sulfur isotope evidence for widespread euxinia and a fluctuating oxycline in early to middle ordovician greenhouse oceans article i nfo despite marine geochemical records indicating widespread oxygenation of the biosphere in the terminal neoproterozoic era late cambrian records point to the persistence of deep water anoxia and potential for development of euxinic conditions the late cambrian spice steptoean positive carbon isotope excursion event however is a globally recognized chemostratigraphic marker that likely represents significant organic carbon burial and subsequent liberation of oxygen to the biosphere here we present high resolution carbon and sulfur isotope profiles from early to middle ordovician carbonate rocks from the argentine precordillera and western newfoundland to constrain oceanic redox conditions in the post spice world marine c isotope profiles record relatively stable behavior excursions b3 that is characteristic of greenhouse climates ma rine s isotope profiles record short term b10 6 yr rhythmic variation superimposed over a longer term

observational evidence for westward propagation of temperature inversions in the southeastern arabian sea 1 a warm pool forms in the southeastern arabian sea seas prior to the onset of the summer monsoon over india in early june the core of this warm pool is in the lakshadweep sea ls xbt and surface salinity data collected in the ls during may 2002 april 2003 show that temperature inversions occur off the southwest coast of india in early december with the arrival of low salinity waters from the bay of bengal the low salinity waters and the inversions propagate westward along with the downwelling rossby waves that constitute the lakshadweep sea level high inversions occur in the western ls 73 e about 40 days after they occur near the coast in the eastern ls 75 5 e they disappear in april when the tropical convergence zone moves over the seas and the warm pool engulfs the region ocean dynamics and air sea fluxes are together responsible for the formation and westward propagation of the inversions

brittle frictional mountain building 2 thermal structure and heat budget this paper describes a simple thermal model of an actively deforming critically tapered fold and thrust belt the model determines the steady state temperature distribution and heat flow as well as the pressure temperature time histories of rocks that outcrop at the surface the main parameters controlling the thermal structure are the accretion and erosion rates the undisturbed geothermal gradient at the toe and the amount of frictional heating both shear heating on the decollement fault and internal strain heating within the deforming brittle wedge are incorporated in a mechanically consistent manner and they dominate the effect of radiogenic heating except in fold and thrust belts with significantly overpressured pore fluids the mean stresses temperatures and surface heat flow all increase with an increase in the basal and internal coefficients of friction and this dependence is used to constrain the level of friction on the decollement fault beneath the steady state fold and thrust belt in taiwan rocks outcropping in the core of the central mountain range of taiwan experience maximum theoretical temperatures in excess of 400 c and maximum mean pressures in excess of 500 mpa if the coefficient of basal friction is b 0 5 qualitatively these conditions are in good agreement with the observed high greenschist facies metamorphism the theoretical surface heat flow which increases from 95 mw m2 at the front of the fold and thrust belt to 240 mw m2 at the rear is in excellent agreement with the results of a recent geothermal survey of taiwan and theoretical cooling histories are in good agreement with fission track and other geochronologic studies taken together these results provide strong evidence that sliding on the basal decollement fault beneath taiwan is governed by a coefficient of friction in the range of typical laboratory measurements b 0 5 0 2 approximately 35 of the total surface heat flux of 3 gw is heat conducted into the base of the wedge from the top of the basal decollement fault and somewhat more than 30 is heat advected into the toe by accretion the remaining heat is generated internally about 25 by internal strain heating and about 10 by radiogenic heating either an increase in the coefficient of basal friction b or a reduction in the pore fluid pressure ratio b leads to an increase in the surface heat flow because of the increased frictional heating within the wedge and on the basal decollement fault the overall balance of energy in a steady state fold and thrust belt is described by the equation e w g q where is the rate at which both mechanical and heat energy are added from external sources w g is the rate at which work is performed against gravitational body forces in a reference frame attached to the overriding plate and q is the rate at which waste heat flows out of both the upper and lower boundaries the total power input into the taiwan fold and thrust belt is approximately 4 2 gw the mechanical work being done on the base and front of the fold and thrust belt accounts for 3 of these 4 2 gw in addition 0 9 gw of heat are being advected from the subducting plate into the toe by accretion the remaining 0 3 gw are being supplied by in situ radiogenic heating the outgoing energy is dominated by the 3 gw of heat conducted out the top in the surface heat flow however another 0 8 gw are conducted down beneath the rear portion of the basal decollement fault to heat the underlying subducting slab only 0 4 of the incoming 4 2 gw do useful mechanical work against gravity within the wedge the efficiency of brittle frictional mountain building in taiwan is therefore 10

load induced seismicity near tsaoling taiwan abstract recent work shows erosion reduces vertical normal stress triggering thrust earthquakes whether deposition will inhibit earthquake triggering or favor different types of earthquakes is not clear here we address this question using earthquake catalogs earthquake focal mechanisms and a point load stress transfer model to study induced seismicity at the tsaoling impoundment after the 1999 mw7 6 chi chi earthquake this impoundment formed when a river was blocked by the mainshock induced landslide temporally we found a marked increase and decrease in earthquake activities in this location at the beginning and end of this impoundment life cycle spatially the number of earthquakes decreased away from the impoundment in a pattern correlated with surface point load models based on both the boussinesq and westergaard equations using first motions and sh to p amplitude ratios we derived focal mechanisms and found the point source on surface weakens the strike slip and normal fault planes mostly occurred during our study period some of the chi chi aftershocks not explained by dislocation stress transfer studies can be explained by such load induced seismicity previous studies show earthquakes can affect surficial geological processes this study suggests that surficial processes can create positive loading that feedbacks and triggers seismicity

role of narrow mountains in large scale organization of asian monsoon convection abstract the asian summer monsoon is organized into distinct convection centers but the mechanism for this organization is not well understood analysis of new satellite observations reveals that narrow mountain ranges are an important organizing agent anchoring monsoon convection centers on the windward side the bay of bengal convection in particular features the heaviest precipitation on its eastern coast because of orographic lifting as the southwest monsoon impinges on the coastal mountains of myanmar also known as burma this is in contrast to the widely held view that this convection is centered over the open ocean as implied by coarse resolution datasets a view that would require an entirely different explanation for its formation narrow in width and modest in height 1 km these mountains are hardly mentioned in conceptual depictions of the large scale monsoon and poorly represented in global climate models the numerical simulations of this study show that orographic rainbands are not a

spatial and temporal distribution of slip for the 1999 chi chi taiwan earthquake we investigated the rupture process of the 1999 chi chi taiwan earth quake using high quality near source strong motion records broadband teleseismic displacement waveforms and well distributed global positioning system gps data the near source strong motion displacement waveforms recorded significant static offsets of up to 8 m the teleseismic displacement records show a significant pulse with duration of about 18 to 20 sec taking into account the surface displacements observed along the chelungpu fault we considered two fault geometries a single planar fault and a two segment fault with a northeast striking section near the north ern end using the finite fault model with variable slip vectors we derived two models of the temporal and spatial slip distribution of the earthquake the gps data provided good surface displacement constraints for the slip distribution determina tion the spatial slip distribution is generally consistent with field observations the results for the simple fault model show a large asperity located in the region about 25 to 55 km north of the hypocenter with maximum slip of about 15 m when we use the two segment model the asperity further extends to the region where the fault bends toward the northeast with a maximum slip of up to 20 m a large amount of right lateral slip beneath station tcu068 is necessary to explain its observed large west movement it implies a local converging slip at the corner where the fault bends to the northeast the slip amplitude near the hypocenter is about 3 to 6 m the seismic moments determined from the various data sets are within the range of 2 to 4 10 27 dyne cm most of the slip concentrated at shallow depths less than 10 km the total rupture duration is about 28 sec and the rupture velocity is 75 to 80 of the shear wave velocity the slip vector shows a clockwise rotation during the fault rupture the static stress drop of the large asperity region is comparable with the dynamic stress drop as observed directly from the slip velocity at the station near the large slip region

inference of postseismic deformation mechanisms of the 1923 kanto earthquake 1 coseismic slip associated with the m7 9 1923 kanto earthquake is fairly well understood involving slip of up to 8 m along the philippine sea honshu interplate boundary under sagami bay and its onland extension postseismic deformation after the 1923 earthquake however is relatively poorly understood we revisit the available deformation data in order to constrain possible mechanisms of postseismic deformation and to examine the consequences for associated stress changes in the surrounding crust data from two leveling lines and one tide gage station over the first 7 8 years postseismic period are of much greater amplitude than the corresponding expected interseismic deformation during the same period making these data suitable for isolating the signal from postseismic deformation we consider both viscoelastic models of asthenosphere relaxation and afterslip models a distributed coseismic slip model presented by pollitz et al 2005 combined with prescribed parameters of a viscoelastic earth model yields predicted postseismic deformation that agrees with observed deformation on mainland honshu from tokyo to the izu peninsula elsewhere southern miura peninsula boso peninsula the considered viscoelastic models fail to predict observed deformation and a model of 1 m shallow afterslip in the offshore region south of the boso peninsula with equivalent moment magnitude mw 7 0 adequately accounts for the observed deformation using the distributed coseismic slip model layered viscoelastic structure and a model of interseismic strain accumulation we evaluate the post 1923 stress evolution including both the coseismic and accumulated postseismic stress changes and those stresses contributed by interseismic loading we find that if account is made for the varying tectonic regime in the region the occurrence of both immediate first month post 1923 crustal aftershocks as well as recent regional crustal seismicity is consistent with the predicted stress pattern this suggests that the influence of the 1923 earthquake on regional seismicity is fairly predictable and has persisted for at least seven decades following the earthquake

integrated geophysical characteristics of the 2015 illapel chile earthquake on 16 september 2015 a mw 8 3 earthquake ruptured the subduction zone offshore of illapel chile generating an aftershock sequence with 14 mw 6 0 7 0 events a double source w phase moment tensor inversion consists of a mw 7 2 subevent and the main mw 8 2 phase we determine two slip models for the mainshock one using teleseismic broadband waveforms and the other using static gps and insar surface displacements which indicate high slip north of the epicenter and west northwest of the epicenter near the oceanic trench these models and slip distributions published in other studies suggest spatial slip uncertainties of 25 km and have peak slip values that vary by a factor of 2 we relocate aftershock hypocenters using a bayesian multiple event relocation algorithm revealing a cluster of aftershocks under the chilean coast associated with deep 20 45 km depth mainshock slip less vigorous aftershock activity also occurred near the trench and along strike of the main aftershock region most aftershocks are thrust faulting events except for normal faulting events near the trench coulomb failure stress change amplitudes and signs are uncertain for aftershocks collocated with deeper mainshock slip other aftershocks are more clearly associated with loading from the mainshock these observations reveal a frictionally heterogeneous interface that ruptured in patches at seismogenic depths associated with many aftershocks and with homogeneous slip and few aftershocks up to the trench this event likely triggered seismicity separate from the main slip region including along strike events on the megathrust and intraplate extensional events

