crops out in northern jutland denmark the diatomite is locally termed moler frustules of marine diatoms constitute c 65 by weight of the diatomite and clay minerals chiefly montmorillonite make up the remainder slight variations in the relative supply of diatom frustules and clay minerals are preserved undisturbed in laminated diatomite while lamination is partly destroyed by burrowing organisms in weakly laminated diatomite and obliterated by total bioturbation in structureless diatomite these three facies alternate throughout the sequence the presence or absence of infaunal burrowing organisms is interpreted as a record of the content of dissolved oxygen in the water above the sediment water interface and hence of the position of the redox potential discontinuity interspaced in the diatomite are 179 identifiable layers of volcanic ash these ash layers provide a means of precise lateral correlation they show that levels of laminated diatomite may be followed throughout the basin and therefore that changes between anoxic and oxic conditions occurred simultaneously across the area the laminated diatomites may consequently be interpreted as representing short term anoxic events of which twelve have been recognized

monitoring velocity variations in the crust using earthquake doublets an application to the calaveras fault california we present a technique that greatly improves the precision in measuring temporal variations of crustal velocities using an earthquake doublet or pair of microearthquakes that have nearly identical waveforms and the same hypocenter and magnitude but occur on different dates we compute differences in arrival times between seismograms recorded at the same station in the freqency domain by cross correlation of short windows of signal a moving window analysis of the entire seismograms including the coda gives t the difference in arrival times versus running time along the seismogram the time resolution of the method is an order of magnitude better than the digitization interval the t technique is illustrated with a pair of microearthquakes m 1 7 and 2 0 that occurred before and after the coyote lake california earthquake m 5 9 of august 6 1979 and on the same segment of the calaveras fault that ruptured during the earthquake the coda wave arrivals for some stations are progressively delayed for the second earthquake in the doublet so that its seismogram appears as a stretched version of the earlier event we interpret this systematic variation in t along the coda as a change in the average s velocity in the upper crust in the time interval between the two doublets s wave velocities appear to have decreased by 0 2 in an oblong region 5 10 km in radius at the south end of the aftershock zone

postseismic fault healing on the rupture zone of the 1999 m 7 1 hector mine california earthquake we probed the rupture zone of the october 1999 m 7 1 hector mine earthquake using repeated near surface explosions in october 2000 and november 2001 three dense linear seismic arrays were deployed across the north and south lavic lake faults llfs that broke to the surface in the mainshock and across the bullion fault bf that experienced minor slip in that event two explosions each year were detonated in the rupture zone one on the middle and one on the south llf we found that p and s velocities of fault zone rocks increased by 0 7 1 4 and 0 5 1 0 between 2000 and 2001 respectively in contrast the velocities for p and s waves in surrounding rocks increased much less this trend indicates that the hector mine rupture zone has been healing by strengthening after the main shock most likely due to the closure of cracks that opened during the 1999 earth quake the observed fault zone strength recovery is consistent with an apparent crack density decrease of 1 5 within the rupture zone the ratio of travel time decrease for p to s waves was 0 72 consistent with partially fluid filled cracks near the fault zone this restrengthening is similar to that observed after the 1992 m 7 4 landers earthquake which occurred 25 km to the west li and vidale 2001 we also find that the velocity increase with time varies from one fault segment to another at the hector mine rupture zone we see greater changes on the llfs than on the bf and the greatest change is on the middle llf at shallow depth we tentatively conclude that greater damage was inflicted and thus greater healing is observed in regions with larger slip in the mainshock

temporal variations of the coda decay rate on kamchatka are they real and precursory temporal variations of the decay rate of the coda of local earthquakes are studied for four kamchatka stations to suppress any bias caused by the coda q versus lapse time dependence i use the deviation of an individual coda decay function from the empirical reference decay function and the analysis is done over a fixed lapse time window for each individual coda record a decay rate parameter is determined by the following procedure first the reference log coda decay function is subtracted from an individual one giving a residual function second the slope of this residual versus lapse time is determined over the fixed lapse time window giving an estimate the data used are records of 150 250 kamchatka earthquakes of 1967 1990 ml 3 5 5 obtained by four stations employing three component 1 s seismographs the processed data show apparent temporal variation of with high statistical significance several possibilities are then investigated of mimicking the genuine temporal variation of by systematic variation of other parameters first of all epicenter wandering and nodal plane rotation this analysis does not reveal any strong bias and thus suggests that the observed variation is genuine then i show the significance of five apparently precursory anomalies identified retroactively also i describe briefly the real time prediction experiment conducted on kamchatka for 1982 1990 using coda decay aimed at the intermediate term forecasting of large benioff zone earthquakes this experiment resulted in the successful forecast of the august 17 1983 mw 7 0 event with an accuracy of 2 months in time and 100 200 km in location magnitude was overestimated by 0 5 later a false alarm was also issued thus the experiment confirms the reality of the coda decay rate precursor but also shows that it needs further improvement to become reliable physically the variations of the coda decay rate are associated with time dependent local variations of scatterer density in the lithosphere

characteristic repeating earthquakes in an arc continent collision boundary zone the chihshang fault of eastern taiwan abstract creeping crustal faults often generate a large number of microearthquakes and less commonly they may also produce major earthquakes that rupture the brittle crust the chihshang fault in eastern taiwan is characterized by such behavior and has been known to undergo 2 3 cm yr surface creep making it one of the most active creeping thrust faults known in the world it gives an excellent opportunity to study how a creeping fault can generate large earthquakes however the understanding of fault behavior at depth in this area has been limited due to sparse sampling from seismic and geodetic stations in this study we determine and evaluate a population of repeating earthquakes to improve the understanding of deep fault deformation we propose a repeating sequence identification scheme in the region where the station coverage is sparse and one sided using this identification method we found 30 m 2 3 characteristic repeating sequences under the northern half of the chihshang fault at 7 23 km depth slip estimates from these sequences indicate an average slip rate of 3 cm yr which is consistent with the rate inferred from surface geodetic measurements we infer that the 30 km long chihshang fault is creeping along its northern half and locked in the south consistent with the occurrence of the 2003 ml 6 4 earthquake on the southern fault section

