retardation of 238u and 232th decay chain radionuclides in long island and connecticut aquifers abstract knowledge of the ability of an aquifer to retard the groundwater transport of toxic or radioactive ions can be inferred from the analysis of groundwater for the radionuclides of the 238u and 232th decay chains groundwaters of varying chemical composition were analyzed from wells in long island new york and connecticut aquifer lithologies are arkose quartz sandstone granite and glacial drift adsorption k1 and desorption k2 coefficients for ra and pb were calculated using 222rn activity as a measure of the supply of other recoil nuclides laboratory tests of the validity of this assumption were made by measuring the flux of 224ra and 222rn from aquifer solids the ratio k 1 k 2 is the distribution coefficient k which is effectively equal to r the retardation factor the average value of k for ra is 6 102 in long island aquifers and 5 104 in connecticut the distribution coefficient for pb is 104 in long island and 105 in connecticut results from this and other studies reveal a strong dependence of retardation on ph o 2 and ionic strength that tends to overwhelm any dependence of retardation on lithology

large intermediate depth earthquakes and the subduction process this study provides an overview of intermediate depth earthquake phenomena placing emphasis on the larger tectonically significant events and exploring the relation of intermediate depth earthquakes to shallower seismicity especially we examine whether intermediate depth events reflect the state of interplate coupling at subduction zones and whether this activity exhibits temporal changes associated with the occurrence of large underthrusting earthquakes historic record of large intraplate earthquakes m b 7 0 in this century shows that the new hebrides and tonga subduction zones have the largest number of large intraplate events regions associated with bends in the subducted lithosphere also have many large events e g altiplano and new ireland we compiled a catalog of focal mechanisms for events that occurred between 1960 and 1984 with m 6 and depth between 40 and 200 km the final catalog includes 335 events with 47 new focal mechanisms and is probably complete for earthquakes with m b 6 5 for events with m 6 5 nearly 48 of the events had no aftershocks and only 15 of the events had more than five aftershocks within one week of the mainshock events with more than ten aftershocks are located in regions associated with bends in the subducted slab focal mechanism solutions for intermediate depth earthquakes with m 6 8 can be grouped into four categories 1 normal fault events 44 and 2 reverse fault events 33 both with a strike nearly parallel to the trench axis 3 normal or reverse fault events with a strike significantly oblique to the trench axis 10 and 4 tear faulting events 13 the focal mechanisms of type 1 events occur mainly along strongly or moderately coupled subduction zones where a down dip extensional stress prevails in a gently dipping plate in contrast along decoupled subduction zones great normal fault earthquakes occur at shallow depths e g the 1977 sumbawa earthquake in indonesia type 2 events with strike subparallel to the subduction zone and most of them with a near vertical tension axis occur mainly in regions that have partially coupled or uncoupled subduction zones and the observed continuous seismicity is deeper than 300 km the increased dip of the downgoing slab associated with weakly coupled subduction zones and the weight of the slab may be responsible for the near vertical tensional stress at intermediate depth and consequently the change in focal mechanism from type 1 to type 2 events events of type 3 occur where the trench axis bends sharply causing horizontal parallel to the trench strike extensional or compressional intraplate stress type 4 are hinge faulting events for strongly coupled zones we observed temporal changes of intermediate depth earthquake activity associated with the occurrence of a large underthrusting event after the occurrence of a large underthrusting event the stress axis orientation of intermediate depth earthquakes changes from down dip tensional to down dip compressional e g 1960 chile 1974 peru 1982 tonga and 1952 kamchatka earthquakes or the number of large intermediate events decreases for a few years e g 1964 alaska and 1985 valparaiso earthquakes we conclude that even though the stress changes induced by slab pull and slab distortion control the general pattern of intermediate depth seismicity spatial and temporal variations of the intraplate stress associated with interplate coupling are important in controlling the global occurrence of large intermediate depth events