stress sensitivity of fault seismicity a comparison between limited offset oblique and major strike slip faults we present a new three dimensional inventory of the southern san francisco bay area faults and use it to calculate stress applied principally by the 1989 m 7 1 loma prieta earthquake and to compare fault seismicity rates before and after 1989 the major high angle right lateral faults exhibit a different response to the stress change than do minor oblique right lateral thrust faults seismicity on oblique slip faults in the southern santa clara valley thrust belt increased where the faults were unclamped the strong dependence of seismicity change on normal stress change implies a high coefficient of static friction in contrast we observe that faults with significant offset 50 100 km behave differently microseismicity on the hayward fault diminished where right lateral shear stress was reduced and where it was unclamped by the loma prieta earthquake we observe a similar response on the san andreas fault zone in southern california after the landers earthquake sequence additionally the offshore san gregorio fault shows a seismicity rate increase where right lateral oblique shear stress was increased by the loma prieta earthquake despite also being clamped by it these responses are consistent with either a low coefficient of static friction or high pore fluid pressures within the fault zones we can explain the different behavior of the two styles of faults if those with large cumulative offset become impermeable through gouge buildup coseismically pressurized pore fluids could be trapped and negate imposed normal stress changes whereas in more limited offset faults fluids could rapidly escape the difference in behavior between minor and major faults may explain why frictional failure criteria that apply intermediate coefficients of static friction can be effective in describing the broad distributions of aftershocks that follow large earthquakes since many of these events occur both inside and outside major fault zones

proterozoic early paleozoic evolution in western south america a discussion in tectonophysics 354 121 137 2002

evidence for a weak and overpressured decollement beneath sediment dominated accretionary prisms we present structural data from several underthrust sequences in southwest alaska that suggest that the deformation regime beneath accretionary prisms is characterized by a subvertical 1 and regionally extensive layer parallel shearing these results contrast markedly with the results obtained from the more shallow parts of accretionary prisms and fold and thrust belts where 1 plunges gently seaward or toward the deformation front on the basis of mohr circle diagrams we propose that these different stress directions are consistent only with a model in which the two regimes are separated by a very weak decollement and calculate coefficient of friction values that range from 0 25 to 0 08 these values are significantly lower than coefficient of friction values calculated for the basal decollement in the taiwan fold and thrust belt e g 0 2 barr and dahlen 1989a and they are also lower than laboratory friction measurements for most rocks e g 0 6 0 8 byerlee 1978 consequently these low values suggest the presence of rocks with unusual material properties e g montmorillonite logan and rauenzahn 1987 or anomalous fluid pressures along the decollement on the basis of the occurrence of shale diapirs the distribution of early veins and the absence of montmorillonite in the underthrust sequences we propose that there was an anomalously high fluid pressure along the decollement fault instead of there being such a low coefficient of friction these results emphasize the importance of dewatering and fluid flow in controlling the position of the decollement which in turn determines how the accretionary prism grows in essence it is the development of a weak overpressured decollement within a thick sediment pile that allows plate convergence to be partitioned into offscraping and underthrusting regimes

coulomb stress interactions among m 5 9 earthquakes in the gorda deformation zone and on the mendocino fault zone cascadia subduction zone and northern san andreas fault 1 the gorda deformation zone a 50 000 km2 area of diffuse shear and rotation offshore northernmost california has been the site of 20 m 5 9 earthquakes on four different fault orientations since 1976 including four m 7 shocks this is the highest rate of large earthquakes in the contiguous united states we calculate that the source faults of six recent m 5 9 earthquakes had experienced 0 6 bar coulomb stress increases imparted by earthquakes that struck less than 9 months beforehand control tests indicate that 0 6 bar coulomb stress interactions between m 5 9 earthquakes separated by 9 months are unlikely to occur by random chance suggesting that the multiple short term stress interactions observed among the recent gorda zone earthquakes are not an apparent effect in all well constrained 0 2 bar coulomb stress interactions between earthquakes that occurred within 4 years of each other the second earthquake is promoted on longer timescales calculated stress changes imparted by the 1980 mw 7 3 trinidad earthquake are consistent with the locations of m 5 9 earthquakes in the gorda zone until at least 1995 as well as earthquakes on the mendocino fault zone in 1994 and 2000 coulomb stress changes imparted by the 1980 earthquake are also consistent with its distinct elbow shaped aftershock pattern from these observations we derive generalized static stress interactions among right lateral left lateral and thrust faults near triple junctions

a viscoelastic earthquake cycle model for taiwan 1 while recent geophysical measurements including global positioning system gps surveys have clarified the mechanisms of active mountain building in the taiwan arc continent collision the geometry of faults beneath the island remains unclear in this paper we use gps data collected before the 1999 chi chi earthquake to image the geometry of active faults under taiwan and estimate slip rates on these faults we invert the data using kinematic and mechanical earthquake cycle models consisting of faulting in an elastic lithosphere overlying a viscoelastic asthenosphere in the kinematic model uniform slip is prescribed on the faults at a constant rate while in the mechanical model the faults slip in response to shear stresses in the lithosphere these models explain the essential features of both the horizontal and vertical gps velocity fields we find a 27 39 km thick elastic lithosphere overlying a viscoelastic asthenosphere with maxwell relaxation time of 15 80 years or viscosity of 0 5 4 1019 pa s the models produce a combined slip rate of 30 50 mm yr on the frontal thrusts in western taiwan and 60 80 mm yr of slip on the longitudinal valley fault inversion of the data using both models indicates a decollement dipping 10 under western taiwan and increasing in dip by no more than 10 under the central ranges and eastern taiwan

crustal deformation in taiwan results from finite source inversions of six mw 5 8 chi chi aftershocks 1 we inverted strong motion data for the finite source parameters of six large aftershocks of the 1999 chi chi taiwan earthquake to investigate seismogenic structure in taiwan for each event we derived a preferred model by testing different focal mechanisms hypocenters rupture velocities and dislocation risetimes as well as different combinations of stations in more than 1000 inversions we documented how the fits between the waveforms and the corresponding synthetics deteriorated as the hypocenter and focal mechanism deviate from those of the preferred model if the deviation in focal mechanisms and hypocenters is less than 20 and 5 km respectively we generally recovered 80 of the preferred model s synthetic waveform fit unlike the dislocation risetime the rupture velocity used in the inversion had a strong influence on the waveform fits in this study we also used the slip models to study fault geometry two of the aftershocks ruptured on the southern extension of the main shock slip area one strike slip aftershock nucleated within the basement but ruptured mainly within the overlying sedimentary strata suggesting that seismogenic deformation in the basement can influence shallow structures p axes of the derived models have azimuths consistent with current plate motion finally gps displacement derived from the six slip models can explain 80 of the postseismic deformation observed in the aftershock regions indicating that studies of postseismic deformation must take into account the cumulative effects of large shallow aftershocks

the impact of static stress change dynamic stress change and the background stress on aftershock focal mechanisms the focal mechanisms of earthquakes in southern california before and after four m 6 7 main shocks provide insight into how fault systems respond to stress and changes in stress the main shock static stress changes have two observed impacts on the seismicity changing the focal mechanisms in a given location to favor those aligned with the static stress change and changing the spatial distribution of seismicity to favor locations where the static stress change aligns with the background stress the aftershock focal mechanisms are significantly aligned with the static stress changes for absolute stress changes of 0 02 mpa for up to 20 years following the main shock the dynamic stress changes have similar although smaller effects on the local focal mechanisms and the spatial seismicity distribution dynamic stress effects are best observed at long periods 30 60 s and for metrics based on repeated stress cycling in the same direction this implies that dynamic triggering operates at least in part through cyclic shear stress loading in the direction of fault slip the background stress also strongly controls both the preshock and aftershock mechanisms while most aftershock mechanisms are well oriented in the background stress field 10 of aftershocks are identified as poorly oriented outliers which may indicate limited heterogeneity in the postmain shock stress field the fault plane orientations of the outliers are well oriented in the background stress while their slip directions are not implying that the background stress restricts the distribution of available fault planes

on the resolution of shallow mantle viscosity structure using postearthquake relaxation data application to the 1999 hector mine california earthquake 1 most models of lower crust mantle viscosity inferred from postearthquake relaxation assume one or two uniform viscosity layers a few existing models possess apparently significant radially variable viscosity structure in the shallow mantle e g the upper 200 km but the resolution of such variations is not clear we use a geophysical inverse procedure to address the resolving power of inferred shallow mantle viscosity structure using postearthquake relaxation data we apply this methodology to 9 years of gps constrained crustal motions after the 16 october 1999 m 7 1 hector mine earthquake after application of a differencing method to isolate the postearthquake signal from the background crustal velocity field we find that surface velocities diminish from 20 mm yr in the first few months to 2 mm yr after 2 years viscoelastic relaxation of the mantle with a time dependent effective viscosity prescribed by a burgers body provides a good explanation for the postseismic crustal deformation capturing both the spatial and temporal pattern in the context of the burgers body model which involves a transient viscosity and steady state viscosity a resolution analysis based on the singular value decomposition reveals that at most two constraints on depth dependent steady state mantle viscosity are provided by the present data set uppermost mantle viscosity depth 60 km is moderately resolved but deeper viscosity structure is poorly resolved the simplest model that explains the data better than that of uniform steady state mantle viscosity involves a linear gradient in logarithmic viscosity with depth with a small increase from the moho to 220 km depth however the viscosity increase is not statistically significant this suggests that the depth dependent steady state viscosity is not resolvably different from uniformity in the uppermost mantle

formation and spreading of arabian sea high salinity water mass the formation and seasonal spreading of the arabian sea high salinity water ashsw mass were studied based on the monthly mean climatology of temperature and salinity in the arabian sea north of the equator and west of 80 e on a 2 2 grid the ashsw forms in the northern arabian sea during winter and spreads southward along a 24 sigma t surface against the prevailing weak zonal currents the eastern extent of the core is limited by the strong northward coastal current flowing along the west coast of india during the southwest monsoon the northern part of the core shoals under the influence of the findlater jet while the southern part deepens throughout the year the southward extent of the ashsw is inhibited by the equatorial currents the atmospheric forcing that leads to the formation of ashsw was delineated using the monthly mean climatology of heat and freshwater fluxes monsoon winds dominate all the flux fields during summer june september while latent heat release during the relative calm of the winter november february monsoon driven by cool dry continental air from the north results in an increased density of the surface layer thus excess evaporation over precipitation and turbulent heat loss exceeding the radiative heat gain cool the surface waters of the northern arabian sea during winter and drive convective formation of ashsw

stress transfer among en echelon and opposing thrusts and tear faults triggering caused by the 2003 mw 6 9 zemmouri algeria earthquake stress transfer among en echelon and opposing thrusts and tear faults triggering caused by the 2003 m w 6 9 zemmouri algeria earthquake 1 the essential features of stress interaction among earthquakes on en echelon thrusts and tear faults were investigated first through idealized examples and then by study of thrust faulting in algeria we calculated coseismic stress changes caused by the 2003 m w 6 9 zemmouri earthquake finding that a large majority of the zemmouri afterslip sites were brought several bars closer to coulomb failure by the coseismic stresses while the majority of aftershock nodal planes were brought closer to failure by an average of 2 bars further we calculated that the shallow portions of the adjacent thenia tear fault which sustained 0 25 m slip were brought 2 bars closer to failure we calculated that the coulomb stress increased by 1 5 bars on the deeper portions of the adjacent boumerdes thrust which lies just 10 20 km from the city of algiers both the boumerdes and thenia faults were illuminated by aftershocks over the next 6 years the entire south dipping thrust system extending 80 km to the southwest experienced an increased rate of seismicity the stress also increased by 0 4 bar on the east sahel thrust fault west of the zemmouri rupture algiers suffered large damaging earthquakes in a d 1365 and 1716 and is today home to 3 million people if these shocks occurred on the east sahel fault and if it has a 2 mm yr tectonic loading rate then enough loading has accumulated to produce a m w 6 6 6 9 shock today thus these potentially lethal faults need better understanding of their slip rate and earthquake history among en echelon and opposing thrusts and tear faults triggering caused by the 2003 m w 6 9 zemmouri algeria earthquake

