coseismic coulomb stress changes caused by the mw6 9 yutian earthquake in 2014 and its correlation to the 2008 mw7 2 yutian earthquake abstract a three dimensional viscoelastic finite element model of the qinghai tibet plateau and its adjacent regions was used to explore the relationship between the mw7 2 yutian earthquake in 2008 and the 2014 mw6 9 yutian earthquake we further analyzed the coulomb failure stress change caused by the yutian mw6 9 earthquake on faults surrounding the bayan har block and discussed the relationship between the coulomb stress change and aftershock distribution the preliminary results showed that 1 the coulomb failure stress change caused by the mw7 2 yutian earthquake in 2008 which was projected on the slip direction of the 2014 yutian mw6 9 earthquake exceeded the earthquake triggering threshold of 0 01 mpa implying an apparent triggering effect specifically the coseismic coulomb failure stress changes were 0 0167 mpa and 0 0170 mpa when assuming apparent friction coefficients of 0 4 and 0 6 respectively the coulomb failure stress changes including viscoelastic relaxation effects were 0 0187 mpa and 0 0194 mpa respectively combined with the tectonic stressing rate the mw6 9 yutian earthquake in 2014 was advanced 21 4 24 9 years by the 2008 mw7 2 yutian earthquake 2 the stress changes derived from the viscoelastic relaxation effect were much less than that from the coseismic effect which was possibly interpreted as the shorter elapsed time between the two earthquakes relative to the characteristic time 3 the coulomb failure stresses increased on the mid northern segment of the altyn tagh fault the middle segment of the mani yushu fault and the western segment of the eastern kunlun fault which demonstrated increasing seismic potential in particular the coulomb failure stress on the middle segment of the altyn tagh fault increased by 2 8 103 pa the most significant increase in contrast the coulomb failure stress decreased on the western segment of the mani yushu and minjiang faults with the western segment of the mani yushu fault dropping by 3 6 103 pa the coulomb stress changes on most faults surrounding the bayan har block were calculated to have increased while the minjiang fault on the eastern boundary was inhabited due to the enhancement of coseismic compressive stresses 4 combining the relocation of the aftershock sequence with the coseismic failure stress change we concluded that the mw6 9 yutian main shock triggered the aftershock sequences

the thermal and mineral springs of aitoloakarnania prefecture function mechanism and origin of groundwater the study area is located in the northwestern part of greece in aitoloakarnania prefecture in this region where no volcanic activity exists thermal springs such as kremasta and kokkino stefani well known for their healing properties occur the objective of this study was the investigation of these springs as well as the study of the chemical composition and origin of water relationships between these springs were also examined the geological setting of the area comprises sedimentary rocks of the pindos gavrovo tripolis and ionian geotectonic zones deformed by orogenic movements followed by neogene extensional tectonism the thermal and mineral springs were classified into three main groups the first group is characterized by ca hco3 water type and low water temperatures it corresponds to the springs that are hosted in the ionian zone and their possible enrichment in so4 is mainly attributed to the evaporites the other two groups consist of alkaline thermal water mainly hosted in the formations of gavrovo tripolis zone in these two groups the very strong reducing conditions that prevail are expressed by high amounts of nh3 and h2s moreover na f li sr and ba display elevated concentrations the second includes mineral waters of ca na hco3 type that are depleted in calcium their residence time is rather long and they originate from deep water circulation through siliceous rocks the third group includes thermal waters of ca mg na hco3 water type of higher water temperatures that reveal characteristics of deep circulation directly associated with the underlain limestones

geochemical evolution of groundwater beneath mexico city abstract the geochemical evolution of groundwaters along a 24 km flow beneath central mexico city path from the sierra de las cruces towards lake texcoco has been investigated using stable isotopes radiocarbon and major and trace elements to determine the natural baseline conditions the extent of any contamination and the effectiveness of the overlying aquitard seal modern groundwaters of low salinity 1 are found up to 11 km from the outcrop area and groundwater ages of up to 6000 yr bp occur in the middle part of the section groundwater stable isotope ratios 18 o and 2 h lie close to the global meteoric water line indicating that the groundwater originates from local rainfall the groundwater chemistry may be interpreted as the result of inputs from the source area with progressive water rock interaction down the horizontal flow gradient a redox boundary is found at 9 km along the line of section coincident with the start of the confined section relatively low nitrate concentrations below 9 mg l 1 no 3 n are found in the aerobic waters low concentrations of no 3 in the aerobic waters and low cl reflect inputs prior to the modern development some elements cr u as se sb increase their concentration with distance time as far as the redox boundary but low concentrations occur in the reducing aquifer section the chemistry of several major ions mg na cl k as well as trace elements such as li rb ba reflect the weathering of the basaltic mineral assemblage feldspars and mafic minerals and their increases are generally proportional to residence time phosphate f and i concentrations indicate a probable source from apatite in the basaltic or rhyolitic rocks a borehole in the east of the city some 17 km downgradient intercepted thermal water si geothermometry indicates 163 c at depth this water gives a distinctive composition indicating possible addition of metamorphic co 2 which has then reacted with the igneous rocks increases in b and cl are derived from volatiles trapped in the glass or vesicular basalt a thermal anomaly found in the middle section of the heavily pumped aquifer is interpreted as the up coming of warmer water from medium to greater depth mainly from basalts rhyolites and possibly limestones the geochemistry indicates that groundwater beneath mexico city is of good quality and there is no obvious evidence of leakage of inorganic compounds from surface sources of contamination through the aquitard the younger groundwater drawn from the western outcrop area is generally of good inorganic quality increased drawdowns in the confined aquifer have induced flow of warmer water with higher cl from depth the resources in the aquifer represent an important reserve of good quality water which need to be properly managed as a high quality resource as part of integrated plans for the city s future supplies

a multiscale study of the permeability of a thick clayey till we describe and evaluate two previously undocumented methods of bulk permeability k determination and compare results obtained with results of conventional smaller scale tests on the same thick clayey till deposit analysis of the downward propagation of seasonal water table fluctuations yielded bulk k of approximately 10 10 m s 1 in close agreement with results of laboratory consolidation and permeameter tests slug tests and distribution of vertical hydraulic gradient with depth in the deposit this agreement suggests that for such materials high gradient tests conducted at small scales of distance and time can provide reasonable estimates of bulk k the results also imply groundwater residence times in the till of thousands of years the flow pattern observed near a large excavation in the till was consistent with the initial recoveries of piezometers installed in much smaller boreholes assuming perturbation of hydraulic head in the formation due to borehole excavation time scales of these perturbations which prevent interpretation of measured hydraulic head using conventional well hydraulic methods varied from months to tens of years

hydrogeology of two saskatchewan tills i fractures bulk permeability and spatial variability of downward flow groundwater flow patterns were compared and contrasted in two deposits of clayey till of different thicknesses both of which overlie regional aquifers in a flat upland recharge area analysis of 67 field and laboratory hydraulic conductivity determinations indicated that conductive fractures in the unoxidized zone of the thinner till dalmeny site give rise to an isotropic bulk permeability of approximately 5 10 9 ms 1 which is two orders of magnitude greater than the bulk permeability of unoxidized till in the thicker deposit warman site vertical pore water velocities estimated using hydraulic and isotopic evidence vary greatly at dalmeny and appear to be of the order of 8 m in 10 000 yr at warman the association of vertical groundwater flux depth to water table and depth of oxidized zone at a given location is referred to as the flow regime there spatially detailed water level monitoring and test drilling indicated substantial variation of the flow regime at dalmeny on a horizontal scale of tens of meters no such variation exists at warman the differentiation of the flow regime at dalmeny is due to higher till permeability which permits a spatially variable response to microtopographically focussed infiltration of surface water recognition of the causes and symptoms of flow regime variability beneath flat till landscapes has implications for environmental assessment procedures agricultural practices wetland management and waste disposal

the 2008 yutian normal faulting earthquake mw 7 1 nw tibet non planar fault modeling and implications for the karakax fault abstract the ene striking altyn tagh fault and the wnw striking karakax fault are two major strike slip fault systems in northern tibet and form a prominent 2000 km long fault system the 2008 yutian normal faulting earthquake mw 7 1 struck near the southern edge of the tarim basin where the two fault systems converge while there are numerous ns trending normal faults particularly in southern tibet their tectonic origins have remained contentious based on crustal deformation data sets obtained from synthetic aperture radar sar as well as aftershock distribution we developed a non planar fault source model for the 2008 yutian earthquake that exhibits a large normal fault slip on a west dipping surface with a nearly ns strike thus suggesting a localized ew trending extensional stress field the extensional stress was presumably generated at a step over region of two ne trending left lateral strike slip faults which would probably belong to the altyn tagh and longmu gozha co fault systems in the epicentral area there exists a fault scarp that coincides with the top edge of our fault model and thus similar earthquakes must have occurred over geological time such normal faulting earthquakes must have been repeatedly suppressed the left lateral slip behavior of the karakax fault in addition if the slip along the altyn tagh fault is transferred to the longmu gozha co fault which is se across the normal fault system the slip rate of the karakax fault would be less than that of the adjoining altyn tagh fault