stability of coda wave attenuation during the loma prieta california earthquake sequence the loma prieta california earthquake occurred in a densely instrumented region with a history of microearthquake recording beginning more than a decade before the october 1989 mainshock this affords an unprecedented opportunity to detect changes in seismic wave propagation in the earth s crust associated with a major earthquake in this study we use pairs of nearly identical earthquakes doublets to search for temporal changes of coda attenuation in the vicinity of the loma prieta earthquake we analyze 21 earthquake doublets recorded from 1978 to 1991 that span the preseismic coseismic and postseismic intervals and measure the change in coda q using a running window ratio of the doublet spectral amplitudes in three frequency bands from 2 to 15 hz this method provides an estimate of changes in coda q that is insensitive to other factors that influence coda amplitudes our observations place an upper bound of about 5 on preseismic coseismic and postseismic changes of coda q in the epicentral region of the loma prieta earthquake even at this low level the changes are neither spatially coherent nor correlated between adjacent frequency bands the only hint of a signal is in the preseismic data where there is a possible precursory increase in coda q of approximately 5 in the two years before the mainshock the stability of coda q throughout the loma prieta sequence is in sharp contrast to other studies that have reported much larger precursory changes in coda q for other earthquakes

differential analysis of coda q using similar microearthquakes in seismic gaps part 1 techniques and application to seismograms recorded in the anza seismic gap abstract a large but conflicting literature exists that suggests that the scattering properties of the earth in seismogenic areas may be temporally variable and that this variability could be correlated with the nucleation times of moderate to large earthquakes to search for temporal variations in coda q in the vicinity of an expected moderate earthquake nucleation region along the san jacinto fault zone pairs and clusters of earthquakes recorded by the anza seismic network between 1982 and 1992 are examined to minimize differences arising from path and source variability only the most similar examples of microearthquake wave fields are intercompared to assess coda q differences between similar microearthquake signals we search for systematic temporal changes in moving window spectral amplitude ratios for a single scattering coda model with an envelope decay function of the form t m e ft qc the relationship between the natural log amplitude spectral ratio and q 1 q 1 c 2 q 1 c 1 is linear i e ln r f t ft q 1 f w f where w f is a time independent relative source term to obtain robust uncertainty estimates of q 1 f and w f we utilize multi taper spectral analysis coupled with a nonparametric monte carlo confidence interval estimation procedure for the most similar events in the anza region coda cross correlation values of approximately 0 7 for 16 sec of coda we find that coda q as a function of time is stable at a typical 1 resolution of only between 25 and 50 of the reference coda q value for the area even in the best constrained frequency bands comparison of similar microearthquake coda signals with both short and long temporal separations indicates that all of this variability can be reasonably attributed to random fluctuations in the coda that are driven by source variability and is not indicative of any systematic temporal variability in coda q to obtain estimates of relative coda q that are more precise than this level the spectral ratio analysis technique requires significantly more similar earthquakes than are produced by seismogenic processes in the anza region such earthquakes have been observed to occur in other regions of the san andreas fault system and are analyzed in antolik et al 1996

a precisely dated lake level rise marked by diatomite formation in northeastern ireland the intercorrelation of palaeoclimate events from various studies is often hindered by a lack of precise chronological control tephra isochrons can overcome this problem by providing direct site linkages this paper outlines a study of holocene peat and diatomite deposits that accumulated within the floodplain of lough neagh northern ireland the icelandic hekla 4 tephra has been identified at the base of diatomite deposits at a number of sites and provides firm dating evidence for a widespread flooding event in the area at ca 2300 bc the evidence is consistent with other studies in ireland and elsewhere for increased wetness at this time the results demonstrate that the terrestrial deposits around lough neagh contain an important record of holocene lake level change dendrochronological evidence from the lough neagh area provides additional information about lake level fluctuations over the past two millennia copyright 2004 john wiley sons ltd

seismic waveform attributes before and after the loma prieta earthquake scattering change near the earthquake and temporal recovery during 1987 1995 several clusters of nearly identical seismic events multiplets occurred near the loma prieta source region these multiplets allow us to investigate and demonstrate spatial and temporal changes in seismic wave character associated with the 1989 loma prieta main shock for seismogram pairs we use a moving window technique to compute coherencies depending on lapse time and frequency post loma prieta events have reduced coherencies with pre loma prieta events in a spatially limited region close to the loma prieta hypocenter while other paths remain nearly unaffected these changes gradually recover within a time interval of 5 years after the loma prieta earthquake a possible explanation for the time dependence is coseismically opened cracks which cause scattering increase for wavefields after the loma prieta event postseismic relaxation processes such as crack healing fluid diffusion or after deformations lead to progressive closure of these cracks with time after the main shock thus the scattering properties of the local crust approach the pre main shock state

advanced insights into magmatism and volcanism of the mozambique ridge and mozambique basin in the view of new potential field data summary a new plate tectonic model is presented which describes the emplacement of the mozambique ridge off southeast africa as the result of long lasting volcanic activity 140 122 ma during the initial opening between africa and antarctica thus an oceanic origin for the mozambique ridge is proposed this model is based on a new and systematic high resolution magnetic anomaly data set acquired across the mozambique ridge and throughout the mozambique basin data from the mozambique basin allow the identification of mesozoic magnetic anomalies from m0r to m26 124 61 155 3 ma with previously unmatched accuracy small scale fracture zones are recognized by offsets of magnetic anomalies in the westernmost part of the basin additionally a bend in the major fracture zones f and e between m17r and m18n 142 84 144 04 ma and a recognized sinusoidal change in spreading direction with an amplitude of about 15 indicate that the basin experienced several small scale changes in spreading direction through time a maximum change in spreading direction to almost 0 at around m11n 135 69 ma is followed by a short lived increase in spreading half rate from 23 5 km ma 1 to about 27 5 km ma 1 in the time frame from m10r to m9n 134 30 132 83 ma we propose that this is related to the initial opening of the south atlantic ocean represented by the onset of seafloor spreading between the falkland plateau and africa in the conjugate georgia and natal basins across the mozambique ridge high amplitude magnetic anomalies at major structural boundaries suggest that the different plateaus of the ridge were formed at different times a simple 2 d gravity and magnetic model for the ridge supports the hypothesis of multiple volcanic episodes which formed the ridge though long lasting volcanic activity between about 140 and 122 ma together with new and verified rotation parameters from the mozambique basin and its conjugate the riiser larsen sea antarctica a series of plate tectonic reconstructions are presented which demonstrate when and how the different parts of the ridge evolved through time