how did the 2013 lushan earthquake ms 7 0 trigger its aftershocks insights from static coulomb stress change calculations the lushan earthquake ms7 0 occurred at 08 02 beijing time 00 02 utc on 20 april 2013 located on the eastern margin of the tibetan plateau where the 2008 ms8 0 wenchuan earthquake took place the other strong shock following the wenchuan earthquake occurred on the longmen shan thrust fault a large number of aftershocks were recorded after the mainshock to investigate how the aftershocks were triggered we firstly relocated the 6 month aftershock sequence by the double difference algorithm next we calculated coulomb failure stress changes imparted by the lushan mainshock both on the optimally oriented plane and on the aftershock nodal planes then we examined the correspondence between the coulomb failure stress changes and the spatial distribution of the aftershocks the computed results show that most of the aftershock hypocenters did not occur on the region with the positive coulomb stress changes caused by the lushan mainshock moreover the majority of aftershock nodal planes were brought away from failure by coseismic coulomb stresses this implies that coseismic static coulomb stress changes may not be the governing process for aftershock genesis in contrast postseismic stress transfer by processes of afterslip and pore fluid flow are assumed to play an important part in aftershock triggering based upon the spatiotemporal distribution of the aftershocks

impact of remote equatorial winds and local mesoscale eddies on the existence of river in the sea along the east coast of india inferred from satellite smap

gps measurement of postseismic deformation following the 1999 chi chi taiwan earthquake 1 in the 15 month period september 1999 to december 2000 after the 1999 chi chi taiwan earthquake mw 7 6 about 80 campaign surveyed gps stations in central taiwan were occupied up to seven times furthermore seven continuous gps stations were put into operation in the epicentral region mostly within three weeks after the main shock gps data from these near field stations and some pre existing far field stations were utilized to study the postseismic deformation following the chi chi earthquake the postseismic displacements of the gps stations were approximated by a combination of an exponential transient decay function with a relaxation time of 86 days and a postseismic linear rate change stations on the hanging wall displayed west northwesterly horizontal displacements of up to 252 mm and uplift as large as 229 mm postseismic displacements are much larger to the south near the epicenter of chi chi main shock than in the north where the maximum coseismic displacement occurred stations on the footwall show only small displacements the postseismic gps data were inverted to infer the deeper fault geometry and afterslip distribution based on a four segment fault model with its shallow part constrained by coseismic fault geometry deduced from gps and seismological studies the optimal model requires the lower fault segment to be a horizontal plane at a depth of 10 4 km consistent with a model based on gps data taken during the first three months after the main shock and other geological or geophysical studies the maximum afterslip 459 mm occurs in the hypocentral region and in the northern part of the decollement slip of about 430 mm also occurs in the southern part of the decollement the afterslip moment inferred from 15 months of gps data is 4 7 1019 n m approximately 2 3 times the seismic moment released by aftershocks this implies that a major part of the postseismic deformation is aseismic

finite source modeling of the 1999 taiwan chi chi earthquake derived from a dense strong motion network the 1999 chi chi earthquake m w 7 6 20 september 1999 17 47 15 9 utc located at 23 853 n 120 816 e and depth of 7 5 km inicted severe re gional scale damage to taiwan the strong motion waveeld was captured by a dense network of stations with average station spacing of 5 km which represents the most complete strong motion dataset to date to use to study the kinematic source process of an earthquake we inverted velocity waveforms recorded by 21 stations for the spatial variation in slip on a planar fault model composed of 416 subfaults each with a dimension of 3 5 km by 3 5 km the planar model has a strike of n5 e and a dip of 30 e and the inversion solves for the direction and magnitude of the slip to account for possible temporal source complexity we allowed each subfault to slip within 10 overlapping time windows each with a duration of 3 sec the results show that the source is composed primarily of three major asperities the rst of which is mainly dip slip extending from the hypocenter to the northern end of the surface rupture in this asperity slip occurred in two pulses separated in time by 5 sec the dislocation rise time for each pulse is short 3n4 sec yielding an approximate av erage slip velocity of 80 cm sec the second asperity is located at shallow depth near the northern end of the rupture where very large ground velocities were observed this asperity is on average oblique and shows a temporal rake rotation from pure dip slip to strike slip the rotating rake suggests a low initial shear stress on the northern end of the fault slip in this asperity is dominated by a large pulse with a dislocation rise time of 8 sec a station near the northern end of the surface rupture recorded a peak velocity of 390 cm sec which we nd to be due to the constructive interference of energy radiated from the rst two asperities the third asperity is located south of the epicenter the total moment from the three asperities is 4 1 10 27 dyne cm which was released over a period of 30n35 sec within an area of 900 km 2 synthetics calculated from the three asperity model explain 85 of the data and represent 98 of the total variance reduction our results indicate that slip is conned to the shallow region of the fault deep slip patches are less constrained and that the slip distribution may be representative of fault segmentation along the chelungpu fault system

structural geology of a classic thrust belt earthquake the 1999 chi chi earthquake taiwan mw 7 6 abstract we document the structural context of the 1999 chi chi earthquake m w 7 6 in western taiwan which is one of the best instrumented thrust belt earthquakes the main surface break and large slip 3 10 m is on two segments of the shallow otherwise aseismic bedding parallel chelungpu sanyi thrust system which shows nearly classic ramp flat geometry with shallow detachments 1 6 km in the pliocene chinshui shale and mio pliocene kueichulin tungkeng formations however rupture is complex involving at least six faults including a previously unknown deeper thrust 8 10 km on which the rupture began we compare the coseismic displacements with a new 3d map of the chelungpu sanyi system the displacements are spatially and temporally heterogeneous and well correlated with discrete geometric segments of the 3d shape of the fault system geodetic displacement vectors are statistically parallel to the nearest adjacent fault segment and are parallel to large scale oblique fault corrugations the displacement magnitudes are heterogeneous at several scales which requires in the long term other non chi chi events or significant aseismic deformation the chelungpu thrust has a total displacement of 14 km but the area of largest chi chi slip 10 m is on a newly propagated north chelungpu chinshui detachment 0 3 km total slip which shows abnormally smooth rupture dynamics

kinematics of convergence deformation and stress distribution in the taiwan collision area 2 d finite element numerical modelling abstract using a 2 d plane stress finite element model with elastic and elasto plastic rheologies appropriate for deformation within the brittle upper crust we analyse the relationship between kinematics of convergence deformation and stress distribution in the present taiwan collision occurring between the ryukyu and luzon subduction zones the distribution of stress trends calculated in our models is compared with a synthetic map of actual stress trajectories based on the most recent data available in the collision zone these data combine present day sources from borehole breakouts and earthquake focal mechanism with the reconstruction of quaternary palaeostress from fault slip data analyses resulting in a complete map of compressional stress trajectories which is used to constrain our models we show that the distribution of stress trajectories in the active taiwan collision is principally controlled by 1 the geometric configuration of the boundary between eurasia and the philippine sea plate 2 the shape and rheological properties of major structural units 3 the direction of convergence of the philippine sea plate relative to eurasia and 4 the influence of the opening of the okinawa trough the study of a two dimensional elastic and elasto plastic finite element modelling of the subduction collision in and around taiwan allows us to estimate the influences of these different parameters in the stress pattern taking into account both the simplifying assumptions of the numerical modelling and the angular uncertainties of field determinations the fit between the calculated stress pattern of the finite element model and that determined based on the geometrical synthesis of field analyses is rather good in general indicating that our model is valid to first approximation misfits remain minor and can be explained by data uncertainties and simplifying modelling assumptions for instance the shape of the corner of the collision zone is critical but is not accurately known also limited decoupling in the longitudinal valley collision zone was not considered in our models although it certainly plays a role some interesting features of our model are 1 the greater influence of the shape of the collision zone in comparison with that of the direction of convergence 2 the requirement for a trench retreat related to suction force in the ryukyu arc and 3 the crucial role of the interaction between okinawa trough opening and collision at the sharp northwestern corner of the philippine sea plate including its influence on the geological evolution of northeastern taiwan

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

statistical analysis of earthquakes after the 1999 mw 7 7 chi chi taiwan earthquake based on a modified reasenberg jones model abstract we investigated the temporal spatial hazard of the earthquakes after the 1999 september 21 m w 7 7 chi chi shock in a continental region of taiwan the reasenberg jones rj model reasenberg and jones 1989 1994 that combines the frequency magnitude distribution gutenberg and richter 1944 and time decaying occurrence rate utsu et al 1995 is conventionally employed for assessing the earthquake hazard after a large shock however it is found that the b values in the frequency magnitude distribution of the earthquakes in the study region dramatically decreased from background values after the chi chi shock and then gradually increased up the observation of a time dependent frequency magnitude distribution motivated us to propose a modified rj model mrj to assess the earthquake hazard to see how the models perform on assessing short term earthquake hazard the rj and mrj models were separately used to sequentially forecast earthquakes in the study region to depict the potential rupture area for future earthquakes we further constructed relative hazard rh maps based on the two models the receiver operating characteristics roc curves swets 1988 finally demonstrated that the rh map based on the mrj model was in general superior to the one based on the original rj model for exploring the spatial hazard of earthquakes in a short time after the chi chi shock

response of the san andreas fault to the 1983 coalinga nunez earthquakes an application of interaction based probabilities for parkfield 1 the parkfield cholame section of the san andreas fault site of an unfulfilled earthquake forecast in 1985 is the best monitored section of the world s most closely watched fault in 1983 the m 6 5 coalinga and m 6 0 nunez events struck 25 km northeast of parkfield seismicity rates climbed for 18 months along the creeping section of the san andreas north of parkfield and dropped for 6 years along the locked section to the south right lateral creep also slowed or reversed from parkfield south here we calculate that the coalinga sequence increased the shear and coulomb stress on the creeping section causing the rate of small shocks to rise until the added stress was shed by additional slip however the 1983 events decreased the shear and coulomb stress on the parkfield segment causing surface creep and seismicity rates to drop we use these observations to cast the likelihood of a parkfield earthquake into an interaction based probability which includes both the renewal of stress following the 1966 parkfield earthquake and the stress transfer from the 1983 coalinga events we calculate that the 1983 shocks dropped the 10 year probability of a m 6 parkfield earthquake by 22 from 54 22 to 42 23 and that the probability did not recover until about 1991 when seismicity and creep resumed our analysis may thus explain why the parkfield earthquake did not strike in the 1980s but not why it was absent in the 1990s we calculate a 58 17 probability of a m 6 parkfield earthquake during 2001 2011 index terms 7223 seismology seismic hazard assessment and prediction 7230 seismology seismicity and seismotectonics 7260 seismology theory and modeling keywords coalinga earthquake parkfield stress change earthquake probability seismicity rate