stress in the contorted nazca plate beneath southern peru from local earthquakes we study earthquake focal mechanisms in a region of highly contorted subducting lithosphere to identify dominant sources of stress in the subduction process we observe a stress pattern in the contorted nazca plate beneath southern peru from an analysis of hypocentral trend and focal mechanisms of intermediate depth earthquakes expanding on previous studies we examine the hypocentral trend using 1673 of 2178 well located local events from the nine station arequipa network the dip of the plate beneath southern peru averages 25 30 from 25 to 100 km depth below this depth there is an 80 to 100 km wide contortion between a zone of increasing dip convex to the southeast and a flat lying concave zone to the northwest using more than 6000 p wave first motions of events deeper than 50 km we derive stress orientations from a moving average of composite focal mechanisms across a 200 by 350 km region including the contortion the in plate distribution of tension t and compression p axes reveals a coherent stress pattern the trend is most clear beneath south central peru nw section and below 100 km depth in southernmost peru se section both t and p axes tend to be dominantly in plate especially below 100 km depth t axes orient toward the contortion in a fan shaped trend which suggests that the deepest part of the seismic zone within the convex se section is sinking and pulling the more buoyant nw section we conclude that from 50 to 200 km depth slab pull forces are dominant in the observed stress our results suggest that a significant amount of plate extension occurs in this region of intermediate depth subduction

intermediate and deep seismicity and lateral structure of subducted lithosphere in the circum pacific region in this paper we present a region by region review of the wadati benioff zone structure of most of the world s seismically active subduction zones focusing primarily on the intermediate and deep seismicity lateral changes in wadati benioff zone structure are common in every major subduction zone in this study we use these changes to define possible boundaries between portions or segments of lithosphere with differing subduction geometries although earthquake data seldom have the resolution to show conclusively whether these boundaries separate independent blocks of lithosphere the available data indicate that the active process at most of these segment boundaries is ductile deformation of the subducting plate rather than tearing we found the strongest evidence for the existence of tears where tears are geometrically necessary such as where a transform boundary terminates a trench as at the new hebrides tonga and south sandwich trenches weak evidence suggesting other tears does exist in some regions such as taiwan the japan izu bonin corner and the philippines the causes of these changes in structure are in most cases unclear only about 34 of the possible segment boundaries coincide with subducting bathymetric features some boundaries occur where there is apparent lateral strain caused by anomalous trench geometry we have designed a simple modeling procedure which incorporates published plate motion and the observed geometry of trenches and wadati benioff zones to estimate the lateral strain in subduction zones throughout the circum pacific region although no observed subduction zone has a perfectly strain free geometry there is a broad range of geometries for which the lateral strain is small indeed the observed geometry of most subduction zones involve relatively little lateral strain comparison with centroid moment tensor focal mechanisms indicates that in zones where the modeling predicted little lateral strain the mechanisms of intermediate and deep earthquakes show no effects of lateral stress and downdip stresses are clearly dominant in regions such as the mariana arc where the model predicts very large lateral extension lateral tension is very evident in the focal mechanisms in regions such as the hokkaido corner where the modeling predicts large compressional strains the plate appears to buckle and bending stresses parallel to the trench are evident in general this study finds that subducted lithosphere is remarkably cohesive and rigid and only rarely deforms by breaking or stretching

deep earthquakes in the southwest pacific a tectonic interpretation this paper examines the spatial distribution of deep earthquakes in the tonga fiji new hebrides region of the southwest pacific our interpretation emphasizes the complex cenozoic tectonics of the pacific indo australian plate boundary as a primary control on the distribution and deformation of subducted lithosphere most deep earthquakes in the interarc region are associated with the contorted pacific plate lithosphere subducted at the tonga trench however anomalous groups of deep earthquakes located west of the tonga zone are unrelated to the present plate configuration tectonic reconstructions of the region to 8 m y b p provide circumstantial evidence that 1 the anomalous events west of the tonga zone occur in two pieces of detached lithosphere subducted at the vitiaz and proto new hebrides trenches during the late miocene 2 the flattening of the inclined seismic zone in northernmost tonga is related to the rapid opening of the lau basin since 4 m y b p and 3 the sharp westward curvature of the tonga seismic zone in this area coincides with a preexisting bend in the late miocene vitiaz arc the sharpness of its present curvature appears to be a secondary effect of shear flow in the lower mantle and compression between detached vitiaz and attached tonga lithosphere thus much of the contortion of the subducted lithosphere beneath tonga fiji may be produced by local tectonic interactions rather than collision of the slab with an impenetrable boundary in the midmantle