triggering of the ms 5 4 little skull mountain nevada earthquake with dynamic strains abstract we have developed an approach to test the viability of dynamic strains as a triggering mechanism by quantifying the dynamic strain tensor at seismogenic depths we focus on the dynamic strains at the hypocenter of the m s 5 4 little skull mountain lsm nevada earthquake this event is noteworthy because it is the largest earthquake demonstrably triggered at remote distances 280 km by the m s 7 4 landers california earthquake and because of its ambiguous association with magmatic activity our analysis shows that if dynamic strains initiate remote triggering the orientation and modes of faulting most favorable for being triggered by a given strain transient change with depth the geometry of the most probable lsm fault plane was favorably oriented with respect to the geometry of the dynamic strain tensor we estimate that the magnitude of the peak dynamic strains at the hypocentral depth of the lsm earthquake were 4 strain 2 mpa which are 50 smaller than those estimated from velocity seismograms recorded at the surface we suggest that these strains are too small to cause mohr coulomb style failure unless the fault was prestrained to near failure levels the fault was exceptionally weak and or the dynamic strains trigger other processes that lead to failure

earthquake triggering along the xianshuihe fault zone of western sichuan china western sichuan is among the most seismically active regions in southwestern china and is characterized by frequent strong m 6 5 earthquakes mainly along the xianshuihe fault zone historical and instrumental seismicity show a temporal pattern of active periods separated by inactive ones while in space a remarkable epicenter migration has been observed during the last active period starting in 1893 the sinistral strike slip xianshuihe fault of 350 km total length was entirely broken with the epicenters of successive strong earthquakes migrating along its strike this pattern is investigated by resolving changes of coulomb failure function cff since 1893 and hence the evolution of the stress field in the area during the last 110 years coulomb stress changes were calculated assuming that earthquakes can be modeled as static dislocations in an elastic halfspace and taking into account both the coseismic slip in strong m 6 5 earthquakes and the slow tectonic stress buildup associated with major fault segments the stress change calculations were performed for faults of strike dip and rake appropriate to the strong events we evaluate whether these stress changes brought a given strong earthquake closer to or sent it farther from failure it was found that all strong earthquakes and moreover the majority of smaller events for which reliable fault plane solutions are available have occurred on stress enhanced fault segments providing a convincing case in which coulomb stress modeling gives insight into the temporal and spatial manifestation of seismic activity we extend the stress calculations to the year 2025 and provide an assessment for future seismic hazard by identifying the fault segments that are possible sites of future strong earthquakes

evolution of the stress field in the northern aegean sea greece summary the evolution of the stress field in the area of the northern aegean sea during the 20th century has been studied the area is dominated by dextral strike slip faulting and is characterized by frequent strong earthquakes coulomb stress changes dcff were calculated assuming that earthquakes can be modelled as static dislocations in an elastic half space and taking into account both the coseismic slip in large mi7 0 earthquakes and the slow tectonic stress build up along the major fault segments the stress change calculations were performed for strike slip faults of strike dip and rake appropriate to the large events we evaluate whether these stress changes brought a given large earthquake closer to or farther from failure it was found that each of the large events occurred in regions of increased calculated coulomb stress moreover the majority of smaller events for which reliable fault plane solutions are available were also located in areas of positive dcff by extending the calculations to 2020 and assuming that no additional large mi7 0 earthquake occurs between 1999 and 2020 possible sites of future large earthquakes are identified

normal and oblique slip of the 2008 yutian earthquake evidence for eastward block motion northern tibetan plateau abstract the 2008 yutian mw 7 1 earthquake occurred in a junction area of the altyn tagh karakax and kunlun fault systems and becomes a most northerly normal faulting event in the northern tibetan plateau high resolution satellite image interpretation and field investigation indicate that the surface rupture zone produced by the yutian earthquake is 31 km long along the yulong kashgar fault a ns trending fault at the western piedmont of a snow covered mountain at the headwater of the yulong kashgar river about 20 km south of the ashikule volcanoes the surface rupture zone along the yulong kashgar fault consists of four different types of surface ruptures with both normal and oblique slip components the maximum left lateral slip and vertical offset we measured in the field are 3 6 m and 3 3 m respectively this ns trending seismogenic fault belongs to one of the boundary faults between the qaidam qilian and the western kunlun blocks thus the yutian earthquake is as a result of abrupt normal and oblique slip faulting which fits the eastward escape of the qaidam qilian block relative to the western kunlun block this surface rupture pattern supports the eastward block like motion model whose deformation takes place mainly along the block boundaries delineated by mega strike slip faults in the northern tibetan plateau more important may be that the yutian earthquake has implication on future earthquake risk along the boundary faults of the qaidam qilian block

changes in static stress on southern california faults after the 1992 landers earthquake the magnitude 7 5 landers earthquake of 28 june 1992 was the largest earthquake to strike california in 40 years the slip that occurs in such an earthquake would be expected to induce large changes in the static stress on neighbouring faults these changes in stress should in turn affect the likelihood of future earthquakes stress changes that load faults towards failure have been cited as the cause of small1 5 moderate6 and large7 earthquakes conversely those that relax neighbouring faults have been related to a decrease in seismicity5 here we use an elastic half space model8 to estimate the stress changes produced by the landers earthquake on selected southern california faults including the san andreas we find that the estimated stress changes are consistent with the triggering of four out of the five aftershocks with magnitude greater than 4 5 and that the largest changes 1 10 bar occurring on part of the san bernardino segment of the san andreas fault may have decreased the time to the next magnitude 8 earthquake by about 14 years

role of static stress transfer in earthquake occurrence in the himalaya abstract coulomb failure hypothesis suggests that earthquake interaction can lead to earthquake sequences and clustering this implies that the phenomenon should be considered a fundamental feature in any description of seismicity and evaluation of the seismic hazard and risk of a region we translate this idea to the himalaya and investigate how significantly the past earthquakes of the region may have influenced the present day seismicity and seismic hazard potential along its different segments for this we estimate separately the change in coulomb failure stress at the source of the most recent moderate magnitude earthquake along the kumaon garhwal segment the march 29 1999 chamoli earthquake due to two possible major sources these are i the process of subduction of the indian plate beneath the himalaya and ii some selected preceding moderate and small earthquakes in and around the region during their post seismic phase our results indicate that the change due to the former source completely overshadows that due to the latter the implication of our calculations is that the plate subduction processes in the himalaya i actively promote large moderate and small earthquake activity and ii also indirectly influence the regional seismicity through the occurrence of the past earthquakes

frictional constraints on crustal faulting we consider how variations in fault frictional properties affect the phenomenology of earthquake faulting in particular we propose that lateral variations in fault friction produce the marked heterogeneity of slip observed in large earthquakes we model these variations using a rate and state dependent friction law where we differentiate velocity weakening behavior into two fields the strong seismic field is very velocity weakening and the weak seismic field is slightly velocity weakening similarly we differentiate velocity strengthening behavior into two fields the compliant field is slightly velocity strengthening and the viscous field is very velocity strengthening the strong seismic field comprises the seismic slip concentrations or asperities the two intermediate fields weak seismic and compliant have frictional velocity dependences that are close to velocity neutral these fields modulate both the tectonic loading and the dynamic rupture process during the interseismic period the weak seismic and compliant regions slip aseismically while the strong seismic regions remain locked evolving into stress concentrations that fail only in main shocks the weak seismic areas exhibit most of the interseismic activity and aftershocks but can also creep seismically this mixed frictional behavior can be obtained from a sufficiently heterogeneous distribution of the critical slip distance the model also provides a mechanism for rupture arrest dynamic rupture fronts decelerate as they penetrate into unloaded complaint or weak seismic areas producing broad areas of accelerated afterslip aftershocks occur on both the weak seismic and compliant areas around a fault but most of the stress is diffused through aseismic slip rapid afterslip on these peripheral areas can also produce aftershocks within the main shock rupture area by reloading weak fault areas that slipped in the main shock and then healed we test this frictional model by comparing the seismicity and the coseismic slip for the 1966 parkfield 1979 coyote lake and 1984 morgan hill earthquakes the interevent seismicity and aftershocks appear to occur on fault areas outside the regions of significant slip these regions are interpreted as either weak seismic or compliant depending on whether or not they manifest interevent seismicity

a landslide in till near warman saskatchewan canada little information is available about typical shear strength parameters of tills in southern saskatchewan even though till is the most common earth material used for construction in this region the warman landslide in the south saskatchewan river valley provides some insight into the shear strength characteristics of a till and the results are compared with laboratory tests the till is from the upper till of the sutherland group which has a high clay content relative to the underlying and overlying tills a back analysis of the landslide produced 27 assuming c 0 comparison with laboratory test data and results from a similar landslide near lebret saskatchewan suggests that 22 5 with c 7 kpa may be appropriate residual shear strength parameters a rising water table appears to have been the main contributing factor to instability between 1969 and 1984 there is a possibility however that at the 1 50 return interval for flood levels on the river erosion at the toe of the landslid