postseismic relaxation driven by brittle creep a possible mechanism to reconcile geodetic measurements and the decay rate of aftershocks application to the chi chi earthquake taiwan we evaluate the effect of coseismic stress changes on the fault slip at midcrustal depth assuming a velocity strengthening brittle creep rheology we show that this model can help reconcile the time evolution of afterslip as measured from geodesy with aftershocks decay we propose an analytical expression for slip of the brittle creeping fault zone bcfz that applies to any dynamic or static stress perturbation including shear stress and normal stress changes the model predicts an initial logarithmic increase of slip with time postseismic slip rate decays over a characteristic time t r a that does not depend on the amplitude of the stress perturbation and it asymptotically joins the long term creep imposed by interseismic stress buildup given that the seismicity rate might be considered proportional to the sliding velocity of the bcfz the model predicts a decay rate of aftershocks that follows omori s law with a mathematical formalism identical to that of dieterich 1994 although based on a different mechanical rationale our model also differs from dieterich s model in that it requires that aftershock sequences and deep afterslip as constrained from geodetic measurements should follow the same temporal evolution we test this for the 1999 chi chi earthquake m w 7 6 and find that both sets of data are consistent with a model of afterslip due to the response of the bcfz the inferred relaxation time t r 8 5 years requires a value for a log v being the coefficient of friction in the range between 1 3 10 3 and 10 2

imaging the ramp decollement geometry of the chelungpu fault using coseismic gps displacements from the 1999 chi chi taiwan earthquake abstract we use coseismic gps data from the 1999 chi chi taiwan earthquake to estimate the subsurface shape of the chelungpu fault that ruptured during the earthquake studies prior to the earthquake suggest a ramp decollement geometry for the chelungpu fault yet many finite source inversions using gps and seismic data assume slip occurred on the down dip extension of the chelungpu ramp rather than on a sub horizontal decollement we test whether slip occurred on the decollement or the down dip extension of the ramp using well established methods of inverting gps data for geometry and slip on faults represented as elastic dislocations we find that a significant portion of the coseismic slip did indeed occur on a sub horizontal decollement located at 8 km depth the slip on the decollement contributes 21 of the total modeled moment release we estimate the fault geometry assuming several different models for the distribution of elastic properties in the earth homogeneous layered and layered with lateral material contrast across the fault it is shown however that heterogeneity has little influence on our estimated fault geometry we also investigate several competing interpretations of deformation within the e w trending rupture zone at the northern end of the 1999 ground ruptures we demonstrate that the gps data require a 22 to 35 km long lateral ramp at the northern end contradicting other investigations that propose deformation is concentrated within 10 km of the chelungpu fault lastly we propose a simple tectonic model for the development of the lateral ramp

the chi chi earthquake sequence active out of sequence thrust faulting in taiwan we combined precise focal depths and fault plane solutions of more than 40 events from the 20 september 1999 chi chi earthquake sequence with a synthesis of subsurface geology to show that the dominant structure for generating earthquakes in central taiwan is a moderately dipping 20 to 30 thrust fault away from the deformation front a second subparallel seismic zone lies about 15 kilometers below the main thrust these seismic zones differ from previous models indicating that both the basal decollement and relic normal faults are aseismic

analysis of the b values before and after the 23 october 2011 mw 7 2 van ercis turkey earthquake abstract the b values from the gutenberg richter frequency magnitude relation are investigated in the lake van region in eastern turkey where accommodated the 23 october 2011 m w 7 2 van ercis earthquake earthquake catalog is provided by the kandilli observatory and earthquake research institute koeri the catalog consists of 6709 well located events with local magnitudes m l from 2 6 to 5 8 the time span of the catalog covers the period 2002 and 2012 the b values were significantly lower at the van ercis mainshock area prior to 23 october 2011 the mainshock occurred near to the high stress low b value asperity in the east of lake van the decrease in b values supports low b value high stress hypothesis prior to the van ercis earthquake the temporal change of b value before the mainshock exhibits a strong trend of decreasing which could be used as an indicator of the approaching mainshock this b value analysis provides an evidence for significant difference that exist between pre seismic and post seismic periods of the mainshock area distribution of low b values for van ercis aftershock sequence relates the termination of the mainshock rupture to the east may be the precursory phenomena for the future moderate or large magnitude earthquakes in the lake van region

paleogeographic significance of clavohamulus hintzei miller conodonta and other ibexian conodonts in an early paleozoic carbonate platform facies of the argentine precordillera pre tremadocian conodonts and trilobites and tremadocian conodonts are reported from the cambrian and ordovician la silla formation in the cerro la silla section in east southeast jachal san juan province argentina a shallow marine conodont fauna contains elements of clavohamulus hintzei miller a common species in north america but reported for the first time from the early paleozoic platform carbonates of the western argentine precordillera the presence of this species suggests a correlation with the clavohamulus hintzei conodont subbiozone of the cordylodus intermedius conodont biozone in north america considered early ordovician skullrockian stage ibexian series in north america but by south american and european standards this biozone would be of latest cambrian age c hintzei and associated conodonts of the la silla formation are typical of the tropical faunas of the north american midcontinent faunal province late cambrian trilobites from lower in the formation also are typical north american taxa the presence of these faunas in the platform carbonates is consistent with plate reconstructions suggesting that the precordillera was in a tropical or subtropical position close to laurentia during the late precambrian and early paleozoic these new paleontological data provide one more argument for recent models of the precordillera as a displaced terrane derived from the ouachita embayment at the southern margin of laurentia

large and primarily updip afterslip following the 2012 mw 7 6 nicoya costa rica earthquake we present detailed surface measurements of the first 3 5 years of postseismic deformation following the 5 september 2012 moment magnitude mw 7 6 nicoya costa rica earthquake the dominant signal in the first 2 5 years is uniform horizontal trenchward motion totaling 7 26 cm across 40 stations trenchward velocity is strongly diminished by mid 2014 and appears by 2016 to have begun reversing we invert the first 2 5 years to determine the corresponding afterslip on a detailed 3d interface results show significant afterslip both up and down dip of the main coseismic rupture zone with as much as 1 7 m of offset in two patches at 15 20 km depth and immediately up dip of the maximum coseismic slip this updip slip represents an important mechanism to address unrelieved interseismic locking although sufficient strain energy remains to generate up to a mw 7 1 event near the coastline the afterslip patches are anticorrelated with strongly clustered aftershocks at the same depth which is indicative of varying frictional behavior along strike an additional patch of slip is colocated with reoccurring slow slip events beneath the gulf of nicoya the magnitude of the observed slip however cannot be sufficiently explained by the known slow slip events ongoing measurements will be crucial to understanding the relocking process in nicoya

response of seismicity to coulomb stress triggers and shadows of the 1999 mw 7 6 chi chi taiwan earthquake 1 the correlation between static coulomb stress increases and aftershocks has thus far provided the strongest evidence that stress changes promote seismicity a correlation that the chi chi earthquake well exhibits several studies have deepened the argument by resolving stress changes on aftershock focal mechanisms which removes the assumption that the aftershocks are optimally oriented for failure here one compares the percentage of planes on which failure is promoted after the main shock relative to the percentage beforehand for chi chi we find a 28 increase for thrust and an 18 increase for strike slip mechanisms commensurate with increases reported for other large main shocks however perhaps the chief criticism of static stress triggering is the difficulty in observing predicted seismicity rate decreases in the stress shadows or sites of coulomb stress decrease detection of sustained drops in seismicity rate demands a long catalog with a low magnitude of completeness and a high seismicity rate conditions that are met at chi chi we find four lobes with statistically significant seismicity rate declines of 40 90 for 50 months and they coincide with the stress shadows calculated for strike slip faults the dominant faulting mechanism the rate drops are evident in uniform cell calculations 100 month time series and by visual inspection of the m 3 seismicity an additional reason why detection of such declines has proven so rare emerges from this study there is a widespread increase in seismicity rate during the first 3 months after chi chi and perhaps many other main shocks that might be associated with a different mechanism

a physical model for strain accumulation in the san francisco bay region stress evolution since 1838 1 understanding of the behavior of plate boundary zones has progressed to the point where reasonably comprehensive physical models can predict their evolution the san andreas fault system in the san francisco bay region sfbr is dominated by a few major faults whose behavior over about one earthquake cycle is fairly well understood by combining the past history of large ruptures on sfbr faults with a recently proposed physical model of strain accumulation in the sfbr we derive the evolution of regional stress from 1838 until the present this effort depends on 1 an existing compilation of the source properties of historic and contemporary sfbr earthquakes based on documented shaking geodetic data and seismic data bakun 1999 and 2 a few key parameters of a simple regional viscoelastic coupling model constrained by recent gps data pollitz and nyst 2004 although uncertainties abound in the location magnitude and fault geometries of historic ruptures and the physical model relies on gross simplifications the resulting stress evolution model is sufficiently detailed to provide a useful window into the past stress history in the framework of coulomb failure stress we find that virtually all m 5 8 earthquakes prior to 1906 and m 5 5 earthquakes after 1906 are consistent with stress triggering from previous earthquakes these events systematically lie in zones of predicted stress concentration elevated 5 10 bars above the regional average the sfbr is predicted to have emerged from the 1906 shadow in about 1980 consistent with the acceleration in regional seismicity at that time the stress evolution model may be a reliable indicator of the most likely areas to experience m 5 5 shocks in the future index terms 1206 geodesy and gravity crustal movements interplate 8155 1236 geodesy and gravity rheology of the lithosphere and mantle 8160 1243 geodesy and gravity space geodetic surveys keywords crustal deformation plate boundary zones viscoelastic relaxation san francisco bay region

the upper ocean response to monsoonal forcing in the arabian sea seasonal and spatial variability abstract observations from four towed profiler surveys undertaken between december 1994 and october 1995 examine the seasonal and spatial variability of the upper ocean response to the monsoon cycle in the arabian sea although observed atmospheric forcing agrees well with modern climatologies cross basin patterns of mixed layer depth and water properties observed in 1994 1995 are not entirely consistent with an upper ocean response dominated by ekman pumping during the winter monsoon the mixed layer deepens dramatically with distance offshore surface cooling intensifies with offshore distance and a one dimensional response dominated by convective overturning could explain observed wintertime mixed layer depths except for waters associated with a filament extending offshore from the omani coast mixed layer depths and water properties show only modest cross basin contrasts during the southwest monsoon filament waters differ from surrounding mid basin waters having shallow mixed layers and water properties similar to those of waters upwelled near the omani coast in september following the southwest monsoon waters within 1000 km of the omani coast have cooled and freshened with marked changes in stratification extending well into the pycnocline estimates of ekman pumping and wind driven entrainment made using the southampton oceanographic center 1980 1995 surface flux and the levitus mixed layer climatologies indicate that during the southwest monsoon wind driven entrainment is considerably stronger than ekman pumping inshore of the windstress maximum ekman pumping partially counters wind driven entrainment while offshore the two processes act together to deepen the mixed layer as ekman pumping is too weak to counter wind driven mixed layer deepening inshore of the windstress maximum another mechanism must act to maintain the shallow mixed layers seen in our observations and in climatologies offshore advection of coastally upwelled water offers a mechanism for maintaining upper ocean stratification that is consistent with observed changes in upper ocean water properties ekman upwelling will modulate wind driven entrainment but these results indicate that the primary mechanisms acting inshore of the windstress maximum are wind driven mixing and horizontal advection