movement of lithospheric plates relative to subduction zones formation of marginal seas and active continental margins abstract subduction zones with deep seismicity are believed to be associated with the descending branches of convective flows in the mantle and are subordinated to them therefore the position of subduction zones can be considered as relatively fixed with respect to the steady state system of convective flows the lithospheric plate overhanging a subduction zone as a rule of continental type may 1 1 either move away from the subduction zone or 2 2 move onto it in the first case extensional conditions originate behind the subduction zone and the new oceanic crust of back arc basins forms in the second case active andean type continental margins with thickening of the crust and lithosphere are observed behind the majority of volcanic island arcs along the boundary with marginal sea basins independent shallow seismicity belts can be traced they are parallel to the main seismicity belts coinciding with the benioff zones the seismicity belts frame island arc microplates island arc microplates are assumed to be a frame of reference to calculate relative movements of the consuming and overhanging plates using slip vector azimuths for shallow seismicity belts in the frontal parts of the kurile japan izu bonin mariana and tonga kermadec arcs the position of the pole of rotation of the pacific plate with respect to the western pacific island arc microplates was computed its coordinates are 66 1 n 119 2 w from the global closure of plate movements it has been determined that for the past 10 m y the eurasian and indian plates have been moving away from the western pacific island arc system both rotating clockwise around poles at 31 1 n 164 2 w and 1 3 s 157 5 w respectively this provides for the opening of the back arc basins at the same time south america is moving onto the subduction zone at the rate of 4 cm yr some hot spots such as hawaiian tibesti and those of the south atlantic are moving relative to the island arc system at a very low rate viz 0 5 0 7 cm yr presumably the western pacific subduction zone and hot spots form a single frame of reference which can generally be used for the analysis of absolute motions

source characteristics of the kunashiri strait earthquake of december 6 1978 as deduced from strain seismograms abstract a great intermediate depth earthquake m jma 7 7 occurred on december 6 1978 beneath the kunashiri strait in the southern part of the kurile arc strain seismograms recorded by the three components of a fused quartz extensometer at the erimo geophysical observatory are used to study the source process of this earthquake the seismograms show a notable sequence of pulses with a short period of several seconds the sequence follows the first p wave arrival and lasts for 60 s after the first s wave arrival furthermore a long period 60 s wave arriving at the first p time appears on each seismogram from the short period pulse sequence at least 18 distinct events which have a total seismic moment of 2 5 10 27 dyne cm can be identified during 60 s by comparing the synthetic seismograms to the observed ones however the amplitude of the observed long period wave cannot be explained enough by the sequence of short period events from this fact it may be concluded that this earthquake contains in addition to short period events a main long period event with a source process time rupture time of 60 s and a seismic moment of 4 5 10 27 dyne cm the observed strain steps in both magnitude and shape are also explained well by the complex multiple shock sequence

the behavior of u and th series nuclides in groundwater groundwater has long been an active area of research driven by its importance both as a societal resource and as a component in the global hydrological cycle key issues in groundwater research include inferring rates of transport of chemical constituents determining the ages of groundwater and tracing water masses using chemical fingerprints while information on the trace elements pertinent to these topics can be obtained from aquifer tests using experimentally introduced tracers and from laboratory experiments on aquifer materials these studies are necessarily limited in time and space regional studies of aquifers can focus on greater scales and time periods but must contend with greater complexities and variations in this regard the isotopic systematics of the naturally occurring radionuclides in the u and th decay series have been invaluable in investigating aquifer behavior of u th and ra these nuclides are present in all groundwaters and are each represented by several isotopes with very different half lives so that processes occurring over a range of time scales can be studied table 1 within the host aquifer minerals the radionuclides in each decay series are generally expected to be in secular equilibrium and so have equal activities see bourdon et al 2003 in contrast these nuclides exhibit strong relative fractionations within the surrounding groundwaters that reflect contrasting behavior during release into the water and during interaction with the surrounding host aquifer rocks radionuclide data can be used within the framework of models of the processes involved to obtain quantitative assessments of radionuclide release from aquifer rocks and groundwater migration rates the isotopic variations that are generated also have the potential for providing fingerprints for groundwaters from specific aquifer environments and have even been explored as a means for calculating groundwater ages view this table table 1 radionuclides important in groundwater studies the highly fractionated nature of the

