inversion for seismic slip rate history and distribution with stabilizing constraints application to the 1986 andreanof islands earthquake we develop and present a method of inverting seismograms including their absolute amplitudes to obtain the complete seismic slip time history and distribution using the method of linear programming a rectangular fault is discretized into cells and the source duration into time steps the inversion for the slip rate distribution on the fault as a function of time reduces to the solution of a system of linear equations the influence on the solution of constraining different source parameters such as the sign of the slip rate and the seismic moment is investigated it is found that the constraint of positiveness of slip rate and the constraint that the seismic moment determined from the solution should be close to that obtained from the centroid moment tensor cmt solution are very strong constraints and are essential to obtaining a physically reasonable solution since the inverse problem is unstable instead of presenting only one particular solution we examine several solutions equally well fitting the data for all practical purposes we find that when both constraints are applied the solution becomes quite stable the method is applied to a data set consisting of the vertical component of available digitally recorded teleseismic p waves at stations within a chosen distance window for the 1986 andreanof islands earthquake mw 8 0 the preferred solution is the one in which the slip rate is constrained to be positive and the moment is set to the cmt value the average rupture speed in the 90 km segment to the east of the hypocenter was 3 2 km s and in the 190 km section to the west was 2 km s the maximum slip in the area of large moment release to the west was 5 m and the maximum slip rate which occurs in the same place as the maximum slip was 6 5 m s the slip distribution on the fault shows a corrugated pattern with the long axis in the direction of the plate convergence rather than normal to the arc which was the slip vector direction for the earthquake with most of the slip occurring pn a region of the fault to the west of the rupture nucleation zone this region continued to slip for about 55 s the total source duration being about 80 s the results of this analysis suggest that the long term motion on this portion of the aleutian arc is indeed in the direction of plate convergence

the effect of redox conditions and bioirrigation on nitrogen isotope fractionation in marine sediments abstract nitrogen isotopic signatures of sources and sinks of fixed nitrogen n can be used to constrain marine nitrogen budgets however the reported fractionation during benthic n 2 production varies substantially to assess the range and mechanisms responsible for such observations we conducted a model study to evaluate the extent to which nitrification denitrification and anaerobic ammonium oxidation contribute to the isotopic composition of in situ n 2 production different hydrodynamic regimes were taken into account ranging from bioirrigation to diffusion dominated transport the benthic redox conditions were found to control the n isotope effect which under reducing conditions is driven by fractionation during nitrification and anaerobic ammonium oxidation and under oxidizing conditions by fractionation during denitrification environmental parameters such as the mineralization rate the bioirrigation intensity and chemical composition of the overlying water affect the benthic redox zonation and therefore also the benthic n isotope effect the n isotope effect of benthic n 2 production was computed for a wide range of bioirrigation intensities and mineralization rates and found to be approximately 3 for commonly encountered conditions this value is similar to previous estimates of the global n isotope effect of benthic n 2 production and further constrains the relative importance of water column vs benthic n 2 production

on the origin of the chemical properties of the arctic ocean halocline profiles of chemical constituents in seawater above the alpha ridge in the central arctic ocean obtained during the canadian expedition to study the alpha ridge cesar support a model in which the halocline is maintained by dense saline water produced on the continental shelves within the arctic ocean by the formation of sea ice it is argued that nutrient regeneration on the continental shelves largely accounts for the nutrient maximum that has been observed in the upper halocline over a wide region of the arctic ocean and that while the bering sea and north altantic can be recognized as origins of halocline water water masses from these regions undergo sufficient changes in their chemical properties within the arctic ocean significant that they no longer retain their identity in the halocline

horizontal subduction and truncation of the cocos plate beneath central mexico 1 based on analysis of data from a trans mexico temporary broadband seismic network centered on mexico city we report that the subducting cocos plate beneath central mexico is horizontal and tectonically underplates the base of the crust for a distance of 250 km from the trench it is decoupled from the crust by a very thin low viscosity zone the plate plunges into the mantle near mexico city but is truncated at a depth of 500 km probably due to an e w propagating tear in the cocos slab unlike the shallow slab subduction in peru and chile there is active volcanism along the trans mexican volcanic belt tmvb that lies much further inland than regions to either side where subduction dip is not horizontal geodynamical modeling indicates that a thin weak layer such as imaged by the seismic experiment can explain the flat subduction

seasonal and spatial patterns of sedimentary denitrification rates in the chukchi sea abstract recent constructions of the global nitrogen budget estimate that at least half of the ocean s fixed nitrogen is lost by sedimentary denitrification the majority of which occurs in continental shelves the arctic contains approximately 20 of the world s continental shelf suggesting it is a substantial contributor to the global sedimentary denitrification rate during two cruises in the summer and spring of 2002 and 2004 respectively denitrification rates were calculated from the downward diffusive flux of nitrate in the shelf and slope sediments of the chukchi sea in the western arctic additionally in the spring of 2004 denitrification rates were determined by whole core incubations in which the flux of nitrogen gas out of the sediments was measured measurements were made along three transects crossing the shelf and slope 50 3000 m each transect having different overlying water characteristics denitrification rates generally decreased with increasing water depth rates varied from about 1 6 mmol n m 2 d 1 for the shallow water sediments to undetectable in deep water sediments rates showed little variation between the two seasons however rates were found to correspond with differences in annual overlying primary productivities and particulate organic carbon export fluxes an extrapolation to the whole arctic yielded an average arctic sedimentary denitrification rate of 13 tg n yr 1 taken in the context of the global nitrogen budget it is about 4 13 of the total sink of fixed nitrogen in the ocean

determination of earthquake energy release and ml using terrascope we estimated the energy radiated by earthquakes in southern california using on scale very broadband recordings from terrascope the method we used involves time integration of the squared ground motion velocity and empirical determination of the distance attenuation function and the station corrections the time integral is typically taken over a duration of 2 min after the p wave arrival the attenuation curve for the energy integral we obtained is given by q r cr n exp kr r 2 2 h ref 2 with c 0 49710 n 1 0322 k 0 0035 km 1 and h ref 8 km where is the epicentral distance a similar method was used to determine m l using terrascope data the station corrections for m l are determined such that the m l values determined from terrascope agree with those from the traditional optical wood anderson seismographs for 1 5 6 5 m l saturates the ratio e s m 0 m 0 seismic moment a measure of the average stress drop for six earthquakes the 1989 montebello earthquake m l 4 6 the 1989 pasadena earthquake m l 4 9 the 1990 upland earthquake m l 5 2 the 1991 sierra madre earthquake m l 5 8 the 1992 joshua tree earthquake m l 6 1 and the 1992 landers earthquake m w 7 3 are about 10 times larger than those of the others that include the aftershocks of the 1987 whittier narrows earthquake the sierra madre earthquake the joshua tree earthquake and the two earthquakes on the san jacinto fault the difference in the stress drop between the mainshock and their large aftershocks may be similar to that between earthquakes on a fault with long and short repeat times the aftershocks which occurred on the fault plane where the mainshock slippage occurred had a very short time to heal hence a low stress drop the repeat time of the major earthquakes on the frontal fault systems in the transverse ranges in southern california is believed to be very long a few thousand years hence the events in the transverse ranges may have higher stress drops than those of the events occurring on faults with shorter repeat times such as the san andreas fault and the san jacinto fault the observation that very high stress drop events occur in the transverse ranges and the los angeles basin has important implications for the regional seismic potential the occurrence of these high stress drop events near the bottom of the seismogenic zone strongly suggests that these fault systems are capable of supporting high stress that will eventually be released in major seismic events characterization of earthquakes in terms of the e s m 0 ratio using broadband data will help delineate the spatial distribution of seismogenic stresses in the los angeles basin and the transverse ranges

modification of no po and no po during flow across the bering and chukchi shelves implications for use as arctic water mass tracers the no and po tracers 9 no3 o2 and 135 po4 o2 respectively and their derivative no po have found increasing use in arctic water mass analyses for identifying the specific basin or shelf areas from which surface waters originate based upon assumed differences in pacific and atlantic derived content and basin to basin differences within the arctic following shipboard sampling in june september 1993 and may june 1994 both north and south of bering strait we have found evidence that pacific derived waters flowing north to bering strait do not necessarily have any unique no po or no po identity that would permit unequivocal use as a water mass tracer in particular no po ratios in the bering sea continental shelf 150 m waters varied from 0 7 to 1 1 which encompasses ratios previously reported for arctic continental shelf and atlantic origin waters in the arctic ocean the highest no po ratios 1 in the bering sea were observed to the southwest of st lawrence island close to where high nutrient waters are first upwelled onto the shelf and seasonally early in the biological production cycle by contrast later in the summer north of bering strait at the depth of the arctic ocean nutrient maximum the highest concentrations of silica 60 m were associated with low no po ratios 0 7 apparent increases in the proportions of sea ice melt in these waters inferred from 18o and salinity regressions were associated with lower no po ratios this pattern the potential for sea air exchange and a significant relationship between decreases in nitrate phosphate ratios and both no po ratios and silica concentrations indicate that biological and physical processes north and south of bering strait affect the fidelity of these nutrient based tracers these results indicate the need for consideration of shelf based processes before no po ratios and other nutrient based tracers can be successfully applied as arctic circulation tracers

frictional melting during the rupture of the 1994 bolivian earthquake the source parameters of the 1994 bolivian earthquake magnitude m w 8 3 suggest that the maximum seismic efficiency was 0 036 and the minimum frictional stress was 550 bars thus the source process was dissipative which is consistent with the observed slow rupture speed only 20 of the locals wave velocity the amount of nonradiated energy produced during the bolivian rupture was comparable to or larger than the thermal energy of the 1980 mount st helens eruption and was sufficient to have melted a layer as thick as 31 centimeters once rupture was initiated melting could occur which reduces friction and promotes fault slip

slab pull and slab push earthquakes in the mexican chilean and peruvian subduction zones we studied intermediate depth earthquakes in the chile peru and mexican subduction zones paying special attention to slab push down dip compression and slab pull down dip extension mechanisms although slab push events are relatively rare in comparison with slab pull earthquakes quite a few have occurred recently in peru a couple slab push events occurred in 1991 and one slab pull together with several slab push events occurred in 1970 near chimbote in mexico several slab push and slab pull events occurred near zihuatanejo below the fault zone of the 1985 michoacan event in central chile a large m 7 1 slab push event occurred in october 1997 that followed a series of four shallow mw 6 thrust earthquakes on the plate interface we used teleseismic body waveform inversion of a number of mw 5 9 slab push and slab pull earthquakes in order to obtain accurate mechanisms depths and source time functions we used a master event method in order to get relative locations we discussed the occurrence of the relatively rare slab push events in the three subduction zones were they due to the geometry of the subduction that produces flexure inside the downgoing slab or were they produced by stress transfer during the earthquake cycle stress transfer can not explain the occurence of several compressional and extensional intraplate intermediate depth earthquakes in central chile central mexico and central peru it seemed that the heterogeneity of the stress field produced by complex slab geometry has an important influence on intraplate intermediate depth earthquakes 2002 elsevier science b v all rights reserved

on the processes controlling shelf basin exchange and outer shelf dynamics in the bering sea abstract we use a 9 km pan arctic ice ocean model to better understand the circulation and exchanges in the bering sea particularly near the shelf break this region has historically been undersampled for physical chemical and biological properties very little is known about how water from the deep basin reaches the large shallow bering sea shelf to address this we examine here the relationship between the bering slope current and exchange across the shelf break and resulting mass and property fluxes onto the shelf this understanding is critical to gain insight into the effects that the bering sea has on the arctic ocean especially in regard to recent indications of a warming climate in this region the bering sea shelf break region is characterized by the northwestward flowing bering slope current previously it was thought that once this current neared the siberian coast a portion of it made a sharp turn northward and encircled the gulf of anadyr in an anticyclonic fashion our model results indicate a significantly different circulation scheme whereby water from the deep basin is periodically moved northward onto the shelf by mesoscale processes along the shelf break canyons along the shelf break appear to be more prone to eddy activity and therefore are associated with higher rates of on shelf transport the horizontal resolution configured in this model now allows for the representation of eddies with diameters greater than 36 km however we are unable to resolve the smaller eddies