seismic behavior in central taiwan response to stress evolution following the 1999 mw 7 6 chi chi earthquake abstract following the 1999 mw 7 6 chi chi earthquake a large amount of seismicity occurred in the nantou region of central taiwan among the seismic activities eight mw 5 8 earthquakes took place following the chi chi earthquake whereas only four earthquakes with comparable magnitudes took place from 1900 to 1998 since the seismicity rate during the chi chi postseismic period has never returned to the background level such seismicity activation cannot simply be attributed to modified omori s law decay in this work we attempted to associate seismic activities with stress evolution based on our work it appears that the spatial distribution of the consequent seismicity can be associated with increasing coseismic stress on the contrary the stress changes imparted by the afterslip lower crust upper mantle viscoelastic relaxation and sequent events resulted in a stress drop in most of the study region understanding seismogenic mechanisms in terms of stress evolution would be beneficial to seismic hazard mitigation

present day crustal motion within the tibetan plateau inferred from gps measurements 1 using the measurements of 726 gps stations around the tibetan plateau we determine the rigid rotation of the entire plateau in a eurasia fixed reference frame which can be best described by an euler vector of 24 38 0 42 n 102 37 0 42 e 0 7096 0 0206 ma the rigid rotational component accommodates at least 50 of the northeastward thrust from india and dominates the eastward extrusion of the northern plateau after removing the rigid rotation to highlight the interior deformation within the plateau we find that the most remarkable interior deformation of the plateau is a glacier like flow zone which starts at somewhere between the middle and western plateau goes clockwise around the eastern himalayan syntaxis ehs and ends at the southeast corner of the plateau with a fan like front the deformation feature of the southern plateau especially the emergence of the flow zone could be attributed to an eastward escape of highly plastic upper crustal material driven by a lower crust viscous channel flow generated by lateral compression and gravitational buoyancy at the later developmental stage of the plateau the first order feature of crustal deformation of the northeastern plateau can be well explained by a three dimensional elastic half space dislocation model with rates of dislocation segments comparable to the ones from geological observations in the eastern plateau although gps data show no significant convergence between the eastern margin of the plateau and the sichuan basin a small but significant compressional strain rate component of 10 5 2 8 nstrain yr exists in a relatively narrow region around the eastern margin in addition a large part of the eastern plateau northeast of the ehs is not undergoing shortening along the northeastward convergence direction of the ehs but is stretching

residence time indicators in groundwater the east midlands triassic sandstone aquifer the east midlands triassic sherwood sandstone aquifer which has been the subject of detailed radiometric age studies is used to investigate both inert and reactive constituents of groundwater as indicators of residence time detailed resampling of the aquifer in 1992 has provided a considerable body of new inorganic geochemistry data though without radiocarbon several inert indicators are defined including the isotopic ratios 18o 2h 36cl noble gas ratios as well as the halogen elements cl br f i and their element ratios these form a group of essentially unreactive tracers primarily reflecting changing rainfall inputs and palaeoclimatic conditions except at outcrop where human impacts are also seen clearly the concentrations of cl mainly from atmospheric sources remain below 25 mg l 1 cl over a distance of some 30 km from outcrop reactive indicators the result of time dependent water rock interactions include 13c mg ca sr ca na cl and show diagnostic trends along the flow lines however the concentrations of certain trace elements li rb cs mn and mo which are not limited by solubility constraints show linear trends along the present day flow gradient this water rock interaction is taking place in groundwaters with low total mineralisation and it can be demonstrated that reactions involving these elements and isotopes are occurring entirely within the aquifer since high salinity groundwaters are found below the sherwood sandstone there is good correlation between some of the elements and 14c activities in the 1977 data set and this has been used to derive a concentration age relationship for the 1992 set of data a chemical timescale for the aquifer is then established using a li and b a combination of five trace elements the age of the fresh groundwater is thus shown to be up to 100 ka bp indicating the likelihood of semi continuous recharge during the devensian glacial period preceding the glacial maximum when no recharge occurred 10 20 ka bp this approach may be of value in extending groundwater dating beyond the radiocarbon timescale as well as interpreting sites where no radiocarbon data are available the scatter from the mean age line indicates those groundwaters which are derived from either rapid or slower than average flow zones within the aquifer as well as age stratification this stratification is borne out by a depth profile of groundwater from a new borehole near to outcrop which is shown by a combination of chemical and isotopic tracers to contain holocene overlying late pleistocene water

triggering of earthquake aftershocks by dynamic stresses it is thought that small static stress changes due to permanent fault displacement can alter the likelihood of or trigger earthquakes on nearby faults many studies of triggering in the near field particularly of aftershocks rely on these static changes as the triggering agent and consider them only in terms of equivalent changes in the applied load on the fault here we report a comparison of the aftershock pattern of the moment magnitude mw 7 3 landers earthquake not only with static stress changes but also with transient oscillatory stress changes transmitted as seismic waves that is dynamic stresses dynamic stresses do not permanently change the applied load and thus can trigger earthquakes only by altering the mechanical state or properties of the fault zone these dynamically weakened faults may fail after the seismic waves have passed by and might even cause earthquakes that would not otherwise have occurred we find similar asymmetries in the aftershock and dynamic stress patterns the latter being due to rupture propagation whereas the static stress changes lack this asymmetry previous studies have shown that dynamic stresses can promote failure at remote distances but here we show that they can also do so nearby

three dimensional finite element modeling of stress evolution around the xiaojiang fault system in the southeastern tibetan plateau during the past 500 years abstract the left lateral strike slip xiaojiang fault system is a main boundary between the southeastern tibetan plateau and the south china block and accumulates several large m 7 earthquakes in the past 500 years and a dozen of moderate 5 7 earthquake ruptures occurred in the past 500 years are simulated with split node technique in seismogenic upper crust we investigate the coulomb failure stress changes mainly on vertical planes parallel to the different strands of the xiaojiang fault system and at the epicenters of large m 7 earthquakes in the past 500 years and of moderate 5 7 earthquakes that occurred in the past 500 years both from their coseismic and postseismic effects among which the postseismic effect results in the stressing rate being obviously accelerated near epicenters of the large m 7 earthquakes after their occurrence thus the percentage of positive coulomb failure stress changes of 0 01 mpa at the epicenters increases from 37 to 50 for the large m 7 earthquakes and from 43 to 64 for the moderate 5

evidence of instantaneous dynamic triggering during the seismic sequence of year 2000 in south iceland 1 we analyze the coseismic stress perturbation during the 17 june 2000 south iceland seismic sequence the main shock ms 6 6 was followed by three large events within a few tens of seconds 8 26 and 30 s located within 80 km the aim of this paper is to investigate short term fault interaction and instantaneous triggering this happens when a fault perturbed by a stress change fails before the end of the transient stress perturbation we compute the shear normal and coulomb stress changes as functions of time in a stratified elastic half space by using discrete wave number and reflectivity methods we calculate dynamic stresses caused by the main shock at the hypocenters of these three subsequent events our numerical results show that the onset of the last two events is slightly delayed with respect to the arrival time of the second positive peak of coulomb stress variation while the first event occurred after the first positive stress peak we have also analyzed the response of a spring slider system representing a fault governed by a rate and state dependent friction law perturbed by shear and normal stress variations caused by the main shock the fault response to the computed stress perturbations is always clock advanced we have found suitable constitutive parameters of the modeled fault that allow the instantaneous dynamic triggering of these three earthquakes if the initial sliding velocity is comparable with the tectonic loading velocity we obtained failure times close to the observed origin times for low values of the initial effective normal stress

investigation of the chemical processes controlling the groundwater quality of ilia prefecture neogene formations display complex structure and host confined aquifers that are alternated and very often interrupted by impermeable clay layers in many cases the lack of hydrogeological information and data on those aquifers hinders research and planning of exploitation projects this study attempts to exploit the available information that can be derived from a sampling and the corresponding chemical analyses to reconstruct the hydrogeological image of such areas thus the chemical composition is used as a tool for natural tracing of the groundwater origin and movement in a previously unknown environment such as that of ilia region which was selected as the application area the distribution maps of certain hydrochemical parameters combined with the appropriate hydrochemical and isotopic data contributed to the classification of the groundwater samples by linking the hydrochemical processes with the samples locations as well as the origin of the aquifers recharge water at the same time the effectiveness of the used methodology was evaluated the use of r mode factor analysis led to the determination of the basic hydrochemical processes that shape the groundwater quality the analysis showed that the reducing environment is the defining factor that regulates the presence of the various chemical compounds finally the conceptual model of the aquifers emerged from the analysis of all the data this analysis also pointed out that the recharge of the local aquifers takes place mainly through the river beds and laterally from their contact with the carbonate rocks

the petrofka landslide saskatchewan both abutments and all piers of the petrofka bridge are on a landslide the bridge has performed satisfactorily for 23 years indicating the factor of safety of the petrofka landslide is greater than unity this stability is abnormal because landslide slopes on bedrock clays shale in this region are notoriously unstable a dormant landslide is difficult to analyze because the input parameters must represent a condition of limiting equilibrium the petrofka landslide is 70 m high 880 m long 3000 m wide and up to 100 m thick the shear zone is in highly plastic montmorillonitic clays in an unnamed formation of probable tertiary age the upper cretaceous lea park judith river and bearpaw formations and the unnamed formation underly the landslide and are restricted to a collapse structure resulting from dissolution of devonian evaporites a retrogressive mechanism developed as valley downcutting progressed the pore water pressures could be estimated from the hydraulic head in the artesian aquifer of th