tectonic implications of the microearthquake seismicity and fault plane solutions in southern peru using recordings from 15 portable instruments and six permanent stations operated during two field investigations of microearthquakes in southern peru we determined locations of 888 shallow and intermediate depth earthquakes and 56 fault plane solutions a series of tests evaluating the effects of elevation corrections erroneous velocity structure and random errors in arrival times on calculated locations suggest that the locations of most of the events are uncertain by less than 7 km at shallow depths 60 km the seismicity is concentrated in three belts one offshore one beneath the coast and one beneath the western cordillera there is also scattered but infrequent shallow activity near the altiplano the belts offshore and beneath the coast are apparently associated with slip along the nazca south american plate boundary although the style of deformation is complicated one can infer that beneath the coast the active boundary between the nazca and south american plates is within the mantle rather than at the base of the south american crust the belt beneath the western cordillera is about 30 km in width and dips southwest from near the surface to a depth of 60 km fault plane solutions suggest that crustal shortening is taking place in a northwest southeast direction in this region the significance of this activity is difficult to explain but these earthquakes provide no clear evidence of tectonic erosion in this area a composite fault plane solution from the few events near the altiplano shows normal faulting which is typical of the active tectonics of that region the results of the investigation of intermediate depth earthquakes shows that the downgoing slab in southern peru is warped and not torn in the region where the dip changes from nearly horizontal to a dip of about 30 they also suggest that the axis of the contortion is not perpendicular to the coast but parallel to the direction of plate motion moreover the volcanic line ends at the contortion which suggests that a relatively thick wedge of aesthenospheric material is necessary for the formation of subduction related volcanism fault plane solutions for events between 60 and 100 km depth show a large scatter but some suggest a detachment of a sliver of the downgoing slab solutions for deeper events are more consistent with one another with the t axes for most events being nearly horizontal and generally normal to the projections of the seismic zone

lateral structure of the subducting pacific plate beneath the hokkaido corner from intermediate and deep earthquakes we present a study of the lateral structure and mode of deformation in the transition between the kuril and honshu subduction zones we begin by examining the source characteristics of the january 19 1969 intermediate depth earthquake north of hokkaido in the framework of slab tearing which for the december 6 1978 event has been well documented by previous studies we use a least squares body wave inversion technique and find that its focal mechanism is comparable to the 1978 event to understand the cause of these earthquakes which in the case of the 1978 event occurred on a vertical tear fault but does not represent hinge faulting we examine the available international seismological centre isc hypocenters and harvard centroid moment tensor cmt solutions to determine the state of stress and lateral structure and segmentation in the kuril and northern honshu slabs these data are evaluated in the framework of two models model a requires the subducting slab at the hokkaido corner to maintain surface area model b requires slab subduction to be dominated by gravity with material subducting in the down dip direction the distribution of ics hypocenters shows a gap in deep seismicity down dip of the hokkaido corner supporting model b from the cmt data set we find that three types of earthquake focal mechanisms occur the first type a represents dip slip mechanisms consistent with down dip tension or compression in the slab in a direction normal to the strike of the trench these events occur throughout the honshu and kuril slabs with focal mechanisms beneath hokkaido showing nnw plungingp andt axes consistent with the local slab geometry the second type b occurs primarily at depths over 300 km in the southern part of the kuril slab with a few events in the northern end of the honshu deep seismicity these earthquakes have focal mechanisms with p axes oriented roughly e w highly oblique to the direction of compression found in the type a events with which they are spatially interspersed the third type c group of earthquakes are those events which do not fit in either of the first two groups and consist of either strike slip focal mechanisms such as the tearing events or oddly oriented focal mechanisms examination of the stress axes orientations for these three types reveals that the compressional axes of the type c events are consistent with those of type b the slab tearing events are just differential motion reflecting the e w compressive states of stress which is responsible for the type b family of events there is no need to invoke down dip extension which does not fit the slab geometry we conclude that these two states of stress can be explained as follows 1 the type a events and the seismicity distribution support model b 2 the type b and c events upport model a the solution is that the slab subducts according to model b but the flow in the mantle maintains a different trajectory possibly induced by the plate motions which produces the second state of e w compressive stress