the proportion of remineralized nitrate on the ice covered eastern bering sea shelf evidenced from the oxygen isotope ratio of nitrate 1 we present measurements of nitrate and its natural abundance oxygen isotope composition 18o 16o in the water column of the broad and shallow eastern bering sea shelf in the late winter and early spring of 2007 and 2008 in both years nitrate concentrations showed a characteristic decrease from 25 m at the slope to 5 m inshore the 18o 16o ratio of nitrate 18ono3 versus smow also decreased from 3 2 at the slope to 1 5 inshore in 2007 and to as low as 1 inshore in 2008 indicating that nitrate inshore was nitrified at least once since having been entrained as nitrate from the slope the shoreward decrease was less pronounced in 2007 due to 18o enrichment of nitrate from incident phytoplankton assimilation in the ice covered water column whereas little to no algal growth in the water column was evident in 2008 by comparing the 18o of nitrate to that of ambient water in spring 2008 we estimate the fraction of nitrate that was remineralized in situ rather than recently advected from the slope these estimates indicate that 20 100 of the nitrate in winter water of the middle and inner shelves derives from regeneration directly on the shelf rather than from the seasonal entrainment of slope waters with recycling being the dominant mode of seasonal nitrate recharge from the 70 m isobath shoreward these observations indicate substantial nutrient recycling on the shallow shelf which has direct implications for the extent of fixed n loss to benthic denitrification and the fertility of the eastern shelf

a source and wave propagation study of the copalillo mexico earthquake of 21 july 2000 mw 5 9 implications for seismic hazard in mexico city from inslab earthquakes the copalillo earthquake of 21 july 2000 m w 5 9 is the closest well located inslab event to mexico city ever to be recorded in this study we analyze local and regional broadband and accelerometric recordings to determine the source parameters of the earthquake and the attenuation of ground motion with distance and to obtain a preliminary estimate of the seismic hazard posed to the city by such events our results show that the earthquake occurred at a depth of about 50 km most probably in the subducted oceanic crust the waveform inversion discriminates between the two nodal planes the fault plane defined by the following strike 305 dip 32 and rake 80 the rupture propagated nearly unilaterally along the strike toward northwest with a small downdip component the observed source spectrum can be well explained by an 2 source model with m 0 6 0 10 25 dyne cm and a stress drop of 360 bar we find that high frequency ground motion f 3 hz which is related to a max during inslab earthquakes is not amplified at ciudad universitaria cu a hill zone site in the valley of mexico that is known to suffer amplification at low frequencies 0 1 f m w 7 0 event could give rise to an a max value of 30 40 gal the cu recordings indicate that the a max value of 30 gal could have a return period of about 40 yr about the same as from shallow dipping thrust earthquakes along the mexican subduction zone which have been regarded as posing the highest hazard for the city an inslab earthquake with an a max value of about 40 gal could cause heavy damage to small buildings at certain locations of the city we conclude that seismic hazard from inslab earthquakes to mexico city has so far been underestimated

isotopic constraints on glacial interglacial changes in the oceanic nitrogen budget early deglacial maximum in nitrate 15 n 14 n in suboxic zones and no significant glacialto late holocene change in global ocean nitrate 15 n 14 n consistent with the work of brandes and devol 2002 we find that the steady state 15 n 14 n of oceanic nitrate is controlled primarily by the fraction of total denitrification that occurs in the water column therefore a deglacial peak in the ratio of water column to sediment denitrification caused by either a strong feedback between water column denitrification and the n reservoir or by an increase in sediment denitrification due to sea level rise can explain the observed deglacial 15 n 14 n maximum in sediments underlying water column denitrification zones the total denitrification rate and the mean ocean nitrate concentration are also important determinants of steady state nitrate 15 n 14 n for this reason modeling a realistic deglacial 15 n 14 n maximum further requires that the combined negative feedbacks from n2 fixation and denitrification are relatively strong and n losses are relatively small our results suggest that the glacial oceanic n inventory was at most 30 greater than today s and probably less than 10 greater index terms 4267 oceanography general paleoceanography 4805 oceanography biological and chemical biogeochemical cycles 1615 4845 oceanography biological and chemical nutrients and nutrient cycling 4870 oceanography biological and chemical stable isotopes keywords feedback nitrogen isotopes paleoceanography

reconciling teleseismic and regional estimates of seismic energy estimates of the radiated seismic energy based on teleseismic and regional data often differ by up to an order of magnitude with a tendency for regional estimates to be larger than teleseismic estimates for the same event in this study we compare the velocity spectrum determined from teleseismic data after correction for radiation pattern and propagation effects with the velocity spectrum determined from regional data after the corresponding corrections for nine earthquakes in the middle america subduction zone of mexico this comparison of the corrected spectra is used to identify and reduce the sources of the regional versus teleseismic energy discrepancy which is about an order of magnitude for these events we find that the teleseismic attenuation operator needs to be calibrated in our case for the tectonic environment of the mexican subduction zone we need a teleseismic attenuation operator that is stronger at high frequencies than the global average a larger factor however is the correction needed to account for site amplification this correction has an impact on both regional and teleseismic data but it has a larger influence on the regional estimates because the angle of incidence for teleseismic waves is steep and the stations are located on more competent rock by modifying the teleseismic attenuation operator and applying site corrections based on a generic site model we essentially eliminate the order of magnitude discrepancy between teleseismic and regional estimates of the radiated seismic energy for these events

can observations of earthquake scaling constrain slip weakening summary we use observations of earthquake source parameters over a wide magnitude range m w 0 7 to place constraints on constitutive fault weakening the data suggest a scale dependence of apparent stress and stress drop both may increase slightly with earthquake size we show that this scale dependence need not imply any difference in fault zone properties for different sized earthquakes we select 30 earthquakes well recorded at 2 5 km depth at cajon pass california we use individual and empirical green s function spectral analysis to improve the resolution of source parameters including static stress drop and total slip s we also measure radiated energy e s w ecompare the cajon pass results with those from larger california earthquakes including aftershocks of the 1994 northridge earthquake and confirm the results of abercrombie 1995 e s m 0 where rigidity and both e s m 0 and increase as m 0 and s increases uncertainties remain large due to model assumptions and variations between possible models and earthquake scale independence is possible within the resolution assuming that the average trends are real we define a quantity g 2 es m 0 s 2w hich is the total energy dissipation in friction and fracture minus 1 s w here 1 is the final static stress if 1 d the dynamic shear strength during the last increments of seismic slip then g g the fracture energy in a slip weakening interpretation of dissipation

crustal structure below the valley of mexico estimated from receiver functions abstract receiver functions rfs from two permanent stations cuig and zaig of the mexican broadband network were calculated to infer the crustal structure at both sites the data were stacked to allow uncertainty estimations and inverted using two global optimization algorithms simulated annealing and genetic algorithms strong and systematic azimuthal dependence was observed in the stacked functions at cuig site which is located in the mexican volcanic belt mvb assuming a one dimensional five layer model per azimuthal stack these variations were explained with the same upper crustal structure including a superficial 2 km thick low velocity layer associated with the mvb and a well defined conrad interface around 15 km depth different depths for the moho discontinuity and variations in both the s wave velocity and poisson s ratio for the upper mantle our inversion of observed rfs at cuig and theoretical considerations suggest an eastward crustal thickening of at least 5 km below the valley of mexico we tested the sensitivity of the modeling procedure by applying it to the recordings at zaig station in this case no azimuthal dependence was noticed in the stacked functions after a simultaneous inversion of three different azimuthal stacks a single four layer model satisfactorily explained all data thus revealing a flat one dimensional crustal model below the mexican mesa central where zaig station is located these results support the resolution and robustness of the applied method at both sites

isotopic fractionation of oxygen and nitrogen in coastal marine sediments abstract the bulk and isotopic exchanges of o2 n2 no3 and nh4 between sediments and the overlying water benthic flux were measured at three shallow locations in puget sound using a benthic tripod oxygen consumption by the sediments averaged 4 4 mmol m 2 d 1 while n2 gas fluxes averaged 1 1 mmol m 2 d 1 the nitrogen gas flux out of the sediments was always greater than the corresponding no3 flux into the sediments indicating a large nitrification contribution to the n2 flux isotopic changes in overlying water 18o of o2 were small with an average apparent sediment respiration fractionation factor e of 3 this fractionation factor is much smaller than that measured in either the open ocean or in the laboratory the low degree of fractionation is hypothesized to occur because of the interaction of diffusive layers around reactive microsites within these sediments there was no apparent isotopic fractionation of downward diffusing no3 e 0 while the ammonium that diffused out of the sediments averaged 4 5 o heavier than both source organic matter within the sediments and overlying water no3 the large discrepancy in isotopic composition between the source organic material and nh4 can be attributed to significant isotopic fractionation of ammonium during nitrification in the oxic zone finally the 15n of n2 did not change significantly over the course of the incubations in agreement with the nitrogen isotopic budget for these sediments

nitrogen cycling in the sediments of santa barbara basin and eastern subtropical north pacific nitrogen isotopes diagenesis and possible chemosymbiosis between two lithotrophs thioploca and anammox riding on a glider abstract this article presents data on the isotopic composition of pore water ammonium and n org in the anoxic sediments of the santa barbara basin and the eastern subtropical north pacific esnp region ammonium 15 n behaves differently in the two regions reflecting variability in bacterial processes of sedimentary nitrogen cycling in santa barbara basin stoichiometry of tco 2 and nh 4 production defined by pore water chemistry indicates that the only reaction influencing pore water ammonium concentration in these sediments appears to be organic matter decomposition pore water ammonium was found to be 1 3 heavier than decomposing organic matter in contrast the upper 15 30 cm of esnp pore waters have ammonium with 15 n that is more than 10 heavier than decomposing organic matter at greater depth the 15 n of deeper pore waters approaches that of decomposing organic matter mass balance calculations indicate that the observed isotopic enrichment is not due to the preferential loss of isotopically enriched organic matter pore water profiles show that the zone with heavy isotopes is characterized by a tco 2 nh 4 inputs ratio of 4 or less we attribute this to the presence of thioploca bacteria who transport no 3 to depth and reduce it to nh 4 while oxidizing sulfide based on pore water stoichiometry and reaction diffusion modeling of pore water isotopic composition we propose that the 15 n enriched ammonium is produced as the result of chemosymbiosis between thioploca and anammox like bacteria in these sediments our work provides the new insights into benthic microbial ecology and the role of coupled individual metabolic pathways in sedimentary nitrogen cycling in santa barbara basin factors which contribute to the isotopic fractionation of 1 3 between pore water ammonium and solid n org were also considered in this sediments 23 of the rain of organic nitrogen is lost to diagenesis therefore the isotopic effect of degradation on the residual n org is estimated to be small 0 7 the most likely cause of fractionation is preferential degradation of an isotopically heavier more labile marine fraction relative to an isotopically lighter more refractory terrestrial component of the organic matter but other possibilities cannot be ruled out when corrected for the observed fractionation it appears that the 15 n of these sediments may represent non steady state conditions

a global marine fixed nitrogen isotopic budget implications for holocene nitrogen cycling 1 a nitrogen stable isotopic model was constructed in order to constrain the holocene marine fixed nitrogen budget the primary sources and sinks considered were riverine and atmospheric sources nitrogen fixation sedimentary and water column denitrification and sediment burial the source budget was found to be insensitive to changes in nitrogen fixation rates and thus could not be used to constrain this term however the isotopic value of fixed nitrogen losses was very sensitive to the amount of sedimentary denitrification if the isotopic value of marine fixed nitrogen has not changed during the holocene as supported by sedimentary records then in order to balance the isotopic value of sinks and sources approximately 280 tg n yr 1 of sedimentary denitrification is required if such a high rate of denitrification has been sustained throughout the holocene it implies that present day estimates of marine nitrogen fixation are grossly underestimated it also implies that the marine nitrogen budget has a residence time of less than 2000 years

interannual variability of summer biochemical enhancement in relation to mesoscale eddies at the shelf break in the vicinity of the pribilof islands bering sea abstract this study examined variability of the eddy field and primary production over the bering sea s eastern shelf break area during summer using a satellite multi sensor data set the eddy field and on shelf nutrient flux below 50 m water depth were also investigated using a numerical model the satellite altimeter data analysis and numerical experiments indicated that the eddy field was induced by perturbations of the bering slope current system and or instabilities in the bering sea basin via flow through the aleutian passes the distribution of high primary production roughly coincided with the high variability area of the eddy field numerical experiments showed a 70 increase in net on shelf flux and 54 increase in net on shelf transport by the generation propagation of eddies along the shelf break at pribilof canyon towed ctd fluorometer measurements during the summer of 2003 revealed a stable water column and high fluorescence 10 g l 1 in the area between the 200 and 1000 m isobaths at a time when there was a low level of on shelf flux regardless of the eddy field conversely the eddy induced on shelf flux and stable water column can enhance primary production from spring to summer at the shelf break moreover the eddy induced on shelf nutrient flux probably contributes to the primary productivity at the western shelf of the pribilof islands where the bering sea ecosystem is very dynamic this eddy related shelf break process can be applied to not only the bering sea shelf break but also all shelf break areas in which propagating eddies are found