temporal changes in coda q 1 and b value due to the static stress change associated with the 1995 hyogo ken nanbu earthquake the seismicity in the tamba region northeast of the hyogo ken nanbu earthquake in japan january 17 1995 mjma 7 2 increased significantly following this earthquake this increase suggests that the static stress change due to a large earthquake causes a change in the crustal condition or dynamics in order to reveal the changes quantitatively we investigate the temporal variation in coda q l and b value in the tamba region we analyze the waveform data of many shallow microearthquakes m 1 5 3 0 in the region recorded in a period from 1987 to 1996 coda q 1 is estimated in 10 frequency bands in a range of 1 5 24 hz based on the single isotropic scattering model at frequencies between 1 5 and 4 0 hz the temporal variation in coda q 1 shows significant correlation with the occurrence of the hyogo ken nanbu earthquake coda q l increases after the event a variation in b value whose sign is opposite to that of coda q 1 is recognized the fracture dimensions of microearthquakes that contribute to the variation in b value are estimated to be 400 m this scale length is consistent with the characteristic scale length of scatterer 300 600 m which contributes effectively to a temporal variation in coda q 1 the crustal activity in the tamba region is possibly controlled by the heterogeneity with dominant scale of 102 m the stress sensitivity of the coda q 1 change is estimated to be 10 mpa 1 this value is an order of magnitude larger than the stress sensitivity of seismic velocity reported before

the supply and accumulation of silica in the marine environment rivers and submarine hydrothermal emanations supply 6 1 1014g sio2 yr to the marine environment approximately two thirds of the silica supplied to the marine environment can be accounted for in continental margin and deep sea deposits siliceous deep sea sediments located beneath the antarctic polar front convergence account for over a fourth 1 6 1014g sio2 yr of the silica supplied to the oceans deep sea sediment accumulation rates beneath the polar front are highest in the south atlantic with values as large as 53cm kyr during the last 18 000 yr siliceous sediments in the bering sea sea of okhotsk and subarctic north pacific accumulate 0 6 1014g sio2 yr or 10 of the dissolved silica input to the oceans the accumulation of biogenic silica in estuarine deposits removes a maximum of 0 8 1014g sio2 yr although estuarine silica versus salinity plots indicate extensive removal of riverine silica from surface waters the removal rates must be considered as maximum values because of dissolution of siliceous material in estuarine sediments and bottom waters siliceous sediments from continental margin upwelling areas e g gulf of california walvis bay or peru chile coast have the highest biogenic silica accumulation rates in the marine environment 69 g sio2 cm2 kyr despite the rapid accumulation of biogenic silica upwelling areas account for less than 5 of the silica supplied to the marine environment because they are confined laterally to such small areas

short and long term variations in the reykjanes geothermal reservoir from seismic noise interferometry the reykjanes geothermal system rgs is a high temperature geothermal system located on the reykjanes peninsula a transtensional plate boundary zone located on the southwestern tip of iceland the area is characterized by high seismicity recent volcanism and high temperature geothermal fields we use seismic noise records from april 2014 to august 2015 to study stress changes and potential deformation of the subsurface caused by injection and production operations at rgs through seismic interferometry we retrieve continuous time series of waveform similarity values and seismic velocity changes during this period the s transform of the similarity values allows us to clearly identify three variations in the mechanical properties of the reykjanes peninsula related to rapid changes of rgs production in addition we observe a slow seismic velocity decrease of 0 36 year in the reservoir due to the water deficit and seasonal variations associated with the energy production demand

changes in source and site effects compared to coda q 1 temporal variations using microearthquakes doublets in california we analyse spectral ratio of the coda of doublets of microearthquakes our purpose is to find evidence for temporal changes of the attenuation in the crust before a large magnitude earthquake a moving window cross spectral analysis of the coda of doublets gives a plot of the spectral ratio as a function of lapse time along the seismogram for several frequency bands sr t f plot from a certain pattern in thesr t f plot we should infer a temporal change in coda attenuation several doublets recorded in central california by the usgs network are analysed

site amplification of coda waves from local earthquakes in central california the goals of this study are two fold 1 to examine the site effect of coda waves believed to be the average site effect of shear waves in order to understand its spatial and frequency dependent behavior and 2 to learn what we can about the processes that generate the coda itself we use digital data from over 90 earthquakes each recorded by a subset of some 150 stations in the coast ranges of central california between san francisco and san luis obispo results from the band 1 5 to 24 hz indicate that site amplification depends strongly on surface geology and frequency at low frequencies 1 5 to 3 0 hz changes in amplification up to a factor of 20 are observed amplification generally varies inversely with deposit age at sediment sites and is lowest at granite and franciscan basement sites at high frequencies 6 to 24 hz the pattern changes many granite sites the majority in the gabilan range exhibit increasing amplification with frequency relative to the average station this behavior differs strikingly from that observed at sediment and franciscan sites which roll off at an intermediate rate and many sediment sites adjacent to the san andreas fault zone which decay away even more rapidly with frequency these results can be explained qualitatively by appealing to variable near site impedance and attenuation however complicating phenomena such as mode excitation in valley sediments must exist at low frequencies peaked behavior at a few hard rock sites at high frequency 12 hz currently lacks explanation at a few sediment sites the low frequency excitation is so strong that the coda envelopes begin to take on different shapes this contradicts the assumption based on previous observations that the coda shape is independent of source receiver location thus some site measurements may be artificially high the working model of coda waves as body wave energy backscattered from randomly distributed inhomogeneities must be modified to include the possibility of efficient near site resonance excited by incident direct and coda wave energy