seismotectonics and fault geometries of the 2019 ridgecrest sequence insight from aftershock moment tensor catalog using 3 d green s functions the 2019 m w 7 1 ridgecrest earthquake occurred on july 6th preceded by the m w 6 4 foreshock on july 4th 2019 these two earthquakes occurred close in space and time with partially overlapping surface ruptures and aftershock patterns raising the question of the relationship between the two events geological surveys and satellite observations provide important constraints on the surface traces of faulting however the subsurface fault geometries which are important for understanding the regional stress field earthquake initiation propagation and termination are not well resolved in this study moment tensor solutions for 256 earthquakes in the 2019 ridgecrest sequence were determined by waveform inversion using 3d velocity model the obtained moment tensor solutions show rotations of the stresses after mainshock indicating the ratio of mainshock stress drop to the background stress to be 0 5 0 9 the obtained moment tensor catalog also facilitates a better understanding of the sub surface fault geometries including 1 splay faults and antithetic faults in the northwest aftershock zone 2 shallow flower structures near the m w 7 1 epicenter 3 sub parallel faults in the southeast aftershock zone the aftershocks studies suggest the very complex surface ruptures near the 2019 ridgecrest m w 7 1 epicenter are near surface features that linked to a simple large through going fault at 5 km depth we also found that the southeastern most aftershocks which are located less than a kilometer from the garlock fault trace have significant different strike directions from that of the garlock fault indicating the central garlock fault remains seismically quiet

mm a variable period mantle magnitude this paper lays the theoretical groundwork for a variable period mantle magnitude mm based on the measurement of the spectral amplitude x of very long period rayleigh waves we retain the concept of magnitude by restricting ourselves to single station measurements ignoring the focal mechanism and the exact depth of the shallow earthquakes considered our measurements are made at a series of periods in all cases greater than 40 s and the largest value is retained this procedure effectively avoids the well known interference effects leading to saturation of magnitude scales defined at a fixed period two corrections are used a period dependent distance correction cd and a source correction cs also period dependent compensating for the variation of the excitation of rayleigh waves with period both of these corrections are fully predictable on the basis of standard surface wave excitation and dispersion theory the result is a formula of the type mm log10 x cd cs c0 in which all coefficients including the constant c0 are justifiable on sound theoretical grounds the analysis of a data set of 256 records from the broadband seismograph at papeete tahiti the ultra long period system at pasadena and stations of the geoscope network shows that the mean error in the estimation of the seismic moment is on the order of 0 1 0 2 units of magnitude with the standard deviation at each station also on the order of 0 2 units of magnitude no significant trend with either distance period or station can be identified the method can also be transposed to the time domain under some simple assumptions which are justifiable theoretically for typical teleseismic distances across the pacific basin both versions of the method lend themselves well to automation thus by providing a real time estimate of the seismic moment of distant earthquakes mm has considerable potential for tsunami warning purposes its concept can easily be extended to love waves and also to intermediate and deep earthquakes

spherical shell tectonics effects of sphericity and inextensibility on the geometry of the descending lithosphere the shape of a deep seismic zone is thought to represent that of the descending slab of lithosphere the lithosphere before subduction is a spherical shell and the shape of the descending slab is the result of the deformation of the spherical lithosphere at the subduction zone upon bending a spherical shell often deforms in a very different way from a simple plate we examine whether the actual shape of the descending slab can be explained by a simple bending of an inextensible spherical shell which shows little surface deformation under moderate stress this examination is made region by region for most of the subduction zones in the world by means of an analogue method the lithosphere is simulated by an inextensible spherical shell made of polyvinyl chloride resin the wadati benioff zone is shaped by plaster by referring to the reliable hypocentral data selected from the international seismological centre isc bulletins the spherical shell is forced to fit the miniature of the wadati benioff zone fitting is first attempted only by bending if a good fit is not attainable and if a discontinuity or gap in seismic activity is observed in the relevant region the spherical shell is torn along this discontinuity or gap and the goodness of fit is reexamined the results of the analysis are summarized as follows 1 the shape of the wadati benioff zone can be simulated largely by a simple bending of a spherical shell without surface deformation 2 in almost all of the regions of poor fit with bending a good fit can be achieved by tearing the spherical shell along the trace of low seismicity the sphericity of the lithosphere and the inextensibility upon deformation are the two essential factors in controlling the slab shape this means that the lateral constraint is most important for understanding the geometry of the downgoing slab of lithosphere and the stress state within it further several problems related to the deformational characteristics of the spherical lithosphere are also reviewed and discussed in connection with subduction tectonics