dynamic source inversion of the m6 5 intermediate depth zumpango earthquake in central mexico a parallel genetic algorithm we introduce a method for imaging the earthquake source dynamics from the inversion of ground motion records based on a parallel genetic algorithm the source model follows an elliptical patch approach and uses the staggered grid split node method to simulate the earthquake dynamics a statistical analysis is used to estimate errors in both inverted and derived source parameters synthetic inversion tests reveal that the average rupture speed vr the rupture area and the stress drop may be determined with formal errors of 30 12 and 10 respectively in contrast derived parameters such as the radiated energy er the radiation efficiency r and the fracture energy g have larger errors around 70 40 and 25 respectively we applied the method to the mw 6 5 intermediate depth 62 km normal faulting earthquake of 11 december 2011 in guerrero mexico inferred values of 29 2 6 2 mpa and r 0 26 0 1 are significantly higher and lower respectively than those of typical subduction thrust events fracture energy is large so that more than 73 of the available potential energy for the dynamic process of faulting was deposited in the focal region i e g 14 4 3 5 1014j producing a slow rupture process vr vs 0 47 0 09 despite the relatively high energy radiation er 0 54 0 31 1015 j and energy moment ratio er m0 5 7 10 5 it is interesting to point out that such a slow and inefficient rupture along with the large stress drop in a small focal region are features also observed in both the 1994 deep bolivian earthquake and the seismicity of the intermediate depth bucaramanga nest

seismic site effects and the spatial interpolation of earthquake seismograms results using aftershocks of the 1986 north palm springs california earthquake abstract we address the following two questions can a microearthquake9s ground motions be modeled by incident p and s waves that excite a site transfer function that is a smooth function of incidence angle given recorded ground motions from a set of earthquakes having known locations and mechanisms can we derive such a site transfer function and use it to obtain the ground motions that would result from an earthquake source occurring somewhere in the same volume but having a location and mechanism that are different from the recorded events although many factors will cause two distinct microearthquake sources to have different seismograms at a common station in this paper we concentrate only upon the differences caused by source mechanisms p and s wave travel time variations and by variations in the site transfer function we specifically exclude the effects of waves scattered from heterogeneities in the geologic structure away from the seismic site we express the site transfer function as a sum of several terms having simple dependences upon incidence angle and azimuth each term is an independent function of time given a set of seismograms observed at the site we solve a linear system of equations for the time dependences of each term these time series may be used to calculate the seismograms that would have resulted from an earthquake having arbitrary mechanism and location this step is an interpolation we have applied this technique to seismograms after aftershocks of the 1986 north palm springs earthquake our interpolation technique works fairly well within the volume occupied by the recorded events but the method is not very successful at providing accurate seismograms for sources located outside the aftershock volume the primary causes of the inaccuracy are the inadequacy of our chosen angular functions to model the site response fully and the likely scattering of seismic waves by geological heterogeneities in this case the banning and mission creek faults near the seismic stations our methods could be used to determine the effects of single scattering from lateral heterogeneities in geologic structure

aspects of the marine nitrogen cycle of the chukchi sea shelf and canada basin abstract as a highly productive seasonally ice covered sea with an expansive shallow continental shelf the chukchi sea fuels high rates of sedimentary denitrification this contributes to its fixed nitrogen n deficit relative to phosphorus p which is among the largest in the global ocean making the chukchi sea severely n limited during the phytoplankton growth season here we examine aspects of the n cycle on the chukchi sea shelf and the downstream canada basin using nutrients dissolved oxygen o 2 and the stable isotopes of nitrate no 3 in the northward flow path across the chukchi shelf bottom waters experienced strong o 2 drawdown from which we calculated a nitrification rate of 1 3 mmol m 2 d 1 this nitrification was likely primarily in sediments and directly fueled sedimentary denitrification historically measured at similar rates we observed significant accumulations of ammonium nh 4 in bottom waters of the chukchi shelf up to 5 m which were inversely correlated with 15 n no 3 indicating a sediment source of 15 n enriched nh 4 this is consistent with a process of coupled partial nitrification denitrification cpnd which imparts significant 15 n enrichment and 18 o depletion to pacific origin no 3 this cpnd mechanism is consistent with a significant decrease in 18 o no 3 relative to bering sea source waters indicating that at least 58 of no 3 populating the pacific halocline was regenerated during its transit across the north bering and chukchi shelves rather than arriving preformed from the bering sea slope this pacific origin no 3 propagates into the canada basin and towards the north atlantic being significantly 15 n enriched and 18 o depleted relative to the underlying atlantic waters

benthic nitrogen fluxes and fractionation of nitrate in the mauritanian oxygen minimum zone eastern tropical north atlantic we present sedimentary geochemical data and in situ benthic flux measurements of dissolved inorganic nitrogen din no3 no2 nh4 and oxygen o2 from 7 sites with variable sand content along 18 n offshore mauritania nw africa bottom water o2 concentrations at the shallowest station were hypoxic 42 m and increased to 125 m at the deepest site 1113 m total oxygen uptake rates were highest on the shelf 10 3 mmol o2 m 2 d 1 and decreased quasi exponentially with water depth to 3 2 mmol o2 m 2 d 1 average denitrification rates estimated from a flux balance decreased with water depth from 2 2 to 0 2 mmol n m 2 d 1 overall the sediments acted as net sink for din observed increases in 15nno3 and 18ono3 in the benthic chamber deployed on the shelf characterized by muddy sand were used to calculate apparent benthic nitrate fractionation factors of 8 0 15eapp and 14 1 18eapp measurements of 15nno2 further demonstrated that the sediments acted as a source of 15n depleted no2 these observations were analyzed using an isotope box model that considered denitrification and nitrification of nh4 and no2 the principal findings were that i net benthic 14n 15n fractionation eden was 12 9 1 7 ii inverse fractionation during nitrite oxidation leads to an efflux of isotopically light no2 22 1 9 and iii direct coupling between nitrification and denitrification in the sediment is negligible previously reported eden for fine grained sediments are much lower 4 8 we speculate that high benthic nitrate fractionation is driven by a combination of enhanced porewater seawater exchange in permeable sediments and the hypoxic high productivity environment although not without uncertainties the results presented could have important implications for understanding the current state of the marine n cycle

subducting slab ultra slow velocity layer coincident with silent earthquakes in southern mexico great earthquakes have repeatedly occurred on the plate interface in a few shallow dipping subduction zones where the subducting and overriding plates are strongly locked silent earthquakes or slow slip events were recently discovered at the down dip extension of the locked zone and interact with the earthquake cycle here we show that locally observed converted sp arrivals and teleseismic underside reflections that sample the top of the subducting plate in southern mexico reveal that the ultra slow velocity layer usl varies spatially 3 to 5 kilometers with an s wave velocity of 2 0 to 2 7 kilometers per second most slow slip patches coincide with the presence of the usl and they are bounded by the absence of the usl the extent of the usl delineates the zone of transitional frictional behavior

a test of the nitrogen limitation hypothesis for retarded eukaryote radiation nitrogen isotopes across a mesoproterozoic basinal profile abstract nitrogen limitation caused by trace metal scarcity under euxinic ocean conditions has been proposed as an explanation for the delayed radiation of eukaryotes until at least the late mesoproterozoic however evidence for how the nitrogen cycle was operating during the middle precambrian is so far rare more specifically it is unknown which steps in the biogeochemical nitrogen cycle e g nitrogen fixation nitrification or ammonification were rate limiting and thus controlling microbial community structures the mesoproterozoic belt supergroup in western montana hosts a variety of facies ranging from shallow to deep water and thus offers the opportunity to address this issue bulk 15 n values show a clear trend from 1 in the deepest part of the basin to 5 along basin margins which suggests that coastal areas were sufficiently oxygenated for aerobic nitrogen cycling the total fractionation of carbon isotopes between carbonate and organic carbon 13 c increases from 20 to 32 in the same direction possibly indicating an ecological response to redox stratification and nitrogen speciation evidence from the belt supergroup is thus consistent with the idea that nutrient availability may have restricted early eukaryotic organisms to a narrow range of habitats which thus prevented a global rise to ecological dominance until concentrations of fixed nitrogen increased in the global open ocean

water column iron dynamics in the subarctic north pacific ocean and the bering sea 1 we measured water column iron concentrations from west to east along 47 n in the subarctic north pacific and in the bering sea in the north pacific dissolved fe d fe showed surface depletion mid depth maxima at 1000 1500 m west 1 3 1 6 nm east 0 9 1 1 nm and a gradual decrease with depth below 3500 4000 m depth west 1 1 1 4 nm east 0 6 0 7 nm d fe and total soluble fe t fe in deep water showed a decreasing trend eastward the higher iron concentrations in western deep waters probably result from higher inputs of dissolved fe through atmospheric deposition or lateral transport in contrast d fe throughout the bering sea showed a consistent depth regime characterized by a rapid increase with depth to mid depths a gradual increase with depth in intermediate water to a maximum of 1 6 1 7 nm at 1500 2250 m and a gradual decrease with depth to 1 3 1 4 nm at 3700 m higher iron concentrations and deeper d fe maxima in the bering sea are likely due to higher biological productivity and greater and deeper d fe input from the decomposition of sinking particulate organic matter in deep water we suggest that the higher concentrations and deeper input of d fe as well as po4 and humic type fluorescent dissolved organic matter in the bering sea probably results from the longer time for the accumulation of decomposition products resulting from iron supply from the organic rich downslope sediment along the steep continental slopes and slow replacement of the deep water in the bering sea basin

tracing pacific water in the north atlantic ocean 1 in the arctic ocean pacific source water can be distinguished from atlantic source water by nitrate phosphate concentration relationships with pacific water having higher phosphate concentrations relative to those of nitrate furthermore pacific water originally from the inflow through bering strait is clearly recognizable in the outflows of low salinity waters from the arctic ocean to the northern north atlantic ocean through the canadian arctic archipelago and through fram strait in the canadian arctic archipelago we observe that almost all of the waters flowing through lancaster and jones sounds most of the water in the top 100 m in smith sound containing the flow through nares strait and possibly all waters in hudson bay contain no water of atlantic origin significant amounts of pacific water are also observed along the western coast of baffin bay along the coast of labrador and above the 200 m isobath of the grand banks there is a clear signal of pacific water flowing south through fram strait and along the east coast of greenland extending at least as far south as denmark strait pacific water signature can be seen near the east coast of greenland at 66 n but not in data at 60 n temporal variability in the concentrations of pacific water has been observed at several locations where multiple year observations are available

spatial and temporal trends in precambrian nitrogen cycling a mesoproterozoic offshore nitrate minimum abstract fixed nitrogen is an essential nutrient for eukaryotes as n 2 fixation and assimilation of nitrate are catalyzed by metalloenzymes it has been hypothesized that in mesoproterozoic oceans nitrate was limited in offshore environments by low trace metal concentrations and high rates of denitrification in anoxic and episodically euxinic deep water masses restricting eukaryotes to near shore environments and limiting their evolutionary innovation to date this hypothesis has only been tested in the belt supergroup 1 4 ga with results that support an onshore offshore nitrate gradient as a potential control on eukaryote ecology here we present bulk nitrogen and organic carbon isotopic data from non isochronous cross basinal facies across the bangemall 1 5 ga and the roper 1 4 1 5 ga basins to better understand the extent and variability of onshore offshore nitrogen isotope gradients in the mesoproterozoic both basins show an average 1 2 enrichment in 15 n bulk from deep to shallow facies with a maximum range from 1 offshore to 7 5 onshore unlike the belt basin the bangemall and roper basins show some offshore 15 n bulk values that are enriched beyond the isotopic range associated with biological n 2 fixation alone this suggests a mixture of aerobic and anaerobic metabolisms offshore in shallow waters where 15 n bulk enrichment peaks an aerobic nitrogen cycle was evidently operating even though isotopic signatures of aerobic nitrogen cycling are seen in all parts of the bangemall and roper basins our data are consistent with a lateral gradient in nitrate availability within the photic zone with higher concentrations in near shore environments than offshore the variability in 15 n bulk values in each depositional environment and the consistently low 15 n bulk values from mesoproterozoic units compared to the paleoproterozoic and neoproterozoic suggest that nitrate concentrations in the global ocean were likely low this trend is now seen in all three mesoproterozoic basins so far examined and contrasts with the paleoproterozoic and neoproterozoic where nearly all 15 n bulk data plot above the n 2 fixation window thus we propose that the mesoproterozoic ocean was characterized by a nitrate minimum with the lowest concentrations in offshore environments this inference is consistent with a mesoproterozoic o 2 decline following a temporary paleoproterozoic o 2 peak and it further supports the idea that nitrate limitation offshore may have contributed to the restriction of photosynthetic eukaryotes to near shore environments delaying their rise to ecological dominance until the neoproterozoic era