viscoelastic afterslip concurrence a possible mechanism in the early post seismic deformation of the mw 7 6 1999 chi chi taiwan earthquake summary observed coseismic data as well as 97 days of post seismic gps data for the chi chi earthquake are used as constraints in the modelling of crustal evolution using the 3 d finite element method first the coseismic gps data are used to justify the use of the elastic earth model and the source rupture model subsequently the most likely rheological model is determined by analysing several modelled time dependent displacements for various viscosity structures the range of viscosities of the lower crust in central taiwan is determined in advance from laboratory measurements and the long term strain rate the estimated viscosity of 5 0 10 17 p as seems to be very low and a relaxation time of 116 days seems very short but the latter approximates the gps measurement of 86 days since earlier studies have indicated that both the viscoelastic response model and the afterslip model may affect post seismic deformation we compare theoretical surface displacements for each of the two models that we evaluate the results reveal that there is little doubt that while neither of these models alone is able to predict the gps measurements well in a 97 day period the combination of the two models improves the predictions considerably we conclude that the afterslip mainly dominated chi chi post seismic deformation in the rupture area while the viscoelastic model did so elsewhere

stress evolution following the 1999 chi chi taiwan earthquake consequences for afterslip relaxation aftershocks and departures from omori decay summary we explore how coulomb stress transfer and viscoelastic relaxation control afterslip and aftershocks in a continental thrust fault system the 1999 september 21 m w 7 6 chi chi shock is typical of continental ramp dsystems throughout the world and so inferences drawn from this uniquely well recorded event may be widely applicable first we find that the spatial and depth distribution of aftershocks and their focal mechanisms are consistent with the calculated coulomb stress changes imparted by the coseismic rupture some 61 per cent of the m 2 aftershocks and 83 per cent of the m 4 aftershocks lie in regions for which the coulomb stress increased by 0 1 bars and there is a 11 12 per cent gain in the percentage of aftershocks nodal planes on which the shear stress increased over the pre chi chi control period second we find that afterslip occurred where the calculated coseismic stress increased on the fault ramp and dsubject to the condition that friction is high on the ramp and low on the d ecollement third viscoelastic relaxation is evident from the fit of the post seismic gps data on the footwall fourth we find that the rate of seismicity began to increase during the post seismic period in an annulus extending east of the main rupture the spatial extent of the seismicity annulus resembles the calculated 0 05 bar coulomb stress increase caused by viscoelastic relaxation and afterslip and we find a 9 12 per cent gain in the percentage of focal mechanisms with 0 01 bar shear stress increases imparted by the post seismic afterslip and relaxation in comparison to the control period thus we argue that post seismic stress changes can for the first time be shown to alter the production of aftershocks as judged by their rate spatial distribution and focal mechanisms

equivalent strike slip earthquake cycles in half space and lithosphere asthenosphere earth models by virtue of the images used in the dislocation solution the deformation at the free surface produced throughout the earthquake cycle by slippage on a long strike slip fault in an earth model consisting of an elastic plate lithosphere overlying a viscoelastic half space asthenosphere can be duplicated by prescribed slip on a vertical fault embedded in an elastic half space for the case in which each earthquake ruptures the entire lithosphere thickness h the half space equivalent slip rate is as follows depth interval 0 h slip identical to that in lithosphere asthenosphere model i e abrupt coseismic slip and no subsequent slip depth interval 2n 1 h to 2n 1 h n 1 2 slip rate uniform in space and dependent upon time as fn t exp t a where fn is a n 1 degree polynomial in t a is twice the asthenosphere relaxation time and t is measured from the instant after the preceding earthquake the slip rate averaged over the seismic cycle in each depth interval equals the secular rate of relative plate motion for reasonable values of a slip rates below 5h do not vary much from that mean value and can be treated as constant thus the surface deformation due to the earthquake cycle in the lithosphere asthenosphere model can be calculated very simply from the half space model with time dependent slip in the two depth intervals h 3h and 3h 5h and uniform slip at a rate equal to the secular relative plate velocity below depth 5h inversion of 1973 1988 geodetic measurements of deformation across the segment of the san andreas fault in the transverse ranges north of los angeles for the half space equivalent slip distribution suggests no significant slip on the fault above 30 km and a uniform slip rate of 36 mm yr below 30 km one equivalent lithosphere asthenosphere model would have a 30 km thick lithosphere and an asthenosphere relaxation time greater than 33 years but other models are possible

observed stress state for the iodp site c0002 and implication to the stress field of the nankai trough subduction zone abstract conducting a stress state analysis is one of major challenges for evaluating earth crust and formation conditions stress magnitude is especially essential to comprehend stress heterogeneity significant structural anisotropy and or pre existing fracture systems stress state is influenced by rock strength structural properties and near field principal stresses which impact borehole integrity during drilling factors that we can measure through logging while drilling lwd and measure while drilling mwd to better understand borehole conditions include mud weight overburden weight and velocity the lwd resistivity images in holes c0002a and c0002f of the nankai trough seismogenic zone experiment nantroseize indicate that most of the drillings in stable environments however the bottom hole assembly became stuck around 3 km depth in hole c0002p and did not reach the mega thrust fault we evaluated the stress state for the site c0002 by the geomechanical model with the breakout width and rock strength estimation we constrain the stress profiles in the drilled depth our analysis inferred that the instability of hole c0002p had been caused by an overpressure drilling fluid state and low horizontal principal stress furthermore we developed an optimally oriented plane oop model to exam the stress state in the site c0002 from nankai accretionary prism to the nankai trough subduction zone the normal faulting stress regime modeled from oop is consistent with the geomechanical model in site c0002 the horizontal principal stresses magnitude appears to less than the overburden weight above this subduction zone

source mechanisms and stress fields of the 15 16 june 2013 crete earthquake sequence along hellenic subduction zone 15 june 2013 mw 6 1 off shore southern crete earthquake and its aftershock sequence along hellenic subduction zone are examined centroid moment tensors cmts for 40 earthquakes with moment magnitudes mw between 3 5 and 6 1 are determined by applying a waveform inversion method the mainshock is shallow focus thrust event with a minor strike slip component at a depth of 20 km the seismic moment mo of the mainshock is estimated as 2 07 1018 nm and rupture duration of the mainshock is 4 s the focal mechanisms of aftershocks are mainly thrust faulting with a strike slip component the geometry of the moment tensors mw 3 5 reveals a thrust faulting regime with n s trending direction of p axis in the entire activated region according to high resolution cmt solutions of the off shore southern crete earthquake sequence one main cluster consisting of 40 events is revealed the aftershock activity in the observation period between 15 june and 15 july 2013 extends from n to s and nw to se directions seismic cross sections indicate a complex pattern of the hypocenter distribution with the activation of two segments the subduction interface is clearly revealed with high resolution hypocenter source relocation and moment tensor solution the best constrained focal depths indicate that the aftershock sequence is mainly confined in the upper plate depth 30 km and is ranging from about 5 28 km depth a stress tensor inversion of focal mechanism data is performed to obtain a more precise picture of the off shore southern crete stress field the stress tensor inversion results indicate a predominant thrust stress regime with a ne sw oriented maximum horizontal compressive stress sh according to variance of the stress tensor inversion to first order the southern crete region is characterized by a homogeneous interplate stress field we also investigate the coulomb stress change associated with the mainshock to evaluate any significant enhancement of stresses along southern crete and surrounding regions positive lobes with stress more than 0 3 bars are obtained for the mainshock indicating that these values are large enough to increase the coulomb stress failure toward wnw ese direction

stress triggering in thrust and subduction earthquakes and stress interaction between the southern san andreas and nearby thrust and strike slip faults 1 we argue that key features of thrust earthquake triggering inhibition and clustering can be explained by coulomb stress changes which we illustrate by a suite of representative models and by detailed examples whereas slip on surface cutting thrust faults drops the stress in most of the adjacent crust slip on blind thrust faults increases the stress on some nearby zones particularly above the source fault blind thrusts can thus trigger slip on secondary faults at shallow depth and typically produce broadly distributed aftershocks short thrust ruptures are particularly efficient at triggering earthquakes of similar size on adjacent thrust faults we calculate that during a progressive thrust sequence in central california the 1983 mw 6 7 coalinga earthquake brought the subsequent 1983 mw 6 0 nunez and 1985 mw 6 0 kettleman hills ruptures 10 bars and 1 bar closer to coulomb failure the idealized stress change calculations also reconcile the distribution of seismicity accompanying large subduction events in agreement with findings of prior investigations subduction zone ruptures are calculated to promote normal faulting events in the outer rise and to promote thrust faulting events on the periphery of the seismic rupture and its downdip extension these features are evident in aftershocks of the 1957 mw 9 1 aleutian and other large subduction earthquakes we further examine stress changes on the rupture surface imparted by the 1960 mw 9 5 and 1995 mw 8 1 chile earthquakes for which detailed slip models are available calculated coulomb stress increases of 2 20 bars correspond closely to sites of aftershocks and postseismic slip whereas aftershocks are absent where the stress drops by more than 10 bars we also argue that slip on major strike slip systems modulates the stress acting on nearby thrust and strike slip faults we calculate that the 1857 mw 7 9 fort tejon earthquake on the san andreas fault and subsequent interseismic slip brought the coalinga fault 1 bar closer to failure but inhibited failure elsewhere on the coast ranges thrust faults the 1857 earthquake also promoted failure on the white wolf reverse fault by 8 bars which ruptured in the 1952 mw 7 3 kern county shock but inhibited slip on the left lateral garlock fault which has not ruptured since 1857 we thus contend that stress transfer exerts a control on the seismicity of thrust faults across a broad spectrum of spatial and temporal scales

frictional afterslip following the 2005 nias simeulue earthquake sumatra continuously recording global positioning system stations near the 28 march 2005 rupture of the sunda megathrust moment magnitude m w 8 7 show that the earthquake triggered aseismic frictional afterslip on the subduction megathrust with a major fraction of this slip in the up dip direction from the main rupture eleven months after the main shock afterslip continues at rates several times the average interseismic rate resulting in deformation equivalent to at least a m w 8 2 earthquake in general along strike variations in frictional behavior appear to persist over multiple earthquake cycles aftershocks cluster along the boundary between the region of coseismic slip and the up dip creeping zone we observe that the cumulative number of aftershocks increases linearly with postseismic displacements this finding suggests that the temporal evolution of aftershocks is governed by afterslip

redox fluctuations in the early ordovician oceans an insight from chromium stable isotopes abstract chromium cr stable isotopes are a useful tracer of changes in redox conditions because changes in its oxidation state are accompanied by an isotopic fractionation recent co precipitation experiments have shown that cr vi is incorporated into the calcite lattice suggesting that carbonates are a potential sink for dissolved cr in the ocean and could effectively capture the cr isotope composition of ambient seawater thus the cr isotopic composition 53cr of carbonates has the potential to record changes in the oxygenation state of ancient oceans to test the potential of this system we measured 53cr values of limestones and dolostones from an early ordovician marine carbonate platform cerro la silla section of the precordillera san juan argentina an increasing number of studies support the hypothesis that the early ordovician oceans were stratified with respect to oxygen and that the global ocean experienced multiple episodes of anoxic expansion and contraction making this time period ideal for testing the usefulness of the cr isotope system the most prominent feature of the cerro la silla cr isotope profile is a positive excursion of 0 5 during the late tremadocian accompanied by exceptionally low cr concentrations