stress evolution on active faults in the southwestern yunnan region southeastern tibetan plateau and implications for seismic hazard abstract southwestern yunnan is expressed as a seismically active region bordering the southeastern tibetan plateau however the recent seismicity in this region has been relatively quiet considering some highly populated cities nearby and the historical record in this region understanding the stress changes on the main active faults is crucial for seismic hazard analysis in this study we calculated the co and post seismic coulomb failure stress change cfs on these faults caused by historical earthquakes in and around the region then by integrating the inter seismic tectonic loading estimated using a geodynamic model and gps data we analyzed the total stress change on all of these faults the results reveal that the cfs patterns caused by events on different segments of the sagaing fault vary the combined cfs increased on the middle segment of the longling lancang fault the heihe fault and the nantinghe fault among which the maximum change reached 3 5 kpa the great 1950 assam earthquake in the eastern himalayan syntax increased the cfs on almost all faults and the maximum change reached 20 kpa more importantly the cfs caused by earthquakes in the southwestern yunnan region was significantly larger than that caused by earthquakes in the surrounding region because of the geographical proximity the inter seismic tectonic stress loading rate reached 2 kpa yr 1 a significant total stress build up occurred on five segments distributed among four faults namely the nantinghe fault the longling lancang fault the heihe fault and the menglian fault making them possible candidates for the next major earthquake in this region

compression characteristics and index properties of tills and intertill clays in southern saskatchewan canada compression characteristics and index properties of unweathered tills and clays from repeated glaciations follow a predictable pattern at 10 sites along a 1300 km transect across southern saskatche

a review of the source behaviour and distribution of arsenic in natural waters the range of as concentrations found in natural waters is large ranging from less than 0 5 g l 1 to more than 5000 g l 1 typical concentrations in freshwater are less than 10 g l 1 and frequently less than 1 g l 1 rarely much higher concentrations are found particularly in groundwater in such areas more than 10 of wells may be affected defined as those exceeding 50 g l 1 and in the worst cases this figure may exceed 90 well known high as groundwater areas have been found in argentina chile mexico china and hungary and more recently in west bengal india bangladesh and vietnam the scale of the problem in terms of population exposed to high as concentrations is greatest in the bengal basin with more than 40 million people drinking water containing excessive as these large scale natural as groundwater problem areas tend to be found in two types of environment firstly inland or closed basins in arid or semi arid areas and secondly strongly reducing aquifers often derived from alluvium both environments tend to contain geologically young sediments and to be in flat low lying areas where groundwater flow is sluggish historically these are poorly flushed aquifers and any as released from the sediments following burial has been able to accumulate in the groundwater arsenic rich groundwaters are also found in geothermal areas and on a more localised scale in areas of mining activity and where oxidation of sulphide minerals has occurred the as content of the aquifer materials in major problem aquifers does not appear to be exceptionally high being normally in the range 1 20 mg kg 1 there appear to be two distinct triggers that can lead to the release of as on a large scale the first is the development of high ph 8 5 conditions in semi arid or arid environments usually as a result of the combined effects of mineral weathering and high evaporation rates this ph change leads either to the desorption of adsorbed as especially as v species and a range of other anion forming elements v b f mo se and u from mineral oxides especially fe oxides or it prevents them from being adsorbed the second trigger is the development of strongly reducing conditions at near neutral ph values leading to the desorption of as from mineral oxides and to the reductive dissolution of fe and mn oxides also leading to as release iron ii and as iii are relatively abundant in these groundwaters and so4 concentrations are small typically 1 mg l 1 or less large concentrations of phosphate bicarbonate silicate and possibly organic matter can enhance the desorption of as because of competition for adsorption sites a characteristic feature of high groundwater as areas is the large degree of spatial variability in as concentrations in the groundwaters this means that it may be difficult or impossible to predict reliably the likely concentration of as in a particular well from the results of neighbouring wells and means that there is little alternative but to analyse each well arsenic affected aquifers are restricted to certain environments and appear to be the exception rather than the rule in most aquifers the majority of wells are likely to be unaffected even when for example they contain high concentrations of dissolved fe

hydrogeology of two saskatchewan tills ii occurrence of sulfate and implications for soil salinity abstract the occurrence of sulfate salts in groundwater and solids was documented at two sites named dalmeny and warman where clayey tills overlie regional aquifers at dalmeny the till is thinner and approximately two orders of magnitude more permeable than at warman samples for sulfur and sulfate determinations were collected to depths of approximately 14 m and at horizontal spacings of a few tens of meters large numbers of indirect measurements of bulk salt content were obtained with a noncontacting earth conductivity meter results showed that the bulk sulfur and sulfate contents of solids could not in general be adequately characterized with point or grab samples from boreholes at dalmeny measurements representing larger averaging volumes revealed a spatially intricate pattern of sulfate abundance and depletion which extends vertically through the oxidized and unoxidized zones of the till to the underlying aquifer similar measurements at warman suggest that sulfate variability there is limited to the oxidized zone these findings and the observed coincidence of sulfate patterns with the groundwater flow patterns documented in part i2 show that the bulk permeability of the till controls the manner in which the subsurface sulfate reservoir is affected by microtopographic focussing of surface water input conceptual analysis of the responses of these systems to various land management practices indicates that where tills are relatively permeable these practices would primarily affect the quality of groundwater recharge to underlying aquifers where tills have low permeability the same practices would have no effect on aquifer water quality but they would affect the potential for surface and soil salinization

investigation of the rupture process of the 28 june 1992 landers earthquake utilizing terrascope abstract displacement seismograms recorded by terrascope for the 28 june 1992 landers earthquake m w 7 3 are deterministically modeled using a forward point source summation technique although the data set is sparse it was possible to robustly determine important rupture parameters such as gross slip distribution rupture velocity rise time and total source duration the relatively simple approach lends itself to rapid application following large earthquakes provided that a catalog of green9s functions appropriate for the region is available the fault used in the modeling of the landers mainshock has a length of 70 km along strike and a width of 15 km along dip a model was found in which the distribution and amplitude of slip at the surface matches the observed surface slip and provides a very good level of fit to the seismic data seismically the landers earthquake is characterized as two subevents the peak slip of the first subevent is 10 km north of the epicenter and the second is 40 km northwest along strike from the epicenter the seismic moment is distributed as 2 10 26 dyne cm to the first and 6 10 26 to the second subevent respectively it was assumed in our modeling that the distribution of seismic moment along strike was the same at all depths this assumption implies that slip at depth is 69 of that at the surface as a result of differences in the material properties in the velocity model the sensitivities of the source model to rupture velocity and dislocation rise time were examined a rupture velocity of 2 9 km sec 80 of the shear wave velocity and a rise time of 1 to 3 sec were found to satisfy the data the rise time is only a fraction of the total source process time of 24 sec and implies that slip on the fault occurred within a narrow band 3 to 10 km at any instant during the rupture

stress coupling between earthquakes in northwest turkey and the north aegean sea we have investigated the coulomb stress interactions of 29 earthquakes m s greater than or equal to 6 0 that have occurred in the region of northwest turkey and north aegean sea since 1912 of these events 23 may be related to earlier events and 16 are clearly related to earlier events all events after 1967 are related to previous events events in the early part of our time interval that show no correlation could be related to historical events as yet unidentified in some cases faults that have received a stress reduction from earlier events are prepared for an event by an earthquake occurring a few years before that creates a local coulomb stress rise thus regions of coulomb stress shadow can become regions where a damaging earthquake may occur the relation between smaller events and the coulomb stress distribution is less clear but may be related to poor data quality and practical limitations of our modeling technique nonetheless there are 4 times as many events per unit area in regions of enhanced stress than where stress is reduced we discuss the contemporary distribution of coulomb stress and argue that it is possible to identify the likely locations of future damaging earthquakes including identifying the most likely candidate faults

influence of the 2008 wenchuan earthquake mw 7 9 on the occurrence probability of future earthquakes on neighboring faults abstract the longquan shan fault and the huya fault are two major neighboring faults of the longmen shan fault zone where the 12 may 2008 wenchuan earthquake m w 7 9 occurred to study the influence of the wenchuan event on these two active faults we calculate changes of coulomb stress on the longquan shan fault and the huya fault caused by the wenchuan mainshock our results indicate that the coulomb stress in the northern section zone a of the longquan shan fault is increased by 0 07 0 10 bars that in the middle section zone b by 0 04 0 11 bars and that in the southern section zone c shows almost no change for the huya fault the coulomb stress is decreased by 0 01 0 03 bars in the northern section zone a 0 10 0 35 bars in the middle section zone b and nearly 0 5 bars in the southern section zone c the epicenter distribution of small earthquakes m l 1 5 on the longquan shan fault and the huya fault after the wenchuan earthquake is consistent with the distribution of the coulomb stress change this implies that the wenchuan earthquake may have triggered small events on the longquan shan fault but inhibited those on the huya fault we then use the rate state friction law to calculate the occurrence probability of future earthquakes in the study region for the next decade they include the distribution of b values magnitude of completeness m c the background seismicity rate a value of a n and the duration for the transient effect t a in the study region we also estimate the earthquake occurrence probabilities on the neighboring faults after the wenchuan earthquake our results show that the occurrence probability of future earthquakes in the longquan shan has a slight increase being 7 for m 5 0 shocks during the next decade but the earthquake probability in the huya region is reduced obviously being 5 20 7 26 and 3 9 for m 5 0 shocks during the next decade in sections a b and c of the huya fault respectively