kinematic and dynamic inversion of the 2008 northern iwate earthquake we perform kinematic and dynamic inversion of the 24 july 2008 m w 6 8 northern iwate intermediate depth earthquake in japan using strong motion records from the k net and kik net networks the rupture of this moderate magnitude earthquake is modeled as a simple elliptical patch the optimal solutions are found comparing observed and synthetic records using an l 2 norm and the neighborhood algorithm to search for the best solution followed by an exploration of solution space with a monte carlo technique the geometry of the rupture rupture velocity and slip distribution are estimated by kinematic inversion the rupture geometry stress and friction parameters are obtained by dynamic inversion both approaches converge to very similar source models with semiminor axes of 4 km maximum slip of about 4 m and large stress drops in the 30 45 mpa range rupture duration was less than 3 s because of very high sub rayleigh rupture propagation speeds energy release rate for the best models was in the range 23 36 mj m2 a rather large value for events of this size for both kinematic and dynamic inversion we found families of solutions that fit the strong motion data within a certain error confirming the strong trade off among inverted parameters finally we demonstrate that dynamic inversion solutions are controlled by the dynamic similarity parameter and by seismic moment m these two parameters define a region of model space where dynamically similar models fit the observations with approximately the same misfit online material figures that compare observed and synthetic waveforms for all available stations and comparison of fourier spectrum of observed and synthetic seismograms

estimation of the breakdown slip from strong motion seismograms insights from numerical experiments fukuyama and mikumo 2007 presented a method to extract informa tion about the friction controlling earthquake rupture directly from near field seismo grams recorded out to several kilometers from a fault this method computes a parameter d 00 approximating the breakdown slip dc from the rake parallel dis placement at the time of the peak particle velocity the validation of the method was based on simple 2d steady state green s functions an approach lacking sufficient physics of the rupture process to support their conclusions here we use 3d simula tions of subshear strike slip spontaneous rupture propagation to demonstrate that d 00 is almost always controlled by rupture and wave propagation effects rather than the fault friction process only if rupture reaches the earth s surface and within a short distance from the fault rc is it possible to extract information about dc from strong motion data due to the fast decay with distance from the fault of the seismic energy related to the stress breakdown process beyond rc d 00 is controlled by the dynamic stress drop from the earthquake rupture without information about dc rc is comparable to the length of the fault cohesive zone where the breakdown process takes place during rupture and approximately equal to 80 of the wavelength associated with the break down frequency defined as the reciprocal of the time span required by the stress to drop to the dynamic level our findings suggest that the dc estimate by fukuyama and mikumo 2007 for the 2000 mw 6 6 tottori earthquake is unrelated to the breakdown slip moreover the very narrow widths for rc obtained from our simulations are in agreement with previous studies challenging the resolution of bandlimited strong motion data to estimate dc from kinematic rupture models guatteri and spudich 2000 spudich and guatteri 2004

pervasive aerobic nitrogen cycling in the surface ocean across the paleoproterozoic era abstract nitrogen isotope ratios in marine sedimentary rocks have become a widely used biogeochemical proxy that records information about nutrient cycling and redox conditions in earth s distant past while the past two decades have seen considerable progress in our understanding of the precambrian sedimentary nitrogen isotope record it is still compromised by substantial temporal gaps furthermore quantitative links between nitrogen isotope data marine redox conditions and nutrient availability are largely lacking in a precambrian context here we present new nitrogen isotope data from a suite of marine sedimentary rocks with ca 2 4 to 1 8 ga ages spanning the great oxidation event in the paleoproterozoic to better constrain the response of the nitrogen cycle to the first major redox transition in earth s history we further construct a simple box model to describe the major pathways that influenced the nitrogen isotope mass balance of the precambrian ocean and use this as a platform to evaluate the precambrian nitrogen isotope record within this framework we find that consistently positive nitrogen isotope values ranging from 1 1 to 7 7 across the early paleoproterozoic are strong evidence for an expansion of oxygenated surface waters since the isotopic signature of aerobic nitrogen cycling is recorded in the biomass of nitrate assimilating organisms this implicates widespread nitrate bioavailability in this time interval the decline in offshore nitrogen isotope ratios in the mesoproterozoic is consistent with the contraction of oxic waters which could have inhibited the expansion of nitrate fueled ecosystems to pelagic waters until the widespread oxygenation of the ocean in the latest neoproterozoic to early phanerozoic

geometry of subduction and depth of the seismogenic zone in the guerrero gap mexico the western coast of guerrero in southern mexico has been identified as a seismic gap on the middle american trench in which no large earthquakes have occurred at least since 1908 it has been suggested that the seismic energy accumulated since 1908 will eventually be released by a large earthquake a permanent seismic network was installed to monitor the seismicity of this mature seismic gap and to understand the geometry of the subducted slab beneath this region the seismicity defines an unusual distribution along two bands of seismic activity parallel to the coast the resulting geometry of the subduction zone shows that the cocos plate dips at a shallow angle beneath the north american plate to a depth of 40 km from there the subducted slab is bent upward following a subhorizontal trajectory extending inland at a depth of 50 km this plate geometry is reminiscent of that found in peru and central argentina two other regions where a young oceanic plate is being subducted in mexico however the slab underplates an overriding plate which is only half as thick as that observed in south america a possible oceanic origin of the allochtonous terranes comprising southern mexico may explain the presence of the anomalously thin lithosphere in this region

teleseismic estimates of radiated seismic energy the e m 0 discriminant for tsunami earthquakes we adapt the formalism of boatwright and choy for the computation of radiated seismic energy from broadband records at teleseismic distances to the real time situation when neither the depth nor the focal geometry of the source is known accurately the analysis of a large data set of more than 500 records from 52 large recent earthquakes shows that this procedure yields values of the estimated energy ee in good agreement with values computed from available source parameters for example as published by the national earthquake information center neic the average logarithmic residual being only 0 26 units we analyze the energy to moment ratio by defining log10 ee m0 for regular earthquakes this parameter agrees well with values expected from theoretical models and from the worldwide neic catalogue there is a one to one correspondence between values of that are deficient by one full unit or more and the so called tsunami earthquakes previously identified in the literature as having exceedingly slow sources and believed due to the presence of sedimentary structures in the fault zone our formalism can be applied to single station measurements and its coupling to automated real time measurements of the seismic moment using the mantle magnitude mm should significantly improve real time tsunami warning

transport through unimak pass alaska we examined inflow through unimak pass 200 m deep which is the only major connection between the shelves of the north pacific ocean and the eastern bering sea geostrophic transport was generally northward from the gulf of alaska into the bering sea the flow through the pass appeared to be modulated by the seasonal cycle of freshwater discharge on shorter time scales transport also was affected by semi daily variations in tidal mixing this effect was significant and not anticipated near bottom currents measured from moorings were maximum during winter and significantly correlated r 0 7 with the alongshore winds although the flow through unimak pass transported some nutrients from the north pacific ocean the gulf of alaska shelf is not the major source of nutrients to the bering sea shelf

spring and fall phytoplankton blooms in a productive subarctic ecosystem the eastern bering sea during 1995 2011 abstract the timing and magnitude of phytoplankton blooms in subarctic ecosystems often strongly influence the amount of energy that is transferred through subsequent trophic pathways in the eastern bering sea spring bloom timing has been linked to ice retreat timing and production of zooplankton and fish a large part of the eastern bering sea shelf 500 km wide is ice covered during winter and spring four oceanographic moorings have been deployed along the 70 m depth contour of the eastern bering sea shelf with the southern location occupied annually since 1995 the two northern locations since 2004 and the remaining location since 2001 chlorophyll a fluorescence data from the four moorings provide 37 realizations of a spring bloom and 33 realizations of a fall bloom we found that in the eastern bering sea if ice was present after mid march spring bloom timing was related to ice retreat timing p n 11 a fall bloom also commonly occurred usually in late september average day 274 se 4 2 n 33 and its timing was not significantly related to the timing of storms p 0 88 df 1 27 or fall water column overturn p 0 49 df 1 27 the magnitudes of the spring and fall blooms were correlated p 0 011 df 28 the interval between the spring and fall blooms varied between four to six months depending on year and location we present a hypothesis to explain how the large crustacean zooplankton taxa calanus spp likely respond to variation in the interval between blooms spring to fall and fall to spring

origin of the deep bering sea nitrate deficit constraints from the nitrogen and oxygen isotopic composition of water column nitrate and benthic nitrate fluxes 1 on the basis of the normalization to phosphate a significant amount of nitrate is missing from the deep bering sea bs benthic denitrification has been suggested previously to be the dominant cause for the bs nitrate deficit we measured water column nitrate 15n 14n and 18o 16o as integrative tracers of microbial denitrification together with pore water derived benthic nitrate fluxes in the deep bs basin in order to gain new constraints on the mechanism of fixed nitrogen loss in the bs the lack of any nitrate isotope enrichment into the deep part of the bs supports the benthic denitrification hypothesis on the basis of the nitrate deficit in the water column with respect to the adjacent north pacific and a radiocarbon derived ventilation age of 50 years we calculate an average deep bs 2000 m water depth sedimentary denitrification rate of 230 mol n m 2 d 1 or 1 27 tg n yr 1 more than 3 times higher than high end estimates of the average global sedimentary denitrification rate for the same depth interval pore water derived estimates of benthic denitrification were variable and uncertainties in estimates were large a very high denitrification rate measured from the base of the steep northern slope of the basin suggests that the elevated average sedimentary denitrification rate of the deep bering calculated from the nitrate deficit is driven by organic matter supply to the base of the continental slope owing to a combination of high primary productivity in the surface waters along the shelf break and efficient down slope sediment focusing along the steep continental slopes that characterize the bs

nonlinear dynamic rupture inversion of the 2000 western tottori japan earthquake 1 we have developed a systematic nonlinear inversion method for estimating rupture propagation and the underlying dynamic parameters for large historical earthquakes the rupture modeling is carried out using a three dimensional finite difference method and the inversion is implemented by a neighbourhood algorithm minimizing the misfit between computed and observed near fault seismograms we test the method by estimating the stress drop within 32 regions on the causative fault for the 2000 magnitude 6 6 western tottori japan earthquake while the dynamic models show both similarities and differences with the conventional kinematic models our method provides an ensemble of physically correct models with plausible rupture propagation for the earthquake

seismic energy release in mexican subduction zone earthquakes we estimated radiated seismic energy es from 18 shallow thrust mexican subduction zone earthquakes 4 1022 m 1 1 1028 dyne cm 11 h 37 km using digital accelerograms from the guerrero accelerograph array and neglecting stations with large site effects es is computed by integrating squared velocity spectra after applying geometrical spreading and q corrections we discarded epicentral recordings for the largest michoacan event we find that log es m 4 152 0 275 which gives a median value of es m and apparent stress a of 7 1 10 5 and 24 bars respectively the median es m value is in accordance with gutenberg and richter s g r 1956 formula for es in which es m 5 10 5 is implicit worldwide es m data where es is computed from local records mostly fall between 5 10 5 and 5 10 4 for events with m 1022 dyne cm on the other hand es m values generally lie between 5 10 6 and 5 10 5 if es is estimated from teleseismic records especially anomalous are the es m data from kikuchi and fukao 1988 for large and great earthquakes which fall near 5 10 6 thus while es from local data suggests that the g r relation seldom overestimates seismic energy release the teleseismic data point to the contrary the cause of this discrepancy may lie in the difficulty of resolving incoherent radiation from the fault and inappropriate choice of t in the analysis of teleseismic data