evolution of stress in southern california for the past 200 years from coseismic postseismic and interseismic stress changes summary the seismicity of southern california results from stresses that arise from the relative motion of the pacific and north american plates being accommodated along the san andreas fault saf system and the eastern california shear zone ecsz here we calculate how the stress field in southern california has evolved over the past two centuries due to interseismic loading as inferred from current gps observations of surface velocities from redistributions of static stress induced by large m w 6 5 earthquakes since the 1812 wrightwood quake and postseismic viscoelastic relaxation associated with these events that serves to transfer coseismic stresses from the deep warm lower crust and upper mantle to the overlying seismogenic upper crust we calculate coulomb stress changes on vertical strike slip faults striking parallel to the saf and at the hypocenters on the rupture planes of all m w 6 events over the past two centuries our results suggest that the 1857 m w 8 2 fort tejon earthquake by far the largest event to have occurred in the region over the past two centuries had a profound influence on the state of stress in southern california during the 19th century inducing significant stress increases to the north parkfield region and adjoining creeping saf and south southern saf and san jacinto fault and stress relief across the southern ecsz these stress changes were then greatly magnified by postseismic relaxation through the early part of the 20th century slow interseismic build up of stress further loads all major strike slip faults and works to reload the areas of the ecsz where stress was relieved by the 1857 quake our calculations suggest that only 56 of hypocenters were pushed closer to failure by preceding coseismic stress changes suggesting that the occurrence of large earthquakes is not strongly determined by coseismic coulomb stress changes this percentage rises to 70 when postseismic stress changes are also considered our calculations demonstrate the importance of postseismic viscoelastic relaxation in the redistribution of stress following large earthquakes we find however that postseismic processes associated with events more than about a decade old are near completion and thus do not significantly influence the regional velocity field presently observed in southern california

postseismic deformation following the 1999 chi chi earthquake taiwan implication for lower crust rheology on 1999 september 21 the mw 7 6 chi chi earthquake ruptured a segment of the chelungpu fault a frontal thrust fault of the western foothills of taiwan the stress perturbation induced by the rupture triggered a transient deformation across the island which was well recorded by a wide network of continuously operating gps stations the analysis of more than ten years of these data reveals a heterogeneous pattern of postseismic displacements with relaxation times varying by a factor of more than ten and large cumulative displacements at great distances in particular along the longitudinal valley in eastern taiwan where relaxation times are also longer we show that while afterslip is the dominant relaxation process in the epicentral area viscoelastic relaxation is needed to explain the pattern and time evolution of displacements at the larger scale we model the spatiotemporal behavior of the transient deformation as the result of afterslip on the decollement that extends downdip of the chelungpu thrust and viscoelastic flow in the lower crust and in the mid crust below the central range we construct a model of deformation driven by coseismic stress change where afterslip and viscoelastic flow are fully coupled the model is compatible with the shorter relaxation times observed in the near field which are due to continued fault slip and the longer characteristic relaxation times and the reversed polarity of vertical displacements observed east of the central range our preferred model shows a viscosity of 0 5 1 x 10 19 pa s at lower crustal depths and 5 x 10 17 pa s in the mid crust below the central range between 10 and 30 km depth the low viscosity zone at mid crustal depth below the central range coincides with a region of low seismicity where rapid advection of heat due to surface erosion coupled with underplating maintain high temperatures estimated to be between 300 c and 600 c from the modeling of thermo chronology and surface heat flow data

formation mechanisms of temperature inversions in the southeastern arabian sea numerical simulations using a high resolution 0 25 0 0 25 0 40 levels ogcm of the indian ocean are used to study the formation mechanisms of temperature inversions ti in the southeastern arabian sea seas during winter to the west and south of india haline stratification is a necessary pre requisite for ti formation off the southern tip of india low salinity water advected from the bay of bengal is cooled by strong winds and leads to ti formation in the southern part of the seas advection of this cooler low salinity water over warm salty seas water leads to the formation of ti and occur at a deeper depth sim 80 m due to the downwelling within the lakshadweep high in the northern part ti occur at a much shallower depth sim 20 m and are caused by shortwave radiation penetrating below the mixed layer of about 12 m

arabian sea and bay of bengal exchange of salt and tracers in an ocean model exchanges of mass and salt between the mixed layers in the arabian sea and the bay of bengal are examined in a model of the indian ocean using passive tracers as a tool to map the pathways inflow of high salinity water from the arabian sea into the bay of bengal is significant and occurs after the mature phase of the southwest monsoon freshwater transport out of the bay of bengal is southward throughout the year along the eastern boundary of the indian ocean low salinity water transport into the arabian sea occurs in the somali current during the southwest monsoon closing a clockwise path of water mass transport only a small fraction of low salinity water is advected into the eastern arabian sea from the bay of bengal

static stress transferred by the 1999 chi chi taiwan earthquake effects on the stability of the surrounding fault systems and aftershock triggering with a 3d fault slip model we investigated the effect of changes in static stress caused by the 1999 m l 7 3 chi chi earthquake on the surrounding fault systems in taiwan we examined this problem from two perspectives first we applied a statistical test for the hypothesis of static stress triggering by comparing the distributions of the coulomb failure stress changes on the focal mechanism nodal planes both before and after the mainshock since preshocks could obviously not have been affected by the mainshock the deviation of these distributions determined the significance of static stress triggering second we calculated the coulomb failure stress changes on the chukou and meishan faults in specified slip directions to evaluate the stability of these two active faults after the chi chi earthquake the static stress changes were obtained by applying the theory of dislocation in an elastic half space to a 3d rupture model derived from the surface displacements associated with the chi chi earthquake our results showed that the distributions of the coulomb failure stress changes for events preceding and following the chi chi earthquake were significantly different yet consistent with the hypothesis of static stress triggering we also found that both the chukou and the meishan faults were in stress shadows at depths shallower than 10 km however because a number of aftershocks fell in these stress shadows it is possible that other mechanisms such as dynamic stress were responsible for triggering these events manuscript received 6 november 2000

early palaeozoic brachiopods and associated shelly faunas from western gondwana their bearing on the geodynamic history of the pre andean margin

possible stress states adjacent to the rupture zone of the 1999 chi chi taiwan earthquake abstract we explore regional stress states in the vicinity of the rupture area of the 1999 chi chi taiwan earthquake by reconciling data from the taiwan chelungpu fault drilling project tcdp in situ measurements and earthquake focal mechanisms given the background deviatoric stress in the range of 10 50 mpa and the horizontal nw se directed maximum principal stress axis the predicted fault types show strike slip and normal faulting near the coseismic surface rupture and thrust and strike slip faulting in central taiwan such predictions are able to fit tcdp in situ observations in a local scale and aftershock earthquake focal mechanisms in a regional scale additionally the proposed stress state explains remarkable rotations of the maximum stress axes observed near the northern segment of the chelungpu fault this result provides key information for forecasting of consequent earthquakes and evaluation of focal mechanisms after the occurrence of a large earthquake

nonlinear strain buildup and the earthquake cycle on the san andreas fault two contrasting models of the earthquake deformation cycle on strike slip faults predict significant temporal declines in shear strain rate near the fault accompanied by a progressive broadening of the zone of deformation adjacent to it in the thin lithosphere model transient deformation results from flow in the asthenosphere due to stress relaxation following faulting through most or all of the lithosphere for an earth model with a thick elastic lithosphere plate thickness depth of seismic slip transient motions are due to postearthquake aseisrnic slip below the coseismic fault plane data from the san andreas fault indicate a long term temporal decrease in strain rate that persists for at least 30 years and may extend through the entire earthquake cycle observations support a cycle long rate decrease and a temporal spreading of the deformation profile only if movement cycles on the northern and southern locked sections of the fault are basically similar if so the usually lower strain rates and broader deformation zone currently observed on the southern san andreas represent a later evolutionary stage of the northern locked section where a great earthquake is a more recent occurrence although the data allow some extreme models to be discarded no sufficiently strong constraints exist to decide between the thin and thick lithosphere models regardless of the appropriate model the geodetic observations themselves indicate that strain buildup is sufficiently nonlinear to cause significant departures from recurrence estimates based on linear strain accumulation and the time predictable model

coulomb stress perturbation after great earthquakes in the sumatran subduction zone potential impacts in the surrounding region abstract the sumatran subduction zone has been seismically very active compared with other major subduction zones in the world since 2004 a sequence of large earthquakes ruptured along the trench including the 2004 mw 9 2 sumatra andaman the 2005 mw 8 6 nias simeulue the 2007 mw 8 4 bengkulu the 2010 mw 7 8 mentawai tsunami earthquakes and numerous moderate and small events these earthquakes released stress over tens of seconds to minutes disturbing the lithosphere and asthenosphere across a broad region as earthquake introduced stress disturbance can trigger tremendous aftershocks and sometimes precipitate large earthquakes in their vicinity gaining a detailed picture of spatial and temporal stress evolution on the fault and its neighboring faults is crucial for seismic forecasting here we have developed spatiotemporal coulomb stress models for the sumatran region which includes the sumatran subduction zone and the sumatran fault using a well studied postseismic model this postseismic model is constrained by decade long time series of geodetic observations from the sumatran gps array sugar and from tide gauge data in southeast asia we show that coulomb stress changes imparted by co and post seismic processes of previous great ruptures could well explain the temporal and spatial connection between seismicity and stress evolution most importantly our results reveal that the stress in the mentawai seismic gap of the sumatran subduction zone was loaded by more than two bars these stress perturbations could potentially trigger the rupture of the mentawai seismic gap which was already overdue in the last seismic cycle now the likelihood for the failure of this gap is even higher than before 2004 we also find that along the sumatran fault stress has increased to more than one bar which may explain the surrounding seismicity consequently we highlight the considerable seismic hazard and associated tsunami threat to the neighboring regions

a seismicity burst following the 2010 m 6 4 jiashian earthquake implications for short term seismic hazards in southern taiwan abstract following the 4 march 2010 mw 6 4 jiashian earthquake the seismicity rate in southern taiwan was determined to be significantly higher than before the quake and aroused seismic hazard awareness in this work seismic hazards were investigated in terms of earthquake activity the coulomb stress change the rate and state friction model and short term seismic hazard assessments the significantly higher seismicity rate that followed the 2010 jiashian earthquake was found to mainly be attributed to aftershock decay in terms of the modified omori formula the results suggest that aftershock duration may continue until the end of 2012 the spatial migration of seismicity was modeled using the coulomb stress changes of large earthquakes most of the consequent events were distributed in the vicinity of large earthquakes the observations corresponded to a remarkable stress increase within the same area additionally large events were located within regions with stress increases promoted by previous earthquakes the results confirm interactive relationships between large events by considering time dependency the seismicity rate evolution was estimated using the rate and state friction model the results indicated that a high seismic rate will persist at least until the end of 2012 short term probabilistic seismic hazard assessments were also applied in terms of the probability of strong ground motion using this application a sudden jump in seismic hazards in southern taiwan was accompanied by each large earthquake at the end of 2012 it is expected that hazards will return to a background level our results may be valuable in the future to decision makers and public officials engaged in seismic hazard mitigation