carbon cycling within an east african lake revealed by the carbon isotope composition of diatom silica a 25 ka record from lake challa mt kilimanjaro the carbon cycle of a lake is a balance between supply from the atmosphere and catchment and the net demand exerted by primary producers minus losses back to the atmosphere and to sediment storage evaluating the sum of these processes and reconstructing them from sediment records of lake history requires a range of methods and a multi proxy approach one promising technique is to explore the carbon isotope composition 13cdiatom of organic matter incorporated within the silica frustules of diatom algae here we present a 25 000 year record of 13cdiatom from the sediments of crater lake challa on the eastern flank of mt kilimanjaro and along with other proxy data we make inferences about the three major phases in the history of the lake s carbon cycle from 25 ka to 15 8 ka years bp 13cdiatom is positively correlated with the 13c of bulk sediment organic matter 13cbulk indicating that high diatom productivity as recorded by high biogenic silica at this time was preferentially removing 12c and enriching the 13c of lake water dissolved inorganic carbon from 15 8 to 5 5 ka the correlation between 13cdiatom and 13cbulk breaks down suggesting carbon supply to the lake satisfied or exceeded the demand from productivity from 5 5 ka bp the positive correlation resumes indicating an increase in the internal demand for carbon relative to external supply diatom frustule bound carbon isotopes offer an original tool in examining long term fluctuations in a lake s carbon budget and how the balance between supply and demand has changed through time

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

near source modeling of the loma prieta earthquake evidence for heterogeneous slip and implications for earthquake hazard i have analyzed records from 20 strong motion instruments that recorded the 1989 loma prieta earthquake ml 6 9 to determine the spatial and temporal distribution of slip using a tomographic back projection technique i find that the earthquake ruptured bilaterally at approximately 80 of the local shear wave velocity over a distance of slightly more than 30 km from 13 km northwest to 20 km southeast of the hypocenter slip within the rupture zone was highly variable both in magnitude and direction slip at the hypocenter and immediately up dip was relatively low about 1 m there were two main areas of concentrated slip one centered 7 km northwest of the hypocenter at a depth of 14 km and the other centered 6 km southeast of the hypocenter at a depth of 12 km peak slip amplitude on these high slip patches exceeded 4 5 m a surprising aspect of the rupture model is that rake varies from being predominately strike slip to the southeast of the hypocenter to being predominately reverse slip to the northwest despite this variation in rake the average rake determined from the strong motion data is in agreement with that determined teleseismically there is a correlation between areas of high slip and areas of low aftershock activity a simple model to explain this observation is that the areas of high slip are also areas of high strength which slip only during infrequent large mainshocks other areas of the fault are weaker and may slip both seismically and aseismically most of the slip in the loma prieta earthquake occurred between 9 and 16 km depth on a structure that dips to the southwest and runs underneath the surface trace of the san andreas fault at a depth of 8 to 10 km this observation complicates the assessment of seismic hazard on the southern santa cruz mountains segment of the san andreas fault

repeating earthquakes in western corinth gulf greece implications for aseismic slip near locked faults an extensive search for repeating earthquakes was performed in the western corinth gulf by applying waveform cross correlation on 22 000 earthquakes that occurred in 2008 2014 event pairs with high correlation coefficient cc 0 95 recorded by two or more stations are classified as multiplets of repeating events the highly similar event pairs have typically interevent distances less than a quarter wavelength 150 m for a dominant frequency of 10 hz and are used to estimate the accuracy of the relocated catalog a detailed analysis of the spatiotemporal properties of the repeating sequences revealed two types of repeating events namely burst like and continuous type repeaters burst like repeaters are widespread in the entire study area mostly associated with seismic excitations located at depths between 5 and 9 km triggered either by fluid intrusion or stress transfer their duration is short with high values of coefficient of variation in recurrence intervals cov 1 and high slip rates the continuous type repeaters which last 1 to 7 years with cov 1 and slip rates almost 0 26 cm yr form a very narrow shallow north dipping seismic zone at 10 km depth along the psathopyrgos and aigion faults that kind of activity provides strong evidence for aseismic slip in the western corinth gulf and defines the boundaries between brittle ductile transition in the area

origins of amplitude variations in seismic doublets source or attenuation process summary accurate estimates of spectral ratios of two doublets i e similar seismic events occurring along the san andreas fault in the hollister region have been computed using a cross spectral method both events of each doublet occurred within a 24 hr interval although both waveforms of a doublet are remarkably similar computed spectral ratios exhibit strong but steady variations with frequency in the first arrivals the attenuation process can explain neither the sign nor the order of magnitude of these variations on the contrary they are closely related to the source mechanism we show that the slope computed from the linear and coherent part of the spectral ratio is an estimation of the variation in the apparent rupture duration between the two events one doublet exhibits an azimuthal distribution of the spectral ratio variations that is interpreted as the result of a change in the position of the hypocentres relative to the rupture area for this doublet location of hypocentres varies only from 10 to 30 per cent of the source length this points out that small changes in rupture kinematics can lead to significant variations in the spectral ratio of earthquake doublets i e in the amplitude of each event for the other doublet the variations are related to a change in source length we also have computed spectral ratios all along the seismograms using a moving window technique when an azimuthal distribution of the spectral ratio is observed in the first arrivals we notice a decrease of the spectral ratio all along the seismograms the doublet with strong but non azimuthal variations of the spectral ratio in the first arrivals does not indicate such a decrease of the spectral ratio along the seismogram this implies that the variations of the coda decay are related not only to temporal or spatial changes of attenuation but also to source parameters these variations have the same order of magnitude as those provoked by a temporal change in coda q of 10 per cent they can occult time changes of spectral ratio for instance in case the attenuation in the crust would vary before a strong earthquake