nailing down the slip rate of the altyn tagh fault 1 previous estimates of the geodetic and geologic slip rates of the 1500 km long altyn tagh fault bordering the northern edge of the tibetan plateau vary by a factor of five proposed reasons for these discrepancies include poor gps geometry interpretative errors in terrace morphology and changes in fault slip rate over time here we present results from a new dense gps array orthogonal to the fault at 86 2 e that indicates a velocity of 9 0 3 2 4 4 mm yr in close agreement with geomorphologic estimates at the same location our estimated geodetic slip rate is consistent with recent geological slip rates based on terrace offsets the resulting mean combined geological and geodetic slip rate 9 0 4 0 mm yr is remarkably uniform for the central 800 km of the altyn tagh fault significantly lower than early kinematic estimates and consistent with deformation elsewhere in tibet and central asia

potential of future seismogenesis in hebei province ne china due to stress interactions between strong earthquakes abstract northeast china a densely populated area is affected by intense seismic activity which includes large events that caused extensive disaster and tremendous loss of life for contributing to the continuous efforts for seismic hazard assessment the earthquake potential from the active faults near the cities of zhangjiakou and langfang in hebei province is examined we estimate the effect of the coseismic stress changes of strong m 5 0 earthquakes on the major regional active faults and mapped coulomb stress change onto these target faults more importantly our calculations reveal that positive stress changes caused by the largest events of the 1976 tangshan sequence make the xiadian and part of daxing fault thus considered the most likely sites of the next strong earthquake in the study area the accumulated static stress changes that reached a value of up to 0 4 bar onto these faults were subsequently incorporated in earthquake probability estimates for the next 30 years

importance of the vertical component of groundwater flow a hydrogeochemical approach in the valley of san luis potosi mexico fractured volcanics exert a control on groundwater flow in the san luis potosi slp valley the chemical composition and temperature of water pumped from boreholes partially penetrating the fractured volcanics indicate that the produced water originates from an upward vertical flow most of the thermal groundwater has been detected in areas related to regional faults and lineaments intensive and uncontrolled pumping from the upper 14 of the aquifer total depth 1500 m causes the rise of water from a deep regional flow system that mixes with the shallower waters the deep waters contain high fluoride concentrations that contaminate the mixture and cause substantial health related effects the recharge controls on the regional flow system require further research however hydrogeochemical evidence supports the view that the origin of this recharge is limited to the western bounding sierra madre occidental higher levels of dissolved na li f and so4 2 derived from tertiary volcanics have been introduced into the exploited region the concentrations indicate lengthy and deep circulation flow li concentration was used as an indicator of groundwater residence time and therefore of the length of the groundwater flow path hydrogeochemical interpretation indicates the presence of three flow systems a shallow local one controlled by a clay layer that subcrops most of the valley floor an intermediate system in which water infiltrates just beyond the boundary of the clay layer and a deep regional system which originates outside the surface catchment the local and intermediate systems circulate through materials with comparatively low hydraulic conductivity low cl concentrations suggest rapid flow in the regional system concentrations of li and f can be used to calculate percentages of waters in mixtures of regional and intermediate flows concentrations of na ca2 and so4 2 appear to be controlled by water rock reactions uncontrolled pumping increases will tend to enhance fluoride concentration and its undesirable effects lack of management of aquifers in basins similar to the slp valley can result in groundwater contamination not only from surface anthropogenic sources but also from natural water rock interactions

the evolution of alkaline groundwaters in the continental intercalaire aquifer of the irhazer plain niger this paper discusses the hydrochemical evolution of groundwaters in the multilayer artesian aquifer of the continental intercalaire the mainly jurassic agadez dabla sandstones aquifer of northern niger isotopic and chemical changes in relation to the geological setting suggest that the increasing hco3 alkalinity of the groundwaters is caused by interaction of deep co2 with the aquifer matrix alteration of feldspars by co2 with a 13c value of 3 0 accounts for the observed alkalinity and isotopic and trends carbon 14 ages have been estimated for recent and holocene groundwaters which have a distinct stable isotopic 2h and 18o signature compared with 14c dead groundwaters from further west on the irhazer plain recharge temperatures estimated by analysis of noble gas contents show that the 14c dead waters were recharged under conditions cooler than present averages in the region possibly at the beginning of deglaciation 16 ka b p or during the period 23 30 ka b p trends in the hydrochemistry of u are related to the deep co2 alteration process radiogenic he contents increase along the flow direction and extremely high values occur in the proximity of u ores denitrification in the paleowaters of the irhazer was assessed by measurements of nitrogen argon ratios

changes in state of stress on the southern san andreas fault resulting from the california earthquake sequence of april to june 1992 the april to june 1992 landers earthquake sequence in southern california modified the state of stress along nearby segments of the san andreas fault causing a 50 kilometer segment of the fault to move significantly closer to failure where it passes through a compressional bend near san gorgonio pass the decrease in compressive normal stress may also have reduced fluid pressures along that fault segment as pressures are reequilibrated by diffusion that fault segment should move closer to failure with time that fault segment and another to the southeast probably have not ruptured in a great earthquake in about 300 years

post 1906 stress recovery of the san andreas fault system calculated from three dimensional finite element analysis 1 the m 7 8 1906 san francisco earthquake cast a stress shadow across the san andreas fault system inhibiting other large earthquakes for at least 75 years the duration of the stress shadow is a key question in san francisco bay area seismic hazard assessment this study presents a three dimensional 3 d finite element simulation of post 1906 stress recovery the model reproduces observed geologic slip rates on major strike slip faults and produces surface velocity vectors comparable to geodetic measurements fault stressing rates calculated with the finite element model are evaluated against numbers calculated using deep dislocation slip in the finite element model tectonic stressing is distributed throughout the crust and upper mantle whereas tectonic stressing calculated with dislocations is focused mostly on faults in addition the finite element model incorporates postseismic effects such as deep afterslip and viscoelastic relaxation in the upper mantle more distributed stressing and postseismic effects in the finite element model lead to lower calculated tectonic stressing rates and longer stress shadow durations 17 74 years compared with 7 54 years all models considered indicate that the 1906 stress shadow was completely erased by tectonic loading no later than 1980 however the stress shadow still affects present day earthquake probability use of stressing rate parameters calculated with the finite element model yields a 7 12 reduction in 30 year probability caused by the 1906 stress shadow as compared with calculations not incorporating interactions the aggregate interaction based probability on selected segments not including the ruptured san andreas fault is 53 70 versus the noninteraction range of 65 77

deformation of the deer creek bridge by an active landslide in clay shale two parallel concurrently active slip surfaces of a landslide in clay shale of the cretaceous lea park formation are causing deformation of a bridge structure across the north saskatchewan river n

the 1992 landers earthquake sequence seismological observations the m w 6 1 7 3 6 2 1992 landers earthquakes began on april 23 with the m w6 1 1992 joshua tree preshock and form the most substantial earthquake sequence to occur in california in the last 40 years this sequence ruptured almost 100 km of both surficial and concealed faults and caused aftershocks over an area 100 km wide by 180 km long the faulting was predominantly strike slip and three main events in the sequence had unilateral rupture to the north away from the san andreas fault the m w6 1 joshua tree preshock at 33 n58 and 116 w19 on 0451 ut april 23 was preceded by a tightly clustered foreshock sequence m 4 6 beginning 2 hours before the mainshock and followed by a large aftershock sequence with more than 6000 aftershocks the aftershocks extended along a northerly trend from about 10 km north of the san andreas fault northwest of indio to the east striking pinto mountain fault the m w7 3 landers mainshock occurred at 34 n13 and 116 w26 at 1158 ut june 28 1992 and was preceded for 12 hours by 25 small m 3 earthquakes at the mainshock epicenter the distribution of more than 20 000 aftershocks analyzed in this study and short period focal mechanisms illuminate a complex sequence of faulting the aftershocks extend 60 km to the north of the mainshock epicenter along a system of at least five different surficial faults and 40 km to the south crossing the pinto mountain fault through the joshua tree aftershock zone towards the san andreas fault near indio the rupture initiated in the depth range of 3 6 km similar to previous m 5 earthquakes in the region although the maximum depth of aftershocks is about 15 km the mainshock focal mechanism showed right lateral strike slip faulting with a strike of n10 w on an almost vertical fault the rupture formed an arclike zone well defined by both surficial faulting and aftershocks with more westerly faulting to the north this change in strike is accomplished by jumping across dilational jogs connecting surficial faults with strikes rotated progressively to the west a 20 km long linear cluster of aftershocks occurred 10 20 km north of barstow or 30 40 km north of the end of the mainshock rupture the most prominent off fault aftershock cluster occurred 30 km to the west of the landers mainshock the largest aftershock was within this cluster the m w6 2 big bear aftershock occurring at 34 n10 and 116 w49 at 1505 ut june 28 it exhibited left lateral strike slip faulting on a northeast striking and steeply dipping plane the big bear aftershocks form a linear trend extending 20 km to the northeast with a scattered distribution to the north the landers mainshock occurred near the southernmost extent of the eastern california shear zone an 80 km wide more than 400 km long zone of deformation this zone extends into the death valley region and accommodates about 10 to 20 of the plate motion between the pacific and north american plates the joshua tree preshock its aftershocks and landers aftershocks form a previously missing link that connects the eastern california shear zone to the southern san andreas fault