stress breakdown time and slip weakening distance inferred from slip velocity functions on earthquake faults we estimate the critical slip weakening distance on earthquake faults by using a new approach which is independent of the estimate of fracture energy or radiated seismic energy the approach is to find a physically based relation between the breakdown time of shear stress tb the time of peak slip velocity tpv and the slip weakening distance dc from the time histories of shear stress slip and slip velocity at each point on the fault which can be obtained from dynamic rupture calculations using a simple slip weakening friction law numerical calculations are carried out for a dynamic shear crack propagating either spontaneously or at a fixed rupture velocity on a vertical fault located in a 3d half space and a more realistic horizontally layered structure with finite difference schemes the results show that tpv is well correlated with tb for faults even with a heterogeneous stress drop dis tribution except at locations near strong barriers and the fault edges we also inves tigate this relation for different types of slip weakening behavior we have applied the method to two recent strike slip earthquakes in western japan the 2000 tottori and the 1995 kobe events we integrated the slip velocity functions on the vertical fault obtained from kinematic waveform inversion of strong motion and teleseismic records from the arrival time of rupture tr to the time of the peak slip velocity tpv and we then corrected the slip obtained at tpv for the errors expected from the dynamic calculations it was found that the slip weakening dis tance d c estimated in the frequency window between 0 05 and 0 5 hz ranges between 40 and 90 cm on the two earthquake faults however if we consider the limited frequency resolution of the observed waveforms probable time errors in the slip velocity functions obtained from kinematic inversion and the uncertainty of the slip weakening behavior the above estimates may be those located between the minimum resolvable limit and the upper bound of their real values the estimated dc values do not necessarily seem to indicate larger values in the shallower part and smaller values in the deeper part of the fault but rather a spatially heterogeneous distribution that appears to be dependent on the local maximum slip this possible dependence might be interpreted by the frictional properties of the fault such as the degree of roughness or the thickness of gouge layers in addition to stress heterogeneities

on the inference of absolute stress levels from seismic radiation abstract spudich p k p 1992 on the inference of absolute stress levels from seismic radiation in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 99 106 this paper determines the conditions under which it is possible to learn the absolute stress level at a subset of points on a fault from observation of the fault kinematics specifically the points on a rupturing fault can be divided into two groups those points at which the rake rotates during the rupture process and those points at which the rake does not rotate if it is true that sliding frictional traction is collinear with the instantaneous velocity of one side of a fault then at the former group of points having rotating rakes there is a unique absolute stress consistent with the motion while at the latter group of points having rakes that do not rotate the absolute stress is unspecified by the motion published dislocation solutions for the 1979 imperial valley 1986 north palm springs 1987 whittier narrows and 1989 loma prieta california earthquakes are examined for evidence of temporal rotations of rake at the hypocenters and largest asperities the 1986 north palm springs earthquake shows evidence of a possible rake rotation at the hypocenter this may imply a low absolute stress level

oxygen isotopic composition of bottom seawater and tunicate cellulose used as indicators of water masses in the northern bering and chukchi seas oxygen isotopic composition of bottom seawater and tunicate cellulose were used as short term and long term indicators respectively of water mass characteristics in the northern bering and chukchi seas oxygen isotopic composition of northeastern bering sea waters is influenced by yukon river inflows of sup 18 o depleted continental water mixing with relatively sup 18 o enriched waters contributed by the anadyr current tunicate cellulose sampled under alaska coastal water is more depleted in sup 18 o than that collected under bering shelf and anadyr waters which reflects the oxygen isotopic composition of these waters tunicate cellulose collected under the mixed bering shelf water displays intermediate delta sup 18 o values oxygen isotopic analyses of bottom seawater were used to determine the spatial location and influence of continental and coastal derived precipitation and of sea ice formation on water mass structure on the continental shelf of the northern bering and chukchi seas results indicate that the oxygen isotopic composition of tunicate cellulose averaged over multiple seasons may serve as a long term biochemical indicator of water mass patterns in ice covered polar regions where continuous sampling is impractical

seismic evidence for thermal runaway during intermediate depth earthquake rupture 1 intermediate depth earthquakes occur at depths where temperatures and pressures exceed those at which brittle failure is expected there are two leading candidates for the physical mechanism behind these earthquakes dehydration embrittlement and self localizing thermal shear runaway a complete energy budget for a range of earthquake sizes can help constrain whether either of these mechanisms might play a role in intermediate depth earthquake rupture the combination of high stress drop and low radiation efficiency that we observe for mw 4 5 earthquakes in the bucaramanga nest implies a temperature increase of 600 1000 c for a centimeter scale layer during earthquake failure this suggests that substantial shear heating and possibly partial melting occurs during intermediate depth earthquake failure our observations support thermal shear runaway as the mechanism for intermediate depth earthquakes which would help explain differences in their behavior compared to shallow earthquakes

dynamic inversion of the 2000 tottori earthquake based on elliptical subfault approximations 1 we propose a simplified nonlinear method for the kinematic and dynamic inversion of near field strong motion data at low frequencies using a few elliptical patches we reduce the number of independent parameters of the inverse problem we apply this method to the dynamic inversion of the western tottori japan earthquake mw 6 6 6 8 of 6 october 2000 using unfiltered records we relocated the hypocenter close to 14 km in depth fifteen records obtained by the kik net and k net accelerometer networks were then filtered to the 0 1 0 5 hz frequency range and integrated to displacement we compare observed and synthetic records using the 2 norm a nonlinear kinematic inversion for the elliptical subfaults is first computed using the neighborhood algorithm na inversion converges to a slip distribution modeled by just two elliptical patches we then propose a dynamic inversion method based on the same simple geometrical ideas dynamic rupture propagation is computed by finite differences on a coarse numerical grid rupture propagation is controlled by a classical slip weakening friction law inversion is implemented with the na for a barrier model in this model prestress is uniform and rupture propagation is arrested by a simple distribution of barriers inversion converges to a model with two elliptical barriers synthetics computed for the dynamic inversion fit the observed data reducing the variance by nearly 60 by making different assumptions about the rupture process we illustrate the nonuniqueness of the solution to dynamic inversion

kinematic and spontaneous rupture models of the 2005 tarapaca intermediate depth earthquake summary the 2005 june 13 an intraplate intermediate depth earthquake m w 7 8 98 km depth occurred in northern chile previous studies show that this earthquake is a slab pull event with down dip extensional source mechanism however the physical origin and the role in seismic cycle of this kind of event are still under debate we present a seismological source study from strong motion data based on the simplified slip patch kinematic parametrization we find two distinct sources the main source is almost centred around the hypocentre and triggers the secondary one situated deeper in the slab the weaknesses and uncertainties of the kinematic models are partly resolved and discussed by constructing dynamic spontaneous rupture models with two radiative asperities and one embedding zone of non radiative slip process a required feature to fulfil the seismic moment constraint two kinds of models are compatible with observations a crack mode model combined with fast rupture velocity and small zone of non radiative slip and a pulse mode model combined with slower rupture velocity and large zone of non radiative slip the fast rupture model is preferred for its physical simplicity replacing our result in the geometry and thermal contexts of the subduction we conclude that the rupture has probably broken the whole lithosphere by encountering little resistance and without being very sensitive to the presence of the double seismic zone this suggests that the rupture happens along a weak plane on which large slab instabilities can take place

benthic fluxes and pore water studies from sediments of the central equatorial north pacific nutrient diagenesis benthic exchange rates of radon 222 oxygen nitrate ammonia and silica were determined using an in situ benthic flux chamber and by modeling pore water profiles at three sites in the central equatorial north pacific a comparison of these results reveals several artifacts of pore water collection and processing whole core squeezer wcs silica profiles are influenced by adsorption during squeezing and yield calculated fluxes that are too large pore water ammonia profiles show near surface maxima that appear to be an artifact of core recovery near surface nitrate measurements may also be suspect due to oxidation of the ammonia released causing anomalously large nitrate gradients that yield overestimates of benthic exchange rates fluxes of radon oxygen and nitrate calculated from wcs profiles agree with chamber fluxes to better than 40 at all sites fluxes of silica and nitrate calculated from pore water data collected at coarser scales 1 cm intervals agree within 50 with chamber measurements previous flux estimates from pore water and solid phase models established at two of these sites using data collected 6 years prior to this work differ from these chamber measurements in some cases by up to a factor of 5 due to modeling uncertainties and temporal variabilities in the delivery of organic matter to a site the benthic oxygen consumption rates measured at these sites are similar they average 0 36 0 03 mmol m 2 day 1 and are consistent with a trend of oxygen uptake vs water depth previously established by others on a transect through the oligotrophic north pacific gyre

earthquake fracture energy inferred from kinematic rupture models on extended faults 1 we estimate fracture energy on extended faults for several recent earthquakes by retrieving dynamic traction evolution at each point on the fault plane from slip history imaged by inverting ground motion waveforms we define the breakdown work wb as the excess of work over some minimum traction level achieved during slip wb is equivalent to seismological fracture energy g in previous investigations our numerical approach uses slip velocity as a boundary condition on the fault we employ a three dimensional finite difference algorithm to compute the dynamic traction evolution in the time domain during the earthquake rupture we estimate wb by calculating the scalar product between dynamic traction and slip velocity vectors this approach does not require specifying a constitutive law and assuming dynamic traction to be collinear with slip velocity if these vectors are not collinear the inferred breakdown work depends on the initial traction level we show that breakdown work depends on the square of slip the spatial distribution of breakdown work in a single earthquake is strongly correlated with the slip distribution breakdown work density and its integral over the fault breakdown energy scale with seismic moment according to a power law with exponent 0 59 and 1 18 respectively our estimates of breakdown work range between 4 105 and 2 107 j m2 for earthquakes having moment magnitudes between 5 6 and 7 2 we also compare our inferred values with geologic surface energies this comparison might suggest that breakdown work for large earthquakes goes primarily into heat production

shakeout d ground motion estimates using an ensemble of large earthquakes on the southern san andreas fault with spontaneous rupture propagation 1 we simulate ground motion in southern california from an ensemble of 7 spontaneous rupture models of large mw7 8 northwest propagating earthquakes on the southern san andreas fault shakeout d compared to long period spectral accelerations from the next generation attenuation nga empirical relations shakeout d predicts similar average rock site values i e within roughly their epistemic uncertainty but significantly larger values in los angeles and ventura basins due to wave guide focusing effects the shakeout d ground motion predictions differ from those of a kinematically parameterized geometrically similar scenario rupture 1 the kinematic rock site predictions depart significantly from the common distance attenuation trend of the nga and shakeout d results and 2 shakeout d predictions of long period spectral acceleration within the basins of the greater los angeles area are lower by factors of 2 3 than the corresponding kinematic predictions we attribute these differences to a less coherent wavefield excited by the complex rupture paths of the shakeout d sources

determination of the friction law parameters of the mw 6 7 michilla earthquake in northern chile by dynamic inversion 1 we perform a full dynamic inversion at low frequencies of the 16 december 2007 mw 6 7 northern chile earthquake that we model as a simple elliptical patch we use two different stress friction end member models asperities and barriers finding similar results the inversions are performed for strong motion data filtered between 0 02 and 0 5 hz eleven geometrical and stress and friction parameters are inverted using the neighbourhood algorithm the optimum solutions have relative errors lower than 0 21 the earthquake rupture has duration of less than 5 s and propagates at sub shear speed the rupture area is similar to that of the aftershock distribution and the seismic moment is 0 95 10 19 nm we derive the friction law parameters from the models situated close to the optimum solution using a monte carlo technique the results show a strong trade off between applied stress and frictional resistance we find that the distribution of friction models collapses into a finite zone of the space of moment and non dimensional parameter k we conclude that it is possible to determine the friction law from near field seismograms but there is a strong trade off between friction and initial stress

aerobic respiration along isopycnals leads to overestimation of the isotope effect of denitrification in the ocean water column abstract the nitrogen n isotopes provide an integrative geochemical tool for constraining the fixed n budget of the ocean however n isotope budgeting requires a robust estimate for the organism scale nitrogen isotope effect of denitrification in particular as it occurs in water column denitrification zones ewcd ocean field data interpreted with the rayleigh model have typically yielded estimates for ewcd of between 20 and 30 however recent findings have raised questions about this value in particular culture experiments can produce a substantially lower isotope effect 13 under conditions mimicking those of ocean suboxic zones in an effort to better understand prior field estimates of ewcd we use a geochemical multi box model to investigate the combined effects of denitrification aerobic respiration and isopycnal exchange on the 15n of nitrate in the context of this admittedly simplistic model we consider three isopycnals extending from the southern ocean to the eastern tropical north pacific etnp we show that the data from the etnp suboxic zone can be reproduced with an ewcd of 13 given a rate of aerobic respiration consistent with the nutrient data on these isopycnals and a plausible range in the 15n of the sinking flux being remineralized we discuss the limitations of our analysis additional considerations as well as possible data based tests for the proposal of a lower ewcd than previously estimated all else held constant a lower ewcd would imply a lower global ocean rate of denitrification that is more similar to the estimated rate of n input to the global ocean providing a major impetus for further investigation