mapping the northern portion of the chelungpu fault taiwan by shallow reflection seismics 1 the chelungpu fault was activated by the 1999 chi chi earthquake mw 7 6 taiwan this fault exhibited extraordinarily large surface ruptures up to 9 8 m as well as underground fault slippages up to 12 m during the earthquake these large displacements were concentrated along the northern portion of the fault 40 km north of the epicenter to prepare data for the future drilling of deep wells in this area many shallow seismic reflection surveys were conducted to investigate the sites an approximate 3d structure of the fault surface can be deduced by this cost effective approach although the depth penetration may be limited e g 3 km the method still provides reliable information to study large ruptures and to better plan future deep wells

cumulative coulomb stress triggering as an explanation for the canterbury new zealand aftershock sequence initial conditions are everything using 2 years of aftershock data and three fault plane solutions for each of the initial m7 1 darfield earthquake and the larger m 6 aftershocks we conduct a detailed examination of coulomb stress transfer in the canterbury 2010 2011 earthquake sequence moment tensor solutions exist for 283 of the events with m ge 3 6 while 713 other events of m ge 3 6 have only hypocentre and magnitude information available we look at various methods for deciding between the two possible mechanisms for the 283 events with moment tensor solutions including conformation to observed surface faulting and maximum delta cff transfer from the darfield main shock for the remaining events imputation methods for the mechanism including nearest neighbour kernel smoothing and optimal plane methods are considered fault length width and depth are arrived at via a suite of scaling relations a large 50 70 proportion of the faults considered were calculated to have initial loading in excess of the final stress drop the majority of faults that accumulated positive delta cff during the sequence were encouraged by the main shock failure but on the other hand of the faults that failed during the sequence more than 50 of faults appeared to have accumulated a negative delta cff from all preceding failures during the sequence these results were qualitatively insensitive to any of the factors considered we conclude that there is much unknown about how coulomb stress triggering works in practice

transient rheology of the upper mantle beneath central alaska inferred from the crustal velocity field following the 2002 denali earthquake the m7 9 2002 denali earthquake alaska is one of the largest strike slip earthquakes ever recorded the postseismic gps velocity field around the 300 km long rupture is characterized by very rapid horizontal velocity up to 300 mm yr for the first 0 1 years and slower but still elevated horizontal velocity up to 100 mm yr for the succeeding 1 5 years i find that the spatial and temporal pattern of the displacement field may be explained by a transient mantle rheology representing the regional upper mantle as a burghers body 1 infer steady state and transient viscosities of 1 2 8 x 10 18 pa s and 2 1 0 x 10 17 pa s respectively corresponding to material relaxation times of 1 3 and 0 05 years the lower crustal viscosity is poorly constrained by the considered horizontal velocity field and the quoted mantle viscosities assume a steady state lower crust viscosity that is 7 1 systematic bias in predicted versus observed velocity vectors with respect to a fixed north america during the first 3 6 months following the earthquake is reduced when all velocity vectors are referred to a fixed site this suggests that the post denali gps time series for the first 1 63 years are shaped by a combination of a common mode noise source during the first 3 6 months plus viscoelastic relaxation controlled by a transient mantle rheology

joint inversion of coseismic and early postseismic slip to optimize the information content in geodetic data application to the 2009 m w 6 3 l aquila earthquake central italy when analyzing the rupture of a large earthquake geodetic data are often critical these data are generally characterized by either a good temporal or a good spatial resolution but rarely both as a consequence many studies analyze the coseismic rupture with data that also include one or more days of early postseismic deformation here we invert simultaneously for the coseismic and postseismic slip with the condition that the sum of the two models remains compatible with data covering the two slip episodes we validate the benefits of this approach with a toy model and an application to the 2009 m w 6 3 l aquila earthquake using a bayesian approach and accounting for epistemic uncertainties for the l aquila earthquake we find that if early postseismic deformation is not an explicitly acknowledged coseismic signal coseismic slip models may overestimate the peak amplitude while long term postseismic models may largely underestimate the total postseismic slip amplitude this example illustrates how the proposed approach could improve our comprehension of the seismic cycle fault frictional properties and the spatial and temporal relationship between seismic rupture afterslip and aftershocks

fault rupture process of the 20 september 1999 chi chi taiwan earthquake we modeled the source rupture process of the 20 september 1999 chi chi taiwan earthquake mw 7 6 using an integrated dataset of near field strong ground motion and global positioning system observations this large inland thrust earthquake produced a surface break more than 80 km long along the chelungpu fault the event triggered over 700 strong motion instruments on the island of tai wan this large ground motion dataset provides the best opportunity to date to un derstand earthquake source processes in a major earthquake first we develop a 3d curved fault model consistent with the surface rupture data and aftershock distribu tion the subsurface fault plane has an average dip of 30 to the east and measures 96 km along strike and 40 km down dip next we applied a genetic algorithm to reconstruct the source rupture process the inversion results indicate a complex slip distribution with a generally broad triangular shaped slip zone that points down dip most of the displacement occurred over a 15 km depth range in the northern segment of the fault with the maximum slip exceeding 20 m the average slip is 3 8 m and the average rupture velocity is 2 6 km sec the mainshock slip distribution clearly complements the aftershock location distribution slip rise times range from 8 to 6 sec over most of the primary slip area and decrease slightly as rupture propagates south to north this relatively long rise time resulted in a low dynamic stress drop and low peak ground accelerations observed at surface stations the rake angle rotates across the rupture plane from predominantly dip slip in the southern segment to mostly left lateral oblique slip in the north we find that geometrical irregularities over the fault plane played an important role in controlling the temporal rupture behavior at fault bends rupture tends to decelerate the rake angle rotates to a near dip slip orientation and the shear stress increases as rupture tries to overcome the bending barriers the total seismic moment from our inversion is 2 9 10 27 dyne cm in good agreement with the harvard and the u s geological survey moment estimation

evidence for the existence of persian gulf water and red sea water in the bay of bengal the high salinity water masses that originate in the north indian ocean are arabian sea high salinity water ashsw persian gulf water pgw and red sea water rsw among them only ashsw has been shown to exist in the bay of bengal we use ctd data from recent cruises to show that pgw and rsw also exist in the bay the presence of rsw is marked by a deviation of the salinity vertical profile from a fitted curve at depths ranging from 500 to 1000 m this deviation though small of the order of similar to 0 005 psu and therefore comparable to the ctd accuracy of 0 003 psu is an order of magnitude larger than the similar to 0 0003 psu fluctuations associated with the background turbulence or instrument noise in this depth regime allowing us to infer the existence of rsw throughout the bay pgw is marked by the presence of a salinity maximum at 200 450 m in the southwestern bay pgw can be distinguished from the salinity maximum due to ashsw because of the intervening arabian sea salinity minimum this salinity minimum and the maximum associated with ashsw disappear east and north of the south central bay 85a degrees e 8a degrees n owing to mixing between the fresher surface waters that are native to the bay bay of bengal water or bbw with the high salinity ashsw hence ashsw is not seen as a distinct water mass in the northern and eastern bay and the maximum salinity over most of the bay is associated with pgw the surface water over most of the bay is therefore a mixture of ashsw and the low salinity bbw as a corollary we can also infer that the weak oxygen peak seen within the oxygen minimum zone in the bay at a depth of 250 400 m is associated with pgw the hydrographic data also show that these three high salinity water masses are advected into the bay by the summer monsoon current which is seen to be a deep current extending to 1000 m these deep currents extend into the northern bay as well providing a mechanism for spreading ashsw pgw and rsw throughout the bay

preseismic deformation and coseismic displacements associated with the 1999 chi chi taiwan earthquake the destructive 1999 chi chi taiwan earthquake m w 7 5 produced an approximately 100 km long surface rupture mostly along the previously recognized north south trending chelungpu fault preseismic deformation in central taiwan is realized from annually repeated global positioning system gps data acquired during the 1992 1999 period the total wnw ese shortening rate in the vicinity of the epicentral region that is from the west coast to the western boundary of the central range is up to 25 mm yr the crustal deformation before the chi chi earthquake was essentially a uniaxial compressional strain of 0 36 strain yr in the direction of 114 the gps measurements taken 0 2 2 7 yr before and within 3 months after the mainshock were utilized to estimate the coseismic displacements horizontal movements of 1 1 9 1 m in the nw nnw directions are observed on the hanging wall eastern side of the fault there is a northward increasing trend in the magnitude of the displacement vectors and a dramatic change in the direction of about 50 toward the east along the fault strike in contrast much smaller se see movements of 0 1 1 5 m are found on the footwall western side of the fault the gps data show 2 4 10 1 m of total horizontal offsets across the chelungpu fault vertical offsets of 1 2 4 4 m with the eastern side up are also observed along the surface rupture the uplift on the hanging wall decreases rapidly toward the east it becomes subsidence at sun moon lake and in the puli wushe area the stations on the footwall show subsidence of 0 02 0 26 m the width of the uplift zone increases from about 10 km in the south to approximately 30 km in the north manuscript received 13 october 2000

correlation between surface damage and the coseismic displacement and stress relaxation of the 1999 chi chi taiwan earthquake various types of damage caused by the devastating 1999 chi chi taiwan earthquake were not only experienced in the hanging wall and footwall but also in the southern and northern parts of the chelungpu fault in this paper we determine the coseismic displacements and determine stress relaxation using a rupture model and compare the values with surface damage the patterns of damage can be clearly correlated with the calculated coseismic displacement and stress relaxation caused by the event it is found that the surface damage was predominately caused by coseismic deformation and that large scale landslides were mainly a result of steep coseismic displacements

cambrian ordovician conodonts from slump deposits of the argentine precordillera new insights into its passive margin development fil voldman gustavo gabriel consejo nacional de investigaciones cientificas y tecnicas centro cientifico tecnologico conicet cordoba centro de investigaciones en ciencias de la tierra universidad nacional de cordoba facultad de ciencias exactas fisicas y naturales centro de investigaciones en ciencias de la tierra argentina

a summer monsoon pump to keep the bay of bengal salty the bay of bengal receives a large influx of freshwater from precipitation and river discharge outflow of excess freshwater and inflow of saltier water is required to prevent the bay from freshening relatively fresh water flows out of the bay along its boundaries and inflow of saltier water occurs via the summer monsoon current smc which flows eastward from the arabian sea into the bay this saltier water however slides under the lighter surface water of the bay maintaining the salt balance of the bay therefore demands upward mixing of this saltier subsurface water here we show that an efficient mechanism for this mixing is provided by upward pumping of saltier water in several bursts during the summer monsoon along the meandering path of the smc advection by currents can then take this saltier water into the rest of the basin allowing the bay to stay salty despite a large net freshwater input