single link cluster analysis of earthquake aftershocks decay laws and regional variations using single link cluster analysis the authors investigate how various properties of aftershock sequences depend on their tectonic regime and focal depth for international seismological centre earthquakes of m sub b ge 4 8 they find that earthquakes deeper than 70 km have the fewest and smallest aftershock sequences even after accounting for differences in detectability and maximum magnitude they find that ridge transform earthquakes have smaller aftershock sequences that shallow subduction zone earthquakes among different subduction zones they find that zones with high moment release rates possess larger aftershock sequences comparing ridge transform zones they find those with slower spreading rates possess larger aftershock sequences by transposing origin times of several different aftershock sequences as if all had main shocks occurring at time zero they are able to study the properties of aftershock sequences which individually have too few aftershocks to study by other means secondary events determined by single link cluster analysis follow a modified omori s power law decay for time separations of 0 1 day to 20 days from the parent event with p values ranging from 0 539 plus minus 0 022 intermediate and deep focus earthquakes to 0 928 plus minus 0 024 ridge transform earthquakes they find that earthquake foreshocks and multiplets also follow a modified omori s law atmore greater times from the main shock the decay is steeper than a power law decay more like an exponential decay aftershocks in the adak catalog m sub b ge 2 0 show a marked decrease in activity between 40 and 50 km depth they speculate that the observed differences in number of aftershocks and p values may be caused by variations in fault heterogeneity or in fluid pressures less

tectonic stressing in california modeled from gps observations 1 what happens in the crust as a result of geodetically observed secular motions in this paper we find out by distorting a finite element model of california using gps derived displacements a complex model was constructed using spatially varying crustal thickness geothermal gradient topography and creeping faults gps velocity observations were interpolated and extrapolated across the model and boundary condition areas and the model was loaded according to 5 year displacements results map highest differential stressing rates in a 200 km wide band along the pacific north american plate boundary coinciding with regions of greatest seismic energy release away from the plate boundary gps derived crustal strain reduces modeled differential stress in some places suggesting that some crustal motions are related to topographic collapse calculated stressing rates can be resolved onto fault planes useful for addressing fault interactions and necessary for calculating earthquake advances or delays as an example i examine seismic quiescence on the garlock fault despite a calculated minimum 0 1 0 4 mpa static stress increase from the 1857 m 7 8 fort tejon earthquake results from finite element modeling show very low to negative secular coulomb stress growth on the garlock fault suggesting that the stress state may have been too low for large earthquake triggering thus the garlock fault may only be stressed by san andreas fault slip a loading pattern that could explain its erratic rupture history

dynamic stress variations due to shear faults in a plane layered medium summary a complete set of expressions is presented for the computation of elastic dynamic stress in plane layered media we use a discrete wavenumber reflectivity method to compute the stress field radiated by arbitrary moment tensor sources the expressions derived here represent an interesting tool for both the observational and theoretical analysis of dynamic stress changes associated with earthquake phenomena dynamic stress changes associated with a strike slip fault having unilateral rupture are shown this modelling which is similar to the 1992 landers california earthquake illustrates the effects of distance directivity and depth on transient stress changes

extension on the tibetan plateau recent normal faulting measured by insar and body wave seismology we use insar and body wave modelling to determine the faulting parameters for a series of five mw 5 9 7 1 normal faulting earthquakes that occurred during 2008 including the march 20 yutian earthquake mw 7 1 one of the largest normal faulting events to have occurred recently on the continents we also study three earlier normal faulting earthquakes that occurred in southern tibet between 1992 and 2005 coseismic deformation for each of these eight events is measured with ascending and descending interferograms from envisat ers and alos sar data elastic dislocation modelling of the line of sight insar displacements and body wave seismological modelling of p and sh waves are used to estimate fault parameters and are found to be in good agreement for all the events studied the use of insar to measure deformation allows a relatively precise determination of the fault location in addition to resolving the focal plane ambiguity only five of the eight events are associated with a clear surface topographic feature suggesting that an underestimation of the amount of extension would result from using the surface expressions of normal faulting alone the observations in all cases are consistent with slip on planar surfaces with dips in the range 40 50 that penetrates the uppermost crust to a depth of 10 15 km we find no evidence for active low angle dip less than 30 normal faulting the contribution of the normal faulting to overall extension estimated by summing seismic moments over earthquakes for the past 43 yr is 3 4 mm yr 1 or 15 20 per cent of the rates of extension measured across the plateau using gps 85 per cent of the moment release in normal faulting over the past 43 yr has occurred in regions whose surface height exceeds 5 km this observation adds weight to the suggestions that the widespread normal faulting on the plateau is the result of variations in the gravitational potential energy of the lithosphere

geomorphic evidence for active faulting in the altyn tagh and northern tibet and qualitative estimates of its contribution to the convergence of india and eurasia a brief traverse across the altyn tagh and kunlun in northern tibet revealed abundant evidence for recent crustal deformation there from ridges offset roughly 10 m and from fresh mole tracks 40 cm or more high along the altyn tagh strike slip fault we conclude that a large earthquake occurred in the past few hundred years accordingly the rate of slip could be relatively high 30 20 mm yr active reverse or thrust faults mark the bases of several mountain ranges in the altyn tagh and kunlun where the neighboring basins lie at elevations less than 4000 m therefore roughly north south crustal shortening appears to be occurring there probably at a rate of several millimetres per year the crustal shortening accommodated by strike slip and reverse or thrust faulting appears to account for a significant part 10 25 of india s convergence with eurasia the broad zone of crustal shortening in northern tibet indicates that the areal extent of the plateau is increasing northward by crustal thickening on its northern margin

transient triggering of near and distant earthquakes we demonstrate qualitatively that frictional instability theory provides a context for understanding how earthquakes may be triggered by transient loads associated with seismic waves from near and distance earthquakes we assume that earthquake triggering is a stick slip process and test two hypotheses about the effect of transients on the timing of instabilities using a simple spring slider model and a rate and state dependent friction constitutive law a critical triggering threshold is implicit in such a model formulation our first hypothesis is that transient loads lead to clock advances i e transients hasten the time of earthquakes that would have happened eventually due to constant background loading alone modeling results demonstrate that transient loads do lead to clock advances and that the triggered instabilities may occur after the transient has ceased i e triggering may be delayed these simple clock advance models predict complex relationships between the triggering delay the clock advance and the transient characteristics the triggering delay and the degree of clock advance both depend nonlinearly on when in the earthquake cycle the transient load is applied this implies that the stress required to bring about failure does not depend linearly on loading time even when the fault is loaded at a constant rate the timing of instability also depends nonlinearly on the transient loading rate faster rates more rapidly hastening instability this implies that higher frequency and or longer duration seismic waves should increase the amount of clock advance these modeling results and simple calculations suggest that near tens of kilometers small moderate earthquakes and remote thousands of kilometers earthquakes with magnitudes 2 to 3 units larger may be equally effective at triggering seismicity our second hypothesis is that some triggered seismicity represents earthquakes that would not have happened without the transient load i e accumulated strain energy would have been relieved via other mechanisms we test this using two new seismicity models that 1 are inherently unstable but slide at steady state conditions under the background load and 2 are conditionally stable such that instability occurs only for sufficiently large perturbations for the new seismicity models very small amplitude transients trigger instability relative to the clock advance models the unstable steady state models predict that the triggering delay depends inversely and nonlinearly on the transient amplitude as in the clock advance models we were unable to generate delayed triggering with conditionally stable models for both new seismicity models the potential for triggering is independent of when the transient load is applied or equivalently of the prestress unlike in the clock advance models in these models a critical triggering threshold appears to be inversely proportional to frequency further advancement of our understanding will require more sophisticated quantitative models and observations that distinguish between our qualitative yet distinctly different model predictions