lithification of biogenic siliceous sediments evidence from neogene diatomaceous sequences of northeast japan abstract biogenic siliceous deposits are a significant component of neogene stratigraphic sequences of the northeast japan arc field observations demonstrate that the siliceous rocks are classified into five lithologic types diatomite opaline porcelanite opaline chert quartzose porcelanite and black shale stratigraphic data suggest that the siliceous formations were deposited continuously from the middle miocene to early pliocene diatomite occurs throughout the sedimentary sequences and changes laterally into chert and porcelanite multivariate statistical analysis of geochemical data indicates that lithification and mineralogical transformation of silica rich deposits was governed not only by compaction and cementation during burial but also by sorting of grains by biological sediment seawater mixing diatomite is composed of frustules without fine ornamentations and delicate structures fine and delicate tests were more susceptible to dissolution than coarse and solid ones and dissolved in the course of biological sediment disturbance ubiquitous occuerences of diatomite in the stratigraphic sequences are attributed to residual frustules which were more resistant to the diagenetic dissolution precipitation reaction of silica biogenic silica is the main source of silica to form chert diagenetic precipitation of silica in diatomaceous beds through progressive sediment burial formed porcelanitic rocks with opaline porcelanite being the precursor of quartzose porcelanite chert was not subjected to biological sediment reworking and fragile shells were preserved during accumulation silicification of precursor deposits of chert was promoted by intraformational dissolution precipitation reaction of silica with increasing sediment compaction and dehydration

spatial and temporal correlation between coda q 1 and seismicity and its physical mechanism from the analysis of local earthquake coda waves recorded by the benioff short period vertical seismograph at riverside california we found a surprisingly large and systematic temporal variation in coda q 1 in the southern california during the 55 year period from 1933 to 1987 comparison of the coda q 1 variation with the b value obtained for earthquakes with m 3 occurred in the area within 180 km from riverside corresponding to the area for which q 1 is estimated by the coda method led to an extraordinary result that coda q 1 correlates positively with b value with the correlation coefficient of 0 79 the significance of correlation cannot be rejected at the confidence level of 0 974 since the data used for measuring q 1 namely the coda part of seismograms and the data used for measuring b namely the seismicity catalog are totally independent the observed correlation supports strongly the physical reality of temporal change in both coda q 1 and b value the correlation between coda q 1 and b value has been reported for several other cases but the correlation was positive in some cases and negative in others our current preferred model for explaining the observed correlation is the following aseismic creep is going on from time to time over various sizes of cracks in the ductile part and in some cases in the brittle part of lithosphere at and near the plate boundary creep activities tend to increase the crack density and coda q 1 observed in the region if the creek occurs over distributed cracks with a certain predominant scale length spatial stress concentration may enhance seismicity for earthquakes with a similar size if this characteristic size is in the lower part of the magnitude range for which the b value is estimated the enhanced seismicity will increase b on the other hand if it is in the upper part of the magnitude range it will decrease b thus we can explain the observed strong correlation between coda q 1 and b value for both positive and negative cases the existence of such a characteristic crack size in a seismic region has been supported by various observations on earthquake scaling

complexities in high frequency seismic waveforms due to three dimensional structure in the new madrid seismic zone effects of three dimensional structure on 2 to 10 hz seismic signals in the new madrid seismic zone nmsz are investigated using waveforms from similar microearthquakes that occurred near ridgely tennessee slowness power spectral sps analysis is used to study the plane wave composition of p s and coda waves the observed sps at four stations suggest that the early s coda is composed mainly of wavelets leaving the source with slownesses of the direct s wave at greater lapse times coda is increasingly affected by random scattering sps observed on the vertical components and or near the s wave nodes are more comlplex due to practical limitations of the algorithm p coda and early s coda at a fifth station located southwest of the junction show the most complex sps analysis of that complexity together with waveform synthetics and travel time analysis strongly suggests that scattering multipathing due to source zone velocity heterogeneities occurs near the path probably due to dilatancy occurring southwest of the ridgely junction or due to internal fault zone complexities coda q when averaged over three components show no station variations we infer that 1 coda at 1 hz is mainly caused by scattering in the sedimentary layer where q is frequency independent 2 at higher frequencies the coda contains an increasing amount of scattered body waves and 3 the source radiation pattern and path variable scattering multipathing effects in early coda do not affect coda q

do big and little earthquakes start differently can the occurrence of earthquakes be predicted this question has profound social and scientific implications but little is known about what triggers earthquakes or what precursor events might indicate one is about to happen in his perspective vidale reviews recent findings that attempt to resolve the question of whether large earthquakes are initiated by different mechanisms than smaller ones

classification of deep sea fine grained sediments abstract most deep sea sediments contain one or more biogenic components and one dominant nonbiogenic component usually clay or silty clay we present a descriptive classification scheme in which deep sea fine grained sediments are placed within a three component system of calcareous biogenic siliceous biogenic and nonbiogenic components in a three step procedure the user assesses whether the dominant component is biogenic or nonbiogenic whether the dominant biogenic component is siliceous or calcareous and what the relative abundances of the biogenic components are within limits of 10 25 and 50 the terminology proposed is that commonly used by many sedimentologists with some refinements and greater precision in the use of terms

coda wave interferometry for estimating nonlinear behavior in seismic velocity in coda wave interferometry one records multiply scattered waves at a limited number of receivers to infer changes in the medium over time with this technique we have determined the nonlinear dependence of the seismic velocity in granite on temperature and the associated acoustic emissions this technique can be used in warning mode to detect the presence of temporal changes in the medium or in diagnostic mode where the temporal change in the medium is quantified