subduction of the nazca plate beneath peru as determined from seismic observations a detailed reinvestigation of seismicity in the southern half of peru using data from a local seismic network and a focal mechanism study using p wave first motions reveal that for the first 100 km of descent the slab as defined by this seismicity enters at a normal dip angle near 30 and that below this depth it is bent to a nearly horizontal angle this horizontal slab extends eastward for about 300 km and then dips steeply below the thick continental lithosphere scsp observations are found to be consistent with the profile of the subducting plate as determined from the local seismicity beneath the region from southern peru to northern chile the nazca plate descends with an almost constant dip angle 30 down to at least 300 km depth between the 30 dipping plate in southern peru and the horizontal profile beneath central peru there is a contortion over a 100 km wide continuous lateral section of the subducting plate we infer that at least in the upper 150 km depth range the descending nazca plate is contorted rather than torn

238u and 232th series nuclides in groundwater from the j 13 well at the nevada test site implications for ion retardation the activities of members of the 238u and 232th decay chains have been determined in water from well j 13 at the nevada test site near yucca mountain using 222rn as a measure of recoil flux adsorption k1 and desorption k2 rate constants for ra and pb have been determined the ratio k1 k2 defined as the dimensionless distribution coefficient k is 1 3 104 for ra and 2 7 104 for pb and is estimated to be about 104 for th

global seismicity of 1981 centroid moment tensor solutions for 542 earthquakes abstract centroid moment tensor solutions are presented for 542 earthquakes that occurred during 1981

collision rotation and back arc spreading in the region of the okhotsk and japan seas the india eurasia collision caused extensive deformation in the eurasia continent since late eocene time we propose that back arc spreading in the okhotsk and japan seas was related to the movement of microplates relevant to the india eurasia collision the kuril basin of the okhotsk sea and the basins of the japan sea were formed as back arc basins simultaneously their late oligocene to middle miocene age is constrained by sediment stratigraphy basement depth and heat flow data the movements of two microplates the okhotsk and amuria plates are significant for the tectonics of the okhotsk and japan seas simultaneous retreat of both microplates from trench hinge lines caused back arc spreading in the kuril basin and the japan sea the amuria plate moved north northeastward due to the india eurasia collision this caused pull apart opening of the baikal rift along a transform boundary between the siberia and amuria plates and a collision along the stanovoy range at its northern margin this movement triggered a clockwise rotation of the okhotsk plate with a component of dextral collision between the okhotsk and amuria plates which is well observed along the central zone of the sakhalin hokkaido islands between the two back arc plates the clockwise rotation of the okhotsk plate is suggested by the southwestward fan shaped opening of the kuril basin the kuril basin narrows to the northeast and terminates south of the kamchatka peninsula where a relative rotation pole between the okhotsk and kuril forearc plates is located the kamchatka peninsula was a zone of collision on the other side of the clockwise rotating okhotsk plate during the opening of the kuril basin the sredinny range in the kamchatka peninsula is one of the results of the collision between the okhotsk plate and the forearc plate of the kuril arc the bending of the japanese islands occurred in association with the back arc spreading of the yamato basin of the japan sea

m m a variable period mantle magnitude for intermediate and deep earthquakes we extend to the case of intermediate and deep earthquakes the mantle magnitude developed for shallow shocks byokal andtalandier 1989 specifically from the measurement of the spectral amplitude of rayleigh waves at a single station we obtain a mantle magnitude mm theoretically related to the seismic moment of the event through m m log 10 m 0 20 the computation ofmminvolves two corrections the distance correction is the same as for shallow shocks for the purpose of computing the frequency dependent source correction we define three depth windows intermediate a 75 to 200 km intermediate b 200 400 km and deep over 400 km in each window the source correctioncs is modeled by a cubic spline of log10t