dynamic rupture processes on a dipping fault and estimates of stress drop and strength excess from the results of waveform inversion summary the dynamic rupture processes on a dipping fault in a horizontally layered medium are investigated on 3 d spontaneous shear crack models the wave equations for the 3 d space are solved numerically by a finite difference method under appropriate boundary conditions the displacement components right above and right below the dipping fault plane have been obtained by solving the boundary conditions across the fault it was found that the final fault slip on the hanging wall side is appreciably larger than that on the foot wall side and that their amount is quite sensitive to the fault depth and the elastic heterogeneities of the medium particularly of the existence of low velocity surface layers on the basis of the above model the dynamic rupture process of the 1961 kita mino earthquake in central japan has been investigated with constraints of the fault parameters derived from the results of waveform inversion analysis for this purpose a locking fracture criterion was introduced for rupture propagation from which a lower bound of the maximum shear stress before the rupture and hence of the strength excess has been estimated the dynamic stress drop has also been evaluated by repeated iterations to minimize the difference between the dynamic and kinematic fault slips the results revealed quite heterogeneous distribution of strength excess indicating large values near the sw section of the fault at shallow depths and in the central bottom section in contrast to small values at mid depths in the ne section around the rupture nucleus zone the distribution of stress drop was also found to be heterogeneous with the existence of high stress drop zones in the ne shallow and bottom sections and a low stress drop zone at most of mid depths except in the sw section these results suggest that the dynamic rupture of this earthquake initiates at a small nucleus zone with low strength excess and small stress drop then broke the shallower and deeper fault sections with moderate strength excess and high stress drop and finally ruptured barrier zones with high strength excess these zones may be interpreted as weak and strong asperities and barriers respectively

a fast and simple diagnostic method for identifying tsunamigenic earthquakes an analysis of regional broadband seismograms of moderate and large subduction zone earthquakes in mexico shows that earthquakes which occur near the trench are abnormally depleted in high frequency radiation this observation leads to a simple and fast method to assess regional tsunami potential from earthquakes which occur along the pacific coast of mexico a significant advantage of the method is that a single broadband seismograph is sufficient for the purpose the method is based on the ratio of the total energy to the high frequency energy between 1 and 5 hz er computed from the seismograms for earthquakes with the same seismic moment er is an order of magnitude greater for a source area near the trench as compared to those near the coast the same seismograms are used to compute energy magnitude me which is tied to the moment magnitude mw a regional tsunami may be expected along the coast if me 6 5 and er corresponds to a source near the trench if however er corresponds to a near coast source then me may have to be greater than about 7 3 for a tsunami of similar size to occur along the coast the method holds promise for a fast regional tsunami warning in many pacific basin countries which lack an adequate seismic network

coupled nitrification denitrification in sediment of the eastern bering sea shelf leads to 15n enrichment of fixed n in shelf waters 1 we studied the nitrogen biogeochemistry of the ice covered eastern bering sea shelf using the isotope ratios 15n 14n and 18o 16o of no3 and other n species the 15n 14n of late winter no3 on the shelf decreases inshore and is inversely correlated with bottom water nh4 consistent with an input of low 15n 14n no3 from partial nitrification of nh4 remineralized from the sediments an inshore 15n 14n increase in total dissolved n tdn suggests that 1 the sediment derived nh4 is elevated in 15n due to the same partial nitrification that yields the low 15n 14n no3 and 2 15n deplete no3 from partial nitrification within the sediments is denitrified to n2 the proportion of newly nitrified no3 on the shelf evidenced by an inshore decrease in no3 18o 16o is correlated with the n deficit further implicating nitrification coupled to denitrification however a simple n isotope budget indicates a comparable rate of denitrification supported by diffusion of no3 into the sediments the isotopic impact of benthic n loss is further demonstrated by a correlation between the 15n 14n of shelf surface sediment and the n deficit of the overlying water column both of which increase inshore and northward as well as by arctic no3 isotope data indicating that the fixed n transported through bering strait has a 15n 14n higher than is found in the open bering sea the significant net isotope effect of benthic n loss on the bering shelf 6 8 is at odds with previous assumptions regarding the global ocean s n isotope budget

methane and nitrous oxide distributions across the north american arctic ocean during summer 2015 we collected arctic ocean water column samples for methane ch4 and nitrous oxide n2o analysis on three separate cruises in the summer and fall of 2015 covering a 10 000 km transect from the bering sea to baffin bay this provided a three dimensional view of ch4 and n2o distributions across contrasting hydrographic environments from the oligotrophic waters of the deep canada basin and baffin bay to the productive shelves of the bering and chukchi seas percent saturation relative to atmospheric equilibrium ranged from 30 800 for ch4 and 75 145 for n2o with the highest concentrations of both gases occurring in the northern chukchi sea nitrogen cycling in the shelf sediments of the bering and chukchi seas likely constituted the major source of n2o to the water column and the resulting high n2o concentrations were transported across the arctic ocean in eastward flowing water masses methane concentrations were more spatially heterogeneous reflecting a variety of localized inputs including likely sources from sedimentary methanogenesis and sea ice processes unlike n2o ch4 was rapidly consumed through microbial oxidation in the water column as shown by the 13c enrichment of ch4 with decreasing concentrations for both ch4 and n2o sea air fluxes were close to neutral indicating that our sampling region was neither a major source nor sink of these gases our results provide insight into the factors controlling the distribution of ch4 and n2o in the north american arctic ocean and an important baseline data set against which future changes can be assessed

late spring nitrate distributions beneath the ice covered northeastern chukchi shelf measurements of late springtime nutrient concentrations in arctic waters are relatively rare due to the extensive sea ice cover that makes sampling difficult during the subice cruise in may june 2014 an extensive survey of hydrography and pre bloom concentrations of inorganic macronutrients oxygen particulate organic carbon and nitrogen and chlorophyll a was conducted in the northeastern chukchi sea cold 1 5 c winter water was prevalent throughout the study area and the water column was weakly stratified nitrate no3 concentration averaged 12 6 1 92 m in surface waters and 14 0 1 91 m near the bottom and was significantly correlated with salinity the highest no3 concentrations were associated with winter water within the central channel flow path no3 concentrations were much reduced near the northern shelfbreak within the upper halocline waters of the canada basin and along the eastern side of the shelf near the alaskan coast net community production ncp estimated as the difference in depth integrated no3 content between spring this study and summer historical varied from 28 38 g c m 2 a 1 this is much lower than previous ncp estimates that used no3 concentrations from the southeastern bering sea as a baseline these results demonstrate the importance of using profiles of no3 measured as close to the beginning of the spring bloom as possible when estimating local ncp they also show that once the snow melts in spring increased light transmission through the sea ice to the waters below the ice could fuel large phytoplankton blooms over a much wider area than previously known

micro and macronutrients in the southeastern bering sea insight into iron replete and iron depleted regimes abstract surface transects and vertical profiles of macronutrients dissolved iron d fe and dissolved manganese d mn were investigated during august 2003 in the southeastern bering sea we observed iron limited hnlc surface waters in the deep basin of the bering sea 15 20 mol kg nitrate 0 07 nmol kg d fe and 1 0 nmol kg d mn nitrate limited iron replete surface waters over the shelf

multiple oscillations in neoarchaean atmospheric chemistry abstract the great oxidation event goe represents a crucial juncture in earth history signifying the rise in atmospheric oxygen from parts per million to percent levels at 2 45 2 32 billion years ago ga although planetary oxygenation undoubtedly led to the inception of the contemporary earth system the trigger s and mechanism s controlling this chemical reorganisation remain elusive quantitative estimates of the atmosphere s composition in the prelude to the goe are central to understanding this oxygenation event previous analyses of 2 65 2 5 ga sediments from the griqualand basin south africa invoke a tantalising picture of an unusual earth environment alluding to an atmosphere periodically dominated by a layer of organic particles haze formed from methane photolysis however as yet this hypothesis has remained untested here we present four new coupled carbon and quadruple sulphur isotope records from distal time equivalent 2 7 2 5 ga sedimentary successions from south africa and western australia these extended records reveal similar chemostratigraphic trends supporting a dynamic terminal neoarchaean atmosphere oscillating between a hazy state at elevated methane concentrations and a haze free anoxic background state we suggest these atmospheric aberrations were related to heightened biogenic methane fluxes fuelled by enhanced nutrient delivery from climatically or weathering induced feedbacks these data question the canonical view of a simple unidirectional planetary oxygenation and signify that the overture to the goe was governed by complex feedbacks within the earth biosphere system

spatio temporal evolution of ocean redox and nitrogen cycling in the early cambrian yangtze ocean abstract the early cambrian was a critical interval for the earth system during which a rise in oceanic and atmospheric oxygen levels coincided with the rapid diversification of metazoans a variety of contrasting models have been proposed for the spatiotemporal redox evolution of the early cambrian ocean these include the development of a well oxygenated deep ocean at the base of cambrian stage 3 commencing at 521 ma or alternatively persistent and widespread anoxic ferruginous conditions throughout the early cambrian ocean here we present redox sensitive trace element rste fe speciation and n and c isotope 15nsed and 13corg data for samples from a section zhongnancun of the early cambrian niutitang formation which was deposited on the outer shelf of the yangtze block south china the fe speciation and rste data provide evidence of a transition from euxinic through ferruginous to oxic conditions during deposition of the niutitang formation the combination of these new data with existing data from the inner shelf to basin environment implies regional redox stratification across the yangtze block during cambrian stages 2 and 3 with oxic shallow waters above ferruginous deep waters and spatial variability in the degree of mid depth euxinia oxygenation of deeper waters may have occurred by early cambrian stage 4 514 ma a compilation of 15n values from multiple early cambrian sections of the yangtze block indicate that n2 fixation dominated the nitrogen cycle during late cambrian stage 2 low 15n values

the 1992 nicaragua earthquake a slow tsunami earthquake associated with subducted sediments the 1992 nicaragua earthquake was a tsunami earthquake that is it generated tsunamis disproportionately large for its surface wave magnitude m s the moment magnitude m w determined from long period 250 s surface waves was 7 6 significantly larger than the 20 s m s of 7 this m s m w disparity is also characteristic of tsunami earthquakes the nicaragua earthquake is the first tsunami earthquake to be captured by modern broadband seismic networks allowing us to present here seismograms of sufficiently high quality to make inferences about the rupture mechanisms we conclude that the nicaragua earthquake was a slow thrust earthquake which occurred on the subduction interface between the cocos and north american plates and because of the absence of sediments on the trench floor offshore of nicaragua the slip propagated up dip all the way to the ocean bottom exciting large tsunamis the occurrence of slip on a plate interface filled with soft subducted sediments caused the rupture process to be slower than in ordinary subduction zone thrust earthquakes our results reinforce the idea that tsunami warning systems using long period 100 s waves are necessary to reduce the hazard from this type of earthquake

geometry of the benioff zone and state of stress in the overriding plate in central mexico analysis of depths and focal mechanisms of 16 small earthquakes in central mexico along with previous data shows that a the subducted cocos plate becomes subhorizontal between 110 and 275 km from the trench reaching a depth of about 50 km and b the bottom part of the overriding continental plate is in tensional stress regime neotectonic structures in central mexico and stress orientations estimated from borehole elongations cinder cone alignments and fault slip analysis in the trans mexican volcanic belt which lies in the northeastern portion of the area under study also indicate the same stress regime absolute motion of the north american plate in the region has a component normal to the trench of 20 mm yr which if true would result in compressional stress in the upper plate if the trench position is fixed in an absolute frame of reference if we allow for seaward retreat of the trench then the tensional stress in the overriding plate and the observed geometry of the benioff zone can be explained this however implies that seaward retreat of a trench is possible even for a young 15 m y old subducting slab alternatively tectonic erosion of the leading edge of the continent which has been proposed to explain truncated igneous and metamorphic continental terrane could give rise to the tensional stress but would not explain the geometry of the benioff zone