temporal change in attenuation intensity before and after the eastern yamanashi earthquake of 1983 in central japan a crutal earthquake of magnitude 6 0 took place on august 8 1983 in the middle of the kanto tokai microearthquake observation network of the national research center for disaster prevention in central japan by analyzing vertical seismograms of microearthquakes occurring in a small volume in the aftershock region we measured temporal change in attenuation intensity for a period from 22 months before to 9 months after the main shock attenuation change was estimated by using two different kinds of analyses in the first analysis station magnitude calculated from the maximum amplitude at each station was compared with the average of station magnitudes deviations of station magnitude from the average magnitude at five stations in the vicinity of the aftershock area are found to have increased after the main shock compared with that before in the second analysis s coda duration from the origin time was measured and was compared with the average magnitude the increase of s coda duration for the same average magnitude is found after the main shock at two stations close to the main shock epicenter the simplest interpretation of the results is that the attenuation intensity in the upper crust in the close vicinity of the focal region decreased after the main shock for frequencies f from 2 to 20 hz qs 1 1 4 10 2f 1 since other precursors appeared in the preparation stage it is possible to interpret the change in qs 1 as a precursor the attenuation intensity would have been anomalously large before the main shock

coseismic and postseismic velocity changes measured by repeating earthquakes 1 repeating earthquakes that rupture approximately the same fault patch and have nearly identical waveforms are a useful tool for measuring temporal changes in wave propagation in the earth s crust since source and path effects are common to all earthquakes in a repeating earthquake sequence multiplet differences in their waveforms can be attributed to changes in the characteristics of the medium we have identified over 20 multiplets containing between 5 and 40 repeating events in the aftershock zones of the 1989 loma prieta and 1984 morgan hill california earthquakes postmain shock events reveal delays of phases in the early s wave coda of as much as 0 2 s relative to premain shock events the delay amounts to a path averaged coseismic velocity decrease of about 1 5 for p waves and 3 5 for s waves since most of the multiplets are aftershocks and follow omori s law we have excellent temporal sampling in the immediate postmain shock period we find that the amplitude of the velocity decrease decays logarithmically in time following the main shock in some cases it returns to the premain shock values while in others it does not similar results are obtained for the morgan hill main shock because the fractional change in s wave velocity is greater than the fractional change in p wave velocity it suggests that the opening or connection of fluid filled fractures is the underlying cause the magnitude of the velocity change implies that low effective pressures are present in the source region of the velocity change our results suggest that the changes are predominantly near the stations and shallow but we cannot exclude the possibility that changes occur at greater depth as well if the variations are shallow we may be detecting the lingering effects of nonlinearity during main shock strong ground motion if the variations are deep it suggests that pore pressures at seismogenic depths are high which would likely play a key role in the earthquake process

time space variation of seismic s wave coda attenuation qc 1 and magnitude distribution b values for the petatlan earthquake average s wave coda q qc at 6 hz and to about 10 sec after the s wave arrival shows 1 temporal variation positively correlated with the magnitude distribution parameter b for foreshocks and aftershocks of the 1979 petatlan mexico earthquake 2 spatial variation through the aftershock area positively correlated with b high qc b values at the two activity centers intermediate values in between and landward and low values toward the middle america trench which remain low for two weeks after the main shock and 3 five foreshock average qc 135 1 6 and eleven aftershock average qc 175 1 3 for the eastern asperity the b values and possibly qc values seem to correlate with the level of aftershock activity

coda q before and after the 1983 misasa earthquake of m 6 2 tottori prefecture japan to detect any temporal change in coda q associated with an earthquake of m 6 2 digitized data from analog magnetic records of microearthquakes for a small confined region just around its hypocenter have been processed with special attention to measuring errors the m 6 2 earthquake took place at the misasa town tottori prefecture in southwest japan with preceding anomalous seismicity changes theq values were elevated 20 around three years before the m 6 2 event for frequencies of 5 to 20 hz and tended to decrease around two years before data with high quality show undulated temporal variation with a period of 5 6 years before the earthquake which is correlated with the regional seismic activity there is a possibility that the observedq change stands for precursory effect of the medium scale earthquake the aftershock sequence also shows an undulated temporal variation ofq with a period of 150 days for around 10hz correlating also with the seismic activity the fracturing processes by numerous microearthquakes may be responsible for the attenuation property of randomly scattered seismic waves

high precision relative locations of earthquakes at mount st helens washington several earthquake sequences have occurred at mount st helens as clusters of shallow events with nearly identical waveforms multiplets we present a technique that allows the relative relocation of events within a multiplet to at least an order of magnitude higher precision than is typically possible with traditional techniques a multiplet is studied in pairs of events called doublets each doublet is made up of a reference event and one other event from the multiplet the cross spectrum of a short window beginning just before the p arrival is used to compute differences in arrival times between seismograms from the same station for each doublet these time differences computed with a precision of 1 2 ms are then used to locate each earthquake relative to the reference event engineering explosions with well known locations are used to test the relocation technique explosions can be relocated with accuracy better than 20 m for events within 250 m of the reference explosion the relocation technique is applied to earthquakes preceding the september 10 1984 and the may june 1985 dome building eruptions of mount st helens forty events that occurred during a 12 hour period in september 1984 are located in a volume approximately 30 m in diameter beneath the lava dome the earthquakes following the extrusion do not occur in multiplets and appear to be distributed over a much larger volume in the may june 1985 eruption we recognize three multiplet sets that took place successively in time the multiplets are interpreted as repeated failure or slip within a relatively small volume due to high strain rates around the magma supply conduit

seismological studies at parkfield iii microearthquake clusters in the study of fault zone dynamics abstract more than half of the microearthquakes that occur near parkfield california when located with high resolution methods are seen to define some 80 small clusters of 2 to 12 similar events each cluster occupies a patch typically 100 to 200 m in length within the fault zone cluster members have nearly identical waveforms correlation coefficient of 0 9 or greater to frequencies of 50 to 100 hz as recorded by the borehole installed seismographic network the clusters are distributed throughout the fault zone around the presumed nucleation region in the locked section to the se and in the creeping part of the nw of the previous m 6 hypocenter they are also found in the salinian block several kilometers sw of the fault zone the total area occupied by all of the clusters constitutes only a small fraction 1 to 2 of the fault zone approaching failure at parkfield such clusters provide insight into the dynamics of the failure process through their spatial temporal characteristics and their mechanisms they also serve as highly repetitive sources distributed throughout the fault zone suitable for monitoring the nucleation zone for possible precursory changes in physical properties that affect wave propagation in this article we demonstrate these applications with a detailed analysis of selected clusters