regional tectonics of the coso geothermal area along the intracontinental plate boundary in central eastern california three dimensional vp and vp vs models spatial temporal seismicity patterns and seismogenic deformation we synthesize the tectonics of the southern walker lane belt and coso range in central eastern california using regional earthquake data first we invert for three dimensional models of the v p and v p v s structure of the upper and middle crust using these models we also determine three dimensional vs and poisson s ratio models the changes in seismic velocities across the region are small except for low velocities in sedimentary basins and a 2 km positive elevation of the basement velocities v p 6 km s beneath the southern sierra nevada localized low v p and low v s zones beneath the central coso range image a geothermal reservoir at 0 to 3 km depth as well as distinct low velocity anomalies in the depth range of 8 to 12 km because the v p v s has average crustal values within this broader zone we interpret the anomaly to indicate a zone of few percent geothermal brines extending from 8 to 12 km depth in addition an embedded highly localized poorly resolved zone possibly as small as 1 km 3 of slightly above average v p v s and higher poisson s ratio is a tentative suggestion of a small volume percent of magma present at depth of 10 km second we relocated the seismicity in the region using absolute traveltimes and differential traveltimes determined from waveform cross correlation the relocated seismicity forms several spatially clustered lineaments along the southeast side of the sierra nevada and in the indian wells valley and vicinity of the coso geothermal field which coincide with mapped late quaternary faults in the region the base of seismicity shallows from a regional depth of about 11 to 5 km beneath the central coso range which we interpret as evidence for shallowing of the brittle ductile transition zone beneath the geothermal field in addition to abundant background seismicity two large earthquake swarms located 5 to 8 km to the west of coso occurred in april to may 1992 and may to june 2001 two dual main shock aftershock sequences also occurred as follows the 1994 sequence near ridgecrest and the later coso earthquake sequence from late 1996 to early 1998 with the pairs of main shocks spaced 47 days and 16 months apart respectively kinematic analysis of the focal mechanisms indicates that the crustal stress loading process varies across the region the low vp anomaly abundant seismicity and crustal thinning provide quantitative evidence for the coso region being an extensional releasing step over between two northwest striking dextral faults the little lake and airport lake fault zones to the south and the owens valley fault to the north

a method for determining the extent of bulk 210po and 210pb adsorption and retardation in aquifers abstract the naturally occurring radionuclides 210 po and 210 pb are normally found in very low concentrations in the environment however under some conditions 210 po contributes significantly to the radioactivity in ground waters and can constitute a health risk naturally occurring 210 pb can be used in predicting the behavior of contaminant pb while studies have identified the processes controlling 210 po and 210 pb behavior the extent of adsorption and rates of removal in an aquifer cannot be determined without quantifying the rates of supply of these isotopes from decay of parent nuclides on and within aquifer solids this study demonstrates how the isotope systematics of the 238 u decay series which includes 210 po and 210 pb can be used to quantify the adsorption and retardation factors of 210 po and 210 pb in aquifers the rates of 210 po and 210 pb supply to ground waters are determined from the concentrations of other radionuclides in the series especially 222 rn and so the distribution coefficients between aquifer solids and groundwater can be calculated the effects of different supply rates due to different distributions of parent radionuclides in the aquifer can then be separated from differences in adsorption which is controlled by aquifer water chemistry a review of groundwaters where 210 po 210 pb and 222 rn data are available indicates that systematic variations can be found in bulk effective adsorption coefficients and retardation factors overall a method is provided for future studies of 210 po and 210 pb behavior and greater understanding of the occurrence and migration of these radionuclides

an experimental study of subduction and slab migration results of fluid dynamical experiments on the behavior of subducted slabs are presented that address two important characteristics of subduction slab penetration through the transition zone and horizontal slab migration cold negatively buoyant molded slabs of concentrated sucrose solution with viscosities of 3 5 106 p were introduced into a more dilute two layered sucrose solution representing the upper and lower mantle the transition zone was modeled by a step increase in both density and viscosity ratios of slab thickness to upper layer depth the relative viscosities in each layer and the effective rayleigh number were close to mantle values the initial configuration consisted of two plates separated by a trench gap with one plate attached to a dipping slab simulating a developing subduction zone with an overriding plate two different boundary conditions were used at the trailing ridge end of the subducting plate 1 fixed end corresponding to zero ridge motion and 2 free end corresponding to zero ridge resistance based on results from 15 experiments with various combinations of upper layer lower layer and slab densities u l s and viscosities we find that slab penetration depth is sensitive to both the normalized slab density anomaly r s l s u and the dip angle three penetration regimes are recognized r 0 5 in which the slab sinks into the lower layer without distortion 0 2 r 0 5 which permits limited penetration the amount dependent on dip angle in this regime the slab develops a thick root just beneath the transition zone and r 0 2 in which the slab is deflected by the transition and little or no penetration occurs retrograde slab motion and trench migration occurred in nearly every case with the overriding plate moving parallel to but faster than the trench causing the gap between subducting and overriding plates to close with time prograde trench motion was never observed and the trench remained stationary only in the extreme case free ridge condition with equal upper and lower layer densities these results suggest that the transient extension found in many back arc basins may be a direct consequence of the tendency for retrograde subduction and the amount of extension may be governed in part by the degree of slab penetration through the transition zone