deep crustal deformation of the longmen shan eastern margin of the tibetan plateau from seismic reflection and finite element modeling rivaling the himalaya in relief the longmen shan is probably one of the most enigmatic mountain ranges in the world high mountains reach more than 4000 m relief but without adjacent foreland subsidence and with only slow active convergence what are geological and geodynamic processes that built the longmen shan coseismic deformation associated with the 2008 wenchuan earthquake could hold clues to answer these questions the primary features associated with the 2008 wenchuan earthquake rupture have been narrowly distributed coseismic deformation and predominantly vertical displacements that could be interpreted as the result of slips on high angle listric seismogenic faults deep sounding seismic reflection profiling across the seismogenic faults indeed reveals high angle listric reverse faulting in the brittle upper crust and east dipping reflectors that we interpret as ductile shearing in the viscous lower crust in conjunction with a visco elastic finite element modeling of coseismic displacements associated with the wenchuan earthquake we show that the high angle listric nature of earthquake faults produces insignificant horizontal shortening across the fault and facilitates upward slips along the fault that both explain the localized coseismic deformation and vertical displacement as well as the presence of high mountains without adjacent foreland flexure we suggest that the formation of the longmen shan may be better understood in terms of partitioned lithospheric pure shear thickening in which upward high angle listric faulting of brittle upper crust is linked to thickening of the more viscous lithospheric mantle through downward ductile shearing of rheologically deformable lower crust

coulomb stress change pattern and aftershock distributions associated with a blind low angle megathrust fault nepalese himalaya abstract on april 25th 2015 the magnitude mw 7 8 gorkha earthquake ruptured the main himalaya thrust mht in central nepal as a result of a thrust type event comprehensive evaluations of this event and its aftershocks have been undertaken to understand the locations and mechanisms of the aftershocks here we model co seismic displacement and strain caused by the gorkha earthquake using different slip models and then compare those results with the focal information we have the following conclusions 1 previous work suggests that there are two aftershock belts after the gorkha earthquake and we further suggest that the negative cfs lobe s beneath the maximum slip led to the seismic gap between the two seismic belts 2 aftershocks associated with the gorkha earthquake occurred not only along the mht fault plane but also 10 20 km deeper than the depth of the maximum slip zone triggered by the cfs increase this stress change pattern and distribution of aftershocks coincide with the recently proposed top down effect that the stress pulses produced by earthquake in upper crust can drive aftershocks in the lower crust 3 we suggest that the normal fault aftershock 40 km north of the maximum slip zone was triggered by the gorkha earthquake the cfs on the nodal plane strike 178 dip 52 rake 78 driven from different slip models all increase more than 1 bar 4 the aftershocks mechanisms are strongly related to the deformation field caused by the mainshock the displacement directions always nearly parallel to the maximum principal stress directions or the shear directions of the aftershocks focal mechanism solutions our studies give suitable explanations to the distribution of the aftershocks accompanied with the gorkha earthquake which is also of great significance for the analysis of aftershocks under the similar seismogenic environments

the role of salinity on the dynamics of the arabian sea mini warm pool warmer 28 c sea surface temperature sst occurs in the south eastern arabian sea seas 5 13 n 65 76 e during march april and is known as the arabian sea mini warm pool asmwp in this study we address the role of salinity and the upper layer heat and salt budgets in the formation and collapse of this asmwp an assessment of level 3 sea surface salinity sss data from the soil moisture and ocean salinity smos satellite mission for the year 2010 shows that smos is able to capture the sss variability in the seas analysis of temperature salinity and currents from the hybrid coordinate ocean model hycom during 2003 06 and in situ temperature and salinity data from argo floats during 2003 06 for the seas revealed that low salinity waters cap the top 60 m of the seas in january february this minimum salinity was concurrent with the formation of a barrier layer and with the time when the seas gained little net heat flux and the equatorward flowing east india coastal current eicc fed low saline waters into the seas subsequently the net heat flux increased to a peak value under the increased salinity stratification leading to the formation of the asmwp in march april the asmwp collapsed by may due to increase in sss and the associated weakening of the salinity stratification the monsoon onset vortex in may 2004 could be related to the minimum sss that occurred in february 2004 followed by higher sst and heat content of the asmwp in april 2004

observations of basin ground motions from a dense seismic array in san jose california we installed a dense array of 41 digital seismographs in san jose california to evaluate in detail the effects of a deep sedimentary basin and shallow sedimentary deposits on earthquake ground motions this urban array is located near the eastern edge of the santa clara valley and spans the evergreen sedimentary basin identified by gravity data average station spacing is 1 km with three stations initially spaced 110 m apart despite the high noise urban environment the stations of the array successfully triggered on and recorded small local earthquakes m 2 5 2 8 at 10 25 km distance and larger regional events such as the m 5 0 bolinas earthquake 90 km distance m 4 6 5 6 earthquakes near mammoth lakes 270 km distance m 4 9 5 6 events in western nevada 420 km distance and the m 7 1 hector mine earthquake 590 km distance maps of spectral ratios across the array show that the highest amplitudes in all frequency bands studied 0 125 8 hz are generally observed at stations farther from the eastern edge of the santa clara valley larger spectral amplitudes are often observed above the western edge of the evergreen basin snapshots of the recorded wavefield crossing the array for regional events to the east reveal that large low frequency 0 125 0 5 hz arrivals after the s wave travel from south to north across the array a moving window cross correlation analysis finds that these later arrivals are surface waves traveling from the south the timing and propagation direction of these arrivals indicates that they were likely produced by scattering of incident s waves at the border of the santa clara valley to the south of the array it is remarkable that the largest low frequency phases at many of the valley sites for regional events to the east are basin surface waves coming from a direction about 70 degrees different from that of the epicenters basin surface waves emanating from the eastern edge of the valley are also identified by the cross correlation analysis

thermal modeling of continental subduction and exhumation constrained by heat flow and seismicity in taiwan abstract the thermal evolution of crustal exhumation subsequent to the subduction of the continental margin is modeled through numerical solutions to the two dimensional heat conduction equation the boundary conditions used in the modeling are basically constrained by available geophysical and geological observations in the taiwan area temperature distributions are calculated at one million year intervals over a 5 ma duration although this study presents a tentative estimation based on some reasonable assumptions the preliminary results not only explain the sharp change in seismicity within the crust but also agree with the general heat flow pattern on the surface of taiwan more specifically both the high heat flow and the aseismic belt beneath the eastern central range which are probably attributed to higher temperatures within the crust are modeled by crustal exhumation after the subduction of the continental margin however the occurrence of deeper earthquakes east of the central range is a result of the lower thermal gradient which is due to the subduction of the cold continental crust into the uppermost mantle in summary preliminary thermal modeling supports the possibility of a tectonic model of active continental subduction and crustal exhumation for the taiwan orogen

global omori law decay of triggered earthquakes large aftershocks outside the classical aftershock zone 1 triggered earthquakes can be large damaging and lethal as evidenced by the1999 shocks in turkey and the 2001 earthquakes in el salvador in this study earthquakes with ms 7 0 from the harvard centroid moment tensor cmt catalog are modeled as dislocations to calculate shear stress changes on subsequent earthquake rupture planes near enough to be affected about 61 of earthquakes that occurred near defined as having shear stress change 0 01 mpa the ms 7 0 shocks are associated with calculated shear stress increases while 39 are associated with shear stress decreases if earthquakes associated with calculated shear stress increases are interpreted as triggered then such events make up at least 8 of the cmt catalog globally these triggered earthquakes obey an omori law rate decay that lasts between 7 11 years after the main shock earthquakes associated with calculated shear stress increases occur at higher rates than background up to 240 km away from the main shock centroid omori s law is one of the few time predictable patterns evident in the global occurrence of earthquakes if large triggered earthquakes habitually obey omori s law then their hazard can be more readily assessed the characteristic rate change with time and spatial distribution can be used to rapidly assess the likelihood of triggered earthquakes following events of ms 7 0 i show an example application to the m 7 7 13 january 2001 el salvador earthquake where use of global statistics appears to provide a better rapid hazard estimate than coulomb stress change calculations

enhanced stress and changes to regional seismicity due to the 2015 mw 7 8 gorkha nepal earthquake on the neighbouring segments of the main himalayan thrust abstract in this study we evaluate stress evolution and change in seismic hazard after the 2015 gorkha earthquake sequence we take a methodology usually used in areas with well established seismic monitoring and apply it to an area with a sparse dataset and a limited time observation window our goal is to validate this approach as a rapid response tool for seismic forecasting after large earthquakes we propose a long term seismic forecasting model of the main himalayan thrust using the historical earthquake catalogue and regional paleo seismicity through application of the rate and state friction model we evaluate short term rate evolution after the gorkha earthquake the long elapsed time since the last megathrust event and the mainshock coseismic stress increase on the main himalayan thrust suggest high seismic potential in the lalitpur and lamjung areas along the fault system we also calculate the stress change on optimally oriented planes in the region and model the regional seismicity rate using a smoothing kernel method and seismicity since 1921 the location of the consequent earthquakes coincides with areas of high background seismicity rate and areas where stress was enhanced by the mw 7 8 mainshock and mw 7 3 aftershock we model the change of seismic rate over time and project a fast decrease due to the short aftershock duration assumption we use

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

fault structural control on earthquake strong ground motions the 2008 wenchuan earthquake as an example continental thrust faulting earthquakes pose severe threats to megacities across the world recent events show the possible control of fault structures on strong ground motions the seismogenic structure of the 2008 wenchuan earthquake is associated with high angle listric reverse fault zones its peak ground accelerations pgas show a prominent feature of fault zone amplification the values within the 30 to 40 km wide fault zone block are significantly larger than those on both the hanging wall and the footwall the pga values attenuate asymmetrically they decay much more rapidly in the footwall than in the hanging wall the hanging wall effects can be seen on both the vertical and horizontal components of the pgas with the former significantly more prominent than the latter all these characteristics can be adequately interpreted by upward extrusion of the high angle listric reverse fault zone block through comparison with a low angle planar thrust fault associated with the 1999 chi chi earthquake we conclude that different fault structures might have controlled different patterns of strong ground motion which should be taken into account in seismic design and construction

slip history and dynamic implications of the 1999 chi chi taiwan earthquake we investigate the rupture process of the 1999 chi chi taiwan earthquake using extensive near source observations including three component velocity waveforms at 36 strong motion stations and 119 gps measurements a three plane fault geometry derived from our previous inversion using only static data ji et al 2001 is applied the slip amplitude rake angle rupture initiation time and risetime function are inverted simultaneously with a recently developed finite fault inverse method that combines a wavelet transform approach with a simulated annealing algorithm ji et al 2002b the inversion results are validated by the forward prediction of an independent data set the teleseismic p and sh ground velocities with notable agreement the results show that the total seismic moment release of this earthquake is 2 7 10 20 n m and that most of the slip occurred in a triangular shaped asperity involving two fault segments which is consistent with our previous static inversion the rupture front propagates with an average rupture velocity of 2 0 km s 1 and the average slip duration risetime is 7 2 s several interesting observations related to the temporal evolution of the chi chi earthquake are also investigated including 1 the strong effect of the sinuous fault plane of the chelungpu fault on spatial and temporal variations in slip history 2 the intersection of fault 1 and fault 2 not being a strong impediment to the rupture propagation and 3 the observation that the peak slip velocity near the surface is in general higher than on the deeper portion of the fault plane as predicted by dynamic modeling