spatial and temporal distribution of slip for the 1992 landers california earthquake we have determined a source rupture model for the 1992 landers earthquake m w 7 2 compatible with multiple data sets spanning a frequency range from zero to 0 5 hz geodetic survey displacements near field and regional strong motions broadband teleseismic waveforms and surface offset measurements have been used explicitly to constrain both the spatial and temporal slip variations along the model fault surface our fault parameterization involves a variable slip multiple segment finite fault model which treats the diverse data sets in a self consistent manner allowing them to be inverted both independently and in unison the high quality data available for the landers earthquake provide an unprecedented opportunity for direct comparison of rupture models determined from independent data sets that sample both a wide frequency range and a diverse spatial station orientation with respect to the earthquake slip and radiation pattern in all models consistent features include the following 1 similar overall dislocation patterns and amplitudes with seismic moments of 7 to 8 10 26 dyne cm seismic potency of 2 3 to 2 7 km 3 2 very heterogeneous unilateral strike slip distributed over a fault length of 65 km and over a width of at least 15 km though slip is limited to shallower regions in some areas 3 a total rupture duration of 24 sec and an average rupture velocity of 2 7 km sec and 4 substantial variations of slip with depth relative to measured surface offsets the extended rupture length and duration of the landers earthquake also allowed imaging of the propagating rupture front with better resolution than for those of prior shorter duration strike slip events our imaging allows visualization of the rupture evolution including local differences in slip durations and variations in rupture velocity rupture velocity decreases markedly at shallow depths as well as near regions of slip transfer from one fault segment to the next as rupture propagates northwestward along the multiply segmented fault length the rupture front slows as it reaches the northern limit of the johnson valley landers faults where slip is transferred to the southern homestead valley fault an abrupt acceleration is apparent following the transfer this process is repeated and is more pronounced as slip is again passed from the northern homestead valley fault to the emerson fault although the largest surface offsets were observed at the northern end of the rupture our modeling indicates that substantial rupture was also relatively shallow less than 10 km in this region

stress strain changes and triggered seismicity following the mw 7 3 landers california earthquake calculations of dynamic stresses and strains constrained by broadband seismograms are used to investigate their role in generating the remotely triggered seismicity that followed the june 28 1992 mw7 3 landers california earthquake i compare straingrams and dynamic coulomb failure functions calculated for the landers earthquake at sites that did experience triggered seismicity with those at sites that did not bounds on triggering thresholds are obtained from analysis of dynamic strain spectra calculated for the landers and mw6 1 joshua tree california earthquakes at various sites combined with results of static strain investigations by others i interpret three principal results of this study with those of a companion study by gomberg and davis this issue first the dynamic elastic stress changes themselves cannot explain the spatial distribution of triggered seismicity particularly the lack of triggered activity along the san andreas fault system in addition to the requirement to exceed a coulomb failure stress level this result implies the need to invoke and satisfy the requirements of appropriate slip instability theory second results of this study are consistent with the existence of frequency or rate dependent stress strain triggering thresholds inferred from the companion study and interpreted in terms of earthquake initiation involving a competition of processes one promoting failure and the other inhibiting it such competition is also part of relevant instability theories third the triggering threshold must vary from site to site suggesting that the potential for triggering strongly depends on site characteristics and response the lack of triggering along the san andreas fault system may be correlated with the advanced maturity of its fault gouge zone the strains from the landers earthquake were either insufficient to exceed its larger critical slip distance or some other critical failure parameter or the faults failed stably as aseismic creep events variations in the triggering threshold at sites of triggered seismicity may be attributed to variations in gouge zone development and properties finally these interpretations provide ready explanations for the time delays between the landers earthquake and the triggered events

geotechnical model for great plains hummocky moraine formed by till deformation below stagnant ice abstract hummocky or disintegration moraine is a widespread late wisconsin glacial terrain type across the glaciated great plains of mid continent north america this terrain consists of chaotic hummocks and enclosed kettle hollows that are widely underlain by clay rich till the prevailing interpretation of this terrain is one of supraglacial deposition from debris covered stagnant ice and is based largely on geomorphologic comparisons with modern valley glacier margins recently in contrast it has been argued that hummocky moraine can form subglacially by the pressing of fine grained deformable till under the overburden pressure of stagnant ice in this paper we test the postulated subglacial origin of hummocky moraine using a geotechnical model of a deforming soil till the model confirms that hummocky topography can be the result of failure and deformation of fine grained till by differential loading below stagnant ice differential pressures at the ice bed contact arise from the complete or partial perforation of thin stagnant ice sheet lobes by numerous water filled doline like depressions the growth and decay of ponds as ice down wastes result in repeated cycles of subglacial till failure and landform development resulting in the observed random linear circular elongate and other patterns of hummocks ridges and hollows formerly streamlined preferentially oriented subglacial landforms such as drumlins and rogen moraines can also be overprinted forming washboard moraine

evolution of the stress field in southern california and triggering of moderate size earthquakes a 200 year perspective changes in stress in southern california are modeled from 1812 to 2025 using as input 1 stress drops associated with six large 7 0 7 5 earthquakes through 1995 and 2 stress buildup associated with major faults with slip rates 3 mm yr as constrained by geodetic paleoseismic and seismic measurements evolution of stress and the triggering of moderate to large earthquakes are treated in a tensoffal rather than a scalar manner we present snapshots of the cumulative coulomb failure function acff as a function of time for faults of various strike dip and rake throughout southern california we take acff to be zero everywhere just prior to the great shock of 1812 we find that about 95 of those well located m 6 earthquakes whose mechanisms involve either strike slip or reverse faulting are consistent with the coulomb stress evolutionary model that is they occurred in areas of positive acff the interaction between slow moving faults and stresses generated by faster moving faults significantly advanced the occurrence of the 1933 long beach and 1992 landers events in their earthquake cycles coulomb stresses near major thrust faults of the western and central transverse ranges have been accumulating for a long time future great earthquakes along the san andreas fault especially if the san bernardino and coachella valley segments rupture together can trigger moderate to large earthquakes in the transverse ranges as appears to have happened in the santa barbara earthquake that occurred 13 days after the great san andreas shock of 1812 maps of current acff provide additional guides to long term earthquake prediction

stress strain changes and triggered seismicity at the geysers california the principal results of this study of remotely triggered seismicity in the geysers geothermal field are the demonstration that triggering initiation of earthquake failure depends on a critical strain threshold and that the threshold level increases with decreasing frequency or equivalently depends on strain rate this threshold function derives from 1 analyses of dynamic strains associated with surface waves of the triggering earthquakes 2 statistically measured aftershock zone dimensions and 3 analytic functional representations of strains associated with power production and tides the threshold is also consistent with triggering by static strain changes and implies that both static and dynamic strains may cause aftershocks the observation that triggered seismicity probably occurs in addition to background activity also provides an important constraint on the triggering process assuming the physical processes underlying earthquake nucleation to be the same gomberg this issue discusses seismicity triggered by the mw 7 3 landers earthquake its constraints on the variability of triggering thresholds with site and the implications of time delays between triggering and triggered earthquakes our results enable us to reject the hypothesis that dynamic strains simply nudge prestressed faults over a coulomb failure threshold sooner than they would have otherwise we interpret the rate dependent triggering threshold as evidence of several competing processes with different time constants the faster one s facilitating failure and the other s inhibiting it such competition is a common feature of theories of slip instability all these results not surprisingly imply that to understand earthquake triggering one must consider not only simple failure criteria requiring exceedence of some constant threshold but also the requirements for generating instabilities

the tectonic setting and earthquake ground hazards of the 1993 pyrgos earthquake peloponnese greece the pyrgos earthquake in western greece 26 march 1993 m l 5 2 was caused by oblique normal slip on a north dipping and wnw trending fault and produced significant damage in the town of pyrgos several lines of evidence from the epicentral area suggest that faults within two active graben in the pyrgos basin the grape and alfios grabens were reactivated during this earthquake and subsequent aftershocks evaluation of morphotectonic data the fault slip rate between 0 05 and 0 35 mm a 1 and seismic activity associated with the 1993 earthquake suggest that the alfios graben is the most active structure in the basin this graben which is 25 km long and crosses the pyrgos basin is segmented along its strike by transfer zones analysis of the seismic activity in this graben shows that transfer zones are the focus for the concentration of seismic activity and thus the concept of transfer zones is important for understanding the pyrgos earthquake the 1993 earthquake caused landslides at 47 locations and liquefaction phenomena at 7 localities within an area of 145 km 2 the lower threshold level to induce landslides was m l 4 0 and to induce liquefaction was 5 0 the majority of landslides occurred along fault scarps and either caused the slope to retreat and or acted to protect the slope gradient

the dirt hills structure an ice thrust feature in southern saskatchewan canada the dirt hills structure is expressed at the surface as a broad arcuate moraine 7 5 km wide 120 m high and 40 km long the structure is composed mainly of bedrock and drift consisting of three slabs stacked to form a single block 215 m thick the shear zones that bound the slabs are mainly in clays of the bearpaw formation the dirt hills structure originated during the last deglaciation when the glacier under compressive flow readvanced up the missouri coteau escarpment and stacked the subglacial slabs of bedrock and drift the subparallel arcuate ice thrust ridges are formed presumably by imbricate thrust faults originating from the uppermost shear zone