application of fluid mechanic principles to the study of geodynamic processes at trench arc back arc systems geological processes at trench arc back arc systems are some of the most complex tectonic processes in need of study a large amount of data based on geological and geophysical observations has been accumulated to synthesize these data mathematical models have proven to be very useful because geological processes are directly related to the dynamical behavior of the solid earth many of them can be investigated in terms of fluid flow models some applications of fluid flow principles in studying tectonic processes at convergent plate boundaries are discussed in this paper when mantle processes are modeled as convective systems they are found to have direct implications for the determination of slab dip angles additionally they can also account for the high heat flows in the back arc basins provide a mechanism for back arc opening and resolve the question whether subducted oceanic crust can reach melting at shallow depth for island arc magma generation besides mantle processes flow models can also be used to study surface processes a simple one parameter plane flow theory is used to model the evolution of trench geometries this model is able to fit simulataneously the trench curvature and the differential paleomagnetic rotation between volcanic islands for the mariana despite the simplicity of many of these models their ability to synthesize geological and geophysical data at convergent plate boundaries is quite remarkable

implications of paleomagnetic data on the collision related bending of island arcs paleomagnetic studies from the marianas central philippines ryukyus sulawesi fiji new hebrides all show major differences in the declination values observed from individual islands within the same arc comparable differences are not observed in the inclination values a simple plate tectonic model would predict that each island arc should experience the same rotation along its entire length as the plate moves around its rotation axis bathymetry from each of these arcs show that the differentially rotated segments of the upper plate occur in areas where the downgoing plate carries a feature such as a seamount chain rifted continental fragment or an island arc the coincidence of the bathymetric feature and the discordant paleomagnetic declination data support an earlier hypothesis of vogt 1973 that these features are buoyant zones on the downgoing plate which interact with the margin of the upper plate as a result of this interaction the upper plate boundary is locally deformed in addition to the paleomagnetic evidence other geologic and geophysical data from these areas suggest such interactions for example there appears to be a good correlation between the deflections in major structures around the collision zone and the paleomagnetic directions that are reported as this deformation proceeds eventually the stresses resulting from the collision may result in any of the following tectonic processes 1 slip line style strike slip faults allowing for the sideward extrusion of portions of the upper plate tapponnier et al 1983 2 changes in the polarity of the subduction zone 3 development of strike slip faults around the margin of the indenter or 4 reorganization of the entire plate margin in addition data from the marianas southern end of the ryukyu arc and the lau basin suggest that back arc basin development may result from the collision related deformation

an experiment in systematic study of global seismicity centroid moment tensor solutions for 201 moderate and large earthquakes of 1981 data from the global digital seismograph network were used to obtain centroid moment tensor solutions using the method of dziewonski et al 1981 results were obtained for 201 earthquakes ranging in seismic moment from 7 1023 to 3 1027 dyne cm the wide dynamic range of the sro asro stations allows us to investigate using the same algorithm series of events among which the smallest and the largest may differ in moment by a factor as large as 1000 among the events studied is a particularly interesting series in january 1981 off the island of honshu the main shock was preceded by three foreshocks and followed by four aftershocks all falling within the range of our analysis there is an indication that the source mechanism changes with the size of the event statistical analysis of the results for all earthquakes reveals that the half duration of shallow and intermediate events is approximated by 1 6 10 8 m01 3 the shifts in origin time for deep focus earthquakes are indicative of the general complexity of these events which depends only weakly on the seismic moment the extreme departures from a double couple mechanism seem to depend both on the moment and on the focal depth while for shallow earthquakes these departures decrease from a maximum at about 1025 dyne cm with increasing moment the reverse appears to be true with respect to deep focus earthquakes shallow earthquakes north of new guinea and along the solomon islands show systematic and substantial departures from the double couple mechanism

deep earthquakes and the thermal assimilation of subducting lithosphere two problematic aspects of deep seismic zones are investigated the maximum depth about 700 km of seismic activity and the relative maximum in seismic energy release in the 500 700 km depth range it is shown that both features can be accounted for by a model based on thermal assimilation this implies that no barrier is required at a depth of 650 700 km to explain the observations