the dynamics of the marine nitrogen cycle across the last deglaciation 1 we use a geochemical box model to investigate the changes in marine n fixation and denitrification required to match the observed sedimentary 15n changes between 30 kyr b p and the late holocene this is achieved by optimizing a set of seven parameters that describe the strengths of three ocean internal n feedbacks and the response of the oceanic n cycle to four external forcings scenarios that best match the 15n constraints indicate a strong transient decrease in n fixation in the early deglacial in response to the decrease in iron input by dust around 15 kyr b p n fixation rebounds primarily in response to an abrupt increase in water column denitrification caused by an expansion of anoxia benthic denitrification is not well constrained by our model but tends to increase in sync with water column denitrification as a result of the transient imbalance between n fixation and denitrification we infer a glacial to interglacial decrease in the marine n inventory of between 15 and 50 the model diagnoses this reduction in order to simultaneously fit the data from all ocean basins requiring it to reduce the degree by which water column denitrification in the oxygen minimum zones is influencing the 15n of nitrate of the whole ocean dilution effect our optimal solution suggests a glacial n cycle that operated at nearly the same rates as that in pre industrial times but sensitivity cases with substantially lower rates fit the data only marginally worse an important caveat of our study is the assumption of an unchanging ocean circulation an initial sensitivity experiment shows that this affects primarily the magnitude of the change in the n inventory while the diagnosed deglacial dynamics with global marine n fixation taking a dip before the onset of denitrification remains a robust result

inslab earthquakes of central mexico peak ground motion parameters and response spectra we developed equations to predict pseudoacceleration response spectra 5 damping peak ground acceleration and peak ground velocity at free field rock sites for intermediate depth normal faulting inslab earthquakes of central mexico the data set comprises 16 earthquakes 5 2 m w 7 4 35 h 138 km recorded at local and regional distances r 400 km it represents a homogeneous catalog with respect to tectonic regime fault mechanism and soil class our results show larger amplitudes in the epicentral area from inslab events than from interplate events a consequence of higher stress drops during the former type of earthquakes peak ground accelerations from moderate to large m w 6 0 inslab events significantly exceed those from interplate events with similar magnitude reaching almost three times for the largest events the ground motion due to inslab events however decays faster than for the thrust events our results are in reasonable agreement with other studies based on japanese and the worldwide data

3d finite difference dynamic rupture modeling along nonplanar faults properunderstandingofseismicemissionsassociatedwiththe growth of complexly shaped microearthquake networks and larger scale nonplanar fault ruptures both in arbitrarily heterogeneousmedia requiresaccuratemodelingoftheunderlyingdynamic processes we present a new 3d dynamic rupture finitedifference model called the finite difference fault element fdfe method it simulates the dynamic rupture of nonplanar faults subjected to regional loads in complex media fdfe is based on a 3d methodology for applying dynamic rupture boundary conditions along the fault surface the fault is discretizedbyasetofparallelepipedfaultelementsinwhichspecific boundary conditions are applied these conditions are applied to the stress tensor once transformed into a local fault reference frame numerically determined weight functions multiplying particle velocities around each element allow accurate estimates offaultkinematicparametersi e slipandsliprateindependent of faulting mechanism assuming a coulomb like slip weakeningfrictionlaw aparametricstudysuggeststhatthefdfemethod converges toward a unique solution provided that the cohesive zone behind the rupture front is well resolved i e four or more elements inside this zone solutions are free of relevant numerical artifacts for grid sizes smaller than approximately 70 m results yielded by the fdfe approach are in good quantitative agreement with those obtained by a semianalytical boundaryintegralmethodalongplanarandnonplanarparabola shaped faults thefdfemethodthusprovidesquantitative accurateresults for spontaneous rupture simulations on intricate fault geometries

the effect of bandwidth limitations on the inference of earthquake slip weakening distance from seismograms numerous researchers have obtained estimates of slip weakening distance dc and fracture energy for recent earthquakes dc is often observed to be a significant fraction of the total slip and tends to correlate with total slip although these observations may well be true of real earthquakes we show that low pass filtering of strong motion seismograms can also produce some of these effects in inverted rupture models we test the accuracy of dc estimates by calculating them in low pass filtered versions of models a and b of guatteri and spudich 2000 models a and b are two different rupture models for a hypothetical m 6 5 earthquake and they have nearly identical rupture time slip and stress drop distributions and nearly identical predicted seismograms but dc for model b is about twice that for model a by low pass filtering slip models a and b at 1 0 hz we simulate the blurring effects of band limited waveform inversions on these slip models at each point on a fault graphic 1 is defined to be the slip at the time of the peak slip speed at that point low pass filtering the slip models causes an upward bias in dc inferred from stress slip curves and it causes an artificial correlation between graphic 2 and the total slip low pass filtering might also bias fracture energy high and radiated energy low these biases should be considered when interpreting dc derived from band limited slip models of real earthquakes 1 embed inline graphic 1 gif 2 embed inline graphic 2 gif

low frequency current regimes over the bering sea shelf abstract using direct current measurements made during the period 1975 81 we describe the general circulation over the southeastern bering sea and differentiate it by regimes related to depth and forcing mechanisms three regimes are present delineated by water depth z the coastal z 50 m the middle shelf 50 z 100 m and the outer shelf z 100 m these are nearly coincident with previously described hydrographic domains statistically significant mean flow 1 to 10 cm s 1 exists over the outer shelf generally directed toward the northwest but with a cross isobath component flow of similar magnitude 1 6 cm s 1 occurs in the coastal regime paralleling the 50 m isobath in a counterclockwise sense around the shelf mean flow in the middle shelf is insignificant kinetic energy at frequencies 0 5 cycle per day cpd is greater over the outer shelf than in the other two regimes suggesting that oceanic forcing is important there but does not affect the remainder of the shelf kinetic

the δ15n of nitrate in the southern ocean nitrogen cycling and circulation in the ocean interior we report analyses of the nitrogen isotopic composition of nitrate in the eastern indian and pacific sectors of the southern ocean in this paper we focus on the subsurface data as well as data from the deep waters of other ocean basins nitrate 15n is relatively invariant in much of the abyssal ocean i e below 2 5 km with a value of 4 8 0 2 observed in lower circumpolar deep water north atlantic deep water and central pacific deep water the isotopic invariance of deep ocean nitrate stems fundamentally from the completeness of nitrate utilization in most of the global surface ocean the southern ocean surface being an important exception in the subantarctic zone north of the polar frontal zone the nitrate 15n of upper circumpolar deep water is 0 7 greater than that of lower circumpolar deep water this isotopic enrichment appears to result from denitrification in the low latitude water masses with which upper circumpolar deep water communicates the isotopic enrichment of upper circumpolar deep water is diminished in the antarctic probably because of the remineralization of sinking organic n which has a low 15n in the antarctic relative to the other water masses of the southern ocean the subantarctic thermocline has a very low nitrate 15n for its nitrate concentration because of exchange with the low latitude thermocline where this isotopic signature appears to originate this signature of the low latitude thermocline has two probable causes 1 mixing with low nitrate surface water and 2 the oxidation of newly fixed n

observational constraints on the fracture energy of subduction zone earthquakes we relate seismologically observable parameters such as radiated energy seismic moment rupture area and rupture speed to the dynamics of faulting to achieve this objective we computed the radiated energy for 23 subduction zone earthquakes recorded between 1992 and 2001 most of these earthquakes have a magnitude m w 7 5 but we also included some smaller m w 6 7 well studied subduction zone earthquakes and six crustal earthquakes we compiled static stress drop estimates for these 29 earthquakes from literature and used a slip weakening model to determine the radiation efficiency of these earthquakes we also determined the rupture speed of these earthquakes from literature from fracture mechanics fracture energy and hence radiation efficiency can be related to the rupture speed the radiation efficiencies estimated from the partitioning of energy as given by the slip weakening model are consistent with the rupture speed estimated for these earthquakes most earthquakes have radiation efficiencies between 0 25 and 1 and are hence efficient in generating seismic waves but tsunami earthquakes and two deep earthquakes the 1994 bolivia and the 1999 russia china border earthquakes have very small radiation efficiencies 0 25 and hence dissipate a large amount of energy during faulting we suggest that differences in the radiation efficiencies of different types of earthquakes could be due to fundamental differences in their rupture mechanics in deep events the energy is probably dissipated in thermal processes in the fault zone while it is possible that the morphology of the trench causes large energy dissipation during the rupture process of tsunami earthquakes

enhanced stratification and seasonality in the subarctic pacific upon northern hemisphere glaciation new evidence from diatom bound nitrogen isotopes alkenones and archaeal tetraethers abstract coincident with the intensification of northern hemisphere glaciation nhg around 2 73 million years ma ago sediment cores from both the open subarctic north pacific and the antarctic indicate a rapid decline in diatom opal accumulation flux to the seabed representing one of the most abrupt and dramatic changes in the marine sediment record associated with the development of pleistocene glacial cycles in the north pacific bulk sediment nitrogen isotope data and alkenone derived sea surface temperature sst estimates suggest that the productivity decline was driven by reduced exchange between surface and deep water due to weaker wind driven upwelling and or a strengthening of the halocline i e stratification in this study of the 2 73 ma transition at ocean drilling program odp site 882 in the western subarctic north pacific diatom bound nitrogen isotopes 15 n db alkenone mass accumulation rate and alkenone and archaeal tetraether based sst reconstructions support the stratification hypothesis indicating perennially lower export production generally higher nitrate consumption and greater inter seasonal variation in sst after the 2 73 ma transition in addition the 15 n db of large and small size fractions of coscinodiscus spp suggest that these diatoms grew mostly during the spring bloom during the late pliocene switching to their current fall to winter growth period at the 2 73 ma transition this view is consistent with their decline in dominance and provides further evidence for increased stratification reduced vertical exchange in the north pacific after 2 73 ma the 15 n db data indicate that over the 100 kyr period after the 2 73 ma transition studied here nitrate consumption did not reach late pleistocene ice age levels and that nitrate consumption in post 2 73 ma warm stages was similar to that before the transition even though productivity was greatly reduced we tentatively attribute this to relatively weak dust borne iron inputs in the early post 2 73 ma period

supershear mach waves expose the fault breakdown slip abstract mikumo et al 2003 showed that it is possible to estimate the breakdown slip dc as the slip at the time of the peak slip rate for rupture propagation with subshear speeds cruz atienza et al 2009 later attempted to extend this method to extract information about dc as the displacement at the time of the peak particle velocity from seismic strong motion records however a reasonably accurate estimate of dc was only possible in a narrow zone adjacent to the fault typically on the order of hundreds of meters due to the fast decay with distance from the fault of the seismic energy related to the stress breakdown process when the rupture propagates with supershear speeds on the other hand this energy is carried much farther away from the fault by mach waves in particular rayleigh mach waves when rupture reaches the earth s surface dunham and bhat 2008 here we present a new approach to estimate dc from strong motion records containing mach waves first we show that the method by mikumo et al is valid for supershear rupture propagation this method is then used to estimate dc via an asymptotic approximation of the slip and slip rate time histories from the mach waves using spontaneous rupture simulations we demonstrate that for a visco elastic half space model dc can be estimated within an error of 40 from mach waves that have propagated a distance of at least 3 km from the fault the method is applied to estimate dc for the 2002 mw7 9 denali alaska earthquake 1 5 m and for the 1999 mw7 6 izmit turkey earthquake 1 7 m

sinking organic matter spreads the nitrogen isotope signal of pelagic denitrification in the north pacific 1 culture studies of denitrifying bacteria predict that denitrification will generate equivalent gradients in the 15n and 18o of deep ocean nitrate a depth profile of nitrate isotopes from the hawaii ocean time series station aloha shows less of an increase in 18o than in 15n as one ascends from abyssal waters into the denitrification impacted mid depth waters a box model of the ocean nitrate n and o isotopes indicates that this is the effect of the low latitude nitrate assimilation regeneration cycle organic n sinking out of the surface spreads the high 15n signal of pelagic denitrification into waters well below and beyond the suboxic zone whereas the nitrate 18o signal of denitrification can only be transmitted by circulation in the interior