temporal changes in p wave attenuation in the loma prieta rupture zone 1 clusters of recurring nearly identical microearthquakes have been recorded near loma prieta california such events make possible direct measurement of temporal changes in the attenuation operator t along fixed paths under what are tantamount to laboratory conditions analysis of almost 10 years of p wave data following the 18 october 1989 loma prieta california main shock m 7 0 reveals a large amplitude t increase within the first 3 weeks of the main shock a gradual t recovery toward its level prior to the main shock is seen in the years ensuing the results point to changes in porosity and fluid saturation taking place in parts of the main shock source volume stemming from coseismic crack opening and postseismic relaxation processes such as postseismic fluid flow into the cracks crack healing and rock compaction

temporal stability of coda q 1 in southern california some studies of coda q 1 have found temporal changes that may be associated with earthquake activity but these analyses are subject to biases due to differences in source locations and other nonstationary behavior in earthquake catalogs these biases can be greatly reduced by using clusters of repeating earthquakes studies using this approach have generally found no resolvable changes in coda q 1 we examine coda q 1 variations across southern california using 22 similar event clusters identified from a recent large scale waveform cross correlation project to improve earthquake locations these clusters are found across the region and span the time period between 1981 and 2005 we apply the method of beroza et al 1995 to compute differential coda q 1 using waveforms from similar earthquake pairs and analyze the results to constrain any possible temporal variations results from individual event pairs show a great deal of scatter in differential coda q 1 but exhibit no clear temporal variations or changes associated with large earthquakes application of a median filter to smooth the results shows that any persistent large scale changes in coda q 1 during this time period are less than about 30

seimological studies at parkfield v characteristic microearthquake sequences as fault zone drilling targets studies at very high resolution of microearthquakes at parkfield california since 1987 reveal a systematic organization in space and time dominated by clustering of nearly identical regularly occurring microearthquakes characteristic events on 10 to 20 m wide patches within the fault zone more than half of the 4000 events in our 1987 to 1996 catalog exhibit this trait in general recurrence intervals 0 5 to 2 yr scale with the magnitude of the repeating events for the on scale range mw 0 2 to 1 3 in this study the similar waveforms superimposed locations quasi periodic recurrence and uniform size of these characteristic events permit relative hypocenter location accuracy of meters and predictable occurrence times within windows of a few months clustered characteristic events occur at depths as shallow as about 3 km and these are feasible targets for deep scientific drilling and observation at the focus of a subsequent small earthquake within an active plate boundary fault zone at parkfield the achievable location accuracy to which a hypocenter can be specified as well as the predictability of its occurrence time appear to be uniquely favorable for in situ fault zone measurements

mechanism diversity of the loma prieta aftershocks and the mechanics of mainshock aftershock interaction the diverse aftershock sequence of the 1989 loma prieta earthquake is inconsistent with conventional models of mainshock aftershock interaction because the aftershocks do not accommodate mainshock induced stress changes instead the sense of slip of the aftershocks is consistent with failure in response to a nearly uniaxial stress field in which the maximum principal stress acts almost normal to the mainshock fault plane this orientation implies that i stress drop in the mainshock was nearly complete ii mainshock induced decreases of fault strength helped were important in controlling the occurrence of after shocks and iii mainshock rupture was limited to those sections of the fault with preexisting shear stress available to drive fault slip

properties of scattered elastic waves in the lithosphere of kamchatka parameters and temporal variations abstract records of near earthquakes from the shipunsky station were used to estimate s wave absorption for seven frequency bands ranging from 0 4 to 24 hz three techniques were employed and three rows of consistent estimates were obtained two of the three techniques give estimates that are independent of q anisotropic scattering was studied by observing the broadening of the s wavelet the estimates obtained show that anisotropic scatter may be significant the time variation of the coda envelope shape was studied using the records of 1 s regional seismographs three epicentral regions were chosen in the kurile kamchatka zone where m 8 earthquakes have occurred time series of the parameter which is the anomaly of the logarithmic steepness of coda envelope were constructed for vertical and horizontal channels of several stations pronounced forerunner type variations of were revealed we show that the probable cause of these variations is a qs decrease in the vicinity of impending m 8 earthquake this decrease is about 20 with a lead time of about a year the forerunner anomaly is not of the bay type and recovers 1 2 years after a large earthquake

spatial and temporal stress drop variations in small earthquakes near parkfield california 1 we estimate source parameters from spectra of 42367 earthquakes between 1984 and 2005 that occurred in the parkfield segment of the san andreas fault in central california we use a method that isolates the source term of the displacement spectra based on a convolutional model and correct the observed p wave source spectra with a spatially varying empirical green s function egf our brune type stress drop estimates vary from 0 1 to over 100 mpa with a median value of 6 75 mpa which is nearly constant with moment implying self similarity over the ml 0 5 to 3 0 range of our data the corner frequency decreases for earthquakes at shallower depths consistent with slower rupture velocities and reduced shear wave velocities in local velocity models the estimated median stress drops show significant lateral variations we find lower stress drops in the middle mountain asperity and along the creeping fault section and higher stress drops in the hypocentral region of the 2004 m6 0 parkfield earthquake the main shock did not alter the overall pattern of high and low stress drop regions however a statistical test reveals areas with significant changes in computed stress drops after the main shock which we compare to estimated absolute shear stress changes from a main shock slip model by calculating t from the spectral egf ratio we also identify areas with increased attenuation after the main shock and we are able to distinguish source effects and near source attenuation effects in the spectral analysis these results are confirmed independently from spectral ratios of repeating microearthquake clusters