single link cluster analysis as a method to evaluate spatial and temporal properties of earthquake catalogues summary single link cluster analysis is a straightforward method to quantitatively measure the degree of clustering or isolation of groups of elements in a set such as a catalogue of earthquakes to apply single link cluster analysis to a set of n earthquakes individual earthquakes are first linked to their nearest neighbours to form event sub groups the process is then repeated and each sub group linked to its nearest neighbour recursively until n 1 links are found to join all the earthquakes this paper shows how knowledge of these links can be used to divide a group of earthquakes into any number of spatial clusters with various modifications this analysis can identify earthquake nests isolated events aftershock sequences and zones of seismic quiescence to illustrate this method we apply it to i a global data set of 2178 earthquakes having mb of 5 8 or greater reported by the international seismological centre isc between 1964 and february 1986 and ii sets of earthquakes having mb of 4 9 and greater as reported by the isc occurring in central america and in the aleutians to facilitate comparison with real data this study also investigates the distribution of link lengths for synthetic events placed randomly on 1 d circular line 2 d circular area and 3 d spherical volume geometries if we remove links between nuclear explosions from the isc data the resulting link distribution is quite similar to that for synthetic events placed along a 1 d fault for the shortest links the similarity improves markedly if the synthetic fault has a finite width or if relative location errors cause events to be located away from the fault trace with a standard deviation of about 15 km

a constitutive law for rate of earthquake production and its application to earthquake clustering seismicity is modeled as a sequence of earthquake nucleation events in which the distribution of initial conditions over the population of nucleation sources and stress ing history control the timing of earthquakes the model is implemented using solutions for nucleation of unstable fault slip on faults with experimentally derived rate and state dependent fault properties this yields a general state variable constitutive formulation for rate of earthquake production resulting from an applied stressing history to illustrate and test the model some characteristics of seismicity following a stress step have been explored it is proposed that various features of earthquake clustering arise from sensi tivity of nucleation times to the stress changes induced by prior earthquakes the model gives the characteristic omori aftershock decay law and interprets aftershock parameters in terms of stress change and stressing rate earthquake data appear to support a model prediction that aftershock duration defined as the time for rates to return to the back ground seismicity rate is proportional to mainshock recurrence time observed spatial and temporal clustering of earthquake pairs arises as a consequence of the spatial depen dence of stress changes of the first event of the pair and stress sensitive time dependent nucleation applications of the constitutive formulation are not restricted to the simple stress step models investigated here it may be applied to stressing histories of arbitrary complexity the apparent success at modeling clustering phenomena suggests the possibil ity of using the formulation to estimate short to intermediate term earthquake probabil ities following occurrence of other earthquakes and for inversion of temporal variations of earthquake rates for changes in driving stress

stress transferred by the 1995 mw 6 9 kobe japan shock effect on aftershocks and future earthquake probabilities the kobe earthquake struck at the edge of the densely populated osaka kyoto corridor in southwest japan we investigate how the earthquake transferred stress to nearby faults altering their proximity to failure and thus changing earthquake probabilities we find that relative to the pre kobe seismicity kobe aftershocks were concentrated in regions of calculated coulomb stress increase and less common in regions of stress decrease we quantify this relationship by forming the spatial correlation between the seismicity rate change and the coulomb stress change the correlation is significant for stress changes greater than 0 2 1 0 bars 0 02 0 1 mpa and the nonlinear dependence of seismicity rate change on stress change is compatible with a state and rate dependent formulation for earthquake occurrence we extend this analysis to future mainshocks by resolving the stress changes on major faults within 100 km of kobe and calculating the change in probability caused by these stress changes transient effects of the stress changes are incorporated by the state dependent constitutive relation which amplifies the permanent stress changes during the aftershock period earthquake probability framed in this manner is highly time dependent much more so than is assumed in current practice because the probabilities depend on several poorly known parameters of the major faults we estimate uncertainties of the probabilities by monte carlo simulation this enables us to include uncertainties on the elapsed time since the last earthquake the repeat time and its variability and the period of aftershock decay we estimate that a calculated 3 bar 0 3 mpa stress increase on the eastern section of the arima takatsuki tectonic line attl near kyoto causes fivefold increase in the 30 year probability of a subsequent large earthquake near kyoto a 2 bar 0 2 mpa stress decrease on the western section of the attl results in a reduction in probability by a factor of 140 to 2000 the probability of a mw 6 9 earthquake within 50 km of osaka during 1997 2007 is estimated to have risen from 5 6 before the kobe earthquake to 7 11 afterward during 1997 2027 it is estimated to have risen from 14 16 before kobe to 16 22

seismicity in the western great basin apparently triggered by the landers california earthquake 28 june 1992 within 24 hr after the landers earthquake there were three magnitude 3 4 events in western nevada and an unexpected widespread increase in the rate of small events based on combined catalogs for northern nevada southern nevada and southern california and a model that assumes statistical independence of events in these regions the probability of this happening in a 24 hr period by random chance is less than 10 12 per day therefore there is high statistical confidence that the increased seismicity was triggered by the landers event based on the statistical model we develop a list of 227 earthquakes in the first 83 days following the landers earthquake each of which has no more than 10 probability of occurring by random chance the suspect events are broadly distributed in regions that correlate with historical activity in the great basin the events are not uniquely associated with known volcanic activity or with zones that were previously active with microearthquakes or aftershock sequences the magnitudes of the largest triggered events appear to decrease with distance with time the number of suspect events decreases at a rate comparable to the rate of decrease of aftershocks of the landers and big bear earthquakes three of the most significant triggered events that that occurred were as follows mina 500 km from landers m 4 36 min after landers smith valley 590 km from landers m 3 4 56 min after landers and little skull mountain 280 km from landers m 5 6 22 3 hr after landers the evidence for triggering is particularly strong in the case of the little skull mountain event where an increased rate of microseismicity was evident as soon as small events could be identified in the coda of the landers earthquake we evaluate the seismic history for the past 3 yr to understand what was unique about the landers earthquake this includes identifying the previous earthquakes most likely to have caused the strongest shaking in various frequency bands based on a simple screening model the strains from the landers earthquake were uniquely large at low frequencies but at high frequencies they were exceeded frequently by small more local events thus we hypothesize that the cause of triggering is low frequency dynamic strains at periods of about 10 sec or greater and that there is a regional threshold that must be exceeded since previous events with calculated strains that were almost as strong were not causes of widespread triggering this mechanism for triggering satisfies the criterion of being a relatively rare phenomenon since it is likely to occur only when a large long period wave is radiated into an area where strain has been building slowly toward the point where faults are unstable

spatial temporal evolutions of early aftershocks following the 2013 mw 6 6 lushan earthquake in sichuan china we perform a comprehensive detection of early aftershocks following the 2013 mw 6 6 lushan earthquake which occurred in the southern longmenshan fault zone in sichuan province china about 5 years after the 2008 mw 7 9 wenchuan earthquake we use events in both standard and relocated catalogs as templates to scan through continuous waveforms 2 days before and 3 days after the main shock we successfully reduce the magnitude of completeness mc by more than 1 order and obtain up to 6 times more events than listed in both catalogs aftershocks in the first hour mostly occur around the main shock slip region and aftershocks at later times show systematic expansions in the along strike perpendicular strike and updip directions although postseismic deformation following the lushan main shock has not been clearly identified we suggest that early aftershock expansions are likely driven by afterslip of the lushan main shock this is consistent with the observations that most aftershocks were in the stress shadow of the lushan main shock and that there was significant slip deficit in the top 10 km of the crust we also find that seismicity on the back thrust fault was activated as soon as 20 min after the main shock earlier than previously reported we are unable to detect any clear foreshocks in the last 2 days before the lushan main shock

stratigraphy and paleomagnetism of late pliocene and pleistocene sediments in the wellsch valley and swift current creek areas southwestern saskatchewan canada the late pliocene and pleistocene are represented in southern saskatchewan by sequences of preglacial glacial and nonglacial deposits these have been studied in surface exposures and bore cores and have been subdivided and correlated on the basis of their lithologies in this study new observations of magnetic polarity are presented they reveal a lower preglacial sequence empress group with reversed polarity and an upper normally magnetized sequence which contains glacial deposits of pre illinoian illinoian and wisconsinan age and which are correlative with the saskatoon and sutherland groups of central saskatchewan the reversed preglacial sequence is referred to the matuyama zone late pliocene to early pleistocene 2 58 0 78 ma and the normal glacial sequence to the brunhes zone middle to late pleistocene and holocene 0 78 ma to present in southern saskatchewan there is no evidence of glaciation during the late pliocene and early pleistocene matuyama chron which is in marked contrast w

red deer hill a drumlinized glaciotectonic feature near prince albert saskatchewan canada red deer hill is 5 km long 3 km wide and 70 m high the hill is a glaciotectonic structure formed by a single 106 m thick thrust block presumably derived from the 80 m deep upglacier holmes depression the thrust block became dislodged by displacement along a presheared horizontal gouge zone where the shearing resistance of the clay was reduced to a residual state by the first mennon glaciation the hill depression structure was formed by the battleford glacier which drumlinized red deer hill before depositing a veneer of till over the structure the holmes depression was filled with glacial lake saskatchewan lacustrine and deltaic sediments horizontal displacement along a decollement in the direction of the activating force resulted in a stress environment changing from extension to compression in the zone of extension holmes depression an active rankine state developed and beds stretched and thinned in the zone of compression red deer hill conversely a passive rankine state developed an