geometry and seismic properties of the subducting cocos plate in central mexico the geometry and properties of the interface of the cocos plate beneath central mexico are determined from the receiver functions rfs utilizing data from the meso america subduction experiment mase the rf image shows that the subducting oceanic crust is shallowly dipping to the north at 15 for 80 km from acapulco and then horizontally underplates the continental crust for approximately 200 km to the trans mexican volcanic belt tmvb the crustal image also shows that there is no continental root associated with the tmvb the migrated image of the rfs shows that the slab is steeply dipping into the mantle at about 75 beneath the tmvb both the continental and oceanic moho are clearly seen in both images and modeling of the rf conversion amplitudes and timings of the underplated features reveals a thin low velocity zone between the plate and the continental crust that appears to absorb nearly all of the strain between the upper plate and the slab by inverting rf amplitudes of the converted phases and their time separations we produce detailed maps of the seismic properties of the upper and lower oceanic crust of the subducting cocos plate and its thickness high poisson s and vp vs ratios due to anomalously low s wave velocity at the upper oceanic crust in the flat slab region may indicate the presence of water and hydrous minerals or high pore pressure the evidence of high water content within the oceanic crust explains the flat subduction geometry without strong coupling of two plates this may also explain the nonvolcanic tremor activity and slow slip events occurring in the subducting plate and the overlying crust

seismicity and state of stress in guerrero segment of the mexican subduction zone 1 we take advantage of a relatively dense network of seismic stations in the guerrero segment of the mexican subduction zone to study seismicity and state of stress in the region we combine our results with recent observations on the geometry of the subducted cocos plate imaged from receiver function rf analysis an ultraslow velocity layer mapped in the upper crust of the subducted slab and episodic slow slip events sses and nonvolcanic tremors nvts reported in the region to obtain a comprehensive view of the subduction process seismicity and focal mechanisms confirm subduction of the cocos plate below mexico at a shallow angle reaching a depth of 25 km at a distance of 65 km from the trench the plate begins to unbend at this distance and becomes horizontal at a distance of 120 km at a depth of 40 km some of the highlights of the inslab seismicity are as follows 1 a cluster of earthquakes in the depth range of 25 45 km immediately downdip from the strongly coupled part of the plate interface revealing both downdip compressional and extensional events this seismicity extends from 80 to 105 km from the trench and may be attributed to the unbending of the slab 2 the slab devoid of seismicity in the distance range of 105 160 km 3 sparse inslab seismicity revealing downdip extension in the distance range of 160 240 km the nvts are also confined to this distance range the episodic sses occur on the horizontal segment in the distance range of 105 240 km where an ultraslow velocity layer in the upper crust of the slab has been mapped from waveform modeling of converted sp phases thus in the distance range of 105 160 km sses occur but nvts and inslab earthquakes are absent this suggests that metamorphic dehydration reactions in the subducting oceanic crust and upper mantle begin at a distance of 160 km giving rise to both the inslab earthquakes and nvts no inslab earthquake occurs beyond 240 km the receiver function images and p wave tomography suggest that the slab begins a steep plunge at a distance of 310 km reaching a depth of 500 km around 340 km from the trench the negative buoyancy of such a slab should give rise to large extensional stress in the slab yet inslab seismicity is remarkably low which may be explained by a slab that is not continuous up to a depth of 500 km but is broken at a shallower depth the resulting slab window may permit subslab material to flow through the gap this may provide an explanation for the recent rift related basalts found near mexico city the fore arc upper plate seismicity which during the period of study 1995 2007 consisted of a moderate earthquake mw5 8 near the coast h 12 km its numerous aftershocks and two shallow events farther inland demonstrates a trench normal extension in the upper plate near this convergent margin a state of stress that may be explained by tectonic erosion and or seaward retreat of the trench seismicity location of the mantle wedge and rupture areas of mexican earthquakes suggest that the downdip limit of rupture during large great earthquakes in guerrero may be 105 15 km shallow dipping interplate thrust earthquakes are not the only type of events that affect the seismic hazard in the region the magnitudes of inslab downdip compressional and extensional earthquakes that occur within 20 km inland from the coast in the depth range of 25 45 km may reach 6 5 and 7 5 respectively in addition we now identify normal faulting earthquakes in the upper plate these sources need to be taken into account in the hazard estimation

spectral scaling of the aftershocks of the tocopilla 2007 earthquake in northern chile summary we study the scaling of spectral properties of a set of 68 aftershocks of the 2007 november 14 tocopilla m 7 8 earthquake in northern chile these are all subduction events with similar reverse faulting focal mechanism that were recorded by a homogenous network of continuously recording strong motion instruments the seismic moment and the corner frequency are obtained assuming that the aftershocks satisfy an inverse omega square spectral decay radiated energy is computed integrating the square velocity spectrum corrected for attenuation at high frequencies and for the finite bandwidth effect using a graphical approach we test the scaling of seismic spectrum and the scale invariance of the apparent stress drop with the earthquake size to test whether the tocopilla aftershocks scale with a single parameter we introduce a non dimensional number that should be constant if earthquakes are self similar for the tocopilla aftershocks cr varies by a factor of 2 more interestingly cr for the aftershocks is close to 2 the value that is expected for events that are approximately modelled by a circular crack thus in spite of obvious differences in waveforms the aftershocks of the tocopilla earthquake are self similar the main shock is different because its records contain large near field waves finally we investigate the scaling of energy release rate gc with the slip we estimated gc from our previous estimates of the source parameters assuming a simple circular crack model we find that gc values scale with the slip and are in good agreement with those found by abercrombie and rice for the northridge aftershocks

on the recent warming of the southeastern bering sea shelf abstract during the last decade the southeastern bering sea shelf has undergone a warming of 3 c that is closely associated with a marked decrease of sea ice over the area this shift in the physical environment of the shelf can be attributed to a combination of mechanisms including the presence over the eastern bering sea shelf of a relatively mild air mass during the winter especially from 2000 to 2005 a shorter ice season caused by a later fall transition and or an earlier spring transition increased flow through unimak pass during winter which introduces warm gulf of alaska water onto the southeastern shelf and the feedback mechanism whereby warmer ocean temperatures during the summer delay the southward advection of sea ice during winter while the relative importance of these four mechanisms is difficult to quantify it is evident that for sea ice to form cold arctic winds must cool the water column sea ice is then formed in the polynyas during periods of cold north winds and this ice is advected southward over the eastern shelf the other three mechanisms can modify ice formation and melt and hence its extent in combination these four mechanisms have served to temporally and spatially limit ice during the 5 year period 2001 2005 warming of the eastern bering sea shelf could have profound influences on the ecosystem of the bering sea from modification of the timing of the spring phytoplankton bloom to the northward advance of subarctic species and the northward retreat of arctic species

interpretation of the nitrogen isotopic composition of precambrian sedimentary rocks assumptions and perspectives nitrogen isotope compositions in sedimentary rocks d 15 n sed are routinely used for reconstructing cenozoic n biogeochemical cycling and are also being increasingly applied to understanding the evolution of ancient environments here we review the existing knowledge and rationale behind the use of d 15 n sed as a proxy for the precambrian n biogeochemical cycle with the aims of i identifying the major uncertainties that affect analyses and interpretation of nitrogen isotopes in ancient sedimentary rocks ii developing a framework for interpreting the precambrian d 15 n sed record iii testing this framework against a database of precambrian d 15 n sed values compiled from the literature and iv identifying avenues of focused research that should increase confidence in interpreting precambrian d 15 n sed data this review highlights the intrinsic complexity of the d 15 n sed proxy and the significant effort that remains to realize its potential specifically it is crucial to gain a better understanding of how and when diagenesis and metamorphism affect the d 15 n of bulk and kerogen bound nitrogen ultimately more data are required to apply statistics to interpreting d 15 n sed variability within given geological time intervals finally numerical modeling of the d 15 n sed variability expected in different environments under varying redox scenarios is necessary to establish a predictive template for interpreting the ancient nitrogen isotope record in spite of the challenges facing the application of this proxy to the precambrian the existing d 15 n sed record shows several features possibly related to the stepwise oxygenation of the surface environment underlining the potential for nitrogen isotopes to reveal clues about the evolution of early earth

modeling dynamic rupture in a 3d earthquake fault model we propose a fourth order staggered grid finite difference method to study dynamic faulting in three dimensions the method uses an implementation of the boundary conditions on the fault that allows the use of general friction models including slip weakening and rate dependence because the staggered grid method defines stresses and particle velocities at different grid points we preserve symmetry by implementing a two grid row thick fault zone slip is computed between points located at the borders of the fault zone while the two components of shear traction on the fault are forced to be symmetric inside the fault zone we study the properties of the numerical method comparing our simulations with well known properties of seismic ruptures in 3d among the properties that are well modeled by our method are full elastic wave interactions frictional instability rupture initiation from a finite initial patch spontaneous rupture growth at subsonic and supersonic speeds as well as healing by either stopping phases or rate dependent friction we use this method for simulating spontaneous rupture propagation along an arbitrarily loaded planar fault starting from a localized asperity on circular and rectangular faults the shape of the rupture front is close to elliptical and is systematically elongated in the in plane direction of traction drop this elongation is due to the presence of a strong shear stress peak that moves ahead of the rupture in the in plane direction at high initial stresses the rupture front becomes unstable and jumps to super shear speeds in the direction of in plane shear another interesting effect is the development of relatively narrow rupture fronts due to the presence of rate weakening friction the solutions for the thick fault boundary conditions scale with the slip weakening distance do and are stable and reproducible for do greater than about 4 in terms of 2t t ax finally a comparison of scalar and vector boundary conditions for the friction shows that slip is dominant along the direction of the prestress with the largest deviations in slip rate direction occurring near the rupture front and the edges of the fault

imaging coseismic rupture in far field by slip patches summary since the end of the 1970s teleseismic waves are used routinely to infer the first order characteristics of the event location depth duration focal mechanism but finding the second order kinematic parameters spatial distribution of slip rupture velocity and more basically the discrimination between the fault plane and the other nodal plane of distant events remains a difficult task classically these events are studied by two different methods either they are seen as a succession of subevents each of which is considered as a point source or like in near field as extended sources where the kinematic parameters are retrieved on a grid the first approach is not physically satisfactory and can lead to erroneous interpretations of the

variability of physical and chemical characteristics along the 70 m isobath of the southeastern bering sea abstract from observations of ice cover temperature salinity currents and nitrate it is evident that along shelf variability was significant over the middle shelf of the eastern bering sea but less distinct than that observed in the cross shelf domains along the 70 m isobath three zones were evident in the summer the southeastern cold pool centered at 57 n an intermediate zone consisting of warmer water with weaker stratification and the northern cold pool extending northward from 58 n small scale 20 km horizontal features that persisted for months were common nutrient concentrations were related to salinity and were replenished more uniformly over the southern shelf than north of the pribilof islands although mean currents were weak 1 cm s 1 short energetic advective events impacted the temperature and salinity structure

characteristics and variability of the inner front of the southeastern bering sea the inner front of the southeastern bering sea shows marked spatial variability in frontal characteristics created by regional differences in forcing mechanisms differences in forcing mechanisms sea ice advance retreat and storm strength and timing and early spring water properties result in strong interannual variability in biological chemical and physical features near the front we have developed a simple model based on surface heat flux and water column mixing to explain the existence of cold belts cont shelf res 19 14 1999 1833 associated with such fronts hydrography fluorescence and nutrient observations show that pumping of nutrients into the euphotic zone occurs and this can prolong primary production at the inner front the effectiveness of this process depends on two factors the existence of a reservoir of nutrients in the lower layer on the middle shelf and the occurrence of sufficient wind and tidal energy to mix the water column

determination of constitutive relations of fault slip based on seismic wave analysis constitutive laws define the boundary conditions on fault plane and govern many aspects of earthquake failure although several constitutive laws have been formulated based on laboratory rock experiments and applied to theoretical studies in various fields no actual relation during a natural earthquake has been determined the 1995 kobe earthquake is suitable for detailed kinematic analysis and this enables the first evaluation of constitutive relations for a natural earthquake in this study we determine spatiotemporal slip distribution on an assumed fault plane of the 1995 kobe earthquake by waveform inversion and then solve elastodynamic equations using a finite difference method to determine the stress distribution and constitutive relations on the fault plane an inversion method based on bayes theorem is employed to obtain a spatiotemporal slip distribution and enables us to ensure the objective uniqueness of the solution with numerous parameters and smoothing constraints this slip distribution is then used as part of the boundary condition in the finite difference calculation the time histories of slip and shear stress obtained then provide a constitutive relation at each point on the fault plane they show slip weakening relations almost everywhere on the fault plane while slip rate dependency is not clear the slip weakening behavior has a clear depth dependency indicating that the slip weakening rate d du is smaller in the shallow crust than that in the deep crust this may be associated with the paucity of shallow seismicity observed in the source region of this earthquake as reported for many mature fault systems