fault healing inferred from time dependent variations in source properties of repeating earthquakes we analyze two sets of repeating earthquakes on the calaveras fault to estimate in situ rates of fault strengthening healing earthquake recurrence intervals tr range from 3 to 803 days variations in relative moment and duration are combined to study changes in stress drop rupture dimension rupture veloc ity and particle velocity as a function of t healing rates and source variations are compared with predictions of laboratory derived friction laws two interpretations of event duration z are used one in which is given by the ratio of slip to particle ve locity and one in which it scales as rupture dimension divided by rupture velocity our data indicate that faults strengthen during the interseismic period we infer that source dimension de creases with t r due to aseismic creep within the region surround ing the repeating events stress drop increases 1 3mpa per dec ade increase in twhich represents an increase of a factor of 2 3 relative to events with tbetween 10 and 100 days this rate of fault healing is consistent with extrapolations of laboratory meas urements of healing rates if fault strength is high on order of 60mpa and stress drop is roughly 10 of this value

temporal change in scattering and attenuation associated with the earthquake occurrence a review of recent studies on coda waves the study of coda waves has recently attracted increasing attention from seismologists this is due to the fact that it is viewed as a new means by which the stress accumulation stage preceding a large earthquake can be measured since the scattering paths nearly uniformly cover a fairly large region around the focus and observation stations compared with the direct ray paths to date we have had many reports on the temporal variation of the relation between coda duration and amplitude magnitude and that of the coda attenuationq 1 which is estimated from coda amplitude decay some of these have shown a precursor like behavior however others seem to have shown a coseismic change we have critically reviewed these reports and discussed what these observational facts tell us about the change in the heterogeneous crust we found significant temporal variations not only in the mean but also in the scatter ofq 1 associated with the mainshock occurrence the formation of new cracks the reopening and growing of existing cracks the interaction of these cracks and the pore water movement through these cracks might correspond to such variations in addition we may expect an inhomogeneous distribution of crack clusters in a fairly large region compared with the aftershock region the gradual appearance of such crack clusters seems to be the most plausible mechanism by which coda decay gradients are caused to largely scatter in the stress accumulation stage

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

quantifying the impact of freshwater diatom productivity on silicon isotopes and silicon fluxes lake myvatn iceland diatom productivity in the oceans plays a crucial role in the carbon cycle but is strongly dependent upon the continental silicon supply however the relative influence of weathering and biological processes on continental si fluxes remains poorly constrained this study aims to quantify the impact of terrestrial diatom productivity on si fluxes to the ocean lake myvatn in north iceland is one of the most productive lakes in the northern hemisphere with nutrient rich waters almost uniquely sourced by groundwater the primary production is mainly controlled by diatom growth but also by cyanobacteria and the lake output is via a single river thereby providing a relatively simple natural laboratory to quantify the impact of diatom growth on the chemistry and si budget of lake waters silicon stable isotopes o 30si provide a tracer of this biocycling and have been measured in groundwater inputs to the lake and in time series monitoring of waters at the lake outlet the o 30si values at the outlet range from 0 70 0 08 to 1 42 0 06 which is significantly heavier than the groundwater input average cold and hot springs 0 50 0 17 2sd and consistent with the preferential uptake of light si isotopes by diatoms the o 30si value at the outlet increases by up to 0 9 in spring and autumn relative to the si isotope composition of the inflow these seasonal diatom blooms can be modeled by an open system of si uptake and affect si fluxes at the outlet of the lake by up to 79 or 53 integrated over the year in the summer a shift to lighter o 30si values is correlated with a higher ph which results in dissolution of diatoms releasing light si isotopes from mass balance this seasonal diatom dissolution affects si fluxes by up to 33 but is limited to 3 7 integrated over the year these results clearly illustrate that biological activity can have a significant impact on both isotope composition and elemental abundance of continental derived si they also demonstrate the ph dependency of diatom dissolution and or preservation which is likely to affect not only the continental si fluxes to the ocean but also the si recycling in the oceans themselves

differential analysis of coda q using similar microearthquakes in seismic gaps part 2 application to seismograms recorded by the parkfield high resolution seismic network abstract nearly two thirds of the microseismicity occurring at parkfield california consists of repeating clusters of closely located events exhibiting waveform cross correlation values exceeding 0 95 this observation coupled with a high probability for an m 6 earthquake within the next decade working group on california earthquake probabilities 1988 make this segment of the san andreas fault system a prime location to search for temporal variations in coda q associated with crustal changes that may occur during the earthquake cycle the coda comparison analysis method described in part 1 of this study aster et al 1996 is applied to data from four borehole stations of the parkfield high resolution seismic network hrsn for the period january 1987 through february 1994 we examine 21 of the nearly 300 repeating clusters observed to date the restriction of the data set to only the most highly similar sources is observed to be critical small differences in the source process even for event pairs with centroid locations within approximately 20 m significantly increase the spurious noise level in the parameter estimates absolute 68 confidence bound estimates obtained on the repeatability of coda q for all microearthquake pairs are 10 for frequencies below 20 hz tighter constraints are obtainable by reconstructing the temporal history of coda q from the first difference measurements between microearthquake pairs these functional reconstructions show that the mean value of coda q has not varied by more than approximately 5 from 3 to 30 hz at the 1 level during the study period however other observables such as an increased level of microseismicity suggest that a preparatory stage for the next parkfield earthquake has begun indicating that coda q is so far showing no sign of sensitivity to preparatory processes at parkfield

postseismic response of repeating aftershocks the recurrence intervals of repeating earthquakes on the san andreas fault in the loma prieta aftershock zone follow the characteristic 1 t decay of omori s law a model in which these earthquakes occur on isolated patches of the fault that fail in stick slip with creep around them can explain this observation in this model the recurrence interval is inversely proportional to the loading rate due to creep logarithmic velocity strengthening friction predicts 1 t decay in creep rate following the mainshock the time dependence of recurrence is inconsistent with a simple viscous constitutive relationship which predicts an exponential decay of loading rate thus our observations imply postseismic slip at seismogenic depth under a power law rheology the time dependence of postseismic deformation measured geodetically may be diagnostic of whether postseismic deformation is caused by creep or possible viscoelastic deformation at greater depths